DIFFERENTIAL TRACE GAS CONCENTRATIONS IN CAVE AIR COMPARED TO THE ATMOSPHERE

The interactions between the atmosphere and mineral surfaces are poorly understood. One place where this interface is widely present on earth is in caves. Despite the fact that most caves are open to the atmosphere and have a vast surface area on which chemical reactions can occur, little is known about the processes controlling the abundances of trace gases in cave air. Atmospheric volatile organic compounds (VOCs), one class of trace gases, pose concerns for human health either directly or through their breakdown products, namely ozone, and are important in regulating the reducing capacity of the atmosphere. Little is known about the behavior of VOCs in caves.

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.