Reversibility controls on extreme methane clumped isotope signatures from anaerobic oxidation of methane

Microbial anaerobic oxidation of methane (AOM) substantially mitigates atmospheric emissions on Earth and is a process to consider for astrobiological targets where has been detected. The measurement doubly substituted, or “clumped”, isotopes proven useful in tracing processes formation oxidation. Both near-equilibrium extreme disequilibrium clumped isotope signatures can be attributed AOM, but, date, understanding the mechanistic environmental controls those lacking. We report measurements compositions residual AOM-active microbial incubations using sediment slurries from Svalbard Santa Barbara Channel seeps. Incubation experiments resulted with very high ?13CH3D ?12CH2D2 values up 19.5‰ 65.1‰, respectively. found similarly fluid samples Chamorro Seamount, serpentinite mud volcano Mariana forearc, suggesting that minimal reversibility AOM intracellular reactions leads kinetic fractionation isotopologues. When conditions were consistent low thermodynamic drive however, isotopologues approached intra-species quasi-equilibrium. This was clearly observed exchange methyl-coenzyme M reductase (Mcr) slurries. Using an isotopologue model, we highlight critical role controlling trajectory gases vs. space during AOM. are generalized as result Mcr-catalyzed operating under near-threshold free energy conditions, shown deep-biosphere incubations. Our results show reaction central meaning ratios affected by production