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3 January 2009
Did a comet explode over North America 12,000 years ago?

Roughly 12,900 years ago, massive global cooling kicked in abruptly, along with the end of the line for some 35 different mammal species, including the mammoth, as well as the so-called Clovis culture of prehistoric North Americans. Various theories have been proposed for the die-off, ranging from abrupt climate change to overhunting once humans were let loose on the wilds of North America. But now nanodiamonds found in the sediments from this time period point to an alternative: a massive explosion or explosions by a fragmentary comet, similar to but even larger than the Tunguska event of 1908 in Siberia.
     Sediments from six sites across North America yielded such teensy diamonds, which only occur in sediment exposed to extreme temperatures and pressures, such as those from an explosion or impact, according to new research published today in Science. "The nanodiamonds that we found at all six locations exist only in sediments associated with the Younger Dryas Boundary layers, not above it or below it," said Doug Kennett of the University of Oregon in Eugene. "These discoveries provide strong evidence for a cosmic impact event at approximately 12,900 years ago that would have had enormous environmental consequences for plants, animals and humans across North America."
     Last year a 26-member team from 16 institutions proposed that a cosmic impact event, possibly by multiple airbursts of comets, set off a 1,300-year-long cold spell known as the Younger Dryas, fragmented the prehistoric Clovis culture and led to the extinction of a large range of animals, including mammoths, across North America. The cooling period interrupted an extended warming out of an ice age predicted by slight changes in Earth's orbit (known as Milankovitch cycles) that continues today. And it remains an unexplained anomaly in the climate record.
     But a series of cometary fragments exploding over North America might explain a layer of soil immediately prior to the cooling containing unusually high levels of iridium an element more common in cosmic wanderers like meteoroids than in Earth's crust. Paired with the fact that this layer occurs directly before the extinction of at least 35 genera of large mammals, including mammoths, it is strong circumstantial evidence for a cosmic event. "Very strong impact indicators are found in the sediments directly above, and often shrouding, the remains of these animals and the people who were hunting them," says Kennett. "Is it a comet? Is it a carbonaceous chondrite? Was it fragmented? Was it focused? Based on the distribution of the diamonds, it was certainly large scale."
     Many argued that the one-page paper in Science did not provide enough evidence to support the authors' claim. "Nanodiamonds could be a good indicator of an impact event... but after reading the paper, I wasn't convinced they found diamonds," said physicist Tyrone Daulton of Washington University in St. Louis. Spectroscopist Peter Buseck of Arizona State University said that he "wouldn't question that they saw nanodiamonds," but added that for such a potentially important discovery, he "would like to have it well supported." Archaeologist Douglas Kennett conceded that the restrictive format of the rapid publication limited the amount of data the team could incorporate into their paper. But he said the presence of nanodiamonds had been confirmed in three separate laboratories. "There are going to be a lot of follow-up papers that will clearly demonstrate that these are diamonds," he said.
     Preliminary searches further afield Europe, Asia and South America have turned up similar minerals and elements in sediments of the same age, Kennett says, and his own work on California's Channel Islands tells a tale of a massive burn-off, followed by erosion and a total change in the flora of the region. "It's consistent with a fragmentary body breaking up with air shocks and possible surface impacts in various parts of North America. It could be above the ice sheet or offshore in the ocean," he says, explaining why no impact crater(s) has been found to date. "Immediate effects on the ground include high temperatures and pressures triggering major transformations of the vegetation, knocking trees over but also burning."
     And that would make the climate shift of the Younger Dryas a closer cousin to the massive asteroid impact that wiped out the dinosaurs 65 million years ago. "This is an event that happened on one day," Kennett notes. "We're going to need high-resolution climate records, archaeological records, paleontological records to try to explore the effects."

Sources: Scientific American, ScienceDaily, Los Angeles Times, San Francisco Chronicle (2 January 2009)

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