VARIATIONS IN THE OSMIUM ISOTOPE COMPOSITION OF SEA-WATER OVER THE PAST 200,000 YEARS

The first osmium isotope data are presented for marine sediments spann ing the past 200,000 years (two full glacial cycles) and establish tha t there have been ocean-wide variations in sea-water osmium isotope co mposition over this time. While the osmium isotope composition of sea water has been relatively invariant over much of this interval (Os-187 /Os-186 of 8.5 to 8.6), excursions to lower isotope ratios (Os-187/Os- 186 approximate to 8.1) occur late in the last two glacial periods at 20 ka and 160 ka. These are times of high global aridity, low global t emperature and maximum ice extent. While various interpretations are p ossible, a likely explanation is that the low sea-water ratios reflect reduced chemical weathering of the continents during times of extreme cold and aridity. There is no evidence that there is a net enhancemen t of chemical weathering during periods of continental glaciation. The response of ocean composition to changes in input flux constrain the oceanic residence time of osmium to be less than 12,000 years. (C) 199 8 Elsevier Science B.V. All rights reserved.