the hans tausen drill design performance further developments and some lessons learned in the mid 1990s excellent results from the grip and gisp2 deep drilling projects in greenland opened up funding for continued ice coring efforts in antarctica epica and greenland northgrip the glaciology group of the niels bohr institute university of copenhagen was assigned the task of providing drilling capability for these projects as it had done for the grip project the group decided to further simplify existing deep drill designs for better reliability and ease of handling the drill design decided upon was successfully tested on hans tausen ice cap peary land greenland in 1995 the 5 0 m long hans tausen ht drill was a prototype for the 11 m long epica and northgrip versions of the drill which were mechanically identical to the ht drill except for a much longer core barrel and chips chamber these drills could deliver up to 4 m long ice cores after some design improvements had been introduced the berkner island antarctica drill is also an extended ht drill capable of drilling 2 m long cores the success of the mechanical design of the ht drill is manifested by over 12 km of good quality ice cores drilled by the ht drill and its derivatives since 1995

technical innovations and optimizations for intermediate ice core drilling operations the british antarctic survey in collaboration with laboratoire de glaciologie et geophysique de l environnement has in recent years successfully drilled to bedrock on three remote sites around the antarctic peninsula based on the experience from the multi season project at berkner island 948m depth 2002 05 we optimized the drill set up to better suit two subsequent single season projects at james ross island 363m depth 2008 and fletcher promontory 654m depth 2012 the adaptations as well as the reasons for them are discussed in detail and include a drill tent set up without a trench drilling without a borehole casing with a relatively low fluid column height and using a shorter drill these optimizations were aimed at reducing cargo loads and installation time while maintaining good core quality productivity and a safe working environment in addition we introduce a number of innovations ranging from a new lightweight cable tensioning device and drill head design to core storage and protection trays to minimize the environmental impact all the drill fluid was successfully recovered at both sites and we describe and evaluate this operation

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

climate records covering the last deglaciation the oxygen 18 oxygen 16 ratio of molecular oxygen trapped in ice cores provides a time stratigraphic marker for transferring the absolute chronology for the greenland ice sheet project gisp ii ice core to the vostok and byrd ice cores in antarctica comparison of the climate records from these cores suggests that near the beginning of the last deglaciation warming in antarctica began approximately 3000 years before the onset of the warm bolling period in greenland atmospheric carbon dioxide and methane concentrations began to rise 2000 to 3000 years before the warming began in greenland and must have contributed to deglaciation and warming of temperate and boreal regions in the northern hemisphere

holocene glacial and climate history of prince gustav channel northeastern antarctic peninsula the antarctic peninsula is one of the most rapidly warming regions on earth as evidenced by a recent increase in the intensity and duration of summer melting the recession of glaciers and the retreat and collapse of ice shelves despite this only a limited number of well dated near shore marine and lake sediment based palaeoenvironmental records exist from this region so our understanding of the longer term context of this rapid climate change is limited here we provide new well dated constraints on the deglaciation history and changes in sea ice and climate based on analyses of sedimentological proxies diatoms and fossil pigments in a sediment core collected from an isolation basin on beak island in prince gustav channel ne antarctic peninsula 63 36 s 57 20 w twenty two radiocarbon dates provided a chronology for the core including a minimum modelled age for deglaciation of 10 602 cal yr bp following the onset of marine sedimentation conditions remained cold and perennial sea ice persisted in this part of prince gustav channel until c 9372 cal yr bp this was followed by a seasonally open marine environment until at least 6988 cal yr bp corresponding with the early retreat and disintegration of the ice shelf in southern prince gustav channel following isolation of the basin from 6988 cal yr bp a relatively cold climate persisted until 3169 cal yr bp a mid late holocene climate optimum occurred between 3169 and 2120 cal yr bp inferred from multiple indicators of increased biological production this postdates the onset of the mid late holocene climate optimum in the south shetland islands 4380 cal yr bp and the south orkney islands 3800 cal yr bp suggesting that cooler climate systems of the weddell sea gyre to the east of the peninsula may have buffered the onset of warming climate deterioration is inferred from c 2120 cal yr bp until 543 cal yr bp this was followed by warming superimposed on this warming trend the instrumental record of recent warming at nearby hope bay is mirrored by a recent increase in the lake s primary production and a shift in the diatom communities in the uppermost 3 cm of sediments suggesting that this is amongst the first records to show an ecological response to recent rapid temperature increase these new constraints on glaciological and climate events in prince gustav channel are reviewed in the context of wider changes in the antarctic region

