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Why did oxygen produced by early organisms take so long to accumulate in the 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.
The delay in the accumulation of atmospheric oxygen, despite its production by early organisms like cyanobacteria, was primarily due to the presence of massive "oxygen sinks" on Earth’s surface.12
When primitive organisms first began releasing oxygen through photosynthesis, the gas was consumed almost immediately by abundant oxidizable materials in the environment. Key factors included:12
- Chemical Sinks: Vast quantities of ferrous iron, sulfides, and other organic compounds reacted with and absorbed the released oxygen.12
- Crustal Recycling: The rock cycle—involving erosion, sedimentation, and tectonic uplift—continuously exposed fresh, unoxidized material from the Earth's crust, which acted as a persistent buffer against oxygen buildup.2
- Iron Precipitation: Much of the early oxygen was sequestered into iron formations, where soluble ferrous iron was converted into insoluble ferric iron and deposited in the oceans.13
It took hundreds of millions of years for these chemical sinks to be exhausted. Only after the capacity of the crust and oceans to absorb oxygen was overwhelmed did the gas begin to accumulate significantly in the atmosphere, eventually enabling the evolution of complex life.123