Oxygen cycle
From Wikipedia, the free encyclopedia
The oxygen cycle is the biogeochemical cycle that describes the movement of oxygen within and between its three main reservoirs: the atmosphere (air), the biosphere (living things), and the lithosphere (Earth's crust). The main driving factor of the oxygen cycle is photosynthesis, which is responsible for the modern Earth's atmosphere and life.
Contents |
[edit] Reservoirs
By far the largest reservoir of Earth's oxygen is within the silicate and oxide minerals of the crust and mantle (99.5%). Only a small portion has been released as free oxygen to the biosphere (0.01%) and atmosphere (0.36%). The main source of atmospheric oxygen is photosynthesis, which produces sugars and oxygen from carbon dioxide and water:
- 6CO2 + 6H2O + energy → C6H12O6 + 6O2
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.[1]
An additional source of atmospheric oxygen comes from photolysis, whereby high energy ultraviolet radiation breaks down atmospheric water and nitrite into component atoms. The free H and N atoms escape into space leaving O2 in the atmosphere:
- 2H2O + energy → 4H + O2
- 2N2O + energy → 4N + O2
The main way oxygen is lost from the atmosphere is via respiration and decay, mechanisms in which animal life and bacteria consume oxygen and release carbon dioxide.
Because lithospheric minerals are oxidised in oxygen, chemical weathering of exposed rocks also consumes oxygen. An example of surface weathering chemistry is formation of iron-oxides (rust):
- 4FeO + O2 → 2Fe2O3
- Main article: Mineral redox buffer
Oxygen is also cycled between the biosphere and lithosphere. 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 sea floor and buried over time to create the limestone 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.
[edit] 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.)[2]:
Table 1: Major reservoirs involved in the oxygen cycle
| Reservoir | Capacity (kg O2) |
Flux In/Out (kg O2 per year) |
Residence Time (years) |
|---|---|---|---|
| Atmosphere | 1.4 * 1018 | 30,000 * 1010 | 4,500 |
| Biosphere | 1.6 * 1016 | 30,000 * 1010 | 50 |
| Lithosphere | 2.9 * 1020 | 60 * 1010 | 500,000,000 |
Table 2: Annual gain and loss of atmospheric oxygen (Units of 1010 kg O2 per year)
| Gains | |
| 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 |
Arif
[edit] Ozone
The presence of atmospheric oxygen has led to the formation of ozone and the ozone layer within the stratosphere. The ozone layer is extremely important to modern life as it absorbs harmful ultraviolet radiation:
- O2 + uv energy → 2O
- O + O2 → O3
[edit] References
- ^ Steve Nadis, The Cells That Rule the Seas, Scientific American, Nov. 2003 [1]
- ^ Walker, J. C. G. (1980) The oxygen cycle in the natural environment and the biogeochemical cycles, Springer-Verlag, Berlin, Federal Republic of Germany (DEU)
- Cloud, P. and Gibor, A. 1970, The oxygen cycle, Scientific American, September, S. 110-123
- Fasullo, J., Substitute Lectures for ATOC 3600: Principles of Climate, Lectures on the global oxygen cycle, http://paos.colorado.edu/~fasullo/pjw_class/oxygencycle.html
- Morris, R.M., OXYSPHERE - A Beginners' Guide to the Biogeochemical Cycling of Atmospheric Oxygen, http://seis.natsci.csulb.edu/rmorris/oxy/Oxy.htm
|
|||||
