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 as recent as 2017 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. The Hiawatha Glacier impact crater in Greenland is offered as evidence for the Younger Dryas impact hypothesis, due to its location and the speculative possibility that could be simultaneous with the start of the Younger Dryas cold period and megafauna extinctions which occurred approximately around the same era.
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.[excessive citations]:Details should be specified and individually cited
Critics of the hypothesis have repeatedly addressed the responses, and have published counterarguments.        [excessive citations]:Details should be specified and individually cited
- Hiawatha Glacier Crater
- Holocene extinction event
- Murray Springs Clovis Site
- Pleistocene megafauna
- Tollmann's hypothetical bolide
- Tunguska event
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