SR07

This report highlights the scientific results of the first funding period addressing the following objectives: -

Characterization of the chemical and physical properties of methane hydrates Interaction of gas hydrates with the natural environment including seafloor stability and global climate Characterization of the unique biological communities dependent on methane hydrate occurrences Technologies for an improved survey of methane hydrates in both the laboratory and the field Technologies for the safe and commercial production of methane from hydrates

The papers published in this report offer a comprehensive insight into the present status of gas hydrate research in Germany and reflects the multidisciplinary approach of the programme.

Science Report GEOTECHNOLOGIEN

In Germany a National Gas Hydrate Programme has been initiated in 2001 as part of the R&D-Programme GEOTECHNOLOGIEN. Between 2001 and 2004, 15 joint projects have been funded with 15 Million Euros by the Federal Ministry of Education and Research. All projects were carried out in close cooperation with various national and international partners from academia and industry.

Gas Hydrates in the Geosystem (200-2004)

Gas Hydrates in the Geosystem

GEOTECHNOLOGIEN Science Report

Gas Hydrates in the Geosystem The German National Research Programme on Gas Hydrates

Report on the First Funding Period (2000 - 2004)

The GEOTECHNOLOGIES programme is financed by the Federal Ministry for Education and Research (BMBF) and the German Research Council (DFG)

No. 7

ISSN: 1619-7399

No. 7

GEOTECHNOLOGIEN Science Report

Gas Hydrates in the Geosystem The German National Research Programme on Gas Hydrates Results from the First Funding Period (2001 - 2004)

Number 1

No. 7

Impressum

Schriftleitung Dr. Ludwig Stroink © Koordinierungsbüro GEOTECHNOLOGIEN, Potsdam 2006 ISSN 1619-7399 The Editors and the Publisher can not be held responsible for the opinions expressed and the statements made in the articles published, such responsibility resting with the author.

Die Deutsche Bibliothek – CIP Einheitsaufnahme GEOTECHNOLOGIEN; Gas Hydrates in the Geosystem, The German National Research Programme on Gas Hydrates Report on the First Funding Period (2001 - 2004) – Potsdam: Koordinierungsbüro GEOTECHNOLOGIEN, 2006 (GEOTECHNOLOGIEN Science Report No. 7) ISSN 1619-7399 Bezug / Distribution Koordinierungsbüro GEOTECHNOLOGIEN Telegrafenberg 14473 Potsdam, Germany Fon +49 0331-620 14 800 Fax +49 0331-620 14 801 www.geotechnologien.de geotech@gfz-potsdam.de Bildnachweis Titel / Copyright Cover Picture: G. Bohrmann, Universität Bremen / RCOM Bremen (Februar 2006)

Preface

In Germany, the National Research Programme on Gas Hydrates »Gas Hyrates in the Geosystem« has been initiated in 2001 as part of the R&D-Programme GEOTECHNOLOGIEN. After a public call, more than 40 project proposals have been evaluated in an international two-step review procedure, involving 14 experts from five countries. Finally 15 joint projects were recommended and funded by the Federal Ministry of Education and Research (BMBF) with more than 15 Million Euro for a three year funding period (2001 – 2004). The research projects involving 15 institutional partners from academia and industry covered a balanced portfolio of laboratory and field studies, tool design and testing and computer model development.

Funding by the government and the private sectors has strongly accelerated the progress of gas hydrate research in Germany. Rapid progress has been made concerning an improved characterization of the chemical and physical properties of gas hydrates and how they interact with the unique biological communities dependent on hydrate occurrences. A scientific breakthrough was the successful characterisation of biogeochemical and microbial processes of methane turnover in hydrate-bearing sediments. Beside the scientific results, which received worldwide recognition, a number of novel technologies to improve the investigations of

hydrates in both the laboratory and the field have been developed. A new autoclave technology allows sampling and investigating gas hydrates under in-situ conditions: an important precondition to understand the formation and structure of marine gas hydrates. Lander systems as long term seafloor observatories were developed for a wide spectrum of oceanographic applications. New software programs for numerical simulation for controlled extraction of methane by thermal destabilisation of gas hydrates, could be a first step towards a safe production of gas from hydrates.

To maintain the momentum of gas hydrate research in Germany and to enlarge the existing scientific and technological Know-how, the international review committee recommended a second funding period. Recently four joint projects are funded by the Federal Ministry of Education and Research with 7.6 Million Euros.

Like in the first funding period all projects are carried out in close cooperation between various national and international partners. All who are interested in these activities – from Germany, Europe or overseas – are welcome to share their ideas and results.

Ludwig Stroink Detlev Leythaeuser

Table of Contents

Shallow Marine Gas Hydrates: Dynamics of a Sensitive Methane Reservoir (OMEGA) Bohrmann G., Abegg F., Amann H., Brückmann W., Drews M., Gust G., Hohnberg H.-J.,

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19

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39

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57

Treude T., Niemann H., Orcutt B., Joye S., Witte U., Jørgensen B. B., Boetius A.

Microbial Methane Turnover at Marine Methane Seeps (MUMM – SPI) . . . . . . . . . . . . 58

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63

Microsensor Measurements in Gas Hydrate Bearing Sediments (MUMM – SPII) De Beer D. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

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Biomarker Signatures of the Anaerobic Oxidation of Methane (MUMM – SPIII) Elvert M., Niemann N., Orcutt B., Jørgensen B.B. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

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Distribution and Diversity of Microorganisms in Gas Hydrate Bearing Sediments (MUMM – SP IV) Knittel K., Lösekann T., Amann R. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

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Physiology of Microorganisms in Gas Hydrate Bearing and other Methane-Rich Marine Sediments (MUMM – SP V) Krüger M., Nauhaus K., Meyerdierks A., Widdel F. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

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85

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97

Kipfstuhl J., Klaucke I., Rehder G., Suess E., Wallmann K., Weinrebe W.

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Long-term Observatory for the Study of Control Mechanisms for the Formation and Destabilisation of Gas Hydrates (LOTUS) Linke P., Abegg C., Eisenhauer A., Gubsch S., Gust G., Greinert J., Keir R., Liebetrau V., Luff R., Pfannkuche O., Sommer S., Spiess .V, Wallmann K.

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Gas Hydrates: Occurrence, Stability, Transformation, Dynamics, and Biology in the Black Sea (GHOSTDABS) Michaelis W., Seifert R., Blumenberg M., Pape T., Lüdmann T., Wong H.K., Konerding P., Zillmer M., Petersen J., Flüh E. ,Reitner J., Reimer A.

High resolution imaging and physical properties of hydrate and gas-bearing sediments within the INGGAS project Reston T.J., Bialas J., Breitzke M., Flueh E.R., Kläschen D., Klein G., Talukder A., Zillmer M.

. . . . . 86

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Table of Contents

An in-situ laboratory to study terrestrial, permafrost related gas hydrates (Mallik 2002) Weber M., Bauer K., Kulenkampff J., Henninges J., Huenges E.,

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98

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Gas hydrate induced submarine slides - An engineering geological approach Grupe B., Kreiter S., Feeser V., Hoffmann K., Becker H. J., Savidis S., Rackwitz F., Schupp J. . . . . 118

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Microstructure, thermodynamics, formation- and decompositionkinetics of gas hydrates Itoh H., Klapproth A., Goreshnik E., Techmer K., Kuhs W.F. . . . . . . . . . . . . . . . . . . . . . . . . 134

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New perspectives for the extraction of oceanic gas hydrates Schultz H.J., Deerberg G., Fahlenkamp H. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138

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Experimental determination of the petrophysical and thermodynamic properties of gas hydrates and hydrate bearing sediments Schicks J., Spangenberg E., Naumann R., Kulenkampff J., Erzinger J. . . . . . . . . . . . . . . . . . . 152

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GASHYDRATES – Paleoatmospheric archive Reconstruction of paleoclimatic changes in the source strength of potential methane sources using the methane isotopic signature in bubble enclosures in polar ice cores Fischer H. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166

- 169

Wiersberg T., Erzinger J., Löwner, R.

Gas Hydrates in Hemipelagic Sediments – CONGO Spieß V., Zühlsdorff L., Villinger H., Flueh E., Bialas J., Kasten S.,

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Techniques and Instruments for Gas Hydrates Exploration and Research (TIGER) Degenhardt A., Hanken T., Helmke J., Jaguttis J., Masson M., Poppen B. . . . . . . . . . . . . . . . 186

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Detailed seismic study of a gas hydrate deposit at the convergent continental margin off Costa Rica – DEGAS Müller C., Bönnemann C., Neben S. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198

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Schneider R., Bohrmann G., Sahling H.

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Shallow Marine Gas Hydrates: Dynamics of a Sensitive Methane Reservoir (OMEGA) Bohrmann G. (1), Abegg F. (1), Amann H. (2), Brückmann W. (3), Drews M. (3), Gust G. (4), Hohnberg H.-J. (1), Kipfstuhl J. (5), Klaucke I. (3), Rehder G. (3), Suess E. (3), Wallmann K. (3), Weinrebe W. (3) (1) RCOM Forschungszentrum Ozeanränder an der Universität Bremen, Klagenfurter Str., GEO-Gebäude, D-28359 Bremen. Email: gbohrmann@uni-bremen.de (2) Technische Universität Berlin; Maritime Technik, Müller-Breslau Str., D-10623 Berlin (3) Leibniz-Institut für Meereswissenschaften (IFM-GEOMAR), Wischhoftsr. 1-3, D-24148 Kiel (4) Technische Universität Hamburg-Harburg, Arbeitsbereich Meerestechnik 1, Lämmersieth 72, D-22305 Hamburg (5) Alfred-Wegener-Institut für Polar- und Meeresforschung, Bürgermeister-Smidt-Straße 20, D-27568 Bremerhaven

1. Introduction Methane as a greenhouse gas is twenty times more effective than CO2, but its concentration within the atmosphere is smaller. In contrast, methane generated by microbial decay and thermogenic breakdown of organic matter seems to be a large pool in geological reservoirs. Numerous features such as shallow gas accumulations, pockmarks, seeps, and mud volcanoes are present in a wide variety of oceanographic and geological environments. Such methane sources may provide positive and negative feedback to global warming and/or cooling and are therefore focal points of current research. Studying methane emission sites will elucidate how stable these reservoirs are and how the pathways to the atmosphere are working. Because of their high methane density, gas hydrates are of special interest when occurring close to the seafloor. Previous investigations have shown that such hydrates generate extremely high and variable fluxes of methane to the overlying water column due to their exposed position close to the sediment/ water interface. They do not only influence their immediate environment, but they may also contribute substantially to the transfer of methane to the atmosphere.

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2. Objectives The objective within the framework of the OMEGA project was to investigate near-surface methane and methane hydrates in the Black Sea, on Hydrate Ridge (Cascadia Margin) and the Gulf of Mexico in order to understand their origin, structure, and behavior as well as their interaction with the sedimentary and oceanic environment. This is crucial for evaluating and quantifying their importance in the global carbon cycle. Past studies on these known occurrences were limited because of the lack of appropriate pressurized sampling techniques.

Since gas hydrates react rapidly to changes in pressure and temperature, pressurized autoclave sampling technology as well as investigations and experiments under in situ conditions are essential. The technical development of these capabilities and their application to the improved understanding of gas hydrate dynamics was the main focus of our collaborative research project. This new autoclave technology has the following objectives: - To quantify gas, gas hydrate and pore water in the grain framework in sediment cores using autoclave sampling and detailed computer-based tomographic imaging. A 3dimensional density model will be developed to distinguish the components of the gas-hydrate/sediment/pore-fluid system.

- To study gas hydrate formation and dissociation by using chemical, physical and isotope data derived from the solid hydrate phase, the pore water and from the host sediment on samples acquired with the autoclave technology.

SP 3: Tracking mechanisms of gas hydrate formation and dissociation through chemical and isotopic studies on hydrates and associated fluids.

Prof. Dr. E. Suess, PD Dr. K. Wallmann, Dr. M. Drews (GEOMAR)

- To examine and quantify the areal extent of methane and methane hydrate deposits, as well as associated carbonates using a variety of mapping techniques and systems.

SP 4: Mapping and quantification of surface gas hydrates and related carbonates Dr. W. Weinrebe, Prof. Dr. G. Bohrmann, Dr. I. Klaucke (GEOMAR)

- To conduct controlled pressurized laboratory studies of methane-unsaturated fluids and methane fluxes at the sediment-water interface during gas hydrate decomposition.

Experiments will be run with pressurized gas hydrate-bearing sediment samples.

3. Scientific and technical background The OMEGA project was funded by the Federal Ministry of Education and Research (BMBF) in the frame of the special programme »GEOTECHHNOLOGIEN«. The proposal was submitted in response to the call for proposals on »Gas hydrates in the geosystem – a research strategy« by the Ministry of Education and Research of the Federal Republic of Germany.

The project was structured in 5 subprojects:

SP 5: Gas hydrate pressure laboratory Prof. Dr. G. Gust (TU-Hamburg-Harburg), Dr. G. Rehder (GEOMAR) The total running time of the OMEGA project was 39 month (Fig.1). Workshops, meetings and cruises have been conducted in close cooperation with other projects (MUMM, LOTUS, GHOSTDABS, INGGAS). Specific cooperation occurred in joint research cruises (Table 2).

Coordination: Prof. Dr. G. Bohrmann, S. Schenk SP 1: Autoclave sampling and in situ preservation system, ASAP Prof. Dr. H. Amann, Dipl.-Ing. H.-J. Hohnberg (TU Berlin) SP 2: Formation and quantification of gas hydrates: structural analyses of gas hydrates and host sediments Prof. Dr. G. Bohrmann, Dr. J. Kipfstuhl, Dr. W. Brueckmann, Dr. F. Abegg (GEOMAR)

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Figure 1: Time table of the project documenting various milestones, workshops and research cruises.

Table 1: Overview of research cruises

4. Results of the subprojects 4.1 Autoclave sampling and in situ preservation systems (SP1) Prerequisites for the investigation of gas hydrate containing gas rich sediments are sampling tools which preserve the in situ pressure and temperature. This is necessary to keep the hydrates within their in situ stability field and avoid sediment structure damage through generation and ebullition of free gas.

To fulfill these requirements two different tools have been developed. The first is the Multi Autoclave Corer (MAC) which is in function and size like a multiple corer and operated on the deep sea cable of the ship (Fig. 2). This allows the use of a video system for high precision deployments. In general the MAC consists of a frame which can hold up to four removable pressure vessels (Laboratory Transfer Chambers, LTC). Below the LTCs is the core cutting unit. When deployed on the seafloor the core cutting unit with the LTCs on top penetrates the seafloor, driven by gravity but

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damped in speed. Upon retrieval the cores, now in a core liner, are moved into the LTCs and sealed. After this process the whole device is recovered. The core length is limited to 550 mm with a core diameter of 100 mm. To preserve the temperature each of the LTCs is enclosed by a mantel tube. This tube contains seawater which prevents heating during retrieval, supported by the glass-fiber reinforced plastic material of the pressure chamber. Back on deck the mantel tube can be filled with ice to keep the cores cool. When completely recovered and disconnected from the deep sea cable the LTCs are dismantled and either directly investigated or stored in an appropriate cold storage. To preserve the pressure inside the LTCs over longer time or to counteract small leakage, each LTC is equipped with a pressure accumulator. Up to now the pressure vessel is designed for a pressure of 140 bar (1400 m water depth). Use of the MAC in greater depth is possible but the pressure will not be stable until 140 bar. This keeps gas hydrates within their stability filed but free gas bubbles inside the sediment would expand and dissolved gas

would exsolve. The weight of the MAC is 800 kg, the diameter of the frame is 2.55 m and the height is 3 m. The system has been tested by the German TĂœV and the safety certificate is on hand. Finally it is possible to connect the LTCs to the pressure laboratory (s. description below) and to transfer cores into that laboratory. The special design and choice of materials of the LTCs allows the MAC cores to be directly investigated with computed X-ray tomography (CT, see below).

The second autoclave tool developed here is the Dynamic Autoclave Piston Corer (DAPC).

This tool consists of one pressure chamber with attached core cutting-tube (Fig. 3).

Deployment and release is analogue to a conventional piston corer. The core cutting tube is specially designed to penetrate gas hydrate bearing sediments in a free fall mode. When the device is released by the trigger weight the pressure chamber and the core cutting tube with the liner penetrate and sample the sediment. Similar to the MAC, the first step of recovery is the transfer of the core liner into the pressure chamber by lifting the deep sea cable. The next step, before pulling the whole device out of the seafloor, is the sealing of the pressure chamber by closing a ball valve. Back on deck of the vessel, the core cutting unit is dismantled to reduce the size of the system for further work on the core. The DAPC also has a mantel tube. In order to increase the stability of the pressure chamber it is made out of steel.

Similar to the MAC, the mantel tube is filled with cold water or ice or a mixture of both.

This autoclave coring device is likewise equipped with a pressure accumulator using a larger volume than the MAC system. The maximum core length to be achieved with the DAPC is 230 cm with a core diameter of 84 mm. The safety approval by the German TĂœV is available. The certified pressure is also 140 bar, but as with the MAC, deployments in greater depth are possible with the already mentioned restraints. The weight of the DAPC amounts to 500 kg and the outer diameter is 40 cm. Due to the design and handling of the DAPC, a video controlled deployment is not possible.

Figure 2: Multi Autoclave Corer on deck of RV SONNE.

Figure 3: Dynamic Autoclave Pisten Corer (DAPC) during deployment from RV SONNE.

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4.2 Structure and quantification of gas hydrate and host sediment (SP2) The tools described in the previous chapter, especially the MAC, together with Computerized Tomography (CT) are forming a milestone in the investigation of natural shallow gas hydrate. For the first time it becomes possible to sample, recover and investigate sediments containing gas hydrate while in situ conditions of pressure and temperature are preserved.

This means that hydrate is kept in its stability field and free gas bubbles will keep their size.

One prerequisite was that the LTCs were designed to be translucent for X-ray (Fig. 4) beams.

This has been achieved with a combination of aluminum and glass fiber reinforced plastic.

One other aspect also was important: the weight of the LTC containing the core and some water had to be limited to less than about 200 kg. This allows using medical CT scanners, which are available in almost any hospital and can also be hired as a mobile system. After dismantling of the LTCs, one after the other is placed on the table of the CT scanner.

The MAC has been used during the RV SONNE cruises 165 and 174 (SO165, SO174) to Hydrate Ridge and the Gulf of Mexico, respec-

Figure 4: Image of a CT slice from an autoclave core.

White colors are dense material (carbonate), light grey indicates mud, dark grey represents gas hydrae and black displays free gas. It is easily to be recognized that the gas hydrate in this core contains a lot of free gas while still under pressure.

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tively. The cruise SO165 was the first deployment of the MAC system and after several improvements finally two pressurized cores were taken and shipped to port. These cores were CT-investigated four days after recovery in a clinic close to San Francisco. First thing done when a LTC is on the table of the CT is to scan an overview to determine the location of the core within the LTC and to control core length and quality. Based on this overview the core is virtually sliced up perpendicular to the core axis to generate a 3-dimensional dataset of the density variation of the material inside the liner. Slice thickness was set to 1 mm.

The two pressurized cores, taken with one deployment of the MAC system during the cruise SO165 to the southern summit of Hydrate Ridge, show distinct variations in either horizontal and vertical distribution of gas hydrate. In both cores the main compound of the sedimentary matrix consists of mud with several clasts of carbonate. One core (LTC 3) hardly reaches a maximum hydrate volume of 5 vol % and most of the hydrate is located in the uppermost 13 cm. In the other core (LTC 4) a distinct gas hydrate horizon at a depth ranging between 25 and 31 cm below seafloor was detected. The highest measured gas hydrate in one slice is 47 vol %. An important outcome of this study is the direct proof for free gas inside the gas hydrate layer of this core. The free gas volume reaches up to 2.4 vol % in one slice. Considering the depth interval from 25 to 31 cm of LTC 4 the gas hydrate volume amounts to 19 vol % and the free gas reaches 0.8 vol %.

The existence of free gas is a hint on its source to be in greater depth below the BSR and that free gas percolates through the subsurface and reaches the seafloor as observed by acoustic measurements and video observation (Heeschen, 2003) without being converted into gas hydrate.

This result is supported by investigations carried out in the Gulf of Mexico during RV SONNE cruise 174. At the site Green canyon Block 415 at the Louisiana Slope, free gas leaving the seafloor was also detected by video observation in a water depth of 1000 m. A MAC deployment (MAC 07) and the corresponding CT-analysis with a mobile CT scanner revealed free gas but no gas hydrate. Pore water analyses revealed a pore water salinity that is several times higher than normal seawater concentrations (Bohrmann & Schenk, 2004). Calculation of the thermodynamic balances shows that gas hydrate formation is inhibited at these high pore water salinities (Heeschen et al., subm.)

4.3 Tracking mechanism of gas hydrate formation and dissociation through chemical and isotopic studies (SP3) The samples for the geochemical analyses within the OMEGA research project have been collected during three cruses: the RV METEOR cruise 52/1 to the Black Sea, the already mentioned cruises SO165 to Hydrate Ridge and the SO174 to the Gulf of Mexico. During METEOR cruise M52/1 (MARGASCH, January 2002) mud volcanoes (MV) from the central part of the Black Sea and the Sorokin Trough were sampled and investigated, from which the Dvurechenskii MV was sampled in more detail (Bohrmann and Schenck 2002; Bohrmann et al., 2003; Blinova et al. 2003; Krastel et al.

2003; Aloisi et al. 2004a and 2004b) Mud of higher temperature (Fig. 5) and fluids enriched in chloride and other chemical constituents appear to ascend from deeper stratigraphic levels and most probably from the Late Oligocene to Miocene Maikopian Formation.

Figure 5: Mud volcanoes in the Black Sea. Shaded relief map of mud volcanoes in the Sorokin Trough, SE of the Crimean Peninsula. Most mud volcanoes lie on the crest of an ENE-WSW morphological ridge which is the bathymetric expression of diapiric ridges formed in the compressional regime between the Tetyaev and Shatskii Rises (Bohrmann et al., 2003).

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Figure 6: Location map of seafloor temperature measurements taken on the DMV with recorded mud temperatures (left). Down core temperature gradients from in-situ temperature measurements (TGC-2, -3, -5, -7, and -8 stations are from Dvurechenskii MV; Bohrmann et al. 2003).

Figure 7: Hydrate stability field calculated according to Sloan (1998) for pure methane and Black Sea water chlorinity of 355 mM (1) and pore water chlorinity of 900 mM in sediments from Dvurenchenskii MV (2). Temperature measurements were taken during the M52/1 cruise. Thickness of the hydrate stability zone at 2,000 m water depth in the Sorokin Trough as graphically inferred from the bottom water temperature of 9째C and a constant temperature gradient in sediments of 29째C km-1.

The findings document that Dvurechenskii mud volcano is presently active.

Although temperatures as high as 16.5째C are reached in sediments close to the surface, gas hydrates are within their stability field (Fig. 7) and their presence was proved by gas hydrate sampling. Sediments from Odessa MV, Yalta MV and an unnamed mud volcano in the Sorokin Trough also revealed gas hydrates. This was documented either by direct gas hydrate sampling or by gas hydrate proxies like cold temperatures measured on opened sediment cores, or by negative chloride anomalies in the pore water profiles. High methane concentrations in the sediments lead to a methane flux

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into the bottom water, which provokes anaerobic oxidation of methane (AOM). Evidence for AOM was shown by pore water data and carbonate precipitation.

Five gravity cores (TGC-2, -3, -5, -7 and -8) and three short cores from multicorers (MIC-3, MIC4 and MIC-5) were obtained from the summit of Dvurechenskii MV during the M52/1 cruise.

All cores are composed of very fluid, dark grey mud which contains mm to cm-sized rounded rock clasts. Most clasts are mudstones probably originating from the Maikopian formation.

Figure 8: Origin of gas hydrates and the Cl--δ18O signature of the Dvurechenskii fluids. (a) Comparison of Cl-δ18O data with the model closed system evolution of Cl-δ18O values during silicate alteration processes (path 1), smectite-illite transformation (path 2), gas hydrate formation ( path 3) and evaporation (path 4); (b) Model evolution of fluid Cl--δ18O values during silicate alteration (path 1) and smectite – illite transformation (path 2); (c) Model evolution of fluid Cl--δ18O values during silicate alteration (path 1), smectite – illite transformation (path 2) and gas hydrate formation (path 3). The starting fluid in all calculations is supposed to be similar to modern Black Sea water. The final fluid in simulations (b) and (c) is the Dvurechenskii fluid. BS – Black Sea water; SW – Seawater; DV – Dvurechenskii fluid (Aloisi et al., 2004a).

The fluids expelled from Dvurechenskii MV are particularly rich in dissolved Cl and Na. Several processes including seawater evaporation, gas hydrate formation, ash diagenesis, and dissolution of halite (NaCl) can result in the formation of hypersaline pore fluids. Three of these (evaporation, gas hydrate formation and ash diagenesis) enhance fluid salinity by consuming water, while only halite dissolution increases salinity by addition of dissolved ions. The later process is probably not responsible for the observed fluid chemistry because the Na/Cl ratio of the fluids is close to the seawater ratio but significantly smaller than unity. Because the water-consuming processes mentioned above produce changes in the oxygen stable isotope composition of water, a further discrimination can be made based on the δ18O of the expelled fluids. Aloisi et al. (2004a) have plotted the data from Dvurechenskii MV on a δ18O-Cl- diagram and compared them with calculated fluid δ18O-Cl- compositions produced by the above processes of water consumption, considering a starting fluid with δ18O and Clsimilar to present day Black Sea bottom waters (Fig. 8a). The evolution of fluid δ18O and Clwas modeled assuming a closed system Raleigh fractionation behavior. In addition, they plotted the δ18O-Cl- path of a fluid experiencing smectite-to-illite transformation, assuming that the interlayer water has a δ18O of 17 ‰ and applying a two end member mixing model. Amongst the processes that increase salinity evaporation leads to an increase in the δ18O of fluids while gas hydrate formation and ash alteration lead to a decrease in fluid δ18O. The smectite-to-illite transformation on the other hand, freshens pore water and results in an increase in δ18O.

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The Dvurechenskii δ18O-Cl- data do not plot on any one of the model curves, but lie between the evaporation and the ash diagenesis and gas hydrate formation curves (Fig. 8a). This suggests that a combination of processes results in a net increase in salinity and in δ18O of pore fluids, producing the δ18O-Cl- signature of the Dvurechenskii fluids. We have applied the closed system Raleigh fractionation model and the two end-member mixing model to reproduce two possible scenarios of Cl-- δ18O evolution, one which considers silicate alteration and smectite-to-illite transformation (Fig. 8b) and one which considers silicate alteration, smectite-to-illite transformation and gas hydrate formation (Fig. 8c). In the scenario of Fig. 8b only one possible Cl--δ18O path that joins modern Black Sea waters to the Dvurechenskii fluids exists for the chosen oxygen isotope fractionation factor and smectite oxygen isotope composition. In the scenario of Fig. 8c on the other hand, an infinite number of Cl--δ18O paths are possible, depending on the relative importance of the three considered processes. For clarity, only one of the many Cl--δ18O paths has been shown in Fig. 8c. It is not possible to discriminate between the two scenarios of Figs. 8b and 8c. Recent results from ODP Leg 204, however, show that an elevated pore water chlorinity is associated to rapidly forming gas hydrates, if methane is transported upwards as gas bubbles (Haeckel et al., 2004). Acoustic flares attributed to intense methane venting at Dvurechenskii MV has been observed recently during echo-sounder surveys, making the scenario of Fig. 8c likely.

4.4 Mapping and Quantification of surface gas hydrates and related carbonates (SP4) Besides small-scale quantification of gas hydrates by sampling at discrete sites, remote sensing technologies reveal the distribution of hydrates and other vent-indicators as vesicomyid clams or carbonates on a larger scale. A vital part of this approach within the OMEGA project is the deep-towed sidescan sonar system DTS-1. The system contains a dual-fre-

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quency sidescan sonar (chirp system with 75 or 410 kHz center frequencies) and a subbottom profiler (chirp system, 2-15 kHz). The 75 kHz allows for a 1500 m wide swath and is routinely processed with a 1 m pixel size. In this case objects that are several meters across can be detected. The 410 kHz sidescan sonar allows for about 300 m of coverage and data are processed with a 0.25 cm pixel size. 410 kHz sidescan sonar is difficult to handle, because it requires towing the instrument ideally 20 m above the seafloor with several km of cable behind the ship. Precise control of the navigation of the tow-fish is essential. Sidescan sonar systems register the backscattered acoustic signal of the seafloor.

The following factors contribute to the backscattering strength in decreasing order: the regional slope, the microroughness of the seafloor and the physical properties of the material on the seafloor. In areas of constant regional slope the microroughness of the seafloor becomes the dominant factor contributing to the backscattering strength. Microroughness is strongly related to the lithology and sidescan sonar consequently allows imaging fluid escape structures or gas content of the uppermost sediments, even and in particular if those structures that do not have a bathymetric expression.

All geoacoustic data such as multibeam bathymetry and in particular sidescan sonar require ground-truthing in order to achieve meaningful geologic interpretation. Ground-truthing has been carried out in part through TV-grabs and coring but mainly through towed video mapping using the Ocean Floor Observation System (OFOS). In general, methane seeps can be readily detected by the occurrence of authigenic carbonates or chemosynthetic communities on the seafloor The DTS-1 has been utilized during the RV SONNE cruise 165 on Hydrate Ridge/Oregon and three different facies could be identified in this region. The first facies consists of an association of gas hydrates and carbonate crusts

Figure 9: Illustration showing the "zoom-in" approach using multibeam bathymetry, sidescan sonar and video observation. For example, at the northern summit of Hydrate Ridge large authigenic carbonates (chemoherms) cause typical backscatter signals. By combining the sidescan sonar image with the visual observations a geological interpretation of the area can be derived.

(Bohrmann et al., 2002). The second facies is build up of chemoherms (large blocks of carbonate, Fig. 9) and the third is identified through pockmarks. The hydrate-carbonate association is mostly present on top of summits of Hydrate ridge, which is also confirmed by the OFOS records. The hydrate-carbonate associations are flanked by the blocks of carbonate. Most probably they are generated from strong outflow of methane-containing fluids (Johnson et al., 2003). The pockmarks concentrate on the southern flank of Hydrate Ridge and have not been observed previously.

Based on this facies differentiation and their regional distribution as well as the quantitative analyses of the methane fluxes the first time a regional calculation of the methane flux rate is possible (Klaucke et al., subm.).

During the RV METEOR cruise 52/1 in the Black Sea different acoustic facies have been recorded. Due to closing of the Thetys many mud volcanoes have been formed. High backscatter signatures at the flanks of the mud volcanoes are not only caused from the relief but also are caused by mud flows and carbonates, formed at the top of the mud volcanoes. The mudflows are connected to the methane seeps (Bohrmann et al., 2003).

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4.5 Design of a mobile pressure laboratory (SP5) One of the main tasks of the pressure laboratory was to allow the transfer of cores, containing sediment samples with gas hydrate taken with the MAC, without pressure loss. Based on this demand many design-engineering basic components had to be considered.

The pressure laboratory consists of several different parts. The main part is the pressure vessel which holds the sample material and is the place to conduct experiments. Connected to the pressure vessel is a tank with a volume of 40 l. The water inside this tank can be mixed with substances, for instance with methane, and can be fed into the pressure vessel.

Additionally there is a cylinder of water which allows a singular exchange of the fluids inside the pressure vessel. Together with a temperature control this combination allows adjustment of different chemical and thermodynamical settings.

Transfer of sediment cores taken with the MAC is possible through a pressurized lock.

The lock is mounted on top of the pressure vessel and the LTC is mounted on top of the lock. When the pressure is equilibrated, the LTC is opened mechanically with an axle-drive shaft and the core will move into the pressure vessel. After the core transfer the pressure vessel is closed and LTC and lock are dismounted.

The whole pressure laboratory has been developed for the use at sea. It is mounted in a 20-

feet container. Because of the height of the laboratory including the base plate it is a High Cube container. The roof of the container can partially be opened for mounting a crane. The crane is used to safely handle all heavy parts of the pressure laboratory. Last but not least the container is equipped with air conditioning to allow work also in low latitudes.

The whole pressure laboratory is certified by the German TÜV and the container has been approved for sea transportation according to the regulations of the ‘Germanischer Lloyd’.

5. Conclusions Multidisciplinary research within the framework of the OMEGA project lead to many results in near-surface gas hydrate deposits.

Major highlights are: - Construction and operation of two autoclave systems that sampled gas hydrates under in-situ pressure.

- The co-existence of free-gas and gas hydrates have been documented in autoclave cores taken on Hydrate Ridge. Gas bubbles are surrounded by gas hydrate, which seem to be encapsulated. By this formation the hydrate separates the pore water from the free gas phase and explains how free gas can stay in the gas hydrate stability zone.

- A deep-tow sidescan sonar system operating in two frequencies (75 kHZ and 410 kHz was purchased, installed with other tools and successfully used in hydrate-hosting areas such as Hydrate Ridge, the Black Sea and along the Pacific Continental Margin.

Table 2: Technical data of the pressure laboratory.

Operating pressure Temperature Volume Inner diamter Free inner height Main hole diameter Interconnections to pressure vessel Weight

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0-55 Mpa -2°C bis 30°C 99 l 300 mm 1400 mm 110 mm Hydraulics, electrical, mechanically (axle drive shaft), video, view glasses 1500 kg

-

-

-

-

-

-

-

Deeply altered fluids have been collected and investigated from mud volcanoes in the Sorokin Trough of the Black Sea and have been compared to model data in order to interpret the origin of the fluids.

The presence of gas hydrate has been documented from various mud volcanoes in the Sorokin Trough of the Black Sea. In a mud flow on Odessa mud volcano a carbonate crust currently forms in association with anaerobic methane oxidation by microbial colonies.

At Dvurechenskii mud volcano in the Black Sea high sediment temperatures of up to 16.5°C in close contact to the ambient bottom water of 9°C suggest strong mud volcanic activity.

In seismic records over the mud volcanoes bottom simulating reflectors are not present, but pronounced lateral amplitude variations and bright spots in the approximate depth of the gas hydrate stability zone may indicate the occurrence of gas hydrates and free gas.

Hydrocarbon gases were determined in sediments from mud volcanoes in the Sorokin Through. In comparison to a reference station outside the mud volcano area, the deposits are characterized by an enrichment of high-molecular hydrocarbons (C2 – C4), an absence of unsaturated homologues, a predominance of isobutane in comparison with n-butane and the presence of gas hydrate.

The pressure laboratory has been successfully developed and the transfer from sediment cores to the laboratory was realized trough a pressure lock.

Acknowledgements The project and the cruises were financed by the German Federal Ministry of Education and Science (Bundesministerium für Bildung und Forschung; grants 03G0165A; 03G0174A; 03G0566A) and the German Research Foundation (Deutsche Forschungsgemeinschaft; grant Su 114/11-1). Special thanks go to our technicians and engineers.

References Abegg F, Freitag J and Bohrmann G (submitted) Free gas within the gas hydrate stability zone of Hydrate Ridge – signs of water exhaustion due to rapid hydrate formation.

Aloisi G, Drews M, Wallmann K, and Bohrmann G (2004a) Fluid expulsion from Dvurechenskii mud volcano (Black Sea) Part I: fluid sources and relevance to Li, B, Sr and dissolved inorganic nitrogen cycles. Earth and Planetary Science Letters, 225: 347-363. GEOTECH-72 Aloisi G, Wallmann K, Drews M, and Bohrmann G (2004b) Evidence for the submarine weathering of silicate minerals in Black Sea sediments: possible implications for the marine Li and B cycles. Geochemistry, Geophysics, Geosystem 5 (1) doi: 10.1029/ 2003GC000639.

GEOTECH-71 Blinova V, Ivanov M, Bohrmann G (2003) Hydrocarbon gases in deposits from mud volcanoes in the Sorokin Trough, north-eastern Black Sea. Geo-Marine Letters 23 (3-4) 250257. GEOTECH-26 Boetius A, Suess E (2004) Hydrate Ridge: a natural laboratory for the study of microbial life fueled by methane from near-surface gas hydrates. Chemical Geology 205: 291-310.

GEOTECH-28 Bohrmann G and Torres M (in press) Gas hydrates in marine sediments. In Schulz HD and Zabel M., Marine Geochemistry. 481- 512, Springer. GEOTECH-194 Bohrmann G, (2005) Gashydrate der Ozeane – feste Verbindungen aus Gas und Wasser. PdNChemie, 54 (6): 2-8.

Bohrmann G, Schenck S (2004) GEOMAR Cruise Report SO 174, OTEGA II, RV »SONNE«.

GEOMAR Report 117, Kiel.

Bohrmann G, Suess E (2004) Gashydrate der Meeresböden: Ein dynamischer Methanspeicher im Ozean. AKE-Tagungsband: 16 Seiten.

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Bohrmann G, Tréhu A, Rack F, Torres M and ODP Leg 204 Shipboard Scientific Party (2003).

Drilling gas hydrates on Hydrate Ridge, Cascadia Continental Margin. Energy, exploration and exploitation 21 (4): 333-334

Greinert J, Bohrmann G and Suess E (2001) Gas hydrate-associated carbonates and methan-venting at Hydrate Ridge (Cascadia): Their classification, distribution and origin. In: Paull C (Editor), AGU Monograph 124, 99-113.

Bohrmann G, Ivanov M, Foucher JP, Spiess V, Bialas J, Weinrebe W, Abegg F, Aloisi G, Artemov Y, Blinova V, Drews M, Greinert J, Heidersdorf F, Krastel S, Krabbenhöft A, Polikarpov I, Saburova M, Schmale O,Seifert R, Volkonskaya A, Zillmer, M (2003) Mud volcanoes and gas hydrates in the Black Sea – new data from Dvurechenskii and Odessa mud volcanoes. Geo-Marine Letters 23 (3-4) 239-249.

GEOTECH-25

Gutt C, Press W, Bohrmann G, Greinert J, and Hüller A, 2001, Brennendes Eis: Methanhydrat - Energiequelle der Zukunft oder Gefahr fürs Klima: Physikalische Blätter, v. 59: p. 1-6.

Bohrmann, G., Suess, E., Greinert, J., Teichert, B., and Naehr, T., 2002. Gas hydrate carbonates from Hydrate Ridge, Cascadia Convergent Margin: indicators of near-seafloor clathrate deposits. Fourth Int. Conf. Gas Hydrates, Yokohama, Japan: 102–107.

Haeckel, M., Suess, E., Wallmann, K., and Rickert, D. (2004) Rising methane gas bubbles form massive hydrate layers at the seafloor.

Geochimica et Cosmochimica Acta 68 (21), 4335-4345.

Bohrmann, G., Suess, E., Greinert, J., Teichert, B. Nähr, T., 2002. Gas Hydrate Carbonates from Hydrate Ridge, Cascadia Convergent Margin.

Indicators of near-seafloor clathrate deposits.

Proceedings of the Fourth International Conference on Gas Hydrates, Yokohama: 18-21.

Bohrmann G, Schenck S (2002) GEOMAR Cruise Report M52/1, MARGASCH, RV »Meteor«, Marine Gas Hydrates of the Black Sea. GEOMAR Report 108, Kiel.

Bohrmann G, Greinert J, and Suess E, 2001, Methanhydrate, Enzyklopädie Naturwissenschaften und Technik, 7. Erg.-Lfg.10/01, p. 1-8.

Drews M., Wallmann K., Aloisi G., and Bohrmann G. (submitted) Fluid expulsion from the Dvurechenskii mud volcano (Black Sea).

Part II: Methane fluxes and their relevance to the Black Sea methane cycle. Earth and Planetary Science Letters.

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Hovland, M., MacDonald, I.R., Rueslåtten, H., Johnsen, H.K., Naehr, T., and Bohrmann, G., 2005, Chapopote asphalt volcano, may have been generated by supercritical water. EOS 86 (42): 397, 402 GEOTECH-182

Heeschen KU, Tréhu AM, Collier RW, Suess E, Rehder G (2003) Distribution and height of methane bubble plumes on the Cascadia Margin offshore Oregon from acoustic imaging.

Geophysical Research Letters, 30(12), 1643,doi: 10.1029/2003GL016974.

Heeschen KU, RW. Collier, MA de Angelis, E Suess, G Rehder, P Linke and Klinkhammer GP (2005) Methane sources, distributions, and fluxes from cold vent sites at Hydrate Ridge, Cascadia Margin. Global Biogeochemical Cycles 19, GB2016, doi:10.1029/2004GB00 2266 GEOTECH-40 Heeschen KU, Hohnberg J, Drews M, Abegg F, and Bohrmann G (submitted) In-situ hydrocarbon inventory from pressurized cores in surface sediments, Northern Gulf of Mexico.

Marine Geology.

Johnson JE, Goldfinger C, Suess E (2003) Geophysical constraints on the surface distribution of authigenic carbonates across the Hydrate Ridge region, Cascadia Margin.

Marine Geology 202: 79-120.

Luff R, Greinert J, Wallmann K, Klaucke I and Suess E (2005) Simulation of long-term feedbacks from authigenic carbonate crust formation at cold vent sites. Chemical Geology 216, 157-174. GEOTECH-99

Klaucke I, Bohrmann G, Weinrebe W (submitted) Estimation of regional methane efflux on Hydrate Ridge, Oregon. Geochemistry, Geophysics, Geosystems.

MacDonald IR, Bohrmann G, Escobar E, Abegg F, Blanchon P, Blinova V, Brückmann W, Drews M, Eisenhauer A, Han X, Heeschen K, Meier F, Mortera C, Naehr T, Orcutt B, Bernard B, Brooks J, de Farágo M, (2004) Ashalt volcanism and chemosynthetic life in the Campeche Knolls.

Science, 304, p.999-1002. GEOTECH-62

Klaucke I, Sahling H, Bürk D, Weinrebe W, Bohrmann G (2005) Mapping deep-water gas emissions with high-resolution sidscan sonar. EOS, 86 (38): 341, 346. GEOTECH-189 Krastel S, Spiess V, Ivanov M, Weinrebe W, Bohrmann G, Shaskin P (2003) Acoustic images of mud volcanoes in the Sorokin Trough. GeoMarine Letters 23 (3-4) 230-238. GEOTECH-30 Kuhs, WF, Genov GY, Goreshnik E, Zeller A, Techmer K, Bohrmann G (2004), The impact of porous microstructures of gas hydrates on their macroscopic properties. Proceedings of the Fourteenth International Offshore and Polar Engineering Conference Toulon, France, May 23-28, 2004, 31-35. GEOTECH-100 Luff R. and Wallmann K. (2003) Fluid flow, methane fluxes, carbonate precipitation and biogeochemical turnover in gas hydrate-bearing sediments at Hydrate Ridge, Cascadia Margin: Numerical modeling and mass balances. Geochimica et Cosmochimica Acta 67(18), 3403-3421. GEOTECH-8 Luff R., Wallmann K., and Aloisi G. (in press) Physical and biogeochemical constraints on carbonate crust formation at cold vent sites: significance for fluid flow and methane budgets and chemosynthetic biological communities. Earth and Planetary Science Letters. GEOTECH-32

Pfannkuche O und Fahrtteilnehmer (2001) Cruise report ALKOR No. 192: Test of novel instrumentation for gas hydrate research at methane seeps in the Skagerrak online: http:// www.geomar.de /~jgreiner/web_LOTUS/ index.htm.

Pfannkuche O, Eisenhauer A, Linke P, Utecht C (2003) RV SO165 cruise report OTEGA-I, GEOMAR Report 112, GEOMAR Forschungszentrum.

Rehder G, Kirby S, Durham B, Stern L, Peltzer ET, Pinkston J, Brewer PG, (2003) Dissolution rates of pure methane and hydrate and carbon-dioxide hydrate in undersaturated seawater at 1000-m depth. Geochimica Cosmochimica Acta, 68 (2): 285-292.

Reed A, Abegg F, Grader A, Winters W, (2004) Gas Hydrates in Detail. NETL Newsletter Fire in the Ice, Spring 2004, 6-9.

Suess E, Torres M, Bohrmann G, Collier RW, Rickert D, Goldfinger C, Linke P, Heuser A, Sahling H, Jung C, Nakamura K, Greinert J, Pfannkuche O, Trehu A, Klinkhammer G, Whiticar MJ, Eisenhauer A, Teichert B, Elvert M, (2001) Dynamics of sea floor hydrate at Hydrate Ridge. In: Paull C (Editor), AGU Monograph 124, 87-98.

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Shoji H., Hachikubo A., Miyamoto A., Hyakutake K., Abe K., Bohrmann G., Kipfstuhl S., (2002) Construction of a Pressure Cell for Visual Observations on formation Processes of Globular Gas Hydrate. Proceedings of the Fourth International Conference on Gas Hydrates, Yokohama: 320-323.

Suess E., Bohrmann G., Rickert D., Kuhs W., Torres M., Trehu A., Linke P., (2002) Physical Properties and Fabric of Near-surface Methane Hydrates at Hydrate Ridge, Cascadia Margin. Proceedings of the Fourth International Conference on Gas Hydrates, Yokohama: 185-188.

Shipboard Scientific Party, 2002. Leg 204 Preliminary Report. ODP Prelim. Rpt. (Online).

Available from World Wise Web: http:// w w w - o d p . t a m u . e d u / p u b l i c a t i o n s / p re lim/204_prel/204PREL.PDF Tishchenko P., Hensen C., Wallmann K. Wong C.S. (2005) Calculation of the stability and solubility of methane hydrate in seawater.

Chemical Geology. 219: 37-52.

Torres M.E., Wallmann K., Tréhu A.M., Bohrmann G., Borowski W.S., Tomaru H. (2004) Gas hydrate growth, methane transport, and chloride enrichment at the southern summit of Hydrate Ridge, Cascadia Margin. Earth and Planetary Science Letters, 226:225-241.

Tréhu A, Long PE, Torres ME, Bohrmann G, Rack F, Collett TS, Goldberg DS, Milkov A, Reidel M, Schultheiss P, Bangs NL, Barr SR, Borowski WS, Claypool GE, Delwiche ME, Dickens GR, Gracia E, Guerin G, Holland M, Johnson JE, Lee Y-J, Liu C-S, Su X, Teichert B, Tomaru H, Vanneste M, Watanabe M, Watanabe M, Weinberg JL (2004) Threedimensional distribution of gas hydrate beneath southern Hydrate Ridge: constraints from ODP Leg 204. Earth and Planetary Science Letter.222, 845-862.

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Tréhu A.M., Bangs N.L., Arsenault M.A., Bohrmann G., Goldfinger C., Johnson J.E., Nakamura Y., Torres M.E., (2002) Complex subsurface plumbing beneath southern Hydrate Ridge, Oregon continental margin, from highresolution 3-D seismic reflection and OBS Data.

Fourth Int. Conf. Gas Hydrates, Yokohama, Japan: 90–96.

Tréhu A, Bohrmann G, Rack F, Torres M and ODP Leg 204 Shipboard Scientific Party (2003) Drilling gas hydrates on Hydrate Ridge, Cascadia Continental Margin. ODP Initial Rpt., 204.

Tréhu A, Bohrmann G, Rack F, Torres M and ODP Leg 204 shipboard Scientific Party (2003) Gas hydrate distribution and dynamics beneath Southern Hydrate Ridge. JOIDES Journal vol. 29 (2): 5-8.

Teichert BM, Eisenhauer A, Bohrmann G, Haase-Schramm G, Bock B, Linke P (2003) U/Th systematics and ages of authigenic carbonates from Hydrate Ridge, Cascadia Margin: Recorders of fluid flow variations.

Geochimica et Cosmochimica Acta 67 (20) 3845-3857. GEOTECH-2 Teichert B, Gussone N, Eisenhauer A, Bohrmann G (2005) Clathrites – Archives of near-seafloor pore water evolutions (δ44Ca, δ13C,δ18O) in seep environments. Geology, 33(3): 213-216.

Teichert BM, Bohrmann G, Suess E (2005) Microbially mediated carbonate build ups in cold seep environments. Palaeogeography, Palaeoclimatology, Palaeoecology, 227: 67-85.

Treude T, Boetius A, Knittel K, Wallmann K, Jørgensen BB (2003) Anaerobic oxidation of methane above gas hydrates (Hydrate Ridge, OR).

Marine Ecology Series 264, 1-14. GEOTECH-20

Winckler G, Aeschebach-Hertig W, Holoucher J, Kipfer R, Levin I, Poss C, Rehder G, Suess E, Schlosser P (2002) Noble gases and radiocarbon in natural gas hydrates. Geophysical Research Letters, 29(10), 0.1029/2001, correction printed in 29 (15), 1029/2002GL01573.

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Long-term Observatory for the Study of Control Mechanisms for the Formation and Destabilisation of Gas Hydrates (LOTUS) Linke P. (1), Abegg C. (2), Eisenhauer A. (1), Gubsch S. (3), Gust G. (3), Greinert J. (1), Keir R. (1), Liebetrau V. (1), Luff R. (1), Pfannkuche O. (1), Sommer S. (1), Spiess V. (4), Wallmann K. (1) (1) Leibniz-Institut für Meereswissenschaften (IFM-GEOMAR), Wischhofstr. 1-3, 24148 Kiel, Germany, E-Mail: plinke@ifm-geomar.de (2) L3-Communications ELAC-Nautik GmbH, Kiel, Germany (3) Technische Universität Hamburg-Harburg, Arbeitsbereich Meerestechnik 1, Lämmersieth 72, 22305 Hamburg, Germany (4) Fachbereich Geowissenschaften, Universität Bremen, Klagenfurterstraße, 28334 Bremen, Germany

1. Introduction It is generally recognized that destabilization of gas hydrates and the resulting release of methane may be one of the most powerful influences on past abrupt climatic changes of the earth system. However, in climate research the release of methane from gas hydrates has hardly been considered in model calculations since little information exists concerning the geochemical cycle of methane from marine hydrate deposits. It is not clear whether the entire amount of methane released at the sediment-water interface reaches the atmosphere or whether the exit of large amounts of methane to the atmosphere is prevented by oxidization in the benthic boundary layer or in the overlying water column by methane-oxidizing bacteria. Stimulated by the injection of reduced chemical species, a significant autotrophic production with an enormous oxygen consumption develops at vent sites, i.e. by methane from gas hydrates and hydrogen sulfide and ammonia from vent fluids. The efficiency and relevance of this »benthic filter or reactor« in comparison to »normal« heterotrophic systems dependent on pelagic POC fluxes needs to be determined and quantified. Furthermore it is not known whether the exhalation of methane from the sediments into the

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water column represents a constant flux or if variations occur that are controlled by environmental factors. In addition, little information exists concerning the lifetime and temporal activity of gas hydrate deposits and methane vents, and therefore no quantitative evaluation of temporal oscillations in gas hydrate source strengths has been possible to date. Even the residence time of methane in the form of gas hydrate is totally unknown. These complex dynamics must be understood before mechanisms responsible for hydrate formation and destruction at the sea floor can be quantified and modelled.

2. Objectives In view of these gaps in our knowledge, quantification and modelling of methane, formation and release can only be achieved by in situ observatories. This approach of LOTUS will enable fundamental new insights in the longterm temporal variability of the controlling physico-chemical and biogeochemical parameters in the sediment and in the water column as well as their impact on the temporal and spatial variability of venting. Previous measurements with short-term sampling intervals have shown differences on the orders of magnitude which indicate a significant variability in fluid

and gas fluxes. A long-term observation of methane fluxes and their control mechanisms is necessary for a better resolution of the temporal variability of fluxes and their relation to the benthic autotrophic communities. The objective of the multi-disciplinary programme LOTUS was to monitor in situ the complex trigger mechanisms of formation and destabilisation of gas hydrates on different time and space scales and to contribute to improved mass balances and diagenetic and prognostic modelling. This task included the interrelation between fluxes associated with gas hydrates and biogeochemical reactions at the benthic boundary layer. This was realised by novel long-term observatories for the sediment water interface and the water column, by dating and interpretation of the natural geoarchives as well as by process-oriented modelling of the benthic processes.

3. Scientific and technical background of the project The LOTUS project was proposed and funded within the first funding period of the special programme »GEOTECHNOLOGIEN« funded by the BMBF. Scientifically the project is based on the results and experiences, which have been obtained during two RV SONNE cruises to Hydrate Ridge within the TECFLUX project (BMBF). The project was composed of four scientific subprojects with the participation of four companies (Tab. 1).

LOTUS was integrated into a network of national projects of the »BMBF Gashydrat Initiative« and international EU-projects (Fig. 2). The projects LOTUS and OMEGA in combination with the MPI project »MUMM« represent a core within this network with intensive scientific and technical exchange. For example, the in situ measurements of bacterial turnover rates

Table 1: Structure of LOTUS

Co-ordination: Dr. P. Linke (GEOMAR) SP 1: In situ long-term observatories for the study of control mechanisms of fluid and methane flux during formation and destabilisation of near-surface marine gas hydrates.

Dr. P. Linke & Dr. O. Pfannkuche (GEOMAR), Prof. Dr. G. Gust (TUHH) with participation of the Fa. Oktopus SP 2: The fate of methane in the water column Dr. R. S. Keir (GEOMAR) Prof. Dr. V. Spiess (Universität Bremen) with participation of the Fa. CAPSUM, ELAC-Nautik, STN Atlas SP 3: Chronology and geochemical dynamics of near-surface gas hydrate deposits Prof. Dr. A. Eisenhauer, Dr. G. Bohrmann (GEOMAR) Dr. J. Scholten (Institut für Geowissenschaften, Kiel) SP 4: Modeling methane fluxes and biogeochemical processes in hydrate-bearing surface sediments Dr. K. Wallmann (GEOMAR)

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in the sediment (sulphate reduction) have been investigated by the Max-Planck-Institute for Marine Microbiology in Bremen within MUMM. Joint planning and accomplishment of expeditions as well as an exchange of samples and data were performed. Data from the SONNE cruises were transferred to the PANGAEA data center. Furthermore, in the field of sensor development we conducted a close cooperation with the company Unisense, Aarhus.

Major stepping stones for LOTUS were the two expeditions with RV SONNE, SO165 – OTEGA-I (Pfannkuche et al., 2002) to the Cascadia continental margin (Fig. 2) and SO174 – OTEGA-II (Bohrmann and Schenck, 2003) to the Gulf of Mexico (Fig. 4). Furthermore, members of LOTUS participated on the METEOR cruise M52/1 - MARGASCH and two cruises with RV Prof. Vodyanitsky (EU project CRIMEA) to the Black Sea.

Scientific and technical input was gained from EU programmes (ESONET, CRIMEA) and from previous BMBF projects (BIGSET, TECFLUX).

With our experience in long-term observatories LOTUS will provide a substantial input to the planned ESONET and Neptune Network.

Technical developments within LOTUS will impact the innovation and competitiveness of small companies which partly co-operate as partners in LOTUS.

Research on cold seeps of the Cascadia continental margin began with the discovery of fluid venting and associated chemosynthetic life (Suess et al. 1985; Kulm et al. 1986) and was propelled forward by the findings of deep (Kastner et al. 1998) and shallow gas hydrate deposits (Suess et al. 1997; 1999). After more than a decade of research at Hydrate Ridge by international and interdisciplinary scientific expeditions, this is probably the best-studied convergent margin with intense fluid flow and large-scale gas hydrate deposits (Boetius and Suess 2004). Active venting of fluids and gases (Linke et al. 1994; Torres et al., 2002; Tryon et al., 1999, 2002; Heeschen et al., 2005), exposure of methane hydrates at the seafloor (Suess et al. 2001; Haeckel et al. 2004), distribution, composition and activity of chemosyn-

The project started in February 2001 with the technical development of the various in-situ observatories and instrumentation, which was tested on two cruises with RV ALKOR in October 2001 and May 2002. In parallel to this, the laboratory and computing facilities were established (Fig. 1).

2001 F

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P/E Planing and evaluation seminar E Evaluation seminar Project phase

Figure 1: Schedule of the LOTUS project displaying the different working phases, cruise and planing and evaluation seminars of the project.

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thetic communities (Boetius et al. 2000; Knittel et al. 2003; Tryon and Brown 2001; Sahling et al. 2002; Sommer et al. 2003; Heinz et al., in press), authigenic carbonates forming chemoherms (Bohrmann et al. 1998; Greinert et al.

