Carbonatation in palaeosols  formed on terraces of the Tormes river basin (Salamanca, Spain)

Alonso, P.*; Dorronsoro, C.**; Egido, J. A.*

* Departamento de Edafología. Universidad de Salamanca. Spain
** Departamento de Edafología y Química Agrícola. Universidad de Granada. Spain.


1. Introduction

Accumulations of secondary carbonates are common in palaeosols formed in the fluvial terraces of the Tormes river basin (Salamanca, central-western Spain). These soils formed fundamentally from deposits of eroded granites, sandstones, slates and quartzites, but notably lack carbonate materials. Secondary carbonates commonly fill cracks, forming laminae in preferentially horizontal disposition (though also tipped and vertical), and give rise to distinctive patterns that have long attracted scientific attention (Huguet del Villar, 1937; Glinka, 1963; Roquero and Ontañon, 1966).

In the present study, we characterize the paleosols of the Tormes river basin in relation to the presence of carbonate accumulations in these soils. To do so, we identify the origin and trace the genesis of the carbonates of these soils, describe the micromorphological characteristics of the accumulations, determine the processes of formation and transformation affecting the soils, and finally establish relationships between carbonatation and soil age and paleoclimatic conditions.


2. Area description, materials and methods

A total of 21 soils were studied in fluvial terraces of the Tormes river basin (Table 1). Over geological time this river has left a typical steeped relief, with abundant horizontal surfaces between abrupt scarps, which according to Vreeken (1975) are post-incisive sequences. The river terraces analysed are located near the villages of Macotera (UTM coordinates: TL 03014/45212 and 03125/45240 of National Grid Map 479), Fresno Alhándiga (TL 02817/45111 and 02932/45060 of National Grid Map 503), Calvarrasa de Abajo (TL 02594/45511 and 02605/4548 of National Grid Map 451) and San Pedro del Valle (UL 03084/45270 and 03124/45232 of National Grid Map 479).

In general, the terraces are well preserved, occupying a few km2 of surface area, sometimes dissected by water courses of surface runoff.  These terraces are formed by non-consolidated deposits some 1.5 to 5 m thick of coarse and very coarse materials (2 to 200 mm in diameter) embedded in a fundamentally sandy matrix.  In the rock fragments, quartz and quartzite predominate, with subangular forms and without showing alteration, accompanied by granite (very rounded and strongly altered), and, in lesser proportions, cobbles of slate, phyllites, flattened angular, moderately altered schists (sometimes encrusted with Mn compounds), and sandstone fragments.  The granite gravels are more abundant in the younger terraces and in their superficial horizons, disappearing almost completely in the soils of the high terraces.  The sands have a mineralogical composition of quartz, feldspars (potassium and plagioclases of albite), and micas (moscovite and biotite); the clays were composed of smectite, illite and kaolinite (Dorronsoro, 1994; Dorronsoro and Alonso, 1994; Alonso et al., 1994).

The descriptions of the morphological properties of the soils were made by conventional methods (Soil Survey Staff, 1999). All of the samples were air-dried and screened to 2 mm, and the percentages of gravels (>2mm) and fine earth (<2mm) were determined. The laboratory analyses were made with the fine-earth fraction. Particle-size distribution was measured by the pipette method after eliminating organic matter with H2O2 and dispersion with sodium hexametaphosphate (Loveland and Whalley, 1991). The pH was measured potentiometrically in a 1:2.5 soil-water suspension, in a CRISON Digit 501 instrument. The CaCO3 equivalent was determined manometrically by the Barahona method (1984). Total carbon was measured by dry combustion with a LECO mod. SC-144DR instrument. Organic carbon was calculated as the difference between total carbon and inorganic carbon from CaCO3. The cation-exchange capacity (CEC) was determined with 1N Na-acetate at pH 8.2 (Rhoades, 1982), measuring the sodium in a METEOR NAK-II flame-photometer. For the micromorphological study, the polymerized samples, previously embedded in polyester resin (chronolite 1108, with an activating and catalysing solvent) were cut and polished to a thin layer, using a Logitech PM2A instrument.

