Evidence
The evidence claimed for an impact event includes a charred carbon-rich layer of soil that has been found at some 50 Clovis-dated sites across the continent. The layer contains unusual materials (nanodiamonds, metallic microspherules, carbon spherules, magnetic spherules, iridium, charcoal, soot, and fullerenes enriched in helium-3) interpreted as evidence of an impact event, at the very bottom of the black mat of organic material that marks the beginning of the Younger Dryas, and is claimed that it cannot be explained by volcanic, anthropogenic, and other natural processes. However, nearly all of this evidence was found to be non-reproducible and has been dismissed as support for the hypothesis.
Recent research has been reported that at Lake Cuitzeo in the central Mexican state of Guanajuato evidence supporting the Younger Dryas impact hypothesis was found in lake bed cores dating to 12,900 BP. The evidence included properly identified nanodiamonds, carbon spherules, and magnetic spherules. Multiple analyses demonstrated the presence of three allotropes of nanodiamond: n-diamond, i-carbon, and hexagonal nanodiamond (lonsdaleite). 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, including the Tunguska air burst event of 1908. The analysis of the study has not been confirmed or repeated by other researchers. Lonsdaleite has been made artificially in laboratories. Several researchers have criticized the conclusions for incorrect aging of the sediments, an inconsistent hypothesis that made it difficult to predict the type and size of bolide, lack of proper identification of lonsdaleite, confusing an extraterrestrial impact with other causes such as fire, and for inconsistent use of the carbon spherule "proxy."
Naturally occurring lonsdaleite has also been identified in non-bolide diamond placer deposits in the Sakha Republic. Nanodiamonds can be created in laboratories from tequila, which implies that they can develop in the absence of bolide impacts.
Read more about this topic: Younger Dryas Impact Hypothesis
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