reynolds stress and sand transport over a foredune reynolds shear stress rs u w and sand transport patterns over a vegetated foredune are explored using three dimensional velocity data from ultrasonic anemometers at 0 2 and 1 2 m and sand transport intensity from laser particle counters at 0 014 m a mid latitude cyclone on 3 4 may 2010 generated storm force winds exceeding 20 m s 1 that shifted from offshore to obliquely alongshore quadrant analysis was used to characterize the spatial variation of rs quadrant components q1 through q4 and their relative contributions were parameterized using the flow exuberance relation exfl q1 q3 q2 q4 the magnitudes of rs and sand transport varied somewhat independently over the dune as controlled by topographic forcing effects on flow dynamics a flow exuberance effect was evident such that q2 ejection like and q4 sweep like quadrants that contribute positively to rs dominated on the beach dune toe and lower stoss whereas q1 and q3 that contribute negatively to rs dominated near the crest this exuberance effect was not expressed however in sand transport patterns instead q1 and q4 with above average streamwise velocity fluctuations u were most frequently associated with sand transport q4 activity corresponded with most sand transport at the beach toe and stoss locations 52 60 100 at the crest 25 to 86 of transport was associated with q1 while q4 corresponded with most of the remaining transport 13 to 59 thus the relationship between sand transport and rs is not as straightforward as in traditional equations that relate flux to stress in increasing fashion generally rs was poorly associated with sand transport partly because q1 and q4 contributions offset each other in rs calculations thus large amounts of transport can occur with small rs turbulent kinetic energy or reynolds normal stresses u 2 w 2 may provide stronger associations with sand transport over dunes although challenges exist on how to normalize and compare these quantities copyright 2013 john wiley sons ltd

equilibrium versus disequilibrium of barchan dunes abstract barchans are crescentic dunes which occur in mainly mono directional winds shape aspect ratios and velocities of these dunes have been studied as if they were in equilibrium however following a study of the shape and migration of 11 barchans of different sizes for 18 months in the field on moroccan atlantic sahara we show that they only appear to be in a stationary state if studied over a long timeframe at the scale of the year or several years but are never in equilibrium at the scale of weeks or months rather they are always trying to reach a possible equilibrium state but never have enough time to accomplish this this may be the main reason for the large variation observed in previous measurements and justifies some caution in what can be deduced from them

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

simulation and measurement of surface shear stress over isolated and closely spaced transverse dunes in a wind tunnel topographic interactions generate multidirectional and unsteady air ow that limits the application of velocity pro le approaches for estimating sediment transport over dunes results are presented from a series of wind tunnel simulations using irwin type surface mounted pressure sensors to measure shear stress variability directly at the surface over both isolated and closely spaced sharp crested model dunes findings complement existing theories on secondary air ow effects on stoss transport dynamics and provide new information on the in uence of lee side air ow patterns on dune morphodynamics for all speeds investigated turbulent unsteadiness at the dune toe indicates a greater more variable surface shear despite a signi cant drop in time averaged measurements of streamwise shear stress at this location this effect is believed suf cient to inhibit sediment deposition at the toe and may be responsible for documented intermittency in sand transport in the toe region on the stoss slope streamline compression and ow acceleration cause an increase in ow steadiness and shear stress to a maximum at the crest that is double that at the toe of the isolated dune and 60 70 per cent greater than at ow reattachment on the lower stoss of closely spaced dunes streamwise ow accelerations rather than turbulence have greater in uence on stress generation on the stoss and this effect increases with stoss slope distance and with incident wind speed reversed ow within the separation cell generates signi cant surface shear 30 40 per cent of maximum values for both spacings this supports eld studies that suggest reversed ow is competent enough to return sediment to the dune directly or in a de ected direction high variability in shear at reattachment indicates impact of a turbulent shear layer that despite low values of time averaged streamwise stress in this region would inhibit sediment accumulation downwind of reattachment shear stress and ow steadiness increase within 6 h h dune height of reattachment and approach upwind values by 25 h a distance of at least 30 h is suggested for full boundary layer recovery which is comparable to uvial estimates the irwin sensor used in this study provides a reliable means to measure skin friction force responsible for sand transport and its robust simple and cost effective design shows promise for validating these ndings in natural dune settings copyright 2003 john wiley sons ltd

metamorphism and deformation along the emplacement thrust of the samail ophiolite oman abstract the base of the samail ophiolite is a metamorphic shear zone that provides key information about ophiolite emplacement the classic exposure of this metamorphic sole at wadi tayin consists of a few meters of garnet clinopyroxene mafic gneiss and 80 m of upper amphibolite facies mafic and quartzose tectonite overlying 150 m of lower amphibolite facies mafic and sedimentary schist the sole is bounded by faults but is internally coherent peak metamorphic temperatures decrease exponentially from 825 c at the top of the sole to 500 c at its base as documented by changes in grain size hornblende composition the disappearance of chlorite and epidote garnet clinopyroxene element partitioning and garnet hornblende partitioning the inverted peak temperature gradient decreases from 4000 c km in the upper 50 m to 100 c km in the lower 150 m contrary to earlier reports greenschist facies tectonites are not present at wadi tayin although the entire sole underwent greenschist facies retrogression driven by the influx of fluids from the overlying peridotite or underlying carbonate calculations based on an amphibolite constitutive relation reveal that although the section represents a lithologic continuum the lower 200 m of the sole is a relatively undeformed block whereas the uppermost 25 m represent material initially separated by tens to a couple of hundreds of kilometers and now integrated into a single column

saltation and wind flow interaction in a variable slope wind tunnel abstract in a 15 m long horizontal wind tunnel we studied the internal boundary layer over fixed arrays with roughness z0 similar to the roughness of saltation of a sand bed utilizing turbulence spires the logarithmic overlap layer is at least 75 mm thick some 5 m downwind of the entry this permits a precise determination of u from wind profile data over a sand bed in a variable slope wind tunnel with a 6 m long working section we studied how slope grain size and friction speed influence bed roughness owen 1964 proposed that z 0 c u 2 2 g where c is a constant but we found that c depends in a complex manner on grain size and friction speed c increases from very low values near the threshold to an apparently limiting value at large friction speeds for an almost uniform 125 m sand the limiting value of c is 0 06 the limiting value decreases however with particle size for a uniform 544 m sand c only approaches 0 03 for friction speeds as high as 1 m s with the measured values of friction speed corrected for the effects of slope our results appear to show that the limiting values of c do not depend on the slope angle even though the slope affects the individual shapes of particle trajectory c is perhaps independent of slope angle because the roughness height and therefore c is more directly related to the amount of momentum extracted from the air by the particles i e the shear stress rather than to the manner of extraction particle trajectory shapes

the effects of slope and slope position on local and upstream fluid threshold friction velocities in deserts dunes are common aeolian landforms and parallel ridges are common in cultivated land a computational fluid dynamics cfd model is used to simulate a stable wind blowing over slope beds of varying height and coupled with the slope compensating fluid threshold friction velocity formula the model accurately reproduced patterns of flow deceleration at the slope toe and stoss flow acceleration based on the cfd based model quantitative analyses of slope gradient and particle position on the initiation of particle movement are performed results indicate that the slope has a great influence on particle saltation in the windward slope and the initiation of particle movement is particularly sensitive to particle position with respect to the slope copyright 2008 john wiley sons ltd

characterization of hyperalkaline fluids produced by low temperature serpentinization of mantle peridotites in the oman and ligurian ophiolites a regional survey of alkaline springs in oman and ligurian ophiolites shows that the alkaline water compositions significantly vary from one ophiolite to the other and within the same ophiolite the first order correlation between the na and k and cl concentrations points to fluid compositions only partly due to evaporation the scatter around the evaporation line implies that na and cl may not be conservative during the alteration of the ultramafic rocks mg is almost entirely depleted at ph 10 5 as a result of serpentine formation within the ultramafic body and of brucite and minor hydrotalcite precipitation at the springs ca accumulates in the high ph fluids and is consumed by ca carbonate formation at the springs by mixing with river waters or by the co2 supply from the atmosphere thermodynamic calculations show that brucite saturation is reached at ph values around 10 5 which triggers major changes in the water composition the waters evolve from a quartz saturated low ph continental environment to a brucite dominated high ph serpentinizing system at low temperature the highest water salinities are found in springs located along the basal thrust plane of the ophiolite the highest al concentrations are found in some springs located on the crustal side of the mantle crust boundary this poses the question of the hydrologic pathways and of the role of the mineralogical composition of the altered formations key points new temperature and composition of the alkaline waters brucite formation at ph 10 5 triggers major chemical and mineralogical changes relationship between the water compositions and the hydrologic pathways

