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We studied fairy circles 20 km west of Sesriem at one of the driest locations of fairy circles in Namibia, at the foot of the popular Sossusvlei dunes. These fairy circles lack the typical hexagonal order of the Namibian fairy circles. After years of drought, their pattern is more similar to that of vegetation rings, due to the sparse vegetation in the area between the circles. Cross-section measurements of the soil water content (SWC) show that the upper layer (12 cm) is very dry (~1%) and much below the wilting point of Stipagrostis ciliata grasses, whereas the deeper soil layer is wetter (4%). The grain size distribution of soil samples taken from inside and outside the fairy circles reveals considerable heterogeneity in the size fractions due to aeolian (wind-driven) sand sorting. The bare soil inside the fairy circles contains coarser grains, and the ground surface is covered by sand megaripples. There is a linear trend between the vertical soil moisture gradient and the median grain diameter. Fine particles trapped on the vegetated edges of the fairy circle result in small nebkhas that increase the soil water retention at the surface. The dry and loose coarser topsoil inside the fairy circles may prevent the recolonization of new seedlings with short root lengths inside the fairy circles. Our results highlight the role of aeolian sand transport and deposition in desert vegetation environments and seem to support the notion that fairy circle formation may be affected by the interplay between sand sorting and soil moisture gradients. [ABSTRACT FROM AUTHOR]

Aeolian sand transport is a major process shaping landscapes on Earth and on diverse celestial bodies. Conditions favoring bimodal sand transport, with fine-grain saltation driving coarse-grain reptation, give rise to the evolution of megaripples with a characteristic bimodal sand composition. Here, we derive a unified phase diagram for this special aeolian process and the ensuing nonequilibrium megaripple morphodynamics by means of a conceptually simple quantitative model, grounded in the grain-scale physics. We establish a well-preserved quantitative signature of bimodal aeolian transport in the otherwise highly variable grain size distributions, namely, the log-scale width (Krumbein phi scale) of their coarse-grain peaks. A comprehensive collection of terrestrial and extraterrestrial data, covering a wide range of geographical sources and environmental conditions, supports the accuracy and robustness of this unexpected theoretical finding. It could help to resolve ambiguities in the classification of terrestrial and extraterrestrial sedimentary bedforms. Megaripples are sand landforms found in wind-blown environments. A newly identified characteristic signature of the underlying bimodal sand transport process is found in the grain-size distribution on megaripples and could lend insight into transport conditions on Earth and other planetary bodies. [ABSTRACT FROM AUTHOR]

Soil-derived dust particles produced by aeolian (wind) processes have significant impacts on humans and the Earth's systems. The soil particle size distribution is a major soil characteristic in dust emission models. Yet empirical information on the dependence of dust emission thresholds on soil particle size distribution is still lacking. The main goal of this study was to explore the dust emission threshold from semi-arid loess soil samples by a targeted wind-tunnel experiment. The results clearly show that the dust emission threshold is associated with the saltation threshold with no distinct direct aerodynamic lifting of the loose dust particle. The dust flux depends on the amount of the clay-silt fraction in the soil, the shear velocity, and the saltation flux under certain shear velocity. The study aimed to advance our understating of the dust emission processes, and to provide empirical information for parametrization in dust emission models and for management strategy of soils in preventing dust emission. [ABSTRACT FROM AUTHOR]

Dust emission resulted from soil erosion by wind with significant impacts of soil (nutrient) loss and air pollution of particulate matter (PM). The ejection of dust from soil aggregates due to saltation has been hypothesized to play a major role in dust emission. Yet empirical information on the role of different aggregate sizes in dust emission is still lacking. The main goal of this study was to explore the dust emission threshold in different aggregate sizes of a semiarid loess soil. To this end, we conducted targeted wind-tunnel experiment on dust emission. The results show that dust emission from aggregate at size of 63–250 µm, 250–500 µm, and 500–1000 µm is enabled only under the conditions of saltation. The dust-PM threshold at shear velocities of 0.24–0.52 m/s depends on the aggregates size. Aggregates at the size of saltators (125–500 µm) were the most productive in dust generation by the mechanism of aggregate disintegration. In our bulk sample, the aggregate group of 63–250 µm has the highest contribution to the total dust emission. The study aimed to advance our capability in soil resources management and for model parameterization in dust emission schemes. [ABSTRACT FROM AUTHOR]

This Special Issue on soil erosion invites novel and original articles based on physical and chemical theories, field and laboratory experimental, soil analyses, and/or statistical and mathematical modeling that advance our knowledge on dust control and sand stabilization. [ABSTRACT FROM AUTHOR]

Dust emission by aeolian (wind) soil erosion depends on the topsoil properties of the source area, especially on the nature of the aggregates where most dust particles are held. Although the key role of soil aggregates in dust emission, the response of soil aggregation to aeolian processes and its implications for dust emission remain unknown. This study focuses on aggregate size distribution (ASD) analyses before and after in-situ aeolian experiments in semiarid loess soils that are associated with dust emission. Wind tunnel simulations show that particulate matter (PM) emission and saltation rates depend on the initial ASD and shear velocity. Under all initial ASD conditions, the content of saltator-sized aggregates (63–250 μm) increased by 10–34% due to erosion of macro-aggregates (> 500 μm), resulting in a higher size ratio (SR) between the saltators and macro-aggregates following the aeolian erosion. The results revealed that the saltator production increases significantly for soils that are subjected to short-term (anthropogenic) disturbance of the topsoil. The findings highlight a decrease in soil aggregation for all initial ASD's in response to aeolian erosion, and consequently its influence on the dust emission potential. Changes in ASD should be considered as a key parameter in dust emission models of complex surfaces. [ABSTRACT FROM AUTHOR]

