twenty first century warming of a large antarctic ice shelf cavity by a redirected coastal current the redirection of warm water under the filchner ronne ice shelf during the second half of this century could cause the ice shelf base to melt at a rate 20 times higher than at present warm ocean currents are known to erode ice shelves from below but changes in currents can be forced by many different mechanisms leading to uncertain outcomes this study highlights the vulnerability to climate change of a small antarctic coastal region which has potentially severe consequences for the mass balance of a large antarctic ice shelf hellmer et al use climate modelling to show that the projected loss of sea ice in the weddell sea east of the antarctic peninsula leads to an increase in wind stress which in turn accelerates a warm ocean current far underneath the vast filchner ronne ice shelf the authors predict that the increased warmth could increase melt by a factor of 20 with possible consequences for ice stream dynamics in the east antarctic ice sheet the antarctic ice sheet loses mass at its fringes bordering the southern ocean at this boundary warm circumpolar water can override the continental slope front reaching the grounding line1 2 through submarine glacial troughs and causing high rates of melting at the deep ice shelf bases3 4 the interplay between ocean currents and continental bathymetry is therefore likely to influence future rates of ice mass loss here we show that a redirection of the coastal current into the filchner trough and underneath the filchner ronne ice shelf during the second half of the twenty first century would lead to increased movement of warm waters into the deep southern ice shelf cavity water temperatures in the cavity would increase by more than 2 degrees celsius and boost average basal melting from 0 2 metres or 82 billion tonnes per year to almost 4 metres or 1 600 billion tonnes per year our results which are based on the output of a coupled ice ocean model forced by a range of atmospheric outputs from the hadcm35 climate model suggest that the changes would be caused primarily by an increase in ocean surface stress in the southeastern weddell sea due to thinning of the formerly consolidated sea ice cover the projected ice loss at the base of the filchner ronne ice shelf represents 80 per cent of the present antarctic surface mass balance6 thus the quantification of basal mass loss under changing climate conditions is important for projections regarding the dynamics of antarctic ice streams and ice shelves and global sea level rise

antarctic snow accumulation mapped using polarization of 4 3 cm wavelength microwave emission different parts of antarctica receive different amounts of snowfall each year in this paper we map the variations of the mean annual snow accumulation across the ice sheet we also quantify the uncertainty in our estimates more objectively than has been possible for earlier maps the new map is produced using observations from satellites and ground based measurements after a logarithmic transformation these are combined using the geostatistical method of continuous part universal kriging to give an estimate of the snow accumulation within each cell of a rectangular grid covering antarctica we also derive spatial averages over the major drainage systems of the ice sheet along with their confidence intervals we obtain a value of 143 4 kg m 2 a 1 for the average rate of snow accumulation upon the grounded ice sheet of antarctica

assessing the ability of numerical ice sheet models to simulate grounding line migration grounding line migration is a key process affecting the stability of marine ice sheets such as the west antarctic ice sheet wais grounding line motion is often included in numerical models simulating the past and future evolution of the wais however little attention has been paid to the numerical consistency of these models the aim of this paper is to assess the ability of simple versions of existing marine ice sheet models to simulate grounding line migration in particular we investigate the response of the grounding line to external forcing and the sensitivity of the models predictions to their numerics and the mechanical coupling between ice sheet and shelf from the model comparison there is no consensus on how the grounding line should react to changes in boundary conditions a crucial finding is the strong dependency of models using a fixed grid on numerical details such as the horizontal grid size this implies that we should be very wary about grounding line predictions from such models including mechanical coupling at the grounding line does not seem to change the qualitative behavior of the models this suggests that the way the grounding line is treated in marine ice sheet models dominantly determines the grounding line dynamics we find that models that employ a moving grid to explicitly track the grounding line do not share many of the deficiencies of the fixed grid models we conclude that at present no reliable model of the grounding line is available and further model development is urgently needed

