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Locations of oxygen

Location	% oxygenby volume	Notes
Atmosphere	21%	This equates to a total of roughly 3.4×1019 mol of oxygen (O2).7 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	22%	Present mainly as a component of organic 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 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 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.^9101112 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.¹³¹⁴¹⁵

Biological production

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

6   C O 2 + 6 H 2 O + e n e r g y ⟶ C 6 H 12 O 6 + 6   O 2 {\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 oceans.¹⁶¹⁷

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:

2   H 2 O + e n e r g y ⟶ 4   H + O 2 {\displaystyle \mathrm {2\ H_{2}O+energy\longrightarrow 4\ H+O_{2}} } 2   N 2 O + e n e r g y ⟶ 4   N + 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.):¹⁸ More recent research indicates that ocean life (marine primary production) is actually responsible for more than half the total oxygen production on Earth.¹⁹²⁰

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

Annual gain and loss of atmospheric oxygen (Units of 1010 kg O2 per year)²¹

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	~30,000
Losses - weathering
Chemical weathering	50
Surface reaction of O3	12
Total losses	~30,000

Ozone

Main article: Ozone-oxygen cycle

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

O 2 + u v   l i g h t ⟶ 2   O ( λ ≲ 200   nm ) {\displaystyle \mathrm {O_{2}+uv~light\longrightarrow 2~O} \qquad (\lambda \lesssim 200~{\text{nm}})} O + O 2 ⟶ O 3 {\displaystyle \mathrm {O+O_{2}\longrightarrow O_{3}} }

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

O 3 + u v   l i g h t ⟶ O 2 + O ( λ ≲ 300   nm ) {\displaystyle \mathrm {O_{3}+uv~light\longrightarrow O_{2}+O} \qquad (\lambda \lesssim 300~{\text{nm}})}

See also

• Carbon cycle
• Nitrogen cycle
• Hydrogen cycle
• Dark oxygen

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. PMID 5459721.
• 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.

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. 978-0-08-098300-4 ↩

3. Knoll AH, Canfield DE, Konhauser K (2012). "7". Fundamentals of geobiology. Chichester, West Sussex: John Wiley & Sons . pp. 93–104. ISBN 978-1-118-28087-4. OCLC 793103985. 978-1-118-28087-4 ↩

4. 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. 978-0-08-098300-4 ↩

5. Falkowski PG, Godfrey LV (August 2008). "Electrons, life and the evolution of Earth's oxygen cycle". Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences. 363 (1504): 2705–16. doi:10.1098/rstb.2008.0054. PMC 2606772. PMID 18487127. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2606772 ↩

6. Falkowski PG (January 2011). "The biological and geological contingencies for the rise of oxygen on Earth". Photosynthesis Research. 107 (1): 7–10. Bibcode:2011PhoRe.107....7F. doi:10.1007/s11120-010-9602-4. PMID 21190137. https://doi.org/10.1007%2Fs11120-010-9602-4 ↩

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8. "hydrosphere - Origin and evolution of the hydrosphere | Britannica". www.britannica.com. Retrieved 2022-07-03. https://www.britannica.com/science/hydrosphere/Origin-and-evolution-of-the-hydrosphere ↩

9. 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. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1578726 ↩

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11. Sigman DM, Haug GH (December 2003). "The biological pump in the past.". Treatise on geochemistry. Vol. 6 (2nd ed.). p. 625. doi:10.1016/b978-0-08-095975-7.00618-5. ISBN 978-0-08-098300-4. 978-0-08-098300-4 ↩

12. Falkowski PG (January 2011). "The biological and geological contingencies for the rise of oxygen on Earth". Photosynthesis Research. 107 (1): 7–10. Bibcode:2011PhoRe.107....7F. doi:10.1007/s11120-010-9602-4. PMID 21190137. https://doi.org/10.1007%2Fs11120-010-9602-4 ↩

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17. Morris JJ, Johnson ZI, Szul MJ, Keller M, Zinser ER (2011). "Dependence of the Cyanobacterium Prochlorococcus on Hydrogen Peroxide Scavenging Microbes for Growth at the Ocean's Surface". PLOS ONE. 6 (2): e16805. Bibcode:2011PLoSO...616805M. doi:10.1371/journal.pone.0016805. PMC 3033426. PMID 21304826. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3033426 ↩

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19. Roach, John (June 7, 2004). "Source of Half Earth's Oxygen Gets Little Credit". National Geographic News. Archived from the original on June 8, 2004. Retrieved 2016-04-04. https://web.archive.org/web/20040608065449/http://news.nationalgeographic.com/news/2004/06/0607_040607_phytoplankton.html ↩

20. Lin, I.; Liu, W. Timothy; Wu, Chun-Chieh; Wong, George T. F.; Hu, Chuanmin; Chen, Zhiqiang; Wen-Der, Liang; Yang, Yih; Liu, Kon-Kee (2003). "New evidence for enhanced ocean primary production triggered by tropical cyclone". Geophysical Research Letters. 30 (13): 1718. Bibcode:2003GeoRL..30.1718L. doi:10.1029/2003GL017141. S2CID 10267488. https://digitalcommons.odu.edu/cgi/viewcontent.cgi?article=1335&context=oeas_fac_pubs ↩

21. Knoll AH, Canfield DE, Konhauser K (2012). "7". Fundamentals of geobiology. Chichester, West Sussex: John Wiley & Sons . pp. 93–104. ISBN 978-1-118-28087-4. OCLC 793103985. 978-1-118-28087-4 ↩