Laurent Dezileau

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 231 Pa/ 230 Th ratios of surface sediments are primarily determined by the mass flux of particles. In this study we compare the 230 Th normalized vertical fluxes of both total sediments and opal to the 231 Pa/ 230 Th ratios measured from six sediment cores sampled across the Antarctic Circumpolar Current (ACC). We observe a better correlation between the 231 Pa/ 230 Th 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 231 Pa. 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 231 Pa. Finally, our results suggest that the 231 Pa/ 230 Th 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. D 2002 Elsevier Science B.V. All rights reserved.

Enrichissement en uranium authigène dans les sédiments glaciaires de l'océan Austral

Enrichments in authigenic uranium in glacial sediments of the Southern Ocean. Four sediment cores from the Polar frontal zone and the Antarctic zone in the Indian sector of the Southern Ocean present an increase of authigenic uranium during glacial periods. We show that this increase in uranium is due to a combination of (i )a n increase in the lateral transport of organic matter, (ii) a decrease in the oxygen in deep waters, and (iii) a process of diagenesis. It appears that uranium concentration cannot be used as a proxy of palaeoproductivity in the Southern Ocean, as previously suggested by