Marine chemical structure during the Cambrian explosion

The Early Cambrian Late Stage 2 to Stage 3 (ca. 526-514 Ma) was characterized by the peak of the Cambrian Explosion and a large-scale global transgression that resulted in the deposition of organic-rich black shales, which have important implications for Earth-system science and petroleum/economic geology. However, there are many uncertainties regarding the spatiotemporal evolution of structural model, and driving mechanisms of the paleo-oceanic environment during this period, requiring more examples and research constraints. To fill the knowledge gap, high-resolution elemental, carbon isotope, and biogeochemical data were obtained from two continuous drill cores, along with redox-sensitive proxy data from additional 21 cores/sections of different sedimentary facies in the Yangtze Platform, South China, provide a comprehensive and detailed spatiotemporal window into this issue. It demonstrates that the dynamic evolution process of redox conditions varied between shallow- and deep-water regions, and temporal variations in chemical weathering intensity and spatiotemporal differences in upwelling currents controlled the redox state of the ocean, which was closely related to the relative contents of organic carbon (TOC), SO42-, and Fe2+. The mechanisms that controlled the scale of sulfidic wedges differed among different regions and stages. During intervals I (Late Stage 2) and II (Early-Middle Stage 3), shallow and deep waters were controlled mainly by TOC and SO42- contents, respectively. Deep waters during interval III (Late Stage 3) were controlled mainly by TOC contents. The spatiotemporal heterogeneity of the Early Cambrian paleo-ocean redox state partly controlled the spatiotemporal distribution of the fossil record. Primary productivity was the main controlling factor on the spatiotemporal variations in hydrocarbon source rock quality, followed by the organic matter preservation conditions. This study highlights the Earth-system science research on the deep-time ocean environment and its resource effects.