NATURAL VENTILATION OF A CAVE, IMPACTS OF SEASONAL AND DIURNAL TEMPERATURES VARIATIONS ON AIR EXCHANGES

Caves are the interface in between atmospheric and rock environments. Part of the exchanges between these two air masses occurs through these cavities. In this line, it is interesting to have a better understanding on how these exchanges function. For instance, it can help in estimating the cave air quality (Pco₂ and temperature) which is a key parameter in conserving archeological artifacts inside these cavities. In addition, it can facilitate in recognizing a cave as a sink or a source of CO₂ in the global carbon cycle. Accordingly, cave air quality is an essential factor to be examined. It is being influenced by the exchanges between the cave per se and external atmosphere. Cussac cave is a good example of a cavity with important archeological artifacts and parietal art. This cave has a single entrance and is considered as a cold air trap. In this study, air mass exchanges were assessed. Measurements of temperature and Pco₂ were made in several locations close to the cave entrance area with a 30 minutes frequency. Results show that cave air Pco₂varies from 0.18% to 3.33%.

Speed of the air flow was also measured at the door of cave entrance. It ascertains that the cave never stops in communicating with the external atmosphere. Furthermore, seasonal and diurnal behavior are identified: (i) in cold season, the cave inhale at night and blow a little at the warmest hours and (ii) in warm season, cave blow at night, but a convection loop takes place during the day in the entrance area, preventing the external air from entering the cave.

For the year 2009, the cave blew and inhaled, 33% and 34% of the year, respectively. For the rest of the year, 34%, a loop took place in the entrance area. The cave appears as a source of CO₂; a net mass of CO₂ blown was 2 319 tons in 2009. Air sampling results explain that carbon stable isotope of CO₂ (¹³Cco₂) ranges from ‑20.6‰ in cold season to ‑23.8‰ in warm season. Cave air is interpreted as a result from a mix between external atmosphere and an airisotopically depleted, coming from the rock environment. The isotopic value of the light member varies through time, from -23.9‰ to -22.5‰.