Uranium diagenesis in sediments underlying bottom waters with high oxygen content

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 mu M), but become reducing below the sediment-water interface due to the relatively high organic carbon oxidation rates in sediments (EN433-1: 212 mu mol C/cm(2)/y; OC426: 120 +/- 10 mu mol C/cm(2)/y; EN433-2: 33 mu mol 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 (similar to 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 Eire 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 similar to 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 mu M 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 front relationships developed from results that include locations where [O-2](bw) < 150 mu M. (C) 2009 Elsevier Ltd. All rights reserved.