Isotope fractionation in the systems CH4–H2O and CH4–CO2 during microbial methane genesis in the Earth's crust

Distribution of hydrogen and carbon isotopes (D/H and 13C/12C) in the microbial systems CH4–H2O and CH4–CO2 was studied in different world's regions. According to the distribution of these isotopes in the above systems, two types of microbial methane are recognized in the Earth's crust: (1) resulting from CO2 reduction and (2) produced through acetate fermentation. In the case of methane 1, the biologic distribution of hydrogen and carbon isotopes in the above systems corresponds to the thermodynamic isotope exchange equilibrium at a given temperature of the methane genesis medium. In the case of methane 2, the same systems show mainly a nonequilibrium distribution of these isotopes. We have revealed a linear relationship between the isotopic compositions of CH4, H2O, and CO2: δD(CH4) = αDbDδD(H2O) + b and δ13C(CH4) = αCbCδ13C(CO2) + d, where αDb and αCb are the general factors of biologic nonequilibrium fractionation of hydrogen and carbon isotopes, respectively, in the systems CH4–H2O and CH4–CO2. These factors are determined from the equations 103 ln αDb = −477.357(106/T2) + 3458.55 and 103 ln αCb = −277.954(106/T2) + 1988.677, where T(K) is the temperature of the acetate methanogenic medium. The values of αDb and αCb do not depend (in contrast to the values of b and d) on the kind of bacteria and the temperature of the methane genesis medium. Based on thermodynamic data, we proposed a model for the formation of the isotopic composition of microbial methane in nature. Variations in the hydrogen and carbon isotope compositions of microbial methane in various geologic objects are due mainly to the variations in the temperature of the methanogenic medium and the mixing (in different proportions) of methane 1 with methane 2. The portions of acetate fermentation methane in the total balance of microbial methane in different geologic objects vary over a wide range of values: 52 to 100% in marine deposits of Cape Lookout Bight, North Carolina, US; 65 to 100% in surface fresh waters of Lake Wuermsee, Germany; and 35% in Lake Kivu, East Central Africa.