Carbon isotope exchange during anaerobic oxidation of methane (AOM) in sediments of the northeastern South China Sea

The major processes that determine the distribution of methane (CH₄) in anoxic marine sediments are methanogenesis and the anaerobic oxidation of methane (AOM), with organoclastic sulfate reduction exerting an important secondary control. However, the factors leading to the distribution of stable carbon isotopes (δ¹³C) of CH₄ are currently poorly understood, in particular the commonly-observed minimum in δ¹³C-CH₄ at the sulfate-methane transition (SMT) where AOM rates reach maximum values. Conventional isotope systematics predict ¹³C-enrichment of CH₄ in the SMT due to preferential ¹²CH₄ consumption by AOM. Two hypotheses put forward to explain this discrepancy are the addition of ¹²C-enriched CH₄ to porewaters by methanogenesis in close proximity to AOM, and enzymatically-mediated carbon isotope equilibrium between forward and backward AOM at low concentrations of sulfate. To examine this in more detail, field data including δ¹³C of CH₄ and dissolved inorganic carbon (DIC) from the continental margin offshore southwestern Taiwan were simulated with a reaction-transport model. Model simulations showed that the minima in δ¹³C-CH₄ and δ¹³C-DIC in the SMT could only be simulated with carbon isotope equilibrium during AOM. The potential for carbon cycling between methanogenesis and AOM in and just below the SMT was insignificant due to very low rates of methanogenesis. Backward AOM also gives rise to a pronounced kink in the δ¹³C-DIC profile several meters below the SMT that has been observed in previous studies. We suggest that this kink marks the true base of the SMT where forward and backward AOM are operating at very low rates, possibly sustained by cryptic sulfur cycling or barite dissolution.