Abstract The early Cambrian ocean was marked by significant redox changes, but the detailed redox evolution of seawater and its relationship to biological expansion are not fully understood. Widespread chert successions are present in lower Cambrian deposits on the Yangtze Block, South China, which have great potential to record oceanic chemistry. Here, we analyze rare earth elements (REE) and Si and Fe isotopes along three regional chert sections to constrain the origin of lower Cambrian cherts and the redox conditions of the early Cambrian ocean. The REE patterns and Si isotopes illustrate that lower Cambrian cherts at the Muyang (MY; Yanjiahe Formation) and Zunyi (ZY; Niutitang Formation) locales originated from the replacement of precursor carbonates and black shales, respectively, whereas cherts at Chuanyanping (CYP; Liuchapo Formation) locale originated from direct chemical precipitation from seawater and/or hydrothermal fluids. All cherts, except for several lower CYP cherts with hydrothermal contributions, primarily preserved seawater chemical signals. The Ce/Ce* and Y/Ho ratios indicate that these cherts were deposited near a suboxic/anoxic interface, where seawater may have been ferruginous overall, while more subtle redox changes, namely, ferruginous (less reducing) and ferruginous (more reducing), were identified by Fe isotopes. Based on new data and previous results, we propose that the early Cambrian ocean on the Yangtze Block was characterized by a stratified redox framework. Before ~535 Ma in the Cambrian, oxic seawater occurred only along the inner shelf (MSC section), below which seawater was primarily ferruginous (less reducing). During ~535–526 Ma, oxic seawater gradually expanded to the outer shelf (MY section) and a metastable ferruginous (more reducing) zone dynamically existed at shelf–slope locations. Temporal and spatial comparisons revealed that the water column on the Yangtze Block may have experienced a progressive deepening of the redoxcline during the early Cambrian (~535–526 Ma). The metastable redox zone may have been an analogue of the modern oxygen minimal zone (OMZ), which is associated with biological activity and high productivity. The occurrence and spatial fluctuation of the metastable ferruginous (more reducing) zone may have been associated with the evolution of small shelly fossils from assemblage 1 (SSF1) to assemblage 3 (SSF3) and their spatial expansion from shelf to slope. Therefore, accompanied by oceanic oxygenation, the occurrence and spatial fluctuation of an OMZ-like metastable zone may have regulated the biological diversification and distribution during the early Cambrian (~535–526 Ma).

中文翻译：

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华南早期寒武纪爆发期间燧石记录的海洋化学

摘要 早寒武世海洋以显着的氧化还原变化为标志，但海水的详细氧化还原演化及其与生物扩张的关系尚未完全了解。华南扬子地块下寒武统沉积物中存在广泛的燧石序列，具有巨大的海洋化学记录潜力。在这里，我们沿着三个区域燧石剖面分析了稀土元素 (REE) 和 Si 和 Fe 同位素，以限制下寒武统燧石的起源和早寒武纪海洋的氧化还原条件。REE 模式和 Si 同位素表明牧羊（MY；燕家河组）和遵义（ZY；牛蹄塘组）的下寒武统燧石分别来源于前驱体碳酸盐岩和黑色页岩的置换，而川岩坪（CYP；六岔坡组）起源于海水和/或热液流体的直接化学沉淀。所有的燧石，除了几个具有热液贡献的低 CYP 燧石外，主要保留了海水化学信号。Ce/Ce* 和 Y/Ho 比率表明这些燧石沉积在低氧/缺氧界面附近，其中海水总体上可能是含铁的，而更细微的氧化还原变化，即含铁（还原较少）和含铁（还原较多） , 由 Fe 同位素鉴定。基于新的数据和先前的结果，我们提出扬子地块早寒武世洋具有分层氧化还原框架的特征。在寒武纪约 535 Ma 之前，含氧海水仅沿着内陆架（MSC 部分）出现，在该区域以下海水主要是铁质的（还原较少）。在~535-526 Ma 期间，含氧海水逐渐扩展到外陆架（MY 部分），并且在陆架-斜坡位置动态存在亚稳态铁质（更多还原）带。时空比较表明扬子地块上的水体可能在寒武纪早期（~535-526 Ma）经历了氧化还原线的逐渐加深。亚稳态氧化还原区可能是现代氧最小区 (OMZ) 的类似物，它与生物活性和高生产力有关。亚稳铁质（更多还原）带的发生和空间波动可能与小壳化石从组合 1 (SSF1) 到组合 3 (SSF3) 的演化及其从陆架到斜坡的空间扩展有关。因此，伴随着海洋的氧化作用，