the factors influencing the abrasion efficiency of saltating grains on a clay crusted playa the entrainment and subsequent transport of pm10 particulate matter 10 m has become an important and challenging focus of research for both scientific and practical applications arid and semi arid environments are important sources for the atmospheric loading of pm10 although the emission of this material is often limited by surface crusts it has been suggested that the primary mechanisms through which pm10 is released from a crusted surface are abrasion by saltating grains or disturbance by agricultural and recreational activities to examine the importance of saltation abrasion in the emission of pm10 a series of field wind tunnel tests were conducted on a clay crusted surface near desert wells arizona in a previous part of this study it was found that the emission rate varies linearly with the saltation transport rate although there can be considerable variation in this relationship this paper more closely examines the source of the variability in the abrasion efficiency the amount of pm10 emitted by a given quantity of saltating grains the abrasion efficiency was found to vary with the susceptibility of the surface to abrasion the ability of the sand to abrade that surface and the availability of material with a caliper size 10 m within the crust specifically the results of the study show that the abrasion efficiency is related to the crust strength the amount of surface disturbance and the velocity of the saltating grains it is concluded that the spatial and temporal variability of these controls on the abrasion efficiency imposes severe contextual limitations on experimentally derived models and can make theoretical models too complex and impractical to be of use copyright 2001 john wiley sons ltd

the effects of soluble salts on the threshold shear velocity of fine sand wind tunnel tests were carried out to investigate the effects of two soluble salts nacl and kci on the threshold shear velocity of a well sorted fine sand a sensitive laser monitoring system was used to detect initial grain motion during the tests results indicate that even small amounts of soluble salt can significantly increase the threshold velocity of the sand because of cement like bonds formed between grains that tend to hold individual particles in place however with increasing concentration the salt crystal growth causes the initially smooth sand surface to develop an irregular frothy texture which also directly affects the threshold shear velocity this is indicated by increased scatter of data on the threshold shear velocity surface salt concentration plots for both nacl and kcl although data scatter is evident significant exponential relationships do exist between threshold shear velocity and surface salt concentration in each case

the physics of wind blown loess implications for grain size proxy interpretations in quaternary paleoclimate studies abstract loess deposits are recorders of aeolian activity during past glaciations since the size distribution of loess deposits depends on distance to the dust source and environmental conditions at the source during transport and at deposition loess particle size distributions and derived statistical measures are widely used proxies in quaternary paleoenvironmental studies however the interpretation of these proxies often only considers dust transport processes to move beyond such overly simplistic proxy interpretations and toward proxy interpretations that consider the range of environmental processes that determine loess particle size distribution variations we provide a comprehensive review on the physics of dust particle mobilization and deposition furthermore using high resolution bulk loess and quartz grain size datasets from a last glacial interglacial sequence we show that because grain size distributions are affected by multiple often stochastic processes changes in these distributions over time allow multiple interpretations for the driving processes consequently simplistic interpretations of proxy variations in terms of only one factor e g wind speed are likely to be inaccurate nonetheless using loess proxies to understand temporal changes in the dust cycle and environmental parameters requires i a careful site selection to minimize the effects of topography and source distance and ii the joint use of bulk and quartz grain size proxies together with high resolution mass accumulation rate calculations if possible

wind tunnel evaluation of a wedge shaped aeolian sediment trap abstract a detailed set of tests in a wind tunnel investigated the characteristics of flow and sampling efficiency of a low cost sediment trap that can be used in wind tunnels and field applications the trap is constructed of thin sheet aluminum and has a wedge shape 32 internal inlet angle with a 20 500 mm rectangular inlet orifice that extends 40 mm beyond the wedge form relatively unimpeded airflow through the trap is achieved by the large exit area 130 500 mm which is covered with fine stainless steel mesh 62 5 m openings 60 porosity and collected all but the finer silt sediment sizes despite its angular shape the trap has good characteristics of flow and a very high sampling efficiency 90 over a wide range of wind velocities because of its near isokinetic characteristics of flow results also indicate that this trap and others that have wide side profiles are very sensitive to incident wind angle incident angles 5 significantly reduce the sampling efficiency and promote scour around the trap inlet

