flux of debris transported by ice at three alaskan tidewater glaciers the stability of tidewater terminus is controlled by glacial dynamics calving processes and sedimentary processes at the grounding line an investigation of grounding line sediment dynamics and morainal bank sediment budgets in glacier bay alaska u s a has yielded data that enable us to determine the debris fluxes of grand pacific margerie and muir glaciers debris flux ranges from 10 5 to 10 6 m 3 a 1 one to two orders of magnitude lower than the glacifluvial sediment fluxes 10 6 10 7 m 3 a 1 combined these fluxes represent the highest yields known for glacierized basins large debris fluxes reflect the combined effecrs of rapid glacier flow driven by the maritime climate of southeast alaska and highly erodible bedrock englacial debris distribution is affected by valley width and relief both of which control the availability of sediment the number of tributaries controls the distribution and volume of debris in englacial and supraglacial moraines at the terminus iceberg rafting removes up to two orders of magnitude more sediment from the ice proximal environment than is deposited by melt out or is dumped during calving events rough estimates of the sediment flux by deforming beds suggests that soft bed deformation may deliver up to an order of magnitude more sediment to the terminus than is released from within the glacier ice

origin of unusually thick heinrich layers in ice proximal regions of the northwest labrador sea ten piston cores and a 25 m long giant piston core from the ice proximal region off hudson strait contain heinrich layers 1 and 2 identified by their sedimentary structure and high detrital carbonate content both layers are unusually thick on the upper labrador slope 3 8 2 1 m and on the lower slope and rise 1 5 1 0 m heinrich layers 1 and 2 can be subdivided into three units unit a is restricted to the upper slope and overlies hemipelagic sediment with a gradational boundary it is 6 1 m thick dark black to dark gray in color and made up of coarse ice rafted sand and granules dispersed in hemipelagic mud with a few faint laminations towards the top the overlying unit b is up to 3 35 m thick and consists of two facies i centimeter thick graded carbonate rich mud layers containing dispersed coarser grains in the mud interpreted as nepheloid flow deposits with coarse ice rafted debris and ii carbonate rich finely laminated mud layers which alternate with millimeter thick laminae of ice rafted debris unit c up to 0 90 m thick is devoid of sedimentary structures and consists of hemipelagic sediment with dispersed dropstones that increase in abundance towards the top of the unit data suggest the following sequence of processes could have occurred during the deposition of heinrich layers in ice proximal sites as the laurentide ice sheet grew it extended through the hudson strait ice stream outlet to a floating ice margin near the shelf edge and perhaps beyond to the upper slope and deposited unit a by releasing dropstones from the basal debris rich layer unit b is interpreted to be the deposits of the combined processes of nepheloid layer flow low density turbidity currents and massive ice rafting maximum carbonate content s 50 the lightest n 18 o values in very sparse planktonic foraminifera and low magnetic susceptibility are characteristic of unit b unit c is inferred to represent the time of waning supply of fines as nepheloid flow deposition ceased while the relative concentration of dropstones increased as a result of continuing intense ice rafting due to the reduction of fine grained sediment supply the carbonate concentration decreased to the labrador sea background level this finding was further supported by the appearance of darker colored sediments and heavier n 18 o values in neogloboquadrina pachyderma s which indicates the reduction of meltwater discharge

stabilizing feedbacks in glacier bed erosion glaciers often erode transport and deposit sediment much more rapidly than nonglacial environments1 with implications for the evolution of glaciated mountain belts and their associated sedimentary basins but modelling such glacial processes is difficult partly because stabilizing feedbacks similar to those operating in rivers2 3 have not been identified for glacial landscapes here we combine new and existing data of glacier morphology and the processes governing glacier evolution from diverse settings to reveal such stabilizing feedbacks we find that the long profiles of beds of highly erosive glaciers tend towards steady state angles opposed to and slightly more than 50 per cent steeper than the overlying ice air surface slopes and that additional subglacial deepening must be enabled by non glacial processes climatic or glaciological perturbations of the ice air surface slope can have large transient effects on glaciofluvial sediment flux and apparent glacial erosion rate

hydrological characteristics of the drainage system beneath a surging glacier a rare combination of natural circumstances permits assessment of current theories on water flow beneath glaciers outburst floods from the subglacial lake grimsvotn in iceland took place before during and after surging of skei ararjokull the glacier beneath which the outburst floods drain the observable drainage patterns associated with these floods show the different nature of the basal water conduit system of the glacier during surge and non surge phases during surge conditions basal water is dispersed slowly across the bed in a distributed drainage system but when the glacier is not surging water is transported rapidly through a system of tunnels

hudson bay hudson strait jokulhlaups and heinrich events a hypothesis abstract abrupt temperature changes in the northern north atlantic occurred frequently throughout the last glaciation as shown by proxy records from greenland ice cores deepsea cores and norway speleothems many of these variations occurred in irregular sawtooth cooling cycles which sometimes ended with the deposition of thick layers of ice rafted debris ird in the deep ocean that are known as heinrich events the lithologies of most of the ird deposits are consistent with widespread surges of the laurentide ice sheet that may have resulted from the accumulation of deformable sediments in portions of the ice sheet bed an alternative conceptual model proposed here to explain the sawtooth coolings and the surges involves repetitive jokulhlaups from a hudson bay lake dammed by ice at the mouth of hudson strait the slow sawtooth coolings may be explained by storm track diversion due to progressive coverage of the lake by icebergs and the surges by abrupt losses of water pressure and buttressing ice shelves at all ice sheet fronts in the lake when the ice dams failed

post stagnation behavior in the upstream regions of ice stream c west antarctica the region where two active tributaries feed into the now stagnant ice stream c isc west antarctica is thickening in this region we observe a correlation between faster ice flow the tributaries and elevated topography we conclude that stagnation of isc resulted in compression and thickening along the tributaries eventually forming a bulge on the ice sheet surface modern hydraulic potential gradients would divert basal meltwater from isc to ice stream b isb these gradients are primarily controlled by the bulge topography and so likely formed subsequent to trunk stagnation as such we argue against water piracy as being the cause for isc s stagnation kinematic wave theory suggests that thickness perturbations propagate downstream over time but that kinematic wave speed decreases near the stagnant trunk this and modest diffusion rates combine to trap most of the tributary fed ice in the bulge region using interferometric synthetic aperture radar velocity measurements we observe that half of the ice within isc s southern tributary flows into isb that flow pattern and other observations of non steady flow in the region likely result from stagnation induced thickening along upper isc combined with a longer period of thinning on upper isb if current trends in thickness change continue more ice from upper isc will be diverted to isb

