Methane in marine sediments

Abstract

Methane forms primarily from the breakdown of organic matter, which can be mediated by microbial activity or occur through thermochemical processes. Thus highest methane contents occur in rapidly accumulating sediments that underlie regions of high primary productivity, such as those bordering most continental margins. Methane in marine sediments occurs in dissolved, gaseous, hydrated, or adsorbed states, as determined by thermodynamic constraints as well as by sediment structure and composition. In the presence of geologic traps, methane accumulates in the sediments as natural gas, and these deposits are commonly exploited commercially. However, the biggest methane reservoir in marine sediments is thought to exist in the form of methane hydrate, where methane molecules are trapped inside a water cage. In the absence of geologic or gas hydrate traps, methane tends to migrate through the sediment via diffusion or advection. During this migration, methane can be effectively consumed by anaerobic microbial oxidation, a process which is thought to remove ~90 % of sedimentary methane, most likely via sulfate reduction mediated by Deltaproteobacteria in conjunction with anaerobic methanotrophic archaea (ANMEs). Methane oxidation in marine sediments is an area of active research, and various alternate electron acceptors and microbial pathways have been identified. At high advective rates, the large flux of methane overwhelms the microbial filter, and it is discharged at the seafloor in what is known as cold seeps, the most impressive of which are imaged as bubble plumes in the water column. Notwithstanding this massive discharge, the global methane input from marine sediments, including that from marine gas hydrate, accounts for a few percent of the global atmospheric methane flux.