Britannica Sources used in this answer

Why would weakened ocean circulation reduce oxygen levels in the ocean?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Line graph showing the number of marine animal families from the Precambrian era to the present, with illustrations of key groups such as trilobites, ammonoids, marine reptiles, and diatoms. The graph highlights major extinction events and a rise to 1,900 living families today. — Permian extinction The diversity of marine animal families since late Precambrian time. The data for the curve comprise only those families that are reliably preserved in the fossil record; the 1,900 value for living families also includes those families rarely preserved as fossils. The several pronounced dips in the curve correspond to major mass-extinction events. The most catastrophic extinction took place at the end of the Permian Period.

Weakened ocean circulation reduces oxygen levels in the ocean because it disrupts the process of ventilation, which is essential for maintaining healthy oxygen concentrations at depth.12

Under normal conditions, thermohaline circulation acts as a global conveyor belt. Cold, dense surface waters in polar regions sink and spread into the deep ocean, effectively transporting oxygen-rich water from the surface to the seafloor. When this circulation weakens, the rate at which oxygenated surface water is replenished at depth decreases.32

During the Permian-Triassic transition, this reduction in circulation is believed to have contributed to widespread oceanic anoxia—a state of oxygen depletion—which devastated marine life, particularly bottom-dwelling organisms. By slowing the movement of water, the ocean loses its ability to efficiently distribute dissolved oxygen, leading to stagnant, low-oxygen conditions that cannot support many marine species.14