Benchmark soils on alluvial , fluvial and fluvio-glacial formations of the upper-Segre valley

The upper reaches of the Segre river, flowing through the Pyrenees, offers a variety of geomorphic surfaces that allow us to study soil chronosequences. The objective of this work is to widen the knowledge about the main characteristics and formation processes of some benchmark soils developed on fluvioglacial, alluvial-fan and terrace materials of Pleistocene and Holocene age related to the Segre river, either siliceous or carbonatic. This knowledge will allow us to identify soil forming processes, commonly found in Mediterranean environments such as carbonate redistribution, clay formation and mobilization and rubefaction, all as functions of parent material and age. Five profiles, ranging from the Lower Pleistocene to the Holocene were classified according to Soil Taxonomy /WRB. The Montferrer profile (Calcic Palexeralf /Calcic Cutanic Luvisol (Chromic) is a deep, partly decarbonated soil, with calcium carbonate accumulation in depth covering glacial features. The Torre del Remei profile (Typic Paleustalf /Cutanic Luvisol) developed on silicic moraines and shows an extreme clay formation and illuviation. The Alp (Typic Haplustalf /Cutanic Luvisol) and Tartera (Petric Calciustept /Petric Calcisol) soils are developed on alluvial fans with calcium carbonate sources. The former is partly decarbonated, whilst the latter is rubefacted on top and shows speleothem-like carbonate pendants with superposition of clay illuviation. The youngest profile, Abellerols, (Typic Calciustept /Typic Calcisol) shows only a partial decarbonation and calcite accumulation at depth. The results show that soil development is determined by the age of the surface and the source of calcite, either in the parent rock or brought by subsurface flow: clay illuviation is extreme in absence of it. Special morphologies of carbonate pendants are indicators of environmental conditions. The coexistence of clay coatings and secondary calcite can be explained by recarbonatation or by spatial differentiation of soil environments in the profile. One of the implications of this research is the inconsistence of using soil development indices based on morphological indicators when soils are formed on different parent materials and are subjected to different geomorphic dynamics.


Introduction
In Mediterranean environments leaching and accumulation of carbonates, clay illuviation and rubefaction are some of the most common morphological soil features which have been widely reported (Yaalon 1997;Torrent and Barrón 2003;Verheye and De la Rosa 2006).Some authors studying these processes have highlighted that a moderate weathering with illuviation of mostly 2:1 clays into B horizons (Xeralfs/Luvisols) takes place in semiarid regions, together with a hematite-induced reddening of the clays due to summer dehydration of free iron oxyhydroxides, carbonate dissolution and reprecipitation (Yaalon 1997)

Material and Methods
The Five profiles were selected from a soil survey for further description, analyses and classification (Figure 1, Table 1).The profile Montferrer was described in a house foundation trench, Tartera and Abellerols in quarry walls and Alp and Torre del Remei in a soil pit.They were described and analyzed according to the methods of Porta et al. (1986).Thin sections were studied according to Stoops (2003).SEM-EDAX analyses were done at the EM service of the Autonomous University of Barcelona.Locations of the five profiles are shown in Figure 1.*Profile in a proximal position of the alluvial fan, far from the river.

Results
Full descriptions of the profiles may be found in Poch et al. (2011).Table 2 and Figure 2 contain  a summary of the morphological features, and   Table 3 the main physico-chemical characteristics of the profiles.
All micromass is found as oriented clay infillings and coatings.
All micromass is found as oriented clay infillings and coatings.Aggregated impregnative nodules of Fe and Mn oxi-hydroxides in the micromass.Absent.

Alp
Clay coatings along fissures of coarse gravels.
Generalized microlaminated clay coatings and infillings around packing pores and coarse gravels, some of them argilloturbated.Generalized microlaminated clay coatings and infillings around packing pores and coarse gravels, some of them argilloturbated.Aggregated nodules of Fe-oxihydroxides.The micromorphological study of the Montferrer profile reveals that the micromass of the rubefacted horizons is mainly sericite silt, partly coming from the weathering of feldspars, which also forms the silt coatings in the deepest horizons.Clay coatings are generalized, some of them are allophanic with varying Fe contents, evidenced by SEM-EDAX (Figure 3).Hydrated Fe-hydroxides coatings are also present in crystalline fan-like form.The recarbonatation of the Btk horizon is very strong, and proceeds mainly from the dissolution of the limestones, breaking down the previous features (Figure 4).

