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The carbonate mountainous landscape around most of the Mediterranean is karstic, is almost barren, and has thin soils. Erosion of preexisting thicker soils is a common hypothesis used to explain this bare terrain. An alternative hypothesis is that in the Mediterranean region, thin soils are attributed to long-distance transport of very fine, silty clay dust, resulting in low mass accumulation rates. Even if accreted over millennia, such dust cannot produce thick, highly productive soils. A pronounced anomaly in the Mediterranean is the thick, more productive soil of the semiarid southern Levant (SL). These soils contain order-of-magnitude coarser grains than the characteristic thin soils in the Mediterranean and a high proportion (>70%) of coarse silt quartz sourced from the nearby Sinai-Negev erg, the primary contributor of the Negev loess. This proximal intense dust supply produced greatly thicker soils. However, influx of coarse silt quartz loess is a geologically recent phenomenon in the SL. Pre-loess (i.e., older than 200 ka, pre-coarse-silt influx) SL soils are much finer and were generated by long-distance dust from the Sahara and Arabia like most other Mediterranean soils. Thus, we hypothesize that the geologically recent Negev Desert loess interval caused a drastic change in mountainous soil properties within the SL, enriching the Levant’s ecology and affecting early human development. The high amounts of coarse silt deposited on the landscape have contributed to the unique sustainable agriculture in the SL, which assisted in transforming the Levant into “the land of milk and honey” and a cradle of civilizations.

Quaternary loess covers desert margins and vast areas of the Negev, southern Israel. The Negev loess is among the best-studied desert loess, with research going back to the early 20th century. The contrast between carbonate rocks of the Negev and its silicate-rich coarse-silt loess allows determining the loess sources, learning the synoptic-scale paleoclimatology, and exploring processes of coarse silt formation. Here, we present an overview of new perspectives on the origins and climatic significance of the Negev loess, expand on how (a) coarse silts affected soils farther downwind, and (b) how the loess has now turned into an active dust source.The sources of the Negev loess are the (a) distal Sahara and Arabia delivering fine silts and clays, transported over thousands of kilometers, and (b) proximal sand dunes in Sinai and Negev, advancing and concurrently supplying the coarse silts to the loess accretion through eolian abrasion of sand grains. It was found that the coarse silts which compose the majority of the loess, commenced during the late middle Pleistocene – early late Pleistocene, coeval with the appearance of the advancing Sinai/Negev sand dunes and the first coarse silt accretion in regional soils; The main loess formation episode is ~95-10 ka, when the dunes appeared in the Negev. Within the loess, the dust mass accumulations rates (MAR), and consequently, soil formation rates, spatiotemporally vary according to specific site location and distance relative to the proximal sources. With increasing distance beyond the loess zone, both dust MARs and grain size gradually decrease; thus, whereas Mediterranean mountains located in central Israel, tens of kilometers downwind the loess, exhibit thick soils on top of the carbonate bedrock, the even wetter regions in northern Israel, located hundreds kilometers away from the loess, exhibit only thin soils. Thus, in Mediterranean regions located at the desert fringe, coarse silt influx is one of the main factors in determining the environmental sustainability, rather than only the precipitation amount.During the Holocene, dust MARs in the Negev were much lower than late Pleistocene ones, and loess was not formed. Recently, the Negev loess became a prime source of dust mainly due to anthropogenic interferences, contributing to the regional dust cycle, and thus, at present the loess zone is a dust source rather than a dust sink. Today, the Negev loess is a non-replenishable natural resource that is slowly eroding and disappearing from the landscape.

Hyperarid (< 80 mm yr-1) soils in hot deserts are characterized by accumulations of soluble salts (gypsum and halite) in diagnostic horizons as a result of limited moisture availability. In most desert terrains, the source for pedogenic gypsum and halite is atmospheric dust and rainwater. The interplay between climatic properties such as frequency and intensity of rain events, rainfall composition, dust flux, and evaporation rates, govern the depth and concentration of these salts. Better understanding of these relationships can improve our estimation of regional paleoenvironmental and paleoclimate conditions. Up to date, only empirical correlations between annual rainfall and pedogenic salt horizons are available.The goals of this study are to: 1) quantify rates of pedogenic gypsum accumulation with time and the role of controlling climatic conditions that govern its accumulation, 2) estimate the most likely climatic scenarios that led to the formation of the diagnostic gypsic horizon developed in late Pleistocene (~ 60 ka) abandoned alluvial fan surfaces in the hyperarid Negev desert, southern Israel. To achieve these goals, we constructed a compartment model that simulates gypsum accumulation in soil and tests its sensitivity to various changes in the long-term climate properties. The model predicts gypsum content and depth of accumulation in the soil profile over thousands of years and more. The input parameters are stochastically simulated rainstorms, evaporation, dust flux, and sulfate concentration in rainwater, at daily time steps. The model was tested and calibrated using data of Holocene (< 11 ka) soil profiles developed on stable alluvial fans in the hyperarid Negev. With the assumption that the climate during the Holocene was not much different than today (i.e., mean annual rainfall < 50 mm). Sensitivity analyses indicate that gypsum accumulation is highly sensitive to mean annual rainfall and sulfate concentration in rainwater. Synthetic gypsum profiles were calculated using different climate scenarios and compared to late Pleistocene soils. Our results suggest that: (a) gypsum accumulation in late Pleistocene soils cannot occur simply by extending current climate conditions for a much longer duration. (b) The plausible climate scenarios for the late Pleistocene must include additional rain input (1.5 – 2.0 times than mean annual rainfall today) and increased sulfate concentration in rainwater (2.0 – 2.5 times than today) to successfully reconstruct the observed accumulated gypsum in mature (60 – 12 Ka) soil profiles.

Sampling approach, soil data, and landscape and soil profile photos.

