Thursday, August 2, 2007
Ancient Megaflood Made Britain an Island, Study Says
Sometime between 450,000 and 200,000 years ago, waters from a glacial lake burst through a natural chalk dam, flooding the area that is now the English Channel and cutting off the land that is now Britain from mainland Europe, a new study says. Sonar mapping of the seabed revealed scours and streamlined "islands" that could only have been created by gushing floodwaters, scientists report. Graphic by Miranda Mulligan/NGS
Ancient Megaflood Made Britain an Island, Study Says
for National Geographic News
July 18, 2007
A flood of biblical proportions cut the British Isles off from mainland Europe sometime between 450,000 and 200,000 years ago, according to a new study.
The research, based on three-dimensional sonar mapping of the English Channel, provides the strongest evidence yet that a catastrophic megaflood broke a land bridge that once connected what is now Britain and France.
"It is probably one of the largest floods ever identified," said Phillip Gibbard, a geographer at the University of Cambridge who wasn't involved in the study.
At its peak, the flood would have discharged water at a rate of about 264 million gallons (a million cubic meters) a second, gushing at speeds of up to 62 miles (100 kilometers) an hour, the researchers say. This is roughly equivalent to ten times the combined flow rate of all the rivers in the world.
In addition to making Britain an island, the authors add, the huge flood had wide-ranging environmental consequences.
For example, the gigantic pulse of freshwater entering the Atlantic Ocean likely caused a period of climate cooling in the Northern Hemisphere, Gibbard said.
"The introduction of ice and freshwater into an ocean drives climate oscillations and causes marked cooling events," he explained.
The flood also marooned many animals and plants, so those species gradually evolved into different forms than their mainland cousins.
And humans appear to have avoided the newly made island altogether, leaving it unoccupied for over a hundred thousand years.
Researchers have long known that a narrow ridge of chalk once connected Dover in southeast England to Calais in northwest France.
During the ice ages, when sea levels were low, the ridge held back a glacial lake from inundating a large valley between the two regions.
But during warm interglacial periods, sea levels rose and the chalk ridge was the only link.
At some point the ridge crumbled. Theories as to why have included river or glacial erosion, tidal scraping, and—most controversial of all—a megaflood.
Now Sanjeev Gupta of Imperial College London and colleagues say they have found the first concrete evidence to support the megaflood theory.
A 3-D map of part of the English Channel reveals features that could only have been created by a massive flood, the team says.
"We have identified huge scours on the seafloor and streamlined islands," said Gupta, whose results will appear in tomorrow's issue of the journal Nature.
These features are very unusual, he said, and have previously been found only in regions where megafloods are known to have occurred.
Similar features exist, for example, in the Channeled Scabland in eastern Washington State, which was deluged when the glacial Lake Missoula burst its banks about 12,000 years ago.
Based on their analysis, Gupta and colleagues say the most likely source of all this water was a huge glacial lake sitting in what is now the southern North Sea off the east coast of Britain.
The water was probably held back by the chalk ridge, and a small earthquake could have caused the first few cracks to appear.
"Chalk is not very strong, and eventually the water probably just started to over-spill," Gupta said.
Determining exactly when the megaflood took place is difficult.
But the divergence of plant and animal species between Britain and mainland Europe suggest that the event must have occurred sometime between 450,000 and 200,000 years ago.
"We now need to drill into the sediments to get an accurate date," Gupta said.
The great flood could help explain why Britain remained an uninhabited region for a large chunk of the archaeological record.
"There seems to be a large gap in the evidence for human occupation [of Britain] during cold and warm phases from about 180,000 until about 60,000 years ago," said Nicholas Ashton, an archaeologist at the British Museum in London.
(Related: "Humans Sped to U.K. After Ice Age, Study Says" [November 3, 2003].)
When the climate was warm, sea level between the island and the mainland was too high for humans to cross, Ashton said.
And during the much colder ice ages, humans could have crossed, but seem to have preferred to live in sunnier regions such as modern-day Italy and Spain.
"It wasn't until 60,000 years ago," Ashton said, "that humans—late Neanderthals—had the technological capabilities, such as more effective clothing and shelter, to survive the cold conditions."
Humans Sped to U.K. After Ice Age, Study Says
James Owen in England
for National Geographic News
November 3, 2003
Humans hotfooted it to Britain after the last ice age, scientists say. The new research, which challenges previous studies, suggests these early settlers advanced rapidly as the glaciers melted away.
