North American megafaunal extinctions considered

Submitted by Sammy Smith (sammy@thesga.org)

Firestone_2007_Fig_3.jpg

Figure 3 from Firestone et al. paper.

Have you read (heard?) about the hypothesis that an extraterrestrial impact lead to the extinction of Pleistocene megafauna in North America? This hypothesis has been raised in opposition to hypotheses that posit that Paleoindians and/or climate did in the megafauna.

The first paper advancing the extraterrestrial impact hypothesis was by RB Firestone and a slew of coauthors, and published in October 2007. Here’s the abstract of that paper, “Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling,” published in the Proceedings of the National Academy of Sciences of the United States of America:

A carbon-rich black layer, dating to ≈12.9 ka [ka = thousand years], 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 idea caused quite a furor.

A year after the initial Firestone et al. publication, Todd A. Surovell and eight co-authors published a paper where they tested the spherules hypothesis. In the abstract, they baldly stated: “We were unable to reproduce any results of the Firestone et al. study and find no support for Younger Dryas extraterrestrial impact.” Instead, they posit (pg. 18157):

Alternatively, it may be that the presence, absence, and relative abundance of magnetic materials, especially the spherules, is due to characteristics of the parent material and depositional environment instead of some sort of continent-wide extraterrestrial process. The characteristics of the local depositional setting before, during, and after 12.9 ka have not been addressed by the proponents of the impact hypothesis. The zones producing the YDB [Younger Dryas boundary] “impact markers” are typically associated with soils (stable surfaces) or shifts in the depositional environment (e.g., alluvial to lacustrine conditions at Blackwater Draw, Lubbock Lake, Murray Springs, and Lake Hind; buried soils in the Carolina Bays and at Lommel, Belgium).

Follow the links below and download both papers (they’re free!). Perhaps you’d be interested in reading this Wikipedia entry, and doing further research. What do you think? Are you pursuaded by either hypothesis?

References cited

Firestone, R.B., A. West, James P. Kennett, L. Becker, T.E. Bunch, Z.S. Revay, P.H. Schultz, T. Belgya, Douglas J. Kennett, Jon M. Erlandson, O.J. Dickenson, Albert C. Goodyear, R.S. Harris, G.A. Howard, J.B. Kloosterman, P. Lechler, Paul A. Mayewski, J. Montgomery, R. Poreda, T. Darrah, S.S. Que Hee, A.R. Smith, A. Stich, W. Topping, J.H. Wittke, and W.S. Wolbach. 2007. Evidence for an Extraterrestrial Impact 12,900 Years Ago That Contributed to the Megafaunal Extinctions and the Younger Dryas Cooling. Proceedings of the National Academy of Sciences of the United States of America 104(41):16016–16021.

Surovell, Todd A., Vance T. Holliday, Joseph A.M. Gingerich, Caroline Ketron, C. Vance Haynes, Jr., Ilene Hilman, Daniel P. Wagner, Eileen Johnson, and Philippe Claeys. 2009. An Independent Evaluation of the Younger Dryas Extraterrestrial Impact Hypothesis. Proceedings of the National Academy of Sciences of the United States of America 106:18155–18158.