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

late quaternary submarine bedforms and ice sheet flow in gerlache strait and on the adjacent continental shelf antarctic peninsula geophysical data from gerlache strait croker passage bismarck strait and the adjacent continental shelf reveal streamlined subglacial bedforms that were produced at the bed of the antarctic peninsula ice sheet apis during the last glaciation the spatial arrangement and orientation of these bedforms record the former drainage pattern and flow dynamics of an apis outlet up flow and feeding into a palaeo ice stream in the western bransfield basin evidence suggests that together they represent a single ice flow system that drained the apis during the last glaciation the ice sheet outlet flowed north northeastwards through gerlache strait and croker passage and converged with a second more easterly ice flow tributary on the middle shelf to form the main palaeo ice stream the dominance of drumlins with low elongation ratios suggests that ice sheet outlet draining through gerlache strait was comparatively slower than the main palaeo ice stream in the western bransfield basin although the low elongation ratios may also partly reflect the lack of sediment progressive elongation of drumlins further down flow indicates that the ice sheet accelerated through croker passage and the western tributary trough and fed into the main zone of streaming flow in the western bransfield basin topography would have exerted a strong control on the development of the palaeo ice stream system but subglacial geology may also have been significant given the transition from crystalline bedrock to sedimentary strata on the inner mid shelf in the broader context the apis was drained by a number of major fast flowing outlets through cross shelf troughs to the outer continental shelf during the last glaciation copyright 2004 john wiley sons ltd

spatial variability of antarctic peninsula net surface mass balance 1 measurements from ice cores and snow pits collected over the last 50 years are used to examine how net surface mass balance varies across the antarctic peninsula to give the first detailed map of mass balance for the region a total of 211 reliable mass balance measurements were available for the preparation of the map but some areas were found to be very data sparse the analysis suggests that the largest values of mass balance are found along the spine of the northern part of the peninsula where over 2 5 m yr 1 water equivalent we has been measured a secondary peak of more than 2 0 m yr 1 we is determined along the mountains of eastern alexander island precipitation minus evaporation p e fields from the european centre for medium range weather forecasts reanalysis project are compared with our analysis of in situ data the model fields are found to have peak values of p e of only half the amounts found from the measurements the greatest model values are located on the western side of the peninsula areas where a high density of in situ data is available including king george vi sound and the high south central plateau part of the peninsula show a high spatial variability of net surface mass balance suggesting that local orographic features play a major part in dictating the mass balance

reassessment of net surface mass balance in antarctica abstract recent in situ measurements of surface mass balance and improved calculation techniques are used to produce an updated assessment of net surface mass balance over antarctica a new elevation model of antarctica derived from ers 1 satellite altimetry supplemented with conventional data was used to delineate the ice flow drainage basins across antarctica the areas of these basins were calculated using the recent digital descriptions of coastlines and grounding lines the delineation of drainage basins was achieved using an automatic procedure which gave similar results to earlier hand drawn catchment basins more than 1800 published and unpublished in situ measurements of net surface mass balance from antarctica were collated and then interpolated a net surface mass balance map was derived from passive microwave satellite data being employed as a forcing field to control the interpolation of the sparse in situ observations basinwide integrals of net surface mass balance were calculated using t

sedimentation associated with antarctic peninsula ice shelves implications for palaeoenvironmental reconstructions of glacimarine sediments abstract recent disintegration of a number of antarctic peninsula ice shelves has given us a unique opportunity to investigate sub ice shelf sediments we characterize three sediment facies associations of two antarctic peninsula ice shelves larsen a and larsen inlet subglacial facies consist mainly of basal till diamicton with high shear strengths deposited under a grounded ice sheet progressive upward decrease in shear strength reflects a gradual decrease in the confining vertical effective pressure of grounded ice during till deposition proximal ice shelf glacimarine facies diamicton gravel rich and sand rich facies gravelly mud dropstone mud and sandy muds were deposited by sub ice shelf rain out bottom current activity and sediment gravity flows following decoupling of grounded ice from the sea floor distal ice shelf glacimarine and or open marine facies comprise terrigenous and diatom bearing bioturbated muds and gravelly muds that contain limited ice rafted debris these accumulated after recession of the grounding line to the coast with coarse grained surface sediments possibly documenting most recent ice shelf break up antarctic peninsula ice shelf sediments are more heterogeneous than ice shelf facies deposited elsewhere in antarctica cold polar antarctic ice shelves can be differentiated from temperate and sub polar marine terminated glacial sedimentary systems by the dominance of coarse grained proximal ice shelf glacimarine facies and an absence of subaqueous outwash meltwater sediment facies

the evolution of the patagonian ice sheet from 35 ka to the present day patice abstract we present patice a gis database of patagonian glacial geomorphology and recalibrated chronological data patice includes 58 823 landforms and 1 669 geochronological ages and extends from 38 s to 55 s in southern south america we use these data to generate new empirical reconstructions of the patagonian ice sheet pis and subsequent ice masses and ice dammed palaeolakes at 35 ka 30 ka 25 ka 20 ka 15 ka 13 ka synchronous with the antarctic cold reversal 10 ka 5 ka 0 2 ka and 2011 ad at 35 ka the pis covered of 492 6 x103 km2 had a sea level equivalent of 1 496 mm was 350 km wide and 2090 km long and was grounded on the pacific continental shelf edge outlet glacier lobes remained topographically confined and the largest generated the suites of subglacial streamlined bedforms characteristic of ice streams the pis reached its maximum extent by 33 28 ka from 38 s to 48 s and earlier around 47 ka from 48 s southwards net retreat from maximum positions began by 25 ka with ice marginal stabilisation then at 21 18 ka which was then followed by rapid irreversible deglaciation by 15 ka the pis had separated into disparate ice masses draining into large ice dammed lakes along the eastern margin which strongly influenced rates of recession glacial readvances or stabilisations occurred at least at 14 13 ka 11 ka 6 5 ka 2 1 ka and 0 5 0 2 ka we suggest that 20th century glacial recession a 1 is occurring faster than at any time documented during the holocene

