Cerium anomaly variations in Ediacaran-earliest Cambrian carbonates from the Yangtze Gorges area, South China: Implications for oxygenation of coeval shallow seawater

摘要

The late Neoproterozoic ocean witnessed the naissance of animals which is believed to have been stimulated by an increase in oxygen levels in the ocean. However, apart from the emergence of animal fossils, little supporting evidence has been found for the rise of oxygen in shallow seawater. Here we present Ce anomaly data, a redox proxy with a higher reduction potential than many other redox proxies, for carbonates from well-preserved marine successions (the Doushantuo Formation, Dengying Formation and lower Yanjiahe Formation of the Ediacaran Period, and the upper Yanjiahe Formation of early Cambrian) on the Neoproterozoic Yangtze Platform in the Yangtze Gorges area, in order to constrain the redox evolution of the shallow marine environment Calculated Ce anomalies were screened, using the following criteria: Al. < 0.35% and Fe < 0.45% in bulk rock, Th < 0.5. ppm, Sc < 2. ppm, ΣREE < 12. ppm and Y/Ho > 36 in acetic-acid-leached carbonate, to target samples that preserve primary seawater REE features and Ce anomalies. The samples satisfying this screening show seawater-like REE distribution patterns in leached carbonates and may have recorded Ce anomalies of the seawater from which the carbonates precipitated. These data show that the Doushantuo Formation has Ce/Ce~* values between 0.92 and 0.71, the Dengying Formation between 0.90 and 0.40, and the Yanjiahe Formation between 0.52 and 0.72. The Ce/Ce~* values of the Doushantuo Formation suggest that shallow waters during the Doushantuo stage (635-551. Ma) were anoxic to suboxic. From the bottom to the top of the Dengying Formation, Ce/Ce* values decrease systematically, which cannot be explained by a change in depositional depth of the carbonate but suggests that the shallow waters became more oxygenated during the Dengying stages (from 551. Ma to Ediacaran-Cambrian boundary). These results may provide direct evidence for an increase in oxygen levels in the shallow marine environment during the Ediacaran Period.