Permafrost is degrading under current warming conditions, disrupting infrastructure, releasing carbon from soils, and altering seasonal water availability. Therefore, it is important to quantitatively map the change in the extent and depth of permafrost. We used satellite images of land-surface temperature to recognize and map the zero curtain, i.e., the isothermal period of ground temperature during seasonal freeze and thaw, as a precursor for delineating permafrost boundaries from remotely sensed thermal-infrared data. The phase transition of moisture in the ground allows the zero curtain to occur when near-surface soil moisture thaws or freezes, and also when ice-rich permafrost thaws or freezes. We propose that mapping the zero curtain is a precursor to mapping permafrost at shallow depths. We used ASTER and a MODIS-Aqua daily afternoon land-surface temperature (LST) timeseries to recognize the zero curtain at the 1-km scale as a "proof of concept. " Our regional mapping of the zero curtain over an area around the 7000 m high volcano Ojos del Salado in Chile suggests that the zero curtain can be mapped over arid regions of the world. It also indicates that surface heterogeneity, snow cover, and cloud cover can hinder the effectiveness of our approach. To be of practical use in many areas, it may be helpful to reduce the topographic and compositional heterogeneity in order to increase the LST accuracy. The necessary finer spatial resolution to reduce these problems is provided by ASTER (90 m). Fil: Batbaatar, Jigjidsurengiin. University of Washington; Estados Unidos Fil: Gillespie , Alan R.. University of Washington; Estados Unidos Fil: Sletten, Ronald S.. University of Washington; Estados Unidos Fil: Mushkin , Amit. University of Washington; Estados Unidos Fil: Amit, Rivka. Geological Survey Of Israel; Israel Fil: Trombotto, Dario Tomas. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Provincia de Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Universidad Nacional de Cuyo. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales; Argentina Fil: Liu , Lu. University of Washington; Estados Unidos Fil: Petrie, Gregg. University of Washington; Estados Unidos

A common spatial feature within loess deposits worldwide is a downwind decrease in thickness and grain size, trends that are powerful tools for reconstructing paleowinds and past atmospheric circulation. Although such trends have been identified, there is limited knowledge of similar trends farther downwind from the loess region, where eolian influx can influence soil formation and hydrological processes. To examine these impacts we studied Quaternary sequences in prehistoric sites in Jerusalem, located only ∼50 km downwind from the edge of the Negev loess. All sequences are composed of two units separated by an unconformity. The lower unit is of middle Pleistocene age, it is composed of unimodal clay to silty clay dust deposits with chert clasts and Lower-Middle Paleolithic artifacts. A non-deposition interval characterized the middle to late Pleistocene transition, when dust accumulation rates were low. The upper unit age is late Pleistocene to Holocene; it is composed of bimodal silty clay to silty clay loam. Quartz, K-feldspars, and plagioclase contents together with the location and amplitude of grain-size coarse mode increase up-sequence. The addition of coarse silts to the upper unit of the sequences was coeval with episodes of loess accretion in the Negev. These silts were generated most likely by eolian abrasion of sand grains in the upwind dunes. Similar to the Negev, the addition of silts resulted in burial of the drainage network and reduced runoff and soil erosion rates. We stress the importance of desert loess in determining soil composition and surficial hydrology in wetter areas located in adjacent downwind regions.

Meso-scale weather systems, such as convective haboobs, are considered to be an important dust generation mechanism. In Israel, however, rather than of meso-scale weather systems, most dust storms are generated by synoptic-scale systems, originating from Sahara and Arabia. Consequently, only distal sources of suspended and deposited dust in Israel are currently reported. Here we report the first detailed study on the merging of synoptic- and meso-scale weather systems leading to a prominent dust outbreak over the Negev, Israel. During the afternoon of May 2 nd , 2007, a massive dust storm covered the northern Negev, forming a one kilometer high wall of dust. The haboob was associated with PM 10 concentrations of 1000–1500 μg m −3 that advanced at a speed of 10–15 m s −1 and caused temporary closure of local airports. In contrast to most reported haboobs, this one was generated by a sea breeze front acting as a weak cold front enhanced by a cold core cyclone positioned over Libya and Egypt. The sea breeze that brought cold and moist marine air acted as a gravity current with strong surface winds. The sources for the haboob were the loessial soils of the northwestern Negev, especially agricultural fields that were highly disturbed in late spring to early summer. Such surface disturbance is caused by agricultural and/or intensive grazing practices. Our study emphasizes the importance of local dust sources in the Negev and stresses loess recycling as an important process in contemporary dust storms over Israel.

Sequences of abandoned alluvial surfaces are ubiquitous desert landscapes considered as responding to Quaternary-timescale climatic fluctuations and tectonic activity. The post-depositional evolution surficial properties of these alluvial surfaces change with time: (a) surface clast breakdown and development of desert pavement, (b) dust and salts accumulation (c) Reg (gravelly) desert soil evolution. In the present study, we focus on the hydrological response and the resulting erosional processes, to these changes in surface properties over a sequence of late-Pleistocene-Holocene, abandoned alluvial surfaces in Nahal Evrona catchment at the hyperarid southern Arava Valley north of the towns of Eilat (Israel) and Aqaba (Jordan). The methods used in the study are: (a) simulated rainfall experiments to determine hydrological characteristics, (b) Terrestrial Laser Scanning (TLS) to determine surface roughness, (c) OSL dating for the chronology, (d) surface and soil descriptions and measurements and grain-size analyses to determine texture of the uppermost Av horizon of the Reg soil (e) GIS analysis of the new drainage network developing over the older abandoned alluvial surfaces. The results show that with time, roughness decreases, infiltration decreases to about 8–9% relative to present rates, point runoff generation increases >5 fold, and as a result, rill and gully erosion initiated, finally leading to the destruction of the alluvial surface. Under the current hyper-arid climate in the southern Negev desert, between ~30 and 50 ka of surface weathering and soil development are required before significant runoff is generated and >100 ka until effective runoff initiates rill and gully erosion over these abandoned alluvial surfaces. This local-source runoff and gully erosion essentially destroys the alluvial surfaces in the southern Arava valley within a period of 400–500 ka. Roughness, extracted from spaceborne Lidar scanning, which is inversely related to surface age, directly reduces friction and resistance to runoff, can provide an estimate for the hydrological characteristics of the surfaces. This can support applications such as rainfall-runoff models, risk assessment, etc.

The Estimation of the relative intensity of different cold periods occurring during the Late Quaternary is a difficult task, particularly in non-glaciated mountain landscapes and where high- to medium-resolution archives for proxy data are lacking. In this paper, we study a Holocene polycyclic soil sequence in the central Alps (Val Cavargna, Northern Italy) to estimate climatic parameters (specifically Temperature) changes in non-glaciated, high altitude environments. We investigate this key site through palaeopedological and micromorphological analyses in order to understand phases of soil development and detect hidden evidence of cold conditions during its formation. Three phases of pedogenesis can be recognized and attributed in time to different periods during the Holocene. Pedogenetic phases were separated by two truncation and deposition episodes related to the reactivation of slope processes under cold conditions at the onset of the Neoglacial and the Iron Age Cold Epoch, respectively. Micromorphological evidence of frost action in the soil can instead relate to pedogenetic processes acting in the Little Ice Age. The different expression of these three cold periods corresponds to changes in climatic conditions, pointing to the Little Ice Age as a cooler/drier period in comparison to the preceding ones.

