Which event was a major source of atmospheric oxygen? photosynthesis

volcanic eruptions

decomposition of water

outgassing

Photosynthesis was a major source of atmospheric oxygen.

Explanation:

Earth's atmosphere was not the same as it is in the past. For a larger portion of Earth's history, the atmosphere actually didn't have an atmosphere or the one it had was not suitable for complex lifeforms to develop. This all changed because of one major event, the process of photosynthesis.

Initially, it was the cyanobacteria that started to use this process in order to produce their own food and energy. Basically, this process meant that the cyanobacteria, and a lot of later organisms, were taking in carbon dioxide, use it for their own needs, and release the oxygen. The oxygen was released into the ocean and into the atmosphere, which resulted in a gradual change in the composition of the atmosphere to a degree where it was suitable for larger organisms to start to develop.