The role of rare rainstorms in the formation of calcic soil horizons on alluvial surfaces in extreme deserts

Quaternary Research 74 (2010) 177–187

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The role of rare rainstorms in the formation of calcic soil horizons on alluvial surfaces in extreme deserts Rivka Amit a,⁎, Yehouda Enzel b, Tamir Grodek c, Onn Crouvi a,b, Naomi Porat a, Avner Ayalon a a b c

The Geological Survey of Israel, 30 Malkhe Israel St., Jerusalem, 95501, Israel The Fredy and Nadine Herrmann Institute of Earth Sciences, The Hebrew University of Jerusalem, Edmond J. Safra campus, Givat Ram, Jerusalem 91904, Israel Department of Geography, The Hebrew University of Jerusalem, Mt. Scopus, Jerusalem 91905, Israel

a r t i c l e

i n f o

Article history: Received 9 March 2010 Available online 1 July 2010 Keywords: Namib Desert Negev Desert Calcic soils Gypsic-salic soils Carbonate nodules Oxygen and carbon isotopes Rainfall characteristics

a b s t r a c t Soils in similar geomorphic settings in hyperarid deserts (b 50 mm yr−1) should have similar characteristics because a negative moisture balance controls their development. However, Reg soils in the hyperarid southern Negev and Namib deserts are distinctly different. Soils developed on stable alluvial surfaces with only direct input of rainfall and dust depend heavily on rainfall characteristics. Annual rainfall amount can be similar (15–30 mm), but storm duration can drastically alter Reg soil properties in deserts. The cooler fall/ winter and dry hot summers of the southern Negev Desert with a predominance brief (≤1 day) rainstorms result in gypsic-saline soils without any calcic soil horizon. Although the Namib Desert receives only 50–60% of the southern Negev annual rainfall, its rainstorm duration is commonly 2–4 days. This improves leaching of the top soil under even lower annual rainfall amount and results in weeks-long grass cover. The long-term cumulative effect of these rare rain-grass relationships produces a calcic-gypsic-saline soil. The development of these different kinds of desert soils highlights the importance of daily to seasonal rainfall characteristics in influencing soil-moisture regime in deserts, and has important implications for the use of key desert soil properties as proxies in paleoclimatology. © 2010 University of Washington. Published by Elsevier Inc. All rights reserved.

Introduction Soils of hyperarid deserts (b50 mm yr−1) in similar geomorphic settings should have similar characteristics, as the main controlling factors for their development are an extremely negative moisture balance and atmospheric dust input (e.g., Bao et al., 2001; Ewing et al., 2006; Amit et al., 2006; Quade et al., 2007). In the southern Negev Desert and in parts of Sinai and northern Arabia deserts, stable Quaternary alluvial surfaces that have received only direct rainfall and dust input are characterized by Reg soils with diagnostic gypsic (By) and salic (Bz) soil horizons at depths of 10–30 and 80–120 cm, respectively (Dan et al., 1982; Amit et al., 2006; Matmon et al., 2009). These diagnostic gypsic and salic Reg soil horizons are characteristic of extremely arid climates (b80 mm yr−1) and are ubiquitous in the Negev in areas having precipitation of 25–50 mm yr−1 or even less. No calcic soil horizons typical of arid environments (e.g., Gile, 1975; McFadden and Tinsely, 1985) are found on any of these surfaces. In the Negev Desert calcic horizons are widespread in soils developed in the arid-semi-arid (100–250 mm yr−1) central and

⁎ Corresponding author. E-mail addresses: [email protected] (R. Amit), [email protected] (Y. Enzel), [email protected] (T. Grodek), [email protected] (O. Crouvi), [email protected] (N. Porat), [email protected] (A. Ayalon).

northern Negev Desert (Dan et al., 1964; Magaritz, 1986; Goodfriend and Magaritz, 1988; Amit et al., 2006). This indicates that if soils have calcic horizons they were not developed under hyperarid conditions (Dan and Yaalon, 1982; Birkeland, 1999; Amit et al., 2006). These observations and inferences raise a question: can we apply the total annual rainfall threshold of ∼ 80 mm yr−1 to distinguish soil characteristics in other deserts in the world? Answering this question requires the identification of potential causes for the differences in soil properties in different deserts. This study compares soil properties to rainfall patterns in the Namib and southern Negev deserts and is a step toward a resolution of this question. Both deserts currently have large areas experiencing precipitation of 50 mm yr−1 or less and have been hyperarid throughout the Quaternary (Heine, 1998; Brook et al., 1999; Bao et al., 2001; Lancaster, 2002; Amit et al., 2006). The study will contribute to a better understanding of climatic controls over soilformation process in extreme environments. In both deserts, we attempt to characterize the soils formed at the edge of moisture availability and to identify parameters that may explain similarities and dissimilarities between them. Amit et al. (2006) argued that soils are indicators of the cumulative effect of climate only if they formed in response to direct rain and dust input. Therefore, we use diagnostic horizons of Reg soils developed on stable, relatively flat alluvial surfaces, which are excellent proxies for regional-scale climate conditions (e.g., Yaalon, 1971; Birkeland, 1999) and avoided soil from sites that potentially could have received runoff from adjacent slopes