timing of the antarctic cold reversal and the atmospheric co2increase with respect to the younger dryas event the transition from the last glacial to the holocene is a key period for understanding the mechanisms of global climate change ice cores from the large polar ice sheets provide a wealth of information with good time resolution for this period however interactions between the two hemispheres can only be investigated if ice core records from greenland and antarctica can be synchronised accurately and reliably the atmospheric methane concentration shows large and very fast changes during this period these variations are well suited for a synchronisation of the age scales of ice cores from greenland and antarctica here we confirm the proposed lead of the antarctic cold reversal on the younger dryas cold event the antarctic cooling precedes the younger dryas by at least 1 8 kyr this suggests that northern and southern hemispheres were in anti phase during the younger dryas cold event a further result of the synchronisation is that the long term glacial interglacial increase of atmospheric co 2 was not interrupted during the younger dryas event and that atmospheric co 2 changes are not necessarily dominated by changes in the north atlantic circulation

modeling the evolution of antarctic ice sheet over the last 420 000 years implications for altitude changes in the vostok region a new thermomechanical three dimensional model designed to simulate the evolution of the antarctic ice sheet over long time periods is presented this model incorporates the various types of ice flow found in antarctica relatively slow inland ice flow that is essentially due to ice deformation fast ice flow in the regions with ice streams and ice shelf flow by coupling these three types of flow it is possible to predict grounding line migration simulations covering four glacial interglacial cycles have been conducted by forcing this model with a temperature record from vostok and a sea level record from marine cores our findings indicate that grounding line migration induced by sea level changes is the primary factor governing the evolution of the antarctic ice volume on the other hand the altitude of the ice sheet surface at vostok is driven by accumulation rate variations the amplitude of the altitude change does not exceed 150 m and is very similar for all the sites located on the antarctic plateau

core handling and processing for the wais divide ice core project on 1 december 2011 the west antarctic ice sheet wais divide ice core project reached its final depth of 3405 m the wais divide ice core is not only the longest us ice core to date but is also the highest quality deep ice core including ice from the brittle ice zone that the us has ever recovered the methods used at wais divide to handle and log the drilled ice the procedures used to safely retrograde the ice back to the us national ice core laboratory nicl and the methods used to process and sample the ice at the nicl are described and discussed

the james ross island and the fletcher promontory ice core drilling projects following on from the successful project to recover an ice core to bedrock on berkner island similar drilling equipment and logistics were used on two further projects to recover ice cores to bedrock in the antarctic peninsula at james ross island a ship and helicopter supported project drilled to bedrock at 363m depth in a single season while a twin otter supported project drilled to bedrock at 654m depth again in a single season from fletcher promontory in both new projects drilling was from the surface with the infrastructure enclosed in a tent using an uncased partially fluid filled borehole

reconstructions of antarctic topography since the eocene oligocene boundary abstract accurate models of past antarctic ice sheet behaviour require more realistic reconstructions of the evolution of bedrock topography however other than a preliminary attempt to reconstruct antarctic topography at the eocene oligocene boundary the long term evolution of antarctica s subglacial topography throughout its glacial history has not previously been quantified here we derive new reconstructions of antarctic topography for four key time slices in antarctica s climate and glacial history the eocene oligocene boundary ca 34 ma the oligocene miocene boundary ca 23 ma the mid miocene climate transition ca 14 ma and the mid pliocene warm period ca 3 5 ma to reconstruct past topography we consider a series of processes including ice sheet loading volcanism thermal subsidence horizontal plate motion erosion sedimentation and flexural isostatic adjustment and validate our models where possible using onshore and offshore geological constraints our reconstructions show that the land area of antarctica situated above sea level was 25 larger at the eocene oligocene boundary than at the present day offshore sediment records and terrestrial constraints indicate that the incision of deep subglacial topographic troughs around the margin of east antarctica occurred predominantly in the oligocene and early miocene whereas in west antarctica erosion and sedimentation rates accelerated after the mid miocene changes to the topography after the mid pliocene were comparatively minor our new palaeotopography reconstructions provide a critical boundary condition for models seeking to understand past behaviour of the antarctic ice sheet and have implications for estimating changes in global ice volume temperature and sea level across major cenozoic climate transitions

