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Fractures in rock are ubiquitous; from cold dry planetary bodies to the hottest, wettest climates on Earth, and from km-scale tectonic fractures deep in Earth’s crust to microcracks in surficial rocks. Yet, many of these fractures propagate progressively over geologic timescales, making their development complex and enigmatic. Therefore, to measure how fractures have developed in rocks exposed at Earth’s surface over millennia, and how this consequently changes rock physical properties, we collected ten ~25 cm diameter granitic boulders from two sites in the Eastern Sierra, California, USA. The boulders were deposited on the surface of alluvial terraces and fans during geologically instantaneous glacial and alluvial events at different times since about 148ka BP, then the depositional surfaces were subsequently abandoned. The chronosequences of geomorphic surfaces provide a natural laboratory in which rocks of consistent lithology have been exposed to similar environmental conditions for different lengths of time, allowing us to compare rock property evolution on the order of 0 to 105 years of environmental exposure; an approach that allows us to better understand and characterize mechanical weathering processes, especially long-term changes in rock fracturing. Note that fresh (time-zero) rocks in this study are represented by boulders found within active channels, and that the measured changes in rocks with longer exposure times are interpreted by comparison with the fresh rocks. Focusing only on similarly sized boulders removes any ambiguities in tectonic and exhumation history that might arise in outcrop samples, thus ensuring that rocks from each site have experienced similar stress conditions; namely those restricted to the environment.We performed laboratory measurements on 10 granitic boulders (four from Lundy Canyon, with exposure ages of ~0 to ~148 ka; six from Shepherd Creek, with exposure ages of ~0 to ~117 ka) to quantify how rock physical properties changed as a function of environmental exposure age. We measured key parameters commonly used as proxies for crack damage, including porosity, compressional wave velocity (Vp), and shear wave velocity (Vs). We hypothesize that changes in crack damage are likely to affect rock mechanical properties, so we also measured tensile strength, uniaxial compressive strength (UCS), and Young’s modulus (E). We find that all measured parameters evolve as a function of exposure age, with systematic increases in porosity, and systematic decreases in Vp, Vs, tensile strength, UCS, and E. For example, porosity increases from 0.5 – 1.0 % in the fresh rock to 2.6 – 3.2 % in the oldest rocks. We interpret these changes as reflecting progressive subcritical crack growth that arises due to ubiquitous, but relatively low magnitude, environmental stresses continuously acting on the boulders, as opposed to differences inherited before their erosion from bedrock.Apart from demonstrating the importance of environmentally driven cracking in rock weathering, these observations of progressive crack damage accumulation also have significant implications for the interpretation of any measurements made on rocks exposed at Earth’s surface, even if the age of exposure is relatively short compared to the age of the geologic deposit itself.

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.

Direct measurements of boulder entrainment in desert wadis are not available. The 2004 flood (peak discharge - 470 m3 s-1; recurrence interval - 120 years) in the hyperarid, ungauged Nahal Hatzera ephemeral stream (45 km2), which drains the Small Erosional Crater (SEC; Makhetesh in Hebrew) in the Negev Desert Israel, transported and deposited 0.85-2.1 m concrete boulders and slabs detached from infrastructure upstream as well as natural boulders. EDM and drone air-photographic surveys documented the geometry of the study reach and the location of boulders. Analyses of flood slackwater deposits established a paleoflood record of 23 floods with peak discharges of 200-760 m3 s-1, during the last 600 years. 1-D HEC-RAS hydraulic analysis provided water surface profiles, discharges and hydraulics, along the study reach and velocity, shear stress and stream power for each boulder Xsection.. MAX program and Pearson 3 distribution were used for flood frequency analysis. Most of the concrete boulders were deposited in the sub-critical backwater of channel constrictions where velocities were 1.5-2.1 m s-1. The largest boulders were deposited in super-critical flow where velocity was 8-9.2 m s-1. The sandy alluvial channel enables to transport these concrete boulders, reflecting the unstable, active sandy layer of the channel bed over which the boulders moved. The maximum flood shear stress and stream power characterize medium-large floods with return period of 20-120 years rather than the largest floods, which produce larger extension of backwater ponding upstream of constrictions. The combination of the flood frequency analysis and the hydraulic parameters of the flood enables to reconstruct the frequency of the tractive forces that transported these boulders. Boulders

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.

