Radionuclides in the Weddell Sea@en

228Ra and its granddaughter 228Th were measured on a N-S transect from 45's to the Antarctic continent across the Antarctic Circumpolar Current (ACC) and the Weddell Sea. The distributions of 230Th, 228Th and 228Ra show that southward transport across the ACC of Circumpolar Deep Water (CDW), the source of Warm Deep Water (WDW) in the Weddell Sea, occurs on a time scale between 8 and 30 years, in qualitative agreement with estimates of the upwelling rate of WDW. The distribution of 228Ra in deep waters is controlled by advection and isopycnal mixing rather than diapycnal mixing. In the Weddell Sea, deep-water 228Ra activities reach 15-20 dpm/m**3. Enrichment in deep water is controlled by the production in the deep-sea floor, favoured by low biogenic sediment accumulation rates and consequently high 232Th contents in the surface sediment (3 to 5 dpm/g). The highest 228Ra value (73 dpm/m**3) was observed near the sea floor in a channel where an eastern outflow of Weddell Sea Bottom Water (WSBW) is suspected. It is not yet known whether this value is produced in-situ by accumulation in the stratified bottom water, or contains a Signal of enrichment in shelf- and Ice Shelf Water. High 228Ra activities on the south-eastem shelf (22 dpm/m**3) and low activities offshore yield an estimated residente time of 1.5 years on this shelf and imply slow exchange with offshore waters.