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Cliff bands are common in drylands and their evolution is often influenced by hydrogeomorphic processes. It has been previously suggested that cliff retreat patterns and morphology are affected by the properties and frequency-magnitude relations of rainstorms. However, basic questions on this topic persist because landscape evolution models typically do not account for the surface processes like runoff generation and sediment transport that occur under short-duration (sub-hourly) intense rainfall. Here we test the hypothesis that changes in rainstorm properties can systematically alter cliff retreat patterns and morphology. We developed a novel numerical model that simulates the response of cliffs and associated sub-cliff slopes to various rainstorm regimes to (1) identify dominant cliff morphologies, and (2) examine if extreme rainstorm properties are encoded in the topography. The new model utilizes the Landlab modeling toolkit and includes an explicit novel representation of surface processes that occur during short-duration rainstorms, including cliff-weathering, infiltration, runoff generation, clast fragmentation, and size-dependent sediment transport. Using a suite of numerical experiments, we vary model parameters and rainfall types and simulate changes in cliff retreat patterns and morphology. Our model results agree well with analytical predictions for cliff morphology under a control case of no transport on the sub-cliff slope, indicating a good representation of processes. Furthermore, sensitivity analyses on cases where sediment transport is explicitly included show that cliff evolution is highly dependent on both the grain size of sediment derived from the cliff and the rainfall intensities. These two factors can alter retreat patterns and determine whether and how fast the cliff can be buried under its own sediment. Numerical experiments based on rainfall and field measurements from the central Negev desert (eastern Mediterranean) demonstrate that including the dynamics of high-intensity rainfall and sediment grain size can help explain observed topographic trends. In addition, for a given imposed storm depth, we find that the rainstorm intensities pattern strongly influences both the cliff retreat and its morphology. Short rainstorms with higher intensities are much more erosive than longer storms with lower intensities. This latter case frequently triggers cliff burying. Taken as a whole, our results demonstrate that cliff evolution and morphology are significantly affected by storm-scale sediment transport dynamics and thus highlight the importance of incorporating high-resolution rainfall forcing into landscape evolution models of dryland landforms.

Deciphering aspect-related hillslope asymmetry can enhance our understanding of the influence of climate on Earth’s surface morphology and the linkage between topographic morphology and erosion processes. Although hillslope asymmetry is documented worldwide, the role of microclimatic factors in the evolution of dryland cliffs has received little attention. Here, we address this gap by quantifying aspect-dependent bedrock weathering, slope-rill morphology, and sub-cliff clast transport rates in the hyperarid Negev desert, Israel, based on light detection and ranging (LiDAR)-derived topography, clast-size measurements, and cosmogenic 10Be concentrations. Cliff retreat rates were evaluated using extrapolated profiles from dated talus flatirons and 10Be measurements from the cliff face and sub-cliff sediments. We document systematic, aspect-dependent patterns of south-facing slopes being less steep and finer-grained relative to east- and north-facing aspects. In addition, cliff retreat and clast transport rates on slopes of the south-facing aspect are faster compared to the other aspects. Our data demonstrate that bedrock weathering of the cliff face and the corresponding grain size of cliff-derived clasts delivered to the slopes constitute a first-order control on cliff retreat and sediment transport rates. We demonstrate that the morphology of the cliff and the pattern of bedrock weathering co-vary with the solar radiation flux and hence that cliff evolution in hyperarid regions is modulated by aspect-dependent solar radiation. These results help to better understand interactions between climate and dryland surface processes.

The timing of vertical motions adjacent to the Dead Sea Transform plate boundary is not yet firmly established. We utilize laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb geochronology of carbonate cave deposits (speleothems) to constrain paleo-groundwater levels along the western margin of the Dead Sea Transform and provide a proxy for the timing of large-scale incision and tectonic uplift. Phreatic speleothems can form in caves that are located slightly below the groundwater level. Tectonic uplift and/or base level subsidence can trigger incision of canyons and induce a drop in the groundwater table. This can cause dewatering of the caves, cessation of the deposition of phreatic speleothems, and initiation of growth of vadose speleothems. The transition between deposition of phreatic and vadose speleothems can therefore reflect tectonic or erosive events. We obtained 102 U-Pb ages from 32 speleothems collected from three cave complexes across a 150-km-long, north-to-south transect. These ages indicate that phreatic deposition began between 14.68 ± 1.33 and 11.34 ± 1.62 and ended by 6.21 ± 0.59 Ma. Later, vadose speleothems grew intermittently until the Quaternary. These results suggest an abrupt drop in the water table starting at ca. 6 Ma with no re-submergence of the caves. We interpret this to indicate river incision of ~150–200 m that was driven by uplift and folding of the western margin of the Dead Sea Transform and by inland morpho-tectonic, base-level subsidence in the Dead Sea area. The observed timing corresponds with a change in the Euler pole of the plates motion along the Dead Sea Transform. The growth period of phreatic speleothems suggests groundwater level stability and limited vertical tectonic motions between 14 Ma and 6 Ma.

