Uranium Precipitation in Black Sea Sediments

Abstract

Previous studies have shown that organic-rich sediments of anoxic marine basins like the Black Sea are enriched in uranium, and that the seawater itself is the source of the uranium. An earlier study of uranium in the water column of the Black Sea showed that the processes of uranium precipitation are not occurring in the water column, from which it was inferred that the precipitation process occurs in the sediments. Concentrations of U in sediments and associated pore waters were measured at three sites in the Black Sea: an abyssal plain site in the central eastern basin, and two sites on the shelf along a transect out from the Bay of Sinop: one shallow site underlying oxygenated bottom waters and one deeper site underlying sulfidic bottom waters. Similar pore water uranium profiles were found at all three sites, indicating that similar processes of U precipitation are occurring at all of these sites. Authigenic U contents of the sediments are highest in the abyssal sediments, lowest in sediments from the outer shelf, and intermediate at the inner shelf site. Chemically reducing conditions of the sediments themselves, rather than the condition of the bottom waters or the composition of the sediments, lead to precipitation of uranium. The concentration of authigenic uranium that builds up in the sediments is inversely related to sediment accumulation rate.