Regime switch in karstic caves atmosphere; possible consequence on annual speleothem growth.

Speleothem are usually considered as records of past climate, and are supposed to present annual growth rings. Yet, they grow inside caves that benefit from very stable environment. However, Bourges et al. 2001, have shown that the atmosphere of Aven d'Orgnac (South East France), was characterized by drop of CO2 concentration and 222Rn activity at the end of autumn and presented each year the succession of a winter and a summer regime. Temperature data are now used to constrain the climate of this cave system. Our data consist in 5 years monitoring with 0.01°C accuracy, three short thermal profiling campaigns, and sparser data gathered in different French painted caves. Near the opening of Aven d'Orgnac, the Salle de Jolys room records each year at the end of autumn the onset of the winter regime that is shown to be triggered by the inverse density stratification induced by the decrease of the outside night temperature. Comparison of summer and winter vertical temperature profiles point to a thermoconvective destabilization of this room atmosphere, involving the downward flow of cold outside bearing air near the cave floor during winter nights. The winter regime propagates then stepwise inside the Aven d'Orgnac cave system. In Salle Plane, which is situated more than one kilometer away from the entrance, the winter regime has never been observed. Our thermal profiling experiment shows there low amplitude (0.03°C) temperature changes, with major daily and half daily components, that are strongly correlated with the pressure first time derivative. Comparison with temperature records from other rooms of the Aven d'Orgnac cave system and with other caves monitored by our team suggest that a strong correlation between temperature changes and the pressure first time derivative could be considered as a clue to the confined character of a given cave room. We propose therefore that the Aven d'Orgnac cave system could be divided in two parts : the open system, where the winter regime sets on each year, and that is submitted to yearly compositional changes of the atmosphere, and the confined rooms that never turn to the winter regime. Between the open and the confined sub-systems only the coldest winters are able to induce the switching to winter regime. This could provide a valuable climatic information. This suggest that speleothems should be gathered after thermal monitoring in winter. If the inner temperature is strongly correlated with the pressure first time derivative, the cave room is in a confined state and benefits from a stable atmosphere, whose compositional changes cannot control the annual growth of speleothems. If the inner temperature is controlled by the outside one, the onset of a winter regime each year associated with a drop of CO2 concentration may be suspected and could be involved in the annual growth of speleothems. Reference Bourges, F., Mangin, A., D'Hulst, D., Le gaz carbonique dans la dynamique de l'atmosphere des cavites karstiques, l'exemple de l'Aven d'Orgnac (Ardoche), C.R. Acad. Sci. Paris, 333, 685-692, 2001.