Redox fluctuations and organic accumulation on the outer shelf of the Early Cambrian (Ages 2–3) Yangtze sea: Geochemical records on a seaward submarine sill

Abstract

The Early Cambrian was a critical interval marked by dramatic oceanic, geochemical, and biological changes in Earth's history. Although numerous studies have documented oceanic redox changes and revealed a heterogeneous pattern from the inner shelf to outer shelf margin in the Yangtze Block, less attention has been paid to their palaeogeographic‐specific occurrences, particularly in the deeper basinal regime during that time. To better understand marine redox variations and organic accumulation across the entire outer shelf‐basin transect, we selected a coeval organic‐rich section (i.e., Guantang section, western Hunan) deposited on a farther seaward submarine sill for a comparison. Integrated geochemical data of iron speciation, redox‐sensitive trace element [i.e., molybdenum (Mo) and uranium (U)] concentrations and total organic carbon (TOC) contents as well as P/Al and Ba/Al ratios, in combination with previously published data from the outer shelf slope‐basin, reveal an overall high TOC content on the sill and further attest to the occurrence of a fluctuating euxinic water wedge overlying the deep ferruginous water on the outer shelf slope to basin transect in the presence of oceanic upwelling system. The fluctuating euxinic watermass could be dynamically advanced upslope and retreated downslope in the pace of sea‐level rise and fall. All these signify the critical roles of palaeogeographic setting and eustatic changes in controlling the spatiotemporal productivity and redox fluctuations in the Yangtze sea during the Early Cambrian.