Coupled carbonate-shale thallium isotopes reveal the evolution of deep-marine anoxia during the end-Permian mass extinction

Ocean anoxia played a key role in the end-Permian mass extinction, the most severe biotic crisis in Earth's history, yet reconstructions of oxygenation through this event have lacked a focus on deep-water oxygen loss. Thallium isotopes, which track deep-ocean oxygenation via manganese oxide burial, offer a potential solution but have only been applied to siliciclastic sections, limiting spatial and temporal coverage. Here, we present a combined carbonate-shale Tl isotope record, spanning four geographically distant regions and providing a near-continuous record of global-scale redox change during the extinction interval. Both lithologies record deep-marine anoxia beginning 1 million years before the extinction, demonstrating that carbonate- and shale-hosted Tl isotopes preserve consistent redox signals. Late Permian oxygen depletion likely remained largely constrained to the deep ocean and was distinct from the shelf- and slope-depth euxinia of the Early Triassic that was coupled with climate warming and central to the loss of animal diversity.