Particulate organic matter (POM) that rains to the seafloor and is incorporated into marine sediment provides essential elements and energy to the microbes living in the deep biosphere. Chemical profiles in pore water of organic matter-rich sediment indicate that microbes use chemical species that accept electrons to oxidize POM in the order of how much energy they provide. Deeper microbes use methane rather than POM for energy in a reaction called anaerobic oxidation of methane (AOM). AOM by sulfate reduction is an important sink for methane, regulating methane fluxes and is critical in maintaining greenhouse gas stability. Deeper in the sediment than AOM by sulfate reduction, AOM also occurs through the reduction of iron (Fe). It is not clear how deep this reaction extends and if it is prevalent globally. If this microbial process is ubiquitous in continental margin sediment, it could impact our estimates of methane stored in methane hydrates and our understanding of what reactions are sustaining the deep microbial community. We propose to evaluate this process using samples collected by IODP at the Hikurangi subduction zone (New Zealand) and the Gulf of Mexico through pore water chemical analyses, sediment analyses, and numerical reaction transport modeling.