expansion of ice marginal lakes at the vatnajokull ice cap iceland from 1999 to 2009 abstract outlet glaciers from the vatnajokull ice cap in iceland europe s largest ice cap have been retreating since the end of the little ice age only interrupted by short intervals of advances the net mass balance has been negative since 1995 and ice marginal lakes in front of the outlets have been expanding since then the switch from land based glacier termini to lake terminating glaciers is an eye catching environmental change the heat capacity of the ice marginal lakes and the change into calving glacier fronts accelerates ice loss from vatnajokull landsat 7etm satellite imagery from 1999 to 2009 reveals high resolution snapshots of the lake development during the last decade ice marginal lakes are growing at many outlet glaciers draining the southern sector of vatnajokull this is most pronounced at the large brei amerkurjokull glacier where the jokulsarlon lake expanded from 15 km 2 in 2000 to more than 21 km 2 in 2009 the lakes act as traps for sediment that was earlier transported across outwash plains toward the atlantic coast hence the glacier retreat and sediment trapping enhance coastline retreat and river incision in the outwash plains similar changes although at a much larger scale occurred in many past glaciated regions during the last glacial termination thus the lakes in front of vatnajokull provide important modern analogues for processes that likely impacted global climate in the past

outlet glacier response to forcing over hourly to interannual timescales jakobshavn isbrae greenland the loss of the floating ice tongue on jakobshavn isbrae greenland in the early 2000s has been concurrent with a pattern of thinning retreat and acceleration leading to enhanced contribution to global sea level these changes on decadal timescales have been well documented here we identify how the glacier responds to forcings on shorter timescales such as from variations in surface melt the drainage of supraglacial lakes and seasonal fluctuations in terminus position ice motion and surface melt were monitored intermittently from 2006 to 2008 dual frequency gps were deployed 20 50 km upstream of the terminus along the glacier center line gaps in surface melt measurements were filled using a temperature index model of ablation driven by surface air temperatures recorded during the same time period our results corroborate the premise that the primary factors controlling speeds on jakobshavn isbrae are terminus position and geometry we also observe that surface speeds demonstrate a complex relationship with meltwater input on diurnal timescales velocities closely match changes in water input however on seasonal timescales a longer more intense melt season was observed to effectively reduce the overall ice flow of the glacier for the whole year

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

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

the effect of the subglacial water pressure on the sliding velocity of a glacier in an idealized numerical model in order to interpret observed short term variations of the sliding velocity of a glacier the effect of a variable subglacial water pressure on the sliding velocity has been studied using an idealized numerical model in particular the transient stages of growing or shrinking water filled cavities at the ice bedrock interface were analysed it was found that the sliding velocity was larger when cavities were growing than when they had reached the steady state size for a given water pressure the smallest sliding velocities occurred while cavities were shrinking when cavitation is substantial a small drop of water pressure below the steady state value e g by 0 5 bar can temporarily cause backward sliding a limiting water pressure at which sliding becomes unstable is derived the consequences of more realistic assumptions than those of the model are discussed

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

ice front variation and tidewater behavior on helheim and kangerdlugssuaq glaciers greenland we used satellite images to examine the calving behavior of helheim and kangerdlugssuaq glaciers greenland from 2001 to 2006 a period in which they retreated and sped up these data show that many large iceberg calving episodes coincided with teleseismically detected glacial earthquakes suggesting that calving related processes are the source of the seismicity for each of several events for which we have observations the ice front calved back to a large pre existing rift these rifts form where the ice has thinned to near flotation as the ice front retreats down the back side of a bathymetric high which agrees well with earlier theoretical predictions in addition to the recent retreat in a period of higher temperatures analysis of several images shows that helheim retreated in the 20th century during a warmer period and then re advanced during a subsequent cooler period this apparent sensitivity to warming suggests that higher temperatures may promote an initial retreat off a bathymetric high that is then sustained by tidewater dynamics as the ice front retreats into deeper water the cycle of frontal advance and retreat in less than a century indicates that tidewater glaciers in greenland can advance rapidly greenland s larger reservoir of inland ice and conditions that favor the formation of ice shelves likely contribute to the rapid rates of advance

seismic reflection evidence for a deep subglacial trough beneath jakobshavns isbrae west greenland seismic reflection methods were used to determine the ice thickness and basal topography of jakobshavns isbrae a large fast moving ice stream outlet glacier in west greenland a method of data analysis was developed which involves the pointwise migration of data from a linear seismic array and a single explosive source the method yields the depth horizontal position and slope of the basal reflector a deep u shaped subglacial trough was found beneath the entire length of the well defined ice stream the trough is incised up to 1500 m into bedrock and its base lies 1200 1500 m below sea level for at least 70km inland center line ice thickness along most of the channel is about 2500 m or about 2 5 times that of the surrounding ice sheet this prominent bedrock trough was not apparent in existing radio echo sounding data reflection coefficients indicate that much of the basal interface is probably underlain by compacted non deforming sediment the large ice thickness coupled with relatively steep surface slopes leads to high basal shear stresses 200 300 kpa along the ice stream the large shear stresses and lack of a deformable bed imply that internal deformation plays a dominant role in the dynamics of jakobshavns isbrae

relating crevassing to non linear strain in the floating part of jakobshavn isbrae west greenland jakobshavn isbr is a major ice stream that drains the west central greenland ice sheet and becomes afloat in jakobshavn isfjord 69 n 49 w where it has maintained the world s fastest known sustained velocity and calving rate 7kma 1 forat least four decades thefloating portion isapproximately12km longand 6kmwide surface elevations and motion vectors were determined photogrammetrically for about 500 crevasses on the floating ice and adjacent grounded ice using aerial photographs obtained 2 weeks apart injuly l985 surface strain rates were computed from a mesh of 399 quadrilateral elements having velocity measurements ateach corner it is shownthat heavycrevassing of floating ice invalidatesthe assumptions of linear straintheory that i surface strain in the floating ice is homogeneous in both space and time ii the squares and products of strain components are nil and iii first and second order rotation com ponents are small compared to strain components therefore strain rates and rotation rates were also computed using non linear strain theory the percentage difference betweencomputedlinearandnon linear secondinvariantsof strainrateperelementwere greatest mostly in the range 40 70 where crevassing is greatest isopleths of strain rate parallel and transverse to flow and elevation isopleths relate crevassing to known and inferred pinning points

force perturbation analysis of recent thinning and acceleration of jakobshavn isbrae greenland observations between 1997 and 2001 of a 30 velocity increase and up to 60 m thinning of downstream parts of jakobshavn isbrae greenland immediately following calving of about 4 km of its 15 km floating ice tongue suggest that acceleration may have been initiated by the calving assuming that the force perturbation associated with such weakening is swiftly transmitted far up glacier i develop equations to estimate the perturbation initially the observed changes are consistent with the comparatively small perturbation associated with the calving thereafter it was probably sustained by thinning of the remaining ice tongue at rates of about 80 m a 1 otherwise the force perturbation would soon have been balanced by reduction in the hydrostatic driving force for longitudinal creep as the glacier thinned with velocities dropping to their former values the calculated force perturbation increases to a maximum about 10 km inland of the grounding line consistent with decreasing weight forces as the glacier thins over bedrock that slopes uphill seawards further inland it progressively decreases probably because marginal drag increased as the glacier accelerated both here and on the floating tongue marginal ice appears to have been softened by the influence of locally intense shear on ice temperature and or fabric more recent observations show continued acceleration and thinning and most of the remaining ice tongue calved away in april 2003 so thinning is likely to continue

