Multiple observations have shown that widespread oceanic anoxia may have played an important role in the end-Permian mass extinction and delayed biotic recovery in the Early Triassic. However, it has not been reached a consensus on the temporal and spatial changes in oceanic redox conditions at that time. Chromium (Cr) isotopes have been widely used to trace redox processes in surface environments. Here, we present the Cr isotope compositions (${\delta }^{53}$Cr) of the Upper Permian–Lower Triassic carbonate ramp from the Xiakou section in South China (eastern Paleotethys). We observed largely fractionated ${\delta }^{53}$Cr values (1.15‰ to 2.16‰) in the latest Permian, which are higher than most values among Phanerozoic sedimentary carbonates. Such high values may result from Cr reduction facilitated by the development of a reducing environment during the latest Permian. The high Cr isotope ratios were followed by an abrupt negative excursion of ${\delta }^{53}$Cr values (from average 1.78‰ to average −0.13‰) across the extinction horizon, which may be attributed to a rapid expansion of oceanic anoxia in the eastern Paleotethys. Persistent negative ${\delta }^{53}$Cr values covering the Early Triassic indicate that the ocean may not have recovered from anoxia for an extended period. Our Cr isotope results support a stepwise deterioration of oceanic redox environments, which has been previously proposed as a possible cause of the most severe biotic catastrophe in the Phanerozoic.

多项观察表明，广泛的海洋缺氧可能在早三叠世的二叠纪末大灭绝和生物恢复延迟中发挥了重要作用。然而，对于当时海洋氧化还原条件的时空变化尚未达成共识。铬 (Cr) 同位素已被广泛用于追踪表面环境中的氧化还原过程。在这里，我们介绍了 Cr 同位素组成（${\delta }^{53}$华南夏口剖面（古特提斯东部）上二叠统-下三叠统碳酸盐岩斜坡的 Cr)。我们观察到大部分分馏${\delta }^{53}$晚二叠世 Cr 值（1.15‰～2.16‰），高于显生宙沉积碳酸盐岩中的大多数值。如此高的值可能是由于最近二叠纪期间还原环境的发展促进了 Cr 的减少。高 Cr 同位素比率之后是突然的负偏移${\delta }^{53}$灭绝范围内的 Cr 值（从平均 1.78‰ 到平均 -0.13‰），这可能归因于古特提斯东部海洋缺氧的快速扩张。持续消极${\delta }^{53}$覆盖早三叠世的 Cr 值表明海洋可能在很长一段时间内都没有从缺氧中恢复过来。我们的 Cr 同位素结果支持海洋氧化还原环境的逐步恶化，这之前曾被认为是显生宙最严重的生物灾难的可能原因。