erosion of the laurentide region of north america by glacial and glaciofluvial processes collection of seismic reflection data from continental margins and ocean basins surrounding north america makes it possible to estimate the amount of material eroded from the area formerly covered by laurentide ice sheets since major glaciation began in north america a minimum estimate is made of 1 62 106 km3 or an average 120 m of rock physically eroded from the laurentide region this figure is an order of magnitude higher than earlier estimates based on the volume of glacial drift cenozoic marine sediments and modern sediment loads of rivers most of the sediment produced during laurentide glaciation has already been transported to the oceans the importance of continental glaciation as a geomorphic agency in north america may have to be reevaluated evidence from sedimentation rates in ocean basins surrounding greenland and antarctica suggests that sediment production sediment transport and possibly denudation by permanent ice caps may be substantially lower than by periodic ice caps such as the laurentide low rates of sediment survival from the time of the permo carboniferous and precambrian glaciations suggest that predominance of marine deposition during some glacial epochs results in shorter lived sediment because of preferential tectonism and cycling of oceanic crust versus continental crust

effects of groundwater level on seismic response of soil foundation systems abstracta set of dynamic centrifuge experiments was performed to assess the effect of the depth of the groundwater table on the seismic site response and kinematic interaction of a foundation on un

joint bounded crescentic scars formed by subglacial clast bed contact forces implications for bedrock failure beneath glaciers glaciers and ice sheets are important agents of bedrock erosion yet the precise processes of bedrock failure beneath glacier ice are incompletely known subglacially formed erosional crescentic markings crescentic gouges lunate fractures on bedrock surfaces occur locally in glaciated areas and comprise a conchoidal fracture dipping down ice and a steep fracture that faces up ice here we report morphologically distinct crescentic scars that are closely associated with preexisting joints termed here joint bounded crescentic scars these hitherto unreported features are ca 50 200 mm deep and involve considerably more rock removal than previously described crescentic markings the joint bounded crescentic scars were found on abraded rhyolite surfaces recently exposed 20 years beneath a retreating glacier in iceland as well as on glacially sculpted precambrian gneisses in nw scotland and various precambrian rocks in ontario glaciated during the late pleistocene we suggest a common formation mechanism for these contemporary and relict features whereby a boulder embedded in basal ice produces a continuously migrating clast bed contact force as it is dragged over the hard bedrock bed as the ice embedded boulder approaches a preexisting joint in the bedrock stress concentrations build up in the bed that exceed the intact rock strength resulting in conchoidal fracturing and detachment of a crescentic wedge shaped rock fragment subsequent removal of the rock fragment probably involves further fracturing or crushing comminution under high contact forces formation of joint bounded crescentic scars is favoured by large boulders at the base of the ice high basal melting rates and the presence of preexisting subvertical joints in the bedrock bed we infer that the relative scarcity of crescentic markings in general on deglaciated surfaces shows that fracturing of intact bedrock below ice is difficult but that preexisting weaknesses such as joints greatly facilitate rock failure this implies that models of glacial erosion need to take fracture patterns of bedrock into account

were deforming subglacial beds beneath past ice sheets really widespread abstract the concept of widespread large strain deformation of subglacial unconsolidated unfrozen sediments during pleistocene glaciations has inconsistencies in the geological record numerous properties of tills and related sediments are difficult to reconcile with pervasive strains as predicted by the deforming bed theory while accepting glacier bed deformation as a geological process occurring under certain circumstances we propose that it was much less widespread than believed by some instead we believe that basal sliding and englacial transport are major ice movement and debris transport mechanisms

the evolution of the subglacial landscape of antarctica the aim is to investigate the evolution of the subglacial landscape of antarctica using an ice sheet and erosion model we identify different stages of continental glaciation and model the erosion processes associated with each stage the model links erosion to the basal thermal regime and indicates that much of the antarctic interior may have been subject to less than 200 m of erosion the depth of erosion reflects the presence or absence of warm based ice and the consistency of ice flow direction this information linked with knowledge about landscapes of glaciation in the northern hemisphere and some simple but robust assumptions about initial topography is used to generate a map of the subglacial antarctic landscape in which much of the lowland interior resembles the landscapes of areal scouring typical of the laurentian and scandinavian shields near the continental margins selective linear erosion has overdeepened pre existing river valleys by as much as 2 8 km high elevation plateaus adjacent to such drainage systems have survived largely unmodified under cold based ice high erosion rates result from steep thermal gradients in basal ice mountain regions such as the gamburtsev mountains uplands in dronning maud land and massifs in west antarctica are likely to bear features of local alpine glaciation such landscapes may have been protected under cold based ice for the last 34 myrs or possibly longer

effect of fines and confining stress on undrained shear strength of silty sands a silty sand is considered as a delicate composite matrix containing a sand grain matrix and a silt matrix large strain undrained shear strength sus data in triaxial compression for a particular host sand mixed with different amounts of nonplastic fines are presented results indicate that the intergranular void ratio es which is the void ratio of the sand grain matrix given by e fc 1 fc where fc is the silt content fraction by weight plays an important role on sus of silty sands at the same void ratio e a silty sand shows low sus compared to that of the host sand however when compared at the same es provided that it is less than the maximum void ratio of the host sand emax hs both the silty sand and the host sand show similar sus that is fairly independent of the initial confining stress when es of the silty sand is in the vicinity of or exceeds emax hs the sus depends on the initial effective confining stress at such loose states sus normalized with respect to the initial con

regelation of ice through debris at glacier beds implications for sediment transport results from laboratory experiments show that ice at its melting temperature regelated through dense three dimensional arrays off clasts abrading a rock bed regelation theory for an isolated stationary array of clasts successfully approximates periods when ice separated from the bed thermal gradients and clast bed contact forces despite the relative motion of clasts and incoming heat from the bed the process may entrain sediment at glacier beds and restrict deposition to zones where sediment in ice is concentrated effective pressure is low and basal melting is rapid

the relative efficacy of fluvial and glacial erosion over modern to orogenic timescales it has been debated whether rivers or glaciers are more effective agents of erosion a global compilation of erosion rates reveals that both are capable of generating rates of erosion that match or exceed the highest rates of rock uplift since the late nineteenth century it has been debated whether rivers or glaciers are more effective agents of erosion1 the dramatic landscapes associated with glaciated terrain have often led to the argument that glaciers are more erosive than rivers and recent studies have documented the topographic signature of an ice controlled limit of mountain height known as the glacial buzz saw 2 3 here we present a new global compilation of erosion rates which questions the conventional view of glaciers and erosion in regions of rapid tectonic uplift erosion rates from rivers and glaciers both range from 1 to over 10 mm yr 1 indicating that both are capable of generating erosion rates matching or exceeding the highest rates of rock uplift moreover a comparison of erosion rates over timescales ranging from 101 to 107 years indicates that glacial erosion tends to decrease by one to two orders of magnitude over glacial cycles whereas fluvial erosion rates show no apparent dependence on time we conclude that tectonics controls rates of both fluvial and glacial erosion over millennial and longer timescales and that the highest rates of erosion 10 mm yr 1 generally result from a transient response to disturbance by volcanic eruptions climate change and modern agriculture

numerical modeling of advective transport of saturated deforming sediment beneath the lake michigan lobe laurentide ice sheet abstract we investigated the dynamics of advective transport of saturated fine grained deforming sediment beneath the late wisconsinan lake michigan lobe lml of the laurentide ice sheet using a numerical flowline model sediment rheology is modeled using a general constitutive law for mass movement of fine grained material geotechnical tests on selected tills deposited by the lml in northern illinois suggest an effective viscosity of about 5 109 pa s for fully saturated sediment and mild nonlinearity in the stress strain relation numerical calculations of sediment flux based on conservative parameters and assumptions yield estimates on the order of 100 m3 y meter width of the lobe suggesting that advective transport could account directly for at least 25 of the total sediment flux estimated on the basis of field evidence less restrictive parameters and assumptions would increase sediment flux attributable to both advective transport as well as other ice velocity dependent sediment transport processes modeling results also suggest that maximum advective transport rates might be associated with viscosities up to an order of magnitude higher than the viscosity exhibited by fully saturated sediment these results thus suggest that subglacial sediment deformation could account for a substantial portion of total sediment flux in areas with fine grained glacial till and that transport rates are strongly sensitive to the subglacial hydrologic processes that control basal water pressure and sediment viscosity

sediment evacuation and glacial erosion rates at a small alpine glacier 1 glacial erosion is known to be driven by sliding which occurs in brief episodes in the melt season what is not known is how well this sliding history is reflected in the sediment output from a glacier and to what degree temporary sediment storage in the glacier smoothes the history of sediment production we document sediment evacuation rates for three seasons at bench glacier a small temperate alpine glacier in the chugach range of alaska two lines of evidence suggest that subglacial sediment storage is small strong hysteresis in the relationship between sediment and water discharge argues for sediment exhaustion from the subglacial system on seasonal and flood event timescales uplift of the glacier surface during periods of enhanced basal motion reflects intimate contact of the glacier sole with up glacier inclined stoss slopes of bedrock bumps sediment evacuation rates at bench glacier should therefore closely track glacial erosion rates over annual timescales much of the sediment emerges in a major pulse that closely follows the termination of enhanced sliding each year suggesting that the conduits and well connected cavities that allowed removal of englacial water to terminate sliding also promoted efficient transport of sediment this small glacier efficiently modifies the landscape lowering its bed by 1 2 mm yr while sliding only 1 2 m yr our sparse measurements of instantaneous bed load discharge average 41 of concurrent suspended sediment discharge assuming that half of the suspended load derives from abrasion of subglacially quarried clasts roughly two thirds of the sediment evacuated can be attributed to quarrying