new constraints on greenland ice sheet dynamics during the last glacial cycle evidence from the uummannaq ice stream system this paper presents the first assessment of the uummannaq ice stream system uiss in west greenland the uiss drained 6 of the greenland ice sheet gris at the last glacial maximum lgm the onset of the uiss is a function of a convergent network of fjords which feed a geologically controlled trough system running offshore to the shelf break mapping cosmogenic radiogenic nuclide crn dating and model output reveal that glacially scoured surfaces up to 1266 m above sea level asl in fjord head areas were produced by warm based ice moving offshore during the lgm with the elevation of warm based ice dropping westwards to 700 m asl as the ice stream trunk zone developed marginal plateaux with allochthonous blockfields suggest that warm based ice produced till and erratics up to 1200 m asl but crn ages and weathering pits suggest this was pre lgm with only cold based ice operating during the lgm deglaciation began on the outer shelf at 14 8 cal kyrs b p with ubekendt ejland becoming ice free at 12 4 ka the uiss then collapsed with over 100 km of retreat by 11 4 ka 10 8 cal kyrs b p a rapid and complex response to bathymetric deepening trough widening and sea level rise coinciding with rapidly increasing air temperatures and solar radiation but which occurred prior to ocean warming at 8 4 cal kyrs b p local fjord constriction temporarily stabilized the unzipped uiss margins at the start of the holocene before ice retreat inland of the current margin at 8 7 ka

short term flow dynamics of a retreating tidewater glacier leconte glacier alaska u s a an analysis of motion in the terminus region of leconte glacier alaska u s a delineates mechanisms that are important to tidewater glacier stability this glacier is undergoing rapid retreat since 1995 it has retreated 2 km and thinned 125 m at the present location of the terminus ice velocities in this region are quite high at the terminus they exceed 27 m d 1 our analysis reveals that fluctuations in speed and surface elevation are forced by ocean tides surface melt and precipitation the near terminus ice speed is 180 out of phase with the tide such that high tide corresponds to low speed smaller melt forced diurnal variations in speed are present throughout the lowermost 7 km speed ups in excess of 10 of the mean often occur after rainstorms as a result of increased basal water pressure and storage but the relation is not simple the velocity does not vary over the spring and summer seasons

the role of lateral drag in the dynamics of ice stream b antarctica the partitioning of resistive force between the bed and sides of ice stream b antarctica is obtained for three large areas that have been measured using repeat aerial photogrammetry problems associated with data errors and local variations in ice strength and velocity are reduced by considering the areally averaged budget of forces for each photo block results indicate that the bed under ice stream b must be very weak and unable to provide muche resistance mechanical control on this ice stream emanates almost entirely from the lateral margins

subglacial hydrology in north western germany during the last glaciation groundwater flow tunnel valleys and hydrological cycles abstract numerical modelling of groundwater dynamics in an overpressured system of subglacial aquifers and aquitards under the marginal portion of the scandinavian ice sheet in northwestern germany has been coupled with calculations of basal meltwater production rates and with field evidence of meltwater erosional features largely fine grained subglacial sediments with relatively low hydraulic conductivities had a capacity to drain only about 25 of all basal meltwater produced in the relevant subglacial catchment area the rest of the meltwater was evacuated through tunnel valleys in spontaneous outburst events subglacial drainage cycles are proposed in which each cycle begins with meltwater drainage through the substratum groundwater flow followed by water ponding at the ice bed interface and ends in catastrophic meltwater releases through tunnel valleys

surficial glaciology of jakobshavns isbrae west greenland part i surface morphology jakobshavns isbrae is a large fast moving ice stream outlet glacier in west greenland which ends at a floating calving front it drains about 6 5 of the area of the greenland ice sheet studies of its surface morphology are described in this paper the surface is relatively steep 0 01 0 03 and the thickness is large up to 2600 m along the center line clarke and echelmeyer 1989 indicating very high driving stresses 200 300 kpa the ice stream is about 6 km wide and 85 90 km long all of which is in an area of surface melting the base of the ice stream and of much of the drainage area is below sea level marginal crevasse zones have a width on the order of the width of the ice stream itself unique surficial features are ice blisters and lakes the latter have a sequence of ogive like features on their floating ice cover which can be used to determine velocity there is a pinning point near the terminus which may act as a stabilizing influence possibly playing a role in halting at least temporarily a recent retreat of the terminus ice thickness estimates at the terminus lead to a flux which is less than previously assumed by others e g bindschadler 1984 pelto and others 1989 when estimating jakobshavns isbrae s drainage basin to be nearly in balance the driving stresses on jakobshavns isbrae are an order of magnitude higher than those of the ice streams of west antarctica its crevasse patterns are much less localized its relatively unconfined terminus is more comparable to that of relatively unbuttressed ice streams such as pine island and thwaites glaciers than it is to other west antarctic ice streams which terminate in large confined ice shelves

drainage networks lakes and water fluxes beneath the antarctic ice sheet antarctica bedmap2 datasets are used to calculate subglacial hydraulic potential and the area depth and volume of hydraulic potential sinks there are over 32 000 contiguous sinks which can be thought of as predicted lakes patterns of subglacial melt are modelled with a balanced ice flux flow model and water fluxes are cumulated along predicted flow pathways to quantify steady state fluxes from the main basin outlets and from known subglacial lakes the total flux from the continent is 21 km3 a 1 byrd glacier has the greatest basin flux of 2 7 km3 a 1 fluxes from subglacial lakes range from 1 10 4 to 1 5 km3 a 1 lake turnover times are calculated from their volumes and fluxes and have median values of 100 a for known active lakes and 500 a for other lakes recurrence intervals of a 0 25 km3 flood range from 2 months to 2000 a median 130 a for known active lakes and from 2 to 2400 a median 360 a for other lakes thus several lakes that have recently been observed to fill and drain may not do so again for many centuries and several lakes that have not so far been observed to fill and drain have the potential to do so even at annual to decadal timescales

drainage from water filled crevasses along the margins of jakobshavn isbrae a potential catalyst for catchment expansion 1 saturated crevasses occur in local depressions within the shear margins of jakobshavn isbrae at inflections in the ice stream s flow direction spatio temporal variability of seven distinctive saturated crevasse groups was examined during the 2007 melt season the area of saturated crevasses reached its maximum extent 1 8 km2 in early july and remained largely constant until early august filling rates are correlated with regional melt production while drainage rates are highly correlated with areal extent estimates on potential drainage volume from the largest crevasse system are 9 23 10 3 km3 2 15 10 8 km3 and 4 92 10 2 km3 3 58 10 8 km3 respectively over a 16 day interval and are more than required for a distributed basal hydrologic system across this area to temporarily flood bedrock obstacles believed to control basal sliding future drainage events likely extending farther inland with warming could result in enhanced lateral mass discharge into the ice stream with implications for the dynamic evolution of the entire basin

jakobshavns glacier drainage basin a balance assessment maximum and minimum estimates are made of the drainage basin feeding the jakobshavns glacier by using surface elevation maps derived from seasat altimetry benson s 1962 net balance measurements are used to calculate the balance flow within the basin comparisons of the balance flux at the terminus with estimates of actual flux suggest the basin is in overall equilibrium or slightly thickening this agrees with measurements along the nearby egig traverse balance velocities accelerate rapidly within 100 km of the coast farther upstream balance velocities are consistent with both measured velocities along the egig traverse and calculated deformation velocities it is estimated that jakobshavns glacier discharges between 4 8 and 7 6 of the annual net balance over greenland and drains between 3 7 and 5 8 of the ice sheet area

