Banded iron formation

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2.1 billion year old rock with black-band ironstone
2.1 billion year old rock with black-band ironstone

Banded iron formations (also known as banded ironstone formations, or BIFs) are a distinctive type of rock often found in old sedimentary rocks. The structures consist of repeated thin layers of iron oxides, either magnetite or hematite, 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 conventional concept is that the banded iron layers were formed in water as the result of oxygen released by photosynthetic cyanobacteria, 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. The banding is assumed to result from cyclic variations in available oxygen. It is unclear whether these banded formations were seasonal 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

Water flowing over iron-rich beds
Water flowing over iron-rich beds

[edit] Later banded iron formations

Until fairly recently, it was assumed that the rare later 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 rare deposits is undergoing much research as part of the Snowball Earth hypothesis — wherein it is believed 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) which corresponds to evidence that 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) (See Cryogenian period, from 800 million years ago (mya, boundary defined chronometrically) to approximately 635 mya) prior to the Ediacaran wherein the earliest multicellular lifeforms appear. Alternatively, some geochemists suggest that BIFs could form by direct oxydation of iron by autotrophic (non-photosynthetic) microbes.

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.

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