equifinality of basal ice facies from an antarctic cold based glacier threedebris bearingicefacieswererecognizedatthebaseofsuessglacier a cold basedglacierdamminga lakeintaylorvalley southvictoria land antarctica these faciesaretermed amber ice solidfacies and basal stratifiedfacies thispaper uses stable isotope composition dd and d 18 o gas content and gas composition co 2 o2 and n2 to develop an understanding of the processes responsible for the formation of these facies the basal iceischaracterizedbya strikingdifferencein icepropertiesbetweentheinnermostend of a 25m long tunnel dug 200m upstream fromthe glacier front and the front itself at the glacier front co isotopic data plot along a well defined freezing slope s 5 6 whereas insidethetunnel the isotopic data offset fromthefreezing slope and fromthe local meteoric water line which has a slope of 8 2 co 2 concentrations rise from a minimum of about 1000ppmv inthetunnelto about 220000ppmvatthe front takentogether these character istics strongly suggest an increasing contribution of liquidwater inthe formation of basal ice towards the glacier terminus we therefore conclude that visually similar basal ice facies can have differentorigins

bedform genesis in bedrock substrates insights into formative processes from a new experimental approach and the importance of suspension dominated abrasion bedrock channels are common in the natural environment and bedrock channel erosion sets the pace of denudation in many river catchments however in comparison to the large number of studies concerning the formation of alluvial bedforms relatively few investigations have concerned bedrock bedform genesis field based analysis of sculptured forms within bedrock channels has been restricted notably by the slow rate of bedform development in such environments furthermore only a limited number of flume scale experiments have been conducted that attempt to simulate the genesis of sculpted bedforms in bedrock channels this study demonstrates that optimisation of clay beds through analysis of clay strength enables the development of features analogous to bedrock river channel bedforms even at a scale that is orders of magnitude smaller than some natural examples three sets of suspended sediment laden experiments were carried out using hard medium and soft clay bed substrates a suite of erosive bedforms including potholes flutes and furrows developed on all experimental beds all observed erosional features have clear equivalents to those observed in natural bedrock rivers bed shear strength was found to be a significant factor for the genesis of different types of simulated bedrock bedforms in our experiments with other factors such as flow velocity bed slope and flow depth held approximately constant importantly in a subset of experiments performed with an absence of suspended sediment fluid flow did not result in the erosion and development of bedforms in the clay bed hence this work illustrates that abrasion by suspended sediments is the key process required for the formation of these simulated bedrock bedforms in our experiments in the absence of bedload abrasion other processes such as plucking cavitation and dissolution will have been negligible

an isotopic model for basal freeze on associated with subglacial upward flow of pore water 1 subglacial freezing in polar glaciers can have a significant dynamical effect recent studies have shown that freezing of pore water flowing upward through subglacial fine grained sediments at the freezing interface and progression of this freezing front downward are responsible for fast ice flow stoppage in ice streams the upward pore water flow leads to the formation of debris bearing basal ice layers a model for stable isotope composition both in d and 18o is developed for predicting the isotopic composition of the ice segregated by such a mechanism the development of this isotopic model for water films present along the grains of the subglacial sediment predicts the absence of apparent fractionation for the ice formed this prediction is tested against two east antarctic outlet glaciers by studying the d 18o relationships in the basal ice layers of these glaciers

strain induced phase changes within cold basal ice from taylor glacier antarctica indicated by textural and gas analyses this paper reports detailed textural and gas measurements conducted in cold basal ice 178c from the margin of taylor glacier an outlet glacier of the east antarctic ice sheet the analyzed samples were retrieved from a basal ice sequence excavated at the end of a subglacial tunnel dug near the glacier snout the basal sequence exhibits two contrasting ice facies defined as the englacial and stratified facies on the one hand analysis of ice crystal textures from the basal ice sequence provides evidence for localized ductile deformation especially within the stratified facies where significant dynamic recrystallization was detected on the other hand high resolution gas analyses reveal that strong changes in gas composition occurred at the structural interfaces of the stratified facies these gas composition changes are typical of melting refreezing processes but are not associated with any significant loss of gas volume given the specific subglacial thermal conditions at the margin of taylor glacier we interpret this phenomenon as resulting from microscopic phase changes involving selective gas redistribution through the pre melt phase it is argued that such processes may play an important role in the post genetic geochemical evolution of cold debris laden ice and may be enhanced through intense strain conditions

basal ice accretion and debris entrainment within the coastal ice margin law dome antarctica basal ice stratigraphy in coastal ice cliffs at the law dome margin has revealed the basal accretion of clean and debris bearing ice marine congelation ice and granular marine ice inland of the margin co isotopic analysis of 18 o and d isotopes together with solute chemistry were applied to determine the modes of accretion and debris entrainment

