The geochemistry of rhenium and osmium in recent sediments from the Black Sea

Abstract The Os isotopic composition of recent sediments from the Black Sea are reported with the concentrations of Re, Os, and major and minor elements. Osmium concentrations range from 0.23 to 0.69 ppb and Re concentrations range from 21 to 85 ppb. Concentrations of both elements are large relative to average crustal material and are positively correlated with organic carbon. 187 Os 186 Os ratios range from 5.7 to 7.1; all of these values are less radiogenic than the estimated 187 Os 186 Os ratio of seawater and average continental crust material. Variations in the Os isotopic composition of these sediments are attributed to mixing of a hydrogenous component ( 187 Os 186 Os ratio between 7 and 9) and a less radiogenic detrital component ( 187 Os 186 Os ratio between 1 and 4). A mixing model is used to determine the fraction of Os in the sediment associated with each component, and the burial fluxes of Re and Os in the Black Sea are calculated. In addition the data are used to estimate the magnitude of systematic error which such initial Os isotopic heterogeneity may introduce into Re-Os age determinations of ancient organic-rich sediments.