Subterranean Karst Environments as a Global Sink for Atmospheric Methane

Abstract

Abstract:The air in subterranean karst cavities is often depleted in methane (CH4) relative to the atmosphere. Karst is considered a potential sink for the atmospheric greenhouse gas CH4 because its subsurface drainage networks and solution-enlarged fractures facilitate atmospheric exchange. Karst landscapes cover about 14 % of earth's continental surface, but observations of CH4 concentrations in cave air are limited to localized studies in Gibraltar, Spain, Indiana (USA), Vietnam, Australia, and by incomplete isotopic data. To test if karst is systematically acting as a global CH4 sink, we measured the CH4 concentrations, δ 13 CCH 4 , and δ 2 HCH 4 values of cave air from 33 caves in the USA and three caves in New Zealand. We also measured CO2 concentrations, δ 13 CCO 2 , and radon (Rn) concentrations to support CH4 data interpretation by assessing cave air residence times and mixing processes. Among these caves, 35 exhibited subatmospheric CH4 concentrations in at least one location compared to their local atmospheric backgrounds. CH4 concentrations and δ 13 CCH 4 and δ 2 HCH 4 values suggest that microbial methanotrophy within caves is the primary CH4 consumption mechanism as the atmosphere exchanges with subsurface air. The pattern of δ 13 CCH 4 and δ 2 HCH 4 values along CH4 concentration gradients in cave air provides evidence for incomplete oxidation by methanotrophy. Only 5 locations from 3 caves showed elevated CH4 concentrations compared to the atmospheric background and could be ascribed to local CH4 sources from sewage and outgassing swamp water. Several associated δ 13 CCH 4 and δ 2 HCH 4 values point to carbonate reduction and acetate fermentation as biochemical pathways of limited methanogenesis in karst environments and suggest that these pathways occur in the environment over large spatial scales. Our data show that karst environments function as a global CH4 sink. Estimates of CH4 flux in karst landscapes are needed in order to include the subterranean CH4 sink in climate models.

Fundamental Science and Engineering Questions in Planetary Cave Exploration

“…For example, microalgae in Atacama Desert coastal caves produce oxygen as a photosynthetic byproduct (Azua‐Bustos et al., 2009, 2010), while Webster et al. (2015) and Webster (2019) have shown that chemical and isotopic constituents in cave air are distinguishable from the surrounding ambient air. While this approach seems promising, more research will be required to examine other gases (e.g., methane, hydrogen, and carbon monoxide; Lyu et al., 2018; Voordouw, 2002; Wang & Wan, 2009) produced by different microbial metabolisms.…”