The Cambrian Miaolingian and Furongian (509–485 Ma) represent a critical transitional interval in evolutionary history of animals, bridging the Cambrian explosion to Great Ordovician Biodiversification Event (GOBE). Despite its significance, scant attention has been directed toward understanding the shifts in marine redox conditions and biogeochemical cycles during this epoch, hampering the broader understanding of linkages between marine environment and early animal radiation. Here we present new paired sulfur isotope records from carbonate-associated sulfate (δ34SCAS) and pyrite (δ34Spy), along with nitrogen isotopes (δ15Ndecarb) from a continuous carbonate succession in South China, aiming to better constrain marine redox states and biogeochemical cycles during the middle-late Cambrian (ca. 509–495 Ma). Overall low sulfur isotope offsets Δ34SCAS-py (Δ34SCAS-py = δ34SCAS-δ34Spy, 1.1–46.6‰，median = 21.0‰) indicate continuously low seawater sulfate concentrations during middle-late Cambrian. The consistently low δ15Ndecarb values (−1.4–2.3‰，median = 0.4‰) suggest that this period was featured by active nitrogen fixation and a small nitrate reservoir in the shallow ocean. Taken together, the sulfur and nitrogen isotopic data can be best explained by persistently low global oxygenation levels of the middle-late Cambrian oceans. Meanwhile, high variabilities of Δ34SCAS-py and δ15Ndecarb recorded by the studied section also suggest fluctuated marine redox conditions on continental margins during this period. In combination with paleontological records, we propose that persistently low marine oxygenation levels and frequent expansion of anoxic seawater on continental margins may have induced a trough period in animal biodiversification after the Cambrian explosion.

中文翻译：

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寒武纪大爆发后大陆架海水的低氧水平和波动的氧化还原态


寒武纪苗陵纪和芙蓉纪（509-485 马）代表了动物进化史上的关键过渡期，将寒武纪大爆发与大奥陶纪生物多样性事件 （GOBE） 联系起来。尽管它很重要，但很少有人关注了解这一时期海洋氧化还原条件和生物地球化学循环的变化，阻碍了对海洋环境与早期动物辐射之间联系的更广泛理解。在这里，我们展示了来自碳酸盐伴生硫酸盐 （δ34SCAS） 和黄铁矿 （δ34Spy） 的新配对硫同位素记录，以及来自华南连续碳酸盐演替的氮同位素 （δ15Ndecarb），旨在更好地限制寒武纪中晚期（约 509-495 马）的海洋氧化还原状态和生物地球化学循环。总体低硫同位素偏移 Δ34SCAS-py （Δ34SCAS-py = δ34SCAS-δ34Spy，1.1–46.6‰，中位数 = 21.0‰）表明寒武纪中晚期海水硫酸盐浓度持续较低。持续较低的 δ15Ndecarb 值 （-1.4–2.3‰，中位数 = 0.4‰） 表明这一时期的特点是活性固氮和浅海中的小型硝酸盐储层。综上所述，硫和氮同位素数据可以用寒武纪中晚期海洋持续较低的全球氧合水平来最好地解释。同时，研究部分记录的 Δ34SCAS-py 和 δ15Ndecarb 的高变异性也表明在此期间大陆边缘的海洋氧化还原条件波动。结合古生物学记录，我们提出，持续的低海洋氧合水平和大陆边缘缺氧海水的频繁扩张可能诱导了寒武纪大爆发后动物生物多样性的低谷期。