2001; Teichert et al. 2003), and gas plumes in the water column have been well documented during high-resolution seismic and side-scan sonar surveys (Tréhu et al., 1995; Johnson et al., 2003; Heeschen et al., 2003), supplemented by submersible and ROV dives (Torres et al., 2002; Linke 2003), camera surveys, as well as by video-guided sediment sampling and deployment of benthic landers. The vent sites and related topographic features and the tectonic convergence setting of the mid-slope margin are easily accessible from the Pacific coast of the U.S. (Fig. 2). Furthermore, this location straddles the stability limit of gas hydrates and thus makes Hydrate Ridge an

ideal natural laboratory for the study of active fluid flow, gas hydrate formation and dissociation, and its impact on the environment (Boetius and Suess 2004). Most of the data on Hydrate Ridge so far were obtained during the TECFLUX programmes (TECtonically induced FLUXes) in 1999-2000, followed by the programmes LOTUS, OMEGA and MUMM in 2001-2003, and the international Ocean Drilling Programme (e.g. Schlüter et al. 1998; Carson et al. 2003; Tréhu et al. 2004).

In the Gulf of Mexico some of the best-documented gas hydrate occurrences in the world are situated (Fig. 3). In the northern part of the Gulf gas hydrate has been found at more than 50 locations in combination with the discharge of hydrocarbons from the seafloor (Brooks et al. 1984, Milkov and Sassen 2001, Sassen et al.

2001). The sediments in the northern Gulf of

Figure 2: Shaded relief bathymetry of the Hydrate Ridge region. Contour interval is 100 m and bathymetric grid is 100-m pixel resolution.

Inset shows Pacific Northwest bathymetry and topography and the location of Hydrate Ridge region on the lower continental slope of the Cascadia accretionary prism.

Hydrate Ridge is a NE-SWtrending thrust ridge with northern and southern summits; (NHR) Northern Hydrate Ridge; (SHR) Southern Hydrate Ridge. The ridge is located V10 km from the deformation front (DF) and bordered on the west and east by slope basins (HRB-W) Hydrate Ridge Basin-West and (HRB-E) Hydrate Ridge Basin-East. ODP (Ocean Drilling Programme) site 891 on the crest of the first accretionary ridge (FAR) and site 892 on NHR are shown. Daisy Bank (DB) is also shown (Johnson et al. 2003).

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Mexico overlie enormous reservoirs of liquid and gaseous hydrocarbons that rest upon Jurassic-age salt deposits (Kennicutt et al. 1988, Roberts et al. 1999). These allochthonous salt bodies and sediment-filled minibasins are the most obvious features of the regional geology.

Their geometry, distribution, and structural evolution through time influence the occurrence of gas hydrate in the nothern Gulf of Mexico.

Milkov and Sassen (2001) have developed a conceptual model of gas hydrate occurrence with two types of gas hydrate accumulations in the NW GOM: (1) structurally focused thermogenic and bacterial gas hydrate on the rims of minibasins, and (2) disseminated bacterial methane hydrates (~100% CH4) that resides within minibasins. Thermogenic structure II hydrates are containing methane (44%), ethane (11%), propane (32%), iso-butane (9.5%), butane (3%) and pentane (0.5%) (Sassen et al.

1998, Orcutt et al. 2003).

Figure 3: Map of the northern Gulf of Mexico with documented locations of gas hydrates, oil and gas seeps (taken from Milkov and Sassen 2001).

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Salt-driven tectonics creates fault networks that serve as conduits for the rapid transfer of oil, gas and brines from deep reservoirs through the overlying sediments and ultimately into the water column (Kennicutt et al. 1988a and 1988b, Aharon 1994, Roberts and Carney 1997). On the seafloor, such conduits give rise to gas vents and seeps, subsurface and sediment surface-breaching gas hydrates, brine pools, and mud volcanoes (Roberts and Aharon 1994, Sassen et al. 1994).

The oil and gas escaping at seeps rises through the water column and form long linear layers on the ocean surface (MacDonald et al. 1996).

These layers of floating oil can be detected in satellite images and provide a means for finding seeps (De Beukelaer, MacDonald et al. 2003).

Oil slicks that form over seeps are typically long, linear features, broadest at the point of origin where the oil drops reach the surface, and tape-

ring away in the direction of prevailing wind and current. By comparing the locations of slicks in multiple images, it is possible to predict the seafloor location of a seep. On the other hand, these natural oil slicks provide a dramatic demonstration that some fraction of seeping hydrocarbons escapes the water column and reaches the atmosphere (Leifer and MacDonald, 2003).

In-situ instrumentation of shallow or exposed deposits of gas hydrates indicate that they alternately form and decompose as bottom water temperature fluctuates (MacDonald et al.

1994). Gas hydrate deposits have also been found to generate irregular bathymetry (MacDonald et al. 2003) and to support colonies of unusual annelid worms (Fisher, MacDonald et al. 2000). Several authors have suggested that presence of gas hydrate enables or facilitates formation and maintenance of tube worm aggregations (Carney 1994; Sassen et al. 1999). In any event, mounded and irregular bathymetry and chemosynthetic communities have been accepted as reliable indicators of active hydrocarbon seepage (MacDonald et al. 1996; Roberts and Carney 1997).

Sediments in and around areas of active seepage are characterized by elevated concentrations of simple (C1-C5) and complex (oils) hydrocarbons and hydrogen sulfide (H2S). Complex chemosynthetic communities comprised of a variety of microorganisms and bacteria-metazoan symbioses thrive around hydrocarbon seeps in the Gulf of Mexico (Kennicutt et al. 1985, MacDonald et al. 1989, 1990 and 1996, Fisher 1990, Ferrel and Aharon 1994, Larkin et al. 1994).

These communities proliferate in a cold, highpressure environment by exploiting the abundance of energy rich reduced substrates, such as methane and H2S. While the diversity and distribution of seep macrofauna has been the focus of intense study, the activity of free-living bacteria in seep sediments and around gas hydrates has received little attention so far (Joye et al.

2004). This lack of information is surprising given that microbial activity may impact the flux and composition of both liquid and gaseous

hydrocarbons and oils as they transit the seep ecosystem (Kennicutt et al. 1988, Sassen et al.

1998) and may even be responsible for the formation of seep deposits, such as carbonate reefs, chimneys, and mounds (Ferrel and Aharon 1994, Suess et al. 1999, Michaelis et al. 2002).

In a recent study on anaerobic oxidation of methane (AOM) and sulfate reduction (SR) in sediments from Gulf of Mexico cold seeps only a weak coupling between AOM and SR was observed and that AOM accounts for only a small fraction of SR activity in the methane-rich sediments (Joye et al. 2004). In this system, CH4 is just one of a diverse suite of seep-derived organic substrates that could fuel sulfate reduction (Brooks et al. 1984, Aharon 2000). A variety of long-chain alkanes, complex aliphatic and aromatic compounds, and oils can be consumed by sulfate reducing bacteria in this system.

The lack of strong (1:1) coupling between AOM and SR means that SR in Gulf of Mexico seep sediments must be driven by the oxidation of other organic compounds more so than by AOM (Joye et al. 2004).

The magnitude of spatial and temporal variation in fluid flow at Gulf of Mexico seeps is presently unknown. Fluid flow data from Hydrate Ridge show that temporal and spatial variations in seepage occur (Torres et al. 2002, Tyron et al. 2002) and similarly variable flow rates might be expected at the Gulf of Mexico cold seeps. The inherent variability in seepage rates, and hence in substrate supply, may select for a metabolically plastic microbial community that is adept at consuming a variety of organic substrates when CH4 is limiting but one that is also poised to take advantage of periods when CH4 is available.

Furthermore, the presence of other, more energetically favorable substrates could generate competition for available sulfate between microbes involved in AOM and microbes oxidizing other hydrocarbons and oil. Such competition could serve to structure the microbial community at mixed substrate (gas- and petroleumrich) cold seeps. The capacity for AOM in such sediments and the impact of other reduced car-

25

bon substrates on AOM rates should be examined carefully to better elucidate the biogeochemical and microbiological controls on AOM in situ. Despite the presence of alternate reduced carbon substrates, Gulf of Mexico seep sediments harbor anaerobic methane consuming microorganisms that act as an efficient bio-filter preventing methane emission to the hydrosphere, except at a few sites where hydrate dissociation and fluid flow creates so much gas pressure that free gas emanates from the sediments in the form of gas bubbles (Joye et al. 2004).

The working area of the second SONNE expedition (Fig. 4) has been chosen out of logistic reasons, since the cruise schedule of the vessel made it impossible to reach Cascadia a second time in 2003. These logistic constrains shifted the second cruise into the Gulf of Mexico towards the end of the funding period of LOTUS. This is the main reason that some of

the analytical procedures, evaluation and modeling of data as well as publication of results could not be finished within the funding period. Most of this work is done at present and will be finalised with the next funding period.

Nevertheless, most of the working programme which was scheduled in the LOTUS proposal is completed.

4. Results of the subprojects Measurement of in-situ benthic fluxes by 2 novel benthic observatories Microbial methanotrophy in surface sediments represents an important sink for methane and acts as a biological barrier for methane before it enters the water column. These microbial processes embedded within a complicated network of biogeochemical reactions, control the emission of methane across the sediment

Figure 4: Cruise track of R/V SONNE during SO 174 (OTEGA-II) with the major working area of the LOTUS project in the northern Gulf of Mexico during Leg 1. Detailed investigations were performed at Bush Hill and two sites in the Green Canyon (GC 234 and GC 415).

26

water boundary layer and contribute towards the regulation of the susceptible balance of the greenhouse gas contents in the earth’s atmosphere.

Direct in situ measurements of emission rates of dissolved methane from gas hydrate containing sediments are very scarce, which is due to a limited availability of appropriate in situ technology in marine sciences.

Within subproject 1 two novel observatories, BIGO and FLUFO have been developed as scientific modules which fit into the basis of the GEOMAR Lander System. Latter has been successfully deployed during several projects (ALIPOR, EU; BIGSET, BMBF; TECFLUX, BMBF) for the measurement of benthic turnover and to conduct in situ experiments (Pfannkuche and Linke 2003).

As to the modules, the FLUFO has been developed by TUHH/MT1, while the BIGO module is based on a combination of contributions from GEOMAR, TUHH and Hydro Data Inc. which holds the patent.

The FLUFO module was developed on the basis of an assessment of the performance of existing devices. In particular, improvements were enacted related to: 1. test of leakage during deployment 2. identification of emanating gas-liquid mixtures 3. identification of direction (inflow/outflow) 4. calibration in a range of exchange velocities from 10 cm/yr to 74 km/yr.

The BIGO contains a particularly adapted version of the patented microcosm (Hydro Data Inc.) with an added control system to generate the hydrodynamic inside the chamber as prevailing in the boundary layer flow (TUHH). The chamber was connected with a gas exchange system which permits control of the oxygen contents and additional fluid sampling units (GEOMAR). Both units are mounted into the GEOMAR Lander System and are unique as combined tools.

During both research-cruises both observatories have been deployed successfully. Their deployment enabled to obtain a unique data-

set of in situ benthic material fluxes from sediments with shallow gas hydrates at Hydrate Ridge, Cascadia subduction zone and in the Gulf of Mexico. The presented measurements are so far the only existing direct measurements of the seabed methane emission under natural conditions.

Particularly at Hydrate Ridge, which belongs to an extensive oxygen minimum zone with bottom water concentrations around 50 µmol l-1, the novel gas exchange system enabled measurements under natural in situ conditions by maintaining ambient oxygen concentrations inside the benthic chambers. By use of this system we detected that seabed methane emission apparently is sensitive for oxygen availability. Under natural oxic conditions average methane emission from sediments covered with microbial mats overlying shallow gas hydrates was low with 5.7 mmol m-2 d-1. When inside the benthic chambers oxygen became depleted methane emissions increased 27.5 fold reaching maximum values of 156.7 mmol m-2 d-1. Thus, apart from anaerobic methane oxidation aerobic oxidation of methane, which takes place down to oxygen concentrations of 6.3 µmol l-1 (Heyer 1990) appears to be an important process in the methane cycle within the sediment water boundary layer. Availability of oxygen might further indirectly affect other biogeochemical processes involved in benthic methane turnover and contribute towards the efficiency of the benthic filter (Sommer et al. in review).

From the amount of methane, entering at the bottom of the modelled sediment column in 20 cm depth and the measured methane emission rates across the sediment water interface an estimate of the efficiency of the benthic filter at the different sites can be calculated. At clam field sites about 83 % of the incoming methane is consumed by methane oxidation. In sediments covered with bacterial mats 66 % of all methane is consumed. So far the efficiency of the benthic filter has been estimated indirectly based on the ex situ determination of the anaerobic oxidation of

27

methane, which is believed to be the major methane consuming process in these sediments. Apparently, the investigated sites at Hydrate Ridge, which are characterised with low pore water velocities (10 – 20 cm yr-1), are under the present environmental conditions in a quiescent state. There are, however, strong implications that under altered environmental conditions such as enhanced surface water productivity and warming of bottom water enhancing overall oxygen uptake in the sediment water boundary layer this benthic filter might loose its efficiency and high concentrations of methane might be injected into the water column. There it will be oxidised aerobically when oxygen is present and affect the overall oxygen inventory of regional water masses as it was also postulated by Hinrichs et al. (2003). Environmental conditions and their threshold levels beyond which the benthic filter fails remain to be investigated. With the Fluid-Flux-Observatory, aqueous fluid flows in vertical direction up to 150 m per year (minimum threshold 10 cm/yr) were detected at Hydrate Ridge. The distribution pattern of these flows reveals in addition to temporal periodicity high variations in intensity, direction and phase (relative to the tidal signal). All aqueous fluid flow crossing the sedimentwater boundary were strongest with increased bottom currents. Gas flows were not detected.

The origin of the flow pattern is traced not to the venting from deep sources but to a shallow circulation pattern of fluid flow in the surfacenear sediment. It is driven by a combination of (tidal) bottom currents, topography and device exposure (Gubsch et al., in review).

At the Gulf of Mexico total oxygen uptake (up to 56.7 mmol m-2 d-1) and seabed methane emission (up to 7.0 mmol m-2 d-1) was similar to that measured at Hydrate Ridge. The porewater profiles have not been modelled so far, thus the efficiency of the benthic filter cannot be estimated presently. At the Gulf of Mexico we also discerned oxygen sensitivity of methane efflux but not to such a strong extent as at Hydrate Ridge, which was due to the higher bottom water oxygen concentrations. In both

28

regions oxygen availability in the sediment water boundary layer apparently represents an important mechanism controlling the dissipation of methane from sediment and indirectly affects formation and degradation kinetics of shallow gas hydrates.

The newly developed observatories represent an important step from static to dynamic systems enabling measurements of material fluxes of the natural environmental system.

Their experimental possibilities are urgently needed for process-orientated studies and to test and validate prognostic models.

The fate of methane in the water column Subproject 2 of LOTUS studied the amount of free gas released from cold vents, its distribution in the water column as well as the isotopic signature of the methane carbon. Many technical developments have been part of the project which are: the enhancement of the METS methane sensor, the development of a WINDOWS-based software for the data storage of the hydroacoustic water column data of the PARASOUND system, the development of a lander based system for the monitoring of bubble release (GasQuant), and the setup of an isotope laboratory for the tracking of the methane source and its isotopic fractionation during decomposition. All technological developments were completely implemented, tested successfully and used during several cruises at Hydrate Ridge, the Gulf of Mexico and the Black Sea (Bohrmann et al. 2003).

The METS sensors detected during two cruises in the Black Sea extreme variations at one spot (either in a mooring or fixed to the FLUFO lander) which are caused by varying current directions and the additional non-continuous methane release form vents. This show in a dramatic way that single geochemical methane analyses from water casts close to vent sites have to be interpreted very carefully and cannot be used for larger budget modelling purposes. The variations sometimes exceed more than a magnitude.

Of great use is the hydroacoustic detection of free gas in the water column for the mapping of active vent sites. The new software for the PARASOUND system and the general possibility of the digital data recording of the 18 kHz signal enhance the accuracy to detect active vents and allow the post processing and 3D visualization of flares together with other data (e.g. bathymetry, side scan sonar, geochemical data). Flare Imaging was conducted during SO165, SO174 and provided essential information for cruise and station planning. It further shows that the relation between a disappearing hydroacoustic flare and assumed increasing methane concentrations is not as simple as supposed at the beginning of the project. A strong oxidation of methane in the water column, the current driven distribution of dissolved gas and additional stripping effects of the gas phases have a strong impact in the fate of methane in the water column. Thus, a detailed monitoring of the environmental parameters such as currents with low frequency ADCPs is of special importance.

Another complication is the non continuous release of free gas from vent sites. The successful development of the GasQuant system can be used now to monitor this release over extended periods of time. Furthermore, it provides a spatial view of the bubble-site distribution within a larger area (1900 m2) which can not be monitored simultaneously by ROVs or smaller video observation systems. The use of more specific echosounders which can detect the real target strength (e.g. via split beam technique) allows the determination of bubble sizes and their variations during the bubble rise (shrinking rate: important for the methane flux from the free gas phase into the dissolved gas phase) and finally enable flux estimates of a larger area (Greinert and Nützel 2004). This is of great importance as deployments in the Black Sea showed that the temporal variability is extreme, varying between »bursts« of several minutes followed by hours or »silence« to a more continues bubbling which nevertheless is not active the entire day (e.g. the most active bubble spots in the studied seep area of the

Black Sea are only active for 25% of the day).

Despite the temporal delay of nearly one year for the setup of the isotopic laboratory, we are now able to analyse a greater quantity of samples in a standard procedure of very high accuracy per day. Measurements of samples from Hydrate Ridge show in combination with the geochemically determined methane concentrations that high methane concentrations coincide with low δ13C values of -40‰. Unfortunately the shelf area itself is also a very strong methane source with similar isotopic signals as those detected next to vents. Thus we cannot for sure define the source of rather high methane concentrations above 500 m water depth observed between the shelf and the area west of Hydrate Ridge. Nevertheless our data indicate that extensive methane oxidation occurs close to vents sites with a strong enrichment of 13C in the residual methane (δ13C values up to -10‰). The extended water sampling grid over the Hydrate Ridge area further shows that even the very active vent site at the northern summit in 'only' 600 m water depth have no impact on the water surface concentration of methane. This demonstrates the great filter and dilution potential of the water body which has to be studied in more detail in the course of the COMET project.

The chronology and geochemical dynamics of near surface gas hydrate deposits Although occasional gas and fluid venting can be seen at Hydrate Ridge the massive chemoherms are predominantly fossil relicts of past extensive fluid flow during glacials when sea level was much lower than today. For the investigated glacial periods fluid advection rates were calculated in the order about 40 cm/a and up to more than 150 cm/a, respectively.

The coincidence of massive carbonate build-up and glacial climatic intervals point to the possibility that the formation of the chemoherm carbonates and, hence, the activity of the cold seep vent sites are directly related to the height of sea level via the pressure difference bet-

29

ween the height of the seawater column and the hydraulic head and buoyancy of the upward advecting fluids in the plumbing system of the sediments. If the fluid control of the Hydrate Ridge vent sites via the height of the sea level is a general phenomena then their active gas and fluid venting did not contribute to the increasing concentrations of greenhouse gases at glacial/interglacial transitions during the Late Quaternary. As a base for a more detailed deciphering of geochemical archives from cold seeps an improved U/Thmeasurement technique for geochronological purpose has been developed (MIC-ICP-MS) resulting in a five time higher precision (Fietzke et al. 2005).

The combination of high precision U/Th-geochronology on smallest sample amounts with laser ablation element profiles opened recently new perspectives for the reconstruction of paleo-activity and compositional changes of cold seeps in high resolution. A detailed interpretation is topic of the current project progress.

During the course of the LOTUS project temperature proxies (Ba/Ca, Sr/Ca, etc) have been calibrated as chemical and isotopic indicators for fluid temperature changes and to indicate whether chemoherm are of pure inorganic origin or their trace element budget was actively altered by microbial activity. First measurements indicate an active control of microbial activity on the trace element distribution in these carbonates. Important steps towards the use of Ca- and Sr-isotope proxies for cold seep environments were the development of the socalled cool plasma MC-ICP-MS measurement technique (Fietzke et al. 2004) for Ca isotopes and the highly innovative temperature calibration of the δ88Sr isotope ratio of carbonates.

Although the chemoherm carbonates originate from the anaerobe activity of sulphate reducing and methane oxidizing microbes they extend well into oxygenated water. Latter observation may indicate that the massive chemoherm build-ups provide a micro-niche for housing of anaerobe microbial communities within a well oxygenated marine environment.

30

In order to contribute to the determination of recent fluid flux in cold seep environments the improvement of radiochemical methods (Purkl and Eisenhauer 2004) is further in progress.

Modelling methane fluxes and biogeochemical processes in hydrate-bearing surface sediments The numerical model C. CANDI was applied to investigate and to quantify biogeochemical processes and methane turnover in gas hydrate-bearing surface sediments from a cold vent site situated at Hydrate Ridge, Cascadia Margin (Luff and Wallmann 2003). Steady state as well as non steady state simulations, based on measurements from the center of an active vent site were carried out to obtain a comprehensive overview on the activity in these sediments which are covered with a bacterial mat and are affected by strong fluid flow from below. A fit of the model to the dataset allowed the determination of different unknown parameters. With this method the fluid flow velocitiy could be isolated to values around 10 cm a-1. The turnover rate of Anaerobic Methane Oxidation (AOM) in these sediments is tremendously high, all methane that reaches the surface sediment from below is oxidized in the surface sediments. Thus, AOM is the major process, proceeding at a depth-integrated rate of 872 µmol cm-2 a-1. A significant fraction (14 %) of bicarbonate produced by anaerobic methane oxidation is removed from the fluids by precipitation of authigenic aragonite and calcite. The total rate of carbonate precipitation (120 µmol cm-2 a-1) allows for the build-up of a massive carbonate layer with a thickness of 1 m over a period of 20,000 years. Aragonite is the major carbonate mineral formed by anaerobic methane oxidation if the flow velocity of methane-charged fluids is high enough (≥ 10 cm a-1) to maintain super-saturation with respect to this highly soluble carbonate phase. Non-steady state simulations using measured fluid flow velocities as forcing demonstrate a rapid respond of the sediments within a few days to changes in advective flow. At flow rates exceeding appro-

ximately 100 cm a-1, dissolved methane break through the sediment surface to induce large fluxes of up to 5000 Âľmol CH4 cm2 a-1 into the overlying bottom water.

To investigate the conditions that induce carbonate crust formation in the sediment and the effect of crust formation on sediment porosity and fluid flow rate the porosity has been formulated as an inert component, defined by the amount of terrigenous matter and a temporal variable component reflecting the amount of authigenic CaCO3 (Luff et al. 2004).

Starting with the conditions prevailing at a previously investigated reference site located on Hydrate Ridge, off Oregon, several parameters are systematically varied in a number of numerical experiments. The simulations show that carbonate crusts in the sediments only form if the fluids contain sufficient dissolved methane (>50 mM) and if bioturbation coefficients are low (<0.05 cm2 a-1). Moreover, high sedimentation rates (>50 cm ka-1) inhibit crust formation.

Bioirrigation induces a downward displacement of the precipitation zone and accelerates the formation of a solid crust. Crusts only form over a rather narrow range of upward fluid flow velocities (20 - 60 cm a-1), which is somewhat enlarged (up to 90 cm a-1) if the overlying bottom waters are supersaturated with respect to calcite. Moreover, using a non steady state model approach we simulate the aragonite and calcite precipitation and dissolution in a 2 m long sediment column (Luff et al. 2005).

Assuming constant conditions for 7,000 years, fluid flow, anaerobic oxidation of methane (AOM) rates and carbonate precipitation/dissolution rates show strong oscillations evoked by the changes in permeability and fluid flow over time and depth. The simulation predicts cycles of crust formation and dissolution with a duration of 2,000 to 2,700 years resulting in several distinct carbonate layers. The oscillations are dampened so that fluid flow and biogeochemical turnover slowly approach a steady state after about 7,000 years towards the end of the simulation period.

Other models have been developed to simulate the formation of gas hydrates at Hydrate Ridge. These simulations show that near-surface gas hydrate deposits can only be formed by the ascent of gas bubbles (Torres et al. 2004).

Moreover the models also has been used to derive AOM rates from sulfate pore water profiles (Treude et al. 2003), fluid flow velocities from dissolved chloride concentrations (Hensen et al. 2004), rates of barite and carbonate precipitation in the Derugin Basin (Aloisi et al. 2004a), fluid release at a mud volcano located in the Black Sea (Aloisi et al.

2004b), the uptake of 14C in carbonate crusts formed at mud volcanoes in the Mediterranean Sea (Aloisi et al. 2004c) and the methane and sulfide turnover in benthic chambers placed on active vent sites located in the central American subduction zone (Linke et al. 2005).

5. Conclusions and outlook Most of the technology and expertise has been developed and successfully been used within the first funding period of the GEOTECHNOLOGIEN Programme. The engineering and construction of the complex deep-sea instrumentation for long-term measurements and specific experiments has been conducted in close co-operation with numerous small and mediumsized companies in northern Germany. As visible in the LOTUS project, some of these companies have realised that this project offered an ideal scenario for technology transfer and first user applications and were willing to invest a significant share in the development costs of the specific instrumentation. Furthermore, the transfer of expertise in the use of these and other instruments and hence the qualification of leased personnel within this project has strengthened the international position of the SMEs in marine technology in northern Germany. IFM-GEOMAR and a number of SMEs close to the institute have gained a leading role in lander technology, which was recently presented on several international workshops and on the largest marine technology exhibition ÂťOceanology 2004ÂŤ in London.

In the future, landers will be also incorporated

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as modules into glass-fibre optical cable systems. Autonomous lander clusters connected by optical cable and with data transmission to the surface and further on by satellite link to the shore are envisioned as an important contribution to future sea floor observatories.

Other highlights with technological, economic and scientific outreach are:

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The development of a sensor that responds rapidly and quantitatively to the dissolved methane concentration in seawater would be of considerable use in the exploration for oil and gas offshore, especially off the shelf margin. The oil and gas industry uses so-called ÂťsniffersÂŤ for this purpose, but in this system the ambient water must be pumped on board the ship, and therefore sniffers have a limited depth utility. The successful development of the methane sensor would likely open up a new market in the offshore exploration industry and LOTUS provided the framework for the improvement of this sensor.

Software and system development may lead to a system that can be implemented in future version of a Parasound system, which will include digital data acquisition. Ongoing communication with the Parasound manufacturer assured this mutual benefit.

The upgrade of the ParaDigMA is now available on all three large German research vessels (METEOR, SONNE, POLARSTERN).

The visualisation and /or an easy-to-handle method for quantifying gas ebullitions by vertical echosounders can now be used in the exploration of marine gas fields, pipeline observation surveys and the success control of closing submarine gas field boreholes (STN-Atlas, ELAC Nautik).

Hardware and software developments can be introduced into the market or used for other products.

Participating companies may now market their products with the label 'tested in research' and may cite scientific institutes for reference (ELAC-Nautik, STN-Atlas, K.U.M., Oktopus, Capsum).

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The combination of hydrogen and carbon isotope measurement capability in a single IRMS mass spectrometer system is a new development in the industry. This system is marketable to several industrial and research sectors, including the oil and gas industry and other laboratories concerned with isotopic analysis of hydrocarbons.

As a base for the deciphering of geochemical archives from cold seeps the U/Th-measurement technique has been improved resulting in a five time higher precision.

A combination of high precision U/Th-geochronology on smallest sample amounts with laser ablation element profiles on laminated chemoherm carbonates allowed recently the reconstruction of paleo-activity and compositional changes of cold seeps in high resolution.

The overall age distribution implies a close correlation of increasing fluid flux with periods of sea level low stands.

For Ca-isotope measurements an alternative MC-ICP-MS technique (so-called cool plasma) was developed for future studies about fluid source, mixing processes and the understanding of temperature dependent isotope fractionation processes.

Also one highly innovative isotopic proxie (δ88Sr) has been calibrated for fluid temperature reconstructions.

During the course of the LOTUS project new methods for chemical purification of radioisotopes have been developed in order to determine disperse and advective fluid flow out of the sediment.

The modeling approaches developed within LOTUS will enhance our understanding of methane and carbon fluxes at the seafloor in continental margin settings with gasand hydrate-bearing sediments. It predicts orders of magnitude in the turnover at these specific locations for comparison with other environments to determine the importance in the global C-budget. More specifically, the formation of hydrate induced carbonates through ascending fluids and gases as well as the dissolution rates

have be illuminated and quantified.

Moreover, the anoxic oxidation of methane (AOM) rates and the release of the by-products into the water column have been determined and the complex network of induced biogeochemical processes has been deciphered.

The existing technology will now be refined and applied to fill the gaps in our scientific knowledge within the COMET project (Controls on methane fluxes and their climatic relevance in marine gas hydrate-bearing environments). The results and their modeling will enhance our understanding of methane and carbon fluxes at the seafloor in continental margin settings with gas- and hydrate-bearing sediments. More specifically, the formation of hydrates through ascending fluids and gases as well as the dissolution rates of exposed and buried hydrates will be illuminated and quantified. Moreover, the methane oxidation and release in the water column will be determined and the complex net of induced biogeochemical processes will be deciphered.

Acknowledgements We are indebted to the expertise and enthusiasm of our engineers (M. Pieper, M. Poser, M. Türk, T. Viergutz), technicians (B. Bannert, A. Bleyer, B. Domeyer, A. Gerriets, A. Kähler, A. Petersen, W. Queisser, R. Suhrberg) and students (D. Hägele, S. Kriwanek, B. Mählich, P. Orlinsky, M. Rohleder, M. Treitschke) who worked in the workshops and laboratories, at home and at sea and made the project a success. Special thanks to Christine Utecht who supported the coordinator and the subprojects whenever 2 helping hands were required. The project and the SONNE cruises (SO165 and SO174) were financed by the Federal Ministry of Education and Research (BMBF) (grants no.

03G0565A-E; 03G0165A; 03G0174A) within the programme GEOTECHNOLOGIEN. Project reviews and scheduling of the SONNE Cruises was handled efficiently by the Projektträger Jülich-Warnemünde. On behalf of all partici-

pants we wish to thank these agencies, departments, and staff for their support. The Reedereigemeinschaft Forschungsschifffahrt Bremen provided technical support on the vessel in order to accommodate the variety of technological, electronic, and navigational challenges required for the complex sea-going operations. We would like to especially acknowledge the vessel’s master H. Andresen (SO165 and SO174) and his crews for their continued interest, flexibility, patience, and their contribution to provide an always pleasant and professional atmosphere aboard.

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Aharon P (2000) Microbial processes and products fueled by hydrocarbons at submarine seeps. In: Riding, R. E., Awramik, S. M. (Eds.) Microbial Sediments, Springer-Verlag, Berlin, pp. 270-281.

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Gas Hydrates: Occurrence, Stability, Transformation, Dynamics, and Biology in the Black Sea (GHOSTDABS) Michaelis W. (1), Seifert R (1), Blumenberg M. (1), Pape T. (1), Lüdmann T. (1), Wong H.K. (1), Konerding P. (1), Zillmer M. (2), Petersen J. (2), Flüh E. (2) Reitner J. (3) Reimer A. (3)

(1) University of Hamburg, Institute of Biogeochemistry and Marine Chemistry, Bundesstrasse 55, 20146 Hamburg, Germany (2) IFM-GEOMAR, Leibniz-Institut für Meereswissenschaften, Wischhofstrasse 1-3, 24148 Kiel, Germany (3) University of Göttingen, Section Geobiology, Goldschmidtstrasse 3, 37077 Göttingen, Germany

Introduction Methane has attracted high attention since long time as an energy source and a main greenhouse gas. This interest was even intensified by recent discoveries. Vast amounts of methane were found in gas hydrates present within marine sediments and permafrost soils (Kvenvolden et al., 1993; Kvenvolden & Lorenson, 2001), indications for large-scale methane releases substantially effecting the global atmosphere and hydrosphere were observed in the geological record (Dickens et al., 1995; Haq, 1998; Beerling & Berner, 2002; de Wit et al., 2002), and the anaerobic oxidation of methane was found to be a highly relevant process in marine sediments (Barnes & Goldberg, 1976; Reeburgh, 1976). Though considerable progress could be achieved during the last 30 years in elucidating the geochemistry and microbiology of methane dynamics on earth, many questions remained unanswered including those concerning anoxic marine habitats.

The secluded ecosystem of the Black Sea represents an extraordinary research area for studies of methane related processes. It comprises the world’s largest anoxic marine water body with strongly elevated concentrations of hydrocarbons. Moreover, multiple seeps of methane rich gas occur within a large water depth

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range and gas hydrates are found at the outer continental margin. We here report on the results obtained within the project GHOSTDABS that was funded as part of the GEOTECHNLOGIEN program to study the biogeochemistry of methane in the north-western Black Sea. With a multidisciplinary approach combining geophysical, organic-geochemical, geological, and microbiological components the project was dedicated to a comprehensive investigation of gas hydrates and associated gas/fluid seeps in the northern Black Sea with the following topics: - Inventory of gas hydrate occurrences - Localisation and detailed investigation of escape structures of gases and fluids at the seafloor - Biogeochemical conversion and turnover of methane released from the sediment in either an oxic or an anoxic bottom water environment. Studies on seep carbonates and sediment samples.

- Characterisation of the fauna (meio- and micro-fauna) in the vicinity of seeps and vents. Relation between the biological community and the physical and chemical milieu at the sediment/water boundary.

Between June 30th and July 22nd, 2001 a research cruise was undertaken using the Russian

RV »Professor Logachev« and the German submersible »JAGO«. The work program included reflection seismic studies, investigations of the water column and of sediment cores, and detailed biogeochemical and microbiological surveys at locations of active gas seepage. A first impressive glimpse on huge methane seep associated chimney structures at the seafloor was captured during a TV-Grab station already in the late night of July 2nd, at about 230m water depth. Surveys by the submersible JAGO at that area displayed a fantastic view on a field of active gas seeps covered by a forest of structures built by microbial communities thriving at methane gas seeps that protrude up to 4m into the anoxic waters (Michaelis et al., 2002). Moreover, a comprehensive set of samples could be obtained also with keeping the microbial mats active for in vitro experiments.

The excellent samples and data that could be gained have already given rise to numerous per reviewed publications (Michaelis et al., 2002; Gulin et al., 2003; Krüger et al., 2003; 2005;

Thiel et al., 2003; Blumenberg et al., 2004; 2005; Lüdmann et al., 2004; Nauhaus et al., 2005; Kube et al., 2005; Meyerdierks et al., 2005; Pape et al., 2005; Pimenov & Ivanova, 2005; Reitner et al., 2005; Seifert et al., 2005; Treude et al., 2005; Zillmer et al., 2005). This paper gives a comprehensive overview on selected aspects of the results achieved.

The GHOSTDABS field Located at 44°46´N 32°00´E the GHOSTDABS field still harbours the most impressive seep related microbial structures so far observed in the Black Sea (Fig. 1).

The extraordinary size of the numerous pillars extending up to about 4m from the seafloor that are erected by methane consuming microbial consortia indicates specific conditions for that area. Looking at the low growth rate to be assumed for these organisms living on anaerobic methanotrophy, a process that delivers small amounts of free energy, it must have

Figure1: Image of microbial reef structures (as seen from the submersible). A) Tip of a chimney-like structure. Free gas emanates in constant streams from the microbial structures into the anoxic sea water. B) Broken structure of approx. 1 m height. The surface of the structure consists of grey-black coloured microbial mat, the interior of the massive mat is pink. The greenish-greyish inner part of the structure consists of porous carbonate which encloses microbial mats and forms irregular cavities.

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Figure 2: left: Concentrations of dissolved organic and inorganic carbon at a deep water station, middle: Concentrations of methane and δ13CH4 at a deep water station, right: turbidity, concentration of methane and δ13CH4 above the GHOSTDABS-field, transition zone between oxic and anoxic water body at a water depth of 120 m accompanied by a turbidity maximum and maximum of methane consumption (AOM)

needed a long time period of more or less undisturbed growth with permanent supply of methane gas from the sediments to establish such huge structures. The upper age limit is given by the transition of the Black Sea from limnic to marine conditions at about 7.5 ky ago and the subsequent development of permanent anoxic water body. Dating of carbonates by 14C (Gulin et al., 2003) resulted in ages from 2.9 to 5.3 ky. However, the GHOSTDABS field is protected from slumping that is common along the shelf by its position morphological ridge. Moreover, it is located directly on a fault that channelled gas supply from deeper sediments most probably during thousands of years.

Water and gas chemistry In addition to the determination of dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) measurements of concentrations, δ13C and δ2H of methane in the water column and sediments were performed (Fig. 2).

Figure 2 shows an increase of concentrations of dissolved methane with increasing water depth and concentrations of up to 18 µmol l-1

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in a depth of 1800 m. The deep water CTDprofiles shown in Figure 2 observed at a station above a mud volcano in the central Black Sea. Therefore, the concentration maximum of methane at 1800m water depth may be interpreted as a plume of water masses flowing out of the tectonic ridge. δ13Cmethane values show large variations between –35 to –55 d (vs. VPDB) indicating different microbial production and consumption processes within the water column. Additionally, the biogeochemical work within GHOSTDABS focused on the major seep-area found during the cruise (GHOSTDABS-field). On the right side of Figure 1 concentrations and δ13C of dissolved methane within this area were shown. The steepest gradient of 13C was observed within the turbidity maximum at a water depth of 120 m pointing to the maximum of methane consumption at that depth. In addition to dissolved methane, concentrations and δ13C of emanating gas from sediments and microbial formations were analysed. The sampling was accomplished with the use of the submersible and a special gas sampling device. Methane concentrations vary between 95 and 99% (δ13C from –62.4 to –68.3 ‰). No trends were

Figure 3: left: δ13C-Werte of emanating gas sampled from the sediment and microbial structures at the GHOSTDABSfield, right: differences in δ13C of dissolved and free gas at different water depths. (filled circles = dissolved methane; open boxes = emanating gas)

Figure 4: Concentrations of methane and δ13C Methane obtained from a push-core within the GHOSTDABS-field (Treude et al., 2005)

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Table 1: Biomarkers found and the respective δ13C-values of a Black Sea mat and likely source organisms (the 2nd column gives the original stable carbon isotope signatures (Michaelis et al., 2002). The 3rd to 5th column give the results of an incorporation experiment using 13C enriched methane (discussion see below; Blumenberg et al., 2005).

Bulk mat Biomarker Archaea crocetane archaeol sn2-hydroxyarchaeol biphytane (C40:0) Bacteria ai-15-DAGE ai-pentadecanoic acid hexadecanoic acid 11-hexadecenoic acid 11-octadecanoic acid

start -72.2

month 3 -65.5

month 7 -62.6

month 12 n.a.

-94.7 -87.9 -90.0 -91.3

-88.8 -78.8 -87.0 n.a.

-88.4 -62.2 -89.9 n.a.

-89.5 -63.3 -88.3 -72.2

-89.5 -83.9 -82.6 -82.9 -86.2

-85.1 -64.2 -30.2 -31.5 -49.9

-83.7 -47.2 -26.8 +69.9 +25.4

-80.9 +3.8 +33.7 +84.5 +48.2

observed concerning the sampling location (seep, sediment, microbial constructions; Fig. 3).

However, significant differences exist between dissolved and free gas at the same sampling area indicating either a different gas source for both compartments or a larger degree of consumption of the water dissolved methane. Sediment push-cores have also been analysed for methane dynamics (Fig. 4).

The clear trends observed in concentration and δ13C allowed for a first estimation of the fractionation factor αC for the anaerobic oxidation of methane in Black Sea sediments (αC = 1.009; Seifert et al., 2005).

Microbial communities and biomarkers Geochemical investigations on microbial mats were performed, which had been sampled with a TV grab. Detailed lipid analyses have shown high concentrations of strongly 13Cdepleted biomarkers of AOM-performing microorganisms. These associates comprise of anaerobic methanotrophic archaea and sulfate-reducing bacteria. Characteristic archaeal biomarkers at AOM-sites are irregular isoprenoid hydrocarbons (crocetane and crocetenes, 2,6,10,15,19-pentamethylicosane and -cosenes), isoprenoid dialkylglyceroldiethers (archa-

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eaol, sn-2-hydroxyarchaeol) and glyceroldialkylglyceroltetraethers (GDGT). Lipids of the sulfate-reducing bacterial partners are terminal branched fatty acids (e.g. 12-methyltetradecanoic acid, ai-C15:0) and uncommon non-isoprenoidal monoalkylglycerolethers (MAGE) and dialkylglyceroldiether (DAGE). These biomarkers have been found within the Black Sea mat sample accompanied with strong depletions in δ13C-values showing an involvement in the methane-cycling (see first and second column of Table 1).

Additionally, a very uncommon component was observed within the massive pink mats (α,β‚-bishomohopanoic acid, δ13C = –78.4‰).

Although the source organism is still unclear, this is the first proof of hopanoids produced by organism thriving in an anaerobic environment. Moreover, the configuration of the hopanoic acid points to the direct microbial biosynthesis of the so-called geological α,β‚-form (Thiel et al., 2003).

In order to characterise variances in the microbial populations within the large bioherms, macroscopically different areas were sampled and separately analysed for their biomarker distributions and microbial interior by molecular microbiology. Interesting results of this

approach have shown, that – in addition to the ANME-1 dominated massive pink mats (Michaelis et al., 2002) – specific areas are ruled by members of the ANME-2 cluster, accompanied by obviously also different sulfate-reducing bacteria. Although, their biomarkers show strong discrepancies (Fig. 5) both bacterial associates belong to the Desulfosarcina/Desulfococcus-group.

The ANME-2/SRB associates appear to dominate in the outer parts of the chimney-like constructions, whereas ANME-1/SRB comprise the majority of micro organisms in the massive pink mats. The appearance of highly diverse but structured microbial populations within the Black Sea structures lead to the identification of group specific biomarkers for archaea of the ANME-1 and the ANME-2 cluster. Thus, ANME-1 archaea are the exclusive producers of specific GDGT, whereas members of the ANME-2 contain high concentrations of sn-2hydroxyarchaeol and crocetane (Blumenberg et al., 2004). These results underline the phylogenetic separation of both groups and hint to a closer relation of the ANME-1 to methanogens of the Methanomicrobiales as previously thought. Moreover, ANME-1 associated SRB mainly produce ether-bound lipids (MAGE and DAGE), which are minor compound classes in the ANME-2 associated SRB. These first results of the parallel approaches of biomarker and molecular microbiological ana-

lyses point to high diversities of AOM-performing micro organisms in the Black Sea, leading to large amounts of microbial biomass.

However, further detailed work is needed for a better understanding of the microbial ecologies and processes regulating the populations in the bioherms.

Moreover, for studies of methane uptake and metabolic and biosynthetic processes occurring in the Black Sea mats in vitro-experiments with 13CH4 were performed. The one-year lasting experiments have shown methane-uptake under laboratory conditions (bulk mat changed ~10‰). Very interesting are differences in the uptake of 13C into individual lipids. Table 1 gives the changes in δ13C of selected archaeal and bacterial biomarkers. In general, uptake rates of bacteria are higher than archaeal components. A maximum uptake of ~160‰ was observed for the ω5-hexadecenoic acid. Remarkable are the strong differences of the lipid compounds indicating different biosynthesis rates of the individual biomarkers or a high heterogenity of micro organisms growing in the mat (e.g. differences between ANME-1 and ANME-2; Blumenberg et al., 2005).

TEM analyses of fixed mat samples covering a lenticular carbonate concretion revealed abundant cylinder-shaped micro organisms having external sheaths. The sheaths seem to consist of a resistant biopolymer, as empty sheaths tend to become enriched and make up a major portion of the mats (> 80 vol.% in some sec-

Figure 5: Differences in fatty acid distributions between ANME-1 and ANME2 dominated mat samples normalised to i-C15:0 FA (=1), (Blumenberg et al., 2004)

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tions). The cylindrical to rod-like shape of single filaments was visualised by FE-SEM. Single cells have a diameter between 0.6 and 1 µm, and are variable in length (mostly about 3 µm).

They form multicellular filaments in which the single cells are separated by distinct invaginations. FISH analyses of these samples revealed strong signals from Archaea of the ANME-1 cluster, which has been previously related to AOM (Hinrichs et al., 1999). We assume that these ANME-1 Archaea are identical to those reported from this seep area using microscopy (Pimenov et al., 1997), FISH (Michaelis et al., 2002), and a 16S rRNA survey (Tourova et al., 2002). The ANME-1 probe also responded to extremely long multicellular chains, sometimes exceeding 100 µm in length. Similar filamentous forms were previously recognized microscopically by Pimenov et al. (1997), and interpreted as different growth stages of the shorter, cylinder-shaped microbes. ANME-1 Archaea with a like morphology were also visualized by FISH of microbial consortia from methane-rich sediments of the Eel River Basin (Orphan et al., 2002).

Intriguingly, the cylinder-shaped ANME-1 cells contain complex arrays of stacked internal membranes, a feature which has so far not been reported from Archaea. Similar stacks of intracytoplasmic membranes are the site of the C1-metabolism in Type I and Type X methanotrophic bacteria (for instance in the genus Methylococcus), and it has recently been reported that common gene coding for C1-transfer enzymes does exist among methanogenic Archaea and aerobic, methanotrophic bacteria (Chistoserdova et al., 1998). We thus suggested a function of the membranes in the methane metabolism of ANME-1 Archaea, which awaits verification in forthcoming studies.

Among other yet unidentified microbiota, a further component of the mat population are microbes forming large, localized colonies of some tenths of µm in diameter. FISH analyses revealed a strong response to the DSS 658 probe, suggesting that the colony-building

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organisms are SRB belonging to the Desulfosarcina/Desulfococcus (DSS) group. DSS have previously been reported as the predominant SRB in a microbial mat sampled from a near-by carbonate tower (Michaelis et al., 2002).

However, these authors reported coccoid forms with an internal diameter of about 0.6 µm. The bacteria reported here show similar internal diameters (0.5 to 1 µm), but longitudinal cell sections in TEM, and FE-SEM on native samples reveal that they are vibrioform, with cell lengths of 3 µm and more. It is therefore likely that these bacteria belong to a different DSS-taxon as those previously described (Reitner et al., 2005b).

The concretion-associated SRB were found in association with ANME-1 Archaea. The ANME-1 cells are frequent close to the periphery of SRB-colonies, but isolated clusters of ANME-1 Archaea have been observed as well.

This observation has to be checked up on other samples, but it may indicate that a spatial proximity to SRB is not required for the archaeal metabolism. A remarkable feature of the SRB is that they contain abundant granules resembling internal sulphur spherules common in some sulphide-oxidizing bacteria.

GC-MS analyses of apolar solvent extracts from two samples revealed elemental sulphur in its eight-membered ring structure as the main compound, with concentrations of 360 and 400 µg/g -1 carbonate, respectively.

Longitudinal cell sections also reveal intracellular aggregates of crystallites showing a somewhat blotchy diffraction contrast in the TEM. By size (~30 to 80 nm i.d.) and arrangement (chains, clusters), they strongly resemble magnetosomes formed by the so-called magnetotactic bacteria (Pósfai et al., 1998; Schüler, 1999). Indeed, FE-SEM/EDX analyses confirmed that the crystallites consist of iron sulphide. Notably the accumulation of intracellular iron sulphides characterises anaerobic magnetotactic bacteria, with ferrimagnetic greigite (Fe3S4) being the principal mineral (Pósfai et al., 1998; Schüler, 1999). It is therefore very likely, though remains to be confir-

med, that the iron sulphide aggregates of the Black Sea SRB consist of greigite as well (Reitner et al., 2005a).

Our observations raise questions on the nature of the biogeochemical pathways used by the magnetotactic SRB. Possibly, elemental sulphur is generated through the oxidation of H2S (being the product of sulphate-dependant AOM) by ferric iron. This could produce ferrous iron contributing to the formation of greigite in the magnetosome chains. Iron reduction, with an energy yield strongly exceeding that of sulphate reduction, may serve as an additional energy source. However, FeIII+ shows extremely low concentrations in marine waters. Some marine bacteria overcome iron shortage by synthesizing extracellular organic substances known as ÂťsiderophoresÂŤ. Some of these compounds consist of a peptidic headgroup with a fatty acid acyl appendage, and are excreted as micells that scavenge reactive iron in the extracellular environment (Martinez et al., 2000). In the mats associated with the carbonate concretions, we observed conspicious globular

structures, being 20 to 100 nm in diameter and surrounded by a lipid bilayer. Thus, these globules exactly match the size range and organization of amphiphilic siderophores (marinobactin) in their vesicle stage, that were reported from laboratory experiments with Marinobacter sp. (Martinez et al. 2000). Cell walls of concretion-associated SRB frequently invaginate upon contact with the globules and it seems as if they are deliberately transferred into the cells.

Reflection seismic The reflection seismic studies in the framework of the GHOSTDABS project were dedicated to locate possible occurrences of gas hydrates and free gas in the Black Sea and to evaluate the amount of methane confined to these occurrences. Worldwide, gas hydrates have been found in very different environments (e.g., Kvenvolden 1988; Ginsburg & Soloviev 1998; Kvenvolden & Lorenson 2001).

Considering the depositional environment, the semi-enclosed Black Sea is a potential candidate for gas hydrate accumulation on account of

Figure 6: Map showing the locations of MCS profiles and the OBS and OBH locations as well as the mapped fault system. OBS profiles are numbered.

Inset shows the position of the study area (dashed frame).

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its anoxic water regime (the chemocline lies at 110-140 m depth, CTD measurements of this cruise) which favours the preservation of organic matter in the sediments. Regions outside the stability field of gas hydrates, namely the shelf and upper slope, show a high gas content in the sediments. According to our observations, these areas are characterised seismically by extensive acoustic blanking as in the southeastern Black Sea (Ergün et al. 2002) and by numerous methane seeps (Polikarpov et al.

1992; Luth et al. 1999; Kutas et al. 2002; Michaelis et al. 2002). Gas hydrates were discovered in surface samples at different locations in the Black Sea below a water depth of 700 m where they are stable (Yefremova & Zhizhchenko 1974; Ginsburg et al. 1990; Limonov et al. 1994; Ivanov et al. 1998; Bohrmann et al. 2003).

Our study area is located in the northwestern Black Sea southwest of the Crimean Peninsula (Fig. 6), where the continental shelf is exceptionally wide (100-150 km) and the shelf break occurs at a water depth of approximately 130 m. Our seismic profiles document that this structural change is marked by normal faulting and a high slope gradient (Fig. 6).

1130 km of high-resolution multi-channel seismic (MCS) reflection profiles as well as three wide-angle seismic transects using ocean bottom seismometers (OBS) and ocean bottom hydrophones (OBH) were acquired in the study area (Fig. 6). The MCS system used consisted of a Geo-Prakla 8-channel mini-streamer with an active length of 100 m and a Seismic System Inc. mini-GI gun as a seismic source (total volume 0.98 l and frequencies up to 300 Hz). Data processing for identification of the bottom simulating reflector (BSR) includes filtering, stacking, signature deconvolution and time domain f-k migration. To determine the velocity structure of the uppermost sedimentary column, wide-angle measurements of the reflected and refracted waves were carried out. A Kirchhoff migration was applied to obtain an (depth) image of the subsurface and

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a P-wave as well as S-wave velocity model.

Seismic reflection data document for the first time the existence of a BSR in a limited area west of the Dnieper Canyon in the northwestern Black Sea (Fig. 7) (Lüdmann et al. 2004).

The quantification of methane associated with gas hydrates by mapping of the BSR has been applied by many authors (e.g., Holbrook et al.

1996; Bouriak et al. 2000; Tinivella & Accaino 2000; Milkov & Sassen 2000; Vanneste et al.

2001; Lodolo et al. 2002; Hornbach et al.

2003; Lüdmann & Wong 2003). However, studies on the Blake Ridge and the Hydrate Ridge show that the computed values, especially for the free gas zone, differ from direct measurements of methane concentration in drill holes (Dickens et al. 1997; Milkov et al. 2003). This may be related to a large number of poorly constrained variables and the assumptions implicit in the calculations based on seismic data. Hence, the calculations of methane carbon presented here should be regarded only as order-of-magnitude estimates.

The estimation of total methane relies on the following equation: gas content is equal to hydrate area multiplied by hydrate thickness, sediment porosity, fractional pore fill, and the gas expansion factor for methane (164 at STP) (Sloan 1990).

Seismic wide-angle data suggest that gas hydrates occupy an average of about 15±2 % of the pore space in a zone of 100 m thickness Latest analysis of P- and S-wave velocities by Zillmer et al. (2005) show a maximum gas hydrate saturation of 38±10 % at the base of the gas hydrate zone (BGHSZ). A conservative quantification of the amount of methane associated with this gas hydrate occurrence is therefore about 12±3 x 1011 m3 (0.6±0.2 gt of methane carbon).

Figure 7: Part of the processed MCS profile 35 which crosses the northern Dnieper Canyon (see Fig. 6 for location). Above the BSR (labelled) is a zone of weak reflections (GHZ); below it the amplitudes of the reflectors are enhanced. In addition, a montage (OBS profile 3) of the OBS stations (black triangles) along MCS profile 35 is shown.

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Additionally to the quantification of the methane, the local conductive heat flow was estimated from the BSR depth. This was done by assuming a linear temperature gradient and the following simple conductive heat transport relationship:

in which the bottom temperatures (Tbottom) were determined from CTD measurements of the GHOSTDABS cruise, and temperatures at the BSR depths (TBSR) were estimated using the experimental thermobaric stability conditions for the methane-seawater system of Dickens and Quinby-Hunt (1994). The gas hydrates found in sediment cores from the Black Sea contain mainly methane (99.1-99.9 %, Soloviev & Ginsburg 1994; Ginsburg & Soloviev 1998; Ivanov et al. 1998) and their isotopic composition points to a biogenic origin. The thermal conductivity (k) as a function of depth was taken from results of DSDP Leg 42B Site 379A in the central Black Sea (Ross et al. 1978). To calculate the lithostatic pressure at the BSR depth, we used the theoretical rockphysics model of Helgerud et al. (1999) and Ecker et al. (2000) for gas hydrate which replaced the pore fluids. The input parameters for the model are the average sediment composition and our measured P-wave velocities. The porosity as a function of depth was taken from the results of DSDP Leg 42B (Ross et al. 1978).

For the calculations, we assumed a sediment composition of 55 % clay, 30 % quartz and 15 % calcite (DSDP Leg 42B, Ross et al. 1978), with gas hydrate in the pore space. The same model was applied to the calculation of the gas hydrate concentration. For the determination of the BSR depth (ZBSR) as well as the bulk density, we used a velocity function deduced from the wide-angle measurements and the rock-physics model.

The cumulative error in BSR depth, thermal conductivity as well as bottom water salinity and temperature yields an uncertainty in the

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heat flow of approximately 27 %. Conductive heat flow deduced from the BSR depth is in the range of 21Âą6 to 55Âą15 mW m-2.

The study documents that although gas hydrates theoretically are present in the entire Black Sea below a water depth of about 700 m, a BSR rarely occurs. This implies that the conditions in the Black Sea allow only locally a substantial accumulation of free gas and the formation of gas hydrates in the HSZ. Figure 8 shows that a BSR is absent in the distal fan below a water depth of about 1400 m, in the deep basin, and on the continental slope where drift sediments occur. This could be attributed to intrinsically gas-poor (low Corg content) sediments, or to the fact that gas concentrations below the gas hydrate stability zone (GHSZ) are so low that the impedance contrast is insufficient to produce a BSR. In addition, low sediment permeability may inhibit the upward migration of gas and fluids into the GHSZ. In the upper Dnieper Canyon close to the prograding lowstand deltas at the shelf break, the sediments are presumably coarser, and the fluids and gas which migrated into the GHSZ allow gas hydrates to form (Fig. 8). Their ascent is probably promoted by mud diapirism triggered by tectonic movements along the NE-SW oriented normal faults or the NNW-SSE to NW-SE trending strike-slip system. The diapirism destroyed the original layering, thereby contributing to an overall higher flux rate. A continuation of these faults from the shelf edge into the basin is obscured by the limited penetration of the seismic signal due to acoustic masking below the upper layers of the mud diapirs. However, along the Dnieper Canyon east of the area of mud diapirism, our profiles show a fault which reaches the seafloor, suggesting that it is still active (Fig. 7).

This fault follows a major tectonic element (the West Crimean Fault, Okay 1994) which offsets the Alpine Crimean Mountains against the Cimmerian Balkanide Kalamit Ridge (Robinson 1997). Thus, we speculate that the faults on the shelf continue into the mud diapir area and that in addition to geological factors such as

Figure 8: Mapped seismic facies and outline of the areas of BSR occurrence, showing that the BSR is confined largely to the proximal fan.

disequilibria compaction, tectonic movements may have triggered the diapirism.

