Younger Dryas impact hypothesis

The Younger Dryas impact hypothesis (YDIH) proposes that the onset of the Younger Dryas (YD) cool period (stadial) at the end of the Last Glacial Period, around 12,900 years ago was the result of some kind of extraterrestrial event with specific details varying between publications. The hypothesis is controversial and not widely accepted by relevant experts.

It is an alternative to the long-standing and widely accepted explanation that it was caused by a significant reduction in, or shutdown of the North Atlantic Conveyor due to a sudden influx of freshwater from Lake Agassiz and deglaciation in North America. A 1997 analysis suggested that to create continent-wide damage a 4 km  comet  direct impact would be required, or that the same damage could be caused by a smaller disintegrating comet airburst. In 2007, the first YDIH paper speculated that a comet airburst over North America  created a Younger Dryas boundary (YDB) layer; however, inconsistencies have been identified in other published results. And authors have not yet responded to requests for clarification and have never made their raw data available. Some YDIH proponents have also proposed that this event triggered extensive biomass burning, a brief impact winter that destabilized the Atlantic Conveyor and triggered the Younger Dryas instance of abrupt climate change, contributed to extinctions of late Pleistocene megafauna, and resulted in the disappearance of the Clovis culture.

Comet research group
The Comet research group (CRG), dedicated to investigating the YDIH, was established in 2016. The credibility and motivations of individual CRG researchers have been questioned by critics of the impact hypothesis, including their specific claims for evidence in support of the YDIH and/or the effects of meteor air bursts or impact events on ancient settlements, people, and environments. Doubts have been raised about several of the CRG's other claims.; for example a 2021 paper suggested that a Tunguska-sized or larger airburst destroyed Tall el-Hammam, a Middle Bronze Age city located in the Jordan Valley near the Dead Sea around 1650 BCE. Image forensics expert Elisabeth Bik discovered evidence for digital alteration of images used as evidence for the claim that the village of Tall el-Hammam was engulfed by an airburst. CRG members initially denied tampering with the photos but eventually published a correction in which they admitted to inappropriate image manipulation. Five of the paper's 53 images received retouching to remove labels and arrows present in other published versions of the photos, which Bik believed to be a possible conflict with Scientific Reports' image submission guidelines but was not in itself a disproval of the Tall el-Hammam airburst theory. Subsequent concerns that have been brought up in PubPeer have not yet been addressed by the CRG, including discrepancies between claimed blast wave direction compared to what the images show, unavailability of original image data to independent researchers, lack of supporting evidence for conclusions, inappropriate reliance on young Earth creationist literature, misinformation about the Tunguska explosion, and another uncorrected example of an inappropriately altered image. On February 15, 2023, the following editor’s note was posted on this paper, "Readers are alerted that concerns raised about the data presented and the conclusions of this article are being considered by the Editors. A further editorial response will follow the resolution of these issues." On August 30, 2023, a paper authored by a CRG member and leading YDIH advocate was retracted by Scientific Reports. The journal's Retraction Note cited a publication "indicating that the study does not provide data to support the claims of an airburst event or that such an event led to the decline of the Hopewell culture."

