Thomas Holler Tobias Goldhammer Gunter Wegener John W. Pohlman Benjamin Brunner Marcel Kuypers Kai-Uwe Hinrichs Marcus Elvert Marcos Y. Yoshinaga 2013 Collectively, marine sediments comprise the largest reservoir of methane on Earth. The flux of methane from the sea bed to the overlying water column is mitigated by the sulphate-dependent anaerobic oxidation of methane by marine microbes within a discrete sedimentary horizon termed the sulphate–methane transition zone. According to conventional isotope systematics, the biological consumption of methane leaves a residue of methane enriched in ¹³C (refs 1–3). However, in many instances the methane within sulphate–methane transition zones is depleted in ¹³C, consistent with the production of methane, and interpreted as evidence for the intertwined anaerobic oxidation and production of methane^4–6. Here, we report results from experiments in which we incubated cultures of microbial methane consumers with methane and low levels of sulphate, and monitored the stable isotope composition of the methane and dissolved inorganic carbon pools over time. Residual methane became progressively enriched in ¹³C at sulphate concentrations above 0.5 mM, and progressively depleted in ¹³C below this threshold. We attribute the shift to ¹³C depletion during the anaerobic oxidation of methane at low sulphate concentrations to the microbially mediated carbon isotope equilibration between methane and carbon dioxide. We suggest that this isotopic e ect could help to explain the ¹³C-depletion of methane in subseafloor sulphate–methane transition zones. application/pdf 10.1038/ngeo2069 en Carbon isotope equilibration during sulphate-limited anaerobic oxidation of methane article