Everything about The Oxygen Cycle totally explained
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. Because of the vast amounts of oxygen in the atmosphere, even if all photosynthesis were to cease it would take between 5,000 to 2.4
million years (unknown reference) to strip out more or less all oxygen.
Reservoirs and Fluxes
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 oxygen within the biosphere and atmosphere is photosynthesis, which breaks down carbon dioxide and water to create sugars and oxygen:
» 6CO
2 + 6H
2O + energy → C
6H
12O
6 + 6O
2
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.
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 O
2 in the atmosphere:
» 2H
2O + energy → 4H + O
2
2N
2O + energy → 4N + O
2
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 + 3O
2 → 2Fe
2O
3
Main article: Mineral redox buffer
Oxygen is also cycled between the biosphere and lithosphere. Marine organisms in the biosphere create
calcium carbonate shell material (CaCO
3) that's 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.
Oxygen reservoir 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.):
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 10
10 kg O
2 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 |
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:
» O
2 + uv energy → 2O
O + O
2 → O
3
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 hasn't resulted in a corresponding increase in oceanic photosynthesis. This is because 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 phosphorus available to photosynthetic life thus buffering their total population as well as the levels of O
2.
Further Information
Get more info on 'Oxygen Cycle'.
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