title: Evidence for deposition of 10 million tonnes of impact spherules across four continents 12,800 y ago.
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.
authors.affiliation: Geology Program, School of Earth Science and Environmental Sustainability, Northern Arizona University, Flagstaff, AZ 86011, USA.
authors.forename: James H
authors.initials: JH
authors.lastname: Wittke
authors.forename: James C
authors.initials: JC
authors.lastname: Weaver
authors.forename: Ted E
authors.initials: TE
authors.lastname: Bunch
authors.forename: James P
authors.initials: JP
authors.lastname: Kennett
authors.forename: Douglas J
authors.initials: DJ
authors.lastname: Kennett
authors.forename: Andrew M T
authors.initials: AM
authors.lastname: Moore
authors.forename: Gordon C
authors.initials: GC
authors.lastname: Hillman
authors.forename: Kenneth B
authors.initials: KB
authors.lastname: Tankersley
authors.forename: Albert C
authors.initials: AC
authors.lastname: Goodyear
authors.forename: Christopher R
authors.initials: CR
authors.lastname: Moore
authors.forename: I Randolph
authors.initials: IR
authors.lastname: Daniel
authors.forename: Jack H
authors.initials: JH
authors.lastname: Ray
authors.forename: Neal H
authors.initials: NH
authors.lastname: Lopinot
authors.forename: David
authors.initials: D
authors.lastname: Ferraro
authors.forename: Isabel
authors.initials: I
authors.lastname: Israde-Alcántara
authors.forename: James L
authors.initials: JL
authors.lastname: Bischoff
authors.forename: Paul S
authors.initials: PS
authors.lastname: DeCarli
authors.forename: Robert E
authors.initials: RE
authors.lastname: Hermes
authors.forename: Johan B
authors.initials: JB
authors.lastname: Kloosterman
authors.forename: Zsolt
authors.initials: Z
authors.lastname: Revay
authors.forename: George A
authors.initials: GA
authors.lastname: Howard
authors.forename: David R
authors.initials: DR
authors.lastname: Kimbel
authors.forename: Gunther
authors.initials: G
authors.lastname: Kletetschka
authors.forename: Ladislav
authors.initials: L
authors.lastname: Nabelek
authors.forename: Carl P
authors.initials: CP
authors.lastname: Lipo
authors.forename: Sachiko
authors.initials: S
authors.lastname: Sakai
authors.forename: Allen
authors.initials: A
authors.lastname: West
authors.forename: Richard B
authors.initials: RB
authors.lastname: Firestone
mesh_terms: D019015:Geologic Sediments; D005844:Geology / Q000379:methods*; D049690:History, Ancient; D019110:Meteoroids; D008855:Microscopy, Electron, Scanning; D018543:Minor Planets; D013052:Spectrometry, X-Ray Emission / Q000379:methods; D014934:Wood
keywords: Clovis–Folsom; lechatelierite; tektite; wildfires