an experimental study on the susceptibility of crusted surfaces to wind erosion a comparison of the strength properties of biotic and salt crusts abstract penetrometry has been employed in a number of studies investigating the strength of crusted sediments as related to their stability to particle impact in aeolian systems despite the early success of this procedure questions remain concerning the effect of the angle of penetration the role of fatigue and the relative importance of the ultimate strength as compared to the elasticity of the crust this paper reports on a series of penetrometer and wind tunnel experiments that consider these effects in the context of both salt and biotic crusts the results demonstrate that the angle of penetration is not important but the density of penetrations is the decreased strength observed with high densities of penetration repeated loading and continual impact in wind tunnel experiments provides further confirmation that sedimentary crusts progressively become weaker during mass transport events the elasticity of the surface appears to be at least as important to consider as the ultimate strength although much stronger the salt crusts were found to break down and erode sooner than biotic crusts in wind tunnel studies the notable elasticity of biotic crusts appears to afford them some protection against abrasion

evolution of double diffusion convection in a felsic magma chamber abstract the onset of double diffusion convection ddc is modeled in a two dimensional case in respect to magma chambers the viscosity model for the melt takes into account the effects of temperature and concentration of the dissolved component h 2 o the upper boundary of the convecting magma chamber is assumed to be anhydrous and at constant temperature whereas the lower boundary is treated as being hydrous permeable with a temperature greater than that within the upper boundary the case of positive compositional and thermal buoyancy of melt is studied assuming a h 2 o diffusion coefficient small in comparison with thermal diffusivity the ddc has been modeled using a system of equations solved by the finite difference method on a square grid the convective pattern evolution has been studied for fixed boundary conditions as well as for cooling and degassing due to the higher viscosity in the upper zone the upper boundary layer is thicker than the lower one the variation of water concentration in this zone of the convective cell can be significant in nature the high gradient of water concentration can be responsible for the observed variations of water content in minerals crystallized from a granite melt e g biotite because of a high lewis number 100 temperature variations in the magma chamber decay much faster than the water concentration in this case the intensive convection can continue at a constant temperature due to the non zero water content in the chamber in principle the effect can be applied to the formation of magmatic bodies if the cooling and degassing system reaches a uniform temperature distribution prior to the crystallization temperature water content throughout the body may still remain variable

double diffusive convection in magma chambers single or multiple layers an approximate set of equations applicable to double diffusive convection in magma chambers has been solved in the steady state for a range of geologically relevant parameters no evidence for the steady state existence of layered convection for conditions appropriate to magma chambers has been found at large ratios of the thermal to chemical diffusivity lewis number le the heat transport is independent of the lewis number and equal to that found for thermal convection in a single component fluid the vertical upward transport of light component varies directly with both the rayleigh and lewis numbers nuc ra0 23 le0 37

an experimental study of wind abrasion rocks were abraded by a sand laden air jet in the laboratory to examine the quantitative relationships between wind abrasion and wind velocity quantity and grain size of windblown sand particles and physical properties of the target rocks the results of 101 runs are expressed by an experimental equation with dimensionless constant which indicates that the rate of abrasion is directly related to the kinetic energy of the sand projectiles and inversely proportional to the compressive strength of the target rocks a tentative extension of the experimental equation to field condition leads to an equation for the wind abrasion in fields an application of the latter equation to the natural settings is preliminarily examined by comparing it with sharp s 1964 field experiments and a plausible result is obtained

evolutionary structures in double diffusive convection in magma chambers finite amplitude solutions to subcritical time dependent double diffusive convection d d c applicable for magma chambers are obtained by a two dimensional finite element method based on stream function temperature and compositional fields grid refinement is used for resolving the disparately scaled thermal and chemical boundary layers present for large ratios of the thermal to chemical diffusivity lewis number characteristic of magmas the occurrence of layered convection depends strongly on the initial conditions of the temperature and composition it is shown that in the infinite prandtl number limit a local transition from a diffusive to a finger like regime can take place for large enough lewis numbers this time dependent finding suggests that a strict categorization of d d c into the finger and diffusive regimes requires modification in geological applications

induced compositional layering in a convecting fluid layer by an endothermic phase transition abstract an endothermic phase change in the presence of chemical heterogeneities can act as an efficient filter such that heavy material is sequestered below the phase change and light material is trapped above the phase change due to the fact that the endothermic phase change damps thermal convection it can act as a trap door for chemical heterogeneities ascending flows which are chemically dense will need more thermal buoyancy to rise through the transition than ascending flows whose composition is closer to the background composition likewise descending flows which are chemically light need more thermal buoyancy to sink through the transition the results of a suite of numerical calculations of two dimensional double diffusive convection in a fluid layer with an endothermic phase change suggests that if the 670 km seismic discontinuity has a steep enough clapeyron slope compositional layering may be induced and dynamically maintained by the transition furthermore these calculations also show that the phase change and compositional boundaries will approximately coincide