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.

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.

Rectangular drainage networks are characterized by right-angle channel bends and confluences. The formation of the rectangular pattern is commonly associated with orthogonal sets of fractures, making rectangular drainages an outstanding example of structurally controlled landform evolution. However, the association between geologic structures and rectangular patterns remains circumstantial. So far, no specific mechanisms were suggested to explain the linkage between the emergent right-angle bends and confluences and the preexisting fracture system. This gap is particularly significant for planetary rectangular drainages, where the association with preexisting structures can not be directly observed.We investigated the mechanistic linkages between geologic structures and the geomorphic drainage pattern in the hyper-arid Ami'az Plain located within the Dead Sea Basin in SE Israel. The Ami'az Plain is incised by a seemingly rectangular canyon system and is also penetrated by hundreds of sub-vertical clastic dikes (mode-I opening cracks infilled with sedimentary material), that reach a width of up to 0.18 m. Additionally, many caves and cavities extend from the banks and heads of the canyon system. Based on field surveys and analysis of a high resolution LiDAR based DEM, we mapped and characterized the Ami’az Plain drainage network and associated geomorphic structures including sinkholes. Our analysis revealed that the canyon system exhibits rectangular characteristics and its tributaries share dominant orientations with the strike of the clastic dikes. Surface and subsurface mapping assisted by Ground scanning LiDAR, together with field experiments, demonstrated that the caves and sinkholes are spatially associated with clastic dikes and that the caves formed by piping erosion along dikes.Based on these findings, we propose a three-component hydrologic-geomorphic model for the formation of the Ami’az Plain rectangular drainage network: First, clastic dikes act as efficient infiltration pathways for surface runoff into the subsurface, where subsurface flow along clastic dikes induces internal erosion and forms piping caves. Second, collapses of cave roofs create sinkholes. Coalescence of sinkholes and seepage erosion in places where dikes intersect canyon banks and canyon heads generate new tributaries and extend existing ones. Finally, fluvial erosion and bank collapse modify the drainage network. Our observations and model emphasize the critical role of subsurface erosion and the formation of caves and sinkholes in linking fractures to drainage pattern evolution. This linkage could be highly consequential for our understanding of rectangular drainage evolution on planetary and terrestrial surfaces.

Slip partitioning along the northern boundary of the Tibetan Plateau is essential for understanding regional deformation and the seismic potential of the different faults that accommodate it. Within this framework the Altyn Tagh Fault (ATF) is commonly viewed as the primary structure that separates the Tibetan Plateau from the stable Gobi-Alashan block to the north. Late Quaternary sinistral slip rates of 8-12 mm/yr across the central ATF between 86° and 93°E decrease eastwards to zero as the fault approaches its mid-continental termination at ~97°E. To better understand how late Quaternary slip is partitioned along the ATF’s eastern termination we obtained new slip-rate measurements for the ~200-km-long left-lateral ENE striking Sanweishan Fault (SSF) located ~60 km north of the ATF between 94°-96°E near the town of Dunhuang.Multiple sinistral offsets ranging up to 600 m were identified by linking the clast assemblage of offset alluvial fan remnants with their provenance upstream of the fault. Luminescence dating revealed depositional ages ranging from 100 - 200 ka for the offset features and time-invariant minimum sinistral slip of 2.5±1 mm/yr during the last ~200 ka, which is approximately an order of magnitude higher than previously reported slip-rates for the SSF. Our results indicate that the SSF and the eastern segment of the ATF accommodate comparable magnitudes of late Quaternary slip. Considering this substantial transfer of lateral slip as far as 60 km north of the eastern ATF we propose that the SSF may represent juvenile northeastward expansion of the Tibetan Plateau into previously stable parts of the Gobi-Alashan block.

Water volume estimates of shallow desert lakes are the basis for water balance calculations, important both for water resource management and paleohydrology/climatology. Water volumes are typically...

