Deriving the Eustatic Sea Level Component in the Kattaegatt Sea

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Deriving the Eustatic Sea Level Component in the Kattaegatt Sea Nils‐Axel Mörner Paleogeophysics & Geodynamics, Stockholm, Sweden morner@pog.nu Received 9 May 2014; Accepted 18 June 2014; Published 26 June 2014 © 2014 American Society of Science and Engineering

Abstract Changes in global sea level is an issue of much controversy. In the Kattegatt Sea, the glacial isostatic component factor is well established and the axis of tilting has remained stable for the last 8000 years. At the point of zero regional crustal movements, there are three tide gauges indicating a present rise in sea level of 0.8 to 0.9 mm/yr for the last 125 years. This value provides a firm record of the regional eustatic rise in sea level in this part of the globe.

Keywords Sea Level Changes; Eustasy; Glacial Isostatic Uplift; Shorelines; Benchmarks; Tide Gauges; Zone of Zero Uplift; The Kattegatt Sea

Introduction Changes in global sea level have been quite vigorously debated, not least in connection with the Global Warming scenarios [1]. Fig. 1 illustrates the spectrum of divergence when it concerns the recording of proposed global sea level changes. It this paper I will purposely stay out of this debate and only discuss the evidence available in the Kattegatt Sea area between Sweden and Denmark. The reason for this is that we here have an unusually strict knowledge of the glacial isostatic uplift component where the axis of tilting (hinge between uplift to the NE and subsidence to the SW) has remained stationary for the last 8000 years [2‐7]. Personally, the sea level changes in the Kattegatt region have been in the focus of my own research for 60 years [2‐16].

FIG. 1. SPECTRUM OF RATES OF SEA LEVEL CHANGES IN RELATION TO THE DISTRIBUTION OF RATE VALUES OF THE NOAA TIDE GAUGE STATIONS [18, 22, 24]. ESTIMATES OF RISE BY THE IPCC FOR YEAR 2100 (GREEN ARROWS) [1], SATELLITE ALTIMETRY (+3.2 mm/yr) [23], MEAN OF 182 NOAA TIDE GAUGE STATIONS (+1.6 mm/yr) [24], THE NEW DATA FROM THE KATTEGATT SEA HERE PRESENTED (+0.8‐0.9 mm/yr), AND THE VALUE FROM SOME KEY SITES (±0.0 mm/yr) [22, 24].

The Kattegatt Region The Kattegatt region occupies the marginal zone of postglacial uplift (Fig. 2). A total of 40 synchronous shorelines have been identified and followed over a distance of 250 km in the direction of tilting (Fig. 3) [2, 3]. The isobase of

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zero amount of uplift (i.e. the hinge between uplift to the NE and subsidence to the SW) is closely defined to the NW‐side of the Straight of the Great Belt in Denmark [2, 3]. Furthermore, the location of the zero isobase has remained stable for 8000 years (the PTM‐2 shoreline dated at 7000 C14 yrs BP has a calibrated age of about 7850 cal.yrs BP) [4‐7]. Consequently, this point in the shoreline spectrum (Fig. 3, ‐200 km curve) should give the eustatic component of sea level changes [2, 3], or what I defined as the regional eustatic changes in sea level in the northwest European region [3‐11].

FIG. 2. GEOMETRY OF THE TOTAL AMOUNT OF GLACIAL ISOSTATIC UPLIFT OF FENNOSCANDIA [8, 9, 16]. THE KATTEGATT SEA REGION OCCUPIES THE MARGINAL ZONE AND TRANSITION FROM UPLIFT (YELLOW) TO SUBSIDENCE (BLUE). IT IS MARKED WITH AN OPEN BOX CORRESPONDING TO FIG. 4.

FIG. 3. SHORELINE SPECTRUM OF THE KATTEGATT SEA [2, 3, 8, 9, 16]. THE SEA LEVEL CURVE OF THE LAST 8500 C14‐YEARS BP (≈9.500 cal. yrs BP) OF THE ‐200 KM POINT LIES CLOSE TO THE ZERO UPLIFT POINT (at ‐207 km) AND HENCE GIVES A CLOSE REPRESENTATION OF THE REGIONAL EUSTATIC CHANGES [2, 5].

The direction of uplift (tilting) has a firm course in N40°E over the Swedish West Coast and the Kattegatt Sea [2, 4, 7, 8]. The tilting is affected by a perpendicular fault zone running SE–NW direction along the north side of the Hallandsåsen horst and across the Kattegatt [2‐9, 17]. It acted as a shoreline bend.

For recent changes in sea level, there are a number of tide gauge stations along the Swedish and Danish coasts. Those attached to the solid rock (in Sweden) provide fine records. The same seems to apply for stations anchored in glacial

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till in Denmark. Those attached to harbour constructions resting on soft sediment are problematic as they may include a significant component of local site‐specific subsidence.

