The 231Pa/230Th ratio as a proxy for past changes in opal fluxes in the Indian sector of the Southern Ocean

Published scavenging models generally assume that the 231Pa/230Th ratios of surface sediments are primarily determined by the mass flux of particles. In this study we compare the 230Th normalized vertical fluxes of both total sediments and opal to the 231Pa/230Th ratios measured from six sediment cores sampled across the Antarctic Circumpolar Current (ACC). We observe a better correlation between the 231Pa/230Th ratios and the vertical opal fluxes corrected for dissolution than with total sediment vertical fluxes. This observation indicates that opal may explain the enhanced scavenging of 231Pa. This result is consistent with the studies of Loeff and Berger [Deep-Sea Res. 40 (2) (1993) 339], Kumar et al. [Nature 378 (1995) 675] and Walter et al. [Earth Planet. Sci. Lett. 149 (1997) 85] who speculated that opal may explain enhanced scavenging of 231Pa. Finally, our results suggest that the 231Pa/230Th ratio is a reliable indicator of opal mass flux and can be used, taking some precautions, as a proxy for opal paleoproductivity in the Indian sector of the Southern Ocean.