   The local climate is classified as subhumid (mean annual rainfall 412 mm) and mesic (mean annual T, 11°C), within the continental Mediterranean type. The soils lie on a supramediterranean bioclimatic base with an alkaline substratum. The climax vegetation is composed of oaks (Genisto-Histricis-Quercetum rotundifoliae sigmetum association). In certain areas this vegetation has been altered by cultivation.

   The soil surfaces have been dated by Santonja et al. (1976, 1982, 1984) and by the “Instituto Geológico y Minero de España” (1982), mainly by archaeological and stratigraphic methods. Dorronsoro and Alonso (1994) reported very good results on correlating the age and the evolution of the soils of the chronosequence of Macotera, providing comparisons with other chronosequences having similar soil-forming factors and dated with radiometric methods.

3. Results

3.1. The soils

In all cases, the soils were Xeralfs (Soil Survey Staff, 1999), the youngest being Haploxeralfs and the oldest, Palexeralfs. Of the latter, the subgroup Calcic corresponded to the oldest soils of each series. Table 2 summarizes the characteristics of the three types of most representative soils. These were 21 relict palaeosols that on 3 occasions buried (fossil soils. according to Reuter, 2000). Following the classification of Nettleton et al. (2000) these are Truncated, Oxidized, Unleached, Enduric Paleoevolvisol, residual, extensive and, in the case of the 3 buried soils, Truncated, Oxidized, Unleached, Kryptic Paleoevolvisol, residual, extensive.

The soils were characterized by: substantial thicknesses, high gravel contents, differing textures, an argillic horizon of a clayey texture, acidic pH, low soluble-salt contents and high degree of base saturation.

3.2. The carbonates in the soil profile

Pedogenic carbonates were widely distributed in these palaeosols, although their contents were generally not extraordinarily high (mean content 11%, maximum 51 %; Table 3).

3.2.1. Forms. The carbonates appeared in varied forms, both macro- and microscopic, and were either isolated or massive. When examined individually, they appeared as pseudomycelia, coatings, hypo-coatings, nodules and laminae (Table 3). The pseudomycelia (Fig. 1a) were comprised generally of small needle-fibre calcite—that is, elongated, highly developed crystals forming fine fibres of some 2µm in diameter by 30µm in length, creating a fine interlacing partially filling in pores, (which were invariably intrapedal and in discrete cavities, channels and chambers). The needle-fibre calcite and micro-rod forms are well known in terms of morphology and crystallography; they are thought to originate from fungal biomineralization (Callot et al., 1985; Verrecchia and Verrecchia, 1994; Verrecchia and Dumont, 1996; Bezce –Deak et al., 1997; Loisy et al., 1999) and are also considered to be purely physico-chemical features (Riche et al., 1982; Verges et al., 1982; Verrecchia and Verrecchia, 1994).

The coatings and hypo-coatings covered the surfaces of the aggregates, gravels and the walls of the pores. Macroscopically, these appeared with some frequency, covering the undersides of the gravel pebbles and reaching great thicknesses (up to 1 cm), with thicker coatings appearing at the bottom of larger clasts (Treadwell-Steitz and McFadden, 2000). Microscopically, the coatings and hypo-coatings were abundant on the walls of the pores, the former type more common than the latter. Both types of coatings appearing within any type of void (most commonly within channels, cavities and cracks) were irregularly distributed, with some coated pores occurring next to others completely lacking any calcareous accumulation. In rare cases, the coatings of neighbouring pores fused (Fig. 1b) and filled in the entire pore (infillings).

The size of the crystals varied markedly with, micrites being more abundant than sparry calcite. At times, the coatings were formed by needle-fibre calcite crystals precipited parallel to each other and to the walls of the pores, which were in turn covered with illuvial-clay coatings. (Fig. 1c).

