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Oxygen cycle
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<p>The oxygen cycle refers to the various movements of <a href="/facts/Oxygen/acyn6o8r">oxygen</a> through the <a href="/facts/Earth/HLLmDfj0">Earth</a>'s <a href="/facts/Earth%2527s_atmosphere/FV97Am25">atmosphere</a> (<a href="/facts/Air/6IyFHIpG">air</a>), <a href="/facts/Biosphere/UpKAe1DB">biosphere</a> (<a href="/facts/Flora/2xkcTn2d">flora</a> and <a href="/facts/Fauna/sMqPFxVD">fauna</a>), <a href="/facts/Hydrosphere/RbJmcSX6">hydrosphere</a> (<a href="/facts/Water_bodies/MoDAUuW4">water bodies</a> and <a href="/facts/Glacier/8GrKVUSS">glaciers</a>) and the <a href="/facts/Lithosphere/1mqaxASd">lithosphere</a> (the <a href="/facts/Earth%2527s_crust/VSy2u31r">Earth's crust</a>). The oxygen cycle demonstrates how free oxygen is made available in each of these regions, as well as how it is used. It is the <a href="/facts/Biogeochemical_cycle/3vG5KBu4">biogeochemical cycle</a> of oxygen <a href="/facts/Atom/69syB8dH">atoms</a> between different <a href="/facts/Oxidation_state/W53W6s6V">oxidation states</a> in <a href="/facts/Ion/wrbwhF3z">ions</a>, <a href="/facts/Oxide/DXtoVe1C">oxides</a> and <a href="/facts/Molecule/N9ljSfFq">molecules</a> through <a href="/facts/Redox/Pyd5RVcj">redox</a> <a href="/facts/Chemical_reaction/GpqWlKrz">reactions</a> within and between the <a href="/facts/Outline_of_Earth_sciences/sGLvE4Qh">spheres/reservoirs</a> of the planet Earth. The word oxygen in the literature typically refers to the most common <a href="/facts/Allotropes_of_oxygen/UVNYnKUQ">oxygen allotrope</a>, elemental/diatomic oxygen (O2), as it is a common <a href="/facts/Product_(chemistry)/8uCoDUdv">product</a> or <a href="/facts/Reagent/c3GLInCu">reactant</a> of many biogeochemical redox reactions within the cycle. Processes within the oxygen cycle are considered to be <a href="/facts/Biology/1M1ee3pB">biological</a> or <a href="/facts/Geology/WGvVRIYF">geological</a> and are evaluated as either a <a href="/facts/Source%25E2%2580%2593sink_dynamics/DLopYt2n">source</a> (O2 production) or sink (O2 consumption). </p><p>Oxygen is one of the most common elements on Earth and represents a large portion of each main reservoir. By far the largest reservoir of Earth's oxygen is within the <a href="/facts/Silicate_minerals/WclrQW2I">silicate</a> and <a href="/facts/Oxide_minerals/B4ajLX61">oxide</a> <a href="/facts/Mineral/LkUFbNmv">minerals</a> of the <a href="/facts/Crust_(geology)/FYG1kP4F">crust</a> and <a href="/facts/Mantle_(geology)/RpDdtjCr">mantle</a> (99.5% by weight). The Earth's atmosphere, <a href="/facts/Hydrosphere/RbJmcSX6">hydrosphere</a>, 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 <a href="/facts/Biomass_(ecology)/A5rDsUax">biomass</a>, <a href="/facts/Properties_of_water/FFsl65l1">H2O</a>, <a href="/facts/Carbon_dioxide/xGzpppEp">CO2</a>, <a href="/facts/Nitric_acid/UDbwGp6s">HNO3</a>, <a href="/facts/Nitric_oxide/UvhdvDzp">NO</a>, <a href="/facts/Nitrogen_dioxide/FoLvApPI">NO2</a>, <a href="/facts/Carbon_monoxide/CKuLtEMJ">CO</a>, <a href="/facts/Hydrogen_peroxide/vju6HBIc">H2O2</a>, <a href="/facts/Ozone/o0E5gn2p">O3</a>, <a href="/facts/Sulfur_dioxide/jDK1fIDs">SO2</a>, <a href="/facts/Sulfuric_acid/Wom6fymJ">H2SO4</a>, <a href="/facts/Magnesium_oxide/0GfLn47t">MgO</a>, <a href="/facts/Calcium_oxide/Wh9C8l7j">CaO</a>, <a href="/facts/Aluminium_oxide/wXM9P6pI">Al2O3</a>, <a href="/facts/Silicon_dioxide/A3WILp6J">SiO2</a>, and <a href="/facts/Phosphate/FDGVCxUG">PO3−4</a>. </p>