0033-5894/$ – see front matter © 2010 University of Washington. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.yqres.2010.06.001

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(Yair and Berkowicz, 1989) or had any relationship with ephemeral channels (Amit et al., 2007). We show that key properties of soils that develop on stable alluvial surfaces in a sustained hyperarid climate depend predominantly on the long-term cumulative effect of the characteristics of individual rainstorms over thousands of years and not simply on the average annual rainfall. The differences in the soil properties of these similar desert settings highlight subtle differences in the moisture regimes in hyperarid deserts that can result in very different pedogenic features that when simplistically overlooked can lead to misinterpretation of soils of the past. Climates of the Namib and the Negev deserts Namib Desert Namibia lies along the southwestern coast of Africa with the northflowing cold Benguela current producing the coastal Namib Desert (Fig. 1). The dry climate of Namibia is the outcome of the blocking by the Southern Hemisphere subtropical high-pressure zone (e.g., Mendelsohn et al., 2002). Shifts to the south of this zone in the austral summer reduce this blocking effect and allow southward movement of moist tropical air. The rare summer rainstorms in the central Namib Desert are in part incursions of moisture from the Intertropical Convergence Zone (ITCZ) under the influence of the Angola Low centered just north of Namibia (Nicholson, 2000; Mendelsohn et al., 2002). In the Namib Desert, the moist air converges in the zone of low pressure to produce non-localized, longer-duration rains (e.g., Tyson, 1986; Nicholson, 2000). Another source for rare summer rainfall in the Namib Desert is transKalahari moisture, advected from the Indian Ocean. The decrease in the Benguela upwelling associated with weakening of the coastal highpressure inversion allows Indian Ocean air masses to reach and produce rainfall in the western Namib Desert (Brook et al., 2007). The direction of the relative long-distance transport of moisture to the Namib Desert by these systems determine the spatial distribution of rainfall and the low total annual rainfall amounts (Mendelsohn et al., 2002). Rainstorms occur in the Namib Desert during the summer, between January and April, with total seasonal rainfall ranging from only 1–5 mm

along the Atlantic Ocean coast increasing to ∼85 mm along the eastern limits of the Namib Desert (Fig. 2) (Lancaster, 1984; Lovegrove, 1993; Lancaster, 2002). Temperatures are moderate relative to other desert regions, reflecting the influence of the cold Benguela current. Mean annual daily maximum temperatures range from 17 °C at the coast to 28°–33 °C inland, while minimum daily temperatures average 13°–16 °C throughout the region. The mean annual evaporation in the central Namib is N2000 mm with relative humidity b20% during the least humid months and 50–60% in the most humid months (Mendelsohn et al., 2002). Negev Desert The Negev Desert, at the northern edge of the large Saharo-Arabian desert belt, is located south of the semiarid Mediterranean climatic zone (Fig. 3). The area encompassing the Negev, Sinai, and northern Arabia is currently among the driest places on Earth: in 75% of the area the precipitation averages b80 mm yr−1 and half of that area is hyperarid (b50 mm yr−1) (Fig. 3). Mean annual rainfall in the southern Negev Desert is 25–30 mm yr−1. The synoptic-scale system responsible for most of the annual precipitation in the Negev Desert is an eastern Mediterranean extratropical cyclone, the Cyprus Low (Sharon and Kutiel, 1986; Alpert et al., 1990). The southern boundary of the winter rains during wettest winters is usually associated with the continuation to the east of the northern Sinai coastline, commonly located 10–20 km to its south (Enzel et al., 2008). Convective rainfall by the active Red Sea Trough contributes a higher proportion of the annual rainfall in the southern Negev than in the northern Negev (e.g., Kahana et al., 2002). Rainfall is usually in ∼2–3 high-intensity showers (N20–30 mm/hr) during the late fall or winter. The showers are localized, scattered, and short-lived, thus having limited effect on the regional mean rainfall (Sharon, 1972): 15%–25% and 10%–15% in the southern and northern Negev, respectively (Sharon and Kutiel, 1986; Morin et al., 1998). Thus the eastern Mediterranean extratropical cyclone, the Cyprus Low rainfall is probably responsible for most of the moisture available for soil development in the hyperarid parts of the Negev Desert during most of the Quaternary (Amit et al., 2006). In general, summers in the Negev Desert are extremely hot and winters are cold (Atlas of Israel, 1985). Soil surface temperature in the

Figure 1. Location map and mean annual rainfall over the central Namib Desert.

