Timing of the Permian-Triassic biotic crisis: implications from new zirconU/Pb age data (and their limitations)

The most profound biotic crisis in the Earth's history, causing the near ex tinction of both terrestrial and marine life, occurred at the end of the Pe rmian period about 253 Myr ago and marks the Paleozoic-Mesozoic era boundar y. The cause of this event is still a matter of vigorous debate, with both brief and catastrophic as well as gradual mechanisms having been proposed. Similar to a recent landmark study, this study uses the U-Pb method on zirc ons from the uppermost Permian/lowermost Triassic ash fall deposits at Meis han (Zhejiang Province, SE China) in order to examine time and rate constra ints for these events. The results of both this study and previous work sho w that for these ash layers, the effects of Pb loss are combined with varyi ng amounts and sources of inheritance, resulting in an age scatter which pr ohibits the extraction of a statistically robust age in many cases. Though the effects of Pb loss on the zircons analyzed in this study were reduced b y leaching the grains in hydrofluoric acid las opposed to commonly applied air abrasion) prior to analysis, the presence within a single ash layer of multiple generations of older xenocrysts tin many cases only slightly older than the depositional age) has made quantitative interpretation even more difficult. When these combined phenomena bias individual zircon ages by les s than a percent, they are extremely difficult to deconvolute, and, if mult i-grain analyses are used, can become impossible to recognize (because of t he resulting age averaging). Monte Carlo simulations using actual measureme nts of individual zircon crystals show that age excursions due to Pb loss a nd xenocrystic contamination for the Meishan bentonites are easily homogeni zed to the point of undetectability when replicate analyses of multi-grain zircon samples are compared. Thus this study uses only high-precision analy ses of single crystals, whether from our work or that of previous studies. Three main conclusions have emerged. First, our data require a significant increase in the age of the Permian-Triassic boundary by more than 2 myr com pared to the previous study, which shifts the age to a value older than 253 Ma. Second, neither our data nor those from previous work can confirm or n egate the possibility of a very abrupt biotic crisis. Third, even large sui tes of very high-quality, single-zircon U-Pb analyses for these tuffs canno t, in most cases, yield objective, reliable, and robust dates with accuraci es at the sub-myr level - though the temptation to perform arbitrary select ion of subsets of the analyses for that purpose is almost irresistible. The last conclusion is not an indictment of zircon U/Pb dating in general (oth er rocks and other zircon populations can - and do - behave very differentl y), and further technical advances will likely improve our ability to prepa re grains or sub-grains of adequately enhanced quality for analysis. Conseq uently, the results of the present study strongly suggest that for problems requiring time-scale accuracy, inferences from zircon U-Pb dating must be based on sufficiently large suites of single-crystal or crystal domain, hig h-precision analyses (< 1% error) that are realistically interpreted. <(c)> 2001 Elsevier Science B.V. All rights reserved.