early stage aeolian protodunes bedform development and sand transport dynamics early stage aeolian bedforms or protodunes are elemental in the continuum of dune development and act as essential precursors to mature dunes despite this we know very little about the processes and feedback mechanisms that shape these nascent bedforms whilst theory and conceptual models have offered some explanation for protodune existence and development until now we have lacked the technical capability to measure such small bedforms in aeolian settings here we employ terrestrial laser scanning to measure morphological change at the high frequency and spatial resolution required to gain new insights into protodune behaviour on a 0 06 m high protodune we observe vertical growth of the crest by 0 005 m in two hours our direct measurements of sand transport on the protodune account for such growth with a reduction in time averaged sediment flux of 18 observed over the crestal region detailed measurements of form also establish key points of morphological change on the protodune the position on the stoss slope where erosion switches to deposition is found at a point 0 07 m upwind of the crest this finding supports recent models that explain vertical dune growth through an upwind shift of this switching point observations also show characteristic changes in the asymmetric cross section of the protodune flow form feedbacks result in a steepening of the lee slope and a decline in lower stoss slope steepness by 3 constituting a reshaping of protodune form towards more mature dune morphology the approaches and findings applied here a demonstrate an ability to quantify processes at requisite spatial and temporal scales for monitoring early stage dune evolution b highlight the crucial role of form flow feedbacks in enabling early stage bedform growth alluding to a fluctuation in feedbacks that require better representation in dune models and c provide a new stimulus for advancing understanding of aeolian bedforms

influence of topography on wind speed over a coastal dune and blowout system at jockey s ridge nc usa this study examines the spatial distribution of wind speed across a coastal dune system located at jockey s ridge state park north carolina the study area consists of a trough blowout through a foredune ridge and the landforms that have developed behind the foredune wind speed and direction were measured simultaneously with single sensors placed at a fixed height in 13 locations across the blowout dune complex fractional wind speed up is computed for sampling stations using data from a mast located on the beach as the reference results show that wind speeds were generally accelerated across the study site the highest speeds were recorded on the foredune ridges adjacent to the blowout wind was accelerated through the center of the blowout throat and along the downwind lateral wall further into the blowout at the base of the ramp to the depositional lobe higher wind speeds shifted to the upwind lateral wall and continued to accelerate up the ramp as air exited to the rear significant variations in the wind speed up pattern were associated with different wind approach angles with greater speed up occurring when the winds were aligned normal to the dune system the speed up decreased as the angle of approach became increasingly oblique to the ridge the patterns of wind speed up across the site point to the influence of topography on airflow to quantify the relationship measures of several topographic variables were obtained along sample transects running upwind from each sample station along flow lines representing different wind approach angles examination of correlation coefficients between wind speed up and topographic variables suggests that for groups of stations with similar topographic characteristics 30 50 of the variations in speed up may be explained by the upwind topographic variability copyright 2014 john wiley sons ltd

a field study of mean and turbulent flow characteristics upwind over and downwind of barchan dunes field measured patterns of mean velocity and turbulent airflow are reported for isolated barchan dunes turbulence was sampled using a high frequency sonic anemometer deriving near surface reynolds shear and normal stresses measurements upwind of and over a crest brink separated barchan indicated that shear stress was sustained despite a velocity reduction at the dune toe the mapped streamline angles and enhanced turbulent intensities suggest the effects of positive streamline curvature are responsible for this maintenance of shear stress this field evidence supports an existing model for dune morphodynamics based on wind tunnel turbulence measurements downwind the effect of different dune profiles on flow re attachment and recovery was apparent with transverse incident flow a re attachment length between 2 3 and 5 0h h is dune brink height existed for a crest brink separated dune and 6 5 to 8 6h for a crest brink coincident dune the lee side shear layer produced elevated turbulent stresses immediately downwind of both dunes and a decrease in turbulence with distance characterized flow recovery recovery of mean velocity for the crest brink separated dune occurred over a distance 6 5h shorter than the crest brink coincident form as the application of sonic anemometers in aeolian geomorphology is relatively new there is debate concerning the suitability of processing their data in relation to dune surface and streamline angle this paper demonstrates the effect on reynolds stresses of mathematically correcting data to the local streamline over varying dune slope where the streamline angle was closely related to the surface windward slope time averaged shear stress agreed best with previous wind tunnel findings when data were rotated along streamlines in the close lee however the angle of downwardly projected separated flow was not aligned with the flat ground surface here shear stress appeared to be underestimated by streamline correction and corrected shear stress values were less than half of those uncorrected

airflow up the stoss slope of sand dunes limitations of current understanding the windward stoss side of a sand dune acts as a streamlined obstacle in the path of the wind continuity principles necessitate compression of the flow field up the stoss slope of a dune and shear stress must progressively increase as the flow accelerates measurements in transverse flow over thirteen dunes at padre island texas the algodones california and white sands new mexico confirm that velocity profiles on the stoss slope are not log linear and that flow acceleration occurs very close to the surface within an internal boundary layer as a consequence in the overlying flow where measurements have historically been made an overall decrease in shear stress occurs up the slope the actual shape of the velocity profiles and the identification of the appropriate segment of the profile from which to derive the shear stress that drives saltation represent major problems not approachable by traditional means

thermodynamic constraints on mineral carbonation of serpentinized peridotite abstract carbonation of serpentinite appears to be a promising process to mitigate rising concentrations of atmospheric co2 however given that chemical affinities of mineral carbonation reactions in serpentinized peridotite change drastically with temperature protolith and the activities of dissolved constituents this process is incompletely understood in this report numerical models are used to investigate the heterogeneous phase equilibria and the speciation of intergranular fluids during mineral carbonation of serpentinite dunite at 100 200 300 and 400 c the results suggest that temperature dependent activity gradients of both aqueous carbon dioxide aco2 aq and aqueous silica asio2 aq determine the carbonation potential of serpentinite at low to moderate temperatures there is ample thermodynamic driving force for the carbonation of serpentinite as the gradients in asio2 aq and aco2 aq set between talc quartz magnesite and serpentine brucite equilibria are steepest with increasing temperature these activity gradients become smaller because the stability fields of brucite serpentine and talc expand toward higher aco2 aq and asio2 aq while the stability field of magnesite contracts in addition the enhanced solubility of quartz toward higher temperatures shifts the talc quartz magnesite equilibrium to very high aco2 aq field observations of carbonated serpentinite reveal a rather simple sequence of arrested carbonation reactions which is typically fossilized as spatially distinct metasomatic zones a zone of serpentinized peridotite is followed by a zone of serpentine carbonate then talc carbonate and finally quartz carbonate our modeling results not only reproduce these observations they also suggest that complete carbonation is favored at low to moderate temperatures since complete carbonation of serpentinite is achieved at relatively small fluid to rock ratios at temperatures 200 c prolonged fluid influx at low temperatures can result in decarbonation and silicification of the altered rock in contrast at temperatures 200 c decarbonation and silicification reactions are unlikely even at very high fluid to rock ratios since serpentinized peridotite is highly reactive in the presence of co2 aq its widespread exposure at slow and ultraslow spreading mid ocean ridges may represent a significant sink for co2 in the global carbon cycle our modeling results can be used to evaluate fluid rock interactions in active or fossil mineral carbonation systems if the mineralogy is known and the temperature of formation can be constrained independently the asio2 aq aco2aq oxygen fugacity fo2 g and ph can be estimated in turn if the alteration temperature asio2 aq and aco2 aq are known governing phase equilibria can be approximated the latter is a promising tool to monitor the reaction progress during co2 sequestration into serpentinized peridotite

relations between dune morphology air flow and sediment flux on reversing dunes silver peak nevada relations between wind speed sediment flux and dune morphology were measured for two reversing dunes situated in the south western part of the silver peak dunefield in clayton valley west central nevada the larger dune was 120 m in length with a height of 12ae5 m and the smaller dune 80 m long and 6 m high both dunes were sharp crested aligned approximately e w perpendicular to the dominant wind direction and had slightly concave stoss profiles twenty seven rotating cup anemometers were placed 0ae3 m elevation along n s transects on each of the dunes at each anemometer site a passive wedge shaped sediment trap was used to measure sediment flux amplification of wind speed was observed towards the crest on the stoss side of both dunes with speed up factors ucrest ubase ranging from 1ae50 3ae19 with a corresponding increase in sediment flux by 1 2 orders of magnitude in general the ratio of crest flux to base flux qc qb increased with increasing incident basal wind speed on both dunes direct measurements of the stoss slope variation in sediment flux relative to the dune crest are in good agreement with owen s transport model friction speed u was approximated from near surface 0ae3 m point wind speed although not all assumptions of the owen model are upheld the modified model performance is sufficiently robust to predict short term variation in stoss sediment flux on the study dunes improved models that adequately account for variation in sediment flux under changing air flow and transport conditions are necessary for the prediction of longterm evolution of dunes in this regard further progress in model development will require increased understanding of the spatial and temporal variability of airflow and the short term response of sediment flux to these flow conditions

