Broecker 9314674 The potential of osmium isotopic ratios of marine sediments as a geochemical tracer was demonstrated by Pegram et. al. (1992). They showed that there has been a large increase in the 187 Os/186 Os ratio of osmium leached from Pacific red clays over the past 58 million years. They proposed that the leachable osmium is hydrogenous in origin, and reflects the isotopic composition of sea water at the time of precipitation. The observed change in 187 Os/186 Os was attributed to changes in continental weathering rates; higher values corresponding to increased supply of radiogenic 187 Os via rivers. The presumed short residence time of osmium in sea water potentially allows it to respond to rapid (<10 Ka) changes in weathering rates. At present, no tracer with this capacity exists. Since osmium isotopes have not been homogeneous in the oceans in the past, inferences drawn from the record at a single site do not necessarily apply to the whole ocean. However, the heterogeneity of osmium in the oceans may facilitate evaluation of the importance of local changes and their effect on mean ocean water chemistry. This study will evaluate the spatial and temporal variability of osmium in sea water by analyzing the hydrogenous (leachable) fraction of marine carbonate sediments and elucidate the processes that control osmium isotope ratios in the oceans on various time scales, and thus help to lay the groundwork for the use of osmium as a tracer for these processes.