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

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

preliminary results of tritium analyses in basal ice matanuska glacier alaska u s a evidence for subglacial ice accretion

breakup and conditions for stability of the northern larsen ice shelf antarctica the breakup of ice shelves has been widely regarded as an indicator of climate change1 with observations around the antarctic peninsula having shown a pattern of gradual retreat associated with regional atmospheric warming and increased summer melt and fracturing processes2 3 4 5 6 7 8 9 the rapid collapse of the northernmost section of the larsen ice shelf larsen a over a few days in january 1995 indicated that after retreat beyond a critical limit ice shelves may disintegrate rapidly here we use a finite element numerical model that treats ice as a continuum without fracture10 to examine the breakup history2 between 1986 and 1997 of the two northern sections of larsen ice shelf larsen a and larsen b from which we establish stability criteria for ice shelves analysis of various ice shelf configurations reveals characteristic patterns in the strain rates near the ice front which we use to describe the stability of the ice shelf on larsen a only the initial and final ice front configurations show a stable pattern larsen b at present exhibits a stable pattern but if the ice front were to retreat by a further few kilometres it too is likely to enter an irreversible retreat phase

binge purge oscillations of the laurentide ice sheet as a cause of the north atlantic s heinrich events ice rafted debris in sediment cores from the north atlantic suggests that the laurentide ice sheet lis periodically disgorged icebergs in brief but violent episodes which occurred approximately every 7 000 years here i propose that heinrich events i e what these episodes are called were caused by free oscillations in the flow of the laurentide ice sheet which arose because the floor of hudson bay and hudson strait is covered with soft unconsolidated sediment that forms a slippery lubricant when thawed the proposed heinrich event cycle has two phases the growth phase occurs when the sediment is frozen and the lis is stranded immobile on a rigid bed the volume of the lis slowly grows during this phase at a rate dictated by snow accumulation the purge phase occurs when the basal sediment thaws and a basally lubricated discharge pathway i e an ice stream such as those which occur in west antarctica today developes through hudson strait the volume of the lis rapidly equilibrates to the reduced basal friction during this phase by dumping icebergs into the labrador sea the periodicity t k sl2g 2 7000 years of the proposed heinrich event cycle is a function of the thermal conductivity and diffusivity of ice k and respectively the atmospheric sea level temperature sl in degrees celsius and the excess geothermal heat flux defined by g g k where is the atmospheric lapse rate and g is the geothermal heat flux agreement between the predicted t and the apparent periodicity implied by the marine record is the main virtue of the free oscillation mechanism i propose an alternative mechanism in which heinrich events are forced by periodic variations in external climate is implausible because periodic atmospheric temperature perturbations are strongly attenuated with depth in an ice sheet

sedimentologic evidence for outburst floods from the laurentide ice sheet margin in wisconsin usa implications for tunnel channel formation abstract clast supported boulder gravel in outwash fans along the glacial maximum margin of the laurentide ice sheet in wisconsin indicates the occurrence of outburst floods these sediments with clast intermediate axes of up to 2 m are located downstream of tunnel channels and were deposited shortly before cessation of glaciofluvial activity on each fan since tunnel channels with fans are widespread along the ice sheet margin in the western great lakes region these outburst floods were probably common paleodischarge estimates derived from the boulder deposits are poorly constrained but values of at least several hundred m3 s 1 are likely four potential water sources for the floods exist an extreme surface melt event an extreme precipitation event drainage of supraglacial lakes or drainage of stored subglacial meltwater we focus on the storage of subglacial meltwater behind the ice sheet margin as proglacial permafrost was present as ice advanced to its maximum extent and a frozen bed zone upstream from the margin probably impeded drainage through groundwater aquifers decay of this permafrost seal would have eventually allowed trapped water to drain through the tunnel channels we suggest that the 2 m boulders were entrained in an outburst of subglacial water that enlarged a pre existing channel cut by ablation derived flows

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

the morphology of ice streams a b and c west antarctica and their environs airborne radar soundings of the ice sheet surface made in 1984 1985 together with elevations measured by oversnow traverses between 1957 and 1964 have been used to produce a surface elevation map of ice streams a b and c and much of the region around them the flight positions and elevations were tied to many ground control stations whose positions in three dimensions were fixed by satellite tracking ice thicknesses were also measured along the flight lines but have not yet been mapped the surfaces of active ice streams a and b exhibit a longitudinal ridgetrough topography of uncertain origin prominent surface valleys are associated with most but not all of their marginal shear zones there is a deep subglacial trough beneath the grid northeastern side of ice stream a that connects to the subglacial trough beneath reedy glacier the boundary between ice streams bl and b2 the two tributaries of ice stream b persists as a suture zone for 250 km downstream from their merger ice stream bl overlies a relatively deep subglacial trough that continued under the corresponding part of ice stream b until it disappears close to the grounding line between ice streams bl and b2 there is a complex zone containing several regions of undisturbed ice separated by bands of disturbed ice which suggest to us that rafts of ice are being incorporated into the ice streams the previously reported negative mass balance for the drainage system of ice stream b may arise from the expansion of the ice stream into the interior of the west antarctic ice sheet crevasse bands at some places on ridge ab together with high glacial velocities reported elsewhere also suggest this expansion close to the ross ice shelf ice stream b widens into a nearly flat area where the mean surface slope is only 3 5 10 4 and the surface elevations are only a few tens of meters above hydrostatic equilibrium inactive ice stream c differs from active ice streams a and b in surficial and basal characteristics no elongated ridges and troughs are observed instead the ice stream surface exhibits several terraces including some maxima and minima in elevation radar sounding reveals areas where basal echoes are strong and steady indicating subglacial water alternating with areas of weaker echoes with short fading lengths calculations indicate that the strong reflections require a water layer at least several centimeters to several meters thick depending on the electrical conductivity of the water due to reversals in the surface slope the hydrologic potential gradient changes sign several times along ice stream c which implies that subglacial water should collect in some locations