lgm ice sheet extent in the weddell sea evidence for diachronous behavior of antarctic ice sheets abstract inaccessibility due to harsh weather conditions and perennial sea ice has resulted in the weddell sea being one of the more under studied regions of antarctica re examination of existing cores and unpublished multibeam bathymetric data provide new insight into the lgm configuration of the ice sheet in the region sediment cores from the eastern continental shelf where the east antarctic ice sheet eais drains display a retreat stratigraphy with distal glacimarine sediments overlying proximal glacimarine sediments and till the distinction between distal and proximal glacimarine sediments is based on degrees of marine influence based on grain size distributions and foraminiferal abundance and diversity glacimarine sediments yielded ams radiocarbon ages that span the past 30 476 cal yr bp indicating that retreat of the eais from the continental shelf occurred prior to the lgm multibeam swath bathymetry data show mega scale glacial lineations in two troughs on the southern continental shelf which indicates the existence of two ice streams draining the west antarctic ice sheet wais more subtle lineations within filchner trough imply more sluggish flow across bedrock and older strata within this trough although there are no radiocarbon ages to absolutely constrain the timing of this grounding event on the southern continental shelf we interpret the lineations as lgm age based on their pristine nature this interpretation is consistent with results from other sectors of the west antarctic continental shelf where compelling evidence for significant wais advance during the lgm exists in the weddell sea this apparent diachronous behavior of the eais and wais is consistent with results from onshore studies

influence of tides on melting and freezing beneath filchner ronne ice shelf antarctica an isopycnic coordinate ocean circulation model is applied to the ocean cavity beneath filchner ronne ice shelf investigating the role of tides on sub ice shelf circulation and ice shelf basal mass balance including tidal forcing causes a significant intensification in the sub ice shelf circulation with an increase in melting 3 fold and refreezing 6 fold the net melt rate and seawater flux through the cavity approximately doubles with tidal forcing the spatial pattern and magnitude of basal melting and freezing generally match observations the 0 22 m a 1 net melt rate is close to satellite derived estimates and at the lower end of oceanographic values the ice shelf water outflow mixes with shelf waters forming a cold 1 9 degrees c dense overflow 0 83 sv that spills down the continental slope these results demonstrate that tidal forcing is fundamental to both ice shelf ocean interactions and deep water formation in the southern weddell sea citation makinson k p r holland a jenkins k w nicholls and d m holland 2011 influence of tides on melting and freezing beneath filchner ronne ice shelf antarctica geophys res lett 38 l06601 doi 10 1029 2010gl046462

evaluating the mechanical compaction of quartzarenites the importance of sorting llanos foreland basin colombia abstract mechanical compaction is the main porosity reducing process in sandstones including high reservoir quality rigid grain sandstones for such sandstones the extrapolation of theoretical or experimental compaction algorithms needs calibration with rocks having well constrained burial histories evaluating the compaction of these rocks is achieved by comparing current intergranular volume igv with depositional igv which is strongly dependent on sorting however because sandstone sorting is difficult to measure accurately its impact on depositional porosity and compaction state is largely underestimated we use the quartzarenites of the oligocene carbonera formation in the subsurface of the hydrostatically pressured llanos basin to illustrate the importance of sorting when evaluating the compaction of rigid grain sandstones igv and sorting were measured in core samples using a combination of transmitted light and cathodoluminescence images resulting in improved accuracy over standard procedures the compaction state of clean quartzarenites at given depths is best described using igv versus sorting plots which are used to derive compaction curves for specified sorting values the igv versus sorting trends are displaced to lower igv values with increasing burial depth the differences in igv caused by differences in sorting exceed the differences in igv resulting from 1000 m of burial illustrating the high impact of sorting when evaluating compaction contrasting with published experimental results the compaction of the llanos basin ductile grain poor quartzarenites is independent of grain size and grain rearrangement is the main compaction mechanism during the first 1 6 km of burial based on the llanos data we have generated igv versus depth curves for clean pure quartzarenites of specific sorting which can be used to predict their maximum primary porosity up to moderate burial depths differences with other published burial curves are probably related to unaccounted variations in sorting ductile grain content and framework strengthening cements however the llanos basin quartzarenites contain virtually no cements explaining their high degree of compaction relative to other rigid grain sandstones and making them ideal to isolate the effects of compaction on the igv of quartzarenites the llanos basin data suggest that below 2 5 km of depth clean well to moderately well sorted quartzarenites continue reducing their igv by mechanical compaction below the 26 limit which should apply only to extremely well sorted rigid grain uncemented sandstones

