Paper No. 11
Presentation Time: 11:05 AM


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, DROBNIAK, Agnieszka, Indiana Geological Survey, Indiana University, 611 North Walnut Grove, Bloomington, IN 47405, MASTALERZ, Maria, Indiana Geological Survey, Indiana University, Bloomington, IN 47405, ETIOPE, Giuseppe, Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma 2, Italy, SAUER, Peter, Geoloigcal Sciences, Indiana University, 1001 E. 10th St, Bloomington, IN 47405 and LENNON, Jay T., Department of Biology, Indiana University, 1001 E 3rd. St, Bloomington, 47405,

Karst environments, which commonly host caves, cover 20 % of the Earth’s surface. The biogeochemistry of caves is only beginning to be understood and significant gaps remain in our knowledge. For example, cave atmospheres are generally uncharacterized with respect to trace gases such as methane (CH4) – a greenhouse gas. CH4 may enter caves from the outside atmosphere, microbial CHproduction (methanogenesis) in wet sediments or waters within the cave or overlying aquifers and soils, or thermogenic CH4 from underlying geologic rock formations such as shales or coals. As cave atmospheres exchange with outside air, caves may function as either sources or sinks of atmospheric CH4.

We measured CH4 in 20 limestone caves in Indiana, Kentucky, Pennsylvania, Virginia, and Tennessee to gain more insight into cave CH4 cycling. CH4 concentrations were quantified using in-situ spectroscopic methods and/or discrete sampling methods followed by laboratory quantification. Carbon stable isotope ratios of methane (δ13CCH4 values) from cave air were also measured using a gas chromatograph linked to a combustion interface and a stable isotope ratio mass-spectrometer.

Nearly all of the caves (19 of 20) showed sub-atmospheric CH4 concentrations and one showed elevated CH4 concentrations compared to the outside atmosphere. CH4 concentration in the caves showed spatial heterogeneity. Less ventilated rooms further from cave entrances showed the lowest CH4 concentrations (~ 0.30 to 0.08 ppm). Additionally, less ventilated rooms showed decreased δ13CCH4 values (~ -55 ‰) compared to more ventilated cave rooms (~ -45 ‰).

Our data suggest both microbial methane consumption (methanotrophy) and methanogenesis in karst environments may account for the patterns of CH4 concentrations and δ13CCH4 values present in the studied caves. For instance, in well ventilated rooms atmospheric sources may dominate the CH4 signal. In poorly ventilated cave rooms (i.e. rooms without significant atmospheric inputs), a slow, continuous seepage or in-situ production and consumption of 13C-depleted CH4 may dominate the signal. Additionally, caves circulate significant volumes of air during the course of the year and may represent a non-trivial sink for CH4 based on the measured sub-atmospheric concentrations.