Younger Dryas impact hypothesis
The Younger Dryas impact hypothesis, also known as the Clovis comet hypothesis, is one of the competing scientific explanations for the onset of the Younger Dryas cold period. The hypothesis, which scientists continue to debate, proposes that the climate of that time was cooled by the impact or air burst of one or more comets..
The general hypothesis states that about about 12,900 BP calibrated (10,900 14C uncalibrated) years ago, air burst(s) or impact(s) from a near-Earth object(s) set areas of the North American continent on fire, disrupted climate and caused the extinction of most of the megafauna in North America and the demise of the North American Clovis culture after the last glacial period. 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 major impact crater has been identified, 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 wildfires would have suffered due to the disrupted ecologic relationships affecting the continent.
The evidence claimed for an impact event includes carbon-rich layers of soil that have been found at some 50 Clovis sites across the continent. The proponents report that layers contain unusual materials (nanodiamonds, metallic microspherules, carbon spherules, magnetic spherules, iridium, charcoal, soot, and fullerenes enriched in helium-3) that they interpret as evidence of an impact event, at the very bottom of black mats of organic material that they say marks the beginning of the Younger Dryas, and is claimed that it cannot be explained by volcanic, anthropogenic, and other natural processes.
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. Researchers have criticized the conclusions of various studies for incorrect age-dating of the sediments, contamination by modern carbon, and inconsistent hypothesis that made it difficult to predict the type and size of bolide, lack of proper identification of lonsdaleite, confusing an extraterrestrial impact with other causes such as fire, and for inconsistent use of the carbon spherule "proxy". Naturally occurring lonsdaleite has also been identified in non-bolide diamond placer deposits in the Sakha Republic.
A 100-fold spike in the concentration of platinum has also been found in Greenland ice cores, dated to 12890 BP with 5 year accuracy. The source of the platinum has not yet been identified, but the researchers ruled out either earth's mantle or stony meteorites (chondrites). The researchers said the source could be from an iron-rich impactor that probably would have left a crater of "few kilometers" in diameter, but none has so far been identified.
Consequences of hypothetical impact
It is conjectured that this impact event brought about the extinction of many North American Pleistocene megafauna. These animals included camels, mammoths, the giant short-faced bear and numerous other species that the proponents suggest died at this time. The proposed markers for the impact event are claimed to appear at the end of the Clovis culture. However, some large animals survived that time period.
History of the hypothesis
The initial description of this hypothesis was published in a 2006 book. 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.
Additional data purported to support the synchronous nature of the black mats was published. The author stated that the data required further analysis, and independent analysis of other Clovis sites for verification of this evidence. Haynes stated that he remained skeptical of the bolide impact hypothesis as the cause of the Younger Dryas and the megafaunal extinction, concluding that "However (...) something major happened at 10,900 B.P. (14C uncalibrated) that we have yet to understand."
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. Also, in the same issue, D.J. Kennett reported that the nano-diamonds 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. This study has been strenuously disputed by mainstream scientists for a variety of technical and professional reasons. Scientific 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.
The disputing scientists claim that the study's conclusions could not be repeated, that further research suggests that no nanodiamonds were found, and that the supposed carbon spherules were, in fact, either fungus or insect feces and included modern contaminants.
A re-evaluation published by the original proponents in June 2013 of spherules from 18 sites worldwide is seen by them as supporting their hypothesis.
In 2014, Kinzie, and an international collaboration of scientists published in The Journal of Geology research arguing that they had found what they interpreted to be "a thin layer over three continents, particularly in North America and Western Europe, that contain a rich assemblage of nanodiamonds, the production of which can be explained only by cosmic impact," 
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. They suggested that the hypothesis would probably need to be revised. There is also no evidence of continent-wide wildfires at any time during terminal Pleistocene deglaciation, along with evidence that most larger wildfires had a human origin, 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 or non-catastrophic factors.
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. 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, Russia, until 3700 BP, and the survival of ground sloths in the Antilles, the Caribbean, until 4700 cal BP. The Australian megafaunal extinctions occurred approximately 30,000 years earlier than the hypothetical Younger Dryas event.
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. 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. 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, possibly from anthropogenic activities, including hunting.
Scientists have asserted that the carbon spherules originated as fungal structures and/or insect fecal pellets, and contained modern contaminants and that the claimed nanodiamonds are actually misidentified graphene and graphene/graphane oxide aggregates. 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. Researchers have also have not found any extraterrestrial platinum group metals in the boundary layer which would be inconsistent with the hypothesized impact event. 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.
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.
Research published in 2012 has shown that the so-called "black mats" are easily explained by typical earth processes in wetland environments. 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.
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."
Recent evidence continues to oppose the YDB impact hypothesis. New research, which analyzed sediments claimed, by the hypothesis proponents, to be deposits resulting from a bolide impact were, in fact, dated from much later or much earlier time periods than the proposed date of the cosmic impact. The researchers examined 29 sites that are commonly referenced to support the impact theory to determine if they can be geologically dated to around 13,000 years ago. Crucially, only 3 of the sites actually date from that time. According to the researchers, the Younger Dryas impact event evidence "fails the critical chronological test of an isochronous event at the YD onset, which, coupled with the many published concerns about the extraterrestrial origin of the purported impact markers, renders the YDIH unsupported. There is no reason or compelling evidence to accept the claim that a cosmic impact occurred ∼12,800 y ago and caused the Younger Dryas."
- Bloody Creek crater
- Charity Shoal Crater
- Corossol crater
- Pleistocene megafauna
- Holocene extinction event
- Tollmann's hypothetical bolide
- 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.
- 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 1994902. PMID 17901202.
- 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. PMID 22711809.
- 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.
- Napier WM (July 2010). "Palaeolithic extinctions and the Taurid Complex". Monthly Notices of the Royal Astronomical Society 405 (3): 1901_1906. Bibcode:2010MNRAS.405.1901N. doi:10.1111/j.1365-2966.2010.16579.x.
- Napier WM (March 2010). "Palaeolithic extinctions and the Taurid Complex". arXiv. arXiv:1003.0744.
- 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
- Wittke, James H. (2013-05-20). "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): E2088–97. doi:10.1073/pnas.1301760110. PMC 3677428. PMID 23690611.
- 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 3324006. PMID 22392980.
- 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 3427088. PMID 22829673.
- 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 3427067. PMID 22829675.
- 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 3427052. PMID 22829671.
- 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 3427112. PMID 22829674.
- 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 3427080. PMID 22829672.
- 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.
- Simon Redfern (2013-08-01). "Ice core data supports ancient space impact idea". BBC.
- 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. doi:10.1073/pnas.1303924110.
- Haynes, G (5 November 2010). "The catastrophic extinction of North American mammoths and mastodonts". World Archeology 33 (3): 391–416. doi:10.1080/00438240120107440.
- 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.
- 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.
- 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 2373324. PMID 18436643.
- Kerr, Richard A. (2009). "Did the Mammoth Slayer Leave a Diamond Calling Card?". Science 323 (5910): 26. doi:10.1126/science.323.5910.26. PMID 19119192.
- Dalton R (2011). "Comet Theory Comes Crashing to Earth". Miller-McCune. Retrieved 2012-04-15.
- 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.
- 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, and A. J. T. Jull (2012). "Arguments and Evidence Against a Younger Dryas Impact Event". GEOPHYSICAL MONOGRAPH SERIES 198: 13–26. doi:10.1029/2012gm001209.
- 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.
- Cohen, Julie (2014-08-27). "Study examines 13,000-year-old nanodiamonds from multiple locations across three continents". Physorg.com. Retrieved 2014-08-29.
- "Nanodiamond-Rich Layer across Three Continents Consistent with Major Cosmic Impact at 12,800 Cal BP". The Journal of Geology 122. 2014. JSTOR 21104101928971. Retrieved 2014-08-29.
- 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 2012-04-20.
- 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.
- Gary Haynes (2009). American megafaunal extinctions at the end of the Pleistocene. Springer. p. 125. ISBN 978-1-4020-8792-9. Retrieved 2012-04-20.
- 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.
- Perkins S (2012-04-23). "No Love for Comet Wipeout - ScienceNOW". Retrieved 2012-04-28.
- Pinter N., Ishman S.E (2008). "Impacts, mega-tsunami, and other extraordinary claims". GSA Today 18 (1): 37–38. doi:10.1130/GSAT01801GW.1.
- 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.
- 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.
- 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.
- 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.
- 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 2556404. PMID 18695222.
- 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.
- 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.
- Kerr, Richard A. (2010-10-30). "Mammoth-Killer Impact Rejected". Science NOW. AAAS. Retrieved 2010-08-31.
- 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 3017148. PMID 21173270.
- 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 2799824. PMID 20007789.
- Surovell TA, Holliday VT, Gingerich JA, et al. (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 2775309. PMID 19822748. Retrieved 2012-04-20.
- 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 3356666. PMID 22547791.
- 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. PMID 22529347.
- Meltzer DJ, Holliday VT, Cannon MD, Miller DS (May 2014). "Chronological evidence fails to support claim of an isochronous widespread layer of cosmic impact indicators dated to 12,800 years ago". Proc. Natl. Acad. Sci. U.S.A. 111 (21): E2162–71. doi:10.1073/pnas.1401150111. PMID 24821789.
- James Kennett, UC Santa Barbara, May 21, 2013, Comprehensive Analysis of Impact Spherules Supports Theory of Cosmic Impact 12,800 Years Ago
- Holliday, V. T., 2011, A Cosmic Catastrophe: The Great Clovis Comet Debate: A personal perspective on an Outrageous Hypothesis. Argonaut Archaeological Research Fund, Department of Anthropology at the University of Arizona, University of Arizona, Tucson, Arizona.
- Pringle, H., 2008, Firestorm from space wiped out prehistoric Americans. The New Scientist. vol. 194, no. 2605, pp. 8–9.
- West, A., and A. Goodyear, 2008, The Clovis Comet: Part I:Evidence for a Cosmic Collision 12,900 Years Ago. Mammoth Trumpet. v. 23, no. 1, pp. 1–4.
- "Younger Dryas Boundary: Extraterrestrial Impact or Not" (pdf). www.georgehoward.net. Retrieved 2012-04-15.
- "Younger Dryas Boundary: Extraterrestrial Impact or Not?" (pdf). www.georgehoward.net. Retrieved 2012-04-15.
- Hoffman, Carey (2008-07-02). "Exploding Asteroid Theory Strengthened by New Evidence Located in Ohio, Indiana". University of Cincinnati. Retrieved 2008-08-05.
- "Science & Environment: Diamond clues to beasts' demise". BBC NEWS. Retrieved 2012-04-15.
- "Sciency Thoughts: Evidence for a Younger Dryas impact event?". Retrieved 2012-04-15.
- "The Younger Dryas Impact Hypothesis". Scientific American Blog Network. Retrieved 2012-04-15.
- "New Clovis-Age Comet Impact Theory". Retrieved 2012-04-15.