thickness and extent of the subglacial till layer beneath an antarctic paleo ice stream fast flowing ice streams and outlet glaciers currently account for as much as 90 of the discharge from the antarctic and greenland ice sheets although the deformation of subglacial material has been proposed as the mechanism for this rapid motion such sediment is usually hidden under several kilometers of ice marine geophysical records have allowed reconstruction of the three dimensional thickness of the sedimentary bed beneath a large antarctic paleo ice stream for the first time fast flow is indicated by streamlined seafloor lineations that form the surface of a layer of low shear strength unsorted sediment averaging 4 6 m thick rapid motion of the paleo ice stream was a result of subglacial deformation within this layer

the overdeepening hypothesis how erosional modification of the marine scape during the early pliocene altered glacial dynamics on the antarctic peninsula s pacific margin abstract a new synthesis of diatom assemblage data from ocean drilling program odp leg 178 suggests that the pacific margin of the antarctic peninsula underwent a transition from a shallow shelf to an overdeepened shelf in the early pliocene this modification of the marine scape was due to a relatively brief interval of erosion begun at 5 2 ma the erosion was caused by high frequency advances of a super inflated antarctic peninsula ice sheet apis the frequent advances of the higher elevation ice sheet were a consequence of abundant moisture delivered to the region as the polar front migrated southward by 5 12 ma ice streams incised foredeepened glacial troughs into basement on the inner shelf sediment eroded from the inner shelf was transported through cross shelf troughs and deposited in large trough mouth fan depocenters on the upper slope overdeepened shelf conditions became widespread as troughs widened and intra trough banks beveled by 4 25 ma trough mouth fan construction ceased and subsequent advances of the apis have been infrequent we propose that the reduced frequency of grounding events signaled the transition to a modern foredeepened and overdeepened shelf we hypothesize that a new glacial dynamic emerged in the early pliocene because overdeepening led to accelerated heat exchange between the ocean and apis in two ways firstly the overdeepened shelf required that a larger area of the grounded ice sheet s marine terminus be in contact with the ocean secondly erosional deepening of the outer shelf was equivalent to lowering a shelf edge sill that permitted frequent and voluminous intrusion of warm circumpolar deep waters that upwell in the region the resultant accelerated melting at the apis marine terminus caused the super inflated apis to deflate on the mainland which further decreased the possibility that grounded ice could advance on the overdeepened shelf

problems and possible solutions concerning radiocarbon dating of surface marine sediments ross sea antarctica abstract radiocarbon accelerator mass spectrometric ams dates on the acid insoluble fraction from 38 core tops from the western ross sea antarctica are used to address these questions 1 what are the apparent ages of sediments at or close to the present sediment water interface 2 is there a statistically significant pattern to the spatial distribution of core top ages and 3 is there a correction factor that can be applied to these age determinations to obtain the best possible holocene downcore chronologies ages of core top sediments range from 2000 to 21 000 14c yr b p some old core top dates are from piston cores and probably represent the loss of sediment during the coring process but some core top samples 6000 14c yr b p may represent little or no holocene deposition four possible sources of variability in dates 6000 14c yr b p n 28 are associated with 1 different sample preparation methods 2 different sediment recovery systems 3 different geographic regions and 4 within sample lateral age variability statistical analysis on an a posteriori design indicates that geographic area is the major cause of variability there is a difference in mean surface sediment age of nearly 2000 yr between sites in the western ross sea and sites east of ross bank in south central ross sea the systematic variability in surface age between areas may be attributed to a variable sediment accumulation rates sar surface age is inversely related to sar b differences in the percentage of reworked dead carbon between each area and or c differences in the co2 exchange between the ocean and the atmosphere

the death mask of the antarctic ice sheet comparison of glacial geomorphic features across the continental shelf abstract multibeam swath bathymetry data seismic lines and shallow cores were collected seaward of all of the major drainage outlets of the antarctic ice sheet from the pennell coast of north victoria land to the northwestern weddell sea the results show that during the last glacial maximum the ice sheet extended onto the outer shelf in all areas to the shelf break in many swath bathymetry and deep tow sidescan sonar data from the continental shelf also record the geomorphology left by the retreating ice sheet glacial troughs occur offshore of all major glacial outlets in each drainage area except for parts of the ross sea the inner shelf is characterized by acoustic basement interpreted as crystalline substrate the geomorphology of these inner shelf areas consists of erosional features such as grooves that show that ice flow tended to follow the structural grain of the bedrock outer shelf areas are floored by sedimentary substrate where the direction of ice flow was more directly offshore and depositional features characterize the seafloor in these areas the signature of the grounded ice consists of till deposits and large scale geomorphic features mega scale glacial lineations drumlins occur within the region of contact between crystalline and sedimentary substrates meltwater channels also have been imaged in both inner and outer shelf settings while the presence of meltwater across the shelf has long been suspected we present here the first multibeam image of a meltwater channel on the outer continental shelf of antarctica at the shelf break gullies are in all areas where we know ice reached the shelf break we note the remarkable similarity in form of mega scale glacial lineations observed on the outer shelf over sedimentary substrate across all areas surveyed typically measuring 200 600 m crest to crest this suggests that the shape of these landforms is governed by the same process in each drainage area and that process was occurring under a large area of the ice sheet the erosional and transitional features show much variability in form and distribution

