SEARCH FOR EVIDENCE OF IMPACT AT THE PERMIAN-TRIASSIC BOUNDARY IN ANTARCTICA AND AUSTRALIA

Life on Earth was almost destroyed some 250 m.y. ago in the most profo und of all known mass extinction events. We investigated the possible role of impact by an extraterrestrial bolide through chemical and mine ralogical characterization of boundary breccias, search for shocked qu artz, and analysis for iridium in Permian-Triassic boundary sections a t Graphite Peak and Mount Crean, Antarctica, and Wybung Head, Australi a. Thin claystone breccias at the isotopically and paleobotanically de fined boundary at all three locations are interpreted as redeposited s oil rather than impact ejecta, The breccias at all three locations als o yielded shocked quartz, but it is an order of magnitude less abundan t (0.2 vol%) and smaller (only as much as 176 mu m diameter) than shoc ked 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-Ter tiary 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 demonstrate d convincingly.