comment on subglacial erosion and englacial sediment transport modeled for north american ice sheets by d h d hildes g k c clarke g e flowers s j marshall

experimental rock on rock frictional wear application to subglacial abrasion 1 rock on rock wear under known normal loads was measured experimentally using 62 mm diameter rotating discs machined from a range of silicic rock types one granite two metamorphic rocks and three sandstones of porosities varying between 7 and 28 wear debris was removed continuously from the contacting surfaces by water we obtained a wear law of the form dt dx a n m where is contact normal stress mpa dt dx is a dimensionless wear rate meter wear t per meter shear displacement x is porosity and a n and m are empirical constants 10 8 11 8 33 and 4 50 respectively low porosity rocks displayed surface polishing and abraded particle size was substantially smaller than the starting grain size whole grains were plucked from high porosity rocks without any surface polishing so that the wear product particle size remained the same as that of the starting material the empirical wear law describes abrasion beneath a hard bedded temperate glacier provided we can estimate both the in situ normal stress between entrained clasts and the bed and the horizontal velocity of the clasts in the flowing ice normal stress concentration is expected to develop at clast bed contacts from drag against ice flowing toward the bed where pressure melting occurs these experimental data are consistent with wear rates inferred from silt outflow in subglacial streams provided a tenfold stress concentration can occur relative to the ice overburden pressure the rate of production of fine grained wear products under subglacial conditions is substantially less than by tectonic faulting

cyclic volumetric strain behavior of sands with fines of low plasticity abstractthis work investigates the seismic compression characteristics of nonplastic and low plasticity silty sands with varying fines content 0 fc 60 cyclic simple shear testing was performed on various sand fines mixtures at a range of modified proctor relative compaction levels and degrees of saturation aside from the expected strong influence of relative compaction increasing fines content is found to generally decrease volume change for fines fractions consisting of silts and clayey silts of moderate to low plasticity index pi 10 with truly nonplastic fines rock flour cyclic volume change increases with fc some materials also exhibit an effect of as compacted saturation in which moderate saturation levels associated with high matric suction cause volume change to decrease additionally the data consistently demonstrate that vertical strains decrease as overburden pressure is increased in a manner consistent with a previous clean sands model this paper presents empirical equations to ca

deglaciation of a soft bedded laurentide ice sheet we present a series of numerical reconstructions of the laurentide ice sheet during the last deglaciation 18 7 14c ka that evaluates the sensitivity of ice sheet geometry to subglacial sediment deformation these reconstructions assume that the laurentide ice sheet flowed over extensive areas of water saturated deforming sediment soft beds corresponding to the st lawrence lowland the great lakes region the western prairies of the u s and canada and the hudson bay and hudson strait regions sediment rheology is based on a constitutive law that incorporates experimental results from late wisconsin till deposited by the laurentide ice sheet which suggest only mildly nonlinear viscoplastic behavior by varying the effective viscosity of till we produced four reconstructions for the ice sheet during the last glacial maximum 18 14c ka and two reconstructions each of the ice sheet at 14 13 12 11 and 10 14c ka we also produced one reconstruction for 9 8 4 8 and 7 14c ka reconstructions that assume a low effective viscosity for all areas of deforming sediment show a multidomed ice sheet with a large bowl shaped depression over hudson bay and thin ice 1000 m above modern sea level over the western and southern margins those reconstructions that assume a higher effective viscosity of till in the hudson bay region than for the western and southern margins also show a multidomed ice sheet but with considerably thicker ice over hudson bay and a more northerly position of the central ice divide these two different geometries may represent ice sheet orographic changes associated with a heinrich event further increases in effective viscosity of till approaching the effective viscosity of ice would result in a high monolithic ice dome centered over hudson bay reinforcing the notion that a multidomed ice sheet reflects the distribution of substrate geology modeled ice surface geometry at the last glacial maximum shows many of the same general features as previous reconstructions that incorporate deformable beds our reconstructions with higher effective till viscosities in hudson bay also agree with the ice 4g reconstructions peltier 1994 which are based on inversion of relative sea level data for the early part of the last deglaciation 18 13 14c ka but then depart significantly from ice 4g beginning at about 12 14c ka due to differing assumptions of the history of deglaciation modeled ice volume for the last glacial maximum suggests a glacioeustatic change of 50 55 m by a soft bedded laurentide ice sheet this would increase as the effective viscosity of till increases subsequent ice volume changes through the last deglaciation generally parallel the trend of eustatic rise recorded at barbados new guinea and tahiti but suggest that the laurentide ice sheet was not the source of meltwater pulse 1a

numerical modeling of the development of u shaped valleys by glacial erosion the steep sided valleys and overdeepened basins of alpine landscapes are well known products of glaciation yet relatively little is known about how the dynamics of ice flow and glacial erosion interact to give rise to such landforms by linking a finite element model for ice flow through a glacier cross section with an erosion model it is possible to investigate the development of one of the most striking glacial landforms the u shaped valley in addition to providing a detailed understanding of landform development such modeling provides a way to test current understanding of the controls on glacial sliding and erosion to simulate valley development i first model flow through an initial glacier cross section and calculate the glaciological parameters that govern erosion i then numerically simulate erosion to produce a modified transverse profile for which a new flow field and erosion pattern are computed a number of iterations permits examination of the progressive transformation of cross section form which can be compared with field data model predictions of the cross section flow field are in close accord with data from the athabasca glacier and include marked lateral variations in sliding velocity with an erosion law dependent on basal velocity the model predicts the rapid transformation of a v shaped cross section into a recognizably glacial form over a time period on the order of 10 4 yr and the eventual development of a steady state quasi parabolic glacier cross section better agreement with empirical data from glaciated valleys is obtained by including temporal variations in ice discharge in order to mimic the characteristics of 100 000 yr glacial cycles the high discharge phase dominates form development and at low discharges cross section form is essentially inherited from the central part of the form that developed during the preceding high discharge phase

alpine subglacial hydrology abstract the size and shape of subglacial flow pathways exert a strong influence over the pressure and speed at which meltwaters are routed through glaciers these characteristics in turn influence glacier sliding velocity and the quantity and quality of bulk meltwaters discharging from glaciers in this paper we summarise four empirical research approaches that have been developed to improve our understanding of the subglacial hydrology of predominantly temperate based alpine glaciers investigations of bulk meltwater discharge and chemistry tracer studies proglacial bedrock investigations and borehole investigations as a result of linking these studies to conceptual or numerical models three arborescent and four non arborescent or distributed subglacial drainage configurations are described these drainage networks may be neither spatially nor temporally persistent but may compete with and replace each other on a seasonal basis a qualitative model of the temporal and spatial evolution of a typical alpine subglacial drainage system is advanced on the basis of research conducted at haut glacier d arolla switzerland

ice rafted debris associated with binge purge oscillations of the laurentide ice sheet the north atlantic sediment record suggests quasi periodic 7000 to 12 000 year period ice rafted debris ird depositions during at least the last glacial period the cause of these heinrich events as they are commonly known is not fully understood however they may point to surges of the ice stream that drained the hudson bay hudson strait region of the laurentide ice sheet we investigate a simple conceptual model of ice stream instability the binge purge model to suggest ways in which the ice stream could have entrained sufficient debris to account for the estimated mass of ird associated with a typical heinrich ird layer in the north atlantic 1 0 0 3 1015 kg we find that freezing of debris laden ice at the bed of the ice stream during the brief 750 years surge phase of the ice stream s hypothesized binge purge cycle can incorporate up to 5 1 1015 kg this amount is sufficient to meet the constraints of the north atlantic sediment record but by no means verifies the binge purge model as the cause of heinrich events

using inherited cosmogenic 36cl to constrain glacial erosion rates of the cordilleran ice sheet cosmogenic 36 cl cl ratios measured from glacially eroded bedrock provide the first quantitative constraints on the magnitude rate and spatial distribution of glacial erosion over the last glacial cycle of 23 36 cl cl ratios 8 yield exposure ages that predate the well constrained deglaciation of the puget lowland washington and are inferred to result from 36 cl inherited from prior exposure during the last interglaciation where ice did not erode enough rock 1 80 2 95 m to reset 36 cl cl ratios to background levels surfaces possessing inherited 36 cl

