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Iron Isotope Fractionation during Pedogenesis in Redoximorphic Soils

^a Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, CHN, 8092 Zurich, Switzerland, and Institute of Isotope Geochemistry and Mineral Resources, ETH Zurich, NW, 8092 Zurich, Switzerland
^b Institute of Isotope Geochemistry and Mineral Resources, ETH Zurich, NW, 8092 Zurich, Switzerland, and Institute of Earth Sciences, Hebrew Univ. of Jerusalem, 91904 Jerusalem, Israel
^c Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, CHN, 8092 Zurich, Switzerland, and Department of Environmental Geosciences, Univ. of Vienna, Althanstrasse 14, 1090 Vienna, Austria
^d Institute of Isotope Geochemistry and Mineral Resources, ETH Zurich, NW, 8092 Zurich, Switzerland, and Dep. of Earth Sciences, Univ. of Oxford, Parks Road, Oxford, OX1 3PR, UK
^e Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, CHN, 8092 Zurich, Switzerland

View larger version (113K): [in this window] [in a new window]   Fig. 1. Photographs of the Stagnic Cambisol (near Rafz, Switzerland) and the Haplic Gleysol (near Tettnang, Germany). Scale bars indicate 10-cm sections. Both profiles exhibit clear redoximorphic features. Horizon designations correspond to the collected samples and follow FAO (2006): O = organic surface layer, i = slightly decomposed organic material, e = moderately decomposed organic material, Ah = organic-rich mineral horizon, B = subsurface mineral horizon altered by pedogenic processes, w = weathered, C = soil substrate, c = concretions or nodules, g = stagnic conditions, l = capillary fringe mottling (gleying), r = strong reduction; prefix numbers indicate lithogenic discontinuities, suffix numbers indicate vertical subdivision of horizon.

View larger version (16K): [in this window] [in a new window]   Fig. 2. Depth profiles of total Fe concentration in the two soil profiles. The redoximorphic features of the Bcg2 and the CBg horizon of the Stagnic Cambisol and of the 2Blc1 and the 2Blc2 horizon of the Haplic Gleysol were sampled separately. Open symbols indicate the calculated sum of the three Fe fractions that were separated by sequential extractions (Fig. 4). Horizon designations follow FAO (2006): B = subsurface mineral horizon altered by pedogenic processes, C = soil substrate, c = concretions or nodules, g = stagnic conditions, l = capillary fringe mottling (gleying); suffix numbers indicate vertical subdivision of horizon.

View larger version (15K): [in this window] [in a new window]   Fig. 3. Depth profiles of 57Fe values in total soil digests of the two soil profiles relative to the international Fe isotope standard IRMM-014. Error bars represent 2 SD of replicate measurements. The redoximorphic features of the Bcg2 and the CBg horizon of the Stagnic Cambisol and of the 2Blc1 and the 2Blc2 horizon of the Haplic Gleysol were sampled separately. Open symbols indicate the calculated isotope mass balance of the three Fe fractions that were separated by sequential extractions (Fig. 4 and 5). Horizon designations follow FAO (2006): B = subsurface mineral horizon altered by pedogenic processes, C = soil substrate, c = concretions or nodules, g = stagnic conditions, l = capillary fringe mottling (gleying); suffix numbers indicate vertical subdivision of horizon.

View larger version (20K): [in this window] [in a new window]   Fig. 4. Depth profiles of Fe concentrations for the three Fe fractions poorly crystalline Fe oxyhydroxides (FeHCl), crystalline Fe oxyhydroxides (FeNH2OH-HCl), and silicate-bound Fe (Feresidue) for bulk soil samples and depleted and enriched zones. Note different concentration scale bars.

View larger version (20K): [in this window] [in a new window]   Fig. 5. Depth profiles of 57Fe for the three Fe fractions poorly crystalline Fe oxyhydroxides (FeHCl), crystalline Fe oxyhydroxides (FeNH2OH-HCl), and silicate-bound Fe (Feresidue) for bulk soil samples and depleted and enriched zones relative to the international Fe isotope standard IRMM-014. Error bars represent 2 SD of replicate measurements. Note different 57Fe scale in Fig. 5C.

View larger version (18K): [in this window] [in a new window]   Fig. 6. Difference in Fe concentration and Fe isotope ratios in the enriched and depleted zones of the Stagnic Cambisol profile relative to bulk soil samples. 57Fe represents Fe isotope fractionation relative to the bulk soil sample (57Fe = 57Fedepleted – 57Febulk or 57Fe = 57Feenriched – 57Febulk); FeHCl represents poorly crystalline Fe oxyhydroxides, FeNH2OH-HCl represents crystalline Fe oxyhydroxides, and Feresidue represents silicate-bound Fe. Error bars of 57Fe were adapted according to Eq. [4–6]. Horizon designations follow FAO (2006): B = subsurface mineral horizon altered by pedogenic processes, C = soil substrate, c = concretions or nodules, g = stagnic conditions; suffix numbers indicate vertical subdivision of horizon.

View larger version (15K): [in this window] [in a new window]   Fig. 7. Difference in Fe concentration [%] and Fe isotope ratios in the enriched zones of the Haplic Gleysol profile relative to bulk soil samples. 57Fe represents Fe isotope fractionation relative to the bulk soil sample (57Fe = 57Feenriched – 57Febulk) ; FeHCl represents poorly crystalline Fe oxyhydroxides, FeNH2OH-HCl represents crystalline Fe oxyhydroxides, and Feresidue represents silicate-bound Fe. Error bars of 57Fe were adapted according to Eq. [4–6]. Horizon designations follow FAO (2006): B = subsurface mineral horizon altered by pedogenic processes, C = soil substrate, c = concretions or nodules, l = capillary fringe mottling (gleying); suffix numbers indicate vertical subdivision of horizon.