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

• Published in: Geochimica et Cosmochimica Acta
• Main Authors: Liu, Jiarui, Harris, Rachel L., Ash, Jeanine L., Ferry, James G., Krause, Sebastian J. E., Labidi, Jabrane, Prakash, Divya, Sherwood Lollar, Barbara, Treude, Tina, Warr, Oliver, Young, Edward D.
• Other Authors: Institut de Physique du Globe de Paris (IPGP (UMR_7154)), Institut national des sciences de l'Univers (INSU - CNRS)-Université de La Réunion (UR)-Institut de Physique du Globe de Paris (IPG Paris)-Centre National de la Recherche Scientifique (CNRS)-Université Paris Cité (UPCité)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2023
• Subjects: Methane isotopologues; Methyl-coenzyme M reductase; Kinetic fractionation; Equilibrium fractionation; Isotopic bond re-ordering; Earth Science; [SDU]Sciences of the Universe [physics]; Svalbard
• Online Access: https://insu.hal.science/insu-04155736; https://insu.hal.science/insu-04155736/document; https://insu.hal.science/insu-04155736/file/1-s2.0-S0016703723000972-main.pdf; https://doi.org/10.1016/j.gca.2023.02.022

International audience Microbial anaerobic oxidation of methane (AOM) substantially mitigates atmospheric methane emissions on Earth and is a process to consider for astrobiological targets where methane has been detected. The measurement of doubly substituted, or "clumped", methane isotopes has proven useful in tracing processes of methane formation and oxidation. Both near-equilibrium and extreme disequilibrium methane clumped isotope signatures can be attributed to AOM, but, to date, understanding the mechanistic and environmental controls on those signatures has been lacking. We report measurements of methane clumped isotope compositions of residual methane in AOM-active microbial incubations using sediment slurries from Svalbard and Santa Barbara Channel methane seeps. Incubation experiments of Svalbard sediment slurries resulted in residual methane with very high Δ 13 CH 3 D and Δ 12 CH 2 D 2 values up to 19.5‰ and 65.1‰, respectively. We found similarly high Δ 13 CH 3 D and Δ 12 CH 2 D 2 values in fluid samples from the Chamorro Seamount, a serpentinite mud volcano in the Mariana forearc, suggesting that minimal reversibility of AOM intracellular reactions leads to kinetic fractionation of clumped isotopologues. When conditions were consistent with a low thermodynamic drive for AOM, however, methane isotopologues approached intra-species quasi-equilibrium. This was clearly observed in isotope exchange experiments with methyl-coenzyme M reductase (Mcr) and in microbial incubations of the Santa Barbara Channel sediment slurries. Using an isotopologue fractionation model, we highlight the critical role of reversibility in controlling the trajectory of gases in Δ 13 CH 3 D vs. Δ 12 CH 2 D 2 space during AOM. The near-equilibrium methane isotopologue signatures are generalized as a result of the Mcr-catalyzed intracellular isotope exchange operating under near-threshold free energy conditions, as shown in the deep-biosphere incubations. Our results show that the reversibility of the Mcr-catalyzed reaction is ...