on the duration of west antarctic ice sheet grounding events in ross sea during the quaternary abstract a back stepping succession of three seismically defined grounding zone wedges gzws in the glomar challenger basin palaeo ice stream trough is usually assigned to the short time that elapsed since the west antarctic ice sheet retreat began at 11 ka 14c bp recent radiocarbon dates have however suggested an alternate interpretation in which the youngest of these three gzws the gray unit gzw on the middle shelf corresponds to deposition during the last glacial maximum lgm if so then the gray unit must represent an amalgamation of erosion and deposition spanning a much longer time interval i e the 100 ky interval between the last interglacial and the lgm to test these conflicting interpretations the gray unit sediment volume was mapped from seismic and multibeam data two end member durations were calculated because flux during ice sheet retreat is significantly higher than flux during ice sheet advance using the retreat mode flux the 1 47 ky grounding event estimate shows that the middle shelf gzw could have been deposited during the post lgm retreat however the 147 34 ky grounding event estimate based on the advance mode flux also demonstrates that the gzw might reasonably represent an amalgamation of erosion and deposition as grounded ice gradually advanced between mis5e and mis2 i e from the last interglacial to the last glacial maximum these data thus require that both interpretations of how the near surface stratigraphy relates to grounding line translations in the last glacial cycle be considered feasible working hypotheses

fjords as temporary sediment traps history of glacial erosion and deposition in muir inlet glacier bay national park southeastern alaska glacimarine sedimentary deposits within the basins of muir inlet a 48 km long silled fjord are interpreted from complimentary sets of high resolution seismic reflection profiles using known glacial advance and retreat history two prominent glacial erosion surfaces are identified the lowest attributed to the last glacial maximum lgm advance and the upper coincident with the little ice age lia advance the lgm ice sheet which advanced onto the continental shelf was 1700 m thick in muir inlet and eroded bedrock whereas the thinner lia ice did not lgm deposits 300 m thick occur beneath the lia erosion surface in the deepest basins evidence for earlier neoglacial advances is present in subaerial deposits however neoglacial sediments preserved within the marine record are restricted to one depositional package on the entrance sill volumes of lia retreat sediments were calculated within basins an average annual sediment flux was calculated by modeling the duration of sediment contributed from muir glacier and from tributary glaciers and side entry sources the annual sediment flux ranged from 1 3 10 6 m 3 yr to 4 6 10 7 m 3 yr and increases logarithmically with increasing drainage basin area similar to fluvial systems this sediment flux does not only represent bedrock erosion additional sediment is contributed from persistent tributary glaciers and from lgm sediment stored within deeper basins basin wide reflections characterize the most common seismic facies and indicate that strata are horizontal and continuous across each basin confirming the importance of sediment gravity flows originating from sills and sloping fjord walls

milankovitch theory of ice ages hypothesis of ice sheet linkage between regional insolation and global climate abstract j d hays j imbrie and n j shackleton 1976 science 194 1121 1132 showed that the astronomical theory explained many features of late quaternary ice age climates but they did not specify the physical mechanisms involved here it is proposed that interlocked variations of ice sheet heat sinks in both polar hemispheres amplified and transmitted milankovitch summer half year insolation changes a version of the astronomical theory between 45 and 75 n into the globally synchronous climate changes recorded in geologic records it is suggested that late quaternary ice sheets had terrestrial components grounded above sea level melting margins fluctuations controlled by climate and marine components grounded below sea level drained largely by ice streams limited melting margins fluctuations controlled primarily by sea level and secondarily by climate interior surface elevations coupled to downdraw through ice streams northern hemisphere ice sheets were largely marine with minor melting margins in the arctic and terrestrial with major melting margins in the midlatitudes west antarctic and peripheral east antarctic ice was marine based and lacked melting margins because of their geographic array these terrestrial and marine components formed an ice sheet system whose variations were coupled on a global scale milankovitch summer isolation changes near midlatitude northern hemisphere melting margins controlled most variations of this system because advance or retreat of melting margins initiated concurrent eustatic sea level change such sea level change afforded the critical interlocking mechanism between terrestrial and marine components because it forced simultaneous expansion or contraction of marine margins in both polar hemispheres this initiated an amplifying feedback loop among all marine components and influenced interior downdraw through ice streams arctic summer insolation change was less important because northern melting margins were relatively minor its greatest influence was on surface ablation of ice streams that controlled interior downdraw this affected eustatic sea level and activated global linkage of marine sectors by analogy with present day antarctica late quaternary ice sheets were enormous planetary heat sinks due to their reflective and radiative surface characteristics it is suggested that the effectiveness of these ice sheet heat sinks varied with their areal extent and interior surface elevation thus it is postulated that concurrent growth or decay of these interlocked ice sheet heat sinks in both polar hemispheres served as the global amplifier of regional milankovitch summer insolation

pliocene pleistocene glaciomarine sedimentation in eastern prydz bay and development of the prydz trough mouth fan odp sites 1166 and 1167 east antarctica abstract lithostratigraphy grain sizes and down hole logs of site 1166 on the continental shelf and site 1167 on the upper slope are analyzed to reconstruct glacial processes in eastern prydz bay and the development of the prydz trough mouth fan in eastern prydz bay upper pliocene lower pleistocene glaciomarine sediments occur interbedded with open marine muds and grade upward into waterlaid tills and subglacial tills lower pleistocene sediments of the trough mouth fan consist of coarse grained debrites interbedded with bottom current deposits and hemipelagic muds indicating repeated advances and retreats of the lambert glacier amery ice shelf system with respect to the shelf break systematic fluctuations in lithofacies and down hole logs characterize the upper pliocene lower pleistocene transition at sites 1166 and 1167 and indicate that an ice stream advanced and retreated within the prydz channel until the mid pleistocene the record from site 1167 shows that the grounding line of the lambert glacier did not extend to the shelf break after 0 78 ma published ice rafted debris records in the southern ocean show peak abundances in the pliocene and the early pleistocene suggesting a link between the nature of the glacial drainage system as recorded by the trough mouth fans and increased delivery of ice rafted debris to the southern ocean

