Editing Proterozoic (section)

==The accumulation of oxygen==
{{Main|Great Oxygenation Event|Neoproterozoic Oxygenation Event}}
One of the most important events of the Proterozoic was the [[oxygen catastrophe|accumulation of oxygen]] in the Earth's atmosphere. Though oxygen is believed to have been released by [[photosynthesis]] as far back as the Archean Eon, it could not build up to any significant degree until mineral sinks of unoxidized [[sulfur]] and [[iron]] had been exhausted. Until roughly 2.3&nbsp;billion years ago, oxygen was probably only 1% to 2% of its current level.<ref name=Stanley/>{{rp|323}} The [[banded iron formation]]s, which provide most of the world's [[iron ore]], are one mark of that mineral sink process. Their accumulation ceased after 1.9&nbsp;billion years ago, after the iron in the oceans had all been [[oxidized]].<ref name=Stanley/>{{rp|324}}

[[Red bed]]s, which are colored by [[hematite]], indicate an increase in atmospheric oxygen 2&nbsp;billion years ago. Such massive iron oxide formations are not found in older rocks.<ref name=Stanley/>{{rp|324}} The oxygen buildup was probably due to two factors: Exhaustion of the chemical sinks, and an increase in [[carbon sequestration]], which sequestered [[organic compound]]s that would have otherwise been oxidized by the atmosphere.<ref name=Stanley/>{{rp|325}}

The first surge in atmospheric oxygen at the beginning of the Proterozoic is called the [[Great Oxygenation Event]], or alternately the [[Oxygen Catastrophe]] – to reflect the mass extinction of almost all life on Earth, which at the time was virtually all [[obligate anaerobe|obligate anaerobic]]. A second, later surge in oxygen concentrations is called the [[Neoproterozoic Oxygenation Event]],<ref name=Shields-Zhou2011>{{cite journal |last1=Shields-Zhou |first1=Graham |last2=Och |first2=Lawrence |title=The case for a Neoproterozoic Oxygenation Event: Geochemical evidence and biological consequences |journal=GSA Today |date=March 2011 |volume=21 |issue=3 |pages=4–11 |doi=10.1130/GSATG102A.1 |bibcode=2011GSAT...21c...4S |url=https://discovery.ucl.ac.uk/id/eprint/1354478/1/1354478_Shields-Zhou%2520and%2520Och_2011.pdf }}</ref> occurred during the Middle and Late Neoproterozoic<ref name="Och2012">{{cite journal |last1=Och |first1=Lawrence M. |last2=Shields-Zhou |first2=Graham A. |title=The Neoproterozoic oxygenation event: Environmental perturbations and biogeochemical cycling |journal=Earth-Science Reviews |date=January 2012 |volume=110 |issue=1–4 |pages=26–57 |doi=10.1016/j.earscirev.2011.09.004 |bibcode=2012ESRv..110...26O }}</ref> and drove the rapid evolution of multicellular life towards the end of the era.<ref>{{cite journal |last1=Canfield |first1=Donald Eugene |last2=Poulton |first2=Simon W. |last3=Narbonne |first3=Guy M. |date=5 January 2007 |title=Late-Neoproterozoic deep-ocean oxygenation and the rise of animal life |journal=Science |volume=315 |issue=5808 |pages=92–95 |doi=10.1126/science.1135013 |pmid=17158290 |s2cid=24761414 |doi-access=free |bibcode=2007Sci...315...92C }}</ref><ref name=EdiacaranOxygenationIronIsotopes>{{cite journal |last1=Fan |first1=Haifeng |last2=Zhu |first2=Xiangkun |last3=Wen |first3=Hanjie |last4=Yan |first4=Bin |last5=Li |first5=Jin |last6=Feng |first6=Lianjun |title=Oxygenation of Ediacaran Ocean recorded by iron isotopes |journal=Geochimica et Cosmochimica Acta |date=September 2014 |volume=140 |pages=80–94 |doi=10.1016/j.gca.2014.05.029 |bibcode=2014GeCoA.140...80F }}</ref>