Methoxyl stable isotopic constraints on the origins and limits of coal-bed methane

Description Coaling in the deep Despite our current reliance on fossil carbon for energy, the biogeochemical reactions that produce coal and natural gas aren’t entirely understood. Lloyd et al. tested the chemistry and isotope composition in samples ranging from wood to hard, mature coal (see the Perspective by Keppler). Methyoxyl groups in this organic material, which are a potential source of methane, declined with maturity, whereas the carbon-13 fraction increased gradually. The most plausible explanation for this observed pattern is biological demethylation under substrate-limited conditions. These results help us understand the processes that form coal and natural gas on geologic time scales. —MAF Site-specific isotope abundances are consistent with natural gas formation by deep biosphere microbes. Microbial coal-bed methane is an important economic resource and source of a potent greenhouse gas, but controls on its formation are poorly understood. To test whether the microbial degradability of coal limits microbial methane, we monitored methoxyl group demethylation—a reaction that feeds methanogenesis—in a global sample suite ranging in maturity from wood to bituminous coal. Carbon isotopic compositions of residual methoxyl groups were inconsistent with a thermal reaction, instead implying a substrate-limited biologic process. This suggests that deep biosphere communities participated in transforming plant matter to coal on geologic time scales and that methoxyl abundance influences coal-bed methane yield. Carbon isotopic enrichments resulting from microbial methylotrophy also explain an enigmatic offset in the carbon-13 content of microbial methane from coals and conventional hydrocarbon deposits.