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How did high global temperatures contribute to oxygen-poor Devonian bottom waters?

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

During the Devonian Period, high global temperatures contributed to oxygen-poor bottom waters by slowing the mixing rate between the ocean’s surface and its deeper layers.12

Normally, the circulation of ocean water helps transport oxygen from the surface to the depths. However, when global temperatures are high, this vertical mixing process is inhibited. As a result, the re-oxygenation rate of bottom waters is significantly reduced, leading to hypoxic or anoxic conditions—environments with little to no dissolved oxygen. These low-oxygen conditions are often evidenced in the geological record by layers of black shale and are associated with significant environmental stress and the extinction of many marine species.12

It remains a subject of scientific debate whether these climatic extremes were driven by an increase in solar energy, an amplified greenhouse effect, or other Earth-bound processes, such as increased nutrient runoff from the colonization of land by plants, which may have further increased the susceptibility of continental seas to anoxia.12