Oxygen cycle

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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]

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]

Reservoirs[edit]

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 weigh less than 0.05% of the Earth's total mass. 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[edit]

The Atmosphere is ~20.9% oxygen by volume which equates to a total of roughly 34x1018 mol of oxygen.[2] Other oxygen-containing molecules in the atmosphere include ozone (O3), carbon dioxide (CO2), water vapor (H2O), and sulfur and nitrogen oxides (SO2, NO, N2O, etc.).

Biosphere[edit]

The Biosphere is 22% oxygen by volume present mainly as a component of organic molecules (CxHxNxOx) and water molecules.

Hydrosphere[edit]

The Hydrosphere is 33% oxygen by volume present mainly as a component of water molecules with dissolved molecules including free oxygen and carbonic acids (HxCO3).

Lithosphere[edit]

Lithosphere is 46.6% oxygen by volume present mainly as silica minerals (SiO2) and other oxide minerals.

Reservoir Dynamics[edit]

Free Oxygen (O2) from the atmosphere forms an equilibrium concentration by gas exchange with the hydrosphere as a dissolved gas in aqueous solution according to Henry's law. According to this law, O2 saturates in water at 450µM at 0ºC and 270µM at 25ºC, but other dissolved solutes in seawater can reduce this saturation concentration.[2] Oxygen concentrations in the hydrosphere can be influenced locally by the presence or absence of turbulent mixing or local production or consumption of O2 by biological metabolism. Oxygen concentration in the soil and groundwater of the pedosphere is determined by gas diffusion through soil pore space in air and rainwater and can also be influenced locally by biological processes.

Oxygen is cycled between the biosphere and lithosphere within the context of the calcium cycle, marine organisms in the biosphere create calcium carbonate shell material (CaCO3) that is rich in oxygen. When the organism dies, its shell is deposited on the shallow seafloor and buried over time to create the limestone sedimentary rock of the lithosphere. Weathering processes initiated by organisms can also free oxygen from the lithosphere. Plants and animals extract nutrient minerals from rocks and release oxygen in the process.

Seasonal high latitude O2 level fluctuations of +/- 15 p.p.m. in the northern hemisphere have been observed and attributed to seasonal cycles of primary production and respiration.[3] Human combustion of fossil fuels has been linked to a measured decrease of around 1x1015 mol per year in O2 concentrations in recent decades.[8]

Sources and sinks[edit]

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[edit]

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

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.[15]

Abiotic production[edit]

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[clarify] escape into space, leaving O2 in the atmosphere:

Biological consumption[edit]

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.

Abiotic consumption[edit]

The lithosphere also consumes atmospheric free oxygen by chemical weathering and surface reactions. An example of surface weathering chemistry is formation of iron oxides (rust):

Capacities and fluxes[edit]

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]

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)
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[edit]

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

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

References[edit]

  1. ^ a b c d 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.
  2. ^ a b c d e 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. ^ a b 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. doi:10.1016/S0016-7037(02)00950-X.
  5. ^ Lasaga AC, Ohmoto H (2002). "The oxygen geochemical cycle: dynamics and stability". Geochimica et Cosmochimica Acta. 66 (3): 361–381. doi:10.1016/S0016-7037(01)00685-8.
  6. ^ 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.
  7. ^ a b 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. ^ Manning A, Keeling RF (January 2006). "Global oceanic and land biotic carbon sinks from the Scripps atmospheric oxygen flask sampling network". Tellus B: Chemical and Physical Meteorology. 58 (2): 95–116. Bibcode:2006TellB..58...95M. doi:10.1111/j.1600-0889.2006.00175.x. ISSN 1600-0889.
  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.
  10. ^ a b Walker JC (1980). "The Natural Environment and the Biogeochemical Cycles". The Oxygen Cycle. The Handbook of Environmental Chemistry. Springer Berlin Heidelberg. pp. 87–104. doi:10.1007/978-3-662-24940-6_5. ISBN 9783662229880.
  11. ^ 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.
  12. ^ 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.
  13. ^ 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.
  14. ^ 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.
  15. ^ 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.

Further reading[edit]