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Banded iron formation

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Banded iron formations (also known as banded ironstone formations or BIFs) are distinctive units of sedimentary rock that are almost always of Precambrian age.

Banded iron formation
Sedimentary rock
Banded iron formation Dales Gorge.jpg
Composition
Primary iron oxides, shales and cherts
Secondary Other
2.1 billion year old rock showing banded iron formation

A typical banded iron formation consists of repeated, thin layers (a few millimeters to a few centimeters in thickness) of silver to black iron oxides, either magnetite (Fe3O4) or hematite (Fe2O3), alternating with bands of iron-poor shales and cherts, often red in color, of similar thickness, and containing microbands (sub-millimeter) of iron oxides.[1]

Some of the oldest known rock formations, formed over 3,700 million years ago, include banded iron layers.[2] Banded iron beds are an important commercial source of iron ore, such as the Pilbara region of Western Australia and the Animikie Group in Minnesota.

Contents

OriginsEdit

 
Close-up of banded iron formation specimen from Upper Michigan.

The formations are abundant around the time of the great oxygenation event,[3] 2,400 million years ago (mya or Ma), and become less common after 1,800 mya[4] with evidence pointing to intermittent low levels of free atmospheric oxygen.[5] 750 million years ago new banded iron formations formed that are associated with Snowball Earth[6].

 
An ashtray carved out of a soft form of banded ironstone from the 3.5 billion year old Barbeton Supergroup in South Africa. The red layers were laid down during the daylight hours when Archaean photosynthesizing cyanobacteria produced oxygen that immediately reacted with dissolved iron compounds in the water, to form insoluble iron oxide (rust). The white layers are sediments that settled during the night when there was no oxygen in the water.[7]

The conventional hypothesis is that the banded iron layers were formed in sea water as the result of oxygen released by photosynthetic cyanobacteria. The oxygen then combined with dissolved iron in Earth's oceans to form insoluble iron oxides, which precipitated out, forming a thin layer on the ocean floor, which may have been anoxic mud (forming shale and chert). Each band is similar to a varve, to the extent that the banding is assumed to result from cyclic variations in available oxygen. It is unclear whether these banded ironstone formations were seasonal, followed some feedback oscillation in the ocean's complex system or followed some other cycle.[8] It is assumed that initially the Earth started with vast amounts of iron and nickel dissolved in the world's acidic seas. As photosynthetic organisms generated oxygen, the available iron in the Earth's oceans precipitated out as iron oxides. At a suspected tipping point where the oceans became permanently oxygenated, small variations in oxygen production produced periods of free oxygen in the surface waters, alternating with periods of iron oxide deposition.

Snowball EarthEdit

 
Water flowing over iron-rich beds

Until 1992[9] it was assumed that the rare, later (younger) banded iron deposits represented unusual conditions where oxygen was depleted locally. Iron-rich waters would then form in isolation and subsequently come into contact with oxygenated water. The Snowball Earth hypothesis provided an alternative explanation for these younger deposits. In a Snowball Earth state the continents, and possibly seas at low latitudes, were subject to a severe ice age circa 750 to 580 million years ago (mya) that nearly or totally depleted free oxygen. Dissolved iron then accumulated in the oxygen-poor oceans (possibly from seafloor hydrothermal vents). Following the thawing of the Earth, the seas became oxygenated once more causing the precipitation of the iron.

An alternative mechanism for banded iron formations in the Snowball Earth era suggests the iron was deposited from metal-rich brines in the vicinity of hydrothermally active rift zones.[10] Alternatively, some geochemists suggest that banded iron formations could form by direct oxidation of iron by microbial anoxygenic phototrophs.[11]

Sudbury Basin impactEdit

Banded iron formations in Northern Minnesota were found directly underneath a thick layer of ejecta from the Sudbury Basin impact. At the time of formation Earth had a single supercontinent called Columbia with substantial continental shelves. An asteroid (estimated at 10 km across) slammed into waters about 1,000 m deep some 1.85 billion years ago. Computer models suggest that the tsunami would have been at least 1,000 metres high at the centre, and 100 metres high about 3,000 kilometres away. Those immense waves and large underwater landslides triggered by the impact stirred the ocean, bringing oxygenated waters from the surface down to the ocean floor.[4]