dynamic implications of discontinuous recrystallization in cold basal ice taylor glacier antarctica crystallographic investigations have been conducted of cold 17 c debris bearing ice from the base of an antarctic outlet glacier taylor glacier the 4 m thick sequence studied has been retrieved from a 20 m long tunnel dug from the glacier snout and has been analyzed with an automatic ice fabric analyzer aifa the top and bottom of the sequence consists of clean meteoric ice englacial facies whereas alternating debris rich and clean bubbly ice layers are found in the middle part stratified facies ice from the englacial facies displays a polygonal texture and a strong c axis clustering toward the vertical denoting recrystallization through subgrain rotation sgr in contrast clean ice from the stratified facies shows sgr fabrics which are delimited at the contact with debris rich layers by large interlocking grains organized in ribbons these two distinct textures within the stratified facies are associated with looser c axis patterns at the scale of single thin sections which is interpreted as resulting from migration recrystallization mr the change from sgr to mr trends marks a clear increase in grain boundary and nucleation kinetics hence the term discontinuous recrystallization and may be associated with strain localization at rheological interfaces during basal ice genesis analogies with bottom ice from deep polar ice sheets where temperature is commonly higher than at the studied site are highlighted two recrystallization scenarios are proposed accounting for the development of both types of fabrics it is shown that by controlling the repartition of stress and strain energy within basal ice the rheology of debris bearing ice layers plays a decisive role in recrystallization dynamics at structural interfaces we also demonstrate how the same recrystallization regimes may occur in cold glaciers and temperate ice sheets provided that strain accumulation has been high enough in the former this challenges the common belief that migration fabrics observed in bottom ice from deep ice sheets are exclusive to warm stagnant annealed ice copyright 2008 by the american geophysical union

preservation of miocene glacier ice in east antarctica antarctic climate during the pliocene has been the subject of considerable debate one view holds that during part of the pliocene east antarctica was largely free of glacier ice and that vegetation survived on the coastal mountains1a 4 an alternative viewpoint argues for the development of a stable polar ice sheet by the middle miocene which has persisted since then5a 10 here we report the discovery of buried glacier ice in beacon valley east antarctica which appears to have survived for at least 8 1 million years we have dated the ice by 40ar 39ar analysis of volcanic ash in the thin overlying glacial till which we argue has undergone little if any reworking isotope and crystal fabric analyses of the ice show that it was derived from an ice sheet we suggest that stable polar conditions must have persisted in this region for at least 8 1 million years for this ice to have avoided sublimation

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

the educational value of the history of geomorphology abstract the history of geomorphology can be a valuable tool for educating students of geomorphology the largest and most available record of what geomorphologists thought and did in the past is their original published work also available are some fine papers on the history of geomorphology that review and analyze primary sources and past events in the discipline in order to generalize explain trends and reveal historical associations interest in the history of the discipline varies widely of course among geomorphologists nevertheless several educational benefits in addition to explaining the academic affiliation of geomorphology with two separate disciplines can be derived from incorporating the history of geomorphology into university level geomorphology courses history is a popular subject and presenting some geomorphic concepts using a historical approach can help to maintain or stimulate student interest because of the tendency for older literature to contain more qualitative description and methodological detail than more recent literature undergraduate students may comprehend some concepts better from older papers by reading the original literature students determine for themselves what previous practitioners accomplished rather than relying solely on the interpretations of others reading original literature also helps students realize that older does not mean less intelligent and that like the critical reading of recent geomorphic literature the critical reading of historic geomorphic papers can provide a wealth of new research ideas a thematic set of the historic literature can be used to demonstrate to students the scientific method the origin testing and evolution of hypotheses how explanations develop in this field and that science proceeds through individuals working in a sociological context including history of geomorphology in the curriculum helps to place contemporary research in the perspective of the past as well as the perspective of the future

reply to comment by a w rempel et al on a quantitative framework for interpretation of basal ice facies formed by ice accretion over subglacial sediment 1 we are grateful to rempel et al 2007 hereinafter referred to as rww2007 for giving us this opportunity to discuss in greater detail the fascinating problem of developing quantitative models of subglacial freeze on and basal ice formation we find many of their comments valuable but disagree with their general negative assessment of our previous work on the subject christoffersen et al 2006 hereinafter referred to as ctcb2006 we find it particularly disappointing that rww2007 dismiss our model on the basis of misconstrued model physics rww2007 criticize our assumed treatment of the freezing process but never really give specific examples of how changing these assumptions would improve the ability of our model to realistically reproduce different facies of debris bearing basal ice here we lay out the details of our disagreement with their dismissal of our work 2 despite the claim made by rww2007 there is no sign error in equation 1b of christoffersen and tulaczyk 2003 hereinafter referred to as ct2003 if we switch the sign in this equation as rww2007 imply should be done then we would get pore water pressures higher than ice pressure at the freezing ice water interface this would result in water being driven away from the ice water interface rather than being drawn toward it as the phenomenon of frost heaving requires o neill and miller 1985 their claim that our equation is erroneous is the more surprising since it is straightforward to show that their equation for interfacial pressure rempel et al 2004 hereinafter referred to as rww2004 takes on the same form as equation 1b of ct2003 once the assumption is made that ice saturation is zero in the till underlying a freezing ice base i e no frozen fringe under this assumption the upper limit of integration in equation 3 4 of rww2004 is zl zf and the equation for interfacial pressure piw at a freezing icewater interface can be expanded to a form similar to our equation 1b ct2003

