oxygen isotope analyses and pleistocene ice volumes abstract the oxygen isotope record from fossil foraminifera in deep sea sediments is commonly used as a proxy for global ice volume the linkage between 18 o and ice volume however is probably nonlinear we have developed a simple numerical model of the isotopic response of the oceans to ice volume change the major features it simulates are 1 the changing mean isotopic composition of snow as a function of ice volume colder snow temperatures forced by climate change and higher elevation accumulation areas imply more negative mean 18 o 2 the nonequilibrium isotopic composition of ice sheets the past history of an ice sheet is integrated into its mean isotopic composition which introduces a lag of isotopic ice volume i e the measured 18 o record scaled to ice volume units behind true ice volume 3 selective preservation of isotopically more negative colder higher latitude ice this geographic effect can selectively amplify or dampen the isotopic response to the ice volume signal we illustrate the response of our model to simple hypothetical ice volume transitions of ice growth and ice decay sensitivity tests are illustrated for all model parameters the results suggest that oxygen isotope records reproduce the general patterns of ice volume change fairly accurately the foraminiferal isotope record however may misrepresent the true amplitude of the ice volume signal and lag true ice volume by 1000 to 3000 yr

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

a continuum mixture model of ice stream thermomechanics in the laurentide ice sheet 1 theory we employ a continuum mixture framework to incorporate ice streams in a three dimensional thermomechanical model of the laurentide ice sheet the ice mass is composed of a binary mixture of sheet ice which deforms by viscous creep and stream ice which flows by sliding and or sediment deformation at the bed dynamic and thermal evolutions are solved for each component in the mixture with coupling rules to govern transfer between flow regimes we describe two different transfer mechanisms 1 creep exchange the nourishment of ice streams by viscous creep inflow from the surrounding ice sheet and 2 bed exchange the activation growth and deactivation of ice streams perpetrated by transfers of bed area between flow constituents this paper develops the underlying mixture theory we express the governing equations for mass momentum and energy balance in a form suitable for direct incorporation in existing numerical models of ice thermomechanics a companion paper in this issue explores mixture and ice stream behavior in applications with the laurentide ice sheet

modelled atmospheric temperatures and global sea levels over the past million years the climate has passed through a series of glacials and interglacials over the past million years but the nature of this cyclicity in terms of temperature ice volume and sea level and the underlying causes are not well known bintanja et al use a new method to deduce a one million year time series of these variables the reconstructed records are much longer than other methods have provided for any of these variables individually the most intense glacial stages were 17 c colder than today and most of the continental ice was present in north america strong cooling in the beginning of glacials was found to precede ice sheet build up these findings may shed light on the causes of ice age cycles marine records of sediment oxygen isotope compositions show that the earth s climate has gone through a succession of glacial and interglacial periods during the past million years but the interpretation of the oxygen isotope records is complicated because both isotope storage in ice sheets and deep water temperature affect the recorded isotopic composition1 2 3 4 5 separating these two effects would require long records of either sea level or deep ocean temperature which are currently not available here we use a coupled model of the northern hemisphere ice sheets6 and ocean temperatures forced to match an oxygen isotope record for the past million years compiled from 57 globally distributed sediment cores to quantify both contributions simultaneously we find that the ice sheet contribution to the variability in oxygen isotope composition varied from ten per cent in the beginning of glacial periods to sixty per cent at glacial maxima suggesting that strong ocean cooling preceded slow ice sheet build up the model yields mutually consistent time series of continental mean surface temperatures between 40 and 80 n ice volume and global sea level we find that during extreme glacial stages air temperatures were 17 1 8 c lower than present with a 120 10 m sea level equivalent of continental ice present

predicted bounds on peak global mean sea level during marine isotope stages 5a and 5c abstract estimates of marine isotope stage mis 5c and 5a global mean sea level gmsl based on marine terraces and coastal indicators range from 15 37 m and 9 30 m below present respectively tectonic displacement and glacial isostatic adjustment gia complicate efforts to refine this range we revisit this issue using numerical predictions of post glacial sea level change and updated estimates of site specific tectonic signals based upon these predictions laurentide and cordilleran peripheral bulge dynamics and variations in the gravitational effect of these ice masses dominate the gia signal along the east and west coasts of north america published compilations suggest that mis 5 sea level indicators extending from virginia to the caribbean lie on the outer flank of the peripheral bulge while those along a transect from oregon to baja california sit on the inner flank of the bulge our gia modeling reconciles these data by adopting a significantly weaker i e lower viscosity upper mantle in analyzing data on the pacific coast relative to the atlantic an inference supported by seismic tomography we present a sensitivity analysis that compares the observed elevation of globally distributed high stand markers to a suite of gia simulations that vary the earth model and gmsl from the last interglaciation mis 5e 130 115 ka to 70 ka we conclude that gmsl peaked at 8 5 4 6 m 1 during mis 5a and 9 4 5 3 m during mis 5c a more restrictive analysis yields slightly wider bounds corresponding to 10 5 5 5 m and 11 1 6 6 m respectively

application of a polythermal three dimensional ice sheet model to the greenland ice sheet response to steady state and transient climate scenarios abstract steady state and transient climate change computations are performed with the author s three dimensional polythermal ice sheet model simulation code for polythermal ice sheets for the greenland ice sheet the distinctive feature of this model is the detailed consideration of the basal temperate ice layer in which the water content and its impact on the ice viscosity are computed its transition surface to the cold ice region is accounted for by continuum mechanical jump conditions on this interface the simulations presented include steady states subject to a range of physical parameters and two different climates present and glacial conditions as well as three types of transient scenarios namely i sinusoidal milankovic period forcing ii paleoclimatic forcing from the greenland ice core project core reconstruction and iii future greenhouse warming forcing