Tartera
The Torre del Remei soil is developed on the frontal moraines of glaciers coming down into the valley, on the south facing side.Their lithology, considering the source area, is mainly granite and schist, without carbonates.They are considered weathered fluvio-glacial formations (Calvet 2004).The soils located on these formations allow us to observe the parent material consisting of gravel and large boulders of gritty granites and schists, highly weathered, that infers the age of the soil, since alteration must have taken place in situ.The attempts to date these surfaces result in high value dispersion due to the weathering degree, which gives low ages to such deeply weathered glacial and glaciofluvial materials ( 10 Be dating, Calvet et al. 2011).
The processes of soil genesis have been primarily formation and mobilization of clay from the alteration of feldspars and micas, as shown by the coatings observed through the microscope in the Bt (argillic) horizons (Figure 5).This clay illuviation is a common feature in the soils of the northern part of La Cerdanya that are located in older geomorphic surfaces.In those cases clay illuviation is extreme (Simó 2005).When this accumulation is very intense, it modifies the texture of the horizons so that the loose surface horizons become more sandy, overlying with a sharp boundary on the more clayey horizons.In our case, the abrupt textural change between A and B horizons seems not to be fully due to this process, but to a lithological discontinuity (a new material supplied on the surface), as shown by The Alp soil is also characterized by clay illuviation, remarkably it is not decalcified.The upper B horizon shows clay coatings that continue with depth.The most noticeable micromorphological features are the decarbonated micromass in spite of the high pH that is indicative of a car-bonate supply through the overland flow of the alluvial fan, the presence of pressure faces (granostriation) around gravels and sands, and an extreme clay illuviation similar to that of Torre del Remei.Other soils in this unit show pressure faces and development of slickensides (data not presented, Boixadera pers.comm.).
Tartera soil shows a rubefied A horizon, masked by organic matter in some non cultivated profiles  (data not presented, Boixadera, pers.comm.).
Other features are (i) the crystalline nature of calcite pendants underneath the gravels, (ii) the coexistence of clay illuviation together with secondary calcite, and (iii) evidences of dissolution of limestone fragments.These features can be observed both in the field and microscopically.Besides calcitic pendants formed by micrite or by layers of micrite and microsparite, many pendants consist of large calcite crystals, perpendicular to the gravel surface, with interspersed dark reddish layers within the crystals, showing a continuity subparallel to the gravel surface, crossing the crystals (Figure 6).The presence of clay coatings in some layers of the deposit allows us to describe Bkt or Bkmt horizons in the field (we propose this suffix order to be coherent with the process chronology).In thin section they appear as clay coatings with different degrees of orientation, mostly covering previous calcite coatings, and more rarely around the voids of cemented masses of secondary calcite (Figure 7).The Abellerols soil is located in the youngest surface.In spite of the fact that it is a Segre terrace formed by silicatic materials, the presence of some limestone fragments at the top horizons show that there has probably been a supply of carbonates by the distal parts of the alluvial fans at the top.This soil is partly decarbonated in the top 50 cm, although there is some biogenic calcite (biosparite, calcareous spherulites).In the 2Bwk1 horizon there are few clay coatings (Figure 8) coexisting with micritic orthic nodules.Calcite accumulation was highest in the 3Bk horizon, with very frequent infillings and coatings of acicular calcite, together with calcite nodules in a cristallitic micritic micromass.In this horizon, clay coatings sometimes covered the calcitic pedofeatures (Figure 9).

Clay illuviation and carbonate redistribution
In three of the studied soils (Montferrer, Tartera, Abellerols) clay illuviation occurred with carbonate accumulation, either before or after (microlaminated clay coatings overlying or underlying secondary carbonates).The absence of such features in the other two soils is explained by the low carbonate supply (Alp), or by the absence of carbonate in the parent material (Torre del Remei).Alonso et al. ( 2004) explain clay illuviation on secondary calcite in siliceous Pleistocene river terraces by a combined process of dissolution of formerly coated carbonate crystals and clay illuviation filling the pores.In our case, although limestone dissolution is evident (Montferrer, Tartera), the presence of remains of a decarbonated, rubefacted A horizon allows us to suggest a process of clay illuviation from this horizon, which would accumulate in the pores of the underlying Bk or Bkm horizon, in a process similar to that of the formation of beta horizons (Mathieu and Stoops 1974).In agreement with the concepts of Wieder and Yaalon (1978) and Aguilar et al. (1983), carbonate prevents the dispersion of clay but does not prevent the movement of already dispersed clay (in A horizons) if pores are large enough (Pazos 1990).We have to think about both processes (clay illuviation and calcite redistribution) as alternating, since in some cases, coatings of calcite, mainly acicular, cover the former clay coatings.The origin of dispersed clay is evident in the Montferrer and Abellerols profiles; also the rubefacted, partly decarbonated A horizon is still visible in the Tartera soil.In this case, elluviation takes place in the A horizon and clay floculates in the Bk horizon.Successive episodes of alluvial fan aggradation in the Tartera profile would erode the previous decarbonated A horizon, and start again the processes in the newly deposited material.In this profile, these flooding episodes of the alluvial fan could supply the fine silt and clay particles that would remain at the surface of the continuously growing sparite crystals of the pendants.Their crystalline nature could then be explained by the fairly constant supply of bicarbonate subsurface water flowing through the coarse, skeleton-supported alluvial fan deposit.The difference in age between Montferrer (the oldest soil) and Abellerols (the youngest soil) is shown only by the thickness of horizons, suggesting a prevalence of conditions favourable for carbonate leaching down to a given depth during the Quaternary.