Sediments deposited on the seafloor before the impact, including banded iron formations, contained little if any oxidized iron (Fe(III)), but were high in reduced iron (Fe(II)). This Fe(III) to Fe(II) ratio suggests that most parts of the ocean were relatively devoid of oxygen. Marine sediments deposited after the impact included substantial amounts of Fe(III) but very little Fe(II). This suggests that sizeable amounts of dissolved oxygen were available to form sediments rich in Fe(III). Following the impact dissolved iron was mixed into the deepest parts of the ocean. This would have choked off most of the supply of Fe(II) to shallower waters where banded iron formations typically accumulated.

The geological record suggests that environmental changes were happening in oceans worldwide even before the Sudbury impact. The role of the Sudbury Basin impact in temporarily shutting down banded iron formation accumulation is not fully understood.

See alsoEdit

NotesEdit

  1. ^ Katsuta, N.; Shimizu, I.; Helmstaedt, H.; Takano, M.; Kawakami, S.; Kumazawa, M. (1 June 2012). "Major element distribution in Archean banded iron formation (BIF): influence of metamorphic differentiation". Journal of Metamorphic Geology. 30 (5): 457–472. doi:10.1111/j.1525-1314.2012.00975. 
  2. ^ Minik T. Rosing, et al., Earliest part of Earth's stratigraphic record: A reappraisal of the >3.7 Ga Isua (Greenland) supracrustal sequence, Geology, 1996, v. 24 no. 1 p. 43-46
  3. ^ Cloud, P. (1973). "Paleoecological Significance of the Banded Iron-Formation". Economic Geology. 68 (7): 1135–1143. doi:10.2113/gsecongeo.68.7.1135. 
  4. ^ a b Slack, J. F.; Cannon, W. F. (2009). "Extraterrestrial demise of banded iron formations 1.85 billion years ago". Geology. 37 (11): 1011. Bibcode:2009Geo....37.1011S. doi:10.1130/G30259A.1. 
  5. ^ Lyons, T. W.; Reinhard, C. T. (2009). "Early Earth: Oxygen for heavy-metal fans". Nature. 461 (7261): 179–181. Bibcode:2009Natur.461..179L. PMID 19741692. doi:10.1038/461179a. 
  6. ^ Hoffman, P. F.; Kaufman, A. J.; Halverson, G. P.; Schrag, D. P. (1998). "A Neoproterozoic Snowball Earth" (PDF). Science. 281 (5381): 1342–1346. Bibcode:1998Sci...281.1342H. PMID 9721097. doi:10.1126/science.281.5381.1342. 
  7. ^ Margulis, L., Sagan, D. (1995). ‘’What is Life?’’. p. 81-83. Weidenfeld and Nicolson, London.
  8. ^ Good discussions for the layman are in Cesare Emiliani, Plant Earth 1992:407f, and Tjeerd van Andel, New Views on an Old Planet 2nd ed. 1994:303-05.
  9. ^ Kirschvink, Joseph (1992). "Late Proterozoic low-latitude global glaciation: the Snowball Earth", in J. W. Schopf; C. Klein: The Proterozoic Biosphere: A Multidisciplinary Study. Cambridge University Press.
  10. ^ Eyles, N.; Januszczak, N. (2004). "’Zipper-rift’: A tectonic model for Neoproterozoic glaciations during the breakup of Rodinia after 750 Ma". Earth-Science Reviews 65 (1-2): 1-73 Archived 28 November 2007 at the Wayback Machine.. Retrieved on 2008-02-04.
  11. ^ Andreas Kappler et al.: Deposition of banded iron formations by anoxygenic phototrophic Fe(II)-oxidizing bacteria. Geology, November 2005, v. 33, no. 11, p. 865–868. (pdf, 250 Kb) (doi:10.1130/G21658.1 Abstract)

ReferencesEdit

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