Oxygen cycle


The oxygen cycle is the biogeochemical transitions of oxygen atoms between different oxidation states in ions, oxides, and molecules through redox reactions within and between the spheres/reservoirs of the planet Earth. The word oxygen in the literature typically refers to the most common oxygen allotrope, elemental/diatomic oxygen, as it is a common product or reactant of many biogeochemical redox reactions within the cycle. Processes within the oxygen cycle are considered to be biological or geological and are evaluated as either a source or sink.

Reservoirs

Oxygen is one of the most abundant elements on Earth and represents a large portion of each main reservoir. By far the largest reservoir of Earth's oxygen is within the silicate and oxide minerals of the crust and mantle. The Earth's atmosphere, hydrosphere, and biosphere together hold less than 0.05% of the Earth's total mass of oxygen. Besides O2, additional oxygen atoms are present in various forms spread throughout the surface reservoirs in the molecules of biomass, H2O, CO2, HNO3, NO, NO2, CO, H2O2, O3, SO2, H2SO4, MgO, CaO, AlO, SiO2, and PO4.

Atmosphere

The atmosphere is 20.9% oxygen by volume, which equates to a total of roughly 34 × 1018 mol of oxygen. Other oxygen-containing molecules in the atmosphere include ozone, carbon dioxide, water vapor, and sulphur and nitrogen oxides.

Biosphere

The biosphere is 22% oxygen by volume present mainly as a component of organic molecules and water molecules.

Hydrosphere

The hydrosphere present mainly as a component of water molecules with dissolved molecules including free oxygen and carbonic acids.

Lithosphere

The lithosphere is 46.6% oxygen by volume present mainly as silica minerals and other oxide minerals.

Biological production

The main source of atmospheric free oxygen is photosynthesis, which produces sugars and free oxygen from carbon dioxide and water:
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 of the open ocean.

Abiotic production

An additional source of atmospheric free oxygen comes from photolysis, whereby high-energy ultraviolet radiation breaks down atmospheric water and nitrous oxide into component atoms. The free H and escape into space, leaving O2 in the atmosphere:

Biological consumption

The main way free oxygen is lost from the atmosphere is via respiration and decay, mechanisms in which animal life and bacteria consume oxygen and release carbon dioxide.

Capacities and fluxes

The following tables offer estimates of oxygen cycle reservoir capacities and fluxes. These numbers are based primarily on estimates from :
ReservoirCapacity
Flux in/out
Residence time
Atmosphere
Biosphere50
Lithosphere

Table 2: Annual gain and loss of atmospheric oxygen

Ozone

The presence of atmospheric oxygen has led to the formation of ozone and the ozone layer within the stratosphere:
The ozone layer is extremely important to modern life as it absorbs harmful ultraviolet radiation: