spatial and temporal distribution of cyanobacterial soil crusts in the kalahari implications for soil surface properties localised patterns of erosion and deposition in vegetated semi arid rangelands have been shown to influence ecological change and biogeochemical cycles in the flat vegetated kalahari rangelands of southern africa the factors regulating erodibility of the fine sand soils and the erosivity of wind regimes require further investigation this paper reports on the spatial and temporal patterns of cyanobacterial soil crust cover from ten sites at five sampling locations in the semi arid kalahari and discusses the likely impact on factors regulating surface erodibility and erosivity cyanobacterial soil crust cover on kalahari sand varied between 11 and 95 of the ground surface and was higher than previously reported cover was inversely related to grazing with the lowest crust cover found close to boreholes and the highest in the game reserve and wildlife management zone in grazed areas crusts form under the protective canopies of the thorny shrub acacia mellifera fenced plot data showed that crusts recover quickly from disturbance with a near complete surface crust cover forming within 15 months of disturbance crust development is restricted by burial by wind blown sediment and by raindrop impact crusts had significantly greater organic matter and total nitrogen compared to unconsolidated surfaces crusts also significantly increased the compressive strength of the surface and thus decreased erodibility and changed the surface roughness establishing exactly how these changes affect aeolian erosion requires further process based studies the proportion of shear velocity acting on the surface in this complex mixed bush grass crust environment will be the key to understanding how crusts affect erodibility

growth by extension and reworking of a south western kalahari linear dune optically stimulated luminescence osl dating studies of linear longitudinal dunes have been used extensively to elucidate late quaternary environments and climates in arid or formerly arid regions yet understanding of the development of such dunes is incomplete in particular conflicting opinions have been presented regarding the propensity of linear dunes to migrate laterally the degree to which they rework their own sediment during accumulation and whether they form primarily by extension as opposed to lateral sand movement from adjacent interdunes this study focuses on this last point although the importance of the other controversies is discussed in context a simple linear dune in the south western kalahari which has a prominent termination on a pan playa surface provides an opportunity to directly test hypotheses of dune extension chronostratigraphy along a 600 m transect along the crest of the dune constrained by 42 osl ages reveals that the dune grew by extension on occasions in the late pleistocene and early holocene but has also been subject to reworking along its length which has continued until recent times dune development by extensional growth is suggested to operate under environmental conditions more conducive to net accumulation whereas reworking is largely independent of conditions throughout the last 18 ka and may represent seasonal fluctuations in the position of the dune crest the relative significance of these two modes of development is suggested to be a key control on the efficacy of linear dunes as archives of environmental and climatic change copyright 2011 john wiley sons ltd

holocene aeolian activity in the southwestern kalahari desert southern africa significance and relationships to late pleistocene dune building events little is known about the timing of dune building episodes in the southwestern kalahari desert previously arid phases when dune activity occurred have been inferred from gaps in chronologies derived from information that attests to former humid periods to redress this optically stimulated luminescence osl dating was applied to sediment samples taken from a range of aeolian landforms in the sw kalahari four periods of dune building are identified in the late pleistocene and holocene holocene aeolian activity at 6 ka and 1 2 ka led to the construction of localised dune forms in contrast to late pleistocene times when the extensive field of linear dunes that dominates the region was emplaced late holocene aridity was however sufficient to allow rapid emplacement of the lunette dune at witpan the record of aeolian activity is evaluated in the context of humid chronologies and an ocean atmosphere model of holocene climate change with good agreement obtained it is concluded that the application of

frequency and magnitude biases in the fryberger model with implications for characterizing geomorphically effective winds abstract the fryberger model uses standardized wind data to estimate regional aeolian sediment drift potential dp which allows for interpretation and classification of aeolian dune landscapes this paper identifies systematic frequency and magnitude biases in the model that result from i variations in wind direction sector range which affects frequency bin size for determining percent occurrence of transporting winds and ii use of wind speed class mid point values over more statistically representative values e g mean median or minimally classified whole knot wind speeds in the magnitude weighting factor for dp calculations the significance of these biases is tested using unclassified data from stanhope prince edward island canada frequency bias resulting from wind data aggregation into direction sectors of varying size produces statistically significant discrepancies in dp and rdp estimates for this dataset when 36 point 10 s of degree data are reduced to 16 sectors cardinal directions receive one extra direction class and the magnitude of dp and rdp values decreases by 1 4 and 2 8 respectively compared to 16 equal 22 5 sectors derived from original data this results from under representation of intermediate direction classes for this dataset the effects on rdd less than 1 and directional variability ratio rdp dp are insignificant magnitude biases result from the use of the wind speed class mid point values instead of other statistically representative measures of wind speed in dp calculations in that wind speed distributions are often positively skewed mid point values yield an over estimate of total dp by as much as 34 and rdp by up to 22 for this dataset over those derived using mean median and whole knot minimally classified values this difference is statistically significantly at both the individual wind speed direction category level and at the aggregate directionally summed level and causes rdp vectors to shift 3 5 to the south though the impacts of these frequency magnitude biases are site specific and modest the effects on rdp estimation may be more significant in complex i e multi modal wind regimes and or in environments with more frequent high magnitude winds from the cardinal directions given the increased availability of more precise unclassified wind data most of these systematic frequency magnitude biases can be avoided recommendations on reducing inaccuracies imposed by these biases include i using to the degree wind data where available and categorizing into 16 equal 22 5 direction sectors ii using either wind speed class statistical mean values or minimally classified whole knot values in dp calculations and iii recognizing that converting 36 point 10 s of degrees data to 16 direction classes may introduce a frequency bias toward the cardinal directions and will cause inaccuracies in dp and rdp estimates of an amount that depends on the wind regime further research is needed to assess the implications of such biases in different wind regimes and into the influence of localized supply and transport limiting factors on regional scale assessments of dune morphodynamics and mobility these findings are also relevant for other applications that use categorized wind speed direction data such as dust or contaminant plume dispersion modelling

interpreting and modelling late quaternary dune accumulation in the southern arabian peninsula abstract one of the fundamental questions in the reconstruction of quaternary dryland environmental conditions is what do dune osl ages mean in terms of palaeoenvironmental change this paper investigates the relationship between dune chronological records and external environmental forcing in the southern arabian peninsula during the last 30 ka aeolian records from the region are reviewed with reference to other regional palaeoenvironmental proxies and these findings are related to a modelled assessment of dune response to the aeolian system state of the northeastern rub al khali the model replicates a range of features of the regional dune chronology when forced by three external parameters sediment supply sediment availability and transport capacity including extensive regional dune accumulation during the late pleistocene holocene transition and reduced activity during the early holocene humid period dune chronologies incorporate the influence of both external forcing parameters and localised short term factors included as stochastic processes thick sedimentary units most likely represent external forcing conditions conducive to aeolian sediment transport and deposition but at the same time these factors may be represented by periods of erosion of sediment at different sites confident interpretation of past environmental conditions is therefore not possible based on results from individual stratigraphic records sampling at multiple locations is needed in order to distinguish dune accumulation and preservation due to external forcing from stochastic processes of dune development

relationships of aeolian erosion and deposition with lidar derived landscape surface roughness following wildfire the reduction of vegetation by wildfire can subject stable soil surfaces to increased aeolian transport vegetation and associated microtopography function as surface roughness elements that influence the entrainment transport and deposition of sediment by wind in burned and unburned semiarid shrublands we examined whether surface roughness derived from lidar can explain variability in aeolian surface change following wildfire in loess soils of a cold desert shrub steppe in se idaho usa erosion bridges were installed in fall 2007 following a late summer wildfire to monitor soil surface change at sites in burned and downwind unburned areas surface elevation measurements were made when the erosion bridges were installed and again in fall 2008 surface change was determined from the difference in relative elevation between the two dates airborne lidar data were acquired in fall 2007 following erosion bridge installation surface roughness was calculated at 2 m raster resolution using the standard deviation of all lidar elevations within the 2 m cells after elevations were detrended to remove the effects of topographic slope surface change varied as a function of surface roughness among burned and unburned surfaces with net erosion occurring on the relatively smooth burned surfaces and net deposition occurring on the rough unburned surfaces site mean surface change decreased linearly as a function of the inverse of site mean surface roughness r2 0 77 p 0 00 quantile regression analysis indicated that changes in surface roughness were related to proportionally greater changes in erosion compared to deposition analysis of surface change at finer spatial scales suggested that aeolian processes occurred with strong spatial patterns on burned but not unburned surfaces future research to examine relationships between aeolian transport and fine spatial resolution topographic variability for example from ground based lidar systems is recommended

dune mobility and vegetation cover in the southwest kalahari desert as part of a wider project investigating the palaeoenvironmental significance of partially vegetated linear dunes in the southwest kalahari data collected in the latter part of 1992 concerning dune movement and vegetation cover suggest that sediment transport is occurring on some dune surfaces and that the majority of surface activity occurs on the crests and upper slopes of the dunes the data suggest that the limiting variables on surface sediment movement vary on different parts of a dune on interdunes and lower dune slopes the primary limiting variable is available wind energy while on dune crests and upper slopes it is vegetation cover ground cover by litter has much greater importance in protecting the surface sediment from erosion than rooted vegetation from individual data points no evidence is found to support a threshold vegetation cover below which sediment movement occurs rather a gradient of activity is suggested whereby a reduction in vegetation cover increases the potential for sediment movement and surface change however dunes with differing amounts of mean vegetation cover display differing degrees of surface activity and at this scale a vegetation cover threshold in the region of 14 per cent may be recognized

secondary flow deflection in the lee of transverse dunes with implications for dune morphodynamics and migration measurements of lee side airflow response from an extensive array of meteorological instruments combined with smoke and flow streamer visualization is used to examine the development and morphodynamic significance of the lee side separation vortex over closely spaced transverse dune ridges a differential deflection mechanism is presented that explains the three dimensional pattern of lee side airflow structure for a variety of incident flow angles these flow patterns produce reversed along dune and deflected surface flow vectors in the lee that are inferred to result in net lateral diversion of sand transport over one dune wavelength for incident angles as small as 10 from crest transverse i e 80 from the crest line this lateral displacement increases markedly with incident flow angle when expressed as the absolute value of the total deflection in degrees reversed and multi directional flow occurs for incident angles between 90 and 50 these results document the three dimensional nature of flow and sand transport over transverse dunes and provide empirical evidence for an oblique migration model copyright 2013 john wiley sons ltd

