The “breathing spots” in karst areas—the sites of advective exchange of gases between soils and adjacent underground cavities

Abstract

Microclimatic processes were studied in the soil of the well-defined “breathing spot” named Půlhodina (BSP) in the Hranice Karst (Czech Republic). The results have indicated that air is exchanged between the external atmosphere and a supposed underground cavity under air-buoyancy control. Upward airflow ventilation mode (UAF mode) and downward airflow ventilation mode (DAF mode) were distinguished. The virtual temperature of switching between both modes was about 16.5 °C. At the UAF mode, the underground air is rising from the cavity through the BSP, warms soil, and carries out the underground CO2. At the DAF mode, the external air is sucked into the cavity through the BSP soil, promoting the advective flux of the soil-respired CO2 deeper into the cavity. Whereas the warming of the soil is a dominant process in the UAF mode, the CO2 advective flux into the cavity is the main process in the DAF mode. The simplified mathematical model was derived as a function of the external air temperature and the soil air temperature to simulate the time evolution of CO2 concentration in the BSP. Despite simplifying assumptions, this model showed a good agreement with the collected data. The advective CO2 flux from a breathing spot soil may significantly contribute to the total CO2 flux into the adjacent underground cavity. Its total values may reach 4 × 10−3 mol s−1 or even more, based on the actual CO2 concentration, the exchanged airflow magnitude (both controlled by external temperature), and the breathing spot area. This conclusion is important for karsologists studying the cave CO2 budget, CO2 sources, and the mechanisms of CO2 transport into caves.