fracture propagation to the base of the greenland ice sheet during supraglacial lake drainage surface meltwater that reaches the base of an ice sheet creates a mechanism for the rapid response of ice flow to climate change the process whereby such a pathway is created through thick cold ice has not however been previously observed we describe the rapid 2 hours drainage of a large supraglacial lake down 980 meters through to the bed of the greenland ice sheet initiated by water driven fracture propagation evolving into moulin flow drainage coincided with increased seismicity transient acceleration ice sheet uplift and horizontal displacement subsidence and deceleration occurred over the subsequent 24 hours the short lived dynamic response suggests that an efficient drainage system dispersed the meltwater subglacially the integrated effect of multiple lake drainages could explain the observed net regional summer ice speedup

the subglacial thermal organisation sto of ice sheets abstract this paper examines ice sheet wide variations in subglacial thermal regime and ice dynamics using the landform record exposed on the beds of former mid latitude ice sheets the laurentide cordilleran fennoscandian and british irish ice sheets we compare the landform patterns beneath these former ice sheets to the flow organisation beneath parts of the contemporary antarctic ice sheet inferred from radarsat 1 antarctic mapping project ramp data the evidence preserved in the landform record and observed on contemporary ice masses can be grouped into four major ice dynamical components that collectively define the subglacial thermal organisation sto of ice sheets these ice dynamical components are frozen bed patches ice streams ice stream tributaries and lateral shear zones frozen bed patches appear at a wide range of spatial scales spanning four orders of magnitude in some areas frozen bed zones comprise large proportions of the bed e g near the ice divide in continental areas whilst in other areas they constitute isolated islands in areas dominated by thawed bed conditions ice streams narrow zones of fast flow in ice sheets that are otherwise dominated by slow sheet flow are also common features of quaternary ice sheets tributaries to ice streams flow at velocities intermediate between full ice stream and sheet flow and may divert ice drainage from one primary ice stream corridor to an adjacent one sharp lateral boundaries between landforms indicate sliding and non sliding conditions respectively these lateral boundaries represent important discontinuities in the glacial landscape and mark the location of shear zones between thawed bed ice streams and intervening frozen bed areas we use the landform evidence in the area around great bear lake canada to trace the evolution of an ice stream web through time demonstrating that frozen bed patches are integral components of this complex system we conclude that frozen bed patches are important for the stability of ice sheets because they laterally constrain and isolate peripheral drainage basins and their ice streams

the impact of parametric uncertainty and topographic error in ice sheet modelling ice sheet models isms developed to simulate the behaviour of continental scale ice sheets under past present or future climate scenarios are subject to a number of uncertainties from various sources these sources include the conceptualization of the ism and the degree of abstraction and parameterizations of processes such as ice dynamics and mass balance the assumption of spatially or temporally constant parameters such as degree day factor atmospheric lapse rate or geothermal heat flux is one example additionally uncertainties in ism input data such as topography or precipitation propagate to the model results in order to assess and compare the impact of uncertainties from model parameters and climate on the glimmer ice sheet model a parametric uncertainty analysis pua was conducted parameter variation was deduced from a suite of sensitivity tests and accuracy information was deduced from input data and the literature recorded variation of modelled ice extent across the pua runs was 65 for equilibrium ice sheets additionally the susceptibility of ism results to modelled uncertainty in input topography was assessed resulting variations in modelled ice extent in the range of 0 8 6 6 are comparable to that of ism parameters such as flow enhancement basal traction and geothermal heat flux

deglaciation of a marine based ice sheet late weichselian palaeo ice dynamics and retreat in the southern barents sea reconstructed from onshore and offshore glacial geomorphology abstract marine and terrestrial datasets swath bathymetry 2d and 3d seismic and remote sensing data are used to map the megascale glacial geomorphology of the southern barents sea seafloor northern fennoscandia and the kola peninsula giving new insights into the dynamics and behaviour of the barents sea ice sheet during retreat the glacial geomorphology indicates clear spatial and temporal variations in ice dynamics with evidence for both active ice streaming and frozen bed conditions at the maximum and during deglaciation a five stage reconstruction of the late weichselian deglaciation is proposed at the late weichselian maximum ice covered the whole of the barents sea shelf and the configuration and dynamics of the ice sheet were controlled by the bjornoyrenna bear island trough ice stream this was fed by source areas in the central and north eastern barents sea and in the fennoscandian mainland deglaciation was initiated along the western continental margin available dating results show that this occurred after 19 cal ka bp the bjornoyrenna ice stream retreated rapidly ice margin retreat rates of 275 m a 1 but was periodically grounded as evidenced by a well developed series of grounding zone wedges this was followed by an advance of two short lived ice streams from the eastern sector of the ice sheet into the south western barents sea these ice streams advanced and retreated very rapidly with no evidence for ice margin standstills correlation to dated cores suggests an age of around 16 000 cal ka bp for this readvance the period of operation of these two ice streams was coincident with maximum ice extent along the eastern margin in russia the western barents sea ice margin then retreated onshore with deglaciation continuing at a reduced rate the initiation of deglaciation coincided with rising global eustatic sea level acceleration of calving due to grounding line floatation rapidly opened up a major calving bay in bjornoyrenna encouraging rapid loss of the marine based ice the retreat rate stabilised only once the ice sheet retreated onshore and calving ceased we note that the observed pattern of rapid retreat with high variability in ice streaming for the marine based barents sea ice sheet is similar to that of the marine based sectors of the late weichselian british laurentide and west antarctic ice sheets

influence of glacier hydrology on the dynamics of a large quaternary ice sheet the influence of glacier hydrology on the time dependent morphology and flow behaviour of the late weichselian scandinavian ice sheet is explored using a simple one dimensional ice sheet model the model is driven by orbitally induced radiation variations ice albedo feedback and eustatic sea level change the influence of hydrology is most marked during deglaciation and on the southern side of the ice sheet where a marginal zone of rapid sliding thin ice and low surface slopes develops such a zone is absent when hydrology is omitted from the model and its formation results in earlier and more rapid deglaciation than occurs in the no hydrology model the final advance to the glacial maximum position results from an increase in the rate of basal sliding as climate warms after 23000 yr bp channelised subglacial drainage develops only episodically and is associated with relatively low meltwater discharges and high hydraulic gradients the predominance of iceberg calving as an ablation mechanism on the northern side of the ice sheet restricts the occurrence of surface melting lack of meltwater penetration to the glacier bed in this area means that ice flow is predominantly by internal deformation and the ice sheet adopts a classical parabolic surface profile

greenland ice sheet motion coupled with daily melting in late summer 1 we use ground based and satellite observations to detect large diurnal and longer period variations in the flow of the greenland ice sheet gris during late summer that are strongly coupled with changes in its surface hydrology the diurnal signals are associated with periodic changes in surface melting and the longer period signals are associated with the episodic drainage of supra glacial lakes ice velocity doubles around 2 hours after peak daily melting and returns approximately to wintertime levels around 12 hours afterwards demonstrating an intimate link between the surface and basal hydrology during late summer the ice sheet accelerates by 35 per positive degree day of melting the observed link between surface melting and enhanced flow is typical of alpine glaciers which may provide an appropriate analogue for the evolution of the gris in a warming climate

