Clumped isotopes of methane constrain the global magnitude of microbial methane oxidation in wetlands

Wetlands contribute one-third of global methane emissions, but the role of microbial methane consumption in regulating these emissions and their isotopologue signatures remains unclear. Here, we show that anaerobic oxidation of methane (AOM) in a coastal wetland imparts a distinct signature to the clumped isotopes of methane, characterized by a substantial positive shift in Δ13CH3D and a comparatively smaller shift in Δ12CH2D2. The consistent positive departure from the methanogenic endmember serves as a powerful tracer for cryptic methane cycling. Applying this finding to the global methane budget reveals a paradox: the globally averaged source lacks this AOM signature, and its isotopologue composition can be explained by a simple mixture of thermogenic and microbial sources. Using a Bayesian Markov Chain Monte Carlo model, we constrain the permissible global rate constants to upper limits of ∼0.1−1 yr−1 for the anaerobic sink and ∼0.1 yr−1 for the aerobic sink. This work develops methane clumped isotopologues as a novel tool for refining the global methane budget and presents a top-down framework to quantify the impact of microbial oxidation on global methane fluxes and their resulting atmospheric signatures.