Many of the world’s drylands are characterized by interior drainage systems that terminate at shallow desert lakes or playas. Except for episodic flooding these largely ephemeral water bodies, remain mostly dry. Surveying and mapping their respective floor topography in a suitable resolution for calculating water balance is a difficult and laborious task. As this is crucial for water resources management and reconstructing paleoenvironmental conditions, diverse methods and efforts were applied. However, detailed, high-quality bathymetric surveys in such environments are rare and have only been conducted in a few such lakes. This is primarily due to their shallow, low-relief, large areas, and often remote characteristics, which complicate application of conventional topographic surveying techniques. Therefore, satellite-based remote sensing is an essential complementary approach for deriving bathymetry of such lakes.Global digital elevation models, such as NASA’s Shuttle Radar Topography Mission (SRTM) or ASTER’s GDEM, are unsuitable for accurate measurements of these ephemeral lakes, mainly because of their impenetrability to water and their high signal-to-noise ratio in the low-relief environments. Recent studies addressed these complications by combining remote sensing data with local calibrations of in-situ measurements, or alternatively, by relating shoreline altitudes with precise altimetry. This approach requires a spatial interpolation of individual measurements. Therefore, it is prone to errors that demand intensive efforts to be reduced; even then the errors may remain larger than the actual depth of a flooded lake. Moreover, such methods are hard to apply in complex lakes with multiple sub-basins.To tackle these problems, we developed a simple methodology to derive a high-resolution (30 m per pixel) bathymetry of shallow desert lakes. In this new method we combine two sources of data: a >30-yr record of Landsat-derived surface water occurrence data; and accurate high-resolution elevation data, acquired by the NASA’s recently launched ICESat-2 satellite (Ice, Cloud, and Land Elevation Satellite-2). We test the proposed new method over three ephemeral lakes around the world: Lake Eyre, Australia, with its complex shallow lake system, consisting of a few sub-basins; Sabkhat Al-Mellah, Algeria; Lago Coipasa, Bolivia. The accuracy of the resulted bathymetric maps was evaluated using cross-validation of ICESat-2 scans, yielding low RMSD values of ~0.3 m, versus ~2.5 m of the SRTM data (validated through other ICESat-2 scans). At Lago Coipasa, we show that bathymetry was effectively determined even when the lake was full of water (up to a few meters depth). This high-resolution, low-error bathymetry mapping complement other globally available topographic data.

Direct measurements of boulder entrainment in desert wadis are not available. The 2004 flood (peak discharge – 470 m s; recurrence interval – 120 years) in the hyperarid, ungauged Nahal Hatzera ephemeral stream (45 km), transported and deposited 0.85–2.1 m concrete boulders and slabs detached from infrastructure upstream and natural boulders. EDM and drone air-photographic surveys documented the geometry of the study reach and the location of boulders. Analyses of flood slackwater deposits established a paleoflood record of 23 floods with peak discharges of 200–760 m s, during the last 600 years. 1-D HEC-RAS hydraulic analysis provided water surface profiles, discharges and hydraulics, along the study reach and velocity, shear stress and stream power for each boulder. MAX program and Pearson 3 distribution were used for flood frequency analysis. Most of the concrete boulders were deposited in the sub-critical backwater of channel constrictions where velocities were 1.5–2.1 m s. The largest boulders were deposited in super-critical flow where velocity was 8–9.2 m s. The alluvial channel enabled to transport these concrete boulders, reflecting the unstable, active sandy layer of the channel bed over which the boulders moved. The maximum flood shear stress and stream power characterize medium-large floods with return period of 20–120 years and not for the largest floods, as expected. Boulders about 2.1 m and weighing about 15 t can be transported at least once in 120 years. The shear stress and stream power indicate that the moderate-large floods are the most geomorphically effective floods rather than the largest floods in Nahal Hatzera basin. Nevertheless, the ‘geomorphic effectiveness’ of the 2004 flood – a typical desert flash flood with high peak and short duration, was small based on the minor changes along the channel and banks indicating that their resistance thresholds were not exceeded and energy expenditure was mainly on boulders entrainment and transport., The authors wish to thank the Royal Geographical Society Thesiger-Oman International Fellowship UK, for fund no. THES03/16, to P.A.

