SCIENCE. When the Mediterranean lost 70% of its water… 5.5 million years ago

5.5 million years ago, the Mediterranean lost 70% of its water due to the temporary closure of the Strait of Gibraltar, a recent study reveals. This phenomenon has profoundly transformed the Mediterranean landscape and ecosystem. Between spectacular drying and geological upheavals, a look back at a significant episode in the history of our planet.

A spectacular drying of the Mediterranean occurred around 5.5 million years ago, following the temporary closure of the Strait of Gibraltar. This phenomenon, documented in a study published Monday in Nature Communications, reveals the colossal impact of this geological event on the Mediterranean ecosystem.

The Strait of Gibraltar, a narrow passage separating Spain from Morocco, plays a crucial role in regulating water exchanges between the Atlantic Ocean and the Mediterranean. On the surface, water from the Atlantic enters the Mediterranean Sea, while at depth, the saltier water from the latter flows towards the Atlantic. These exchanges are essential to compensate for the significant evaporation of the Mediterranean, the rivers not being enough to fill this natural deficit.

The researchers point out that if this passage were to close today, the level of the Mediterranean Sea would drop “by around 0.5 meters per year”. But between 5.97 and 5.33 million years BCE, this imbalance took on a dramatic magnitude when the Strait of Gibraltar closed due to tectonic movements. This episode, known as the “Messinian salinity crisis”, led to an extreme concentration of salt and the progressive drying of the Mediterranean.

The study, led by Giovanni Aloisi, geochemist at the CNRS and the Institute of Globe Physics, shows that this drying took place in two distinct phases. During the first, about 35,000 years ago, the Mediterranean was still “full of water, like now”, but the narrowing of the strait slowed the evacuation of salt water towards the Atlantic, which made the sea brackish , explains the researcher.

The second phase, much briefer – around 10,000 years – saw the strait completely close. Trade with the Atlantic ceased, leading to a massive accumulation of salts throughout the basin. The Mediterranean has dried up, its level falling by 1.7 to 2.1 kilometers in its eastern part and by around 850 meters in the western part. “In total, the Mediterranean basin has lost 70% of its water volume,” explains Giovanni Aloisi.

The drying up has left visible traces today: “The bottom of the Mediterranean is covered with a layer of salt which is up to 2-3 km thick”, or a million cubic kilometers, explains the researcher. . The consequences on biodiversity were dramatic, with only microscopic life forms able to survive in such a saline environment.

The lowering of sea levels would also have allowed the formation of a land bridge between Africa and Europe in the western part of the Mediterranean. This passage would have facilitated “the colonization of the Balearics by mammals” such as goats, rodents or rabbits, underlines Giovanni Aloisi, based on previous studies.

In addition to ecological changes, this drying would also have influenced the local climate by modifying atmospheric circulation. In addition, the reduction in pressure exerted by the mass of water on the lithosphere would have favored an intensification of volcanic activity. “70% of the volume of the Mediterranean represents an enormous mass of water, which exerts pressure on the earth's crust. When this pressure decreases, the formation of magma and its migration to the surface are facilitated,” explains the geochemist.

After approximately 10,000 years, the Strait of Gibraltar reopened, allowing the Mediterranean to fill again, marking the end of this exceptional crisis.

Written with AFP.