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Comparison of Experimental vs Theoretical Abundances of ¹³CH₃D and ¹²CH₂D₂ for Isotopically Equilibrated Systems from 1 to 500 °C

Eldridge, Daniel L. and Korol, Roman and Lloyd, Max K. and Turner, Andrew C. and Webb, Michael A. and Miller, Thomas F., III and Stolper, Daniel A. (2019) Comparison of Experimental vs Theoretical Abundances of ¹³CH₃D and ¹²CH₂D₂ for Isotopically Equilibrated Systems from 1 to 500 °C. ACS Earth and Space Chemistry, 3 (12). pp. 2747-2764. ISSN 2472-3452.

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Methane is produced and consumed via numerous microbial and chemical reactions in atmospheric, hydrothermal, and magmatic reactions. The stable isotopic composition of methane has been used extensively for decades to constrain the source of methane in the environment. A recently introduced isotopic parameter used to study the formation temperature and formational conditions of methane is the measurement of molecules of methane with multiple rare, heavy isotopes (‘clumped’) such as ¹³CH₃D and ¹²CH₂D₂. In order to place methane clumped-isotope measurements into a thermodynamic reference frame that allows calculations of clumped-isotope based temperatures (geothermometry) and comparison between laboratories, all past studies have calibrated their measurements using a combination of experiment and theory based on the temperature dependence of clumped isotopologue distributions for isotopically equilibrated systems. These have previously been performed at relatively high temperatures (>150˚C). Given that many natural occurrences of methane form below these temperatures, previous calibrations require extrapolation when calculating clumped-isotope based temperatures outside of this calibration range. We provide a new experimental calibration of the relative equilibrium abundances of ¹³CH₃D and ¹²CH₂D₂ from 1–500˚C using a combination of γ-Al₂O₃ and Ni-based catalysts and compare them to new theoretical computations using Path Integral Monte Carlo (PIMC) methods and find 1:1 agreement (within ± 1 standard error) for the observed temperature dependence of clumping between experiment and theory over this range. This demonstrates that measurements, experiments, and theory agree from 1–500°C providing confidence in the overall approaches. Polynomial fits to PIMC computations, which are considered the most rigorous theoretical approach available, are given as follows (valid T ≥ 270 K): ∆¹³CH₃D≅1000×ln(K¹³CH₃D)= 1.47348×10¹⁹/T⁷ - 2.08648×10¹⁷/T⁶ + 1.19810×10¹⁵/T⁵ - 3.54757×10¹²/T⁴ +5.54476×10⁹/T³ – 3.49294×10⁶/T² + 8.89370×10₂/T ∆¹²CH₂D₂≅1000×ln(8/3×K¹²CH₂D₂)= -9.67634×10¹⁵/T⁶ + 1.71917×10¹⁴/T⁵ - 1.24819×10¹²/T⁴ + 4.30283×10⁹/T3 -4.48660×10⁶/T² + 1.86258×10³/T. We additionally compare PIMC computations to those performed utilizing traditional approaches that are the basis of most previous calibrations (Bigeleisen, Mayer, and Urey model, BMU) and discuss the potential sources of error in the BMU model relative to PIMC computations.

Item Type:Article
Related URLs:
URLURL TypeDescription
Eldridge, Daniel L.0000-0001-7340-0652
Korol, Roman0000-0001-9307-6351
Lloyd, Max K.0000-0001-9367-2698
Turner, Andrew C.0000-0003-1187-2560
Webb, Michael A.0000-0002-7420-4474
Miller, Thomas F., III0000-0002-1882-5380
Stolper, Daniel A.0000-0003-3299-3177
Additional Information:© 2019 American Chemical Society. This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes. Received: September 16, 2019; Revised: October 26, 2019; Accepted: October 28, 2019; Published: October 28, 2019. DLE and DAS acknowledge research support from the Laboratory Directed Research and Development Program of Lawrence Berkeley National Laboratory under U.S. Department of Energy Contract No. DE-AC02-05CH11231. DAS acknowledges support for ACT from the National Science Foundation under Grant No. EAR-1911296 and the Donors of the American Chemical Society Petroleum Research Fund. MKL acknowledges support from the Agouron Institute. TFM acknowledges support from the National Science Foundation under Grant No. CHE-1611581. The 253 Ultra Mass Spectrometer was funded by the Heising-Simons Foundation and the University of California, Berkeley. Mark Conrad and Markus Bill (LBNL) are thanked for helpful conversations regarding standardization. Xuecheng Tao (Caltech) is thanked for helpful conversations regarding PIMC calculations. The authors declare no competing financial interest.
Funding AgencyGrant Number
Lawrence Berkeley National LaboratoryUNSPECIFIED
Department of Energy (DOE)DE-AC02-05CH11231
American Chemical Society Petroleum Research FundUNSPECIFIED
Agouron InstituteUNSPECIFIED
Heising-Simons FoundationUNSPECIFIED
University of California, BerkeleyUNSPECIFIED
Subject Keywords:Methane Clumped Isotopes, Methane Isotope Equilibration, Methane Geochemistry, Path Integral Monte Carlo Calculations, 253 Ultra
Issue or Number:12
Record Number:CaltechAUTHORS:20191029-105305218
Persistent URL:
Official Citation:Comparison of Experimental vs Theoretical Abundances of 13CH3D and 12CH2D2 for Isotopically Equilibrated Systems from 1 to 500 °C. Daniel L. Eldridge, Roman Korol, Max K. Lloyd, Andrew C. Turner, Michael A. Webb, Thomas F. Miller, III, and Daniel A. Stolper. ACS Earth and Space Chemistry 2019 3 (12), 2747-2764. DOI: 10.1021/acsearthspacechem.9b00244
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:99532
Deposited By: Tony Diaz
Deposited On:29 Oct 2019 18:12
Last Modified:07 Jan 2020 22:38

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