Subterranean karst environments as a global sink for atmospheric methane

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 CH4because its subsurface drainage networks and solution-enlarged fractures facilitate atmospheric exchange. Karst landscapes cover about 14% of earth’s continental surface, but observations of CH4concentrations 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 acting as a global CH4sink, we measured the CH4concentrations, δ13CCH4, and δ2HCH4values of cave air from 33 caves in the USA and three caves in New Zealand. We also measured CO2concentrations, δ13CCO2, and radon (Rn) concentrations to support CH4data interpretation by assessing cave air residence times and mixing processes. Among these caves, 35exhibited subatmospheric CH4concentrations in at least one location compared to their local atmospheric backgrounds. CH4concentrations, δ13CCH4, and δ2HCH4values suggest that microbial methanotrophy within caves is the primary CH4consumption mechanism. Only 5 locations from 3 caves showed elevated CH4concentrations compared to the atmospheric background and could be ascribed to local CH4sources from sewage and outgassing swamp water. Several associated δ13CCH4and δ2HCH4values 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 CH4sink.