seismic detection of a subglacial lake near the south pole antarctica 1 seismic reflection data are analyzed to verify radar identification of a subglaical lake near the geographic south pole the seismic amplitude variation with offset avo technique is applied to confirm the presence of extensive free water and seismic imaging of the subsurface constrains lake depth and deeper subglacial structure in the region this lake is at least 4 2 km wide and likely as much as 10 km in diameter is up to 32 10 m deep and occupies a basin of thick sedimentary strata these results imply that extensive water storage is occurring in the south pole region the proximity of this lake to the amundsen scott south pole station makes research drilling to sample the lake and underlying sediments feasible and supportable

roman comagmatic province central italy evidence for subduction related magma genesis geochemical data on mafic volcanics show that important affinities exist between the roman and the calc alkaline rocks from the aeolian arc south tyrrhenian sea these affinities together with the close association of calc alkaline and k rich volcanics in the aeolian arc and in the naples area the continuity in the variation of abundances of incompatible elements from calc alkaline to potassic suites and the similarity in terms of major element geochemistry support a genetic relationship of the roman magmatism and the subduction processes that affected the apennines in tertiary time and are still active under the aeolian arc in the genetic model presented here both calc alkaline and k rich magmas were generated within a mantle heterogenously enriched in lil elements composition of the mantle was modified by addition of material probably sediments dragged down by the undergoing slab the geochemical and petrological differences displayed by the calc alkaline and k rich volcanics are accounted for by the different conditions of melting as well as by chemical and isotopic heterogeneities of the source 26 references 3 figures 1 table

continued deceleration of whillans ice stream west antarctica 1 earlier observations indicated that whillans ice stream slowed from 1973 to 1997 we collected new gps observations of the ice stream s speed in 2003 and 2004 these data show that the ice stream is continuing to decelerate at rates of about 0 6 yr2 with faster rates near the grounding line our data also indicate that the deceleration extends over the full width of the ice plain extrapolation of the deceleration trend suggests the ice stream could stagnate sometime between the middle of the 21st and 22nd centuries

anatomy of a meltwater drainage system beneath the ancestral east antarctic ice sheet the east antarctic ice sheet was larger than present during past cold periods seafloor geophysical data show that in the ross sea the extended ice sheet was underlain by an active hydrologic system during the glacial termination

basal characteristics of the main sticky spot on the ice plain of whillans ice stream antarctica abstract understanding the processes that affect streaming ice flow and the mass balance of glaciers and ice sheets requires sound knowledge of their subglacial environments previous studies have shown that an extensive deformable subglacial sediment layer favors fast ice stream flow however areas of high basal drag termed sticky spots are of particular interest because they inhibit the fast flow of the overriding ice the stick slip behavior of whillans ice stream wis is perhaps the most conspicuous manifestation of a subglacial sticky spot we present new ice thickness and seismic reflection measurements collected over the main sticky spot in the ice plain of wis allowing us to elucidate its role in the behavior of the ice stream ice thickness and surface elevation data show that the sticky spot occupies a subglacial topographic high water flow in response to the hydrological potential gradient will be routed around the sticky spot if effective pressures are similar on the sticky spot and elsewhere the seismic experiment imaged a laterally continuous basal layer approximately 6 m thick having compressional wave velocities of greater than 1800 m s 1 and density greater than 1800 kg m 3 indicative of a till layer that is stiffer than corresponding till beneath well lubricated parts of the ice stream this layer likely continues to deform under the higher shear stress of the sticky spot and some water may be pumped up onto the sticky spot during motion events

seismic measurements reveal a saturated porous layer beneath an active antarctic ice stream seismic reflection studies recently conducted on ice stream b part of the marine ice sheet of west antarctica show a metres thick layer immediately beneath the ice in which both compressional p and shear s wave speeds are very low these low wave speeds imply that the material in the layer is highly porous and is saturated with water at a high pore pressure from this and from arguments presented in an accompanying paper1 to the effect that the layer would be too weak to support the shear stress exerted by the overlying ice we conclude that the layer is deforming and that the ice stream probably moves principally by such deformation

the role of subglacial water in ice sheet mass balance subglacial water can significantly affect the velocity of ice streams and outlet glaciers of ice sheets depending on the geometry and capacity of the subglacial hydrologic system increased surface melting in greenland over the coming decades may influence the ice sheet s mass balance furthermore subglacial lakes in antarctica can modulate ice velocities and act as nucleation points for new fast flowing ice streams in the coming decades significant changes in the polar regions will increase the contribution of ice sheets to global sea level rise under the ice streams and outlet glaciers that deliver ice to the oceans water and deformable wet sediments lubricate the base facilitating fast ice flow the influence of subglacial water on fast ice flow depends on the geometry and capacity of the subglacial hydrologic system water moving rapidly through a well connected system of conduits or channels will have little impact on ice sheet velocities but water injected into a spatially dispersed subglacial system may reduce the effective pressure at the base of the ice sheet and thereby trigger increased ice sheet velocities in greenland the form of the subglacial hydrologic system encountered by increasing surface melt water will determine the influence of changing atmospheric conditions on ice sheet mass balance in antarctica subglacial lakes have the capacity to both modulate velocities in ice streams and outlet glaciers and provide nucleation points for new fast ice flow tributaries climate models of ice sheet responses to global change remain incomplete without a parameterization of subglacial hydrodynamics and ice dynamics

seismic evidence for a water filled canal in deforming till beneath rutford ice stream west antarctica 1 seismic data from rutford ice stream west antarctica show high amplitude reversed polarity reflections from parts of the bed only interfaces between ice and water or between water and sediment have the acoustic properties to produce such strong reflections ice sediment interfaces produce much weaker echoes the evidence suggests that an area of the ice stream measuring at least 1 km by 0 2 km is underlain by water with a depth of less than 1 m the data are consistent with the presence of a sub ice drainage system comprising shallow canals cut into the sedimentary substrate as predicted by theory

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

approximations to seismic ava responses validity and potential in glaciological applications amplitude variation with angle ava methods establish the seismic properties of material either side of a reflective interface and their use is growing in glaciology the ava response of an interface is defined by the complex knott zoeppritz k z equations numerous approximations to which we typically assume weak interface contrasts and isotropic propagation inconsistent with the strong contrasts at glacier beds and the vertically transverse isotropic vti fabrics were associated with englacial reflectivity we considered the validity of a suite of approximate k z equations for the exact p wave reflectivity rp of ice overlying bedrock sediment and water and englacial interfaces between isotropic and vti ice we found that the approximations of aki richards shuey and fatti match exact glacier bed reflectivity to within rp 0 05 smaller than the uncertainty in typical glaciological ava analyses but only for maximum incident angle i limited to 30 a stricter limit of i 20 o ffered comparable accuracy to a hydrocarbon benchmark case of shale overlying gas chargedsand the vti compliant rugerapproximation accurately described englacial reflectivity to within rp 0 01 and it can be modified to give a quadratic expression in sin 2 i suitable for curve matching operations having shown the circumstances under which ava approximations were valid for glaciological applications we have suggested that their interpretative advantages can be exploited in the future ava interpretations