a new bedrock and surface elevation dataset for modelling the greenland ice sheet a new digital elevation model of the surface of the greenland ice sheet and surrounding rock outcrops has been produced from a comprehensive suite of satellite and airborne remote sensing and cartographic data sets the surface model has been regridded to a resolution of 5 km and combined with a new ice thickness grid derived from ice penetrating radar data collected in the 1970s and 1990s a further data set the international bathymetric chart of the arctic ocean was used to extend the bed elevations to include the continental shelf the new bed topography was compared with a previous version used for ice sheet modelling near the margins of the ice sheet and in particular in the vicinity of small scale features associated with outlet glaciers and rapid ice motion significant differences were noted this was highlighted by a detailed comparison of the bed topography around the north east greenland ice stream

the flow of a polythermal glacier mccall glacier alaska u s a we have analyzed the flow of polythermal mccall glacier in arctic alaska using measurements of surface velocity from the 1970s and 1990s together with measurements of ice thickness and surface slope we have investigated both the present flow and seasonal and long term flow variations our analysis of the present flow reveals that i longitudinal stress coupling is important along the entire length of the glacier and ii there is significant basal sliding beneath a 2 km long section of the lower glacier this sliding exists year round and it accounts for more than 70 of the total motion there we have developed a numerical model which shows that such a sliding anomaly causes an asymmetric decrease in ice thickness accompanying this decrease in thickness is a decrease in surface slope at the center of the anomaly and an increase in slope up glacier from it both effects are reflected in the observed surface profile of mccall glacier the longitudinal stress coupling length of mccall glacier is three times the ice thickness almost twice that typical of temperate glaciers this is a direct effect of lower strain rates which themselves are associated with the smaller mass balance gradients of arctic and continental glaciers long term variations in surface velocity between the 1970s and 1990s are explained solely by the effects of changes in glacier geometry on the deformational flow contribution this means that long term variations in the spatial patterns of longitudinal stresses and basal sliding must have been small seasonally velocities reach their annual minimum in spring and increase during the short summer melt season by up to 75 above mean winter values however the extra motion associated with the period of elevated velocities is only about 5 of the total annual motion the speed up is due to an increase in basal sliding this implies that most of the glacier bed is at the melting point the zone affected by the melt season speed up extends well up glacier of any moulins or other obvious sources for meltwater at the bed

waves of accelerated motion in a glacier approaching surge the mini surges of variegated glacier alaska u s a periods of dramatically accelerated motion in which the flow velocity increases suddenly from about 55 cm d to a peak of 100 300 cm d and then decreases gradually over the course of a day occurred repeatedly during june and july 1978 81 in variegated glacier alaska a surging type glacier that surged in 1982 83 these mini surges appear to be related mechanistically to the main surge the flow velocity peak propagates down glacier as a wave at a speed of about 0 3 km h over a reach of about 6 km in length it is accompanied by a propagating pressure wave in the basal water system of the glacier in which after a preliminary drop the pressure rises rapidly to a level greater than the ice overburden pressure at the glacier bed and then drops gradually over a period of 1 2 d usually reaching a new low for the summer the peak velocity is accompanied by a peak of high seismic activity due to widespread fresh crevassing it is also accompanied by a rapid uplift of the glacier surface amounting to 6 11 cm which then relaxes over a period of 1 2 d maximum uplift rate coincides with the peak in flow velocity the peak in accumulated uplift lags behind the velocity peak by 2 h the uplift is mainly due to basal cavitation driven by the high basal water pressure although the strain wave associated with the mini surge motion can also contribute basal cavitation is probably responsible for the pulse of high turbidity that appears in the terminal outflow stream in association with each mini surge in the down glacier reach where the mini surge waves are attenuating the observed strain wave corresponds to what is expected for the propagating pulse in flow velocity but in the reach of maximum mini surge motion the strain wave has a form quite different possibly related to special features in the mini surge initiation process from that point up stream the flow acceleration in the mini surges is due to enhanced basal sliding caused by the high basal water pressure and the consequent reduction of bed friction a preliminary velocity increase shortly before the pressure wave arrives is caused by the forward shove that the main accelerated mass exerts on the ice ahead of it and the resulting preliminary basal cavitation causes the drop in water pressure shortly before the pressure wave arrives the mini surge wave propagation is controlled by the propagation of the water pressure wave in the basal water conduit system the propagation characteristics result from a longitudinal gradient up glacier increase in hydraulic conductivity of the basal water system in response to the up glacier increase of the basal water pressure in the mini surge wave the mini surge waves are initiated in a succession of areas situated generally progressively up glacier during the course of the summer season in these areas presumably melt water that has accumulated in subglacial reservoirs is released suddenly into the basal water system immediately below generating a pressure rise that propagates down stream from there relationships of the mini surges to the main surge are seen in the role of high basal water pressure in causing the rapid glacier motion in both phenomena in the pulse propagation features of both and in the high outflow turbidity associated with both the mini surges of variegated glacier have a strong resemblance to movement and uplift events observed in unteraargletscher and findelengletscher switzerland this bears on the question whether the mini surges are a particular characteristic of surge type glaciers prior to surge

submarine melting of the 1985 jakobshavn isbrae floating tongue and the triggering of the current retreat 1 photogrammetric reanalysis of 1985 aerial photos has revealed substantial submarine melting of the floating ice tongue of jakobshavn isbrae west greenland the thickness of the floating tongue determined from hydrostatic equilibrium tapers from 940 m near the grounding zone to 600 m near the terminus feature tracking on orthophotos shows speeds on the july 1985 ice tongue to be nearly constant 18 5 m d 1 indicating negligible dynamic thinning the thinning of the ice tongue is mostly due to submarine melting with average rates of 228 49 m yr 1 0 62 0 13 m d 1 between the summers of 1984 and 1985 the cause of the high melt rate is the circulation of warm seawater thermal forcing of up to 4 2 c beneath the tongue with convection driven by the substantial discharge of subglacial freshwater from the grounding zone we believe that this buoyancy driven convection is responsible for a deep channel incised into the sole of the floating tongue a dramatic thinning retreat and speedup began in 1998 and continues today the timing of the change is coincident with a 1 1 c warming of deep ocean waters entering the fjord after 1997 assuming a linear relationship between thermal forcing and submarine melt rate average melt rates should have increased by 25 57 m yr 1 sufficient to destabilize the ice tongue and initiate the ice thinning and the retreat that followed

seasonal surface velocity variations on a sub polar glacier in west greenland huit balises implantees dans les zones d accumulation et d ablation d un glacier de groenland ont ete relevees tous les dix jours pendant les etes 1982 et 1983 l analyse des vitesses laisse penser qu un reseau sous glaciaire permet a l eau de fusion produite dans la zone d ablation de remonter vers l amont

grounding line migration and high resolution calving dynamics of jakobshavn isbrae west greenland 1 we performed three field campaigns in 2004 2007 and 2010 at the southern margin of the jakobshavn isbrae west greenland in order to infer flow velocities and their changes from photogrammetric time lapse imagery with a temporal resolution of 20 min and a spatial spacing of about 30 m on the glacier surface area wide analysis of more than 3000 three dimensional trajectories at individual glacier positions allow for both the mapping of the grounding line and the detailed observation of flow variations during major calving events from 2004 to 2010 the grounding line of jakobshavn isbrae retreated 3 5 0 2 km considering previously published results the grounding line retreat amounts to 6 km since 1985 the glacier has an ephemeral floating tongue that can establish during the readvance of the glacier front and break apart after large calving events observations of a major calving event show that an acceleration of flow velocities coincides with the onset of the break up during which flow velocities of up to 70 m d can be reached moreover large vertical displacements of the glacier front in the order of 15 m and lowering of 8 m at positions 500 m beyond the calving front were observed 2 days before the calving event after the break up the glacier slowly adjusts to the new boundary conditions within the next 4 5 days flow velocity variations caused by calving were detected up to 1 km upstream only which indicates that individual calving events have no immediate effect on the large scale glacier dynamics

