Editing Banded iron formation (section)

=====Abiogenic mechanisms=====
The lack of organic carbon in banded iron formation argues against microbial control of BIF deposition.<ref name="klein-beukes-1989"/> On the other hand, there is [[fossil]] evidence for abundant photosynthesizing cyanobacteria at the start of BIF deposition<ref name="trendall-blockley-2004"/> and of [[Biosignature|hydrocarbon markers]] in shales within banded iron formation of the Pilbara craton.<ref name="brocks-etal-1999">{{cite journal |last1=Brocks |first1=J. J. |first2=Graham A. |last2=Logan |first3=Roger |last3=Buick |first4=Roger E. |last4=Summons |title=Archean Molecular Fossils and the Early Rise of Eukaryotes |journal=Science |date=13 August 1999 |volume=285 |issue=5430 |pages=1033–1036 |doi=10.1126/science.285.5430.1033|pmid=10446042 |bibcode=1999Sci...285.1033B }}</ref> The carbon that is present in banded iron formations is enriched in the light isotope, <sup>12</sup>C, an [[Carbon isotope ratio|indicator]] of a biological origin. If a substantial part of the original iron oxides was in the form of hematite, then any carbon in the sediments might have been oxidized by the decarbonization reaction:<ref name="trendall-2002"/>

:{{chem2|6 Fe2O3 + C <-> 4 Fe3O4 + CO2}}

Trendall and J.G. Blockley proposed, but later rejected, the hypothesis that banded iron formation might be a peculiar kind of Precambrian [[evaporite]].<ref name="trendall-blockley-2004"/>  Other proposed abiogenic processes include [[radiolysis]] by the [[radioactive isotope]] of [[potassium]], <sup>40</sup>K,<ref name="draganic-etal-1991">{{cite journal |last1=Draganić |first1=I.G. |last2=Bjergbakke |first2=E. |last3=Draganić |first3=Z.D. |last4=Sehested |first4=K. |title=Decomposition of ocean waters by potassium-40 radiation 3800 Ma ago as a source of oxygen and oxidizing species |journal=Precambrian Research |date=August 1991 |volume=52 |issue=3–4 |pages=337–345 |doi=10.1016/0301-9268(91)90087-Q|bibcode=1991PreR...52..337D }}</ref> or annual turnover of basin water combined with upwelling of iron-rich water in a stratified ocean.<ref name="klein-beukes-1989">{{cite journal |last1=Klein |first1=Cornelis |last2=Beukes |first2=Nicolas J. |title=Geochemistry and sedimentology of a facies transition from limestone to iron-formation deposition in the early Proterozoic Transvaal Supergroup, South Africa |journal=Economic Geology |date=1 November 1989 |volume=84 |issue=7 |pages=1733–1774 |doi=10.2113/gsecongeo.84.7.1733|bibcode=1989EcGeo..84.1733K }}</ref>

Another abiogenic mechanism is [[photooxidation]] of iron by sunlight. Laboratory experiments suggest that this could produce a sufficiently high deposition rate under likely conditions of pH and sunlight.<ref name="braterman-etal-1983">{{cite journal |last1=Braterman |first1=Paul S. |author-link1=Paul Braterman |last2=Cairns-Smith |first2=A. Graham |author-link2=Graham Cairns-Smith |last3=Sloper |first3=Robert W. |title=Photo-oxidation of hydrated Fe2+—significance for banded iron formations |journal=Nature |date=May 1983 |volume=303 |issue=5913 |pages=163–164 |doi=10.1038/303163a0|bibcode=1983Natur.303..163B |s2cid=4357551 }}</ref><ref name="braterman-cairns-smith-1987">{{cite journal |last1=Braterman |first1=Paul S. |last2=Cairns-Smith |first2=A. Graham |title=Photoprecipitation and the banded iron-formations — Some quantitative aspects |journal=Origins of Life and Evolution of the Biosphere |date=September 1987 |volume=17 |issue=3–4 |pages=221–228 |doi=10.1007/BF02386463|bibcode=1987OrLi...17..221B |s2cid=33140490 }}</ref> However, if the iron came from a shallow hydrothermal source, other laboratory experiments suggest that precipitation of ferrous iron as carbonates or silicates could seriously compete with photooxidation.<ref name="konhauser-etal-2007">{{cite journal |last1=Konhauser |first1=Kurt O. |last2=Amskold |first2=Larry |last3=Lalonde |first3=Stefan V. |last4=Posth |first4=Nicole R. |last5=Kappler |first5=Andreas |last6=Anbar |first6=Ariel |title=Decoupling photochemical Fe(II) oxidation from shallow-water BIF deposition |journal=Earth and Planetary Science Letters |date=15 June 2007 |volume=258 |issue=1–2 |pages=87–100 |doi=10.1016/j.epsl.2007.03.026 |bibcode=2007E&PSL.258...87K |url=https://www.sciencedirect.com/science/article/abs/pii/S0012821X07001823 |access-date=23 June 2020}}</ref>