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Evidence for deposition of 10 million tonnes of impact spherules across four continents 12,800 y ago

Proceedings of the National Academy of Sciences of the United States of America

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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.

Additional Publication Details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
Evidence for deposition of 10 million tonnes of impact spherules across four continents 12,800 y ago
Series title:
Proceedings of the National Academy of Sciences of the United States of America
Volume
110
Issue:
23
Year Published:
2013
Language:
English
Publisher:
National Academy of Sciences
Publisher location:
Washington, D.C.
Description:
10 p.
Larger Work Type:
Article
Larger Work Subtype:
Journal Article
Larger Work Title:
Proceedings of the National Academy of Sciences of the United States of America
First page:
E2088
Last page:
E2097
Other Geospatial:
Europe;Middle East;North America