Hi Moose,
This is not to gripe at you, but to gripe at the Science By Press Release that these articles represent. (Here's the link to NSF:
Comet May Have Exploded Over North America 13,000 Years Ago | Beta site for NSF - National Science Foundation ).
Impacts have been invoked for every mass extinction since the discovery of the K-T impact, and it's gone overboard. It's a shame, especially when some top-notch science has been done on extinction routes other than impact. For instance, the discovery of iridium at the Triassic-Jurassic boundary resulted in a paper in Science: Ascent of dinosaurs linked to an iridium anomaly at the Triassic-Jurassic boundary by Olsen et al. 2002. However, iridium does not an impact make. Other evidence for a T-J impact is lacking (
http://www.lpi.usra.edu/meetings/metsoc2007/pdf/5130.pdf ), but negative evidence is harder to get published, and a lot less sexy.
To prove there was an impact, several things are needed:
http://palaeo.gly.bris.ac.uk/Palaeofiles/Triassic/bolide.htm ,
the single most important of which is an impact crater. Currently these researchers have some of the geochemical evidence, but squat as far as impacts go. There definitely should be an impact crater, given that 1) the amount of material they're invoking should leave a crater about 1/4-1/2 the size of the Yucatan impact, or 50-100 km in diameter, and 2) since it happened only 12,000 years ago, it should still be around and should be pretty dang easy to spot.
There is a bit of shoddy science being invoked- they state that a comet (made of ice) hit an ice sheet (made of ice). The inferred connotation is that ice striking ice wouldn't leave a crater. However, a body of material 10^13 kg striking the Earth at escape velocity will completely ignore whatever ice sheet is in its way, independent of its starting composition. There should be an impact!
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