Mediterranean dries out

Between about 5.97 and 5.33 million years ago, the Mediterranean Sea nearly dried out in an event called the Messinian salinity crisis. Tectonic uplift and falling sea levels cut the basin off from the Atlantic at the Strait of Gibraltar region. A sea in a hot, dry climate loses more water to evaporation than rivers supply, so the water level fell, perhaps by more than a kilometer in the deepest areas, and salt, gypsum and other minerals were left behind in layers up to a kilometer or more thick. Scientists first found the salt in 1970, in cores drilled from the Mediterranean seabed by the research ship Glomar Challenger.

How dry it became is still debated. Some researchers argue the basin held brine lakes and shallow water rather than a full desert. Rivers such as the Nile and Rhône cut deep canyons into the exposed margins. Salt-loving life survived and many marine species vanished. The basin refilled about 5.33 million years ago, when Atlantic water broke through at Gibraltar in what is called the Zanclean flood. Computer models suggest the water poured in at rates far above the Amazon River, and the sea may have refilled in as little as a few months to a couple of years.