In general, gas hydrates are finely disseminated in the sediments and if their concentrations are low, they are probably of little economic interest. However, areas of mud diapirism, especially when they occur in medium water depths on the continental slope and when the mud diapirs penetrate high porosity strata in the GHSZ, could be future exploitation targets.

Carbonates During the GHOSTDABS expedition, several types of authigenic carbonates and microbial mats were sampled with the TV-guided grab and the submersible JAGO. Up to now, analyses have been focussed on the geobiology of intrasedimentary precipitates that appear to make up the quantitatively most significant portion of the methane carbonates in the Black Sea. The results obtained during

these studies are subject of two publication (Reitner et al., 2005a and b) and are briefly outlined below.

Among the distinctive carbonate types studied are porous plates of lithified and semi-lithified sediments. These plates are cemented by automicritic, high Mg-calcite. Methane microseepage is thought to have induced the formation of large elongated (»stromatactoid«) and spherical voids, with the latter probably representing ‘lithified’ gas bubbles. These structures resemble the »birds eye« structures often observed in fossil tidal flat carbonates (Fig. 9).

Due to rapid lithification, the cavities are not compacted. Some of them are cemented by granular high Mg-calcite, and most internal surfaces are covered by biofilms. All carbonates are strongly depleted in 13C. δ13C values range from ˜-27 to –41‰ PDB for high Mg-calcite and from –26 to –38‰ for aragonite phases, indicating that a large portion of the carbonate derives from AOM (Fig. 8).

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Figure 9: Morphology and fabrics of seepage-related carbonates. (A) Cemented carbonate sediment with abundant cavities eventually representing former gas bubbles. Due to early cementation the carbonate crust has not been affected by compaction. The coin is 19 mm in diameter. (B) Thin section of the crust shown in A (air dried sample), showing voids within the carbonate that formed by gas seepage. Internal surfaces of the cavities are commonly covered with biofilms. (C) UV-Fluorescence micrograph showing birdâ&#x20AC;&#x2122;s eye-like voids within the concretionâ&#x20AC;&#x2122;s matrix formed of high Mg-calcite.

The strongly fluorescent rim at the contact between sediment and cement may result from organic matter derived from a mineralized biofilm. The coin is 19 mm in diameter. (D) Lenticular carbonate concretions embedded in lithified, well-bedded background sediments. Note that the lenticular concretions in the upper left are orientated sub-vertical to bedding. Insert shows a thick microbial mat surrounding the carbonate concretion. (E) Thin section of a carbonate concretion showing that bakkground sediment predominates in the central part of the concretion, whereas the outer regions consist of virtually pure authigenic carbonate. Assuming a radial growth mode, this suggests that the initial stage of concretion formation involves the cementation of sediment, whereas further growth is characterized by displacement of the surrounding material. (F). Lenticular concretions are commonly surrounded by biofilms. The fluorescence micrograph using a Zeiss no. 487709 fluorescence filter (resulting in a green fluorescence) indicates enrichment of organic matter within the enclosing sediment as well as in the peripheral, newly forming portions of the concretion.

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A further carbonate variety sampled comprises lenticular concretions forming within the sediments. They consist of high Mg-calcite and range in size from several centimetres to a few decimetres. Several concretions are commonly interconnected and then form larger aggregates. The lenticular concretions consist of ca.

100 µm sized, elongated rod- to dumbbellshaped crystal aggregates of fibrous calcite.

Staining techniques revealed dense populations of micro organisms surrounding the crystallites, indicating that microbes mediate the formation of these precipitates. The aggregates showed a conspicuous near-rectangular orientation. We suggest that AOM triggers carbonate precipitation through an increase in carbonate alkalinity, whereby the regular fabric of the precipitates may be controlled by the meta-structures of organic matrices provided by excreted EPS (extracellular polymeric substances).

The high Mg-calcite yield δ13C values between –25.5 and –28.2‰ PDB, indicating that a significant portion of the carbonate carbon derives from AOM. In organic extracts of bulk carbonate and a microbial mat sample, we found isoprene-based membrane lipids derived from Archaea, and carbon skeletons of putative bacterial origin. Strong depletions in 13C, with δ13C values in the range of –70 to –100‰ allow to distinguish methane-related compounds from allochthonous marine lipids (δ13C ~ –20 to –30‰), and imply that methane carbon is transferred into the biomass of the source biota. Similar biomarkers have been observed in other mats covering Black Sea seep-carbonates (Thiel et al., 2001; Michaelis et al., 2002) and in fossil, methane-rich environments (Thiel et al., 2001; Peckmann & Thiel, 2004). Due to their structures and 13C-depletions, these compounds could be consistently related to contributions from methane-consuming Archaea and metabolically associated SRB, although their precise taxonomic affiliation still remains to be elucidated.

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Orphan VJ, House CH, Hinrichs KU, McKeegan KD, DeLong EF (2002) Multiple archaeal groups mediate methane oxidation in anoxic cold seep sediments. Proc. Natl. Acad. Sci. USA 99: 7663-7668 Pape T, Blumenberg M, Seifert R, Egorov VN, Gulin SB, Michaelis W (2005) Lipid geochemistry of methane-seep-related Black Sea carbonates. Palaeogeogr Palaeoclimatol Palaeoecol 227: 31-47 Peckmann J, Thiel V (2004) Carbon cycling at ancient methane-seeps. Chem Geol 205: 443-467

Pimenov NV, Ivanova AE (2005) Anaerobic methane oxidation and sulfate reduction in bacterial mats on coral-like carbonate structures in the Black Sea. Mikrobiologiya 74: 420-429.

Pimenov NV, Rusanov II, Poglazova MN, Mityushina LL, Sorokin DY, Khmelenina VN, Trotsenko YA (1997) Bacterial mats on corallike structures at methane seeps in the Black Sea. Mikrobiologiya 66: 354-360 Polikarpov GG, Egorov VN, Gulin SB, Gulin MB, Stokozov NA (1992) Gas sseps from the bottom of the Black Sea – a new object of molismology. In: Polikarpov GG (ed) Molismology of the Black Sea. Nauka, Kiev:10-28 (in Russian) Pósfai M, Buseck PR, Bazylinski DA, Frankel RB (1998) Iron sulfides from magnetotactic bacteria: Structure, composition, and phase transition. American Mineralogist 83: 1469-1481 Reeburgh WS (1976) Methane consumption in Cariaco Trench waters and sediments. Earth Plan Sci Lett 28: 337-344 Reitner J, Peckmann J, Reimer A, Schumann G, Thiel V (2005a) Methane-derived carbonate build-ups and associated microbial communities at cold seeps on the lower Crimean shelf (Black Sea), Facies (in press)

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Reitner J, Peckmann J, Blumenberg M, Michaelis W, Reimer A, Thiel V (2005b) Concretionary methane-seep carbonates and associated microbial communities in Black Sea sediments. Palaeogeogr Palaeoclimatol Palaeoecol 227: 18-30 Robinson AG (1997) Introduction: tectonic elements of the Black Sea region. In: Robinson AG (Ed.), Regional and Petroleum Geology of the Black Sea and Surrounding Regions. Am Assoc Petr Geol Mem 68:1-6 Ross DA, Neprochnov YP and the Scientific Party of DSDP Leg 42B (1978) Initial Reports of the Deep Sea Drilling Project Leg 42B. Washington DC Schüler, D (1999) Formation of magnetosomes in magnetotactic bacteria. J Mol Microbiol Biotechnol 1: 79-86 Seifert R, Nauhaus K, Blumenberg M, Krüger M, Michaelis W (2005) Methane dynamics in a microbial community of the Black Sea traced by stable carbon istotopes in vitro. Org Geochem (subm) Sloan ED Jr (1990) Clathrate Hydrates of Natural Gases. Marcel Dekker, Inc., New York Soloviev V, Ginsburg GD, (1994) Formation of submarine gas hydrates. Bull Geol Soc Denmark 41(1):86-94 Thiel V, Blumenberg M, Pape T, Seifert R, Michaelis W (2003) Unexpected occurrence of hopanoids at gas seeps in the Black Sea. Org Geochem 34: 81-87 Thiel V, Peckmann J, Richnow HH, Luth U, Reitner J, Michaelis W (2001) Molecular signals for anaerobic methane oxidation in Black Sea seep carbonates and a microbial mat. Mar Chem 73: 97-112

Tinivella U, Accaino F (2000) Compressional velocity structure and Poissonâ&#x20AC;&#x2122;s ratio in marine sediments with gas hydrate and free gas by inversion of reflected and refracted seismic data (South Shetland Islands, Antarctica). Mar Geol 164:13-27 Tourova TP, Kolganova TV, Kuznetsov BB, Pimenov NV (2002) Phylogenetic diversity of the archaeal component in microbial mats on corallike structures associated with methane seeps in the Black Sea. Mikrobiologiya 71: 230-236 Treude T, Knittel K, Blumenberg M, Seifert R, Boetius A (2005) Subsurface microbial methanotrophic mats in the Black Sea. Appl Environ Microbiol 71: 6375-6378 Vanneste M, De Batist M, Golmshtok A, Kremlev A, Versteeg W (2001) Multi-frequency seismic study of gas hydrate-bearing sediments in Lake Baikal, Siberia. Mar Geol 172:1-21 Winguth C, Wong HK, Panin N, Dinu C, Georgescu P, Ungureanu G, Krugliakov VV, Podshuveit V (2000) Upper Quaternary water level history and sedimentation in the northwestern Black Sea. Mar Geol 167 (1-2):127-146 Yefremova AG, Zhizhchenko BP (1974) Occurrence of crystal hydrates of gas in sediments of the modern marine basins. Doklady Akademii Nauk SSSR 214:1179-1181 Zillmer M, Flueh ER, Petersen J (2005) Seismic investigation of a bottom simulating reflector and quantification of gas hydrate in the Black Sea. Geophys Res Int: doi:10.1111/j.1365246X.2005.02/2635.x

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Microbial Methane Turnover at Marine Methane Seeps (MUMM – SPI) Treude T. (1), Niemann H. (1), Orcutt B. (2), Joye S. (2) Witte U. (1), Jørgensen B. B. (1), Boetius A. (1, 3, 4) (1) Max Planck Institute for Marine Microbiology, Celsiusstr. 1, 28359 Bremen, Germany, aboetius@mpi-bremen.de (2) Department of Marine Sciences, University of Georgia, Athens, Georgia, USA (3) Alfred Wegener Institute for Polar and Marine Research, 27515 Bremerhaven, Germany (4) International University Bremen, 28759 Bremen, Germany

1. Introduction Methane is an important energy source for society, but it is also a greenhouse gas which has influenced earth’s climatic history substantially. Methane can be produced by three processes: abiotically through hydrothermal processes, thermogenically by alteration of organic matter, and microbially as the end-product of degradation processes in various environments such as natural wetlands, rice fields, ruminants and aquatic sediments. Understanding the evolution and development of earth’s methane budget is a relevant problem in the earth sciences. A main sink for methane on earth is a microbial process called anaerobic oxidation of methane (AOM). In marine sediments, the majority of produced methane is consumed by AOM before it can enter the hydrosphere or the atmosphere (Reeburgh, 1996). Instead of oxygen, the responsible microbes use sulfate, a compound of seawater, to oxidize the methane anaerobically (Barnes and Goldberg, 1976):

According to current knowledge, the process is mediated by a syntrophic consortium of methanotrophic archaea and sulfate-reducing bacteria (Zehnder and Brock, 1980; Hoehler et al., 1994; Boetius et al., 2000). The first investigation of AOM dates back to the year of 1974, when Martens and Berner (1974) speculated about the cause for conspicuous metha-

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ne and sulfate profiles in organic rich sediments. The scientists observed that methane was not accumulating before sulfate was exhausted. From the decrease of methane concentrations in the sulfate-reducing zone, they concluded that methane must be consumed with sulfate. Zehnder and Brock (1979 and 1980) were the first who demonstrated methane oxidation under anoxic conditions and hypothesized a coupled two-step mechanisms of AOM. They postulated that methane is first activated by methanogenic archaea, working in reverse, leading to the formation of intermediates, e.g. acetate or methanol. In a second step, the intermediates are oxidized to CO2 under concurrent sulfate reduction by other non-methanogenic members of the microbial community. Since then, the knowledge of AOM increased substantially with biogeochemical, microbiological, and molecular investigations adding one peace after the other to the big puzzle. Measurements with radiotracers enabled the first direct quantification of AOM and concurrent sulfate reduction rates in anoxic marine sediments (Reeburgh, 1976; Iversen and Blackburn, 1981; Devol, 1983). By this technique, traces of 14CH4 and 35 SO4-2 are added to the sediment and their conversion into 14CO2 and H235S is quantified.

Including the total methane and sulfate concentration of the sediment, turnover rates can be calculated. Applying radiotracer techniques, it was then for the first time possible to demonstrate a 1:1 ratio of AOM and sulfate

reduction in the sulfate-methane transition zone, confirming the close coupling between the two processes (Iversen and Blackburn, 1981). Only few measurements of AOM rates in marine sediments existed prior to the MUMM project (for a review see Hinrichs and Boetius, 2002). Estimates for AOM at methane seeps were missing, although seeps generally exhibit much higher methane fluxes compared to systems controlled by molecular diffusion (Wallmann et al. 1997). The methane flux is defined as the transport of methane per time through a given area. At methane seeps, forces like tectonic displacements or gas overpressure cause an advective transport of methane and fluids to the sediment surface. At the beginning of the MUMM project only a few measurements of sulfate reduction rates at methane seeps were published, which suggested extremely high methanotrophic activity in these kinds of environments (Boetius et al., 2000; Aharon and Fu, 2000). Hence, the quantification of methane turnover at gas seeps was one of the main goals of the project.

2. Objectives of the Project The aim of TP-I within the GEOTECHNOLOGIEN Project MUMM was the determination of methane turnover rates in methane seep environments applying radiotracer techniques.

Methane in diffusion controlled systems is quantitatively removed by AOM before it reaches the sediment-water interface. Our study investigated the functioning of the microbial barrier against methane in advective systems, which are characterized by high methane fluxes including ebullition of free gas or hydrate.

In collaboration with the other TPs of the MUMM Project, turnover rates were compared with the abundance, diversity and physiology of the methanotrophic community at the different study sites. Finally, we wanted to compare the biogeochemistry of different methanebearing environments such as mud volcanoes (Hรฅkon Mosby Mud Volcano), methane seeps comprising gas hydrates (Hydrate Ridge and Gulf of Mexico), methane seeps in anoxic basins (Black Sea), gassy organic-rich sedi-

ments with actual methane production (e.g.

Eckernfรถrde Bay, German Baltic), and diffusive systems (e.g. Chilean continental margin).

3. Present Status and Results A considerable AOM activity was detected at all methane-bearing environments investigated during the MUMM Project. The sites investigated within the MUMM project included as diverse habitats as coastal and deep-sea sediments, oceanic and brackish environments as well as surface and subsurface sediments.

Hence, our study demonstrated that AOM is a ubiquitous process in marine systems. A direct comparison of the different sites revealed that the magnitude of AOM correlated with the methane fluxes. Accordingly, turnover rates were found to be several orders of magnitude higher at advective methane seeps compared to diffusive sites (Table 1). The studies further showed that AOM leads to a substantial removal of dissolved methane before it reaches the hydrosphere even in systems controlled by advection. The main escape route for methane is as free gas, in the form of rising gas bubbles.

Our findings therefore confirm earlier assumptions that AOM controls methane emission from the ocean over a wide range of methane fluxes. Another interesting observation was to find the hot spots of AOM activity situated closer to the sediment surface when the methane flux is high. In diffusive systems (Chilean continental margin), AOM hot spots were situated deeper than 1 m below the sediment surface.

In gassy sediments of Eckernfรถrde Bay, the zone was closer to the surface between 20 and 30 cm sediment depth. At methane-seep sites (Hydrate Ridge, Gulf of Mexico, Haakon Mosby Mud Volcano), methane was consumed already within the top 10 cm of the sediment, below which sulfate was depleted. Hence, another main factor determining the positioning of AOM zones thereby appears to be availability of sulfate. In anoxic parts of the Black Sea, this results in the formation of methanotrophic reefs above methane seeps.

Here the methanotrophic consortia grow along gas bubble pathways into the water column

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Table 1: Methane turnover in different methane-bearing habitats.

for an optimized access to methane and sulfate (Fig. 1). Besides turnover rate and depth of AOM hot spots, also the biomass of the methanotrophic community correlated with the methane flux. The highest methanotrophic biomass is represented by the microbial mats of Black Sea seeps. The mats comprise about 1011 methanotrophic cells per cm3 of mat. Still high in methanotrophic biomass are sediments of methane seeps such as Hydrate Ridge, where more than 90% of the total microbial biomass was found to be methanotrophic. Cell numbers of methanotrophs than further decrease with decreasing methane flux. In subsurface sediments we found numbers as low as 105 methanotrophic cells per cm3 of sediment, which were however still able to control methane diffusion.

Another characteristic of AOM in sediments driven by advective processes is the extreme small-scale variability (cm-m scale). Replicates of rate measurements varied by up to one order of magnitude within a sampled area of not more than 0.25 m2. The biogeochemistry of methane-seep sediments appears very variable over space and time due to permanent changes in methane migration pathways, bioturbation of animals as well as geological disturbances in the sediment (Fig. 2). Main problems for an accurate quantification are the artifacts introduced by sampling and recovery

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of gas-laden sediments from deep waters.

Sampling and ex situ measurement procedures terminate natural processes such as flow and interaction with other organisms, and may affect turnover rates considerably. Most likely, chemoautotrophic communities, which re-oxidize the sulfide produced during AOM are important in controlling the microenvironment at seeps. Beggiatoa, a sulfide-oxidizing filamentous microbe, removes sulfide efficiently and may also introduce sulfate into deeper sediment layers via vertical migration. An even stronger mixing effect may result from the digging activity of clams like Calyptogena and Acharax, which harbor symbiotic sulfide-oxidizing bacteria in their gills. These large animals may even introduce oxygen into deeper sediment layers. During our investigations of methane-seep sediments, we combined AOM and sulfate reduction rate measurements with vertical profiling of oxygen and sulfide, indicating such impacts of the chemoautotrophic community. Furthermore interesting in this context are correlations observed between the diversity of the methanotrophic community, whose investigation is part of the TP-IV, and the presence of different chemoautotrophic organisms. Small-scale variations and correlations of both biogeochemical gradients and microbial diversity will be a major objective in the second phase of the MUMM Project (2005-2008).

Figure 1: A microbial methanotrophic reef structure (ca. 1 m high) growing above methane seeps in the Black Sea. The inner carbonated core is externally covered by pure microbial biomass. Very soft nodules (close-up) of microbial biomass grow on top of the reefs and bear pure methane gas.

4. Conclusions With the results gained during TP-I of the MUMM Project the knowledge about the global role of AOM in marine sediments increased considerably. Turnover rates measured in methane-seep sediments revealed an important role of methanotrophic communities in the control of methane emission. This microbial »filter« seems to be able to adjust to a wide scale of methane and sulfate fluxes. It will be one objective of MUMM II to characterize the microbial habitats at methane-seeps on smaller scales and with in situ technologies.

Acknowledgements We thank our collaborators in the GEOTECHNOLOGIEN program »Gashydrate im Geosystem«, projects GHOSTDABS, LOTUS, and OMEGA. Further thanks are due to the following institutions for cooperation in field and lab work: IFM-GEOMAR (Germany), AWI-Bremerhaven (Germany), University of Bremen (Germany), RCOM (Germany), IFREMER (France), University of Georgia, Athens (USA), We also thank the crews and shipboard scientific parties of the expeditions on the research vessels »Sonne«, »Meteor«, »Polarstern«, »Heincke«, »L´Atalante«, »Littorina«, »Seaward Johnson« and »Professor Logachev« as well as the crews of the submersibles and ROVs »Jago«,

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Figure 2: Scheme illustrating heterogeneity of surface sediments at methane seeps causing deviations in methane turnover rates. The sediments are covered by different chemoautotrophic organisms, i.e. Beggiatoa, Calyptogena and Acharax (see text).

»Johnson Sealink«, »Trieste«, and »Victor 6000«. The study was funded by the Bundesministerium für Bildung und Forschung and the Deutsche Forschungsgemeinschaft in the frame of the GEOTECHNOLOGIEN project MUMM (FKZ 03G0554A). We thank the Max Planck Society for supplementary funding of this project.

References Aharon, P. and Fu, B. (2000). Microbial sulphate reduction rates and sulphur and oxygen isotope fractionations at oil and gas seeps in deepwater Gulf of Mexico. Geochim. Cosmochim.

Acta 64, 233-246

rently mediating anaerobic oxidation of methane. Nature 407, 623-626.

Devol, A.H. (1983). Methane oxidation rates in the anaerobic sediments of Saanich Inlet.

Limnol. Oceanogr. 28(4), 738-742.

Hinrichs, K.-U., Boetius, A. (2002). The anaerobic oxidation of methane: new insights in microbial ecology and biogeochemistry. In: G. Wefer, D. Billett, D. Hebbelnet al (Eds.), Ocean Margin Systems. Springer-Verlag, Berlin, pp. 457-477.

Barnes, R.O., Goldberg, E.D. (1976). Methane production and consumption in anoxic marine sediments. Geology 4, 297-300.

Hoehler, T.M., Alperin, M.J., Albert, D.B., Martens, C.S. (1994). Field and laboratory studies of methane oxidation in an anoxic marine sediments: evidence for methanogen-sulphate reducer consortium. Global Biochem. Cycles 8(4), 451-463.

Boetius, A., Ravenschlag, K., Schubert, C.J., Rickert, D., Widdel, F., Giesecke, A., Amann, R., Jørgensen, B.B., Witte, U., Pfannkuche, O. (2000). A marine microbial consortium appa-

Iversen, N., Blackburn, T.H. (1981). Seasonal rates of methane oxidation in anoxic marine sediments. Appl. Environ. Microbiol. 41(6), 1295-1300.

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Martens, C.S., Berner, R.A. (1974). Methane production in the interstitial waters of sulphate-depleted marine sediments. Science 185, 1167-1169.

Reeburgh, W.S. (1976). Methane consumption in Cariaco Trench waters and sediments. Earth Planet. Sci. Lett. 28, 337-344.

Reeburgh, W. S. (1996). »Soft spots« in the global methane budget. Microbial Growth on C1 Compounds. L. M.E. and F. R. Tabita. Intercept, Andover, UK, Kluwer Academic Publishers: 334-342.

Wallmann, K., Linke, P., Suess, E., Bohrmann, G., Sahling, H., Schlüter, M., Dählmann, A., Lammers, S., Greinert, J., Von Mirbach, N. (1997). »Quantifying fluid flow, solute mixing, and biogeochemical turnover at cold vents of the eastern Aleutian subduction zone.« Geochim.

Cosmochim. Acta 61(24): 5209-5219.

Zehnder, A.J.B., Brock, T.D. (1979). Methane formation and methane oxidation by methanogenic bacteria. J. Bacteriol. 137(1), 420-432.

Zehnder, A.J.B., Brock, T.D. (1980). Anaerobic methane oxidation: occurrence and ecology.

Appl. Environ. Microbiol. 39(1), 194-204.

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Microsensor Measurements in Gas Hydrate Bearing Sediments (MUMM â&#x20AC;&#x201C; SPII) De Beer D. Max-Planck-Institute for Marine Microbiology, Bremen, Germany, E-Mail: dbeer@mpi-bremen.de

1. Introduction Methane hydrates form transient reservoirs between methane seeping from deep sediments and the oxidized upper sediments and overlying water phase. The methane released from hydrates is transported upwards by diffusion and advection, and then oxidized by microorganisms. The main microbiological processes associated with gas hydrates are anaerobic methane oxidation (AOM) coupled to sulfate reduction (SR) (Boetius et al. 2000) and sulfide oxidation by Beggiatoa or related bacteria (Schulz and Jørgensen 2001). As result of this efficient oxidative microbial filter only a small fraction of the enormous amount of reducing power residing in the sediments reaches the seawater. Instead, areas rich in gas hydrates are highly diverse oases teeming with fauna (Boetius and Suess 2004). The basis of the food chain is the chemosynthetic production of microbial biomass by methane and sulfide oxidation. These are exclusively prokaryotic processes. Consortia of Archaea and sulfate reducing bacteria (SRB) oxidize methane under the formation of sulfide. The sulfide is further oxidized by sulfide oxidizers. Thus the methane seeps are typically covered with veils of Beggiatoa, i.e. giant filamentous bacteria that oxidize sulfide with nitrate that is stored in their vacuoles. A second common path is the oxidation of methane by aerobic symbioses between methane oxidizing bacteria and tube worms or bivalves. Also symbiotic relations with aerobic sulfide oxidizers are found.

Although anaerobic oxidation of methane is essential for the removal of methane, most biomass production occurs by the energetic

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much more favorable sulfide and aerobic methane oxidation. Thus these oases of life are not autarkic: essential is the supply of electron donors oxygen and sulfate, which formation is finally dependent on photosynthesis. Our central hypothesis is that the transport of the electron donors governs the development of different habitats.

2. Objectives of the Project Our goal is to learn why so many different habitats are present in and around cold seeps, and which conditions leads to the development of a certain microbial and faunal community. We measured (1) the process rates, (2) their stratification inside the methane rich sediments and (3) the mass transfer phenomena occurring in these sediments with combinations of in situ techniques. In situ measurements are more difficult than ex situ analyses on retrieved sediment, but they are necessary due to the side effects of pressure release.

Retrieval of pressurized sediments with gas hydrates or dissolved gasses is impossible without severe disturbance due to outgassing (see Fig. 1). The work is organized in 3 experimental units that complement each other. The integration of in situ rate measurements of AOM, characterization of the microenvironments in the sediments with high spatial resolution and the mass transfer phenomena result in actual turnover rates of methane and give insight in the regulatory mechanisms.

Figure 1: The retrieval of cold seep sediments is often highly disturbing (see bottom photo). In situ measurements with microsensors (top left) result in different microprofiles than those measured on these retrieved cores, although the essential features are still present. Note that the left and right graphs are identical, but differently scaled, to better display the phenomena near the sediment-water interface.

3. Present Status and Results / Methods & Results / Results The main tools for our research are instruments that measure on the seafloor 1) the exchange of oxygen and methane (benthic chambers), 2) sulfate reduction rate measurements based on injecting 35SO42- into a sealed volume of sediment, 3) microprofiles of O2, H2S, pH and temperature (see Fig. 1). We are also working on a microsensor for CH4 measurements, which is a major technological challenge that so far has not been solved. Technical development is in progress on the in situ sulfate reduction rate experiments. We have performed 2 cruises on which we were able to deploy the instruments in situ. Highly successful were the microsensor measurements, with which we characterized the actual microenvironments of AOM. This allowed conclusions on mass transfer phenomena and rates.

First successful in situ measurements with pH, H2S and O2 microsensors were done in shallow water in the Baltic Sea near Kiel (Eckernfรถrder Bucht). As the AOM zone was located between 25-40 cm below the sediment-water interface, very long microsensors were used (60 cm). Indeed a sulfide peak was observed in the zone of high AOM activity. Furthermore, the measurements showed a separation between the oxic and sulfidic zone of ca 10 cm.

This zone was inhabited by gliding Beggiatoa, a common companion to be found above AOM sediments. Most interesting were the pH profiles, inside the Beggiatoa inhabited zone.

Two explanations for these unusual profiles are possible: 1) sulfide oxidation occurs in two spatially separated steps: sulfide to sulfur and sulfur to sulfate, 2) sulfide oxidation is coupled to iron and manganese cycling. We will further investigate both hypotheses by detailed analy-

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ses of sulfur intermediates and Fe/Mn distributions. In the AOM zone no increased pH was observed. This was speculated as AOM is often associated with calcite precipitation; however, the measurements showed that AOM does not lead to a shift in the carbonate chemistry that can explain calcium precipitation.

A second cruise led to the Haakon Mosby Mud Volcano, on the continental slope between Norway and Spitsbergen. Combined in situ and on-board measurements were done with all microsensors available for seawater studies.

All 7 deployments with the free-falling lander produced valuable data. The profiles were the first recorded in this mud volcano and the first in sediments with high AOM activity. Thus these data represent the actual microenvironment of AOM. We collaborated with Dr. M. Schl端ter and Dr. E. Sauter (Alfred Wegener Institute for Polar Research), who used our sensors on their profiling unit that was deployed with an ROV. Essential was that they could position, using the ROV, the profiling unit exactly in the middle of the volcano, an area that was not reached by the free-falling lander.

Also new mats were discovered near seeps that could only be investigated with the ROV positioned equipment. The combined results gave a good overview of the different habitats of the volcano, the main microbial processes and the controlling factors. The volcano can roughly be divided in 3 concentric areas: Area 1) The central area consisting of grey subsurface mud. Here no AOM was observed and no sulfide was detected. The high oxygen

uptake can be attributed to aerobic methane oxidation. The retrieved sediments showed lower interfacial oxygen gradients, which is highly unusual, as normally the oxygen influx strongly increases upon retrieval. The difference between in situ and ex situ profiles can only be explained by upward advection: the porewater flows upward through the sediments with a velocity of ca 12 m per year.

Area 2) A surrounding area with extensive Beggiatoa mats (Fig. 2). This is a site of high AOM activities and thus of sulfide production.

The sulfide is quantitatively oxidized by Beggiatoa, large nitrate storing bacteria that are typically found on sediments with high sulfidogenic activity. Remarkable is the clear sulfide peak at a depth of ca 2-3 cm, which coincides with the AOM zone. Our in situ measurements showed that the AOM zone is ca 0.5 cm thick, which is supported by other observations, e.g. FISH studies and fatty acid distributions. Remarkable is the sulfide decrease below the AOM zone. This phenomenon is compatible with an upwards advection of ca 3 m per year: sulfide is produced in the AOM zone, part of it diffuses downwards but is blown upwards by advection with equal flux rate. The difference in oxygen gradients at the sediment-water interface indicated a similar porewater flow velocity. The upflow of sulfate free porewater limits the sulfate penetration to several cm. It seems most likely that the downward diffusion of sulfate against an upflow of porewater determines the narrow zone of AOM. Indeed, retrieved cores showed a sharp

Figure 2: Beggiatoa are large filamentous bacteria that can form white sheets on sediments. In their vacuoles upto 0.5 M nitrate is stored that is used to oxidize sulfide to sulfur (visible as small granules in right panel). Their energy reserves (sulfur and nitrate) and gliding motility allows them to commute over large distances (cm to m). (below)

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decrease of sulfate with depth. All produced sulfide was oxidized within the sediment. No overlap between the oxic and sulfidic zone was observed. The mass balance showed that far too little oxygen diffused into the sediments to oxidize all sulfide. Beggiatoa seems responsible for the sulfide oxidation. Nitrate is actively accumulated into their vacuoles and transported downwards within the gliding bacteria to the sulfidic zone, where it is used to oxidize sulfide. No remarkable pH changes were found in the AOM zones.

Area 3) An outer area dominated by Pogonophora. These are symbiotic tube worms, known to oxidize methane aerobically. It can not be excluded that also sulfide oxidation occurs. The temperature profile showed that at least the top 5-10 cm were efficiently ventilated by the activity of the worms. Due to this ventilating activity, sulfate is transported deep into the sediments.

Therefore AOM can also take place very deep, at ca 70 cm sediment depth.

4. Conclusions We found that microbial processes in gassy sediments are ruled by transport of the main reactants: methane, sulfate, sulfide and nitrate. Three transport mechanisms were recognized as important: advection, diffusion and nitrate uptake and transport inside gliding bacteria. The high upflow of porewater in the center prevents significant penetration of sulfate into the sediments, thus AOM is not possible.

In the Beggiatoa fields upflow is lower, but significant, limiting the sulfate penetration to 2-3 cm depth. We speculate that the upflow prevents settling of tube worms: their tubes would form channels of preferred upflow and they would suffocate quickly. In the Pogonophora fields upflow is probably very low, just sufficient to bring methane upwards. Here AOM occurs at 70 cm depth, fuelled by sulfate pumped down by the ventilating activity of the worms. The cold seeps are highly interesting areas, with a diverse mosaic of habitats.

Thus they form a natural laboratory for micro-

bial ecologists. The research must be done by in situ equipment, thus we will focus our efforts in further technological development, especially modules for use as payloads of ROVs.

Acknowledgements We are grateful for the technical assistance with the deployments by Gaby Eickert, AnneKatrin Schlesier and Axel Nordhausen, as well as for the excellent microsensors from Gaby Eickert, Ines Schröder, Vera Hübner, Karin Hohmann and Cecilia Wiegand. We thank Lubos Polerecky for mathematical modelling, and Bo Barker Jørgensen, Norbert Kaul and Jean-Paul Foucher for helpful comments. The team from GENAVIR/IFREMER is acknowledged for the expert operations with ROV »Victor«. We thank the crew and officers of the research vessel »Polarstern« for their excellent support. The study was funded by the Bundesministerium für Bildung und Forschung and the Deutsche Forschungsgemeinschaft in the frame of the GEOTECHNOLOGIEN project MUMM (FKZ 03G0554A).

References Boetius, A., K. Ravenschlag, C. Schubert, D. Rickert, F. Widdel, A. Gieseke, R. Amann, B. B. Jørgensen, U. Witte, and O. Pfannkuche. 2000.

A marine microbial consortium apparently mediating anaerobic oxidation of methane.

Nature 407: 623-626.

Boetius, A., and E. Suess. 2004. Hydrate Ridge: a natural laboratory for the study of microbial life fuelled by methane from near-surface gas hydrates. Chem. Geol. 205: 291-310.

Schulz, H. N., and B. B. Jørgensen. 2001. Big Bacteria. Ann. Rev. Microbiol. 55: 105-137.

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Biomarker Signatures of the Anaerobic Oxidation of Methane (MUMM – SPIII) Elvert M. (1,2), Niemann N. (1), Orcutt B. (3), Jørgensen B.B. (1) (1) Max Planck Institute for Marine Microbiology, 28359 Bremen, Germany (2) Research Center Ocean Margins, University of Bremen, 28359 Bremen, Germany, E-Mail: melvert@uni-bremen.de (3) Department of Marine Sciences, University of Georgia, Athens, GA, 30602, USA

1. Introduction and Objectives of the Project This subproject of the MUMM-project focused on the identification and quantification of anaerobic and aerobic methanotrophs in the marine environment using characteristic organic molecules of microbial origin, so-called biomarkers, and the carbon isotopic signature of those biomarkers as indicators for the function of the microorganisms involved. The study was performed on sediments from various marine habitats world wide (Hydrate Ridge - Cascadia subduction zone, Håkon Mosby mud volcano – Barents Sea, Gulf of Mexico). Repetitive patterns of specific microbial biomarkers in these methane-rich sediments were used to trace the microbial carbon flow from methane and to identify the zones of its oxidation in situ.

Moreover, by analyzing biomarkers of microorganisms from enrichment studies, we investigated which types were specifically enriched and whether those microbes are important in the environments studied. The aim of this approach was particularly powerful in combination with process measurements and cultivation-independent molecular techniques performed by other subprojects within MUMM.

2. Present Status and Results 2.1. Biomarker diversity Specific archaeal and bacterial biomarkers indicative of syntrophic AOM-consortia were present in all sediments investigated. As an example, a biomarker fraction containing high

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amounts of such lipids in Hydrate Ridge sediments is shown in Figure 1. The incorporation of methane carbon via AOM into microbial biomass is indicated by very depleted carbon isotope values of the specific biomarkers derived from methanotrophic archaea of -135‰ PDB (Pee Dee Belemnite) and from sulfatereducing bacteria (SRB) of -103‰ (Elvert et al., submitted). 13C-signatures of the biomass of anaerobic methanotrophs are always lighter than δ13C-values of the substrate methane, resulting in heavier δ13C-values in environments dominated by thermogenic methane δ13C = -30 to -60‰) compared to environments which contain biogenic methane (δ13C = -50 to 110‰) (Whiticar, 1999). Accordingly, Hydrate Ridge seeps and the Håkon Mosby mud volcano (HMMV) (Niemann et al., submitted a) with their high proportion of biogenic methane host biomarkers which are more 13Cdepleted those from the Gulf of Mexico (Orcutt et al., in press).

All sites investigated are dominated by specific biomarker lipids such as fatty acids (e.g., FAs C16:1ω5c, C17:1ω6c, cyC17:0ω5,6) derived from SRB and phytanyl glycerol diethers (archaeol and sn-2-or sn-3-hydroxyarchaeol) produced by the methanotrophic archaea. Other prominent biomarkers found at the various environments are irregular isoprenoidal hydrocarbons (e.g. crocetane, Cr:1, PMI:3, PMI:4, PMI:5) also indicative of the anaerobic methanotrophs and series of short chain n-alcohols, sn-1-mono alkyl glycerol ethers (MAGEs), and sn-1/sn-2-di alkyl glycerol ethers (DAGEs) presumed to be speci-

Figure 1: GC chromatogram of the alcohol fraction in the 2-4cm sediment horizon from the Beggiatoa site at Hydrate Ridge. Archaeal biomarkers: Ar: Archaeol, sn2-OH-Ar: sn-2Hydroxyarchaeol; Bacterial biomarkers: short chain n-alcohols, MAGEs: sn-1-mono alkyl glycerol ethers, DAGEs: sn-1/sn-2-di alkyl glycerol ethers (sum of carbon atoms of both side chains and number of double bonds are given); Planktonic biomarkers: Phytol, Cholesterol.

fic for the SRB. Nevertheless, depending on the environment studied the overall abundance of the specific biomarkers varies according to the AOM-consortia present. At Hydrate Ridge, which is dominated by a consortium of ANME-2 archaea and SRB affiliated with the Desulfosarcina/Desulfococcus group (DSS), biomarkers include highly abundant SRB-derived FAs C16:1ω5c and cyC17:0ω5,6 (Elvert et al., 2003) and archaeal-derived archaeol and sn-2-hydroxyarchaeol. The same distribution has been found in enrichments of AOM-consortia from this environment (Nauhaus et al., submitted).

In contrast, sediments from HMMV are indicated by FAs specific for SRB of the genus Desulfobulbus (C16:1ω5c and C17:1ω6c) and archaeal-derived irregular isoprenoidal hydrocarbons PMI:4 and PMI:5 in combination with archaeol and sn-2-hydroxyarchaeol. The exclusive abundance of PMI:4 and PMI:5 and the absence of all other irregular isoprenoids usually detected at other AOM-sites points to unknown archaea involved in AOM at HMMV. Indeed, this is in full accordance to results obtained by Niemann et al. (submitted b) which found aggregates at HMMV that consist of a newly discovered archaeal group (ANME-3) and SRB related to the genus Desulfobulbus.

2.2. Investigation area Hydrate Ridge Surface sediments: The spatial distribution of methanotrophic archaea and SRB sampled from sediments above outcropping methane hydrate were investigated by lipid biomarkers and combined with independent results obtained by other groups during the project period (Elvert et al., 2003; Elvert et al., submitted). The abundance of the specific biomarkers indicative of AOM performed by ANME-2/DSS-consortia seems to be strongly correlated to methane flux and thus to the specific environmental characteristics of the different chemosynthetic provinces found at Hydrate Ridge (Beggiatoa sites, Calyptogena fields, Acharax fields). The abundance of specific biomarkers at the Beggiatoa site is between 5 and 9 times higher than at the Calyptogena field and two orders of magnitude higher than at the Acharax field. Whereas there have been pronounced maxima observed at the Beggiatoa site (3 and 9 cm sediment depth) and the Calyptogena field (5 cm sediment depth) no obvious concentration maximum has been detected at the Acharax field, suggesting that AOM is lowest in this chemosynthetic province in the surface sediments of Hydrate Ridge, which has also been evident

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from rate measurements and aggregate counts at this location (Treude et al., 2003).

Positive correlations of specific biomarkers with counts of AOM-consortia at the Beggiatoa site enabled an estimation of the specific biomarker content per viable archaeal or bacterial cell at this high methane flux site. Cellspecific values obtained for the most dominant specific biomarkers range between 0.62 to 0.90 x 10-15 g for sn-2-hydroxyarchaeol and 0.22 to 0.30 x 10-15 g for archaeol as biomarkers indicative for the ANME-2 archaea and between 0.46 to 0.58 x 10-15 g for C16:1ω5c and 0.10 to 0.14 x 10-15 g for cyC17:0ω5,6 as biomarkers indicative for SRB of the DSS cluster (Elvert et al., 2003). These estimates may be used in the future to calculate the number of ANME-2 or DSS cells in AOM aggregates obtained from sediments or enrichment studies by the use of biomarker analyses alone.

Deep gas hydrate layers: Depth profiles of ANME-2/DSS specific biomarkers were analysed in a 1.2 m gravity core from southern Hydrate Ridge, which contained thick layers of methane gas hydrates, (Elvert et al., submitted). Biomarker carbon isotope values in the core are more 13C-depleted in sediment horizons just above methane hydrate layers accompanied by an apparent concentration increase (Fig. 2). This finding indicates that some of the methane stored in the hydrates is

available for the microbial community even though the sediments at Hydrate Ridge are in the hydrate stability filed (water depth 770 m; 4°C bottom water temperature). The methane available might be supplied by the continuous dissociation of gas hydrates due to diffusion caused by the intense concentration gradient between the gas hydrates and the surrounding sediment, or might be derived from the free gas present in some sediment layers.

2.3. Investigation area Håkon Mosby mud volcano At Håkon Mosby mud volcano (HMMV), an Arctic deep-sea cold seep, where warm, methane-rich subsurface mud is transported to the surface in a central conduit, the change in community structure in response to biological and physical gradients have been investigated in detail. Aerobic methanotrophs (Methylobacter sp.), evident from lipid biomarker analyses by the high abundance of the FA C16:1ω8c specific for type-I methanotrophs (Table 1), colonize the surface horizon where oxygen is present at the centre of HMMV (Niemann et al., submitted b). Over time, i.e. along the mud flows towards the outer volcano, aerobic methanotrophy is outcompeted by AOM performed by aggregates of SRB (Desulfobulbus sp. indicated

by C17:1ω6c) and novel archaea (ANME-3 group indicated by the sole abundance of PMI:4 and

Figure 2: Depth profiles of pore water sulfide and chloride, ANME-2 archaea specific lipids (archaeol, sn-2-hydroxyarchaeol), and lipids diagnostic for DSS (FAs C16:1ω5c and cyC17:0ω5,6) in a gravity core containing distinctive layers of gas hydrates (gray shaded areas).

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Table 1: Selected biomarker δ13C-signatures obtained from surface and deep sediment as well as whole tissue extracts. Indicated are the most likely microbial sources of the biomarkers at HMMV

Figure 3: Concentration (solid bar, scale to left) and isotopic composition (black dot, scale to right) of lipid biomarkers extracted from a core of oily Gulf of Mexico sediment (5 cm sediment depth) collected in a white Beggiatoa spp. microbial mat. »Archaeol« and »sn-2-hydroxy« refer alcoholderivatives of archaeal biomarkers; the remaining labels indicate bacterial fatty acids.

PMI:5) that develop in a shallow Beggiatoa covered sub-surface horizon. At this location, comparative analysis of sulfate concentration measurements and direct counts of ANMEaggregates show a correlating vertical distribution pattern with a sharp decline of sulfate concentration and aggregate numbers below the AOM horizon. In the oldest sediments outside of the HMMV center, tubeworms (Pogonophora sp) harbouring endosymbiontic sulfide-oxidizing and aerobic methane-oxidizing bacteria relocate the AOM horizon to a depth of approximately 80 cm by ventilation.

2.4. Investigation area Gulf of Mexico

In accordance with molecular biological, geochemical and radiotracer evidence, the isotopic signatures of lipid biomarkers extracted from Gulf of Mexico cold seep sediments demonstrate the occurrence of AOM. Archaeal biomarkers – namely, archaeaol and sn-2-hydroxyarchaeol – are strongly depleted in 13C (δ13C = 80‰ to -115‰; Fig. 3), revealing the incorporation of methane-derived carbon into microbial biomass. Differences in the carbon isotopic signatures of these biomarkers by approximately 35‰ are correlated with different sources of the methane, i.e. thermogenic versus biogenic, at the various sampling sites (δ13C-CH4 = ~

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-45‰ at hydrate sites, ~ -85‰ at brine sites).

Additionally, lipid biomarkers suggested to be indicative of DSS involved in AOM (C16:1ω5c and cyC17:0ω5,6) also show strong 13C-depletion (δ13C = -44‰ to -90‰), supporting evidence for the role of these bacteria in methane cycling in Gulf of Mexico seep sediments.

Interestingly, the abundance of DSS-derived biomarkers is up to an order of magnitude higher than that of biomarkers derived from methanotrophic archaea which is not evident from aggregate counts of the respective microorganisms (Orcutt et al., in press).

Acknowledgments We thank Gabi Klockether for help with instruments and laboratory analyses. The study was funded by the Bundesministerium für Bildung und Forschung and the Deutsche Forschungsgemeinschaft in the frame of the GEOTECHNOLOGIEN project MUMM (FKZ 03G0554A).

References Elvert M., Boetius A., Knittel K., Jørgensen B.B. (2003). Characterization of specific membrane fatty acids as chemotaxonomic markers for sulfate-reducing bacteria involved in anaerobic oxidation of methane. Geomicrobiol. J., 20, 403-419.

Elvert M., Boetius A., Hopmans E. C., Suess E. (submitted) Spatial variations of methanotrophic consortia in gas hydrate-bearing sediments: Implications from a high resolution molecular and isotopic approach. Geobiology.

Nauhaus K., Albrecht M., Elvert M., Boetius A.,

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Widdel F. (submitted) In vitro growth of anaerobic consortia of archaea and sulfate-reducing-bacteria with methane as the sole electron donor. Environmental Microbiology.

Niemann H., Elvert M., Lösekann T., Jakob J., Nadalig T., Boetius A. (submitted a) Distribution of methanotrophic guilds at Håkon Mosby Mud Volcano, Barents Sea. Geobiology.

Niemann H., Lösekann T., de Beer D., Elvert M., Knittel K., Amann R., Sauter E., Schlüter M., Klages M., Foucher J. P., Boetius A. (submitted b) Microbial colonization of a submarine mud volcano: how subsurface fluid flow structures methanotrophic communities. Nature.

Orcutt B., Joye S. B., Boetius A., Elvert M., Samarkin V. (in press) Molecular biogeochemistry of sulfate reduction, methanogenesis and the anaerobic oxidation of methane at Gulf of Mexico methane seeps. Geochimica et Cosmochimica Acta.

Treude T., Boetius A., Knittel K., Wallmann K., Jørgensen B. B. (2003) Anaerobic oxidation of methane above gas hydrates at Hydrate Ridge, NE Pacific Ocean. Marine Ecology Progress Series 264, 1-14.

Whiticar M.J. (1999) Carbon and hydrogen isotope systematics of bacterial formation and oxidation of methane. Chemical Geology 161, 291-314.

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Distribution and Diversity of Microorganisms in Gas Hydrate Bearing Sediments (MUMM – SPIV) Knittel K. (1), Lösekann T. (1), Amann R.(1) (1) Max Planck Institute for Marine Microbiology, Dept. Molecular Ecology, Celsiusstr. 1, 28359 Bremen, Germany, E-Mail: kknittel@mpi-bremen.de

1. Introduction Massive reservoirs of natural methane are present in the deep subsurface of the oceans and are considered as a promising source of energy for the future. But methane also endangers the global climate: each mol entering the atmosphere contributes more than 30 times to the greenhouse effect than carbon dioxide (CO2). However, more than 90% of the methane rising from the subsurface is oxidized anaerobically to CO2 with sulfate as electron acceptor before it reaches the oxidized layers.

This process of anaerobic oxidation of methane (AOM) is carried out by anaerobic methanotrophic archaea (ANME) and sulfate reducing bacteria (SRB) as syntrophic partners. For more than three decades researchers throughout the world have been trying to isolate these microorganisms but neither the ANME groups nor their sulfate-reducing partners have been isolated yet. In 1999, when applying molecular techniques on methane seep sediments for the first time, two new groups of archaea (ANME-1 and ANME-2) had been discovered as potential candidates for AOM (Hinrichs et al., 1999). In the year 2000, just before starting MUMM project, our group provided the first microscopic evidence for a structured consortium of ANME-2 archaea and SRB (»ANME-2/DSS aggregates«; Boetius et al., 2000). The consortia oxidize methane with sulfate, yielding equimolar amounts of carbonate and sulfide (Nauhaus et al., 2002, Orphan et al., 2001).

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2. Objectives of the Project Our group focused on the identification and quantification of anaerobic and aerobic methanotrophic microorganisms. We investigated different methane seeps and a variety of other gassy sediments, from deep subsurface cores to the Baltic Sea. Another aim was the development of new nucleic acid probes for dominant microorganisms to identify guilds linked to the turnover of methane and sulfate in the sediment.

3. Present Status, Methods, and Results Only the minority of marine bacteria can be cultivated under laboratory conditions. Thus, we used cultivation-independent ribosomal RNA (rRNA) based methods for the identification of microbial communities. The rRNA of the small subunit, in prokaryotes the 16S rRNA, is currently the molecule of choice for bacterial molecular systematic studies. The analysis of microbial diversity was accordingly done by constructing 16S rRNA gene libraries followed by sequencing and phylogenetic analysis.

Another 16S rRNA based method was the fluorescence in situ hybridization (FISH), which makes it possible to stain bacteria depending on their taxonomic origin with small nucleic acid probes carrying a fluorescent dye.

Nine methane-rich sites were investigated: Gas hydrates and the overlaying sediments at Hydrate Ridge (Oregon, USA), methane seeps in the Gulf of Mexico and the Guaymas Basin, the Haakon Mosby Mud Volcano in the Barents Sea, anoxic sediments in the Wadden Sea, Baltic Sea, and the Congo Basin, and thick

microbial mats from the anoxic Black Sea. During MUMM project we built up large databases including more than 5000 new AOM related 16S rRNA gene sequences from our own and other groups. We found a greater diversity of methanotrophic archaea and SRB as previously assumed (Knittel et al., 2003, Knittel et al., 2005, Lemke, 2001, Lösekann, 2002).

A comparison of these sequences showed the ubiquitous presence of methanotrophic archaea in almost all anoxic methane environments investigated so far. Furthermore, ANME/SRB consortia were detected in all habitats (Fig. 1, Knittel et al., 2005). However, the consortia differed in abundance, size, ratio of ANME:SRB, and their morphology. Two different types of consortia had been identified: structured »shell-type« consortia (Knittel et al., 2005), consisting of an inner core of ANME archaea partially or fully surrounded by SRB (Hydrate Ridge, Haakon Mosby mud volcano, Black Sea, Guaymas Basin, Wadden Sea) and »mixed type« consortia in which ANME and SRB are completely mixed (Hydrate Ridge, Haakon Mosby Mud Volcano,

Gulf of Mexico). In addition, consortia consisting of only archaeal cells (Eckernförder Bight) or just two archaeal cells and a single SRB cell (Congo Basin) were detected.

It has been shown that the three groups ANME1, ANME-2, and ANME-3 often co-occur, however, quantitative analysis of the distribution indicated dominance of particular groups in certain environments (Knittel et al., 2003; 2005; Michaelis et al., 2002; Lösekann, 2002; Lösekann et al., in prep.; Treude et al., in press). In the following three sites are described which are dominated by one ANME group each.

Black Sea Microbial MatsDominance of ANME-1 During submersible dives to methane seeps in the permanently anoxic Black Sea giant microbial structures were discovered, composed of massive microbial mats of centimeter to decimeter thickness, producing large carbonate columns. The mat is streaked with a system of microchannels (ca. 30% of mat volume; Knittel

Figure 1: Anaerobic methane oxidizers in different habitats

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et al., in prep.), allowing an exchange with the surrounding seawater. Epifluorescence microscopy of mat sections revealed that the mat is composed of densely aggregated archaea and bacteria (Michaelis et al., 2002). The cell numbers ranged between 150 and 300 billions per cm-3 mat, which is two orders of magnitude higher than those in rich coastal sediments and still one order of magnitude higher than numbers in highly active Hydrate Ridge sediments above gas hydrates (see below). Quantitative analysis showed that ANME-1 and ANME-2 groups co-occur in the mats, however, ANME1 dominated strongly. The most abundant bacterial population was a group of SRB affiliated with Desulfosarcina/Desulfococcus. These SRB occurred in larger clusters of 10-50 µm in diameter or as smaller clusters and single cells spread throughout the ANME-1 biomass.

Hydrate Ridge Sediments Above Gas HydratesDominance of ANME-2 At Hydrate Ridge a large quantity of methane is constantly released from decomposing gas hydrates. The surface sediments were dominated by ANME-2/DSS aggregates accounting for up to 40 billion cells per cm-3 and more than 90% of total cell biomass (Boetius et al., 2000; Elvert et al., 2003; Knittel et al., 2003; Treude et al., 2003). ANME-1 cell numbers were at least one order of magnitude lower. The microbial community living in pure gas hydrates did not differ remarkably from those in the surrounding sediment. However, the AOM mediating groups ANME-1 and ANME-2 were less active and occurred in much lower numbers.

Chemosynthetic organisms populate the seafloor. Giant sulfur-oxidizing bacteria (e.g.

Beggiatoa species) form thick mats. Also specific mussels (Calyptogena species), which harbor symbiotic sulfur oxidizers in their gills live from the hydrogen sulfide formed during AOM. Surprisingly, the relative abundance of ANME-2 subgroups ANME-2a and ANME-2c was remarkably different at Beggiatoa (80% ANME-2a, 20% ANME-2c) and Calyptogena sites (20% ANME-2a, 80% ANME-2c) indicating a selection of either group by the location (Knittel et al., 2005).

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Haakon Mosby Mud Volcano anoxic sediments- Dominance of ANME-3 The Haakon Mosby Mud Volcano (HMMV) is an active cold seep expelling methane enriched mud from a zone 2-3 km below the seafloor. Here, methane is oxidized anaerobically by a new type of ANME/SRB consortium. In contrast to all other sampling sites (see Fig. 1) the archaea in these consortia could be assigned to a new group, ANME-3 (Lösekann et al., in prep.). ANME-3 is affiliated with Methanococcoides species and is distinct from known phylogenetic groups involved in AOM. The sulfate-reducing partner belongs to Desulfobulbus (DBB) spp. The finding of the new ANME3/DBB aggregates is supported by biomarker signatures at the same sampling sites (Niemann et al., in prep.). Aggregate abundance was highest in sediments covered with mats of sulfide-oxidizing Beggiatoa. At this site, AOM is the dominant methane consuming process (Fig. 2). In contrast, no AOM aggregates were found in center sediments, where methane is emitted to the hydrosphere and aerobic methanotrophic Gammaproteobacteria dominate the microbial community (46% of total cells). In sediments colonized with Pogonophora worms AOM rates were below the detection limit, indicating an only minor importance of AOM at this site. Here, endosymbiotic bacteria living in the worms are probably the major methane-consumers.

4. Conclusions A comparison of 16S rRNA gene sequences showed the ubiquitous presence of methanotrophic archaea in almost all methane environments investigated so far independent of in situ temperature, depth, pressure, methane and sulfate concentrations. ANME-1 and ANME-2 co-occur at all seep systems investigated, however, the microscopic analysis of the distribution shows a dominance of certain types within particular niches. This is in line with the famous statement: »Everything is everywhere, the environment selects!« (BaasBecking, 1934; Beijerinck, 1913). To single out the selection factors environmental conditions

Figure 2: Overview of the dominant methane-consuming processes at different sampling sites at Håkon Mosby mud volcano

and geochemical gradients need to be analyzed in situ at a high-resolution. This would require parallel investigations with microsensors in the field or experimental studies in flow-through microcosms.

Acknowledgements We thank the officers, crews and shipboard scientific parties of RV SONNE during TECFLUX cruises SO143 and SO148 (Grant 03G0143A and 03G0148A) to Hydrate Ridge and during GHOSTDABS cruise (Grant 03G0559A) with RV Prof. LOGACHEV in summer 2001 for excellent support. We greatly acknowledge the GEOTECHNOLOGIEN projects OMEGA (Grant 03G0566A), LOTUS (Grant 03G0565) and GHOSTDABS for providing access to samples and infrastructure. This study was part of the program MUMM (Mikrobielle UMsatzraten von Methan in gashydrathaltigen Sedimenten, 03G0554A) supported by the Bundesministerium für Bildung und Forschung (BMBF, Germany). Further support was provided by the Max Planck Society, Germany.

References Baas-Becking, L. G. M. 1934. Geobiologie of Inleiding Tot de Milieukunde. In W. P. van Stockum and Zoon N. V. (ed.), The Hague, The Netherlands.

Beijerinck, M. W. 1913. De infusies en de ontdekking der backteriën, Jaarboek van de Koninklijke Akademie v. Wetenschappen. Müller, Amsterdam,The Netherlands.

