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

palaeoenvironmental change recorded in the palaeodunefields of the western murray basin south australia new data from single grain osl dating this study is concerned with the reconstruction of the palaeoenvironmental history of southeastern australia for the last 300 ka by establishing a luminescence chronology of dune sand deposition in the western murray basin south australia in the study area vast fields of palaeodunes stabilised by vegetation provide evidence of past environmental change in total 98 samples were collected from dune sand layers at 13 different dune sections the time of their deposition was determined using optically stimulated luminescence dating of single quartz grains accounting for the impact of post depositional mixing by the use of a finite mixture model the oldest depositional phase demonstrates that dune sand layers of great antiquity are preserved in the western murray basin ranging up to at least 380 ka phases of substantial dune sand deposition were identified for the periods 18 38 ka and 63 72 ka older depositional phases also exist but are poorly resolved due to relatively large errors of the luminescence ages aeolian deposition during the last termination and the holocene is relatively limited with a slight clustering of ages at the time of the antarctic cold reversal and from 5 8 ka two modern ages give evidence of very recent dune sand deposition comparison with other palaeoclimate records from the region suggests that phases with high aeolian sedimentation coincide with more arid conditions and breaks in the dune record with more humid phases thus although dune records are often discontinuous and their interpretation in palaeoclimatic terms is not always straightforward the palaeodunes of the western murray basin show a good preservation of phases of aeolian activity and provide useful information for palaeoenvironmental reconstruction

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

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

interpreting geoproxies of late quaternary climate change in african drylands implications for understanding environmental change and early human behaviour abstract climate and environmental change in the late quaternary are widely regarded as key drivers of early human development and dispersal in africa robust records of change are required to assess the nature of potential impacts today s late quaternary palaeoenvironmental palaeoclimate reconstructions from the interior southern african basin kalahari are more spatially detailed have a stronger chronometric underpinning and span a longer time range than those of 10 20 years ago but are no less controversial reconstructions for the last 150 ka are usually interpreted in terms of changes in hydrological balance from conditions more humid or more arid than today and expressed in climatological terms as the changing latitudinal interplay between monsoon easterlies and tropical and mid latitude westerlies with more data than ever before why then is the record not yet delivering a more coherent picture of regional climate change this paper deliberately addresses current challenges and controversies including issues often ignored or neglected with the view that it is essential to do so if more robust interpretations of the past are to be generated in the future sources of palaeo data from the interior are predominantly geoproxies reconstructions not based on highly resolved organic or isotope records but on interpretation of spatially extensive geomorphological proxies of past environments despite being increasingly datable using osl their inclusion in syntheses has proved complex not least when interpretations appear to conflict with records from other sources consequently they are sometimes removed from regional syntheses of climate change these omissions are often replaced by spatially exaggerated interpretations of higher quality local records allowing more spatially coherent timeslice interpretations to be presented yet this is counter intuitive to the spatial complexity of conditions present in the palaeoenvironmental proxies or in climatic and environmental systems today in addition the quest to link regional records to master records from ice and ocean cores can force interpretation within a temporal scale that may be inappropriate for their analysis underplaying the reality of complexity within the environmental system there are not necessarily good or bad records of terrestrial palaeoenvironments in interior southern africa rather there are records that contain different suites of information about past land surface hydrological and climate conditions preserved at temporal scales which can sometimes be incomparable consideration of these factors may resolve apparent conflicts between datasets resulting in a better understanding of past environmental dynamics and revealing the nature of environmental variability which may be a key facet to which human development has responded

