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

High 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.

Evidence for Highly Complex Redox Conditions and Strong Water‐Column Stratification in an Early Cambrian Continental‐Margin Sea

“…A theoretical model of similar spatial chemical zonation for early Cambrian and Proterozoic oceans was proposed by , in which the distribution of multiple redox zones conforms to the energy-yield sequence from oxygen, nitrate, Mn 4+ , Fe 3+ , and sulfate reductions during the remineralization of organic matter. If these redox zones all existed in early Cambrian oceans, the marine redox heterogeneity must have been more complex than the three redox layers presented in earlier studies (e.g., Feng et al, 2014;Jin et al, 2016;Zhang et al, 2017).…”

“…For example, some studies have argued that strong oceanic oxygenation occurred in the early Cambrian (e.g., Chen et al, 2015;Li et al, 2017), whereas others have inferred only limited oxygenation during the same interval (e.g., Sperling et al, 2015;Xiang et al, 2017). Spatial heterogeneity of early Cambrian oceanic redox conditions was recently hypothesized to reconcile these conflicting views, with redox stratification in the form of an oxic surface layer, a euxinic intermediate layer, and a ferruginous deep layer (e.g., Feng et al, 2014;Jin et al, 2016;Zhang et al, 2017). In this scenario, oxygenation of the ocean would have led to an expansion of the oxic ocean-surface layer at the expense of the euxinic intermediate layer with limited effects on the anoxic deep ocean (Jin et al, 2016;Li et al, 2017;Tostevin et al, 2016;Wen et al, 2015).…”

Comparative Analysis of the Siliceous Source and Organic Matter Enrichment Mechanism of the Upper Ordovician–Lower Silurian Shale in the Upper-Lower Yangtze Area

“…The relation between the Mo element and TOC content can reflect the closure degree of a water body [39,47,48]. According to the relative relationship between the Mo element and the TOC content obtained by previous studies [24,39]., the method of judging the closure of the water body can be shown in Figure 4. In the Figure 4, the pink region represents the strong water closure, the blue region represents the medium water closure, and the yellow region represents the weak water closure.…”

“…Also collected are excess barium (Ba XS ) data, which reflects the bio-productivity of the Upper Ordovician Xinkailing Formation-Lower Silurian no. 1 section of the Lishuwo Formation in the Jiangye-1 well [38,39], and excess zinc (Zn XS ) data, which reflects the bio-productivity of the Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation [37]. Finally, log data for U, Si, Al, and Th from two wells provided by the Schlumberger Corp. are also collected.…”

Mechanism Analysis of Organic Matter Enrichment of Upper Ordovician-Lower Silurian Shale in Upper Yangtze Area-Taking Jiaoye-1 Well in Jiaoshiba Block as an Example

“…Influenced by the collision between the Yangtze Plate and the Cathaysian Plate, during the deposition of the Wufeng Formation-the lower section of the Longmaxi Formation 1st member, volcanic activity was frequent and intense, leading to dramatic climate changes, improving the reducibility of the lower water [37,38,[76][77][78][79][80]. After the ash materials formed by volcanic eruption subsiding into surface water, minerals [68] and Zhang et al [30]. See Figure 1 for the well location.…”