Boetius, A., K. Ravenschlag, C. Schubert, D. Rickert, F. Widdel, A. Gieseke, R. Amann, B. B. Jørgensen, U. Witte, and O. Pfannkuche. 2000.

A marine microbial consortium apparently mediating anaerobic oxidation of methane.

Nature 407:623–626.

Elvert M., Boetius A., Knittel K., Jørgensen B.B. (2003). Characterization of specific membrane fatty acids as chemotaxonomic markers for sulfate-reducing bacteria involved in anaerobic oxidation of methane. Geomicrobiology Journal, 20, 403-419.

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Hinrichs, K. U., J. M. Hayes, S. P. Sylva, P. G. Brewer, and E. F. DeLong. 1999. Methane-consuming archaebacteria in marine sediments.

Nature 398: 802–805.

Knittel, K., Boetius, A., Lemke, A., Eilers, H., Lochte, K., Pfannkuche, O., Linke, P., Amann, R. (2003). Activity, distribution, and diversity of sulfate reducers and other bacteria in sediments above gas hydrate (Cascadia Margin, OR). Geomicrobiol. J. 20, 269-294.

Knittel, K., Lösekann, T., Boetius, A., Kort, R., Amann, R. Diversity and Distribution of Methanotrophic Archaea at Cold Seeps (2005). Appl. Environ. Microbiol. 71: 467-479 Knittel, K., Treude, T., Gieseke, A., Boetius, A., Amann, R. (in prep.) In situ quantification of methanotrophic communities in massive microbial mats (Black Sea) Krüger, M., Meyerdierks, A., Glöckner, F.O., Amann, R., Widdel, F., Kube, M., Reinhardt, R., Kahnt, J., Böcher, R., Thauer, R.K. and Shima, S. (2003) A conspicuous nickel protein in microbial mats that oxidize methane anaerobically. Nature, 426, 878-881.

Lemke, A. (2001). Mikrobielle Diversität in gashydrathaltigen Sedimenten. Diploma thesis, University Bremen.

Lösekann, T. (2002). Molekularbiologische Untersuchungen der Diversität und Struktur mikrobieller Lebensgemeinschaften in methanreichen, marinen Sedimenten (Haakon-MosbySchlammvulkan). Diploma thesis, University Bremen.

Lösekann, T., Knittel, K., Nadalig, T., Niemann, H., Boetius, A., Amann, R. (in prep.). Identification of a new cluster of anaerobic methane oxidizers at an Arctic mud volcano (Haakon Mosby Mud Volcano, Barents Sea).

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Lösekann, T., Knittel., K., Boetius, A., Amann, R. (in prep.). Microbial diversity in pure gas hydrates (Hydrate Ridge, OR) Michaelis, W., Seifert, R., Nauhaus, K., Treude, T., Thiel, V., Blumenberg, M., Knittel, K., Gieseke, A., Peterknecht, K., Pape, T., Boetius, A., Amann, R., Jørgensen, B.B., Widdel, F., Peckmann, J., Pimenov, N.V., Gulin, M.B.. Microbial reefs in the Black Sea fueled by anaerobic oxidation of methane. Science 297, 1013-1015.

Nauhaus, K., A. Boetius, M. Krüger, and F. Widdel. 2002. In vitro demonstration of anaerobic oxidation of methane coupled to sulphate reduction in sediment from a marine gas hydrate area. Environ. Microbiol. 4:296–305.

Niemann, H., Elvert, M., Lösekann, T., Jakob, J., Nadalig, T., Boetius, A. (in prep.). Lipid biomarker of methanotrophic guilds at haakon Mosby Mud Volcano, Barents Sea. Orphan, V. J., C. H. House, K.-U. Hinrichs, K. D. McKeegan, and E. F. DeLong. 2001. Methaneconsuming archaea revealed by directly coupled isotopic and phylogenetic analysis. Science 293:484–487.

Treude, T., Boetius, A., Knittel, K., Wallmann, K., Jørgensen, B.B. (2003). Anaerobic oxidation of methane above gas hydrates at Hydrate Ridge, NE Pacific Ocean. Mar. Ecol. Prog. Ser. 264, 1-14.

Treude, T., Knittel, K., Blumenberg, M., Seifert, R., Boetius, A. (in press). Subsurface microbial methanotrophic mats in the Black Sea. Appl.

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Physiology of Microorganisms in Gas Hydrate Bearing and other Methane-Rich Marine Sediments (MUMM – SP V) Krüger M. (1,2), Nauhaus K. (2,3), Meyerdierks A. (2), Widdel F. (2) (1) Federal Institute for Geosciences and Resources (BGR), Stilleweg 2, 30655 Hannover, Germany, E-Mail: M.Krueger@bgr.de (2) Max Planck Institute for Marine Microbiology, Celsiusstrasse 1, 28359 Bremen, Germany (3) LMU Munich, Department Biology I, Maria-Ward Strasse 1A, 80638 Munich, Germany

1. Introduction The anaerobic oxidation of methane (AOM) is an important microbial process representing one of the major sinks for methane on earth, oxidising up to 90 % of the methane produced in marine sediments prior to their release into the atmosphere (Reeburgh, 1996). Biogeochemical evidence for AOM was first based on depth profiles of methane and sulfate/sulfide, 13 C/12C ratios in CO2 and CH4 in sediment profiles, and 14CH4-labeling studies with sediment samples (e. g. Alperin and Reeburgh, 1984; Hoehler et al., 1984; Iversen and Blackburn, 1981; Iversen and Jørgensen, 1985). The hypothesis that the biochemical process underlying AOM might be a reversal of methanogenesis has been discussed in connection with field (Hoehler et al., 1994) and laboratory studies with growing methanogens, in which a fraction of added 14C-methane was converted to 14 CO2 during methanogenesis (Harder, 1997; Zehnder and Brock, 1979).

The finding of highly 13C-depleted isoprenoid biomarkers and archaeal (Methanosarcinalesrelated) 16S rRNA-gene sequences in the zone of anaerobic methane oxidation provided further evidence for the theory of a »reverse methanogenesis« coupled to the reduction of sulfate to sulfide as the terminal electron-accepting process (Elvert et al., 1999; Hinrichs et al., 1999; Orphan et al., 2001). Molecular studies indicate that AOM is mediated by assemblages

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of archaea (ANME-1, ANME-2, and ANME-3) related to the Methano-sarcinales, and sulfatereducing bacteria (SRB) of the Desulfosarcina/ Desulfococcus group (Hinrichs et al., 1999; Boetius et al., 2000; Orphan et al., 2001b, Knittel et al,. 2005). However, microorganisms that grow with methane under strictly anoxic conditions have not been isolated so far. Consequently, a detailed mechanistic understanding of the biochemistry and an in vitro study of AOM has not yet been achieved (Nauhaus et al., 2002 & 2005; Sørensen et al., 2001; Hoehler et al., 1994).

Therefore, a combination of molecular and biochemical approaches in connection with simultaneous microbiological work such as growth of biomass was applied in this project to obtain an in-depth understanding of the globally important process of AOM.

2. Objectives of the Project The main objective of this project was the study of the physiology of methane-oxidising microorganisms in gas hydrate bearing sediments. This included the isolation of microrganisms involved in AOM in marine habitats, the characterisation of their metabolism, growth behaviour and genetic function. For the study of microorganisms involved in AOM new cultivation techniques had to be developed, to account for the slow growth and unknown

interactions of different types of anaerobes. In addition, marine microcosms with methane supply were established as a source for microorganisms and the study of chemical gradients. In the future these incubations should provide adequate material for subsequent biochemical and molecular biological investigations of the mechanism of AOM.

3. Present Status and Results This project aimed at an understanding of individual types of microorganisms and their physiological capacities in methane consumption and production in marine environments, with special focus on two sites: The gas hydrate-bearing sediments from Hydrate Ridge (Pacific, Oregon; Boetius et al., 2000) and microbial mats from the north-western part of the Black Sea (Michaelis et al., 2002).

Environmental regulation of AOM at different sites In sediments from the marine gas hydrate area at Hydrate Ridge (HR, NE Pacific) the anaerobic oxidation of methane (AOM) is predominantly carried out by a consortium of ANME-II-Archaea, and sulfate-reducing bacteria of the Desulfococcus/Desulfosarcina group (Boetius et al., 2000; Knittel et al., 2003 & 2005). An in vitro method to investigate AOM in the laboratory was established with HR samples, confirming the 1:1 stoichiometry for methane and sulfate (Nauhaus et al., 2002). The optima of environmental parameters, including temperature, salinity, pH, and sulfate concentration, for AOM were found to be close to the values observed in situ (Kr체ger et al., 2005; Nauhaus et al., 2005). This demonstrates the optimal adaptation of the microorganisms to the prevailing conditions in the environment.

Reef-forming microbial mats were collected at methane seeps in the north-western Black Sea (BS, Michaelis et al., 2002). These microbial mats consist mainly of archaea (ANME-1 cluster) and sulfate-reducing bacteria (Desulfosarcina/Desulfococcus group) (Knittel et al., 2005).

Laboratory incubations with homogenised subsamples of the mats revealed their ability for AOM. Methane oxidation is coupled to sulfate reduction in a 1:1 stoichiometry. Elevated methane partial pressures (0.1 to 1.1 MPa) increased the sulfate reduction rates in the Black Sea samples only two- fold in contrast to 5-fold in HR samples. The optimal temperature for the BS samples was between 10 and 25 째C.

The mechanism of AOM At Hydrate Ridge, AOM appeared to be exclusively coupled to the use of sulfate as terminal electron acceptor. Other alternative electron acceptors, including nitrate, ferric iron, sulfur, fumarate, manganese oxide or AQDS, were reduced, but this was not coupled to AOM (Nauhaus et al., 2005). Oxygen-dependent methane oxidation was restricted to the top few millimeters of the sediment. The addition of a large number of possible intermediates of AOM to the sediment did not result in elevated sulfide production in the absence of methane, providing no evidence for one of these compounds being the intermediate exchanged in the consortia (Figure 1). Also, the addition of known electron shuttles, like AQDS, humic acids or different phenazines, did not result in a stimulation of AOM (Nauhaus et al., 2005).

Isolation of axenic cultures for the elucidation of the physiology and biochemistry of AOM has not been achieved. Nevertheless, Girguis et al. (2003) and Nauhaus et al. observed the increase of AOM-catalyzing biomass with methane in the laboratory (see below).

However, sufficient biomass for the first biochemical studies was obtained directly from a natural habitat, the northwestern Black Sea shelf (Michaelis et al., 2002). An abundant protein was purified directly from these mats, which closely resembled methyl-coenzyme M reductase, the terminal enzyme in methanogenesis (Kr체ger et al., 2003). This protein could be assigned to anaerobically methane oxidising archaea of the ANME-I group, and represents a likely candidate for the initial step in AOM.

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Figure 1: Effect of different possible intermediates of AOM as electron donors on sulfate reduction rates in Hydrate Ridge1 and Black Sea samples (mean ± sd, n = 3). SR: Sulfate reduction

Figure 2: Activities§ (marked with *) and genes$ for methanogenic enzymes found in methanotrophic archaea (ANME). § after Krüger et al., (2003), $ Hallam et al., (2004).

Figure 3: Effect of the specific inhibitors bromoethanesulfonate (BES) and molybdate on methane dependent sulfate reduction in Hydrate Ridge and Black Sea samples (mean ± sd, n = 3). SR: Sulfate reduction

These findings as well as in situ analyses of genes and enzymes (Figure 2) (Hallam et al., 2003 & 2004; Krüger et al., 2003) suggest that AOM might be in principle a reversal of methanogenesis. Further support for this hypothesis was obtained in experiments with specific inhibitors (Figure 3). The addition of bromoethanesulfonate (BES), a specific inhibitor for methanogenic archaea, completely inhibited AOM (Nauhaus et al., 2005).

ria (Desulfosarcina/Desulfococcus) were observed and quantified in a long-term laboratory experiment lasting more than 2 years. As inoculum samples from Hydrate Ridge (HR) with high in situ activity of AOM and a high biomass content of presumably methane oxidizing consortia were used. This sediment was incubated repeatedly under elevated methane partial pressure of 1.37 MPa. In 60 subsequent incubation periods methane-dependent sulfate reduction rates increased continuously from 0.035 to 0.24 mmol d-1 gdw-1. Increasing activity was accompanied by increasing biomass observed as tenfold higher numbers of consortia and up to 150times higher concentrations of biomarkers specifically assigned to orga-

Growth of AOM-microorganisms Growth and enrichment of anaerobically methane oxidizing consortia composed of archaea (ANME-2) and sulfate-reducing-bacte-

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nisms performing AOM. Fluorescence in situ hybridization with specific oligonucleotide probes revealed that the consortia grown in the enrichment were the same as those present in situ. Microscopic examination indicates that Archaea and SRB grow together as a consortium from the early stage of a few cells, resulting from the division of large aggregates, and developing again into big spherical aggregates of several micrometers in diameter.

4. Conclusions AOM is the most important process in the turnover of the greenhouse gas methane in marine environments. The physiological and biochemical characterization as well as the cultivation and isolation of the involved microorganisms are essential tasks for an in-depth understanding of this process. Many examples of past environmental research have shown that a broad functional understanding of biologically mediated large-scale processes needs integrated approaches ranging from habitat studies to investigations on the cellular and molecular level.

The results obtained in this project provided first insights into the physiology and the mechanism of AOM. Furthermore, the successful enrichment of the respective microorganisms as well as the discovery of naturally enriched samples provide a promising fundament for further, detailed studies. The increase in knowledge should then be used to more accurately estimate the global significance of AOM for the mitigation of methane emissions.

Furthermore, reactions of methane as the most abundant natural hydrocarbon are also of technological and chemical interest. This longterm perspective ranges from an understanding of possible processes in gas storage caverns to the development of catalysts for controlled use of methane in chemical syntheses. Furthermore, the project provides case studies for the advancement of protein- and RNAbased methods for the in situ-study of microbial communities.

Acknowledgements The project was carried out in collaboration with partners in the Gas Hydrate Initiative of the BMBF, especially the projects GHOSTDABS (University of Hamburg), TECFLUX I and II, LOTUS and OMEGA (all IFM-GEOMAR Kiel) as well as the GenoMik Network (Göttingen).

Furthermore, Prof. R. K. Thauer and Dr. S. Shima from the Department of Biochemistry at the MPI for Terrestrial Microbiology (Marburg) significantly contributed to the biochemical studies. The study was funded by the Bundesministerium für Bildung und Forschung and the Deutsche Forschungsgemeinschaft in the frame of the GEOTECHNOLOGIEN project MUMM (FKZ 03G0554A).

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Boetius, A., Ravenschlag, K., Schubert, C.J., Rickert, D., Widdel, F., Giesecke, A., Amann, R., Jørgensen, B.B., Witte, U., Pfannkuche, O. (2000). A marine microbial consortium apparently mediating anaerobic oxidation of methane. Nature 407, 623-626.

Elvert, M., & Suess, E. (1999). Anaerobic methane oxidation associated with marine gas hydrates: superlight C-isotops from saturated and unsaturated C20 and C25 irregular isoprenoids. Naturwissenschaften 86, 295-300.

Girguis, P.R., Orphan, V.J., Hallam, S.J. and DeLong, E.F. (2003) Growth and methane oxidation rates of anaerobic methanotrophic archaea in a continuous-flow bioreactor. Appl.

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Hallam, S.J., Putnam, M., Preston, C.M., Detter, J.C., Rokhsar, C., Richardson, P.M., DeLong, E.F. (2004). Reverse methanogenesis: Testing the hypothesis with environmental genomics.

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Hallam, S.J., Girguis, P.R., Preston, C.M., Richardson, P.M., DeLong, E.F. (2003). Identification of methyl coenzyme M reductase A (mcrA) genes associated with methane-oxidizing archaea.

Appl. Environ. Microbiol. 69, 5483-5491.

Harder, J. (1997). Anaerobic methane oxidation by bacteria employing 14C-methane uncontaminated with 14C-carbon monoxide. Marine Geol.

137, 13-23.

Hinrichs, K. U. & Boetius, A. B. (2002) The anaerobic oxidation of methane: new insights in microbial ecology and biogeochemistry. In Ocean Margin Systems. Wefer, G.; Billet, D.; Hebbeln, D.; Jørgensen, B. B.; Schlüter, M.; van Weering, T. (eds.) Heidelberg: Springer-Verlag, 457-477.

Hinrichs, K.-U., Hayes, J.M., Sylva, S.P., Brewer, P.G., De Long, E.F. (1999). Methane-consuming archaebacteria in marine sediments.

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Hoehler, T.M., Alperin, M.J., Albert, D.B., Martens, C.S. (1994). Field and laboratory studies of methane oxidation in an anoxic marine sediments: evidence for methanogen-sulphate reducer consortium. Global Biochem. Cycles 8, 451-463.

Iversen, N., & Blackburn, & T.H. (1981).

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Iversen, N., & Jørgensen, B.B. (1985). Anaerobic methane oxidation rates at the sulphate-methane transition in marine sediments from Kattegat and Skagerrak (Denmark). Limnol. Oceanogr.

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Knittel, K., T. Lösekann, A. Boetius, R. Kort, & R. Amann. (2005). Diversity and distribution of methanotrophic archaea at cold seeps. Appl.

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Knittel, K., Boetius, A., Lemke, A., Eilers, H., Lochte, K., Pfannkuche, O., Linke, P. (2003).

Activity, distribution, and diversity of sulfate reducers and other bacteria in sediments above gas hydrates (Cascadia Margin, Oregon).

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Krüger M., T. Treude, H. Wolters, K. Nauhaus, and A. Boetius. (2005) Microbial methane turnover in different marine habitats. Palaeogeography, Palaeoclimatology, Palaeoecology.

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Michaelis, W., Seifert, R., Nauhaus, K., Treude, T., Thiel, V., Blumenberg, M., Knittel, K., Gieseke, A., Peterknecht, K., Pape, T., Boetius, A., Amann, R., Jorgensen, B.B., Widdel, F., Peckmann, J., Pimenov, N.V., Gulin, M.B. (2002) Microbial Reefs in the Black Sea Fueled by Anaerobic Oxidation of Methane.

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Nauhaus K., T. Treude, A. Boetius, and M. Krüger*. 2005. Environmental regulation of the anaerobic oxidation of methane in ANMEI or –II dominated communities: A comparison.

Environmental Microbiology 7, 98-106.

Nauhaus, K., A. Boetius, M. Krüger, Widdel F. 2002. In vitro demonstration of anaerobic oxidation of methane coupled to sulfate reduction in sediment from a marine gas hydrate area.

Environmental Microbiology 4 (5), 296-305.

Orphan, V.J., House, C.H., Hinrichs, K.-U., McKeegan, K.D. and DeLong, E.F. (2001) Methane-consuming archaea revealed by directly coupled isotopic and phylogenetic analysis.

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High resolution imaging and physical properties of hydrate and gas-bearing sediments within the INGGAS project Reston, T. J. (1), Bialas, J. (1), Breitzke, M. (2), Flüh, E. (1), Kläschen, D. (1), Klein, G. (1, 3); Talukder, A. (1), Zillmer, M. (1) (1) IFM-GEOMAR, Leibniz Institute of Marine Sciences, Wischhofstraße 1-3, D24148 Kiel, treston@ifm-geomar.de (2) Alfred-Wegener Institut for Polar and Marine Research, Postfach 12 0161, D-27515 Bremerhaven, mbreitzke@awi-bremerhaven.de (3) Institute of Geosciences, Kiel University, Olshausenstraße, Kiel.

Abstract Methane hydrate has been invoked as a possible climate-killer (through the release of the greenhouse gas methane), as a natural hazard (through the influence of hydrate and of underlying free gas on the physical properties of the slope), and as a future energy resource (due to the large volumes of carbon sequestered within such hydrates). The investigation of each of these scenarios requires knowledge of the amount and distribution of hydrate beneath the continental slopes, of the processes of hydrate formation and dissociation, and of the thicknes of the free gas zone at the base of the hydrates.

However, neither the fine structure of the hydrate- and gas-bearing sediments nor the links between hydrate formation / dissociation and fluid flow are fully understood. Addressing these issues requires high resolution imaging techniques, determination of the physical properties of the hydrate-bearing and underlying sequences, and the thermal regime. The INGGAS (Integrated Geophysical Characterization and Quantification of Gas Hydrates) project set out primarily to develop equipment suitable to develop such equipment for future hydrate research, including high frequency seismic sources, a deep tow seismic system, ocean bottom seismometers and heat flow probes. In this contribution we concentrate on the development of the deep-tow streamer, referring briefly to

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the complementary results from the ocean bottom seismometers and heat flow probe.

High resolution imaging of the shallow subsurface The resolution of subsurface structures in reflection seismic images depends in part on the seismic acquisition system used. Whereas vertical resolution in fundamentally controlled by wavelength (and can be improved by a sharp wavelet - a high frequency but broad bandwidth source - and to a lesser extent by deconvolution), the lateral resolution is determined by the size of the Fresnel zone, itself dependent on the source frequency, and on the velocity and distance between the source and streamer and the reflector. 2D migration shrinks the Fresnel zone in the in-line direction by calculating the response as the source and streamer are lowered towards the target, but has no influence on the cross-line resolution (Yilmaz, 2001). The latter can however be improved by lowering the streamer and/or the source towards to the sea floor. The philosophy adopted within INGGAS (Integrated Geophysical Characterization and Quantification of Gas Hydrates) was to achieve this improved resolution by combining a conventional marine seismic surface source (airgun or GI-gun) with a deep-tow streamer and /or with ocean bottom seismometers.

Figure 1: Derivation of p- and s-wave velocity structure from the dispersion characteristics of Scholte waves recorded on OBS in the Arkona Basin (Klein et al., in press).

Ocean bottom seismometers By deploying closely-spaced ocean bottom instruments on the seafloor, it is not only possible to determine the velocity structure (both p-wave and s-wave) but also to image the structure beneath the instruments. Within INGGAS, ocean bottom seismometers were used to determine the p-wave and s-wave velocity of gas-bearing sediments within the Arkona Basin (Baltic Sea) â&#x20AC;&#x201C; Klein et al., in press â&#x20AC;&#x201C; see Figure 1) and of hydrate and gasbearing sediments at a variety of continental margins (e.g. Figure 2).

The work in the Arkona Basin relied on the identification and inversion of Scholte waves travelling just beneath the seafloor. A single OBS (including both geophones and a hydrophone) recorded shots from a 0.3 l airgun

towed at 8 m depth. The energy travelling in the acoustic waveguide of the water column showed 6 distinct modes, controlled by the physical properties of the immediate subsurface. Inversion of these in the tau-p domain led to a detailed Vp and Vs velocity model of the shallow subsurface, producing an excellent match in terms of amplitude, slowness and frequency. Applied to hydrate bearing sediments, such analysis promises to reveal the physical properties and shear strength of the seafloor with high resolution.

The first BSR to be detected in the Black Sea was imaged using closely-spaced ocean bottom seismometers (c. 300m separation) and a GI-gun source. The survey in the northwestern Black Sea (Zillmer et al., 2005) was in conjunction with the GHOSTDABS project. The BSR

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Figure 2: One of the first ever images from the Black Sea of the BSR at the base of hydrate bearing sediments, made using closely spaced OBS (Zillmer et al., 2005).

occurs at depths of 205-235 m below the seafloor itself at 1.1-1.2 km. Kirchhoff migration was used to determine both the sub-surface velocity structure and an image. S-wave velocities were estimated using converted waves.

A sharp drop in p-wave velocity at the BSR from 1850-1600 m/s is consistent wit the presence of free gas beneath a 150 m thick layer with 49% porosity and 30% hydrate saturation (Zillmer et al., 2005).

Deep-tow system The use of OBS for high resolution seafloor imaging is however time consuming and thus of necessarily limited extent. An alternative method is continuous profiling using a surface source and a deep-towed streamer (Figure 3), although this does not deliver the complementary information on physical properties.

The IFM-GEOMAR deep-tow system comprises a multichannel digital streamer (High

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Tech), towed behind either directly behind a 2 ton depressor weight, which keeps the deeptowed system at depth and as close to the towing ship as possible or behind an intervening towfish connected to the depressor by a 30m umbilical. The towfish contains a dual frequency sidescan sonar system (DTS-1; 75 kHz and 410 kHz; Edge Tech Full Spectrum) and a chirp subbottom profiler (2-15 kHz, Edge Tech) (Figures 3, 4). The controlling electronics and a USBL transponder for instrument location are mounted either on the depressor directly (if the towfish is not deployed) or on the towfish. The streamer itself consists of a 50 m. lead in section and mainly acoustic nodes (containing a single hydrophone, bandpass filter, pre-amplifier and A/D converter) and several engineering nodes (each also containing depth and motion sensors, and a magnetic compass).

The nodes can be connected by either 1m or 6.25 m cable sections, allowing a variety of

Figure 3: Sketch of the deep-tow streamer and sidescan sonar system (after Breitzke and Bialas, 2003). Note that the reflection points at a single channel do not form a vertical line, but rather a hyperbola, meaning that standard CMP processing will not work. Also note that the acquisition geometry allows the undershooting of locally hard seafloor.

different deployment geometries. Full details of the system are given by Breitzke and Bialas (2004).

First test â&#x20AC;&#x201C; Sonne 162 The deep-tow streamer was first deployed during RV Sonne cruise SO162 over the Yaquina basin off Peru in 2002 (Reston and Bialas, 2002). The Yaquina basin is a forearc basin associated with the subsidence of the margin due to subduction erosion at the interface between the Nazca and South American plates. It is known to contain hydrates, as evidenced by a well-developed BSR (Bialas and Kukowski, 2000a, b). The pre-existing database allowed the choice of well-defined targets as well as a comparative seismic dataset.

The intention of this survey was to gather experience in the handling and operation of the deep-tow system. We found that in 1000 m water depth, the online knowledge of the

position of towfish and streamer meant that turns between profiles of 500â&#x20AC;&#x201C;600 m spacing were feasible. Even using an 18 mm coaxial deep sea cable, data transfer rates were sufficient to allow online transmission of all seismic (and sidescan sonar) data from the bottom- to the top-side, to be displayed as common shot and offset gathers and stored on DLT tape.

Quality control of the seismic data is through display of both shot gathers and a single channel. The former shows if all the channels are functioning; the latter provides a first glimpse of the geology along the section (Figure 5), showing that here signal penetration is about 0.4 s TWT or 300 m depth, respectively and that imaging three chemoherms embedded within a weakly reflecting sequence of hemipelagic or turbiditic sediments. A spatially limited, weak BSR can also be observed. Some of the strong reflections seem to continue across the chemoherms but are difficult to trace

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Figure 4: Photograph of the deep-tow system components,prepared for deployment from the ship's stern during RV Sonne cruise SO162. Below left: the entire system on deck before deployment, showing in the foreground the streamer nodes and cables, in the middle the 2 ton depressor weight and at the back the sidescan sonar towfish. Top: detail of the acoustic (AM) and engineering modules (EM). Right: the streamer during deployment, showing the individual modules connected by 1m sections.

because much of the incident signal energy is scattered at the top and trace spacing of about 7.7 m provides only coarse lateral resolution.

We collected a grid of 10 closely spaced profiles of 5 km length and 100 m spacing in an area where the »Max«, »Moritz« and »Witwe Bolte« chemoherms occur at about 1000 m water depth (Bialas and Kukowski, 2000a, b).

The streamer consisted of 22 acoustic and three engineering nodes each spaced 1 m apart to allow very high resolution imaging of subsurface structures by close subsurface reflection points and had an overall length of 74 m (including 50m lead-in). Source was a 1.6 litre air gun, generating frequencies between 50-300 Hz. Data recording parameters were 3.072 s recording time and 0.25 ms sample interval. Shot interval was 5 s and average ship velocity 3 kn resulting in an average shot point spacing of 7.7 m.

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Deep-tow multichannel seismic data processing The first step in processing the data is the accurate determination of the location of each hydrophone node relative to the ship during data acquisition. This is accomplished using a combination of the USBL and engineering node data. The asymmetric source and receiver geometry of the hybrid deep-tow system causes subsurface reflection points to lie on a hyperbola even in the case of a plane-layered subsurface (cf. Figure 3), so that standard CMP methods (NMO correction and stack) are not applicable. Instead, the various single channels were combined into a final image using a Kirchhoff 3D-prestack depth migration, specifying directly the unusual acquisition geometry.

The migration (Figure 6), described more thoroughly in the Figure caption, shows a high resolution image of the structures within hydrate bearing sediments, in particular of those related to the transport of fluids and the construction of carbonate build-ups.

Figure 5: Single-fold deep tow seismic profile recorded in the »Max & Moritz« chemoherm area off Peru (Breitzke and Bialas, 2003). Each trace represents the recordings of each shot by one channel (5), after bandpass-filtering between 55/110 - 500/1000 Hz. No depth corrections have been applied. VE = vertical exaggeration for a velocity of 1500 m /s. Notice the »dithered« appearance of the direct arrival, of the seafloor reflection and of the sub-surface reflection. This is due to variations in the gun depth of ~ 1 m. and needs to be removed by a residual statics correction. After such removal and after the incorporation of the correct geometry (from Posidonia and from the pressure sensors on the streamer, calibarated and checkaed using the time of the direct wave), the data can be prestack migrated.

Central America A second major deployment of the system was made during So-173 in summer 2003 offshore Nicaragua (Figure 7), where the Cocos plate is subducted beneath Central America, a classic example of an erosive margin. ODP drilling as well as seismic velocities and images indicate that the »margin wedge« between the shelf and the trench consists of basement material overlain by 0.5-1.5 km of slope sediments.

Only a small frontal prism is observed and consists of reworked slope sediments rather than those scraped off the downgoing plate; rapid subsidence of the margin is evidence for the removal of material from the base of the margin wedge and the transport of that material to depth.

Previous surveys had identified a widespread and well-developed BSR and a variety of fluid expulsion features, including mound-like structures (e.g. Pecher et al., 1998; Bohrmann et al., 2002), thought to represents dominantly mud diapiric structures (Figure 7). Apart from local chemosynthetic carbonate caps, the sediments within the mounds are dominantly from

the slope sequence. The presence or mud diapirs is however anomalous considering the thinness of the slope sequence above top basement. Chemical and isotopic data indicate that the fluids come from dewatering at the plate boundary, leading to the suggestion that the mounds are formed by the remobilisation of sediments at the base of the slope sequence by high pressure fluids coming from below.

Of particular interest is the relationship of the mounds to the underlying BSR. On surface seismic, the BSR is not imaged beneath the mounds, possible due to the absence of gas or the elevation of the base of the hydrate stability zone (due to high heat flow) to the surface, although, complementary heat flow studies carried out by Bremen (using in part equipment developed within the INGGAS project) showed that heat flow was not sufficiently raised to expect this (Grevemeyer et al., 2004).

Alternatively, the absence of the BSR beneath the mounds may be due to signal penetration problems through the carbonate cap characteristic of such mounds. The deep-tow streamer provides a way to test this as the long lay-back

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Figure 6: Prestack shot migration of Profile 12. By migrating prestack, the resolution and the signal:noise ration are improved. In particular note a small positive polarity feature ~ 20 ms (15 m) beneath the seafloor above a steeply dipping negative polarity reflection (Arrow A). We interpret the small feature as a carbonate build-up (~ 10 m across) in the sediment where methane has been anaerobically oxidised and the steep reflection is a conduit along which the methane has moved upwards. The fine detail of other larger carbonate mounds is also apparent; these mounds occur above significant upward distortion of the underlying reflectors, which may be related to the upward movement of warm fluids. The mounds have also formed by the anaerobic oxidation of methane. The deep-tow streamer thus has revealed the fine details of the fluid plumbing system within the hydrate stability zone.

distance of the streamer behind the ship allows the undershooting of the mound â&#x20AC;&#x201C; raypaths reaching reflection point beneath the mound enter the subsurface one side of the mound and exit the other and so are not affected by cap rocks.

The source was a 1.7 l GI-gun, providing a cleaner source than used during Sonne 162 in the frequency range 50-300 Hz. The streamer was towed 100m above the seafloor, consisted of 17 channels (14 acoustic nodes and 3 engineering nodes), connected by sections varying between 1 (twelve sections) and 6.5 (four sections) metres, giving a total active length of 38 metres.

Occurrence of BSR and its relationship to the mud mounds The BSR in the area is characterized by: i) reverse polarity relative to the sea floor reflector ii) cross-cutting relationship with the sedimentary stratigraphy and iii) roughly parallelism to the seafloor. The amplitude of the BSR is variable on the deep-tow profiles, with high to moderate amplitudes near the mud mounds but appearing to fade away from them. Directly

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beneath the mounds, the BSR either rises or disappears (Figures 8 and 9). The former cases seems to occur where the mounds are not associated with an offset of the seafloor (i.e. where the mounds are not obviously fault controlled); the latter where such an offset (and probable fault control) exists (Figures 8 and 9).

Elsewhere, the BSR also appears to disappear where the seafloor is offset by normal faults.

Although it is always problematic interpreting amplitude variations in terms of subsurface properties, the variations described above can be interpreted in terms of the fluid flow regime of the subsurface. It is generally, thought that the presence of the BSR indicates local concentrations of free gas beneath the base of the hydrate stability field, and that a brighter BSR indicates increasing amounts of free gas to a few % of the water saturation. The relative slow upward flow of warm (but not hot â&#x20AC;&#x201C; heat flow at the mounds is only elevated above the regional by 10-20 mW/m2 - Grevemeyer et al., 2004) fluids beneath the mounds would result in an upward displacement of the base of the hydrate stability field, the partial dissociation of hydrate and the increase in the concentra-

Figure 7. Bathymetric relief map (illumination from NW) showing two of the physiographic elements off Nicaragua Pacific margin: submarine mud mounds (arrowed) and deeply incised canyons.

Thick segments along track lines refer the positions of Figures 8 and 9. Star symbol indicates the location of Mound Iguana (Figure 9) which has only a slight topographic expression.

tion of free gas. This may explain the brighter BSR in the vicinity of the mounds). However increased permeability where normal faults offset the seafloor allows such gas to escape, resulting in no clear BSR reflection (Holbrook et al., 1996; Gorman et al. 2002).

The relatively minor deflection of the BSR where it is observed indicates that fluid fluxes even at the more active vents such as Mound Iguana (Figure 9 - Sahling et al., 2003) may be considerably less than at well-studied mud volcanoes such as Hakon Mosby (Eldholm et al., 1999) and those off Barbados (e.g. Henry et al., 1996). In both of these other cases, the fluids are derived from the dewatering of a thick sedimentary pile rather than from the dewatering of the thinner subducted sequence inferred here (Hensen et al., 2003).

Conclusions The deep tow streamer developed within INGGAS has already proved useful for characterizing structures related to fluid flow through

hydrate-impregnated sediment. Off Peru, the system imaged small fluid conduits feeding local carbonate build-ups, confirming the idea that such build-ups are related to the transport of deep fluids towards the surface. Off Nicaragua, the images show that the BSR rises beneath the mud mounds, implying a moderately elevated heat flow associated with the upward passage of warm fluids. A local brightening of the BSR near the mounds may be related to increase in the amount of free gas beneath the hydrate zone, perhaps associated with the dissociation of the hydrate near the warmer conduit.

Acknowledgements The development of the deep-tow MCS system, and of the heat flow probe and the work in the Arkona Basin were funded by the German Ministry of Education, Science, Research and Technology (BMBF) within the gas hydrate initiative of the program GEOTECHNOLOGIEN, project INGGAS (grants 03G0564A, C, D and E).

Other generous funding for work off Central

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Figure 8: Seismic interpretation of profile DTMCS-P05 (see the location in fig. 7). Note the mounds are associated with a vertical offset of the sea floor and sedimentary wedges (indicated by black dots) suggesting asymmetric growth controlled by normal faults. Gray dots indicate the unconformity. The strong inclined reflectors masking the SE side of the mound walls are artefacts produced by the asymmetric geometry of the deep- tow reflection system. Beneath these mounds the BSR disappears: we infer that free gas has escaped upwards along the normal faults that both offset the seafloor and control the location of these mounds. After Talukder et al., (in review).

Figure 9. Seismic interpretation of profile DTMCS-P07 (see the location in fig.7). The BSR is clearly imaged beneath the mound, where it is displaced upwards. Note also that the BSR is brightest below the flanks of the mound, suggesting increased concentrations of free gas beneath the hydrate zone. We infer that the slow upward flow of warm fluids associated with the mud diapir has displaced the base of the hydrate stability field upwards, perhaps causing the dissociation of some hydrate to free gas, but that no conduits exist for such free gas to escape upwards. After Talukder et al., (in review).

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America came from the DFG through SFB 574.

We are indepted to Captain Papenhagen and his crew for their excellent support during RV Sonne cruise SO162 off Peru and to Captain Kull and his crew for their efficient help during RV Sonne cruise SO173-1.

Grevemeyer I, Kopf AJ, Fekete N, Kaul N, Villiner HW, Heesemann M, Wallmann K, Spiess V, Gerrerich HH, Mueller M, Weinrebe W (2004): Fluid flow through active mud dome Mound Culebra offfshore Nicoya Peninsula, Costa Rica: evidence from heat flow surveying: Marine Geology 207, 145-157.

References cited Bohrmann G, Heeschen K, Jung C, Weinrebe W, Baranov B, Cailleau B, Heath R, Huehnerbach V, Hort M, Masson D (2002): Widespread fluid expulsion along the seafloor of the Costa Rica convergent margin: Terra Nova 14: 69-79.

Henry P, Le Pichon X, Lallemant S, Lance S, Martin JB, Foucher JP, Alina FM, Rostek F, Guilhaumou N, Pranal VA, Castrec M (1996): Fluid flow in and around a mud volcano field seaward of the Barbadoes accretionary wedge: Results from Manon cruise. Journal of Geophysical Research 101, 20,297-20,323.

Bialas J, Kukowski N (2000a): RV Sonne Cruise Report SO146-1&2. GEOPECO (Geophysical Experiments at the Peruvian Continental Margin - Investigations of Tectonics, Mechanics, Gas Hydrates and Fluid Transport). Arica - Talcahuano.

March 1 - May 4, 2000. Geomar Report 96.

Bialas J, Kukowski N (2000b): Peruvian cruise provides fresh insights into gas hydrates. First Break 18(8), 360- 362.

Breitzke M, Bialas J (2003): A deep-towed multichannel seismic streamer for very high-resolution surveys in full ocean depth: Marine Seismic 21, 59-64.

DeMets C, Gordon RG, Argus DF, Stein S (1994): Effect of recent revisions to the geomagnetic reversal time scale on estimates of current plate motions. Geophysical Research Letters 21, 2191-2194.

Eldholm O, Sundvor E, Vogt PR, Hjelstuen BO, Crane K, Nilsen AK, Gladczenko TP (1999): SW Barents Sea continental margin heat flow and Hakon Mosby volcano. Geo-Marine Letter 19, 29-37.

Gorman AR, Holbrook WS, Hornbach MJ, Hackwith KL, Lizarralde D and Pecher I (2002): Migration of methane gas through the hydrate stability zone in a low-flux hydrate province: Geology 30, 327-330.

Hensen C, Wallmann K, Schmidt M, Ranero CR, Sahling H, Suess E (2003): Fluid expulsion related to mud volcanism at Costa Rica continental margin - a window to the subducting slab. Geology 32, 201-204.

Holbrook WS, Hoskins H, Wood WT, Stephen RA, Lizarralde D, Leg 164 Science Party (1996): Methane Hydrate and Free Gas on the Blake Ridge from Vertical Seismic Profiling. Science 273 (5283), p.1840-1843.

Kimura G, Silver EE, Blum P, Shipboard scientific party leg 170 (1997): Proceedings of the Ocean Drilling Program Initial reports 170, College Station, TX, Ocean Drilling Program, pp. 458.

Pecher IA, Ranero CR, von Huene R, Minshull TA, Singh SC (1998): The nature and distribution of bottom simulation reflectors at the Costa Rican convergent margin. Geophys. J. Int. 133, 219-229.

Ranero CR, von Huene R, Flueh ER (2000): A cross section of the convergent Pacific margin of Nicaragua. Tectonics 19, 335-357.

Ranero CR, von Huene R (2000): Subduction erosion along the Middle America convergent margin. Nature 404, 748-752.

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Reston TJ, Bialas J (2002) RV Sonne Cruise Report SO162. INGGAS Test (Integrated Geophysical Characterisation and Quantification of Gas Hydrates - Instrument Test Cruise). Valparaiso - Balboa. February 21- March 12, 2002.

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Sahling H, Echeverria-Saenz S, Corrales-Cordero EM, Soeding E, Suess E (2003): Sea floor observation by OFOS. Kiel, IFM-Geomar, pp. 492.

Talukder AR, Bialas J, Klaeschen D, Brueckman W, Reston TJ, Breitze M (in review): High-resolution, deep towed, multichannel seismic survey of submarine mounds and associated BSR off Nicaragua pacific margin. Submitted to Geology.

Yilmaz Ă&#x2013;. (2001) Seismic Data Analysis. Society of Exploration Geophysicists, Tulsa, OK, USA. Zillmer M, Flueh, ER, Petersen J (2005): Seismic investigations of a bottom simulating reflector and quantification of gas hydrate in the Black Sea. Geophys. J. Int 161, 662-678

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An in-situ laboratory to study terrestrial, permafrost related gas hydrates (Mallik 2002) Weber M., Bauer K.*, Kulenkampff J., Henninges J., Huenges E., Wiersberg T., Erzinger J., Lรถwner, R. GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany, *E-Mail: klaus@gfz-potsdam.de

1. Introduction of the Mallik research drilling project From December 2001 to March 2002 fieldwork was conducted on a new gas hydrate research well program at the northeastern edge of the Mackenzie Delta, Northwest Territories, Canada (Fig. 1). The program was initiated by the Geological Survey of Canada (GSC), the Japan National Oil Corporation (JNOC/JAPEX), the United States Geological Survey (USGS), and the GeoForschungsZentrum Potsdam (GFZ). The main objective of the drilling program was to investigate gas hydrates formed under permafrost conditions in one of the most prominent occurrences of this kind (see Dallimore et al., 2002, for overall description of the project). The major questions that were addressed in the research program were 1) the in-situ geological, geochemical, and petrophysical properties of the gas hydrate bearing sediments, 2) the response of the gas hydrates to controlled production tests, in which the gas hydrates were destabilized by de-pressurization and thermal stimulation, and 3) the effects of these stimulation tests on the insitu material properties of the affected regions.

The experiments included a wide variety of geoscientific investigations including coring, petrophysical analysis, downhole geophysical measurements, and gas geochemical logging.

After a long review process the drill site near Imperial Oil Mallik L-38, an industry exploration well drilled in 1972 (Bily and Dick, 1974), was selected for the location of the gas hydrate research well program (Figure 1). The Mallik site was chosen as it offered favorable logistics

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and had the thickest known occurrence of gas hydrate in the region. Detailed geological, geophysical and engineering data were available from the original industry well and from the Mallik 2L-38 research well program conducted in 1998 (Dallimore et al., 1999). Well log interpretations and core samples from the 1998 research well revealed a strong lithological control on gas hydrate occurrences at Mallik.

For the most part, gas hydrate occurred within coarse-grained sandy sediments that were typically interbedded with non-gas-hydratebearing, or very low gas hydrate content, finegrained silty sediments. On the basis of the well-log interpretations, more than 110 m of well defined gas-hydrate-bearing sands and silty sands were found between 897 and 1110 m (Collett et al., 1999a). Quantitative well-logderived estimates suggested that in situ gas hydrate concentrations were very high, with gas hydrates filling more than 60% of the pore space in most gas hydrate layers, and in many cases more than 80%.

The Mallik 2002 research well program included the drilling of a 1200 m deep main production research well (Mallik 5L-38) and, for the first time, two 1150 m deep scientific observation wells offset 45 m from the main well (Fig. 2). A wide-ranging research program was conducted, involving extensive geophysical studies, core studies, and the application of several technologies to investigate in-situ formation conditions. Field-scale experiments were conducted to monitor the physical behaviour of the gas hydrate deposits and the adjacent sediments during depressurization and

Figure 1: top) The location map shows the site of the Mallik gas hydrate research well program. bottom) The detailed site layout shows locations for main well Mallik 5L-38 and the two scientific observation wells.

thermal stimulation. The science program has been substantially expanded through the acceptance of research proposals that involved over 100 researchers from more than 30 research institutions. The GEOTECHNOLOGIEN research initiative provided funding for several sub-projects within the Mallik program, which were carried out by research groups at the GeoForschnungsZentrum Potsdam. In the following text each of the sub-projects is described and the main results are summarized.

2. Results 2.1 Crosshole seismic experiment Seismic methods are widely used to detect, characterize, and eventually quantify gas hydrate accumulations. In the Mackenzie Delta, the distribution of gas hydrates has been mapped by evaluation of available drilling information,

and extrapolation based on 2-D and 3-D industry seismic data (e.g., Bily and Dick, 1974, Collett et al., 1999b). The Mallik 2002 project included a series of seismic experiments covering a wide range of scales: from core studies, through downhole sonic measurements, crosshole experiments, vertical seismic profiling (VSP), and surface seismic experiments (Fig. 1).

These experiments were originally proposed in order to improve the understanding of the relationships between the seismic observations at different observation scales. Crosshole experiments, which were carried out in Mallik for the first time in gas hydrate research, correspond to scales between those obtained from borehole-sonic and VSP and surface seismic experiments. Additional to this scaling aspect, the crosshole seismic experiments were designed to image possible changes in the seismic

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Figure 2: Perspective view showing the true 3-D location of the three boreholes, and the major stratigraphic sequences. The scaling of the horizontal axis is exaggerated by a factor of 5 against the scaling of the vertical axis. The gray shaded region corresponds with the coverage of the crosswell seismic experiments. The depth range of the permafrost and the gas-hydrate-bearing interval are also depicted.

properties during hydrate production, potentially providing data on the effects of the gas hydrate dissociation tests. These changes may provide important post-production information for the understanding of gas hydrate dissociation processes and stability conditions.

The crosshole seismic measurements made use of two 1160 m deep observation wells (Mallik 3L-38 and 4L-38), both located 42.5 m from and co-planar with the 1188 m production research well (5L-38). Boreholes 3L-38 and 4L38 served as source and receiver wells, respectively. Four complete surveys were conducted between boreholes 3L-38 and 4L-38: one baseline experiment to provide the reference seismic structure, and three monitor, or repeat surveys carried out after the initiation of the thermal stimulation test. Each survey required approximately 24 hours of acquisition time.

The baseline survey covered a depth range between 800 and 1150 m, bracketing the proposed gas hydrate interval between 900 and

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1100 m depth, as well as portions of the surrounding sedimentary sequences on top of and below the primary imaging target. It was anticipated that these sequences might host significant amounts of free gas. Such free gas zones have been encountered in many gas hydrate fields, particularly in deep sea environments. The repeat experiments were limited to a depth range from 800 m to 1050 m, providing sufficient coverage of the target regions of the thermal stimulation tests, between 907 â&#x20AC;&#x201C; 920 m depth (Fig. 2).

The four time-lapse crosswell seismic surveys in the vicinity of the production experiment at Mallik 5L-38 provide a unique opportunity to examine the in-situ seismic properties of arctic gas hydrates, and the evolution of these properties during thermal dissociation. The four surveys are high quality with sufficient signalto-noise ratios. The data exhibit clear direct Pwave arrivals; first arrival times were identified on more than 90% of the traces. The other

Figure 3: a) Sub-set of the data from the crosswell seismic experiment. b) One-dimensional P-wave velocities estimated from horizontal crosswell seismic data. c) Logarithmic presentation of the spectral amplitudes calculated by wavelet analysis for the first-arrival signals of the horizontal crosswell seismic data. d) One-dimensional P-wave attenuation estimated from horizontal crosswell seismic data. e) Lithological profile from coring and gas hydrate saturation estimated from the difference of density and NMR porosity at Mallik 5L-38.

dominant signals present on the surveys are tube-waves and tube-wave related signals, that can be largely removed by f-k filtering of the data in source and receiver domains. The four surveys are highly repeatable, however the effects of the small, experimental thermal dissociation test are expected to be very subtle. The data analysis was done in two sequential steps: The first step comprised the determination of the seismic background structure before the thermal production test was started (Bauer et al., 2005b, Pratt et al. 2005).

These results were used as reference model for the second step, in which modelling studies were used to determine the feasibility of monitoring the changes induced by the thermal production test (Bauer et al., 2005a, Watanabe et al., 2005).

The determination of the seismic background structure was based on data as shown exemplarily in Figure 3. Ray-based tomographic me-

thods were applied to the complete data set of the baseline experiment to derive 2-D models of isotropic and anisotropic velocities, and attenuation at depths between 800 and 1150 m (Fig. 4). The seismic results image the major litho-stratigraphic environment of the target region. The delta front/shallow marine Mackenzie Bay Sequence is characterised by smaller velocities and slightly smoother variability, as compared with the fluvio-deltaic Kugmallit Sequence underneath. The gas hydrates formed preferentially within several sandy layers at the lowermost base of the Mackenzie Bay and the upper part of the Kugmallit Sequence. The entire gas hydrate interval covers the depth range between 890 and 1107 m and consists of several layers of very high gas hydrate concentration (between 30 to 80%). These layers are characterized by high P velocities (up to and slightly larger than 3.5 km/s), strong transverse isotropy (between 5-15% faster horizontal velocities than vertical

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velocities), and strong attenuation (with the smallest Q values between 15 and 8). Lateral variations in these properties indicate that the hydrates occur in more lense-like, rather than stratified, structures reflecting the fluvio-deltaic deposition of the host material. The bottom of the hydrate interval coincides approximately with the lower theoretical boundary of the hydrate stability field (e.g., Dallimore et al., 1999, Henninges et al., 2005). Thus, accumulations of gas, which migrated from the same sources as responsible for the hydrate formation, could be expected below the gas-hydrate-bearing interval. However, the tomographic results show no indications for the presence of free gas, taking into account similar velocity and attenuation values observed in the nonhydrated sediments on top of the section, and the low degree of transverse isotropy. This is also in agreement with the results from the mud gas logging (Wiersberg et al., 2005). The lack of free gas could be responsible for the

difficulty to detect bottom simulating reflections in the regional seismic data.

The observed characteristic values of seismic velocity, attenuation, and anisotropy for the hydrate-bearing sediments may provide important constraints for the microscopic structure and interaction between the grain frame, hydrates and co-existing pore filling. Especially, the association of velocity with attenuation is not intuitive for porous sediments, but the results from the crosswell data are also confirmed by the sonic data. Based on a more tentative argumentation, both the cementation model and the no-contact-cement model (Ecker et al., 1998) partly could explain the observations, but neither model appears to be completely consistent. We speculate, that also the microporous structures (Kuhs et al., 2000) could be important in terms of petrophysical modelling of the internal stucture of hydrate bearing sediments.

Figure 4: Two-dimensional images of the seismic P-wave velocity, anisotropy, and attenuation distribution derived from from the crosswell seismic data. The gas-hydrate-bearing interval is indicated by a red box. Lithostratigraphic units and gas hydrate saturation are also shown.

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Modelling studies were carried out to investigate the possible effects of the gas hydrate dissociation on the repeat crosswell data. From this study we conclude that a small dissociation region will introduce only very weak changes in the crosswell seismic wavefields. The most significant effects occur when the direct P-wave is transmitted through the dissociation region, resulting in a phase shift for these arrivals. Other effects, such as diffractions and mode conversions from the top and the bottom of the anomaly are likely to be masked by other secondary arrivals, and would exhibit very weak amplitudes as a result of the strong attenuation of the gas hydrates. We calculated difference data for one example receiver gather at 915 m depth. Small phase changes in the direct P-waves were identified on these difference data. We ascribed these phase changes to real changes in formation properties as a result of the gas hydrate dissociation. Full waveform inversion of the differential wavefields, as carried out by Watanabe et al. (2005) is considered to be the most suitable method to make use of these observed phase changes, in order to image the dissociated regions.

2.2 Petrophysical properties from laboratory studies Estimates of the total amount of methane hydrates in the earth's crust are highly speculative because detection and quantification algorithms for gas hydrate deposits are based on imprecise empirical observations and assumptions. Quantitative relations between gas hydrate occurrences and geophysically observable parameters have to be derived from physical principles and laboratory measurements. Up to now such relations could not be established, because testing of natural gas hydrate bearing sediments poses experimental challenges: Stability conditions have to be maintained from coring until the laboratory test as far as possible.

In the frame of the Mallik 2002 gas hydrate production research well program, we succeeded in taking into account the fragility of gas hydrates as far as possible and minimizing the time lapse between core retrieval and laboratory investigations and the time spent at nonstability conditions.

Cores were taken from the Mallik 5L-38 gas hydrate research well over the complete gas hydrate interval from 890 to 1100 m with a wireline coring system. The mud was chilled to about 0째C, providing gas hydrate stability conditions below about 300 m. From core temperature records it was estimated that the gas hydrate was outside of the stability zone for about 20 minutes during retrieval. At the surface the cores were rapidly frozen at arctic conditions (less than -30째C), but for some minutes they stayed at conditions of anomalous preservation. The loss of gas hydrate during the coring trip was estimated to be less than 10%.

Core sections were quickly put into transport vessels that were pressurized with methane to 5 MPa. The deep frozen samples were then at stability conditions for transport to the laboratory and for storage over a period of hours to some days. 20 samples were prepared from the core sections at arctic conditions, with minimal thermal impact.

A versatile Field Laboratory Experimental Core Analysis System (FLECAS) for measurements at controlled temperature, confining and pore pressure, was developed and built at the GFZ in Potsdam for the investigation of gas hydrates under simulated in situ conditions (Fig. 5).

It was installed at the field laboratory at Inuvik.

The measuring system consists of P- and Swave transducers, temperature sensors, 6 electrodes for resistivity measurements, a length sensor, a flow sensor, and pressure transducers for pore pressure at both ends of the sample and for the confining pressure.

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decrease of the sample length, the recovery of ultrasonic amplitudes and the endothermal cooling of the samples, causing a temperature depression of 1 to 3 째C over about 20 minutes. A volume of roughly 13 l of gas at ambient conditions was produced out of the sample.

5) After gas hydrate decomposition, the sample was again flushed with N2 to extract gas hydrate water. Then N2 was flowed through the sample at a constant rate to estimate permeability of the gas hydrate-free sample. These permeability values of the host sediments were rather high, in the order of 1 Darcy.

6) An optional procedure was to decompose the gas hydrate by heating above the stability zone. Then the decomposition appears to be less brisk. The reaction of the measuring parameters was not as strong, because the gas remained in the pore space.

Figure 5: Schematic diagram of the internal setup and measuring facilities of FLECAS

A typical test is divided into five parts (Fig. 6): 1) The deep frozen (< -20 째C) samples were placed into the main pressure vessel which was previously chilled to less than 10 째C. Then the confining and pore pressure were increased to in situ conditions.

2) Within 3-4 hours the temperature was increased to in situ conditions. During this heating period resistivity and sonic velocities decreased gradually. A strong decrease of the ultrasonic amplitudes was observed, but no significant response from the length sensor.

3) After reaching in situ conditions resistivity, ultrasonic P- and S-wave velocities and amplitudes were recorded.

4) Then we usually triggered gas hydrate decomposition by pore pressure release. At the same time, pore water was extracted from the sample. It was intended to measure permeability during this period, but the decomposition process started immediately. This was obvious by the immediate

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Matrix densities around 2.65 g/cm3 and grain size distribution (well sorted fine sand to silt) of the sample remnants were determined after the experiments. Bulk density, porosity, water saturation were determined, considering volume, sample mass before and after the experiment, as well as the gas hydrate saturation that were determined on the preparation remnants.

The gas hydrate content and the physical properties of all samples were quite similar, with insignificant variations. These small variations reflect that the gas hydrate content and the type of the host sediment are quite uniform.

Thus, an empirical universal relationship between gas hydrate content and resistivity, respectively ultrasonic parameters, can not be derived from the data set. However, some conclusions for the structure of gas hydrates can be drawn: Melting of the ice in the pores significantly changes the mechanical and transport parameters, although the gas hydrate remains stable. The decrease of resistivity and sonic velocity is caused by ice melting.

Figure 6: Example of a FLECAS measuring record (sample from Mallik 5L-38, 839 m).

Decomposition of the gas hydrate changes the physical properties less significantly; only the mechanical strength is lost completely, causing the strong decrease in length during depressurization and a further decrease of ultrasonic velocities.

No significant change in resistivity is observed when the sample is heated above the stability threshold. This is because the number of conducting ions remains constant although the conducting pore volume becomes larger when the gas hydrate is replaced by water and gas.

