Editing Oxygen (section)

===Occurrence===
{{see also|Silicate minerals|Category:Oxide minerals|Stellar population|Cosmochemistry|Astrochemistry}}
{| class="wikitable sortable" style="float:left; margin-right: 20px"
|+Ten most common elements in the [[Milky Way Galaxy]] estimated spectroscopically (not to scale)<ref name="croswell">{{cite book | last = Croswell | first = Ken | title = Alchemy of the Heavens | publisher = Anchor | year = 1996 | url = http://kencroswell.com/alchemy.html | isbn = 978-0-385-47214-2 | access-date = December 2, 2011 | archive-date = May 13, 2011 | archive-url = https://web.archive.org/web/20110513233910/http://www.kencroswell.com/alchemy.html | url-status = live }}</ref>
|-
![[Atomic number|Z]] !! Element !! colspan="2"|Mass fraction in parts per million
|-
| 1 || [[Hydrogen]] || align="right"|{{bartable| 739,000||0.001}}
|-
| 2 || [[Helium]] || align="right"|{{bartable| 240,000||0.001}}
|-
| 8 || Oxygen || align="right"|{{bartable| 10,400||0.005||background:red;}}
|-
| 6 || [[Carbon]] || align="right"|{{bartable| 4,600||0.005}}
|-
| 10 || [[Neon]] || align="right"|{{bartable| 1,340||0.005}}
|-
| 26 || [[Iron]] || align="right"|{{bartable| 1,090||0.005}}
|-
| 7 || [[Nitrogen]] || align="right"|{{bartable| 960||0.005}}
|-
| 14 || [[Silicon]] || align="right"|{{bartable| 650||0.005}}
|-
| 12 || [[Magnesium]] || align="right"|{{bartable| 580||0.005}}
|-
| 16 || [[Sulfur]] || align="right"|{{bartable| 440||0.005}}
|}

Oxygen is the most abundant chemical element by mass in the Earth's [[biosphere]], air, sea and land. Oxygen is the third most abundant chemical element in the universe, after hydrogen and helium.<ref name="NBB297">[[#Reference-idEmsley2001|Emsley 2001]], p. 297</ref> About 0.9% of the [[Sun]]'s mass is oxygen.<ref name="ECE500" /> Oxygen constitutes 49.2% of the [[Earth's crust]] by mass<ref name="lanl">{{cite web |url=http://periodic.lanl.gov/elements/8.html|publisher=Los Alamos National Laboratory|title=Oxygen|access-date=December 16, 2007|archive-url=https://web.archive.org/web/20071026034224/http://periodic.lanl.gov/elements/8.html|archive-date=October 26, 2007}}</ref> as part of oxide compounds such as [[silicon dioxide]] and is the most abundant element by mass in the [[crust (geology)#Earth's crust and mantle|Earth's crust]]. It is also the major component of the world's oceans (88.8% by mass).<ref name="ECE500" /> Oxygen gas is the second most common component of the [[Earth's atmosphere]], taking up 20.8% of its volume and 23.1% of its mass (some 10<sup>15</sup> tonnes).<ref name="ECE500" /><ref name="NBB298">[[#Reference-idEmsley2001|Emsley 2001]], p. 298</ref><ref group="lower-alpha">Figures given are for values up to {{convert|80|km|mi|abbr=on}} above the surface</ref> Earth is unusual among the planets of the [[Solar System]] in having such a high concentration of oxygen gas in its atmosphere: [[Mars]] (with 0.1% {{chem|O|2}} by volume) and [[Venus]] have much less. The {{chem|O|2}} surrounding those planets is produced solely by the action of ultraviolet radiation on oxygen-containing molecules such as carbon dioxide.<ref>{{cite book |author1=Richard Peer Wayne |title=Chemistry of Atmospheres |date=2006 |publisher=Oxford University Press |isbn=9780198503750 |language=en |pages=562–584}}</ref>

[[File:WOA09 sea-surf O2 AYool.png|thumb|right|Cold water holds more dissolved {{chem|O|2}}.|alt=World map showing that the sea-surface oxygen is depleted around the equator and increases towards the poles.]]

The unusually high concentration of oxygen gas on Earth is the result of the [[oxygen cycle]]. This [[biogeochemical cycle]] describes the movement of oxygen within and between its three main reservoirs on Earth: the atmosphere, the biosphere, and the [[lithosphere]]. The main driving factor of the oxygen cycle is [[photosynthesis]], which is responsible for modern Earth's atmosphere. Photosynthesis releases oxygen into the atmosphere, while [[Cellular respiration|respiration]], [[Decomposition|decay]], and combustion remove it from the atmosphere. In the present equilibrium, production and consumption occur at the same rate.<ref>{{Greenwood&Earnshaw2nd|page=602}}</ref>

Free oxygen also occurs in solution in the world's water bodies. The increased solubility of {{chem|O|2}} at lower temperatures (see [[#Physical properties|Physical properties]]) has important implications for ocean life, as polar oceans support a much higher density of life due to their higher oxygen content.<ref>From The Chemistry and Fertility of Sea Waters by H.W. Harvey, 1955, citing C.J.J. Fox, "On the coefficients of absorption of atmospheric gases in sea water", Publ. Circ. Cons. Explor. Mer, no. 41, 1907. Harvey notes that according to later articles in ''Nature'', the values appear to be about 3% too high.</ref> [[Water pollution|Water polluted]] with plant nutrients such as [[nitrate]]s or [[phosphate]]s may stimulate growth of algae by a process called [[eutrophication]] and the decay of these organisms and other biomaterials may reduce the {{chem|O|2}} content in eutrophic water bodies. Scientists assess this aspect of water quality by measuring the water's [[biochemical oxygen demand]], or the amount of {{chem|O|2}} needed to restore it to a normal concentration.<ref name="NBB301">[[#Reference-idEmsley2001|Emsley 2001]], p. 301</ref>