Carbonate hillslopes are often soil mantled and display a classic convex morphology. In this study we examine controls on carbonate hillslope denudation and morphology using a modified regolith mass balance equation to account for chemical weathering and dust input—two fluxes that are commonly neglected in settings with silicate-dominated bedrock. We utilize seven study sites in the Eastern Mediterranean across a significant gradient in the mean annual rainfall and dust deposition flux. Combining cosmogenic 36Cl-derived hilltop denudation rates with an estimate of the regolith chemical depletion and the quantified fraction of dust in the regolith we predict hilltop curvature and compare our predictions with observations based on high-resolution airborne LiDAR (light detection and ranging). Denudation rates vary from 5 to 210 mm/k.y. and increase with mean annual rainfall. Less resistant carbonates (chalk) experience faster denudation rates relative to more resistant dolo-limestone and are less prone to chemical weathering. Soil production exhibits a humped dependency on soil thickness. The observed hilltop curvature varies as a function of rainfall and dust flux with a minimum at sub-humid sites. While trends in hilltop convexity are often solely attributed to variations in erosion rate, our results illustrate the additional effects of dust production and chemical depletion. Our mass balance model implies that drier sites in the south probably experienced a more intricate history of regolith production due to dust flux fluctuations. Thus, by incorporating dust flux and chemical weathering to the classic hillslope evolution model we are able to identify a complex relation between hilltop curvature, soil production, and climate.

Climatic impact on landscape morphology was previously demonstrated under pronounced gradients in average climatic properties such as mean annual precipitation or temperature. However, in arid areas, where both meteorological observations and rainfall measurements are scarce and the latter is meager, short-term and highly variable in space and time, the determination of meaningful “average climatic” conditions and their variability is challenging. Although it is generally acknowledged that surface processes in arid landscapes should be effected by short-duration rainfall intensities and their extremes, the topographic sensitivity to storm-scale properties were rarely quantified. Here, we attempted to bridge this gap by documenting systematic precipitation variations along a 40 km arid escarpment (Ramon crater) in the central Negev desert (Israel) and their associated topographic signature.We used 0.5 m pixel-1 LiDAR-derived topographic data coupled with field measurements to characterize the morphology of cliffs and slopes along the entire Ramon crater. Sub-hourly rainfall intensities were characterized using an 8-year record of high-resolution, convection-permitting, numerical weather model prediction (NWP). Frequency analyses of rainfall intensity and its spatial variation were conducted using a novel statistical method and used to determine runoff and sediment transport along sub-cliff slopes, through grid-based hydrological simulations of synthetic rainstorms with different frequencies.Our results indicate that due to a pronounced decreasing gradient in the number of rain storms per year, the mean annual rainfall decreases from ~100 mm in the southwest (SW) cliff segment to ~40 mm in the northeast (NE) segment. However, in the drier NE cliff segment, extreme rainfall intensities such as the ones occurring during a storm with a 100-year return period are higher. Topographic cliff gradients and the percentage of exposed bedrock over the cliffs increase toward the drier NE cliff section. Sub-cliff slopes in the NE are systematically straighter, shorter, and associated with a smaller clast sizes relative to the wetter (SW) part of the escarpment. Hydrological simulations reveal that under extreme storms, sediment is mobilized by sheetwash on the NE slopes but is less mobile on the wetter SW slopes. In addition, incised gullies and disconnected talus-flatirons are more frequent in the NE and correlate with the higher erosion efficiency of extreme rainstorms in this zone. Our results indicate that significant morphologic differences can be imprinted in arid landforms due to spatial gradients in the properties of extreme rainstorms.