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

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

greenland ice sheet response to external forcing kinematic wave modeling is used to evaluate possible responses of the greenland ice sheet to changes in its surface mass balance in the approach followed here the reference state is defined based on measured velocity and discharge flux along the central flow line of petermann glacier in the northwest and perturbations on this state are considered the results indicate that significant rates of thickness change can occur immediately after the prescribed change in surface mass balance but adjustments in flow rapidly diminish these rates to a few centimeters per year at most full adjustment of the ice sheet requires times of the order of 1000 years the instability mechanism known as the jakobshavn effect is discussed and based on observational evidence as well as results from prior modeling studies it is concluded that this is an unlikely mechanism for destabilizing major drainage basins of the greenland ice sheet

sedimentological record of subglacial conditions and ice sheet dynamics of the vistula ice stream north central poland during the last glaciation abstract deposits of the vistula ice stream draining the scandinavian ice sheet during the last glaciation were investigated at four field sites in north central poland using micro and macroscale features the study reveals several till units with specific structural textural and lithological characteristics the individual till units are either macroscopically massive or bedded and the contacts between the units are either sharp or transitional the nature of the contacts with the underlying sediments ductile deformation structures largely undeformed clayey clasts tectonic lamination thin horizontal stringers of sorted sediments ploughing marks boulder pavements and striated upper surfaces of pebbles in the till indicate both bed deformation and enhanced basal sliding under high subglacial water pressure conditions it is suggested that the till is a hybrid deposit generated by some combination of lodgement deformation and ploughing punctuated by periods of basal decoupling the depth of deformation at any point in time was thinner up to several decimetres than the maximum till thickness c 2 5 m the ice sliding velocity estimations indicate velocities of less than 100 to over 2000 m yr 1 which suggests an unstable and highly dynamic ice lobe consistent with spatial variability of till characteristics sand wedges in the deposits beneath the till and the nature of the till bed interface indicate permafrost under the advancing ice sheet we suggest that under the increasing ice thickness a layer of thawed water saturated sediment formed on top of the still frozen ground due to inefficient drainage and contributed to ice streaming by promoting pervasive deformation and basal sliding

the interpretation and classification of subglacially deformed materials abstract a general classification of subglacially deformed materials is proposed based on sediment properties and their relationship to styles of subglacial strain deformation till is defined as homogenized usually diamictic material formed by glacially induced shear of subsole materials three types are recognized type a formed by pervasive ductile deformation type b formed by brittle shear and comminution till produced by the reduction of void space by in situ crushing and abrasion the term glacitectonite is adopted for materials that have undergone subglacial shear but retain some of the structural characteristics of the parent material original structures may be truncated by glacitectonic fabric elements type a or distorted but not truncated type b the principles of strain and material response particle orientation mechanisms fabric development and drainage conditions are reviewed examples of deformation tills and glacitectonites from breidamerkurjokull iceland slettmarkbreen norway loch lomond scotland and east yorkshire england are used to illustrate the nature of subglacially deformed materials and their relationship to former subglacial strain and drainage conditions the geologic evidence suggests a continuum of bed strengths can be recognized in deforming substrata ranging from high strength low strain type b deformation till and comminution till to low strength high strain type a deformation till

evolution of variegated glacier alaska u s a prior to its surge on presente l evolution saison apres saison des changements dans la geometrie et la vitesse du glacier et on tente de rattacher ces variations a la deformation de la glace et au glissement basal

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

oceanic boundary conditions for jakobshavn glacier part i variability and renewal of ilulissat icefjord waters 2001 14 jakobshavnglacier west greenland has respondedtotemperaturechangesinilulissaticefjord intowhich it terminates thisstudycollectedhydrographicobservationsinsideilulissaticefjordandfromadjacentdiskobay between2001and2014 thewarmestdeepdiskobaywaterswereblockedbytheentrancesillanddidnotreach jakobshavn glacier in the fjord basin the summer mean temperature was 2 88c from 2009 to 2013 excluding 2010 whenitwas 18c cooler despitethisvariability summerpotential densities in the basinwereinthe narrow range of 27 20 su 27 31kgm 23 and basin water properties matched those of disko bay in this layer each summer this relation has likely held since at least 1980 basin waters from 2009 and 2011 13 were therefore similar tothosein1998 99 whenjakobshavn glacier began toretreat whilebasin watersin2010wereascoolas in the 1980s the 2010 basin temperature anomaly was advected into disko bay not produced by local atmospheric variability this anomaly also shows that ilulissat icefjord basin waters were renewed annually or faster time series fragments inside the fjord did not capture the 2010 anomaly but show that the basin temperatures varied little subannually outside of summer fjord velocity profiles from summer 2013 implied a basin renewal time scale of about 1 month in model simulations of the fjord circulation subglacial discharge from jakobshavn glacier could drive renewal of the fjord basin over a single summer while baroclinic forcing from outside the fjord could not because of the sill at the mouth

growth and collapse of the distributed subglacial hydrologic system of kennicott glacier alaska usa and its effects on basal motion nearly 100 days of hourly glacier motion hydrology and hydrochemistry measurements on kennicott glacier alaska usa demonstrate the complicated relationship between water and motion at the glacier bed our observations capture the transient glacier response to seasonal and daily melt cycles and to a jokulhlaup that prompts a sixfold increase in glacier speed sliding is promoted whenever the water inputs to the glacier exceed the capacity of the subglacial hydrologic system to transmit the water sensitivity of sliding to daily meltwater inputs varies strongly through the season implying that the state of the hydrologic system governs the sensitivity of basal sliding a numerical model constructed to explore these relationships reveals the roles of the effective pressure the exponent to which this is taken in the sliding law 0 1 0 6 glacier macroporosity 2 and the cavity generating capacity of the glacier bed which encapsulates the sizes and spacing of bed roughness elements temporal changes in the effective pressure associated with evolution of both water inputs and subglacial water transmission capacity can explain the varying strength of diurnal velocity fluctuations of kennicott glacier spatial patterns of glacier macroporosity and of basal roughness govern variation in sensitivity of sliding to water inputs

subglacial decoupling at the sediment bedrock interface a new mechanism for rapid flowing ice abstract in undetermined on millennial or even centennial time scales the activity of rapid flowing ice can affect climate variability and global sea level through release of meltwater into the ocean and positive feedback loops to the climate system at the surge type glacier bruarjokull an outlet of the vatnajokull ice cap eastern iceland extremely rapid ice flow was sustained by overpressurized water causing decoupling beneath a thick sediment sequence that was coupled to the glacier this newly discovered mechanism has far reaching consequences for our understanding of fast flowing ice and its integration with sediment discharge and meltwater release c 2006 elsevier ltd all rights reserved

multi decadal record of ice dynamics on daugaard jensen gletscher east greenland from satellite imagery and terrestrial measurements this is the publisher s version also available electronically from http www ingentaconnect com

