Messinian Salinity Crisis: When the Mediterranean Sea Dried Up

The Messinian salinity crisis was a geological event, roughly 5.96 to 5.33 million years ago, in which the Mediterranean Sea was cut off from the Atlantic Ocean and much of its water evaporated. It left behind salt and gypsum layers more than a kilometre thick beneath the sea floor. The crisis ended with the Zanclean flood, when Atlantic water poured back through the Strait of Gibraltar, and it remains one of the most dramatic episodes in the history of Europe’s inland sea.

Background: a sea that depends on the Atlantic

The Mediterranean is an almost enclosed sea. In today’s climate it loses far more water through evaporation than it receives from rivers such as the Nile, Rhône and Po and from rainfall. The deficit is made up by Atlantic water flowing in through the Strait of Gibraltar, which is only about 14 kilometres wide at its narrowest.

About six million years ago, in the late Miocene epoch, that connection looked different. Atlantic water reached the Mediterranean through seaways across what is now southern Spain (the Betic corridor) and northern Morocco (the Rifian corridor). Slow collision between the African and Eurasian plates, along with uplift of the land, gradually narrowed and closed these passages.

The name of the crisis comes from the Messinian age, the last stage of the Miocene, which the Swiss geologist Karl Mayer-Eymar named in 1867 after the Sicilian city of Messina. Its end is marked by the Zanclean age, named after Zancle, the ancient Greek name of Messina.

What happened

Geologists usually divide the crisis into three stages, although the exact timing and interpretation of each are still argued over.

Stage one: gypsum in the margins

From about 5.97 million years ago, the restricted inflow made the Mediterranean saltier. In shallow marginal basins, such as those in Sicily, the northern Apennines and the Sorbas basin of south-east Spain, thick beds of gypsum (calcium sulphate) began to crystallise. These are known as the Primary Lower Gypsum.

Stage two: the great salt layers

Around 5.6 to 5.55 million years ago the crisis reached its peak. In a short interval, perhaps around 50,000 years, enormous quantities of halite (rock salt) were deposited in the deep central basins. Seismic surveys show salt bodies more than a kilometre thick in parts of the western and eastern Mediterranean. Most researchers think the sea level inside the basin fell far below that of the ocean, perhaps by hundreds of metres or more, although the amount is disputed.

As the sea level dropped, rivers cut deep canyons to reach the new, much lower shoreline. The Nile and the Rhône both carved valleys that are now buried under later sediments. The Soviet geologist Ivan Chumakov reported the buried Nile canyon in the 1960s, from boreholes drilled during preparations for the Aswan High Dam.

Stage three: the Lago-Mare

In the final stage, roughly 5.55 to 5.33 million years ago, the basin held a patchwork of brackish lakes. Their fossils include shellfish and tiny crustaceans related to those of the Paratethys, a former inland sea to the north-east. Geologists call this phase the Lago-Mare, Italian for “lake-sea”.

The Zanclean flood

About 5.33 million years ago, at the boundary between the Miocene and Pliocene epochs, Atlantic water broke back into the Mediterranean through the area of the Strait of Gibraltar. Sediments above the salt suddenly change to normal deep-sea muds containing open-ocean fossils.

In 2009 a team led by Daniel Garcia-Castellanos published a study in Nature arguing that the flood cut an erosion channel roughly 200 kilometres long beneath the present strait. Their model suggested that most of the water may have arrived within a few months to two years, with the sea level rising by metres per day at the peak. Other researchers favour a slower refill or an earlier, more gradual reconnection. The idea of a catastrophic waterfall pouring over Gibraltar is popular, but the details remain uncertain.

Key people and discoveries

Strong reflections deep beneath the Mediterranean floor, later called the M-reflector, were noticed in seismic surveys during the 1960s, but their meaning was unclear. The breakthrough came in 1970 with Leg 13 of the Deep Sea Drilling Project. Aboard the research ship Glomar Challenger, the co-chief scientists Kenneth J. Hsü and William B. F. Ryan recovered cores of gypsum, salt and other evaporites, together with structures such as algal mats and nodules that typically form in very shallow water or on exposed salt flats.

Hsü, Ryan and the Italian geologist Maria Bianca Cita proposed the “deep desiccated basin” model: a Mediterranean that dried out into a hot, salty plain several kilometres below sea level, refilled from time to time. Hsü popularised the idea in his book The Mediterranean Was a Desert (1983). Since then rival models have argued for a deep basin that never fully dried, or for salt formed in brines under substantial water. Today most specialists accept a major drawdown, while still debating its scale.

By the numbers

Consequences

The salt removed from the oceans during the crisis amounted to several per cent of all the dissolved salt in the world’s seas, a large enough share to have slightly lowered global ocean salinity. Some researchers have explored possible effects on climate and ocean circulation, although these are hard to measure.

On land, the lowered sea and exposed shelves may have made it easier for some animals to cross between Africa and southern Europe. Fossils show exchanges of mammals between North Africa and Iberia around this time, but how far they depended on the crisis is debated. The episode also shaped modern geology and industry: the Messinian salt acts as a seal above major natural gas fields in the eastern Mediterranean.

The crisis took place long before the first humans existed. It belongs to the same deep past as other turning points in Earth history, such as the Chicxulub asteroid impact some 60 million years earlier, and it set the stage for the Ice Age world in which later creatures such as the woolly mammoth would appear.

Timeline

• c. 7.25 million years ago: The Messinian age begins; Atlantic gateways through southern Spain and Morocco are narrowing.
• c. 5.97 million years ago: The salinity crisis begins, with gypsum forming in marginal basins.
• c. 5.6 to 5.55 million years ago: Peak of the crisis; thick halite is deposited in the deep basins.
• c. 5.55 to 5.33 million years ago: Lago-Mare phase of brackish lakes.
• c. 5.33 million years ago: The Zanclean flood refills the Mediterranean, opening the Pliocene epoch.
• 1867: Karl Mayer-Eymar names the Messinian stage after Messina, Sicily.
• 1960s: Ivan Chumakov reports a buried Nile canyon from boreholes near Aswan.
• 1970: Deep Sea Drilling Project Leg 13 recovers Messinian evaporites from the Mediterranean floor.
• 2009: A study in Nature proposes a rapid, catastrophic refill through Gibraltar.

Why it matters

The Messinian salinity crisis shows how a narrow gateway can control the fate of an entire sea. It explains the hidden salt beneath the Mediterranean, the buried canyons of rivers such as the Nile, and the shape of the Strait of Gibraltar. It is also a vivid reminder that the familiar map of Europe and Africa is only a snapshot in a much longer, changing story.