The Banded Iron Formation:
Biological Origin
By Jason Triplett
(http://www.adrian.edu/chemistry/th/Somelinks/Spages/ttackett/08773_08.jpg)
Banded Iron Formations (BIF’s) are Precambrian aged, chemical precipitates distinguished by alternating iron-rich and silica-rich layers and, are probably one of the most controversial geological deposits known. The iron-rich layers primarily consist of hematite (Fe2O3) and magnetite (Fe3O4), while the silica-rich layers are silica usually in the form of chert (SiO2) and/or jadeite (NaAlSi2O6).
In 1954, H. L. James began to study these formations. He wrote a paper titled, “Sedimentary facies of iron formation” which was published in Economic Geology, volume 49. Since then, the questions and ideas regarding the origins of these chemical precipitates have been almost never ending.
The predominate explanation/hypothesis of the day for the origin of the BIF’s is that they formed on partially isolated, submerged platforms on the continental shelves of Archean cratons. Pulsed output from distal mid-ocean ridges, believed to be the iron source, supplied the hydrothermal waters which were brought onto the shelf by upwelling currents and minerals precipitated uniformily throughout the basin. The more iron-rich layers formed during major episodes of hydrothermal input, where as the silica-rich layers formed during periods of relative hydrothermal inactivity (Konhauser et al., 2002).
Until recently, the primary oxidation of the Fe2+ and Fe 3+, which forms the hematite (Fe2O3) and magnetite (Fe3O4) layers, supplied by the ocean ridges has been explained by photochemical or inorganic reactions with the aid of photosynthesis generated O2. It had been speculated that primitive O2 producing photosynthetic bacteria lacked the oxygen mediating enzymes to play a significant role in the oxidation of Fe2+ and Fe 3+ (Konhauser et al., 2002).
But
now, after present day research, it is believed that oxidation of iron by chemolithoautotrophic species could have possibly accounted
for the precipitation of ferric iron over the wide ocean basins.
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http://www.ldeo.columbia.edu/edu/dees/U4735/projections/bif_histo.html
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http://www.brunel.ac.uk/depts/bl/project/bb1041S02/5
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