how glaciers entrain and transport basal sediment physical constraints abstract simple insights from the physics of ice water and sediment place constraints on the possible sediment transport behavior of glaciers and ice sheets because glaciers concentrate runoff streams generated by glaciers transport much sediment and may erode bedrock rapidly deforming glacier beds also can transport much sediment particularly in marginal regions rapid sediment entrainment producing thick debris rich basal zones may occur by regelation into subglacial materials and by freeze on from rising supercooled waters numerous other mechanisms may be important but primarily near ice margins especially those of advancing or fluctuating glaciers several sediment entrainment mechanisms may be active beneath a single glacier but one process is likely to be dominant at any place and time

bathymetry and geological structures beneath the ross ice shelf at the mouth of whillans ice stream west antarctica modeled from ground based gravity measurements 1 new gravity data reveal important geologic controls on the location and behavior of the grounding zone of whillans ice stream wis west antarctica grounding zones of ice sheets and contiguous ice shelves are important for understanding ice sheet dynamics as key processes that influence the grounded ice and its discharge into the ocean occur in these regions here we model the bathymetry and shallow geological structures beneath the ross ice shelf in an embayment of the wis grounding zone using gravity data collected on the ground in conjunction with seismic and radar data we find that the region of shallow ocean water 50 m is extensive oceanographic models suggest that grounding zones exhibiting thin ocean cavities with gently sloping ice ocean interfaces are likely to be tidally well mixed leading to slower basal melting than would occur in a thicker stratified water column beneath the ocean water column we model a fault and a sedimentary basin in a half graben filled with two layers of sediments the total thickness of the sediment layer is 900 to 1250 m in the half graben and 600 to 800 m on the upthrown block and the basement depth is no more than 2000 m we observe that the upper softer sediment is thinnest near the modern grounding line and may possibly pinch out near our grid and that the modeled fault is roughly parallel to part the grounding we therefore hypothesize that the wis grounding line stabilized in its current location in part due to the subglacial geology

theoretical framework and diagnostic criteria for the identification of palaeo subglacial lakes abstract the antarctic ice sheet is underlain by numerous subglacial lakes which comprise a significant and active component of its hydrological network these lakes are widespread and occur at a range of scales under a variety of conditions at present much glaciological research is concerned with the role of modern subglacial lake systems in antarctica another approach to the exploration of subglacial lakes involves identification of the geological record of subglacial lakes that once existed beneath former ice sheets this is challenging both conceptually in identifying whether and where subglacial lakes may have formed and also distinguishing the signature of former subglacial lakes in the geological record in this work we provide a synthesis of subglacial lake types that have been identified or may theoretically exist beneath contemporary or palaeo ice sheets this includes a discussion of the formative mechanisms that could trigger onset of or drain subglacial lakes these concepts provide a framework for discussing the probability that subglacial lakes exist ed beneath other palaeo ice sheets indeed we conclude that the former mid latitude ice sheets are likely to have hosted subglacial lakes although the spatial distribution frequency and type of lakes may have differed from today s ice sheets and between palaeo ice sheets given this possibility we propose diagnostic criteria for identifying palaeo subglacial lakes in the geological record these criteria are derived from contemporary observations hydrological theory and process analogues and provide an observational template for detailed field investigations

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

changes in the velocity structure of the greenland ice sheet using satellite radar interferometry observations of greenland we detected widespread glacier acceleration below 66 north between 1996 and 2000 which rapidly expanded to 70 north in 2005 accelerated ice discharge in the west and particularly in the east doubled the ice sheet mass deficit in the last decade from 90 to 220 cubic kilometers per year as more glaciers accelerate farther north the contribution of greenland to sea level rise will continue to increase

rapid discharge connects antarctic subglacial lakes a hidden subglacial world consisting of more than 140 lakes lies beneath the antarctic ice sheet it is widely assumed that these lakes are a static ecosystem cut off from each other and the rest of the world far from it according to a 16 month study of surface elevation changes in the region changes indicating a rapid discharge of water from one lake to another were detected and it is possible that the entire subglacial drainage basin is flushed through periodically by massive water transfers observation of surface elevation changes over 16 months in an area of the east antarctic ice sheet overlying subglacial lakes suggests a rapid discharge of one lake to at least two other lakes involving a water volume of 1 8 km3 in conflict with previous expectations that subglacial lakes have long residence times and slow circulations the existence of many subglacial lakes1 provides clear evidence for the widespread presence of water beneath the east antarctic ice sheet but the hydrology beneath this ice mass is poorly understood2 such knowledge is critical to understanding ice flow basal water transfer to the ice margin glacial landform development and subglacial lake habitats here we present ice sheet surface elevation changes in central east antarctica that we interpret to represent rapid discharge from a subglacial lake our observations indicate that during a period of 16 months 1 8 km3 of water was transferred over 290 km to at least two other subglacial lakes while viscous deformation of the ice roof above may moderate discharge the intrinsic instability of such a system3 suggests that discharge events are a common mode of basal drainage4 if large lakes such as lake vostok or lake concordia1 are pressurizing it is possible that substantial discharges could reach the coast5 6 our observations conflict with expectations that subglacial lakes have long residence times and slow circulations2 7 8 and we suggest that entire subglacial drainage basins may be flushed periodically the rapid transfer of water between lakes would result in large scale solute and microbe relocation and drainage system contamination from in situ exploration is therefore a distinct risk

interfacial water in polar glaciers and glacier sliding at 17 c we have observed sliding at a cold 17 oc ice rock interface beneath meserve glacier antarctica and the segregation of ice into clean lenses amidst the dirty basal layers of this glacier we interpret these as manifestation s of thin water films at ice rock interfaces we use shreve s theory for sub freezing sliding to esti mate the nominal film thickness to be at least tens of nanometers such water films should exist around rocks in most polar ices and likely have high solute concen trations due to solute rejection during regelation and due to exchange with veins and grain boundaries where impurities reside

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

changes in speed near the onset of bindschadler ice stream west antarctica ice stream velocities can change rapidly understanding the spatial and temporal pattern of these changes and the forcings responsible is essential for predicting ice sheet mass balance inland migration of the onset location will lead to more efficient drainage of inland ice one way to monitor the stability of the onset location is to investigate changes in the velocity field we report on the velocity near the onset of bindschadler ice stream west antarctica in 2002 and compare these data to the velocity measured in 1996 mean annual velocities were determined by measuring the gps position of markers during consecutive seasons we compare our results with similar measurements from 1996 to investigate temporal changes in this ice stream onset our results indicate that only minimal changes have occurred in the speed of the ice stream between 1996 and 2002

in situ experiments on an excavation disturbed zone induced by mechanical excavation in neogene sedimentary rock at tono mine central japan abstract the excavation disturbed zone edz is defined as the rock zone where rock properties and conditions have been changed due to the processes induced by excavation such as fracturing stress redistribution and desaturation based on our results of the in situ blasting excavation disturbance experiment at the tono mines the rock mass around the drift behaved elastically and the excavation damage induced by blasting was considered to be a primary factor in causing a change in rock properties the excavation damage was considered to depend on the excavation method so we performed an excavation disturbance experiment with mechanical excavation using a boom header in the neogene sedimentary soft rock to compare the rock property change and width of the edz induced by blasting geological observation rock mass displacement and vibration measurement were performed during the drift excavation seismic refraction and tomographic surveys borehole expansion test and hydraulic test were performed before and after the drift excavation a low seismic velocity layer of the edz with a width of ca 0 3 m and a seismic velocity of 65 70 of the intact rock was detected by a seismic refraction survey along the drift wall deformability and hydraulic conductivity were not changed in the rock mass up to 0 3 m from the drift wall the change in rock properties and width of the edz was shown to depend on the excavation method in this geological setting the mechanical excavation method is more effective in limiting the extent of the edz and change in rock mass properties

