Micromorphology and development of loess-derived surface and buried soils along a precipitation gradient in Northern Iran

Quaternary International 234 (2011) 109e123

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Micromorphology and development of loess-derived surface and buried soils along a precipitation gradient in Northern Iran Farhad Khormali a, Martin Kehl b, * a b

Department of Soil Science, Gorgan University of Agricultural Sciences and Natural Resources, Gorgan, Iran INRES e Soil Science, University of Bonn, Nussallee 13, 53115 Bonn, Germany

a r t i c l e i n f o

a b s t r a c t

Article history: Available online 5 November 2010

The northern footslopes of the Alborz Mountains and an extensive hill area in eastern Golestan Province are covered by thick (∼70 m) loess deposits, divided by different types of interstadial and interglacials paleosols. The area shows a precipitation gradient of about 800 mm over 80 km north-south. Along this gradient, eight pedons of modern soils and six paleosols were selected in order to investigate the types of modern and past soil forming processes and to compare the degree of soil development of surface and buried soils. The soils were described and sampled using standard sedimentological/pedological laboratory analysis. The soil pH and calcium carbonate contents show a decreasing trend with precipitation, while soil organic carbon, clay content and cation exchange capacity increase with rainfall. The silt content, however, shows a decreasing trend with rainfall. Clay mineralogy of parental loess is illite > chlorite > smectite > kaolinite. From north to south, the relative proportion of smectite in soil horizons increases, reaching almost dominance in regions with typic xeric soil moisture regime (SMR; ∼600 mm of rainfall). In the areas with udic SMR, vermiculite increases and sometimes dominates. Micromorphology provides evidence for the formation of Bt horizons and intensified decalcification and increase of clay mobilisation as indicated by b-fabrics with increasing rainfall. Soils within the xeric and udic SMRs are either Alfisols or Mollisols, both showing clay illuviation features, whereas clay mobilisation is very limited in the arid part of the area. The paleosols show differential weathering degrees indicated by trends of increasing carbonate depletion, clay mobilisation, and clay enrichment with assumed paleo-precipitation. Pedogenesis, micromorphological properties and clay minerals in the last interglacial paleosols suggest similar climate controlled trends to those reflected in the modern soils. Ó 2010 Elsevier Ltd and INQUA. All rights reserved.

1. Introduction Climate is one of the main factors of soil genesis (Jenny, 1941; Birkeland, 1999). Among the main physico-chemical properties affected by climate is the soil organic carbon (SOC) content. Its quantity and composition depend on several moisture and temperature sensitive processes, including the rate and type of litter production and the balance between mineralisation and humification processes as a function of microbial decay and turnover of the organic matter. The SOC content of topsoil horizons often increase with annual precipitation but is further related to other soil properties. For instance, it shows a close correlation with the fine grain

* Corresponding author. Present address: Department of Geography, University of Cologne, Albertus-Magnus-Platz, 50923 Cologne, Germany. E-mail addresses: [email protected] (F. Khormali), [email protected] (M. Kehl). 1040-6182/$ e see front matter Ó 2010 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2010.10.022

size fractions such as the clay content (e.g., Nichols, 1984) or the clay and silt content (Buschiazzo et al., 1991), because the fines increase the water holding capacity of the soil, the biological activity, and therefore, the deposition rates of organic residues to the soil. The amount of fines may itself be related to climate with clay contents of soils derived from the same parent material showing an increase with precipitation (e.g., McDaniel and Hipple, 2010). In arid to subhumid areas the degree of depletion in soil inorganic carbon (SIC) and its subsequent enrichment in Bk horizons of the subsoil increases with precipitation, whereas loess soils in humid areas are often leached in SIC and carbonate accumulation either occurs in lower strata of the underlying loess (loess kindle) or the dissolved bicarbonate is leached to the groundwater. The clay mineralogical composition of soils may also be affected by climate. The origins of clay minerals in a soil profile have been explained by i) the inheritance of clay minerals directly from parent materials (Beavers et al., 1955; Gunal and Ransom, 2006a); ii) sequential weathering of clay minerals including, e.g., the

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transformation of mica to the 2:1 type clay minerals due to the removal of Kþ from interlayers of mica (c.f. Jackson et al., 1952; Fanning et al., 1989); iii) the addition of clay minerals through dust fall (Smith et al., 1970); and iv) the neoformation of clay minerals (Borchardt, 1989), where the pedogenic formation of smectite occurs in solutions high in Si, Al and Mg under restricted drainage conditions. The latter three of these clay formation mechanisms are related to climate. However, the clay mineralogy of soils formed on loess deposits may also be related to time, with mica as the dominant clay mineral in the younger loess and vermiculite in the older ones (McDaniel and Hipple, 2010). With clay content and mineralogy, the soil physical properties and soil structure change. Grossman et al. (1968) reported that the coefficients of linear extensibility (COLE) of soil samples dominated by kaolinite and mica are less than 0.03, and those dominated by smectite have values higher than 0.03. Soils formed on loess in Kansas (Gunal and Ransom, 2006a,b) showed significant increases in COLE values and bulk densities with increasing precipitation. The mineralogy also plays an important role on the range of shrinkswell potential and the kind of b-fabric of soils (Franzmeier and Ross, 1968; Nettleton et al., 1969). Climate-related changes in soil structure can be identified using micromorphology. This approach also gives unequivocal evidence of soil processes, for instance, of the formation of clay coatings resulting from clay illuviation. The micromorphological index of soil development MISECA suggested by Khormali et al. (2003) evaluates the degree of argillic horizon development in highly calcareous soils of arid and semiarid regions. Argillic horizons according to this index can be classified from weakly to welldeveloped. Micromorphological criteria of the MISECA are microstructure, b-fabric, clay coating, decalcified zone, Fe/Mn hydroxide and alteration degree of mineral grains. With increasing degree of soil development, the MISECA values range from 0 to 24. There was a good correlation between MISECA and soil properties, mainly the amount of available potassium, along a precipitation gradient in western Golestan Province in Northern Iran on soils formed mainly from loess material (Ghergherechi et al., 2009). The b-fabric has been used to distinguish development stages of surface and buried soils (e.g., Gerasimova et al., 1996; Günster and Skowronek 2001; Kehl et al., 2005a) with increasing degree of development reflected in the sequence from crystallitic to speckled and striated b-fabrics. However, the expression of clay coatings and b-fabrics are also related to shrink-swell potential as affected by clay mineralogy and climate. Soil horizons with high COLE values were mostly missing clay coatings on the faces of peds, because of high shrink-swell activity of the groundmass (Griffin and Buol, 1988). In the loess derived soils of Kansas, the b-fabric of the upper part of argillic horizons was generally dominated by crossstriated and grano-striated b-fabrics. Thick and continuous clay coatings were not observed when the cross-striated b-fabric dominated. In the lower part of the argillic horizons, less affected by shrink and swell, parallel striated and grano-striated b-fabrics dominated (Gunal and Ransom, 2006a,b). Another micromorphological approach was presented by Kühn et al. (2006) who used micromorphological feature-sets to characterize the sequence of soil forming processes within paleosols of loess-like sediments in Italy. The feature-set approach provided an opportunity to refine the formerly established pedostratigraphy and could also be helpful to distinguish development degrees of soils. In order to study the effect of climate on soil formation other factors of pedogenesis including parent material, relief, vegetation and time must be kept constant. For this purpose, soils derived from the same parent material and developed on level relief positions far above the groundwater table have to be found. These soils have been classified as climaphytomorphic (Jenny, 1961), i.e. their formation

