Iron isotope fractionation by microbial iron reduction in modern chemically precipitated sediments

Details Iron isotope fractionation by microbial iron reduction in modern chemically precipitated sediments Iron isotope fractionation by microbial iron reduction in modern chemically precipitated sediments

Dec 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.p53d..06r&link_type=abstract

American Geophysical Union, Fall Meeting 2008, abstract #P53D-06

Biology

0424 Biosignatures And Proxies, 0448 Geomicrobiology, 0463 Microbe/Mineral Interactions, 1041 Stable Isotope Geochemistry (0454, 4870), 5225 Early Environment Of Earth

Scientific paper

Laboratory experiments have demonstrated that dissimilatory microbial iron oxide reduction (DIR) can produce Fe(II) phases that have low 56Fe/54Fe ratios similar to those found in Neoarchean and Paleoproterozoic banded iron formations (BIFs) and shales. Direct application of these experiments to BIF formation has been hindered by the lack of Fe isotope data from modern environments that are analogous to BIFs. Here we report Fe inventories and isotopic compositions for chemically precipitated sediments in the Spring Creek Arm of Keswick Reservoir (SCAKR) downstream of the Iron Mountain acid mine drainage site in northern California, USA. The high concentration of reactive Fe(III) (ca. 50-100 mmol of amorphous Fe(III) oxyhydroxides per liter of bulk sediment) allows dissimilatory iron-reducing bacteria (DIRB) to predominate over dissimilatory sulfate-reducing bacteria in sediment carbon metabolism, making the SCAKR a better analog for BIFs compared to modern marine environments. DIR has generated millimolar concentrations of aqueous Fe(II) (Fe(II)aq) in SCAKR sediments. The Fe(II)aq has lower 56Fe/54Fe values than bulk HCl-extractable Fe; δ56Fe values for bulk HCl-extractable Fe fall within the range previously defined for mafic- to intermediate-composition terrestrial igneous rocks, modern marine sediments, suspended river sediments, Proterozoic-Phanerozoic shales, loess, aerosols, and soils. After removal of pore fluid, sediment was reacted for 1 hr with 0.1M HCl to isolate solid-phase Fe(II) (Fe(II)s), which was likely a mixture of sorbed Fe(II) and amorphous surface-precipitated Fe(II) hydroxide. Subsequent 24-hr extraction with 0.5M HCl recovered amorphous Fe(III) oxide (Fe(III)am). Sediment incubation experiments with SCAKR sediment produced trends in in Fe isotopic fractionations between Fe(II)aq, Fe(II)s, and Fe(III)am analogous to those observed in situ. Collectively the data suggest an equilibrium 56Fe/54Fe isotope fractionation between Fe(II)aq and Fe(III)am on the order of -2 per mil, and a smaller but significant fractionation between Fe(II)aq and Fe(II)s of -0.4 per mil. Our results illustrate how DIR could have produced large quantities of mobile Fe with low-δ56Fe values during early sediment diagenesis in Neoarchean and Paleoproterozic BIFs.

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