Differential water distribution over dune slopes as affected by slope position and microbiotic crust,Negev Desert,Israel

Runoff is one of the main water sources responsible for water redistribution within a given ecosystem. Water redistribution is especially important in arid regions, and may be of great importance on sandy dunes, where the likelihood of runoff is low owing to the high infiltration rates of sand. Redistribution of water may significantly affect plant and animal distribution, and may explain vegetation patterns within an ecosystem. Runoff yield over sandy dune slopes in the western Negev Desert was measured under natural conditions during 1990–1994. The magnitude of runoff yield on different slope sections and on north and south exposures was established. The results demonstrate that while slope position controlled the microbiotic crust cover, crust cover and crust biomass controlled the amounts of runoff obtained. Whereas no runoff was measured on the upper dune sections devoid of crust, only meagre quantities were measured on the midslope sections, characterized by discontinuous crust cover. Substantially larger amounts were, however, obtained at the bottoms of the slopes, characterized by continuous crust cover. North‐facing slopes, usually characterized by a chlorophyll a content of 29–41 mg m⁻², yielded on average 3·2 times more runoff than south‐facing footslopes, characterized by a 17 mg m⁻² chlorophyll a content. Whereas microbiotic crust was found to be responsible for runoff generation, additional water supply owing to runoff may also explain the occurrence of a high biomass crust and the dense vegetation belt at the dune–interdune interface of the northern exposure, where runoff tends to collect. Thus, whereas crust may reduce infiltration in certain habitats, runoff generated by crust may also be responsible for the promotion of crust growth in other habitats. Runoff may also be used to promote vegetation growth at the dune footslopes. The possibility of using runoff to facilitate agroforestry is discussed. Copyright © 1999 John Wiley & Sons, Ltd.     

A high magnitude storm and flood in a hyperarid catchment,Nahal Zin,Negev Desert,Israel

In October 1991 a high magnitude rainstorm flood, estimated return period 40 years, occurred in Nahal Zin, a 1400 km² catchment in the hyperarid Negev Desert. The meso-scale structure of the storm was a curved squall line that developed from a thunderstorm in accordance with the topography of the catchment divide, by which it was strongly affected. Tropical moisture reached the area via the subtropical jet stream, in conjunction with a lower level northward intrusion of the Red Sea trough (RST-N) into the Mediterranean Sea. Rainfall, as measured at the few and sparse gauging stations, was much too small to account for the resulting large flood. Peak flow and other hydraulic characteristics of the flood were indirectly reconstructed. The techniques of palaeoflood hydrology used were based on sedimentological evidence of fine-grained flood sediments deposited in back-flooded tributaries, as well as on other stage indicators. The HEC-2 procedure was employed to determine water surface profiles. The spatial and temporal characteristics of the event were studied through a combination of rainstorm analysis, remote sensing, hydrological and sedimentological data; they jointly explain the magnitude and timing of tributary contributions producing the integrated flood in the main channel. The flood as reconstructed reveals a three-peak hydrograph: two peaks were generated by the same storm but had different floodwave arrival times in the main channel; the third resulted from a local rainstorm which occurred on the following day and covered only one tributary. The curved structure of the storm and its dynamics in relation to catchment orientation resulted in storm move- ment in tandem with the floodwave. The synchronous contribution from all main tributaries preserved evidence of the floodwave both in stage and volume by replacing the transmission losses in the sections with thick alluvium. Other high magnitude floods on record for the large Negev Desert catchments are caused by a cold upper air incursion associated with the RST-N. Most of them occur in the autumn and are caused by storms with high-intensity rainfall. This is in stark contrast with the flooding behaviour of the semi-arid zone further north, which is linked primarily to the core of the Mediterranean winter. The complexities involved in the generation of a large desert flood, as revealed by this study, illustrate the fallacy of applying routine hydrological modelling to such events, and underline the need to study the processes involved in adequate detail. © 1998 John Wiley & Sons, Ltd.     