mainly depends on climate characteristics and related vegetation. Climatophytomorphic soils showing different development degrees can then be used to set up a soil climosequence and to reconstruct past climatic conditions during former periods of soil formation. This approach can only be reliable, if the duration of soil development is known. As a first assumption, soils developed during the same interglacial can be assigned the same age, if there development was not interrupted by erosion or deposition of younger sediments. As the physical and chemical properties of unweathered loess deposits are often very homogenous over large areas, loess is well suited as a parent material for studying soil properties across climatic gradients. This study considered the loess-covered southern Caspian lowlands in Northern Iran in order to (1) examine the morphological, chemical, physical and mineralogical characteristics of soils formed in loess along a precipitation gradient; (2) investigate the relationship between the clay mineralogy and other soil properties such as birefringence fabric and COLE values in different climatic conditions; and 3) to compare the weathering degree of loess-derived interglacial paleosols with those of the surface soils. 2. Study area The extensive loess deposits of Northern Iran reach a thickness of about 30 m on the northern footslopes of the Alborz Mountain and of about 70 m in the Iranian loess plateau extending to the northeast of the city of Gonbad-e Kaboos (Fig. 1). Previous studies have shown that the granulometric, mineralogical and geochemical properties of unweathered Last-glacial loesses of Northeastern Iran are quite homogenous (Kehl, 2010). Based on geochemical and mineralogical composition as well as geomorphological reasoning, their most likely source areas are the alluvial plains of Atrek and Gorgan rivers. The alluvial plains are mostly covered by silty deposits which probably originate from the loess-covered areas and can be classified as loess-derived alluvium according to Pye (1995). Primary and secondary or reworked loesses are thus widespread parent materials of the surface soils in the area. The study area is characterised by a pronounced precipitation gradient of about 800 mm year1 over 80 km from north to south. In the lowlands, the mean annual air temperature (MAAT) decreases from about 18  C to 16  C, while a further decrease in temperature is observed with increasing altitude in the Alborz Mountains. Data on the mean annual potential evaporation (ET0) is sparse. According to estimates of Dinpashoh (2006) the ET0-values in the eastern Caspian lowlands range from 1000 to 1200 mm per year. The precipitation gradient is reflected in the sequence of aridic, dry xeric and typic xeric SMR in the lowlands and udic SMR on the forested northern hillslopes of Alborz Mountains (SWRI, 2000). Present land use gradually shifts from scattered pastures in the arid northern regions, over rainfed agriculture in the central part to dense forest in the southern subhumid regions. Land use changes during the past are poorly documented, but it is assumed that modern grazing areas in the Turkmen steppe and the natural forest of Alborz Mountains footslopes have never been used for cultivation. To date, there is no comprehensive study on the formation, physico-chemical properties, micromorphology and clay mineralogy of modern soils in this important agroecological area. Some sedimentological/paleopedological information is available for the loess key sections at Neka, Now Deh and Agh Band (Figs. 1 and 2). The loesses are divided by paleosols showing different degrees of weathering probably related to paleo-rainfall and duration of soil development (Lateef, 1988; Kehl et al., 2005b). Based on pedostratigraphy and luminescence dating, the paleosols were correlated with interglacial and interstadial phases of the Middle to Upper Quaternary (Frechen et al., 2009). For comparison with modern soils, the paleosols of former interglacial periods are most suitable,

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Fig. 1. Location map of the study area and the sites of the modern soils (numbers 1, 2, .,8) and loess sections at Agh Band, Now Deh and Neka, where the paleosols were sampled.

assuming that the duration of soil formation was comparable to that of the present interglacial. Luminescence age estimates of Frechen et al. (2009) indicate that loess deposition at the footslopes of Alborz Mountains lasted at least until about 21 ka and 18 ka as measured at the profiles of Neka and Now Deh and until about 9e10 ka in the Iranian loess plateau (section at Agh Band). This implies that soil formation might have started during the Late Glacial and Lower Holocene on the footslopes of Alborz Mountains and in the loess plateau, respectively. Since the overall climatic cyclicity reflected in the loess-soil sequences correlates well with the climatic cycles of the Northern hemisphere, it is very much likely, that formation of the modern soils started with the onset of the Holocene (MIS 1). However, there is evidence that in W Iran the Lower Holocene was considerably dryer than the Middle and Upper Holocene (cf. Kehl, 2009 for discussion). This climatic trend would have equally affected the whole study area in NE Iran. Overall, it is assumed that the duration of pedogenesis is the same for the loessderived modern soils of the study area. 3. Materials and methods 3.1. Modern soils 3.1.1. Sampling Eight representative pedons were selected in a south-north direction on loess deposits (Fig. 1). Special attention was paid to omit soils affected by groundwater or by soil erosion on slopes. All pedons were under pasture and/or forest cover. There was no indication of former cultivation of these soils. As presented in Table 1, the climate data show that precipitation varies from 200 mm in the upper north

regions (Pedon 1) to up to 900 mm in the southern areas on Alborz north facing slopes (Pedon 8) in less than 80 km latitudinal distance. The soil moisture regime (SMR) is aridic in the northern regions shifting to dry xeric, typic xeric and udic towards south (SWRI, 2000; Soil Survey Staff, 2010). The soil temperature regime (STR) also changes from thermic in the north to mesic in the Alborz heights in south. The studied soils therefore present a unique precipitation gradient. The soils were described and classified according to the Soil Survey Manual (Soil Survey Staff, 1993), Keys to Soil Taxonomy (Soil Survey Staff, 2010), and WRB (2006) respectively. 3.1.2. Physico-chemical analyses Particle-size distribution was determined after dissolution of CaCO3 with 2N HCl and decomposition of organic matter with 30% H2O2. After repeated washing to remove salts, samples were dispersed using sodium hexametaphosphate for determination of sand, silt and clay fractions by the pipette method (Day, 1965). Alkaline-earth carbonate was measured by acid neutralisation and expressed as calcium carbonate equivalent, CCE (Salinity Laboratory Staff, 1954). The Coefficient of linear extensibility (COLE) was measured directly as the change of the clod dimension from moist to dry conditions (Soil Survey Staff, 1999). Organic carbon was measured by wet oxidation with chromic acid and back titration with ferrous ammonium sulphate (Nelson, 1982). Gypsum (CaSO4$2H2O) was determined by precipitation with acetone (Salinity Laboratory Staff, 1954). Soil pH was measured in a saturation paste and electrical conductivity (total soluble salts) was determined in a saturation extract (Salinity Laboratory Staff, 1954). Cation exchange capacity (CEC) was determined using sodium acetate (NaOAc) at pH 8.2 (Chapman, 1965).

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Fig. 2. Loess-soil sequences at the sections at Neka, Now Deh and Agh Band showing thick layers of primary loess and intercalated moderately to strongly developed paleosol horizons. The correlation of loess accumulation phases with oxygen isotope stages (MIS) is based on pedostratigraphic reasoning and physical dating using the luminescence method (Kehl et al., 2005b; Frechen et al., 2009). The presumably Last Interglacial paleosols of MIS 5e are used for chronostratigraphical correlation of the sections. Paleosol selected for this study are underlined. For locations of the sections see Fig. 1.

3.1.3. Mineralogical analyses Chemical cementing agents were removed and clay fractions separated according to Mehra and Jackson (1960), Kittrick and Hope (1963) and Jackson (1975). Iron-free samples were centrifuged at 750 rpm for 5.4 min to separate total clay (<2 mm; Kittrick

and Hope, 1963). The total clay fractions were analysed mineralogically by X-ray diffractometry (Jackson, 1975). The same concentration of clay suspensions was used for all samples to give reliable comparisons between relative peak intensities. Two drops of the prepared suspension were used on each glass slide.

