Seasonal pressurization of a coastal karst: the paleolithic decorated Cosquer cave (SE France)

Résumé En

The Cosquer cave is a paleolithic decorated cave, in a coastal karst linked to the sea. Stability of climatic parameters in caves is known to be one main condition for conservation of art. Hydroclimate data are measured since several years at a 5 minutes time step: karst air pressure, water level in the karst, atmospheric pressure and sea level. Data shows an unusual behaviour for a karst: the karst air pressure is nearly always higher than the atmospheric pressure. As a result, water level in the karst is below the sea level. Some rock art figures present on walls near water level undergo wash out and fading but limited thanks to the karst pressurization. A stop of this mechanism due to rising sea-level, an increase of the massif permeability or changes in climatic conditions would lead to the loss of arts near water bodies.The cave air overpressure is related to the rock permeability that should be low. The pressure time series show that three main processes drive the cave pressure. The daily variations of the sea tide provide an assessment of the cave air volume above the pools water level. Although the cave air is confined by the rock and the seawater, there are external air inflows during short pressurization events, that can be deduced from pressure data. Then, the low cave air pressure decrease over the summer season is explained by air outflow through the rock. A bulk permeability is then calculated using Darcy law, assuming a gas permeability in a non-saturated medium. Three theoretical cases are evaluated: an equivalent porous medium, a single fracture, and a single karst conduit. The time series give an observation database to assess future changes in the pressure behaviour of this decorated paleolithic cave, and to detect water level increase and adjust conservation choices.