Messinian salinity crisis

The Messinian salinity crisis was a very 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 know about this crisis from studying samples taken from deep below the sea floor. These samples contain 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 at the time helped the sea to 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 studied fossils found between layers of gypsum, brackish, and freshwater sediments. He noticed these fossils were from just before the end of the Miocene Epoch and named this time period the Messinian after the city of Messina in Sicily, Italy. Since then, many other layers rich in salt and gypsum across the Mediterranean have been linked to this same time.

Further evidence and confirmation

In 1961, special sound waves used to study the ocean floor found a special layer under the Mediterranean Sea. This layer was thought to be made of salt. Later, scientists drilled deep into the sea floor and found real salt and other minerals that form when water dries up. These discoveries helped prove that the Mediterranean Sea had dried up long ago.

Drillers also found evidence of dry land, like wind-blown dust and cracks in the ground, mixed with layers of sea fossils. This showed that the sea dried up and filled up 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 have since been brought above sea level by tectonic activity, the salinity crisis started at the same time over all the Mediterranean basin, at 5.96 ± 0.02 million years ago. This episode was part of the "Messinian" age of the Miocene epoch.

The Mediterranean-Atlantic strait closed tight time and time again, and the Mediterranean Sea partially dried up repeatedly. The basin was finally isolated from the Atlantic Ocean for a longer period, between 5.59 and 5.33 million years ago, resulting in a large lowering of the Mediterranean sea level. About 5.33 million years ago, at the start of the Zanclean age (at the start of the Pliocene epoch), the barrier at the Strait of Gibraltar broke, re-flooding the Mediterranean basin in the Zanclean flood. The basin has not dried up since.

The amount of Messinian salts has been estimated as around 4×10¹⁸ kg. This suggests either a succession of drying and refilling cycles or a long period of extra saltiness during which incoming water from the Atlantic Ocean was evaporated. Research since then has suggested that the drying-flooding cycle may have repeated several times during 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. Scientists think that cooler periods reduced the amount of water flowing into the Mediterranean Sea, helping it dry up. Also, 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, and 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 difficult to know because there is nothing on Earth today quite like it. Scientists use computer models to guess what might have happened. Some think that large, shallow lakes of salty water likely 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 marine fish and other sea animals to disappear forever. After this event, the Mediterranean Sea’s animal life changed, with fewer species found toward the east. Land animals also faced challenges, and some were able to move between Iberian Peninsula and North Africa. This even allowed animals to reach places like the Balearic Islands, where they stayed isolated for millions of years.

The Mediterranean Sea dried up so much that it might have looked almost completely empty of water. The Strait of Gibraltar was closed off, and other water paths like the Betic corridor and Rifean corridor were also shut. The water that remained was extremely salty and hot, making it hard for many creatures to survive. Rivers such as the Nile carved deeper valleys because there was less water to fill them.

Replenishment

Main article: Zanclean flood

When the Strait of Gibraltar finally opened again, a huge amount of water from the Atlantic Ocean rushed into the Mediterranean Sea. Scientists think this flood might have looked like a giant waterfall, much taller and stronger than famous waterfalls like Angel Falls, Iguazu Falls, or Niagara Falls. However, studies show the water likely flowed down a gentle slope rather than 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.