The nodules varied considerably in size and shape, and exhibited a certain rounded tendency. In profile, these were distinctively white, at times with a sharp border, but generally indistinct. Some of these were soft and crumbly whereas others were hard, the latter commonly showing signs of having undergone surface dissolution. Microscopically, these were far less abundant than the coatings, had a micritic composition (in somecases sparry), and generally had distinct borders. Some nodules fused, giving rise to broad carbonate-rich zones with a typical crystalline texture.

Horizontal laminae were common in the carbonates of these soils, although tipped and vertical laminaeare also present. In some cases the intersection of various directions of lamination caused distinctive interlacing (Fig. 2a y 2b), but this occurred only in the C horizon, in sandy materials without gravel. Generally, the laminae were thin (< 1 cm). Microscopically, these were micritic and massive in distribution, although in somecase laminar fragments intruded from the Bt horizon immediately above. The laminae exhibited moderate porosity in the form of cracks, channels and cavities. As in the previous cases, the borders were generally clearly distinguishable.

Figure 2. (2a) Distinctive networks of horizontal lamination of carbonates in the C horizon of profile TP38a. (2b) Detail of the intersection of various directions of lamination in carbonates causing distinctive interlacing in the C  horizon of profile AM64.

Finally, in the horizons with the highest carbonate content, the CaCO3 distribution was massive.

3.2.2. Distribution of the carbonates in the profile. The pedogenic carbonates in these soils followed a classical distribution, with several leached surface horizons to 100 cm in depth overlying horizons containing carbonate accumulations, and these, in turn, overlying deeper leached horizons. Generally, the uppermost part of the calcic horizon abruptly registered the highest values, then declined gradually with depth until reaching the leached zone.

3.3. Carbonates and parent material

The fluvial materials that comprise the terraces was derived from the erosion of siliceous rocks, primarily granite and sandstone, together with minor proportions of slates and quartzites. It is important to emphasize that in no case was any carbonate detrital grain found, either in the parent material of the terrace soils, or in any of the fluvial deposits that formed the present-day riverbeds and floodplains of this hydrographic basin.

3.4. The carbonates and underlying material

No  carbonate minerals were found in the substratum of the series of terraces at Fresno Alhándiga, Calvarrasa de Abajo and San Pedro del Valle, in which the parent materials were Tertiary sands and sandstone, as well as Palaeozoic slates. In the case of the terrace series of Macotera, the fluvial surfaces were deposited over feldspathic (potassium and plagioclases of albite), muddy sands of Tertiary age, in which carbonate levels appeared with a certain frequency.

3.5. The carbonates in the hydrographic basin

Acidic rocks occupied great expanses in the field area, and were comprised of adamellitic granites, slates, sandstones, quartzites, graywackes, schists, mica schists, phiyllites and conglomerates. There were only small outcrops of more or less calcareous composition (Palaeo- or Neogene Tertiary sediments, such as marls, calcarenites and limestones). The small sizes, together with the higher  weatherability of these materials, explains their complete absence in the fluvial deposits comprising the terraces.


4. Discussion

4.1 Carbonate precipitation and leching  processes

The presence, morphology and distribution of carbonates in the soil profile are governed by complex conditions of solubility.

4.1.1. Crystallization. The most common size of the carbonates of these soils was micritic, indicating rapid crystallization (Barthurst, 1971: Folk, 1974; Bal, 1975). In this sense, the nucleation effect exerted by clays in the soil has been recognized by numerous authors (Wieder and Yaalon, 1974 and 1978; Elbersen, 1982). Because clay particles hamper the formation of large crystals, close relationships have generally been found between the soil texture and the crystal size of the carbonates formed. Nevertheless, in these soils, we did not find this correlation, either in the microscopic or in the analytical studies, presumably because in most cases the carbonates in the soils studied underwent later transformation processes.

Needle-fibre calcite was common in certain settings, invariably within discrete pores (channels and cavities), and forming pseudomycelia of more or less loose interlaced fine crystals inside the pores. Needle-fibre calcites  formed preferentially in the upper levels of the calcic horizons and represent a strongly diagnostic indicator of crystallization of soil carbonates. Needle-fibre calcite constitutes the first stage in the precipitation of the carbonates in the soil, which many researchers have attributed to oversaturated solutions (Buckley, 1951; James, 1972; Folk, 1974; Elbersen, 1982; Magaldi, 1983).