sedimentation beneath ice shelves the view from ice stream b abstract ice shelf development is favored by rapid flow of cold ice from outlet glaciers or ice streams into protected embayments with localized high spots basal melting of ice shelves is rapid near the ice front and may occur near the grounding line ice from outlet glaciers may contain significant englacial debris that is deposited as a dropstone diamicton in regions of basal melting englacial debris is sparse or absent in ice streams evidence from ice stream b draining into the ross ice shelf of west antarctica suggests that the rapid ice velocity arises from deformation of a several meter thick water saturated basal till layer that is eroding an unconformity on sediments beneath and that has deposited a till delta tens of meters thick and tens of kilometers long at the grounding line sea level fall would cause conveyor belt recycling of this till delta and grounding line advance across the ross sea to the edge of the continental shelf forming an ice sheet with a low ice stream profile resting on a several meter thick deforming till layer eroding an unconformity the modern ross sea is characterized by a regional unconformity overlain by a diamicton of probable latest pliocene pleistocene age measuring several meters to tens of meters thick we hypothesize that this diamicton is a deformed glacial till and that the ross sea sediments record one or more expansions of the till lubricated west antarctic ice sheet to the edge of the continental shelf

piston core based biostratigraphic constraints on pleistocene oscillations of the west antarctic ice sheet in western ross sea between north basin and and 1b drill site abstract new diatom biostratigraphic age data derived from piston core were used to constrain the chronology for a seismic based interpretation of west antarctic ice sheet wais advance and retreat from the north basin outer shelf in western ross sea the data demonstrates that grounded ice occupied the outer shelf on at least ten occasions during the last 2 0 ma of the pleistocene although the biostratigraphic resolution is low the constraints permit correlations to coeval section recently drilled in south mcmurdo sound at the and 1b drill site our interpretation of regional scale correlations suggests that the pleistocene dynamics of the wais involved grounding line translations in excess of 600 km however the data also suggests that only two major advances of grounded ice occurred since 0 71 ma in other words several of the late pleistocene sediment cycles at and 1b must represent only minor advance and retreat of grounded ice to and from south mcmurdo sound the results prove false the long standing tacit assumption that the wais experienced major advances in association with peak glacials and eustatic lowstands during the last 700 ka of the pleistocene these results show that piston cores can be used to constrain the timing of pre holocene grounding events these data also highlight the need for regional scale correlations to distinguish between major versus minor grounding line translations of grounded ice in the marine setting

ice sheet stability and sea level rise a wedge of sediments appears to stabilize the whillans ice stream suggesting that sea level rise may not destabilize ice sheets as much as previously feared

extent and dynamics of the west antarctic ice sheet on the outer continental shelf of pine island bay during the last glaciation abstract swath bathymetry and topas sub bottom profiler acoustic data reveal the presence of a major cross shelf bathymetric trough containing streamlined subglacial bedforms formed in soft till that extends to the continental shelf edge of outer pine island bay amundsen sea west antarctica the new data indicate that the west antarctic ice sheet wais extended to the shelf edge of pine island bay during the last glaciation and was drained by a grounded fast flowing palaeo ice stream through this outer shelf trough topography and subglacial geology are likely to have exerted a strong control on the development and location of palaeo ice streams in pine island bay topas records show that mega scale glacial lineations are formed in an acoustically transparent sediment layer interpreted to be soft till and are inferred to be the product of subglacial sediment deformation beneath the palaeo ice stream the flat to irregular nature of the basal reflector of the acoustically transparent layer of soft till implies that a combination of groove ploughing of the substrate and subglacial sediment deformation were important processes beneath the palaeo ice stream in the outer trough of pine island bay as well as palaeo ice streams elsewhere in antarctica therefore the formation of the soft till layer and possibly associated subglacial bedforms are likely to be a function of these palaeo ice stream processes in a regional context the wais extended to the continental shelf edge of the bellingshausen amundsen and ross seas and was drained by a number of palaeo ice streams during the last glaciation

geological constraints on antarctic palaeo ice stream retreat submarine landforms preserved in bathymetric troughs on the antarctic continental shelf show that the style of ice stream retreat across the shelf following the last glacial maximum varied between different troughs three styles of retreat are inferred from the geological evidence rapid episodic and slow rapid retreat by ice stream floatation and calving is recorded by the preservation of a landform assemblage of unmodified streamlined subglacial bedforms including mega scale glacial lineations msgls that record streaming flow along these troughs these elongate bedforms are not overprinted by recessional glacial landforms formed transverse to ice flow such as moraines or grounding zone wedges and overlying deglacial sediments are thin a second type of landform assemblage consists of msgls overprinted or interrupted by transverse grounding zone wedges this assemblage implies episodic retreat between successive grounding zone positions the third type of landform assemblage is that of numerous closely spaced recessional moraines and intermittent grounding zone wedges that overlie and interrupt msgls this assemblage records the slow retreat of grounded ice across the shelf variation in the style of ice stream retreat between the different bathymetric troughs indicates that antarctic palaeo ice streams did not respond uniformly to external forcing at the end of the last glacial cycle rather their diachronous retreat reflects the dominance of local controls in the form of bathymetry and drainage basin size more broadly these data show that retreat of marine based ice sheets in areas of reverse bed slope is not necessarily catastrophic and they provide important constraints for numerical models that attempt to predict the dynamics of large polar ice sheets copyright 2008 john wiley sons ltd

