Oxygen cycle
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224788778
2008-07-10T12:34:50Z
Joriki
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/* Reservoirs and Fluxes */ photosynthesis is not a source of oxygen in the biosphere
[[Image:Oxygen_Cycle.jpg|thumb|350px|right|The oxygen cycle.]]
The '''oxygen cycle''' is the [[biogeochemical cycle]] that describes the movement of [[oxygen]] within and between its three main reservoirs: the [[Earth's atmosphere|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 as we know it.
==Reservoirs and Fluxes==
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%).
The main source of atmospheric oxygen is photosynthesis, which produces sugars and oxygen from carbon dioxide and water:
:6CO<sub>2</sub> + 6H<sub>2</sub>O + energy → C<sub>6</sub>H<sub>12</sub>O<sub>6</sub> + 6O<sub>2</sub>
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>
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 O<sub>2</sub> in the atmosphere:
:2H<sub>2</sub>O + energy → 4H + O<sub>2</sub>
:2N<sub>2</sub>O + energy → 4N + O<sub>2</sub>
The main way 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.
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-oxide]]s (rust):
:4FeO + 3O<sub>2</sub> → 2Fe<sub>2</sub>O<sub>3</sub>
:''Main article: [[Mineral redox buffer]]''
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]] 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.
== 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:
:O<sub>2</sub> + uv energy → 2O
:O + O<sub>2</sub> → O<sub>3</sub>
== References ==
<references/>
*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
{{Biogeochemical cycle}}
[[Category:Ecology]]
[[Category:Geochemistry]]
[[Category:Chemical oceanography]]
[[Category:Photosynthesis]]
[[cs:Koloběh kyslíku]]
[[es:Ciclo del oxígeno]]
[[et:Hapnikuringe]]
[[fr:Cycle de l'oxygène]]
[[fi:Hapen kiertokulku]]
[[th:วัฏจักรออกซิเจน]]
[[zh:氧循環]]