Spatial variability of cave-air carbon dioxide and methane concentrations and isotopic compositions in a semi-arid karst environment

Abstract

There is insufficient information on the movement of air in karst environments to constrain the uncertainty associated with quantifying sources and sinks of methane (CH4) and carbon dioxide (CO2) within caves for global carbon accounting. We analysed cave-air samples for their CO2 and CH4 concentrations ([CO2] and [CH4]) and carbon isotopic compositions from sampling campaigns in winter (August 2014) and summer (February 2015) at numerous heights and locations throughout Gaden and Cathedral caves, in a semi-arid environment, Wellington Caves, NSW, Australia. Ventilation is the dominant control on cave-air CO2 and CH4, with the highest cave-air CO2 concentrations ([CO2]cave) occurring in summer, in association with the lowest cave-air CH4 concentrations ([CH4]cave). Analyses show that the cave-air CO2 has both atmospheric and soil sources. Soil air and cave air in both caves undergo methanogenesis and methanotrophy, but we identify cave-specific differences in cave-air CH4 and CO2. [CH4]cave in Cathedral Cave shows an inverse relationship to [CO2]cave, particularly in areas separated from the main cave passage. In contrast, Gaden Cave has near-atmospheric [CH4]cave and isotopic ratios present at all locations sampled in winter. Where no ventilation is occurring in summer, [CH4]cave in Gaden Cave decreases, but remains reasonably high compared to Cathedral Cave. Our research shows adjacent caves vary in their ability to act as a net sink for CH4, and highlights the need for further studies before global generalisations can be made about the carbon budget of karst environments.