The Mediterranean Sea dried up almost completely during the Messinian Salinity Crisis

About six million years ago, almost all the water of the Mediterranean Sea evaporated as it became cut off from the global ocean. In a geological blink of an eye, the sea level dropped until only a few seething lagoons remained, at a depth of roughly 1,500 to 3,000 meters below mean sea leve, causing a huge ecological crisis. But then great natural floodgates opened in the Strait of Gibraltar and through a mega-flood the basin refilled with seawater. In this article we explain why the Mediterranean, actually a dying ocean, dried up so suddenly and what evidence was left behind in the surrounding regions.

Author: Kathelijne Bonne.

When looking out over the glistening surface of the Mediterranean Sea, while holding a glass of wine, it's easy to believe the gods had meant it this way since the beginning of time. White-plastered, bougainvillea-overgrown houses lie scattered across the landscape. Here and there lies an old amphora. Fichi d'india cling to the rocks. Along these coasts, enjoyment was elevated to an art, practiced since Antiquity. But was the Mediterranean always so rich, lovely and generous? Heaven on earth? No, in nature everything is transient. Also the Greeks knew this. Panta rhei. Everything flows. Heaven was once Hades.

Salt layers

The Messinian event or crisis was in any sense a low point in the geological history of the Mediterranean. The name refers to the age in which it occurred, the Messinian, after the city of Messina in Sicily where sedimentary layers from that time were first described. The discovery of massive deposits of halite (rock salt/kitchen salt), gypsum and lime (also salts) across the coasts of Spain, Italy, Greece, Turkey, Israel and North Africa led scientists in the 1950s to suspect that there had been a phase of intense evaporation. (Dissolved salt in water precipitates when the salt concentration becomes too high, which happens when water evaporates.) The resulting salty layers are called evaporites. 

The ambitious Deep Sea Drilling Project in the 1960s confirmed that the evaporites continue in the rocks below the bottom of the sea, and are more than three kilometers thick in some places. This made researchers hypothesize that all of the Mediterranean Sea had once dried up. The Messinian Salinity Crisis has since been studied extensively. Different scenarios and many unresolved details still fuel heated debates in research papers today, which makes it all the more fascinating.

Deep canyons

On land, scientists also discovered that many rivers had carved out inexplicably deep river gorges, well below the current mean sea level. Large canyons (now filled with sediment) once cut through the landscape, channeling large rivers that flowed into deep depressions, as demonstrated for the Rhone, the Po, and the Nile rivers. The Nile Valley was a 1,000-meter-long canyon stretching as far inland as Aswan, and when the seawater rose, the Nile became a wide ria.

Messinian world: where were the humans?

The Messinian is the last age of the Miocene Epoch. The Miocene lasted from 23 to 5 million years ago, and the Messinian takes up the last two million years. This is fairly recent relative to the total geological history of the Earth. During the Miocene the climate gradually became cooler and drier, heading towards the Ice Ages that would later follow. The fauna and flora were fairly similar to those of today. The dinosaurs had long been erased from the map, and mammals had taken their place as the main large animals.

Several species of ancient humans (genus Homo) roamed the Messinian world, and their ancestors had only just split off from the ancestors of the chimpanzees (genus Pan), to whom we are most closely related. The humans then inhabited East Africa, so they did not see the spectacular downdraw and refill of the Mediterranean with their own eyes. But they may have noticed climate and ecosystem changes.

During the Miocene, the African tectonic plate drifted towards to the Eurasian tectonic plate. But there was still a large sea, the Tethys, between both continents. The Tethys Sea was connected to the Atlantic and Indian Oceans, and was the precursor of the Mediterranean. But as Africa drifted northward, those marine connections narrowed. A collision between Africa and Eurasia was imminent.

Cut off from the world ocean

The continents collided 15 million years ago. A land bridge first formed in the east (the Arabian Peninsula, connected to Asia), cutting off the connection to the Indian Ocean. To the west, Africa collided with Iberia and mountains rose. The Betic Cordillera in southern Spain and the Rif Mountains in Morocco were connected to each other forming a great mountainous arc (see map below). The Strait of Gibraltar now lies between these mountains, but it didn't exist back then. There were marine connections to the Atlantic, across the interior of Spain and of Morocco. But due to the action of plate tectonics these two corridors closed as well. And by 5,97 million years ago, the Mediterranean was completely cut off from the global ocean.

The large desiccated basin and Lago Mare

The water evaporated and was no longer efficiently replenished by water from the Atlantic Ocean. Sea level dropped and salinity increased. Thick layers of salt deposited on the exposed slopes, and later also in the deepest parts. 5 to 6% of all the world's sea salt is stored in those layers. At the peak of the crisis, the water level was so low that the sea had shrunk to a few large, hypersaline lagoons. Summer temperatures may have reached 80° Celsius at the basin floor. Only extremophilic bacteria survived.

