VDid you like the disaster scenario of the killer asteroid falling on Mexico's Yucatan Peninsula 66 million years ago? You will love the sequence of episodes collected around the Mediterranean less than 6 million years ago. Remove the dinosaurs that the asteroid caused vaporized, replace them with terrestrial and aquatic fauna that frolicked in the landscape more than 2 million years before the Australopithecus Lucy set foot in the depression of the Afar, and don't miss a drop of the images that unfold over 640,000 years, from -5.971 to -5.333 million years.
Compacted by the clash between the African plate and the Eurasian plate, the two straits which, at that time, united the Atlantic and the Mediterranean became blocked – one crossed what is today the south of Spain , the other is northern Morocco. Deprived of ocean water, the Mediterranean evaporates for thousands of years and declines until it completely or partially dries up. Erosion digs canyons in the bed of the rivers contributing to the sea. That of the Rhône probably goes back as far as Lyon. That of the Nile, up to Aswan.
Controversial case
The rupture of the land barrier near Gibraltar abruptly ends the episode. In the space of two years, a formidable cataract causes the sea to rise back to its original level. The increase reaches several meters per day depending on the location. This entire chronology corresponds to the “Messinian salinity crisis”. Then, over the course of subsequent climatic and geological variations, the Mediterranean acquired its definitive shape, with its strait which is today 14 kilometers wide and 300 meters deep.
Jean-Denis Renard
Emmanuelle Ducassou and Laurent Londeix in front of a map of the Mediterranean in the Epoc premises, on the campus of the University of Bordeaux.
Emmanuelle Ducassou and Laurent Londeix retrace this spectacular film in the offices full of documentation that they occupy on the campus of the University of Bordeaux. Teacher-researchers at the Epoc laboratory (University of Bordeaux/CNRS/Bordeaux INP/École Pratique des Hautes Études), they unravel its mysteries in the company of some of their colleagues.
The matter has been controversial for half a century. “Was the Mediterranean completely closed or was there still a passage? If water continued to enter, would water exit in the opposite direction towards the Atlantic? How much has the level of the Mediterranean dropped, by a few hundred meters, 1,200, 1,500 meters, or even more? What influence did these events have on the salinity of the Atlantic, on the currents, on the climate? » list Emmanuelle Ducassou.
6% of global ocean salt trapped
The puzzles have multiplied since the discovery of enormous deposits of sea salt in the Mediterranean region. On land around its edges but also, much more disturbing, under the floor of its deep pools, kilometers from the surface. Their thickness can reach hundreds of meters. During the Messinian crisis, 6% of the salt in the global ocean would have been trapped there, a phenomenal quantity for a sea which represents less than 1% of the planet's ocean surface.
Faced with falling water levels and soaring salinity, aquatic organisms have succumbed in numbers
Au XXe century, “the hypothesis according to which the Mediterranean would have dried up and produced such massive deposits seemed so strange that all sorts of explanations were attempted. We thus sought to know if quantities of salt could fall from the surface and reach the bottom without dissolving. But it doesn’t work,” relates Laurent Londeix. The only plausible explanation: on all or part of the marine surface, the funds have been exposed. And the evaporation left behind mountains of salt, at a height of 12 meters per kilometer of evaporated water column.
An essential Atlantic passage
The story holds up all the better because the mathematics is implacable: without water from the Atlantic, the water balance of the Mediterranean is in deficit by around 1,800 km³ per year. Precipitation, contributions from rivers (the Rhône, the Po, the Nile, etc.) and that of the Black Sea and the Sea of Marmara, via the Dardanelles Strait, do not compensate for evaporation. In the hypothesis of a hermetic closure of Gibraltar, it would take a little more than 2,000 years for the Mediterranean in its current format to disappear and leave behind it, as during the Messinian crisis, a lake landscape where marshes and small inland seas would be close to the emerged lands.
Erick Bravo / IODP / JRSO
The 62-meter derrick of the oceanographic vessel “Joides”, from which drilling on the seabed was carried out a year ago.
