scratching technique for the study and analysis of soil surface abrasion mechanism aeolian abrasion is the most fundamental and active surface process that takes place in arid and semi arid environments its nature is a wear process for wind blown grains impinging on a soil or sediment surface which causes particles and aggregates to fracture from the soil surface through a series of plastic and brittle cracking deformation such as cutting ploughing and brittle fracturing using a universal micro tribometer umt a scratching test was carried out on six soil surfaces sandy soil sand loam silt loam loam silt clay loam and silt clay the results indicate that traces of normal and tangential force vs time show a jagged curve which can reflect the plastic deformation and brittle fracturing of aggregates and particles of various sizes fractured from the soil surfaces the jagged curve peaks and the area enclosed underneath may represent the bonding forces and bonding energies of some aggregates and grains on the soil surface respectively connecting the scratching test with an impact abrasion experiment furthermore demonstrates that soil surface abrasion rates are proportional to the square of speeds of impacting particles and to the 2 6 power of mean soil grain size and inversely proportional to the 1 5 power of specific surface abrasive energy or to the 1 7 power of specific surface hardness

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

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

microbiotic crusts in the high equatorial andes and their influence on paramo soils although microbiotic crusts are known from many lowland deserts they have apparently never been reported from alpine areas this paper describes microbiotic crusts from an equatorial high andean location paramo de piedras blancas in venezuela the main objectives are to describe the basic characteristics of the crusts to examine the properties of the soils on which they occur and to discuss their possible geomorphic significance for paramo soils properties of crusts and of the soils beneath them were compared with those of adjacent bare soils at three sites between 4360 and 4550 m altitude measurements on unconfined soil compressive strength and infiltration rates were taken in the field crust specimens were also gathered and later examined and sectioned under a microscope to ascertain micromorphological details laboratory data included soil texture organic matter content color ph water storage at field capacity and stability of soil aggregates crusts formed discontinuous garland like patches parallel to contours or covered small round soil buds produced by frost sorting microbiotic crusts contained many plants including an unclassified hepatic genus marsupella dumort a moss grimmia longirostris hook and several lichen species crusts consisted of a thin organic layer 5 35 mm overlying mineral soil and showed three parallel bands with diffuse boundaries the outer layer had green reddish brown live plant shoots and stems with few fine mineral grains the middle layer had anatomically recognizable dark brown plant material and a greater content of mineral grains the light colored basal layer had little organic matter mixed with coarse mineral particles it also contained many small orthovughs vesicles and some larger arched chambers caused mainly by frost activity soils with a microbiotic crust were darker than bare soils organic matter content in crust areas was two to three times higher than in adjacent bare soil ph dropped 0 3 units below crusts soils with crust had finer texture than bare ground the content of fines in crusts was 1 5 times higher than in bare soils but gravel in these was four times higher than below cryptogams indices of particle concentration showed that grains 0 5 1 mm were more abundant in crusts finer soils in crusts may result from a combination of a prior random texture variation b cryptogam colonization of fine soil patches sorted by frost c interception and capture of aeolian dust by cryptogams especially by mosses and d reduction of fine particle erosion by crusts as coarse grains are lost mean particle size within crusts drops geomorphic effects of crusts were also significant a infiltration rates were 70 greater in crust than in noncrust soils 67 0 vs 40 4 mm min b crust soils stored more water at field capacity 48 7 88 4 than bare areas 20 5 42 c crust soils had a higher resistance to raindrop erosion than bare areas 47 5 drops 0 1 g vs 10 2 drops 0 1 g undisturbed crust specimens were even more effective in resisting drop impact surviving up to 1100 drops 0 1 g d cryptogams increased soil compressibility more than 200 from 54 g cm2 in bare soils to 118 1 g cm2 in crust areas all these geomorphic changes should result in lower rates of soil erosion in paramo areas with dense crusts

