magnetic basement depth inversion in the space domain we present a total field anomaly inversion method to determine both the basement relief and the magnetization direction inclination and declination of a 2d sedimentary basin presuming negligible sediment magnetization our method assumes that the magnetic intensity contrast is constant and known we use a nonspectral approach based on approximating the vertical cross section of the sedimentary basin by a polygon whose uppermost vertices are forced to coincide with the basin outcrop which are presumably known for fixed values of the x coordinates our method estimates the z coordinates of the unknown polygon vertices to obtain the magnetization direction we assume that besides the total field anomaly information about the basement s outcrops at the basin borders and the basement depths at a few points is available to obtain stable depth to basement estimates we impose overall smoothness and positivity constraints on the parameter estimates tests on synthetic data showed that the simultaneous estimation of the irregular basement relief and the magnetization direction yields good estimates for the relief despite the mild instability in the magnetization direction the inversion of aeromagnetic data from the onshore almada basin brazil revealed a shallow eastward dipping basement basin

non linear flow modelling of a martian lobate debris apron the martian mid latitudes contain numerous small water ice deposits collectively termed viscous flow features vffs the shape and topography of the deposits contain information on their past flow history and formation process in order to access this information it is imperative to get information on their deformational properties here we use a high resolution digital topography map and ice penetrating radar data in combination with an inverse method to constrain the deformational properties of a lobate debris apron a class of vff in the southern hemisphere of mars we find that while the creep parameter and accumulation rates are not well constrained in absolute values their ratio is robust we also find that the creep exponent is most likely n 3

flow dynamics of the rutford ice stream ice drainage basin west antarctica from radar stratigraphy rutford ice stream west antarctica drains a catchment of 45 000 km 2 into the ronne ice shelf through a 26 km wide 2 4 km deep subglacial trough adjacent to the ellsworth mountains forty two per cent of its catchment boundary is common with pine island glacier where rapid change to ice dynamics is currently underway these changes may eventually affect adjacent catchments such as rutford radar sounding data were acquired over the rutford ice drainage basin that show the internal structure in particular distinctive reflector groups were identified that mark the boundaries between four different flow elements the flow margin reflector groups include curvilinear events that cross cut isochrones and are therefore likely to be post depositional these reflections may arise from crystal orientation fabrics generated by localized strain in a flow margin one of the sectors of the ice drainage basin supplies the largest share 38 of the ice volume flux through the main trunk of rutford ice stream this sector may be preferentially affected by continuing surface lowering in the pine island glacier catchment

greenland flow variability from ice sheet wide velocity mapping using radarsat synthetic aperture radar data we have mapped the flow velocity over much of the greenland ice sheet for the winters of 2000 01 and 2005 06 these maps provide a detailed view of the ice sheet flow including that of the hundreds of glaciers draining the interior the focused patterns of flow at the coast suggest a strong influence of bedrock topography differences between our two maps confirm numerous early observations of accelerated outlet glacier flow as well as revealing previously unrecognized changes the overall pattern is one of speed up accompanied by terminus retreat but there are also several instances of surge behavior and a few cases of glacier slowdown comprehensive mappings such as these at regular intervals provide an important new observational capability for understanding ice sheet variability

methodology of the last climatic cycle reconstruction in france from pollen data abstract a numerical method has been devised to reconstruct the climate of the last climatic cycle from pollen data reliable annual reconstructions have been published recently for the two french long sequences of la grande pile and les echets covering the last 140 millennia this paper gives more details on the method used it proposes new reconstructions based on an extended modern pollen data base these reconstructions are consistent with the previous ones especially detailed information is obtained on the seasonal characteristics of climate computations of gcm on the climate variations from 125 kyr to 115 kyr b p are confirmed for western europe the insolation variations have induced at that time a decrease of seasonality accompanied by an increased cyclonic activity in winter this has been the beginning of the first ice growth after the last interglacial other glacier growth phases are detected at the end of substage 5c and particularly at the end of substage 5a

full depth englacial vertical ice sheet velocities measured using phase sensitive radar we describe a geophysical technique to measure englacial vertical velocities through to the beds of ice sheets without the need for borehole drilling using a ground based phase sensitive radio echo sounder pres during seven antarctic field seasons we measure the temporal changes in the position of englacial reflectors within ice divides up to 900 m thick on berkner island roosevelt island fletcher promontory and adelaide island recorded changes in reflector positions yield full depth profiles of vertical ice velocity that we use to examine spatial variations in ice flow near the divides we interpret these variations by comparing them to the results of a full stokes simulation of ice divide flow qualitatively validating the model and demonstrating that we are directly detecting an ice dynamical phenomenon called the raymond effect using pres englacial vertical ice velocities can be measured in higher spatial resolution than is possible using instruments installed within the ice we discuss how these measurements could be used with inverse methods to measure ice rheology and to improve ice core dating by incorporating pres measured vertical velocities into age modeling

snow accumulation rate on qomolangma mount everest himalaya synchroneity with sites across the tibetan plateau on 50 100 year timescales annual layer thickness data spanning ad 1534 2001 from an ice core from east rongbuk col on qomolangma mount everest himalaya yield an age depth profile that deviates systematically from a constant accumulation rate analytical model the profile clearly shows that the mean accumulation rate has changed every 50 100 years a numerical model was developed to determine the magnitude of these multi decadal scale rates the model was used to obtain a time series of annual accumulation the mean annual accumulation rate decreased from 0 8m ice equivalent in the 1500s to 0 3m in the mid 1800s from 1880 to 1970 the rate increased however it has decreased since 1970 comparison with six other records from the himalaya and the tibetan plateau shows that the changes in accumulation in east rongbuk col are broadly consistent with a regional pattern over much of the plateau this suggests that there may be an overarching mechanism controlling precipitation and mass balance over this area however a record from dasuopu only 125 km northwest of qomolangma and 700 m higher than east rongbuk col shows a maximum in accumulation during the 1800s a time during which the east rongbuk col and tibetan plateau ice core and tree ring records show a minimum this asynchroneity may be due to altitudinal or seasonal differences in monsoon versus westerly moisture sources or complex mountain meteorology

migration of the siple dome ice divide west antarctica the non linearity of the ice flow law or a local accumulation low over an ice divide can cause isochrones internal layers to be shallower under the divide relative to the flanks forming a divide bump in the internal layer pattern this divide signature is analyzed using ice flow models and inverse techniques to detect and quantify motion of the siple dome ice divide west antarctica the principal feature indicating that migration has occurred is a distinct tilt of the axis of the peaks of the warped internal layers beneath the divide the calculated migration rate is 0 05 0 50 m a 1 toward ice stream d and depends slightly on whether the divide bump is caused by the non linearity of ice flow or by a local accumulation low our calculations also suggest a strong south north accumulation gradient of 5 10 x 10 6 a 1 in a narrow zone north of the divide a consequence of divide migration is that pre holocene ice is thickest about 0 5 km south of the present divide position divide motion indicates that non steady processes possibly associated with activity of the bounding ice streams are affecting the geometry of siple dome the migration rate is sufficiently slow that the divide bump is maintained in the internal layer pattern at all observable depths this suggests that major asynchronous changes in the elevation or position of the bounding ice streams are unlikely over at least the past 10 3 10 4 years

biostratigraphy of the last glacial period in europe the european early weichselian early wurmian is characterized by two long and relatively warm interstadials brorup and odderade which can be traced and correlated with certainty across large parts of europe the amersfoort interstadial described earlier from the netherlands has been included in the brorup and represents the older part of it both great interstadials were forested in north west central and southern europe in the north conifer forests prevailed giving way towards the south to deciduous forests the climatic gradient was distinctly steeper mainly due to southward displacement of the gulf stream the two great interstadials are synchronous with the isotopic stages 5a and 5c in the deep sea cores subsequent to eemian brorup and odderade and from within the context of a continuous series two additional and clearly delimited interstadials have been recorded in a new standard profile from oerel nw germany these are now described as the oerel and glinde interstadials they were not forested and belong to the early pleni weichselian the lower one oerel can probably be correlated with the 3rd weichselian wurmian interstadial in southern central europe ognon ii durnten samerberg 3 while the upper one glinde has so far no known counterpart for the younger pleni glacial it is assumed that there was a long phase during which it was not too cold the formation of humic layers during this phase took place at different times partly due to local edaphic factors so that the relevant 14c dates show a wide scatter the hengelo and denekamp interstadials represent only slight ameliorations which facilitated the formation of shrub tundra and can be recorded in certain regions only

