The big spill: Flood could have filled Mediterranean in less than two years

Discovery of distinctive channel, calculations of possible water movement suggest fast and furious flow formed the sea

A cataclysmic flood could have filled the Mediterranean Sea — which millions of years ago was a dry basin — like a bathtub in the space of less than two years. A new model suggests that at the flood’s peak water poured from the Atlantic into the Mediterranean basin at a rate one thousand times the flow of the Amazon River, according to calculations published in the Dec. 10 Nature.

ANCIENT FLOODWAY Millions of years ago, a torrent of water through the Strait of Gibraltar, which separates Spain (left) from Morocco (right), could have filled the dried-up Mediterranean Sea basin in less than two years. NASA Gateway to Astronaut Photography of Earth
CLASS-IV RAPIDS Rushing water wore down the sill that separated the Mediterranean from the Atlantic millions of years ago, which allowed more water to pour in and encouraged still more erosion. Daniel Garcia-Castellanos

“In an instantaneous flash, the dry Mediterranean became a normal Mediterranean like we see it today,” says lead author Daniel Garcia-Castellanos of Spain’s Consejo Superior de Investigaciones Cientificas (CSIC) in Barcelona.

He and his colleagues calculate that at the height of the flood, water levels rose more than 10 meters and more than 40 centimeters of rock eroded away per day. The model also shows that 100 million cubic meters of water flowed through the channel per second, with water gushing at speeds of 100 kilometers an hour. Rather than a Niagara Falls-esque cascade from the Atlantic into the Mediterranean, the team’s results imply a torrent several kilometers wide at a fairly gradual slope.

“It would be an exciting rafting place,” Garcia-Castellanos says.

“As a hypothesis it makes sense, though it’s still in early stages,” says Sanjeev Gupta of Imperial College London. “There’s lots more to be done to explore this idea. It’s quite exciting, and I think it will get people interested in this topic.”

Although the Mediterranean features in many placid tourist spots around Europe and northern Africa today, it narrowly escaped becoming a desert. The sea separated from the world’s oceans 5.6 million years ago and was desiccated by evaporation in a period geologists call the Messinian salinity crisis.

Luckily, 5.3 million years ago water from the Atlantic Ocean found a way back in to the drying seabed through what is now the Strait of Gibraltar between Spain and Morocco. Geologists figured the resulting flood must have been impressive, but their estimates for how long it took have varied wildly, from 10 years to several thousand years.

“The record of the Mediterranean tells us that the transition from the dry, high salinity situation to the normal open water situation we have nowadays was very rapid,” Garcia-Castellanos says. “But ‘rapid’ in geology could mean many tens of thousands of years.”

Early models couldn’t resolve the flood’s timescale because they couldn’t tell how the volume of water flowing through the Strait of Gibraltar changed with time, Garcia-Castellanos says. Earlier studies, including work by Gupta, had concluded that England was separated from Europe in a similar cataclysmic flood 450,000 years ago based on the U-shaped valley at the bottom of the Strait of Dover (SN: 7/21/07, p. 35). But because of how long ago the flood that filled the Mediterranean occurred, the geological record of erosion from rushing waters was thought long buried.

But Garcia-Castellanos and colleagues found it, thanks to plans for an underground train. Cores drilled in the seafloor as part of preparations for the Africa-Europe tunnel project, which hopes to run trains under the Strait of Gibraltar from Spain to Morocco, revealed a deep channel filled with loose sediment. Using the drilling data and previously collected seismic data, the researchers determined that the channel is 200 kilometers long, between 6 and 11 kilometers wide, and between 300 and 650 meters deep.

Other geologists who had noticed the channel thought it had formed through erosion by rainwater in a river network, like in the Rhone or Nile rivers. But while those river channels are V-shaped, the new data show that the Strait of Gibraltar channel has the distinctive U-shape of the seafloor beneath the Strait of Dover. This shape is a hint that the strait formed in a torrential flood.

Using equations derived from observations of mountain rivers, the team of researchers modeled how the flood might have progressed: The flood started gradually, but as the sill between the Atlantic and the dry Mediterranean wore down, the rate of water flowing and rock eroding increased exponentially. As more water flowed over the sill, more rock wore away, allowing ever more water to spill in.

The calculations show an upper limit of two years for how long it took to fill the Mediterranean. But Garcia-Castellanos says it could have been as short as a few months. The energy carried in such a flood is comparable to the heat transport along the Gulf Stream in a year, or 4 percent of the kinetic energy of the meteorite impact thought to have killed the dinosaurs.

“I was very satisfied with their explanation, I found it quite exciting,” says Philip Gibbard of the University of Cambridge in England. “It’s a really important development.”

The flood would have had a dramatic effect on local ecosystems, and could even have affected the global climate. The model suggests that global sea level dropped 9.5 meters as a result of the flood. The team points out that a much smaller flood in North America 12,000 years ago has been linked to a worldwide cold snap, and suggests that the Mediterranean flood may have had similarly significant effects.

Lisa Grossman is the astronomy writer. She has a degree in astronomy from Cornell University and a graduate certificate in science writing from University of California, Santa Cruz. She lives near Boston.

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