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

the thermomechanical response of the greenland ice sheet to various climate scenarios using a three dimensional numerical model for land based ice sheets in the shallow ice approximation simulations are performed to determine the velocity and temperature distributions within the greenland ice sheet through time for various climate scenarios the ice is treated as a rheologically nonlinear heat conducting viscous fluid and the substrate is a heat conducting rigid solid this system is fed from above by prescribing as the climatic inputs the atmospheric temperature and the accumulation ablation rate functions at the free surface and from below by the geothermal heat we present the governing equations in the shallow ice approximation discuss the parameterizations used in the descriptions of the ice surface temperature and accumulation ablation functions briefly state how the complicated initial boundary value problem is numerically solved and how the input data that are available from measurements are implemented results of preliminary calculations disclose how the model performs and delimit its validity we study the role played by basal sliding and make clear that sliding should be accounted for where ever the basal ice is temperate and that the frictional heat generated in this sliding is thermomechanically significant we also study the reaction of the greenland ice sheet to various climate scenarios and make clear that today s thermal regime depends significantly upon the prior climate history moreover the thermomechanical properties of the ice are equally significant as is the thermal interaction of the ice sheet with the rockbed beneath

palaeoecology of ostracods from the late cretaceous from northeastern brazil and its relation to sequence stratigraphy abstract ostracods from the turonian campanian interval of jandaira formation potiguar basin ne brazil were analysed in terms of diversity palaeocology and their potential application in sequence stratigraphy ternary plots of cytheroidea cytherelloidea bairdioidea and cypridoidea ccbc plots were assembled to determine the palaeobathymety purposes our results demonstrate the existence of four major palaeoenvironments as follows mixohaline marine 1 marine 1 marine 2 and mixohaline marine 2 which differ in the taxonomic composition abundance and ecological preferences of their assemblages in horizons where the fauna had consistent recovery it was possible to infer system tracts and stratigraphic surfaces the inferred sequence boundaries are indicated by important faunal changes expressed mainly by reduction in diversity differences in the assemblages compositions and their ecological preferences the maximum flooding surfaces mfs were marked by high diversity of marine stenohaline taxa the results of this study reinforce the usefulness of ostracod assemblages in sequence stratigraphy interpretation in the marine and marginal marine successions

greenland subglacial lakes detected by radar 1 subglacial lakes are an established and important component of the basal hydrological system of the antarctic ice sheets but none have been reported from greenland here we present airborne radio echo sounder res measurements that provide the first clear evidence for the existence of subglacial lakes in greenland two lakes with areas 8 and 10 km2 are found in the northwest sector of the ice sheet 40 km from the ice margin and below 757 and 809 m of ice respectively the setting of the greenland lakes differs from those of antarctic subglacial lakes being beneath relatively thin and cold ice pointing to a fundamental difference in their nature and genesis possibilities that the lakes consist of either ancient saline water in a closed system or are part of a fresh modern open hydrological system are discussed with the latter interpretation considered more likely

basal conditions and ice dynamics inferred from radar derived internal stratigraphy of the northeast greenland ice stream we analyze the internal stratigraphy in radio echo sounding data of the northeast greenland ice stream to infer past and present ice dynamics in the upper reaches of the ice stream we propose that shear margin steady state folds in internal reflecting horizons irhs form due to the influence of ice flow over spatially varying basal lubrication irhs are generally lower in the ice stream than outside likely because of greater basal melting in the ice stream from enhanced geothermal flux and heat of sliding strain rate modeling of irhs deposited during the holocene indicates no recent major changes in ice stream vigor or extent in this region downstream of our survey irhs are disrupted as the ice flows into a prominent overdeepening when combined with additional data from other studies these data suggest that upstream portions of the ice stream are controlled by variations in basal lubrication whereas downstream portions are confined by basal topography

distinct patterns of seasonal greenland glacier velocity predicting greenland ice sheet mass loss due to ice dynamics requires a complete understanding of spatiotemporal velocity fluctuations and related control mechanisms we present a 5 year record of seasonal velocity measurements for 55 marine terminating glaciers distributed around the ice sheet margin along with ice front position and runoff data sets for each glacier among glaciers with substantial speed variations we find three distinct seasonal velocity patterns one pattern indicates relatively high glacier sensitivity to ice front position the other two patterns are more prevalent and appear to be meltwater controlled these patterns reveal differences in which some subglacial systems likely transition seasonally from inefficient distributed hydrologic networks to efficient channelized drainage while others do not the difference may be determined by meltwater availability which in some regions may be influenced by perennial firn aquifers our results highlight the need to understand subglacial meltwater availability on an ice sheet wide scale to predict future dynamic changes

seasonal and universal time distribution of patches in the northern and southern polar caps ion density measurements from the dynamics explorer 2 and defense meteorological satellite program f8 and f9 satellites are used to examine hemispherical differences in the occurrence patterns of polar ionization patches as a function of season universal time ut and interplanetary magnetic field imf when bz 0 the greatest frequency of patch occurrence in the northern hemisphere is in the winter in the 1000 2200 ut range this time corresponds to the interval when the northern magnetic pole and hence the cusp lies the farthermost toward the dayside this fact is often used to explain the creation of patches in terms of the entrainment of dayside plasma into the cusp by high latitude convection in the southern hemisphere we see that the occurrence frequencies peak at over twice the northern hemisphere values in the same general ut region 1000 2300 ut however the southern hemisphere cusp is most dayward at approximately 0300 ut a time of minimum patch formation examination of the relationship of the terminator to the polar cap boundaries in each hemisphere leads to a simple explanation in terms of the differing offset of the magnetic poles and the ionospheric convection pattern from the geographic poles

polythermal three dimensional modelling of the greenland ice sheet with varied geothermal heat flux computations over 50 000 years into steady state with greve s polythermal ice sheet model and its numerical code are performed for the greenland ice sheet with today s climatological input surface temperature and accumulation function and three values of the geothermal heat flux 42 54 6 29 4 mw m 2 it is shown that through the thermomechanical coupling the geometry as well as the thermal regime in particular that close to the bed respond surprisingly strongly to the basal thermal heat input the most sensitive variable is the basal temperature field but the maximum height of the summit also varies by more than 100m furthermore some intercomparison of the model outputs with the real ice sheet is carried out showing that the model provides reasonable results for the ice sheet geometry as well as for the englacial temperatures

seasonal acceleration of inland ice via longitudinal coupling to marginal ice we use an ice flow model to demonstrate how flow variations initiated in the marginal zone of an ice sheet affect flow farther inland through longitudinal along flow coupling our findings allow for an alternate interpretation of seasonal accelerations observed near the equilibrium line of the greenland ice sheet zwally and others 2002 we demonstrate that these observations can be explained by accelerations initiated up to 12 km closer to the margin where the ice is 40 thinner is heavily crevassed experiences a seasonal doubling of velocity and where the ablation rate surface meltwater flux and ice temperature are likely higher our modeling and observations suggest that conditions and processes normally found near ice sheet margins are adequate for explaining the observations of zwally and others 2002 this and considerations of the likely subglacial hydrology in the marginal zone lead us to suggest that seasonal accelerations may have limited impact on ice sheet mass balance even in the face of climate warming