setting a chronology for the basal ice at dye 3 and grip implications for the long term stability of the greenland ice sheet abstract the long term stability of the greenland ice sheet gis is an important issue in our understanding of the climate system limited data suggest that the northern and southern sections extend well back into the pleistocene but most age constraints do not definitively date the ice here we re examine the grip and dye 3 ice cores to provide direct ice core observations as to whether the gis survived previous interglacials known to be warmer 130 ka or longer 430 ka than the present interglacial we present geochemical analyses of the basal ice from dye 3 1991 2035 m and grip 3020 3026 m that characterize and date the ice we analyzed the elemental and isotopic composition of o2 n2 and ar of trapped air in these two cores to assess the origin of trapped gases in silty ice dating of the trapped air was then achieved by measuring the paleoatmospheric 40ar 38ar and the 17o anomaly 17 of o2 the resulting age is a lower limit because the trapped air may be contaminated with crustal radiogenic 40ar the oldest average age of replicates measured at various depths is 970 140 ka for the grip ice core and 400 ka 170 ka for dye 3 17 data from dye 3 also argue strongly that basal ice in this core predates the eemian this confirms that the greenland ice sheet did not completely melt at southern greenland during the last interglacial nor did it completely melt at summit greenland during the unusually long interglacial 430 kyr before present

a quantitative framework for interpretation of basal ice facies formed by ice accretion over subglacial sediment 1 we have constructed a numerical model of basal ice formation for glacier ice in contact with subglacial sediment the model predicts four different ice facies whose formation is controlled by availability of subglacial water to satisfy the basal freeze on rate clean or clotted facies may result from congelation or frazil ice growth occurring when supercooled meltwater separates ice base from substrate or if a groundwater source can supply water to the ice base at a velocity equal to the freezing rate laminated facies develops when the supply of subglacial water is sufficiently constrained for cryostatic suction to raise the subglacial effective stress above a threshold for intrusion of ice into sediment by regelation debris laminas 1 mm are entrained by short periodic regelation events of a few hours separated by longer periods days to months of congelation further meltwater limitation produces a massive dirty ice facies due to stacking of debris laminas the model predicts growth of solid dirty ice facies when the bed is meltwater depleted with fast freezing 5 mm yr 1 causing enhanced erosion 30 mm yr 1 we find that basal ice facies and sediment entrainment are controlled mainly by the ratio of freezing rate to water supply rate the predicted ice facies compare favorably with borehole camera imagery of the basal ice layer in kamb ice stream west antarctica facies variability in this layer suggests complex hydrologic history for the west antarctic ice sheet with significant changes occurring over a period of several thousand years

highly deformed basal ice in the vostok core antarctica 1 our paper documents the build up of a highly deformed basal ice layer in the basal part of the vostok ice core this is done mainly on the basis of an isotopic composition investigation of the ice complex deformation in the lower 228 m of the ice sheet has resulted in folding and intermixing of ice at a submetric scale and for the upper part of this basal sequence in interbedding of ice layers from distinct origins at a larger scale this complex deformation occurred at a temperature largely below the pressure melting point the basal ice layer has built upwards and size selective incorporation of bed material into the ice has taken place the documentation of this complex basal deformation has implications for the maximum age of ice that will be useful in paleoclimate studies and for ice sheet dynamics

thresholds of erosion and sediment movement in bedrock channels abstract this study examines the erosive resistance of the discontinuous loose joint blocks of various sizes that comprise the bed material of the etobicoke creek an incised bedrock channel found in an urbanized setting within the city of toronto ontario canada in order to accomplish this both field and laboratory experiments were conducted on the bed material found in this channel the field work involved repeated detailed surveying of the channel bed and estimations of distances of movement of various clasts during many sediment transport events we found that rates of erosion and or deposition can be significant during the snowmelt spring events and that there was a greater loss of bed material over the study reach than was gained in several spots with bed lowering 0 10 m in some locations the field work also showed that the distances moved by the rock clasts scaled directly with shear stresses observed during rain events and that average distances of transport tend to be short the frequency distributions of distance of transport scaled with particle size is log normally distributed flume experiments demonstrated that the clasts offered different levels of resistance to movement depending on their orientation the platy nature of the rock fragments clast orientation and their imbrication significantly affect the threshold of erosion the results have significant implications for the understanding of bedrock rivers from a management perspective especially in the case of those rivers experiencing the effects of urban development in their drainage basins