Validation of emissivity (&epsilon, ) retrievals from spaceborne thermal infrared (TIR) sensors typically requires spatial extrapolations over several orders of magnitude for a comparison between centimeter-scale laboratory &epsilon, measurements and the common decameter and lower resolution of spaceborne TIR data. In the case of NASA&rsquo, s Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) temperature and &epsilon, separation algorithm (TES), this extrapolation becomes especially challenging because TES was originally designed for the geologic surface of Earth, which is typically heterogeneous even at centimeter and decameter scales. Here, we used the airborne TIR hyperspectral Mako sensor with its 2.2 m/pixel resolution, to bridge this scaling issue and robustly link between ASTER TES 90 m/pixel emissivity retrievals and laboratory &epsilon, measurements from the Algodones dune field in southern California, USA. The experimental setup included: (i) Laboratory XRD, grain size, and TIR spectral measurements, (ii) radiosonde launches at the time of the two Mako overpasses for atmospheric corrections, (iii) ground-based thermal measurements for calibration, and (iv) analyses of ASTER day and night &epsilon, retrievals from 21 different acquisitions. We show that while cavity radiation leads to a 2% to 4% decrease in the effective emissivity contrast of fully resolved scene elements (e.g., slipface slopes and interdune flats), spectral variability of the site when imaged at 90 m/pixel is below 1%, because at this scale the dune field becomes an effectively homogeneous mixture of the different dune elements. We also found that adsorption of atmospheric moisture to grain surfaces during the predawn hours increased the effective &epsilon, of the dune surface by up to 0.04. The accuracy of ASTER&rsquo, s daytime emissivity retrievals using each of the three available atmospheric correction protocols was better than 0.01 and within the target performance of ASTER&rsquo, s standard emissivity product. Nighttime emissivity retrievals had lower precision (&lt, 0.03) likely due to residual atmospheric effects. The water vapor scaling (WVS) atmospheric correction protocol was required to obtain accurate (&lt, 0.01) nighttime ASTER emissivity retrievals.

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

Quantitative geomorphic field studies and modeling efforts have focused on the margins of southwestern Africa as an example for landscape evolution in prolonged aridity conditions and tectonic quiescence of passive margins. These efforts concluded that this region is a prime example of a steady state landscape, in which relief changes extremely slowly. Using cosmogenic isotopes, these studies suggested overall landscape exhumation rates of 5–10 m Ma−1 over the past 105–106 yrs. Slightly slower rates on flat-lying exposed bedrock surfaces and faster exhumation rates along the Namibian Great Escarpment as well as on steep slopes of granitic inselbergs, such as the Gross Spitzkoppe are also documented. Here we explore the state of “steady state” in central Namibia. Concentrations of 10Be were measured in bedrock and sediment samples collected throughout the watershed of the Ugab River (~29,000 km2), which drains the highlands of central Namibia and flows to the Atlantic Ocean. Samples were collected from the main stem of the ephemeral Ugab River, from the slopes and streams draining the Brandberg, which is the largest inselberg in the Namib, and from smaller inselbergs around it. We also sampled several other formerly large, but currently subdued, inselbergs such as the Messum Crater. 10Be concentrations in sediment transported along the axial Ugab River indicate that its drainage basin erodes uniformly at 5–6 m Ma−1 and sediment transport from its headwaters source to the ocean is rapid. 10Be concentrations measured in sediment transported in ephemeral streams draining the Brandberg indicate its erosion at ~4 m Ma−1. However, slower rates of 1–3 m Ma−1 were measured for bedrock samples collected from (a) flat lying bedrock surfaces within the Brandberg, (b) top of small tors that rise only a few meters above their surroundings, and (c) exhumed and denuded large magmatic complexes such as the Messum Crater. Furthermore, we found that bedrock buried under grus in the hyperarid zone of Namib ( We therefore suggest that the “steady state” landscape along the Namibian passive margin be viewed as follows: The entire landscape erodes slowly, generally at ~5 m Ma−1 and this maintains the view of steady state. Small differences in erosion rates between the landscape elements result in very slow and only small changes in relief over time scales ≥106 yrs. We find that the large inselbergs and the Great Escarpment erode primarily by retreat of steep slopes and cliffs within the drainage basins while preserving relief over considerable timescales. In the wetter upper reaches of the Namibian drainage systems, erosion of buried rock is most likely increased by the vegetation-covered soil.

We study in this paper land degradation processes in arid environments that are leading to a major environmental change. These processes, which are mostly the outcome of gully development, have direct impact on agriculture potential, biomass degradation, and escalating desertification. Three representative sites within the Negev Highland, Israel, are studied by utilizing terrestrial laser scans to gain detailed spatial information, accurate estimations, and concisely document overarching trends. The resulting information allows us to thoroughly characterize and quantify the geomorphic and vegetative changes and to calibrate rates and trends that have only been roughly estimated thus far. The scans, which were taken over a four-year span (2009–2013), cover three different types of hydrological seasons and provide unique insights on development rates and subsequent ramifications. The characteristic progression of natural land degradation in the Negev Highlands and the techniques used to document and analyze it can be extended globally to regions undergoing similar transitions.