The isotopic composition of Sr and Nd and elemental concentrations (e.g., Na, K, Ca, Mg, Al) of three primary Negev loess sequences trace the relative contributions of desert dust sources over the past ~180,000 yr to the southeastern Mediterranean. We focused on the geochemical signature of the fine (20 μm) grain-size modes of the loess to identify their respective provenances. The isotopic composition of the coarse fraction falls on a mixing line of sources feeding the Nile and its delta. The chemical compositions of the coarse fraction and the Sinai–Negev sand dunes are similar, indicating that this fraction is the product of aeolian abrasion of the adjacent Sinai–Negev sands. In turn, these sands are derived from the Nile delta and transported inland. The fine fraction has less negative εNd values and slightly deviates from the mixing line, indicating an additional end-member. The higher εNd values and Mg/Al ratios of the fine fraction are relatively well correlated (R2 = 0.64) and point to Arabian dust as an additional source. Temporal changes in the geochemical composition of loess over time reflect changes in climates of source areas and the Eastern Mediterranean. Shifts from Saharan to Arabian fine dust-dominated intervals indicate wetter conditions in the southern Sahara that weaken dust fluxes from this source relative to the Arabian fluxes.

The evolution of roughness as a function of surface age was used to quantify weathering rates on rocky desert surfaces. Surface topography on eight late Quaternary alluvial terraces, which record the weathering of Holocene (5 ± 1 ka) boulder-strewn deposits into mature (87 ± 2 ka) desert pavements in the Negev desert of Israel, was measured with ground-based lidar. Roughness on each terrace was characterized with power spectral density (PSD) analysis, and changes in PSD as a function of length scale (λ ∼ 0.04–1.50 m) and surface age were used to estimate diminution/weathering rates of the surface rocks. We found PSD values that systematically increase as a power-law function of λ (roughness exponent of ∼2.0) and decrease as an inverse power-law function of surface age. This PSD evolution indicates a fragmentation rock weathering process driven by salt shattering throughout the 87 k.y. period examined. PSD analysis of the lidar data also revealed weathering rates that increase with rock size and decrease as an inverse power-law function of time, from initial values >20 mm/k.y. to

The Chinese Loess Plateau (CLP) is a large, spatially well defined and persistent zone of loess accumulation developed near the fluctuating northwest margin of the East Asian monsoon. Many studies have analyzed its loess sediments to provide insights into paleoclimatic conditions. Although spatial and temporal variations in the grain sizes of CLP sediments are fundamental to this effort, controversy over the origin of the dominant coarse quartz silt has limited interpretations. Reexamination of the spatial pattern of grain-size distribution across the CLP and a field-scale experiment conducted in the Gobi Desert revealed a genetic association between the coarse silt fraction of the loess and primary production of coarse silt through eolian abrasion of sand in the proximal Mu-Us, Tengger, and Badain Jaran sandy deserts. Our results demonstrate the effectiveness of eolian abrasion of quartz sand in primary coarse silt production in Central Asia and identify this process as the most consistent with the wellrecognized systematic northwest-southeast depositional pattern of the CLP. We suggest that only abraded coarse quartz grains transported short distances by long-term persistent eolian activity can build up thick loess sequences to form a massive and spatially well defined loess plateau. These results decouple the production and transport of coarse silt and finer silt and clay particles, which have a more distant and wider provenance, changing the constraints on previous paleoclimatic reconstructions. © 2013 Geological Society of America.

Colluvial sediments of talus relicts (“talus flatirons”) around mesas preserve a record that sheds light on slope-forming processes at temporal scales > 103 yr. The sedimentology and soil stratigraphy of two groups of talus flatirons in the northeastern hyperarid Negev desert reveal four deposition events in the younger talus and at least two in the older one. Numerical modeling of high-resolution 10Be depth profiles suggests that these taluses were deposited during the middle Pleistocene; the younger talus group first depositional event occurred at 551 − 142+ 80 ka and its abandonment occurred at 270 − 38+ 17 ka. The abandonment of the older talus group and stabilization of its surface occurred at 497 − 114+ 176 ka. These ages indicate that the development of the studied talus sequence is not specifically associated with Pleistocene glacial–interglacial cycles. The 10Be modeled concentrations indicate significant differences in the average inheritance of talus flatirons of different groups. These differences can be attributed to variability in the transport distance and duration of gravel exposure during transport but could also reflect some temporal variability in cliff retreat. Our results also demonstrate that talus slopes in hyperarid areas, despite their steepness, can store sediment for long periods (~ 500 ka) and thus constitute a valuable archive.

The Batsheva de Rothschild Seminar on Atmospheric Dust, Dust Deposits (Loess) and Soils in Deserts and Desert Fringe: The Sahara-Sinai-Negev as an Analogue for the Global Arid Regions; Jerusalem and the Negev Desert, 14–19 October 2015

In their seminal paper in 1979, Bull and Schick proposed a conceptual model for the geomorphic response to Pleistocene to Holocene climate change, based on the hyperarid Nahal Yael watershed in the southern Negev Desert. In this model, the change from semiarid late Pleistocene to hyperarid early Holocene climates reduced vegetation cover, increased the yield of sediment from slopes, and accelerated aggradation of terraces and alluvial fans. The model is now over 30 yr old, and during this time, chronologic, paleoenvironmental, and hydrogeomorphic research has advanced. Here, we reevaluate the model using data acquired in Nahal Yael over the 30yr since the original model was proposed. Recent studies indicate that the late Pleistocene climate was hyperarid, and a transition from semiarid to hyperarid climates did not occur. The revised chronology reveals a major 35–20 ka episode of accelerated late Pleistocene sediment production on slopes (with lower rates probably already at ca. 50ka) due to increased frequency of wetting-drying cycles caused by frequent extreme storms and floods between 35 and 27 ka. Without lag time, these sediments were transported and aggraded in depositional landscape components (fluvial terraces and alluvial fans). This intensified sediment production and delivery phase is unrelated to the Pleistocene-Holocene transition. The depositional landforms were rapidly incised between 20 and 18 ka. Since and/or soon after this Last Glacial Maximum (LGM) incision, most material leaving the basin originated from sediments stored in depositional landforms and was not produced from bedrock. Using these new data, we propose a revision to the Bull and Schick model in this hyperarid environment. Our revision suggests that the model should include the frequent storms and floods responsible for a late Pleistocene pulse of intense weathering due to numerous cycles of wetting and drying on slopes and coeval sediment transport to fluvial terraces and alluvial fans. We also discuss the common use and pitfalls of using the Bull and Schick conceptual model to explain observations in diverse arid environments, usually without sufficient data on basin-specific stratigraphic, chronologic, paleoenvironmental, and paleoclimatic information.

