Enhanced scavenging of super(231)Pa relative to super(230)Th in the South Atlantic south of the Polar Front: Implications for the use of the super(231)Pa/ super(230)Th ratio as a paleoproductivity proxy

The fractionation of super(230)Th and super(231)Pa was investigated throughout the Atlantic sector of the Southern Ocean. Published scavenging models generally assume that the super(231)Pa/ super(230)Th 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 super(231)Pa/ super(230)Th - sub(xs)( super(231)Pa/ super(230)Th) - in surface sediments below the production ratio of both radionuclides in the water column. However, we observed high super(2) sub(x) super(3) sub(s) super(1)Pa / super(230)Th 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 super(231)Pa and super(230)Th 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 degree S. This observation clearly indicates that the high super(2) sub(x) super(3) sub(s) super(1)Pa/ super(230)Th ratios in surface sediments south of the Antarctic Circumpolar Current are produced by a N-S increase in the relative scavenging efficiency of super(231)Pa relative to super(230)Th, most probably due to a change in the chemical composition of particulate matter, and not by a high mass flux. It is speculated that biogenic opal, suggested not to significantly fractionate super(231)Pa and super(230)Th, may explain the enhanced scavenging of super(231)Pa to the south. This assumption is further supported by extremely high super(231)Pa/ super(230)Th ratios up to 0.34 in material collected with sediment traps south of the Polar Front, where fluxes are primarily determined by biogenic opal. Based on these results we conclude that, in regions where the sedimenting flux is dominated by biogenic opal, the super(231)Pa / super(230)Th ratio is not a reliable indicator for the mass flux of particles, thus limiting its use as a paleoproductivity proxy in the Southern Ocean.