late holocene and younger dryas glaciers in the northern cairngorm mountains scotland we present 17 cosmogenic 10be ages of glacial deposits in coire an lochain cairngorm mountains which demonstrate that glacial and nival deposits cover a longer timescale than previously recognis

implications of till deformation on glacier dynamics the dynamics of glacier motion are governed to a large extent by the properties of the basal interface in this paper we address the interaction of a glacier with a layer of till at its bed in an attempt to test whether our physical understanding of till is sufficient to explain general features of the observed flow field and changes in geometry of black rapids glacier alaska u s a we also investigate whether or not a till layer has a clear surface observable signature in the dynamics of the glacier towards this end we use a finite element ice flow model with a coulomb failure criterion within the basal till layer we find that simple till physics can be used to describe decadal seasonal and short term hours to days velocity variations and possibly uplift events mechanisms for each of these variations involve an increase in the extent of till at failure a transfer of shear stress across the bed and a consequent increase in ice deformation effective shape factors are calculated that permit a simple incorporation of this boundary condition into glacier response models our analyses however have not resulted in the identification of a clear and unique signature of a till layer in the surface dynamics of a glacier

ice rafting from the british irish ice sheet since the earliest pleistocene 2 6 million years ago implications for long term mid latitudinal ice sheet growth in the north atlantic region abstract the plio pleistocene intensification of northern hemisphere continental ice sheet development is known to have profoundly affected the global climate system evidence for early continental glaciation is preserved in sediments throughout the north atlantic ocean where ice rafted detritus ird layers attest to the calving of sediment loaded icebergs from circum atlantic ice sheets so far early pleistocene ird deposition has been attributed to the presence of high latitudinal ice sheets whereas the existence and extent of ice accumulation in more temperate mid latitudinal regions remains enigmatic here we present results from the multiproxy provenance analysis of a unique pleistocene holocene ird sequence from the irish ne atlantic continental margin there the challenger coral carbonate mound iodp expedition 307 site u1317 preserved an early pleistocene record of 16 distinctive ird events deposited between ca 2 6 and 1 7 ma strong and complex ird signals are also identified during the mid pleistocene climate transition ca 1 2 to 0 65 ma and throughout the middle late pleistocene interval radiogenic isotope source fingerprinting in combination with coarse lithic component analysis indicates a dominant sediment source in the nearby british irish isles even for the oldest early pleistocene ird deposits hence our findings demonstrate for the first time repeated and substantial i e marine terminating ice accumulation on the british irish isles since the beginning of the pleistocene contemporaneous expansion of both high and mid latitudinal ice sheets in the north atlantic region is therefore implied at the onset of the pleistocene moreover it suggests the recurrent establishment of climatically favourable conditions for ice sheet inception growth and instability in mid latitudinal regions even in the earliest stages of northern hemisphere glacial expansion and in an obliquity driven climate system

modelling the coupling of flood discharge with glacier flow during jokulhlaups we explore a mathematical model that couples together a thermomechanically evolving subglacial channel distributed cavity drainage and basal sliding along a subglacial flood path fed by a jokulhlaup lake it allows water transfer between channel and cavities and a migrating subglacial water divide or seal to form between floods notably it accounts for full coupling between the lake and subglacial drainage in terms of both discharge and pressure unlike models that neglect the pressure coupling by imposing a known history of lake discharge at the channel inlet this means that flood hydrographic evolution and its impact on glacier motion are consistently determined by our model numerical simulations for a model alpine lake yield stable limit cycles simulating repeating jokulhlaups with the channel drawing water from the cavities at a varying rate that modulates basal sliding during each flood a wave of fast sliding propagates down glacier at flood initiation followed by deceleration as the growing channel sucks water from the cavities these behaviours cannot be correctly simulated without the full coupling we show that the flood s peak discharge its initiation threshold and the magnitude of the fast sliding wave decrease with the background water supply to the cavities

hydrological characteristics of the drainage system beneath a surging glacier a rare combination of natural circumstances permits assessment of current theories on water flow beneath glaciers outburst floods from the subglacial lake grimsvotn in iceland took place before during and after surging of skei ararjokull the glacier beneath which the outburst floods drain the observable drainage patterns associated with these floods show the different nature of the basal water conduit system of the glacier during surge and non surge phases during surge conditions basal water is dispersed slowly across the bed in a distributed drainage system but when the glacier is not surging water is transported rapidly through a system of tunnels

origin and morphology of limestone caves limestone caves form along ground water paths of greatest discharge and solutional aggressiveness flow routes that acquire increasing discharge accelerate in growth while others languish with negligible growth as discharge increases a maximum rate of wall retreat is approached typically about 0 01 0 1 cm yr determined by chemical kinetics but nearly unaffected by further increase in discharge the time required to reach the maximum rate is nearly independent of kinetics and varies directly with flow distance and temperature and inversely with initial fracture width discharge gradient and pco2 most caves require 104 105 yr to reach traversable size their patterns depend on the mode of ground water recharge sinkhole recharge forms branching caves with tributaries that join downstream as higher order passages maze caves form where 1 steep gradients and great undersaturation allow many alternate paths to enlarge at similar rates or 2 discharge or renewal of undersaturation is uniform along many alternate routes flood water can form angular networks in fractured rock anastomotic mazes along low angle partings or sponge work where intergranular pores are dominant diffuse recharge also forms networks and spongework often aided by mixing of chemically different waters ramiform caves with sequential outward branches are formed mainly by rising thermal or h2s rich water dissolution rates in cooling water increase with discharge co2 content temperature and thermal gradient but only at thermal gradients of more than 0 01 c m can normal ground water co2 form caves without the aid of hypogenic acids or mixing artesian flow has no inherent tendency to form maze caves geologic structure and stratigraphy influence cave orientation and extent but alone they do not determine branch work versus maze character

tectonic controls on rates and spatial patterns of glacial erosion through geothermal heat flux abstract glacial erosion has shaped many mountain belts during the cold periods of the late cenozoic the rate of glacial erosion is sensitive to the subglacial environment including both the subglacial hydrology and the basal thermal regime geothermal heat from underlying bedrock is a major contributor to glacier energy budgets controlling ice dynamics at the ice bed interface by changing the basal temperature and the supply of meltwater despite the known influence of geothermal heat on the subglacial environment its impact on glacial erosion has received little study the geothermal heat flux in glaciated mountain ranges varies widely as a function of the tectonic setting therefore if glacial erosion is sensitive to geothermal heat flux the evolution of glaciated landscapes may depend upon tectonically controlled geothermal gradients we explore the impact of geothermal heat flux on the rates and spatial patterns of glacial erosion in mountain ranges using numerical models we couple a sliding dependent glacial erosion model with the parallel ice sheet model pism to simulate the evolution of a synthetic glacial landscape we find a robust tendency for increasing glacial erosion with increasing geothermal heat flux the spatial pattern of erosion also varies with the magnitude of geothermal heat flux at low geothermal heat flux glacial erosion is consistently focused in major valleys as geothermal heat flux increases the area of significant glacial erosion expands into higher elevations and the rate of erosion increases the location of maximum erosion migrates up valley as geothermal heat flux increases suggesting that glacial erosion tends to produce distinct landscapes as a function of geothermal heat flux our finding suggests that active mountain belts with high geothermal heat flux will express the glacial buzzsaw effect in which high elevation topography is preferentially removed by glacial erosion glaciers at passive margins with low geothermal heat flux in contrast will tend to incise deep valleys at relatively low elevations previous work on the interaction between tectonics and landscape evolution has focused on relief generation and fracturing of rocks our results introduce a novel potential linkage between tectonics and erosion based on the sensitivity of glacial erosion to geothermal heat