article i nfo Talus flatiron sequences are ubiquitous landforms in arid and semiarid regions characterized by horizontal erodible rocks capped by more resistant rocks. Alternating phases of deposition and erosion lead to the formation of generations of talus flatirons in which ancient ones are located farther from the source cliff. The existing conceptual model of this systematic spatial distribution of talus flatirons is related to glacial- interglacial climatic cycles and to relatively high rates of cliff retreat. Three groups of talus flatirons were analyzed in the northeastern hyperarid Negev desert. The analyses include mapping, age determination using optically stimulated luminescence and cosmogenic nuclide exposure dating, electrical resistivity tomography and sedimentological and soil analyses. All talus flatirons contain gypsic-salic soil catena typical to hyperarid climate (b80 mm yr −1 ). No pedogenic indicators of past wetter environments such as buried calcic soil 10 Be exposure ages of the talus flatirons suggest that they were deposited during the middle Pleistocene; ~610 ka and ~170 ka for the oldest and intermediate-aged talus flatiron groups, respectively. These ages, combined with the present location of the talus flatirons relative to the source cliff yield retreat rates of 6-12 and ~200 m Ma −1 for the bedrock cliff and talus flatiron apex, respectively, and in opposite directions. Our results show that (a) climatic changes at glacial-interglacial time scales are not the main controls over the formation of talus flatiron generation, and (b) that significant cliff retreat is not the main cause for the systematic spatial distribution of talus flatiron generations relative to the cliff. Our results indicate that the process of talus flatiron formation in the Negev desert must be associated with the balance between production, deposition, and removal of clasts under hyperarid conditions with only a minor cliff retreat.

The time at which deserts established their current arid or hyper-arid conditions remains a fundamental question regarding the history of Earth. Cosmogenic isotope exposure ages of desert pavement and welded, calcic–gypsic–salic Reg soils that developed on relatively flat alluvial surfaces ∼2 Ma ago in the Negev Desert indicate long geomorphic stability under extremely dry conditions. Over a short interval during their initial stage of development between 1–2 Ma, these cumulative soils are characterized by calcic soils reaching maximum stage III of carbonate morphology. This interval is the only period when calcic soil horizons formed on stable abandoned alluvial surfaces in the southern Negev Desert. Since ∼1 Ma pedogenesis changed toward more arid soil environment and the formation of gypsic–salic soil horizons that were later followed by dust accumulation. The dichotomy of only moderately-developed calcic soil (stages II–III) during a relatively long time interval (105–106 years) indicates an arid environment that does not support continuous development but only occasional calcic soil formation. The very low δ18O and relatively high δ13C values of these early pedogenic carbonates support soil formation under arid climatic conditions. Such an environment was probably characterized by rare and relatively longer duration rainstorms which occasionally allowed deeper infiltration of rainwater and longer retention of soil moisture. This, in turn enabled the growth of sparse vegetation that enhanced deposition of pedogenic carbonate. At ∼1 Ma these rare events of slightly wetter conditions ceased and less atmospheric moisture reached the southern Negev Desert leading to deposition of soluble salts and dust deposited in the soils. The combination of long-term hyperaridity, scarcity of vegetation and lack of bioturbation, salts cementation, dust accumulation and tight desert pavement cover, has protected the surfaces from erosion forming one of the most remarkably stable landscapes on Earth, a landscape that essentially has not eroded, but accumulated salt and dust for more than 106 yr.

The late middle Pleistocene initiation of loess accretion in the Negev Desert, Israel, resulted from a chain of events starting with the exposure of shallow offshore Nile delta sands beginning ∼180 ka, during a period of glacially lowered eustatic sea level. This exposure allowed the formation of the dunes of the Sinai-Negev erg and the eastward movement of the dunes under the gusty glacial-time winds in northern Sinai. This eastward dune advance occurred by eolian saltation that generated the coarse silts that accumulated downwind as loess. We postulate that the absence of earlier Negev loess is not a result of interglacial erosion as previously proposed but is due to the relatively recent convergence of the above conditions necessary for loess formation. We based our interpretation on analyses of two long-term natural dust traps and the association of coarse quartz silt production with the advancing quartz-rich dunes over this carbonate terrain. Although the Nile delta is considered an early to middle Pleistocene feature, and therefore could have been the source of sand throughout the Quaternary, it attained the shallow depth range of glacial-interglacial sea-level changes only during late marine oxygen isotope stage 7 to early stage 6, allowing its wide exposure. In Quaternary times prior to erg formation, Negev dust was finer, composed of silt and clays derived from distal sources in the Sahara and Arabian deserts.

Soils in similar geomorphic settings in hyperarid deserts (< 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.

Dating the time of abandonment of geomorphic surfaces in the arid mid latitudes is necessary for studies ranging from tectonics, landscape evolution and paleoclimate. It has often been hampered by the limited material suitable for conventional isotopic methods and the uncertainties inherent in cosmogenic radionuclide methods. We propose luminescence dating as a suitable method for dating the time of abandonment of aggradational geomorphic surfaces in the hyperarid regions. We dated the top of such surfaces with different age ranges and geomorphic settings in a sequence of alluvial fans in Nahal Shehoret in the southern Negev and in a sequence of terraces in Nahal Ze'elim and in Nahal Zin, two adjacent drainage basins in the Judean Desert, Israel. Samples were collected from beneath the gypsic horizon at a depth of 0.3–0.7 m, below which sand grains do not penetrate. Depositional ages for the uppermost beds of the landforms, which are proxies for abandonment, range from ∼90 ka to ∼5 ka. In all three basins, the ages are in morphostratigraphic order and agree well with relative age estimates based on soil chronosequences and on Lake Lisan levels. Abandonment ages for an individual alluvial fan cluster within ±10–20%, therefore it is possible to distinguish between surfaces with ages differing by more than 20%. Thus, in hyperarid areas, the luminescence methods can be used for surface dating.

article i nfo Accurate estimation of expected peak ground acceleration (PGA) in seismically active regions is a challenging task. The best way to estimate, quantitatively, expected PGA is by investigating instrumental data of past strong earthquakes in a given area. In some regions of the world however recorded data are scarce, and if they exist, they are typically available only since the late 19th century. As such they are hardly representative of the true seismicity in the studied region. We propose here an analytical approach to constrain the lower threshold of paleoseismic PGA on the basis of back analysis of old landslides. To perform the analysis we need a mapped landslide with geomorphic features that have been preserved in the field, the slip surface, a good reconstruction of the slope geometry and ground water level prior to failure, and the mechanical properties of the sheared material. We perform static and pseudo-static limit equilibrium analyses using standard solution procedures to obtain lower bounds of paleoseismic PGA. Back analyses of three different landslides around the Sea of Galilee (SOG) return similar results that range between 0.15 and 0.5 g, thus constraining the threshold paleoseismic PGA range for this region. The analytically inferred regional PGA is supported by results of an independent numerical analysis of toppled columns in a nearby Byzantine church. Using results from a recent paleoseismic trenching study performed on one of the studied landslides and a modified attenuation relationship for the study area we localize the loci of moment magnitude Mw=7.0 earthquakes that can explain the studied failures along the boundaries of the SOG, and find that they coincide with traces of the Eastern and Western Margin faults of the Dead Sea transform. The temporal relationships between the observed failures are discussed on the basis of dated colluvial sediments, geomorphologic constraints, and archeological evidence.

