The Ocean redox state evolution and its controls during the Cambrian Series 1–2: Evidence from Lijiatuo Section, South China

Abstract

Well-exposed Lijiatuo Section was chosen to explore the temporal evolution and controls of the oceanic redox state, primary productivity and seawater sulfate levels during the Cambrian Series 1–2, South China. This section consists of Xiaoyanxi Formation (Fm.) mudstones and Liuchapo Fm. cherts that deposited in the slope and basin environment. Five oxic-anoxic cycles were identified based on V/Sc, Th/U and the enrichment factors of Mo, U, V, Ni and Cu. The Middle-Upper Liuchapo Fm. and the Middle Xiaoyanxi Fm. were deposited under oxic-suboxic conditions, and the rest of the strata were under anoxic conditions. The Re/Mo ratio demonstrated that the oxic-suboxic conditions in the Middle Xiaoyanxi Fm. were accompanied by transient sulfidic conditions, and the rest of the section was underanoxic and non-sulfidic conditions. All the TOC and the enrichment factors of Ba, Ni, Cu, Zn and Cd demonstrated that both the sinking and burial flux of organic matter (OM) in Liuchapo Fm. were lower than that in the overlying Xiaoyanxi Fm. The highest sinking and burial flux of OM in the Xiaoyanxi Fm. appeared at its lower parts; however, the lowest sinking and burial flux of OM in the Xiaoyanxi Fm. appeared in its middle parts. TOC/TS, TS and the vertical trend of δ34Spy demonstrated that the seawater was dominated by low oceanic sulfate levels, which resulted in the absence of free H2S. The rise of the atmospheric oxygen content may be the principal driver for the associated, transient suboxic-oxic and nearly sulfidic environment in the middle Xiaoyanxi Fm.