active lakes of recovery ice stream east antarctica a bedrock controlled subglacial hydrological system a connected system of active subglacial lakes was revealed beneath recovery ice stream east antarctica by icesat laser altimetry here we combine repeat track analysis of icesat 2003 09 operation icebridge laser altimetry and radio echo sounding 2011 and 2012 and modis image differencing 2009 2011 to learn more about the lake activity history the surface and bedrock topographic setting of the lakes and the constraints on water flow through the system we extend the lake activity time series to 2012 for the three lower lakes and capture twomajor lake drainages one lake underwent a large deflation between 2009 and 2011 while another lake which had been continuously filling between 2003 and 2010 started to drain after 2011 most of the active lakes are located in a 1000 km long bedrock trough under the main trunk of recovery ice stream whose base is 1500 2000 m below present day sea level the hydrologic system beneath recovery ice stream is controlled by this unusually pronounced bedrock topography in contrast to most antarctic systems studied to date which are controlled by the ice surface topography hydrologic connections among the lakes appear to be direct and responsive and we reproduce the lake activity using a simple subglacial water model we discuss potential causes of non steady hydrologic behavior in major antarctic catchments

significant groundwater contribution to antarctic ice streams hydrologic budget satellite observations have revealed active hydrologic systems beneath antarctic ice streams but sources and sinks of water within these systems are uncertain here we use numerical simulations of ice streams to estimate the generation flux and budget of water beneath five ice streams on the siple coast we estimate that 47 of the total hydrologic input 0 98 km3 yr 1 to whillans wis mercer mis and kamb kis ice streams comes from the ice sheet interior and that only 8 forms by local basal melting the remaining 45 comes from a groundwater reservoir an overlooked source in which depletion significantly exceeds recharge of the total input to bindschadler bis and macayeal macis ice streams 0 56 km3 yr 1 72 comes from the interior 19 from groundwater and 9 from local melting this contrasting hydrologic setting modulates the ice streams flow and has important implications for the search for life in subglacial lakes

geomorphology under ice streams moving from form to process ice streams are integral components of an ice sheet s mass balance and directly impact on sea level their flow is governed by processes at the ice bed interface which create landforms that in turn modulate ice stream dynamics through their influence on bed topography and basal shear stresses thus ice stream geomorphology is critical to understanding and modelling ice streams and ice sheet dynamics this paper reviews developments in our understanding of ice stream geomorphology from an historical perspective with a focus on the extent to which studies of modern and palaeo ice streams have converged to take us from a position of near complete ignorance to a detailed understanding of their bed morphology during the 1970s and 1980s our knowledge was limited and largely gleaned from geophysical investigations of modern ice stream beds in antarctica very few palaeo ice streams had been identified with any confidence during the 1990s however glacial geomorphologists began to recognise their distinctive geomorphology which included distinct patterns of highly elongated mega scale glacial lineations ice stream shear margin moraines and major sedimentary depocentres however studying relict features could say little about the time scales over which this geomorphology evolved and under what glaciological conditions this began to be addressed in the early 2000s through continued efforts to scrutinise modern ice stream beds at higher resolution but our current understanding of how landforms relate to processes remains subject to large uncertainties particularly in relation to the mechanisms and time scales of sediment erosion transport and deposition and how these lead to the growth and decay of subglacial bedforms this represents the next key challenge and will require even closer cooperation between glaciology glacial geomorphology sedimentology and numerical modelling together with more sophisticated methods to quantify and analyse the anticipated growth of geomorphological data from beneath active ice streams

georods the development of a four element geophone for improved seismic imaging of glaciers and ice sheets active seismic imaging of glaciers and ice sheets is important for constraining inputs to climate models such as englacial ice fabric and the nature of the basal interface however acquiring high quality seismic data is time consuming and resource intensive using traditional single element geophones requires ideal weather conditions e g light winds and excellent source coupling in addition deploying and retrieving these geophones is slow and cumbersome we have developed a four element georod that enhances signal levels by 20 30 db in a variety of conditions including blowing snow and poorly coupled source detonations the long slender design of these georods makes them easy to deploy and retrieve allowing researchers to acquire greater line kilometers of seismic data during field campaigns that are commonly time constrained

assessment of crack initiation and propagation in rock from explosion induced stress waves and gas expansion by cross hole seismometry and fem dem method abstract single hole blast induced damage in a granitic outcrop has been assessed through both controlled experiments and numerical simulations with a combined finite discrete element method fem dem damage tomographies from decoupled short and long explosive charges in flooded boreholes were obtained through a high resolution cross hole system by measuring pre and post blast seismic velocities around the blastholes damage was assessed through crack density which was calculated by inversion of p wave velocity measurements through an effective medium theory emt method the resulting damage was found to be highly asymmetrical around each blast hole both along the vertical and horizontal planes despite the apparent isotropy and homogeneity of the granitic rock mass experimental results combined with numerical simulations carried out to assess damage from stress waves alone showed that most damage from experiments was caused by the expansion of gases while its magnitude and extension were strongly dependent on confining conditions along the blasthole the difficulty of quantifying the relative contribution of stress waves and gas expansion and the problems inherent in prediction of blast induced damage are described

superimposition of ribbed moraines on a palaeo ice stream bed implications for ice stream dynamics and shutdown the sediments and landforms preserved on palaeo ice stream beds can provide important information about their subglacial conditions and flow mechanisms and the processes accompanying their shutdown in this paper detailed observations of an intriguing subglacial landform assemblage of ribbed moraines superimposed on glacial lineations on the dubawnt lake ice stream bed north west canadian shield are presented including their morphometry internal structure from ground penetrating radar gpr surveys and from glaciogeological analysis and sedimentological characteristics from sediment architecture and lithofacies analysis the observations suggest an abrupt change in ice dynamics that correlates with two phases of glacial landform development this hypothesis is based on evidence from a deformed lodgement till which subsequently underwent brittle deformation and developed prominent thrust shear structures and tension fractures tension fractures are observed in a sediment exposure and thrust structures are observed in gpr surveys where they occur most prominently in the ribbed moraine ridge crests the presence of the fractures and their association with a population of clasts in the till that are orientated with their a axes transverse to the inferred ice flow direction suggests a compressional flow regime it is therefore inferred that the glacial lineations were formed under an extensional flow regime during ice stream activity but that at some point patches of till under the ice stream stiffened through dewatering the subsequent increase in basal shear stress resulted in compressional flow and the development of subglacial thrusting and the building of ribbed moraines we therefore suggest that ribbed moraines may form in areas of compressional flow under ice streams i e sticky spots and or at the transition between slow and fast ice flow along parts of an ice stream the general absence of ribbed moraines on most other palaeo ice stream beds suggests that either these ice streams continued operating during deglaciation or processes other than the development of localized compressional flow sticky spots led to their shutdown e g ice depletion

