oxygen cycle

File:Global Oxygen Cycle.jpg (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, and the purple arrow is related to space (not a reservoir, but also contributes to the atmospheric O₂).

The value of photosynthesis or net primary productivity (NPP) can be estimated through the variation in the abundance and isotopic composition of atmospheric O₂.{{cite journal | vauthors = Keeling RF, Shertz SR | title = Seasonal and interannual variations in atmospheric oxygen and implications for the global carbon cycle. | journal = Nature | date = August 1992 | volume = 358 | issue = 6389 | pages = 723–727 | doi = 10.1038/358723a0 | bibcode = 1992Natur.358..723K | s2cid = 4311084 }}

The rate of organic carbon burial was derived from estimated fluxes of volcanic and hydrothermal carbon.{{Cite journal|last=Holland|first=Heinrich D. | name-list-style = vanc |date=2002|title=Volcanic gases, black smokers, and the great oxidation event|journal=Geochimica et Cosmochimica Acta|language=en|volume=66|issue=21|pages=3811–3826|doi=10.1016/S0016-7037(02)00950-X|bibcode=2002GeCoA..66.3811H }}{{Cite journal|last1=Lasaga|first1=Antonio C.|last2=Ohmoto|first2=Hiroshi| name-list-style = vanc |date=2002|title=The oxygen geochemical cycle: dynamics and stability|journal=Geochimica et Cosmochimica Acta|language=en|volume=66|issue=3|pages=361–381|doi=10.1016/S0016-7037(01)00685-8|bibcode=2002GeCoA..66..361L}}

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The oxygen cycle refers to the various movements of oxygen through the Earth's atmosphere (air), biosphere (flora and fauna), hydrosphere (water bodies and glaciers) and the lithosphere (the Earth'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 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.{{cite book | first1 = Andrew H | last1 = Knoll | first2 = Donald E | last2 = Canfield | first3 = Kurt | last3 = Konhauser | name-list-style = vanc |title=Fundamentals of geobiology|date=2012|publisher=John Wiley & Sons .|isbn=978-1-118-28087-4|location=Chichester, West Sussex|pages=93–104|chapter=7|oclc=793103985}} 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.{{cite book | vauthors = Petsch ST | chapter = The Global Oxygen Cycle|date=2014 | title =Treatise on Geochemistry|pages=437–473|publisher=Elsevier |doi=10.1016/b978-0-08-095975-7.00811-1|isbn=978-0-08-098300-4 }} 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).

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 the silicate and oxide minerals of the crust and mantle (99.5% by weight).{{cite journal | vauthors = Falkowski PG, Godfrey LV | title = Electrons, life and the evolution of Earth's oxygen cycle | journal = Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences | volume = 363 | issue = 1504 | pages = 2705–16 | date = August 2008 | pmid = 18487127 | pmc = 2606772 | doi = 10.1098/rstb.2008.0054 }} 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, Al₂O₃, SiO₂, and {{chem2|PO4(3-)|link=Phosphate}}.

Locations of oxygen

class=wikitable !Location!!% oxygen by volume!!Notes
Atmosphere	21%	This equates to a total of roughly Scientific notation mol of oxygen (O₂). 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%{{Cite web \|title=hydrosphere - Origin and evolution of the hydrosphere {{!}} Britannica \|url=https://www.britannica.com/science/hydrosphere/Origin-and-evolution-of-the-hydrosphere \|access-date=2022-07-03 \|website=www.britannica.com \|language=en}}	Present mainly as a component of water molecules, with dissolved molecules including free oxygen and carbonic acids (H_xCO₃).
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.{{cite journal | vauthors = Holland HD | title = The oxygenation of the atmosphere and oceans | journal = Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences | volume = 361 | issue = 1470 | pages = 903–15 | date = June 2006 | pmid = 16754606 | pmc = 1578726 | doi = 10.1098/rstb.2006.1838 }}{{cite book | vauthors = Walker JC | title = The Natural Environment and the Biogeochemical Cycles| chapter = The Oxygen Cycle|date=1980|pages=87–104|publisher=Springer Berlin Heidelberg|doi=10.1007/978-3-662-24940-6_5|isbn=9783662229880|series=The Handbook of Environmental Chemistry}}{{cite book | vauthors = Sigman DM, Haug GH | chapter = The biological pump in the past. | title = Treatise on geochemistry. | edition = 2nd | date = December 2003 | volume = 6 | page = 625 | doi = 10.1016/b978-0-08-095975-7.00618-5 | isbn = 978-0-08-098300-4 }}{{cite journal | vauthors = Falkowski PG | title = The biological and geological contingencies for the rise of oxygen on Earth | journal = Photosynthesis Research | volume = 107 | issue = 1 | pages = 7–10 | date = January 2011 | pmid = 21190137 | doi = 10.1007/s11120-010-9602-4 | doi-access = free | bibcode = 2011PhoRe.107....7F }} 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.{{cite journal | vauthors = Fischer WW, Hemp J, Johnson JE | title = Evolution of oxygenic photosynthesis. | journal = Annual Review of Earth and Planetary Sciences | date = June 2016 | volume = 44 | issue = 1 | pages = 647–83 | doi = 10.1146/annurev-earth-060313-054810 | bibcode = 2016AREPS..44..647F | doi-access = free }}{{cite journal | vauthors = Lyons TW, Reinhard CT, Planavsky NJ | title = The rise of oxygen in Earth's early ocean and atmosphere | journal = Nature | volume = 506 | issue = 7488 | pages = 307–15 | date = February 2014 | pmid = 24553238 | doi = 10.1038/nature13068 | bibcode = 2014Natur.506..307L | s2cid = 4443958 }}{{cite journal | vauthors = Reinhard CT, Planavsky NJ, Olson SL, Lyons TW, Erwin DH | title = Earth's oxygen cycle and the evolution of animal life | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 113 | issue = 32 | pages = 8933–8 | date = August 2016 | pmid = 27457943 | pmc = 4987840 | doi = 10.1073/pnas.1521544113 | bibcode = 2016PNAS..113.8933R | doi-access = free }}