Quaternary desert loess and sandstone–loessite relationships in the geological record raise questions regarding causes and mechanisms of silt formation and accretion. In the northern Sinai–Negev desert carbonate terrain, only sand abrasion in active erg could have produced the large quantities of quartzo-feldspathic silts constituting the late Quaternary northwestern Negev loess. In the continuum of source (medium to fine sand of dunes) to sink (silts in loess) the very fine sand is unaccounted for in the record. This weakens the sand abrasion model of silt formation as a global process. Here, we demonstrate that, as predicted by experiments, abrasion by advancing dunes generated large quantities of very fine sand (60–110 μm) deposited within the dune field and in close proximity downwind. This very fine sand was generated 13–11 ka, possibly synchronous with the Younger Dryas under gusty sand/dust storms in the southeastern Mediterranean and specifically in the northern Sinai–Negev erg. These very fine sands were washed down slope and filled small basins blocked by the advancing dunes; outside these sampling basins it is difficult to identify these sands as a distinct product. We conclude that ergs are mega-grinders of sand into very fine sand and silt under windy Quaternary and ancient aeolian desert environments.

Katz, O., Amit, R., Yagoda-Biran, G., Hatzor, Y.H., Porat, N., Medvedev, B. 2009/2010. Quaternary earthquakes and landslides in the Sea of Galilee area, the Dead Sea Transform: Paleoseismic analysis and implication to the current hazard. Isr. J. Earth Sci. 58: 275–294. A few destructive large (Mw >6) earthquakes occurred close to the Sea of Galilee (SOG) in the last millennia and, along with recorded current seismicity, underscore the seismic hazard to populated areas in the region. In this study we investigate the Late-Pleistocene–Holocene seismic history of the southeastern SOG area, around the northern extension of the Jordan Valley Fault. We integrate results from paleoseismic trenching of recent fault segments rupturing the surface with slope stability analysis of a large landslide that spatially coincides with the faults, to develop a better understanding of the seismic hazard. We have dated fault rupture as well as sliding events with optically stimulated luminescence (OSL). In a series of paleoseismic trenches we found evidence for at least five Mw >6 earthquakes that were dated to ca. 45, 40, 35, 10, and 5 ka. A younger event of 6 and Mw >6.5, respectively. Such large earthquakes are expected to result in high ground acceleration of up to 0.5 g and earthquake-induced landslides around the SOG.

The Hula Valley, located in northeastern Israel, is a 4 million-year old pull-apart basin bounded by two left stepped segments of the Dead Sea Transform. Basic models of pull-apart basins suggest four faults at all sides of the basin, a situation which is not always the case. In the Hula pull-apart basin the longitudinal (N–S) faults are clearly evident while the transversal (E–W) faults are less explicit. An NW oriented lineament, expressed as a north-facing continuous scarp 1–3 m high, was recognized in the southern part of the valley. Morphological, seismic and sedimentary considerations suggest that this scarp is the surface expression of a deep-seated fault. This fault, termed the Yesod HaMa'ala fault, forms the present southern boundary of the Hula pull-apart basin, and appears to be the southern diagonal fault, which links the Jordan fault in the east with the Yammunneh fault in the west. It is suggested that this fault scarp formed a shallow morphological barrier that partly blocked the southern outlet of the valley, enabling a permanent lake in the southern part of the Hula Valley. Thus, the uplifted southern block determined the SW shoreline of the lake. It is proposed that the Hula basin is a version of the classic pull-apart model. It is also suggested that in the absence of upwelling mantle material, and extension along the length of the basin, a crustal thinning and subsidence might have occurred through normal faulting of the upper brittle crust. The thick depositional sequence within the southern part of the basin shows that the present margins have been a major depositional site for a long time, and that the Hula Valley, has thus developed as an asymmetric, pull-apart (rhomb-shaped) basin, bounded by steep north-striking boundary normal (as well as strike-slip) faults, W–NW striking and northward-dipping normal faults, and a south-facing bending flexure. As activity within the basin continued, and northward migration of the faulting occurred, a new fault developed. According to this perception, the present active fault is the Yesod HaMa'ala fault, while the future fault will be located northward, as can be presumed from the present micro-seismic activity in the valley.

Evidence for unchanging slip rate and a Gutenberg–Richter relation for earthquake distribution along the Dead Sea fault during the past 60,000 yr are presented. The evidence comes from three different segments, approximately 100 km apart, and from three different timescales: prehistoric–paleoseismic, historical, and modern (instrumental) records. The paleoseismic data are based on two different methods. In the southern Arava Valley and the northern Jordan Valley segments, the amount of normal displacement along several faults is used, while in the Dead Sea basin the appearance of brecciated beds, which are considered as seismites, is used. We found that for the southern Arava Valley segment a constant dip-slip rate of 0.5 mm/yr can explain the cumulative normal slip during the past 45,000 yr. This suggests that normal faulting is only ∼10% of the total left-lateral strike-slip motion. We also found that for all three segments, the paleoseismic and historical records of strong earthquakes lie on the linear extrapolation of the frequency–magnitude relation of the instrumental record. The calculated b-values for all three segments are between 0.85 and 1, similar to other major strike-slip faults in the world. It is concluded that the Gutenberg–Richter distribution is a stable mode in the tectonic setting of the Dead Sea fault during the past 60,000 yr.

