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

Journal Article · Wed Dec 01 00:00:00 EST 2021 · Geochimica et Cosmochimica Acta

DOI:https://doi.org/10.1016/j.gca.2021.04.026· OSTI ID:1863786

Turner, Andrew C. ^[1]; Korol, Roman  ^[2]; Eldridge, Daniel L. ^[3]; Bill, Markus ^[4]; Conrad, Mark E. ^[4]; Miller, III, Thomas F. ^[2]; Stolper, Daniel A. ^[3]

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1. Univ. of California, Berkeley, CA (United States)
2. California Institute of Technology (CalTech), Pasadena, CA (United States)
3. Univ. of California, Berkeley, CA (United States); Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
4. Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)

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) (^Dα_{CH4(g)–H2(g)}) based on experiments using γ-Al₂O₃ and Ni catalysts from 3 to 200°C. Results were regressed as a 2^nd order polynomial of 1000 × ln^Dα_{CH4(g)–H2(g)} vs. 1/T (K^-1) yielding: $1000\times {\mathrm{l}\mathrm{n}}^{\mathrm{D}}{\alpha }_{{\mathrm{C}\mathrm{H}}_4\left(\mathrm{g}\right)-{\mathrm{H}}_2\left(\mathrm{g}\right)}=\frac{{3.5317\times 10}^7}{T^2}+\frac{{2.7749\times 10}^5}{T}-179.48$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^Dα_{CH4(g)–H2O(l)} from 3 to 200°C. Our resulting 4th order polynomial is the following equation: $1000\times \mathrm{l}\mathrm{n}{}^{\mathrm{D}}{\alpha }_{{{\mathrm{C}\mathrm{H}}_4\left(\mathrm{g}\right)-\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}=-\frac{{7.9443\times 10}^{12}}{T^4}+\frac{{8.7772\times 10}^{10}}{T^3}-\frac{3.4973{\times 10}^8}{T^2}+\frac{{5.4398\times 10}^5}{T}-382.05$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 CH₄H₂O(l) hydrogen and CH₄CO₂ carbon isotopic equilibrium. In contrast, microbial terrestrial and pure culture gases are not consistent with both CH₄H₂O(l) hydrogen and CH₄CO₂ carbon isotopic equilibrium. Overall, 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.

• Research Organization: Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES). Chemical Sciences, Geosciences & Biosciences Division; USDOE Office of Science (SC), Basic Energy Sciences (BES)
• Grant/Contract Number: AC02-05CH11231; CHE-1611581
• OSTI ID: 1863786
• Alternate ID(s): OSTI ID: 1868683
• Journal Information: Geochimica et Cosmochimica Acta, Vol. 314; ISSN 0016-7037
• Publisher: Elsevier; The Geochemical Society; The Meteoritical Society
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
58 GEOSCIENCES
experimental calibration
methane isotopic equlibrium
methane geochemistry
Path Integral Monte Carlo calculations