sequence stratigraphy and evolution of the paleoproterozoic intracontinental baker lake and thelon basins western churchill province nunavut canada abstract the baker lake and thelon basins lie along the border between the rae and hearne domains of the western churchill province basin fill comprises the dubawnt supergroup a ca 1 85 1 70 ga succession of predominantly continental clastic and intercalated volcanic rocks that nonconformably and unconformably overlies granitic and supracrustal rocks most of late archean age the dubawnt supergroup comprises three second order sedimentary sequences that record deposition within a rift basin a modified rift basin and a thermal sag basin respectively the baker sequence is inferred to represent the principal phase in the development of baker lake basin a series of generally elongate northeast striking half graben and fault bounded troughs filled with continental redbeds and coeval voluminous ultrapotassic volcanic rocks faults parallel to the basin margin exhibit evidence for strike slip ca 1 83 1 81 ga the nature and timing of basin development and associated magmatism allows that it may be a transtensional basin that formed by lateral escape in response to crustal thickening during the trans hudson orogeny similar to the formation of mesozoic and cenozoic strike slip and associated rift basins on the margins of the tibetan plateau the overlying whart sequence was deposited in small basins formed by block faulting and tilting of the baker sequence basin fill comprises eolian and alluvial redbeds with intercalated rhyolite flows and epiclastic rocks distribution of facies and stratigraphy suggest that faulting occurred throughout deposition of the whart sequence but ceased before deposition of the overlying barrens sequence ca 1 72 ga deformation and basin formation can be linked to a regional episode of granite emplacement and associated thermal metamorphism ca 1 76 1 75 ga the barrens sequence represents deposition over a broader area primarily in thelon basin strata generally are thinner and flatter than underlying sequences and display lateral continuity indicating minimal influence from syndepositional faulting the depositional record reflects progressive upward fining and eventually the first record of marine transgression within the dubawnt supergroup these features suggest that the barrens sequence was deposited over a broad region of thermal subsidence likely related to cooling of previously attenuated continental lithosphere the barrens sequence may be a remnant of a huge cratonic sand sheet that included the thelon athabasca amundsen and elu basins a regional subsidence mechanism to account for these basins may be mantle downwelling linked to the late stage amalgamation of laurentia the tripartite sequence evolution of the baker lake and thelon basins represents a framework for future sequence correlation among late paleoproterozoic basins in laurentia and shares characteristics with the evolution of contemporaneous intracontinental basins from north central australia and southeastern brazil

glacial erosion and bedrock properties in nw scotland abrasion and plucking hardness and joint spacing subglacial erosion beneath glaciers occurs predominantly by abrasion and plucking producing distinct erosional forms the controls on the relative importance of abrasion vs plucking are poorly understood on the one hand glacial conditions that favour or suppress cavity formation ice velocity ice thickness and water pressure are thought to favour plucking or abrasion respectively conversely bedrock properties are also known to control landforms but this has rarely been analysed quantitatively in this study we compare landforms and bedrock properties of sandstone and quartzite at the bed of a palaeo ice stream near ullapool in nw scotland the boundary between the rock types is at right angles to the westward palaeo ice flow and palaeoglacial conditions on both rock types were similar we report quantitative parameters for bedrock properties schmidt hammer hardness and joint spacing and use morphometric parameters to analyse the landforms torridon sandstone is soft but thick bedded and with a wide joint spacing erosional bedforms include roche moutonnees with smoothed tops and concave stoss sides whalebacks and elongate p forms indicating a high proportion of abrasion over plucking cambrian quartzite is hard but thin bedded with narrow joint spacing erosional landforms are angular to subangular with abundant plucked lee faces suggesting a high proportion of plucking over abrasion hardness and joint spacing thus exert a strong control on subglacial erosional landforms and the mechanisms that formed them thus glacial conditions ice velocity ice thickness can only be inferred from glacial erosional landforms if the effects of bedrock properties of the substrate are considered

fjord insertion into continental margins driven by topographic steering of ice fjords line mountainous continental margins where icesheets and glaciers once stood a two dimensional model simulation suggests that fjords can be eroded within one million years primarily in response to topographic ice steering and erosion from ice discharge subsequent glaciers that form on these landscapes are smaller and exhibit greater responses to climate change

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

intrusion of ice into porous media by regelation a mechanism of sediment entrainment by glaciers unlithified sediment at glacier beds should be entrained in ice as a result of melting above individual grains and refreezing below grains regelation to test this hypothesis a device that simulates subglacial conditions was used to push ice downward through idealized and natural porous media in contact with a flat bed ice regelated toward the bed at speeds proportional to the gradient in ice pressure across the thickness of particles ice temperatures responded predictably to changes in driving stress and demonstrated that the ice particle mixtures were at or very near the pressure melting temperature a theory of pore ice motion by regelation predicted the measured speed usually to within a factor of two although at low driving stresses the speed was less than the predicted value by as much as a factor of five the low speeds were probably caused by imperfect temperature control that resulted either in heat loss from the ice particle mixtures or in slightly subfreezing pore ice temperatures that were not detected by thermistors ice should intrude the pores of underlying sediment to a steady depth at which the downward regelation speed equals the rate of basal melting departures from the steady state should cause release of debris from ice or further entrainment an approximate calculation of the steady intrusion depth yields values comparable to typical thicknesses of debris in basal ice a few centimeters to more than a meter intrusion of sediment by regelation is consistent with the isotopic composition of debris bearing ice if melting exceeds refreezing

the subglacial mosaic of the laurentide ice sheet a study of the interior region of southwestern hudson bay abstract reconstructions of past ice flow provide useful insights into the long term behaviour of past ice sheets and help to understand how glaciated landscapes are shaped here we present reconstruction of a 10 phase ice flow history from southwestern hudson bay in northeastern manitoba canada a dynamic region situated between two major ice dispersal centres of the laurentide ice sheet we utilize a diverse geologic dataset including 1900 field based erosional indicators 12 streamlined landform flowsets esker and meltwater corridor orientations 103 till fabrics analyses and 1344 till clast lithology counts our reconstruction suggests that both pre mis 2 and mis 2 glaciations followed similar growth patterns where ice advanced into study area from ice centered to the east probably in northern quebec followed by a switch in ice flow direction indicating flow from the keewatin ice centre to the northwest and north the cause for this switch in ice flow orientation is uncertain but the youngest switch may relate to retreat of ice during mis 3 that then left space for keewatin sourced ice to advance over the study area while modelling experiments indicate widespread cold based conditions in the study area during the last glacial cycle uniformly relict landscapes are not common instead the glaciated landscape is palimpsest and commonly fragmented forming a subglacial bed mosaic of erosional and depositional assemblages that record both shifting ice flow direction through time and shifting subglacial conditions each assemblage formed or modified during times of dynamic warm based ice and later preserved under conditions below or close to the pressure melting point slow and sluggish or cold based

sedimentary record of ice divide migration and ice streams in the keewatin core region of the laurentide ice sheet abstract the aberdeen lake region of central mainland nunavut is a former core region of the laurentide ice sheet that is characterized by streamlined glacial landforms classified into multiple crosscutting flow sets and near continuous till blanket the presence of widespread till near the centre of the keewatin ice dome raises questions about its origin detailed drillcore logging revealed a complex stratigraphy consisting of at least 6 till units variably preserved across the study area till provenance analysis indicates deposition by near opposite trending ice flow phases interpreted as evidence of reconfiguration of the keewatin ice divide at the surface large north northwesterly aligned landforms are present across the study area the till stratigraphy within these landforms indicates the same nnw ice flow phase is responsible for considerable till production this ice flow phase is also correlated to a long regional dispersal train of erratics toward the gulf of boothia the production of an extensive thick 12 m till sheet during the nnw trending ice flow phase occurred far from the ice margin at a time of extensive ice cover of mainland nunavut likely from an east west oriented ice divide a deglacial westerly trending ice flow phase formed small drumlins atop the larger nnw streamlined till ridges and deposited a surficial till unit that is too thin to mask the nnw flow set across the study area it is proposed that the boothia paleo ice stream catchment area propagated deep into the laurentide ice sheet and contributed to significant till production in this core region of the keewatin sector prior to the westerly ice flow shift the apparent relationship between till thickness and the size of the associated or correlated drumlins flow sets and dispersal trains indicates complex erosion deposition interplay is involved in the formation of streamlined subglacial landforms

the influence of tectonic deformation on facies variability in stratified debris rich basal ice facies variability displayed within basal ice has hitherto been widely interpreted as a re ection of spatial variation in the processes of ice formation the basal boundary conditions or a combination of both recent literature has consequently been characterised by attempts to derive a genetic classi cation of basal ice facies applicable to di erent types of glacier we present new observations made at the russell glacier greenland which suggest that any such genetic classi cation must take account of post formational deformation occurring within basal ice all the ice sub facies occurring within the strati ed basal ice are explained in terms of two sets of processes the initial entrainment of a debris rich basal layer and subsequent ow related tectonic deformation of that layer to produce distinctive tectonically derived sub facies the evolution appearance and composition of some elements of the basal ice layer are therefore argued to be controlled primarily by post formational ow related deformation any useful genetic classi cation of basal ice facies must therefore include criteria able to distinguish between characteristics derived from the mechanism of ice formation and those that are tectonically derived 2000 elsevier science ltd all rights reserved

