Ra-228 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 Ra-228. When Ra-228 of surface samples from the Arctic interior is plotted against the river water component f(r), derived from salinity, delta(18)O 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 Ra-228 on the shelves in order to determine the full potential of Ra-228 as a tracer for the origin and transport rates of shelfwater in the Arctic Basin.Apart from the usual Ra-228 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 Ra-228 and Th-228 down to more than 1200 m depth, in agreement with the enrichment observed in Cs-137. Deeper maxima around 1900-2500 m in the Nansen Basin are interpreted as inflow of bottom water from the Norwegian Sea through Fram Strait.