Oxygen cycle

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]

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]

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 (C_xH_xN_xO_x) and water molecules.

Hydrosphere

The hydrosphere is 33% oxygen by volume 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 more than half of the photosynthesis 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 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	1.4×10^18	3×10^14	4500
Biosphere	1.6×10^16	3×10^14	50
Lithosphere	2.9×10^20	6×10^11	500000000

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

Photosynthesis (land) Photosynthesis (ocean) Photolysis of N2O Photolysis of H2O	16,500 13,500 1.3 0.03
Total gains	~ 30,000
Losses - respiration and decay
Aerobic respiration Microbial oxidation Combustion of fossil fuel (anthropogenic) Photochemical oxidation Fixation of N2 by lightning Fixation of N2 by industry (anthropogenic) Oxidation of volcanic gases	23,000 5,100 1,200 600 12 10 5
Losses - weathering
Chemical weathering Surface reaction of O3	50 12
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. 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.
3. Keeling RF, Shertz SR (August 1992). "Seasonal and interannual variations in atmospheric oxygen and implications for the global carbon cycle". Nature. 358 (6389): 723–727. Bibcode:1992Natur.358..723K. doi:10.1038/358723a0.
4. Holland HD (2002). "Volcanic gases, black smokers, and the great oxidation event". Geochimica et Cosmochimica Acta. 66 (21): 3811–3826. Bibcode:2002GeCoA..66.3811H. doi:10.1016/S0016-7037(02)00950-X.
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7. Falkowski PG (January 2011). "The biological and geological contingencies for the rise of oxygen on Earth". Photosynthesis Research. 107 (1): 7–10. doi:10.1007/s11120-010-9602-4. PMID 21190137.
8. Holland HD (June 2006). "The oxygenation of the atmosphere and oceans". Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences. 361 (1470): 903–15. doi:10.1098/rstb.2006.1838. PMC 1578726. PMID 16754606.
9. Walker JC (1980). "The Oxygen Cycle". The Natural Environment and the Biogeochemical Cycles. The Handbook of Environmental Chemistry. Springer Berlin Heidelberg. pp. 87–104. doi:10.1007/978-3-662-24940-6_5. ISBN 9783662229880.
10. Sigman DM, Haug GH (December 2003). "The biological pump in the past.". Treatise on geochemistry. 6 (2nd ed.). p. 625. doi:10.1016/b978-0-08-095975-7.00618-5. ISBN 978-0-08-098300-4.
11. Fischer WW, Hemp J, Johnson JE (June 2016). "Evolution of oxygenic photosynthesis". Annual Review of Earth and Planetary Sciences. 44 (1): 647–83. Bibcode:2016AREPS..44..647F. doi:10.1146/annurev-earth-060313-054810.
12. Lyons TW, Reinhard CT, Planavsky NJ (February 2014). "The rise of oxygen in Earth's early ocean and atmosphere". Nature. 506 (7488): 307–15. Bibcode:2014Natur.506..307L. doi:10.1038/nature13068. PMID 24553238.
13. Reinhard CT, Planavsky NJ, Olson SL, Lyons TW, Erwin DH (August 2016). "Earth's oxygen cycle and the evolution of animal life". Proceedings of the National Academy of Sciences of the United States of America. 113 (32): 8933–8. Bibcode:2016PNAS..113.8933R. doi:10.1073/pnas.1521544113. PMC 4987840. PMID 27457943.
14. Nadis S (November 2003). "The Cells That Rule the Seas". Scientific American. 289 (6): 52–53. Bibcode:2003SciAm.289f..52N. doi:10.1038/scientificamerican1203-52. PMID 14631732.

Further reading

• Cloud P, Gibor A (September 1970). "The oxygen cycle". Scientific American. 223 (3): 110–123. Bibcode:1970SciAm.223c.110C. doi:10.1038/scientificamerican0970-110.
• Fasullo J. "Substitute Lectures for ATOC 3600". Principles of Climate, Lectures on the global oxygen cycle.
• Morris RM. "OXYSPHERE - A Beginners' Guide to the Biogeochemical Cycling of Atmospheric Oxygen". Archived from the original on 2004-11-03.