glacial interglacial ice stream stability in the weddell sea embayment antarctica the resilience of the antarctic ice sheet and its effect on global sea level depends on the dynamics of ice streams antarctic ice streams are known to be responsive to changes at the ocean interface and as expected most have thinned in response to ocean warming and sea level rise since the last glacial maximum lgm here we provide direct and unexpected evidence that points toward the glacial interglacial stability of the slessor and recovery glaciers ice streams of the east antarctic ice sheet eais which merge with the west antarctic ice sheet wais to form the filchner ice shelf in the weddell sea embayment cosmogenic nuclide measurements in the shackleton range suggest that the slessor and recovery ice streams were not significantly thicker than today during the lgm we hypothesise that the glaciers did not thicken because the grounding line was not able to migrate seaward beyond the deep thiel crary trough beneath the filchner ice shelf immediately offshore this discovery reveals how a topographic threshold can affect the dynamics of ice streams it also reduces uncertainties on the thickness extent and volume of the antarctic ice sheet in a large but unknown sector of the antarctic ice sheet it constrains the potential sea level rise from antarctica it helps explain observed anomalies in glacio isostatic adjustment above all it suggests that the behaviour of the atlantic facing weddell sea sector of the wais contrasts with that of the pacific facing ross and amundsen sea sectors

model experiments on the evolution and stability of ice streams this is the publisher s version also available electronically from http www igsoc org annals 23

a community based geological reconstruction of antarctic ice sheet deglaciation since the last glacial maximum a robust understanding of antarctic ice sheet deglacial history since the last glacial maximum is important in order to constrain ice sheet and glacial isostatic adjustment models and to explore the forcing mechanisms responsible for ice sheet retreat such understanding can be derived from a broad range of geological and glaciological datasets and recent decades have seen an upsurge in such data gathering around the continent and sub antarctic islands here we report a new synthesis of those datasets based on an accompanying series of reviews of the geological data organised by sector we present a series of timeslice maps for 20 ka 15 ka 10 ka and 5 ka including grounding line position and ice sheet thickness changes along with a clear assessment of levels of confidence the reconstruction shows that the antarctic ice sheet did not everywhere reach the continental shelf edge at its maximum that initial retreat was asynchronous and that the spatial pattern of deglaciation was highly variable particularly on the inner shelf the deglacial reconstruction is consistent with a moderate overall excess ice volume and with a relatively small antarctic contribution to meltwater pulse 1a we discuss key areas of uncertainty both around the continent and by time interval and we highlight potential priorities for future work the synthesis is intended to be a resource for the modelling and glacial geological community

late holocene ice flow reconfiguration in the weddell sea sector of west antarctica abstract here we report late holocene ice sheet and grounding line changes to the weddell sea sector of west antarctica internal radio echo layering within the bungenstock ice rise which comprises very slow flowing ice separating the fast flowing institute and moller ice streams reveals ice deformed by former enhanced flow overlain by un deformed ice the ice rise surface is traversed by surface lineations explicable as diffuse ice flow generated stripes which thus capture the direction of flow immediately prior to the creation of the ice rise the arrangement of internal layers can be explained by adjustment to the flow path of the institute ice stream during either a phase of ice sheet retreat not longer than 4000 years ago or by wholesale expansion of the grounding line from an already retreated situation not sooner than 400 years ago some combination of these events involving uplift of the ice rise bed during ice stream retreat and reorganisation is also possible whichever the case the implication is that the ice sheet upstream of the bungenstock ice rise which currently grounds over a 1 5 km deep basin has been and therefore may be susceptible to significant change