Evidence
Proponents believe that certain microscopic debris is evidence of impact and that "black mats" of sediment are evidence of widespread fires. They contend that extinction of megafauna was synchronous with associated effects on prehistoric human societies. They say that their observations and interpretations cannot be adequately explained by volcanic, anthropogenic, or other natural processes. They argue that there is a synchronous Younger Dryas boundary layer that should be used as a local, or even global stratigraphic marker. Archaeologist Stuart J Fiedel has remarked that "The bolide and its effects have been characterized inconsistently from one paper to the next, which makes this hypothesis difficult to refute." In 2011, a review of the evidence led researchers to state "The YD impact hypothesis provides a cautionary tale for researchers, the scientific community, the press, and the broader public." as "none of the original YD impact signatures have been subsequently corroborated by independent tests. Of the 12 original lines of evidence, seven have so far proven to be non-reproducible. The remaining signatures instead seem to represent either (1) non-catastrophic mechanisms, and/or (2) terrestrial rather than extraterrestrial or impact-related sources. In all of these cases, sparse but ubiquitous materials seem to have been misreported and misinterpreted as singular peaks at the onset of the YD. Throughout the arc of this hypothesis, recognized and expected impact markers were not found, leading to proposed YD impactors and impact processes that were novel, self-contradictory, rapidly changing, and sometimes defying the laws of physics." Additionally, a comprehensive refutation of the Younger Dryas Impact Hypothesis was published in 2023, stating "There is no support for the basic premise of the YDIH that human populations were diminished, and individual species of late Pleistocene megafauna became extinct or were diminished due to catastrophe. Another example is that of extensive wildfires claimed by some YDIH proponents that has been refuted by experts.   "Evidence and arguments purported to support the YDIH involve flawed methodologies, inappropriate assumptions, questionable conclusions, misstatements of fact, misleading information, unsupported claims, irreproducible observations, logical fallacies, and selected omission of contrary information."

Hypothetical impact markers
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. One of the most widely publicized discoveries (nanodiamonds in Greenland) has never been verified and is disputed.

Some 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. A patent application by Allen West and James Kennett in 2009 for methods of forming nanodiamonds based on research in support of the impact hypothesis also likely misidentified Copper and Copper oxides and appears to have since been abandoned. 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. An independent group of researchers reported much lower concentrations of platinum group metals in the purported boundary layer (by a factor of 30 for iridium). The original authors argued that these concentrations were still >300% (a factor of 3) above background in 2 of their samples. Another group was unable to confirm prior claims of magnetic particles and microspherules in 2009, Other studies involving YDIH proponents found concentrations of magnetic spherules but not all were associated with the YDB and not all were attributed to an ET event.

"Black mats"
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 about 50 archaeological sites across North America. Using statistical analysis and modeling, James P. 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.

Marlon et al. suggest that wildfires were a consequence of rapid climate change. "The changes in woody biomass, fire frequency, and biomass burning are not coincident with changes in CO2, although increasing CO2 may have contributed to woody biomass production during the early part of the Bølling–Allerød. Clovis people appeared in North America between 13.4 and 12.8 ka, broadly coincident with the sharp increase in biomass burning at 13.2 ka, and then rapidly spread out across the continent."

Radiocarbon dating, microscopy of paleobotanical samples, and analytical pyrolysis of fluvial sediments in Arlington Canyon on Santa Rosa Island by another group found no evidence of lonsdaleite or impact-induced fires. Research published in 2012 has shown that the so-called "black mats" are easily explained by typical earth processes in wetland environments. This 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, they likely arise from processes common to wetland systems and 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.

Extinction of megafauna
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 of 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 a 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.

Impact on human societies
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. A 2022 study by an independent group presents genomic evidence that a previously unidentified pre-18,000 BP South American population suffered a major disruption at the Younger Dryas onset, resulting in a significant loss of lineages and a Y chromosome bottleneck.

Hiawatha crater
A 2018 paper reported the discovery of an impact crater under the Hiawatha Glacier in Greenland of unknown age. Kurt Kjær, the lead author of the paper, speculated that it might date to the Pleistocene (2.58 million to 11,700 years ago), and mentioned a possible connection to the Younger Dryas.

However, in 2022 the crater was dated to around 58 million years ago, the late Paleocene, using Argon–argon dating combined with uranium–lead dating of shocked zircon crystals.

Other explanations
A number of other hypotheses have been put forward about the cause of the Younger Dryas climate event.

Mainstream explanation
The most widely accepted explanation is that it began because of a significant reduction or shutdown of the North Atlantic "Conveyor" – which circulates warm tropical waters northward – as the consequence of deglaciation in North America. Geological evidence for such an event is not fully secure, but recent work has identified a pathway along the Mackenzie River that would have spilled fresh water from Lake Agassiz into the Arctic and thence into the Atlantic. The global climate would then have become locked into the new state until freezing removed the fresh water "lid" from the North Atlantic.

