Georgia Institute of Technology Ocean Science and Engineering - OSE

The characterization of sediment biogeochemistry at high spatial and temporal resolution is a necessary step in predicting the overall pathways and extent of hydrocarbon degradation in areas affected during and after an oil spill. However, geochemical data for sediments from deeper environments are scarce, and most studies do not measure the full suite of terminal electron acceptors involved in sediment diagenesis. In this study, sediments from the northern Gulf of Mexico were profiled using voltammetric Au/Hg microelectrodes to determine the vertical redox distribution of the main terminal electron acceptors or their reduced metabolites with a high spatial resolution. Drastically different profiles were obtained for shelf, shelf-break, and continental slope sediments with respect to pore water and solid phase chemistries suggesting oil spilled from the Deepwater Horizon may have been exposed to different conditions after sedimentation. Surprisingly, microbial metal reduction dominated on the continental slope, further away from terrestrial sources, whereas sulfate reduction was much more prevalent on the shelf. On the other hand, microbial metal reduction may be an important process in salt marsh sediments subject to tidal irrigation and thus intermittent regeneration of Fe(III) oxides via oxygenation. These findings have potentially important implications. In laboratory incubations with the dissimilatory iron-reducing bacterium Shewanella oneidensis, high concentrations of organic contaminants were degraded rapidly via Fenton reaction during controlled oxic/anaerobic cycling with iron(III) as a substrate. In this process, anaerobic conditions promote the microbial formation of Fe(II) whereas oxic conditions generate reactive oxygen species via Fenton oxidation of Fe(II) by peroxide produced bacterially to degrade the organic contaminants. These results highlight the potential importance of this process near the sediment-water interface in continental margin sediments dominated by Fe(III) reduction, as well as over larger vertical intervals in hydrodynamic environments such as estuaries and salt marshes, where sediment irrigation is significant.