Erosion and inland retreat of coastal cliffs present one of the most dynamic earth-surface processes presently challenging coastlines. The rate of cliff retreat is central to coastal planning and protection of shoreline infrastructure. Until now, the majority of retreat rate estimates have been based on aerial photos from the past century, and therefore do not provide multicentury estimates of retreat rates. Here, we studied Bronze Age to Crusader archeological sites (∼3700 years) located on Israel's coastal cliff and used their spatial relation to the cliff to estimate the long-term centurial-millennial retreat rates of the cliff. To accomplish this, original layouts of partially eroded archaeological structures were reconstructed and compared to the modern coastal cliff line. The eroded parts of the studied structures and their original age constrained the maximum timing of the retreat. The resulting retreat rates are significantly lower than those previously calculated using observations of around 100 years. The archeological data also record the episodicity of the cliff failure events. The research highlights both the issue of the loss of valuable archaeological cultural resources, and simultaneously the usefulness of eroding coastal archaeological features to resolve questions of modern significance.

We quantify the volumetric erosion of Israel's actively retreating Mediterranean coastal cliffs between 2006 and 2015 with airborne LiDAR. Our results reveal annual sand contribution of ~ 45,000 m3 from the coastal cliffs into the Nile's littoral cell (NLC), which amounts to ~ 50% of the annual long-shore sand transport previously measured at the northern termination of the NLC. Accordingly, we find that implementation of cliff-retreat mitigation engineering efforts planned for Israel's coastline can lead to a sand deficit in the NLC that may result in increased beach erosion.

We present a new dating approach for desert alluvial surfaces that utilizes radar image data to obtain regional-scale correlations between dated surfaces and surfaces of unknown ages. The study was carried out along the Arava segment of the Dead Sea Transform north of the Gulf of Elat (Aqaba) using Advanced Land Observation Satellite (ALOS) Phased Array type L-band Synthetic Aperture Radar (PALSAR) data with 6–12 m/pixel resolutions. Nineteen independently dated Quaternary morphostratigraphic alluvial units (5 to 1910 ka) from eight different sites with variable lithologies (carbonate/magmatic/chert) were examined in order to develop, test and validate the proposed approach. Expanding on previous studies that established the characteristic smoothing of desert alluvial surface through time and the capability of measuring such roughness variations with radar, we employed a pixel-aggregation approach that yielded a robust (R 2 = 0.95) power-law relation between the average radar backscatter value from the surface (‘ AR ’) and the abandonment age (‘ T ’) of the unit as measured with independent methods such as luminescence and/or cosmogenic radionuclide dating. We found that: 1) This AR - T correlation can be inverted to obtain calibrated radar-based T estimates as old as ~ 1.5 Ma with 35% uncertainty; 2) ALOS PALSAR 6 m/pixel images acquired at incidence angles of 34–38° yielded optimal results; 3) a sample of at least 70 contiguous radar pixels was required to capture the natural roughness variability of the surfaces examined, and 4) surface lithology did not exert a primary effect on T inversions. Validation experiments yielded radar-based ages of 3 ± 1, 91 ± 32, 484 ± 169 and 1604 ± 561 ka that compare to Holocene, 56 ± 14, 540 ± 60 and 1590 ± 250 ka ages previously determined for the same surfaces, respectively. We propose that with region-specific calibrations, spaceborne radar data can be used to quantitatively estimate the age of abandoned alluvial units across regional scales.

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.

