Evolution of reg soils in Southern Israel and Sinai

Geoderma, 28 (1982) 173--202 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

173

EVOLUTION OF REG SOILS IN SOUTHERN ISRAEL AND SINAI*

J. DAN ~, D.H. YAALON 2, R. MOSHE s and S. NISSIM 1

1Institute o f Soils and Water, Volcani Center, Bet Dagan (Israel) 2Department o f Geology, The Hebrew University o f Jerusalem (Israel) 3 Division o f Soil Conservation and Drainage, Ministry o f Agriculture, Be'er Sheva (Israel) (Received October 26, 1981; accepted March 1, 1982)

ABSTRACT Dan, J., Yaalon, D.H., Moshe, R. and Nissim, S., 1982. Evolution of Reg soils in southern Israel and Sinai. Geoderma, 28: 173--202. Reg soils (mostly Camborthids and Gypsiorthids) cover some 15% of the Negev and Sinai deserts. Analytical data of seven profiles on depositional surfaces of increasing age in the Negev show clear relationships between ages of surfaces and soil profile features, exemplified in the development of the cambic, salic and gypsic horizons. The youngest soil, a Coarse Desert Alluvium soil (Typic Torriorthent) of dry wadi beds, up to a few thousand years old, has little profile differentiation and is generally non-saline or slightly saline. On the higher terraces connected with the Lisan Formation (about 70 000--12 000 years B.P.) clear profile differentiation and the beginning of development of cambic horizons can be discerned, evident in their colour-textural differences. The soils are already saline and somewhat gypsiferous. Both soils are not yet defined as Regs because of their negligible or restricted profile differentiation. Reg soils on the older and higher geomorphic surfaces in the Arava Rift Valley, several hundred thousand years old, have well-differentiated profiles, with cambic, salic and gypsic horizons. Below their stony desert pavements, typical Reg soils have light coloured vesicular horizons, over mellow, very pale brown loam, frequently with laminar structure, almost completely stone-free layers. The reddish brown loamy to clay loam B horizons are very saline and show intensive salt weathering of gravels. Gypsic and petrogypsic horizons occur at depth. Such Reg soils represent the stable surfaces and soils of deserts and were developed over a long period of desert weathering.

INTRODUCTION

Reg soils are the typical soils of the gravelly deserts of Israel (Ravikovitch et al., 1956; Dan et al., 1981, chapters 2 and 11). In contrast to most other desert soils, they exhibit soil horizons that are related to soil genesis. These horizons include an upper, shallow, greyish vesicular horizon and a somewhat deeper, yellowish red saline dusty layer (Ravikovitch et al., 1956; Dan et al., 1981). The underlying parent material is usually reached at a depth of 0.5--1 meter. A well-developed desert pavement characterizes these soils. They are, *Contribution from the Agricultural Research Organization, Israel, No. 182, 1981 Series. 0016-7061/82/0000--0000/$02.75 © 1982 Elsevier Scientific Publishing Company

174

as a rule, saline and gypsiferous; some Regs are also characterized by petrogypsic horizons. The Regs cover the large desert gravel plains in the Middle East, (Zohary, 1940; Moormann, 1959; Veenenbos and Ghaith, 1964; Dewan and Famouri, 1964; Dan and Raz, 1970) North Africa, and other extremely arid regions .I of the world (D'Hoore, 1964). The typical Regs differ from the other semidesert or zonal desert soils such as the Grey, Grey brown, and Red desert soils by the restricted depths of their horizons and their higher salt contents. Most of the Regs may be correIated with the Orthids (Camborthids and Gypsiorthids) of the American Soil Taxonomy (Soil Survey Staff, 1975); however, many relatively young Regs may not be deep enough and thus they are included among the Entisols (Torripsamments or Torriorthents). The object of this paper is to present data of several profiles on a sequence of depositional surfaces of increasing age and to discuss briefly the evolution of the main pedogenetic horizons and features. DESCRIPTION OF THE AREA

In Israel the Regs are confined to the extremely arid zone where the annual rainfall does not usually exceed 65 mm (Dan and Raz, 1970; Dan, 1979; Dan et al., 1981). They cover some 15% of the desert surface. The zone includes all of the Arava Valley, the southern Negev, and most of the Sinai Peninsula. Three broad geographic-soil regions characterize these desert areas (Dan, 1979, Fig. 1). (1) The mountainous zone (southern Sinai, some parts of central Sinai and the slopes of the Negev mountains). (2) The sandy areas {northern Sinai). (3) The gravelly plains and the large valleys (the Arava Valley and the plains of Paran and the central Sinai). Soil development in the first two geographic regions is restricted by severe water or wind erosion (Dan, 1979; Dan et al., 1981). The gravelly plains, on the other hand, are mostly flat and the gravel cover protects the soil from accelerated erosion (Ravikovitch et al., 1956; Dan and Raz, 1970). The surface stability is such that soil development occurs, despite its slow rate. Stones and gravel in the plains and valleys of southern Israel and Sinai have been deposited since Neogene times (Bentor and Vroman, 1951, 1954, 1957; Horowitz, 1979). These stones and gravel consist usually of a mixture of limestone, dolomite and flint fragments mixed with some fine soil material. The relative proportions of the various stones are related mainly to the source ,i Extremely arid regions are defined according to Meigs (1953) as areas where the moisture index, defined by Thornthwaite, is less than --57. In Israel these include mainly areas where the annual rainfallis less than 65 m m and where the vegetation is restricted to favourable ecological sites which receive additional runon water from the surroundings.

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177 nature of most of these formations or because of the occurrence of thin flint beds among the carbonate rocks. In m a n y places several depositional surfaces can be detected. These depositional surfaces are clearly distinguished in the northern and central Arava Valley, where most of the soil samples were collected (Figs. 2, 3). These various surfaces will be described briefly, due to the importance of relationships between their ages and the soil characteristics.

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(1) The lowest depositional surface -- The Dead Sea surface -- is connected with the Recent Dead Sea and the saline-alluvial sediments of Arvat Sedom (the wet saline playa south of the Dead Sea). South of Arvat Sedom this surface is confined to the Nahal HaArava wadi bed and to a small Recent terrace near the wadi. This surface rises in a gradient of about 1% to the vicinity of En Yahav, where the wadi beds attain a more gentle gradient, and merge with the higher surfaces. (2) The second surface -- The Lisan surface -- is connected with the former Lisan Lake or the Pleistocene Dead Sea. The lacustrine sediments of this ancient lake merge and interfinger, in the vicinity of Hazeva, with the saline, clayey, silty and sandy deposits of the ancient wet playa that bordered the lake proper. Further south, these sediments merge and interfinger with gravelly sediments which cover the whole upper terrace of Nahal HaArava north of En Yahav; in the settlement of En Yahav itself, the Lisan surface may be correlated with the ancient saline gypsiferous salt-marsh sediments that surrounded a former spring; these sediments are found nowadays several meters higher than the Recent springs and wadi beds. The depositional Lisan surface continues south of En Yahav to the vicinity of Zofar where the wadi beds and the lower terraces may be correlated with that surface.