The data provide a basis for the development of petrophysical models which can be used to evaluate the influence of gas hydrate on ultrasonic and electrical rock properties. These properties are strongly related to the structure and location of the gas hydrate in the pore space.

Our observations imply that the gas hydrate is filling larger sediment pores rather than

cementing grains. Otherwise the gas hydrate together with the host sediment grains would build a rigid frame that would inhibit strong reactions of the elastic properties on the melting of the ice. The answer of the elastic parameters on gas hydrate decomposition is much smaller than on ice melting. This is more an indication for a loose contact between the sediment grains and the gas hydrate.

A strong decrease of the ultrasonic amplitudes occurs when the ice is melting. The amplitudes usually recover when the gas hydrate decomposes. This happens in spite of the partial gas saturation that fills the pores after gas hydrate decomposition â&#x20AC;&#x201C; and not so much before.

Therefore this absorption effect is a mere gas hydrate effect and not primary caused by gas in the pore space. A possible reason is a Âťshock absorber effectÂŤ of the fluids filling microporous structures.

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Figure 7: The Mallik 2002 Gas Hydrate Production Research Well Program drilling rig and a schematic cross section of the field experiment. Note the permanent installation of fibre-optic distributed temperature sensing cables behind the borehole casing in the cement annulus.

2.3 Fiber-optic distributed temperature monitoring The size and distribution of natural methane hydrate occurrences and the release of gaseous methane through the dissociation of methane hydrate are predominantly controlled by the subsurface pressure and temperature conditions. Because of the related change in enthalpy, both the formation and dissociation of gas hydrate in nature are inevitably coupled to the transport of heat within the surrounding formation. Knowledge

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about the thermal properties of hydrate bearing rocks (i.e. thermal conductivity, specific heat, and latent heat of phase transition) is therefore of crucial importance.

Until now, only a very limited amount of thermal data related to gas hydrate occurrences exists. Analysis of the geothermal conditions and the derivation of the stability field for methane hydrate are often based on the interpolation of single, thermally disturbed bottom-hole temperature measurements

Figure 8: Temperature profiles (T) and 20-m average temperature gradient (dT/dz) of the Mallik 3L-38 observation well for successive times after completion of the well (ts). The base of the ice-bearing permafrost (IBPF) and gas hydrate occurrences are respectively marked by a sinusoidal change of the temperature gradient, which gradually diminishes with time. The gamma-response (GR) is affected by the casing (cased-hole log).

and drill-stem test data from petroleum exploration wells and/or assumptions about the thermal properties of the formation. One aim of the performed borehole temperature measurements was therefore to create a detailed database for the investigation of the geothermal field in the area of the Mallik gas hydrate occurrence and during the field-scale destabilization of methane hydrate.

Distributed temperature measurements at Mallik In the framework of the Mallik 2002 an innovative method for the measurement of continuous temperature profiles in boreholes was developed and its applicability under extreme arctic conditions was proven (Henninges et al., 2005). Three 1180 m deep wells, spaced at 40 m, were equipped with permanent fiber-optic sensor cables (Fig. 7). A special feature of the experiment design is the permanent installation of the sensor cables outside the borehole casing. After completion of the well, the sensor cables are located in the cement annulus between casing and borehole wall. The fiber-

optic cables were attached to the outer side of the casing at every connector, within intervals of approx. 12 m, using custom-built cable clamps.

After the completion of the wells, continuous monitoring of the well temperatures was performed over a period of up to 61 days from January to March 2002. The DTS logging was started one to two days after completion of the respective well. Temperature profiles were recorded with sampling intervals of 0.25 m and 5 minutes. After completion of this initial observation period, the surface ends of the fiber-optic sensor cables were stored in special containers at the wellheads to allow for future temperature measurements at later times. In October 2002 and September 2003 repetitive measurements were carried out successfully during subsequent field trips to the Mallik site.

Effects of phase transitions and thermal properties The analysis of the disturbed well temperatures after drilling revealed a strong effect of phase transitions on temperature changes (Henninges

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et al., 2005). For the first time, the effects of induced temperature changes within a gas hydrate deposit were monitored in-situ. The resulting temperature gradient anomalies could be successfully utilized to determine the base of the gas hydrate occurrences and the permafrost layer at about 1103-1104Âą3.5 m and 599604Âą3.5 m below ground level respectively (Fig. 8). The joint interpretation of the geothermal data and the geophysical well-log data indicates that variations of thermal conductivity are mainly lithologically controlled (Wright et al., 2005).

The influence of hydrate saturation is only of minor significance for the effective thermal conductivity of the formation.

Thermal stimulation test The thermal stimulation test conducted at the Japex/JNOC/GSC Mallik 5L-38 well in March of 2002, was designed to increase the in-situ temperature of a well defined and constrained gas hydrate reservoir above the gas hydrate stability point, while maintaining constant pressure. During the thermal stimulation experiment, the temperature variations along the Mallik 5L-38 wellbore were measured (Henninges et al., 2005). DTS logging started one day after installation of the production casing, and continued for a period of 17 days during the entire thermal production testing program.

The thermal stimulation test was successful in that the bottomhole temperature was increased and held constant in excess of 50 °C; gas from dissociated gas hydrate was produced, sampled, and flared at surface; and significant amounts of real-time downhole temperature and pressure data, as well as other scientific measurements, were obtained (Hancock et al., 2005). Data collected during the thermal stimulation test, including surface and downhole instrumentation readings, as well as advanced logging and seismic programs, was used to calibrate numerical gas hydrate reservoir simulation models, and determine the kinetic and thermodynamic properties of the in-situ gas hydrate (e.g. Moridis et al., 2003).

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2.4 On the geochemistry of gases at Mallik from real-time gas analysis Within the Mallik 2002 Gas Hydrate Production Research Well Program, we have carried out investigations on the geochemistry of gases in real-time. Our on-site studies combined 1) on-line mud gas monitoring during drilling of the JAPEX/JNOC/GSC et al. Mallik 4L-38 and 5L-38 wells, 2) on-line analysis of gas released during a thermal production test in the Mallik 5L-38 well, and 3) decomposition experiments on gas-hydrate-bearing drill cores (Wiersberg et al., 2005).

On-line mud gas monitoring The main objectives of the on-line mud gas monitoring at Mallik were to identify gas hydrate-bearing horizons while drilling and to distinguish them from other gas sources such as gas accumulations below nonpermeable strata. For this, a gas-water separator was installed directly above the mudflow line to extract the gas phase mechanically out of the returning drill mud. The extracted gas was pumped into a laboratory trailer and analyzed in real-time with a quadrupole mass spectrometer, a gas chromatograph, and a radon detector. A complete analysis of Ar, He, N2, O2, CO2, CH4, and H2 within detection limits of less than 1ppmv (parts per million by volume) was achieved after an integration time of 12s. The on-line mud-gas-monitoring method yields gas depth profiles for many different gases and is, in contrast to commercial mud-gas logging, not limited to combustible gases.

From the Mallik 2L-38 well it was known that gas hydrate in Mallik wells consists predominantly of methane gas hydrate (Lorenson et al., 1999; Uchida et al., 1999). It is, however, not possible to identify gas hydrate during drilling based only on high methane concentration in the drill mud. Additional gas data are necessary to identify the source of gas dissolved in the mud during drilling. Figure 9 shows mud gas versus depth profiles of Mallik 4L-38 well for the methane concentration, the helium concentration, and the 222Rn activity. The

Figure 9: Mallik 4L-38 gas versus depth profiles for CH4 (vol.%), He (ppmv) and 222Rn (Bq/m3). Depth below kelly bushing (4.6 m).

principle nonatmospheric gas found in the drill mud was methane, with up to 70 vol.% in the main gas hydrate section at Mallik, however, even at shallow depth the methane concentration reached up to 35vol.% in some cases.

Elevated methane concentration were observed at about 107m, 646m, 766m, and 827m depths and in ten distinct layers within an interval at 890–1150m depth that encompasses the main gas-hydrate-bearing section (890–1110m). Some, but not all are associated with gas-hydrate-bearing strata. In contrast to non-hydrate bearing strata a decrease of helium with increasing methane concentration has been observed while drilling through hydrate intervals. Subsequently, they were identified by geophysical logging after drilling.

For example, mud gas from the depth interval 766–779 m showed increasing helium concentration at increasing methane content. Indeed, geophysical logging yields no evidence for gas hydrate occurrence in this section. Therefore, the combination of high methane concentrations with low helium concentration in the mud gas can be used as diagnostic tool for real-time identification of gas hydrate. The helium concentration in natural gas hydrate is

very low, because helium is too small to be accommodated in the hydrate lattice at ambient pressure and temperature conditions.

On-line analysis of gas released during a thermal production test During the Mallik 5L-38 gas hydrate production research well thermal test program (Takahashi et al., 2003) gases were also analysed in real-time. A gas-hydrate-rich depth interval from 907–920 m was selected for thermal stimulation. The 13m interval of the cased hole was perforated, then hot brine was injected for about 124 hours through a pipe down to the base of the target zone. The hot brine decomposed the gas hydrate, resulting in a fluid flow through the perforation and uplift between the inlet string and the well tubing.

At the surface, the gas was extracted from the water phase with a two-stage oilfield gaswater separator. The brine was reheated and reinjected. A portion of the separated gas was piped out to the laboratory trailer for on-line gas analysis. At 2.15 hours after circulation started (March 5th, 2002, 20:24h), a first methane response was detected. During the following 6 hours, the methane concentration

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Figure 10: Carbon dioxide concentration and fluid temperature versus time after start of the thermal production test at Mallik 5L-38 well. Fluid temperature data after Hancock et al. (2005).

increased to about 90vol.%. The variation in methane concentration was relatively small ±2%) after reaching maximal values, whereas significant variations in the concentrations of other hydrocarbons, He, and CO2, were detected, sometimes on a relatively short time scale.

CO2 concentrations ranged from <0.01vol.% to 1 vol.%, whereas the He concentration varied between 5ppmv and 7ppmv. Values of CH4 /(C2H6 +C3H8) ranged between less than 100 and 2500, and sometimes changed rapidly within one hour.

The variations in CO2 correlate positively with the temperature of the circulating brine (Fig. 10), which is due to the temperature-dependent solubility of CO2 in the brine. Generally, the CO2 solubility decreases with increasing temperature, leading to higher CO2 concentration in the corresponding gas phase. The temperature dependance of the solubility of other gases in brines is distinctly smaller, therefore, the distribution coefficient between gas and water phase for other gases is less sensitive for temperature variations. As shown by the mud-

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gas data of Mallik 4L-38, gas hydrate is characterised by elevated CH4 and CH4/He ratios.

This was confirmed by data from the thermal production test, particularly during the second half of the test, where a positive correlation between CH4 / He as well as CH4 /(C2H6+C3H8) was indicated (Fig. 11). Gases released from different depths within the perforated interval might explain the heterogeneity in the gas composition during the thermal stimulation.

While drilling of the Mallik 4L-38 well an increase of the CH4/He ratio was detected throughout the interval 907–920m.

Decomposition experiments on gas-hydrate-bearing drill cores Data on the composition of gas-hydrate-derived gas were obtained in the field from gas hydrate decomposition of gas-hydrate-bearing sediment core samples from the depth interval 891–923m. These experiments also yield information about the gas-water-solid properties, and physical conditions of gas hydrate decomposition. For this, 20–30g of gas-hydrate-bea-

Figure 11: CH4 / (C2H6+C3H8) (red) and CH4 / He (x10-4) (black) versus time after the start of the thermal production test at Mallik 5L-38 well.

ring core material was deposited in an aluminium sample container and placed into a Teflon vessel equipped with a thermocouple, a pressure sensor, and a connector to the gas mass spectrometer. After loading, the vessel was quickly closed and cooled with liquid nitrogen to minimise uncontrolled gas hydrate decomposition. The temperature in the vessel increased gradually after the liquid nitrogen was completely evaporised. At approximately 130K, the vessel was evacuated to remove air.

Between about 155–160K, beginning of gas hydrate decomposition under pressure conditions less than 100 mbar was indicated by a fast increase of pressure and methane concentration. After the temperature in the vessel exceeded 283K, a gas sample was taken, then the vessel was evacuated, opened, and the gas-free sample mass was determined. After removing water from the residual wet sand, the dry sand was weighed.

All investigated hydrate bearing core samples had a relatively constant gas composition. The principle gas found in the samples was metha-

ne (>99 vol.%). Minor amounts of heavier hydrocarbons, released at less than 293K, can be explained by either the presence of structure II gas hydrate or heavier hydrocarbons trapped in structure I in trace quantities. The water to solid matter ratio is relatively constant for all samples, while amount of gas is more variable.

The latter might be a result of gas got lost during core recovery and handling. The beginning of gas hydrate decomposition is indicated by a significant increase of the pressure and the methane concentration at a temperature of approximately 160K within a pressure range of 40–70mbar.

2.5 Mallik 2002 Data and Information System The »Mallik Data and Information System« was developed in cooperation with the ICDP Operational Support Group and the GeoForschungsZentrum Potsdam (GFZ) Data Centre. It was based on concepts and tools of the ICDP Information Network and adapted to two conditions specific to the Mallik project. First, the core required special handling to preserve features relevant to formation and stability of gas

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Figure 12: Internal network structure used at the Inuvik Research Centre.

hydrate in the sediment. Second, the Mallik science team was divided into three scientific subgroups and a group of industrial partners.

Access rights had to be defined for each science team member according to the confidentiality rules for each of the groups (Loewner R. and Conze R., 2005).

The Mallik-DIS database structure The database structure for the Mallik Drilling Information System (Mallik-DIS) was designed in the fall of 2001 in preparation for integration of parameters acquired at the drill site and at the IRC field lab with results from participating labs.

The starting-point was the basic DIS - an electronic toolbox to develop on-site information systems for scientific drilling projects. Its central part is a set of data templates typical for scientific drilling purposes deriving from a number of earlier ICDP projects. These templates were adapted according to specific demands of the Mallik project, e.g. the greater variety of sampling types. New data structures were added, for example, for the first core description made in the field at the JAPEX/JNOC/

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GSC et al. Mallik 5L-38 production well. Based on the generated data model, input forms and print reports were built automatically using the DIS Graphical-User-Interface-Builder (GUIBuilder). The Data-Pump-Builder was used to create data import facilities for borehole measurements and the online gas monitoring data.

The Web-Info-Builder was used to prepare and to provide project information updates on the Internet day-by-day.

Data Acquisition and Dissemination A small local area network was set up at the Inuvik Research Centre (IRC). It included the DIS server machine, one client and the ICDP core scanner unit (Fig. 12). The Mallik-DIS was installed to acquire data and scan cores without any loss of time, close to the drill rig and the IRC core handling operations. Primary data were captured in February 2002 during the coring period of the production well (Mallik 5L-38). This dataset included information from the drill site: the initial description of the lithology of the recovered core from the 48 core runs, drilling notes for each core run and

Figure 13: Examples from the Mallik Data Warehouse: detail of the lithological profile of Mallik 5L-38.

a record of samples taken at the drill rig for pressure and/or temperature sensitive tests.

Subsequently, the core was documented as it arrived at the IRC in metre lengths contained in plastic or aluminum liners. High-resolution images of the split surfaces of the frozen and unfrozen drill cores from each of the recovered 210 liners were obtained using the ICDP core scanner. The core scanner unit manufactured for ICDP by DMT (Deutsche Montan Technologie, Essen, Germany) is capable of scanning cores ranging from 1 to 15 cm diameter up to a maximum length of one metre. Through a special DIS interface the resulting images were converted to jpg-files and stored together with the corresponding metadata in the Mallik-DIS. Each day, a Web Info update of all data, reports, and images generated within the last 24 hours was produced and sent to the Mallik Web site in the ICDP Information Network via file transfer protocol (FTP). Thereby, it was possible to provide worldwide information access for the participating scientists on the progress of the drilling in near real-time during the active operational phase under restricted data access.

After the drilling period ended in mid-March 2002, the third phase, quality control, data integration, modification of access rules and acquisition of secondary data and results began. The data sets collected, prepared and incorporated into the Mallik data warehouse during the pre-publication phase of the project amounted to several megabytes.

Users may access and integrate lithological descriptions, all kinds of borehole geophysical measurements, monitoring data and an archive of all the core runs and samples. Users may compose data profiles according to specific interests (Fig. 13). The integration of multidisciplinary data sets enables individual dynamic visualization and comparison of different kinds of information.

Developing these data acquisition and management systems Âťon-the-jobÂŤ is usual for data management of ICDP scientific drilling projects, because some features can only be determined under real on-site conditions.

Therefore, the underlying data model of the Mallik-DIS was adapted several times due to particular new or changing requirements, such as the mapping of the ICDP and Geological Survey of Canada (GSC) naming conventions

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for samples or depths correlation. The DIS toolbox provided the necessary means for shortterm modifications of the operational database model, or the user interfaces, e.g. input forms and print reports.

The importance of the core archive and the initial lithological description of the cores are apparent in the production of a stratigraphic column, which was a very effective and useful basic measure of progress for the Mallik project. As it was used as a reference system for all subsequent parameters measured on the cores or downhole data obtained at the drill rig, it immediately enabled the detection of, for instance, errors in depth assignment or liner interpolation. The GFZ Mallik working group used the following entities for discussions, talks and publications during the evaluation phase: - correlation of sedimentary features with geophysical and geochemical measurements, - correlation between the technical parameters, the logging data, and the gas parameters - comparison of the core scans and the lithological data for sub-sampling, - correlation of sedimentary characteristics with the gas hydrate content.

Important issues for the Mallik project were the complexity of the science teams and the corresponding confidentiality rules for each of the groups. Especially in the starting phase, there were some uncertainties concerning collaboration and access security. This showed how important it is to negotiate, communicate, and promote the appropriate rights and duties as early as possible. The standard policies used for ICDP scientific drilling projects can be seen at http://www.icdp-online.de/about.

3. Summary The achievements of the Mallik 2005 Gas Hydrate Production Research Well Program near Inuvik in the Northwest Territorries, Canada, were the first controlled in-situ destabilization tests of terrestrial gas hydrates,

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accompanied by a challenging experimental program. The research program was conducted by an international collaboration, including the GeoForschungsZentrum Potsdam. Two observation wells and one production well were drilled in horizontally aligned configuration. The observation wells served for temperature monitoring and seismic measurements during the thermal stimulation test in the production well.

The GeoForschungsZentrum took part in all major subjects of the research program: Drilling, logging, and core analysis data from all partners were collected and made globally available to participants in the Mallik Drilling Information System. The system had to make allowances for the scientific complexity of the project, its internationality, and special confidentiality rules.

A geochemical gas analysis was conducted during the drilling and production test phase. In the drilling phase, the specialized mud gas analysis allowed to detect gas hydrate layers and to determine the origin of the gas. During the thermal destabilisation test, the monitoring of the composition produced gas composition gave insights into the processes during destabilisation and transport of the gas to the surface.

All three wells were equipped with distributed temperature sensors that served for monitoring the temperature field during the gas hydrate destabilisation test and a long equilibration period thereafter. Data collected during the thermal stimulation test, including surface and downhole instrumentation readings, as well as advanced logging and seismic programs, was used to calibrate numerical gas hydrate reservoir simulation models, and determine the kinetic and thermodynamic properties of the in-situ gas hydrate.

Similar experiments as in situ have been conducted on gas hydrate bearing cores from the procuction well in the field laboratory. Core analysis data provide a basis for the development of petrophysical models, which can be used for calibration of simulation models and formation evaluation with well logging and geophysical field methods.

The direct imaging of the crosshole section between the observation wells with seismic methods gave insights into the structure of the gas hydrate occurrences and their physical properties. The impact of the gas hydrate destabilisation experiment on seismic data was detectable. Nevertheless, the spatial extent of the induced changes at the production well was too small for a quantitative evaluation.

The »Mallik Data and Information System« provided a database for most project data-types and its secure and restricted distribution during the pre-publishing phase. It operated as a communication platform between the project members through the Web portal of the ICDP Information Network (http://www.icdp-online.de).

Detailed evaluation of gas hydrate reservoir properties using JAPEX/JNOC/GSC Mallik 2L38 gas hydrate research well down hole welllog displays; in Scentific results from JAPEX/ JNOC/GSC Mallik 2L-38 Gas Hydrate Research Well, Mackenzie Delta, Northwest Territories, Canada, (ed.) S.R. Dallimore, T. Uchida, and T.S. Collett, 295-312.

References Bauer, K., Pratt, R.G., Weber, M.H., Ryberg, T., Haberland, C., and Shimizu, S., 2005a: Mallik 2002 cross-well seismic experiment: project design, data acquisition, and modelling studies; in Scientific Results from the Mallik 2002 Gas Hydrate Production Research Well Program, Mackenzie Delta, Northwest Territories, Canada, (ed.) S.R. Dallimore and T.S. Collett; Geological Survey of Canada, Bulletin 585, 14 p.

Dallimore, S.R., Uchida, T., and Collett, T.S., eds., 1999: Scientific results from JAPEX/JNOC/ GSC Mallik 2L-38 Gas Hydrate Research Well, Mackenzie Delta, Northwest Territories, Canada; Geological Survey of Canada Bulletin 544, 403 p.

Bauer, K., Haberland, C., Pratt, R.G., Hou, F., Medioli, B.E., and Weber, M.H., 2005b: Raybased tomography for P-wave velocity, anisotropy, and attenuation structure around the JAPEX/JNOC/ GSC et al. Mallik 5L-38 gas hydrate production research well; in Scientific Results from the Mallik 2002 Gas Hydrate Production Research Well Program, Mackenzie Delta, Northwest Territories, Canada, (ed.)

S.R. Dallimore and T.S. Collett; Geological Survey of Canada, Bulletin 585, 21 p.

Bily, C., and Dick, J.W.L., 1974: Natural occurring gas hydrates in the Mackenzie Delta, Northwest Territories; Bulletin of Canadian Petroleum Geology, v. 22, no. 3, 340-352.

Collett, T.S., Lewis, R., Dallimore, S.R., Lee, M.W., Mroz, T.H., and Uchida, T., 1999a:

Collett, T.S., Lee, M.W., Dallimore, S.R., and Agenda, W.F., 1999b: Seismic and well-loginferred gas hydrate accumulations on Richard Island; in Scentific results from JAPEX/JNOC/ GSC Mallik 2L-38 Gas Hydrate Research Well, Mackenzie Delta, Northwest Territories, Canada, (ed.) S.R. Dallimore, T. Uchida, and T.S. Collett, 357-376.

Dallimore S.R., Collett, T.S., Weber, M.H., and Uchida, T. , 2002: Drilling program investigates permafrost gas hydrates; EOS, v. 83, p. 193/198.

Ecker, Ch., Dvorkin, J., and Nur, A., 1998 : Sediments with gas hydrates: Internal structure from seismic AVO; Geophysics, v. 63, p. 1659-1669.

Hancock, S., Collett, T.S., Dallimore, S.R., Satoh, T., Huenges, E., and Henninges, J., 2005: Overview of thermal stimulation production test results for the Japex/JNOC/GSC Mallik 5L-38 Gas Hydrate Research Well; in Scientific Results from the Mallik 2002 Gas Hydrate Production Research Well Program, Mackenzie Delta, Northwest Territories, Canada, (ed.) S.R. Dallimore and T.S. Collett; Geological Survey of Canada, Bulletin 585, 15 p.

Henninges, J., Schrötter, J., Erbas, K., and Huenges, E., 2005: Temperature field of the Mallik gas hydrate occurrence – implications on phase changes and thermal properties; in

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Scientific Results from the Mallik 2002 Gas Hydrate Production Research Well Program, Mackenzie Delta, Northwest Territories, Canada, (ed.) S.R. Dallimore and T.S. Collett; Geological Survey of Canada, Bulletin 585, 14 p.

Kuhs, W.F., Klapproth, A., Gotthardt, F., Techmer, K., and Heinrichs, T., 2000: The formation of meso- and macroporous gas hydrates; Geophysical Research Letters, v. 27, p.

2929-2932.

Kulenkampff, J., Spangenberg, E., 2005: Physical properties of cores from the Mallik 5L-38 gas hydrate production research well under simulated in situ conditions using the Field Laboratory Experimental Core Analysis System (FLECAS); in Scientific Results from the Mallik 2002 Gas Hydrate Production Research Well Program, Mackenzie Delta, Northwest Territories, Canada, (ed.) S.R. Dallimore and T.S. Collett; Geological Survey of Canada, Bulletin 585, 16 p.

interval in the 5L-38 Mallik Research Well; Mallik International Symposium (Chiba, Japan), Program & Abstracts, Japan National Oil Corporation, Technology Research Center, p. 58.

Pratt, R.G., Hou, F., Bauer, K., and Weber, M.H., 2005: Waveform tomography images of velocity and inelastic attenuation from the Mallik 2002 crosshole seismic surveys; in Scientific Results from the Mallik 2002 Gas Hydrate Production Research Well Program, Mackenzie Delta, Northwest Territories, Canada, (ed.) S.R. Dallimore and T.S. Collett; Geological Survey of Canada, Bulletin 585, 14 p.

Riedel, M., Kulenkampff, J., Spangenberg, E., and Dallimore, S.R., 2005: Geophysical Properties of Sediments from Mallik 5L-38; in Scientific Results from the Mallik 2002 Gas Hydrate Production Research Well Program, Mackenzie Delta, Northwest Territories, Canada, (ed.) S.R. Dallimore and T.S. Collett; Geological Survey of Canada, Bulletin 585, 10 p.

Loewner, R., and Conze, R., 2005: The Mallik Data and Information System - Development of a Scientific Data Exchange Platform; in Scientific Results from the Mallik 2002 Gas Hydrate Production Research Well Program, Mackenzie Delta, Northwest Territories, Canada, (ed.) S.R. Dallimore and T.S. Collett; Geological Survey of Canada, Bulletin 585, 9 p.

Takahashi, H., Yanezawa, T., and Fercho, E., 2003: Operation overview of the 2002 Mallik Gas Hydrate Production Research Well Program at the Mackenzie Delta in the Canadian Arctic; Offshore Technology Conference, May 2003, Houston, Texas, 10 p.

Lorenson, T.D., Whiticar, M.J., Waseda, A., Dallimore, S.R., and Collett, T.S., 1999: Gas composition and isotopic geochemistry of cuttings, core, and gas hydrate from the JAPEX/JNOC/ GSC Mallik 2L-38 gas hydrate research well; in Scientific Results from the JAPEX/JNOC/GSC Mallik 2L-38Gas Hydrate ResearchWell, MackenzieDelta,Northwest Territories,Canada, (ed.) S.R.Dallimore, T. Uchida, and T.S. Collett;Geological Survey of Canada, Bulletin 544, p. 143â&#x20AC;&#x201C;163.

Uchida, T., Matsumoto, R., Waseda, A., Okui, T., Yamada, K., Uchida, T., Okada, S., and Takano, O., 1999: Summary of physicochemical properties of natural gas hydrate and associated gas hydrate-bearing sediments, JAPEX/ JNOC/GSC Mallik 2L-38 gas hydrate research well, by the Japanese research consortium; in Scientific Results from the JAPEX/JNOC/GSC Mallik 2L-38 Gas Hydrate Research Well, Mackenzie Delta, Northwest Territories, Canada, (ed.) S.R. Dallimore, T. Uchida, and T.S. Collett; Geological Survey of Canada, Bulletin 544, p. 205â&#x20AC;&#x201C;228.

Moridis, G.J., Seol, Y., Collett, T.S., Dallimore, S.R., Inoue, T., Mroz, T.H., and Henninges, J., 2003: Thermal properties of hydrates from temperature data analysis of an isolated formation

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Watanabe, T., Shimizu, S., Asakawa, E., Kamei, R., and Matsuoka, T., 2005: Preliminary assessment of the waveform inversion method for

interpretation of crosswell seismic data from the thermal production test, JAPEX/JNOC/GSC et al.

Mallik 5L-38 gas hydrate production research well; in Scientific Results from the Mallik 2002 Gas Hydrate Production Research Well Program, Mackenzie Delta, Northwest Territories, Canada, (ed.) S.R. Dallimore and T.S. Collett; Geological Survey of Canada, Bulletin 585, 14 p.

Wiersberg, T., Erzinger, J., Zimmer,M., Schicks, J., and Dahms, E., 2005: Real-time gas analysis at the JAPEX/JNOC/GSC et al. Mallik 5L-38 gas hydrate production research well; in Scientific Results from the Mallik 2002 Gas Hydrate Production Research Well Program, Mackenzie Delta, Northwest Territories, Canada, (ed.) S.R. Dallimore and T.S. Collett; Geological Survey of Canada, Bulletin 585, 15 p.

Wright, J.F., Nixon, F.M., Dallimore, S.R., Henninges, J., and C么t茅, M.M., 2005: Thermal conductivity of sediments within the gas-hydrate-bearing interval at JAPEX/JNOC/GSC et al.

Mallik 5L-38, Mackenzie Delta, Canada; in Scientific Results from the Mallik 2002 Gas Hydrate Production Research Well Program, Mackenzie Delta, Northwest Territories, Canada, (ed.) S.R. Dallimore and T.S. Collett; Geological Survey of Canada, Bulletin 585, 10 p.

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Gas hydrate induced submarine slides - An engineering geological approach Grupe B. (1), Kreiter S. (1), Feeser V. (2), Hoffmann K. (2), Becker H.J. (2), Savidis S. (3), Rackwitz F. (3), Schupp J. (3) (1) TU Berlin, Arbeitsgebiet WUM, Müller-Breslau-Straße/Schleuseninsel, 10623 Berlin, E-Mail: grupe@vws.tu-berlin.de (2) Christian-Albrechts-Universität zu Kiel, Institut für Geowissenschaften, Ludewig-Meyn-Straße 10, 24118 Kiel (3) TU Berlin, Institut für Bauingenieurwesen, Gustav-Meyer-Allee 25, 13355 Berlin

Introduction Submarine slides have been considered as a major geohazard. They can be easily triggered by dissociating of gas hydrates. Specifically with regard to their impact on the release of tsunamis as well as on submarine engineering structures submarine slides are a highly relevant research topic for geotechnical engineers and marine geoscientists alike. Despite this, our knowledge about the triggering processes involved and the mechanical parameters controlling slope stability in gas hydrate bearing sediments is still very limited. To address some of the fundamental scientific and engineering questions in this field, researchers of the Technical University Berlin and the University Kiel worked together closely within the framework of the research project GASSTAB: »Slope Stability and Land Slides in the Deep Sea: Influence Parameter Gas Hydrates«. Three subprojects, »Sediment Dynamics« (Technical University Berlin, WUM), »Sediment Mechanics« (University Kiel) and »Soil Dynamics« (Technical University Berlin), dealt with gas hydrate induced failure mechanisms of submarine slides from an engineering geological point of view.

Objectives of the Sediment Dynamics sub-project Since real laboratory tests with gas hydrates are generally cost intensive and time consuming a virtual laboratory was created parallel to a real test system GTS (Gashydrate Test

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System) developed and built in the frame of the sub-project Sediment Mechanics. As a numerical tool of the virtual laboratory the distinct element method (DEM) was employed using the commercial software Particle Flow Code (PFC2D) from ITASCA. Up to now the formation of gas hydrate lenses could not be observed neither in soil specimens nor in computer simulations. The main task of this study was to simulate the formation and the decomposition of hydrates in marine sediments and to perform different virtual soil mechanical consolidation tests to study the mechanical reaction of the sediment under conditions of geostatic stresses during gas hydrate formation.

Simulation of the formation of gas hydrate crystals The distinct element model was used as a base for the simulation (Cundall 1979, ITASCA 2002) because this method is the only one which allows the simulation of the sediment behavior on the grain scale, large enough to measure the bulk properties of such a virtual sediment. In contrast to the classical finite difference or finite element simulation this simulation is very stable and allows large arbitrary displacements. The growing gas hydrate crystals could be simulated, and the reaction and the properties of the sediment during hydrate growth where measured in virtual equivalents to classical geotechnical experiments.

Figure 1: Potential and force over the distance of the two centres of the inÂŹteracting circles (balls) from 10 % overlap to the double of the contact distance

For the simulation of the growing gas hydrate crystals the focus was laid on nucleated crystals which have a surface energy and exert surface forces. The very base of surface forces is the physical interaction between multiple molecules. The behavior of surfaces has been successfully modeled by molecular dynamics (MD) simulations. The approach chosen in this simulation was to simplify the MD models and to scale them up to the size of sediment pores.

Therefore an appropriate interaction between gas hydrate simulating balls was implemented, where the Mie potential was chosen for the attractive part and the repulsion was simulated by a standard spring potential. The interaction potential and the resulting force is shown in Figure 1. As can be seen in this figure the growing crystal starts as one disk (PFC ball) but this one disk is then divided into several disks depending on the forces acting on it.

The main focus in this part of the project was the implementation of gas hydrate growth into the numerical model. Therefore the chosen virtual sediment used in most of the experiments

consists of simple disks with the standard spring contact model and a standard friction.

A material composed of such distinct elements shows characteristics like a cohesionless granular sediment.

First experiments were made to check the influence of the surface tension on the gas hydrate crystals growth within the sediment.

Figure 2 demonstrates the situation after the gas hydrate growth with growing surface tension in two sediments with a different grain size; the geometry on the left hand side is five times smaller than on the right. While the hydrate crystal in the fine grained sediment keeps its round shape and pushes away the surrounding sediment grains the crystal in the coarse grained sediment intrudes into the edges of the pores and does not push the sediment grains away until the connected pore space is completely filled up. This experiment illustrates the influence of surface tension when gas hydrate interacts with the sediment grains.

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Figure 2: Hydrate growth in fine and coarse grained sediment. Dark gray balls are hydrate, light gray balls are sediment particles. Lines in hydrate balls mark the direction of the applied force. Scale has the same absolute length on the left hand side and the right hand side.

Gas hydrate growth during consolidation To get a first impression of the influence of gas hydrate on the state and the history of a sediment sample, the interaction of sediment and gas hydrate growth was simulated. The gas hydrate was growing before consolidation in a first experiment, and the gas hydrate was growing after consolidation in a second experiment.

Figure 3 shows one result of the experiment: The deviatoric stress is plotted over the effective isotropic stress. The deviatoric stress of the first experiment from the normal consolidation is reversed by the growth of gas hydrate. In the second experiment the gas hydrate hinders the deviatoric strain to build up normally. This means that the stress state of gas hydrate bearing sediment is dependent on the history of load application and the history of gas hydrate growth in the sediment.

Simulation of surface near gas hydrate decay Most of the gas hydrate research is done on surface near gas hydrates, because they are accessible with standard research ships and because they are related to interesting structures like mud volcanoes or active faults. In such

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active environments methane bubbles escape into the free water probably being released from gas hydrate by the warmth of an ascending fluid (Wood 2002). A melting procedure was developed to simulate the surface near decay of gas hydrate to methane. Figure 4 shows the virtual sediment during the process of hydrate decay and the resulting methane bubbles in a later stage of the experiment. In the later stage the methane bubbles are moving through the sediment and some have escaped in the meantime.

Sedimentological work For the design of the virtual and substitute sediment a study of gas hydrate host sediment from the Blake Ridge and the Costa Rica margin was conducted. The microstructure, the clay mineralogy and the grain size were examined. The samples had been taken in regions where gas hydrates were found. The results do not match with the study of Ginsburg (2000), who proposed coarser grain size as the key parameter allowing gas hydrate to grow. The grain size data (Fig. 5) shows coarser grain size only in the upper 100 m of the core, which correlates with the onset of the Pleistocene glaciations.

Figure 3 : Deviatoric stress over isotropic stress during gas hydrate growth and consolidation

Figure 4: Simulation of gas hydrate decay. Light gray balls are methane, medium gray balls sediment, and dark gray balls gas hydrate. Left: early stage, right: later stage. Ambient pressure equivalents 1,000 m water depth.

SEM (Scanning Electron Microscope) studies found no further evidence for the assumption that siliceous shells provide sheltered space in which nucleation of gas hydrates occur preferentially (Kraemer et al. 2000). As a conclusion there were no parameters which indicate a preference for the growth of gas hydrate.

Therefore it is supposed, that as the nucleation of a single gas hydrate crystal is a random process the success of the nucleation is only dependent on the conditions in one particular

pore. SEM studies of the Blake Ridge sediment showed strong variability of the microstructure in mm distance in various samples, suggesting that the possibility of hydrate nucleation can not easily be determined from bulk sediment properties.

Conclusions The experiments have shown that the simulation of surface tension in the DEM is possible with methods of the molecular dynamic simu-

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Figure 5: Grain size data from the Blake Ridge.

Circles indicate the grain size of the log-normal peak of the grain population. Circle size shows the width of the log-normal distributions. Blue color indicates evidence of gas hydrate in close proximity to the sample.

lation. The influence of gas hydrate growth on the sediment state was modeled as well as the escape of methane bubbles into the water. It is possible to extend the application of surface tensed ball ensembles to other processes like the freezing of soil or other processes in polyphase mixtures.

and constitutive laws describing the strength and deformability of the sediments. Strength and deformability are basically controlled by the stress history that the sediments have undergone as well as the stresses within the grain skeleton and the pore pressure to which the sediments are currently subjected.

Experimental data is needed to validate the way the gas hydrate related processes are simulated.

Calibrated sediment-hydrate ensemble can then be used for other numerical experiments, which measure the shear strength of the sediment like biaxial tests.

Although more and more international working groups are recently engaged in mechanical gas hydrate research (Yang et al. 2003, Winters et al. 2004), soil mechanical parameters and constitutive laws of gas hydrate bearing sediments are widely undefined theoretically as well as experimentally (Sloan 2002).

But still more seriously is the fact that no information is available concerning the basic controlling mechanism of the mechanical soil behaviour, i.e. the stress history and the pore pressure regime of marine sediments which follow gas hydrate formation and decay.

Objectives of the Sediment Mechanics sub-project Quality, plausibility, and transferability of numerical stability calculations of gas hydrate bearing deep sea slopes are highly depending on the significance of the input parameters

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To address this lack of knowledge, the main objective of the Sediment Mechanics sub-project was to provide a basis for understanding and quantifying the stress history, stress and pressure interaction of grain skeleton, water, gas and hydrate in sediments during formation and decomposition of gas hydrates.

Gas Hydrate Test System Common soil mechanical standard experiments and test procedures could not respond the conceptual formulation. As a consequence of the experimental lack, GTS (Gas hydrate Test System) has been designed, constructed and installed within the GASSTAB research project.

Thereby the following questions took centre stage of the design engineering: i. Which mechanical reactions of the sediment (anisotropy of the in-situ stress condition, water and gas pressure regime, state of (over/under) consolidation, changing of pore space and stiffness) will follow the growth and decay of gas hydrates? ii. Which role does the factor time play in the decomposition process as well as the static / dynamic stress condition with respect to sediment structural changes, i.e. collapse (micro seismic excitation, over/under conso-

lidation, increase and decrease of pore water and/or gas pressures, anisotropy of the residual stress conditions)?

Experiments to be carried out for answering these questions have to keep strict thermodynamic and soil mechanical boundary conditions as well as marine deep sea conditions.

GTS conforms to these stringent requirements. It enables the generation and decomposition of gas hydrates under real marine conditions and simultaneously allows to measure the geostatic stress-strain-pressure behaviour of the sediment.

The experimental set-up consists of a 300 kN loading frame (Fig. 6). In order to simulate micro seismic events handling of cyclic forces up to 10 Hz is feasible. Geostatic experiments could be carried out with an oedometric highpressure consolidation cell (Fig. 7) which has a pressure capacity up to 200 bars, and a temperature range from 263 to 303 K. Integrated arrays of transducers enable the system to measure total vertical and lateral stresses, axial strain, pore water, and pore gas pressure separately, as well as temperature at different locations within the soil sample. Pore water pressure and temperature equivalent to sub-seabed conditions in which gas hydrates occur are

Figure 6: Gas hydrate Test System GTS. General view of the experimental set up.

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Figure 7: Gas hydrate Test System GTS. Oedometric highpressure consolidation cell.

generated using a hydrostatic pressure system and a heat-exchange system respectively. The flow of gas charged water or pure gas is controlled and monitored (Feeser et al. 2003).

Pilot research In the run-up to the design of GTS a variety of pilot tests was carried out in order to improve and optimize the engineering layout of the test system.

The literature notes that a significant memory effect has been observed during the repeated formation of hydrates (Parent, Bishnoi 1996).

This means, if hydrates are dissociated and then reformed within hours, nucleation occurs at far less pressure. Initial nucleation pressures of about 4 MN /m2 are typical for methane hydrates whereas if methane hydrate is formed again using the same water the pressure to initiate hydrate drops to about 2 MN /m2. This effect is traced back to substantial energy savings from initial hydrate formation equilibrium. Our investigations confirm these observations, even using THF (Tetrahydrofurane) as guest molecules where the hydrate formation solely occurs under atmospheric pressure conditions. Further tests under overpressure were carried out using propane. Both, THF and propane results show an explicit interrelationship between the kinetics of hydrate formation and

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the history of water. The tests have circumstantiated that meta-stable clusters of water affect the hydrate nucleation catalytically. This interpretation is consistent with neutron diffraction analyses of water clusters in water/gas solutions (Koh et al. 2000). Hence the memory effect could not be originated physically but chemically. This insight has resulted in a substantial change of the original design and process engineering of GTS. Now, a system of prereactors provides clustered, gas charged water which is discharged into the sediment under conditions of deep sea water pressure. The nucleation of hydrate is subsequently triggered by lowering of the temperature. No additional supplying of high-pressured gas is necessary.

With the implemented process technology gas hydrates can be reproducibly formed in the laboratory under natural marine soil stress and water pressure conditions.

Planning the sensor details of GTS we initially assumed a mechanical analogy between ice and hydrate bearing sediments. Pilot tests were focussed on K0-stress-strain, creep and yielding behaviour (Feeser, Hoffmann 2004).

The results show that the mechanical behaviour of frozen sediments cannot be transferred to gas hydrate bearing sediments (Fig.8 and 9). So the multitude of literature data referring to the mechanical behaviour of fro-

Figure 8: Mechanical behaviour of sand, sand ice, and sand THF-hydrate compounds. K0-stress paths in the principle stress field.

Successive loading, creep, unloading, and creep.

Figure 9: Mechanical behaviour of sand ice and sand THF-hydrate compounds.

Visco-plastic yielding. Derived from oedometric loading under constant vertical stress Ď&#x192;1 = 4.0 MNm-2.

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zen sediments could not be involved into the final sensor dimensioning of GTS. Moreover, it remains questionable whether the obtained behaviour of THF-hydrate is transferable to methane hydrate. The pilot tests have revealed a considerable gap of knowledge concerning the mechanical behaviour of hydrate bearing sediments.

Conclusions The stress-strain history is the authoritative magnitude governing sediments strength and deformability. For the experimental research of the stress-strain history of hydrate bearing sediments GTS (Gas hydrate Test System) has been constructed. For the first time GTS enables the generation and decomposition of gas hydrates in sediments under real marine conditions and allows to measure the oedometric stress-strain behaviour of the sediment, the water and gas pressure as well as the temperature regime simultaneously. Future application of GTS is provided for the study of the trigger mechanisms of sediment collapse due to the dissociation of gas hydrate.

Objectives of the Soil Dynamics sub-project The Soil Dynamics sub-project within GASSTAB mainly refers to the laboratory investigation of soil mechanical and soil dynamical behavior of marine sediments and the understanding as

well as analyzing of the complex failure mechanism of submarine slopes. Substantial amounts of marine sediments are necessary to perform the soil dynamical and soil mechanical laboratory tests. Therefore sediment substitutes as a result of the Sediment Dynamics subproject are used throughout all tests, as planned from the beginning of the project. The sediment substitute consists of 70 % clay minerals (50 % illite, 17 % chlorite, 33 % kaolinite) and 30 % quartz with parts of feldspar.

In a second part slope stability analyses based on McIverâ&#x20AC;&#x2122;s (1982) model and by using analytical considerations for single block failure were performed. A number of variations give a first impression of the possible failure behavior of submarine slopes.

Soil mechanical laboratory tests Soil mechanical index and element tests are necessary to classify the material and to correlate the resulting indices and parameters with other values from natural soil material. The following indices and parameters tests are of special interest: - unit weight of soil grains ÎłS - Atterbergâ&#x20AC;&#x2122;s limits (liquid and plastic limit, wL and wP) and plasticity index IP - permeability kf - soil stiffness in one dimensional compression (constraint/oedometer modulus) ES

Table 1: Results from soil mechanical index and element tests

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- effective shear parameters from drained direct shear tests (cohesion c’DS and friction angle φ’DS) - effective shear parameters from drained triaxial shear tests (cohesion c’TX and friction angle φ’TX) All tests were performed according to German standards for soil mechanical laboratory tests. The main results from these tests are given in Table 1.

The grain composition and mineral characteristics of the substitute material is well revealed in the measured unit weight of soil grains. Atterberg’s limits reflect the clay content and clay type of a soil and here they characterize a material of medium plasticity. The measured permeability reveals the large amount of fines content. The soil stiffness in Table 1 is specified for virgin loading in oedometric compression. Reloading after completely unloading of the specimen resulted in about double the stiffness compared to first loading.

The measured values of cohesion and friction angles evaluated from direct shear as well as

drained triaxial compression tests are in close agreement.

Soil dynamical laboratory tests The dynamical behavior of soils is mainly influenced by their stiffness at small strains of about 10-5 % and the damping of the material. Both parameters can be measured in the Resonant Column (RC) apparatus (Richart et al.

1970, Hardin & Drnevich 1972). For this purpose a new RC device was designed and built

by GDS Ltd. (UK) and first used in the frame of this research project. In Figure 10 the main components of the RC device are shown. The RC apparatus consists of a fixed heavy bottom plate and a free movable top plate with four connected magnets in radial symmetrical arrangement and associated coils. The cylindrical soil sample, with 50 mm of diameter and 100 mm of height, is placed on the bottom plate and the top plate construction can then be mounted on the top of sample. The magnets can free rotate within the coils, which are fixed on a supporting cylinder. Finally a triaxial chamber, which is not shown in Figure 10, is placed to allow the application of cell pressures up to 1 MPa to the sample.

Figure 10: Resonant Column device (fixed-free type) for testing the dynamical material behavior

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a)

b) Figure 11: Shear modulus degradation (a) and damping build-up (b) with inÂŹcreasing shear strain for RC tests subjected to an isotropic stress state of 700 kPa

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In the test a very low voltage is applied to the fixed coils, which leads via the magnets to vibration torsion of the sample with very small shear strains. A variation of the input frequency with constant voltage leads to resonance motion of the sample, which determines the characteristic values of the test â&#x20AC;&#x201C; resonance frequency, shear wave velocity and small strain shear modulus. Continuous increase of the induced electro-magnetic forces increases the torsional angle and as a result the shear strain amplitude. Associated is the transition from elastic to plastic material behavior of the soil sample.

Figure 11 clearly indicates the softening of the soil sample, i.e. the shear modulus degradation, with increasing shear strain. Associated is a strong build-up of material damping.

Analytical calculations of slope failure mechanisms McIver (1982) first published a model to describe the complex behavior of submarine slopes containing gas hydrates. It assumes that a

large block of hydrated sediment breaks off and slides down the slope. The single block partly slides on the layer of dissociated gas hydrate and partly through the hydrated zone itself.

Based on the model proposed by McIver (1982) a rigid body failure mechanism is applied for the analyses of slope failure (Fig. 12). It consists of a single block which moves on a sliding plane. A failure plane is assumed to exist in the case of a slope collapse. Three dimensional effects and reaction forces in the sliding plane are neglected in this model. Effective shear strength parameters according to the Mohr-Coulomb failure criterion are applied in the failure plane.

The static problem can be solved using the equilibrium of vertical and horizontal acting forces and the moment as well. Among all parameters enclosed in the analysis the following of special interest: slope angle, sliding plane angle, sliding plane length, friction angle, cohesion, unit weight of soil. Figure 13 exhibits the influence of slope and sliding

Figure 12: Assumed failure mechanism for analytical calculations

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Figure 13: Stability analysis for a slope with variation of slope angle as well as sliding plane angle

plane angle as well on slope stability. All remaining parameters are set to constants in this analysis. The bold line in Figure 13 corresponds to a safety factor of one for slope stability.

Figure 14 represents lines with a slope stability safety factor of one depending on the value of friction angle and slope angle as well as sliding plane angle. The bold lines in Figures 13 and 14 respectively are associated. Regarding one separate line with constant friction angle, the safe region of slope and sliding plane angles is on the left side of each line, whereas the unsafe region lies to the right.

A large number of additional variations with the above mentioned parameters were done (Schupp et al. 2003).

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Conclusions A number of soil mechanical and dynamical laboratory tests were performed to investigate the material behavior of submarine-like sediment material in detail. Along with these laboratory investigations slope stability analyses were done using simple models. The analyses clearly indicate the importance of the use of appropriate material parameters in order to determine slope stability satisfactorily.

Further analyses using more sophisticated methods and more realistic modeling of the complex material behavior and dynamic loading input still have to be done. Also the thermodynamic processes of gas hydrate formation and gas release in the surrounding sediment structure should be involved into the modeling of the entire coupled multi-physics mechanisms. It has to be clarified, whether local failure on element level leads to global failure of continental slopes, what causes slope failure and what are the effects.

Figure 14: Slope stability chart for different friction angles with variation of slope angle as well as sliding plane angle

References Cundall, P.A. and Strack, O.D.L., 1979. A Discrete Numerical Model for Granular Assemblies. GĂŠotechnique, 29: 47-65.

Feeser V., Hoffmann K. (2004): Gashydrat in Sedimenten. Bildung und Zerfall im Licht der Ingenieurgeologie (Gas hydrate in sediments.

Formation and decomposition from an engineering geological point of view). - Proc. 15th Conference on Engineering Geology, Erlangen (Germany), 349-354 Feeser V., Becker H.J., Grupe B., Hoffmann K., Kreiter S., Savidis S., Schupp J. (2003): GTS for soil mechanical research of gas hydrate bearing sediments. - EGS-AGU-EUG Joint Assembly, Nice, Geophysical Research Abstracts Vol. 5, 06978, 2003 Ginsburg, G., Soloviev, V.A., Matveeva, T. and Andreeva, I., 2000. SEDIMENT GRAIN-SIZE CONTROL ON GAS HYDRATE PRESENCE, SITES 994, 995 AND 997. In: K. Paull Charles, R. Matsumoto, J. Wallace Paul and W.P. Dillon

(Editors), Proceedings of the Ocean Drilling Program, Scientific Results, pp. 237-245.

Hardin, B.O., Drnevich, V.P. (1972): Shear modulus and damping in soils: measurement and parameter effects. Proc. ASCE, SM6, pp. 603-624.

Itasca Consulting Group, I., 2002. PFC2D Particle Flow Code in 2 Dimensions (Version 3.0 Manual). ICG, Minneapolis.

Koh C.A., Wisbey R.P., Wu X., Westacott R.E. (2000): Water ordering around methane during hydrate formation. - J. Chem. Phys.

113, 6390 - 6397 Kraemer, L.M., Owen, R.M. and Dickens Gerald, R., 2000. LITHOLOGY OF THE UPPER GAS HYDRATE ZONE, BLAKE OUTER RIDGE: A LINK BETWEEN DIATOMS, POROSITY, AND GAS HYDRATE. In: K. Paull Charles, R. Matsumoto, J. Wallace Paul and W.P. Dillon (Editors), Proceedings of the Ocean Drilling Program, Scientific Results, pp. 229-236.

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McIver, R.D. (1982): Role of naturally occurring gas hydrates in sediment transport. - AAPG Bull., 66, pp. 789-792.

Parent J.S., Bishhnoi P.R. (1996): Investigations into the nucleation behavior of methane gas hydrates. - Chemical Engineering Communications, 144, 51-64 Richart, F.E., Woods, R.D., Hall, J.R. (1970): Vibrations of soils and foundations. PrenticeHall, 414 p.

Schupp, J., Savidis, S., Grupe, B., Feeser, V., Hoffmann, K., Becker, H.J., Kreiter, S. (2003): Gas hydrates and slope stability at continental margins â&#x20AC;&#x201C; a mechanical approach. Geophysical Research Abstracts, Vol. 5, EGS-AGU-EUG Joint Assembly, Nice.

Sloan E. D. (2002): Hydrate Properties. Seafloor Stability Workshop, Houston/Texas, March 14-15, 2002 Winters W.J., Pecher I.A., Waite W.F., Mason D.H. (2004): Physical properties and rock physics models of sediment containing natural and laboratory-formed methane gas hydrate: American Mineralogist, 89, 8-9, 1221-1227 Wood, W.T., Gettrust, J.F., Chapman, N.R., Spence, G.D. and Hydman, R.D., 2002.

Decreased stability of methane hydrates in marine sediments owing to phase-boundary roughness. Nature, 420: 656-660.

Yang J., Llamedo M., Tohidi B. (2003): Experimental Investigation of Gas Hydrate Formation and Dissociation in Unconsolidated Porous Media. - EGS-AGU-EUG Joint Assembly, Nice, Geophysical Research Abstracts Vol. 5, 08460, 2003

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Acknowledgements The joint project GASSTAB was funded by the German Federal Ministry for Education and Research (BMBF) under (Grants 03G0560A+B).

The authors are solely responsible for the contents of this paper.

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Microstructure, thermodynamics, formationand decomposition- kinetics of gas hydrates Itoh H. (1), Klapproth A. (1), Goreshnik E. (1), Techmer K. (1), Kuhs W.F. (1)* (1) GZG Abteilung Kristallographie, Universitaet Goettingen, Goldschmidtstrasse 1, 37077 Goettingen, Germany, *E-Mail: wkuhs1@gwdg.de

1. Introduction The aim of this project was the determination of unknown properties of gas hydrates concerning their microstructure, their thermodynamics and kinetics. They were investigated in laboratory experiments at environmental conditions of geological interest aiming to provide basic parameters for a geophysical modelling of natural gas hydrates. In addition, it was planned to develop new methods for a structural characterisation of natural gas hydrates on a sub-microscopic scale, which are also of potential interest to many aspects of natural gas hydrates.

2. Objectives of the Project Based on the state of knowledge in the field and considering our specific methodological knowledge we had defined the following objectives for our project: • The experimental determination of fundamental properties of gas hydrates at variable pressure/fugacity and temperature, i.e. the determination of hydration number, density, and compressibility for pure and some selected mixed hydrates.

• The experimental determination of the regrowth kinetics upon changing pressure/ fugacity or temperature conditions.

• The experimental determination of the low temperature stability limits of gas hydrates in order to advice on appropriate sample storage and handling conditions.

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• The verification of the current statistical thermodynamic model of van der Waals & Platteeuw and its modifications on the basis of our experimental data. It is expected that suggestions for an improvement of this very widespread theory will emerge from this work, which will have applications in a geophysical context.

• The first morphological and textural characterization of gas hydrates on a sub-microscopic scale with emphasis on the sub-micron porosity discovered by us earlier on; this work should give important insights into the mechanisms for gas hydrate formation.

• Based on the morphological characterisation we will perform a modelling of the mechanical as well as the transport properties of microporous hydrates, specifically concerning the formation and decomposition processes.

3. Present Status and Results / Methods & Results / Results Thermodynamics A number of neutron and synchrotron scattering experiments were performed to determine the cage filling of methane and CO2 hydrate.

For the first time we could determine the absolute cage fillings as a function of fugacity and in this way provide critical tests for checking the wide-spread statistical thermodynamic theory of van der Waals & Platteeuw. Clearly, the agreement is far from being perfect (Klapproth et al. 2003) and partly based on our results attempts are now under way worldwide to

improve this situation by using free-energy minimisation methods or ab initio calculated water-gas interaction potentials. Using neutron- and Raman-spectroscopy and comparing the results with molecular dynamics simulations we could determine the modes of guest molecules in the hydrate cages. The quantitative agreement between experiment and computer simulations depends on the water-water interaction potential; consequently a number of widespread potentials (SPC, TIP4P and KKY) were employed; the best agreement was found by using the KKY water interaction potential. It was found that the degree of dynamic coupling of water cages and guest molecules depend strongly on the type of guest molecule encaged (Chazallon et al. 2002, Itoh et al. 2003, Schober et al. 2003). Consequently, a variation of thermal conductivities can be expected for different gas hydrates.

Bearing in mind that most natural gas hydrates contain different types of guest molecules in variable proportions and in order to shed some light on the complex behaviour of mixed hydrate systems we have performed experiments on the N2-CH4 and CH4-C2H6 system using diffraction and Raman-spectroscopy.

