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

evidence for shock metamorphism in sandstones from the libyan desert glass strewn field microscopic analysis of sandstones from the southern libyan desert glass strewn field reveals a sequence of progressive deformation features which range from unaffected rock to extreme brecciation the quartz grains show a wide range of deformation features crushing and fracturing undulatory extinction mosaicism oriented cleavage partial isotropization and multiple sets of planar deformation features this preliminary report provides the first description of these shock indicators which are typical for hypervelocity impact in bedrock material that is assumed to be related to the libyan desert glass

anatomy of a meltwater drainage system beneath the ancestral east antarctic ice sheet the east antarctic ice sheet was larger than present during past cold periods seafloor geophysical data show that in the ross sea the extended ice sheet was underlain by an active hydrologic system during the glacial termination

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

retreat of the west antarctic ice sheet from the western amundsen sea shelf at a pre or early lgm stage recent palaeoglaciological studies on the west antarctic shelf have mainly focused on the wide embayments of the ross and amundsen seas in order to reconstruct the extent and subsequent retreat of the west antarctic ice sheet wais since the last glacial maximum lgm however the narrower shelf sectors between these two major embayments have remained largely unstudied in previous geological investigations despite them covering extensive areas of the west antarctic shelf here we present the first systematic marine geological and geophysical survey of a shelf sector offshore from the hobbs coast it is dominated by a large grounding zone wedge gzw which fills the base of a palaeo ice stream trough on the inner shelf and marks a phase of stabilization of the grounding line during general wais retreat following the last maximum ice sheet extent in this particular area referred to as the local last glacial maximum llgm reliable age determination on calcareous microfossils from the infill of a subglacial meltwater channel eroded into the gzw reveals that grounded ice had retreated landward of the gzw before 20 88 cal ka bp with deglaciation of the innermost shelf occurring prior to 12 97 cal ka bp geophysical sub bottom information from the inner mid and outer shelf indicates grounded ice extended to the shelf edge prior to the formation of the gzw assuming the wedge was deposited during deglaciation we infer the timing of maximum grounded ice extent occurred before 20 88 cal ka bp this could suggest that the wais retreat from the outer shelf was already underway during or even prior to the global lgm 23 19 cal ka bp our new findings give insights into the regional deglacial behaviour of this understudied part of the west antarctic shelf and at the same time support early deglaciation ages recently presented for adjacent drainage sectors of the wais if correct these findings contrast with the hypothesis that initial deglaciation of antarctic ice sheets occurred synchronously at 19 cal ka bp

past and future grounding line retreat of the west antarctic ice sheet the history of deglaciation of the west antarctic ice sheet wais gives clues about its future southward grounding line migration was dated past three locations in the ross sea embayment results indicate that most recession occurred during the middle to late holocene in the absence of substantial sea level or climate forcing current grounding line retreat may reflect ongoing ice recession that has been under way since the early holocene if so the wais could continue to retreat even in the absence of further external forcing

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

geological record of ice shelf break up and grounding line retreat pine island bay west antarctica the catastrophic break ups of the floating larsen a and b ice shelves antarctica in 1995 and 2002 and associated acceleration of glaciers that flowed into these ice shelves were among the most dramatic glaciological events observed in historical time this raises a question about the larger west antarctic ice shelves do these shelves with their much greater glacial discharge have a history of collapse here we describe features from the seafloor in pine island bay west antarctica which we interpret as having been formed during a massive ice shelf break up and associated grounding line retreat this evidence exists in the form of seafloor landforms that we argue were produced daily as a consequence of tidally influenced motion of mega icebergs maintained upright in an iceberg armada produced from the disintegrating ice shelf and retreating grounding line the break up occurred prior to ca 12 ka and was likely a response to rapid sea level rise or ocean warming at that time

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

ross sea paleo ice sheet drainage and deglacial history during and since the lgm onshore and offshore studies show that an expanded grounded ice sheet occupied the ross sea embayment during the last glacial maximum lgm results from studies of till provenance and the orienta

