Iron species in soils on a mofette site studied by Fe K-edge X-ray absorption near-edge spectroscopy

Geogenic CO2, which ascends on so-called mofette sites, may affect development and properties of soils. Therefore, we studied soils on a mofette site in the Czech Republic at three spots differing in the partial pressures of soil CO2 (p(CO2) = 0.04, 1, and 1). We recorded 69 Fe K-XANES spectra in 5 regions of interest on 3 thin sections in fluorescence mode. Iron was enriched in filled pores, e.g., former root channels. Generally, most of the Fe was identified in different clay minerals (smectites, illites and chlorites). Minor pedogenic Fe contributors were ferrihydrite, green rust, magnetite, maghemite, vivianite and siderite, whereas typical soil Fe (hydr)oxides such as goethite, hematite or lepidocrocite were never doubtlessly identified. The Fe(II)-containing minerals were particularly detected at p(CO2) = 1 indicating an effect of soil CO2 on the formation of secondary Fe phases. Organic Fe complexes were less important, and their formation seemed to be inhibited at p(CO2) = 1. We thus conclude that geogenic CO2 affects pedogenesis and causes quantitative and qualitative variations of Fe speciation.