Extreme methane clumped isotopologue bio-signatures of aerobic and anaerobic methanotrophy: Insights from the Lake Pavin and the Black Sea sediments

Thomas Giunta (1) , Edward D. Young , Jabrane Labidi (2) , Pierre Sansjofre (3) , Didier Jézéquel (2) , Jean-Pierre Donval (1) , Christophe Brandily (1) , Livio Ruffine (1)

1 IFREMER - Institut Français de Recherche pour l'Exploitation de la Mer
2 IPGP (UMR_7154) - Institut de Physique du Globe de Paris
3 IMPMC - Institut de minéralogie, de physique des matériaux et de cosmochimie

Microbial methane oxidation - or methanotrophy - is a key control of the global methane budget on Earth, and perhaps in other planetary systems. Here, we explore the potential role of mass-18 isotopologues of methane, expressed as Δ¹³CH₃D and Δ¹²CH₂D₂ values, in tracking both aerobic and anaerobic methanotrophy in nature. We examine two well documented methanotrophic environments: the Lake Pavin (France) water column, where methane degradation is dominated by aerobic methanotrophy (AeOM), and the Black Sea sediments (offshore Romania), dominated by anaerobic methanotrophy (AOM) coupled to sulfate-reduction. In both settings, lighter isotopologues are preferentially consumed, generating elevated ¹³CH₄/¹²CH₄, ¹²CH₃D/¹²CH₄, ¹³CH₃D/¹²CH₄ and ¹²CH₂D₂/¹²CH₄ ratios. This results in increasing of δ¹³C and δD values in the residual methane for both settings, as observed commonly in systems dominated by methanotrophy. As a result, AeOM and AOM cannot be easily distinguished by the development of δ¹³C and δD. In contrast, the Δ¹³CH₃D and Δ¹²CH₂D₂ (departure from stochastic) values have opposite trajectories, with minimal decreases in the case of the AeOM-dominated system, but dramatic increases in the case of AOM, with Δ¹³CH₃D and Δ¹²CH₂D₂ reaching values as high as 15.7 ‰ and 76.6 ‰, respectively. This contrasting behavior of clumped isotopologues signatures illustrates fundamental distinction between the two processes and the way they segregate methane isotopologues. These data demonstrate that both AeOM and AOM have distinctive kinetic isotope effects in natural settings, consistent with preliminary laboratory work. In particular, we find that γ-values (which measure the deviation to the product of 'normal' bulk isotope fractionation factors) are close to unity in the case of AeOM (i.e. a negligible clumped isotope effect), but significantly below unity in the case of AOM (i.e. strong clumped isotope effect). In addition, our data also illustrate how AOM under low-sulfate conditions may promote methane isotopologue equilibration. Taken together, we suggest these data and apparent isotopologue fractionation factors extrapolated from these two environments may help refine the potential bio-signatures of methane affected by methanotrophy.

Mots clés: Anaerobic oxidation of methane; Aerobic oxidation of methane; Methane clumped isotopes

Domaines: Planète et Univers [physics]

Thomas Giunta, Edward D. Young, Jabrane Labidi, Pierre Sansjofre, Didier Jézéquel, et al.. Extreme methane clumped isotopologue bio-signatures of aerobic and anaerobic methanotrophy: Insights from the Lake Pavin and the Black Sea sediments. Geochimica et Cosmochimica Acta, 2022, 338, pp.34-53. ⟨10.1016/j.gca.2022.09.034⟩. ⟨insu-03839235⟩