Large-scale engineering projects such as mines and dams cause ecological damage that can persist after construction is complete. The Qinghai–Tibet Railway QTR from Golmud to Lhasa was constructed entirely during the Landsat era and began operating in 2006. Therefore, it presents an opportunity for determining both the extent of ecological damage and the time required for some level of natural restoration after a major construction project. We have studied the effect of the QTR construction on vegetation abundance measured using multi-endmember spectral mixture analysis MESMA of a time series of Landsat TM/ETM+ images 2001, 2007, 2010 covering the 231 km stretch of the railway from south of Kaixinling Station to north of Chumaerhe Station. We found that the effect of QTR construction on vegetation abundance was limited to within 5.0 km of the tracks, the largest decrease, 2.9%, occurring within 0.125 km of the tracks from 2001 to 2007. There was only 0.4% further decrease in vegetation abundance from 2007 to 2010. We attributed the decrease in vegetation abundance within 0.125 km of the tracks mostly to the accumulation of drifting sand resulting from the barrier of the railway and the measures adopted to keep the sand away from the railway under the prevailing west or northwest winds. It appears that in this sensitive cold desert, vegetation damage was limited to the period of railway construction.

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.

Namibia, with its passive margin setting, long-term tectonic stability, and long-lasting arid climate is a typical granitic landscape characterized by numerous dominating inselbergs. The Namib has been the focus of quantitative studies on the overall rates of bedrock exhumation, escarpment retreat, and sediment generation, transport, and deposition. Results from these studies indicate steady bedrock erosion rates ranging between 1 and 5 mm kyr− 1 over the last 105–106 yr. This rate was determined from samples collected mostly from small inselbergs with low relief. How fast the large and “classic” inselbergs, which dominate the Namib Desert landscape, erode, generate sediment, and contribute to the overall sediment supply has yet to be systematically examined. We document erosion rates of the Gross Spitzkoppe (GS), one of the largest inselbergs in the Namib Desert, Namibia, based on measured concentrations of 10Be in samples collected from bedrock, boulders, and surface grus. The results suggest a slow lowering rate (1–2 mm kyr− 1), while the overall slope processes deliver sediment to the base of the cliff with 5–6 × 105 atoms g− 1 quartz. This concentration is equivalent to an average erosion rate of ~ 8 mm kyr− 1, 2–3 times faster than the bedrock lowering rate. Thus, the inselberg has been eroding by cliff retreat. The concentration of cosmogenic isotopes in slope sediment is controlled by exposure time on the slope, weathering of exposed bedrock, and weathering of subsurface bedrock. Sediment continues to accumulate cosmogenic isotopes while being transported on the flat pediments surrounding the inselberg. Within 1–2 km from the base of the GS, the concentration doubles to ~ 10 × 105 atoms g− 1 quartz. The increase is achieved by dosing due to exposure and mixing with highly dosed sediment eroded from near subsurface bedrock. From this area, grus is transported through a network of low gradient channels to the Atlantic Ocean, ~ 100 km away. Cosmogenic isotope concentration in sediment increases to ~ 30 × 105 atoms of 10Be g− 1 quartz, which is 3–4 times higher than that measured in the grus close to the GS and is controlled by the erosion rate of stable bedrock throughout the entire drainage basin. Dosing due to exposure during transport obviously contributes to the overall measured concentration in the sediment as it approaches the ocean.

Emissivity spectra recovered from spectral radiance images may have lowered spectral contrast due to irradiance from nearby surface elements ‘cavity effect’. For analysis based only on photointerpretation or Reststrahlen band identification, it is not always necessary to account for cavity effects, but for full spectral analysis it may be desirable. We present an approach to compensate thermal infrared TIR images for cavity radiation. This approach is based on optical estimates of subpixel surface roughness and estimation of cavity contribution for different natural surfaces using a TIR radiosity model. It was tested using tripod-mounted Hyper-Cam Telops, Inc., Quebec City, Canada hyperspectral TIR images of natural targets from the Mojave Desert, California, USA, along with centimetre-scale digital elevation models of similar targets measured by ground lidar. For remote subpixel roughness estimation, Advanced Spaceborne Thermal Emission and Reflection Radiometer ASTER nadir-and aft-looking 27.6° near-infrared NIR brightness ratios, as well as synthetic aperture radar SAR images calibrated to roughness root mean square RMS, were used. The TIR compensation approach is adaptable for different spectral resolutions, including hyperspectral.