high resolution record of climate stability in france during the last interglacial period the last interglacial period 127 110 kyr ago has been considered to be an analogue to the present interglacial period the holocene which may help us to understand present climate evolution but whereas holocene climate has been essentially stable in europe variability in climate during the last interglacial period has remained unresolved because climate reconstructions from ice cores1 2 continental records3 4 and marine sediment cores5 6 give conflicting results for this period7 here we present a high resolution multi proxy lacustrine record of climate change during the last interglacial period based on oxygen isotopes in diatom silica diatom assemblages and pollen climate transfer functions from the ribains maar in france contrary to a previous study8 our data do not show a cold event interrupting the warm interglacial climate instead we find an early temperature maximum with a transition to a colder climate about halfway through the sequence the end of the interglacial period is clearly marked by an abrupt change in all proxy records our study confirms that in southwestern europe the last interglacial period was a time of climatic stability and is therefore still likely to represent a useful analogue for the present climate

implications of selected subsurface data on the structural form and evolution of some basins in the northern basin and range province nevada and utah

recumbent folding of divide arches in response to unsteady ice divide migration arches in stratigraphic layers directly under a flow divide raymond bumps are predicted by models of steady ice sheet flow and have been observed in several ice domes here we model the evolution of these layers when a formerly stationary divide migrates rapidly to a new position then again becomes stationary leaving the arched layers in a flank position as they are then carried downstream with the flow these abandoned arches can develop into recumbent folds these folds can occur over a wide range of divide migration speeds the shearing flow that produces these recumbent folds also distributes the folded layers over a wide distance downstream from the original divide location if the divide offset is abrupt pre cores or material lines comprising core relative isochrones can be used to quickly identify portions of an abandoned raymond bump that would be overturned at any future ice core site downstream if as appears to be the case in greenland the divide is never stable long enough to produce a mature arch folds of this type would not occur the most likely place to find such folds might be the flank of an ice ridge bounded by unsteady ice streams

was there an abrupt cold climatic event in the middle eemian pollen record from a palaeolake at the hieronimowo site ne poland abstract seventy samples of lacustrine sediments from a palaeolake at the hieronimowo site ne poland were investigated by pollen analysis method the obtained results indicate that the examined sequence spans the whole eemian interglacial mis 5e the beginning of the early vistulian weichselian mis 5d and some undefined period of the plenivistulian middle weichselian mis 2 4 the specific feature of the studied profile is the great thickness of deposits representing the hornbeam phase of the eemian interglacial e5 r paz which was the younger part of the climatic optimum of this interglacial and the very well developed pollen record of this phase that is absent in other profiles in ne poland due to this our study allowed for the first time in ne poland the detailed reconstruction of changes in the vegetation and in the mean january and july temperature during the middle part of the eemian interglacial this reconstruction unequivocally rules out the existence of an abrupt short lasting cold event at that time only a gradual slight increase in temperature was apparent during the older part of the hornbeam phase to 21 c for july and to 0 c for january and gradual decrease during its younger part to 17 c for july and to 5 c and

ross sea paleo ice sheet drainage and deglacial history during and since the lgm onshore and offshore studies show that an expanded grounded ice sheet occupied the ross sea embayment during the last glacial maximum lgm results from studies of till provenance and the orienta

in situquantification of ice rheology and direct measurement of the raymond effect at summit greenland using a phase sensitive radar 1 the glen exponent ncharacterizes the stress dependence of ice deformation directly influencing the rate at which ice masses respond to external forcing the slow deformation in large ice sheets makes laboratory rheometry at representative strain rates difficult we develop a new technique to estimaten in situ deploying a phase sensitive radar to measure vertical strain rates of around 10 4 yr 1within the top 1000 m of ice across ice divides at summit and neem greenland a fluid dynamical feature the raymond effect predicts strong vertical strain rate variation across divides over distances of a few ice thicknesses we achieve sufficient resolution to show this pattern enabling us to estimaten 4 5 by inverting our observations with flow modelling this is higher than values previously used but consistent with other indirect measurements implying laboratory measurements do not explore the full range of ice rheology and the consequent possibility of a greater sensitivity and responsiveness in ice sheet dynamics

basal melt at northgrip modeled from borehole ice core and radio echo sounder observations from temperature measurements down through the 3001 m deep borehole at the north greenland icecore project northgrip drill site it is now clear that the ice at the base 3080 m below the surface is at the pressure melting point this is supported by the measurements on the ice core where the annual layer thicknesses show there is bottom melting at the site and upstream from the borehole surface velocity measurements internal radio echo layers borehole and ice core data are used to constrain a time dependent flow model simulating flow along the north northwest trending ice ridge flow line leading to the northgrip site also time dependent melt rates along the flowline are calculated with a heat flow model the results show the geothermal heat flow varies from 50 to 200 mw m 2 along the 100km section of the modeled flowline the melt rate at the northgrip site is 0 75 cm a 1 but the deep ice in the northgrip core originated 50 km upstream and has experienced melt rates as high as 1 1 cm a 1

the crossover stress anisotropy and the ice flow law at siple dome west antarctica we used observations and modeling of siple dome west antarctica a ridge ice divide to infer the importance of linear deformation mechanisms in ice sheet flow we determined the crossover stress a threshold value of the effective deviatoric stress below which linear flow mechanisms dominate over nonlinear flow mechanisms by combining measurements of ice properties with in situ deformation rate measurements and a finite element ice flow model that accounts for the effects of viscous anisotropy induced by preferred crystal orientation fabric we found that a crossover stress of 0 18 bar produces the best match between predicted and observed deformation rates for siple dome this means that including a linear term in the flow law is necessary but generally the flow is still dominated by the nonlinear glen n 3 term the pattern of flow near the divide at siple dome is also strongly affected by crystal fabric measurements of sonic velocity which is a proxy for vertically oriented crystal fabric suggest that a bed parallel shear band exists several hundred meters above the bed within the ice age ice

inversion of gravity and magnetic data for the lower surface of a 2 5 dimensional sedimentary basin gravity and magnetic data have been inverted to obtain the continuous lower surface of a 2 5 dimensional sedimentary basin the non linear problem is linearized and a solution is calculated through a recursive process until the predicted data matches the observed data an average model is then calculated and a resolution analysis shows which features are uniquely determined the results of individual inversion indicate that a final solution is initial model dependent but the average models are independent of the initial model except at the margins the average model for the magnetic solutions have uniformly smaller spreads than the gravity solutions the algorithms were applied to data from the sanford basin in north carolina the results indicate that the basin is asymmetrical in shape with a maximum depth of 3 2 km comparing these results with those obtained from a generalized linear inverse gli algorithm indicate that the higher frequency features determined from the gli algorithm are not resolved

the transition from temperate phases to stadials in the long upper pleistocene sequence from les echets france abstract from two sections of the long sequence of les echets corresponding to the end of the eemian and to the end of st germain ii interstadial respectively detailed pollen diagrams have been established which provide information about the climatic conditions prevailing during the transitions from very temperate periods to the following cold stadials

climate environment and stratigraphy of the last pleistocene glacial stage in poland abstract the aim of the paper is to present general characteristics of the climate in poland during the vistulian weichselian expressed by changes of its continentality during warm and cold intervals this last glacial stage in poland is commonly subdivided into early vistulian mis 5d a lower plenivistulian mis 4 interplenivistulian mis 3 and upper plenivistulian and late vistulian mis 2 main climatic features of this glacial stage could be reconstructed based on compilation from published data concerning characteristics of glacial fluvial and aeolian sedimentary environments geomorphology analysis of indicator plant species coleoptera and geographical distribution of periglacial structures these data were indicative especially for evaluation of mean temperature of the warmest and the coldest months but could be also helpful in determination of drier intervals and some aspects of general atmospheric circulation paleoclimatic characteristics of the last glacial stage from the territory of poland were put into the european context during the early vistulian in eastern poland higher continentality was characteristic for interstadials amersfoort brorup and odderade and was considerably lower during the intervening colder stadials herning a cooling between amersfoort and brorup rederstall among the interstadials the most continental climate occurred during brorup similar continentality as at present considerably less during amersfoort and the least during odderade a decreasing trend of continentality for the cold stadials of the early weichselian eastwards in europe could result from a remarkably less dynamic gulf stream in the north atlantic when the ocean was covered with vast sea ice during winters whereas the adjoining continent was occupied by permafrost and the atmospheric circulation was presumably driven also by the scandinavian ice sheet