178 (3) The third surface, or surfaces, are less uniform. Actually, they consist of Pleistocene depositional surfaces composed of flat terraces, hilltops and plains which may be detected in the vicinity of Hazeva between the Arava Conglomerate (see below) at an elevation of --100 m near Hazeva and the Lisan Lake surface at an elevation o f - - 1 5 0 m in the same area. The ages of these surfaces are n o t well-established, b u t it is evident that they were formed during the Pleistocene, inasmuch as they are younger than the Arava Conglomerate which is related to the late Pliocene (Bentor and Vroman, 1957). The regional distribution of these surfaces is not readily evident. The surfaces are quite clear and widespread in the vicinity of Hazeva but they become m o r e diffuse southwards. In these areas the surfaces may merge gradually with the older Arava Conglomerate surface. (4) The oldest depositional surface, which is of great extent, is the Late Neogene Arava Conglomerate surface, which may be covered by gravel of the Meshar Formation (Horowitz, 1979). The Arava Conglomerate and the Meshar Formations covered large areas in the past (Bentor and Vroman, 1951, 1954, 1957). In the vicinity of Hazeva they characterize the tops of small eroded mesas (table mountains). The area of this surface and its sediments become more continuous on the western edge of the Arava Valley. Toward the south, the vertical distance between this surface and the younger ones diminishes until it is negligible and reaches only a few meters in the central Arava. This surface continues westward toward the plains of Paran and central Sinai where they apparently include most of the higher gravelly plains of this region (Horowitz, 1979; Dan et al., 1981). In these areas, however, this surface is again dissected due to relative uplift at the beginning of the Pleistocene and some younger depositional surfaces and terraces may be found. CLIMATE AND VEGETATION

The climate of the areas concerned is extremely arid and the moisture index is lower than --57 (Meigs, 1953). Temperatures may differ according to elevation; the rainfall is very scarce and the annual average of most of the area does not exceed 50 mm (Rosenan, 1970; Israel Meteorological Service, 1967). This figure has really very little meaning, however, because the yearly variations are very large (Ganor et al., 1973). In a single severe thunderstorm the rainfall m a y exceed the annual average. The Reg plains are, as a rule, bare of vegetation (Zohary, 1955; Dan and Raz, 1970). Vegetation is usually confined to the stream-beds, where the plants take advantage of the runoff water that reaches these places. In exceptionally wet years, however, some scattered Mesam bryan them um plants m a y be found in some of the Reg plains. METHODS

AND PROCEDURES

Seven soil profiles from several different depositional surfaces were sam-

179 pled. Three of these profiles were collected in the vicinity of Hazeva. The first profile was sampled from the Recent Arava river terrace (the Dead Sea surface), the second one from the somewhat higher gravelly terrace that is connected with the Lisan marl sediments (the Lisan surface), and the third one was sampled from a dissected higher terrace that is related to the intermediate Pleistocene surfaces. The samples of the profile that characterizes the highest surface were taken from central Sinai; because near Hazeva this surface is already eroded and dissected. The samples of profile nos. 5--7 were collected from surfaces near Bir Thamada in central Sinai. Profile no. 5 characterizes the lower dry wadi bed, profile no. 6 an intermediate terrace, and profile no. 7 the higher flat upland areas. The soils were described in detail (according to the FAO system; FAO, 1968), sampled, and classified according to the Israeli and American systems (Soil Survey Staff, 1975; Committee on Soil Classification in Israel, 1979). The pH, electrical conductivity, features of the exchangeable complex, gypsum and soluble ions were analyzed according to the standard methods followed in the Salinity Laboratory at Riverside, California (Richards, 1954). Lime content was determined by the calcimeter m e t h o d and mechanical composition by the Beam m e t h o d (Wright, 1939). Exchangeable Na and K were determined in profiles 5--7 after removal of the soluble salts by alcohol. In profiles 1--3 exchangeable potassium was determined from solution which was gained due to removal of the K ions by ammonium acetate solution; in profiles 4--7, the K ions were removed by sodium acetate solution. PROFILE DESCRIPTIONS Profile No. 1 - - Coarse Desert A l l u v i u m

Location: Site:

Profile description: C l l 0--8 cm

1 km east of Hazeva, near Nahal HaArava wadi bank at coordinate 1780/0196. The pit was located on a young, Recent terrace, 50 m west of the dry wadi bank. The terrace is only 1--2 m higher than the wadi bed. The vegetation consisted mainly of H a l o x y l o n persicum and some Salsola shrubs. Most of the vegetation was restricted to shallow depressions, where the rainwater usually concentrated. The ground near the pit was bare of vegetation. The area was nearly flat, with a small slope of 1% eastwards. The soil surface was covered by limestone and flint gravel. Many limestone, flint and some quartz porphyry cobbles and gravel were found also in the various soil layers. Very pale brown (10YR 7/3) dry, light yellowish brown (10YR 6/4) moist, gravelly and stony (about 20% gravel

180

C12 8--30 cm C13 30--60 cm

C14 60--82 cm

C15 82--100 cm

and stones) sand; single grained, loose, non-sticky and non-plastic; smooth, clear boundary. Similar sand with only 1% gravel; clear alluvial layering; smooth clear boundary. Similar sand with a considerable amount of gravel (about 20%) and stones; no clear alluvial layering; smooth clear boundary. Similar layer with much (about 70%) gravel and stones; the sand among the stones was very coarse; smooth clear boundary. Similar layer with only 5% gravel, clear alluvial layering and finer sand; the underlying layer is again more stony.

Profile No. 2 -- Young Regosolic Reg, a transition to Coarse Desert Alluvium

Location: Site:

Profile description: Ao A 0--1 cm

B

1--4 cm

Clcs 4--15 cm

C2

15--70 cm

2 km north of En Yahav, at coordinate 1742/0106. Pleistocene gravel terrace that inter fingers with the upper sediments of the Lisan marl sediments further northward. The area is flat, with a very small slope toward the northeast. The vegetation was restricted to the dry wadi beds, and consisted mainly of Acacia tortilis and Anabasis articulata. Some other desert plants, mainly Fagonia arabica, were also seen. The pit was located on a bare surface, 10 m from the nearby dry wadi bed. The soil was covered by flint and limestone gravel. A few stones were also found in the area. Distinct desert varnish stains this gravel. The whole soil is gravelly and highly calcareous. The gravel in the various soil layers consist mainly of limestone fragments with admixture of about 30% flint. Superficial cover of gravel with desert varnish. Very pale brown (10YR 7/3) dry, light yellowish brown (10YR 6/4) moist, sandy loam; vesicular structure; soft, non-sticky but slightly plastic; smooth, clear boundary. Reddish yellow (7.5YR 6/6) dry, reddish yellow to strong brown (7.5YR 5.5/6) moist, slightly gravelly (10% gravel) loamy sand; massive, soft, non-sticky but slightly plastic; smooth clear boundary. Very pate brown (10YR 7.5/3 dry, 10 YR 7/4 moist) gravelly (50% gravel) sand with a few scattered gypsum crystals and some disintegrating stones; single grained, loose, non-sticky and non-plastic; gradual boundary. Similar layer without gypsum crystals; the deeper layer (beyond 50 cm) was less gravelly (only 30% gravel).