three dimensional surface velocities of storstrommen glacier greenland derived from radar interferometry and ice sounding radar measurements non steady state vertical velocities of up to 5 m y 1 exceed the vertical surface parallel flow components over much of the ablation area of storstrommen a large outlet glacier from the east greenland ice sheet neglecting a contribution to the vertical velocity of this magnitude results in substantial errors up to 20 also on the south north component of horizontal velocities derived by satellite synthetic aperture radar interferometry insar measurements in many glacier environments the steady state vertical velocity component required to balance the annual ablation rate is 5 to 10 m y 1 or more this indicates that the surface parallel flow assumption may be problematic also for glaciers in steady state here we derive the three dimensional surface velocity distribution of storstrommen by using the principle of mass conservation to combine insar measurements from ascending and descending satellite tracks with airborne ice sounding radar measurement of ice thickness the results are compared to insar velocities previously derived by using the assumption of surface parallel flow and to velocities obtained by inin situ gps measurements the velocities derived by using the principle of mass conservation are in better agreement with the gps velocities than the previously calculated velocities derived with the assumption of surface parallel flow

river terrace systems in north west europe an archive of environmental change uplift and early human occupation abstract staircases of large scale aggradational river terraces are a notable feature of many valleys in the temperate lattitudes particularly in areas beyond the reach of the erosive activities of pleistocene ice sheets it is now recognized that the cyclic fluctuations of climate during the quaternary have driven the generation of terraces through the direct and indirect influence of both temperature and precipitation on fluviatile activity where fossiliferous deposits are preserved within terrace sequences it is often possible to date these and to correlate them with the oceanic record thus providing an important framework for the evidence of environmental change on land middle and late pleistocene terraces in different areas can commonly be seen to have formed in synchrony with glacial interglacial cycles or with longer periodicity megacycles climatic forcing alone is insufficient to cause terraces to form however uplift is also necessary so that terrace sequences can provide a useful record of crustal movement in northwest europe where some of the best known studies of river terrace sequences have been carried out the fluviatile deposits are also an important repository for palaeolithic artefacts from which a record of early human occupation can be reconstructed

little ice age glaciers in britain glacier climate modelling in the cairngorm mountains it is widely believed that the last glaciers in the british isles disappeared at the end of the younger dryas stadial 12 9 11 7 cal kyr bp here we use a glacier climate model driven by data from local weather stations to show for the first time that glaciers developed during the little ice age lia in the cairngorm mountains our model is forced from contemporary conditions by a realistic difference in mean annual air temperature of 1 5 c and an increase in annual precipitation of 10 and confirmed by sensitivity analyses these results are supported by the presence of small boulder moraines well within younger dryas ice limits and by a dating programme on a moraine in one cirque as a result we argue that the last glaciers in the cairngorm mountains and perhaps elsewhere in upland britain existed in the lia within the last few hundred years rather than during the younger dryas

u pb dating of a speleothem of quaternary age we demonstrate that u pb dating is a promising method for secondary carbonate materials of quaternary age and older by obtaining a 206pb 238u age for a speleothem with high u 10 g g 1 and very low pb 10 ng g 1 that is supported by an independent 230th age thermal ionisation mass spectrometry was used to determine the u and pb isotopic ratios and concentrations for subsamples of a stalactite from winnats head cave peak district uk we obtained 206pb 204pb ratios up to 50 and determined a 206pb 238u age of 248 10 ka which is within error of the 207pb 235u age of 333 79 ka and a spectrometric u th age of 255 ka for samples of tertiary and quaternary age the initial state of u series disequilibrium is an important consideration and as with most radiometric dating techniques the mineral must have remained closed to u th pb and all intermediate daughters we show that dense calcite speleothems are ideal in this respect and that no loss of rn has occurred unlike u series disequilibrium methods u pb dating has no upper limit and hence materials of quaternary age older than 0 6 ma can be analysed to investigate landscape development paleoclimate hominid evolution or hydrogeochemistry in carbonate terrains

a coupled sheet conduit mechanism for jokulhlaup propagation 1 the largest glacier outburst flood jokulhlaup ever recorded in iceland occurred in 1996 and came from subglacial lake grimsvotn in vatnajokull ice cap among other noteworthy features this flood was characterized by an unprecedentedly high lake level prior to flood initiation extremely rapid linear rise in lake discharge delay between the onset of lake drainage and floodwater arrival at the glacier terminus formation of short lived supraglacial fountains and initially unchannelized outbursts of floodwater at the terminus observations suggest that the 1996 flood propagation mechanism was fundamentally different than that of previously observed floods from grimsvotn we advance a new model whereby floodwater initially propagates in a turbulent subglacial sheet which feeds a nascent system of conduits this model is able to explain key observations made of the 1996 jokulhlaup and may shed light on other outburst floods that do not conform to the standard model

observations of ice sheet motion in greenland using satellite radar interferometry we present interferometric observations of ice sheet motion in western greenland based on pairs of ers 1 synthetic aperture radar sar images these observations provide the first detailed regional view of ice motion associated with dynamically supported topography near the margin of an ice sheet the interferograms of this area are much more complicated than other interferograms of ice sheets presented to date we devote the largest part of this paper to explaining the source of the complexity in these interferograms a synthetic interferogram based on a simple model helps to illustrate the effects of different components of the ice velocity field in interferometric data and suggests a method for estimating the large scale ice velocity field from such data

satellite radar interferometry for monitoring ice sheet motion application to an antarctic ice stream satellite radar interferometry sri provides a sensitive means of monitoring the flow velocities and grounding line positions of ice streams which are indicators of response of the ice sheets to climatic change or internal instability the detection limit is about 1 5 millimeters for vertical motions and about 4 millimeters for horizontal motions in the radar beam direction the grounding line detected by tidal motions where the ice goes afloat can be mapped at a resolution of 0 5 kilometer the sri velocities and grounding line of the rutford ice stream antarctica agree fairly well with earlier ground based data the combined use of sri and other satellite methods is expected to provide data that will enhance the understanding of ice stream mechanics and help make possible the prediction of ice sheet behavior