is there evidence for global scale forcing of southern hemisphere quaternary desert dune accumulation a quantitative method for testing hypotheses of dune system development luminescence dating of desert dune sediments has generated many hundreds of ages many used in reconstructions of quaternary environmental changes others in attempts to elucidate dune processes environmental and climatic interpretations of these records have proved problematic and it remains challenging to test hypotheses of the systematic response of dunefields to changes in external forcing in the past and to make predictions of the future we use a method that quantifies dune sediment accumulation to interpret dune luminescence age datasets rather than simply using the ages themselves as proxies of change the accumulation intensity method allows periods of dune sediment accumulation here over the timescale 102 105 years to be identified from compilations of dated sand sea stratigraphic sequences we apply this approach to two of the largest dune age datasets from southern africa and australia testing whether or not dunefield accumulation has co varied in the late quaternary and whether systematic relationships to external drivers at global hemispheric regional and local scales can be identified

late quaternary aeolian and fluvial interactions on the cooper creek fan and the association between linear and source bordering dunes strzelecki desert australia abstract the innamincka dome and associated low gradient fan in the strzelecki desert is the product of cenozoic crustal warping that has aided formation of an extensive array of palaeochannels source bordering transverse dunes and superimposed linear dunes these dunes have impeded the course of cooper creek and provided a repository of evidence for quaternary climate change as well as the interactive processes between transverse and linear dune formation at turra gidgealpa and sites nearby are extensive fluvial and aeolian sand bodies that date from marine isotope stages mis 8 3 and the last glacial maximum lgm and are now surrounded or buried by overbank mud the sandy alluvium was deposited on the downstream slope of the dome by large channels transporting abundant bedload subsequently blown northward to form transverse dunes from what were probably seasonally exposed bars in a palaeo cooper system thermoluminescence tl and optically stimulated luminescence osl ages demonstrate that the base of the dune complex is at least mis 7 in age 250 ka but that it has been subsequently reworked by wind with additional sand blown from the river source bordering dunes formed during a period of enhanced river flow and sand supply from 120 to 100 ka with another short episode of the same at 85 80 ka and from 68 to 53 the lgm was associated with enhanced flows and the supply of dune sediment from 28 to 18 ka pronounced river flow and dune activity occurred in the early to mid holocene but there is no evidence of dunes being supplied from cooper creek since the lgm the dunes forming the oldest basal sand units appear to be largely transverse in form and are aligned roughly parallel to adjacent east west trending palaeochannels linear dunes have formed from and over these and yield basal ages ranging from mis 5 or mis 4 but continuing to accrete and rework through to the holocene the study results in one of the few detailed chronological investigations of the interaction between transverse and linear dunes it is apparent that long distance sand transport has played no significant role in dune formation here for the linear dunes show no significant downwind decline in ages linear dunes appear to have accreted vertically from underlying transverse dunes a wind rift vertical accretion model with only minor lengthwise extension is the dominant mode of linear dune formation in this section of the strzelecki desert the bulk of dune sediment being sourced from adjacent swales since the lgm

reconstructing paleoenvironments and palaeoclimates in drylands what can landform analysis contribute quaternary period palaeoenvironmental and palaeoclimatic reconstructions are based on a wide and diverse array of proxy data sets some of which are geomorphological in nature in drylands where organic proxies may be limited the use of landforms is particularly important but challenging the capacity to establish the age of depositional forms particularly through the use of luminescence dating has advanced the use of landforms in dryland palaeo research though interpretation of these geoproxy records can be complex especially at the nexus of palaeoclimate and palaeoenvironmental interpretations of past conditions in this paper the use of aeolian and lacustrine forms in quaternary research is considered focusing on the relationships between dynamics form and climate and on the essential linkage between process research and palaeoenvironmental research it is concluded that landform analysis is a critical part of dryland palaeoenvironmental climate reconstruction contributing a different set of data compared to other data sources in terms of the elements of past conditions that are revealed five principles are identified to improve the use of geoproxy records in quaternary research 1 greater use of geomorphic process studies by quaternary scientists to better inform palaeolandform interpretation 2 further development of the use of chronometric data especially in terms of interpreting large data 3 interpret landform records in location specific contexts not in general terms 4 capitalise of the complexity of spatially extensive landform records which may offer better representations of real quaternary environmental complexity than at a point proxies 5 establish ways of integrating spatially extensive geoproxy records with other palaeoenvironmental records these challenges are major but not insurmountable and should represent goals for geomorphologists chronologists and quaternary scientists alike copyright 2011 john wiley sons ltd

