Sedimentary iron-phosphorus-sulfur dynamics along a redox transect in the Black Sea

Iron (Fe) plays a crucial role in the biogeochemical cycling of the essential nutrient phosphorus (P) in marine systems; Fe(III) oxides are important carrier phases for phosphate (PO4). Due to this coupling, PO4 burial in sediments is often regulated by sedimentary Fe redox chemistry. It is generally assumed, however, that Fe-associated P is negligible in strongly reducing sediments due to the absence of Fe(III) oxides that can adsorb PO4. Recent findings of Fe-P associations in sulfidic marine sediments, however, have forced us to reconsider this notion. Here, we investigate the coupled sedimentary cycles of Fe, P and S in sediments from the Black Sea, along a depth transect from the oxic shelf to the euxinic deep basin. We show preservation in the sediment at deep sites of reactive Fe(II) that is not sulfidized, despite high concentrations of free sulfide. Furthermore, Fe-P is shown to be an important P burial phase under contrasting redox conditions, from Fe oxide-bound P on the shelf to reduced Fe-P mineral associations in the deep sea. These findings are of great importance to understanding marine P burial fluxes in the past and present ocean.