greenland ice sheet higher nonlinearity of ice flow significantly reduces estimated basal motion in times of warming in polar regions the prediction of ice sheet discharge is of utmost importance to society because of its impact on sea level rise in simulations the flow rate of ice is usually implemented as proportional to the differential stress to the power of the exponent n 3 this exponent influences the softness of the modeled ice as higher values would produce faster flow under equal stress we show that the stress exponent which best fits the observed state of the greenland ice sheet equals n 4 our results which are not dependent on a possible basal sliding component of flow indicate that most of the interior northern ice sheet is currently frozen to bedrock except for the large ice streams and marginal ice ice in the polar ice sheets flows towards the oceans under its own weight knowing how fast the ice flows is of crucial importance to predict future sea level rise the flow has two components 1 internal shearing flow of ice and 2 basal motion which is sliding along the base of ice sheets especially when the ice melts at this base to determine the first component we need to know how soft the ice is by considering the flow velocities at the surface of the northern greenland ice sheet and calculating the stresses that cause the flow we determined that the ice is effectively softer than is usually assumed previous studies indicated that the base of the ice is thawed in large parts up to about 50 of the greenland ice sheet our study shows that that is probably overestimated because these studies assumed ice to be harder than it actually is our new assessment reduces the area with basal motion and thus melting to about 6 13 in the greenland study area

the climate in europe during the eemian a multi method approach using pollen data the last interglacial period the eemian offers a testbed for comparing climate evolution throughout an interglacial with the current warm period we present here results from climatic reconstructions from 17 sites distributed across the european continent allowing an assessment of trends and regional averages of climate changes during this period we use a multi method approach to allow for an improved assessment of the uncertainties involved in the reconstruction in addition the method takes into account the errors associated with the age model the resulting uncertainties are large but allow a more robust assessment of the reconstructed climatic variations than in previous studies the results show a traditional three part eemian with an early optimum followed by slight cooling and eventually a sharp drop in both temperatures and precipitation this sequence is however restricted to the north as this latter change is not observed in the south where temperatures remain stable for longer these variations led to marked variation in the latitudinal temperature gradient during the eemian the difference between the two regions is also noticeable in the magnitude of changes with greater variations in the north than the south some evidence is found for changes in lapse rates however a greater number of sites is needed to confirm this

climate dependent contrast in surface mass balance in east antarctica over the past 216 ka documenting past changes in the east antarctic surface mass balance is important to improve ice core chronologies and to constrain the ice sheet contribution to global mean sea level change here we reconstruct past changes in the ratio of surface mass balance smb ratio between the epica dome c edc and dome fuji df east antarctica ice core sites based on a precise volcanic synchronization of the two ice cores and on corrections for the vertical thinning of layers during the past 216 000 a this smb ratio denoted smb edc smb df varied between 0 7 and 1 1 being small during cold periods and large during warm periods our results therefore reveal larger amplitudes of changes in smb at edc compared with df consistent with previous results showing larger amplitudes of changes in water stable isotopes and estimated surface temperature at edc compared with df within the last glacial inception marine isotope stages mis 5c and mis 5d the smb ratio deviates by up to 0 2 from what is expected based on differences in water stable isotope records moreover the smb ratio is constant throughout the late parts of the current and last interglacial periods despite contrasting isotopic trends

melting at the base of the greenland ice sheet explained by iceland hotspot history basal melting is widespread in the north central greenland ice sheet geophysical data and numerical modelling suggest a geothermal anomaly in this region resulting from the earlier passage of greenland over the iceland mantle plume

stress gradient coupling in glacier flow i longitudinal averaging of the influence of ice thickness and surface slope for a glacier flowing over a bed of longitudinally varying slope the influence of longitudinal stress gradients on the flow is analyzed by means of a longitudinal flow coupling equation derived from the vertically cross sectionally integrated longitudinal stress equilibrium equation by an extension of an approach originally developed by budd 1968 linearization of the flow coupling equation by treating the flow velocity u vertically averaged ice thickness h and surface slope in terms of small deviations u h and a from overall average datum values u 0 h 0 and 0 results in a differential equation that can be solved by green s function methods giving u x as a function of h x and x x being the longitudinal coordinate the result has the form of a longitudinal averaging integral of the influence of local h x and x on the flow u x u x u0 2l l 0 ln n nh n l exp x x l dx where the integration is over the length l of the glacier the operator specified deviations from the datum state and the term on which it operates which is a function of the integration variable x represents the influence of local h x x and channel shape factor x at longitudinal coordinate x on the flow u at coordinate x the influence being weighted by the influence transfer function exp x x l in the integral the quantity l that appears as the scale length in the exponential weighting function is called the longitudinal coupling length it is determined by rheological parameters via the relationship l 2h n n 3n where n is the flowlaw exponent n the effective longitudinal viscosity and n the effective shear viscosity of the ice profile n is an average of the local effective viscosity n over the ice cross section and n 1 is an average of n 1 that gives strongly increased weight to values near the base theoretically the coupling length l is generally in the range one to three times the ice thickness for valley glaciers and four to ten times for ice sheets for a glacier in surge it is even longer l 12h it is distinctly longer for non linear n 3 than for linear rheology so that the flow coupling effects of longitudinal stress gradients are markedly greater for non linear flow the averaging integral indicates that the longitudinal variations in flow that occur under the influence of sinusoidal longitudinal variations in h or with wavelength are attenuated by the factor 1 1 2 l 2 relative to what they would be without longitudinal coupling the short intermediate and long scales of glacier motion raymond 1980 over which the longitudinal flow variations are strongly partially and little attenuated are for 2l 2l 20l and 20l for practical glacier flow calculations the exponential weighting function can be approximated by a symmetrical triangular averaging window of length 4l called the longitudinal averaging length the traditional rectangular window is a poor approximation because of the exponential weighting the local surface slope has an appreciable though muted effect on the local flow which is clearly seen in field examples contrary to what would result from a rectangular averaging window tested with field data for variegated glacier alaska and blue glacier washington the longitudinal averaging theory is able to account semi quantitatively for the observed longitudinal variations in flow of these glaciers and for the representation of flow in terms of effective surface slope values exceptions occur where the flow is augmented by large contributions from basal sliding in the ice fall and terminal zone of blue glacier and in the reach of surge initiation in variegated glacier the averaging length 4l that gives the best agreement between calculated and observed flow pattern is 2 5 km for variegated glacier and 1 8 km for blue glacier corresponding to l h 2 in both cases if l varies with x but not too rapidly the exponential weighting function remains a fairly good approximation to the exact green s function of the differential equation for longitudinal flow coupling in this approximation l in the averaging integral is l x but is not a function of x effects of longitudinal variation of l are probably important near the glacier terminus and head and near ice falls the longitudinal averaging formulation can also be used to express the local basal shear stress in terms of longitudinal variations in the local stope stress with the mediation of longitudinal stress gradients

deformation in the vicinity of ice divides calculs numeriques aux elements finis de la variation de la vitesse horizontale avec la profondeur au voisinage d une difference de la glace symetrique isotherme et ou glissement sur un lit plat

recent contributions to the climatology of the last glacial interglacial cycle based on french pollen sequences abstract new pollen data derived from three long french sequences and methodological improvements in the methods used to reconstruct past climates are summarized here after a short presentation of the study sites and pollen diagrams estimated climate parameters during the major episodes of the last 140 000 years are shown to be coherent between sites analysis of the seasonality variations provides much additional information in particular it is shown that the advection from the ocean has governed the last climatic cycle on the european continent

an analysis of eemian climate in western and central europe on the basis of 31 pollen diagrams and additional data for botanical macrofossils an analysis is made of the last interglacial eemian climatic history in western and central europe the main tool for this analysis is the climatic indicator species method only selected woody species are used for the quantification of data partial climatic range diagrams are presented for abies alba acer monspessulanum acer tataricum buxus sempervirens tilia tomentosa the problem of time correlation and pollen zonation of the eemian is discussed the climatic analysis itself is based on an improved version of the indicator species method in this version not every site is analysed for its climatic values instead maps and tables on the migrational history of hedera ilex buxus abies and species of acer tilia and abies are the basis for climatic maps showing respectively january and july isotherms for the periods of the corylus zone e4a and the carpinus zone e5 it is concluded that mean january temperatures were as much as 3 c higher at amsterdam the netherlands than at present and mean temperatures in july were 2 c higher however the thermal maximum in winter was later zone e5 than the summer thermal maximum zone e4a winter temperatures changed parallel to rise and fall of global sea level precipitation changes are more difficult to estimate in the first part of the eemian precipitation must have been relatively low but from zone e4b onward it increased to higher values reaching 800 mm and probably substantially more in zones e5 and e6 hence the eemian climate was in its beginning relatively more contintental and later from e4b onward more oceanic however as compared with the holocene the eemian climate was generally speaking more oceanic

