Sources of oxygen in the atmosphere:
(i) Early in the evolution of the Earth, oxygen is believed to have been released from water vapour by ultra-violet radiations and accumulated in the atmosphere as the hydrogen escaped into the outer layer of the Earth's atmosphere.
(ii) The main driving factor of the oxygen cycle is when water and carbon dioxide combine in the presence of sunlight and chlorophyll to form glucose and oxygen by the process of photosynthesis.
(iii) Molecular oxygen is mainly contained in rocks and minerals.
(iv) Molecular oxygen is also released in the atmosphere by the process of photolysis.
(v) The weathering process initiated by organisms can also free the oxygen from the lithosphere.
(vi) Plants and animals extract nutrient minerals from rocks and release oxygen in the process.