glacial effects limiting mountain height the height of mountain ranges reflects the balance between tectonic rock uplift crustal strength and surface denudation tectonic deformation and surface denudation are interdependent however and feedback mechanisms in particular the potential link to climate are subjects of intense debate spatial variations in fluvial denudation rate caused by precipitation gradients are known to provide first order controls on mountain range width crustal deformation rates and rock uplift moreover limits to crustal strength are thought to constrain the maximum elevation of large continental plateaus such as those in tibet and the central andes there are indications that the general height of mountain ranges is also directly influenced by the extent of glaciation through an efficient denudation mechanism known as the glacial buzzsaw here we use a global analysis of topography and show that variations in maximum mountain height correlate closely with climate controlled gradients in snowline altitude for many high mountain ranges across orogenic ages and tectonic styles with the aid of a numerical model we further demonstrate how a combination of erosional destruction of topography above the snowline by glacier sliding and commensurate isostatic landscape uplift caused by erosional unloading can explain observations of maximum mountain height by driving elevations towards an altitude window just below the snowline the model thereby self consistently produces the hypsometric signature of the glacial buzzsaw and suggests that differences in the height of mountain ranges mainly reflect variations in local climate rather than tectonic forces

timescale dependence of glacial erosion rates a case study of marinelli glacier cordillera darwin southern patagonia 1 erosion rates have been estimated for a number of glaciated basins around the world mostly based on modern observations last few decades of sediment fluxes to fjords we use time constrained sediment volumes delivered by marinelli glacier 55 s an outlet glacier of the cordillera darwin ice cap southern patagonian andes tierra del fuego to determine erosion rates across different timescales sediment volumes are derived using a dense grid of high and low frequency single channel seismic data and swath bathymetry data along with piston and kasten cores our results show dramatic differences in erosion rates over different timescales erosion rates at marinelli glacier diminish about 80 or by factor of 5 with each ten fold increase in the time span over which erosion rates are averaged 29 3 mm yr for the last 45 years 5 3 mm yr for the last 364 years and 0 5 mm yr for the last 12 500 years these results indicate that modern sediment yields and erosion rates from temperate tidewater glaciers can exceed long term values over the time of deglaciation after the last glacial maximum centennial and millennial timescales by up to 2 orders of magnitude in view of the low exhumation rates of cordillera darwin 0 07 mm yr average for the last 30 myr modern erosion rates could be up to 3 orders of magnitude higher than rates over geological time we conclude that the pattern of erosion rate variation with time reflects the sensitivity of glaciers to climate variability

rheological nonlinearity and flow instability in the deforming bed mechanism of ice stream motion contrary to what has recently been assumed in modeling the proposed deforming bed mechanism for the rapid motion of antarctic ice streams the rheology of water saturated till is probably highly nonlinear according to information from soil mechanics and preliminary experiments on till from the base of ice stream b the equivalent flow law exponent n is probably as high as 100 and the nonlinearities of the shear stress and effective pressure dependences are closely linked the high nonlinearity has important consequences for the deforming bed mechanism a flow system operating by this mechanism can be unstable as a result of feedback from the generation of basal water by shear heating of basal till the short term feedback effect is analyzed for a perturbation in a model ice stream in which the basal meltwater is transported through a distributed system of narrow gap conduits at the ice till interface although the analysis is approximate and some of the system parameters are poorly known the results suggest that the deforming bed mechanism is unstable for n 20 the apparent lack of such an instability in the currently active ice streams implies that their motion is controlled not by the deforming bed mechanism but by some other as yet unidentified mechanism

post little ice age record of coarse and fine clastic sedimentation in an alaskan proglacial lake many assessments of glacial sediment yield rely solely on mea surements of fine grained suspended sediment we show that sus pended sediment contributes 20 of the total clastic sediment discharge into iceberg lake an alaskan proglacial lake drainage of this lake in 1999 exposed outcrops of varved lacustrine sediment that record suspended sediment deposition since a little ice age highstand two centuries ago lake level has dropped in four abrupt spillway controlled events that are recorded stratigraphically as transient increases in the basinwide deposition of unusually fine grained sediments varve counting constrains ages of these events and hence of each abandoned shoreline and associated delta vol umes of deltaic and lake floor sediments constrain specific yields from four multidecadal intervals between a d 1825 and 1999 to tal yield ranges from 3 8 to 4 9 3 10 3 t km 22 yr 21 and most of the sediment 81 86 is sand and gravel now in the deltas a range of evidence suggests that sediment yield faithfully records up stream sediment production in this small 66 km 2 and heavily gla cierized basin implying an effective subglacial erosion rate of 1 6 mm yr these data show that estimates of total yield based upon suspended sediment alone are subject to large errors and suggest that quarrying accounts for a large fraction of the glacial erosion in this system

evidence against pervasively deformed bed material beneath rapidly moving lobes of the southern laurentide ice sheet abstract during the wisconsin glaciation the ice lobes of the southern part of the laurentide ice sheet moved rapidly as the result of elevated subglacial water pressure the rapid movement was probably not accompanied by wide spread pervasive deformation of unfrozen material under the ice the till stratigraphy of much of this area is well known and it is clear that the stratigraphic sequence is generally intact therefore such deformation if it occurred would have been restricted to the till sheet that was currently being deposited however this seems unlikely as well because the till of the region commonly contains lenses and clasts of unlithified bedded sand that should have been destroyed by pervasive shearing if unfrozen deformed till layers were widely present they must have been confined to the thin upper parts of till sheets

a data calibrated distribution of deglacial chronologies for the north american ice complex from glaciological modeling article i nfo past deglacial ice sheet reconstructions have generally relied upon discipline specific constraints with no at tention given to the determination of objective confidence intervals reconstructions based on geophysical inversion of relative sea level rsl data have the advantage of large sets of proxy data but lack ice mechanical constraints conversely reconstructions based on dynamical ice sheet models are glaciologically self consistent but depend on poorly constrained climate forcings and sub glacial processes as an example of a much better constrained methodology that computes explicit error bars we present a dis tribution of high resolution glaciologically self consistent deglacial histories for the north american ice com plex calibrated against a large set of rsl marine limit and geodetic data the history is derived from ensemble based analyses using the 3d mun glacial systems model and a high resolution ice margin chronol ogy derived from geological and geomorphological observations isostatic response is computed with the vm5a viscosity structure bayesian calibration of the model is carried out using markov chain monte carlo methods in combination with artificial neural networks trained to the model results the calibration provides a posterior distribution for model parameters and thereby modeled glacial histories given the observational data sets that takes data uncertainty into account final ensemble results also account for fits between com puted and observed strandlines and marine limits given the model including choice of calibration parameters input and constraint data sets and vm5a earth rhe ology we find the north american contribution to mwp1a was likely between 9 4 and 13 2 m eustatic over a 500 year interval this is more than half of the total 16 to 26 m meltwater pulse over 500 to 700 years with lower values being more probable indicated by the barbados coral record fairbanks 1989 peltier and fairbanks 2006 if one assumes a 5 meter living range for the acropora palmata coral 20 ka ice volume for north america was likely 70 1 2 0 m eustatic or about 60 of the total contribution to eustatic sea level change we suspect that the potentially most critical unquantified uncertainties in our analyses are those related to model structure especially climate forcing deglacial ice margin chronology and earth rheology

numerical modelling of subglacial erosion and sediment transport and its application to the north american ice sheets over the last glacial cycle abstract present day sediment distribution offers a potentially strong constraint on past ice sheet evolution glacial system models gsms however cannot address this constraint while lacking appropriate representations of subglacial sediment production and transport incorporating these elements in gsms is also required in order to quantify the impact of a changing sediment cover on glacial cycle dynamics towards these goals we present a subglacial process model hereafter referred to as the sediment model that incorporates mechanisms for sediment production entrainment transport and deposition bedrock erosion is calculated by both hallet s and boulton s abrasion laws separately and by a novel quarrying law parametrized as a function of subglacial cavity extent these process oriented erosion laws are compared against a simple empirical relationship between erosion rate and the work done by basal stress sediment entrainment is represented by philip s law for regelation intrusion and soft bed deformation is included as a subglacial sediment transport mechanism the model is driven by the data calibrated mun 3d gsm and a newly developed subglacial hydrology module the sediment model is applied to the last north american glacial cycle and predicts sediment thickness and cumulative erosion patterns results are obtained in the context of a sensitivity analysis and are compared against the present day distribution of glacigenic sediment and geological estimates of laurentide ice sheet erosion given plausible ranges for the sensitivity parameters chosen a priori based on available literature or on heuristic arguments the calculated erosion depths overlap with the geological estimates of laurentide erosion most of the runs in the sensitivity set produce unrealistically thick and continuous moraines along the eastern southern and western margins of the north american ice complex which suggests that the model overestimates sediment entrainment and thus englacial transport a realistic sediment distribution is only obtained when the entrainment rate is capped at the average basal melting rate which suggests that modelled entrainment and basal melting rates should be of the same order of magnitude

