Reconstruction of the Ediacaran sulfur cycle and oceanic redox evolution in shallow-water regions of the Yangtze platform, South China

Abstract

The emergence and proliferation of metazoan lineage in the Ediacaran Period is commonly hypothesized to reflect an increase in the oxygen content of marine environments. However, recent studies suggest that the Ediacaran oceanic redox conditions were spatially heterogeneous and temporally dynamic. In this study, we report high-resolution pyrite S-isotope data (δ34Spy) for a continuous Ediacaran drill core that was deposited in a very shallow setting from the Yangtze platform, South China. Combined with petrographic observations, statistics of pyrite framboid diameters, total carbon (TC), total sulfur (TS), trace element, and published data, we attempt to characterize the details of sulfur cycle as well as oceanic redox change in the shallow-water regions of the Yangtze platform during the Ediacaran. The δ34Spy profile exhibits a large amplitude of variation, ranging from –13.7‰ to 41.4‰ throughout the drill core. Both global and regional changes in environment could have exerted effects on δ34Spy values. For the Doushantuo Formation, distinct δ34Spy records from shelf to basin may reflect spatial heterogeneity in sulfate concentrations, depositional environments, and availability of organic matter. Notably, periods of upwelling may cause oxygen-deficient zones within shallow water column and facilitate persistent burial of organic matter and pyrite in shelf regions. Subsequent exposure and weathering of these reduced carbon and sulfur may contribute to the “Shuram-Wonoka” event. The prevalence of high δ34Spy values in the terminal Ediacaran (Dengying Formation) likely indicates an enhanced pyrite burial flux at a global scale, during which extensive marine anoxia might take place. In addition, enrichments of V and U in three shale intervals of the drill core support periods of oxygenation for the Ediacaran oceans.