We must imagine the enormity of the shock to the ecosystems when the passage(s) to the Atlantic were closed and then opened. Faced with falling water levels and soaring salinity, aquatic organisms have succumbed in numbers. According to a study published in August in the scientific journal “Science”, in which Laurent Londeix participated, only 11% of endemic marine species survived the crisis. “The corals were the first to suffer, due to a lack of oxygen,” he sketches.
And 640,000 years later, those who had taken their place suddenly saw a wall of water melting from the horizon. It first swept over the western Mediterranean, stumbled upon the rising sea level near Sicily, then flowed into the eastern Mediterranean. In the meantime, many land animals had been able to use the land passage between reunified Africa and Europe. It was also the discovery on the Iberian Peninsula of fossils of gerbils, small rodents of African origin, which convinced the scientific community that the waterways between the Atlantic and the Mediterranean had once been filled.
Drilling to find out more
If the Messinian salinity crisis had a major regional impact, it is assumed that its consequences were perceptible on a global scale. “During the crisis, the average surface water temperature of all oceans fell by 5°C, which is enormous,” notes Emmanuelle Ducassou. The link between the two phenomena could be explained by the impact of the closure of the Mediterranean on the circulation of currents in the Atlantic.
When it is expelled towards the Atlantic by Gibraltar, the salt-laden water of the Mediterranean is transported to high latitudes where, cooled and denser, it plunges towards the depths and fuels what is called the “thermohaline circulation” oceans, essential for distributing heat from the tropics to the poles. The crisis may have stopped the mechanics of this treadmill – the warm currents on the surface towards the poles, the cold currents at depth towards the Equator.
To find out more about Mediterranean-Atlantic trade before and during the crisis, an ambitious underwater drilling project took place between December 2023 and February 2024. It targeted target areas on both sides of the Strait of Gibraltar using the skills of the “Joides”, an American oceanographic vessel that looks like a floating laboratory. It was carrying International Ocean Discovery Program (IODP) Expedition 401. Emmanuelle Ducassou was its co-leader. It brought together 26 scientists from several nationalities.
Simon George / IODP
The drilling tube and its re-entry cone descend towards the seabed through the hole in the hull of the ship.
Thanks to its 62 meter high derrick, the “Joides” was able to collect rock and sediment cores up to 1,400 meters below the seabed. More than 6,300 samples were brought back to shore. They are carefully preserved in Germany, the United States and Japan. Scientists did not drill directly into the salt deposits, which would have been instructive. Salt layers can be close to pockets of pressurized hydrocarbons, posing insurmountable safety concerns.
“A human adventure above all”
The story is far from over. Drilling on land, in Spain and Morocco, is planned by the end of 2027. And the exploitation of cores taken from the seabed is only just beginning. “Other drilling took place in the Gulf of Cádiz twelve years ago. I am still working on the data collected. Everything we collected during this 401 expedition takes up 90% of our time, it's very dense. It’s a magnificent scientific adventure, but above all it’s a human adventure,” enthuses the researcher.
Erin Winick Anthony / IODP
Emmanuelle Ducassou examines foraminifera, tiny fossil shells, aboard the “Joides” during the winter of 2024.
70% of the water could have evaporated
According to a new French study signed by CNRS researcher Giovanni Aloisi and published on November 18 in the scientific journal “Nature Communications”, 70% of the water in the Mediterranean basin disappeared during the Messinian salinity crisis. This would explain the accumulation of a million km³ of salt on the Mediterranean ocean floor.
In a first phase, lasting around 35,000 years, the salts would have been deposited in the eastern part of the Mediterranean, due to a partial restriction of its flow towards the Atlantic Ocean. During a second, shorter phase, less than 10,000 years ago, they would have dispersed throughout the basin, which would have caused its rapid drying. The fall in sea level is estimated between 1.7 and 2.1 kilometers in the eastern Mediterranean. It would have been approximately 850 meters in its western part, closest to communication with the Atlantic. As a corollary effect of the temporary lightening of the earth's crust, volcanic eruptions would have occurred in the region.
