Habitability at the edge of the redox boundary during the Permian-Triassic mass extinction.

Global superanoxia is widely accepted as one of the main drivers of the end–Permian Mass Extinction (EPME) alongside, oceanic acidification, productivity collapse, and toxification. However, modeling and paleontological studies suggest spatial heterogeneity, with parts of the Tethys Ocean remaining oxygenated. To assess water–column oxygenation in the central Tethys, we studied two shallow–marine Permian–Triassic sections in equatorial paleolatitudes of central Iran; one with terrestrial input, the other fully marine. Continuous sedimentation across the EPME enables reconstruction of the latest Permian environment. U, Th, Mo, and Mn concentration data indicate well–oxygenated conditions until the EPME horizon, followed by Mn concentration peaks in microbialite/black shale intervals that reflect fluctuating oxic–anoxic conditions across the EPME. Micronutrient decline preceding the extinction suggests reduced local productivity. Thus, oxic conditions in microbialite–bearing shallow–marine settings were likely sustained by photosynthetic O₂ production and/or wave agitation. Low productivity also implies limited oxygen demand for organic matter remineralization, minimizing redox stress in these environments. We highlight shallow–marine Tethyan settings as potential oxygenated habitat during deep–sea anoxia, although a fluctuating chemocline repeatedly introduced Mn into marine environments, restricting oxidized habitat to the surface layer in contact with the atmosphere and/or oxygen–producing microbial mats.