Redox-dependent fractionation of iron isotopes in suspensions of a groundwater-influenced soil

Abstract

Redox conditions control the release of iron (Fe) into soil pore waters. A fluctuating groundwater table in soils results in significant changes in redox conditions with both time and depth. The effects of short-term differences in redox conditions on the stable isotope inventory of dissolved Fe in such soils have not yet been studied.Bulk Fe isotope compositions of a Gleysol yielded δ57Fe values from +0.3‰ (humic topsoil, Ah horizon) to −0.2‰ (Fe-enriched subsoil, CrBg horizon). In microcosm experiments, soil suspensions of the Ah and CrBg horizons were subjected to controlled redox conditions ranging from high redox potential (EH) (>430mV, pH5.1 to 5.6), moderate EH (~330mV, pH4.9 to 5.9), to low EH (≤170mV, pH5.2 to 6.7). Membrane-filtered (0.45μm) solutions taken from the suspensions were analysed for their Fe concentrations (Fe2+, and total Fe: Fetot) and isotopic compositions. The microcosm experiments demonstrated that the ferrihydrite- and organic-rich Ah horizon is a highly dynamic and rapidly responding reservoir with respect to Fe mobilization and isotopic fractionation at low EH. Iron concentrations and isotope ratios of the solutions from the Ah horizon varied depending on EH with negative δ57Fe values (−0.4‰) and Fetot (~1.6mgL−1) at moderate EH, and even lower δ57Fe values (−1.1‰) but high Fetot (~7.8mgL−1) at low EH. At high EH, δ57Fe values slowly increased from +0.3‰ to +1.0‰ and Fetot decreased to ~0.2mgL−1 within six weeks. The goethite-rich CrBg horizon constitutes a stable redox-insensitive pool with very low amounts of mobilized Fe and a small degree of isotopic fractionation, even after exposure to low EH over several weeks.In a natural open system, removal of Fe from the dynamic Ah horizon will result in progressively higher soil δ57Fe values due to preferential release of 54Fe. Vertical movement of a low-δ57Fe solution from the topsoil may result with time in the formation of a subsoil with δ57Fe values that are lower than the topsoil after repeated low and high EH cycles. At high EH, Fe mobility in the Ah horizon is much lowered, but release of Fe with high δ57Fe values is in agreement with earlier studies for the formation of Fe pools (Fe oxides, colloids, organic complexes) with high δ57Fe signatures.