But it was probably not terribly hot and inhospitable at all times, the climate varied from region to region, and fluctuated as time passed. Warmer and cooler episodes alternated. For the rocks show a succession of very saline to less saline layers. Sometimes some water flowed in, through the surrounding river networks, a temporary opening to the ocean, or rain.

At some point, the water suddenly became much less saline, it became brackish to fresh, due to rain and inflowing rivers or maybe a connection to the Para-Tethys, the huge water basin in the region of the Black Sea (worthy of another article). This last brackish to freshwater phase is called the Lago-Mare, literally the lake-sea. Various (fresh) water animals could now survive, such as mollusks and fish.

Worldwide effects: Thermohaline circulation

Because so much salt was locked away and no longer circulating in the world ocean, the global salt concentration had decreased. That in turn affected the thermohaline circulation, which is the big system of ocean currents that redistribute the planet's water masses that rise and sink and move laterally due to differences in temperature and salinity. This huge conveyor belt keeps the Earth's climate relatively mild and any disturbance in this system can have profound climate repercussions (Today, addition of cold fresh water from the melting ice caps will exacerbate climate change).

Another consequence of the oceans' lowered salinity was that it freezes at a higher temperature (it freezes faster, that is). The Messinian Crisis may therefore have sped up the onset of the Ice Ages.

And because all the water of the Mediterranean had evaporated, more rain fell elsewhere. Mean sea level rose by 10 meters and coastal areas were flooded.

Hippos explore new lands

In the Mediterranean, the hypersaline water and the drought on the exposed lands killed many animals, plants and microorganisms. But for some land animals of the area, this was an opportunity to explore new territories. They descended into the basin and walked out on the other side. Faunal diversification and exchange occurred between Eurasian and African species that had previously been separated by the sea. Hippos, antelopes, camels, rodents and others, migrated in different directions across the dry sea bed. Some of them colonized the islands (Crete, Malta, etc.) which then stood like mountains in the plain. When the sea level later rose, some populations got trapped on these islands, where they started their own evolution (although many later became extinct).

The Zanclean megaflood: end of the crisis

All extreme events eventually came to an end. Steep rivers incised the edges of the basin, causing it to recede in many places (through the process called headward erosion). Near Gibraltar, river erosion caused scarps to retreat to the west, until at a given time, the Atlantic Ocean flowed over the edge. Water spilled over gradually at first, but soon the huge water masses carved out a deep channel, forming great thundering rapids. The great refill - named the Zanclean Flood - had started. It ended the 600,000-year long salinity crisis. This great flood marks the end of the Messinian Age and the beginning of the Zanclean Age (after Zancle, Greek for, again, the city of Messina. The Zanclean also happens to be the first age of the Pliocene).

The flow rate along the great cataracts in the Strait of Gibraltar may have been a thousand times greater than that of the Amazon, and water may have flowed at 300 kilometers per hour! And the sea level rose by as much as 10 meters per day! First the western Mediterranean basin was filled, until water spilled over and flowed into the eastern basin through a big canyon. There, too, the water raged in a tremendous torrent. Sedimentary deposits 'dumped' in a chaotic manner near Sicily bear witness to this event. There are many estimations of how quickly the sea was full again, about two years has been proposed recently. Swimming and floating along with the great currents, Atlantic fauna and flora repopulated the sea. Among new arrivals were dolphins and seals. Would they have had butterflies in their stomachs when they rode the great slide?

The Pillars of Hercules

The mythical Pillars of Hercules stand on either side of the narrow Strait of Gibraltar (with the Rock of Gibraltar on one side and Monte Hacho or Jebel Musa on the other). In ancient times this was the gateway to the unknown world beyond the Classical world. The pillars even appear on the Spanish flag. Coincidentally or not, Pliny the Elder wrote about the myth (in Historia Naturalis, 77 AD) in which Hercules slashes the mountains at that spot, creating the narrow passage. Would the gods have known that mother nature could be even more savage?

The future of the Mediterranean

As the collision between Africa and Eurasia continues, it is highly likely that the Strait of Gibraltar will close again, probably more than once. It is unsettling to see on a map how narrow that strait really is. When it will close the sea will evaporate unless there is another supply of water. And someday the closure will be final, in a very distant future, within ten million years. A great mountain range will then rise where the Mediterranean is now, and the marine creatures alive today, and the great constructions of civilization, will get compressed into a layer of fossils. If you are perplexed by this image of the future, consider that new seas and beaches - and worlds and possibilities - will emerge elsewhere in the world due to plate tectonics. If humanity chooses to continue to exist and to feel delight, future civilizations may flourish again in other beautiful places on earth.

Watch the video with explanations: 

Read more about salt and sea on GondwanaTalks:

• Tethys Ocean: a vanished realm now lying in the mountains.
• Soil salinization: putting food security at risk.

Or read about the basics and plate tectonics:

• Plate tectonics: the caterpillars of the planet.
• Africa: the long voyage of an ancient continent.

Article written by Kathelijne Bonne, geologist and soil scientist. I also write on Good Climate News. 

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