Changes in the CO2 dynamics in near-surface cavities under a future warming scenario: Factors and evidence from the field and experimental findings

This study is based on field monitoring of a cave-soil-atmosphere system validated with laboratory experiments. CO2 and 222Rn dynamics in the cavity are dependent on climatic parameters, mainly on the differences between the outdoor and indoor temperature. The annual cycles in the cave are characterized by two outstanding phenomena: cave gas recharge and ventilation when the cave acts as a gas sink or source. A permanent relationship with soil above the cave exists. The soil temperature and moisture are responsible for CO2 production on various time scales. Soil CO2 at the Rull site reaches values higher than 3000ppm in April–May, but falls to nearly 1000ppm during the summer. Maximum CO2 values in the cave are reached in the warmest months and are in accordance with soil CO2 values. The maximum CO2 concentration in the cave is 3470ppm on average, while the minimum is 623ppm. To describe the field findings, CO2 production and diffusion experiments related to the soil behaviour were developed. The results show that the soil CO2 production increases as the soil temperature and moisture increase according to a calculated logarithmic equation until the soil water content exceeds the saturation value. The soil-produced CO2 reaches the Rull cave by diffusion, which in Rull soil is reduced to approximately 60% when the soil water content increased from 0 to 30%. We estimated that 57kg of CO2 was emitted from the cave to the atmosphere in an annual cycle, considering a cave volume of 9915m3. Finally, projections of the future climate at the study site confirm a general tendency for annual-mean conditions to be warmer and drier, which will directly affect the soil CO2 production. In this situation, the Rull cave will experience changes in the stored and subsequently exchanged annual amount of CO2.