Methane oxidation in sediment and water column environments—Isotope evidence

Microbial anaerobic oxidation of methane in sediments is a kinetic process associated with a carbon isotope effect which enriches the remaining methane in ¹³C. Three, models: % residual methane, higher hydrocarbon enrichment, and CO₂-CH₄ coexisting pairs are used to independently calculate fractionation factors (αc) in the range of 1.002–1.014, which overlap the range determined by culture studies, αc is smaller than that associated with methanogenesis by CO₂ reduction or by acetate-type fermentation, and comparison of the coexisting CO₂-CH₄ pairs can distinguish between the formation and consumption processes. Methane oxidation in sediments continues to a threshold concentration of ca. 0.2 mM; the residual methane is either unavailable or unattractive to consumption. Minor amounts of methane may also be produced simultaneously in the methane consumption zone, influencing the apparent fractionation factor in this zone.