Abstract
The Precambrian banded iron formations (BIFs) are the major iron ore sources on the Earth. They consist of extensive iron-rich and iron-poor layers within siliceous sedimentary rocks1,2. The banding has been attributed to variations in the conditions for precipitation of Fe2+ in ancient seas. The most favoured precipitating agent is oxygen3–8; this would lead in the first place to insoluble FeOOH. The variations might then arise from fluctuations in low levels of oxygen in the atmosphere8. Similar fluctuations could arise through the in situ photosynthetic activity of phytoplankton5. Alternatively, oxygen might have been a constant factor, the periodicity arising from a varying supply of iron to the zone of precipitation8–10. Another suggestion, that UV photons might have been the precipitating agent, without oxygen11, was based on the conversion of Fe2+ to Fe3+ by UV light (254 nm) in rather strongly acid conditions12,13. We have now tested this idea by photolysing Fe2+ in morerealistic near-neutral conditions. We report here that the presence of Fe(OH)+ becomes important, greatly enhancing previous estimates of the iron-precipitating power of early Precambrian sunlight, and suggesting that this sunlight would have been a sufficient precipitating agent for the iron found in BIFs.
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Braterman, P., Cairns-Smith, A. & Sloper, R. Photo-oxidation of hydrated Fe2+—significance for banded iron formations. Nature 303, 163–164 (1983). https://doi.org/10.1038/303163a0
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DOI: https://doi.org/10.1038/303163a0
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