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

volume changes of vatnajokull ice cap iceland due to surface mass balance ice flow and subglacial melting at geothermal areas 1 we present observed changes in the geometry of western vatnajokull over a period of about ten years which are caused by the surface mass balance ice flow both during surges and quiescent periods and basal melting due to geothermal and volcanic activity comparison of two digital elevation models shows that from 1985 to 1998 the outlet glaciers have lost 14 5 km3 on the average 1 m yr 1 evenly distributed over the area of 1360 km2 however the marginal areas of four outlets have gained up to 80 m in elevation and the uppermost parts have subsided similarly during surges altogether 26 3 km3 were transported from the accumulation areas to the ablation areas of which 19 3 km3 are attributed to surges comparison of dem s from august 1997 and august 1998 revealed 10 ice cauldrons produced by subglacial geothermal activity of which some were previously unknown

effects of glaciations upon karst aquifers in canada abstract in canada there are 5 7 10 5 km 2 of limestone and marble outcrop 6 10 5 km 2 of dolomite and 8 10 4 km 2 of sulfate rocks in addition halite subscrops beneath 5 10 5 km 2 of the central prairie region and interacts with modern groundwater circulation to a varying extent more than 90 of the total outcrop subcrop has been repeatedly glaciated the most recent glaciation late wisconsin terminated 13 000 5000 yrs b p in different areas carbonate terrains still partly buried by glacier ice may be inspected in the rocky mountains nine distinct effects of glacial action upon the propagation and efficiency of karst aquifers are recognized destructive effects include erasure dissection infilling and injection bedrock solution may be inhibited during non glacial periods by a cover of glacial drift rich in carbonate clasts glacial burial may also preserve an aquifer and enhance its storage characteristics aquifer expansion may be stimulated by raising the hydrostatic head or steepening hydraulic gradients or by crustal flexure during deglacial isostatic rebound the differing effects make general prediction of the state of an aquifer in glaciated terrains difficult

thames river sediments of the british early and middle pleistocene abstract the thames drainage system contains the most complete record of fluvial sediments in the british quaternary the kesgrave sands and gravels form the main body of this sediment having been deposited over an interval that spans the later part of the early pleistocene and most of the middle pleistocene conventional stages pre pastonian a through to early anglian ca 1 2 ma this deposit provides a stratigraphic context for related bio morpho and pedostratigraphic evidence and for the recognition of pre anglian pre oi stage 12 upland glaciation in the welsh mountains and intense periglaciation in lowland england it also facilitates correlation with the stratigraphy of northern europe and the global pattern of orbitally forced climatic change this paper describes the first catchment wide correlation of thames river sediments and proposes a formal redefinition of the lithostratigraphic units concerned the scheme outlined here is based on two lithologically distinctive formations defined at type areas reflecting significant changes in catchment dimensions the earlier sudbury formation was formed when the thames catchment extended into wales and drained northwards across east anglia the later colchester formation was formed after the upper catchment had been reduced in extent to that of the present river but continued to flow across east anglia the whole sediment body is now named the kesgrave group

quaternary glaciations of northern europe this paper presents the outcome of a workshop held in berlin in february 2009 concerned with current research on the glacial history of northern europe including the british isles the methodologies presently used to resolve this topic are outlined particular attention is given to new analytical methods deriving from high resolution remote imaging of glacial terrain both on land and on the sea bed key new stratigraphic sections higher resolution results from conventional geochronological methods like radiocarbon and more recently developed technologies such as luminescence and cosmogenic radionuclide dating the relationships between the results derived from these two methods are discussed in further detail along with possible explanations for these differences an outline of a most likely glacial history of the scandinavian and british and irish ice sheets is presented along with possible links to global climate change as represented by the marine isotope mis record tentative evidence for glaciation is identified in mis 22 16 12 10 8 6 4 and 2 and correlations of phases within the last glaciation are also explored for both the scandinavian and british and irish ice sheets the results show that the character and extent of glaciation in different parts of the region are not synchronous and much more geochronological work is required before regional correlations can be established with confidence

speleothems from the earliest quaternary snapshots of paleoclimate and landscape evolution at the northern rim of the alps abstract ancient cave systems in the northern calcareous alps today located well above the timberline at altitudes of 2400 2500 m host u rich speleothems that preserved growth layers on the microscopic scale of presumably annual origin two flowstone samples were dated to 2 019 0 037 0 069 ma and 1 730 0 032 0 068 ma respectively using u pb isochron techniques these ages are corroborated by the late pliocene to early pleistocene pollen spectrum extracted from one of the samples we use a multiproxy approach and exploit laminated speleothem sequences to tie high resolution stable isotope data to a floating lamina counted chronology o isotope values of growth intervals when calcite deposition was close to isotopic equilibrium are low compared to modern and holocene speleothems from other alpine caves and are inconsistent with the current altitudinal setting of the caves a vegetated but geomorphologically stable alpine catchment i e 2000 m asl no peri glacial processes combined with a deep seated cave the thickness of the vadose zone might have exceeded 1000 m is required in order to reconcile the isotopic data with the pollen record and the petrographic evidence furthermore the data can be used to constrain the rate for quaternary rock uplift to 0 8 mm annum for this frontal part of the european alps collectively the data suggest that these speleothems formed both during interglacials mis 59 or 61 and interglacial glacial transitions mis 75 74 or 77 76 but the seasonal precipitation pattern was arguably markedly different from today s provided that the highly regular microscopic laminae are indeed annual lamina counts suggest a minimum length of ca 6 ka for interglacials during the earliest pleistocene

role of sediment in speleogenesis sedimentation and paragenesis although the effects of sedimentation in caves have been recognised for many years its role in speleogenesis is frequently overlooked influxes of sediment into a cave system fundamentally alter the way cave passages develop either by alluviation in a vadose environment forcing lateral corrosion and the development of notches or by upwards dissolution in a phreatic environment through a process known as paragenesis sediment influxes affect the hydrological functioning of a karst aquifer by changing the way conduits behave and subsequently develop both in plan and long section here we give an overview of the mechanisms of cave sedimentation and describe how the process of alluviation and paragenesis affect speleogenesis a characteristic suite of meso and micro scale dissolutional features can be used to recognise paragenetic development which is reviewed here in a vadose environment these include alluvial notches whilst in a phreatic environment half tubes anastomoses and pendants bedrock fins and paragenetic dissolution ramps result using these to identify phases of sedimentation and paragenesis is crucial for reconstructing denudation chronologies from cave deposits we suggest that sedimentation and paragenesis are most likely to occur in certain geomorphological situations such as ice marginal and periglacial environments beneath thick residual soils and where rivers can transport fluvial sediment into a cave either via stream sinks or back flooding

rapid glacier sliding reverse ice motion and subglacial water pressure during an autumn rainstorm measurements of basal water pressure from 15 boreholes located at both local tens of meters and regional kilometers length scales were used to elucidate the pressure sliding relationship during an autumn rapid motion event on bench glacier alaska usa the 8 day event had two distinct phases each with a ten fold speed up with respect to winter velocity the water pressure in all 15 boreholes varied synchronously during the speed up the first phase of rapid sliding began after a peak in basal water pressure and continued while the pressure was elevated and stable or decreasing the second phase of rapid sliding occurred when the basal water pressure was low but increasing and terminated before the pressure peaked pressure and velocity do not appear unrelated but the pressure sliding relationship was not consistently linked to increasing decreasing or a critical water pressure the pressure variations and sliding accelerations are a response to a warm rainstorm although equally large input events occurred in weeks prior with no apparent response drainage system evolution therefore appears to play a key role in both the acceleration and the pressure velocity relationship basal cavity dynamics are likely responsible for three episodes of reverse up valley motion observed after enhanced sliding

