Mechanism of Salt Contamination of Karstic Springs Related to the Messinian Deep Stage. The Speleological Model of Port Miou (France)

摘要

Submarine karstic springs are frequent on the Mediterranean shore but most of them are brackish which prohibits their use. The numerous attempts to catch these springs ended in failure. Recent studies on the development of karst systems and the paleogeography of the Mediterranean sea explain these failures. Studies on the shores of south-eastern France have enabled us to propose an operating model that explains the mechanism of salt contamination. The Port Miou system (Cassis, France) is a two kilometers long submarine gallery developped in the limestone series of Calanques (Marseille, France). The average discharge is between 2 to 5 m3/s but the water is brackish. In the 70's a dam was built to prevent sea intrusion in the cave but the water remained brackish upstream of the dam. The use of helium, and then rebreathers by cave divers, made it possible to explore a vertical pit down to -179 m below the sea level at the end of the cave. At that depth, the water is still brackish. Important quantity of titanium was observed at the surface of the cave sediment upstream of the dam and at the end of the cave. The titanium comes from the residual product of a factory of alumina that is discharged in the Cassidaigne submarine canyon, at a depth of 300 m b.s.l., a few kilometers south to the spring. This residual product locally called ?red mud? is very rich in titanium. This supports the model of a sea water aspiration in a deep gallery connected to the canyon. The Cassidaigne canyon that cuts a limestone plateau with dolines and caves is probably a pocket valley. Its presence is related to the several stages of lowering of the Mediterranean sea at the Messinian Deep Stage that allowed the existence of cave networks up to several hundreds of meters below the present sea level. The sea water is now sucked into the system. A similar example exists in Kefalonia island (Greece) where a sea intrusion is observed in coastal sinkholes. This model explains why the different attempts to diminish the salinity of these brackish springs, by construction of dams close to the outlets, have failed.