Younger Dryas impact hypothesis

The Younger Dryas impact hypothesis or Clovis comet hypothesis posits that fragments of a large (more than 4 kilometers in diameter), disintegrating asteroid or comet struck North America, South America, Europe, and western Asia about 12,800 years ago. Multiple airbursts/impacts produced the Younger Dryas (YD) boundary layer (YDB), depositing peak concentrations of platinum, high-temperature spherules, meltglass, and nanodiamonds, forming an isochronous datum at more than 50 sites across about 50 million km² of Earth’s surface. Some scientists have proposed that this event triggered extensive biomass burning, a brief impact winter, the Younger Dryas abrupt climate change, contributed to extinctions of late Pleistocene megafauna, and resulted in the end of the Clovis culture. 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 dozens of Clovis culture archaeological sites in North America. Proponents have reported materials including nanodiamonds, metallic microspherules, carbon spherules, magnetic spherules, iridium, platinum, 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.. Proponents of the hypothesis claim that these data cannot be adequately explained by volcanic, anthropogenic, or other natural processes. It is hypothesized 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 have contributed to the transition from Clovis culture to subsequent patterns. This supposed event is claimed to have triggered extensive biomass burning, a brief impact winter, and an abrupt climate change. The original hypotheses about a comet impact that had a widespread effect on human populations can be attributed to Edmund Halley, who in 1694 suggested that a worldwide flood had been the result of a near-miss by a comet. The issue was taken up in more detail by William Whiston, a protegé of and popularizer of the theories of Isaac Newton, who argued in his book A New Theory of the Earth (1696) that a comet impact was the probable cause of the Biblical Flood of Noah. Whiston also attributed the origins of the atmosphere and other significant changes in the Earth to the effects of comets. This hypothesis was subsequently popularized by Minnesota congressman and pseudoarchaeology writer Ignatius L. Donnelly in his book Ragnarok: The Age of Fire and Gravel (1883), which followed his better-known book Atlantis: The Antediluvian World (1882). In Ragnarok, Donnelly argued that an enormous comet struct the earth approximately 12,000 years ago, destroying an advanced civilization on the 'lost continent' of Atlantis. Donnelly, following Halley and Whiston, attributed the Biblical Flood to this event, which he hypothesized had also resulted in catastrophic fires and significant climate change. Shortly after the publication of Ragnarok, one commenter noted, 'Whiston ascertained that the deluge of Noah came from a comet's tail; but Donnelly has outdone Whiston, for he has shown that our planet has suffered not only from a cometary flood, but from cometary fire, and a cometary rain of stones.' In 2006, this hypothesis was revived in 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 published by Inner Traditions - Bear & Company and marketed in the category of Earth Changes, a phrase coined by psychic Edgar Cayce. It 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. In 2007, Firestone, West, and twenty-four other authors suggested that the impact event may have led to an immediate decline in human populations in North America at that time. 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 B.P. (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. http://www.swanet.org/2008_pecos_conference/related.html