181

Profile No. 3 -- Petrogypsic Regosolic Reg

Location: Site:

1 km southeast of Hazeva, at coordinate 1758/0182. The profile was sampled on a dissected Pleistocene terrace. The whole area is already somewhat hilly, but flat areas about 20--40 m wide still characterize the water divides. The soil was sampled on a typical flat hilltop. The terrace is 10--15 m above the wadi beds and lower plain that are connected with the Lisan surface. The flat areas were bare of vegetation. Some scattered small shrubs of Anabasis articulata and Zygophyllum d u m o s u m were found along the dry small wadi beds on the terrace slopes. The vegetation of the dry wadi beds in the lower plain consisted of the Acacia tortilis-Anabasis articulata plant association. The soil surface was covered by flint gravel with a distinct coating of desert varnish; few limestone and quartz porphyry gravel were also found. Stones and gravel were found in the various soil horizons although the relative proportions of limestone gravel increased in the deeper soil layers. The whole soil profile was highly calcareous.

Profile description: Ao A 0---1 cm

B21sa

1--5 cm

B22cs

5--12 cm

B3sa

12--20 cm

Cllsacs

20--39 cm

C12

39--52 cm

Superficial cover of flint gravel with desert varnish. Very pale brown {10YR 7/3} dry, light yellowish brown (10YR 6/4} moist, slightly gravelly, fine sandy loam; vesicular structure; soft, non-sticky but slightly plastic; smooth clear boundary. Reddish yellow (7.5YR 6/6) dry, strong brown (7.5YR 5/6) moist, very saline gravelly {about 20% gravel) loam to clay loam; loose, sticky and plastic; wavy clear boundary. A layer of white dusty gypsum of very low density concentrated mainly in large chunks; somewhat gravelly; wavy clear boundary. Light brown to light yellowish brown (9YR 6/4) dry, strong brown (7.5YR 5/6) moist, gravelly (30% gravel) loam to sandy loam with many white mottles of gypsum and salt crystals; loose, slightly sticky and slightly plastic; clear to gradual boundary. White (10YR 8/1) dry, very pale brown (10YR 7/3) moist, massive, gravelly (30% gravel) loamy sand to sandy loamy layer indurated by gypsum; extremely hard, non-sticky and non-plastic; gradual boundary. Very pale brown (10YR 7/3) dry, very pale brown to

182 light yellowish brown (10YR 6.5/4) moist, very gravelly (50% gravel) sand with many gypsum crystals;massive, soft, non-sticky and non-plastic; clear to gradual boundary. C13cs 52--100 cm Similar to above layer, with more gypsum crystals that indurate part of the layer; clear boundary. C14 100--117 cm Similar to above layer with fewer gypsum crystals that diminish with increasing depth; gradual boundary. C2 117--135 cm Similar to above layer with very few gypsum crystals. Profile No. 4 -- Deep Regosolic Reg (Fig. 4)

Location: Site:

Profile description: Ao All 0--3 cm

A12

B1

B2cs

B3sacs

C11cs

Central Sinai, 35 km southeast of Qal'at en Nakhl. The profile was sampled on an old, somewhat dissected gravel plateau. The area was flat. The soil was covered by flint gravel with distinct coating of desert varnish. The surface was bare of vegetation.

A complete cover of flint gravel with desert varnish. Very pale ' brown (10YR 7/3) dry, yellowish brown (10YR 5/5) moist, sandy loam; vesicular structure; slightly hard, non-sticky but slightly plastic; smooth, clear boundary. Very pale brown (10YR 7/3) dry, light yellowish brown 3--9 cm (10YR 6/4) moist, slightly gravelly loamy sand, with some small white mottles; massive to laminar structure; slightly hard, non-sticky but slightly plastic; smooth clear boundary. 9--19 cm Strong brown (7.5YR 5/6} dry and moist, slightly gravelly sandy clay loam; columnar structure; the column margins are pale brown, massive and sandy and the insides are strong brown, loose and loamy; slightly hard, non-sticky and non-plastic at the column margins and loose, slightly sticky and plastic inside the columns; diffuse boundary. 19--35 cm Strong brown (7.5YR 5/6) dry and moist, gravelly sandy loam (about 30% gravel); many disintegrating stones; loose, slightly sticky and plastic; wavy, clear boundary. 35--47 cm Light brown (7.5YR 6/4) dry, strong brown (7.5YR 5/6) moist, gravelly (50% gravel) sandy loam; massive layer indurated somewhat by gypsum; wavy, clear boundary. 47--100 cm Very pale brown (10YR 8/3) dry, light yellowish-brown (10YR 6/4) moist, massive layer indurated by gypsum; part of this induration is in a vesicular form; gradual boundary.

183

Fig. 4. Typical profile of a deep regosolic Reg (profile No. 4).

C12cs 100--110 cm Similar to above layer with vesicular gypsum induration that decreases gradually with depth; gradual boundary. C13 110--120 cm Very pale brown (9YR 8/4 dry, 9YR 7/4 moist) loose gravelly loamy sand with some gypsum crystals that decrease with depth;single-grained, loose, non-sticky and non-plastic; gradual boundary. C2 120--130 cm Similar to above layer without gypsum crystals. Profile No. 5 -- Coarse Desert A l l u v i u m

Location: Site:

1 km north of the road junction near Bir-eth Thamada in Central Sinai at coordinate 54365/33940. The pit was in a dry broad (300 m wide) wadi bed between the stream beds where the vegetation is concentrated; the distance to the nearest plant is 10 m. Some small scattered shrubs of Z y g o p h y l l u m d u m o s u m were found in the wadi bed. The area was flat and limestone gravel covered the soil surface. The gravel in the various soil layers consist also of limestone fragments.

184 Profile description: A 0--10 cm

AC 10--24 cm C l l 24--40 cm

C12 40--56 cm C13 56--110 cm

Very pale brown (10YR 7/4) dry, light yellowish brown (10YR 6/4) moist, very gravelly (about 50% gravel) loamy sand to sandy loam; weak fine subangular blocky structure;loose to soft, non-sticky and non-plastic; clear boundary. Similar to above layer but only slightly gravelly (about 10% gravel) and sandy loam texture; clear boundary. Similar to above layer but very gravelly (about 70% gravel) sand to loamy sand texture without any structure and loose consistency in dry conditions; clear boundary. Similar to above layer but only slightly gravelly (about 10% gravel) and sandy texture; clear boundary. Similar to above layer but very gravelly (about 60-70% gravel).

