Demonstration of significant abiotic iron isotope fractionation in nature

Field and laboratory studies reveal that the mineral ferrihydrite, formed a s a result of abiotic oxidation of aqueous ferrous to ferric Fe, contains F e that is isotopically heavy relative to coexisting aqueous Fe. Because the electron transfer step of the oxidation process at pH >5 is essentially ir reversible and should favor the lighter Fe isotopes in the ferric iron prod uct, this result suggests that relatively heavy Fe isotopes are preferentia lly partitioned into the readily oxidized Fe(II)(OH)(x(aq)) species or thei r transition complexes prior to oxidation. The apparent Fe isotope fraction ation factor, alpha (ferrihydrite-water), depends primarily on the relative abundances of the Fe(II)((aq)) species. This study demonstrates that abiot ic processes can fractionate the Fe isotopes to the same extent as biotic p rocesses, and thus Fe isotopes on their own do not provide an effective bio signature.