Editing Snowball Earth (section)

===Banded iron formations===
[[Image:Black-band ironstone (aka).jpg|thumb|2.1 billion-year-old rock with black-band ironstone]]
Banded iron formations (BIF) are sedimentary rocks of layered [[iron oxide]] and iron-poor [[chert]]. In the presence of oxygen, [[iron]] naturally [[Rust|rusts]] and becomes insoluble in water. The banded iron formations are commonly very old and their deposition is often related to the oxidation of Earth's atmosphere during the [[Palaeoproterozoic]] era, when dissolved iron in the ocean came in contact with photosynthetically produced oxygen and [[Precipitation (chemistry)|precipitated]] out as iron oxide.

The bands were produced at the [[Tipping points in the climate system|tipping point]] between an [[Anoxic waters|anoxic]] and an oxygenated ocean. Since today's atmosphere is [[oxygen]]-rich (nearly 21% by volume) and in contact with the oceans, it is not possible to accumulate enough iron oxide to deposit a banded formation. The only extensive iron formations that were deposited after the Palaeoproterozoic (after 1.8 billion years ago) are associated with [[Cryogenian]] glacial deposits.

For such iron-rich rocks to be deposited there would have to be [[Anoxic event|anoxia]] in the ocean, so that much dissolved iron (as [[Iron(II) oxide|ferrous oxide]]) could accumulate before it met an [[Oxidizing agent|oxidant]] that would precipitate it as [[Iron(III) oxide|ferric oxide]]. For the ocean to become anoxic it must have limited gas exchange with the oxygenated atmosphere. Proponents of the hypothesis argue that the reappearance of BIF in the sedimentary record is a result of limited oxygen levels in an ocean sealed by sea-ice.<ref name="Kirschvink">{{cite book
| last=Kirschvink
| first=Joseph
| editor=J. W. Schopf
| editor2=C. Klein
| title=The Proterozoic Biosphere: A Multidisciplinary Study
| year=1992
| publisher=Cambridge University Press
| chapter=Late Proterozoic low-latitude global glaciation: the Snowball Earth}}</ref> Near the end of a glaciation period, a reestablishment of gas exchange between the ocean and atmosphere oxidised seawater rich in ferrous iron would occur.<ref>{{cite journal |last1=Wu |first1=Chang-Zhi |last2=Zhao |first2=Fei-Fan |last3=Yang |first3=Tao |last4=Lei |first4=Ru-Xiong |last5=Ye |first5=Hui |last6=Gao |first6=Bing-Fei |last7=Li |first7=Feiqiang |date=15 July 2022 |title=Genesis of the Fulu Cryogenian iron formation in South China: Synglacial or interglacial? |url=https://www.sciencedirect.com/science/article/abs/pii/S0301926822001334 |journal=[[Precambrian Research]] |volume=376 |page=106689 |doi=10.1016/j.precamres.2022.106689 |bibcode=2022PreR..37606689W |s2cid=248622156 |access-date=20 May 2023}}</ref> A positive shift in δ<sup>56</sup>Fe<sub>IRMM-014</sub> from the lower to upper layers of Cryogenian BIFs may reflect an increase in ocean acidification, as the upper layers were deposited as more and more oceanic ice cover melted away and more carbon dioxide was dissolved by the ocean.<ref>{{cite journal |last1=Zhu |first1=Xiang-Kun |last2=Sun |first2=Jian |last3=Li |first3=Zhi-Hong |date=1 September 2019 |title=Iron isotopic variations of the Cryogenian banded iron formations: A new model |url=https://www.sciencedirect.com/science/article/abs/pii/S0301926818303243 |journal=[[Precambrian Research]] |volume=331 |page=105359 |doi=10.1016/j.precamres.2019.105359 |bibcode=2019PreR..33105359Z |s2cid=189975438 |access-date=21 May 2023}}</ref> 

Opponents of the hypothesis suggest that the rarity of the BIF deposits may indicate that they formed in inland seas. Being isolated from the oceans, such lakes could have been stagnant and anoxic at depth, much like today's [[Black Sea]]; a sufficient input of iron could provide the necessary conditions for BIF formation.<ref name=Eyles2004 /> A further difficulty in suggesting that BIFs marked the end of the glaciation is that they are found interbedded with glacial sediments;<ref name=Young1995/> such interbedding has been suggested to be an artefact of [[Milankovitch cycles]], which would have periodically warmed the seas enough to allow gas exchange between the atmosphere and ocean and precipitate BIFs.<ref>{{cite journal |last1=Mitchell |first1=Ross N. |last2=Gernon |first2=Thomas M. |last3=Cox |first3=Grant M. |last4=Nordsvan |first4=Adam R. |last5=Kirscher |first5=Uwe |last6=Xuan |first6=Chuang |last7=Liu |first7=Yebo |last8=Liu |first8=Xu |last9=He |first9=Xiaofang |date=7 July 2021 |title=Orbital forcing of ice sheets during snowball Earth |journal=[[Nature Communications]] |volume=12 |issue=1 |page=4187 |doi=10.1038/s41467-021-24439-4 |pmid=34234152 |pmc=8263735 |bibcode=2021NatCo..12.4187M |hdl=20.500.11937/90462 |hdl-access=free }}</ref>