Timing of the Permian–Triassic biotic crisis: implications from new zircon U/Pb age data (and their limitations)
Timing of the Permian–Triassic biotic crisis: implications from new zircon U/Pb age data (and their limitations)
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DOI：
10.1016/s0012-821x(01)00274-6
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发表时间：
2001-04
影响因子：
5.3
通讯作者：
R. Mundil;I. Metcalfe;K. Ludwig;P. Renne;F. Oberli;R. Nicoll
中科院分区：
文献类型：
--
作者：
R. Mundil;I. Metcalfe;K. Ludwig;P. Renne;F. Oberli;R. Nicoll
地球历史上最深刻的生物危机，导致陆地和海洋生物几乎灭绝，发生在大约2.53亿年前的二叠纪末期，标志着古生代-中生代的边界。这一事件的原因仍然是一个激烈辩论的问题，有人提出了短暂和灾难性以及渐进的机制。类似于最近的一个里程碑式的研究，这项研究使用的U-Pb方法从最高的二叠纪/最低的三叠纪灰落沉积物在梅山（浙江省，中国东南部），以检查这些事件的时间和速度的限制。这项研究和以前的工作的结果表明，对于这些灰层，铅损失的影响相结合的不同数量和来源的继承，导致年龄分散，禁止提取的统计上强大的年龄在许多情况下。虽然铅损失对本研究中分析的锆石的影响减少了氢氟酸（而不是常用的空气磨损）分析之前，在一个单一的灰层内的多代老捕虏晶的存在（在许多情况下，只有略老于沉积年龄）的颗粒浸出的定量解释更加困难。当这些综合现象使单个锆石年龄偏差小于百分之一时，它们就很难去卷积，如果使用多颗粒分析，就不可能识别（因为由此产生的年龄平均值）。Monte Carlo模拟使用实际测量的单个锆石晶体显示，年龄偏移由于铅损失和捕虏晶污染的梅山榴辉岩很容易均匀化的点时，多颗粒锆石样品的重复分析进行比较，无法检测。因此，这项研究只使用高精度的单晶分析，无论是从我们的工作或以前的研究。得出了三个主要结论。首先，我们的数据需要一个显着增加的年龄的二叠纪-三叠纪边界超过200万年相比，以前的研究，年龄转移到一个值大于253马。其次，我们的数据和以前的工作都不能证实或否定一个非常突然的生物危机的可能性。第三，在大多数情况下，即使是对这些凝灰岩进行了大量高质量的单锆石U-Pb分析，也不能得出客观、可靠和可靠的年代，其精度达到百万年以下--尽管出于这一目的而任意选择分析子集的诱惑几乎是不可抗拒的。最后一个结论并不是对锆石U/Pb测年法的一种控诉（其他岩石和其他锆石群可以--而且确实--表现得非常不同），进一步的技术进步可能会提高我们制备质量足够高的颗粒或亚颗粒的能力，以供分析。因此，本研究的结果强烈表明，对于需要时间尺度准确性的问题，锆石U-Pb定年的推断必须基于足够大的单晶或晶体域，高精度分析（<1%的误差），现实的解释。
The most profound biotic crisis in the Earth’s history, causing the near extinction of both terrestrial and marine life, occurred at the end of the Permian period about 253 Myr ago and marks the Paleozoic–Mesozoic era boundary. 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 zircons from the uppermost Permian/lowermost Triassic ash fall deposits at Meishan (Zhejiang Province, SE China) in order to examine time and rate constraints for these events. The results of both this study and previous work show that for these ash layers, 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. Though the effects of Pb loss on the zircons analyzed in this study were reduced by leaching the grains in hydrofluoric acid (as opposed to commonly applied air abrasion) prior to analysis, the presence within a single ash layer of multiple generations of older xenocrysts (in 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 less than a percent, they are extremely difficult to deconvolute, and, if multi-grain analyses are used, can become impossible to recognize (because of the resulting age averaging). Monte Carlo simulations using actual measurements of individual zircon crystals show that age excursions due to Pb loss and xenocrystic contamination for the Meishan bentonites are easily homogenized to the point of undetectability when replicate analyses of multi-grain zircon samples are compared. Thus this study uses only high-precision analyses 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 compared 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 negate the possibility of a very abrupt biotic crisis. Third, even large suites of very high-quality, single-zircon U–Pb analyses for these tuffs cannot, in most cases, yield objective, reliable, and robust dates with accuracies at the sub-myr level – though the temptation to perform arbitrary selection 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 (other rocks and other zircon populations can – and do – behave very differently), and further technical advances will likely improve our ability to prepare grains or sub-grains of adequately enhanced quality for analysis. Consequently, 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, high-precision analyses (<1% error) that are realistically interpreted.