Osmium Concentration and Composition in the Indian Ocean Water

Variations in the isotopic composition of osmium in seawater, recorded by marine sediments, are a proxy of the relative variations of continent, mantle and meteorites contributions and potentially provide a record of continental weathering. Analyses have been made on several kind of sediment samples but until recently (Sharma et aL, 1997; Koide et aL, 1996) no direct measurement of the present day composition of seawater osmium was carried out because of analytical difficulties. This study presents two depth profiles of the concentration and isotopic composition of osmium in the Indian ocean. Osmium concentration in seawater is a key parameter to determine the residence time of osmium in ocean. This parameter is in tum quite important to under~ stand the significance of the osmium proxy in the ocean as recorded in marine sediments: the residence time determines the response of the ocean to sudden variations in the input of osmium and consequently the shortest period and smallest amplitude of change resolvable by our present analytical precision of osmium measurements.