LIMITED IRON ISOTOPE FRACTIONATION IN CONCRETIONS PRODUCED BY IRON-OXIDIZING BACTERIA, NAVAJO SANDSTONE, UTAH (USA)

The search for biosignatures for Fe(II)-oxidizing bacteria (FeOB) has included measurement of both laboratory and natural fractionation of Fe isotopes. Fe(III)-oxides produced by FeOB in the lab have significantly higher δ⁵⁶Fe values (by approximately 2‰) than reduced precursors. Values of δ⁵⁶Fe_{Fe(III)mineral}-δ⁵⁶Fe_Fe(II)aq in modern settings are less than expected based on experiment. We have examined Fe-oxide cements that we interpret to be products of FeOB in an evolving groundwater system; an exhumed Navajo SS aquifer. These Fe-oxide cements exhibit extreme variations in δ⁵⁶Fe values; values published previously are as low as -1.4‰ and we have measured values as low as -0.86‰. Although these values are unusual for Fe-oxides, they are typical for reduced species such as siderite or pyrite. In our model, mantle-derived CO₂ and trace CH₄ migrated to Navajo SS aquifers sealed by the Jurassic Carmel Formation. The CH₄ and CO₂ dissolved in formation waters, reduced Fe(III)-oxide that coated sand grains and bleached the rock. The dense, CO₂-charged waters descended in the aquifer, degassed locally and produced siderite. The Colorado Plateau was uplifted and the Navajo SS exhumed by erosion. Oxidizing waters invaded the aquifer and oxidized the siderite. FeOB colonized redox interfaces between siderite-cemented sandstone and uncemented sandstone. The FeOB oxidized aqueous Fe(II) liberated from dissolving siderite. Fe-oxide rhombohedra that we interpret to be psuedomorphs after siderite cement have δ⁵⁶Fe values of -0.59‰. The Fe-oxide rind that comprises the large box-work concretion that encloses these rhombohedra has a δ⁵⁶Fe value of -0.37‰. We interpret the rind to have been generated by microbial oxidation of Fe(II) that diffused from dissolving siderite in the core of the concretion; whereas the Fe-oxide rhombohedra were produced by abiotic oxidation of siderite (oxidation that occurred with no cm-scale migration of Fe). We interpret the limited fractionation to be the product of near-closed system oxidation of Fe(II). The Fe-oxide mineralization of the Colorado Plateau is an ideal environment to test hypotheses regarding Fe isotope fractionation by FeOB. The concretions have a geometry that lends itself to such tests and the δ⁵⁶Fe values of reduced precursors are preserved.