rotating micro structures in antarctic cold basal ice implications for glacier flow and its interpretation structural analyses were conducted in the basal zone of an antarctic glacier the studied basal ice sequence was retrieved from a 20 m long subglacial tunnel dug at the margin of the glacier and is at the temperature of 17 c for the first time rotating clast systems embedded within debris rich ice were thin sectioned using specially designed cutting techniques the observed structures reflect the occurrence of pervasive shearing at the base of the glacier and can be used as shear sense indicators in addition some of these structures provide evidence for the presence of thin liquid films at the time of formation despite the marked freezing temperature of the ice it is showed here that cautious analysis of deformation structures present in debris bearing ice may bring insights not only into the flow dynamics of the embedding matrix but also into the behaviour of the interstitial fluid network at the base of cold glaciers and ice sheets

isotopic signature of debris rich ice formed by regelation into a subglacial sediment bed among the processes that may entrain sediment in the bases of glaciers and ice sheets basal ice may regelate downward into the pores of unlithified sediment laboratory studies suggest that this process leaves interstitial ice with a distribution of 18o and d that is distinct from that developed due to downward freezing of a semi closed water reservoir although as a result of both processes these isotopes lie on a freezing slope in a d 18o diagram debris layers entrained by regelation past grains are both enriched in heavy isotopes with depth and display a downward decrease in deuterium excess the converse of that due to downward freezing of a water reservoir moreover the extent of enrichment can be significantly larger in the case of regelation into the bed reevaluated field data from west greenland are consistent with regelation into the bed as an entrainment mechanism

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

submerged and unsubmerged natural hydraulic jumps in a bedrock step pool mountain channel high resolution digital elevation models dems for natural submerged and unsubmerged jump regions were tested against the classical hydraulic jump chj and engineering analogues that have dominated previous geomorphic and engineering research dems were compared for two discharge conditions the data showed bed and water surface features of the natural jump regions differ significantly from chj conditions and engineering analogues with respect to boundary conditions roughness elements were highly irregular in frequency spacing size and orientation and were comprised of form obliquity and bed scour in boulder bed and pure bedrock conditions the data also showed bed and water surface features of natural jump regions have similarities and deviations from chj conditions and engineering analogues with respect to hydraulic characteristics several variations were observed in the hydraulic characteristics between the ballistic and sloping jets revised conceptual models based on idealized corollaries were developed for the natural jump regions based on upstream energy head relative to the downstream tailwater depth and step geometry the dems highlight the importance of recognizing the interrelationship between spatial transcritical flow structures and localized topographic heterogeneities in bedrock channels

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

on the isotopic composition in δd and δ18o of water and ice during freezing experiments on progressive unidirectional freezing are coi1 ducted to determine rhe evol ution in 00 and 51 bo of successive water samples and ice layers ta ken during the course of freezing r es ults indicate t hal lhi s evoluti on takes pl ace in a bd bibo diagram a long a strai ght line wi th a characteri sti c slope this slope different from that due to the precip itation effec t gives a finger print of the occurrence of a freezing or of a melt ing refreez ing process in the stud ied reservoir residual water was sampled at the beginning of the experiment after 40 80 85 90 and 95 of the material had frozen when the freezing process was complete the ice core was recovered and sectioned the samples covered the freezing ranges from 0 to 10 10 to 20 20 to 50 50 to 60 60 to 70 70 to 80

thermodynamics of basal freeze on predicting basal and subglacial signatures of stopped ice streams and interstream ridges we have constructed a numerical model that simulates the response of subglacial sediments to basal freeze on the model is set up to emulate the basal zone of drilling sites in the ross sea sector of thewest antarctic ice sheet we treatbasal freeze on atanice sedimentinterfaceasathermodynamicprocessthatcouplestheflowofwater heat and solutes inunfrozensubglacial sedimentsunderlyinga freezingicebase thecouplingof these flows occurs through the clapeyron equation which specifies the dependence of the basal freezing melting temperature on ice pressure water pressure solute concentration and surface tension effects thermally driven water flow is induced when an ice base becomes supercooled below the pressure melting point because ice water surface tension inhibits ice growth in small pore spaces of fine grained subglacial sediments our model results show that basal freeze on is capable of inducing considerable changes in the basal zone of both ice streams and interstream ridges these changes are associated with specific signatures that compare with borehole observations and geophysical surveys water pres sure levels are reduced and thick layers of debris laden basal ice develop these basal ice layers andunderlying sedimentscontain a distinct isotopic signal the predicted stable iso tope ratios reflect rayleigh type isotopic fractionation whose significance increases with increasing freezing rates supercooling of the ice base induces also measurable changes in the ice temperature profile of the glacier till porosity represents another quantity whose evolution is influenced strongly by basal freeze on in particular measurements of vertical porositydistributionbeneathstoppedicestreamscouldbeusedtoback calculatethetiming of the onsetof basal freezing our modelresults show thatthebasal zoneof ice streams and interstream ridges responds sensitively to changes inbasal melting freezing rates this sen sitivity may allow reconstruction of past conditions beneath ice streams and interstream ridges from measurements made on basal ice samples and subglacial sediment samples our model resultsalso indicate thatmeltwater from fast flowing ice streams may be driven towardsthe freezing icebase of interstream ridges

