Evidence for deposition of 10 million tonnes of impact spherules across four continents 12,800 y ago
James H. Wittkea, James C. Weaverb, Ted E. Buncha,1, James P. Kennettc, Douglas J. Kennettd, Andrew M. T. Mooree, Gordon C. Hillmanf, Kenneth B. Tankersleyg, Albert C. Goodyearh, Christopher R. Moorei, I. Randolph Daniel, Jr.j, Jack H. Rayk, Neal H. Lopinotk, David Ferrarol, Isabel Israde-Alcántaram, James L. Bischoffn, Paul S. DeCarlio, Robert E. Hermesp,2, Johan B. Kloostermanq,2, Zsolt Revayr, George A. Howards, David R. Kimbelt, Gunther Kletetschkau, Ladislav Nabeleku,v, Carl P. Lipow, Sachiko Sakaiw, Allen Westx, and Richard B. Firestoney
Author Affiliations
Edited* by Steven M. Stanley, University of Hawaii, Honolulu, HI, and approved April 9, 2013 (received for review January 28, 2013)
Significance
We present detailed geochemical and morphological analyses of nearly 700 spherules from 18 sites in support of a major cosmic impact at the onset of the Younger Dryas episode (12.8 ka). The impact distributed ∼10 million tonnes of melted spherules over 50 million square kilometers on four continents. Origins of the spherules by volcanism, anthropogenesis, authigenesis, lightning, and meteoritic ablation are rejected on geochemical and morphological grounds. The spherules closely resemble known impact materials derived from surficial sediments melted at temperatures >2,200 °C. The spherules correlate with abundances of associated melt-glass, nanodiamonds, carbon spherules, aciniform carbon, charcoal, and iridium.
Abstract
Airbursts/impacts by a fragmented comet or asteroid have been proposed at the Younger Dryas onset (12.80 ± 0.15 ka) based on identification of an assemblage of impact-related proxies, including microspherules, nanodiamonds, and iridium. Distributed across four continents at the Younger Dryas boundary (YDB), spherule peaks have been independently confirmed in eight studies, but unconfirmed in two others, resulting in continued dispute about their occurrence, distribution, and origin. To further address this dispute and better identify YDB spherules, we present results from one of the largest spherule investigations ever undertaken regarding spherule geochemistry, morphologies, origins, and processes of formation. We investigated 18 sites across North America, Europe, and the Middle East, performing nearly 700 analyses on spherules using energy dispersive X-ray spectroscopy for geochemical analyses and scanning electron microscopy for surface microstructural characterization. Twelve locations rank among the world’s premier end-Pleistocene archaeological sites, where the YDB marks a hiatus in human occupation or major changes in site use. Our results are consistent with melting of sediments to temperatures >2,200 °C by the thermal radiation and air shocks produced by passage of an extraterrestrial object through the atmosphere; they are inconsistent with volcanic, cosmic, anthropogenic, lightning, or authigenic sources. We also produced spherules from wood in the laboratory at >1,730 °C, indicating that impact-related incineration of biomass may have contributed to spherule production. At 12.8 ka, an estimated 10 million tonnes of spherules were distributed across ∼50 million square kilometers, similar to well-known impact strewnfields and consistent with a major cosmic impact event.
Clovis–Folsom lechatelierite tektite wildfires
Footnotes
1To whom correspondence should be addressed. E-mail: tbear1@cableone.net.
2Retired.
Author contributions: J.H.W., J.C.W., T.E.B., J.P.K., D.J.K., A.M.T.M., G.C.H., K.B.T., A.C.G., D.F., I.I.-A., R.E.H., J.B.K., Z.R., D.R.K., G.K., C.P.L., S.S., A.W., and R.B.F. designed research; J.H.W., J.C.W., T.E.B., J.P.K., D.J.K., A.M.T.M., G.C.H., K.B.T., A.C.G., C.R.M., I.R.D., J.H.R., N.H.L., D.F., I.I.-A., J.L.B., P.S.D., R.E.H., J.B.K., Z.R., G.A.H., D.R.K., G.K., L.N., C.P.L., S.S., A.W., and R.B.F. performed research; J.H.W., J.C.W., T.E.B., J.P.K., D.J.K., A.M.T.M., K.B.T., A.C.G., D.F., I.I.-A., P.S.D., R.E.H., J.B.K., Z.R., G.K., L.N., C.P.L., S.S., A.W., and R.B.F. analyzed data; and J.H.W., J.C.W., T.E.B., J.P.K., D.J.K., A.M.T.M., K.B.T., A.C.G., C.R.M., I.R.D., J.H.R., N.H.L., D.F., I.I.-A., J.L.B., P.S.D., R.E.H., J.B.K., G.A.H., D.R.K., G.K., A.W., and R.B.F. wrote the paper.
The authors declare no conflict of interest.
*This Direct Submission article had a prearranged editor.
†Baker DW, Miranda PJ, Gibbs KE, Montana evidence for extra-terrestrial impact event that caused Ice-Age mammal die-off, American Geophysical Union Spring Meeting, May 27–30, 2008, Ft. Lauderdale, FL, abstr P41A-05.
‡Scruggs MA, Raab LM, Murowchick JS, Stone MW, Niemi TM, Investigation of sediment containing evidence of the Younger Dryas Boundary (YPB) Impact Event, El Carrizal, Baja California Sur, Mexico, Geological Society of America Abstracts with Programs, vol 42, no. 2, p 101 (abstr).
§Elkins-Tanton LT, Kelly DC, Bico J, Bush JWM, Microtektites as vapor condensates, and a possible new strewn field at 5 Ma. Thirty-Third Annual Lunar and Planetary Science Conference, March 11–15, 2002, Houston, TX, abstr 1622.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1301760110/-/DCSupplemental.
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