Younger Dryas impact hypothesis
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. 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.
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. 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 given by proponents of an impact event includes "black mats" of organic-rich soil that have been found at some 50 Clovis sites across the continent. Proponents have reported materials (nanodiamonds, metallic microspherules, carbon spherules, magnetic spherules, iridium, platinum, charcoal, soot, and fullerenes enriched in helium-3), which they interpret 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, and it is claimed these cannot be explained by volcanic, anthropogenic, or other natural processes.
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. Lonsdaleite has also been made artificially in laboratories.
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. A much weaker Pt anomaly was subsequently reported with approximate age dating at 11 continental Younger Dryas sites.
Another study, related to this hypothesis, by Antonio Zamora provides a model of the formation of the Carolina Bays as an indirect consequence of an impact of a comet-like body on the Laurentide Ice Sheet that ejected ice boulders in ballistic trajectories that created the Bays all heading to the Great Lakes Region. It also provides an explanation about the formation of Nebraska's rainwater basins and why they are all pointing to the Lakes Region too. However, this study does not apply the widely accepted standards for identifying and confirming terrestrial impact structures. 
In 2018, evidence of a 31km diameter impact crater along with evidence of impact shock quartz, was found below the Hiawatha Glacier in Greenland, which - it was speculated - might have been connected with the Younger Dryas period.
Consequences of hypothetical impact
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. The proposed markers for the impact event are claimed to appear at the end of the Clovis culture.
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 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."
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 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. This study has been strenuously disputed by some scientists for a variety of technical and professional reasons.
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.
Some of the original proponents published a re-evaluation in June 2013 of spherules from 18 sites worldwide which they interpret to support their hypothesis. Further analysis of Younger Dryas boundary sediments at 9 sites, released in June 2016, found no evidence of an extraterrestrial impact at the YDB. In December 2016, an analysis of nanodiamond evidence failed to uncover lonsdaleite or a spike in nanodiamond concentration at the YDB. 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." 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."
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, though there is 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.
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, 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 they 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. Analysis of fluvial sediments on Santa Rosa Island by another group also found no evidence of lonsdaleite, impact-induced fires, or extraterrestrial impact.
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." But they write that the large Pt anomaly "hints for an extraterrestrial source of Pt," showing that any disagreement with the proponents of the original YDIH is over the nature of the extraterrestrial object, not whether there was one, and it is much more likely that the Greenland Pt anomaly was caused by a small local iron meteorite fall without any widespread consequences. 
Finally, researchers have criticized the conclusions of various studies for incorrect age-dating of the sediments, contamination by modern carbon, 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.
It should be noted that proponents of the hypothesis have responded to defend their findings, disputing the accusation of irreproducibility or replicating their findings. Critics of the hypothesis have repeatedly addressed the responses and continue to publish counterarguments.         
- Pleistocene megafauna
- Holocene extinction event
- Tollmann's hypothetical bolide
- Murray Springs Clovis Site
- Hiawatha Glacier Crater
- Firestone R, West A, Warwick-Smith S (4 June 2006). The Cycle of Cosmic Catastrophes: How a Stone-Age Comet Changed the Course of World Culture. Bear & Company. p. 392. ISBN 978-1591430612.
- Firestone RB, West A, Kennett JP, Becker L, Bunch TE, Revay ZS, et al. (October 2007). "Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling". Proceedings of the National Academy of Sciences of the United States of America. 104 (41): 16016–21. Bibcode:2007PNAS..10416016F. doi:10.1073/pnas.0706977104. PMC 1994902. PMID 17901202.
- Bunch TE, Hermes RE, Moore AM, Kennett DJ, Weaver JC, Wittke JH, DeCarli PS, Bischoff JL, Hillman GC, Howard GA, Kimbel DR, Kletetschka G, Lipo CP, Sakai S, Revay Z, West A, Firestone RB, Kennett JP (July 2012). "Very high-temperature impact melt products as evidence for cosmic airbursts and impacts 12,900 years ago". Proceedings of the National Academy of Sciences of the United States of America. 109 (28): E1903–12. Bibcode:2012PNAS..109E1903B. doi:10.1073/pnas.1204453109. PMC 3396500. PMID 22711809.
