Shelf–Basin interaction along the Laptev – East Siberian Seas

Extensive biogeochemical transformation of organic matter takes place in the shallow continental shelf seas of Siberia. This, in combination with brine production from sea-ice formation, results in cold bottom waters with relatively high salinity and nutrient concentrations, as well as low oxygen and pH levels. Data from the SWERUS-C3 expedition with icebreaker Oden, July to September 2014, show the distribution of such nutrient rich cold bottom waters along the 20 continental margin from about 140 to 180 oE. The water with maximum nutrient concentration, classically named the upper halocline, is absent over the Lomonosov Ridge at 140 oE while it appears in the Makarov Basin at 150 oE to intensify further eastwards. At the intercept between the Mendeleev Ridge and the East Siberian continental shelf slope, the nutrient maximum is still intense, but distributed across a larger depth interval. The nutrient rich water is found at salinities up to~34.5. East of 170 oE transient tracers show significantly less ventilated waters below about 150 m water depth. This likely 25 results from a local isolation of waters over the Chukchi Abyssa l Plain as the boundary current from the west is steered away from this area by the bathymetry of the Mendeleev Ridge. The water with salinities of ~34.5 has high nutrients and low oxygen concentrations as well as low pH, typically indicating decay of organic matter. A deficit in nitrate relative to phosphate suggests that this process partly occurs under hypoxia. We conclude that the high nutrient water with salinity ~34.5 are formed on the shelf slope in the Mendeleev Ridge region from interior basin water that is trapped for enough time 30 to achieve its signature.