a study on velocity of aeolian sand particles over leeward slope by particle image velocimetry abstract current studies on the movement of saltation sand grains in aeolian flow over a complex surface are not satisfactory this is mainly because the landform conditions of most aeolian sand transport models are based on flat sand beds without considering the effect of complex surface and slope on the structure of aeolian sand flow in this paper we used particle image velocimetry piv to measure the state of motion of aeolian sand particles over the leeward slope area analyzed the characteristics of these sands particles with height h above the surface on the leeward slope including the relationship of their mean speed to the slope height and the incoming wind speed and the relationship of particle size with height finally we proposed a formula to describe the linear relationship of the dust emission rate to the incoming wind speed on the bed surface in the leeward slope area our conclusions are of significance for the study of wind driven sand flow in the leeward slope

an experimental study of multiple grain size ejecta produced by collisions of saltating grains with a flat bed collision data are presented from coloured high speed films of three size fractions of sand grains saltating over a bed of the total grain population each fraction was colour tagged and the proportion of each size ejected by grains colliding with the surface was recorded on a number of films taken as the bed was progressively eroded the results confirm earlier findings that v3 v1 0 5 0 6 vn v1 08 and the rebound angle increases with decreasing grain size ejected grains are examined in relation to their size the impactor size ejection speed and angle and the number of ejecta per collision in addition changes in grain parameters are observed with time for fine impactors ejection speeds generally increase with a decrease in ejecta size but the fine fraction does not follow this trend for the coarse and medium impactors ejection angles are typically between 40 and 60 with coarse grains having shallower mean angles than fine ejecta the number of ejections per collision increases with a decrease in particle size for each impactor size the general tendency for coarse particles to be ejected at lower speeds and shallower angles than fine particles will lead to sorting of the grain sizes there is poor correlation between the forward momentum loss of the saltating grams at collision and both the forward momentum of the ejected grains and the number of ejected grains much of the forward momentum of the saltating grains is transfered to creeping grains the composition and geometry of the bed are considered to be important factors in the evolution of the saltation cloud

sedimentary evidence for a rapid kilometer scale crustal doming prior to the eruption of the emeishan flood basalts biostratigraphic and sedimentologic investigations in 67 sections have been carried out for the middle permian maokou formation that immediately underlies the emeishan flood basalts in southwest china the results suggest a domal crustal thinning before the emplacement of the emeishan large igneous province variably thinned carbonates in the maokou formation are capped by a subaerial unconformity which is generally manifested by karst paleotopography paleoweathering zone or locally by relict gravels and basal conglomerates provenance analysis indicates that these gravels and conglomerates were mainly derived from the uppermost maokou formation therefore the stratigraphic thinning likely resulted from differential erosion due to regional uplift iso thickness contours of the maokou formation delineate a subcircular uplifted area in accordance with the crustal doming caused by a starting mantle plume as predicted by experimental and numerical modeling the duration of this uplift is estimated to be less than 3 myr and the magnitude of uplift is greater than 1000 m the sedimentary records therefore provide independent supporting evidence for the mantle plume initiation model for the generation of the emeishan flood basalts

eolian sediments generated by anthropogenic disturbance of playas human impacts on the geomorphic system and geomorphic impacts on the human system abstract in many of the earth s arid and semiarid lands saline lakes playas and similar landforms are disturbed as a result of human activity diversion and or consumptive use of surface or groundwaters has created the effect of a climate change in numerous drainage basins resulting in the desiccation of lakes and reactivation of eolian processes at many locations playas are natural sites for extensive eolian activity because of the deposition of clastic and chemical sediments in basins by surface water via fluvial transport and groundwater via efflorescence wind erosion and deposition of playa sediments has had a major role in the development of landforms and sedimentary units in the present lunette fields worldwide simpson desert australia and geological past from the triassic mercia mudstone england to the quaternary lahontan basin and cima volcanic field usa anthropogenic disturbance or desiccation of playa systems has resulted in the eolian transport of sand e g lop nor china konya basin turkey rajasthan india kappakoola australia several sites in west africa and or dust e g aral sea kazakhstan uzbekistan old wives lake canada kara bogaz gol ex ussr lake texcoco mexico owens dry lake mono lake and other playas usa typically this is accomplished by abstraction of water and or removal of vegetation from terminal lake basins an extensive review of the literature documents many examples and or potential examples of such phenomena in numerous nations the reactivation of eolian processes from closed basins produces air pollution in the form of fugitive dust naturally occurring compounds released into the atmosphere by human actions and has significant environmental and economic impacts on human activities in the surrounding areas restoration or mitigation of degraded land on or surrounding playas has been accomplished at lake texcoco kara bogaz gol and the konya basin and is being actively implemented at mono lake owens dry lake and the aral sea

