HYDROGEN ISOTOPE PARTITIONING IN THE NATURAL CH4-H2O SYSTEM DURING MICROBIAL METHANE GENESIS

Abstract

Distribution of hydrogen and carbon isotopes (D/H and 13C/12C) in the microbial systems CH4–H2O and CH4–CO2 was studied in differentworld’s regions. According to the distribution of these isotopes in the above systems, two types of microbial methane are recognized in theEarth’s crust: (1) resulting from CO2 reduction and (2) produced through acetate fermentation. In the case of methane 1, the biologic distributionof hydrogen and carbon isotopes in the above systems corresponds to the thermodynamic isotope exchange equilibrium at a given temperatureof 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 andδ13C(CH4) = αCbCδ13C(CO2) + d, where αDb and αCb are the general factors of biologic nonequilibrium fractionation of hydrogen and carbonisotopes, 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 ofαDb 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. Variationsin the hydrogen and carbon isotope compositions of microbial methane in various geologic objects are due mainly to the variations in thetemperature of the methanogenic medium and the mixing (in different proportions) of methane 1 with methane 2. The portions of acetatefermentation 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.© 2008, IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved.Keywords: Isotopes; geochemistry; microbial methane genesisIntroductionIn nature, microbial methane is produced in water-saturatedsediments in two main ways — through acetate fermentationand CO2 reduction (Koyama et al., 1973; Schoell, 1980; Takai,1970; Winfrey and Zeikus, 1977) — by the reactionsCH3COOH → CH4 + CO2,(1)CO2 + 4H2→ CH4 + 2H2O.(2)About 70% of natural methane result in process (1), and30%, in process (2) (Jeris and McCarty, 1965; Koyama, 1963;Schlegel et al., 1976; Smith and Mah, 1966). Both processescan run simultaneously, which often yields mixtures ofmethane of both types (Burke et al., 1988; Martens et al.,1986). It is difficult to distinguish between acetate fermenta-tion methane and CO2 reduction methane in geologic objects,because the effect of different factors during the microbialformation of CH4 has been poorly studied.Using stable isotopes of hydrogen (D/H) and carbon(13C/12C) to distinguish a difference between the above waysof microbial methane formation in geologic objects is basedon some empirical factors and is restricted by some poorlysubstantiated assumptions. For example, Nakai et al. (1974)and Erokhin (1978) revealed an empirical linear relationshipbetween the hydrogen isotope compositions of methane andassociated water:δD(CH4) = aδD(H2O) + b,(3)