Abstract The Late Ordovician was a time of major climatic and sea-level changes linked to the Hirnantian Glaciation, marked by large positive excursions of δ13Ccarb and δ13Corg globally known as the Hirnantian carbon isotopic excursion (HICE). However, some Upper Ordovician sections in South China exhibit negative δ13Ccarb shifts within the Hirnantian-age Guanyinqiao Bed. We hypothesize that these unusual carbon-isotope patterns were due to influx of 13C-depleted dissolved inorganic carbon (DIC) from the underlying Wufeng and overlying Longmaxi black shales as a result of greater compaction of these fine-grained siliciclastic units compared to the coarse-grained carbonate Guanyinqiao Bed. The isotopically light DIC was generated through oxidation of organic matter by either microbial sulfate reduction (MSR) or methanogenesis (with subsequent anaerobic oxidation of methane, AOM) in the black shales. This hypothesis is supported by petrographic and geochemical evidence of vertical fluid migration within the Guanyinqiao Bed and across its contacts. In the Shiqiao drillcore, the Guanyinqiao Bed exhibits a V-shaped depression at its top that is connected to vertical channels filled with secondary carbonate cements having low δ13C values. In the Huanggexi section, both the base and top of the Guanyinqiao Bed are characterized by strong U enrichments, despite unambiguously oxic depositional conditions as inferred from other proxies (Fe speciation, Mo and V concentrations). These anomalous U enrichments resulted from oxidative remobilization of U along the Guanyinqiao-black shale contacts and subsequent transport into the Guanyinqiao Bed through compactional flow. This finding demonstrates the importance of recognizing remobilized trace metals in sedimentary successions in order to avoid potential errors in redox interpretations.

中文翻译：

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压实驱动流体垂直运移进入华南上奥陶统（Hirnantian）观音桥层的新证据

摘要 晚奥陶世是与 Hirnantian 冰川作用相关的主要气候和海平面变化时期，其标志是 δ13Ccarb 和 δ13Corg 的大规模正偏移，被称为 Hirnantian 碳同位素偏移（HICE）。然而，华南部分上奥陶统剖面在海南天观音桥层内表现出δ13Ccarb负位移。我们假设这些不寻常的碳同位素模式是由于这些细粒硅质碎屑单元与粗-粒状碳酸盐观音桥层。同位素轻 DIC 是通过微生物硫酸盐还原 (MSR) 或产甲烷作用（随后厌氧氧化甲烷，AOM）在黑色页岩中氧化有机物产生的。这一假设得到了观音桥层内及其接触层内流体垂直运移的岩石学和地球化学证据的支持。在石桥钻芯中，观音桥层顶部呈V形凹陷，与充满低δ13C值的次生碳酸盐胶结物的垂直通道相连。在黄阁西剖面，观音桥层的底部和顶部均以强烈的 U 富集为特征，尽管从其他替代物（Fe 形态、Mo 和 V 浓度）推断出的含氧沉积条件明确。这些异常的 U 富集是由于 U 沿观音桥-黑色页岩接触面发生氧化再动员，随后通过压实流输送到观音桥层所致。这一发现证明了识别沉积序列中再迁移的痕量金属以避免氧化还原解释中的潜在错误的重要性。