late quaternary grounded ice extent in the filchner trough weddell sea antarctica new marine geophysical evidence the last glacial maximum lgm ca 23 19 ka bp extent of grounded ice in the filchner trough a major cross shelf trough extending seaward from the filchner ice shelf in the southern weddell sea has been much debated here we present data from the first extensive multibeam swath bathymetry and sub bottom acoustic profiling surveys in the filchner trough that include several parallel survey lines with overlapping swaths we interpret these new data combined with published observations and radiocarbon dates from sediment cores as indicating that the grounding line in the filchner trough during the lgm advanced beyond the middle shelf probably to within 40 km of the shelf break and possibly reached the shelf break three different hypotheses are discussed that could reconcile this interpretation with interpretations based on ice coring and surface exposure age data that lgm ice surface elevations in areas draining into the filchner and ronne ice shelves were no more than a few hundred metres higher than today 1 ice plain conditions extended along most of the filchner trough 2 the ice shelf advanced and thickened so that it touched down on the continental shelf for a short period 3 lgm ice drainage pathways in the interior of the weddell sea embayment were different from those observed today

millennial scale variability in antarctic ice sheet discharge during the last deglaciation two well dated high resolution records of iceberg rafted debris are presented that document variability in antarctic ice sheet discharge during the last deglaciation global sea levels have risen by more than 100 metres since the last glacial maximum around 20 000 years ago with several meltwater pulses of several metres or more in the most dramatic of these meltwater pulse 1a sea level rose by about 16 metres at 14 600 years ago this magnitude of sea level rise strongly suggests major antarctica contributions but to date there has been no firm physical evidence now michael weber and colleagues present a record of iceberg rafted debris from the scotia sea and show clear signals of pulsed iceberg release from antarctica as early as 19 000 years ago the largest iceberg release occurred during meltwater pulse 1a providing the long sought confirmation of antarctic contributions to this major jump in sea level rise our understanding of the deglacial evolution of the antarctic ice sheet ais following the last glacial maximum 26 000 19 000 years ago 1 is based largely on a few well dated but temporally and geographically restricted terrestrial and shallow marine sequences2 3 4 this sparseness limits our understanding of the dominant feedbacks between the ais southern hemisphere climate and global sea level marine records of iceberg rafted debris ibrd provide a nearly continuous signal of ice sheet dynamics and variability ibrd records from the north atlantic ocean have been widely used to reconstruct variability in northern hemisphere ice sheets5 but comparable records from the southern ocean of the ais are lacking because of the low resolution and large dating uncertainties in existing sediment cores here we present two well dated high resolution ibrd records that capture a spatially integrated signal of ais variability during the last deglaciation we document eight events of increased iceberg flux from various parts of the ais between 20 000 and 9 000 years ago in marked contrast to previous scenarios which identified the main ais retreat as occurring after meltwater pulse 1a3 6 7 8 and continuing into the late holocene epoch the highest ibrd flux occurred 14 600 years ago providing the first direct evidence for an antarctic contribution to meltwater pulse 1a climate model simulations with ais freshwater forcing identify a positive feedback between poleward transport of circumpolar deep water subsurface warming and ais melt suggesting that small perturbations to the ice sheet can be substantially enhanced providing a possible mechanism for rapid sea level rise

integrating satellite observations with modelling basal shear stress of the filcher ronne ice streams antarctica using inverse methods constrained by recent satellite observations we have produced a comprehensive estimate of the basal shear stress beneath the filchner ronne ice streams the inversions indicate that a weak bed approx 4 20 kpa underlies much of these ice streams compared to the ross ice streams the distribution of weak subglacial till is more heterogeneous with sticky spots providing much of the resistance to flow a weak bed beneath recovery ice stream extends several hundred kilometres inland with flow along this ice stream discrepancies between thickness measurements and flux estimates suggest the existence of a deep 1400 m trough not resolved by existing maps of subglacial topography we hypothesize that the presence of this and other deep troughs is a major influence on this sector of the ice sheet that is not fully incorporated in current models of ice sheet evolution

