Carbon isotope equilibration during sulphate-limited anaerobic oxidation of methane

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.