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 CH(4) and CO(2). Bacterial uptake by methanogens of either CO(2) or "preformed" reduced carbon substrates such as acetate, methanol or methylated amines can be tracked with the aid of carbon ((13)C/(12)C) and hydrogen (D/H=(2)H/(1)H) isotopes. The bacterial reduction of CO(2) to CH(4) is associated with a kinetic isotope effect (KIE) for carbon which discriminates against (13)C. This leads to carbon isotope separation between CO(2) and CH(4) (epsilon(C)) exceeding 95 and gives rise to delta(13)C(CH4) values as negative as -110 parts per thousand vs. PDB. The carbon KIE associated with fermentation of methylated substrates is lower (epsilon(C), is ca. 40 to 60, with delta(13)C(CH4) values of -50 parts per thousand to -60 parts per thousand). Hydrogen isotope effects during methanogenesis of methylated substrates can lead to deuterium depletions as large as delta D(CH4) = -531 parts per thousand vs. SMOW, whereas, bacterial D/H discrimination for the CO(2)-reduction pathway is significantly less (delta D(CH4) ca. -170 parts per thousand to -250 parts per thousand). 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 CH(4), both aerobic and anaerobic, is also associated with KIEs for C and H isotopes that enrich the residual CH(4) in the heavier isotopes. Carbon fractionation factors related to CH(4) oxidation are generally less than 6, = 10, although values > 20 are known. The KIE for hydrogen (epsilon(H)) during aerobic and anaerobic CH(4) oxidation is between 95 and 285. The differences in C and H isotope ratios of CH(4), in combination with the isotope ratios of the coexisting H(2)O and CO(2) pairs, differentiate the various bacterial CH(4) generation and consumption pathways, and elucidate the cycling of labile sedimentary carbon. (C) 1999 EIsevier Science B.V. All rights reserved.
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