title: Coupled Fe(II)-Fe(III) electron and atom exchange as a mechanism for Fe isotope fractionation during dissimilatory iron oxide reduction.
abstract: Microbial dissimilatory iron reduction (DIR) is an important pathway for carbon oxidation in anoxic sediments, and iron isotopes may distinguish between iron produced by DIR and other sources of aqueous Fe(II). Previous studies have shown that aqueous Fe(II) produced during the earliest stages of DIR has delta56Fe values that are 0.5-2.0%o lowerthan the initial Fe(III) substrate. The new experiments reported here suggest that this fractionation is controlled by coupled electron and Fe atom exchange between Fe(II) and Fe(III) at iron oxide surfaces. In hematite and goethite reduction experiments with Geobacter sulfurreducens, the 56Fe/54Fe isotopic fractionation between aqueous Fe(II) and the outermost layers of Fe(III) on the oxide surface is approximately -3%o and can be explained by equilibrium Fe isotope partitioning between reactive Fe(II) and Fe(III) pools that coexist during DIR. The results indicate that sorption of Fe(II) to Fe(III) substrates cannot account for production of low-delta56Fe values for aqueous Fe(II) during DIR.
authors.affiliation: Department of Geology and Geophysics, University of Wisconsin-Madison, 1215 West Dayton Street, Madison, Wisconsin 53706, USA. heidic@geology.wisc.edu
authors.forename: Heidi A
authors.initials: HA
authors.lastname: Crosby
authors.forename: Clark M
authors.initials: CM
authors.lastname: Johnson
authors.forename: Eric E
authors.initials: EE
authors.lastname: Roden
authors.forename: Brian L
authors.initials: BL
authors.lastname: Beard
mesh_terms: D000327:Adsorption; D000693:Anaerobiosis; D004583:Electrons; D005290:Ferric Compounds / Q000737:chemistry / Q000378:metabolism*; D005296:Ferrous Compounds / Q000737:chemistry / Q000378:metabolism*; D019015:Geologic Sediments / Q000737:chemistry / Q000382:microbiology*; D007503:Iron Isotopes / Q000737:chemistry*; D010084:Oxidation-Reduction; D013997:Time Factors; D014874:Water Pollutants, Chemical / Q000378:metabolism
chemical_list: D005290:Ferric Compounds; D005296:Ferrous Compounds; D007503:Iron Isotopes; D014874:Water Pollutants, Chemical; C000499:ferric oxide