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

distributions of dissolved titanium in chesapeake bay and the amazon river estuary abstract total dissolved ti in chesapeake bay and the amazon river estuary exists at nanomolar to subnanomolar concentrations in both estuaries dissolved ti is removed at low salinities in the course of estuarine mixing most likely by coagulation of colloidal metal perhaps with stabilization of the aggregates by organic material or by adsorption onto other particles titanium concentrations in 0 2 m filtered surface samples obtained on a summer survey of chesapeake bay range from 0 52 0 87 nm at 0 1 salinity decreasing to 0 07 0 28 nm at higher salinities the extent of removal is 65 variable but low levels in the middle estuary probably reflect inputs from tributaries to the bay a comparison of ti concentrations in oxic surface waters and suboxic and anoxic bottomwaters shows no consistent evidence for ti release from dissolving fe or mn phases on a summer survey of the amazon river estuary ti concentrations in 0 22 m filtered surface waters decrease from 3 3 7 1 nm at salinities of 0 3 0 8 to 0 27 0 62 nm above a salinity of 5 for a removal of 90 slight elevations in ti concentrations at salinities greater than 34 may result from resuspension of sediments on the dynamic amazon shelf using the amazon river estuary as an example at least 90 of the global flux of dissolved ti from rivers is removed to particles at low salinities considering this the effective global flux of dissolved ti from rivers to the oceans here estimated at 2 107 mol y 1 is 10 100 times lower than what has been previously estimated based on river inputs

a study of particle splash on developing ripple forms for two bed materials a custom designed particle tracking velocimetry ptv system was developed to measure particle splash dynamics in two materials mixed quartz sand and ground acrylic particles approximating the density of snow the strengths and limitations of the ptv system are described in the context of five experiments carried out at friction velocities slightly greater than the threshold friction velocity for particle entrainment and saltation analysis of particle collisions on the bed surface splash demonstrates that the simulated snow particles impact the surface at a smaller angle than quartz sand they also eject a greater number of particles though at lower velocities and angles than observed for the sand particles these relations appear to be highly sensitive to the porosity of the bed surface while particle shape and elasticity play a secondary role with regard to ptv measurements carried out along the wind aligned axis of migrating aeolian ripples particles ejected from the lee slope tend to be slightly larger than those on the stoss slope and in general are ejected with a lower speed and greater angle to the surface irrespective of the type of granular material when the average impact angle is close to or greater than the negative slope of the evolving bedform the particle ejection rate from the stoss slope was measured to be approximately twice that for the lee slope this empirical observation confirms the assumption made in models of ripple development that the ejection flux governs the bedform migration rate however systematic under prediction of this rate by an early model suggests that additional processes inclusive of surface creep must also be accounted for

geochemical evidence of a massive slide in the southern norwegian sea during the 1981 cruise of the ms tyro to the southern norwegian basin several piston cores were collected for sedimentological research some of these were available for geochemical study fig 1 in laboratory analyses the pore water composition of one of these cores showed evidence of input of non saline water whereas pore water in other cores from the same basin did not give any indication of dilution with fresh water careful examination of the chemical constituents of the pore water and some parameters of the sediment gives convincing evidence for the origin of the fresh water the presence of non saline water can be attributed to a massive slide of a coastal fresh to brackish sediment slab with preservation of structural and pore water composition this sliding is thought to have taken place about 12 000 to 15 000 years bp

a wind tunnel study of the resilience of three fungal crusts to particle abrasion during aeolian sediment transport abstract this study is a comparison of the strength stability and resistance to abrasion of monospecific sand crusts formed by three species of free living fungi aureobasidium pullulans trichoderma harzianum and absidia corymbifera compared to three photoautotrophs examined in a previous study the fungal crusts were thicker and stronger in flexure by a factor of two or more in addition to the morphological properties of these fungi the organic substrate required for growth contributes to the total crust strength wind tunnel tests in which the crusts were subjected to particle impact from an upwind sediment source nominally 0 014 kg m 1 s 1 and varied levels of freestream velocity u showed that crusts comprised of trichoderma and absidia were stable at wind velocities under 10 m s 1 crusts of aureobasidium generally ruptured at velocities well below this level and to a much greater extent the distinctive morphologies and metabolic rates of the three fungi appear to explain these differences as compared to the wind tunnel experiments independent tests of crust strength in flexure which were intended to simulate crust flaking were found to be less sensitive indicators of relative crust stability in aeolian settings analysis of the time rate of expansion of the perimeter of crust loss indicates that this relation is approximately exponential with the exception of low wind velocities near the threshold for grain ejection where the momentum of the saltators at impact appears to be of prime importance