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

mechanisms of fast flow in jakobshavn isbrae west greenland part iii measurements of ice deformation temperature and cross borehole conductivity in boreholes to the bedrock at a site on the ice sheet adjacent to the jakobshavn ice stream in west greenland ice deformation rates and temperatures have been measured in boreholes to the bedrock at 830 m depth enhanced deformation rates were recorded just below the holocene wisconsin transition at 680 m depth a 31 m layer of temperate ice and the temperature minimum of 22 c at 520 m depth were detected the good agreement of these data with results of a two dimensional thermomechanically coupled flow model implies that the model input is adequate discrepancies between modelled and measured temperature profiles on a flowline at the ice stream centre have been attributed to effects not accounted for by the model we have suggested that the convergent three dimensional flow leads to a vertical extension of the basal ice entering the stream a thick basal layer of temperate and wisconsin ice would explain the fast flow of this ice stream as a test of this hypothesis the new core borehole conductivity cbc method has been used to compare conductivity sequences from the ice stream to those of the adjacent ice sheet the correlation thus inferred suggests that the lowest 270 m of the ice sheet correspond to the lowermost 1700 m of the stream and consequently that the lower part of the ice stream has experienced a very large vertical extension

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

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

deeply incised submarine glacial valleys beneath the greenland ice sheet the bed topography beneath the greenland ice sheet controls the flow of ice and its discharge into the ocean a combination of sparse radar soundings of ice thickness and high resolution ice motion data suggest that many submarine ice covered valleys extend significantly deeper below sea level and farther inland than thought

recovery of subglacial water extent from greenland radar survey data subglacial water threatens the stability of a deep ice sheet the greenland ice sheet has been the subject of extensive radar surveys under nasa s parca program and others yet the extent of basal water has not previously been explored in detail this paper provides a method for recovering basal radar reflection characteristics that yield robust discrimination of subglacial water using parca data by narrowing the spread of measured echo intensities sources of validation are provided and they support the procedure and its results the method can be extended to the entire greenland radar dataset it is not dependent on details of radar characteristics but relies on sufficient penetration performance high fidelity data acquisition and appropriate treatment of signals and interface statistics

evaluation of cryo hydrologic warming as an explanation for increased ice velocities in the wet snow zone sermeq avannarleq west greenland 1 wintertime satellite derived ice surface velocities from 2001 through 2007 suggest an increase in ice velocity in the wet snow zone of southwest greenland we present a thermomechanical model to evaluate the influence of surface meltwater runoff on englacial temperatures via cryo hydrologic warming chw as a possible mechanism to explain this velocity increase at sermeq avannarleq the model incorporates chw through a previously published dual column parameterization we compare model simulations with i chw active over the entire ice thickness base case chw ii chw active only in the surface 80 m of the ice sheet surface chw and iii no chw to represent a traditional thermomechanical model the horizontal extent of chw is prescribed based on equilibrium line altitude position and thus incorporates the upstream expansion of the ablation zone over the past decade the base case chw simulations reproduce the observed increase in inland ice velocity between 2001 and 2007 reasonably well the no chw and surface chw simulations significantly underestimate observed ice surface velocities in both epochs the higher ice velocities in the base case chw simulations are attributable to both decreased basal ice viscosities associated with increased basal ice temperatures and an increase in the extent of basal sliding permitted by temperate bed conditions only the temperate bed extent predicted by the base case chw simulation is consistent with independent observations of basal sliding based on our sensitivity analysis of chw we evaluate alternative explanations for an increase in inland ice velocity and suggest chw is the most plausible mechanism

dilatant till facilitates ice stream flow in northeast greenland abstract we present radio echo sounding res global positioning system gps and active source seismic data across the central portion of the northeast greenland ice stream negis negis widens downglacier from a small region of high geothermal flux near the ice divide our data reveal high porosity 40 water saturated till lubricating the ice stream ice accelerates and thins as it flows into negis producing marginal troughs in surface topography these troughs create steep gradients in the subglacial hydropotential that generate parallel sticky and slippery bands beneath the shear margins the low porosity sticky sediment bands limit ice entrainment across the margins and thus restrict further widening producing the long narrow and relatively stable ice stream however the observed relations among surface elevation basal water routing broad sedimentary drape and till dilatancy suggest that rapid shifts in ice dynamics are possible including rapid transmission of ocean forcing inland the source and routing of the subglacial till are unclear but our data help constrain hypotheses

greenland ice sheet surface properties and ice dynamics from ers 1 sar imagery c band synthetic aperture radar sar imagery from the european space agency s ers 1 satellite reveals the basic zonation of the surface of the greenland ice sheet the zones have backscatter signatures related to the structure of the snowpack which varies with the balance of accumulation and melt at various elevations the boundaries of zones can be accurately located with the use of this high resolution imagery the images also reveal a large flow feature in northeast greenland that is similar to ice streams in antarctica and may play a major role in the discharge of ice from the ice sheet

seasonal and diurnal variations in ampere observations of the birkeland currents compared to modeled results we reduce measurements made by the active magnetosphere and planetary electrodynamics response experiment ampere to give the total birkeland field aligned current flowing in both hemispheres in monthly and hourly bins we analyze these totals using 6 years of data 2010 2015 to examine solar zenith angle driven variations in the total birkeland current flowing in both hemispheres simultaneously for the first time a diurnal variation is identified in the total birkeland current flowing consistent with variations in the solar zenith angle a seasonal variation is also identified with more current flowing in the northern southern hemisphere during bartels rotations in northern southern summer for months close to equinox more current is found to flow in the northern hemisphere contrary to our expectations we also conduct the first test of the milan 2013 model for estimating birkeland current magnitudes with modifications made to account for solar contributions to ionospheric conductance based on the observed variation of the birkeland currents with season and time of day the modified model using the value of d averaged by bartels rotation scaled by 1 7 is found to agree with the observed ampere currents with a correlation of 0 87 in the northern hemisphere and 0 86 in the southern hemisphere the improvement over the correlation with dayside reconnection rate is demonstrated to be a significant improvement to the model the correlation of the residuals is found to be consistent with more current flowing in the northern hemisphere this new observation of systematically larger current flowing in the northern hemisphere is discussed in the context of previous results which suggest that the northern hemisphere may react more strongly to dayside reconnection than the southern hemisphere

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

greenland ice sheet volume sensitivity to basal surface and initial conditions derived from an adjoint model we extend the application of control methods to a comprehensive three dimensional thermomechanical ice sheet model sicopolis simulation code for polythermal ice sheets lagrange multipliers i e sensitivities are computed with an exact efficient adjoint model that has been generated from sicopolis by rigorous application of automatic differentiation the case study uses the adjoint model to determine the sensitivity of the total greenland ice volume to various control variables over a 100 year period the control space has of the order 1 2 10 6 elements consisting of spatial fields of basal flow parameters surface and basal forcings and initial conditions reliability of the adjoint model was tested through finite difference perturbation calculations for various control variables and perturbation regions ascertaining quantitative inferences of the adjoint model as well as confirming qualitative aspects of ice sheet sensitivities e g expected regional variations we detect regions where model sensitivities are seemingly unexpected or counter intuitive albeit real in the sense of actual model behavior an example is inferred regions where sensitivities of ice sheet volume to basal sliding coefficient are positive i e where a local increase in basal sliding parameter increases the ice sheet volume similarly positive generally negative ice temperature sensitivities in certain parts of the ice sheet are found the detection of which seems highly unlikely if only conventional perturbation experiments had been used the object of this paper is largely a proof of concept available adjoint code generation tools now open up a variety of novel model applications notably with regard to sensitivity and uncertainty analyses and ice sheet state estimation or data assimilation