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

submarine glacial landforms and rates of ice stream collapse the rate of deglacial ice sheet retreat across polar continental shelves and possible ice stream collapse and sea level rise has been much debated high resolution imagery of seafloor morphology is available for many polar shelves and fjords the rapidity of ice retreat is inferred from diagnostic assemblages of submarine landforms produced at ice stream sedimentary beds these landforms exposed by ice retreat across high latitude shelves demonstrate that deglaciation occurs in three main ways rapidly by flotation and breakup episodically by still stands and or grounding events punctuating rapid retreat or by slower retreat of grounded ice submarine landform assemblages imply through the presence of grounding zone wedges overprinting mega scale glacial lineations on many polar shelves that ice stream retreat is more often episodic than catastrophic these observations provide a robust test of the ability of numerical models to predict the varied response of ice sheet basins to environmental changes

measuring glacier surface roughness using plot scale close range digital photogrammetry glacier roughness at sub metre scales is an important control on the ice surface energy balance and has implications for scattering energy measured by remote sensing instruments ice surface roughness is dynamic as a consequence of spatial and temporal variation in ablation to date studies relying on singular and or spatially discrete two dimensional profiles to describe ice surface roughness have failed to resolve common patterns or causes of variation in glacier surface morphology here we demonstrate the potential of close range digital photogrammetry as a rapid and cost effective method to retrieve three dimensional data detailing plot scale supraglacial topography the photogrammetric approach here employed a calibrated consumer grade 5 mpix digital camera repeatedly imaging a plot scale 25 m2 ice surface area on midtre lovenbreen svalbard from stereo pair images digital surface models dsms with sub centimetre horizontal resolution and 3 mm vertical precision were achieved at plot scales 4 m2 extraction of roughness metrics including estimates of aerodynamic roughness length z 0 was readily achievable and temporal variations in the glacier surface topography were captured close range photogrammetry with appropriate camera calibration and image acquisition geometry is shown to be a robust method to record sub centimetre variations in ablating ice topography while the dsm plot area may be limited through use of stereo pair images and issues of obliquity emerging photogrammetric packages are likely to overcome such limitations

ice flow of the antarctic ice sheet we present a reference comprehensive high resolution digital mosaic of ice motion in antarctica assembled from multiple satellite interferometric synthetic aperture radar data acquired during the international polar year 2007 to 2009 the data reveal widespread patterned enhanced flow with tributary glaciers reaching hundreds to thousands of kilometers inland over the entire continent this view of ice sheet motion emphasizes the importance of basal slip dominated tributary flow over deformation dominated ice sheet flow redefines our understanding of ice sheet dynamics and has far reaching implications for the reconstruction and prediction of ice sheet evolution

rapid erosion beneath the greenland ice sheet the pleistocene ice sheets left a clear signature of erosion but the rate at which ice sheets erode is difficult to determine from either paleolandscapes or observations of contemporary processes here we use two years of sediment flux data derived from meltwaters emerging from an outlet glacier in west greenland to calculate an average rate of subglacial erosion across a catchment extending 50 km inland from the ice margin erosion in this zone occurs at 4 8 2 6 mm a 1 a rate 1 2 orders of magnitude greater than previous estimates of erosion rate beneath the greenland ice sheet our results suggest that where surface meltwaters are able to access the bed the rate of erosion by ice sheets is in keeping with the rapid erosion observed at temperate alpine glaciers during deglacial phases when meltwater was abundant ice sheet margins should therefore have acted as highly efficient agents of erosion

spreading pulses of the tyrrhenian sea during the narrowing of the calabrian slab the opening of the tyrrhenian sea has been punctuated by short lived episodes of oceanic accretion on separate small backarc basins during early pliocene vavilov basin and early pleistocene marsili basin time these spreading pulses are related to slab rollback and are synchronous with the reduction of the subduction zone width during the formation of the narrow calabrian arc using laboratory models we investigated the long term and transient effects of the reduction of slab width on the subduction kinematics we found that the abrupt reduction in slab width results in a pulse of acceleration of the trench retreat velocity as the balance between driving and resisting forces acting on the slab is temporarily modified our findings also show that the time scale and amplitude of spreading observed in the tyrrhenian sea can be experimentally fitted if the scaled viscosity of the uppermost part of the mantle ranges between 10 19 and 10 20 pa s

retreat of the east antarctic ice sheet during the last glacial termination the east antarctic ice sheet retreated at the end of the last glacial period terrestrial and marine data suggest that the retreat began 14 000 years ago indicating that the east antarctic ice sheet probably did not contribute to meltwater pulse 1a 14 700 years ago

provenance of lgm glacial till sand fraction across the ross embayment antarctica abstract the provenance of late quaternary ross embayment till was investigated by comparing the coarse sand composition of east and west antarctic source area tills with till samples from across the ross sea the west antarctic samples from beneath the whillans b and kamb c ice streams are petrologically distinct from samples of lateral moraines flanking several east antarctic outlet glaciers the characteristic assemblage of four west antarctic samples includes felsic intrusive and detrital sedimentary lithic fragments plagioclase and abundant quartz in contrast most of the ten east antarctic till samples contains abundant mafic intrusive and detrital sedimentary lithic fragments as well as less abundant quartz the distinctive composition of these source areas can be linked to 33 samples from 20 cores of last glacial maximum lgm age till distributed across the ross sea western ross sea till samples exhibit mineralogic and lithological similarities to east antarctic till samples although these western ross sea tills contain higher percentages of felsic intrusive and extrusive lithic fragments eastern ross sea till samples are compositionally similar to west antarctic till particularly in their abundance of quartz and dearth of mafic and extrusive lithic components central ross sea till exhibits compositional similarities to both east and west antarctic source terranes including a mafic lithic component and marks the confluence of ice draining from east and west antarctica during the lgm thus west antarctic derived ice streams did not advance into the western ross sea this indicates that even if pre lgm equivalents of the present siple coast ice streams existed they did not simply expand allowing west antarctic derived ice to dominate the lgm ross ice sheet