challenges in aeolian geomorphology investigating aeolian streamers abstract this paper reviews the challenges involved with instrumentation data acquisition analysis and the application of theory when conducting field experiments in natural aeolian environments this is presented in the context of a field investigation into the formation and behaviour of aeolian sand streamers also known as sand snakes streamers are a familiar manifestation of pronounced spatial and temporal variability in wind blown sand over beaches dunes and other sedimentary surfaces streamers represent complex and dynamic transport patterns that change over temporal scales on the order of 0 1 s and over a range of spatial scales from 0 1 to 10 m these transport features are likely governed by near surface turbulent structures in the wind necessitating detailed measurements of turbulence dynamics in association with sediment transport patterns this paper reviews all stages of the investigation from field site selection to the evaluation of theoretical models the discussion covers a range of topics including research strategy site selection instrumentation requirements and limitations experimental design and measurement techniques scale and precision of data acquisition data processing and visualization description and parameterisation of results challenges of spatio temporal data analysis turbulence measurements in the saltation layer scaling issues and non dimensionalisations in fluid dynamics theory and the application of theory and conceptual models to the complexities of real natural environments the challenges discussed here apply to many geomorphological field investigations in general and three principal issues are identified 1 the need for inexpensive compact and robust instruments with high spatio temporal resolution 2 the lack of extended quantitative methods for the computational analysis of shapes forms and patterns and 3 the conceptual discrepancies and ambiguities of applying scaling parameters from experimental engineering and idealized theoretical models to complex natural geomorphic systems

measurements of the aeolian sand transport saturation length the wavelength at which a dune pattern emerges from a flat sand bed is controlled by the sediment transport saturation length which is the length needed for the sand flux to adapt to a change in wind strength the influence of the wind shear velocity on this saturation length and on the subsequent dune initial wavelength has remained controversial in this paper we present direct measurements of the saturation length performed in a wind tunnel experiment complementary to this initial dune wavelengths are measured under different wind conditions in particular after storms using the linear stability analysis of dune formation it is then possible to deduce the saturation length from field data both direct and indirect measurements agree that the saturation length is almost independent of the wind strength this result supports the idea that grain inertia is the dominant dynamical mechanism limiting sediment transport saturation on dunes

parabolic dune development modes according to shape at the southern fringes of the hobq desert inner mongolia china since the 1970s parabolic dunes at the southern fringe of the hobq desert inner mongolia china have exhibited many different shapes v shaped u shaped and palmate each with a unique mode of development in the study area parabolic dunes are mainly distributed in regions a b and c with an intermittent river running from the south to the north we used high resolution remote sensing images from 1970 to 2014 and rtk gps measurements to study the development modes of different dune shapes the modes are characterized by the relationship between the intermittent river and dunes formation of the incipient dune patterns the predominant source supply of dunes and the primary formation of different shapes v shaped u shaped and palmate most parabolic dunes in region a are v shaped and closer to the bank of the river the original barchans in this region exhibit disconnected arms behavior with the sand blown out of the riverbed through gullies the nebkhas on the disconnected arms acquire the external sand source through the fertile island effect thereby developing into triangular sand patches and further developing into v shaped parabolic dunes most parabolic dunes in regions b and c are palmate the residual dunes cut by the re channelization of river from transverse dune fields on the west bank are the main sand source of region b the parabolic dunes in region c are the original barchans having then been transformed the stoss slopes of v shaped parabolic dunes along the riverbank are gradual and the dunes are flat in shape the dune crest of v shaped parabolic dune is the deposition area which forms the arc shaped sand ridge their two arms are non parallel the lateral airflow of the arms jointly transport sand to the middle part of dunes resulting in a narrower triangle that gradually becomes v shaped palmate parabolic dunes have a steeper stoss slope and height the dune crest of the palmate parabolic dune is the erosion area which forms a long and narrow trough between nebkhas by the funnelling effect this process forces sand towards lee slopes which transform from concave original barchans into convex ultimately resulting in the formation of palmate parabolic dunes

the influence of topography and approach angles on local deflections of airflow within a coastal blowout the spatial variability of air flow through complex topography is an important but not fully understood component of dune development and dynamics this study examines the spatial variability of the wind field in a linear blowout in coastal dunes at jockey s ridge state park on the outer banks of north carolina a spatial array of single height anemometers and wind vanes were placed within the blowout topography exerted a significant steering effect when onshore winds approached from directions within 50 of the blowout axis under those conditions wind flow in the blowout aligned to the axis regardless of approach angle maximizing the potential for erosion and transport in the trough in other locations aspect variations caused deflection both proportional and disproportional to changes in the approaching wind when prevailing winds approached from directions more oblique than 50 to the blowout axis topographic steering through the blowout trough was reduced and secondary flow generated by flow separation over the trough became more prominent during those approach angles wind directions and speeds within the upper blowout trough became erratic as vortices and turbulence dominated the flow minimizing transport potential the changing characteristics of airflow in the blowout relative to differing approach angles has implications on dune development and variations in transport potential under changing conditions copyright 2013 john wiley sons ltd

grainfall processes in the lee of transverse dunes silver peak nevada grainfall deposition and associated grainflows in the lee of aeolian dunes are important in that they are preserved as cross beds in the geological record and provide a key to the interpretation of the aeolian rock record despite their recognized importance there have been very few field laboratory or numerical simulation studies of leeside depositional processes on aeolian dunes as part of an ongoing study the relationships among grainfall wind speed and direction stoss sand transport rates and dune morphometry height and aspect ratio were investigated on four relatively small straightcrested transverse dunes at silver peak nevada between 55 and 95 of the total grainfall was found to be deposited within 1 m of the crest and 84 99 within 2 m depending primarily on dune size and shape grainfall decay rates on high dunes of large aspect ratio were observed to be very consistent with a weak positive dependence on wind speed for small dunes with low aspect ratios grainfall deposition was more varied and decreased rapidly within 1 m of the dune crest whereas at increased distance from the dune crest it eventually approached the smaller decay rates observed on the large dunes no dependence of grainfall on wind speed was observed for these small dunes comparison of field data with predictions from anderson s 1988 saltation model of grainfall based on the computation of saltation path lengths indicates lack of agreement in the following areas 1 deposition rate magnitude 2 variation in decay rate with wind speed and 3 the magnitude and location of the localized lee slope depositional maxima the silver peak field results demonstrate the importance of dune aspect ratio and related wake effects in determining the rate and pattern of grainfall this work confirms earlier speculation by mcdonald anderson 1995 that temporary turbulent suspension or modified saltation of relatively large grains does occur within the dune wake so that transport distances generally are larger than predicted by numerical simulations of true saltation

the geomorphological significance of airflow patterns in transverse dune interdunes abstract the interdunes between aeolian dunes have been relatively ignored when compared with the research attention on the morphodynamics of the dune bodies themselves this neglect is in spite of the possible significance of interdune dynamics for the geomorphology of the sand dune system as a whole especially with regard to dune spacing this paper considers the mean airflow within four relatively simple transverse dune interdunes the study locations were chosen in order to sample interdunes with different size and surface characteristics the dynamics of which were investigated for when incident flow was normal to the upwind crest the findings confirm existing models of flow reattachment length and recovery for aeolian dune lee side flow and show a consistent pattern of increasing near surface velocity downwind of reattachment that supports a mechanism for interdunes as sand free features flow dynamics are characterised for the different types of interdune observed where two groups are recognised the flow patterns in relatively short interdunes where dunes are closely spaced with a sandy surface were accordant with those of the flow response model in extended interdunes where bounding dunes were spaced with a length well over that for flow separation evidence at the downwind edge of the interdunes suggested that flow reacted to the subsequent dune for the case of these extended interdunes a new descriptive model is presented to characterise their dynamics in this model the variation in near surface flow allowed process zones to be identified through the interdune the geomorphological significance of the processes dominating each zone is discussed and comparisons are made between the flow response case and the new interdune model from this study in a discussion on the controls of spacing between dunes where reattachment length exerts a fundamental control the role of sediment availability is also highlighted as a significant factor the presence of a sandy bed can in some circumstances determine whether dune development and therefore spacing is controlled primarily by elements of flow response

near surface diagenesis of ophiolite derived conglomerates of the barzaman formation united arab emirates a natural analogue for permanent co2 sequestration via mineral carbonation of ultramafic rocks carbon capture and storage by mineralization is a potential method for storing anthropogenic co2 emissions and is based on the reaction between mg silicate and co2 to form mg carbonate the conglomerates of the barzaman formation exposed in the eastern united arab emirates represent an excellent natural analogue of this process these conglomerates were deposited as a series of alluvial fans along the western margin of the hajar mountains part of the oman uae ophiolite and are composed largely of ultramafic and lesser mafic clasts the clasts and matrix have been extensively altered to dolomite during diagenetic processes analysis and interpretation of rock textures provide evidence for the various factors that influenced the diagenetic processes and shed light on the silicate carbonate transformation process all the reactions have taken place in the near surface environment the silicate carbonate conversion reaction is exothermic and occurs spontaneously at near ambient pressure and temperature probably no greater than 50 c estimates of the amount of co2 stored in this way can be obtained from considerations of outcrop area formation thickness and percentage of dolomite replacement and show that c 150 billion tonnes equivalent to about 4 years of worldwide co2 emissions at current rates are stored

