About 6 million years ago, a mile-high field of salt formed across the entire Mediterranean seafloor, sucking up 6 percent of the oceans’ salt.
Now, new research has pinpointed when key events during the formation of that “salt giant” occurred. The new research, presented here Dec. 11 at the annual meeting of the American Geophysical Union, could help unravel the mystery behind the great salt crisis.
Every so often, huge accumulations of the world’s salt form in one place. The most recent salt crisis happened during the Miocene Epoch, which lasted from about 23 million to 5 million years ago.
About 6 million years ago, the Strait of Gibraltar linking the Mediterranean with the Atlantic Ocean was closed and instead, two channels — one in Northern Morocco and another in Southern Spain — fed the sea with salty water and let it flow out, said study co-author Rachel Flecker, a geologist at the University of Bristol in England.
But during the Messinian Salinity Crisis, as this particular event is known, Eurasia was colliding with Africa, squishing the outlet flow for the Mediterranean Sea. But tectonic shifts left the basin floor below the outlet channel between the two water bodies intact. Dense salty water from the Atlantic rushed in, but couldn’t leave the sea. Water evaporated; salt piled high; and sea life collapsed.
The team used climate simulations to understand rainfall, evaporation and water flow into and out of the Mediterranean for a period spanning 22,000 years around the crisis onset, and tied that to sediment data. Ancient rivers in North Africa dumped huge pulses of freshwater into the sea in late summer, leaving traces of surging biological activity in the fossil record, the models show.
Based on their simulations, the researchers found the freshwater pulses happened at a time in the Earth’s orbital rotation when the Northern Hemisphere would experience colder winters and hotter summers. That, in turn, meant that the evaporation must have started much later in the Earth’s orbital cycle.