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The biogeochemical cycle of oxygen within its four main reservoirs: the atmosphere, the biosphere, the hydrosphere, and the lithosphere
Main reservoirs and fluxes (in unit 1012 mol/yr) of the modern global O2 cycle on Earth. There are four main reservoirs: terrestrial biosphere (green), marine biosphere (blue), lithosphere (brown), and atmosphere (grey). The major fluxes between these reservoirs are shown in colored arrows, where the green arrows are related to the terrestrial biosphere, blue arrows are related to the marine biosphere, black arrows are related to the lithosphere, purple arrow is related to space (not a reservoir, but also contributes to the atmospheric O2). The value of photosynthesis or net primary productivity (NPP) can be estimated through the variation in the abundance and isotopic composition of atmospheric O2. The rate of organic carbon burial was derived from estimated fluxes of volcanic and hydrothermal carbon.
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 oxideminerals 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 × 1018mol of oxygen. Other oxygen-containing molecules in the atmosphere include ozone (O3), carbon dioxide (CO2), water vapor (H2O), and sulphur 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 hydrosphereis 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.
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 cyanobacteriumProchlorococcus 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 N atoms[clarify] escape into space, leaving O2 in the atmosphere:
The main way free oxygen is lost from the atmosphere is via respiration and decay, mechanisms in which animal life and bacteria consume oxygen and release carbon dioxide.
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.):
Capacity (kg O2)
Flux in/out (kg O2 per year)
Residence time (years)
Table 2: Annual gain and loss of atmospheric oxygen (Units of 1010 kg O2 per year)
Photosynthesis (land) Photosynthesis (ocean) Photolysis of N2O Photolysis of H2O
16,500 13,500 1.3 0.03
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