analysis of late quaternary linear dune development in the thar desert india abstract linear dunes are the most widespread dune type worldwide and act as important geoproxies of late quaternary environmental change in deserts they are less common in the thar desert india than other dune types especially parabolic forms and to date their development history is poorly understood here we investigate a linear dunefield in the northern thar through analysis of a series of excavated full dune profile sites and the application of optically stimulated luminescence osl dating to provide chronometric control results show that linear dunes have been present since at least 58 ka and were active throughout much of the holocene the early holocene a period marked by strong indian summer monsoon winds e g gill et al 2017 has been shown as the last major phase of dune accumulation recorded in all sites with ages ranging from 11 6 8 6 ka two spatially restricted records of dune accumulation have also been identified during the later part of holocene 4 3 ka and 2 1 ka attributed to localised reworking within the dunes the suite of luminescence ages generated from the study region confirms the principle recognised in other dunefields e g the south western kalahari stone and thomas 2008 and theoretically bailey and thomas 2014 that age records from individual dunes are unlikely to capture the developmental history of dunefields

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

multiple sources supply eolian mineral dust to the atlantic sector of coastal antarctica evidence from recent snow layers at the top of berkner island ice sheet the sr and nd isotopic composition of dust extracted from recent snow layers at the top of berkner island ice sheet located within the filchner ronne ice shelf at the southern end of the weddell sea enables us for the first time to document dust provenance in antarctica outside the east antarctic plateau eap where all previous studies based on isotopic fingerprinting were carried out berkner dust displays an overall crust like isotopic signature characterized by more radiogenic 87sr 86sr and much less radiogenic 143nd 144nd compared to dust deposited on the eap during glacial periods differences with eap interglacial dust are not as marked but still significant indicating that present day berkner dust provenance is distinct at least to some extent from that of the dust reaching the eap the fourteen snow pit sub seasonal samples that were obtained span a two year period 2002 2003 and their dust sr and nd isotopic composition reveals that multiple sources are at play over a yearly time period southern south america patagonia in particular likely accounts for part of the observed spring summer dust deposition maxima when isotopic composition is shifted towards younger isotopic signatures in the spring possible additional inputs from australian sources would also be supported by the data most of the year however the measured isotopic signatures would be best explained by a sustained background supply from putative local sources in east antarctica which carry old crust like isotopic fingerprints whether the restricted east antarctic ice free areas produce sufficient eolian material has yet to be substantiated however the fact that large 5 mu m particles represent a significant fraction of the samples throughout the entire time series supports scenarios that involve contributions from proximal sources either in patagonia and or antarctica possibly including snow free areas in the antarctic peninsula and other areas as well this also indicates that additional dust transport which does not reach the eap must occur at low tropospheric levels to this coastal sector of antarctica

the onset of dune formation in the strzelecki desert south australia abstract this study is concerned with the late quaternary climatic chronology of the strzelecki desert dunefields in central australia the sand ridges comprise layers of quartz sand some of which include palaeosol horizons with carbonated rootlets providing excellent opportunity for dating of alternations of dune building and stability by using optically stimulated luminescence osl deduced from the osl age of the oldest aeolian layer dated we conclude that the onset of aridity dates back to at least 65 ka older phases of aeolian activity though following a fluvial depositional phase 160 ka ago cannot be excluded although no aeolian layers giving evidence for this have been found in the two dunes dated here unconsolidated dune sands in the upper part of one section with late holocene 4 ka to modern depositional ages indicate a reactivation of the dunefield in recent times from the crosscheck of 14 c ages of the carbonated rootlets with osl results it is concluded that under the given environmental conditions radiocarbon dating of the calcareous rootlets is not able to provide reliable ages for the phase of soil development

