228Ra as a tracer for shelf water in the arctic ocean

During its formation on the vast Siberian shelves, Arctic surface water is strongly enriched in 228Ra. When 228Ra of surface samples from the Arctic interior is plotted against the river water component fr, derived from salinity, δ180 and silicate as tracers, a shelfwater end-member can be calculated by extrapolation. Highest values occur in the core of the Transpolar Drift, indicating rapid transport of surface water, comparable to the known ice-drift pattern. Low values at Ice Island T3 are explained by radioactive decay (5.8 year half-life) during the long residence time of fresh and shelf-influenced water in the Beaufort Gyre. Some evidence of decay and, consequently, prolonged transit times is also observed in the southern Nansen Basin. Future research is required to establish the seasonal, interannual and spatial variability of 228Ra on the shelves in order to determine the full potential of 228Ra as a tracer for the origin and transport rates of shelfwater in the Arctic Basin. from the usual 228Ra signature of bottom waters, the tracer is also observed in intermediate layers where it gives evidence of recent contact with slope or shelf sediments. The Atlantic Inflow along the Barents slope is enriched down to 2000 m. The return flow over the Amundsen Basin and Lomonosov Ridge carries a shelf signature of 228Ra and 228Th down to more than 1200 m depth, in agreement with the enrichment observed in 137Cs. Deeper maxima around 1900–2500 m in the Nansen Basin are interpreted as inflow of bottom water from the Norwegian Sea through Fram Strait.