The Younger Dryas impact hypothesis (YDIH) or Clovis comet hypothesis posits that fragments of a large (more than 4 kilometers in diameter), disintegrating asteroid or comet struck North America, South America, Europe, and western Asia around 12,800 years ago. Multiple meteor air bursts and/or impacts produced the Younger Dryas (YD) boundary layer (YDB), depositing peak concentrations of platinum, high-temperature spherules, meltglass, and nanodiamonds, forming an isochronous datum at more than 50 sites across about 50 million km2 of Earth's surface. Some scientists have proposed that this event triggered extensive biomass burning, a brief impact winter and the Younger Dryas abrupt climate change, contributed to extinctions of late Pleistocene megafauna, and resulted in the end of the Clovis culture.
Evidence
The evidence given by proponents of a bolide or meteorite impact event includes "black mats", or strata of organic-rich soil that have been identified at over 50 archaeological sites across four continents, primarily in North America and Greenland. Proponents have reported materials including nanodiamonds, metallic microspherules, carbon spherules, magnetic spherules, iridium, platinum, platinum/palladium ratios, charcoal, soot, and fullerenes enriched with helium-3 that they interpret as evidence for an impact event that marks the beginning of the Younger Dryas. Proponents of the hypothesis claim that these data cannot be adequately explained by volcanic, anthropogenic, or other natural processes. It has been suggested that this boundary layer should be used as a local stratigraphic marker.
Consequences of such an impact
It is hypothesized 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 have contributed to the transition from Clovis culture to subsequent patterns. This supposed event is claimed to have triggered extensive biomass burning, a brief impact winter, and an abrupt climate change.
History of the hypothesis
Early speculative hypotheses
The Younger Dryas was first recognized in Denmark in 1901 by Hartz and Mithers, although the term was not created until 1912, Hartz. However, there were several speculative hypotheses connecting cometary impacts with climatic events dating to about the correct time (circa 10000 BCE) that predate the discovery of the Younger Dryas. The original hypotheses about a comet impact that had a widespread effect on human populations can be attributed to Edmond Halley, who in 1694 suggested that a worldwide flood had been the result of a near-miss by a comet. The issue was taken up in more detail by William Whiston, a protégé of and popularizer of the theories of Isaac Newton, who argued in his book A New Theory of the Earth (1696) that a comet impact was the probable cause of the Biblical Flood of Noah. Whiston also attributed the origins of the atmosphere and other significant changes in the Earth to the effects of comets.
This hypothesis was subsequently popularized by Minnesota congressman and pseudoarchaeology writer Ignatius L. Donnelly in his book Ragnarok: The Age of Fire and Gravel (1883), which followed his better-known book Atlantis: The Antediluvian World (1882). In Ragnarok, Donnelly argued that an enormous comet struck the Earth approximately 12,000 years ago, destroying an advanced civilization on the "lost continent" of Atlantis. Donnelly, following Halley and Whiston, attributed the Biblical Flood to this event, which he hypothesized had also resulted in catastrophic fires and significant climate change. Shortly after the publication of Ragnarok, one commenter noted, "Whiston ascertained that the deluge of Noah came from a comet's tail; but Donnelly has outdone Whiston, for he has shown that our planet has suffered not only from a cometary flood, but from cometary fire, and a cometary rain of stones."
Scientific impact hypothesis
In 2006, this hypothesis was revived in The Cycle of Cosmic Catastrophes: How a Stone-Age Comet Changed the Course of World Culture, a trade book by Richard Firestone, Allen West and Simon Warwick-Smith published by Inner Traditions – Bear & Company and marketed in the category of Earth Changes. It proposed that a large meteor air burst or impact of one or more comets initiated the Younger Dryas cold period about 12,900 BP calibrated (10,900 14C uncalibrated) years ago.
In May 2007, at a meeting of the American Geophysical Union in Acapulco, Firestone, West, and around twenty other scientists made their first formal presentation of the hypothesis. Later that year, the group published a paper in the Proceedings of the National Academy of Sciences (PNAS) that suggested the impact event may have led to an immediate decline in human populations in North America.
