COULD BACTERIA FORM PRECAMBRIAN BANDED IRON FORMATIONS?

Most models explaining the primary oxidation of Fe(II) to Fe(III) in Precambrian banded iron formations (BIFs) have focused on photochemical processes or passive inorganic reactions using photosynthetically-generated O2. Other biological processes can also account for BIF deposition. In the model proposed here, two bacterial genera are considered which directly oxidize Fe(II) as an energy source, namely Gallionella and Chromatium. Both are likely to have existed in Precambrian oceans. Intact microbial fossils have not yet been identified in Fe-rich BIF bands. Consequently, any attempt to prove a direct microbial role in iron biomineralization must rely on circumstantial evidence. One such example is using the concentrations of trace metals/nutrients in BIF to establish the plausibility of bacteria oxidizing sufficient iron in the water column of the depositional basin. Sections of fresh core were selected from BIF macrobands of the Dales Gorge Member, Hamersley Group, Western Australia. The cores are dominated by hematite, magnetite and quartz, and they experienced only low grade metamorphism. Based on available U-Pb dating of zircons in bulk Hamersley samples and measured iron concentrations in core, an annual rate of 45.3 moles of Fe/m2 is estimated during periods of peak iron deposition. Using documented growth rates for both bacteria, and assuming continuous growth during sustained hydrothermal activity, it is further calculated that only 4.3 x 104 cells/cm3 of Gallionella and 5.7 x 104 cells/cm3 of Chromatium were needed over a 100m photic zone throughout the basin. Both cell population numbers are much less than the mixed bacterioplankton assemblages growing in the photic zone of modern marine waters. Interestingly, those ancient populations would also have been easily supported by the concentrations of P, V, Mn, Co, Zn and Mo measured within the same iron-rich bands. Clearly, microorganisms had the means to be important agents in iron deposition, and must now be considered as a demonstrable mechanism for BIF development in the Precambrian.