relative importance of horizontal and vertical transports to the formation of ionospheric storm enhanced density and polar tongue of ionization there are still uncertainties regarding the formation mechanisms for storm enhanced density sed in the high and subauroral latitude ionosphere in this work we deploy the thermosphere ionosphere electrodynamic general circulation model tiegcm and gps total electron content tec observations to identify the principle mechanisms for sed and the tongue of ionization toi through term by term analysis of the ion continuity equation and also identify the advantages and deficiencies of the tiegcm in capturing high latitude and subauroral latitude ionospheric fine structures for the two geomagnetic storm events occurring on 17 march 2013 and 2015 our results show that in the topside ionosphere upward e b ion drifts are most important in sed formation and are offset by antisunward neutral winds and downward ambipolar diffusion effects in the bottomside f region ionosphere neutral winds play a major role in generating seds sed signature in tec is mainly caused by upward e b ion drifts that lift the ionosphere to higher altitudes where chemical recombination is slower horizontal e b ion drifts play an essential role in transporting plasma from the dayside convection throat region to the polar cap to form tois inconsistencies between model results and gps tec data were found 1 gps relative tec difference between storm time and quiet time has holes in the dayside ion convection entrance region which do not appear in the model results 2 the model tends to overestimate electron density enhancements in the polar region possible causes for these inconsistencies are discussed in this article

sensitivity of the frozen melted basal boundary to perturbations of basal traction and geothermal heat flux isunnguata sermia western greenland a full stress thermomechanically coupled numerical model is used to explore the interaction between basal thermal conditions and motion of a terrestrially terminating section of the west greenland ice sheet the model domain is a two dimensional flowline profile extending from the ice divide to the margin we use data assimilation techniques based on the adjoint model in order to optimize the basal traction field minimizing the difference between modeled and observed surface velocities we monitor the sensitivity of the frozen melted boundary fmb to changes in prescribed geothermal heat flux and sliding speed by applying perturbations to each of these parameters the fmb shows sensitivity to the prescribed geothermal heat flux below an upper threshold where a maximum portion of the bed is already melted the position of the fmb is insensitive to perturbations applied to the basal traction field this insensitivity is due to the short distances over which longitudinal stresses act in an ice sheet

draping or overriding the effect of horizontal stress gradients on internal layer architecture in ice sheets internal isochronic layers in ice sheets sensed by radar show two characteristic relationships to the basal topography either they override it with layers above the crests of rises lying essentially flat or they drape over it with the layers following rises and falls in basal topography a mechanical theory is presented which shows that overriding is the expected behavior when topographic wavelengths are comparable with or less than the ice thickness while draping occurs at longer wavelengths this is shown with analytical perturbation solutions for newtonian fluids numerical perturbation solutions for nonlinear fluids and finite element solutions for nonlinear fluids and large amplitude variations bed variation from topography and changes in the basal boundary condition are considered for fixed bed and sliding beds as well as three dimensional flows and thermomechanically coupled flows in all cases the dominant effect on draping overriding is the wavelength of the topography or variation in basal boundary conditions results of these full mechanical system calculations are compared with those from the shallow ice approximation and the longitudinal stress approximation some calculations are carried out for zero accumulation where the age of the ice and therefore isochrone geometry is not defined it is shown that there is a close relationship between isochrones and streamlines and that they behave similarly when bed wavelength divided by the ice thickness is small compared with the ratio of ice velocity and accumulation rate which is a useful approximation numerical comparisons of isochrones and streamlines show them to be virtually coincident

ecophenotypic variation of the devonian benthic ostracod species cavellina rhenana krommelbein 1954 a paleoenvironmental proxy for the ardenne france belgium and rheno hercynian realm abstract the morphological variation of the carapace of the ostracod species cavellina rhenana krommelbein 1954 is explored here quantitatively this species existed during the late givetian to the earliest frasnian and it is characteristic of the rheno hercynian area five morphological characters and several linear measurements of the carapace were analyzed on 260 complete carapaces coming from three different areas each of them representing a different paleoenvironment of the devonian ardennean platform the original species description and diagnosis are emended based on this newly observed variability in ornamentation sulcation curvature of the ventral margin marginal flattening and asymmetry of valves it is established herein that the morphological variation expressed in these five characters is independent of the ontogenetic growth morphological variation appears to be correlated with different types of facies reflecting a distinct depositional environment a high degree of morphological variation is found in facies of inner platform semi restricted environments while a low degree of variation is found in facies of outer platform open marine environments thus the degree of morphological variation of c rhenana is an indicator of the stability of the environment in the neritic realm while environmentally cued polymorphism is well documented in mesozoic to recent ostracods our study demonstrates the first devonian case of ecomorphism and suggests that intraspecific polymorphism of the ostracod carapace can be a useful tool for refining paleoenvironmental reconstructions

committed sea level rise for the next century from greenland ice sheet dynamics during the past decade we use a three dimensional higher order ice flow model and a realistic initial condition to simulate dynamic perturbations to the greenland ice sheet during the last decade and to assess their contribution to sea level by 2100 starting from our initial condition we apply a time series of observationally constrained dynamic perturbations at the marine termini of greenland s three largest outlet glaciers jakobshavn isbrae helheim glacier and kangerdlugssuaq glacier the initial and long term diffusive thinning within each glacier catchment is then integrated spatially and temporally to calculate a minimum sea level contribution of approximately 1 0 4 mm from these three glaciers by 2100 based on scaling arguments we extend our modeling to all of greenland and estimate a minimum dynamic sea level contribution of approximately 6 2 mm by 2100 this estimate of committed sea level rise is a minimum because it ignores mass loss due to future changes in ice sheet dynamics or surface mass balance importantly 75 of this value is from the long term diffusive response of the ice sheet suggesting that the majority of sea level rise from greenland dynamics during the past decade is yet to come assuming similar and recurring forcing in future decades and a self similar ice dynamical response we estimate an upper bound of 45 mm of sea level rise from greenland dynamics by 2100 these estimates are constrained by recent observations of dynamic mass loss in greenland and by realistic model behavior that accounts for both the long term cumulative mass loss and its decay following episodic boundary forcing

interhemispheric study of polar cap patch occurrence based on swarm in situ data the swarm satellites offer an unprecedented opportunity for improving our knowledge about polar cap patches which are regarded as the main space weather issue in the polar caps we present a new robust algorithm that automatically detects polar cap patches using in situ plasma density data from swarm for both hemispheres we compute the spatial and seasonal distributions of the patches identified separately by swarm a and swarm b between december 2013 and august 2016 we show a clear seasonal dependency of patch occurrence in the northern hemisphere nh patches are essentially a winter phenomenon as their occurrence rate is enhanced during local winter and very low during local summer although not as pronounced as in the nh the same pattern is seen for the southern hemisphere sh furthermore the rate of polar cap patch detection is generally higher in the sh than in the nh especially on the dayside at about 77 magnetic latitude additionally we show that in the nh the number of patches is higher in the postnoon and prenoon sectors for interplanetary magnetic field imf by 0 respectively and that this trend is mirrored in the sh consistent with the ionospheric flow convection overall our results confirm previous studies in the nh shed more light regarding the sh and provide further insight into polar cap patch climatology along with this algorithm we provide a large data set of patches automatically detected with in situ measurements which opens new horizons in studies of polar cap phenomena

basal temperature conditions of the greenland ice sheet during the glacial cycles a high resolution three dimensional thermomechanical ice sheet model which includes isostasy the possibility of ice sheet expansion on the continentalshelf and refined climatic parameterisations was used to investigate the basal thermal regime of the greenland ice sheet the thermodynamic calculations takeinto account the usual terms of heat flow within the ice a thermal active bedrock layer in transient situations and all of the effects associated with changes inice thickness and flow pattern basal temperature conditions are documented with respect to glacial interglacial shifts in climatic boundary conditions both insteady state as during simulations over the last two glacial cycles using the grip d180 record it is found that the basal temperature field shows a largesensitivity in steady state experiments but that during a glacial cycle basal temperature variations are strongly damped in particular in central areas acomparison has been made with measured data from deep ice cores and implications are discussed

