Gehalte an Haupt- und Nebenelementen in Konkretionen aus Oberböden von Marschen – Untersuchungen mit einer Mikrosonde

Contents of major and trace elements in concretions of surface samples from marsh soils – Electron microprobe analysis The amounts of 14 elements (Ti, Fe, Mn, Si, Al, Ca, and P, S, Co, Ni, Cu, Zn, Cd, Pb) in concretions of four A horizons of marsh soils were determined by electron microprobe analysis on thin sections. According to the major element compositions the following types of concretions can be distinguished: Ti-Fe(-Mn) concretions (31–39% Ti; 18–29% Fe; 0,3–5,5% Mn); Fe(-Mn) concretions (31–58% Fe; ≤ 1,8% Mn); Fe-Si(-Mn) concretions (6–28% Fe; 8–42% Si; ≤ 5% Mn); Mn-Fe and Mn-Fe-Si concretions (16–40% Mn; 2–9% Fe; 2–15% Si). The formation of Ti-Fe(-Mn) concretions can be explained by adsorption of Fe²⁺ ions under reducing conditions on surfaces of Ti minerals followed by oxidation to Fe(III) oxide. In this way pseudorutile may be formed. Concretions with high Fe and/or Mn content often contain occlusions of carbonates, silicates and/or SiO₂ (bioopal) which may act as nuclei for their growth. Redox processes in marsh soils result in the formation of concretions with dominating Fe or Mn accumulation. Among the group of trace elements, P and S (like Ti and Si) reach their highest concentrations in Fe rich concretions (up to 54.000 mg P/kg and 4.200 mg S/kg). The accumulation of these elements is obviously influenced by anion competition, especially of P and Si, and probably also Ti. Likewise, high amounts of Pb were found in Fe(-Mn) concretions (up to 12.000 mg/kg). Co, Ni, Zn, and Cd show the highest amounts in Mn rich concretions (Co up to 3.400; Ni 1.800; Zn 13.200; Cd 1.000 mg/kg). Cu is also accumulated in some concretions (up to 1140 mg/kg) but no clear affinity to one of the different types of concretions was found.