late pleistocene holocene retreat of the west antarctic ice sheet system in the ross sea part 1 geophysical results in the 1994 1995 and 1998 austral field seasons geophysical research efforts in the ross sea focused on acquiring a high resolution database designed to permit 1 reconstruction of the maximum extent and configuration of the ice sheet during the last glacial maximum i e oxygen isotope stage 2 2 reconstruction of conditions at the base of the ice sheet and 3 assessment of the relative retreat history of the ice sheet following the last glacial maximum five seismic facies are distinguished on the basis of external geometry relationships among features bounding surface amplitude intensity of internal acoustic signature and geometry of internal reflectors seismic facies 1 is a transparent draping unit correlated to diatomaceous mud interpreted as being deposited under open marine conditions seismic facies 2 and 3 are comprised of units displaying subdued massive internal signatures with smooth lower bounding surfaces and hummocky upper surfaces characterized by glacial lineations these units are interpreted to be grounding zone proximal deposits overridden by the expanded ice sheet seismic facies 2 may be a deforming till unit seismic facies 4a is characterized by an internally massive to chaotic signature an erosional often flat lower surface and a hummocky upper surface displaying glacial lineations this unit is interpreted to be till and is divided into deposits associated with the most recent glacial expansion 4a and with an earlier pre last glacial maximum expansion 4b seismic facies 5 is an acoustically laminated ponded and draping deposit interpreted to contain proglacial and sub ice shelf materials deposited continuously since the last interglacial period the associations of these units provide context for the interpretation of the ice edge maximum position conditions at the base of the ice sheet and the relative retreat history of the region there is compelling evidence for a much expanded ice sheet in the ross sea during the last glacial maximum in the western ross sea the maximum grounding position is marked by an isolated grounding zone wedge and placed in the vicinity of coulman island approximately 150 km from the continental shelf edge in the central ross sea the maximum grounding position is close to the continental shelf break based primarily on the presence of an extensive 60 m thick sheet like deposit with a fluted upper surface streaming ice may have occupied bathymetric lows on the continental shelf as suggested by 1 the configuration of bathymetry 2 the presence of glacially eroded troughs 3 the concentration of sediment interpreted to be deforming till within the middle to outer shelf reaches of the troughs and 4 the fluted nature of the upper surface absolute rates of streaming ice flow relative to inter ice stream areas are not implied the western ross sea continental shelf deposits record the retreat history of ice derived predominantly from the east antarctic ice sheet and glaciers of the transantarctic mountains ice flow on the continental shelf is interpreted to have remained fixed in position restricted by the walls of the troughs on the inner shelf of the western ross sea the ice flowed over and eroded lithified sedimentary strata slower moving ice occupied flat topped banks a single grounding zone wedge occurs on the western ross sea central shelf no substantial deposits are observed on the inner continental shelf this lack of grounding zone features reflects a restricted sediment supply and a relatively steady rapid retreat of the ice sheet during retreat ice flow acted independently in each major trough and on the bank tops ice retreated from victoria land basin drygalski trough before it retreated from joides basin ice remained on the bank tops shedding material into the abandoned troughs grounded ice in the central ross sea was derived predominantly from an expanded west antarctic ice sheet the ice sheet remained grounded on the continental shelf edge after retreat of ice from the western ross sea expanded ice eroded the inner shelf and deposited sheets of till on the central and outer shelf ice stream drainage shifted laterally as recorded by rounded laterally accreting ridges that separate bathymetric troughs the central ross sea is the repository for larger volumes of sediment derived from ice flow across basins of relatively thick unlithified sedimentary deposits two till sheets mark grounding zone positions in the central ross sea reflecting the higher sediment supply and a stepped deglaciation ice remained grounded on the pennell bank to the west a series of moraines marks the retreat of ice across the bank mega scale glacial lineations and other streamlined subglacial geomorphic features occur across the continental shelf primarily within the troughs the lineations substantiate the maximum reconstruction and support the interpretation of a deforming substrate beneath the outer reaches of the expanded ice sheet this deforming substrate may have contributed to the onset of deglaciation features associated with meltwater are rare or absent suggesting that basal meltwater played a minor role in retreat of the ice sheet

post lgm grounding line positions of the bindschadler paleo ice stream in the ross sea embayment antarctica the west antarctic ice sheet wais retreated more than 1000 km since last grounding at the ross sea outer continental shelf here we show an interpretation of former grounding line positions from a new large area multibeam survey and a regional grid of chirp cross section data from the whales deep basin in eastern ross sea the basin is a paleo glacial trough that was occupied by the bindschadler ice stream when grounded ice advanced to the shelf edge during the last glacial maximum lgm these new geophysical data provide unambiguous evidence that the wais occupied at least seven grounding line positions within 60 km of the shelf edge four of seven grounding zone wedges gzws are partly exposed over large areas of the trough the overlapping stratal arrangement created a large volume compound gzw cgzw some of the grounding events involved local re advance of the grounding line subsequent to these seven outer continental shelf groundings the ice sheet retreated more than 200 km toward roosevelt island on the middle continental shelf the major retreat across the middle continental shelf is recorded by small scale moraine ridges that mantle the top of gzw7 and these are suggestive of relatively continuous grounding line recession the results indicate that retreat was considerably more complex than was possible to reconstruct with reconnaissance level data the added details are important to climate models which must first be able to reproduce the recent retreat pattern in all of its complexities to improve confidence in model predictions of the system s future response

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

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

the influence of alpine glaciation on the relief of tectonically active mountain belts a surface processes model incorporating large scale fluvial processes local hill slope processes and glacial erosion is used to investigate the effect of glaciation on the relief of a fast growing narrow orogen the results of several simulations show that glaciers concentrate erosion near peaks fluvial erosion is reduced downstream of glaciers glaciers with frozen bases can lead to an increase in relief and the net affect of glaciation in the absence of widespread basal freezing is to reduce relief

