2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 15-6
Presentation Time: 9:30 AM

SEASONAL FLUCTUATIONS IN THE METHANE CONCENTRATION OF CAVE AIR


WEBSTER, Kevin D., Department of Geological Sciences, Indiana University, 1001 E 10th St, Bloomington, IN 47405, SCHIMMELMANN, Arndt, Department of Geological Sciences, Indiana University, Bloomington, IN 47405-1405 and SAUER, Peter E., Geological Sciences, Indiana University, 1001 E 10th Street, Bloomington, IN 47405-1405, kevdwebs@indiana.edu

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.