Constraints on the timing of the Permian-Triassic biotic crisis: New U/Pb zircon-ages

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1035 Geochronology, 8404 Ash Deposits, 9320 Asia, 9609 Mesozoic, 9614 Paleozoic

Scientific paper

New IDTIMS U/Pb single-zircon data from ash layers intercalated within the Permo-Triassic (P-T) deposits of the Shangsi section (Sichuan Province, Central China) in combination with recently published ages from the GSSP in Meishan (Zhejiang Province, E China) constrain the most profound biotic crisis in the Earth's history, causing the near extinction of both terrestrial and marine life, to an age of >253 Myr (Mundil et al., 2001). In addition, our data indicate that either the sedimentation rate for the uppermost Permian/lowermost Triassic in the Meishan section drastically decreases or that there was a depositional hiatus. Both of these conclusions are inconsistent with the concept of extreme rapidity for the end Permian extinction as well as the hypothesis of a very short duration of the δ 13C excursion near the P-T boundary. The results of both our study and previous work (Bowring et al., 1998) show that for the Permian-Triassic ash layers in SE China, the effects of Pb loss are combined with varying amounts and sources of inheritance, resulting in an age-scatter which prohibits the extraction of a statistically robust age in many cases. The presence within a single ash layer of multiple generations of older xenocrysts has made quantitative interpretation even more difficult. If multi-grain analyses are used, these combined phenomena are extremely difficult to resolve and can become impossible to recognize (due to age-averaging). As a result multi-crystal analyses may be inaccurate by 1% or more and implications may be misleading. Single-zircon analyses from an ash layer 6 m below the first appearance of {\Hindeodus parvus} (the biostratigraphic definition for the P-T boundary) from the Shangsi section yield a preliminary mean 238U/206Pb age of 254.7 +/- 0.7 Myr (95% conf.). Also, initial analyses from an ash layer just below the final stage of the Permian (Changshingian) yield an age of 260 +/- 0.4 Ma. Both ages are consistent with the results obtained on the ash layers from Meishan. Our estimate of 253 Myr for the P-T boundary (based on U/Pb ages) is in agreement with Ar/Ar ages of 250 Myr presented in Renne et al. (1995) for the Siberian Trap basalts and ash falls at the P-T boundary in Meishan and Shangsi, as well as new Ar/Ar ages from Shangsi, if systematic errors are taken into consideration. In addition, we find that Ar/Ar ages are systematically younger, which agrees with ongoing research suggesting that there is a systematic bias of 1-2% between U/Pb and Ar/Ar isotopic systems (Min et al., 2000). Our combined evidence is at variance with recent studies (Rampino et al., 2000, Becker et al., 2001) which suggest a rapid extinction caused by a bolide impact. Although we cannot exclude an impact at or near the P-T boundary, a more gradual scenario and a causal relation between the massive continental basalt volcanism in South China at the end of the Permian and Siberia at the Permian-Triassic boundary - possibly in combination with other factors - and the biotic crisis must be considered. Becker et al., Science 291, 1530-1533, (2001). Bowring et al., Science 280, 1039-1045, (1998). Min et al., Geochim.et Cosmochim. Acta 64: 73-98, (2000). Mundil et al., Earth Planet. Sci. Lett. 187, 133-147, (2001). Rampino et al., Geology 28, 643-646, (2000). Renne et al., Science 269, 1413-1413, (1995).

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