Iron isotope fractionation of redox and geochemical cycling in the typical Gleysols in Mun River Basin, Northeast Thailand

Abstract

Iron (Fe) isotope is a potential tool for tracking redox process and geochemical cycling in terrestrial environment. In this study, Fe concentration and its isotopic composition (δ56Fe) in two typical Gleysol profiles (M1 and M2) were investigated to distinguish the processes which influence the variation of Fe isotopic composition during redox regimes in the Mun River Basin (MRB). Under oxidizing condition, Fe(II) was oxidized and re-precipitated to form Fe(III) (hydr)oxides zone (Fe nodule-containing zone) in two Gleysol profiles, leading to extremely light Fe isotopes in these zones. The results revealed that the lowest δ56Fe value in Fe(III) (hydr)oxides zone was derived from the migration of light Fe isotopes in upper zone, and Fe(II) was retained and oxidized to Fe(III) (hydr)oxides. Proton-promoted dissolution and leaching were two critical factors leading to a decrease in Fe concentration, which were accompanied by the accumulation of heavy Fe isotopes in the upper zone of M1 profile. In M2 profile, light Fe induced by soil organic matter was accumulated in the topsoil with abundant organic matter. These findings provide comprehensive information of Fe isotopic fractionation and Fe cycling in soil profiles, which would contribute to the understanding of biogeochemical elemental cycling in terrestrial ecosystems.