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Why did oxygen made by photodissociation not quickly build up in the early atmosphere?
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evolution of the atmosphere
A “best guess” reconstruction of the abundance of O2 in Earth's atmosphere as a function of time. The O2 abundance axis is logarithmic.
Oxygen produced by the photodissociation of water vapor in the early atmosphere did not accumulate significantly for two primary reasons:
- Limited Production: The rate of photochemical oxygen production was constrained because water vapor had to reach high altitudes to be cleaved by solar ultraviolet radiation. This process was severely impeded by a cold atmospheric layer that caused water vapor to freeze before it could reach those altitudes.1
- Abundant Oxygen Sinks: The early Earth contained vast quantities of reduced, oxidizable materials—such as ferrous iron, sulfides, and organic compounds—that acted as potent sinks for oxygen. These chemicals reacted with and consumed any released oxygen almost immediately. Furthermore, the rock cycle continuously exposed fresh, unoxidized crustal material to the surface, ensuring that these oxygen-absorbing sinks remained replenished for a long period.23
Because the strength of these sinks exceeded the rate of photochemical production, the residence time of oxygen was too short to allow for any meaningful buildup until the later establishment of oxygenic photosynthesis.13