ICONOX
Environmental change in the ocean: past and present
Iron cycling in continental margin sediments and the nutrient and oxygen balance of the ocean
Oxygen concentrations in the world’s oceans are decreasing, which is affecting nutrient recycling from sediments and thus the nutrient balance in the water column. We spoke to Dr Florian Scholz about his research into iron cycling in marine sediments, which has important implications for nutrient release to the water column and the health of marine ecosystems more generally. The sediments on the ocean floor can act as both a source and a sink of nutrients. This depends largely on the chemical composition of the surrounding seawater, in particular the oxygen concentration and the pH level, which have changed significantly over recent years. “These parameters are changing because of anthropogenic perturbations, so nutrient fluxes from the sea floor into the water column – and vice-versa – are likely to change as well,” says Dr Florian Scholz. As the Principal Investigator of the ICONOX project, Dr Scholz aims to investigate the impact of these changes in seawater on nutrient fluxes from the ocean floor. “The major nutrients we’re looking at are phosphate, iron and other bio-essential trace metals like cobalt, zinc and copper,” he outlines. “We’re looking at what factors control whether sediments act as a source or a sink of nutrients. We’re also interested in what happens upon release very close to the ocean floor, in the nearbottom water.” Nutrients are mobilized in the surface sediment through the re-mineralization of organic material or dissolution of terrigenous minerals. Some of these nutrients are released into the bottom water, while others are reprecipitated and buried. Metals released at the seafloor are partly re-precipitated in the bottom water. It is currently possible to sample around 10 centimetres of the bottom water overlying sediments, while the open water column can also be sampled at a safe distance, several metres above the seafloor. However, Dr Scholz says it is the layer in between the bottom water and the sediments that is crucial to building a more complete picture. “In a way it is the missing link. You’ve got to understand what’s happening in this layer in order to understand how fluxes from the sediments
to the bottom water translate into nutrient transport throughout the open water column,” he explains. Sampling this layer is a challenging task, as it’s difficult to generate meaningful trace metal data without contaminating the sample in some way. Researchers in the project have built a device, the Benthic Trace Profiler, to sample particles and water from this layer, which will enable Dr Scholz and his colleagues to gain deeper insights into how much of the sedimentderived micro-nutrients can be transported to the surface of the ocean. “The Benthic Trace Profiler is mainly used for collecting particles and water samples, which are then analysed in the laboratory,” he continues.
an instrument called a benthic lander on the sea floor. The lander contains a glass chamber, which is driven into the surface sediment, thereby incubating a sample of bottom-water and surface sediment. “We then take water samples from these chambers in regular time intervals,” outlines Dr Scholz. “By measuring the evolution of nutrient concentrations in these chambers over time, we can determine the flux of phosphate or iron from the sediments into the bottom water.” “We use concentration profiles in pore waters and benthic chamber incubations to quantify fluxes and to determine which environmental parameters and processes control the magnitude of these fluxes,” continues Dr Scholz.
We need to understand how nutrient fluxes will respond to declining oxygen concentrations in order to predict how marine ecosystems that rely on these nutrients will be affected by
global environmental change. Sampling techniques This device is being used in the project to gather samples from areas of the oceans in which oxygen concentrations in the water column are low, where these sedimentary sourcesink fluxes are particularly important, such as around the eastern boundary upwelling regions of the world’s oceans. A variety of other techniques are also being used in the project to take samples of sediment, pore water and bottom water. “We can recover sediment by sediment coring. The water in between the solid sediment particles is called pore water. The composition of this pore water provides information on water-solid interactions. Pore water is recovered from sediment cores using various techniques,” explains Dr Scholz. Nutrient fluxes from the sediment into the bottom water can be determined by putting
Researchers are also investigating the extent to which the magnitude of fluxes between the sediments and the bottom water depends on parameters, like the concentration of oxygen or other oxidizing agents. Once a relationship has been established, Dr Scholz says it’s possible to look towards predicting how the iron flux will evolve in future. “The interesting thing here is that there are feedback connections. If oxygen concentrations in the ocean go down, more iron would be released from the sea floor into the water column,” he explains. “As iron is often the limiting nutrient for primary production, there is a chance of higher rates of primary production, and increased fluxes. This in turn leads to the export of more organic carbon, and to more oxygen consumption. In a way this closes the feedback loop.”