glacier surge propagation by thermal evolution at the bed bakaninbreen southern svalbard began a prolonged surge during 1985 in 1986 an internal reflecting horizon on radio echo sounding data was interpreted to show that the position of the surge front coincided with a transition between areas of warm unfrozen and cold frozen bed ground penetrating radar lines run in 1996 and 1998 during early quiescence show that the basal region of the glacier is characterized by a strong reflection interpreted as the top of a thick layer of sediment rich basal ice down glacier of the present surge front features imaged beneath the basal reflection are interpreted as the bottom of the basal ice layer the base of a permafrost layer and local ice lenses this indicates that this region of the bed is cold up glacier of the surge front a scattering zone above the basal reflection is interpreted as warm ice there is no evidence for this warm zone down glacier of the surge front nor do we see basal permafrost up glacier of it thus as in early surge phase the location of the surge front is now at the transition between warm and cold ice at the glacier bed we suggest that the propagation of the front is associated with this basal thermal transition throughout the surge because propagation of the front occurs rapidly and generates only limited heat basal motion during fast flow must have been restricted to a thin layer at the bed and occurred by sliding or deformation localized at the ice bed interface

regional reconstruction of subglacial hydrology and glaciodynamic behaviour along the southern margin of the cordilleran ice sheet in british columbia canada and northern washington state usa abstract subglacial landsystems in and around okanagan valley british columbia canada are investigated in order to evaluate landscape development subglacial hydrology and cordilleran ice sheet dynamics along its southern margin major landscape elements include drumlin swarms and tunnel valleys drumlins are composed of bedrock diamicton and glaciofluvial sediments their form truncates the substrate tunnel valleys of various scales km to 100s km length incised into bedrock and sediment exhibit convex longitudinal profiles and truncate drumlin swarms okanagan valley is the largest tunnel valley in the area and is eroded 300 m below sea level over 600 m of late wisconsin age sediments consisting of a fining up sequence of cobble gravel sand and silt fill okanagan valley landform substrate relationships landform associations and sedimentary sequences are incompatible with prevailing explanations of landsystem development centred mainly on deforming beds they are best explained by meltwater erosion and deposition during ice sheet underbursts during the late wisconsin glaciation okanagan valley functioned as part of a subglacial lake spanning multiple connected valleys few 100s km of southern british columbia subglacial lake development started either as glaciers advanced over a pre existing sub aerial lake catch lake or by incremental production and storage of basal meltwater high geothermal heat flux geothermal springs and or subglacial volcanic eruptions contributed to ice melt and may have triggered along with priming from supraglacial lakes subglacial lake drainage during the underburst s sheetflows eroded drumlins in corridors and channelized flows eroded tunnel valleys progressive flow channelization focused flows toward major bedrock valleys in okanagan valley most of the pre glacial and early glacial sediment fill was removed a fining up sequence of boulder gravel and sand was deposited during waning stages of the underburst s and bedrock drumlins in okanagan valley were enhanced or wholly formed by this underburst s subglacial lake development and drainage had an impact on ice sheet geometry and ice volumes the prevailing conceptual model for growth and decay of the cis suggests significantly thicker ice in valleys compared to plateaus subglacial lake development created a reversal of this ice sheet geometry where grounded ice on plateaus thickened while floating valley ice remained thinner due to melting and enhanced sliding with significant transfer of ice toward the ice sheet margin subglacial lake drainage may have hastened deglaciation by melting ice lowering ice surface elevations and causing lid fracture this paper highlights the importance of ice sheet hydrology its control on ice flow dynamics distribution and volume in continental ice masses

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

ice cover landscape setting and geological framework of lake vostok east antarctica abstract lake vostok located beneath more than 4 km of ice in the middle of east antarctica is a unique subglacial habitat and may contain microorganisms with distinct adaptations to such an extreme environment melting and freezing at the base of the ice sheet which slowly flows across the lake controls the flux of water biota and sediment particles through the lake the influx of thermal energy however is limited to contributions from below thus the geological origin of lake vostok is a critical boundary condition for the subglacial ecosystem we present the first comprehensive maps of ice surface ice thickness and subglacial topography around lake vostok the ice flow across the lake and the landscape setting are closely linked to the geological origin of lake vostok our data show that lake vostok is located along a major geological boundary magnetic and gravity data are distinct east and west of the lake as is the roughness of the subglacial topography the physiographic setting of the lake has important consequences for the ice flow and thus the melting and freezing pattern and the lake s circulation lake vostok is a tectonically controlled subglacial lake the tectonic processes provided the space for a unique habitat and recent minor tectonic activity could have the potential to introduce small but significant amounts of thermal energy into the lake

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

sub catchment melt and long term stability of ice stream d west antarctica 1 the apparent long term persistence and short term variability of the siple coast ice streams of west antarctica are tied to regional thermal conditions and local basal lubrication numerical simulations indicate that the flux of latent heat in a throughgoing hydrologic system fed by melt beneath thick inland ice maintains the lubrication of fast moving modern and ice age ice streams despite their tendency to freeze to the bed and would allow additional thinning and grounding line retreat

evidence for iceberg armadas from east antarctica in the southern ocean during the late miocene and early pliocene abstract sediments from ocean drilling program site 1165 in the indian ocean sector of the southern ocean off prydz bay contain a series of layers that are rich in ice rafted debris ird here we present evidence that ird rich layers at site 1165 at 7 4 8 and 3 5 ma record short lived massive discharges of icebergs from wilkes land and adelie land more than 1500 kilometers to the east of the depositional site this distant source of icebergs is clearly defined by the presence of ird hornblende grains with 40ar 39ar ages of 1200 1100 ma and 1550 1500 ma ages that are not found on the east antarctic continent in locations closer to site 1165 this observation requires enormous amounts of detritus carrying drifting icebergs most likely in the form of large icebergs these events probably reflect destabilization surge and break up of ice streams on the wilkes land and adelie land margins of the east antarctic ice sheet in the vicinity of the low lying aurora and wilkes basins they occurred under warming conditions but each coast seems to have produced ice rafting events independently at different times the data presented here constitute the first evidence of far traveled icebergs from specific source areas around the east antarctic perimeter launch of these icebergs may have happened during quite dramatic events perhaps analogous to heinrich events in the north atlantic

