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Banded iron formation
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Banded iron formations (also known as banded ironstone formations or BIFs) are a distinctive type of rock often found in primordial (Precambrian) sedimentary rocks. The structures consist of repeated thin layers of iron oxides, either magnetite (Fe3O4) or hematite (Fe2O3), alternating with bands of iron-poor shale and chert.
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Banded iron formations (also known as banded ironstone formations or BIFs) are a distinctive type of rock often found in primordial (Precambrian) sedimentary rocks. The structures consist of repeated thin layers of iron oxides, either magnetite (Fe3O4) or hematite (Fe2O3), alternating with bands of iron-poor shale and chert. Some of the oldest known rock formations, formed around three thousand million years before present (3 Ga), include banded iron layers, and the banded layers are a common feature in sediments for much of the Earth's early history. Banded iron beds are less common after 1.8 Ga, although some are known that are much younger.
The total amount of oxygen locked up in the banded iron beds is estimated to be perhaps twenty times the volume of oxygen present in the modern atmosphere. Banded iron beds are an important commercial source of iron ore, such as the Pilbara region of Western Australia and the Mesabi Range in Minnesota.
Origins
The conventional concept is that the banded iron layers were formed in sea water as the result of oxygen released by photosynthetic cyanobacteria (bluegreen algae), combining with dissolved iron in Earth's oceans to form insoluble iron oxides, which precipitated out, forming a thin layer on the substrate, 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. It is assumed that initially the Earth started out with vast amounts of iron dissolved in the world's acidic seas. Eventually, as photosynthetic organisms generated oxygen, the available iron in the Earth's oceans was precipitated out as iron oxides. At the tipping point where the oceans became permanently oxygenated, small variations in oxygen production produced pulses of free oxygen in the surface waters, alternating with pulses of iron oxide deposition.
Snowball Earth scenario
Until 1992, it was assumed that the rare, later (younger) banded iron deposits represent unusual conditions where oxygen was depleted locally and iron-rich waters could form then come into contact with oxygenated water. An alternate explanation of these later deposits is undergoing much discussion as part of the Snowball Earth hypothesis. This hypothesis states that an early equatorial supercontinent (Rodinia) was totally covered in an ice age (implying the whole planet was frozen at the surface to a depth of several kilometers). In this case the Earth's free oxygen may have been nearly or totally depleted during a severe ice age circa 750 to 580 million years ago (mya).
Another mechanism for BIF-formatíon, also proposed in the context of the Snowball Earth discussion, is by deposition from metal-rich brines in the vicinity of hydrothermally active rift zones. Alternatively, some geochemists suggest that BIFs could form by direct oxidation of iron by (non-photosynthetic) autotrophic microbes.
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