the influence of basal ice debris on patterns and rates of glacial erosion abstract glaciers have played a key role for shaping much of earth s high topography during the cold periods of the late cenozoic however despite of their distinct influence on landscapes the mechanisms of glacial erosion and the properties that determine their rate of operation are still poorly understood theoretical models of subglacial erosion generally highlight the influence of basal sliding in setting the pace of erosion but they also point to a strong influence of other subglacial properties such as effective bed pressure and basal ice debris concentration the latter properties are however not easily measured in existing glaciers and hence their influence cannot readily be confirmed by observations in order to better connect theoretical models for erosion to measurable properties in glaciers we used computational landscape evolution experiments to study the expected influence of basal ice debris concentration for subglacial abrasion at the scale of glaciers the computational experiments couple the two erosion processes of quarrying and abrasion and furthermore integrate the flow of ice and transport of debris within the ice thus allowing for the study of dynamic feedbacks between subglacial erosion and systematic glacier scale variations in basal ice debris concentration the experiments explored several physics based models for glacial erosion in combination with different models for basal sliding to elucidate the relationship between sliding speed erosion rate and basal ice debris concentration the results demonstrate how differences in debris concentration can explain large variations in measured rates the experiments also provide a simple explanation for the observed dependence of glacier averaged rate of erosion on glacier size that large glacier uplands feed more debris into their lower elevation parts thereby strengthening their erosive power

response of sands to multidirectional dynamic loading in centrifuge tests abstractdynamic centrifuge tests were performed to investigate multidirectional loading effects on the shear and volumetric response of saturated sands under partially drained level ground conditi

basal mechanics of ice stream b west antarctica 1 till mechanics data from laboratory geotechnical tests on till recovered from beneath ice stream b west antarctica at the upstream b camp hereinafter the upb till show that failure strength of this till is strongly dependent on effective stress but is practically independent of strain and strain rate these data support use of a coulomb plastic rheology in modeling of ice stream behavior and subglacial till deformation our testing program combined triaxial ring shear and confined uniaxial tests to investigate till strength and compressibility results show that the upb till follows closely coulomb s equation in which shear strength is a linear function of normal effective stress apparent cohesion near zero and internal friction angle equal to 24 till compressibility is best described by a logarithmic function that relates void ratio to normal effective stress in general the behavior of the upb till is consistent with other experimental evidence regarding mechanical behavior of granular materials based on our laboratory results we formulate the compressible coulomb plastic till model in which there are three interrelated primary state variables shear strength void ratio and normal effective stress this model is used in the second part of our study to simulate response of subglacial till to realistic effective stress forcings these simulations demonstrate that the model is capable of reproducing fundamental aspects of subglacial till kinematics 1 occurrence of tilt rate oscillations and negative tilt rates in tiltmeter records and 2 distributed till deformation to depths of 0 1 1 0 m beneath the ice base our laboratory and modeling results substantiate application of the compressible coulomb plastic model in simulations of the motion of ice stream b over its weak till bed

role of transient water pressure in quarrying a subglacial experiment using acoustic emissions 1 probably the most important mechanism of glacial erosion is quarrying the growth and coalescence of cracks in subglacial bedrock and dislodgement of resultant rock fragments although evidence indicates that erosion rates depend on sliding speed rates of crack growth in bedrock may be enhanced by changing stresses on the bed caused by fluctuating basal water pressure in zones of ice bed separation to study quarrying in real time a granite step 12 cm high with a crack in its stoss surface was installed at the bed of engabreen norway acoustic emission sensors monitored crack growth events in the step as ice slid over it vertical stresses water pressure and cavity height in the lee of the step were also measured water was pumped to the lee of the step several times over 8 days pumping initially caused opening of a leeward cavity which then closed after pumping was stopped and water pressure decreased during cavity closure acoustic emissions emanating mostly from the vicinity of the base of the crack in the step increased dramatically with repeated pump tests this crack grew with time until the step s lee surface was quarried our experiments indicate that fluctuating water pressure caused stress thresholds required for crack growth to be exceeded natural basal water pressure fluctuations should also concentrate stresses on rock steps increasing rates of crack growth stress changes on the bed due to water pressure fluctuations will increase in magnitude and duration with cavity size which may help explain the effect of sliding speed on erosion rates

subglacial erosion and englacial sediment transport modelled for north american ice sheets abstract reconstructions of north american glacial history have either been based on interpretation of the geologic record or on numerical models of continental ice dynamics because such models make no explicit reference to the geological products of glaciation the full potential of a combined approach has not been realized we present the results of a process based model of sediment production entrainment transport and deposition that represents a first step toward uniting these two lines of investigation in this model basal ice processes are dependent on ice sheet geometry hydrology and dynamics a three dimensional thermomechanical ice sheet model yields the ice geometry flow and thermal conditions that are used by a model of subglacial hydrology together these drive the basal process model comparisons of simulation results with mapped debris distributions of the hudson bay paleozoic carbonates and of the distinctive dubawnt group are used to test the validity of the transport model dispersion of debris radially out of hudson bay is greatly overestimated by the model and dispersion toward the bay equally underestimated this is because the modelled ice sheet has a single dome over hudson bay at glacial maxima including last glacial maximum rather than a saddle separating ice domes over quebec labrador and keewatin estimated rates of glacial erosion for the cordilleran and laurentide ice sheets are compared against those predicted by the model the model underestimates these rates because it underestimates the vigour of the quarrying process

mass balanced paleogeographic reconstructions paleogeography paleotopography and paleobathymetry of a closed erosion depositional system can be reconstructed by restoring sedimentary masses to elevated surfaces in a drainage basin based on the inverse of present erosion equations and adjusting for isostasy sea level changes sediment compaction and thermal subsidence the erosion deposition history of the northwestern gulf of mexico margin and its western central north american source area during the cenozoic is used to explore the sensitivity of mass balance reconstructions and changes in assumptions concerning erosion rate parameters and sea level

lithological and structural controls on the surface wear characteristics of glaciated metamorphic bedrock surfaces ossian sarsfjellet svalbard although there is an established relationship between geological structure and the morphology of certain glacial erosional landforms the role of lithology is less clear this is particularly true of the surface wear characteristics of glaciated bedrock in order to examine this relationship the surface wear characteristics of eight recently deglaciated metamorphic bedrock slabs in the kongsfjorden area of svalbard were mapped and recorded using detailed micro maps features recorded included lee side fracture surfaces lee side cavities and the location and depth of open joints and quartz veins on schist glacial erosion is favored by situations where ice movement is parallel to the trend of the bedrock foliation in these situations cavities may be elongated in the direction of ice flow on more homogeneous lithologies such as marble cavity formation is suppressed and more uniform glacially abraded rock surfaces develop on all the metamorphic rocks examined glacial abrasion is favored in situat

origin of the middle pleistocene transition by ice sheet erosion of regolith the transition in the middle pleistocene 0 9 ma seen in 18o deep sea core records from relatively low amplitude high frequency 41 kyr to high amplitude low frequency 100 kyr ice volume variations under essentially the same orbital forcing can be attributed to a change from an all soft bedded to a mixed hard soft bedded laurentide ice sheet through glacial erosion of a thick regolith and resulting exposure of unweathered crystalline bedrock a one dimensional ice sheet and bedrock model which includes transport of sediment and ice by subglacial sediment deformation demonstrates that a widespread deforming sediment layer maintains thin ice sheets before the transition which respond linearly to the dominant 23 and 41 kyr orbital forcing progressive removal of the sediment layer eventually causes a transition to thicker ice sheets whose dominant timescale of change 100 kyr reflects nonlinear deglaciation processes in model simulations over the last 3 ma initialized with no ice and a uniform 50 m sediment layer the time series of ice volume and extent agree in several important aspects with the observed records

laboratory simulations of glacial abrasion comparison with theory glacial abrasion was simulated in experiments in which a small artificial glacier bed was pushed beneath a fixed ice block under pressure the experiments provide a means of testing theoretical models of abrasion particularly those factors that govern the magnitude of stress concentrations beneath abrading rock fragments in preliminary experiments vertical ice flow around a sphere mounted on the bed was studied in subsequent experiments marble tablets were pushed beneath granitic rock fragments frozen into the base of the ice block unlike previous abrasion experiments the sliding velocity was realistic 25 mm d 1 and ice near the bed was at the pressure melting temperature resultant striations closely resemble those observed on glaciated bedrock as predicted by hallet 1979 the component of the ice velocity towards the bed strongly influenced stresses beneath fragments and classical regelation and creep theory provided an approximate estimate of the downward drag force on fragments half of the rock fragments rotated significantly accounting for 10 50 of their motion relative to the bed and influencing abrasion rates and the shear stress supported along the ice bed interface striation patterns indirectly suggest that fragment rotations were inhibited by increases in ice pressure which presumably increased the drag on roughness elements on fragment surfaces this may have resulted from a reduction in the thickness of the water film around fragments facilitated by leakage of water from the bed

assessing the paradigm shift deformable glacier beds abstract during the mid 1980s glaciologists recognised that many glaciers and ice masses overlie soft sediments rather than hard bedrock these sediments are typically weaker than glacier ice and may therefore deform this realisation was heralded as a paradigm shift in glaciology since this realisation a wide body of theoretical and field based research has been undertaken to assess the mechanical and hydrological significance of such substrates we review the techniques and results of this work and attempt to assess this paradigm shift in glaciology and in particular its importance for quaternary scientists we suggest areas of research that remain unanswered and identify those in which glacial geology in particular may be able to inform glaciologists

ice cap erosion patterns from bedrock 10be and 26al southeastern tibetan plateau quantifying glacial erosion contributes to our understanding of landscape evolution and topographic relief production in high altitude and high latitude areas combining in situ be 10 and al 26 ana