multiproxy climate reconstructions for the eemian and early weichselian palaeobotanical coleopteran and periglacial data from 106 sites across northwestern europe have been analysed in order to reconstruct palaeoclimatic conditions during the eemian and early weichselian three time slices in the eemian and four in the early weichselian have been considered in the pinus quercetum mixtum corylus phase of the eemian summer temperatures were probably at their highest and the botanic evidence suggests a southeast to northwest gradient for both the warmest and coldest month coleoptera indicate that the summers in southern england were several degrees warmer than those of present day the climate during thecarpinus picea phase was uniform and oceanic without obvious gradients in the final time slice of the eemian the pinus picea abies phase temperatures of the warmest month seem to drop slightly with some indication of a shift towards a more boreal and suboceanic climate the reconstruction of the palaeoclimate in the herning stadial and rederstall stadial is hampered by the limited number of sites but botanical evidence suggests a gradient in temperature of the coldest month from east to west coleoptera from the herning stadial in central england and eastern germany suggest similarly cold and continental climates during the brorup interstadial and the odderade interstadial the botanical evidence suggests that the minimum mean july temperatures rose to 15 16 c but during the coldest month these temperatures show a gradient between 13 c in the east and 5 c in the west 1998 john wiley sons ltd

was the climate of the eemian stable a quantitative climate reconstruction from seven european pollen records the aim of the present study is to estimate the range of the climatic variability during the eemian interglacial which lasted about 10 000 years marine isotopic stage 5e the modem pollen analogue technique is applied to seven high resolution pollen records from france and poland to infer the annual precipitation and the mean temperature of the coldest month the succession of pollen taxa and the reconstructed climate can be interpreted coherently the warmest winter temperatures are centred in the first three millennia of the eemian interglacial during the mixed oak forest phase with quercus and corylus as dominant trees a rapid shift to cooler winter temperatures of about 6 degrees to 10 degrees c occurred between 4000 and 5000 years after the beginning of the eemian related to the spread of the carpinus forest this shift is more obvious for the reconstructed temperatures than for precipitation and is unique and irreversible for the whole eemian period following this climatic shift of the eemian variations of temperature and precipitation during the fast 5000 years were only slight with an amplitude of about 2 degrees to 4 degrees c and 200 to 400 mm yr the estimated temperature changes were certainly not as strong as those reconstructed for the stage 6 5e termination or the transition 5e 5d this is consistent with the constantly high ratio of tree pollen throughout the eemian indicative of a succession of temperate forest types this gradual transition between different forest landscapes can be related to intrinsic competition between the species rather than to a drastic climatic change c 1998 elsevier science b v all rights reserved

deformation warming and softening of greenland s ice by refreezing meltwater when basal meltwater refreezes the resulting warm ice can influence the flow dynamics of the ice sheet above an analysis of airborne gravity and radar data identifies extensive basal ice units across the northern greenland ice sheet that coincide with areas of deformed ice and fast ice flow

the tertiary creep of polycrystalline ice experimental evidence for stress dependent levels of strain rate enhancement laboratory creep deformation experiments have been conducted on initially isotropic laboratory made samples of polycrystalline ice steady state tertiary creep rates ter were determined at strains exceeding 10 in either uniaxial compression or simple shear experiments isotropic minimum strain rates min determined at 1 strain provide a reference for comparing the relative magnitude of tertiary creep rates in shear and compression through the use of strain rate enhancement factors e defined as the ratio of corresponding tertiary and isotropic minimum creep rates i e e ter min the magnitude of strain rate enhancement in simple shear was found to exceed that in uniaxial compression by a constant factor of 2 3 results of experiments conducted at octahedral shear stresses of o 0 04 0 80 mpa indicate a creep power law stress exponent of n 3 for isotropic minimum creep rates and n 3 5 for tertiary creep rates the difference in stress exponents for minimum and tertiary creep regimes can be interpreted as a o stress dependent level of strain rate enhancement i e e 1 2 o the implications of these results for deformation in complex multicomponent stress configurations and at stresses below those used in the current experiments are discussed

early weichselian interstadial mis 5c summer temperatures were higher than today in northern fennoscandia abstract only fragmentary biostratigraphical interstadial data exist from northern european high latitudes the palaeoenvironmental interpretations for the early part of the last glaciation in northern fennoscandia are mainly based on palynological evidence that suggests open birch woodland and a sub arctic climate plant macrofossils from the sokli sediment sequence in finnish lapland provide different evidence of interstadial climate conditions the assemblage includes several species that currently have considerably more southern distribution ranges this indicates that ca 100 000 years ago summer temperatures were warmer than today the mean minimum july temperature may have been as high as 16 c and the effective temperature sum may have been 1000 in day degree units d d the modern values being 13 c and 650 d d respectively the contemporary astronomical forcing mechanisms may have resulted in a weaker north south temperature gradient and a longer growing period creating more favourable climate conditions compared with today

occurrence of slender naiad najas flexilis willd rostk w l e schmidt during the eemian interglacial an example of a palaeolake from the hieronimowo site ne poland abstract nowadays najas flexilis is believed to be extinct in poland germany and switzerland while in other european countries it is classified as endangered or vulnerable during the early and middle holocene the species occurred throughout much of europe but in the late holocene it underwent a significant decline and reduction in range following as it is believed the gradual cooling of the climate and or the eutrophication and acidification of lakes caused by human activity the article recognizes the existence and disappearance of slender naiad in the palaeolake at the hieronimowo site ne poland during the eemian interglacial and at the beginning of the last glaciation vistulian weichselian when lake transformations were caused solely by natural factors without human impact results of palynological and macrofossil analyses reveal that the appearance of slander naiad in the studied water body was probably triggered by the change of its ph and trophic status resulting from the start of spruce and pine expansion stimulated by climate worsening during the younger part of the hornbeam phase e5 r paz in the middle eemian the appearance of spruce and pine in the area of the lake led to its acidification and oligotrophication and that caused the strong and rapid development of n flexilis the species existed in the palaeolake at the hieronimowo site through the spruce e6 r paz and pine e7 r paz phases until the end of the eemian interglacial these observations suggest that regressive transformations of the environment typical for the late part of the interglacial cycle unsupported by human activity do not lead the extinction of n flexilis quite the opposite these processes promote this species its disappearance started only at the beginning of the early vistulian weichselian and it was probably caused by a drastic decrease in temperatures and or by the development of green algae which limited light availability on the bottom of the water body

ice flow in greenland for the international polar year 2008 2009 a digital representation of ice surface velocity is essential for a variety of glaciological geologic and geophysical analyses and modeling here we present a new reference comprehensive high resolution digital mosaic of ice motion in greenland assembled from satellite radar interferometry data acquired during the international polar year 2008 to 2009 by the envisat advanced synthetic aperture radar asar the advanced land observation system alos s phase array l band sar palsar and the radarsat 1 sar that covers 99 of the ice sheet in area the best mapping performance is obtained using alos palsar data due to higher levels of temporal coherence at the l band frequency but c band frequency sar data are less affected by the ionosphere the ice motion map reveals various flow regimes ranging from patterned enhanced flow into a few large glaciers in the cold low precipitation areas of north greenland to diffuse enhanced flow into numerous narrow fast moving glaciers in the warmer high precipitation sectors of northwest and southeast greenland we find that the 100 fastest glaciers v 800m yr drain 66 of the ice sheet in area marine terminating glaciers drain 88 of greenland and basal sliding motion dominates internal deformation over more than 50 of the ice sheet this view of ice sheet motion provides significant new constraints on ice flow modeling copyright 2012 by the american geophysical union

calibration of the climatic signal in a new pollen sequence from la grande pile a new core gpxx from la grande pile vosges france has enabled a more precise evaluation of pollen data concerning the last climatic cycle at this site and has enabled reconstruction of the monthly temperature and precipitation this paper shows that the various components of the reconstructed climatic signal are clearly separated and that they are coherent with the vegetation dynamics the eemian interglacial was influenced first by an oceanic climate and second by a more continental one the two interstadials st germain i and ii were mainly continental these three temperate periods ended with a cool and humid transition period dominated by boreal forests which may have been favourable to ice accretion the dansgaard oeschger oscillations from 55 to 25 kyr bp are not significantly recorded in la grande pile

glaciological and climatic significance of hercules dome antarctica an optimal site for deep ice core drilling 1 we present glaciological and climatological characteristics of hercules dome antarctica 86 s 105 w which demonstrate its potential as a deep ice core site annual layering in d ratios from a 72 m ice core collected by the us itase 2002 traverse indicate accumulation rates of 0 16 0 20 m yr ice equivalent over the last 300 years age control from stratigraphy seen in the radio echo sounding data collected during the same traverse suggests a rate of 0 09 0 11 m yr averaged over the past 18 000 years ice stratigraphy also indicates that the ice divide position has been stable through at least this period comparison of satellite derived temperature anomalies with atmospheric reanalysis data show that the site is sensitive to the two dominant patterns of climate variability in the high latitude southern hemisphere climate proxy data from a deep ice core at hercules dome would be indicative of changes in pacific southern hemisphere climate variability and may provide new information on rapid climate change events in antarctica the sensitivity of the site and the combination of relatively high accumulation rates low temperatures mean annual 35 c to 40 c and simple ice flow suggest that hercules dome is an ideal site for a future deep ice core