overturn and evolution of a crystallized magma ocean a numerical parameter study for mars 1 early in the history of terrestrial planets the fractional crystallization of a magma ocean can lead to a mantle stratification characterized by a progressive enrichment in heavy elements from the core mantle boundary to the surface such configuration is gravitationally unstable it causes mantle overturn and the formation of a stable chemical layering using simulations of thermo chemical convection we analyzed the consequences of overturn and subsequent layering on mantle dynamics assuming mars scaling parameters we found that the time needed to achieve chemical homogenization via convective mixing scales exponentially with the buoyancy ratio inline image which measures the relative importance of chemical to thermal buoyancy in addition when using a strongly temperature dependent viscosity the formation of a stagnant lid prevents the uppermost crystallized layers from sinking into the mantle in order to obtain their subduction an yielding mechanism must be invoked 2 in the context of mars evolution our results suggest that complete chemical mixing is unlikely to take place within time scales comparable with the planet s age magma ocean freezing could be thus responsible for the long term preservation of compositional heterogeneities as required by meteoritic evidence the lack of a surface highly enriched in incompatible elements and of a high density lid is difficult to reconcile with a stagnant lid regime operating throughout mars history an episode of surface mobilization induced by compositional overturn can resolve this difficulty provided that inline image is large enough too large buoyancy ratios however tend to suppress convective heat transport rendering it problematic to explain the late volcanic history of mars

a model for the interior structure evolution and differentiation of callisto abstract the recently measured dimensionless moment of inertia moi factor for callisto of 0 3549 0 0042 anderson et al 2001 icarus 153 157 161 poses a problem its value cannot be explained by a model in which callisto is completely differentiated into an ice shell above a rock shell and an iron core such as its neighboring satellite ganymede nor can it be explained by a model of a homogeneous undifferentiated ice rock satellite we show that callisto may be incompletely differentiated into an outer ice rock shell in which the volumetric rock concentration is close to the primordial one at the surface and decreases approximately linearly with depth an ice mantle mostly depleted of rock and an about 1800 km rock ice core in which the rock concentration is close to the close packing limit the ice rock shell thickness depends on uncertain rheology parameters and the heat flow and can be roughly 50 to 150 km thick we show that if callisto accreted from a mix of metal bearing rock and ice and if the average size of the rocks was of the order of meters to tens of meters then callisto may have experienced a gradual but still incomplete unmixing of the two components an ocean in callisto at a depth of 100 200 km is difficult to obtain if the ice is pure h2o and if the ice rock lithosphere is 100 km or more thick a water ocean is more plausible for ice contaminated by ammonia methane or salts or for pure h2o at a depth of 400 600 km

nonlinear physics of double diffusive convection in geological systems double diffusive convection d d c is a nonlinear fluid dynamical phenomenon driven by thermal and compositional effects on the density we discuss the nature of subcritical finite amplitude double diffusive convection for infinite prandtl number and large lewis number le which have applications in magma chambers and at the d layer at the core mantle boundary cmb numerical solutions by two dimensional finite element method are presented to portray the nature of time dependent d d c in both the diffusive and the finger regimes the compositional heterogeneities at the cmb are far more complex than the local thermal structure because of the largele and may have important implications for the scattering of seismic waves off the cmb the effects of increasing the buoyancy ratior are to suppress time dependent d d c even at highra in narrow slots shear heating instabilities can be triggered in the finger regime by the abrupt overturn of the compositional interface this phenomenon may occur in tall magma chambers with low dissipation numbers of of order 0 0 01

dust sources in arid and semiarid china and southern mongolia impacts of geomorphological setting and surface materials abstract analysis of the geomorphological settings and surface materials in arid and semiarid china and southern mongolia revealed that the dominant dust sources are gobi deserts and alluvial fans adjacent to gobi deserts in the piedmonts of the kunlun qilian helan and gobi altai mountains these areas have persistent supplies of dust sized particles deposited by intermittent floods from the mountains and fine materials are also generated by weathering and aeolian processes acting on the coarse fractions of gobi surfaces clear spatial differences in mineral types and contents and in the salt contents of surface materials also strongly affected the intensity of dust emissions inconsistencies in monitoring simulation and calculation of dust emissions in previous studies of northern china and mongolia may have been caused by insufficiently detailed considerations of variations in the geomorphological setting where the geomorphological settings in these areas are not considered in sufficient detail deviations arise in the estimation and prediction of dust using current models although the present study did not quantify the persistence and replenishment of dust sources in the study area our conclusions about the importance of geomorphology can improve future studies of dust sources and controlling factors improve assessments of the effects of climate change on dust emissions and facilitate the interpretation of paleoclimate records

