Banded Iron Formations (BIFs) are large, iron-rich sedimentary rock deposits. They are indicators of conditions on early Earth AND the source of Australia’s iron deposits. In the past 45 years, our understanding of BIFs has changed tremendously, and a much more complex picture of the ancient oceans has been revealed.
Kalmina gorge in the Pilbara region has cliffs of red banded iron formations (Gypsy Denise 2013, Creative Commons 4.0)
An old story
In 1973, Preston Cloud published an influential paper suggesting that the evolution of oxygenic photosynthesis (which produces oxygen as a waste product) was responsible for the majority of these iron deposits. The early Earth had very little free oxygen and large amounts of dissolved iron in the oceans. As oxygen was produced, it reacted with dissolved iron to produce insoluble iron oxides that dropped to the sea floor, forming BIFs.
The 1973 story of BIFs has persisted in the Australian syllabus (from Tompkins & Watkins 2016 Exploring Earth and Environmental Science Year 12)
Cloud’s theory accounts for enormous changes to ocean chemistry, as the oceans were gradually depleted of iron. Atmospheric carbon dioxide greatly decreased before the build-up of oxygen and carbon was transferred into the biosphere. This resulted in large quantities of cyanobacteria, the first organisms to carry out oxygenic photosynthesis.
The Great Oxygenation Event
The Great Oxygenation Event (GOE) occurred 2.4 – 2.35 Ga (billion years ago). At this time, atmospheric oxygen finally built up to a level 0.001% of that we experience today. Although there was only a little oxygen, it had a profound effect. Many microbes were driven into marginal habitats or became extinct. Aerobic respiration evolved and evolution began to pick up pace. There is not an exact time for the GOE, as this appears to have happened in pulses. However, by 2.3 Ga, oxygen was a permanent feature of the atmosphere and upper oceans. Deposition of BIFs gradually decreased as oceanic iron was depleted and oxygen concentrations increased.
Our current understanding
In the 45 years since Cloud proposed his theory, many new ways of examining early life have been developed. Scientists examine rock using scanning electron microscopes, carry out sophisticated geochemical analyses and have found an array of modern microbes that carry out photosynthesis without producing oxygen. Ongoing research suggests that the old story needs to be modified.
BIFs were common BEFORE oxygenation of the atmosphere and well before most evidence for evolution of oxygenic photosynthesis in cyanobacteria. Most researchers now consider early BIFs to be the result of either chemical processes, that did not require life or a unique type of photosynthesis called photoferrotrophy. In photoferrotrophy, bacteria use ferrous iron (Fe2+) as an electron donor. Modern purple bacteria and green sulfur bacteria use this type of photosynthesis today.
The role of oxygen from photosynthesis has now been pushed back to the late stages of BIF deposition. Even then, the photoferrotrophs may be responsible for much of the iron deposition in lower layers of the ocean. Investigations into ancient ocean chemistry help us to understand the enormous changes that Earth has been through that were triggered by life, billions of years before humans evolved.
- AusEarthEd teacher notes provide summaries of recent scientific literature: Rethinking Banded Iron Formations, Evidence for Photosynthesis, Effect of Cyanobacteria
- Learn about the effects of the Great Oxygenation Event from PBS Eons (5:52).
- Geologists discuss possible ways of forming BIFs through the action of oxygenic photosynthesis and photoferrtrophs (4:23). This explains some of the chemistry.
- BHPs Geology of the Pilbara explains how initial iron deposits in BIFs were from volcanic activity and iron-poor layers during times of reduced activity. Go to the section ‘Formation of the Pilbara Craton’ to learn about this.