R. Amit et al. / Quaternary Research 74 (2010) 177–187

Figure 2. Time series of annual, monthly, and daily rainfall in Eilat, southern Israel (1949–2004) and Gobabeb, Namibia (1963–2006). Thin dashed lines mark the mean annual rainfall.

southern Negev may reach 50 °C at midday and drop to 30 °C at night (Ashbel et al., 1965). Mean annual temperature is 25 °C. The potential evaporation is 2600–3600 mm/year, and the mean annual humidity is 40–45%. Methods Field survey was focused on locating stable alluvial surfaces of late Pleistocene age in the Namib hyperarid environment, similar to those studied in the southern Negev Desert by Amit et al. (2006). Soil profiles were documented and classified using standard sedimentological and pedologic methods (Dan et al., 1964; Birkeland, 1999; Soil Survey Staff, 1999) with emphasis on the soil properties and horizons that specifically characterize soils such as those common in this region (Amit and Gerson, 1986; Amit et al., 1993; Gerson et al., 1985). Samples were collected at ∼ 10 cm intervals and according to soil horizons. Major ion concentrations were measured in soil solution by ICP AES Optima 3300, PerkinElmer (2% analytical error) and Ion Chromatograph ICS-2000 (5% analytical error), Dionex. Samples were analyzed for particle-size

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distribution (PSD) using laser diffraction (Malvern Mastersizer MS2000). Measurement procedures included: sieving (b2 mm), dispersion using sodium hexametaphosphate solution, stirring for 5 minutes and ultrasonication for 30 s. Three to six replicate samples of each soil sample were then subjected to three consecutive 5-s runs at a pump speed of 1800 RPM. The raw laser diffraction values were transformed into PSD using the Mie scattering model, with optical parameters of RI = 1.52 and A = 0.1. Carbonate nodules (∼2–3 mm size) were ground, and ∼ 500 μg were taken for stable isotope analysis. δ18O and δ13C measurements were performed using a Gas Bench system that is attached directly to a Finnigan Delta Plus mass spectrometer. All δ18O and δ13C values were calibrated against the international standard NBS-19, and are reported in the δ notation permil (‰), relative to the VPDB standard. Analytical reproducibility of duplicates is better than 0.1‰ both for δ18O and for δ13C. The ages of the alluvial surfaces were determined by optically stimulated luminescence (OSL) dating, with samples taken at 20- to 30-cm intervals, from the top of the soil to a depth of 70 cm (Table 1). The quartz is an original component of the pediment sediment and is assumed to have been bleached during fluvial transport (Porat et al., 2010). 88–125 μm quartz was separated from the samples using standard laboratory procedures (Porat, 2006, 2007). Luminescence measurements were carried out on Risø DA-12 or DA-15 TL/OSL readers. Equivalent doses (De) were determined using the standard single-aliquot regenerative dose (SAR) protocol with preheating in the range of 220–260 °C (Murray and Wintel, 2000). De values do not vary within this temperature range, indicating that the measured signal is thermally stable. Samples have recycling ratios within 1.0 ± 0.1, indicating that the SAR protocol is appropriate. Although the samples are feldspar-rich, negligible IR signals were measured for the purified quartz. The average and errors on the De were calculated using the central-age model (Galbraith et al., 1999) and the natural scatter in the sample is given by the over-dispersion value. Alpha and beta dose rates were calculated from the concentrations of the radioelements U, Th, and K in the sediment, measured by ICP-MS. Cosmic dose rates were calculated from burial depths. Moisture contents were estimated at 2 ± 1%. For the analyses of annual, seasonal and daily rainstorms in the Namib and Negev deserts we used data from the rain stations closest to our study sites in the respective deserts: Gobabeb (1963–2006) with mean annual rainfall of 19 mm is ∼ 50 km south of our study site and is representive of the central Namib Desert, and Eilat (1949– 2004) with mean annual rainfall of 29 mm is ∼ 8 km south of our study site in the southern Negev Desert. Namib soil characteristics The analyzed Reg soils developed on abandoned late Pleistocene alluvial surfaces that are part of a pediment (Fig. 4A,B). The sediments originate from the weathered low Gongochuab ridge composed of the Damara granites and schist of the Khomas Group. The alluvial surfaces slope between 1° and 3° and are covered by well-developed to moderately developed desert pavement (70% pavement cover: the rest is granitic grus and fine sand and silt). No perennial vegetation grows on the abandoned surfaces, with very sparse vegetation in the shallow streams which incise the pediment. However, thin root remnants of annual grasses are sparsely scattered in the upper 20 cm of the soil. For example, in September 2007 we identified remnants of dry grasses which covered the surface in the extremely rainy year 2006 (Fig. 2). The soil developed in gravelly coarse alluvia composed of 60% granite, schist and quartz gravel of up to ∼ 15 cm in diameter, and 10% pebbles of 3 cm diameter with grus (grains 1–0.5 cm) and sandy silty matrix. It has the following pedogenic characteristics (Fig. 5; Table 1): A vesicular horizon ∼ 0.5 cm thick, slightly

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Figure 3. Climatic zones of the Negev, Israel including isohyets and characteristic soils.