subglacial basins their origin and importance in glacial systems and landscapes closed topographic basins are found beneath contemporary ice masses and within the footprint of former ice masses in all glaciated regions we present the first integrated review of subglacial basin occurrence and formation and the implications of such basins for glaciological processes and the evolution of landscape our purpose is to motivate research in areas where understanding of basin origin and process significance is weak basins on the order of 10 102 m deep and 102 103 m long are produced by glacial erosion of subglacial rock and or sediment and are known as overdeepenings outlet and valley glaciers can overdeepen their beds far below sea level or local fluviatile base level larger basins typically in ice sheet contexts may have a pre glacial usually tectonic origin subglacial basins are important glaciologically because they require ice water and sediment to ascend an adverse subglacial slope in order to exit the glacial system the efficiency of which is dependent upon the gradient of the adverse slope and that of the ice surface basins thus influence subglacial drainage system morphology and transmissivity the thickness and distribution of basal ice and sediment layers and the mechanisms and dynamics of ice flow adverse gradients that exceed 11 times that of the ice surface may even permit the formation of subglacial lakes we speculate that in comparison to ice masses with few or no subglacial basins those with numerous or very large basins may respond to climatic changes with unexpected vigour in addition erosion rates and transport pathways of water and sediment through the glacial system and the expression of these processes in the sediment and landform record may be unexpectedly complex further our review shows that in a warming climate ice masses resting on adverse slopes will be vulnerable to rapid and potentially catastrophic retreat new lakes in subglacial basins exposed by mountain glacier retreat will present an increasing hazard and subglacial lakes may drain catastrophically on even longer time scales we speculate that the glacial excavation and post glacial filling of basins in mountainous regions should contribute importantly to climate related changes in isostasy and relief although the controls on overdeepening and their influence on other glacial and landscape processes remain uncertain we hypothesise that overdeepened glacial systems reflect an equilibrium ice bed geometry that maximises the efficiency of ice discharge improved understanding of overdeepening processes especially overdeepened bed hydrology is therefore necessary to understand fully the dynamic behaviour of valley and outlet glaciers and thus the fate of earth s largest ice masses

estimating the excavation disturbed zone in the permanent shiplock slopes of the three gorges project china abstract the stability and deformation of the permanent shiplock slopes are among the key issues in the design and construction of the three gorges project the permanent shiplock slopes formed by deeply and steeply cutting into weathered and fresh granites are 1607 m in length and 50 170 m in height the disturbed zone in the permanent cut slopes induced by excavation is therefore one of the most important aspects in slope stability and deformation evaluation a comprehensive investigation including non linear finite element analysis in situ testing instrumentation and monitoring back analysis and rock mass quality rating has been carried out for the identification and evaluation of the excavation disturbed zone in the permanent cut slopes the results of the investigation confirmed the existence of such an excavation disturbed zone in the permanent cut slopes this zone is characterized by a considerable weakening in the mechanical properties of the rock mass from the cut surface to the deeper region of the permanent cut slopes the excavation disturbed zone can be further divided into a damaged zone an affected zone and a slightly affected zone according to the extent of weakening in the rock mass the damaged zone and the affected zone have thicknesses of 5 10 m and 10 20 m respectively the exact pattern of the excavation disturbed zone is variable in different parts of the permanent cut slopes and are generally similar to that of the plastic zones estimated by using the non linear finite element analysis the present investigation has provided both factual data and insights for the stability and deformation evaluation of the permanent shiplock slopes the approach and methodology developed in the paper can be used to assess similar excavation disturbed zones in other large cut slopes

englacial seismic reflectivity imaging crystal orientation fabric in west antarctica abrupt changes in crystal orientation fabric cof and therefore viscosity are observed near the base of the ice sheet throughout west antarctica we report on active source seismic observations from wais divide mid stream and downstream on thwaites glacier and the onset region of bindschadler ice stream these data reveal a prevalence of englacial seismic reflectivity in the bottom quarter of the ice sheet the observed seismic reflectivity is complex but largely bed conformable with long spatial wavelength features observed in the flow direction and short wavelength features observed across flow a correspondence of englacial structures with bed features is also observed we determine the origin of the reflectivity to be abrupt changes in the cof of ice based on the following 1 observations of englacial reflectivity are consistent with current knowledge of cof within ice sheets 2 englacial reflectivity caused by cof contrasts requires the simplest genesis especially at ice divides and 3 amplitude analysis shows that the observed englacial reflectivity can be explained by contrasts in seismic velocity due to cof changes we note that the downstream increase in the quantity and complexity of observations indicates that direct observations of cof at ice divides likely underestimate the role that fabric plays in ice sheet dynamics

recent dramatic thinning of largest west antarctic ice stream triggered by oceans a growing body of observational data suggests that pine island glacier pig is changing on decadal or shorter timescales these changes may have far reaching consequences for the future of the west antarctic ice sheet wais and global sea levels because of pig s role as the ice sheet s primary drainage portal we test the hypothesis that these changes are triggered by the adjoining ocean specifically we employ an advanced numerical ice flow model to simulate the effects of perturbations at the grounding line on pig s dynamics the speed at which these changes are propagated upstream implies a tight coupling between ice sheet interior and surrounding ocean

seismic detection of transient changes beneath black rapids glacier alaska u s a ii basal morphology and processes using changes observed in daily seismic reflections we have investigated the basal morphology of black rapids glacier alaska u s a the englacial drainage of ice marginal lakes caused significant changes in the daily reflections as well as dramatic increases in basal motion changes in reflection arrival times and amplitudes indicate that there is a basal till layer at least 5 m thick at some locations beneath this surge type glacier rapid changes in the observed reflection coefficients during the drainage events indicate that changes in till properties must occur throughout the entire 5 m thick layer they must last for several days following the lake drainages and they must be completely reversible over as little as 36 min our seismic analysis shows that changes in effective pressure of the till are unlikely to cause the required changes in the reflection coefficients but that a decrease in till saturation is likely we therefore interpret the cause ofthe seismic anomalies as being a temporary decrease in saturation as water is input to the subglacial hydraulic system and propose that such a change may occur quickly and reversibly by a redistribution ofoverburden pressure higher water pressures within the hydraulic system cause that region to support more of the glacier s weight leaving the remaining areas to support less any till within these areas of decreased normal stress would experience a consequent decrease in pore water pressure causing gas to exolve thus decreasing saturation this decrease in saturation would cause a change in the strength of the basal layer and may affect basal dynamics

