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

Riverine Fe input is the primary Fe source for 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 mu m), colloidal (0.22 mu m-1 kDa), and truly dissolved (< 1 kDa) fractions of Fe was carried out. The total Fe concentrations ranged from 0.2 to 57 mu M with Fe dominantly as particulate Fe. The loss of > 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 delta Fe-56 values (relative to IRMM-14). The colloidal Fe phase showed negative delta Fe-56 values close to the river mouth (about -0.20 parts per thousand) and positive delta Fe-56 values in the outermost stations (about +0.10 parts per thousand). We suggest that the shelf zone acts as a sink for Fe particles and colloids with negative delta Fe-56 values, representing chemically reactive ferrihydrites. The positive delta Fe-56 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 delta Fe-56 composition in the Arctic Ocean.