Banded iron formations (also known as banded ironstone formations or BIFs) are distinctive units of sedimentary rock that are almost always of Precambrian age. A typical BIF consists 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 over 3,700 million years ago, include banded iron layers.[1] Banded layers rich in iron were a common feature in sediments for much of the Earth's early history but for reasons that will be explained below, are now rare.
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The formations are abundant around the time of the great oxygenation event,[2] 2,400 million years ago (mya or Ma), and become less common after 1,800 mya. The reappearance of BIF conditions at 1,900 million years ago,[3] and in association with Snowball Earth 750 million years ago,[4] is problematic to explain (see below).
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 Animikie Group in Minnesota.
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.[5] 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.
Until 1992,[6] it was assumed that the rare, later (younger) banded iron deposits represented 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 was undergoing much discussion as part of the Snowball Earth hypothesis. This hypothesis stated that following the breakup of the early equatorial supercontinent (Rodinia), the earth's continents were totally covered in an ice age (implying the whole planet was frozen at the surface to a depth of several kilometers).
If this was the case, Earth's free oxygen may have been nearly or totally depleted during a severe ice age circa 750 to 580 million years ago (mya). 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.
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.[7] Alternatively, some geochemists suggest that BIFs could form by direct oxidation of iron by microbial anoxygenic phototrophs.[8]
Northern Minnesota's banded iron formations lie directly underneath a thick layer of material only recently recognized as ejecta from the Sudbury Basin impact. At the time of formation the earth had a single supercontinent with substantial continental shelves.
An asteroid (estimated at 10 km across) that slammed into in waters about 1000 m deep some 1.85 billion years ago. Computer models suggest that the tsunami would have been at least 1000 m at the epicentre, and 100 m high about 3000 km 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.[1]
Sediments deposited on the seafloor before the impact, including BIFs 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 BIFs typically accumulated.
The geological record suggests that environmental changes were happening in oceans worldwide even before the Sudbury impact. The role the Sudbury Basin impact played in temporarily shutting down BIF accumulation is not fully understood.
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