Theoxygen cycle refers to the various movements ofoxygen through theEarth'satmosphere (air),biosphere (flora andfauna),hydrosphere (water bodies andglaciers) and thelithosphere (theEarth'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 thebiogeochemical cycle of oxygenatoms between differentoxidation states inions,oxides andmolecules throughredoxreactions within and between thespheres/reservoirs of the planet Earth.[1] The word oxygen in the literature typically refers to the most commonoxygen allotrope, elemental/diatomic oxygen (O2), as it is a commonproduct orreactant of many biogeochemical redox reactions within the cycle.[2] Processes within the oxygen cycle are considered to bebiological orgeological and are evaluated as either asource (O2 production) or sink (O2 consumption).[1][2]
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 thesilicate andoxideminerals of thecrust andmantle (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 ofbiomass,H2O,CO2,HNO3,NO,NO2,CO,H2O2,O3,SO2,H2SO4,MgO,CaO,Al2O3,SiO2, andPO3−4.[7]
| Location | % oxygen by volume | Notes |
|---|---|---|
| Atmosphere | 21% | This equates to a total of roughly3.4×1019mol of oxygen (O2).[2] Other oxygen-containing molecules in the atmosphere includeozone (O3),carbon dioxide (CO2),water vapor (H2O), andsulphur andnitrogen oxides (SO2,NO,N2O, etc.). |
| Biosphere | 22% | Present mainly as a component oforganic molecules and water. |
| Hydrosphere | 33%[8] | Present mainly as a component of water molecules, with dissolved molecules including free oxygen and carbonic acids (HxCO3). |
| Lithosphere | 46.6% | Present mainly assilica minerals (SiO2) and otheroxide minerals. |
While there are manyabiotic sources and sinks for O2, the presence of the profuse concentration of free oxygen in modernEarth's atmosphere andocean is attributed to O2 production in thebiological process ofoxygenic photosynthesis in conjunction with a biological sink known as thebiological pump and a geologic process ofcarbon burial involvingplate tectonics.[9][10][11][7] Biology is the main driver of O2flux on modern Earth, and theevolution of oxygenic photosynthesis bybacteria, which is discussed as part ofthe Great Oxygenation Event, is thought to be directly responsible for the conditions permitting the development and existence of allcomplexeukaryoticmetabolism.[12][13][14]
The main source of atmospheric free oxygen is photosynthesis, which producessugars and free oxygen from carbon dioxide and water:
Photosynthesizing organisms include the plant life of the land areas, as well as thephytoplankton of the oceans. The tiny marinecyanobacteriumProchlorococcus was discovered in 1986 and accounts for up to half of the photosynthesis of the open oceans.[15][16]
An additional source of atmospheric free oxygen comes fromphotolysis, whereby high-energyultraviolet 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:
The main way free oxygen is lost from the atmosphere is viarespiration anddecay, mechanisms in whichanimal life andbacteria consume oxygen and release carbon dioxide.
The following tables offer estimates of oxygen cycle reservoir capacities and fluxes. These numbers are based primarily on estimates from (Walker, J. C. G.):[10] More recent research indicates that ocean life (marine primary production) is actually responsible for more than half the total oxygen production on Earth.[17][18]
| Reservoir | Capacity (kg O2) | Flux in/out (kg O2 per year) | Residence time (years) |
|---|---|---|---|
| Atmosphere | 1.4×1018 | 3×1014 | 4,500 |
| Biosphere | 1.6×1016 | 3×1014 | 50 |
| Lithosphere | 2.9×1020 | 6×1011 | 500,000,000 |
| Process | Amount |
|---|---|
| Gains | |
| Photosynthesis (land) | 16,500 |
| Photosynthesis (ocean) | 13,500 |
| Photolysis of N2O | 1.3 |
| Photolysis of H2O | 0.03 |
| Total gains | ~30,000 |
| Losses - respiration and decay | |
| Aerobic respiration | 23,000 |
| Microbial oxidation | 5,100 |
| Combustion of fossil fuel (anthropogenic) | 1,200 |
| Photochemical oxidation | 600 |
| Fixation of N2 by lightning | 12 |
| Fixation of N2 by industry (anthropogenic) | 10 |
| Oxidation of volcanic gases | 5 |
| Total losses by respiration and decay | ~29,927 |
| Losses - weathering | |
| Chemical weathering | 50 |
| Surface reaction of O3 | 12 |
| Total losses | ~30,000 |
The presence of atmospheric oxygen has led to the formation ofozone (O3) and theozone layer within thestratosphere:
The ozone layer is extremely important to modern life as it absorbs harmfulultraviolet radiation:
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