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/cm2/y;
OC426: 120±10 ?mol C/cm2/y; EN433-2: 33 ?mol C/cm2/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/cm2/y) and less at EN433-2
and OC426 (0.24±0.05 and 0.13±0.05 nmol/cm2/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/cm2/y), but the
average among the three sites on the MAB is ~0.2 nmol/cm2/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 [O2]bw < 150 ?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 [O2]bw have lower authigenic U accumulation
rates than would be predicted from relationships developed from results that include
locations where [O2]bw < 150 ?M.