morphometry of bedrock meltwater channels on antarctic inner continental shelves implications for channel development and subglacial hydrology abstract expanding multibeam bathymetric data coverage over the last two decades has revealed extensive networks of submarine channels incised into bedrock on the antarctic inner continental shelf the large dimensions and prevalence of the channels implies the presence of an active subglacial hydrological system beneath the past antarctic ice sheet which we can use to learn more about inaccessible subglacial processes here we map and analyse over 2700 bedrock channels situated across 100 000 km2 of continental shelf in the western antarctic peninsula and amundsen sea to produce the first inventory of submarine channels on the antarctic inner continental shelf morphometric analysis reveals highly similar distributions of channel widths depths cross sectional areas and geometric properties with subtle differences between channels in the western antarctic peninsula compared to those in the amundsen sea at 75 3400 m wide 3 280 m deep 160 290 000 m2 in cross sectional area and typically 8 times as wide as they are deep the channels have similar morphologies to tunnel valleys and meltwater channel systems observed from other formerly glaciated landscapes despite differences in substrate geology and glaciological regime we propose that the antarctic bedrock channels formed over multiple glacial cycles through the episodic drainage of at least 59 former subglacial lakes identified on the inner continental shelf

climate swings come into focus the last glacial period was far from quiet during so called heinrich events large armadas of icebergs were shed from the ice sheet that covered much of north america the tracks of debris left by the melting icebergs can still be seen in sediment cores from the north atlantic in their perspective broecker and hemming 1 report from a recent miniconference that attempted to chart the climatic impacts of these events 1 http www sciencemag org cgi content short 294 5550 2308

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

subglacial thermal balance permits ongoing grounding line retreat along the siple coast of west antarctica changes in the discharge of west antarctic ice streams are of potential concern with respect to global sea level the six relatively thin fast flowing ross ice streams are of interest as low slope end members among antarctic ice streams extensive research has demonstrated that these rivers of ice have a history of relatively high frequency asynchronous discharge variations with evolving lateral boundaries amidst this variability a 1300 km grounding line retreat has occurred since the last glacial maximum numerical studies of ice stream d parizek and others 2002 indicate that a proposed thermal regulation mechanism clarke and marshall 1998 hulbe and macayeal 1999 tulaczyk and others 2000a b which could buffer the west antarctic ice sheet against complete collapse may be over ridden by latent heat transport within melt water from beneath inland ice extending these studies to ice stream a whillans ice stream and ice stream c suggests that further grounding line retreat contributing to sea level rise is possible thermodynamically however the efficiency of basal water distribution may be a constraint on the system because local thermal deficits promote basal freeze on especially on topographic highs observed short term variability is likely to persist

depositional facies of late pleistocene heinrich events in the labrador sea late pleistocene heinrich ice rafting events produced layers rich in ice rafted debris in major parts of the north atlantic north of 40 n a high detrital carbonate content points to the hudson strait outlet of the laurentide ice sheet as a dominant source of the icebergs heinrich events were coupled with short term climate fluctuations during the last and penultimate glaciations and provide evidence for cryosphere hydrosphere atmosphere interaction in pleistocene climate change an unsolved problem with heinrich layers has been their high concentration of fine grained detrital carbonate 80 of the total detrital carbonate which cannot have been delivered by icebergs alone we propose combinations of different processes that deposited four sedimentologically different types of heinrich layers ice rafting alone for the coarser sand to gravel sized fractions and the fine fractions in distal regions type iv heinrich layers whereas nepheloid flows deposited the bulk of the fine sediment in regions proximal to the hudson strait type i heinrich layers on the labrador slope turbidity currents spilling over from canyons were also involved in transporting the fine grained carbonate rich material causing an alternation of mud turbidites and thin laminae of ice rafted debris in type ii heinrich layers on the levees of the northwest atlantic mid ocean channel the thickness relationship is reversed mud turbidites deposited by occasional spillover of currents from the channel are thin and alternate with thicker laminae of ice rafted debris type iii heinrich layers

response of subglacial sediments to basal freeze on 2 application in numerical modeling of the recent stoppage of ice stream c west antarctica 1 ross ice streams supply over 90 of the ice volume flowing out of the ross sector of the west antarctic ice sheet wais stoppage of ice stream c isc ca 150 years ago appears to have pushed this sector of wais from negative into positive mass balance joughin and tulaczyk 2002 we propose an explanation for the unsteady behavior of isc using a new numerical ice stream model which includes an explicit treatment of a subglacial till layer when constrained by initial conditions emulating prestoppage geometry dynamics and mass balance of isc the model yields a rapid 100 years stoppage of the main ice stream trunk the stoppage is triggered by basal freeze on which consolidates and strengthens the subglacial till our numerical simulations produce results consistent with a number of existing observations for example continuing activity of the two tributaries of isc the model always yields rapid stoppage unless we specify ice stream width that is smaller than its prestoppage values maximum of 80 km we conjecture that if isc was active for at least a few thousand years before slowdown its width was significantly smaller than today to sustain the long active phase ice stream width is a key control that helps determine whether ice stream flow is sustainable over a long term our work indicates that the recent stoppage of ice stream c could have been part of inherent ice stream cyclicity and it leaves open the possibility that other active ice streams may evolve in the future toward rapid shutdowns

a numerical investigation of ice lobe permafrost interaction around the southern laurentide ice sheet permafrost existed around and under marginal parts of the southern laurentide ice sheet during the last glacial maximum the presence of permafrost was important in determining the extent form and dynamics of ice lobes and the landforms they produced because of influences on resistance to basal motion and subglacial hydrology we develop a two dimensional time dependent model of permafrost and glacier ice dynamics along a flowline to examine i the extent to which permafrost survives under an advancing ice lobe and how it influences landform development and hydrology and ii the influence of permafrost on ice motion and surface profile the model is applied to the green bay lobe which terminated near madison wisconsin during the last glacial maximum simulations of ice advance over permafrost indicate that the bed upstream of the ice sheet margin was frozen for 60 200 km at the glacial maximum permafrost remained for centuries to a few thousand years under advancing ice and penetrated sufficiently deep tens of meters into the underlying aquifer that drainage of basal meltwater became inefficient likely resulting in water storage beneath the glacier our results highlight the influence of permafrost on subglacial conditions even though uncertainties in boundary conditions such as climate exist

