Sediment iron dynamics along a shelf to deep basin transect in the Black Sea

Iron (Fe) is an essential nutrient to sustain life in marine environments. Continental shelf sediments act as a major source of dissolved iron to marine waters. However, the exact mechanisms controlling mobilization of Fe in shelf sediments and its release to the overlying water are still incompletely understood.Here, we present geochemical data for sediments at eight sites along a shelf-to-basin transect in the north-western Black Sea collected during cruises with RV Pelagia in 2013 and 2015. A strong surface enrichment in highly reactive Fe oxides was observed at all sites on the oxic shelf and hypoxic shelf break, implying a potential for Fe release to the pore water. Depth profiles of pore water Fe indeed show evidence for mobilization in the surface sediments through reductive dissolution of Fe-oxides. A distinct gradient in pore water concentrations was observed, with high Fe concentrations at stations in the coastal zone (> 200 µM) and very low concentrations (< 5 µM) on most of the shelf. We will discuss the processes contributing to the release of Fe in the pore water, the potential for release to the overlying water and the implications for the Black Sea system.