dynamic distributed drainage implied by the flow evolution of the 1996 1998 adventure trench subglacial lake discharge abstract the transport of subglacial water beneath the east antarctic ice sheet is an enigmatic and difficult to observe process which may affect the flow of the overlying ice and mixing of the oceans in the sub ice shelf cavities and ultimately global climate periodic outbursts are a critical mechanism in this process recent analysis of satellite data has inferred a subglacial hydraulic discharge totaling 2 km 3 traveling some 260 km along the ice bed interface of the adventure subglacial trench between 1996 and 1998 wingham et al 2006 rapid discharge connects antarctic subglacial lakes nature 440 1033 1036 using radar echo sounding data from the adventure subglacial trench region in conjunction with the previously reported satellite observations along with some basic modeling we calculate a mass budget and infer a flow mechanism for the 1996 1998 event the volume released from the source lake exceeded the volume received by the destination lakes by 1 1 km 3 this discrepancy indicates that some water must have escaped downstream from the lowest destination lake from 1997 onward the downstream release of water from the destination lakes continued until at least 2003 several years after the 1998 cessation of surface subsidence at the source lake by 2003 a total of 1 5 km 3 or nearly 75 of the water released by the source lake had traveled downstream from the destination lakes the temporal evolution of discharge from the outlet can be simulated with the classic ice walled semicircular channel model if and only if the retreat of the source lake shoreline is taken into account further downstream the ice bedrock geometry along the inferred flow path downstream includes many sections where thermal erosion of the overlying ice would not be sustainable along these reaches mechanical lifting of the ice roof and or erosion of a sedimentary substrate by a broad shallow water system would be most effective means of sustaining the discharge a distributed system is also consistent with the 3 month delay between water release at the source lake and water arrival at the destination lake observations of intermittent flat bright bed reflections in radar data acquired along the flow path are consistent with the presence of a broad shallow water system ultimately the presence of large subglacial lakes along the flow path of the 1996 1998 adventure subglacial trench flow path delayed the arrival of water to points downstream by approximately 12 months

petrogenesis of monte vulture volcano italy inferences from mineral chemistry major and trace element data the paper presents major and trace element data and mineral compositions for a series of foiditic tephritic to phonolitic rocks coming from monte vulture southern italy and investigates their origin evolution and relationship with the other centres of the roman province

an ice grounding line wedge from the ghaub glaciation 635 ma on the distal foreslope of the otavi carbonate platform namibia and its bearing on the snowball earth hypothesis our detailed examination of the ghaub formation possibly 635 ma on the distal foreslope of the otavi carbonate platform is part of a regional study of the congo paleocontinental margin in northwestern namibia detrital carbonates of the ghaub formation disconformably overlie the franni aus member of the ombaatjie formation a coarsening upward stack of carbonate turbidites and oolite clast debris flow breccias interpreted to be a glacioeustatic falling stand wedge within the main ghaub formation carbonate diamictites are interleaved with mesoscale laminated to cross laminated climbing rippled grainstones and mudstones and conglomeratic carbonates amalgamation of diamictite units is observed where interleaved facies grainstones mudstones are laterally discontinuous due to reactivation of erosion followed by renewed deposition the diamictite package is progradational overall and 80 m thick on average it is overlain by the 5 15 m thick bethanis member which is unique in its lateral continuity composite fining upward trend and distinctive interbedding of turbidite grainstones argillaceous siltstones climbing rippled mudstones and meter scale stromatolite dropstones dropstones are ubiquitous within the finer grained ghaub lithofacies and their presence along with the facies context for subglacial and near grounding line deposition indicates a glacigenic origin for the ghaub formation despite its subtropical paleolatitude and distal foreslope setting we infer a glacial maximum represented by the sub ghaub disconformity followed by the main ghaub interval when an ice grounding line on the distal foreslope experienced abrupt step backs and readvances of limited magnitude terminated by the bethanis episode of unusually widespread iceberg calving and slope instability the bethanis member is overlain conformably by the keilberg member of the maieberg formation reconstruction of the foreslope places the ghaub grounding line wedge 1 3 km vertically below the rim of the platform implying an enormous base level change upon deglaciation when the platform was drowned below wave base for a period far exceeding the time scale for isostatic adjustment the magnitude of base level change supports the pan glacial hypothesis that dynamic thick ice sheets existed simultaneously on virtually all continents the snowball hypothesis that the oceans were also covered by glacial ice sea glacier provides a simple explanation for the main ghaub to bethanis transition terminal deglaciation was triggered by collapse of the sea glacier

considerations and observations of stress induced and construction induced excavation damage zone in crystalline rock abstract bsed on the recent observations from two underground laboratories the construction induced excavation damage zone edz ci caused by explosion or by mechanical excavation and stress induced excavation damage zone edz si are distinguished the key location for in situ testing of edz and method development has been the tbm tunnel in aspo hard rock laboratory in sweden since the 1990s the research and development has been continued between 2010 and 2012 with in situ stress measurement campaign and in 2013 with ground penetrating radar investigations with new gpr edz method the new gpr edz method based on frequency analysis of the reflected signal indicates mean depth for edz ci of 120 mm the same zone is indicated also by various other geophysical and laboratory methods and is distinguished as construction induced and continuous around the tunnel perimeter fracture mechanics modelling suggests a lowered hydraulic conductivity of 1 8 10 13 m s in edz si and 4 0 10 11 m s in edz ci biaxial testing suggests a zone in which young s modulus values are lowered and which is not continuous around the tunnel this zone is distinguished as an observable part of stress induced edz si

subglacial sediments as a control on the onset and location of two siple coast ice streams west antarctica laterally continuous subglacial sediments are a necessary component for ice streaming in the modern onset regions of the ice streams draining the siple coast of west antarctica on the basis of new seismic data combined with previous results we present geophysical results from seismic reflection and refraction experiments in the upper reaches of ice streams c and d that highlight continuous sedimentary basins within and upstream of the current onset regions of both ice streams with streaming ice overlying these sedimentary packages the subglacial environment changes from no sediment to discontinuous sediment to continuous sediment cover along a longitudinal profile from the ice sheet to tributary c1b along this same profile we observe a speedup of ice flow and then full development of the ice stream tributary ice stream d flows above a thick sedimentary package with an uppermost low seismic velocity zone indicative of soft till and the upglacier and lateral extensions of ice stream d are tightly constrained by the extent of continuous sediments the inland termination of these sediments suggests that future migration of high velocity low shear stress ice flow in these regions appears unlikely

seismic attenuation in glacial ice a proxy for englacial temperature 1 seismic attenuation or internal friction q 1 in glacial ice is highly sensitive to temperature particularly near the melting point here we detail a technique to estimate q and apply it to active source seismic data from jakobshavn isbrae greenland we compare our results to measured and modeled temperature profiles of the ice in the region we find an excellent match with differences between seismically estimated and modeled temperatures of less than 2 c mapping variations in seismic q through glacial ice thus is shown to allow detailed estimation of englacial temperature profiles which may be of special value in regions where in situ measurements are logistically difficult

subglacial lake whillans seismic observations of a shallow active reservoir beneath a west antarctic ice stream abstract active subglacial lakes concentrate the distribution of water beneath ice sheets in both space and time seismic and surface observations from subglacial lake whillans slw west antarctica reveal that this active lake forms a persistent albeit fluctuating reservoir beneath whillans ice stream imaging and phase observations using active source seismic data show that slw is a perpetually shallow feature when surveyed near its low stand a water column was resolvable by seismic techniques along only 5 km of the 45 km profiled with a maximum depth of less than 8 m satellite altimetry shows that the high stand adds no more than 3 4 m to this this water column presents a suitable drill site at s 84 240 w 153 694 elsewhere the majority of the bed appears wet with soft sediment or water thicknesses of less than the imaging resolution of our data of approximately 2 m the surface expression of the active lake previously revealed by icesat elevation data and image differencing generally corresponds to the seismic estimate of soft sediment or water with notable exceptions occurring at the upstream and downstream ends of the lake these exceptions indicate that slw s water column is very shallow or absent in places at low stands or has disconnected or transiently active and inactive portions

