METHANE DYNAMICS IN LIMESTONE CAVES

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 CH_4­ production (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 (δ¹³C_CH4 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 δ¹³C_CH4 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 δ¹³C_CH4 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.