position and variability of complex structures in the central east antarctic ice sheet although the flow of the east antarctic ice sheet is well constrained from surface measurements and altimetry our knowledge of the dynamic processes within the ice sheet remains limited recent high resolution radar data from the gamburtsev subglacial mountains in central east antarctica reveal a series of anomalous englacial reflectors in the lower half of the ice column that cannot be explained by conventional ice flow expanding on previous analyses we describe the geometrical and morphological features of 12 of these anomalous reflectors our description reveals a previously unacknowledged diversity in size geometry and internal structure of these reflectors we are able to identify four distinct morphological features 1 fingers 2 inclusions 3 sheets and 4 folds the fingers and inclusions probably form by shear instabilities at the boundary between the reflectors and the surrounding meteoric ice the sheets highlight that basal ice can be uplifted off of the bed and above surrounding meteoric ice and the folds may have formed in local regions of converging flow associated with subglacial topography the study provides key insights into the rheology stress and deformational regimes deep within the central east antarctic ice sheet

stochastic perturbation of divide position perturbation of divide position is considered by a linearization about the vialov nye solution and also about related solutions with 0 1 relief relaxation times of one sixteenth the fundamental thickness accumulation rate time scale are found for the vialov nye configuration while substantial basal topography can halve the rate of relaxation steady divide position is most sensitive to anti symmetric accumulation rate distributions near the divide but transient divide motion is most strongly excited by anti symmetric accumulation rate variations halfway between the margin and the divide relaxation times for the antarctic peninsula divide position are estimated to be around 200 years while the larger greenland ice sheet has a divide position relaxation time of around 600 years modelling accumulation rate as a white noise process permits analysis of divide perturbation as a stochastic ornstein uhlenbeck process where the standard deviation of the response is proportional to the standard deviation of the forcing if observed accumulation rate variability in the antarctic peninsula were antisymmetric about the divide it would be sufficient to force the divide position to fluctuate with standard deviation 10 20 times the depth of the ice sheet there appears to be sufficient noise to cause raymond bumps to be spread significantly more data on the statistical variation of accumulation with position are needed random forcing will increase the complexity of any fold structures created in the divide region and in particular the number of such structures intersecting any borehole

eemian and early weichselian temperature and precipitation variability in northern germany abstract dramatic changes in european vegetation occurred during the transition from the eemian interglacial to weichselian glacial climates correlative with major changes in global ice core and marine records quantitative knowledge of climate change is important for understanding of the climate system and for climate modelling for which reconstructions of this transitional period are of special interest however it has been difficult to quantify the climatic changes involved in the eemian to early weichselian transition from terrestrial archives due to the lack of modern vegetation analogues to circumvent this problem we applied a suitable multivariate probabilistic approach to pollen and plant macrofossil assemblages to reconstruct temperature and precipitation for this transition in central europe our reconstructions span the interval from the beginning of the eemian marine isotope stage mis 5e to the odderade interstadial mis 5a they indicate a relatively stable eemian with increasing precipitation reducing the continentality of the climate with time during the transition from the eemian to the herning stadial mean july and january temperatures decreased by 4 c and by as much as 20 c respectively temperatures remained high enough to support forests during the stadials and we infer that the reconstructed decrease of precipitation below 500 mm per year caused the extirpation of forests during these periods thus we conclude that precipitation although difficult to reconstruct is of vital importance for explaining vegetation change during the eemian and eemian early weichselian transition

high geothermal heat flow basal melt and the origin of rapid ice flow in central greenland age depth relations from internal layering reveal a large region of rapid basal melting in greenland melt is localized at the onset of rapid ice flow in the large ice stream that drains north off the summit dome and other areas in the northeast quadrant of the ice sheet locally high melt rates indicate geothermal fluxes 15 to 30 times continental background the southern limit of melt coincides with magnetic anomalies and topography that suggest a volcanic origin

ice stream sticky spots a review of their identification and influence beneath contemporary and palaeo ice streams rapidly flowing ice streams are the arterial drainage routes in continental ice sheets and exert a major influence on ice sheet mass balance recent observations have revealed that ice stream flow exhibits considerable variability with relatively rapid changes taking place in speed and direction this spatial and temporal variability is intimately linked to the conditions at the base of the ice streams and the distribution of localised patches of basal friction known as sticky spots in this paper we provide a detailed review of sticky spot observations from both contemporary and palaeo ice stream beds in order to better understand their nature and influence observations and theoretical considerations reveal four primary causes of stickiness i bedrock bumps ii till free areas iii areas of strong well drained till and iv freeze on of subglacial meltwater these may act together in one location or in isolation and a progressive increase in their distribution could lead to ice stream shut down bedrock bumps are influential under active ice streams where they provide form drag and can create thinner ice which increases the likelihood of basal freeze on increased bed roughness may prevent the lateral migration of some ice streams but bedrock bumps are unlikely to cause ice stream shut down because over long time scales ice stream erosion might be expected to reduce their amplitude the influence of till free areas beneath an ice stream will depend critically on the amount of water that might be drawn out of the surrounding till to lubricate such areas they are likely to be most important in ice stream onset zones but their identification has proved difficult beneath active ice streams if an ice stream operates solely by till deformation it is conceivable that a progressive increase in the exposure of till free areas could lead to shut down through a process of sediment exhaustion areas of strong well drained till have been identified beneath both active and ancient ice streams and are most likely to result from the reorganisation of subglacial meltwater the collapse of an inefficient cannalised system to a more efficient channelised system can occur rapidly and this mechanism has been hypothesised as a candidate for ice stream shut down in both contemporary and palaeo settings basal freeze on has also been observed and inferred from beneath modern and palaeo ice streams and a reduction in basal meltwater supply coupled with ice stream drawdown and the advection of cold ice increases the likelihood of switching off an ice stream a paucity of data from ice stream sticky spots limits a better understanding of their nature distribution and evolution beneath ice streams future technological advances are likely to improve the resolution of the data collected from the beds of modern ice streams but well preserved palaeo ice stream beds also hold potential for investigating their influence on ice stream flow and we present simple landsystems models to aid their identification such data will considerably enhance the basal boundary condition in ice stream models which will ultimately refine our predictions of the response of contemporary ice sheets to future changes in climate

joint inversion of geophysical data summary by jointly inverting several different kinds of geophysical measurements at a site we avoid some of the ambiguity inherent in the individual methods we show how this can be done for the combination of dc resistivity and magnetotelluric measurements on a layered medium by considering a simple 3 layer model the combination resolves the resistivity of the thin resistive second layer even though neither of the two methods can do so alone the method is then applied to field data from a shallow sedimentary basin a blind zone occurs beneath a thick near surface conductive shale by a study of the eigenvalue structure of the model it can be seen that resolution in this zone would be slightly enhanced by higher frequency magnetotelluric data but additional dc data at larger spacing would yield no improvement

summer temperatures and lake development during the mis 5a interstadial new data from the unterangerberg palaeolake in the eastern alps austria abstract investigations of interstadials during early stages of glacial periods are of special interest because they featured large scale transformations of the climate system and the build up of land based ice sheets lacustrine sediment sequences are considered to be important natural archives that register past climate and environmental signals here we present new multi proxy data obtained from a palaeolake succession preserved in the inner alpine terrace of unterangerberg eastern alps these sediments formed during the second early wurmian interstadial equivalent to marine isotope stage mis 5a and were used to reconstruct changes in lake conditions and to infer past air temperatures the sediment geochemical data and subfossil aquatic biota provide evidence of a cyclic lake fen lake development during this interstadial the proxy records reveal stable lacustrine conditions with dense charophyte meadows and abundant aquatic fauna during the early part of the interstadial a progressive shallowing of the lake resulting in the spreading out of fen vegetation in the middle part and a transition from wetland to a renewed shallow lake stage towards the end of the interstadial chironomids were used to reconstruct mean july air temperatures employing a combined norwegian swiss chironomid temperature inference model the reconstruction indicates a temperature close to present day values of ca 18 c in the middle part of the record while temperatures of ca 13 14 c are recorded for the lower and upper parts the proxy data from this palaeolake provide evidence of heat and drought in the middle part of mis 5a supported by the chironomid based temperature reconstruction our reconstruction shows a climate pattern broadly similar to that found in pollen based estimates of mean july air temperatures from sites in the northern alpine foreland and compares well to other european palaeoclimatic reconstructions of mis 5a climate

