Abstract
New iron isotope data document open-system formation of terrestrial iron oxide concretions and the potentially important role of iron-reducing bacteria in mobilizing iron. These terrestrial insights can provide valuable models for understanding extraterrestrial hematite spherules and their diagenetic history at Meridiani Planum, Mars. Whole-rock samples of Jurassic Navajo Sandstone host rock have δ56Fe values near 0 per mil (‰), whereas concretions typically have negative δ56Fe values. Negative δ56Fe values can be explained by complete oxidation and precipitation from aqueous fluids that had δ56Fe values of −0.5‰ to −1.5‰. The low δ56Fe values for the majority of concretions overlap those of Fe(II)aq and reactive ferric oxides in modern marine sediments where iron-reducing bacteria are actively cycling Fe, suggesting that Fe mobilization in the Navajo Sandstone occurred through bacterial reduction of Fe oxides. Variations in δ56Fe values support an open-system model of concretion formation where Fe is recycled via different chemical reactions involving reduction, mobilization, and precipitation. If the Mars concretions formed in a similarly open system during Fe mobilization and precipitation, their δ56Fe values should also deviate from δ56Fe = 0, dependent upon the pathway, but positive δ56Fe values would be expected for oxides in the absence of a role for microbial redox cycling.
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