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

subglacial processes on an antarctic ice stream bed 2 can modelled ice dynamics explain the morphology of mega scale glacial lineations mega scale glacial lineations msgls are highly elongate subglacial bedforms associated with ice streaming however the link between msgls and rapid ice flow is largely qualitative and there have been few attempts to quantitatively link their formation to ice flow characteristics e g ice velocity thickness basal shear stress we take measurements of msgls from a palaeo ice stream that once occupied marguerite trough antarctic peninsula and explore a range of possible correlations with ice dynamics generated from an ensemble of numerical modelling experiments that reproduce the deglaciation of the ice stream our results confirm that high mean ice velocities and a weak bed correlate with longer msgls furthermore the height of msgls are low 2 3 m where modelled basal shear stress is low but their height tends to be higher and more variable where basal shear stress is larger the mean density of msgls decreases as ice flux increases our analysis further suggests that the length of msgls is a function of basal ice velocity and time although our data model correlations confirm the importance of ice velocity in msgl formation a significant challenge remains if we are to employ msgls as a quantifiable measure of past ice stream velocity

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

palaeo ice stream pathways and retreat style in the easternmost amundsen sea embayment west antarctica revealed by combined multibeam bathymetric and seismic data multibeam swath bathymetry data sets collected over the past two decades have been compiled to identify palaeo ice stream pathways in the easternmost amundsen sea embayment we mapped 3000 glacial landforms to reconstruct ice flow in the 250 km long cross shelf abbot trough this bathymetric feature was occupied by a large ice stream which was fed by two tributaries cosgrove and abbot and reached the continental shelf edge during the last maximum ice sheet advance geomorphological mapping has enabled a clear differentiation between subglacial landforms indicating warm e g megascale glacial lineations and cold based e g hill hole pairs ice conditions on the continental shelf during the last glaciation grounding zone wedges and recessional moraines mapped within the palaeo ice stream troughs and on adjacent sea floor highs referred to as inter ice stream ridges indicate grounding line stillstands or re advances of the west antarctic ice sheet during the last deglaciation of the shelf we observe that the locations of grounding zone wedges coincide with trough constrictions as well as local topographic highs of harder substrate this combination of trough bottlenecks and local pinning points on an otherwise retrograde slope is likely to have modified the pace of grounding line retreat causing the grounding zone to pause and deposit grounding zone wedges the episodic retreat recorded within abbot trough corresponds to post glacial episodic retreat interpreted for the neighbouring pine island thwaites palaeo ice stream trough thus suggesting a uniform pattern of retreat across the eastern amundsen sea embayment locally indications are strong that a change in basal thermal regime of the ice from warm to cold based conditions occurred prior to final retreat as hill hole pairs overprint megascale glacial lineations further the correlation of grounding zone wedges with geological boundaries emphasises the influence of subglacial geology on ice stream flow our new geomorphological map of the easternmost amundsen sea embayment resolves the pathways of palaeo ice streams that were probably all active during the last maximum extent of the ice sheet and the extent of adjacent inter ice stream ridges it reveals information about the style of and the basal thermal regime during the subsequent grounding line retreat such information provides an important empirical framework by which the accuracy of ice sheet models can be gauged

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

flow and retreat of the late quaternary pine island thwaites palaeo ice stream west antarctica multibeam swath bathymetry and sub bottom profiler data are used to establish constraints on the flow and retreat history of a major palaeo ice stream that carried the combined discharge from the parts of the west antarctic ice sheet now occupied by the pine island and thwaites glacier basins sets of highly elongated bedforms show that at the last glacial maximum the route of the pine island thwaites palaeo ice stream arced north northeast following a prominent cross shelf trough in this area the grounding line advanced to within similar to 68 km of and probably reached the shelf edge minimum ice thickness is estimated at 715 m on the outer shelf and we estimate a minimum ice discharge of similar to 108 km 3 yr 1 assuming velocities similar to today s pine island glacier similar to 2 5 km yr 1 additional bed forms observed in a trough northwest of pine island bay likely formed via diachronous ice flows across the outer shelf and demonstrate switching ice stream behavior the style of ice retreat is also evident in five grounding zone wedges which suggest episodic deglaciation characterized by halts in grounding line migration up trough stillstands occurred in association with changes in ice bed gradient and phases of inferred rapid retreat correlate to higher bed slopes supporting theoretical studies that show bed geometry as a control on ice margin recession however estimates that individual wedges could have formed within several centuries still imply a relatively rapid overall retreat our findings show that the ice stream channeled a substantial fraction of west antarctica s discharge in the past just as the pine island and thwaites glaciers do today

continental collision gravity spreading and kinematics of aegea and anatolia we have carried out experiments using a layered medium of sand and silicone to investigate the lateral extrusion of a material which spreads over its own weight while being compressed by the advance of a rigid indenter boundary conditions in the box mimic those prevailing in the anatolian aegean system both shortening in front of the rigid piston which models the northward motion of arabia and extension resulting from the gravity spreading of the sand silicone layer are necessary to initiate the lateral extrusion strike slip faults accommodate the lateral escape and link the normal faults accompanying gravity spreading with the thrust faults in front of the rigid indenter strike slip faults begin to accommodate extrusion at a late stage in the experiments after the normal and thrust faults have developed experiments also show that the initial geometry of the boundary of the spreading layer may result in the formation of two arcs behind which material extends in a manner analogous to the hellenic and cypriot arcs without invoking a rheological change at the junction of the two arcs the experiments also suggest that southward motion of the eastern part of the spreading region is compensated by the northward advance of the piston which is a possible explanation for the slower movement of the cypriot arc compared to the aegean arc

