Arizona geologist Allan West has been forwarding a theory for the past four years that a giant comet struck Canada 12,900 years ago, wiping out many Ice Age megafauna and ushering in a 1,000 year global cooling period. There are lots of skeptics.
However, a new study, co-authored by West, came out in Proceedings of the National Academy of Sciences last week, with evidence of microscopic ("nano") diamonds on Santa Rosa Island off the coast of California, that the authors call a smoking gun proving the idea that a massive impact, possibly into the Hudson Bay region, covered the continent with a cloud of super-hot ejecta.
The authors state that this form of nanodiamond, lonsdaleite, "is known on Earth only in meteorites and impact craters, and its presence strongly supports a cosmic impact event."
They note that "These shock-synthesized diamonds are also associated with proxies indicating major biomass burning (charcoal, carbon spherules, and soot). The only previously known co-occurrence of nanodiamonds, soot, and extinction is the Cretaceous-Tertiary (K/T) impact layer" which is blamed for the end of the dinosaurs.
Ref: Shock-synthesized hexagonal diamonds in Younger Dryas boundary sediments,