The Younger Dryas impact hypothesis, also known as the Clovis comet hypothesis, is one of the competing scientific explanations for the onset of the Younger Dryas cold period after the last glacial period. The hypothesis, which scientists continue to debate, proposes that the climate of that time was cooled by the impact or air burst of one or more comets.
The general hypothesis states that about 12,900 BP calibrated (10,900 14C uncalibrated) years ago, air burst(s) or impact(s) from a near-Earth object(s) set areas of the North American continent on fire, disrupted climate and caused the Quaternary extinction event in North America. This resulted in the extinction of most of the megafauna, and the rapid demise of the North American Clovis culture. The Younger Dryas ice age lasted for about 1,200 years before the climate warmed again. These events are also seen as part of the Holocene extinction phenomenon.
One or more big explosions may have occurred above or possibly on the Laurentide Ice Sheet in the region of the Great Lakes. Though no major impact crater has been identified, the proponents argue 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 wildfires would have suffered due to the disrupted ecologic relationships affecting the continent.
The impact of this postulated event (or series of events) goes beyond the Americas. A number of studies reported evidence of this impact around the world. For example, James Wittke et al. argued for the deposition of impact spherules 12,800 years ago across four continents, including Europe and the Middle East.
The evidence in favour of a meteoric impact initiating the Younger Dryas cooling period includes carbon-rich layers of soil have been found at some 50 Clovis sites across North America. Dalton and Wittke reported that layers containing anomalous materials (nanodiamonds, metallic microspherules, carbon spherules, magnetic spherules, iridium, charcoal, soot, and fullerenes enriched in helium-3) were discovered at the very bottom of the ‘black mats’ of organic material that mark the beginning of the Younger Dryas. It has been claimed that these anomalous materials cannot be explained by volcanic, anthropogenic, or other natural processes, and were therefore evidence of an impact event.
Prof Richard Firestone performed a similar analysis of seven ‘black mat’ sites and fifteen Carolina Bay sites, which all showed increased concentrations of carbon spherules, vitreous charcoal, magnetic microspherules, nanodiamonds, iridium, fullerenes and extraterrestrial helium, which are all indicative of an extraterrestrial impact and associated biomass burning at the onset to the Younger Dryas. Although Firestone did acknowledge that previous OSL dating of the Carolina Bays suggested that they predated the Younger Dryas. At the Carolina Bay sites vitreous charcoal and carbon spherules were concentrated in the upper layers of the Bay formations, and the dates derived were found to be much younger than the equivalent OSL dates. Carbon dating of the carbon spherules mostly gave dates in the future, suggesting an unusual enrichment of the 14C at the time these deposits were laid down. Carbon dating of the vitreous charcoal and charcoal gave dates from 8,450 BP to 510 in the future, again suggesting 14C enrichment. Firestone suggested that this enrichment was connected to the sudden 14C increase that has been observed at the beginning of the Younger Dryas 12.9 kyr ago (see his fig 9), which might suggest a link between these impact indicators and the Younger Dryas. In addition, a small piece of yellow pine wood was found to be vitrified on one side, which indicated exposure to a very brief temperature approaching 3,200 °C that ‘melted’ one side of the wood sample. Firestone suggested that this may have been caused by an intense high temperature shock-wave associated with an extraterrestrial impact.
Additional data supporting the synchronous nature of the black mats, plus the simultaneous and catastrophic nature of the American Megafauna and Clovis Man extinctions was published the following year by Prof Vance Haynes.
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. 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.
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. However, some large animals survived that time period.
The genesis of the Younger Dryas impact hypothesis (or early Holocene impact hypothesis as it was sometimes called) goes back to the 1930s. The first to highlight this possibility Melton, Schriever, MacCarthy and Prouty in the 1930s; they were followed by Wells, Boyce and Houry in the 1950s; Eyton and Parkhurst in the 1970s; and then Howard, West, Firestone and Kennett in the 2000s.
In his work on the Lehner Mammoth-Kill Site near Hereford, Arizona, Emil Haury found Clovis point artifacts buried by a distinctive black clay layer, which was named ‘Lehner swamp soil’. This black soil was associated with a subhumid climate and ponding. Prof Vance Haynes subsequently studied this phenomenon and renamed this strata the ‘black mat’. Over 60 geoarchaeological sites bridging the Pleistocene-Holocene transition (the last deglaciation) exhibit this ‘black mat’, which is a black organic-rich layer containing mollic paleosols, aquolls, and diatomites. This layer typically overlies strata within which the last remnants of the terminal Pleistocene megafauna are recorded, and it has been accurately dated to 12.9 ky ago. (Haynes quotes 10.9 kyr ago, uncalibrated.)
The full description and extension of this hypothesis was published in a 2006 book entitled The Cycle of Cosmic Catastrophes by Firestone et al. In the following year, a paper by the same authors suggested that the onset of Younger Dryas cooling, the extinction of the megafauna, and the appearance of a black mat, strongly suggested that these events were directly related. Additional data supporting the synchronous nature of the black mats, plus the simultaneous and catastrophic nature of the American Megafauna and Clovis Man extinctions was published the following year by Prof Vance Haynes (not to be confused with Prof. Gary Haynes). Haynes demonstrated that the catastrophic events that marked the beginning of the Younger Dryas cooling were almost instantaneous. However, Haynes remained skeptical of the meteoric impact hypothesis as the cause of the Younger Dryas and the megafaunal extinction, but concluded by saying: “However, I reiterate, something major happened at 10,900 B.P. (14C uncalibrated) that we have yet to understand.”
In 2009 further 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.
James Kennett, UC Santa Barbara, May 21, 2013, “Comprehensive Analysis of Impact Spherules Supports Theory of Cosmic Impact 12,800 Years Ago”
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Simon Redfern (2013-08-01). “Ice core data supports ancient space impact idea”. BBC.
Michail I. Petaev, Shichun Huang, Stein B. Jacobsen, Alan Zindler (2013). “Large Pt anomaly in the Greenland ice core points to a cataclysm at the onset of Younger Dryas”. Proc. Natl. Acad. Sci. U.S.A. 110 (32): 12917–12920.
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