DISTRIBUTION OF MAJOR AND TRACE ELEMENTS AT THE AGGREGATE SCALE IN A SOIL NATURALLY RICH IN TRACE ELEMENTS: A SPATIAL APPROACH USING ELECTRON MICROPROBE AND X-RAY MICROFLUORESCENCE ANALYSES.

Major and trace element concentrations have been determined from thin sections of an unpolluted and well-structured B horizon that presents macroscopic heterogeneous aggregates. Electron scanning microprobe and X-ray microfluorescence analyses have enabled the characterization of Fe, Si, Al, Ca, K, Cu, Cr, Co, Zn, and Ni concentrations. The structure of the aggregate was demonstrated as being complex, with an inner part richer in Fe and fine material and an outer part with abundant small quartz minerals. Iron, Cr, and Cu concentrations were much higher in the inner part of the soil aggregates, whereas Zn and Ni concentrations were not related to any location in the aggregate. Water logging during winter has been identified as the major process that modifies the spatial location of Fe in the aggregates. Swelling and shrinking caused by seasonal changes have been demonstrated as responsible for falls of allochthonous material from the upper part of the soil profile that results in modifications of trace elements concentrations in the outer part of the aggregates. This study showed the ability of quantitative microscopic techniques in describing the trace element spatial distribution at the aggregate scale in soils.