Very high-temperature impact melt products as evidence for cosmic airbursts and impacts 12,900 years ago
- aGeology Program, School of Earth Science and Environmental Sustainability, Northern Arizona University, Flagstaff, AZ 86011;
- bLos Alamos National Laboratory (retired), Los Alamos, NM 87545;
- cCollege of Liberal Arts, Rochester Institute of Technology, Rochester, NY 14623;
- dDepartment of Anthropology, Pennsylvania State University, University Park, PA 16802;
- eWyss Institute for Biologically Inspired Engineering, Harvard University, Cambridge, MA 02138;
- fSRI International, Menlo Park, CA 94025;
- gUS Geological Survey, Menlo Park, CA 94025;
- hInstitute of Archaeology, University College London, London, United Kingdom;
- iRestoration Systems, LLC, Raleigh, NC 27604;
- jKimstar Research, Fayetteville, NC 28312;
- kFaculty of Science, Charles University in Prague, and
- lInstitute of Geology, Czech Academy of Science of the Czech Republic, v.v.i., Prague, Czech Republic;
- mInstitute for Integrated Research in Materials, Environments, and Society (IIRMES), California State University, Long Beach, CA 90840;
- nForschungsneutronenquelle Heinz Maier-Leibnitz (FRM II), Technische Universität München, Munich, Germany;
- oGeoScience Consulting, Dewey, AZ 86327;
- pLawrence Berkeley National Laboratory, Berkeley, CA 94720; and
- qDepartment of Earth Science and Marine Science Instititute, University of California, Santa Barbara, CA 93106
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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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¶Buchner E, Schmeider M, Strasser A, Krochert L, Impacts on spherules, 40th Lunar and Planetary Science Conference. March 26, 2009, abstr 1017.
∥Svetsov VV, Wasson JT, Melting of soil rich in quartz by radiation from aerial bursts—A possible cause of the formation for Libyan Desert Glass and layered Australasian tektites, 38th Lunar and Planetary Science Conference. March 13, 2007, abstr 1499.
**Petaev ML, Jacobsen SB, Basu AR, Becker L, Magnetic Fe,Si,Al-rich impact spherules from the P-T Boundary Layer at Graphite Peak, Antarctica, 35th Lunar and Planetary Science Conference. March 16, 2004, abstr 1216.
††Buchner E, Schmeider M, Strasser A, Krochert L, Impacts on spherules, 40th Lunar and Planetary Science Conference. March 26, 2009, abstr 1017.
Freely available online through the PNAS open access option.