Messinian salinity crisis

The Messinian salinity crisis was a big event that happened a long time ago, between 5.96 and 5.33 million years ago, during the Messinian age of the Miocene epoch. During this time, the Mediterranean Sea almost dried up completely.

This happened because the Strait of Gibraltar, which connects the Mediterranean to the Atlantic Ocean, closed for a while. When this happened, the sea could no longer get new water from the Atlantic. Because the climate was very dry, the water evaporated and left behind lots of salt.

Scientists have found evidence of this drying up in the deep seafloor of the Mediterranean. They discovered minerals called evaporite, soils, and even fossil plants that grew when the sea was dry. After the strait closed for the last time about 5.6 million years ago, the whole basin became almost completely dry in just a thousand years. The deepest parts of the basin were as deep as 5 kilometers below what we now think of as sea level! There were only a few small, very salty lakes left, similar to the Dead Sea today.

Later, around 5.5 million years ago, the climate became wetter, and fresh water from rivers started to fill the basin again. This turned the salty lakes into larger pockets of brackish water, like the Caspian Sea. Finally, about 5.33 million years ago, the Strait of Gibraltar opened up again in an event called the Zanclean flood. This allowed the Atlantic Ocean to rush back in and fill the Mediterranean Sea once more.

Even today, the Mediterranean Sea is saltier than the North Atlantic because it is almost enclosed by the Strait of Gibraltar and loses a lot of water through evaporation. If the strait were to close again, the Mediterranean could mostly dry up in about a thousand years.

Naming and first evidence

In the 1800s, a scientist named Karl Mayer-Eymar studied old remains found between layers of minerals and soil. He saw that these remains were from a time just before the end of the Miocene Epoch. In 1867, he named this time the Messinian after the city of Messina in Sicily, Italy. Since then, scientists have found many layers with lots of salt and minerals like gypsum in the Mediterranean area from that same time.

Further evidence and confirmation

In 1961, special sound waves helped scientists study the sea floor. They found a special layer under the Mediterranean Sea. This layer, called the M reflector, looked like it was made of salt. Scientists thought this layer was from a time long ago when the sea almost dried up.

In 1970, scientists on a ship called the Glomar Challenger took samples from deep in the sea. These samples had minerals like salt and gypsum. These minerals usually form when water dries up. They also found layers showing the sea dried up and filled with water many times over hundreds of thousands of years. Rivers cut deep valleys into the dry sea floor, showing how low the water level got.

Chronology

The Messinian salinity crisis began around 5.96 million years ago when the connection between the Mediterranean Sea and the Atlantic Ocean closed. This caused the Mediterranean to dry up in stages, creating large canyons and lots of salt. The sea level dropped between 5.59 and 5.33 million years ago.

Finally, about 5.33 million years ago, the barrier at the Strait of Gibraltar broke, and a huge flood of water from the Atlantic refilled the Mediterranean basin. This event, called the Zanclean flood, ended the crisis, and the Mediterranean has stayed filled ever since. Evidence suggests there may have been several cycles of drying and refilling during this time.

Causes

The Messinian salinity crisis happened because of changes in Earth’s climate and its shifting plates. Cooler climates caused less rain to fall into the Mediterranean Sea. This meant the sea lost water faster than it gained. Sometimes the water nearly dried up.

The shape of the land also mattered. The area where the Mediterranean meets the Atlantic Ocean, near the Strait of Gibraltar, is where two large pieces of Earth’s crust—the African plate and the European plate—come close together. When these plates moved, they could close the passage between the seas. When this happened, the Mediterranean could not get new water from the Atlantic, and it began to dry out. These changes caused the sea to fill up and dry out many times.

Relationship to climate

We do not know exactly what the climate was like when the Mediterranean Sea dried up. There is nothing on Earth today quite like this, so scientists use computer models to guess what might have happened. Some think the Mediterranean may have still held a few big lakes of salty water.

If the whole sea had dried up, the air in the summer might have gotten very hot—perhaps up to around 80 °C (176 °F) in the lowest areas. This would make it hard for most life to survive, except for very tough kinds of tiny organisms. However, these ideas might be too extreme.

Without the Mediterranean Sea adding moisture to the air, places like Italy, Greece, and the Levant would have had a drier climate. Areas such as the eastern Alps, the Balkans, and the Hungarian plain would also have been much drier. Nearby water bodies, such as the Paratethys ocean and the Pannonian Sea, may have provided some moisture to nearby lands.

Effects

The Messinian salinity crisis caused many sea creatures to disappear from the Mediterranean Sea. After this time, the number of different animals and plants in the sea changed, with fewer types found farther east. Land animals also lost some variety. Because land areas shifted, animals could move between places like the Iberian Peninsula and North Africa, and some even reached faraway islands such as the Balearic Islands.

The Mediterranean did not have any water for a time, which made it very hard for sea life to survive. The area became very hot, and rivers dug deeper into the ground as they flowed. Some think the Red Sea was also dry during this period, but it was not connected to the faraway Indian Ocean.

Replenishment

Main article: Zanclean flood

When a connection formed at the Strait of Gibraltar, a lot of water from the Atlantic Ocean flowed into the Mediterranean. Scientists think this refill might have looked like a giant waterfall, much taller and more powerful than famous waterfalls today. But newer research shows the water probably flowed more slowly, not all at once.

Underwater, near the southeast corner of Sicily, scientists found a large pile of mixed materials. They think this was left behind when the flood filled the Mediterranean with water again.