Oxygen cycle

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. If all photosynthesis were to cease, the Earth's atmosphere would be devoid all but trace amounts of oxygen within 5000 years. The oxygen cycle would no longer exist.

Contents

1 Reservoirs and Fluxes 2 Ozone 3 Phosphorus 4 References

Reservoirs and Fluxes

The vast majority of molecular oxygen is contained in rocks and minerals within the Earth (99.5%). Only a small fraction has been released as free oxygen to the biosphere (0.01%) and atmosphere (0.5%). The main source of oxygen within the biosphere and atmosphere is photosynthesis which breaks down carbon dioxide and water to create sugars and oxygen:

CO₂ + H₂O + energy → CH₂O + O₂ <p> An additional source of atmospheric oxygen comes from photolysis, whereby high energy ultraviolet radiation breaks down atmospheric water and nitrite into component molecules. The free H and N atoms escape into space leaving O₂ in the atmosphere: <p> 2H₂O + energy → 4H + O₂
2N₂O + energy → 4N + O₂ <p> The main way oxygen is lost from the atmosphere is via respiration and decay mechanisms in which animal life consumes oxygen and releases carbon dioxide. Because lithospheric minerals are reduced in oxygen, surface weathering of exposed rocks also consumes oxygen. An example of surface weathering chemistry is formation of iron-oxides (rust) such as that found in the red sands of Australia: <p> 4FeO + 3O₂ → 2Fe₂O₃ <p> Oxygen is also cycled between the biosphere and lithosphere. Marine organisms in the biosphere create carbonate shell material (CaCO₃) that is rich in molecular oxygen. When the organism dies its shell is deposited on the shallow sea floor and buried over time to create limestone rock in 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. <p>
The following tables offer estimates of oxygen cycle reservoir capacities and fluxes. These numbers are based primarily on estimates from Walker, J.C.G. <p>
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

<p>
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 lead to the formation of the ozone layer within the stratosphere. The ozone layer is extremely important to modern life as it absorbs harmful ultraviolet radiation: <p> O₂ + uv energy → 2O <P> O + O₂ + uv energy → O₃ <p> The absorbed ultraviolet 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).

Phosphorus

An interesting theory is that phosphorus (P) in the ocean helps regulate the amount of atmospheric oxygen. Phosphorus disolved in the oceans is an essential nutrient to photosynthetic life and one of the key limiting factors. High oxygen levels in the oceans promote P removal by increasing the productivity of certain types of bacteria which uptake dissolved P to make their tissues. The decreasing phosphorus availability subsequently suppresses the population growth of the photosynthesizers and O₂ production diminishes. Low oxygen levels lead to increased mortality of oceanic organisms which, during decomposition, release P back into the ocean. This increasing phosphorus availability subsequently elevates the population growth of photosynthesizers and O₂ production increases.

References

Cloud, P. and Gibor, A. 1970, The oxygen cycle, Scientific American, September, S. 110-123 <p> 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 <p> Morris, R.M., OXYSPHERE - A Beginners' Guide to the Biogeochemical Cycling of Atmospheric Oxygen,
http://seis.natsci.csulb.edu/rmorris/oxy/Oxy.htm <p>

Walker, J. C. G., 1980, The oxygen cycle in The natural environment and the biogeochemical cycles, Springer-Verlag, Berlin, Federal Republic of Germany (DEU)