Geochemical and paleontological evidence of early Cambrian dynamic ocean oxygenation and its implications for organic matter accumulation in mudrocks at the Three Gorges area, South China

• Published in: Marine and petroleum geology 2022-12, Vol.146, p.105958, Article 105958
• Main Authors: Zhang, Yu, Zhang, Tongwei, Huang, Dejiang, Shao, Deyong, Luo, Huan
• Format: Article
• Language: English
• Subjects: Agglutinated foraminifera; Cambrian; Organic matter; Redox condition; Shuijingtuo formation; Trace elements
• ISSN: 0264-8172; 1873-4073
• DOI: 10.1016/j.marpetgeo.2022.105958

The evolution of global ocean oxygenation during the early Cambrian remains highly controversial, making it difficult to evaluate how environmental triggers play a role in controlling the organic matter (OM) accumulation in black shales. In this study, an integrated approach, including total organic carbon (TOC) content, major and trace element geochemistry, and microscope images, was systematically conducted in a continuous core well that penetrated through the Lower Cambrian Yanjiahe (YJH)–Shuijingtuo (SJT, subdivided into SM Ⅰ, SM Ⅱ, SM Ⅲ, and SM Ⅳ members) successions (∼541-514Ma) at the Three Gorges area, Yangtze Platform, South China. Our results showed that the ocean stepwise oxidation evolution in shelf-slope environments occurred even though the marine redox state showed highly dynamic fluctuations during the early Cambrian. Based on geochemical proxies and sedimentological and faunal criteria, the OM-rich mudrocks in the SJT Formation can be divided into two units. The lower part (SM Ⅰ) contains a higher TOC content (average value 5.71 wt%) than the upper part (SM II, average TOC value of 2.73 wt%). SM Ⅰ are characterized by high MoEF, UEF, Corg/P, and U/Th ratios, indicating that SM I was deposited under constantly euxinic conditions. The relatively declining MoEF, UEF, Corg/P, and U/Th ratios at the transition interval between the SM Ⅰ and SM II records a change in bottom-water to predominantly anoxic conditions with intermittent oxygenation. Correspondingly, the biological record evolved from extremely limited benthic organisms to diverse sponge spicules. The high paleoproductivity, sustained euxinic conditions, and limited dilution linked to sea-level rising are responsible for the OM enrichment in SM I black shales. In comparison, SM II was deposited under mainly suboxic/dysoxic conditions with the lower Corg/P, U/Th ratios, and moderate enrichment of Mo (