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

radar attenuation and temperature within the greenland ice sheet 2015 american geophysical union all rights reserved the flow of ice is temperature dependent but direct measurements of englacial temperature are sparse the dielectric attenuation of radio waves through ice is also temperature dependent and radar sounding of ice sheets is sensitive to this attenuation here we estimate depth averaged radar attenuation rates within the greenland ice sheet from airborne radar sounding data and its associated radiostratigraphy using existing empirical relationships between temperature chemistry and radar attenuation we then infer the depth averaged englacial temperature the dated radiostratigraphy permits a correction for the confounding effect of spatially varying ice chemistry where radar transects intersect boreholes radar inferred temperature is consistently higher than that measured directly we attribute this discrepancy to the poorly recognized frequency dependence of the radar attenuation rate and correct for this effect empirically resulting in a robust relationship between radar inferred and borehole measured depth averaged temperature radar inferred englacial temperature is often lower than modern surface temperature and that of a steady state ice sheet model particularly in southern greenland this pattern suggests that past changes in surface boundary conditions temperature and accumulation rate affect the ice sheet s present temperature structure over a much larger area than previously recognized this radar inferred temperature structure provides a new constraint for thermomechanical models of the greenland ice sheet

antarctic subglacial conditions inferred from a hybrid ice sheet ice stream model abstract subglacial conditions of large polar ice sheets remain poorly understood despite recent advances in satellite observation major uncertainties related to basal conditions such as the temperature field are due to an insufficient knowledge of geothermal heat flow here a hybrid method is presented that combines numerical modeling of the ice sheet thermodynamics with a priori information using a simple assimilation technique additional data are essentially vertical temperature profiles measured in the ice sheet at selected spots as well as the distribution of subglacial lakes in this way geothermal heat flow datasets are improved to yield calculated temperatures in accord with observations in areas where information is available results of the sensitivity experiments show that 55 of the grounded part of the antarctic ice sheet is at pressure melting point calculated basal melt rates are approximately 65 gt year 1 which is 3 of the total surface accumulation although these sensitivity experiments exhibit small variations in basal melt rates the impact on the ice age of basal layers is quite important in areas that are characterized by relatively low melt rates this may lead to standard deviations up to 50 in a model run over a period of 10 6 years subglacial water flow is concentrated underneath the large outlet glaciers of antarctica the larger subglacial lakes are perched at the head of these subglacial water systems an exception however is the lake system of the recovery ice stream catchment that receives a substantial amount of subglacial melt water from the upstream catchment

caterpillar like ice motion in the ablation zone of the greenland ice sheet current understanding of ice dynamics predicts that increasing availability and variability of meltwater will have an impact on basal motion and therefore on the evolution and future behavior of the greenland ice sheet we present measurements of ice deformation subglacial water pressure and surface velocity that show periodic and episodic variations on several time scales seasonal multiday and diurnal these variations observed with gps and sensors at different depths throughout the ice column are not synchronous but show delayed responses of ice deformation with increasing depth and basal water pressure in antiphase with surface velocity with the help of a full stokes ice flow model these observations are explained as ice motion in a caterpillar like fashion caused by patches of different basal slipperiness horizontal stress transfer through the stiff central part of the ice body leads to spatially varying surface velocities and ice deformation patterns variation of this basal slipperiness induces characteristic patterns of ice deformation variability that explain the observed behavior ice flow in the ablation zone of the greenland ice sheet is therefore controlled by activation of basal patches by varying slipperiness in the course of a melt season leading to caterpillar like ice motion superposed on the classical shear deformation

an enthalpy formulation for glaciers and ice sheets polythermal conditions are ubiquitous among glaciers from small valley glaciers to ice sheets conventional temperature based cold ice models of such ice masses cannot account for that portion of the internal energy which is latent heat of liquid water within temperate ice so such schemes are not energy conserving when temperate ice is present temperature and liquid water fraction are however functions of a single enthalpy variable a small enthalpy change in cold ice is a change in temperature while a small enthalpy change in temperate ice is a change in liquid water fraction the unified enthalpy formulation described here models the mass and energy balance for the three dimensional ice fluid for the surface runoff layer and for the subglacial hydrology layer together in a single energy conserving theoretical framework it is implemented in the parallel ice sheet model results for the greenland ice sheet are compared with those from a cold ice scheme this paper is intended to be an accessible foundation for enthalpy formulations in glaciology

dependence of century scale projections of the greenland ice sheet on its thermal regime observations show that the greenland ice sheet has been losing mass at an increasing rate over the past few decades which makes it a major contributor to sea level rise here we use a threedimensional higher order ice flow model adaptive mesh refinement and inverse methods to accurately reproduce the present day ice flow of the greenland ice sheet we investigate the effect of the ice thermal regime on 1 basal sliding inversion and 2 projections over the next 100 years we show that steady state temperatures based on present day conditions allow a reasonable representation of the thermal regime and that both basal conditions and century scale projections are weakly sensitive to small changes in the initial temperature field compared with changes in atmospheric conditions or basal sliding we conclude that although more englacial temperature measurements should be acquired to validate the models and a better estimation of geothermal heat flux is needed it is reasonable to use steady state temperature profiles for short term projections as external forcings remain the main drivers of the changes occurring in greenland 2014 publishing technology

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

patches in the polar ionosphere ut and seasonal dependence the seasonal and ut dependencies of patches in the polar ionosphere are simulated using the utah state university time dependent ionospheric model tdim patch formation is achieved by changing the plasma convection pattern in response to temporal changes in the interplanetary magnetic field imf b sub y component during periods of southward imf this mechanism redirects the plasma flow from the dayside high density region which is the source of the tongue of ionization toi density feature through the throat and leads to patches rather than a continuous toi the model predicts that the patches are absent at winter solstice northern hemisphere between 0800 and 1200 ut and that they have their largest seasonal intensity at winter solstice between 2000 and 2400 ut between winter solstice and equinox patches are strong and present all day patches are present in summer as well although their intensity is only tens of percent above the background density these winter to equinox findings are also shown to be consistent with observations the model was also used to predict times at which patch observations could be performed to determine the contributions from other patch mechanisms this observational window is 20 days about winter solstice between 0800 and 1200more ut in the northern hemisphere in this observational window the toi is either absent or reduced to a very low density hence the time dependent electric field mechanism considered in this study does not produce patches and if they are observed then they must be due to some other mechanism 32 refs 10 figs less

recharge of a subglacial lake by surface meltwater in northeast greenland observations of rapid persistent elevation gains that occur on the ice surface above a subglacial lake as the lake is refilled with surface meltwater during the summer melt period in greenland show that surface meltwater may be trapped and stored at the bed of an ice sheet affecting ice dynamics downstream the discovery of a 70 metre deep collapse basin near the summit of the largest and most northerly peripheral ice cap of greenland and the use of high resolution stereo satellite imagery to monitor its evolution suggests that surface meltwater can be trapped and stored beneath the greenland ice sheet previously meltwater from the surface of the ice sheet had been assumed to drain efficiently through the basal hydrologic system during the summer melt period rapid persistent elevation gains occur on the ice surface above a subglacial lake as the lake is refilled with surface meltwater and stored at least on annual time scales these findings have potential implications for our understanding of ice sheet temperature and velocity in a warming climate surface meltwater production on large ice sheets is expected to increase if this water is delivered to the ice sheet base it may have important consequences for ice dynamics for example basal water distributed in a diffuse network can decrease basal friction1 2 and accelerate ice flow3 4 5 6 7 8 whereas channelized basal water can move quickly to the ice margin where it can alter fjord circulation and submarine melt rates9 10 less certain is whether surface meltwater can be trapped and stored in subglacial lakes beneath large ice sheets here we show that a subglacial lake in greenland drained quickly as seen in the collapse of the ice surface and then refilled from surface meltwater input we use digital elevation models from stereo satellite imagery and airborne measurements to resolve elevation changes during the evolution of the surface and basal hydrologic systems at the flade isblink ice cap in northeast greenland during the autumn of 2011 a collapse basin about 70 metres deep and about 0 4 cubic kilometres in volume formed near the southern summit of the ice cap as a subglacial lake drained into a nearby fjord over the next two years rapid uplift of the floor of the basin which is approximately 8 4 square kilometres in area occurred as surface meltwater flowed into crevasses around the basin margin and refilled the subglacial lake our observations show that surface meltwater can be trapped and stored at the bed of an ice sheet sensible and latent heat released by this trapped meltwater could soften nearby colder basal ice11 and alter downstream ice dynamics12 13 heat transport associated with meltwater trapped in subglacial lakes should be considered when predicting how ice sheet behaviour will change in a warming climate