a new ice thickness and bed data set for the greenland ice sheet 1 measurement data reduction and errors ice thickness data collected between 1993 and 1999 using a coherent ice penetrating radar system developed at the university of kansas have been combined with data collected by the technical university of denmark in the 1970s to produce a new ice thickness grid for greenland crossover analysis was used to assess the relative accuracy of the two data sets and they were weighted accordingly and interpolated onto a regular 5 km spacing grid using a kriging interpolation procedure a high resolution land ice mask was used to help constrain the interpolation of the ice thickness data near the ice sheet margins where in the past the relative errors have been largest the ice thickness grid was combined with a new digital elevation model of the ice sheet and surrounding rock outcrops to produce a new bed elevation data set for the whole of greenland the ice thickness grid was compared with the currently available data set differences in the center of the ice sheet where the ice is thickest were of the order of a few percent near the margins however large differences of as much as a factor of 10 were found the total volume of ice contained in the ice sheet was reestimated and found to have a value of 2 93 x 10 6 km 3 the ice thickness grid was used to calculate the spatial pattern of gravitational driving stress over the ice sheet anomalous patterns of stress were found in areas that appeared to be associated with areas of rapid flow

dynamical processes involved in the retreat of marine ice sheets marine ice sheets with mechanics described by the shallow ice approximation by definition do not couple mechanically with the shelf such ice sheets are known to have neutral equilibria we consider the implications of this for their dynamics and in particular for mechanisms which promote marine ice sheet retreat the removal of ice shelf buttressing leading to enhanced flow in grounded ice is discounted as a significant influence on mechanical grounds sea level rise leading to reduced effective pressures under ice streams is shown to be a feasible mechanism for producing postglacial west antarctic ice sheet retreat but is inconsistent with borehole evidence warming thins the ice sheet by reducing the average viscosity but does not lead to grounding line retreat internal oscillations either specified or generated via a macayeal payne thermal mechanism promote migration this is a noise induced drift phenomenon stemming from the neutral equilibrium property of marine ice sheets this migration occurs at quite slow rates but these are sufficiently large to have possibly played a role in the dynamics of the west antarctic ice sheet after the glacial maximum numerical experiments suggest that it is generally true that while significant changes in thickness can be caused by spatially uniform changes spatial variability coupled with dynamical variability is needed to cause margin movement

controls on the early holocene collapse of the bothnian sea ice stream new high resolution multibeam data in the gulf of bothnia reveal for the first time the subglacial environment of a bothnian sea ice stream the geomorphological record suggests that increased meltwater production may have been important in driving rapid retreat of bothnian sea ice during deglaciation here we apply a well established one dimensional flowline model to simulate ice flow through the gulf of bothnia and investigate controls on retreat of the ice stream during the post younger dryas deglaciation of the fennoscandian ice sheet the relative influence of atmospheric and marine forcings are investigated with the modelled ice stream exhibiting much greater sensitivity to surface melting implemented through surface mass balance and hydrofracture induced calving than to submarine melting or relative sea level change such sensitivity is supported by the presence of extensive meltwater features in the geomorphological record the modelled ice stream does not demonstrate significant sensitivity to changes in prescribed ice stream width or overall bed slope but local variations in basal topography and ice stream width result in non linear retreat of the grounding line notably demonstrating points of short lived retreat slowdown on reverse bed slopes retreat of the ice stream was most likely governed by increased ice surface meltwater production with the modelled retreat rate less sensitive to marine forcings despite the marine setting

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

800 000 years of abrupt climate variability we constructed an 800 000 year synthetic record of greenland climate variability based on the thermal bipolar seesaw model our greenland analog reproduces much of the variability seen in the greenland ice cores over the past 100 000 years the synthetic record shows strong similarity with the absolutely dated speleothem record from china allowing us to place ice core records within an absolute timeframe for the past 400 000 years hence it provides both a stratigraphic reference and a conceptual basis for assessing the long term evolution of millennial scale variability and its potential role in climate change at longer time scales indeed we provide evidence for a ubiquitous association between bipolar seesaw oscillations and glacial terminations throughout the middle to late pleistocene

seismic precursory patterns before a cliff collapse and critical point phenomena 1 we analyse the statistical pattern of seismicity before a 1 2 103 m3 chalk cliff collapse on the normandie ocean shore western france we show that a power law acceleration of seismicity rate and energy in both 40 hz 1 5 khz and 2 hz 10khz frequency range is defined on 3 orders of magnitude within 2 hours from the collapse time simultaneously the average size of the seismic events increases toward the time to failure these in situ results are derived from the only station located within one rupture length distance from the rock fall rupture plane they mimic the critical point like behavior recovered from physical and numerical experiments before brittle failures and tertiary creep failures our analysis of this first seismic monitoring data of a cliff collapse suggests that the thermodynamic phase transition models for failure may apply for cliff collapse

the extreme melt across the greenland ice sheet in 2012 1 the discovery of the 2012 extreme melt event across almost the entire surface of the greenland ice sheet is presented data from three different satellite sensors including the oceansat 2 scatterometer the moderate resolution imaging spectroradiometer and the special sensor microwave imager sounder are combined to obtain composite melt maps representing the most complete melt conditions detectable across the ice sheet satellite observations reveal that melt occurred at or near the surface of the greenland ice sheet across 98 6 of its entire extent on 12 july 2012 including the usually cold polar areas at high altitudes like summit in the dry snow facies of the ice sheet this melt event coincided with an anomalous ridge of warm air that became stagnant over greenland as seen in melt occurrences from multiple ice core records at summit reported in the published literature such a melt event is rare with the last significant one occurring in 1889 and the next previous one around seven centuries earlier in the medieval warm period given its rarity the 2012 extreme melt across greenland provides an exceptional opportunity for new studies in broad interdisciplinary geophysical research

seasonal speedup along the western flank of the greenland ice sheet it has been widely hypothesized that a warmer climate in greenland would increase the volume of lubricating surface meltwater reaching the ice bedrock interface accelerating ice flow and increasing mass loss we have assembled a data set that provides a synoptic scale view spanning ice sheet to outlet glacier flow with which to evaluate this hypothesis on the ice sheet these data reveal summer speedups 50 to 100 consistent with but somewhat larger than earlier observations the relative speedup of outlet glaciers however is far smaller 15 furthermore the dominant seasonal influence on jakobshavn isbrae s flow is the calving front s annual advance and retreat with other effects producing outlet glacier speedups an order of magnitude larger seasonal melt s influence on ice flow is likely confined to those regions dominated by ice sheet flow

a time dependent glaciological model of the weichselian ice sheet a time dependent model is used to study the evolution of the weichselian ice sheet the results indicate three stages of the glaciation a mountain centered ice sheet between 110 000 and 80 000 bp followed by a phase with an extension similar to or slightly larger than the younger dryas zone the last and maximum extent was reached about 30 000 20 000 bp and the main deglaciation began about 15 000 bp the model is based on a finite element method solution of the continuity equation the climatic records from the camp century and summit ice cores are used for climate control and the model is calibrated using geological deglaciation evidence in scandinavia the assumption of a general sliding condition for elevations below 100 m with an enhanced sliding zone through the center of the baltic sea and gulf of bothnia yield broad agreement for marginal positions in sweden and finland during a simulated termination

