Editing Banded iron formation (section)

====Oxidation====
Although Cloud argued that microbial activity was a key process in the deposition of banded iron formation, the role of oxygenic versus [[anoxygenic photosynthesis]] continues to be debated, and nonbiogenic processes have also been proposed.

=====Oxygenic photosynthesis=====
[[Image:CSIRO ScienceImage 4203 A bluegreen algae species Cylindrospermum sp under magnification.jpg|thumb|Cyanobacteria species ''Cylindrospermum sp.'' under magnification]]
Cloud's original hypothesis was that ferrous iron was oxidized in a straightforward manner by molecular oxygen present in the water:<ref name= "Cloud_1973"/><ref name="cox-etal-2013"/>

:{{chem2|4 Fe(2+) + O2 + 10 H2O -> 4 Fe(OH)3 + 8 H+}}

The oxygen comes from the photosynthetic activities of cyanobacteria.<ref name="cox-etal-2013"/> Oxidation of ferrous iron may have been hastened by aerobic iron-oxidizing bacteria, which can increase rates of oxidation by a factor of 50 under conditions of low oxygen.<ref name="cox-etal-2013"/>

=====Anoxygenic photosynthesis=====
[[Image:Iron bacteria burn.JPG|right|thumb|A [[Burn (landform)|burn]] in Scotland with iron-oxidizing bacteria]]

Oxygenic photosynthesis is not the only biogenic mechanism for deposition of banded iron formations. Some geochemists have suggested that banded iron formations could form by direct oxidation of iron by microbial [[Anoxygenic photosynthesis|anoxygenic phototrophs]].<ref>{{cite journal |last1 = Kappler |first1=A. |last2=Pasquero |first2=C. |last3=Konhauser |first3=K.O. |last4=Newman |first4=D.K. | title = Deposition of banded iron formations by anoxygenic phototrophic Fe (II)-oxidizing bacteria. | journal = Geology | date = November 2005 | volume = 33 | issue = 11 | pages = 865–8 | url = http://www.ess.uci.edu/~cpasquer/papers/kappleretal_GEO2005.pdf | archive-url = https://web.archive.org/web/20081216220557/http://www.ess.uci.edu/~cpasquer/papers/kappleretal_GEO2005.pdf | archive-date=16 December 2008 | doi = 10.1130/G21658.1 | bibcode = 2005Geo....33..865K }}</ref> The concentrations of phosphorus and trace metals in BIFs are consistent with precipitation through the activities of iron-oxidizing bacteria.<ref name="konhauser-etal-2002">{{cite journal |last1=Konhauser |first1=Kurt O. |last2=Hamade |first2=Tristan |last3=Raiswell |first3=Rob |last4=Morris |first4=Richard C. |last5=Grant Ferris |first5=F. |last6=Southam |first6=Gordon |last7=Canfield |first7=Donald E. |title=Could bacteria have formed the Precambrian banded iron formations? |journal=Geology |date=2002 |volume=30 |issue=12 |pages=1079 |doi=10.1130/0091-7613(2002)030<1079:CBHFTP>2.0.CO;2|bibcode=2002Geo....30.1079K }}</ref>

Iron isotope ratios in the oldest banded iron formations (3700-3800 Ma), at Isua, Greenland, are best explained by assuming extremely low oxygen levels (<0.001% of modern O<sub>2</sub> levels in the photic zone) and anoxygenic photosynthetic oxidation of Fe(II):<ref name="czaja-etal-2013"/><ref name="cox-etal-2013"/>

:{{chem2|4 Fe(2+) + 11 H2O + CO2 + hv → CH2O + 4 Fe(OH)3 + 8 H+}}

This requires that dissimilatory iron reduction, the biological process in which microorganisms substitute Fe(III) for oxygen in respiration, was not yet widespread.<ref name="czaja-etal-2013"/> By contrast, Lake Superior-type banded iron formations show iron isotope ratios that suggest that dissimilatory iron reduction expanded greatly during this period.<ref name="johnson-etal-2008">{{cite journal |last1=Johnson |first1=Clark M. |last2=Beard |first2=Brian L. |last3=Klein |first3=Cornelis |last4=Beukes |first4=Nic J. |last5=Roden |first5=Eric E. |title=Iron isotopes constrain biologic and abiologic processes in banded iron formation genesis |journal=Geochimica et Cosmochimica Acta |date=January 2008 |volume=72 |issue=1 |pages=151–169 |doi=10.1016/j.gca.2007.10.013|bibcode=2008GeCoA..72..151J }}</ref>

An alternate route is oxidation by anaerobic [[denitrifying bacteria]]. This requires that [[nitrogen fixation]] by microorganisms is also active.<ref name="cox-etal-2013"/>

:{{chem2|10 Fe(2+) + 2 NO3- + 24 H2O → 10 Fe(OH)3 + N2 + 18 H+}}

=====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>