F. Khormali, M. Kehl / Quaternary International 234 (2011) 109e123 Table 1 Climate data of the studied areas.a Pedon

Region

SMR

STR

Land Use

P, mm y1

MAAT,  C

1 2 3 4 5 6 7 8

Korand Ghoijegh Shoordareh Agh Su Gorgan Neka Narmab Shast Kola

Aridic Dry Xeric Dry Xeric Typic Xeric Typic Xeric Typic Xeric Udic Udic

Thermic Thermic Thermic Thermic Thermic Thermic Mesic Mesic

Pasture Pasture Pasture Forest Forest Forest Forest Forest

200 350 450 630 650 750 800 900

18.0 17.5 16.0 16.0 17.0 16.5 13.0 13.0

SMR: Soil moisture regime; STR: Soil temperature regime; P: Precipitation; MAAT: Mean annual air temperature. a SWRI e Soil and Water Research Institute Iran (2000). The climatic data are the means of the period 1987e2009.

The (001) reflections were obtained following Mg-saturation, ethylene glycol solvation and K-saturation. The K-saturated samples were studied both after drying and after being heated at 330  C and 550  C for 4 h. To identify kaolinite in the presence of tri-octahedral chlorite, samples were also treated with 1 N HCl at 80  C overnight. Clay minerals were estimated semiquantitatively from the relative first order X-ray peak areas of glycol-treated samples (Johns et al., 1954). 3.1.4. Micromorphological analysis Thin sections of about 60 and 30 cm2 were prepared from airdried, undisturbed clods using standard techniques (Murphy, 1986). Carbonate was removed from some thin sections by placing them in a 1 N HCl solution for 3 min before mounting a cover slip (Wilding and Drees, 1988). Micromorphological descriptions were made according to Bullock et al. (1985) and Stoops (2003), while the interpretation of micromorphological features followed criteria of Stoops et al. (2010). 3.2. Paleosols Six moderately to strongly developed paleosols exposed at the loess sections at Agh Band, Now Deh and Neka were selected for comparison with the modern soils (Fig. 1). Modern rainfall at the sections was estimated at 350, 600, and 750 mm per year (Kehl et al., 2005b). The loess sections and paleosols are described in detail by Kehl et al. (2005b), while results of luminescence dating and geochemical and mineralogical analyses are discussed in Frechen et al. (2009). For this study, the moderately developed paleosol of the section at Agh Band (AB 1, Bw horizon) and the three strongly developed paleosols from Now Deh section, exposed at 13.5 m (ND 1, Bt), 20 m (ND 2, Bt), and 24 m (ND 3, Bt) depth below surface were chosen (Fig. 2). The two well developed paleosols from Neka section exposed at about 12 m (NK 1, Bt) and 15 m (NK 2, ABt) below the present land surface were also included. The paleosols AB 1, ND 1 and NK 2 probably correlate with the last Interglacial sensu stricto or marine isotope stage (MIS) 5e. NK 1 may correlate either with interstadials of MIS 5a and/or 5c or represent MIS 5e (Frechen et al., 2009). In the latter case, paleosol NK 2 would represent MIS 7. The chronological positions of the paleosols ND 2 and ND 3 is unknown but is older than MIS 5e. The most strongly developed horizons of the paleosols were chosen for comparison with the corresponding soil horizons of the modern soils. The paleosols ND 1 and ND 2 are erosional remnants of the original soil profiles. However, it is most likely that their Bt horizons reflect the most strongly developed soil horizons in respect to clay enrichment, micromorphological characteristics and clay mineralogy. Furthermore, the lower part of this argillic horizon

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may have been protected from shrink and swell. In upper parts of argillic horizons, this process might have destroyed clay coatings and affected b-fabrics as reported from loess soils in Kansas (cf. Gunal and Ransom, 2006b). Paleosol horizons and parental loesses were analysed for inorganic carbon (IC) using the gas volumetric method (Schlichting et al., 1995). IC was expressed as CaCO3 equivalent (CCE). Organic carbon (OC) was calculated by substracting the total amounts of carbon before and after dry combustion of organic matter for 5 h at 550  C. In both cases, total carbon was measured with a C/N/S analyzer (Forno EA of Fisons Instruments, Italy). The pH was recorded in a suspension of 10 g of soil and 25 ml of 0.01 M CaCl2 solution. Electrical conductivity was determined in a 1:5 soilewater extract. For the analyses of gypsum, the acetone method was applied (Van Reeuwijk, 1995). Herein, reference is made to corresponding results presented in Kehl et al. (2005b), and new data on the grainsize distribution is presented, as measured after destruction of carbonates with hydrochloric acid and using a combined wetsieving pipette method after dispersion with Na2P4O7  10 H2O (Schlichting et al., 1995). In addition, results of micromorphological investigations as studied in thin sections (maximum 5 cm  3 cm) will be presented using the terminology of Bullock et al. (1985) and Stoops (2003).

4. Results and discussions 4.1. Modern soils 4.1.1. Soil classification The classifications of the soils are shown in Table 2. Soils of the aridic SMR are mainly classified as weakly developed Entisols lacking any developed horizons. In the dry xeric regions, Haploxerepts with weakly developed cambic horizons and also Calcixerepts with calcic horizons were observed showing the initiation of brunification and structure formation as soil forming processes. In the typic xeric regions (pedon 4), however, the conditions were favorable for the downward decalcification and the subsequent clay illuviation and formation of moderately developed argillic horizons and formation of Calcic Argixerolls. In the udic SMR, well developed Alfisols with deep Bt horizons were formed. The depth of carbonate accumulation is about 1 m (pedon 8) and deeper than in soils of the xeric region (45 cm, pedon 4, Table 2). Pedons 7 and 8 were both classified as Hapludalfs, and there was not any significant difference in the soil development expressed by the classification even up to the family level except for the physico-chemical properties such as higher clay content for pedon 6, which can probably be explained by higher precipitation (900 mm, compared to 800 mm for pedon 7). Comparison of the three soils of the typic xeric region, pedons 4, 5, 6 in an east-west direction, shows the absence of any significant pedogenic differences resulted in the similar classification of Calcic Argixerolls. The horizonation and genesis of the soils therefore follow a developmental trend with increasing precipitation as shown below:

A=C / A=Bw=C / A=Bk= C / A=Bt=Bk=C / A=Bt1=Bt2=Bk=C Aridic

dry Xeric

dry Xeric

typic Xeric

Udic

4.1.2. Soil physico-chemical properties As seen in Table 2, clay and silt contents of modern soils along the precipitation gradient show a trend of increasing clay and decreasing silt contents with rainfall. In the North, near the source area of loess, the sand content is higher and less clay was formed by weathering processes (Fig. 3). Silt is the dominant particle size in all the soils. As seen in the C horizons of pedons 2 and 3, the typical

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Table 2 Classification and selected physico-chemical properties of the studied pedons. Pedon

Depth

Horizon

CCE

Clay

Silt

Sand

SOC

CEC

EC

Gypsum

Reaction

%

%

%

%

%

Cmolc kg1

dSm1

%

HCl 1N

0.5 0.3 0.2

11.0 10.2 9.6

0.9 2.1 10.6

2 3 3.5

þ þ þ

Fine-silty, mixed, calcareous, thermic, Typic Haploxerept (Haplic Cambisol) 0e18 A 7.7 17 20 70 10 18e35 Bw1 7.4 20 20 64 16 35e65 Bw2 7.8 20 16 74 10 65e120 C 7.8 22 16 73 11

0.6 0.5 0.4 0.2

14.1 16.2 13.0 13.2

0.9 1.8 2.3 3.0

0.7 0.9 1.2 4.0

þ þ þ þ

Fine-silty, mixed, thermic, Typic Calcixerept (Haplic 0e30 A 7.4 20 30e85 Bk 7.7 18 85e130 C 7.8 20

