Younger Dryas impact hypothesis

From Wikipedia, the free encyclopedia
  (Redirected from Younger Dryas impact event)
Jump to navigation Jump to search

The Younger Dryas impact hypothesis or Clovis comet hypothesis originally proposed that a large air burst or earth impact of one or more comets initiated the Younger Dryas cold period about 12,900 BP calibrated (10,900 14C uncalibrated) years ago.[1][2][3] The hypothesis has been contested by research showing that most of the conclusions cannot be repeated by other scientists, and criticized because of misinterpretation of data and the lack of confirmatory evidence.[4][5][6][7]

The current impact hypothesis states that the air burst(s) or impact(s) of a swarm of carbonaceous chondrites or comet fragments set areas of the North American continent on fire, causing the extinction of most of the megafauna in North America and the demise of the North American Clovis culture after the last glacial period.[8] The Younger Dryas ice age lasted for about 1,200 years before the climate warmed again. This swarm is hypothesized to have exploded above or possibly on the Laurentide Ice Sheet in the region of the Great Lakes, though no impact crater has yet been identified and no physical model by which such a swarm could form or explode in the air has been proposed. Nevertheless, the proponents suggest that it would be physically possible for such an air burst to have been similar to, but orders of magnitude larger than, the Tunguska event of 1908. The hypothesis proposed that animal and human life in North America not directly killed by the blast or the resulting coast-to-coast wildfires would have likely starved on the burned surface of the continent.


The evidence for an impact event includes charred carbon-rich layers of soil that have been found at some 50 Clovis sites across the continent. The layers contain unusual materials (nanodiamonds, metallic microspherules, carbon spherules, magnetic spherules, iridium, platinum, charcoal, soot and fullerenes enriched in helium-3), which are interpreted to be potential evidence of an impact event, at the very bottom of black mats of organic material that marks the beginning of the Younger Dryas,[9][10] and it is claimed these cannot be explained by volcanic, anthropogenic, or other natural processes.[3]

Recent research has been reported that at Lake Cuitzeo, in the central Mexican state of Guanajuato, evidence supporting a modified version of the Younger Dryas impact hypothesis—involving a much smaller, non-cometary impactor—was found in lake bed cores dating to 12,900 BP. The reported evidence included nanodiamonds (including the hexagonal form called lonsdaleite), carbon spherules, and magnetic spherules. Multiple hypotheses were examined to account for these observations, though none were believed to be terrestrial. Lonsdaleite occurs naturally in asteroids and cosmic dust and as a result of extraterrestrial impacts on Earth. The analysis of the study has not been confirmed or repeated by other researchers.[11] Lonsdaleite has also been made artificially in laboratories.[12][13]

A 100-fold spike in the concentration of platinum has also been found in Greenland ice cores, dated to 12,890 BP with 5-year accuracy. This is interpreted as evidence against the Younger Dryas impact hypothesis by the study’s authors, but cited as evidence for the hypothesis by its proponents.[14][15]

Consequences of hypothetical impact[edit]

It is conjectured that this impact event brought about the extinction of many species of North American Pleistocene megafauna. These animals included camels, mammoths, the giant short-faced bear and numerous other species that the proponents suggest died out at this time.[16] The proposed markers for the impact event are claimed to appear at the end of the Clovis culture.[17]

History of the hypothesis[edit]

Forest destroyed by the prototypical Tunguska airburst event

The initial description of this hypothesis was published in a 2006 book.[1] The following year, a paper with the same principal authors suggested that the impact event may have led to an immediate decline in human populations in North America at that time.[2]

Additional data purported to support the synchronous nature of the black mats was published. The authors stated that the data required further analysis, and independent analysis of other Clovis sites for verification of this evidence. The authors stated that they remained skeptical of the bolide impact hypothesis as the cause of the Younger Dryas and the megafaunal extinction. They also concluded that "...something major happened at 10,900 B.P. (14C uncalibrated) that we have yet to understand."[18]

