Published online 12 August 1999 | Nature | doi:10.1038/news990812-10

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When the Mediterranean was a desert

Tourists who flock to the shores of the Mediterranean Sea may not appreciate how precarious those warm waters are. If the connection to the Atlantic Ocean through the Strait of Gibraltar were to become blocked, the entire sea would evaporate within something like a thousand years.

However unlikely that might sound, researchers discovered in the early 1970s that such a thing did indeed happen around five and a half million years ago. Now Wout Krijgsman from Utrecht University in the Netherlands and colleagues have been able to establish exactly when and how the Mediterranean's vanishing act, the so-called 'Messinian salinity crisis', occurred. A finding which should lead to a better understanding of the global events that precipitated this bizarre chapter of the Earth's history.

The desiccation of the Mediterranean was discovered when oceanographers drilled into the sediments at the sea bed, and found very few fossils and thick layers of minerals such as halite (salt) and gypsum, which are formed when salty water evaporates. If all of the Mediterranean water were to be removed, the salt would form a layer dozens of metres thick. Yet the deposits observed were a few kilometres thick, suggesting that the Mediterranean basin must have repeatedly dried out and then refilled with salty water.

The Mediterranean loses more water by evaporation that it gains from rivers and rain. But at present, the shortfall can be made up by the flow across the narrow, shallow Strait of Gibraltar. If this lip were raised, or if the sea water were to fall far enough, the connection to the Atlantic would be cut, and the water's edge would gradually retreat. The sea would become a dense, salty lake, then a salt-encrusted marsh, and finally a desert. Rivers reaching the former coast from Europe and Africa would face a plunge to the floor of this desert two thousand metres below global mean sea level.

This, it seems, is what happened repeatedly between 6 and 5.3 million years ago. But what provoked the Messinian salinity crisis, and what course did it take?

To answer these questions, scientists need to know the exact timing of the episodes of drying, so that they can be linked to changes occurring elsewhere on the planet. This is what Krijgsman's team now report in the journal Nature [12 August]. They find that the onset of the crisis period seems to be tied to changes in the rate of formation of new ocean crust, suggesting that the link to the Atlantic was weakened as the Strait was pushed up by the expanding sea floor.

Once that had happened, the whole Mediterranean basin became liable to desiccation. Each layer of salt minerals, precipitated from over-salty waters, coincided, Krijgsman's group explains, with changes in the tilt angle of the Earth's orbit around the Sun, which would have resulted in changes in the seasonal distribution of heat being received from the Sun that in turn caused a drier Mediterranean climate. Thus For half a million years, scattered bodies of water more saline than the Dead Sea, retreated across the isolated, salt-encrusted Mediterranean basin.