0.7 0.6 0.4

18 20 17

0.8 0.5 0.4

0.1 0.2 0.1

þ þ þ

2.5 1.0 0.8 0.2

34 24 26 30

0.2 0.3 0.3 0.5

   

  þ þ

cm 1

2

3

4

5

6

7

8

pH

Fine-loamy, mixed, active, calcareous, thermic, Typic Torriothent (Haplic 0e20 A 7.9 18 18 57 20e55 C1 8.0 19 20 63 55e110 C2 8.1 17 19 60

Fine-silty, mixed, superactive, 0e22 A 22e45 Bt 45e90 Bk 90e125 C

Calcisol) 22 19 20

65 73 70

Reglosol) 25 17 21

13 8 10

thermic, Calcic Argixeroll (Luvi-Calcic Chernozem) 7.3 1 32 63 5 7.4 3 39 55 6 7.4 18 25 63 12 7.5 20 23 65 13

Clayey, mixed, 0e26 26e48 48e87 88e135

superactive, thermic, Calcic Argixeroll (Luvi-Calcic Chernozem) A 7.0 2 32 58 10 Bt 7.1 3 40 50 10 Bk 7.4 16 35 57 8 C 7.5 21 24 64 12

2.2 1.1 0.6 0.1

35 27 27 30

0.1 0.2 0.3 0.5

   

  þ þ

Clayey, mixed, 0e34 34e67 67e100 100e140

superactive, thermic, Calcic Argixeroll (Luvi-Calcic Chernozem) A 6.9 2 35 50 15 Bt 7.1 4 45 48 7 Bk 7.1 17 35 52 13 C 7.4 23 25 60 15

2.0 1.0 0.7 0.1

39 28 26 25

0.1 0.1 0.1 0.3

   

  þ þ

20 7 8 15 16

3.5 1.5 1.3 0.5 0.2

62 50 48 35 23

0.1 0.1 0.1 0.2 0.3

    

   þ þ

16 4 4 18 10

6.6 1.0 0.8 0.4 0.1

55 48 43 35.1 25

0.1 0.1 0.1 0.1 0.3

    

   þ þ

Fine-silty, mixed, superactive, 0e19 A Bt1 19e38 38e88 Bt2 88e135 Bk >135 C Clayey, mixed, 0e15 15e50 50e100 100e150 >150

mesic, Mollic Hapludalf (Haplic Luvisol) 6.5 3 18 62 6.4 8 31 62 6.7 8 42 50 7.2 26 25 60 7.5 20 20 64

superactive, mesic, Typic Hapludalf (Haplic Luvisol) A 6.4 5 32 52 Bt1 6.4 8 48 48 Bt2 6.9 8 42 44 Bk 7.2 17 27 55 C 7.5 22 25 65

parent material loess consists of about 20, 70 and 10% clay, silt and sand, respectively. The higher sand content in pedon 1 is mainly due to its nearby source area. By increasing precipitation, the silt content increases (Fig. 3, Table 2) reaching a maximum of 74% in pedons 2 and 3. In the typic xeric regions (pedon 4), by increasing available soil moisture and favorable conditions for soil formation, the clay content increases while the sand and silt fractions decrease. Towards the udic regions (pedons 7 and 8) where the conditions are suitable for decalcification and clay illuviation, and also clay neoformation through weathering of coarser fractions, the clay content increases significantly (∼48% in the Bt horizon of pedon 8, Table 2). COLE values show a significant relationship with clay content of the soils increasing towards humid regions with higher P/ET (ratio of precipitation to evapotranspiration ¼ index of soil available moisture, Khormali et al., 2003) (Fig. 4). In a preliminary study of loess-derived soils in Golestan Province, the COLE values also showed a good correlation to the percentage of smectite (Ghergherechi et al., 2007). Climate driven trends are also observed for other physicochemical properties, i.e. cation exchange capacity, organic carbon, pH and electrical conductivity (Table 2, Fig. 3). Soil pH decreases towards the humid regions where there is a decreasing trend observed for carbonate. The lowest pH is

observed in the A and Bt horizons of pedons 5 and 6. The lowest CCE is 3% determined for the A horizon of pedon 7. The depth of the upper boundary of carbonate accumulation increases from 30 cm in pedon 3 to almost 100 cm in pedon 8 which indicates increased leaching related to precipitation totals. 4.1.3. Clay mineralogy of the soils studied As shown in Table 3, chlorite, illite, smectite, kaolinite, vermiculite and Hydroxy-interlayer vermiculite (HIV) are observed in the clay fraction of the studied pedons. In the aridic and dry xeric SMRs the dominant clay minerals are illite and chlorite (pedons 1, 2, 3). These clay minerals also dominate the clay fraction of the parental material loess as seen in the C horizon of these pedons (Fig. 5). Illite and chlorite are two commonly observed clay minerals occurring mainly in areas where soil formation is limited (Fanning et al., 1989; Wilson, 1999). Their abundance in the soils is largely due to their presence in parent materials. There is some evidence that illite may form pedogenically from K fixation in pre-existing smectites because of the hot and dry soil conditions (Mahjoory, 1975). However, illite (mica) constitutes the main part of all studied parental loesses, and its presence in soils is mainly of detrital origin.

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Fig. 3. Variations of clay, silt, sand fractions (a), organic carbon (b), calcium carbonate (c) and pH (d) as an average in soil solum along the studied precipitation gradient.

In the typic xeric regions where there is higher soil available moisture (P/ET > 0.4), smectite content increases (Table 3, Fig. 5, pedons, 4, 5, and 6). This is in line with the findings of DeAnn Ricks Presley et al. (2010) for the central plains of Northern America. Smectite is present in the parental loess but with small amounts. The increase in smectite especially in the Bt horizons of these pedons therefore is believed to be mainly of transformed origin. Smectite constitutes the major portion of the clay minerals in welldrained Alfisols of southern Iran (Khormali and Abtahi, 2003). Increasing soil available moisture, and consequently a relative leaching environment for the release of Kþ from micaceous minerals and mainly illite, in the calcareous environment with high Mg2þ and high Si4þ mobility may provide favorable conditions for the formation of smectite through transformation. In the more humid areas (udic SMR, pedons 7 and 8), vermiculite appears and constitutes the major portion of the clay fraction besides illite (Table 3, Fig. 5). Vermiculite is present in small amounts in the loess material as reported by Khormali et al. (2006). The occurrence of vermiculite in the forest land is due to

0.90 0.80 0.70

P/ET

O

0.60

higher leaching conditions and the removal of K mainly from mica (Douglas, 1989). According to Boettinger and Southard (1995), moisture availability for chemical weathering and dampened temperature fluctuations provide favorable conditions for vermiculite stability, which may explain the high vermiculite contents in pedons 7 and 8. Egli et al. (2008) believe that higher precipitation rates and the production of organic chelating compounds in the soil promote the appearance of vermiculite by organic acid weathering rather than carbonic acid weathering of primary mica. In high chemical weathering conditions, hydroxy-interlayer vermiculite can be formed (Fig. 5). Hydroxy-interlayer vermiculite is present in considerable amounts in the argillic horizons of pedon 8.

Table 3 Semiquantitative analyses of clay minerals in the studied soils. Pedon SMR

Hor. Illite

1

Aridic

A C

2

3

0.7825

y = 5.6024x 2 R = 0.7093

0.50 0.40

þ þ

þþ þ

 

 

Dry Xeric A Bw C

þþþ þþþ þþþ

þþ þþ þþ

þ þ þ

þ þ þ

  

  

Dry Xeric A Bk C

þþþ þþþ þþþ

þþ þþþ þþþ

þ þ þ

þ þ þ

  

  

4, 5, 6 Xeric

A Bt C

þþ þþ þþ

þþ þþ þþ

þþ þþþ þþ

þ þ þ

 þ 

  

7

Udic

A Bt C

þþ þþ þþ

þ þ þ

þþ þþ þþ

þ þ þ

þþ þþ þþ

  

8

Udic

A Bt C

þþ þþ þþ

þ  þ

þ þ þ

þ þ þþ

þþ þ þ

 þþ 

0.30 0.20 0.10 0.00 0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

COLE Fig. 4. Relationship between COLE and P/ET in the study area.

