During World War II, German submarines used a peculiarity of ocean physics to pass through the Strait of Gibraltar without being detected by the British: Those who wanted to enter the Mediterranean went up to the shallowest waters, turned off their engines and let themselves be carried by the current. Those who wanted to cross the Atlantic, descended until the deeper currents took them out, also silently. What they took advantage of were the differences in density. The greater concentration of salt in Mediterranean waters makes them sink, while Atlantic waters, poorer in sodium chloride, float on top of the others. This exchange between the sea and the ocean, vital for the former, was cut off six million years ago, causing, according to a new study published in Sciencethe almost total extirpation of marine life. When the passage was reopened, the biodiversity of the Our Sea changed forever.

The Mediterranean already had a configuration similar to that of today six million years ago. It had long since lost its connection with what would become the Indian Ocean and then separated from the Paratethys when what is now the Anatolian peninsula rose. It would be a closed sea if it were not for the strait that connected it to the current Atlantic. But it did not connect it to what is now the Campo de Gibraltar. If it had a name, it would be called the Strait of Cazorla, since the passage opened through lands of what is now Jaén and most of Baetica. In general terms, the salinity differential between Mediterranean and Atlantic waters is four grams of salt per litre more in the former than in the latter (39 gr/l versus 35 gr/l). But geological dynamics began to contradict fluid dynamics.

Geologist Daniel García-Castellanos, from the Institute of Geosciences of Barcelona (GEO3BCN) of the CSIC, explains what happened then: “At the end of the Messinian, due to geodynamic processes, this region began to rise, initiating the disconnection between both sides.” The process takes time, tens of thousands of years, a sigh in geological terms. During this period, the entry of more superficial Atlantic waters continued. But as the depth of the passage decreased, “the exit of hypersaline water was blocked,” explains García-Castellanos, senior author of the research published in Science. Although scientists still debate whether that strait was completely closed or not, what is known is that the Mediterranean at that time was accumulating more and more salt, in a process accelerated by the negative balance between evaporation and the water supplied by rivers and rain.

The salt concentration in the Mediterranean must have equaled or even exceeded that in the Dead Sea.Westend61 (Getty Images)

The process led to the so-called Messinian salt crisis, which began 5.97 million years ago, according to the geological record. For thousands of years (the exact duration is still debated), the Mediterranean was transformed into a giant brine. “Salt is entering through the Atlantic seawater, but there is no outlet. The salt concentration will continue to increase until it reaches saturation and begins to precipitate,” explains García-Castellanos. When it reaches a concentration of 371 grams per litre, the brine precipitates, with the salt emerging in a solid state. This figure is almost ten times the normal concentration in the Mediterranean and equals that present in the Dead Sea. This was helped by accelerated evaporation, which led to the widespread retreat of water by hundreds of metres. It is estimated that the sea level dropped by up to one kilometre.

The first result of the crisis in the form of salt giants was discovered under the seabed in the 1970s: “They are not mountains of salt, but rather a more or less horizontal layer that is largely intact, having been covered by subsequent sediment for five million years.” Measurements using seismic waves have revealed that the layer is actually a gigantic mass of salt measuring between 1,000 and 2,000 metres, with a volume of more than one million cubic kilometres of sodium chloride in the form of halite. It is one of the largest accumulations of this mineral on the planet.

Only 11% of Mediterranean species survived the salt crisis, including many bivalve species. Pictured here are shells of a mollusc fossilised 6.5 million years ago in present-day Crete.Konstantina Agiadi

The second consequence of such salt accumulation was that it nearly wiped out life in the Mediterranean. University of Vienna geologist and first author of the study, Konstantina Agiadi, points out that “although it was a regional event, the effect of the Messinian salinity crisis on Mediterranean marine biota was as massive as the K/T event.” She is referring to the asteroid or meteorite that wiped out most of life 66 million years ago, starting with non-winged dinosaurs. “Of those marine organisms that lived exclusively in the Mediterranean before the crisis (i.e. endemic), only 11% (86 species) could have survived in some way,” she explains. But species, entire genera, such as corals, were wiped out, according to the count of almost 5,000 species found in the fossil record of three large areas of the Mediterranean basin. And the percentage is not higher because up to 30% of the species were also present in the Atlantic and, although they disappeared during the crisis, they returned when the waters opened again.

There are several hypotheses about how the Atlantic and the Mediterranean met again. But, according to the geologist from the University of Salamanca and also co-author of the study, Francisco Javier Sierro, “the data suggest that, once again, tectonic dynamics led to a sinking of the region.” [sur de la península y norte del actual Marruecos] giving rise to the Strait of Gibraltar.” According to current bathymetry, there is a huge ravine on the Mediterranean side of the strait that must have caused the arrival of Atlantic waters to a Mediterranean that was then at a much lower level. Sierro, like Agiadi and García-Castellanos, highlights the enormous impact on biodiversity. “The species that survived, such as foraminifera or bivalves, are accustomed to extreme environments or with great changes in salinity,” he says. Others, as García-Castellanos argues, “were able to survive in river deltas.” But for the most part, the marine life that exists today between Algeciras and Istanbul came from outside.

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