Soil-parent material relationship in a mountainous arid area of Kopet Dagh basin, North East Iran

Catena 152 (2017) 252–267

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Soil-parent material relationship in a mountainous arid area of Kopet Dagh basin, North East Iran Hossein Tazikeh a, Farhad Khormali a,⁎, Arash Amini b, Mojtaba Barani Motlagh a, Shamsollah Ayoubi c a b c

Department of Soil Sciences, Gorgan University of Agricultural Sciences and Natural Resources, Gorgan, Iran Department of Geology, Golestan University, Gorgan 49138-15759, Iran Department of Soil Sciences, College of Agriculture, Isfahan University of Technology, Isfahan 84156-83111, Iran

a r t i c l e

i n f o

Article history: Received 21 July 2016 Received in revised form 27 December 2016 Accepted 13 January 2017 Available online xxxx Keywords: Parent material Soils Arid Kopet Dagh Iran

a b s t r a c t The effects of parent rock types on soil evolution in arid areas were studied in a sequence of soils, derived from different lithologies in the Kopet Dagh basin (Northeastern Iran) using micromorphology, clay mineralogy, magnetic susceptibility and physico-chemical properties. The selected parent rocks and associated soils were shale (Haplocalcids), claystone (Haplotorrerts), gypsiferous marl (Haplogypsids), limestones (Haplocalcids and Torriorthents), siltstone and sandstones (Torriorthents). The results showed that the properties and development of soils were mainly affected by grain size and mineralogy of parent materials. Soil magnetic susceptibility (χlf) variations were attributed to the types of parent material and pedogenic processes. Redistribution of calcite and gypsum in soil profiles and natural and pedogenic formation of ferrimagnetic minerals were responsible for χlf variations. The soils clay mineral origins were found to be mainly of inheritance from parent materials. Smectite was the dominant clay mineral of the most soils. Based on the micromorphological index of soil evolution (MISECA), the soils studied were categorized into weakly developed Orthents, weakly to moderately developed Aridisols (Gypsids and Calcids) and moderately developed soils including Calcids and Torrerts. The degree of microstructure development, alteration of weatherable minerals and calcitic features were the most important criteria influencing assessment of soil development degree by MISECA index. The vertic features were only observed in soils of claystone in which there were considerable amounts of clay and smectite. High amounts of gypsum and low smectite content were mainly responsible for the lack of vertic behavior in other fine grained soils derived from shale and marl. © 2017 Elsevier B.V. All rights reserved.

1. Introduction The Kopet Dagh depositional basin, located in northeastern Iran and southern Turkmenistan, is a mountainous area dominated by sedimentary rocks and arid climate. The mean annual precipitation, temperature and potential evapotranspiration are 255 mm, 13 °C and 764 mm, respectively. The geomorphic surfaces of the area are young and have a close relation with geological structure in which anticlines make mountains and synclines form intermountain basins (Afshar-Harb, 1979). Parent materials play a key role in soilscape diversity in arid and semiarid areas (Badía et al., 2013). Yousefifard et al. (2015) have observed different pedogenetic paths related to types of parent material under semi-arid condition in northwestern Iran. Soil parent material characterization is of paramount importance in soil genesis studies in arid and semi-arid conditions and soils on young surfaces (Schaetzl and Anderson, 2005). Parent materials affect many soil properties that may be critical for soil series separation and to develop soil mapping ⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (F. Khormali).

http://dx.doi.org/10.1016/j.catena.2017.01.020 0341-8162/© 2017 Elsevier B.V. All rights reserved.

models. Soil properties and landscape evolution could indicate the type of underlying parent materials in arid regions as well. It has been stated that the mineralogy of soils in arid area is largely affected by parent material (Woodruff et al., 2009). Inheritance of clay minerals from parent rocks is the most important factor influencing the clay mineral composition of soils which is influenced by weathering intensity and relief (Graham and O'Geen, 2010), climate (Ruffell et al., 2002), vegetation, drainage (Abtahi and Khormali, 2001) and different response of bedrocks to chemical weathering, can lead to different landscapes and weathering products (Dultz, 2000; Yousefifard et al., 2012). Although the clay mineralogy of sedimentary rocks in the Kopet Dagh basin has recently been investigated (Khormali and Amini, 2015), no study has been made on the effects of mineralogy of parent rocks on soil evolution. The presence of different forms of Fe oxides in soils is highly affected by the type of parent material under variable condition such as moisture, temperature, pH, Eh, organic matter and iron release from iron bearing minerals (Filho et al., 2015). Fed (free Fe oxides) indicates the amount of fine-grained secondary Fe oxides released from Fe-bearing minerals during pedogenic processes (Mehra and Jackson, 1960). Fed

H. Tazikeh et al. / Catena 152 (2017) 252–267

(a)

253

(b)

Kopet Dagh N

Fault Road Village Neogene Khangiran FM Pesteligh FM Kalat FM Abderaz FM Aitamir FM Sanganeh FM

5 km

Study area Sorkhezoo Hamami Ali khan Tuplanlu

Sheikh

(c) 37°° 31′ 55.1″ 57°° 25′ 39.57″

37°° 38′ 10.32″ 57°° 29′ 35.11″

P6

N

P5

P1 Sheikh Village

P3

P4 P7

P2

P8 37°° 34′ 11.11″

37°° 35′ 3.49″

57°° 36′ 14.27″

57°° 36′ 13.26″

Fig. 1. Location map of the study area, in Kopet Dagh basin, in northeastern Iran (a), simplified geological map of the study area as a part of Sheikh syncline (b), Google earth image of Sheikh syncline showing locations of soil profiles formed on selected lithologies of different formations: P1: shale of Sanganeh formation, P2: glauconitic sandstone of Aitamir formation, P3: limestone of Abderaz formation, P4: limestone of Kalat formation, P5: claystone of Pesteligh formation, P6: sandstone of Khangiran formation, P7: siltstone of Neogene and P8: gypsiferous marl of Neogene (c).

to total secondary Fe oxides and its correlation with Fed, Feo and Feo/Fed are indicators to evaluate the relations between magnetic susceptibility, Fe oxide crystallinity and pedogenesis (Hu et al., 2009). The main objective of the present study is to investigate soil-parent material relationship using free and active Fe oxide, clay mineralogy, micromorphology as proxies for degree of soil development and soilparent material homogeneity.

content of soils, which is strongly affected by parent material, could represent the degree of pedogenesis (Filho et al., 2015; Hu et al., 2004). Feo reflects the concentration of poorly crystalline Fe oxides in soils (Schwertmann, 1973). The ratio of Feo/Fed reflects Fe oxides crystallinity in soils and has been used to evaluate genetic processes and for assessing degree of profile development (Hu et al., 2009). The magnetic susceptibility of soils, can be measured by fast, cheap and nondestructive methods and has been used in different aspects of pedogenic studies such as climate change records in loess-paleosol sequences, description and diagnosis of parent material uniformity and chronosequence of soils (Jia et al., 2012; Fine et al., 1992; Maher, 1998; Singer et al., 1992; Williams and Cooper, 1990). The magnetic enhancement of soils is mainly related to in situ pedogenic formation of ferrimagnetic minerals and strongly affected by soil-forming factors (Feng and Johnson, 1995; Fine et al., 1992; Singer and Fine, 1989). Slight changes of ferrimagnetic mineral will significantly influence low-field magnetic susceptibility (χ lf values) (Maher, 1998). The χlf/Fed ratio reflects the proportion of ferrimagnetic mineral