compaction of wilcox and carrizo sandstones paleocene eocene to 4420 m texas gulf coast abstract changes in packing and porosity resulting from compaction were studied in 40 wilcox and carrizo sandstone samples with burial depths from 792 to 4420 m as measured by packing indices and loss of intergranular porosity and pre cement porosity sands compacted rapidly to approximately 1200 m and more slowly and variably at greater depths contact index average number of contacts grain and tight packing index average number of long concavo convex and sutured contacts grain increase and intergranular porosity and pre cement porosity decrease logarithmically with burial depth because of a scarcity of deep samples compaction behavior deeper than 3000 m is uncertain compaction appears to have stopped at 3000 m which is 500 m shallower than the present depth of hard overpressure neither quartz cement x 11 nor carbonate cement x 2 was introduced shallow enough or insufficient quantity to halt compaction although quartz cement may have retarded compaction compaction was the major cause of porosity reduction with depth r 0 79 the total whole rock porosity lost by compaction for individual samples ranges from 9 to 31 at all depths the amount of porosity lost by grain rearrangement 9 to 27 is more than twice the amount of porosity lost by either measureable ductile grain deformation 0 to 8 3 or pressure solution 0 to 7 3 although the amount of pressure solution and its influence on grain rearrangement is probably underestimated depth and thermal maturation the latter represented by time temperature best explain compaction in these sandstones

diagenesis compaction and fluid chemistry modeling of a sandstone near a pressure seal lower tuscaloosa formation gulf coast petrographic isotopic and fluid inclusion evidence from normally and overpressured sand stones of the lower tuscaloosa formation upper cretaceous in the gulf coast documents quartz overgrowth precipitation at 90 c or less calcite cement precipitation at approximately 100 and 135 c and prismatic quartz cement precipitation at about 125 c textural evidence suggests that carbonate cement dissolution occurred before the second phases of calcite and quartz precipitation and was followed by precipitation of grain rimming chlorite and pore filling kaolinite geochemical calculations demonstrate that present day lower tuscaloosa formation water from 5500 m depth could either dissolve or precipitate calcite cements in model simulations of upward water low calcite dissolution or precipitation depends on pco2 variability with depth i e whether there is one or two phase flow or on the rate of generation of co2 with depth calculations suggest that 105 106 rock volumes of water are required to flow through the section to precipitate 1 10 calcite cement compaction analysis suggests that late stage compaction occurred in normally pressured sandstones after dissolution of carbonate cements but was hindered in overpressured sandstones despite the presence of high porosity these results document the inhibition of compaction by overpressured fluids and constrain the timing of pressure seal formation modeling results demonstrate that the proposed paragenesis used to constrain timing of pressure seal formation is feasible and that most of the cement diagenesis occurred before the pressure seal became effective as a permeability barrier

heterogeneous packing and quartz cementation of the eureka quartzarenite middle ordovician utah and nevada u s a abstract the eureka sandstone is a quartzarenite 200 m thick that was deposited on the eastern shelf of the cordilleran foreland basin from canada to california it has many characteristics of other lower paleozoic quartzarenites in terms of its sheet like geometry mineralogical and textural maturity 99 5 detrital quartz almost free of detrital clay shallow marine environment of deposition and abundance of quartz cement 10 26 however it displays wide variations in compaction as measured by intergranular volume igv range of 16 that are not all related to differences in burial depth plus igv values as low as 10 sutured grain contacts and fractured grains cannot account for such abnormally low igv values which suggests that dissolution of the edges of grains where they overlapped their neighbors the fuchtbauer effect leaving minimal evidence was a major process of compaction the normal evolution of microquartz overgrowths 10 mm with time was retarded consequently microquartz and mesoquartz cement 80 dominate over macroquartz 20 and are the only cements in the poorest cemented beds and laminations illite co precipitated with micro and mesoquartz and impeded quartz cementation by coating quartz crystal faces typical macrocrystalline quartz was the final cement only in illite poor samples heterogeneous illite distribution may reflect heterogeneous abundance of k feldspar or detrital illite that possibly served as seed crystals for authigenic illite in spite of reaching temperatures 135 c for 100 my many eureka sandstones especially in utah remain incompletely and unevenly cemented consequently beds range from weakly friable to pervasively quartz cemented quartzite in the same outcrop porosity was generally reduced to less than 2 except in rare laminations where it reaches 22 grain size had some influence on the amount of quartz cement locally but the chief cause of cement heterogeneity appears to have been illite abundance the most likely major source of silica for quartz cement was quartz grain dissolution at shale beds and clay drapes although documentation is absent intergranular grain dissolution was a minor source of silica overlap quartz averages less than 4 oxygen isotopic data are compatible with the precipitation of quartz either from diagenetically evolved meteoric water that invaded the sandstones from tectonic highlands to the west or evaporative brines