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

bathymetry of the amundsen sea continental shelf implications for geology oceanography and glaciology the amundsen sea continental shelf is one of the most remote areas of coastal antarctica and was relatively unexplored until the late 1980s over the last two decades increased oceanographic and geological interest has led to several cruises that resulted in sufficient bathymetric data to compile a fairly detailed regional map of the amundsen continental shelf we have combined available multibeam and single beam bathymetry data from various sources and created a new regional bathymetry of the amundsen sea continental shelf and margin deep trough systems that dominate the inner shelf are aligned with present glaciers and separated by shallower ridges shaped by paleo ice streams these features merge into a small number of broader troughs on the middle shelf and shoal seaward they now serve as conduits and reservoirs for relatively warm circumpolar deep water this new compilation is a major improvement over previously available regional maps and should aid the numerical modeling of ocean circulation the reconstructions of paleo ice streams and the refinement of ice sheet models

the norwegian greenland sea continental margins morphology and late quaternary sedimentary processes and environment the continental margins surrounding the norwegian greenland sea are to a large degree shaped by processes during the late quaternary the paper gives an overview of the morphology and the processes responsible for the formation of three main groups of morphological features slides trough mouth fans and channels several large late quaternary slides have been identified on the eastern norwegian greenland sea continental margin the origin of the slides may be due to high sedimentation rates leading to a build up of excess pore water pressure perhaps with additional pressure caused by gas bubbles triggering might have been prompted by earthquakes or by decomposition of gas hydrates trough mouth fans tmf are fans at the mouths of transverse troughs on presently or formerly glaciated continental shelves in the norwegian greenland sea seven tmfs have been identified varying in area from 2700 km2 to 215 000 km2 the trough mouth fans are depocentres of sediments which have accumulated in front of ice streams draining the large northwest european ice sheets the sediments deposited at the shelf break upper slope by the ice stream were remobilized and transported downslope mostly as debris flows the trough mouth fans hold the potential for giving information about the various ice streams feeding them with regard to velocity and ice discharge two large deep sea channel systems have been observed along the norwegian continental margin the lofoten basin channel and the inbis channel along the east greenland margin several channel systems have been identified the deep sea channels may have been formed by dense water originating from cooling sea ice formation and brine rejection close to the glacier margin or they may originate from small slides on the upper slope transforming into debris flows and turbidity currents

successive pore fluid generations in a lower permian brine aquifer palo duro basin texas panhandle u s a abstract the successive presence of four compositionally distinct pore fluids in lower permian carbonate strata of the palo duro basin texas panhandle is interpreted from 1 lithological minerlogical and petrographic evidence for depositional conditions 2 isotopic compositions of c o and sr in limestone and dolomite 3 limited data on fluid inclusions in sphalerite and 4 chemical and isotopic analyses of formation water wolfcamp carbonate mudstone containing normal marine fauna was initially deposited on an open shallow shelf the c and o isotopic compositions indicate precipitation from permian seawater with minor terrestrial and meteoric influence sr isotopic compositions document a leonardian seawatersource of material conditions gradually became more restricted and seawater was concentrated by evaporation resulting in deposition of penecontemporaneous dolomite and anhydrite in overlying wichita strata reflux of wichita brine dolomitized underlying wolfcamp carbonate mudstone and generated the upper wolfcamp aquifer isotopic compositions of sr in wolfcamp dolomite suggest that the reflux brine was evaporatively concentrated leonardian seawater expulsion of warm saline formation water from deeper in the basin through the upper wolfcamp aquifer is recorded in fluid inclusions in sphalerite this change in pore fluid composition occured before and during regional tertiary uplift and tilting of the texas panhandle more recently meteoric water has recharged at least the western one third of the upper wolfcamp aquifer this modern flow regime was established within the past 10 15 ma

late quaternary glacial history flow dynamics and sedimentation along the eastern margin of the antarctic peninsula ice sheet geological and geophysical data from the ne antarctic peninsula continental shelf are used to reconstruct the glacial history flow dynamics and sedimentation of the antarctic peninsula ice sheet apis along its eastern margin during the late quaternary ice advanced to the shelf edge during the last glacial maximum lgm and deposited a stiff till across the shelf the presence of highly attenuated bedforms indicates that fast flowing outlets drained the apis through cross shelf troughs to the outer shelf after this ice advance the bedforms are formed in deformation till in response to deforming bed processes deglaciation of robertson trough and the troughs of prince gustav channel larsen a and larsen inlet was continuous and possibly rapid on account of the absence of ice margin recessional features in contrast grounding zone wedges across the shelf of northern larsen a and south of prince gustav channel indicate that ice retreat was gradual and was punctuated by stillstands the transition from a grounded ice sheet to ice shelf conditions was completed before 11 12 14c ka bp on the shelf south of prince gustavchannel and is marked across the shelf by a change from subglacial till to a transitional heterogeneous unit dominated by coarse grained facies transitional sediments record mainly sub ice shelf rain out and restricted bottom current and sediment laden plume activity as well as localised debris flows meltwater derived facies are largely absent indicating that release of meltwater was not significant beneath polar ice shelves or during deglaciation of the apis in the broader context the colder eastern side of the apis was extensive and was drained mainly through fast flowing outlets palaeo ice streams therefore the eastern apis would have been an important contributor to sediment and iceberg flux to the weddell sea embayment during the lgm and subsequent deglaciation

seismic evidence for a water filled canal in deforming till beneath rutford ice stream west antarctica 1 seismic data from rutford ice stream west antarctica show high amplitude reversed polarity reflections from parts of the bed only interfaces between ice and water or between water and sediment have the acoustic properties to produce such strong reflections ice sediment interfaces produce much weaker echoes the evidence suggests that an area of the ice stream measuring at least 1 km by 0 2 km is underlain by water with a depth of less than 1 m the data are consistent with the presence of a sub ice drainage system comprising shallow canals cut into the sedimentary substrate as predicted by theory

the melting history of the late pleistocene antarctic ice sheet spatial and temporal variations in the sea levels of the past 20 000 years around the globe place constraints on the melting history of the major late pleistocene ice sheets the antarctic ice sheets provided a significant contribution to the sea level rise at a rate that was approximately synchronous with the melting of the laurentide ice sheet except for the interval 9 000 6 000 years ago when it may have lagged behind minor melting of the antarctic ice sheet continued throughout the holocene

late quaternary architecture of trough mouth fans debris flows and suspended sediments on the norwegian margin abstract trough mouth fans are the main marine depocentres for glacier derived sediments in the polar north atlantic but their growth through the late quaternary is complex glacigenic debris flows gdfs are sourced from a common and homogeneous part of the upper fan and only develop as coherent individual flows after downslope transport their genesis and mode of deposition mean that gdfs are confined to particular areas of trough mouth fans accumulation of these subglacial sediments is controlled by a combination of margin glaciology and fan morphology although most of the fan sediment is deposited as gdfs during glacials considerable areas of trough mouth fans are dominated by sedimentation of suspension deposits associated with extensive meltwater release from a warm based ice sheet and probable contour current activity the depositional sequence of these two sediment types may be important in generating the long run out distances of gdfs which are initiated and sustained over low gradients furthermore emplacement of gdfs is interpreted to be a relatively low frequency event and temporally at least fans are not dominated by this mode of sediment emplacement whilst ice sheets are at the shelf break large scale trough mouth fan development is therefore asynchronous and non uniform a result of the interaction between glaciology morphology and oceanography