4.1.2. Recrystallization. Recrystallization was substantially developed in these soils. In one step of the recrystallization, needle-fibre calcite interlaced to form micritic masses, and in another step these masses recrystallized to sparry calcite crystals (Loisy et al., 1999) by a mechanism of neomorphic aggradation (Bathurst, 1971). The distinctive features of carbonate recrystallization in these soils, in accord with Bathurst (1971), were:

i) Highly variable sizes of the crystals, grouped in domains; with micrite, microsparite aggregates (Ø= 4 to 10µm), pseudosparite (Ø=10 to 50µm) and sparitic. There were transition zones between micritic and sparry calcite domains (Ringrose et al, 2002)(Fig. 1d).

ii) Central masses of microsparite or pseudosparite formed by radiating spar crystals (Fig. 1e).

iii) Contact surfaces of the spar crystals being rounded or undulated (Fig. 1f).

Figure 1. (1a) Random mesh of needle-fibre calcite, profile TC45, horizon 2Ck, crossed polarized light (XPL). (1b) Abundant carbonate coatings on the walls of pores, with rare fusing of the coatings of neighbouring pores, profile AM14a, horizon 3Ck2, XPL. (1c) Coatings formed by needle-fibre calcite crystals oriented parallel to each other and to the walls of the pores, which were in turn intercalated between illuvial-clay coatings, profile TP42, horizon 2CBtk3, XPL. (1d) The recrystallization process is evident due to the presence of highly variable sizes of the crystals, grouped in domains, with transition zones between micritic and sparry domains, profile TF63a, horizon 2CBtk1, XPL. (1e) The presence of central masses of microsparite formed by radiating sparry crystals is also a distinctive feature of recrystallization, profile TF63a, horizon 2Ck/Btb, XPL. (f) Contact surfaces of the sparry calcite crystals that are rounded also distinguish the process of recrystallization, profile TF63a, horizon 2Ck/Btb, XPL.

iv) Coatings of elongated spar crystals, parallel, with radiating fibrous shapes and frequently with undulating edges.

In addition, it is noteworthy that in the youngest soils from the Late Pleistocene, (lower terraces, <50,000 years) the carbonates appeared almost exclusively in the form of micrite, whereas the sparry calcite was more abundant in the oldest soils from the Middle and Early Pleistocene (>300,000 years, medium and high terraces), especially in the upper levels of the calcic horizon. Recrystallization rarely occurred in the youngest soils from the Late Pleistocene (<40,000 years), but was more common in the Middle Pleistocene soils (medium terraces, 350,000 years), and increased in development with the age of the soil (Table 3).

4.1.3. Replacement. The replacement of silicate material by carbonates has been discussed by numerous authors (e.g., Degens and Rutte, 1960; Gile et al., 1966; Multer and Hoffmeister, 1968; Reeves, 1970; Millot et al., 1977; Ruellan et al., 1978; Millot, 1979; Bech et al., 1980; Ruellan, 1980; Watts, 1980; Paquet and Ruellan, 1997), and is referred to as epigenesis by Nahon and Ruellan (1975). In these soils, this process is quite generalized, because replacement affects not only the coatings of illuvial clay and other clay masses, but also detrital quartz grains, feldspars and micas.

The illuvial-clay coatings underwent intense replacement that began in the soils of the low terraces (Late Pleistocene) and reached extraordinary development in the medium terraces (Middle Pleistocene). The process was represented in all its phases from the initial stage of disorganization of the coatings to complete replacement by the carbonates, with the only trace of the clay being the yellowish and reddish colorations of the carbonates impregnated by iron compounds from the clay. Typically, this transformation appears in the intermediate stages, in which half-replaced illuvial-clay coatings are extremely common.