development of deflation lag surfaces a series of wind tunnel tests were carried out to investigate the development of deflation lags in relation to the non erodible roughness element concentration glass spheres 18 mm in diameter were placed along the complete length of the wind tunnel working section in regular staggered arrays using three different spacings d 18 30 and 60 mm and completely covered with a 0 27 mm erodible sand a pre selected free stream velocity above threshold 8m s 1 was established above the surface and the sediment transport measured at 2 s intervals using a wedge shaped trap in which an electronic balance is incorporated throughout each test the emerging lag surface was periodically photographed from above at two locations upwind of the trap the photographs were electronically scanned and analysed to calculate the lag element coverage and location as well as mean height and frontal area for each time period test results indicate that lag development has a profound effect on both the sediment flux and wind profile characteristics initially there is an increase in sediment flux above that for a rippled sand bed because of increased erosion around and reduced kinetic energy loss in highly elastic collisions with the emerging roughness elements with further emergence a dynamic threshold is reached whereupon the sediment flux decreases rapidly tending towards zero at this point the supply of grains to the air stream through fluid drag follows a reduction in aerodynamic roughness and therefore surface shearing stress at least as important is the lesser potential for grain ejection at impact because of reduced momentum imparted from the air stream during saltation although recent shear stress partitioning models indicate when particle movement may commence on varying surfaces our experimental results demonstrate that this partitioning has a further direct bearing upon the saltation flux ratio

a wind tunnel study of particle kinematics during crust rupture and erosion abstract sediments with protective crusts of varying type were subjected to particles fed from an upwind source during wind tunnel experiments carried out to compare their ability to resist erosion and alter the kinematics of the saltation cloud a laser doppler anemometer measured the distribution of particle velocity for saltators impacting each crusted surface and for particles ejected from this same surface inclusive of ricochets biotic crusts grown on sand in an environmental chamber were able to withstand erosion over several hours of continuous particle impact as compared to brittle salt crusts which regardless of wind speed or sodium chloride concentration eroded fully within one half hour despite the appearance of deep pits and grooves on the ruptured crusts loss of mass dominated over particle trapping on these rough surfaces while decreasing salt concentration between 320 g kg 1 and 80 g kg 1 generally was found to be associated with an increase in the mass flux of particles ejected from the surface little to no correlation with either wind speed or particle impact speed was observed the range over which varying salt concentration affects the momentum of ejected particles is rather narrow within 160 g kg 1 temporal changes in the velocity distribution of ejected particles with crust rupture and deflation are complicated by variations in crust strength with depth this is especially true of biotic crusts and weak salt crusts fast moving particles associated with the upper 40 of the cumulative velocity distribution generally demonstrate little variation from the control surfaces and probably represent ricochets in comparison those within the lower 60 of the distribution are significantly affected by crust rupture and erosion and may include low energy rebounds as well as the entrainment of new particles loosened from within the crust while such measurements are exceedingly rare they are needed for the validation of physically based models of crust erosion

transition to chaos and implications for time scales of magma hybridization during mixing processes in magma chambers abstract in this contribution numerical simulations of magma mixing are carried out with the aim of understanding the physico chemical conditions that trigger the development of chaotic mixing dynamics in detail the mixing process is simulated in a magma chamber where a less evolved magma underplates a more evolved magma the chaoticity of this system is quantitatively estimated via computed finite time lyapunov exponents it is shown that magma chamber dynamics are strongly controlled by the rayleigh number ra whereas other parameters such as buoyancy ratio and viscosity ratio play a subordinate role results indicate that magma chambers with ra of the order of 10 6 are dominated by non chaotic behaviour increasing ra shifts the system towards an increasing chaotic state and in particular above ra 10 7 chaotic dynamics are fully developed in the magma chamber estimates of possible time scales of hybridization of magmas during chaotic mixing suggest for example that a blob of mafic magma with an initial diameter of 100 m within a magma chamber with a width of 1000 m is stretched to a filament like structure with a thickness of 15 m after 320 years and after 1900 years its thickness is reduced to 4 0 mm using typical chemical diffusion coefficients for magmas of the order of 10 14 m 2 s it can be calculated that a 4 0 mm thick filament would homogenize by diffusion in 10 years such homogenization time scales are orders of magnitude lower that typical life times estimated for magma chambers even in the volcanic environment indicating that fast hybridization of magmas can be rapidly achieved when chaotic dynamics govern the evolution of the magmatic system

