Earth:Oxygen cycle

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Short description: Biogeochemical cycle of oxygen
Main reservoirs and fluxes — in the 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, and the 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]

Oxygen cycle refers to the movement of oxygen through the atmosphere (air), biosphere (plants and animals) 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. 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]

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).[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, Al2O3, SiO2, and PO4.[7]

Atmosphere

The atmosphere is 21% 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 (CxHxNxOx) and water.

Hydrosphere

The hydrosphere is 33% oxygen by volume[8] present mainly as a component of water molecules, with dissolved molecules including free oxygen and carbolic acids (HxCO3).

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.[9][10][11][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.[12][13][14]

Biological production

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

[math]\displaystyle{ \mathrm{6 \ CO_2 + 6H_2O + energy \longrightarrow C_6H_{12}O_6 + 6 \ O_2} }[/math]

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.[15][16]

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:

[math]\displaystyle{ \mathrm{2 \ H_2O + energy \longrightarrow 4 \ H + O_2} }[/math]
[math]\displaystyle{ \mathrm{2 \ N_2O + energy \longrightarrow 4 \ N + O_2} }[/math]

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.):[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 4500
Biosphere 1.6×1016 3×1014 50
Lithosphere 2.9×1020 6×1011 500000000


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

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
Losses - weathering
Chemical weathering 50
Surface reaction of O3 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:

[math]\displaystyle{ \mathrm{O_2 + uv~light \longrightarrow 2~O}\qquad(\lambda \lesssim 200~\text{nm}) }[/math]
[math]\displaystyle{ \mathrm{O + O_2 \longrightarrow O_3} }[/math]
O + O2 :- O3

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

[math]\displaystyle{ \mathrm{O_3 + uv~light \longrightarrow O_2 + O}\qquad(\lambda \lesssim 300~\text{nm}) }[/math]

See also

References

  1. ↑ 1.0 1.1 1.2 1.3 Knoll, Andrew H; Canfield, Donald E; Konhauser, Kurt (2012). "7". Fundamentals of geobiology. Chichester, West Sussex: John Wiley & Sons .. pp. 93–104. ISBN 978-1-118-28087-4. OCLC 793103985. 
  2. ↑ 2.0 2.1 2.2 2.3 "The Global Oxygen Cycle". Treatise on Geochemistry. Elsevier. 2014. pp. 437–473. doi:10.1016/b978-0-08-095975-7.00811-1. ISBN 978-0-08-098300-4. 
  3. ↑ "Seasonal and interannual variations in atmospheric oxygen and implications for the global carbon cycle.". Nature 358 (6389): 723–727. August 1992. doi:10.1038/358723a0. Bibcode: 1992Natur.358..723K. 
  4. ↑ Holland, Heinrich D. (2002). "Volcanic gases, black smokers, and the great oxidation event" (in en). Geochimica et Cosmochimica Acta 66 (21): 3811–3826. doi:10.1016/S0016-7037(02)00950-X. Bibcode: 2002GeCoA..66.3811H. 
  5. ↑ Lasaga, Antonio C.; Ohmoto, Hiroshi (2002). "The oxygen geochemical cycle: dynamics and stability" (in en). Geochimica et Cosmochimica Acta 66 (3): 361–381. doi:10.1016/S0016-7037(01)00685-8. Bibcode: 2002GeCoA..66..361L. 
  6. ↑ "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. August 2008. doi:10.1098/rstb.2008.0054. PMID 18487127. 
  7. ↑ 7.0 7.1 "The biological and geological contingencies for the rise of oxygen on Earth". Photosynthesis Research 107 (1): 7–10. January 2011. doi:10.1007/s11120-010-9602-4. PMID 21190137. 
  8. ↑ "hydrosphere - Origin and evolution of the hydrosphere | Britannica" (in en). https://www.britannica.com/science/hydrosphere/Origin-and-evolution-of-the-hydrosphere. 
  9. ↑ "The oxygenation of the atmosphere and oceans". Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences 361 (1470): 903–15. June 2006. doi:10.1098/rstb.2006.1838. PMID 16754606. 
  10. ↑ 10.0 10.1 "The Oxygen Cycle". The Natural Environment and the Biogeochemical Cycles. The Handbook of Environmental Chemistry. Springer Berlin Heidelberg. 1980. pp. 87–104. doi:10.1007/978-3-662-24940-6_5. ISBN 9783662229880. 
  11. ↑ "The biological pump in the past.". Treatise on geochemistry.. 6 (2nd ed.). December 2003. p. 625. doi:10.1016/b978-0-08-095975-7.00618-5. ISBN 978-0-08-098300-4. 
  12. ↑ "Evolution of oxygenic photosynthesis.". Annual Review of Earth and Planetary Sciences 44 (1): 647–83. June 2016. doi:10.1146/annurev-earth-060313-054810. Bibcode: 2016AREPS..44..647F. 
  13. ↑ "The rise of oxygen in Earth's early ocean and atmosphere". Nature 506 (7488): 307–15. February 2014. doi:10.1038/nature13068. PMID 24553238. Bibcode: 2014Natur.506..307L. 
  14. ↑ "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. August 2016. doi:10.1073/pnas.1521544113. PMID 27457943. Bibcode: 2016PNAS..113.8933R. 
  15. ↑ Nadis, Steve (November 2003). "The Cells That Rule the Seas". Scientific American 289 (6): 52–53. doi:10.1038/scientificamerican1203-52. PMID 14631732. Bibcode: 2003SciAm.289f..52N. 
  16. ↑ "Dependence of the Cyanobacterium Prochlorococcus on Hydrogen Peroxide Scavenging Microbes for Growth at the Ocean's Surface". PLOS ONE 6 (2): e16805. 2011. doi:10.1371/journal.pone.0016805. PMID 21304826. Bibcode: 2011PLoSO...616805M. 
  17. ↑ Roach, John (June 7, 2004). "Source of Half Earth's Oxygen Gets Little Credit". National Geographic News. http://news.nationalgeographic.com/news/2004/06/0607_040607_phytoplankton.html. 
  18. ↑ Lin, I.; Liu, W. Timothy; Wu, Chun-Chieh; Wong, George T. F.; Hu, Chuanmin; Chen, Zhiqiang; Wen-Der, Liang; Yang, Yih et al. (2003). "New evidence for enhanced ocean primary production triggered by tropical cyclone". Geophysical Research Letters 30 (13): 1718. doi:10.1029/2003GL017141. Bibcode: 2003GeoRL..30.1718L. https://digitalcommons.odu.edu/cgi/viewcontent.cgi?article=1335&context=oeas_fac_pubs. 

Further reading



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