A predominantly ferruginous condition in the Ediacaran deep ocean: Geochemistry of black shales in the Ediacaran Doushantuo Formation, South China

摘要

The redox condition of the ocean played an important role in the evolution of the Ediacaran biota, but it remains largely unexplored, particularly for the deep ocean environment. In this study, we present a comprehensive study of Fe speciation and trace elements of the Doushantuo black shales from the Longe and Jinjiadong sections in South China, to investigate the redox condition of basinal environments in Ediacaran. In general, both sections show high FeHR/FeT ratios (>0.38), low Fe-py/Fe-HR ratios (<0.7), suggesting a predominately anoxic and ferruginous condition in the deep water of Nanhua basin during the Doushantuo period. A few high Fe-py/Fe-HR ratios (>0.7) in the Jinjiadong section indicating occasional euxinia in the basin fades. Fepy/FeHR ratios show an increase in the middle part both in the Longe and Jinjiadong sections, indicating enhanced production of H2S via bacteria sulfate reduction (BSR) in the deep anoxic water, which was likely responsible to the upper Doushantuo negative carbon isotope excursion.