Runoff generation in a sandy area—the nizzana sands,Western Negev,Israel

This paper presents the results of sprinkling experiments conducted over the lower part of vegetated and crusted linear dunes as well as over flat playa surfaces that appear in the interdune corridors. Data obtained show that these two surface units respond quickly to rainstorms. Runoff generation can be expected for any storm exceeding 2-3 mm and runoff coefficients are high. When the topsoil algal crust, 1 to 2 mm thick, is removed from the surface of the dune, infiltration increases drastically and eliminates any possibility of runoff generation under present-day rainfall conditions. This data may be of great help in the understanding of the geomorphology and sedimentary sequence of the corridors separting linear dunes.     

Holocene fluvial valley fill sources of atmospheric mineral dust in the Skeleton Coast,Namibia

Western Namibia is a significant global source of atmospheric mineral dust. We investigate the relationship between dust and source sediments, assessing the sustainability of dust flux. Remote sensing studies have highlighted specific ephemeral fluvial systems as important contributors to dust flux, including highlighting sections of valleys that are the origins of dust plumes in the period 2005–2008. Little is known however about the specific within‐valley dust sediment sources, particularly whether dust is derived from modern ephemeral channel floors or older valley fill sediments, many of which have been reported in the region. As part of a region‐wide analysis of aeolian dust flux, we investigate the sediment properties of atmospheric dust samples and valley sediments from the Huab valley, one of the principal regional dust sources. Trapped dust samples contain up to 88% very fine sand and silt when collected samples are disaggregated prior to analysis. Valley fill surface samples comprise 80% very fine sand and silt, and the surface of the modern ephemeral channel 30%. Valley fill sediments were sampled at depths up to 3.6 m below the present surface and reveal Holocene depositional ages from 0.6 ± 0.03 ka back to 9.79 ± 0.73 ka. These sediments contain 30% to 6% very fine sand and silt, with levels decreasing with depth and age. Aeolian bedforms in the valley system (nebkhas on the fill surface and climbing dunes on valley margins) indicate that aeolian processes under the influence of strong seasonal easterly winds likely result in dust being winnowed out of the valley fill surfaces, with sandy bedforms being constructed from the coarser component of the fill sediments. The volume of valley fill sediment suggests dust sourced from Holocene sediments is likely to continue into the future regardless of flow conditions in the modern channel system. Copyright © 2017 John Wiley & Sons, Ltd.     

The effect of shrub canopy upon surface temperatures and evaporation in the Negev Desert

Although known as ‘islands of fertility’ or ‘resource islands’, information regarding the effect of shrubs upon microclimate in deserts is scarce. Here we report on measurements of evaporation and temperatures that were carried out in and around a pair of shrubs at the Nizzana research site in the western Negev Desert during 1993–94 and during the growing season (November–March) of 1994–95 and 1996–1997. Whereas evaporation was measured monthly using mini‐atmometers (10 cm diameter and 10 cm tall) at an exposed site and under and around the shrub (at the eastern, southern, western and northern aspects), temperature was measured under a shrub canopy, at its northern aspect, and at an exposed habitat. Evaporation was aspect dependent with increasing rates in the following order: exposed > south‐facing > west‐facing ≈ east‐facing > north‐facing > under canopy. Except from the northern aspect, the under‐canopy habitat showed substantially lower rates of evaporation in comparison with all other habitats. The differences between the under‐canopy and the exposed habitat were larger during wintertime (with the under‐canopy habitat having 0·53 times the evaporation rate than that of the exposed habitat) although higher differences in temperatures characterized both habitats in summertime (up to 14·4 °C in summer as compared with 6·9 °C only in winter). The results were explained by extended surface wetness that characterized the under‐canopy habitat following rainstorms. While already being dried out at the exposed habitat, surface wetness at the under canopy habitat persisted for several days afterwards, resulting, following one rainstorm, in vapour pressure of 2·15–2·39 kPa in comparison with only 0·82–0·83 kPa of the exposed habitat. The substantially lower evaporation rates that characterize the under‐canopy habitat may thus play a pivotal role in providing preferential conditions for lush under‐canopy annual growth. Copyright © 2008 John Wiley and Sons, Ltd.     

