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1 Department of Geological Sciences, University of Oregon, Eugene, Oregon 97403-1272
2 Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California 90095-1567
Life on Earth was almost destroyed some 250 m.y. ago in the most profound of all known mass extinction events. We investigated the possible role of impact by an extraterrestrial bolide through chemical and mineralogical characterization of boundary breccias, search for shocked quartz, and analysis for iridium in Permian-Triassic boundary sections at Graphite Peak and Mount Crean, Antarctica, and Wybung Head, Australia. Thin claystone breccias at the isotopically and paleobotanically defined boundary at all three locations are interpreted as redeposited soil rather than impact ejecta. The breccias at all three locations also yielded shocked quartz, but it is an order of magnitude less abundant (0.2 vol%) and smaller (only as much as 176 micrometers m diameter) than shocked quartz at some Cretaceous-Tertiary boundary sites. Faint iridium "anomalies" were detected (up to 134 pg
g–1). These values are an order of magnitude less than iridium anomalies at some Cretaceous-Tertiary boundary sites. Furthermore, peak iridium values are as much as 1 m below the isotopically and paleobotanically defined boundary. The idea that impact caused the extinctions thus remains to be demonstrated convincingly.
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