late weichselian submarine debris flow deposits on the bear island trough mouth fan sedimentary processes on the bear island trough mouth fan during the last glacial were studied using high resolution reflection seismics and gravity cores the fan succession is dominated by large debris flow deposits of up to 24 km in width and 50 m thick debris lobes can be followed over 100 km downslope and the largest cover an area of 1880 km2 the large debris flows were generated when the barents sea ice sheet reached the shelf break glacigenic sediments transported to the grounding line were temporarily stored on the upper slope due to the high sedimentation rate the sediments were unstable and earthquakes oversteepening and or build up of excess pore pressure triggered sediment release generating large debris flows sediments were also eroded and incorporated during the downslope flow debris flows were the most important sediment distributing process due to the relatively high shear strength low flow mobility of the glacigenic diamicton and the low fan gradient the weichselian evolution of the fan was characterized by a relatively low sedimentation rate during the early and mid weichselian the barents sea ice sheet might have reached the shelf break twice during the late weichselian on average one large debris flow comprising 18 km3 of sediments was released between every 35 to 75 years during the last glacial maximum

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

flow of the west antarctic ice sheet on the continental margin of the bellingshausen sea at the last glacial maximum geophysical data show that during the last glaciation the west antarctic ice sheet wais drained to the continental shelf edge of the bellingshausen sea through a cross shelf bathymetric trough belgica trough as a grounded fast flowing ice stream the drainage basin feeding this ice stream probably encompassed southwestern palmer land parts of southern alexander island and the bryan coast of ellsworth land with an area exceeding 200 000 km2 on the inner continental shelf streamlined bedrock and drumlins mapped by swath bathymetry show that the ice stream was fed by convergent ice flow draining from eltanin bay and bays to the east as well as by ice draining the southern part of the antarctic peninsula ice sheet through the ronne entrance the presence of a paleoice stream in belgica trough is indicated by megascale glacial lineations formed in soft till and a trough mouth fan on the continental margin grounding zone wedges on the inner and midshelf record ice marginal stillstands during deglaciation and imply a staggered pattern of ice sheet retreat these new data indicate an extensive wais at the last glacial maximum lgm on the bellingshausen sea continental margin which advanced to the shelf edge in conjunction with ice sheet reconstructions from the antarctic peninsula and pine island bay this implies a regionally extensive ice sheet configuration during the lgm along the antarctic peninsula bellingshausen sea and amundsen sea margins with fast flowing ice streams draining the wais and antarctic peninsula ice sheet to the continental shelf edge

the sedimentary legacy of a palaeo ice stream on the shelf of the southern bellingshausen sea clues to west antarctic glacial history during the late quaternary a major trough belgica trough eroded by a palaeo ice stream crosses the continental shelf of the southern bellingshausen sea west antarctica and is associated with a trough mouth fan belgica tmf on the adjacent continental slope previous marine geophysical and geological studies investigated the bathymetry and geomorphology of belgica trough and belgica tmf erosional and depositional processes associated with bedform formation and the temporal and spatial changes in clay mineral provenance of subglacial and glaciomarine sediments here we present multi proxy data from sediment cores recovered from the shelf and uppermost slope in the southern bellingshausen sea and reconstruct the ice sheet history since the last glacial maximum lgm in this poorly studied area of west antarctica we combined new data physical properties sedimentary structures geochemical and grain size data with published data shear strength clay mineral assemblages to refine a previous facies classification for the sediments the multi proxy approach allowed us to distinguish four main facies types and to assign them to the following depositional settings 1 subglacial 2 proximal grounding line 3 distal sub ice shelf sub sea ice and 4 seasonal open marine in the seasonal open marine fades we found evidence for episodic current induced winnowing of near seabed sediments on the middle to outer shelf and at the uppermost slope during the late holocene in addition we obtained data on excess pb 210 activity at three core sites and 44 ams c 14 dates from the acid insoluble fraction of organic matter aio and calcareous micro fossils respectively at 12 sites these chronological data enabled us to reconstruct for the first time the timing of the last advance and retreat of the west antarctic ice sheet wais and the antarctic peninsula ice sheet apis in the southern bellingshausen sea we used the down core variability in sediment provenance inferred from clay mineral changes to identify the most reliable aio c 14 ages for ice sheet retreat the palaeo ice stream advanced through belgica trough after similar to 36 0 corrected c 14 ka before present b p it retreated from the outer shelf at similar to 25 5 ka b p the middle shelf at similar to 19 8 ka b p the inner shelf in eltanin bay at similar to 12 3 ka b p and the inner shelf in ronne entrance at similar to 6 3 ka b p the retreat of the wais and apis occurred slowly and stepwise and may still be in progress this dynamical ice sheet behaviour has to be taken into account for the interpretation of recent and the prediction of future mass balance changes in the study area the glacial history of the southern bellingshausen sea is unique when compared to other regions in west antarctica but some open questions regarding its chronology need to be addressed by future work c 2010 elsevier ltd all rights reserved

large scale sedimentation on the glacier influenced polar north atlantic margins long range side scan sonar evidence long range side scan sonar gloria imagery of over 600 000 km of the polar north atlantic provides a large scale view of sedimentation patterns on this glacier influenced continental margin high latitude margins are influenced strongly by glacial history and ice dynamics and linked to this the rate of sediment supply extensive glacial fans up to 350 000 km were built up from stacked series of large debris flows transferring sediment down the continental slope the fans were linked with high debris inputs from quaternary glaciers at the mouths of cross shelf troughs and deep fjords where ice was slower moving but still extended to the shelf break large scale slide deposits are observed where ice failed to cross the continental shelf during full glacials the continental slope was sediment starved and submarine channels and smaller slides developed a simple model for large scale sedimentation on the glaciated continental margins of the polar north atlantic is presented