carbonate clumped isotope bond reordering and geospeedometry carbonate clumped isotope thermometry is based on the preference of 13c and 18o to form bonds with each other at elevated temperatures such bond ordering is susceptible to resetting by diffusion of c and o through the solid mineral lattice this type of bond reordering has the potential to obscure primary paleoclimate information but could also provide a basis for reconstructing shallow crustal temperatures and cooling rates we determined arrhenius parameters for solid state reordering of c o bonds in two different calcites through a series of laboratory heating experiments we find that the calcites have different susceptibilities to solid state reordering reaction progress follows a first order rate law in both calcites but only after an initial period of non first order reaction that we suggest relates to annealing of nonequilibrium defects when the calcites are first heated to experimental temperature we show that the apparent equilibrium temperature equations or closure temperature equations for carbonate clumped isotope reordering are analogous dodson s equations for first order loss of daughter isotopes for each calcite the sensitivity of apparent equilibrium temperature to cooling rate is sufficiently high for inference of cooling rates within a factor of 5 or better for cooling rates ranging from tens of degrees per day to a few degrees per million years however because the calcites have different susceptibilities to reordering each calcite defines its own cooling rate apparent equilibrium temperature relationship the cooling rates of carrara marble inferred from carbonate clumped isotope geospeedometry are 10 6 10 3 degrees per annum and are in broad agreement with rates inferred from thermochronometric methods cooling rates for 13c depleted calcites from the late neoproterozoic doushantou cap carbonates in south china are on the order of 102 104 degrees per annum consistent with rapid cooling following formation of these calcites by a short lived hydrothermal event bristow et al 2011 nature v 474 p 68 71 most of the uncertainty in these estimates relates to uncertainty in arrhenius parameters for different calcites thus while the carbonate clumped isotope geospeedometer shows promise for recording cooling rates in settings and lithologies where other geospeedometers may not be applicable the uncertainty in cooling rate will be large without independent knowledge of the reordering kinetics of each study material thus the full potential of the method will only be realized if reordering kinetics can be accurately determined for each study material or predicted on the basis of mineral composition texture or other observable parameters

uranium lead isotopic ages of the samail ophiolite oman with applications to tethyan ocean ridge tectonics plagiogranites are a minor but widespread component of the samail ophiolite plutonic member they crystallized from the most fractionated melts generated by magmatic crystallization and differentiation of a steady state magma chamber beneath the tethyan spreading ocean ridge and their ages are thought to mark the time of ocean crust formation isotopic u pb ages of zircons from 13 plagiogranites collected along a 270 km segment of the samail ophiolite subparallel to the regional trend of the sheeted dike complex the former spreading ridge axis direction define a narrow time interval of 93 5 97 9 m y with a pronounced clustering about 95 m y the zircon ages of the plagiogranites agree remarkably well with the early cenomanian to early turonian biostratigraphic ages of sediments that are intercalated within the ophiolite pillow lavas and that lie just above them tippit et al 1981 the agreement of radiometric and biostratigraphic ages provides strong support for the conclusion that the plagiogranite u pb ages closely date the time span of ocean crust formation no step changes in age patterns are observed along the ridge axis sheeted dike direction suggesting that there are no major internal offsets of the ophiolite by transform or other faults along most of the traverse one possible exception occurs at the southeastern end of the sampled interval ibra area where a 3 m y discontinuity might be caused by an unmapped fault assuming that the regional trend of the sheeted dikes n10 25 w marks the direction of the former spreading ridge axis the present array of sample localities spans a distance of 130 to 195 km normal to that axis i e in the spreading direction the data as a whole do not define a clear cut age trend normal to the spreading axis but by eliminating samples that may be aberrant due to faulting the data array suggests a pattern of increasing ages from east to west this would indicate that the samail ophiolite is derived from the western southwestern side of the tethyan spreading system a conclusion reached independently from geological evidence also the gentle age gradient in the spreading direction i e between 130 and 195 km of ocean crust were formed within an age span of no more than 2 m y suggests a very fast spreading rate this conclusion also is supported by independent geologic evidence

flow dynamics and sediment transport over a reversing barchan changli china abstract the flow and sediment transport over a reversing 6 9 m high barchan dune is examined on the changli gold coast in hebei province china wind velocity profiles are non logarithmic exhibit rapid accelerations up slope and display near surface jets at the dune crest and downwind lee slope from the windward lower slope to dune crest the wind speed at 5 cm height increased by 55 the windward slope is a slipface and therefore has a significantly greater gradient than most normal windward slopes and the magnitude of speed up is significantly greater speed up ratio of 5 25 the majority of sediment was transported in the 0 4 cm height above the bed range maximum total sediment flux occurred at the dune crest and was greater than the total flux from three other sites combined the leeward mid slope sedimentation rate is strongly influenced by the sediment plume streaming upwards and downwind from the actively reversing dune crest dunes in the process of reversing display quite different wind flow and sedimentation patterns than when the dune is in normal flow conditions

mineralogical assemblages forming at hyperalkaline warm springs hosted on ultramafic rocks a case study of oman and ligurian ophiolites 1 we report on the mineralogical assemblages found in the hyperalkaline springs hosted on liguria and oman ophiolites based on exhaustive x ray diffraction and scanning electron microprobe analyses in liguria hyperalkaline springs produce a thin brownish calcite precipitate that covers the bedrock due to the concomitant atmospheric co2 uptake and neutralization of the hyperalkaline waters no brucite and portlandite minerals are observed the discharge of alkaline waters in oman ophiolite forms white orange precipitates calcium carbonate minerals calcite and or aragonite are the most abundant and ubiquitous precipitates and are produced by the same mechanism as in liguria this process is observed as a thin surface crust made of rhombohedral calcite morphological features of aragonite vary from needle bouquet dumbbell spheroidal like habitus according to the origin of carbon temperature and ionic composition of the hyperalkaline springs and the biochemical and organic compounds brucite is observed both at hyperalkaline springs located at the thrust plane and at the paleo moho the varying mixing proportions between the surface runoff waters and the hyperalkaline ones control brucite precipitation the layered double hydroxide minerals occur solely in the vicinity of hyperalkaline springs emerging within the bedded gabbros finally the dominant mineralogical associations we found in oman ca bearing carbonates and brucite in a serpentinizing environment driven by the meteoric waters are surprisingly the same as those observed at the lost city hydrothermal site in a totally marine environment

lee slope sediment processes leading to avalanche initiation on an aeolian dune 1 in order to detail the governing conditions through which a slipface matures to the point of failure dry sand avalanches were observed in the dune simulation wind tunnel on a 1 1 replica transverse dune with a crest height of approximately 1 2 m areal distributions of grainfall and reptation were measured using traps changes in the slipface elevation were observed using 3 d laser scanning with a vertical accuracy of 0 096 mm for approximately every 1 mm2 of surface area grainfall decayed exponentially from the brink with a constant rate across all wind velocities reptation removed sediment from areas close to the brink and deposited it downslope creating low amplitude cross slope ripples on the slipface a critical length scale separating grainscale and bulk sediment behavior is identified and it defines the lower limit to the validity of angle of repose measurements avalanche initiation occurred in an area of steep surface slope below a sediment bulge with distance from the brink independent of wind velocity the time between avalanches was found to be constant for constant wind velocity

tectonic development of the samail ophiolite high precision u pb zircon geochronology and sm nd isotopic constraints on crustal growth and emplacement 1 new high precision single grain u pb zircon geochronology and whole rock nd isotopic data provide insight into the magmatic and tectonic development of the samail ophiolite the analyzed rocks can be broadly divided into two groups based on their structural position dates and isotopic composition an older group related to on axis magmatism and a younger group of post ridge dikes sills and stocks on axis gabbros tonalites and trondhjemites yielded th corrected 206pb 238u dates from 96 441 0 062 to 95 478 0 056 ma these dates combined with dates from rioux et al 2012 suggest that most of the ophiolite crust formed at an oceanic spreading center in 1 ma the post ridge intrusions come from all depths in the crust the upper mantle and the metamorphic sole post ridge gabbros tonalites and trondhjemites from the crust and mantle yielded th corrected 206pb 238u dates of 95 405 0 062 to 95 077 0 062 ma a small trondhjemitic pod from the metamorphic sole yielded younger th corrected 206pb 238u dates of 94 90 0 38 to 94 69 0 12 ma isotopic data suggest two distinct sources for the post ridge magmas five of the gabbros and tonalites from the crust have end 96 ma 6 90 0 12 to 7 88 0 16 and two trondhjemites from the upper mantle and metamorphic sole have end 96 ma 7 77 0 08 and 7 01 0 16 the negative end t and u pb dates from the mantle dike require that subduction or thrusting was established below the ophiolite 0 25 0 5 ma after formation of the crust the bimodal isotopic composition of post ridge magmas may reflect coeval decompression and or fluid fluxed melting of the mantle and melting dehydration or assimilation of sediment in the down going plate at this time the new data place temporal constraints on mid ocean ridge and supra subduction zone models for ophiolite formation

cross sectional profiles of sand ripples megaripples and dunes a method for discriminating between formational mechanisms cross sectional profiles of sand ripples megaripples and sand dunes provide a useful tool for discriminating between formation by ripple and dune processes feature width defined as the basal break in slope along the profile to either side of the crest represents a good standard for comparison of profile attributes across more than three orders of magnitude aspect ratio height width as a function of log width separates measurements into clusters representing differing mechanisms of formation scaling both height and distance for individual profiles by feature width facilitates comparison of profile shapes across three orders of magnitude in width the data presented here should prove useful for evaluating possible mechanisms of origin for aeolian features observed remotely including on planetary bodies copyright 2012 john wiley sons ltd

