The Oxygen Cycle

The oxygen cycle is the biogeochemical cycle that describes the movement of oxygen within and between its three main reservoirs: the atmosphere (air), the biosphere (living things) and the lithosphere (earth’s crust). The main driving factor of the oxygen cycle is photosynthesis, which is responsible for the modern earth’s atmosphere and life. The largest reservoir of the earth’s oxygen is within the silicate and oxide minerals of the crust and mantle (99.5 per cent). Only a small portion has been released as free oxygen to the biosphere (0.01 per cent) and atmosphere (0.36 per cent). The main source of atmospheric oxygen is photosynthesis, which produces sugar and oxygen from carbon dioxide and water

6CO₂ + 6H₂O + energy → C₆H₁₂O₆ + 6O₂

Photosynthesizing organisms include the plant life of the land areas as well as the phytoplankton of the oceans. The tiny marine cyanobacterium Prochlorococcus was discovered in 1986 and accounts for more than half of the photosynthesis of the open ocean. An additional source of atmospheric oxygen is photolysis, whereby high-energy ultraviolet radiation breaks down atmospheric water and nitrite into component atoms. The free H and N atoms escape into space leaving O₂ in the atmosphere

2H₂O + energy → 4H + O₂

2N₂O + energy → 4N + O₂

The main way oxygen is lost from the atmosphere is via respiration and decay, mechanisms in which animal life and bacteria consume oxygen and release carbon dioxide. Lithospheric minerals are oxidized in the presence of oxygen and chemical weathering of exposed rocks also consumes oxygen. An example of surface weathering chemistry is the formation of iron oxides (rust)

4FeO + O₂ → 2Fe₂O₃

Oxygen is also cycled between the biosphere and lithosphere. Marine organisms in the biosphere create calcium carbonate shell material (CaCO₃) that is rich in oxygen. When the organism dies, its shell is deposited on the shallow seafloor and buried over time to create the limestone rock of the lithosphere. Weathering processes initiated by organisms can also free oxygen from the lithosphere. Plants and animals extract nutrient minerals from rocks and release oxygen in the process.