An article in Journal of Cosmology volume 2, pages 256-285 Oct 27th 2009, ‘The Case for the Younger Dryas Extraterrestrial Impact Event: Mammoth, Megafauna, and Clovis Extinctions 12,900 Years Ago’ by Richard Firestone can now be downloaded in pdf format (17 pages of print) at http://journalofcosmology.com/Extinction105.html . Firestone, of Lawrence Berkeley National Laboratory in California, outlines his arguments in a pretty effective manner – with copious references. The onset of the YD cooling event is marked by a carbon rich black layer at many locations in North America. Included in this are extraterrestrial markers such as magnetic grains enriched by iridium, magnetic microspherules, vesicular carbon sperules enriched by nano-diamonds, glass like carbon containing fullerenes and nano-diamonds, charcoal, soot, and polycyclic aromatic hydrocarbons. He then claims these same markers have been found in sediments of fifteen Carolina Bays, elliptical depressions along the Atlantic coast.
Various organic constituents are enriched by C14 and these are responsible for highly variable C14 dates at this period of time. The concentration of impact markers near the Great Lakes caused the researchers to think a comet may have disintegrated above the Laurentide ice sheet in NE America and created craters that are now found at the bottom of the lakes. A common explanation for the YD cooling period is the shutdown of the North Atlantic thermohaline circulation system as a result of a sudden influx of fresh water from deglaciation (Broecker 2006) but what caused the influx? In addition, why would cool weather cause mammalian extinctions when those same animals lived through the Ice Age. In the final section of the article Firestone addresses the various objections that have been made to the impact theory. Some geologists have a uniformitarian bias, which he does not address as such but does reply to more recent criticism – such as the idea that statistically, there are too many extraterrestrial events than current wisdom allows. He also addresses the lack of a crater and generally takes the line that the ice sheet would have borne the brunt of the impact and not the surface of the earth.