Clearly, the predictions from statistical thermodynamic theory are insufficient and additional problems occur due to the formation of metastable phases belonging to van Stackelberg’s type II structure (Staykova et al. 2003).

In cooperation with Prof.Bohrmann (GEOMAR/RCOM) a number of natural gas hydrate samples from several locations were investigated by X-ray diffraction at laboratory sources as well as at the synchrotron source at HASYLAB/ Hamburg. X-ray diffraction proved to be essential to determine the structure type and the degree of preservation of these recovered samples (Kuhs et al. 2004a).

Kinetics A large number of experimental runs to determine the formation and decomposition kine-

tics were performed using both neutron diffraction at D20/ ILL(Grenoble) and synchrotron radiation at BW5/ HASYLAB (Hamburg), both for CH4 and CO2 hydrate. The analysis was performed using a newly developed multi-stage model (Salamatin & Kuhs 2002) in which a fast first reaction stage was distinguished from a later diffusion-limited stage (Staykova et al.

2003). These experiments were complemented by in-house runs in which the gas consumption/ release was measured during formation/ decomposition. Using the model we can now determine the activation energy, diffusion coefficients of the process and predict the hydrate formation and decomposition behaviour over a large range of temperatures and pressures (Genov et al. 2004). Particular attention was given to the effect of »anomalous preservation«, also named »self-preservation«, which describes to the unexpected phenomenon of a long-term stability of gas hydrates outside their field of stability at temperatures below the ice melting point. A physical understanding of this technologically and geologically important phenomenon did not exist. We could quantitatively confirm the effect and in a combination of diffraction and scanning electron microscopy give for the first time a full physical explanation of the phenomenon (Kuhs et al. 2004b). The onset of self-preservation arise because of the annealing of stacking faults in the ice layer initially formed on the hydrate surface; at higher temperatures the ice reorganizes into larger crystallites in an Ostwald-ripening process leading to a maximum of self-preservation just a few degrees below the melting point of ice.

Microstructure Cryo scanning electron microscopy (SEM) has become a standard tool in the microstructural characterisation of gas hydrates for both natural and laboratory-made samples. A large number of samples were investigated using this technique, including samples from Gas Hydrate Ridge, Black Sea, Congo basin and the Mallik research well in N.W.T./ Canada. The

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Figure 1: Cryo scanning electron micrograph of gas hydrate - sediment contacts of a natural sample from sub-permafrost gas hydrates of the Mallik research well (N.W.T. Canada). Left: Quartz-gas hydrate contact with intermediate (frozen) water layer of several µm thickness. Right: Clay mineral particles with intercalated microporous gas hydrates (see arrow).

sub-micron porosity of gas hydrates has been confirmed to be characteristic for gas hydrates and is now used as a fast means of identifying gas hydrates in complex samples (Staykova et al. 2003, Genov et al. 2004, Kuhs et al.

2004a). The detailed physico-chemical origin of this microstructure is not yet understood, however. Of particular importance were studies of the sediment – gas hydrate contact using cryo SEM (Techmer et al. 2005). For the Mallik samples we could show that clay minerals form intimate contacts while quartz grains seem to be separated from gas hydrates by a liquid layer (see Fig.1.). This has important consequences for the elasto-mechanical behaviour of gas hydrates and their seismic response.

Systematic studies of the sediment – gas hydrate contacts are underway in the follow-up project in the framework of the GEOTECHNOLGIEN program, which should lead to a first quantitative description of the elastic response of natural gas hydrate bearing sediments.

4. Conclusions This project was largely devoted to open questions concerning the thermodynamics, kinetics and microstructure of gas hydrates as far as they are of importance in a geological setting.

Due to the lack of well-preserved natural samples, the investigations were mainly conducted on laboratory-made material. Important insights were gained concerning the molecular interactions between guest and host molecules responsible for the anomalously low thermal conductivity. Likewise, the first determination of the absolute cage fillings for methane hydrate was obtained. The formation and decomposition kinetics were established experimentally and a model was constructed which has predictive power over a large range of pressures and temperatures. Finally, the first physically sound and complete explanation of the self-preservation effect was given.

Acknowledgements The work was funded by the grant 03G0553A. We thank ILL/Grenoble for beamtime and support. Likewise we thank Drs. Viorel Chihaia, Georgi Genov, Till Heinrichs, Helmut Klein and Doroteya Staykova (all Göttingen) for their help in various aspects of the work.

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References Chazallon, B., H. Itoh, M. Koza, W. F. Kuhs, and H. Schober (2002). Anharmonicity and guest-host coupling in clathrate hydrates.

Phys. Chem. Chem. Phys. 4, 4809-4816.

Genov, G., W. F. Kuhs, D. K. Staykova, E. Goreshnik, and A. N. Salamatin (2004) Experimental studies on the formation of porous gas hydrates. Am. Miner. 89, 1228-1239.

Itoh, H., B. Chazallon, H. Schober, K. Kawamura, and W. F. Kuhs (2003). Inelastic neutron scattering and molecular-dynamics studies on low-frequency modes of clathrate hydrates.

Can. J. Phys. 81, 493-501.

Staykova, D. K., W. F. Kuhs, A. N. Salamatin, and T. Hansen (2003). Formation of porous gas hydrates from ice powders: Diffraction experiments and multi-stage model. J. Phys. Chem. B 107, 10299-10311.

Techmer, K., T. Heinrichs und W. F. Kuhs (2005). Cryo-electron microscopic studies on the structures and composition of Mallik gashydrate-bearing samples. Scientific Results from the Mallik gas Hydrate Production Research Well Program, Mackenzie Delta, Northwest Territories, Canada. Eds. S. R. Dallimore und T. S. Collett, ÂťGeological Survey of Canada BulletinÂŤ 585.

Klapproth, A., E. Goreshnik, D. K. Staykova, H. Klein, and W. F. Kuhs (2003). Structural studies of gas hydrates. Can. J. Phys. 81, 503-518.

Kuhs, W. F., G. Y. Genov, E. Goreshnik, A. Zeller, K. Techmer, and G. Bohrmann (2004a). The impact of porous microstructures of gas hydrates on their macroscopic properties. J.Offshore and Polar Engineering 14, 305-309.

Kuhs, W.F., G.Genov, D.K. Staykova and T. Hansen (2004). Ice perfection and the onset of anomalous preservation of gas hydrates. Phys.

Chem. Chem. Phys. 6, 4917-4920.

Salamatin, A. N. and W. F. Kuhs (2002). Formation of porous gas hydrates. Proceedings of the Fourth International Conference on Gas Hydrates, Yokohama, May 19-23, 2002, Yokohama, 766-770.

Schober, H., H. Itoh, A. Klapproth, V. Chihaia, and W. F. Kuhs (2003). Guest-host coupling and anharmonicity in clathrate hydrates. Eur. Phys. J. E 12, 41-50.

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New perspectives for the extraction of oceanic gas hydrates Schultz H.J. (1), Deerberg G. (2); Fahlenkamp H. (3) (1) Celanese Chemicals Europe GmbH, Werk Ruhrchemie, Otto-Roelen-Str. 3, D-46147 Oberhausen, Heyko-Juergen.Schultz@celanese.de (2) Fraunhofer Institute for Environmental, Safety, and Energy Technology UMSICHT, Osterfelder Straße 3, D-46047 Oberhausen, Germany, de@umsicht.fhg.de (3) Universität Dortmund, Fachbereich Bio- und Chemieingenieurwesen, Emil Figge Straße 70, D-44221 Dortmund, H.Fahlenkamp@bci.uni-dortmund.de

Introduction Over the last two decades, natural gas has gained increasing importance in the energy technology. Without natural gas (generally methane-CH4 ), technical systems like peak load power plants, gas and steam cogeneration plants with an unachieved low emission of CO2 per kWhel., and fuel cells powered by hydrogen (H2), yielded from natural gas would often not be feasible. But even in other fields of technology, natural gas has attained significant relevance as reservoir of hydrogen or as hydrogen supplier. Facing today’s population growth and the increase in agricultural productivity and the resulting sweeping demand of ammoniac fertilizers would be hardly met without natural gas as hydrogen resource, and would entail a manifold of carbon dioxide emissions.

These characteristics – highly valued, not only in ecological respect – are in contrast to the forecast that natural gas, as one of the fossil primary energy carriers, will have only a short exploration period of 60 years [1]. Accordingly, the detection of the so-called gas hydrate materials (briefly: gas hydrates), detected over the lengths of the submarine area, in the ridges of the continental shelf, in a water depth of about 500 to 1500 m, has given rise to the hope that the dilemma forecasted above may be prevented. However, this hope is only realistic if the exploitation of gas hydrates on a

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technical scale will be mastered successfully without significant losses. As natural gas, particularly methane, is characterized by an increased infrared activity (higher by a 23 factor), these significant losses are not only an economical drawback, but make part of the detrimental impacts on the climate. As a consequence, the above mentioned, uniquely low specific CO2 emission of a GuD power plant is affected.

Gas hydrates are solid, icelike (Fig.1 for example) physical inclusion compounds between water and small gas molecules, as is methane, ethane, carbon dioxide, or mixtures among these gas molecules. Research on gas hydrates is gaining increasing importance because of the above outlined factors. According to conservative estimates, the deposits contain more than double the energy content than the fossil energy resources coal, conventional natural gas and crude oil together (about 10.000 Gt), [2, 3] (Fig. 2). Even if actual published and pessimistic values are approved in future (5002.500 Gt) [4] there is much more carbon kept in gas hydrates worldwide than is kept in the proved (state end 2002 [1]) worldwide natural gas reserves. Vast majority of gas hydrates has been detected in submarine deposits, whereas the permafrost soils of Canada and Siberia, which are equally suitable soils for the forma-

Figure 1: A gas hydrate manufactured in vitro at the laboratory of Fraunhofer UMSICHT, Oberhausen (here: methane hydrate)

tion of gas hydrates, hold only few deposits of these substances, as has been proven.

To develop a safe and sustainable access to the to date non-used gas hydrates, a technology for the extraction of hydrates and a simulation model for the device has been developed and analyzed for the first time in the research project. Due to the high cost for offshore pilot plants, the »dynamic simulation« was chosen as working and research approach. On the basis of a complete mathematical device model in interaction with the exactly described destabilization of the gas hydrates and the controlled extraction of the natural gas emitted from the deposit, this approach permits the development and scientific verification of an extraction method.

This novel method allows –provided a testing phase in a large-scale experiment is conducted– the extraction of oceanic and also of permafrost gas hydrates.

Properties of Gas Hydrates Gas hydrates belongs to the group of real clathrates [5] without chemical link between host and guest molecule. They are stable at low temperatures and high pressure conditions. Small guest molecules, such as methane, are embadded in a cage of water molecules

Figure 2: Simplified sketch of carbon resources on the earth, according to Kvenvolden [3]

(Fig. 3). Each cage can usually take exactly one guest molecule [2]. Through the interaction between the guest molecules and water molecules in the lattice structure, the formed cavities are strengthened, which would be thermodynamically unstable without guest molecule [6]. This requires a minimum size of the guest molecules (approx. 0,35 nm, [2]) to enable a guest molecule to stabilize a hydrate cage. The upper limit value for the guest molecule diameter determines the specific cage size.

Three different gas hydrate structures have been detected in natural systems (Fig. 3). Two further structures have been manufactured in the laboratory [7, 8], others are predicted theoretically. Basically, the cage structures differ in size, number and distribution of the water molecules. The structures I and II found in nature display a cubic form, structure H has a hexagonal form. The structure formed depends strongly on the size of the guest molecules. Fig. 3 illustrates the cage structures I, II and H. The cage type with the title 512 exists in all three cage structures. It is characterized by a cage consisting of 12-pentagons. The different gas hydrate structures consist of different cage types which span the cavities, in which the gas molecules are included.

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Figure 3: Cage structure of the three natural gas hydrate structures I, II and H

Hydrates of different gases have been detected in nature, also in extra terrestric systems. Much more than 90 percent of natural gas hydrates in the terrestric system are methane hydrates, which due to their physical stabilization conditions exist in ocean floor sediments and in permafrost soils of the polar regions.

Whereas the physical parameters pressure and temperature describe the generally possible distribution, another limiting factor is the general availability of a sufficient amount of gas, preferably CH4. In the deep sea, major part of the methane comes from the organic components respectively from the fermentative decomposition of organic compounds or the bacterial CO2 reduction in the sediments. Part of it also derives from the thermocatalytic transformation processes formed in deeper sediments. Most part of CH4 forms in the area of continental shelves, where, due to a high plankton productivity of the oceans and a high sedimentation rate, large amounts of organic material are available in the sediment for the formation of gas. As a result, gas hydrates are to be found not only at all passive and active continental shelves, but also in the Caspic Sea, in the Black Sea, in the Mediterranean and in the Baikal sea [9].

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Verified and partly sampled gas hydrate deposits are to be found worldwide (Fig. 4). For a fictive deposit in more than 1000 m of water depth, first the gas hydrate stability zone (HSZ) is shown in Fig. 5. The gas hydrate balance curve of a representative natural gas is represented by the continuous line. Below this line, the gas hydrate formation conditions are generally given. The hydro chemical (decreasing temperature at increasing depth) and geothermal (increasing temperature from geothermal power at increasing depth) temperature gradients with the increase of â&#x20AC;&#x201C;0.5°C /100 m or +2.0°C/100 m as well as the ocean floor, limit the hydrate stability zone (HSZ) of about 600 m thickness. Above the hydrate stability zone, there is the oceanic water column, below the zone there is sediment with free circulating gas.

The verification of gas hydrates is possible by the geophysical registration of the so-called bottom-simulating reflector (BSR). The BSR is a seismic reflector which forms at the interface of hydrate containing sediments with those containing free methane gas. The reflector is found in depth up to several hundred meters below the ocean floor and indicates the lower boundary of the gas hydrate stability zone.

Accordingly, gas hydrates are to be expected principally above the BSR, below there is free circulating methane. The exact mechanism of

Figure 5: Gas hydrate stability zone (HSZ) in a fictive deposit Figure 4: Gas hydrate reservoirs worldwide [11] Figure 6: Gas hydrate / gas extraction device for the oceanic extraction based on the mammoth pump system

the BSR localization is not the subject of this article. It has to be mentioned, nevertheless, that the gas hydrate sediment layers cause a weaker reflection of the seismic waves than do typically sediment layers and gas-rich floor sections. Gas hydrate floor sections without fields with free gas underneath are not exactly detectable today, therefore the worldwide gas hydrate resources may be larger than assumed today.

Extraction Method and Device The novel processing method for the extraction of gas hydrates is based on the mammoth-pump-principle (Fig. 6) and works like an overdimensioned coffee machine.

The method includes the feeding of heated ocean water into the gas hydrate deposit through a concentric double pipe system, which leads to the thermal destabilization of the gas hydrate and to the release of the con-

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tained gas (especially methane). If salty ocean water is used, the dispersion of the gas hydrates is supported by the inhibiting influence of the salt.

By the ascending gas bubbles (»airlift«) which accelerate the rising of the surrounding fluid, and the density difference between the both twin pipe sectors of the extraction system – downcoming sector: fluid phase (+ solid phase), upstreaming section: fluid and gas phase (+ solid phase) – a self maintaining circulation process is induced. The released natural gas is fed into the device head via the upstreamer (external annular space in Fig. 6) – a collector (drilling ship or platform), from which it may be transported to a subsequent utilization. The released gas volumes in the deposit may be controlled via temperature and volume of the circulating water. A part of the gained gas flow may be used for the heating of the circulating water.

Cell net models are considered highly flexible tools for the modeling of multiphase fluid flows, and comprise the serial linking of ideally backmixed volumes (»cells«) which are in material and energetic interaction among each other. By a hypothetical backflow contrarious to the convective mass flow assumed for each cell, the deviation from the ideal plug flow is considered. The underlying multiphase system (index m for physical phases) the NC-linked cells are passed through by the volume flows . The backflows circulating between the cells are described as backflow ratio in equation 1: (1) The practical application is performed in a manner to associate these model parameters with those of the well known continuous dispersion model.

(2)

Mathematical Model of the device The complex technical system of the described mammoth loop system is described in a detailed rigorous mathematical model for the dynamic simulation in order to provide significant evaluation possibilities on feasibility, operational safety, and efficiency properties over the entire extraction process. With regard to the model set-up, the following physical phenomena are considered: - fluid dynamics, - thermodynamics, - hydro kinetics (dispersion and formation kinetics), - mass transfer, - heat transfer.

The contribution of these aspects to the model of the apparatus (Fig. 6) are formulated separately as modules, implemented and unified in a suitable program structure. The modeling of the system is performed on the basis of a multi-sector cell net model with backflow (Fig. 7, Fig. 8).

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That means that the related backflow ratio may be determined as a function of the dispersion coefficient at a freely selected number of cells NC. The backflows in this case are no physically measurable units (therefore termed hypothetical). They have to be interpreted always together with the associated number of cells NC. The selected number of cells is only relevant for the discretization of the flow sectors and has no impact on the backmixing characteristics.

Due to the differing flow regimes and physical phenomena in single device sectors (Fig. 6), the model structure developed for the extraction system is primary based on the subdivision of the plant into four separately modeled macroscopical sectors groups. These are suitable balance volumes, which are each partitioned into a net of NC,k sector-dependant, ideally mixed cells: 1. upstreamer 2. downcomer 3. head 4. bottom

Figure 7: Schematic layout of the used multi-sector cell net model

Figure 8: Schematic layout of the balance cell system (convective flow: dashed arrows, backflow: dotted arrows)

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Fig. 7 shows the simplified and schematic image of the four coupled sectors of the used multi-sector cell net model. In Fig. 8 the schematic linking of the cells by the balance cell system is presented. The vertical, dashed arrows indicate the main or primary convective flow, the vertical, dotted double arrows represent the superimposed or backflows as a measure for the consideration of deviations from the ideal plug flow. The horizontal arrows represent the transfer flow crossing the interfacial area between the phases. Additionally to the above mentioned three flow types, also feed and removal as well as flows by reactions (chemical rates) may be considered, which are not listed here for a better schematic view.

For the modeling of the thermal destabilization of gas hydrate, the complex thermodynamics of gas hydrates have to be investigated fundamentally. The impact of deposit parameters, such as sediment properties, porosity and hydrate contents on the destabilization processes is essential. Since the extraction process may include the feeding of solid particles from the deposit site to the head of the device, the dispersion and formation kinetics are another factors not to be neglected.

Possible impacts of the mass transfer among the involved phases, such as the absorption of the released gases in the liquid phase, as well as of heat transport processes regarding heat losses, relevant for the assessment of economical working conditions, are included in the model.

The balancing of the device sectors is performed cell by cell. The connection of cells and device sectors is given by the subsystem-linking macroscopic component (mass flows), energy, momentum and information flows.

The balance equations are complemented by algebraic approximation equations, resulting in a differential-algebraic equation system of thousands of equations depending on the number of cells NC. The solution of the system is performed simultaneously, using numerical methods. The simu-

144

lation provides, among other things, pressure, concentration, temperature, and velocity profiles /parameters over the geometry and the entire process time.

Construction Study for the Extraction System Example details for the construction design and dimensioning of the described extraction system are developed for the technical evaluation and verification of the feasibility and safety of the extraction device. Due to the basic and unique character of the extraction system, suggestions are presented which are fed into the simulation tool as a geometric basis describing the extraction process. During further investigations, system components, building parts and other parts will have to be subjected to a critical review and optimization. However, the simplified layout provides a realistic basis for the conducted evaluation of economic efficiency, safety and feasibility of the extraction process and device.

Pressure profiles, simultaneously determined, indicate that the pressure difference between the two twin pipe sectors or to the ambiance do not exceed 7 bar (6.16 bar) in the example system. During various further simulations, which are not explicitly presented here, a maximal excess pressure value of 15 bar was not surpassed. Due to the high climatic effect of the methane exploited and the safe working conditions at the extraction process according to the model suggested, a worst-case internal and external excess pressure value of pExcess = 36 bar at a 1000 m long twin pipe set may be assumed for the estimate layout of the construction. This calculated excess pressure may also be used for the dimensioning of the head of the device. The internal diameter of the external pipe is calculated to 1.5 m, the internal diameter of the internal pipe is 0.75 m. The construction material for the twin pipe arrangement is considered to be from steel (St 37 = worst-case).

In compliance with the general regulations for the manufacturing of machinery and pipe con-

Ë&#x153;

Figure 9: Example 20 m part of the Twin pipe system, with flange connection and spacers at the upper side

struction, the wall thickness of 30 mm for the external pipe and 21 mm for the internal pipe are calculated.

The twin pipe may be manufactured in parts of 20 m for example, as is illustrated in Fig. 9.

The pipe parts are coupled via flanges, with the internal pipe being mounted first, followed by external pipe, and finally the new pipe part concentrically fixed through a spacer. The flanges of the external parts are connected with 40 pieces of M-48 screws, including 11 screws for safety reasons. For internal pipe flanges, 30 screws M-24 are sufficient, 14 screws of them for safety reasons.

Selecting a pipe connection type, a flange connection that is at least partly form-adapted and removable, is the most suitable one. This ensures the reusability of the twin pipe sections by simple dismounting of the parts, after the exploitation of a deposit has been finished.

The twin pipe parts do not necessarily have to be 20 m long, but may also be adjusted to the specific manufacturing parameters. It is in addition theoretically possible and recommendable not to screw the parts together with a flange connection, since particularly the mounting takes more time than a conventional welding connection. According to the practical manageability and transport features, only each second or third twin pipe part may there-

fore be connected by flange connection, the others by welding, which may be more easily done ashore requiring lower costs. Connection parts such as flanges, screws and gaskets, have to be considered in the calculations always in dependency of each other.

The bottom section of the twin pipe system, which is protruding to the deposit, is designed different. As is displayed in Fig. 10, the length of the last part of the external pipe equals 30 m, the internal pipe may be 10 m longer for example, in order to achieve a better circulation of the flow in the formed cavity and a reduced short circuit flow.

A perforation of the external pipe in the bottom sector of the device is possible, for example with holes of 15 cm diameter (Fig. 10). As a result, the lateral gas collection is increased according to the water dispersion pump principle.

The head coupling part, serving as link between head and twin pipe device, has also a specific construction. As depicted in Fig. 11, in the 2.5 m high head part the internal pipe is fed from the centering part to the surrounding area, via a 90째 bow which is welded to the external pipe. The downcomer is led laterally from the head sector in horizontal direction, which permits an easy-construction, separate heating of the downstreaming fluid. The hea-

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Figure 10: Bottom section of the twin pipe system

Figure 11: Twin pipe system with head sector coupling part and schematic of an example joint compensator for the twin pipe system

ted ocean water is fed into the downstreamer (here: internal pipe) to the downcomer via the 90째-bow. Consequently, upstreaming and downcoming flows are separated in the head connection unit, because the upstreaming multiphase mixture is fed to the head sector vertically.

For the reduction of the bending stress caused by the ambient oceanic flow, the twin pipe system may be constructed elastically, to be mounted in a certain spacing, e.g. each 60 m distance, for example by compensators.

Fig. 11 gives a schematic picture of an exemplary compensator with a height of 1 m and a wall thickness of 30 mm for the external and internal pipes.

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The bending stress here separates the compensators in the 60 m long pipe system, so that the extraction pipe / device may be considered roughly as a rope, and not as a bending bridge. As the ocean currents here usually move only in one direction (horizontally), the compensator is flexible only in this direction.

Discussion of Selected Simulation Results In the following, selected simulation results for the extraction of oceanic gas hydrates on the basis of an exemplary realistic system are presented. The worldwide distribution of gas hydrate reservoirs discloses various potential exploitation sites, each with differing local

ambient conditions. One of the well investigated and intensively sampled offshore deposit sites of gas hydrates is the »Hydrate Ridge« alongside the U.S. west coast which is chosen to be a model deposit for the simulation.

position of the gas hydrate. The gas product mass flow amounts to approx. 1.54 kg/s.

If this value is set into relation with the related caloric value of the product gas (here

The hydrate ridge is an extended geological formation, extending approx. 30 km in north-south direction, with a width of about 15 km, alongside the American continental margin near Oregon and Washington. It is a submarine mountain massif in the range of the German Harz mountain chain and has emerged from the subduction of the Juan de Fuca-plate under the North American continent to the east. Large quantities of gas hydrate samples could be taken particularly in the southern peak of the hydrate ridge, at a sea depth of approx. 780 m [9]. 30 Vol.-% of the deposit consist of hydrate, 70 Vol% are sediments. For the composition of the included gas, the compounds methane (98 %), ethane (1,5 %) and propane (0,5 %) are assumed. The temperature decreases with increasing depth from 15°C in the surrounding air (surface) to 5°C in the deposit.

), a fictive heating or product gas enthalpy flow is yielded, as shown in equation 3.

(3) With the product enthalpy flow, a first fundamental measure for the assessment of the economic efficiency of the extraction method is provided. The energetic (yield) coefficient α, taking into account the heating enthalpy flow fed to the system ( equation 4), constitutes the most important target value and measure for a more detailed and deeper evaluation of the efficiency of the extraction technique.

(4)

The liquid flow velocity of the feed into the deposit via the downcomer is 0.2 m3/s. The device is partitioned into 35 cells, 16 cells are taken each by the up- and the downstreamer, 2 cells by the head sector, and 1 cell is taken by bottom sector.

Fig. 12 displays the progress of the energetic (yield) coefficient α for the system described with a gas hydrate contents of 30 Vol.-% in the deposit, assuming different comprehensive heat losses in the deposit. At a heat loss of 50 % in the deposit, it amounts to 33.42 W/W, at a heat loss of only 25 % it is 46.92 W/W, and at a heat loss of 75% assumed, it is still 16.8 W/W. This means that the extraction of product gas supplies a manifold of 17 to 47 times quantity of enthalpy than is fed to the system as heating energy. As is expected, the heat losses in the deposit have a great impact on the gas yields. In all of the three calculated scenarios, however, the fictive enthalpy flow, released during the heating, is significantly higher than the heating enthalpy flow that is fed to the system.

For the extraction process under investigation, the simulation results show a steady state after about 15 000 s (approx. 4 h) of the start up phase, which is owing to the thermal decom-

With high quality (stainless) steel (1.4359, 1.4571) as material for the extraction system, the efficiency of the method is reduced compared to a GRP construction (glass fibre pla-

The hypothetical extraction device, which is simulated, is designed to explore a gas hydrate field in a depth of 800 m. The diameter of the external pipe (upstreamer) is assumed to be 1.25 m, of the internal pipe (downcomer) to be 0.50 m. Alternatively glass fibre reinforced plastic material is taken as material for the extraction device. The heat input, amounting to 2.5 MW constantly over the entire process, is fed in the first downcomer cell.

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Figure 12: Yielding coefficients Îą against time for different heat loss values (HLV) in the deposit

stics). Besides the higher specific density, the heat transfer capacity of stainless steel with roughly

has a fundamentally different dimension than that of GRP, amounting

This property has significant impacts on the heat transfer between the flow regimes and the heat loss to the ambience. A second analysis of the defined system with a heating energy fed of constantly 2.5 MW yields the result, that when using steel as material at unchanged conditions, the heat loss to the ambience increases that much, that the hydrate decomposition temperature is not reached. This is due to the intense cooling of the downcoming fluid on the way to the deposit. Consequently, the heating energy fed in has to be increased.

In the case of an increase of the heating enthalpy flow to 5.0 MW, however, an energetic coefficient Îą of 15.98 W/W is still reached with stainless steel as material for the extraction device. Again, a 17 fold quantity of energy is

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released than is supplied to the system.

The influence of the ocean current velocity on the efficiency of the technique is given particularly when using steel as material. Table 1 shows the product gas enthalpy flows and energetic coefficients in the steady state, relating to the different sea current velocities as a measure for the heat loss to the ambience of the twin pipe device (stainless steel as material). The higher the sea current velocity, the higher is the heat transfer coefficient for circumfluent pipes, according to the implemented calculations of the VDI-Wärmeatlas [10]. As a result, the heat transfer coefficient and the heat losses increase at constant conditions.

The effects may be seen in Table 1.

The product gas enthalpy flow and the energetic coefficient are given for a sea current velocity of 0.00 m /s with 104.64 MW or maximum 19.93 W/W respectively and continually decrease at increasing sea current velocity, thus reducing the efficiency of the extraction method. The vast impact on heat losses to the ambience is thus clearly elucidated. Due to the low material-specific heat transmission via the twin pipe device when using glass fibre reinfor-

Table 1: Product gas enthalpy and energetic (yield) coefficient in steady state. Dependency on different oceanic current velocities, for stainless steel as construction material

Ocean current velocity Product gas enthalpy flow Energetic (yielding) coefficient α

[m/s]

0,00

0,25

[MW]

104,64

[W/W]

19,93

ced materials, here the product enthalpy flow and energetic coefficient are nearly independent of the sea current velocity and nearly neglectable.

Conclusion and Outlook In the view of the forthcoming shortage of conventional crude oil and natural gas reservoirs in a mid term period, the present research study on the exploration of the immense, worldwide spread gas hydrate deposits and their utilization as a natural gas resource is a fundamental contribution to the development of a gas hydrate extraction system. Traditional forecasts predicting a natural gas resources depletion within a period of 60 years, given a constant consumption and constant quantities [1], are hence contradicted. The chemical properties of natural gas, the classical hydrogen carrier among the carbon dioxides and minerals, justify the classification of the presented extraction technique as a sustainable process development. The implementation of this technique does not only permit the long term utilization of peak load plants and GuD, steam and power cogeneration plants, but also provides the required quantities of hydrogen, if the fuel cell technology, as for now still in the testing phase (1 MW scale), is to be scaled up to the dimension of today’s power plants. Furthermore, gas hydrate reservoirs are expected to be roughly as widely spread as carbon resources, according to current estimates, which consequently helps to avoid the dependency on supply monopolies, valued detrimentally in political terms.

0,50

0,75

1,00

1,25

1,50

87,19

84,92 78,81 75,02

73,42

68,97

16,44

15,98 14,76 14,00

13,68

12,79

In this study, a new gas hydrate production respectively extraction device has been developed theoretically. Using a complete, detailed mathematical system model the feasibility of new technology has been investigated. Based on a »multi-sector cell net model with backflow« including physical-chemical (transport) phenomena for the exploitation device of gas hydrate deposits could be successfully developed and implemented. The utilization of a multi-sector cell net system, proven for the simulation of multi-phase (fluid) flows, has the advantage of high flexibility, an easy to use underlying balance equation system, easy scale-up, and the easy adaptation of model depth. Via the linking of the system-describing differential-algebraic equation system to a numerical equation solver, the simulation tool provides, among others, parameters on concentration, temperature and velocity profiles as well as material and heat transport values over the entire device area. As a result, practical calculation values for the dimensioning and verification of the feasibility, economic efficiency, and safety of the method are presented.

The simulation results allow the determination of optimum operating conditions and operation modes, allowing the conclusion that the mammoth pump principle may be applied for the controlled thermal destabilization of oceanic and permafrost gas hydrates, yielding a high exploitation rate of natural gas. The simulation tool implemented may be further used for the layout, dimension planning and project planning of an extraction system and for the optimization and planning of field tests.

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However, due to the estimates for the construction costs of the plant designed, coming to the cost for a drilling platform, the practical application of the concept is still a unsolved problem.

The support of a partnering company from the energy, specifically, the power plant sector would be very helpful in this regard.

The extraction process according to the suggested, novel process technology via mammoth loop device is considered to be technically feasible, energetically efficient and safe for man and environment as a result of the principally foreseen leakage-free extraction of natural gas.

Facing the ever increasing shortage of the energy carriers crude oil and natural gas, the immense reservoirs of gas hydrates constitute a

great alternative energy resource that may be exploited in a promising and innovative way using the mammoth pump system. Until the development of efficient and entirely CO2 free (coal-fired) power plants will be realized, gas hydrates may be used as primary energy resource to be efficiently exploited using the extraction system presented in this contribution.

With regard to speculative scenarios from the ecological and social points of view, the preventive removal of unstable gas hydrate fields with the suggested extraction device may be considered as active climate protection measure.

Table 2: Symbols

Symbols

Unit

Parameter, meaning, description

[m2/s]

effective dispersion coefficient

[W]

product gas enthalpy flow

[W]

heating enthalpy flow

[1]

sector index (upstreamer, downcomer, head, and bottom)

[kg/s]

product gas mass flow

[1]

phase index (gas, fluid or solid phase)

[1]

total number of cells

[m3/s]

volume flow

[m3/s]

backflow

[1]

molar percentage of a component in the gas phase

[W/W = 1]

energetic yield coefficient

[J/kg]

heating value = upper heating value, product gas enthalpy

[1]

150

backflow relation

Literature [1] BP; Statistical review of world energy 2003, 2003 [2]

Sloan, E. D. Jr.: Clathrate Hydrates of Natural Gases, Marcel Dekker Inc., New York 1998

[3]

Kvenvolden, K. A.: Methane hydrate – a major reservoir of carbon in the shallow geosphere?, Chemical Geology, Vol. 71, pp. 41-51, 1988

[4]

Milkov V. A.: Global estimates of hydratebound gas in marine sediments: how much is really out there?, Earth-Science Reviews, Vol. 66, pp. 183-197, 2004

[5]

Nixdorf, J.: Experimentelle und theoretische Untersuchung der Hydratbildung von Erdgasen unter Betriebsbedingungen, Dissertation TH Karlsruhe, 1996

[6]

Parrish, W. P.; Prausnitz, J. M.: Dissociation pressures of gas hydrates formed by gas mixtures, Ind. Eng. Chem. Proc. Des. Develop. 11 (1972) 1, S. 26-35

[7]

Kurnosov, A. V.; Manakov, A. Yu.; Komarov, V. Yu.; Voronin, V. I.; Teplykh, A. E.; Dyadin, Yu. A.: A new gas hydrate structure, Doklady Physical Chemistry, Vol. 381, Nos. 4-6, pp. 303-305, 2001

[8]

Konstantin, A.; Uchadin, K.A.; Ripmeester, J. A.: A complex clathrate clathrate hydrate structure showing bimodal guest hydration, Nature, 397, S. 420-423, 1999

[9]

Forschungszentrum für marine Geowissenschaften (GEOMAR), Homepage der Gashydrate, http://www.gashydrate.de, 2003

[11] Bundesanstalt für Geowissenschaften u.

Rohstoffe, Hannover/Berlin (BGR): Reserven, Ressourcen und Verfügbarkeit von Energierohstoffen 1998, 1998 [12] Takahashi, H.; Yonezawa, T.; Fercho, E.: Operation Overview of the 2002 Mallik Gas Hydrate Production Research well Program at the Mackenzie Delta in the Canadian Arctic, Offshore Technology Conference, Houston, Texas, U.S.A., 05.08.05.2003

[10] VDI-Hrsg.; VDI-Wärmeatlas, 8. erweiterte Auflage, VDI-Verlag Düsseldorf, 1997

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Experimental determination of the petrophysical and thermodynamic properties of gas hydrates and hydrate bearing sediments Schicks J., Spangenberg E., Naumann R., Kulenkampff J., Erzinger J.* GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany, *E-Mail: erz@gfz-potsdam.de

Introduction and objectives of the project The majority of global carbon is bound up in gas hydrate occurrences in continental permafrost regions or below the sea floor continental margins. The potential exist that methane gas could be released from these occurrences, which would have disastrous consequences. Thermodynamic conditions control the formation of gas hydrates and their decomposition, however the role of the properties and the influence of fluid compositions are only vaguely known. The determination of the physical and chemical parameters affecting the stability of gas hydrate occurrences provide a sound basis for: - understanding the kinetics of gas hydrate formation and decomposition - the quantitative determination of the gas hydrate content of sediments with help of geophysical well logging and field measurements - understanding seismic signatures and features of gas hydrates (absorption, BSR) The relevant parameters for stability, formation and decomposition processes of (methane) gas hydrates (in sediments) were focus of this work. The project was divided in two sections: 1) Thermodynamic properties of (mixed) gas hydrates: the aim of this work was establish precise phase for the fundamental thermodynamic properties of pure methane hydrates and mixed gas hydrates containing CO2, C2H6, C3H8 and H2S beside methane. These phase diagrams establish stability fields, decompositions lines, miscibility gaps, composition of the gas phase and the hydrate

152

phases. In addition, they predict specific conditions for the coexistence of hydrate phases with different structures and phase transition which can be caused by changing environmental conditions. Composition as well as structurs have been determined by Raman spectroscopy.

2) Petrophysical properties of gas hydrate bearing sediments: The physical properties of gas hydrate bearing sediments depend on the gas hydrate content and the distribution of the hydrate phase within the host sediment. The aim of this project was to simulate the natural process of pore space hydrate formation in the lab and to measure the physical sediment properties as a function of hydrate content. These data provide a basis both for improving the existing models or for developing new methods for the characterization of naturally occurring hydrate deposits based on geophysical surface and borehole measurements.

Present status and results 1. Determination of thermodynamic properties of (mixed) gas hydrates A key milestone was designing and constructing a pressure cell for the experimental set up which can be used over a temperature range between -27 °C and + 80 °C. The temperature of the sample cell is controlled by a cryostat and the temperature is determined with a precision of ± 0.1 °C and the accessible pressure range is between 0.1 and 10.0 MPa. A pressure controller adjusts the sample pressure with a

precision of 2% rel.. The small sample volume (0.393 cm3) and the all-around cooling of the sample prevent temperature gradients. At a gas flow of 1 ml/min, it takes 17 sec for the incoming gas flow to pass the cell body and to enter the inner cell space; this time is sufficient to allow the gas to attain the cell temperature.

A quartz window permits the analysis of the phases by Raman spectroscopy as well as visual observation and the recording of microscopic photo-documentation of formation and decomposition processes. The experimental setup with the confocal Raman spectrometer (LabRam, JobinYvon) permits the focus of the laser beam on a precise point, e.g. the surface of a hydrate crystal, thus assuring that only selected volume is analyzed.

The experiments were carried out using the following procedure: 150 µl of pure and degassed water were placed in the sample cell.

The cell was carefully sealed and flushed with the appropriate gas before pressurization. The system was cooled as rapidly as possible until hydrates formed. After that, the system is warmed at constant pressure in order to melt most

of the hydrate. When only a few crystals are left, the temperature is lowered about 0.5 °C and the euhedral crystals of gas hydrate grow under steady state conditions. After the water phase has completely transformed into hydrate crystals Raman spectroscopic investigation at defined pressures and temperatures were performed on the following systems: CH4-H2O, CH4-CO2-H2O. For testing the functionality of the experimental set-up the first experiments were done with the system CH4-H2O because numerous investigations have already been performed on this system. The determined decomposition data correspond well to literature data (shown in Fig. 2) An interesting observation made by Raman spectroscopy is that not all the analysed crystals show the expected structure I spectra.

Some of the analysed crystals show different spectra which indicate of structure II. Until now it was generally accepted that guests like CH4 or CO2 form structure I hydrates, either individually or in combination, under moderate conditions (P≤100 MPa and T ≤ +20°C) and that structure I hydrate is the

Figure 1: Design of the sample cell

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Figure 2: P-T-diagram for methane hydrate

Figure 3: Coexistence of structure I (s I) and structure II (s II) methane hydrates. The width of the image is equivalent to 450 Âľm.

thermodynamically most stable phase for such guest.

The experimental set-up allows the in-situ Raman spectroscopic measurements on different crystals with different structures which are coexi-

154

sting under identical P-T conditions as is shown in Figure 3: It is known that the C-H symmetric stretch vibrational frequency for methane can be correlated with cage size â&#x20AC;&#x201C; with vibrational frequencies for small cavities shifted to higher

Figure 4: Raman spectra of structure I (a) and structure II (b) methane hydrates

frequencies relative to those for methane molecules in large cavities according to the Pimentel-Charles (1963) Âťtight-cage-loosecage modelÂŤ as recently elaborated by Subramanian and Sloan (2002). Since the ratios of large to small cavities for structure I and structure II hydrate are 3:1 and 1:2, respectively, the relative peak areas in the experimental spectra should readily distinguish between the two structures. It turns out that the ratio for the Raman bands of most of the crystals is close to

3:1 (Figure 4a), indicative of structure I as would be expected for methane hydrate.

Nevertheless, the Raman spectra of a few crystals (Figure 4b) gave spectra with peak intensities 1:2, indicating the presence of structure II hydrate.

It is clear that structure II methane hydrate is less stable than structure I methane hydrate under these conditions: if the conditions are changed, for instance, to lower temperature at

155

Figure 5: Raman spectra documenting the transformation of structure II methane hydrates into structure I methane hydrates.

constant pressure, or to higher pressure at constant temperature, the structure II crystals transform. First the crystals become less welldefined. A rounding of the crystals, reminiscent of a melting process, is followed by a transformation to a fine-grained hydrate mass.

This exothermic transformation process -

156

beginning with the structure II crystals - induces a recrystallization of the entire hydrate phase until a new steady state is reached. All of the hydrate phase now has the fine-grained texture before new well-defined crystals starts to grow again.

Raman spectroscopic measurements (shown in Figures 5a-d) on this hydrates during the change in morphology show a loss of the structure II character: the intensity of the methane band at 2904 cm-1 (large cavities) increases, while the intensity of the methane band at 2916 cm-1 (small cavities) decreases.

Finally, structure II methane hydrate can no longer be detected by Raman spectroscopy.

These results have been published elsewhere (Schicks and Ripmeester, 2004) A similar process was observed for the system CH4-CO2-H2O. Analogous to the behaviour of the pure methane hydrates, CH4-CO2-hydrates grow as euhedral crystals at conditions close to the decomposition line (Figure 6a). Decreasing the temperature induces a change in morphology of the crystals as it is shown in Figure 6b.

In the end only a gas hydrate mass with a finegrained texture remain (Figure 6c).

Raman spectroscopic measurements indicate that a small amount of such euhedral crystals have the unstable structure II (approx. 5-7%) whereas a large majority of the crystals have structure I. Figure 7 presents the pressure and temperature conditions where a coexistence of structure I and structure II hydrate crystals is possible. Please note that â&#x20AC;&#x201C; compared to the pure methane hydrate - the decomposition line of the mixed CH4-CO2-hydrates is shifted to higher temperatures or lower pressures.

Figure 6a-c: Growth and transformation process of CH4 -CO2-hydrates

Hitherto no evidence has been reported for the coexistence of structure I and structure II hydrate phases in natural gas hydrate samples.

Raman spectroscopic investigations of gas hydrate samples from the Mallik 5L-38 drilling project indicate only structure I methane hydrate (see also Figure 8). It should be noted that during the recovery of natural samples changes in pressure and temperature are inevitable, such changes could induce a transformation process.

In order to study the influence of other gases such as H2S on the stability and composition of

157

Figure 7: P-T-diagram of CH4-CO2-H2O System.

Figure 8: Raman spectrum of natural methane hydrate from Mallik 5L-38 project

158

gas hydrates, Raman spectroscopic investigations were conducted on C-H-N-S bearing gas inclusions in fluorite. The data lead to the conclusion that H2S is preferentially incorporated into gas hydrates whereas nitrogen was not detected in the hydrate phase. Furthermore, such mixed gas hydrates show a high thermal stability; the decomposition temperature was detected at 300 K.

2. Determination of the petrophysical properties of gas hydrate bearing sediments: Our experimental concept is based on a model of the natural process of pore space hydrate formation. Methane solved in water migrates upwards into a formation which provides lower pressure and temperature conditions. In marine environments or in arctic areas such upward migrating, methane charged water will pass through a zone with pressure temperature conditions under which methane hydrate is stable. Whether or not hydrate forms at this depth range will depend on the methane supply and the sediment properties.

To simulate this process in the lab we have

designed and built an apparatus which consists of a thermal insulated box with two compartments (Figure 9).

The temperature in both compartments can be controlled independently. The first compartment is kept at a temperature above the hydrate stability. It represents the deep subsurface where methane is formed and water is charged with methane. It contains a methane volume which is separated from the methane cylinder and the rest of the system by two valves.

The volume can be charged with methane by opening valve v1. The exact amount of gas in the volume can be calculated from the ideal gas law. Via valve v2 the gas volume is connected to the rest of the system which is filled with degassed NaCl solution. By opening valve v2 the water reservoir is charged with methane.

From the pressure drop in the gas volume we can calculate how much gas was transferred to the water reservoir where it starts to dissolve in the aqueous solution. To speed up this process the water is circulated trough the reservoir with the bypass valve v3 opened until the system pressure is constant. Then the bypass is closed and the regulation valve RV is opened

Figure 9: Experimental system used to generate methane hydrate in the pore space of a sediment sample and which can measure the petrophysical sediment properties.

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so that methane charged water can flow to the sample cell in the second compartment representing the depth range in the subsurface that is within the hydrate stability field. The pressure and temperature is measured at the fluid inlet and outlet of the sediment cell. The closures of the cell are made of stainless steel and act as current electrodes A and B. They contain a Pt100 temperature sensor and ultrasonic P- and S-wave transducers. The cell consists of three high tensile strength Plexiglas rings separated by two thin stainless steel rings which act as potential electrodes M and N. The inner diameter and the length of the cell is 50mm. Measurements of sonic wave velocities and electrical resistivity are recorded at predetermined time intervals. The methane charged water coming from the first compartment cools and enters the sample which is in the hydrate stability field. When hydrate forms in the sediment it consumes water but excludes the salt ions, hence the salt content of the remaining solution increases. The increasing

Figure 10: Resistance of the circulating water and hydrate saturation of a glass bead sample versus time.

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salt content results in an increasing electrical conductivity of the water which is measured with the conductivity sensor CS. The water passing the sample cell looses a part of the solved methane due to the hydrate formation process. When the water enters the reservoir again, it is recharged with methane. The methane consumption results in a drop of system pressure. To prevent the pressure from decreasing to the stability boundary where hydrate formation stops additional methane must be periodically feed into the system. The amount of hydrate formed can be calculated from the change in the electrical conductivity of the water. Because the solubility of methane is low (about 10-3mol/mol at 5MPa and 20째C) the formation of hydrate from a solution of methane saturated water is a very slow process.

Due to the absence of empirical data on the influence of hydrate saturation on the petrophysical properties of hydrate-bearing sediments, it is a challenge to estimate the hydra-

te content of a formation based on geophysical field and well log data. Of particular interest would be the use of sonic velocities and electric resistivities because they are more strongly affected by the presence of gas hydrate as compared to other geophysical properties.

To determine the amount of hydrate in the pore space from physical in-situ measurements Pearson et al (1986) suggested the use of Archie’s Law (Archie, 1942). Archie’s Law consists of two equations. The first Archie equation is for fully water saturated rocks with a conductivity σ0

(1)

where F0 is the formation resistivity factor of the fully water saturated rock, σw is the conductivity of the pore water, φ is the porosity of the rock, and a and m are the empirical Archie parameters.

The second Archie equation is for partly saturated rocks with a conductivity of σt

(2)

where Sw is the water saturation, σt is the conductivity of the partly saturated rock, Ft is its formation resistivity factor, and n is the empirical saturation exponent.

For practical applications equation (1) is often used with the resistivity index I,

pore space occupied by gas hydrates has been estimated from resistivity measurements in gas hydrate research wells e.g. ODP Leg 164 site 994 (Paul et al., 1996) and Mallik 2L-38 (Dallimore et al., 1999). The empirical saturation exponent in both studies was chosen to be n= 1.9386 (Pearson et al. 1983).

The empirical saturation exponent is controlled by the distribution of the conductive brine in the pore space, thus it depends on wetting properties, saturation history, and the rock microstructure. The influence of different types of hydrate occurrences on the resulting electrical properties has been studied theoretically by Spangenberg (2001). The formation of pore space hydrate was investigated based on a sphere packing model. For the situation that the pore water is the wetting phase and the hydrate forms as non-cementing material in the pore space, the model predicts a saturation exponent that depends on the saturation itself.

Our measurements confirm this theoretical prediction. Figure 11 shows the saturation exponent of the hydrate-bearing glass bead sediment together with the resulting errors a constant saturation exponent is assumed.

Most attempts to predict hydrate contents from velocity data are based on derivates of the time-average relation (Wyllie et al., 1958), which relates the velocity of a fluid saturated consolidated rock to the velocity of the solid phase, the velocity of the fluid phase and the volume fractions of both phases. To apply this approach to ice- or hydrate-bearing formations a three phase time average relation version has been used (Timur, 1968; Pearson et al., 1986) shown by the following equation:

(3) (4)

which is the ratio of conductivities when the rock is fully vs. partially saturated. In equations (1) - (3), and throughout this paper, brine is assumed to be the only conducting phase. Following this suggestion the fraction of the total

where Vtar is the p-wave velocity of the hydrate-bearing sediment, Vh is the p-wave velocity of pure hydrate, VW is the compressional wave velocity of the pore fluid, Sh is the hydrate saturation , and φ is the porosity containing the

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(5)

where ρ is the bulk density of the sediment in the form (6) ρw is the density of the pore water, ρh is the density of pure hydrate, and ρm is the density of the matrix material. This equation pertains to particles in suspension and can sometimes underestimate the true velocity porosity relationship in marine sediments. Lee et al. (1996) use a weighted combination of the time average relation (4) and Wood’s equation (5) to predict the velocity of hydrate-bearing sediments:

(7)

Figure 11: Measured and predicted saturation exponent as a function of water saturation and the Archie prediction for the hydrate content with constant n.

hydrate and pore fluid. A drawback of the time average approach is that its predictions can be wildly inaccurate if the rock is unconsolidated (Wyllie et al., 1958). In such a situation an artificially low matrix velocity can be used (Hoyer et. al, 1975) to adjust for the unconsolidated state of the porous medium. For marine sediments sometimes the equation of Wood (1941) is used, which can also be adjusted for hydrate-bearing sediments to

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A comparison of the time average relation with an adjusted matrix velocity, Lee’s weighted 3-phase equation and our measurements of ultrasonic p-wave velocities are shown in Figure 12.

With increasing hydrate content we observed an increase in signal attenuation. At a hydrate saturation of about 40% we detected a new first arrival which is generally weak and only just above the noise level. This observation explains the sudden increase of velocity seen in Figure 12.

By increasing hydrate saturation still further this first arrival becomes more pronounced. This behaviour is related to the special situation of wave propagation within a medium which is composed of two frameworks, a grain framework and a hydrate framework. Certainly, further investigations are necessary to understand the peculiarities of wave propagation in hydrate bearing sediments.

Figure 12: Comparison of the measured p-wave velocities with the time average relation with an adjusted matrix velocity and Leeâ&#x20AC;&#x2122;s equation with W=1.51 and n=1.

Conclusion Until now the reason for the formation of structure II CH4-(CO2)-hydrates under the conditions where only structure I CH4-(CO2)-hydrate was thought to be stable is not clear. Subramanian and Sloan (1999) observed during real-time Raman spectra monitoring a transformation of dissolved CH4 to CH4 in the large and small cages of structure I methane hydrates. In the initial stages of hydrate formation the large to small cages ratio was 0.5, which correspond well with the large to small cages ratio of structure II clathrate hydrates. They assumed that the formation of the large cavity is the rate-limiting factor in the structure I methane hydrate formation. Staykova et al. (2003) observed the transient formation of structure II CO2-hydrates during the growth of pure CO2-hydrates on icegrains. The 129Xe NMR experiments of Moudrakovski et al. (2001), who determined the cage occupancy ratio as a function of time during the

early stages of the formation and growth of hydrate also observed a predominance of small cages during in the initial stages of the reaction, indicating a major role for the small cavities. In these studies the presence of large numbers of guest molecules in small cages at early times in the hydrate growth process suggests that structure II hydrate may be the kinetically favoured structure as it contains the small cage as the major building block for the structure II framework. The observation of a coexistence of structure I and structure II hydrates at the conditions mentioned above suggests that the initial product is determined by kinetics in which structure II appears as a metastable phase.

We quantified for the first time the relationship between electrical resistivity and sonic pwave velocity on hydrate saturation. The measurement of electrical resistivity is in good agreement with a theoretical prediction for

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spherical packing. However, for the sonic wave velocities, the observed data show that the dependence of velocity on hydrate saturation is more complex than what is predicted by the currently used models.

Acknowledgement The GEOTECHNOLOGIEN programme of BMBF and DFG provided funding of this work through the Research Grant G0555A. Further we would like to thank John Ripmeester, National Research Council of Canada for the fruitful cooperation.

References Archie, C.E., The electrical resistivity log as an aid in determining some reservoir characteristics, Trans. Am. Inst. Min. Metall. Pet. Eng., 146, 54-62, 1942 Dallimore, S. R., T. Uchida, and T. S. Collett, Scientific Results from JAPEX/JNOC/GSC Mallik 2L-38 Gas Hydrate Research Well, Mackenzie Delta, Northwest Terretories, Canada. Geological Survey of Canada, Bulletin 544, 1999 Hoyer, W.A., S.O. Simmons, M.M. Spann, and A.T. Watson, Evaluation of permafrost with logs, Trans. SPWLA Annu. Logging Symp., 16th paper 15pp., 1975 Lee, M.W., D.R. Hutchinson, T.S. Collett, and W.P. Dillon, Seismic velocities for hydrate-bearing sediments using weighted equation, J. Geophys. Res., 1001, B9, 20,347-20,358, 1996 Moudrakovski, I. L.; Sanchez, A. A.; Ratcliffe, C. I.; Ripmeester, J. A. (2001) J. Phys. Chem. B, 105, 12338 Paull, C.K., R. Matsumoto, P.J. Wallace, Proceedings of the Ocean Drilling Program, Initial Reports, Vol. 164, 6. Site 994, pp 142144, 1996

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Pearson, C.F., Halleck, P.M., McGire, P.L., Hermers, R.E., and Mathews, M.A., Natural Gas Hydrate Deposits, a Review of in situ Properties, J. Phys. Chem., v. 87, no. 21, pp.

4180-4185, 1983 Pimentel, G.C.; Charles, S.W. (1963) Pure Appl. Chem., 35, 111 Schicks, J.M.; Ripmeester, J.A. (2004) Angew.

Chem. Int. Ed., 43, 3310 Spangenberg, E., Modeling of the influence of gas hydrate content on the electrical properties of porous sediments, J. Geophys. Res., 106, B4, 6535-6548, 2001 Staykova, D.K.; Kuhs, W.F.; Salamantin, A.N.; Hansen, T. (2003) J. Phys. Chem., 107, 10299 Subramanian, S.; Sloan, E.D. (1999) Fluid Phase Equilib., 158-160, 813 Subramanian, S.; Sloan, E.D. (2002) J. Phys.

Chem. B, 106, 4348 Timur, A., Velocity of compressional waves in porous media at permafrost temperature, Geophysics, 33, 584-594, 1968 Wood, A.B., A Text Book of Sound, 578 pp., Macmillan, New York, 1941 Wyllie, M.R.J., A.R. Gregory, and G.H.F. Gardener, An experimental investigation of factors affecting elastic wave velocities in porous media, Geophysics, 23, 459-493, 1958

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GASHYDRATES – Paleoatmospheric archive Reconstruction of paleoclimatic changes in the source strength of potential methane sources using the methane isotopic signature in bubble enclosures in polar ice cores Hubertus Fischer Alfred Wegener Institute for Polar and Marine Research, Columbusstrasse, 27568 Bremerhaven, Germany, E-Mail: hufischer@awi-bremerhaven.de

Objectives Methane concentration records in polar ice cores [Blunier and Brook, 2001] show prominent variations in parallel to rapid climate variations during the last glacial period, the so called Dansgaard/Oeschger events [Johnsen, et al., 1992]. The origin of this additional methane on the one side and the trigger for the temperature shifts on the other side is still a matter of debate, where especially the role of a potential methane release by destabilization of marine gas hydrates remains uncertain. The hydrogen and carbon isotopic signature of the corresponding CH4 changes in ice cores can resolve this puzzle. Accordingly the long-term perspective of this project is the quantitative differentiation of the contribution of different CH4 sources to the changes observed in ice cores.

Such measurements, however, have not been possible so far because of the strong limitations in available sample size and the requirements on the analytical precision. The only previous ice core δ13CH4 analyses used 10 kg of ice, representing about 1.5 meters of ice core [Craig, et al., 1988]. Accordingly, the goal of this project has been the technical development of an efficient air extraction for air enclosed in clathrate and bubble ice as well as of a

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high precision gas chromatography isotope ratio mass spectrometry method (GCirmMS) on small ice samples (200- 400 g) to enable such measurements.

