glaciohydraulic supercooling in iceland we present evidence of glaciohydraulic supercooling under jokulhlaup and ablation dominated conditions from two temperate icelandic glaciers observations show that freezing of sediment laden meltwater leads to intraglacial debris entrainment during normal and extreme hydrologic regimes intraglacial frazil ice propagation under normal ablation dominated conditions can trap copious volumes of sediment which forms anomalously thick sections of debris rich ice glaciohydraulic supercooling plays an important role in intraglacial debris entrainment and should be given more attention in models of basal ice development extreme jokulhlaup conditions can result in significant intraglacial sediment accretion by supercooling which may explain the concentration of englacial sediments deposited in heinrich layers in the north atlantic during the last glaciation

weertman regelation multiple refreezing events and the isotopic evolution of the basal ice layer a simple model is developed to s imulate the isotopic fractionation which accompanies weertman regelation at the bed of temperate based glaciers the fractionation equations of jouzel and souchez 1 982 are applied to multiple refreezing events over measured glacier bed profiles and mass balance is maintained as the basal ice and meltwater produced at one bedrock hummock enter the next s imulation results indicate that undeformed regelation ice layers are on the order of millimetres to centimetres thick often being completely melted at the stoss face of certain hummocks and exceptionally reaching a thickness in excess of 1 0 cm neither the internal morphology nor the isotopic composition of these layers is constant but both vary down glacier in accordance with bedrock configuration a glacier wide fractionation process is identified whereby heavy isotopes are preferentially removed fro m the basal meltwater film and incorporated into the basal ice this process might go some way to explaining the anomalously light isotopic composition measured in base flow waters leaving some glaciers vertical isotope profiles through undeformed basal ice layers are reconstructed and show that significant isotopic excursions can occur at a scale of millimeteres while the range of isotopic compositions within such m ulti layered regelation ice is greater than that which would occur in ice produced by a single refreezing event in circumstances where the regelation system is disrupted by removal of film waters into a network of linked cavities i t is found that the remaining basal ice may be significantly enriched in heavy isotopes relative to the composition of the initial mass inputs to the system heavy isotope enrichment of this m agnitude and consideration of the thickness of the basal ice layers concerned may explain the absence of recorded basal ice samples heavy enough to h ave been formed in equilibrium with subglacial precipitates sampled at one of the sites glacier de tsanfleuron and reported in an earlier paper

glaciohydraulic supercooling a freeze on mechanism to create stratified debris rich basal ice ii theory simple theory supports field observations lawson and others 1998 that subglacial water flow out of overdeepenings can cause accretion of layered debris bearing ice to the bases of glaciers the large meltwater flux into a temperate glacier at the onset of summer melting can cause rapid water flow through expanded basal cavities or other flow paths if that flow ascends a sufficiently steep slope out of an overdeepening the water will supercool as the pressure melting point rises and basal ice accretion will occur diurnal occasional or annual fluctuations in water discharge will cause variations in accretion rate debris content of accreted ice or subsequent diagenesis producing layers under appropriate conditions net accretion of debris bearing basal ice will allow debris fluxes that are significant in the glacier sediment budget

hydro acoustic and tsunami waves generated by the 2012 haida gwaii earthquake modeling and in situ measurements detection of low frequency hydro acoustic waves as precursor components of destructive tsunamis can enhance the promptness and the accuracy of tsunami early warning systems tews we reconstruct the hydro acoustic wave field generated by the 2012 haida gwaii tsunamigenic earthquake using a 2 d horizontal numerical model based on the integration over the depth of the compressible fluid wave equation and considering a mild sloped rigid seabed spectral analysis of the wave field obtained at different water depths and distances from the source revealed the frequency range of low frequency elastic oscillations of sea water the resulting 2 d numerical model gave us the opportunity to study the hydro acoustic wave propagation in a large scale domain with available computers and to support the idea of deep sea observatory and data interpretation the model provides satisfactory results compared with in situ measurements in the reproduction of the long gravitational waves differences between numerical results and field data are probably due to the lack of exact knowledge of sea bottom motion and to the rigid seabed approximation indicating the need for further study of poro elastic bottom effects

