Dynamic seawater redox status in the early Mesoproterozoic: Evidence from trace elements and sulfur isotopic compositions of carbonate associated sulfate in multiple sections

Abstract

The early Mesoproterozoic (1.6–1.4 Ga) is a critical stage for oxygenation of Earth’s surface and early eukaryote evolution. However, the relationship between both remains uncertain. Here, we provide new carbonate associated sulfate (CAS) sulfur isotopic compositions (δ34SCAS) and trace elements from the 1.6 to 1.55 billion-year-old Gaoyuzhuang Formation in multiple sections within the Yanshan Basin, North China, to investigate the temporal and spatial variation of ocean redox conditions. New and collected δ34SCAS data from five sections (Pingquan, Kuancheng, Qianxi, Jixian and Gan'gou sections) reveal similar sulfur isotopic excursions, but the differences in absolute values indicate spatial heterogeneity of sulfur isotopic compositions, which may reflect marine low-sulfate concentrations and oceanic redox stratification. The most elevated δ34SCAS values appeared at the top of the Second Member of the Gaoyuzhuang Formation, followed by distinct negative excursions at the bottom of the Third Member, suggesting a dynamic sulfur cycle and seawater redox status during this period. Combined with previous reported carbon and sulfur isotopes and some redox proxies (e.g., Ce anomalies, manganese-rich deposit, I/(Mg + Ca) and Fe speciation), we suggest that at least two possible oxygenation events may have occurred in the Gaoyuzhuang Formation: (1) the bottom of the Second Member and (2) the bottom or middle part of the Third Member. Different sections show asynchronous timing of these events, indicating that the redox status was spatial heterogenous within the Yanshan Basin. The spatial heterogeneity may be controlled by facies and paleogeographic locations. Considering that decimeter‐scale multicellular eukaryotes are preserved in the middle part of the Third Member, we argue that oxic bottom water condition may be a critical factor for early eukaryote evolution.