2. Materials and methods 2.1. Description of the study area The Kopet Dagh Basin covers an area of approximately 500 km2 in northeast Iran (Fig. 1a). The basin formed as a result of the enclosure of the Paleotethys Ocean induced by convergence of the Iranian and Turanian plates during early Cimmerian orogeny. The final collision folded the entire rocks that had been deposited in the basin from the Jurassic to the Miocene and formed the Kopet Dagh Mountains. (Berberian and

Table 1 Selected parent rocks of studied formations in Sheikh Syncline. Formations, Ages

Sanganeh, Albian

Aitamir, Albian-cenomanian

Abderaz, Turonian

Kalat, Maastrichtian

Pesteligh, lower paleocene

Khangiran, Eocene

Neogene, Miocene

Selected lithologies

Shale

Glauconitic sandstone

Limestone

Limestone

Claystone

Sandstone

Siltstone

37° 33′ 41.7″ 57° 33′ 47.3″

37° 33′ 57.6″ 57° 33′ 37.8″

37° 34′ 4.2″ 57° 33′ 43.2″

37° 34′ 27.4″ 57° 33′ 11.8″

37° 34′ 36.9″ 57° 33′ 9.1″

37° 34′ 36.4″ 57° 34′ 57.4″

Gypsiferous Marl 37° 34′ 17.6″ 57° 36′ 4.7″

1236

1230

1251

1300

1303

1318

1494

Soil profile North: 37° 33′ 20.5″ locations* East: 57° 31′ 53.2″ Elevations 1110 (m above sea level)

Neogene, Miocene

254

H. Tazikeh et al. / Catena 152 (2017) 252–267

Table 2 Some morphological and physico-chemical properties of the studied pedons. Parent rocks Horizons

Depth (cm)

Color (moist)

Stra

Shale: Fine, mixed, active, mesic, Typic Haplocalcids A 0–20 10YR4.5/2 3,C,abk AB 20–35 10YR4.5/2 3,C,abk Bk1 35–60 10YR5/3 2,m,abk Bk2 60–90 10YR5/3 2,m,abk Bw 90–125 10YR5/4 2,m,abk By1 125–150 10YR5/4 2,m,abk By2 150–175 10YR5/4 2,m,abk C N175 5Y4/2 m

CCE

Clay

Silt

Sand

OC

pH 1:5

Gyp g·kg−1

CEC Cmol+·kg−1

Fed (%)

Feo (%)

χlf 10−8 m3·kg−1

(%) 53 57 56 57 55 54 66 –

30 23 29 25 32 32 19 –

17 20 15 16 13 14 15 –

1.9 0.78 0.39 0.70 0.46 0.39 0.19 0.55

7.2 8.6 8.7 8.7 8.5 7.7 7.2 7.2

– 0.4 1.2 3.4 0.5 1.0 5.5 16

25 26 25 41 24 23 25 24

0.88 0.67 0.68 0.70 0.62 0.66 0.56 0.45

0.05 0.053 0.052 0.044 0.048 0.053 0.053 0.050

28 21 22 21 22 23 23 12

Ab Limestone: Loamy-skeletal, carbonatic, mesic, Xeric Torriorthents A 0–20 10YR 6/3 1,f,sbk 51 23 20–70 10YR 6/3 m 50 15 C1 C2 70–120 10YR 6/3 m 55 18

40 22 38

37 60 43

0.35 0.06 0.06

7.2 7.5 7.5

– – –

10 9 10

0.21 0.14 0.18

0.047 0.014 0.028

15 7 10

Gl Sandstone: Fine-loamy, smectitic, calcareous, mesic, Xeric Torriorthents A 0–6 10YR5/6 1,f,sbk 9.5 20 34 AC 6–20 10YR5/6 1,f,sbk 8.5 18 25 C 20–60 10YR4/4 m 1.5 12 37

46 57 52

2.08 1.56 0.12

7.1 7.4 7.6

– – –

12 10 10

0.5 0.6 0.7

0.074 0.062 0.052

10.9 11.2 7.1

Ka Limestone: Fine-loamy, carbonatic, mesic, Typic Haplocalcids A 0–20 10YR4/6 1,f,sbk 31 Bk 20–35 7.5YR4/6 1,f,sbk 41 Cr 35–60 7.5YR4/4 m 44.5

20 24 25

43 40 42

37 36 33

1.75 1.44 1.17

7.2 7.3 7.2

– – –

14 14 17

0.63 0.65 0.67

0.052 0.032 0.028

32.1 27 25.5

Claystone: Fine, smectitic, mesic, Chromic Haplotorrerts Ap 0–35 2.5YR4/6 3,m,abk Bss1 35–80 2.5YR5/6 2,m,abk Bss2 80–120 2.5YR5/6 2,m,abk Cy 120–150 2.5YR5/6 2,m,abk

47 52 52 40

30 40 33 33

23 8 15 27

0.78 0.58 0.35 0.2

7.3 8.2 8.5 7.3

0.9 0.5 0.6 25

25 25 24 19

0.76 0.48 0.6 0.52

0.034 0.02 0.019 0.013

18 14 12 13

Kh Sandstone: Fine-loamy, smectitic, calcareous, mesic, Xeric Torriorthents A 0–13 10YR6/2 1,f,gr 5 30 19 13–20 10YR6/2 m 6 23 13 Cr1 20–60 10YR6/1 m 4.5 34 26 Cr2

51 64 40

1.10 0.81 0.39

7.4 8 8

– – –

19 17 17

0.049 0.044 0.045

0.011 0.018 0.019

10 10.6 10.3

Siltstone: Fine-loamy, smectitic, calcareous, mesic, Xeric Torriorthents A 0–10 10YR5/4 1,f,gr 21.5 22 Cr1 10–35 10YR4.5/4 m 26 25 Cr2 35–70 10YR4/4 m 25.5 20

55 52 54

23 23 26

1.22 0.75 0.17

7.4 7.4 7.4

– – –

16 10 11

0.343 0.29 0.16

0.020 0.011 0.006

35 30 31

Marl: Very fine, smectitic, mesic, Typic Haplogypsids Ap 0–35 10YR4.5/4 1,f,sbk By1 35–80 2.5Y 6/2 2,m,abk By2 80–120 2.5Y5/2 3,m,abk Cssy 120–150 2.5Y4/3 m

21 5 20 18

6 29 14 12

0.47 0.27 0.19 0.08

7.7 7.3 7.3 7.4

1 8 8.1 10

32 46 33 35

0.8 0.71 0.85 0.78

0.026 0.030 0.034 0.042

34 31 28.9 24

6.5 7 8.5 15 10 9 9.5 11.5

13 11 7 11.5

9.5 7 7.5 5

70 66 66 70

a Str = structure; abbreviation for structure are based on field book for describing and sampling soils (National Soil Survey Center, 2012), CCE = calcium carbonate equivalent, OC = organic carbon, CEC = cation exchange capacity.

