Biogenic methane formation in marine and freshwater environments: CO2 reduction vs. acetate fermentation—Isotope evidence

Two primary methanogenic pathways can be distinguished using the carbon and hydrogen stable isotope composition of the methane as a function of the coexisting carbon dioxide and formation water precursors. Although both pathways may occur in both marine and freshwater sediments. CO2 reduction is dominant in the sulphate-free zone of the former, while acetate fermentation is the major pathway in freshwater sediments. Methane in marine sediments can be defined isotopically by δ13C −110 to −600/%., and δD −250 to −1700‰. In contrast, methane from freshwater sediments ranges from δ13C −65 to −500/%. and δD −400 to −2500/%.. Carbon isotope fractionations (αcCO2-CH4) are generally between 1.05 and 1.09 for marine sediments, while lower in freshwater sediments (1.04 to 1.06). The relationship of the methane to the formation water indicates the source of the hydrogen for CO2 reduction to be the water directly with an associated hydrogen fractionation of −180 ± 200/%.. The CH4-H2O hydrogen fractionation is larger for acetate fermentation due to the transfer of the methyl group during methanogenesis which is depleted in deuterium and accounts for 34 of the hydrogen in the methane. A model is presented showing that the fourth hydrogen via acetate fermentation may ultimately come from the formation water but is isotopically fractionated. Combination of the carbon and hydrogen isotope fractionations (αC, αD) from CH4 with CO2 and H2O respectively, can clearly delineate the CO2 reduction and acetate fermentation environments. Defining the character of the methanogenic types with carbon and hydrogen isotopes not only provides information about the environment of formation, it is also most useful in distinguishing biogenic from thermogenic methane gases.