late quaternary ice sheet history of northern eurasia the maximum limits of the eurasian ice sheets during four glaciations have been reconstructed 1 the late saalian 140 ka 2 the early weichselian 100 80 ka 3 the middle weichselian 60 50 ka and 4 the late weichselian 25 15 ka the reconstructed ice limits are based on satellite data and aerial photographs combined with geological field investigations in russia and siberia and with marine seismic and sediment core data the barents kara ice sheet got progressively smaller during each glaciation whereas the dimensions of the scandinavian ice sheet increased during the last ice age the barents kara ice sheet attained its maximum size as early as 90 80 000 years ago when the ice front reached far onto the continent a regrowth of the ice sheets occurred during the early middle weichselian culminating about 60 50 000 years ago during the late weichselian the barents kara ice sheet did not reach the mainland east of the kanin peninsula with the exception of the nw fringe of taimyr a numerical ice sheet model forced by global sea level and solar changes was run through the full weichselian glacial cycle the modeling results are roughly compatible with the geological record of ice growth but the model underpredicts the glaciations in the eurasian arctic during the early and middle weichselian one reason for this is that the climate in the eurasian arctic was not as dry then as during the late weichselian glacial maximum

eurasian ice sheet dynamics and sensitivity to subglacial hydrology ice stream dynamics are strongly controlled by processes taking place at the ice bed interface where subglacial water both lubricates the base and saturates any existing underlying sediment large parts of the former eurasian ice sheet were underlain by thick sequences of soft marine sediments and many areas are imprinted with geomorphological features indicative of fast flow and wet basal conditions here we study the effect of subglacial water on past eurasian ice sheet dynamics by incorporating a thin film model of basal water flow into the ice sheet model sicopolis and use it to better represent flow in temperate areas the adjunction of subglacial hydrology results in a smaller ice sheet building up over time and generally faster ice velocities which consequently reduces the total area fraction of temperate basal ice and ice streaming areas minima in the hydraulic pressure potential governing water flow are used as indicators for potential locations of past subglacial lakes and a probability distribution of lake existence is presented based on estimated lake depth and longevity

deglacial rapid sea level rises caused by ice sheet saddle collapses a simulation of ice sheet retreat reveals two meltwater pulses coinciding with mwp 1a and the 8 200 year event caused by saddle collapses between ice domes during the last deglaciation the retreating ice sheets occasionally released massive amounts of fresh water these meltwater pulses are often implicated as important influences on abrupt climate change but the underlying triggering mechanisms are not clear here lauren gregoire et al show that saddle zones that connect adjacent high domes on ice sheets can be lowered by ongoing warming to an elevation at which they rapidly degrade leading to separate domes and a meltwater pulse that is consistent with geological observations although the mechanism itself is not novel the authors establish that two important sudden cooling events meltwater pulse 1a and the 8 200 year event were probably caused by saddle separation the last deglaciation 21 to 7 thousand years ago was punctuated by several abrupt meltwater pulses which sometimes caused noticeable climate change1 2 around 14 thousand years ago meltwater pulse 1a mwp 1a the largest of these events produced a sea level rise of 14 18 metres over 350 years3 although this enormous surge of water certainly originated from retreating ice sheets there is no consensus on the geographical source or underlying physical mechanisms governing the rapid sea level rise4 5 6 here we present an ice sheet modelling simulation in which the separation of the laurentide and cordilleran ice sheets in north america produces a meltwater pulse corresponding to mwp 1a another meltwater pulse is produced when the labrador and baffin ice domes around hudson bay separate which could be associated with the 8 200 year event the most pronounced abrupt climate event of the past nine thousand years7 for both modelled pulses the saddle between the two ice domes becomes subject to surface melting because of a general surface lowering caused by climate warming the melting then rapidly accelerates as the saddle between the two domes gets lower producing nine metres of sea level rise over 500 years this mechanism of an ice saddle collapse probably explains mwp 1a and the 8 200 year event and sheds light on the consequences of these events on climate

implications of increased greenland surface melt under global warming scenarios ice sheet simulations abstract the greenland ice sheet is likely to make a faster contribution to sea level rise in a warming world than previously believed based on numerical modelling using a parameterization of recent results showing surface meltwater lubrication of ice flow zwally et al science 297 557 2002 218 documented correlation between increased ice velocity and increased surface melt as parameterized by positive degree days they argued that surface water is piped directly to the bed with little delay causing increased basal water pressures and basal sliding velocities an effect not included in recent greenland ice sheet models known to the authors using the pennsylvania state university university of chicago thermomechanical flowline model numerous simulations were conducted to test a wide range of parameter space linking surface melt with a new sliding law based on the zwally et al data under three different global warming scenarios 2 co2 4 co2 and 8 co2 comparisons to reconstructions generated with a traditional sliding parameterization illustrate an enhanced sensitivity of the ice sheet to surface warming resulting in higher ablation rates additional thinning and retreat of the margin and a reduction in ice volume leading to an increased contribution to global sea level rise

intra annual and intra seasonal flow dynamics of a high arctic polythermal valley glacier measurements of surface dynamics onpolythermaljohn evans glacier nunavut canada over two winter periods and every 7 10 days throughout two melt seasons june july 2000 2001 provide new insight into spatio temporal patterns of high arctic glacier dynamics in the lower ablation zone mean annual surface velocities are 10 21ma 1 but peak velocities up to 50 higher are attained during latejune earlyjuly in the upper ablation zone and lower accumulation zone mean annual surface velocities are typically10 18ma 1 and peak velocities up to 40 higher occur during latejuly in the upper accumulation zone mean annual surface velocities are 2 9ma 1 and motion in mid to latejuly exceeds this by up to 10 rapid drainage of ponded supraglacial water in the upper ablation zone to an initially distributed subglacial drainage system in mid june may force excess surface motion in the warm based lower glacier the data indicate thatthe durationof the velocity response maybe relatedto the rate of channeliz ation of the basal drainage and the velocity response may be transmitted up glacier by longitudinal coupling an increase in surface velocities in the middle glacier in latejuly occurs in conjunction with the opening of two further moulins in the accumulation zone

modelling the delivery of supraglacial meltwater to the ice bed interface application to southwest devon ice cap nunavut canada the transfer of surface generated meltwater to the subglacial drainage system through full ice thickness crevassing may lead to accelerated glacier velocities with implications for ice motion under future climatic scenarios accurate predictions of where surface meltwater accesses the ice bed interface are therefore needed in fully coupled hydrodynamic ice sheet models we present a spatially distributed modelling routine for predicting the location and timing of delivery of surface derived meltwater to the ice bed interface through moulins and supraglacial lake drainage the model is explained as it is applied to the croker bay glacial catchment of devon ice cap canada the formation of moulins drainage of lakes and the transfer of meltwater through the full ice thickness are modelled for the 2004 and 2006 ablation seasons through this case study we assess the model s sensitivity to degree day factors fracture toughness tensile strength and crevasse width and confirm that parameters influencing the rate at which water fills a crevasse are the most significant controls on the ability of a crevasse to reach the bed increased surface melt production therefore has the potential to significantly influence the spatial and temporal transfer of meltwater through surface to bed connections in a warmer climate

high resolution record of northern hemisphere climate extending into the last interglacial period high resolution record of northern hemisphere climate extending into the last interglacial period

