Iron enrichments and Fe isotopic compositions of surface sediments from the Gotland Deep, Baltic Sea

Recent sediments from the Gotland Deep display enrichments in reactive Fe, associated with elevated Fe/Al ratios and light Fe isotopic signatures of the bulk sediments that are indicative of euxinic (anoxic and sulfidic) conditions. These enrichments can be explained by the Fe shuttle model where benthic Fe is transported from the shelf to the euxinic basin and transferred to the sediments by pyrite precipitation in the sulfidic water. The data provide evidence that the Fe shuttle at present results in accumulations of Fe that are larger compared to Fe enrichments during the Litorina Sea stage in the Gotland Deep probably caused by an increase of the benthic Fe flux from the shelf to the basin. The derived Fe enrichments are also larger compared to those in recent Black Sea sediments, which likely reflects the larger shelf to basin ratio of the Gotland Deep compare to the Black Sea. The Fe isotope data show no correlation with the organic C content of the samples indicating that the negative Fe isotope signatures are not associated with organic materials, as was suggested as an alternative explanation for the origin of the isotopically light Fe in sediments from the Litorina Sea stage. Conversely, pyrites carry the negative Fe isotopic signature of the sediments, which supports the Fe shuttle model. Variations in the abundance and Fe isotopic signature of reactive Fe and pyrite with depth suggest that syngenetically formed pyrite in the sulfidic water column has a less negative Fe isotopic composition compared to diagenetically produced pyrite.

Research Highlights
► High Fe/Al and negative δ56Fe in sediments reflect euxinic conditions.
► Iron enrichments occur likely by Fe shuttling from the shelf to the basin.
► Larger Fe excesses in the present compared to Litorina time due to higher Fe flux.
► Larger shelf to basin ratio causes larger Fe excesses than in the Black Sea.
► Syngenetically formed pyrite has less negative δ56Fe than diagenetic pyrite.