The Dead Sea Transform (DST) accounts for ~105 km of left-lateral slip between the Arabian plate and the Sinai subplate since the Miocene. Paleoseismic studies along the Arava Valley segment of the DST suggest that late Quaternary deformation has been primarily concentrated along the axis of the transform valley. Here, we examine late Quaternary changes in drainage system characteristics and attribute them to recent tectonic deformation in this region. Field-based geomorphic mapping, topographic cross sections, and optically stimulated luminescence (OSL) dating of fluvial deposits were used to map and date recent changes in the fluvial characteristics of catchments along the western margin of the southern Arava. Our results reveal coeval migration of channels, consistent with tectonically induced surface tilting caused by north–south compressional deformation along the western margin of the transform valley. OSL dating indicates this tilting was initiated in the late Pleistocene and continued at least into the mid-Holocene. The late Quaternary tectonic deformation along the southern Arava segment of the DST is distributed across a wider zone than previously considered and extends out to the margins of the transform valley. We associate the inferred wider deformation zone to possible changes in the geometry of motion along the DST.

An hyperspectral imaging spectrometer measuring in the longwave thermal infrared (7.6-11.6 mum), with a spatial resolution less than 5 mm at a range of 10 m, was used in the field to observe the variability of emissivity spectra of individual rock surfaces. The rocks were obtained commercially, were on the order of 20 cm in size, and were selected to have distinct spectral features: they include alabaster (gypsum), soapstone (steatite with talc), obsidian (volcanic glass), norite (plagioclase and orthopyroxene), and ldquojasperrdquo (silica with iron oxides). The advantages of using an imaging spectrometer to characterize these rocks spectrally are apparent. Large spectral variations were observed within individual rocks that may be attributed to roughness, surface geometry, and compositional variation. Nonimaging spectrometers would normally miss these variations as would small samples used in laboratory measurements, spatially averaged spectra can miss the optimum spectra for identification of materials, and spatially localized components of the rock can be obscured.

Virtually all remotely sensed thermal infrared (IR) pixels are, to some degree, mixtures of different materials or temperatures: real pixels are rarely thermally homogeneous. As sensors improve and spectral thermal IR remote sensing becomes more quantitative, the concept of homogeneous pixels becomes inadequate. Quantitative thermal IR remote sensors measure radiance. Planck's Law defines a relationship between temperature and radiance that is more complex than linear proportionality and is strongly wavelength-dependent. As a result, the area-averaged temperature of a pixel is not the same as the temperature derived from the radiance averaged over the pixel footprint, even for blackbodies. This paper uses simple linear mixing of pixel elements (subpixels) to examine the impacts of pixel mixtures on temperature retrieval and ground leaving radiance. The results show that for a single material with one temperature distribution and with a subpixel temperature standard deviation of 6 K (daytime images), the effects of subpixel temperature variability are small but can exceed 0.5 K in the 3-5 µm band and about a third of that in the 8-12 µm band. For pixels with a 50 : 50 mixture of materials (two temperature distributions with different means) the impact of subpixel radiance variability on temperature retrieval can exceed 6 K in the 3-5 µm band and 2 K in the 8-12 µm band. Sub-pixel temperatures determined from Gaussian distributions and also from high-resolution thermal images are used as inputs to our linear mixing model. Model results are compared directly to these broadband thermal images of plowed soil and senesced barley. Finally, a theoretical framework for quantifying the effect of non-homogeneous temperature distributions for the case of a binary combination of mixed pixels is derived, with results shown to be valid for the range of standard deviations and temperature differences examined herein.

The (550‐530Ma) alkaline magmatic suite of the Amram Massif, southern Israel, was emplaced during the transition from an orogenic to an intra-plate tectonic setting in the northeastern Arabian‐Nubian Shield (ANS). The suite ranges from 45 6t o 788 wt % SiO2, and consists of rhyolites, alkali quartz syenites, quartz syenites, monzonites, and co-magmatic mafic to felsic alkaline dikes. These rocks define a continuous chemical evolutionary trend and reveal a correlation between decreasing stratigraphic age and increasing silica content. The felsic members of the suite display A-type characteristics and are genetically linked through fractionation to the more mafic ones. Moderately positive initial eNd values (a2 05), low initial 87 Sr/ 86 Sr values (07036 2), high MgO and Fe2O3 concentrations (410‐895 and 100‐125 wt %, respectively) and relatively flat rare earth element patterns [(La/Yb)na 64 09] in the Amram mafic dikes (456‐495 wt % SiO2), suggest their derivation from the sub-continental lithospheric mantle, above the garnet stability zone. The MELTS program was used to quantitatively model the chemical evolution of the suite. Extensive anhydrous fractionation (490%), of plagioclase, alkali-feldspar, clinopyroxene, olivine, and minor Timagnetite and apatite from parental mafic magmas, represented by the Amram mafic dikes, produced the rhyolitic compositions as well as the intermediate members of the suite. This suggests the presence of a large unexposed body of cumulate rocks at depth, as well as fusion of a large source-region (equivalent to an 5km layer) in the lithospheric mantle. Regarded as a representative example for similar A-type outcrops in this region, this petrogenetic model further suggests that Neoproterozoic‐Early Cambrian A-type magmatism in the northeastern ANS represents a significant post-orogenic addition of mantlederived material to the juvenile crust. This magmatic episode was of a similar magnitude to that of the Cenozoic, extensionrelated, alkaline volcanism of the Arabian plate.

