Dissolved osmium in Bengal plain groundwater: Implications for the marine Os budget

Abstract

We report osmium concentrations and isotopic compositions of 40 groundwater samples from the Bengal plain. Groundwaters have Os concentrations (16.9–191.5 pg/kg), about 5–10 times higher than those published for most rivers or seawater. 187Os/ 188Os varies widely (from 0.96 to 2.79) and is related to the isotopic signatures of the sediments constituting local aquifers. Os contents are correlated with those of soluble elements such as Sr, Mg, and Ca, suggesting that differing extents of solid-solution interaction explain most of the variation in measured Os concentrations. The covariation between Os and Sr allows us to estimate the mean Os content of Bengal groundwater (∼70 pg/kg). This concentration is too low to allow Bengal groundwater to significantly influence the marine Os isotopic composition, if likely fresh groundwater discharge rates to the Bay of Bengal are assumed. However, if Bengal groundwater Os concentrations are typical, the global Os groundwater flux would be expected to be around 180 kg/year, making it the second largest input of Os to the ocean after the river flux. Including this flux in the current Os marine budget, and assuming that this and other fluxes have remained constant with time, would decrease the calculated residence time of Os in the ocean by about 30%.