Clumped ¹³CH₂D and ¹²CHD₂ compositions of methyl groups from wood and synthetic monomers: Methods, experimental and theoretical calibrations, and initial results

Journal Article · Fri Oct 16 00:00:00 EDT 2020 · Geochimica et Cosmochimica Acta

DOI:https://doi.org/10.1016/j.gca.2020.10.008· OSTI ID:1837044

Lloyd, Max K. ^[1]; Eldridge, Daniel L. ^[2]; Stolper, Daniel A. ^[2]

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1. Univ. of Californiaca, Berkeley, CA (United States)
2. Univ. of Californiaca, Berkeley, CA (United States); Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)

Methyl groups are found in numerous biogenic and synthetic materials including geologically preserved materials such as wood. The carbon and hydrogen isotope compositions of methyl groups are used as tracers in biogeochemical cycles, as paleothermometers, and to determine the hydrogen isotopic composition of ancient rain. Here we present analyses of resolved ¹³C–D (¹³CH₂D) and D–D (¹²CHD₂) clumped isotope compositions of methyl groups as new variables for the study of methyl groups in the present and past. We first present chemical methods to extract, purify, and derivatize methyl groups from methoxyl (R–O–CH₃) groups as CH₃F and CH₃Cl, and high-resolution mass spectrometric techniques to determine the clumped isotope compositions of these species. We achieve precisions for ¹³C–D clumping of ±0.25‰ and D–D clumping of ±2.5‰. We anchor our clumped isotopic measurements to a thermodynamic reference frame by first calculating the theoretical temperature dependences of ¹³C–D and D–D clumping in CH₃Cl, then placing our measurements onto this reference frame through experimental internal isotopic equilibration of CH₃Cl at 200 °C. Finally, we provide and analyze an initial dataset of clumped ¹³C–D and D–D compositions of methyl groups from various commercial/synthetic monomers and environmental woods. We observe ranges in clumped isotope compositions of ~11‰ in ¹³C–D and ~48‰ in D–D, and systematic differences within these ranges between methyl groups from commercial monomers and wood. Specifically, commercial clumped ¹³C–D compositions are between 0 and 3‰, which correspond to apparent equilibrium temperatures between 170 °C and the infinite temperature limit. In contrast, the clumped ¹³C–D compositions of wood methoxyl groups are distinctively high (9.50–11.25‰) and 3–6‰ higher than would be expected if formed in internal isotopic equilibrium at Earth-surface temperatures. Commercial/synthetic methyl and wood methoxyl clumped D–D compositions are also distinct: -5 to +13‰ in commercial monomers vs. -35 to -8‰ in wood—such negative values cannot result from formation in isotopic equilibrium and require kinetic processes to have occurred. Overall, these results indicate that wood methoxyl groups are formed out of isotopic equilibrium and that clumped isotope compositions of methyl groups may be useful tracers of methyl group sources and sinks in the environment. For instance, isotopic clumping in methyl groups may be useful for understanding controls on isotopic clumping in methane produced by methylotrophic methanogens.

• 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
• Grant/Contract Number: AC02-05CH11231
• OSTI ID: 1837044
• Alternate ID(s): OSTI ID: 1775806
• Journal Information: Geochimica et Cosmochimica Acta, Vol. 297; 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
Clumped isotope
Methyl groups
Methoxyl groups
Lignin