Talus-pediment slopes are a common morphologic feature in arid areas and constitute a prominent runoff and sediment source at the watershed and channel scales. The evolution of talus-pediment sequences (talus flatirons) was often linked to climatic cycles, although the physical processes that may account for such a link remained obscure. Our approach is to integrate field measurements, high-resolution radar rainfall data and numerical modeling to link the frequency of storms and the resulted hillslope runoff and sediment transport. We present a quantitative hydrometeorological analysis of rainstorms and their geomorphic impact, potentially involved in the evolution of arid talus-pediment slopes in the Negev desert (Israel). Artificial rainstorms were designed based on intensity-duration-frequency curves and simulated in the field using a rainfall simulator. Then, the obtained experimental results were up-scaled to the entire slope length using a fully distributed hydrological model. In addition, natural storms and their hydro-geomorphic impacts were monitored using X-band radar and time-lapse cameras.These integrated analyses constrain the rainfall threshold for local runoff generation at rain intensity of 14-22 mm h-1 for a duration of 5 min for the study area conditions. We characterized small-scale runoff-generating convective rain cells using an X-band radar and found that small convective cells (~30 km2), having extremely high internal spatial gradients in rainfall intensity and low velocity (-1), have the potential to generate local hillslope runoff. The frequency of local runoff-producing rainstorms is ~1-3 per year, but most of these storms activate only small parts of the hillslope. Modeling results indicate that a full extent hillslope runoff occurs under much rarer rainstorms of at least 100-years return interval (1% or less). During such rainstorms, the shear stress produced by the runoff flow (sheetwash) is capable of transporting surface clasts at a distance of ~80 m downslope. However, transport of coarse clasts in the upper parts of the slopes is most probably gravitationally controlled. The erosion efficiency of discrete rare events (1% or less) on the lower part of the slopes highlights their potential to trigger incision and lead to cliff dissection. This study results support the hypothesis that a climatic shift in terms of the properties and frequency of extreme rainstorms, rather than the common views of it as changing precipitation means, can play an important role in shaping and in transforming landscapes in such arid setting.

Convex, soil-covered hillslopes are ubiquitous in various tectonic and climatic settings and their morphologic characteristics can be significantly altered by dust flux and chemical weathering. Carbonate hillslopes across the Eastern Mediterranean display a classic convex morphology despite high rates of dust deposition, which are apparently similar in magnitude to denudation rates and despite a highly soluble bedrock lithology. These high rates of dust flux and bedrock dissolution result in soil composition which is strikingly different from the underlying bedrock. In this study we apply a modified regolith mass balance equation which includes dust input and chemical depletion and utilize a combination of field work and geochemical methods (immobile and major elements, XRD) to estimate the fraction of dust-derived materials in the soil and the fraction of the soil that was lost via dissolution. Soil and bedrock were sampled at 7 Eastern Mediterranean study sites, across a prominent north-to-south gradient in both precipitation (250 to 900 mm yr−1) and dust flux (150 to 40 g m−2 yr−1) and compared with dust composition. Our results indicate that the dust fraction in the soil correlates with modern measurements of dust deposition rates and decrease from nearly 100% of the 2 mm) indicating that mechanical weathering of the underlying bedrock contributes to regolith formation and plays a role in shaping carbonate hillslopes. Using a commonly used soil transport model, we show that the observed dust fraction and chemical weathering can potentially account for a 50% change in hillslope curvature. Furthermore, the predicted steady state hilltop curvature, based on our results, fits well the observed curvature based on high-resolution LiDAR-derived topographic data. Our results highlight the potential importance of dust and chemical weathering in soil production formulations and hillslope evolution models and we propose that these contributors to soil mass balance could also be relevant for silicate terrain. In addition, we also demonstrate that the source material of Mediterranean soils varies with environmental factors such as rainfall, dust deposition rates and bedrock mechanical properties.

