The evolution of marine redox conditions across the Permian-Triassic (P-T) boundary is a critical scientific issue in paleoenvironmental research, with particular controversy surrounding redox conditions during the microbialite deposition period. This study focused on microbialites near the P-T boundary in Bianyang Town, Luodian, Guizhou. Through systematic environmental magnetic analysis and mineralogical observations, we reassessed the changes in paleo-marine redox conditions during this period. The results show that the concentration of magnetic minerals in the Early Triassic microbialites and micritic limestones above the extinction boundary is significantly higher than that in the underlying Permian clastic limestones. The Upper Permian limestones exhibit lower magnetic mineral content. The basal part is dominated by antiferromagnetic goethite and hematite, which is consistent with stable oxic conditions, whereas the sample near the extinction boundary shows an abrupt shift to magnetite dominance, a pattern that strongly supports a rapid expansion of anoxia prior to the extinction. In contrast, in the Early Triassic microbialites, the middle-lower part is dominated by hematite and goethite, while the upper part is dominated by magnetite. This alternation in magnetic mineral assemblages provides strong evidence for redox fluctuations within the microbialite unit. At the Dajiang section, the anoxic conditions that developed in the Late Permian did not persist into the microbialite unit, and anoxia was reestablished only in the middle-upper part of the microbialites. The co-occurrence of superparamagnetic particles and ankerite in post-extinction microbialites further suggests that microbial activity, through its metabolic byproducts, may have played a role in both the aggregation of fine-grained magnetite and the precipitation of ankerite.
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