surges of glaciers in iceland surges are common in all the major ice caps in iceland and historical reports of surge occurrence go back several centuries data collection and regular obser vationover thelast several decadeshave permitteda detailed descriptionof several surges fromwhich it is possible to generalize onthe nature of surging in icelandic glaciers com bining the historical records of glacier front variations and recent field research we sum marize the geographic distribution of surging glaciers their subglacial topography and geology the frequency andduration of surges changes in glacier surface geometry during the surge cycle and measured velocity changes comparedto calculatedbalance velocities wenotetheindicatorsofsurgeonsetanddescribechangesinice waterandsedimentfluxes duringa surge surgesaccomplishasignificantfractionofthetotalmasstransportthrough the main outlet glaciers of ice caps in iceland and have important implications for their hydrology ouranalysisof the data suggeststhat surge type glaciersin icelandarecharac terizedbygentlysloping surfacesandthatthey movetoo slowlytoremain inbalance given their accumulation rate surge frequency is neither regular nor clearly related to glacier size or massbalance steeply sloping glaciers whetherhard or soft bedded seemto move sufficiently rapidly to keep inbalance with the annual accumulation

evidence for subglacial water transport in the west antarctic ice sheet through three dimensional satellite radar interferometry 1 radarsat data from the 1997 antarctic mapping mission are used interferometrically to solve for the 3 dimensional surface ice motion in the interior of the west antarctic ice sheet wais an area of 125 km2 in a tributary of the kamb ice stream slumped vertically downwards by up to 50 cm between september 26 and october 18 1997 areas in the bindschadler ice stream also exhibited comparable upward and downward surface displacements as the uplift and subsidence features correspond to sites at which the basal water apparently experiences a hydraulic potential well we suggest transient movement of pockets of subglacial water as the most likely cause for the vertical surface displacements these results and related lidar observations imply that imaging the change in ice surface elevation can help reveal the key role of water in the difficult to observe subglacial environment and its important influence on ice dynamics

pre weichselian quaternary glaciations of the british isles the netherlands norway and adjacent marine areas south of 68 n implications for long term ice sheet development in northern europe within this paper we review the pre weichselian glacial history of northern europe focussing on evidence from the british isles netherlands norway and adjacent marine areas that record the activity of the british bis and scandinavian sis ice sheets the objective of the paper is to examine the long term evolution of the two ice sheets in order to determine their level of synchronicity and their relationship to the plio pleistocene record of northern hemisphere glaciation geological evidence demonstrates striking long term similarities between the behaviour of the bis and sis with a step wise intensification in the glacial signal 1 first ice rafted debris ird input onto the continental margin at ca 2 7 2 6 ma and restricted glaciation prior to ca 1 1 ma 2 initiation of more lowland style glaciation from 1 1 ma 3 repeated shelf edge glaciations from 0 45 ma these steps coincide with a major intensification of the northern hemisphere climate signal at the beginning of the quaternary and the mid pleistocene transition temporal and spatial variabilities in the behaviour of different sectors of the bis and sis appear to reflect regional scale geographic latitude elevation climatic moisture temperature and glaciological deformable beds controls that modulate the build up of ice volume in the main ice accumulation areas relative to global climate forcing and its subsequent lateral expansion 2010 nerc

hydraulics of subglacial cavities modele theorique du comportement a l etat stationnaire des cavites remplies d eau et interconnectees sur le lit du glacier

what controls dead ice melting under different climate conditions a discussion abstract in the geological record hummocky dead ice moraines represent the final product of the melt out of dead ice processes and rates of dead ice melting in ice cored moraines and at debris covered glaciers are commonly believed to be governed by climate and debris cover properties here backwasting rates from 14 dead ice areas are assessed in relation to mean annual air temperature mean summer air temperature mean annual precipitation mean summer precipitation and annual sum of positive degree days the highest correlation was found between backwasting rate and mean annual air temperature however the correlation between melt rates and climate parameters is low stressing that processes and topography play a major role in governing the rates of backwasting the rates of backwasting from modern glacial environments should serve as input to de icing models for ancient dead ice areas in order to assess the mode and duration of deposition a challenge for future explorations of dead ice environments is to obtain long term records of field based monitoring of melt progression furthermore many modern satellite borne sensors have high potentials for recordings of multi temporal digital elevation models dems for detection and quantification of changes in dead ice environments in recent years high accuracy dems from airborne laser scanning altimetry lidar are emerging as an additional data source however time series of high resolution aerial photographs remain essential for both visual inspection and high resolution stereographic dem production

isostatic compensation of quaternary vertical crustal motions coupling between uplift of britain and subsidence beneath the north sea new data from offshore and onshore regions confirm the view that the quaternary uplift of britain and subsidence beneath the north sea are coupled mediated by lower crustal flow induced by the lateral pressure gradient caused by climate driven surface processes most measured onshore uplift rates in both upland and lowland localities are only applicable since the mid pleistocene revolution beforehand generally lower rates prevailed earlier oligocene pliocene phases of uplift and subsidence had typically even lower rates furthermore the western margin of the north sea depocentre was 200 km farther from the modern coastline of north east england in the earliest cenozoic complex isostatic adjustments occurred in response to the magmatism associated with the iceland mantle plume the contemporaneous land surface relative to the present day rock column in northern england was typically 1500 m od however this activity died out during the palaeogene the main effect of these events on the modern isostatic configuration was via the emplacement of thick mafic underplating which now constricts the mobile lower crustal layer explaining the late cenozoic ultra stability of ireland the evidence enables interpretations of the present day topography of britain as static caused by the iceland mantle plume or actively developing in response to plate motions or plate boundary forces to be excluded

geologic and geochemical evidence for the nature and development of the middle proterozoic keweenawan midcontinent rift of north america abstract general models of active continental rifting must take into account the large plateau basalt provinces many of which have been precursors of or associated with continental breakup the keweenawan middle proterozoic midcontinent rift mcr of north america 2300 km long is one such major continental rift but it aborted before significant crustal separation was achieved these plateau basalt rifts including the mcr differ from the east african type in being dominated by broad subsidence rather than rift valleys tholeiitic flood basalts rather than alkalic central volcanoes and large positive gravity anomalies geologic and geophysical evidence is reviewed which contradicts some recent models which have been proposed for the development of the mcr the basic causes of the pre rift subsidence the u shaped nature of the structure and the abrupt termination of its activity are obscure a triple junction in the usual sense does not appear to have been formed