The Suez Rift is the northern extension of the Red Sea Rift and is an ideal location to study young continental breakup tectonics. Yet, despite much effort to constrain the timing of rift flank uplift and extensional exhumation in this region, the emerging picture remains uncertain. Here we present new thermochronologic constraints including apatite (U–Th)/He and fission track data from a transect across the eastern flank of the Suez Rift in SW Sinai. The use of two different thermochronometers in conjunction with thermal history modeling and well constrained kinetic parameters delineates a pronounced episode of relatively rapid cooling at 25-18 Ma followed by subdued post ∼18 Ma cooling. In contrast to previous estimates, this cooling pattern suggests that the most rapid uplift and exhumation phase along the Gulf of Suez took place during the early stage of rift formation, prior to the middle Miocene, with over 2.4 km of exhumation occurring during this stage. Uplift and exhumation along the Suez Rift flank slowed substantially post ∼18 Ma, as deformation migrated to the newly formed plate boundary along the Dead Sea Transform. This finding complements and strengthens previous indications for the initiation timing of the Dead Sea Transform plate boundary. The inferred late Oligocene–early Miocene timing of rapid exhumation, associated with tectonic extension and rift flank uplift coincides with the onset of regional alkali-basalt magmatism, suggesting lithospheric heating prior to the rift formation. This observation, along with the elevated topography around the Sinai triple junction and indications for some pre-rifting uplift, supports a more active mantle plume role in the rifting process.

Recurring Slope Lineae (RSL) are dynamic, low-albedo, slope-parallel surface features on Mars that occur mainly on steep (>25°) slopes. RSL typically display seasonal dynamics as they appear during late Martian spring, progressively grow during summer, and subsequently fade as summer ends. RSL formation mechanisms remain under debate with proposed mechanisms involving either water/brines (‘wet theories’) vs. dry granular flows within a surficial dust layer (‘dry theories’). In an attempt to distinguish between plausible RSL mechanisms, this study compares the topographic and morphologic characteristics of hillslopes with and without RSL. We suggest that a distinct topographic signature for RSL hillslopes would argue against the ‘dry’ RSL mechanisms, as RSL dynamics within a thin dust layer are not expected to significantly impact the hillslope-scale topography. In contrast, the presence of fluids on RSL hillslopes could conceivably accelerate rock weathering rates, which in turn may impact the hillslope-scale topography. Our analyses are based on HiRISE, CTX and HRSC digital terrain models (DTMs) together with geomorphic mapping using high-resolution orbital images. We focus on inner crater hillslopes and compare the topographic characteristics of RSL vs. non-RSL slopes. In addition, in order to account for the potential influence of aspect-dependent solar irradiation on hillslope processes, we also applied our analysis on adjacent ‘control’ craters that are devoid of RSL activity. Preliminary results from Palikir (-41.6°/ 202.1°E) and Rauna (35.2°/ 328°E) craters reveal that the topographic slope distribution along crater walls with RSL activity is distinct from the slope distribution along crater walls which are devoid of RSL activity. Our results appear to support increased rock-weathering rates on crater walls that presently experience RSL activity.

Convex soil-covered hillslopes are ubiquitous in various tectonic and climatic settings and are often modeled based on a mass balance relating hillslope convexity to regolith transport and soil production. In order to account for chemical weathering of carbonate rocks and dust input to the regolith, two fluxes that are commonly neglected in settings with silicate-dominated bedrock, we modify this mass balance.We studied 7 study sites in carbonate rocks across an Eastern Mediterranean gradient in the mean annual rainfall (250 to 900 mm yr-1) and dust flux (150 to 40 g m-2 yr-1). Combining cosmogenic 36Cl-derived hilltops denudation rates with an estimate of the regolith chemical depletion and dust fraction based on immobile elements, we predict the hillslope curvature and compare our predictions with observations based on high-resolution airborne LiDAR.Our results demonstrate that soft carbonates (chalk) experience faster denudation rates relative to resistant dolo-limestone. However, the harder carbonates are more prone to chemical weathering, which systematically constitutes around half of their total denudation. Soil production rates exhibit a humped dependency on soil thickness, with an apparent maximum at a depth of 8-16 cm.The observed hillslope curvature vary as function of rainfall and dust flux with a minimum at sub-humid sites with intermediate rainfall of 500-600 mm/yr. The predicted curvature based on our new mass balance is not far from the observed curvature, illustrating the prominent effects of dust flux and chemical weathering on hillslope morphology. Our model also implies that drier sites in the south probably experienced a more complex history of regolith production due dust flux fluctuations.By incorporating dust flux and chemical weathering to the classic hillslope evolution model we identify a complex relation between hillslope curvature, soil production, and climate. These two fluxes are not unique to carbonate bedrock and should be incorporated in hillslope evolution models.