A team of European scientists estimated the speed and timing of human resettlement in late glacial Britain by comparing radiocarbon dated remains with ice-core climate records. Their findings, now published in the Journal of Quaternary Science, suggest a wave of migration coincided with a sudden rise in temperature and the northwards spread of herd animals such as wild horse and deer.
Previously, scientists thought repopulation had been a drawn-out affair, preceded by centuries of sporadic forays from mainland Europe.
"The big question has always been how quickly, and in what number, did people return once the glaciers had retreated," said research team leader Nick Barton, from the anthropology department of Oxford Brookes University, Oxford, England. "Now with the benefit of larger numbers of radiocarbon dates corrected against a highly accurate record of global climatic change from the Greenland ice record, it seems reoccupation was an almost instantaneous event across northern and central Europe."
Early modern humans reached Britain by around 30,000 years ago, but within 3,000 years they were driven out by the advance of the last ice age.
The archaeologists looked for evidence of their return in ancient caves in western and northern England. The team radiocarbon dated bits of butchered bone from animals the settlers hunted such as red deer, and wild horse and cattle. The data reveal repopulation began as far back as 16,000 years ago.
Roger Jacobi, from the paleontology department of the Natural History Museum, London, said: "When you compare the pattern of radiocarbon dating against the Greenland ice core, humans get back when the ice cores are showing quite a sharp temperature rise."
Jacobi says the oldest bones came from a cave at Cheddar Gorge, Somerset. He added: "They were a group of neck vertebrae from wild horses that had been butchered and were therefore covered in cut marks. It's very clear humans had been instrumental in dismembering them."
The bones are only slightly younger than earliest dated human-modified remains from countries such as Belgium and Germany, suggesting a rapid advance from mainland Europe. Their progress was helped by the fact Britain was a peninsula, not an island.
"Most of the English Channel and southern North Sea would have been dry land," added Jacobi. "So Britain would have been joined eastwards to the northern tip of Denmark. It was a huge land connection."
Jacobi and his colleagues suggest it was the movement of animals across this same land connection that triggered the wave of human migration.
"It seems clear that people were following herds of large animals, like horse, which expanded to occupy the continent," said co-researcher Martin Street, from the Römisch-Germanisches Zentralmuseum, Neuwied, Germany. "Humans can be seen as part of that pulse."
They brought with them a tool kit of projectile points and knives used to kill and dismember their prey. Analysis of flint tools specific to the late glacial period shows these early settlers were highly mobile and covered large distances.
"For instance, we know from flint evidence at Cheddar Gorge that hunters were moving at least 70 kilometers (43 miles) on a regular basis," Jacobi added.
One of the strongest patterns to emerge from the study is the correlation between the location of early settlement sites and the edges of upland areas. Jacobi says this again highlights the importance of prey animals as a catalyst for human repopulation.
"It looks as if there was a whole range of micro-environments at the interface between uplands and lowlands," said Jacobi. "The joy of living on an upland edge is that you are able to exploit a whole range of environments and with it a more diverse fauna. The typography of places like Cheddar Gorge also make them ideal for trapping game against rock walls."
Remains found at Creswell Crags in Derbyshire suggest these early Britons were also partial to Arctic hare. "As well as its meat, they were probably going for the white winter pelts which were thick and look so attractive," Jacobi added.
The recent discovery at Creswell Crags of the world's northernmost ice age cave art lends weight to the study's findings. Though no match for the artistry of examples from caves in France and Spain, one of the animals depicted hints at the speed at which migrants spread from mainland Europe.
Archaeologists made out the outline of an ibex, an animal which is thought to have been absent from Britain.
"It's possible evidence that these people came from an area like Belgium, as ibexes certainly occurred in places like the Ardennes," said Jacobi. "Some researchers have interpreted this as indicating that groups had seen ibex on the continent and were drawing them from memory."
Yet these early colonizers secured only a temporary foothold in Britain, as Jacobi explains. "Interestingly, radiocarbon dating seems to show that humans, having resettled Britain in the late glacial period, then go away again for several hundred years, when it gets very cold again."
So it appears humans were cleaned out of Britain one last time, around 12,000 years ago. But they soon returned, hot on the heels of those deer and wild horses. And this time they were there to stay.
Posted by lmurx at 8:53 PM