[[File:Iron banding 01.jpg|thumb|right|An ashtray carved out of a soft form of banded ironstone from the [[Archean life in the Barberton Greenstone Belt|Barbeton Supergroup]] in South Africa. The red layers were laid down when Archaean [[photosynthesis|photosynthesizing]] [[cyanobacteria]] produced oxygen that reacted with dissolved iron compounds in the water, to form insoluble iron oxide (rust). The white layers are sediments that settled when there was no oxygen in the water, or when dissolved Fe<sup>2+</sup> was temporarily depleted.<ref name=Margulis>{{cite book | last1=Margulis |first1=L |author-link1=Lynn Margulis |last2=Sagan |first2=D |author-link2=Dorion Sagan | date = August 2000 | title = What is Life? | pages = 81–83 | publisher = University of California Press | isbn = 978-0-520-22021-8 }}</ref>]]
Banded iron formation provided some of the first evidence for the timing of the [[Great Oxygenation Event]], 2,400 Ma.4 billion years ago.<ref name= "Cloud_1973">{{cite journal |last1 = Cloud |first1=P. |author-link=Preston Cloud | doi = 10.2113/gsecongeo.68.7.1135| pages = 1135–1143| volume = 68| journal = Economic Geology| title = Paleoecological Significance of the Banded Iron-Formation| year = 1973 | issue = 7 }}</ref><ref name="holland-2006">{{cite journal |last1=Holland |first1=Heinrich D |title=The oxygenation of the atmosphere and oceans |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |date=19 May 2006 |volume=361 |issue=1470 |pages=903–915 |doi=10.1098/rstb.2006.1838|pmid=16754606 |pmc=1578726 }}</ref> With his 1968 paper on the early atmosphere and oceans of the earth,<ref name="cloud-1968">{{cite journal |last1=Cloud |first1=Preston E. |title=Atmospheric and Hydrospheric Evolution on the Primitive Earth. |journal=Science |volume=160 |number=3829 |year=1968 |pages=729–736 |doi=10.1126/science.160.3829.729 |jstor=1724303 |pmid=5646415 }}</ref> [[Preston Cloud]] established the general framework that has been widely, if not universally,<ref name="ohmoto-etal-2006">{{cite journal |last1=Ohmoto |first1=H. |last2=Watanabe |first2=Y. |last3=Yamaguchi |first3=K.E. |last4=Naraoka |first4=H. |last5=Haruna |first5=M. |last6=Kakegawa |first6=T. |last7=Hayashi |first7=K. |last8=Kato |first8=Y. |title=Chemical and biological evolution of early Earth: Constraints from banded iron formations |journal=Geological Society of America Memoir |date=2006 |volume=198 |pages=291–331 |doi=10.1130/2006.1198(17) |isbn=9780813711980 |url=https://books.google.com/books?id=U_QqAd1QlIgC&pg=PA291 |access-date=19 June 2020}}</ref><ref>{{cite book |title=Banded iron formations, to iron ore : an integrated genesis model |isbn=978-1536109719 |first1=Desmond Fitzgerald |last1=Lascelles |date=2017 |publisher=Nova Science Publishers}}</ref> accepted for understanding the deposition of BIFs.<ref name="trendall-blockley-2004"/><ref name="condie-2015"/>
Cloud postulated that banded iron formations were a consequence of anoxic, iron-rich waters from the deep ocean welling up into a [[photic zone]] inhabited by cyanobacteria that had evolved the capacity to carry out oxygen-producing photosynthesis, but which had not yet evolved enzymes (such as [[superoxide dismutase]]) for living in an oxygenated environment. Such organisms would have been protected from their own [[Reactive oxygen species|oxygen waste]] through its rapid removal via the reservoir of reduced ferrous iron, Fe(II), in the early ocean. The oxygen released by photosynthesis oxidized the Fe(II) to ferric iron, Fe(III), which precipitated out of the [[sea water]] as insoluble iron oxides that settled to the ocean floor.<ref name="cloud-1968"/><ref name= "Cloud_1973"/>