decadal slowdown of a land terminating sector of the greenland ice sheet despite warming ice flow along land terminating margins of the greenland ice sheet gis varies considerably in response to fluctuating inputs of surface meltwater to the bed of the ice sheet such inputs lubricate the ice bed interface transiently speeding up the flow of ice greater melting results in faster ice motion during summer but slower motion over the subsequent winter owing to the evolution of an efficient drainage system that enables water to drain from regions of the ice sheet bed that have a high basal water pressure however the impact of hydrodynamic coupling on ice motion over decadal timescales remains poorly constrained here we show that annual ice motion across an 8 000 km 2 land terminating region of the west gis margin extending to 1 100 m above sea level was 12 slower in 2007 14 compared with 1985 94 despite a 50 increase in surface meltwater production our findings suggest that over these three decades hydrodynamic coupling in this section of the ablation zone resulted in a net slowdown of ice motion not a speed up as previously postulated increases in meltwater production from projected climate warming may therefore further reduce the motion of land terminating margins of the gis our findings suggest that these sectors of the ice sheet are more resilient to the dynamic impacts of enhanced meltwater production than previously thought

on the nature of the dirty ice at the bottom of the gisp2 ice core abstract we present data on the triple ar isotope composition in trapped gas from clean stratigraphically disturbed ice between 2800 and 3040 m depth in the gisp2 ice core and from basal dirty ice from 3040 to 3053 m depth we also present data for the abundance and isotopic composition of o2 and n2 and abundance of ar in the basal dirty ice the ar n2 ratio of dirty basal ice the heavy isotope enrichment reflecting gravitational fractionation and the total gas content all indicate that the gases in basal dirty ice originate from the assimilation of clean ice of the overlying glacier which comprises most of the ice in the dirty bottom layer o2 is partly to completely depleted in basal ice reflecting active metabolism the gravitationally corrected ratio of 40ar 38ar which decreases with age in the global atmosphere is compatible with an age of 100 250 ka for clean disturbed ice in basal ice 40ar is present in excess due to injection of radiogenic 40ar produced in the underlying continental crust the weak depth gradient of 40ar in the dirty basal ice and the distribution of dirt indicate mixing within the basal ice while various published lines of evidence indicate mixing within the overlying clean disturbed ice excess ch4 which reaches thousands of ppm in basal dirty ice at grip is virtually absent in overlying clean disturbed ice demonstrating that mixing of dirty basal ice into the overlying clean ice if it occurs at all is very slow order of magnitude estimates indicate that the mixing rate of clean ice into dirty ice is sufficient to maintain a steady thickness of dirty ice against thinning from the mean ice flow the dirty ice appears to consist of two or more basal components in addition to clean glacial ice a small amount of soil or permafrost plus preglacial snow lake or ground ice could explain the observations

limits to future expansion of surface melt enhanced ice flow into the interior of western greenland moulins are important conduits for surface meltwater to reach the bed of the greenland ice sheet it has been proposed that in a warming climate newly formed moulins associated with the inland migration of supraglacial lakes could introduce surface melt to new regions of the bed introducing or enhancing sliding there by examining surface strain rates we found that the upper limit to where crevasses and therefore moulins are likely to form is 1600m this is also roughly the elevation above which lakes do not drain completely thus meltwater above this elevation will largely flow tens of kilometers through surface streams into existing moulins downstream furthermore results from a thermal ice sheet model indicate that the 1600m crevassing limit is well below the wet frozen basal transition 2000m together these data sets suggest that new supraglacial lakes will have a limited effect on the inland expansion of melt induced seasonal acceleration key points greenland ice sheet meltwater volumes are increasing notably at high elevations low strain rates limit the likelihood of moulin formation at high elevations high elevation meltwater will reach an already wet bed at lower elevations

decoding the dipstick thickness of siple dome west antarctica at the last glacial maximum ice thickness in west antarctica at the last glacial maximum lgm is poorly known yet is key information for understanding ice streams and interpreting ice cores although trim lines moraine limits and exposure age dating provide geologic constraints on ice thickness near the transantarctic mountains and in marie byrd land lack of exposed bedrock hampers traditional geologic methods in a central 2 3 10 6 km 2 region here we infer ice sheet thickness changes in the central ross sea embayment by using a tran sient ice flow model to find combinations of accumulation rate and ice sheet thickness histories that match the depth age relationship and the measured layer thickness pattern in the siple dome ice core after we reject unreasonable accumulation rate histories the remaining history pairs indicate thinning of 200 400 m since the lgm our estimate is lower than previous reconstructions that were constrained by geologic evidence from the transantarctic mountains and by marine data from the ross sea floor which indicate that a grounded ice sheet extended to the continental shelf margin during the last glacial period low surface elevations in the central ross sea embayment during the lgm do not preclude thicker ice along its boundaries however if this grounded ice sheet came over 1000 km from interior west antarctica as is usually assumed then it had very low surface slope requiring a very slippery bed alternatively the grounded ross sea ice might have flowed from the transantarctic mountains and western marie byrd land

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

transmission of basal variability to a glacier surface 1 transmission of basal variability to a glacier surface is investigated using analytical models for a linearly viscous medium the three dimensional transient response of the surface to both bedrock undulations and spatial variations in basal slipperiness for perturbations of arbitrary wavelengths is determined using perturbation methods both information transfer toward the surface and lateral transmission of horizontal stresses are strongly affected by the slip ratio that is the ratio of basal sliding to deformational velocity for any mean bedrock slope and above a minimum value of slip ratio the amplitude transfer of bedrock undulations toward the surface has a local maximum at undulation span corresponding to about 3 8 times the mean ice thickness the transmission of basal variability to a glacier surface increases quite significantly with increasing slip ratio this explains why the surfaces of fast flowing ice streams are more undulating than the slower moving bordering areas at slip ratios higher than about 100 the flow of glaciers and ice sheets becomes nonlocal in the sense that surface velocities and buildup and propagation of surface undulations cannot be calculated accurately on the basis of local thickness and slope using linearized long wave theories at these slip ratios instead of the more accurate arbitrary wavelength theory gives estimates of decay times that are an order of magnitude too small and phase velocities several times too large the problem of the propagation and decay of small amplitude surface undulations on glaciers in three dimensions is solved small amplitude surface waves on glaciers are strongly diffusive and dispersive redistribution of mass on ice sheets and glaciers is a diffusion process and it is misleading albeit not mathematically incorrect to describe the reaction of glaciers to surface perturbations in terms of a wave propagation