Using ground-penetrating radar, optically stimulated luminescence dating, particle-size distribution and morphological analysis, the study of the construction phases of a vegetated linear dune in the arid north-western Negev dunefield of Israel during the last millennium improves current knowledge about vegetated linear dunes that developed in the late Pleistocene. Vertical accretion in rapid pulses forming horizontally bedded units along the axis of vegetated linear dunes, regardless of their age, was found to be characteristic of vegetated linear dunes. The combination of the unique topographic feature of a longitudinal 5 m step-like fall in dune crest elevation with the substantial narrowing of dune width constitutes a distinct morphological marker for interpreting local dune growth and stabilization of the last, albeit localized, dune mobilization episode at ca 0·5 ka. Evidence for lateral dune migration was not observed. Where local sediment supply exists, short episodes of powerful winds within the Holocene (with recurrence intervals separated by hundreds of years) can lead to the construction of vegetated linear dunes. The spatially constrained extent of such young dunes in the north-western Negev may be due to limited sand availability because most of the Negev dunes were stable during the Holocene. These findings imply that vegetated linear dune construction can occur in glacial and interglacial (including Holocene) environments in semi-arid to arid climates if certain conditions are met. In times of increased wind power during the Anthropocene, a period characterized by simultaneous rises in the human impact on the landscape and in climate variability (i.e. drought), local growth of vegetated linear dunes can be expected. This study demonstrates that ground-penetrating radar is a reliable tool for interpreting the shallow internal structure of young vegetated linear dunes. [ABSTRACT FROM AUTHOR]

Eolian sand fractions along the west-east transport system of the northern Sinai Peninsula--northwestern Negev erg of Egypt and Israel were analyzed in this study with regard to source, dune geomorphology, eolian transport, and paleoclimate. The studied erg is composed of active linear (seif) dunes in northern Sinai (western part), and stabilized vegetated linear dunes in the NW Negev dune field (eastern part). Linear seif dunes differ from vegetated linear dunes in their vegetation cover, linearity, internal structure, and dynamics. Sand samples were analyzed for sand-grain morphology and particle-size distribution. Although both dune types are continuous landforms with similar orientations and sand-grain roundness values, the linear dunes of Sinai are coarser grained than the Negev vegetated linear dunes. The vegetated linear dunes have a variable but higher proportion of very fine sand (50-125 µm) content and a varying but lower sand fining ratio (defined as the ratio of fine sand percentage to very fine sand percentage). From these observations, we infer that fractionation of sand occurred along the studied eolian transport path during periods of enhanced windiness. Very fine sands are suggested to have been transported by saltation and low suspension from source deposits and sand sheets. We suggest that a significant proportion of the very fine sand fraction of Nile Delta sands has been transported downwind through northern Sinai during the late Pleistocene, especially when linear dunes reached the NW Negev due to last-glacial period windiness and probably larger sediment supply. Generally decreasing wind velocities and increasing precipitation along the west-east dune transport path enhanced vegetative cover in the northern Negev and enabled deposition of the very fine sand component within the dunes and probably further downwind. Our results suggest that particle-size distribution can elucidate much about erg history over time scales of a glacial-interglacial cycle, especially in cases where the sand provenance is of a single dominant source. [ABSTRACT FROM AUTHOR]

Abstract Mineral dust emission is a major process in determining the global dust cycle. At a global scale there is still uncertainty about the absolute and relative contribution of dust from different source areas and landforms. Dust sources are mainly considered as surfaces containing relatively high percentages of fine particles (e.g. Playas). Yet, active (dune) sands have been identified recently, by remote sensing studies, as dust sources in northern Africa, China, and elsewhere. Previous studies on dust emission from active sands suggested that dust can be generated by different aeolian mechanisms that are related to (i) re-emission of settled dust particles, (ii) clay coating removal, and (iii) abrasion of the sand grains. However, there is only limited information of the relative importance of the different mechanisms for producing this dust under natural aeolian (wind) conditions. This study integrates wind tunnel experiments and high resolution laboratory sand analyses to explore aeolian dust emission from active sands with conditions simulating the natural processes of saltation and explores the role of the different dust emission mechanisms. Sand samples from three sites with different characteristics of grain size, dust content, morphology, and mineralogy were used in the experiments. No dust emission was recorded for shear velocities below the saltation threshold. The initial content of dust-sized particles (>63 μm) in the sand sample was found to influence PM 10 emission. PM 10 concentrations were increased with the initial content of dust-sized particles in the sand. The experiments identify clay coatings removal as the dominant mechanism over time of dust emission in typical active sand dunes (<2% dust content) with an addition of re-emission of existing dust-sized particles (<63 μm). The results also suggest that aeolian abrasion play only a minor role in PM 10 dust generation from active sands, producing mostly relatively course dust-sized particles (>40 μm). The dust emission observed in this study indicates that, in addition to the classic dust sources of non-sandy soils, sand bodies should also be taken into consideration in determining global dust emission. Highlights • Dust emission from sands is enabled only under shear velocities that are above the saltation threshold. • The initial content of dust-sized particles in the sand sample influence the rate of PM 10 flux. • Clay coating removal and re-emission of settled dust were found as the key mechanisms in dust emission from active sands. • Sand bodies should be taken into consideration in global dust emission models. [ABSTRACT FROM AUTHOR]