The process of carbonatation in the luvisols in Fresno-Alh‡ndiga (Salamanca, Spain)


Alonso P.1, Dorronsoro C.2 & Egido JA. 1


1. Dpto. de Edafologia. Facultad de Ciencias Agrarias y Mediambientales. Universidad de Salamanca. Espa–a.

2. Dpto. de Edafologia. Facultad de Ciencias. Universidad de Granada. Granada. Espa–a.


The process of carbonatation is widespread in the soils of a post-incisive chronosequence of fluvial terraces formed by the Tormes River in Fresno Alh‡ndiga (Salamanca, Spain). These include 4 surfaces composed of deposits from the erosion of granites and slates dated to c. 40,000 (Aquiultic Haploxeralf), 200,000 (Calcic Palexeralf), 600,000 (Thapto-alfic Typic Rhodoxeralf ) and 1,200,000 years (Ultic-aquic Palexeralf).


The carbonates present various pedofeatures, such as hypo-coatings, infillings, and nodules. In addition, they take the form of mottling and layers and also appear as groundmass. The layers are arrayed horizontally, vertically and on a slope, forming attractive networks. The carbonates of these soils have been affected by a broad range of such processes as crystallization, recrystallization, replacement, displacement, dissolution and recarbonatization. The crystallization gives rise generally to microcrystal and acicular crystals. The recrystallization is well developed and characterized primarily by: crystals varying greatly in size and with a gradual transition between them; the presence of curved or undulating contact between sparite grains; star-like masses having a central zone of microsparite surrounded by sparite crystals; and elongated parallel sparite crystals having a radial fibrous habit and frequently with undulating extinction forming coatings. Replacement affected the silicate materials of the detrital grains of quartz, feldspars, micas (brecciation, floating grains and ghost features), clay domains and clay coatings; and also affected the remains of soil organic matter. Displacement is an uncommon process in these soils. Meanwhile, dissolution is recognized by grains with well-rounded shapes and grains with cone-like tips. Sometimes, when the carbonate grains coated by illuvial clay dissolve, the illuvial clay coatings appear to form a mesh.


The process of recarbonatization reflects the polycyclic character of these soils, forming coatings composed of alternating layers of carbonates and illuvial clay. A positive linear correlation was found between the age of the soils and their content in carbonates.


The origin of these carbonates appears to be an allochthonous supply from vadouse waters, given: i) the absence of carbonates in the original material, ii) the low presence of minerals that by alteration could give rise to substantial accumulations of carbonates, ii) the association of carbonates with the accumulations of Mn compounds, and iv) the network-like distribution of the carbonates, formation that is frequent in the marls and clays of the region.