conditions for thrust faulting in a glacier 1 dipping arcuate bands of debris rich ice outcropping near the margins of glaciers are often interpreted as thrust faults assumed to originate in zones of longitudinal compression identification of thrusts is typically based either on the geometry and sedimentology of the debris bands or on the crystal fabric of surrounding ice but the physical processes necessary to generate thrusts are rarely evaluated herein we combine a numerical model of compressive ice flow near a glacier margin with theoretical stress and strain rate criteria for ice fracture and stress criteria for frictional slip to determine the conditions necessary for thrust faulting in glaciers this model is applied to two different glaciological settings where longitudinal compression has been documented 1 the transition between warm based and cold based ice near the terminus of storglaciaren sweden and 2 the downglacier extent of the 1983 surge front of variegated glacier where surging ice encountered stagnant ice simulations representing the margin of storglaciaren indicate that peak compressive strain rates are six orders of magnitude too small to induce fracture whereas at variegated glacier strain rates were an order of magnitude too small for compressive fracture in both groups of simulations preexisting fractures governed by coulomb friction are susceptible to slip if they span the ice thickness are oriented close to the optimal fracture angle and in the case of storglaciaren are subject to water pressures that are a large fraction of ice overburden pressure variations about the optimal fracture orientation low or zero water pressure high sliding friction coefficient and limited vertical or lateral fracture extent each tend to suppress thrusting

positive feedbacks associated with erosion of glacial cirques and overdeepenings the principal points of water input to a glacier are the bergschrund in cirques and crevasse fields lower on the glacier crevasse fields commonly occur over convexities at the heads of overdeepenings in glacier beds the amplitude of subglacial water pressure fluctuations is large just down glacier from these points of water input erosion by quarrying is likely in such areas erosion is thus inferred to be localized on the headwalls of cirques and overdeepenings in the case of overdeepenings this leads to a positive feedback process in which a perturbation in the bed causes crevassing at the surface resulting in erosional forces that accentuate the perturbation when subglacial water flows up an adverse bed slope leading out of a cirque or overdeepening much of the viscous energy dissipated is used to warm the water to keep it at the pressure melting temperature as the ice thins and the pressure decreases in such situations subglacial conduits are maintained by high water pressures rather than by melting of conduit walls in the limit water pressures apparently become so high that water is forced out along the ice bed interface and the conduits collapse the products of erosion are then no longer flushed out and a protective till layer accumulates by limiting erosion on such adverse bed slopes this till layer controls the geometry of these overdeepened basins

basal crevasse fill origin of laminated debris bands at matanuska glacier alaska u s a the numerous debris bands in the terminus region of matanuska glacier alaska u s a were formed by injection of turbid meltwaters into basal crevasses the debris bands are millimeter s thick layers of silt rich ice cross cutting older debris poor englacial ice the sediment grain size distribution of the debris bands closely resembles the suspended load of basal waters and of basal and proglacial ice grown from basal waters but does not resemble supraglacial debris till or the bedload of subglacial streams most debris bands contain anthropogenic tritium 3h in concentrations similar to those of basal meltwater and ice formed from that meltwater but cross cut englacial ice lacking tritium stable isotopic ratios 18 o and d of debris band ice are consistent with freezing from basal waters but are distinct from those in englacial ice ice petrofabric data along one debris band lack evidence of active shearing high basal water pressures and locally extensional ice flow associated with overdeepened subglacial basins favor basal crevasse formation

