Iron Isotope Fractionation during Pedogenesis in Redoximorphic Soils

Abstract

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 δ57Fe 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 δ57Fe 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.