Deposition in organic-rich sediments is a major sink for uranium in the ocean. Uranium concentrations and234U/238U ratios of paniculate matter collected by sediment traps and U concentrations in sediment pore waters collected in situ in Saanich Inlet were measured to study the process of U deposition in anoxic marine basins. The results provided direct evidence for two previously hypothesized mechanisms of U deposition in organic-rich sediments. Dissolved U is incorporated into particles in surface waters, possibly in association with autochthonous organic matter. Fluxes of paniculate authigenic U ranged between 0.025 and 0.4 dpm238U cm-2yr-1, with lowest fluxes occurring during winter months. Particulate U fluxes collected in the upper oxic water column and the lower anoxic waters were not significantly different, indicating that chemical scavenging of U by particles settling through the anoxic H2S-bearing bottom waters of Saanich Inlet contributes negligibly to the total paniculate U flux. Dissolved U also diffuses into the sediments where it is precipitated. The calculated diffusive flux of U into the sediments (5.3 · 10-3dpm238U cm-2yr-1) is much smaller than the particulate U flux from the water column, and the sum of these two fluxes is less than the rate of authigenic U burial in the sediments (0.48 dpm238U cm-2yr-1), suggesting that an additional mechanism of U precipitation may be taking place at, or near, the sediment-water interface. The pore water data are consistent with thermodynamic predictions that U should be reduced and precipitated under the Eh and pH conditions of the sediments and suggest that previous studies which have indicated a flux of dissolved U out of anoxic marine sediments suffered from serious sampling artifacts. © 1989.
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