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Experimental and theoretical determinations of hydrogen isotopic equilibrium in the system CH₄-H₂-H₂O from 3 to 200°C

Turner, Andrew C. and Korol, Roman and Eldridge, Daniel L. and Bill, Markus and Conrad, Mark E. and Miller, Thomas F., III and Stolper, Daniel A. (2021) Experimental and theoretical determinations of hydrogen isotopic equilibrium in the system CH₄-H₂-H₂O from 3 to 200°C. Geochimica et Cosmochimica Acta . ISSN 0016-7037. doi:10.1016/j.gca.2021.04.026. (In Press)

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The stable isotopic composition of methane (CH₄) is commonly used to fingerprint natural gas origins. Over the past 50 years, there have been numerous proposals that both microbial and thermogenic CH₄ can form in or later attain hydrogen isotopic equilibrium with water (H₂O) and carbon isotopic equilibrium with carbon dioxide (CO₂). Evaluation of such proposals requires knowledge of the equilibrium fractionation factors between CH₄ and H₂O or CO₂ at the temperatures where microbial and thermogenic CH₄ form in or are found in the environment, which is generally less than 200°C. Experimental determinations of these fractionation factors are only available above 200°C, requiring extrapolation of these results beyond the calibrated range or the use of theoretical calculations at lower temperatures. Here, we provide a calibration of the equilibrium hydrogen isotopic fractionation factor for CH₄ and hydrogen gas (H₂) (^DαCH₄(g)-H₂(g)) based on experiments using γ-Al₂O₃ and Ni catalysts from 3 to 200°C. Results were regressed as a 2nd order polynomial of 1000×ln^DαCH₄(g)-H₂ (g) vs. 1/T (K⁻¹) yielding: 1000×ln^DαCH₄(g)-H₂(g) = 3.5317×10⁷/T² + 2.7749×10⁵/T-179.48. We combine this calibration with previous experimental determinations of hydrogen isotope equilibrium between H₂, H₂O(g), and H₂O(l) and we provide an interpolatable experimental calibration of 1000×ln^DαCH₄(g)-H₂O(l) from 3 to 200°C. Our resulting 4th order polynomial is the following equation: 1000×ln^DαCH₄(g)-H₂Ol = -7.9443×10¹²/T⁴ + 8.7772×10¹⁰/T³-3.4973×10⁸/T²+5.4398×10⁵/T-382.05 At 3°C, the value from our calibration differs by 93‰ relative to what would be calculated based on the extrapolation of the only experimental calibration currently available to temperatures below its calibrated range (lowest temperature of 200°C; Horibe and Craig, 1995). We additionally provide new theoretical estimates of hydrogen isotopic equilibrium between CH₄ (g), H₂(g), and H₂O(g) and carbon isotopic equilibrium between CH₄ (g) and CO₂(g) using Path Integral Monte Carlo (PIMC) calculations. Our PIMC calculations for hydrogen isotopic equilibrium between CH₄ and H₂ agree 1:1 with our experiments. Finally, we compile carbon and hydrogen isotopic measurements of CH₄, CO₂, and H₂O from various environmental systems and compare observed differences between carbon and hydrogen isotopes to those expected based on isotopic equilibrium. We find that isotopic compositions of some microbial gases from marine sedimentary, coalbed, and shale environments are consistent with those expected for CH4-H₂O(l) hydrogen and CH₄-CO₂ carbon isotopic equilibrium. In contrast, microbial terrestrial and pure culture gases are not consistent with both CH₄-H₂O(l) hydrogen and CH₄-CO₂ carbon isotopic equilibrium. These results are explained qualitatively using previously developed conceptual models that link free energy gradients available to microorganisms to the degree that their enzymes can promote isotope-exchange reactions between CH₄, CO₂, and H₂O.

Item Type:Article
Related URLs:
URLURL TypeDescription
Turner, Andrew C.0000-0003-1187-2560
Korol, Roman0000-0001-9307-6351
Eldridge, Daniel L.0000-0001-7340-0652
Miller, Thomas F., III0000-0002-1882-5380
Stolper, Daniel A.0000-0003-3299-3177
Alternate Title:Experimental and theoretical determinations of hydrogen isotopic equilibrium in the system CH4-H2-H2O from 3 to 200°C
Additional Information:© 2021 Published by Elsevier Ltd. Received 15 June 2020, Revised 20 April 2021, Accepted 21 April 2021, Available online 29 April 2021. DAS acknowledges support from the National Science Foundation under Grant No. EAR-1911296 and the Donors of the American Chemical Society Petroleum Research Fund. Work at Lawrence Berkeley National Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division, under Award Number DE-AC02-05CH11231. TFM acknowledges support from the National Science Foundation under Grant No. CHE-1611581. RK acknowledges Michael Webb (Princeton) and Xuecheng Tao (Caltech) for valuable discussions, and Ryan DiRisio (U. Washington) for sharing his code that obtains harmonic frequencies from a potential energy surface. We thank Margarida Costa Gomes (ENS Lyon) for clarification on the CD₄/CH₄ gas-liquid partitional experiments. We also thank Edward Hornibrook for handling this paper and David T. Wang and an anonymous reviewer for constructive reviews. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Funding AgencyGrant Number
American Chemical Society Petroleum Research FundUNSPECIFIED
Department of Energy (DOE)DE-AC02-05CH11231
Subject Keywords:experimental calibration; methane isotopic equilibrium; methane geochemistry; Path Integral Monte Carlo calculations
Record Number:CaltechAUTHORS:20210507-093059787
Persistent URL:
Official Citation:Andrew C. Turner, Roman Korol, Daniel L. Eldridge, Markus Bill, Mark E. Conrad, Thomas F. Miller, Daniel A. Stolper, Experimental and theoretical determinations of hydrogen isotopic equilibrium in the system CH4-H2-H2O from 3 to 200°C, Geochimica et Cosmochimica Acta, 2021, ISSN 0016-7037, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:109003
Deposited By: Tony Diaz
Deposited On:07 May 2021 17:06
Last Modified:07 May 2021 17:06

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