Oxygen cycle
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
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
Biological production
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
Abiotic production
An additional source of atmospheric free oxygen comes from
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:
- O + O2 :- O3
The ozone layer is extremely important to modern life as it absorbs harmful ultraviolet radiation:
See also
References
- ^ OCLC 793103985.
- ^ ISBN 978-0-08-098300-4.
- S2CID 4311084.
- .
- .
- PMID 18487127.
- ^ PMID 21190137.
- ^ "hydrosphere - Origin and evolution of the hydrosphere | Britannica". www.britannica.com. Retrieved 2022-07-03.
- PMID 16754606.
- ^ ISBN 9783662229880.
- ISBN 978-0-08-098300-4.
- .
- S2CID 4443958.
- PMID 27457943.
- PMID 14631732.
- PMID 21304826.
- ^ 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.
- S2CID 10267488.
Further reading
- Cloud P, Gibor A (September 1970). "The oxygen cycle". Scientific American. 223 (3): 110–123. 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.