modeling north american freshwater runoff through the last glacial cycle the northern hemisphere ice sheets decayed rapidly during deglacial phases of the ice age cycle producing meltwater fluxes that may have been of sufficient magnitude to perturb oceanic circulation the continental record of ice sheet history is more obscured during the growth and advance of the last great ice sheets ca 120 000 20 000 yr b p but ice cores tell of high amplitude millennial scale climate fluctuations that prevailed throughout this period these climatic excursions would have provoked significant fluctuation of ice sheet margins and runoff variability whenever ice sheets extended to mid latitudes giving a complex pattern of freshwater delivery to the oceans a model of continental surface hydrology is coupled with an ice dynamics model simulating the last glacial cycle in north america meltwater discharged from ice sheets is either channeled down continental drainage pathways or stored temporarily in large systems of proglacial lakes that border the retreating ice sheet margin the coupled treatment provides quantitative estimates of the spatial and temporal patterns of freshwater flux to the continental margins results imply an intensified surface hydrological environment when ice sheets are present despite a net decrease in precipitation during glacial periods diminished continental evaporation and high levels of meltwater production combine to give mid latitude runoff values that are highly variable through the glacial cycle but are two to three times in excess of modern river fluxes drainage to the north atlantic via the st lawrence hudson and mississippi river catchments averages 0 356 sv for the period 60 000 10 000 yr b p compared to 0 122 sv for the past 10 000 yr high amplitude meltwater pulses to the gulf of mexico north atlantic and north pacific occur throughout the glacial period with ice sheet geometry controlling intricate patterns of freshwater routing variability runoff from north america is staged in the final deglaciation with a stepped sequence of pulses through the mississippi st lawrence arctic and hudson strait drainages

areal distribution of the oxygen isotope ratio in greenland mean values of the oxygen isotope ratio relative to standard mean ocean water 8 180 in 0 reported for 46 sites on the greenland ice sheet are compiled together with data on mean annual surface temperature latitude 8180 elevation and mean annual shor test distance to the open ocean denoted by the 10 sea ice concentration boundary stepwise regression analyses with 8180 as the dependent variable define two robust models in the forward mode at the 99 9 confidence level only temperature enters the model in the backward mode at the 95 con fidence level only temperature latitude and distance to the open ocean remain in the model inversions of the models on the basis of 160 gridpoint locations 100 km apart in the area delimited by the surface equilibrium line produce four contoured distributions of b iho two distributions are based on the bivariate model and two on the multivariate modcl the sccond distribution for each model is obtained substituting mean annual surfa ce tempe ratu re values obtained from the nim bus 7 temperature humidity infrared radiometer thir database all four distribu tions are considered valid and differences between them are evaluated using contoured anomaly maps it is suggested that the inversion o the multivariate model using thir data pro ides the more reliable pattern for stu dies of atmospheric advection or for the derivation of ice flow adjustments for 81ho series obt ained from deep core or ablation zone sites

calving processes and the dynamics of calving glaciers abstract calving of icebergs is an important component of mass loss from the polar ice sheets and glaciers in many parts of the world calving rates can increase dramatically in response to increases in velocity and or retreat of the glacier margin with important implications for sea level change despite their importance calving and related dynamic processes are poorly represented in the current generation of ice sheet models this is largely because understanding the calving problem involves several other long standing problems in glaciology combined with the difficulties and dangers of field data collection in this paper we systematically review different aspects of the calving problem and outline a new framework for representing calving processes in ice sheet models we define a hierarchy of calving processes to distinguish those that exert a fundamental control on the position of the ice margin from more localised processes responsible for individual calving events the first order control on calving is the strain rate arising from spatial variations in velocity particularly sliding speed which determines the location and depth of surface crevasses superimposed on this first order process are second order processes that can further erode the ice margin these include fracture propagation in response to local stress imbalances in the immediate vicinity of the glacier front undercutting of the glacier terminus by melting at or below the waterline and bending at the junction between grounded and buoyant parts of an ice tongue calving of projecting submerged ice feet can be regarded as a third order process because it is paced by first or second order calving above the waterline first order calving can be represented in glacier models using a calving criterion based on crevasse depth which is a function of longitudinal strain rate modelling changes in terminus position and calving rates thus reduces to the problem of determining the ice geometry and velocity distribution realistic solutions to the problem of modelling ice flow therefore depend critically on an appropriate choice of sliding law models that assume that basal velocities are controlled by basal drag can replicate much of the observed behaviour of calving glaciers with grounded termini but an important limitation is that they cannot be used to model floating glacier termini or ice shelves alternative sliding laws that parameterise drag from the glacier margins provide more flexible and robust ways of representing calving in ice sheet models such models can explain a remarkable range of observed phenomena within a simple unifying framework including downglacier increases in velocity and strain rates where basal and or lateral drag diminishes flow acceleration in response to thinning through time the tendency for glaciers to stabilise at pinning points in relatively shallow water or fjord narrowings the constraints on ice shelf stability and the contrasts in calving rates between tidewater and freshwater calving glaciers many unresolved issues remain however including the role played by the removal of backstress in the acceleration of retreating calving glaciers and the controls on melting at and below the waterline