a quantitative approach to understanding dated dune stratigraphies attempts to reconstruct past changes in climate related forcing of dryland landscapes are hampered by the lack of an adequate quantitative framework for understanding the production and interpretation of dated sedimentary records in drylands as in other environments information on past forcing conditions is progressively modified degraded and removed from the available stratigraphic record by a series of filters involving changes in the primary forcing factors themselves geomorphological processes and the sampling dating procedures in this paper we describe a quantitative model that includes these effects and use the model to examine the nature of preserved dryland sedimentary records and their relationships to primary forcing conditions thicker preserved sedimentary records reflect periods of more intense aeolian activity localized switching between erosion and deposition results in discontinuous and highly variable stratigraphic sequences a preservation bias towards younger deposits is observed potentially leading to a continuum of accumulation that decays approximately in proportion to time periods not represented by deposition can in some cases be interpreted as periods of higher precipitation and or lower wind energy an asymmetry exists between the efficiency with which past drier and wetter episodes can be identified which relates to the time separation of depositional periods and the correct distinction between hiatuses due to forcing conditions and those due to under sampling relevant to this is the effect of random dating errors statistical uncertainty which increasingly with age filter out higher frequency events from the record a new data treatment method termed accumulation rate variability provides an efficient proxy for accumulation rates and therefore the intensity of aeolian activity with significant improvements over existing date frequency methods the filtering problem discussed applies to all attempts at understanding the timing and nature of past events independent of the proxies and dating methods employed further explicit analysis of these issues would be beneficial copyright 2013 john wiley sons ltd

analysis of linear sand dune morphological variability southwestern kalahari desert abstract linear dunes are the most common desert dune form usually occurring in extensive dunefields rather than as isolated individuals as part of a wider project investigating the dynamics and environmental significance of linear dunes the extensive linear dunefield of the southwestern kalahari desert southern africa was investigated for planimetric pattern variability considerable intradunefield variability was identified through aerial photograph analysis of a 4000 km 2 area leading to the development of a five class classification scheme this scheme was validated statistically utilising data for key planimetric pattern variables y junctions termini orientation range and wavelength the application of the classification scheme thoughout the dunefield permits the identification of trends in planimetric patterns this provides a basis for first attempts to explain aspects of planimetric variability in terms of the behaviour of linear dunes and their responses to key environmental variables

a field quantification of coastal dune perennial plants as indicators of surface stability erosion or deposition dune plants both modify the wind field around them and are impacted by various stress factors among them sand erosion and sand deposition as coastal dunes are being either stabilized or remobilized in response to the changes in the rates of sand and dune movement the vegetation cover and composition are expected to vary reflecting the differences in the sedimentary conditions in this field study 315 quadrats of 100 m2 in which the perennial plant species were sampled were analysed with respect to annual rates of sand erosion and deposition that were measured using erosion pins a visual exploratory data analysis was introduced based on selective filtering of samples according to their vegetation cover this method combined with established statistical tools enabled the authors to uncover the inclination and indicative power of nine perennial dune plants to either a stabilized or a mobile environment and to establish whether they are more prevalent in places undergoing sand erosion or sand burial two species were found to be clear indicators of a stabilized environment stipagrostis lanata and retama raetam of the species indicating a mobile environment only one may be stated as a clear indicator of sand erosion silene succulenta with cyperus macrorrhizus coming close to being an indicator of a less mobile erosive environment the best indicator species for sand burial was found to be as expected ammophila arenaria with artemisia monosperma also indicating high rates of sand mobility especially when its relative cover is higher than 80 such information can be used to monitor natural processes of dune stabilization or reactivation or to assess the success of a management plan that aims at stabilizing a dune or at remobilizing it by removing vegetation

influence of vegetation on aeolian sand transport rate from a backshore to a foredune at hasaki japan the influence of vegetation on aeolian sediment transport rate in the region from a backshore to a foredune was investigated at the hasaki coast in japan where an onshore wind was predominant and the creeping beach grasses carex kobomugi and calystegia soldanella were major species the comparison of cross shore distributions of the cross shore component of aeolian sand transport rate with and without vegetation which were estimated on the basis of the beach profile changes and a mass conservation equation showed that the creeping grasses influenced the aeolian sand transport rate the landward aeolian sand transport rate rapidly decreased landward from the seaward limit of vegetation when the grasses grew the aeolian sand transport rate reduced by 95 with a vegetation cover of 28 on the other hand when the grasses were absent the landward aeolian sand transport rate did not decrease near the seaward vegetation limit but near the foot of the foredune

nonuniform response of the major surface currents in the nordic seas to insolation forcing implications for the holocene climate variability 1 high resolution sediment cores from the voring plateau the north iceland shelf and the east greenland shelf have been studied to investigate the stability of major surface currents in the nordic seas during the holocene results from diatom assemblages and reconstructed sea surface temperatures ssts indicate a division of the holocene into three periods the holocene climate optimum 9500 6500 calendar cal years bp the holocene transition period 6500 3000 cal years bp and the cool late holocene period 3000 0 cal years bp the overall climate development is in step with the decreasing insolation on the northern hemisphere but regional differences occur regarding both timing and magnitude of sst changes sites under the direct influence of the norwegian atlantic current and the irminger current indicate sst cooling of 4 5 c from early holocene to present compared to 2 c recorded under the east greenland current superimposed on the general holocene cooling trend there is a high frequency sst variability which is in the order of 1 1 5 c for the voring plateau and the east greenland shelf and 2 5 3 c on the north iceland shelf

aeolian system responses to global change challenges of scale process and temporal integration aeolian sediment systems are extensive components of the earth s surface not just in today s active desert environments but in coastal locations and as presently stable loess and peri desert ergs all these systems are subject to potentially significant 21st century changes in dynamics due to global warming impacts on atmospheric circulation sediment moisture and vegetation cover but do we know enough in the critical areas of science to allow robust models of likely future dynamics to be developed since the establishment of modern aeolian science research has largely taken two divergent routes 1 small scale investigations of the processes of wind shear sediment entrainment and transportation and 2 landform and landscape analyses with bedform systems described and relationships to circulation explained an intermediate sedimentological route has generated attempts towards a more integrated understanding of system behaviour at a range of scales four recent developments provide the potential for more robust multiscale integrations of aeolian system behaviour these are 1 advances in empirical measurement that allow wind power and sediment flux relations to be better described 2 modelling of dune and dust dynamics and interactions with key controls 3 monitoring systems that allow high resolution observations of sediment flux variations to be recognized and aeolian deposit structures to be understood and 4 chronometric advances that permit the assessment of the frequency and drivers of dune system activity in the past we explore and analyse these methodological advances and their potential contribution to the multiscale integrations that are essential for better informed predictions of future global aeolian system dynamics considering whether the integration of these advances in reductionist and holistic elements of aeolian science represent an example of the effective application of the principles of earth system science copyright 2008 john wiley sons ltd

quantifying relationships of burning roughness and potential dust emission with laser altimetry of soil surfaces at submeter scales abstract decrease in surface roughness by the reduction of vegetation is one mechanism by which fire can promote aeolian transport in a variety of landscapes the extent to which fire might alter the roughness of the soil surface at fine spatial scales and the effect of this alteration on post fire aeolian response is not well known we examined relationships in the field between dust emissions and subcentimeter level soil surface roughness at submeter spatial scales with a terrestrial laser scanner and portable wind tunnel analog based on aeolian theory we hypothesized that observed relationships would differ from those determined in previous studies with laser altimetry at landscape scales meter kilometer length scales we examined four semiarid shrublands in southern idaho usa containing a distinct pattern of undershrub and interspace microsites including recently burned and unburned conditions mixed models were used to determine effects of burning on surface roughness and the response of dust emissions to changes in surface roughness results indicated that burned soil surfaces were rougher than unburned soil surfaces dust emissions were enhanced by increases in roughness on burned soil surfaces and in the absence of nonerodible roughness elements i e plants this finding is expectedly in contrast to previous work that demonstrated an inverse relationship between soil erosion and surface roughness determined with lidar light detection and ranging at landscape scales vegetated and unvegetated surfaces relationships of lidar derived roughness and aeolian emissions are scale dependent and vary with environmental factors of fire and vegetation these findings are integral for future research that attempts to parameterize model based predictions of aeolian emissions with lidar derived roughness

little ice age clearly recorded in northern greenland ice cores four ice cores drilled in the little investigated area of northern and northeastern greenland were evaluated for their isotopic 18o and chemical content from these rather uniform records a stable isotope temperature time series covering the last 500 years has been deduced which reveals distinct climate cooling during the 17th and the first half of the 19th century timing of the preindustrial temperature deviations agrees well with other northern hemisphere temperature reconstructions however their extent 1 c significantly exceeds both continental records as well as previous southern and central greenland ice core time series a 20 30 increase in the sea salt aerosol load during these periods supports accompanying circulation changes over the north atlantic comparison with records of potential natural climate driving forces points to an important role of the long term solar influence but to only episodically relevant cooling during years directly following major volcano eruptions

u th he dating of apatite a potential thermochronometer abstract we found a gem quality crystal of durango fluorapatite to have a 4he content consistent with complete retention of radiogenic helium since its formation at about 31 ma isothermal heating and step heating analysis reveal 4he loss to occur systematically by volume diffusion at low temperatures the linear low temperature portion of the diffusion data yields an activation energy of 38 5 8 1 kcal mol and a frequency factor of in d 0 a 2 16 4 2 8 sec 1 corresponding to a closure temperature of 105 c 30 c cooling rate 10 c m y it appears that u th he dating of apatite might represent a useful new thermochronometer with a range similar to that of fission track dating of apatite from these results we infer that a number of the too young u th he dates reported in the literature on minerals such as zircon and magnetite may in fact represent valuable records of the low temperature thermal history of their host rocks

holocene sea ice variations in greenland onshore evidence the oldest dated driftwood log from northern greenland is c 9300 cal years old which is about 2000 years younger than the beginning of the last deglaciation and 2000 years younger than the oldes