variations in the strontium isotopic ratio of seawater during the miocene stratigraphic and geochemical implications a composite strontium isotopic seawater curve was constructed for the miocene between 24 and 6 ma by combining 87sr 86sr measurements of planktonic foraminifera from deep sea drilling project sites 289 and 588 site 289 with its virtually continuous sedimentary record and high sedimentation rates 26 m m y was used for constructing the oligocene to mid miocene part of the record which included the calibration of 63 biostratigraphic datums to the sr seawater curve using the timescale of cande and kent 1992 across the oligocene miocene boundary a brief plateau occurred in the sr seawater curve 87sr 86sr values averaged 0 70824 which is coincident with a carbon isotopic maximum cm o m from 24 3 to 22 6 ma during the early miocene the strontium isotopic curve was marked by a steep rise in 87sr 86sr that included a break in slope near 19 ma the rate of growth was about 60 ppm m y between 22 5 and 19 0 ma and increased to over 80 ppm m y between 19 0 and 16 ma beginning at 16 ma between carbon isotopic maxima cm3 and cm4 of woodruff and savin 1991 the rate of 87sr 86sr growth slowed and 87sr 86sr values were near constant from 15 to 13 ma after 13 ma growth in 87sr 86sr resumed and continued until 9 ma when the rate of 87sr 86sr growth decreased to zero once again the entire miocene seawater curve can be described by a high order function and the first derivative d87sr 86sr dt of this function reveals two periods of increased slope the greatest rate of 87sr 86sr change occurred during the early miocene between 20 and 16 ma and a smaller but distinct period of increased slope also occurred during the late miocene between 12 and 9 ma these periods of steepened slope coincide with major phases of uplift and denudation of the himalayan tibetan plateau region supporting previous interpretations that the primary control on seawater 87sr 86sr during the miocene was related to the collision of india and asia the rapid increase in 87sr 86sr values during the early miocene from 20 to 16 ma imply high rates of chemical weathering and dissolved riverine fluxes to the oceans in the absence of another source of co2 these high rates of chemical weathering should have quickly resulted in a drawdown of atmospheric co2 and climatic cooling through a reversed greenhouse effect the paleoclimatic record however indicates a warming trend during the early miocene culminating in a climatic optimum between 17 and 14 5 ma we suggest that the high rates of chemical erosion and warm temperatures during the climatic optimum were caused by an increase in the contribution of volcanic co2 from the eruption of the columbia river flood basalts crfb between 17 and 15 ma the decrease in the rate of crfb eruptions at 15 ma and the removal of atmospheric carbon dioxide by increased organic carbon burial in monterey deposits eventually led to cooling and increased glaciation between 14 5 and 13 ma the crfb hypothesis helps to explain the significant time lag between the onset of increased rates of organic carbon burial in the monterey at 17 5 ma as marked by increased 13c values and the climatic cooling and glaciation during the middle miocene as marked by the increase in 18o values which did not begin until 14 5 ma

the effect of the salt viscosity on future evolution of the gorleben salt diapir germany abstract the gorleben diapir which has been targeted for radioactive waste disposal contains large blocks of anhydrite numerical models that depict the geometrical configuration of the gorleben diapir are used to understand internal structure of diapir caused by movement of the anhydrite blocks for various salt rheologies it is shown that the rheology of the salt plays a significant role in how and at which rate the anhydrite blocks sink within the diapir the mobility of anhydrite blocks depends on the effective viscosity of salt which has to be lower than threshold value of around 1018 1019 pa s decreasing salt viscosity allows the previously stationary anhydrite blocks to sink if the effective viscosity of salt in post depositional stage of the gorleben diapir falls below this threshold value induced internal flow due to the present anhydrite layer might disturb any repository within the diapir