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

a sedimentological and isotopic study of the origin of supraglacial debris bands kongsfjorden svalbard hubbard b glasser n f hambrey m j etienne j l 2004 a sedimentological and isotopic study of the origin of supraglacial debris bands kongsfjorden svalbard journal of glaciology 50 169 157 170 rae2008

terminal zone glacial sediment transfer at a temperate overdeepened glacier system continuity of sediment transfer through glacial systems is essential to maintain subglacial bedrock erosion yet transfer at temperate glaciers with overdeepened beds where subglacial fluvial sediment transport should be greatly limited by adverse slopes remains poorly understood complex multiple transfer processes in temperate overdeepened systems has been indicated by the presence of large frontal moraine systems supraglacial debris of mixed transport origin thick basal ice sequences and englacial thrusts and eskers at svinafellsjokull thrusts comprising decimetre thick debris rich bands of stratified facies ice of basal origin with a coarser size distribution and higher clast content than that observed in basal ice layers contribute substantially to the transfer of subglacial material in the terminal zone entrainment and transfer of material occurs by simple shear along the upper surface of bands and by strain induced deformation of stratified and firnified glacier ice below thrust material includes rounded and well rounded clasts that are also striated indicating that fluvial bedload is deposited as subglacial channels approach the overdeepening and then entrained along thrusts substantial transfer also occurs within basal ice with facies type and debris content dependent on the hydrological connectedness of the adverse slope a process model of transfer at glaciers with terminal overdeepenings is proposed in which the geometry of the overdeepening influences spatial patterns of ice deformation hydrology and basal ice formation we conclude that the significance of thrusting in maintaining sediment transfer continuity has likely been overlooked by glacier sediment budgets and glacial landscape evolution studies

ice core evidence for the thickness and character of clear facies basal ice at glacier de tsanfleuron switzerland five ice cores have been retrieved from a transect close to the terminus of glacier de tsanfleuron switzerland the cores extend from the ice surface to the glacier bed and are 3 5 44 8 m long stratigraphic logging based on bubble size and density reveals the presence of a highly metamorphosed basal ice layer about 10 m thick from which all traces of bubble rich ice have been removed this bubble poor ice which corresponds closely with clear facies ice observed in cavities beneath numerous temperate based glaciers contrasts with the overlying bubble rich or bubble foliated englacial ice and the underlying debris rich and bubble free dispersed facies basal ice down core patterns in major ion composition stable isotope composition and total gas content and composition are generally consistent with formation of clear facies ice by deformation related metamorphism of bubbly englacial ice in addition isotopic data suggest that storage of downward percolating meltwaters occurs close to the upper surface of the clear facies ice layer perhaps reflecting a local variation in ice permeability across the transition from englacial to clear facies ice enrichment in crustally derived ionic species is noted in the lowermost decimetres of the debris free clear facies ice that immediately overlies debris rich dispersed facies basal ice this ionic enrichment in debris free ice is interpreted in terms of active inter granular meltwater flow within some decimetres of the glacier bed

ice ages and nuclear waste isolation abstract the greatest natural threats to the integrity of the geological barriers to nuclear wastes isolated in cavities mined at depths between 400 and 800 m are likely during rapid retreats of future ice sheets the next major glacial retreat is expected at ca 70 ka well within the lifetime of high grade nuclear waste but it is not yet clear how long man s greenhouse effect may delay it this contribution discusses the potential problems posed to european waste isolation sites during erosion by ice and over pressurizing of meltwater and gasses in a lithosphere flexed by major ice sheets these depend on the target rocks and the location of the site with respect to the ice streams and margins of future ice sheets of particular size no sites are planned under the centres of future ice sheets in europe where end glacial earthquakes can be expected to reactivate major faults nor where ice can be expected to deepen and lengthen fjords along the atlantic coast sites in the alps may be vulnerable to radical changes in the patterns of glacial troughs the stability and geohydrology of sites in coastal areas beyond future ice margins are threatened by river gorges when sea level falls ca 125 m or in enclosed basins like the mediterranean ever lower the greatest problems are likely in lowland regions exposed by the rapid retreat of thick ice fronts where large lakes on or under thick warm based ice are dammed by more distal cold based ice groundwater in subhorizontal fractures dilated by glacial unloading may reach over pressures capable of hydraulically lifting megablocks of bedrock with fracture permeability and or the ice damming them so that less permeable substrates are susceptible to incisions eroded to depths of ca 360 at locations controlled mainly by ice topography kinematics and history