Profile No. 6 -- Young Regosolic Reg

Location:

200 m north of profile 7 and 150 m south of profile 5 near Bir-eth Thamada in central Sinai, at coordinate 54338/33914. The soil is on a terrace about 20 m from the terrace escarpment. The area is flat, and the soil is covered by a desert pavement consisting of flint and limestone gravel that are coated by a distinct desert varnish. This pavement covers about 50% of the ground. The gravel found in the various soil layers, especially in the deeper ones, consist mainly of limestone fragments. The soil is bare of vegetation. The whole profile is highly calcareous.

Site:

Profile description: Ao A 0--4 cm

B2sa

4--15 cm

B3cs

15--22 cm

Superficial cover of gravel with desert varnish. Very pale brown (10YR 7/4) dry, yellowish brown (10YR 5/6) moist, slightly gravelly (10% gravel) sandy loam; vesicular structure; soft to slightly hard, nonsticky but slightly plastic; some narrow cracks, 5 cm apart, were seen in this layer; wavy clear boundary. Reddish yellow (7.5YR 6/6) dry, strong brown (7.5YR 4/6) moist, slightly gravelly (about 10% gravel) loam to sandy clay loam with common (10--20%) fine distinct white mottles and some disintegrating stones; loose, sticky and plastic; some penetration of A horizon materials occurs beneath the cracks; gradual boundary. Reddish yellow (7.5YR 6/6) dry, strong brown (7.5YR 5/6) moist, gravelly (about 20% gravel) sandy loam with

185 many (about 30%) fine distinct white mottles (apparently gypsum); loose, slightly sticky and slightly plastic; wavy, clear boundary. Cllsacs 22--34 cm Reddish yellow (7.5YR 6/6) dry, strong brown (7.5YR 5/6) moist, gravelly (about 40% gravel), coarse sandy loam with many (about 30%) fine distinct white mottles (apparently partly gypsum); loose, slightly sticky and slightly plastic; gradual boundary. C12cs 34--78 cm Light yellowish brown (10YR 6/4) dry, yellowish brown (10YR 5/4) moist, very gravelly (about 60% gravel), slightly indurated sand; some parts of this horizon are more indurated while others are less; the induration is caused by gypsum; massive, slightly hard, non-sticky and non-plastic; gradual boundary. C13 78--130 cm Light yellowish brown (10YR 6/4) dry, yellowish brown (10YR 5/4) moist, very gravelly (about 30% gravel) coarse sand; some gypsum crystals were seen; single grained, loose, non-sticky and non-plastic; gradual boundary. C2 130--150 cm Similar to above layer without gypsum crystals. Profile No. 7 - - Mature Regosolic R e g

Location: Site:

Profile description: A0 A 0--7 cm

B2sa

7--16 cm

200 m south of profile 6, near Bir-eth Thamada in central Sinai, at coordinate 54305/33888. The soil characterizes the upper part of the plain. The area is flat, with a westward slope of 1.5%. The soil was paved by flint and limestone gravel that covers about 50% of the ground surface. Distinct desert varnish covered this gravel. The relative amount of flint gravel decreases with depth and from 25 cm downwards only few flint fragments were visible. The soil is bare of vegetation. All the soil is highly calcareous. The pit was about 50 m from the escarpment to a lower terrace. A complete cover of flint gravel with strong desert varnish. Light yellowish brown (10YR 6/4) dry, yellowish brown (10YR 5/4) moist, sandy loam; vesicular structure; soft to slightly hard, non-sticky but slightly plastic;some very small cracks about 5--10 cm apart; wavy clear boundary. Yellowish red (5YR 4/5) dry and moist, gravelly (30% gravel) saline clay loam with numerous small white mottles; loose, sticky and plastic; some intrusions of A horizon materials are underneath the small cracks to a depth of 15 cm; clear boundary.

186

B3sa

16--25cm

BCsa

25--39 cm

C11sa 39--50 cm

C12cs 50--80 cm C13

80--95 cm

C14

95--120 cm

Yellowish red (6YR 5/6 dry, 5YR 5/6 moist) gravelly (30% gravel) saline loam to clay loam; loose, sticky and plastic; clear boundary. Pink (7.5YR 7/4) dry, reddish-yellow (7.5YR 6/6) moist, very gravelly (about 50% gravel) sandy loam; loose, slightly sticky and slightly plastic; clear boundary. Very pale brown (10YR 7/3) dry, yellowish brown to brownish yellow (10YR 6/5) moist, very gravelly (50--60% gravel) loamy sand to sandy loam; single grained, loose, non-sticky and non-plastic; clear boundary. Similar to above layer but more gravelly (60--70% gravel) somewhat indurated by gypsum; clear boundary. Pink (7.5YR 7/4) dry, reddish yellow (7.5YR 6/6) moist, very gravelly (50--60% gravel) sand; single grained, loose, non-sticky and non-plastic; clear boundary. Strong brown (7.5YR 5/6) dry and moist, very gravelly (60% gravel), slightly indurated sandy loam; massive, hard, non-sticky but slightly plastic.

A n a l y t i c a l data

The analytical data are presented in Tables I and II. Table I shows pH, the amounts of gypsum and lime, CEC and the a m o u n t of exchangeable sodium and potassium as well as the particle-size distribution. In Table II electrical conductivity and values of the soluble anions and cations are presented. TABLEI Analytical resultsof pH, gypsum, carbonate, CEC, exchangeable Na and K, and mechanical composition Profile Depth and (cm) horizon

pH of Gypsum saturated CaSOdo2H20 paste (%)

Carbonates Cation Exchangeable cations as CaCO3 exchange (meq./100 g soil) (%) capacity Na K (meq./100 g)

8.15 7.60 8.05 7.90 8.35

0.077 0.21 0.34 0.12 0.13

30.8 24.5 20.3 19.4 8.4

5.87 4.56 2.72 3.26 1.74

1.20 1.55 0.85 1.05 0.55

0.75 0.79 0.31 0.36 0.22

7.65 7.60 7.65 7.50

0.01 0.017 0.075 0.83

33.6 39.5 51.0 45.1

6.20 7.17 2.72 2.39

0.95 0.80 0.65 0.70

1.0 1.0 0.38 0.45

Profile 1

Cll C12 C13 C14 C15

0---8 8--30 30--60 60--82 82--100

Profile 2

A B Clcs

C21

0-1 1--4 4--15 15- -30

187 DISCUSSION

Formation of desert pavement Soil formation in the extremely arid zone is restricted to areas that are stabilized and protected from erosion {Fig. 5). The erosion in the desert is generally severe because of the absence of a protective cover of vegetation

Fig. 5. A typical desert p a v e m e n t o f Reg soils.

