Why the water in the Cosquer cave remains below sea level

The karstic system of the Cosquer cave

The Cosquer cave in Marseille is a semi-drowned Upper Paleolithic cave where some engravings and paintings are temporarily saved from rising sea levels. The preservation of these caves is highly dependent on the flow of air in the karst system and through the surrounding rock. Karst is a system of cavities and conduits formed naturally by the hydrochemical dissolution of carbonate rock. Air flows are governed by the pressure gradient and influenced by the shape of the karstic conduits and the permeability of the limestone massif. The Cosquer cave is half-submerged in coastal karst, where conservation also depends on water levels connected to the sea. 

Hydroclimatic data, such as air pressure, air temperature and water level inside and outside the cave, were measured over several years to identify the main processes governing water level variations, air flow and air renewal. The data show an unusual behavior for a karst: the air pressure in the karst is almost always higher than atmospheric pressure. As a result, the water level in the cave is below sea level. Daily tidal variations allow us to estimate the volume of the cave above water level.

Waves and permeable carbonate rocks

Although the air in the cave is confined by the rock and seawater, there are external air entries during short wave-related pressurization events that can produce and force air bubbles to flow into the massif, along open fissures or karst conduits beneath the sea. In addition, the effective permeability of carbonate rocks to air at massif scale is deduced from the decrease in air pressure in the cave during summer by applying Darcy's law in a partially saturated environment. 

Six years of data show that permeability varies from year to year and as a function of cumulative precipitation during spring and summer. Drier years are correlated with higher permeability, a more rapid decrease in air pressure in the cave and a more rapid rise in water level. In the future, in the context of climate change, a disruption of rock permeability is therefore expected in caves close to the surface, which will have an impact on air flows in decorated caves and could alter their fragile hydroclimatic stability.

The results of this study are the fruit of collaboration between Aix Marseille Université, the research laboratory for historic monuments (Ministry of Culture), the CNRS and the Direction Régionale des Affaires Culturelles.