The Younger Dryas impact event or Clovis comet hypothesis refers to the hypothesized large air burst or earth impact of an object or objects from outer space that initiated the Younger Dryas cold spell about 12,900 BP calibrated (10,900 BP uncalibrated).

One scenario proposes that an air burst and/or earth impact with a rare swarm of carbonaceous chondrites or comets set vast areas of the North American continent on fire, causing the extinction of most of the large animals in North America and the demise of the North American Clovis culture at the end of the last glacial period. This swarm would have exploded above or even into the Laurentide Ice Sheet north of the Great Lakes. An airburst would have been similar to but many orders of magnitude larger than the Tunguska event of 1908. Animal and human life not directly killed by the blast or the resulting coast to coast wildfires would have starved on the burned surface of the continent.

The scenario is the product of a team of geologists at American universities, among them James Kennett of the University of California Santa Barbara, Richard Firestone of Lawrence Berkeley National Laboratory, as well as archaeologists Douglas Kennett and Jon Erlandson of the University of Oregon.

Evidence

The evidence claimed for an impact event includes a charred carbon-rich layer of soil that has been found at some 50 Clovis-age sites across the continent. The layer contains unusual materials (nanodiamonds, metallic microspherules, carbon spherules, magnetic spherules, iridium, charcoal, soot, and fullerenes enriched in helium-3) interpreted as evidence of an impact event, at the very bottom of the "black mat" of organic material that marks the beginning of the Younger Dryas.

Results

It is conjectured that this impact event brought about the extinction of many North American large mammals. These animals included camels, mammoths, the giant short-faced bear and numerous other species. The markers for the impact event also appear at the end of the Clovis culture.

History of the hypothesis

The British science journal Nature addressed the theory in a news story on 17 May 2007. On 24 May 2007, a session at the spring 2007 joint assembly of the American Geophysical Union in Acapulco, Mexico was held to discuss this hypothesis and reveal the evidence. On 27 September 2007, a paper presenting the findings of the Acapulco group was pre-published online at the Proceedings of the National Academy of Sciences website. According to the study, the impact event may have led to an immediate decline in human populations in North America at that time.

Less than a year later substantial support for the synchronous nature of the black mat was provided by leading Clovis archaeologist, C. Vance Haynes, also in the PNAS. Says Haynes:

Further analysis is in progress and other Clovis sites need independent study and verification of this evidence.

Until then I remain skeptical of the ET impact hypothesis as the cause of the YD onset and the megafaunal extinction.

However, I reiterate, something major happened at 10,900 B.P. that we have yet to understand.

The theory drew new scrutiny in March 2008 at the annual meeting of the Society for American Archaeology in Vancouver, Canada. In August 2008, at the annual Pecos Archaeological Conference Allen West, a lead proponent of the Clovis Comet theory, and Ted Bunch, a co-author of the original PNAS paper and former NASA chief of exobiology, presented new evidence, and participated in a panel discussion of the findings with Sandia Labs asteroid impact modeler Mark Boslough, and comet hunter Carolyn Shoemaker.Independent verification of Firestone and West's identification of ET material in Clovis stratigraphy was presented by Mustafa Fayek and Sharon Hull of the University of Manitoba.

Most recently, In January 2009, transmission electron microscopy evidence showing nanodiamonds from the geologic moment of the event was published in the journal Science and reviewed in the International Herald Tribune. Also, in the same issue, D.J. Kennett reported that:These diamonds provide strong evidence for Earth's collision with a rare swarm of carbonaceous chondrites or comets at the onset of the Younger Dryas cool interval, producing multiple airbursts and possible surface impacts, with severe repercussions for plants, animals, and humans in North America.

Criticisms of the hypothesis

A study of Paleoindian demography published in August 2008 (almost a year after the first publication in PNAS) states "The results of the analyses were not consistent with the predictions of extraterrestrial impact hypothesis. No evidence of a population decline among the Paleoindians at 12,900 ± 100 calBP was found. Thus, minimally, the study suggests the extraterrestrial impact hypothesis should be amended."

Since the effects of the putative impact on Earth's biota would have been brief, all extinctions caused by the impact should have occurred simultaneously. However, there is evidence that the megafaunal extinctions that occurred across northern Eurasia, North America and South America at the end of the Pleistocene were not synchronous. The extinctions in South America appear to have occurred at least 400 years after the extinctions in North America. The extinction of woolly mammoths in Siberia also appears to have occurred later than in North America. A greater disparity in extinction timings is apparent in island megafaunal extinctions that lagged nearby continental extinctions by thousands of years; examples include the survival of woolly mammoths on Wrangel Island until 3700 BP, and the survival of ground sloths in the Antilles until 4700 cal BP.

Some extant megafaunal species such as bison and grizzlies seem to have been little affected by the extinction event, while the environmental devastation caused by a bolide impact would not be expected to discriminate.

See also

• Pleistocene megafauna
• Holocene extinction event

References

1. News article in Nature
2. Includes links to abstracts.
3. "Blast from the Past? A controversial new idea suggests that a big space rock exploded on or above North America at the end of the last ice age," by Rex Dalton, Nature, vol. 447, no. 7142, pages 256-257 (17 May 2007). Available on-line at: http://www.geo.arizona.edu/~reiners/blackmat.pdf
4. Younger Dryas “black mats” and the Rancholabrean termination in North America, C. Vance Haynes, Jr., PNAS May 6, 2008 vol. 105 no. 18 6520-6525
5. Video of the August, 2008, Pecos Conference presentations on the Clovis Comet
6. Fayek and Hull, Pecos Archaeological Conference, Flagstaff, Arizona, August 10, 2008
7. B Buchanan, M Collard & K Edinborough 2008. "Paleoindian demography and the extraterrestrial impact hypothesis " PNAS August 19, 2008 vol. 105 no. 33 11651-11654 doi: 10.1073/pnas.0803762105 [1]
8. [2]

External reading

• THE CLOVIS COMET Part I:Evidence for a Cosmic Collision 12,900 Years Ago In the Mammoth Trumpet, Volume 23 Number 1, by Allen West GeoScience Consulting and Albert Goodyear South Carolina Institute of Archaeology and Anthropology. Accessed August 2008
• Clovis Comet at Real Science blog. Accessed August 2008
• New Clovis-Age Comet Impact Theory Newswise 2008
• New Clovis-Age Comet Impact Theory SpaceDaily.com May 23, 2007. Accessed August 2008.
• New Scientist - Heather Pringle Firestorm from space wiped out prehistoric Americans 26 May 2007

External links

• PDF of abstracts of Oral Presentations for the 2009 AGU Fall Meeting, December 16, 2009, session titled "Younger Dryas Boundary: Extraterrestrial Impact or Not
• PDF of abstracts of Poster Presentations for the 2009 AGU Fall Meeting, December 16, 2009, session titled "Younger Dryas Boundary: Extraterrestrial Impact or Not
• Video of the 2007 AGU Acapulco Joint Assembly, and the 2008 Pecos Archeological Conference, where the theory and the evidence were presented
• Hayek and Hull present independent verification at the annual Pecos Conference
• Video of the AGU Press Conference announcing the discovery
• BBC: Diamond clues to beasts' demise
• Nova episode on the impact and its ramifications.