Iron isotope biosignatures

The Fe-56/Fe-54 of Fe-bearing phases precipitated in sedimentary environmen ts varies by 2.5 per mil (delta(56)Fe values of +0.9 to -1.6 per mil). In c ontrast, the Fe-56/Fe-54 of Fe-bearing phases in igneous rocks from Earth a nd the moon does not vary measurably (delta(56)Fe = 0.0 +/- 0.3 per mil). E xperiments with dissimilatory Fe-reducing bacteria of the genus Shewanella algae grown on a ferrihydrite substrate indicate that the delta(56)Fe of fe rrous Fe in solution is isotopically Lighter than the ferrihydrite substrat e by 1.3 per mil. Therefore, the range in delta(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 an cient Earth.