Table 3 The relative abundance of minerals in the clay fraction of studied soils. Mineralogical composition of clay fraction %a

Parent rocks, Pedons

Horizons

Smectite

Illite

Chlorite

Kaolinite

Vermiculite

Ch/I

Shale Typic Haplocalcids

A Bk2 C

ND ND ND

+++ +++ +++

+ + +

+ + +

+ + +

0.43 0.84 0.77

Ab Limestone Xeric Torriorthents

A C

+++ +++

+ +

+ +

+ +

ND ND

0.61 0.63

Gl Sandstone Xeric Torriorthents

A C

+++ +++

+ +

+ +

+ +

ND ND

– –

Ka Limestone Typic Haplocalcids

A C

+++ +++

++ ++

+ +

+ +

ND ND

0.36 0.62

Claystone Chromic Haplotorrerts

Bss1 C

+++ +++

+ +

+ +

+ +

ND ND

0.66 1.24

Kh Sandstone Xeric Torriorthents

A C

++++ ++++

ND ND

ND ND

ND ND

ND ND

– –

Siltstone Xeric Torriorthents

A C

+++ +++

+ +

+ +

+ +

ND ND

1.08 1.06

Marl Typic Haplogypsids

By1 C

+++ +++

++ ++

+ +

+ +

ND ND

0.6 0.5

a

Relative abundance of clay minerals is shown by: + (b25%), ++ (25–50%), +++ (50–75%), ++++ (N75%), ND: not detected and Ch/I: Chlorite/Illite ratio.

H. Tazikeh et al. / Catena 152 (2017) 252–267

King, 1981). The study area, located in North East of Bojnurd city, is a part of Sheikh Syncline which is a unique area concerning various outcroppings of Kopet Dagh formations. This site has rock outcrops of different formations from Cretaceous to Neogene (Fig. 1b). Eight dominant lithologies of seven formations including Sanganeh (shale), Aitamir (Gl Sandstone, glauconitic), Abderaz (Ab limestone), Kalat (Ka limestone), Pesteligh (claystone), Khangiran (Kh sandstone) and Neogene (siltstone and gypsiferous marl) were selected for soil genesis studies (Fig. 1c). Table 1 shows the selected parent rocks and ages of studied formations. Soil moisture and temperature regimes of the study area calculated by Newhall simulation model (version 1.4.4– 20,110,329) are Aridic and Mesic, respectively (Soil Survey Staff, 2014). The natural vegetation is mainly ephemeral grasses and xerophytic shrubs.

(a)

0

255

2.2. Soil sampling Eight soil profiles derived from selected parent rocks were described using the Soil Survey Manual (Soil Survey Staff, 1993) and classified according to Keys to Soil Taxonomy (Soil Survey Staff, 2014). Soils were mainly selected on the most stable landforms with minimum slope gradient in order to reduce the effect of topography on soil formation and highlight the soil-parent material relationship. The locations of soils are shown in Table 1. Disturbed samples from genetic horizons and parent materials were taken for physico-chemical and clay mineralogical analysis. Undisturbed clods were also taken for micromorphological studies. Except for pedon formed on Ab limestone which was sampled at each horizon due to skeletal nature, other soil profiles were sampled at 5–10 cm intervals for A horizons and 10 cm intervals for deeper parts

0

(b)

20

0

A

20

AB

40

40

Bk1

60

Depth (cm)

Depth (cm)

Bk2

60 Clay/10 Clay/10

80

Bw

Gypsum (%) Gypsum (%)

Bss2

CCE (%) CCE (%)

100

Cy

0

5

Clay/10 Clay/10 Gypsum Gypsum(%) (%) CCE CCE(%) (%) EC EC(dS/m) (dSm-1)

80 100 120

180

C

(c)

20

160

By2

120

15

140

By1

EC (dSm-1) EC (dS/m)

10

20

Ap

Bss1

5

0

10

0

(d)

A

0

60

A

40

10

Depth (cm)

Depth (cm)

By1

Clay/10 Clay/10 Gypsum (%) Gypsum (%) CCE (%) CCE (%) ECEC (dS/m) (dSm-1)

40

20

0

20

Bk

80

Clay /10

20

Gypsum (%) CCE (%)

30

EC(dS/m) (dSm-1) EC

By2 40

Cr

100

Cssy 50

120

(e)

0 1 2 3 4 5 6 7 8 9 0

(f)

5

Deoth (cm)

C1

C2

10 15 20 25

(g)

0

20

A

5

AC

10

Cr

Clay/10 Clay/10

0

35

EC(dS/m) (dSm-1) EC

40

(h)

0

50

CCE CCE(%) (%)

60

EC EC(dS/m) (dSm-1)

70

C2

Gypsum Gypsum(%) (%)

80 90

Cr1

Cr2

Depth (cm)

Depth (cm)

C1

40

5

10 15 20 25

5

20 Clay/10 Clay/10

Clay/10 Gypsum (%) CCE (%) EC(dS/m) (dSm-1) EC

0

A

10 30

25

CCE(%) (%) CCE

0

A

20 30

60

10

15

Gypsum(%) (%) Gypsum

40

5

0

Depth (cm)

A

10

Clay/10

15

Gypsum (%)

20

CCE (%)

25

EC(dS/m) (dSm-1) EC

30 35 40

Fig. 2. Depth trends of selected physico-chemical parameters in pedons studied; Torrerts over claystone (a), Calcids over shale (b), Gypsids over gypsiferous marl (c), Calcids over Ka limestone (d), Torriorthents over Kh sandstone (e), Torriorthents over Gl sandstone (f), Torriorthents over Ab limestone (g) and Torriorthents over siltstone (h).

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H. Tazikeh et al. / Catena 152 (2017) 252–267

(a) Cps

(b) Cps

2-Theta-Scale

2-Theta-Scale

(c) Cps

(d) Cps

2-Theta-Scale

2-Theta-Scale

Fig. 3. X-ray diffraction patterns of the clay fraction of studied pedons and associated parent materials (EG treatments).

H. Tazikeh et al. / Catena 152 (2017) 252–267

(e)

Cps

257

(f)

Cps

2-Theta-Scale

2-Theta-Scale

(g)

Cps

(h)

Cps

2-Theta-Scale

2-Theta-Scale Fig. 3 (continued).

