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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 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 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 document this impact around the world. For example, James Wittke ''et al''. document deposition of impact spherules 12,800 years ago across four continents, including Europe and the Middle East. == Evidence == 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,〔 News article in ''Nature''〕 and claim that this 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, there is 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. 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Younger Dryas impact hypothesis」の詳細全文を読む スポンサード リンク
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