response of subglacial sediments to basal freeze on 1 theory and comparison to observations from beneath the west antarctic ice sheet 1 we have constructed a high resolution numerical model of heat water and solute flows in sub ice stream till subjected to basal freeze on the model builds on quantitative treatments of frost heave in permafrost soils the full version of the clapeyron equation is used hence ice water phase transition depends on water pressure osmotic pressure and surface tension the two latter effects can lead to supercooling of the ice base this supercooling in turn induces hydraulic gradients that drive upward flow of pore water which feeds the growth of segregation ice onto the freezing interface this interface may progress into the till and form ice lenses if supercooling is sufficiently strong hence the ice segregation process develops a stratified basal ice layer in our model a high basal temperature gradient 0 054 c m 1 triggers ice stream stoppage and the loss of basal shear heating leads to relatively high basal freeze on rates 3 5 mm a 1 in response the subglacial till experiences comparatively rapid consolidation till porosity can decrease from 40 to 120 kpa approximately within one century basal supercooling arising from redistribution of solutes and ice water interfacial effects amounts to ca 0 35 c below the pressure melting point fine grained till is in our model associated with widely spaced thick ice lenses coarse grained till yields thinner ice lenses that are more closely spaced our model results compare favorably although not in all details with available observational evidence from borehole studies of west antarctic ice streams

the geography of basal ice and its relationship to glaciohydraulic supercooling svinafellsjokull southeast iceland abstract glaciohydraulic supercooling is potentially an important mechanism of basal ice formation with significant implications for quaternary science but remains controversial some studies cite similarities between basal ice and the ice forming at vents of upwelling supercooled subglacial water as evidence that where supercooling occurs it dominates basal ice formation other studies find no evidence linking supercooling to basal ice even at glaciers where supercooling occurs questioning the supposed genetic link between basal ice and vent ice at svinafellsjokull southeast iceland we compare the physical characteristics and geographical distribution of stratified basal ice with the characteristics and distribution of phenomena such as upwellings and anchor ice terraces associated with supercooling two physically and geographically distinct basal ice populations emerge one occurs only where there is evidence for glaciohydraulic supercooling and has physical characteristics consistent with an origin by supercooling the other is ubiquitous even at locations where there is no evidence for supercooling and has characteristics consistent with non supercooling modes of origin we suggest that glaciohydraulic supercooling does not account for all of the basal ice at svinafellsjokull and that other mechanisms of basal ice formation are also significant

distribution of glaciofluvial sediment within and on the surface of a high arctic valley glacier marthabreen svalbard marthabreen is a 7 8km long valley glacier in sw spitsbergen the glacier is partially covered by a layer of angular debris derived from rockfall in its accumulation area pierced in places by pinnacles and ridges of glaciofluvial sediment these concentrations of glaciofluvial sediment fall into three categories 1 debris pinnacles 2 longitudinal sediment dykes 3 longitudinal ridge accumulations debris pinnacles are slabs of sediment predominantly sands gravels and cobbles elevated to the glacier surface along thrusts longitudinal sediment dykes are low 1m high ridges of sorted sand and gravels which are not associated with penetrative ice structures their occurrence downglacier of sediment dykes and debris pinnacles suggests that they originate as supraglacial or englacial channels or tunnels filled by sediment derived from the dykes or thrusts the presence of large quantities of glaciofluvial sediment on the surface of marthabreen does not imply englacial water flow at high levels within the glacier but is related to ice deformational processes such as thrusting and folding of debris rich stratification copyright 1999 john wiley sons ltd