numerical reconstruction of a soft bedded laurentide ice sheet during the last glacial maximum we used a numerical ice sheet model to reconstruct the north american laurentide ice sheet during the last glacial maximum our model simulates ice sheet conditions that can be specified experimentally as either a rigid substrate hard bed or a wet deformable till soft bed basal sliding is excluded we use geologic records of former basal ice sheet processes to prescribe the distribution of hard and soft beds our reconstruction of the laurentide ice sheet is significantly lower in ice surface height and contains less ice volume than the climap maximum reconstruction in contrast our reconstruction agrees well with the ice 4g reconstruction both in height and volume because the ice 4g reconstruction is based on the inversion of relative sea level data whereas our reconstruction is based on glacial geology and ice mechanics this agreement suggests that soft beds provide a glaciological mechanism to explain the shape and volume of the laurentide ice sheet that is most consistent with observations of relative sea level change and other geodynamic considerations

potential effects of subglacial water pressure fluctuations on quarrying water pressure fluctuations beneath glaciers may accelerate rock fracture by redistributing stresses on subglacial bedrock and changing the pressure of water in bedrock cracks to study the potential influence of water pressure fluctuations on the fracture of subglacial bedrock ice flow over a small bedrock step with a water filled cavity in its lee is numerically modeled and stresses on the bedrock surface are calculated as a function of transient water pressures in the cavity stresses on the bed are then used to calculate principal stress differences within the step rapid reductions in cavity water pressure increase principal stress differences in the bed increasing the likelihood of crack growth in the step and the formation of predominantly vertical fractures relatively impermeable bedrock may be most susceptible to fracturing during water pressure reductions because high water pressure in cracks within the rock can be maintained as water pressure decreases in cavities these results when considered in conjunction with the strong likelihood that increases in water pressure accelerate the removal of rock fragments loosened from the bed suggest that in zones of ice bed separation where water pressure fluctuations typically are large rates of quarrying may be higher than along other parts of glacier beds

late glacial ice margins and deglacial chronology for southeastern baffin island and hudson strait eastern canadian arctic radiocarbon dates from marine piston cores and from onshore raised marine stratigraphic sections in the hudson strait region were used to reconstruct deglacial isochrons for 9900 9500 8800 8500 and 8000 bp at the culmination of the gold cove readvance 9900 bp labrador ungava ice flowed northeastward across hudson strait and outer frobisher bay and stood for the last time on the baffin island continental shelf subsequent retreat by calving was rapid and profound opening the entire hudson strait marine trough by 9500 bp at this time ice dispersal from foxe basin labrador ungava and local ice on meta incognita peninsula supported tidewater margins along much of the coastline with the exception of northernmost ungava peninsula where the ice margin stabilized onshore this onshore margin remained in place throughout the cockburn substage while a major northeastward readvance of ungava bay ice the noble inlet readvance from 8800 to 8500 bp crossed outer hudson strait grounding on the hudson st

the concept of transport capacity in geomorphology the notion of sediment transport capacity has been engrained in geomorphological and related literature for over 50 years although its earliest roots date back explicitly to gilbert in fluvial geomorphology in the 1870s and implicitly to eighteenth to nineteenth century developments in engineering despite cross fertilization between different process domains there seem to have been independent inventions of the idea in aeolian geomorphology by bagnold in the 1930s and in hillslope studies by ellison in the 1940s here we review the invention and development of the idea of transport capacity in the fluvial aeolian coastal hillslope debris flow and glacial process domains as these various developments have occurred different definitions have been used which makes it both a difficult concept to test and one that may lead to poor communications between those working in different domains of geomorphology we argue that the original relation between the power of a flow and its ability to transport sediment can be challenged for three reasons first as sediment becomes entrained in a flow the nature of the flow changes and so it is unreasonable to link the capacity of the water or wind only to the ability of the fluid to move sediment secondly environmental sediment transport is complicated and the range of processes involved in most movements means that simple relationships are unlikely to hold not least because the movement of sediment often changes the substrate which in turn affects the flow conditions thirdly the inherently stochastic nature of sediment transport means that any capacity relationships do not scale either in time or in space consequently new theories of sediment transport are needed to improve understanding and prediction and to guide measurement and management of all geomorphic systems

theory of glacial erosion transport and deposition as a consequence of subglacial sediment deformation a theory of erosion transport and deposition of unlithified sediments by glaciers is presented it predicts the large scale areal distribution of zones and rates of erosion and deposition in time and space through a complete glacial cycle together with the resultant intensity of large scale lincations drumlins which will be incised in the landscape the theory also predicts the dispersal patterns of subglacial lithologies together with the form of dispersal trains derived from distinctive sources and the vertical and horizontal distribution of lithologies within a till it predicts major erosional discontinuities within tills and the formation of boulder pavements it suggests that the dominant proportion of the lowland tills produced by pleistocene mid latitude ice sheets was generated by subglacial deformation and explains why they are predominantly fine grained the theory is based on an analysis of glacier dynamic processes and therefore can be used to infer the dynamic behaviour of former ice sheets from the distribution of tills and their lithologic composition

dynamics of the north american ice sheet complex during its inception and build up to the last glacial maximum the north american ice sheet complex played a major role in global sea level fluctuations during the late quaternary but our knowledge of its dynamics is based mostly on its demise from the last glacial maximum lgm a period characterised by non linear behaviour in the form of punctuated ice margin recession episodic ice streaming and major shifts in the location of ice divides in comparison knowledge of the pre lgm ice complex is poorly constrained largely because of the fragmentary nature of the evidence relating to ice sheet build up in this paper we explore the inception and growth of ice 120 20 ka using a glacial systems model which has been calibrated against a large and diverse set of data relating to the deglacial interval we make use of calibration data prior to the lgm but its scarcity introduces greater uncertainty which is partly alleviated by our large ensemble analysis results suggest that following the last interglaciation oxygen isotope stage ois 5e the ice complex initiated over the north eastern canadian arctic and in the cordillera within a few thousand years it then underwent rapid growth to an ois 5 maximum at 110 ka 5d and covered 70 of the area occupied by the lgm ice cover although only 30 by volume an ois 5 minimum is modelled at 80 ka 5a before a second phase of rapid growth at the start of ois 4 which culminated in a large ice complex at 65 ka almost as large as at the lgm subsequent deglaciation was rapid maximum modelled sea level contribution of 16 cm per century and resulted in an ois 3 minimum between ca 55 60 ka thereafter the ice complex grew towards its lgm configuration interrupted by several phases of successively less significant mass loss our results support and extend previous inferences based on geological evidence and reinforce the notion of a highly dynamic pre lgm ice complex e g with episodes of 10 s m of eustatic sea level equivalent in 50 but even the thin ois 5 ice sheets exhibit fast flow features several 1000 m a 1 in major topographic troughs notwithstanding the severe limitations imposed by the use of the shallow ice approximation we note that most fast flow features generated prior to the lgm correspond to the location of known ice streams during deglaciation i e in major topographic troughs and over soft sediments at the southern and western margins moreover the modelled flux of these ice streams sensu lato appears to be non linearly scaled to ice sheet volume i e there is no evidence that decay phases were associated with significantly increased ice stream activity this hypothesis requires testing using a model with higher order physics and future modelling would also benefit from additional pre lgm constraints e g dated ice free margin positions to help reduce and quantify uncertainties

reply to the comments by g s boulton k e dobbie s zatsepin on deforming soft beds under ice sheets how extensive were they

ice streaming in the laurentide ice sheet a first comparison between data calibrated numerical model output and geological evidence 1 despite the importance of rapidly flowing ice streams to ice sheet mass balance their incorporation into numerical ice sheet models is a major scientific challenge this introduces large uncertainties in model output and inhibits a more complete understanding of the role of ice streams in overall ice sheet stability recent computational advances have enabled more realistic representations of ice streaming but few studies have attempted to compare model output against known locations of ice streams this paper compares predictions of ice streaming derived from a large ensemble analysis of a glacial systems model of the laurentide ice sheet against independent geological evidence compiled from previously published studies although the precise dating of paleo ice stream locations is problematic our analysis includes comparisons at six different time steps 18 to 10 cal ka bp during deglaciation results indicate that the model is successful in predicting all of the major marine terminating ice streams but there is mixed success in simulating terrestrial ice streams in the right place and at the right time which is vital in guiding future model development the model also reveals that whilst some ice streams persist throughout deglaciation the focus of mass loss associated with ice streaming switches through time with dynamic changes in ice stream catchments and tributaries this implies that major changes in ice stream activity are to be expected in a deglaciating ice sheet with important implications for contemporary ice sheet dynamics

the basal ice layer of glaciers and ice sheets abstract in many glaciers and ice sheets there is a basal ice layer bil in which the ice is conditioned primarily by processes operating at the bed the bil is chemically and physically distinctive and is characterised by a component of basally derived sediment the bil is a rheological control on ice sheet dynamics an indicator of subglacial conditions and processes an agent of subglacial geologic processes the source of a substantial proportion of glacial sediments a limit to the downward extension of the climate record from deep ice cores if debris characteristics of the bil are preserved in glacial sediments former glacier conditions can be inferred analysis of stable isotopes gas and cation composition ice crystallography debris composition and structural glaciology has elucidated many mechanisms for bil formation common elements can be identified from a range of locations key issues are entrainment of ice and debris from the bed formation of new ice metamorphism of existing ice in the bil and thickening of the bil by deformation a distinction can be made between basal ice formed by accretion of material from the bed stratified facies and basal ice formed by metamorphic processes within the ice close to the bed dispersed facies