basal hydraulic system of a west antarctic ice stream constraints from borehole observations pressure and tracer measurements in boreholes drilled to the bottom of ice stream b west antarctica are used to obtain information about the basal water conduit system in which high water pressures are developed these high pressures presumably make possible the rapid movement of the ice stream pressure in the system is indicated by the borehole water level once connection to the conduit system is made on initial connection here also called breakthrough to the basal water system the water level drops in a few minutes to an initial depth in the range 96 117 m below the surface these water levels are near but mostly somewhat deeper than the flotation level of about 100m depth water level at which basal water pressure and ice overburden pressure are equal which is calculated from depth density profiles and is measured in one borehole the conduit system can be modelled as a continuous or somewhat discontinuous gap between ice and bed the thickness of the gap has to be about 2 mm to account for the water level drop on breakthrough and about 4 mm to fit the results of a salt tracer experiment indicating downstream transport at a speed of 7 5 mm s 1 the above gap conduit model is however ruled out by the way a pressure pulse injected into the basal water system at breakthrough propagates outward from the injection hole and also by the large hole to hole variation in measured basal pressure which if present in a gap conduit system with 2 or 4 mm would result in unacceptably large local water fluxes an alternative model that avoids the e objections called the gap opening model involves opening a gap as injection proceeds starting with a thin film the injection of water under pressure lifts the ice mass a round the borehole creating a gap 3 or 4 mm wide at the ice bed interface evaluated quantitatively the gap opening model accounts for the volume of water that the basal watery system accepts on breakthrough which obviates the gap conduit model in order to transport basal meltwater from upstream it is then necessary for the complete hydraulic model to contain also a network of relatively large conduits of which the most promising type is the canal conduit proposed theoretically by walder and fowler 1994 flat low conduits incised into the till 0 1 m deep and perhaps 1 m wide with a flat ice roof the basal water pressure data suggest that the canals are spaced 50 300 m apart much closer than r tunnel would be the deepest observed water level 117 m i the most likely to reflect the actual water pressure in the canals corresponding to a basal effective pressure of 1 6 bar in this interpretation the shallower water levels are affected by loss of hydraulic head in the narrow passageway s that connect along the bed from borehole to canal s once a borehole has frozen up and any passageways connecting with canals have become closed a pressure sensor in contact with the unfrozen till that underlies the ice will measure the pore pressure in the till given enough time for pressure equilibration this pressure varies considerably with time over the equivalent water level range from 100 to 113 m basal pressure sensors 500 m apart report uncorrelated variations whereas sensors in boreholes 25 m apart report mostly but not entirely well correlated variations of unknown origin in part of the record remarkable anticorrelated variations are interspersed with positively correlated one and there are rare abrupt excursions to extreme water levels as deep as 125m and as shallow as 74 m a diurnal pressure fluctuation intermittently observed may possibly be caused by the ocean tide in the ross sea the lack of any observed variation in ice stream motion when large percentagewise variations in basal effective pressure were occurring according to our data suggests that the observed pressure variations are sufficiently local and so randomly variable from place to place that they are averaged out in the process by which the basal motion of the ice stream is determined by an integration over a large area of the bed

antarctic palaeo ice streams we review the geomorphological sedimentological and chronological evidence for palaeo ice streams on the continental shelf of antarctica and use this information to investigate basal conditions and processes and to identify factors controlling grounding line retreat a comprehensive circum antarctic inventory of known palaeo ice streams their basal characteristics and minimum ages for their retreat following the last glacial maximum lgm is also provided antarctic palaeo ice streams are identified by a set of diagnostic landforms that nonetheless display considerable spatial variability due to the influence of substrate flow velocity and subglacial processes during the lgm palaeo ice streams extended via bathymetric troughs to the shelf edge of the antarctic peninsula and west antarctica and typically to the mid outer shelf of east antarctica the retreat history of the antarctic ice sheet since the lgm is characterised by considerable asynchroneity with individual ice streams exhibiting different retreat histories this variability allows antarctic palaeo ice streams to be classified into discrete retreat styles and the controls on grounding line retreat to be investigated such analysis highlights the important impact of internal factors on ice stream dynamics such as bed characteristics and slope and drainage basin size whilst grounding line retreat may be triggered and to some extent paced by external atmospheric and oceanic forcing the individual characteristics of each ice stream will modulate the precise timing and rate of retreat through time

retreat history of the ross ice sheet shelf since the last glacial maximum from deep basin sediment cores around ross island abstract radiocarbon dated sediment cores from deep basins beneath the mcmurdo ice shelf and seasonally open water north of ross island mcmurdo sound display a characteristic succession of sedimentary facies that document the retreat of the ross ice sheet at the last glacial maximum to its present ice shelf configuration the facies succession records a transition from a nearly grounded ice sheet to open marine environments north of ross island that comprises in ascending stratigraphic order 1 slightly consolidated clast rich muddy diamict dominated by basement clasts from the transantarctic mountains and interpreted as melt out from the basal layer debris proximal to a retreating grounding zone 2 sparsely fossiliferous reworked diatom frustules and non bioturbated mud lacking lonestones interpreted as a sub ice shelf facies and 3 diatom mud and diatom ooze indicative of open marine conditions with evidence of iceberg rafting the succession in the open marine lewis basin north of ross island is similar though the diamict is much sandier and sedimentation rates 1 2 orders of magnitude higher we present a radiocarbon chronology from total organic carbon which implies that lift off of grounded ice in the 900 m deep marine basins surrounding ross island occurred by 10 100 14c years bp following lift off an ice shelf was maintained to the north of ross island until 8900 14c years bp we identify a phase of accelerated retreat at that time between the drygalski trough and ross island immediately preceding the timing of meltwater pulse 1b at 8900 14c years bp the calving line became pinned to ross island significantly decoupling from the grounding line and marking the transition from a retreating ice sheet to the development of the present ice shelf

