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

• Published in: Environmental earth sciences 2016-04, Vol.75 (8), p.1-20, Article 700
• Main Authors: McDonough, L. K., Iverach, C. P., Beckmann, S., Manefield, M., Rau, G. C., Baker, A., Kelly, B. F. J.
• Format: Article
• Language: English
• Subjects: Air sampling; Arid environments; Biogeosciences; Carbon dioxide; Carbon footprint; Carbon sequestration; Caves; Earth and Environmental Science; Earth Sciences; Environmental accounting; Environmental Science and Engineering; Geochemistry; Geology; Hydrology/Water Resources; Karst; Methane; Methanogenesis; Original Article; Semiarid environments; Spatial analysis; Summer; Sustainability reporting; Terrestrial Pollution; Ventilation; Winter
• ISSN: 1866-6280; 1866-6299
• DOI: 10.1007/s12665-016-5497-5

There is insufficient information on the movement of air in karst environments to constrain the uncertainty associated with quantifying sources and sinks of methane (CH 4 ) and carbon dioxide (CO 2 ) within caves for global carbon accounting. We analysed cave-air samples for their CO 2 and CH 4 concentrations ([CO 2 ] and [CH 4 ]) 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 CO 2 and CH 4 , with the highest cave-air CO 2 concentrations ([CO 2 ] cave ) occurring in summer, in association with the lowest cave-air CH 4 concentrations ([CH 4 ] cave ). Analyses show that the cave-air CO 2 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 CH 4 and CO 2 . [CH 4 ] cave in Cathedral Cave shows an inverse relationship to [CO 2 ] cave , particularly in areas separated from the main cave passage. In contrast, Gaden Cave has near-atmospheric [CH 4 ] cave and isotopic ratios present at all locations sampled in winter. Where no ventilation is occurring in summer, [CH 4 ] 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 CH 4 , and highlights the need for further studies before global generalisations can be made about the carbon budget of karst environments.