On the island of Sicily
near the straits
are immense deposits of evaporite
s, minerals that form when
they drop out of solution from evaporating water.
In the 1970s, geologists using seismic profiling to perform a geological
survey of the Mediterranean basin found a curious phenomenon: A highly-reflective
layer (dubbed the "M Reflector") buried beneath the sea's floor.
Later, the Glomar Challenger, sent to drill cores from the seabed
found the astonishing cause of the reflector: Directly underneath
sea-floor sediments in the deepest part of the Mediterranean was a mixture
of gravel and crystals of several minerals associated with evaporating
seawater, notably anhydrite, which changes into gypsum in the presence
of water. Further investigation revealed that these deposits
were 6000 feet (nearly 2 km) thick. The cores also revealed evidence
of stromatolites, which only grow in sunlight.
It was realized that the Mediterranean has a negative moisture balance:
Although rainfall and rivers add 1830 km3 to the Mediterranean
every year, it loses 4690 km3 every year to evaporation.
The balance has to be made up from the Atlantic Ocean. If water were
not constantly flowing in through the Straits of Gibraltar, the Mediterranean
Sea would dry up in 1500 years.
Finally, excavation in the Rhone and Nile
river beds revealed that those rivers flowed through mile-deep gorges, since
filled in with sediment. The Nile's gorge caused the Soviet Union
much trouble while building the Aswan High Dam, 1200 miles from the sea!
All of this evidence drew investigators to the astonishing conclusion:
Beginning 7-12 million years ago, the opening between the Mediterranean
and the Atlantic was blocked by tectonic uplift from the African Plate
pushing into the Eurasian plate, combined with a drastic drop in sea-level
caused by an Ice Age.
The sea dried up over a period of 1000 years.
About 5-5.5 million years ago, tectonic shifts created a new opening
between the empty basin and the Atlantic Ocean. An unimaginably large
waterfall 2600 feet (~1 km) high allowed the Atlantic to pour in and
refill the Mediterranean over the course of 100 years.
Climactic consequences abounded: The Mediterranean Sea floor was
a desert 10,000 feet below sea level. The final stages of the sea's
evaporation would have looked like the Dead Sea recreated on a vastly larger
scale. The part of Europe that wasn't covered by ice was a dry steppe.
Not only that, about 6% of the Earth's salt deposits were now locked
up under the Mediterranean sea floor: Today's oceans are somewhat fresher
than the oceans of millions of years ago. Fresher water means a higher
freezing point, and makes the formation of sea ice easier, and more difficult
to melt once it has formed. This "Messinian Salinity Crisis" led
to global cooling, and a cascade of mass exctinction and evolutionary adaptation
that had earlier led geologists to divide the Miocene epoch of the
Cenozoic Era from the Pliocene epoch.
The Mediterranean's drying up has in turn led to the cycle of Ice Ages
that dominate Earth's climatological history to this day.