stable climatic conditions in central germany during the last interglacial stable isotopic data from the high resolution limnic sequence in the opencast mine at grobern central germany confirm the relatively stable climate of the last interglacial deduced from pollen analyses after significant warming during the transition from the saalian to the eemian stable warm climatic conditions set in once the climatic optimum was reached in the course of biozones e4 to e5 from the middle of biozone e6b a gradual temperature decrease can be observed reaching a minimum at the start of biozone e7 the division of the early weichselian glacial in central germany into two stadials herning rederstall and two interstadials brorup odderade is also documented by the findings copyright 2000 john wiley sons ltd

the microstructure of polar ice part ii state of the art an important feature of natural ice in addition to the obvious relevance of glaciers and ice sheets for climate related issues is its ability to creep on geological time scales and low deviatoric stresses at temperatures very close to its melting point without losing its polycrystalline character this fact together with its strong mechanical anisotropy and other notable properties makes natural ice an interesting model material for studying the high temperature creep and recrystallization of rocks in earth s interior after having reviewed the major contributions of deep ice coring to the research on natural ice microstructures in part i of this work faria et al 2014 here in part ii we present an up todate view of the modern understanding of natural ice microstructures and the deformation processes that may produce them in particular we analyze a large body of evidence that reveals fundamental flaws in the widely accepted tripartite paradigm of polar ice microstructure also known as the three stage model cf part i these results prove that grain growth in ice sheets is dynamic in the sense that it occurs during deformation and is markedly affected by the stored strain energy as well as by air inclusions and other impurities the strong plastic anisotropy of the ice lattice gives rise to high internal stresses and concentrated strain heterogeneities in the polycrystal which demand large amounts of strain accommodation from the microstructural analyses of ice cores we conclude that the formation of many and diverse subgrain boundaries and the splitting of grains by rotation recrystallization are the most fundamental mechanisms of dynamic recovery and strain accommodation in polar ice additionally in fine grained high impurity ice layers e g cloudy bands strain may sometimes be accommodated by diffusional flow at low temperatures and stresses or microscopic grain boundary sliding via microshear in anisotropic ice sheared at high temperatures grain boundaries bulged by migration recrystallization and subgrain boundaries are endemic and very frequent at almost all depths in ice sheets evidence of nucleation of new grains is also observed at various depths provided that the local concentration of strain energy is high enough which is not seldom the case as a substitute for the tripartite paradigm we propose a novel dynamic recrystallization diagram in the three dimensional state space of strain rate temperature and mean grain size which summarizes the various competing recrystallization processes that contribute to the evolution of the polar ice microstructure dedicated to sepp kipfstuhl on occasion of his 60th birthday

the ice flow velocity profile for dye 3 greenland uniaxial compression tests were made on 72 deep ice core specimens taken from 14 depth intervals between 247 m and 2021 m on the dye 3 greenland ice core test results give stress exponent values ranging from 2 7 to 3 2 which are in close agreement to the 3 0 measured on laboratory manufactured polycrystalline ice specimens these test results are compared with similar measurements made in a field laboratory immediately after the ice core was recovered the data sets showed no significant differences over the four year time span the horizontal velocity profile is calculated for dye 3 using a strain rate enhancement factor relationship and using stress exponent and activation energy values measured for artificial laboratory ice the calculated profile is in good agreement with the results from direct borehole tilting measurements this is the first time such a close correlation has been affected between laboratory generated experimental data and field measurements in the analysis of a horizontal velocity profile for a large ice sheet

similarity of organized patterns in driving and basal stresses of antarctic and greenland ice sheets beneath extensive areas of basal sliding the rate of ice transport from the interior of ice sheets to their margins and hence the rate with which it contributes to sea level is determined by the balance of driving stress basal resistance and ice internal deformation using recent high resolution observations of the antarctic and greenland ice sheets we compute driving stress and ice deformation velocities inferring basal traction by inverse techniques the results reveal broad scale organization in 5 20 km band like patterns in both the driving and basal shear stresses located in zones with substantial basal sliding both ice sheets experience basal sliding over areas substantially larger than previously recognized the likely cause of the spatial patterns is the development of a band like structure in the basal shear stress distribution that is the results of pattern forming instabilities related to subglacial water the similarity of patterns on the greenland and antarctic ice sheets suggests that the flow of ice sheets is controlled by the same fundamental processes operating at their base which control ice sheet sliding and are highly variable on relatively short spatial and temporal scales with poor predictability this has far reaching implications for understanding of the current and projection of the future ice sheets evolution

a synthesis of the basal thermal state of the greenland ice sheet the basal thermal state of an ice sheet frozen or thawed is an important control upon its evolution dynamics and response to external forcings however this state can only be observed directly within sparse boreholes or inferred conclusively from the presence of subglacial lakes here we synthesize spatially extensive inferences of the basal thermal state of the greenland ice sheet to better constrain this state existing inferences include outputs from the eight thermomechanical ice flow models included in the searise effort new remote sensing inferences of the basal thermal state are derived from holocene radiostratigraphy modern surface velocity and modis imagery both thermomechanical modeling and remote inferences generally agree that the northeast greenland ice stream and large portions of the southwestern ice drainage systems are thawed at the bed whereas the bed beneath the central ice divides particularly their west facing slopes is frozen elsewhere there is poor agreement regarding the basal thermal state both models and remote inferences rarely represent the borehole observed basal thermal state accurately near northgrip and dye 3 this synthesis identifies a large portion of the greenland ice sheet about one third by area where additional observations would most improve knowledge of its overall basal thermal state

periodic lake peatland shifts under the eemian and early weichselian climate changes in central europe on the basis of multi proxy studies abstract long term palaeoecological sequences dated to the eemian interglacial and weichselian glaciation are crucial for understanding cyclic climate changes and ecosystem responses this article presents a multi proxy palaeobotanical zoological and geochemical study considering variable aspects of the eemian early weichselian lake mire succession in central poland the investigated sequence collected from a former small kettle hole revealed many ecosystem alterations i e warm stages led to the terrestrialisation of the lake whereas stadial coolings to its reappearance hence we examined whether i the eemian warming influenced the ecological succession of the lake ii lakes which reappeared during the herning and rederstall stadial coolings developed alike and iii the beginning of peatland functioning interglacial or interstadial had an impact on further plant succession the results show that thermal amelioration during the eemian interglacial supported domination of tetraedron minimum and pediastrum boryanum var boryanum which contributed to the sedimentation of bituminous shales the expansion of salvinia natans in the final stage of lake existence middle eemian was probably a result of the mild climate characterised by the lack of late spring frosts algae in contrast to cladocera revealed bipartition of the herning and rederstall stadials older sections were colder younger warmer however geochemical and isotopic data point to the very low productivity of the lakes during those stadials the lake that existed during the herning stadial revealed very low taxonomical diversity of cladocera whereas that during the rederstall was characterised by relatively high diversity of this group during the late eemian and brorup interstadial rich fens with some oligotrophic patches with sphagnum sp developed this study revealed that small water bodies due to their very local character and often uniqueness of palaeo records may be a very valuable source of information about climate and palaeoecology of different groups of organisms during the eemian interglacial and the weichselian glaciation

the down stress transition from cluster to cone fabrics in experimentally deformed ice abstract during plastic deformation of polycrystalline ice 1h ice crystals become crystallographically aligned due to dislocation glide primarily on the basal slip system such crystallographic preferred orientation cpo introduces a viscous anisotropy in ice and thus strongly influences the kinematics of the flow of glaciers and ice sheets two key mechanisms exert different controls on cpo in axial compression recrystallization dominated by lattice rotation yields a cluster of c axes parallel to compression and recrystallization dominated by grain boundary migration gbm yields a cone shaped distribution of c axes with the cone axis parallel to compression the transition between these dominant mechanisms of cpo formation has not been well quantified in this study we explore how this transition varies with stress ice deformation experiments were conducted using a high pressure gas medium apparatus to prevent fracturing of samples at relatively high stresses samples were deformed in uniaxial compression at a temperature of 10 c and a confining pressure of 10 mpa fabricated ice samples with starting average grain sizes of either 0 23 mm or 0 63 mm were each deformed to an axial strain of 0 2 at a nominally constant strain rate in the range 1 2 10 6 to 2 4 10 4 s 1 yielding flow stresses of 1 17 to 4 31 mpa high quality electron backscatter diffraction reveal the grain size shape subgrain structure and cpos formed at different stresses all deformed samples have strong non random cpos with c axes concentrated in cones the cone angle and cpo strength are observed to decrease with increasing stress as stress increases the fraction of grains with highly curved or lobate grain boundaries decreases and the fraction of polygonal grains with straight grain boundaries increases based on these observations we propose that a transition in the dominant mechanism of cpo formation occurs with increasing stress from gbm which consumes grains with low schmid factors at low stress to lattice rotation caused by slip primarily on the basal slip system which causes c axes to rotate to become parallel to the shortening direction at high stress mapping out the transition from cluster rotation dominated to cone gbm dominated cpos as a function of stress this study and temperature future studies allows for a robust extrapolation to and a fundamental understanding of the cpos formed at glaciological stresses and temperatures

