Scientists may have solved the mystery of how the Mediterranean Sea almost vanished millions of years ago, saying that this extreme evaporation of the sea happened in two phases, according to a recent study.
Over five million years ago, almost three quarters of the Mediterranean Sea evaporated into the air, which researchers call the Messinian Salinity Crisis. When the sea dried up, a nearly 5-kilometer thick layer of salt built up under the basin.
According to the international team behind the recent study published in Nature Communications in November, it presents “evidence for a two-phase accumulation of the Mediterranean salt level, based on the chlorine stable isotope composition of halite.”
How the two-phase extreme evaporation unfolded, according to scientists
The first phase spanned 35,000 years, when the Mediterranean became basically cut off from water flow from the Atlantic Ocean via what is now the Strait of Gibraltar. During that time, salt deposits were accumulated on the sea bed while the sea was also evaporating.
During the second phase, which lasted for the following 10,000 years, the whole process accelerated. The Mediterranean Sea was completely isolated and salt deposits built up, driven by a rapid drop in sea level of between 1.7 and 2.1 kilometers in its eastern arm, and about 0.85 kilometers in its most easterly part.
As water was evaporating in phase two, the underwater ridge across the Strait of Sicily became exposed, the research team suggests, effectively splitting the Mediterranean in two and forming a land bridge between Africa and Europe. That led to faster evaporation rates in the eastern Mediterranean where the biggest drops in sea level would have been and where most salt deposits were found.
In their detailed study, researchers led by Earth system scientist Giovanni Aloisi from the French National Center for Scientific Research (CNRS) came to these conclusions by analyzing chlorine isotopes in the salt deposited on the seabed, and building mathematical models and simulations. Their findings led them to chart when, how and why the Mediterranean Sea once lost 70 percent of its water.
The salinity crisis severely affected marine life in the Mediterranean
Geologists have long known about the Mediterranean salinity crisis that temporarily turned the sea to salt. Yet Konstantina Agiadi, a geologist at the University of Vienna who led a team of colleagues researching the extinction and subsequent recovery of marine species after the salinity crisis, tells Science that “many mysteries still exist so bringing together many types of scientists allows pieces of the puzzle to be put together.”
Agiadi and her international team scoured museum collections and old published records for fossils from the region, collecting sufficient data to paint a stark picture. As salt deposits increased, biodiversity began to drop. Coral reefs disappeared, never to return. Out of 800 species once found only in the Mediterranean, only 86 survived. When water from the Atlantic Ocean began to flow again 50,000 years later, the Mediterranean became repopulated again by almost 2,700 species. Only a few were returning fish while most, like the great white shark and dolphins were newcomers.
The study, published recently in Science Advances, concluded that the Mediterranean marine biodiversity was severely impacted by the extinction of the water in the sea, with only 11% of the endemic species surviving the crisis and the Mediterranean’s biodiversity needing at least 1.7 million years to recover. Scientists say that such studies help them to better understand marine biodiversity recovery after an ecological crisis.
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