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

debris entrainment and transfer in polythermal valley glaciers hambrey mj bennett mr dowdeswell ja et al 1999 debris entrainment and transfer in polythermal valley glaciers journal of glaciology 45 149 69 86 published 1999

effect of a cold margin on ice flow at the terminus of storglaciaren sweden implications for sediment transport the cold based termini of polythermal glaciers are usually assumed to adhere strongly to an immobile substrate and thereby supply significant resistance to the flow of warm based ice up glacier this compressive environment is commonly thought to uplift basal sediment to the surface of the glacier by folding and thrust faulting we present model and field evidence from the terminus of storglaci aren sweden showing that the cold margin provides limited resistance to flow from up glacier ice temperatures indicate that basal freezing occurs in this zone at 10 1 1 0 2 ma 1 but model results indicate that basal motion at rates greater than 1 m a 1 must nevertheless persist there for surface and basal velocities to be consistent with measurements estimated longitudinal compressive stresses of 20 25 kpa within the terminus further indicate that basal resistance offered by the cold based terminus is small these results indicate that where polythermal glaciers are underlain by unlithified sediments ice flow trajectories and sediment transport pathways may be affected by subglacial topography and hydrology more than by the basal thermal regime

sedimentary signatures of basal ice formation and their preservation in ice marginal sediments reconstruction of subglacial processes from sedimentological evidence is central to our understanding of glaciological conditions beneath former ice masses at svinafellsjokull southeast iceland we assess the extent to which two different processes of basal ice formation regelation and glaciohydraulic supercooling can be identified from ice marginal sediments our results indicate that the sedimentary characteristics of deposits produced by these two processes can be distinguished from one another and that it may be possible to recognise evidence of these processes in quaternary sediments and to reconstruct their spatial pervasiveness sediments derived from the melting of regelation basal ice have i a massive structure ii a sediment matrix 0 to 10 dominated by coarse sand and iii a higher proportion of angular clasts than supercool basal ice and associated sediments sediments derived from supercool basal ice i can be either massive or layered ii tend to have a silt dominated matrix and iii contain a slightly higher proportion of rounded and well rounded clasts than regelation basal ice and sediments previous studies indicate that the dominance of silt within supercool basal ice may be unique to this process and hence supercooling should leave a readily recognisable signature in the sedimentary record our results from svinafellsjokull lend support to that idea although we suggest that further work is required to assess whether silt dominance is a process signature diagnostic of supercooling and in particular the extent to which subglacial sediment supply determines the sedimentary character of basal ice facies and associated sediments

glaciohydraulic supercooling a freeze on mechanism to create stratified debris rich basal ice i field evidence debris laden ice accretes to the base of matanuska glacier alaska u s a from water that supercools while flowing in a distributed drainage system up the adverse slope of an overdeepening frazil ice grows in the water column and forms aggregates while other ice grows on the glacier sole or on substrate materials sediment is trapped by this growing ice forming stratified debris laden basal ice growth rates of 0 1 m a 1 of debris rich basal ice are possible the large sediment fluxes that this mechanism allows may have implications for interpretation of the widespread deposits from ice that flowed through other overdeepenings including heinrich events and the till sheets south of the laurentian great lakes

controlled moraines origins characteristics and palaeoglaciological implications abstract controlled moraines are supraglacial debris concentrations that become hummocky moraine upon de icing and possess clear linearity due to the inheritance of the former pattern of debris rich folia in the parent ice linearity is most striking wherever glacier ice cores still exist but it increasingly deteriorates with progressive melt out as a result moraine linearity has a low preservation potential in deglaciated terrains but hummocky moraine tracts previously interpreted as evidence of areal stagnation may instead record receding polythermal glacier margins in which debris rich ice was concentrated in frozen toe zones recent applications of modern glaciological analogues to palaeoglaciological reconstructions have implied that a controlled moraine development can be ascribed to a specific process e g englacial thrusting or supercooling and b controlled moraine preservation potential is good enough to imply the occurrence of the specific process in former glacier snouts e g ancient polythermal or supercooled snouts these assumptions are tested using case studies of controlled moraine construction in which a wide range of debris entrainment and debris rich ice thickening mechanisms are seen to produce the same geomorphic features polythermal conditions are crucial to the concentration of supraglacial debris and controlled moraines in glacier snouts via processes that are most effective at the glacier permafrost interface end moraines lie on a process form continuum constrained by basal thermal regime the morphological expression of englacial structures in controlled moraine ridges is most striking while the moraines retain ice cores but the final deposits landforms tend to consist of discontinuous transverse ridges with intervening hummocks preserving only a weak impression of the former englacial structure these are arranged in arcuate zones of hummocky moraine up to 2 km wide containing ice walled lake plains and lying down flow of streamlined landforms produced by warm based ice a variety of debris entrainment mechanisms can produce the same geomorphic signature spatial and temporal variability in process form relationships will lead to the sequential development of different types of end moraines during the recession of a glacier or ice sheet margin