how nonlinear is the creep deformation of polar ice a new field assessment most analyses of glacial systems require a relationship between strain rates and stresses for ice deforming in creep for conditions relevant to glacier flow much evidence shows that strain rates increase approximately as deviatoric stress raised to a power n field and laboratory experiments suggest that n 3 but values span a wide range and controversy persists most field efforts to determine n seek clarity by examining situations with simple stress states we instead use a fully three dimensional model to interpret measurements of flow of an antarctic outlet glacier that follows a tortuous path through a mountain range by comparing observed flow to models with different parameter values we conclude that the effective n must be between 2 6 and 5 1 99 confidence the best match occurs with n 3 5 we conclude that use of the traditional value is appropriate for polar ice deforming at stresses of 0 6 1 2 10 5 pa

3d gravity modeling of buyuk menderes basin in western anatolia using parabolic density function abstract a method to model 3d sedimentary basins with parabolic density contrast is applied to buyuk menderes basin in western anatolia the measured gravity fields reduced to a horizontal plane are assumed to be available at grid nodes of a rectangular square mesh juxtaposed 3d vertical prisms with their geometrical epicenters on top coincide with grid nodes of a mesh to approximate a sedimentary basin the algorithm based on newton s forward difference formula automatically calculates the initial depth estimates of a sedimentary basin assuming that 2d infinite horizontal slabs can generate the measured gravity fields and among these slabs the density contrast varies with depth the lower boundary of a sedimentary basin is formulated by estimating the depth values of the 3d prisms with in predetermined limits measured gravity fields pertaining to the buyuk menderes basin turkey where the density contrast varies with depth are interpreted to show the applicability of the method

reconstruction of quaternary temperature fields by dynamically consistent smoothing conventional methods of palaeoclimate reconstruction provide estimates of climatic parameters using proxy data which have originated from individual sites these reconstructions yield information on the local environment but only limited information on spatial scales that are required for model data intercomparisons we present here a new approach that connects these different scales by an upscaling of the local palaeoinformation together with a dynamically consistent spatial smoothing a probabilistic data based method for local reconstructions is combined with a dynamic constraint on the reconstructed climate parameter which stabilises the reconstruction on the target scale the variational analysis leads to climatological fields being optimised with respect to the proxy data and to the prescribed dynamics in a statistically consistent way this method allows a probabilistic approach of quality control of the palaeodata in terms of their spatial consistency and homogeneity and for an estimation of reconstruction errors the method was applied to palaeobotanical data to reconstruct near surface temperature fields constrained by simple linear dynamics an approximate approach was used to estimate the magnitude of reconstruction errors in terms of standard deviations reconstructed january and july mean temperature of the early eemian 125 000 years bp have errors with a median value of about 1 8 c in january and about 1 1 c in july reconstructions across europe show positive temperature anomalies for scandinavia and near the east coast of the baltic sea in contrast early eemian temperatures were apparently quite similar to those found today in central europe as no drastic differences were reconstructed between the eemian and modern 1961 1990 climate this implies somewhat stronger temperature gradients in the eemian than are observed today

increased seasonality in middle east temperatures during the last interglacial period the last interglacial period about 125 000 years ago is thought to have been at least as warm as the present climate owing to changes in the earth s orbit around the sun it is thought that insolation in the northern hemisphere varied more strongly than today on seasonal timescales which would have led to corresponding changes in the seasonal temperature cycle here we present seasonally resolved proxy records using corals from the northernmost red sea which record climate during the last interglacial period the late holocene epoch and the present we find an increased seasonality in the temperature recorded in the last interglacial coral today climate in the northern red sea is sensitive to the north atlantic oscillation a climate oscillation that strongly influences winter temperatures and precipitation in the north atlantic region from our coral records and simulations with a coupled atmosphere ocean circulation model we conclude that a tendency towards the high index state of the north atlantic oscillation during the last interglacial period which is consistent with european proxy records contributed to the larger amplitude of the seasonal cycle in the middle east

seasonal changes in ice sheet motion due to melt water lubrication abstract a numerical model is used to calculate how the motion of an idealized ice sheet margin is affected by the subglacial drainage of melt water from its surface the model describes the evolution of the drainage system and its coupling with ice flow through a sliding law that depends on the effective pressure the results predict ice acceleration during early summer when the inefficient drainage system is temporarily overwhelmed the growth of a more efficient drainage system leads to a subsequent slowdown of the ice very close to the margin but high water pressure and ice velocity are maintained through much of the summer further inland annual mean ice velocity increases with the total quantity of melt water except close to the margin where it is almost insensitive to the amount of melting short term variability of melt water input leads to rapid changes in ice velocity that result in a slight increase in the mean velocity relative to a smoother input linked cavity and poroelastic models for the distributed drainage system are compared and their relative merits discussed two different sliding laws are considered and the need for a holistic description of hydraulically controlled sliding is highlighted

evolution of derwael ice rise in dronning maud land antarctica over the last millennia ice rises situated in the ice shelf belt around antarctica have a spatially confined flow regime with local ice divides beneath the divides ice stratigraphy often develops arches with amplitudes that record the divide s horizontal residence time and surface elevation changes to investigate the evolution of derwael ice rise dronning maud land antarctica we combine radar and gps data from three consecutive surveys with a two dimensional full stokes thermomechanically coupled transient ice flow model we find that the surface mass balance smb is higher on the upwind and lower on the downwind slopes near the crest the smb is anomalously low and causes arches to form in the shallow stratigraphy observable by radar in deeper ice arches are consequently imprinted by both smb and ice rheology raymond effect the data show how arch amplitudes decrease as along ridge slope increases emphasizing that the lateral positioning of radar cross sections is important for the arch interpretation using the model with three rheologies isotropic with n 3 4 5 and anisotropic with n 3 we show that derwael ice rise is close to steady state but is best explained using ice anisotropy and moderate thinning our preferred albeit not unique scenario suggests that the ice divide has existed for at least 5000 years and lowered at approximately 0 03 m a 1 over the last 3400 years independent of the specific thinning scenario our modeling suggests that derwael ice rise has exhibited a local flow regime at least since the mid holocene

the last climatic cycle at la grande pile vosges france a new pollen profile a new pollen diagram has been made from an old unanalyzed core from la grande pile gp xx correlations with the previous work of woillard 1975 1978 leads to the following conclusions the end of the eemian was marked by a minor climatic change prior to the melisey i stadial the ognon ii and ognon iii interstadials are not consistent and correspond to the level of reworking of mesophilous taxa during the last pleniglacial the limitations of the botanical interpretation of pollen spectra from the last pleniglacial are underlined lastly bruckenthalia and larix are identified for the first time at la grande pile the respective roles of these taxa following the eemian are additional arguments for correlating the st germain i and ii with the early weichsel interstadials in the north of middle europe

vegetation and climate in the early and pleni weichselian in northern central europe analysis of numerous pollen diagrams from north and central germany and from the adjacent lowlands of the netherlands denmark poland and belarus have facilitated a regional comparison of the vegetation and climatic evolution from the beginning of the early weichselian glaciation to the weichselian pleniglacial data from geological studies and analysis of fossil beetles and plant macroremains have been used to supplement the palaeoclimatic reconstruction up to the end of the oerel interstadial the palaeoclimate was characterised by increasing continentality the winter temperatures in particular tended to fall continuously during the stadials and the interstadials of the early weichselian and early pleniglacial in the brorup and odderade interstadials summer temperatures where sufficiently high to enable boreal forests to grow whereas in the oerel interstadial summer temperatures were such that tree growth was inhibited it is probable that falling sea level and the consequent extension northwards of the north sea coast were the main causes of increasing continentality in the latter part of the pleniglacial peat accumulation frequently took place and humic silts were deposited under lacustrine conditions correlation between various sites is extraordinarily difficult pollen diagrams from the so called interstadials of the glinde moershoofd complex hengelo and denekamp are similar to those of the known early weichselian stadials it is proposed therefore that these interstadials should be called intervals and that the term interstadial be reserved for climatic variations that result in distinctive pollen assemblages and which in turn reflect distinctive vegetation dynamics interstadials should be capable of being characterised on a biostratigraphical basis and it should be possible to establish correlations over considerable distances according to this definition the first three warm oscillations of the weichselian glaciation in the central european lowlands are the brorup odderade and oerel interstadials all other pleniglacial peat layers should be assigned the rank of interval copyright 2001 john wiley sons ltd