on the influence of post lgm ice shelf loss and grounding zone sedimentation on west antarctic ice sheet stability abstract the whales deep basin in eastern ross sea was occupied by the paleo bindschadler ice stream during the last glacial maximum lgm previous studies showed that megascale glacial lineations msgls extend to the continental shelf edge in a landward direction these lineations are buried by an overlapping stack of backstepped grounding zone wedges gzws the constraints require that the west antarctic ice sheet wais grounded at least seven times within 60 km of the shelf edge but the morphological relationships in the whales deep basin cannot be used to uniquely reveal the magnitudes of either grounding line and or calving front retreat and re advance between the successive groundings here we show a new regional transect of cores from the basin which demonstrates that there were no major back and forth oscillations of either the grounding line or calving front during the overall retreat of grounded and floating ice instead the new sedimentologic data show that a small ice shelf formed as the grounding line retreated from the shelf edge and backstepped over the outer continental shelf an event bed records breakup of the ice shelf that covered the outer shelf the stratal relationships indicate that the ice shelf collapse occurred during the fourth grounding and that the grounding line remained on the outer continental shelf for three additional groundings we infer that accelerated ice stream flow due to loss of buttressing increased sediment flux to the grounding line and thinned the wais at the grounding line following the seventh grounding thinning contributed to an abrupt 200 km retreat of the grounding line to the middle continental shelf and a large ice shelf re formed over the middle continental shelf a series of small scale moraine ridges formed as the grounding line shifted south towards roosevelt island the facies relationships on the middle continental shelf indicate that the calving front then shifted abruptly to its modern position the ground truth of the seafloor geomorphology presented here is important because it is a necessary framework for constraining retreat chronology for a central part of the wais that was not influenced by east antarctic ice sheet overflow

the antarctic ice sheet during the last glacial maximum and its subsequent retreat history a review abstract an emerging body of evidence from studies of the last glacial interglacial cycle suggests that the east and west antarctic ice sheets have not advanced and retreated in concert the west antarctic ice sheet advanced to the outer shelf in most regions during the last glacial maximum lgm the retreat from the shelf commenced shortly after the lgm and continued into the late holocene the west antarctic ice sheet in ross and amundsen seas slid across a deforming bed at least during the final phases of extended glaciation this implies that at this time the ice sheet had a low profile differences in the number and locations of grounding zone wedges and smaller grounding zone features from trough to trough imply that individual west antarctic ice sheet ice streams retreated episodically details concerning the expansion retreat and behavior of the east antarctic ice sheet eais are more sparse the picture emerging is that the eais did not expand to the continental shelf edge during the lgm rather it achieved a maximum extent of a mid shelf position in some locations while in other areas the ice terminus was situated near its present location the timing of retreat along sectors within the eais appears diachronous and in places occurred prior to the lgm the antarctic peninsula shelf contained considerably more ice during the lgm than previously proposed the results presented in this paper support more recent published ice sheet models that call for greater contributions of melting ice from west antarctica including the antarctic peninsula to the post glacial rise in sea level

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

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

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

glaciology and geological signature of the last glacial maximum antarctic ice sheet dynamical changes in contemporary ice sheets account for significant proportions of their current rates of mass loss but assessing whether or not these processes are a natural part of ice sheet evolution requires inference from palaeo glaciological records however a robust mechanism for translating sparse geological data into meaningful interpretations of past glacier dynamics at the continental scale is lacking since geological archives can be ambiguous and often their chronology is only poorly constrained to address this we combine the interpretation of high resolution antarctic ice sheet model results with continent wide geological evidence pertinent to the dynamical configuration of the ice sheet during the last and possibly preceding glacial maxima we first focus on the thermal regime of the ice sheet its pattern and velocity of flow variability in likely subglacial erosion and sediment transport and how these characteristics evolve during glacial transitions we show that rapid basal sliding was restricted to discrete outlets that eroded and advected sediment toward and across the continental shelf primarily during the early stages of advance and retreat of the ice sheet highlighting the need to consider time transgressive behaviour in the interpretation of geological archives secondly we present new modelling that attempts to improve the fit of our numerical model to geologically based reconstructions in the ross sea by accounting for locally enhanced ablation in mcmurdo sound our new simulation achieves a much closer fit to empirically derived flow patterns than previously growth of the modelled last glacial maximum ice sheet takes place primarily by marine ice accretion in the major embayments as a consequence of cooler ocean temperatures and reduced sub ice shelf melting and at its maximal extent represents a grounded ice volume excess above present of approximately 8 3 m sea level equivalent this figure thus provides an upper bound on the possible antarctic contribution to deglacial meltwater pulses

