Iron oxide mineralogy and stable iron isotope composition in a Gleysol with petrogleyic properties

Abstract

Purpose Properties of Fe oxides are poorly understood in soils with fluctuating water tables and variable redox conditions. The objective of this research was to (a) characterize the mineralogical composition of Fe oxides and (b) determine the relationship to the stable Fe isotope ratio in a soil with temporally and spatially sharp redox gradients. Materials and methodsThe lowland Gleysol (Petrogleyic) is in Northwest Germany and consists of oximorphic soil horizons (Ah 0-15, Bg 15-35, and CrBg 35-70 cm) developed from Holocene fluvial loam overlaying glaciofluvial sand with reductomorphic properties (2Cr horizon, +70 cm). Field measurements during the course of 28 months included the monitoring of groundwater table, soil redox potential, and analysis of the soil solutions. Solid Fe phases were studied by room temperature and cryogenic 57 Fe Mössbauer spectroscopy, and stable Fe isotope compositions by multiple collector inductively coupled plasma mass spectrometry. Results and discussion The groundwater table ranged from −83 cm below to +8 cm above soil surface (median −27 cm). Permanent reducing conditions occurred in the 2Cr horizon with dissolved Fe concentrations of 44.8 mg L −1 (median). The duration of oxidizing conditions increased in the order CrBg < Bg < Ah. Total Fe increased from 50 (Ah) over 316 (Bg) up to 412 g kg −1 (CrBg) and was lowest in the 2Cr horizon (7 g kg −1 ). Ferrihydrite (51% of total Fe) was dominant over goethite (24%) in the Ah horizon. Conversely, nanogoethite dominated both the Bg (94%) and CrBg (86%) horizons. Iron in siderite amounted to 7% in the CrBg horizon. Iron isotope compositions yielded a range of δ 57 Fe values from +0.29‰ (Ah horizon) to −0.30‰ (Bg horizon). In contrast to the overlying CrBg (δ 57 Fe=−0.19‰) and Bg horizons, the 2Cr horizon is characterized by a relatively high δ 57 Fe value of +0.22‰. Conclusions Lasting water saturation and frequent reducing conditions lead to the enrichment of goethite in subsoil. Once formed, goethite remains stable compared to ferrihydrite because it is less available for microbial mediated reductive dissolution. High δ 57 Fe values in the topsoil primary result from fast ferrihydrite precipitation during aeration immediately after reducing conditions. In contrast,

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Phytic Acid-to-Iron Molar Ratio Rather than Polyphenol Concentration Determines Iron Bioavailability in Whole-Cowpea Meal among Young Women ,

“…Iron isotopic analyses were performed by using a high-resolution, multicollector, inductively coupled plasma mass spectrometer [Thermo-Finnigan Neptune; University of Bonn, Germany; see (24) for details]. Copper was added (1 mg/g) to the solution immediately prior to analysis to correct for mass bias (23,25). Each isotopically enriched solution was measured in triplicate by using standard sample bracketing (23,25,26).…”

“…Copper was added (1 mg/g) to the solution immediately prior to analysis to correct for mass bias (23,25). Each isotopically enriched solution was measured in triplicate by using standard sample bracketing (23,25,26). One-third of the samples were remeasured as external duplicates for quality control.…”

Iron and silicon isotope behaviour accompanying weathering in Icelandic soils, and the implications for iron export from peatlands

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