The Doushantuo negative carbon isotope excursion (DOUNCE) is the largest known marine inorganic carbon isotope anomaly. The origin of this pronounced negative excursion is still an enigmatic issue that attracts geologists. Time constraints on the excursion are the critical information that would provide insight into its genesis. In previous decades, the timing of its termination has been constrained by the widely cited zircon U-Pb age of 550.5 ± 0.8 Ma for the tuff at the top of the Miaohe Member at the Jiuqunao section in the Yangtze Gorges area, South China. However, results of recent studies indicate that the reliability of this time constraint needs to be re-evaluated. Here, a geochronological study was carried out using two K-bentonites from Fanglong in South China. A K-bentonite in the lower Dengying Formation yielded a U-Pb age of 557 ± 3 Ma, while a K-bentonite in the basal Liuchapo Formation yielded an age of 550 ± 3 Ma. Based on regional correlations between the Ediacaran successions in South China, the age (557 ± 3 Ma) for the K-bentonite in the lower Dengying Formation may serve as a second critical timing constraint for the ending of the DOUNCE. Combined with available estimates of the DOUNCE duration, our new data indicate that the DOUNCE has a maximum onset age ∼570 Ma.

陡山沱负碳同位素偏移（DOUNCE）是已知最大的海洋无机碳同位素异常。这种明显的负面偏移的起源仍然是一个吸引地质学家的神秘问题。游览的时间限制是提供深入了解其起源的关键信息。在过去的几十年里，它的终止时间受到广泛引用的华南长江三峡地区九曲脑剖面苗河段顶部凝灰岩锆石 U-Pb 年龄 550.5±0.8 Ma 的限制。然而，最近的研究结果表明，需要重新评估此时间限制的可靠性。在这里，使用来自华南方龙的两种钾膨润土进行了年代学研究。下灯影组钾膨润土U-Pb年龄为557±3 Ma，而六岔坡组底部钾膨润土U-Pb年龄为550±3 Ma。基于华南埃迪卡拉纪层序的区域相关性，灯影组下部钾膨润土的年龄（557±3 Ma）可作为DOUNCE结束的第二个关键时间约束。结合对 DOUNCE 持续时间的可用估计，我们的新数据表明 DOUNCE 的最大发作年龄约为 570 Ma。