The latest Proterozoic to early Cambrian is characterized by major global phosphogenic events and the diversification of complex multicellular life. However, the feedback mechanism of atmospheric oxidation and upwelling processes to form voluminous phosphorite deposits is unclear. South China is a key example of the sedimentary spectrum related to the Neoproterozoic to Early Cambrian bloom of oxygenic photosynthesis, with alginite deposits (sapropelite; “stone coal”), barite, polymetallic black shales and phosphorites along the passive continental paleo-margin of the Yangtze craton. Here, we present a comprehensive geochemical analysis of drill core samples from the Zhijin phosphorite deposit, including Cr isotope, Fe speciation, and bulk elemental composition of dolostone, phosphorite and black shale from the Ediacaran to Early Cambrian sequence. The positively fractionated δ53Cr values of 0.11 to 1.81 ‰ of leachates from Ediacaran dolostone and Terreneuvian phosphorite indicate intensive oxidative terrestrial weathering. The phosphorite samples exhibit REE distribution patterns similar to modern seawater and strongly negative Ce anomalies (0.47–0.61), suggesting an oxic, open ocean environment. In contrast, the absence of Ce anomaly, high FeHR/FeT, and Fepy/FeHR ratios, enhanced redox-sensitive element concentrations, high Mo/TOC and TS/TOC, and high molar Corg:P ratios suggest euxinic to anoxic conditions for the overlying black shale sedimentation in a restricted sedimentary marine environment. Bulk-rock shale samples retain the positively fractionated Cr isotope pattern. The dramatic change from oxic to anoxic conditions aligns with the fossil record, which shows a proliferation of small shelly fossils in the phosphorite and their extinction in the overlying black shales, likely by hydrogen sulfide poisoning. We infer that nutrient-rich, oxidized upwelling seawater transported phosphorus and other nutrients to the photic zone, from where organic matter settled down to the seafloor and formed thick phosphorite beds at the oxic-anoxic boundary at and below the seafloor. Our study highlights that atmospheric and seawater oxygenation accelerated the Early Cambrian bio-diversification, and the subsequent restricted basin situation is responsible for the extinction at the Neoproterozoic-Cambrian boundary.

中文翻译：

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大陆的氧化风化作用和沿被动大陆边缘的海水上升流促进了华南新元古代-寒武纪边界磷矿的广泛形成


最新的元古代至寒武纪早期的特点是主要的全球磷酸化事件和复杂多细胞生命的多样化。然而，大气氧化和上升流过程形成大量磷矿床的反馈机制尚不清楚。华南地区是与新元古代至寒武纪早期含氧光合作用有关的沉积光谱的一个关键例子，海藻矿床（蓝宝石岩;“石煤”）、重晶石、多金属黑色页岩和磷矿沿着长江克拉通的被动大陆古边缘。在这里，我们介绍了来自智金磷矿床的钻芯样品的全面地球化学分析，包括 Cr 同位素、Fe 形态以及从埃迪卡拉纪到早寒武纪序列的白云石、磷矿和黑色页岩的体元素组成。来自埃迪卡拉纪白云岩和特雷诺瓦亚磷矿的浸出液的正分馏 δ53Cr 值为 0.11 至 1.81 ‰，表明强烈的氧化陆地风化作用。磷矿样品表现出类似于现代海水的 REE 分布模式和强负 Ce 异常 （0.47-0.61），表明存在氧化、开阔的海洋环境。相比之下，不存在 Ce 异常、高 FeHR/FeT 和 Fepy/FeHR 比率、增强的氧化还原敏感元素浓度、高 Mo/TOC 和 TS/TOC 以及高摩尔 Corg：P 比率表明在受限沉积海洋环境中上覆黑色页岩沉积的 euxin-oxic 条件。块状岩石页岩样品保留了正分馏的 Cr 同位素模式。 从含氧到缺氧条件的巨大变化与化石记录一致，化石记录显示小贝壳化石在磷矿中增殖，而它们在上覆的黑色页岩中灭绝，可能是由于硫化氢中毒。我们推断，营养丰富、氧化的上升流海水将磷和其他营养物质输送到光区，有机物从那里沉降到海底，并在海底及其下方的氧化缺氧边界形成厚厚的磷酸盐床。我们的研究强调，大气和海水氧化加速了早寒武纪生物多样化，随后的限制盆地状况是导致新元古代-寒武纪边界灭绝的原因。