258

H. Tazikeh et al. / Catena 152 (2017) 252–267

of the soil profiles. The collected samples were packed for free Fe oxide (Fed), active iron (Feo) and magnetic susceptibility measurements. 2.3. Laboratory analysis Particle size distribution was determined by hydrometer method (Gee and Bauder, 1986).The dispersion of bulk soils was conducted by chemical treatment with sodium hexametaphosphate followed by mechanical methods. Organic carbon was measured by wet oxidation with chromic acid (Nelson and Sommers, 1982). Gypsum (CaSO4·2H2O) and calcium carbonate equivalent (CCE) were determined by precipitation

A

q

with acetone and acid neutralization (Nelson, 1982), respectively. Soil pH was measured in water using a 1:5 soil/solution ratio (McLean, 1982) and electrical conductivity (total soluble salts) was determined in a saturation extract (Rhoades, 1982). Cation exchange capacity (CEC) was measured using sodium acetate (NaOAc) at pH 8.2 (Chapman, 1965), free iron oxide (Fed) by the citrate–bicarbonate– dithionite (CBD) method (Mehra and Jackson, 1960) and active iron (Feo) by the acid ammonium oxalate method (Schwertmann, 1973). The mass specific low field magnetic susceptibility (χlf) of the airdried samples was measured by Bartington magnetic susceptibility meter with MS2B dual frequency sensor (χlf at 0.47 kHz). The specific

B

o o c C

D

Spt

Cyt

n Cry

E

Sp

F

Fe

Cl

Gyp

G

H

Gyp

Gyp

Spt

Cry

Fig. 4. Represantative photomicrographs of pedons studied: close porphyric c/f related distribution pattern in Torriorthents derived from Kh sandstone (q = quartz and o = organic material) (A); single grain microstructure of Torriorthents frormed on Ab limestone (c = calcite) (B); orthic calcite nodule embedded in speckled and strited b-fabric (spt) in Calcids developed from shale (C); cytomorphic calcite infillings of channel surrounded by decalcified zone (speckled b-fabric) in Bk horizon of Calcids over Ka limestone (D); C horizon (E) and Bss horizon (F) derived from claystone showing presence of gypsum, calcitic crystallitic b-fabric and local concentration of Fe oxide (Fe) and weakly oriented impure clay pedofeature (Cl) in Bss horizon; C horizon (G) and By horizon of Gypsids developed from marl (H) showing presence of gypsum in soil and parent material and partly calcitic crystallitic b-fabric with clayey matrix. All images are presented in cross polarized light except for image A which is in plane polarized light.

H. Tazikeh et al. / Catena 152 (2017) 252–267

or mass susceptibility χ, measured in units of m3 kg−1, is defined as the ratio of the material magnetization, σ (per unit mass), to the weak external magnetic field H: χ ¼ σ=H For clay mineral studies, clay fractions were separated by conventional Stoke's settling velocity principle after removing of cementing agent according to Mehra and Jackson (1960) and Kittrick and Hope (1963). The clay oriented samples of Mg saturated, ethylene glycol (EG) solvated, K saturated in 25, 330 and 550 °C were analyzed by Xray diffraction (XRD) using a D8 ADVANCE diffractometer with CuKα radiation (40 kV, 40 mA). The peak area of the 001 reflections of the main clay minerals (smectite = 17 Å, illite = 10 Å, and kaolinite/chlorite = 7 Å) on EG treated sample were used to semiquantitative estimation of clay minerals (Moore and Reynolds, 1997). To discriminate kaolinite from trioctahedral chlorite, samples were treated with 1 N HCl at 80 °C overnight. For micromorphological studies, thin sections were prepared from undisturbed, oriented and dry clods by standard methods (Murphy, 1986) and described under a polarizing optical microscope according to Bullock et al. (1985). The micromorphological index of soil development (MISECA), suggested by Khormali et al. (2003), was calculated for parent materials and most developed genetic horizons of the soils. These representative horizons are A horizons of Orthents (formed on siltstone, Kh sandstone, Gl sandstone and Ab limestone), By of Gypsids, Bss of Torrerts and Bk of Calcids. The MISECA index was proposed for assessing the degree of soil development in calcareous deposits in arid to semi-arid environments and range from 0 to 24. Its calculation is based on microstructure, b-fabric and the presence of clay coatings, decalcified zones, and Fe/Mn hydroxide pedofeatures as well on the alteration degree of mineral grains. Each feature was quantified by the application of a simple rating that expresses the degree of pedogenic evolution. With increasing degree of soil development, structure proceeds from massive to blocky/prismatic, fabric evolves from undifferentiated to striated, coatings of clay or Fe/Mn oxide increase by neoformation from primary minerals, and the area of the calcite depletion pedofeatures increases in calcareous soils. The sum of the ratings gives the value for the MISECA index.

259

weakly developed soils formed on sandstones, siltstone and Ab limestone are classified as Entisols using the Soil Taxonomy (Soil Survey Staff, 2014). No genetic B horizon is developed in these soils and accumulation of organic matter and formation of structure led to the development of A horizons directly on parent materials. These soils are similar to their parent materials and are coarse texture (those formed on sandstones and Ab limestone) and medium texture (those formed on siltstone). The more developed soils formed on shale, marl and Ka limestone are classified as Aridisols. Organic matter addition, calcium carbonate transformation and development of calcic horizon are the main features of Calcids formed on Ka limestone. These are shallow soils having loamy texture and reddish brown color (7.5YR4/6). Gypsum dissolution from the surface horizon and its recrystallization in gypsic horizons are the main pedogenic processes in Gypsids derived from marl. The soils formed on shale are classified as Calcids and have clayey texture, well developed structure, distinct difference in color between the soil and parent material (10YR in surface horizon vs. 5Y in parent material). The development of calcic and gypsic subsurface horizons reflects appropriate weathering environments resulting in calcite depletion from the surface horizons and gypsum dissolution from upper meter of the solum. Secondary calcite and gypsum are concentrated in the forms of soft powdery pockets and mycelium in calcic and gypsic horizons, respectively. Soil formation on claystone resulted in the development of Vertisols. Presence of the features such as slickensides, wedge shaped peds and diffuse horizon boundaries implies shrink–swell cycles. The parent material is gypsiferous (25% gypsum). Although gypsum can originate from surrounding geological materials (Khademi et al., 1997), but the source of gypsum is claystone parent rocks as evidenced by Moussavi-Harami (1993). The soil solum shows gypsum dissolution and there is no distinction between red color (2.5YR Hue) of soil and underlying parent material. All the studied soils are nonsaline (EC b 4 dS/m), pH values ranging from neutral (7.2) to alkaline (8.7) and all effervesce with HCl 10%. Calcium carbonate equivalent (CCE) varying from highly calcareous (N 40% in soils derived from limestones) and moderately calcareous (nearly 20% in soils derived from siltstone) to slightly or non-calcareous (b 15%). 3.2. Clay mineralogy

3. Results 3.1. Morphological and physico-chemical properties

15 10

Marl Gl sandstone

20

Ab limestone

25

Kh sandstone

30

Shale

lf (10-8 m3. kg-1)

35

Claystone

40

Siltstone

Ka limestone

The selected physico-chemical, morphological characteristics and classifications of the pedons studied are presented in Table 2. The

X-ray diffraction patterns and clay mineral abundance of the clay fraction for parent materials and representative horizons from each pedon studied are shown in Fig. 3 and Table 3. Illite, chlorite, kaolinite and vermiculite were found in the clay fraction of soils formed on shale. Smectite is the only clay mineral detected in clay fraction of Kh

Parent materials Soils

5 0

Fig. 5. Bar diagram comparing mean magnetic susceptibility of soil sola with their parent materials.