quaternary eolian dunes in the savannah river valley jasper county south carolina usa sand hills in the savannah river valley in jasper county south carolina usa are interpreted as the remnants of parabolic eolian dunes composed of sand derived from the savannah river and stabilized by vegetation under prevailing climate conditions optically stimulated luminescence ages reveal that most of the dunes were active ca 40 to 19 ka ago coincident with the last glacial maximum lgm through early deglaciation modern surface winds are not sufficient for sustained eolian sand transport when the dunes were active winds blew at velocities of at least 4 m s from west to east and some vegetation was present the ratio of annual precipitation to potential evapotranspiration p pe was less than the modern ratio of 1 23 and may have been 0 30 caused by stronger winds which would have resulted in greater evaporation and or reduced precipitation the savannah river dunes are part of a larger assemblage of eolian dunes that were active in the eastern united states during and immediately after the lgm suggesting that eolian sediment behavior in this region has been controlled by regional forcing mechanisms during the quaternary

early holocene history of the west greenland ice sheet and the gh 8 2 event abstract the margin of the greenland ice sheet retreated rapidly during the first few thousand years of the holocene during this period of relative warmth known as the holocene thermal maximum ice core records identify a significant short lived cooling event at approximately 8 4 8 0 ka cal yr bp the gh 8 2 event associated with a 5 7 c fall in mean annual air temperature over the centre of the ice sheet in this paper we constrain the history of the ice sheet margin in disko bugt west greenland and that of a major ice stream jakobshavns isbrae during the early holocene which incorporates the interval of the gh 8 2 event our work is based on a new relative sea level curve and minimum age estimates for the timing of deglaciation from two field sites combined with a review of previously published research from the study area we identify important differences in the chronology of ice margin recession during the early holocene most noticeably the margin of jakobshavns isbrae retreated well inland of the adjacent ice sheet at this time we conclude that the early holocene fjord stade moraines in disko bugt do not record a uniform ice sheet margin response to the gh 8 2 event rather these moraines are diachronous and formed between c 10 8 ka cal yr bp their age varying as a function of the interplay between topography and ice sheet ice stream dynamics we hypothesise that one cause for the lack of an identifiable response to the gh 8 2 event is because topographic controls dominated ice sheet behaviour at this time in lowland areas any increase in ice sheet mass balance was probably associated with an increase in calving rather than any major advance of a grounded ice sheet margin

the climate of scotland over the last 5000 years inferred from multiproxy peatland records inter site correlations and regional variability the mid to late holocene climates of most of scotland have been reconstructed from seven peat bogs located across north south and east west geographical and climatological gradients the main techniques used for palaeoclimatic reconstruction were plant macrofossil colorimetric humification and testate amoebae analyses which were supported by a radiocarbon based chronology aided by markers such as tephra isochrons and recent rises in pine pollen and in spheroidal carbonaceous particles scps field stratigraphy was undertaken at each site in order to show that the changes detected within the peat profiles were replicable proxy climate records were reconstructed using detrended correspondence analysis dca of the plant macrofossil data and a mean water table depth transfer function on the testate amoebae data these reconstructions coupled with the humification data were standardised for each site and used to produce a composite record of bog surface wetness bsw from each site the results show coherent wet and dry phases over the last 5000 years and suggest regional differences in climate across scotland specifically between northern and southern scotland distinct climatic cycles are identified all of which record a millennial scale periodicity which can be correlated with previously identified marine and ice core holocene cycles the key role of the macrofossil remains of sphagnum imbricatum a taxon now extinct on many sites is discussed in relation to the identified climatic shifts

environmental controls morphodynamic processes and ecogeomorphic interactions of barchan to parabolic dune transformations the transformation of barchans into parabolic dunes has been observed in various dune systems around the world precise details of how environmental controls influence the dune transformation and stabilisation mechanism however remain poorly understood a horns anchoring mechanism and a nebkhas initiation mechanism have previously been proposed and selected environmental controls on the transformation have been explored by some modelling efforts but the morphodynamic processes and eco geomorphic interactions involved are unclear and comparison between different dune systems is challenging this study extends a cellular automaton model informed by empirical data from fieldwork and remote sensing to fully explore how vegetation characteristics boundary conditions and wind regime influence the transformation process and the resulting dune morphologies a dynamic growth function is introduced for clump like perennials to differentiate between growing and non growing seasons and to simulate the development of young plants into mature plants over multiple years modelling results show that environmental parameters interact with each other in a complex manner to impact the transformation process the study finds a fundamental power law relation between a non dimensional parameter group so called the dune stabilising index s and the normalised migration distance of the transforming dune which can be used to reconstruct paleo environmental conditions and monitor the impacts of changes in climate or land use on a dune system four basic eco geomorphic interaction zones are identified which bear different functionality in the barchan to parabolic dune transformation the roles of different environmental controls in changing the eco geomorphic interaction zones transforming processes and resulting dune morphologies are also clarified

the dynamic effects of moisture on the entrainment and transport of sand by wind abstract there is an increasing awareness of the influence of surface moisture on aeolian entrainment and transport of sediment existing wind tunnel studies have shown the impact of a limited range of moisture contents on entrainment thresholds but similar investigations are lacking in the field the research reported here investigated the influence of changes in surface moisture content on sand entrainment and transport on a meso tidal beach in anglesey north wales high frequency 1 hz wind velocities measured with hot wire anemometers were combined with grain impact data from a sensit monitor and mass flux measurements from a standard sand trap surface and near surface moisture contents were assessed gravimetrically from surface sand scrapes and also directly by using a thetaprobe critical threshold values for entrainment were specified using a modified form of the time fraction equivalence method stout j e zobeck t m 1996a establishing the threshold condition for soil movement in wind eroding fields proceedings of the international conference on air pollution from agricultural operations mwps c 3 kansas city 7 9 february 1996 pp 65 71 results indicate a time dependent change in dominant control of the sand transport system from moisture to wind speed dependent upon the moisture content of the surface sediment this interchange between controlling parameters on both entrainment and transport was very sensitive to prevailing moisture conditions and took place over a period of minutes to hours under conditions experienced in the experiments presented here the critical moisture threshold for sediment entrainment was determined to be between 4 and 6 higher than the 1 4 specified in previous wind tunnel experiments furthermore a moisture content of nearly 2 where moisture was adhered to transported sediment appeared to have little or no impact on the rate of sand flux

the spatial and temporal variability of sand erosion across a stabilizing coastal dune field this study aimed at quantifying the temporal and spatial variability in sand erosion and deposition over a coastal dune field in israel these were measured monthly over 2 years using 315 erosion pins over four transects that were placed perpendicular to the coastline vegetation cover was estimated based on aerial photographs and landsat satellite images whereas the relative height was based on a digital elevation model these variables were calculated for the area upwind south west of the erosion pins at various lengths ranging from 15 to 400 m nine geomorphologic units were defined five related to active units and four to stabilized units in active units at least 65 of the temporal variance in the annual absolute changes in sand level was explained by the index of resultant drift potential with most of the sand movement occurring during winter storms local rainfall had no apparent impact on sand mobility due to the low coincidence of sand carrying winds and rainfall in israel during the passage of frontal cyclones as for the spatial variables only a weak correlation was found between sand mobility with the distance from the coastline r 2 18 rather sand erosion and deposition were influenced by vegetation cover and the relative height of an area of 100 200 m upwind the values of soil adjusted vegetation index were significantly negatively correlated with annual absolute changes r 2 40 whereas the relative height was significantly positively correlated r 2 36 applying a multiple regression model 68 of the spatial variability in sand mobility was explained the resulting map of sand activity clearly shows that at this stage of the stabilization process most of the dunes are now disconnected and movement of sand grains from the beach or between the dunes is very limited these methods can be applied into spatial and temporal models of sand mobility thus assessing the impact of different management practices on coastal dunes

a decade of surface change on a namib linear dune survey data from return visits to a linear dune in the namib desert provide information about change in dune form over a decade the data demonstrate that although change on these large features is slow there is none the less considerable movement over this time scale the dunes are therefore confirmed to be currently active and not pleistocene relics

understanding glacier flow in changing times subannual lurches of the greenland and antarctic ice sheets may reduce uncertainties about climate change effects on sea level rise

surface properties of stabilizing coastal dunes combining spectral and field analyses the coastal dunes of israel have been undergoing a process of stabilization since 1948 one of the major features of this process is a change in the surface properties of the dunes the development of a biological soil crust bsc and a change in the properties of the sand grains themselves in ashdod at the southern coastal plain of israel sand properties that include the bsc their fines silt and clay content and free iron oxide indicating their degree of rubification have been analysed in detail using field and lab spectroscopy methods in addition sand erosion and deposition were measured using erosion pins to determine their effect on the presence of the above mentioned factors it was found that the bsc over these dunes is comprised of green algae that differs in its reflectance spectra from cyanobacterial crust especially in the blue band the crust was found to be particularly developed in the stable areas mainly the interdunes and on the north facing slopes rather than on the south facing slopes a positive correlation was found between the crust fines and chlorophyll content with stable areas showing more developed bsc the stable areas showed also a lower albedo and slightly more developed reddish colour indicating a slightly higher rate of rubification this study demonstrates that the intensity of sand erosion deposition rates affects soil properties with the bsc being the fastest to react to the stabilization process months to several years followed by the content of fine particles several years to a decade whereas the rubification process is a much weaker marker and may need much longer time periods to develop decades to centuries

anomalously mild younger dryas summer conditions in southern greenland the first late glacial lake sediments found in greenland were analyzed with respect to a variety of environmental variables the analyzed sequence covers the time span between 14400 and 10500 calendar yr b p and the data imply that the conditions in southernmost greenland during the younger dryas stadial 12800 11550 calendar yr b p were characterized by an arid climate with cold winters and mild summers preceded by humid conditions with cooler summers climate models imply that such an anomaly may be explained by local climatic phenomenon caused by high insolation and fohn effects it shows that regional and local variations of younger dryas summer conditions in the north atlantic region may have been larger than previously found from proxy data and modeling experiments

variations in wind velocity and sand transport on the windward flanks of desert sand dunes the magnitudes of increases in wind velocity or speed up factors have been measured on the windward flanks of transverse and linear dunes of varying height on transverse dunes velocity speed up varied with dune shape and height for linear dunes speed up factors varied principally with wind direction relative to the dune with dune shape and dune height the main effect of velocity speed up on the windward flanks of dunes is to increase potential sand transport rates considerably in crestal areas this is greatest for large dunes with winds of moderate velocity blowing at a large angle to the dune changing ratios of base to crest sand transport rates on transverse dunes tend to reduce dune steepness as overall wind velocities increase on linear dunes the tendency for crestal lowering is counteracted by deposition in this area when winds reverse in a bi directional wind regime

