Marine redox stratification during the early Cambrian (ca. 529‐509 Ma) and its control on the development of organic‐rich shales in Yangtze Platform

Abstract

AbstractHigh resolution geochemical data from nine sections representing shelf to basinal environments in the Yangtze Platform were analyzed to reconstruct the marine redox environment during early Cambrian. Based on Fe species and Mo/TOC ratios, we have supplemented marine redox stratification during Stage 4 (late Canglangpuian‐Longwangmiaoan, ∼514‐509 Ma) on basis of the previously studied Stage 2‐Stage 3 (Meishucunian‐Qiongzhusian, ∼529‐514 Ma). A new proposed marine stratified redox model indicates that the middepth “euxinic wedge” developed at the base of slope during ∼514‐509 Ma in contrast to that the “euxinic wedge” prevailed at the shelf margin during ∼529‐514 Ma, even though these middepth euxinic waters both occurred between the oxic surface waters and ferruginous deep waters. This marine redox stratification resulted in high production and good preservation of organic matter during early Cambrian. TOC values in euxinic waters in the middle are generally higher than in ferruginous waters due to upwelling in slope. Therefore, the lower Cambrian organic‐rich shales in the Yangtze Platform are inferred to be deposited under the anoxic‐ferruginous and euxinic bottom waters with moderate‐strong restriction.