geomorphic implications of subglacial drainage configuration rates of basal sediment evacuation controlled by seasonal drainage system evolution abstract although basal sediment evacuation by subglacial meltwater is widely assumed to dominate the sediment budget of most temperate glaciers and ice caps its potential influence for glacial geomorphic processes and landforms has largely been overlooked in the glacial literature data collected at haut glacier d arolla switzerland during the 1998 ablation season are used to test theoretical predictions that subglacial drainage configuration should exert a primary control on rates of basal sediment evacuation by subglacial meltwater seasonal evolution of the subglacial drainage system enables observation of sediment evacuation from both distributed and channelised drainage systems daily suspended sediment load exhibits an extreme nonlinear increase with respect to discharge between winter and summer periods which correspond to periods of predominantly distributed and channelised subglacial drainage respectively these results suggest that subglacial drainage configuration should significantly impact upon a rates of subglacial erosion and debris production b the nature of debris transport and the proportion of sediment in glacial versus fluvioglacial transport pathways and c rates and styles of ice marginal sedimentation in contrast to their distributed counterparts channelised drainage configurations are predicted to encourage subglacial erosion through the removal of basal sediment from the ice bedrock interface and with the exception of supraglacial transport will discourage sediment transport by alternative glacial mechanisms direct sedimentation at the glacier margin may therefore be reduced resulting in smaller moraines despite an overall increase in sediment production an appreciation of the coupling between glacier hydrology and geomorphic processes suggests new frameworks for the understanding of rates and styles of glacial erosion sediment transfer and deposition and the complex geomorphology of ice marginal environments

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

freezing rate effects on the physical characteristics of basal ice formed by net adfreezing a number of theoretical and empirical studies have indicated that many individual characteristics of ice formed by processes of net basal adfreezing may be sensitive to the rate of propagation of the freezing front through the reservoir concerned the effects of freezing rate on the the stable isotope chemistry and crystallography of ice in addition to the disposition and character of included debris and gas are reported unidirectional freezing through a cylindrical reservoir containing various water sediment mixtures has been conducted in the laboratory and the resulting cores analysed for debris and gas disposition and ice crystal size and fabric the data lend support to inferences drawn from studies concerned with specific ice properties and an idealized suite of characteristics is developed which may be diagnostic of basal ice formed by net adfreezing

reply formation and reorientation of structure in the surge type glacier kongsvegen svalbard we welcome the development of the debate about structures at the surge type glacier kongsvegen svalbard our paper woodward et al 2002 tested a number of hypotheses proposed in the literature by glasser et al 1998 and bennett et al 1996 for sediment entrainment in glacier ice we feel that after testing sufficient evidence exists to reject these hypotheses and as a result we have proposed alternative mechanisms for the formation of these features we wish to reply to the issues raised in the above comment by glasser et al

genesis of hummocky moraines by thrusting in glacier ice evidence from svalbard and britain irregular mounds of glacial debris commonly referred to as 9hummocky moraine9 until recently were linked to ice stagnation during rapid climatic amelioration however recent work in scotland has demonstrated that some hummocky moraine dating from the younger dryas event c 10 000 years bp was the product of deposition at active ice margins observations at modern high arctic glacier margins in svalbard 76 80 n indicate that some moraines of this type form under a dynamic glaciological regime mainly by thrusting in active polythermal ice undergoing strong longitudinal compression this situation is particularly common where there is a transition from warm based ice in the interior of a glacier to cold based ice at the margins or snout from their morphological similarity rectilinear slopes facing up glacier and irregular downglacier faces and apparent stacking of slabs of sediment of subglacial derivation it is concluded that some assemblages of british moraines are also the product of thrusting this process appears to be most dominant in polythermal glaciers rather than in temperate glaciers on this basis it is tentatively suggested that the younger dryas in britain was characterized by a climatic regime similar to that in svalbard today mean annual temperature 5 c 400 1000 mm precipitation and that the associated glaciers may have been of polythermal character