rockbed wear at a flow convergence zone in fifteen mile creek niagara peninsula ontario sculpted bedforms developed in rock channels must be considered within the context of temporally and spatially persistent flow dynamics a shallow meandering channel 14 m long 30 50 cm wide 3 4 cm deep with raised margins has been eroded into resistant irondequoit limestone forming the streambed of fifteen mile creek ontario potholes 18 cm deep 25 cm wide 35 cm long within the channel increase in size downstream subaerial solution of the limestone and fracture patterns within the irondequoit limestone are not responsible for this large dolostone blocks within the channel stabilize patterns of vorticity that form in the lee of the blocks during highflows when flow is about 1 3 m deep vortices migrating downstream along a major flow junction have eroded the distinctive channel small irregularities in the bed may enlarge to potholes by attracting vortices and undergoing erosional wear as they pass the feature may have developed within several hundred to several thousand years but likely d

wisconsin and holocene δ18o variations barnes ice cap canada new 18o measurements on ice samples from barnes ice cap show a relatively large shift from about 23 in the blue ice that makes up the bulk of the ice cap to 38 in a basal layer of clean white ice this strongly suggests that the latter is of pleistocene age thus confirming an earlier interpretation based on less comprehensive data the shift between the pleistocene and holocene ice is attributed in part to the change in climate at the end of the pleistocene and in part to a decrease in elevation of the accumulation area of 700 m beneath the clean white ice there is a layer of dirty white ice with less negative 18o values this ice is believed to have been deposited during a climatically warm period in early to middle wisconsin time in addition however the less negative 18o values in it are in part attributed to fractionation at a time in the past when the basal ice was at the melting point and some of the water produced by pressure melting was lost through a permeable bed

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

solute sources and geochemical processes in subglacial lake whillans west antarctica national science foundation nsf grants opp 0838933 1346250 and 1439774 italian national antarctic program barbante graduate fellowships from the nsf integrative graduate education and research traineeship program 0654336 and nsf center for dark energy biosphere investigations dissertation grant from the american association of university women

mechanisms of basal ice formation in polar glaciers an evaluation of the apron entrainment model 1 previous studies of polar glaciers have argued that basal ice can form when these glaciers override and entrain ice marginal aprons that accumulate adjacent to steep ice cliffs to test this idea we have studied the morphology structure composition and deformation of the apron and basal ice at the terminus of victoria upper glacier in the mcmurdo dry valleys which are located on the western coast of the ross sea at 77 si n southern victoria land antarctica our results show that the apron has two structural elements an inner element that consists of strongly foliated ice that has a steep up glacier dip and an outer element that lacks a consistent foliation and has a down glacier slope parallel dip although strain measurements show that the entire apron is deforming the inner element is characterized by high strain rates whereas relatively low rates of strain characterize the outer part of the apron co isotopic analyses of the ice together with analysis of solute chemistry and sedimentary characteristics show that the apron is compositionally different from the basal ice our observations show that aprons may become deformed and partially entrained by advancing glaciers however such an ice marginal process does not provide a satisfactory explanation for the origin of basal ice observed at the ice margin our interpretation of the origin of basal ice is that it is formed by subglacial processes which are likely to include deformation and entrainment of subglacial permafrost

the dynamics of ice sheet outlets a comparison of data from aircraft altimetry landsat imagery and radia echo sounding has shown characteristic surface topographies associated with sheet and stream flow the transition between the two is abrupt and occurs at a step in the subglacial topography this marks the onset of basal sliding and high velocities caused by subglacial water it results in crevassed amphitheatre like basins round the head of outlet glaciers it is also the zone of maximum driving stress beyond which values decline rapidly as velocities increase this abrupt transition appears to be topographically controlled since basal temperatures are at the pressure melting point well inland of the change in regime the marie byrd land ice streams exhibit qualitative differences from other ice sheet outlets however the change to lower driving stresses is much more gradual and occurs several hundred kilometres inland such ice streams have particularly low surface slopes and appear in form and flow regime to resemble confined ice shelves rather than grounded ice the repeated association of the transition to rapid sliding with a distinct subglacial feature implies a stabilizing effect on discharge through outlet glaciers acceleration of the ice is pinned to a subglacial step and propagation of high velocities inland of this feature seems improbable rapid ice flow through subglacial trenches may also ensure a relatively permanent trough through accentuation of the feature by erosion this is concentrated towards the heads of outlet glaciers up stream of the region where significant basal decoupling occurs this may be a mechanism for the overdeepening of fjords at their inland ends and the development of very steep fjord headwalls