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sandstone and associated soils. Dominance of smectite and absence of other clay minerals is confirmed by presence of the sole strong peak in 17 Å in EG treatment (Fig. 3c). Clay mineral composition of other soils and parent materials consists of smectite, illite, chlorite and kaolinite and the compositions are also dominated by smectite as indicated by observation of its strong peaks (16.5–18 Å in EG treatment). Glauconite is the main mica type present in clay fraction of Gl sandstone and related soils and its presence is confirmed by observation of intense 9.9 Å and weak 5 Å peaks (Fig. 3b). Glauconite has also been recognized in thin section of soil and parent materials. Comparison of types and abundance of clay minerals between soils and underlying parent materials reveal similarity in clay mineral types. Small differences in concentration of chlorite and smectite can be recognized in soils developed from

Depth (cm)

10

20

0 5 10 15 20 25 30 35 40 45 50

0

lf (10-8m3kg-1) 20 5

10

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20 25 30 35 40 45 50

(b)

(c)

Feo (%) 0

50

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25

25

30

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30

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(d) lf (10-8m3kg-1) 20

30

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lf / Fe d

0

0.05

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Feo/Fed

(f)

(g)

Feo (%) 150

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5

5

5

120

10

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60

25

25

25

30

30

40 0.02

(i)

10

0

0

(h)

15

5

40

(e)

y = 124.79x + 3.2253 R² = 0.8866

20

10 15

40

Depth (cm)

15

/ Fed

0

5

5

lf

0

0.05

0 10

lf / Fe d

0

Feo (%) 0

30

lf / Fed

Depth (cm)

10

The thin section studies of parent materials and representative genetic horizons are mainly based on soil type and features reflecting soil development according to MISECA index. The degrees of the soil development as indicated by MISECA index of genetic horizons are presented in Fig. 7. The calculated MISECA index ranged from 1 to 5 for parent materials and from 2 to 11 for soils.

0.1

0 5 10 15 20 25 30 35 40 45 50

(a)

0

3.3. Micromorphological observations

Fed (%)

lf (10-8m3kg-1) 0

claystone and Ka limestone (Fig. 3 and Table. 3). The clay mineralogical findings are also in agreement with the results reported by Khormali and Amini (2015) for Kopet Dagh.

(j)

80

y = 668.24x + 63.757 R² = 0.864

0.03

(k)

0.04

0.05

0.06

0.07

Feo/Fed

Fig. 6. Depth trend variations of χlf, Fed, Feo in Torriorthents derived from Kh sandstone (a, b and c); depth trend variations of χlf, χlf/Fed, Feo and Feo/Fed correlation with χlf/Fed in Gl sandstone (d, e, f and g) and siltstone (h, i, j and k).

H. Tazikeh et al. / Catena 152 (2017) 252–267

12

Bk2

10

Bss1

Gl sandstone

A

By1 A Marl

A

Kh sandstone

2

Ka limestone

4

Bk

Siltstone

Soils Ab limestone

6

Parent materials

Claystone

8

Shale

MISECA

261

A

0

Pedons

Fig. 7. Bar diagram comparing MISECA index of representative genetic horizons of soils with their parent materials.

The surface horizon of soils formed on siltstone have single to double spaced porphyric c/f related distribution, crystallitic b-fabric, moderately developed granular microstructure. Calcite and quartz are the dominant minerals in the coarse fraction. The surface horizons of other Entisols are characterized by crystallitic b-fabric and close porphyric c/ f related distributions. Soils derived from Kh sandstone and Ab limestone have single grain to vughy microstructure and their coarse mineral components are dominated by quartz and calcite (Fig. 4a, b). Soils developed from Gl sandstone exhibit vughy to granular microstructure

lf/Fe

lf (10-8 m3 kg-1) 10

30

with quartz and glauconite as major minerals. Low degrees of mineral alteration can be recognized in soils having calcite and glauconite. Micromorphological studies of Aridisols are focused on subsurface diagnostic horizons. The most important feature of calcic horizon of Calcids developed from Ka limestone is redistribution and reorientation of calcite. Different forms of pedogenic calcium carbonate were identified as nodules, infillings of cytomorphic calcite in root channels which is occasionally covered by acicular forms (Fig. 4d). Thin section studies of gypsic horizon of Gypsids show infillings of secondary

Feo (%)

d

30

50

50

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70

0

0

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30

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40

40

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50

50

60

60

60

0.05

0.1

50

lf / Fed

Depth (cm)

60

40 30

y = 375.03x + 23.776 R² = 0.7811

20 10

70

70

(a)

0

10

30

55

0.05

0.1

Feo/Fed

(d)

(c) Feo (%)

d

20

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70

(b) lf/Fe

lf (10-8 m3 kg-1)

0

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80

0

0

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20

20

40

40

40

0.05

60

60

80

80

100

100

/ Fed

70 60 50 40

y = 180.1x + 32.92 R² = 0.897

lf

Depth (cm)

80

30

60

20 10 80

0 0

(e)

100

(f)

(g)

(h)

0.1

0.2

0.3

Feo/Fed

Fig. 8. Depth trend variations of χlf, χlf/Fed, Feo and correlation of χlf/Fed with Feo/Fed in Calcids derived from Ka limestone (a, b, c and d) and Torriorthents derived from Ab limestone (e, f, g and h).

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The parent material is characterized by abundant fine gypsum crystals distributed in the clayey matrix. The main micromorphological features of Vertisols developed from claystone are well separated angular blocky microstructure associated with planes, speckled to striated b-fabric and the presence of coarse calcite nodules impregnated by Fe oxide. The coarse component is dominated by calcite and quartz. The parent material shows accumulation of many lenticular gypsum crystals in voids, calcitic crystallitic b-fabric and local concentration of Fe oxide in matrix (Fig. 4e, f).

lenticular gypsum crystals in voids, open porphyric c/f related distribution, moderately developed angular blocky microstructure and crystallitic b-fabric (Fig. 4h). The matrix of underlying parent material is also clayey and gypsum is found in the form of coarse lenticular crystals embedded in crystallitic and partly striated b-fabric (Fig. 4g). Thin section description of surface and calcic horizons of Calcids formed on shale show calcite depletion. Speckled to striated b-fabrics occur in surface horizon and secondary calcite accumulation in calcic horizon is evident in the calcic horizons. B-fabric of the calcic horizon is both crystallitic and speckled (50%) and calcitic features can be recognized as nodules in the matrix and hypocoatings around voids (Fig. 4c). Calcite depletion zones decrease from the soil surface horizon toward deeper horizons. Microstructure varies from vughy in the surface horizon to moderately developed subangular blocky in the calcic horizon.

