The formation conditions of the early Ediacaran cherts, South China

Abstract

During the early Ediacaran, abundant cherts (nodules and bands) were discontinuously hosted by dolostones and mudstones of the Doushantuo Formation (635–551Ma) in the Yangtze Three Gorges area, South China. The appearance of these cherts correlates with the distribution of acanthomorphic acritarch fossils and is potentially important for the reconstruction of the biogeochemical conditions and oceanic chemistry during the early Ediacaran. However, the formation mechanisms of these cherts are still poorly understood. In this study, silicon isotopes, Ge/Si molar ratios and rare earth element (REE)+Y features are used to constrain the origin mechanism of these cherts, silica geochemical cycling and seawater redox conditions during the Early Ediacaran. Our results suggest that the studied cherts were formed by the replacement of precursor carbonate minerals, which included the dissolution of a siliceous precursor and reprecipitation of microquartz. The positive silicon isotopic composition (typically between 0.4‰ and 1.4‰) and Ge/Si molar ratios (0.04–0.95×10−6) indicate a major seawater source of Ge and Si in these cherts with minor detrital and negligible hydrothermal contamination. In the early Ediacaran Ocean, the dissolved silica may be derived from the river input with high δ30Si values. On the other hand, the output of dissolved silica was controlled by inorganic precipitation, which was subsequently fixed as microquartz in cherts during early diagenesis. During the formation of cherts, the primary REE+Y signals of seawater were preserved with little detrital influence. Meanwhile, the REE+Y patterns, obvious negative Ce anomalies and near modern seawater Y/Ho ratios indicate an oxic marine chemistry during chert formation. This oxic seawater environment may have promoted the observed diversity of Ediacaran biological systems. Finally, temporal variations related to silica sources and silicon isotopic fractionation controls in the cherts and/or ocean were also evaluated.