Can Fe isotope fractionations trace the pedogenetic mechanisms involved in podzolization?

Abstract

Stable Fe isotopes have shown the potential for tracing pedogenetic processes. Large isotopic fractionations were especially observed in Podzols. Nevertheless, a clear link between isotopic fractionation and elementary processes still needs to be established. To spatially distinguish the mechanisms successively involved in pedogenesis, we studied a podzolic chronosequence from Vancouver Island (British Columbia, Canada). We analyzed depth variations in soil properties (pH, particle size fractions, organic carbon, Si/Al ratio, Zr, Ti), Fe concentrations in different Fe pools, and Fe isotopic compositions. The Si/Al ratio, Zr, and Ti demonstrated that the Cox Bay Podzols developed from the same parental material, satisfying the requirements of a chronosequence. We showed that acidification, to a pH of 4.7, was a prerequisite for the start of podzolization. This first phase took place after 270years. Furthermore, we observed that once the required pH was reached (4.7), podzolization occurred rapidly over 50years. We found that Fe isotope fractionation in the A/E horizon was linked to mineral dissolution, while in the podzolic B horizons this fractionation was clearly associated with the accumulation of Fe-organic complexes and poorly crystalline Fe oxyhydroxides.