When this popular vacation spot was reduced to extremely salty lakes
The Mediterranean went through an extreme salt crisis that changed it forever.
Published
If you have ever taken a swim in the Mediterranean, you might have noticed it's saltier than the oceans we're familiar with.
This is true – the Mediterranean is indeed saltier than the Atlantic Ocean.
There is a natural explanation for this: the Mediterranean is fairly isolated and receives a lot of sunlight and heat. This causes a significant amount of water to evaporate from the sea.
As the water evaporates, the salt stays behind, increasing the overall salt concentration.
Fresh seawater constantly flows in from the Atlantic through the Strait of Gibraltar. But what happens if the strait closes?
This actually happened almost six million years ago. It marked the beginning of a major salinity crisis, drastically shrinking the Mediterranean.
Dried up
“When such major changes happen in a relatively short period of time, the consequences are dramatic,” Espen Torgersen tells sciencenorway.no.
He is a geologist at the Geological Survey of Norway and associate professor at NTNU.
He refers to one of the most significant geological events to recently impact Europe – the isolation of the Mediterranean.
Around six million years ago, southern Europe looked quite similar to how it does today. But then the Mediterranean underwent a drastic transformation: It became cut off from the rest of the world's oceans.
Tectonic movements, which trigger earthquakes and shifts in the Earth's crust around the area we now know as Gibraltar, simply closed off the corridor to the Atlantic Ocean.
“The entire Mediterranean region, from Gibraltar to Turkey and Iran, is an incredibly complex and fascinating area,” says Torgersen.
With numerous tectonic plates in motion, it becomes a true geological puzzle.
Over hundreds of thousands of years, the Mediterranean didn’t receive enough seawater to offset the evaporation, causing it to shrink significantly compared to today.
The changes were so drastic that vast land areas dried up, opening new regions of Europe and Africa for plants and animals to thrive.
This phenomenon also occurred closer to home in the lost region of Europe known as Doggerland.
At its most extreme, it was possible to walk from present-day Tunisia and North Africa across Sicily and the Italian peninsula without crossing water. The Mediterranean was reduced to just two extremely salty lakes when it was at its driest.
Naturally, this had a profound impact on the life forms that thrived there. A recent study estimates that only a few species survived this crisis.
This major event is known as the Messinian salinity crisis. Messinian refers to the time period, and salinity refers to the concentration of salt.
But how do we know this happened?
The salt left behind in the Mediterranean
Researchers have long known about this dramatic geological period in the Mediterranean’s history. In areas where parts of the sea dried up, vast deposits of salt were left behind.
“As water evaporates, the salt concentration increases, eventually forming gypsum crystals. Gypsum is also called evaporite because it forms during evaporation,” says Torgersen.
Nearly a million cubic kilometres of salt accumulated at various locations on the Mediterranean seafloor, showing that these areas were cut off from the ocean and dried up. Gypsum is a mineral made up of calcium, hydrogen, oxygen, and sulphur.
“Evaporation is the only way to form these massive gypsum deposits,” says Torgersen.
In a comprehensive 2024 study, researchers explored how these enormous gypsum layers were created.
This was a complex and lengthy process. While seawater likely flowed into the isolated Mediterranean at different times, it was not enough to halt the intense evaporation.
These thick gypsum layers have made it possible to both date and understand the severity of this salinity crisis.
When the sea was at its lowest point, water levels were hundreds of metres below today’s levels. The landscape was so altered that most of the Greek islands disappeared, and the Italian peninsula resembled a mountain range.
This transformation had significant consequences for life in the region.
Coral reefs that disappeared
The Mediterranean’s closure took place 5.97 million years ago, though the geological process had been in motion for some time.
It took hundreds of thousands of years for the sea to become nearly isolated. In a new study, a research group studied large fossil collections to better understand how life in the Mediterranean adapted to this transition.
Many species that once thrived in the Mediterranean disappeared, unable to withstand the drastic changes. As the sea shrank, entire marine ecosystems vanished. For instance, the deep-sea environment disappeared entirely, wiping out species that depended on it.
Large coral reefs in shallow waters also vanished during this period. Fossil evidence shows that many fish and shellfish species were greatly affected as well.
Despite this, a few species managed to survive the crisis. When the crisis ended, significant changes in wildlife followed.
After 600,000 years, the Mediterranean’s isolation came to an end. While it is difficult to reconstruct the full picture of what happened and how species fit in, the researchers note that many new species emerged in the Mediterranean.
Nearly 70 per cent of the species present before the salinity crisis were replaced afterward.
Dolphins and great white sharks
The most significant changes likely occurred after the Mediterranean was reopened. Species from the Atlantic poured in, some reestablishing themselves or being entirely new to the region.
Examples of the incoming species include various types of dolphins and great white sharks.
As a result, biodiversity in the Mediterranean actually increased after the salinity crisis. However, it took a long time for species to establish themselves and for the ecosystem to recover. Research also shows that biodiversity is greater in the west, near Gibraltar, than in the eastern Mediterranean.
This disparity has persisted for the last five million years, since the end of the crisis.
“These changes were driven by tectonic activity. On a larger scale, it demonstrates how Earth is a living and dynamic planet,” Torgersen tells sciencenorway.no.
He points out that this is a clear example of how deep processes within the Earth directly affect the evolution of life on the surface.
Seas that open and close
This phenomenon is not unique to the Mediterranean – it has happened in several other seas as well. The Red Sea, for instance, also shows evidence of large salt deposits.
Since this is driven by earthquakes and tectonic plate movements, could the Mediterranean become isolated again?
“It’s difficult to predict exactly how the movements of the two large plates and all the microplates will unfold,” says Torgersen.
However, he believes there is a strong possibility that, in geological time, the Mediterranean could be cut off once again.
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Translated by Alette Bjordal Gjellesvik
Read the Norwegian version of this article on forskning.no
