Changes in micro-fabric and re-distribution of Fe and Mn with nodule formation in a floodplain soil

Abstract

Ferromanganese nodules are common features in aquic soils of the temperate climate. Although they are intensively studied due to their pedogenic significance, there is a lack of knowledge on the relationship between their micro-fabric and distribution of their major chemical components. Our aim was to fill this gap and to relate these characteristics to the different stages of the nodule development in the soil. To fulfill our aims, ferromanganese nodules from a gleyic fluvisol profile was separated in which nodules with strongly varying appearance and fabric are present in a wide depth interval, so they are expected to represent the different stages of nodule formation. Micro-chemical analyses were carried out on the polished surface of 28 nodules. Micro-X-ray fluorescence spectrometry was used to produce Fe and Mn elemental maps of the whole nodules as well as line scan analyses along perpendicular cross sections. Additionally, the spatial distribution and major element associations of Fe and Mn within the individual nodules were studied by point analyses at 775 spots by electron micro-probe analysis. Typic and concentric nodules in the soil exhibited both similar (presence of outer coating band) and different (undifferentiated and banded interiors, respectively) characteristics in their micro-fabric. These were related to the rate of hydromorphism in the soil which was found to determinate the major processes (accretion vs. impregnation) forming the fabric of the nodules. The following stages of the nodule development were distinguished: (1) cementation, (2) formation of outer band, (3) re-arrangement and slow impregnation of nodules’ interior, and (4) fast impregnation of the interior and exfoliation of outer band. We found that separation of Fe and Mn is characteristic of each stage of nodule formation. However, as long as spatial segregation occurs in the first stages, displacement of Mn by Fe is rather typical later. Fabric and appearance of nodules form by varying rate and dominance of accretion and impregnation relatively slowly. However, distribution pattern of Fe and Mn within the nodules may exhibit much faster changes simultaneously. Complex micro-chemical analyses support a powerful tool to follow such changes and to get a deeper insight into the genesis of ferromanganese nodules.