new ways of studying ice sheet flow directions and glacial erosion by computer modelling examples from fennoscandia a computer ice sheet model has been used to study regional ice flow directions and glacial erosion of the weichselian ice sheet adopting a new method of presenting modelled ice flow directions ice sheet model results from different time periods during the weichselian were extracted for five regions and presented in rose diagrams when comparing these computer generated results with information on flow directions obtained from current conceptual geological models based on field data large similarities were observed in comparing the ice flow directions as such the similarities were strikingly good in many cases there was also a good agreement in the timing of the events while in some cases a certain flow direction was assigned to a different time period by the ice sheet model than in the current interpretation of geological information nevertheless the overall agreement between the data sets shows that results from ice sheet models can aid in placing geological information into a coarse timeframe and chronostratigraphic context and also fill in time gaps in the glacial geological record where chronological control is sparse ice sheet model results thus constitute a new data set against which glacial geological information can be compared and tested a new quantity basal sliding distance has also been calculated from the ice sheet model results describing the over time accumulated length of ice that has passed over the landscape by basal sliding the results show high basal sliding distance values in sw sweden se norway in skagerrak and along the gulf of bothnia implying relatively large amounts of glacial erosion in these regions on elevated parts of the scandinavian mountain range and on adjacent plains in the east the basal sliding distance values are low implying weaker glacial erosion this compares well with different types of geological and morphological data suggesting that basal sliding distance is a useful entity for studying regional patterns of glacial erosion intensity

sedimentation from turbulent jets and plumes theoretical models are developed for the sedimentation from the margins of a particle laden axisymmetric turbulent buoyant plume in a still environment and for an axisymmetric turbulent momentum jet the models assume that the mass of each individual size fraction of sediment carried in a parcel of fluid decreases exponentially with time for relatively coarse particles the fallout models predict that the sediment deposition beyond a distance r on the ground expressed in log units should decay linearly with distance away from the vent for the momentum jet and should decrease with r1 3 for the buoyant plume the exponential decay constant j is proportional to the terminal fall velocity vt of the particles in both cases and inversely proportional to the square root of the initial momentum flux m0 for the jet fallout jj vtmo 1 2 and to the third power of the initial buoyancy flux fo for the plume fallout jp vtfo 1 3 smaller particles are affected by reentrainment caused by the turbulent eddies sweeping ambient fluid back into the plume or jet and thus reincorporating some particles that were released from the flow at greater heights this is taken into account by introducing a reentrainment coefficient into the theoretical models with the assumption that the coefficient has a constant value for a plume of given strength in new experiments fallout occurs from the margins of particle laden fresh water buoyant jets and plumes in a tank of salty water and sedimentation is measured on the tank floor two experiments were weakly affected by reentrainment and show excellent agreement with the simple theory for smaller particles and increasingly buoyant plumes and strong jets particle reentrainment is important the experimental data are fitted by the new reentrainment theory confirming that values of the reentrainment coefficient are approximately constant for a given flow a settling number is defined as the ratio of the characteristic velocity of the jet or plume to the particle settling velocity for 1 reentrainment seems to reach an equilibrium state for which the reentrainment coefficient is a constant of value 0 1 for jets and 0 4 for plumes irrespective of flow strength or particle size the plume experiments indicate that the value of the reentrainment coefficient is strongly dependent on plume strength and particle size for slightly less than 1 the general principles of sedimentation from turbulent plumes and jets are applied to the fallout of pumice from volcanic eruption columns and of metalliferous particles from black smokers on the ocean floor for volcanic eruptions the results provide an explanation for the near vent overthickening of tephra fall deposits and imply that lithic and pumice fragments from small lapilli up to at least 1 m diameter blocks are efficiently reentrained into eruption columns the size of particles reentrained in hydrothermal plumes is predicted to vary from less than 100 m in weakly buoyant plumes up to over 1000 m in megaplumes

seasonal speedup of a greenland marine terminating outlet glacier forced by surface melt induced changes in subglacial hydrology we present subdaily ice flow measurements at four gps sites between 36 and 72 km from the margin of a marine terminating greenland outlet glacier spanning the 2009 melt season our data show that 35 km from the margin seasonal and shorter time scale ice flow variations are controlled by surface melt induced changes in subglacial hydrology following the onset of melting at each site ice motion increased above background for up to 2 months with resultant up glacier migration of both the onset and peak of acceleration later in our survey ice flow at all sites decreased to below background multiple 1 to 15 day speedups increased ice motion by up to 40 above background these events were typically accompanied by uplift and coincided with enhanced surface melt or lake drainage our results indicate that the subglacial drainage system evolved through the season with efficient drainage extending to at least 48 km inland during the melt season while we can explain our observations with reference to evolution of the glacier drainage system the net effect of the summer speed variations on annual motion is small 1 this in part is because the speedups are compensated for by slowdowns beneath background associated with the establishment of an efficient subglacial drainage system in addition the speedups are less pronounced in comparison to land terminating systems our results reveal similarities between the inland ice flow response of greenland marine and land terminating outlet glaciers

large scale changes in greenland outlet glacier dynamics triggered at the terminus the recent dramatic retreat of many outlet glaciers of the greenland ice sheet has raised concerns over greenland s contribution to future sea level rise simulations with a numerical ice flow model indicate that the recent rates of mass loss in greenland s outlet glaciers are transient and should not be extrapolated into the future

frozen bed fennoscandian and laurentide ice sheets during the last glacial maximum the areal extents of the laurentide and fennoscandian ice sheets during the last glacial maximum about 20 000 years ago are well known1 but thickness estimates range widely from high domed2 to thin3 with large implications for our reconstruction of the climate system regarding for example northern hemisphere atmospheric circulation and global sea levels this uncertainty stems from difficulties in determining the basal temperatures of the ice sheets and the shear strength of subglacial materials4 a knowledge of which would better constrain reconstructions of ice sheet thickness here we show that in the absence of direct data the occurrence of ribbed moraines in modern landscapes can be used to determine the former spatial distribution of frozen and thawed bed conditions we argue that ribbed moraines were formed by brittle fracture of subglacial sediments induced by the excessive stress at the boundary between frozen and thawed bed conditions resulting from the across boundary difference in basal ice velocity maps of glacial landforms from aerial photographs of canada and scandinavia reveal a concentration of ribbed moraines around the ice sheet retreat centres of quebec keewatin newfoundland and west central fennoscandia together with the evidence from relict landscapes that mark glacial areas with frozen bed conditions the distribution of ribbed moraines on both continents suggest that a large area of the laurentide and fennoscandian ice sheets was frozen based and therefore high domed and stable during the last glacial maximum

modelling late weichselian evolution of the eurasian ice sheets forced by surface meltwater enhanced basal sliding we simulated the late weichselian extent and dynamics of the eurasian ice sheets using theshallow ice approximation ice sheet model sicopolis our simulated last glacial maximum ice sheetextents cl

surface melt induced acceleration of greenland ice sheet flow ice flow at a location in the equilibrium zone of the west central greenland ice sheet accelerates above the midwinter average rate during periods of summer melting the near coincidence of the ice acceleration with the duration of surface melting followed by deceleration after the melting ceases indicates that glacial sliding is enhanced by rapid migration of surface meltwater to the ice bedrock interface interannual variations in the ice acceleration are correlated with variations in the intensity of the surface melting with larger increases accompanying higher amounts of summer melting the indicated coupling between surface melting and ice sheet flow provides a mechanism for rapid large scale dynamic responses of ice sheets to climate warming