ice fracturing during jokulhlaups implications for englacial floodwater routing and outlet development theoretical studies of glacial outburst floods jokulhlaups assume that i intraglacial floodwater is transported efficiently in isolated conduits ii intraglacial conduit enlargement operates proportionally to increasing discharge iii floodwater exits glaciers through pre existing ice marginal outlets and iv the morphology and positioning of outlets remains fixed during flooding direct field observations together with historical jokulhlaup accounts confirm that these theoretical assumptions are not always correct this paper presents new evidence for spatial and temporal changes in intraglacial floodwater routing during jokulhlaups secondly it identifies and explains the mechanisms controlling the position and morphology of supraglacial jokulhlaup outlets and finally it presents a conceptual model of the controls on supraglacial outbursts field observations are presented from two icelandic glaciers skei ararjokull and solheimajokull video footage and aerial photographs taken before during and after the skei ararjokull jokulhlaup and immediately after the solheimajokull jokulhlaup reveal changes in floodwater routing and the positioning and morphology of outlets field observations confirm that glaciers cannot transmit floodwater as efficiently as previously assumed rapid increases in jokulhlaup discharge generate basal hydraulic pressures in excess of ice overburden under these circumstances floodwater can be forced through the surface of glaciers leading to the development of a range of supraglacial outlets the rate of increase in hydraulic pressure strongly influences the type of supraglacial outlet that can develop steady increases in basal hydraulic pressure can retro feed pre existing englacial drainage whereas transient increases in pressure can generate hydraulic fracturing the position and morphology of supraglacial outlets provide important controls on the spatial and temporal impact of flooding the development of supraglacial jokulhlaup outlets provides a new mechanism for rapid englacial debris entrainment

the deglaciation of the northern hemisphere a global perspective orbitally induced increase in northern summer insolation after growth of a large ice sheet triggered deglaciation and associated global warming ice albedo sealevel and greenhouse gas feedbacks together with tropical warming from weakening winds in response to polar amplification of warming caused regionalto global near synchronization of deglaciation effects were larger at orbital rather than millennial frequencies because ice sheets and carbon dioxide vary slowly ice sheet linked changes in freshwater delivery to the north atlantic and possibly free oscillations in the climate system forced millennial climate oscillations associated with changes in north atlantic deep water nadw flow the north atlantic typically operates in one of three modes modern glacial and heinrich deglaciation occurred from a glacial mode ocean that in comparison to modern had shallower depth of penetration of nadw formed further south causing strong northern cooling and the widespread cold dry and windy conditions associated with the glacial maximum and the cold phases of the millennial dansgaard oeschger oscillations the glacial mode was punctuated by meltwater forced heinrich conditions that caused only small additional cooling at high northern latitudes but greatly reduced the formation of nadw and triggered an oceanic seesaw that warmed some high latitude southern regions centered in the south atlantic

breaking the seal at grimsvotn iceland of several problems associa ted wit h theoretica l expla nation s of the jokul hla ups which emerge from the outlet gla cie r skeioarar jokull of the ice ca p vatna jokull in sou theast iceland the mechani sm of flood init iation is one that has hi therto defied expl anat ion we provide such an expl ana tion based on a car efu l analysis of the cla ssica l nye ro thlisbe rger model near the subglacial lake grimsvotn the hyd raulic potenti al gradie nt is towards the lake and there is theref ore a catchme nt boun dary under the ice whose location dep ends on the subglacial mel twater drai nage charac terist ics as the con dit ions for a flood approa ch we show that the wat er divi de migra tes towards the lake while at the sa me time the lake pre ssure in creases when the hydra ulic potential gradient to wards the lake is low and the refillin g rate is sl ow the se al will break whe n the cat chment boun dary reaches the lake while the la ke level is st ill below flotati on pressu re whereas if refillin g is rapid flo tation can be achie ved bef ore a flood is initia ted this theory can thus explain why the seal is normally broken when the lake le vel at grimsvotn is still some 60 m below flotation level in addit ion we are abl e to explain why thej okulhla up following the 1996 eruption did not occur until flo tation level was achie ved and we show how the cyeli cit y and magnitude ofjoku lhla ups can be explained wit hin this theory

rapid bottom melting widespread near antarctic ice sheet grounding lines as continental ice from antarctica reaches the grounding line and begins to float its underside melts into the ocean results obtained with satellite radar interferometry reveal that bottom melt rates experienced by large outlet glaciers near their grounding lines are far higher than generally assumed the melting rate is positively correlated with thermal forcing increasing by 1 meter per year for each 0 1 c rise in ocean temperature where deep water has direct access to grounding lines glaciers and ice shelves are vulnerable to ongoing increases in ocean temperature

numerical modelling of ice sheet dynamics across the vostok subglacial lake central east antarctica a numerical model of the ice sheet ice shelf transformation was used to investigate ice sheet dynamics across the large subglacial lake beneath vostok station central east antarctica european remote sensing satellite ers 1 altimetry of the ice surface and 60 mhz radio echo sounding res of the ice sheet base and internal ice sheet layering were used to develop a conceptual flowline across the ice sheet which the model used as input the model calculates horizontal and vertical velocities and stresses from which particle flow paths can be obtained and the ice sheet temperature distribution an inverse approach to modelling was adopted where particle flow paths were forced to match those identified from internal res layering results show that ice dynamics across the inflow grounding line are similar to an ice sheet ice shelf transition model particle flow paths match internal res layering when ice is a taken away from the ice base across the first 2 km of the flowline over the lake and b added to the base across the remainder of the lake we contend that the process causing this transfer of ice is likely to be melting of ice and freezing of water at the ice water interface other explanations such as enhanced rates of accumulation over the grounding line of three dimensional convergent divergent flow of ice are inconsistent with available measurements such melting and refreezing would be responsible for circulation and mixing of at least the surface layers of the lake water our model suggests that several tens of metres of refrozen basal ice would accrete from lake water to the ice sheet before the ice regrounds

observations of the dispersion characteristics and meridional sea level structure of equatorial waves in the pacific ocean spectral techniques applied to altimetry data are used to examine the dispersion relation and meridional sea level structure of wavelike variability with periods of about 20 to 200 days in the equatorial pacific ocean zonal wavenumber frequency power spectra of sea surface height when averaged over about 7 s 7 n exhibit spectral peaks near the theoretical dispersion curves of first baroclinic mode equatorial kelvin and rossby waves there are distinct statistically significant ridges of power near the first and second meridional mode rossby wave dispersion curves sea level variability near the theoretical kelvin wave and first meridional mode rossby wave dispersion curves is dominantly but not perfectly symmetric about the equator while variability near the theoretical second meridional mode rossby wave dispersion curve is dominantly antisymmetric these results are qualitatively consistent with expectations from classical or shear modified theories of equatorial waves the meridional structures of these modes resemble the meridional modes of equatorial wave theory but there are some robust features of the meridional profiles that were not anticipated the meridional sea level structure in the intraseasonal kelvin wave band closely resembles the theoretically expected gaussian profile but sea level variability coherent with that at the equator is detected as far away as 11 75 s possibly as a result of the forced nature of these kelvin waves both first and second meridional mode rossby waves have larger amplitude in the northern hemisphere the meridional sea level structure of tropical instability waves closely resembles that predicted by lyman et al using a model linearized about a realistic equatorial zonal current system