Climatology of yellow sand (Asian sand, Asian dust or Kosa)in East Asia

In order to study climatology of yellow sand (Asian sand, Asian dust or Kosa) in EastAsia, secular fluctuation in China, Korea and Japan in the recent 30 years was presented. Thenumber of days with sand-dust storm at five stations in China-Hotan, Zhangye, Minqin, Jurhand Beijing, decreases a lot at the former three stations, but changed little at the latter two stations.Suggesting that the recent global warming is more evident in Xinjiang and Gansu, where the fre-quency of cold air invasions from the higher latitudes is decreasing. But, the eastern parts ofMongolia, inner Mongolia, and North China encounter stronger cyclones in early spring as a resultof global warming. These cyclones bring cold air from higher latitudes, causing severe duststorms. Secular variation in the annual days with sand-dust storms in China and Kosa days in Ko-rea and Japan show a parallel change with higher frequency from 1975 to 1985. This may be re-lated to the higher frequency of La Nina years. However, different tendency was shown in theperiod from 1986 to 1996. Since 1996 or 1997, a sharp increase is clear, which may be caused bythe developed cyclones in East Asia as well as human activities, and stronger land degradationunder La Nina conditions. Anomalies of the total number of stations with Kosa days were dis-cussed in accordance with some synoptic meteorological conditions such as the differences be-tween Siberian anticyclone and Aleutian cyclone center at 500 hPa level during the previous winter.     

The palaeoflood record of a hyperarid catchment,Nahal Zin,Negev Desert,Israel

The palaeohydrology of Nahal Zin, a 1400 km² catchment in the hyperarid Negev Desert, is inferred from slackwater deposits and palaeostage indicators in a canyon near its lower end. The palaeoflood record, augmented by the instrumental and historical records of the last decade, includes 28 floods ranging from 200 to 1500 m³s⁻¹ over the last 2000 years. This helps to reanalyse the frequency of floods in this drainage system. The clusters of floods around 1000 years BP and again during the last 60 years are characterized by high flow magnitudes. Periods with many floods correspond well to periods with high Dead Sea levels and are probably relatively wet periods, while periods with few floods correspond well to low Dead Sea levels indicating a drier climate. Fluctuations in the frequency of floods are typical of periods of transition from one climate regime to another. Copyright © 2000 John Wiley & Sons, Ltd.     

Superposition of aeolian dust ripple patterns as a result of changing wind directions

The effect of a change of wind direction on the orientation (and some other morphometric characteristics) of aeolian dust ripples is investigated. Ripple formation is simulated in a wind tunnel on surfaces that are already characterized by a previously established rippling. The effect of a wind rotation of 45 degrees, 90 degrees, and 180 degrees is examined. It is found that wind rotations of 45 and 180 degrees will lead towards a ripple alignment perpendicular to the last air flow. A wind rotation of 90 degrees, however, leads towards a ripple alignment perpendicular to the resultant wind direction. The faster the wind blows, the more this effect is expressed. No distinct relationship exists between the asymmetry of a dust ripple (and a dust ripple field) and the direction of the wind blowing as the ripples formed. As the orientation and asymmetry of aeolian dust ripples do not necessarily reflect the air flow direction during which the ripples formed, great care should be taken when reconstructing wind directions from such aeolian ripples.     

