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

pollution and paradigms lessons from icelandic volcanism for continental flood basalt studies grattan j pollution and paradigms lessons from icelandic volcanism for continental flood basalt studies lithos 2005 79 pp 343 353

basal flow characteristics of a non linear flow sliding frictionless over strongly undulating bedrock the flow field of a medium sliding without friction over a strongly undulating surface is calculated numerically the results are used to elucidate the basal flow characteristics of glacier flow and they are discussed with reference to known analytical solutions extrusion flow is found to become increasingly pronounced as the value of n where n is a parameter in glen s flow law becomes larger for sinusoidal bedrock undulations a flow separation occurs if the amplitude to wavelength ratio exceeds a critical value of about 0 28 the main flow then sets up a secondary flow circulation within the trough and the ice participating in this circular motion theoretically never leaves it the sliding velocity is calculated numerically as a function of the mean basal shear stress the amplitude to wavelength ratio and the flow parameter n for moderate and high slope fluctuations the sliding velocity is significantly different from what would be expected from results based on the small slope approximation

high latitude eruptions cast shadow over the african monsoon and the flow of the nile 1 nile river records indicate very low flow following the 1783 1784 laki volcanic eruption as well as after other high latitude volcanic eruptions as shown by climate model simulations of the laki eruption significant cooling 1 to 3 c of the northern hemisphere land masses during the boreal summer of 1783 resulted in a strong dynamical effect of weakening the african and indian monsoon circulations with precipitation anomalies of 1 to 3 mm day over the sahel of africa thus producing the low nile flow future high latitude eruptions would significantly impact the food and water supplies in these areas using observations of the flow of the nile river this new understanding is used to support a date of 939 for the beginning of the eruption of the eldgja volcano in iceland the largest high latitude eruption of the past 1500 years

mortality in england during the 1783 4 laki craters eruption 1783 4 has been recognised as a mortality crisis year in the population history of england this demographic incident coincides with the laki craters eruption iceland which began in june 1783 and fumigated many parts of europe with volcanic gases and particles many reports and proxy climate records implicate the volcanic cloud in meteorological anomalies including notably hot 1783 summer conditions in england and a severe subsequent winter we present here a detailed analysis of the geographical and temporal trends in english mortality data and interpret them in the light of the climatological records and observations of the pollutant cloud we show that there were two distinct crisis periods in august september 1783 and january february 1784 which together accounted for 20 000 extra deaths in both cases the east of england was the worst affected region possible causes for the two crisis periods are considered and we conclude that the timing and magnitude of the winter mortality peak can be explained by the severe cold of january 1784 the late summer mortality followed 1 2 months after the very hot july of 1783 and may also have been related to the weather with the time lag reflecting the relatively slow spread of enteric disease or the contraction of malaria however it is hard to explain the entire late summer anomaly by these high temperature causes we therefore consider that fine acid aerosol and or gases in the volcanic haze may also have contributed to the unusual august september mortality given that complex radiative and dynamical effects of the volcanic cloud are implicated in the climatic anomalies in 1783 4 it is likely that the laki craters eruption did play a role in the english mortality crises of the same period

regional warming of the lower atmosphere in the wake of volcanic eruptions the role of the laki fissure eruption in the hot summer of 1783

illness and elevated human mortality in europe coincident with the laki fissure eruption grattan j durand m taylor s illness and elevated human mortality in europe coincident with the laki fissure eruption in volcanic degassing geological society special publication 213 oppenheimer c pyle d m and barclay j eds geological society london special publications 410 414 2003

is the faroe bank channel overflow hydraulically controlled abstract the overflow of dense water from the nordic seas through the faroe bank channel fbc has attributes suggesting hydraulic control primarily an asymmetry across the sill reminiscent of flow over a dam however this aspect has never been confirmed by any quantitative measure nor is the position of the control section known this paper presents a comparison of several different techniques for assessing the hydraulic criticality of oceanic overflows applied to data from a set of velocity and hydrographic sections across the fbc these include 1 the cross stream variation in the local froude number including a modified form that accounts for stratification and vertical shear 2 rotating hydraulic solutions using a constant potential vorticity layer in a channel of parabolic cross section and 3 direct computation of shallow water wave speeds from the observed overflow structure though differences exist the three methods give similar answers suggesting that the fbc is indeed controlled with a