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

a model of oxygen isotope composition of precipitation implications for paleoclimate data the ratio of oxygen isotopes in glacial ice is often used as an indicator of climatic conditions during the period of time in which the ice was deposited we have developed a model which predicts the isotopic composition of snow given the composition of the source water and the meteorological path between the formation of an air water parcel in the boundary layer and the final deposition of precipitation onto the ice sheets after comparing the model results with isotope data from the present we draw inferences about the past isotopic composition of continental ice sheets our results depend strongly on the details of the meteorological path they are consistent with standard interpretations of isotope data but in some cases they allow alternate interpretations

the middle pleistocene transition characteristics mechanisms and implications for long term changes in atmospheric pco2 abstract the emergence of low frequency high amplitude quasi periodic 100 kyr glacial variability during the middle pleistocene in the absence of any significant change in orbital forcing indicates a fundamental change internal to the climate system this middle pleistocene transition mpt began 1250 ka and was complete by 700 ka its onset was accompanied by decreases in sea surface temperatures ssts in the north atlantic and tropical ocean upwelling regions and by an increase in african and asian aridity and monsoonal intensity during the mpt long term average ice volume gradually increased by 50 m sea level equivalent whereas low frequency ice volume variability experienced a 100 kyr lull centered on 1000 ka followed by its reappearance 900 ka although as a broad band of power rather than a narrow persistent 100 kyr cycle additional changes at 900 ka indicate this to be an important time during the mpt beginning with an 80 kyr event of extreme sst cooling followed by the partial recovery and subsequent stabilization of long term north atlantic and tropical ocean ssts increasing southern ocean sst variability primarily associated with warmer interglacials the loss of permanent subpolar sea ice cover and the emergence of low frequency variability in pacific ssts and global deep ocean circulation since 900 ka ice sheets have been the only component of the climate system to exhibit consistent low frequency variability with the exception of a near universal organization of low frequency power associated with marine isotope stages 11 and 12 all other components show an inconsistent distribution of power in frequency time space suggesting a highly nonlinear system response to orbital and ice sheet forcing most hypotheses for the origin of the mpt invoke a response to a long term cooling possibly induced by decreasing atmospheric pco2 none of these hypotheses however accounts for the geological constraint that the earliest northern hemisphere ice sheets covered a similar or larger area than those that followed the mpt given that the mpt was associated with an increase in ice volume this constraint requires that post mpt ice sheets were substantially thicker than pre mpt ice sheets indicating a change in subglacial conditions that influence ice dynamics we review evidence in support of the hypothesis that such an increase in ice thickness occurred as crystalline precambrian shield bedrock became exposed by glacial erosion of a thick mantle of regolith this exposure of a high friction substrate caused thicker ice sheets with an attendant change in their response to the orbital forcing marine carbon isotope data indicate a rapid transfer of organic carbon to inorganic carbon in the ocean system during the mpt if this carbon came from terrigenous sources an increase in atmospheric pco2 would be likely which is inconsistent with evidence for widespread cooling apparently rapid carbon transfer from terrestrial sources is difficult to reconcile with gradual erosion of regolith a more likely source of organic carbon and nutrients which would mitigate pco2 rise is from shelf and upper slope marine sediments which were fully exposed for the first time in millions of years in response to thickening ice sheets and falling sealevels during the mpt modeling indicates that regolith erosion and resulting exposure of crystalline bedrock would cause an increase in long term silicate weathering rates in good agreement with marine sr and os isotopic records we use a carbon cycle model to show that a post mpt increase in silicate weathering rates would lower atmospheric pco2 by 7 12 ppm suggesting that the attendant cooling may have been an important feedback in causing the mpt

thermal convection in ice sheets we look but do not see un critere de rayleigh generalise de la convection thermique dans les solides polycristallins chauffes par en dessous a ete developpe et applique a la convection dans les calottes glaciaires une analyse du profil de temperature a l aval du forage de byrd station suggere qu environ mille metres de glace en dessous du point d inversion de la densite entretiendrait la convection

tracer transport in the greenland ice sheet three dimensional isotopic stratigraphy abstract polar ice cores can provide both a record of climate history and a sharp test of the performance of numerical ice dynamics models the stratigraphic structure of an ice sheet is an expression of its full depositional and dynamic history and thus presents a greater challenge to computer models than merely matching the contemporary ice thickness and areal extent we describe a coupled model of ice and tracer dynamics that is realized by adding a semi lagrangian tracer transport scheme to a conventional thermomechanical ice dynamics model model skill is demonstrated by using ice core data from the grip site near summit greenland to successfully predict the isotopic stratigraphy of ice cores at other deep drilling sites the success of this effort indicates that when compensated for the effects of ice flow and elevation all the deep cores relate a coherent glacial history over the past 120 000 years according to the simulation results the oldest greenland ice lies beneath the grip gisp2 and northgrip sites although comparably old ice may also be found in north greenland and east central greenland

substantial contribution to sea level rise during the last interglacial from the greenland ice sheet during the last interglacial period the eemian global sea level was at least three metres and probably more than five metres higher than at present1 2 complete melting of either the west antarctic ice sheet or the greenland ice sheet would today raise sea levels by 6 7 metres but the high sea levels during the last interglacial period have been proposed to result mainly from disintegration of the west antarctic ice sheet3 with model studies attributing only 1 2 m of sea level rise to meltwater from greenland4 5 this result was considered consistent with ice core evidence4 although earlier work had suggested a much reduced greenland ice sheet during the last interglacial period6 here we reconsider the eemian evolution of the greenland ice sheet by combining numerical modelling with insights obtained from recent central greenland ice core analyses our results suggest that the greenland ice sheet was considerably smaller and steeper during the eemian and plausibly contributed 4 5 5 m to the sea level highstand during that period we conclude that the high sea level during the last interglacial period most probably included a large contribution from greenland meltwater and therefore should not be interpreted as evidence for a significant reduction of the west antarctic ice sheet

