Paul Vosteen

Reactive iron (Fe) oxides in marine sediments may represent a source of bioavailable Fe to the ocean via reductive dissolution and sedimentary Fe release or can promote organic carbon preservation and long‐term burial. Furthermore, enrichments of reactive Fe (sum of Fe oxides, carbonates and sulfides normalized to total Fe) in ancient sediments are utilized as a paleo‐proxy for anoxic conditions. Considering the general importance of reactive Fe oxides in marine biogeochemistry, it is important to quantify their terrestrial sources and fate at the land‐ocean interface. We applied sequential Fe extractions to sediments from the Amazon shelf to investigate the transformation of river‐derived Fe oxides during early diagenesis. We found that ∼22% of the Amazon River‐derived Fe oxides are converted to Fe‐containing clay minerals in Amazon shelf sediments. The incorporation of reactive Fe into authigenic clay minerals (commonly referred to as reverse weathering) is substantiated by the relationship between Fe oxide loss and potassium (K) uptake from sedimentary pore waters, which is in agreement with the previously reported Fe/K stoichiometry of authigenic clay minerals. Mass balance calculations suggest that widely applied sequential extractions do not separate Fe‐rich authigenic clay minerals from reactive Fe oxides and carbonates. We conclude that the balance between terrestrial supply of reactive Fe and reverse weathering in continental margin sediments has to be taken into account in the interpretation of sedimentary Fe speciation data.

The availability of major nutrients, nitrogen (N) and phosphorus (P), largely controls primary productivity in eastern boundary upwelling systems. The oxygen minimum zone (OMZ) on the Namibian shelf is characterized by high productivity and extraordinarily high particulate organic carbon (POC) contents (up to 19 % dry weight) in the surface sediments. The anaerobic degradation of POC by bacterial sulfate reduction leads to the production of hydrogen sulfide (H2S) that supports extensive communities of large sulfur bacteria Thiomargarita namibiensis in surface shelf sediments. These bacteria oxidize sulfide by reducing nitrate (NO3-) to either ammonium (NH4+) or dinitrogen (N2). Thiomargarita also affect phosphorus cycling by intracellular incorporation of polyphosphates and extracellular formation of hydroxyapatites. In order to understand and quantify the complexity of the coupled benthic cycles of C, N, P, S, Fe in the Benguela Upwelling System, a reaction-transport model (RTM) was used to simulate sediment biogeochemical data collected from the RV Meteor cruise (M157, August 4th-September 16th 2019) off Namibia. This allowed deeper insights into the role of sulfur-oxidizing bacteria on P and N fluxes across the sediment surface. Results are presented that point toward potentially strong feedbacks by Thiomargarita on primary production in response to ongoing global warming and ocean deoxygenation.