The
Oxygen cycle is the
biogeochemical cycleIn ecology and Earth science, a biogeochemical cycle or substance turnover or cycling of substances is a pathway by which a chemical element or molecule moves through both biotic and abiotic compartments of Earth. A cycle is a series of change which comes back to the starting point and which can...
that describes the movement of
oxygenOxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
within its three main reservoirs: the
atmosphereThe atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...
(air), the total content of biological matter within the
biosphereThe biosphere is the global sum of all ecosystems. It can also be called the zone of life on Earth, a closed and self-regulating system...
(the global sum of all ecosystems), and the
lithosphereThe lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's lithosphere :...
(Earth's crust). Failures in the oxygen cycle within the
hydrosphereA hydrosphere in physical geography describes the combined mass of water found on, under, and over the surface of a planet....
(the combined mass of water found on, under, and over the surface of a planet) can result in the development of hypoxic zones. The main driving factor of the oxygen cycle is
photosynthesisPhotosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
, which is responsible for the modern Earth's atmosphere and life.
Reservoirs
By far the largest reservoir of Earth's oxygen is within the silicate and oxide
mineralA mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
s of the
crustIn geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle...
and
mantleThe mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....
(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
phytoplanktonPhytoplankton are the autotrophic component of the plankton community. The name comes from the Greek words φυτόν , meaning "plant", and πλαγκτός , meaning "wanderer" or "drifter". Most phytoplankton are too small to be individually seen with the unaided eye...
of the oceans. The tiny marine cyanobacterium
ProchlorococcusProchlorococcus is a genus of very small marine cyanobacteria with an unusual pigmentation . These bacteria belong to the photosynthetic picoplankton and are probably the most abundant photosynthetic organism on Earth....
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
ultravioletUltraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
radiation breaks down atmospheric water and nitrous oxide into component atoms. The free H and N atoms escape into space leaving O
2 in the atmosphere:
- 2H2O + energy → 4H + O2
- 2N2O + energy → 4N + O2
The main way oxygen is lost from the atmosphere is via
respiration'In physiology, respiration is defined as the transport of oxygen from the outside air to the cells within tissues, and the transport of carbon dioxide in the opposite direction...
and
decayDecomposition is the process by which organic material is broken down into simpler forms of matter. The process is essential for recycling the finite matter that occupies physical space in the biome. Bodies of living organisms begin to decompose shortly after death...
, mechanisms in which
animalAnimals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
life and
bacteriaBacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
consume oxygen and release carbon dioxide.
Because lithospheric consumes oxygen. An example of surface weathering chemistry is formation of iron-oxides (rust):
- 4FeO + O2 → 2Fe2O3
- Main article: Mineral redox buffer
In geology, a redox buffer is an assemblage of minerals or compounds that constrains oxygen fugacity as a function of temperature. Knowledge of the redox conditions at which a rock forms and evolves can be important for interpreting the rock history...
Oxygen is also cycled between the biosphere and lithosphere. Marine organisms in the biosphere create
calcium carbonateCalcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
shell material (CaCO
3) 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
limestoneLimestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
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.
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)
|
>
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
ozoneOzone , or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope...
(O
3) and the
ozone layerThe ozone layer is a layer in Earth's atmosphere which contains relatively high concentrations of ozone . This layer absorbs 97–99% of the Sun's high frequency ultraviolet light, which is potentially damaging to the life forms on Earth...
within the
stratosphereThe stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler...
. The ozone layer is extremely important to modern life as it absorbs harmful
ultravioletUltraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
radiation:
- O2 + uv energy → 2O
- O + O2 → O3