Palaeoclimate research in Villars Cave (Dordogne, SW-France)

Abstract

C to any vegetation and climatic changes. Drip rate monitoring, carried out under four stalactites from the lower and upper galleries, has shown a well marked seasonality of the seepage water with high flow rates during winter and spring. A time delay of about two months is observed between the water excess (estimated from outside meteorological stations) and the drip rate in the cave. A great heterogeneity in the flow rate amplitude variations and in the annual quantity of water between two nearby stalactites is observed, confirming the complexity of the micro-fissure network system in the unsaturated zone. At a daily scale, the air pressure and drip rates are anti-correlated probably because of pressure stress on the fissure network. Cave air CO 2 concentration follows soil CO 2 production and is correlated with its δ 13 C content. Since the beginning of the monitoring, the cave air temperature, in both lower and upper galleries, displays a warming trend of ~+0.4°C±0.1/10yrs. This might be the consequence of the outside temperature increase that reaches the Villars Cave galleries through thermal wave conduction. Chemistry monitoring over a few years has shown that the seepage water of the lower gallery stations is significantly more concentrated in trace and minor elements (i.e. Sr, Mg, Ba, U) than the upper stations, probably due to the 10-20 m depth difference between these galleries, which implies a different seepage pathway and different water/rock interaction durations. There is also, in the elemental concentration (i.e. [Ca]), a seasonal signal which causes variation in the speleothem growth rates. Modern calcite deposit experiments conducted for several years have permitted the calculation of vertical growth rates, which are extremely high in Villars (i.e. 1.0 to 1.75 mm/ yr). Pollen filter experiments in the cave have demonstrated that most of the pollen grain found in the cave comes from the air and not from the water. The specificity of the Villars Cave records is that the climatic variations were well recorded in the calcite δ 13 C whereas the δ 18 O is usually used in such studies. Overall, these results are helpful for the interpretation of speleothem records for palaeoclimatic reconstructions, but more work is needed, especially numerical modelling of the temperature, chemistry and hydrology.