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

water sheet outburst floods from the laurentide ice sheet field evidence and a theoretical model support the concept that during wisconsinan glaciation subglacial water sheet outburst floods issued from a large subglacial lake located in the hudson bay ba

changes in jokulhlaup sizes in grimsvotn vatnajokull iceland 1934 91 deduced from in situ measurements of subglacial lake volume a record of volumes of jokulhlaups from the subglacial grimsvotn lake vatnajokull iceland has been derived for the period 1934 91 the change in lake volume during jokulhlaups is estimated from the lake area ice cover thickness and the drop in lake level the jokulhlaup volumes have decreased gradually during this period of low volcanic activity and declining geothermal power the two jokulhlaups in the 1930s each discharged about 4 5 km peak discharge 25 30 10 3 m 3 s 1 in the 1980s jokulhlaup volumes were 0 6 1 2 km 3 peak discharge 2 10 3 m 3 s 1 the lake level required to trigger a jokulhlaup has risen as an ice dam east of the lake has thickened water flow in a jokulhlaup ceases when the base of a floating ice shelf covering grimsvotn settles to about 1160 m a s l apparently the jokulhlaups are cut off when the base of the ice shelf collapses on to a subglacial ridge bordering the lake on its eastern side the decline in melting rates has resulted in a positive mass balance of the 160 170 km grimsvotn ice drainage basin comparison of maps shows that the average positive mass balance rate was 0 12 km 3 a 1 25 of the total accumulation in the period 1946 87 a gradually increasing positive mass balance has prevailed since 1954 reaching 0 23 km 3 a 1 in 1976 86 48 of total accumulation

tsivat basin conduit system persists through two surges bering piedmont glacier alaska the 1993 1995 surge of bering glacier alaska occurred in two distinct phases phase 1 of the surge began on the eastern sector in july 1993 and ended in july 1994 after a powerful outburst of subglacial meltwater into tsivat lake basin on the north side of weeping peat island within days jokulhlaup discharge built a 1 5 km 2 delta of ice blocks 25 30 m buried in outwash by late october 1994 discharge temporarily shifted to a vent on weeping peat island where a second smaller outburst dissected the island and built two new sandar during phase 2 which began in spring 1995 and ended within five months continuous discharge issued from several vents along the ice front on weeping peat island before returning to the tsivat basin surge related changes include a five to six fold increase in meltwater turbidity the redirection of supercooled water in two ice contact lakes and an increase in the rate of glaciolacustrine sedimentation u s geological survey aerial photos by austin post show large ice blocks in braided channels indicating excessive subglacial discharge in a similar position adjacent to weeping peat island during the 1966 1967 surge during the subsequent three decades of retreat the location of ice marginal subglacial discharge vents remained aligned on a linear trend that describes the position of a persistent subglacial conduit system the presence of a major conduit system possibly stabilized by subglacial bedrock topography is suggested by 1 high level subglacial meltwater venting along the northern side of weeping peat island during the 1966 1967 surge 2 persistent low level discharge between surges and 3 the recurrence of localizing meltwater outbursts associated with both phases of the 1993 1995 surge

outburst floods from glacier dammed lakes the effect of mode of lake drainage on flood magnitude published accounts of outburst floods from glacier dammed lakes show that a significant number of such floods are associated not with drainage through a tunnel incised into the basal ice the process generally assumed but rather with ice marginal drainage mechanical failure of part of the ice dam or both non tunnel floods are strongly correlated with formation of an ice dam by a glacier advancing from a tributary drainage into either a main river valley or a pre existing body of water lake or fiord for a given lake volume non tunnel floods tend to have significantly higher peak discharges than tunnel drainage floods statistical analysis of data for floods associated with subglacial tunnels yields the following empirical relation between lake volume v and peak discharge qp qp 46v0 66 r2 0 70 when qp is expressed in metres per second and v in millions of cubic metres this updates the so called clague mathews relation for non tunnel floods the analogous relation is qp 1100v0 44 r2 0 58 the latter relation is close to one found by costa 1988 for failure of constructed earthen dams this closeness is probably not coincidental but rather reflects similarities in modes of dam failure and lake drainage we develop a simple physical model of the breach widening process for non tunnel floods assuming that 1 the rate of breach widening is controlled by melting of the ice 2 outflow from the lake is regulated by the hydraulic condition of critical flow where water enters the breach and 3 the effect of lake temperature may be dealt with as done by clarke 1982 calculations based on the model simulate quite well outbursts from lake george alaska dimensional analysis leads to two approximations of the form qp vqf hi 0 where q 0 5 to 0 6 hi is initial lake depth 0 is lake temperature and the form of f hi 0 depends on the relative importance of viscous dissipation and the lake s thermal energy in determining the rate of breach opening these expressions along with the regression relations should prove useful for assessing the probable magnitude of breach type outburst floods

