Reduced scavenging of super(230)Th in the Weddell Sea: implications for paleoceanographic reconstructions in the South Atlantic

The scavenging of super(230)Th and super(231)Pa was investigated in the central Weddell Sea by combining results from a sediment trap and three sediment cores. Scavenging of both radionuclides is closely coupled with the annual cycle of particle fluxes. For super(230)Th the mean radionuclide flux measured in the trap is only 40% of its expected flux from production in the water column. This value is in excellent agreement with the long-term record in the sediment cores (33-43%). Similar results were obtained for super(231)Pa, although burial fluxes are generally higher. The data suggest that during the last 130 ka the Weddell Sea has been a net source for both radionuclides, with more than half of the super(230)Th and about half of the super(231)Pa being exported. As a consequence, super(230)Th normalized rain rates (assuming a constant flux equal to the production rate) overestimate the true rain rate in the Weddell Sea by 150%. The laterally transported super(230)Th and super(231)Pa activity exits the Weddell Sea to the north, where it is incorporated into the eastward flowing Antarctic Circumpolar Current (ACC). There it is scavenged in addition to local production. During its residence time in the S-Atlantic sector of the ACC about 3/4 of the dissolved super(230)Th imported from the Weddell Sea is transfer-red onto particles. Whether this particulate super(230)Th is entirely deposited in the S-Atlantic or is distributed over a larger area extending into the Indian and Pacific sectors of the ACC remains an open question. In the ACC, Th-normalization therefore leads to an underestimation of fluxes, but the effect is probably less than 50%. Interglacial--glacial shifts in the position of the productive belt are believed to cause temporal and regional variations in the deposition rate for super(230)Th.