derived bedrock elevations strain rates and stresses from measured surface elevations and velocities jakobshavns isbrae greenland jakobshavns isbrae 69 10 n 49 59 w drains about 6 5 of the greenland ice sheet and is the fastest ice stream known the jakobshavns isbrae basin of about 10 000 km 2 was mapped photogrammetrically from four sets of aerial photography two taken in july 1985 and two in july 1986 positions and elevations of several hundred natural features on the ice surface were determined for each epoch by photogrammetric block aerial triangulation and surface velocity vectors were computed from the positions the two flights in 1985 yielded the best results and provided most common points 716 for velocity determinations and are therefore used in the modeling studies the data from these irregularly spaced points were used to calculate ice elevations and velocity vectors at uniformly spaced grid points 3 km apart by interpolation the field of surface strain rates was then calculated from these gridded data and used to compute the field of surface deviatoric stresses using the flow law of ice for rectilinear coordinates x y pointing eastward and northward and curvilinear coordinates l t pointing longitudinally and transversely to the changing ice flow direction ice surface elevations and slopes were then used to calculate ice thicknesses and the fraction of the ice velocity due to basal sliding our calculated ice thicknesses are in fair agreement with an ice thickness map based on seismic sounding and supplied to us by k echelmeyer ice thicknesses were subtracted from measured ice surface elevations to map bed topography our calculation shows that basal sliding is significant only in the 10 15 km before jakobshavns isbrae becomes afloat in jakobshavns isfjord

ice stream influence on west greenland ice sheet dynamics during the last glacial maximum this paper investigates the processes governing bedrock bedform evolution in ice sheet and ice stream areas in central west greenland and explores the evidence for a cross shelf ice stream at the last glacial maximum lgm to the east of sisimiut the formation of streamlined bedforms with high elongation ratios and high bedform density has been controlled by geological structure and topography in slow flowing ice sheet areas at the coast the effects of regional flow convergence caused by coastal fjord orientation routed ice into the sisimiut itilleq area where it formed an ice stream onset zone this funnelled ice into an offshore trough holsteinsborg dyb resulting in a southwesterly regional ice flow direction and the formation of a topographically routed ice stream holsteinsborg isbrae to the south of this striae and bedform evidence show that local valley glaciers initially flowed east to west across the coast but were later redirected by the itilleq fjord ice which turned southwestward due to diffluent flow and deflection by holsteinsborg isbrae roches moutonnees in this area have low elongation ratios and high bedform density but do not provide unequivocal support for ice streaming as they are a product of both bedrock structure and changes in ice flow direction rather than enhanced flow velocities cosmogenic surface exposure ages limit maximum ice sheet surface elevation to ca 755 810 m above sea level in this region such ice thickness enabled holsteinsborg isbrae to reach the mid outer continental shelf during the lgm and to contribute to the formation of a trough mouth fan and the outer hellefisk moraines initial deglaciation across this region was driven by rising sea level and increasing air temperatures prior to the bolling interstadial at ca 14 5 cal ka bp between 12 and 10 cal ka bp both increased air and ocean temperatures post the younger dryas and peak sea level rise up to the marine limit caused accelerated thinning and marginal retreat through calving although dating evidence suggests ice streams remained along the inner shelf coast boundary until at least ca 10 cal ka bp their longevity maintained by increased ice thickness and ice discharge copyright 2009 john wiley sons ltd

structural evolution of backthrusting in the mentawai fault zone offshore sumatran forearc 1 we present the interpretation of newly acquired high quality industry standard deep seismic reflection and swath bathymetry data to provide insight into the structural style and evolution of the mentawai fault zone mfz the mfz lies along the boundary between the accretionary wedge and the proposed continental backstop this zone exhibits arcuate ridges on the seafloor convex toward the east beneath these ridges the structures developed as landward vergent imbricated backthrusts in the inner part of the accretionary wedge and higher angle backthrusts that deformed the forearc basin sediments in the forearc high anticlines were developed due to the seaward vergent forearc high thrusts originating in the accretionary wedge the imbricated backthrusts may have initiated during the early middle miocene contemporaneously with the slide and back rotation of forearc high thrusts in the late miocene the higher angle backthrusts were initiated continuous contraction induced the frontal higher angle backthrusts and formed a fold thrust belt toward the east during the pliocene the folds and thrusts were disturbed by diapirs and mud volcanoes backthrusting and fold thrust belts developed in the mfz may explain the compressional features observed at the boundary between the accretionary wedge and continental backstop along the southern sumatra margin the backthrusts along the mfz are waning in activity and hence the risk of a large earthquake and associated tsunami at the present time should be small

where glaciers meet water subaqueous melt and its relevance to glaciers in various settings glacier change is ubiquitous but the fastest and largest magnitude changes occur in glaciers that terminate in water this includes the most rapidly retreating glaciers and also several advancing ones often in similar regional climate settings furthermore water terminating glaciers show a large range in morphology particularly when ice flow into ocean water is compared to that into freshwater lakes all water terminating glaciers share the ability to lose significant volume of ice at the front either through mechanical calving or direct melt from the water in contact here we present a review of the subaqueous melt process we discuss the relevant physics and show how different physical settings can lead to different glacial responses we find that subaqueous melt can be an important trigger for glacier change it can explain many of the morphological differences such as the existence or absence of floating tongues subaqueous melting is influenced by glacial runoff which is largely a function of atmospheric conditions this shows a tight connection between atmosphere oceans and lakes and glaciers subaqueous melt rates even if shown to be large should always be discussed in the context of ice supply to the glacier front to assess its overall relevance we find that melt is often relevant to explain seasonal evolution can be instrumental in shifting a glacier into a different dynamical regime and often forms a large part of a glacier s mass loss on the other hand in some cases melt is a small component of mass loss and does not significantly affect glacier response

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

glacial geological studies of surge type glaciers in iceland research status and future challenges abstract surging glaciers are potential analogues for land terminating palaeo ice streams and surging ice sheet lobes and research on surge type glaciers is important for understanding the causal mechanisms of modern and past ice sheet instabilities the geomorphic signatures left by the icelandic surge type glaciers vary and range from glaciotectonic end moraines formed by folding and thrusting crevasse squeeze ridges concertina eskers drumlins and fluted forefields to extensive dead ice fields and even drift sheets where fast ice flow indicators are largely missing we outline some outstanding research questions and review case studies from the surge type outlets of bruarjokull eyjabakkajokull and tungnaarjokull vatnajokull ice cap mulajokull and satujokull hofsjokull ice cap hagafellsjokull and su urjokull langjokull ice cap kaldalonsjokull leirufjar arjokull and reykjarfjar arjokull drangajokull ice cap as well as the surge type cirque glaciers in northern iceland we review the current understanding of how rapid ice flow is sustained throughout the surge the processes that control the development of the surge type glacier landsystem and the geological evidence of surges found in sediments and landforms we also examine if it is possible to reconstruct past surge flow rates from glacial landforms and sediments and scale up present day surge processes landforms and landsystems as modern analogues to past ice streams finally we also examine if there is a climate mass balance control on surge initiation duration and frequency

volume change of jakobshavn isbrae west greenland 1985 1997 2007 following three decades of relative stability jakobshavn isbrae west greenland under went dramatic thinning retreat and speed up starting in 1998 to assess the amount of ice loss we analyzed 1985 aerial photos and derived a 40 m grid digital elevation model dem we also obtained a 2007 40 m grid spot dem covering the same region comparison of the two dems over an area of 4000 km 2 revealed a total ice loss of 160 4k m 3 with 107 0 2 km 3 in grounded regions 0 27 mm eustatic sea level rise and 53 4k m 3 from the disintegration of the floating tongue comparison of the dems with 1997 nasa airborne topographic mapper data indicates that this ice loss essentially occurred after 1997 with 0 7 5 6 km 3 between 1985 and 1997 and 160 7k m 3 between 1997 and 2007 the latter is equivalent to an average specific mass balance of 3 7 0 2 m a 1 over the study area previously reported thickening of the main glacier during the early 1990s was accompanied by similar magnitude thinning outside the areas of fast flow indicating that the land based ice continued reacting to longer term climate forcing

