METHANE DYNAMICS IN LIMESTONE CAVES
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.