the isotopic composition of the laurentide ice sheet and fossil groundwater stable isotopes have been used to study large scale changes in hydrology during the pleistocene epoch many of these efforts have required an estimate of the 18o value of runoff generated by melting ice sheets there is no consensus on representative values here we examine 18o values from fossil groundwater samples and isotope enabled general circulation models gcms to better understand the isotopic composition of the laurentide ice sheet lis groundwater 18o values ranged from 12 5 to 25 3 and tended to increase southward the 18o precipitation values predicted by gcms follow a similar trend but increase more steeply southward the difference in groundwater and gcm output can be explained by invoking movement of glacial ice and meltwater along with mixing within groundwater systems most groundwater 18o values are higher than an average lis 18o value of 25 4 2 5 calculated based on estimated ice sheet volumes and sea level data

comparison of oxygen isotope records from the gisp2 and grip greenland ice cores recent results1 2 from the greenland ice core project grip summit ice core suggest that the climate in greenland has been remarkably stable during the holocene but was extremely unstable for the time period represented by the rest of the core spanning the last two glaciations and the intervening eemian inter glacial here we present the complete oxygen isotope record for the greenland ice sheet project 2 gisp2 core drilled 28 km west of the grip core we observe large rapid climate fluctuations throughout the last glacial period which closely match those reported for the grip core however in the bottom 10 of the cores spanning the eemian interglacial and the previous glacia tion there are significant differences between the two records it is possible that ice flow may have altered the chronological sequences of the stratigraphy for the bottom part of one or both of the cores considerable further work will be necessary to evaluate the likelihood of this and the extent to which it will still be possible to extract meaningful climate information from the lowest sections of the cores

ice sheets and sea level of the last glacial maximum this paper outlines the general issues regarding ice sheets and sea level of the last glacial maximum lgm which formed the basis of an epilog project workshop papers in this special issue of quaternary science reviews provide a comprehensive assessment of these issues from the perspective of geological reconstructions of ice sheet extent records of sea level change ice sheet modelling geophysical models of glacial isostatic adjustment and geochemical proxies of ice volume this new assessment has substantially narrowed the uncertainties in the total changes in ice sheets and sea level and their proxies suggesting a net decrease in eustatic sea level at the lgm ranging from 120 to 135 m r 2001 elsevier science ltd all rights reserved

isotopic balance of the greenland ice sheet modelled concentrations of water isotopes from 30 000 bp to present abstract polar ice cores contain a complex record of climate and ice flow that can be used to decode climate history interpretation of ice core records requires the flow signature to be either simple or removable when the signature is complex computer modelling can become a powerful aid to interpretation by combining an ice dynamics model with tracer deposition and transport models the stratification of individual tracers can be calculated in this way the time evolving three dimensional chemical stratigraphy can be simulated by linking an ice tracer dynamics model with models of atmosphere and ocean tracer dynamics coupled models of the global geochemical cycling of water isotopes can be developed such models hold promise for an integrated interpretation of the marine and ice core climate records as a first step along this path we apply tracer transport modelling to simulate the temporal and spatial variations of water isotopes in the greenland ice sheet over the past 30 000 years and calculate the influence of greenland ice volume and isotopic chemistry on the isotopic content of the ocean we find that the greenland ice sheet maintained its maximum ice volume from 18 000 to 15 000 bp but was isotopically lightest at 11 000 bp the sea level equivalent of ice volume changes occurring over the past 30 000 years did not exceed 2 1 m and the volume averaged influence on ocean chemistry did not exceed 0 025 for 18 o and 0 20 for d and at the last glacial maximum taken as 21 000 bp these values were respectively 1 9 m for ice volume 0 022 for 18 o and 0 17 for d

distortion of isochronous layers in ice revealed by ground penetrating radar in addition to measuring ice sheet thickness ground penetrating radar can be used to delineate reflections in ice sheets1 2 3 these reflections are generally accepted to result from layers of isochronous deposition of snow and can reveal much about the dynamics of the ice flow here we present ground penetrating radar data from fletcher promontory antarctica which show arches and troughs in isochronous ice layers to a depth of 100 m we demonstrate that the origin of these features can be determined by their growth with depth and many features result from local anomalies in accumulation rate which can be correlated with the ice surface slope one arch appears to be the result of a local anomaly in vertical strain rate its proximity to the ice divide width and growth with depth indicate that this arch is one of a class of features postulated by raymond4 but not previously shown to exist in the field such a feature is an indication of the nonlinear rheology of ice and requires that palaeoclimate records from ice cores extracted from the vicinity of ice divides underlain by similar features should be specifically corrected for such effects on ice flow

sensitivity of a greenland ice sheet model to ice flow and ablation parameters consequences for the evolution through the last climatic cycle sensitivity experiments are conducted to test the influence of poorly known model parameters on the simulation of the greenland ice sheet by means of a three dimensional numerical model including the mechanical and thermal processes within the ice two types of experiments are performed steady state climatic conditions and simulations over the last climatic cycle with a climatic forcing derived from the grip record the experiments show that the maximum altitude of the ice sheet depends on the ice flow parameters deformation and sliding law coefficients geothermal flux and that it is low when the ice flow is fast on the other hand the maximum altitude is not sensitive to the ablation strength and consequently during the climatic cycle it is driven by changes in accumulation rate the ice sheet extension shows the opposite sensitivity it is barely affected by ice flow velocity and the ice covered area is smaller for large ablation coefficients for colder climates when there is no ablation the ice sheet extension depends on the sea level an interesting result is that the variations with time of the altitude at the ice divide summit do not depend on the parameters we tested the present modelled ice sheets resulting from the climatic cycle experiments are compared with the present measured ice sheet in order to find the set of parameters that gives the best fit between modelled and measured geometry it seems that compared to the parameter set most commonly used higher ablation rate coefficents must be used