wet subglacial bedforms of the ne greenland ice stream shear margins we describe elongate wet subglacial bedforms in the shear margins of the ne greenland ice stream and place some constraints on their formation lateral shear margin moraines have been observed across the previously glaciated landscape but little is known about the ice flow conditions necessary to form these bedforms here we describe in situ sediment bedforms under the ne greenland ice stream shear margins that are observed in active source seismic and ground penetrating radar surveys we find bedforms in the shear margins that are 500 m wide 50 m tall and elongated nearly parallel to ice flow including what we believe to be the first subglacial observation of a shear margin moraine acoustic impedance analysis of the bedforms shows that they are composed of unconsolidated deformable water saturated till we use these geophysical observations to place constraints on the possible formation mechanism of these subglacial features

joint interpretation of explosive and vibroseismic surveys on cold firn for the investigation of ice properties two seismic surveys were carried out on the high altitude glacier saddle colle gnifetti monte rosa italy switzerland explosive and vibroseismic sources were tested to explore the best way to generate seismic waves to deduce shallow and intermediate properties 100 m of firn and ice the explosive source sissy excites strong surface and diving waves degrading data quality for processing no englacial reflections besides the noisy bed reflector are visible however the strong diving waves are analyzed to derive the density distribution of the firn pack yielding results similar to a nearby ice core the vibrator source elvis used in both p and sh wave modes produces detectable laterally coherent reflections within the firn and ice column we compare these with ice core and radar data the sh wave data are particularly useful in providing detailed high resolution information on firn and ice stratigraphy our analyses demonstrate the potential of seismic methods to determine physical properties of firn and ice particularly density and potentially also crystal orientation fabric

force balance along an inland tributary and onset to ice stream d west antarctica the transition from inland to streaming style ice flow near to and upstream from the onset to ice stream d west antarctica is investigated using the force balance technique basal drag provides the majority of the flow resistance over the study area but is substantially modified by non local stress gradients lateral drag increases with distance downstream balancing 50 100 of the driving stress at the onset longitudinal stress gradients lsg are also found to be significant an observation that distinguishes ice flow in this region from the inland and streaming flow regimes that bound it in which lsg are usually negligible lsg decrease the spatial variability in basal drag and sliding speed and increase the area of the bed over which frictional melting occurs overall lsg decrease the resistive influence of basal stress concentrations and increase the spatial uniformity of basal sliding these observations suggest that streaming flow develops as an integrated response to the physical interaction between the ice and its bed over an extended region upstream from the onset rather than being solely due to changes in basal characteristics at the onset an implication is that non steady flow behavior upstream from the onset may ultimately propagate downstream and result in non steady behavior at the onset

glacial geomorphology of the neutral hills uplands southeast alberta canada the process form imprints of dynamic ice streams and surging ice lobes abstract the neutral hills uplands of southern alberta canada is an area of complex and varied glacial landforms dominated by glacitectonic compressional structures but also containing expansive areas of hummocky terrain and kame and kettle topography it lies between the strongly streamlined trunks of the former central alberta cais and maskwa palaeo ice streams of the sw laurentide ice sheet lis and hence comprises an inter ice stream regional moraine zone constructed at around 15 5 cal ka bp this study aimed to compile a regional map of the glacial geomorphology of central southeast alberta in order to decipher the landform sediment signatures of overprinted ice stream margins in terrestrial continental environments and to refine the palaeoglaciological reconstructions for the southwest lis detailed mapping from lidar and aerial imagery identifies distinctive glacial landsystems diagnostic of the partial overprinting of cross cutting ice stream trunks and fast flow lobes widespread evidence of surge diagnostic features indicates that the ice streams experienced repeated flow instabilities consistent with the broader scenario of a highly dynamic and unstable sw lis characterised by markedly transitory and cross cutting palaeo ice streams the inter ice stream moraine zone is characterised by spectacular glacitectonic compression of bedrock cupola hill construction and mega raft displacement but also displays evidence of multi phase stagnant ice melt out where partially overprinted surge lobes advanced into large areas of buried glacier ice contemporaneous ice melting led to the widespread development of glacier karst and the production of eskers at a range of scales the largest of which record deranged drainage patterns indicative of ice walled channel sedimentation controlled by the regional bedrock slope towards the northeast these process form regimes have created a significant local relief that is a product of not only glacitectonic compression of bedrock but also the creation and melting of a melange of ice and bedrock sediment blocks of variable ice volume which are representative of former buried snout ice with a glacier karst system that was repeatedly proglacially thrust due to surging widespread evidence for subglacial channel cutting is likely strongly linked to the transitory surging and cross cutting nature of the palaeo ice streams in the region whereby ice streams switched on and surged in response to the build up migration and marginal outbursts of subglacial water reservoirs in addition to the reduced basal friction caused by the low permeability of the cretaceous bedrock pressurized groundwater and potentially also shallow biogenic gas deposits were likely important to the process form regimes of surging lobes of soft bedded ice streams in a region where ice flow was against an adverse bed slope a scenario that gave rise to a variety of enigmatic landforms such as doughnuts doughnut chains apparent blow out features and possible till eskers as well as glacitectonic mega rafts

active seismic studies in valley glacier settings strategies and limitations subglacial tills play an important role in glacier dynamics but are difficult to characterize in situ amplitude variation with angle ava analysis of seismic reflection data can distinguish between stiff tills and deformable tills however ava analysis in mountain glacier environments can be problematic reflections can be obscured by rayleigh wave energy scattered from crevasses and complex basal topography can impede the location of reflection points in 2 d acquisitions we use a forward model to produce challenging synthetic seismic records in order to test the efficacy of ava in crevassed and geometrically complex environments we find that we can distinguish subglacial till types in moderately crevassed environments where moderate depends on crevasse spacing and orientation the forward model serves as a planning tool as it can predict ava success or failure based on characteristics of the study glacier applying lessons from the forward model we perform ava on a seismic dataset collected from taku glacier in southeast alaska in march 2016 taku glacier is a valley glacier thought to overlay thick sediment deposits a near offset polarity reversal confirms that the tills are deformable

channelized sandstone bodies cordons in the tassili n ajjer algeria libya snapshots of a late ordovician proglacial outwash plain

extensive storage of basal meltwater in the onset region of a major west antarctic ice stream a major meltwater body exists beneath a tributary of bindschadler ice stream west antarctica in a region where subglacial lakes have not been mapped but near where rapid vertical motion of the ice sheet surface has suggested shifting of a subglacial water body the water is trapped by a local reversal in ice air surface slope arising from ice flow over variable basal topography and from the positive feedback of basal lubrication from the trapped water strong variations in the water content of the sediments upglacier of the water body arise from a similar process these results are revealed by a novel application of the amplitude variation with offset avo seismic technique the existence of such water bodies and of the strong spatial variation in subglacial sediment properties is not captured in current models of subglacial hydrology lubrication of ice stream motion and sediment transport

glacier surge mechanism 1982 1983 surge of variegated glacier alaska the hundredfold speedup in glacier motion in a surge of the kind the kind that took place in variegated glacier in 1982 1983 is caused by the buildup of high water pressure in the basal passageway system which is made possible by a fundamental and pervasive change in the geometry and water transport characteristics of this system the behavior of the glacier in surge has many remarkable features which can provide clues to a detailed theory of the surging process the surge mechanism is akin to a proposed mechanism of overthrust faulting

