Why do banded iron formations form

Because of their great thickness and the enormous areas that they cover, banded iron formations probably accumulated on wide continental shelves at water depths of over metres. The process of iron deposition in the Proterozoic seas, 2. It was the emergence of the earliest forms of life, tiny microbes cyanobacteria that produced oxygen through photosynthesis, that probably saw the composition of the early atmosphere begin to change.

Along with banded iron formations , the rocks of the Pilbara region also contain fossilised remnants of marine communities stromatolites built up by cyanobacteria at this time. Specimen and photograph: A. Most importantly, it is difficult to transport iron very far from these sources today because when iron reacts with oxygen gas , it becomes insoluble it cannot be dissolved in water and forms a solid particle.

This way, it could be transported long distances in seawater from its sources to the locations where BIFs formed. This would be possible only if there were little or no oxygen gas in the atmosphere and ocean at the time the BIFs were being deposited. Cloud recognized that since BIFs could not form in the presence of oxygen , the end of BIF deposition probably marked the first occurrence of abundant oxygen gas on Earth Cloud, Cloud further reasoned that, for dissolved iron to finally precipitate and be deposited, the iron would have had to react with small amounts of oxygen near the deposits.

Small amounts of oxygen could have been produced by the first photosynthetic bacteria living in the open ocean. When the dissolved iron encountered the oxygen produced by the photosynthesizing bacteria, the iron would have precipitated out of seawater in the form of minerals that make up the iron-rich layers of BIFs: hematite Fe 2 O 3 and magnetite Fe 3 O 4 , according to the following reactions:.

The abrupt disappearance of BIFs around 1. Banded Iron Formation Source is unknown. This would have essentially prevented the oceans from interacting with the atmosphere , limiting the supply of oxygen gas in the water and again allowing dissolved iron to be transported throughout the oceans.

When the sea ice melted, the presence of oxygen would have again allowed the iron to precipitate. References: 1. Even though this more-or-less standard approach to teaching about BIFs makes for a nice story I am concerned that it might represent a gross oversimplification.

After all BIFs are one of the least understood and most controversial deposits on Earth. Their temporal distribution is more complicated than stated above: for example BIFs first appeared about 3. Opinions regarding the alternating BIF banding or the role of microbial and other processes in the formation of BIFs vary greatly.

For these reasons I look forward to the opportunity to consult with experts at the workshop about strategies for developing effective approaches for teaching this and other important and fascinating but potentially poorly understood and controversial subjects.

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