steady state characteristics of the greenland ice sheet under different climates the greenland ice sheet is modelled to simulate its extent and volume in warmer climates and also to find out whether the ice sheet would re form on theice free bedrock under present climatic conditions the ice sheet model is a three dimensional thermo mechanical model with a fine resolution grid thebedrock surface beneath the ice sheet was mapped using radio echo sounding measurements by the electromagnetic institute copenhagen the modelexperiments show that increased temperature will result in ice margin retreat but the ice sheet is relatively stable it takes a rather high temperature rise of atleast 6ic for the ice sheet to disappear completely which indicates that the ice sheet probably survived the last interglacial also it appears that the greenlandice sheet is not a mere relict ice mass from a previously colder climate but that the ice sheet will still re form on the bare bedrock under the present or evenslightly warmer climatic conditions

large scale ice sheet modelling as a means of dating deep ice cores in greenland the three dimensional ice sheet model sicopolis is used to simulate the dynamic thermodynamic behaviour of the entire greenland ice sheet from 250 000 a bp until today external forcing consists of a surface temperature history constructed from 18 o data of the grip core a snowfall history coupled linearly to that of the surface temperature a piecewise linear sea level scenario and a constant geothermal heat flux the simulated greenland ice sheet is investigated in the vicinity of summit the position where the maximum elevation is taken and where the two drill sites grip and gisp 2 are situated 28 km apart from each other in the region the aggreement between modelled and observed topography and ice temperature tuns out to be very good computed age depth profiles for grip and gisp2 are presented which can be used to complete the dating of these cores in the deeper regions where annual layer counting is not possible however artififical diffusion influences the computed ages in a near basel boundary lar of approximately 15 of theice thickness so that the age at the bottom of the cores cannot be predicted yet

tracer transport in an isochronal ice sheet model the full history of ice sheet and climate interactions is recorded in the vertical profiles of geochemical tracers in polar ice sheets numerical simulations of these archives promise great advances both in the interpretation of these reconstructions and the validation of the models themselves however fundamental mathematical shortcomings of existing models subject tracers to spurious diffusion thwarting straightforward solutions here i propose a new vertical discretization for ice sheet models that eliminates numerical diffusion entirely vertical motion through the model mesh is avoided by mimicking the real world flow of ice as a thinning of underlying layers a new layer is added to the surface at equidistant time intervals isochronally thus identifying each layer uniquely by its time of deposition and age this new approach is implemented for a two dimensional section through the summit of the greenland ice sheet the ability to directly compare simulations of vertical ice cores with reconstructed data is used to find optimal model parameters from a large ensemble of simulations it is shown that because this tuning method uses information from all times included in the ice core it constrains ice sheet sensitivity more robustly than a realistic reproduction of the modern ice sheet surface

human used upper montane ecosystem in the horton plains central sri lanka a link to lateglacial and early holocene climate and environmental changes abstract this study utilizes radiocarbon dated pollen spores sphagnum spp macrofossils and total organic carbon proxies to examine variability of past climate environment and human activity in montane rainforest grassland and wetland of the horton plains hp central sri lanka since the last glacial maximum lgm the lgm is largely characterized by grasslands and xerophytic herbs dominated open habitats arid lgm punctuated climatic ameliorations which took place in short episodes humans appear to have reached the hp ecosystem after 18 000 cal yrs bp occasionally the first intertropical convergence zone itcz induced changes in south west monsoon swm rains occurred at low latitudes between 16 200 and 15 900 cal yrs bp suggesting an onset of monsoon rains after this event monsoon rains weakened for several millennia except the period 13 700 13 000 cal yrs bp but human activity seems to have continued with biomass burning and clearances by slash and burn very large size grass pollen grains which are morphologically similar to pollen from closer forms of oryza nivara were found after 13 800 cal yrs bp early holocene extreme and abrupt climate changes seem to have promoted the forms of o nivara populations in association with humans new data from the hp would therefore be most interesting to investigate the dispersal and use of domesticated rice in south asia

stable isotope pattern predicted in surge type glaciers the distribution pattern of stable isotope ratio 18o in cold glaciers ice streams and ice sheets has the potential to reveal past changes in flow rate for example those associated with surges in this study we use a time dependent numerical model of ice flow to establish that each surge creates a stratigraphic horizon across which 18o is discontinuous two plausible relations between glacier geometry and 18o in snowfall allow us to bracket the expected magnitude of this isotopic signal these stratigraphic markers could be located by 18o analysis of a longitudinal series of ice cores or by detailed longitudinal sampling of exposed ice calculations for a model with characteristics resembling those of steele glacier yukon territory showed that at most three stratigraphic markers could be detected at any one time the discontinuities in 18o were as large as 0 8 this is an order of magnitude larger than mass spectrometer precision but comparable to observed background noise at steele glacier

using internal layers from the greenland ice sheet identified from radio echo sounding data with numerical models spatially extensive internal layers have been traced in airborne radio echo sounding res data collected over greenland during the late 1990s by linking internal layers within individual flight lines at crossover points it is possible to identify spatiallycontinuouslayersthatareinterpretedas isochronoussurfaces severalof thesur vey lines pass over the grip core site and this allows us to use the published gripage depth relationship to accurately date these surfaces two layers with ages of 3891 and 6956years bp have been traced over a large part of north greenland accurately dated and spatially continuous isochrones are valuable for both assimilation within and verifi cation of numerical models forexample comparison of isochronous surfaces from a nu merical simulation with those layers observed in res data can be used to inform the choice of parameters e g rheology and climate history usedto force a numerical model to demonstrate the potential of the res data two layers for north greenlandwere used to determine palaeo accumulation rates the inversion from layer depthto accumulation rate requires a three dimensional velocity field this velocity field is constructed by com biningatwo dimensionalbalance velocity fieldwithanassumedverticalstructureforthe horizontal velocity the isochronous layer derived accumulation rates were compared with the bales and others 2001 rates a larger east west gradient was found across the central ice divide for the derived accumulation rate suggesting a trend in the holocene accumulation rates for this region the layers were also compared with isochronous sur faces derived from simulations of a three dimensional thermodynamic ice sheet model using the isochronous layer derived accumulation rates to force the model improved the matchbetween modelled and observed layers

