THE DYNAMICS OF CARBON DIOXIDE IN THE INFILTRATION AREA OF A KARST SYSTEM: APPLICATION TO THE LASCAUX CAVE SITE (FRANCE)

Recently, there is a growing interest on the understanding of the carbon dioxide (CO₂) exchanges of the karst and the atmosphere. This study aims to describe the dynamics of CO₂ in different compartments of a karst system such as soils, epikarst and the vadose zone. In order to do so, mechanisms of production including parameters affecting them and the various transfer mechanisms were examined. The study site includes the famous cave of Lascaux (France).

Monthly soil monitoring to get CO₂ partial pressure (Pco₂) was carried out. At various depths in the limestone and in the cavity, continuous measurements were done. This allowed us to acquire every minute climatological accounts and in-situ parameters: CO₂ partial pressure, atmospheric pressure, volumetric water content and temperature in the epikarst, the vadose zone and in the cave itself (over 100 sensors at different depths). CO₂ exchanges between atmosphere and the different karst compartments were analyzed using air density calculations. Isotopic analyses were also carried out to determine CO₂ origin and dynamics.

High CO₂ contents in the soil with 15000-40000 ppm and in epikarst with 25000-65000 ppm (minimum in winter and maximum in summer) were measured. It is found out that Pco₂ increases with depth (maximum were measured from 3 to 8 meters deep). Changes in soil water content considerably influence the Pco₂ in soils and epikarst. Transfers of CO₂ to cave are made in gaseous form through the pores, cracks and fractures. A part may also come from drip-water degassing. The Pco₂ in the different areas of the cave showed significant seasonal and spatial variations. Highest values of Pco₂ are identified in the deeper areas of the cave (25000 ppm to 55000 ppm), corresponding to the Pco₂ of the vadose zone. In shallower areas (12000 to 20000 ppm), Pco₂ values are lower, highlighting possible exchanges with the atmosphere. Isotopic analyses (δ¹³Cco₂) showed biogenic origin and inputs of atmospheric air in the shallowest parts of the cave. The air exchanges are driven by: (1) the density differences between external atmosphere and cave air and (2) atmospheric pressure. At the end, a conceptual scheme of the CO₂ dynamics in the infiltration area of the Lascaux karst including exchanges between atmosphere, soils, epikarst and cave is presented.