a three dimensional numerical model of the confluence area of unteraargletscher bernese alps switzerland with the use of a numerical three dimensional 3 d model the flow dynamics of the confluence area of unteraargletscher bernese alps switzerland are studied previous predictions based on conceptual two dimensional models about flow characteristics at confluence areas are tested against results from the fully 3 d model measured winter velocities are used for model verification despite some consistent systematic differences good overall agreement between measured and calculated surface velocities is obtained the calculated vertical strain rate variation with depth is in good agreement with available measurements from boreholes the ice is found to be almost three times stiffer than standard estimates of rheological parameters for glacier ice would predict the model predicts a complicated yet realistic pattern of vertical velocity variation along the surface the most noticeable features of the vertical velocity distribution across the surface are listed and their relation to topographic surface undulations and the overall dynamics of the confluence discussed in accordance with previous results from analytical models a strongly localized surface trough and a concomitant negative downward orientation vertical velocity anomaly develop at the junction point although depth integrated strain rates are positive extension the basal layer is compressed vertically the ice cored medial moraine is formed by differential ablation the flow mechanics of the confluence area play only an indirect role by enabling transfer of debris covered marginal ice towards the confluence center in the absence of differential ablation an elongated surface depression would be formed in the down glacier direction from the junction point instead of an elevated ice cored medial moraine

modelling mass balance using photogrammetric and geophysical data a pilot study at griesgletscher swiss alps the kinematic boundary condition at the glacier surface can be used to give glacier mass balance at a point as a function of changes in the surface elevation and of the horizontal and vertical velocities vertical velocity can in turn be estimated from basal slope basal ice velocity and surface strain in a pilot study on the tongue of griesgletscher swiss alps the applicability of the relation for modelling area wide ice flow and mass balance distribution is tested the key input of the calculations i e the area wide surface velocity field is obtained using a newly developed photogrammetric technique ice thickness is derived from radar echo soundings error estimates and comparisons with stake measurements show an average accuracy of approximately 0 3 m a for the calculated vertical ice velocity at the surface and 0 7 m a for the calculated mass balance due to photogrammetric restrictions and model inherent sensitivities the applied model appeared to be most suitable for determining area wide mass balance and ice flow on flatlying ablation areas but is so far not very well suited for steep ablation areas and most parts of accumulation areas nevertheless the study on griesgletscher opens a new and promising perspective for the monitoring of spatial and temporal glacier mass balance variations

modeling the distribution of the volcanic aerosol cloud from the 1783 1784 laki eruption 1 we conducted simulations of the atmospheric transformation and transport of the emissions of the 1783 1784 laki basaltic flood lava eruption 64 10 n 17 15 w using the nasa goddard institute for space studies modele climate model coupled to a sulfur cycle chemistry model the model simulations successfully reproduced the aerosol clouds of the 1912 katmai and 1991 mount pinatubo eruptions giving us confidence in the laki simulations simulations of the laki eruption produce peak zonal mean sulfate so4 concentrations of over 70 ppbv during august and into september 1783 in the upper troposphere and lower stratosphere at high latitudes while the majority of the sulfate aerosol was removed during the fall and early winter a significant aerosol perturbation remained into 1784 the peak so2 gas loading was just over 37 megatons mt in late june with the sulfate loading peaking in late august 1783 at 60 mt over the average of 3 runs this yielded a peak sulfate aerosol 75 h2so4 25 h2o loading of over 80 mt with the total aerosol produced during the entire eruption being about 165 mt the resulting sulfate deposition compares well with ice cores taken across greenland the top of atmosphere net radiative forcing peaks at 27 w m2 over the high latitudes during late summer 1783 and produces a global mean forcing of 4 w m2 the model results confirm that northern hemisphere high latitude volcanic eruptions produce aerosols that remain mostly confined north of 30 n latitude

non climatic factors and the environmental impact of volcanic volatiles implications of the laki fissure eruption of ad 1783 studies of the potential of icelandic volcanic eruptions to modify the environment of the british isles have primarily concentrated on the degree of induced climate change but models and temperatu

changes in geometry and subglacial drainage derived from digital elevation models unteraargletscher switzerland 1927 97 digital elevation models of the bed and surface of unteraargletscher switzerland are used to reconstruct the theoretical pattern of basal water drainage for the years 1927 1947 1961 and 1997 during which period the glacier was thinning and receding the theoretical drainage pattern for 1997 compares well in a broad sense with the locations of active moulins and the hydraulic connection status of boreholes drilled to the glacier bed changes in the basal water flow pattern over the period 1927 97 that are revealed by the theoretical reconstructions of the subglacial drainage system structure are likely to have resulted from changes in glacier geometry concurrent with the retreat and thinning of the glacier the height of medial moraines increased probably due to the insulating effect of the debris cover reducing the melt of the underlying ice this increase of moraine heights has led to the formation of hydraulic barriers at the glacier bed such that water flow has become channelized beneath the ice along drainage axes that parallel the course of the medial moraines on the glacier surface