Soil development and surface age
Among the different researches conducted on chronosequences and dating on the terrace systems of the north Ebro tributaries, Lewis et al. (2009) show a positive relationship between surface age and the profile development index (PDI, obtained from morphological properties), and carbonate stage morphology (Gile et al. 1981;Birkeland 1999).In our case, although carbonate morphology of Tartera corresponds to stadium II + (continuous pebble coatings, some interpebble infillings), consistent with the terrace age of Lewis et al. (2009) on the Cinca and Gállego rivers, we should not think that the carbonatation process proceeds in the same way, since the progressive alluvial fan aggradation causes this morphology to occur in a much shorter period of time and to be buried by the next material supply.Indeed, older sectors of the same alluvial fan corresponding to an older terrace show identical morphologies (Boixadera, pers.comm.).We must conclude that, in our sequence, the erosion events that might have occurred during the time span of soil development, the different nature of soil parent materials, the diverse soil formation processes -in particular the lateral carbonate supply-and the high weathering prevent the use of a consistent soil development index for all soils.Indeed, some of the features observed correspond to diverse soil formation paths.As examples we showed the formation of amorphous clays in the oldest profile, that could be the result of extreme granite weathering, as it has been observed in spodosols (García-Rodeja et al. 1987).The presence of podzolisation processes in the region, at higher altitudes (Boixadera et al. 2008), supports this idea.Another indication of soil evolution is the presence of vertic features in the Alp and surrounding soils, which would surely mask clay illuviation.All these facts confirm that the use of age indicators should be carefully used, and only when the soil formation process is well determined.

Conclusions
In the middle altitudes of La Seu-La Cerdanya valley, calcium carbonate and clay leaching and accumulation are the main soil formation processes.In addition to surface age, soil development is also controlled by parent material characteristics.When calcium carbonate is absent from the parent material (Torre del Remei and Alp), clay illuviation is strong; it couples with mottling phenomena, and leaching of cations are noticeable (Torre del Remei) if CaCO 3 is absent in the percolating water.
Profiles Tartera and Montferrer allow us to identify different types of percolation regimes.The existence of alternating morphologies of clay illuviation and calcium carbonate accumulation should be interpreted also in the sense of percolating (clay illuviation) or non-percolating (CaCO 3 accumulation) regimes.Special morphologies of carbonate pendants are indicators of environmental conditions.The coexistence of clay coatings and secondary calcite can be explained by recarbonatation or by spatial differentiation of soil environments in the profile.
Reddening of soil materials is more pronounced in calcareous than in non calcareous materials.Mottling is very much related to clay content and it seems to prevent rubefaction (Torre del Remei).Our results are a clear indication that soil development indices based on morphological features could be meaningless if not applied to uniform sequences.• García-Rodeja E, Silva BM, Macías F. 1987.Andosols developed from non-volcanic materials in Galicia, NW Spain.J Soil Sci.38:573-591.
• Gile LH, Hawley JH, Grossman RB. 1981.Soils and geomorphology in the basin and range area of Southern New Mexico -guidebook to the desert Project.Memoir 39.Socorro, NM: New Mexico Bureau of Mines and Mineral Resources.

Figure 2 .
Figure 2. Sketch of the horizon sequence of the studied profiles.

Figure 4 .
Figure 4. Recarbonatation of a silt capping (Btk, Montferrer).The yellow dotted line shows the recarbonated section by micrite and microsparite.Thin clay coatings are found on the pores of the recarbonated section.
Their morphology is very similar to those of the speleothems, whose laminae have been explained by Paulsen et al. (2003) as annual layers; and by Gradziński et al. (2003) as composed from charcoal and organic particles produced by fires.In our case, in absence of SEM-EDAX evidence, the origin is detrital (clay and fine silt), as shown in thin section, which is in agreement with the formation of the deposit.The large density of bands would then indicate a higher flood frequency in the past, during the period of maximal alluvial fan formation.

Table 2 .
Morphology of the studied profiles

Table 4 (
I). Summary of the micromorphological descriptions

Table 4 (
II). Summary of the micromorphological descriptions