Other alternatives
Although initially sceptical, Wallace Broecker—the scientist who proposed the conveyor shutdown hypothesis—eventually agreed with the idea of an extraterrestrial impact at the Younger Dryas boundary, and thought that it had acted as a trigger on top of a system that was already approaching instability.

Another hypothesis suggests instead that the jet stream shifted northward in response to the melting of the North American ice sheet, which brought more rain to the North Atlantic, which freshened the ocean surface enough to slow the thermohaline circulation.

Another proposed cause has been volcanic activity. However, this 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."

History
The idea that a comet struck North America at the end of the last ice age was first proposed as a speculative premise by the American congressman and pseudohistorian Ignatius Donnelly in 1883, who suggested it formed the Great Lakes and caused a sudden extreme cold period, which devastated animal and human populations.

In 2001, Richard Firestone and William Topping published their first version of the YDIH, "Terrestrial Evidence of a Nuclear Catastrophe in Paleoindian Times" in Mammoth Trumpet, a newsletter of the Center for the Study of the First Americans. They proposed that "the entire Great Lakes region (and beyond) was subjected to a particle bombardment and a catastrophic nuclear radiation..." They argue that this cataclysm generated a shock wave that gouged out the Carolina Bays and reset the radiocarbon clock. Most geologists today interpret the Carolina bays as relict geomorphological features that developed via various eolian and lacustrine processes. Multiple lines of evidence, e.g. radiocarbon dating, optically stimulated luminescence dating, and palynology, indicate that the Carolina bays predate the start of the Holocene. Fossil pollen recovered from cores of undisturbed sediment taken from various Carolina bays in North Carolina by Frey, Watts, and Whitehead  document the presence of full glacial pollen zones within the sediments filling some Carolina bays. The range of dates can be interpreted that Carolina bays were either created episodically over the last tens of thousands of years or were created at time over a hundred thousand years ago and have since been episodically modified. Recent work by the U.S. Geological Survey has interpreted the Carolina bays as relict thermokarst lakes that have been modified by eolian and lacustrine processes. Modern thermokarst lakes are common today around Barrow (Alaska), and the long axes of these lakes are oblique to the prevailing wind direction.

In 2006, 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, was 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. Since this paper was considered too controversial for standard peer review, it was handled by a specially selected 'personal editor' who was friendly to the hypothesis.

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, papers by Kerr and Kennett 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 published a model suggesting 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.

In 2011, Pinter and others challenged the Younger Dryas impact hypothesis on the basis 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, Daulton stated that no nanodiamonds were found and that the supposed carbon spherules could be fungus or insect feces and included modern contaminants as stated by Boslough and others and Roach. In response, in June 2013 Wittke and others published a re-evaluation of spherules from eighteen sites worldwide that they interpret as supporting their hypothesis.

In 2012, a paper by Bunch and others reported the discovery of scoria like objects (SLO) and stated that they were consistent with a extraterrestrial impact or airburst. Post-publication review of this paper suggests that at least some of these SLOs are anthropogenic. 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, Petaev and others reported a hundredfold spike in the concentration of platinum in Greenland ice cores roughly dated to 12,890 YBP. This anomaly was attributed to a small local iron meteorite fall without any widespread consequences. A refutation of the YDIH, by Holliday and others, showed that the Pt spike was not evidence to support the YDIH because it occurred 20 years after the YDB.

In 2016, Holiday and others reported 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, Daulton and others reported an analysis of nanodiamond evidence failed to uncover lonsdaleite or a spike in nanodiamond concentration at the YDB.

In 2017, C R Moore and others reported a Pt anomaly at eleven continental sites dated to the Younger Dryas, which is linked with the Greenland Platinum anomaly.

In 2018, dealing with an "extraordinary biomass-burning episode" associated with the Younger Dryas Impact were reported by Wolbach and others and Lynch. However, these claims of extraordinary fires are disputed by Holliday and others with a response by Wolbach.

