Climate-controlled variability of iron deposition in the Central Arctic Ocean (southern Mendeleev Ridge) over the last 130,000 years

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Distinct brown-coloured Mn-rich layers are common features in Quaternary sediments of the central Arctic Ocean, but their paleoenvironmental significance is disputed. Glacial–interglacial changes in bottom water ventilation, variable riverine input, and diagenetic processes have all been suggested to cause the cyclicity in optical and geochemical parameters in these deposits. To better assess the nature and origin of these sediment intervals and their implications for nutrient cycles, we applied bulk geochemical analyses and a sequential Fe extraction scheme to a sediment record covering the last ~130ka on the southern Mendeleev Ridge. All brown layers in this core are enriched in Mn, but also in Fe (oxyhydr)oxides (e.g., ferrihydrite, lepidocrocite, goethite) that contribute to the characteristic sediment colour. Phosphorus and several trace metals (As, Co, Cu, Mo, Ni) are also enriched, indicating that both Mn and Fe (oxyhydr)oxides adsorbed P and trace metals from the water column. The metal enrichments are most probably caused by a combination of enhanced Mn and Fe input from Arctic rivers and/or the erosion of shelf deposits, coupled with early diagenetic processes leading to precipitation of authigenic Mn and Fe (oxyhydr)oxides close to the sediment–water interface. Both primary supply and diagenetic recycling of metals should have been enhanced by a more intense hydrological cycle under warmer climate conditions, combined with seasonal increases in productivity and reactive organic matter export during ice-free conditions in the Central Arctic Ocean. Consequently, the studied metal (oxyhydr)oxide-rich layers were formed under interglacial/interstadial climatic conditions. In contrast, sheet silicate-bound Fe (e.g., chlorite), a second major Fe pool in these sediments (15–35% of total Fe), is enriched between the oxide-rich layers. Thus, the delivery of Fe-rich sheet silicates from the East Siberian hinterland or the Bering Sea, and subsequent deposition on the Mendeleev Ridge, was enhanced by more intense physical weathering during glacial/stadial periods. These contrasts in Fe speciation between the brown layers and the adjacent sediments document the direct response of biogeochemical element cycles to glacial–interglacial variations in river input, weathering regime, and sea level in the Arctic Ocean.