Ancient Mediterranean flood mystery solved

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. By Victoria Gill

Science reporter, BBC News

Rising Sea Levels Are Increasing Risk Of Flooding Along South Coast Of England

A new study by researchers at the University of Southampton has found that sea levels have been rising across the south coast of England over the past century, substantially increasing the risk of flooding during storms.

The team has conducted a major data collection exercise, bringing together computer and paper-based records from across the south of England, from the Scilly Isles to Sheerness, to form a single data set of south coast sea levels across the years.

Their work has added collectively about 150 years worth of historic data to the existing record of English Channel sea-level change and extended the data along the south coast. Their findings are published in the latest edition of the journal Continental Shelf Research.

The data shows that both average sea levels and extreme sea levels have been rising at a similar rate through the 20th Century. The rate of rise is in the range 1.2 to 2.2 mm per year, with 1.3 mm per year recorded at Southampton.

Coastal engineering expert Professor Robert Nicholls, of the University's School of Civil Engineering and the Environment, who conducted the study, comments: "While these changes seem small, over a century they accumulate and substantially increase the risk of flooding during storms, unless there have been corresponding upgrades to flood defences. A water level that had an average likelihood of occurring once every 100 years in 1900 now has an average likelihood of occurring on average every 10 to 25 years, depending on the site considered. As sea levels continue to rise and probably accelerate, this increase in the likelihood of flooding will continue."

The most significant extension to the records is that of sea level changes at Southampton where the record now begins in 1935.

Paper-based records at St Mary's on the Isles of Scilly, Weymouth, Southampton and Newhaven have been used to greatly extend existing computer-based records, while the records at Devonport and Portsmouth have both been extended and corrected for pervious errors of interpretation.

This new data is feeding into ongoing efforts to increase the understanding and management of flooding.

The work was conducted by Professor Robert Nicholls, Dr Neil Wells from the University's School of Ocean and Earth Science based at the National Oceanography Centre, Southampton and Dr Ivan Haigh, formerly of the University of Southampton and now at the University of Western Australia.

---

Adapted from materials provided by University of Southampton, via EurekAlert!, a service of AAAS.

Post-flood 'dead zone' found off Texas

Fresh water pouring into the Gulf of Mexico after weeks of flooding in Texas has created an oxygen-depleted "dead zone" that threatens sea life, a researcher said Tuesday। Steve DiMarco, a professor of oceanography at Texas A&M University, said freshwater runoff from the swollen Brazos River left an area 1,750 square miles hypoxic, or depleted of oxygen. Sea life is threatened about 35 miles offshore."This can definitely have an effect on organisms living there — fish and shrimp, crabs, clams," DiMarco said. "All these things need oxygen in order to live. If you have this phenomenon occurring, depending on the severity and how long it lasts, you can have mass mortality."It's the first extensive dead zone detected off Texas, he said. The zone is strongest at the river delta off Freeport and San Luis Pass, just west of Galveston, then extends down the coast and dissipates at Matagorda Bay.

The phenomenon has been more identified with the Mississippi River's runoff into the Gulf off Louisiana. The Texas zone is about a quarter of the size of this year's Louisiana zone.In a dead zone, fresh water on the surface caps the oxygen below, affecting organic material at the bottom. The duration of the dead zone depends on currents and winds."The storms come through and the wind mixes up the ocean and breaks down this capping of the fresh water," he said.DiMarco based his findings on data collected recently by researchers aboard a ship from the National Oceanic and Atmospheric Administration National Marine Fisheries Service.Nancy Rabalais, chief scientist for the Northern Gulf of Mexico Hypoxia Studies, said it may not be a long-term problem."That area doesn't usually doesn't have hypoxia but also doesn't usually have heavy rains like that," she said. "The system off Texas is just not conducive for formation of hypoxia."Rabalais, executive director and a professor with the Louisiana Universities Marine Consortium, said that once the sediments and organic matter that came with the fresh water subside, researchers are likely to find the new dead zone along the Texas coast is temporary.