Concentration, oxidation state, and particulate flux of uranium in the Black Sea

Abstract

In contrast to its unreactive behavior in the open ocean, uranium is removed from seawater to sediments of anoxic marine basins. Four biogeochemical processes have been proposed to account for this removal: active uptake by organisms whose remains are preserved by the anoxic conditions of the sediments; complexation of U(VI) by particulate organic matter; chemical reduction of soluble U(VI) to insoluble U(IV) which is scavenged to the sediments by settling particles; and precipitation of uranium within the sediments themselves. The Black Sea is the largest anoxic marine basin in the world today. Waters below 150–200 m are anoxic and H2S levels build up to about 400 μmol 1−1 in the bottom water (maximum depth ~2200 m). Concentrations of total U and the oxidation state of dissolved U were measured throughout the water column while particulate U fluxes were determined from time series sediment trap samples collected in the deep anoxic zone. Bottom waters were depleted in U by ~40% relative to the initial U content of the water. Two independent methods gave residence times of dissolved U in the deep-water column of about 103 years. Uranium occurred throughout the water column as soluble, chemically labile (reducible) U(VI) rather than the thermodynamically favored U(IV). Uranium is neither reduced to U(IV) by the high levels of H2S nor is it rapidly scavenged from the water column as would be expected for strongly hydrolyzed tetravalent actinides like U(IV). Fluxes of particulate authigenic U measured with sediment traps are at least two orders of magnitude less than the rate of authigenic U burial in Black Sea sediments. Therefore, the first three processes listed above contribute negligibly to the deposition of U in the sediments of anoxic marine basins such as the Black Sea. The dominant removal process must involve precipitation of U in the sediments rather than scavenging from the water column.