Iron isotope fractionation in soils - From phenomena to process identification

Abstract

Stable iron isotope ratios in three natural soil profiles were measured by MC-ICPMS to trace pedogenic iron transformation and translocation processes under oxic conditions. The motivation for this work was to elucidate the relationships between geochemical processes, responsible for mineral weathering and pedogenesis in oxic soils, and mass-dependent fractionations of iron isotopes. Two Podzols that are characterized by intense pedogenic vertical iron translocation within the soil profile and one Cambisol without pedogenic iron translocation were investigated. A three-step sequential extraction procedure was used to separate operationally-defined iron mineral pools (poorly-crystalline iron oxyhydroxides, crystalline iron oxides, silicate-bound iron) from the soil samples. Iron isotope ratios of total soil digests were compared to those of the separated iron mineral pools. Mass balance calculations demonstrate excellent agreement between results of sequential extractions and total soil digestions. Systematic variations in the iron isotope signature were found in the Podzol profiles that were formed by pedogenic vertical transport of iron under the influence of organic ligands and acidic conditions (soil pH < 4). An enrichment of light iron isotopes of about 0.6 ‰ in δFe 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 exhibit a wide range of δFe values of more than 3 ‰ in the Podzol profiles. Strong enrichments of heavy iron isotopes in silicate-bound iron, which constitutes the residue of weathering processes, indicate the preferential transformation of light iron isotopes during weathering. In contrast to the Podzol profiles, the Cambisol profile displayed uniform δFe values over soil depth and showed only a small enrichment of light iron