The carbon budget at Earth’s surface determines Earth’s climate; this is because the partitioning of carbon among various surface reservoirs determines how much CO2 is in the atmosphere, where it acts as the dominant greenhouse gas. The fate of organic carbon in shallow sediments, whether it is buried, oxidized, or made into methane, is fundamentally tied to other sensitive biogeochemical cycles such as nitrogen, iron and sulfur, through their redox couplings. In marine sediments, sulfate plays a particularly important role due to its high abundance, and sulfate reduction is responsible for over half the sedimentary organic matter oxidation in the modern ocean. In addition, the majority of the methane produced in marine sediments is oxidized through sulfate reduction. Thus, sulfate-driven anaerobic methane oxidation is the main process that prevents the escape of methane produced within sediments into the atmosphere in the modern ocean. In the terrestrial environment other electron acceptors such as iron, nitrate and oxygen are more important to the overall breakdown of organic carbon.
In recent years, there has been increasing evidence that the microbial use of sulfur is often coupled in complex ways to sedimentary iron, that is sulfate reduction can be coupled to iron oxidation rather than organic carbon oxidation, or intermediate valence state sulfur species are used to shuttle electrons to iron minerals. I will demonstrate, by the use of sulfur and oxygen isotopes in dissolved sulfate, how marine and marginal marine environments, which exhibit a complex interplay of the subsurface iron, sulfur and carbon cycles, are poised to become significant sources of methane if the careful balance between these geochemical species is changed.