Upwelling-Induced Organic Matter Enrichment in Wuchiapingian Shales of the Northeastern Sichuan Basin, China

Late Permian paleoenvironmental instability and recurrent biotic crises coincided with enhanced marine organic-carbon burial, yet ocean-circulation dynamics have remained underappreciated as a key driver. In particular, for the Wuchiaping Formation along the eastern margin of the Paleo-Tethys Ocean, the presence, variability, and mechanistic impact of upwelling—and its coupling with water-column redox structures—have not been systematically constrained, limiting a process-based understanding of organic-matter enrichment. Here, we integrate sedimentological, mineralogical, and multi-proxy geochemical data to investigate the dominant controls on organic matter enrichment in the Wuchiaping Formation shale succession from the northeastern Sichuan Basin. The Lower Wuchiaping Formation consists mainly of clay-rich shales deposited under oxic, shallow-water, and weakly stratified conditions, as indicated by low Ni/Co ratios (average 1.88), limited uranium enrichment (UEF = 0.21), low Ba/Al ratios, and sparse biogenic debris. Biomarker indices (gammacerane index = 0.35; Pr/Ph = 1.91) suggest unfavorable preservation conditions, resulting in a low mean TOC of 0.78%. In contrast, the Upper Wuchiaping Formation is dominated by siliceous shales with elevated Ni/Co ratios (average 15.83), moderate uranium enrichment (UEF = 2.48), abundant framboidal pyrite, radiolarian–planktic foraminiferal assemblages, and laminated apatite. High Ba/Al and Cd/Mo ratios, higher gammacerane values, and low Pr/Ph ratios (<1) indicate enhanced water-column stratification and bottom-water anoxia, leading to efficient organic matter preservation and a high mean TOC of 9.2%. Biomarker compositions reveal a shift from terrestrial-dominated organic matter in the Lower Wuchiaping Formation to algal- and plankton-derived inputs in the Upper Wuchiaping Formation. Collectively, these results indicate that intensified upwelling—rather than tectono-magmatic forcing alone—was the primary driver of enhanced productivity, strengthened redox stratification, and organic matter enrichment in the Upper Wuchiaping Formation. Our findings highlight the importance of upwelling–redox coupling as a key mechanism linking Late Permian ocean-system reorganization to spatially and stratigraphically heterogeneous organic-carbon accumulation along the Paleo-Tethyan margin.