cemented; Bk horizons (Bk1, 7–12 cm; and Bk2, 12–20 cm, Table 1) with weak carbonate cementation around thin roots of dried annuals and soft patchy calcium carbonate nodules forming a stage I calcic horizon (Gile, 1975; Gile et al., 1981; Machette, 1985); By horizons (20–65 cm) with gypsum disseminated in the sandy silty matrix and around the gravels. At a depth of 45–65 cm, a wellcemented to moderately cemented gypsic horizon is discernible. A well-developed to moderately developed petrosalic horizon (Bz) is present at a depth of 65–80 cm. The Bk horizon has 17% calcium carbonate at a depth between 7 and 30 cm. The calcium carbonate decreases with depth to 5% (Fig. 5). Chlorine concentration increases to 250 meq/100 gr soil at a depth of 65–80 cm and the maximum SO4 of ∼ 40 meq/100 gr soil occurs at a depth of 30– 65 cm. The soil matrix is composed mainly of silt and sand with low amount of clay (Fig. 6). Clay content does not exceed 10% and silt and sand may reach maximum values of ∼ 55% and ∼ 70%

respectively. Below 80 cm there is no clear trend in the relative amount of the clay, silt, and sand, and the grain-size distribution represents the parent-material sediment composition. Stable isotopes composition of the pedogenic carbonates Oxygen (δ18O) and carbon (δ13C) values were measured for pedogenic carbonate nodules collected from the Bk horizons from a depth of 7–30 cm (Figs. 4E, 5; Table 1) The carbonate nodules are soft and small (0.5 cm in diameter) composed of micritic carbonate. δ13C values of all the carbonate nodules range from 1.6‰ to 2.8‰ (average 2.4 ± 0.3‰; Fig. 7). No trend could have been observed with depth because the carbonate nodules are concentrated in one distinct horizon at the upper 20 cm of the soil profile. δ18O values spread on a much larger range, varying between −0.3‰ and + 5.8‰ (average 2.9 ± 1.8‰).

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Table 1 Soil properties Namibia. Sample

Horizona Depth (cm)

Dry colorb

Moist colorb

−2 Structurec Cementationd Gravele Sand% Silt% Clay% Stage of carbonate Cl− SO 4 (%) morphologya, (meq/100 gr soil) (meq/100 gr soil) other soluble saltsf

GOGO1-1 GOGO1-2 GOGO1-3 GOGO1-4 GOGO1-5 GOGO1-6 GOGO1-7 GOGO1-8 GOGO1-9 GOGO1-10

Av B Bk1 Bk2 By1 By2 By3 Bz C1 C2

7.5YR 6/4 7.5YR 6/4 7.5YR 6/4 7.5YR 7/4 7.5 YR6/4 7.5YR 7/4 7.5YR 6/4 7.5YR 6/4 7.5YR 6/4 7.5YR6/4

7.5YR5/6 7.5YR 5/4 7.5YR 5/4 10YR 5/6 7.5YR 5/4 7.5YR 5/6 7.5YR 5/6 7.5YR 5/6 7.5YR 5/4 7.5YR 5/4

2,m,gr 2,m,pl 1,sg,c, 2,m,pl 1,sg,c,gr 1,sg,c,gr 1,sg,vc,gr 2,vc,gr 2,m,vc,gr 3,m,vc

a b c d e f

0–0.5 0.5–7 7–12 12–20 20–30 30–45 45–65 65–80 80–90 90–110

cw cw cw cw cw cw/cm f cw cw cw

– 10 10 20 10 20 30 50 40 50

40.1 70.4 47.1 53.2 53.0 41.4 48.7

53.9 29.2 44.2 41.2 42.0 54.6 49.1

7.0 2.8 10.6 8.5 6.9 5.2 4.8

51.8 64.9

41.9 33.8

4.6 3.6

– – stage I stage I stage I cs cs sc sc –

0.1 1.6 15.0 11.3 12.1 23.4 18.6 250.2 41.8 27.1

0.1 0.1 3.5 0.9 5.2 38.7 39.4 5.1 2.7 4.0

Horizon nomenclature and carbonate stage follow Birkeland (1999) and references therein. Horizon color is according to Munsell notation. Structure codes: Strength/Grade: 1–3; m-massive; sg-single grain. m-medium; c-coarse; vc-very coarse. gr-granular; pl- platy. Cementation codes: f-friable; cw-weakly cemented; cm-moderately cemented. Percent out of gravel. Soluble salts: cs-calcium sulfate; sc-sodium chloride.

Alluvial surface ages The OSL ages of the alluvial surfaces in two of the profiles studied in the Namib Desert are similar and overlap at the 1-σ level: 42 ± 3 ka

and 44 ± 2 ka (GOGO-4, GOGO-1-I). A sub-surface sample from a depth of ∼80 cm yielded an age of 63 ± 6 ka. However, due to high dose rates, this sub-surface sample (GOGO-1-II) is close to saturation and its age must be considered as a minimum age.

Figure 4. (A) Google Earth image of the Namib study site. Arrow marks the location of the studied surfaces; (B) the pediment alluvial surface is covered by desert pavement; (C) pediment surface (at the study site vicinity) covered by grasses after extremely rainy year, 2006 (Photo by H. Kolb, Gobabeb); (D) example of a dried Stipagrostis ciliata grass that covered the studied surfaces following the extreme rainy year of 2006. Note the fine material adheres to the fine roots, moderately cemented by calcium carbonate; (E) Calcic-gypsicsalic Reg soil profile (Table 1).

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Figure 5. Sulfates, chlorides (meq/100 gr soil) and calcium carbonate (weight percent) distribution with depth in the sampled Reg soil in the Namib Desert.

