THE CONCENTRATION AND ISOTOPIC COMPOSITION OF OSMIUM IN THE OCEANS

Osmium is one of the rarer elements in seawater. Analytical difficulti es have previously prevented the direct measurement of the osmium conc entration and isotopic composition in seawater. We report a chemical s eparation procedure that yields quantitative extraction of osmium stan dard and of osmium tracer by iron hydroxide precipitation from seawate r doped with osmium standard, osmium tracer, and FeCl3. The iron hydro xide precipitate is processed to extract osmium, using techniques deve loped for iron meteorites. Utilizing this procedure, water samples fro m the Pacific and Atlantic oceans were analyzed for osmium concentrati on and isotopic composition. Direct determination of the osmium concen tration of seawater gives between 15 and 19 fM kg(-1). Detailed experi ments on different aliquots of one seawater sample from the North Atla ntic Ocean, keeping the amounts of reagents constant, yield concentrat ions from 16 to 19 fM kg(-1). The variability in concentration is outs ide the uncertainty introduced because of blanks and indicates a lack of full equilibration between the osmium tracer and seawater osmium. T he most reliable osmium concentration of the North Atlantic deep ocean water is 19 fM kg(-1) with the Os-187/Os-186 ratio being 8.7 +/- 0.2 (2 sigma). Detailed experiments on one seawater sample from the Centra l Pacific Ocean indicate that the most reliable osmium concentration o f the deep ocean water from the Central Pacific is 19 fM kg(-1) with t he Os-187/Os-186 ratio being 8.7 +/- 0.3 (2 sigma). The directly measu red osmium isotopic composition of the oceans is in good agreement wit h that obtained from the analysis of some rapidly accumulating organic rich sediments (Ravizza and Turekian, 1992). A sample of ambient seaw ater around the Juan de Fuca Ridge gave Os-187/Os-186 = 6.9 +/- 0.4. T his is distinctly lower than the deep-sea water value and may reflect local hydrothermal activity or some analytical difficulty with this sa mple. The osmium isotopic composition of the deep oceans indicates tha t similar to 80 % of the osmium is derived from the continents and the rest from extraterrestrial and hydrothermal sources. Using the iridiu m data of Anbar et al. (1996, 1997) we find that the Os/Ir ratio is si milar to 22 for seawater. The mean residence time of osmium (<(tau)ove r bar>(Os)) cannot be derived directly from these data. However, using the Os/Ir ratio of seawater and the Os/Ir ratio of continental rocks, a lower limit can be established of <(tau)over bar>(Os) greater than or equal to 4.4 x 10(4) years. The Os-187/Os-186 in modern deep sea se diments should be relatively constant and is distinctly less radiogeni c than seawater osmium due to limited evaporation (26%) of osmium from infalling cosmic dust. Relative to the cosmic dust infall the hydroth ermal sources may, however, play a smaller but significant role in the bulk economy of dissolved osmium and other platinum Group Elements in the oceans. Copyright (C) 1997 Elsevier Science Ltd.