Article Dans Une Revue Journal of Geophysical Research : Solid Earth Année : 2024

Clumped Isotope Signatures of Abiotic Methane: The Role of the Combinatorial Isotope Effect

Résumé

Methane clumped isotope signatures of abiogenesis may be diagnostic of the origin of methane on Earth and other planetary bodies. We performed synthesis of abiogenic methane in hydrothermal conditions between 130 and 300°C and determined δ$^{13}$C, δD, Δ$^{13}$CH$_3$D, and Δ$^{12}$CH$_2$D$_2$. The experiments were performed by heating water in the presence of Fe$^0$ powder and CO. The reduction of water on metallic iron led to the formation of H$_2$. CO was reacted with both H$_2$ and H$_2$O, generating both CH$_4$ and CO$_2$. Methane δ$^{13}$C values are isotopically depleted by ∼25‰ relative to the CO starting material. This is consistent with carbon isotopic equilibrium between methane, carbon monoxide and carbon dioxide in our experiments. In contrast, D/H ratios are inconsistent with equilibrium isotopic fractionation, as illustrated by δD values of methane fractionated by ∼500‰ relative to starting H2O. This suggests that under our experimental conditions, hydrogen additions to carbon may be governed by kinetics. Δ$^{13}$CH$_3$D values track experimental temperature, with values between +1.5‰ and +5.0‰ for most samples. In contrast, Δ$^{12}$CH$_2$D$_2$ values are displaced from equilibrium. We find exclusively negative Δ$^{12}$CH$_2$D$_2$ values, showing deficits down to 40‰ relative to thermodynamic equilibrium. We interpret the data as evidence for distinct, kinetically induced D/H pools contributing to methane assembly, that is, a combinatorial effect. The cumulative D/H fractionations associated with CO hydrogenation explain the direction and magnitude of Δ$^{12}$CH$_2$D$_2$ values during abiotic methane formation. We suggest that near equilibrium Δ$^{12}$CH$_3$D with negative Δ$^{12}$CH$_2$D$_2$ signatures will help identify methane formed abiotically in nature.

Domaines

Fichier principal
Vignette du fichier
abiotic_methane_HAL.pdf (1.47 Mo) Télécharger le fichier
Origine Fichiers produits par l'(les) auteur(s)
Licence

Dates et versions

hal-04692834 , version 1 (10-09-2024)

Licence

Identifiants

Citer

Jabrane Labidi, Thomas M. Mccollom, Thomas Giunta, Barbara Sherwood Lollar, William D. Leavitt, et al.. Clumped Isotope Signatures of Abiotic Methane: The Role of the Combinatorial Isotope Effect. Journal of Geophysical Research : Solid Earth, 2024, 129 (7), pp.e2023JB028194. ⟨10.1029/2023JB028194⟩. ⟨hal-04692834⟩
108 Consultations
130 Téléchargements

Altmetric

Partager

  • More