theoretical calving rates from glaciers along ice walls grounded in water of variable depths calving has been studied for glaciers ranging from slow polar glaciers that calve on dry land such as on deception island 63 0 s 60 6 w in antarctica through temperate alaskan tide water glaciers to fast outlet glaciers that float in fiords and calve in deep water such as jakobshavns isbrae 69 2 n 49 9 w in greenland calving from grounded ice walls and floating ice shelves is the main ablation mechanism for the antarctic and greenland ice sheets as it was along marine and lacustrine margins of former pleistocene ice sheets and is for tide water and polar glaciers

elevation changes and dynamic provinces of jakobshavn isbrae greenland derived using generalized spatial surface roughness from icesat glas and atm data

water sheet outburst floods from the laurentide ice sheet field evidence and a theoretical model support the concept that during wisconsinan glaciation subglacial water sheet outburst floods issued from a large subglacial lake located in the hudson bay ba

controls on short term variations in greenland glacier dynamics short term ice dynamical processes at greenland s jakobshavn and kangerdlugssuaq glaciers were studied using a 3 day time series of synthetic aperture radar data acquired during the 2011 european remote sensing satellite 2 ers 2 3 day repeat campaign together with modelled meteorological parameters the time series spans the period march july 2011 and captures the first 30 of the summer melting season in both study areas we observe velocity fluctuations at the lower 10 km of the glacier at jakobshavn isbrae where our dataset covers the first part of the seasonal calving front retreat we identify ten calving episodes with a mean calving front area loss of 1 29 0 4km2 significant glacier speed up was observed in the near terminus area following all calving episodes we identify changes in calving front geometry as the dominant control on velocity fluctuations on both glaciers apart from a

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

jakobshavn isbrae west greenland flow velocities and tidal interaction of the front area from 2004 field observations 1 during the summer of 2004 the front area of the jakobshavn isbrae was monitored using a geodetic photogrammetric survey with temporarily coincident precise observations of local ocean tides in the disko bay close to ilulissat the geodetic and photogrammetric observations were conducted at the southern margin of the glacier front the largest observed horizontal flow velocities are in the central part of the front with values up to 45 m d this is a factor of 2 greater than the average velocities at the front area observed in the last century our new observations confirm previous estimates of an acceleration of glacier flow during the last decade the photogrammetric survey provided flow trajectories for 4000 surface points with a time resolution of 30 min these flow trajectories were used to compare the vertical motion of the glacier with the observed tides the existence of a free floating glacier tongue in 2004 was confirmed by these data however it occupied only a small belt of at most a few 100 m width in the central part of the glacier front horizontal motion did not appear to depend on the tidal phase unlike some of the fast moving ice streams of west antarctica

seasonal variability in the dynamics of marine terminating outlet glaciers in greenland recent studies indicate that the dynamics of fast flowing marine terminating outlet glaciers of the greenland ice sheet may be sensitive to climate and ocean forcing on sub annual timescales observations of seasonal behavior of these glaciers at such high temporal resolution however are currently few here we present observations of front position flow speed near surface air temperature and ocean conditions for six large marine terminating glaciers in the uummannaq region of west greenland to investigate controls on short term glacier dynamics as proposed by other studies we find that seasonal front advance and retreat correlates with the formation and disappearance of an ice melange our data suggest that high sea surface temperature anomalously low sea ice concentration and reduced melange formation in early 2003 have triggered multi year retreat of several glaciers in the study area which is consistent with other regions in greenland of the stable glaciers only rink isbrae exhibits a seasonal speed variation that correlates with variations in front position with the others undergoing mid summer deceleration that indicates the effects of subglacial meltwater discharge and drainage system evolution drainage of supraglacial lakes and water filled crevasses results in substantial decreases in speed 40 60 on fast flowing glaciers our results demonstrate that attempts to model ice sheet evolution must take into account short timescale flow dynamics resulting from drainage events and oceanographic conditions

a mini surge on the ryder glacier greenland observed by satellite radar interferometry satellite radar interferometry reveals that the speed of the ryder glacier increased roughly threefold and then returned to normal 100 to 500 meters year over a 7 week period near the end of the 1995 melt season the accelerated flow represents a substantial though short lived change in ice discharge during the period of rapid motion meltwater filled supraglacial lakes may have drained which could have increased basal water pressure and caused the mini surge there are too few velocity measurements on other large outlet glaciers to determine whether this type of event is a widespread phenomenon in greenland but because most other outlet glaciers are at lower latitudes they should experience more extensive melting making them more susceptible to meltwater induced surges

mapping tide water glacier dynamics in east greenland using landsat data landsat multispectral scanner and thematic mapper images were co registered for the kangerdlugssuaq fjord region in east greenland and were used to map glacier drainage basin areas changes in the positions of tide water glacier termini and to estimate surface velocities of the larger tide water glaciers statistics were compiled to document distance and area changes to glacier termini the methodologies developed in this study are broadly applicable to the investigation of tide water glaciers in other areas the number of images available for consecutive years and the accuracy with which images are co registered are key factors that influence the degree to which regional glacier dynamics can be characterized using remotely sensed data three domains of glacier state were interpreted net increase in terminus area in the southern part of the study area net loss of terminus area for glaciers in upper kangerdlugssuaq fjord and a slight loss of glacier terminus area northward from ryberg fjord local increases in the concentrations of drifting icebergs in the fjords coincide with the observed extension of glacier termini positions ice surface velocity estimates were derived for several glaciers using automated image cross correlation techniques the velocity determined for kangerdlugssuaq gletscher is approximately 5 0 km a 1 and that for kong christian iv gletscher is 0 9 km a 1 the continuous presence of icebergs and brash ice in front of these glaciers indicates sustained rates of ice front calving

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

large fluctuations in speed on greenland s jakobshavn isbrae glacier it is important to understand recent changes in the velocity of greenland glaciers because the mass balance of the greenland ice sheet is partly determined by the flow rates of these outlets jakobshavn isbrae is greenland s largest outlet glacier draining about 6 5 per cent of the ice sheet area and it has been surveyed repeatedly since 1991 ref 2 here we use remote sensing data to measure the velocity of jakobshavn isbrae between 1992 and 2003 we detect large variability of the velocity over time including a slowing down from 6 700 m yr 1 in 1985 to 5 700 m yr 1 in 1992 and a subsequent speeding up to 9 400 m yr 1 by 2000 and 12 600 m yr 1 in 2003 these changes are consistent with earlier evidence for thickening of the glacier in the early 1990s and rapid thinning thereafter our observations indicate that fast flowing glaciers can significantly alter ice discharge at sub decadal timescales with at least a potential to respond rapidly to a changing climate

holocene glacimarine sedimentation inner scoresby sund east greenland the influence of fast flowing ice sheet outlet glaciers abstract holocene glacimarine sedimentation associated with fast flowing outlet glaciers draining the greenland ice sheet is investigated using sedimentary and acoustic data from inner scoresby sund east greenland sedimentation in inner scoresby sund is dominated by three processes which are influenced by differences in proximity to fast flowing outlet glaciers extent of glacier ice cover and fjord bathymetry 1 sediment gravity flow principally in the form of turbidity currents and debris flows 2 suspension sedimentation from turbid meltwater plumes and 3 iceberg rafting these processes result in texturally and sedimentologically heterogeneous lithofacies proportionally fine grained muds laminated stratified and massive facies dominate cores recovered from inner scoresby sund accounting for 80 of the total whereas diamict facies account for only 15 abundant fine grained muds demonstrate that meltwater flux and sedimentation is significant in this high arctic glacimarine environment in settings proximal to fast flowing outlet glaciers with increasing distance from these glacier termini muds are replaced progressively by iceberg rafted coarse grained sediment the dominance of this iceberg rafted sediment in outer scoresby sund reflects both its more distal location from fast flowing glacier termini and the high calving flux associated with these ice masses laminated muds deposited by turbidity currents and suspension settling from overflow plumes in inner scoresby sund are similar to lithofacies produced in temperate and subpolar glacimarine systems this implies a similarity in sedimentation processes and resulting facies across a wide spectrum of climatically glaciologically fast flowing and non fast flowing ice masses and oceanographically variable glacimarine settings recognition of laminated fine grained facies in the geological record therefore does not necessarily indicate a temperate palaeo glacial setting however the predominance of iceberg rafted diamict facies in ice distal sedimentary records suggests the former presence of relatively cold environmental conditions in which iceberg sedimentation played a dominant role

