Direct Measurement of Femtomoles of Osmium and the 187 Os/ 186 Os Ratio in Seawater

Two depth profiles of the osmium concentration and the 187 Os/ 186 Os isotopic ratio in the Indian Ocean showed that the osmium concentration seems to be unaltered by chemical or biological processes occuring in seawater; accordingly, osmium is conservative. These data were obtained from an experimental method that eliminated the problems related to osmium preconcentration. This method led to a new evaluation of the concentration of osmium in seawater; the mean concentration of osmium and the 187 Os/ 186 Os ratio are equal to 10.86 ± 0.07 picograms per kilogram and 8.80 ± 0.07, respectively. The results suggest the existence of an organocomplex that dominates the speciation of osmium in seawater.