short term variations in glacier flow controlled by subglacial water pressure at lauteraargletscher bernese alps switzerland short term variations in horizontal and vertical surface motion were studied with high temporal resolution during the ablation season in lauteraargletscher bernese alps switzerland horizontal surface flow speed oscillated diurnally showing a correlation with the water level in a borehole flow speed increased as a function of the water level with an asymptote at the ice overburden level this observation implied that the flow variations were principally controlled by the local water pressure which enhanced basal motions detailed examination of the diurnal variations however showed that the speed was larger when the pressure was increasing than when it was decreasing greater speed with increasing pressure was interpreted by subglacial watercavity opening and or longitudinal stress coupling with the upper reaches of the glacier upward surface movements were observed when the glacier flow speed increased simultaneous measurement of internal vertical strain in a borehole showed that the uplift had two different sources vertical straining of ice and volume increase of subglacial water cavities the vertical surface movement was largely affected by the vertical strain and the uplift events could not be simply attributed to cavity opening

evidence for excess pore water pressure generated in subglacial sediment implications for clast ploughing 1 pore water pressure in front of an object pulled through subglacial sediment and water pressure at the base of a borehole were simultaneously recorded at unteraargletscher switzerland to investigate the generation of excess pore pressures downglacier from ploughing clasts analysis of the strongly correlated pressure records revealed that the amplitude of the pore pressure fluctuations was larger than that of the corresponding borehole pressure fluctuations guided by high temporal resolution measurements of glacier surface motion from a different year this finding suggests that temporal changes in glacier speed lead to changes in the degree of excess pore pressure in front of the ploughing object the generation of excess pore water pressure has the potential to weaken the sediment downglacier from ploughing clasts instead of these clasts providing the roughness at the ice bed interface to resist glacier motion the ploughing process may in fact help to decouple the ice from the bed reducing basal resistance

multispectral imaging contributions to global land ice measurements from space global land ice measurements from space glims is an international consortium established to acquire satellite images of the world s glaciers analyse them for glacier extent and changes and assess change data for causes and implications for people and the environment although glims is making use of multiple remote sensing systems aster advanced spaceborne thermal emission and reflection radiometer is optimized for many needed observations including mapping of glacier boundaries and material facies and tracking of surface dynamics such as flow vector fields and supraglacial lake development software development by glims is geared toward mapping clean ice and debris covered glaciers terrain classification emphasizing snow ice water and admixtures of ice with rock debris multitemporal change analysis visualization of images and derived data and interpretation and archiving of derived data a global glacier database has been designed at the national snow and ice data center nsidc boulder colorado parameters are compatible with and expanded from those of the world glacier inventory wgi these technology efforts are summarized here but will be presented in detail elsewhere our presentation here pertains to one broad question how can aster and other satellite multispectral data be used to map monitor and characterize the state and dynamics of glaciers and to understand their responses to 20th and 21st century climate change our sampled results are not yet glaciologically or climatically representative our early results while indicating complexity are generally consistent with the glaciology community s conclusion that climate change is spurring glacier responses around the world mainly retreat whether individual glaciers are advancing or retreating the aggregate average of glacier change must be climatic in origin as nonclimatic variations average out we have discerned regional spatial patterns in glaciological response behavior these patterns are best attributed to climate change variability and to regional differences in glacier size and response times in many cases glacier length changes under represent the magnitude of glacier ablation because thinning sometimes without immediate length changes is also important an expanded systematic uniform analysis of many more glaciers is needed to isolate the glacier response components due to climatic and nonclimatic perturbations to produce quantitative measures of regional variation in glacier changes and to predict future regional glacier trends relevant to water resources glaciological hazards and global sea level this comprehensive assessment to be completed in stages is expected to lend a critically needed filter to identify successful climate models that explain recent glacier changes and change patterns and hence are apt to describe future changes and to eliminate unsuccessful models

