The Origin and Distribution of Methane in Marine Sediments

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Methane has been detected in several cores of rapidly deposited (> 50 m/my) deep sea sediments. Other gases, such as carbon dioxide and ethane, are commonly present but only in minor and trace amounts, respectively. The methane originates predominantly from bacterial reduction of CO2, as indicated by complimentary changes with depth in the amount and isotopic composition of redox-linked pore water constituents: sulfate-bicarbonate and bicarbonate-methane.

Presently, no precise determination exists of the amount of gas present under in situ conditions in deep sea sediments. Using C13/C12 isotope ratios of the dissolved bicarbonate and methane, and employing kinetic calculations based on Rayleigh distillation equations, the amounts of methane generated by reduction of carbon di-oxide by hydrogen has been estimated. The amounts calculated suggest that a minimum of 20 mmol CH4/kg interstitial water is formed.

A methane concentration of 20 mmol/kg approaches the amount required for the formation of gas hydrates under pressure-temperature conditions corresponding to a water column of about 500 meters, with a temperature of 5°C at the sediment-water interface. Depth of stability of the gas hydrate within the sediment is directly proportional to: hydrostatic pressure, or height of the water column above the sediment, temperature at the sediment surface, the geothermal gradient, and concentration of methane. Under average oceanic conditions, gas hydrates could be stable in sediment under a 3 km water column to depths of approximately 600 meters, if sufficient methane is present.

Gas hydrates have been proposed as the cause of anomalously high acoustic velocities in the upper 500–600 meters of sediment in the Blake-Bahama outer ridge. It is here suggested that acoustic reflectors in gas-rich sediment is associated with temperature-dependent lithologic transitions, which are in part formed by diagenetic processes involving microbiological methane generation.

Under certain conditions, carbonate ion must be removed from solution during methane production to maintain pH equilibrium between the pore water and the sediment. Authigenic carbonates, typically iron-rich nodules and cements, have been observed in the zone of active methane production. This link between methane production and carbonate precipitation may be an important mechanism for lithification of deep sea sediments.