Rhenium geochemical cycling: Insights from continental margins

Abstract

We measured rhenium (Re) in sediments (both pore waters and solid phase) from three locations on the Mid-Atlantic Bight (MAB) from the eastern margin of the United States: a northern location on the continental shelf off Massachusetts (OC426, 75m water depth), and two southern locations off North Carolina (EN433-1, 647m water depth and EN433-2, 2648m water depth). These sediments underlie high oxygen bottom waters (250–270μM), but become reducing below the sediment–water interface due to the relatively high organic carbon oxidation rates in sediments. Greater ‘reduction intensity’ is reflected by shallower oxygen penetration, greater ammonium concentrations at depth, and shallower first appearances of Mn and Fe in pore waters. This comparison suggests that for the three sites along the MAB the ‘reduction intensity’ from greatest to least is: EN433-1>OC426>EN433-2.All of the pore water profiles show removal of Re from pore waters. Modeled pore water Re fluxes follow the trend in reducing conditions (EN433-1: 2.0±0.2pmol/cm2/y, OC426: 0.9±0.1pmol/cm2/y, and EN433-2: 0.60±0.03pmol/cm2/y, respectively). Solid phase profiles show authigenic Re enrichment in sediments from all three locations, and the calculated Re accumulation rates also mirror the trend in reducing conditions, with the values decreasing from EN433-1>OC426>EN433-2 (1.5±0.2pmol/cm2/y; 0.29±0.09pmol/cm2/y; 0.19±0.09pmol/cm2/y, respectively).Controls on Re accumulation are determined from the MAB data and from the literature, and include: 1) the extent of reducing conditions, with greater removal of Re from pore waters with greater anoxic organic carbon oxidation as determined by the net dissolved ammonium production, consistent with the expected removal of pore water Re coincident with Fe(III) and sulfate reduction; 2) irrigation, which results in net removal of Re to sediments, augments the diffusive flux of Re across the sediment–water interface, and is more important in shallow, coastal sediments rather than locations from deeper water depths (MAB sites); and 3) seasonal remobilization of Re from surface sediments due to deepening oxygen penetration depth (from 0.2cm to 1cm), although smaller seasonal changes in oxygen penetration depth (e.g., 0.2cm to 0.6cm) are less conducive for remobilizing Re from sediments.There is an empirical relationship with larger Re accumulation rates present when the oxygen penetration depth is shallow. However, there is a stronger correlation between organic carbon oxidation and Re accumulation rates. This latter relationship is more appropriate, since the oxidation of labile organic matter in sediments dictates the sedimentary reducing conditions that are then conducive for Re accumulation in the solid phase.