effects of glaciations and permafrost upon the development of karst in canada canadian examples suggest that karst landforms may be divided into eight types in terms of their temporal relationships to the record of repeated quaternary glaciations two types are postglacial two are subglacial one type occurs where glacial features are adapted to karstic drainage and three types display sequences of karstic and glacial action glacier effects upon karst landforms and their underlying aquifers display the gamut of possibilities they may destroy inhibit preserve or stimulate karst development where continuous permafrost is maintained when covered by glacier ice postglacial karst is limited to the active layer epikarst where permafrost is thawed beneath ice or during deglaciation there are a variety of postglacial karst developments depending in part upon climate and in part upon local lithologic and relief conditions

a 1 2 ma record of glaciation and fluvial discharge from the west european atlantic margin abstract the correlation of continental sedimentary records with the marine isotope stratigraphy is a challenge of central importance in quaternary stratigraphy particularly in western europe where long records of glaciation on land areas are particularly rare here we demonstrate for the first time the interrelationship of events during the last 1 2 ma in an ocean sediment core from the bay of biscay ne atlantic sw of the channel the identification of discharge variations from tributary river systems to the fleuve manche palaeoriver during glacio eustatic sea level lowstands demonstrates the correlation of the marine sediment stratigraphy to the expansion and recession of the european ice sheets the amplitude and chronology of european ice sheet oscillations since the late early pleistocene is discussed and our results demonstrate that the first coalescence of the fennoscandian and british ice sheets in the north sea basin ca 450 ka ago caused a profound change in lowstand european drainage alignment this change caused a rerouting of fennoscandian and eastern british ice sheets derived meltwaters from northwards into the nordic seas to southwards into the eastern north atlantic thereafter besides allowing a thorough synchronisation of the european ice sheet palaeogeography with the well dated records of palaeoceanographical changes our results improve the stratigraphy of the english channel palaeovalleys and will provide important constraints on paleoclimatic scenarios considering the impact that such rerouting and meltwater surges might have on the stability of the oceanic conveyor belt

icecap and subglacial crustal deformation inferred from sar pixel tracking the 2014 dike intrusion episode in the barðarbunga volcanic system iceland

ice fracturing during jokulhlaups implications for englacial floodwater routing and outlet development theoretical studies of glacial outburst floods jokulhlaups assume that i intraglacial floodwater is transported efficiently in isolated conduits ii intraglacial conduit enlargement operates proportionally to increasing discharge iii floodwater exits glaciers through pre existing ice marginal outlets and iv the morphology and positioning of outlets remains fixed during flooding direct field observations together with historical jokulhlaup accounts confirm that these theoretical assumptions are not always correct this paper presents new evidence for spatial and temporal changes in intraglacial floodwater routing during jokulhlaups secondly it identifies and explains the mechanisms controlling the position and morphology of supraglacial jokulhlaup outlets and finally it presents a conceptual model of the controls on supraglacial outbursts field observations are presented from two icelandic glaciers skei ararjokull and solheimajokull video footage and aerial photographs taken before during and after the skei ararjokull jokulhlaup and immediately after the solheimajokull jokulhlaup reveal changes in floodwater routing and the positioning and morphology of outlets field observations confirm that glaciers cannot transmit floodwater as efficiently as previously assumed rapid increases in jokulhlaup discharge generate basal hydraulic pressures in excess of ice overburden under these circumstances floodwater can be forced through the surface of glaciers leading to the development of a range of supraglacial outlets the rate of increase in hydraulic pressure strongly influences the type of supraglacial outlet that can develop steady increases in basal hydraulic pressure can retro feed pre existing englacial drainage whereas transient increases in pressure can generate hydraulic fracturing the position and morphology of supraglacial outlets provide important controls on the spatial and temporal impact of flooding the development of supraglacial jokulhlaup outlets provides a new mechanism for rapid englacial debris entrainment

effects of subglacial geothermal activity observed by satellite radar interferometry we use one day synthetic aperture radar sar interferograms from data of the earth remote sensing satellites ers 1 and ers 2 to study ice flow and uplift of two surface depressions within the vatnajokull ice cap iceland the ice cauldrons are created by melting at sub glacial geothermal areas meltwater accumulates in a reservoir under the cauldrons over 2 to 3 years until it drains in a jokulhlaup under the ice dam surrounding the reservoir the ice surface in the depressions drops down by several tens of meters during these draining events but rises again as ice flows into the depressions until a jokulhlaup occurs again using sar interferograms we quantify an uplift rate of about 2 to 18 cm day within the jokulhlaup cycle varying with the surface slope of the depressions the uplift rate is high during the first months after a jokulhlaup when the cauldron is relatively deep with steep slopes but the uplift rate decreases as the cauldron is gradually filled a simple axisymmetric model simulating the ice flow into one of the depressions describes quantitatively the filling rate of the cauldron and qualitatively the shape of the ice flow field the best fit model has an ice flow law parameter a0 that is about one order of magnitude lower than typically estimated for temperate glaciers

three dimensional surface motion maps estimated from combined interferometric synthetic aperture radar and gps data 1 we provide a technique to efficiently produce high resolution three dimensional surface motion maps by combining information about the motion of the earth s surface from interferometric observations of synthetic aperture radar images and repeated global positioning system gps geodetic measurements unwrapped interferograms showing pixel wise change in range from ground to satellite and sparse values of threedimensional movements are required as input the problem of finding the full threedimensional motion field is separated into two two dimensional problems initially the vertical component of the deformation field and its horizontal component in the look direction of the satellite are found later the look direction component is resolved into north and east components initial values for the motion fields are assigned to each pixel of interferograms from interpolation of available gps observations these values are then updated and optimized by comparison with the interferograms and the gps observations an additional constraint is an assumption of a smoothly varying motion field markov random field based regularization and simulated annealing algorithm are used for the optimization the technique is applied to create surface motion maps for the reykjanes peninsula sw iceland index terms 0933 exploration geophysics remote sensing 0910 exploration geophysics data processing 1206 geodesy and gravity crustal movements interplate 8155 8150 tectonophysics evolution of the earth plate boundary general 3040 8155 tectonophysics evolution of the earth plate motions general keywords remote sensing motion maps deformation data fusion interferometry gps

glacier volcano interactions deduced by sar interferometry glacier surface displacements produced by geothermal and volcanic activity beneath vatnajokull ice cap in iceland are described by field surveys of the surface topography combined with interferograms acquired from repeat pass synthetic aperture radar images a simple ice flow model serves well to confirm the basic interpretation of the observations the observations cover the period october 1996 january 1999 and comprise a the ice flow field during the infilling of the depressions created by the subglacial gjalp eruption of october 1996 b the extent and displacement of the floating ice cover of the subglacier lakes of grimsvotn and the skafta cauldrons c surface displacements above the subglacier pathways of the jokulhlaups from the gjalp eruption site and the grimsvotn lake d detection of areas of increased basal sliding due to lubrication by water and e detection of spots of temporal displacement that may be related to altering subglacial volcanic activity at the depression created by the gjalp eruption the maximum surface displacement rate away from the radar decreased from 27 cm d 1 to 2 cm d over the period january 1997 january 1999 the observed vertical displacement of the ice cover of grimsvotn changed from an uplift rate of 50 cm d 1 to sinking of 48 cm d 1 and for skafta cauldrons from 2 cm d 1 to 25 cm d 1