ice deformation at the confluence of two glaciers investigated with conceptual map plane and flowline models using analytical and numerical techniques a two dimensional 2 d map plane model and a 2 d flowline model are utilized to elucidate the horizontal and vertical ice deformation at the confluence of two glaciers for a perfectly symmetrical confluence the junction point of the two tributaries can be modeled as a no slip free slip transition a strongly localized surface depression develops around the junction point accompanied by two broadly elevated zones positioned close to the margins of the tributaries facing the junction point the confluence center line is subjected to horizontal longitudinal extension and a transverse compression the compression generally exceeds the concomitant longitudinal extension in magnitude depth integrated vertical strain rates along the center line are positive extension but the strain rate variation with depth depends critically on the type of basal boundary conditions at the glacier bed for a no slip boundary condition vertical strain rates change from positive at the surface to negative close to the base whereas for a free slip boundary condition perfect sliding vertical strain rates are positive throughout the depth these theoretical results are compared with field measurements from unteraargletscher bernese alps switzerland

effects of volcanic activity on mire development case studies from hokkaido northern japan field stratigraphy and macrofossil composition of four mires in hokkaido northern japan were investigated to test whether tephra deposition induces long term vegetation shifts macroscopic tephras of known origin ranging from 0 5 to 25 cm in thickness were found in two or more of the study sites allowing correlation of the stratigraphic development of the mires major peat types and position of macroscopic mineral layers were determined in the field and selected cores were analysed by quantitative macrofossil analysis some macrofossil groups and the content of unidentifiable organic matter showed changes in the vicinity of tephra layers but they did not indicate a clear response pattern of the vegetation or the decomposition dynamics of plant material mosses were expected to be most strongly affected by tephra deposition but even in cases where tephra fell on sphagnum dominated vegetation no fundamental shifts to new plant communities were found this suggests that disturbance by widespread tep

hydrological controls on diurnal ice flow variability in valley glaciers 1 this paper uses a combination of field data and three dimensional modeling to investigate the spatial variability in basal conditions required to induce observed fluctuations in diurnal ice velocity at haut glacier d arolla switzerland a network of surface velocity markers was observed at intervals of as little as four hours over diurnal cycles in both winter and late summer winter motion showed limited diurnal variability presumably due to the absence of supraglacial meltwater inputs by contrast diurnal fluctuations in ice motion were recorded in summer across the lower and upper glacier in the lower glacier surface velocities were intimately linked to hydrological forcing in the vicinity of a subglacial channel previously observed diurnal excursions of meltwater away from the channel should reduce areas of basal drag adjacent to the channel thereby impacting on ice dynamics using a first order ice flow approximation we investigated the distribution of basal shear traction adjacent to the channel necessary to replicate the observed surface velocity field during periods of rapid ice motion the modeling suggests that the observed variations in diurnal velocity will only occur with extensive reductions in basal drag across a transverse zone of up to 560 m across well beyond the immediate vicinity and previously observed extent of diurnal excursions of meltwater away from the subglacial channel

observations of a reversal in vertical and horizontal strain rate regime during a motion event on unteraargletscher bernese alps switzerland during a motion event on unteraargletscher bernese alps switzerland in spring 1996 surface velocities were measured up to eight times a day at four different locations along the central flowline using global positioning system equipment in addition accumulated vertical strains over the uppermost 50 and 100 m were measured at a location where the total ice thickness is 260 m the motion event was accompanied by high horizontal and vertical strain rates as compared to annual mean values a reversal in strain regime was observed with horizontal strain rates changing to extension while vertical strain rates became compressive this strain rate reversal coincided within the temporal resolution of the data with a maximum in vertical ice displacement at the surface within a day variations in vertical strain from 0 04 a 1 to 0 06 a 1 were observed over the uppermost 100 m vertical stretching is estimated to have contributed to at least 20 of the anomalous vertical ice movement at the surface there were significant differences between measured longitudinal strain averaged over a distance corresponding to a few ice thicknesses and measured vertical strain in spring 1997 a similar but more detailed set of measurements was collected at the same measuring site and vertical strain rates were found to vary non uniformly with depth with the largest values closest to the surface

flow and structure in a dendritic glacier with bedrock steps sherpa romeo green journal open access article creative commons attribution 4 0 international cc by 4 0 applies

global surface temperature responses to major volcanic eruptions the impacts of pollution resulting from large explosive volcanic eruptions on the atmospheric heat budget and planetary albedo are not in doubt1 2 however the effects of volcanic aerosols on the surface climate are less clear and still controversial3 4 in a previous study5 it was shown that significant surface cooling occurs over the landmasses of the northern hemisphere in the first few months after a major eruption in that hemisphere here we extend that work using new surface air temperature compilations based on land and marine data6 for both the northern and southern hemispheres our results indicate that major northern hemisphere eruptions have an immediate effect on the northern hemisphere average surface temperature but little or no effect on the southern hemisphere average southern hemisphere eruptions affect both southern and northern hemisphere temperatures after a lag of between six months and a year