till characteristics genesis and transport beneath antarctic paleo ice streams 1 marine geophysical data from the antarctic continental shelf indicate the former presence of ice streams that drained through bathymetric troughs during the last glacial cycle streaming flow is recorded by elongate subglacial bedforms formed in the upper part of an acoustically transparent sediment layer cores from the layer demonstrate that it is a weak and porous subglacial till soft till shear within the soft till was concentrated in zones that were 0 1 0 9 m thick the soft till is a hybrid till formed by a combination of subglacial sediment deformation and lodgment the base of the soft till is marked by a strong reflector which represents the top of a denser and stronger stiff till the form of this reflector ranges from flat and continuous to deeply grooved and irregular implying the operation of different mechanisms of sediment mobilization and incorporation into the soft till layer the irregular form of the basal reflector is consistent with grooving whereas the flat and continuous form is more consistent with sediment mobilization as a subglacial traction carpet related to changes in basal effective stress geophysical evidence for large scale advection of the soft till combined with evidence for deformation partitioning into relatively thin layers up to 0 9 m thick indicates that localized shear zones integrated to transport significant volumes of sediment beneath antarctic paleo ice streams

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

reconstructing the antarctic ice sheet at the last glacial maximum abstract a reconstruction of the antarctic ice sheet at the last glacial maximum lgm constrained by geologic data shows expansion across continental shelves accompanied by little inland change this added peripheral ice volume contributed 14 m to the lowering of eustatic sea level at the lgm of the total expansion one third took place in east antarctica and two thirds in west antarctica

ice sheet extent of the antarctic peninsula region during the last glacial maximum lgm insights from glacial geomorphology we present marine geological and geophysical data for the antarctic peninsula that call for a larger ice sheet reconstruction during the last glacial maximum lgm than suggested by previous studies such glacial reconstructions are important for quantifying post lgm sea level rise and providing boundary conditions for general circulation models megascale glacial lineations on the continental shelf surrounding antarctica provide documentation for grounded ice that was streaming swath bathymetry data nbp02 01 reveal lineations in each of the major glacial troughs except for smith trough which features grooves and bedrock drumlins 0 5 3 km spacing 1 20 elongation ratios on crystalline bedrock we place the maximum extent of ice at the seaward limit of the lineations at or near megascale glacial lineations imaged in this study and other areas surrounding antarctica including the ross sea pine island bay have a consistent morphology with elongation ratios of 80 1 and spacings of 200 600 m mode of 300 m in contrast lineations bundles canals et al 2000 in the gerlache boyd trough exhibit larger spacing 1 5 km and the upstream portions 7 km are carved directly into bedrock rather than till as suggested by previously unpublished air gun 120 hz seismic data these features have yet to be dated however they are believed to have been active during the lgm radiocarbon dates in glaciomarine sediments foraminifera and organic matter from seven glacial troughs indicate that megascale glacial lineations were formed during the lgm other geomorphic features such as the grounding zone wedge in the northwestern weddell sea formed when the ice sheet was retreating the initial retreat of the grounded ice from the outer shelf occurred by 18 500 cal yr b p the inner shelf was mostly ice free by 13 000 cal yr b p this is significantly earlier than numerical models which suggest antarctic deglaciation began at 12 000 cal yr b p this also suggests the peninsula area contributed to global sea level rise associated with meltwater pulse mwp 1a

topographic control on the post lgm grounding zone locations of the west antarctic ice sheet in the whales deep basin eastern ross sea abstract after the last glacial maximum lgm the antarctic ice sheet retreated several hundred kilometers from the outer continental shelf an overall backstepping stratal pattern of grounding zone wedges gzws indicates that the post lgm retreat occurred in a step wise fashion each gzw records a prior interval of ice stream sedimentation during which the location of paleo grounding zone i e the former seaward limit of flowing ice grounded to the seafloor was relatively stationary here we investigate whether the locations of post lgm grounding zone stillstands in the whales deep basin wdb of the eastern ross sea were controlled by the antecedent topography the bindschadler ice stream formerly occupied the wdb we used 7500 km of seismic reflection data to create a depth converted map of the lgm unconformity that was sub glacially eroded when the wais was grounded at the continental shelf edge i e prior to the deposition of the post lgm gzws our mapping shows that the first three paleo grounding zones were located slightly downstream of relatively low relief obstacles on the lgm unconformity the subsequent four grounding zones were located above a low relief saddle on the lgm unconformity at a bottleneck constriction in the width of the paleo ice stream basin these observations support the general hypotheses that post lgm grounding zone locations were at least partly controlled by the antecedent subglacial topography we cannot exclude the possibility that other factors may have also been important however the results suggest that models seeking to accurately predict the changing extent of grounded ice in response to ongoing climate changes should incorporate highly resolved maps of the existing subglacial topography as well as considerations as to how subglacial topography evolves in response to contemporaneous ice stream erosion and sedimentation