11 11 dune morphology and dynamics this chapter discusses the key types of sand dunes and how they can be classified and assesses the major controls on their development the processes of dune dynamics are reviewed leading to a discussion of the concept of equilibrium in solitary dune form recent advances in modeling are reviewed that show how interactions between dunes may result in equilibrium at the dune field scale rather than at the single dune scale

sand transportation and reverse patterns over leeward face of sand dune abstract sand saltation has complex interactions with turbulent flow and dune form most models of wind blown sand consider ideal circumstances such as steady wind velocity and a flat surface and the bulk of data on wind flow and sand transport over an individual dune has focused mostly on the influence of dune shape or inter dune space on the wind flow neglecting the effect of morphology on sand saltation particularly airflow and sand transportation over the leeward slope wind flow structures over the leeward slope of sand dunes have a fundamental influence on the organization of sand dunes in order to understand sand dune dynamics lee face airflow and sediment transportation should be paid more attention previous field observations could not measure turbulent flow structure well because of the limited observation points and the influence of experiment structure on wind field in addition the reverse sand particles over leeward face could not be collected by sand trap in field numerous field observations could not measure turbulent flow structure because of the limited observation points and the influence of experimental structures on the wind field in addition the reverse transport of sand particles over leeward face could not be collected by sand traps in field therefore this paper aims to investigate the turbulent flow structure and sand transport pattern over the leeward slope a numerical model of sand saltation over slope terrain is constructed which also considers the coupling effects between air flow and sand particles the large eddy simulation method is used to model turbulent flow sand transport is simulated by tracking the trajectory of each sand particle the results show that terrain significantly alters the turbulent air flow structure and wind blown sand movement especially over the leeward slope here mass flux increases initially and then decreases with height in the reversed flow region in the direction of wind flow and the mass flux decreases with height in the reversed direction the height of 0 5 h is the height of vortex core in the reversed flow region the vortex core is a critical point in the flow region where few particles are transited in the reversed region the reversed mass flux of sand particles is 25 of the mass flux in the flow direction this research may contribute to scientific understanding of the mechanisms of sand motion and wind flow over leeward of dune and it is likely to be significant in desertification control

dune formation under bimodal winds the study of dune morphology represents a valuable tool in the investigation of planetary wind systems the primary factor controlling the dune shape is the wind directionality however our understanding of dune formation is still limited to the simplest situation of unidirectional winds there is no model that solves the equations of sand transport under the most common situation of seasonally varying wind directions here we present the calculation of sand transport under bimodal winds using a dune model that is extended to account for more than one wind direction our calculations show that dunes align longitudinally to the resultant wind trend if the angle w between the wind directions is larger than 90 under high sand availability linear seif dunes are obtained the intriguing meandering shape of which is found to be controlled by the dune height and by the time the wind lasts at each one of the two wind directions unusual dune shapes including the wedge dunes observed on mars appear within a wide spectrum of bimodal dune morphologies under low sand availability

an empirical model of aeolian dune lee face airflow airflow data gathered over dunes ranging from 60 m tall complex crescentic dunes to 2 m tall simplecrescentic dunes were used to develop an empirical model of dune lee face airflow for straight crested dunes the nature of lee face flow varies and was found to be controlled by the interaction of at least three factors dune shape the incidence angle between the primary wind direction and the dune brinkline and atmospheric thermal stability three types of lee face flow separated attached and deflected along slope or attached and undeflected were found to occur separated flows characterized by a zone of low speed 0 3o of crestal speed back eddy flow typically occur leeward of steep sided dunes in transverse flow conditions unstable atmospheric thermal stability also favours flow separation attached flows characterized by higher flow speeds up to 84 of crestal speed that are a cosine function of the incidence angle typically occur leeward of dunes that have a lower average lee slope and are subject to oblique flow conditions depending on the slope of the lee face attached flow may be either deflected along slope lee slopes greater than about 20 or have the same direction as the primary flow lee slopes less than about 20 neutral atmospheric thermal stability also favours flow attachment as each of the three types of lee face flow is defined by a range of wind speeds and directions the nature of lee face flow is intimately tied to the type of aeolian depositional process i e wind ripple or superimposed dune migration grainflow or grainfall that occurs on the lee slope and the resulting pattern of dune deposits therefore the model presented in this paper can be used to enhance the interpretation of palaeowind regime and dune type from aeolian cross strata

field measurements of mean and turbulent airflow over a barchan sand dune advances in our knowledge of the aeolian processes governing sand dune dynamics have been restricted by a reliance on measures of time averaged airflow such as shear velocity u it has become clear that such measures are incapable of explaining the complete dynamics of sediment transport across dune surfaces past evidence from wind tunnel and modelling studies has suggested that in some regions on a dune s surface the sediment transport might be better explained through investigations of the turbulent nature of the airflow however to date there have been no field studies providing data on the turbulent characteristics of the airflow around dunes with which to support or refute such hypotheses the field investigation presented here provides mean and turbulent airflow measurements across the centre line of a barchan sand dune in namibia data were collected using arrays of sonic anemometers and were compared with sand flux data measured using wedge shaped traps results support previously published data derived from wind tunnels and numerical models the decline in mean wind velocity at the upwind toe of the dune is shown to coincide with a rise in turbulence whilst mean velocity acceleration on the upper slope corresponds with a general decline in measured turbulence analysis of the components of reynold shear stress u w and normal stresses u 2 and w 2 supports the notion that the development of flow turbulence along the dune centre line is likely to be associated with the interplay between streamline curvature and mean flow deceleration acceleration it is suggested that due to the nature of its calculation turbulence intensity is a measure of less practical use than direct assessments of the individual components of reynolds stress particularly the instantaneous horizontal streamwise component u 2 and shear stress u w whilst increases in reynolds shear stress and the horizontal streamwise component of stress in the toe region of the dune may effectively explain the maintenance of sand flux in a region of declining mean velocity they have much less explanatory power for sand flux on the upper windward slope and in the crestal region of the dune here it is suggested that mean flow acceleration is likely to provide the most significant driving force on sand flux possibly augmented by a rise in the horizontal streamwise component of reynolds stress u 2 in the crest brink region therefore although wind turbulence is considered to be of fundamental importance in explaining the sediment transport dynamics across the dune s surface it is recognised that the interaction between mean flow deceleration acceleration streamline curvature and individual components of reynolds stress is complex and the identification of a single element of flow that offers a panacea for accounting for sand flux and dune dynamics is difficult to find

secondary airflow and sediment transport in the lee of a reversing dune lee side windspeed and sediment transport were measured over a small 1 2 m transverse ridge in the silver peak dunefield west central nevada usa using an intensive array of 25 cup anemometers and seven total flux traps during crest transverse and transporting flow conditions u0 3crest 8 4 m s 1 windspeed near the surface of the lee slope averaged half 48 per cent that of crest speeds dimensionless speeds in the separation zone ranged from 0 2 to 0 8 that of the outer flow u12 along the boundary of the separation cell windspeed increased by 10 per cent of the crest speed before separation equilibrium of upper and lower wake regions was not observed by the documented eight dune heights suggesting that wake recovery may not occur over closely spaced dunes sediment transport measured directly on both the lee slope and interdune surfaces averaged approximately 15 per cent of crest inputs this suggests that a significant amount c 70 95 per cent of sediment transported over the crest moved as fallout for this data set flux was approximately proportional to the cube of the near surface windspeed u0 3 and in general there was an order of magnitude difference between flux measured at the crest and that measured within the separation zone transport direction in the separation zone was acutely oblique to the incident direction owing to secondary flow deflection beyond the interdune transport direction progressed from oblique to crest transverse this indicates that an appreciable amount of sediment may move laterally along the lee slope and interdune corridor under crest transverse flows regarding the grain size and sorting properties of transported sediment there was no significant difference in mean grain size over the dune although in general particles were finer and more poorly sorted in the lee copyright 1999 john wiley sons ltd

a wind tunnel simulation of the effects of stoss slope on the lee airflow pattern over a two dimensional transverse dune 1 secondary airflow plays an important role in dune formation and development the lee airflow pattern over transverse dunes is important in determining the shape alignment and spacing of dunes and is influenced significantly by the lee slope angle in this paper we present the results of scaled wind tunnel simulations of the effects of stoss slope on the mean lee airflow patterns of transverse dunes the leeward velocity field was measured nonintrusively using particle image velocimetry to assess the airflow pattern we found that the flow separated over the dunes with a leeward angle close to the slip face angle of natural transverse dunes and was insensitive to stoss angle separation cells were characterized by a back eddy reattachment distance height area and aspect ratio were used to characterize the separation cell the first three parameters increased with increasing stoss angle for angles less than 15 but remained constant or decreased slightly with angles greater than 15 on the basis of the leeward velocity profiles six flow regions corresponding to those in previous models were recognized these regions are initially bounded by kinks but gradually transition downwind and merge to recover a uniform velocity profile after a significant distance we suggest that the separation cell is most developed when the angle of stoss slope reaches 15 but the significance of this angle in the evolution of transverse dunes needs further exploration