Reg soils in the hyperarid southern Negev Desert Stable surfaces in the Negev Desert extend over an area of 2300 km2 (23% of the Negev Desert), are covered by continuous desert pavement and are totally bare of vegetation. The soils developed on such stable surfaces are Reg soils with gypsic and salic/petrosalic soil horizons at depths of 10–30 and 80–120 cm, respectively (Dan et al., 1982; Amit

et al., 1993; Amit and Yaalon, 1996) (Fig. 8) with no calcic soil horizons (Fig. 3) (Amit et al., 2006). The analyzed gypsic/salic soils were documented on many of alluvial surfaces dated by OSL to between 10 ka and N230 ka (Amit et al., 2006; Porat et al., 2010). In the Negev Desert, the arid/semi-arid zone (80–250 mm yr−1) exhibits a transition from gypsic–salic soils to saline–calcic and calcic soils, whereas the mildly arid zone (N250 mm yr–1) is devoid of salic horizons, and calcic and non-

Figure 6. Particle size of Reg soils from Namib and Negev deserts. Part of the wide range is explained by the use of both, the laser diffraction (Mastersizer, MS) and the sieve-pipette methods.

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Discussion Calcic and saline soil formation in hyperarid deserts

Figure 7. δ13C values and δ18O values of the pedogenic carbonate, from two Reg soils in the Namib Desert study area.

calcic soils are abundant (Amit et al., 2006). These studies show that on middle Pleistocene-Holocene alluvial surfaces in the Negev Desert (Porat et al., 2010), salic-gypsic horizons are found only where current rainfall is b80 mm yr−1, i.e., in the southern and eastern Negev Desert. Rainfall characteristics in Gobabeb (Namib) and Eilat (Negev) Annual, monthly, and daily time series indicate that mean annual rainfall and the number of rainy days in Eilat is greater than in Gobabeb (Fig. 2), and the daily rain amounts in Eilat are almost twice those in Gobabeb. Rainfall duration is also very different in these two stations. In Eilat, rainstorm duration is frequently less than a day, probably lasting no more than a few hours (Grodek et al., 1999) and only 1% of the rainstorms were longer than 3 days, whereas in Gobabeb 11% of rainstorms exceeded this duration (Fig. 9). This large difference is expressed also in rainfall amount during each event. In Eilat, most storms with relatively high rainfall amounts were short whereas the opposite occurred in Gobabeb (Figs. 9 and 10). Therefore, Gobabeb and the central Namib are generally characterized by longer duration of rainstorms, whereas the Eilat and southern Negev Desert, with higher mean annual rainfall amount, are characterized by shorter duration of rainstorms (Figs. 9 and 10). We concluded that a combination of high-intensity storms of longer durations increase the total infiltration into the soil and allow much greater portions of the rainfall to leach the central Namib top soil. This enables short episodes of annual grass cover on the alluvial surfaces in Gobabeb, whereas such vegetation is not observed in the Negev.

Soil formation can partly be viewed as a mass balance between inputs and losses integrated over geological time scales (Brimhall et al., 1992; Amundson, 2004). Inputs to the initial parent material (rocks or sediments) are water, organic material, products of root-zone activity, and atmospheric dust and salts, which are enhanced in arid and semiarid environments. Dissolved losses result from chemical weathering and downward transport of solutes from the surficial weathering zone. In humid regions, long-term soil development involves a net mass loss through chemical weathering, and these losses exceed the addition of dust and solutes (Birkeland, 1999). In arid to hyperarid regions, precipitation and evaporation limit water availability for weathering, leaching and transport and lead to net mass gains of salts and dust (Yaalon, 1971; Dan and Yaalon, 1982; McFadden et al., 1987, 1991; Gerson and Amit, 1987; Wells et al., 1987; Harden et al., 1991; Amit et al., 1993; Quade et al., 1995; Capo and Chadwick, 1999; Ewing et al., 2006). This mass gain and the preservation of the soil parent material make desert soils an excellent archive for studying desert paleoenvironments. In the hyperarid southern Negev Desert, soils developed on stable Quaternary alluvial surfaces are saline and gypsic without any secondary soil carbonate accumulation, although the dust is rich in carbonates (Amit et al., 2006). In the Negev, only areas with rainfall N80 mm yr−1 have suitable conditions for secondary carbonate to accumulate in soils and to form calcic horizons (Fig. 2) (Dan et al., 1964; Magaritz, 1986; Goodfriend and Magaritz, 1988; Amit et al., 2006). This is due to the limited leaching of soils when mean annual rainfall is b30 mm, leading to soil salinization which prevents vegetation establishment on these alluvial surfaces. Such limited leaching of the soil profile may indicate a lack of episodes with increased rainfall. This observation led us to propose that a climatic threshold, based on total annual rainfall, determines the formation of calcic horizons in the Negev Desert. This threshold separates the hyperarid desert lacking calcic soils from the arid and semi-arid parts of the Negev Desert with calcic soils (Amit et al., 2006; Matmon et al., 2009). In contrast to the Negev Desert, in the central Namib Desert, in an area with an even lower mean annual rainfall (10–20 mm), soils developed on similar late Quaternary stable alluvial surfaces surprisingly contain pronounced, although weakly developed, calcic soil horizons, in addition to the salic/gypsic horizons. This pronounced difference is not related to the grain-size distributions of the soils, which are similar (Fig. 6). To form a calcic horizon an elevated pCO2 is needed to dissolve carbonate into the soil solution and later to deposit the soil carbonate, mainly by evaporation and degassing (Drever, 1982; Brook et al., 1987; Cerling, 1984; Solomon and Cerling, 1987; Amundson et al., 1989; Quade et al., 1989; McFadden et al., 1991; Birkeland, 1999). The source of this elevated pCO2 is root-zone respiration and/or

