Banded Iron Formations: Ancient Rocks That Record Rising Oxygen

Banded iron formations are striking striped rocks made of thin alternating layers of iron oxide and silica, laid down on the sea floor mostly between about 2.5 and 1.8 billion years ago. They formed when iron dissolved in ancient, oxygen-poor oceans reacted and settled out. Today they are one of the best records of how oxygen rose on early Earth, and they supply most of the world’s iron ore.

What banded iron formations are

A banded iron formation, often shortened to BIF, is a chemical sedimentary rock. It is made of layers rich in iron minerals, such as hematite and magnetite, alternating with layers of chert, a fine-grained form of silica. The iron layers are usually red, black or silvery grey, while the chert layers can be red, white or grey, giving the rocks their dramatic striped look.

In a definition widely used since the American geologist Harold James proposed it in 1954, an iron formation contains at least 15 per cent iron. Typical BIFs contain around 20 to 40 per cent iron. The banding occurs on many scales, from thick bands metres across to layers less than a millimetre thick.

How they formed

Iron in an oxygen-poor ocean

Iron dissolves easily in water only in its reduced, or ferrous, form. In the Archean and early Proterozoic, the oceans contained very little oxygen, so large amounts of ferrous iron could build up. Much of it probably came from hydrothermal vents on the sea floor, where hot water leaches iron from volcanic rock.

When this iron was oxidised to its ferric form, it became insoluble and settled to the sea floor as tiny particles. Over time these deposits were compacted and altered into the iron minerals seen today.

What oxidised the iron?

Scientists debate how the iron was oxidised. The classic explanation, set out by the American geologist Preston Cloud in the late 1960s and early 1970s, is that cyanobacteria in surface waters produced oxygen, which immediately reacted with iron rising from the deep. In this view, the oceans acted as an oxygen sink, and the BIFs record oxygen being consumed before it could build up in the air.

Other mechanisms may also have played a part. Some bacteria can oxidise iron without oxygen, using light energy, a process called photoferrotrophy. Ultraviolet light may also have oxidised some iron directly. Many researchers think different processes dominated at different times.

Why the bands?

The cause of the alternating layers is also debated. The Australian geologist Alec Trendall, who studied the Hamersley Range rocks in detail, proposed that the finest layers might be annual, like tree rings, reflecting seasonal changes. Other explanations involve cycles in volcanic activity, in microbial productivity or in ocean circulation.

Types and ages

Geologists recognise several types. Algoma-type BIFs are relatively small and associated with volcanic rocks in Archean greenstone belts. The oldest known example comes from the Isua Greenstone Belt in Greenland, about 3.8 billion years old.

Superior-type BIFs, named after the Lake Superior region, are much larger and formed on continental shelves, mostly between about 2.5 and 1.85 billion years ago. They include the vast Hamersley Basin deposits of Western Australia, around 2.5 billion years old, and the iron ranges of Minnesota and Michigan. In Karijini National Park, in the Hamersley Range, gorges such as Dales Gorge cut through hundreds of metres of these layers, offering some of the finest natural exposures of BIF in the world and drawing many visitors each year.

A third group, sometimes called Rapitan-type, formed briefly about 750 to 600 million years ago. These are linked to the Snowball Earth glaciations, when ice cover may have cut the oceans off from the air and allowed iron to build up again.

Banded iron and the rise of oxygen

The great age of BIFs overlaps with the Great Oxidation Event, between about 2.4 and 2.1 billion years ago, when oxygen first accumulated in the atmosphere. BIFs show that oxygen was being produced and that the oceans held large reserves of iron capable of absorbing it.

Around 1.85 billion years ago large BIFs largely disappear from the record. One explanation is that the deep ocean became oxygenated enough to prevent iron from accumulating. Another, proposed by Donald Canfield in 1998, is that the deep ocean became rich in hydrogen sulphide, which removed iron as the mineral pyrite. Some researchers have also suggested a link to the Sudbury asteroid impact in Canada, about 1.85 billion years ago, though this is speculative.

Myths and facts

A popular account says that banded iron formations prove the oceans “rusted” all at once when oxygen appeared. The real picture is more gradual. BIFs formed over more than two billion years, began long before oxygen accumulated in the atmosphere, and peaked at different times in different basins. Some were probably produced by microbes that never released oxygen at all.

It is also sometimes said that the red colour of BIFs is ordinary rust. The red layers are usually hematite or iron-stained chert called jasper, minerals that formed and recrystallised as the sediments were buried and slowly heated. Much of the original iron may first have settled as softer iron hydroxides or iron-bearing clays and carbonates, which were later transformed into the minerals seen today.

Finally, BIFs are not unique to Earth’s distant past in their chemistry. Iron-rich sediments still form today in small amounts around some hydrothermal vents and in a few oxygen-poor lakes. These modern settings help scientists test ideas about how the ancient deposits formed, although none comes close to the vast scale of the Precambrian formations.

Iron ore and industry

Banded iron formations are the main source of the world’s iron, and therefore of steel. In many places, natural weathering over millions of years removed silica and enriched the iron, creating high-grade ores with more than 60 per cent iron. The biggest producers, including Western Australia’s Pilbara region and Brazil’s Carajas and Iron Quadrangle districts, mine such deposits.

Lower-grade BIF, known in the United States as taconite, is also mined. On Minnesota’s Mesabi Range, first shipped from in 1892, taconite is crushed and the iron separated and rolled into pellets. Other major BIF-hosted ore districts include Kryvyi Rih in Ukraine, the Kursk Magnetic Anomaly in Russia and the Labrador Trough in Canada.

By the numbers

Timeline

• c. 3.8 billion years ago: Oldest known BIF forms in the Isua belt, Greenland.
• c. 2.5 billion years ago: Vast Hamersley Basin BIFs accumulate in Western Australia.
• c. 2.4-2.1 billion years ago: Great Oxidation Event.
• c. 1.85 billion years ago: Large BIFs largely cease to form.
• c. 750-600 million years ago: Brief return of BIFs during Snowball Earth glaciations.
• 1892: First iron ore shipped from Minnesota’s Mesabi Range.
• 1954: Harold James publishes a widely used definition of iron formation.
• 1973: Preston Cloud publishes his influential paper linking BIFs to early oxygen.

Why it matters

Banded iron formations connect the deep history of life with everyday life today. They record how microbes changed the chemistry of the oceans and air, and the iron they hold is the backbone of modern steel, from cars and bridges to skyscrapers.