Younger Dryas impact hypothesis

Younger Dryas impact hypothesis

The Younger Dryas impact hypothesis or Clovis comet hypothesis was the hypothesized large air burst or earth impact of an object or objects from outer space that initiated the Younger Dryas cold period about 12,900 BP calibrated (10,900 BP uncalibrated).[1]

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.[2] 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 has been the subject of criticism and doubts. Impact specialists have studied the claim and concluded in 2010 that there never was such an impact, in particular because various physical signs of such an impact cannot be found.[3] The evidence for the event has been thoroughly dismissed, and the hypothesis is no longer considered viable in the scientific community.[4]



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.[5] Nearly all of this evidence was found to be non-reproducible and has been dismissed as support for the hypothesis.[4]


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.[6]

History of the hypothesis

Forest destroyed by the prototypical Tunguska airburst event

The British science journal Nature addressed the theory in a news story on 17 May 2007.[5] 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.[6] 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.[1]

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.[7]

The hypothesis 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 hypothesis, 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.[8] Independent verification of Firestone and West's identification of extraterrestrial material in Clovis stratigraphy was presented by Mustafa Fayek and Sharon Hull of the University of Manitoba.[9]

Most recently, In January 2009, transmission electron microscopy evidence showing nanodiamonds from the geologic moment of the event was published in the journal Science[10] and reviewed in the International Herald Tribune.[11] 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.[2]

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." [12][13] Nor is there evidence of continent-wide wildfires at any time during terminal Pleistocene deglaciation,[14] 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.[15]

Since it is assumed 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 much 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.[16][17][18] The extinction of woolly mammoths in Siberia also appears to have occurred later than in North America.[16] 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, Russia, until 3700 BP,[16][17] and the survival of ground sloths in the Antilles, the Caribbean, until 4700 cal BP.[16][17][18]

The megafaunal extinction pattern observed in North America poses a problem for the bolide impact scenario, since it raises the question why large mammals should be preferentially exterminated over small mammals or other vertebrates.[19] 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.[16]

Skeptics of the theory have asserted that the carbon spherules originated as fungal structures and/or insect fecal pellets,[20] and that the claimed nanodiamonds are actually misidentified graphene and graphene/graphane oxide aggregates.[21][22]

See also


  1. ^ a b Firestone, RB; West, A; Kennett, JP; Becker, L; Bunch, TE; Revay, ZS; Schultz, PH; Belgya, T et al. (2007). "Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling". Proc. Natl. Acad. Sci. U.S.A. 104 (41): 16016–21. Bibcode 2007PNAS..10416016F. doi:10.1073/pnas.0706977104. PMC 1994902. PMID 17901202. 
  2. ^ a b Kennett,, D.J.; 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 323 (5910): 94. Bibcode 2009Sci...323...94K. doi:10.1126/science.1162819. PMID 19119227.;323/5910/94. 
  3. ^ Richard A. Kerr (3 September 2010). "Mammoth-Killer Impact Flunks Out". Science 329 (5996): 1140–1. Bibcode 2010Sci...329.1140K. doi:10.1126/science.329.5996.1140. PMID 20813931. 
  4. ^ a b Pinter, N.; Scott, A. C.; Daulton, T. L.; Podoll, A.; Koeberl, C.; Anderson, R. S.; Ishman, S. E. (2011). "The Younger Dryas impact hypothesis: A requiem". Earth-Science Reviews. doi:10.1016/j.earscirev.2011.02.005.  edit
  5. ^ a b Dalton, Rex (2007-05-17). "Archaeology: Blast in the past?" (PDF). Nature 447 (7142): 256–7. Bibcode 2007Natur.447..256D. doi:10.1038/447256a. PMID 17507957.  News article in Nature
  6. ^ a b "Session Information, 2007 Joint Assembly, Paleoceanography and Paleoclimatology". American Geophysical Union. Retrieved 2007-05-22.  Includes links to abstracts.
  7. ^ C. Vance Haynes Jr. (May 6, 2008). "Younger Dryas "black mats" and the Rancholabrean termination in North America". Proc. Natl. Acad. Sci. U.S.A. 105 (18): 6520–5. Bibcode 2008PNAS..105.6520H. doi:10.1073/pnas.0800560105. PMC 2373324. PMID 18436643. 
  8. ^ Video of the August, 2008, Pecos Conference presentations on the Clovis Comet
  9. ^ Fayek and Hull, Pecos Archaeological Conference, Flagstaff, Arizona, August 10, 2008
  10. ^ Kerr, Richard A. (2009-1). "Did the Mammoth Slayer Leave a Diamond Calling Card?". Science 323 (5910): 26. doi:10.1126/science.323.5910.26. PMID 19119192. 
  11. ^ Chang, Kenneth (2009-01-02). "Diamonds linked to quick cooling eons ago". Health and Science, International Herald Tribune (International Herald Tribune). Retrieved 2009-01-03. 
  12. ^ Buchanan B, Collard M, Edinborough K (August 19, 2008). "Paleoindian demography and the extraterrestrial impact hypothesis". Proc. Natl. Acad. Sci. U.S.A. 105 (33): 11651–4. Bibcode 2008PNAS..10511651B. doi:10.1073/pnas.0803762105. PMC 2575318. PMID 18697936. 
  13. ^ Gary Haynes (2009). American megafaunal extinctions at the end of the Pleistocene. Springer. pp. 125–. ISBN 9781402087929. 
  14. ^ Marlon J.R., et al. (2009). "Wildfire responses to abrupt climate change in North America". Proc. Natl. Acad. Sci. U.S.A. 106 (8): 2519–24. Bibcode 2009PNAS..106.2519M. doi:10.1073/pnas.0808212106. PMC 2650296. PMID 19190185. 
  15. ^ Pinter N., Ishman S.E (2008). "Impacts, mega-tsunami, and other extraordinary claims". GSA Today 18 (1): 37–38. doi:10.1130/GSAT01801GW.1. 
  16. ^ 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. 
  17. ^ 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. 
  18. ^ a b Vergano, Dan (2009-01-02). "Study links mammoth extinction, comets". USA Today.Com. Gannett Company. Retrieved 2009-05-10. 
  19. ^ Scott, E. (2010). "Extinctions, scenarios, and assumptions: Changes in latest Pleistocene large herbivore abundance and distribution in western North America". Quat. Int. 217 (1-2): 225. Bibcode 2010QuInt.217..225S. doi:10.1016/j.quaint.2009.11.003. 
  20. ^ Roach, John (2010-06-22). "Fungi, Feces Show Comet Didn't Kill Ice Age Mammals?". National Geographic Daily News. National Geographic Society. Retrieved 2010-06-25. 
  21. ^ Daulton, T. L.; Pinter, N.; Scott, A. C. (2010-08-30). "No evidence of nanodiamonds in Younger–Dryas sediments to support an impact event". Proc. Natl. Acad. Sci. U.S.A. 107 (37): 16043–7. Bibcode 2010PNAS..10716043D. doi:10.1073/pnas.1003904107. PMC 2941276. PMID 20805511. 
  22. ^ Kerr, Richard A. (2010-10-30). "Mammoth-Killer Impact Rejected". Science NOW. AAAS. Retrieved 2010-08-31. 

External reading

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