0.09

Chlorite Smectite Kaolinite Vermiculite HIV

þþþþ þþþ þþþ þþþ

þþþþ: >50%; þþþ: 30e50%; þþ: 10e25%, þ: <10%, : not present.

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Fig. 5. X-ray diffractograms of the clay fraction in (a and b) A and C horizons of pedon 1; (c and d) A and Bt horizons of pedon 4; and (e and f) Bt1 and Bt2 horizons of pedons 7 and 8, respectively.

4.1.4. Micromorphological studies The main micromorphological properties of the studied pedons are summarized in Table 4. The microstructure of the most developed horizons of the soils ranges from structureless in pedon 1 to weakly developed as in pedons 2 and 3 and well developed subangular blocky in pedons 4 to 8 (Table 4, Fig. 6). The c/f related distribution is porphyritic in all the soils, with quartz, feldspar and carbonate grains as the main coarse constituents.

The most characteristic pedofeatures are calcite depletion zones. They can be identified as areas of speckled or striated b-fabric in a micromass dominated by a crystallitic b-fabric. The term calcite depletion, as defined by Bullock et al. (1985), has to be understood as a simple statement that a zone contains less calcite than the surrounding groundmass, and not as an identification of a process (Khormali et al., 2003). These depletion zones are well expressed in pedons 4, 5, and 6 and much more extended in pedons

F. Khormali, M. Kehl / Quaternary International 234 (2011) 109e123

117

Fig. 5. (Continued).

7 and 8 (Fig. 6). The dominant b-fabric varies thus from calcitic crystallitic in pedons 1, 2, and 3 to speckled and crystallitic in pedons 4, 5, and 6 to striated in the Bt horizons of pedons 7 and 8 (Table 4 and Fig. 6). As these decalcification features are localised phenomena in a groundmass with a crystallitic b-fabric, they are

not in contradiction to the presence of relatively high amounts of carbonates revealed by chemical analyses of bulk samples. Various forms of calcitic pedofeatures are present. Most notable are microcrystalline impregnative and pure calcite nodules (Fig. 6), which occur in most of the soils. Nodules of needle shaped calcite

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Fig. 5. (Continued).

F. Khormali, M. Kehl / Quaternary International 234 (2011) 109e123

119

Table 4 Micromorphological properties of the most developed horizons of the studied pedons. Pedon/Horizon

Microstructure

B-fabric

Clay coating

Decalcified zone

Alteration degree

Fe/Mn hydroxide

MISECAa (Degree of soil development)

1. A (Torriorthent) 2. Bw (Haploxerept) 3. Bk (Calcixerept)

Calcitic crystallitic Calcitic crystallitic Calcitic crystallitic

e e e

e e e

0 0 0

e e e

1, non-developed 3, weakly developed 3, weakly developed

4. Bt (Argixeroll) 5. Bt (Argixeroll)

Massive Weakly separated sbk Moderately separated sbk Well separated abk Well separated abk

Speckled and crystallitic Speckled and crystallitic

50% 60%

1 1

Few Few

12, moderately developed 13, moderately developed

6. Bt (Argixeroll)

Well separated abk

Speckled and crystallitic

75%

2

Few

15, moderately developed

7. Bt2 (Hapludalf) 8. Bt2 (Hapludalf)

Well separated abk Well separated abk

Striated and speckled Striated and speckled

Few Few to common few to common Common Common

>90% >90%

2 2

Common Common

18, well developed 19, well developed

abk ¼ angular blocky, sbk ¼ subangular blocky. a Khormali et al. (2003).

and cytomorphic calcite are occasionally observed. Fe/Mn hydroxide nodules occur in volume percentages varying from about 10% in pedons 7 and 8 to as little as 2% in other pedons. In the field, shiny surfaces considered as clay coatings were seen in most of the pedons with Bt horizons, but in thin sections more pronounced clay coatings occur in pedons 7 and 8 of udic regions (Fig. 6). According to Verheye and Stoops (1973), Sobecki and Wilding (1983), Kemp and Zarate (2000) and others, the main factor responsible for the absence or lower occurrence of clay coatings in argillic horizons is their physical disturbance by shrinking and swelling in soils with high clay contents and also smectitic type of clay. The best developed and preserved clay coatings therefore, are seen in pedons 7 and 8, as expected due the higher soil moisture, dominance of vermiculite and therefore lower shrink-swell properties and their better preservation. The limited expanding nature of the vermiculitic type of clay minerals and the stable landscape on which they have formed have provided a suitable environment for the downward translocation of clay and thus formation of thick and more pronounced clay coatings in udic regions and contributed to their preservation (Fig. 6). It may appear surprising that clay illuviation is observed in the loess soils of the xeric SMR showing pH values of 7 to 7.4 and carbonate contents exceeding trace values (pedons 4e6). As bulk samples have been analysed, the pH and CCE represent average values of the horizons. Partial decalcification and lowering of the pH have enabled local dispersion, translocation and illuviation of clay particles to form weak Bt. On the other hand, dust inputs from nearby desert areas of the Turkmen steppe might have supplied fine carbonate particles keeping pH and carbonate comparatively high. This could have resulted in a retardation of soil development in soils located near the dust source. 4.2. Paleosols The brown to reddish brown soil horizons are depleted in carbonate relative to the underlying lighter colored Ck horizons (Table 5). The latter are slightly to strongly enriched in secondary carbonate as compared to the median amount of primary carbonate estimated for the Last Glacial loesses at 12%, 16%, and 17% at Agh Band, Now Deh and Neka, respectively (Frechen et al., 2009). The low carbonate concentrations in the Bt and ABt horizons at Now Deh and Neka are related to slight recalcification from the overlying loess as indicated by small concretions and infillings of carbonate in the larger pores observed in the field. The paleosol horizons show a moderate to strong enrichment in clay particles due to clay neoformation and clay illuviation from overlying horizons as evidenced by clear clay coatings in the Bt and ABt horizons, as discussed below. The percentages of clay are in

