Historical Geology/Banded iron formations
In this article we shall look at what banded iron formations are and how they might have formed. The reader may find it useful to look back at the main article on siliceous ooze and chert.
Banded iron formations, or BIFs are sedimentary rocks consisting of alternating bands iron-rich sediment (typically hematite, Fe2O3, and magnetite, Fe3O4) and iron-poor sediment, typically chert; the size of the bands ranges from less than a millimeter to more than a meter in thickness. The image to the right shows a fairly typical banded iron formation: the red bands are the iron oxides.
While BIFs have a wide geographical distribution, they are localized in time. They start to become common about 3.5 billion years ago, peak around 2.5 billion years ago, vanish about 1.8 billion years ago, make a small comeback around 1 billion years ago, and then essentially vanish from the geological record; none are being produced today.
BIFs and the rise of oxygenEdit
With the exception of saline giants (as explained in the previous article) it is usually very easy to explain the origin of sedimentary rocks, because we can see identical sediments being deposited in the present: chalk looks just like lithified calcareous ooze, tillite looks just like lithified glacial till, aeolian sandstone looks just like lithified desert sand, and so on.
In the case of BIFs, however, no BIFs are being formed in the present, nor even recently. It seems, then, as though in searching for a cause for BIFs we must be looking for an event which could only have happened at in the past.
Fortunately, one comes to mind. According to biologists, the first living organisms neither produced nor consumed oxygen. Indeed, they would not have been able even to tolerate oxygen: oxygen is a very reactive gas, and is toxic to organisms which are not adapted to its presence (for example the modern bacterium Clostridium botulinum, which can only survive in the near-total absence of oxygen).
Biologists are also agreed that in the absence of oxygen-producing organisms, the atmosphere would have been very poor in free oxygen (i.e. the molecule O2).
What does all this have to do with BIFs? Well, one of the interesting things about iron is that elemental iron (Fe) dissolves in water, whereas the various oxides of iron (as found in banded iron formations) precipitate out. The waters of the early Earth would certainly have had sources of iron, such as emissions from submarine volcanoes, and iron liberated from rocks by chemical weathering. It follows that when organisms arose that produced oxygen, iron dissolved in the oceans would combined with dissolved oxygen to form iron oxides which would then have precipitated out, producing the iron oxides that characterize BIFs.
The iron would, indeed, form an "oxygen sink"; only after the iron had been used up in this way would O2 have begun to constitute a large proportion of the atmosphere. The accumulation of oxygen in the atmosphere, which according to geological dating methods started about 2.4 billion years ago, is variously known as the Great Oxygenation Event (GOE), the oxygen catastrophe, and the oxygen crisis.
How do we know?Edit
The scenario given above is plausible according to biologists; indeed, if they're right about the history of early life, we should expect to see this kind of geological evidence of a rise in oxygen as a result of the rise of oxygen-producing organisms. And biology aside, it is certainly chemically very plausible: iron is soluble in water and iron oxides are not, and this at least is something we can check by direct observation. It is also plausible in that it explains the localization in time of BIFs in terms of something that we would expect to happen only once.
But was there in fact a change from an oxygen-poor to an oxygen-rich atmosphere? Studies of minerals before, during, and after the GOE answer this question in the affirmative.
The fact that the production of free oxygen is indeed a plausible explanation for BIFs is a point in favor of this scenario; what is more, this explanation is borne out by the nature of the iron oxides in BIFs: they tend to be iron oxides with a low oxygen to iron ratio such as hematite and magnetite, rather than, for example, goethite (FeO(OH)); which is what we would expect if they formed in conditions in which oxygen was still scarce.
However, we would be verging on circular reasoning if we explained BIFs by the advent of free oxygen, and if our evidence for this event consisted solely of BIFs. Fortunately, this is far from being the case: there are other indications of the GOE in the mineralogy of the early Earth.
For example, before, but not after, the date assigned to the GOE, the minerals uraninite (UO2) and siderite (FeCO3) can be found in river sediments (for information on how to identify such sediments, see the main article on rivers). The significance of this is that these minerals would not survive in waters containing dissolved O2, as all rivers do today; so the rivers that deposited them must have co-existed with an oxygen-poor atmosphere.
After the date assigned to the GOE, on the other hand, we see a great diversification of mineral types in the geological record, as after the GOE new minerals could then be produced from old ones by oxidization; and it is just such minerals that we find after the GOE. (For more details, see Sverjensky and Lee (2010) The Great Oxidation Event and Mineral Diversification, Elements, 6(1).)
So we have abundant data pointing to the rise of free oxygen in the atmosphere; and such a rise would explain, indeed necessitate, the extensive formation of iron oxide deposits such as are found in BIFs.
BIFs: some questionsEdit
It seems, then, as though we have a good explanation for BIFs. However, the reader should bear in mind that what I have sketched here is only a broad outline of a broad consensus. There is still controversy over details.
The reader may well already be puzzling over a couple of these details. Firstly, why is the sediment in the dark bands of BIFs so frequently chert? And why are BIFs banded at all — why were the chert and iron oxides not deposited simultaneously as a mixture?
The chert may have been deposited by silica-forming organisms. It is true that we don't find in it the tests of diatoms and radiolarians, so it probably didn't have its origin as the sort of siliceous ooze produced by these organisms today; but one cannot rule out the possibility of other silica-producing organisms active in the pre-Cambrian and now extinct.
The rhythmic nature of deposition seems to suggest a cyclic variation in conditions. One candidate cause is a repeated cycle of ecological boom and bust. On biological grounds, it would be reasonable to suspect that the earliest oxygen-producing organisms could not tolerate high levels of oxygen. (This is not as paradoxical as it sounds: most organisms can't live in an environment full of their own waste products. Animals, for example, produce CO2 and would suffocate in an atmosphere dominated by it.)
This suggests the following scenario: oxygen producing organisms would grow and flourish until they had produced toxic levels of oxygen; the population would then collapse almost to nothing, surviving only in low-oxygen refuges; the oxygen would be removed from the atmosphere by combining with the iron in the water to produce the iron oxide bands of BIFs; in these low-oxygen conditions the oxygen-producing organisms could once more increase in number, and the cycle would begin again. This would plausibly account for the periodic precipitation of iron oxides. It is even conceivable that the oxygen-producing organisms were identical with the (hypothetical) silica-secreting organisms mentioned above.
But here we have gone into the realms of speculation and controversy. It is quite possible that these questions will remain controversial: since geologists labor under the handicap of not being able to watch BIFs form in the present, BIFs will never be understood with quite the same certainty as other sedimentary rocks.