New explanation for megafauna extinction: extraterrestrial impact

In a compelling and very interesting study, Firestone et al. propose a new explanation for why Clovis technology changed drastically (and disappeared) at about the same time the megafauna of the Americas went extinct.
Published yesterday in PNAS (their paper is open access and can be found here.), the authors present multiple lines of evidence suggesting that a comet either struck the earth at an unknown location or exploded in an airburst at about 12.9 kya. They propose that the ensuing fires and climatic changes caused the end-Pleistocene extinctions and the the cultural changes observed archaeologically at this time. The event is also proposed to explain the Younger Dryas climatic event. Much of their evidence relates to a well-known stratigraphic layer, called the blackmat, which has been observed directly above a number of buried Clovis cultural deposits. The blackmat layer has been found above but never below Clovis containing deposits (i.e., it must be just a bit younger than the time when Clovis people were active on the landscape and we don't see any evidence for Clovis after it was deposited). They demonstrate that the blackmat contains grains associated with the content of extraterrestrial bodies, comets, that are not found above or below it and are also found in other sediments of the same time period. No large now-extinct mammals have been found above this layer.

Here's the info and abstract for their paper:
Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling

R. B. Firestone^a,b, A. West^c, J. P. Kennett^d, L. Becker^e, T. E. Bunch^f, Z. S. Revay^g, P. H. Schultz^h, T. Belgya^g, D. J. Kennettⁱ, J. M. Erlandsonⁱ, O. J. Dickenson^j, A. C. Goodyear^k, R. S. Harris^h, G. A. Howard^l, J. B. Kloosterman^m, P. Lechlerⁿ, P. A. Mayewski^o, J. Montgomery^j, R. Poreda^p, T. Darrah^p, S. S. Que Hee^q, A. R. Smith^a, A. Stich^r, W. Topping^s, J. H. Wittke^f, and W. S. Wolbach^r

A carbon-rich black layer, dating to 12.9 ka, has been previously identified at 50 Clovis-age sites across North America and appears contemporaneous with the abrupt onset of Younger Dryas (YD) cooling. The in situ bones of extinct Pleistocene megafauna, along with Clovis tool assemblages, occur below this black layer but not within or above it. Causes for the extinctions, YD cooling, and termination of Clovis culture have long been controversial. In this paper, we provide evidence for an extraterrestrial (ET) impact event at 12.9 ka, which we hypothesize caused abrupt environmental changes that contributed to YD cooling, major ecological reorganization, broad-scale extinctions, and rapid human behavioral shifts at the end of the Clovis Period. Clovis-age sites in North American are overlain by a thin, discrete layer with varying peak abundances of (i) magnetic grains with iridium, (ii) magnetic microspherules, (iii) charcoal, (iv) soot, (v) carbon spherules, (vi) glass-like carbon containing nanodiamonds, and (vii) fullerenes with ET helium, all of which are evidence for an ET impact and associated biomass burning at 12.9 ka. This layer also extends throughout at least 15 Carolina Bays, which are unique, elliptical depressions, oriented to the northwest across the Atlantic Coastal Plain. We propose that one or more large, low-density ET objects exploded over northern North America, partially destabilizing the Laurentide Ice Sheet and triggering YD cooling. The shock wave, thermal pulse, and event-related environmental effects (e.g., extensive biomass burning and food limitations) contributed to end-Pleistocene megafaunal extinctions and adaptive shifts among PaleoAmericans in North America.


This paper cannot be ignored but I'm sure many will try. There's clearly something going on here and they do a great job putting their lines of evidence together.
Major weakness: Absolutely no link is made between the proposed effects of this impact and the strongly sized-biased pattern of the extinction event they propose to explain.
We know reasons why large animals might have been more prone to extinction - low population densities, low fertility rates, 'expensive' offspring, etc. - but it is not yet clear why the impact would operate on environmental parameters such that primarily large animals would be affected.
This needs to be addressed. In future studies it would also be nice to see a more detailed comparison of this event with other extraterrestrial impacts that caused extinctions. Obviously, this 'YD event' as they call it (YD for Younger Dryas) was of a lower magnitude, but how do the traces of the elements and the links to environmental productivity compare to previous events where the link to an impact event are robust and much more clear?

There has been a lot of buzz about this idea for some time as they've presented the argument at meetings and the like. This is only the first paper and I'm sure we'll see a good deal of debate and further papers in short order.

Just to give a few more details of their argument. Here's one of the their figures showing the microspherules that are found in the blackmat, don't seem to be found above or below it, and are found in lots of different layers that correspond closely to the date of the impact:

Fig. 2. High-titanomagnetite microspherules from Blackwater Draw, NM (120 µm) (a); Chobot, AB, Canada (150 µm) (b), Gainey, MI (90 µm) (c), and Howard Bay, NC (100 µm) (d).

Also, they show how specific the attributes of the impact are in terms of 'spikes' in the profiles of several stratified localities. Here's their figure depicting these spikes:

Fig. 1. Sediment profiles for seven sites. Concentrations are shown for magnetic grains, microspherules, charcoal, soot, glass-like carbon, carbon spherules, Ir, Cr, and Ni, which peak mostly in a narrow stratigraphic section spanning only a few hundred years. Ir open circles indicate values below detection, typically <0.5–1 href="http://www.pnas.org/cgi/content/full/0706977104/DC1">SI Fig. 8. The locations of all sites that were sampled are shown in SI Fig. 9.

I hope you all enjoy thinking about this new provocative argument and I look forward to the commentary that will follow. Its already gotten attention by the popular press and will certainly get lots of scrutiny by scholars in multiple disciplines.

Cheers,

Oskar