[meteorite-list] LATimes- Comet hits North America 13, 000 years ago

From: Howard Wu <freewu2000_at_meteoritecentral.com>
Date: Thu, 1 Jan 2009 18:21:13 -0800 (PST)
Message-ID: <311779.79797.qm_at_web52209.mail.re2.yahoo.com>

E.P. Grondine is going to love this one

mr. wu


http://www.latimes.com/news/science/la-sci-extinction2-2009jan02,0,896970.story
>From the Los Angeles Times
Diamonds show comet struck North America, scientists say
The impact caused an ice age that killed some mammal species and many humans 12,900 years ago, researchers report. They say the discovery of tiny heat-formed diamonds is proof of the catastrophe.
By Thomas H. Maugh II

1:11 PM PST, January 1, 2009

A thin layer of miniature gemstones called nanodiamonds in North American soil provides the strongest evidence yet that a comet struck the continent nearly 13,000 years ago, triggering a mini-ice age that wiped out many species of mammals and interrupted the culture of early humans for hundreds of years.

Researchers had earlier discovered the thin layer of black soil containing iridium and other debris suggestive of a massive comet or meteor impact, but critics had suggested a variety of less dire explanations.

The discovery of the nanodiamonds, however, reported today in the journal Science, provides the most powerful support for the comet theory because the gems can only be created under the extreme temperatures and pressures of a massive explosion, such as a comet striking the Earth's surface.

"There's no other way we can interpret the presence of these diamonds other than an extraterrestrial impact," said paleooceanographer James P. Kennett of UC Santa Barbara, one of the authors of the paper.

Such an impact would be the most likely source of nanodiamonds, critics agreed, but many argued that the one-page paper in Science does 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. "Maybe they found diamonds, and maybe they didn't."

Added spectroscopist Peter Buseck of Arizona State University, "I wouldn't question that they saw nanodiamonds," but for such a potentially important discovery, "I would like to have it well-supported."

Archaeologist Douglas J. Kennett of the University of Oregon, lead author of the report and James Kennett's son, conceded that the restrictive format of the rapid publication limited the amount of data the team could incorporate.

But he said the presence of nanodiamonds had now 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."

The new findings may tie together a variety of hitherto mysterious events in North America that all occurred around the same time.

Beginning about 12,900 years ago, North America -- and perhaps the entire world -- entered a 1,300-year-long period of profound cooling known as the Younger Dryas, and often colloquially called the "Big Freeze."

About that same time, at least 35 species of large mammals, such as the woolly mammoth, camels and mastodons, disappeared forever. The period also saw an end to the Paleo-Indian Clovis culture characteristic of the first inhabitants of the continent.

Other evidence indicates a massive decrease in salinity in the Gulf of Mexico at the beginning of the event, caused by melting of the Laurentide ice sheet in Canada. The influx of water into the gulf sharply changed circulation patterns in the Atlantic Ocean, which may be the proximate cause of the cooling event.

The Kennetts and their colleagues have previously reported the discovery of a thick mat of black soil radiocarbon-dated to 12,900 years ago at 10 archaeological sites scattered around the continent. The layer contains iridium, carbon spherules and fullerenes containing helium-3, all characteristic of an extraterrestrial impact.

Critics, however, said the evidence was insufficient to prove an impact, particularly in the absence of a demonstrable crater.

James Kennett and his colleagues went back to the mats they had collected and performed what he termed the "extremely labor-intensive" process of looking for the nanodiamonds. That involved using acids to dissolve everything else in the samples, then using a variety of techniques to identify the diamond residue.

They found a family of at least five different forms of diamonds, including some that are formed only by impacts, they reported in the Science paper. Moreover, the nanodiamonds were found only at the bottom of the black mat -- not in the soil either below or above it.

What the team now believes is that a comet struck North America 12,900 years ago, breaking into pieces before reaching the ground. Heat from the massive explosions melted the Laurentide glacier and set off wildfires all over the continent, leading to the deaths of large numbers of animals -- and people.

In a series of papers presented last week at a meeting of the American Geophysical Union, the team also reported that at least 15 of the 35 species that became extinct during the Younger Dryas -- including camels, mammoths, mastodons, the short-faced bear, the giant beaver, the dire wolf and the American lion -- disappeared within 100 years of the event.

Their fossils appear "right up to the base of the black mat and never go above it," James Kennett said. The other species may have been hunted to extinction by humans, as has often been speculated, but only after their numbers were greatly reduced by the fires and climate change.

There was also a "major population crash" of humans, he added. In the Great Plains, all traces of humans disappeared for about 200 years, he noted. The effects were more severe in what is now Southern California, with evidence of human occupation disappearing for most of the Younger Dryas.

He is convinced that there was a cometary impact. "I've counted up 15 major things that occurred at 12,900 years ago. I'd like somebody to explain that to me in some other way."

thomas.maugh at latimes.com


      
Received on Thu 01 Jan 2009 09:21:13 PM PST