sedimentary response to mantle plumes implications from paleocene onshore successions west and east greenland the uplift history of the upper cretaceous paleocene sedimentary successions underneath the extensive tertiary flood basalts of west and east greenland supports recently described models for the generation of flood basalt provinces in response to rising hot mantle plumes however the sedimentary development indicates that the period of plume related uplift prior to eruption of volcanic rocks was very short 5 m y substantial regional uplift and faulting took place in west and east greenland in the early paleocene to earliest late paleocene resulting in a basinwide unconformity in west greenland the uplift was associated with fluvial peneplanation valley incision and catastrophic deposition in east greenland it was associated with fluvial peneplanation the cumulative uplift in west greenland is probably on the order of several hundred meters whereas in east greenland the observed uplift cannot be quantified in both west and east greenland this phase was shortly followed by rapid major subsidence associated with extensive volcanism it is suggested that this sequence of events reflects the arrival of the north atlantic mantle plume or plumes

threshold friction velocities and rupture moduli for crusted desert soils for the input of soil particles into the air desert soils having clay crusts mostly from the mojave desert were tested for threshold friction velocity the friction velocity at which soil erosion begins with an open bottomed wind tunnel the soils were also tested for content of clay water soluble material calcium carbonate organic material mineralogy of clay and of salts soil moisture modulus of rupture and crust thickness if no loose material existed on the soil surface crusts having modulus of rupture greater than 0 7 bar and crust thickness of 0 7 cm to 0 3 cm were effective in protecting against wind erosion disturbed clay crusts having modulus of rupture before disturbance greater than 2 bar with thickness less than 1 9 cm did not experience significant wind erosion modulus of rupture was related to composition of soil but was shown to depend mostly on clay content soil composition is related to modulus of rupture in an empirical equation

erodibility of some crust forming soils sediments from the southern aral sea basin as determined in a wind tunnel severe dust storms in the southern aral sea basin have become common with the desiccation of the sea the high incidence of dust in the area has had severe ecological consequences within the framework of efforts to reduce this phenomenon deflatability as well as deflatability related characteristics of some prominent soils sediment surfaces in the southern aral sea basin were examined the materials included a salt crust from a developed solonchak a takyr crust and a takyr like soil and salt crusts from undeveloped solonchaks formed on the exposed bottom of the aral sea characteristics determined were particle size distribution dry aggregate size distribution and salt carbonate and organic carbon contents deflatability was examined using a suction type wind tunnel with a sensit type sensor to detect airborne unconsolidated material on materials treated to different moisture levels and with a chemical stabilizer and on restored crusts created from the unconsolidated materials fine sand dominates in the materials and in the takyr crust and takyr like soils is accompanied by significant amounts of silt and clay all materials contain moderate amounts of carbonate and are low in organic matter all soils sediments contain salts but in the salt crusts of the solonchaks the salt fraction dominates they all have more than 50 per cent pm850 particles with diameter 850 m indicating a relatively high deflatability potential the materials from the takyr crusts and takyr like soil with a high proportion of fine aggregates had the lowest threshold friction velocities while the salt crusts of the solonchaks with a high proportion of coarse aggregates had the highest this suggests that takyrs and takyr like soils are the most deflatable and solonchak soils the least deflatable these differences are attributed to the presence of salts that create stable large aggregates in the solonchak crusts wetting of the materials to three moisture levels considerably increased threshold friction velocity the increase was most prominent in the salt rich materials and was attributed to the rapid formation of surface films by drying in the course of the wind tunnel determinations applications of chemical stabilizers at two levels also considerably increased threshold friction velocity on the restored crusts threshold friction velocity dramatically increased occasionally to non recordable values this increase was monitored with both the salt crusts characteristic for the solonchak soils and the fine grained crusts characteristic for the takyr soils the stability was attributed to the tightly packed salt particles in the salt crusts and to the cohesive properties of the fine grained materials in the takyr crusts once the crusts were ruptured however strong deflation commenced these results suggest that by maintaining moisture in the soils sediments for example by maintaining a high water table in the amu darya river flood plain deflation can be reduced by the same means deflation can be reduced by creating new crusts or by preserving existing crusts copyright 2005 john wiley sons ltd