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

erosion rates during rapid deglaciation in icy bay alaska 1 contemporary glacial erosion rates based on sediment yields in southeast alaska merit considerable attention because they are unsurpassed worldwide and they significantly exceed long term exhumation rates in the region two issues are likely to contribute to these high rates contemporary sediment yields in fjords 1 have generally been overestimated by failing to account for the considerable input of subaerially derived material and 2 are exceptionally high because tidewater glaciers in southeast alaska have been anomalously dynamic and erosive during the past century of rapid retreat to investigate these influences and to quantify the rate at which tyndall glacier erodes its basin we present seismic data defining the volume of sediments in taan fjord icy bay we subtract the contribution of subaerially derived sediments from the fjord sediment package to determine the sediment yield directly from tyndall glacier during the most recent period of retreat 1962 1999 using a numerical model of proglacial glacimarine sedimentation we then calculate the annual sediment yield from and the corresponding erosion rate of tyndall glacier during this period which averages 28 5 mma 1 a strong correlation emerges between glacial retreat rates and glacial sediment yields implying that most contemporary sediment yield data from retreating tidewater glaciers may correspond to contemporary erosion rates that are a factor of 3 5 1 5 higher than in the long term hence we estimate the long term erosion rate for tyndall glacier to be 9 2 mma 1

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

expansion and rapid retreat of the west antarctic ice sheet in eastern ross sea possible consequence of over extended ice streams abstract marine geological data from the eastern ross sea provide compelling evidence that the west antarctic ice sheet extended to the continental shelf break during the last glacial maximum swath bathymetry records show mega scale glacial lineations within each of the four large glacial troughs on the shelf these features extend virtually uninterrupted across the shelf and have sharp lateral boundaries sediment cores that sampled the lineations recovered diamicton that is interpreted as deformation till the combined data indicate streaming ice the great extent of the paleo ice streams up to 1000 km in length is attributed to the fact that they flowed across unconsolidated sedimentary strata geomorphic features that are known to form at the retreating grounding line such as recessional moraines and ridges are restricted to the southeastern portion of the shelf iceberg furrows that occur at depths of up to 650 m overprint mega scale lineations these deep furrows are interpreted as having been formed by icebergs calved from the grounding line of the ice sheet which indicates the absence of a fringing ice shelf sediment cores sampled a thin layer of glaciomarine sediments resting sharply on till sub ice shelf deposits similar to those sampled in the western ross sea were not recovered in the eastern ross sea these combined results imply rapid retreat of the ice sheet from the shelf crosscutting lineations with different orientations indicate changes in flow direction that probably occurred as the ice profile was lowered and the physiography of the sea floor across which the ice sheet was retreating changed grounding zone wedges vary in number and location within individual troughs which indicates diachronous retreat of ice streams due to a lack of carbonate material for radiocarbon dating all radiocarbon ages were derived from acid insoluble organic aio fractions these dates indicate early retreat of the ice sheet relative to the western ross sea and other areas of west antarctica however the older dates must be viewed as suspect due to a paucity of diatomaceous material and contamination from old carbon while we were not successful at constraining the timing of ice sheet retreat from the continental shelf the data do indicate that the ice sheet was grounded on the shelf during the lgm

sea level changes at the lgm from ice dynamic reconstructions of the greenland and antarctic ice sheets during the glacial cycles abstract new experiments were performed with three dimensional thermomechanical models of the greenland and antarctic ice sheets to simulate their behaviour during the glacial cycles to reconstruct their thickness and extent at the last glacial maximum lgm and to estimate their glacio eustatic contribution to the global sea level stand the calculations used improved ice dynamic and isostatic treatments updated datasets on higher grid resolutions and refined climatic treatments based on newly calibrated transfer functions between ice core records and climatic perturbations results are discussed from a reference run with standard parameters that is compared with available glacial geological observations and from a series of sensitivity experiments focusing on isostatic adjustment thermomechanical coupling climatic forcing mass balance changes and basal melting rates and viscosity changes of antarctic ice shelves for the antarctic ice sheet we find that volume changes are closely linked with grounding line changes of the west antarctic ice sheet at the lgm the grounding line extended close to the continental shelf break almost everywhere ice over central east antarctica was generally thinner than today and varied mainly in accordance with accumulation fluctuations for the greenland ice sheet melting is important only during interglacial periods and the most sensitive period concerns the size of the ice sheet during the eemian at the lgm the greenland ice sheet extended beyond the present coastline to cover at least the inner continental shelf and thinned by up to several hundred meters in central areas for a plausible range of parameters the experiments indicate that at the time of maximum sea level depression 21 kyr bp the antarctic ice sheet contributed 14 18 m to the sea level lowering and the greenland ice sheet 2 3 m significantly less than the older climap reconstructions whereas both ice sheets were at 21 kyr bp close to their maximum extent the experiments also indicate that their maximum volumes were reached only by 16 5 kyr bp greenland and 10 kyr bp antarctica equal to an additional sea level lowering of respectively 0 4 and 3 7 m holocene retreat was essentially complete by 5 kyr bp in greenland but is found to still continue today in west antarctica before reversing to growth during the next millenium the models were found to reproduce gross features of the ice sheet s history since the lgm in reasonably good agreement with available glacial geological data although observational control on ice thickness changes remains very limited

chronology of late wisconsin ice retreat from the western ross sea antarctica lithologic data from marine sedimentary cores and accelerator mass spectrometer ams radiocarbon dates indicate that grounded ice did not advance to the western ross sea continental shelf edge during the last glacial maximum lgm a chronology of the timing of ice retreat was provided by 26 ams dates obtained from 12 cores dates ranging from 20 to 29 ka suggest that the outer continental shelf beyond ca lat 74 s was not covered by grounded glacial ice prior to and during the early stages of the lgm 14c dates just above transitions from subglacial diamictons to marine muds indicate that the area around the drygalski ice tongue was deglaciated by at least 11 5 ka the ross ice shelf reached its present day position near ross island by about 7 ka a hiatus of about 15 ka is apparent in radiocarbon dates in cores from the outer continental shelf the hiatus is interpreted to represent an absence of sedimentation caused by the presence of an ice shelf

