IRON ISOTOPE SIGNATURES OF BIOTICALLY AND ABIOTICALLY PRECIPITATED IRON-OXIDES

Delta ⁵⁶Fe values of Fe²⁺_aq, Fe³⁺_aq, and precipitated Fe(III) from both biotic and abiotic laboratory experiments show a significant range of isotope effects. Abiotic experiments includ precipitation of Fe(III) oxides from ferrous chloride and ferrous sulfate solutions under varying pH conditions. Biotic experiments include oxidation of Fe(II) to Fe(III) and subsequent precipitation of Fe(III) oxides using acidophilic and neutrophilic bacteria. Variations in geochemical parameters (Fe speciation, pH, rates of oxidation) result in d⁵⁶Fe values of Fe(III) oxides that can be enriched or depleted in 56Fe by as much as 4 ‰ relative to Fe(II). During oxidation of Fe(II) by neutrophilic Fe(II)-oxidizing bacteria, the d⁵⁶Fe value of Fe(II)_(aq) decreases during the first 30 hours of oxidation. Correspondingly, Fe(III) oxides have d⁵⁶Fe values consistently ~2 ‰ higher than Fe(II)_(aq). Similarly, Fe(III) _aq and Fe(III)ppt produced by oxidation of Fe²⁺ by Thiobacillus ferrooxidans, an acidophilic Fe²⁺-oxidizing bacteria, are isotopically enriched by ~2.3 ‰ relative to Fe(II). There is no difference in the d⁵⁶Fe values of the Fe(III)_aq and Fe(III)_ppt formed by T. ferrooxidans. In contrast, an abiotic control experiment, set up under identical conditions as those with T. ferrooxidans, results in Fe³⁺_ppt that has 3.9 ‰ higher d⁵⁶Fe values relative to dissolved Fe²⁺. These results suggest that bacterial Fe(II) oxidation may result in iron oxides that are less enriched in d⁵⁶Fe than those produced abiotically. However, when other geochemical influences on isotopic fractionation are considered it may not be possible to discern a unique “bacterial” signal in iron oxides produced from bacterial Fe(II)_aq oxidation.