The Mediterranean Sea represents only 1% of the global ocean surface but it houses approximately 7% of global marine biodiversity, taking into consideration the 17.000 marine species described.

21 countries surround it belonging to 3 different continents and the Mediterranean Sea has an average depth of 1430 m, while the deepest recorded point is in the Calypso Deep, near Greece.

The Mediterranean Sea is characterized by a high evaporation rate and the loss of water via evaporation could not be even by the precipitations and contribution of rivers: a huge quantity of Atlantic water constantly enters the Mediterranean basin through the Gibraltar Strait, maintaining salt and water equilibrium.

Have you ever wondered what would happen if the Gibraltar Strait closed and the Mediterranean Sea did not receive water from the Atlantic?

It seems unlikely, but it is exactly what happened between 5.96 and 5.33million years ago in the Messinian Age.

This disconnection with the Atlantic Ocean contributed to increase the evaporation process, which ultimately led to an increasing in the Mediterranean water salinity and caused a large deposition of gypsum and halite (common salt).

In 1971, a deep-sea drilling campaign was undertaken with the Glomar Challenge oceanographic vessel, and samples of gypsum and halite layers were found in the seabed records, somewhere reaching 3 km thick. In these layers, microfossils of shallow-water foraminifera and shells of small molluscs were found, which can be attributed to the presence of a hypersaline lake. These geological records confirmed that the sea level dropped by one or two kilometres below the current level, causing nearly a complete desiccation. The salt and gypsum exploitation mines that exist along the coast of the current Mediterranean, such as those found in Sicily, are considered clear evidence of this strong evaporation event.

What could have been the causes of this event?

Different theories have been elaborated on the causes of what is called the Messinian Salinity Crisis (MSC). One of those is based on the displacement of the tectonic plates: the shift of the African, Arabic, and Eurasian plates caused the Strait of Gibraltar to close, resulting in the isolation of the Mediterranean basin.

Another hypothesis supports the possibility that the global ocean sea level dropped due to an increase in the Antarctic ice sheet in the final stages of the Miocene, thus resulting in a lower water flux from the Atlantic to the Mediterranean basin.

The most likely explanation considers two main factors: the displacement of the tectonic plates that caused the closing of the Strait of Gibraltar, and the effect of regional climate changes that resulted in less rainy seasons, higher temperatures and less river discharge. Altogether, the level of the Mediterranean fell due to the closure of the connection with the Atlantic Ocean and then oscillated depending on regional climatic changes.

Without going any further, the supply of water was reduced or even cut off, in the same way, a faucet is closed in a bathtub. The water left behind evaporated and the salt crystals precipitated at the bottom of the sea. This condition lasted for hundreds of thousands of years! The Mediterranean became almost uninhabitable and the marine species migrated or died because of the extreme environmental conditions (the water was either too salty or too shallow).

And when did the Mediterranean fill up again?

This aspect was also highly debated by the community of geologists, but almost everyone agreed that the Strait of Gibraltar has once again played a key role: water was reintroduced from the Atlantic Ocean, although the causes may have been different. A rise in the global sea level could have occurred, due to the deglaciation, which favoured the entry of the Atlantic water in the Mediterranean basin.

Other studies provide the theory that tectonic movements could allow a path to be opened in the current Strait of Gibraltar, allowing the water entry. Another possibility is that a narrow stream of Atlantic water eroded and opened a breach where now the Gibraltar Straits stands.

What is certain is that the Mediterranean Sea was gradually refilled in what was called “Zanclian Flood” by the Italian geologist Maria Bianca Cita. This Flood ended the Salinity Crisis of the Mediterranean Sea in the Pliocene Age.

The Messinian Age was a key stage for the history of the Mediterranean Sea: it left a valuable raw material that we continue to exploit today and has caused radical changes in the marine fauna and flora.

Could something similar happen again in the future?