Around six million years ago the Mediterranean Sea very nearly dried up
Roughly 5.96 million years ago the forerunner of the Strait of Gibraltar closed, cutting the Mediterranean off from the Atlantic. Evaporation did the rest, shrinking the sea into a basin 3 to 5 km below normal sea level, dotted with salty pools like the Dead Sea.
Without Atlantic inflow, the basin loses around 3,300 cubic kilometres of water a year, enough to drain its 3.7 million cubic kilometres in little more than a thousand years. After the strait shut for the last time, around 5.6 million years ago, that is roughly what happened. The early dry stages carved canyons on the scale of the Grand Canyon; the Nile cut its bed to 2,500 metres below sea level just north of Cairo. From about 5.5 million years ago, wetter weather sent more river water in, turning the briny pools into bigger brackish lakes rather like today's Caspian, a phase known as Lago Mare.
The clues piled up slowly. The Swiss geologist Karl Mayer-Eymar named the Messinian age in 1867, after the Sicilian city of Messina. Seismic surveys in 1961 found a buried layer, the M reflector, 100 to 200 metres under the seafloor, and Ruggieri later coined the phrase Messinian salinity crisis. In the summer of 1970 the drilling ship Glomar Challenger, with William Ryan and Kenneth Hsu as co-chief scientists, hauled up cores of gypsum, rock salt and potash, river gravels, and even deep-sea ooze that had dried to dust and been blown around by sandstorms before settling in a brine lake.
The salt adds up to more than a million cubic kilometres, around 50 times what the sea's water normally holds, which points either to repeated drying and refilling or to long spells of extreme saltiness. Hsu read the cores as cycles: even layers laid down in deep water, choppier ones as it shallowed, then stromatolites and anhydrite as the floor dried out, all suddenly buried by mud from the next deluge. Some recent studies doubt, on geodynamic grounds, that the cycle repeated many times.
The crisis closed 5.33 million years ago, as the Zanclean age opened, when the barrier at Gibraltar broke for good and the Atlantic poured back in. The basin has not dried out since, but it remains saltier than the North Atlantic because it evaporates quickly and connects to the ocean only through a narrow gap. Close the strait again, as is likely on geological timescales, and most of it would vanish in about a thousand years.
Source: Messinian salinity crisis