Size class of particle diameter ( m m )

clay

Water content (%)

silt 0.002--0.05 (%)

fine sand 0.05--0.25 (%)

coarse sand 0.25--2 (%)

at saturation

3.4 1.6 -2.1 0.8

23.5 11.0 1.4 3.8 1.7

42.4 34.3 20.7 23.6 8.4

30.7 53.2 77.9 70.6 89.1

18.3 20.5 23.8 22.3 27.5

6.4 6.6 -1.2

27.2 22.4 3.8 3.3

40.5 24.8 16.7 26.3

25.9 46.2 79.5 69.2

25.5 25.0 22.0 23.5

<0.002 (%)

air-dry

188 T A B L E I (continued)

Profile Depth and (cm) horizon

pH of Gypsum saturated CaSO4o2H20 paste (%)

Exchangeable cations Carbonates Cation exchange (meq./100 g soil) as CaC03 capacity (%) (meq./100 g) Na K

C22 C23

7.55 7.45

44.2 42.5

2.17 2.39

0.80 0.85

0.47 0.51

27.4 33.6 36.0 23.9 14.7 14.2 8.8 13.8 13.9

8.04 9.88 3.93 4.61 4.46 3.55 3.87 2.29 2.39

3.90 1.71 3.25* 9.32* 21.65" 8.09* 4.56* 4.89* 10.30"

0.92 0.83 0.38 0.39 0.31 0.29 0.29 0.28 0.24

10.59 8.63 8.94 10.43 9.17 4.41 4.68 4.69 4.87

2.9* --* 2.9* --* --* 3.1" --* 6.6* 4.3*

0.88 0.45 0.41 0.33 0.27 Tr Tr Tr Tr

24.1 21.6 34.4 37.0 42.9

4.20 3.94 2.19 2.37 2.28

1.04 0.99 0.60 0.60 0.71

0.49 0.45 0.19 0.20 0.15

30--50 50--75

0.23 0.01

Profile 3 A 0--1 B21sa 1--5 B22cs 5--12 B3sa 12--20 C11sacs 2 0 - - 3 9 C12 39--52 C13cs 52--100 C14 100-117 C2 117--135

7.50 7.30 7.85 7.40 7.10 7.60 7.77 7.65 7.75

0.9 0.2 29.2 8.1 20.7 8.0 9.9 7.7 3.1

7.81 7.42 7.35 7.34 7.24 7.77 7.75 7.71 7.66

0.07 0.78 0.72 13.35 9.30 42.96 29.36 32.55 19.80

Profile 4 All A12 B1 B2cs B3sacs Cllcs Cllcs C12cs C13

0--3 3--9 9--19 19--35 35--47 47--75 75--100 100--110 110--120

Profile 5 A AC Cll C12 C13

0--10 10--24 24--40 40--56 56--110

7.90 7.80 8.02 8.18 8.21

0.004 0.009 0.23 0.06 0.27

Profile 6 A B2sa B3cs Cllsacs C12cs C13 C2

0--4 4--15 15--22 22--34 34--78 78--130 130--150

7.80 7.67 7.65 7.51 7.80 7.77 8.00

0.16 1.35 18.8 21.4 4.90 2.71 0.29

28.9 31.1 24.8 20.1 40.0 47.6 60.7

14.06 10.15 8.98 10.38 4.88 7.64 4.12

4.64 2.63 2.20 7.31 1.55 1.55 1.53

0.96 0.42 0.24 0.27 0.16 0.15 0.065

7.95 7.53 7.53 7.45 7.50 7.76 7.78 7.75

0.03 0.30 1.21 6.17 10.17 15.67 8.13 1.46

27.1 34.9 44.5 63.4 66.0 57.6 66.4 59.9

11.13 15.76 12.40 8.07 6.12 5.78 4.45 5.71

3.43 3.13 2.39 5.65 3.51 2.89 1.67 1.79

0.83 0.55 0.36 0.15 0.09 0.05 0.03 0.10

Profile 7 A B2sa B3sa BCsa C11sa C12cs C13 C14

0--7 7--16 16--25 25--39 39--50 50--80 80--95 95--120

*The exchangeable sodium values are meaningless due to the high saltcontent; **the high clay values include a large portion of gypsum.

189 Water content (%)

Size class of particle diameter (ram) day < 0.002 (%)

fine sand 0.05--0.25 (%)

coarse sand 0.25--2

1.3 4.7

12.3 28.3

84.8 66.4

22.5 23.0

9.3 6.4 18.4"* 4.3 18.8"* 1.5 14.3"* 0.8 2.2

43.7 43.6 23.7 12.5 13.8 6.1 11.0 4.8 2.2

29.7 30.4 34.6 28.3 16.3 23.6 12.1 14.6 21.1

17.3 19.6 23.2 44.9 51.1 68.9 62.7 79.8 74.5

28.7 24.5 50.0 25.3 30.5 31.0 39.9 29.9 25.0

11.4 6.0 5.6 11.7"* 22.2**

36.6 25.9 20.6 21.8 35.9

50.9 64.6 66.5 54.0 27.8

1.6 2.9 7.4 12.5 14.0

35.8 30.4 33.6 43.3 37.4 63.6 43.5 47.9 36.0

1.9 1.9 2.0 1.2 1.4

9.9 7.2 2.2 3.3 3.0

59.8 49.7 20.0 48.7 16.8

28.4 41.2 75.8 46.7 78.9

20.6 22.4 21.6 27.3 22.0

0.8 1.1 0.6 0.8 0.6

21.1 9.0 17.8 22.8 9.4 8.0 3.6

22.8 20.9 7.8 12.8 2.6 7.4 2.7

36.6 32.4 45.8 27.9 33.3 15.9 7.6

19.5 37.8 28.6 36.6 54.7 68.7 86.2

33.2 26.2 39.5 39.1 28.9 33.9 27.1

3.4 3.7 6.6 8.4 2.6 2.8 1.1

14.3 4.2 13.2 17.0 15.3"* 22.0** 9.6 11.4

14.4 42.5 21.9 24.2 21.9 17.4 14.8 11.0

53.6 31.4 23.0 20.1 16.0 13.0 16.3 14.4

17.2 22.0 41.8 38.6 46.8 47.6 59.4 63.2

30.3 38.3 38.9 31.1 33.8 36.8 29.2 38.2

2.4 4.6 5.1 3.8 4.5 5.9 2.4 2.4

1.6 0.6

silt 0.002--0.05 (%)

at saturation

air-dry

(%)

