The Permian-Triassic mass extinction (PTME), the most catastrophic biotic crisis of the Phanerozoic, triggered the transition of marine ecosystems from Paleozoic to modern types. Although mechanisms such as volcanism, extreme warming, marine anoxia, and ocean acidification and so on have been proposed, no definitive conclusion has yet been reached. Rather than attributing the collapse of the ecosystem to a single specific cause, it is currently more widely proposed that multiple environmental pressures acted in concert to bring about this catastrophic biotic crisis. Previous studies in South China initially focused on deep-water sections and later expanded to shallow-water settings. However, whether extinction mechanisms and environmental stressors differed between deep- and shallow-water environments remains unresolved.

    Here, we evaluate the role of marine deoxygenation in shallow-water carbonate platforms through geochemical analysis of three P-T boundary sections (Yangou in Jiangxi Province, Cili in Hunan Province, Dajiang in Guizhou Province). A total of 82 micritic limestone samples were analyzed for major and trace elements through in situ laser ablation - inductively coupled plasma - mass spectrometry (LA-ICP-MS). Ce/Ce* ratios exhibit systematic stratigraphic increases across all sections: Yangou (Permian mean 0.84 to Triassic 0.91; range: 0.50–1.26), Cili (Permian mean 0.73 to Triassic 0.92; range: 0.31–1.76), and Dajiang (Permian mean 0.52 to Triassic 0.88; range: 0.26–1.15), signaling oxygen depletion post-extinction but not reach the level of overall anoxia. Episodic Ce/Ce* excursions >1 (e.g., 1.21、1.26 at Yangou; 1.21、1.41 at Cili; 1.12、1.15 at Dajiang) indicate transient anoxia, yet these events lack temporal correlation with the second phase of PTME. It is therefore improbable that these transient, and potentially even ephemeral, marine anoxic events were responsible for the mass extinction of marine organism although they may have contributed to the selective adaptation and elimination of organisms and the suppression of marine recovery during the Early Triassic period. Integrated with the cerium anomaly and other indicators, including petrography, fossil records, and the geochemical index Ce/La, Y/Ho, Th/U, this study suggests that: 1) In the shallow water carbonate platform region of South China, marine oxygen levels decreased following the extinction event. The overall environment was characterized by suboxic conditions and not reach the level of anoxia, despite some strata experiencing transient anoxia. 2) The high temperature linked with volcanic eruptions are likely the primary drivers to the collapse of shallow water ecosystems, while marine deoxygenation plays a supplementary role and suboxic conditions may facilitate the selective adaptation and elimination of organisms, which is one of the contributing factors to the extinction of marine organisms. Combined with previous studies, this study offers additional insights for investigating the factors contributing to the Permian-Triassic mass extinction at varying water depths.