- Kerr RA (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.
- Pinter N, Scott AC, Daulton TL, Podoll A, Koeberl C, Anderson RS, Ishman SE (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.
- Pigati JS, Latorre C, Rech JA, Betancourt JL, Martínez KE, Budahn JR (May 2012). "Accumulation of impact markers in desert wetlands and implications for the Younger Dryas impact hypothesis". Proceedings of the National Academy of Sciences of the United States of America. 109 (19): 7208–12. Bibcode:2012PNAS..109.7208P. doi:10.1073/pnas.1200296109. PMC 3358914. PMID 22529347.
- Boslough, M; Nicoll, K; Holliday, V; Daulton, T. L; Meltzer, D; Pinter, N; Scott, A. C; Surovell, T; Claeys, P; Gill, J; Paquay, F; Marlon, J; Bartlein, P; Whitlock, C; Grayson, D; Jull, A.J.T (2013). "Arguments and Evidence Against a Younger Dryas Impact Event". Climates, Landscapes, and Civilizations. Geophysical Monograph Series. pp. 13–26. doi:10.1029/2012GM001209. ISBN 9781118704325.
- Kennett DJ, Kennett JP, West A, Mercer C, Hee SS, Bement L, Bunch TE, Sellers M, Wolbach WS (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.
- Dalton, Rex (2007). "Blast in the past?". Nature. 447 (7142): 256–257. doi:10.1038/447256a. PMID 17507957.
- Wittke JH, Weaver JC, Bunch TE, Kennett JP, Kennett DJ, Moore AM, Hillman GC, Tankersley KB, Goodyear AC, Moore CR, Daniel IR, Ray JH, Lopinot NH, Ferraro D, Israde-Alcántara I, Bischoff JL, DeCarli PS, Hermes RE, Kloosterman JB, Revay Z, Howard GA, Kimbel DR, Kletetschka G, Nabelek L, Lipo CP, Sakai S, West A, Firestone RB (June 2013). "Evidence for deposition of 10 million tonnes of impact spherules across four continents 12,800 y ago". Proceedings of the National Academy of Sciences of the United States of America. 110 (23): E2088–97. Bibcode:2013PNAS..110E2088W. doi:10.1073/pnas.1301760110. PMC 3677428. PMID 23690611.
- Israde-Alcántara I, Bischoff JL, Domínguez-Vázquez G, Li HC, DeCarli PS, Bunch TE, Wittke JH, Weaver JC, Firestone RB, West A, Kennett JP, Mercer C, Xie S, Richman EK, Kinzie CR, Wolbach WS (March 2012). "Evidence from central Mexico supporting the Younger Dryas extraterrestrial impact hypothesis". Proceedings of the National Academy of Sciences of the United States of America. 109 (13): E738–47. Bibcode:2012PNAS..109E.738I. doi:10.1073/pnas.1110614109. PMC 3324006. PMID 22392980.
- Bundy, F. P. (1967). "Hexagonal Diamond—A New Form of Carbon". Journal of Chemical Physics. 46 (9): 3437–3446. Bibcode:1967JChPh..46.3437B. doi:10.1063/1.1841236.
- Kaminskii FV, Blinova GK, Galimov EM, Gurkina GA, Klyuev YA, Kodina LA, Koptil VI, Krivonos VF, Frolova FN, Khrenov AY (1985). "Polycrystalline aggregates of diamond with lonsdaleite from Yakutian [Sakhan] placers". Mineral. Zhurnal. 7: 27–36.
- Petaev MI, Huang S, Jacobsen SB, Zindler A (August 2013). "Large Pt anomaly in the Greenland ice core points to a cataclysm at the onset of Younger Dryas". Proceedings of the National Academy of Sciences of the United States of America. 110 (32): 12917–20. doi:10.1073/pnas.1303924110. PMC 3740870. PMID 23878232.
