Marine redox evolution and organic matter accumulation in the end Guadalupian in NE Sichuan, South China

Abstract

The Guadalupian-Lopingian (G-L) transition represents a pivotal interval, during which changes in oceanic redox conditions have been proposed to correlate with the end-Guadalupian mass extinction. However, the detailed spatial and temporal variations in marine redox states are still not well-defined. To explore the concurrent oceanic shifts and the factors controlling organic matter accumulation, we investigate an organic-rich succession from the G-L boundary located in an intrashelf basin in the northeastern Sichuan Basin, Southwest China by means of multiple geochemical indicators, including organic carbon isotope, total organic carbon contents, iron speciation contents, mercury contents and major and trace elements contents. Our results delineate four distinct intervals (I-IV) of redox conditions based on Fe-Mo-U-V data, showing a sequence from suboxic to ferruginous, followed by euxinic, and returning to suboxic conditions. The euxinic phase appears to have been driven by a highly restricted basin environment combined with frequent volcanic episodes. Additionally, the primary productivity peaked in the Interval II₂, which may have been a key factor in the organic matter accumulation. Comparative analysis with studies from other regions indicates that the Paleo-Tethys was more anoxic and exhibited greater stagnation than the Panthalassa during the end-Guadalupian, with the severity of anoxia during the Permian-Triassic transition surpassing that of the G-L transition. Overall, the significant marine anoxia was confined to moderate depths, with varying onset times across different areas, and was notably absent in the pelagic basins. This pattern implies that anoxia might not have been the primary cause of the mass extinction event.