a new volcanic province an inventory of subglacial volcanoes in west antarctica abstract the west antarctic ice sheet overlies the west antarctic rift system about which due to the comprehensive ice cover we have only limited and sporadic knowledge of volcanic activity and its extent improving our understanding of subglacial volcanic activity across the province is important both for helping to constrain how volcanism and rifting may have influenced ice sheet growth and decay over previous glacial cycles and in light of concerns over whether enhanced geothermal heat fluxes and subglacial melting may contribute to instability of the west antarctic ice sheet here we use ice sheet bed elevation data to locate individual conical edifices protruding upwards into the ice across west antarctica and we propose that these edifices represent subglacial volcanoes we used aeromagnetic aerogravity satellite imagery and databases of confirmed volcanoes to support this interpretation the overall result presented here constitutes a first inventory of west antarctica9s subglacial volcanism we identified 138 volcanoes 91 of which have not previously been identified and which are widely distributed throughout the deep basins of west antarctica but are especially concentrated and orientated along the 3000 km central axis of the west antarctic rift system

a review of terrestrial radar interferometry for measuring surface change in the geosciences this paper presents a review of the current state of the art in the use of terrestrial radar interferometry for the detection of surface changes related to mass movement different hardware types and acquisition concepts are described which use either real or synthetic aperture for radar image formation we present approaches for data processing procedures paying special attention to the separation of high resolution displacement information from atmospheric phase variations recent case studies are used to illustrate applications in terrestrial radar interferometry for change detection applications range from detection and quantification of very slow moving millimeters to centimeters per year displacements in rock walls from repeat monitoring to rapid processes resulting in fast displacements 50 m yr acquired during single measurement campaigns with durations of only a few hours fast and episodic acting processes such as rockfall and snow avalanches can be assessed qualitatively in the spatial domain by mapping decorrelation caused by those processes a concluding guide to best practice outlines the necessary preconditions that have to be fulfilled for successful application of the technique as well as in areas characterized by rapid decorrelation empirical data from a ku band sensor show the range of temporal decorrelation of different surfaces after more than two years for rock surfaces and after a few seconds to minutes in vegetated areas during windy conditions the examples show that the displacement field can be measured for landslides in dense grassland ice surfaces on flowing glaciers and snowpack creep copyright 2014 john wiley sons ltd

a spectrum of jokulhlaup dynamics revealed by gps measurements of glacier surface motion gps campaigns on glaciers during jokulhlaups show how subglacial floods affect glacier motion and shed light on the dynamics of such floods three such campaigns have been carried out on southern and western vatnajokull southeast iceland over known jokulhlaup paths two slowly rising jokulhlaups from grimsvotn and two rapidly rising jokulhlaups from the western and eastern skafta cauldrons were captured in these campaigns with maximum discharge ranging from 240 to 3300 m 3 s 1 glacier surface movements measured in these campaigns are presented along with the corresponding discharge curves the measurements are interpreted as indicating 1 initiation of rapidly rising jokulhlaups with a propagating subglacial pressure wave 2 decreased glacier basal friction during jokulhlaups 3 subglacial accumulation of water in slowly rising jokulhlaups and 4 lifting of the glacier caused by subglacial water pressure exceeding overburden in both rapidly and slowly rising jokulhlaups the latter two observations are inconsistent with assumptions that are typically made in theoretical and numerical modelling of jokulhlaups both viscous and elastic deformation of the glacier as well as turbulent hydraulic fracture at the ice bedrock interface are important in the dynamics of the subglacial pressure wave at the front of rapidly rising jokulhlaups

a reassessment of the brassington formation miocene of derbyshire uk and a review of related hypogene karst suffosion processes the brassington formation of the pennines is the most extensive onshore miocene succession in the uk it is preserved as outliers in lower carboniferous limestone during the cenozoic central england underwent uplift with erosion of post mississippian strata from the pennine axis in the peak district the brassington formation is hence significant in reconstructing cenozoic geological history it is non marine derived from triassic sandstone and of mid late miocene age the c 60 outliers occur in three clusters over c 220 km2 they are remnants of a sedimentary prism at least 75 m thick and with a volume of c 10 km3 suffosion of bedrock conditioned by aggressive precursor fluids of hypogenic origin was the major control on subsidence the calculated volume of the fills in the bees nest and green clay outliers is 3 7 5 0 106 m3 referenced to a sub miocene surface at c 450 m od in pre subsidence times the original volume of this subsidence complex was c 21 8 106 m3 and the aggregated volume of the late neogene white peak palaeokarst was at least 0 66 km3 subsidence was concomitant with pliocene uplift of the pennine axis suggesting suffosion accommodation of 3 106 m3 per km2 or 244 m3 per annum

surface ice motion deviating toward the margins during speed up events at gornergletscher switzerland 1 high frequency ice flow measurements during speed up events in gornergletscher switzerland revealed intriguing ice motion which has never been reported in detail before during the summer 2005 more than a 100 flow speed increase was observed three times at four gps stations installed across gornergletscher the speed ups were accompanied by a decimeter scale surface uplift two of the events were triggered by intensive surface melt and rainfall while the third one was due to the outburst of gornersee a glacier dammed lake located 2 km upglacier an interesting observation was ice motion deviating toward the side margins during the events as the glacier accelerated a transverse cross glacier velocity component was generated turning the flow direction away from the central flow line toward the margins when the glacier decelerated the transverse velocity component reversed so that the ice flowed back to the azimuth of the initial flow direction in the most significant case the trajectory of the survey stake deviated from the original track by 0 2 m in the transverse direction we hypothesize that the observed lateral ice motion was caused by locally elevated subglacial water pressure when the ice sole decoupled from the bed at a part of the glacier a point source of vertical displacement was transmitted to the surface through viscous ice this caused the transverse as well as vertical surface motion as observed in ground motion during magma intrusion the hypothesis was tested with a two dimensional ice flow model applied to the transverse glacier cross section the model confirmed that the surface ice would move toward the margins as observed in gornergletscher if subglacial water pressure exceeded the ice overburden pressure over a limited part of the bed

