Editing Oxygen (section)

===Build-up in the atmosphere===
{{Main|Geological history of oxygen}}
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[[File:Oxygenation-atm.svg|thumb|left|upright=1.35|{{chem|O|2}} build-up in Earth's atmosphere: 1) no {{chem|O|2}} produced; 2) {{chem|O|2}} produced, but absorbed in oceans & seabed rock; 3) {{chem|O|2}} starts to gas out of the oceans, but is absorbed by land surfaces and formation of ozone layer; 4–5) {{chem|O|2}} sinks filled and the gas accumulates|alt=A graph showing time evolution of oxygen pressure on Earth; the pressure increases from zero to 0.2 atmospheres.]]

Free oxygen gas was almost nonexistent in [[Earth's atmosphere]] before photosynthetic [[archaea]] and [[bacteria]] evolved, probably about 3.5 billion years ago. Free oxygen first appeared in significant quantities during the [[Paleoproterozoic]] era (between 3.0 and 2.3 billion years ago).<ref name="Crowe2013">{{Cite journal | last1 = Crowe | first1 = S. A. | last2 = Døssing | first2 = L. N. | last3 = Beukes | first3 = N. J. | last4 = Bau | first4 = M. | last5 = Kruger | first5 = S. J. | last6 = Frei | first6 = R. | last7 = Canfield | first7 = D. E. | title = Atmospheric oxygenation three billion years ago | journal = Nature | volume = 501 | issue = 7468 | pages = 535–38 | year = 2013 | pmid = 24067713 | doi = 10.1038/nature12426 | bibcode = 2013Natur.501..535C | s2cid = 4464710 }}</ref> Even if there was much dissolved [[iron]] in the oceans when oxygenic photosynthesis was getting more common, it appears the [[banded iron formation]]s were created by anoxyenic or micro-aerophilic iron-oxidizing bacteria which dominated the deeper areas of the [[photic zone]], while oxygen-producing cyanobacteria covered the shallows.<ref>[https://www.sciencedaily.com/releases/2013/04/130423110750.htm Iron in primeval seas rusted by bacteria] {{Webarchive|url=https://web.archive.org/web/20200311023339/https://www.sciencedaily.com/releases/2013/04/130423110750.htm |date=March 11, 2020 }}, ScienceDaily, April 23, 2013</ref> Free oxygen began to [[Outgassing|outgas]] from the oceans 3–2.7&nbsp;billion years ago, reaching 10% of its present level around 1.7&nbsp;billion years ago.<ref name="Crowe2013" /><ref name="Campbell">{{cite book|last1 = Campbell|first1 = Neil A.|last2=Reece|first2=Jane B.|title = Biology|edition = 7th|publisher = Pearson&nbsp;– Benjamin Cummings |date=2005|location = San Francisco|pages = 522–23|isbn = 978-0-8053-7171-0}}</ref>

The presence of large amounts of dissolved and free oxygen in the oceans and atmosphere may have driven most of the extant [[anaerobic organism]]s to [[extinction]] during the [[Great Oxygenation Event]] (''oxygen catastrophe'') about 2.4 billion years ago. [[Cellular respiration]] using {{chem|O|2}} enables [[aerobic organism]]s to produce much more [[Adenosine triphosphate|ATP]] than anaerobic organisms.<ref name="Freeman">{{cite book|last = Freeman|first = Scott|title = Biological Science, 2nd|publisher = Pearson&nbsp;– Prentice Hall|date = 2005|location = Upper Saddle River, NJ|pages = [https://archive.org/details/biologicalscienc00scot/page/214 214, 586]|isbn = 978-0-13-140941-5|url = https://archive.org/details/biologicalscienc00scot/page/214}}</ref> Cellular respiration of {{chem|O|2}} occurs in all [[eukaryote]]s, including all complex multicellular organisms such as plants and animals.

Since the beginning of the [[Cambrian]] period 540 million years ago, atmospheric {{chem|O|2}} levels have fluctuated between 15% and 30% by volume.<ref name="geologic">{{cite journal |title=Atmospheric oxygen over Phanerozoic time |first=Robert A. |last=Berner |issue=20 |pages=10955–57 |date=1999|journal=Proceedings of the National Academy of Sciences of the USA |pmid=10500106 |doi=10.1073/pnas.96.20.10955 |volume=96 |pmc=34224 |bibcode=1999PNAS...9610955B|doi-access=free }}</ref> Towards the end of the [[Carboniferous]] period (about 300&nbsp;million years ago) atmospheric {{chem|O|2}} levels reached a maximum of 35% by volume,<ref name="geologic" /> which may have contributed to the large size of insects and amphibians at this time.<ref name="Butterfield2009">{{Cite journal | last1 = Butterfield | first1 = N. J. | title = Oxygen, animals and oceanic ventilation: An alternative view | doi = 10.1111/j.1472-4669.2009.00188.x | journal = Geobiology | volume = 7 | issue = 1 | pages = 1–7 | year = 2009 | pmid = 19200141 | bibcode = 2009Gbio....7....1B | s2cid = 31074331 }}</ref>

Variations in atmospheric oxygen concentration have shaped past climates. When oxygen declined, atmospheric density dropped, which in turn increased surface evaporation, causing precipitation increases and warmer temperatures.<ref>{{cite journal|url=http://ns.umich.edu/new/releases/22942-variations-in-atmospheric-oxygen-levels-shaped-earth-s-climate-through-the-ages|doi=10.1126/science.1260670|pmid=26068848|journal=Science|title=Long-term climate forcing by atmospheric oxygen concentrations|author1=Poulsen, Christopher J.|author2=Tabor, Clay|author3=White, Joseph D.|volume=348|issue=6240|pages=1238–41|bibcode=2015Sci...348.1238P|year=2015|s2cid=206562386|access-date=June 12, 2015|archive-date=July 13, 2017|archive-url=https://web.archive.org/web/20170713125418/http://ns.umich.edu/new/releases/22942-variations-in-atmospheric-oxygen-levels-shaped-earth-s-climate-through-the-ages|url-status=live}}</ref>

At the current rate of photosynthesis it would take about 2,000&nbsp;years to regenerate the entire {{chem|O|2}} in the present atmosphere.<ref>{{cite journal|title=The Natural History of Oxygen|first=Malcolm|last=Dole |journal=The Journal of General Physiology|volume=49|pages=5–27|date=1965|doi=10.1085/jgp.49.1.5|pmid=5859927|issue=1|pmc=2195461}}</ref>
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It is estimated that oxygen on Earth will last for about one billion years.<ref>{{Cite journal|url=https://www.nature.com/articles/s41561-021-00693-5|title=The future lifespan of Earth's oxygenated atmosphere|first1=Kazumi|last1=Ozaki|first2=Christopher T.|last2=Reinhard|date=March 9, 2021|journal=Nature Geoscience|volume=14|issue=3|pages=138–142|via=www.nature.com|doi=10.1038/s41561-021-00693-5|arxiv=2103.02694|bibcode=2021NatGe..14..138O |s2cid=232083548 }}</ref><ref>{{Cite web|url=https://www.eurekalert.org/news-releases/825455|title=How much longer will the oxygen-rich atmosphere be sustained on Earth?|website=EurekAlert!}}</ref>