seismic detection of transient changes beneath black rapids glacier alaska u s a i techniques and observations to gain new insight into the mechanisms of basal motion we have demonstrated the feasibility of an active seismic technique to measure temporal changes in basal conditions on sub hourly time scales one region of the bed of black rapids glacier alaska u s a was monitored for a period of 45 days using seismic reflections the majority ofthese reflections were nearly identical however three significant anomalies were recorded several days apart these corresponded with the englacial drainage of two ice marginal lakes and one supraglacial pothole each up glacier of the study site as well as dramatic increases in basal motion two of these seismic anomalies revealed identical changes over 1 km 2 of the bed despite the fact that their drainage events occurred at different locations further these two seismic anomalies were followed by records identical to the non anomalous state showing that the seismic changes were reversible in one of these events two records taken 36 min apart revealed that the transition between the anomalous and normal states occurred completely within this short interval

late ordovician climbing dune cross stratification a signature of outburst floods in proglacial outwash environments climbing dune scale cross statification is described from late ordovician paraglacial successions of the murzuq basin sw libya this depositional facies is comprised of medium grained to coarse grained sandstones that typically involve 0 3 to 1 m high 3 to 5 m in wavelength asymmetrical laminations most often stoss depositional structures have been generated with preservation of the topographies of formative bedforms climbing dune cross stratification related to the migration of lower flow regime dune trains is thus identified related architecture and facies sequences are described from two case studies i erosion based sandstone sheets and ii a deeply incised channel the former characterized the distal outwash plain and the fluvial subaqueous transition of related deltaic wedges while the latter formed in an ice proximal segment of the outwash plain in erosion based sand sheets climbing dune cross stratification results from unconfined mouth bar deposition related to expanding sediment laden flows entering a water body within incised channels climbing dune cross stratification formed over eddy related side bars reflecting deposition under recirculating flow conditions generated at channel bends associated facies sequences record glacier outburst floods that occurred during early stages of deglaciation and were temporally and spatially linked with subglacial drainage events involving tunnel valleys the primary control on the formation of climbing dune cross stratification is a combination between high magnitude flows and sediment supply limitations which lead to the generation of sediment charged stream flows characterized by a significant relatively coarse grained sand sized suspension load concentration with a virtual absence of very coarse to gravelly bedload the high rate of coarse grained sand fallout in sediment laden flows following flow expansion throughout mouth bars or in eddy related side bars resulted in high rates of transfer of sands from suspension to the bed net deposition on bedform stoss sides and generation of widespread climbing dune cross stratification the later structure has no equivalent in the glacial record either in the ancient or in the quaternary literature but analogues are recognized in some flood dominated depositional systems of foreland basins

outburst flooding and the initiation of ice stream surges in response to climatic cooling a hypothesis abstract outburst flooding from subglacial reservoirs and associated surging of ice streams are caused by processes following climatic cooling that produce ice shelf grounding on proglacial sills trapping subglacial water according to an hypothesis presented here glaciers often advance into proglacial water bodies cooling may allow ice shelf formation and advance causing grounding on a proglacial sill trapping subglacial water ice shelf freeze on to the sill and development of a local reversal in ice air surface slope over the sill then are likely allowing ice thickening and water overpressurization if basal thawing then occurs as is likely an outburst flood and a surge may be triggered these processes may have been involved in heinrich events generation of antarctic subglacial lakes and meltwater scouring of some regions

simulation of cracking near a large underground cavern in a discontinuous rock mass using the expanded distinct element method abstract the rock masses in a construction site of underground cavern are generally not continuous due to the presence of discontinuities such as bedding joints faults and fractures the performance of an underground cavern is principally ruled by the mechanical behaviors of the discontinuities in the vicinity of the cavern a number of experimental and numerical investigations have demonstrated the significant influences of discontinuities on the mechanical thermal and hydraulic behaviors of discontinuous rock masses indicating that the deformation mechanism and stability of rock structures in the discontinuous rock masses depend not only on the existing discontinuities but also on the new cracks generated and thereafter keep propagating due mainly to the stress redistribution induced by excavation in this study an expanded distinct element method edem was developed for simulating the crack generation and propagation due to the shear and tension failures in the matrix rock blocks using this method excavation simulations of deep underground caverns have been carried out on the models with differing depths of cavern and differing geometrical distributions of the existing discontinuities model experiments by using the base friction test apparatus were conducted to verify the proposed numerical approach furthermore the support effects of rock bolts on controlling the deformations of the rock mass surrounding a cavern and movements of key blocks were evaluated by means of the edem approach

influence of subglacial geology on the position of a west antarctic ice stream from seismic observations ice streams drain much of the interior west antarctic ice sheet and buffer the main ice reservoir from oceanic influences1 2 the slow flowing interior feeds the floating ross ice shelf with ice via fast flowing ice streams3 that are believed to modulate sea level change through their control of inland ice storage understanding ice stream behaviour and predicting the response to climate change4 requires a better knowledge of the subglacial geology5 6 it is known that a thawed ice bed and high pressure basal water are necessary but not sufficient conditions to cause ice streaming7 8 moreover it has been hypothesized that a soft sedimentary bed is also required because of its intrinsic low frictional resistance to flow9 and owing to its high erodibility so as to generate till that can deform and lubricate ice motion10 11 or to bury rough features and smooth the bed for sliding here we use seismic observations to provide evidence that one margin of the upglacier part of an ice stream is directly above the boundary of a basin with such sedimentary fill the ice stream is within the basin and the ice outside the basin is slow flowing the basin fill presents an order of magnitude lower frictional resistance to ice flow than the subglacial material outside the basin we conclude that the ice stream position is dependent on subglacial geology

detecting and characterising an englacial conduit network within a temperate swiss glacier using active seismic ground penetrating radar and borehole analysis englacial hydrology plays an important role in routing surface water to the glacier s bed and it consequently affects the glacier s dynamics however it is often difficult to observe englacial conduit conditions on temperate glaciers because of their short lived nature we acquired repeated active surface seismic data over the rhone glacier switzerland to monitor and characterise englacial conduit conditions amplitude versus angle analysis suggested that the englacial conduit is water filled and between 0 5 and 4 m thick a grid of gpr profiles acquired during the 2018 melt season showed the englacial conduit network persisting and covering 14 000 m 2 in late summer 2018 several boreholes were drilled into the conduit network we observed generally stable water pressure but there were also short sudden increases a borehole camera provided images of a fast flowing englacial stream transporting sediment through the conduit from these observations we infer that the englacial conduit network is fed by surface meltwater and morainal streams the surface and morainal streams merge together enter the glacier subglacially and flow through subglacial channels along the flank these subglacial channels flow into highly efficient englacial conduits traversing the up glacier section of the overdeepening before connecting with the subglacial drainage system

high sensitivity of subglacial hydrological pathways in antarctica to small ice sheet changes 1 the flow of water beneath the antarctic ice sheet has recently been shown to involve drainage from and exchange between subglacial lakes with flow routing over distances of 100km as subglacial water is known to be important to ice sheet dynamics changes to the flow routing of basal water may yield an ice sheet response here we show that the directions of subglacial hydrological flowpaths are highly sensitive to the ice sheet surface topography at certain sites in antarctica surface elevation changes of between 5 and 15 m are found to be sufficient to produce significant changes to the subglacial water flowpaths which affect the potential drainage routes of large subglacial lakes we identify the reasons for this high sensitivity and recognise several locations in east antarctica where changes to the drainage regime could have important effects on large scale basal water distribution

