SEASONAL FLUCTUATIONS IN THE METHANE CONCENTRATION OF CAVE AIR

Uncertainties of individual sources and sinks of the atmospheric greenhouse gas methane (CH₄) remain large, despite progress. Recent observations of subatmospheric methane concentrations in cave air suggest that karst landscapes may be an unaccounted sink for atmospheric CH₄. Karst is estimated to cover 10 – 20 % of earth’s land surface, and presents a large microbial habitat for the subterranean removal of atmospheric CH₄ following air exchange. A greater understanding of the CH₄ dynamics of caves and karst is needed before karst-CH₄ dynamics can be incorporated into global methane budgets and climate models.

In a study extending 18-months, concentrations and the hydrogen and carbon stable isotopic compositions of CH₄ and carbon dioxide (CO₂) were measured at 5 sites in Buckner Cave in Southern Indiana. CO₂ was measured as an established tracer of airflow in caves. Sampling sites ranged from 55 to 527 meters from the cave’s main entrance. Samples were acquired on a monthly basis.

CH₄ and CO₂ concentrations in the cave fluctuated seasonally. Higher CH₄ concentrations occurred in the winter and lower concentrations occurred in the summer when cave air is semi-stagnant. CO₂ and CH₄ concentrations were negatively correlated. Methane concentrations decreased with the distance from the main entrance (p < 10^-13) and concentrations ranged from atmospheric background concentrations of 1.9 ppm to 0.1 ± 0.1 ppm. The carbon and hydrogen stable isotope ratios of CH₄, expressed as conventional δ-values, in the cave ranged from +7 to –58.7 ‰ (VPDB) and +10 to –170 ‰ (VSMOW), respectively.

The isotopic data suggest that the methane dynamics of Buckner Cave can be described by atmospheric CH₄ consumption by methanotrophs in addition to methanogenic sources associated with carbonate reduction and/or acetate fermentation. Methanogenic sources of CH₄ were only noticeable at low CH₄ concentrations. The type and extent of methanogenesis may depend on the amount of water stored in the epikarst. Karst landscapes may be an important year-round sink for atmospheric CH­₄. Restricted airflow during summer months likely allows for greater depletion of CH₄ in cave air. Future studies examining the rate of consumption of CH₄ by karst will assist improved modeling of the global atmospheric CH₄ budget.