T16 Biogeochemical Interactions in Caves and Karst

Sunday, 22 September 2019
8:00 AM - 12:00 PM
Room 224B, North Building

AN OVERVIEW OF METHANE IN CAVE AIR

Paper number: T16-9
Program: Technical Sessions
Time: 10:10 AM – 10:30 AM

Authors

Abstract

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. δ13CCH4 and δ2HCH4 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.

Geological Society of America Abstracts with Programs. Vol. 51, No. 5

doi: 10.1130/abs/2019AM-339807

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