The event which was a major source of atmospheric oxygen is photosynthesis.
What is photosynthesis?
Green plants and other organisms use solar energy to generate nutrition from carbon dioxide and water. Plants need the green pigment chlorophyll to perform photosynthesis, which produces oxygen as a by-product.
- Volcanic eruptions is not the source of atmospheric oxygen as they harm the environment due to producing hot lava from it.
- Decomposition of water produces oxygen but it is not the natural source it is done by electrolysis or any other method.
- Outgassing is also not the source of oxygen it is only a phenomenon.
Hence photosynthesis is the major source.
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