Table 2 Luminescence dating results, Namibia. Sample

Depth

Cosmic

K

U

Th

Ext. αb

Ext. β

(m)

b

Ext. γ

Dose

No. of discs

De

Over-dispersion

Age

(Gy)

(%)

(ka)

(μGy/a)

(%)

(ppm)

(ppm)

(μGy/a)

(μGy/a)

(μGy/a)

(μGy/a)

Profile GOGO-1 GOGO-1-I 0.25 GOGO-1-II 0.88

236 188

2.91 2.74

3.3 3.4

16.5 19

21 24

2828 2784

1824 1911

4909 ± 49 4906 ± 49

11/13 12/13

205 ± 14 309 ± 31

21 34

42 ± 3 63 ± 6

Profile GOGO-4 GOGO-4/up 0.45 GOGO-4-45 0.45

214 214

2.24 3.4

2.3 2.9

8.7 10.6

13 16

2050 2972

1191 1621

3468 ± 38 4824 ± 50

13/13 11/13

153 ± 7 203 ± 16

15 24

44 ± 2 42 ± 3

Field and laboratory results for OSL measurements. Measurements were carried out on 88–125 μm quartz. Water content was estimated at 2%. Cosmic dose was calculated from burial depth. External α, β and γ dose rates were calculated from the concentration of the radioisotope K, U and Th. No. of discs is the number from those measured that was used for calculating the De. The average and errors on the De were calculated using the Central Age Model (Galbraith et al., 1999). The scatter in the sample is provided by the over dispersion value (which does not include measurement errors).

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Figure 8. Sulfates (meq/100 g soil) and chlorides (meq/100 g soil) distribution with depth in Reg soil (Shehoret alluvial fan profiles 1–4; after Amit et al. (1993, 2006), Negev Desert (location of Shehoret alluvial fan Fig 2).

microbiological activity (e.g., Wright and Tucker, 1991); this process presumably registers the vegetation type by the δ13C values of the soil carbonate (Magaritz et al., 1981, 1991; Cerling, 1984; Cerling et al., 1989; Cerling and Quade, 1993), although variation in δ13C isotope values also reflect the time of year when pedogenic carbonate actually forms (Breecker et al., 2009). At present, in the Negev Desert, vegetation covers flat alluvial surfaces after heavy rainstorms only in areas receiving N80 mm yr−1 or in niches in the hyperarid areas where rock outcrops contribute additional runoff (Yair and Berkowicz, 1989; Lekach et al., 1998). In contrast with these southern Negev Desert alluvial surfaces, the Namib Desert alluvial surfaces are occasionally and briefly (up to a few weeks) covered by grasses such as Stipagrostis ciliate (a wider range of grasses is listed by Henschel et al., 2005) following rare rainstorms during rainy years such as 2000 (Henschel et al., 2005) or 2006 (Figs. 2, 4). These findings guided us to search the differences and similarities in the two-hyperarid deserts in the rainfall characteristics such as

Figure 9. Duration vs total rainstorm recorded in Gobabeb (A) and Eilat (B).

Figure 10. Relative frequency of rainstorm duration in Gobabeb and Eilat. Note the relatively frequent ≥2-days long rainstorms Gobabeb, Namib Desert.