The geodetic network of benchmarks began to be installed in 1886, and they were all re‐measured in the 1960s, implying a recording of relative land uplift from about 1890 to 1960. The benchmarks are attached both to the solid bedrock and to big blocks. On the Swedish West Coast there are about 39 re‐measured benchmarks.

The tide gauges give relative sea level changes in mm/yr. The benchmarks give relative uplift in mm/yr. The tilted shorelines give relative uplift in meters and time. Because the isostatic and eustatic components could be identified and separated in the Kattegatt–Swedish West Coast region, absolute uplift and absolute sea level could be recorded [2, 3]. Therefore, the individual shorelines could be converted to lines of absolute uplift in mm/yr allowing direct comparisons with the records from benchmarks and tide gauges[5]. The values of the 39 benchmarks from the Swedish West Coast form a line of relative uplift that runs parallel to – but below – the youngest line of absolute uplift in mm/yr. This line intersects the zero isobase of uplift at a level of ‐1.1 mm/yr, which I took as a good indication of a eustatic rise of 1.1 mm/yr for the period 1890 to 1960 [5, 7‐9, 13].

FIG. 4. THE UPLIFT PROFILE OF THE KATTEGATT REGION: AS ESTABLISHED FROM 40 SYNCHRONOUS SHORELINES [2, 3] AND THE ISOBASES OF THE PRESENT RATE OF UPLIFT (in mm/yr) AS ESTABLISHED FROM A COMBINATION OF GEODETIC BENCHMARKS AND SHORELINE DATA [5]. THE THREE TIDE GAUGES [18] (RED DOTS) LOCATED AT OR CLOSE TO THE ZERO POINT OF UPLIFT (WHICH HAS REMAINED STABLE FOR AT LEAST 8000 YEARS) ARE HERE ANALYSED WITH RESPECT TO PRESENT REGIONAL EUSTASY. THE EXTRAPOLATION OF THE ISOBASES TO THE EAST WOULD IMPLY A TRANSITION TO AN E–W EXTENSION IN THE BALTIC REGION.

During the last 50 years, sea level is often claimed to have accelerated [1]. We can easily test if this is the case in the Kattegatt area. The network of lines (isobases) of absolute uplift is shown in Fig. 4. It now happens so that the Korsör

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tide gauge station lies right at the zero‐isobase, the station of Slipshavn (Nyborg) lies at the ‐0.1 mm/yr isobase and the station of Aarhus at the +2.8 mm/yr isobase. The stations have records from 1888/97 to 2013 [18].

Fig. 5 shows the tide gauge records from Korsör, Nyborg and Aarhus [18]. At Korsör, where uplift has remained zero during the last 8000 years [4, 5], sea level rose by 0.81 ±0.18 mm/yr from 1897 to 2013. This value ought to be an excellent value of local eustasy over the last 115 years. It could hardly be higher, but possibly smaller, if local site‐ specific compaction occurs.

FIG. 5. TIDE GAUGE RECORDS OF KORSÖR, NYBORG AND AARHUS AS PRESENTED BY NOAA [18]. KORSÖR LIES RIGHT AT THE ZERO ISOBASE OF UPLIFT, AND THE SEA LEVEL RECORD (+0.81 ±0.18 mm/yr) SHOULD HENCE REPRESENT REGIONAL EUSTASY. THE NYBORG RECORD LIES AT THE ‐0.1 MM/YR ISOBASE AND THE TIDE GAUGE RECORD MUST HENCE BE CORRECTED BY 0.1 MM/YEAR TO PROVIDE REGIONAL EUSTASY (+0.91 ±0.16 mm/yr). THE TIDE GAUGE AT AARHUS, LOCATED AT THE +2.8 ISOBASE, RECORDS A MEAN RELATIVE RISE OF +0.63 ±0.11 MM/YR, WHICH CORRESPONDS TO A EUSTATIC FACTOR OF +0.91 ±0.11 mm/yr. THIS IMPLIES THAT ALL THREE RECORDS GIVE A CONGRUENT RECORD OF A REGIONAL EUSTATIC RISE IN THE ORDER OF 0.8‐0.9 mm/yr (THE MEAN BEING +0.87 ±0.15 mm/yr). THIS TREND HAS REMAINED STABLE OVER THE LAST 125 YEARS.

The Nyborg record shows a relative rise in sea level of 1.01 ±0.16 mm/yr over the same time period of. Because, this site lies at the ‐0.1 mm/yr isobase, the eustatic component at this site would be a rise in the order of ≈ +0.9 mm/yr.

