Oceanographic and hydroclimatic data explain depressed water level in the coastal karst hosting the decorated...

Paleolithic decorated caves are home to a priceless heritage, but their preservation depends on hydroclimatic conditions within the cave. In coastal areas, changing sea levels pose a further threat to caves, as the sea floods the karst and obliterates Paleolithic artefacts. In this paper, we study the case of the Cosquer Cave, a half-submerged coastal cave located in southeastern France, home to Upper Paleolithic archeological remains. This is a very special case, where the sea represents both an opportunity and a threat for the preservation of an archeological site. The cave is confined, submerged in its lower part, and embedded in a limestone massif with low permeability in the unsaturated zone. Several times a year, mainly in autumn, winter, and spring, air flows through the karstic massif, most likely below sea level, raising the cave's air pressure above atmospheric pressure. The resulting overpressure lowers the cave water level for weeks, keeping it below sea level and temporarily keeping the lowest wall paintings and engravings emerged. However, the oceanographic conditions that cause a pressurization event have not yet been described, although it is a key understanding to help preserve the natural heritage housed in the Cosquer Cave. Based on nine years of in situ continuous monitoring, we use descriptive statistics to decipher the oceanographic conditions controlling air inflow, air outflow, and absence of air flow through the submerged karst. We show that waves are the engine for the pressurization of the cave. The three main factors controlling air entrance are wave height, wave direction and seawater level. 90 % of air inflows coincide with significant wave heights exceeding 0.8 m. Additionally, air inflows are more efficiently caused by SSW and SW waves, propagating in a direction orthogonal to the cliff than by waves from the SE-SSE direction, propagating along the cliff. The minimum wave height required for air inflow to occur increases with sea-level rise, likely because submerged conduits become less accessible for air input. This study establishes a conceptual model of functioning for the natural hydrosystem of the Cosquer Cave, and provides the basis for further modeling and predictions according to scenarios of climate change and sea-level rise.