bending shear the rate controlling mechanism for calving ice walls bending shear was observed to produce nearly vertical shear bands in a calving ice wall standin on dry land on deception island iat 63 0 os long 60 6 w and slabs calved straight downward when shear rupture occurred along these shear bands hughes 1989 a formula for the calving rate was developed from the deception island data and we have attempted to justify generalizing this formula to include ice walls standing along beaches or in water these are environments in which a wave washed groove develops along the base of the ice wall or along a water line above the base the rate of wave erosion provides an alternative mechanism for controlling the calving rate in these environments we have determined that the rate at which bending creep produces nearly vertical shear bands along which shear rupture occurs controls the calving rate in all environments shear rupture occurs at a calving shear stress of about i bar our results justify using the calving formula to compute the calving rate of ice walls in computer models of ice sheet dynamics this is especially important in simulating retreat of northern hemisphere ice sheets during the last deglaciation when marine and lacustrine environments were common along retreating ice margins these margins would have been ice walls standing along beaches or in water because floating ice shelves are not expected in the ablation zone of retreating ice sheets

global budget of water isotopes inferred from polar ice sheets 1 water isotope ratios in ice cores and marine sediments are a key indicator of past temperature and global ice volume quantitative interpretation of these ratios requires understanding of the storage capacity and exchanges among the ocean atmosphere and cryosphere we combine numerical models of ice dynamics and tracer transport to predict bulk ice properties by simulating the fine layering of ice sheets locally validated at ice core sites the 18o 16o content of ice sheets is found to vary between the present and 20 kyr ago from 34 to 37 for greenland from 41 to 42 5 for west antarctica and always remained near 56 5 for east antarctica their combined effect on sea water 18o 16o is a 0 08 0 12 increase 20 kyr ago a 1 11 decrease if ice sheets were to vanish we confirm that ice volume changes in antarctica and greenland linearly affect ocean composition though at different rates

methodology for use of isotopic climate forcings in ice sheet models in ice sheet models an internally consistent calculation of climatic temperature forcing from ice core isotopic records requires that corrections be made for an ice core site s elevation changes such corrections can be important for two reasons first elevation change at an ice divide can be large compared to elevation change at the migrating ice margin resulting in erroneous estimates of marginal melt rate second the of precipitation at an ice divide responds less to regional climatic temperature changes than to local temperature changes of the same magnitude that are caused by elevation changes significant elevation change corrections may also be necessary for inferring the relationships used to calculate climatic temperature from a simple methodology incorporating both of these elevational effects is presented here further i recommend that the to temperature conversion relations used in ice sheet models should derive from independently reconstructed temperature and time series rather than from spatial correlations and justify this recommendation based on current understanding of the atmospheric isotopic distillation system

history of the greenland ice sheet paleoclimatic insights abstract paleoclimatic records show that the greenland ice sheet consistently has lost mass in response to warming and grown in response to cooling such changes have occurred even at times of slow or zero sea level change so changing sea level cannot have been the cause of at least some of the ice sheet changes in contrast there are no documented major ice sheet changes that occurred independent of temperature changes moreover snowfall has increased when the climate warmed but the ice sheet lost mass nonetheless increased accumulation in the ice sheet s center has not been sufficient to counteract increased melting and flow near the edges most documented forcings and ice sheet responses spanned periods of several thousand years but limited data also show rapid response to rapid forcings in particular regions near the ice margin have responded within decades however major changes of central regions of the ice sheet are thought to require centuries to millennia the paleoclimatic record does not yet strongly constrain how rapidly a major shrinkage or nearly complete loss of the ice sheet could occur the evidence suggests nearly total ice sheet loss may result from warming of more than a few degrees above mean 20th century values but this threshold is poorly defined perhaps as little as 2 c or more than 7 c paleoclimatic records are sufficiently sketchy that the ice sheet may have grown temporarily in response to warming or changes may have been induced by factors other than temperature without having been recorded

vostok ice core a continuous isotope temperature record over the last climatic cycle 160 000 years a continuous deuterium profile along the 160 000 year vostok ice core antarctica is interpreted in terms of atmospheric temperature changes this climatic record is the awaited terrestrial complement of the deep sea records supporting the existence of a relation between the pleistocene climate and orbital forcing

tracer transport in the greenland ice sheet constraints on ice cores and glacial history the climate history and dynamics of the greenland ice sheet are studied using a coupled model of the depositional provenance and transport of glacier ice allowing simultaneous prediction of the detailed isotopic stratigraphy of ice cores at all the major greenland sites adopting a novel method for reconstructing the age depth relationship we greatly improve the accuracy of semi lagrangian tracer tracking schemes and can readily incorporate an age dependent ice rheology the larger aim of our study is to impose new constraints on the glacial history of the greenland ice sheet leading sources of uncertainty in the climate and dynamic history are encapsulated in a small number of parameters the temperature and elevation isotopic sensitivities the glacial interglacial precipitation contrast and the effective viscosity of ice in the flow law comparing predicted and observed ice layering at ice core sites we establish plausible ranges for the key model parameters identify climate and dynamic histories that are mutually consistent and recover the past depositional elevation of ice cores to ease interpretation of their climatic records with the coupled three dimensional model of ice dynamics and provenance transport we propose a method to place all the ice core records on a common time scale and use discrepancies to adjust the reconstructed climate history analysis of simulated grip ice layering and borehole temperature profiles confirms that the grip record is sensitive to the dynamic as well as to the climatic history but not enough to strongly limit speculation on the state of the greenland ice sheet during the eemian in contrast our study indicates that the dye 3 and camp century ice cores are extremely sensitive to ice dynamics and greatly constrain eemian ice sheet reconstructions we suggest that the maximum eemian sea level contribution of the ice sheet was in the range of 3 5 4 5 m