west antarctic ice sheet grounding events on the ross sea outer continental shelf during the middle miocene abstract new seismic stratigraphic analysis of the ross sea continental shelf suggests that there were a at least five shelf wide grounding events of the west antarctic ice sheet wais during the middle miocene although the number of wais grounding events generally matches the number of extreme 18 o enrichments and eustatic lowstands these results do not support the long standing assumption that west antarctica was substantially ice free instead seismic stratigraphic evidence from the ross sea shelf documents waxing and waning of a well developed wais in the marine environment at least on the pacific sector of the west antarctic continental shelf

influence of rapid glacial retreat on the rate of erosion by tidewater glaciers glacial erosion rates inferred from total sediment budgets in recently deglaciated fjords which are the highest reported erosion rates worldwide have received considerable attention in fields as diverse as tectonics glacial sedimentation and climate these record rates however are representative only of tidewater glaciers during their extensive retreat of the post little ice age period erosion rates averaged over glacial interglacial cycles and longer periods are likely to be substantially smaller we examine the influence of retreat rate on sediment yields from tidewater glaciers by reconstructing the history of sediment output from retreating glaciers necessary to produce sediment packages observed in contemporary fjords using a simple numerical model of proglacial sedimentation in front of a retreating glacier seismic profiles of proglacial sediments and the history of terminus retreat of muir glacier glacier bay alaska we calculate the sediment flux as a function of time from this glacier between 1900 and 1979 and conclude that sediment flux scales with retreat rate the corresponding basin wide erosion rate during this 79 yr period averages 37 mm yr and exceeds long term erosion rates by a factor of 5 1 for muir glacier and by inference for other calving glaciers the general drastic retreat and the marked regional drawdown of ice since the little ice age are both linked to unusually rapid calving and fast ice motion which is conducive to rapid erosion

antarctic sediment chronology by programmed temperature pyrolysis methodology and data treatment 1 we report a detailed programmed temperature pyrolysis combustion methodology for radiocarbon 14c dating of antarctic sub ice shelf sediments the method targets the autochthonous organic component in sediments that contain a distribution of acid insoluble organic components from several sources of different ages the approach has improved sediment chronology in organic rich sediments proximal to antarctic ice shelves by yielding maximum age constraints significantly younger than bulk radiocarbon dates from the same sediment horizons the method proves adequate in determining isotope ratios of the pre aged carbon end member however the isotopic compositions of the low temperature measurements indicate that no samples completely avoided mixing with some proportion of pre aged organic material dating the unresolved but desired young end member must rely on indirect methods but a simple mixing model cannot be developed without knowledge of the sedimentation rate or comparable constraints a mathematical approach allowing for multiple mixing components yields a maximum likelihood age a first order approximation of the relative proportion of the autochthonous component and the temperature at which allochthonous carbon begins to volatilize and mix with the autochthonous component it is likely that our estimation of the cutoff temperature will be improved with knowledge of the pyrolysis kinetics of the major components chronology is improved relative to bulk acid insoluble organic material ages from nine temperature interval dates down to two but incorporation of inherently more pre aged carbon in the first division becomes more apparent with fewer and larger temperature intervals

early stall of west antarctic ice sheet advance on the eastern ross sea middle shelf followed by retreat at 27 50014cyrbp abstract marine geological and geophysical data suggest that the west antarctic ice sheet wais deposited three thick seismically resolvable units in the eastern ross sea since the last glacial maximum lgm but the chronology of these grounding events is debated existing ice retreat chronologies for the eastern ross sea are considered suspect because nearly all have been developed using radiocarbon dating of acid insoluble organics aio from bulk sediment we used a new strategy to isolate in situ forams from diamict sediment deposited at the middle shelf grounding zone wedge gzw a subaqueous ice marginal landform previously interpreted to represent deposition during the third grounding event since the lgm advance to the shelf edge here we present a synthesis of our new radiocarbon dates of foram tests and aio with previously published radiocarbon dates we propose that the wais s lgm advance in the eastern ross sea sector reached the middle shelf circa 27 500 14c yr bp and that open marine sedimentation began shortly thereafter there is as of yet no clear explanation for why wais retreat in this sector of antarctica apparently began prior to the peak of the last glacial cold period a precipitation deficit localized warm water intrusion an ice dynamic response in the absence of climate forcing or another as of yet unknown forcing mechanism could be responsible

volume of antarctic ice at the last glacial maximum and its impact on global sea level change abstract the volume of antarctic ice at the last glacial maximum is a key factor for calculating the past contribution of melting ice sheets to late pleistocene global sea level change at present there are large uncertainties in our knowledge of the extent and thickness of the formerly expanded antarctic ice sheets and in the timing of their release as meltwater into the world s oceans this paper reviews the four main approaches to determining former antarctic ice volume namely glacial geology glacio isostatic studies glaciological modelling and ice core analysis and attempts to reconcile these to give a best estimate for ice volume in the ross sea there was a major expansion of grounded ice at the last glacial maximum accounting for 2 3 3 2 m of global sea level at some time in the weddell sea a large grounded ice sheet corresponding to c 2 7 m of global sea level extended to the shelf break however this ice expansion has not yet been confidently dated and may not relate to the last glacial maximum around east antarctica there was thickening and advance offshore of ice in coastal regions ice core evidence suggests that the interior of east antarctica was either close to its present elevation or thinner during the last glacial so the effect of east antarctica on sea level depends on the net balance between marginal thickening and interior thinning suggested east antarctic contributions vary from a 3 5 5 m lowering to a 0 64 m rise in global sea level the antarctic peninsula ice sheet thickened and extended offshore at the last glacial maximum with a sea level equivalent contribution of c 1 7 m thus the antarctic ice sheets accounted for between 6 1 and 13 1 m of global sea level fall at the last glacial maximum this is substantially less than has been suggested by most previous studies but the maximum figure matches well with one modelling estimate the timing of antarctic deglaciation is not well known in the ross sea terrestrial evidence suggests deglaciation may have begun at c 13 000 yr bp 1 but that grounded ice persisted until c 6 500 yr bp marine evidence suggests the western ross sea was deglaciated by c 11 500 yr bp deglaciation of the weddell sea is poorly constrained grounded ice in the northern antarctic peninsula had retreated by c 13 000 yr bp and further south deglaciation occurred sometime prior to c 6 000 yr bp many parts of coastal east antarctica apparently escaped glaciation at the lgm but in those areas that were ice covered deglaciation was underway by 10 000 yr bp with existing data the timing of deglaciation shows no firm relation to northern hemisphere driven sea level rise this is probably due partly to lack of antarctic dating evidence but also to the combined influence of several forcing mechanisms acting during deglaciation

