Pyrite morphology and episodic euxinia of the Ediacaran Doushantuo Formation in South China

The Ediacaran Doushantuo Formation with well-preserved fossil record in South China provides a rare window for our understanding of biological evolution, global carbon cycle, and oceanic redox states. Prominent negative delta C-13 anomalies (i.e., the Shuram excursion) in Ediacaran successions worldwide fundamentally challenge the traditional models of isotopic mass balance. Additionally, conflicting opinions of both oxic and anoxic conditions have been proposed for the deep waters during this period. Here, we present a detailed study of pyrite morphology and carbonate carbon isotope data documented from drill core samples at Songtao County, northeastern Guizhou. Framboid aggregates are the dominant pyrite form in black shale and they can transfer to euhedral crystals through continuous growth of the constituent microcrystals. A positive correlation between microcrystal sizes (d) and framboid diameters (D) is observed, while the different D/d ratios of framboids in argillaceous dolostone and black shale reflect different substrate availability. Electron microprobe analyses reveal no consistent compositional patterns between framboidal and euhedral pyrites. Framboid size distributions of the investigated drill core, in combination with previously published redox data from the intra-shelf Jiulongwan section, shelf margin Zhongling section, and lower slope Wuhe section, suggest that three episodes of marine euxinia have been established throughout the deposition of the Doushantuo Formation. The time lag between the uppermost euxinic interval and the Shuram excursion may arise from the depression of sulfate reduction maintained by other oxidants.