In 2008, C. Vance Haynes Jr. published data to support the synchronous nature of the black mats, emphasizing that independent analysis of other Clovis sites was required to support the hypothesis. He was skeptical of the bolide impact as the cause of the Younger Dryas and associated megafauna extinction but concluded "... something major happened at 10,900 YBP (14C uncalibrated) that we have yet to understand." The first debate between proponents and skeptics was held at the 2008 Pecos Conference in Flagstaff, Arizona.
In 2009, a paper in the journal Science asserted that nanodiamonds were evidence for a swarm of carbonaceous chondrites or comet fragments from air burst(s) or impact(s) that set parts of North America on fire, caused the extinction of most of the megafauna in North America, and led to the demise of the Clovis culture. A special debate-style session was convened at the 2009 AGU Fall Meeting in which skeptics and supporters alternated in giving presentations.
In 2010, astronomer William Napier presented evidence that fragments of a comet—initially 50 to 100 kilometers in diameter—could have been responsible for such an impact, and that the Taurid complex is formed of the remaining debris. Napier refined this model and published further research in 2019.
In 2010 the American Quaternary Association held a debate between skeptics and supporters in Laramie, Wyoming.
In 2011, a group of scientists challenged the Younger Dryas impact hypothesis on the basis of claims that most of the conclusions could not be reproduced and were a misinterpretation of data. Skepticism increased when it was reported that one of the lead authors of the original paper had practiced geophysics without a license. Around that time, other articles claimed that no nanodiamonds were found and that the supposed carbon spherules could be fungus or insect feces and included modern contaminants. In response, in June 2013 some of the original proponents published a re-evaluation of spherules from eighteen sites worldwide that they interpret as supporting their hypothesis.
In 2012, another paper in PNAS offered evidence of impact glass that resulted from the impact of a meteorite. Another group of scientists reported evidence supporting a modified version of the hypothesis—involving a fragmented comet or asteroid—was found in lake bed cores dating to 12,900 YBP from Lake Cuitzeo in Guanajuato, Mexico. It 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. Lonsdaleite has also been made artificially in laboratories.
In 2013, scientists reported a hundredfold spike in the concentration of platinum in Greenland ice cores that are dated to 12,890 YBP with 5 year accuracy. They attribute this platinum anomaly to the likely impact of a large (~0.8 km) iron-rich meteorite locally onto Greenland's ice, which would have been "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". An alternative suggestion is that the Greenland Pt anomaly was caused by a small local iron meteorite fall without any widespread consequences, but this is disputed by the paper's authors who claim that a global platinum anomaly is expected due to the ~ 20 year lifetime of the platinum signal.
In 2016, a report on further analysis of Younger Dryas boundary sediments at nine sites found no evidence of an extraterrestrial impact at the Younger Dryas boundary. Also that year, 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."
In 2017, scientists reported a Pt anomaly dating at eleven continental sites dated to the Younger Dryas, which is linked with the Greenland Platinum anomaly.
In 2018, some researchers interpreted the undated Hiawatha Glacier impact crater in Greenland as evidence for the Younger Dryas impact event due to its location. Two papers were published dealing with an "extraordinary biomass-burning episode" associated with the Younger Dryas Impact.
In 2019, scientists reported evidence in sediment layers with charcoal and pollen assemblages both indicating major disturbances at Pilauco Bajo, Chile in sediments dated to 12,800 BP. This included rare metallic spherules, melt glass and nanodiamonds thought to have been produced during airbursts or impacts. Pilauco Bajo is the southernmost site where evidence of the Younger Dryas impacts has been reported. This has been interpreted as evidence that a strewn field from the Younger Dryas impact event may have affected at least 30% of Earth's radius. Also in 2019, analysis of age-dated sediments from a long-lived pond in South Carolina showed not just an overabundance of platinum but a platinum/palladium ratio inconsistent with a terrestrial origin, as well as an overabundance of soot and a decrease in fungal spores associated with the dung of large herbivores, suggesting large-scale regional wildfires and at least a local decrease in ice age megafauna.
