Links between hydrographic restriction, redox conditions, and organic matter accumulation in the Early Cambrian intrashelf basin, South China

There was a strong relationship between paleo-oceanic conditions and organic matter accumulation during the critical period of the Early Cambrian. The Yangtze Platform in South China provides an opportunity to study this relationship, but the basin architecture and paleo-hydrographic dynamics remain poorly understood, which hinders one's understanding of the evolution in oceanic redox state and its relationship to organic matter accumulation. To fill the knowledge gap, a newly obtained data set from the Lower Cambrian Niutitang Formation shales (Late Cambrian Stage 2-Stage 3) of the intrashelf basin is used. These findings reveal that the molybdenum–uranium covariation, along with the abnormally low Mo contents and Mo/TOC ratios in the anoxic (ferruginous and euxinic) Niutitang shales of the intrashelf basin, indicate a hydrographic circulation that is moderately to strongly restricted. The hydrographic evolution at different stages, influencing redox conditions to a certain extent, in conjunction with variations in primary productivity, controls the enrichment of organic matter. In particular, the relatively low sea level during Interval I (Late Stage 2) resulted in weak connectivity between the intrashelf basin and open ocean; this, coupled with high primary productivity and enhanced chemical weathering, formed stable stratified water (euxinic bottom-water) conditions that were favorable for organic matter enrichment. In contrast, Interval II (Early–Middle Stage 3) and Interval III (Late Stage 3) involved less organic matter enrichment due to weakened restricted hydrographic circulation (Interval II) and oxidizing bottom-water conditions (Interval III). This study highlights how hydrodynamic processes affected the marine environment during the Early Cambrian and reveals that sluggish oceanic circulation combined with high primary productivity led to anomalous enrichment of organic matter. This may be a common feature of basins with restricted hydrographic circulation in deep time.