Atmospheric oxygen currently sits at approximately 21% of Earth’s air, a concentration that has remained relatively stable for the last half-billion years. However, the concentration of free oxygen in the atmosphere has fluctuated dramatically over geologic time. Earth’s history contains specific periods when oxygen levels surged far beyond modern measurements, driven by profound shifts in biological activity and planetary-scale geochemistry. Understanding these past peaks requires examining the forces that govern the production and consumption of oxygen over millions of years.
The First Breath: The Great Oxidation Event
The story of atmospheric oxygen begins with the Great Oxidation Event (GOE), a major shift that occurred roughly 2.4 billion years ago. Before this time, Earth’s atmosphere was largely devoid of free oxygen, consisting instead of gases like methane and carbon dioxide. The appearance of oxygen-producing organisms, specifically ancient cyanobacteria, introduced a new element into the planet’s chemistry.
These microbes used sunlight to convert water and carbon dioxide into sugars, releasing oxygen as a waste product through a process called oxygenic photosynthesis. Initially, this oxygen did not accumulate in the atmosphere because it was immediately consumed by sinks in the ocean. The most significant of these was dissolved ferrous iron, which reacted with the new oxygen to form ferric iron oxides.
This reaction led to the precipitation of iron-rich sediments, forming the characteristic Banded Iron Formations. Once the supply of reduced iron in the oceans was oxidized, oxygen began escaping into the atmosphere. The GOE represented the tipping point where the biological production of oxygen finally exceeded the planet’s capacity to absorb it, permanently changing the atmosphere from a reducing to an oxidizing one. Oxygen levels remained low, likely less than 10% of today’s concentration, for a long period afterward.
The Paleozoic Oxygen Maximum
The highest concentration of atmospheric oxygen in Earth’s history occurred much later, during the Late Paleozoic Era, specifically spanning the Carboniferous and early Permian periods, about 359 to 299 million years ago. During this time, oxygen levels are estimated to have peaked significantly higher than they are today, reaching a range of 30% to 35%.
This oxygen surplus had significant biological and environmental consequences. One notable effect was the ability of arthropods, like insects, to grow to enormous sizes, such as dragonflies with wingspans of over two feet. This gigantism was facilitated by high oxygen levels, which allowed their simple, passive respiratory systems to more efficiently deliver oxygen to their tissues.
The highly oxygenated atmosphere also dramatically lowered the combustion point of organic material. This resulted in an environment prone to widespread and intense wildfires, which became a common feature of the landscape during this era.
The Mechanism of Carbon Burial
The primary driver for the Paleozoic Oxygen Maximum was the widespread and rapid burial of organic carbon, creating conditions that released amounts of oxygen. The Carboniferous Period is named for the immense coal deposits that formed globally during this time, which act as a record of this carbon burial event. For every molecule of carbon dioxide a plant converts into organic material via photosynthesis, one molecule of oxygen is released into the atmosphere.
If this organic matter is consumed by decomposition, the oxygen released is consumed again, resulting in no net change to atmospheric composition. However, during the Carboniferous, the forests of vascular land plants, which contained the complex polymer lignin, died and fell into low-lying swamp environments. These waterlogged, anoxic conditions prevented complete decomposition.
Crucially, the organisms necessary to efficiently break down lignin, such as specific types of white-rot fungi, had not yet evolved or were not widespread enough. This evolutionary mismatch meant that quantities of carbon were buried and sequestered underground as peat, which eventually became coal, rather than being oxidized back into the atmosphere. This sequestration left the corresponding oxygen molecules in the air, causing the atmospheric concentration to soar to its maximum level.
Long-Term Controls on Atmospheric Oxygen
The current 21% oxygen level represents a stable long-term equilibrium between sources and sinks. The main source is the burial of organic carbon, while long-term sinks are geological processes that consume oxygen over millions of years, eventually bringing the Paleozoic peak back down to modern levels.
One sink is the oxidative weathering of reduced minerals, such as pyrite and iron-bearing silicates. When these minerals are exposed to the atmosphere and water through tectonic uplift and erosion, they react chemically with oxygen, consuming it in a process similar to rust formation. The rate of this weathering is a major factor in regulating long-term oxygen levels.
Another geological sink is the release of reduced gases, such as hydrogen sulfide and carbon monoxide, from volcanic eruptions and metamorphic activity. These gases react with atmospheric oxygen upon release, scrubbing it from the air. The balance between the rate of carbon burial (source) and the rate of consumption by weathering and volcanic outgassing (sinks) maintains the oxygen level that life depends on today.