Distribution of Fe isotopes in particles and colloids in the salinitygradient along the Lena River plume, Laptev Sea

Riverine Fe input is the primary Fe source to the ocean. This study is focused on the distribution of Fe along the Lena River freshwater plume in the Laptev Sea using samples from a 600 km long transect in front of the Lena River mouth. Separation of the particulate (> 0.22 µm), colloidal (0.22 µm–1 kDa), and truly dissolved (  99 % of particulate Fe and about 90 % of the colloidal Fe was observed across the shelf, while the truly dissolved phase was almost constant across the Laptev Sea. Thus, the truly dissolved Fe could be an important source of bioavailable Fe for plankton in the central Arctic Ocean, together with the colloidal Fe. Fe-isotope analysis showed that the particulate phase and the sediment below the Lena River freshwater plume had negative δ 56 Fe values (relative to IRMM-14). The colloidal Fe phase showed negative δ 56 Fe values close to the river mouth (about −0.20 ‰) and positive δ 56 Fe values in the outermost stations (about +0.10 ‰). We suggest that the shelf zone acts as a sink for Fe particles and colloids with negative δ 56 Fe values, representing chemically reactive ferrihydrites. While the positive δ 56 Fe values of the colloidal phase within the outer Lena River freshwater plume, might represent Fe-oxyhydroxides, which remain in the water column, and will be the predominant δ 56 Fe composition in the Arctic Ocean.