on the suitability of partially clathrated ice for analysis of concentration and δ13c of palaeo atmospheric co2 abstract the stable carbon isotopic signature of carbon dioxide 13co2 measured in the air occlusions of polar ice provides important constraints on the carbon cycle in past climates in order to exploit this information for previous glacial periods one must use deep clathrated ice where the occluded air is preserved not in bubbles but in the form of air hydrates therefore it must be established whether the original atmospheric 13co2 signature can be reconstructed from clathrated ice we present a comparative study using coeval bubbly ice from berkner island and ice from the bubble clathrate transformation zone bctz of epica dome c edc in the edc samples the gas is partitioned into clathrates and remaining bubbles as shown by erroneously low and scattered co2 concentration values presenting a worst case test for 13co2 reconstructions even so the reconstructed atmospheric 13co2 values show only slightly larger scatter the difference to data from coeval bubbly ice is statistically significant however the 0 16 magnitude of the offset is small for practical purposes especially in light of uncertainty from non uniform corrections for diffusion related fractionation that could contribute to the discrepancy our results are promising for palaeo atmospheric studies of 13co2 using a ball mill dry extraction technique below the bctz of ice cores where gas is not subject to fractionation into microfractures and between clathrate and bubble reservoirs

the transition from the last glacial period in inland and near coastal antarctica recent studies suggested that during the transition out of the last glacial period one near coastal site in antarctica showed a response similar to that of greenland and unlike that of central antarctica here we present a new high resolution record of calcium from dome c antarctica changes in flux of calcium an indicator of dust input from other continents should be synchronous across the region and probably the continent using ca to synchronise records we find that the main warming at the near coastal site of taylor dome was slower than suggested previously and similar to that of central antarctica until there is further evidence it is still a reasonable hypothesis that antarctic climate behaved more or less as a single unit during the transition

paleocean circulation during the last deglaciation a bipolar seesaw hughen et al 1998 have documented that during the first 200 years of younger dryas time the 14c content of atmospheric co2 increased by 50 and that during the remainder of this 1200 year duration cold event it steadily declined the initial increase in 14c c was likely the result of a reduction in the atlantic s conveyor circulation however were the subsequent radiocarbon decline due to the rejuvenation of this potent heat pump then it is difficult to understand why the climate conditions in the northern atlantic basin remained cold throughout the younger dryas modeling exercises by stocker and wright 1996 mikolajewicz 1998 and schiller et al 1998 show that if the conveyor is terminated the transfer of radiocarbon into the deep sea shifts to the southern ocean thereby stabilizing the atmospheric 14c c ratio paleoclimatic evidence from the antarctic continent suggests that this model based scenario might have been played out in the real world while the younger dryas cooling has been documented in many places around the world including new zealand denton and hendy 1994 sowers and bender 1995 using their 18o in o2 based correlation between the ice core 18o in ice records for antarctica and greenland have demonstrated that in antarctica the younger dryas was a time of maximum warming the point of this paper is that the steep rise in 18o rise in antarctic ice which commenced close to the onset of the younger dryas might have been caused by heat released to the atmosphere in response to an increase in deep sea ventilation in the southern ocean

synchronous climate changes in antarctica and the north atlantic central greenland ice cores provide evidence of abrupt changes in climate over the past 100 000 years many of these changes have also been identified in sedimentary and geochemical signatures in deep sea sediment cores from the north atlantic confirming the link between millennial scale climate variability and ocean thermohaline circulation it is shown here that two of the most prominent north atlantic events the rapid warming that marks the end of the last glacial period and the bolling allerod younger dryas oscillation are also recorded in an ice core from taylor dome in the western ross sea sector of antarctica this result contrasts with evidence from ice cores in other regions of antarctica which show an asynchronous response between the northern and southern hemispheres

viscosity and density of a two phase drilling fluid many ice core drilling projects expect to reach depths greater than 1000m at such depths it is necessary to fill the borehole with a fluid to compensate the ice pressure and avoid the resulting significant hole closure knowledge of the ice chips and drilling fluid circulation parameters e g flow rate pressure drop velocity is essential for understanding the behaviour of the fluid around the drill and will support drill design through a better parameterization in models a characterization of the dynamic viscosity and of the density of the ice chips and drilling fluid mixture is required to calculate these circulation parameters the goal of this study is to propose a method to calculate to a first approximation both these physical properties the relationships presented here have been established after experimentation building on prior assumptions taken from the literature

