Cosmic impacts and the end-Pleistocene extinction: idea seems not to hold up

A while back I posted a blog on a paper published in PNAS arguing that a comet impact was responsible for the extinction of the megafauna in North America. They also argue that the impact was responsible for the disappearance of the Clovis culture - the earliest recognized archaeological culture in the New World known for large well made and very distinctive fluted spear points.
I was skeptical of their ability to link the proposed impact to the strongly size biased nature of the extinctions and wondered about how it extended to the similar patterns in extinction that we see on other continents. There was a lot missing in terms of how to link the impact to mammals - but aside from that it seemed like they may have demonstrated that lots of stratigraphic layers around the United States contain minerals of extra-terrestrial origin. or maybe not...

A news focus just published in Science (authored by Science writer Richard Kerr) reviews the evidence and points out that many of the claims by the authors of the comet study are not holding up. This is probably not surprising on the whole but it is amazing how many of the claims seem to be really thoroughly refuted and experts in the study of cosmic impacts suggest that the hole's in the study were evident long before it was published - that can happen when papers go to non-specialized audiences. Also note that the paper was published in PNAS...

Magnetic spherules believed to be diagnostic of the impact were a key line of evidence for the study by Firestone et al. (the authors of the PNAS paper) but it turns out that such spherules may be regularly introduced to our atmosphere from space and are not useful markers for a specific impact.

Excerpt from the article:
"One problem is that no one has "any of the classic evidence of an impact," says impact specialist David Kring of the Lunar and Planetary Institute in Houston, Texas. Spurred by the 1980s debate over what killed off the dinosaurs, "the community learned a lot about what the threshold of evidence is" for confirming an impact, he explains. But taking all the evidence offered by the group proposing the mammoth-killer impact, "you end up with [markers] that are not diagnostic of impact," says impact specialist Bevan French of the National Museum of Natural History in Washington, D.C. Proponents, meanwhile, are defending some of their published claims and giving ground on others but promising ultimate vindication.

ET? An impactor (top) may have produced magnetic spherules (lower right), but similar spherules (lower left) continually fall from space. CREDIT: STOCKTREK IMAGES/GETTY IMAGES; L. FRANZÉN, N. PINTER, AND S. ISHMAN, GSA TODAY (2008); FIRESTONE ET AL., PNAS 104, 41 (2007)

Diamonds not forever
Everyone agrees on one point at least. "Obviously, something really interesting happened 13,000 years ago," as Kring puts it. It was 12,900 years ago, to be precise, that a world staggering out of the last Ice Age suddenly plunged back into a millennium of near-glacial climate before emerging into the current warmth. It was also about then--emphasis on the uncertainties summed up by "about"--that the mammoths and other great beasts disappeared from North America. And the Paleo-Indian Clovis culture vanished from the archaeological record around then, too."
Science 7 March 2008:
Vol. 319. no. 5868, pp. 1331 - 1332
DOI: 10.1126/science.319.5868.1331


Experts question the existence of the nanodiamonds that are thought to be traced to the impact and other lines of evidence, such as the iridium spike in the stratigraphic layers in question, have been fairly extensively criticized and questioned as well.

These criticisms really don't sound like the nay-saying that goes with old stodgy types not like their conventional wisdoms being questioned. They seem pretty solid. Perhaps the debate will continue to play out in the literature.

later
O