Paper No. 7
Presentation Time: 9:45 AM
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.