capabilities and limitations of numerical ice sheet models a discussion for earth scientists and modelers the simulation of dynamically coupled ice sheet ice stream and ice shelf systems poses a challenge to most numerical ice sheet models here we review present ice sheet model limitations targeting

a sub millennial sediment record of ice stream retreat and meltwater storage in the baltic ice lake during the bolling allerod interstadial abstract the rapid retreat of the baltic ice stream and the development of the baltic ice lake is assessed using data from sediment cores retrieved from three sub basins in the southern baltic sea hydraulic piston coring by the international ocean discovery program iodp recovered for the first time intact glacial lake sequences overlying diamictons and other ice contact deposits at sites m0063 m0064 and m0065 based on the particle size and bulk sediment chemical composition the glacial lake sediments were subdivided into a proximal and a distal varve sequence the origin of a dark lithologically distinct horizon between the proximal and distal varves is attributed to a lake drainage event following the opening of a spillway in central sweden available age constraints suggest that the baltic ice lake developed during the bolling allerod interstadial and reached its maximum size at 13 ka ice retreat was forced by surface melt and amplified by calving in the upstream deepening lake environment furthermore rapid ice retreat and glacio isostatic processes allowed for the storage of substantial amounts of meltwater in the baltic ice lake during the allerod warm period subsequent lake drainage into the north atlantic took place through a conduit at higher latitude than previous drainage pathways the pronounced changes in meltwater storage and routing caused by the rapid retreat of the baltic ice stream may have contributed to abrupt climate change through the effects of changing freshwater supply on atlantic overturning circulation

flow variability in the scandinavian ice sheet modelling the coupling between ice sheet flow and hydrology abstract there is increasing geologic evidence for periodic flow variability within large ice sheets manifested as spatially and temporally variable areas of fast ice flow and resulting in the very complex patterns of lineations observed in formerly glaciated areas however many ice sheet models do not replicate this behaviour a possible reason for this is that such models do not include a detailed treatment of basal hydrology changes in the character of sub glacial drainage systems are believed to cause surges in valley glaciers recent ice sheet models which have included basal hydrology or at least a link between basal velocity and the presence of water at the bed often show flow variability however these models have typically assumed a deformable bed or have made no assumptions about the nature of the bed whilst these assumptions seem applicable to areas close to the former margins of quaternary ice sheets they are less applicable to interior areas these areas typically show thin or scanty till cover over eroded bedrock and the presence of eskers which are indicative of drainage in sub glacial tunnels we have developed a two dimensional time dependent ice sheet model that includes hard bed basal hydrology this allows calculation of sub glacial water pressures and the use of a water pressure dependent sliding law to calculate ice sheet velocities when used to simulate the weichselian scandinavian ice sheet with late quaternary climate and sea level as forcing functions this model develops localised areas of fast flowing ice which vary in extent and in distance of penetration into the interior of the ice sheet both spatially and temporally the behaviour of these lobes depends crucially on the influence of the evolving ice sheet topography on the routing of subglacial water flow due to the resulting variations in the subglacial hydraulic potential which drive the water flow bedrock topography also has some influence but fast flow areas are not confined to obvious topographic troughs a relatively thin ice sheet with low surface slopes is produced in areas experiencing fast ice flow generally two to four separate areas of fast flow can be recognised and these are similar in size and shape to the lobes identified in some geologically based reconstructions of the scandinavian ice sheet within the fast flowing areas sub glacial drainage is typically in a cavity based system however tunnel based drainage is predicted to have extended up to 150 km from the ice sheet margin particularly during deglaciation because of the changes in ice sheet topography associated with fast flow and the resulting changes in the pattern of sub glacial water flow the model predicts that these fast flowing lobes interacted in complex ways and exhibited quasi periodic switching between fast and slow flow

deglaciation of fennoscandia abstract to provide a new reconstruction of the deglaciation of the fennoscandian ice sheet in the form of calendar year time slices which are particularly useful for ice sheet modelling we have compiled and synthesized published geomorphological data for eskers ice marginal formations lineations marginal meltwater channels striae ice dammed lakes and geochronological data from radiocarbon varve optically stimulated luminescence and cosmogenic nuclide dating this is summarized as a deglaciation map of the fennoscandian ice sheet with isochrons marking every 1000 years between 22 and 13 cal kyr bp and every hundred years between 11 6 and final ice decay after 9 7 cal kyr bp deglaciation patterns vary across the fennoscandian ice sheet domain reflecting differences in climatic and geomorphic settings as well as ice sheet basal thermal conditions and terrestrial versus marine margins for example the ice sheet margin in the high precipitation coastal setting of the western sector responded sensitively to climatic variations leaving a detailed record of prominent moraines and other ice marginal deposits in many fjords and coastal valleys retreat rates across the southern sector differed between slow retreat of the terrestrial margin in western and southern sweden and rapid retreat of the calving ice margin in the baltic basin our reconstruction is consistent with much of the published research however the synthesis of a large amount of existing and new data support refined reconstructions in some areas for example the lgm extent of the ice sheet in northwestern russia was located far east and it occurred at a later time than the rest of the ice sheet at around 17 15 cal kyr bp we also propose a slightly different chronology of moraine formation over southern sweden based on improved correlations of moraine segments using new lidar data and tying the timing of moraine formation to greenland ice core cold stages retreat rates vary by as much as an order of magnitude in different sectors of the ice sheet with the lowest rates on the high elevation and maritime norwegian margin retreat rates compared to the climatic information provided by the greenland ice core record show a general correspondence between retreat rate and climatic forcing although a close match between retreat rate and climate is unlikely because of other controls such as topography and marine versus terrestrial margins overall the time slice reconstructions of fennoscandian ice sheet deglaciation from 22 to 9 7 cal kyr bp provide an important dataset for understanding the contexts that underpin spatial and temporal patterns in retreat of the fennoscandian ice sheet and are an important resource for testing and refining ice sheet models

the late weichselian glacial record in northern poland a new look at debris transport routes by the fennoscandian ice sheet abstract the area of the kashubian coastland and areas adjacent to the northern part of the lower vistula valley northern poland were involved in the major transgressive phases distinguished in poland during the late weichselian due to the small scale of the regressions between the different phases of the late weichselian the ice sheet usually deposited only one till but with a complex vertical profile based on the studies of the directional properties petrographic composition of the fine gravel fraction and the analysis of indicator erratics the dichotomy of the till bed was demonstrated the petrographic composition of the gravel fraction in tills together with the analysis of the indicator erratics and delimitation of the theoretical boulder centres were used to determine the direction of the influx of ice masses this allowed speculation on the reasons for the different dynamics of the fennoscandian ice sheet fis as well as the operation and time variability of the baltic ice stream bis it was emphasised that the delivery system of debris by the ice sheet might have been complex and multidirectional more than one route it has been suggested that changes in the petrographic composition of the gravel fraction of the glacial sediments did not have to be associated only with shifting the centre of glaciation but may have resulted from a change in the thermal regime of the ice sheet s bed in the source area especially with the spread of the cold regime zone and depended on the dynamics of the ice sheet stream or sheet flow as well as the spatial and temporal conditions of its development in the deposition area