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

sustained high basal motion of the greenland ice sheet revealed by borehole deformation ice deformation and basal motion characterize the dynamical behavior of the greenland ice sheet gris we evaluate the contribution of basal motion from ice deformation measurements in boreholes drilled to the bed at two sites in the western marginal zone of the gris we find a sustained high amount of basal motion contribution to surface velocity of 44 73 percent in winter and up to 90 percent in summer measured ice deformation rates show an unexpected variation with depth that can be explained with the help of an ice flow model as a consequence of stress transfer from slippery to sticky areas this effect necessitates the use of high order ice flow models not only in regions of fast flowing ice streams but in all temperate based areas of the gris the agreement between modeled and measured deformation rates confirms that the recommended values of the temperature dependent flow rate factor a are a good choice for ice sheet models

patterns of variability in ostracode species and communities from the late cretaceous carbonate platforms a report for ecozonal modelling and the study of ambient conditions abstract previous palaeoecological and sedimentological analyses have demonstrated the marked variability of ostracode faunas in late cretaceous environments such organisms are generally strongly related to their biotopes but deeply affected by fluctuations even slight of ambient conditions therefore variations of both taxonomic units and communities may be used for a better knowledge and characterization of the main carbonate platform domains an interesting method of investigation is provided by the study of anatomical and or morphological variations within well defined taxonomic units especially ornate species living in the more internal zones of platforms they reflect with accuracy and reliability the impact of both ecological parameters and palaeogeographic settings this is also the case for some genera and even families ostracode type assemblages have been recently erected in the tethyan upper cretaceous microfaunas develop characteristic features such as richness diversity morphological trends according to their position on the carbonate platform system and sedimentary facies patterns furthermore without any consideration of the stratigraphical position of carbonate formations certain morphological variations concerning analogous taxa may be used for palaeogeographical interpretations trophic structures of ostracode communities versus nutrient flux are identifiable major changes occur through the sequences in the abundance specific composition and quantitative ratio between two main sedimentological formations carbonates especially rudist limestones and calcareous marls with benthic foraminifers and proximal terrigenous siliciclastic argillaceous areas finally this paper may be also considered as a useful attempt to synthesize present knowledge of late cretaceous carbonate platform ostracodes particularly intended for non ostracodologists who are perhaps unaware of the potential contribution of these organisms for palaeoenvironmental reconstructions

evidence for variable grounding zone and shear margin basal conditions across thwaites glacier west antarctica abstractdefinitive interpretation of ice sheet basal conditions from radar sounding data beneath outlet glacier grounding zones and shear margins can be problematic due to poorly constrained and spatially variable englacial attenuation rates and losses from propagation through a rough ice surface to correct for spatially variable attenuation rates we developed a novel radar analysis approach that provided improved empirical attenuation correction by fitting linearly variable attenuation rates along radar sounding profiles from the ice sheet interior to the grounding zone we also corrected for ice surface propagation losses by using surface echo amplitude distributions to constrain the loss of coherent power for surface reflections and two way propagation through a rough ice surface by applying this approach to airborne radar sounding observations of the thwaites glacier catchment in west antarctica we produced relative reflectivity profiles which show grounding zone basal conditions varying across t

first in situ measurements of hf radar echoing targets 1 the sounding rocket investigation of cusp irregularities 2 ici 2 was launched into the cusp ionosphere over svalbard to investigate the production of decameter scale irregularities in the electron plasma associated with hf radar backscatter the main mission objective was to obtain high resolution measurements of decameter scale electron plasma irregularities and to quantify the growth rate for the gradient drift instability gdi at the 5 7 khz sampling rate of the absolute density measurements ici 2 has provided the first documentation in terms of absolute electron density measurements of how 10 m structures are located on km scale electron density gradients ici 2 traversed a cusp electron density structure created by ongoing soft precipitation 10 m scale irregularities were generated at km scale density gradients the estimated growth time for the gdi process was 10 50 seconds

large subglacial lakes in east antarctica at the onset of fast flowing ice streams water has been found to gather in lakes underneath the antarctic ice sheet and it can move between lakes and drain catastrophically despite this relatively little is known about how subglacial lakes influence ice sheet dynamics now satellite imagery has identified four subglacial lakes similar in total area to lake vostok right at the onset of the recovery glacier ice stream in east antarctica this suggests that subglacial lakes may initiate and maintain rapid ice flow a mechanism that will need to be considered in future ice sheet mass balance assessments satellite imagery is used to identify four subglacial lakes similar in total area to lake vostok right at the onset of the recovery glacier ice stream in east antarctica this suggests that the subglacial lakes can initiate and maintain rapid ice flow a mechanism that will need to be considered in future ice sheet mass balance assessments water plays a crucial role in ice sheet stability and the onset of ice streams subglacial lake water moves between lakes1 and rapidly drains causing catastrophic floods2 the exact mechanisms by which subglacial lakes influence ice sheet dynamics are unknown however and large subglacial lakes3 4 have not been closely associated with rapidly flowing ice streams here we use satellite imagery and ice surface elevations to identify a region of subglacial lakes similar in total area to lake vostok at the onset region of the recovery glacier ice stream in east antarctica and predicted by ice sheet models5 we define four lakes through extensive flat featureless regions of ice surface bounded by upstream troughs and downstream ridges using ice velocities determined using interferometric synthetic aperture radar insar we find the onset of rapid flow moving at 20 to 30 m yr 1 of the tributaries to the recovery glacier ice stream in a 280 km wide segment at the downslope margins of these four subglacial lakes we conclude that the subglacial lakes initiate and maintain rapid ice flow through either active modification of the basal thermal regime of the ice sheet by lake accretion or through scouring bedrock channels in periodic drainage events we suggest that the role of subglacial lakes needs to be considered in ice sheet mass balance assessments

observation of polar cap patches and calculation of gradient drift instability growth times a swarm case study the swarm mission represents a strong new tool to survey polar cap patches and plasma structuring inside the polar cap in the early commissioning phase the three swarm satellites were operated in

radiostratigraphy and age structure of the greenland ice sheet 2015 the authors several decades of ice penetrating radar surveys of the greenland and antarctic ice sheets have observed numerous widespread internal reflections analysis of this radiostratigraphy has produced valuable insights into ice sheet dynamics and motivates additional mapping of these reflections here we present a comprehensive deep radiostratigraphy of the greenland ice sheet from airborne deep ice penetrating radar data collected over greenland by the university of kansas between 1993 and 2013 to map this radiostratigraphy efficiently we developed new techniques for predicting reflection slope from the phase recorded by coherent radars when integrated along track these slope fields predict the radiostratigraphy and simplify semiautomatic reflection tracing core intersecting reflections were dated using synchronized depth age relationships for six deep ice cores additional reflections were dated by matching reflections between transects and by extending reflection inferred depth age relationships using the local effective vertical strain rate the oldest reflections dating to the eemian period are found mostly in the northern part of the ice sheet within the onset regions of several fast flowing outlet glaciers and ice streams reflections typically do not conform to the bed topography disrupted radiostratigraphy is also observed in a region north of the northeast greenland ice stream that is not presently flowing rapidly dated reflections are used to generate a gridded age volume for most of the ice sheet and also to determine the depths of key climate transitions that were not observed directly this radiostratigraphy provides a new constraint on the dynamics and history of the greenland ice sheet

inference of accumulation rate patterns from deep layers in glaciers and ice sheets the spatial pattern of accumulation rate can be inferred from internal layers in glaciers and ice sheets non dimensional analysis determines where finite strain can be neglected shallow layer approximation or approximated with a local one dimensional flow model local layer approxima tion and where gradients in strain rate along particle paths must be included deep layers we develop a general geophysical inverse procedure to infer the spatial pattern of accumulation rate along a steady state flowband using measured topography of the ice sheet surface bed and a deep layer a variety of thermomechanical ice flow models can be used in the forward problem to calculate surface topography and ice velocity which are used to calculate particle paths and internal layer shapes an objective tolerance criterion prevents over fitting the data after making site specific simplifications in the thermomechanical flow algorithm we find the accumulation rate along a flowband through taylor mouth a flank site on taylor dome antarctica using a layer at approximately 100 m depth or 20 of the ice thickness accumulation rate correlates with ice surface curvature at this site gradients along flow paths critically impact inference of both the accumulation pattern and the depth age relation in a 100 m core

