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Iron isotope fractionation in oxic soils by mineral weathering and podzolization
Authors:Jan G Wiederhold  Nadya Teutsch  Stephan M Kraemer  Alex N Halliday  Ruben Kretzschmar
Institution:a Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, CHN, 8092 Zurich, Switzerland
b Institute of Isotope Geochemistry and Mineral Resources, ETH Zurich, NW, 8092 Zurich, Switzerland
c Institute of Earth Sciences, Hebrew University of Jerusalem, 91904 Jerusalem, Israel
d Department of Environmental Geosciences, University of Vienna, Althanstrasse 14, 1090 Vienna, Austria
e Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK
Abstract:Stable iron isotope ratios in three soils (two Podzols and one Cambisol) were measured by MC-ICPMS to investigate iron isotope fractionation during pedogenic iron transformation and translocation processes under oxic conditions. Podzolization is a soil forming process in which iron oxides are dissolved and iron is translocated and enriched in the subsoil under the influence of organic ligands. The Cambisol was studied for comparison, representing a soil formed by chemical weathering without significant translocation of iron. A three-step sequential extraction procedure was used to separate operationally-defined iron mineral pools (i.e., poorly-crystalline iron oxides, crystalline iron oxides, silicate-bound iron) from the soil samples. Iron isotope ratios of total soil digests were compared with those of the separated iron mineral pools. Mass balance calculations demonstrated excellent agreement between results of sequential extractions and total soil digestions. Systematic variations in the iron isotope signature were found in the Podzol profiles. An enrichment of light iron isotopes of about 0.6‰ in δ57Fe was found in total soil digests of the illuvial Bh horizons which can be explained by preferential translocation of light iron isotopes. The separated iron mineral pools revealed a wide range of δ57Fe values spanning more than 3‰ in the Podzol profiles. Strong enrichments of heavy iron isotopes in silicate-bound iron constituting the residue of weathering processes, indicated the preferential transformation of light iron isotopes during weathering. Iron isotope fractionation during podzolization is probably linked to the ligand-controlled iron translocation processes. Comparison of iron isotope data from eluvial and illuvial horizons of the Podzol profiles revealed that some iron must have been leached out of the profile. However, uncertainties in the initial iron content and iron isotopic composition of the parent materials prevented thorough mass balance calculations of iron fluxes within the profiles. In contrast to the Podzol profiles, the Cambisol profile displayed uniform δ57Fe values across soil depth and showed only a small enrichment of light iron isotopes of about 0.4‰ in the poorly-crystalline iron oxide pool extracted by 0.5 M HCl. This work demonstrates that significant iron isotope fractionations can occur during pedogenesis in oxic environments under the influence of organic ligands. Our findings provide new insights into fractionation mechanisms of iron isotopes and will help in the development of stable iron isotopes as tracers for biogeochemical iron cycling in nature.
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