Results In the course of this project an automated, quantitative and fractionation free method for the extraction and isotope analysis of CH4 using an innovative GCirmMS technique could be established based on previous work by [Merritt, et al., 1995]. For δ13CH4 a reproducibility of 0.1-0.2 ‰ for a sample size of only 20 ml STP of air (equivalent to approximately 200 g of ice) could be established using preconcentration and complete combustion of CH4 to CO2 in a micro-combustion oven. This fulfilled the required accuracy to resolve the expected δ13CH4 changes during Dansgaard/Oeschger events (Figure 1). For δD(CH4) also a new GCirmMS method could be established reaching a reproducibility of about 6 ‰ on 100 ml STP of air. This still falls about a factor of two short of the anticipated accuracy/sample requirements, but is already sufficient to resolve the expected effects in δD in the atmosphere for a significant release of methane from marine gas hydrates (see Figure 3). First results on air and ice core samples reproduced previously publis-

Figure 1: Isotope temperature (δ18O) and CH4 profiles in the GISP2 ice core, Central Greenland [Blunier and Brook, 2001; Grootes, et al., 1993]

Figure 2: Comparison of δ13C values of recent air [Quay, et al., 1999] and preindustrial [Craig, et al., 1988] ice core samples with values determined in this study. Full circles indicate a bubble close off in the firn column at 73m, open circles at 63 m, which influences the age of the air enclosed.

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hed values very well (Figure 2). Routine application of this method on ice core samples will follow in the future.

Model considerations For the interpretation of the ice core data in terms of changes in CH4 concentration and isotopic composition a simple box model of the global methane cycle has been developed, which represents a substantial expansion of previous model calculations for CH4 concentrations [Chappellaz, et al., 1997]. The model reproduced recent and preindustrial CH4 concentrations and carbon isotopic signature very well.

To predict the influence of a methane release by destabilisation of marine gas hydrates a short term CH4 source of 100 Tg CH4/a over 40 years with an isotopic signature of δ13C = -60‰ and δD = -180‰ [Whiticar et al., 1986] was assumed. This leads to a short term increase in CH4 concentration to 650 ppbv, of δD to -40 ‰ and a decrease in δ13C to –52 ‰ (thin lines in Figure 3). Due to the low pass filter induced by the bubble close off, this amplitude is reduced to about 460 ppbv, –100‰ and -49.2‰, respectively (thick lines).

References Blunier, T., and E. J. Brook (2001), Timing of millenial-scale climate change in Antarctica and Greenland during the last glacial period, Science, 291, 109-112.

Chappellaz, J., et al. (1997), Changes in the atmospheric CH4 gradient between Greenland and Antarctica during the Holocene, J Geophys Res, 102, 15987-15997.

Craig, H., et al. (1988), The isotopic composition of methane in polar ice cores, Science, 242, 1535-1539.

Grootes, P. M., et al. (1993), Comparison of oxygen isotope records from the GISP2 and GRIP Greenland ice cores, Nature, 366, 552-554.

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Figure 3: Model results on a disturbance of the global methane cycle by a hypothetical release of CH4 from marine gas hydrates.

Johnsen, S. J., et al. (1992), Irregular glacial interstadials recorded in a new Greenland ice core, Nature, 359, 311-313.

Merritt, D. A., et al. (1995), Carbon isotopic analysis of atmospheric methane by isotope-ratiomonitoring gas chromatography-mass spectrometry, J Geophys Res, 100, 1317--1326.

Quay, P., et al. (1999), The isotopic composition of atmospheric methane, Glob Biogeochem Cyc, 13, 445-461.

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Gas Hydrates in Hemipelagic Sediments – CONGO Spieß V. (1), Zühlsdorff L. (1), Villinger H. (1), Flueh E. (2), Bialas J. (2), Kasten S. (1), Schneider R. (1), Bohrmann G. (1), Sahling H. (1) (1) Department of Geosciences, Bremen University, Klagenfurter Straße, D-28334 Bremen, Germany (2) IFM-GEOMAR, Wischhofstraße 1-3, D-24148 Kiel, Germany

1. INTRODUCTION The nature of sedimentary structures and deformation as well as the type of dewatering and fluid flow is significantly different between active and passive continental margins. While active margins are characterized by subduction, compression, sediment accretion, pronounced faulting, and locally high pore pressure (Moore & Vrolijk, 1992; Hyndman et al., 1992; Carson & Screaton, 1998; Zühlsdorff & Spieß, 2004), passive margins are characterized by continuous sediment accumulation and the development of sedimentary basins, where the creation of vertical permeable pathways requires marginal block faulting, basin subsidence, differential loading, slope instabilities or diapirism of mud or salt (Graue, 2000; Hooper et al., 2002; Babonneau et al., 2002). Thus, the nature of pathways as well as the driving forces controlling vertical fluid flow are different at active and passive margins, and, consequently, the development of gas hydrate systems in the shallow sub-seafloor may be significantly affected.

Permeability as a key parameter controls the supply of fluids (including gas) from greater depth, the fluid transport within the GHSZ, and thus the presence, concentration and distribution of gas hydrate. At active margins, permeability is enhanced by massive deformation, but fine grained passive margin sediments may often act as a hydraulic seal with regard to vertical fluid flow. Thus, larger gas hydrate reservoirs at passive margins can only

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develop if efficient migration pathways towards and into the GHSZ do exist, gas saturation of pore water is reached within or beneath the GHSZ, and sufficient volumes of water are available to allow for the growth of massive hydrate layers. As a consequence of limited permeability, the distribution of gas hydrates on passive margins may be more limited compared to active margins, as it is indicated by the worldwide distribution of known gas hydrate reservoirs and gas hydrate BSRs (Kvenvolden, 1994; Gornitz & Fung, 1994).

However, hydrocarbon systems within thicker sediment packages, that are characterized by a high content of organic matter, as well as areas of massive slumping, provide favorable conditions for gas hydrate accumulation if sedimentary, mechanical or tectonic processes create permeable pathways for hydrocarbon transport.

During Cruise SO 86 with R/V Sonne in 1993, locations of anomalous acoustic columnar blanking features had been identified in few multichannel seismic and sediment echosounder lines from the Lower Congo Basin at 5°S in about 3000 m water depth, and were subsequently studied in detail during R/V Meteor Cruise M47/3 in 2000. Clear indications for fluid venting were derived from samples of carbonate precipitates, shell fragments, living vent fauna and massive gas hydrates, which could be recovered from the sea floor in the vicinity of circular depressions of a few hundred meters diameter and a few to twenty meter depth. Beeing a typical feature found on

some passive margins, and the only so far observed structures on the Congo margin related to fluid venting, the pockmarks were chosen as a primary target for the first phase of research on gas hydrates and gas and fluid venting in the Congo region.

suspended material of the river and high biologic productivity (Jansen et al., 1984), associated with significant flux of organic matter to the sea floor. The hemipelagic sequences thus represent the host facies for gas and gas hydrate occurrences.

The surface expressions of such venting sites may be somewhat characteristic for the driving forces, flux rates, gas focusing mechanism and sediment deformation styles in the region, since only pockmarks have been so far found in the Congo region, while other fluid venting sites are characterized by positive sea floor morphology, as mounds or mud volcanoes. To investigate the relationship between the deeper sources of venting and the surface expressions, an integrated research program was designed for few such pockmark structures, combining multichannel seismics, sediment echosounder surveys and swath bathymetry operated from the vessel with deep tow side scan and seismics, heat flow in situ measurements, video profiling, water sampling, gravity coring and videoguided multicorer and grab sampling.

The overall structure of the continental margin is mainly shaped by salt diapirism and raft tectonics, which are closely related (Emery et al., 1975; Marton et al., 2000; Valle et al., 2001; Rouby et al., 2002). Although major differences regarding sedimentation rates and tectonics exist regionally, pronounced faulting may locally support hydrocarbon and gas migration and favors conditions for shallow gas and gas hydrate accumulation.

The Lower Congo Basin is located off the mouth of the Congo River (Figure 1), which provides significant sediment influx from the continent. A major portion is transported through the Congo Canyon (Heezen et al., 1964; Shepard & Emery, 1973; Droz et al., 1996) directly into the deep sea. In water depths below 3000 m, the narrow and deep channel opens into the Congo Cone, where typical fan deposits and channel/levee systems can be found.

However, drilling of three sites during ODP Leg 1075 (Wefer et al., 1998) as well as the respective seismic pre-site survey data (UenzelmannNeben, 1998) confirmed that the upper few hundred meters of the sediment section north of the Congo Canyon are dominated by hemipelagic sediments, which reveal low reflectivity and only subtle changes in physical properties with depth. The hemipelagic sediment input originates primarily from the fine grained

Three of the largest pockmarks called Hydrate Hole, Black Hole and Worm Hole (Figure 1) as well a number of smaller features were therefore re-visited during R/V Meteor Cruise M56 (2002) for detailed seismoacoustic mapping in two and three dimensions and an extensive video survey and sampling program. A second phase was planned to further investigate key parameters for such systems as the degree of deformation or the facies of host sediments for hydrates and gas.

2. OBJECTIVES Fluid venting systems function on multiple scales, when the pathways caused by large-scale salt-related sediment deformation, the reservoirs of free gas beneath and of gas hydrates within the gas hydrate stability zone are connected to the shallow subsurface, where methane fluxes through focusing chimneys counteract with methane oxidation and local biologically mediated processes create precipitates and gas hydrates and support a large variety of life forms. The overall objective was to study a gas hydrate system, where individual control parameters can be distinguished and investigated by high resolution methods looking at potential migration pathways, gas traps, hydrate reservoirs, seafloor features and any post-sedimentary changes.

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Figure 1: Location map of Congo pockmark area with Hydrate Hole, Black Hole and Worm Hole as the main targets for seismoacoustic surveys and sea floor sampling.

Among the various objectives of such an interdisciplinary studies, we planned to focus on: - Characterization of seismic properties at maximum available resolution within a sediment volume, which extends from the sea floor to beneath the gas hydrate stability zone.

- Integration of seismic data of different frequency content to optimize resolution at any given depth level both for structural imaging with reflection seismics and tomographic characterization.

- Comparison of lateral and vertical changes in seismic properties with evidence from sea floor mapping, in situ physical and chemical measurements, coring transects and subsequent studies on pore water, cores and samples and study of microbiological changes.

- Reconstruction of fluid/gas fluxes through the sediment column as a function of structural disturbance and methane supply.

- Understanding of processes related to fluid

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upflow within the surface zone of steep geochemical and physical gradients.

- Linkage of surface signals expressed in a variety of chemical, physical and biological parameters to seismic properties at depth and the occurrence and distribution of gas hydrates and free gas.

3. PRESENT STATUS AND RESULTS 3.1 METHODS High resolution seismic and acoustic imaging is a major tool to study both principal geological parameters controlling the occurrence of gas hydrate as well as the distribution of free gas and gas hydrate within shallow sediments. The presence of gas hydrate further reveals information about the nature and efficiency of fluid flow pathways (Z端hlsdorff et al., 2000). In the hemipelagic sedimentary sequences off the

Congo, where layering and uniform properties exist at the time of deposition, modification of sediment physical properties due to mixing between water, gas and gas hydrate within pore spaces affects amplitude and phase properties of seismic reflections. Furthermore, fluid flow and gas or gas hydrate accumulations are often associated with sediment deformation or faulting on different scales, partly related to salt diapirism at greater depth.

While multichannel seismics in conjunction with ocean bottom hydrophones, sediment echosounding and swath bathymetry were used for first order structural mapping, deep tow side scan and seismic data allowed to zoom in onto the surface characteristics on the meter scale providing a basis for subsequent sampling. Video sled transects in the vicinity of the pockmarks then allowed to calibrate the different backscatter signals confirming the direct relationship of high backscatter with the massive occurrences of bioherms or chemoherms. Dedicated video guided sampling and densely space gravity corer transects were the last steps in the row of methods and techniques to collect all available information on the processes, which were linked to the presence and migration of methane in soft hemipelagic sediments.

Hydro-acoustic systems A deep-towed DTS-1 side-scan sonar was used to identify active seepage areas. For M56 sidescan sonar mapping, acoustic energy at 75 kHz main frequency, which is back-scattered from targets on the seafloor, is recorded over 750 m wide swaths to both sides. The signal strength is related to both micro-topography and physical properties of these targets. Across-track resolution is 0.1 m whereas along-track resolution is 0.75 m at a typical towing speed of 2.5 knots.

During operation, a sensor on the instrument provided information about heading, roll and pitch. Underwater navigation and depth measurement was provided by a responder-based telemetry system, but not working properly.

Bathymetric data were collected with the Hydrosweep swath sounder onboard R/V Meteor that is operated at 15.5 kHz and produces usable data up to 2 x water depth with a horizontal resolution of the order of 100 m (at 3000 m depth and depending on shipâ&#x20AC;&#x2122;s speed and other factors). To suppress refraction effects on the outer beams, the Hydrosweep system uses a calibration mode to compare depth values of the central and outer beams in order to calculate a mean sound velocity. Using this configuration, residual depth errors are minimized to <0.5% of the water depth, on the order of 15 m for typical water depths in this area (Grant & Schreiber, 1990). However, for high coverage during 3D seismic surveying (up to 100x), noise could be significantly suppressed and vertical resolution appeared to be on the order of 1 meter.

The Parasound echosounder onboard R/V Meteor is a narrow-beam system that is operated at 4 kHz and used to image the uppermost part of the sediment column at very high resolution. Depth penetration depends on the type of sediment and was typically on the order of 80-100 m. Footprint size is only 7% of water depth, diffraction hyperbolae are suppressed, and both lateral and vertical resolution are significantly higher in comparison to conventional sediment profiling systems. Both Hydrosweep and Parasound systems are hull-mounted and compensate for heave, pitch and roll. They are operated simultaneously during multi-channel seismic or DTS-1 data acquisition.

Multi-channel seismics and ocean bottom instruments The multi-channel seismic system of the University of Bremen is optimized for high resolution imaging of small-scale sedimentary structures and closely-spaced layers on a meter to sub-meter scale. The alternating operation of a small chamber watergun (0.16 L, 2001600 Hz, Sodera) and a Generator-Injector Gun (GI-Gun, Sodera) with chamber volumes of 2 x 1.7 L (30-200 Hz) yielded two separately recorded seismic data sets along each seis-

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mic line. Both data sets distinguish with respect to depth penetration and temporal resolution and are further supplemented by simultaneously recorded Parasound data (see above) in order to image each depth level at best possible resolution and to be able to compare seismic signatures on different scales.

Using a Syntron multi-channel streamer with 96/48 recording groups separated by 12.5 m (Cruise M47/3) and 6.25 m (Cruise M56), respectively, trace fold of 8-15 was achieved for a CMP spacing of 10 m, that provided the best compromise between trace density, image quality and noise reduction. Six remotely controlled birds with compass units kept the streamer at ~3+/-0.5 m depth and provided crosstrack distances for all streamer groups.

Positioning was provided differential GPS recordings. Standard data processing included editing, bandpass filtering, correction for geometrical spreading, stacking and time migration. During Cruise M56, an area of about 7 x 2 km size was covered by parallel profiles separated by 25 m for a threedimensional imaging of detailed sediment structures and vent-related features. Individual 2-D lines were binned and stacked for a pre-defined grid of 10 m (inline) and 25 m (crossline) cell size, leading to a cell coverage between 5 and 10 fold. Trace interpolation could be avoided due to the small inline spacing.

Four ocean bottom hydrophones and five ocean bottom seismometers were located in the vicinity of Hydrate Hole during the 3D seismic survey, to collect data for a tomographic analysis of velocity and signal attenuation near the columnar blanking zone and within the gas hydrate stability field. More accurate than from multichannel seismics with the 600 m long streamer, velocity models derived from OBS measurements shall provide the basis for the quantification of gas hydrates and gas reservoir near the migration channel.

During the side scan profiles run with the DTS-1 system, seismic acquisition was opera-

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ted in parallel, collecting GI Gun shots with a 24-channel deep tow streamer attached to the fish. Due to the proximity of the receiver system to the sea floor, the footprint of seismic reflections is much smaller and lateral resolution can be significantly enhanced. However, fish navigation was not available and positioning quality is limited. Static corrections are very difficult, and so far processing was limited to dealing with single channel data sets.

Sea floor observations and heat flow The video sled OFOS, towed close to the sea floor, was used to visually image the sea floor, while slowly cruising with a speed around 0.5 knots, searching for indications of fluid venting, precipitates, gas hydrates or vent fauna. It is equipped with a black-and-white video camera, xenon lights, a still camera and flashlights, externally powered through the coaxial cable. The video signal is transmitted through the cable to a realtime display and is stored on tapes. A memory CTD gives additional information about depth, which is very helpful when no navigation system is available. In addition to this standard configuration, a methane sensor was mounted on the sled during this cruise. Data obtained by this sensor are discussed in the chapter about CTD/rosette deployments.

To determine the regional and local heat flux in situ, miniaturized temperature loggers (MTL) were used. 4 to 6 MTLs were mounted onto the strength member of a 6 m long probe or onto the barrel of the gravity corer. 71 measurements with the 6 m probe and 4 measurements on a gravity corer were carried out along several transects crossing the main pockmark structures. Measurements took 7-10 minutes for penetration and 15 to 45 minutes for transit with gear towed just a few tens of meters above the sea floor. Thermal conductivity was measured on sediment cores for calibration of heat flow profiles.

Sampling For sampling, TV guided grab (TVG) and multi-

corer devices were used, combined with a conventional gravity corer. The TVG is equipped with a camera system and lights that are powered by deep-sea batteries. The TV Grab is able to sample ~1.8 square meters of the sea floor. The TV-MUC, equipped with a camera system and xenon lights, consists of a conventional multicorer with 8 cores of 10 cm in diameter and is designed to recover undisturbed surface sediment sections and the overlying bottom water. It was particularly used to take well-defined surface samples in clam or carbonate fields, and also attempts were made to sample bacterial mats or areas which seemed to show gas hydrates at the sediment surface.

With the gravity corer sediment cores between 3.5 and 14.0 m core length were taken at 17 stations. Altogether about 165 m of sediments were recovered. For dedicated sampling of gas hydrates, instead of a plastic liner a plastic foil was used to lay the sediment on deck and immediately sample all sections for pieces of gas hydrates. This ÂťcrudeÂŤ method allowed us to gain excellent, well-shaped hydrates that showed nearly in-situ morphology and structure.

3.2 PRESENT STATUS Other than most GEOTECHNOLOGIEN projects on gas hydrates, the CONGO project started late during the funding period in spring 2002, since ship time was only available in late 2002, and funding and time for subsequent processing of data and samples was restricted to a year or less. As a consequence, most of the work, which was designed on a long term funding and research program, as was requested by the original gas hydrate program, could not be finished. An extension of the research funding was expected, but was not provided. The following results are therefore preliminary, and publications could not yet be finished within the funding period.

3.3 PRELIMINARY RESULTS

Bathymetric and side-scan data Within the area covered by Cruises M47/3 and M56, bathymetric charting indicated several pockmarks with depressions on the order of 1-30 m and diameters of 50-2000 m. Work, however, was concentrated on three large features called Hydrate Hole, Black Hole and Worm Hole (Figure 1). Since these features were covered by a 3-D seismic grid, swath mapping data coverage in this area was >100.

While Hydrate Hole and Black Hole revealed numerous bathymetric features of different size and at different depth, Worm Hole represents the largest seafloor depression, showing >40 m depth variation (Figure 1). The pronounced morphology may be related to a salt diapir underneath as indicated by seismic data for several pockmark structures in the area (see below), suggesting that the side walls of Worm Hole are not necessarily the result of ejective processes.

At all three locations, the morphology of the pockmarks is characterized by small steps rather than by continuous depth variation, as it could be confirmed from depth versus time records obtained from deep-tow video surveys.

Outside the large depressions, smaller scale features were observed that reveal diameters of the order of 1-10 m.

In general, a limited correlation between sidescan and bathymetric data is observed, indicating that both data sets provide basically different types of information (Figure 2). Side-scan sonar data reveal high backscatter patches within the pockmark structures that usually do not cover the whole pockmark area. The widespread occurrence of clams, shells and mussels observed at the seafloor during video observations indicated active venting at high backscatter locations. Thus, high amplitude patches very likely correlate with enhanced methane flux supporting vent communities. At Hydrate Hole, different levels of morphologic and bakkscatter anomalies are observed, with the highest backscatter associated with the NorthEast patch, whereas the topographic depres-

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sion is much more pronounced at the SouthWest patch (Figure 2). Different characteristics of the different patches may suggest different ages and stages of activity. Furthermore, all gravity corers taken within high backscatter areas contained several layers of gas hydrates, while hydrates were absent in the upper ~10 m near the rim of the venting area.

Parasound data The Parasound record across Hydrate Hole and Black Hole in Figure 3 is characterized by lateral and vertical reflection amplitude variations and shows acoustic signatures that are typical for most of the Congo pockmarks. At the bottom of the seafloor depressions, patches of high reflection amplitudes without sharp boundaries and with only a few coherent reflector elements are observed. Beneath,

Figure 2: Side scan sonar image (top) and high resolution bathymetry (bottom) of Hydrate Hole. The SE depression is appx. 15 m deep, while the high backscatter patch in the NW reveals a depth variation of only few meters.

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almost complete acoustic blanking appears to overprint a layer package of high reflection amplitudes, that otherwise is laterally continuous. All described high amplitude features are located well within the GHSZ, as it is estimated from water depth and standard diagrams for hydrate stability, e.g.

Dickens & Quinby-Hunt (1994).

Parasound data can be analyzed in greater detail at Hydrate Hole, where a dedicated survey with closely spaced (75 m) grid lines was carried out during Cruise M47/3. Due to the high data density, amplitudes and other acoustic attributes can be investigated in a near-3D volume. Digital seismograms were used to determine maximum reflection amplitudes within 3 m thick slices and to map amplitude anomalies as indicator for structural elements, that are not related to the hemipelagic layering (Figure 4). Time slices reveal four units with different characteristics: (1) a seafloor depression associated with a weak seafloor return and the seafloor occurrence of carbonates and gas hydrates within the upper 6 m of the sediments (as it was confirmed by core data), (2) high amplitude patches beneath the pockmark within the uppermost 10 m, a blanking zone beneath the high amplitude patches, and high amplitude patches next to the blanking zone at 30 m depth, (3) a transition of the blanking zone from linear to circular shape at 40-50 m depth, associated with the maximum occurrence of high amplitude patches (10-20 m thick) around the blanking, and (4) most pronounced blanking along a SW-NE trending lineament.

Due to the low wet bulk density of the hemipelagic sediment section, normal reflection coefficients are very low, and high amplitudes therefore may indicate the presence of anomalous physical properties. High reflectivity zones were identified both in the vicinity of gas hydrate findings at the seafloor as well as around a chimney characterized by very low reflection amplitudes, that may indicate the presence of free gas. Using the detailed surfa-

Figure 3: Digital Parasound profile across Hydrate Hole and Black Hole, revealing high amplitudes near the sea floor above columnar blanking zones. Strong reflectors appear at 50 m sub-bottom depth near the blanking zone, decreasing with distance to them.

Figure 4: Time slice through Parasound data volume around Hydrate Hole with maximum amplitudes within 3 m thick layers displayed as dark colors.

The lowermost 4 layers reveal the spatial distribution of high amplitudes around 50 m subbottom depth

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Figure 5: Multichannel seismic line across Hydrate Hole and Black Hole, acquired during R/V Meteor Cruise M47/3. High amplitudes at depth indicate massive free gas occurrences, and several columnar blanking zones connect to depressions at the sea floor. Also, complicated faults patterns are observed.

ce evidence, high reflection amplitudes may thus be attributed to the presence of gas hydrates or carbonate precipitates and can likely be used to trace surface information down to depth. One possible interpretation would be that low reflection amplitudes indicate a potential gas supply conduit and that gas hydrate growth occurred around its rim.

Two-dimensional seismic data Though GI-Gun data are less sensitive for local variations in sediment physical properties than very high resolution Parasound data, the seismic image across Hydrate Hole and Black Hole is also dominated by lateral and vertical variations in reflection amplitude (Figure 5). Higher reflection and scatter amplitudes at ~250 ms TWT sub-bottom depth inhibit imaging of deeper parts of the sediment section. Since the estimated base of the GHSZ is located above this depth, it is likely that free gas is trapped beneath a low permeable cover of hemipelagic sediments. However, the data suggest that the seal is broken at locations of small-scale faulting and beneath the pockmarks.

As for Parasound data, diffuse patches of high reflection amplitudes are observed beneath

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the seafloor depressions of the pockmarks. A layered zone at about 100 ms TWT sub-bottom depth reveals locally increased reflection amplitudes in the proximity of columnar blanking zones that are interpreted as pockmark feeder-channels (see below).

Figure 6 shows a close-up of Black Hole and a comparison of GI-Gun and watergun data. Watergun amplitudes are very weak in this area, however, zones with locally higher amplitude can be better identified and resolved. A high amplitude patch with positive polarity is observed 50-60 ms TWT beneath Black Hole that is interpreted as gas hydrate. A likely scenario is that gas hydrate is clogging the vertical pathway and may subsequently grow laterally into more permeable parts of the sediment section, associated with a local increase of reflection coefficients in the vicinity of the pockmark.

2-D seismic lines from the regional seismic grid were used to create isopach maps for several time intervals. Age information from ODP Site 1075 was used by tracing eight laterally continuous reflectors from the drill site location into the pockmark study area. Sediment thickness between these reflectors was subtracted by average thickness and the residuum was normalized relative to the standard deviation in order to reveal pronounced relative variations

Figure 6: Comparison of GI Gun (left) and watergun seismic data (right) near Black Hole.

in sedimentation rates. For the time interval of about 0.5â&#x20AC;&#x201C;0.88 Ma, a local positive anomaly of the order of three standard deviations is observed (Figure 7), indicating an event of significantly increased sediment accumulation that is not observed in all other studied time intervals between 0 and 2.0 Ma. This suggests that at some time after 0.9 Ma, a massive subsidence event with subsequent sediment fill may have been associated with gas hydrate decomposition and enhanced degassing near the depression, which is now marked by the occurrence of numerous seafloor pockmarks.

Three-dimensional seismic data 3-D seismic data across Hydrate Hole and Black Hole further support the assumption that typically low reflection coefficients of the opal-rich and water-rich sediment section are affected by the presence of hydrocarbons. The high amplitude patch beneath Black Hole, interpreted as a hydrate cap that locally increases seismic impedance, is of limited extent (Figure 8), supporting the assumption that the blanking zone underneath represents a zone of focused fluid upflow, that supplies sufficiently large volumes of gas to

allow for the formation of massive gas hydrates.

The blanking would then be explained by scattering of seismic energy at free gas bubbles.

Though free gas usually is associated with seismic data attenuation, trapping of free gas beneath a low permeable seal may increase the reflection amplitude of the seal layer by increasing the velocity contrast at the seal-gasboundary. The package of high amplitude reflector elements at about 250 ms TWT subbottom depth (Figure 8) may thus be interpreted as trapped gas beneath an low permeable layer (see above). 3-D mapping indicates that this zone is locally elevated beneath Black Hole (Figure 8), probably indicating a deeper salt diapir, that is associated with doming structures, faulting and probably higher permeability above the diapir.

Faulting due to diapirism may further be superimposed by zones of weakness within a regional fault pattern that probably is of polygonal structure. A fault that was mapped east of Black Hole turns out to reveal a clearly defined fault plane that is obliquely oriented to the

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Figure 7: Anomaly in sedimentation rate for the time interval 0.88 to 0.50 Ma, derived from normalized isopach maps. The patch is located SE of the pockmark area.

Figure 8: View at the 3D seismic volume around Hydrate Hole and Black Hole, with shallow gas occurrences, assumed hydrate cap and fault zone.

Figure 9: Time slices through 3D seismic volume revealing several linear orientations, which might be part of a polygonal fracture and fault pattern.

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main strike and parallel to a connecting line between Black Hole and smaller pockmarks to the Northwest (Figure 8). Time slices indicate that other fault orientations are present as well (Figure 9), suggesting that pockmarks may in principle be located at the corners of fault polygons. However, the creation (and subsequent clogging) of pathways beneath the pokkmarks is not yet completely understood.

Sampling and video surveying Primary target of the sampling campaign had been three pockmarks, which were also surveyed in detail with all available geophysical methods. Accordingly, a dense set of echosounder lines as well as high resolution bathymetry and side scan sonar mosaics could be used to pick interesting sampling targets.

Specifically, high backscatter patches were first surveyed by video transects, revealing all kinds

of indications for fluid venting (Figure 10), as tubeworms, clam shells or carbonate precipitates. The pockmarks show a distinct small scale topography with several steps, probably faults, and steep flanks. However, the outline of the morphological pockmark structure and the location and shape of the backscatter patches do not match at all. The active area appears smaller, and in case of Hydrate Hole, venting is no longer active within the depression, but in a shallower part NE of the main structure, in a minor depression of just a few meters only.

Carbonate precipates dominate in most of the area, with tubeworm and clam shell fields appearing in smaller patches.

Few gravity cores were taken in Black Hole and Worm Hole, recovering gas hydrates and carbonate precipitates. A coring transect was carried out in Hydrate Hole with a spacing of 100 m or less, to search for systematic trends in the occurrence of gas hydrates and other vent

Figure 10: Results of an OFOS survey across Hydrate Hole drawn on top of echosounder profile. Vent indications are found, where Parasound surface amplitudes and side scan sonar backscatter strength are high.

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indicators. A clear trend was found in the depth of the shallowest gas hydrate layer, which increased from ~1 m in the center towards ~6 m near the rim (Figure 11).

Gas hydrates did occur macroscopically in 9 gravity cores and show a wide variety in distribution and fabric formation. Pure white hydrate occurred in layers or veins several millimeters to centimeters thick. Gas hydrate layers occur parallel to bedding but also exist as near-vertical veins, some of them over a vertical range of 40 centimeters. The veins have a thickness of a few millimeters. Few examples of veins and gas hydrate pieces were found with open spaces, indicating either channels kept open by continuous flow or trapped gas bubbles.

3.4 DISCUSSION Sites characterized by continuous or episodic fluid venting, associated with the presence of hydrocarbons, often reveal complex surface and internal structures, which are difficult to

image and cause problems to relate results from local sampling with structural reconstruction and estimations of average flux rates.

Detailed seismic and acoustic surveys in both two and three dimensions and at sufficiently high resolution are essentially required as a basis for interpretation. However, the choice of an appropriate environment clearly is as important in order to gain a deeper understanding of flow related processes. A study area may be considered to be appropriate if such processes overprint a well known and clearly defined background and if flow-affected and unaffected areas can be distinguished. Hemipelagic sediments appear to be suitable since the typical layering is associated with coherent seismic reflectors, that show minor lateral amplitude variation and can be continuously traced over long distances with respect to the scales of flow-related features.

The Congo pockmark study area is characterized by a fine-grained sediment cover that pro-

Figure 11: The sampling transect confirms gas hydrates recovered in up to 10 m cores in the center of the high backscatter patch, with decreasing minimum depth from 6 m to 1 m.

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vides an efficient seal for upward fluid and gas migration. Shallow gas occurrence is usually restricted to zones beneath the GHSZ. Thus, a gas hydrate BSR is not observed in the region.

Instead, zones of extremely high reflection amplitudes follow stratigraphic boundaries, that are likely associated with higher permeability, e.g. due to a facies change, more pronounced sediment deformation or slumping.

The survey and sampling work during R/V Meteor Cruises M47/3 and M56 was focused on locations, where sediment deformation or small scale faulting, likely related to salt diapirism at greater depth and associated with local subsidence, created conduits between the deep gas reservoirs and the seafloor. Large scale pokkmarks of several hundred meters diameter and up to 30 m depth are a consequence of vertical fluid transport. Several 2-D seismic lines and the 3-D seismic survey in the vicinity of these pockmarks reveal a clear picture of post-sedimentary enhancement of reflection amplitudes, that can directly be related to the trapping of free gas (below the GHSZ) or the presence of gas hydrate (within the GHSZ).

The characteristics of seismic and acoustic datasets presented in this study are different with respect to acquisition geometry, signal generation, lateral and vertical resolution, and depth penetration. They provide imaging results of different nature and on multiple scales, and they are highly complementary, even though an integrated interpretation is not straightforward. However, the key seismic and acoustic signatures characterizing the system are undisputed: (1) topographic and backscatter anomalies at the seafloor related to observed fluid venting, (2) patches of high reflection amplitudes beneath the pockmarks, that are related to increased reflection coefficients of a layer package within the proximity of the pockmark, (3) columnar amplitude blanking beneath patches of higher reflection amplitudes, and (4) a high reflectivity zone at greater depth, that inhibits further depth penetration and does not reveal a sharp base.

In order to consistently interpret seismic and acoustic features, additional information can be used. On one hand, seafloor sampling, coring and video observations allow to identify active venting areas and to relate shallow high amplitude reflections with the proved occurrence of gas hydrates and carbonates (the latter of which are not expected to occur deeper than a few meters sub-bottom depth). On the other hand, estimates of the expected depth of the base of the GHSZ help to distinguish between features likely related to free gas and features likely related to massive gas hydrates. Thus, constraints are provided for the upper and lower boundaries of the studied system. Furthermore, seafloor observations can likely be translated downward into the sediment section.

A simple conceptual model would describe free gas that is trapped beneath a low permeable boundary and beneath the GHSZ. Were the seal is broken and permeable pathways for vertical fluid transport exist, free gas rises into the GHSZ, where massive hydrates may be formed.

Gas hydrates filling pore spaces may significantly reduce permeability and may therefore clog the vertical pathway, forming a self-trapping intermediate reservoir. Gas may consequently migrate laterally into the more permeable layers of the section forming hydrate and thus increasing the seismic reflection coefficients of the respective layers. On the other hand, seismic energy is scattered at free gas bubbles within the transport channel between the gas reservoir below and the hydrate cap above.

However, this concept does not explain how the pockmarks are initially created, how gas can be transported through the GHSZ in order to form hydrates close to the seafloor, and how free gas can still exist within the GHSZ. As a hypothesis, venting is mostly driven by overpressured gas that was produced from decomposing hydrates during a pronounced subsidence event probably related to sub-bottom salt movement. During the rapid rise of fluids through pre-existing zones of weakness likely

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related to polygonal faulting or diapirism, more gas is released out of solution and an eruptive process is initiated. Collapsing of the seafloor may indicate predominant gas and fluid release but not much material transport.

After the eruption, gas pressure decreases and venting slows down or even completely stops for some time. Gas transported during the eruptive event transforms into hydrate close to the seafloor and clogs the feeder-channel of the pockmark. Free gas may still exist within a narrow zone beneath the pockmark if focused fluid upflow affects the temperature gradient and thus rises the base of the GHSZ or if the permeability of hemipelagic sediments is not sufficient to provide the required volumes of water in order to form gas hydrate. Self-accelerating upflow events may be repeated periodically if some mechanism exists to re-establish gas over-pressure.

4. CONCLUSIONS Sedimentation patterns within the study area indicate a massive subsidence event and subsequent rapid sediment fill that was likely associated with gas hydrate destabilization and enhanced degassing near the depression, which is now marked by the occurrence of numerous seafloor pockmarks.

3-D seismic data across these pockmarks indicate that the typical low-amplitude signature of opal-rich and water-rich sediments is superimposed by zones of high reflection amplitudes near faults and other potential fluid pathways. Since the lateral extent of these high amplitude zones is limited, a direct relationship to fluid and gas migration can be expected. A high amplitude patch with positive polarity is observed at 40-50 m sub-bottom depth and interpreted as a gas hydrate cap that plugs the feeder channel of a pockmark. The upflow zone at greater depth is characterized by amplitude blanking, indicating free gas bubbles that scatter seismic energy. A package of high amplitude reflector elements at 150-200 m sub-bottom depth suggests the presence of

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trapped gas beneath a low permeable layer package. This package is bent upwards at the vicinity of the pockmarks, probably indicating a deeper salt diapir that is associated with faulting and likely higher permeability above the diapir. However, the creation of pathways beneath the pockmarks is not yet completely understood. Preliminary results based on 3-D mapping of fault plane orientations suggest that faulting due to diapirism may be superimposed by zones of weakness within a regional fault pattern that probably is of polygonal structure or derives from salt uplift.

ACKNOWLEDGEMENTS Research project and Meteor cruises were partially funded by the »Bundesministerium für Bildung und Forschung« and »Deutsche Forschungsgemeinschaft«. The proponents greatfully acknowledge the competent support of the crew of R/V METEOR.

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Droz L, Rigaut F, Cochonat P, Tofani R. (1996): Morphology and recent evolution of the Zaire turbidite system (Gulf of Guinea). Geol. Soc. Am. Bull 108: 253-269.

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Hyndman RD, Wang K, Yuan T, Spence GD (1993): Tectonic thickening, fluid expulsion and the thermal regime of subduction zone accretionary prisms: The Cascadia Margin off Vancouver Island. J. Geophys. Res. 98: 21,865-21,876.

Jansen JHF, van Weering TCE, Gieles R, van Iperen J (1984): Middle and late Quaternary oceanography and climatology of the ZaireCongo Fan and the adjacent eastern Angola Basin. Neth. J. Sea Res. 17: 201-249.

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Africa, with emphasis on post-salt structural styles. In: Mohriak W, Talwani M (eds), Atlantic rifts and continental margins. AGU, Washington DC: pp 129-150.

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Rouby D, Raillard S, Guillocheau F, Bouroullec R, Nalpas T (2002): Kinematics of a growth fault/ raft system on the West African margin using 3D restoration. J. Struct. Geol. 24: 783-796.

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Geol. Bull. 57: 1679-1691.

Uenzelmann-Neben G (1998): Neogene sedimentation history of the Congo Fan. Mar. Petrol. Geol. 15: 635-650.

Valle PJ, Gjelberg JG, Helland-Hansen W (2001): Tectonostratigraphic development in the eastern Lower Congo Basin, offshore Angola, West Africa. Mar. Petrol. Geol. 18: 909-927.

Wefer G, Berger WH, Richter C et al.. (1998): Proc. ODP Init. Repts.; 175. College Station, TX (Ocean Drilling Program).

Zühlsdorff L, Spieß V, Hübscher C, Villinger H, Rosenberger A (2000): Implications for focused fluid transport at the northern Cascadia accretionary prism from a correlation between BSR occurrence and near-sea-floor reflectivity anomalies imaged in a multi-frequency seismic data set. Int. J. Earth Sciences 88: 655-667.

Zühlsdorff L, Spieß V. (2004): Three-dimensional seismic characterization of a venting sites reveals compelling indications of natural hydraulic fracturing. Geology 32: 101-104.

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Techniques and Instruments for Gas Hydrates Exploration and Research (TIGER) Degenhardt A. (1), Hanken T. (2), Helmke J. (3), Jaguttis J. (4), Masson M. (5), Poppen B. (6) (1) Technologie Transfer Zentrum Bremerhaven, An der Karlstadt 10, 27568 Bremerhaven, degenhardt@ttz-bremerhaven.de (2) iSiTEC GmbH, Stresemannstr. 46, 27570 Bremerhaven, thanken@isitec.de (3) Gaskatel GmbH, Holl채ndische Str.195, 34127 Kassel, helmke@gaskatel.de (4) de la Motte & Partner GmbH, Am St체b 10, 21465 Reinbek, P.W.delaMotte@t-online.de (5) Capsum Technologie GmbH, Technologiepark 24, 22946 Trittau, m.masson@capsum.com (6) JHK Anlagenbau und Service GmbH & Co. KG, Labradorstr. 5, 27572 Bremerhaven, Anlagenbau-und-service.poppen@jhk.de

1. Introduction The exploration of conditions in deep sea is still primarily restricted to purely scientific institutions. The commercial use of the sea for oil production or fishery has led to the development of simple and effective equipment. These exploration techniques already existing are not suitable for the observation of gas hydrate fields. If gas hydrate fields will be used as raw material or energy source in the future, these scientific equipments have to be adapted and manufactured for the industrial applications. In Japan and the USA the intensive investigation concerning the future use of gas hydrates has already started. Therefore the development of appropriate industrial investigation techniques can be expected, although appropriate activities could not be found by a literature research.

Major aim of the project was the development of a submarine experimental station for the collection of measuring data of sedimentary gas hydrates in the deep sea. Emphasis was placed on long-term data collection on the release and the behaviour of methane at the sea bottom under natural site conditions. A further aim was the integration of regional enterprises, preferably medium-sized companies, into this specific problem of the deep sea exploration in the preliminary stage of a potential utilisation of gas hydrates. From the linkage of research projects

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and economic interests on the one hand potentials for the development of marketable products result, on the other hand available knowledge from the industry can be used for research purposes. The project is divided into the following work packages: preparation of a feasibility study, status quo acquisition / investigation, development and laboratory test, testing phase / preliminary planning of the measuring station, development of a measuring unit suitable for the deep sea (prototype), first field tests and conclusion evaluation and reporting.

The prototype components of the measuring probe were developed in order to adapt it to the employment in a deep-sea measuring station.

The components were intensively tested in the laboratory and field. The results of the tests as well as the requirements of the individual parts among themselves were harmonised and led to the planning of the final prototype. After conclusion of the planning phase the individual units (methane sensor, data logger and pressure-resistant enclosure) were assembled into a fully functional test unit.

2. Objectives of the Project Major aim of the project is to align two aspects of maritime research and development. On the one hand the installation of a measuring sta-

tion on the sea bottom will provide valuable data for the evaluation of gas hydrate fields and the possibilities of their economic use. On the other hand small and medium-sized enterprises in North Germany shall be introduced to the specific problems of the exploration in deep sea and gas hydrate extraction. The knowledge collected in the basic research and the technology used offer a potential which should not be underestimated for the development of marketable products. In return valuable know-how from the industry is made available for the basic research. Originally a project duration of 10 years (in five subprojects) from the exploration to the development of a technology for the use of gas hydrate sources was planned. Devices and equipment concentrating on scientific research will be transferred into innovative products of maritime technology in global demand.

In the context of the further exploration of marine gas hydrate sources regarding a future use the project TIGER I (Techniques and of instrument for gas of hydrate exploration and Research, 03G0561 A), funded by the BMBF, pursues several aspects.

A partner consortium of medium-sized companies under the co-ordination and conceptual management of Technology Transfer Center Bremerhaven, Environmental Institute, was involved in this project. The enterprises involved were assigned to the different tasks as follows: The working group Empting - de la Motte was concerned with the overall feasibility, description and investigation of the development of equipment for the production of electricity from sea currents. In particular the special problem areas for the design and set-up of a prototype with regard to relevant boundary conditions was discussed and results with preview on the further treatment of the research project were presented.

Major task of the company iSiTEC was the development of the data collection system (data logger), which records and stores the measuring signals of the sensors attached and if necessary offers additional control functions.

The substantial requirements on the system

were: processing of analogue measurements, safe data storage with sufficient capacity, read out of the data and parameterisation by PC, low energy consumption, extended temperature range and employment in the sea waterresistant pressure housing until a depth of 2.000 m. In the first development stage the data logger will be employed in a test construction, in order to gain first experiences with the system by short-term measurements.

In the second stage extended functions will be realised, e.g. logging of several sensors, eventcontrolled recording, intelligent energy management, employment duration of up to 1 year, employment depth up to 3.500 m.

Capsum was responsible for the further development of the methane sensor. The original tasks were the reduction of the time constant and increase of responsivity. In the course of the project the increase of stability, the reduction of the energy consumption and increase of the employment depth, which is important for the long-term autonomous employment, were investigated.

Task of Gaskatel was the development of equipment for the production of electricity from fuel cells. This contains the search and type selection of the possible fuel cell types with critical examination of the own systems »low-temperature alkaline« and »high temperature phosphoric acid«, the examination of the existing cells on pressure strength, the examination and search of the electro-chemical reactivity of methane at the electrodes and possibilities of the thermal transformation of methane in hydrogen. Moreover Gaskatel was responsible for production and test of the prototypes after the defaults of performance, temperature, fuel and disposal.

JHK was responsible for the development of a pressure-resistant housing for the components of the deep-sea station as well as the conceptional cooperation for energy storing and selfsufficient energy distribution system. This contains the search, examination and representation of the state of the art, the listing of a requirement catalogue for the solution of the tasks set and evaluation, interpretation and

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interface planning as well as the prototype production and the execution of tests.

Impac Offshore carried out a feasibility study for a deep-sea station for measuring, taking up and collecting gas and gas hydrates.

The Technology Transfer Center Bremerhaven was the project co-ordinator, coordinating the tasks of the individual partners, merging the scientific know-how as well as securing and monitoring of the technology transfer.

3. Present Status and Results / Methods & Results / Results The work was split up in two steps; first step was a small test unit consisting of the methane sensor, the data acquisition unit and the pressure-resistant housing. In a second step the alternative energy supply was developed and the entire deep-sea measuring station was designed. The following paragraphs show the development achieved for the different components.

Methane sensor In the following table the obtained improvements are represented in comparison to the existing commercial version. The improvements of the individual parameter (independently from each other) are shown in the following table.

Different field tests in the river Elbe, in Norway and in the Mediterranean were carried out under normal oceanic conditions and during different seasons, which showed that a stability of at least 6 months without significant drift is possible. On laboratory scale 12 months could be reached, although some sensors fai-

led. Therefore main emphasis of future research is placed on the development of a fast method for the pre-selection of sensors for long-term employments.

Reduction of time constant The investigation of different detector types and electronic configurations showed that the minimum response time, i.e. the time until the first signal rise after a change in concentration, is within seconds. The T90-time, i.e. the time to reach 90% of the final signal lies between 1 and 5 min dependent on turbulences and/or incident flow. The decay time is between 3 and 15 min. These values apply to the full measuring range and are independent from the density gradient. By modifying the sensor head the response time could be improved. The gas volume behind the membrane was reduced significantly. The moisture sensor could be omitted, the membrane surface and the supporting structure were reduced, etc. The last investigations showed that the response time could be reduced further down to 45 seconds, which will most likely be the lowest limit that can be obtained without external devices like agitator or propeller.

Increase of employment depth In a first phase the employment depth could be increased from 2.000 to 3.500m, the corresponding pressure resistance was improved from 250 to 400 bar by an optimisation of the sealing, the housing design and manufacture as well as by the adaptation of the membrane carrier. In a second phase the sensor head design was improved, which was based on the

Table 1: Improvements of the methane sensor

Parameter Time constant Responsivity Electric power consumption 230 mA Pressure resistance 250 bar

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Commercial version 5 min 50 Nm 60-100 mA 494 bar

Improvement 1 min 10 Nm

reduction of the membrane area that is exposed to pressure. Moreover the carrier structure behind the membrane was changed. Static and dynamic pressure tests were successfully accomplished up to 494 bar.

Increase of responsivity A responsivity of 10 nmol/l in some cases even 3 nmol/l could be achieved by accurate adjustment of the heating temperature. However this strongly depends on the detector and detector type. The best reproducibility could be reached with gallium oxide detectors compared to the common tin dioxide detectors.

Further investigations are necessary in order to control all parameters, which affect the responsivity. E.g. the long-term interrelations between the responsivity and the aging of the detector are still not known.

Electric power consumption The power consumption partly depends on the electronics, but mainly on the detector and the heating of the active layer of the semiconductor, which in turn depends on the manufacturing quality. Depending on the manufacturer the power consumption is different. With the best detectors the power consumption for the complete sensor including electronics could be reduced to 60 mA with 12V DC power supply.

Other detectors use approx. 90 to 100 mA, which is still an improvement compared to a consumption of 230 mA by the original sensor.

By a redesign the electronics could be further improved. The indicated values apply to electronics with analogue outputs, the digitisation consumes additionally approx. 30 mA, whereof the A/D transducer consumes 10mA alone.

Operating mode In order to minimise the energy consumption and to allow an event-controlled operation a discontinuous operation of the sensor was assessed. Investigations showed that this affects the long-term stability of the detectors; moreover the switch-off of the heating poses the risk that water vapour condenses in the detector chamber. This condensation water can only

vaporise if the heating is switched on for a certain time. In the long term repeated switch off will lead to more and more condensation water, which would destroy the detector.

However this could not be approved during the tests. Nevertheless a complete disconnection of the electronics is not recommended; the detector heating and the processor control should remain switched on. The switch-off of selected components does not bring much per se (10 mA for the A/D converter), but nevertheless presents more than 10% of the energy consumption. The saving potential could be used for the event controlling of the sensor, whereby data storage capacity can be saved. A version of electronics was developed, which allows the switch on and off of the A/D converter from the outside by specific commands.

Accuracy The solubility of methane theoretically depends on the salinity. Nevertheless the methane sensor is usually calibrated with fresh water.

Only exception is the calibration with seawater at temperatures below 2°C. The effect of the salinity is actually small: at temperatures around 15-20°C the solubility between 0 and 30â&#x20AC;° changes by approx. 10%, i.e. under typical deep-sea employment conditions the measuring error of the methane sensor amounts to only few percent, which gets lost in the normal signal noise.

The general accuracy of the sensor lies between 10 to 30% of the measured value, whereby the worse values occur only sporadically, e.g. at a certain concentration and a certain temperature. The accuracy depends both on the manufacturing quality of the semiconductor detectors and on the calibration procedure.

The following steps have to be considered: Inaccuracy of the mass flow control, which control the gas mixture, temperature and incident flow fluctuations at signal recording as well as aging processes within the semiconductor during the calibration and finally fluctuations of the heating control of the semiconductor and the selection of the parameters setup of the calibration formula. An optimisation

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of the calibration is planned, which aims to increase the accuracy to 5 - 10%, a further improvement seems not realistic. On the one hand the semiconductors are primarily manufactured bulk product for applications, where no high accuracy is required. E.g. the semiconductors used are usually not produced in clean rooms; therefore fluctuations in the chemical composition and thus the characteristics emerge.

Cross sensitivity The cross sensitivity on H2S was examined in air. A concentration of 10 and 100 ppm H2S in air causes a suppressed cross sensitivity, which normally gives no signal. 300 ppm H2S in air gives the same signal as for 10 ppm methane.

Starting from 1% H2S in air the detector breaks down and can possibly be regenerated, although this is more expensive than a simple exchange. The cross sensitivity on hydrogen is clearly visible for concentrations in the range of mol/l. For applications, where methane and hydrogen are coexisting in very high concen-

trations, the signal allocation is more difficult.

Future research and development should target this problem.

Data acquisition In particular the reliable function of the device (storage of the data) is relevant for the employment in a long-term system. It should be ensured that data is not lost during a system failure. A further important point is the realization of energy management functions in order to reduce the energy consumption between the measurements (sleep mode) as well as the shut-down of parts of the system in the case of low energy resources or a failure of the main power supply. Moreover the necessity of an additional stand-by battery / power pack in case of system failure needs to be assessed.

The data acquisition unit essentially consists of two basic components: the controller module and the basis module.

Table 2: Properties of the data acquisition unit

Employment duration Employment depth Number of methane sensors Measurement interval Measurement duration (average) Operation mode Energy supply Power consumption External sensors

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First development step Up to 4 weeks Up to 2000 m 1 1 measurement per hour (variable)

Second development step Up to 1 year Up to 3500 m 1-2 (optional more) 1 measurement per hour (variable)

1 minute (variable) Stand-alone / sleep mode event controlled Internal 0,3 W CTD (optional)

1 minute (variable) Stand-alone / sleep mode / Internal and external 0,3 W CTD and other (optional)

The data acquisition unit essentially consists of two basic components: the controller module and the basis module.

Figure 1: Controller module

Figure 2: Basis module

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Controller module The controller module was manufactured in SMD execution, in order to place an inexpensive, efficient space saving module on smallest area. This module already contains the substantial, for most applications sufficient, functions.

Table 3: Properties of the controller module

Frequency EPROM RAM I/O channels Up to 32 A/D-converter D/A-converter Interfaces Time / calendar Sensors Dimensions

Up to 25 MHz (optional: 100 MHz) 64 kB Flash 4 kB (optional: 8 kB RAM)

12 Bit resolution, up to 8 channels, 100 ksps 8 Bit resolution, up to 8 channels, 500 ksps 12 Bit resolution, 2 channels 2 x RS232/TTL, SPI, SM-BUS, MCU-BUS RTC with calendar function and backup-battery Temperature (internal) Approx. 40 x 50 x 10 mm

Basis module The standard controller module is attached to the custom-designed basis module. Here the periphery components like power supply, plug connections, interfaces, memory expansions, A/D converters and signal adaptation are integrated according to the application.

Specifications of the basis module:

Table 4: Properties of the basis module

Power supply Plug connection Interfaces Data memory A/D-converter I/O functions Analogue signal range 0-2,5V (optional: 0-5V, 0-10V, 0-20mA, 4-20mA)

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DC-DC converter 18 - 36V DC Terminals, strips RS232 (optional: RS 485, USB, Ethernet) SD-Card (up to 512 Mbytes capacity) 24-Bit resolution, 6 sps (optional: 16 Bit, 1ksps) REED-switch for proximity activation Relay / MOSFET output

This results in the following features of the entire data logging unit: Table 5: Properties of the entire data logging unit

Power consumption Operational temperature range Dimensions

Approx. 300 mW -20 to 50째C Approx. 60 x 140 x 20 mm

Figure 3: Fuel cell design

Energy supply by fuel cell Due to the findings from the previous planning of the power supply by fuel cells the main part of the system would be an Alkaline Fuel Cell (AFC). The electrolyte of an AFC is usually made of 30% caustic potash dilution. The reaction water dilutes the electrolyte of the AFC. The discharge of water in a deep-sea measuring station has to be realised via diffusion. The fact that the density of caustic potash solution increases with its concentration can be utilised.

The common working temperature range of an AFC is 60 - 80 째C. If a caustic solution with a concentration of 30% is used the fuel cell can work far below 0 째C, although the AFC has to be bigger at lower temperatures. The AFC is dimensioned in such a way that it sup-

plies a minimal voltage of 5 V and a current of 1 A at a temperature of 4 째C. Hydrogen and oxygen are used as primary energy sources, which are stored as high-purity gases in compressed gas cylinders. The system has to be placed in a pressure resistant housing and operated at atmospheric pressure.

At the given performance data and an efficiency of the AFC of approx. 60% one needs 30.000 l of hydrogen. At a cylinder pressure of 300 bar this means a volume of 100 litres.

15.000 l (corresponds to 50 l at 300 bar) of oxygen are needed to oxidise this amount of hydrogen. Thus altogether three 50 l-bottles are needed, in order to store the appropriate gas masses, which have a net weight of approx. 225 kg.

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The reaction water resulting from the alkaline fuel cell is taken up by the electrolyte (KOH) and dilutes it. In order to ensure a safe operation of the AFC, the KOH concentration must lie between 30% and 10%. Since a discharge of the water is not possible, an adequate amount of KOH solution has to be carried along to prevent that the concentration falls below the minimum. 11 kg KOH are sufficient to take up 19,27 kg of arising water.

The fuel cell stack should consist of 7 single cells, which provide a minimal voltage of 5 V if serially connected. The stack is placed in a container filled with KOH, which takes up the reaction water, too. The container is built as a pressure tank and coupled with the gas pressure. The caustic solution can flow against the electrodes freely; a pump is not needed. The reaction water is discharged from the cell by diffusion.

The stack is operated in dead-end mode on the gas side, i.e. the stack does not have a gas outlet. Thus the problem of a contamination of the gases has to be tackled. This will be solved by placing a container for the inert gases at each outlet of the stack, which prevents the hydrogen and oxygen concentration from falling below 50 vol%. With a hydrogen purity of 6.0 (99.9999%) and an oxygen purity of 5.5 (99.9995%) a container of 21 l is required in order to prevent the cell from damage.

Pressure reducers are used on the gas input side in order to regulate the cylinder pressure to approx. 0,5 bar positive pressure. The system has been designed in a simple way as in the past systems, which work with pumps, and valves caused trouble that led to system malfunction.

Moreover the power requirements of the system itself can be reduced to 0 and therefore reduces the gas volume needed.

The different components of the power supply unit are not high pressure resistant and have to be placed in a pressure resistant housing.

Energy supply by sea currents The enhancement of the efficiency is a very important factor for the use of a sea current plant as power unit for the deep sea measuring station, i.e. existing losses have to be minimised

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and the locally available current has to be used in an efficient way.

For a rotor diameter of 10 m flow rates of at least 2-2,5 m/s are needed. Although the energy demands of the station are quite low the rotor diameter has to be between 3 and 9 m under prevailing flow rates of only 0,1 to 0,2 m/s above the sea bottom.

Single rotors supply higher energy yields compared to several small rotors. Furthermore shadowing effects occur if several rotors are used.

In contrast bigger rotors have a higher inertia as the constructions are heavier and more compact. An independent start-up assuming flow rates of 0,1-0,2 m/s and rotor diameters up to 9 m appears technically not feasible, therefore the optimal ratio between plant size and plant number has to determined in tests.