evolution of subglacial bedforms along a paleo ice stream antarctic peninsula continental shelf geophysical data from the antarctic peninsula continental shelf reveal streamlined subglacial bedforms in a cross shelf trough bedforms exhibit progressive elongation with distance along the trough and record flow of a paleo ice stream from the antarctic peninsula ice sheet during the last glacial maximum downflow evolution of the bedforms indicates increasing flow velocities as the ice stream traversed the shelf this in turn is related to a transition from crystalline bedrock on the inner shelf to a soft sedimentary substrate on the outer shelf although streaming flow operated across both substrates the highest flow velocities occurred over the soft bed spatial variation in the inferred nature of fast flow from sliding to subglacial sediment deformation and or ploughing was also lithologically controlled these data highlight the control of subglacial geology on ice stream dynamics in the geological record and demonstrate a direct relationship between the formation of streamlined subglacial bedforms and paleo ice streams

the antarctic ice sheet during the last glacial interglacial cycle a three dimensional experiment a complete 3 d thermo mechanical ice sheet model for the entire antarctic ice sheet including an ice shelf grounding line dynamics and isostatic bedadjustment is employed to simulate the response of the ice sheet during the last glacial interglacial cycle with respect to changing environmental conditions to do this the vostok temperature signal is used to force changes in surface temperature and accumulation rate and sea level prescribed by a piecewise linearsawtooth function model calculations started at 160 kyears bp in line with glacial geological evidence the most pronounced fluctuations are found in thewest antarctic ice sheet and appear to be essentially controlled by changes in eustatic sea level grounding occurs more readily in the weddell sea than in theross sea and due to the long time scales involved the ice sheet does not reach its full glacial extent until 16 kyears bp the concomitant disintegration of thewest antarctic ice sheet is triggered by a rise in sea level and takes around 6000 years to complete the ice sheet then halts close to the present state and nocollapse takes place this holocene deglaciation appears to have added 6 8 million km3 of ice to the world oceans corresponding with an antarcticcontribution to world wide sea level of 12 15 m

relict sea floor ploughmarks record deep keeled antarctic icebergs to 45 s on the argentine margin abstract this paper reports ploughmarks on the argentine continental margin up to 44 50 s the most northerly morphological evidence of giant icebergs in the southern atlantic more than 2500 ploughmarks up to 32 km long 685 m wide and more than 20 m deep have been observed at modern water depths up to 646 m taking into account these iceberg sizes and ocean current directions which appear to have remained similar from the last full glacial to the present interglacial the icebergs that produced the ploughmarks are interpreted to be calved from fast flowing ice streams draining huge basins of the west antarctic ice sheet and the western antarctic peninsula during the last glacial maximum lgm about 20 000 years ago ploughmarks at the deepest water depths correspond to icebergs more than 500 m thick and at least 2 km wide given a sea level rise of 120 m since the lgm the icebergs after having drifted northeast in the antarctic circumpolar current were transported northward by the falklands malvinas current a total distance of 2000 to 4000 km the waters north of the falkland islands in particular were probably several degrees colder than today to prevent rapid iceberg melting and deterioration

geomorphological criteria for identifying pleistocene ice streams ice streams are critical regulatory mechanisms in contemporary ice sheets it has been inferred that they also had a significant effect on the dynamics of former ice sheets subsequently many people have invoked their widespread occurrence from a variety of formerly glaciated areas hypothesised locations however have often outweighed meaningful evidence this paper addresses the problem using the characteristics of contemporary ice streams as a basis for their identification from former ice sheet beds a convergence of knowledge gained from contemporary ice stream research coupled with theories of glacial geomorphology allows several geomorphological criteria to be identified as suggestive signatures of ice stream activity it is envisaged that the geomorphological criteria developed here will introduce a more objective approach to the study of former ice streams the criteria are used to construct conceptual land system models of the beds of former ice streams and it is hoped such models can provide an observational template upon which hypotheses of former ice streams can be better based

late pleistocene and holocene melting history of the antarctic ice sheet derived from sea level variations abstract spatial and temporal variations of the late pleistocene and holocene sea levels provide important constraints on the melting history of the major ice sheets in this study we have examined the sea level variations at eight sites along the coast of antarctica to investigate the melting history of antarctic ice sheet complexes comparisons of sea level variations between observations and predictions due to glacio hydro isostatic adjustment suggest that the ice thicknesses removed from the last glacial maximum lgm around the weddell sea are significantly thicker than those around the ross embayment sea level observations also indicate that climap derived models predict too much relative sea level change and that consequently ice mass change along the coast has to be scaled down improvement of the late pleistocene antarctic ice model is required to evaluate more accurate predictions of present day geophysical observables associated with this history thus these more accurate predictions can then be subtracted from observables and the residual field can be analyzed to constrain ongoing mass changes from the ice sheet we therefore predicted present day geophysical observables such as the rate of change of solid surface gravity and crustal movement these geophysical observables based on the ice models constructed from observed sea level variations are significantly different from those based on previous models on the other hand detailed sea level observations around syowa indicate the existence of a sea level highstand of about 10 m around 40 kyr bp to investigate these sea level observations we have performed preliminary numerical calculations comparisons between these observations and predictions indicate the synchronous melting of the northern and southern ice sheets suggesting a strong link between the melting histories of both polar ice sheets

flow dynamics and till genesis associated with a marine based antarctic palaeo ice stream geophysical and geological data indicate that during the last glacial cycle a palaeo ice stream drained the antarctic peninsula ice sheet apis through marguerite bay to the edge of the continental shelf via a bathymetric trough marguerite trough mega scale glacial lineations msgl in outer marguerite trough are formed in an acoustically transparent sediment unit consisting of a soft deformation till the association of msgl with deformation till indicates that bed deformation occurred beneath the ice stream and the thickness of the subglacial deforming layer was on the order of several metres msgl within outer marguerite trough are related to a pre existing thick sequence of soft sediments on the outer shelf and are inferred to have formed by a combination of subglacial sediment deformation by attenuation from a point source and groove ploughing morphologically the msgl differ in several respects from those documented elsewhere and this variation suggests a polygenetic origin for this subglacial landform an initial advance of the apis across the marguerite bay continental shelf at the lgm deposited a stiff subglacial till following this advance the ice margin within marguerite trough retreated 70 100 km before stabilising and depositing a grounding zone wedge an ice stream then developed in the trough and extended to the shelf edge forming a soft deformation till development of the ice stream may have been a glacio dynamic response to regional deglaciation subsequent retreat of the marguerite trough ice stream was underway by 13 490 uncorrected 14c years before present based on the geophysical and core data ice stream retreat appears to have been rapid and it contrasts with reconstructions of palaeo ice streams from other locations in antarctica this implies marked regional variations in antarctic ice stream dynamics during late quaternary deglaciation

