The steel in your car almost certainly contains iron mined from a banded iron formation or BIF. These Precambrian sediments are the largest repository of high-grade iron ore on the planet, and nearly all of them formed before about 2 billion years ago, when Earth’s atmosphere and hydrosphere are reckoned by many to have contained very low amounts of free oxygen. The enigma of BIFs is that, as well as vast amounts of iron, they contain equally large amounts of oxygen combined in hematite and magnetite. However they formed, there must have been sufficient iron and oxygen in their environment to make these minerals in astounding quanties. Iron is problematic, because in its Fe-3 form it is almost completely insoluble, and modern sea water contains very little because it is an oxidizing fluid now. Nobody doubts that BIFs formed in a marine environment, and that would have had to contain plenty of soluble Fe-2. So seawater before 2 Ga must have been a reducing fluid so that iron emanating from hydrothermal vents on the basaltic ocean floor could remain in solution and end up in near-surface water. A popular explanation for the oxygen in BIFs is that it was released by the photosynthetic metabolism of blue-green bacteria, near to the basins where BIFs accumulated. So BIFs mopped up any free oxygen that would otherwise have ended up in air or water and made both oxidising. Eventually oxygen production outstripped that of soluble Fe-2 (perhaps by a gradual slowdown of sea-floor spreading) and thereby caused all hydrothermal iron to be precipitated near to ocean floor hydrothermal vents; the oceans became iron-poor after 2 Ga.
There is another plausible scenario for BIF formation, explored by a team from Canada, Britain, Australia and Denmark. Some types of modern bacteria, chemolithoautotrophs and photosynthesisers that do not produce oxygen, are able to fix iron as Fe-3 hydroxides where there is very little oxygen or none at all. The simple chemical equilibria that they exploit provide both energy and carbohydrate (Konhauser and 6 others 2002. Could bacteria have formed the Precambrian banded iron formations? Geology, v. 30, p. 1079-1082). Evidence that such a process might have “grown” the massive BIFs comes from the famous Palaeoproterozoic Hamersley Group of Western Australia, the source of all the steel in cars produced in east Asia. The Hamersley BIFs contain extraordinarily fine layers of iron oxides and silica, which may be annual or even daily records of biological cycles. The key evidence lies in the relative concentrations of other elements in the deposit, phosphorus and trace metals (V, Mn, Co, Zn and Mo), which are close to the nutritional balance needed by the bacteria that Konhauser et al. suggest to have been involved. Experiments with colonies modern bacteria of these kinds show that they are quite capable of depositing iron hydroxide at rates that would easily build vast thicknesses, given time. Around 1022 individual cells could do the job at a rate that would have built the Hamersley BIFs – about 100 metres per million years. That might seem to be an awful lot of bacteria, but it amounts to only about 40 thousand cells per cubic centimetre – far less than the number that build plaque on our teeth!
Phanerozoic marine strontium record throws spanners in the works
Jan Veizer of Ruhr University, Germany and the University of Ottawa is rightly known as “Dr Strontium”. Almost single handedly he has created the record of strontium variation in seawater through geological time, by analysing carbonates that have extracted it along with calcium. Input of strontium to the oceans is through continental weathering and hydrothermal solutions from the oceanic crust, and it has proved tempting to use variations in the Sr/Ca ratio of carbonates as a proxy for the rates of both processes, particularly using Sr isotopes. It is not so simple however, as Thomas Steuber of Ruhr University and Veizer have shown (Steuber, T. & Veizer, J. 2002. Phanerozoic record of plate tectonic control of seawater chemistry and carbonate sedimentation. Geology, v. 30, p. 1123-1126). As in many geochemical cycles, the other important process is burial of strontium in marine sediments, and that depends very much on the type of carbonate that carries it from solution. Aragonite is between 8 and 4 times more efficient at mopping up dissolved strontium than the other common calcium carbonate, calcite. So, if aragonite is the main carbonate that is buried, seawater strontium is likely to fall more rapidly than with calcite burial. Which form dominates in sedimentation depends a great deal on the kind of animal that builds shells – most carbonate buried during the Phanerozoic has been of biogenic origin. Corals and carbonate-secreting algae use aragonite, whereas molluscs, brachiopods, coccoliths and forams have calcite shells.
Other workers have suggested that there have been periods dominated by deposition of one or other form of calcium carbonate, mainly calcite until the mid-Carboniferous, then aragonite up to the mid-Jurassic, calcite through the Cretaceous and most of the Tertiary, and a current tendency for more aragonite. Steuber and Veizer show how there is good correlation between changing ocean-crust formation and seawater Sr, and a negative correlation with the Mg/Ca ratio of seawater. Clearly there are linkages between the three variables, as follows: hydrothermal alteration of new ocean crust exchanges Mg for Ca, so the rate of sea-floor spreading modulates the seawater Mg/Ca ratio; magnesium inhibits the formation of calcite, thereby encouraging aragonite formation; periods of slow spreading therefore favour a higher rate of strontium removal from seawater. This has profound negative implications for the use of strontium isotopes in marine sediments to monitor the pace of continental weathering (the crux for some gross models of global climate change), and using the Mg/Ca ratio as a means of monitoring seawater temperature variations.
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