The incision of bedrock channels is typically modeled through the stream power or the shear stress applied on the channel bed. However, this approach is not valid for quasi-vertical knickpoints (hereafter waterfalls), where water and sediments do not apply direct force on the vertical face and waterfall retreat rate is often modeled as a power function of drainage area. These different incision modes are associated with two measurable exponents: the channel concavity, $\theta$, that is measured from the channel topography and is used to evaluate the exponents of drainage area and slope in the channel incision model, and $p$, that is measured from the location of waterfalls within watersheds, and evaluates the dependency of the waterfall recession rate on drainage area. To better understand the relations between channel incision and waterfall recession we systematically compare between the exponents $p$ and $\theta$. These parameters were computed from digital elevation models (30 m SRTM) of 12 river basins with easily detectable waterfalls. We show that $p$ and $\theta$ are: (1) similar within uncertainty, (2) come from a similar distribution, and (3) covary for networks with a large number of waterfalls (>9). In the context of bedrock incision models this hints that the same processes govern waterfall retreat rate and the incision of non-vertical channel reaches in the analyzed basins, and/or that downstream incision can dictate waterfall retreat rate.

Although hillslope evolution has been subject to much investigation for more than a century, the effect of climate on the morphology of soil-mantled hillslopes remains poorly constrained. In this study, we perform numerical simulations of volcanic cinder cones in the Golan Heights (eastern Mediterranean) to estimate soil transport efficiency across a significant north–south gradient in mean annual precipitation (1100 to 500 mm). We use the initial cinder cone morphology (constrained by stratigraphy), the modern hillslope form (surveyed with sub-meter accuracy) and the eruption age (based on 40Ar–39Ar chronology) to predict the best-fit value of the soil transport coefficient (‘diffusivity’) based on a nonlinear transport model. Our results indicate that the best-fit diffusivity (K) varies from 1 to 6 m2 ka−1 among the five cinder cones in our field area. Diffusivity (K) values vary systematically with precipitation and hillslope aspect; specifically, K is higher on south-facing (drier) hillslopes and decreases with mean annual precipitation. We interpret this climate dependency to reflect vegetation-driven variations in apparent soil cohesion, which increases with root network density, and attenuation of rain splash and overland flow erosion, which increases with vegetative ground cover. To assess how vegetative root mass and ground cover vary with precipitation and aspect, we quantified the spatial distribution of NDVI (normalized difference vegetation index) from ASTER satellite images and observed spatial variations that correlate with our calibrated values of K. Analysis of previously studied cinder cones in the USA can be used to extend our framework to arid domains. This endeavor suggests a humped relationship between K and precipitation with maximum diffusivity at mean annual precipitation of 400–600 mm. Copyright © 2017 John Wiley & Sons, Ltd.

Sinkhole generation and land subsidence are commonly attributed to dissolution of subsurface layers by under-saturated groundwater and formation of cavities. Along the Dead Sea (DS) shorelines, this process also involves seasonal flash floods that are drained into the subsurface by existing and newly formed sinkholes. We quantify the contribution of flash-floods to salt dissolution and land subsidence using high-resolution interferometric synthetic aperture radar (InSAR). Subsidence rates during a 3-year period (2012–2015) were calculated from 57 COSMO SkyMed X-band interferograms bracketing major flood events and intra-flood periods in 21 sinkhole sites. The sites are located within channels and alluvial fans along the western shores of the Dead Sea, Israel. The observed subsidence reaches maximum rates of ~ 2.5 mm/day, accumulating in specific sites to 500 mm/year. In most of the sinkhole sites a gradual increase in the annual subsidence rate is observed during the 3-year study period. Three different modes of response to floods were observed: (1) sites where floodwater is not directly channeled into sinkholes do not respond to floods; (2) sites adjacent to active channels with sinkholes are unaffected by specific floods but their subsidence rates increase gradually from early winter to mid-summer, and decay gradually until the following winter; and (3) sites in active channels with sinkholes are characterized by an abrupt increase in subsidence rates immediately after each flood (by a factor of up to 20) and by a subsequent quasi-exponential subsidence decay over periods of several months. In these latter sites, subsidence rates after each flood are temporally correlated with alternating groundwater levels in adjacent boreholes. The rapid rise in groundwater head following floods increases the hydraulic gradient of the under-saturated groundwater and hence also the groundwater discharge and the dissolution rate of the subsurface salt layer. A subsequent quasi-exponential water level drop results in similar deceleration in dissolution and subsidence rates, with a similar characteristic decay time of about 150 days. The observed subsidence decay pattern may also be explained by viscoelastic relaxation of the overburden in response to instantaneously-formed dissolution cavities. Utilizing a Kelvin viscoelastic model, we show that the contribution of this process is most probably

