Iron speciation in cherts from the Laobao Formation, South China: Implications for anoxic and ferruginous deep-water conditions.

Cherts in the Laobao Formation in the Silikou section, South China were deposited in terminal Ediacaran deep-water. The speciations of iron in cherts, such as pyrite Fe (FeP), HCl-extractable Fe (FeH), and total Fe (FeT), and their interrelationships, are proxies for distinguishing oxic, anoxic but ferruginous, and sulfidic conditions, which is an effective method for studying the terminal Ediacaran ocean environment. For chert samples from the Laobao Formation, pyrite concentrations are generally low (<0.56%), degree of pyritization (DOP) ranges from <0.01 to 0.78 and most are <0.45; (FeP+FeH)/FeT ratios are between 0.27 and 0.84 and most exceed 0.38, and most of FeT/Al ratios range from 0.55 to 5.87 (averaging 1.1). Relatively high content of highly reactive Fe and low content of pyrite indicate a scarcity of H2S and Fe-enriched deep-water. These features suggested that the deposition setting of Laobao cherts was anoxic and ferruginous. Terminal Ediacaran deep-water in the South China Basin was most likely anoxic and ferruginous, which played a key role in the emergence and evolution of early multicellular life. [ABSTRACT FROM AUTHOR]