# Oxygen cycle

Main reservoirs and fluxes (in unit 1012 mol/yr) of the modern global O2 cycle on Earth. There are four main reservoirs: terrestrial biosphere (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, purple arrow is related to space (not a reservoir, but also contributes to the atmospheric O2).[1] The value of photosynthesis or net primary productivity (NPP) can be estimated through the variation in the abundance and isotopic composition of atmospheric O2.[2][3] The rate of organic carbon burial was derived from estimated fluxes of volcanic and hydrothermal carbon.[4][5]

The oxygen cycle is the biogeochemical transitions 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.[1] 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.[2] 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).[1][2]

## Reservoirs

Oxygen is one of the most abundant 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).[6] 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, AlO, SiO2, and PO4.[7]

#### Atmosphere

The atmosphere is ~20.9% oxygen by volume, which equates to a total of roughly 34 × 1018 mol of oxygen.[2] Other oxygen-containing molecules in the atmosphere include ozone (O3), carbon dioxide (CO2), water vapor (H2O), and sulphur and nitrogen oxides (SO2, NO, N2O, etc.).

#### Biosphere

The biosphere is 22% oxygen by volume present mainly as a component of organic molecules (CxHxNxOx) and water molecules.

#### Hydrosphere

The hydrosphere is 33% oxygen by volume[citation needed] present mainly as a component of water molecules with dissolved molecules including free oxygen and carbonic acids (HxCO3).

#### Lithosphere

The lithosphere is 46.6% oxygen by volume present mainly as silica minerals (SiO2) and other oxide minerals.

## 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 from 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.[8][9][10][7] 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.[11][12][13]

#### Biological production

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

${\displaystyle \mathrm {6\ CO_{2}+6H_{2}O+energy\longrightarrow C_{6}H_{12}O_{6}+6\ O_{2}} }$

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 more than half of the photosynthesis[citation needed] of the open ocean.[14]

#### 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 H and N atoms[clarify] escape into space, leaving O2 in the atmosphere:

${\displaystyle \mathrm {2\ H_{2}O+energy\longrightarrow 4\ H+O_{2}} }$
${\displaystyle \mathrm {2\ N_{2}O+energy\longrightarrow 4\ N+O_{2}} }$

#### 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.):[9]

Reservoir Capacity
(kg O2)
Flux in/out
(kg O2 per year)
Residence time
(years)
Atmosphere 1.4×1018 3×1014 4500
Biosphere 1.6×1016 3×1014 50
Lithosphere 2.9×1020 6×1011 500000000

Table 2: Annual gain and loss of atmospheric oxygen (Units of 1010 kg O2 per year)[1]

 Photosynthesis (land)Photosynthesis (ocean)Photolysis of N2OPhotolysis of H2O 16,50013,5001.30.03 Total gains ~ 30,000 Losses - respiration and decay Aerobic respirationMicrobial oxidationCombustion of fossil fuel (anthropogenic)Photochemical oxidationFixation of N2 by lightningFixation of N2 by industry (anthropogenic)Oxidation of volcanic gases 23,0005,1001,20060012105 Losses - weathering Chemical weatheringSurface reaction of O3 5012 Total losses ~ 30,000

## Ozone

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

${\displaystyle \mathrm {O_{2}+uv~light\longrightarrow 2~O} \qquad (\lambda \lesssim 200~{\text{nm}})}$
${\displaystyle \mathrm {O+O_{2}\longrightarrow O_{3}} }$

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

${\displaystyle \mathrm {O_{3}+uv~light\longrightarrow O_{2}+O} \qquad (\lambda \lesssim 300~{\text{nm}})}$

## References

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2. ^ a b c d Petsch ST (2014). "The Global Oxygen Cycle". Treatise on Geochemistry. Elsevier. pp. 437–473. doi:10.1016/b978-0-08-095975-7.00811-1. ISBN 978-0-08-098300-4.
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