Abstract Two pulses of faunal mortality occurred during the Late Ordovician mass extinction (ca. 445 Ma). This biocrisis is recorded in Hirnantian strata of South China as a stepwise extinction of graptolites in both the hydrologically semi-restricted inner and open outer Yangtze Sea. Although expanded marine euxinia is widely regarded as the main cause of the biocrisis, the spatial-temporal pattern and driving mechanisms behind redox changes, as well as the extent to which they influenced marine faunas, remain unclear. Here, we present a study of mid-shelf and outer-shelf sections of the less-studied outer Yangtze Sea, based on an integrated suite of geochemical proxies [i.e., iron speciation, pyrite δ34S (δ34Spy), and major- and trace-element data] to provide insight into changes in marine redox conditions, chemical weathering rates, and primary productivity across the Hirnantian Glaciation. Iron speciation ratios and trace-element enrichment factors show that euxinia appeared in mid-shelf settings during the late Katian, subsequently expanded into deeper waters, and then diminished during the Hirnantian Glaciation and expanded again thereafter. Expansions of euxinia across the outer Yangtze Sea prior to and after the Hirnantian Glaciation may have been the result of both elevated primary productivity, as indicated by elevated organic carbon accumulation rates (OCAR), and increased sulfate weathering inputs due to preglacial and postglacial enhanced fresh-rock exposure on land. However, we propose that the local development of euxinia was controlled mainly by sulfate availability, which depended on continental weathering intensity—a hypothesis supported by strong covariant relationships between the chemical index of alteration (CIA, a weathering proxy), redox proxies, and δ34Spy values. The contraction of oceanic euxinia during the main glacial interval was caused by a reduction in continental weathering intensity; contemporaneously weaker euxinia, as well as higher δ34Spy values, in outer-shelf relative to mid-shelf areas may have been due to limited terrestrial sulfate supply in deeper sites and small sulfate reservoir in the open ocean during the glaciation. Persistence of euxinia in the outer Yangtze Sea after the termination of the main glacial interval is evidence of a growing sulfate reservoir in early Silurian oceans, probably due to increased continental weathering and rising atmospheric O2 levels. Furthermore, the comparision of the redox and fossil records indicates that elevated extinctions among mesopelagic graptolites coincided with enhanced euxinia in the outer Yangtze Sea, supporting the hypothesis that redox changes were the main stressor of both benthic fauna and some zooplankton. Our results highlight the interrelated influences of climate, continental weathering, and riverine sulfate fluxes on marine redox variations and the biotic crisis of the Late Ordovician.

中文翻译：

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外扬子海（华南）通过 Hirnantian 冰川的氧化还原变化及其对奥陶纪末生物危机的影响

摘要 在晚奥陶世大灭绝（约 445 Ma）期间发生了两次动物群死亡。此次生物危机在华南的 Hirnantian 地层中被记录为水文半限制性的长江内海和开放的外海中笔石的逐步灭绝。虽然扩大的海洋euxinia被广泛认为是生物危机的主要原因，但氧化还原变化背后的时空模式和驱动机制，以及它们对海洋动物群的影响程度仍不清楚。在这里，我们基于一套完整的地球化学指标[即铁形态、黄铁矿 δ34S (δ34Spy) 和主要和微量-元素数据] 深入了解海洋氧化还原条件、化学风化率的变化，和整个 Hirnantian 冰川的初级生产力。铁形态比和微量元素富集因子表明，euxinia 出现在晚期 Katian 中陆架环境，随后扩展到更深的水域，然后在 Hirnantian 冰川期间减少，此后再次扩展。在 Hirnantian Glaciation 之前和之后，euxinia 在外长江的扩张可能是初级生产力提高的结果，如有机碳积累率 (OCAR) 升高所示，以及由于冰前和冰后增强的新鲜度增加了硫酸盐风化输入。 - 陆地上的岩石暴露。然而，我们认为euxinia的局部发育主要受硫酸盐可用性控制，这取决于大陆风化强度——这一假设得到了化学变化指数（CIA，风化代理）、氧化还原代理和 δ34Spy 值之间的强协变关系的支持。主冰期期间大洋裸露的收缩是大陆风化强度降低引起的；同时期较弱的euxinia 以及较高的δ34Spy 值，可能是由于冰川作用期间较深地点的陆地硫酸盐供应有限和开阔海洋中的小型硫酸盐储层。主冰期结束后，外长江海藻的持续存在是志留纪早期海洋中硫酸盐库不断增长的证据，这可能是由于大陆风化作用的增加和大气中 O2 水平的升高。此外，氧化还原和化石记录的比较表明，中远洋笔石的灭绝加剧与长江外海的euxinia增强相吻合，支持氧化还原变化是底栖动物和一些浮游动物的主要压力源的假设。我们的研究结果强调了气候、大陆风化和河流硫酸盐通量对海洋氧化还原变化和晚奥陶世生物危机的相互影响。