understanding jokulhlaups from tale to theory the study of jokulhlaups has advanced from describing them qualitatively to measuring systematically on site their discharge curves and source geometry as well as investigating conditions when particular floods were initiated in the early days of this science hypotheses were formulated which attempted to explain the processes responsible for unstable flood growth the general theory of time dependent turbulent water flow through intraglacial conduits marked a breakthrough in understanding jokulhlaup hydraulics and has since remained a classical foundation for jokulhlaup studies this theory described flood evolution in terms of water input into the upper end of a tunnel leading from a reservoir taking account of sensible stored lake heat and geometry such a tunnel expands by positive feedback since its ice walls melt through frictional heat produced by the flowing water while this model successfully described jokulhlaups in which the discharge hydrograph increases more or less exponentially over time other floods were observed which are in stark contrast in them pressurized floodwater propagates in a turbulent subglacial water sheet which opens and expands a system of conduits with an extremely rapid linear rise in discharge the rushing water enlarges its subglacial pathway not only by melting the ice around it but also by lifting deforming and fracturing it subsequent drainage is nourished by two different sources with conduit growth being controlled not only by water input at the upper end of the flood path but also by further water joining in all the way to the edge of the glacier moreover the trigger for releasing meltwater from glacial lakes has been seen to be either of two conduit opening mechanisms which also determine whether a turbulent subglacial water sheet will result or not on the one hand drainage may begin by expanding already existing conduits in which case the pressure remains lower than that of the ice overburden at the dam and the conduits grow slowly on the other hand lake levels may rise until flotation of the ice dam abruptly opening a sufficient gap for water to propagate as a sheet flow in order to explain which of these two initiation mechanisms will act further studies are required into the structure of hydrological drainage systems under ice dams and into their linkage with these two mechanisms although current theoretical models of jokulhlaup drainage have managed to reproduce the observed discharge patterns inconsistencies with the observed behaviour of subglacial drainage systems along the flood path demand further exploration of the actual hydraulic and glaciodynamic processes of jokulhlaups

speleogenetic evidence from ogof draenen for a pre devensian glaciation in the brecon beacons south wales uk the british isles have been affected by as many as 30 glaciations during the quaternary however the evidence for pre devensian glaciations in upland regions is scarce understanding the extent and timing of earlier upland glaciations is essential for modelling the long term evolution and sensitivity of the british ice sheet caves being protected from surface erosion and weathering can preserve evidence of earlier glaciations in the form of speleothem and sediment archives the 70 km long ogof draenen cave system in south wales uk contains multiple cave levels related to changes in the surface topography and drainage during the past 0 5 ma the cave contains evidence of massive influxes of sediment that were sufficient to choke the cave and alter the underground drainage analysis of the cave sediments passage morphology and geometry suggests the cave once acted as a subterranean glacial spill way before being overridden by ice speleothem u series data demonstrate that this sediment influx occurred before marine isotope state mis 9 probably during the anglian glaciation mis 12 evidence from ogof draenen indicates the impact of subsequent glaciations on the landscape evolution of the region was minimal and that much of the surface topography dates from the anglian

uranium series chronology and environmental applications of speleothems an increasing number of scientists recognize the value of speleothems1 as often extremely well preserved archives of information about past climate vegetation hydrology sea level nuclide migration water rock interaction landscape evolution tectonics and human action well constrained data are required to document past changes reconstruct past patterns and predict future responses of the earth system at a wide range of spatial and temporal scales speleothems are particularly useful in this regard because they can be found in many locations of the globe sampled at high resolution and reliably dated using high precision uranium series techniques speleothems are bodies of mineral material formed in caves as the result of chemical precipitation from groundwater flowing or dripping in a cave most speleothems are composed of calcite formed by slow degassing of co2 from supersaturated groundwater but aragonite and gypsum forms are also common particularly near cave entrances where evaporative effects are important a host of different speleothem types decorate the walls ceilings and floors of caves and their mineralogy and morphology is a function of fluid flow and chemistry of waters feeding a particular location as well as the ambient conditions temperature chemistry light in the air or water filled void subaerial forms include the familiar stalagmites stalactites draperies flowstones subaqueous forms include rimstone pools rafts mammillary calcite wall coatings and dog tooth spar for an extensive review of the types of speleothem that have been observed see hill and forti 1997 speleothems are used in a multitude of ways to explore past environmental conditions perhaps the most fundamental of which is their very presence or absence deposition of speleothems relies on sufficient water supply and soil co2 to enable dissolution and transport of reactants in the vadose zone to underlying caves in arid or glacial times conditions may not have been favorable for speleothem formation

borehole water level variations and the structure of the subglacial hydrological system of haut glacier d arolla valais switzerland late summer subglacial water pressures have been measured in a dense array of boreholes in the ablation area of haut glacier d arolla switzerland interpolated surfaces of minimum diurnal water pressure and diurnal water pressure variation suggest the presence of a subglacial channel within a more widespread distributed drainage system the channel flows along the centre of a variable pressure axis vpa some tens of metres wide that is characterized by low minimum diurnal water pressures frequently atmospheric and high diurnal water pressure variations these characteristics are transitional over a lateral distance of c 70 m to higher and more stable subglacial water pressures in the adjacent distributed system water pressure variations recorded in boreholes located close to the centre of the vpa reflect the delivery of surface derived meltwater to the glacier bed and result in a diurnally reversing transverse hydraulic gradient that drives water out from the channel into the distributed system during the afternoon and back to the channel overnight subglacial observations suggest that such flow occurs through a vertically confined sediment layer borehole turbidity records indicate that the resulting diurnal water flows are responsible for the mobilization and transport of fine debris in suspension analysis of the propagation velocity and amplitude attenuation of the diurnal pressure waves suggests that the hydraulic conductivity of the sediment layer decreases exponentially with distance from the channel falling from c 10 4 m s 1 at the channel boundary to c 10 7 ms 1 70m away these apparent hydraulic conductivities are consistent with darcian flow through clean sand and typical glacial till respectively we suggest that fine material is systematically flushed from basal sediments located adjacent to large melt season drainage channels beneath warm based glaciers this process may have important implications for patterns of glacier erosion hydrochemistry and dynamics

drainage through subglacial water sheets 1 subglacial drainage plays an important role in controlling coupling between glacial ice and underlying bed here we study the flow of water in thin macroporous sheets between ice and bed previous work shows that small perturbations in depth of a nearly parallel sided water film grow unstably because these areas have enhanced viscous dissipation that leads to enhanced melting of an ice roof we argue that in the presence of bed protrusions bridging a water sheet downward motion of the ice roof can stabilize this sheet stability results when the rate of roof closure increases faster with water depth than the rate of viscous dissipation we therefore modify existing theory to include protrusions that partially support the overlying glacier differences in the pressure on protrusions relative to water pressure drive roof closure the mechanisms of both regelation and creep normal to the bed close the overlying ice roof and decrease the ice bed gap in order to account for multiple protrusion sizes along the bed for instance resulting from an assortment of various sized sediment grains we incorporate a method of partitioning overburden pressure among different protrusion size classes and the water sheet partitioning is dependent on the amount of ice protrusion contact and therefore water depth this method allows prediction of roof closure rates we then investigate stable steady sheet configurations for reasonable parameter choices and find that these steady states can occur for modest water depths at very low effective pressures as is appropriate for ice streams moreover we find that multiple steady sheet thicknesses exist raising the possibility of switches between low and high hydraulic conductivity regimes for the subglacial water system