a 3 d model for the antarctic ice sheet a sensitivity study on the glacial interglacial contrast on the longer climatic time scales changes in the elevation and extent of the antarctic ice sheet have an important role in modulating global atmospheric and oceanographic processes and contribute significantly to world wide sea levels in this paper a 3 d time dependent thermomechanical model for the entire ice sheet is presented that is subsequently used to examine the effects of glacial interglacial shifts in environmental boundary conditions on its geometry the model takes into account a coupled ice shelf grounding line dynamics basal sliding and isostatic bed adjustment and considers the fully coupled velocity and temperature fields ice flow is calculated on a fine mesh 40 km horizontal grid size and 10 layers in the vertical for grounded and floating ice and a stress transition zone in between at the grounding line where all stress components contribute in the effective stress in the flow law there is free interaction between ice sheet and ice shelf so that the entire geometry is internally generated a simulation of the present ice sheet reveals that the model is able to yield realistic results a series of sensitivity experiments are then performed in which lower temperatures reduced accumulation rates and lower global sea level stands are imposed either singly or in combination by comparing results of pairs of experiments the effects of each of these environmental changes can be determined in agreement with glacial geological evidence we found that the most pronounced changes show up in the west antarctic ice sheet configuration they appear to be essentially controlled by variations in eustatic sea level whereas typical glacial interglacial changes in temperature and ice deposition rates tend to balance one another these findings support the hypothesis that the antarctic ice sheet basically follows glacial episodes in the northern hemisphere by means of sea level teleconnections grounding occurs more readily in the weddell sea than in the ross sea and long time scales appear to be involved it may take up to 30 40000 years for these continental shelf areas to become completely grounded after an initial stepwise perturbation in boundary conditions according to these reconstructions a steady state antarctic ice sheet may contribute some 16 m to global sea level lowering at maximum glaciation

large arctic temperature change at the wisconsin holocene glacial transition analysis of borehole temperature and greenland ice sheet project ii ice core isotopic composition reveals that the warming from average glacial conditions to the holocene in central greenland was large approximately 15 c this is at least three times the coincident temperature change in the tropics and mid latitudes the coldest periods of the last glacial were probably 21 c colder than at present over the greenland ice sheet

hydrographic variations in deep ocean temperature over the mid pleistocene transition abstract during the mid pleistocene transition the dominant 41 ka periodicity of glacial cycles transitioned to a quasi 100 ka periodicity for reasons not yet known this study investigates the potential role of deep ocean hydrography by examining oxygen isotope ratios in benthic foraminifera oxygen isotope records from the atlantic pacific and indian ocean basins are separated into their ice volume and local temperature hydrography components using a piece wise linear transfer function and a temperature calibration although our method has certain limitations the deep ocean hydrography reconstructions show that glacial deep ocean temperatures approached freezing point as the mid pleistocene transition progressed further analysis suggests that water mass reorganisation could have been responsible for these temperature changes leading to such stable conditions in the deep ocean that some obliquity cycles were skipped until precessional forcing triggered deglaciation creating the apparent quasi 100 ka pattern this study supports previous work that suggests multiples of obliquity cycles dominate the quasi 100 ka glacial cycles with precession components driving deglaciations

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

peregrinations of the greenland ice sheet divide in the last glacial cycle implications for central greenland ice cores abstract the superb quality of the climate chronology archived in the summit greenland ice cores grip gisp2 testifies that the greenland ice sheet divide has been generally stable through the last glacial cycle the ice sheet has experienced a broad range of paleoclimate conditions ice sheet margin configurations and internal dynamical adjustments in glacial interglacial transitions however it is unlikely that the summit region escaped shifts in ice divide position geometry elevation and flow characteristics details of this dynamical history are important to several aspects of ice core studies the magnitudes of pure and simple shearing reconstruction of vertical ice velocity the explicit location of the ice divide and the divide residence time at different locations are all of interest in interpretation of climatic variables and physical properties of ice in the ice cores we apply a three dimensional thermomechanical ice sheet model to examine the evolution of these dynamical variables over the last 160 kyr in central greenland while a high elevation ice dome is present in the summit region throughout the simulation ice divide migrations of up to 150 km are predicted all points in the vicinity of the summit ice cores including the modern divide have been subject to flowline shifts and variable non zero shear deformation during the adjustment from glacial to holocene conditions from ca 10 ka to the present modelled divide peregrinations and strain rate history are consistent with the observed disturbance of deep ice in the grip and gisp2 ice cores which has muddled paleoclimate reconstructions for the last interglacial eemian period in greenland dynamical excursions are also evident north of the modern summit where the ngrip ice core is currently being drilled dahl jensen et al j glaciol 43 1997 300 306 however the prevailing flow direction and deformation regime at the ngrip site are much more stable than those at grip and gisp2 in the simulations combined with the greater depth of ice at this site this lends cautious optimism to the hope that eemian ice at ngrip may contain an intact record of eemian climate