magmatic hydrothermal evolution at the lyangar redox intermediate tungsten molybdenum skarn deposit western uzbekistan tien shan insights from igneous petrology hydrothermal alteration and fluid inclusion study abstract the lyangar w mo skarn deposit is situated in a metallogenic belt of w mo cu mo w au and au deposits and is related to an early permian multiphase monzodiorite granodiorite granite leucogranite pluton that was emplaced in a tectonic domain with possible ancient continental crust the latter coupled with an inferred ascent of hot asthenospheric material has defined an enrichment in mo and fluorine together with w and au to form a distinct metallogenic assemblage in a post collisional environment the parental moderately to high k transitional metaluminous to peraluminous weakly oxidized weakly reduced ilmenite titanite series granitoids are accompanied by hydrothermal alteration and mineralization that formed in redox intermediate environment the deposit incorporates prograde and retrograde pyroxene and garnet skarns composed of hedenbergitic pyroxene and grossular to almandine spessartine rich garnet respectively fluorite is present in the retrograde skarn together with scheelite and sulfides the latter including most abundant pyrrhotite and molybdenite hydrosilicate amphibole epidote chlorite quartz locally with albite sulfides and scheelite assemblage has limited development phyllic quartz sericite fe carbonate locally with albite fluorite scheelite and sulfides also bi and au minerals alteration assemblage partially overprints altered skarns and extends over much larger areas overall the deposit represents a superposition of the early w mo mineralization related to retrograde skarn and hydrosilicate alteration and the late au w cu bi mineralization in phyllic alteration assemblages prograde and retrograde skarns were formed from dominantly magnesian sodic chloride moderately saline 11 7 13 9 wt to 8 6 10 5 wt nacl eq high pressure 1500 to 1350 bars hot aqueous fluids that were followed by moderately saline 12 3 14 5 wt nacl eq ca rich lower pressure 600 bars fluids during deposition of most scheelite hydrosilicate propylitic assemblages were formed from boiling fluids at the temperatures of 380 430 c and pressures of 600 300 bars whereas phyllic alteration assemblages from boiling carbonic aqueous low salinity fluids at the temperatures of 300 320 c and pressures of 1000 bars 18oh2o values range from 7 8 to 8 6 for retrograde skarn and from 6 6 to 7 0 for phyllic alteration whereas 13cco2 from 3 3 to 0 5 for retrograde skarn and from 5 5 to 4 5 for phyllic alteration 34s increases from retrograde skarn 34s 2 0 to 2 3 to phyllic alteration 34s 4 3 to 4 6 stages

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

evidence for abundant subglacial meltwater beneath the paleo ice sheet in pine island bay antarctica marine geological and geophysical data collected from the continental shelf in pine island bay antarctica reveal a complex paleo subglacial drainage system controlled by bedrock topography and subglacial meltwater discharge significant amounts of freely flowing meltwater existed beneath former ice sheets in pine island bay subglacial drainage is characterized by descriptions of glacial landforms imaged on the sea floor and sedimentary deposits collected in piston cores bedrock geology is characterized using seismic data large scale landforms on the shelf include channels and cavities incised into impermeable crystalline bedrock there is a transition from randomly oriented channels on the inner shelf to a dendritic pattern of elongate channels on the middle shelf on the outer shelf a change in basal conditions occurs where sedimentary deposits bury crystalline bedrock no evidence for flowing meltwater exists on sedimentary substrates instead meltwater formed a the ice sediment contact was incorporated into the sediments contributing to development of a deforming lied which was sampled in piston cores characterization of subglacial meltwater processes that occurred in the past may aid in understanding the role meltwater plays in stability of the west antarctic ice sheet today

variations in sediment yield over the advance and retreat of a calving glacier laguna san rafael north patagonian icefield abstract bathymetric and sub bottom acoustic data were collected in laguna san rafael chile to determine sediment yields during the little ice age advance and subsequent retreat of san rafael glacier the sediment volumes and subaqueous landforms imaged are used to interpret the proglacial dynamics and estimate erosion rates from a temperate tidewater glacier over a complete advance retreat cycle sediment yields from san rafael glacier averaged 2 7 107 m3 a since the end of the little ice age circa ad 1898 corresponding to average basin wide erosion rates of 23 9 mm a the highest erosion rates 68 23 mm a occurred at the start of the retreat phase and have since been steadily decreasing erosion rates were much lower during glacial advance averaging at most 7 mm a than during retreat such large glacial sediment yields over two centuries of advance and retreat suggest that the contribution of sediments stored subglacially cannot account for much of the sediment being delivered to the terminus today the detailed sub bottom information of a proglacial lagoon yields important clues as to the timing of erosion deposition and transfer of glacigenic sediments from orogens to the continental shelves and the influence of glacier dynamics on this process

