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PHYSICAL SCIENCES / GEOPHYSICS
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

^aLawrence Berkeley National Laboratory, Berkeley, CA 94720; ^cGeoScience Consulting, Dewey, AZ 86327; ^dDepartment of Earth Sciences and ^eInstitute of Crustal Studies, University of California, Santa Barbara, CA 93106; ^fNorthern Arizona University, Flagstaff, AZ 86011; ^gInstitute for Isotope and Surface Chemistry, H-1525, Budapest, Hungary; ^hDepartment of Geological Sciences, Brown University, Providence, RI 02912; ⁱDepartment of Anthropology and Museum of Natural and Cultural History, University of Oregon, Eugene, OR 97403; ^jEastern New Mexico University, Portales, NM 88130; ^kSouth Carolina Institute of Archaeology and Anthropology, University of South Carolina, Columbia, SC 29208; ^lRestoration Systems, LLC, Raleigh, NC 27604; ^mRozenstraat 85, 1018 NN, Amsterdam, The Netherlands; ⁿBureau of Mines and Geology, University of Nevada, Reno, NV 89557; ^oClimate Change Institute, University of Maine, Orono, ME 04469; ^pUniversity of Rochester, Rochester, NY 14627; ^qDepartment of Environmental Health Sciences, University of California, Los Angeles, CA 90095; ^sP.O. Box 141, Irons, MI 49644; and ^rDepartment of Chemistry, DePaul University, Chicago, IL 60614

Communicated by Steven M. Stanley, University of Hawaii at Manoa, Honolulu, HI, July 26, 2007 (received for review March 13, 2007)

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.

comet | iridium | micrometeorites | nanodiamonds | spherules

Author contributions: R.B.F., A.W., J.P.K., L.B., and W.T. designed research; R.B.F., A.W., J.P.K., L.B., T.E.B., Z.S.R., P.H.S., D.J.K., J.M.E., O.J.D., A.C.G., R.S.H., G.A.H., J.B.K., P.L., P.A.M., J.M., R.P., T.D., S.S.Q.H., A.R.S., A.S., W.T., J.H.W., and W.S.W. performed research; R.B.F., A.W., J.P.K., L.B., T.E.B., Z.S.R., T.B., D.J.K., O.J.D., A.C.G., G.A.H., J.B.K., P.L., J.M., R.P., S.S.Q.H., W.T., J.H.W., and W.S.W. contributed new reagents/analytic tools; R.B.F., A.W., J.P.K., L.B., T.E.B., Z.S.R., P.H.S., D.J.K., J.M.E., R.S.H., G.A.H., P.A.M., R.P., T.D., S.S.Q.H., A.R.S., A.S., W.T., J.H.W., and W.S.W. analyzed data; and R.B.F., A.W., J.P.K., and P.H.S. wrote the paper.

The authors declare no conflict of interest.

This article contains supporting information online at www.pnas.org/cgi/content/full/0706977104/DC1.

^t Miura, Y., 37th Annual Lunar and Planetary Science Conference, March 13–17, 2006, League City, TX, Vol. 2441, pp. 1–2 (abstr.).

^u Rösler, W., Hoffmann, V., Raeymaekers, B., Yang, Z. Q., Schryvers, D., Tarcea, N. (2006) First International Conference on Impact Cratering in the Solar System, May 7–12, 2006, Noordwijk, The Netherlands, abstr. 295464.

^bTo whom correspondence should be addressed. E-mail.com rbfirestone{at}lbl.gov

© 2007 by The National Academy of Sciences of the USA

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