0.8 6.1 1.3 2.3 1.7 4.5 0.9

190

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191

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M~NNM

N4d~MN4

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NMNMNMM~

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192 (Dan et al., 1981). Thus erosion is mainly due to the high water runoff in the desert areas during the occasional rainstorms as a result of the low permeability of desert soils (Sharon, 1962; Hunt and Mabey, 1966; Cooke, 1970; Shanan, 1975). Wind deflation may also occur, however, mainly in areas which are n o t protected b y a surface crust and where the surface layer is loose; this happens usually on sandy sites (Dan et al., 1981, chapter 2). The erosion continues rapidly on the various desert landscapes until some kind of gravel or stone protection is concentrated on the surface. This gravel accumulates either because of erosion of the finer materials (Cooke, 1970) or as a result of upward pushing of stones because of soil swelling and shrinking (Springer, 1958; Jessup, 1960; Mabbutt, 1965). It seems that in Israel the formation of the desert pavement is partly due to erosion. It is likely, however, that some upward movement of stones occurs in the well-differentiated Regs (profiles 4 and 7) in which the clay content of the A and B horizons is relatively high and in which the high salinity may enhance this process. The relative proportion of the unweatherable flint gravel increases in the desert pavement of the well-differentiated Reg soils and in some of them, especially in the Arava valley and the plains of Paran and most of central Sinai (Dan et al., 1981, see profiles 3 and 4), no other stones or gravel are found in this layer. The relative proportions of limestone and other carbonate stones increase in the deeper soil layers and in some profiles, such as those near Bir-eth Thamada, they comprise the absolute bulk majority of these fragments. These stones and gravel disintegrate during the process of salt weathering. Some of this weathered material may be eroded later, thus leaving a concentration of the unweatherable flint gravel on the surface. The sizes of the pavement gravel and stones in the pavement decrease somewhat with time; in y o u n g soils some stones are found among the gravelly pavements (see profile No. 2) whereas in the older soils the pavement consists entirely of small gravel. This phenomenon also characterizes similar desert pavements in other countries (Hunt and Mabey, 1966; Cooke, 1970). The disintegration of stones to gravel, both on the surface and in the various soil layers, is affected to a large extent by salt crystal growth (Yaalon, 1970; Goudie, 1974}. Insolation may also affect the splitting of gravel on the surface (Peel, 1974). The gravel that covers the soil is characterized by a desert varnish (Hunt and Mabey, 1966; Cooke, 1970; Evenari et al., 1974; Dan et al., 1981; Dorn and Oberlander, 1981). This varnish develops relatively fast (a few thousand years} as it characterizes also the young Reg soils; it is missing only in the very y o u n g coarse desert Alluvium which is found in stream beds. This varnish helps to keep to a large extent the gravel from further disintegration due to the hardness of this layer; this seems true especially for limestone and dolomite fragments as the flint gravel is already very hard and resistant to weathering.

193

Weathering and formation of mature soil horizons The typical Reg soil has two thin well-differentiated horizons that may be considered as A and B horizons. The upper horizon always consists of a lightcoloured, somewhat loamy, vesicular layer (Evenari et al., 1974) that may be designated as an A horizon. The lower part of it may have a laminar structure. The underlying horizon, as a rule, consists of a mellow, loose yellowish red or strong brown saline loam or even clay loam and is thus a cambic or argiUic horizon. The depths of these horizons are usually restricted to several centimeters. It seems that the depths of these horizons, their relative clay contents and their distinctness may serve as criteria of Reg soil development. The formation of soil horizons in the Regs is very slow. The youngest soil (profiles 1 and 5) does n o t yet have any horizon and the clay c o n t e n t is nearly nil and seems related to the alluvial parent material. Horizon differentiation of the somewhat older soil from the Lisan surface (profile 2) is also very restricted. This soil exhibits only a very shallow vesicular A horizon and the beginnings of a B horizon evident from the colour difference. The clay c o n t e n t of these two shallow layers is also very low and does n o t exceed 6--7%. Horizon formation in the other soils is already clearly evident. The upper horizons of profiles 3 and 6 are quite well differentiated. The total depths of the upper two horizons, however, do not exceed 20 cm in profile 3 but are somewhat deeper in profile 6. Profiles 4 and 7 represent well-differentiated Reg soils in which the various horizons reach what seems to be their maximum development. This stage is best expressed in profile 4. Very few Reg soils reach this stage in the Negev (Ravikovitch et al., 1956). The weathering and formation of the A and B horizon can occur only when the soil is moist; the upper few centimeters may be moistened several times during the winter months so that the formation of a shallow A and B horizon as in profile 2 is relatively fast. Moisture penetration to greater depths happens rarely and as a result a long time may pass before horizon differentiation may be marked in these layers. The maximum depth of water penetration in deep Reg soils reaches 50--60 cm (Gerson and Amit, 1982) but this occurs very rarely; therefore a very long time m a y pass before these relatively deep layers are weathered. Weathering to these depths (50--60 cm) is expressed only in the mature and old Reg softs of profiles 4 and 7. The high salt content of the deep Reg soils enhances the deep weathering because these layers remain moist for a relatively long time after the rainstorms due to the hygroscopic nature of these salts.

Salinization The various Reg soils have been gradually salinized by airborne salts (Yaalon, 1963). These salts concentrate in the soil profile due to the restricted water penetration. The yearly addition of salt is small (Yaalon, 1964b) b u t these salts are gradually concentrated.

194

Tb ~ differences in salt contents of the various soils are significant. The younoCst soil (profiles 1 and 5) is only slightly saline; the salinization increases in profile 2 and reaches maxima in profiles 3 and 4 (Fig. 6). Profiles 6 and 7 are also very saline, although the salinity values are lower than in profiles 3 and 4 (Fig. 7). Chlorides of Na and gypsum are the main soluble salts. Large amounts of CaC12 and MgC12 are also present. Differential salt distribution characterizes these soil profiles. The differences are related to the limited wetting of the soils and the limited mobilization of salts under dry conditions (Yaalon, 1964a). The uppermost layers of 1 or 2 cm contain relatively small amounts of soluble salts. The salt content increases in the next deeper layers (see Figs. 6 and 7}. These layers (profile 2 : 1 - - 5 0 cm; profile 3 : 1 - - 2 0 cm; profile 4 : 9 - - 3 5 cm; profile 6 : 4 - - 2 2 cm; and profile 7 : 7 - - 2 5 cm) are already very saline. The main salts include various chlorides, among which are Ca and Mg chlorides, although in the upper parts of these layers in profile 2 (1--15 cm) the amounts of Ca and Mg chlorides are restricted. Gypsum is generally present and most profiles already have considerable increases in the gypsum content in one of these layers (profile 2 : 1 5 - - 3 0 cm; profile 3 : 5 - - 1 2 cm; profile 4 : 2 - - 9 cm; profile 6 : 1 5 - - 2 2 cm); in profile 3 this increase is most significant and reaches very high values. Nowadays, the gypsum seems to concentrate in this layer due to the very low leaching activity of the rainwater. The most saline layer is found somewhat deeper (profile 2 : 5 0 - - 7 5 cm; profile 3 : 2 0 - - 3 9 cm; profile 4 : 3 5 - - 4 7 cm; profile 6 : 2 2 - - 3 4 cm; profile 7: 25--39 cm). The values of the salt contents in these layers in the older profiles are extremely high. This is probably the deepest layer that is still reached by the rainwater after the heaviest rainfall, as most of the salts are not leached further downward. The depths of this layer in profiles 3, 4, 6 and 7 are more or less the same, an argument that supports this hypothesis. In the younger profiles this layer is found somewhat deeper, due to some increase in leaching as a result of the low water-holding capacities of the various soil layers. The depth of this very saline layer corresponds to a large extent with the depth of maximum water penetration measured during recent years (Gerson and Amit, 1982}. The water penetration after a heavy rainfall of 30--35 mm reached a b o u t 25 cm in the mature Reg soils and a b o u t 40 cm in the young Reg soils and coarse desert Alluvium which is n o t affected by floods and where the beginning of the Reg soil formation m a y be recognized (as in profile 2). The deeper water penetration of the young Reg soils is related to the low water-holding capacity {only a b o u t 10% at field capacity according to measurement near Zofar (D. Russo, personal communication 1980). This depth corresponds with the beginning of the most saline layers in the various profiles. After an exceptionally heavy rainstorm such as that which occurred during 19/2--21/2 1975, when the rainfall in the southern Negev reached 60--80 mm, the water penetration was somewhat deeper but even on this occasion it did n o t exceed 50--60 cm in the mature Reg soils. In the young Reg soils this water penetration is somewhat deeper and