In 2019, a South African team consisting of Francis Thackeray, Louis Scott and Philip Pieterse announced the discovery of a platinum (Pt) spike in peat deposits at Wonderkrater, an artesian spring site in South Africa in the Limpopo Province, near the town of Mookgophong (formerly Naboomspruit) situated between Pretoria and Polokwane. The spike in platinum was documented in a sample dated at 12,744 years BP (calibrated) preceding a decline in a paleo-temperature index based on multivariate analysis of pollen spectra. This drop in temperature is associated with the Younger Dryas. The Wonderkrater platinum spike is in marked contrast to the almost constant low Pt concentrations in adjacent levels. It is consistent with the Younger Dryas Impact Hypothesis and is the first of its kind in Africa, supplementing evidence for platinum anomalies at more than 25 other sites in the world.
The platinum spike at the South African site has been interpreted in terms of global dispersal of platinum-rich dust at the time of the hypothesized asteroid impact, potentially associated with a crater of the kind found beneath the Hiawatha Glacier in Greenland. Thackeray and his colleagues recognise that Terminal Pleistocene megafaunal extinctions in southern Africa (Megalotragus priscus, Syncerus antiquus and Equus capensis) may be attributed to both environmental change and human predation within a period of time before and after 12,800 cal yr BP. However, on the basis of data presented in their study, they state that the consequences of a hypothesised YD cosmic impact (including the dispersal of atmospheric dust) may have contributed to some extent to the process of extinctions not only in southern Africa, but also to that which occurred in North and South America as well as Europe, recognising synchroneity of Pt anomalies that has been cited in support the Younger Dryas Impact Hypothesis. It is noted that in parts of South Africa, the Robberg stone tool technology terminates at about 12,800 cal yr BP, co-terminus with the termination of the Clovis technocomplex in North America, but further work is required to assess this coincidence.
In 2019 research at White Pond near Elgin, South Carolina, conducted by Christopher Moore and 16 colleagues from the University of South Carolina, used a core to extract sediment samples from underneath the pond. The samples, dated by radiocarbon to the beginning of the Younger Dryas, were found to contain a large platinum anomaly, consistent with findings from other sites, A large soot anomaly was also found in cores from the site.
In 2020, a group led by Andrew M. T. Moore found high concentrations of iridium, platinum, nickel, and cobalt at the Younger Dryas boundary in material from Tell Abu Hureyra. They concluded that the evidence supports the impact hypothesis.
In James L. Powell's 2020 book Deadly Voyager: The Ancient Comet Strike that Changed Earth and Human History, he pointed out that "the microspherule evidence reported by [Firestone et al. 2007] has been reproduced by more than two dozen authors at nearly as many [Younger Dryas boundary] sites. It is eminently reproducible". Powell compared the hesitancy in accepting the hypothesis to other initially controversial ideas such as the Alvarez hypothesis (mass extinction of the dinosaurs by an asteroid impact). Geologist Marc J. Defant, who was previously skeptical of the YDIH, stated that "It is a superb book and has absolutely convinced me there were comet airbursts at the Younger Dryas."
In 2021, Martin B. Sweatman, authored a paper published in Earth-Science Reviews, entitled "The Younger Dryas impact hypothesis: Review of the impact evidence". After reviewing the abundant platinum, microspherule, nanodiamond, melt-glass, and soot/charcoal evidence at the Younger Dryas boundary, including its reproducibility and synchroneity on several continents and the cometary scenario for its production, he suggests that the impact hypothesis should now be referred to as a 'theory'. Notably, he concludes "... arguments by a small cohort of researchers against their claims of a major impact are, in general, poorly constructed, and under close scrutiny most of their evidence can actually be interpreted as supporting the impact hypothesis."
Criticism
Chronology and age-dating
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. A critique of the Buchanan paper concluded that these results were an insensitive, low-fidelity population proxy incapable of detecting demographic change. The authors of a subsequent paper described three approaches to population dynamics in the Younger Dryas in North America, and concluded that there had been a significant decline and/or reorganisation in human population early in this period. The same paper also shows an apparent resurgence in population and/or settlements in the later Younger Dryas.
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.
A group in the Netherlands 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 at the Atacama Desert in Chile showed that silicate surface glasses were formed during at least two distinct periods at the end of the Pleistocene, separated by several hundred years.
