Anaerobic oxidation of methane and its impact on iron and phosphorus cycling in marine sediments

Abstract

Methane (CH4) represents one of the key compounds in the global carbon (C) cycle. A major proportion of the CH4 in the Earth system is produced in marine sediments by methanogenesis, the final step in the gradual fermentation of organic matter deposited on the seafloor. Once emitted to the atmosphere, CH4 acts as a powerful greenhouse gas. Despite high rates of methanogenesis in continental shelf and slope environments, the ocean today contributes only a relatively small amount of this potent greenhouse gas to the global atmospheric budget. The low atmospheric CH4 efflux from the ocean is largely due to the effective biological removal of dissolved CH4 through anaerobic oxidation with sulfate (SO42-) in marine sediments. This removal of pore water CH4 occurs within a distinct sulfate/methane transition zone (SMTZ), preventing the large amounts of CH4 generated in marine sediments from escaping to the water column. The relevant pathways and the environmental factors that control the rates of CH4 oxidation in marine sediments are, however, still incompletely understood. In some settings, for example, pore water CH4 is found throughout the SO42--bearing zone, pointing towards an inefficient CH4 oxidation by SO42- in certain marine environments. Other more recent findings further indicate that nitrate and nitrite, as well as metal oxides (e.g. manganese and iron (Fe) oxides) can enhance the conversion of CH4 to CO2 in the absence of oxygen. Sulfate may thus not be the only electron acceptor used by microorganisms to oxidize CH4 in anoxic sediments, but knowledge about the significance of additional electron acceptors for the global CH4 cycle is still lacking. In addition, little is known about how CH4 oxidation may impact the marine cycling of Fe and phosphorus (P), both key nutrients for oceanic phytoplankton. This thesis aims to refine our understanding of the potential impacts and controls of CH4 oxidation in marine sediments. In particular, it discusses how anaerobic oxidation of CH4 affects the sequestration and sedimentary records of Fe and P, using a wide range of geochemical tools. The results presented here also demonstrate that climate change and anthropogenic nutrient loading may alter the position and efficiency of the CH4 oxidation barrier in coastal sediments, which in turn could lead to increased atmospheric CH4 emissions from the coastal ocean.