All exposed rocks on Earth’s surface experience erosion; the fastest rates are documented in rapidly uplifted monsoonal mountain ranges, and the slowest occur in extreme cold or warm deserts—millennial submeterscale erosion may be approached only in the latter. The oldest previously reported exposure ages are from boulders and clasts of resistant lithologies lying at the surface, and the slowest reported erosion rates are derived from bedrock outcrops or boulders that erode more slowly than their surroundings; thus, these oldest reported ages and slowest erosion rates relate to outstanding features in the landscape, while the surrounding landscape may erode faster and be younger. We present erosion rate and exposure age data from the Paran Plains, a typical environment in the Near East where vast abandoned alluvial surfaces (10 2 –10 4 km 2 ) are covered by well-developed desert pavements. These surfaces may experience erosion rates that are slower than those documented elsewhere on our planet and can retain their original geometry for more than 2 m.y. Major factors that reduce erosion converge in these regions: extreme hyperaridity, tectonic stability, fl at and horizontal surfaces (i.e., no relief), and effective surface armoring by a clast mosaic of highly resistant lithology. The 10 Be concentrations in amalgamated desert pavement chert clasts collected from abandoned alluvial surfaces in the southern Negev, Israel (representing the Sahara-Arabia Deserts), indicate simple exposure ages of 1.5–1.8 Ma or correspond to maximum erosion rates of 0.25–0.3 m m.y. –1 . The 36

Colluvial sediments in tectonically active areas are associated with faults, and dating them is important for timing earthquakes and assessing seismic hazards. In hyperarid regions organic matter for radiocarbon is scarce, and the dating method of choice is luminescence. However, colluvial sediments are difficult to date due to the short transport distance that is commonly involved and the resulting incomplete bleaching of the sediment. Single grain and small aliquot measurements of fault-related colluvial sediments may isolate well-bleached grains and provide reliable ages of faulting events. Elat, southern Israel, lies on the active Dead Sea Transform and paleoseismic data are crucial for city planners. A small fresh-looking NNE trending graben within the late Pleistocene to Holocene alluvial fan of Nahal Shehoret near Elat was trenched to expose recently active normal faults with a stack of colluvial wedges on the footwalls. Samples for luminescence dating were collected near the eastern fault, from the alluvial fan and from the heels (less than 1 m from the fault plane) and toes (∼2 m away from the fault plane) of a stack of five colluvial wedges. Single grain measurements of quartz were compared with large (thousands of grains) and small (hundreds of grains) single aliquots of quartz and alkali feldspar (KF). KF was measured using the infrared stimulated luminescence (IRSL) signal and the single aliquot added dose (SAAD) or single aliquot regenerative dose (SAR) protocols. Large aliquots and single grains of quartz were measured using the optically stimulated luminescence (OSL) signal and the SAR protocol. Large and small aliquots show large scatter in equivalent dose values, with a distinct population of young ages. Quartz large aliquot ages of samples taken from the toes of the wedges range from 5200 to 2600 years, however the ages of samples taken from the heels, close to the fault, are in reversed order, from 4300 years at the base to 13,000 years at the upper wedge. This results from progressive exposure and erosion of deeper and older beds on the fault scarp with continued faulting, and incorporation of older poorly bleached grains into the upper wedges. Only single grain measurements could isolate a distinct, small population of grains that were fully bleached at the time of deposition, dating the colluvial wedges to between 500 and 1300 years. Large aliquots ages substantially overestimated the time of faulting. The single grain ages record very young faulting events which may be correlated with the known historical earthquakes of the past 1500 years.

Grain size analyses of three hilltop, primary eolian loess sequences in the Negev desert, southern Israel, show a bimodal grain-size distribution at 50–60 μm and 3–8 μm. Using analyses of mineralogy and OSL ages we demonstrate that the coarse mode is composed mostly of quartz grains and its relative magnitude increases regionally with time, suggesting an enhancement of a time-transgressive proximal dust source compared to a distal, Saharan fine-grain dust. The only proximal dust source for large amount of coarse silt quartz grains is the sands that advanced into Sinai and the Negev concurrently with the loess accretion during the late Pleistocene as a result of the exposure of the Mediterranean shelf. We therefore propose that the coarse silt quartz grains were formed through eolian abrasion within the margins of an advancing sand sea. This relationship between desert sand seas as a source for proximal coarse dust and desert margin loess deposits can be applicable to other worldwide deserts such as Northern Africa, China and Australia.

During the past three decades, the U.S. armed forces have been called on repeatedly to operate in the deserts of the Middle East and southwest Asia. Avoiding locations susceptible to extreme dust emissions and other terrain-related hazards requires the ability to predict soil and terrain conditions, often from limited information and under dynamic environmental conditions. This paper reports the approach used to develop an integrated, predictive tool for forecasting terrain conditions to support military operations in desert environments at strategic, operational, and tactical scales. The technical approach relies on the systematic integration of desert landform parameters in geomorphic models for predicting terrain conditions. This integrated effort is performed in a geographic information system (GIS) framework using expert-based analysis of airborne and spaceborne imagery to identify terrain elements. Advances in earth science research have established that unique, predictable relations exist among landscape position, soils, vegetation, and geology. Furthermore, new instrumentation allows the collection of a wide range of environmental information to characterize surface and subsurface conditions. By integrating models and methods from geomorphology, soil science, climatology, and atmospheric science with remote sensing and other technologies, a predictive model can be developed to support military operations.

Mapping and dating of arid and semi-arid alluvial fans are of great importance in many Quaternary studies. Yet the most common mapping method of these features is based on visual, qualitative interpretation of air-photos. In this study we examine the feasibility of mapping arid alluvial surfaces by using airborne hyperspectral reflective remote sensing methodology. This technique was tested on Late Pleistocene to Holocene alluvial fan surfaces located in the hyperarid southern Arava valley, Israel. Results of spectral field measurements showed that the surface reflectance is controlled by two main surficial processes, which are used as relative age criteria: the degree of desert pavement development (gravel coverage %) controls the absorption feature depths, while the rock coating development influences significantly the overall reflectance of the surface, but its effect on the absorption feature depths is limited. We show that as the percent of the surface covered by gravels increases, the absorption feature depth of the common gravels, in this case carbonate at 2.33 μm, increases as well; whereas the absorption features depth of the fine particle in-between the gravels, decrease (hydroxyl and ferric absorption features at 2.21 μm, and 0.87 μm, respectively), as the fines are removed from the surface. Using these correlations we were able to map the surface gravel coverage (%) on the entire alluvial fan, by calculating the gravel coverage (%) in each pixel of the hyperspectral image. The prediction of gravel coverage (%) is with accuracy of ± 15% (e.g. gravel coverage of 50% can be predicted to be 35% to 65%). Using extensive accuracy assessment data, we show that the spectral based mapping maintained high accuracy degree ( R 2 = 0.57 to 0.83). The quantitative methodology developed in this study for mapping alluvial surfaces can be adapted for other surfaces and piedmonts throughout the arid regions of the world.