Feo (%) 1

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20

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40

40

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(a) lf (10-8 m3 kg-1) 10

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(i)

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60

200

(g)

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180

15 0.01

Fed (%) 0.4

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40

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180 200

/ Fed

0

0.332 y = 75.37x R² = 0.642

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120

lf (10-8 m3 kg-1) 10 15 20 25 30 35

/ Fed

20

Depth (cm)

20

(f)

0.06

(d)

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120

0.04

Feo/Fed

0

(e)

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20 0.02

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120

y =-264.58x + 51.439 R² = 0.0718

50

30

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(b) Fed (%)

60

/ Fed

0

0

lf

40

30

lf

Depth (cm)

0

The comparison of mean soil χlf and underlying parent materials is shown in Fig.5. The mean soil χlf is the average of χlf values measured

Fed (%)

lf (10-8 m3 kg-1) 20

3.4. Magnetic susceptibility

20 10 0 0.05

y = 256.0x + 14.15 R² = 0.685 0.07

0.09

0.11

Feo/Fed

(j)

(k)

Fig. 9. Depth trend variations of χlf, Fed, Feo and correlation of χlf/Fed with Feo/Fed in Gypsids derived from marl (a, b, c and d) and Torrerts derived from Claystone (e, f, g and h), depth trend variations of χlf, Fed and correlation of χlf/Fed with Feo/Fed in calcids derived from shale (i, j and k).

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in soil solum. Except for soil formed on Kh sandstone, the χlf of soils is enhanced as compared with underlying parent materials. The absolute χlf values of parent materials are ranging from 40 to b10 × 10−8 m3 kg−1. Ab limestone and siltstone have the lowest and highest values of χlf, respectively. Figs. 6, 8 and 9 illustrate vertical distribution of χlf for each pedon. The χlf–depth curve of Entisols formed on Gl sandstone, siltstone and limestone show a decreasing trend downwards then reaches constant level (Figs. 6d, h, 8e) but there is almost no variation in χlf curve of Entisols derived from Kh sandstone (Fig. 6a). The χlfvalues for Aridisols formed on shale, gypsiferous marl and limestone gradually decrease downward while χlf–depth variation pattern trend for Vertisols of claystone is irregular decrease from surface horizon to base horizon (Figs. 8a, 9e, i, a).

show the dominance of quartz in coarse mineral composition and absence of any weatherable mineral (Fig. 4a). These soils have the lowest concentration of Fed (Fig. 6b) and the depth curve of Fed and χlf show almost no variation from the soil surface to parent material reflecting no pedogenic formation of Fe oxides and ferrimagnetic minerals (Fig. 6a, b). It implies that inert parent material of Kh sandstone with low concentration of Fe oxide or iron bearing minerals is responsible for weak pedogenic alteration. Lu (2000) also reported minor variation of χlf between Entisols and underlying sandstone parent rocks with low χlf and Fe oxides in China. Thin section studies of Gl sandstone and siltstone shows dominance of quartz in their coarse mineral fractions while calcite with Fe oxides in siltstone and glauconite beside Fe oxides in Gl sandstone are also major components. Decomposition or transformation of these minerals results in a more favorable condition for soil development and also more intensive vegetation providing weathering environment for secondary ferrimagnetic mineral formation. These processes are confirmed by variation pattern of χlf and its correlation with Fed and Feo (Fig. 6). The low χlf values of parent materials indicate low input of ferrimagnetic minerals from rocks to soils (Fig. 5). Vertical distributions of χlf for both soil profiles show elevated level of χlf in the A horizon and increasing trend from parent materials to surface horizon reflecting formation of ferrimagnetic minerals (Fig. 6d, h). The magnetic enhancement of modern soils is related to the formation of nano-sized magnetite and/ or maghemite (Torrent et al., 2010). CBD treatment dissolve maghemite grains of any size, maghemite coats on large magnetite grains and magnetite grains smaller than 1 μm. Coarse-grained magnetite grains (N1 μm) are inherited from parent material to soils and are not affected

4. Discussion 4.1. Soil formation in coarse-medium texture parent materials Soils derived from residuum of sandstones and siltstone are texturally like the clastic particles of rocks. Morphological comparison of Entisols derived from sandstones and siltstone show that Kh sandstone has weak horizon differentiation but soils of Gl sandstone and siltstone show more distinct horizonation and soil development. These variations can be related to the difference in mineralogical composition of their parent materials. Calculated MISECA index for their A horizon represent higher degree of soil development form Gl sandstone and siltstone because they show some degree of mineral alteration and more developed microstructure (Fig. 7). Thin section study of Kh sandstone

(a)

35

y = 7.948x + 20.08 R² = 0.843

30

(b)

20

χlf (10-8 m3. kg-1)

χlf (10-8 m3. kg-1)

40

263

25 20

y = 8.775x + 10.05 R² = 0.738

18 16 14 12 10

15

8

10 0

0.5

1

1.5

0

2

0.2

0.4

18 16 14 12 10 8 6 4 2 0

0.6

0.8

1

OC% 35

(c) χlf (10-8 m3. kg-1)

χlf (10-8 m3. kg-1)

OC %

y = 22.85x + 7.575 R² = 0.867

(d)

30 25

y = 8.773x + 16.01 R² = 0.708

20 15 10

0

0.1

0.2

0.3

0.4

1

1.2

1.4

1.6

1.8

OC %

OC% 14 40

(e)

(f)

10 8

χlf 10-8 m3. kg-1)

χlf (10-8 m3. kg-1)

12

y = 2.143x + 7.004 R² = 0.928

6 4 2 0

35 30

y = 9.765x + 26.2 R² = 0.773

25 20

0

0.5

1

1.5

OC %

2

2.5

0

0.2

0.4

0.6

0.8

1

1.2

OC %

Fig. 10. The correlations of organic carbon with χlf of soils derived from shale (a), claystone (b), Ab limestone (c), Ka limestone (d), Gl sandstone (e) and siltstone (f).

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by CBD treatment (Hunt et al., 1995). Since pedogenic species of Fe oxides are removed by CBD treatment, the comparison of Fed in soils with their parent materials help to evaluate weathering intensity. Higher release of Fe from Fe bearing minerals and its conversion to Fe oxides points to a higher degree of pedogenesis (Hu et al., 2004). The χlf/Fed ratio represents the proportion of ferrimagnetic mineral to total secondary Fe oxides. The variation pattern of χlf/Fed for soils developed from Gl sandstone and siltstone show increasing trend from parent materials to the soil surface indicating ferrimagnetic mineral formation (Fig. 6e, i). The first possible explanation regarding these processes is the transformation of Fe oxide in parent materials in to forms with higher magnetic susceptibility. The formation of microcrystalline maghemite or magnetite from weakly magnetic Fe oxide and hydroxides via oxidation–reduction cycles has been reported (Mullins, 1977; Longworth et al., 1979). This process occurs during normal pedogenic conditions and can change upper soil mineralogy (Lu, 2000). The second mechanism could be neoformation of ferrimagnetic minerals. According to Singer et al. (1996) the weathering of iron-bearing minerals is a major process in magnetic enhancement of eluvial and illuvial horizons through formation of magnetite and maghemite. Glauconite is a susceptible mineral that easily weatherable in soil environment (El-Amamy et al., 1982; Skiba et al., 2014). The release of Fe via glauconite weathering and its oxidation to magnetite or maghemite in soils formed on Gl sandstone could be responsible for increasing χlf of the soil. Feo/Fed index is used to evaluate Fe oxide crystallinity and the similarity of depth variations trends of Feo/Fed with χlf/Fed and also their positive significant correlation coefficient indicate that ferrimagnetic minerals are concentrated more in poorly crystalline portion of free Fe oxide (Fig. 6g, k). This result is consistent with Hu (2004) and Hu et al. (2009) who reported strong correlation between χlf and Feo/ Fed in studies of loess-paleosol sequences in China. Although many soil genesis studies over a wide range of lithologies and climate regimes have indicated that the magnetic susceptibility of surface soil was often higher than that of the parent materials (Maher, 1986; Mullins, 1977; Singer and Fine, 1989; Thompson and Oldfield, 1986), the vertical distribution of χlf for soils derived from Kh sandstone shows that magnetite enhancement of topsoil in arid environment did not occur on parent materials with low concentration of iron-bearing minerals. Dearing et al. (1996) evidenced that Fe supply, via ferrihydrite formation that crystallize to magnetite and maghemite, is a major factor controlling the concentration of pedogenic ferrimagnetic minerals. The similarity in clay mineral composition of soils and underlying parent materials indicate close genetic relations between them. In addition, this similarity implies that weathering intensity is not high enough to change relative abundance of clay minerals to the detectable extent of x-ray diffraction (Fig. 3, Table.3).