new chronological evidence for the nature and timing of linear dune development in the southwest kalahari desert abstract the age of the linear dunes in the southwest kalahari desert has not previously been directly established by applying the optical dating method to samples from the dunefield a number of questions relating to the timing and nature of linear dune development are addressed the last major period of simple linear dune development occurred 17 000 10 000 years ago probably from the reworking of earlier aeolian sediments dune emplacement occurred under more arid probably windier conditions than those prevailing today which are only responsible for episodic surficial dune sand movement as well as simple linear dunes the dunefield also contains more complex linear dune patterns samples from one area in the eastern dunefield demonstrate that multi phase development is more probable for complex dune patterns as opposed to single phase evolution under a complex wind regime

oxygen isotope and palaeotemperature records from six greenland ice core stations camp century dye 3 grip gisp2 renland and northgrip oxygen isotope variations spanning the last glacial cycle and the holocene derived from ice core records for six sites in greenland camp century dye 3 grip gisp2 renland and northgrip show strong similarities this suggests that the dominant influence on oxygen isotope variations reflected in the ice sheet records was regional climatic change differences in detail between the records probably reflect the effects of basal deformation in the ice as well as geographical gradients in atmospheric isotope ratios palaeotemperature estimates have been obtained from the records using three approaches i inferences based on the measured relationship between mean annual 18o of snow and of mean annual surface temperature over greenland ii modelled inversion of the borehole temperature profile constrained either by the dated isotopic profile or iii by using monte carlo simulation techniques the third of these approaches was adopted to reconstruct holocene temperature variations for the dye 3 and grip temperature profiles which yields remarkably compatible results a new record of holocene isotope variations obtained from the northgrip ice core matches the grip short term isotope record and also shows similar long term trends to the dye 3 and grip inverted temperature data the northgrip isotope record reflects i a generally stronger isotopic signal than is found in the grip record ii several short lived temperature fluctuations during the first 1500 yr of the holocene iii a marked cold event at ca 8 2 ka the 8 2 ka event iv optimum temperatures for the holocene between ca 8 6 and 4 3 ka a signal that is 0 6 stronger than for the grip profile v a clear signal for the little ice age and vi a clear signal of climate warming during the last century these data suggest that the northgrip stable isotope record responded in a sensitive manner to temperature fluctuations during the holocene copyright 2001 john wiley sons ltd

late quaternary linear dune accumulation and chronostratigraphy of the southwestern kalahari implications for aeolian palaeoclimatic reconstructions and predictions of future dynamics abstract the linear dunes of the kalahari now largely inactive have long been identified as having potential palaeoenvironmental significance the application of optically stimulated luminescence osl dating to these dunes in the 1990s provided the first chronology of aeolian accumulation in this region though field methodologies and time consuming multiple aliquot laboratory protocols limited both the depth of sampling in dune bodies and the total number of samples dated in order to permit a more thorough investigation of the potential of these dunes to preserve long chronological records this intensive study presents 71 osl ages from the linear dunes of the southwestern kalahari at witpan south africa sampled with coring equipment at regular and frequent intervals down to bedrock the earliest sand accumulation recorded at witpan is at 104 ka and in spatially discrete locations other evidence of dune activity is recorded at 77 76 57 52 and 35 27 ka although an inherently discontinuous archive the linear dunes of the southwestern kalahari have the potential to record multiple phases of dune construction following the last glacial maximum there is near continuous evidence of dune building with a peak of accumulation recorded from 15 to 9 ka at five individual sites this latter period is generally recognised from other proxy evidence as being unusually arid in this region and such periods of dune activity are likely to be related to intensification of the continental anticyclone during the holocene accumulation has continued at most sites sampled albeit at a lesser intensity this may imply that these dunes are presently not far from thresholds of activation

barchan parabolic dune pattern transition from vegetation stability threshold 1 many dune fields exhibit a downwind transition from forward pointing barchan dunes to stabilized backward pointing parabolic dunes accompanied by an increase in vegetation a recent model predicts this pattern transition occurs when dune surface erosion deposition rates decrease below a threshold of half the vegetation growth rate we provide a direct test using a unique data set of repeat topographic surveys across white sands dune field and find strong quantitative support for the model threshold we also show the threshold hypothesis applied to a barchan dune results naturally in its curvature inversion as the point of threshold crossing progresses from the horns to the crest this simple general threshold framework can be an extremely useful tool for predicting the response of dune landscapes to changes in wind speed sediment supply or vegetation growth rate near the threshold a small environmental change could result in a drastic change in dune pattern and activity

dune sand transport as influenced by wind directions speed and frequencies in the ordos plateau china the ordos plateau in china is a region with extensive wind erosion severe desertification and various aeolian sand hazards in order to determine aeolian sand transport in this region the relationship between the sand transport rate and wind speed at 10 min frequencies was established by field observation in both the qubqi sand desert and the mu us sandy land threshold wind speeds 2 m above the ground for mobile semi fixed and fixed dune surfaces were estimated by field observations the sand transport rate increased with the increase of the bare land ratio and near bed wind speed high resolution meteorological 10 min average wind velocity data at 10 m above the ground were converted into velocity values at a height of 2 m to calculate sand transport potential based on three specific parameters decisive for sand transport wind speed duration and direction the quantity of aeolian sand transported was calculated for various wind speed levels and directions and the overall characteristics of sand transport on different dune surface types were determined by vector operation techniques sand transporting winds took place mainly in springtime the prevailing wind directions were w wnw and nw with a frequency of more than 60 in total and sand transport in these directions made up more than 70 of the total transport corresponding to a general southeastward encroachment of aeolian sand in the study area the relationship between wind frequency and speed can be expressed by a power function high magnitude strong winds had a low frequency but they played a dominant role in aeolian sand transport

response of a marine terminating greenland outlet glacier to abrupt cooling 8200 and 9300 years ago greenland s largest outlet glacier using 10 be surface exposure ages and 14 c dated lake sediments our chronology of ice margin change demonstrates that jakobshavn isbrae advanced to deposit moraines in response to abrupt cooling recorded in central greenland ice cores ca 8 200 and 9 300 years ago while the rapid dynamically aided retreat of many greenland outlet glaciers in response to warming is well documented these results indicate that marine terminating outlet glaciers are also able to respond quickly to cooling we suggest that short lag times of high ice flux margins enable a greater magnitude response of marine terminating outlets to abrupt

challenges of calculating dunefield mobility over the 21st century attention has been directed towards both the impacts of future climate change on environmental systems and dunefield activity in the past but there has been relatively little consideration of potential dune mobility in a future and possibly warmer world this paper considers the use and limitations of four global circulation models gcms hadcm3 hadcm2 csiro mk2b and cgcm1 in combination with simple dune mobility indices to predict the activity of the kalahari dunefield it is clear that uncertainties surround gcm resolution and accuracy mobility index robustness for the calculation of intra annual dune mobility and data collection for mobility index calibration macro scale studies that look at large areas of the world over long time scales are well suited to gcm and mobility index use but dune mobility can be variable within a dunefield and it is the extreme sand transporting events occurring at high temporal resolutions that are the most important for short term studies to investigate intra annual changes in dune mobility over a specific dunefield techniques such as downscaling weather generators and probability curve fitting can help provide climate predictions for smaller areas over shorter time frames however these methods introduce uncertainty of their own and they often rely on the accuracy of original gcm predictions or the climate parameter relationships observed at present analysis of intra annual changes also requires mobility indices that can model monthly mobility patterns well although existing indices have only been used for calculating annual dune mobility potential when they are used for intra annual predictions the lack of lag response between precipitation decreases and the assumed vegetation dieback leads to an exaggerated amplitude pattern of dune mobility throughout the year calibration of dune mobility indices to dune mobility observed on the ground is therefore important but is hampered by a lack of observed measurements for individual months solutions are available to overcome some of the outlined problems but they can provide their own set of uncertainties which combine to further reduce the confidence given to future dune mobility predictions

geomorphology of sand dunes in the northeast taklimakan desert three types of sand dunes exist in the taklimakan desert namely compound complex crescent dunes and crescent chains compound dome dunes and compound complex linear dunes besides these three compound complex types single simple dunes are also distributed throughout the sand sea the compound complex linear dunes are developed under acute bimodal wind regimes though the ratios of the resultant drift potential rdp and the drift potential dp are the same as that near the border and adjacent area of the sand sea the compound complex crescent and dome dunes are developed respectively because of divergence of the sand available the stress of the sand moving winds and the time scales of dune formation the sand supply for the dunes is not from lopo nor in the east as previous studies suggested but mainly from local alluvial or lacustrine deposits the grain size component does not correlate evidently to the morphology parameters of the sand dunes analyses of the dp and drift direction suggest that the northeast taklimakan is an area of low wind energy and the resultant drift direction rdd coincides well with the distribution morphology and scales of the dunes

interglacial and holocene temperature reconstructions based on midge remains in sediments of two lakes from baffin island nunavut arctic canada abstract midge remains insecta diptera chironomidae and chaoboridae from two upland lake basins on the cumberland peninsula of baffin island were analyzed and the results used in a transfer function to reconstruct paleotemperatures this upland region remained ice free throughout the last glacial cycle and the two lake basins retain sediments deposited during the previous interglacial period as well as during the holocene period but they did not accumulate sediment during most of the glacial period midge remains are abundant and well preserved at both sites midge inferred summer surface water temperatures and mean july air temperatures were estimated using an inference model built on modern samples spanning a geographic range extending from devon island canada to maine usa data from twenty nine new surface samples from baffin island were added to an existing inference model the new weighted averaging wa model for summer surface water temperature yields r 2 jack 0 88 and rmsep 2 22 c for summer water temperatures and r 2 jack 0 88 and rmsep 1 53 c for mean july air temperatures reconstructions at both sites indicate that summer temperatures during the last interglacial were higher than at any time in the holocene and 5 to 10 c higher than present peak holocene temperatures occurred in the first half of the period and have decreased since about the mid holocene

