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.
Explore
- 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.