seasonal variations in greenland ice sheet motion inland extent and behaviour at higher elevations abstract we present global positioning system observations that capture the full inland extent of ice motion variations in 2009 along a transect in the west greenland ice sheet margin in situ measurements of air temperature and surface ablation and satellite monitoring of ice surface albedo and supraglacial lake drainage are used to investigate hydrological controls on ice velocity changes we find a strong positive correlation between rates of annual ablation and changes in annual ice motion along the transect with sites nearest the ice sheet margin experiencing greater annual variations in ice motion 15 18 than those above 1000 m elevation 3 8 patterns in the timing and rate of meltwater delivery to the ice bed interface provide key controls on the magnitude of hydrologically forced velocity variations at each site in the lower ablation zone the overall contribution of variations in ice motion to annual flow rates is limited by evolution in the structure of the subglacial drainage system at sites in the upper ablation zone a shorter period of summer melting and delayed establishment of a hydraulic connection between the ice sheet surface and its bed limit the timeframe for velocity variations to occur our data suggest that land terminating sections of the greenland ice sheet will experience increased dynamic mass loss in a warmer climate as the behaviour that we observe in the lower ablation zone propagates further inland findings from this study provide a conceptual framework to understand the impact of hydrologically forced velocity variations on the future mass balance of land terminating sections of the greenland ice sheet

ribbed moraine formation ribbed rogen moraines are conspicuous landforms found in interior parts of formerly glaciated areas two major theories for ribbed moraine formation have been suggested in recent years i the shear and stack theory which explains ribbed moraine formation by shearing and stacking of till slabs or englacially entrained material during compressive flow followed by basal melt out of transverse moraine ridges and ii the fracturing theory according to which ribbed moraines form by fracturing of frozen pre existing till sheets at the transition from cold to warm based conditions under deglaciating ice sheets in this paper we present new data on the distribution of ribbed moraines and their close association with areas of frozen bed conditions under ice sheets in addition we show examples of ribbed moraine ridges that fit together like a jig saw puzzle these observations indicate that fracturing and extension of a pre existing till sheet may be a predominant process in ribbed moraine formation in summary we conclude that all described characteristics of ribbed moraines are compatible with the fracturing theory while the shear and stack theory is hampered by an inability to explain many conspicuous features in the distribution pattern and detailed morphology of ribbed moraines one implication of the fracturing theory is that the distribution of ribbed moraines can be used to reconstruct the extent of areas that underwent a change from frozen bed to thawed bed conditions under former ice sheets

glacial and glaciofluvial deposits in the interlobate areas of the scandinavian ice sheet abstract rapid flow within ice streams took place in a broad marginal zone of the scandinavian ice sheet during deglaciation ice streams and interlobate zones have been characterised by analysis of glacial flow patterns and examination of sedimentary features of interlobate glacial and glaciofluvial deposits reconstructed glaciodynamic systems are compared with ice streams of modem ice sheets wide interstream areas of inactive ice were left between some adjacent ice streams where they separated during the retreat of the margin these areas are characterised by morainic hummocks composed of both till and glaciofluvial sediment and sections show indications of subglacial deformation observations from fennoscandia suggest that these interstream areas were sites of net accumulation of glacial and glaciofluvial sediment during deglaciation glaciofluvial sediment was deposited in subglacial tunnels or cavities simultaneously with till deformation and deposition resulting from subglacial melting substantial glaciofluvial complexes were formed in narrow interlobate joints where adjacent ice lobes coalesced a case study made in southern finland shows converging flow patterns of two ice lobes and intervening glaciofluvial deposits concentration of meltwater in interlobate zones was a consequence of ice sheet configuration thinner ice in interlobate zones caused convergence of supraglacial and subglacial drainage the crevassed and strain softened ice may also have allowed surface meltwater to penetrate down to the ice bed interface close to the ice margin steep ice velocity gradients in interlobate zones would have increased subglacial melting rates and this environment was favourable for establishment of a subglacial drainage network and deposition of glaciofluvial sediment

simulation of fast flow features of the fennoscandian ice sheet during the last glacial maximum in order to reconstruct the palaeoglaciation in fennoscandia and north ern asia during the late weichselian ice age phase simulations with the dynamic and thermodynamic ice sheet model sicopolis are carried out our focus is on the last glacialmaximum lgm around20 kyr bp climateforcingisbasedonmeanannualsur face temperature and precipitation derived from present data and palaeoclimatic model ling intercomparison project pmip ukmo21results for the lgm these distributions areinterpolatedvia aglacial indexdefinedby thegreenlandicecoreproject grip d 18 o record the extent of the scandinavian and the barents ice sheets is reproduced in good agreement with the quaternary environments of the eurasian north queen recon struction but the kara sea and taymyr peninsula areas are excessively glaciated the fast flow regions derived from the simulations which are generally connected to regions with a temperate base and temperate ice above are compared to hypothesized palaeo ice stream locations especially in the norwegian channel and the baltic area in the norwegian channel temperate basal conditions with temperate ice above prevail and favour fast flow in the baltic area ice sheet advance is generally accompanied by slow ice velocities 5200ma 1 some temporary fast flow features occur due to transitional temperate base conditions and higher velocities arise in retreat phases

subglacial water storage and drainage beneath the fennoscandian and barents sea ice sheets abstract subglacial hydrology modulates how ice sheets flow respond to climate and deliver meltwater sediment and nutrients to proglacial and marine environments here we investigate the development of subglacial lakes and drainage networks beneath the fennoscandian and barents sea ice sheets over the late weichselian utilizing an established coupled climate ice flow model we calculate high resolution spatio temporal changes in subglacial hydraulic potential from ice sheet build up 37 ka bp to complete deglaciation 10 ka bp our analysis predicts up to 3500 potential subglacial lakes the largest of which was 658 km2 and over 70 of which had surface areas

short term variability in greenland ice sheet motion forced by time varying meltwater drainage implications for the relationship between subglacial drainage system behavior and ice velocity 1 high resolution measurements of ice motion along a 120 km transect in a land terminating section of the gris reveal short term velocity variations 1 day which are forced by rapid variations in meltwater input to the subglacial drainage system from the ice sheet surface the seasonal changes in ice velocity at low elevations 1000 m are dominated by events lasting from 1 day to 1 week although daily cycles are largely absent at higher elevations reflecting different patterns of meltwater input using a simple model of subglacial conduit behavior we show that the seasonal record of ice velocity can be understood in terms of a time varying water input to a channelized subglacial drainage system our investigation substantiates arguments that variability in theduration and rate rather than absolute volume of meltwater delivery to the subglacial drainage system are important controls on seasonal patterns of subglacial water pressure and therefore ice velocity we suggest that interpretations of hydro dynamic behavior in land terminating sections of the gris margin which rely on steady state drainage theories are unsuitable for making predictions about the effect of increased summer ablation on future rates of ice motion

melt induced speed up of greenland ice sheet offset by efficient subglacial drainage fluctuations in surface melting are known to affect the speed of glaciers and ice sheets but their impact on the greenland ice sheet in a warming climate remains uncertain although some studies suggest that greater melting produces greater ice sheet acceleration others have identified a long term decrease in greenland s flow despite increased melting here we use satellite observations of ice motion recorded in a land terminating sector of southwest greenland to investigate the manner in which ice flow develops during years of markedly different melting although peak rates of ice speed up are positively correlated with the degree of melting mean summer flow rates are not because glacier slowdown occurs on average when a critical run off threshold of about 1 4 centimetres a day is exceeded in contrast to the first half of summer when flow is similar in all years speed up during the latter half is 62 16 per cent less in warmer years consequently in warmer years the period of fast ice flow is three times shorter and overall summer ice flow is slower this behaviour is at odds with that expected from basal lubrication alone instead it mirrors that of mountain glaciers where melt induced acceleration of flow ceases during years of high melting once subglacial drainage becomes efficient a model of ice sheet flow that captures switching between cavity and channel drainage modes is consistent with the run off threshold fast flow periods and later summer speeds we have observed simulations of the greenland ice sheet flow under climate warming scenarios should account for the dynamic evolution of subglacial drainage a simple model of basal lubrication alone misses key aspects of the ice sheet s response to climate warming

