The significant increase of abundance and expansion of depositional environments that produced unusual sediments in the Early Triassic indicates stressed ecosystems in the aftermath of the Permian–Triassic (P–Tr) mass extinction. As one of the characteristically common Early Triassic carbonate sediments, ooids provide a potential proxy to refine understanding of the biotic and environmental stresses during this time through analysis of their formation and size variations. A case study from South China and a global review are presented herein to explore the interrelations between occurrences of oolites and ooid size variations with biotic and environmental changes. Correlations between oolites and various biotic and environmental changes suggest a strong correspondence with episodes of euxinia/dysoxia but less so with skeleton abundance and temperature changes, implying complex interactions between multiple biotic and environmental anomalies in the aftermath of the P–Tr extinction. The episodic occurrence pattern of oolites from the end-Permian through the Early Triassic coincides with the multiple crises of the P–Tr mass extinction and its aftermath. The global increase in size of ooids during the early stage of the P–Tr mass extinction aftermath indicates the most severe and extensive conditions of devastation for ecosystems. The single occurrence of giant ooids in the Nanpanjiang Basin within the Olenekian implies local higher ecosystem stress than other areas. This analysis of ooid size variations and the paleoceanographic implications suggests that the size of ooids could be an appropriate quantified sedimentary proxy for ecosystem devastation with varied temporal and spatial ranges.

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