Although marine deoxygenation is widely cited as a primary kill mechanism during the end-Triassic mass extinction, its spatial extent and timing relative to the event's two distinct extinction pulses remain poorly constrained. Specifically, it is unclear whether oxygen depletion was already widespread during the initial phase, or if it emerged later as a delayed response to prolonged environmental forcing. To address this, we reconstructed water-column redox conditions across contrasting basin types in the western Tethys. Integrating new Fe speciation and trace-metal data from the Cleveland and Cheshire basins with existing regional records, we demonstrate that the first extinction pulse coincided with a transition from predominantly oxic to only localized oxygen-deficient conditions. This argues strongly against pervasive marine anoxia during the early extinction stage. Conversely, the second extinction pulse aligned with a significant expansion of oxygen-deficient waters. This phase was characterized by ferruginous to euxinic conditions in restricted epicontinental basins, whereas suboxic to oxic conditions persisted on adjacent carbonate platforms. Ultimately, our results reveal that marine deoxygenation was a spatially selective and temporally delayed response to Central Atlantic Magmatic Province volcanism, heavily modulated by local basin restriction and circulation. Consequently, we propose that distinct environmental mechanisms drove the two phases of this mass extinction.

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