Quartz sands, feldspars and micas were more stable, and are replaced only in Middle Pleistocene and older soils. The mica grains were progressively replaced by carbonates. In the first phase, the mica lamellae / cleavages planes were separated by the crystallization of the carbonates and were finally completely replaced, with only some residual traces remaining. Feldspars and quartz underwent an equally progressive replacement (Figs. 3a and 3b). A highly characteristic result of the replacement process was the brecciation of the grains (Ruellan et al., 1978; Paquet and Ruellan, 1997). The sands and gravels of the detrital minerals were broken into a number of fragments that remained separated but retained the orientation of the original crystal (Fig. 3a and 3c). The replacement generally led to the formation of colourless sparry calcite crystals, some of which showed secondary coatings of distinctive interference colours (Fig. 3c) (Ruellan et al., 1978; Paquet and Ruellan, 1997). In some cases, the original shapes of the replaced detrital grains were recognizable (Fig. 3c).

In comparison with the preceding cases, the replacement of clayey masses by carbonates produced yellowish-brown calcite of thick fibrous crystals, which grew radially, forming aggregates in fan-shaped clusters.

According to several authors (Swineford et al., 1958; Multer and Hoffmeister, 1968; Nagtegaal, 1969; Reeves, 1970; Millot et al., 1977; Reheis, 1988; Paquet and Ruellan, 1997), the replacement of silicate grains is indicated by the presence of floating grains of quartz and feldspar (quartz and feldspar grains separated from the carbonate matrix by a porous spaces. This fabric was common in these soils, and is attributed to the dissolution of the quartz and of the feldspars in an arid paleoclimates; this interpretation is reasonable because in a calcic horizon the pH can be locally quite high and spatially variable (Callot et al., 1978). Under such high pH conditions, the solubility of the silica and the alumina increases, easily reaching oversaturation for the carbonates.

The complex microstructures described previously are highly characteristic features of replacement and recrystallization. In some cases, the roots of plants were replaced by elongated carbonate crystals in parallel arrangements, following the vegetal morphology, commonly  forming tubes with a hollow central part (Jaillard, 1992; Becze-Deak et al., 1997) (Fig. 3d).

Figure 3. (3a) Quartz crystal partially replaced by carbonates, profile TF63a, horizon 2Ck/Btb, XPL. (3b) Plagioclase feldspar crystal partially replaced by carbonate, profile TF63a, horizon 2Ck/Btb, XPL. (3c) The replacement generally led to the formation of colourless sparry calcite crystals, in somes cases with secondary coatings exhibiting distinctive interference colours, whereas in other cases, as in this K-feldspars crystal, the original shapes of the replaced detrital grains were recognizable, profile TF63a, horizon 2CBtk1, XPL. (3d) Roots of plants replaced by elongated carbonate crystals in parallel arrangements, following the vegetal morphology, profile AM25, horizon 2CBtk, XPL.

In general, all these types of replacements were visible beginning with the Middle Pleistocene soils, and developing with steadily greater intensity with the age of the soil; they are prominent in the soils older than 300,000 years (Table 3).

4.1.4. Displacement. Displacement was not found to be a major process in the formation of calcic horizons in the soils studied, perhaps because, in general, the carbonate content of these soils was not high and thus some carbonate zones still maintained appreciable porosity.

4.1.5. Dissolution. Some soils revealed signs of dissolution and mixing of the previously accumulated carbonates (Srivastava, 2001). Commonly, the grains were rounded, (usually ellipsoidal, but sometimes spherical) and widely separated in most cases (Fig. 4b). However, the most distinctive feature of this process was the presence of crystals with pronounced serration as a result of dissolution (Fig. 4a).

In some places, unexpected aggregates of clay coatings with sponge-like porosity appeared without carbonate crystals. In thin section, these took the form of well-defined networks of highly uniform holes corresponding to fine grains of sand (Figs. 4c and 4d). An analysis of other zones of the same horizon revealed that the same clay coatings of similar characteristics covered sparry equidimensional calcite grains (Fig.  4e). Thus, the formation of the networks of empty clay coatings can be attributed to the dissolution of formerly coated carbonate crystals.