the berkner island antarctica ice core drilling project we describe a project to retrieve a 948 m deep ice core from berkner island antarctica using relatively lightweight logistics and a small team the drilling operation over three austral summer seasons used electromechanical drilling technology described in detail from a covered shallow pit and a fluid filled borehole a basal temperature well below pressure melting point meant that no drilling problems were encountered when approaching the bed and the borehole penetrated through to the base of the ice sheet and sediment was retrieved from beneath the ice

drilling to the beds of the greenland and antarctic ice sheets a review we give a review of drilling to the beds of the greenland and antarctic ice sheets with a history of just a century we use a chronological rather than a geographical ordering to emphasize the way drilling has developed the review is divided into two parts core drilling and hole only drilling for access to the beds or the deep interior of the ice sheets this is an overview not a comprehensive report on each project while we have endeavoured to include all projects that fit our selection criteria we have not provided all the technical details

a minimum thermodynamic model for the bipolar seesaw 1 the simplest possible model is proposed to explain a large fraction of the millennial climate variability measured in the isotopic composition of antarctic ice cores the model results from the classic bipolar seesaw by coupling it to a heat reservoir in this thermal bipolar seesaw the heat reservoir convolves northern time signals with a characteristic timescale applying the model to the data of grip and byrd we demonstrate that maximum correlation can be obtained using a timescale of about 1000 1500 years higher correlations are obtained by first filtering out the long term variability which is due to astronomical and greenhouse gas forcing and not part of the thermal bipolar seesaw the model resolves the apparent confusion whether northern and southern climate records are in or out of phase synchronous or time lagged

antarctic ice rises and rumples their properties and significance for ice sheet dynamics and evolution locally grounded features in ice shelves called ice rises and rumples play a key role buttressing discharge from the antarctic ice sheet and regulating its contribution to sea level ice rises typically rise several hundreds of meters above the surrounding ice shelf shelf flow is diverted around them on the other hand shelf ice flows across ice rumples which typically rise only a few tens of meters above the ice shelf ice rises contain rich histories of deglaciation and climate that extend back over timescales ranging from a few millennia to beyond the last glacial maximum numerical model results have shown that the buttressing effects of ice rises and rumples are significant but details of processes and how they evolve remain poorly understood fundamental information about the conditions and processes that cause transitions between floating ice shelves ice rises and ice rumples is needed in order to assess their impact on ice sheet behavior targeted high resolution observational data are needed to evaluate and improve prognostic numerical models and parameterizations of the effects of small scale pinning points on grounding zone dynamics

sea level changes at the lgm from ice dynamic reconstructions of the greenland and antarctic ice sheets during the glacial cycles abstract new experiments were performed with three dimensional thermomechanical models of the greenland and antarctic ice sheets to simulate their behaviour during the glacial cycles to reconstruct their thickness and extent at the last glacial maximum lgm and to estimate their glacio eustatic contribution to the global sea level stand the calculations used improved ice dynamic and isostatic treatments updated datasets on higher grid resolutions and refined climatic treatments based on newly calibrated transfer functions between ice core records and climatic perturbations results are discussed from a reference run with standard parameters that is compared with available glacial geological observations and from a series of sensitivity experiments focusing on isostatic adjustment thermomechanical coupling climatic forcing mass balance changes and basal melting rates and viscosity changes of antarctic ice shelves for the antarctic ice sheet we find that volume changes are closely linked with grounding line changes of the west antarctic ice sheet at the lgm the grounding line extended close to the continental shelf break almost everywhere ice over central east antarctica was generally thinner than today and varied mainly in accordance with accumulation fluctuations for the greenland ice sheet melting is important only during interglacial periods and the most sensitive period concerns the size of the ice sheet during the eemian at the lgm the greenland ice sheet extended beyond the present coastline to cover at least the inner continental shelf and thinned by up to several hundred meters in central areas for a plausible range of parameters the experiments indicate that at the time of maximum sea level depression 21 kyr bp the antarctic ice sheet contributed 14 18 m to the sea level lowering and the greenland ice sheet 2 3 m significantly less than the older climap reconstructions whereas both ice sheets were at 21 kyr bp close to their maximum extent the experiments also indicate that their maximum volumes were reached only by 16 5 kyr bp greenland and 10 kyr bp antarctica equal to an additional sea level lowering of respectively 0 4 and 3 7 m holocene retreat was essentially complete by 5 kyr bp in greenland but is found to still continue today in west antarctica before reversing to growth during the next millenium the models were found to reproduce gross features of the ice sheet s history since the lgm in reasonably good agreement with available glacial geological data although observational control on ice thickness changes remains very limited