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seasonality and intensity-duration of rainstorms. The mean annual winter rainfall amount and the number of rainy days are higher in the Negev Desert than in the Namib Desert, the longer duration of each storm event in the Namib Desert results in reduced rainwater loss to runoff and increased soil-water infiltration and stronger leaching of the upper part of the soil profile than in the southern Negev soils. Thus, we propose that the upper part of the Namib soils is subject to a greater magnitude of leaching, dissolution and deeper translocation of salts relative to the soils in the southern Negev. Accordingly, the key difference in soils developed in these two hyperarid deserts is related to the different long-term synoptic climatology that controls storm duration. In the southern Negev Desert, convective rainfall associated with the Red Sea trough contributes more to the annual rainfall than to the northern Negev (e.g. Kahana et al., 2002). This result in high-intensity, mostly scattered and short-lived showers (N20–30 mm/h) that contribute only slightly to the regional mean annual rainfall (Sharon, 1972; Sharon and Kutiel, 1986; Morin et al., 1998). The longer-duration rainstorms in the southern Negev are associated with the winter eastern Mediterranean cyclones that only occasionally reach the area. The scattered high-intensity short-lived and localized showers have limited effects on soils and most of the water is not available for soil leaching. As a result, the very top soils in the southern Negev accumulate only soluble salts which prevent even occasional vegetation growth on the alluvial surfaces. The lack of vegetation on such surfaces indicates the absence of decades or centuries of intense rainfall episodes capable of leaching the top soil and enhance vegetation growth and secondary calcium carbonate deposition. In the Namib Desert study site, the longer-lasting rainstorms that occur about once a decade trigger short-term episodes of grass cover from dormant seed-banks that germinate along the path of the rainfall cells (Henschel et al., 2005). The roots of the grass occupy the leached top 20–30 cm of the soil that is characterized by relatively low amounts of salts that allow episodic root-zone respiration and soil carbonate deposition close to the surface (Fig. 5). In terms of soil horizon formation, nothing happens in the soil between such storms. The soil we studied in the Namib Desert records thousands of years of such storms, and should be regarded as a cumulative result of these rare but repeated episodes of rainstorms. This is not observed in the soils of the southern Negev Desert which are characterized by limited leaching that prevents the establishment of even occasional grasses. Implications for paleoclimate interpretations of the soils The possibility that the calcic horizons observed in the Namib soils indicate wetter paleoclimatic conditions is ruled out by the δ13C and δ18O values. The Namib high δ13C and δ18O values of the soil carbonate sampled from this horizon range from 1.6‰ to 2.8‰ (average 2.4 ± 0.3‰) and −0.3‰ to + 5.8‰ (average 2.9 ± 1.8‰), respectively (Fig. 7), and are indicative of hyperarid conditions. They signify relatively low biogenic activity and low respiration rates (e.g., Quade et al., 1989). For comparison, these δ13C values are significantly higher than the isotopic composition of the pedogenic carbonates from the Northern Negev Desert in which calcic soils are common and where δ13C ranges between ∼−11‰ and 0‰ (Magaritz and Heller, 1980; Magaritz et al., 1981). The δ13C values of the soils from Namib Desert are similar to those obtained for the eastern Mojave Desert, Nevada (reaching + 4‰, Amundson et al., 1988; Quade et al., 1989), the southern Negev Desert (−0.4 to +2.5‰; Amit et al., 2007) and the Atacama Desert in northern Chile (up to + 7.9‰; Quade et al., 2007). The high average δ13C values of +2.8‰ of the Namib soil carbonates suggests significant contribution of atmospheric CO2 during their formation. Rapid loss of CO2, as suggested by Quade et al. (2007) through advection of soil air near the soil is the most plausible explanation for the elevated δ13C values. Quade et al. (2007) evaluated the impact of exceptionally sparse plant cover (0–20%) and rainfall (2–

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114 mm yr−1) on the stable carbon and oxygen composition of soil carbonate from the Atacama Desert. They suggested that respiration rates are the main predictor of the δ13C value of soil carbonate in the Atacama Desert, whereas the fraction C3 to C4 biomass at individual sites has a subordinate influence. Quade et al. (2007) claim that the high average δ13C value (+4.1‰) of carbonate from the driest study sites indicates that it may have formed abiotically, in the presence of pure atmospheric CO2. We suggest that the somewhat lower δ13C values obtained for the Namib Desert are derived from the sparse annuals growth and clearly support low biotic contribution. Assuming that Namib Desert pedogenic calcite was precipitated at a temperature of 25 °C, and using the calcite–water fractionation equation of O'Neil et al. (1969), the calculated δ18Ow of the soil water is 2.4‰ (SMOW) for the minimum δ18OCc value measured for the Namib Desert carbonate of −0.3‰, and the δ18Ow is ∼+8.4‰ for the maximum δ18OCc of 5.8‰. Available rainwater isotopic composition from Windhoek station in Namibia (number 6811000 of IAEA network, at latitude 22,57 S and longitude 17,10 E, altitude 728 m; 1961–2001), yields a long-term mean δ18Ow value of −2.7‰. The δ18OCc values indicate that the Namib pedogenic carbonates are not in isotopic equilibrium with the local meteoric water composition and soil temperature. All observed δ18OCc values (up to + 5.8‰) are higher than those predicted to be in equilibrium with local rainfall, and show the effects of moderate to strong evaporation of soil water prior to carbonate formation. Our observation is in accordance with other studies (e.g., Quade et al., 1989; Liu et al., 1996) that evaporation, in addition to temperature and the δ18Ow value of rainfall, likely influences the δ18OCc value of soil carbonates in deserts. This pattern indicates that, as suggested by Quade et al. (2007) for the Atacama Desert, soils undergo significant dewatering by evaporation prior to soil-carbonate formation. This pattern must be the result of extreme evaporation due to the hyperaridity of the Namib Desert. Moreover, the distribution of soluble salts with depth supports the hypothesis of deposition according to their solubility, with the most soluble salts at a lower part of the soil profile and the less soluble close to the surface (Yaalon, 1964). No superposition of salts was found in these profiles. Such distribution of salts disproves the possibility that the calcic horizons (Bk) formed prior to salt accumulation and that Bz and By horizons formation is most likely synchronous with the formation of salty horizons. These results suggest that the climate conditions in the Namib Desert have been extremely arid since at least 60 ka: but, unlike the Negev Desert, calcic horizons have formed in this hyperarid desert. This conclusion is also in accordance with other studies which showed that the climate of the Namib Desert has remained arid to hyperarid throughout most of the Quaternary with a possibility for climatic fluctuations for short duration and small amplitude (Deacon and Lancaster, 1988; Vogel, 1989; Heine, 1998; Brook et al., 1999; Lancaster, 2002). The suggested episodes of increased moisture in the currently hyperarid Namib Desert during LGM (e.g., Stuut and Lamy, 2004; Chase and Meadows, 2007) or during the last 100 ka (e.g., Scott et al., 1991; Little et al., 1997; Stokes et al., 1998; Brook et al., 1999; O'Connor and Thomas, 1999; Thomas, 2000; Stone et al., 2010) have no clear expression in the studied soils. The lack of any pedogenic carbonate at greater depth in these soils may indicate that the climate in the study area in the Namib Desert may have not been less arid during the last ∼ 60 ka to form different soil profiles. Moreover, in other arid areas such as the lowermost basins of the Mojave Desert of the southwestern United States, most of the soils developed into late Pleistocene alluvial surfaces are currently in areas with precipitation N50 mm yr−1. Therefore, both the Holocene and the late Pleistocene soils in this region cannot be directly compared with the hyperarid soils we documented in the Namib and the Negev with the exception of the soil of the lowermost Death Valley (e.g., Ku et al., 1998; Lowenstein et al., 1998; Klinger, 2001; Machette et al., 2001; Yang et al., 2005).

