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

Abstract

Abstract. The conservation of decorated caves is highly dependent on airflows in the karst network and through the surrounding host rock. Airflows are driven by the pressure gradient and influenced by the shape of the karst conduits and the permeability of the carbonate rock massif. Cosquer Cave is an Upper Paleolithic decorated cave, half submerged 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 the 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. The data show 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 water level in the pools. Although the cave air is confined by the rock and the seawater, there are also external air inflows during short pressurization events connected with waves that can produce and force air bubbles to flow along submarine open fissures or karst conduits inside the massif. Moreover, the effective permeability of the carbonate rocks 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 cumulative 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 in 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.