ENHANCED SCAVENGING OF PA-231 RELATIVE TO TH-230 IN THE SOUTH ATLANTIC SOUTH OF THE POLAR FRONT - IMPLICATIONS FOR THE USE OF THE PA-231 TH-230 RATIO AS A PALEOPRODUCTIVITY PROXY/

The fractionation of Th-230 and Pa-231 was investigated throughout the Atlantic sector of the Southern Ocean. Published scavenging models ge nerally assume that the Pa-231/Th-230 ratio of surface sediments is pr imarily determined by the mass flux of particles. This relationship ho lds north of the Polar Front, where low primary productivity coincides with ratios of unsupported Pa-231/Th-230-(xs)(Pa-231/Th-230)-in surfa ce sediments below the production ratio of both radionuclides in the w ater column. However, we observed high Pa-231(xs)/Th-239 ratios, conve ntionally interpreted as a high-productivity signal, in surface sedime nts south of the Polar Front, especially throughout the Weddell Sea, i n contradiction with the low particle flux of this region. Measurement s of both dissolved and particulate fractions of Pa-231 and Th-230 in the water column revealed a strong N-S decrease in the Th/Pa fractiona tion factor, from typical open ocean values around 10 north of the Pol ar Front to values between 1 and 2 south of 60 degrees S. This observa tion clearly indicates that the high Pa-231(xs)/Th-230 ratios in surfa ce sediments south of the Antarctic Circumpolar Current are produced b y a N-S increase in the relative scavenging efficiency of Pa-231 relat ive to Th-230, most probably due to a change in the chemical compositi on of particulate matter, and not by a high mass flux. It is speculate d that biogenic opal, suggested not to significantly fractionate Pa-23 1 and Th-230, may explain ihs enhanced scavenging of Pa-231 to the sou th. This assumption is further supported by extremely high Pa-231/Th-2 30 ratios up to 0.34 in material collected with sediment traps south o f the Polar Front, where fluxes are primarily determined by biogenic o pal. Based on these results we conclude that, in regions where the sed imenting flux is dominated by biogenic opal, the Pa-231/Th-230 ratio i s not a reliable indicator for the mass flux of particles, thus limiti ng its use as a paleoproductivity proxy in the Southern Ocean.