Radionuclides of surface sediments and water samples in the South Atlantic south of the Polar Front

Published scavenging models generally assume that the 231Pa/230T ratio of surface sediments is primarily determined by the mass flux of particles. This relationship holds north of the Polar Front, where low primary productivity coincides with ratios of unsupported 231Pa/230Th-xs(231Pa/230Th) - in surface sediments below the production ratio of both radionuclides in the water column.

However, we observed high xs231Pa/230Th ratios, conventionally interpreted as a high-productivity signal, in surface sediments south of the Polar Front, especially throughout the Weddell Sea, in contradiction with the low particle flux of this region. Measurements of both dissolved and particulate fractions of 231Pa and 230Th in the water column revealed a strong N-S decrease in the Th/Pa fractionation factor, from typical open ocean values around 10 north of the Polar Front to values between 1 and 2 south of 60°S. This observation clearly indicates that the high xs231Pa/230Th ratios in surface sediments south of the Antarctic Circumpolar Current are produced by a N-S increase in the relative scavenging efficiency of 231Pa relative to 230Th, most probably due to a change in the chemical composition of particulate matter, and not by a high mass flux.