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 M), but become reducing below the sediment-water interface due to the relatively high organic carbon oxidation rates in sediments (EN433-1: 212 mol C/cm 2 /y; OC426: 12010 mol C/cm 2 /y; EN433-2: 33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 (~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.

Sediment-bound Arsenic and Uranium Within the Bowman–Haley Reservoir, North Dakota

“…The contradicting behavior between these metals suggests that As and U were likely not associated with Fe(II) behavior subsequent to Fe(III) reductive dissolution. Similar iron reduction findings have been reported under similar shallow sediment depths (Andrade et al 2010;Morford et al 2009). BH2 results (Fig.…”

Impact of anaerobic oxidation of methane on the geochemical cycle of redox-sensitive elements at cold-seep sites of the northern South China Sea

“…It is well established that soluble U is reduced to insoluble U under suboxic conditions near the iron reduction zone, and low concentration of dissolved U in pore water would present below the iron reduction zone (e.g. Zheng et al, 2002;Algeo and Tribovillard, 2009;Morford et al, 2009). The low U concentration (o2 nM) almost occurred at or above the depth of 50 cm for D-8, D-7 and D-F cores (Fig.…”

Early diagenesis of redox-sensitive trace metals in the Peru upwelling area – response to ENSO-related oxygen fluctuations in the water column

“…The resulting concentration gradient promotes a downward diffusive flux of U across the benthic boundary. A variety of interlinked factors have been proposed to control the magnitude of this flux, including the bottom water oxygen concentration, the sinking rate of organic carbon and the combined rates of Fe and sulfate reduction (Zheng et al, 2002a;McManus et al, 2005McManus et al, , 2006Morford et al, 2009a). It is generally agreed that most of the U in anoxic sediments is delivered by diffusion across the sediment-bottom water interface (McManus et al, 2005;Morford et al, 2009a).…”

“…A variety of interlinked factors have been proposed to control the magnitude of this flux, including the bottom water oxygen concentration, the sinking rate of organic carbon and the combined rates of Fe and sulfate reduction (Zheng et al, 2002a;McManus et al, 2005McManus et al, , 2006Morford et al, 2009a). It is generally agreed that most of the U in anoxic sediments is delivered by diffusion across the sediment-bottom water interface (McManus et al, 2005;Morford et al, 2009a). Notwithstanding, a few studies have revealed that a significant portion of the U buried in sediments beneath an anoxic water column may be delivered as bioauthigenic U (or particulate non-lithogenic U, PNU) which had originally formed in the photic zone (Anderson et al, 1989a;Zheng et al, 2002a,b).…”

“…Diffusion coefficients for Fe, Mn, Mo (MoO 2À 4 ) and U (UO 2þ 2 ) under standard conditions were taken from Li and Gregory (1974), Morford et al (2009a) pointed out that the D sw of Mo would be more appropriate for calculating diffusive fluxes for UO 2 ðCO 3 Þ 4À 3 , the major U species in seawater. However, in order to ensure consistency with most other studies on the diagenetic behavior of U, we decided to adhere to the D sw of UO 2þ 2 .…”