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

the role of the cryosphere in source to sink systems abstract glacial erosion and sediment production are of interest to diverse scientific communities concerned with the interaction of climatic tectonic and surface processes that influence the evolution of orogens and with the climatic signals archived in glacigenic strata we review the current state of knowledge on the generation transfer and accumulation of glacigenic sediment from land to sea we draw from geomorphology marine geology geochronology numerical modeling of surface processes and landscape evolution and experimental and field observations of glacier erosion and deposition and the interaction of ice with its bed and the ocean boundary our primary goal is to examine glacial systems using a holistic source to sink approach with a focus on describing a how glacial motion produces sediment b how the sediments sink record the dynamic nature of glacial systems under different climatic thermal regimes c the challenges in using the sedimentary record to interpret these dynamics in space and time and d the approaches still needed to further our understanding of how ice and associated sediment fluxes respond to climatic and other perturbations the dynamic state of ice i e the ice flux and ice extent is defined differently between the source and sink communities reflecting the challenges of establishing a stratigraphic signal that volumetrically constrains glacigenic sediment production as a function of the ice response to climate advances in marine geophysics have greatly assisted our understanding of mass transfer pathways and of former ice extents as a measure of ice dynamics and have identified the primary depocenters and key lithofacies of glacial sinks sediment fluxes associated with the dynamic state of the ice are best constrained where sediment volumes derived from key lithofacies and seismic reflection isopachs can be temporally partitioned of which there are few examples rather than from discrete point measures of sediment flux that are subject to sediment transfer biases forward numerical modeling of sediment fluxes as a function of ice dynamics agree with observational data at the continental margin scale but finer time space scale ice dynamic models do not yet recreate observed ice extent or flowpaths future source to sink work in glaciated systems should focus on refining empirical relationships between ice velocity and sediment production and expand the application of existing methods to develop sediment volumes and fluxes in known depocenters of former and modern ice streams

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

modelling west antarctic ice sheet growth and collapse through the past five million years changes in earth s orbit are known to influence climate shifts from cold glacials to warm interglacials how the vast west antarctic ice sheet responds to these fluctuations is uncertain but because its collapse could raise sea levels by about 5 metres of great interest naish et al have analysed the and 1b ocean sediment core extracted from beneath the ross ice shelf as part of the andrill drilling project and find evidence that the ice sheet collapsed periodically during the early pliocene 3 5 million years ago when atmospheric co2 levels were similar to or slightly higher than today s the pattern of collapse suggests an influence of approximately 40 000 year cycles in the tilt of earth s rotational axis obliquity also in this issue of nature in a numerical modelling study focused on the past 5 million years in antarctica david pollard and robert deconto combine ice sheet land supported and ice shelf water supported modelling approaches to simulate the movement of the grounding line the border between land and sea ice their results show that over the past 5 million years the west antarctic ice sheet transitioned between full intermediate and collapsed states in just a few thousand years this means that the ice sheet is likely to disintegrate if ocean temperatures in the area rise by 5 c if the west antarctic ice sheet wais melted sea levels would rise by about 5 m such changes are thought to have occurred in the past but could not be simulated by models pollard and deconto combine ice sheet with ice shelf modelling and show that over the past 5 million years the wais transitioned among full intermediate and collapsed states in only a few thousand years suggesting possible disintegration of the wais if ocean temperatures in the area rise by 5 c the west antarctic ice sheet wais with ice volume equivalent to 5 m of sea level1 has long been considered capable of past and future catastrophic collapse2 3 4 today the ice sheet is fringed by vulnerable floating ice shelves that buttress the fast flow of inland ice streams grounding lines are several hundred metres below sea level and the bed deepens upstream raising the prospect of runaway retreat3 5 projections of future wais behaviour have been hampered by limited understanding of past variations and their underlying forcing mechanisms6 7 its variation since the last glacial maximum is best known with grounding lines advancing to the continental shelf edges around 15 kyr ago before retreating to near modern locations by 3 kyr ago8 prior collapses during the warmth of the early pliocene epoch9 and some pleistocene interglacials have been suggested indirectly from records of sea level and deep sea core isotopes and by the discovery of open ocean diatoms in subglacial sediments10 until now11 however little direct evidence of such behaviour has been available here we use a combined ice sheet ice shelf model12 capable of high resolution nesting with a new treatment of grounding line dynamics and ice shelf buttressing5 to simulate antarctic ice sheet variations over the past five million years modelled wais variations range from full glacial extents with grounding lines near the continental shelf break intermediate states similar to modern and brief but dramatic retreats leaving only small isolated ice caps on west antarctic islands transitions between glacial intermediate and collapsed states are relatively rapid taking one to several thousand years our simulation is in good agreement with a new sediment record andrill and 1b recovered from the western ross sea11 indicating a long term trend from more frequently collapsed to more glaciated states dominant 40 kyr cyclicity in the pliocene and major retreats at marine isotope stage 31 1 07 myr ago and other super interglacials

