 2006 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org. Geology; June 2006; v. 34; no. 6; p. 509–512; doi: 10.1130/G22354.1; 2 figures; Data Repository item 2006099. 509 Permanent Quaternary hyperaridity in the Negev, Israel, resulting from regional tectonics blocking Mediterranean frontal systems Rivka Amit* Geological Survey of Israel, 30 Malkhe Israel Street, Jerusalem 95501, Israel Yehouda Enzel* David Sharon    Institute of Earth Sciences, Hebrew University of Jerusalem, Jerusalem 91904, Israel ABSTRACT Although various episodes of wet Quaternary climates have been suggested in studies of the Negev Desert, here we demonstrate that Reg soils, developed on flat alluvial surfaces and sensitive to minor changes in precipitation, indicate that the southern Negev has been permanently hyperarid at least since the middle Pleistocene. The wetter episodes were restricted to the northern Negev, currently mildly arid. Gypsic-saline Reg soils that de- veloped on stable surfaces during 200 k.y. in the southern Negev are cumulative and polygenetic, but none exhibits even the weakest development of calcic horizons. Under current climate conditions in the Negev, calcic soils are widespread in areas with rainfall 80 mm yr 1 . The fact that the hyperarid (50 mm yr 1 ) southern Negev lacks any calcic horizons indicates that it never experienced an average rainfall of 80 mm yr 1 during the middle-late Pleistocene, whereas the northern Negev was wetter. This Negev dichotomy is explained by the main physical features controlling rainfall over the Negev, i.e, the southward-decreasing depth of the atmospheric boundary layer with distance from the Mediterranean, and the altitude of the central Negev Highland (1000 m). The interaction between these two features often prevents the passage of rain clouds into the southern Negev. The Holocene and Pleistocene gypsic-salic soil distribution across the Negev closely matches the current circumstances. We suggest that hyperaridity has prevailed over the southern Negev since the last stages of the uplift of the central Negev Highland in the late Pliocene–early Pleistocene. Keywords: Negev, Saharo-Arabian, Pleistocene, Holocene, paleosols, hyperaridity. INTRODUCTION The Negev Desert (10,000 km 2 at 29°– 31°N), part of the larger Saharo-Arabian des- ert belt, is south of the semiarid Mediterranean climatic region (Fig. 1). The area encompass- ing the Negev, Sinai, and northern Arabia is currently among the driest places on Earth; 75% of the area is hyperarid (80 mm yr -1 ) of which 50% receives 50 mm yr -1 (Fig. 1). Earlier studies suggested that the Negev ex- perienced much wetter climatic episodes dur- ing the Quaternary (e.g., Horowitz, 1979; Ma- garitz, 1986; Goodfriend and Magaritz, 1988; Goodfriend, 1999), based on data from the semiarid northern Negev: the question re- mains whether that applies to the southern Negev. Here we present results of analyses of gypsic-saline Reg soils that developed on flat alluvial surfaces throughout the southern and eastern Negev. We analyzed the spatial distri- bution of late Holocene to recent rainfall- conditioned salic and calcic soils, compared them with present-day rainfall, and show that their distribution pattern has not altered for at least a few hundred thousand years. We iden- tify controls on the rainfall distribution over *E-mails: rivka@mail.gsi.gov.il; yenzel@vms. huji.ac.il. southern Israel, and propose that regional hy- droclimatic and physiographic settings led to hyperaridity in the southern Negev despite the dramatic climatic variations recorded by spe- leothems (Bar-Matthews et al., 1997) and lake levels (Bartov et al., 2002) 200–300 km to the north. METHODS AND RESULTS Diagnostic soil horizons developed on sta- ble alluvial surfaces are excellent proxies for average regional climate (Yaalon, 1971; Birkeland, 1999). In hyperarid regions, soils are indicative of climate only if they formed in response to direct rain without additional water contribution from adjacent areas and slopes (Yair and Berkowicz, 1989) or ephem- eral channels (Amit et al., 2006). Therefore, we avoided soils with any slope or catenary relationship and refer only to soils developed on stable, relatively flat surfaces that cover 25% of the Negev. Data from soil chrono- sequences from the Negev and Sinai (Dan et al., 1982; Gerson et al., 1985; Amit and Ger- son, 1986; Amit et al., 1993; Amit and Yaa- lon, 1996) were summarized and supplement- ed by observations of hundreds of individual soils outside these sequences. The ages of the alluvial surfaces were determined by optically stimulated luminescence (OSL) methods (Por- at et al., 1997, 2006). Parts of these surfaces are beyond the OSL age range, and are at least a few hundred thousand years old based on either their association with early to middle Pleistocene sedimentary units or their being older than middle Pleistocene human artifacts (Gerson et al., 1985; Dan et al., 1982; Ginat et al., 2002). Using a geographic information system (GIS), we compiled data from earlier studies and our own soil data in the Negev in a map of soil associations (after Dan et al., 1976) based on diagnostic horizons (Fig. 1). Soil samples were treated by standard methods (Dan et al., 1964; Soil Survey Staff, 1999). Chemical and physical properties were deter- mined according to laboratory procedures (Amit et al., 1993, 2000). Figure 2 shows the southeasternmost extent of excessive annual rainfall (100%) in northern and central Israel for each of the 7 wettest rain seasons of the last 23 for which such data were published (Israel Meteorolog- ical Service, 1981–2003). South of each line, rain from Mediterranean systems was consid- erably more restricted. PLEISTOCENE–HOLOCENE SOILS IN THE NEGEV AS RELATED TO RAINFALL Rainfall strongly affects the formation, characteristics, and depths of soil horizons in arid regions; therefore, we superimposed the soil data on the mean annual rainfall map to test and illustrate the case. Figure 1 shows that, except for a few transitional exceptions, the majority of soils related to extremely arid climates plot below the 80 mm/yr isohyet delineating the southern and eastern Negev. The eastern Negev includes gypsic and salic- petrosalic soil horizons (groups X, Y, Z) at depths of 10–30 and 80–120 cm, respectively, regardless of the age of the alluvial surfaces on which these soils developed (Amit et al., 1993). However, no calcic soil horizons have been found on any of these surfaces. The semiarid zone (80–250 mm yr -1 ) exhibits a transition from gypsic-salic soils to saline- calcic and calcic soils (groups R, S, T, V, W). The mildly arid zone (250 mm yr -1 ) is de- void of salic horizons, whereas calcic and noncalcic soils (group A in Fig. 1) are abun- dant. On middle Pleistocene–Holocene allu- on November 13, 2011 geology.gsapubs.org Downloaded from