1000 year ice core records from berkner island antarctica two medium depth ice cores were retrieved from berkner island by a joint project between the alfred wegener institut and the british antarctic survey in the 1994 95 field season a151mdeep core from the northern dome reinwarthho he of berkner island spans 700 years while a 181m deep core from the southern dome thyssenho he spans approximately 1200 years both cores display clear seasonal cycles in electrical conductivity measurements allowing dating by annual layer counting and the calculation of accumulation profiles stable isotope measurements both d18o and dd together with the accumulation data allow us to estimate changes in climate for most of the past millennium the data show multi decadal variability around a generally stable long term mean in addition a full suite of major chemistry measurements is available to define the history of aerosol deposition at these sites again there is little evidence that the chemistry of the sites has changed over the past six centuries finally we suggest that the southern dome with an ice thickness of 950m is an ideal site from which to gain a climate history of the late stages of the last glacial and the deglaciation for comparison with the records from the deep antarctic ice cores and with other intermediate depth cores such as taylor dome and siple dome

late pleistocene and early holocene change in the weddell sea a new climate record from the patriot hills ellsworth mountains west antarctica the transition from the late pleistocene to the holocene 30 000 5000 years ago was a period of considerable climate variability which has been associated with changes in deep water formation and the intensity of the meridional overturning circulation although numerous records exist across the north atlantic region few antarctic ice core records have been obtained from the south here we exploit the potential of upwelling ancient ice so called blue ice areas bias from the patriot hills in the ellsworth mountains to derive the first deuterium isotope record d from continental antarctica south of the weddell sea gas analysis and glaciological considerations provide a first relative chronology inferred temperature trends from the patriot hills bia and snow pit suggest changing climate influences during the transition between the last glacial period and holocene under modern conditions the interplay between the antarctic high pressure system and the southern annular mode appears to play a significant role in controlling katabatic wind flow over the site while the bia record suggests that greater sea ice extent during the last glacial period was a major control our results demonstrate the considerable potential of the patriot hills site for reconstructing past climate change in the south atlantic region

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

flow at ice divide triple junctions 1 three dimensional full stokes modeling 1 ice domes are either axisymmetric high points along ridges or ridge triple junctions we model time dependent isothermal flow near triple junctions solving the full set of mechanical equations with a nonlinear power law rheology forcing is applied through the boundary conditions which affect flow patterns at outlets where such forcing is purely axisymmetric an axisymmetric dome is formed if a threefold symmetry in the forcing is applied the axisymmetric dome breaks up into three ridges subtending angles of 120 sets of experiments where the forcing was not exactly threefold symmetric by angle or by amplitude caused the triple junction to migrate to a new steady state here in steady state the ridges join the triple junction at nearly 120 but one ridge curves to satisfy the boundary forcing the slope pattern in the immediate dome vicinity depends only on a dimensionless parameter which is a function of the ice consistency the accumulation and the rheological power law index attempts to replicate the topography around summit greenland obtained a good fit with n 3 at a triple junction the dome is really distinct from the surrounding ridges contrary to the highest point of a single ridge divide as a consequence the raymond effect is at its strongest at the dome and weakens considerably over one ice thickness as one moves away from the flow center along the ridges leaving the dome the raymond effect is still present and decreases with the ratio of the flow across and along the ridge in the vicinity of the dome horizontal strain rates vary strongly from uniaxial to biaxial large scale effects represented in our model as fluxes at boundaries seem to be the primary controls on dome position and shape