mechanisms of fast flow in jakobshavns isbrae west greenland part i measurements of temperature and water level in deep boreholes several holes were drilled to depths of 1500 1630 m along a profile across jakobshavns isbrae 50 km upstream from the calving front drilling was by hot water and required approximately 20 h the holes were rapidly closed by refreezing but it was possible to instrument them with thermistors and tilt sensors before this occurred near the margins of the ice stream the holes reached the bed and connected with the subglacial drainage system water level changes recorded in these holes are discussed in terms of the basal hydraulic system the temperature measurements show that the glacier is temperate based moreover extrapolation of a measured temperature profile and its curvature suggests that a temperate layer of substantial thickness may exist at the bed near the center of the ice stream there is a striking difference in the shapes of temperature profiles measured at different locations beneath the center line the temperature minimum is at a considerably smaller relative depth than near the margins but it is nearly the same in magnitude 22 1 c this may indicate a disproportionately large vertical stretching of the basal ice in the center of the ice stream since the basal ice is warmer and much less viscous than the ice above a thickening of that layer would cause a corresponding increase of surface velocity we presume that this mechanism contributes to the fast flow of jakobshavns isbrae

surface roughness over the northern half of the greenland ice sheet from airborne laser altimetry 1 surface roughness defined as the standard deviation of small scale elevation fluctuations from the linear trend over 0 5 km can be estimated from high resolution airborne laser altimetry here we present results for the northern half of the greenland ice sheet using laser data collected in may 1995 roughness is smallest in the central region straddling the ice divide increases in amplitude toward the coast and appears to be correlated with slope of the ice surface for most of the study region surface roughness is 8 cm or less 2 5 cm water equivalent in smaller regions associated with fast flow larger values are found comparison of the size of small scale topographic disturbances with the spatial noise estimated from five closely spaced ice cores drilled in northwest greenland shows good agreement similar correspondence was found earlier using nine ice cores from the summit region these results indicate that the airborne laser altimeter provides an efficient platform for characterizing the statistical nature of the snow surface over large areas of the polar ice sheets

transformation of high latitude ionospheric f region patches into blobs during the march 21 1990 storm discrete f region electron density enhancements of a factor of 2 or more have been observed in the high latitude ionosphere these enhancements have been termed patches if they occur within the polar cap and blobs if they occur outside of the polar cap it is important to understand the formation and evolution of these structures because they are associated with large phase and amplitude scintillation in transionospheric radio signals blobs are generally thought to result from the breakup of patches as they exit the polar cap however this process has not previously been observed detailed study of high latitude ionospheric plasma transport is generally difficult because of the sparseness spatial and temporal of electron density and velocity observations in this paper we present electron density enhancements measured from the qaanaaq digisonde the millstone hill incoherent scatter radar and the dmsp f8 satellite during a 5 hour interval of the march 21 1990 storm period and show definitively how a patch is transformed into a blob we present a new trajectory analysis package that is capable of using ionospheric convection patterns to determine the motion of ionospheric plasma over a period of several hours the new package uses convection patterns from the assimilative mapping of ionospheric electrodynamics amie technique to track the motion of observed patches from one site to another and thus determines where the measured electron density enhancements originated and where they went after being observed the trajectory analysis also establishes that there is a direct connection between the enhancements observed by the different instruments at different locations in this case within 4 hours plasma observed by a digisonde near the pole is convected through 35 of latitude to the northeastern united states where it is observed by the millstone hill radar then roughly equal portions are transported westward to alaska and eastward to scandinavia where they are observed by the dmsp satellite this study demonstrates that the changing convection pattern can significantly distort the patch shape and trajectory and illustrates the high degree of mixing of ionospheric plasma by convection the changing convection pattern leads to the simultaneous existence of a boundary blob and a subauroral blob which are both observed by the millstone hill radar this work is very relevant to our future ability to specify and forecast ionospheric conditions at high latitudes it represents a critical step from a merely qualitative ability to model the evolution of patches and blobs to a quantitative ability

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

the platycopid signal deciphered responses of ostracod taxa to environmental change during the cenomanian turonian boundary event late cretaceous in se england abstract a multi proxy investigation of the cenomanian turonian boundary event ctbe near dover in southern england previously demonstrated that coincident with a major global positive carbon stable isotope excursion during oceanic anoxic event 2 oae 2 the diversity of nanoplankton dinoflagellates planktonic and benthonic foraminifera and ostracods was severely reduced this was attributed to decreasing levels of dissolved oxygen consequent on an intensification and expansion of the oceanic oxygen minimum zone omz into shelf seas in the case of the ostracods it was noted that as podocopid taxa became locally extinct platycopids became the dominant component of the fauna subsequently the platycopid signal hypothesis psh claimed that dominance of platycopids in ostracod assemblages could be regarded as a signal of dysaerobic conditions on the sea floor based on the premise that the filter feeding platycopids able to pass more water over their respiratory surface were better equipped than other benthonic ostracods to survive in water of reduced oxygen concentration the psh has been widely accepted and applied in palaeoenvironmental reconstructions of stratigraphic intervals ranging from the palaeozoic to the quaternary however the modern biological and ecological support claimed for the platycopid signal has been challenged platycopids are occasionally dominant in modern omzs but often they are not and in any case the same can be said about some podocopids apparently precise calibration scales published by some authors are not justified by available data furthermore platycopid signal indications of dysaerobic intervals in the english chalk succession often conflict with the evidence of macrofossils and trace fossils here we review old and new data from two ctbe sites in se england dover and eastbourne and advance an alternative interpretation of the platycopid signal based on the concept of the spread of oceanic oligotrophic conditions into the european chalk sea during oae2 we propose that ostracod assemblages overwhelmingly dominated by platycopids signify oligotrophy because living platycopids appear to be adapted to filter feed on nano and picoplankton which are predominant in oligotrophic conditions

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

direct observations of evolving subglacial drainage beneath the greenland ice sheet simultaneous observations of moulins and boreholes in western greenland show that water delivery to the base of the ice sheet via moulins affects short term ice velocity fluctuations but not late season ice velocity decelerations increased meltwater delivery to the base of the greenland ice sheet will increase the ice sheet velocity accelerating its inevitable rush towards the ocean and subsequent sea level rise or will it debate on this topic is at the forefront of cryospheric research but has been hampered by the lack of simultaneous observations of hydraulic head in moulins vertical shafts that deliver water to the base of the ice sheet and boreholes which monitor basal water pressure lauren andrews and colleagues now provide these observations from a small region in western greenland and reveal that water delivery by moulins into channelized basal flow is indeed linked to short term fluctuations in ice velocity late season decelerations in ice velocity however seem to be controlled by changes in unchannelized flow rather than any shifts in the moulin system seasonal acceleration of the greenland ice sheet is influenced by the dynamic response of the subglacial hydrologic system to variability in meltwater delivery to the bed1 2 via crevasses and moulins vertical conduits connecting supraglacial water to the bed of the ice sheet as the melt season progresses the subglacial hydrologic system drains supraglacial meltwater more efficiently1 2 3 4 decreasing basal water pressure4 and moderating the ice velocity response to surface melting1 2 however limited direct observations of subglacial water pressure4 5 6 7 mean that the spatiotemporal evolution of the subglacial hydrologic system remains poorly understood here we show that ice velocity is well correlated with moulin hydraulic head but is out of phase with that of nearby 0 3 2 kilometres away boreholes indicating that moulins connect to an efficient channelized component of the subglacial hydrologic system which exerts the primary control on diurnal and multi day changes in ice velocity our simultaneous measurements of moulin and borehole hydraulic head and ice velocity in the paakitsoq region of western greenland show that decreasing trends in ice velocity during the latter part of the melt season cannot be explained by changes in the ability of moulin connected channels to convey supraglacial melt instead these observations suggest that decreasing late season ice velocity may be caused by changes in connectivity in unchannelized regions of the subglacial hydrologic system understanding this spatiotemporal variability in subglacial pressures is increasingly important because melt season dynamics affect ice velocity beyond the conclusion of the melt season8 9 10