nebkha formation implications for reconstructing environmental changes over the past several centuries in the ala shan plateau china abstract sediments of chinese tamarisk tamarix chinensis lour nebkhas in china s arid ala shan plateau document both the formation processes of these vegetated dunes and the environmental changes and geomorphological evolution in this region dating of the nebkhas showed that after surface water disappeared the wadis and playas evolved into gobi deserts desert pavements within two centuries the 13c trends in the litter deposited within the nebkhas revealed three stages of fluctuating moisture conditions during the past five centuries before the 1580s between the 1580s and the late 1990s and from the early 2000s to the present however changes in the coarse fraction of the nebkha sediments show that this region experienced at least five phases of aeolian sediment transport during the same period the early stages of nebkha formation coincided with the initial stage when the wadis and playas evolved into gobi deserts and nebkha sizes increased rapidly due to the abundant sediment supply provided by the playas however after the playa surfaces stabilized the nebkha sediments documented environmental changes in adjacent regions our results also show that the formation of the tamarix nebkhas in this region was not controlled by the groundwater table because in most periods the water table depth remained above the depth that limits tamarix survival in the ala shan plateau nebkha formation was mainly controlled by aeolian processes and revealed the integrated effects of environmental changes thereby providing possibilities for climate and environment reconstructions with a relatively high resolution in extremely arid areas

monitoring surface change on a namib linear dune in tackling the apparently intractable problem of linear dune initiation and maintenance there has been a move away from large scale deductive models to smaller scale field studies of individual dunes this paper reports a study of surface change on a large complex linear dune in the namib desert southern africa the dune surface responds to a markedly seasonal wind regime in summer westerly winds erode sand from the west flank of the dune and deposit it on the easterly lee side of the dune crest in winter this pattern is reversed easterly winds erode sand from the east slope and deposit it on the west slope the crest therefore moves back and forth some 15 m each year returning at the end of a year s cycle to its position at the beginning the position of the base of the dune appears to remain fixed even though sand is moving throughout the dune system the dune does extend northward along some resultant of the westerly and easterly winds despite relatively high levels of activity especially at the dune crest there is no evidence of the breakdown of the linear dune form the conclusion must therefore be that linear dunes can be maintained in bimodal wind regimes and are not necessarily related to unidirectional parallel regimes as others have suggested

late quaternary environmental change in central southern africa new data synthesis issues and prospects much of the interior of central southern africa is a sand sea within which aeolian and lacustrine landforms and sediments of local and regional extent are preserved closed cave sites are restricted to very few locations while fluvial systems traverse the margins of the interior until the early 1990s chronologies of late quaternary environmental and climatic changes developed for this region were based on only a limited number of proxy data sets derived largely from lacustrine deposits and precipitates in particular directly determined ages from aeolian deposits the most extensive suite of features in the region were absent the application of luminescence dating techniques to dune sediments and the development of further detailed chronologies from cave precipitates is now providing a more comprehensive record for the last 50 ka with some chronologies extending back a further 100 ka we present and review these data assess their contribution to enhanced understanding of late quaternary environmental changes in the region and for the first time assess them against corrected radiocarbon ages from lacustrine sites it is concluded that there is now an enhanced understanding of the spatial and temporal complexity of climate changes affecting the region in the last glacial cycle including a complex record of punctuated aridity but that many issues including data set integration and forcing mechanism controls are imperfectly understood

dunefield activity response to climate variability in the southwest kalahari abstract aeolian activity on the crests of linear dunes in the arid southwest kalahari desert is primarily controlled by vegetation cover cover is temporally variable in response to rainfall with mobility index mi values and field based studies demonstrating the dynamic nature of aeolian processes in this dunefield landsat tm data for a drought 1984 and wetter 1993 year were analysed for two subscenes in the dune field one for a pastoral landscape and one for a conservation area to investigate the spatial dynamics of dune activity using the empirically determined 14 cover threshold as the limit to crestal aeolian activity tm band 3 data were found to best represent vegetation cover including senesced material at the end of the dry season the period when the potential for aeolian activity is normally high due to the wind strength at this time the extent of 0 14 cover areas ranged from 10 1984 to 3 1993 for the pastoral area and 16 1984 to 6 1993 for the conservation landscape the wetter 1993 image residuals have different spatial characteristics in the two subscenes with bare active dunes in the pastoral lands associated with locations of livestock pressure and those in the conservation area related to natural fire events the study demonstrates the spatial dynamics of temporally variable linear dune activity in the southwest kalahari and also shows that areas devegetated by drought can recover sufficient vegetation in wet years to limit the spatial extent of aeolian transport on dune crests

vegetated dune morphodynamics during recent stabilization of the mu us dune field north central china the response of dune fields to changing environmental conditions can be better understood by investigating how changing vegetation cover affects dune morphodynamics significant increases in vegetation and widespread dune stabilization over the years 2000 2012 are evident in high resolution satellite imagery of the mu us dune field in north central china possibly a lagged response to changing wind strength and temperature since the 1970s these trends provide an opportunity to study how dune morphology changes with increasing vegetation stabilization vegetation expansion occurs mainly by expansion of pre existing patches in interdunes as vegetation spreads from interdunes onto surrounding dunes it modifies their shapes in competition with wind driven sand movement primarily in three ways 1 vegetation anchoring horns of barchans transforms them to parabolic dunes 2 vegetation colonizes stoss faces of barchan and transverse dunes resulting in lower dune height and an elongated stoss face with shortening of barchan horns and 3 on transverse dunes the lee face is fixed by plants that survive sand burial along each of these pathways of stabilization dune morphology tends to change from more barchanoid to more parabolic forms but that transformation is not always completed before full stabilization artificial stabilization leads to an extreme case of frozen barchans or transverse dunes with original shapes preserved by rapid establishment of vegetation observations in the mu us dune field emphasize the point that vegetation growth and aeolian sand transport not only respond to external factors such as climate but also interact with each other for example some barchans lose sand mass during vegetation fixation and actually migrate faster as they become smaller and vegetation growth on a barchan s lower stoss face may alter sand transport over the dune in a way that favors more rapid stabilization conceptual models were generalized for the development of vegetation stabilized dunes which should be helpful in better understanding of vegetated dune morphology model verification and prediction and guiding practical dune stabilization efforts

a mid holocene shift in arctic sea ice variability on the east greenland shelf records of iceberg rafting and palaeohydrography from two east greenland shelf cores jm96 1206 1 gc and jm96 1207 1 gc are reported benthic foraminifera stable isotopes and ird e uxes indicate a shift toward colder lower salinity polar conditions c 5 cal ka a new proxy of iceberg rafting on the east greenland shelf is the e ux of calcium carbonate tic thought to be derived from glacial erosion of cretaceous calcareous mudstones a change in the regularity and spacing of carbonate e ux peaks at c 4 7 cal ka in jm96 1207 coincides with the onset of neoglacial cooling in the renland ice core d 18 o record we propose that the carbonate e ux peaks between 4 7 and 0 4 cal ka are related to sea surface coolings associated with increased e ux of polar water and sea ice in the east greenland current these peaks are synchronous with sea surface coolings interpreted from north atlantic deep sea cores but additional peaks centred around 2 4 and 3 8 cal ka in jm96 1207 suggest that the shelf site captures higher frequency events the data indicate that severe arctic sea ice events began in the neoglacial interval and that earlier holocene cool events in deep sea records are associated with other processes such as release of meltwater from residual glacier ice and glacial lakes

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

holocene temperature history at the western greenland ice sheet margin reconstructed from lake sediments abstract predicting the response of the greenland ice sheet to future climate change presents a major challenge to climate science paleoclimate data from greenland can provide empirical constraints on past cryospheric responses to climate change complementing insights from contemporary observations and from modeling here we examine sedimentary records from five lakes near jakobshavn isbrae in central west greenland to investigate the timing and magnitude of major holocene climate changes for comparison with glacial geologic reconstructions from the region a primary objective of this study is to constrain the timing and magnitude of maximum warmth during the early to middle holocene positive anomaly in summer insolation temperature reconstructions from subfossil insect chironomid assemblages suggest that summer temperatures were warmer than present by at least 7 1 ka the beginning of the north lake record ka thousands of years before present and that the warmest millennia of the holocene occurred in the study area between 6 and 4 ka previous studies in the jakobshavn region have found that the local greenland ice sheet margin was most retracted behind its present position between 6 and 5 ka and here we use chironomids to estimate that local summer temperatures were 2 3 c warmer than present during that time of minimum ice sheet extent as summer insolation declined through the late holocene summer temperatures cooled and the local ice sheet margin expanded gradual insolation driven millennial scale temperature trends in the study area were punctuated by several abrupt climate changes including a major transient event recorded in all five lakes between 4 3 and 3 2 ka which overlaps in timing with abrupt climate changes previously documented around the north atlantic region and farther afield at 4 2 ka

multiple episodes of aridity in southern africa since the last interglacial period there is generally a dearth of evidence of the nature of quaternary climate change within desert systems which has limited previous interpretations of past environmental change at low latitudes the last glacial maximum has previously been identified as the peak of late quaternary aridity when desert systems expanded to five times their present extent1 2 3 and low latitude aridity has been described for previous glaciations4 but little evidence has been derived directly for large desert basins particularly southern africa here we report new chronological optical dating evidence of arid episodes recorded in aeolian sediments from the mega kalahari sand sea episodic aeolian activity is recorded at the northeastern desert margin whereas more sustained activity is evident from the southwestern desert core several significant arid events are apparent since the last interglacial period with dune building arid phases at 95 115 41 46 20 26 and 9 16 kyr before present existing atmospheric general circulation model simulations and independent palaeoclimate data indicate that the changes in aridity are related to changes in the northeast southwest summer rainfall gradient which are in turn related to sea surface temperatures in the southeastern atlantic ocean

linear dune accumulation chronologies from the southwest kalahari namibia challenges of reconstructing late quaternary palaeoenvironments from aeolian landforms abstract the linear dunes of the southern kalahari dunefield constitute one of the major palaeoenvironmental proxies in the region the application of optically stimulated luminescence osl dating since the1990s and advancements in the depth of sampling using augering equipment over the past few years have permitted the reconstruction of linear dune accumulation chronostratigraphies for entire dune profiles from base to crest these methods are applied to four dunes in the mariental stampriet region of the southern kalahari dunefield sampled at predominantly 0 5 m intervals individual dunes record multiple phases of dune construction but with only a few phases recorded consistently between two or more of the dunes results from the 48 osl ages produced here extend the aeolian accumulation record for the southern kalahari dunefield through the last three glacial interglacial cycles with two ages from the early part of mis6 a synthesis of all existing luminescence ages for the southern kalahari reveals that the dunefield has been partially active throughout much of the past 120 ka there are no clear clusters of ages within osl age errors this is in contrast to previous syntheses of ages for this region in addition these new data from mariental stampriet dunes show that clusters in grouped dune osl ages can be spuriously produced as a function of reducing the sampling frequency with depth within the dunes from 0 5 to 1 mintervals this has significant implications for previous conclusions regarding discrete phases of aeolian accumulation based on sampling at 1 m intervals and less vertically intensive sampling techniques the total luminescence data set of 136 ages for the southern kalahari implies that this dunefield has been close to the threshold of reactivation throughout much of the late quaternary