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In most of the Mojave Desert soils developed on stable alluvial surfaces, salts are superimposed on calcic horizons enveloping the previous calcic crusts and/or calcic nodules (e.g., Reheis et al., 1989; McFadden et al., 1992; McDonald et al., 1996; Ku et al., 1998; Lowenstein et al., 1998; Klinger, 2001; Machette et al., 2001; Yang et al., 2005). This is usually explained (a) by the relatively arid Holocene climate that followed a much wetter late Pleistocene, and partly (b) as the result of reduced infiltration as part of the accumulation of fine-grained sediments in the top soil (Av and Bw horizons; e.g., McFadden et al., 1992). In the Namib and the Negev deserts no such overprinting of salts over preexisting calcic horizon was observed for the late Pleistocene soils; there were no past soils that exhibited only calcic horizons without salic or gypsic horizons. The results of this study stress that the mean annual rainfall amount is less important than rainfall duration within the range of hyperarid deserts. The duration and long-term seasonal rainfall distribution are the two dominant factors that allow different types of soil to form in these extreme deserts. Therefore the simplistic use of presence of calcic horizons for paleoclimate interpretation can be misleading. In hyperarid areas temporal changes need to be investigated over long time spans to generate meaningful results. Conclusions Despite overall similarities in annual rainfall and temperatures that result in hyperaridity, differences in the timing and duration of storms can profoundly influence the genesis of carbonate and salt horizon in soils developed on abandoned alluvial surfaces. The important and distinct climate-dependent threshold that exists in the Negev desert that separates the gypsic–calcic soils typical of the arid part of the Negev from the gypsic–salic soils of the hyperarid southern Negev, may not exist in other desert regions, such as the Namib. Mean annual precipitation can only drive speculations concerning the general soil properties in hyperarid deserts but cannot be used as the sole predictor of soil characteristics. This has implications for paleoclimate inferences which are too simplistically based only on annual rainfall. The different soil properties in the Namib and in the Negev deserts demonstrate that although soils in hyperarid deserts are always under moisture deficit and are characterized mainly by soluble salts, the soils are sensitive enough to record the slight differences in long-term rainfall characteristics. Calcic horizons can be formed even in hyperarid deserts if the characteristic duration of rainstorms is relatively long. Although rainstorms may be rare and episodic, their longer durations occasionally allow rainwater to penetrate and push soluble salts deeper into the soil profile. This almost unnoticeable difference has a profound impact on soil development. The constant deficit in soil moisture which is a basic characteristic of all hyperarid areas is sometimes of secondary importance in forming calcic horizons. Acknowledgments The research in the Negev was supported by the United StatesIsrael Binational Science Foundation grant 2006-221 and the U.S. Army Research Office grant (DAAD19-03-1-0159). Our research in Namibia was supported in addition by the U.S.–Israel Cooperative Development Research Program, Bureau for Economic Growth, Agriculture, and Trade, U.S. Agency for International Development (Grant No. C24-26). We also were generously assisted by the Gobabeb Research and Training Center and the Desert Research Foundation of Namibia in receiving permits and collecting the soil samples. We thank Dr Mary Seely and Dr Joh Henschel who were particularly helpful. The rainfall data they provided were essential in the understanding the pedologic processes. We particularly thank the editor Alan Gillespie for handling the manuscript and Les McFadden and Bruce Harrison for formal review comments that significantly

improved the manuscript. PSD analyses were conducted by A. Lokshin and R. Krasilshikov. Z. Dolgin prepared the OSL samples. Special thanks for Michael Davis for sample preparation and measurements. We are grateful to Bat-Sheva Cohen and Hanna Netzer Cohen for map drawing.

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