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The Younger Dryas impact hypothesis or Clovis comet hypothesis posits that fragments of a large , disintegrating asteroid or comet struck North America, South America, Europe, and western Asia around 12,850 years ago, coinciding with the beginning of the Younger Dryas cooling event. Multiple meteor air bursts and/or impacts are claimed to have produced the Younger Dryas boundary layer , depositing peak concentrations of platinum, high-temperature spherules, meltglass, and nanodiamonds, forming an isochronous datum at more than 50 sites across about 50 million km of Earth's surface. Some scientists have proposed that this event triggered extensive biomass burning, a brief impact winter and the Younger Dryas abrupt climate change, contributed to extinctions of late Pleistocene megafauna, and resulted in the end of the Clovis culture. The view is a minority, with the prevailing view being that the Younger Dryas was generated by an influx of glacial meltwater into the North Atlantic.