Oxygen cycle
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The oxygen cycle is the biogeochemical cycle that describes the movement of oxygen within and between its three main reservoirs: the atmosphere, the biosphere, and the lithosphere. The main driving factor of the oxygen cycle is photosynthesis, which is responsible for the modern Earth's atmosphere and life as we know it. If all photosynthesis were to cease, the Earth's atmosphere would be devoid of all but trace amounts of oxygen within 5000 years. The oxygen cycle would no longer exist.
Reservoirs and Fluxes
The vast number of molecular oxygen is contained in rocks and minerals within the Earth (99.5%). Only a small fraction has been released as free oxygen to the biosphere (0.01%) and atmosphere (0.49%). The main source of oxygen within the biosphere and atmosphere is photosynthesis which breaks down carbon dioxide and water to create sugars and oxygen:
CO2 + H2O + energy → CH2O + O2
An additional source of atmospheric oxygen comes from photolysis, whereby high energy ultraviolet radiation breaks down atmospheric water and nitrite into component molecules. 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 consumes oxygen and releases carbon dioxide. Because lithospheric minerals are reduced in oxygen, surface weathering of exposed rocks also consumes oxygen. An example of surface weathering chemistry is formation of iron-oxides (rust) such as that found in the red sands of Australia:
4FeO + 3O2 → 2Fe2O3
Oxygen is also cycled between the biosphere and lithosphere. Marine organisms in the biosphere create carbonate shell material (CaCO3) that is rich in molecular oxygen. When the organism dies its shell is deposited on the shallow sea floor and buried over time to create limestone rock in 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.
The following tables offer estimates of oxygen cycle reservoir capacities and fluxes. These numbers are based primarily on estimates from Walker, J.C.G.
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 (anthropologic) Photochemical Oxidation Fixation of N2 by Lightning Fixation of N2 by Industry (anthropologic) 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
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 + uv energy → O3
Phosphorus
An interesting theory is that phosphorus (P) in the ocean helps regulate the amount of atmospheric oxygen. Phosphorus dissolved in the oceans is an essential nutrient to photosynthetic life and one of the key limiting factors. Oceanic photosynthesis contributes approximately 45% of the total free oxygen to the oxygen cycle. The population growth of photosynthetic organisms is primarily limited by the availability of dissolved phosphorus.
One side-effect of mining and industrial activities is a dramatic increase in the amount of phosphorus being discharged to the world's oceans. However, this increase in available phosphorus has not resulted in a corresponding increase in oceanic photosynthesis. Why?
An increase in photosynthesizer population results in increased oxygen levels in the oceans. The elevated oxygen levels promote the growth of certain types of bacteria that compete for uptake of dissolved phosphorus. This competition limits the amount of phosphorous available to photosynthetic life thus buffering their total population as well as the levels of O2.
References
- 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
- Walker, J. C. G., 1980, The oxygen cycle in The natural environment and the biogeochemical cycles, Springer-Verlag, Berlin, Federal Republic of Germany (DEU)
| Biogeochemical cycles |
|---|
| Carbon cycle - Hydrogen cycle - Nitrogen cycle |
| Oxygen cycle - Phosphorus cycle - Sulfur cycle - Water cycle |
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