medial moraines of the haut glacier d arolla valais switzerland debris supply and implications for moraine formation the haut glacier d arolla displays three medial moraines an ablation dominant moraine east of the glacier centre line an ice stream interaction moraine west of the glacier centre line and a medial lateral supraglacial moraine complex along the western margin of the glacier the eastern moraine is formed by the emergence of coarse angular clasts from a series of short longitudinal debris bands 1 km from the glacier terminus this debris is derived from rock falls at the glacier head wall and follows a low level transport path through the glacier the western moraine is formed partly by the direct fall of weathered debris on to the ice surface at a point 2 6 km from the snout but it receives further increments of sediment from extensive longitudinal englacial debris bands developed from the convergence of two ice streams with lateral englacial debris septa the medial lateral moraine complex is formed largely by rock falls from the western valley wall but is also nourished by a debris layer developed at the base of the small tributary glacier de la mitre this layer crops out at the eastern margin of the re sultant inset ice stream and releases more rounded abraded and comminuted rock fragments to supplement the surface debris cover of supraglacial origin

intense mixing of the faroe bank channel overflow 1 the continuous swift flow of cold water across the sill of the faroe bank channel the deepest passage from the nordic seas to the north atlantic ocean forms a bottom attached dense plume overflow the amount and distribution of entrainment and mixing that the overflow encounters during its descent influence the ventilation of the deep north atlantic however remain poorly known due to lack of direct measurements using the first direct turbulence measurements we describe the dynamic properties and mixing of the overflow plume as it descends toward the iceland basin the vigorously turbulent plume is associated with intense mixing and enhanced turbulent dissipation near the bottom and at the plume ambient interface but with a quiescent core our measurements show a pronounced transverse circulation consistent with rotating plume dynamics a strong lateral variability in entrainment velocity and a vertical structure composed of order 100 m thick stratified interface and comparably thick well mixed bottom boundary layer with significant transport and entrainment

insar observations of the 1993 95 bering glacier alaska u s a surge and a surge hypothesis time varying accelerations were observed on bagley icefield during the 1993 95 surge of bering glacier alaska u s a using repeat pass synthetic aperture radar interferometry observations were from datasets acquired during winter 1991 92 pre surge winter 1993 94 during the surge and winter 1995 96 post surge the surge is shown to have extended 110 km up the icefield from bering glacier to within 15 km or less of the flow divide acceleration and step like velocity profiles are strongly associated with an along glacier series of central phase bull s eyes with diameters of 0 5 4 km these bull s eyes are interpreted to represent glacier surface rise fall events of 3 30 cm during 1 3 day observation intervals and indicate possible migrating pockets of subglacial water we present a surge hypothesis that relates late summer climate to englacial water storage and thence to the subglacial water dynamics pressurization hydraulic jacking depressurization and migration suggested by our observations

modelling massive sulphate aerosol pollution following the large 1783 laki basaltic eruption abstract the climatic impact of volcanic eruptions depends mainly on the amount of sulphur rich gases released into the stratosphere these are rapidly converted to sulphate aerosols which result in cooling of the lower troposphere and warming of the lower stratosphere these gases are retained in the stratosphere and then concentrated by atmospheric processes forming acid fog near the earth s surface with potentially dramatic environmental consequences on plants animals and humans we have modelled such emissions in the case of the unusually large 1783 1784 basaltic fissure eruption of laki iceland using an atmospheric general circulation model results show good agreement with historical observations such as typical time taken by individual pulses to reach continental europe or geographical extent of the deadly haze that covered much of the northern hemisphere the model could be adjusted to predict the climatic consequences of very large eruptions up to the scale of massive flood basalts and test the proposal that these are the main agent in most mass extinctions of life on earth

acid loading from icelandic tephra falling on acidified ecosystems as a key to understanding archaeological and environmental stress in northern and western britain abstract this paper proposes that the additional acid loading brought about by the deposition of large quantities of acid aerosols from icelandic volcanic eruptions may be responsible for environmental stress and settlement abandonment postulated by previous authors for northern and western britain in the late 2nd millennium bc the mechanism is predicted to have been of significance only in those ecosystems that were already significantly acidified the paper discusses and counters suggestions that the volcanically induced climatic change was the principal mechanism for both an apparent abandonment of settlement over large areas of northern scotland and the stress experienced by oak trees growing on raised bogs in ireland