The timing and mode of uplift of the Troodos ophiolite are constrained by low-temperature thermochronology combined with geomorphic analysis. Zircon (U-Th)/He and apatite fission track cooling ages in the Troodos plutonic sequence are all Cretaceous (83–106 Ma) and within error of published zircon U-Pb crystallization ages. This indicates early cooling of the oceanic crust and termination of spreading axis magmatism at ~90 Ma. Apatite (U-Th)/He ages decrease with reconstructed crustal depths from ~40 Ma near the top of the sheeted-dike complex to ~4 Ma within the mantle sequence. A prominent inflection point in the age versus depth curve defines the bottom of the exhumed helium partial retention zone and records the onset of rapid exhumation of the main Troodos massif at 6 ± 2 Ma. Inverse thermal modeling supports this conclusion, indicating that the timing of uplift is earlier than previously estimated. The boundaries of the mantle sequence exposed in the core of the Troodos structure closely overlap the boundaries of a concentric zone delineated by high local relief and higher channel steepness indices, indicating differential exhumation and uplift of this area relative to its surroundings. This zone also overlaps with a prominent negative Bouguer gravity anomaly. The timing and pattern of the Troodos ophiolite uplift suggest that it is driven by serpentinite diapirism, possibly triggered by Miocene reactivation of subduction along the Cyprean Arc. The worldwide ubiquity of suprasubduction zone ophiolites may thus reflect the importance of extensive serpentinization at the overthrusting mantle wedge in obduction processes.

We document and analyze the rapid development of a real-time karst system within the subsurface salt layers of the Ze'elim Fan, Dead Sea, Israel by a multidisciplinary study that combines interferometric synthetic aperture radar and light detection and ranging measurements, sinkhole mapping, time-lapse camera monitoring, groundwater level measurements and chemical and isotopic analyses of surface runoff and groundwater. The >1 m/yr drop of Dead Sea water level and the subsequent change in the adjacent groundwater system since the 1960s resulted in flushing of the coastal aquifer by fresh groundwater, subsurface salt dissolution, gradual land subsidence and formation of sinkholes. Since 2010 this process accelerated dramatically as flash floods at the Ze'elim Fan were drained by newly formed sinkholes. During and immediately after these flood events the dissolution rates of the subsurface salt layer increased dramatically, the overlying ground surface subsided, a large number of sinkholes developed over short time periods (hours to days), and salt-saturated water resurged downstream. Groundwater flow velocities increased by more than 2 orders of magnitudes compared to previously measured velocities along the Dead Sea. The process is self-accelerating as salt dissolution enhances subsidence and sinkhole formation, which in turn increase the ponding areas of flood water and generate additional draining conduits to the subsurface. The rapid terrain response is predominantly due to the highly soluble salt. It is enhanced by the shallow depth of the salt layer, the low competence of the newly exposed unconsolidated overburden and the moderate topographic gradients of the Ze'elim Fan.

This work sets out to test the applicability of stable cosmogenic 21Ne for quantifying the rates of surface processes over time scales of 107–108 yr and the potential limitations and pitfalls associated with such time spans. First, we examine several processes in addition to in-situ production during exposure that affect the final measured concentration of 21Ne. We calculate the magnitude of 21Ne produced by interaction with secondary cosmic ray particles after burial (muogenic Ne) and by non-cosmogenic sources (nucleogenic Ne). We also evaluate the fraction of 21Ne lost through diffusion out of the quartz crystal as a function of time and temperature (depth). We then apply our calculations to 21Ne concentrations measured in sediments that were deposited along the northern passive margin of Gondwana during the late Precambrian, Cambrian, and Lower Cretaceous. In light of the measured concentrations in the sediment samples, we discuss the limitations imposed by our calculations and show that 21Ne concentrations measured in Lower Cretaceous samples can be interpreted in terms of surface exposure times or average erosion rates during the time of erosion and transport. In contrast, concentrations measured in Cambrian and Precambrian samples are limited in their use as surface process indicators although they still yield valuable geological information. We conclude that this novel application of in situ stable cosmogenic nuclides holds the potential as a tool for quantifying surface processes and understanding landscape evolution during the deep geological past and provides insight into macro-scale processes that have shaped Earth over the past hundreds of millions of years.