Transmission electron microscopy evidence purported to show nanodiamonds from a layer assumed to correspond to the geologic moment of the event was published in the journal Science.[19] Also, in the same issue, D.J. Kennett reported that the nanodiamonds were evidence for bolide impacts from a rare swarm of carbonaceous chondrites or comets at the start of Younger Dryas, resulting from multiple airbursts and surface impacts. This resulted in substantial loss of plant life, megafauna and other animals.[8] This study has been strenuously disputed by some scientists for a variety of technical and professional reasons. Rex Daulton's skepticism increased with the revelation of documentation demonstrating misconduct and past criminal conduct (conviction for fraud and misrepresentation of credentials) by the researcher who prepared samples for the proponents of the hypothesis.[20] However, those charges were later dismissed and expunged by the court.[21][22]

The disputing scientists claim that the study's conclusions could not be repeated, that further research suggests that no nanodiamonds were found,[23] and that the supposed carbon spherules were, in fact, either fungus or insect feces and included modern contaminants.[7][24]

A re-evaluation published by the original proponents in June 2013 of spherules from 18 sites worldwide supports their hypothesis.[25] Further analysis of Younger Dryas boundary sediments at 9 sites, released in June 2016, found no evidence of an extraterrestrial impact at the YDB.[26] In December 2016, an analysis of nanodiamond evidence failed to uncover lonsdaleite or a spike in nanodiamond concentration at the YDB.[27] Radiocarbon dating, microscopy of paleobotanical samples, and analytical pyrolysis of fluvial sediments "[found] no evidence in Arlington Canyon for an extraterrestrial impact or catastrophic impact-induced fire."[28] Exposed fluvial sequences in Arlington Canyon on Santa Rosa Island "features centrally in the controversial hypothesis of an extra-terrestrial impact at the onset of the Younger Dryas."[28]

In 2018 two new papers were published dealing with a "Extraordinary Biomass-Burning Episode" associated with the Younger Dryas Impact.[29][30]


A study of Paleoindian demography found no evidence of a population decline among the Paleoindians at 12,900 ± 100 BP, which was inconsistent with predictions of an impact event.[31] They suggested that the hypothesis would probably need to be revised.[32][33] There is also no evidence of continent-wide wildfires at any time during terminal Pleistocene deglaciation,[34] though there is evidence that most larger wildfires had a human origin,[34] which calls into question the origin of the "black mat."[35] 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 or non-catastrophic factors.[36]

If it is assumed that the hypothesis supposes that all 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.[37][38][39] The extinction of woolly mammoths in Siberia also appears to have occurred later than in North America.[37] 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,[37][38][40] and the survival of ground sloths in the Antilles,[41] the Caribbean, until 4700 cal BP.[37] The Australian megafaunal extinctions occurred approximately 30,000 years earlier than the hypothetical Younger Dryas event.[42]

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.[43] Additionally, some extant megafaunal species such as bison and Brown bear 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.[37] Also, it appears that there was collapse in North American megafaunal population from 14,800 to 13,700 BP, well before the date of the hypothetical extraterrestrial impact,[44] possibly from anthropogenic activities, including hunting.[17]

Scientists have asserted that the carbon spherules originated as fungal structures and/or insect fecal pellets, and contained modern contaminants[7][24] and that the claimed nanodiamonds are actually misidentified graphene and graphene/graphane oxide aggregates.[23][45] An analysis of a similar Younger Dryas boundary layer in Belgium yielded carbon crystalline structures such as nanodiamonds, but the authors concluded that also did not show unique evidence for a bolide impact.[46] Researchers have also have not found any extraterrestrial platinum group metals in the boundary layer which would be inconsistent with the hypothesized impact event.[47] Further independent analysis was unable to confirm prior claims of magnetic particles and microspherules, concluding that there was no evidence for a Younger Dryas impact event.[48]

Other research has shown no support for the impact hypothesis. One group examined carbon-14 dates for charcoal particles that showed wildfires occurred well after the proposed impact date, and the glass-like carbon was produced by wildfires and no lonsdaleite was found.[49] Analysis of fluvial sediments on Santa Rosa Island by another group also found no evidence of lonsdaleite, impact-induced fires, or extraterrestrial impact.[28]