20 my of nitrogen fixation during deposition of mid cretaceous black shales on the demerara rise equatorial atlantic ocean abstract thick sequences of dark colored organic carbon rich finely laminated santonian cenomanian claystones and homogeneous albian siltstones were recovered from ocean drilling program sites 1257 1258 and 1260 on the demerara rise in the western equatorial atlantic ocean total organic carbon toc concentrations vary from 2 to over 20 wt in the sequences of black shales that were deposited over a period of 20 million years similarly long periods of elevated marine productivity implied by the high toc concentrations are uncommon in the geological record and must have required unusual paleoceanographic conditions the importance of nitrogen fixing bacteria to sustaining the amplified export production of organic matter is indicated by 15n values that remain between 4 and 0 a range that is notably less positive than the average of 5 for modern ocean sediments although containing mostly marine organic matter the black shales have toc tn molar ratios between 20 and 40 that mimic those of land plant organic matter the anomalously large toc tn ratios suggest selective organic matter degradation probably associated with low oxygen conditions in the water column that favored preservation of nitrogen poor forms of organic matter relative to nitrogen rich components deposition of black shales on the demerara rise was likely a consequence of the mid cretaceous warm and wet greenhouse climate that strengthened thermohaline stratification of this part of the atlantic ocean which in turn encouraged bacterial nitrogen fixation enhanced primary production magnified organic matter export and ultimately established anoxic conditions at the seafloor that improved preservation of organic matter for much of the 20 my period represented by these thick sequences

subglacial processes of the scandinavian ice sheet in fennoscandia inferred from flow parallel features and lithostratigraphy abstract subglacial processes of the last scandinavian ice sheet in fennoscandia were investigated on two different scales a detailed case study in southwestern finland and a generalized study in the central parts of the ice sheet previously reconstructed time transgressive glacial flow patterns were correlated with different types of glacial flow parallel features and lithostratigraphic units the comparison of orientations of each flow parallel feature with relative ages of different flow patterns enabled estimations of relative distances of the orientation generating processes in relation to the receding ice margin this method was also used to infer sedimentary environments of each lithostratigraphic facies identified generalized lithostratigraphic sequences were compiled in fennoscandia using sections described previously in areas of ice streams a sandy till showing mature deformation structures forms till sheets drumlins and some ribbed moraines the facies is overlain by occasional glaciofluvial deposits and partly washed deformation till both indicating abundant excess water at the ice bed interface in interlobate areas glaciofluvial material is under and overlain by a silty till showing ductile deformation structures in places uppermost thin clayey till interpreted to be a subglacial melt out till mantles underlying till and subglacial glaciofluvial deposits the characteristics of different units are controlled by ice flow velocities subglacial hydrology and deformation type of the substratum the time transgressive origin of till beds and intervening washed sediment as well as divergent ice flow patterns is emphasized ice flow was concentrated in streams which generated marginal fan shaped lobes interstream areas of low basal velocities and melting rates were left between adjacent ice streams in areas of ice streams fast basal ice flow velocities continuously generated heat and meltwater which flowed at first in the sediment and closer to the margin also in thin discontinuous sheets at the ice bed interface excess water may have subsequently formed a linked cavity system and channelized to a tunnel network initiating esker deposition or incision of tunnel channels most processes operated continuously during deglaciation with spatial and temporal differences in their activity the mode of subglacial drainage changed periodically during deglaciation and was probably controlled by the volume of subglacial meltwater available in some periods sheet flow of basal water deposited major marginal complexes and subsequently tunnel networks were re established esker pattern and sedimentation rates of proglacial varved clays indicate that tunnel drainage of subglacial water was partly episodic

water flow through temperate glaciers understanding water movement through a glacier is fundamental to several critical issues in glaci ology including glacier dynamics glacier induced floods and the prediction of runoff from glacierized drainage basins to this end we have synthesized a conceptual model of water movement through a temper ate glacier from the surface to the outlet stream pro cesses that regulate the rate and distribution of water input at the glacier surface and that regulate water movement from the surface to the bed play important but commonly neglected roles in glacier hydrology where a glacier is covered by a layer of porous perme able firn the accumulation zone the flux of water to the glacier interior varies slowly because the firn tempo rarily stores water and thereby smooths out variations in the supply rate in the firn free ablation zone in con trast the flux of water into the glacier depends directly on the rate of surface melt or rainfall and therefore varies greatly in time water moves from the surface to the bed through an upward branching arborescent net work consisting of both steeply inclined conduits formed by the enlargement of intergranular veins and gently inclined conduits spawned by water flow along the bottoms of near surface fractures crevasses engla cial drainage conduits deliver water to the glacier bed at a limited number of points probably a long distance downglacier of where water enters the glacier englacial conduits supplied from the accumulation zone are quasi steady state features that convey the slowly varying water flux delivered via the firn their size adjusts so that they are usually full of water and flow is pressurized in contrast water flow in englacial conduits supplied from the ablation area is pressurized only near times of peak daily flow or during rainstorms flow is otherwise in an open channel configuration the subglacial drainage system typically consists of several elements that are distinct both morphologically and hydrologically an up glacier branching arborescent network of channels in cised into the basal ice conveys water rapidly much of the water flux to the bed probably enters directly into the arborescent channel network which covers only a small fraction of the glacier bed more extensive spatially is a nonarborescent network which commonly includes cav ities gaps between the glacier sole and bed channels incised into the bed and a layer of permeable sediment the nonarborescent network conveys water slowly and is usually poorly connected to the arborescent system the arborescent channel network largely collapses during winter but reforms in the spring as the first flush of meltwater to the bed destabilizes the cavities within the nonarborescent network the volume of water stored by a glacier varies diurnally and seasonally small temper ate alpine glaciers seem to attain a maximum seasonal water storage of 200 mm of water averaged over the area of the glacier bed with daily fluctuations of as much as 20 30 mm the likely storage capacity of subglacial cavities is insufficient to account for estimated stored water volumes so most water storage may actually occur englacially stored water may also be released abruptly and catastrophically in the form of outburst floods

continued evolution of jakobshavn isbrae following its rapid speedup 1 several new data sets reveal that thinning and speedup of jakobshavn isbrae continue following its recent rapid increase in speed as its floating ice tongue disintegrated the present speedup rate of 5 a 1 over much of the fast moving region appears to be a diffusive response to the initial much larger speedup near the front there is strong seasonality in speed over much of the fast flowing main trunk that shows a good inverse correlation with the seasonally varying length of a short typically 6 km floating ice tongue this modulation of speed with ice front position supports the hypothesis that the major speedup was caused by loss of the larger floating ice tongue from 1998 to 2003 analysis of image time series suggests that the transient winter ice tongue is formed when sea ice bonds glacier ice in the fjord to produce a nearly rigid mass that almost entirely suppresses calving major calving only resumes in late winter when much of this ice clears from the fjord the collapse of the ice tongue in the late 1990s followed almost immediately after a sharp decline in winter sea ice concentration in disko bay this decline may have extended the length of the calving season for several consecutive years leading to the ice tongue s collapse