a 10 000 year record of arctic ocean sea ice variability view from the beach we present a sea ice record from northern greenland covering the past 10 000 years multiyear sea ice reached a minimum between 8500 and 6000 years ago when the limit of year round sea ice at the coast of greenland was located 1000 kilometers to the north of its present position the subsequent increase in multiyear sea ice culminated during the past 2500 years and is linked to an increase in ice export from the western arctic and higher variability of ice drift routes when the ice was at its minimum in northern greenland it greatly increased at ellesmere island to the west the lack of uniformity in past sea ice changes which is probably related to large scale atmospheric anomalies such as the arctic oscillation is not well reproduced in models this needs to be further explored as it is likely to have an impact on predictions of future sea ice distribution

on the origin and age of the great sand dunes colorado abstract over the past 100 yr several hypotheses have been proposed for the origin and age of the great sand dunes these hypotheses differ widely in the descriptions of dune morphometry the immediate source of eolian sand and when sand transport occurred the primary purpose of this paper is to evaluate these hypotheses and where warranted to present new ideas about the origin and age of the great sand dunes to evaluate the previous hypotheses we had to develop more detailed information about the surficial geology of the northern san luis valley thus we mapped the surficial geology of an area extending several tens of kilometers north south and west of the great sand dunes and examined subsurface stratigraphy in more than 200 wells and borings in addition we used relative dating criteria and several radiocarbon and osl ages to establish the chronology of surficial deposits and we determined the u pb ages of detrital zircons to obtain information about the sources of the sand in the great sand dunes the first principal finding of this study is that the lower part of the closed basin north of the rio grande referred to here as the sump is the immediate source of the sand in the great sand dunes rather than the late pleistocene flood plain of the rio grande the most widely accepted hypothesis a second principal finding is that the great sand dunes are older than late pleistocene they postdate the draining of lake alamosa which began 440 ka and predate the time when streams draining the west flank of the sangre de cristo mountains were deflected by incipient dunes that formed near the mountain front geomorphic and stratigraphic evidence indicate that this deflection occurred prior to the end of the next to last glaciation bull lake i e prior to 130 ka

airflow and roughness characteristics over partially vegetated linear dunes in the southwest kalahari desert there is little understanding of the flow field surrounding semi vegetated linear dunes and predictions of dune mobility are hampered by a lack of empirical data concerning windflow in an attempt to characterize the near surface airflow upwind of and over partially vegetated linear dunes in the southwest kalahari desert this study presents measurements of vertical and horizontal wind velocity profiles across cross sectional transects of seven partially vegetated linear dunes vegetation surveys combined with velocity measurements from vertical arrays of cup anemometers placed up to 2 3 m above the ground surface were used to gain information concerning the modification of airflow structure caused by the intrusion of the dunes into the atmospheric boundary layer and to predict the variability of aerodynamic roughness z0 from interdune to crest the results suggest an acceleration of flow up the windward slopes of the dunes and as such the data correspond to classical theory concerning flow over low hills essentially jackson and hunt 1975 principles where the theory is incapable of explaining the airflow structure and acceleration characteristics this is explained in part by the presence of a spatially variable vegetation cover over the dunes the vegetation is important both in terms of the varying aerodynamic roughness z0 and problems concerning the definition of a zero plane displacement d it is considered that any attempts to characterize surface shear stress over the kalahari linear dunes in order to predict sand transport and dune mobility will be hampered by two problems these are the progressively non log linear nature of the velocity profiles over the dunes caused by flow acceleration and the production of thin near surface boundary layers caused by areally variable aerodynamic roughness as a result of the partially vegetated nature of the dunes

mapping and measuring aeolian sand dunes with photogrammetry and lidar from unmanned aerial vehicles uav and multispectral satellite imagery on the paria plateau az usa abstract the paria plateau is a potentially important but relatively unstudied aeolian sand source area in the grand canyon region of arizona usa this study used unmanned aerial vehicle uav based lidar and structure from motion sfm photogrammetry to produce high resolution topographic models of aeolian dunes on the plateau we combined the dune topography data with a high resolution satellite image maximum likelihood classification producer s accuracy 87 2 to quantify potential aeolian sand source areas across the 958 km2 plateau we mapped all the unvegetated active aeolian dunes on the plateau and estimate they contain 24 tg of sand and could therefore be a proportionately important regional sand source considering the annual sand loads of the downwind paria river at its confluence with the colorado river are generally

weathering indices as climate proxies a step forward based on congo and sw african river muds abstract despite the influence of other geological and geomorphological factors chemical weathering at the earth s surface is strongly controlled by climate thus a measure of weathering intensity determined from soils or sediments should provide information about the climatic conditions associated with their formation available geochemical and mineralogical data on modern fluvial and marine muds from different regions of southern africa and its atlantic continental margin are used to review the links between sediment composition and climatic properties together with the possible causes of variance although river muds may not be generated exclusively in a single sedimentary cycle and erosion and weathering processes do not necessarily take place in a spatially homogeneous way significant relationships between mineralogical and geochemical signatures of river mud and rainfall in the corresponding catchment area were recognised our study shows that the composition of clay is strongly influenced by climatically driven weathering whilst coarser mud fractions tend to be more affected by provenance grain size hydraulic sorting and recycling in the marine environment the climatic signal may be lost even in clay because of hydraulic fractionation authigenic mineral growth and mixing with foreign particles given the ubiquitous character of fluvial muds and the easy and non expensive methods available for separating and analysing clay fractions their geochemical fingerprints represent a most precious source of information concerning climate any geochemical parameter used as a regional proxy of climate however still requires that the diversity of geological geomorphological and biological factors that affect its value are cautiously considered

paleomagnetic chronology and paleoenvironmental records from drill cores from the hetao basin and their implications for the formation of the hobq desert and the yellow river abstract reconstructing the cenozoic environmental history of hetao basin in the northern part of the ordos plateau in north china is important not only for revealing the evolution of the yellow river but also for understanding the formation of the hobq desert here we present the stratigraphic framework of drill core dr01 with length of 2503 18 m and the results of magnetostratigraphic and esr dating and multi proxy analyses of drill core wedp05 with length of 274 60 m from the hetao basin the magnetostratigraphic and esr results indicate that core wedp05 spans the last 1 68 ma stratigraphic sequence of core dr01 indicates that the hetao area was uplifted and eroded during the early cenozoic before subsiding to form a sedimentary basin subsequently the basin was a fluvio lacustrine environment during the pliocene and then experienced alternating desert and fluvio lacustrine conditions during the quaternary sedimentary facies and multi environmental proxy analyses of core wedp05 indicate that the basin was occupied by a fluvio lacustrine system during the following intervals 1 47 1 30 ma 1 17 1 07 ma 0 68 0 60 ma and from 0 47 ma to the last interglacial and that a desert environment developed during the lake regression phases of 1 30 1 17 ma 1 07 0 68 ma and 0 60 0 47 ma the presence of aeolian sand at the base of core wedp05 suggests that the origin of the hobq desert can be traced back to the early pleistocene and resulted from the erosion and transportation of exposed fluvio lacustrine sediments by near surface winds associated with the asian winter monsoon a large river channel in the hetao basin may have existed as early as the pliocene which was occupied by the yellow river when its upper reaches formed by at least 1 6 ma subsequently at least since 1 2 ma the yellow river formed its drainage system around the hetao basin and controlled the paleoenvironment evolution of the basin

early holocene climate variability and the timing and extent of the holocene thermal maximum htm in northern iceland the magnitude and timing of holocene maximum warmth in the arctic and sub arctic has been the subject of considerable recent interest particularly in the context of future climate change although lying at a crucial location in the north atlantic close to significant atmospheric and oceanic boundaries terrestrial holocene climatic data from iceland are few and predominantly derive from glacial and palaeoecological evidence here we present new datasets from trollaskagi based on chironomid inferred temperatures ci t using sub fossil chironomids from the same lake sediments supplemented by pollen data july air temperatures have been derived using an icelandic training set and the data suggest optimal temperatures at sea level up to 1 5 c above current levels around 8 k cal yr bp a time when birch woodland was well developed in trollaskagi but when woodland had still not fully developed in the more isolated nw peninsula our data thus suggest that optimal summer warmth did not occur in iceland until 8 kcal yr bp at the earliest possibly lasting until 6 7 kcal yr bp the amount of warming for july was therefore at least 1 5 c but possibly up to 2 3 c higher than the 1961 1990 average on the basis of the tree line data comparison with data from elsewhere in adjacent arctic regions greenland and eastern arctic canada show peak warmth to be later in iceland and less pronounced it also appears that there were enhanced temperature gradients during the first half of the holocene between the two study areas trollaskagi and the nw peninsula and that they influenced patterns of vegetation colonisation with current spatial temperature patterns only developing as holocene climate deteriorated after around 6 kcal yr bp

the dynamics of star dunes an example from the gran desierto mexico observations of patterns of erosion and deposition and surface wind velocity and direction on a 40 m high star dune in the gran desierto sand sea indicate that interactions between dune form and airflow as winds change direction seasonally play a major role in the formation of this dune type such interactions lead to deposition of sand in the central parts of the dune giving rise to its pyramidal shape as well as to some extension of the linear arms the major arms of the dune studied are oriented ne sw or transverse to summer sse and winter nnw winds an avalanche face up to 10 m high develops during the course of each season flow separation at the main crestline gives rise to a wide zone of lee side secondary flow which moves sand along the base of the avalanche face towards the central part of the dune where it is deposited as wind ripples migrate into zones of locally reduced flow velocity reattachment of the separated flow occurs on the lower part of the n or s arms parallel to the flow spring westerly winds move sand obliquely up the s and n arms of the dune and outwards on the e arm large scale flow separation and diversion are replaced by the development of strong helical eddies in the immediate lee of the main crestline which move sand along avalanche faces and into zones of lower flow velocity at the end of dune arms formation of star dunes in the gran desierto follows a sequence in which crescentic dunes migrating into areas of opposed winds first develop a reversing crestal ridge convergent leeside secondary flows are developed which result in the formation of linear elements parallel to each major wind direction and the concentration of sand in the central part of the dune examples of star dunes at different stages of their development can be documented