carbonation rates of peridotite in the samail ophiolite sultanate of oman constrained through 14c dating and stable isotopes abstract detailed 14 c dating as well as stable c and o isotope analyses were conducted on carbonates formed during alteration of the peridotite layer of the samail ophiolite sultanate of oman 14 c results obtained in this and previous studies indicate that surface travertines range in age from modern to 45 000 yr bp indicating long term deposition and preservation travertine deposition rates in two localities were 0 1 to 0 3 mm yr between 30 000 and 45 000 yr bp using an estimate of total travertine area this would result in a maximum of 1000 to 3000 m 3 yr of travertine being deposited throughout the ophiolite during this time period this travertine deposition would have sequestered a maximum of 1 to 3 10 6 kg co 2 yr ca rich carbonate veins that are associated with the surface travertine deposits have ages ranging from 4000 to 36 000 yr bp average 15 000 yr bp mg rich carbonate veins exposed in outcrops have ages ranging from 8000 to 45 000 yr bp average 35 000 yr bp detailed sampling from numerous locations 3 locations in this study and 10 locations in the previous studies indicates that no carbonate veins from the natural peridotite weathering surface are older than the 50 000 yr bp dating limit of 14 c however 14 c dating of mg rich carbonate veins from three roadcut exposures qafeefah fanja and al wuqbah indicates that a significant number of roadcut veins are 14 c dead 50 000 yr bp a location weighted average indicates that 40 of veins sampled at the three roadcuts are 14 c dead an average including veins sampled at both roadcuts and outcrops indicates that overall 8 of mg rich carbonate veins are 14 c dead mg rich carbonate veins are estimated to sequester on the order of 10 7 kg co 2 yr throughout the ophiolite

characteristics of wind blown sand in the region of the crescent moon spring of dunhuang china based on the detailed wind data this paper aims to investigate wind regime drift potential and the movement of mega dunes at the crescent moon spring an important scenic spot in western china and to clarify the dominant factors that control sand transport in this region the results of this paper revealed that the dominant wind directions at the spring were nw and sw the spring region belonged to a low energy wind environment the wind regime and sand drift potential were hugely influenced by local topography and the regional atmospheric circulation intensified the complexity of their variations besides the movement of the southern and northern mega dune towards the spring greatly endangered the preservation of this scenic spot these results provide insights into the characteristics of wind blown sand at the crescent moon spring that will guide efforts to control sand damages

comparison of the coseismic rupture with the aftershock distribution in the hyuga nada earthquakes of 1996 on october 19 1996 a large underthrusting earthquake ms 6 7 the hyuga nada japan earthquake occurred along the southern end of nankai trough about two months later a second large earthquake ms 6 7 occurred in the adjacent region we study the source process of the two large earthquakes in the hyuga nada region and compare the coseismic rupture area with aftershock distribution the main source parameters obtained for the first mainshock are strike dip rake 210 12 81 the seismic moment mo 2 3 1019 nm mw 6 8 the rupture area s 20 15 km and the source duration t 17 s for the second main shock strike dip rake 210 12 87 the seismic moment mo 1 5 1019 nm mw 6 7 the rupture area s 18 18 km and the source duration t 15 s the coseismic rupture areas do not overlap the aftershock area while the aftershock areas of the two main shocks mutually overlap this implies that the common aftershock region takes a role of barriers to dynamic rupture it is also seen that the aftershock area expanded during the first one day

sediment flux and airflow on the stoss slope of a barchan dune abstract measurements of sediment flux on the windward slope of an isolated barchan using an array of 30 sand traps provide new data that can constrain models of dune dynamics the data show that at low wind incident speeds flux increases up the dune exponentially whereas at higher wind speeds the increase with distance approaches linearity wind profile measurements conducted at the same time as the flux measurements indicate that although wind speed at a given height increases by 1 2 times from dune toe to brinkline wind shear velocity derived from the profile data decreases up the dune and is in many cases below transport threshold values this demonstrates that conventional wind profiles derived from anemometry on dunes do not measure the part of the boundary layer that is significant for sediment transport

airflow and aeolian sediment transport patterns within a coastal trough blowout during lateral wind conditions blowouts are depressions that occur on coastal dunes deserts and grasslands the absence of vegetation in blowouts permits high speed winds to entrain and remove sediment whereas much research has examined patterns of wind flow and sediment transport on the stoss slopes and lee of sand dunes no study has yet investigated the connections between secondary air flow structures and sediment transport in a blowout where zones of streamline compression expansion and steering are less clearly delineated in this study we investigated the variability of sediment flux and its relation to near surface wind speed and turbulence within a trough blowout during wind flow that was oblique to the axis of the blowout wind flow was measured using six three dimensional 3d ultrasonic anemometers while sediment flux by eight sand traps all operating at 25 hz results demonstrated that sediment flux rates were highly variable throughout the blowout deflation basin even over short distances 0 5 m where flow was steadiest flux was greatest consequently the highest rates of sediment transport were recorded on the erosional wall crest where flow was compressed and accelerated the strength of correlation between sediment flux and wind parameter improved with an increase in averaging interval from 10 seconds to 1 minute at an interval of 10 seconds however wind speed correlated best with flux at seven of eight traps whereas at an interval of one minute turbulent kinetic energy tke provided the best correlation with flux at six of the eight traps correlation between sediment flux and wind parameters was best in the centre of the blowout and poorest on the erosional wall crest the evidence from this paper suggests for the first time that tke may be a better predictor of sediment transport at minute scale averaging intervals particularly over landforms where wind flow is highly turbulent

meteorological controls on sand transport and dune morphology in a polar desert victoria valley antarctica the hyperarid polar desert of the mcmurdo dry valleys contains an abundance of aeolian sedimentary deposits the largest in antarctica providing a unique setting for studying cold climate aeolian processes and dune morphology meteorological controls on sand transport and changes in dune morphology were monitored for 11 days in november december 2004 on a barchan dune in the victoria valley mcmurdo dry valleys sand transport in this environment was found to be influenced by 1 interparticle cohesion ice bonding 2 diurnal variations in the speed of thermally driven easterly valley winds due to reduced solar heating by night and 3 topographically modified foehn southwesterly winds the threshold entrainment velocity during easterly valleys winds was 5 3 m s at 0 4 m 0 21 m s warm dry and gusty foehn southwesterly winds were found to promote aeolian processes by dislodging surface sediments with dune morphology responding rapidly importantly no diurnal signal in sand transport was observed under these conditions when wind speed and sand transport occurred independently of solar heating measured changes in dune morphology indicate that the dune field of the victoria valley under current environmental conditions is in dynamic equilibrium with frequent up valley sand transport by thermally generated valley winds balanced by less frequent but greater magnitude down valley sand transport by moderate to strong foehn southwesterlies this results in little net movement of dunes and subsequent change in the position of the dune field accordingly we believe that regional scale shifts in climate that cause change in the synoptic circulation patterns of the ross sea region that affect the frequency of either the up valley easterlies or down valley foehn winds will result in a net change in the morphology of the victoria valley s dune field such change in circulation may become apparent early as change in the morphology of the victoria valley sand dunes we therefore suggest this site should be monitored as an indicator site for identifying change in regional weather and climate

the effect of surface slope on saltation threshold a new wind tunnel has been constructed to study the mass transport properties of wind blown sand this report on the first set of experiments in the new wind tunnel concerns the effect of slope on threshold friction speed the results of this series of static threshold experiments and one dynamic threshold experiment for a range of particle diameters and bed slope angles show that provided the effects of reynolds number variation and interparticle cohesive force are accounted for the static friction angle is independent of slope and close in value to the measured static angle of response

oxygen isotope fractionation during synthesis of camg carbonate and implications for sedimentary dolomite formation hydrous camg carbonate was synthesized at temperatures of 40 degrees 60 degrees and 80 degrees c in the laboratory this material has very similar mineralogical characteristics to natural disordered dolomite from the coorong region in south australia besides the dolomite variable amounts of amorphous carbonate are present in all samples the oxygen isotope compositions of synthesized bulk carbonate samples e g amorphous carbonate plus dolomite plot significantly lower than the northrop and clayton 1966 dolomite water equilibrium fractionated degassing of the samples however revealed relatively low oxygen isotope values for fast reacting using 100 h3po4 amorphous carbonate in contrast slow reacting dolomite has more positive oxygen isotope values and calculated carbonate water oxygen isotope fractionation values are close to strongest known dolomite water oxygen isotope fractionation published earlier on variations of reaction stabilization temperatures during synthesis gave evidence for dolomite formation from hypersaline solutions by a dissolution reprecipitation process it is likely that amorphous carbonate has been a problem in defining the dolomite water fractionation in the past moreover dolomite associated amorphous carbonate contents probably led to incorrect speculations about lower oxygen isotope fractionation in a so called protodolomite water system copyright c 2005 elsevier ltd