basal flow characteristics of a linear medium sliding frictionless over small bedrock undulations the basal deformation of a gravity driven linear creeping flow sliding frictionless over slowly varying bed undulations in two dimensions is analysed analytically using results from second order perturbation theory one of the key results is that close to sinusoidal bedrock undulations up to two different spatial regions of local extrusion flow may arise the offset and onset of extrusion flow is controlled primarily by the amplitude to wavelength ratio above the crest of a sinusoidal bed line a local maximum of the surface parallel velocity develops for e ak 0 138 where a is the amplitude and k is the wave number as e increases from zer to this critical value the vertical position of the velocity maximum moves from kz 1 to kz 1 98 where z is the vertical distance above the mean bed line within and above the trough of a sinusoid a region of local minimum of the surface parallel velocity component develops which shifts from kz 1 towards the bed line as e increases front zero to 1 2 below this velocity minimum and for some distance above the velocity maximum the surface parallel velocity increases with depth this type of extrusion flow will cause a reversal of borehole inclination profiles close to the bedrock

glacial abrasion and sliding their dependence on the debris concentration in basal ice a model of bedrock abrasion by a temperate glacier is developed that takes into account the significant impeding effect of scattered rock fragments at the ice rock interface on glacier sliding an increase in the debris concentration above a relatively low value can reduce the abrasion rate by decreasing both the flux of rock fragments and the effective forces which press fragments against the bed abrasion of a simple wavy bed is shown to be most rapid for a relatively low debris concentration above which the abrasion varies inversely with the debris content a similar concentration dependence of abrasion is expected for beds of arbitrary geometry but low roughness sliding glaciers will tend to abrade until the basal ice is nearly stagnant at which point rocks of all sizes start to lodge against the bed

measurements of ice deformation at the confluence area of unteraargletscher bernese alps switzerland an experimental study of ice deformation at the confluence area of unteraargletscher bernese alps switzerland has been carried out surface velocities were measured by repeatedly surveying stakes and with the use of remote sensing methods the variation of the vertical strain rates with depth was measured in boreholes the confluence center line is subjected to longitudinal horizontal extension which is exceeded in magnitude by a concomitant transverse compression vertical strain rates change from positive extension at the surface to negative compression in the lowest layers of the glacier

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

hydraulic and mechanical properties of glacial sediments beneath unteraargletscher switzerland implications for glacier basal motion the force on a ploughmeter and subglacial water pressure have been measured in the same borehole at unteraargletscher switzerland in order to investigate ice sediment coupling and the motion at the base of a soft bedded glacier a strong inverse correlation of the recorded pressure and force fluctuations in conjunction with a significant time lag between the two signals suggests that pore water pressures directly affect the strength of the subglacial sediment the lag is interpreted to reflect the time required for the water pressure wave to propagate through the pores of the sediment to the depth of the ploughmeter analysis of the propagation velocity of this pressure wave yielded an estimate of the hydraulic diffusivity a key parameter necessary to characterize transient pore water flow furthermore the inferred inverse relationship between pore water pressure and sediment strength implies that coulomb plastic deformation is an appropriate rheological model for the sediment underlying unteraargletscher however the sediment strength as derived from the ploughmeter data was found to be one order of magnitude smaller than that calculated for a coulomb frictional material using the water pressure measurements this significant discrepancy might result from pore water pressures in excess of hydrostatic down glacier from the ploughmeter as the ploughmeter is dragged through the sediment sediment is compressed if the rate of this compression is large relative to the rate at which pore water can drain away excess pore water pressures will develop that have the potential to weaken the sediment the same process could lead to highly fluid sediment down glacier from clasts that protrude into the glacier sole and thus would otherwise provide the roughness to couple the glacier to its bed iverson 1999 rapidly sliding glaciers overlying sediments might therefore move predominantly by ploughing which tends to focus basal motion near the glacier sole rather than at depth in the bed

the ice thickness distribution of unteraargletscher switzerland results from 15 years of work on glacier bed mapping by radio echo soundings on finsteraar lauteraar and unteraargletscher switzerland are summarized and a new and greatly improved map of the ice thickness distribution presented in contrast to the tongue of unteraargletscher its two main tributaries lauteraar and finsteraar are both deep and narrow and ice thickness determination depends on the detection of more than just the primary reflection migrating the data led to considerably improved bed determination wherever possible additional information on ice thicknesses gained from numerous hot water drillings to the glacier bed is used as an independent verification of the results of the radar measurements and a fair agreement is found