sedimentary record of a late saalian jokulhlaup case study in siedlątkow outcrop central poland abstract the aim of the research conducted in an outcrop of late saalian sediments in siedl tkow central poland was to reconstruct extreme meltwater flows during a jokulhlaup on the basis of the geological record a combination of field and laboratory lithological methods was used which included lithofacies analysis analyses of grain size distribution shape and roundness of gravelly clasts examination of sediments made it possible to recognise palaeohydrological sedimentation and deformation processes during and immediately after the flood in particular the simultaneous operation of water flows in four paths in the immediate foreland of the ice front was reconstructed mechanisms of transport and sedimentation of deposits were determined characteristic features of the analysed jokulhlaup deposits include erosional and deformational contact with the substrate a high content of very coarse sediments poorly sorted with predominant massive clast supported gravels and boulders the presence of rip up clasts inverse grading in the lower part of the profile normal grading in the upper part and similarities in clast shape and roundness and the ratio of local rocks to northern origin rocks when compared to the surrounding saalian till the documented features allow for the following phases of jokulhlaup to be distinguished initial rising high stage and waning stage during the reconstructed event hyperconcentrated flow was dominant whereas fluvial flow and debris flow were of lesser importance the release of meltwater in the marginal part of the ice sheet came from the depression of the kolo basin to the north of the analysed sites complex deformation processes which occurred in the flood deposits and their substratum were also interpreted the analysed jokulhlaup event resembles some minor glacial floods in the ice contact zone of present day icelandic glaciers further research in marginal zones of pleistocene ice sheets should account for their occurrence as the analysed example indicates that jokulhlaups might have had considerable influence on the course of sedimentation processes in the marginal zones of ice lobes of pleistocene ice sheets as well as in the marginal valleys and sandars

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

sensitivity of pine island glacier west antarctica to changes in ice shelf and basal conditions a model study we use a finite element model of coupled ice stream ice shelf flow to study the sensitivity of pine island glacier west antarctica to changes in ice shelf and basal conditions by tuning a softening coefficient of the ice along the glacier margins and a basal friction coefficient controlling the distribution of basal shear stress underneath the ice stream we are able to match model velocity to that observed with interferometric synthetic aperture radar insar we use the model to investigate the effect of small perturbations on ice flow we find that a 5 5 13 reduction in our initial ice shelf area increases the glacier velocity by 3 5 10 at the grounding line the removal of the entire ice shelf increases the grounding line velocity by 70 the changes in velocity associated with ice shelf reduction are felt several tens of km inland alternatively a 5 reduction in basal shear stress increases the glacier velocity by 13 at the grounding line by contrast softening of the glacier side margins would have to be increased a lot more to produce a comparable change in ice velocity hence both the ice shelf buttressing and the basal shear stress contribute significant resistance to the flow of pine island glacier

hypothesis for mega outburst flooding from a palaeo subglacial lake beneath the east antarctic ice sheet subglacial outburst floods at the margins of the antarctic and laurentide ice sheets have been linked to changes in global ocean circulation and climate the impact of palaeo hydrological systems beneath the east antarctic ice sheet eais has however remained elusive by analysing bed morphology in the wilkes subglacial basin we hypothesise the occurrence of a major palaeo subglacial lake and associated outburst floods in the interior of east antarctica at similar to 70 km wide and over 100 km long the inferred area of outburst flooding is the most extensive in antarctica and the palaeo subglacial lake from which the floods originated is the second largest after lake vostok the scale of inferred outburst flooding is similar to mega floods beneath the laurentide ice sheet we suggest that this major hydrological system developed during expansion of the eais in the middle miocene and probably affected ice sheet stability ocean circulation and climate evolution

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

impacts of an accumulation hiatus on the physical properties of firn at a low accumulation polar site 1 recent field investigations of a megadune region of east antarctica provide evidence that differences in grain size thermal conductivity and permeability across a megadune profile are due to spatial accumulation variability in the absence of significant microclimate variations the megadunes are low amplitude 2 8 m long wavelength 2 5 km bands with perceptible but low accumulation less than 40 mm water equivalent weq yr 1 and accumulation hiatus within several kilometers proximity as determined by remote sensing surface feature classification and ground penetrating radar profiling our hypothesis that accumulation rate impacts the extent of temperature gradient driven metamorphic growth in low accumulation rate sites is supported by measurements of various firn physical properties relatively small differences in accumulation rate less than 40 mm weq yr 1 result in large differences in physical properties including grain size thermal conductivity and permeability which are apparent in satellite based microwave data from both passive and active sensors the differences in physical snow structure between low accumulation areas and accumulation hiatus areas in the near surface are sufficiently distinct that evidence of past accumulation hiatus should be observable in the physical and chemical properties of an ice core record

reconstruction of the west antarctic ice sheet in pine island bay during the last glacial maximum and its subsequent retreat history abstract marine geological and geophysical studies were conducted to determine the late quaternary glacial history of the west antarctic ice sheet in pine island bay antarctica the data set includes sea floor imagery in the form of multibeam swath bathymetry and deep tow side scan sonar surveys distribution of sedimentary deposits and basal surface conditions are revealed through intermediate resolution seismic data and chirp 3 5 khz sub bottom profiles sedimentary deposits were sampled with piston cores and calcareous material was dated using ams radiocarbon techniques results from the study indicate the ice sheet was grounded on the outer shelf during its maximum extent sometime in the past thick ice flowing slowly and resting on crystalline bedrock was influenced by significant amounts of subglacial meltwater on the inner shelf thinner but more rapidly flowing ice characterized the seaward extent of the ice sheet which was grounded on a sedimentary substrate deglaciation occurred in two phases beginning soon after the last glacial maximum the outer most extension of the ice sheet retreated gradually pausing on a bathymetric high on the middle shelf by 16 000 14c years ago between 16 000 and 12 000 14c years ago ice sheet retreat resumed and may have proceeded rapidly to reach a present day position by 10 000 14c years ago

melting and freezing beneath filchner ronne ice shelf antarctica 1 we use remote sensing data sets to evaluate the spatial distribution of melt beneath the filchner ronne ice shelf fris the net melt rate of 83 4 24 8 gtons yr is 2 5 5 times lower than previous glaciological estimates but is similar to existing oceanographic estimates the spatial distribution however differs significantly from standard conceptual and numerical models in which most melt occurs along the grounding lines our results suggest most grounding line melt is refrozen while the dominant ice shelf water isw source is melting near the ice shelf front probably associated with tidal action this suggests that changes in ice shelf extent can impact isw production rates in the weddell sea

