Sulfur and oxygen isotopes of sulfate extracted from Early Cambrian phosphorite nodules: Implications for marine redox evolution in the Yangtze Platform

Abstract

Phosphorite nodule beds are discovered in the black shale of basal Niutitang Formation throughout the Yangtze Platform in South China, recording an important phosphorite-generation event. Platform-wide phosphorite precipitation requires special oceanographic and geochemical conditions, thus the origin of the Niutitang phosphorite nodules may provide valuable information about the ocean chemistry in the Early Cambrian. In this study, we measured sulfur and oxygen isotopic compositions of sulfate extracted from phosphorite nodules collected from the basal Niutitang Formation. Phosphorite associated sulfate (PAS) is a trace amount of sulfate that incorporates into crystal lattice during phosphorite precipitation, accordingly PAS records the geochemical signals during phosphorite nodule formation. Sulfur isotopic composition of PAS (δ³⁴S_PAS) ranges from -1.16‰ to +24.48‰ (mean=+8.19‰, n=11), and oxygen isotopic value (δ¹⁸O_PAS) varies between -5.3‰ and +26.3‰ (mean=+7.0‰, n=8). Most phosphorite nodules have low δ³⁴S_PAS and low δ¹⁸O_PAS values, suggesting PAS mainly derived from anaerobic oxidation of H₂S within suboxic sediment porewater. We propose that phosphate was delivered to the Yangtze Platform by a series of upwelling events, and was scavenged from seawater with the precipitation of FeOOH. The absorbed phosphate was released into suboxic porewater by the reduction of FeOOH at the oxic-suboxic redox boundary in sediments, and phosphorite nodule precipitated by the reaction of phosphate with Ca²⁺ diffused from the overlying seawater. The platform-wide deposition of phosphorite nodules in the basal Niutitang Formation implies the bottom water might be suboxic or even oxic, at least sporadically, in Early Cambrian. We speculate that the intensified ocean circulation as evident with frequent occurrences of upwelling events might be the primary reason for the episodic oxidation of the Yangtze Platform in Early Cambrian.

Keywords

phosphorite nodules / Niutitang Formation / phosphorite associated sulfate / sulfur isotope / oxygen isotope

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