Radiolytic CH4 and CO2 from irradiated organic compounds

Abstract

The chemical composition of sedimentary organic matter is often used to reconstruct Earth’s biogeochemical history. However, few studies have considered that the long-term effects of subterranean natural radioactivity over geologic time can be common enough and provide sufficient activation energy to trigger radiolysis and the generation of methane, carbon dioxide, and other gases. We performed separate α -, β -, and γ -irradiation experiments at low temperatures on chemically diverse and pure model organic compounds in evacuated and sealed glass tubes to test the hypothesis that radiolytic C–C bond cleavage can result in radiolytic methane and carbon dioxide. α -Irradiation utilized uranium oxide powder mixed with organic model compounds, whereas β - and γ -irradiation experiments relied on β -radiation from 90 Sr (370 MBq or 10 mCi) and on delayed γ -ray energy in a TRIGA nuclear research reactor, either 25 kGy typical for biosafety sterilization or 1 MGy. We report initial experimental evidence from ongoing experiments after up to three years of irradiation. Widespread observations of methane in the headspaces of γ -irradiated sealed glass tubes identified radiolytic methanogenesis. Carbon dioxide was observed when organic compounds with carboxyl groups were γ -irradiated. For example, 2.2 % of glycine was converted to methane and carbon dioxide during 1 MGy of γ -irradiation. α -Irradiation produced measurable methane only from squalane which is particularly rich in methyl groups. 90 Sr produced insufficiently intense β -irradiation to yield measurable headspace gases. Our study offers an improved organic-molecular understanding of which functional groups and carbon skeletal configurations lend themselves to radiolytic methanogenesis and generation of carbon dioxide. Graphical abstract

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