195

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.................... i:

o. - .....

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EC VALUES (mmho/cm)

.-

........

.'

V° i

~,. .': II~ ."~

I

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a. : ! '."

"%1 ul

•,~

.~

,/i

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1.0'

Prof, I

'..! '..

COPRSEDESERTALLUVIUM (Recent surface)

......

Prof. 2 VERYYOUNC~EGOSOLIC REG (Lisan surface)

............

Prof. 3 REGOSOLICREG (mid-Pleistocene surface!

. . . . .

Prof. 4

DEEPREGOSOLIC REG (Neogene surface)

1.5" Fig. 6. Conductivity values of the profiles in the vicinity of Hazeva and central Sinai (profiles 1--4).

EC VALUES(mmho/cm)

5o

tpo

i

1,5o

2p0

t~-] I

0.5 i~j/. o

II,. 1.0

l,"i" i' /i I Prof. 5 COARSEDESERTALLUVIL'~

1.5

.....

Prof. 6 YOUNGREGOSOLIC REG

........

Prof. 7 MATUREREGOSOLIC REG

Fig. 7. Conductivity values of the soils near Bir-eth Thamada (profiles 5--7).

196

reached 75 cm. This was followed by the deeper leaching of the salts. The depth of water penetration during this storm may correspond to the deeper part of the most saline layer in the Reg soils. The deeper soil layers of profiles 3, 4, 6 and 7 are characterized by very high gypsum contents (Figs. 8 and 9), whereas the salinity values decrease slowly with depth. Most of this gypsum cannot be related to the present rainfall regime as this layer is found beneath the main sa horizon. The occurrence of this gypsum may be related to a somewhat more rainy period in the past, although some of it may be formed even nowadays after the heaviest rainstorms. This deep gypsum layer is absent in profile 2. Inprofile 6 it is not significant (the main gypsum layer of this profile is found at a depth of 22-34 cm) due to the young age. The deep gypsic layers of the older soils, especially that of profile 4, apparently developed during a more rainy period in the past. Below 1 m the gypsum contents in profiles 3, 4 and 7 decrease again slowly, whereas the salinity values rise again to somewhat higher degrees. This may be the beginning of a saline horizon related to the old gypsum pan, but most of the salt related to that gypsum pan should be found only in still deeper layers, as the total gypsum contents of the various pans are very high, whereas the other salt contents are relatively low in the layers that have been analyzed. No increase in the salinity of the deepest layers of profile 6 was detected, an indication of the younger features of this soil. GYPSUM (P~)

0.1

0,2

,,~_,,~-'

0,5

1

2

5

io

2,0

50

::::::::::::::::::::: ........... '............................................

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150'

Prof. I COARSEDESERTALLUVIUM (Recent surface) ~rcf. 2 VERV YOUNAREGOSOLICREG (Lisan surface) . . . . . . . . . . . Prof. 3 REGOSOLICRE~ (mld-Pleistocene surface) --•--,--•-- Prof. 4 DEEPREGOSOLICREG (Neoeene surface) .....

Fig. 8. Gypsum contents of the profiles in the vicinity of Hazeva and central Sinai (profiles 1--4)•

197 GYPSUM (%)

L

"%.-.

0,1

03.

0,5

1

2

5

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COARSEDESERTALLUVIUM

.......

Prof. 6

YOUNGREGOSOLIC REG

.........

Prof. 7

MATUREREGOSOLIC REG

i

1.5'

4

Fig. 9. Gypsum contents of the soils near Bir-eth Thamada (profiles 5--7).

Age estimate of the various Reg soils from salt accumulation The salinization of the Reg soils may be related to atmospheric automorphic salinization (Yaalon, 1963). The various soils checked show evident increases in salinization, passing from the soils on the youngest, lowest surfaces to the soils on the oldest, highest surfaces. Calculations were made for the seven profiles to test the possibility of estimating the ages of these soils from salt and gypsum contents. The following calculations were based on several assumptions, viz., that the annual fall-out of salts was more or less similar to the prevailing fall-out at Sde Boqer (Yaalon, 1964b), i.e., that the deposition of sodium, on which the calculation of the most soluble salts was based, reached 2 kg/ha and that of SO4 anions {calculated for gypsum) reached 10 kg/ha. (At Sde Boqer the value was 9 kg/ha but the amount at Mizpe Ramon, which is nearer the desert, was much higher, so that a value of 10 kg seemed reasonable.) The actual data may deviate considerably from the calculated figures for the following reasons: (1) The rainfall figure of 50 m m may be too high, as the average rainfall in the desert region (Arava Valley and central Sinai) is somewhat lower. At Nakhl, the nearest point to Bir-eth Thamada, the average rainfall reached 27 mm (Ganor et al., 1973). At Kuntilla and Thamad, the stations in the vicinity