Research published in 2012 has shown that the so-called "black mats" are easily explained by typical earth processes in wetland environments.[6] The study of black mats, that are common in prehistorical wetland deposits which represent shallow marshlands, that were from 6000 to 40,000 years ago in the southwestern USA and Atacama Desert in Chile, showed elevated concentrations of iridium and magnetic sediments, magnetic spherules and titanomagnetite grains. It was suggested that because these markers are found within or at the base of black mats, irrespective of age or location, suggests that these markers arise from processes common to wetland systems, and probably not as a result of catastrophic bolide impacts.[6]

A 2013 study found a spike in platinum in Greenland ice. The authors of that study conclude that such a small impact of an iron meteorite is “unlikely to result in an airburst or trigger wide wildfires proposed by the YDB impact hypothesis."[15]

Finally, researchers have criticized the conclusions of various studies for incorrect age-dating of the sediments,[50] contamination by modern carbon, inconsistent hypothesis that made it difficult to predict the type and size of bolide,[51] lack of proper identification of lonsdaleite,[52] confusing an extraterrestrial impact with other causes such as fire,[53] and for inconsistent use of the carbon spherule "proxy".[54] Naturally occurring lonsdaleite has also been identified in non-bolide diamond placer deposits in the Sakha Republic.[13]

See also[edit]


