ANOTHER LOOK AT IMPACT, METHANE OUTBURSTS AND EXTINCTION MECHANISMS AT THE PERMIAN-TRIASSIC BOUNDARY

Until recently, evidence for impact at the Permian-Triassic boundary included iridium anomalies an order of magnitude smaller than at the Cretaceous-Tertiary boundary, fullerenes with extraterrestrial helium, and rare grains of shocked quartz, Ni-rich spinels and microspherules. Because this evidence was less compelling than for the smaller mass extinction at the Cretaceous-Tertiary boundary, the role of impact at the Permian-Triassic boundary remained controversial. Additional evidence for impact appeared in November 2003, when Asish Basu and colleagues reported fragments of carbonaceous chondrites from boundary beds in Antarctica. Sbsequently, Stein Jacobson also has found microspherules in the same samples. Kevin Pope has found more shocked quartz from the boundary in southeastern Australia. Luann Becker and Bob Poreda have confirmed impact with a new radiometric date, impact breccia, and maskelynite in the Bedout Crater, offshore Canning Basin, Western Australia, first proposed as an impact structure by John Gorter in 1996. This crater is comparable in size with Chicxulub Crater, and is evidence of an impactor at least as large as at the Cretaceous-Tertiary boundary. In 2000 Evelyn Krull reported carbon isotopic excursions at the Permian-Triassic boundary in Australia and New Zealand that were so isotopically light that they could only be explained by methane clathrate outburst. Such uniquely methanogenic excursions were later reported from Australia, Austria, India, Madagascar, and Greenland. Kliti Grice has recently found the methanogenic biomarker crocetane in Permian-Triassic boundary samples from Antarctica. Impact could have released methane catastrophically. Modeling of a large methane outburst by Bob Berner in 2002 shows that combined with death and decay from extinction, and with volcanic reduced gases, methane oxidation could have reduced atmospheric oxygen to 12% from a preexisting high of 35%. Impact-triggered atmospheric methane pollution could have killed by hypoxia, hypercapnia and acidosis.