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Biogeochemical cycling of iron, carbon and sulfur in coastal and continental margin sediments

The objectives of the PhD project are to investigate the carbon, iron and sulfur cycling within sediments during early diagenesis and how these are controlled by sedimentary geochemical processes, the impact of different depositional environments as well as the reactivity of organic matter or mineral phases.

Geochemical analyses of the sediment-water interface are used to characterize sediments. Trace elemental composition, mineral assemblages, stable isotopes and radionuclides are determined to understand sources and transport pathways. In detail, stable Fe isotopes (δ56Fe) of solid phases and pore water, where Fe reduction occurs in different redox zones, are studied in great detail at various depositional environments of the Argentina Continental Margin. We aim to identify how these depositional environments impact biogeochemical and transport processes within sediments during early diagenesis. A further goal is to determine if and how δ56Fe of reactive Fe phases could be used as a proxy. Therefore, it needs to be evaluated if sedimentary δ56Fe signatures are influenced by a) the depositional environment (abiotically) and b) microorganisms (biotically). The working area is located at the Argentina Continental Margin, where samples were collaboratively collected during RV SONNE cruise SO260 in January and February 2018.

Bio-available Fe phases (Fe carbonate, Fe (hydr)oxides, Fe (oxyhydr)oxides and magnetite) have been observed in a variety of geologic settings, where Fe reduction occurs simultaneously in deep sediments. So far it has not been shown as to why these are not completely reduced. Thus, during the PhD project multidisciplinary lab experiments will be set up with experts in organic geochemistry (Kai Uwe Hinrichs Group) and microbial ecology (Michael Friedrich Group) at the University of Bremen and the Center for Marine Environmental Science (MARUM). Abiotic and biotic lab experiments will be carried out to look into the interaction of organic matter and Fe minerals. Further we will investigate redox cycling with flow-through reactors. In detail, the goal is to determine if the effect of adsorption and/or co-precipitation of organic compounds and sulfide compounds onto Fe minerals in marine sediments limits Fe reduction and leads to carbon sequestration and Fe mineral preservation.

Contacts: Anne-Christin Melcher, Sabine Kasten, Susanne Henkel