The Elat fault (a segment of the Dead Sea Transform) runs along the southern Arava valley (part of the Dead Sea Rift, Israel) forming a complex fault zone that displays a time-dependent seismic behaviour. Paleoseismic evidence shows that this fault zone has generated at least 15 earthquakes of magnitude larger than M 6 during the late Pleistocene and the Holocene. However, at present the Elat fault is one of the quietest segments of the Dead Sea Transform, lacking even microsesimicity. The last event detected in the southern Arava valley occurred in the Avrona playa and was strong enough to have deformed the playa and to change it from a closed basin with internal drainage into an open basin draining to the south. Paleoseismological, geophysical and archaeological evidences indicate that this event was the historical devastating earthquake, which occurred in 1068 AD in the eastern Mediterranean region. According to the present study this event was strong enough to rupture the surface, reactivate at least two fault branches of the Elat fault and vertically displace the surface and an early Islamic irrigation system by at least 1 m. In addition, the playa area was uplifted between 2.5 and 3 m along the eastern part of the Elat fault shear zone. Such values are compatible with an earthquake magnitude ranging between M 6.6 and 7. Since the average recurrence interval of strong earthquakes during the Holocene along the Elat fault is about 1.2 ± 0.3 ky and the last earthquake occurred more about 1000 years ago, the possibility of a very strong earthquake in this area in the future should be seriously considered in assessing seismic hazards.

Activities of 26 Al and 10 Be in five chert clasts sampled from two beach ridges of late Pleistocene Lake Lisan, precursor of the Dead Sea in southern Israel, indicate low rates of chert bedrock erosion and complex exposure, burial, and by inference, transport histories. The chert clasts were derived from the Senonian Mishash Formation, a chert-bearing chalk, which is widely exposed in the Nahal Zin drainage basin, the drainage system that supplied most of the material to the beach ridges. Simple exposure ages, assuming only exposure at the beach ridge sampling sites, range from 35 to 354 ky; using the ratio 26 Al/ 10 Be, total clast histories range from 0Ð46 to 4Ð3 My, unrelated to the clasts’ current position and exposure period on the late Pleistocene beach ridges, 160–177 m below sea level. Optically stimulated luminescence dating of fine sediments from the same and nearby beach ridges yielded ages of 20Ð0 s 1Ð4 ka and 36Ð1 s 3Ð3 ka. These ages are supported by the degree of soil development on the beach ridges and correspond well with previously determined ages of Lake Lisan, which suggest that the lake reached its highest stand around 27 000 cal. years BP. If the clasts were exposed only once and than buried beyond the range of significant cosmogenic nuclide production, then the minimum initial exposure and the total burial times before delivery to the beach ridge are in the ranges 50–1300 ky and 390–3130 ky respectively. Alternatively, the initial cosmogenic dosing could have occurred during steady erosion of the source bedrock. Back calculating such rates of rock erosion suggests values between 0Ð4 and 12 m My � 1 .T he relatively long burial periods indicate extended sediment storage as colluvium on slopes and/or as alluvial deposits in river terraces. Some clasts may have been stored for long periods in abandoned Pliocene and early Pleistocene routes of Nahal Zin to the Mediterranean before being transported again back into the Nahal Zin drainage system and washed on to the shores of Lake Lisan during the late Pleistocene. Copyright  2003 John Wiley & Sons, Ltd.

Alluvial units (termed “Red Units”) accumulated along the western margin of the Dead Sea Rift (DSR) valley (Southern Negev Desert, Israel) in the Early Pleistocene. These sedimentary units consist of coarse to fine-grained alluvium of local origin, which contain calcic paleosols. The sediments of the Red Units accumulated in stream valleys and newly-forming semi-closed local tectonic basins during a tectonic phase that deformed the western margin of the DSR. As a result of this deformation the Pliocene, northward-flowing, regional drainage system of the southern Negev disintegrated to the present northeast-flowing, ephemeral drainage systems of Nahal Zihor, Nahal Hiyyon and Nahal Paran, all of them draining to the Arava Valley in the DSR. This change was induced by two deformation episodes: (1) reactivation of the Zihor and the Milhan faults—two major, northeast-trending tectonic lines, and (2) east and northeast tilting of the entire region during upwarp (arching) of the central Negev. The paleosols of the Red Units are characterized by pedogenic features such as rhizoconcretions and Bk horizons. The paleosols developed in the alluvial sediments that were deposited and preserved in local, small structural basins along the main fault lines and in reversed or abandoned westward-flowing Pliocene stream valleys. Formation of paleosols was initiated in the stream valleys when they attained a near horizontal gradient due to gradual eastward-directed regional tilting, followed by low rates of deposition. The sections of the Red Units in the Pliocene valleys were incised due to continuing tilting, which completely reversed the primary westward gradient of the valleys, and they were incised by new east and northeast-flowing streams. The Red Units have paleoclimatic implications. Since the Middle Pleistocene the southern Negev has been characterized by gypsic and salic soils typical of an extremely arid climate, while the calcic soils detected in the Red Units point to a semi-arid climate. This indicates a gradual transition from more humid conditions in the Early Pleistocene to the present extreme aridity. Therefore, the Red Units reflect a distinct tectonic and climatic regime and can serve as regional morphostratigraphic marker of Early Pleistocene age. In this paper, we used this marker to reconstruct the tectonic deformation that established the present configuration of the southern Negev Terrain.

The Elat fault system in the southern Arava Valley (Dead Sea rift, Israel) is a complex fault zone, characterized by marginal normal faults and central sinistral strikeslip faults. Paleoseismic evidence shows that the Elat fault system has generated at least 15 earthquakes of magnitudes (M) larger than 6 during the late Pleistocene and the Holocene. At least two branches of the fault zone were tectonically active simultaneously, indicating that the seismic response over a period of 80 k.y. was time and space dependent. Late Pleistocene earthquakes displaced the surface by 1‐1.5 m; their magnitudes were between M 6.7 and M 7, and their average recurrence interval was 2.8 6 0.7 k.y. Movements along the fault system in the Holocene had a higher frequency and a recurrence interval of 1.2 6 0.3 k.y., but resulted in smaller displacement amounts (0.2‐1.3 m) and smaller earthquake magnitudes (M 5.9‐M 6.7). Historical records document the last seismic event along the Elat fault zone at ;1000 yr ago. The decrease in tectonic activity with time is inferred from the concentration of offset along the fault segments in the central part of the Elat fault zone and the decreased seismicity in the eastern and western margins. The magnitude range determined for the central zone (M 6.1‐M 6.7) was likely not high enough to activate the marginal faults. The average slip rate on the normal faults is 0.2 mm/yr. However, the slip rate has changed through time on different fault segments in the active wide shear zone and between clusters of events related to the same segment. The event-specific slip rates, therefore, have varied from 0.1 to 0.3 mm/ yr. The decrease in earthquake magnitudes with time, combined with the observations that the last large event occurred in A.D. 1068 and that no microseismicity has been detected during the past 15 yr, might signal locking of the Elat fault zone. This effect, if true, may result from episodic global reorganization of the system’s mode of strainenergy release, reflected in the configurational entropy of stress states on the fault. These results have significant implications for seismic hazard assessment in the southern Arava Valley, southern Israel, and underscore the possibility that the Elat fault may be a site of major earthquakes in the near future.

