| ranium can exist in several oxidation states (VI, V, IV) which have strikingly different geochemical behaviors, and transformations among these different oxidation states may exert strong controls on uranium chemistry in the marine environment. To evaluate this possibility, uranium geochemistry was studied with respect to diagenetic redox conditions in a number of sedimentary environments, including the hemipelagic (suboxic) sediments of the NW Atlantic shelf and slope, the anoxic sediments of the Black Sea, Cariaco Trench and San Clemente Basin, and the estuarine sediments of Long Island Sound and the Amazon shelf.;Previous studies of U in the pore waters of anoxic sediments showed high concentrations, possibly caused by oxidation of reduced uranium during sample handling. Comparison of U in pore waters recovered from box cores and in situ by a probe shows elevated pore water U concentrations in box core samples relative to in situ samples. This is consistent with an artifact produced by oxidative release of U to pore water during sampling.;Profiles of U in sediment pore waters collected in situ generally show depletions in U at depth in the sediment. The gradients indicate that removal of U from pore water (to the sediment) occurs in the upper few centimeters in anoxic sediments dominated by sulfate reduction (Black Sea and Cariaco Trench) and over greater depth in the suboxic, hemipelagic sediments of the NW Atlantic characterized by NO;Sediment incubation experiments with Long Island Sound sediments suggest that U removal from pore water occurs by its reduction from +6 to +4. This reduction may be microbially-mediated, either indirectly through the production of biogenic H;Marine sediments are the dominant sinks for U in the oceans. This study has shown that suboxic and anoxic sediments together remove... |
