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Prehistoric Americas

America Ravaged by Ice Age Blast?

Edited from BBC News, 21 May 2007

A controversial idea which was published in 2007 suggested that a large space rock may have exploded over North America around thirteen thousand years ago.

The blast may have wiped out one of America's first Stone Age cultures as well as the continent's big mammals such as the mammoth and the mastodon. The blast, from a comet or asteroid, caused a major bout of climatic cooling which may also have affected human cultures which were emerging in Europe and Asia.

Scientists outlined their evidence at a meeting in Mexico in late May 2007. Their evidence came from layers of sediment at more than twenty sites across North America. These sediments contained exotic materials: tiny spheres of glass and carbon, ultra-small specks of diamond - called nanodiamond - and amounts of the rare element iridium which were too high to have originated on Earth.

All of this, they argued, pointed to the explosion of an extraterrestrial object of up to five kilometres in width a few years either side of 10,900 BC. No crater survives today, possibly because the Laurentide Ice Sheet, which blanketed thousands of square kilometres of North America during the last ice age, was thick enough to mask the impact.

North American large mammals
The Younger Dryas cold spell hit North America hard, just when things were starting to warm up at the end of the ice age - not only did many of the large mammals die out but so did the Clovis culture (click or tap on image to view full sized)

Another possibility is that the comet or asteroid exploded in the air.

Climate cooling

The rocks studied by the researchers had a black layer which, they argued, was the charcoal which was deposited by wildfires which swept the continent after the explosion. The blast would not only have generated enormous amounts of heat which could have given rise to wildfires, but could also have brought about a period of climate cooling which lasted a thousand years - an event known as the Younger Dryas.

Professor James Kennett, from the University of California in Santa Barbara (UCSB), pointed out the possibility that the explosion could have been to blame for the extinction of several large North American mammals at the end of the most recent ice age. All of the elephants, including the mastodon and the mammoth, all of the ground sloths, including the giant ground sloth which, when standing on its hind legs, would have been as big as a mammoth, all of the horses and even all of the North American camels failed to survive.

There were large carnivores such as the sabre-toothed cat and an enormous bear called the short-faced bear which also failed to survive. Professor Kennett mentioned that this could have had an enormous impact on human populations.

Population decline

Humans crossed from north-eastern Asia to America at the end of the most recent ice age, across a land bridge which - at the time - connected Siberia to Alaska. Whether they were the first humans to make the crossing is open to question (and is seeming less and less likely), but they eventually formed the Clovis culture, one of the earliest known cultures on the continent. These proficient hunter-gatherers developed a distinctive thin, fluted spear head known as the Clovis point, which is regarded as one of the most sophisticated stone tools ever developed.

Archaeologists have found evidence from the Topper site in South Carolina, USA, that Clovis populations here went through a population collapse at the time of the proposed ice age blast.

But there is no evidence of a similar decline in other parts of the continent. The Clovis culture does vanish from the archaeological record abruptly, but it is replaced by a myriad local hunter-gatherer cultures.

Jeff Severinghaus, a palaeoclimatologist at Scripps Institution of Oceanography in California, told Nature magazine that the impact theory shouldn't be dismissed; it deserved further investigation. According to the new idea, the comet would have caused widespread melting of the North American ice sheet. The waters would have poured into the Atlantic, disrupting its currents.

This, the scientists said, could have caused the thousand-year-long Younger Dryas cold spell, which also affected Asia and Europe. The Younger Dryas has been linked by some researchers to changes in the living patterns of people living in the Near East - coincidentally or otherwise the beginning of farming is noted very soon after this period.

While unproven, the 'space rock' idea is not without some support in known fact. A massive explosion near the Tunguska river, Siberia, in 1908, is also thought to have been caused by a space rock exploding in the atmosphere. It felled eighty million trees over an area of two thousand square kilometres.



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