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. 
One 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 been yet identified and no physical model by which a 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 claimed for an impact event includes carbon-rich layers of soil that have been found at some 50 Clovis sites across the continent. The proponents report that layers contain unusual materials (nanodiamonds, metallic microspherules, carbon spherules, magnetic spherules, iridium, charcoal, soot, and fullerenes enriched in helium-3) that they interpret as evidence of an impact event, at the very bottom of black mats of organic material that they say marks the beginning of the Younger Dryas, and is claimed that it cannot be explained by volcanic, anthropogenic, and other natural processes.
Recent 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.
Researchers have criticized the conclusions of various studies for incorrect age-dating of the sediments, contamination by modern carbon, and 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.
A 100-fold spike in the concentration of platinum has also been found in Greenland ice cores, dated to 12890 BP with 5 year accuracy. The source of the platinum has not yet been identified, but the researchers ruled out either earth's mantle or stony meteorites (chondrites). The researchers said the source could be from an iron-rich impactor that probably would have left a crater of "few kilometers" in diameter, but none has so far been identified.
Consequences of hypothetical impact
It is conjectured that this impact event brought about the extinction of many North American Pleistocene megafauna. These animals included camels, mammoths, the giant short-faced bear and numerous other species that the proponents suggest died 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 nano-diamonds 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 mainstream scientists for a variety of technical and professional reasons. Scientific skepticism increased with the revelation of documentation demonstrating misconduct and past criminal conduct (conviction for fraud and misrepresentation of credentials) by the researcher who prepared samples for the proponents of the hypothesis.
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.
A re-evaluation published by the original proponents in June 2013 of spherules from 18 sites worldwide is seen by them as supporting their hypothesis.
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, along with 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.
If it is assumed that the hypothesis supposes that all effects of the putative impact on Earth's biota would have been brief, all extinctions caused by the impact should have occurred simultaneously. However, there is much 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 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.
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."
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