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SFB 754:  Climate - Biogeochemistry Interactions in the Tropical Ocean

Subject Area Geosciences
Biology
Medicine
Term from 2008 to 2019
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 27542298
 
Final Report Year 2020

Final Report Abstract

The overall goal of the SFB 754 was to understand the coupling of tropical ocean circulation and climate variability with the ocean’s oxygen and nutrient budgets, to quantitatively evaluate the functioning of oxygen-sensitive microbial processes and their impact on biogeochemical cycles, and to assess potential consequences for the ocean’s future. Oxygen dissolved in seawater was the central chemical element of the project. It is not only essential for higher forms of marine life, but it also controls remineralisation processes and thereby interacts with major nutrient cycles in the ocean and Earth system. This is most prominent in so-called - (OMZs) where low oxygen levels act as “switch” from aerobic to anaerobic processes, such as denitrification and associated loss of fixed nitrogen from the ocean, or the release of the nutrients phosphate and iron from sediments and the associated gain of these nutritive elements upon sediments turning anoxic. OMZs are located in the tropical oceans. The SFB 754 targeted two contrasting regions, the generally anoxic OMZ off Peru and the OMZ off Mauretania, where oxygen concentrations are low, but generally well above the thresholds for switches from aerobic to anaerobic processes. Major scientific findings of the SFB 754 include the identification and detailed quantitative understanding of oxygen supply by small-scale physical processes, in particular temporally varying zonal current bands that transport oxygen to the tropical OMZs. Coarse-resolution climate models cannot resolve these small-scale features and thus tend to systematically underestimate the oxygen supply – and hence overestimate the volume of OMZs. New high-resolution biogeochemical models and parameterizations developed for coarse-resolution models developed within the SFB 754 have helped to considerably improve the representation of OMZs in state-of-the-art climate models. An unexpected finding of the dedicated field work was the discovery of anoxic eddies in the tropical Atlantic. These form regularly in the Cape Verde region and for their many month long lifetime provide stable environments for anoxic microbial processes and special food webs that were not previously thought to occur in the North Atlantic. Novel biogeochemical processes for cycling nutrients in the sediments and in the water column were identified in the quasi-permanent low-oxygen conditions off Peru, and their often substantial role in the nutrient budgets of the OMZs and the overlying productive surface waters was quantified to derive, for the first time, a closed nitrogen budget. Some processes drive feedbacks that may accelerate marine oxygen loss under climate change, and, via the production of greenhouse gases such as nitrous oxide, may also accelerate climate change under expanding anoxia. Parameterisations of these processes were developed and employed in climate models to yield realistic representations of past oceanic anoxic events in Earth’s history. These models also suggest that positive oxygen-climate feedbacks for expected future warming are relatively small compared to uncertainties in other components of current climate models. Perhaps the single most surprising finding of the SFB 754 was the identification of the rapid decline of the marine oxygen inventory, called ocean deoxygenation, a term coined by the SFB 754 and now widely used in science and environmental assessments. Interestingly, our estimate of a 2 % oxygen decline during the past 50 years is 2 to 3 times larger than simulated by current climate models. This discrepancy between observational estimates and model results poses a major challenge for both modellers and observationalists, which is now addressed by a number of research groups worldwide. The comprehensive data set of oxygen and additional biogeochemical and physical parameters obtained by the SFB 754 will continue to help the international scientific community in better understanding the processes and consequences of ocean deoxygenation.

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