Denudation of carbonate terrains dominated by chemical weathering can be treated as a rain gauge proxy, because the mass lost by dissolution over time is proportional to the time-integrated precipitation. Therefore, the denudation history of carbonate landscapes may provide paleoprecipitation data and shed light on the interactions between climate, denudation, and tectonic uplift. We apply this approach to constrain 10 4 –10 7 yr average precipitation along the western Dead Sea Rift (DSR) margin, which is underlain predominantly by carbonate rocks and maintains a climatic gradient from Mediterranean to hyperarid along its axis and between its windward and leeward flanks. The comparison between total denudation (from a Turonian datum), denudation rate calculations (from in-situ 36 Cl measurements), and present-day mean annual precipitation shows that (1) total denudation correlates with denudation rates and both correlate with precipitation, and (2) over the lifetime of the range (10–20 m.y.), the effective precipitation gradient across the range was higher than at present by at least 40% on average. Along the ∼450-km-long range, the denudation gradient is probably compensated by isostatic uplift. We suggest that along the western DSR margin, and possibly in other carbonate terrains, precipitation (to a threshold precipitation of ∼1000 mm yr –1 ) controls the long-term denudation rate and the subsequent compensating uplift.

Carbonate minerals, unlike silicates, have the potential to dissolve almost completely and with high efficiency. Thus, in carbonate terrains denudation rate and style (the governing process of denudation, mechanical or chemical) should be more sensitive to climatic forcing. Using 36 Cl measurements in 39 carbonate bedrock and sediment samples, we calculate long-term denudation rates across a sharp climatic gradient from Mediterranean to hyper-arid conditions. Our samples were collected along the Arugot watershed, which drains the eastern flank of the Judea Range (central Israel) to the Dead Sea and is characterized by a pronounced rain shadow. Denudation rates of flat-lying bedrock outcrops sampled along interfluves differ by an order of magnitude from ∼20 mm ka −1 in the Mediterranean zone to 1–3 mm ka −1 in the hyper-arid zone. These rates are strongly correlated with precipitation, and thus reflect the importance of carbonate mineral dissolution in the overall denudation process. In contrast, denudation rates of steep bedrock surfaces depend on the hillslope gradient, but only in the hyper-arid climate zone, indicating that mechanical processes dominate the overall hillslope denudation within this zone. The dominance of slope-dependent mechanical erosion in the hyper-arid zone is also reflected by an increase in spatially-average denudation rates from 17–19 mm ka −1 in the Mediterranean–semi-arid zones to 21–25 mm ka −1 in the hyper-arid zone. These higher rates are attributed to clast contribution from steep slopes under arid climate. This suggests an increased importance of mechanical processes to the overall denudation in the hyper-arid zone. We demonstrate that the transition between chemically-dominated denudation to mechanically-dominated denudation occurs between 100 and 200 mm of mean annual precipitation. Long-term denudation rates across the Judea Range indicate that between Mediterranean and hyper-arid climates, chemical weathering rates are limited by precipitation. Nevertheless, in more humid climates, chemical weathering rates are apparently limited by the rates of carbonate mineral dissolution. This study demonstrates that carbonate terrains have the capacity to shift between mechanically and chemically dominated denudation in response to changes in precipitation. Similar transitions in response to changes in temperature or the level of tectonic activity have been previously reported. We suggest that the abrupt nature of such transitions can be primarily attributed to the efficiency of carbonate dissolution processes and the competition between surface and subsurface drainage systems in carbonate terrains.