Aeolian dust transport and deposition over crete and adjacent parts of the mediterranean sea

Monitoring of dust deposition at several stations on Crete over a three year period has shown that the present-day depositional flux is of the order of 10-100 gm⁻² yr⁻¹. Most of the dust deposition takes place during a few annual dust [events] which typically last for 1-3 days. Dust haze episodes are usually associated with southerly or southwesterly winds which transport fine sediment from North Africa. Dust is raised by a wide variety of meteorological conditions which generate strong near-surface winds in the source areas, but major long-range transport events are often associated with cold fronts linked to the passage of deep mid-latitude depressions during winter and spring. Dust haze frequency and deposition rates are highest in western Crete and decrease towards the east, suggesting that transport from Tunisia and neighbouring parts of North Africa is particularly important. The measured rate of dust deposition is well below the minimum level required for loess formation. Deposits which have previously been identified as loess are shown to be uncemented marine marls of Tertiary age. Laboratory analysis of red soils, surface sediments, and bedrock samples has confirmed that many of the soils contain an important aeolian dust component, but it is concluded that a more important source of soil parent material is provided by weathering of local rocks. Many of the soils contain significant amounts of quartz sand which cannot have been transported across the sea from North Africa. Reworking of weathered material and deposited dust is extensive, and is accomplished by both aeolian and fluvial processes. Tectonically-controlled depressions in the mountains and parts of the coastal lowlands have acted as long-term sinks in which a thickness of several metres of sediment and soil has accumulated during the Quaternary.     

Modelling dynamic water redistribution patterns in arid catchments in the Negev Desert of Israel

In arid climate regions, redistribution of runoff water is highly relevant for vegetation development. The process of water redistribution at catchment scale is studied with the landscape process model LAPSUS, mainly used for erosion and sedimentation modelling. LAPSUS, formerly applied in Mediterranean climates, is modified to deal with the arid climate of the Negev Desert of Israel. Daily event based model runs were used instead of yearly model runs, and the infiltration module was modified to better represent the spatial diversity in water availability in an arid catchment. The model is calibrated for two small catchments in the Negev Desert of Israel, Halluqim and Avdat. First, a sensitivity analysis of the modified LAPSUS is performed. Pore volume appears to have an especially strong influence on the modelling results. Second, the capability of LAPSUS to deal with varying surface characteristics is assessed by comparing the water redistribution patterns in the two catchments with field data. Simulation results demonstrate that the catchments respond very differently to precipitation. Water redistribution is larger in the dominantly bedrock covered Halluqim compared to the dominantly loess covered catchment of Avdat. Consequently, Halluqim has more positions with water accumulation than Avdat, and can sustain a larger vegetation cover, including Mediterranean species. Finally, the modelled infiltration patterns are compared with vegetation cover in the catchments. The results indicate that there is a broad agreement between infiltration and vegetation patterns, but locally there is a strong mismatch, indicating that some of the involved processes are still missing from the model. Copyright © 2007 John Wiley & Sons, Ltd.     

Microbiotic crusts as biomarkers for surface stability and wetness duration in the Negev Desert

Microbiotic crusts play an important role in arid and semi‐arid regions. Yet, very little information exists regarding the factors that impact their development. In an attempt to assess the main factors that may determine their growth, measurements of the amount of fines (silt and clay), rain, moisture content, wetness duration and wind erosion and deposition were carried out along a 12 station transect within a partially crusted dune field in the western Negev Desert and compared to the crust cover and chlorophyll content. Surface stability was the only variable that exhibited significant relationship with crust cover while daylight wetness duration exhibited strong positive relationship (r² = 0·92–0·99) with the crust's chlorophyll content. The data point out that microbiotic crusts may serve as a useful biomarker for surface stability. While wetness duration and wind will control crust cover and the crust chlorophyll content in semi‐stable habitats (with absolute annual change in sand level of 2–3 mm), stable habitats (absolute change <1 mm) will be controlled primarily by moisture, while habitats with low surface stability (absolute change of tens and hundreds of millimeters) will be primarily controlled by wind. Furthermore, owing to the strong positive relationship between daylight wetness duration and the crust's chlorophyll content, the crust may serve as a useful biomarker for the quantification of surface wetness duration. Copyright © 2009 John Wiley & Sons, Ltd.     

