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Why could oceanic anoxia have been harmful to marine life at the end of the Permian Period?

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At the end of the Permian Period, oceanic anoxia—the widespread lack of dissolved oxygen in seawater—is considered a primary driver of the mass extinction that devastated marine life.1

This condition was likely caused by a weakening of oceanic circulation, which prevented the replenishment of oxygen-rich waters across various shelf depths and latitudes. Because oxygen is essential for the survival of most marine organisms, its depletion created environments that were lethal to many species.123

The impact was particularly severe for bottom-dwelling organisms, known as the benthos, which were unable to survive in these oxygen-depleted zones. This environmental crisis contributed to the loss of 85 to 95 percent of marine invertebrate species, including groups such as trilobites, rugose and tabulate corals, and various brachiopods.1