hydrogen generation from mantle source rocks in oman hydrogen gas associated with ca2 oh rich alkaline groundwaters ph 10 12 is at present emanating from ultramafic rocks of the oman ophiolite isotopic and chemical evidence indicates that hydrogen is formed by low temperature redox reactions in a closed groundwater environment this is normally a cryptic process in the hydrosphere but is fortuitously revealed by the unusual hydrogeological conditions in oman where atmospheric oxygen is totally assimilated it is suggested that hydrogen generation in mantle source rocks at depth and in the early archaean surface environment may be more widespread than has hitherto been realised

geomorphology of desert sand dunes a review of recent progress through the 1980s and 1990s studies of the geomorphology of desert sand dunes were dominated by field studies of wind flow and sand flow over individual dunes alongside these there were some attempts numerically to model dune development as well as some wind tunnel studies that investigated wind flow over dunes as developments with equipment allowed field measurements became more sophisticated however by the mid 1990s it was clear that even these more complex measurements were still unable to explain the mechanisms by which sand is entrained and transported most importantly the attempt to measure the stresses imposed by the wind on the sand surface proved impossible and the use of shear or friction velocity as a surrogate for shear stress also failed to deliver at the same time it has become apparent that turbulent structures in the flow may be as or more important in explaining sand flux in a development paralleled in fluvial geomorphology aeolian geomorphologists have attempted to measure and model turbulent structures over dunes progress has recently been made through the use of more complex numerical models based on computational fluid dynamics cfd some of the modelling work has also suggested that notions of dune equilibrium form may not be particularly helpful this range of recent developments has not meant that field studies are now redundant for linear dunes careful observations of individual dunes have provided important data about how the dunes develop but in this particular field some progress has been made through ground penetrating radar images of the internal structure of the dunes the paradigm for studies of desert dune geomorphology for several decades has been that good quality empirical data about wind flow and sand flux will enable us to understand how dunes are created and maintain their form at least some of the difficulty in the past arose from the plethora of undirected data generated by largely inductive field studies more recently attention has shifted although not completely to modelling approaches and very considerable progress has been made in developing models of dune development it is clear however that the models will continue to require accurate field observations in order for us to be able to develop a clear understanding of desert sand dune geomorphology

formation of surface features on ventifacts modeling the role of sand grains rebounding within cavities surface textural features pits grooves flutes and scallops of sand blasted ventifacts have been attributed to various erosional mechanisms and to variations in material hardness this study develops a numerical model of abrasion to examine feature formation by direct grain impacts we demonstrate that abrasion by a continuous supply of impactors arriving from a single direction each hitting the surface once transforms existing cavities into transient features similar to scallops flutes and pits which shrink and eventually vanish however by incorporating additional impacts within the features from rebounding particles persistent continuously growing features emerge we conclude that 1 rebounding particles concentrate their effect within features and therefore increase their size 2 the development of features is sensitive to material type and 3 the ultimate maximum feature scale is related to the size of the rock and the size and velocity of impactors our model helps organize diverse ventifact field observations into an initial framework of pattern formation and confirms field and laboratory observations that the most common features are linear in wind parallel directions

ultramafic rock carbonation constraints from listvenite core bt1b oman drilling project the occurrence of the quartz carbonate alteration assemblage listvenite in ophiolites indicates that ultramafic rock represents an effective sink for dissolved co2 however the majority of earlier studies of ultramafic rock carbonation had to rely on the surface exposure of reaction textures and field relationships here we present the first observations on ultramafic rock alteration obtained from the 300 m deep bt1b drill hole icdp oman drilling project allowing for a continuous and high resolution investigation hole bt1b recovered continuous drill core intersecting surface alluvium 200 meters of altered ultramafic rock comprising mainly listvenite and minor serpentinite bands at 90 m and 180 m depth and 100 meters of the underlying metamorphic sole textural evidence suggests that the carbonation of fully serpentinized harzburgite commenced by non equilibrium growth of spheroidal carbonate characterized by sectorial zoning resulting from radially oriented low angle boundaries in the serpentinite carbonate spheroids are composed of alternating magnesite cores and dolomite rims whereas texturally similar carbonate in the listvenite is composed of fe rich magnesite cores and ca fe rich magnesite rims the distinct compositions and mineral inclusions indicate that the carbonation extent was controlled by fluid accessibility resulting in the simultaneous formation of limited carbonate in the serpentinite bands and complete carbonation in the listvenite parts of bt1b the presence of euhedral magnesite overgrowing spheroidal carbonate in the listvenite suggests near equilibrium conditions during the final stage of carbonation the carbonate clumped isotope thermometry constrains carbonate crystallization between 50 c and 250 c implying repeated infiltration of reactive fluids during ophiolite uplift and cooling

rapid emplacement of young oceanic lithosphere argon geochronology of the oman ophiolite 40 ar 39 ar dates of emplacement related metamorphic rocks beneath the samail ophiolite in oman show that cooling to

relations between morphology air flow sand flux and particle size on transverse dunes taklimakan sand sea china field experiments on two isolated transverse dunes with different scales in the taklimakan sand sea show that particle size and slope control the threshold shear velocity and sand flows on the stoss of the dunes the decreased threshold shear velocity and the unsaturated sand flows are the main reasons why sands are not deposited at the toe of the dunes the particle size controls sand flux through its influence on the aerodynamic roughness elements and threshold shear velocities the sand flux is different for dune surfaces with different particle size distributions under the control of the medium to low stress of the drifting sand winds the results of the field experiments and particle size analysis showed that sand transport had little influence on the particle size distributions the mean particle size being transported on the stoss is lower than that on the dune surface it is difficult to determine whether or not the wind is fully saturated with sands when sand flux calculation is mainly dependent on the shear velocity and the mean particle size the sands within some range of the particle size are transported and developed into simple transverse dunes and then the interdune surface is coarsened and alternated with the draas in the taklimakan sand sea copyright 2002 john wiley sons ltd

physical and logistical considerations of using ultrasonic anemometers in aeolian sediment transport research abstract recently ultrasonic anemometers uas have become available for precise high frequency measurement of three dimensional velocity and turbulence properties except for a few wind tunnel and computational fluid dynamics cfd simulations advances in aeolian sediment transport and bedform research have been limited to field studies using instrumentation that is either incapable of measuring turbulence e g cup anemometers or unable to withstand sediment laden airflow e g hotfilms in contrast extensive progress has occurred in fluvial research where turbulence instrumentation has been available for some time this paper provides a pragmatic discussion on using uas in aeolian research recent advances using this technology are reviewed and key physical and logistical considerations for measuring airflow properties and near surface shear stress using uas over complex terrain are discussed physical considerations include limitations of applying boundary layer theory to flow over natural surfaces such as non logarithmic velocity profiles resulting from roughness and topographically induced effects and the inability of instrumentation to measure within the thin constant stress region these constraints hinder accurate shear velocity u shear stress and sand transport estimation uas allow measurement of turbulent reynolds stress rs that in theory should equal profile derived shear stress discrepancies often exist between these quantities however due to three dimensional spanwise flow components and rapid distortion effects i e unbalanced production and dissipation of turbulence common in flow over complex terrain while the rs approach yields information on turbulent contributions to near surface stress generation little evidence exists showing that rs is a better measure of forces responsible for sediment transport consequently predictive equations for sediment transport using rs do not exist there is also a need to identify the role of micro turbulent events e g burst sweep cycles and macro turbulent structures e g separation cells shear layers in aeolian dynamics in field settings to validate recent wind tunnel and cfd simulations a conundrum exists regarding whether velocity data should be rotated to correct for potential sensor misalignment effects in unsteady non uniform flow over complex terrain streamline angles vary spatially and temporally with height and location thus determination of and correction to true streamline coordinates is difficult caution should be exercised with correction methods that remove implicit vertical velocity trends as this may preclude detection of geomorphically important flow behaviour e g lift at a dune crest and may complicate interpretations of rs instruments should be aligned with the underlying surface and flow visualization should be used to adjust sensor measurement planes as close as possible to local streamlines logistical considerations include sensor design advantages and operational limitations data communication formats and field deployment strategies each can affect measurement accuracy and are easily overlooked sampling range frequency and period are also important as they limit the range of velocities and scales of turbulence that can be characterized ultrasonic anemometers offer a new sampling resolution to measure turbulent airflow properties in field settings with proper considerations of their limitations uas may allow researchers to close the gap between fluvial research and develop more robust models of aeolian processes and morphodynamics