a comparison of eulerian and lagrangian methods for dating in numerical ice sheet models accurate dating in ice sheets is required for a correct interpretation of palaeoclimatic records and for incorporation of material characteristics in the flow law which depend on ice age in this paper we make a comparison between a lagrangian and eulerian approach to the ice advection problem in numerical ice sheet models this comparison is first performed for a schematic two dimensional ice sheet of nye vialov typewithaprescribed stationary velocity field severalcasesareexaminedwhichincorp orate basal melting basal sliding and an undulating bed a further comparison is made with an analytical solution for the ice divide both methods are also applied in a thermo mechanical model of the antarctic ice sheet for steady state present day conditions our main conclusion is that for similar discretization parameters the lagrangian method produces less error than an eulerian approach using a second order upwinding finite difference scheme thoughthe difference is small 51 forthe largest partof the model domain however problems withthe lagrangian approach areintroducedby the disper sion of tracers necessitating the use of interpolationproceduresthat are a main source of additional error it is also shown that a cubic spline approximation of lagrangian trajec tories improves accuracy but such a method is computationally hardly applicable in large scale ice sheet models

sea level and deep water temperature changes derived from benthic foraminifera isotopic records we show that robust regressions can be established between relative sea level rsl data and benthic foraminifera oxygen isotopic ratios from the north atlantic and equatorial pacific ocean over the last climatic cycle we then apply these regressions to long benthic isotopic records retrieved at one north atlantic and one equatorial pacific site to build a composite rsl curve as well as the associated confidence interval over the last four climatic cycles our proposed reconstruction of rsl is in good agreement with the sparse rsl data available prior to the last climatic cycle we compute bottom water temperature changes at the two sites and at one southern indian ocean site taking into account potential variations in north atlantic local deep water 18o our results indicate that a last glacial maximum lgm enrichment of the ocean mean oxygen isotopic ratio of 0 95 is the lowest value compatible with unfrozen deep waters in the southern indian ocean if local deep water 18o did not increase during glacials with respect to present such a value of the lgm mean ocean isotopic enrichment would impose a maximum decrease in local bottom water 18o at the north atlantic site of 0 30 during glacials

temperature accumulation and ice sheet elevation in central greenland through the last deglacial transition we present a combined heat and ice flow model constrained by measurements of temperature in the greenland ice sheet project 2 gisp2 borehole and by the gisp2 18o record and depth age scale which determines a history of temperature accumulation rate and ice sheet elevation for the past 50 000 years in central greenland important results are that the temperature increase from average glacial to holocene conditions was large approximately 15 c with a 20 c warming from late glacial to holocene that the average accumulation rate during the last glacial maximum between 15 and 30 kyr b p was 5 5 to 7 cm yr 1 approximately 25 of the modern accumulation rate that long term 500 1000 years averaged accumulation rate and temperature have been inversely correlated during the most recent 7 millennia of the holocene and that the greenland ice sheet probably thickened during the deglacial transition the inverse correlation of accumulation rate and temperature in the mid and late holocene suggests that the greenland ice sheet is more prone to volume reduction in a warmed climate than previously thought and demonstrates that accumulation rate is not a reliable proxy for temperature the elevation history of the ice sheet is poorly constrained by the model and independent evidence is needed we also present a simple estimate of the response time for thinning of the interior region of an ice sheet due to retreat of its margins this was approximately 1900 years for central greenland during deglaciation

paleoclimatic tracers an investigation using an atmospheric general circulation model under ice age conditions 2 water isotopes the linear relationship observed between the water isotopic contents of precipitation and surface air temperatures leads to the use of the water isotopes h218o and hdo in paleoclimatology applied to the measurements of the isotopic content of paleowaters like groundwaters and deep ice cores this relationship is used to infer paleotemperatures however this interpretation of paleo isotopic contents is only valid if the isotope temperature relationship is not affected by climate change to address this problem we have developed a water isotope modeling inside an atmospheric general circulation model agcm and performed simulations of both the present day and last glacial maximum lgm climatic conditions agcms are indeed the only appropriate tools able to account the whole complexity of the atmospheric circulation for the present day climate preliminary results for january were presented by joussaume et al 1984 and are complemented by new simulations performed for both february and august climatic conditions with a higher resolution version of the model model results are well corroborated by observations they also exhibit some effects of the atmospheric circulation on the isotopic fields for the simulated lgm climate the model results compare well with paleoclimatic data of water isotopic contents except for a higher than observed spatial variability the overall patterns of the simulated 18o temperature relationship for the lgm climate are practically unchanged which tends to comfort the use of water isotopes in paleoclimatology however concerning the deuterium excess i e the relationship between oxygen 18 and deuterium the model results are not sufficiently valid to allow a discussion of the use of deuterium excess in paleoclimatology

flow law for ice in polar ice sheets theories of glacier flow are based commonly on the assumption that ice is not a newtonian fluid but has a non linear stress dependent viscosity1 5 here we re examine the spreading of antarctic ice shelves and suggest that the data cannot define a unique flow law tilt measurements in four boreholes in both the arctic and antarctic seem to show that a linear flow law e a1 where a1 10 15 s 1 pa 1 at 28 c where e is the effective strain rate and the effective shear stress may be just as appropriate for describing the flow of polar ice sheets

velocity anisotropy in granodiorite and quartzite due to the distribution of microcracks summary the spatial anisotropy of p wave velocities was investigated in spherical samples of granodiorite and quartzite and correlated with the orientation of microcracks and grain boundaries determined optically by universal stage techniques the preferred orientation of normals to cleavage cracks of biotite and amphibole in granodiorite correlates well with the direction of maximum velocity increase under hydrostatic pressure although the minerals form only nine per cent of the volume irregular microcracks in an almost monomineral quartzite have only a very small effect on the velocity anisotropy although they are also preferentially oriented the nature of the microdiscontinuities is of primary importance for the velocity anisotropy observed at atmospheric and low hydrostatic pressures