coupling the flow of ice water and sediment in a glacial landscape evolution model abstract the processes of subglacial erosion depend not only on the sliding motion of glaciers over bedrock but also on the presence of meltwater and sediment in the subglacial environment in particular theoretical models for subglacial quarrying and abrasion as well as hypothesized erosion thresholds associated with subglacial sediment transport include both positive and negative effects of subglacial water and sediment on the rate of erosion in order to incorporate the existing theoretical models for subglacial erosion by quarrying and abrasion in a long term glacial landscape evolution model we here present a coupled computational framework for simulating the simultaneous flow processes of ice water and sediment we supplement a higher order ice sheet model with simple long term models for glacial hydrology and subglacial sediment transport as well as fluvial and hillslope related erosion processes an important strength of the modeling framework presented relates to the morphological detail of the landscapes produced which facilitates direct comparison with existing landforms this improves the connection between glacial landscape evolution models geomorphological observations and the existing knowledge of the physical processes that operate under glaciers we demonstrate with model examples how increased basal meltwater pressure and transport limited subglacial erosion lead to both positive and negative feedbacks related to glacial erosion and the formation of overdeepenings

unstable behavior of the laurentide ice sheet over deforming sediment and its implications for climate change geologic records of fluctuations of the laurentide ice sheet margin following the most recent glacial maximum ca 20 000 14c yr b p identify fundamental differences in ice sheet behavior depending on subglacial bed conditions rapid and irregular icemargin fluctuations occurred only over areas of deforming sediment indicating nonclimatic forcing controlled by the inherent instability of coupled ice sheet deforming sediment dynamics in contrast largely uninterrupted ice margin retreat with no evidence of significant readvance occurred over rigid bed areas indicating stable behavior unstable ice sheet behavior was most pronounced from 15 000 until 10 000 14c yr b p by which time most of the ice margin had retreated onto a rigid bed unstable ice sheet behavior would have been an integral component in controlling variable fluxes of icebergs and meltwater as well as meltwater routing to the north atlantic thus affecting thermohaline circulation the abrupt climate oscillations in the north atlantic region that ended at 10 000 14c yr b p may thus have their origin in the inherently unstable behavior of the laurentide ice sheet overriding deforming sediment

control of glacial quarrying by bedrock joints abstract a principal assumption of models that describe bedrock quarrying by glaciers is that cracks in the bed are small and isolated these cracks are assumed to grow slowly in response to glacial loading to eventually delimit the areas of quarried surfaces if this assumption is correct then quarried surface orientations will be controlled by orientations of principal stresses in the bed associated with sliding and resultant ice bed separation downglacier from rock ledges and roches moutonnees rather than by orientations of joints that predate glacial loading to test this hypothesis we compared orientations of quarried surfaces and preglacial joints in nine recently deglaciated forefields of glaciers in canada and switzerland with different bedrock lithologies and bedforms in all but one forefield quarried surfaces and major joint sets are coincident with orientations of quarried surfaces bearing no systematic relationship to sliding direction other than their obvious tendency to not face upglacier a difference in sliding direction of 64 between two areas of one glacier forefield had no effect on the orientation of quarried surfaces with the same joint set exploited in both areas these observations motivate a new characterization of subglacial bedrock that differs from that of existing quarrying models the bed is idealized as a group of rock blocks separated by preglacial joints slow crack growth necessary for quarrying would be confined to small bodies of rock bridges that connect the faces of joints this model of the bed does not allow it to be treated as an intact elastic solid with hydraulically isolated cracks as is usually assumed which would have a profound influence on characterization of quarrying mechanics

the role of sub milankovitch climatic forcing in the initiation of the northern hemisphere glaciation mechanisms responsible for the initiation of major glaciation in the northern hemisphere at about 2 75 million years ago are poorly understood a laminated terrestrial sequence from pula maar hungary containing about 320 000 years in annual layers between 3 05 and 2 60 million years ago provides a detailed record of rates of climatic change across this dramatic transition an analysis of the record implies that climatic variations at sub milankovitch frequencies less than or equal to 15 000 years were an important driving force during this transitional interval and that as the threshold was approached these increased in frequency and amplitude possibly providing the final trigger for the amplification of northern hemisphere ice sheets

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

distribution and characteristics of overdeepenings beneath the greenland and antarctic ice sheets implications for overdeepening origin and evolution das thanks the national cooperative for the disposal of radioactive waste nagra switzerland for their role in stimulating new research on the topic of focused glacial erosion and das and hp further thank nagra for funding the quantitative study of overdeepening morphology on which this paper is based grant 11 716 fcu bys hp also acknowledges support by the research council of norway through its centres of excellence funding scheme grant 223259 and the petromaks project glaciations in the barents sea area glacibar grant 200672 j

origin and significance of dispersed facies basal ice svinafellsjokull iceland dispersed facies basal ice massive i e structureless ice with dispersed debris aggregates is present at the margins of many glaciers and as a product of internal glacial processes has the potential to provide important information about the mechanisms of glacier flow and the nature of the subglacial environment the origin of dispersed facies is poorly understood with several hypotheses having been advanced for its formation and there is disagreement as to whether it is largely a sedimentary or a tectonic feature we test these established hypotheses at the temperate glacier svinafellsjokull iceland and find that none fully account for dispersed facies characteristics at this location instead dispersed facies physical sedimentological and stable isotope d 18 o dd character istics favour a predominantly tectonic origin that we suggest comprises the regelation and strain induced metamorphism of debris rich basal ice that has been entrained into an englacial position by tectonic processes operating at the base of an icefall further thickening of the resultant dispersed facies may also occur tectonically as a result of ice flow against the reverse bed slope of a terminal overdeepening lack of efficient subglacial drainage in the region of the overdeepening may limit basal melting and thus favour basal ice preservation including the preservation of dispersed facies despite the relatively low sediment content of dispersed facies 1 6 by volume its thickness up to 25 m and ubiquity at svinafellsjokull results in a significant contribution to annual sediment discharge 1635 3270 m 3 a 1 that is 6 5 times that contributed by debris rich stratified facies basal ice

continuous simulations over the last 40 million years with a coupled antarctic ice sheet sediment model abstract much of the knowledge of antarctic ice sheet variations since its inception 34 ma derives from marine sediments on the continental shelf deposited in glacimarine or sub ice environments by advancing and retreating grounded ice and observed today by seismic profiling and coring here we apply a 3 d coupled ice sheet and sediment model from 40 ma to the present with the goal of directly linking ice sheet variations with the sediment record the ice sheet model uses vertically averaged ice dynamics and parameterized grounding line flux the sediment model includes quarrying of bedrock sub ice transport and marine deposition atmospheric and oceanic forcing are determined by uniform shifts to modern climatology in proportion to records of atmospheric co2 deep sea core 18o and orbital insolation variations the model is run continuously over the last 40 myr at coarse resolution 80 or 160 km modeling post eocene ice landscape evolution and off shore sediment packages in a single self consistent simulation strata and unconformities are tracked by recording times of deposition within the model sediment stacks which can be compared directly with observed seismic profiles the initial bedrock topography is initialized to 34 ma geologic reconstructions or an iterative procedure is used that yields independent estimates of paleo bedrock topography preliminary results are compared with recognized cenozoic ice sheet variations modern sediment distributions and seismic profiles and modern and paleo bedrock topographies

sediment deformation beneath glaciers and its coupling to the subglacial hydraulic system abstract the extent and style of shear deformation in sediments beneath modern glaciers and the geological evidence for such deformation in deposited sediments are reviewed new evidence is presented from beneath a modern glacier of the spatial and temporal patterns of water pressure fluctuation and of time dependent patterns of deformation in sediments it is concluded that in most experimental sites beneath soft bedded modern glaciers deformation is a significant or major contributor to glacier movement and the resultant discharge of till is large enough to make sediment deformation a major till forming process particular modes of deformation facilitate incorporation of underlying material into the till whilst the capacity of a deforming till to absorb strain can protect the underlying strata from deformation leading to the commonly found relationship where till overlies other strata with a sharp planar interface it is argued that the almost ubiquitous occurrence of drumlins on the beds of former ice sheets is a reflection of the widespread occurrence of sediment deformation beneath them with important implications for the coupling of ice sheet flow and bed properties it is argued that the mechanical behaviour of the subglacial system is not simply determined by till properties but largely controlled by the subglacial water pressure regime determined by the nature of subglacial drainage results of field experiments show how the nature of the basal hydraulic system can play a vital role in controlling the coupling between the glacier and till deformation processes they show that rapid glacier advances can produce undrained loading of sediments that effective pressure may increase either upwards or downwards in a till according to the direction of drainage and that interstitial water pressures in subglacial sediments can show large and rapid variations producing strong variations in the rate and distribution of strain and in the partitioning of basal movement between sliding and deformation