numerical reconstructions of the eurasian ice sheet and climate during the late weichselian abstract geological investigations undertaken through the quaternary environments of the eurasian north programme established ice sheet limits for the eurasian arctic at the last glacial maximum lgm sedimentary records of palaeo ice streams and uplift information relating to ice sheet configuration and the pattern of deglaciation ice sheet numerical modelling was used to reconstruct a history of the eurasian ice sheet compatible with these geological datasets the result was a quantitative assessment of the time dependent behaviour of the ice sheet its mass balance and climate and predictions of glaciological products including sediments icebergs and meltwater at the lgm ice cover was continuous from scandinavia to the arctic ocean margin of the barents sea to the north and the kara sea to the east in the west along the continental margin between the norwegian channel and svalbard the ice sheet was characterised by fast flowing ice streams occupying bathymetric troughs which fed large volumes of sediment to the continental margin that were deposited as a series of trough mouth fans ice streams may also have been present in bathymetric troughs to the north between svalbard and franz josef land further east however the ice sheet was thinner across the kara sea the ice thickness was predicted to be less than 300 m while on severnaya zemlya the ice cover may have been thinner at the lgm than at present it is likely that the taymyr peninsula was mainly free of ice at the lgm in the south the ice margin was located close to the shoreline of the russian mainland the climate associated with this ice sheet is maritime to the west and in stark contrast desert like in the east atmospheric general circulation modelling has revealed that such a contrast is possible under relatively warm north atlantic conditions because a circulation system develops across the kara sea isolating it from the moisture laden westerlies which are diverted to the south ice sheet decay began through enhanced iceberg calving in the deepest regions of the barents sea which caused a significant ice embayment within the bear island trough by about 12 000 years ago further iceberg calving reduced ice extent to the northern archipelagos and their surrounding shallow seas ice decay was complete by about 10 000 years ago

paleoglaciological reconstructions for the tibetan plateau during the last glacial cycle evaluating numerical ice sheet simulations driven by gcm ensembles abstract the tibetan plateau is a topographic feature of extraordinary dimension and has an important impact on regional and global climate however the glacial history of the tibetan plateau is more poorly constrained than that of most other formerly glaciated regions such as in north america and eurasia on the basis of some field evidence it has been hypothesized that the tibetan plateau was covered by an ice sheet during the last glacial maximum lgm abundant field and chronological evidence for a predominance of local valley glaciation during the past 300 000 calendar years that is 300 ka coupled to an absence of glacial landforms and sediments in extensive areas of the plateau now refute this concept this furthermore calls into question previous ice sheet modeling attempts which generally arrive at ice volumes considerably larger than allowed for by field evidence surprisingly the robustness of such numerical ice sheet model results has not been widely queried despite potentially important climate ramifications we simulated the growth and decay of ice on the tibetan plateau during the last 125 ka in response to a large ensemble of climate forcings 90 members derived from global circulation models gcms using a similar 3d thermomechanical ice sheet model as employed in previous studies the numerical results include as extreme end members as an ice free tibetan plateau and a plateau scale ice sheet comparable in volume to the contemporary greenland ice sheet we further demonstrate that numerical simulations that acceptably conform to published reconstructions of quaternary ice extent on the tibetan plateau cannot be achieved with the employed stand alone ice sheet model when merely forced by paleoclimates derived from currently available gcms progress is however expected if future investigations employ ice sheet models with higher resolution bidirectional ice sheet atmosphere feedbacks improved treatment of the surface mass balance and regional climate data and climate reconstructions

fracture propagation as means of rapidly transferring surface meltwater to the base of glaciers 1 propagation of water filled crevasses through glaciers is investigated based on the linear elastic fracture mechanics approach a crevasse will penetrate to the depth where the stress intensity factor at the crevasse tip equals the fracture toughness of glacier ice a crevasse subjected to inflow of water will continue to propagate downward with the propagation speed controlled primarily by the rate of water injection while the far field tensile stress and fracture toughness determine where crevasses can form once initiated the rate of water driven crevasse propagation is nearly independent of these two parameters thus rapid transfer of surface meltwater to the bed of a cold glacier requires abundant ponding at the surface to initiate and sustain full thickness fracturing before refreezing occurs

fennoscandian palaeoglaciology reconstructed using a glacial geological inversion model the evolution of ice sheet configuration and flow pattern in fennoscandia through the last glacial cycle was reconstructed using a glacial geological inversion model i e a theoretical model that formalises the procedure of using the landform record to reconstruct ice sheets the model uses mapped flow traces and deglacial melt water landforms as well as relative chronologies derived from cross cutting striae and till lineations as input data flow trace systems were classified into four types i time transgressive wet bed deglacial fans ii time transgressive frozen bed deglacial fans iii surge fans and iv synchronous non deglacial event fans using relative chronologies and aggregation of fans into glaciologically plausible patterns a series of ice sheet configurations at different time slices was erected a chronology was constructed through correlation with dated stratigraphical records and proxy data reflecting global ice volume geological evidence exists for several discrete ice sheet configurations centred over the scandinavian mountain range during the early weichselian the build up of the main weichselian fennoscandian ice sheet started at approximately 70 ka and our results indicate that it was characterised by an ice sheet with a centre of mass located over southern norway this configuration had a flow pattern which is poorly reproduced by current numerical models of the fennoscandian ice sheet at the last glacial maximum the main ice divide was located overthe gulf of bothnia a major bend in the ice divide was caused by outflow of ice to the northwest over the lowest part of the scandinavian mountain chain widespread areas of preserved pre late weichselian landscapes indicate that the ice sheet had a frozen bed core area which was only partly diminished in size by inward transgressive wet bed zones during the decay phase