general considerably higher in paleosol horizons than in the underlying parental loesses, while silt shows an inverse trend. Comparatively high clay percentages in C horizons of the sections Neka and Now Deh most probably originate from long-distance transport and accumulation of preweathered fines. The percentage of silicate sand is low in both B and C horizons. The soil organic carbon (SOC) contents of the B and C horizons are considerably lower than those of the modern soils. A slight enrichment due to soil formation is only detected in the dark colored ABt horizon of paleosol NK 2. However, the humus enriched topsoil is probably lost by erosion and/or mineralisation might have degraded the organic material after burial. The pH, EC and gypsum values of paleosols and parental loesses show some relation to the present climatic gradient. The highest pH values are recorded at Agh Band and the lowest at Neka. The EC values, as a proxy for the concentration of readily soluble salts decrease with increasing rainfall. A similar trend is indicated by gypsum, which is found in loess at Agh Band and Now Deh sections with higher concentrations at Agh Band (cf. Kehl, 2010). The mineralogical composition of the clay fraction shows a dominance of illite in all paleosol horizons and parental loesses (Table 6). Chlorite is found in all samples. Since the reflection at 1.4 nm remains stable after saturation with Kþ and heating to 550  C, it is very likely, that it is primary chlorite of detrital origin. Primary chlorite together with vermiculite occurs only in the samples from the section at Neka. Smectite contents are more or less similar at Now Deh and Neka, but significantly lower at Agh Band. Kaolinite is present in small amounts and missing in samples from Now Deh section. Soil formation apparently did not change the relative composition of clay minerals. The clay mineral composition of the paleosols thus reflects climate related trends observed in the modern soils with vermiculite occurring only in the moister part of the spectrum, and reduced amounts of smectite in the dryer areas. Thin sections of the Bt horizons show abundant calcite depletion pedofeatures and some occurrences of secondary calcites including pore infillings, coatings and hypocoatings. The secondary calcite probably sums up to the low CCE values of the Bt horizons. The other micromorphological properties of the paleosols (Table 7) show some resemblances to the climate dependent features of the modern soils. The microstructure ranges from weakly developed subangular blocky in the AB 1 of Agh Band to well separated fine angular blocky in NK 2 at Neka. Accordingly, the b-fabric indicates increasing degrees of soil formation comparing the crystallitic and striated types at Agh Band and Now Deh, respectively. Within the well developed soils, the micromorphological features and the MISECA of NK 1 at Neka show a slightly lower degree of soil formation which corroborates its correlation with an interstadial of MIS 5 (Kehl et al., 2005b). The other B horizons of interglacial paleosols reflect more advanced soil formation reflected by MISECA values of 18 to 20.

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F. Khormali, M. Kehl / Quaternary International 234 (2011) 109e123

Fig. 6. (a, PPL) Massive microstructure of the C horizon of pedon 1 and (b, PPL) well separated subangular blocky microstructure of Bt horizon of pedon 7; (c, XPL) Crystallitic b-fabric in Bk horizon of pedon 3, and (d, XPL) speckled b-fabric in the Bt horizon of pedon 7; (e, XPL) Micritic calcite coatings and hypocoatings and infilling of calcite (f, XPL) along channels in the Bk horizon of pedon 4; (g, XPL) thick coatings of clay along channels of the Bt2 horizon of pedon 8; (h, XPL) Decalcified zones (shown by white arrow) and associated cytomorphic calcite in Bk horizon of pedon 5.

The Bt horizons at the sections Now Deh and Neka show that leaching and subsequent clay illuviation were pronounced during the corresponding phases of soil formation. In the studied modern soils, high degrees of leaching and clay illuviation are found under udic soil moisture regime (SMR) rather than under the xeric SMR. The modern soil at the section at Now Deh is a Typic Calcixeroll not

characterised by clay illuviation. This may be related to truncation caused by soil erosion or slightly lower edaphic moisture and percolation during the present interglacial (Kehl et al., 2005b). However, a Pachic Argixeroll derived from loess and located at a distance of about 1.5 km to the east and about 200 m upslope of the section at Now Deh shows a well developed, about 30 cm thick

F. Khormali, M. Kehl / Quaternary International 234 (2011) 109e123

121

Table 5 Selected physico-chemical properties of paleosol horizons and their parent materials. Paleosol

Horizon

Samplea

Soil color

pH

CCE

Clayb

Siltb

Sandb

SOC

EC

CaCl2

%

%

%

%

%

dS m1

%

Gypsum

AB 1

Bw Ck1

AB 10 AB 11

10YR 4/4 10YR 5/4

8.6 8.8

3.8 20.2

28 18

71 76

2 6

0.2 0.1

2.9 2.0

0.9 0.4

ND 1

Bt Ck1

ND 22 ND 23

7.5YR 3/4 10YR 4/6

7.9 8.0

1.3 20.6

41 35

56 63

4 2

0.2 0.2

0.1 0.3

e 0.2

ND 2

Bt Ck

ND 32 ND 33

7.5YR 3/4 10YR 6/4

8.2 8.2

0.8 41.2

47 37

50 62

2 1

0.2 0.2

1.9 0.9

0.5 e

ND 3

Bt2 Ck

ND 37 ND 39

7.5YR 3/4 10YR 6/4

8.0 8.0

1.3 36.5

41 27

55 66

4 7

0.3 0.1

0.2 0.2

e e

NK 1

Bt Ck2

Nk 14 Nk 16

7.5YR 4/4 10YR 5/4

7.6 7.8

0.6 35.4

39 29

60 70

1 1

0.3 0.1

0.1 0.1

e e

NK 2

ABt1 Ck

Nk 20 Nk 23

10YR 3/4 10YR 5/4

7.7 7.9

1.1 21.4

43 31

56 68

1 1

0.4 0.1

0.1 0.1

e e

a Horizon and sample number acc. to Kehl et al. (2005b). All horizons are buried below at least 10 m of loess. The code Ck represents a C horizon enriched with pedogenic (secondary) carbonate. The meaning is equivalent to Bk horizons, as listed in Tables 2e4. b Grain size distribution after destruction of carbonates using hydrochloric acid.

Table 6 Semiquantitative analysis of clay minerals in parental loess and paleosol horizons. Chlorite þ vermiculite

Paleosol

Horizon

Sample*

Illite

Chlorite

Smectite

Kaolinite

AB 1

Bw Ck1

AB 10 AB 11

þþþþ þþþþ

þþ þþ

þ þ

þ þ

ND 1

Bt Ck1

ND 22 ND 23

þþþþ þþþþ

þþþ þþ

þþ þþ

 

ND 2

Bt C(t)k

ND 32 ND 33

þþþþ þþþþ

þþþ þþ

þþ þþ

 

ND 3

Bt2 Ck

ND 37 ND 39

þþþþ þþþþ

þþ þþ

þþ þþ

 

NK 1

Bt Ck2

Nk 14 Nk 16

þþþþ þþþþ

þþ þþ

þþ þþ

þ þ

NK 2

ABt1 Ck

Nk 20 Nk 23

þþþþ þþþþ

þþ þþ

þþ þþ

þ þ

þþþþ: >50%; þþþ: 30e50%; þþ: 10e25%, þ: <10%, : not present.

Bt horizon (Khormali, unpublished data). Ancient analogues of Argixerolls and Calcixerolls were not found in the loess sequence of Now Deh. Edaphic moisture and percolation during former interglacials were probably higher than today. Alternatively, part of the organic carbon, characteristic of mollic horizons (cf. Nettleton et al., 2000) was reduced by mineralisation after burial. Table 8 presents the correlation of paleosols with modern soils as based on the micromorphological approach of Kühn et al. (2006). The featuresets of the paleosols reflect the general precipitation driven trend of increasing development of modern soils from north to south but do not indicate a further distinction between different Bt horizons at

the section at Now Deh. The paleosol NK 1 appears to reflect similar moisture conditions as today. Overall, the climatic boundary for the onset of clay illuviation in former interglacial times appears to have been located near the studied profiles and the paleosol type may be a good proxy of former climatic conditions in the area. Summarizing the above comparison between modern and ancient soils, the paleosol horizon at Agh Band most probably reflect a dry-xeric soil moisture regime (SMR), whereas paleosol horizons at Neka and Now Deh appear to have developed under (moist) xeric to udic SMR. A better understanding on the genesis of the loess derived modern soils and paleosols is needed for more

Table 7 Micromorphological properties of the most developed horizons of paleosols. Paleosol/ Horizon

Microstructure

B-fabric

Clay coatings

Decalcified zone

Alteration degree

Fe/Mn hydroxide

MISECAa (Degree of development)

AB 1 Bw ND 1 Bt ND 2 Bt ND 3 Bt NK 1 Bt

Weakly developed sbk Well separated sbk Well separated sbk Moderately separated sbk Mod. separated sbk to fine abk Well separated fine abk

Crystallitic, partially speckled Speckled, partially striated Striated Speckled, partially striated Speckled, partially striated

e Common Common Common Few

20% >90% >90% >90% >90%

0 1 2 1 1

Very few Common Few Common Few

5, weakly developed 18, well developed 20, well developed 19, well developed 16, moderately developed

Speckled, partially striated

Common

>90%

2

Common

19, well developed

NK 2 ABt

abk ¼ angular blocky, sbk ¼ subangular blocky. a Khormali et al. (2003).