- Moore CR, West A, LeCompte MA, Brooks MJ, Daniel IR, Goodyear AC, Ferguson TA, Ivester AH, Feathers JK, Kennett JP, Tankersley KB, Adedeji AV, Bunch TE (March 2017). "Widespread platinum anomaly documented at the Younger Dryas onset in North American sedimentary sequences". Scientific Reports. 7 (1): 44031. doi:10.1038/srep44031. PMC 5343653. PMID 28276513.
- Zamora, Antonio (2017). "A model for the geomorphology of the Carolina Bays". Geomorphology. 282: 209–216. doi:10.1016/j.geomorph.2017.01.019.
- French, Bevan M.; Koeberl, Christian (2010). "The convincing identification of terrestrial meteorite impact structures: What works, what doesn't, and why". Earth-Science Reviews. 98 (1–2): 123–170. doi:10.1016/j.earscirev.2009.10.009. ISSN 0012-8252.
- BBC News - Greenland ice sheet hides huge 'impact crater'
- Haynes G (5 November 2010). "The catastrophic extinction of North American mammoths and mastodonts". World Archaeology. 33 (3): 391–416. doi:10.1080/00438240120107440.
- Carrasco MA, Barnosky AD, Graham RW (December 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 2789409. PMID 20016820.
- Haynes CV (May 2008). "Younger Dryas "black mats" and the Rancholabrean termination in North America". Proceedings of the National Academy of Sciences of the United States of America. 105 (18): 6520–5. Bibcode:2008PNAS..105.6520H. doi:10.1073/pnas.0800560105. PMC 2373324. PMID 18436643.
- Kerr RA (January 2009). "Planetary impacts. Did the mammoth slayer leave a diamond calling card?". Science. 323 (5910): 26. doi:10.1126/science.323.5910.26. PMID 19119192.
- Daulton TL, Pinter N, Scott AC (September 2010). "No evidence of nanodiamonds in Younger-Dryas sediments to support an impact event". Proceedings of the National Academy of Sciences of the United States of America. 107 (37): 16043–7. Bibcode:2010PNAS..10716043D. doi:10.1073/pnas.1003904107. PMC 2941276. PMID 20805511.
- 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.
- Holliday V, Surovell T, Johnson E (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. PMC 4938604. PMID 27391147.
- Daulton TL, Amari S, Scott AC, Hardiman M, Pinter N, Anderson RS (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.
- Scott AC, Hardiman M, Pinter N, Anderson RS, Daulton TL, Ejarque A, Finch P, Carter-champion A (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.
- Wolbach WS, Ballard JP, Mayewski PA, Adedeji V, Bunch TE, Firestone RB, 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.
- Wolbach WS, Ballard JP, Mayewski PA, Parnell AC, Cahill N, Adedeji V, 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.
- Holliday VT, Meltzer DJ (2010). "The 12.9-ka ET Impact Hypothesis and North American Paleoindians". Current Anthropology. 51 (5): 575–606. doi:10.1086/656015.
- Buchanan B, Collard M, Edinborough K (August 2008). "Paleoindian demography and the extraterrestrial impact hypothesis". Proceedings of the National Academy of Sciences of the United States of America. 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 20 April 2012.
- Marlon JR, Bartlein PJ, Walsh MK, Harrison SP, Brown KJ, Edwards ME, Higuera PE, Power MJ, Anderson RS, Briles C, Brunelle A, Carcaillet C, Daniels M, Hu FS, Lavoie M, Long C, Minckley T, Richard PJ, Scott AC, Shafer DS, Tinner W, Umbanhowar CE, Whitlock C (February 2009). "Wildfire responses to abrupt climate change in North America". Proceedings of the National Academy of Sciences of the United States of America. 106 (8): 2519–24. Bibcode:2009PNAS..106.2519M. doi:10.1073/pnas.0808212106. PMC 2650296. PMID 19190185.
- Perkins S (23 April 2012). "No Love for Comet Wipeout".
- Pinter N., Ishman S.E (2008). "Impacts, mega-tsunami, and other extraordinary claims". GSA Today. 18 (1): 37–38. doi:10.1130/GSAT01801GW.1.