reproducibility and utility of dune luminescence chronologies abstract optically stimulated luminescence osl dating of dune deposits has increasingly been used as a tool to investigate the response of aeolian systems to environmental change amalgamation of individual dune accumulation chronologies has been employed in order to distinguish regional from local geomorphic responses to change however advances in dating have produced chronologies of increasing complexity in particular questions regarding the interpretation of dune ages have been raised including over the most appropriate method to evaluate the significance of suites of osl ages when local noisy and discontinuous records are combined in this paper these issues are reviewed and the reproducibility of dune chronologies is assessed osl ages from two cores sampled from the same dune in the northeast rub al khali united arab emirates are presented and compared alongside an analysis of previously published dune ages dated to within the last 30 ka distinct periods of aeolian activity and preservation are identified which can be tied to regional climatic and environmental changes this case study is used to address fundamental questions that are persistently asked of dune dating studies including the appropriate spatial scale over which to infer environmental and climatic change based on dune chronologies whether chronological hiatuses can be interpreted how to most appropriately combine and display datasets and the relationship between geomorphic and palaeoclimatic signals chronological profiles reflect localised responses to environmental variability and climatic forcing and amalgamation of datasets with consideration of sampling resolution is required otherwise local factors are always likely to dominate using net accumulation rates to display ages may provide an informative approach of analysing and presenting dune osl chronologies less susceptible to biases resulting from insufficient sampling resolution

the utility of desert sand dunes as quaternary chronostratigraphic archives evidence from the northeast rub al khali abstract the distributions of optically stimulated luminescence osl ages from dunes are affected by palaeoenvironmental changes complex dune dynamics and sampling strategy extracting the relative importance of these factors when interpreting discontinuous osl chronologies from sand dunes has proven difficult and is particularly hindered in contexts where the internal sedimentary structures of dunes are not visible in this study samples for osl dating were taken from three major dune exposures in the rub al khali united arab emirates each showing clear internal structure with the aim of addressing these problems specific objectives were to assess how ages of individual sedimentary units represent dune accumulation as a whole and how the dune record in turn reflects known past environmental changes final ages were calculated using the oxcal software package by incorporating the known relative stratigraphy through the application of bayesian methods the results show that stratigraphy alone is not sufficient at these sites to guide osl sampling that is chrono and lithostratigraphic boundaries do not necessarily coincide where chronological hiatuses are present internal sediment stratigraphy can be a useful tool in identifying potential problems of under sampling the full dune record the implications of these findings for reconstructing quaternary climates from dune chronologies are considered

rates of sedimentation and stratigraphical completeness the most obvious result of sedimentation processes is the change in the thickness of accumulated sediment with time whenever the net change over a period of time is positive a record is left of a sedimentation system s existence a stratigraphical section for the stratigrapher the problem that such a section poses is to determine the detailed correspondence of thickness to time1 for the sedimentologist the problem is to infer the nature of the parent sedimentation system and to estimate its parameters a simple stochastic model is used here 1 to illustrate some basic stratigraphical results of clastic sedimentation processes 2 to offer an explanation for the commonly cited systematic decrease of mean sedimentation rate with increasing time span and 3 to assess the concept of stratigraphical completeness it will be assumed that all post depositional non erosional processes such as compaction can be identified and their effects on stratigraphical thickness allowed for

