Mesoscale permeability variations estimated from natural airflows in the decorated Cosquer Cave (SE France)

Abstract. Conservation of decorated caves is highly dependent on airflows in the karst network and through the surrounding host rock. Airflows are driven by pressure gradient and influenced by the shape of the karst conduits and the permeability of the carbonate rock massif. The Cosquer cave is an Upper Paleolithic decorated cave, half drowned in a coastal karst, where conservation is also dependent on the cave’s pools connected to the sea. Hydroclimatic data, such as air pressure and temperature and water level inside and outside the cave have been measured for several years to identify the main processes governing the water level variations, the airflows and the air renewal. Data show an unusual behavior for a karst: the karst air pressure is nearly always higher than the atmospheric pressure. As a result, the water level in the cave is below the sea level. The daily variations of the sea tide provide an assessment of the cave volume above the pools water level. Although the cave air is confined by the rock and the seawater, there are also external air inflows during short pressurization events. Moreover, the carbonate rocks effective permeability to air at the massif scale is inferred from the cave air pressure decrease over the summer season, by applying Darcy’s law in a partially-saturated medium. Six years of data show that permeability varies from year to year, and according to the cumulated rainfalls during the spring and summer. The driest years are correlated with a higher permeability, a faster air pressure decrease in the cave and a faster rise of the pools water level. In the future, in the context of climate change, a perturbation of the rock permeability is then expected in the near surface caves, which will impact airflows in decorated caves and may alter their fragile hydroclimatic stability.