spatial and temporal variability of holocene temperature in the north atlantic region the early holocene climate of the north atlantic region was influenced by two boundary conditions that were fundamentally different from the present the presence of the decaying laurentide ice sheet lis and higher than present summer solar insolation in order to assess spatial and temporal patterns of holocene climate evolution across this region we collated quantitative paleotemperature records at sub millennial resolution and synthesized their temporal variability using principal components analysis pca the analysis reveals considerable spatial variability most notably in the time transgressive expression of the holocene thermal maximum htm most of the region but especially areas peripheral to the labrador sea and hence closest to the locus of lis disintegration experienced maximum holocene temperatures that lagged peak summer insolation by 1000 3000 years many sites from the northeastern north atlantic sector including the nordic seas and scandinavia either warmed in phase with maximum summer insolation 11 000 9000 years ago or were less strongly lagged than the baffin bay labrador sea region these spatially complex patterns of holocene climate development which are defined by the pca resulted from the interplay between final decay of the lis and solar insolation forcing

the impact of fire on sand dune stability surface coverage and biomass recovery after fires on western australian coastal dune systems from 1988 to 2016 abstract this study aims to determine the common response of coastal sand dunes in western australia wa to fire on decadal time scales in terms of ecological geomorphic climatic interactions to test the hypothesis that fire plays a role in coastal dune destabilisation fires are commonly suggested to have contributed to widespread dune reactivation in australia and globally a hypothesis that is relatively untested we used data from the landsat thematic mapper enhanced thematic mapper plus and operational land imager missions to monitor changes in surface coverage on coastal sand dunes in south west wa after fires we analysed 31 fire scars from 1988 to 2016 in two landsat scenes on the west and south coast of wa recovery ratios derived from the normalised difference vegetation index ndvi were used to monitor patterns in post fire biomass and surface cover recovery ratios are correlated with indices of burn severity and meteorological data to investigate relationships we also used maximum likelihood classification to monitor changes in bare sand area results suggest that recovery followed a strongly consistent pattern and is characterised by rapid vegetation cover re establishment within six to twelve months prior to this some aeolian activity may have occurred but without substantial surface changes initial germination and or resprouting were followed by steady growth up to seven years where ndvi typically neared pre fire values some variation in early recovery occurred between the west and south coast possibly owing to relative proportions of reseeding and resprouting plants a log regression explained 75 of the recovery pattern 79 on the south coast precipitation had some ability to explain recovery up to nine months post fire r 2 0 29 to 0 54 no relationships were observed between estimates of burn severity and recovery after nine months the biggest cause of spatial variation in recovery was the pre fire community composition and related seedbank or resprouting density image classification did not identify any new blowout features except where fires were not the primary cause results suggest that fires are not presently contributing to the destabilisation of coastal dunes in south west wa

late quaternary aeolian dunes on the presently humid blue mountains eastern australia abstract sand dunes on the newnes plateau 1000 m a s l west of sydney were active during the last glacial maximum the scattered sand dunes are forested under the modern humid temperate climate regime dune types range from parabolic to transverse lee dunes and sand sheets or patches all point to the presence of conditions marginal for aeolian activity made possible through wind acceleration on windward slopes ready sand supply from the weathered sandstone of the plateau and sparse vegetation cover modern climate envelopes of sand dune activity in australia predict that unrealistically drier conditions are necessary to allow wind transport at this site only additional impediments to plant growth such as lower temperature and lower atmospheric carbon dioxide concentrations appear to allow the necessary conditions for dune formation these observations and conclusions extend our understanding of the extremes of the lgm climate in humid eastern australia confirming that the widespread treeless vegetation was also sparse even in areas that today have annual rainfall above 1000 mm

modelling palaeosol preservation in aeolian dunes in some of the world s desert and desert marginal areas e g simpson strzelecki australia dunefields preserve well developed palaeosols whereas in other regions with broadly similar climatic regimes and topography e g southwest kalahari the dunes are characterized by very poorly developed internal stratigraphy it has been postulated that dunes such as those in the kalahari may never have had conditions conducive to soil formation or that soils once formed but any evidence of palaeosols has been lost due to reworking this study develops and applies a one dimensional numerical model to simulate dune development soil formation and soil preservation variables in the model allowed experimentation on the influence of sediment supply the time taken for soil to form and the additional resistance to erosion offered by the soil reduced sediment supply plays a vital role in landscape development during periods of initial pedogenesis although re exhumed palaeosols influence sediment supply the effect is minimal although under almost all parameterized conditions more than half and up to 80 90 of those soils initially formed are lost due to reworking evidence of their past formation remains in the large majority of profiles and the dominant factor in controlling the preservation of palaeosols is the frequency of their formation the implication is that where dunes are found without palaeosols the most likely albeit not certain inference is that they have never formed counter intuitively the limited sediment supply means their additional resistance to erosion becomes almost inconsequential to their preservation at least until the unit approaches complete invulnerability short chronostratigraphic hiatuses around palaeosols are normal and although long gaps can occur they are extremely infrequent where such gaps are observed in field studies external forcing factors e g climatic or environmental changes are implied as they are highly unlikely to result from stochastic net preservation copyright 2014 john wiley sons ltd

understanding linear dune chronologies insights from a simple accumulation model abstract dunefields are today often found in regions where environmental conditions are no longer conducive to widespread aeolian transport and deposition and are thus seen as potential archives of palaeoenvironmental information some dune types are more suitable for this purpose than others and linear dunes used here synonymously with longitudinal dunes have been most frequently employed they are commonly found in dryland and dryland marginal regions and they are apparently less migratory than some other dune forms thereby offering the potential for longer environmental histories most recent studies have employed optical osl dating surveys which directly date the emplacement of dune sands to attempt to produce a representative history of aeolian accumulation at a dunefield scale yet such studies have not always given due consideration to the geomorphology of linear dune accumulation although linear dunes do not rework their sand as readily as say barchans dunes it is clear that sand deposited during an aeolian event is likely to have been removed from elsewhere on the dunefield s surface for this reason all dune archives must be assumed to be discontinuous over long timescales the effect of sediment redistribution on dune preservation and thus palaeoenvironmental interpretation is difficult to observe and poorly understood we present a simplified probabilistic model to replicate deposition erosion of sand at a linear dune crest thus simulating the type of profile sampled in many field studies in order to produce a local accumulation history the modelled sand flux incorporates various feedbacks and the simulations are run with varying boundary conditions in which the simulated surface is assumed to be in equilibrium over the multi millennial timescales of the model s duration periodic instabilities in the landscape are generally well recorded by the modelled dune accumulation history particularly when the events involve the system as a whole briefly functioning out of an equilibrium state i e the probabilities of deposition and reworking are not equal however under forcing conditions where the probabilities are unchanged throughout the duration of the model run the results can show apparent structure within the data when sample sizes are low although at large sample sizes the preservation potential decays with burial age according to a power law the model confirms the importance of rigorous sampling strategies for such studies but also suggests that if such precautions are taken the reworking inherent to linear dune aeolian systems does not preclude their use as a quaternary palaeoenvironmental archive of environmental change

punctuated aridity in southern africa during the last glacial cycle the chronology of linear dune construction in the northeastern kalahari abstract the mega kalahari of central southern africa is one of the most extensive quaternary desert basins on a regional scale present day aeolian activity is restricted to episodic dune crest reactivation in the most arid southwestern desert core there is however abundant evidence of former periods of both more arid and more humid conditions many of which have little or no chronological control we have employed optical dating of quartz sand grains to develop a chronology of arid intervals as recorded by phases of linear dune construction in the northeastern sector of the mega kalahari we identify repeated phases of aeolian deposition during the last interglacial glacial cycle at ca 95 115 41 46 20 26 and post 20 ka which are separated by depositional hiatuses that we infer to correspond to more humid periods these aeolian depositional events correlate with and are inferred to relate to millennial scale cold sea surface temperature events in the southeast atlantic which have been linked to sub milankovitch climate changes recognised in northern hemisphere oceanic and cryospheric environmental archives covering the same time period while the present landscape is the product of either post 20 ka hwange national park dune field or 20 30 ka victoria falls dune field aeolian activity and subsequent erosion and reworking much of the vertical expression of the larger dune forms corresponds to the earlier periods of activity the linear ridges of the area are rich archives of late quaternary terrestrial climate change

variable vegetation cover and episodic sand movement on longitudinal desert sand dunes abstract longitudinal linear sand dunes of the simpson and strzelecki dunefields in eastern central australia present a paradox low levels of activity today stand in contrast to luminescence dating which has repeatedly shown deep deposits of sand on dune crests dating to within the late holocene in order to investigate the nature of dune activity in the simpson strzelecki dunefield vegetation and sand mobility were investigated by detailed vegetation survey and measurement of rippled area and loose sand depth of dunes at three sites along a climatic gradient the response of both vegetation and sand movement to inter annual climate variability was examined by repeat surveys of two sites in drought and non drought conditions projected plant cover and plant crust cover were found to have inverse linear relationships with rippled area and the area of deep loose sand no relationship was found between these measures of sand movement and the plant frontal area index a negative exponential relationship between equivalent mobile sand depth on dune surfaces and both vascular plant cover and vascular crust cover was also found there is no simple threshold of vegetation cover below which sand transport begins dunes with low perennial plant cover may form small dunes with slip faces leading to a positive feedback inhibiting ephemeral plant growth in wet years and accelerating sand transport rates the linear dunefields are largely within the zone in which plant cover is sufficient to enforce low sand transport rates and in which there is a strong response of vegetation and sand transport to inter annual variation in rainfall both ephemeral plants mostly forbs and crust were found to respond rapidly to large 20 mm month rainfall events on millennial time scales the level of dune activity is controlled by vegetation cover and probably not by fluctuations of wind strength land use or extreme decadal time scale drought may destabilise dunes by removing perennial plant cover accelerating wind erosion

age and dynamics of linear dunes in the namib desert bristow c s duller g a t lancaster n 2007 age and dynamics of linear dunes in the namib desert geology 35 6 555 558 sponsorship american chemical society

