Iron Isotope Biosignatures

Abstract
The 56 Fe/ 54 Fe of Fe-bearing phases precipitated in sedimentary environments varies by 2.5 per mil (δ 56 Fe values of +0.9 to −1.6 per mil). In contrast, the 56 Fe/ 54 Fe of Fe-bearing phases in igneous rocks from Earth and the moon does not vary measurably (δ 56 Fe = 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 δ 56 Fe of ferrous Fe in solution is isotopically lighter than the ferrihydrite substrate by 1.3 per mil. Therefore, the range in δ 56 Fe 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.