high pressure behaviour of selected boron minerals and the question of boron distribution between fluids and rocks abstract experimental results in the simplest possible systems up to generally 20 50 kbar in a few cases 100 kbar bearing on the pt stability limits of 17 end member borosilicates and borates are summarized except for the sillimanite structure type minerals grandidierite and werdingite which break down near 10 kbar the following phases are stable up to much higher pressures and no influence of boron coordination with oxygen three or fourfold can be recognized alkali free dravite b nepheline boromuscovite danburite dravite dumortierite jeremejevite oh kotoite magnesiodumortierite prismatine pseudosinhalite reedmergnerite szaibelyite sinhalite suanite the framework borosilicates reedmergnerite and danburite remain more stable towards higher pressures than their al analogues albite and anorthite respectively kornerupine stability decreases with decreasing boron content in the mineral from about 37 kbar for near end member prismatine to 12 kbar for the boron free end member with a maximum thermal stability of 1050 c at 30 kbar boron rich kornerupine prismatine is the thermally most stable borosilicate at high pressures thus all the above 15 phases can in principle occur as minerals of ultrahigh pressure metamorphic rocks boron initially enriched in crustal rocks may be subducted to depths exceeding 100 km in al rich systems b 2 o 3 generally behaves as an inert component and enters refractory minerals so that such rocks may act as sinks for boron even in the presence of h 2 o in other systems rich sio 2 na 2 o mgo however boron is more volatile and may fractionate into hydrous fluids that may cause metasomatism elsewhere altogether the behaviour of boron in natural petrogenetic systems is strongly dependent on the local chemical environment

crystal chemistry and origin of grandidierite ominelite boralsilite and werdingite from the bory granulite massif czech republic a veinlet croscutting the host granulite foliation in the bory granulite massif contains mineral assemblage involving grandidierite ominelite boralsilite fe analogue of werdingite dumortierite locally sb ti rich tourmaline and corundum the assemblage formed at medium pressure and moderate temperature during the exhumation of the granulite body

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

minerals mapping of the lunar surface with clementine uvvis nir data based on spectra unmixing method and hapke model the distribution of minerals on the lunar surface is information which could contribute to studying lunar origin and evolution in this paper the distribution of clinopyroxene orthopyroxene olivine ilmenite and plagioclase on the lunar surface has been mapped based on hapke radiative transfer model and linear unmixing of spectra with clementine uvvis nir data the results have been validated on the basis of minerals modal abundance data of the apollo samples and problems in the minerals abundance mapping have been analyzed the validation based on analysis data of apollo samples indicates that plagioclase mapped in this paper represents the total abundance of plagioclase and agglutinitic glass the minerals mapping results show that the lunar surface is mainly composed of pyroxene plagioclase agglutinitic glass and ilmenite basalt in the lunar mare is mainly composed of clinopyroxene and ilmenite and lunar highland is mainly composed of plagioclase and agglutinitic glass orthopyroxene is mainly distributed on the north of mare imbrium on the south of maria and aitken basin according to our results there is probably no large area of olivine distribution on the lunar surface which is different from earlier published results therefore emphasis should be put on the olivine distribution in the minerals mapping using hyperspectral data such as m3 of chandrayaan 1 and iim of change 1

basal ice sequences in antarctic ice stream exposure of past hydrologic conditions and a principal mode of sediment transfer 1 the brightness distribution of sequentially extracted borehole camera imagery shows two distinct sequences of basal ice in kamb ice stream the upper sequence 7 3 m comprises clear ice and layers with dispersed and stratified debris the lower sequence 8 2 m consists of accretion ice with alternating layers of stratified and solid debris the two sequences have volumetric debris contents of about 5 and 20 respectively we infer that the upper sequence formed in a tributary where subglacial meltwater was abundant and that the lower sequence formed on the siple coast plain where basal freezing currently dominates the basal thermal regime the basal ice layer contains the equivalent of 2 1 0 4 m of frozen sediment the high volume of sediment is a result of debris rich layers interpreted to have formed by accretion in the stagnant phase of ice stream on off cycles fast ice streaming punctuated by entrainment of debris during stagnation episodes is consistent with inferred past flow of adjacent ice streams and geologic features in the ross sea our results show that sediment wedges near grounding lines may form by melt out of basal ice debris this is different from previous studies where it was assumed that sediment transfer occurs through sediment deformation within layers of subglacial till cumulative freezing in recurring ice stream cycles shows that entrainment of sediment by basal freeze on is an important erosion mechanism and that high sediment fluxes can occur when ice streams override coulomb plastic substrates

an occurrence of grandidierite kornerupine and tourmaline in southeastern ontario canada grandidierite kornerupine and tourmaline occur in high grade pelitic gneisses from southeastern ontario canada the kornerupine occurs in quartz bearing layers associated with biotite cordierite garnet ilmenite k feldspar magnetite quartz and less commonly sillimanite grandidierite is found in quartz poor cordierite sillimanite segregations in contact with biotite cordierite ilmenite k feldspar magnetite sillimanite and more rarely garnet tourmaline is sporadically distributed in all compositional layers but is not in contact with the other borosilicates there is no textural evidence for a reactive relationship among the three borosilicates neither chemical or textural equilibrium has been achieved on the scale of a thin section

