The Younger Dryas stadial, also referred to as the Big Freeze, was a geologically period of cold climatic conditions and drought that began in 10,800 BC (12,811 years ago). The cause of the Big Freeze has been a controversial subject. Nothing of the size, extent, or rapidity of the climate change has been experienced since. The Big Freeze replaced the forest land in Scandinavia with glacial tundra. It caused the level of snow accumulation in the mountains to increase and the North American Clovis culture disappeared after the event. The climate change is correlated with the extinction of Pleistocene megafauna.
A collection of geologists have claimed the Big Freeze was caused by the collapse of the North American ice sheets, while others have supported the Younger Dryas impact hypothesis. The impact hypothesis claims that a large air burst or impact event initiated the Younger Dryas cold period. The evidence discovered for an impact event includes a charred carbon-rich layer of soil that has been uncovered at some 50 Clovis-age sites across the North American continent. The layer contains unusual materials, including metallic microspherules, carbon spherules, magnetic spherules, iridium, charcoal, soot, and fullerenes enriched in helium. The material was found at the very bottom of the “black mat” of organic material that marks the beginning of the Younger Dryas period.
In January 2009, transmission electron microscopy evidence was recovered showing nanodiamonds in the Earth layer around the time of the Big Freeze. The evidence was published in the journal Science. The article suggests that the diamonds provide strong evidence for the Earth’s collision with a rare swarm of carbonaceous chondrites or comets at the onset of the Younger Dryas cool interval. The event produced multiple airbursts and possible surface impacts, with severe repercussions for plants, animals, and humans in North America. It has been suggested that this impact event brought about the extinction of North American large mammals, including camels, mammoths, the giant short-faced bear and numerous other species.
The evidence for an impact event in North America has been dismissed by most geologists and historians. Specialists have studied the claim and concluded that there was never such an impact, in particular because various physical signs can’t be found. A collection of the impact signatures have not been corroborated by independent tests. Of the twelve original lines of evidence, seven have proven to be non-reproducible. The hypothesis is no longer considered viable in the scientific community. However, it remains a controversial topic.