Anaerobic oxidation of methane (AOM) is an important process of methane (CH 4 ) removal in sediments. Various studies suggest that AOM coupled to iron oxide (Fe(OH) 3 ) reduction (Fe-AOM) may complement sulfate-driven AOM in CH 4 -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 CH 4 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 (SO 22 4 ) concentrations, (2) Fe(OH) 3 availability, and (3) organic matter (OM) loading. In-situ CH 4 production is particularly sensitive to the OM loading and SO 22 4 bottom-water concentration, with higher SO 22 4 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 Fe 21 concentrations in surface sediments and the flux of Fe 21 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.

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[1] Using a box model of organic carbon (C) and phosphorus (P) cycling in the global ocean, we assess the effects of changes in the continental supply of reactive P, oceanic mixing, and sea level on the marine P cycle on glacial-interglacial timescales. Our model results suggest that mixing is a dominant forcing during early glaciation, causing retention of P in the deep ocean, thereby lowering primary production and associated organic C and P burial. Sea level fall is the dominant forcing during late glaciation, when reduced coastal trapping of reactive P enhances its transfer to the open ocean, restoring primary production and P burial. During postglacial periods, changes in circulation and weathering dominate open ocean processes and oceanic primary production peaks. As primary production is reduced upon glaciation, changes in the marine P cycle are unlikely to drive enhanced primary production and CO 2 drawdown during glaciation.Citation: Tsandev, I., C. P. Slomp, and P. Van Cappellen (2008), Glacial-interglacial variations in marine phosphorus cycling: Implications for ocean productivity, Global Biogeochem. Cycles, 22, GB4004,

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