Editing Banded iron formation (section)

===Formation processes===

The microbands within chert layers are most likely [[varve]]s produced by annual variations in oxygen production. [[Diurnal cycle|Diurnal]] microbanding would require a very high rate of deposition of 2 meters per year or 5&nbsp;km/Ma. Estimates of deposition rate based on various models of deposition and [[sensitive high-resolution ion microprobe]] (SHRIMP) estimates of the age of associated tuff beds suggest a deposition rate in typical BIFs of 19 to 270 m/Ma, which are consistent either with annual varves or [[rhythmite]]s produced by tidal cycles.<ref name="trendall-blockley-2004"/>

Preston Cloud proposed that mesobanding was a result of self-poisoning by early cyanobacteria as the supply of reduced iron was periodically depleted.<ref name= "Cloud_1973"/> Mesobanding has also been interpreted as a secondary structure, not present in the sediments as originally laid down, but produced during compaction of the sediments.<ref name="trendall-blockley-2004"/> Another theory is that mesobands are primary structures resulting from pulses of activity along [[mid-ocean ridge]]s that change the availability of reduced iron on time scales of decades.<ref name="morris-horwitz-1983">{{cite journal |last1=Morris |first1=R.C. |last2=Horwitz |first2=R.C. |title=The origin of the iron-formation-rich Hamersley Group of Western Australia — deposition on a platform |journal=Precambrian Research |date=August 1983 |volume=21 |issue=3–4 |pages=273–297 |doi=10.1016/0301-9268(83)90044-X|bibcode=1983PreR...21..273M }}</ref> In the case of granular iron formations, the mesobands are attributed to [[Winnowing (sedimentology)|winnowing]] of sediments in shallow water, in which wave action tended to segregate particles of different size and composition.<ref name="trendall-blockley-2004"/>

For banded iron formations to be deposited, several preconditions must be met.<ref name="cox-etal-2013"/>

# The deposition basin must contain waters that are ferruginous (rich in [[iron]]).
# This implies they are also anoxic, since ferrous iron oxidizes to ferric iron within hours or days in the presence of dissolved oxygen. This would prevent transport of large quantities of iron from its sources to the deposition basin. 
# The waters must not be [[Euxinia|euxinic]] (rich in [[hydrogen sulfide]]), since this would cause the ferrous iron to precipitate out as [[pyrite]].
# There must be an oxidation mechanism active within the depositional basin that steadily converts the reservoir of ferrous iron to ferric iron.

====Source of reduced iron====
[[File:BlackSmoker.jpg|alt=|thumb|upright=1.5|Hydrothermal vents were one important source for the reduced iron that was later oxidized to form banded iron formations.]]
There must be an ample source of reduced iron that can circulate freely into the deposition basin.<ref name="trendall-blockley-2004"/> Plausible sources of iron include [[hydrothermal vents]] along mid-ocean ridges, windblown dust, rivers, glacial ice, and [[seepage]] from continental margins.<ref name="cox-etal-2013"/>

The importance of various sources of reduced iron has likely changed dramatically across geologic time. This is reflected in the division of BIFs into Algoma and Lake Superior-type deposits.<ref name="Nadoll_2014">{{cite journal|last1 = Nadoll |first1=P. |last2=Angerer |first2=T. |last3=Mauk |first3=J.L. |last4=French |first4=D. |last5=Walshe |first5=J |title=The chemistry of hydrothermal magnetite: A review|journal=Ore Geology Reviews|volume=61|pages=1–32|doi=10.1016/j.oregeorev.2013.12.013|year=2014|bibcode=2014OGRv...61....1N }}</ref><ref name="Zhu_2014">{{Cite journal|last1 = Zhu |first1=X.Q. |last2=Tang |first2=H.S. |last3=Sun |first3=X.H. |title=Genesis of banded iron formations: A series of experimental simulations|journal=Ore Geology Reviews|volume=63|pages=465–469|doi=10.1016/j.oregeorev.2014.03.009|year=2014|bibcode=2014OGRv...63..465Z }}</ref><ref name="Li_2015">{{cite journal |last1 = Li |first1=L.X. |last2=Li |first2=H.M. |last3=Xu |first3=Y.X. |last4=Chen |first4=J. |last5=Yao |first5=T. |last6=Zhang |first6=L.F. |last7=Yang |first7=X.Q. |last8=Liu |first8=M.J. |title=Zircon growth and ages of migmatites in the Algoma-type BIF-hosted iron deposits in Qianxi Group from eastern Hebei Province, China: Timing of BIF deposition and anatexis|journal=Journal of Asian Earth Sciences|volume=113|pages=1017–1034|doi=10.1016/j.jseaes.2015.02.007|bibcode=2015JAESc.113.1017L|year=2015}}</ref> Algoma-type BIFs formed primarily in the Archean. These older BIFs tend to show a positive [[europium anomaly]] consistent with a [[Hydrothermal vent|hydrothermal]] source of iron.<ref name="condie-2015"/>  By contrast, Lake Superior-type banded iron formations primarily formed during the [[Paleoproterozoic]] era, and lack the europium anomalies of the older Algoma-type BIFs, suggesting a much greater input of iron weathered from continents.<ref name="klein-2005"/><ref name="Li-etal-2015">{{cite journal |last1=Li |first1=Weiqiang |last2=Beard |first2=Brian L. |last3=Johnson |first3=Clark M. |title=Biologically recycled continental iron is a major component in banded iron formations |journal=Proceedings of the National Academy of Sciences |date=7 July 2015 |volume=112 |issue=27 |pages=8193–8198 |doi=10.1073/pnas.1505515112|pmid=26109570 |pmc=4500253 |bibcode=2015PNAS..112.8193L |doi-access=free }}</ref><ref name="condie-2015"/>

