The vast majority of Earth's oxygen is locked away beneath your very feet

Nature · Published 3 October 2026 Swipe more like this →

In short: While we rely on the air to breathe, the atmosphere holds less than 0.05 percent of the planet's total oxygen. Nearly all of Earth's oxygen is actually trapped within the minerals of the crust and mantle, leaving only a tiny fraction to circulate through our biosphere, hydrosphere, and atmosphere.

The oxygen cycle is the complex biogeochemical movement of oxygen atoms between different oxidation states and reservoirs. While we often think of oxygen as the diatomic gas (O2) we inhale, it exists in many forms, including water, carbon dioxide, and various oxides. The overwhelming majority of this element—99.5 percent by weight—is sequestered within the silicate and oxide minerals that make up the Earth's crust and mantle. The portion that sustains life is a mere sliver of the total, yet it is constantly exchanged through biological and geological processes.

Biology is the primary driver of modern oxygen flux. Oxygenic photosynthesis, a process perfected by ancient bacteria, is the main source of atmospheric free oxygen. By converting carbon dioxide and water into sugars, plants and marine organisms release oxygen as a byproduct. Marine life is particularly vital; for instance, the cyanobacterium Prochlorococcus, discovered in 1986, accounts for up to half of all photosynthesis in the open ocean. Some estimates suggest that marine primary production is responsible for more than half of the total oxygen generated on Earth, with some sources even citing algae as replacing about 90 percent of consumed oxygen.

The cycle is balanced by sinks, primarily respiration and decay, where organisms consume oxygen and release carbon dioxide. Beyond these biological exchanges, geological processes like carbon burial and plate tectonics play a role in long-term regulation. Additionally, photolysis—where high-energy ultraviolet radiation breaks down water and nitrous oxide in the upper atmosphere—provides a secondary, abiotic source of oxygen. This atmospheric oxygen also facilitates the creation of the ozone layer, which shields the planet by absorbing harmful ultraviolet radiation. Despite human activities like burning fossil fuels, atmospheric oxygen levels remain relatively stable, potentially bolstered by increased plant productivity from global agricultural advances.

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