- Haynes, Gary (2009). "Introduction to the Volume". American Megafaunal Extinctions at the End of the Pleistocene. Vertebrate Paleobiology and Paleoanthropology. pp. 1–20. doi:10.1007/978-1-4020-8793-6_1. ISBN 978-1-4020-8792-9.
- Fiedel, Stuart (2009). "Sudden Deaths: The Chronology of Terminal Pleistocene Megafaunal Extinction". American Megafaunal Extinctions at the End of the Pleistocene. Vertebrate Paleobiology and Paleoanthropology. 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 978-0-520-23141-2.
- Barnosky AD (August 2008). "Colloquium paper: Megafauna biomass tradeoff as a driver of Quaternary and future extinctions". Proceedings of the National Academy of Sciences of the United States of America. 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): 225–239. 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. (30 October 2010). "Mammoth-Killer Impact Rejected". Science NOW. AAAS.
- Tian H, Schryvers D, Claeys P (January 2011). "Nanodiamonds do not provide unique evidence for a Younger Dryas impact". Proceedings of the National Academy of Sciences of the United States of America. 108 (1): 40–4. Bibcode:2011PNAS..108...40T. doi:10.1073/pnas.1007695108. PMC 3017148. PMID 21173270.
- Paquay FS, Goderis S, Ravizza G, Vanhaeck F, Boyd M, Surovell TA, Holliday VT, Haynes CV, Claeys P (December 2009). "Absence of geochemical evidence for an impact event at the Bølling-Allerød/Younger Dryas transition". Proceedings of the National Academy of Sciences of the United States of America. 106 (51): 21505–10. Bibcode:2009PNAS..10621505P. doi:10.1073/pnas.0908874106. PMC 2799824. PMID 20007789.
- Surovell TA, Holliday VT, Gingerich JA, Ketron C, Haynes CV, Hilman I, Wagner DP, Johnson E, Claeys P (October 2009). "An independent evaluation of the Younger Dryas extraterrestrial impact hypothesis". Proceedings of the National Academy of Sciences of the United States of America. 106 (43): 18155–8. Bibcode:2009PNAS..10618155S. doi:10.1073/pnas.0907857106. PMC 2775309. PMID 19822748.
- 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". Proceedings of the National Academy of Sciences of the United States of America. 109 (20): 7648–53. Bibcode:2012PNAS..109.7648V. doi:10.1073/pnas.1120950109. PMC 3356666. PMID 22547791.
- Petaev MI, Huang S, Jacobsen SB, Zindler A (August 2013). "Large Pt anomaly in the Greenland ice core points to a cataclysm at the onset of Younger Dryas". Proceedings of the National Academy of Sciences of the United States of America. 110 (32): 12917–20. Bibcode:2013PNAS..11012917P. doi:10.1073/pnas.1303924110. PMC 3740870. PMID 23878232.
- Boslough, M. (2013). "Greenland Pt anomaly may point to noncataclysmic Cape York meteorite entry". Proceedings of the National Academy of Sciences. 110 (52): E5035–E5035. doi:10.1073/pnas.1320328111. ISSN 0027-8424.
- Blaauw M, Holliday VT, Gill JL, Nicoll K (August 2012). "Age models and the Younger Dryas Impact Hypothesis". Proceedings of the National Academy of Sciences of the United States of America. 109 (34): E2240, author reply E2245–7. Bibcode:2012PNAS..109E2240B. doi:10.1073/pnas.1206143109. PMC 3427088. PMID 22829673.
- Boslough M (August 2012). "Inconsistent impact hypotheses for the Younger Dryas". Proceedings of the National Academy of Sciences of the United States of America. 109 (34): E2241, author reply E2245–7. Bibcode:2012PNAS..109E2241B. doi:10.1073/pnas.1206739109. PMC 3427067. PMID 22829675.