  1. ^ a b Firestone, Richard; West, Allen; Warwick-Smith, Simon (4 June 2006). The Cycle of Cosmic Catastrophes: How a Stone-Age Comet Changed the Course of World Culture. Bear & Company. p. 392. ISBN 1591430615. 
  2. ^ a b Firestone RB, West A, Kennett JP; et al. (October 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 1994902Freely accessible. PMID 17901202. 
  3. ^ a b Bunch TE, Hermes RE, Moore AM; et al. (June 2012). "Very high-temperature impact melt products as evidence for cosmic airbursts and impacts 12,900 years ago". Proc Natl Acad Sci U S A. 109 (28): E1903–12. Bibcode:2012PNAS..109E1903B. doi:10.1073/pnas.1204453109. PMC 3396500Freely accessible. PMID 22711809. 
  4. ^ Kerr, R. A. (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. 
  5. ^ Pinter, Nicholas; Scott, Andrew C.; Daulton, Tyrone L.; Podoll, Andrew; Koeberl, Christian; Anderson, R. Scott; Ishman, Scott E. (2011). "The Younger Dryas impact hypothesis: A requiem". Earth-Science Reviews. 106 (3–4): 247. Bibcode:2011ESRv..106..247P. doi:10.1016/j.earscirev.2011.02.005. 
  6. ^ a b c Pigati JS; Latorre C; Rech JA; Betancourt JL; Martínez KE; Budahn JR (April 2012). "Accumulation of impact markers in desert wetlands and implications for the Younger Dryas impact hypothesis". Proc Natl Acad Sci U S A. 109 (19): 7208–12. Bibcode:2012PNAS..109.7208P. doi:10.1073/pnas.1200296109. PMC 3358914Freely accessible. PMID 22529347. Retrieved 7 February 2017. 
  7. ^ a b c Boslough, M.; K. Nicoll; V. Holliday; T. L. Daulton; D. Meltzer; N. Pinter; A. C. Scott; T. Surovell; P. Claeys; J. Gill; F. Paquay; J. Marlon; P. Bartlein; C. Whitlock; D. Grayson & A. J. T. Jull (2012). "Arguments and Evidence Against a Younger Dryas Impact Event" (PDF). Geophysical Monograph Series. 198: 13–26. Retrieved 7 February 2017. 
  8. ^ a b Kennett DJ, Kennett JP, West A; et al. (January 2009). "Nanodiamonds in the Younger Dryas boundary sediment layer". Science. 323 (5910): 94. Bibcode:2009Sci...323...94K. doi:10.1126/science.1162819. PMID 19119227. 
  9. ^ Dalton, Rex (17 May 2007). "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
  10. ^ Wittke, James H. (20 May 2013). "Evidence for deposition of 10 million tonnes of impact spherules across four continents 12,800 y ago" (PDF). Proceedings of the National Academy of Sciences. 110 (23): E2088–E2097. Bibcode:2013PNAS..110E2088W. doi:10.1073/pnas.1301760110. PMC 3677428Freely accessible. PMID 23690611. 
  11. ^ Israde-Alcántara I, Bischoff JL, Domínguez-Vázquez G; et al. (March 2012). "Evidence from central Mexico supporting the Younger Dryas extraterrestrial impact hypothesis". Proc. Natl. Acad. Sci. U.S.A. 109 (13): E738–47. Bibcode:2012PNAS..109E.738I. doi:10.1073/pnas.1110614109. PMC 3324006Freely accessible. PMID 22392980. 
  12. ^ Bundy, F. P. (1967). "Hexagonal Diamond—A New Form of Carbon". Journal of Chemical Physics. 46 (9): 3437. Bibcode:1967JChPh..46.3437B. doi:10.1063/1.1841236. 
  13. ^ a b Kaminskii, F.V., G.K. Blinova, E.M. Galimov, G.A. Gurkina, Y.A. Klyuev, L.A. Kodina, V.I. Koptil, V.F. Krivonos, L.N. Frolova, and A.Y. Khrenov (1985). "Polycrystalline aggregates of diamond with lonsdaleite from Yakutian [Sakhan] placers". Mineral. Zhurnal. 7: 27–36. 
  14. ^ Simon Redfern (1 August 2013). "Ice core data supports ancient space impact idea". BBC. 
  15. ^ a b Michail I. Petaev, Shichun Huang, Stein B. Jacobsen, Alan Zindler (2013). "Large Pt anomaly in the Greenland ice core points to a cataclysm at the onset of Younger Dryas". Proc. Natl. Acad. Sci. U.S.A. 110 (32): 12917–12920. Bibcode:2013PNAS..11012917P. doi:10.1073/pnas.1303924110. PMC 3740870Freely accessible. 
  16. ^ Haynes, G (5 November 2010). "The catastrophic extinction of North American mammoths and mastodonts". World Archeology. 33 (3): 391–416. doi:10.1080/00438240120107440. 
  17. ^ a b Carrasco MA, Barnosky AD, Graham RW (2009). "Quantifying the Extent of North American Mammal Extinction Relative to the Pre-Anthropogenic Baseline". PLOS One. 4 (12): e8331. Bibcode:2009PLoSO...4.8331C. doi:10.1371/journal.pone.0008331. PMC 2789409Freely accessible. PMID 20016820. 