diagenetic controls on evolution of porosity and permeability in lower tertiary wilcox sandstones from shallow to ultradeep 200 6700 m burial gulf of mexico basin u s a abstract reservoir quality is a critical risk factor in deep to ultradeep reservoirs at depths 4 5 km analysis of paleogene wilcox sandstones on the upper texas gulf coast provides insight into the evolution of reservoir quality during shallow to ultradeep burial diagenesis reduction of porosity and permeability with burial in wilcox sandstones was evaluated using subsurface samples from 200 to 6700 m at temperatures of 25 230 c diagenesis and petrophysical properties were interpreted from petrographic data and core analyses wilcox sandstones are mostly lithic arkoses and feldspathic litharenites having an average composition of q59f22r19 provenance did not change significantly during wilcox deposition in this area nor does average sandstone composition vary among lower middle and upper wilcox sandstones however composition does vary with sequence stratigraphic position lowstand slope fan deposits contain more rock fragments than do deposits from highstand or transgressive systems tracts given observations from this onshore dataset wilcox sandstones deposited in deepwater environments in the gulf of mexico are likely to contain more rock fragments than their linked highstand equivalents pore types in wilcox sandstones change from a mix of primary and secondary pores and micropores at shallower depths to predominantly secondary pores and micropores in deeper sandstones wilcox sandstones show steady porosity reduction from an average of 33 at 38 c to 12 at 132 c with little additional loss at higher temperatures by 132 c most primary pores have been lost by mechanical compaction or occluded by quartz cementation volume of secondary pores remains nearly constant despite dissolution of 3 5 average potassium feldspar during deep burial volume of this late dissolution of potassium feldspar is offset by precipitation of ankerite albite illite and minor quartz in secondary pores the proportion of microporosity which occurs in authigenic clays altered grains and matrix increases in the deepest sandstones change in proportion of pore types and reduction in total porosity with increasing temperature changes porosity permeability transforms because most deep to ultradeep wilcox sandstones are dominated by secondary pores and micropores porosity permeability transforms have lower slopes than those for shallower sandstones

tectonic insight based on anisotropy of magnetic susceptibility and compaction studies in the sierras australes thrust and fold belt southwest gondwana boundary argentina the sierras australes fold and thrust belt buenos aires province argentina was in the southwestern gondwanaland margin during the paleozoic the tunas formation permian is exposed along the eastern part of it and continues eastward beneath the claromeco basin anisotropy of magnetic susceptibility ams and compaction studies are described and compared with previous paleomagnetic studies with the aim of determining direction and magnitude of the main stresses acting during the sedimentation of the tunas formation the anisotropy ellipsoids are triaxial with oblate or prolate shapes reflecting different stages of layer parallel shortening during the evolution of the basin kmax axes trend nw se parallel to the fold axes while kmin move from a horizontal base to a vertical orientation at the top of the succession showing a change from a tectonic to almost a sedimentary fabric the magnitude of anisotropy and compaction degree decreases toward the top of the succession the ams results are consistent with the outcrop structural observations and the compaction and paleomagnetic data regional pattern indicates a compression from the sw along this part of gondwana with a migration of the orogenic front and attenuation toward the ne in the foreland basin during the upper paleozoic this deformation locally assigned to the san rafael noncollisional orogenic phase is the result of the latitudinal movements toward the equator of gondwana southern plates and laurentia northern plates during the permian this movement is the result of a rearrangement of the microplates that collided with gondwana during the late devonian to configure pangea during the triassic