the sedimentary record of the last glaciation in the western bellingshausen sea west antarctica implications for the interpretation of diamictons in a polar marine setting upper quaternary marine sediments recovered from the west antarctic continental margin are characterized by a distinct lithological succession allowing the reconstruction of past environmental changes massive homogenous diamictons were deposited elsewhere on the margin during the last glacial period when grounded ice masses advanced across the shelf sedimentological investigations using a multi proxy approach and examination of the margin topography suggest that during this time a deformation till and subsequently a glaciomarine till were deposited on the shelf while glaciogenic debris flows were deposited on the slope and rise the comparison of the clay mineral assemblages in the matrix of the diamictons with clay mineral assemblages of potential source areas reveals distinct pathways of grounded ice flow on the shelf lithogenic sandy muds overlying the diamictons were deposited by meltwater flows and or marine currents on the shelf and by turbidity currents and marine currents on the slope and rise respectively the sedimentation of the sandy muds denotes a deglaciation stage when grounded ice started to retreat from the shelf but semi permanent sea ice coverage still hampered biological productivity bioturbated foraminifer bearing sediments were deposited in a glaciomarine setting during the present interglacial period when sea ice cover was only seasonally present and a marine current related to the southern boundary of the antarctic circumpolar current winnowed the seafloor on the outer shelf slope and rise

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

early deglaciation of the british irish ice sheet on the atlantic shelf northwest of ireland driven by glacioisostatic depression and high relative sea level understanding the triggers and pace of marine based ice sheet decay is critical for constraining the future mass loss and dynamic behaviour of marine based sectors of the large polar ice sheets in greenland and antarctica numerical models which seek to predict this behaviour need to be calibrated against data from both contemporary and palaeo ice sheets and the latter requires accurate reconstruction of former ice sheet extent dynamics and timing marine geophysics sediment cores benthic foraminiferal assemblages and radiocarbon dating are used to reconstruct the extent of the last british irish ice sheet biis and the timing and style of its retreat on the atlantic shelf northwest of ireland shelf edge moraines and subglacial till recovered in cores from the outer continental shelf are dated to younger than 26 3 ka cal bp and indicate an extensive ice sheet at the last glacial maximum lgm that was grounded to the shelf edge nested arcuate moraines record the subsequent episodic retreat of the ice sheet across the shelf lithofacies and associated foraminiferal assemblages demonstrate that this retreat occurred in a glacimarine environment as a grounded tidewater margin and that high relative sea level and cold waters prevailed during retreat radiocarbon dates indicate that the timing of initial ice sheet retreat from the shelf edge occurred in the interval between 26 3 and 24 8 ka cal bp during the period of minimum global eustatic sea level and that the ice sheet had retreated to the mid shelf by 24 8 ka cal bp the donegal bay moraine a large moraine at the mouth of donegal bay records a major stillstand and readvance of the ice sheet during deglaciation between 20 2 and 17 9 ka cal bp estimated retreat rates of 5 5 35 m a 1 across the shelf demonstrate that retreat was slow it is noteworthy that retreat was initiated in the absence of ocean warming and when eustatic sea level was at a minimum the sea level rise that initiated deglaciation from the shelf edge therefore is inferred to have been a product of local glacio isostatic crustal depression rather than external forcing this demonstrates that marine based sectors of ice sheets can trigger their own demise internally through glacio isostatic adjustment and it provides an explanation for the early retreat of the biis on the atlantic shelf during the global lgm glgm

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

abraded rock landforms whalebacks developed under ice streams in mountain areas like many mountain ranges the coast mountains of british columbia canada have undergone both local and ice sheet glaciation effects of ice sheets are concentrated along major valleys and on adjacent spurs and passes which carried strong flows of diffluent ice the major valleys are broad glacial troughs with frequent rock basins their slopes are broken into rounded steep sided bosses whalebacks abraded on all sides they are of the order of 100 m to 1 km long and 10 m high in the southern coast mountains the distribution of these whalebacks is consistent with a proposed pattern of former ice streams 1 0 2 1 km thick within the cordilleran ice sheet they are best developed where geological structures parallel the valley and thus the former ice flow direction but they are found on a range of lithologies and some are transverse to structure the whalebacks provide an impression of glacial streamlining and occasionally grade into rock drumlins roches moutonnees are rare in the major troughs it is hypothesised that these whalebacks and rock drumlins develop under ice streams of greenland or east antarctic type sliding rapidly over bedrock and exploiting rock weaknesses to produce streamlined features lee slopes are abraded when thick ice suppresses bed separation even with rapid flow basal ice of low viscosity would aid this suppression water pressures under the ice streams may have remained high so that lee side plucking was rare such plucking is most likely where pressure fluctuates dramatically and especially when lee cavities under active ice reach atmospheric pressure

dating diagenetic monazite in mudrocks constraining the oil window authigenic monazite nodules increasingly recognized in pre mesozoic mudrock successions provide a means for dating diagenesis by exploiting trace uranium within the monazite structure but dating is difficult in practice because of zonation and abundant host rock inclusions we report here a refined technique that has yielded a date of 419 14 ma 2 mswd 1 2 for a 17 point pb pb regression age from a telychian silurian late llandovery mudrock from central wales shrimp analyses of monazite nodules from the same horizon are consistent with this and additionally demonstrate concordance of the u pb and th pb systems these results open the door to providing robust dates for a phase of burial related diagenesis which in wales seems approximately coincident with and perhaps was related to hydrocarbon expulsion

morphology and sedimentary processes on the continental slope off pine island bay amundsen sea west antarctica morphology and sedimentation on the pine island bay continental margin eastern amundsen sea is investigated using swath bathymetry parametric subbottom profiler topas records and sediment cores a major cross shelf trough extends to the continental shelf edge at 114 w through which fast flowing ice drained the west antarctic ice sheet under full glacial conditions the slope adjacent to the trough mouth is 4 whereas to the east and west it is steeper a bulge in bathymetric contours beyond and east of the trough mouth indicates some slope progradation elsewhere seismic reflection profiles indicate major shelf aggradation and minor slope progradation networks of gullies and channels dominate the slope adjacent to pine island bay in front of the trough mouth gullies at the shelf edge and uppermost slope connect directly with channels that continue to the continental rise other gullies connect indirectly with channels further downslope topas records indicate that gully channel systems acted as conduits for coarse grained sediment transfer cores comprise turbidites representing channel deposits and diamicton facies deposited by debris flows sandy turbidites interbedded with hemipelagic muds occur on the rise and abyssal plain the west antarctic ice sheet extended to the shelf edge during full glacials delivering water saturated till and turbid water directly to the continental slope along a line source gully channel systems are probably a product of sediment gravity flows derived from the release of water saturated till and turbid water along the ice sheet margin perhaps augmented subsequently by downslope flow of dense cold and saline water associated with sea ice production during the holocene the gully channel systems enable sediment to bypass the continental slope spatial variations in gully channel system morphology along the amundsen sea continental slope are probably linked to differences in sediment gravity flow processes and the rate and composition of sediment delivered to the shelf edge from the ice sheet cored sediments on the slope and abyssal plain record the widespread presence of turbidite sands indicating that downslope transfer of glacier derived sediments has taken place this supports our interpretation that submarine channels are derived from and are conduits for mass wasting submarine channels on the upper slope continue into the abyssal plain as far north as 67 s they are separated by sediment drifts and sediment waves resulting from the interaction between downslope turbidity current processes and along slope bottom currents similar deep sea sedimentary processes operate along much of west antarctica and the western side of the antarctic peninsula

