Iron isotope composition of aqueous phases of a lowland environment

Abstract

Environmental context Iron (Fe) isotope analysis is a powerful tool to understand the transport of Fe within and from soils to rivers. We determined Fe isotopes and Fe concentrations of soil solutions at different depths and found that the Fe isotope compositions are modified owing to adsorption onto Fe oxides, especially in the subsoil. Hence Fe-rich capillary rising groundwater or seeping Fe-rich surface water are depleted in Fe and potentially other metals in Fe oxide-rich soil horizons. Abstract The mobility of iron (Fe) in soils is strongly affected by redox conditions, which also affect Fe input into groundwater and rivers. Stable Fe isotope analyses allow further investigation of Fe translocation processes within, into and out of soils. Soil solutions taken from a Gleysol in a lowland area (NW Germany) at different depths revealed that Fe concentration and isotope ratios strongly varied with abundance of solid Fe oxides. Low δ56Fe values of –1.7 ‰ and minimum Fe concentrations of ~0.2mgL–1 were recorded in soil solutions of Fe-rich horizons. Soil solutions of a Fe-poor horizon, however, yielded higher δ56Fe values (–0.39 ‰) and Fe concentrations of up to 68mgL–1. The water of an adjacent drainage ditch featured δ56Fe values of –1.1 ‰, in strong contrast to +0.60 ‰ of short-range ordered Fe oxide deposits in the ditch bed. We attribute the coupled low δ56Fe values and Fe concentrations to combined adsorption and atom exchange between dissolved Fe and Fe oxides. Consequently Fe oxide-poor horizons had higher δ56Fe values and dissolved Fe concentrations. Outflow of Fe-rich groundwater and surface water during rainfall into rivers is responsible for high δ56Fe for Fe-oxide precipitates and low riverine δ56Fe values.