Using cosmogenic isotopes and solute load analysis, we quantify chemical weathering (solutional erosion) and denudation rates over variable time scales in a tectonically stable, moderate-relief, carbonate terrain (Soreq drainage, Judea Hills, Israel), located in a semihumid Mediterranean climate. Long-term (>10 4 yr) denudation rates were calculated from 36 Cl concentrations in 51 bedrock and sediment samples. Bedrock samples range in elevation (340–850 m), hillslope gradient (0°–30°), and mean annual precipitation (MAP; 500–630 mm) and vary in soil cover thickness (0–75 cm), Mg/Ca ratio (0.0–1.0 mol), clay mineral contents (0–6 wt%), and mechanical strength (41–58 Schmidt hammer rebound units). Soil pCO2 values at a single location during the course of 1 yr, range between 0.4 and 9.0 mmol mol –1 . Average long-term denudation rate of exposed bedrock samples is 21 ± 7 mm k.y. –1 . Field observations and 36 Cl measurements indicate that soil pockets undergo cycles in the rate of deepening, and that over 10 5 yr time scale, average denudation rates beneath soil pockets are similar to those of exposed bedrock. Sediment samples yield even higher denudation rates, which are probably anthropogenically induced, but could also indicate that the sediment source is soil pockets. Long-term denudation rates are decoupled from hillslope gradient, elevation, and rock strength. Denudation rates show a positive correlation with present-day MAP values, exhibit a complex relation with rock Mg content, and show a weak correlation with clay content. Annual chemical weathering rates were calculated from modern-day solute load measured in waters of perched springs and the regional carbonate aquifer. Our results indicate that on annual, decadal, and 10 4 yr time scales, chemical weathering and denudation are controlled by carbonate dissolution, while mechanical processes are far less signifi cant. Overlap between the distributions of HCO3 – concentrations measured in runoff, springs, and the regional aquifer water suggests that chemical weathering focuses at the bedrock surface and therefore is comparable with solutional denudation. This result is in contrast to the features of ancient fl uvial and colluvial activity (steep nonconcave hillslopes and stream profi les and knickzones in the streams) preserved in the present landscape. Such features were formed in response to mid-Pleistocene uplift and could have been preserved due to a decrease in stream power following the formation of subsurface drainage and the lowering in abrasive clast supply that followed the stabilization of hillslopes in the drainage. Long-term denudation rates calculated from exposed bedrock samples are higher by factor of 1.4 relative to annual, contemporary chemical weathering rates. Increased precipitation by a similar factor, averaged over the last glacial and present interglacial, can explain this difference.

The largest Kuiseb River floods initiate at the basin's semi-arid headwater. Downstream, along the hyperarid Namib Desert, these floodwaters are feeding shallow alluvial aquifers, the only available water for human activity and for the natural ecology. Here, we characterize the largest floods and their changing frequency and magnitudes using palaeohydrological methods. Along 120 km of the rivers canyon 35 palaeoflood deposit sites were identified. At five of these sites we conducted stratigraphic and geochronological analyses and flood discharge estimations. The upper bound of the largest flood over the late Holocene is 1475 m 3 s 1 . Over the last 1300 years more than ten floods have exceeded 1250 m 3 s 1 . An additional 33 floods exceeded 400 m 3 s 1 . The last millennium was characterized by one large flood every 30-40 years during the periods 1250-1335, 1355- 1565 and 1715 AD to the present. A slight increase in flood frequency (not magnitude) occurred during 1565- 1715 AD (one large flood in 20 years) and during two short episodes, 1185-1205 and 1335-1355 AD (seven and four floods in 20 years, respectively). These episodes of increased flood frequency are associated with other proxy records of higher water availability in the Namib. Copyright # 2013 John Wiley & Sons, Ltd.

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.

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.

The Dead Sea (drainage area of ~42,200 km2) is a terminal lake fed from the north by the Jordan River. The main water source into the Dead Sea is runoff, and its level is highly sensitive to the annual rainfall in the upper Jordan River drainage basin. Here, we summarize relevant data about present and past surface-water hydrology in the drainage basin and water input into the Dead Sea. The lower Jordan River, with a natural mean annual discharge of ~1100 106 m3 yr−1, drains Mediterranean to semiarid areas in northern Israel, Jordan, Syria, and Lebanon. Additional water is contributed from tributaries such as the Zarqa River (64 106 m3 yr−1). The diversion of water from these and other sources since the 1960s to an infl ow of only ~210 106 m3 yr−1 caused a 20 m decline of the Dead Sea level. The ephemeral Nahal Arava drains hyperarid areas south of the Dead Sea and contributes small volumes of water (~5 106 m3 yr−1) but experiences occasional large fl oods (up to 1000 m3 s−1); the smaller, steep ephemeral tributaries west of the Dead Sea produce relatively large fl oods (up to 775 m3 s−1) due to their wetter headwaters, but their volumes are relatively small (