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

sticky dunes in a wet desert formation stabilisation and modification of the australian desert dunefields independent dating evidence and dune morphology indicate great stability of the australian dunefields most dunefields have seen only minor superficial modification since they were formed up to 1 million years ago despite quite large changes in climate conditions this stability may be partly due to the relatively dense vegetation cover on australian dunes under the marginally arid climate but new studies supported by many older observations suggest that sticky dunes where sand grains are bonded or cemented may form under a broader range of wind climates than widely thought and have greater resistance to reworking new mapping of the australian continental dunefields from satellite imagery shows a previously unrecognised diversity of dune morphologies dune orientation continuity connectedness crest planform crest sharpness spacing and setting all show patterns of variation over the continent these are consistent with the overall low sand supply and variable wind climate that contribute to the dominance of longitudinal dunes but also with only superficial modification of the dunes after their initial formation the longevity of the dunes is likely also partly due to the stabilisation of dune sand by pedogenesis the bonding of sand by pedogenic calcium carbonate gypsum silica and translocated clays the extremely low mobility of the sand dunes has led to preservation of dunes of great age with stacked accretionary units and multiple palaeosols

holocene development of multiple dune generations in the northeast rub al khali united arab emirates it has been hypothesised that in sand seas where multiple dune generations occur each generation represents a distinct terrestrial response to contrasting palaeoatmospheric circulation conditions lancaster n kocurek g singhvi a pandey v deynoux m ghienne j f et al 2002 late pleistocene and holocene dune activity and wind regimes in the western sahara desert of mauritania geology 30 991 994 however reconstructing dunefield accumulation and preservation chronologies has often utilised a limited suite of samples because of the difficulties realised in accessing unconsolidated aeolian sands which has limited the capacity to test this hypothesis in the eastern united arab emirates artificial excavation for quarrying and construction have generated a unique opportunity to examine and sample the internal structures of dune systems and to generate data to test the hypothesis of dune generational development this paper presents new data and chronologies from a multigenerational analysis of dune

late quaternary dune accumulation along the western margin of south africa distinguishing forcing mechanisms through the analysis of migratory dune forms abstract the west coast of south africa is situated at a critical location between temperate and subtropical oceanic and atmospheric circulation systems and palaeoenvironmental proxies from this region have the potential to elucidate issues concerning variations within these systems over glacial interglacical cycles while semi arid climates have hindered the preservations of organic proxies a variety of aeolian bedforms have been analysed in an effort to improve our understanding of environmental change in the region optically stimulated luminescence measurements of 51 samples from 15 reticulate dune sites along the west coast have enabled the identification of phases of aeolian activity as well as periods of relative geomorphic stability combined with data derived from previous studies of the region s sediment accumulating deposits and other palaeoenvironmental proxies periods of increased windiness are identified at 16 24 30 33 43 49 and 63 73 ka from approximately 17 12 ka decreasing transport capacity resulted in the stabilisation of the west coast s dune fields during the holocene altithermal 4 8 ka despite reduced wind strength increased aridity resulting from higher temperatures and a reduced influence of moisture bearing westerly systems appears to have trigged widespread remobilisation of the region s dune fields the combination of ages from a variety of dune forms with different development mechanisms and comparisons with a range of proxy data sources have allowed for an enhanced interpretation of the region s aeolian archives moving beyond simple correlations between dune activity and aridity