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Table 8 Correlation of paleosols with modern soils according to their micromorphological feature sets. SMRa/P (mm), Correlated paleosol/ estimated modern soil for paleosols

Horizon Micromorphological properties Microstructure

b-fabric

Clay coating

Decalcified Alteration Fe/MnMISECA zone degree hydroxide

Dry Xeric 350-450

Pedon 2 (Haploxerept) Bw AB 1 Bw

Weakly separated sbk Calcitic crystallitic Weakly separated sbk Crystallitic, partially speckled

e e

e 20%

0 0

e Very few

3, weakly developed 5, weakly developed

Typic Xeric 500e700

Pedon 6 (Argixeroll)

Bt

Well separated abk

75%

2

Few

NK 1

Bt

Mod. separated sbk to fine abk

Few to common Few

>90%

1

Few

15, moderately developed 16, moderately developed

Xeric-Udic > 700

Pedon 7 (Hapludalf) ND (1,2,3)

Bt2 Bt

Well separated abk Moderately separated sbk

Striated and speckled Common >90% Striated and speckled Common >90%

2 1

Common Common

18, well developed 19, well developed

Xeric-Udic > 700

Pedon 8 (Hapludalf)

Bt2

Well separated abk

Common >90%

2

Common

19, well developed

NK 2

ABt

Well separated abk

Speckled, partially striated Speckled, partially striated

Common >90%

2

Common

19, well developed

a

Speckled, partially crystallitic Speckled, partially striated

SMR: Soil Moisture Regime, P: Mean annual precipitation.

precise paleoclimatic reconstructions in the area. Also, more precise information is needed about the duration of soil development for surface and buried soils. 5. Conclusions Morphology, physicochemical properties and clay minerals of modern climatophytomorphic soils show a strong correlation with precipitation along the pronounced climate gradient in the area. This gives a climosequence of modern soils that is partly mirrored by climatophytomorphic paleosols of the last interglacial period indicating a similar climatic gradient from semiarid to subhumid conditions during the past. The genesis of modern soils and paleosols deserves further study, because these may represent erosional remnants rather than fully developed soil profiles. Overall, paleoclimatic reconstructions in the area based on soil properties appear promising. The study stresses that Iranian loesses bear much information on Mid to Late Quaternary climate change and landscape evolution in the south-western periphery of Central Asia. More investigations should include mapping of modern soils and loess-soil sequences in order to identify the boundary between soils with and without clay illuviation as a proxy for paleo-moisture. In addition, more age estimates for the modern and ancient soils are needed in order to set up more reliable chronological frameworks for northern Iranian soil climosequences. Acknowledgements The sedimentological and palaeopedological investigations of northern Iranian loess-soil sequences at Neka, Now Deh and Agh Band were supported by a grant from the German Research Foundation (DFG-Gz. KE 818/4-1). References Beavers, A.H., Johns, W.D., Grim, R.E., Odell, R.T., 1955. Clay minerals in some Illinois soils developed from loess and till under grass vegetation. In: Milligan, W.O. (Ed.), Clays and Clay Minerals. Proceedings of the 3rd National Conference on Clays and Clay Minerals, Publication 395, Houston, TX, 26e29 Oct. 1954. National Academy of Sciences, National Research Council, Washington, DC, pp. 356e372. Birkeland, P.W., 1999. Soils and Geomorphology, third ed. Oxford University Press, 430 p.

Boettinger, J.L., Southard, R.J., 1995. Phyllosilicate distribution and origin in Aridisols on a granitic pediment, Western Mojave Desert. Soil Science Society of America Journal 59, 1189e1198. Borchardt, G., 1989. Smectites. In: Dixon, J.B., Weed, S.B. (Eds.), Minerals in Soil EnvironmentsSoil Science Society of America Book Series, second ed., vol. 1, pp. 675e718. Madison, WI. Bullock, P., Federoff, N., Jongerius, A., Stoops, G., Tursina, T., Babel, U., 1985. Handbook for Soil Thin Section Description. Waine Research Publications, Wolverhampton (U.K). Buschiazzo, D.E., Quiroga, A.R., Stahr, K., 1991. Patterns of organic matter accumulation in soils of the semiarid Argentinian Pampas. Zeitschrift für Pflanzenernährung und Bodenkunde 154, 437e441. Chapman, H.D., 1965. Cation exchange capacity. In: Black, C.A. (Ed.), Methods of Soil Analysis. Part 2. American Society of Agronomy, Madison, WI, pp. 891e901. DeAnn Ricks Presley, Hartley, P.E., Ransom, M.D., 2010. Mineralogy and morphological properties of buried polygenetic paleosols formed in late quaternary sediments on upland landscapes of the central plains, USA. Geoderma 154, 508e517. Day, R., 1965. Particle fractionation and particle size analysis. In: Black, C.A. (Ed.), Methods of Soil Analysis. Part 1, American Society of Agronomy Series No. 9, pp. 545e566. Madison, WI. Dinpashoh, Y., 2006. Study of reference crop evapotranspiration in Islamic Republic of Iran. Agricultural Water Management 84, 123e129. Douglas, L.A., 1989. Vermiculites. In: Dixon, J.B., Weed, S.B. (Eds.), Minerals in Soil Environments, second ed. Soil Science Society of America, Madison, WI, pp. 635e674. Egli, M., Mirabella, A., Sartori, G., 2008. The role of climate and vegetation in weathering and clay mineral formation in late Quaternary soils of the Swiss and Italian Alps. Geomorphology 102, 307e324. Fanning, D.S., Keramidas, V.S., El-Desoky, M.A., 1989. Micas. In: Dixon, J.B., Weed, S.B. (Eds.), Minerals in Soil Environments, second ed. Soil Science Society of America, Madison, WI, pp. 551e624. Franzmeier, D.P., Ross Jr., S.J., 1968. Soil swelling: laboratory measurement and relation to other soil properties. Soil Science Society of America Proceedings 32, 573e577. Frechen, M., Kehl, M., Rolf, C., Sarvati, R., Skowronek, A., 2009. Loess chronology of the Caspian Lowland in Northern Iran. Quaternary International 128, 220e233. Gerasimova, M.I., Gubin, S.V., Shoba, S.A., 1996. Soils of Russia and Adjacent Countries: Geography and Micromorphology. Agricultural University, Wageningen, 204 pp. Ghergherechi, Sh., Khormali, F., Mamoodi, S., Ayoubi, S., 2007. Micromorphology of gypsum formed in soils of a climo-toposequence in western Golestan Province. In: 10th Proceedings of the 10th Iranian Congress of Soil Science, Karaj, August 25e27, 2010. Ghergherechi, Sh., Khormali, F., Mahmoodi, S., Ayoubi, S., 2009. Micromorphology of argillic horizon development in loess derived soils of humid and subhumid regions of Golestan Province, Iran. Journal of Soil and Water Research 40, 130e138. Griffin, R.W., Buol, S.W., 1988. Soil and saprolite characteristics of vertic and aquic Hapludults from Triassic Basin Sandstones. Soil Science Society of America Journal 52, 1094e1099. Grossman, R.B., Brasher, B.R., Franzmeier, D.P., Walker, J.L., 1968. Linear extensibility as calculated from natural-clod bulk density. Soil Science Society of America Proceedings 32, 570e577. Günster, N., Skowronek, A., 2001. Sediment-soil sequences in the Granada Basin as evidence for long- and short-term climatic changes during the Pliocene and Quaternary in the Western Mediterranean. Quaternary International 78, 17e32.

