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SOIL CHEMISTRY

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

^* Corresponding author (wiederhold{at}env.ethz.ch).

Stable Fe isotopes provide a potential new tool for tracing the biogeochemical cycle of Fe in soils. Iron isotope ratios in two redoximorphic soils were measured by multicollector inductively coupled plasma mass spectrometry to study the relationships between pedogenic Fe transformation and redistribution processes, and mass-dependent fractionation of Fe isotopes. Redoximorphic Fe depletion and enrichment zones were sampled in addition to the bulk soil samples. A three-step sequential extraction procedure was used to separate different Fe pools, which were examined in addition to total soil digests. Significant enrichments of heavy Fe isotopes of about 0.3 in ⁵⁷Fe were found in total soil digests of Fe-depleted zones compared with bulk soil samples and were explained by the preferential removal of light isotopes, presumably during microbially mediated Fe oxide dissolution under anoxic conditions. Accordingly, pedogenic Fe enrichment zones were found to be slightly enriched in light Fe isotopes. Distinct Fe isotope variations of >1 in ⁵⁷Fe were found between different Fe pools within soil samples, specifically enrichments of light isotopes in pedogenic oxides contrasting with heavy isotope signatures of residual silicate-bound Fe. Our data demonstrate that pedogenic Fe transformations in redoximorphic soils are linked to isotope fractionation, revealing greater mobility of lighter Fe isotopes compared with heavier isotopes during pedogenesis. No simple quantitative relationship between Fe depletion and isotope fractionation could be inferred, however. Our findings provide new insights into the behavior of Fe isotopes in soils and contribute to the development of Fe isotopes as a tracer for the biogeochemical Fe cycle.