erosion profiles due to particles entrained by wind application of an eolian sediment transport model erosion by impacts of particles entrained in the wind scales with the kinetic energy of impacting grains a recent model of eolian sediment transport permits calculation of expected erosion patterns in objects of simple geometry the modeling of vertical erosion profiles in turn provides an exacting test of the transport model realistic distributions of liftoff velocities for saltating grains give rise to kinetic energy flux profiles characterized by a strong maximum as much as 1 4 m above the bed during strong winds kinetic energy flux due to suspended grains also peaks above the bed the height and strength of the maxima depend very strongly on the grain size distribution as grain size diminishes increased particle deflection by the air flow around an obstacle reduces delivery of kinetic energy to the surface for both saltating and suspended grains kinetic energy flux scales with the fifth power of the wind shear velocity erosion profiles in man made obstacles are well modeled with only slight modification of the saltation model to account for the relatively high elasticity of actual deflationary surfaces lending considerable support for the model the modeling also elucidates several aspects of ventifaction as small ventifacts 1 2 m in diameter and the lower portions of larger ones are predominantly eroded by saltating grains upwind surface irregularities are quickly damped leading to facets that dip upwind and sharply truncate lee surfaces sandblasting by suspended grains dominates along the upper portions of large ventifacts where grain paths are partially deflected by the obstacle and may lead to radially symmetrical flutes and grooves

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

influence of cryptogamic crust disturbance to wind erosion on sand and loam rangeland soils a portable field wind tunnel was used to assess the sediment flux rates of loam and sand textured soils in the mallee region of southeastern australia three levels of crust disturbance nil moderate and severe simulating stock trampling were investigated the results demonstrated the importance of cryptogamic crusts in binding the soil surface and providing roughness after the soil was moderately disturbed on the loamy soil the crust helped maintain sediment flux rates below the erosion control target to 5 g m 1 s 1 for a 65 km h 1 wind measured at 10 m height once the crust was severely disturbed sediment fluxes increased to 1 6 times the erosion target on the sandy soil even with no crust disturbance the sediment flux was 1 6 times the erosion control target disturbing the crust increased sediment fluxes to a maximum of 6 7 times the erosion control target removal of the crust also decreased the threshold wind velocity that resulted in an increase to the ris k of erosion from 5 per cent to 20 per cent 1998 john wiley sons ltd

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

quantifying dust emissions from desert landforms eastern mojave desert usa abstract the measurement of natural dust emissions from desert landforms is crucial in environmental hazard assessment and field checking the accuracy of global dust models more than 500 individual dust measurements from eight common desert landforms in southern california were collected using the pi swerl portable in situ wind erosion lab the largest emitters of dust are dry washes 13 787 to 0 007 mg m 2 s 1 dunes playa margins distal alluvial fans and lacustrine beaches low emitters include salt crusted playas 0 692 to 0 002 mg m 2 s 1 silt clay crusted playas and desert pavements high emissions are a function of saltating sand that bombards the surface liberating dust sized particles for entrainment low dust emissions are primarily a function of surface crusting gravel armoring and vegetation density pi swerl measurements reveal that emission rates can vary by at least three orders of magnitude reflecting local variability in soil texture and continuity of surface crusts shear stress partitioning models can be applied to dust data measured by the pi swerl to account for large surface roughness features such as vegetation the results presented here give an approximation of the contributions to atmospheric dust loading by landforms in the mojave desert and can potentially be used to improve atmospheric dust models

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

crust strength a wind tunnel study of the effect of impact by saltating particles on cohesive soil surfaces a wind tunnel study examined the effect of distributions of saltating particles on sediment surfaces which were characterized by distributions of their tensile strength the sediments consisted of varying proportions of large sand sized particles with a fine particle cement the energies of the impacting particles and the surface strengths were compared with the mass of material lost from the surface it is important to consider distributions of parameters rather than mean values only since abrasion and erosion may occur from surfaces not predicted from average strength and saltation velocities at the impact velocities used in this study mean velocity 4 4 m s 1 with standard deviation of 0 51 surfaces containing less than 12 per cent fine material were easily eroded but insignificant erosion occurred when the fine particle content exceeded 60 per cent small amounts of cementing material were easily ruptured allowing the large sand grains to be moved largely in creep by the bombarding particles a significant amount of energy was lost to the bed as the percentage of fine material increased the surface became more difficult to break up and less energy was lost to the bed the probability that erosion will occur for known energy distributions of impacting particles and surface strength can be calculated and the mass loss increases exponentially with a decrease in the percentage of fine cementing particles copyright 2001 john wiley sons ltd

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

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