field measurement and analysis of climatic factors affecting dune mobility near grand falls on the navajo nation southwestern united states abstract aeolian sand covers extensive areas of the navajo nation in the southwestern united states much of this sand is currently stabilized by vegetation although many drier parts of these native lands also have active and partly active dunes current prolonged drought conditions that started in the mid 1990s are producing significant changes in dune mobility reactivation of regional aeolian deposits due to drought or increasing aridity from rising temperatures resulting from climate change could have serious consequences for human and animal populations agriculture grazing and infrastructure to understand and document the current and future potential for mobility seasonally repeated surveys were used to track the location of multiple active barchan dunes by utilizing real time kinematic gps field surveys and simultaneously collecting in situ meteorological data it is possible to examine climatic parameters and seasonal variations that affect dune mobility and their relative influences through analysis of the recorded data we examined the fit of various climate parameters and demonstrate that under the current prolonged drought wind power is the dominant factor controlling dune mobility

a comparative analysis of nebkhas in central tunisia and northern burkina faso this paper reports a comparative analysis of nebkha morphology in central tunisia and northern burkina faso based on a total of 473 measured nebkhas nebkhas are mounds composed of wind borne sediment that accumulated around shrubs the size of the mounds was measured by height h and a horizontal component l the latter being the mean of the length and width of the mound in the tunisian data set three trends of development of the mounds were distinguished for ziziphus lotus nebkhas initially the height increases linearly with the increase in the horizontal component until it reaches about 17 m when the horizontal component exceeds 17 m the height stabilizes before a decreasing trend of the height occurs these results provided the basis for mathematical modelling of nebkha development a non linear and a linear fit were compared and a second order polynomial function was found to best fit the data on nebkha size from tunisia the results indicate that nebkhas have three stages of development a growing a stabilizing and a degrading stage the model was applied to data on acacia sp nebkhas and balanites aegyptiaca nebkhas from burkina faso with good results the non linear fit was slightly better than the linear fit nebkhas exist at all three stages of development in tunisia whereas burkina faso has nebkhas that are mainly at the growing stage these differences are a function of time lag in land degradation between the two study areas because the wind transport of sediment and the sediment supply are highly related to the density of the vegetation cover in semiarid areas this conclusion in further supported by dating of nebkhas and of historical and recent vegetation changes in the study areas reported in the literature the results show that differences in plant ecologies however are also very important for the rate of development of nebkhas which is demonstrated by the much faster growth of balanites nebkhas as compared to acacia nebkhas even though they occur in the same type of environment

significance of variations in the wind energy environment over the past 50 years with respect to dune activity and desertification in arid and semiarid northern china climate data from 339 meteorological stations collected from the 1950s to the early 2000s was employed to discuss aeolian activity in arid and semiarid northern china the results show that at decadal time scales erosivity varied greatly in this region most of arid and semiarid northern china was characterised by environments with moderate to low wind energy after the 1980s the erosivity was only 20 to 50 of that beforehand and this difference had a significant impact on the environmental changes observed during the two periods the dune mobility index was consistent with the observed dune activity after the 1980s dune activity decreased and in some deserts with vegetated dune systems during the 1970s most of the dune plinths were active until the 1980s after which only the crests were active some mobile dunes that had developed at the margins of mobile deserts were replaced by semi anchored or anchored dunes because most desert areas with vegetated dune systems in arid and semiarid northern china are used for farming or grazing the results of our monitoring show that the desertification trends were consistent with the trends in erosivity and dune activity in this region desertification was controlled much more by climatic changes than has previously been acknowledged and especially by fluctuations in wind energy

palaeoenvironmental reconstructions from linear dunefields recent progress current challenges and future directions abstract this paper reviews recent progress in the use of linear dunes as geoproxies of late quaternary environmental change summarises the challenges facing their use and explores some potential solutions to these challenges large areas of the swathes of linear dunes which occupy the continental interior of southern africa australia and parts of central asia and southern america currently have limited or negligible aeolian activity they have been recognised as offering potential information about past environments for more than a century but only with the widespread application of luminescence dating during the 1990s did they realistically start to offer the prospect of being an extensive dateable proxy of late quaternary palaeoenvironments and possibly palaeoclimates dating aeolian dune sands with luminescence methods is generally although not always relatively straightforward over the past twenty years a large number 1000 of luminescence ages have been added to the global dataset yet there has also been significant criticism of some of the rationale underpinning much of the interpretation of the records derived at the landscape scale developments of arguably equal importance have come from improved geomorphological understanding based on the wider availability of remotely sensed data and the paradigm of dunefield evolution as a self organising complex system current challenges are identified in three key regions incomplete understanding of how the process geomorphology of linear dunes affect the accumulation and preservation of sediment a lack of clarity regarding the temporal and spatial scale of the response in a dynamic environmental setting and uncertainty surrounding the drivers of changing rates of net accumulation solutions to these challenges lie within diverse research methodologies certainly further field study is required with improvement required in understanding system responses to changing environmental stimuli at scales from sedimentological to landscape in parallel the full implications of complex systems approaches to aeolian geomorphology for linear dunes lag behind the adoption of the concept to some other dune forms e g barchans the relationship between observed dune complexity and age from field data complicates previous suggestions that dune patterning may be purely a function of development time parallel to this lies the need to improve interpretation of the results of dune geochronological studies a suggestion is offered which aims to test the statistical significance of dune accumulation time series by comparison with a modelled system with unchanging external forcing despite the initial promise of linear dune records as a revolutionary source of palaeoenvironmental information in drylands the outcomes to date have been coarse although still valuable in resolution perhaps the most valuable realization has been that geomorphological understanding of these widespread landforms is incomplete

development of spatially diverse and complex dune field patterns gran desierto dune field sonora mexico the pattern of dunes within the gran desierto of sonora mexico is both spatially diverse and complex identification of the pattern components from remote sensing images combined with statistical analysis of their measured parameters demonstrate that the composite pattern consists of separate populations of simple dune patterns age bracketing by optically stimulated luminescence osl indicates that the simple patterns represent relatively short lived aeolian constructional events since 25 ka the simple dune patterns consist of i late pleistocene relict linear dunes ii degraded crescentic dunes formed at 12 ka iii early holocene western crescentic dunes iv eastern crescentic dunes emplaced at 7 ka and v star dunes formed during the last 3 ka recognition of the simple patterns and their ages allows for the geomorphic backstripping of the composite pattern palaeowind reconstructions based upon the rule of gross bedform normal transport are largely in agreement with regional proxy data the sediment state over time for the gran desierto is one in which the sediment supply for aeolian constructional events is derived from previously stored sediment ancestral colorado river sediment and contemporaneous influx from the lower colorado river valley and coastal influx from the bahia del adair inlet aeolian constructional events are triggered by climatic shifts to greater aridity changes in the wind regime and the development of a sediment supply the rate of geomorphic change within the gran desierto is significantly greater than the rate of subsidence and burial of the accumulation surface upon which it rests

wind energy variations in the southwestern kalahari desert and implications for linear dunefield activity the southwestern kalahari linear dunefield which displays marked morphological variability possesses a partial but temporally and spatially variable vegetation cover and has frequently been described as a palaeodunefield palaeo status has been ascribed on the basis of several criteria including the presence of vegetation but also because dunes are thought to be out of alignment with modern resultant potential sand moving wind directions and because present day wind energy is regarded as low for the period 1960 1992 wind data from eight dunefield meteorological stations are analysed in detail to examine these assertions potential sand transport directions including spatial and temporal variations and potential drift directions for the windiest three month periods are calculated and explained it is concluded that the present day potential sand transport environment is markedly variable from year to year and from place to place while periods of low sand transport energy do occur it is also noted that the 1980s possessed considerable potential for sand transport in the dunefield directional variability is also relatively high perhaps exceeding that under which linear dunes can be expected to form because linear dune aeolian activity has a number of states however the present day wind environment may allow dune surface aeolian activity to occur which does not alter the overall pattern of the dunes

midges and palaeotemperature reconstruction the north american experience abstract the development of midge based palaeoclimatology in north america is reviewed with a focus on lateglacial and holocene climatic change although relatively few sites have been studied midge inferred palaeotemperatures reveal detailed records of lateglacial climate oscillations and more subtle holocene temperature changes holocene climate changes are often at or near the detection limit using current palaeotemperature inference models interference from other environmental variables including water depth and oxygen concentration changes are discussed as well as the challenges inherent to developing better models the isotopic signature of chitin preserved in midge head capsules has opened new avenues for midge palaeotemperature inference through coordinated research efforts national and international collaborations will provide opportunities to reconstruct holocene palaeoclimate at continental and hemispheric scales

influence of reed stem density on foredune development vegetation density on foredunes exerts an important control on aeolian sediment transport and deposition and therefore on profile development in a long term monitoring field experiment three plots were planted with regular grids of reed bundles in three different densities 4 2 and 1 bundles per m2 this study reports on the differences in profile development under the range of vegetation densities topographic profiles were measured between may 1996 and april 1997 results indicate important differences in profile development for the three reed bundle densities in the highest density plot a distinct steep dune developed while in the lowest density a more gradual and smooth sand ramp was deposited when the stems had been completely buried differences in profile evolution vanished after a second planting of reed stems in january 1997 the process was repeated in may 1997 all plots had gained a sand volume ranging from 11 5 to 12 3 m3 m 1 indicating that the sediment budget is relatively constant regardless of the particular profile evolution the field evidence is compared with simulations of profile development generated by the foredune development model safe the model successfully reproduces the overall profile development but in general the equations used for vegetation transport interaction overestimate the effect of vegetation this causes some deviations between field and model results several reasons for this are discussed based on the experiments reported here recommendations are given for further research copyright 2001 john wiley sons ltd

the timing and environmental significance of late quaternary linear dune development in western zambia optical luminescence dates for 19 samples from the degraded linear dune field of western zambia indicate multiple periods of regional dune building for the late quaternary 32 000 27 000 16 000 13 000 10 000 8000 and 5000 4000 yr ago these dates show that the last glacial maximum was not the only time when dune construction commonly linked to marked aridity occurred in central southern africa during the late quaternary whereas rainfall significantly less than today s ca 1400 mm yr 1 is a prerequisite for dune construction in the area adequate sediment supply also determines dune construction and preservation so that dune building cannot be simply and singularly linked to marked aridity the zambezi river system is proposed as an important source of dune sediments with the nature of linear dune activity explaining why stacked sediments preserve several phases of dune formation chronologies of dune construction in western zimbabwe and the southwest kalahari are in broad agreement with our zambian chronology and support a model of rainfall shifts along a sw ne gradient with some notable disparities these are probably a function of interregional sediment supply differences the number of samples used to delimit constructional periods and the multicausal nature of forcing mechanisms