GEOCHEMISTRY OF URANIUM IN BLACK-SEA SEDIMENTS

The sediment geochemistry of uranium has been studied at four sites in the Black Sea. Samples collected by an in situ pore water sampler show that U is depleted in the upper few centimeters of the sediment column, decreasing from the overlying water value to <0.5 dpm U-238 l-1). Removal of U from sediment pore water supports a diffusional flux of U across the sediment-water interface, and these fluxes range from 15 to 52 dpm cm-2 ky-1. Authigenic solid phase U concentrations are found to increase with depth at the central basin stations, reaching activities of 10-14 dpm g-1. An outer shelf site, characterized by a high sediment accumulation rate, shows no apparent U accumulation in the solid phase. Fluxes calculated based on solid phase authigenic U at depth in the sediment and on sedimentation rates derived from Pb-210 data range from 47 to 149 dpm cm-2 ky-1. Differences between the fluxes derived from pore water and solid phase data reflect different accumulation processes, sampling constraints and uncertainties associated with each method/calculation. These fluxes are comparable to those reported for other anoxic basin sediments which, together, account for about 20% of the oceanic U balance.