Copper and Zinc isotopes trace the evolution of the Ediacara-Early Cambrian paleo-ocean redox condition in the Tarim Basin, China

Abstract

The Ediacaran-early Cambrian transition is a critical period in the Earth's evolutionary history. A series of important geological and biological events are likely related to environmental change. Ocean oxygenation levels during the early Cambrian are still controversial and barely constrained although abundant geochemical studies have been conducted. The black shale widely developed in the Early Cambrian can record paleo-environmental information of contemporaneous seawater. The early Cambrian Yuertusi Formation black shale in the Luntan 3 well from the Tarim Basin was studied in terms of Cu and Zn isotopic composition and redox sensitive trace element to improve the knowledge of the extent of ocean oxygenation during the Ediacaran-Early Cambrian. The Yuertusi Formation is divided into four stages according to lithology variation and records of Cu and Zn isotopes and redox sensitive elements, indicating the fluctuation of ocean redox condition. The total organic carbon (TOC) varied widely from 0.10% to 18.25% with an average of 6.34%. Enrichment of Mo, U and V elements which have a synergistic relationship with TOC and redox proxies (ratios of U/Th and V/Cr, EF(Mo)-EF(U)) of the four stages in the Yuertusi Formation, indicate an anoxic even euxinic deposition environment, especially in stages Ⅰ and Ⅲ. Most of the Cu isotope values, lower than the modern seawater values, also indicate that the ocean state was anoxic and did not reach the current ocean oxidation level, which shows the same characteristics as the other profiles in different areas. The variation in the Cu isotope ranges from −3.10‰ to +1.23‰, and the Zn isotope ranges from 0.09‰ to 1.46‰. Relatively large Cu and Zn isotopic fractionation in stages Ⅰ and Ⅲ occur with an intense reduction environment, which is attributed to the combination of sulfide deposits that are eventually immobilized in sediments. Under the overall background of an anoxic environment, the four stages show a cycle of relative anoxic-weak oxidation-anoxic-weak oxidation, which is inferred to be connected with the transgressive-regressive cycle and weathering enhancement variation based on Ce anomaly, Mo/Toc ratios and C stable isotope data. The Early Cambrian transgression provided an anoxic even euxinic environment conducive to organic matter preservation for the development of black shale during stage Ⅰ. With the sea level fluctuation and weathering enhancement, the sea retreated gradually, productivity gradually increased, and the marine environment was relatively oxidized. The increasing trend of the whole Cu and Zn isotope values from bottom to top in the Yuertusi Formation is interpreted to be gradual relative oxidizing.