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 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.^[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[]

Part of a series on
Biogeochemical cycles
show Water cycle Water cycle deep water cycle
show Carbon cycle Global atmospheric terrestrial oceanic Sequestration carbon sink deep carbon cycle soil carbon mycorrhizal fungi Boreal forests
hide Nutrient cycle Hydrogen cycle Nitrogen cycle human impact nitrification nitrogen and lichens fixation assimilation Oxygen cycle Phosphorus cycle assimilation Sulfur cycle assimilation
show Rock cycle Rock cycle Calcium cycle Silica cycle Carbonate–silicate cycle
show Marine cycle Marine biogeochemical cycles Biological pump microbial loop viral shunt Calcareous ooze Siliceous ooze
show Methane cycle Atmospheric methane Methane clathrate clathrate gun hypothesis Arctic methane emissions
show Other cycles aluminum arsenic boron bromine cadmium chlorine chromium copper fluorine gold iodine iron lead lithium manganese mercury ozone–oxygen selenium vanadium zinc
show Related topics Biogeochemistry geochemical cycle chemical cycling environmental chemistry Biosequestration Ocean acidification acid rain Biogeochemical planetary boundaries
show Research groups DAAC GEOTRACES IMBER NOBM SOLAS
v

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 (C_xH_xN_xO_x) 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 (H_xCO₃).

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 up to half of the photosynthesis of the open ocean.^[14][15]

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]

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

Ozone[]

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

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

${\displaystyle \mathrm {O+O_{2}\longrightarrow O_{3}} }$

O + O₂ :- O₃

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

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

See also[]

• Carbon cycle
• Nitrogen cycle

References[]

Further reading[]