CARBON AND HYDROGEN ISOTOPE SYSTEMATICS OF BACTERIAL FORMATION AND OXIDATION OF METHANE

Abstract

The diagenetic cycling of carbon within recent unconsolidated sediments and soils generally can be followed more effectively by discerning changes in the dissolved constituents of the interstitial fluids, rather than by monitoring changes in the bulk or solid organic components. The major dissolved carbon species in diagenetic settings are represented by the two carbon redox end-members CH4 and CO2. Bacterial uptake by methanogens of either CO2 or ''preformed'' reduced carbon substrates such as acetate, methanol or methylated amines can be tracked with the aid of carbon (13C/12C) and hydrogen (D/H=2H/1H) isotopes. The bacterial reduction of CO2 to CH4 is associated with a kinetic isotope effect (KIE) for carbon which discriminates against 13C. This leads to carbon isotope separation between CO2 and CH4 (#C) exceeding 95 and gives rise to δ13CCH4 values as negative as -110%% vs. PDB. The carbon KIE associated with fermentation of methylated substrates is lower (#C is ca. 40 to 60, with δ13CCH4 values of -50%% to -60%%). Hydrogen isotope effects during methanogenesis of methylated substrates can lead to deuterium depletions as large as δDCH4=-531%% vs. SMOW, whereas, bacterial D/H discrimination for the CO2-reduction pathway is significantly less (δDCH4 ca. -170%% to -250%%). These field observations have been confirmed by culture experiments with labeled isotopes, although hydrogen isotope exchange and other factors may influence the hydrogen distributions. Bacterial consumption of CH4, both aerobic and anaerobic, is also associated with KIEs for C and H isotopes that enrich the residual CH4 in the heavier isotopes. Carbon fractionation factors related to CH4 oxidation are generally less than #C=10, although values >20 are known. The KIE for hydrogen (#H) during aerobic and anaerobic CH4 oxidation is between 95 and 285. The differences in C and H isotope ratios of CH4, in combination with the isotope ratios of the coexisting H2O and CO2 pairs, differentiate the various bacterial CH4 generation and consumption pathways, and elucidate the cycling of labile sedimentary carbon.