Oxygen cycle

alt=|360px|thumb|Main reservoirs and fluxes — in the [[biosphere (green), marine biosphere (blue), lithosphere (brown), and atmosphere (grey).

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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).

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The rate of organic carbon burial was derived from estimated fluxes of volcanic and hydrothermal carbon.

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The oxygen cycle refers to the various movements of oxygen through the Earth's atmosphere (air), biosphere (flora and fauna), hydrosphere (water bodies and glaciers) 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 atoms between different oxidation states in ions, oxides and molecules 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 reactant of many biogeochemical redox reactions within the cycle. Processes within the oxygen cycle are considered to be biological or geological and are evaluated as either a source (O₂ production) or sink (O₂ consumption). 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, CO₂, HNO₃, NO, NO₂, CO, H₂O₂, O₃, SO₂, H₂SO₄, MgO, CaO, Al₂O₃, SiO₂, and .||Present mainly as a component of water molecules, with dissolved molecules including free oxygen and carbonic acids (H_xCO₃).

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|Lithosphere|| 46.6%||Present mainly as silica minerals (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 oxygenic photosynthesis in conjunction with a biological sink known as the biological pump and a geologic process of carbon burial involving plate tectonics. 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 eukaryotic metabolism.

Biological production

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

:<math>\mathrm{6 \ CO_2 + 6H_2O + energy \longrightarrow C_6H_{12}O_6 + 6 \ O_2}</math>

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.

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:

:<math>\mathrm{2 \ H_2O + energy \longrightarrow 4 \ H + O_2}</math>

:<math>\mathrm{2 \ N_2O + energy \longrightarrow 4 \ N + O_2}</math>

Biological consumption

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

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.):

{| class="wikitable"

|----- align="center"

! Reservoir

! Capacity<br />(kg O₂)

! Flux in/out<br />(kg O₂ per year)

! Residence time<br />(years)

|----- align="right"

| align="left" | Atmosphere ||

| || 4,500

|----- align="right"

| align="left" | Biosphere ||

| || 50

|----- align="right"

| align="left" | Lithosphere ||

| || 500,000,000

|}

{| class="wikitable col2right"

|+Annual gain and loss of atmospheric oxygen<br /> (Units of 10¹⁰ kg O₂ per year)