the chronology of the last deglaciation implications to the cause of the younger dryas event it has long been recognized that the transition from the last glacial to the present interglacial was punctuated by a brief and intense return to cold conditions this extraordinary event referred to by european palynologists as the younger dryas was centered in the northern atlantic basin evidence is accumulating that it may have been initiated and terminated by changes in the mode of operation of the northern atlantic ocean further it appears that these mode changes may have been triggered by diversions of glacial meltwater between the mississippi river and the st lawrence river drainage systems we report here accelerator mass spectrometry ams radiocarbon results on two strategically located deep sea cores one provides a chronology for surface water temperatures in the northern atlantic and the other for the meltwater discharge from the mississippi river our objective in obtaining these results was to strengthen our ability to correlate the air temperature history for the northern atlantic basin with the meltwater history for the laurentian ice sheet

landscape evolution of the dry valleys transantarctic mountains tectonic implications there are different views about the amount and timing of surface uplift in the transantarctic mountains and the geophysical mechanisms involved our new interpretation of the landscape evolution and tectonic history of the dry valleys area of the transantarctic mountains is based on geomorphic mapping of an area of 10 000 km2 the landforms are dated mainly by their association with volcanic ashes and glaciomarine deposits and this permits a reconstruction of the stages and timing of landscape evolution following a lowering of base level about 55 m y ago there was a phase of rapid denudation associated with planation and escarpment retreat probably under semiarid conditions eventually downcutting by rivers aided in places by glaciers graded valleys to near present sea level the main valleys were flooded by the sea in the miocene during a phase of subsidence before experiencing a final stage of modest upwarping near the coast there has been remarkably little landform change under the stable cold polar conditions of the last 15 m y it is difficult to explain the sirius group deposits which occur at high elevations in the area if they are pliocene in age overall denudation may have removed a wedge of rock with a thickness of over 4 km at the coast declining to 1 km at a point 75 km inland which is in good agreement with the results of existing apatite fission track analyses it is suggested that denudation reflects the differences in base level caused by high elevation at the time of extension due to underplating and the subsequent role of thermal uplift and flexural isostasy most crustal uplift 2 4 km is inferred to have occurred in the early cenozoic with 400 m of subsidence in the miocene followed by 300 m of uplift in the pliocene

glacial erosion status and outlook glacier erosion rates range across orders of magnitude and much of this variation cannot be attributed to basal sliding rates subglacial till acts as lubricating fault gouge or sawdust and must be removed for rapid subglacial bedrock erosion such erosion occurs especially where and when moulin fed streams access the bed and are unconstrained by supercooling or other processes streams also may directly erode bedrock likely with strong time evolution erosion is primarily by quarrying aided by strong fluctuations in the water system driven by variable surface melt and by subglacial earthquakes debris bed friction significantly affects abrasion quarrying and general glacier flow frost heave drives cirque headwall erosion as winter cold air enters bergschrunds creating temperature gradients to drive water flow along premelted films to growing ice lenses that fracture rock and the glacier removes the resulting blocks recent subglacial bedrock erosion and sediment flux are in many cases much higher than long term averages over glacial cycles evolution of glacial valley form feeds back strongly on erosion and deposition most of this is poorly quantified with parts open to argument glacial erosion and interactions are important to tectonic and volcanic processes as well as climate and biogeochemical fluxes motivating vigorous research

heinrich events massive late pleistocene detritus layers of the north atlantic and their global climate imprint 1 millennial climate oscillations of the glacial interval are interrupted by extreme events the so called heinrich events of the north atlantic their near global footprint is a testament to coherent interactions among earth s atmosphere oceans and cryosphere on millennial timescales heinrich detritus appears to have been derived from the region around hudson strait it was deposited over approximately 500 250 years several mechanisms have been proposed for the origin of the layers binge purge cycle of the laurentide ice sheet jokulhlaup activity from a hudson bay lake and an ice shelf buildup collapse fed by hudson strait to determine the origin of the heinrich events i recommend 1 further studies of the timing and duration of the events 2 further sedimentology study near the hudson strait and 3 greater spatial and temporal resolution studies of the layers as well as their precursory intervals studies of previous glacial intervals may also provide important constraints

basal perturbations under ice streams form drag and surface expression classical sliding theories consider ice sliding over obstacles which are much shorter than the thickness of overlying ice here we present a theory which considers form drag generated under ice streams by large obstacles such as subglacial bedforms which may have lengths comparable to ice thickness we also investigate how perturbations at the surface of an ice stream can be generated by such bedforms and develop a mathematical framework for separating the effects of such local kilometre scale variations in ice flow from the bulk flow of the ice stream

distribution thickness and origin of heinrich layer 3 in the labrador sea abstract the presence of heinrich layer 3 hl 3 in the northwest labrador sea has been debated in the literature calypso giant piston core md99 2233 five new standard piston cores and re interpretation of 34 cores from previous cruises confirm the presence of hl 3 in the labrador sea it is identified by high total carbonate concentration up to 45 an increase in coarse fraction content and lighter 18o values in polar species planktonic foraminifer neogloboquadrina pachyderma left coiling as low as 3 1 the age of hl 3 of 27 ka was bracketed in the various cores by about 50 14c accelerator mass spectrometer dates where it is present in ice proximal regions it consists of nepheloid flow deposits at the base and mud turbidites at the top the thickness of hl 3 varies between 4 8 m proximal to hudson strait and 0 9 m distal decreasing rapidly seaward on the upper continental slope hl 3 was too deeply buried to be sampled elsewhere hl 3 is absent in some cores probably due to slumping or erosion associated with sandy turbidity currents or debris flows

the link between climate warming and break up of ice shelves in the antarctic peninsula a review of in situ and remote sensing data covering the ice shelves of the antarctic peninsula provides a series of characteristics closely associated with rapid shelf retreat deeply embayed ice fronts calving of myriad small elongate bergs in punctuated events increasing flow speed and the presence of melt ponds on the ice shelf surface in the vicinity of the break ups as climate has warmed in the antarctic peninsula region melt season duration and the extent of ponding have increased most break up events have occurred during longer melt seasons suggesting that meltwater itself not just warming is responsible regions that show melting without pond formation are relatively unchanged melt ponds thus appear to be a robust harbinger of ice shelf retreat we use these observations to guide a model of ice shelf flow and the effects of meltwater crevasses present in a region of surface ponding will likely fill to the brim with water we hypothesize building on weertman 1973 hughes 1983 and van der veen 1998 that crevasse propagation by meltwater is the main mechanism by which ice shelves weaken and retreat a thermodynamic finite element model is used to evaluate ice flow and the strain field and simple extensions of this model are used to investigate crack propagation by meltwater the model results support the hypothesis