Factors controlling the formation of coppice dunes (nebkhas) in the Negev Desert

Due to their role in increasing fertility, coppice dunes (nebkhas) are regarded by many researchers as important contributors to aridland ecosystems. Yet, despite their frequent occurrence, little information exists regarding the rate and factors that control their formation. The goal of the current study is to examine the formation rate and factors that determine the establishment of coppice dunes in the Hallamish dune field in the western Negev Desert. The rate in which sand and fines, hereafter aeolian input (AI) was trapped and its particle size distribution (PSD) were examined by means of the solidification of 2 m × 2 m plots using surface stabilizers, and by the installation of three pairs of artificial shrubs (SH), three pairs of artificial trees (TR), and a pair of control (CT) plots. Measurements were annually conducted during June 2004 and June 2008, with monthly collection during June 2004 and May 2006. The PSD was compared to coppice dunes located on the fine‐grained playa surface. AI was trapped at SH, while it was not trapped at TR and CT. The annual rate of AI accretion under the canopy was highly variable ranging between 1405 and 13 260 g m^?2, with a four‐year average of 5676 g m^?2, i.e. 3.8 mm a^?1. It depended upon the wind power, with drift potential having a threshold velocity of U_t > 10 m s^?1 yielding the higher correlations with the monthly AI (r² = 0.59–0.84). No significant relations were obtained between the monthly AI and shrub height. Sand saltation, suspension and creep are seen responsible for mound formation, which based on the current rates of sand accretion are relatively fast with a 60 cm‐high coppice dune forming within ~150–160 years. The current data highlight the problematic design of some previous research using conventional traps and confining the measurements only to certain seasons. Copyright © 2015 John Wiley & Sons, Ltd.     

Dust emission from wet and dry playas in the Mojave Desert,USA

The interactions between playa hydrology and playa‐surface sediments are important factors that control the type and amount of dust emitted from playas as a result of wind erosion. The production of evaporite minerals during evaporative loss of near‐surface ground water results in both the creation and maintenance of several centimeters or more of loose sediment on and near the surfaces of wet playas. Observations that characterize the texture, mineralogic composition and hardness of playa – surfaces at Franklin Lake, Soda Lake and West Cronese Lake playas in the Mojave Desert (California), along with imaging of dust emission using automated digital photography, indicate that these kinds of surface sediment are highly susceptible to dust emission. The surfaces of wet playas are dynamic surface texture and sediment availability to wind erosion change rapidly, primarily in response to fluctuations in water‐table depth, rainfall and rates of evaporation. In contrast, dry playas are characterized by ground water at depth. Consequently, dry playas commonly have hard surfaces that produce little or no dust if undisturbed except for transient silt and clay deposited on surfaces by wind and water. Although not the dominant type of global dust, salt‐rich dusts from wet playas may be important with respect to radiative properties of dust plumes, atmospheric chemistry, windborne nutrients and human health. Published in 2007 by John Wiley & Sons, Ltd.     