= Biological production =

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

: $\mathrm{6 \ CO_2 + 6H_2O + energy \longrightarrow C_6H_{12}O_6 + 6 \ O_2}$

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.{{cite journal | first = Steve | last = Nadis | name-list-style = vanc | title = The Cells That Rule the Seas | journal = Scientific American | volume = 289 | issue = 6 | pages = 52–53 | date = November 2003 | doi = 10.1038/scientificamerican1203-52 | bibcode = 2003SciAm.289f..52N | pmid = 14631732 }}{{cite journal | vauthors = Morris JJ, Johnson ZI, Szul MJ, Keller M, Zinser ER | year = 2011 | title = Dependence of the Cyanobacterium Prochlorococcus on Hydrogen Peroxide Scavenging Microbes for Growth at the Ocean's Surface | journal = PLOS ONE | volume = 6 | issue = 2 | pages = e16805 | doi = 10.1371/journal.pone.0016805 | pmid = 21304826 | pmc = 3033426| bibcode = 2011PLoSO...616805M | doi-access = free }}

= 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:

: $\mathrm{2 \ H_2O + energy \longrightarrow 4 \ H + O_2}$

: $\mathrm{2 \ N_2O + energy \longrightarrow 4 \ N + O_2}$

= 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.): More recent research indicates that ocean life (marine primary production) is actually responsible for more than half the total oxygen production on Earth.{{cite news|last=Roach|first=John|date=June 7, 2004|title=Source of Half Earth's Oxygen Gets Little Credit|work=National Geographic News|url=http://news.nationalgeographic.com/news/2004/06/0607_040607_phytoplankton.html|archive-url=https://web.archive.org/web/20040608065449/http://news.nationalgeographic.com/news/2004/06/0607_040607_phytoplankton.html|url-status=dead|archive-date=June 8, 2004|access-date=2016-04-04}}{{cite journal|last1=Lin|first1=I.|last2=Liu|first2=W. Timothy|last3=Wu|first3=Chun-Chieh|last4=Wong|first4=George T. F.|last5=Hu|first5=Chuanmin|last6=Chen|first6=Zhiqiang|last7=Wen-Der|first7=Liang|last8=Yang|first8=Yih|last9=Liu|first9=Kon-Kee|year=2003|title=New evidence for enhanced ocean primary production triggered by tropical cyclone|url=https://digitalcommons.odu.edu/cgi/viewcontent.cgi?article=1335&context=oeas_fac_pubs|journal=Geophysical Research Letters|volume=30|issue=13|page=1718|bibcode=2003GeoRL..30.1718L|doi=10.1029/2003GL017141|s2cid=10267488 |doi-access=free}}

class="wikitable"

---- align="center"

! Reservoir

! Capacity
(kg O₂)

! Flux in/out
(kg O₂ per year)

! Residence time
(years)

---- align="right"

| align="left" | Atmosphere

| {{val|3|e=14}}

4,500

---- align="right"

| align="left" | Biosphere

| {{val|3|e=14}}

---- align="right"

| align="left" | Lithosphere

| {{val|6|e=11}}

500,000,000

colspan="2" style="background:#b0b0e0;"\|Gains
class="wikitable col2right" \|+Annual gain and loss of atmospheric oxygen (Units of 10¹⁰ kg O₂ per year) !Process!!Amount
Photosynthesis (land)	16,500
Photosynthesis (ocean)	13,500
Photolysis of N₂O	1.3
Photolysis of H₂O	0.03
style="border-top:2px solid blue;border-bottom:2px solid blue; \| Total gains	~30,000
colspan="2" style="background:#e0b0b0;"\|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
style="border-top:2px solid red;border-bottom:2px solid red; \|Total losses by respiration and decay	~30,000
colspan="2" style="background:#b0e0b0;"\|Losses - weathering
Chemical weathering	50
Surface reaction of O₃	12
style="border-top:2px solid black;border-bottom:2px solid black; \| 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:

: $\mathrm{O_2 + uv~light \longrightarrow 2~O}\qquad(\lambda \lesssim 200~\text{nm})$

: $\mathrm{O + O_2 \longrightarrow O_3}$

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

: $\mathrm{O_3 + uv~light \longrightarrow O_2 + O}\qquad(\lambda \lesssim 300~\text{nm})$

oxygen cycle

Locations of oxygen

Sources and sinks

= Biological production =

= Abiotic production =

= Biological consumption =

Capacities and fluxes

Ozone

See also

References

Further reading