Subterranean karst environments as a global sink for atmospheric methane

Abstract

The air in subterranean karst cavities is often depleted in methane (CH$_4$) relative to the atmosphere. Karst is considered a potential sink for the atmospheric greenhouse gas CH$_4$ 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 CH$_4$ 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 acting as a global CH$_4$ sink, we measured the CH$_4$ concentrations, $\sigma^13$ C$_{CH4}$, and $\sigma^2$ H$_{CH4}$ values of cave air from 33 caves in the USA and three caves in New Zealand. We also measured CO$_2$ concentrations, $\sigma^13$ C$_{CO2}$, and radon (Rn) concentrations to support CH$_4$ data interpretation by assessing cave air residence times and mixing processes. Among these caves, 35 exhibited subatmospheric CH$_4$ concentrations in at least one location compared to their local atmospheric backgrounds. CH$_4$ concentrations, $\sigma^13$ C$_{CH4}$, and $\sigma^2$ H$_{CH4}$ values suggest that microbial methanotrophy within caves is the primary CH$_4$ consumption mechanism. Only 5 locations from 3 caves showed elevated CH$_4$ concentrations compared to the atmospheric background and could be ascribed to local CH$_4$ sources from sewage and outgassing swamp water. Several associated $\sigma^13$ C$_{CH4}$ and $\sigma^2$ H$_{CH4}$ 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 CH$_4$ sink.