Iron-dependent anaerobic oxidation of methane in coastal surface sediments: Potential controls and impact

Anaerobic oxidation of methane (AOM) is an important process of methane (CH4) removal in sediments. Various studies suggest that AOM coupled to iron oxide (Fe(OH) 3) reduction (Fe-AOM) may complement sulfate-driven AOM in CH4-rich sediments. Here, we apply a transient reaction-transport model to depth profiles of key porewater and sediment constituents for a site in the Bothnian Sea where Fe-AOM has been suggested to occur. At the site, increased eutrophication has led to an upward shift of the sulfate-methane transition zone, submerging Fe(OH) 3 in a zone with high CH4 concentrations. Fe-AOM is thought to lead to a strong accumulation of dissolved iron (Fe 21) in the porewater. Results of a sensitivity analysis identify three potential controls on the occurrence of Fe-AOM in coastal surface sediments: (1) bottom-water sulfate (SO42-) concentrations, (2) Fe(OH)(3) availability, and (3) organic matter (OM) loading. In-situ CH4 production is particularly sensitive to the OM loading and SO42- bottom-water concentration, with higher SO42- concentrations significantly inhibiting methanogenesis and decreasing the potential rates of Fe-AOM. We find that only environments with a low salinity and a relatively high Fe(OH)(3) loading allow for Fe-AOM to occur in surface sediments. This suggests that Fe-AOM in surface sediments is restricted to areas with relatively high rates of sediment deposition such as estuaries and other nearshore systems. By enhancing porewater Fe2+ concentrations in surface sediments and the flux of Fe2+ from sediments to the overlying water, Fe-AOM may contribute to the lateral transfer of iron ("iron shuttling") from the coastal zone to deep basins.