Fluctuation of shelf basin redox conditions in the early Ediacaran: Evidence from Lantian Formation black shales in South China

• Lamina-by-lamina pyrite framboid measurements were conducted for Lantian black shales.
• The Ediacaran Lantian black shales were deposited in fluctuant redox conditions.
• The frequent redox condition changes are not clearly indicated by RSTE data.

Finely laminated, pyrite- and organic-rich black shales that bear a euxinic interpretation dominate the Ediacaran Lantian Formation in southern Anhui, South China. However, these black shales preserve benthic sessile algae and possible metazoans in situ, suggesting that the Lantian shelf basin may have had at least episodically oxidized bottom water during the early Ediacaran Period. Redox conditions of the fossiliferous Lantian black shales were examined by petrological analysis, pyrite framboid and δ34S measurements, and redox-sensitive trace element (RSTE) analysis. Lamina-by-lamina pyrite framboid measurements show that Lantian black shales were deposited under fluctuating redox conditions. The δ34S values of pyrites range from −20.5‰ to 2.3‰ for different laminae and display less negative values than the earliest Ediacaran black shales, supporting a return to a relatively low seawater sulfate concentration after the early Ediacaran oxidation event. Similarly, the Lantian black shale RSTE concentrations (e.g., Mo, U, and V) are low compared to previously published results from the early Ediacaran and constant from lamina to lamina, suggesting a drop in ocean wide RSTE concentrations occurring independently of frequent local redox condition changes. These frequent redox condition changes in the Lantian shelf basin may have facilitated the productivity and preservation of macroscopic Lantian biota.