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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.[1] 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[1] and Jon Erlandson of the University of Oregon.

Forest destroyed by the similar Tunguska airburst event

Contents

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.[2]

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.[3]

History of the hypothesis

The British science journal Nature addressed the theory in a news story on 17 May 2007.[4] 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.[3] 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.[5]

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.[6]

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.[7] 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.[8]

Most recently, In January 2009, transmission electron microscopy evidence showing nanodiamonds from the geologic moment of the event was published in the journal Science[9] and reviewed in the International Herald Tribune.[10] 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.[1]

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." [11][12] Nor is there evidence of continent-wide wildfires at any time during terminal Pleistocene deglaciation,[13] which calls into question the origin of the "black mat". Iridium, magnetic minerals, microspherules, carbon, and nanodiamonds are all subject to differing interpretations as to their nature and origin, and may be explained in many cases by purely terrestrial and/or non-catastrophic factors.[14]

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.[15][16][17] The extinction of woolly mammoths in Siberia also appears to have occurred later than in North America.[15] 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,[15][16] and the survival of ground sloths in the Antilles until 4700 cal BP.[15][16][17]

The megafaunal extinction pattern observed in North America is not explained by the bolide impact scenario, since there is no demonstrable reason why large mammals should be preferentially exterminated over small mammals or other vertebrates.[18] Additionally, 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.[15]

See also

References

  1. ^ a b c Kennett,, D. J. Kennett; J. P. Kennett, A. West, C. Mercer, S. S.Que Hee, L. Bement, T. E. Bunch, M. Sellers, W. S. Wolbach (2009-1). "Nanodiamonds in the Younger Dryas Boundary Sediment Layer". Science (American Association for the Advancement of Science) 323. (5910): 94. doi:10.1126/science.1162819. ISSN 1095-9203. http://www.sciencemag.org/cgi/content/abstract/sci;323/5910/94. 
  2. ^ Dalton, Rex (2007-05-17). "Archaeology: Blast in the past?". Nature 447: 256–257. doi:10.1038/447256a.  News article in Nature
  3. ^ a b "Session Information, 2007 Joint Assembly, Paleoceanography and Paleoclimatology". American Geophysical Union. http://www.agu.org/cgi-bin/sessions5?meeting=sm07&part=PP41A&maxhits=400. Retrieved 2007-05-22.  Includes links to abstracts.
  4. ^ "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
  5. ^ "Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling". The National Academy of Sciences. http://www.pnas.org/cgi/content/abstract/0706977104v1. Retrieved 2007-09-30. 
  6. ^ 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
  7. ^ Video of the August, 2008, Pecos Conference presentations on the Clovis Comet
  8. ^ Fayek and Hull, Pecos Archaeological Conference, Flagstaff, Arizona, August 10, 2008
  9. ^ Kerr, Richard A. (2009-1). "Did the Mammoth Slayer Leave a Diamond Calling Card?". Science (American Association for the Advancement of Science) 323. (5910,): 26. doi:10.1126/science.323.5910.26. ISSN 1095-9203. http://www.sciencemag.org/cgi/content/summary/323/5910/26. 
  10. ^ Chang, Kenneth (2009-01-02). "Diamonds linked to quick cooling eons ago". Health and Science, International Herald Tribune (International Herald Tribune). http://www.iht.com/articles/2009/01/02/healthscience/02impact.php. Retrieved 2009-01-03. 
  11. ^ 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]
  12. ^ [2]
  13. ^ J.R. Marlon et al. (2009) Wildfire responses to abrupt climate change in North America, Proceedings of the National Academy of Sciences 106 (8), 2519–2524
  14. ^ N. Pinter, S.E Ishman (2008) Impacts, mega-tsunami, and other extraordinary claims, GSA Today 18 (1), 37–38.
  15. ^ a b c d e Haynes, Gary (2009), "Introduction to the Volume", in Haynes, Gary, American Megafaunal Extinctions at the End of the Pleistocene, Springer, pp. 1–20, doi:10.1007/978-1-4020-8793-6_1, ISBN 978-1-4020-8792-9, http://www.springerlink.com/content/w314m76738r91g35/?p=5af1eb7387d443a2b514b284c646efa7&pi=0 
  16. ^ a b c Fiedel, Stuart (2009), "Sudden Deaths: The Chronology of Terminal Pleistocene Megafaunal Extinction", in Haynes, Gary, American Megafaunal Extinctions at the End of the Pleistocene, Springer, pp. 21–37, doi:10.1007/978-1-4020-8793-6_2, ISBN 978-1-4020-8792-9, http://www.springerlink.com/content/l225628681672725/?p=5af1eb7387d443a2b514b284c646efa7&pi=1 
  17. ^ a b Vergano, Dan (2009-01-02). "Study links mammoth extinction, comets". USA Today.Com. Gannett Company. http://www.usatoday.com/tech/science/2009-01-01-mammothimpact_N.htm. Retrieved 2009-05-10. 
  18. ^ Scott, E. (2010). "Extinctions, scenarios, and assumptions: Changes in latest Pleistocene large herbivore abundance and distribution in western North America". Quat. Int.. doi:10.1016/j.quaint.2009.11.003. 

External reading

External links

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