basal conditions on rutford ice stream west antarctica from seismic observations a seismic reflection profile perpendicular to the ice flow direction has been acquired on rutford ice stream antarctica an interpretation of both the amplitude and phase of the seismic reflections from the ice bed interface has been made to investigate the properties of the sub ice material the interpretation assumes that acoustic impedance can be used to imply subglacial sediment porosity multiple reflections from the ice bed interface on a number of seismic wide angle lines allowed a calibration of the reflection coefficient at the bed this enabled the acoustic impedance of the bed material to be calculated from the seismic reflection data mean acoustic impedance in the different sections of the bed ranges from 2 70 106 kg m 2 s 1 to 4 31 106 kg m 2 s 1 almost three quarters of the ice stream bed at this site appears to be saturated deforming sediments the rest of the bed is probably also saturated sediments but they are not deforming to any significant degree ice flow could include a combination of subglacial deformation and basal sliding localized regions which support disproportionately high amounts of basal shear stress may also occur from the seismic data it is not possible to apportion relative amounts of restraint to these different processes there does not appear to be any correlation between the different sections identified on the ice stream bed with either satellite images or nearby surface velocity data a feature which is interpreted as a subglacial drumlin is seen on the seismic section

longitudinal variations in glacial flow theory and test using data from the byrd station strain network antarctica application de la theorie de budd 1970 des effets du lit sur les variations d ecoulement du glacier aux observations de la calotte de glace a la station byrd on utilise une rheologie visqueuse lineaire et on resout une equation biharmonique des variations des efforts et des vitesses de deformation les conditions aux limites comportent une topographie du fond avec ondulations glissements et cisaillements

combined finite discrete element modelling of rock fracture and fragmentation induced by contour blasting during tunnelling with high horizontal in situ stress a combined finite discrete element method fdem parallelized on the basis of gpgpu is implemented to model the rock fracture and fragmentation process and the resultant excavation damaged zone edz development induced by the controlled contour blasting which was conducted in the tasq tunnel with high in situ stresses in the aspo hard rock laboratory in sweden the combination of in situ stress field equation of state based blast loading fracturing in tension and shear with gas flow loading of fractures enables the modelling of complex dynamic interactions from multiple blast rounds for the contour blasting under high horizontal in situ stresses blasting induced fractures initially propagate horizontally even though the holes are decoupled later these fractures coalesce into larger cracks which prevents the formation of smooth tunnel walls and increases the edz smoother surfaces are created at the crown and invert by the propagation of long fractures in the horizontal direction due to the combined effect of the free surfaces provided by the adjacent blast holes and the maximum principal stress induced by in situ stresses the fractures at the lower part of the tunnel sidewalls have the tendency to propagate upwards with a diagonal direction about 60 relative to vertical in accordance with the field test results removing the in situ stresses results in smoother sidewall fracturing with more damage in the crown and invert increase in rock heterogeneity above a threshold induces more fractures increasing the detonation timing between blast holes induces more damage into the rock mass and fragmentation in the burden outcomes of this study show that the gpgpu parallelized y hfdem ide provides a powerful tool to replicate the mechanisms of rock fracture and fragmentation induced by blasting

excavation induced damage studies at the underground research laboratory the underground research laboratory url is situated in the lac du bonnet granite batholith in southeastern manitoba canada the url was developed to study issues related to the deep geologic disposal of used fuel from nuclear reactors and has developed into an international atomic energy agency recognized geotechnical center of excellence extensive rock mechanics research has been conducted including work to understand the character and extent of excavation damage experience at the url has shown that damage exists around underground openings and that the damage develops from the energy imparted to the rock by the excavation method and by redistribution of the in situ stress field around the excavated openings subsequent near by excavations removal of loose material from the existing tunnel thermal loads and pore pressure changes will all influence the development and extent of rock damage as does the rock type and its fabric studies at the url have shown that in highly stressed rock damage will develop around underground openings even when a low energy excavation method is used and that properly designing the excavation geometry and orientation plays a major role in constructing stable openings two zones of excavation affected rock have been shown to exist from studies at the url 1 a zone of irreversibly damaged rock surrounding the excavation which may include failed zones inner and outer damage zones and 2 a zone of excavation disturbance where the in situ stresses are influence by the excavation no damage is measurable measurements of the properties of the damaged rock ultrasonic velocity transmissivity using a variety of instruments have shown that a less intensively damaged outer zone surrounds a more highly damaged inner zone the inner damage zone may have zones of failed rock depending on stress concentrations relative to rock strength and damage imparted by excavation method the inner damage zone is attributed to the effects of the excavation method and stress redistribution and the outer damage zone is attributed to the effects of stress redistribution alone in lower stressed rock the extent of the outer zone is much less than for an opening in highly stressed rock with the same excavation shape and orientation relative to in situ stresses the excavation disturbed zone describes the volume of rock surrounding the damage zone where the stress redistribution is too small to permanently change rock properties unless some other stress increase such as thermal loading is added and it is also characterized non permanent but potentially long term changes in hydraulic head

genesis evolution and tectonic significance of k rich volcanics from the alban hills roman comagmatic region as inferred from trace element geochemistry trace element data are reported in 21 lava samples from the alban hills one of the most important volcanic complexes of the roman comagmatic region the samples consist mostly of tephritic leucitites with minor phonolitic tephrites and tephritic phonolites emplaced during two distinct phases of activity separated by a caldera collapse

what controls the location of ice streams ice streams influence ice sheet mass balance and stability but key aspects of their behaviour remain poorly understood this paper reviews and discusses one very important aspect what controls their location in an ice sheet seven potential controls on ice stream location are identified from the literature topographic focusing topographic steps macro scale bed roughness calving margins subglacial geology geothermal heat flux and subglacial meltwater routing for each control the theoretical basis for its link to rapid ice flow is introduced followed by discussion of the evidence of its influence on the location of both contemporary and palaeo ice streams based on this new synthesis topographic focusing subglacial geology meltwater routing and calving margins appear to be most commonly associated with fast ice flow it is clear that rather than a single control however there exist a number of potential controls of varying influence we propose a hierarchy of factors with those occurring at the top of the hierarchy exerting a stronger influence on ice stream location and where present beneath an ice sheet are very likely to be associated with fast flow those factors occurring lower down the hierarchy are less commonly associated with ice streaming but appear to be influential in the absence of more common controls in such a hierarchy topographic focusing in the presence of a calving margin is the primary control in the absence of this ice streams will preferentially occur in areas with favourable subglacial meltwater routing and subglacial geology in the absence of these bed roughness geothermal heat flux and topographic steps may promote ice streaming significantly the primary controls on a given ice stream location are likely to influence its spatial and temporal dynamics ice streams governed by the presence of meltwater routing and or calving processes might exhibit more variable behaviour because these controls can vary over relatively short time scales compared to controls that vary over longer time scales e g geothermal heat flux subglacial roughness geology and topography identifying the controls on ice stream location is therefore of paramount importance when understanding ice stream longevity and their past and future activity