  18. ^ Haynes CV (May 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 2373324Freely accessible. PMID 18436643. 
  19. ^ Kerr, Richard A. (2 January 2009). "Did the Mammoth Slayer Leave a Diamond Calling Card?". Science. 323 (5910): 26. doi:10.1126/science.323.5910.26. PMID 19119192. 
  20. ^ Dalton R (2011). "Comet Theory Comes Crashing to Earth". Miller-McCune. Archived from the original on 7 April 2012. Retrieved 15 April 2012. 
  21. ^ "Allen West smeared by Dalton, former Nature writer". 
  22. ^ "2010 Court document" (PDF). 
  23. ^ a b Daulton, T. L.; Pinter, N.; Scott, A. C. (30 August 2010). "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 2941276Freely accessible. PMID 20805511. 
  24. ^ a b Roach, John (22 June 2010). "Fungi, Feces Show Comet Didn't Kill Ice Age Mammals?". National Geographic Daily News. National Geographic Society. Retrieved 25 June 2010. 
  25. ^ Wittke; et al. (June 2013). "Evidence for deposition of 10 million tonnes of impact spherules across four continents 12,800 y ago". Proc. Natl. Acad. Sci. U.S.A. 110 (23): E2088–E2097. Bibcode:2013PNAS..110E2088W. doi:10.1073/pnas.1301760110. PMC 3677428Freely accessible. PMID 23690611. 
  26. ^ Holliday, Vance; Surovell, Todd; Johnson, Eileen (2016-07-08). "A Blind Test of the Younger Dryas Impact Hypothesis". PLOS One. 11 (7): e0155470. Bibcode:2016PLoSO..1155470H. doi:10.1371/journal.pone.0155470. ISSN 1932-6203. PMC 4938604Freely accessible. PMID 27391147. 
  27. ^ Daulton, Tyrone L.; Amari, Sachiko; Scott, Andrew C.; Hardiman, Mark; Pinter, Nicholas; Anderson, R. Scott (2017-01-01). "Comprehensive analysis of nanodiamond evidence relating to the Younger Dryas Impact Hypothesis". Journal of Quaternary Science. 32 (1): 7–34. Bibcode:2017JQS....32....7D. doi:10.1002/jqs.2892. ISSN 1099-1417. 
  28. ^ a b c Scott, Andrew C.; Hardiman, Mark; Pinter, Nicholas; Anderson, R. Scott; Daulton, Tyrone L.; Ejarque, Ana; Finch, Paul; Carter-champion, Alice (2017-01-01). "Interpreting palaeofire evidence from fluvial sediments: a case study from Santa Rosa Island, California, with implications for the Younger Dryas Impact Hypothesis". Journal of Quaternary Science. 32 (1): 35–47. Bibcode:2017JQS....32...35S. doi:10.1002/jqs.2914. ISSN 1099-1417. 
  29. ^ Wolbach, Wendy S; Ballard, Joanne P; et al. (2018). "Extraordinary Biomass-Burning Episode and Impact Winter Triggered by the Younger Dryas Cosmic Impact ∼12,800 Years Ago. 1. Ice Cores and Glaciers". Journal of Geology. 126 (2): 165–184. Bibcode:2018JG....126..165W. doi:10.1086/695703. 
  30. ^ Wolbach, Wendy S; Ballard, Joanne P; et al. (2018). "Extraordinary Biomass-Burning Episode and Impact Winter Triggered by the Younger Dryas Cosmic Impact ∼12,800 Years Ago. 2. Lake, Marine, and Terrestrial Sediments". Journal of Geology. 126 (2): 185–205. Bibcode:2018JG....126..185W. doi:10.1086/695704. 
  31. ^ Holliday VT, Meltzer DJ (2010). "The 12.9-ka ET Impact Hypothesis and North American Paleoindians" (pdf). Current Anthropology. 51 (5): 575–606. doi:10.1086/656015. Retrieved 10 April 2012. 
  32. ^ 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 2575318Freely accessible. PMID 18697936. 
  33. ^ Gary Haynes (2009). American megafaunal extinctions at the end of the Pleistocene. Springer. p. 125. ISBN 978-1-4020-8792-9. Retrieved 20 April 2012. 
  34. ^ a b 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 2650296Freely accessible. PMID 19190185. 
  35. ^ Perkins S (23 April 2012). "No Love for Comet Wipeout - ScienceNOW". Retrieved 28 April 2012. 
  36. ^ Pinter N., Ishman S.E (2008). "Impacts, mega-tsunami, and other extraordinary claims". GSA Today. 18 (1): 37–38. doi:10.1130/GSAT01801GW.1. 
  37. ^ 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. 
  38. ^ a b 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. 
  39. ^ Hubbe A, Hubbe M, Neves W (2007). "Early Holocene survival of megafauna in South America". Journal of Biogeography. 34 (9): 1642–1646. doi:10.1111/j.1365-2699.2007.01744.x. 