198 of profile 4, the figures reached 32 and 37 mm, respectively, whereas the corresponding figure for En Yahav, near Hazeva, was 42 mm (Israel Meteorological Service, 1967). Moreover, these figures include the salts in runoff water, which is very significant in the desert. The real average airborne salinization rate may thus reach only 50% or even less of the salt content figures, so that the calculated time scale might be at least doubled. (2) The calculation was based on the present rainfall figures. As already mentioned, rainfall may have been somewhat higher during some periods in the past (Bull and Schick, 1979; Gat and Magaritz, 1980). The salt accretion figures should thus be somewhat higher; they may have reached or even exceeded these for the Sde Boqer area. (3) The rainfall salinity figures were based on those for the Negev mountains. The figures for the Arava Valley, and especially those for central Sinai, might be somewhat different inasmuch as the rainfall regime in these regions includes some spring and fall storms connected with depressions in the Dead Sea region (Ganor et al., 1973). (4) The calculations were based only on the salt and gypsum contents in the soil profile and did not include the amounts of these c o m p o u n d s in deeper layers. This might be especially important for the profiles where the salt and gypsum contents are quite high in the deepest soil layers, such as profiles 3, 4 and 7. The real salt accumulation figures for these profiles may thus be much greater. The same may hold true for the gypsum values of profile 4 and also for profile 7, as some gypsum pans were found even in the deepest soil layers. (5) The calculation of SO4 accumulation was based only on the gypsum contents, but some other sulphate salts are also present. The calculation based on gypsum might thus be somewhat too low, which would affect especially the calculations from. the younger profiles where the gypsum content is low. The age-estimates based on the salt and gypsum contents of the various profiles are given in Table III. TABLE III E s t i m a t i o n o f age o f the various profiles* Profile no. and surface

E s t i m a t i o n based o n salinity c o n t e n t s (years)

E s t i m a t i o n based on gypsum contents (years)

2. 3. 4. 5. 6. 7.

9 50 61 2 20 19

3 500 127 000 190 000 1 500 57 100 65 5 0 0

Lisan surface Mid-Pleistocene surface Early P l e i s t o c e n e - N e o g e n e surface H o l o c e n e surface Mid-Pleistocene surface Mid t o early P l e i s t o c e n e surface

500 700 200 900 100 700

*Calculated ages are r o u n d e d t o the nearest 1 0 0 years.

199 The age figures calculated according to the salinity contents differ from those calculated according to the gypsum contents. In profiles 2 and 5 the values for sulphates would have been much higher if the total contents of sulphates had been calculated and these figures would then approach those of the salt-age calculation figures. For the other profiles, the age-estimates based on salt c o n t e n t are much lower than those based on gypsum, especially for profile 4 and to a somewhat lesser degree also for profiles 3 and 7. The gypsum values therefore seem better for these cases, as the salt-age figures may be related mainly to the recent more arid cycle. But even the gypsum figures generally indicate t o o low ages, suggesting that some gypsum had been leached into deeper layers or that the supply was lower in the past. Horowitz (1979) claims that several changes in climate during the Pleistocene occurred in all of Israel, including the deserts; therefore, sometime in the past, the area may have received somewhat higher rainfall and, as a result, soluble salts were leached to great depths. It is possible that this somewhat more rainy period corresponded more or less with the beginning of the last glaciation, i.e., a b o u t 90 000 B.P. It is not feasible to estimate the rainfall during that more rainy period in the past. The annual amount could not have exceeded 200 mm, or the gypsum would have been leached, as this is the present limit for gypsum concentrations in medium-textured softs in the northern Negev (Dan and Yaalon, 1980). Moreover, this limit for stony and coarse textured softs such as the Regs is much lower because of the low waterholding capacity of such soils. Thus, the average annual rainfall figures for the rainy period in the Arava and central Sinai probably could n o t have exceeded 100 mm. The total absence of loess also confirms this low figure, as loess deposits would have been found in the desert if it had received more than 100 or 150 mm rainfall (Dan et al., 1981). The differences between the salt and gypsum profile figures of the older soils may also help us to evaluate their relative age and their salinization history. Profile 6 seems to be the youngest of these profiles, as only one definite salt and gypsum horizon was found and the values of both compounds dropped to low values at depths greater than 34 cm. It thus seems that this profile was affected only by the present arid climatic cycle; otherwise the gypsum contents of the deeper soil layers would be much more signigicant. The gypsum horizons in profiles 3 and 7 reach deeper layers, but even in these profiles the gypsum values in the deepest layers fall to quite low figures. These profiles probably enjoyed a somewhat more rainy period in the past, b u t very little gypsum penetrated deeper, and their age estimates would thus correspond, approximately, to the figures calculated from the gypsum content. In profile 4, on the other hand, high gypsum values were still found in the deepest soil layer, and this profile probably enjoyed, during the Middle Pleistocene, another moist period when the rainfall leached some of the gypsum to the still deeper layers.

Classification and correlation Differences in the ages of the soils are well expressed by the soil charac-

200 teristics a n d this is also r e f l e c t e d in classification o f t h e soils. T h e y o u n g e s t soils (profiles 1 a n d 5) d o n o t y e t s h o w a n y h o r i z o n d e v e l o p m e n t a n d w o u l d t h u s be classified as Coarse d e s e r t A l l u v i u m a c c o r d i n g t o t h e Israeli Classificat i o n ( C o m m i t t e e o n Soil Classification in Israel, 1 9 7 9 ) or as T y p i c T o r r i o r t h ents a c c o r d i n g t o t h e A m e r i c a n classification s y s t e m (Soft S u r v e y Staff, 1975). Profile 2 a l r e a d y has m i n i m a l p r o f i l e d e v e l o p m e n t a n d s h o u l d t h u s be classifted as a v e r y y o u n g Reg, b u t t h e w e a t h e r i n g z o n e is v e r y shallow, so t h a t even a s h a l l o w p l o u g h i n g w o u l d d e s t r o y a n d e l i m i n a t e t h e s e p r o f i l e characteristics. This p r o f i l e w o u l d t h u s still be classified as a Coarse desert Alluvial soil or a T y p i c T o r r i o r t h e n t . Profiles 3, 4, 6 a n d 7 a l r e a d y h a v e e i t h e r well or faintly e x p r e s s e d c a m b i c .1 a n d gypsic h o r i z o n s as well as all o t h e r p r o p e r t i e s w h i c h c h a r a c t e r i z e t h e Reg soils. T h e s e softs w o u l d t h u s be classified as T y p i c a l Regs a c c o r d i n g to t h e Israeli classification a n d as G y p s i o r t h i d s o r e v e n p a r t l y as P e t r o g y p s i c G y p siorthids a c c o r d i n g t o t h e A m e r i c a n classification s y s t e m . I t s e e m s t h a t s o m e o f t h e y o u n g e r R e g soils, in w h i c h t h e g y p s u m c o n t e n t is n o t v e r y high, a n d w h e r e t h e p e t r o g y p s i c h o r i z o n has n o t y e t d e v e l o p e d , w o u l d h a v e b e e n classifted as T y p i c or C a m b i c G y p s i o r t h i d s in t h e A m e r i c a n s y s t e m . ACKNOWLEDGEMENT T h e a u t h o r s wish t o t h a n k Dr. H a n n a K o y u m d j i s k y f o r h e r g r e a t h e l p a n d h e r useful r e m a r k s . REFERENCES

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