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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

202410194
10.1073/pnas.2410194121
replyReplyearth-sciEarth, Atmospheric, and Planetary Sciences413
42
Letters
Physical Sciences
Earth, Atmospheric, and Planetary Sciences
Reply to Jautzy et al.: Considerations on methane equilibrium—do not forget to add the pepper
Chowdhury Anirban a
Ventura Gregory T. todd.ventura@smu.ca
a 1 https://orcid.org/0009-0001-5217-2758

Owino Yaisa a
Lalk Ellen J. b
MacAdam Natasha c
Dooma John M. a
Ono Shuhei b https://orcid.org/0000-0002-1348-9584

Fowler Martin d
MacDonald Adam c
MacRae R. Andrew a https://orcid.org/0009-0004-4636-814X

Hubert Casey R. J. e https://orcid.org/0000-0002-8691-8116

Bentley Jeremy N. a
Kerr Mitchell J. a https://orcid.org/0009-0003-0042-7178

aDepartment of Geology, Saint Mary's University, Halifax, NS B3H 3C3, Canada
bDepartment of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139
cNova Scotia Department of Natural Resources and Renewables, Government of Nova Scotia, Halifax, NS B3J 3J9, Canada
dApplied Petroleum Technology Ltd., Calgary T3A 2M3, Canada
eGeomicrobiology Group, University of Calgary, Calgary, AB T2N 1N4, Canada
1To whom correspondence may be addressed. Email: todd.ventura@smu.ca.
3 9 2024
10 9 2024
3 9 2024
121 37 e2410194121Copyright © 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).

Research Nova Scotia (RNS) 501100020170 2142 Gregory Todd Ventura Canadian Government | Natural Sciences and Engineering Research Council of Canada (NSERC) 501100000038 RGPIN-2018- 06147 Gregory Todd Ventura access-typefree
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pmcThe letter from Jautzy et al. (1) focuses on the clumped isotope values of seep-collected gas reported as being in equilibrium and therefore usable for methane geothermometry (2). Using various rationale, the authors conclude that the values are likely out of equilibrium and therefore give spurious formation depths when mapped to the local geothermal gradient (3). In doing so, they question whether deep biosphere oases exist above the underlying salt diapirs, which represents the larger focus of the paper.

While the arguments are interesting, the letter misses the broader point of our study’s data analysis and reporting. Our model does not solely rest on the apparent formation temperatures of the methane from isotopologue measurements in tracking the origin of the gas. Apparent formation temperatures are considered in tandem with other geochemical data, including i) ebullient gas composition, ii) radiocarbon data of methane pointing to carbon sources more deeply buried than the seep structure (i.e., radiocarbon dead or nearly so), and iii) calculation of high seepage rates. Most importantly, our model is corroborated by geophysical evidence that led to the remotely operated vehicle surveys of this area; indicating, for example, a fault-influenced gas chimney extending down 250 to 450 mbsf, which intersects a bright seismic anomaly (inferred to be a direct hydrocarbon indicator) that is fed from below by migrating gas as evidenced by amplitude washouts in the seismic reflection data (2). These anomalies are traceable down to 800 to 1,000 m below the seep carbonate platform, which is the same depth determined when independently applying the 13CH3D formation temperature to calculations of the basin’s localized geothermal gradient. Apparent methane formation temperatures derived from the 13CH3D isotopologue technique are therefore consistent with and supported by other lines of evidence.

Additionally, Jautzy et al. argue we established the seep gas methane as being in equilibrium because the derived formation temperatures match the growth conditions of methanogenic archaea. This is not the case. These conditions are instead based on the data’s numerical fit to empirically derived equilibrium ranges (4) as measured from D/H fractionation in methane–hydrogen–water systems (5). While obtaining porewater hydrogen isotope values for the ~1 km deep basin groundwater would be helpful, such collection is not possible and may be irrelevant as marine-sourced basinal brines typically range +10/−25‰ from standard mean ocean water (6–9), which does not significantly shift our data toward kinetic isotope effects.

We therefore rejected a hypothesis that methanotrophy appreciably altered the ebullient seep gas given the additional consideration that the seepage rates vastly differ at both sites, yet their measured molecular and isotopic compositions are nearly the same. Nonetheless, microbial processes associated with microseepage in these sediments are evident from the large range of isotope values found in void space and sediment headspace sourced gas across both seep sites. In this regard, the prospect of doing CH2D2 spectroscopy (10) is indeed an attractive analytical direction, and we agree that hunting for complex, near-surface isotope effects and overprints is an exciting opportunity for future research. We do, however, caution that exclusionary bottlenecks will arise for many scientists if such high-end analyses are made preconditions for otherwise well-grounded research outcomes.

Author contributions

A.C., G.T.V., N.M., S.O., and R.A.M. designed research; A.C., G.T.V., Y.O., E.J.L., N.M., J.M.D., and R.A.M. performed research; G.T.V., N.M., J.M.D., S.O., and J.N.B. contributed new reagents/analytic tools; A.C., G.T.V., Y.O., E.J.L., N.M., J.M.D., S.O., M.F., A.M., R.A.M., C.R.J.H., and M.J.K. analyzed data; and A.C., G.T.V., E.J.L., N.M., and R.A.M. wrote the paper.

Competing interests

The authors declare no competing interest.
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