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

distribution of glacial geomorphic features on the antarctic continental shelf and correlation with substrate implications for ice behavior surveys were conducted seaward of all the major drainage outlets of the antarctic ice sheet from the pennell coast north victoria land to marguerite bay antarctic peninsula the results show that the ice sheet extended onto the outer shelf glacial troughs occur offshore of all major glacial outlets where the substrate is crystalline bedrock ice flow tended to follow the structural grain of the bedrock deposited little sediment and eroded the underlying bedrock where ice flowed over relatively soft more easily eroded sedimentary strata the direction of ice flow was more directly offshore and depositional features characterize the sea floor in these areas the signature of the grounded ice consists of till deposits and large scale geomorphic features drumlins occur within the region of contact between crystalline and sedimentary substrates the different geological substrates are interpreted to have exerted a fundamental control on the behavior of past ice sheets the troughs in the areas of bedrock composed of sedimentary substrate are interpreted to have been occupied by relatively fast flowing ice ice streams and the troughs in the areas of crystalline substrate are interpreted to have been occupied by slower moving ice the area between these two zones was characterized by ice acceleration and is marked by drumlins

mechanisms of fast flow in jakobshavns isbrae west greenland part i measurements of temperature and water level in deep boreholes several holes were drilled to depths of 1500 1630 m along a profile across jakobshavns isbrae 50 km upstream from the calving front drilling was by hot water and required approximately 20 h the holes were rapidly closed by refreezing but it was possible to instrument them with thermistors and tilt sensors before this occurred near the margins of the ice stream the holes reached the bed and connected with the subglacial drainage system water level changes recorded in these holes are discussed in terms of the basal hydraulic system the temperature measurements show that the glacier is temperate based moreover extrapolation of a measured temperature profile and its curvature suggests that a temperate layer of substantial thickness may exist at the bed near the center of the ice stream there is a striking difference in the shapes of temperature profiles measured at different locations beneath the center line the temperature minimum is at a considerably smaller relative depth than near the margins but it is nearly the same in magnitude 22 1 c this may indicate a disproportionately large vertical stretching of the basal ice in the center of the ice stream since the basal ice is warmer and much less viscous than the ice above a thickening of that layer would cause a corresponding increase of surface velocity we presume that this mechanism contributes to the fast flow of jakobshavns isbrae

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

ice 3g a new global model of late pleistocene deglaciation based upon geophysical predictions of post glacial relative sea level change a new high resolution global model of late pleistocene deglaciation is inferred on the basis of geophysical predictions of postglacial relative sea level variations in which the ice ocean solid earth interaction is treated in a gravitationally self consistent fashion for the purpose of these analyses the radial viscoelastic structure of the planet is assumed known on the basis of previously published sensitivity tests on solutions of the forward problem only radiocarbon controlled relative sea level histories from sites that were actually ice covered with one or two additions are employed to constrain the model leaving relative sea level rsl data from sites that were not ice covered to be employed to confirm its consistency results for these confirmatory analyses are reported elsewhere here the new deglaciation model referred to as ice 3g is compared to previous models derived by several independent means and tested against a number of additional observations other than sea level histories including geologically controlled retreat isochrones oxygen isotope data from deep sea sedimentary cores and coral terrace elevations the latter two observations strongly constrain the net sea level rise that has occurred since the onset of deglaciation and therefore the mass of ice that melted during the last glacial interglacial transition

laurentide ice streaming on the canadian shield a conflict with the soft bedded ice stream paradigm elucidating the controls on the location and vigor of ice streams is crucial to understanding the processes that lead to fast disintegration of ice flows and ice sheets in the former north american laurentide ice sheet ice stream occurrence appears to have been governed by topographic troughs or areas of soft sediment geology this paper reports robust evidence of a major paleo ice stream over the northwestern canadian shield an area previously assumed to be incompatible with fast ice flow because of the low relief and relatively hard bedrock a coherent pattern of subglacial bedforms drumlins and megascale glacial lineations demarcates the ice stream flow set which exhibits a convergent onset zone a narrow main trunk with abrupt lateral margins and a lobate terminus variations in bedform elongation ratio within the flow set match theoretical expectations of ice velocity in the center of the ice stream extremely parallel megascale glacial lineations tens of kilometers long with elongation ratios in excess of 40 1 attest to a single episode of rapid ice flow we conclude that while bed properties are likely to be influential in determining the occurrence and vigor of ice streams contrary to established views widespread soft bed geology is not an essential requirement for those ice streams without topographic control we speculate that the ice stream acted as a release valve on ice sheet mass balance and was initiated by the presence of a proglacial lake that destabilized the ice sheet margin and propagated fast ice flow through a series of thermomechanical feedbacks involving ice flow and temperature

deep sea floor evidence of past ice streams off the antarctic peninsula the past existence of a giant ice stream off the northern end of the antarctic peninsula is suggested by a convex upward elongated sediment body now at a mean water depth of 1000 m because of its morphological characteristics i e a set of parallel to subparallel ridges and grooves to 100 km long with an overall width of 25 km we call this depositional body a bundle structure we hypothesize that bundle structures form by accumulation of basal deformation till under their parent ice streams from its location size and overall characteristics the bundle structure described here constitutes the best preserved largest deepest and relatively low latitude evidence of giant ice streams that flowed offshore antarctica during glacial maxima bundle structures reveal the very dynamic behavior of ice caps in the northern antarctic peninsula during the last glacial maximum with catchment areas draining rapidly under marine based critical subglacial conditions bundle structures represent a new megascale streamlined glacial landform

