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| [[Image:Oxygen Cycle.jpg|thumb|350px|right|The oxygen cycle.]]
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| The '''oxygen cycle''' is the [[biogeochemical cycle]] that describes the movement of [[oxygen]] within its three main reservoirs: the [[Earth's atmosphere|atmosphere]] (air), the total content of biological matter within the [[biosphere]] (the global sum of all ecosystems), and the [[lithosphere]] (Earth's crust). Failures in the oxygen cycle within the [[hydrosphere]] (the combined mass of water found on, under, and over the surface of planet Earth) can result in the development of [[hypoxic zone]]s. The main driving factor of the oxygen cycle is [[photosynthesis]], which is responsible for the modern Earth's atmosphere and life on earth (see the [[Great Oxygenation Event]]).
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| ==Reservoirs==
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| By far the largest reservoir of Earth's oxygen is within the silicate and oxide [[mineral]]s of the [[Crust (geology)|crust]] and [[Mantle (geology)|mantle]] (99.5%). Only a small portion has been released as free oxygen to the biosphere (0.01%) and atmosphere (0.36%).
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| The main source of atmospheric free oxygen is photosynthesis, which produces sugars and free oxygen from carbon dioxide and water:
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| :<math>\mathrm{6 \ CO_2 + 6H_2O + energy \longrightarrow C_6H_{12}O_6 + 6 \ O_2}</math> | |
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| 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.<ref>Steve Nadis, ''The Cells That Rule the Seas'', Scientific American, Nov. 2003 [http://sciam.com/article.cfm?chanID=sa022&articleID=0005BE47-0078-1FA8-807883414B7F0000]</ref>
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| 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 N atoms escape into space leaving O<sub>2</sub> in the atmosphere:
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| :<math>\mathrm{2 \ H_2O + energy \longrightarrow 4 \ H + O_2}</math>
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| :<math>\mathrm{2 \ N_2O + energy \longrightarrow 4 \ N + O_2}</math>
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| The main way free oxygen is lost from the atmosphere is via [[Respiration (physiology)|respiration]] and [[Decomposition|decay]], mechanisms in which [[animal]] life and [[bacteria]] consume oxygen and release carbon dioxide.
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| The lithosphere also consumes free oxygen via chemical weathering and surface reactions. An example of surface weathering chemistry is formation of [[iron-oxide]]s (rust):
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| :<math>\mathrm{4 \ FeO + O_2 \longrightarrow 2 \ Fe_2O_3}</math>
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| {{Main|Mineral redox buffer}}
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| Oxygen is also cycled between the biosphere and lithosphere. Marine organisms in the biosphere create [[calcium carbonate]] shell material (CaCO<sub>3</sub>) 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]] sedimentary 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.
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| ==Capacities and fluxes==
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| The following tables offer estimates of oxygen cycle reservoir capacities and fluxes. These numbers are based primarily on estimates from (Walker, J.C.G.):<ref>Walker, J. C. G. (1980) The oxygen cycle in the natural environment and the biogeochemical cycles, Springer-Verlag, Berlin, Federal Republic of Germany (DEU)</ref>
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| '''Table 1''': Major reservoirs involved in the oxygen cycle
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| {| border="1" cell padding="10"
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| |----- align="center"
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| ! Reservoir
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| ! Capacity<br>(kg O<sub>2</sub>)
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| ! Flux In/Out<br>(kg O<sub>2</sub> per year)
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| ! Residence Time<br>(years)
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| |----- align="right"
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| | align="left" | Atmosphere || 1.4 * 10<sup>18</sup>
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| | 30,000 * 10<sup>10</sup> || 4,500
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| |----- align="right"
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| | align="left" | Biosphere || 1.6 * 10<sup>16</sup>
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| | 30,000 * 10<sup>10</sup> || 50
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| |----- align="right"
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| | align="left" | Lithosphere || 2.9 * 10<sup>20</sup>
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| | 60 * 10<sup>10</sup> || 500,000,000
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| |}
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| <br>
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| '''Table 2''': Annual gain and loss of atmospheric oxygen (Units of 10<sup>10</sup> kg O<sub>2</sub> per year)
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| {| border="1" cell padding="10"
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| |-----
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|
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| |-----
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| | Photosynthesis (land)<br>Photosynthesis (ocean)<br>Photolysis of N<sub>2</sub>O<br>Photolysis of H<sub>2</sub>O
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| | align="center" | 16,500<br>13,500<br>1.3<br>0.03
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| |-----
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| | align="right" | Total Gains || align="center" | ~ 30,000
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| |-----
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| | colspan="2" | <u>''Losses - Respiration and Decay''</u>
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| |-----
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| | Aerobic Respiration<br>Microbial Oxidation<br>Combustion of Fossil Fuel (anthropogenic)<br>Photochemical Oxidation<br>Fixation of N<sub>2</sub> by Lightning<br>Fixation of N<sub>2</sub> by Industry (anthropogenic)<br>Oxidation of Volcanic Gases
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| | align="center" | 23,000<br>5,100<br>1,200<br>600<br>12<br>10<br>5
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| |-----
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| | colspan="2" | <u>''Losses - Weathering''</u>
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| |-----
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| | Chemical Weathering<br>Surface Reaction of O<sub>3</sub>
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| | align="center" | 50<br>12
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| |-----
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| | align="right" | Total Losses || align="center" | ~ 30,000
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| |}
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| == Ozone ==
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| {{main|Ozone-oxygen cycle}}
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| The presence of atmospheric oxygen has led to the formation of [[ozone]] (O<sub>3</sub>) and the [[ozone layer]] within the [[stratosphere]]. The ozone layer is extremely important to modern life as it absorbs harmful [[ultraviolet]] radiation:
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| :<math>\mathrm{O_2 + uv \ energy \longrightarrow 2O}</math>
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| :<math>\mathrm{O + O_2 \longrightarrow O_3}</math>
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| == References ==
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| <references/>
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| *Cloud, P. and Gibor, A. 1970, The oxygen cycle, Scientific American, September, S. 110-123
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| *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
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| *Morris, R.M., OXYSPHERE - A Beginners' Guide to the Biogeochemical Cycling of Atmospheric Oxygen, http://seis.natsci.csulb.edu/rmorris/oxy/Oxy.htm
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| {{Biogeochemical cycle}}
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| [[Category:Ecology]]
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| [[Category:Chemical oceanography]]
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| [[Category:Photosynthesis]]
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| [[Category:Biogeochemical cycle]]
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