interhemispheric ice sheet synchronicity during the last glacial maximum the timing of the last maximum extent of the antarctic ice sheets relative to those in the northern hemisphere remains poorly understood we develop a chronology for the weddell sea sector of the east antarctic ice sheet that combined with ages from other antarctic ice sheet sectors indicates that the advance to and retreat from their maximum extent was within dating uncertainties synchronous with most sectors of northern hemisphere ice sheets surface climate forcing of antarctic mass balance would probably cause an opposite response whereby a warming climate would increase accumulation but not surface melting our new data support teleconnections involving sea level forcing from northern hemisphere ice sheets and changes in north atlantic deep water formation and attendant heat flux to antarctic grounding lines to synchronize the hemispheric ice sheets

seismic data from the northern basin ross sea record extreme expansions of the east antarctic ice sheet during the late neogene abstract approximately 3700 km of intermediate resolution single channel seismic data from the northern basin were analyzed to investigate the late neogene history of east antarctic ice sheet eais grounding events on the shelf and to evaluate how glacial unconformities on the shelf are manifested on the upper slope the northern basin was chosen as the site of this study because ice sheet reconstructions show that the basin received sediment by ice emanating from east antarctica in addition seismic correlations to dsdp site 273 suggest that a relatively thick late neogene section exists on the basin s outer shelf and upper slope on the northern basin shelf glacial unconformities exhibiting broad low angle relief topset truncation and cross cutting relationships reveal a dynamic history of expansions and contractions during which the eais was larger than present on at least eight occasions during the late neogene on the upper slope the correlative conformities of the glacial unconformities are indistinct reflections within thick trough mouth fan tmf depocenters at the mouths of drygalsky and joides basins the glacial unconformities and correlative conformities define tmf sequences and each tmf sequence contains several topset truncated prograding slope reflectors we infer that the correlative conformities on the continental slope correspond to the interface between prograding glaciogenic deposits glacial maximum and diatomaceous glacial marine sediments glacial minimum the seismic stratigraphic analysis and regional mapping indicate that the upper slope does not contain a more complete late neogene section than that which exists on the shelf we infer that diatomaceous glacial marine sediments on the slope may be relatively undisturbed and hence may provide a means of dating the tmf sequences it is hoped that these results will stimulate efforts to core the late neogene section in the northern basin tmfs to investigate how these eais expansions and contractions relate to other records of late neogene climate and eustasy

rates of sediment delivery from the fennoscandian ice sheet through an ice age sediment delivery rates through an entire ice age are investigated using seismic records of glacial erosion products from the fennoscandian ice sheet deposited offshore of mid norway in the 2 7 m y old naust formation volume 100 000 km 3 the mean sedimentation rate over the ice age is 0 24 m k y 1 with bedrock lowering of 520 m in the ice sheet catchment the mean sediment delivery is 2 3 times higher for the most recent 600 k y than for earlier naust sequences the hypothesis that glacial erosion is most rapid early in an ice age when there was presumably much weathered bedrock and preglacial sediment available is not accepted for our study area more important are changing ice sheet dimensions and dynamics varying intensity of individual glacial cycles and complex ice sheet dynamics in single deglaciations the mean rate of sediment delivery from ice sheets is an order of magnitude higher than from fluvial activity in the huge amazon and mississippi systems implying that ice is a very effective agent of long term denudation during cold periods of earth history

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

discovery of till deposition at the grounding line of whillans ice stream we report on the discovery of a grounding line sedimentary wedge till delta deposited by whillans ice stream west antarctica our observation is that grounding line deposition serves to thicken the ice and stabilize the position of the grounding line the ice thickness at the grounding line is greater than that of floating ice in hydrostatic equilibrium thus the grounding line will tend to remain in the same location despite changes in sea level until sea level rises enough to overcome the excess thickness that is due to the wedge further our observation demonstrates the occurrence of rapid subglacial erosion sediment transport by distributed subglacial till deformation and grounding line sedimentation which have important implications for ice dynamics numerical modeling of ice flow and interpretation of the sedimentation record

climate exposed source rock lithologies crustal uplift and surface erosion a theoretical analysis calibrated with data from the alps north alpine foreland basin system paleofloristic data imply that paleoclimate changed in the swiss alps at the oligocene miocene boundary from humid and hot conditions toward a climate with high temperature and low humidity the aridization is associated with a change in depositional pattern from alluvial fans to lakes and floodplains suggesting decreasing sediment discharge a further 25 40 decrease of sediment discharge occurred at ca 20 ma when the orogenic core of the alps became exposed to the surface we applied a surface processes model to explore potential controls on the pattern of sediment discharge and on the evolution of the alpine drainage basin the model is based on the presumption that the rates of fluvial incision into bedrock are proportional to shear stress exerted by the flowing water the model results imply that the paleoclimate change resulted in an instantaneous decrease of sediment discharge and a vertical topographic growth until steady state conditions between erosional and crustal mass flux are established however exposure of the crystalline core of the alps at ca 20 ma is likely to have resulted in the 25 40 decrease of sediment discharge and the reorganization of the drainage pattern from an orogen normal to an orogen parallel orientation of dispersion

west directed flow of the west antarctic ice sheet across eastern basin ross sea during the quaternary abstract detailed seismic stratigraphic correlations and contour mapping time structure and time thickness of near surface units in eastern basin ross sea outer continental shelf suggest that strata outcropping at ross bank were deposited as a terminal moraine in association with west directed advance of the west antarctic ice sheet grounding zone from marie byrd land during the grounding event there was a major shift from west directed ice sheet flow to north directed ice stream flow seismic correlation to age control at dsdp leg 28 sites 271 and 272 suggests a pre last glacial maximum quaternary age for this major grounding event