DIFFERENTIAL TRACE GAS CONCENTRATIONS IN CAVE AIR COMPARED TO THE ATMOSPHERE
We measured the concentration of 24 trace gases in 25 caves across the United States with a Fourier Transform Infrared Spectrometer (FTIR, GASMET DX4030). Cave air was sampled in situ along gradients from the entrance of the cave to the cave interior. The measured gases included but were not limited to dichloromethane, benzene, and acetonitrile. Carbonyl sulfide (OCS) and hydrogen chloride were not detected in any locations by the GASMET and were removed from further analysis.
FTIR measurements showed that the trace gas composition of cave air was different than the composition of the outside atmosphere. For example, dichloromethane, benzene, and acetonitrile were all depleted in cave air compared to the atmosphere. The differences in the composition of cave air and the atmosphere appear to be due to metabolic processes. Benzene and acetonitrile are reduced gases, and microorganisms may be able to consume these gases for energy or these gases may be oxidized as a byproduct of ongoing metabolism. Conversely, N2O was enriched in cave air, likely as a result of soil gas entering the caves. Further research is needed to determine if the depletion of gases like benzene and dichloromethane is a result of microbial activity.