Banded iron formations (also known as
banded ironstone formations or
BIFs) are distinctive units of
sedimentary rockSedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....
that are almost always of
PrecambrianThe Precambrian is the name which describes the large span of time in Earth's history before the current Phanerozoic Eon, and is a Supereon divided into several eons of the geologic time scale...
age. A typical BIF consists of repeated, thin layers of
iron oxideIron oxides are chemical compounds composed of iron and oxygen. All together, there are sixteen known iron oxides and oxyhydroxides.Iron oxides and oxide-hydroxides are widespread in nature, play an important role in many geological and biological processes, and are widely utilized by humans, e.g.,...
s, either
magnetiteMagnetite is a ferrimagnetic mineral with chemical formula Fe3O4, one of several iron oxides and a member of the spinel group. The chemical IUPAC name is iron oxide and the common chemical name is ferrous-ferric oxide. The formula for magnetite may also be written as FeO·Fe2O3, which is one part...
(Fe
3O
4) or
hematiteHematite, also spelled as haematite, is the mineral form of iron oxide , one of several iron oxides. Hematite crystallizes in the rhombohedral system, and it has the same crystal structure as ilmenite and corundum...
(Fe
2O
3), alternating with bands of iron-poor
shaleShale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...
and
chertChert is a fine-grained silica-rich microcrystalline, cryptocrystalline or microfibrous sedimentary rock that may contain small fossils. It varies greatly in color , but most often manifests as gray, brown, grayish brown and light green to rusty red; its color is an expression of trace elements...
. Some of the oldest known rock formations, formed over , include banded iron layers. Banded layers rich in iron were a common feature in
sedimentSediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....
s for much of the Earth's early history but for reasons that will be explained below, are now rare.
Relation to atmospheric oxygenation
The formations are abundant around the time of the
great oxygenation eventThe Great Oxygenation Event , also called the Oxygen Catastrophe or Oxygen Crisis or Great Oxidation, was the biologically induced appearance of free oxygen in Earth's atmosphere. This major environmental change happened around 2.4 billion years ago.Photosynthesis was producing oxygen both before...
, 2,400 million years ago (mya or Ma), and become less common after 1,800 mya. The reappearance of BIF conditions at , and in association with
Snowball EarthThe Snowball Earth hypothesis posits that the Earth's surface became entirely or nearly entirely frozen at least once, some time earlier than 650 Ma . Proponents of the hypothesis argue that it best explains sedimentary deposits generally regarded as of glacial origin at tropical...
, 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 AustraliaWestern Australia is a state of Australia, occupying the entire western third of the Australian continent. It is bounded by the Indian Ocean to the north and west, the Great Australian Bight and Indian Ocean to the south, the Northern Territory to the north-east and South Australia to the south-east...
and the
Animikie GroupThe Animikie Group is a geologic group composed of sedimentary and metasedimentary rock, having been originally deposited between 2,500 and 1,800 million years ago within the Animikie Basin. This group of formations are geographically divided into the Gunflint Range, the Mesabi and Vermilion...
in
MinnesotaMinnesota is a U.S. state located in the Midwestern United States. The twelfth largest state of the U.S., it is the twenty-first most populous, with 5.3 million residents. Minnesota was carved out of the eastern half of the Minnesota Territory and admitted to the Union as the thirty-second state...
.
Origins
The conventional concept is that the
banded iron layers were formed in sea water as the result of
oxygenOxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
released by
photosyntheticPhotosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
cyanobacteria (bluegreen algae), combining with dissolved
ironIron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
in Earth's oceans to form insoluble iron oxides, which precipitated out, forming a thin layer on the substrate, which may have been
anoxicHypoxia, or oxygen depletion, is a phenomenon that occurs in aquatic environments as dissolved oxygen becomes reduced in concentration to a point where it becomes detrimental to aquatic organisms living in the system...
mud (forming shale and chert). Each band is similar to a
varveA varve is an annual layer of sediment or sedimentary rock.The word 'varve' is derived from the Swedish word varv whose meanings and connotations include 'revolution', 'in layers', and 'circle'. The term first appeared as Hvarfig lera on the first map produced by the Geological Survey of Sweden in...
, 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
feedbackFeedback describes the situation when output from an event or phenomenon in the past will influence an occurrence or occurrences of the same Feedback describes the situation when output from (or information about the result of) an event or phenomenon in the past will influence an occurrence or...
oscillationOscillation is the repetitive variation, typically in time, of some measure about a central value or between two or more different states. Familiar examples include a swinging pendulum and AC power. The term vibration is sometimes used more narrowly to mean a mechanical oscillation but sometimes...
in the ocean's
complex systemA complex system is a system composed of interconnected parts that as a whole exhibit one or more properties not obvious from the properties of the individual parts....
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 pointA climate tipping point is a point when global climate changes from one stable state to another stable state, in a similar manner to a wine glass tipping over. After the tipping point has been passed, a transition to a new state occurs...
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 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 EarthThe Snowball Earth hypothesis posits that the Earth's surface became entirely or nearly entirely frozen at least once, some time earlier than 650 Ma . Proponents of the hypothesis argue that it best explains sedimentary deposits generally regarded as of glacial origin at tropical...
hypothesis. This hypothesis stated that following the breakup of the early equatorial
supercontinentIn geology, a supercontinent is a landmass comprising more than one continental core, or craton. The assembly of cratons and accreted terranes that form Eurasia qualifies as a supercontinent today.-History:...
(
RodiniaIn geology, Rodinia is the name of a supercontinent, a continent which contained most or all of Earth's landmass. According to plate tectonic reconstructions, Rodinia existed between 1.1 billion and 750 million years ago, in the Neoproterozoic era...
), the earth's continents were totally covered in an
ice ageAn ice age or, more precisely, glacial age, is a generic geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental ice sheets, polar ice sheets and alpine glaciers...
(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
brineBrine is water, saturated or nearly saturated with salt .Brine is used to preserve vegetables, fruit, fish, and meat, in a process known as brining . Brine is also commonly used to age Halloumi and Feta cheeses, or for pickling foodstuffs, as a means of preserving them...
s in the vicinity of hydrothermally active
rift zoneA rift zone is a feature of some volcanoes, especially the shield volcanoes of Hawaii, in which a linear series of fissures in the volcanic edifice allows lava to be erupted from the volcano's flank instead of from its summit...
s. Alternatively, some geochemists suggest that BIFs could form by direct oxidation of iron by microbial anoxygenic phototrophs.
Effect of asteroid impact
Northern Minnesota's banded iron formations lie directly underneath a thick layer of material only recently recognized as ejecta from the
Sudbury BasinThe Sudbury Basin, also known as Sudbury Structure or the Sudbury Nickel Irruptive, is a major geologic structure in Ontario, Canada. It is the second-largest known impact crater or astrobleme on Earth, as well as one of the oldest....
impact. At the time of formation the earth had a single supercontinent with substantial continental shelves.
An
asteroidAsteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...
(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
tsunamiA tsunami is a series of water waves caused by the displacement of a large volume of a body of water, typically an ocean or a large lake...
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.
http://geology.gsapubs.org/content/37/11/1011.short
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.