The tide gauge at Aarhus lies at an isobase latitude of +2.8 mm/yr. It gives a mean relative rise in sea level of +0.63 ±0.11 mm/yr for the period 1888‐201318. This suggests that the eustatic rise in the order of ≈ +0.9 mm/yr.

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The mean of eustatic component in all three sites (Korsör, Nyborg and Aarhus) is a mean sea level rise over the last 115‐125 years of +0.87 ±0.15 mm/yr.

Taken together, these three sites indicate a mean eustatic rise component in the Kattegatt region of 0.8 to 0.9 mm/yr for the last 125 years. Tide gauges are known to overestimate, rather than underestimate, actual sea level changes due to their vulnerability to effects of local compaction [19]. The regional crustal movements seem to be well recorded [3, 7]. No local crustal irregularities have been reported in the vicinity of these three sites [17, 20]. Therefore, it seems reasonable to conclude that the regional eustatic component in the Kattegatt area can be set at a rise in the order of 0.8‐0‐9 mm/yr (Figure 1). This value is not far off the value of +1.1 mm/yr, previously established for the 1890‐1960 period [3].

The mean trends of the three tide gauge records (Fig. 5) have remained virtually stable though the last 125 years.

Consequently, the present analysis provides no signs of any acceleration in the regional eustatic sea level rise over the past 50 years.

The inter‐annual variations [18] are very similar in the three records (Fig. 5). The changes are likely to represent variations in air pressure, currents, prevailing wind, and interchanges of water between the Kattegatt and the Baltic.

Decadal cycles are not clearly identified.

I have purposely excluded the tide gauge stations at Fredrikshavn (+0.14 ±0.15 mm/yr), Hornbaek (+0.37 ±0.22 mm/yr), Copenhagen (+0.67 ±0.21 mm/yr) and Klagshamn at Malmö (+0.64 ±0.40 mm/yr) [18], because there are problems with isobase curving when diverging from the main profile zone (Fredrikshavn, Copenhagen and Klagshamn) and local crustal movements (Copenhagen, Hornbaek). At Klagshamn (Malmö), the repeated levelling data [7] suggest a present rate of uplift of +0.3 mm/yr, which implies a eustatic component of +0.94 ±40 mm/yr, in good agreement with the Fig. 5 analysis. It also indicates a significant curvature of the isobases towards the Baltic (Fig. 4), which is consistent with a known NS tilt direction in the Baltic [7‐9].

Conclusions The eustatic changes in sea level were originally held to be the same all over the globe [21]. We now know that this is not correct [3, 22] and that sea level changes significantly over the globe. The search for a mean rate of sea level change has become almost illusive as illustrated in Fig. 1. The pros and cons in this debate lie outside the scope of the present paper.

In this paper, focus is set on one single region: the Kattegatt Sea. The reason for this is that we here have a condensed record of the sea level changes and postglacial isostatic uplift [2‐9]. The direction of uplift and the location of the zero isobase (hinge between uplift to the NE and subsidence to the SW have remained stable over the past 8000 years [4, 5] as evidenced by 12 individual shorelines (Fig. 3) [2‐9], 39 benchmarks along the Swedish West Coast [3, 7] and available tide gauges [3, 16‐18].

At, and close to, the line of zero uplift over the last 8000 years, there are three tide gauge stations [13], which are here used in order to define the regional eustatic component in the Kattegatt region. The three sites give a converging picture: a eustatic component indicating a rise in the order of 0.8‐0.9 mm/yr.

Whatever sea level may be doing in other part of the world, the mean regional eustatic value of the last 125 years is herby shown to have been about 0.8‐0.9 mm/yr in the Kattegatt region (Fig. 1).

A second outcome of the analysis is that there are no signs of any acceleration in the last 50 years.

REFERENCES [1]

IPCC. “Fifth Assessment Report” (2014), “Fourth Assessment Report” (2007), “Third Assessment Report” (2001).

[2] Mörner, N.‐A. “The Late Quaternary history of the Kattegatt Sea and the Swedish West Coast: deglaciation, shorelevel displacement, chronology, isostasy and eustasy.” Sveriges Geol. Undersökn. C‐640 (1969):1‐487.

[3] Mörner, N.‐A. “Eustatic changes during the last 20,000 years and a method of separating the isostatic and eustatic factors in an uplifted area.” Palaeogeogr. Palaeoclim. Palaeoecol. 9 (1971):153‐181.

[4] Mörner, N.‐A. “Isostasy, eustasy and crustal sensivity.” Tellus 24 (1972): 586‐592.

[5] Mörner, N.‐A. “Eustatic change during the last 300 years.” Palaeogeogr. Palaeoclim. Palaeoecol. 13 (1973): 1‐14.

[6] Mörner, N.‐A. “Eustatic changes during the last 8,000 years in view of radiocarbon calibration and new information from the

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Kattegatt region and other northwestern European coastal areas.” Palaeogeogr. Palaeoclim. Palaeoecol. 19 (1976): 63‐85.

