Oxygen cycle

 ( Ecology ) 

(green), marine biosphere (blue), lithosphere (brown), and atmosphere (grey).
The major fluxes between these reservoirs are shown in colored arrows, where the green arrows are related to the terrestrial biosphere, blue arrows are related to the marine biosphere, black arrows are related to the lithosphere, and the purple arrow is related to space (not a reservoir, but also contributes to the atmospheric O₂).
The value of photosynthesis or net primary productivity (NPP) can be estimated through the variation in the abundance and isotopic composition of atmospheric O₂.
The rate of organic carbon burial was derived from estimated fluxes of volcanic and hydrothermal carbon. ]]

The oxygen cycle refers to the various movements of oxygen through the Earth's atmosphere (air), biosphere (flora and fauna), hydrosphere (water bodies and ) and the lithosphere (the Earth's crust). The oxygen cycle demonstrates how free oxygen is made available in each of these regions, as well as how it is used. It is the biogeochemical cycle of oxygen between different in , and 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 (O₂), as it is a common product or Reagent of many biogeochemical redox reactions within the cycle.

(2025). 9780080983004, Elsevier. ISBN 9780080983004

Processes within the oxygen cycle are considered to be Biology or Geology and are evaluated as either a source (O₂ production) or sink (O₂ consumption).

Oxygen is one of the most common 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 (99.5% by weight). 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, Carbon dioxide, Nitric acid, Nitric oxide, Nitrogen dioxide, Carbon monoxide, H₂O₂, Ozone, Sulfur dioxide, Sulfuric acid, Magnesium oxide, Calcium oxide, Aluminium oxide, Silicon dioxide, and .

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Locations of oxygen

This equates to a total of roughly mol of oxygen (O2). Other oxygen-containing molecules in the atmosphere include ozone (O3), carbon dioxide (CO2), water vapor (H2O), and Sulfur and Nitrogen oxide (Sulfur dioxide, Nobelium, Nitrous oxide, etc.).

Present mainly as a component of Organic compound and water.

Present mainly as a component of water molecules, with dissolved molecules including free oxygen and carbonic acids (HxCO3).

Present mainly as Silicon dioxide (SiO2) and other oxide minerals.

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Sources and sinks

While there are many abiotic sources and sinks for O₂, the presence of the profuse concentration of free oxygen in modern Earth's atmosphere and ocean is attributed to O₂ production in the biological process of Photosynthesis in conjunction with a biological sink known as the biological pump and a geologic process of carbon burial involving plate tectonics.

(1980). 9783662229880, Springer Berlin Heidelberg. ISBN 9783662229880

(2003). 9780080983004 ISBN 9780080983004

Biology is the main driver of O₂ flux on modern Earth, and the evolution of oxygenic photosynthesis by bacteria, which is discussed as part of the Great Oxygenation Event, is thought to be directly responsible for the conditions permitting the development and existence of all complex Eukaryote metabolism.

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Biological production

The main source of atmospheric free oxygen is photosynthesis, which produces and free oxygen from carbon dioxide and water:

$\backslash mathrm\{6\; \backslash \; CO\_2\; +\; 6H\_2O\; +\; energy\; \backslash longrightarrow\; C\_6H\_\{12\}O\_6\; +\; 6\; \backslash \; O\_2\}$

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 up to half of the photosynthesis of the open oceans.

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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 hydrogen and nitrogen atoms escape into space, leaving O₂ in the atmosphere:

$\backslash mathrm\{2\; \backslash \; H\_2O\; +\; energy\; \backslash longrightarrow\; 4\; \backslash \; H\; +\; O\_2\}$

$\backslash mathrm\{2\; \backslash \; N\_2O\; +\; energy\; \backslash longrightarrow\; 4\; \backslash \; N\; +\; O\_2\}$

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Biological consumption

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

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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.): More recent research indicates that ocean life (marine primary production) is actually responsible for more than half the total oxygen production on Earth.

	4,500
	50
	500,000,000

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Ozone

The presence of atmospheric oxygen has led to the formation of ozone (O₃) and the ozone layer within the stratosphere:

$\backslash mathrm\{O\_2\; +\; uv~light\; \backslash longrightarrow\; 2~O\}\backslash qquad(\backslash lambda\; \backslash lesssim\; 200~\backslash text\{nm\})$

$\backslash mathrm\{O\; +\; O\_2\; \backslash longrightarrow\; O\_3\}$

The ozone layer is extremely important to modern life as it absorbs harmful ultraviolet radiation:

$\backslash mathrm\{O\_3\; +\; uv~light\; \backslash longrightarrow\; O\_2\; +\; O\}\backslash qquad(\backslash lambda\; \backslash lesssim\; 300~\backslash text\{nm\})$

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See also

• Carbon cycle
• Nitrogen cycle
• Hydrogen cycle
• Dark oxygen

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Further reading

Categories: Ecology, Chemical Oceanography, Photosynthesis, Biogeochemical Cycle, Cycle

+Annual gain and loss of atmospheric oxygen (Units of 1010 kg O2 per year) !Process!!Amount
16,500
13,500
1.3
0.03
~30,000
50
12

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