Experimental and theoretical determinations of hydrogen isotopic equilibrium in the system CH4H2H2O from 3 to 200 °C

Abstract

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Controls on the isotopic composition of microbial methane

“…As suggested in previous studies, this apparent hydrogen isotope equilibrium concurrent with carbon isotope disequilibrium may be explained by diffusive mixing of CO 2 and/or methane from different depths within the sediment or isotopic (Rayleigh) distillation ( Fig. 5B ) ( 41 , 71 , 72 ). Rayleigh distillation is not expected to be relevant for H 2 O, as its pool is large enough so that its composition will not change under environmentally plausible conditions.…”

“…3, A and B ). These larger-than-equilibrium 13 ε CO 2 -CH 4 values have been observed in several experimental and environmental datasets and have been suggested to reflect a switch from an equilibrium to a kinetic fractionation in one of the network steps, with a KFF that is larger than the EFF ( 41 ). Our model reveals that this ΔG net – 13 ε CO 2 -CH 4 relation is indeed controlled by the landscape of combined departure from equilibrium of several reactions in the pathway.…”

“…S10). Recent reevaluations of slowly forming biogenic methane sources such as marine sediments, coal beds, or shale gas deposits revealed that apparent CH 4 -CO 2 and CH 4 -H 2 O isotopic equilibrium is common (20,41,43,(63)(64)(65). We used our model to study the isotopic effects in these settings by adopting enzyme kinetic parameters obtained from methanogens grown under energy limitation.…”

Controls on the isotopic composition of microbial methane

“…The opposite situation (i.e., near-equilibrium 2 ε CH 4 -H 2 O and disequilibrium 13 ε CO 2 -CH 4 ) is common in natural environments, though csMR is usually unknown (fig.S9). As suggested in previous studies, this apparent hydrogen isotopic equilibrium concurrent with carbon isotopic disequilibrium may be explained by diffusive mixing of CO 2 and CH 4 , isotopic (Rayleigh) distillation, or diagenetic isotope exchange without net methane production (Fig.4B;8,38,46,47). If disequilibrium 13 ε CO 2 -CH 4 is explained as above rather…”

“…S9). Recent reevaluations of slowly forming biogenic methane sources such as marine sediments, coalbeds or shale gas deposits, revealed that apparent CH 4 -CO 2 and CH 4 -H 2 O isotopic equilibrium is common (8,25,(37)(38)(39)(40). There are currently no laboratory cultures that reproduce the isotopic effects associated with these conditions.…”