The oxygen cycle is the biogeochemical transitions 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. 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. 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).
Oxygen is one of the most abundant elements on Earth and represents a large portion of each main reservoir. By far the largest reservoir of Earth's oxygen is within the silicate and oxide minerals of the crust and mantle (99.5% by weight). The Earth's atmosphere, hydrosphere, and biosphere together hold less than 0.05% of the Earth's total mass of oxygen. Besides O2, additional oxygen atoms are present in various forms spread throughout the surface reservoirs in the molecules of biomass, H2O, CO2, HNO3, NO, NO2, CO, H2O2, O3, SO2, H2SO4, MgO, CaO, AlO, SiO2, and PO4.
The atmosphere is ~20.9% oxygen by volume, which equates to a total of roughly 34 × 1018 mol of oxygen. Other oxygen-containing molecules in the atmosphere include ozone (O3), carbon dioxide (CO2), water vapor (H2O), and sulfur and nitrogen oxides (SO2, NO, N2O, etc.).
The biosphere is 22% oxygen by volume present mainly as a component of organic molecules (CxHxNxOx) and water molecules.
The hydrosphere is 33% oxygen by volume present mainly as a component of water molecules with dissolved molecules including free oxygen and carbonic acids (HxCO3).
The lithosphere is 46.6% oxygen by volume present mainly as silica minerals (SiO2) and other oxide minerals.
Free oxygen (O2) from the atmosphere forms an equilibrium concentration by gas exchange with the hydrosphere as a dissolved gas in aqueous solution according to Henry's law. According to this law, O2 saturates in water at 450 µM at 0 °C and 270 µM at 25 °C, but other dissolved solutes in seawater can reduce this saturation concentration. Oxygen concentrations in the hydrosphere can be influenced locally by the presence or absence of turbulent mixing or local production or consumption of O2 by biological metabolism. Oxygen concentration in the soil and groundwater of the pedosphere is determined by gas diffusion through soil pore space in air and rainwater and can also be influenced locally by biological processes.
Oxygen is cycled between the biosphere and lithosphere within the context of the calcium cycle, where 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 seafloor and buried over time to create the limestone sedimentary 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.
Seasonal high latitude O2 level fluctuations of ±15 p.p.m. in the northern hemisphere have been observed and attributed to seasonal cycles of primary production and respiration. Human combustion of fossil fuels has been linked to a measured decrease of around 1 × 1015 mol per year in O2 concentrations in recent decades.
Sources and sinksEdit
While there are many abiotic sources and sinks for O2, the presence of the profuse concentration of free oxygen in modern Earth's atmosphere and ocean is attributed to O2 production from the biological process of oxygenic photosynthesis in conjunction with a biological sink known as the biological pump and a geologic process of carbon burial involving plate tectonics. Biology is the main driver of O2 flux on modern Earth, and the evolution of oxygenic photosynthesis by bacteria, which is discussed as part of The Great Oxygenation Event, is thought to be directly responsible for the conditions permitting the development and existence of all complex eukaryotic metabolism.
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 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 free oxygen comes from photolysis, whereby high-energy ultraviolet radiation breaks down atmospheric water and nitrous oxide into component atoms. The free H and [clarify] escape into space, leaving O2 in the atmosphere:
The lithosphere also consumes atmospheric free oxygen by chemical weathering and surface reactions. An example of surface weathering chemistry is formation of iron oxides (rust):
Capacities and fluxesEdit
The following tables offer estimates of oxygen cycle reservoir capacities and fluxes. These numbers are based primarily on estimates from (Walker, J. C. G.:
(kg O2 per year)
Table 2: Annual gain and loss of atmospheric oxygen (Units of 1010 kg O2 per year)
Photolysis of N2O
Photolysis of H2O
|Total gains||~ 30,000|
|Losses - respiration and decay|
Combustion of fossil fuel (anthropogenic)
Fixation of N2 by lightning
Fixation of N2 by industry (anthropogenic)
Oxidation of volcanic gases
|Losses - weathering|
Surface reaction of O3
|Total losses||~ 30,000|
The ozone layer is extremely important to modern life as it absorbs harmful ultraviolet radiation:
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