aeolian sand transport a wind tunnel model wind sand transport is an important geological process on earth and some other planets formulating the wind sand transport model has been of continuing significance majority of the existing models relate sand transport rate to the wind shear velocity based on dynamic analysis however the wind shear velocity readapted to blown sand is difficult to determine from the measured wind profiles when sand movement occurs especially at high wind velocity moreover the effect of grain size on sand transport is open to argument detailed wind tunnel tests were carried out with respect to the threshold velocity threshold shear velocity and transport rate of differently sized loose dry sand at different wind velocities to reformulate the transport model the results suggest that the relationship between threshold shear velocity and grain size basically follow the bagnold type equation for the grain size d 0 1 mm however the threshold coefficient a in the equation is not constant as suggested by bagnold but decreases with the particle reynolds number the threshold velocity at the centerline height of the wind tunnel proved to be directly proportional to the square root of grain diameter attempts have been made to relate sand transport rate to both the wind velocity and shear velocity readapted to the blown sand movement the reformulated transport model for loose dry sand follows the modified o brien rindlaub type equation q f1 d 1 ru 2 g v3 or the modified bagnold type equation q f2 d 1 rt 0 25 g u 3 where q is the sand transport rate the sand flux per unit time and per unit width in kg m 1 s 1 is the air density 1 25 kg m 3 g is the acceleration due to gravity 9 81 m s 2 ru vt v rt u t u v is the wind velocity at the centerline of the wind tunnel in m s 1 vt is the threshold velocity measured at the same height as v in m s 1 u is the shear velocity with saltating flux in m s 1 u t is threshold shear velocity in m s 1 f1 d 1 475 24 93 62d d f2 d 1 41 4 98exp 0 5 ln d 1 55d 0 57 2 d is the grain diameter in mm and d is the reference grain diameter equals 0 25 mm the bagnold s equation that asserts for a given wind drag the rate of movement of a fine sand is less than that of a coarse sand is not supported by the reformulated models

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 new look at sedimentation rates and the completeness of the stratigraphic record several recent papers have suggested that completeness of stratigraphic sections can be estimated by using the ratio of long term sedimentation rates to short term rates we show that the median short term sedimentation rates used in these papers are erroneous and are mostly an artifact arising from improper data manipulation to demonstrate the artificial character of short term median rates we have generated a series of plots assuming a constant sedimentation rate with fixed variance and defined limits of measurement precision these plots demonstrate that increasing variance in the time required to deposit sediments will result in spuriously high short term median sedimentation rates a new method for calculating median sedimentation rates which eliminates the problem of spurious median rates is then applied to make an improved estimate of expected completeness recalculated median values show that sedimentation rates for pelagic sediments are close to identical for both short term and long term tim

aeolian system sediment state theory and mojave desert kelso dune field example the sediment state of aeolian dune fields and sand seas at a basinal scale is defined by the separate components of sediment supply sediment availability and the transport capacity of the wind the sediment supply for aeolian systems is the sediment that contemporaneously or at some later point serves as the source material for the aeolian system numerous factors impact the susceptibility of grains on a surface to transport but these are cumulatively manifested by the actual transport rate which serves as a proxy for sediment availability transport capacity is the potential sediment transport rate of the wind because the three aspects of sediment state can be given as a volumetric rate they are directly comparable plotted simultaneously against time the generated curves define nine possible classes of sediment state sediment supply that is stored occurs because it is transport or availability limited or generated at a rate greater than the potential or actual transport rates respectively contemporaneous or lagged influx to an aeolian system may be limited by sediment availability but cannot exceed the transport capacity of the wind for the kelso dune field in the mojave desert of california a variety of stratigraphic and geomorphic evidence is used to approximate the sediment state of the system the sediment supply was generated during the latest pleistocene and earliest holocene during humid periods of enhanced discharge by the mojave river to form the lake mojave fan delta or terminal fan and has been calculated over time from the sedimentation rate and the frequency of floods estimation of transport capacity over time was based upon modern wind data with an allowance for greater winds during the pleistocene based upon climatic models sediment availability was approximated by calculation of a modern dune mobility index with variation over time based upon climatic inferences while quantifying the kelso or any natural system is subject to numerous uncertainties the sediment state approach reflects the temporal and spatial disjointed nature of accumulations at kelso as well as illuminating questions for future research

