30 31The purpose of this research is to characterize the mobilization and 32 immobilization processes that control the authigenic accumulation of uranium (U), 33 rhenium (Re) and molybdenum (Mo) in marine sediments. We analyzed these redox-34 sensitive metals (RSM) in benthic chamber, pore water and solid phase samples at a site 35 in Buzzards Bay, Massachusetts, U.S.A., which has high bottom water oxygen 36 concentrations (230-300 mol/L) and high organic matter oxidation rates (390 mol 37 C/cm 2 /y). The oxygen penetration depth varies from 2-9 mm below the sediment-water 38 interface, but pore water sulfide is below detection (< 2 M). The RSM pore water 39 profiles are modeled with a steady-state diagenetic model that includes irrigation, which 40 extends 10-20 cm below the sediment-water interface. To present a consistent 41 description of trace metal diagenesis in marine sediments, RSM results from sediments in 42Buzzards Bay are compared with previous research from sulfidic sediments (Morford et 43 al., GCA 71). 44Release of RSM to pore waters during the remineralization of solid phases occurs 45 near the sediment-water interface at depths above the zone of authigenic RSM formation. 46This release occurs consistently for Mo at both sites, but only in the winter for Re in 47Buzzards Bay and intermittently for U. At the Buzzards Bay site, Re removal to the solid 48 phase extends to the bottom of the profile, while the zone of removal is restricted to ~2-9 49 cm for U and Mo. Authigenic Re formation is independent of the anoxic 50 remineralization rate, which is consistent with an abiotic removal mechanism. The rate 51 of authigenic U formation and its modeled removal rate constant increase with increasing 52 anoxic remineralization rates, and is consistent with U reduction being microbially 53

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.

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