Pressure-resistant housing The test unit was built to allow the examination of gas hydrates by the methane sensor and the corresponding data acquisition unit. The data acquisition unit including the power supply were integrated in a pressure pipe. During the design phase emphasis was put on the weight reduction. For this reason titanium was chosen as pipe material. The pressure pipe is suitable for an employment depth of up to 6000 m. Pressure pipe and methane sensor were fastened to a framework. The framework consists of square pipe 1.4571. Next step will be the development of a pressure housing for additional devices.

One of the main technical problems of the exploration and future extraction of gas hydrates is the vaporisation of the gas during outcrop. Due to rising temperatures and decreasing pressure the gas discharges from the icy structure of the gas hydrates. In order to prevent this, equipment has to be developed which keeps the surrounding of the gas hydrates stable, especially during outcrop.

In the basic set-up the material to be examined (later outcropped) is placed in a chamber of 14 litres made of plexiglass, which is placed itself in a pressure housing filled with tap water. The bottom of this housing has both feed-through bores as well as ports. Thus the inside and the outside chamber can be accessed from outside.

This is necessary since pressure and temperature are controlled via the tap water and the inner chamber has to be accessed for taking samples.

Many investigations, which have to be accomplished with this equipment, are based on the sampling from the inner plexiglass chamber.

Partly these samples shall be examined under pressure in a separate external chamber (by pressure-resistant sensors or optical techniques).

The samples can partly be released by a throttle valve and submitted to analysis. Main aim is to design and build a universal device, with enables both types of sampling. Hereby a multitude of boundary conditions have to be considered.

Test system (first development step) The first development step of the data acquisition unit was integrated in a test structure in order to gain first experiences with the different system components. This test structure consists of the methane sensor, the data acquisition unit, the pressure housing and the autonomous power supply with battery cells. The following pictures show the test unit (first development step) before its first employment on the research ship Polarstern.

Field testing During a joined AWI-IFREMER cruise (19th of June to 26th of July 2003) with RV Polarstern,

Figure 4: Test system

Figure 5: Data logger and battery cell of test system

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equipped with the Remotely Operated Vehicle (ROV) Victor, the Haakon Mosby Mud Volcano was investigated. The HMMV is located at the Barents Sea continental margin between Norway and Svalbard in a water depth of ~1280 m.

One of the research topics was the measurement of CH4 in the lower bottom water. For this purpose the METS CH4 Sensor (Capsum, Geesthacht) and the data logger developed by ISITEC (Bremerhaven) was applied.

Due to the diving schedule of the ROV it was not possible to deploy the lander system, developed in cooperation with the partners of the project TIGER, at a single spot over a period of several days, since the recovery could not be assured. Nevertheless, the deployment of the CH4 sensor during ROV surveys covering large parts of the Mud Volcano worked successful.

For this purpose the sensor was installed in the basket of the ROV. The data logger was pre-programmed to measure in 5 minute intervals over the 48 hours ROV dive. The dive covered large parts of the Mud Volcano including spots where free gas was released from the seafloor. The stability of the Sensor system including the energy consumption provided a very interesting data set.

The spatial pattern of the recorded CH4 signal shows, although not in every detail, a similarity with the pattern of CH4 concentration derived by Hydrocast and measurements by Gas-Chromatography onboard ship and visual observation of active sites of gas release. Improving technical aspects as the incoming flow reaching the membrane of the Sensor and decreasing memory effects the Sensor-Data Logger System seems to be suitable for monitoring the spatial distribution of CH4 concentrations in deep sea environments enriched in CH4 as mud volcanoes or seep sites.

The track lines of ROV Victor during a survey of 48 hours covering large parts of the Haakon Mosby Mud Volcano were recorded. The signal measured by the METS Sensor was transferred to geographic coordinates via the time step of the data logger and of the ROV (synchronized prior to deployment). By application of GIS the data were mapped on the bathymetric chart of HMMV.

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4. Conclusions The different components of the prototype unit were developed in order to adapt them to the employment in the deep-sea measuring station. The component units were subject to intensive laboratory and field tests. The results of the tests as well as the requirements of the individual parts among each other were harmonised and led to the planning of the prototype. After completion of the planning phase the different components (methane sensor, data logger and pressure housing) were assembled into a fully functional test unit.

For the methane sensor the time constant could be reduced to approx. 45 seconds by the modification of the sensor head. This will probably be the lowest limit, which can be achieved without additional external devices like agitators or propellers. Attempts with different membranes (e.g. thinner membrane, which theoretically has a higher gas diffusion rate), did not lead to the anticipated success. The pressure resistance of the sensor could be increased in a first phase from 250 to 400 bar, which corresponds to an increase of employment depth from 2000 to 3500 m. After further modifications static and dynamic tests up to 494 bar were carried out successfully. Moreover the sensitivity was increased up to 3 nmole/l. However further tests will be necessary to define the detection limit.

The modular data acquisition unit developed is a multifunctional data logger using the latest energy-saving components as well as commercial memory cards (SD Card), which fulfils as well intelligent control tasks. First of all the unit can be employed in scientific-technologic systems under harsh site conditions and at extreme temperatures (e.g. deep sea, polar areas). The hard and software integration of the memory card and its function as safe mass storage offers the application for various acquisition and control problems in the area of microcontrollers. Particularly good experiences were made with the trouble-free operation of these technologies under extreme conditions.

A fuel cell was recommended for the power supply of the deep-sea probe TIGER. The power

output will be a current of 1 A at a voltage of 5 V. The operating temperature is 4 °C and the operating lifetime is 8760 hours. The storage of the reaction gasses should be done in pressure vessels. They should be of good purity.

Commonly used gas qualities and pressure vessels fit to these requirements. Different fuel cell systems were discussed as power supply. The system that fits best to the requirements of the TIGER power supply is an alkaline fuel cell system with simple peripherals. It was shown, how such a system could be realised. It was also shown that the AFC is able to work under the given conditions. The electrodes that were used for the first time under these conditions showed sufficient results. It should be possible to reduce the active area of the electrodes to 100 cm2 by optimisation.

A prototype system fitting to the power output needs should be built and tested in a long-term range to prove the reliability of the system.

A pressure resistant pipe for the incorporation of the methane sensor, the data logger and the power supply was developed and manufactured. During the design emphasis was placed on a weight minimisation. For this reason titanium was used for the pipe material. The pressure pipe can be used in depths of up to 6000 m.

Pressure pipe and methane sensor were fastened to a base frame made of steel 1.4571.

In order to start with the first tests as early as possible, the test unit was supplied by conventional batteries. In parallel to the development of the prototype a power supply by fuel cell, planned for the future deep sea measuring station was developed. Concerning the fuel supply of the cell the further development of established processes as well as the use of alternative sources in the application area of the deep-sea measuring station were considered. The consideration of local fuel extraction partly led to interesting approaches, but has not proved satisfactory during the project duration and was therefore rejected.

Further studies concerning possible alternatives to the power supply by fuel cells using resources available in the surrounding of the deep sea measuring station (utilisation of sea currents,

salinity, etc.) could not be realised within the project / could not guarantee an error-free operation of the deep sea measuring station due to insufficient information on the local condition in the deep sea. However there are interesting approaches, which could be further investigated in subsequent projects.

The prototype unit was tested on a cruise of the research ship Polarstern during a period of several days in the area of the Haakon Mosby Mud Volcano (Norway). Initial results could be obtained, but further research is required to prove the deep sea measuring station and its components.

Acknowledgements This research and development project was supported by the BMBF (reference number 03G0561A).

Special thanks go to Prof. Schlüter from the Alfred-Wegner-Institut, who has supported the project especially during the field testing phase.

References Duan et al. (1992) »The prediction of methane solubility in natural waters to high ionic strength from 0 to 250°c and from 0 to 1600 bar«, Geochimica et Cosmochimica Acta, 56, pp1451-1460 Energie Perspektiven, Ausgabe 4 /2003 www.freenet.de/Freenet/Wissenschaft/Innovat ionen/Hightech www.ipp.mpg.de/ippcms/de/pr/publikationen Marine Current Turbines Ltd., Präsentationen zu Seaflow Marine Current Turbines Ltd.– Press Release Yamamoto et al (1976) »Solubility of methane in distilled water and seawater«, Journal of Chemical Engineering Data, 21(1), pp78-80

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Detailed seismic study of a gas hydrate deposit at the convergent continental margin off Costa Rica – DEGAS Müller C., Bönnemann C., Neben S. Bundesanstalt für Geowissenschaften und Rohstoffe (BGR), Geozentrum Hannover, Stilleweg 2, D-30655 Hannover, Germany, E-Mail: Boennemann@bgr.de

ABSTRACT In previous studies, amplitude variation with offset (AVO) analysis and waveform inversion techniques have been applied to determine qualitative or quantitative information on gas hydrates and free gas in the sediment.

However, the quantitative contribution of gas hydrates to the acoustic impedance contrast observed at the Bottom-Simulating Reflector (BSR) and the reliability of quantitative AVO analyses are still topics of discussion. In this study, common midpoint gathers from multichannel wide-angle reflection seismic data acquired from offshore Costa Rica were processed to preserve true amplitude information at the BSR for a quantitative AVO analysis incorporating incidence angles up to 60°. Corrections were applied for effects that significantly impact the observed amplitude such as source directivity. AVO and rock physics modelling indicate that free gas immediately beneath the gas hydrate stability zone can be detected and low concentrations can be quantified from AVO analysis, whereas the offset dependent reflectivity is not sensitive to gas hydrate concentrations of less than about ten percent at the base of the gas hydrate stability zone.

Patchy BSRs are observed southeast of the Nicoya Peninsula on the continental margin offshore Costa Rica. AVO analysis indicates that this phenomenon is related to the presence of free gas saturations less than 5% beneath the gas hydrate stability zone, pro-

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bably related to focused vertical fluid flow. In areas without BSRs, the results indicate that no free gas is present.

INTRODUCTION Research on gas hydrates is of worldwide scientific and industrial interest. Gas hydrates are studied to prevent their formation in natural gas pipelines. In offshore gas exploration and production, gas hydrates increase the risk for shallow gas blowouts and submarine slumps. Paleoclimate research suggests that methane from gas hydrates played an important role in climate change (Kennett et al., 2003). Finally, gas hydrates are a potential energy resource for the future (Collett, 2000).

The importance of gas hydrates to climate change and as a potential future energy resource depends strongly on the worldwide amount of methane trapped in gas hydrates or accumulated as free gas beneath the gas hydrate stability zone (GHSZ). Although estimates of the global amount of methane in hydrates are speculative, there is general agreement that the quantities are very large (Kvenvolden and Lorenson, 2001).

Most oceanic occurrences of gas hydrates are inferred from observations of BSRs on marine seismic reflection profiles. The BSR coincides with the base of the GHSZ. BSRs generally mark the interface between higher P-wave velocity (hydrate bearing sediment) and lower P-wave velocity (free gas-bearing sediment).

The quantitative relationships between parameters derived from seismic investigation of gas hydrates, and the in situ quantity of gas hydrates and free gas in the sediment are still uncertain and define one of the outstanding research problems.

Under the scope of the German research and development programme GEOTECHNOLOGIEN and it’s research theme »Gas Hydrates in the Geosystem« the present study focuses on quantification of the amount of gas hydrates and free gas trapped inside an isolated gas hydrate patch, outlined by the areal extent of the BSR in reflection seismic sections. No wells are present in the area under investigation, located southeast of the Nicoya Peninsula offshore Costa Rica (Fig. 1). Determining the gas hydrate concentration and free gas saturation in the sediments from an amplitude variation with offset (AVO) analysis was a major objective of this study.

There is still controversy about the contribution of gas hydrates at the base of the GHSZ to the observed negative impedance contrast at the

BSR (Collett, 2001). The accuracy of quantitative results from AVO analysis is not yet proven (Cambois, 2000), and widely used empirical relationships for the elastic moduli of rocks (e.g. Hamilton, 1979) are often inappropriate for quantitative studies. Therefore, this study treats quantitative AVO analysis with considerable emphasis on the open question about the contribution of the gas hydrates to the impedance contrast at the BSR.

NATURE OF THE BSR While seismic reflections generally are caused by impedance contrasts due to lithology changes, the BSR is caused by a thermobaric transition from solid gas hydrates in the sediment pore space within the GHSZ to free gas in the pore space beneath the GHSZ. Therefore, the BSR mimics the seafloor reflection and often crosscuts the stratigraphy. Although scientific drilling and seismic studies agree on the dominant effect of free gas on the formation of the BSR

Figure 1: Offshore Costa Rica the oceanic Cocos Plate is subducted underneath the continental Carribean Plate at the Middle America Trench (MAT). Seismic lines from cruise SO81 (green lines) and BGR99 (black lines) are indicated as well as the 3-D survey area of cruise BGR92-3D. An overall BSR distribution of about 7700 km2 has been inferred from BGR99 data (blue areas). The area under investigation is represented by the BGR92-3D box.

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(Bangs et al., 1995), there is still debate about the quantitative contribution of gas hydrate to the observed negative impedance contrast (Collett, 2001; Diaconescu et al., 2001).

The formation of gas hydrates in the subsurface can basically be described by two models (Hyndman and Spence, 1992): In the first model (Model A), the methane that goes into forming the hydrate is assumed to be generated in situ from organic material. This model has free gas being generated within and beneath the GHSZ. It is assumed that if enough methane is present to form substantial amounts of hydrate within the GHSZ, there should also be enough for substantial amounts of free gas beneath the GHSZ, and this excess gas migrates upward and forms the BSR. In the second model (Model B), hydrate forms from methane that is removed from rising pore fluids expelled from deeper in the sedimentary column. The highest concentrations of gas hydrate are expected at the base of the gas hydrate stability zone. In Model B it is not necessary that free gas is present beneath the GHSZ. The Ocean Drilling Program, usually accompanied and followed up by reflection seismic studies, found evidence for both of the above models.

ODP Leg 164 on the Outer Blake Ridge, offshore South Carolina, drilled two holes (site 995 and 997) through a pronounced BSR in an area of prominent acoustic blanking and strong lateral changes in BSR reflection amplitude (Matsumoto et al., 1996). Sonic logs measured in these holes show a significant decrease in compressional wave speed beneath the BSR indicating the presence of free gas to at least 250 m beneath the BSR (Dickens et al., 1997; Helgerud et al., 1999). A third hole (site 994), where a BSR is absent, shows a less pronounced decrease in velocity, probably due to the absence of significant amounts of free gas beneath similar gas hydrate concentrations (Lu and McMechan, 2002). Holbrook et al. (1996) confirm these sonic log observations from seismic traveltime inversion studies that indicate a significant decrease in seismic velocities directly beneath the BSR at site 995 and 997. This

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study also concluded, more generally, that both methane hydrate and free gas exist even where a clear BSR is absent.

A BSR is regionally distributed throughout much of the Chile Triple Junction region.

Downhole temperature and logging data collected during ODP Leg 141 suggest that the seismic BSR is generated by low seismic velocities associated with the presence of a few percent of free gas in a ~10 m thick zone beneath the GHSZ (Bangs et al., 1993; Brown et al., 1996), indicating that the formation of the BSR is dominated by the presence of free gas.

Offshore Peru (ODP leg 112), a prominent BSR is observed on most of a reflection seismic line along with a significant weakening of the BSR reflection amplitude close to its landward termination. Pre-stack waveform inversion suggests the main contribution to the strong BSR comes from a free gas layer and low P-wave velocities rather than elevated velocities caused by the presence of gas hydrates at the bottom of the GHSZ (Pecher et al., 1996a). However, waveform inversion also indicated that the weak BSR reflection amplitudes at the landward termination of the BSR were mainly caused by elevated P-wave velocities at the base of the GHSZ suggesting very little, if any, free gas is trapped beneath the GHSZ there (Pecher et al., 1996b).

Multichannel reflection seismic data were analyzed from an area with a clear BSR on the northern Cascadia subduction zone margin off Vancouver Island. Results from AVO analysis, high-resolution velocity analysis, and modeling of vertical-incidence data indicate the formation of the BSR is related to a 10 to 30 m thick high-velocity layer immediately above the BSR, without any seismically detectable free gas beneath the BSR (Hyndman and Spence, 1992). On the other hand, Singh et al. (1993) concluded from waveform inversion that the BSR in this region is formed by hydrate-bearing sediment overlying gas-saturated sediment.

Analyses of data from two drill sites from ODP Leg 146 offshore Vancouver Island confirm the main contribution from a free gas layer to the observed BSR (MacKay et al., 1994).

Like any conventional gas reservoir, a reservoir model with substantial amounts of free gas requires an effective top seal. However, the lithology at the base of the GHSZ on continental margins is often homogeneous due to weakly consolidated sediment without structural traps. In this case, only the gas hydrate in the pore space contributes to a significant reduction of the vertical permeability, and thus acts as a trap under which methane can accumulate (Kvenvolden and Barnard, 1983). However, the quantitative contribution of the gas hydrate to the contrast in acoustic impedances at the BSR remains unclear.

Free gas in the pore space is indicated when seismic velocities are below the background velocity trend for brine saturated sediment.

AVO analysis offers a second tool to detect free gas, if data quality enables the estimation of the contrast in Poissonâ&#x20AC;&#x2122;s ratio. A decrease in Poissonâ&#x20AC;&#x2122;s ratio across an interface can produce a significant AVO anomaly.

The present study addresses the influence of gas hydrates at the base of the GHSZ to the observed acoustic impedance contrast. For this purpose, an AVO study on high-resolution long offset reflection seismic data from offshore Costa Rica was carried out.

THE STUDY AREA The study area is located on the convergent continental margin offshore Costa Rica, where the oceanic Cocos Plate is subducted underneath the continental Caribbean Plate. The margin consists of a margin wedge covered by slope sediments, underthrust by trench sediments, and is fronted by a small accretionary prism. The dominant structural feature is the buried margin wedge, which is a wedge-shaped unit with relatively high seismic velocities of more than 4 km/s. Offshore Nicoya Peninsula wide-angle seismic data indicate a landward increase of margin wedge velocities up to more than 6 km/s (Christeson et al., 1999). The impedance contrast between lowvelocity slope sediments and the margin wedge creates a pronounced seismic reflec-

tion, often referred to as the rough surface or BOSS (bottom of slope sediments) reflector.

Since ODP-Leg 170 (Kimura et al., 1997) it is generally agreed that the margin wedge is composed of older oceanic igneous and associated sedimentary rock. The structure of the margin was modified during the subduction of oceanic crust near the Cocos Ridge and the seamount segment along the northern flank of the ridge as observed on deep reflection seismic data by Hinz et al. (1996).

Scientific drilling offshore Costa Rica The first coring on the Costa Rica Margin was at Site 565 on Deep Sea Drilling Project (DSDP) Leg 84 (von Huene et al., 1985). The presence of gas hydrates and the stickiness of the mud in the upper part of the section ended drilling at this site before the primary objective (sampling the high-amplitude reflection below the sedimentary apron) could be accomplished.

Gas hydrates were recovered at 285 and 318 m below sea level. During this leg, a 1.05 m long core of massive hydrates was recovered off Guatemala (Kvenvolden and McDonald, 1985) and downhole well-logging suggest the presence of a 15 m thick hydrated zone containing a 4 m thick nearly pure hydrate section.

During Ocean Drilling Program (ODP) Leg 170 in 1996, five holes were drilled in the vicinity of DSDP site 565 (Fig. 1) to determine mass- and fluid-flow paths through a well-constrained accretionary complex and calculate mass and fluid balances. Shipboard results (Kimura et al., 1997) demonstrated that the Costa Rica margin did not undergo frontal accretion and that the bulk of the small, deformed sedimentary wedge at the toe of the margin was not formed by scraping off the incoming sediment from the Cocos Plate. Gas hydrates were found at site 1041, concentrated between ~100 and 280 m below seafloor, the zone of highest total organic carbon (TOC) content.

The primary mode of gas hydrate occurrence is disseminated, as indicated by the almost constant salinity in this depth interval, with thin sheets of gas hydrate filling microfractures.

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Below about 280 m below seafloor, just beneath a lithologic transition, the concentration of volatile hydrocarbons are highest and then begin to decrease downhole (Kimura et al., 1997). In 2002, during ODP Leg 205, another three holes were drilled in this area. The scientific objectives for this leg were to study seismogenic zone and subduction factory questions.

BSRs and gas hydrates offshore Costa Rica In addition to scientific drilling, gas hydrates have been inferred from the presence of BSRs along the convergent continental margin of Costa Rica as reported by several authors (Pecher et al., 1998; Shipley et al., 1979; Yamano et al., 1982).

In 1999, the German Federal Institute for Geosciences and Natural Resources (BGR) carried out a high-resolution reflection seismic survey from offshore Costa Rica. One objective of this cruise was to map the overall gas hydrate distribution deduced from BSR occurrence along the convergent margin off Costa Rica.

Results show a total area of about 7700 km2 underlain by BSRs (Fig. 1) with different characteristics regarding occurrence, reflection strength, and depth below the seafloor that indicate a large variability of heat flow and likely a strong variability of fluid flux through the sediment. These variations are related to the different oceanic crustal segments subducting under Costa Rica. In the northernmost area NW of Nicoya Peninsula BSRs are continuous in water depths from 700 to 2900 m (Fig. 1). In the area of ODP Leg 170, no BSRs were found in the seismic data. The absence of BSRs might be explained by the subduction of oceanic crustal segments of different ages, leading to differences in the thickness of the gas hydrate stability zone. North of the paleo plate boundary (PPB) about 1 m.y. older, and thus oceanic crust is subducted cooler, (Barckhausen et al., 2001), and therefore the thickness of the GHSZ should be considerably larger than south of the PPB. The GHSZ is probably thicker than the sedimentary column over the margin wedge and thus no free gas is

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present to generate a pronounced BSR. In a previous project, BSRs were mapped in depth in the BGR92-3D box (Fig. 1). These BSRs show a small-scale patchy distribution (Hinz et al., 1999). In the area of the proposed IODP-drilling sites NW of Osa Peninsula, BSRs are found at shallow subbottom depths, which can be related to the influence of the subduction of the igneous Cocos Ridge and the younger and thus warmer oceanic crust under Osa Peninsula.

FIELD DATA The second objective of the BGR99 cruise was to collect high resolution profiles over the previously mapped 3-D box (Fig. 1) for a detailed analysis of the BSRs. Core data from site 1041 (ODP-Leg 170) were used for rock physics modelling.

Reflection seismic data For this part of BGRs 1999 survey, the volume of the tuned airgun array was reduced from 3800 in3 (62.3 l) to 2000 in3 (32.8 l) at a towing depth of 5 m for both the array and the streamer. This produced a much broader spectrum with greater energy at higher frequencies and a center frequency of about 60 Hz (Fig. 2).

The acquisition parameters were set to 25 m shotpoint interval, 1 ms sample rate and 7 s record length. In total 420 channels, at an increment of 12.5 m, with a maximum offset of 5225 m were recorded on profiles BGR9959 to BGR99-69, resulting in approximately 550 km of 105-fold data.

A section of the central seismic line BGR99-60 in the area under investigation, with a clear BSR is shown in Fig. 4. The thickness of the gas hydrate stability zone increases seaward due to higher pressure and lower temperature at the seafloor. The landward termination of the BSR is observed near shotpoint 2400, where the depth of the BSR below seafloor decreases rapidly. On a parallel line five kilometres from this line, the BSR is very weak and difficult to identify as shown in Fig. 5 (line BGR99-61).

The weakness or absence of the BSR is an example of the previously described patchy

Figure 2: Amplitude spectrum calculated for five CMPs from line BGR99-60. The centre frequency is at about 60 Hz

occurrence. Whether gas hydrates are absent or the concentration of gas hydrate and free gas saturation is too low to be recognized as a BSR cannot be determined from stacked seismic sections alone. A slump is observed in this section between shotpoint 1340 and 1380.

The head scarp of the slump coincides with the minimum water depth for gas hydrate stability of about 580 m for a methane-seawater system in this area, indicating mass wasting related to the destabilization of gas hydrates.

The weak BSR might be explained by the escape of free gas from beneath the GHSZ that led to a significant reduction in the acoustic impedance contrast. A similar process was suggested by Delisle & Berner (2002). They observed numerous gas seeps at water depths less than the minimum water depth for hydrate stability in the Makran accretionary prism and proposed that the gas hydrate layers act as an effective cap rock to upward-directed flow of fluids containing significant amounts of gas.

Well data In the 3-D box area, no information from wells is available. Therefore elastic parameters and physical rock properties determined at site 1041 were used to model the AVO response of

shallow unconsolidated sediments at the BSR (Fig.6). The mineralogical composition and the porosity versus depth measurements on extracted core specimens provided parameters for an effective medium theory (EMT), described below, to calculate the P-wave velocity and density versus depth. These modelled parameters fit laboratory measurements of the p-wave velocity (PWS3) and the density measurements on extracted core specimens (Kimura et al., 1997) at site 1041 reasonably well (Fig. 6). The EMT was also used to calculate the S-wave velocity that was not available for any of the sites drilled offshore Costa Rica.

METHODS Offset-dependent reflectivity Ostrander (1984) showed that the reflection coefficient from gas saturated sands varies in an anomalous fashion with increasing offset and used this anomalous behaviour as a direct hydrocarbon indicator. This work popularized the methodology known as amplitude variation with offset analysis (AVO). The anomalous behaviour of reflections from gas saturated sediments can be explained from Gassmannâ&#x20AC;&#x2122;s (1951) equations, that predict a significant de-

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Figure 3: The area under investigation is located on the middle slope of the convergent continental margin offshore Costa Rica, southeast of the Nicoya Peninsula. Eight wide-angle reflection seismic lines have been acquired in 1999, located in a 3-D survey area from 1992 (15 x 30 km2) in order to study a gas hydrate deposit using pre-stack seismic methods.

crease in P-wave velocity and a small increase in S-wave velocity (due to the decrease in density) when even a small amount of gas is present in the pore space. This decrease changes the Pwave reflection coefficient and causes a decrease in Vp/Vs ratio and thus in Poissonâ&#x20AC;&#x2122;s ratio, that results in an AVO anomaly. AVO analysis provides information about the S-wave velocity or Poissonâ&#x20AC;&#x2122;s ratio contrast from P-wave data. In exploration geophysics this method is applied qualitatively to look for AVO anomalies that might indicate the presence of hydrocarbons.

The angle-dependent reflection coefficient at an interface separating two semi-infinite isotropic elastic media is fully described by the Knott and Zoeppritz equations (Knott, 1899; Zoeppritz, 1919), which are given in concise matrix form by Aki & Richards (1980). The reflection coefficient at any given angle of incidence is completely determined by the density and the P-wave and S-wave velocities of each medium. Therefore, the major task in AVO analysis is to restore relative amplitudes (AVO processing) in seismic pre-stack records before extraction and interpretation of the amplitude information.

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The effective medium theory The angle-dependent reflection and transmission coefficients at an interface separating two semi-infinite isotropic elastic media are described by the Knott and Zoeppritz equations, which are functions of the elastic properties of the sediment. These elastic properties depend on the physical properties of the sediment, e.g. the mineralogy, the porosity, the effective pressure, and the type of pore fluid. For gas hydrates, P-wave velocities for hydrate bearing and gas-saturated sediments that have been determined from logging vary significantly with location, gas hydrate concentration and gassaturation. For example, Prensky (1995) found that P-wave velocities for hydrate-bearing sediments range between 2.1 km/s and 4.5 km/s.

In many cases, reflection seismic studies lack in situ-information about the seismic velocities.

When available, well logs often provide only Pwave velocities. Shear-wave velocities are usually unavailable. Therefore, for AVA and waveform inversion modelling, empirical relationships for calculating propagation velocities are often required, e.g. Hamilton (1979) for Pwave velocities and Castagna et al. (1985) for S-wave velocities (Adriansyah and McMechan,

2001; Pecher et al., 1998; Tinivella and Accaino, 2000). These methods do not consider in-situ physical properties of the sediment, and thus provide only a rough estimate of gas hydrate concentration and free gas saturation.

Dvorkin et al. (1999) developed an effective medium theory that allows to calculate the elastic moduli of shallow unconsolidated marine sediment from mineralogy, porosity, effective pressure, and pore fluid compressibility. The model assumes that the modulus-pressure behaviour of the sediment at 36 to 40% porosity (critical porosity) is described by a dense random pack of identical elastic spheres. The effective bulk and shear modulus of this pack is then given by the Hertz-Mindlin contact theory (Mindlin, 1949). The calculation of the porosity-dependent effective moduli for porosities above critical porosity is performed using a modification of the Hashin-Shtrikman (Hashin and Shtrikman, 1963) upper bound.

Finally, the moduli of the saturated sediment is calculated from Gassmannâ&#x20AC;&#x2122;s equations (Gassmann, 1951). Dvorkin et al. (1999) gave a complete description of the theory including verification at an ODP site.

For weak, highly porous rocks, porosity, effective pressure and fluid properties play a major role in the sedimentâ&#x20AC;&#x2122;s elastic moduli, that are considered in the EMT theory (e.g. Helgerud, 2001). Ecker et al. (1998) found from AVA analysis of one CMP on the Outer Blake Ridge that the gas hydrate is located away from the grain contacts and does not affect the stiffness of the sediment frame.

For AVO analyses, this theory allows elastic

moduli to be calculated from an analytical set of equations using in situ physical properties of the sediment. The physical properties used here represent the sediment mineralogy and porosity of shallow marine sediments offshore Costa Rica from measurements acquired on ODP Leg 170. The mineralogy is composed of 5% calcite, 70% clay, and 25% quartz. The elastic parameters for minerals, pore water, and gas hydrate used in our calculation is shown in Table 1. Hydrostatic pore pressure and a porosity of 60% were assumed.

AVO PROCESSING The following describes the processing sequence. Corrections were applied to amplitudes extracted from raw CMP gathers. The travel paths of the seismic signal were estimated by 1-D raytracing.

Raytracing Seismic reflectivity at an interface is a function of incidence angle. A seismic trace, however, is recorded at a fixed offset, where the reflection angle changes with time. Source and receiver directivity are also a function of angle, and the spherical divergence is a function of the distance between source and receiver. Therefore it is appropriate to apply all amplitude corrections in the angle-domain. In order to determine the travel path and the relationship between source-to-receiver offset and incidence angle at the reflecting interface, a 1-D ray tracing following the derivation of Dahl & Ursin (1991) has been

Table 1: Elastic properties of sediment solid phase components, brine, hydrate, and methane used for the effective medium theory calculations (after Helgerud, 2001).

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Figure 4: Time-migrated seismic section of line BGR99-60 across the middle continental margin of Costa Rica, southeast of Nicoya Peninsula. A clear BSR indicates the base of the gas hydrate stability zone with a seaward increasing thickness. The amplitude variation with angle analyses are performed on CMPs from this line. CMP locations where quantitative AVO analyses have been performed are indicated.

implemented. In this approach, the ray parameter is iteratively determined. The seafloor is reasonable smooth and dips less than 3°. The 1-D earth model is characterized by thickness, Pand S-wave velocities, and density of each layer.

The ray path is determined obeying Snell’s law.

Any of the corrections described below are based on this ray tracing approach, where Pwave velocities and layer thicknesses are based on semblance-based velocity analysis.

Geometrical spreading In a homogeneous medium, the energy transmitted outwards from the source is distributed over a spherical shell. Therefore the energy density decays proportional to r2, where r is the radius of the wavefront. The wave amplitude, which is proportional to the square root of the energy density, falls off proportional to r. When the velocity increases with depth, the decay in amplitude occurs even more rapid with distance due to ray bending. In this study, the correction for spherical divergence is based on the raypath, derived from the 1-D ray tracing.

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Absorption When the signal propagates through the subsurface, wave energy is also transformed into another form of energy. Wave motion is gradually absorbed by the medium and reappears in form of heat. In absorbing media the amplitude decay occurs exponential with r (Luh, 1993). In this study the absorption has been restored for the center frequency of 60 Hz and an effective quality factor Qp of 200 for P-waves was used for layers between seafloor and BSR.

Receiver directivity The directivity of the seismic streamer is calculated as the directional response of a linear array (Keary and Brooks, 1984) implementing the cable geometry for the BGR99 cruise. Amplitude compensation due to receiver directivity has been performed for the center frequency of 60 Hz. The amplitude reduction due to the receiver directivity amounts to about 28% at an incidence angle of 60°.

Figure 5: Time-migrated seismic section of line BGR99-61 located at a distance of five kilometres from line BGR99-60. A weak and discontinuous bottom simulating reflector outlines the base of the gas hydrate stability zone. The prominent feature in this section is a slump with its head scarp located at the minimum water depth for hydrate stability (580 m, 770 ms TWT) suggesting an origin of the slumping related to gas hydrate destabilization.

Source directivity In reflection seismic surveys the source array is usually designed to focus energy downward into the subsurface. This implies more or less distinctive source directivities leading to energy emissions that are dependent on the angle of emergence. For wide-angle AVO analyses the source directivity is a critical factor which significantly alters the observed amplitude variation with offset and thus may lead to misinterpretation. In many studies only little, if any, dataindependent information about the source directivity is available (e.g. from modelling using appropriate software packages). In many studies, authors simply estimate the directivity from waveform modelling (e.g. Hyndman & Spence, 1992) or calibrate their data using the seafloor AVO response as a reference (e.g.

Ecker et al., 1998).

During cruise BGR99, lines BGR99-59 to BGR99-69 have been acquired using a tuned airgun array consisting of four sub-arrays. Each sub-array consisted of seven airguns with volumes varying between 0.33 l and 1.64 l. A source directivity plot modelled with the NUCLEUSÂŽ software from PGS Seres AS was available. The source directivity has been extracted for 40

and 60 Hz, respectively. With respect to the center frequency of 60 Hz, the source directivity of the BGR99 array was found to be best represented by a cos2-function (Fig. 7). At an angle of incidence of 60° the amplitude reduction due to the source directivity amounts to 75%. Compared to the receiver directivity (28%), the source has a significantly stronger effect on the amplitudes.

Transmission loss Another type of effect that influences the wave amplitude as the signal propagates through the overlying media is the loss due to energy conversion and reflection/transmission in the overburden. Castagna (1993) showed that these angle dependent effects are most significant when the reflectivity above the target horizon is very strong. Each reflection is transmitted through the shallower interfaces twice, once on the way down and once on the way up. On the return trip the changes in Vp, Vs, and r have reversed sign. Therefore, the angle-dependent effects can be neglected in a first order (e.g. Spratt et al.,1993). As shown in Fig. 4, the BGR99 data show only low reflecti-

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Figure 6: Elastic moduli are calculated from physical rock properties (mineral composition and porosity) at site 1041 of ODP Leg 170 (Kimura et al., 1997) implementing the effective medium theory from Helgerud et al. (2001). The porosity measurements on extracted core specimens (left, circles) are represented by a linear trend (left, solid line) decreasing from 65% at the seafloor to 45% at 400 mbsf. The calculated P-wave velocity and bulk density (middle and right, solid line) adequately represent the measured parameters (middle and right, circles). The mineral composition of the un-consolidated slope sediments is assumed to be constant with depth and is represented by 70% clay, 25% quartz and 5% calcite at this site. Gas hydrates were found between 100 m and 280 m below seafloor (grey area).

vity between the BSR and the seafloor reflection. Regarding the seafloor AVA response, the amplitude variation with offset for the seafloor reflection is very small, only about 3.5% from vertical-incidence to the maximum angle of incidence. The transmission loss has been considered by correcting for the vertical-incidence seafloor reflection/transmission coefficient only.

Reflection coefficients Assuming that the sea bottom is horizontal and smooth, and that the sea bed is a reflector between two half-spaces, the vertical-incidence seafloor reflection coefficient RSeafloor can be estimated from the primary seafloor reflection amplitude and the amplitude of the first seafloor multiple (Warner, 1990) from:

(1) ,

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where Am and Ap are the amplitudes of the multiple and primary seafloor reflection, respectively. The water depth must be large compared to source-to-receiver offset and the variation in reflection amplitude at near-vertical incidence must be negligible. The observed seafloor primary and multiple amplitudes were corrected for absorption (Qp = 10000, fc = 60 Hz) and spherical divergence. Angle-dependent transmission loss is not of concern within the water layer. After determining the seafloor reflection coefficient, the BSR amplitude response was corrected for spherical divergence, absorption, source and receiver directivities.

The vertical incidence reflection coefficient at the BSR RBSR can then be derived from:

(2)

where ABSR and ASeafloor are the amplitudes of the primary BSR and seafloor reflection,

Figure 7: The directivity of the airgun array used on the BGR99 cruise has been derived from a directivity plot created with the NUCLEUS software from PGS Seres AS (symbols). The directivity is best represented by a cos2-function (solid line). The effect of the source directivity on the amplitudes is significantly larger than the effect of the receiver directivity (dashed line).

respectively. The angle dependent reflectivity is then derived from scaling the BSR AVA amplitude response to the derived vertical-incidence BSR reflection coefficient.

AVO MODELING The full Zoeppritz equations were used to calculate synthetic angle-dependent reflection coefficients versus offset (RVO). In order to study the effect of hydrate-saturated and freegas saturated sediment on the RVO response of the BSR, and to interpret the real data with respect to gas hydrate concentration and free gas saturation, synthetic RVA responses were calculated for a »hydrate- over brine-saturated sediment« and a »brine- over free gas-saturated sediment« model of the subsurface. These simplified models have been chosen in order to have only one variable in each model, i.e. the hydrate concentration in the first model and the free gas saturation in the second model.

The gas hydrate was considered to be homo-

geneously distributed in the pore space, following Ecker et al. (1998) results for high-porosity hydrate-bearing sediment at the Outer Blake Ridge.

Synthetic RVA curves were calculated for bulk gas hydrate concentrations and bulk free-gas saturations increasing from 1 to 10% at an increment of 1%, respectively. The results in Fig. 8 clearly show the dominant effect of the free gas on the vertical-incidence reflection coefficient and on the shape of the RVA response. The red array of curves that represents the gas-free BSR case have vertical-incidence reflection coefficient magnitudes of less than 0.05, even at bulk gas hydrate concentrations of 10%. Furthermore, these curves are characterized by almost constant reflection coefficients at intermediate angles of incidence due to almost no contrast in Poisson’s ratio (Fig. 9), while a significant increase in reflection coefficient magnitude is only observed at incident angles greater than 60°, due to the increasing P-wave velocity with increasing gas

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Figure 8: Synthetic AVA responses representing the gas-free BSR model and the free gas BSR model. The red array of curves represent the gas-free model with hydrate-bearing over brine-saturated sediment, were the bulk gas hydrate concentration increases from 1 to 10%. The blue array of curves represent the free gas model with brine-saturated sediment over gas-saturated sediment, were the bulk gas-saturation increases from 1 to 10%.

hydrate concentration (Fig. 9). In contrast, the RVA responses representing the free gas BSR case (brine-saturated over free gas-saturated sediment and blue curves in Fig. 8) are characterized by strong vertical-incidence reflection coefficients that significantly increase in magnitude with increasing saturation of free gas. Moreover, the reflection coefficients also significantly increase at intermediate angles of incidence due to the increasing contrast in Poissonâ&#x20AC;&#x2122;s ratio (Fig. 10). Since we assume that the mineral frame does not change across the BSR, the above mentioned different shapes of the AVO curve represent the different pore fillings. Fig. 8 shows that low concentrations of gas hydrate cannot be resolved using AVO analysis. It is also evident that the differentiation of high saturations of free gas is not possible because of the major drop in P-wave velocity and Poissonâ&#x20AC;&#x2122;s ratio that occur at low gas saturations. However, the synthetic data indicate that AVO analysis using the full Zoeppritz equations and an effective medium theory

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enables free gas to be detected beneath the BSR and provides estimates of free-gas saturation at low saturation levels. The contribution of up to 10% bulk gas hydrate concentration to the observed RVA response is very small and cannot be resolved. Thus a model with hydrate-bearing sediment overlying gas-saturated sediment cannot be distinguished from a model with brine-saturated sediment overlying gas-saturated sediment using this method.

RESULTS Extraction of amplitudes The next step in AVO analysis was the extraction of amplitudes from pre-stack seismic CMP gathers. From the raw pre-stack seismic CMP gathers, the main lobe of the BSR reflection (trough) and the seafloor reflection (peak) was picked as a function of offset. Fig. 11a shows CMP 4520 from line BGR99-60 with a clear BSR reflection characterized by reversed polari-

Figure 9: Elastic properties calculated from the effective medium theory as a function of bulk gas hydrate concentration. Note, the Poisson’s ratio, density, and S-wave velocity are almost unaffected by the change in gas hydrate concentration.

ty with respect to the seafloor reflection. In addition to the source-to-receiver offset, the angles of incidence for the seafloor and the BSR reflections are indicated in Fig. 11b and 11c, based on 1-D raytracing. In this record, the high signal-to-noise ratio allowed amplitudes of the BSR to be extracted up to incidence angles of about 80°. Beyond 60°, interference between the BSR reflection and other reflections corrupts the amplitude information. This behaviour was observed on all CMPs in this study. Therefore, amplitudes beyond incidence angles of 60° were not used for AVO analysis.

Waveform modelling that includes the reflectivity of the overburden is needed to account for interference.

Reflection coefficient versus angle of incidence Line BGR99-60 with a clear BSR and strong variations of the post-stack BSR reflection amplitude along the line (Figure 4) was selected for AVO analysis. 40 CMP gathers were extracted from the pre-stack data at locations with weak, intermediate and strong post-stack

BSR reflection amplitudes. Unfortunately, it was not possible to use the amplitude information up to incidence angles of 60° in most of the gathers. The deterioration of BSR reflection amplitudes was often caused by interference, probably related to faulting or conflicting dips between reflections from the BSR and stratigraphic interfaces. If the maximum incidence angle on a selected CMP for which the BSR reflection amplitude could be extracted was much less than 60°, the CDP was disregarded for AVO analysis.

In Fig. 10, CMP 4500, CMP 4530, and CMP 4720 are CMPs with high signal-to-noise ratio from areas of intermediate, high, and low post-stack BSR reflection amplitudes, respectively (ref. Fig. 4). In Figure 12, the derived reflection coefficients versus angle (RVA) are plotted with synthetic RVA responses for free gas saturations varying between 1 and 10% underneath brine saturated sediment (blue array of curves). The CMP data match the synthetic free gas curves. Due to convergence of

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Figure 10: Elastic properties calculated from the effective medium theory as a function of bulk free gas saturation. Note, the Poisson’s ratio, density, and S-wave velocity changes significantly with increasing free gas saturation compared to the gas hydrate case (Fig. 9). The brine-saturated situation is represented by 0% bulk free gas saturation. With increasing free gas saturation, the contrast in Poisson’s ratio ∆σ to the brine-saturated case is increasing.

the synthetic RVA curves, determination of free gas saturation becomes less accurate with increasing free gas saturation. AVO analysis predicted free gas concentrations of about 2% (CMP 4500, intermediate BSR reflection amplitude), 4 to 5% (CMP 4530, strong BSR reflection amplitude), and about 1% (CMP 4720, weak BSR reflection amplitude). It is not possible to accurately estimate the concentration of gas hydrate with this method, since the reflection coefficient is not sensitive to concentration of gas hydrate as shown before.

DISCUSSION AND CONCLUSIONS Using an analytical relationship between physical properties and elastic moduli of the rock and the full Zoeppritz equations to model AVO responses avoids uncertainties introduced from empirical relationships and approximations to the Zoeppritz equations. The use of an explicit source directivity function is uncommon, becau-

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se of lack of information. As shown here, a tuned airgun array can introduce significant amplitude reduction at high angles of incidence. This study benefited from a data set with high signal-to-noise-ratio and a broad frequency spectrum. However, the results show that quantitative predictions can only be achieved to a limited degree from AVO analysis, even with the information available for this study.

This is mainly because of the low sensitivity of the AVO response to changes in the gas hydrate concentration and to free gas saturations above about 5%.

The measured amplitudes were limited to incidence angles of up to about 60°. Interference between the BSR reflection and other reflections led to a significant deterioration of amplitudes at high angles of incidence. Deconvolution and multiple suppression were not applied to the data to avoid alteration of the amplitudes. Information from high angles of incidence can be used with full waveform inversion,

Figure 11: The amplitude information for AVO analysis is extracted from pre-stack CMP gathers.

(A) Raw CMP gather 4520 from line BGR99-60 showing a normal-polarity sea-floor reflection and a reversed-polarity BSR reflection. The reflectivity within the hydrate stability zone is very weak compared to the seafloor and BSR reflectivity. The record is displayed with a reduction velocity of 2.5 km/s. (B) The flattened seafloor reflection indicates an amplitude decrease without any significant alteration of the wavelet with increasing offset. Angles of incidence based on 1-D raytracing are indicated. (C) The flattened BSR reflection interferes with reflection hyperbolas from shallower and deeper events at high offsets. Since these effects cannot be considered in AVO analysis, the useful amplitude information is often limited to incidence angles of about 60째 or less.

which includes AVO effects, the source signature, and the reflectivity of the overburden. But this approach also requires calibration wells in the area under investigation.

AVO analysis on extracted amplitudes from raw data was time consuming. Many CMPs were unusable for AVO analysis due to deteriorated amplitudes at the seafloor or the BSR reflection. Therefore, reliable 2-D information on the free gas distribution can not be obtained. For this purpose a 2-D waveform inversion including wide-angle AVO effects is required, which has not yet been done. Nevertheless, the approach followed in this study emphasises the advantage of AVO analysis on multichannel seismic data compared to oceanbottom hydrophone (OBH) and ocean-bottom seismometer (OBS) data. In the multichannel case a huge number of CMPs are available to

select AVO responses with adequate signal-tonoise ratio, while only a few records are available in OBH/OBS studies. The consideration of synthetic AVO responses is important to estimate the sensitivity of the AVO responses to the gas hydrate concentration and free gas saturation. This understanding prevents overfitting the real data and drawing incorrect conclusions. This is why no inversion of the extracted AVO responses was performed.

This study also indicates that the patchy BSRs southeast of the Nicoya Peninsula, i.e. the area with a BSR in seismic sections, are related to the presence of free gas beneath the GHSZ. The area without BSRs can be considered as not containing free gas beneath the GHSZ. In conclusion, the method presented in this study emphasises opportunities and drawbakks regarding quantitative AVO analysis. With

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Figure 12: Reflection coefficient versus angle of incidence for three CMPs from line BGR99-60 representing areas with intermediate (CMP 4500), strong (CMP4530), and weak (CMP4720) post-stack BSR reflection amplitudes. Synthetic curves are calculated for bulk free gas saturations from 1 to 10% (blue curves).

reference to the gas hydrate occurrence on the continental margin offshore Costa Rica and the origin of the bottom simulating reflector in seismic sections, the following results have been obtained: - A strong BSR in reflection seismic sections with vertical incidence reflection coefficients of more than -0.1, and increasing reflection coefficients magnitude with increasing angle of incidence, is a clear indication of free gas beneath the GHSZ. - A BSR can also be caused by the exclusive presence of gas hydrate at the base of the GHSZ. In this case, the RVA response is characterized by a small vertical incidence reflection coefficient of less than about 0.05 for bulk hydrate concentrations of up to 10%, and a minor increase of reflection coefficients with increasing angle of incidence.

- Quantitative AVO analysis enables free gas saturations up to 5% to be estimated.

Above these concentrations, differences in reflection coefficients are small.

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- Quantitative AVO analysis does not allow the estimation of the gas hydrate concentration when free gas is present beneath the GHSZ, because the variation in reflection response is small in comparison to the range of variation in gas hydrate concentration.

- Quantitative predictions from AVO analysis alone are limited, even when an analytical relationship between the physical properties and the elastic moduli of the rock, and an explicit source directivity function are considered.

- This study indicates that the patchy occurrence of the BSRs southeast of Nicoya Peninsula is caused by the presence of free gas beneath the GHSZ, which may have migrated through deep-reaching faults and become trapped beneath the GHSZ. Even at locations with a very strong BSR, the results indicate free gas saturations of less than about 5%. Outside the area of BSR occurrences, gas hydrate may be present.

ACKNOWLEDGMENTS The work presented in this paper has been supported by the Federal Ministry for Education and Research (BMBF) and the German Research Council (DFG) under the scope of the research and development programme GEOTECHNOLOGIES.

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217

Index

A Abegg C. . . . . . . . . . . . . . . . . . . . . . 20 Abegg F. . . . . . . . . . . . . . . . . . . . . . . . 4 Amann H.11 . . . . . . . . . . . . . . . . . . . . 4 Amann R. . . . . . . . . . . . . . . . . . . . . . 74 B Bauer K. . . . . . . . . . . . . . . . . . . . . . . 98 Becker H.J. . . . . . . . . . . . . . . . . . . . 118 Bialas J.. . . . . . . . . . . . . . . . . . . 86, 170 Blumenberg M. . . . . . . . . . . . . . . . . . 40 Boetius A. . . . . . . . . . . . . . . . . . . . . . 58 Bohrmann G. . . . . . . . . . . . . . . . 4, 170 Bönnemann C. . . . . . . . . . . . . . . . . 198 Breitzke M. . . . . . . . . . . . . . . . . . . . . 86 Brückmann W. . . . . . . . . . . . . . . . . . . 4 D De Beer D. . . . . . . . . . . . . . . . . . . . . 64 Deerberg G. . . . . . . . . . . . . . . . . . . 138 Degenhardt A. . . . . . . . . . . . . . . . . 186 Drews M. . . . . . . . . . . . . . . . . . . . . . . 4 E Eisenhauer A. . . . . . . . . . . . . . . . . . . 20 Elvert M. . . . . . . . . . . . . . . . . . . . . . . 68 Erzinger J. . . . . . . . . . . . . . . . . . 98, 152 F Fahlenkamp, H.. . . . . . . . . . . . . . . . 138 Feeser, V.. . . . . . . . . . . . . . . . . . . . . 118 Fischer, H. . . . . . . . . . . . . . . . . . . . . 166 Flüh, E. . . . . . . . . . . . . . . . . 40, 86, 170

218

G Goreshnik E . . . . . . . . . . . . . . . . . . 134 Greinert J . . . . . . . . . . . . . . . . . . . . . 20 Grupe B. . . . . . . . . . . . . . . . . . . . . . 118 Gubsch S . . . . . . . . . . . . . . . . . . . . . 20 Gust G . . . . . . . . . . . . . . . . . . . . . 4, 20 H Hanken T . . . . . . . . . . . . . . . . . . . . 186 Helmke J. . . . . . . . . . . . . . . . . . . . . 186 Henninges J. . . . . . . . . . . . . . . . . . . . 98 Hoffmann K . . . . . . . . . . . . . . . . . . 118 Hohnberg H.-J. . . . . . . . . . . . . . . . . . . 4 Huenges E . . . . . . . . . . . . . . . . . . . . 98 I Itoh, H. . . . . . . . . . . . . . . . . . . . . . . 134 J Jaguttis J. . . . . . . . . . . . . . . . . . . . . 186 Jørgensen B. B. . . . . . . . . . . . . . . 58, 68 Joye S. . . . . . . . . . . . . . . . . . . . . . . . 58 K Kasten S . . . . . . . . . . . . . . . . . . . . . 170 Keir R.. . . . . . . . . . . . . . . . . . . . . . . . 20 Kipfstuhl J. . . . . . . . . . . . . . . . . . . . . . 4 Klapproth A. . . . . . . . . . . . . . . . . . . 134 Kläschen D.. . . . . . . . . . . . . . . . . . . . 86 Klaucke I. . . . . . . . . . . . . . . . . . . . . . . 4 Klein G. . . . . . . . . . . . . . . . . . . . . . . 86 Knittel K. . . . . . . . . . . . . . . . . . . . . . 74 Konerding P. . . . . . . . . . . . . . . . . . . . 40 Kreiter S.. . . . . . . . . . . . . . . . . . . . . 118 Krüger M. . . . . . . . . . . . . . . . . . . . . . 80 Kuhs W.F. . . . . . . . . . . . . . . . . . . . . 134 Kulenkampff J. . . . . . . . . . . . . . 98, 152

Index

L Liebetrau V.. . . . . . . . . . . . . . . . . . . . 20 Linke P. . . . . . . . . . . . . . . . . . . . . . . . 20 Lösekann T. . . . . . . . . . . . . . . . . . . . . 74 Löwner, R.. . . . . . . . . . . . . . . . . . . . . 98 Lüdmann T. . . . . . . . . . . . . . . . . . . . . 40 Luff R.. . . . . . . . . . . . . . . . . . . . . . . . 20 M Masson M. . . . . . . . . . . . . . . . . . . . 186 Meyerdierks A. . . . . . . . . . . . . . . . . . 80 Michaelis W. . . . . . . . . . . . . . . . . . . . 40 Müller C. . . . . . . . . . . . . . . . . . . . . 198 N Nauhaus K. . . . . . . . . . . . . . . . . . . . . 80 Naumann R. . . . . . . . . . . . . . . . . . . 152 Neben S. . . . . . . . . . . . . . . . . . . . . . 198 Niemann H. . . . . . . . . . . . . . . . . 58, 68 O Orcutt B.. . . . . . . . . . . . . . . . . . . 58, 68 P Pape T. . . . . . . . . . . . . . . . . . . . . . . . 40 Petersen J. . . . . . . . . . . . . . . . . . . . . 40 Pfannkuche O. . . . . . . . . . . . . . . . . . 20 Poppen B . . . . . . . . . . . . . . . . . . . . 186 R Rackwitz F. . . . . . . . . . . . . . . . . . . . 118 Rehder G. . . . . . . . . . . . . . . . . . . . . . . 4 Reimer A. . . . . . . . . . . . . . . . . . . . . . 40 Reitner J.. . . . . . . . . . . . . . . . . . . . . . 40 Reston T.J.. . . . . . . . . . . . . . . . . . . . . 86

S Sahling H. . . . . . . . . . . . . . . . . . . . . 170 Savidis S.. . . . . . . . . . . . . . . . . . . . . 118 Schicks J. . . . . . . . . . . . . . . . . . . . . 152 Schneider R. . . . . . . . . . . . . . . . . . . 170 Schultz H.J. . . . . . . . . . . . . . . . . . . . 138 Schupp J. . . . . . . . . . . . . . . . . . . . . 118 Seifert R. . . . . . . . . . . . . . . . . . . . . . . 40 Sommer S. . . . . . . . . . . . . . . . . . . . . 20 Spangenberg E. . . . . . . . . . . . . . . . 152 Spiess .V . . . . . . . . . . . . . . . . . . 20,170 Suess E. . . . . . . . . . . . . . . . . . . . . . . . 4 T Talukder A. . . . . . . . . . . . . . . . . . . . . 86 Techmer K. . . . . . . . . . . . . . . . . . . . 134 Treude T. . . . . . . . . . . . . . . . . . . . . . . 58 V Villinger H. . . . . . . . . . . . . . . . . . . . 170 W Wallmann K. . . . . . . . . . . . . . . . . . 4, 20 Weber M . . . . . . . . . . . . . . . . . . . . . 98 Weinrebe W. . . . . . . . . . . . . . . . . . . . . 4 Widdel F.. . . . . . . . . . . . . . . . . . . . . . 80 Wiersberg T. . . . . . . . . . . . . . . . . . . . 98 Witte U. . . . . . . . . . . . . . . . . . . . . . . 58 Wong H.K. . . . . . . . . . . . . . . . . . . . . 40 Z Zillmer M. . . . . . . . . . . . . . . . . . . 40, 86 Zühlsdorff L. . . . . . . . . . . . . . . . . . . 170

219

Notes

Notes

Notes

Notes

This report highlights the scientific results of the first funding period addressing the following objectives: -

Characterization of the chemical and physical properties of methane hydrates Interaction of gas hydrates with the natural environment including seafloor stability and global climate Characterization of the unique biological communities dependent on methane hydrate occurrences Technologies for an improved survey of methane hydrates in both the laboratory and the field Technologies for the safe and commercial production of methane from hydrates

The papers published in this report offer a comprehensive insight into the present status of gas hydrate research in Germany and reflects the multidisciplinary approach of the programme.

Science Report GEOTECHNOLOGIEN

In Germany a National Gas Hydrate Programme has been initiated in 2001 as part of the R&D-Programme GEOTECHNOLOGIEN. Between 2001 and 2004, 15 joint projects have been funded with 15 Million Euros by the Federal Ministry of Education and Research. All projects were carried out in close cooperation with various national and international partners from academia and industry.

Gas Hydrates in the Geosystem (200-2004)

Gas Hydrates in the Geosystem

GEOTECHNOLOGIEN Science Report

Gas Hydrates in the Geosystem The German National Research Programme on Gas Hydrates

Report on the First Funding Period (2000 - 2004)

The GEOTECHNOLOGIES programme is financed by the Federal Ministry for Education and Research (BMBF) and the German Research Council (DFG)

No. 7

ISSN: 1619-7399

No. 7