Mediterranean Sea Was Once a Mile-High Salt Field

About 6 million years ago, a mile-high field of salt formed across the entire Mediterranean seafloor.

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.

"It wasn't a nice place," Flecker said.

In a series of pulses over about 600,000 years, the sea dried out, and a 1-mile-high (1.5 kilometer) salt wall grew across the Mediterranean seafloor, a "bit like the Dead Sea, a huge brine field," Flecker told LiveScience. (In places, it might have been even higher.)

Then, in a geologic flash of time just 200 years' long, waters from the Atlantic cut through the Strait of Gibraltar and flooded the Mediterranean, refilling the sea. [50 Amazing Facts About the Earth]

Precise dates Though scientists understood some of what triggered the great salinity crisis, they still don't fully understand the climatic changes that may have been involved.

The Earth wobbles like a top around its axis as it spins, in a roughly 20,000-year cycle. That shift affects how much sunlight certain parts of the Earth receive at different points in the cycle, thereby changing the climate. In the Mediterranean Sea area, sediments are striped with dark and light bands that correspond to surges and die-offs of sea life as a result of those climactic shifts.

Flecker and her colleagues with the Medgate project, a European Union project that is studying the salinity crisis, looked at those sediments to understand how the salt crisis began.

Unfortunately, they didn't know what part of each band corresponded to a particular position of Earth's axis, making it difficult to sequence events in the crisis.

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.

In addition, revised dating can now tie the onset of the salt crisis with the formation of massive ice sheets in the Arctic, which lowered sea levels and reduced water flowing from the Atlantic ocean into the Mediterranean Sea. Combined with dryer weather conditions in Africa, that may have helped trigger the conditions that formed the salt giant.

Original article on LiveScience.

History's 10 Most Overlooked Mysteries The World's Biggest Oceans and Seas Infographic: Tallest Mountain to Deepest Ocean Trench Copyright 2013 LiveScience, a TechMediaNetwork company. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

The Mediterranean Sea.

During the rainy season, which lasts from January to April, the world's largest salt flat -- Salar de Uyuni, in Bolivia -- turns into the world's largest mirror, reflecting heaven on Earth. Here a 4x4 car drives across the desert.

The rare cacti (Echinopsis tarijensis) sit in profile on Isla Inkahuasi with star trails in the sky during a long night exposure.

A rock formation, better known as the Stone Tree, lies in the middle of the Siloli desert and volcanic region of Potosi in southwestern Bolivia, on the Andes mountain range, near the Chilean border.

Tours are headed by guides who traverse the region without any roads to access geographic areas, such as lagoons, salt mines, rock formations and volcanoes. The region is known for its unpredictable climate and temperature changes, often providing snow in a small 1 kilometer area.

Reflection of Tunupa volcano and four-wheels drive trek across Salar de Uyuni, Bolivia.

Laguna Colorada at the Eduardo Avaroa Andean Fauna National Reserve, with James' flamingos. The salt lake contains borax islands, whose white color contrasts with the reddish color of its waters, which is caused by red sediments and pigmentation of some algae.

A hotel in the southwest Bolivian desert. Elevation is 3,653 meters (nearly 12,000 feet).

During the rainy season, Salar De Uyuni becomes saturated with over several centimeters water.

Salt workers break up the surface salts for collection.

At sunset, the salt sits in mounds to dry.

Floating crystals of salt.

Crystals of salt.

Salt Workers Feet

Cacti on Isla de los Pescadores in the Salar de Uyuni salt flats.

Salar de Uyuni salt flats.

Flamingoes on salt flats.