responses of three dimensional flow to variations in the angle of incident wind and profile form of dunes greenwich dunes prince edward island canada abstract this study reports the responses of three dimensional near surface airflow over a vegetated foredune to variations in the conditions of incident flow during an 8 h experiment two parallel measurement transects were established on morphologically different dune profiles i a taller concave convex west foredune transect with 0 5 m high densely vegetated 45 seaward incipient foredune and ii a shorter concave straight east foredune transect with lower sparsely vegetated 14 seaward incipient foredune five stations on each transect from the incipient dune to the crest were equipped with ultrasonic anemometers at 0 6 and 1 65 m height and logged at 1 hz incident conditions were recorded from a 4 m tower over a flat beach winds increased from 6 m s 1 to 20 m s 1 and were generally obliquely onshore ene 73 three sub events and the population of 10 minute averages of key properties of flow u w s cvu from all sample locations on the east transect n 235 are examined to identify location and profile specific responses over 52 of the incident direction of flow from 11 to 63 onshore topographic steering and forcing cause major deviations in the properties and vectors of near surface flow from the regional wind topographic forcing on the concave straight dune profile increases wind speed and steadiness toward the crest with speed up values to 65 in the backshore wind speed and steadiness of flow are least responsive to changes in incident angle in the backshore because of stagnation of flow and are most responsive at the lower stoss under pronounced streamline compression on the steeper concave convex profile speed and steadiness decrease toward the crest because of stagnation of flow at the toe and flow expansion at the slope inflection point on the lower stoss net downward vertical velocity occurs over both profiles increases toward the crest and reflects enhanced turbulent momentum conveyance toward the surface all of these flow responses are enhanced with faster speeds of incident flow and or more onshore winds significant onshore steering of near surface vectors of flow to 37 occurs and is greatest closer to the surface and during highly oblique winds 15 onshore therefore even subtle effects of streamline compression and amplification of flow under alongshore conditions effectively steer flow and sand transport toward the dune as topographic forcing and steering cause significant three dimensional deviations in near surface properties of flow most regional scale and or two dimensional models of dune process response dynamics are insufficient for characterizing coastal and desert dune sediment budgets and morphodynamics in particular deflection of sand transport vectors with greater fetch distances than those derived from regional winds may occur coincident flow transport and morphological response data are required to better quantitatively model these processes

turbulence fields in the lee of two dimensional transverse dunes simulated in a wind tunnel the turbulence field of airflow in the lee of a dune has significant impacts on dune dynamics and related processes we used particle image velocimetry in a wind tunnel simulation to obtain detailed velocity measurements in the lee of two dimensional transverse dune models then used the results to analyse their turbulence fields the dune models used in this study had a single lee angle of 30 and a total of six stoss angles 3 5 10 15 20 and 25 we used vorticity turbulence intensity reynolds stress and turbulent kinetic energy to characterize the turbulence fields these parameters were functions of stoss angle wind velocity distance from the dune crest and height above the ground surface the stoss angles could generally be divided into two groups based on the profiles of mean velocity turbulence and reynolds stress stoss angles of 3 and 5 usually had similar profiles and angles of 15 20 and 25 formed a second group with similar profiles the profiles for the stoss angle of 10 were usually transitional and were intermediate between the two groups vorticity reynolds stress and turbulent kinetic energy increased monotonically with increasing free stream wind velocity but their variations with respect to the stoss angle were complex the stoss angles of 15 and 20 had the maximum values of these three parameters thus these angles may have special significance in dune development given the characteristics of the mean velocity fields and turbulence fields they produce within the lee airflow it is the streamwise velocity component and its turbulence that determine the surface shear stress copyright 2008 john wiley and sons ltd

particle size and sorting characteristics of sand in transport on the stoss slope of a small reversing dune studies of the particle size and sorting characteristics of sand on the stoss slope of a 6 m high reversing dune show that the sand in transport is generally finer and better sorted than surface sand at the same position on the slope the sand in transport becomes coarser and more poorly sorted as wind speed and rates of mass transport increase toward the dune crest these patterns reflect changes in the competence of the wind which is capable of transporting larger grains and a wider range of grain sizes as its speed increases in space and time our field observations suggest that the particle size and sorting characteristics of surface sand are highly dependent on antecedent wind conditions and are not an invariant property of the dune as is widely assumed the wide range of particle sizes on the surface as well as its change through time also has important implications for modeling sediment transport on dunes transport thresholds may vary by as much as 30 on the stoss slope of the study dune

dynamic processes of a simple linear dune a study in the taklimakan sand sea china samples of dune sands surveys of the morphology and field measurements of wind velocity and direction of a simple linear dune in taklimakan sand sea show that the airflow and sand flux vary with the change of wind direction on the dune surface decrease of the airflow stress on the lee flank does not result in much decrease of the sand flux because of the low threshold shear velocities and the airflow conditions there are no significant relations between the sand flux on the lee flank and the angle of incidence of the airflow the low threshold shear velocities and the maintenance of the sand flux at the lee flank are the main mechanisms keeping the linear shape of the dunes measurements of the sand flux shows that it reaches a maximum on the crest of the dune the grain size of the transported sands has some differences compared to that of the dune surface the sands transported are finer than that on the dune surface but better sorted under the influence of the medium to low wind activity the field experiment results exhibit that it is possible for the dunes to be shaped as linear dunes during the processes of accumulation and elongation

transverse aeolian ridges on mars first results from hirise images abstract three images obtained by the high resolution imaging science experiment hirise were analyzed for the information they could provide regarding transverse aeolian ridges tars on mars tars from five locations in a hirise image of the floor of ius chasma show remarkably symmetric cross sectional profiles with average slopes for the entire feature of 15 these results apply to tars that span an order of magnitude in wavelength and a factor of 6 in height a hirise image of gamboa impact crater in the northern lowlands shows low albedo sand patches

rapid emplacement of the oman ophiolite thermal and geochronologic constraints in understanding of ophiolite emplacement requires knowledge of the elapsed time between igneous crystallization and intraoceanic thrusting and the rate and duration of that thrusting hornblende 4oar 39ar ages demonstrate that the igneous oceanic crust in oman crystallized and cooled to 825 k in 1 2 m y hornblende ages from metamorphic rocks and from cross cutting dikes require that the basal metamorphic thrust fault beneath the ophiolite also cooled below 825 k in 1 2 m y motion along the sole thrust accounted for 200 km of displacement at a rate of 100 200 mm yr on the basis of age relationships and thermal considerations we favor a two stage model for the initial stages of samall ophiolite emplacement intraoceanic thrusting over 2 m y old lithosphere at 150 km m y parallel to a spreading ridge for 1 2 m y followed by equally rapid and brief thrusting over cold and old lithosphere preservation of the samail ophiolite is a direct result of its young age and positive buoyancy at the time of ocean closure and we propose that all ophiolites that originated near spreading centers and were emplaced onto continents were young at the time of intraoceanic thrusting

an integrated model for dune morphology and sand fluxes on mars the transport and deposition of sand is the most prevalent agent of landscape modification on mars today with fluxes comparable to some sand dunes on earth until now the relationship between sand flux and dune field morphology has been poorly constrained by tracking dune movement over 10 km long dune fields in herschel crater and nili patera representative of many dune fields on mars we find a downwind flux decrease that correlates with a sequence of changing morphology from barchans to barchanoids and seifs longitudinal dunes to isolated dome dunes and ending with sand sheets we show empirical consistency with atmospheric internal boundary layer ibl theory which can describe these broad flux and morphology changes in martian dune fields deviations from ibl flux predictions are from wind streamline compressions up slopes leading to a speedup effect by establishing a dune field morphology type example and correlating it with measured and predicted flux changes we provide an integrated morphology and flux model that can be applied to other areas of mars and be used to infer paleo environmental conditions from preserved sandstone

field evidence for surface wave induced instability of sand dunes barchans are crescent shaped dunes that move faster over desert surfaces than most other dune types a three year field study shows how they do it collisions and changes in wind direction destabilize the dunes generating surface waves that form new smaller barchans this prevents the formation of a single giant dune so plays a fundamental role in the development of dune patterns field studies of barchans crescent shaped dunes that propagate over solid ground under conditions of unidirectional wind1 have long focused on the investigation of an equilibrium between sand transport by wind and the control of air flow by dune topography2 3 4 which are thought to control dune morphology and kinematics5 6 7 because of the long timescale involved however the underlying dynamic processes responsible for the evolution of dune fields remain poorly understood8 here we combine data from a three year field study in the moroccan sahara with a model study to show that barchans are fundamentally unstable and do not necessarily behave like stable solitary waves as suggested previously9 10 11 12 we find that dune collisions and changes in wind direction destabilize the dunes and generate surface waves on the barchans because the resulting surface waves propagate at a higher speed than the dunes themselves they can produce a series of new barchans of elementary size by breaking the horns of large dunes the creation of these new dunes provides a mechanism for sand loss that prevents dune fields from merging into a single giant dune and therefore plays a fundamental role in the control of size selection and the development of dune patterns

flow dynamics over a foredune at prince edward island canada time averaged windspeed profiles at eight locations over a relatively high 8 m vegetated topographically simple foredune were measured using cup and ultrasonic anemometry during an onshore wind event in may 2002 the experiment was part of a largerstudyonthesedimentarydynamicsofabeach dunecomplexinthegreenwichdunes princeedwardisland pei canada the foredune is vegetated with ammophila breviligulata and vegetation density ranged from 0 on the foredune ramp to upward of 70 at the dune crest topographic forcing and resulting flow acceleration was observed distinctly in windspeed profiles and linear speedup occurred for all elevations above the vegetation canopy a distinct inflection point is evident in normalized windspeed profiles over the dune indicating a net momentum sink below the vegetation canopy top and systematic speed down within the vegetation was observed up the foredune stoss slope wind speed profile structure was consistent for the range of incident wind speeds measured a comparison of normal and slightly oblique incident winds indicated that for similar approach winds wind speeds are consistently higher during oblique flow conditions the implications for foredune morphodynamics and sedimentation are discussed d 2004 elsevier b v all rights reserved