Evidence for methane isotopic bond re-ordering in gas reservoirs sourcing cold seeps from the Sea of Marmara

T Giunta (1) , Jabrane Labidi (2) , I E Kohl (3) , L Ruffine (4) , J P Donval (1) , L Géli (1) , M N Çağatay (5) , H Lu (6) , E D Young (3)

1 IFREMER - Institut Français de Recherche pour l'Exploitation de la Mer
2 IPGP (UMR_7154) - Institut de Physique du Globe de Paris
3 EPSS - Department of Earth, Planetary and Space Sciences [Los Angeles]
4 GM - Unité Géosciences Marines
5 ITÜ - Istanbul Technical University
6 Peking University [Beijing]

The measurement of methane clumped isotopologues (Δ 13 CH3D and Δ 12 CH2D2) allows exploring isotope bond ordering within methane molecules, and may reveal equilibrium temperatures. Whether such temperature reflects the formation or re-equilibration temperature of the methane is not well understood, but would have critical implications for the use of methane clumped isotopologues as geothermometers. Here we investigate gas bubbles from vigorous emissions at cold seeps (n = 14) in the Sea of Marmara, Turkey. These cold seeps are sourced from deeper sedimentary reservoirs. Conventional geochemical tracers such as carbon and hydrogen bulk isotopic ratios (13 C/ 12 C and D/H) or n-alkane molecular ratios, suggest these gases reflect various degrees of mixing between thermogenic and microbial sources. Some samples would generally be considered purely microbial in origin (C1/C2+ > 1500; δ 13 C <-60 ‰). We report measurements of Δ 13 CH3D and Δ 12 CH2D2 showing that a fraction of those gases are in internal thermodynamic equilibrium, with the abundances of the two mass-18 isotopologues indicating concordant temperatures of ~90 °C and ~130 °C. These concordant temperatures are recorded by gases of putative microbial and thermogenic origin; the temperatures of equilibration are irrespective of the formation mechanism of the gases. We conclude that the two high-temperatures recorded by Δ 13 CH3D and 1 *Manuscript Click here to view linked References Δ 12 CH2D2 are best explained by non-enzymatic re-equilibration at two local subsurface temperatures. First principles suggest that unequal rates of exchange is possible. Disequilibrium signatures where the two isotopologues yield discordant apparent temperatures are exhibited by other samples. In those cases the data define a trend of variable D 13 CH3D at nearly constant D 12 CH2D2. These signatures are enigmatic, and we investigate and reject multiple possible explanations including mixing, diffusion or Anaerobic Oxidation of Methane. Different rates of re-equilibration between the two rare isotopologues is implied, although lacks experimental foundation at present. In general, all of these data point towards reequilibration of the mass-18 methane isotopologues as an important process.

Domaines: Océanographie

T Giunta, Jabrane Labidi, I E Kohl, L Ruffine, J P Donval, et al.. Evidence for methane isotopic bond re-ordering in gas reservoirs sourcing cold seeps from the Sea of Marmara. Earth and Planetary Science Letters, 2021, 553, pp.116619. ⟨10.1016/j.epsl.2020.116619⟩. ⟨hal-03209518⟩