Pre-Phoenix

Did a Comet Hit Earth 12,000 Years Ago?

Nanodiamonds found across North America suggest that major climate change could have been cosmically instigated

By David Biello on January 2, 2009

Did a Comet Hit Earth 12,000 Years Ago?
Credit: Courtesy of Doug Kennett

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—Murray Springs, Ariz.; Bull Creek, Okla.; Gainey, Mich.; Topper, S.C.; Lake Hind, Manitoba; and Chobot, Alberta—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 discovery lends support to a theory first advanced last year in that some type of cosmic impact or impacts—a fragmented comet bursting in the atmosphere or raining down on the oceans—set off the more than 1,300-year cooling period in the Northern Hemisphere known as the Younger Dryas for the abundance of an alpine flower's pollen found during the interval.

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 in the case of Murray Springs, the remains of these animals and the people who were hunting them," says archaeologist and study co-author Doug Kennett of the University of Oregon in Eugene, the son in the father–son team helping to advance the new impact theory. "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."

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."

Evolution
Did a Comet Cause a North American Die-Off around 13,000 Years Ago?

Tiny hexagonal diamonds suggest a massive impact during the late Pleistocene that could have wiped out the Clovis people, mastodons and other continental inhabitants--but the geologic evidence falls short for some skeptics

By Brendan Borrell on July 20, 2009

Did a Comet Cause a North American Die-Off around 13,000 Years Ago?
Credit: UC SANTA BARBARA

Researchers have found shock-synthesized hexagonal diamonds on one of California's Channel Islands, which they say is the strongest evidence yet that a comet exploded in the atmosphere above North America, causing widespread extinctions there around 12,900 years ago. Skeptics, however, say the debate is far from over.

In 2007 researchers theorized that a comet set off continental fires that led to the mysterious disappearance of the Clovis people and the extermination of 35 mammal genera, including mammoths, mastodons, ground sloths and camels. The team documented a "black mat" of charcoal throughout North America that contains high levels of iridium, magnetic spheres, and nano-diamonds, which are consistent with such an airburst. The controversial theory also gibes with the 1908 Tunguska atmospheric detonation (also thought to be from a comet or meteorite) that leveled trees in Siberia, and it echoes the extraterrestrial impact widely believed to have wiped out the dinosaurs 65 million years ago.

Today, in Proceedings of the National Academy of Sciences, the same team reports on shock-synthesized hexagonal diamonds, known only from meteorite and other impact events, in a soot layer from Arlington Canyon on Santa Rosa Island in California. The canyon is famous for containing the earliest human remains in North America, dating back to 13,000 years, and the soot layer coincides with the disappearance of the pygmy mammoth from the island. In a documentary shown earlier this year on the Public Broadcasting Service's NOVA science show, the team also claimed that they discovered similar diamonds from the Greenland Ice Sheet dating to the same period.

But the evidence does not convince everyone. "I don't think much of this whole story," says geochemist Christian Koeberl of the University of Vienna in Austria, "Diamonds of any sort are not uniquely characteristic of impact events." He says that the major lines of evidence are still missing, including the presence of shocked minerals, including breccias and tektites as well as an impact crater. "At least three other groups searched for similar evidence in the same or similar samples and found none," he adds.

Briggs Buchanan, an archaeologist from Simon Fraser University in Burnaby, British Columbia, disputes the notion that humans declined following the purported impact. "We have shown that in California, specifically, that there [was] no severe decline in the resident population." He adds that other researchers have shown that the black mat varies in age across the continent and appears to have a variety of geologic origins.

What does the research team have to say about their doubters? "I'm so skeptical about the skeptics," says marine geologist James Kennett of the University of California, Santa Barbara. "We work in a different paradigm where different materials result from different kinds of impacts."