Figure 4. Dissolution process, profile TP50b, horizon 2BCtk. (4a) The most distinctive feature of this process was the presence of crystals with pronounced serration as a result of dissolution, XPL. (4b) Rounded carbonate grains embedded in a mass of illuvial clay, XPL. (4c) Unexpected aggregates of clay coatings with sponge-like porosity appearing without carbonate crystals in plane polarized light (PPL). (4d) The same microscopic field as c), with XPL. (4e) In the centre of the image, clay coatings with characteristics similar to those of the preceding image but covered now by sparry, equidimensional calcite grains; on the left, calcite grains appear without clay coatings (having not undergone dissolution, the coatings did not form hollows and the clay was not illuviated), XPL.

These networks would have formed in three stages: i) partial dissolution of the carbonate grains, forming intergranular pores; ii) clay illuviation filling the pores; iii) dissolution of carbonate grains and consequent formation of clay-coating networks. These three stages correspond to climatic shifts towards consistently wetter conditions.

Evidence of carbonate dissolution was also apparent at the macroscopic level of the soil profile based on in the presence of hard calcareous nodules with clean surfaces, as well as with clear borders, and commonly with elongated forms vertically arrayed in the profile. This dissolution was active only in the oldest Middle Pleistocene soils (≥200,000 years), in the Late Pleistocene soils and in the upper levels of the calcic horizons.

4.1.6. Recarbonatation. The late development of secondary carbonate was evident in carbonate coatings that covered the illuvial-clay coatings (Figs. 5a and 5b) (Bronger et al., 1998), at times cross-cutting through the previously formed clay coatings. These horizons are called compound soil horizons with mixed calcic and argillic properties (Kleber, 2000).

Occasionally, there were coatings composed of various alternating layers of carbonates and illuvial clay, which are unequivocal evidence of various phases of successive secondary carbonate precipitation (Fig. 5c). These multiple processes took place only in the oldest soils, beginning  with the Middle Pleistocene soils (≥200,000 years), and represent additional proof of the various climatic changes that these soils had undergone.

Figure 5. (5a) The precipitation of secondary carbonate was evident in carbonate coatings that covered the illuvial-clay coatings, profile TF63a, horizon 2CBtk1, PPL. (5b) Carbonate coatings covering illuvial clay coatings, profile TF115, horizon 2Btk2, XPL. (5c) Coatings composed of various alternating layers of carbonates and illuvial clay, unequivocal evidence of various phases of successive secondary carbonate precipitation, profile TF115, horizon 2Btk2, XPL. (5d) Many carbonate accumulations, are accompanied by Mn accumulations, profile AM 25, horizon 2Ck1, PPL.

4.2. Origin of the carbonates

The origin of carbonates in soil forming over non-calcareous parent material, as in the present case, constitutes a controversial issue. The possibility of a geological origin for our carbonates can be dismissed on the basis of the observations analysed above: carbonate coatings in the lower part of the gravel; highly irregular and heterogeneous distribution of carbonate accumulations; and carbonate coatings over illuvial-clay coatings. Thus, it appears evident that pedological processes caused the distribution and morphology of the carbonate accumulations in these soils. These pedological carbonates can be autochthonous or allochthonous.

4.2.1. Autochthonous origin. The carbonates may have come from parent material either directly (being present within that material) or indirectly (forming from components of the parent material). The possibility of inheritance (direct origin) can be discarded on the basis of a lack of carbonates in the present alluvial deposits, as well as the lack of parent material in any terrace. If mineral alteration (indirect origin) of the parent material were responsible, the mineral would have to be feldspar, but feldspars of these soils are albite and oligoclase—sodium-rich silicate minerals that are calcium-poor. These minerals present diverse degrees of alteration, invariably becoming clay phyllosilicates (illite and kaolinite) and gibbsite. In addition, we found no relationship between the intensity of the alteration of the feldspars (expressed by the quartz/feldspar ratio of the sand fraction) and the carbonate content of the soils. Finally, if this process were responsible for the pedogenic carbonate accumulations in these soils, we might expect to find pedogenic carbonate  accumulations in the broad and diverse neighbouring granite regions (the origin of the fluvial materials of these terraces), but no such accumulations were found.

