When the Mediterranean dried up, a gigantic layer of salt was formed

Murderously hot salt desert as far as the eye can see: This is what the Mediterranean once looked like. Researchers are now uncovering the secrets of the so-called Messinian salinity crisis – and also explaining what happened when water rushed into the basin again.

As unimaginable as it may seem, the Mediterranean Sea is largely drying up. The water became increasingly salty as a result of what happened around 5.97 to 5.33 million years ago, and countless animal and plant species died out. Only around eleven percent of the species that only occur there survived the crisis, an international research team now reports in the journal “Science”.

It took at least 1.7 million years for biodiversity to recover when seawater from the Atlantic once again rushed into the massive basin.

Even today, there is only one passage to the Mediterranean: the Strait of Gibraltar. The channel, which is only a few kilometers wide, allows fresh water from the Atlantic to flow into the saltier Mediterranean.

The passage was made possible by movements in the upper mantle around six million years ago, which is consistent with the theory currently in force. Areas of what is now southern Spain and northern Morocco rose in the late Miocene. However, around 7.6 million years ago, as the passage narrowed, there were already major fluctuations in salinity and temperature in the Mediterranean.

Without an inflow from the Atlantic, the water in the Mediterranean eventually largely evaporated – the inflow from rivers such as the Nile or the Ebro was far too small to keep the basin lying between Africa, Europe and the Middle East full. Salt lakes were formed, similar to the Dead Sea today.

Even today, a kilometer-thick layer of salt under the sea bears witness to the state it was in 5.5 million years ago: around one million cubic kilometers of salt accumulated, as the team led by Konstantina Agiadi from the University of Vienna and Daniel García-Castellanos from the Geosciences Barcelona research institute writes.

In order to investigate the influence of the extreme changes on life in the Mediterranean, the 29 researchers include twelve to 3.6 million year old fossils from sites in the neighboring countries as well as from sediment cores from the deep sea in their analysis.

According to the study, a good two-thirds of the species in the Mediterranean were no longer there after the so-called Messinian salinity crisis. Tropical reef-building corals are dying out, according to the study. Of the 779 species originally only found in the Mediterranean, only 86 survived – how and where they managed to do this is still unclear.

After the Atlantic Ocean flowed back into the basin around 5.33 million years ago, earlier and previously non-native species conquered the habitat. Great white sharks and dolphins also came to the Mediterranean. In an animated film lasting around two minutesThe link will open in a new tab Daniel García-Castellanos shows how one can imagine the process – both drying out and reflux.

Overall, the recovery of the ecosystem took a surprisingly long time, as the team led by Agiadi and García-Castellanos writes. This led to the pattern that still exists today of a steadily decreasing biodiversity from west to east. Today, biodiversity is disproportionately high in relation to the size of the basin due to the large number of species that only occur in the Mediterranean region.

In his book “Primeval Worlds,” paleontologist and evolutionary biologist Thomas Halliday describes how researchers imagine the sequence of events. In the Mediterranean region, islands became mountains when the water receded, he says. Valleys four kilometers below sea level were created – the lowest-lying land in the world.

Downdrafts crashed down the rock walls, Halliday said. “Although this is a relatively cool period in Earth's history, the maximum air temperature on a hot summer day four kilometers down on the floor of the canyon can reach a hellish 80 degrees Celsius – about 25 degrees more than the highest temperature ever recorded in modern times in Death Valley, California.”

A shiny mixture of gypsum and sodium chloride, more than three kilometers thick in places, was deposited on the floor of the Mediterranean basin. Then the water from the Atlantic shot back into the western basin. Later, the eastern part was also filled – possibly via the “most powerful waterfall the earth has ever seen.”

According to the book, the assumptions based on these were 1,500 meters high. The water could possibly have shot over the steep slope at a speed of almost 250 kilometers per hour and largely turned into mist before it reached the bottom.

“Despite this continuing deluge, which is raising the eastern Mediterranean by one metre every two and a half hours, it will take more than a year for the eastern Mediterranean to be filled, for Malta, Gozo and Sicily to be finally cut off from Africa and Italy and for Gargano to become an island again,” says Halliday.

Movements in the earth's crust have repeatedly led to the isolation of regional sea areas from oceans and to massive salt accumulations throughout the earth's history, as the research team explains. At least 138 such dried-out basins have formed worldwide over time from the Proterozoic to the Miocene.

Salt deposits covering thousands of cubic kilometers – called salt giants – have been found in Australia, Siberia, northwestern Europe, the Middle East and elsewhere. They have been used to extract salt from ancient times to the present day, for example in the Hallstatt mine in Austria and the Khewra salt mine in Pakistan.

dpa/krei

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