[7] Mörner, N.‐A. “Past and present uplift in Sweden: glacial isostasy, tectonism and bedrock influence.” Geol. Fören. Stockh.

Förh. 99 (1977): 48‐54.

[8] Mörner, N.‐A. “The Fennoscandian uplift and Late Cenozoic geodynamics: geological evidence.” GeoJournal 3 (1979): 287‐318.

[9] Mörner, N.‐A. “The Fennoscandian uplift: Geological data and their geodynamic implication.” In: Earth Rheology, Isostasy and Eustasy, edited by N.‐A. Mörner, p. 251‐284. John Wiley & Sons (1980).

[10] Mörner, N.‐A. “The northwest European “sea‐level laboratory” and regional Holocene eustasy.” Palaeogeogr. Palaeoclim.

Palaeoecol. 29 (1980): 281‐300.

[11] Mörner, N.‐A. “Late Quaternary sea‐level changes in northwestern Europe: a synthesis.” Geol. Fören. Stockh. Förh. 100 (1980): 381‐400.

[12] Mörner, N.‐A. “Course and origin of the Fennoscandian uplift: the case of two different mechanisms.” Terra Nova 3 (1991): 408‐413.

[13] Mörner, N.‐A. “Sea level variability.” Z. Geomorphology N.S. 102 (1996): 223‐232.

[14] Mörner, N.‐A. “Sea level changes in western Europe”, Integrated Coastal Zone Management 1/2 (2000): 31‐36.

[15] Mörner, N.‐A. “Estimating future sea level changes.” Global Planet. Change 40 (2004): 49‐54.

[16] Mörner, N.‐A. “Paleoseismicity and uplift of Sweden.” Excursion Guide, 33rd IGC Excursion No 11, Oslo (2008): 1‐107.

[17] Hansen, J.M., Aagaard, T. and Binderup, M. “Absolute sea levels and isostatic changes of the eastern North Sea to central Baltic region during the last 900 years.” Boreas 41 (2011): 180‐208. DOI 10.1111/j.1502‐3885.2011.00229.x [18] NOAA, “National Oceanic and Atmospheric Administration, Tides & Currents, Sea Level Online” (2012). http://tidesandcurrents.noaa.gov/sltrends/sltrends.shtml [19] Mörner, N.‐A. “Some problems in the reconstruction of mean sea level and its changes with time.” Quaternary International 221 (2010): 3‐8.

[20] Gregersen, S. and Voss, P.H. “Review of some significant claimed irregularities in Scandinavian postglacial uplift on timescales of tens to thousands of years – earthquakes in Denmark?” Solid Earth 5 (2014): 109‐118.

[21] Fairbridge, R.W. “Eustatic changes in sea level.” Physics Chemistry of the Earth 4 (1961): 99‐185.

[22] Mörner, N.‐A. “Sea level changes: past records and future expectations.” Energy & Environment 24 (2013): 509‐536.

[23] Sea Level Research Group of University of Colorado (2014). http://sealevel.colorado.edu/ [24] Mörner, N.‐A. “Sea level changes – a short comment.” Global Climate Status Report 2013 (2013): 5‐9.

Mörner was born in Stockholm, Sweden. He took his Ph.D. in Quaternary Geology in 1969. He became associate professor in Quaternary Geology in 1969, and in General and Historical Geology in 1981. He held a personal Associate Professorship at the Swedish Rsearch Council in Paleogeophysics & Geodynamics from 1978. He was head of the institute on Paleogeophysics & Geodynamics at Stockholm University (1991‐2005). Major fields: sea level changes, paleoseismology and neotectonics, Sun‐Earth interaction.

He was postdoctoral fellow in Canada in 1969/70, consultant in in Physics in Brazil in 1979, and guest scientists at Centre Luminy in France in 1980/81. Fellow of GSA in 1975. He has personal field experiences from 49 different countries scattered all over the globe. He has an extensive publication. This is his paper no. 589. He has given over 550 papers at international meetings. Among his books, one may note: Earth Rheology, Isostasy and Eustasy (Wiley & Sons, 1980), Climate Change on a Yearly to Millennial Basis (Kluwer, 1984) and Patterns in solar variability, their planetary origin and terrestrial impact (Pattern Recognition in Physics, 2013). Current and previous research interests: sea level changes, glacial isostasy, neotectonics, paleoseismology, geoid deformation, Earth’s rate of rotation, ocean circulation, climate, paleomagnetism, geomagnetism, Solar Wind, planetary‐solar‐terrestrial interaction.

Prof. Mörner was, in 2008, awarded “The Golden Condrite of Merits” from Algarve University “for his irreverence and contribution to our understanding of sea‐level change”.

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