an analysis of soil crust strength in relation to potential abrasion by saltating particles wind erosion depends on the ease with which particles can be detached from the soil surface but suitable tests to characterize this property are not available two possible methods to determine surface soil strength in the field were therefore compared on a range of artificially crusted surfaces these were made by spraying or tension wetting aggregates 10 2 2 0 5 and 0 5 mm from a structurally unstable sandy loam followed by drying each test involved measuring the force exerted on a probe driven at a steady rate into the surface using either a flat tipped 0 6 mm diameter penetrometer or a flat ended cylindrical punch with inner and outer diameters of 5 and 6 mm respectively both probes showed that crusts could be produced reproducibly depending on the probe and aggregate size penetration mainly occurred either as a result of aggregates being deflected out of the pathway of the probe or by genuine rupture of aggregates or of the crusted surface the penetrometer because it was comparable to the size of sand grains gave results that can be used to characterize surface erodibility to saltating particles the punch gave results that would be unsuitable for this purpose as would other strength tests that are on too large a scale penetrometer results were analyzed to calculate the energy required for penetration it was thus possible to demonstrate that only the spray wetted fine aggregates had a surface that could undergo large scale rupture by saltating sand grains for all other surfaces saltating particles would be unable to supply sufficient energy to rupture aggregates or the crusted surface erosion could only occur by a slower process of abrasion in which smaller particles or aggregates are chipped away from the surface however it is shown that saltating particles could rupture the interaggregate bonding in the 2 0 5 mm aggregate surfaces thus permitting creep an alternative and potentially simpler way of characterizing surface erodibility by using a surface modulus of elasticity is also discussed our results demonstrate that the small diameter penetrometer is a promising technique for characterizing erodibility of aggregated and crusted surfaces 1997 by john wiley sons ltd

salts in the clay playas of china s arid regions gone with the wind in china s arid regions aeolian salts are transported along with dust aerosols but their contents and the emission rates are poorly understood using samples with undisturbed surfaces and artificially disrupted surfaces to simulate human disturbances such as reclamation planting trampling by animals and other activities the transport rates for windblown material and the salt contents of clay playa surfaces were investigated in a wind tunnel the results showed that under wind velocities ranging from 8 to 22 m s wind erosion rates ranged between 0 009 and 0 066 g m2 s for the undisturbed surfaces and between 0 011 and 0 048 g m2 s for the artificially disrupted playa surfaces although the amount of wind transported material that was captured was too small to allow an analysis of individual salt contents using the available equipment electrical conductivity measurements and correlation analysis revealed mean total salt contents of 9 59 g kg in the transported materials collected above the sealed playa surfaces versus 3 38 g kg in materials from the same surfaces that were artificially cracked after testing the sealed surfaces given the large areas containing clay playas in china s arid regions these results will improve our understanding of aeolian salt transport in arid china and will facilitate interpretation of the sources of aeolian salts

wind erosion of crusted soil sediments saltating particles increase the rate of dust release from sediments in arid and semi arid areas they also break interparticle bonds in aggregated and crusted soils thereby increasing the number of particles available for entrainment this pilot study examines rates of erosion in relation to the flux of saltating grains for three crusted sediments of different strengths dislodgement of surface particles decreases with increasing crust strength as measured by a cylindrical flat ended penetrometer in addition initial dust release from craters formed by single impactors in unaggregated soil is examined in relation to the associated saltator the volume of material removed depends linearly on the kinetic energy of the abraders

evolution of the yardangs at rogers lake california yardangs are streamlined wind eroded hills common to most deserts yardangs at rogers lake mojave desert california have streamlined forms characteristic of objects eroded by moving fluids a teardrop shape that approaches an ideal 1 4 width to length ratio in wind tunnel simulations miniature forms of various shapes changed sequentially by 1 erosion of the windward corners 2 erosion of the windward slope 3 erosion of the leeward corners and flanks and 4 erosion of the leeward slope prominent mechanisms in yardang evolution apparently are abrasion at the windward end and deflation and reverse air flow near the middle and at the downstream end width to length ratios of yardangs are grossly similar to those of some fluvial and glacial streamlined landforms the low kinetic energy of wind relative to ice and water the erosional resistance to wind of most rocks the rarity of long term unidirectional winds and the presence of running water topographic roughness and vegetation all limit the abundance of yardangs