Lateral migration of linear dunes in the Strzelecki desert,Australia

Linear dunes in the Strzelecki Desert trend roughly south-north. Sand transport, which is toward the NNE, has caused the dunes to migrate eastward while they extend or migrate northward. Eastward lateral migration is evidenced by: (1) asymmetrical shape of the dunes; east-facing slopes are several times as steep as west-facing slopes; (2) asymmetrical accumulation of loose recently transported sand (relatively abundant on east-facing slopes); (3) asymmetrical outcropping of older semiconsolidated aeolian sand on the dune surface (more abundant on west-facing slopes); and (4) east-dipping foreset beds that underly the west-facing flanks of some dunes. Dunes in the Strzelecki Desert are still active in the sense that sand is transported along and across many dune crests. However, the dunes are composed primarily of Pleistocene strata, indicating that the trend of the dunes was established before the Holocene. The obliquity of the dunes to the transport direction is not merely an aberration of the wind regime of the last few decades. Preferential accumulation of sand on east-facing flanks indicates that the dunes migrated eastward several metres during the Holocene. Moreover, the west-facing flanks of some dunes have experienced a minimum of tens of metres of erosion. This asymmetric erosion and deposition were caused by dune obliquity and lateral migration that may have begun as early as the Pleistocene. Dunes in the Strzelecki Desert and in the adjacent Simpson Desert display a variety of grossly different internal structures. Computer graphics experiments demonstrate that many of these differences in structure can be explained by different angles of climb of the dunes.     

Evolution and degradation of flat‐top mesas in the hyper‐arid Negev,Israel revealed from 10Be cosmogenic nuclides

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.     

Runoff‐induced sediment yield over dune slopes in the Negev Desert. 1: Quantity and variability

Although extensive data exist on runoff erosion and rates for non‐sandy hillslopes, data for arid dune slopes are scarce, owing to the widespread perception that the high infiltrability of sand will reduce runoff. However, runoff is generated on sandy dunes in the Hallamish dune field, western Negev Desert, Israel (P ≈ 95 mm) due to the presence of a thin (usually 1–3 mm) microbiotic crust. The runoff in turn produces erosion. Sediment yield was measured on ten plots (140–1640 m²) on the north‐ and south‐facing slopes of longitudinal dunes. Two plots facing north and two facing south were subdivided into three subplots. The subplots represented the crest of the active dune devoid of crust, the extensively crusted footslope of the dune, and the midslope section characterized by a patchy crust. The remaining plots extended the full length of the dune slope. No runoff and consequently no water‐eroded sediments were obtained from the crest subplots devoid of crust. However, runoff and sediment were obtained from the mid‐ and footslope crusted subplots. Sediment yield from the footslope subplots was much higher than from the midslopes, despite the higher sediment concentration that characterized the midslope subplots. The mean annual sediment yield at the Hallamish dune field was 432 g per metre width and was associated with high average annual concentrations of 32 g l^?1. The data indicate that owing to the presence of a thin microbiotic crust, runoff and water erosion may occur even within arid sandy dune fields. Copyright © 2001 John Wiley & Sons, Ltd.     

The role of playas in pedogenic gypsum crust formation in the Central Namib Desert: a theoretical model

The formation of Namibia's extensive pedogenic gypsum crusts (CaSO₄·2H₂O) is interpreted in a new light. It is suggested that gypsum primarily precipitates at isolated points of evaporitic concentration, such as inland playas, and that deflation of evaporitic‐rich gypsum dust from these playas contributes to the formation of pedogenic gypsum duricrusts on the coastal gravel plains of the Namib Desert surrounding these playas. This study establishes the nature, extent and distribution of playas in the Central Namib Desert and provides evidence for playa gypsum deflation and gravel plain deposition. Remote sensing shows the distribution of playas, captures ongoing deflation and provides evidence of gypsum deflation. It is proposed that, following primary marine aerosol deposition, both inland playas and coastal sabkhas generate gypsum which through the process of playa deflation and gravel plain redeposition contributes to the extensive pedogenic crusts found in the Namib Desert region. Copyright © 2001 John Wiley & Sons, Ltd.     

