Radiolysis via radioactivity is not responsible for rapid methane oxidation in subterranean air

Abstract

Atmospheric methane is rapidly lost when it enters humid subterranean critical and vadose zones (e.g., air in soils and caves). Because methane is a source of carbon and energy, it can be consumed by methanotrophic methane-oxidizing bacteria. As an additional subterranean sink, it has been hypothesized that methane is oxidized by natural radioactivity-induced radiolysis that produces energetic ions and radicals, which then trigger abiotic oxidation and consumption of methane within a few hours. Using controlled laboratory experiments, we tested whether radiolysis could rapidly oxidize methane in sealed air with different relative humidities while being exposed to elevated levels of radiation (more than 535 kBq m-3) from radon isotopes 222Rn and 220Rn (i.e., thoron). We found no evidence that radiolysis contributed to methane oxidation. In contrast, we observed the rapid loss of methane when moist soil was added to the same apparatus in the absence of elevated radon abundance. Together, our findings are consistent with the view that methane oxidizing bacteria are responsible for the widespread observations of methane depletion in subterranean environments. Further studies are needed on the ability of microbes to consume trace amounts of methane in poorly ventilated caves, even though the trophic and energetic benefits become marginal at very low partial pressures of methane.

Microbial Activity in Subterranean Ecosystems: Recent Advances

“…In fact, one of the important •OH sources in cave air may be from radioactive 222 Rn decay [34]. However, further research verified that the mechanism of CH 4 consumption was seasonally changing and methane-oxidizing bacteria were primarily responsible for the widespread observations of CH 4 depletion in subterranean environments, discarding any evidence of radiolysis contribution [37][38][39].…”

“…Schimmelmann et al [37] tested, in controlled laboratory experiments, whether radiolysis could rapidly oxidize CH 4 in sealed air with different relative humidity and elevated levels of radiation from Rn isotopes. No evidence of CH 4 oxidation by radiolysis was found.…”

Scientific novelty beyond the experiment

“…The authors followed up with practical experiments which demonstrated that rates of microbial methane oxidation in caves are in fact high and could reduce concentrations of gaseous methane (Lennon et al, 2017 ). In a later paper, Lennon and colleagues collaborated with two authors of the Fernandez‐Cortes et al ( 2015 ) paper to experimentally test whether radiation can oxidise methane at rates sufficient to support the original suggestion by Fernandez‐Cortes et al (Schimmelmann et al, 2018 ). The Lennon et al ( 2017 ) study was useful for understanding the contributions of microorganisms to the overlooked sink of greenhouse gases that may be important for local and regional climate‐change modelling.…”

Radioactive Thoron 220Rn Exhalation From Unfired Mud Building Material Into Room Air of Earthen Dwellings

“…A computer fan drew air from the space between the mud wall and a wooden board through a central hole in the board into a perforated polyethylene enclosure that contained the SARAD R Thoron Scout. Although the Thoron Scout is a passive thoron detection device without an internal pump, the active air flow from an external fan makes its thoron data compatible with those from the SARAD R RTM 2200 with an internal pump (Schimmelmann et al, 2018). Hourly average 220 Rn isotope concentrations in room air were automatically logged by the Thoron Scout from November 8, 2017 until January 14, 2018, with a small hiatus in early January.…”