Geochemistry

Abiotic Iron

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Science  03 Aug 2001:
Vol. 293, Issue 5531, pp. 765
DOI: 10.1126/science.293.5531.765b

Iron, an essential nutrient, can be fractionated isotopically by bacteria during its metabolic reduction from Fe(III) to Fe(II). Consequently, iron isotope fractionation may be useful as a tracer of early life or as a signature of biological activity in extreme environments, but theoretical arguments and recent empirical evidence have shown that iron isotopes can be fractionated by nonbiological processes.

Bullen et al. investigated the potential of abiotic chemical reactions to fractionate iron isotopes with samples from the field and the laboratory. During the oxidation of aqueous Fe(II) to Fe(III), abiotic processes form a precipitate of ferrihydrate, Fe(II)(OH)x, with a 56Fe/54Fe nearly 1 0/00 higher than that of the coexisting disssolved Fe(III). This effect is comparable in magnitude to the fractionation observed in microbial systems and supports the argument that Fe isotopic fractionation in geological samples cannot be taken as an unequivocal indicator of biologic activity. — HJS

Geology29, 699 (2001).

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