estimation of completeness of stratigraphical sections using empirical data and theoretical models the completeness of a stratigraphical section is the fraction of time intervals of some specified length t that have left a record a record is left when some sediment is deposited during the interval and is not subsequently eroded a complete section contains no hiatuses longer than t the completeness of a section varies with t and its accumulation rate varies with the length of the time span over which it is measured plots of measured accumulation rate against time span are an empirical means of estimating completeness simple theoretical models help extrapolate meagre data and identify bias completeness at time scale t can depend upon three general properties of the accumulation history the age of the section the long term net accumulation rate drift and the unsteadiness of the sedimentation rate the way unsteadiness is measured depends upon the kinds of fluctuations that can be recognized in the accumulation rate to describe random fluctuations a standard deviation of rates is sufficient one dimensional brownian motion is a model of random fluctuations that explains many aspects of stratigraphical completeness regular periodic fluctuations in accumulation rate may be described in terms of wavelength and amplitude completeness is an increasing function of t drift and in the periodic case the wavelength of fluctuations it decreases with increasing standard deviation of accumulation rate and the amplitude of periodic fluctuations the completeness and thickness of stratigraphical sections are weakly positively associated

evaluating the use of dune sediments as a proxy for palaeo aridity a southern african case study abstract the dominance of dryland environments in the southern hemisphere makes the study of these regions of critical importance for the development of regional hemispheric and global models of environmental change unfortunately the wetting and drying cycles associated with semi hyperarid climates are not conducive to the preservation of traditional organic proxy data sources the last decade however has seen the development of a number of alternative archives including the application of luminescence dating techniques to dunes and other aeolian deposits in continental situations the existence of relict dune fields has long been thought to be evidence of drier conditions during the pleistocene and direct ages from these features have been interpreted almost exclusively as indicating phases of aridity however an increasing number of ages from a broader range of environments are calling into question the assumption that aeolian activity can be simply equated with aridity presented here is a comparison of dune ages that have been obtained from across southern africa with a range of proxies from both terrestrial and marine records taken as a whole three primary phases of activity can be identified at 60 40 35 20 and 17 4 ka the frequent discordance with other terrestrial records indicating coeval increases in humidity and the close correlation of these phases with wind strength proxies suggest that aridity is unlikely to be the sole or even primary forcing mechanism for aeolian activity in the region and the palaeoclimatic significance of these sedimentary archives needs to be reassessed

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

multi decadal periods of enhanced aeolian activity on the north eastern tibet plateau during the last 2ka abstract the north eastern tibetan plateau is regarded as key area for the understanding of the holocene paleoclimate in central asia during the last decade a special emphasis has been placed on multi decadal to millennial scale climate fluctuations especially in the context of the recent climate change however most reconstructions are based on lake sediments tree rings and speleothems whereas only little information from terrestrial archives is included this study presents multi decadal scale climate fluctuations based on optical stimulated luminescence osl ages from aeolian sediments from three catchment areas six phases of enhanced aeolian accumulation during the last 2000 years each lasting around 80 200 years were identified the first three phases 1630 1725 ce 1450 1530 ce and 1250 1350 ce occurred during the little ice age the other three 750 950 ce 390 540 ce 50 225 ce during the so called dark ages cooling aeolian processes were strongly reduced during the medieval climate anomaly a comparison with other proxy records indicates that the formation of aeolian archives on the north eastern tibetan plateau during the late holocene is facilitated by cool and dry climate conditions during times of weaker asian summer monsoon and probably enhanced westerlies the results show that short term climate fluctuations can be reconstructed from non continuous and heterogeneous terrestrial archives in a semi arid environment provided a sufficient number of osl ages from aeolian sediments is available