Alluvial fans constitute important recorders of tectonic and climatic signals. Thus, determining the age of alluvial deposits is a common and pivotal component in many quantitative studies of recent tectonic activity, past climatic variations and landscape evolution processes. In this study we build on the established relation between surface age and surface roughness and examine the use of radar backscatter data as a calibrated proxy for constraining the age of alluvial surfaces in such environments. This study was conducted in the hyper-arid environment of the southern Arava rift valley north of the Gulf of Aqaba. ALOS-PALSAR L-Band dual-polarized (i.e., HH, HV) data with different incidence angles (24°, 38°) and resolutions (6.25m, 12.5m) were examined for 11 alluvial surfaces, for which surface ages ranging from 5-160 ka were previously determined. As expected, radar backscatter in such low-relief hyper-arid desert environments responded primarily to SR at pixel-scales and below. Nonetheless, measured backscatter values for single pixels were found to be unsuitable proxies for surface age because of the natural variability in SR across alluvial units of a given age. Instead, we found the statistical properties of radar pixel populations within a given unit to be the most effective proxies for surface age. Our results show that the mean backscatter value within representativeROI’s (region of interest) provided the best predictor for surface age: Lower mean backscatter values correlated well with older and smoother alluvial surfaces. The HHpolarized image with ~38° incidence angle and 6.25 m/pixel resolution allowed the best separation of surface ages. This radar-based approach allows us to quantitatively constrain the age of alluvial surfaces in the studied region at comparable uncertainty to that of “conventional” surface dating techniques commonly used.

The formation of low-albedo slope streaks represents one of the most active surface processes presently observed on Mars. Ongoing debate about the origin of such streaks centers on whether they are the products of: 1) erosional processes related to ‘dry’ mass-wasting exposing a dark-toned substrate beneath a thin dust mantle; or 2) ‘wet’ processes associated with liquid seeps. Here, we employ hyperspectral CRISM images to determine the spectral properties of individual slope streaks, constrain their composition, and test possible formation mechanisms. Our results demonstrate that the slope streaks analyzed here form a spectrally distinct and compositionally unique class of active Martian surface features. Darkening through exposure of a pre-existing substrate, textural effects and/or persistent soil moisture are all unambiguously ruled out by the spectral observations, which instead point towards a transparent surface coating or enrichment in low-albedo ferric oxides as the most likely and spectrally permissible mechanisms.

The unresolved topographic expression of surfaces, surface roughness (SR), is a fundamental surface property that conveys useful information for a wide range of Earth and planetary sciences. Yet, this information is difficult to measure remotely because most spaceborne imagers have resolutions on the order of meters to hundreds of meters and SR can vary significantly below these scales. One way to measure SR is to exploit differential shadowing in stereo images, and in particular, Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), with its nadir (3N) and backward-looking (3B) near-infrared channels. We have proposed a simple ratio of land-leaving radiance in those two channels as a measure of relative SR at scales

Emissivity spectra recovered from spectral radiance images may have lowered spectral contrast due to irradiance from nearby surface elements (“cavity effect”). For analyses based only on photointerpretation or Reststrahlen band identification, it is not always necessary to account for cavity effects, but for full spectral analyses, including spectral unmixing, it may be desirable. We present a method that is under development for compensating thermal infrared images for cavity radiation based on optical estimates of surface roughness and model inversion for percent cavity contribution. The approach is adaptable for different spectral resolutions, including hyperspectral. It will be tested on tripod-mounted Telops HyperCam hyperspectral thermal-infrared images of natural targets, LiDAR DEMs of similar targets, and optical estimates of shadowing, related to roughness.