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

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

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.

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

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

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

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

Although various episodes of wet Quaternary climates have been suggested in studies of the Negev Desert, here we demonstrate that Reg soils, developed on flat alluvial surfaces and sensitive to minor changes in precipitation, indicate that the southern Negev has been permanently hyperarid at least since the middle Pleistocene. The wetter episodes were restricted to the northern Negev, currently mildly arid. Gypsic-saline Reg soils that de- veloped on stable surfaces during .200 k.y. in the southern Negev are cumulative and polygenetic, but none exhibits even the weakest development of calcic horizons. Under current climate conditions in the Negev, calcic soils are widespread in areas with rainfall .80 mm yr 21 . The fact that the hyperarid (,50 mm yr 21 ) southern Negev lacks any calcic horizons indicates that it never experienced an average rainfall of $80 mm yr 21 during the middle-late Pleistocene, whereas the northern Negev was wetter. This Negev dichotomy is explained by the main physical features controlling rainfall over the Negev, i.e, the southward-decreasing depth of the atmospheric boundary layer with distance from the Mediterranean, and the altitude of the central Negev Highland (1000 m). The interaction between these two features often prevents the passage of rain clouds into the southern Negev. The Holocene and Pleistocene gypsic-salic soil distribution across the Negev closely matches the current circumstances. We suggest that hyperaridity has prevailed over the southern Negev since the last stages of the uplift of the central Negev Highland in the late Pliocene-early Pleistocene.