School of Earth and Space Exploration, Arizona State University, PO Box 871404, Tempe, AZ 85287-1404, USA.
Proc Natl Acad Sci U S A. 2011 Oct 25;108(43):17631-4. doi: 10.1073/pnas.1106039108. Epub 2011 Oct 10.
Periods of oceanic anoxia have had a major influence on the evolutionary history of Earth and are often contemporaneous with mass extinction events. Changes in global (as opposed to local) redox conditions can be potentially evaluated using U system proxies. The intensity and timing of oceanic redox changes associated with the end-Permian extinction horizon (EH) were assessed from variations in (238)U/(235)U (δ(238)U) and Th/U ratios in a carbonate section at Dawen in southern China. The EH is characterized by shifts toward lower δ(238)U values (from -0.37‰ to -0.65‰), indicative of an expansion of oceanic anoxia, and higher Th/U ratios (from 0.06 to 0.42), indicative of drawdown of U concentrations in seawater. Using a mass balance model, we estimate that this isotopic shift represents a sixfold increase in the flux of U to anoxic facies, implying a corresponding increase in the extent of oceanic anoxia. The intensification of oceanic anoxia coincided with, or slightly preceded, the EH and persisted for an interval of at least 40,000 to 50,000 y following the EH. These findings challenge previous hypotheses of an extended period of whole-ocean anoxia prior to the end-Permian extinction.
海洋缺氧期对地球的进化历史有重大影响，通常与大规模灭绝事件同时发生。全球（而非局部）氧化还原条件的变化可以使用 U 系代用指标进行潜在评估。通过在中国南方大汶口碳酸盐岩剖面中（238）U/（235）U（δ（238）U）和 Th/U 比值的变化，评估了与二叠纪末灭绝事件（EH）相关的海洋氧化还原变化的强度和时间。EH 的特征是δ（238）U 值降低（从-0.37‰降至-0.65‰），表明海洋缺氧范围扩大，以及 Th/U 比值升高（从 0.06 升至 0.42），表明海水中 U 浓度降低。利用质量平衡模型，我们估计这种同位素变化代表 U 通量向缺氧相增加了六倍，这意味着海洋缺氧范围相应增加。海洋缺氧的加剧与 EH 同时发生，或略早于 EH，并在 EH 之后至少持续了 40,000 到 50,000 年。这些发现挑战了先前关于整个海洋在二叠纪末灭绝前缺氧时间延长的假说。
