AN OVERVIEW OF METHANE IN CAVE AIR (Invited Presentation)

Methane (CH₄) is a potent greenhouse gas in earth’s atmosphere and is 20 times more effective at trapping infrared radiation than carbon dioxide per molecule. Observations of CH₄ concentrations in cave air have generally been lower than those in the background atmosphere (~ 1.8 ppmv). Most caves are hosted in subaerial limestone outcrops, or karst, and karst is estimated to cover 10 – 20 % of earth’s land surface. Karst currently represents an unaccounted sink for atmospheric CH₄ in global models.

Cave air CH­₄ concentrations from the U.S., New Zealand, and Vietnam were sampled to understand its behavior in the subsurface. The sources and sinks of CH₄ in caves were also investigated by measuring the carbon and hydrogen stable isotopic compositions of CH₄ and through mesocosm experiments. All but one of the studied caves showed subatmospheric CH₄ concentrations in at least one location. CH₄ concentrations in the caves ranged from < 0.1 to 5 ppmv. Only three caves showed elevated CH₄ concentrations relative to the atmospheric background concentration. CH₄ concentrations generally decreased with the distance from a cave entrance and were at or below the detection level of the instruments in the least ventilated rooms. δ¹³C_CH4 and δ²H_CH4 values in the caves ranged from +7 to –58.7 ‰ (VPDB) and from +10 to −195.6 ‰ (VSMOW) respectively. The mesocosm experiments showed that mesocosms with live microorganisms removed CH₄ from the experiments, while control and sterile mesocosms did not.

The concentrations, stable isotopic compositions of CH₄, and mesocosm experiments show that microorganisms are responsible for the subatmospheric CH₄ concentrations in caves. Additionally, two minor sources of CH₄ from microbial production pathways, acetate fermentation and carbonate reduction, were observed entering caves. Future studies need to examine rates of CH₄ influx into caves and CH₄ production by caves to understand net rates of CH­₄ consumption by karst.