Disputed evidence
Claims for impact debris
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. 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. 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 found no extraterrestrial platinum group metals in the boundary layer, which is inconsistent with the hypothesized impact event. Further independent analysis was unable to confirm prior claims of magnetic particles and microspherules.
Evidence for widespread fires
Marlon et al. suggest that wildfires were a consequence of rapid climate change. Analysis of fluvial sediments on Santa Rosa Island by another group 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.
Researchers have also 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.
Dating of boundary layers
Using statistical analysis and modeling, Kennett and others concluded that widely separated organic-rich layers, including black mats, were deposited synchronously across multiple continents as an identifiable Younger Dryas boundary layer. In 2019, Jorgeson and others tested this conclusion with the simulation of radiocarbon ages. They accounted for measurement error, calibration uncertainty, "old wood" effects, and laboratory measurement biases, and compared against the dataset of radiocarbon ages for the Laacher See eruption. They found the Laacher See 14C dataset to be consistent with expectations of synchroneity. They found the Younger Dryas boundary layer 14C dataset to be inconsistent with the expectations for its synchroneity, and the synchronous global deposition of the hypothesized Younger Dryas boundary layer to be extremely unlikely.
Reproducibility of results
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 have published counterarguments.
Impact crater in Greenland
A 2018 paper presented evidence for an impact crater of unknown age (some point during the Pleistocene) under the Hiawatha Glacier in Greenland. Kurt Kjær, the lead author of the paper, mentioned that the team had considered a link to the Younger Dryas impact but dropped the idea in the final paper because of the controversy around the hypothesis. Other scientists also speculated about such a link in news reports. Skeptics reject this connection because it would require an improbably recent impact—an impact of this size should occur only once every few million years—and it would leave evidence, such as a young ejecta blanket.
Christian Koeberl, an impact crater expert from the University of Vienna, has disagreed with the assessment of the site. He was quoted in Popular Science saying: "[Kjær et al.] report on some interesting phenomena, but the 'definitive' interpretation and conclusion that a large impact crater underneath the ice has been discovered is a severe over-interpretation of the existing data."
Alternative hypotheses
Other hypotheses have been proposed to explain the onset of the Younger Dryas, with the most accepted one being that it was caused by a significant reduction or shutdown of the North Atlantic "Conveyor" in response to a sudden influx of fresh water from Lake Agassiz and deglaciation in North America. However, Wallace Broecker, the scientist who originally proposed the conveyor shutdown hypothesis, also ended up accepting the idea of an extraterrestrial impact at the Younger Dryas boundary. Although Broecker disagreed with the premise that the impact caused extinctions, he did come to believe that it acted as a trigger on top of a system that was already approaching instability.
Another proposed cause has been volcanic activity. However, this too has been challenged recently due to improved dating of the most likely suspect, the Laacher See volcano. In 2021, research by Frederick Reinig et al. precisely dated the eruption to 200 ± 21 years before the onset of the Younger Dryas, therefore ruling it out as a culprit. The same study also concluded that the onset took place synchronously over the entire North Atlantic and Central European region. A press release from the University of Mainz stated, "Due to the new dating, the European archives now have to be temporally adapted. At the same time, a previously existing temporal difference to the data from the Greenland ice cores was closed."
In popular culture
The impact hypothesis has been the subject of a number of documentaries, including Mammoth Mystery on National Geographic Explorer (2007), Journey to 10,000 BC on the History Channel (2008), and Megabeasts' Sudden Death on PBS Nova (2009).
Graham Hancock's 2015 pseudoarchaeology book Magicians of the Gods argued that the Younger Dryas comet destroys the earth in a time cycle and that it was responsible for the Noahide flood myth, then universalizes the myth by comparing it with that of other peoples. These claims were criticized for their inaccuracy by various independent reviewers, including Jason Colavito and Marc J. Defant.
In 2017, a debate was held on the Joe Rogan Experience between proponents Graham Hancock, Randall Carlson, and Malcolm A LeCompte, and opponents Michael Shermer, and Marc J. Defant. The week that the podcast was released, the network was reportedly averaging over 120 million downloads a month.