Project Objectives
Iron and other trace metals are essential micronutrients for biological productivity in the ocean. Depending on environmental conditions such as oxygen concentration and pH, marine sediments can represent an important source or sink for these compounds. Within the ICONOX project, observational, experimental and numerical approaches are applied to constrain how changing micronutrient fluxes across the sediment-seawater interface affect biogeochemical cycling in the ocean.
Project Funding Schematic sketch illustrating how oxygen-dependent nutrient fluxes across the sediment-water interface amplify primary production, organic matter export and oxygen consumption in the water column. Dr. Scholz and his collaborators use benthic landers and the Benthic Trace Profiler to quantify chemical fluxes at the seafloor.
There are other links between nutrient cycles to consider, and it is not clear how future changes will affect ocean chemistry and marine ecosystems. Analysis of past changes can help researchers assess how the oceans are likely to change in future, so there is also a paleoceanographic dimension to the project, in which Dr Scholz and his colleagues are investigating the geological past. “We’re looking at how those feedbacks may have affected environmental change in the ocean during past periods of global warming,” he outlines. This work involves not only using the seafloor as a record of past environmental change, but also investigating its role in driving that change, via nutrient release or burial in sediments. “Surface sediments are an active player in ocean biogeochemical dynamics,” continues Dr Scholz. “The sea floor acts as a kind of lynchpin.” The aim here is to investigate how past changes in sedimentary fluxes have affected oxygen concentrations in the oceans over longer timescales. Sedimentary records have been retrieved from the later part of the Cretaceous period (~93 Million years ago), during which substantial areas of the ocean floor were covered by anoxic bottom water. Researchers are analysing these records to try and build a fuller picture of past changes. “We do detailed observational research in different ocean regions to understand processes on the sea floor and in the layer above it. We then extrapolate from our findings to look
at past periods of global warming, to try and understand the biogeochemical dynamics that took place during these periods,” outlines Dr Scholz. Models are also being used in the project to build a deeper picture. “We use global biogeochemical models to evaluate how sedimentary nutrient fluxes affect the dynamics of marine biogeochemical cycles, both today and in Earth’s history” says Dr Scholz.
Ocean change This research is being conducted against a backdrop of growing concern over how the oceans are changing. Ocean warming, acidification and de-oxygenation can all affect nutrient exchange between the seafloor and the water column. “As the ocean becomes warmer, it also gets more stratified. There is less transport of oxygenated water masses into the ocean interior, so less vertical exchange,” explains Dr Scholz. This has significant consequences for marine ecosystems, underlining the wider importance of this research. Samples have been taken from several different ocean regions, and Dr Scholz has further expeditions planned over the coming years, which will allow him to place their findings in the wider context. “We will participate in expeditions to the Benguela upwelling off Namibia this summer, and there will be a cruise in the Arabian Sea next year. We are trying to find recurring patterns in different areas,” he outlines.
The ICONOX project is funded by the Emmy Noether Program of the Deutsche Forschungsgemeinschaft.
Contact Details
Project Coordinator, Dr Florian Scholz GEOMAR Helmholtz Centre for Ocean Research Kiel Wischhofstr. 1-3 24148 Kiel | Germany T: +49 (0)431 600 2113 E: fscholz@geomar.de W: https://www.geomar.de/en/research/ ongoing-projects/project-details/prj/99900085/ W: https://www.geomar.de/en/mitarbeiter/fb2/ mg/fscholz/
Scholz, F., 2018. Identifying oxygen minimum zone-type biogeochemical cycling in Earth history using inorganic geochemical proxies. Earth-Science Reviews 184, 29-45.
Dr Florian Scholz
Florian Scholz is a sediment geochemist and paleobiogeochemist at GEOMAR Helmholtz Centre for Ocean Research Kiel. His research centers on geochemical processes and fluxes at the seafloor and their role in global biogeochemical cycles, today and through Earth’s history.
Recovery of a sediment core in the Gulf of California. The sedimentary record is used to reconstruct biogeochemical cycling in the Eastern Pacific over the last 10,000 years.
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