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

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

magmatic hydrothermal evolution of rare metal pegmatites from the mesoproterozoic orange river pegmatite belt namaqualand south africa abstract the 450 km long orange river pegmatite belt intruded the namaqua sector of the mesoproterozoic namaqua natal province southern africa at ca 1 ga the western part of the belt is characterized by the occurrence of lct pegmatites locally mineralized in li ta nb be and bi most of these mineralized pegmatites display zonation with a quartz feldspar muscovite garnet beryl bearing aplite border and wall zone a quartz feldspar spodumene lepidolite bearing intermediate zone and a quartz k feldspar core the pegmatites and their granodioritic country rocks commonly show evidence of albitization and greisenization i e secondary muscovitization in this study detailed petrographic observations and the compositions of mica feldspar spodumene and nb ta oxide minerals are reported this information is combined with bulk country rock compositions and new u pb zircon and monazite ages plus sm nd isotope compositions of monazite to constrain the origin and magmatic hydrothermal evolution of weakly and strongly mineralized lct pegmatites the li ta be kokerboomrand i pegmatite emplaced at ca 985 ma during the main stage of pegmatite emplacement in the orange river belt and regional strike slip deformation it has end t monazite 14 3 and potentially formed by partial melting of paleoproterozoic rocks from the richtersveld magmatic arc alternatively mesoproterozoic sediments of the bushmanland subprovince may also provide a suitable magma source for the kokerboomrand i pegmatite the rare metal contents of muscovite and columbite group minerals cgm increase from the weakly mineralized to strongly mineralized pegmatites suggesting early stage fractional crystallization or variable partial melting conditions mica nb ta values of rare metal bearing minerals crystallized throughout the magmatic hydrothermal evolution of mineralized pegmatites during the magmatic dominated stage i constitutional zone refining likely induced a decrease of spodumene fe contents and increase of cgm ta contents toward the pegmatite core stage ii marks the pervasive circulation of an immiscible hydrosaline melt this stage is characterized by the replacement of the primary mineral assemblage by lamellar albite cleavelandite fluor calciomicrolite and cs ta rich li muscovite and zinnwaldite stage iii represents the progressive transition from a magmatic hydrothermal system dominated by a li cs ta rich hydrosaline melt toward a system mainly involving li cs ta bi rich aqueous fluids this stage is characterized by the crystallization of lepidolite and zero valence dominant pyrochlore as well as strong localized greisenization and bi mineralization magmatic hydrothermal alteration of the immediate country rock of the mineralized pegmatites is marked by the crystallization of albite li ta cs rich mica various nb ta oxide minerals and led to a strong decrease of the whole rock nb ta ratio plus significant rare metal enrichment our findings from the orange river belt demonstrate that an important role can be ascribed to melt melt and melt fluid immiscibility and metasomatism in the formation of pegmatite related rare metal deposits greisens and albitites associated with pegmatites and their immediate country rocks can be considered as favorable targets for li ta mineral exploration strong and pervasive metasomatism across the mineralized pegmatites and their country rocks suggests involvement of a parental medium that was highly enriched in water

debris entrainment at the ice bedrock interface in sub freezing temperature conditions terre adelie antarctica the debris rich ice from the bottom 6 m of the 82 m deep caroline coastal antarctic record of last interglacial natural environment ice core reaching bedrock and from five 2 m long surface cores at moraine prudhomme in terre adelie antarctica is described and compared to debris laden ice from the core drilling site d10 isotopic total gas content co2 concentration and sem investigations of embedded particles together with ice textures and fabrics rule our pressure melting regelation around bed obstacles or freezing on as possible mechanisms for the debris entrainment at the ice bedrock interface it is suggested that the debris entrapment by purely mechanical means e g shearing is an efficient process in forming basal ice layers bil at sub freezing temperatures from authors

the presence of thrust block naled after a major surge event kuannersuit glacier west greenland thrust block naled in front of kuannersuit glacier west greenland appears to have formed during the termination of a terrestrial surge event by a combination of enhanced winter runoff rapid advance of the glacier terminus and proglacial stress release by thrusting and stacking of naled blocks this process is equivalent to the formation of thrust block moraines the thrust block naled consists of at least seven thrust sheets which are characterized by stratified ice with beds composed of a lower debris rich lamina an intermediate dispersed lamina and a top clean ice lamina and underlain by frozen outwash deposits the thrust block naled differs from basal stratified ice in the absence of internal deformation structures a relatively low debris concentration a clay rich particle size distribution and a preferential sorting of lighter minerals the oxygen isotope composition of the thrust block naled is indistinguishable from d 18 o values from meteoric glacier ice and bulk meltwater but different from basal stratified ice facies the d dd relationship indicates that thrust block naled has been formed by freezing of successive thin layers of bulk waters with variable isotopic composition whereas basal stratified ice has developed in a subglacial environment with regelation this work shows that the association between proglacial naled and rapidly advancing glaciers may have significant consequences for the proglacial geomorphology and the interpretation of basal ice layers

