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

Title	Evidence for the deposition of 10 million tonnes of impact spherules across four continents 12,800 years ago.
Authors	James H Wittke, James C Weaver, Ted E Bunch, James P Kennett, Douglas J Kennett, Andrew M T Moore, Gordon C Hillman, Kenneth B Tankersley, Albert C Goodyear, Christopher R Moore, I Randolph Daniel, Jack H Ray, Neal H Lopinot, David Ferraro, Isabel Israde-Alcántara, James L Bischoff, Paul S DeCarli, Robert E Hermes, Johan B Kloosterman, Zsolt Revay, George A Howard, David R Kimbel, Gunther Kletetschka, Ladislav Nabelek, Carl P Lipo, Sachiko Sakai, Allen West, and Richard B Firestone
Magazine	Proceedings of the National Academy of Sciences of the United Kingdom
Date	06/04/2013
DOI	10.1073/pnas.1301760110
Introduction	A hypothesis involving airbursts or impacts from a fragmented comet or asteroid at the Younger Dryas onset (12.80 ± 0.15 ka) rests on evidence from impact-related markers such as microspherules, nanodiamonds, and iridium. While spherule concentrations at the Younger Dryas boundary (YDB) have been verified in multiple studies across four continents, conflicting reports have fueled ongoing debate regarding their presence, spread, and source. To contribute to this discussion and refine the identification of YDB spherules, this study presents findings from an extensive investigation into spherule geochemistry, morphology, origins, and formation mechanisms. The research encompassed 18 locations spanning North America, Europe, and the Middle East, involving almost 700 analyses of spherules using energy dispersive X-ray spectroscopy for chemical profiling and scanning electron microscopy for surface detail. A dozen of these locations are prominent end-Pleistocene archaeological sites, where the YDB signifies a pause in human activity or significant shifts in site utilisation. The findings align with the melting of sediments at temperatures exceeding 2,200 °C due to thermal radiation and atmospheric shockwaves generated by an extraterrestrial object's atmospheric transit. These results rule out volcanic, cosmic dust, human-caused, lightning, or natural geological sources. Laboratory experiments also generated spherules from wood at temperatures above 1,730 °C, suggesting that biomass combustion linked to impacts could have contributed to spherule formation. Approximately 10 million tonnes of spherules were distributed over an area of about 50 million square kilometres at 12.8 ka, consistent with known impact strewnfields and supporting a significant cosmic impact scenario.
Quote	James H Wittke, James C Weaver and Ted E Bunch et al. 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 Kingdom. 2013. Vol. 110(23). DOI: 10.1073/pnas.1301760110
Element	Iridium (Ir) , Carbon (C)
Materials	Crystals
Topics	Environmental and Green Materials
Industry	Research & Laboratory , Space

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