Terrace pediments in Makhtesh Ramon,central Negev,Israel

Terrace pediments occupy approximately 30 per cent of the bottom of the Makhtesh Ramon erosional cirque in the central Negev Desert, Israel. River terraces and terrace pediments are genetically connected landforms, where each terrace pediment corresponds with a fluvial terrace of the same relative height. A pediment and river terrace constitute a geomorphic pair and should be regarded as chronometrically synchronous morphological elements. The formation of the terrace pediment staircases is controlled mainly by local base level changes. The present‐day configuration and overall morphology of Makhtesh Ramon formed in the early stages of its development by both stream erosion and subsequent pedimentation. Less significantly, modification by intermittent erosion alternating with periods of stability, resulted in deepening of the Makhtesh Ramon bottom. The present‐day stepped relief throughout the Makhtesh valley is, thus, a composite feature. The overall rate of terrace pediment formation in Makhtesh Ramon ranges from 0·05 to 0·10 mm a⁻¹. Copyright © 2000 John Wiley & Sons, Ltd.     

Runoff‐induced sediment yield over dune slopes in the Negev Desert. 2: Texture,carbonate and organic matter

Runoff‐induced sediments were collected in the Hallamish dune field for four years (1990–1994). Runoff and consequently water‐transported sediments were generated on the dunes owing to the presence of a thin microbiotic crust. These sediments were analysed for their particle‐size distribution and carbonate content. In addition, the organic matter content was calculated by measuring the chlorophyll content within the runoff. The results were compared to the slope parent material, i.e. the crust and the underlying sand, as well as to playa sediments, which are scattered within the Hallamish interdunal areas, and which were previously hypothesized to originate from runoff‐induced sediments. Higher amounts of fines (silt and clay) and carbonate characterize the footslopes in comparison to the midslopes. Intermediate contents of fines (17 per cent) and carbonate (8 per cent) characterized the sediments in comparison to the fines (27 per cent) and carbonate (15 per cent) of the crust and to the fines (4 per cent) and carbonate (4 per cent) of the underlying sand. The runoff‐induced fines and carbonate contents were significantly different from those of the playas, suggesting that the playa flats do not originate from runoff‐induced sediments. The sediments were enriched with organic matter. Organic matter which originates from the crust amounted to 0·3–0·4 per cent as compared to ≤0·1 per cent in the bare sand. Nevertheless, the crust was found to be relatively resilient to water flow. Only 0·1–0·5 per cent of the crust was annually eroded off the slope by water, with south‐facing crusts showing higher resilience than north‐facing crusts. The data may thus assist in the evaluation of the crust's residence time. Copyright © 2001 John Wiley & Sons, Ltd.     

Test of high sampling density OSL dating of aeolian samples from the south margin of the Tengger Desert using the global standardised growth curve (gSGC) method

Chronostratigraphic records in the drylands of north China provide basic archives to reveal the dynamic connections between climate changes and the behaviour of aeolian systems. However, the interpretation of aeolian chronostratigraphy is not straightforward and may be associated with significant uncertainty due to a number of external and localised forces. Taking into account this complexity of aeolian systems, excluding preservation and sampling bias from palaeoclimatic signals requires that the interpretation of chronostratigraphic records be based on as many dates from as many sites as possible. Optically stimulated luminescence (OSL) dating provides a direct way to date aeolian sediments. However, the determination of equivalent dose (D_e) using the commonly applied single-aliquot regenerative dose (SAR) protocol is relatively time-consuming for samples with burial doses exceeding 50 or 100 Gy.In this study, the global standardised growth curve (gSGC) method is applied to multi-grain aliquots of coarse quartz from various sites covering a wide region of the south margin of the Tengger Desert in north China to rapidly determine D_e for a large number of aliquots and samples. The large between-aliquot variability in the shape of the dose response curve (DRC) is significantly reduced using a least-squares normalisation procedure. The results of a leave-one-out cross-validation (LOOCV) protocol demonstrates that D_e values determined using the gSGC and SAR methods are very consistent with each other up to at least 200 Gy and 400 Gy, for the 90–150 μm and 63–90 μm fractions, respectively. It suggests that the gSGC protocol can be used as an efficient procedure for D_e determination of a large number of aeolian samples from multiple sites. This in turn provides a better constraint for the interpretation of Late Quaternary aeolian chronostratigraphic records in drylands of north China.