deglaciation of the eurasian ice sheet complex the eurasian ice sheet complex eisc was the third largest ice mass during the last glacial maximum with a span of over 4500 km and responsible for around 20 m of eustatic sea level lowering whilst recent terrestrial and marine empirical insights have improved understanding of the chronology pattern and rates of retreat of this vast ice sheet a concerted attempt to model the deglaciation of the eisc honouring these new constraints is conspicuously lacking here we apply a first order thermomechanical ice sheet model validated against a diverse suite of empirical data to investigate the retreat of the eisc after 23 ka bp directly extending the work of patton et al 2016 who modelled the build up to its maximum extent retreat of the ice sheet complex was highly asynchronous reflecting contrasting regional sensitivities to climate forcing oceanic influence and internal dynamics most rapid retreat was experienced across the barents sea sector after 17 8 ka bp when this marine based ice sheet disintegrated at a rate of 670 gigatonnes per year gt a 1 through enhanced calving and interior dynamic thinning driven by oceanic atmospheric warming and exacerbated by eustatic sea level rise from 14 9 to 12 9 ka bp the eisc lost on average 750 gt a 1 peaking at rates 3000 gt a 1 roughly equally partitioned between surface melt and dynamic losses and potentially contributing up to 2 5 m to global sea level rise during meltwater pulse 1a independent glacio isostatic modelling constrained by an extensive inventory of relative sea level change corroborates our ice sheet loading history of the barents sea sector subglacial conditions were predominately temperate during deglaciation with over 6000 subglacial lakes predicted along with an extensive subglacial drainage network moreover the maximum eisc and its isostatic footprint had a profound impact on the proglacial hydrological network forming the fleuve manche mega catchment which had an area of 2 5 106 km2 and drained the present day vistula elbe rhine and thames rivers through the seine estuary during the bolling allerod oscillation after c 14 6 ka bp two major proglacial lakes formed in the baltic and white seas buffering meltwater pulses from eastern fennoscandia through to the younger dryas when these massive proglacial freshwater lakes flooded into the north atlantic ocean deglaciation temporarily abated during the younger dryas stadial at 12 9 ka bp when remnant ice across svalbard franz josef land novaya zemlya fennoscandia and scotland experienced a short lived but dynamic re advance the final stage of deglaciation converged on present day ice cover around the scandes mountains and the barents sea by 8 7 ka bp although the phase lagged isostatic recovery still continues today

hydrological controls on patterns of surface internal and basal motion during three spring events haut glacier d arolla switzerland three early melt season high velocity events or spring events occurred on haut glacier d arolla switzerland during the melt seasons of 1998 and 1999 the events involve enhanced glacier velocity during periods of rapidly increasing bulk discharge in the proglacial stream and high subglacial water pressures however differences in spatial patterns of surface velocity internal ice deformation rates the spatial extent of high subglacial water pressures and in rates of subglacial sediment deformation suggest different hydrological and mechanical controls the data from two of the events suggest widespread ice bed decoupling particularly along a subglacial drainage axis creating the highest rates of basal motion and plug flow in the overlying ice the other event showed evidence of less extensive ice bed decoupling and sliding along the drainage axis with more mechanical support for ice overburden transferred to areas adjacent to decoupled areas we suggest that 1 plug flow may be a common feature on glaciers experiencing locally induced reductions in basal drag 2 under certain circumstances enhanced surface motion may be due in part to non locally forced enhanced bed deformation and 3 subglacial sediment deformation is confined to a depth of the order of centimetres to decimetres

calibrated prediction of pine island glacier retreat during the 21st and 22nd centuries with a coupled flowline model a flowline ice sheet model is coupled to a box model for cavity circulation and configured for the pine island glacier an ensemble of 5000 simulations are carried out from 1900 to 2200 with varying inputs and parameters forced by ocean temperatures predicted by a regional ocean model under the a1b business as usual emissions scenario comparison is made against recent observations to provide a calibrated prediction in the form of a 95 confidence set predictions are for monotonic apart from some small scale fluctuations in a minority of cases retreat of the grounding line over the next 200 yr with huge uncertainty in the rate of retreat full collapse of the main trunk of the pig during the 22nd century remains a possibility

the greenland ice core chronology 2005 15 42 ka part 1 constructing the time scale the greenland ice core chronology 2005 gicc05 is extended back to 42 ka b2k before 2000 ad i e to the end of greenland stadial 11 the chronology is based on independent multi parameter counting of annual layers using comprehensive high resolution measurements available from the north greenland ice core project ngrip these are measurements of visual stratigraphy conductivity of the solid ice electrolytical melt water conductivity and the concentration of na c a 2 s o 4 n o 3 n h 4 a n uncertainty estimate of the time scale is obtained from identification of uncertain annual layers which are counted as 0 570 5 years the sum of the uncertain annual layers the so called maximum counting error of the presented chronology ranges from 4 in the warm interstadial periods to 7 in the cold stadials the annual accumulation rates of the stadials and interstadials are on average one third and half of the present day values respectively and the onset of the greenland interstadials 2 3 and 8 based on 20 year averaged d 18 o

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

evolution of drainage system morphology at a land terminating greenlandic outlet glacier 1 the influence of meltwater on the dynamics and geomorphic impact of the greenland ice sheet is strongly controlled by the morphology of the ice sheet s drainage system however this system and its evolution through the melt season remain poorly understood here we present the results of an intensive programme of dye tracing experiments undertaken along the lower 14 km of a land terminating greenlandic outlet glacier over a period of four months during the 2010 melt season these data are interpreted in conjunction with observations of proglacial discharge englacial water storage surface melt rates and ice velocity to produce a detailed picture of the changing hydrology of the glacier following the onset of melt in the spring inputs to the drainage system regularly exceed outputs causing the englacial water level to rise to the ice sheet surface during this time there is a rapid transition from distributed to channelized drainage in those parts of the drainage system closed by ice deformation over winter as the melt season progresses channel efficiency increases and englacial storage and ice velocity decrease high velocity events continue to be observed following the channelization of the drainage system however indicating that hydrological forcing of ice velocity occurs despite the existence of channels during periods when meltwater inputs exceed the capacity of the subglacial drainage system

millennial scale variability during the last glacial the ice core record the oxygen isotope records from greenland ice cores show a very strong reproducible pattern of alternation between warm greenland interstadials gi and cold greenland stadials gs at millennial scale during the last glacial period here we summarise what is known about this variability from ice core records the typical cycle has a sawtooth pattern with a very rapid warming event occurring in a few decades a slow cooling trend and then a final fast cooling 25 such events have been numbered in the last glacial the recent gicc05 age scale provides the best available age scale that can be directly applied to this stratigraphy and we summarise the timing of the warming events and the length and strength of each event the greenland stratigraphy can be transferred to other records if we make assumptions about the contemporaneous nature of rapid events in different archives other parameters such as the snow accumulation rate and the concentration of terrestrial dust and sea salt recorded in the greenland cores also show a strong contrasting pattern between gi and gs methane concentrations are generally high during gi and lower during gs with the increase from gs to gi occurring within a century antarctic ice cores show a different pattern each gi has an antarctic counterpart but antarctica appears to warm while greenland is in a gs and cool during gi these changes are consistent with a mechanism involving ocean heat transport but the rapid nature of warmings poses a challenge for modellers while the rapid methane changes pose questions about the pattern of land biosphere emissions during the glacial that are also relevant for understanding glacial interglacial methane variability

evidence from fram strait 78 n for early deglaciation recent syntheses of the history of the last northern hemisphere glaciation and deglaciation illustrate that understanding of the mechanisms and timing of deglaciation before approximately 12 000 years bp1 5 is limited after 12 000 yr bp however there is sufficient evidence from radiocarbon dated moraines raised beaches varved lake sediments and pollen records to provide a reasonable temporal and geographic picture of the decay of the ice sheets here we report on the first oxygen isotope record from the norwegian greenland sea that is radiocarbon dated directly by accelerator mass spectrometry a significant light oxygen isotope event is recorded at approximately 15 000 years bp which suggests that the marine based barents shelf ice sheet disintegrated rapidly at this time recent studies6 7 have estimated that the decay of this ice sheet could have contributed as much as 15 metres to eustatic sea level rise the decay of the barents shelf ice sheet is the earliest major deglacial event yet dated and may have triggered subsequent deglacial events through eustatic sea level effects