Very high-temperature impact melt products as evidence for cosmic airbursts and impacts 12,900 years ago

Ted E. Bunch; Robert E. Hermes; Andrew M.T. Moore; Douglas J. Kennett; James C. Weaver; James H. Wittke; Paul S. DeCarli; James L. Bischoff; Gordon C. Hillman; George A. Howard; David R. Kimbel; Gunther Kletetschka; Carl P. Lipo; Sachiko Sakai; Zsolt Revay; Allen West; Richard B. Firestone; James P. Kennett

doi:10.1073/pnas.1204453109

Edited by* Steven M. Stanley, University of Hawaii, Honolulu, HI, and approved April 30, 2012 (received for review March 19, 2012)

Abstract

It has been proposed that fragments of an asteroid or comet impacted Earth, deposited silica-and iron-rich microspherules and other proxies across several continents, and triggered the Younger Dryas cooling episode 12,900 years ago. Although many independent groups have confirmed the impact evidence, the hypothesis remains controversial because some groups have failed to do so. We examined sediment sequences from 18 dated Younger Dryas boundary (YDB) sites across three continents (North America, Europe, and Asia), spanning 12,000 km around nearly one-third of the planet. All sites display abundant microspherules in the YDB with none or few above and below. In addition, three sites (Abu Hureyra, Syria; Melrose, Pennsylvania; and Blackville, South Carolina) display vesicular, high-temperature, siliceous scoria-like objects, or SLOs, that match the spherules geochemically. We compared YDB objects with melt products from a known cosmic impact (Meteor Crater, Arizona) and from the 1945 Trinity nuclear airburst in Socorro, New Mexico, and found that all of these high-energy events produced material that is geochemically and morphologically comparable, including: (i) high-temperature, rapidly quenched microspherules and SLOs; (ii) corundum, mullite, and suessite (Fe₃Si), a rare meteoritic mineral that forms under high temperatures; (iii) melted SiO₂ glass, or lechatelierite, with flow textures (or schlieren) that form at > 2,200 °C; and (iv) particles with features indicative of high-energy interparticle collisions. These results are inconsistent with anthropogenic, volcanic, authigenic, and cosmic materials, yet consistent with cosmic ejecta, supporting the hypothesis of extraterrestrial airbursts/impacts 12,900 years ago. The wide geographic distribution of SLOs is consistent with multiple impactors.

Footnotes

↵¹To whom correspondence should be addressed. E-mail: tbear1{at}cableone.net.

Author contributions: T.E.B., R.E.H., A.M.M., D.J.K., J.H.W., G.K., A.W., R.B.F., and J.P.K. designed research; T.E.B., R.E.H., A.M.M., D.J.K., J.C.W., J.H.W., J.L.B., G.C.H., G.A.H., D.R.K., G.K., C.P.L., S.S., Z.R., A.W., R.B.F., and J.P.K. performed research; T.E.B., R.E.H., A.M.M., D.J.K., J.C.W., J.H.W., P.S.D., J.L.B., D.R.K., G.K., C.P.L., S.S., Z.R., A.W., R.B.F., and J.P.K. analyzed data; and T.E.B., R.E.H., A.M.M., J.H.W., P.S.D., J.L.B., A.W., R.B.F., and J.P.K. wrote the paper.
The authors declare no conflict of interest.
*This Direct Submission article had a prearranged editor.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1204453109/-/DCSupplemental.
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