====Absence of oxygen or hydrogen sulfide====
The absence of hydrogen sulfide in anoxic ocean water can be explained either by reduced sulfur flux into the deep ocean or a lack of [[dissimilatory sulfate reduction]] (DSR), the process by which microorganisms use sulfate in place of oxygen for respiration. The product of DSR is hydrogen sulfide, which readily precipitates iron out of solution as pyrite.<ref name="holland-2006"/>

The requirement of an anoxic, but not euxinic, deep ocean for deposition of banded iron formation suggests two models to explain the end of BIF deposition 1.8 billion years ago. The "Holland ocean" model proposes that the deep ocean became sufficiently oxygenated at that time to end transport of reduced iron. [[Heinrich Holland]] argues that the absence of [[manganese]] deposits during the pause between Paleoproterozoic and Neoproterozoic BIFs is evidence that the deep ocean had become at least slightly oxygenated. The "[[Canfield ocean]]" model proposes that, to the contrary, the deep ocean became euxinic and transport of reduced iron was blocked by precipitation as pyrite.<ref name="holland-2006"/>

Banded iron formations in northern [[Minnesota]] are overlain by a thick layer of ejecta from the [[Sudbury Basin]] impact. An [[asteroid]] (estimated at {{convert|10|km|abbr=on}} across) [[Impact event|impacted]] into waters about {{convert|1000|m|abbr=on}} deep 1.849 billion years ago, coincident with the pause in BIF deposition. Computer models suggest that the impact would have generated a [[tsunami]] at least {{convert|1000|m|abbr=on}} high at the point of impact, and {{convert|100|m|abbr=on}} high about {{convert|3000|km|abbr=on}} away. It has been suggested that the immense waves and large underwater landslides triggered by the impact caused the mixing of a previously stratified ocean, oxygenated the deep ocean, and ended BIF deposition shortly after the impact.<ref name="Slack_2009"/>

====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>

====Diagenesis====
Regardless of the precise mechanism of oxidation, the oxidation of ferrous to ferric iron likely caused the iron to precipitate out as a [[ferric hydroxide]] gel. Similarly, the silica component of the banded iron formations likely precipitated as a hydrous silica gel.<ref name="trendall-blockley-2004"/> The conversion of iron hydroxide and silica gels to banded iron formation is an example of [[diagenesis]], the conversion of sediments into solid rock.

There is evidence that banded iron formations formed from sediments with nearly the same chemical composition as is found in the BIFs today. The BIFs of the Hamersley Range show great chemical homogeneity and lateral uniformity, with no indication of any precursor rock that might have been altered to the current composition. This suggests that, other than dehydration and decarbonization of the original ferric hydroxide and silica gels, diagenesis likely left the composition unaltered and consisted of crystallization of the original gels.<ref name="trendall-blockley-2004"/> Decarbonization may account for the lack of carbon and preponderance of magnetite in older banded iron formations.<ref name="trendall-2002"/> The relatively high content of hematite in Neoproterozoic BIFs suggests they were deposited very quickly and via a process that did not produce great quantities of biomass, so that little carbon was present to reduce hematite to magnetite.<ref name="cox-etal-2013"/>

However, it is possible that BIF was altered from carbonate rock<ref name="kimberley-1974">{{cite journal |last1=Kimberley |first1=M. M. |title=Origin of iron ore by diagenetic replacement of calcareous oolite |journal=Nature |date=July 1974 |volume=250 |issue=5464 |pages=319–320 |doi=10.1038/250319a0|bibcode=1974Natur.250..319K |s2cid=4211912 }}</ref> or from hydrothermal mud<ref name="krapez-etal-201">{{cite journal |last1=Krapez |first1=B. |last2=Barley |first2=M.E. |last3=Pickard |first3=A.L. |title=Banded iron formations: ambient pelagites, hydrothermal muds or metamorphic rocks? |journal=Extended Abstracts 4th International Archaean Symposium |date=2001 |pages=247–248}}</ref> during late stages of diagenesis. A 2018 study found no evidence that magnetite in BIF formed by decarbonization, and suggests that it formed from thermal decomposition of [[siderite]] via the reaction
::{{chem2|3 FeCO3 + H2O → Fe3O4 + 3 CO2 + H2}}
The iron may have originally precipitated as [[greenalite]] and other iron silicates. Macrobanding is then interpreted as a product of compaction of the original iron silicate mud. This produced siderite-rich bands that served as pathways for fluid flow and formation of magnetite.<ref>{{cite journal |last1=Rasmussen |first1=Birger |last2=Muhling |first2=Janet R. |title=Making magnetite late again: Evidence for widespread magnetite growth by thermal decomposition of siderite in Hamersley banded iron formations |journal=Precambrian Research |date=March 2018 |volume=306 |pages=64–93 |doi=10.1016/j.precamres.2017.12.017|bibcode=2018PreR..306...64R }}</ref>