In 2019, Pino and others 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, CR Moore and others reported 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, Thackery and others reported that a ~10 ppb platinum (Pt) enrichment in peat deposits at Wonderkrater in South Africa was associated with the YDB, although the age uncertainty range of the anomaly exceeded 2 thousand years.

In 2019 research at White Pond near Elgin, South Carolina, conducted by CR Moore from the University of South Carolina and 16 colleagues, 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. However, samples from the site no longer exist so these results cannot be confirmed.

In 2022, a paper by geologist James L. Powell, a YDIH proponent, claimed that opponents had prematurely rejected YDIH, detailing the example of research published by Firestone and others in 2001 and the inability of a later study by Surovell and others in 2009 that was unable to reproduce these results leading a number of other scientists to reject YDIH. Powell argues that since then, many independent studies have reproduced that evidence at dozens of YD sites.

A March 2023 article by planetary impact physicist Mark Boslough and YDIH opponent stated that "...the YDIH has never been accepted by experts in any related field" because it is "plagued by self contradictions, logical fallacies, basic misunderstandings, misidentified impact evidence, abandoned claims, irreproducible results, questionable protocols, lack of disclosure, secretiveness, failed predictions, contaminated samples, pseudoscientific arguments, physically impossible mechanisms, and misrepresentations".

In July 2023 Holliday and others published a comprehensive refutation of the YDIH that collected and summarized many of the positions from opponents to YDIH publications mentioned in the above history. Sections in this article refute the areas of evidence regarding Hypothetical impact markers, "Black mats,"  Extinction of megafauna,  Impact on human societies,  the Hiawatha crater. Also criticized were fundamental assumptions, flawed sampling,  inadequate dating,  Pseudoarchaeological divined date of the impact event,  pseudoscience (fringe) evidence and conjecture,  issues with other YDIH claims, such as the Carolina bays,  contradictory results when different groups have examined the same sample specimens,  and unparalleled promotion of YDIH outside of scientific literature. The paper also responded to and critiqued assertions from Powell. The paper concludes that since "YDIH evolved directly from pseudoscience, the initial publication in scientific literature was seriously plagued by poorly documented interpretations and baseless assertions." and lists 11 serious flaws that persist in YDIH.

In a December 2023 article by CR Moore and others stated that "anomalous peak abundances of platinum and Fe-rich microspherules with high-temperature minerals have previously been demonstrated to be a chronostratigraphic marker for the lower Younger Dryas Boundary (YDB) dating to 12.8 ka," was found in sediments at Wakulla Springs, Florida. "The study confirms the utility of this YDB datum layer for intersequence correlation and for assessing relative ages of Paleoamerican artifacts, including those of likely Clovis, pre-Clovis, and post-Clovis age and their possible responses to environmental changes known to have occurred during the Younger Dryas cool climatic episode."

In popular culture
The impact hypothesis has been the subject of documentaries, including Mammoth Mystery on National Geographic Explorer (2007), Journey to 10,000 BC on the History Channel (2008), Survival Earth on Channel 4 (2008), and Megabeasts' Sudden Death on PBS Nova (2009).

Graham Hancock argued in his 2015 book Magicians of the Gods that the Younger Dryas comet destroyed the earth in a time cycle and that it was responsible for the Noahide flood myth. He inferred that this myth was widespread elsewhere on earth by comparing it with the flood mythology of other peoples. These claims were criticized as inaccurate by independent reviewers, including Jason Colavito, Michael Shermer, and Marc J. Defant. Hancock expanded on his claims in a subsequent book, America Before: The Key to Earth's Lost Civilization (2019), in which he claimed that the Younger Dryas catastrophe had wiped out all traces of a sophisticated Ice Age civilization in North America.

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.

A 2021 episode of the Science Channel series Ancient Unexplained Files had a segment on the evidence from Abu Hureyra; geoscientist Sian Proctor also described the impact hypothesis as a whole.

In 2022 Graham Hancock presented in a Netflix series titled Ancient Apocalypse, with Episode 8 specifically covering the YDIH. In March 2023 Mark Boslough published a commentary in Skeptic magazine with conclusion that many attributes of the series are pseudoscience.