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Proc Natl Acad Sci U S A
Proc Natl Acad Sci U S A
PNAS
Proceedings of the National Academy of Sciences of the United States of America
0027-8424
1091-6490
National Academy of Sciences

202408793
10.1073/pnas.2408793121
letterLetterearth-sciEarth, Atmospheric, and Planetary Sciences413
42
Letters
Physical Sciences
Earth, Atmospheric, and Planetary Sciences
Diapiric controls on deep-biosphere oases: Take with a grain of salt
Jautzy Josué J. josue.jautzy@nrcan-rncan.gc.ca
a 1 https://orcid.org/0000-0003-3194-3143

Campbell Calvin b https://orcid.org/0000-0002-6319-6536

Desiage Pierre-Arnaud b
Douglas Peter M. J. c
Fosu Benjamin R. a https://orcid.org/0000-0002-3802-5348

Larmagnat Stéphanie a
Savard Martine M. a https://orcid.org/0000-0002-1687-5448

aNatural Resources Canada, Geological Survey of Canada, Québec, QC G1K 9A9, Canada
bNatural Resources Canada, Geological Survey of Canada, Dartmouth, NS B2Y 4A2, Canada
cMcGill University, Department of Earth and Planetary Science, Montréal, QC H3A 0E8, Canada
1To whom correspondence may be addressed. Email: josue.jautzy@nrcan-rncan.gc.ca.
3 9 2024
10 9 2024
3 9 2024
121 37 e2408793121Copyright © 2024 the Author(s). Published by PNAS.
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ This article is distributed under Creative Commons Attribution-NonCommercial-NoDerivatives License 4.0 (CC BY-NC-ND).

access-typefree
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pmcChowdhury et al. (1) claim to find evidence for the existence of deep-seated diapir-related microbial oases for methanogenesis. They present a suite of geophysical (2–4) and geochemical data to support this claim, with the strongest support for their proposition based on gas isotopic data. The authors employ the nondestructive laser spectroscopy-based technique (5) to determine a relatively novel isotopic proxy, the clumped isotopic abundance in methane (CH4). This proxy can be utilized as a geothermometer if CH4 formed at isotopic equilibrium (6). The authors base their main conclusions and overarching model on temperatures derived from measurements of a single clumped isotopic species (i.e., 13CH3D isotopologue) on seep gas CH4 samples, which are assumed to have formed at isotopic equilibrium based on agreement with the temperature range for microbial methanogenesis and assumptions of near H-isotope equilibrium between H2O and CH4.

We argue that these justifications for isotopic equilibrium are insufficient for interpreting the apparent clumped isotope-derived temperature as primary temperatures of formation for the following reasons. First, the agreement of the derived temperature with the microbial methanogenetic range does not support isotopic equilibrium, as this range is wide and disequilibrium could lead to an apparent temperature higher than the original temperature of CH4 formation while still within the microbial range. In addition, only the 13CH3D isotopologue was measured, rather than two CH4 isotopologues that provide access to two independent geothermometers. This single characterization leaves the possibility for isotopic disequilibrium of the 12CH2D2 isotopologue as observed elsewhere (7 and references therein). Moreover, the H-isotope fractionation factor between H2O and CH4 is poorly constrained, as no δ2H measurement of the porewater is reported and no explanation is provided for the selection of the assumed value. Furthermore, extensive CH4-derived authigenic carbonates and shallow CH4-hydrates (0 to 10 m below seafloor) have been previously reported at this site by the authors and others (8, 9). The carbonates provide direct evidence that methanotrophy is acting as a major sink for CH4, with a strong potential for fractionation of the CH4 bulk and clumped isotopes (7). With regard to the CH4-hydrates, the CH4 generated from their dissociation can mix with the proposed deeper CH4 source overprinting any primary isotopic equilibrium signatures, further biasing derived temperatures (10).

In other words, the isotopic systematics of CH4 in this system is not fully captured by the analyses presented here, with a distinct possibility that the CH4 sampled does not reflect isotopic equilibrium. Consequently, the derived temperatures reported may only be apparent rather than accurate representations of the actual environmental temperatures at the microbial CH4 formation depth. The claims made by Chowdhury et al. (1) are therefore highly speculative and require further investigation. As the clumped isotopic measurement employed by the authors is nondestructive and provided that the samples have been properly archived, we propose that a full isotopic investigation of CH4 production and consumption using mass spectrometric dual clumped isotopic measurements (10) should be undertaken to verify the authors conclusions prior to the costly deep seafloor exploration of other speculative microbial oases.

Author contributions

J.J.J., C.C., P-.A.D., P.M.J.D., B.R.F., S.L., and M.M.S. wrote the paper.

Competing interests

The authors declare no competing interest.
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