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The age and timing of the Permian-Triassic mass extinction have been difficult to determine because zircon populations from the type sections are typically affected by pervasive lead loss and contamination by indistinguishable older xenocrysts. Zircons from nine ash beds within the Shangsi and Meishan sections (China), pretreated by annealing followed by partial attack with hydrofluoric acid, result in suites of consistent and concordant uranium/lead (U/Pb) ages, eliminating the effects of lead loss. The U/Pb age of the main pulse of the extinction is 252.6 ± 0.2 million years, synchronous with the Siberian flood volcanism, and it occurred within the quoted uncertainty.
Despite intensive study, the cause and timing of events associated with the Permian-Triassic (P-T) extinction-the most profound such extinction known over the history of life on Earth-remain uncertain. Most scenarios proposed in the past few years have invoked a catastrophic event, which almost by definition requires unusually precise geochronology for a meaningful test. Geochronology on the volcanic deposits intercalated with the sediments at the Global Stratotype Section and Point (GSSP) section in Meishan is difficult using the U/Pb method on zircon (1)-the most accurate available method and the only suitable mineral for U/Pb isotopic dating in these ashes. Zircon populations from these beds are typically contaminated with slightly older xenocrysts indistinguishable in appearance and corrupted by variable amounts of postdepositional Pb loss. Age lowering from the latter is pervasive (2) even when the surfaces of the crystals are removed by air abrasion (2, 3). A previous U/Pb-zircon study (4), using analyses of both single- and multicrystal samples of air-abraded zircons, as well as subsequent research on extinction patterns (5) and using the ages in (4), proposed (i) an age of 251.4 ± 0.3 million years (Ma) for the main pulse of the extinction (5), (ii) a duration of no more than 0.165 Ma (from the sedimentation rate implied by the bracketing ages) for the carbon isotope (δ^sup 13^C) excursion, and (iii) an age of 251 Ma for the P-T boundary (base Triassic) defined by the First Appearance Datum (FAD) of the conodont Hindeodus parvus (at the GSSP section in Meishan, eastern China). This data set was complex (such that most of the data were excluded from consideration). Recent time-scale compilations and most subsequent...