- Daulton TL (August 2012). "Suspect cubic diamond "impact" proxy and a suspect lonsdaleite identification". Proceedings of the National Academy of Sciences of the United States of America. 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, Marlon JR, Bartlein PJ, Nicoll K, Scott AC, Whitlock C (August 2012). "Paleoecological changes at Lake Cuitzeo were not consistent with an extraterrestrial impact". Proceedings of the National Academy of Sciences of the United States of America. 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 (August 2012). "Inconsistent redefining of the carbon spherule "impact" proxy". Proceedings of the National Academy of Sciences of the United States of America. 109 (34): E2244, author reply E2245–7. Bibcode:2012PNAS..109E2244H. doi:10.1073/pnas.1206108109. PMC 3427080. PMID 22829672.
- Bement LC, Madden AS, Carter BJ, Simms AR, Swindle AL, Alexander HM, Fine S, Benamara M (February 2014). "Quantifying the distribution of nanodiamonds in pre-Younger Dryas to recent age deposits along Bull Creek, Oklahoma panhandle, USA". Proceedings of the National Academy of Sciences of the United States of America. 111 (5): 1726–31. doi:10.1073/pnas.1309734111. PMC 3918833. PMID 24449875.
- Israde-Alcántara I, Bischoff JL, DeCarli PS, Domínguez-Vázquez G, Bunch TE, Firestone RB, Kennett JP, West A (2012-08-21). "Reply to Blaauw et al., Boslough, Daulton, Gill et al., and Hardiman et al.: Younger Dryas impact proxies in Lake Cuitzeo, Mexico". Proceedings of the National Academy of Sciences. 109 (34): E2245–E2247. doi:10.1073/pnas.1209463109. PMC 3427057.
- Kennett JP, Kennett DJ, Culleton BJ, Aura Tortosa JE, Bunch TE, Erlandson JM, et al. (December 2015). "Reply to Holliday and Boslough et al.: Synchroneity of widespread Bayesian-modeled ages supports Younger Dryas impact hypothesis". Proceedings of the National Academy of Sciences of the United States of America. 112 (49): E6723–4. doi:10.1073/pnas.1520411112. PMC 4679043. PMID 26604309.
- LeCompte MA, Goodyear AC, Demitroff MN, Batchelor D, Vogel EK, Mooney C, Rock BN, Seidel AW (October 2012). "Independent evaluation of conflicting microspherule results from different investigations of the Younger Dryas impact hypothesis". Proceedings of the National Academy of Sciences of the United States of America. 109 (44): E2960–9. doi:10.1073/pnas.1208603109. PMC 3497834. PMID 22988071.
- Napier WM, Bunch TE, Kennett JP, Wittke JH, Tankersley KB, Kletetschka G, Howard GA, West A (November 2013). "Reply to Boslough et al.: Decades of comet research counter their claims". Proceedings of the National Academy of Sciences of the United States of America. 110 (45): E4171. doi:10.1073/pnas.1315467110. PMC 3831498. PMID 24350338.
- Wittke JH, Bunch TE, Kennett JP, Kennett DJ, Culleton BJ, Tankersley KB, et al. (October 2013). "Reply to van Hoesel et al.: Impact-related Younger Dryas boundary nanodiamonds from The Netherlands". Proceedings of the National Academy of Sciences of the United States of America. 110 (41): E3897–8. doi:10.1073/pnas.1313207110. PMC 3799356. PMID 24244962.
- Boslough, M.; Harris, A. W.; Chapman, C.; Morrison, D. (2013). "Younger Dryas impact model confuses comet facts, defies airburst physics". Proceedings of the National Academy of Sciences. 110 (45): E4170–E4170. doi:10.1073/pnas.1313495110. ISSN 0027-8424.
- Boslough, M. (2013). "Faulty protocols yield contaminated samples, unconfirmed results". Proceedings of the National Academy of Sciences. 110 (18): E1651–E1651. doi:10.1073/pnas.1220567110. ISSN 0027-8424.
- Reimold, Wolf Uwe; Ferrière, Ludovic; Deutsch, Alex; Koeberl, Christian (2014). "Impact controversies: Impact recognition criteria and related issues". Meteoritics & Planetary Science. 49 (5): 723–731. doi:10.1111/maps.12284. ISSN 1086-9379.