  40. ^ Stuart AJ, Kosintsev PA, Higham TF, Lister AM (October 2004). "Pleistocene to Holocene extinction dynamics in giant deer and woolly mammoth". Nature. 431 (7009): 684–9. Bibcode:2004Natur.431..684S. doi:10.1038/nature02890. PMID 15470427. 
  41. ^ Martin, Paul (2005). "4 Ground Sloths at Home Cryptozoology, Ground Sloths, and Mapinguari National Park". Twilight of the mammoths: ice age extinctions and the rewilding of America. Berkeley: University of California Press. ISBN 0-520-23141-4. 
  42. ^ Barnosky AD (August 2008). "Colloquium paper: Megafauna biomass tradeoff as a driver of Quaternary and future extinctions". Proc. Natl. Acad. Sci. U.S.A. 105 Suppl 1: 11543–8. Bibcode:2008PNAS..10511543B. doi:10.1073/pnas.0801918105. PMC 2556404Freely accessible. PMID 18695222. 
  43. ^ 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. 
  44. ^ Gill JL, Williams JW, Jackson ST, Lininger KB, Robinson GS (November 2009). "Pleistocene megafaunal collapse, novel plant communities, and enhanced fire regimes in North America". Science. 326 (5956): 1100–3. Bibcode:2009Sci...326.1100G. doi:10.1126/science.1179504. PMID 19965426. 
  45. ^ Kerr, Richard A. (30 October 2010). "Mammoth-Killer Impact Rejected". Science NOW. AAAS. Retrieved 31 August 2010. 
  46. ^ Tian H, Schryvers D, Claeys P (January 2011). "Nanodiamonds do not provide unique evidence for a Younger Dryas impact". Proc. Natl. Acad. Sci. U.S.A. 108 (1): 40–4. Bibcode:2011PNAS..108...40T. doi:10.1073/pnas.1007695108. PMC 3017148Freely accessible. PMID 21173270. 
  47. ^ Paquay FS, Goderis S, Ravizza G; et al. (December 2009). "Absence of geochemical evidence for an impact event at the Bølling-Allerød/Younger Dryas transition". Proc. Natl. Acad. Sci. U.S.A. 106 (51): 21505–10. Bibcode:2009PNAS..10621505P. doi:10.1073/pnas.0908874106. PMC 2799824Freely accessible. PMID 20007789. 
  48. ^ Surovell TA; Holliday VT; Gingerich JA; Ketron C; Haynes CV, Jr.; Hilman I; Wagner DP; Johnson E & Claeyse P (October 2009). "An independent evaluation of the Younger Dryas extraterrestrial impact hypothesis". Proc. Natl. Acad. Sci. U.S.A. 106 (43): 18155–8. Bibcode:2009PNAS..10618155S. doi:10.1073/pnas.0907857106. PMC 2775309Freely accessible. PMID 19822748. 
  49. ^ van Hoesel A, Hoek WZ, Braadbaart F, van der Plicht J, Pennock GM, Drury MR (May 2012). "Nanodiamonds and wildfire evidence in the Usselo horizon postdate the Allerod-Younger Dryas boundary". Proc. Natl. Acad. Sci. U.S.A. 109 (20): 7648–53. Bibcode:2012PNAS..109.7648V. doi:10.1073/pnas.1120950109. PMC 3356666Freely accessible. PMID 22547791. 
  50. ^ Blaauw M, Holliday VT, Gill JL, Nicoll K (July 2012). "Age models and the Younger Dryas Impact Hypothesis". Proc Natl Acad Sci U S A. 109 (34): E2240; author reply E2245–7. Bibcode:2012PNAS..109E2240B. doi:10.1073/pnas.1206143109. PMC 3427088Freely accessible. PMID 22829673. 
  51. ^ Boslough M (July 2012). "Inconsistent impact hypotheses for the Younger Dryas". Proc Natl Acad Sci U S A. 109 (34): E2241; author reply E2245–7. Bibcode:2012PNAS..109E2241B. doi:10.1073/pnas.1206739109. PMC 3427067Freely accessible. PMID 22829675. 
  52. ^ Daulton TL (July 2012). "Suspect cubic diamond "impact" proxy and a suspect lonsdaleite identification". Proc Natl Acad Sci U S A. 109 (34): E2242; author reply E2245–7. Bibcode:2012PNAS..109E2242D. doi:10.1073/pnas.1206253109. PMC 3427052Freely accessible. PMID 22829671. 
  53. ^ Gill JL, Blois JL, Goring S; et al. (July 2012). "Paleoecological changes at Lake Cuitzeo were not consistent with an extraterrestrial impact". Proc Natl Acad Sci U S A. 109 (34): E2243; author reply E2245–7. Bibcode:2012PNAS..109E2243G. doi:10.1073/pnas.1206196109. PMC 3427112Freely accessible. PMID 22829674. 
  54. ^ Hardiman M, Scott AC, Collinson ME, Anderson RS (July 2012). "Inconsistent redefining of the carbon spherule "impact" proxy". Proc Natl Acad Sci U S A. 109 (34): E2244; author reply E2245–7. Bibcode:2012PNAS..109E2244H. doi:10.1073/pnas.1206108109. PMC 3427080Freely accessible. PMID 22829672. 

External links[edit]