4.2.2. Allochthonous origin. The carbonates in question must be of external origin, derived either from the air or from water. No features indicate aeolian transport (for example, substantial increases in the silt or fine-sand fraction in the surface horizons, soils containing abundant fine and translocated dust in the matrix have continuous coatings, Treadwell-Steitz and McFadden, 2000). The derivation of carbonates from surface runoff does not appear probable either, as there are no calcareous zones near the terraces.

   As indicated above, at times carbonates appear in preferentially horizontal laminae which criss-crossed with more or less vertical grains, creating distinctive patterns with the following characteristics:

i) The interlacing is not limited to the soils of these terraces but appears also in numerous surfaces throughout the region and is occurs preferentially in high-altitude surfaces composed of sand, sandstone and clay.

ii) In some cases these patterns appear immediately beneath an argillic horizon, but  in other cases, appearing at subsoil depths, they are difficult to relate to soils (3 to 5 m; Fig. 2a).

iii) In rare cases, these patterns appear in Tertiary sands, forming thick (10 cm), well-defined laminae and with a slope that does not correspond to the present topographical surface.

iv) In some soils, the upper levels of the calcic horizon show a clear horizontal lamination consisting of carbonate laminae alternating with layers of leached material. This lamination could be attributed to the fluctuations of the water table.

vi) In some soils the distribution of the carbonates with depth is irregular and leached horizons appear among carbonate horizons. This fact may also indicate oscillations in a water table, at the time the carbonates were deposited in the soils.

vii) In nearly all cases, carbonate accumulation is associated with distinctive hydromorphic processes (Table 3), generally involving hypo-coatings, coatings and black nodules of Mn compounds (Fig. 5d).

In light of all these observations, we interpret that these patterns of carbonate distribution  have a vadose origin. The laminae must have been the result of the temporal shifts of the water table, which periodically provided carbonates during certain geological stages

Given the great tectonic stability of the zone during the Pleistocene, the formation of the sequence of terraces studied should be attributed to climatic changes (Insituto Geológico  y Minero de España, 1982).  Thus, during cold and wet periods (episodes of resystaxia) the fluvial courses of the Tormes basin were eroded progressively until reaching a new equilibrium at a lower level.  The low temperatures of these episodes led to a greater bicarbonate concentration in the vadose waters (Arkley, 1963), which in wet periods would increase the carbonate content in the soils of  the fluvial deposits, primarily as a consequence of suction and evaporation processes.  This scenario appears to be corroborated by the fact that the soils high in carbonate content formed in colder periods of the isotope stages described by Bassinot  et al. (1994); specifically, the carbonates could have accumulated during substages 6.2, 7.4, 10.2, 12.2, 13.2 and 16.2 (Tables 1 and 3).  In warmer periods (episodes of biostaxia), which alternated with the colder ones, the fluvial deposits (terraces) that would have been left exposed after the waters receded underwent pedological processes such as weathering, leaching, carbonates precipitation and clay illuviation.  The presence of carbonate dissolutions in the soils AM25 and AM47 appear to indicate the development of the two wettest periods in the isotope substages 7.4 and 13.2, occurring around 200,000 to 500,000 years B.P.

The complexity of the climatic changes that these soils have undergone, especially the oldest ones, could be manifested by the recrytallization, replacement, dissolution and secondary carbonate precipitation found in the carbonates in these soils (Table 3), as well as the alternation of several phases of clay illuviation  with episodes of carbonate accumulation.

In each chronosequence, and in the overall composition of the soils, there was a trend of the carbonate contents to diminish the younger the soils (Table 3), apparently indicating  that the climate of this geographical region has been changing in the last 1,200,000 years, towards warmer and presumably drier periods (Vidal, 1979; Gutiérrez, 1986; Simón et al., 2000; Ortiz et al., 2002).


5. Conclusions


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