basal ice facies a review and unifying approach abstract over the past 30 years numerous basal ice facies have been identified named and classified however the resulting facies descriptions and names are inconsistent and no single scheme encompasses all of the different ice types that exist at different ice masses in this paper we review and critique existing basal ice facies names descriptions and classification schemes and propose a new non genetic approach that has the capacity to name and describe all basal ice types we define six fundamental basal cryofacies and a further 12 composite cryofacies which can all be defined on the basis of a cursory evaluation of debris disposition and concentration more detailed cryofacies description is based on characterizing three sets of ice properties that can also be estimated visually in the field a the thickness of the basal ice facies and its constituent sub layers b the concentration and texture of debris included within any or all of those layers and c the concentration and size of bubbles included within any or all of those layers we also propose a shorthand method for the presentation of this descriptive information here codes for the layer thicknesses are presented as standard text and codes for the included debris characteristics and included bubble characteristics are presented as superscripts and subscripts respectively sub layers are characterized similarly but within a nested sequence of brackets we evaluate the effectiveness of these two schemes by using them to rename and reclassify several existing basal ice facies results indicate that the schemes are robust and that they provide a coherent non genetic framework for the effective naming and description of basal cryofacies

detecting biogeochemical activity in basal ice using fluorescence spectroscopy dissolved organic matter dom is an important component of aquatic carbon and nutrient budgets and is a metabolic substrate for organisms at the base of aquatic food chains active microbial communities in glaciers affect the abundance and characteristics of organic matter om that is exported to downstream ecosystems however how om is biogeochemically altered in glaciers remains unknown and studies documenting active microbial activity by detecting in situ biogeochemical modifications of om are lacking due to difficulties characterizing om and the low concentrations of dom typical of glacier environments to address this issue we measure the abundance and fluorescence characteristics of dom in basal ice at victoria upper glacier vug mcmurdo dry valleys antarctica we compare these observations with the results of microbial incubations from the same basal ice samples to determine whether the occurrence of fluorophores indicative of recent microbial activity is linked to the presence of culturable microbial communities containing organisms that could have produced them psychrotolerant bacteria were isolated from basal ice samples and were associated with marine humic like fluorescence this is interpreted as being indicative of in situ microbial degradation of om within basal ice at vug marine humic like material is a recalcitrant form of om and its biogeochemical transformation from a relatively labile form of om in glacier ice may function as a carbon sink

geochemistry and origin of cordierite orthoamphibole orthopyroxene phlogopite rocks from namaqualand south africa abstract a suite of cordierite orthoamphibole cordierite orthopyroxene and cordierite phlogopite gneisses are described from the amphibolite and granulite facies region of the western namaqualand metamorphic complex the gneisses are directly associated with underlying and overlying metapelitic rocks to which they show certain geochemical affinities using overall geochemical trends and comparisons between amphibolite and granulite facies samples particularly for na 2 o feo and tio 2 a residual origin by extraction of granite melt can be excluded and a premetamorphic origin proposed for the mg al rich bulk compositions the formation of these specific cordierite orthoamphibole orthopyroxene rocks by metamorphism of carbonate associated evaporite sequences is not supported by their field relationships or by their al fe rich character the bulk of the cordierite orthoamphibole orthopyroxene rocks have trace element contents that strongly resemble those of underlying mafic rocks an al rich group of cordierite phlogopite bearing rocks shows marked enrichment in zr ce and th suggesting residual concentration following weathering major element modelling of postulated premetamorphic constituents indicates that the majority of cordierite orthoamphibole orthopyroxene rocks comprise former mixtures of chlorite with lesser quartz and other minor clay components phlogopite rich rocks can be modelled as mixtures of chlorite illite and kaolinite the suite probably formed by weathering of mafic volcanic material and its local redeposition in an environment where relatively warm saline conditions favoured the development of chlorite rich assemblages

entrainment at cold glacier beds here we present measurements of the gas content and isotopic composition of debris rich basal layers of a polar glacier meserve glacier antarctica which has a basal temperature of 17 c these measurements show that debris entrainment has occurred without alteration of the glacial ice and provide the most direct evidence to date that active entrainment occurs at the beds of cold glaciers without bulk freezing of water entrainment at subfreezing temperatures may have formed the u shaped trough containing meserve glacier in addition to possibly allowing some cold based glaciers to be important geomorphic agents entrainment at subfreezing temperatures provides a general mechanism for formation of the dirty basal layers of polar glaciers and ice sheets which are rheologically distinct and can limit the time span of ice core analyses furthermore accumulating evidence suggests that geomorphologists should abandon the assumption that cold based glaciers do not slide and abrade their beds