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The 56Fe/54Fe of Fe-bearing phases precipitated in sedimentary environments varies by 2.5 per mil (δ56Fe values of +0.9 to −1.6 per mil). In contrast, the56Fe/54Fe of Fe-bearing phases in igneous rocks from Earth and the moon does not vary measurably (δ56Fe = 0.0 ± 0.3 per mil). Experiments with dissimilatory Fe-reducing bacteria of the genus Shewanella algae grown on a ferrihydrite substrate indicate that the δ56Fe of ferrous Fe in solution is isotopically lighter than the ferrihydrite substrate by 1.3 per mil. Therefore, the range in δ56Fe values of sedimentary rocks may reflect biogenic fractionation, and the isotopic composition of Fe may be used to trace the distribution of microorganisms in modern and ancient Earth.
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