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Why did reduced ocean circulation lower oxygen levels in warm marine habitats?

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Permian paleogeography — Pangea Distribution of landmasses, mountainous regions, shallow seas, and deep ocean basins near the end of the Permian Period. Included in the paleogeographic reconstruction are the locations of the interval's subduction zones.

During the formation of the supercontinent Pangea, the assembly of large landmasses significantly altered global geography, which in turn disrupted ocean circulation patterns.1

As Pangea formed, the development of land barriers inhibited the movement of cooler polar waters into tropical regions. This restriction prevented the mixing of water masses, leading to sluggish circulation and the isolation of warm marine basins. In these warm, shallow habitats, the lack of influx from cooler, oxygen-rich polar currents—combined with the reduced efficiency of ocean circulation—resulted in the depletion of dissolved oxygen levels. This widespread anoxia in warm marine environments is considered a contributing factor to the mass extinction events observed at the end of the Permian Period, as the resulting conditions exceeded the tolerance limits of many marine organisms.12