characteristics of subglacial drainage systems deduced from load cell measurements rock tunnels beneath engabreen northern norway permit access to the ice bedrock interface beneath a 210 m thick glacier eight load cells have been installed in the bedrock of the glacier sole along a 22 m transect with some interruptions the load cells have been logged at 15 min intervals since december 1992 here we analyse the records until 2003 load cell signals measure stresses acting normal to the bedrock and usually log the pressure of the thin water film between the basal ice and the bed occasionally there are distinct pressure events characterized by short lived hours local minima often followed by a maximum before decaying to background load cell pressure the amplitudes of these pressure events are of the order of 0 01 1 mpa and depend on the placement of the sensor and the state of the subglacial drainage system we identify winter and summer pressure regimes the winter regime is characterized by few pressure events of large pressure amplitude influencing all load cells a lag of 0 6 days is observed between surface forcing rain and pressure events during winter the summer regime typically has periods of daily pressure events of low amplitude no delay is seen between surface forcing and pressure events during the summer regime in summer the onset of a pressure event is correlated with a local maximum of the derivative of the subglacial discharge record whereas no such relation is found during the winter regime the transition from winter to summer may june is easily detectable and is strongly correlated with a rapid increase in subglacial discharge and the transition to a dominating r channel system the autumn transition is less clearly defined but has usually occurred by the beginning of november stress bridging an increase in bed pressure at the edge of low pressure channels is recorded during the summer regime water pressures at the bed are connected or unconnected to the drainage system pressure increases in the connected system beyond local normal stress values lead to an uplift of the connected system and a pressure drop in the unconnected system the occurrence of pressure events is determined by the capacity of the drainage system uplift is controlled by local normal stress values and not mean ice overburden pressure

the impact of jokulhlaups on basal sliding observed by sar interferometry on vatnajokull iceland we have analyzed insar data from the ers 1 ers 2 tandem mission to study the ice dynamics of vatnajokull iceland during jokulhlaups from the skaftacauldrons and the grimsvot n geothermal area which drained under the tungnaarjokull and skei ararjokull outlets respectively during the initial phase of a grimsvotn jokulhlaup in march 1996 the velocity of skei ararjokull increased up to three fold relative to observed velocities in december 1995 over an area up to 8 km wide around the subglacial flood path accumulation of water was observed at one location in the flood path during a small jokulhlaup from the skaftacauldrons in october 1995 the velocity on tungnaarjokull increased up to four fold over a 9 km wide area the velocity increase was observed 1 5 days before the floodwater was detected in the river skafta a reduced glacier speed as the flood peaked in skaftaindicates evolution of the subglacial drainage system from sheet to tunnel flow the glacier acceleration and local uplift observed in the early phase of both jokulhlaups supports the concept that increased water inflow in a narrow tunnel system causes water pressure to rise and forces water into areas outside the channels thus reducing the coupling of ice with the glacier bed

early melt season velocity fields of langjokull and hofsjokull central iceland we infer the horizontal velocity fields of the ice caps langjokull and hofsjokull central iceland using repeat pass interferometric synthetic aperture radar insar nasa s uninhabited aerial vehicle synthetic aperture radar uavsar acquired airborne insar data from multiple vantage points during the early melt season in june 2012 we develop a bayesian approach for inferring three dimensional velocity fields from multiple insar acquisitions the horizontal components generally agree with available gps measurements wherever ice motion is well constrained by insar observations we provide evidence that changes in volumetric moisture content near the glacier surface induce phase offsets that obfuscate the vertical component of the surface velocity fields an effect that could manifest itself on any glacier that experiences surface melt spatial patterns in the insar derived horizontal speeds are broadly consistent with the results of a simple viscous flow model and the directionality of the insar derived horizontal flow field is nearly everywhere consistent with the ice surface gradient significant differences between the insar derived horizontal speed and the speed predicted by the viscous flow model suggest that basal slip accounts for more than half the observed outlet glacier flow

funnel shaped surface depressions indicator or accelerant of rapid glacier disintegration a case study in the tyrolean alps abstract alpine glaciers have been retreating at extreme and historically unprecedented rates while the general course of regional retreat rates reflects long term climatic change individual extreme events are closely related to the geomorphological settings and processes of the specific glacier nevertheless these extreme events also influence the regional means and might be an important feedback mechanism accelerating the response of glaciers to climate change in 2009 during the recent disintegration of the terminus of gepatschferner 46 52 30 n 10 45 25 e a shallow circular depression appeared at the glacier tongue with a decrease of surface ice flow velocity to almost nil in 2015 the area was ice free during a heavy precipitation event in august 2012 a subglacial sediment layer of 10 m was flushed out which accelerated the subsidence of the ice surface the development of this 15 to 30 m deep depression was monitored with a combination of methods in high detail including direct ablation measurements and a time series of seven high resolution airborne laser dems plus recordings of ice flow velocity and surface elevation with dgps the thickness of ice and sediment layers was measured with vibroseismic soundings in 2012 and 2013 similar developments were observed at three other glaciers with extreme retreat rates our investigation suggests that this mechanism has a major impact on and can be read as an indicator of a nonlinear increased response of glaciers to climate change

subglacial till a physical framework for its properties and processes the continuum theory of mixtures provides a framework around which present knowledge of subglacial till can be organized the proposed framework is provisional in two senses the constitutive description of till will require improvement and not all potentially significant processes have been included till is treated as a three component mixture of water clasts coarse grained solids and fine grained matrix solids these components obey mass balance equations and their behavior is subject to constitutive relations that describe such processes as darcian water transport fine grained sediment transport consolidation shear deformation dilatancy and comminution five state variables uniquely determine the compressibility permeability shear strength and other physical properties numerical solutions of the state evolutions equations illustrate that the properties of till are highly responsive to changes in the subglacial environment

flow mechanism of glaciers on soft beds subhourly measurements of bed deformation bed shear strength subglacial water pressure and surface speed at storglaciaren a glacier in northern sweden showed that the shear strain rates of the bed decrease during periods of high water pressure and surface speed high water pressures appear to be accompanied by a reduction in the coupling of ice with the bed that is sufficient to reduce or eliminate shearing the instability of large ice masses may result from similar decoupling rather than from pervasive bed deformation as has been commonly thought

plastic bed beneath hofsjokull ice cap central iceland and the sensitivity of ice flow to surface meltwater flux author s minchew b simons m bjornsson h palsson f morlighem m seroussi h larour e hensley s abstract the author s 2016 the mechanical properties of glacier beds play a fundamental role in regulating the sensitivity of glaciers to environmental forcing across a wide range of timescales glaciers are commonly underlain by deformable till whose mechanical properties and influence on ice flow are not well understood but are critical for reliable projections of future glacier states using synoptic scale observations of glacier motion in different seasons to constrain numerical ice flow models we study the mechanics of the bed beneath hofsjokull a land terminating ice cap in central iceland our results indicate that the bed deforms plastically and weakens following incipient summertime surface melt combining the inferred basal shear traction fields with a coulomb plastic bed model we estimate the spatially distributed effective basal water pressure and show that changes in basal water pressure and glacier accelerations are non local and non linear these results motivate an idealized physical model relating mean basal water pressure and basal slip rate wherein the sensitivity of glacier flow to changes in basal water pressure is inversely related to the ice surface slope