cyclic climate fluctuations during the last interglacial in central europe differentiating natural climate change from anthropogenic forcing is a major challenge in the prediction of future climates in this context the investigation of interglacials provides valuable information on natural climate variability during periods that resemble the present this paper shows that natural cyclic changes in winter climates affected central european environments during the last interglacial i e the eemian 126 110 ka as a result of the extraordinarily high counting sums performed at eemian pollen samples it was possible to reveal a robust presence absence pattern of the insect pollinated and therefore in the pollen rain underrepresented taxon hedera this plant is known to require the influence of oceanic winter climates i e moist and mild in northwest and central europe by analogy with recent findings from the north atlantic s holocene interglacial the trigger of the eemian climate variability may have been changes in solar activity possibly amplified by changes in north atlantic ocean currents and or in the north atlantic oscillation our findings suggest natural cyclic changes to be a persistent feature of interglacial climates

traveling slippery patches produce thickness scale folds in ice sheets large complex stratigraphic folds that rise as high as 60 of the local ice thickness have been observed in ice sheets on antarctica and greenland here we show that ice deformation caused by heterogeneous and time variable basal sliding can produce the observed structures we do this using a thermomechanical ice sheet model in which sliding occurs when the base approaches the melting point and slippery patches develop these slippery patches emerge and travel downstream because of a feedback between ice deformation vertical flow and temperature our model produces the largest overturned structures comparable to observations when the patches move at about the ice column velocity we conclude that the history of basal slip conditions is recorded in the ice sheet strata these basal conditions appear to be dynamic and heterogeneous even in the slow flowing interior regions of large ice sheets

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

eemian climate fluctuations observed in a european pollen record recent ice core data from greenland1 2 suggest that the climate during the last interglacial period the eemian was more unstable than that of the holocene about 10 000 years ago to the present being characterized in particular by a series of cold episodes each lasting about 70 to 750 years subsequent analysis of a second greenland ice core3 4 however failed to corroborate the details of these eemian climate fluctuations a result that may be attributable to the effects of ice flow4 to resolve this discrepancy it is imperative to seek alternative sources of information about the eemian climate here we present climate reconstructions from pollen data from the annually laminated eemian lake sediment record at bispingen5 and from the eemian and holocene peat records at la grande pile6 the former record indicates that an initially warm period of 2 900 yr was followed by cooling and a series of colder episodes one of which had winter temperatures comparable to those at the end of the preceding cold stage the latter records show greater climate instability during the eemian than the holocene these results are in broad agreement with those from the grip ice core but contrast both with the gisp2 core3 4 and with recent high resolution marine records from the north atlantic7 8

full field predictions of ice dynamic recrystallisation under simple shear conditions understanding the flow of ice on the microstructural scale is essential for improving our knowledge of large scale ice dynamics and thus our ability to predict future changes of ice sheets polar ice behaves anisotropically during flow which can lead to strain localisation in order to study how dynamic recrystallisation affects to strain localisation in deep levels of polar ice sheets we present a series of numerical simulations of ice polycrystals deformed under simple shear conditions the models explicitly simulate the evolution of microstructures using a full field approach based on the coupling of a viscoplastic deformation code vpfft with dynamic recrystallisation codes the simulations provide new insights into the distribution of stress strain rate and lattice orientation fields with progressive strain up to a shear strain of three our simulations show how the recrystallisation processes have a strong influence on the resulting microstructure grain size and shape while the development of lattice preferred orientations lpo appears to be less affected activation of non basal slip systems is enhanced by recrystallisation and induces a strain hardening behaviour up to the onset of strain localisation and strain weakening behaviour simulations demonstrate that the strong intrinsic anisotropy of ice crystals is transferred to the polycrystalline scale and results in the development of strain localisation bands than can be masked by grain boundary migration therefore the finite strain history is non directly reflected by the final microstructure masked strain localisation can be recognised in ice cores such as the edml from the presence of stepped boundaries microshear and grains with zig zag geometries

zooming on northern cap de creus shear zones abstract the northern cap de creus shear belt provides fine examples of shear zones developed at different scales in rocks bearing pre existing penetrative foliations the shear zones developed under retrograde metamorphic conditions and they are preferentially located in crystalline schists lower grade metasedimentary rocks accommodated deformation by folding the shear zones are linked in an anastomosing framework with self similar properties they are interpreted to have formed as the result of a progressive wrench dominated deformation affecting anisotropic rocks a progressive non coaxial deformation regime is considered responsible for the development of the complex kinematic pattern wherein shear zones with opposite senses of shear may come to lie in close parallelism in such a framework individual shear zones are characterised by marked strain gradients not only across the shear zone but also along it these features together with the geometrical analysis of shear zone marginal domains indicate that these structures developed with a component of shortening across the shear zones the shear zones are interpreted to have nucleated as buckling instabilities that gradually evolved to become shear zones approaching the simple shear model microstructural and quartz c axis fabric analyses indicate that shear zone formation is associated with strong strain partitioning on all scales

depositional environment and climate changes during the late pleistocene as recorded by the netiesos section in southern lithuania the netiesos section in southern lithuania exposes a late pleistocene sedimentary sequence at a depth of between 17 and 4 m an interdisciplinary study of the section investigated the environmental changes that occurred in the study area during the greater part of marine isotope stage mis 5 geochemical thermoluminescence palaeontological plant macro remain diatoms fishbones and magnetic susceptibility analyses were performed on numerous sediment samples chronological control of the sequence was provided by electron spin resonance esr infrared optically stimulated luminescence ir osl and conventional radiocarbon dating methods this interdisciplinary approach enabled the subdivision of the section into stratigraphic units reflecting environmental changes according to chronological data the development of the netiesos palaeolake began at the end of the medininkai saalian glaciation which is thought to correlate with mis 6 and continued up to the thermophilous deciduous forests phase of the subsequent last interglacial of mis 5 sediments of the final phase of the interglacial are missing as are the initial and final phases of the following early nemunas weichselian cooling mis 4 the palaeomagnetic blake event was recorded in the interglacial sediments one of the richest palaeofloras in the east baltic region was observed representing the second half of mis 5 the implications of the results for the regional late pleistocene climato chronostratigraphy are discussed

greenland s contribution to global sea level rise by the end of the 21st century the greenland ice sheet holds enough water to raise the global sea level with 7 m over the last few decades observations manifest a substantial increase of the mass loss of this ice sheet both enhanced melting and increase of the dynamical discharge associated with calving at the outlet glacier fronts are contributing to the mass imbalance using a dynamical and thermodynamical ice sheet model and taking into account speed up of outlet glaciers we estimate greenland s contribution to the 21st century global sea level rise and the uncertainty of this estimate boundary fields of temperature and precipitation extracted from coupled climate model projections used for the ipcc fourth assessment report are applied to the ice sheet model we implement a simple parameterization for increased flow of outlet glaciers which decreases the bias of the modeled present day surface height it also allows for taking into account the observed recent increase in dynamical discharge and it can be used for future projections associated with outlet glacier speed up greenland contributes 0 17 cm to global sea level rise by the end of the 21st century this range includes the uncertainties in climate model projections the uncertainty associated with scenarios of greenhouse gas emissions as well as the uncertainties in future outlet glacier discharge in addition the range takes into account the uncertainty of the ice sheet model and its boundary fields

quantitative reconstruction of climate variability during the eemian merkinė and weichselian nemunas in lithuania abstract little is known concerning climate changes in the eastern baltic region during the last interglacial glacial cycle and in particular climate changes during the weichselian in this study a quantitative reconstruction of the mean january and july temperature for the medininkai 117 site in lithuania is presented the reconstruction is based on pollen and plant macrofossils from this site which reveal that the vegetation was characteristic of many northern europe sites during the eemian and early weichselian gradual evolution of the vegetation suggests that relatively uniform climate conditions existed during the eemian our reconstructions support the view of a relatively stable eemian with short cooling phases of low amplitude a strong increase in temperature was apparent during the beginning of the interglacial and decrease during the transition to the weichselian reconstructed july temperatures of the eemian interglacial were approximately 2 c higher than today 18 5 c today 16 2 c and were similar to today for january 5 2 c today 5 1 c july temperatures during the early weichselian were only 2 c lower than during the eemian whereas the january temperatures gradually decreased winter temperatures were relatively high above 10 c during the early weichselian