Iron compounds in soil are multifunctional, providing physical structure, ion sorption sites, catalytic reaction-centers, and a sink for respiratory electrons. Basaltic soils contain large quantities of iron that reside in different mineral and organic phases depending on their age and redox status. We investigated changes in soil iron concentration and its solid-phase speciation across a single-aged (400ky) lava flow subjected to a gradient in precipitation (2200-4200mmyr-1) and hence redox history. With increasing rainfall and decreasing Eh, total Fe decreased from about 25% to <1% of the soil mass. Quantitative speciation of soil solid-phase iron was constrained by combining 57Fe Mössbauer spectroscopy (MBS) at 295 and 4.2K with powder X-ray diffraction, selective chemical extractions, and magnetic susceptibility measurements. This approach allowed us to partition iron into (1) nanoparticulate and microcrystalline FeIII-(oxy)hydroxides, (2) microcrystalline and bulk FeIII-oxides, (3) organic/silicate bound FeIII, and (4) ferrous iron. The FeIII-(oxy)hydroxide fraction dominated solid-phase Fe, exhibiting a crystallinity continuum based on magnetic ordering temperature. The continuum extended from well-ordered microcrystalline goethite through nanocrystalline FeIII-(oxy)hydroxides to a nano FeIII-(oxy)hydroxide phase of extremely low crystallinity. Magnetic susceptibility was correlated (R2=0.77) with FeIII-oxide concentration, consistent with a contribution of maghemite to the otherwise hematite dominated Fe-oxide fraction. The FeIII-(oxy)hydroxide fraction of total Fe decreased with increasing rainfall and was replaced by corresponding increase in the organic/silicate FeIII fraction. The crystallinity of the FeIII-(oxy)hydroxides also decreased with increasing rainfall and leaching, with the most disordered members of the crystallinity continuum, the nano FeIII-(oxy)hydroxides, gaining proportional abundance in the wetter sites. This finding runs counter to the conventional kinetic expectation of preferential removal of the most disordered minerals in a reductive dissolution-dominated environment. We suggest the persistence of highly disordered Fe phases reflects the dynamic redox conditions of these upland soils in which periods of anoxia are marked by high water-throughput and Fe2+(aq) removal, while periodic Fe oxidation events occur in the presence of high concentrations of organic matter. Our 57Fe Mössbauer study shows basalt-derived nano-scale FeIII phases are more disordered than current synthetic analogs and have nano-structural characteristics that are linked to their formation environment.
ASJC Scopus subject areas
- Geochemistry and Petrology