Here:
http://ie.lbl.gov/mammoth/mammoth.html Firestone paper links
If adequate funding for geological research had of been provided, the debate on this would be over now, and the precise type impactor and impact mechanisms well documented.
I would like to add that while Firestone sees supernova as generating the neutrons that throw off 14C dates, my GUESS is that they may have been freed in the impacts themselves.
E.P. Grondine
Man and Impact in the Americas
Links to Firestone's team's papers
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