Uranium diagenesis in sediments underlying bottom waters with high oxygen content

Abstract

We measured U in sediments (both pore waters and solid phase) from three locations on the middle Atlantic Bight (MAB) from the eastern margin of the United States: a northern location on the continental shelf off Massachusetts (OC426, 75 m water depth), and two southern locations off North Carolina (EN433-1, 647 m water depth and EN433-2, 2648 m water depth). These sediments underlie high oxygen bottom waters (250–270 μM), but become reducing below the sediment–water interface due to the relatively high organic carbon oxidation rates in sediments (EN433-1: 212 μmol C/cm 2/y; OC426: 120 ± 10 μmol C/cm 2/y; EN433-2: 33 μmol C/cm 2/y). Pore water oxygen goes to zero by 1.4–1.5 cm at EN433-1 and OC426 and slightly deeper oxygen penetration depths were measured at EN433-2 (∼4 cm). All of the pore water profiles show removal of U from pore waters. Calculated pore water fluxes are greatest at EN433-1 (0.66 ± 0.08 nmol/cm 2/y) and less at EN433-2 and OC426 (0.24 ± 0.05 and 0.13 ± 0.05 nmol/cm 2/y, respectively). Solid phase profiles show authigenic U enrichment in sediments from all three locations. The average authigenic U concentrations are greater at EN433-1 and OC426 (5.8 ± 0.7 nmol/g and 5.4 ± 0.2 nmol/g, respectively) relative to EN433-2 (4.1 ± 0.8 nmol/g). This progression is consistent with their relative ordering of ‘reduction intensity’, with greatest reducing conditions in sediments from EN433-1, less at OC426 and least at EN433-2. The authigenic U accumulation rate is largest at EN433-1 (0.47 ± 0.05 nmol/cm 2/y), but the average among the three sites on the MAB is ∼0.2 nmol/cm 2/y. Pore water profiles suggest diffusive fluxes across the sediment–water interface that are 1.4–1.7 times greater than authigenic accumulation rates at EN433-1 and EN433-2. These differences are consistent with oxidation and loss of U from the solid phase via irrigation and/or bioturbation, which may compromise the sequestration of U in continental margin sediments that underlie bottom waters with high oxygen concentrations. Previous literature compilations that include data exclusively from locations where [O 2] bw < 150 μM suggest compelling correlations between authigenic U accumulation and organic carbon flux to sediments or organic carbon burial rate. Sediments that underlie waters with high [O 2] bw have lower authigenic U accumulation rates than would be predicted from relationships developed from results that include locations where [O 2] bw < 150 μM.