Spatial distributions of iron and manganese in surface waters in the Arctic’s Laptev and East Siberian seas

Abstract. The Arctic’s Laptev and East Siberian Seas (LESS) is a region with high biogeochemical activity. Nutrient inputs associated with river runoff and shelf sediment-water exchange processes are vital for supporting primary production in the LESS. However, the dynamics of trace metals such as iron (Fe) and manganese (Mn), which are essential micronutrients for primary producers, remain unknown in the LESS. Here, we present data on Fe and Mn in surface waters in the late summer of 2021 and discuss the factors controlling their concentrations and distributions on the surface of the LESS continental margins. Property of surface waters in the East Siberian Sea and Chukchi Abyssal Plain differed significantly from the Makarov and Amundsen Basins. Nutrient-rich Pacific-sourced water exists in the East Siberian Shelf with a high dissolved Mn (dMn) concentration. Pacific-sourced water also receives a large sedimentary flux that releases dMn onto the continental shelf. Dissolved Fe (dFe) could have been released on the shelf as well; however, dFe remained low in the Pacific-sourced water. This is because dFe re-precipitated more rapidly than dMn because of the difference in removal kinetics. In contrast, relatively nutrient-poor Atlantic-sourced water exists in the Makarov and Amundsen Basins. A positive correlation between the fraction of meteoric water (river water and precipitation), dFe, and humic-like colored dissolved organic matter (CDOM) in Atlantic-sourced water confirmed a common freshwater source for dFe and CDOM. Terrigenous organic ligands likely stabilized Fe in the dissolved phase, which was not the case for Mn. The fraction of sea ice meltwater was not correlated with dFe and dMn in any part of the sampling domain. Our results indicate that the major factors controlling these metal concentrations in the LESS continental margins are river discharge and the input of shelf sediment.