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The 'oxygen cycle' is the biogeochemical cycle that describes the movement of oxygen within and between its three main reservoirs: the atmosphere, the biosphere, and the lithosphere. The main driving factor of the oxygen cycle is photosynthesis, which is responsible for the modern Earth's atmosphere and life as we know it. Because of the vast amounts of oxygen in the atmosphere, even if all photosynthesis were to cease it would take between 5,000^[1] to 2.5 million years (unknown reference) to strip out more or less all oxygen.

Contents

Reservoirs and Fluxes

Oxygen reservoir capacities and fluxes

Ozone

Phosphorus

References

Reservoirs and Fluxes

By far the largest reservoir of Earth's oxygen is within the silicate and oxide minerals of the crust and mantle (99.5%). Only a small fraction has been released as free oxygen to the biosphere (0.01%) and atmosphere (0.49%).
The main source of oxygen within the biosphere and atmosphere is photosynthesis, which breaks down carbon dioxide and water to create sugars and oxygen:
: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 cyanobacteria Prochlorococcus was discovered in 1986 and accounts for more than half of the photosynthesis of the open ocean.^[2]
An additional source of atmospheric oxygen comes from 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.
Because lithospheric minerals are oxidised in oxygen, chemical weathering of exposed rocks also consumes oxygen. An example of surface weathering chemistry is formation of iron-oxides (rust):
:4FeO + 3O₂ → 2Fe₂O₃
:''Main article: Mineral redox buffer''
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 sea floor 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.

Oxygen reservoir capacities and fluxes

The following tables offer estimates of oxygen cycle reservoir capacities and fluxes. These numbers are based primarily on estimates from (Walker, J.C.G.):
'Table 1': Major reservoirs involved in the oxygen cycle

Reservoir	Capacity (kg O2)	Flux In/Out (kg O2 per year)	Residence Time (years)
Atmosphere	1.4 ★ 1018	30,000 ★ 1010	4,500
Biosphere	1.6 ★ 1016	30,000 ★ 1010	50
Lithosphere	2.9 ★ 1020	60 ★ 1010	500,000,000

'Table 2': Annual gain and loss of atmospheric oxygen (Units of 10¹⁰ kg O₂ per year)

''Gains''
Photosynthesis (land) Photosynthesis (ocean) Photolysis of N2O Photolysis of H2O	16,500 13,500 1.3 0.03
Total Gains	~ 30,000
''Losses - Respiration and Decay''
Aerobic Respiration Microbial Oxidation Combustion of Fossil Fuel (anthropologic) Photochemical Oxidation Fixation of N2 by Lightning Fixation of N2 by Industry (anthropologic) Oxidation of Volcanic Gases	23,000 5,100 1,200 600 12 10 5
''Losses - Weathering''
Chemical Weathering Surface Reaction of O3	50 12
Total Losses	~ 30,000

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:
:O₂ + uv energy → 2O
:O + O₂ → O₃
The absorbed solar energy also raises the temperature of the atmosphere within the ozone layer creating a thermal barrier that helps trap the atmosphere below (as opposed to bleeding out into space). Also see ozone-oxygen cycle.

Phosphorus

An interesting theory is that phosphorus (P) in the ocean helps regulate the amount of atmospheric oxygen. Phosphorus dissolved in the oceans is an essential nutrient to photosynthetic life and one of the key limiting factors. Oceanic photosynthesis contributes approximately 45% of the total free oxygen to the oxygen cycle. The population growth of photosynthetic organisms is primarily limited by the availability of dissolved phosphorus.
One side-effect of mining and industrial activities is a dramatic increase in the amount of phosphorus being discharged to the world's oceans. However, this increase in available phosphorus has not resulted in a corresponding increase in oceanic photosynthesis. This is because an increase in photosynthesizer population results in increased oxygen levels in the oceans. The elevated oxygen levels promote the growth of certain types of bacteria that compete for uptake of dissolved phosphorus. This competition limits the amount of phosphorus available to photosynthetic life thus buffering their total population as well as the levels of O₂.

References

1. Walker, J. C. G. (1980) The oxygen cycle in the natural environment and the biogeochemical cycles, Springer-Verlag, Berlin, Federal Republic of Germany (DEU)
2. Steve Nadis, ''The Cells That Rule the Seas'', Scientific American, Nov. 2003 [1]


★ Cloud, P. and Gibor, A. 1970, The oxygen cycle, Scientific American, September, S. 110-123

★ Fasullo, J., Substitute Lectures for ATOC 3600: Principles of Climate, Lectures on the global oxygen cycle, http://paos.colorado.edu/~fasullo/pjw_class/oxygencycle.html

★ Morris, R.M., OXYSPHERE - A Beginners' Guide to the Biogeochemical Cycling of Atmospheric Oxygen, http://seis.natsci.csulb.edu/rmorris/oxy/Oxy.htm