- van Hoesel, Annelies; Hoek, Wim Z.; Pennock, Gillian M.; Drury, Martyn R. (2014). "The Younger Dryas impact hypothesis: a critical review". Quaternary Science Reviews. 83: 95–114. doi:10.1016/j.quascirev.2013.10.033. ISSN 0277-3791.
- Meltzer, D. J.; Holliday, V. T.; Cannon, M. D.; Miller, D. S. (2014). "Chronological evidence fails to support claim of an isochronous widespread layer of cosmic impact indicators dated to 12,800 years ago". Proceedings of the National Academy of Sciences. 111 (21): E2162–E2171. doi:10.1073/pnas.1401150111. ISSN 0027-8424.
- Holliday, Vance T. (2015). "Problematic dating of claimed Younger Dryas boundary impact proxies". Proceedings of the National Academy of Sciences. 112 (49): E6721–E6721. doi:10.1073/pnas.1518945112. ISSN 0027-8424.
- Thy, P.; Willcox, G.; Barfod, G.H.; Fuller, D.Q. (2015). "Anthropogenic origin of siliceous scoria droplets from Pleistocene and Holocene archaeological sites in northern Syria". Journal of Archaeological Science. 54: 193–209. doi:10.1016/j.jas.2014.11.027. ISSN 0305-4403.
- van der Hammen, T.; van Geel, B. (2016). "Charcoal in soils of the Allerød-Younger Dryas transition were the result of natural fires and not necessarily the effect of an extra-terrestrial impact". Netherlands Journal of Geosciences. 87 (04): 359–361. doi:10.1017/S0016774600023416. ISSN 0016-7746.
- Scott, Andrew C.; Hardiman, Mark; Pinter, Nicholas; Anderson, R. Scott; Daulton, Tyrone L.; Ejarque, Ana; Finch, Paul; Carter-champion, Alice (2017). "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. doi:10.1002/jqs.2914. ISSN 0267-8179.
- Roperch, Pierrick; Gattacceca, Jérôme; Valenzuela, Millarca; Devouard, Bertrand; Lorand, Jean-Pierre; Arriagada, Cesar; Rochette, Pierre; Latorre, Claudio; Beck, Pierre (2017). "Surface vitrification caused by natural fires in Late Pleistocene wetlands of the Atacama Desert". Earth and Planetary Science Letters. 469: 15–26. doi:10.1016/j.epsl.2017.04.009. ISSN 0012-821X.
- Kennett J (May 21, 2013). "Comprehensive Analysis of Impact Spherules Supports Theory of Cosmic Impact 12,800 Years Ago". UC Santa Barbara.
- Holliday VT (2011). "A Cosmic Catastrophe: The Great Clovis Comet Debate: A personal perspective on an Outrageous Hypothesis". Argonaut Archaeological Research Fund. Tucson, Arizona: Department of Anthropology at the University of Arizona, University of Arizona.
- Pringle H (2008). "Firestorm from space wiped out prehistoric Americans". The New Scientist. 194 (2605): 8–9.
- West A, Goodyear A (2008). "The Clovis Comet: Part I:Evidence for a Cosmic Collision 12,900 Years Ago" (PDF). Mammoth Trumpet. 23 (1): 1–4.
- "Younger Dryas Boundary: Extraterrestrial Impact or Not" (pdf). www.georgehoward.net. Retrieved 15 April 2012.
- Hoffman C (2 July 2008). "Exploding Asteroid Theory Strengthened by New Evidence Located in Ohio, Indiana". University of Cincinnati. Retrieved 5 August 2008.
- "Science & Environment: Diamond clues to beasts' demise". BBC NEWS. Retrieved 15 April 2012.
- "Sciency Thoughts: Evidence for a Younger Dryas impact event?". Retrieved 15 April 2012.
- "The Younger Dryas Impact Hypothesis". Scientific American Blog Network. Retrieved 15 April 2012.
- "New Clovis-Age Comet Impact Theory". Retrieved 15 April 2012.