Recurrent anoxia recorded in shallow marine facies at Zhangcunping (western Hubei, China) throughout the Ediacaran to earliest Cambrian

Abstract

In order to reconstruct the redox history of surface seawater for Ediacaran shallow marine facies on the Yangtze Platform, as well as to understand its implications for geological events (oxidation and phosphatization) and early evolution of life, we have conducted a systematic high-resolution investigation on microfacies analyses, pyrite morphology as well as sulfur and carbon contents in a continuous carbonate Ediacaran–Cambrian succession (drilled 407-core) in the Zhangcunping area of western Hubei (South China). The framboidal pyrites and SNAV/TOC (NAV = non-acid volatile sulfur) based redox reconstruction show a dynamic history of recurrent shallow marine anoxia. The early aftermath of the Marinoan Glaciation (cap dolomite to phosphorites) was characterized by either absence of pyrite or dominated by large framboids, low TOC contents and phosphorite deposition as well as well-preserved microfossils, indicating a minor oxygenated (dysoxic to oxic) photic shallow marine ecosystem. Meanwhile, the uppermost Doushantuo and Dengying formations were potentially fully oxygenated due to the lack of pyrites as well as low SNAV and TOC contents. Correlating with macrofossil assemblages (Miaohe biota and Ediacara biota) found in western Hubei, a major oxygenated marine ecosystem can be outlined for the late Ediacaran. In addition, a complicated diagenetic microfabric overprints the fine microstructure of microfossils preserved in phosphorites, leaving remarkable taphonomic puzzles for future study.