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.
The depth of oxygen penetration into marine sediments differs considerably from one region to another. In areas with high rates of microbial respiration, O2 penetrates only millimetres to centimetres into the sediments, but active anaerobic microbial communities are present in sediments hundreds of metres or more below the sea floor. In areas with low sedimentary respiration, O2 penetrates much deeper but the depth to which microbial communities persist was previously unknown. The sediments underlying the South Pacific Gyre exhibit extremely low areal rates of respiration. Here we show that, in this region, microbial cells and aerobic respiration persist through the entire sediment sequence to depths of at least 75 metres below sea floor. Based on the Redfield stoichiometry of dissolved O2 and nitrate, we suggest that net aerobic respiration in these sediments is coupled to oxidation of marine organic matter. We identify a relationship of O2 penetration depth to sedimentation rate and sediment thickness. Extrapolating this relationship, we suggest that oxygen and aerobic communities may occur throughout the entire sediment sequence in 15–44% of the Pacific and 9–37% of the global sea floor. Subduction of the sediment and basalt from these regions is a source of oxidized material to the mantle.