subglacial topography and geothermal heat flux potential interactions with drainage of the greenland ice sheet 1 many of the outlet glaciers in greenland overlie deep and narrow trenches cut into the bedrock it is well known that pronounced topography intensifies the geothermal heat flux in deep valleys and attenuates this flux on mountains here we investigate the magnitude of this effect for two subglacial trenches in greenland heat flux variations are estimated for idealized geometries using solutions for plane slopes derived by lachenbruch 1968 it is found that for channels such as the one under jakobshavn isbrae topographic effects may increase the local geothermal heat flux by as much as 100

large and rapid melt induced velocity changes in the ablation zone of the greenland ice sheet continuous global positioning system observations reveal rapid and large ice velocity fluctuations in the western ablation zone of the greenland ice sheet within days ice velocity reacts to increased meltwater production and increases by a factor of 4 such a response is much stronger and much faster than previously reported over a longer period of 17 years annual ice velocities have decreased slightly which suggests that the englacial hydraulic system adjusts constantly to the variable meltwater input which results in a more or less constant ice flux over the years the positive feedback mechanism between melt rate and ice velocity appears to be a seasonal process that may have only a limited effect on the response of the ice sheet to climate warming over the next decades

the platycopid signal of oxygen depletion in the ocean a critical evaluation of the evidence from modern ostracod biology ecology and depth distribution abstract the use of relative abundances of filter feeding platycopid ostracods in estimating past levels of dissolved oxygen in the ocean the platycopid signal hypothesis is examined paying particular attention to evidence from faunistic biological and ecological studies the degree of support for the hypothesis has been overstated available data on ostracod depth distribution show that previous claims for platycopid dominance in the oxygen minimum zone in different oceanic regions are misleading and based on only a few studies observations and experimental studies on living platycopids are limited in scope and should be treated with caution we conclude that previously published and widely applied calibrations of platycopid abundance against dissolved oxygen levels are inadequately justified by the available data any further application of the platycopid signal technique should necessarily address these criticisms

insights into spatial sensitivities of ice mass response to environmental change from the searise ice sheet modeling project i antarctica atmospheric oceanic and subglacial forcing scenarios from the sea level response to ice sheet evolution searise project are applied to six three dimensional thermomechanical ice sheet models to assess antarctic ice sheet sensitivity over a 500 year timescale and to inform future modeling and field studies results indicate i growth with warming except within low latitude basins where inland thickening is outpaced by marginal thinning ii mass loss with enhanced sliding with basins dominated by high driving stresses affected more than basins with low surface slope streaming ice and iii mass loss with enhanced ice shelf melting with changes in west antarctica dominating the signal due to its marine setting and extensive ice shelves cf minimal impact in the terre adelie george v oates and victoria land region of east antarctica ice loss due to dynamic changes associated with enhanced sliding and or sub shelf melting exceeds the gain due to increased precipitation furthermore differences in results between and within basins as well as the controlling impact of sub shelf melting on ice dynamics highlight the need for improved understanding of basal conditions grounding zone processes ocean ice interactions and the numerical representation of all three key points sensitivity study of antarctica to atmospheric oceanic and subglacial forcings different sectors of antarctica are vulnerable to the forcings atmospheric forcing lead to a growth but dynamic forcing lead to a mass loss 2013 american geophysical union all rights reserved

heat flux variations beneath central greenland s ice due to anomalously thin lithosphere the thermal state of the earth s surface is usually influenced more by climate than by heating from the earth s interior numerical models show that in the oldest and thickest part of the greenland ice sheet geothermal heat flux through an anomalously thin lithosphere leads to strong regional variations in basal melting

a search in north greenland for a new ice core drill site a new deep ice core drilling site has been identified in north greenland at 75 12 n 42 30 w 316 km north northwest nnw of the grip drill site on the summit of the ice sheet the ice thickness here is 3085 m the surface elevation is 2919 m the north grip ngrip site is identified so that ice of eemian age 115 130 ka bp calendar years before present is located as far above bedrock as possible and so the thickness of the eemian layer is as great as possible an ice flow model similar to the one used to date the grip ice core is used to simulate the flow along the nnw trending ice ridge surface and bedrock elevations surface accumulation rate distribution and ratio echo sounding along the ridge have been used as model input the surface accumulation rate drops from 0 23 m ice equivalent year 1 at grip to 0 19 m ice equivalent year 1 50 km from grip over the following 300 km the accumulation is relatively constant before is starts decreasing again further north ice thickness up to 3250 m bring the temperature of the basal ice up to the pressure melting point 100 250 km from grip the ngrip site is located 316 km from grip in a region where the bedrock is smooth and the accumulation rate is 0 19 m ice equivalent year 1 the modeled basal ice here has always been a few degrees below the pressure melting point internal radio echo sounding horizons can be traced between the grip and ngrip sites allowing us to date the ice down to 2300 m depth 52 ka bp an ice flow model predicts that the eemian age ice will be located in the depth range 2710 2800 m which is 285 m above the bedrock this is 120 m further above the bedrock and the thickness of the eemian layer of ice is 20 m thicker than at the grip ice core site

mapping basal melt under the northern greenland ice sheet ice sheets in greenland and antarctica are based on continental rock and are coupled strongly to changes in global sea level their beds may be frozen or thawed but it has not been known how much or where basal melt occurs except in special cases where a borehole has reached the bed or where airborne radar has revealed subglacial lakes and reflective ice stream beds in antarctica we have used a previously published technique to detect subglacial melt water in the general case and have here applied it to radar sounder data collected over northern and central greenland we have found extensive subglacial water along between 13 and 20 of the flight paths this paper provides maps of the measured locations and probable extent of subglacial water

recent advances in understanding ice sheet dynamics abstract glaciers and ice sheets play a dynamic role in earth s climate system influencing regional and global scale climate and responding to climate change on time scales from years to millennia they are also an integral part of earth s landscape in alpine and polar regions where they are an active agent in isostatic tectonic and earth surface processes this review paper summarizes recent progress in understanding and modelling ice sheet dynamics from the microphysical processes of ice deformation in glaciers to continental scale processes that influence ice dynamics based on recent insights and research directions it can be expected that a new generation of ice sheet models will soon replace the current standard improvements that can be foreseen in the near future include i the addition of internally consistent evolutionary equations for ice crystal fabric anisotropic flow laws ii more generalized flow laws that include different deformation mechanisms under different stress regimes iii explicit incorporation of the effects of chemical impurities and grain size dynamic recrystallization on ice deformation iv higher order stress solutions to the momentum balance stokes equation that governs ice sheet flow and v the continued merger of ice sheet models with increasingly complex earth systems models which include fully coupled subglacial hydrological and geological processes examples from the greenland ice sheet and vatnajokull ice cap iceland are used to illustrate several of these new directions and their importance to glacier dynamics