glacier velocities from time lapse photos technique development and first results from the extreme ice survey eis in greenland automated digital cameras were installed in may june 2007 beside major west greenland marine terminating glaciers as part of the extreme ice survey eis eis cameras began imaging the lowest 4 km 2 of the glacier at hourly intervals throughout sunlit periods of the year this study presents the development of techniques for quantifying glacier velocity from a single camera perspective a multi image multi chip matching procedure yields higher matching skill than conventional matching and facilitates false match rejection via a clustering scheme the matching of motionless on land features facilitates compensating camera motion ray projection to a known terrain elevation allows the assigning of scale to convert pixel displacements to velocity units with the 10 2 megapixel camera system velocities on relatively fast glaciers can be resolved at distances up to 4 km at a distance of 2 km a demonstrated precision of 0 5 pixels yields a 0 5 m footprint size daily velocities indicate progressive multi day velocity accelerations associated with calving deceleration trends are associated with the regrowth of resistive stress after major calving the higher observation frequency available to terrestrial photogrammetry indicates higher observed intra seasonal velocity range than observable by the at best weekly satellite snapshots

spatiotemporal variations in surface velocity of the gangotri glacier garhwal himalaya india study using synthetic aperture radar data the gangotri glacier is the largest in the garhwal himalaya india and its melt water forms the main source stream of the ganga river yet its dynamics are poorly understood its long record of terminus retreat measurements 1800 present shows multi decadal oscillations and more recently a slowing down of the retreat its complex dynamics are also indicated by studies of proglacial melt water at the terminus however there have been no systematic measurements of its surface velocity or how it changes seasonally or over longer term time scales here i have characterized the spatiotemporal variations of surface velocity of the gangotri glacier using synthetic aperture radar sar data spanning nearly two decades 1991 2011 the main findings are as follows 1 the glacier undergoes seasonal fluctuations in surface velocities there was a clear summer speedup of similar to 57 126 compared to winter velocities between 0 and 12 6 km from the terminus with peak summer speeds of 63 1 5 4 m yr in 1992 66 6 6 0 m yr in 1999 58 2 4 5 m yr in 2004 and 42 8 4 2 m yr in 2007 whereas winter speeds were relatively stable 25 30 m yr during the same period this summer speedup is indicative of increased basal sliding known to occur when melt waters penetrate to the glacier bed resulting in reduced friction 2 these summer velocities exhibited an inter annual reduction which was manifest as a reduction in summer speed up from 120 in 2004 to 60 in 2007 this pattern continued into 2011 despite the availability of abundant melt water during those years this inter annual reduction indicates the formation of increasingly efficient melt water channels year on year resulting in lesser lubrication at the glacier bed interface my results show that the gangotri glacier is dynamic throughout its length with systematic spatiotemporal variations in surface velocity that elucidate its subglacial processes based on my results and other evidence in the literature i propose that the subglacial drainage system of the gangotri glacier is subject to seasonal and inter annual hydrodynamic coupling between winters and melt seasons akin to other glaciers c 2016 elsevier inc all rights reserved

seasonal variation in velocity before retreat of jakobshavn isbrae greenland 1 using repeat pass satellite feature tracking the dynamic behaviour of jakobshavn isbrae which drains around 7 of the greenland ice sheet is investigated in improved detail through its recent period of thinning acceleration and retreat the departure in 1995 from the normal seasonal invariance in velocity near the grounding line suggests a possible change in dynamics well before the subsequent thinning phase the initiation of the acceleration and retreat is pinpointed to spring 1998 and may have been prompted by the same mechanism that gave rise to the spring 1995 flow increase

seasonal to decadal scale variations in the surface velocity of jakobshavn isbrae greenland observation and model based analysis 1 using new data we build upon the nearly two decade long record of observations from jakobshavn isbrae to investigate the processes driving its dynamic evolution while winter flow speed has not increased substantially over the last three winters there remains a strong seasonal variation in flow speed that coincides with a cycle of summer thinning and winter thickening we relate changes in glacier speed to geometry through variations in basal traction and horizontal stresses using ice flow models constrained by satellite and airborne observations these results suggest that the bed provides little flow resistance along the main trough within about 20 km of the terminus while the loss of buttressing from the retreat of grounded and floating ice likely contributed to the initial speedup other processes are of comparable significance at seasonal to decadal time scales from analysis of the models we hypothesize that thinning induced change in basal effective pressure is the dominant process influencing near terminus behavior while diffusive processes drive the upstream response the apparent need for the terminus to thin to near flotation before it can calve may limit the rate at which retreat occurs our analysis of the processes controlling the speed suggests little potential for further large acceleration thinning and elevated speeds may continue at rates similar to present however putting the glacier on course to retreat to the head of its deep trough in about a century at which point it likely would stabilize with a thinner terminus

geostatistical analysis of glacier roughness data in most glaciological and hydrological models surface roughness of snow and ice is an important parameter however roughness is generally used only as an estimated parameter for lack of available observations in this paper we present a method to collect and analyze ice surface roughness data using a specially designed instrument for survey and geostatistical methods for analysis the glacier roughness sensor grs built at the university of trier records variations in microtopography at 0 2 m 0 1 m resolution when pulled across an ice surface global positioning system data are used for location after several processing steps the data are analyzed using geostatistical methods the mathematical tool used to achieve a morphological characterization of ice surface types is the variogram grs data variograms and surface roughness analysis are ideal matches for morphological characterization because none of them requires or provides absolute elevation values morphology is described not by absolute elevation values but by the change of elevation in space which is the derivative of elevation surface roughness values the variogram is calculated from incremental values parameters extracted from variograms of grs data serve to distinguish lake surfaces wind structures ridge and vauey systems melting structures and blue ice areas examples are from jakobshavn isbrae drainage basin west greenland

intermittent thinning of jakobshavn isbrae west greenland since the little ice age rapid thinning and velocity increase on major greenland outlet glaciers during the last two decades may indicate that these glaciers became unstable as a consequence of the jakobshavn effect hughes 1986 with terminus retreat leading to increased discharge from the interior and consequent further thinning and retreat to assess whether recent trends deviate from longer term behavior we measured glacier surface elevations and terminus positions for jakobshavn isbrae west greenland using historical photographs acquired in 1944 1953 1959 1964 and 1985 these results were combined with data from historical records aerial photographs ground surveys airborne laser altimetry and field mapping of lateral moraines and trimlines to reconstruct the history of changes since the little ice age lia we identified three periods of rapid thinning since the lia 1902 13 1930 59 and 1999 present during the first half of the 20th century the calving front appears to have been grounded and it started to float during the late 1940s the south and north tributaries exhibit different behavior for example the north tributary was thinning between 1959 and 1985 during a period when the calving front was stationary and the south tributary was in balance the record of intermittent thinning combined with changes in ice marginal extent and position of the calving front together with changes in velocity imply that the behavior of the lower parts of this glacier represents a complex ice dynamical response to local climate forcings and interactions with drainage from the interior

combined measurements of subglacial water pressure and surface velocity of findelengletscher switzerland conclusions about drainage system and sliding mechanism interpretation des mesures de pression dans les forages et des vitesses superficielles la comparaison avec les modeles suggere que l on a neglige le frottement du aux debris dans les modeles et qu il n existe jamais de chenaux cylindriques rectilignes