subglacial processes on an antarctic ice stream bed 1 sediment transport and bedform genesis inferred from marine geophysical data the spatial pattern and morphometry of bedforms and their relationship to sediment thickness have been analysed in the marguerite bay palaeo ice stream trough western antarctic peninsula over 17 000 glacial landforms were measured from geophysical datasets and sediment thickness maps were generated from acoustic sub bottom profiler data these analyses reveal a complex bedform pattern characterised by considerable spatial diversity influenced heavily by the underlying substrate the variability in length and density of mega scale lineations indicates an evolving bedform signature whereby landforms are preserved at different stages of maturity lineation generation and attenuation is associated with regions of thick soft till where deformation was likely to be the greatest the distribution of soft till and the localised extent of grounding zone wedges gzws indicate a dynamic sedimentary system characterised by considerable spatio temporal variability in sediment erosion transport and deposition formation of gzws on the outer shelf of marguerite trough within the error range of the radiocarbon dates requires large sediment fluxes upwards of 1000 m3 a 1 m grounding line width 1 and a 1 m thick mobile till layer or rapid basal sliding velocities upwards of 6 km a 1

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

late pleistocene holocene retreat of the west antarctic ice sheet system in the ross sea part 2 sedimentologic and stratigraphic signature sedimentologic geotechnical geochemical and accelerator mass spectrometer ams radiocarbon data from two marine geologic cruises in the ross sea have allowed us to constrain facies relationships and temporal changes in the west antarctic ice sheet the selection of core sites was facilitated by the use of multibeam bottom imagery and a good quality chirp subbottom reflection system a complex but consistent succession of facies documents a number of environments from subglacial to open marine massive mud rich diamictons have low water contents contain in places a calcareous microfossil assemblage show minimal textural variation and contain low and uniform total organic carbon values this reflects a subglacial setting this unit passes upward into a stratified thin granulated facies consisting of pelletized sandy muddy gravel that is loosely compacted and contains a variable water content and concentrated horizons of pebble sized clasts this facies reflects the lift off zone or thin water film between the basal debris and sea floor overlying this unit are silty clays that contain a well sorted very fine grained sand component there are no coarse grains within this unit this facies is in part laminated and reflects deposition beneath an ice shelf near and away from the grounding line zone the ice shelf facies passes upward into a siliceous mud and ooze unit that represents deposition in an open marine setting a sandy volcaniclastic rich subfacies marks the transition from ice shelf to open marine environments found at the calving line the 86 ams radiocarbon dates on organic matter provide an accurate chronology for 19 cores ice shelf conditions were established in the outer drygalski trough by 11 0 25 ka and perhaps earlier this transition took place in the joides basin by 10 8 ka the calving front of the ross ice shelf passed over the drygalski trough at 74 s by 9 5 0 25 ka the timing of deglaciation in the ross sea calls into question current models for the contribution of antarctic glacial ice to holocene sea level rise and suggests that recession was relatively gradual and more closely aligned with northern hemisphere deglaciation and its associated eustatic pulse

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

mass balance of polar ice sheets recent advances in the determination of the mass balance of polar ice sheets show that the greenland ice sheet is losing mass by near coastal thinning and that the west antarctic ice sheet with thickening in the west and thinning in the north is probably thinning overall the mass imbalance of the east antarctic ice sheet is likely to be small but even its sign cannot yet be determined large sectors of ice in southeast greenland the amundsen sea embayment of west antarctica and the antarctic peninsula are changing quite rapidly as a result of processes not yet understood

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

characteristics of the deforming bed till properties on the deglaciated antarctic continental shelf contemporary ice stream flow is directly linked to conditions at the ice bed interface yet this environment is logistically difficult to access instead we investigate subglacial processes important for ice stream flow by studying tills on the deglaciated antarctic continental shelf we test currently accepted hypotheses surrounding subglacial processes and till properties with a ross sea dataset till shear strengths indicate a continuum of simultaneous processes acting at the bed rather than discrete deformation and lodgement end members we identify a threshold water content representing saturated pore spaces leading to basal sliding and meltwater channelization based on observations of till properties relative to glacial landforms we challenge the assumption that low shear strength is linked to intense deformation spatial variability in landform morphology reflects variability in deforming processes at the sub ice stream scale and suggests a maximum deforming bed thickness of 2 m at the grounding line regional till properties generally correlate with seafloor geology and deglacial history the western ross sea is characterized by higher and more variable shear strengths and water contents while lower shear strength till was preserved in the eastern basin these observations inform till interpretation and provide context for deforming beds beneath the modern ice sheet and on glaciated continental shelves

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