Imma Perfetto
Cosmos science journalist
Between 5.97 and 5.33 million years ago, the Mediterranean transformed from a thriving sea to a giant salt basin.
The catastrophic event, known as the Messinian Salinity Crisis, was triggered by the movement of the European and African continental plates. They pushed against each other and closed what is now known as the Strait of Gibraltar, cutting the Mediterranean off from the Atlantic Ocean.
Without incoming seawater to balance evaporation, water levels dropped by 70%. This transformed the Sea into 2 hypersaline lakes separated by a land-bridge spanning from the toe of Italy to Sicily, to north Africa.
Nearly 1 million cubic kilometres of salt accumulated in less than 700,000 years.
Scientists have now been able to identify the 2 phases by which this extreme evaporation event occurred, according to a study published in the journal Nature Communications.
They analysed the composition of chlorine isotopes in salt from the Mediterranean seabed to estimate the rate at which halite (sodium chloride salt) precipitated – changed to a solid form from a solution – during the Messinian Salinity Crisis.
They found that during the first phase, which lasted for about 35,000 years, salt deposition occurred only in the eastern Mediterranean “…triggered by the restriction of Mediterranean outflow to the Atlantic, in an otherwise brine-filled Mediterranean basin.”
“After this first phase,” the authors write, “the deep outflow at Gibraltar is blocked and the inflow of Atlantic water into the Mediterranean is stopped, leading to the complete isolation of the Mediterranean Sea from the Atlantic Ocean.
“As a result, the water level in the entire Mediterranean basin drops until the depth of the Sicily sill is reached, and then continues to fall independently in the western and eastern Mediterranean basins, forced by their respective freshwater budgets.”
The rate of this evaporation was rapid, occurring in less than 10,000 years. Sea levels dropped by 1.7 to 2.1km in the east and by about 0.85 km in the western Mediterranean.
The researchers write that during this phase “…the eastern Mediterranean basin lost up to 83% of its water volume, and large parts of its margins were desiccated, while its deep … sub-basins remained filled with [less than] 1 km-deep brine.”
The Messinian Salinity Crisis was ended when erosion allowed water to trickle, and then flood back from the Atlantic. It’s estimated that the Mediterranean Sea would have risen more than 10 metres per day at the peak of this event, known as the Zanclean megaflood.
