Iron isotope fractionation during the formation of ferromanganese nodules under different conditions of hydromorphism

Abstract

Hydromorphic soils are often characterized by redoximorphic pedofeatures like ferromanganese nodules. As the highly varying properties of the nodules are sensitive indicators of the pedogenic processes and redox history of the soil, their isotopic study may get a deeper insight into the soil genesis. In this study, the Fe isotopic characteristics, fabric, and mineralogy of ferromanganese nodules formed in six hydromorphic soil profiles were investigated using MC-ICP-MS, EPMA, and XRD analyses. We aimed to relate the Fe isotopic composition of the bulk soils and nodules to the varying conditions of hydromorphism and the various properties of the nodules.Although the extent of hydromorphism could be related to the nodules' properties, such a relationship was not found with their Fe isotopic characteristics. The reductive dissolution of primary Fe minerals resulted in similar Fe isotope distribution characteristics for the bulk soils. However, specific features of hydromorphic conditions (like frequency of the redox cycles, relocation of water fluctuation zone, external Fe input, and inhibition of leaching) affected the Fe isotope characteristics of both bulk soils and nodules. The minimum δ 56Fe value within the soil profile indicated the deepest part where precipitation of light Fe may dominate over its leaching. Above this depth, the isotopically light Fe precipitated to form the nodules. However, the higher frequency of the reductive conditions was responsible for the higher mobilization of isotopically light Fe below this depth. The isotopic characteristics of the nodules supported that authigenic Fe-oxyhydroxides were enriched in the light Fe isotope.The Fe isotope composition of a given soil horizon and its nodules result from a complex interaction of different processes whose effect varies from profile to profile. The integrated study of the Fe isotope characteristics of hydromorphic soils and their nodules may help follow several pedogenic processes and conditions.