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

stochastic and deterministic controls on stratigraphic completeness and fidelity compilations of sedimentation rates determined across a large range of time spans and different sedimentary environments attest to the discontinuous nature of sedimentation and support the reasoning that hiatuses pervade sedimentary successions at multiple spatial and temporal scales the relationship between sedimentation rate and time span of observation also emphasises how stratigraphic completeness is a time span dependent concept the scaling of stratigraphic completeness with time span alludes to the likely stochastic and fractal nature of the stratigraphic record it also implies that the relationship between stratal thickness and time is inherently non linear and this has obvious implications for the way we should expect temporal processes to be recorded stratigraphically order within the stratigraphic record may arise from deterministic processes in stratigraphy most notably through astronomically influenced sedimentation intuitively a strongly ordered stratigraphic record due to the presence of cyclicity in sedimentation increases stratigraphic completeness at time spans equal to and longer than the time span of the cyclicity and minimises the non linearity between stratal thickness and time here geological data and simple modelling are used to support this supposition but these results also highlight how stochastic variability inherent in depositional processes may sensitise certain successions to missed cycles and incompleteness even in the presence of strong cyclicity this stochastic variability has the potential to mask evidence of astronomical forcing of sedimentation it is also shown how hiatuses affect the ability of a succession to faithfully record temporal processes and this is true regardless of whether hiatuses are deterministically controlled or random or indeed whether a succession is considered complete or not consequently stratigraphic completeness and stratigraphic fidelity are not synonymous terms

inter annual variability of southerly winds in a coastal area of the atacama desert implications for the export of aeolian sediments to the adjacent marine environment the analysis of the aeolian content of marine cores collected off the coast of the atacama desert mejillones bay chile suggests that marine sediments can record inter annual to inter decadal variations in the regional southerly winds responsible for particle entrainment at the surface of the nearby desert however the establishment of a simple and direct correlation between the sediment and wind records is complicated by the difference of time scales between the erosion and accumulation processes the aim of this work is to i assess the inter annual variability of the surface winds responsible for the sand movements and ii determine whether the integration over periods of several months completely smoothes the rapid changes in characteristics of the transported and deposited aeolian material to accomplish this aim 14 years of 10 m hourly wind speed measured at the cerro moreno antofagasta airport between 1991 and 2003 and at the orica station between 2000 and 2004 were analyzed for each year the wind speed statistical distribution can be represented by a combination of two to three weibull functions winds of the lowest weibull mode are too weak to move the sand grains at the surface of the pampa this is not the case for the intermediate mode and especially for the highest speed mode which are able to erode the arid surface and transport particles to the bay in each individual year of the period of study the highest speed mode only accounted for a limited number of strong erosion events quantitative analysis of the distribution of the friction velocities and of their impact on erosion using a saltation model suggests that although all wind speeds above threshold produce erosion events values around 0 45 m sec 1 contribute less to the erosion flux this gap allows separation of the erosion events into low and high saltation modes the correlation r 0 997 between the importance of the third weibull mode and the extent of higher rate saltation indicates that the inter annual variability of the erosion at the surface of the pampa as well as the transport of coarse particles 100 m are directly related to inter annual variations in the prevalence of the strongest winds finally a transport and deposition model is used to assess the possible impact of the wind inter annual variability on the deposition flux of mineral particles in the bay the results suggest that inter annual differences in the wind speed distributions have a quantifiable effect on the intensity and size distribution of this deposition flux this observation suggests that a detailed analysis of the sediment cores collected from the bay could be used for reconstructing the inter annual variability of past winds

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

degradation of linear dunes in northwest ngamiland botswana and the implications for luminescence dating of periods of aridity the long morphodynamic history of the linear dune system of northwest ngamiland comprises both construction and degradation processes here we describe features resulting from the latter crest lowering during wet periods is associated with lateral transfer of sand into the straats weathering and leaching have mobilised iron to the base of the dune profiles and have corroded the quartz grains satellite image interpretation indicates that dune sand has been diluted by bioturbation which transferred damaran saprolite from below the dunes to the surface flooding and waterlogging have modified and in places completely destroyed the original crisply linear morphology small linear collapse features across the linear dunes are associated with open fractures or faults where water infiltration is facilitated the implications for luminescence dating of dune sands to identify periods of aridity are discussed the dunes cannot simply be regarded as providing a stacked record of such periods they are very old and much of the record is missing as a result of crest lowering the chronostratigraphy includes degradational components which must be distinguished from those of construction bioturbation cannot be disregarded as a complication which overprints the luminescence signals

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

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