F. Khormali, M. Kehl / Quaternary International 234 (2011) 109e123 Gunal, H., Ransom, M.D., 2006a. Genesis and micromorphology of loess-derived soils from central Kansas. Catena 65, 222e236. Gunal, H., Ransom, M.D., 2006b. Clay illuviation and calcium carbonat accumulation along a precipitation gradient in Kansas. Catena 68, 59e69. Jackson, M.L., 1975. Soil Chemical Analysis, Advanced Course. University of Wisconsin, College of Agriculture, Department of Soil Science, Madison, WI. Jackson, M.L., Hseung, Y., Corey, R.B., Evans, E.J., Van den Heuvel, R.C., 1952. Weathering sequence of clay-size minerals in soils and sediments: II. Chemical weathering of layer silicates. Soil Science Society of America Proceedings 17, 3e6. Jenny, H., 1941. Factors of Soil Formation. A System of Quantitative Pedology. McGraw-Hill, New York, 109 p. Jenny, H., 1961. Derivation of state factor equations of soils and ecosystems. Soil Science Society of America Proceedings 25, 385e388. Johns, W.D., Grim, R.E., Bradley, W.F., 1954. Quantitative estimation of clay minerals by diffraction methods. Journal of Sedimentary Petrology 24, 242e251. Kehl, M., 2009. Quaternary climate change in Iran e the state of knowledge. Erdkunde 63, 1e17. Kehl, M., 2010. Late Quaternary Loesses, Loess-Like Sediments, Soils and Climate Change in Iran. Relief, Boden, Paläoklima, E. Schweizerbart Science Publishers, Stuttgart, Germany, 210 pp. Kehl, M., Frechen, M., Skowronek, A., 2005a. Paleosols derived from loess and loess-like sediments in the Basin of Persepolis, Southern Iran. Quaternary International 140/141, 135e149. Kehl, M., Sarvati, R., Ahmadi, H., Frechen, M., Skowronek, A., 2005b. Loess paleosolsequences along a climatic gradient in Northern Iran. Eiszeitalter u. Gegenwart 55, 149e173. Kemp, R.A., Zarate, M.A., 2000. Pliocene pedosedimentary cycles in the southern Pampas, Argentina. Sedimentology 47, 3e14. Khormali, F., Abtahi, A., 2003. Origin and distribution of clay minerals in calcareous arid and semi-arid soils of Fars Province, southern Iran. Clay Minerals 38, 511e527. Khormali, F., Abtahi, A., Mahmoodi, S., Stoops, G., 2003. Argillic horizon development in calcareous soils of arid and semiarid regions of southern Iran. Catena 53, 273e301. Khormali, F., Ajami, M., Ayoubi, S., 2006. Genesis and micromorphology of soils with loess parent material as affected by deforestation in a hillslope of Golestan province, Iran. In: 18th International Soil Meeting (ISM) on Soil Sustaining Life on Earth, Managing Soil and Technology, May 22e26, 2006, pp. 149e151. Kittrick, J.A., Hope, E.W., 1963. A procedure for particle size separation of soils for X-ray diffraction analysis. Soil Science 96, 312e325. Kühn, P., Terhorst, B., Ottner, F., 2006. Micromorphology of Middle Pleistocene paleosols in northern Italy. Quaternary International, 156e166. Lateef, A.S.A., 1988. Distribution, provenance, age and paleoclimatic record of the loess in Central North Iran. In: Eden, D.N., Furkert, R.J. (Eds.), Loess e Its Distribution, Geology and Soils. Balkema, Rotterdam, pp. 93e101. McDaniel, P.A., Hipple, K.W., 2010. Mineralogy of loess and volcanic ash eolian mantles in Pacific Northwest (USA) landscapes. Geoderma 154, 438e446. Mahjoory, R.A., 1975. Clay mineralogy, physical and chemical properties of some soils in arid regions of Iran. Soil Science Society of America Proceedings 39, 1157e1164.

123

Mehra, O.P., Jackson, M.L., 1960. Iron oxide removal from soils and clays by a dithionite citrate system with sodium bicarbonate. Clays and Clay Minerals 7, 317e327. Murphy, C.P., 1986. Thin Section Preparation of Soils and Sediments. A B Academic Publishers, Berhamsted, UK. Nelson, R.E., 1982. Carbonate and gypsum. In: Page, A.L. (Ed.), Methods of Soil Analysis. Part 2. American Society of Agronomy, Madison, WI, pp. 181e199. Nettleton, W.D., Flach, K.W., Brasher, B.R., 1969. Argillic horizons without clay skins. Soil Science Society of America Proceedings 33, 121e125. Nettleton, W.D., Olsen, C.G., Wysocki, D.A., 2000. Paleosol classification: problems and solutions. Catena 41, 61e92. Nichols, J.D., 1984. Relation of organic carbon to soil properties and climate in the Southern Great Plains. Soil Science Society of America Journal 48, 1382e1384. Pye, K., 1995. The nature, origin and accumulation of loess. Quaternary Science Reviews 14, 653e667. Salinity Laboratory Staff, 1954. Diagnosis and Improvement of Saline and Alkali Soils. United States Department of Agriculture Handbook No. 60 Washington, DC. Schlichting, E., Blume, H.-P., Stahr, K., 1995. Bodenkundliches Praktikum. Blackwell, Wien, 295 pp. Smith, R.M., Twiss, P.C., Krauss, R.K., Brown, M.J., 1970. Dust deposition in relation to site, season, and climate variables. Soil Science Society of America Proceedings 34, 112e117. Sobecki, T.M., Wilding, L.P., 1983. Formation of calcic and argillic horizons in selected soils of the Texas Coast Prairie. Soil Science Society of America Journal 47, 707e715. Soil Survey Staff, 1993. Soil Survey Manual. United States Department of Agriculture Handbook No. 18 Washington, DC. Soil Survey Staff, 1999. Soil Taxonomy, United States Department of Agriculture and Natural Resources Conservation Service, Agriculture Handbook 436, second ed.. Soil Survey Staff, 2010. Keys to Soil Taxonomy, 11th ed. U.S. Department of Agriculture, Natural Resources Conservation Service. Stoops, G., 2003. Guidelines for the Analysis and Description of Soil and Regolith Thin Sections. Soil Science Society of America, Madison, WI. Stoops, G., Marcelino, V., Mees, F., 2010. Interpretation of Micromorphological Features of Soils and Regoliths, first ed. Elsevier Science, 752 pp. SWRI - Soil and Water Research Institute Iran, 2000. Soil Resources and Use Potentiality Map of Iran (1:1 000 000), Tehran. Van Reeuwijk, L.P. (Ed.), 1995 Procedures for Soil Analysis. Technical Paper No. 9, ISRIC, Wageningen. Verheye, W., Stoops, G., 1973. Micromorphological evidences for the identification of an argillic horizon in Terra Rossa soils. In: Rutherford, G.K. (Ed.), Soil Microscopy. Canada, Kingston, pp. 817e831. Wilding, L.P., Drees, L.R., 1988. Removal of carbonate from thin sections for microfabric interpretations. In: Fedoroff, N., Bresson, L.M., Courty, M.A. (Eds.), Soil Micromorphology, Proceedings of the 7th International Working Meeting on Soil Micromorphology. AFES, Paris, pp. 653e665. Wilson, M.J., 1999. The origin and formation of clay minerals in soils: past, present and future perspectives. Clay Minerals 34, 7e24. WRB e World Reference Base for Soil Resources 2006. Food and Agriculture Organization of the United Nations, Rome.