4.2. Soil formation from limestone The soils developed on Ab and Ka limestone have loamy texture and are highly calcareous. Orthents formed on Ab limestone are in early stage of pedogenesis showing initial development of A horizon with single grain microstructure over a massive C horizon (Fig. 4b). In contrast, Calcids of Ka limestone formed on more stable landscape and denser vegetation show higher degree of development as reflected by higher MISECA index. Higher degree of development is confirmed by formation of the A horizon with well separated granular microstructure over calcic horizon with high concentration of secondary calcite. Precipitation of secondary calcite can be recognized by formation of nodules, infillings of cytomorphic calcite crystals in channels covered with needle shaped calcite surrounded by decalcified zones indicating dissolution and recrystallization of calcite induced by chemical and biological processes (Fig. 4d). These types of calcitic features are frequently reported in semi-arid calcareous soils (Jaillard et al., 1991; Herrero et al., 1992; Khormali et al., 2006) and the origin of needle fiber calcite has been

interpreted as fungal biomineralisation (Verrecchia and Verrecchia, 1994; Loisy et al., 1999; Milliere et al., 2011). The χlf of these soils are very low and their χlf–depth curve show gradual decrease by depth (Fig. 8a, e). These results are in accordance with the findings of Lu (2000) and Yu and Lu (1991) who reported similar χlf–depth curve and low χlf values for soils derived from limestone in China. Decrease in χlf/Fed by depth indicates the higher production of ferrimagnetic mineral at the surface horizons due to more intensive weathering environment resulted from biological processes (Fig. 8b, f). Feo has also displayed similar variation (Fig. 8c, g) and significant correlation present between Feo/Fed with χlf/Fed indicates that poorly crystalline Fe oxide could retain higher ferrimagnetic mineral (Fig. 8d, h). Clay minerals distributions of both parent materials are the same (Fig. 3a, e and Table. 3). Comparison of clay mineral abundance of soils with their parent material show no variation for soils of Ab limestone while a little decrease in amount of chlorite can be detected in soils of Ka limestone as revealed by increase of illite/chlorite in soil relative to parent material (Table. 3). Chlorite is more susceptible to weathering than illite (Biscaye, 1965) and their ratio can reflect weathering intensity. Zhao et al. (2005) used this index to record weathering intensity related to paleoclimate variation in loess-paleosol sequences in China. Minor decrease in chlorite content in soils derived from Ka limestone is attributed to more favorite weathering condition in these soils. Chlorite weathering results in the release of iron and in production of Fe oxides and also ferrimagnetic minerals, leading to enhancement of magnetite susceptibility (Guo et al., 2000; Maher, 1998; Ji et al., 2002). Trace amounts of ferrimagnetic mineral in soils dominate magnetic signals and slight change in ferrimagnetic mineral can significantly affect χlf (Maher, 1998). 4.3. Soil formation in fine textured parent materials The properties of soils derived from claystone, gypsiferous marl and shale, are affected mainly by their clayey texture and mineralogy. Thin section studies of residuum of claystone show many gypsum crystals and formation of calcitic crystallitic b-fabric with local concentration of Fe oxides (Fig. 4e). Absence of gypsum in micromorphological observations of soil solum could be due to dissolution of gypsum as compared with gypsum rich parent material. In addition, the low calcite and very high clay contents caused the appearance of speckled and striated bfabric (Fig. 4f). The formation of oriented clay domains, planes as dominant type of void, wedge-shaped structural elements and cracks during dry seasons in field observation imply the strong shrink-swell processes. The clayey texture and dominance of smectitic clay minerals favors the condition for argillipedoturbation. The MISECA index is higher for Torrerts of claystone relative to Gypsids of marl indicating a higher degree of soil development (Fig. 7). Decalcified fabric and higher degree of mineral alteration (gypsum and calcite) in Torrerts are responsible factors to increase this index. The main pedogenic process in Gypsids of marl is gypsum dissolution from surface horizon and its accumulation in gypsic horizons. The main features of gypsic horizon are infillings of gypsum crystals in voids with calcitic crystallitic b-fabric (Fig. 4h). In comparison with Torrerts of claystone, vertic behavior of Gypsids was not enough to qualify it as a Vertisols in spite of having clayey texture and high proportion of smectite in clay mineral composition. It has been shown that shrink-swell potential of soils is negatively correlated with the abundance of carbonate, gypsum, Fe/Mn oxides, low activity clays and with electrolyte concentration (Mermut et al., 1991). The results of some studies show that application of gypsum can reduce vertic behaviors of Vertisols for instance Wild et al. (1992) reported that application of gypsum as an amendment reduced shrink-swell properties and modified physical characteristics of sodic Vertisols in Australia. On the other hand, sometimes swelling restriction of clays is related to the presence of calcite as documented by Rimmer and Greenland (1976). Substantial amount of gypsum in the soils developed from

