Near-surface CO₂ dynamics in a karst vadose zone: insights from a tectonically stable setting

The aim of this study was to investigate the behaviour of CO₂ in the karst vadose zone in a volcanically inactive and relatively stable tectonic setting. Temporal variations of CO₂ gas migration were measured in near-surface using standard micrometeorological techniques over a karst area in NW Sardinia (Italy). The results show that cave CO₂ concentration does not exhibit a daily cycle as observed at the surface; instead, in the underground atmosphere, peaks are observed, sometimes isolated, other times occurring as a continuous series of smaller peaks. The increase and decrease in gas concentration during these plumes are always sharp, while their duration is variable. The peaks are sometimes synchronized with the change in density between the air mass inside the cave and the external air, but more often they are negatively correlated with atmospheric pressure.