Page last updated at 18:05 GMT, Wednesday, 9 December 2009

Ancient Mediterranean flood mystery solved

By Victoria Gill
Science reporter, BBC News

Gibraltar strait
The team made a reconstruction of the Mediterranean during the "megaflood"

Research has revealed details of the catastrophic Zanclean flood that refilled the Mediterranean Sea more than five million years ago.

The flood occurred when Atlantic waters found their way into the cut-off and desiccated Mediterranean basin.

The researchers say that a 200km channel across the Gibraltar strait was carved out by the floodwaters.

Their findings, published in Nature, show that the resulting flood could have filled the basin within two years.

The team was led by Daniel Garcia-Castellanos from the Research Council of Spain (CSIC).

He explained that he and his colleagues laid the foundations for this study by working on tectonic lakes.

This... may have involved peak rates of sea level rise in the Mediterranean of more than 10m per day
Daniel Garcia-Castellanos
Research Council of Spain

They developed a model of how the mountain lakes quickly "cease to exist" when erosion produces "outlet rivers" that drain them.

This same principle, Dr Garcia-Castellanos said, could be used to explain the Zanclean flood that reconnected the Mediterranean with the rest of the World's oceans.

"We could for the first time link the amount of water crossing the channel with the amount of erosion causing it to grow over time," he told BBC News.

New approach

Using existing borehole and seismic data, his team showed how the flood would have begun with water spilling over a sill.

The water would have gradually eroded a channel into the strait, eventually triggering a catastrophic flood, Dr Garcia-Castellanos explained.

He and his colleagues created a computer model to estimate the duration of the flood, and found that, when the "incision channel" reached a critical depth, the water flow sped up.

In a period ranging from a few months to two years, the scientists say that 90% of the water was transferred into the basin.

"This extremely abrupt flood may have involved peak rates of sea level rise in the Mediterranean of more than 10m per day," he and his colleagues wrote in the Nature paper.

Previous estimates of the duration of the flood were very variable, said Dr Garcia-Castellanos, because scientists "had to assume the size of the channel" rather than measure it.

Some estimates suggested that the flood continued for as long as 10,000 years.

Rob Govers, a geoscientist from Utrecht University in the Netherlands, who was not involved in this study, said that the findings were important.

"I think the authors have been very creative using existing data and making sense of it in a completely new way," he said.

Dr Govers said the next important step would be to measure the volume of breccia, or ancient eroded material, in the strait, to confirm whether there was enough material there to have filled the flood channel.

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