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- Title
Iron oxide mineralogy and stable iron isotope composition in a Gleysol with petrogleyic properties.
- Authors
Mansfeldt, Tim; Schuth, Stephan; Häusler, Werner; Wagner, Friedrich; Kaufhold, Stephan; Overesch, Mark
- 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 methods: The 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 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 (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 (CrBg) and was lowest in the 2Cr horizon (7 g kg). 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 δFe values from +0.29‰ (Ah horizon) to −0.30‰ (Bg horizon). In contrast to the overlying CrBg ( δFe = −0.19‰) and Bg horizons, the 2Cr horizon is characterized by a relatively high δFe 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 δFe values in the topsoil primary result from fast ferrihydrite precipitation during aeration immediately after reducing conditions. In contrast, the low δFe values of the Fe-rich horizons (Bg, CrBg) promote adsorption of dissolved Fe with a light isotope composition onto goethite during capillary rise. The high δFe value of the Fe-poor subsoil (2Cr) is related to silicate-bound Fe rather than dissolution and precipitation of Fe oxides.
- Publication
Journal of Soils & Sediments: Protection, Risk Assessment, & Remediation, 2012, Vol 12, Issue 1, p97
- ISSN
1439-0108
- Publication type
Academic Journal
- DOI
10.1007/s11368-011-0402-z