Main drivers of diffusive and advective processes of CO2-gas exchange between a shallow vadose zone and the atmosphere

Abstract

A multiparametric study of Altamira cave conditions was performed to identify mechanisms that affect CO2. A daily survey was used to better understand the role of the shallow vadose system as a source/sink of this gas. Airborne particles were monitored to distinguish the air movement that was joined to δ13CO2 and were also used as a proxy of the origin of the CO2. A gas transport model has been created based on the interaction of three air masses (soil–cave–exterior), which is driven by soil-derived CO2 diffusion to the cave and by the advective mixing of the cave with exterior air. The diffusive process increases cave CO2 and decreases δ13CO2. The advective mixing induces a decrease in CO2 and an increase in the isotopic signal. The diffusive flux depends on soil CO2 production; the advective flux is driven by outer–inner density gradients, and both depend on the degree of exchange between air masses. Consequently, external conditions, such as temperature and humidity, regulate gas interchange. The created process-based model permits the quantification of CO2 fluxes. The consequence of the degassing stage is the release of light CO2 (δ13C quantified in −24.82‰) into the exterior air (δ13C measured in −11.34‰). The migration of gas in the vadose zone may influence many environmental processes, and therefore, the contribution of shallow underground systems to surface CO2 exchange and to the isotopic signal of troposphere should be accounted for.