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Banded Iron Formations (BIFs)

The banded iron formations were laid down in the early days, when the planet was almost devoid of oxygen. They were deposited on the bed of shallow seas.

The upper picture of an outcrop of Banded Iron Formation comes from ‘Earth Story’ by Lamb & Sington (BBC Books). The rather dead-looking tree, in the far right-hand-side foreground gives an idea of scale. The lower picture, showing more detail, is by Preston Cloud.

The BIFs are huge deposits of incredibly rich iron ore. In essence they are pure iron oxide – rust. But they comprise alternate thin layers of two different kinds of rust; one more ‘rusty’ than the other. Although they are found on land today, the BIFs were produced under water.

Theories differ about how the BIFs were formed. But as far as I know, all the theories blame life – and life that has invented photosynthesis to boot. The idea is that photosynthetic ‘cyano’-bacteria were plying their trade and producing spare oxygen, just as photosynthesis does today.

(Actually the above is not quite true. The very earliest real estate ever found has some rather peculiar BIFs in it. Few people believe that life could have got so far as early as that. So these particular BIFs are thought to have some kind of ‘inorganic’ explanation.)

Now the planet’s original environment had no oxygen in it, so neither did the oceans. You never meet water with no oxygen in it these days. But it can dissolve a certain amount of iron. And it dissolved masses or iron out of the rock on the ocean floor. So the oceans were ‘iron rich’.

Then along came the cyanobacteria and started producing oxygen. This combined with the iron and settled out on the sea floor just beneath. We must emphasise though that the cyanos were nowhere near in full production at this stage (see below). They were producing very small amounts of oxygen, and only in a few isolated places. BIFs can only form when there is very little oxygen around.

But why the bands? This is where theories differ. Perhaps winter came and the cyanos produced less oxygen. Or perhaps they had not yet found a way of protecting themselves against this deadly poisonous oxygen. They produced so much (locally) that they suffered a population crash; and had to start again.

Either way, where BIFs were being laid down, there was life, and fairly advanced life at that. Bacterial life to be sure, but photosynthesis is a pretty sophisticated technique. The organisms that invented it deserve our respect.

The earliest true BIFs were laid down around 3-3½ thousand million years ago(?), during the Archaean period. But not a huge amount. So the cyanos had clearly not exactly taken over the planet at that time.

But they did later, with a vengeance. The BIFs heyday came in the early Proterozoic, between 2.5 and 1.8 thousand million years ago. These dates may conflict with what we report elsewhere. Opinions differ widely over when the planet became sufficiently oxygenated to prevent BIF formation.