geometry and development of glacial continental margin depositional systems in the bellingshausen sea we present multi channel seismic mcs reflection profiles and bathymetry data acquired across a remote and poorly surveyed part of the antarctic continental margin in the bellingshausen and amundsen seas this new information has been combined with published data and used to interpret the style of sedimentation on the continental shelf slope and rise and to describe sedimentation processes which have been active in this region most seismic reflection profiles crossing the continental margin show prograded sequences beneath the outer shelf and upper slope and we infer that the stratal characteristics of these sequences indicate that grounded ice sheets reached the shelf edge during previous glacial times although there are general similarities in stratal geometry on these profiles in detail they reveal significant longitudinal variations in sediment input from the shelf to the upper slope on several profiles we found evidence of mass wasting of the continental slope in the form of slump and debris flow deposits at greater depth turbidity flows bottom currents and coriolis force have controlled the further transportation and deposition of sediment which has resulted in the development of mounds channels and sediment wave fields the distribution and variations in the size and geometry of the mounds reflect sediment input and the relative contribution of these other factors which control sedimentation on the continental rise

deformation of till beneath ice stream b west antarctica the behaviour and possible instability of the west antarctic ice sheet depend fundamentally on the dynamics of the large ice streams which drain it model calculations show that most ice stream velocity arises at the bed1 2 and radar sounding has shown the bed to be wet3 but the basal boundary condition is not well understood seismic evidence from the upstream b camp upb on the siple coast of west antarctica4 shows that the ice stream there rests on a layer of unconsolidated sediment averaging 5 or 6 m thick in which the water pressure is only 50 kpa less than the overburden pressure because this thin layer occurs well inland beneath an active ice sheet and rests on a surface showing flutes4 characteristic of glacial erosion5 we presume that it is glacial till we propose here that deformation within the till is the primary mechanism by which the ice stream moves and we discuss implications of this hypothesis

relict subglacial deltas on the antarctic peninsula outer shelf the first deep tow boomer survey on the antarctic continental margin has revealed relict subglacial deltas on the outer continental shelf off the antarctic peninsula progradation of subglacial deltas is thought to take place at the grounding lines of ice streams which flow on deforming subglacial till acoustic characteristics and estimation of likely sediment transport rates suggest that these features were produced by late stage readvance of grounding lines during the waning of the last ice sheet that covered the shelf this readvance could have taken place during the younger dryas 12 9 11 6 ka but changes in sea level and climate may not be the only important controls on subglacial delta formation the discovery of relict subglacial deltas on the outer shelf is consistent with the hypothesis that the grounded ice sheet in these areas was low profile and fast flowing if low profile grounded ice extended to the shelf edge in many places around antarctica at times of glacial maximum this could explain the greater glacial maximum ice sheet extent in interpretations based on offshore data compared with reconstructions based on onshore data and numerical glaciological models

glacigenic sediments on a passive continental margin as exemplified by the barents sea abstract reflection seismics data borehole data and gravity cores have been used to map the sedimentary facies of the epicontinental southwestern barents sea its continental margin and adjacent coastal areas an upper unconformity with a glacially eroded morphology extends over most of the continental shelf up to 300 m of stratiform glacigenic sediments overlie this unconformity these sediments can be grouped into seismic units which are separated by smooth or irregular erosional surfaces each sequence may be up to 150 m thick normally with a lateral extent of 100 300 km three dominant seismic signatures may be discerned semi transparent stratified and chaotic reflection patterns these sediments are partly glacifluvial in origin but on the whole they represent more or less tectonized glacimarine deposits the deeper troughs on the continental shelf contain an undeformed postglacial glacimarine marine trough fill the greatest thickness up to 1000 m of glacigenic sediments occurs at the shelf break and below the upper slope the depocentres are situated at trough mouth fans the sediments comprise several prograding sequences with a complex sigmoid oblique character indicating alternating upbuilding and depositional bypass erosion in the topset segment small and large scale slide scars are observed on the upper slope and debris lobes on the lower slope a marked progradation seems to have occurred during the plio pleistocene glacial periods this progradation also resulted in an unstable slope with various types of sediment gravity flows currents flowing along the slope were established during interglacials and gullying by cold dense shelf water and downslope transport occurred maximum net sedimentation rates are 0 2 m ka continental slope 0 4 m ka shelf break 0 1 m ka shelf stratiform diamictons 1 m ka shelf trough fills and 60 m ka fjord trough fills

future of the west antarctic ice sheet a collapse of the antarctic ice sheets would raise the sea level by 5 meters and depending on how quickly it occurred would pose an enormous threat to global economy and ecology how stable are these volumes of ice in his perspective bindschadler discusses recent efforts to model the behavior of the west antarctic ice sheet on the basis of data on past ice sheet movement including results reported in the same issue by wingham et al consideration of these past data suggests that the ice sheets will remain stable for the next 4000 to 7000 years but an accurate assessment awaits more data on ice sheet history and better understanding of ice sheet dynamics

acoustic impedance and basal shear stress beneath four antarctic ice streams the acoustic impedance of the subglacial material beneath 7 2 km profiles on four ice streams in antarctica has been measured using a seismic technique the ice streams span a wide range of dynamic conditions with flow rates of 35 464 in a 1 the acoustic impedance indicates that poorly lithified or dilated sedimentary material is ubiquitous beneath these ice streams mean acoustic impedance across each profile correlates well with basal shear stress and the slipperiness of the bed indicating that acoustic impedance is a good diagnostic not only for the porosity of the subglacial material but also for its dynamic state deforming or non deforming beneath two of the ice streams lodged non deforming and dilated deforming sediment coexist but their distribution is not obviously controlled by basal topography or ice thickness their distribution may be controlled by complex material properties or the deformation history beneath rutford ice stream lodged and dilated sediment coexist and are distributed in broad bands several kilometres wide while on talutis inlet there is considerable variability over much shorter distances this may reflect differences in the mechanism of drainage beneath the ice streams the material beneath the slow moving carlson inlet is probably lodged but unlithified sediment this is consistent with the hypothesis that carlson inlet was once a fast flowing ice stream but is now in a stagnant phase which could possibly be revived by raised basal water content the entire bed beneath fast flowing evans ice stream is dilated sediment

distinguishing subglacial till and glacial marine diamictons in the western ross sea antarctica implications for a last glacial maximum grounding line analyses of lithology stratigraphy and tephra from marine sediment cores collected from the western ross sea during cruises eltanin 32 and 52 and deep freeze 80 and 87 indicate that subglacial till does not extend to the continental shelf edge subglacial till occurs as the lowest unit in most cores landward south of approximately 74 s while seaward of approximately 74 s the lowest diamicton units are glacial marine diamictons glacial marine diamictons are distinguished from subglacial tills by the presence of higher and more variable total organic carbon content downcore distinct tephra layers stratification higher diatom and foraminifera abundances higher sand content and radiocarbon dates in chronological order downcore sand sized tephra layers from two cores on the outer continental shelf are interpreted as single eruptive events one likely to have been derived from the mount melbourne volcano and the other from the pleiades volcano radiocarbon dates from sediment above and below the tephra layer in one of these cores df87 32 show that deposition indicative of open water conditions occurred between 22 and 26 ka in the western ross sea

