Abstract The Ordovician-Silurian transition (OST) coincided with global environmental and biological changes, including perturbations of the carbon and sulfur cycles, oceanic redox changes, and a mass extinction event, but their effects on the contemporaneous marine nitrogen cycle remain poorly known. In this study, we measured bulk nitrogen isotopes in the Datianba and Shuanghe OST sections of South China, which accumulated in the semi-restricted inner Yangtze Sea. The uniformly low δ15Nbulk (mostly –1 to +1 ‰) of Wufeng and Longmaxi Formation black shales (representing the pre- and post-Hirnantian intervals) imply large losses of bioavailable nitrogen (i.e., nitrate and ammonium) via denitrification/anammox as well as strong nitrogen fixation by diazotrophs. Persistence of low δ15Nbulk values within the Guanyinqiao Bed (representing the Hirnantian glacial interval) indicates that denitrification/anammox remained strong even during this major glacio-eustatic fall and deep-ocean ventilation event. We hypothesize that elevated primary productivity caused by upwelling was a key factor producing a shallow redoxcline in the inner Yangtze Sea. By comparing our nitrogen data with records from other cratons, we suggest that oceanic anoxia was prevalent globally during the Late Katian to early Rhuddanian, and that ocean-redox conditions were spatially heterogeneous during the Hirnantian glaciation. These considerations imply that bioavailable nitrogen was the limiting factor on marine primary productivity during the OST, and that oceanic anoxia may have contributed to the Late Ordovician mass extinction, especially on the Yangtze Platform.

中文翻译：

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奥陶纪-志留纪过渡期海洋生产力提升引发扬子地台（华南）氮限制

摘要 奥陶纪-志留纪过渡（OST）与全球环境和生物变化相吻合，包括碳和硫循环的扰动、海洋氧化还原变化和大规模灭绝事件，但它们对同期海洋氮循环的影响仍知之甚少。在这项研究中，我们测量了华南大田坝和双河 OST 剖面的大量氮同位素，这些同位素聚集在长江半限制性内陆海域。五峰组和龙马溪组黑色页岩的 δ15Nbulk 均一低（主要是 –1 到 +1 ‰）（代表前和后 Hirnantian 层段）意味着生物可利用的氮（即硝酸盐和铵）也通过反硝化/厌氧氨氧化大量损失如固氮菌的强固氮作用。观音桥床（代表 Hirnantian 冰川区间）内低 δ15Nbulk 值的持续存在表明，即使在这次主要的冰川 - 浮沉和深海通风事件期间，反硝化/厌氧氨氧化仍然很强。我们假设上升流引起的初级生产力提高是在长江内海产生浅层氧化还原反应的关键因素。通过将我们的氮数据与其他克拉通的记录进行比较，我们认为在卡德阶晚期到鲁丹阶早期，海洋缺氧在全球范围内普遍存在，并且在赫南特冰川期间海洋氧化还原条件在空间上是异质的。这些考虑意味着生物可利用的氮是 OST 期间海洋初级生产力的限制因素，并且海洋缺氧可能导致了晚奥陶世大灭绝，