Periodic Reporting for period 1 - BioGeoMetal (Nutrients in anoxic oceans – Trace metals in modern and ancient environments)
Reporting period: 2019-12-01 to 2021-11-30
The global oceanic oxygen content has decreased by more than 2% since 1960 and is projected to drop further in the near future. This decline will drastically impact ocean nutrient cycles and marine life, but the exact progression of ocean anoxia and its impacts are hard to predict. Intervals of low-oxygen conditions are relatively common in Earth's long history. The study of such past climate disturbances can improve projections of future ocean anoxia and associated environmental change, but this requires reconstruction of past environmental parameters based on what is left in the sedimentary record. This project aims to develop and apply new ways of using geochemical tracers for past ocean environments. The focus is on the role of trace elements in anoxic waters and their impact on marine life and biogeochemical cycles.
Trace elements are essential to life and are taken up in phytoplankton cells. Many of these elements are also less soluble in waters with less oxygen, such that high enrichments of these elements in the sedimentary record are used as geochemical tracers for low-oxygen conditions. Ongoing ocean deoxygenation may lower the availability of some bio-essential trace metals and this could ultimately affect marine ecosystems. The aims of this study are three-fold: 1) Improve reconstructions of past low-oxygen conditions using trace-element based geochemical tracers, 2) Evaluate the behaviour of bio-essential trace elements in low-oxygen conditions, and 3) Reconstruct the relative availability of trace element micronutrients over Earth's history.
(These aims represent a slight deviation from the original proposal as a consequence of the outbreak of the SARS-CoV-2 pandemic. The planning and content of the project was affected by the pandemic and associated measures, which started ~3 months into the project. However, the revised aims and approach serve the same overall goal: to better understand trace element cycling in low-oxygen waters. Originally, the objectives (also) included study of intervals of global perturbations to the Earth's environment and the development of new laboratory methods. These targets have been replaced with a more detailed study of local and regional trace-element cycling and the compilation and use of large datasets that can substitute for the more specific analyses.)
Trace elements are essential to life and are taken up in phytoplankton cells. Many of these elements are also less soluble in waters with less oxygen, such that high enrichments of these elements in the sedimentary record are used as geochemical tracers for low-oxygen conditions. Ongoing ocean deoxygenation may lower the availability of some bio-essential trace metals and this could ultimately affect marine ecosystems. The aims of this study are three-fold: 1) Improve reconstructions of past low-oxygen conditions using trace-element based geochemical tracers, 2) Evaluate the behaviour of bio-essential trace elements in low-oxygen conditions, and 3) Reconstruct the relative availability of trace element micronutrients over Earth's history.
(These aims represent a slight deviation from the original proposal as a consequence of the outbreak of the SARS-CoV-2 pandemic. The planning and content of the project was affected by the pandemic and associated measures, which started ~3 months into the project. However, the revised aims and approach serve the same overall goal: to better understand trace element cycling in low-oxygen waters. Originally, the objectives (also) included study of intervals of global perturbations to the Earth's environment and the development of new laboratory methods. These targets have been replaced with a more detailed study of local and regional trace-element cycling and the compilation and use of large datasets that can substitute for the more specific analyses.)
1. Mediterranean sapropels represent recent intervals of anoxia in the Mediterranean Sea. As such, these organic-rich sediment layers provide an excellent case study to test trace-element (isotope) based proxies for past low-oxygen environments. Mo- and U-isotope studies were performed on several Quaternary sapropels. This has led to better constraints on the redox conditions during sapropel deposition and improved interpretative frameworks for the application of these isotope tracers further back in Earth’s history.
2. Trace-element nutrient cycles were studied in detail in the high-productive Namibian Margin. Pore water, sediment, and seawater samples were analysed from four stations with different depositional redox conditions. These analyses highlight organic-rich continental-margin sediments as a relevant source or sink of trace elements to the ocean that contributes to closing the global isotopic mass balance.
3. A large dataset of trace element concentration data was compiled to assess long-term trends in micronutrient availability. The compiled data show that the relative availability of different bio-essential trace elements varied over time in association with the oxygenation of the ocean. These trends may have contributed to changes in the dominant primary producers in the ocean over the last few 100 Myrs.
2. Trace-element nutrient cycles were studied in detail in the high-productive Namibian Margin. Pore water, sediment, and seawater samples were analysed from four stations with different depositional redox conditions. These analyses highlight organic-rich continental-margin sediments as a relevant source or sink of trace elements to the ocean that contributes to closing the global isotopic mass balance.
3. A large dataset of trace element concentration data was compiled to assess long-term trends in micronutrient availability. The compiled data show that the relative availability of different bio-essential trace elements varied over time in association with the oxygenation of the ocean. These trends may have contributed to changes in the dominant primary producers in the ocean over the last few 100 Myrs.
The BioGeoMetal project contributes to a better understanding of the broader scale impact of ongoing climate and environmental change. It has allowed for an improved framework for the interpretation of Mo- and U-isotope signatures from the sedimentary record, to better reconstruct the oxygen concentrations of past oceans. The study has gained better insight into the global oceanic Ni cycle, a key micronutrient, and reveals long-term trends in the micronutrient availability of the last 100s of million years and its link to the evolution of marine life. The research performed here will ultimately improve projections of future deoxygenation by providing historical perspective to ongoing changes. Additionally, it provides better constraints on the possible impact of further deoxygenation on availability of micronutrients.