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

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

late quaternary ice flow and sediment delivery through hinlopen trough northern svalbard margin submarine landforms and depositional fan abstract the morphology and distribution of submarine landforms in hinlopen trough northern svalbard margin and the upper continental slope beyond are investigated using swath bathymetric data together with side scan sonar acoustic profiler records and sediment cores sun illuminated images reveal a number of landform assemblages on the sea floor of hinlopen trough and confirm that this depression was occupied by an ice stream during the late weichselian glaciation around 20 ka before present the geomorphology of the inner shelf is characterised by bedrock drumlins and ice sculpted bedrock there is a middle shelf transition from a rock bed to an unconsolidated sedimentary bed that contains highly attenuated drumlins and megaflutes formed within a dark grey matrix supported diamict interpreted as subglacial till outer shelf landforms are mainly iceberg ploughmarks the geomorphological imprint identified from hinlopen trough is similar to that of many former ice streams the trough is dominated by elongate landforms orientated parallel to the inferred flow direction and is lacking in transverse features indicative of inter ice stream locations a characteristic down flow progression in landform elongation is also observed suggesting an increase in ice velocity across the continental shelf the lack of grounding zone features imaged from swath bathymetric data implies that deglaciation was probably rapid within the trough a large depositional fan exists on the upper continental slope beyond the trough characterised by debris flow deposits relating to the down slope transport of glacigenic sediment order of magnitude calculations of the volume of glacigenic sediment on the continental slope indicate that the ice stream provided high rates of debris delivery of around 5 7 5 m ka this suggests that hinlopen trough ice stream represented a major route for the transfer of ice and debris to the hinlopen fan on the northern continental margin of svalbard during the late weichselian glaciation

influence of subglacial geology on the position of a west antarctic ice stream from seismic observations ice streams drain much of the interior west antarctic ice sheet and buffer the main ice reservoir from oceanic influences1 2 the slow flowing interior feeds the floating ross ice shelf with ice via fast flowing ice streams3 that are believed to modulate sea level change through their control of inland ice storage understanding ice stream behaviour and predicting the response to climate change4 requires a better knowledge of the subglacial geology5 6 it is known that a thawed ice bed and high pressure basal water are necessary but not sufficient conditions to cause ice streaming7 8 moreover it has been hypothesized that a soft sedimentary bed is also required because of its intrinsic low frictional resistance to flow9 and owing to its high erodibility so as to generate till that can deform and lubricate ice motion10 11 or to bury rough features and smooth the bed for sliding here we use seismic observations to provide evidence that one margin of the upglacier part of an ice stream is directly above the boundary of a basin with such sedimentary fill the ice stream is within the basin and the ice outside the basin is slow flowing the basin fill presents an order of magnitude lower frictional resistance to ice flow than the subglacial material outside the basin we conclude that the ice stream position is dependent on subglacial geology

mechanisms of holocene palaeoenvironmental change in the antarctic peninsula region the antarctic peninsula is one of the three fastest warming regions on earth here we review holocene proxy records of marine and terrestrial palaeoclimate in the region and discuss possible forcing mechanisms underlying past change with a specific focus on past warm periods our aim is to critically evaluate the mechanisms by which palaeoclimate changes might have occurred in order to provide a longer term context for assessing the drivers of recent warming two warm events are well recorded in the holocene palaeoclimate record namely the early holocene warm period and the mid holocene hypsithermal mhh whereas there are fewer proxy data for the mediaeval warm period mwp and the recent rapid regional rrr warming we show that the early holocene warm period and mhh might be explained by relatively abrupt shifts in position of the southern westerlies superimposed on slower solar insolation changes a key finding of our synthesis is that the marine and terrestrial records in the ap appear to show markedly different behaviour during the mhh this might be partly explained by contrasts in the seasonal insolation forcing between these records circumpolar deep water cdw has been implicated in several of the prominent changes through the holocene but there are still differences in interpretation of the proxy record that make its influence difficult to assess further work is required to investigate contrasts between marine and terrestrial proxy records east west contrasts in palaeoclimate the history of cdw to retrieve a long onshore high resolution record of the holocene and determine the role of sea ice in driving or modulating palaeoclimate change along with further efforts to study the proxy record of the rrr and the mwp

mega scale glacial lineations and cross cutting ice flow landforms landsat images reveal a previously unsuspected large scale pattern of streamlining within drift that is assumed to reflect former phases of ice flow such a glacial grain can be regarded as a landform assemblage comprised of a number of components drumlins and megaflutes form part of the pattern but in addition there are two previously undocumented ice moulded landform elements streamlined lineations of much greater proportions referred to as mega scale glacial lineations and a distinctive cross cutting topology within the grain the ice moulded landform assemblage is described and illustrated with reference to examples from canada possible modes of genesis of such landforms are discussed and their glaciological implications outlined the discovery of this pattern indicates the pervasive nature of subglacial deformation of sediment and demands a radical re interpretation of ice sheet dynamics

trough mouth fans palaeoclimate and ice sheet monitors abstract trough mouth fans are fans at the mouth of transverse troughs channels on glaciated continental shelves on the northwest european glaciated continental margin eight trough mouth fans varying in size between 2700 and 215 000 km 2 have been identified the trough mouth fans are depocentres dominated by debris flows accumulated in front of ice streams draining the former large northwest european ice sheets the debris flow units are separated by hemmipelagic interglacial interstadial sediments it is inferred that the number of debris flow units record the number of shelfbreak positions of the ice sheet margins the number found varies between three in the south and eight in the north typical trough mouth fans and related debris flow units seem to have been formed later than the early mid pleistocene thus the north european ice sheets did not form ice streams extending to the shelfbreak in any appreciable length before the mid pleistocene besides being loci for sediment deposition the trough mouth fans were also the main sites of fresh water supply to the ocean in the form of icebergs during the mid late pleistocene ice ages