Messinian salinity crisis

The Messinian salinity crisis was an important event that happened in the Mediterranean Sea millions of years ago. During this time, the sea dried up partly or almost completely for a while. This happened during the late Miocene epoch, between 5.96 and 5.33 million years ago.

We learn about this crisis by looking at samples from deep below the sea floor. These samples have special minerals called evaporite, soils, and fossils of plants. They show that the Strait of Gibraltar, which now connects the Mediterranean to the Atlantic Ocean, was closed about 5.96 million years ago. With no new water coming in, the Mediterranean began to dry up. The dry climate helped the sea evaporate almost completely within a thousand years, leaving a very deep, dry basin.

Finally, around 5.33 million years ago, the Strait of Gibraltar opened again. This caused a huge flood called the Zanclean flood, when water from the Atlantic rushed in and filled the basin once more. Even today, the Mediterranean is saltier than the North Atlantic because it loses a lot of water through evaporation and only a small amount of water flows in from the Atlantic. If the Strait of Gibraltar were to close again in the future, the Mediterranean could dry up in a similar way.

Naming and first evidence

In the 1800s, a scientist named Karl Mayer-Eymar looked at fossils found between layers of gypsum, brackish, and freshwater sediments. He saw that these fossils were from just before the end of the Miocene Epoch. He named this time period the Messinian after the city of Messina in Sicily, Italy. Since then, many other layers with salt and gypsum across the Mediterranean have been connected to this same time.

Further evidence and confirmation

In 1961, special sound waves studied the ocean floor and found a special layer under the Mediterranean Sea. This layer looked like salt. Later, scientists drilled deep and found real salt and other minerals that form when water dries up. These discoveries showed that the Mediterranean Sea had dried up long ago.

Drillers also found signs of dry land, like dust blown by wind and cracks in the ground, mixed with sea fossils. This proved that the sea dried up and filled with water many times over hundreds of thousands of years. Big rivers like the Nile also cut deep valleys when the sea was dry.

Chronology

Based on palaeomagnetic datings of Messinian deposits that are now above sea level, the salinity crisis began at the same time across the Mediterranean basin, about 5.96 million years ago. This event happened during the "Messinian" age of the Miocene epoch.

The connection between the Mediterranean and the Atlantic Ocean closed many times, causing the Mediterranean Sea to dry up partially again and again. The basin was cut off from the Atlantic for a longer time, between 5.59 and 5.33 million years ago. This caused the Mediterranean sea level to drop a lot.

About 5.33 million years ago, at the start of the Zanclean age (the beginning of the Pliocene epoch), the barrier at the Strait of Gibraltar broke. This allowed water to rush back into the Mediterranean basin in what is called the Zanclean flood. Since then, the basin has stayed filled with water and has not dried up again.

Researchers estimate that about 4×10¹⁸ kg of salt was left behind. This suggests that the area may have dried up and filled with water many times over the last 630,000 years of the Miocene epoch.

Causes

The Messinian salinity crisis happened because of changes in Earth’s climate and the movement of its plates. Cooler periods reduced the amount of water flowing into the Mediterranean Sea, helping it dry up. The way Earth’s plates moved may have closed off the area where the Mediterranean connects to the Atlantic Ocean, like the Strait of Gibraltar.

The area around the Strait of Gibraltar is where the African plate and the European plate meet. Their movements could have opened and closed passages. These changes, along with shifts in climate, helped cause the sea to dry up and refill many times.

Relationship to climate

The climate of the deep Mediterranean during its drying period is hard to know because there is nothing like it on Earth today. Scientists use computer models to guess what might have happened. Some think that large, shallow lakes of salty water may have remained, even when much of the sea was dry.

Without the Mediterranean Sea to provide moisture, the climate around it would have been much drier. Places like Italy, Greece, and the Levant might have been similar to the dry conditions found today in the Iberian Peninsula and the western Maghreb. Areas such as the eastern Alps, the Balkans, and the Hungarian plain would also have been drier than they are now. Nearby water bodies, such as the Paratethys ocean and the Pannonian Sea, may have had some effect on the climate north of the Mediterranean basin.

Effects

The Messinian salinity crisis caused many sea animals to disappear. After this, the Mediterranean Sea’s animal life changed, with fewer species in the east. Land animals had a hard time, but some could move between the Iberian Peninsula and North Africa. This even let animals reach the Balearic Islands, where they stayed alone for millions of years.

The Mediterranean Sea dried up a lot, so it might have looked almost empty. The Strait of Gibraltar was closed, and other water paths like the Betic corridor and Rifean corridor were shut too. The water that was left was very salty and hot, making it hard for many creatures. Rivers such as the Nile cut deeper valleys because there was less water.

Replenishment

Main article: Zanclean flood

When the Strait of Gibraltar opened again, a lot of water from the Atlantic Ocean rushed into the Mediterranean Sea. Scientists think this flood might have looked like a giant waterfall, much bigger than famous waterfalls like Angel Falls, Iguazu Falls, or Niagara Falls. The water likely flowed down a gentle slope, not a steep drop.

Under the sea near the southern tip of Sicily, researchers found a large pile of mixed-up rocks and dirt. They believe this was left behind by the powerful flood that refilled the Mediterranean.