subglacial till formation sedimentary characteristics and classification we review the major subglacial till forming processes as presently understood by glacial researchers and define the parameters within which tills are produced and reconcile them with sedimentary end members processes of deformation flow sliding lodgement and ploughing coexist at the base of temperate glacier ice and act to mobilize and transport sediment and deposit it as various end members ranging from glacitectonically folded and faulted stratified material to texturally homogeneous diamicton the dominance of any one subglacial process varies both spatially and temporally giving rise to the possibility that a till or complex till sequence contains a superimposed signature of former transportation deposition at the ice bed interface we recommend that while glacial geologists and geomorphologists should be able to recognize the sedimentary imprints of various subglacial processes genetic fingerprinting of subglacial tills should be less process specific and till classification must reflect the range of products encompassed by the subglacial till production continuum glacial geologists can presently unequivocally identify a glacitectonite rock or sediment that has been deformed by subglacial shearing deformation but retains some of the structural characteristics of the parent material b subglacial traction till sediment deposited by a glacier sole either sliding over and or deforming its bed the sediment having been released directly from the ice by pressure melting and or liberated from the substrate and then disaggregated and completely or largely homogenised by shearing and support the theoretical case for c melt out till sediment released by the melting of stagnant or slowly moving debris rich glacier ice and directly deposited without subsequent transport or deformation because observations on contemporary glaciers reveal that their beds are most likely to be mosaics of deformation and sliding and warm based and cold based conditions the patterns of which change temporally and spatially it is extremely unlikely that subglacial till end members in the geological record will be anything but hybrids produced by the range of processes operative in the subglacial traction zone

modelling landscape evolution under ice sheets we use an ice sheet model glimmer with an erosion component to examine the evolution of landscapes under ice sheets over long time scales in hypothetical situations glimmer is a fully coupled thermomechanical 3 dimensional ice sheet model with a sliding component the model thus has the ability to predict ice thickness the distribution of areas at pressure melting point and basal velocities the erosion rate is assumed to be a simple linear function of the basal velocity and therefore erosion only occurs where the bed is not frozen and basal sliding is possible in addition sliding velocities are assumed to be related to basal water production thus giving a mechanism for smooth temporal and spatial transitions between eroding and non eroding portions of the bed the eroding glacial landscape system is modelled for long term persistent glaciation assuming a variety of initial landscapes results indicate that without the inclusion of basal water production in the calculation of sliding velocities any thermal feedbacks are exaggerated because of steep horizontal basal temperature gradients additionally bed morphology exerts a greater influence under low basal slip conditions because ice cannot respond readily to thermal instabilities we suggest that erosion and valley overdeepening may aid stabilisation of the thermal regime of an ice sheet and that it is possible to generate glacial signals in previously fluvial systems after only 100 kyr of glaciation at conservative erosion rates

seismic compression of two compacted earth fills shaken by the 1994 northridge earthquake seismic compression is defined as the accrual of contractive volumetric strain in unsaturated soil during strong shaking by earthquakes we document and analyze two case histories denoted school site and site a of ground deformation from seismic compression in canyon fills strongly shaken by the northridge earthquake site a had ground settlements up to about 18 cm which damaged a structure while the school site had settlements up to about 6 cm for each site we perform decoupled analyses of shear and volumetric strain shear strain is calculated using one dimensional and two dimensional ground response analyses while volumetric strain is evaluated from shear strain using material specific models derived from simple shear laboratory testing that incorporates important effects of fines content and as compacted density and saturation analyses are repeated using a logic tree approach in which weights are assigned to multiple possible realizations of uncertain model parameters at the school site predicted settlements appear to be unbiased at site a the analyses successfully predict the shape of the settlement profile along a section but the weighted average predictions are biased slightly too low we speculate that the apparent site a bias can be explained by limited resolution of the site stratigraphy bias in laboratory derived volumetric strain models and or uncertainty in the estimated earthquake induced settlements

antarctic topography at the eocene oligocene boundary we present a reconstruction of the antarctic topography at the eocene oligocene ca 34 ma climate transition this provides a realistic key boundary condition for modeling the first big antarctic ice sheets at this time instead of using the present day bedrock topography which has changed significantly from millions of years of tectonism and erosion we reconstruct topography using a set of tools including ice sheet erosion models models of thermal subsidence and plate movement erosion estimates are constrained with offshore sediment volumes estimated from seismic stratigraphy maximum and minimum topographic reconstructions are presented as indicators of the range of uncertainty our results point to a significant upland area in the ross sea marie byrd land and weddell sea sectors in addition east antarctic coastal troughs are much shallower than today due to the restoration of material that has been selectively eroded by the evolving ice sheets parts of east antarctica have not changed since the e o boundary because they were protected under non erosive cold based ice the reconstructions provide a better defined boundary condition for modeling that seeks to understand interaction between the antarctic ice sheet and climate along with more robust estimates of past ice volumes under a range of orbital settings and greenhouse gas concentrations

bedrock fracture control of glacial erosion processes and rates the importance of rock type and tectonic history on rates of glacial erosion and the relative roles of glacial quarrying and abrasion are poorly understood we use concentrations of cosmogenic 10be in glacial polish and measurements of bedrock fracture spacing to explore the relationship between erosion rates and rock fracturing at 23 sites along tuolumne river valley and five sites in tenaya canyon in yosemite national park california usa most sites yield 10be concentrations that can be best explained as reflecting solely postglacial nuclide accumulation six sites however display anomalously high concentrations implying incomplete removal of the pre glacial nuclide inventory during the last glaciation these require that erosion in the last glacial cycle was 2 3 m these low erosion sites occur preferentially in massive units of the cathedral peak granodiorite and the el capitan granite fractures in high erosion sites are more closely spaced than in low erosion sites with spacings that average 1 1 0 03 m and 3 3 0 1 m respectively our data suggest that the distance between fractures in the rock dictated in part by the original spacing in a particular pluton and in part by the specific tectonic history of the sierra nevada governs the pace of glacial erosion in yosemite

the meltwater hypothesis for subglacial bedforms this paper presents an historical and in places informal account of the meltwater hypothesis which invokes enormous outburst floods for the formation of subglacial bedforms it begins with a brief discussion of the difficulties of determining processes of formation for landforms which are not seen in formation analogy provides a solution to these difficulties analogy between erosional marks at the bases of turbidites and drumlins which were the starting point for this hypothesis rests on the idea that inverted erosional marks at the ice bed are subsequently infilled to form drumlins field tests on the sedimentology architecture and landform associations of drumlins in the livingstone lake drumlin field are outlined before more extensive work on bedrock erosional forms and flood routes is introduced bedrock erosional forms played a central part in establishing the hypothesis since their form and ornamentation are confidently interpreted as fluvial their form and genesis are discussed mainly with reference to sites at french river and wilton creek ontario though some remarkable bedrock erosional forms in antarctica support their regional extent initially in the meltwater hypothesis drumlins were thought to be cavity fills and erosional drumlins were recognized later this development is shown to be central to the realization that drumlin composition may be inferred from drumlin form the scale of drumlin fields measured at about 10 3 km 2 and the magnitude of the inferred floods require that the flood events were regional regional scale flood tracts in ontario quebec alberta and the northwest territories extending over 1000 km in length and several hundred kilometers in width support this suggestion floods had they occurred would have caused rapid rates of sea level rise and may have changed climate through their effects on ocean stratification and sea surface temperatures the meltwater hypothesis covers a range of bedforms besides drumlins and bedrock erosional marksffluting rogen moraine hummocky terrain and transverse ridges recent work shows how these forms are best explained by the meltwater hypothesis the roles of water storage and release which underpin the theory of the meltwater hypothesis remain poorly understood r 2002 published by elsevier science ltd

deformation till in the great lakes region implications for rapid flow along the south central margin of the laurentide ice sheet structural and lithologic data indicate that while deposited under actively moving ice considerable portions of three muddy calcareous subglacial tills in the great lakes region probably experienced some component of ductile deformation viscous till flow and ductile shear are invoked to explain a combination of features such as recumbent isoclinal folds unlithified sediment clasts mixed ostracode shells reversed stone lee ends girdle and transverse fabrics irregular stone pavements fine striae following stone curves and inconsistent stone striae and a axes deforming fine textured subglacial till is considered as a subhorizontal shear zone rheologically layered with associated structures in descending order ductile e g isoclinal folds brittle ductile e g fissility and brittle e g till wedges rheology would be controlled mainly by till pore water content matrix texture and stone content spatial and temporal superposition of rheologies and subglacial processes probably occur

volumetric strains of clean sands subject to cyclic loads we utilize simple shear testing to investigate the volume change of clean sands subject to cyclic loads we examine the effects of a number of compositional and environmental factors on the vertical strain at 15 uniform shear strain cycles and on the cycle to cycle variation of vertical strain the compositional factor found to principally affect seismic compression susceptibility is relative density compositional factors found to not significantly affect cyclic volume change include gradation parameters mean grain size and uniformity coefficient particle angularity soil fabric mineralogy and void ratio breadth e emin an environmental factor found to affect seismic compression susceptibility is confining stress with volumetric strains decreasing with increasing stress environmental factors that do not significantly affect seismic compression susceptibility for clean sands are saturation and age stress history can decrease vertical strains from seismic compression for certain conditions but