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

mantle plume uplift in the sedimentary record origin of kilometre deep canyons within late neoproterozoic successions south australia in south australia canyons 200 250 km long are incised to depths of c 1 5 km in late neoproterozoic after c 590 ma marine deposits of the adelaide fold belt and canyons 90 km long and 700 m deep occur in correlative marine strata of the officer basin 1000 km to the nw in the adelaide fold belt the isotopic signature of limestone veneering canyon walls canyon sinuosity and sedimentary structures in canyon fill suggest that the canyons were cut subaerially and filled by shallow water marine deposits during coastal onlap eustacy cannot account for the kilometre scale change in sea level required for subaerial canyon erosion and chemostratigraphy and the lack of evaporites at that stratigraphic level in southern australia argue against a messinian style drawdown in each region uplift and crustal extension preceding or coeval with canyon incision are indicated by a regional unconformity turbidite or mass flow deposition and extensional faults these features and canyon incision may be explained by regional uplift above a rising mantle plume over a distance of 1000 km in southern australia flood basalt volcanism in the officer basin tentatively dated at 563 40 ma may have followed this uplift mantle plume uplift is expressed in the sedimentary record by emergent trends in facies regional thinning regional unconformities turbidites gravity slides normal faults and incised canyons and palaeovalleys

wind transport of sand surfaces crusted with photoautotrophic microorganisms abstract wind tunnel experiments show that while surfaces inoculated with photoautotrophic organisms are stable in winds up to 19 m s 1 only filamentous cyanobacteria especially nostoc commune afford considerable protection against abrasion from windborne particles crusts formed by the polysaccharide secreting alga chlamydomonas acidophila break down easily under impact the entrainment of large low density aggregates possibly contributing to higher transport rates than would otherwise be expected for untreated sediment surface disintegration encompasses a suite of complex scale dependent processes which vary with the structure thickness and strength of the crust independent measurements of crust strength based on flexure testing are in good agreement with wind tunnel evaluation of crust stability the accommodation of a relatively large amount of deformation without rupture is an important property of fibrous photoautotrophic mats perhaps outweighing consideration of their peak strength

the significance of gobi desert surfaces for dust emissions in china an experimental study a series of experiments to determine the direct emission of dust sized particles from gobi surfaces by clean wind wind without sand and the potential for aeolian abrasion of gobi surfaces and beds of gravel and mobile sand to produce fine 100 m in diameter and then pm10 by sedimentation was acquired the gobi surface provided most of the emitted fine particles during the initial dust emission processes but subsequently release of the clay coatings of particles by abrasion becomes the dominant source of fine materials under sand laden winds pm10 production rates produced by aeolian abrasion of gobi surfaces ranged between 0 002 and 0 244 of blown materials after removal of sand silt or clay with low resistance to erosion from the gobi surfaces by the wind the pm10 production rates caused by aeolian abrasion were similar to those from gravel and sand beds the results also indicated that after the dust sized particles with low resistance to erosion were removed the production of dust sized particles was unrelated to wind velocity under aeolian processes gobi deserts in this region therefore play a major role in dust emissions from arid and semiarid china

the grain bed collision in sand transport by wind in the last decade much progress has been made toward the development of a comprehensive model of aeolian sediment transport in which the grain bed collision has been identified as having a significant role the grain bed collision has been studied by both physical experiments and numerical simulation a principal objective of these studies has been to gather sufficient data to characterize the collision in order that it may be represented as an empirical function in numerical models of the sand transport system thus the study of the exact physical mechanism of the collision has to some extent been neglected the transport of larger particles in saltation over a loose surface is known to promote the release of finer particles from that surface into suspension thus the precise physics of the grain bed collision become highly significant with regard to the physics of dust release from a surface this paper re examines previous collision data and compares them with a simple collision model this model proposed that the impinging grain strikes a single surface grain and rebounds an alternative mechanism might be that the impinging grain ploughs through the surface striking a number of bed grains before rebounding the collision data are shown to support the model first because the duration of the collisions observed on the high speed film supports a short contact time with the surface and second because the data fit well with the model

a conceptual model of wind erosion of soil surfaces by saltating particles a conceptual model is described for the prediction of wind erosion rates dependent on the distribution of impact energy delivered to the surface by saltating grains p ei and the distribution of local surface strength p es methods are presented for the measurement of both distributions and consequent loss of material from the bed it is concluded that saltating sand grains can rupture weak crusts under even moderate wind conditions and that the rate of erosion will depend on the shape of the distribution tails copyright 1999 john wiley sons ltd