Younger Dryas impact hypothesis

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 protegé 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.[9]

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."[10]

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, a phrase coined by psychic Edgar Cayce. It 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.[11]

In 2007, Firestone, West, and twenty-four other authors suggested that the impact event may have led to an immediate decline in human populations in North America at that time.[12]

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."[13] The first debate between proponents and skeptics was held at the 2008 Pecos Conference in Flagstaff, Arizona.[14]

In 2009, a paper in the Proceedings of the National Academy of Sciences offered evidence of impact glass that resulted from the impact of a meteorite.[6] Another asserted that a swarm of carbonaceous chondrites or comet fragments from air burst(s) or impact(s) 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.[15] Another article in Science reported evidence of nanodiamonds in a geological deposit thought to correspond to the event.[16] In the same issue, another article reported that the nanodiamonds were evidence for carbonaceous chondrites or comets at the start of Younger Dryas whose airbursts and surface impacts caused widespread damage.[15] A special debate-style session was convened at the 2009 AGU Fall Meeting in which skeptics and supporters alternated in giving presentations.[17]

In 2010 the American Quaternary Association held a debate between skeptics and supporters in Laramie, Wyoming.[18]

In 2011, a large 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.[19][20][21] The article claimed that no nanodiamonds were found[22] and that the supposed carbon spherules could be fungus or insect feces and included modern contaminants.[21][23] 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.[5] Skepticism increased with the revelation of documentation demonstrating misconduct and past criminal conduct.[lower-alpha 1][24]

In 2012, scientists reported 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 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. The results of the study have not been replicated by other researchers.[25] Lonsdaleite has also been made artificially in laboratories.[26][27]

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.[28] 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."[29] But they write that the large Pt anomaly "hints for an extraterrestrial source of Pt". This interpretation fails to support the 2007 YDIH because it is much more likely that the Greenland Pt anomaly was caused by a small local iron meteorite fall without any widespread consequences.[30]

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.[31] Also that year, an analysis of nanodiamond evidence failed to uncover lonsdaleite or a spike in nanodiamond concentration at the YDB.[32] 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."[33] 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."[33] The nanodiamond evidence was criticized further in 2017.[34]

In 2017, scientists reported a Pt anomaly dating at eleven continental sites dated to the Younger Dryas.[35]

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.[36] Two papers were published dealing with an "extraordinary biomass-burning episode" associated with the Younger Dryas Impact.[37][38]

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.[3] This included rare metallic spherules, melt glass and nanodiamonds thought to have been produced during airbursts or impacts.[3] 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.[3] 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.[39]

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.[40] 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 also was also found in cores from the site.[41][42]

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.[43] They suggested that the hypothesis would probably need to be revised.[44][45] A critique of the Buchanan paper concluded that these results were an insensitive, low-fidelity population proxy incapable of detecting demographic change.[46] The authors of a subsequent paper that described three approaches to population dynamics in the Younger Dryas in North America 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.[47] There is also no evidence of continent-wide wildfires at any time during terminal Pleistocene deglaciation,[48] though there is evidence that most larger wildfires had a human origin,[48] which calls into question the origin of the "black mat."[49] 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.[50]

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.[51][52][53] The extinction of woolly mammoths in Siberia also appears to have occurred later than in North America.[51] 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,[51][52][54] and the survival of ground sloths in the Antilles,[55] the Caribbean, until 4700 cal BP.[51] The Australian megafaunal extinctions occurred approximately 30,000 years earlier than the hypothetical Younger Dryas event.[56]

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.[57] 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.[51] 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,[58] possibly from anthropogenic activities, including hunting.[8]

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.[59]

Scientists have asserted that the carbon spherules originated as fungal structures and/or insect fecal pellets, and contained modern contaminants[21][23] and that the claimed nanodiamonds are actually misidentified graphene and graphene/graphane oxide aggregates.[22][60] 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.[61] Researchers have also found no extraterrestrial platinum group metals in the boundary layer, which is inconsistent with the hypothesized impact event.[62] 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.[63]

Analysis of fluvial sediments on Santa Rosa Island by another group also found no evidence of lonsdaleite, impact-induced fires, or extraterrestrial impact.[33] Research published in 2012 has shown that the so-called "black mats" are easily explained by typical earth processes in wetland environments.[20] 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.[20]

Researchers have also criticized the conclusions of various studies for incorrect age-dating of the sediments,[64] contamination by modern carbon, inconsistent hypothesis that made it difficult to predict the type and size of bolide,[65] lack of proper identification of lonsdaleite,[66] confusing an extraterrestrial impact with other causes such as fire,[67] and for inconsistent use of the carbon spherule "proxy".[68] Naturally occurring lonsdaleite has also been identified in non-bolide diamond placer deposits in the Sakha Republic.[27]

Using statistical analysis and modeling, Kennett and others[69] concluded that widely separated organic-rich layers, including black mats, were deposited synchronously across multiple continents as an indentifiable Younger Dryas boundary layer. In 2019, Jorgeson and others[70] 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.[70]

Proponents of the hypothesis have responded to defend their findings, disputing the accusation of irreproducibility or replicating their findings.[69][71][72][73][74][75]^{:Details should be specified and individually cited} Critics of the hypothesis have repeatedly addressed the responses, and have published counterarguments.[76][77][78][79][80][81][82][83][33][84]

Even though a 2018 paper that presented evidence for an impact crater of unknown age under the Hiawatha Glacier in Greenland did not suggest any connection to the Younger Dryas,[85] some scientists speculated without evidence about such a link in news reports.[86][87] 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.[87] Moreover, this has not yet been accepted as a confirmed impact crater. Christian Koeberl, an impact crater expert from the University of Vienna, was quoted in Popular Science saying: “The authors 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.”[88]