H. Tazikeh et al. / Catena 152 (2017) 252–267

marl can reduce soil extensibility. Dissolving of gypsum crystals in wet seasons can modify soil swelling while its crystallization in dry season leads to modification of soil shrinkage. The Calcids formed on shale display more advanced degree of development as reflected by their highest value of MISECA index (Fig. 7). Calcic, cambic and gypsic are subsurface diagnostic horizons. Gypsum has been dissolved from the upper meter of the solum and formation of calcic horizon with calcite depletion pedofeatures implies probably to a more appropriate paleo-environment in a geomorphologically stable landscape (Fig. 4c). In spite of other studied soils, smectite is not present in clay mineral composition, therefore, they do not show any sign of vertic behavior (Fig. 3h, Table. 3). Bhattacharyya et al. (1997) showed that soil vertic behavior is a function of smectite content. Shirsath et al. (2000) reported a strong relation between shrink–swell properties and smectite content in the clay fraction (b 2 μm). However, shrinkswell magnitude is restricted in soils of dry climate as compared to subhumid environment (Pal et al., 2009). The χlf curve of Gypsids, presented in Fig. 9a, shows a decline with depth. The absence of meaningful variation and correlation for Fed, Feo and χlf/Fed indicate no considerable formation of pedogenic Fe oxides (Fig. 9b, c, d). The data suggest that higher values of χlf can not be interpreted to higher concentration of secondary Fe oxide or ferrimagnetic minerals. The declining trend of χlf from soil surface to parent material could be attributed to increase of gypsum content (Fig. 2c). Gypsum and carbonates are diamagnetic and mobile minerals that can mask soil magnetic properties due to their pronounced dilution effect (Karimi et al., 2013). The variation patterns of χlf for Calcids over shale show increasing trend from parent material to gypsic horizon and reach a nearly constant level in gypsic, cambic and calcic horizons followed by a sharp increase toward the soil surface (Fig. 9i). The variation pattern of χlf is strongly consistent with the Fed curve (Fig. 9j). In contrast, the χlf– depth curve of Torrerts over claystone show a gradual decrease downward and its variation is also consistent with the Fed curve (Fig. 9e, f). The minor variations observed in depth trend of χlf and Fed in Torrerts might be attributed to pedoturbation and homogenization processes. Magnetic susceptibility enhancement of surface horizons is interpreted to more intensive weathering environment evident from higher concentration of free Fe oxide and also depletion of calcite and gypsum as two major diamagnetic minerals. Dependence of magnetic susceptibility variation on Fe oxide crystallinity is confirmed by significant correlation between χlf/Fed and Feo/Fed in both the claystone and shale derived soils (Fig. 9h, k). The more intensive weathering environment of Torrerts and Calcids is reflected in higher differentiation of clay mineral composition of soils with their underlying parent materials (Table. 3). While clay mineral abundances in Gypsids are similar to their parent material, Calcids and Torrerts show differences in their illite/chlorite ratio compared to the parent materials suggesting chlorite weathering. The minor increase in smectite with decreasing chlorite in Torrerts in relation to parent material can be interpreted as weathering and transformation of chlorite to smectite. Chlorite weathering has been reported in soils of arid and semi-arid area of Iran (Burnett et al., 1972). Recent studies show that chlorite weathering through releasing of iron can influence effectively the Fe oxide concentration and magnetic susceptibility enhancement of soils (Peng et al., 2014).

4.4. Possible source of ferrimagnetic minerals in topsoils As discussed above, pedogenic formation of ferrimagnetic minerals play an important role in magnetic enhancement of soils developed from Gl sandstone, siltstone, limestones, shale and claystone. Magnetic enhancement associated with pedogenesis under aerobic condition is mainly related to the formation of nano-sized magnetite and/or maghemite as the two major ferrimagnetic minerals.

265

As illustrated in Fig. 10, there are significant correlations between organic carbon contents and χlf of pedons studied. Piepenbrock et al. (2011) showed that humic substances through constraining interaction between ferrihydrite and Fe2+ (via complexing Fe2+ and blocking sorption sites on ferrihydrite) can inhibit magnetite formation therefore this mechanism cannot explain positive correlations between organic carbon and χlf in pedons studied. In contrast, bacterially mediated pathway of magnetite formation either intercellular by microaerophilic assimilatory bacteria (magnetotactic bacteria) or extracellular by dissimilatory iron reducer bacteria are stimulated by organic matter content (Paasche et al., 2004; Snowball et al., 1999). Biosynthesis of magnetite might account for magnetite enhancement of topsoils especially in soils derived from fine textured parent rocks (shale and claystone) where anaerobic micro-zones are believed to be more frequent than soils derived from coarser texture parent rocks (limestones, siltstone and Gl sandstone). The magnetic enhancement of topsoils can also be attributed to maghemite formation. Maghemite can be formed through solid state transformation of ferrihydrite which is later transformed to hematite. Increasing values of Feo, which is an approximate of ferrihydrite concentration, from parent materials to surface horizons of soils derived from Gl sandstone, limestones and claystone (Fig. 6: f, j; Fig. 8: c, g and Fig. 9: g) might indicate higher pedogenic formation of maghemite associated with magnetic enhancements. Higher organic ligands of surface horizons lead to blocking surface of ferrihydrite and can undergo internal rearrangement and slow dehydration of maghemite to hematite (Torrent et al., 2006). Since magnetite in aerobic condition readily oxidize to maghemite (Torrent et al., 2010), maghemite is a more probable phase controlling magnetic signals of studied soils. Thermal transformation of weakly magnetic iron oxides and hydroxides to ferrimagnetic magnetite may be produced by natural fire or crop burning. Fire affects the iron minerals to a depth of no more than several centimeters (Singer et al., 1996), while in some studied soils, enhancement exceeds 40 cm. In addition, we found no evidence of fire (such as charcoal) both in field and thin section studies of surface horizons. Therefore, fire could not be responsible for magnetic enhancement in the region. However, it is likely that bioturbation (and argillipedoturbation in Torrerts) processes could mix high and low susceptibility horizons in the deeper sola. 5. Conclusion Parent material is a main soil forming factor in the area studied. The close genetic relation between soils and parent materials is indicated by micromorphological and clay mineralogical studies. Texture and mineralogy of parent materials are two most important factors affecting the degree of soil development. Examination of the MISECA development index showed that microstructure development, calcite depletion pedofeatures and alteration of more susceptible minerals such as gypsum and calcite are main factors to assess soil development. Our results showed that magnetic susceptibility beside MISECA index are useful tools to differentiate weak weathering intensity of genetic horizons of soils developed under arid and semi-arid climate. Very low lithogenic contribution of ferrimagnetic minerals is indicated by low χlf of parent materials derived from sedimentary rocks. Depletion and redistribution of diamagnetic minerals (calcite and gypsum), pedogenic formation of magnetite and/or maghemite through weathering and transformation of iron-bearing minerals such as glauconite, chlorite and Fe oxide are processes responsible for χlf variation in the soils. This interpretation is based on the studies of calcite and gypsum distribution in soil profiles, depth trend variations of χlf, χlf/Fed, Fed, Feo and correlations between organic carbon with χlf and χlf/Fed with Feo/Fed. Soil genesis studies on soils derived from clayey parent materials (claystone, marl and shale) reveal that vertic properties are not formed in the absence of smectitic clay minerals in soils developed from shale and that the presence of gypsum plays a key role in impeding vertic

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properties in smectite rich soils derived from marl. In comparison, depletion of gypsum and dominance of smectite favors the condition for Vertisol formation on claystone.

Acknowledgements The present study is a part of the PhD thesis done at Gorgan University of Agricultural Sciences and Natural Resources, Iran. The constructive comments of the reviewers are highly appreciated. Appendix A. Supplementary data Supplementary data associated with this article can be found in the online version, at http://dx.doi.org/10.1016/j.catena.2017.01.020. These data include the google map of the most important areas described in this article.

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