sediments and landforms at kviarjokull southeast iceland a reappraisal of the glaciated valley landsystem abstract eyles identified kviarjokull an outlet of the icecap oraefajokull southeast iceland as an exemplar of the glaciated valley landsystem emphasising rockfall debris supply passive transport and reworking of a thick cover of supraglacial morainic till in this paper we take a fresh look at the sediments and landforms of kviarjokull focusing on clast form analysis to reconstruct primary transport pathways new data demonstrate the importance of active transport of debris derived from the glacier bed substantial portions of the supraglacial debris cover can be traced back to englacial channel fill deposits or anomalously thick exposures of basal ice these two types of sediments also make up much of kviarjokull s large neoglacial moraine rampart we attribute the abundance of these non rockfall sediments to the presence of a terminal overdeepening and the switch from predominantly subglacial to predominantly englacial drainage that this induces this hydrological switch enhances retention of debris within ice transport preventing it from being washed away and making it available to feed ice marginal moraine formation the complexity of sediment transport processes observed at kviarjokull and the contrast between it and alpine glaciers undermines any single concept of the glaciated valley landsystem accordingly we propose kviarjokull as the type example of a distinct subclass defined by its moderate relief high debris turnover and drainage behaviour characteristic of an overdeepened basin

high resolution record of geomagnetic excursions in the matuyama chron constrains the ages of the feiliang and lanpo paleolithic sites in the nihewan basin north china the nihewan basin 40 degrees n in north china is a rich source of early pleistocene paleolithic sites and thus a key area for studying early human evolution in high latitude from an early human perspective east asia here a high resolution magnetostratigraphic investigation is carried out on a fluvio lacustrine section in the northeastern nihewan basin which contains the feiliang and lanpo paleolithic sites paleomagnetic results suggest that this section records the lower portion of the brunhes polarity chron and the upper matuyama polarity chron furthermore the jaramillo polarity subchron and seven of the nine validated geomagnetic excursions within the matuyama polarity chron are identified including the kamikatsura santa rosa intra jaramillo cobb mountain bjorn gardar and gilsa excursions the feiliang artifact layer is located just at the bottom of the cobb mountain excursion thus its age is estimated to be similar to 1 2ma the lanpo artifact layer appears to be coeval with the gilsa excursion yielding an estimated age of similar to 1 6 ma this study provides new evidence for the presence of early humans in north china before 1 5 ma and documents the powerful role of geomagnetic excursions they provide valuable age control points for ongoing efforts to date the early paleolithic sites

transverse englacial debris rich ice bands at kviarjokull southeast iceland thick exposures of debris rich ice at various icelandic glaciers are central to the debate over the prevalence of glacial sediment transfer by glaciohydraulic supercooling we present physical analyses of ice and debris at kviarjokull a temperate glacier in southeast iceland with a terminal glacier bed overdeepening where stratified debris rich ice forms up to metre thick transverse englacial bands our results are not consistent with debris rich ice formation predominantly by supercooling because 1 137cs was absent from sediment filtered from debris rich ice 2 isotopic analysis d and 18o demonstrated no clear pattern of isotopic enrichment of debris rich ice with respect to englacial ice and 3 melt out debris from debris rich ice included large striated clasts from both fluvial and basal sources we support transverse englacial debris rich ice band formation by the thickening and elevation of basal materials in a region of longitudinally compressive ice flow situated between the reverse slope of the overdeepening and the base of an ice fall debris band form and distribution are likely to be controlled by thrusting along transverse englacial foliae associated with the formation of band ogives on the glacier surface