Mesas are ubiquitous landforms in arid and semiarid regions and are often characterized by horizontal stratified erodible rocks capped by more resistant strata. The accepted conceptual model for mesa evolution and degradation considers reduction in the width of the mesa flat-top plateau due to cliff retreat but ignores possible denudation of the mesa flat-top and the rates and mechanism of erosion. In this study we examine mesas in the northeastern hyperarid Negev Desert where they appear in various sizes and morphologies and represent different stages of mesa evolution. The variety of mesas within a single climatic zone allows examination of the process of mesa evolution through time. Two of the four sites examined are characterized by a relatively wide (200–230 m) flat-top and a thick caprock whereas the other two are characterized by a much narrower remnant flat-top (several meters) and thinner caprock. We use the concentration of the cosmogenic nuclide 10Be for: (a) determining the chronology of the various geomorphic features associated with the mesa; and (b) understanding geomorphic processes forming the mesa. The 10Be data, combined with field observations, suggest a correlation between the width of flat-top mesa and the denudation and cliff retreat rates. Our results demonstrate that: (a) cliff retreat rates decrease with decreasing width of the flat-top mesa; (b) vertical denudation rates increase with decreasing width of the flat-top mesa below a critical value (~60 m, for the Negev Desert); (c) the reduction in the width of the flat-top mesa is driven mainly by cliff retreat accompanied by extremely slow vertical denudation rate which can persist for a very long time (>106 Ma); and (d) when the width of the mesa decreases below a certain threshold, its rate of denudation increases dramatically and mesa degradation is completed in a short time. Copyright © 2014 John Wiley & Sons, Ltd.

Modern sediments from a variety of depositional environments in the hyperarid southern Arava region, Israel, were analyzed for their residual equivalent dose (D % ). Colluvial and #uvial sediments from drainage systems of di!erent sizes and parent materials were collected and alkali feldspars were measured using the single-aliquot additive-dose protocol. All samples showed a large scatter in the D % s. For most types of the sediments, the average D % is between 2 and 11 Gy, equivalent to residual ages of 1}5 ka. The lowest D % is usually in the range of 0.1}4 Gy, con"rming that the single-aliquot protocol can identify the best bleached grains. Newly weathered material from active channels gave the largest D % ’s of up to 30 Gy. Reworked #uvial sediments, derived from terraces and fans, gave progressively smaller D % ’s with increasing level of recycling. The degree of bleaching of colluvial sediments strongly depends on the face of the slope. The high D % ’s and the large scatter result from the mode of sediment transport. Brief rain episodes result in #ash #oods, where large volumes of sediment are transported over short distances and only a small part of the sediment is exposed at any time to the sun. Only repeated cycles of transportation can homogenize the sediment and fully expose all its particles to the sun. Single-grain or small-aliquot protocols can overcome the high and variable residual D % values and enable better dating of sediments in hyperarid environments. ( 2000 Elsevier Science Ltd. All rights reserved.

Paleoseismic analysis of the Azaz fault segment of the Dead Sea Fault (northern Israel), indicates that this region was subjected to intermittent periods of strong seismicity and quiescence during the latest Pleistocene and the Holocene. The Azaz fault forms the eastern margin of the Hula Valley, a pull-apart basin between two left-stepping segments of the Dead Sea Fault. The fault is currently inactive, but it displaces a 25–30 ka travertine in the northeastern corner of the Hula Valley and near-surface marsh sediments inside the valley. Two trenches excavated across a 15–20 m high fault scarp near Kefar Szold expose fluvial and colluvial sequences showing clear evidence of recent tectonics. One trench exposes well sorted, bedded alluvial sediments comprising two upward-fining units, each capped by a weakly developed paleosol. The sediments were deposited on the down-faulted block and are bounded in the east by the fault scarp. The second trench uncovers fine colluvial sediments cross-cut by a wide fault-parallel fissure filled by collapsed sediments. In both trenches, the lower sedimentary unit is capped by a coarse colluvium containing boulders up to 1.5 m in size. The colluvium shows no clear bedding and has a weakly developed paleosol on top. Optically stimulated luminescence dating of the lower fluvial sequence shows that its age ranges between 12 ka at the base to 6 ka at the top, while the middle part of the overlying colluvium is ca. 4.8 ka. The relation between the lower fluvial units and the fault indicates that at least two large-scale earthquakes occurred in the Early Holocene, each one resulting in a 1–1.5 m high fault scarp. During this seismically active period, no coarse colluvial sediments were deposited along the fault trace and thus the region must have had low tectonic relief. The clear contact with the overlying coarse colluvium reflects a Middle Holocene rapid change to the present high, steep relief. This change was achieved by frequent, strong seismic events, which triggered colluvial processes and prevented stratification and soil formation on the slope. The soil on the present slope reflects a quiescent period that has lasted at least several hundred years, during which stress has accumulated and seismic risk increased.

The Arava Valley section of the Dead Sea Transform (DST) in southern Israel is characterized by the absence of seismic activity in recent times. However, paleoseismic analysis of sediments in the Avrona Playa, a pull-apart basin along the DST, reveals that at least six M > 6 tectonic events have affected the Avrona playa in the last 14,000 yr. The recurrence interval of the events is approximately 2000 yr. Trenched normal faults and push-up ridges in the playa show that the upper 2 m of the deformed sedimentary sequence consists of playa deposits with uniform soil development. The deformed sediments and the soil are typical of basins with an endoreic fluvial system. Based on the limiting age of the sequence and the extent of soil development, faulting in the playa, followed by compression and uplift, occurred in the last 1000 yr. This most recent tectonic event displaced the surface by at least 1 m, consistent with a M > 6.5 earthquake. This earthquake changed the morphology of the Avrona Playa from a closed system with internal drainage to an open basin, resulting in relief inversion. The seismic quiescence in the Arava may indicate a seismic gap in this segment of the DST.

Sediments from Lunkaransar dry lake in northwestern India reveal regional water table and lake level fluctuations over decades to centuries during the Holocene that are attributed to changes in the southwestern Indian monsoon rains. The lake levels were very shallow and fluctuated often in the early Holocene and then rose abruptly around 6300 carbon-14 years before the present ( 14 C yr B.P.). The lake completely desiccated around 4800 14 C yr B.P. The end of this 1500-year wet period coincided with a period of intense dune destabilization. The major Harrapan-Indus civilization began and flourished in this region 1000 years after desiccation of the lake during arid climate and was not synchronous with the lacustral phase.