steep reverse bed slope at the grounding line of the weddell sea sector in west antarctica the bed of the west antarctic ice sheet is in places more than 1 5 km below sea level1 2 it has been suggested that a positive ice loss feedback may occur when an ice sheet s grounding line retreats across a deepening bed1 2 3 applied to the west antarctic ice sheet this process could potentially raise global sea level4 by more than 3 m hitherto attention has focussed on changes at the siple coast5 6 7 and amundsen sea embayment8 9 10 sectors of west antarctica here we present radio echo sounding information from the ice sheet s third sector the weddell sea embayment that reveals a large subglacial basin immediately upstream of the grounding line the reverse bed slope is steep with about 400 m of decline over 40 km the basin floor is smooth and flat with little small scale topography that would delay retreat indicating that it has been covered with marine sediment5 11 and was previously deglaciated upstream of the basin well defined glacially carved fjords with bars at their mouths testify to the position of a former ice margin about 200 km inland from the present margin evidence so far suggests that the weddell sea sector of the west antarctic ice sheet has been stable but in the light of our data we propose that the region could be near a physical threshold of substantial change

recent antarctic ice mass loss from radar interferometry and regional climate modelling observed estimates of ice losses in antarctica combined with regional modelling of ice accumulation in the interior suggest that east antarctica is close to a balanced mass budget but large losses of ice occur in the narrow outlet channels of west antarctic glaciers and at the northern tip of the antarctic peninsula

potential antarctic ice sheet retreat driven by hydrofracturing and ice cliff failure abstract geological data indicate that global mean sea level has fluctuated on 103 to 106 yr time scales during the last 25 million years at times reaching 20 m or more above modern if correct this implies substantial variations in the size of the east antarctic ice sheet eais however most climate and ice sheet models have not been able to simulate significant eais retreat from continental size given that atmospheric co2 levels were relatively low throughout this period here we use a continental ice sheet model to show that mechanisms based on recent observations and analysis have the potential to resolve this model data conflict in response to atmospheric and ocean temperatures typical of past warm periods floating ice shelves may be drastically reduced or removed completely by increased oceanic melting and by hydrofracturing due to surface melt draining into crevasses ice at deep grounding lines may be weakened by hydrofracturing and reduced buttressing and may fail structurally if stresses exceed the ice yield strength producing rapid retreat incorporating these mechanisms in our ice sheet model accelerates the expected collapse of the west antarctic ice sheet to decadal time scales and also causes retreat into major east antarctic subglacial basins producing 17 m global sea level rise within a few thousand years the mechanisms are highly parameterized and should be tested by further process studies but if accurate they offer one explanation for past sea level high stands and suggest that antarctica may be more vulnerable to warm climates than in most previous studies

quantification of brittle deformation in burial compaction frio and mount simon formation sandstones abstract brittle deformation together with grain rearrangement and ductile grain deformation is a key mechanism of compaction in sandstones quartz cementation by virtue of its impact on sandstone mechanical properties is expected to affect the compaction progress by these various mechanisms sandstone samples of different ages and compositions taken from two basins with contrasting burial histories are used to quantify the relationship between brittle deformation and quartz cementation in the context of burial compaction exponential trends of increasing deformation by microfracturing are observed in both the lithic rich frio formation from the gulf of mexico basin r2 0 81 and in the quartz rich mount simon formation from the illinois basin r2 0 78 the two formations contrast in terms of the observed rate of grain fracture increase with depth a larger number of quartz grains in the mount simon formation undergo fracturing at shallow burial 3 km a larger number of quartz grains are fractured in the frio sandstones cathodoluminescence cl imaging conducted on an sem shows that grain fracturing and subsequent cementation of fractures by quartz can occur before during and after normal quartz overgrowth precipitation although fracturing is generally concurrent with cementation the intensity of brittle deformation for a single fractured grain is independent of depth as illustrated by plotting the ratio of the number of particles in fractured grains versus depth r2 0 21 for frio and 0 03 for mount simon suggesting that once fractured grains do not tend to become the locus of subsequent fracturing apparent fracture apertures in the frio grains are slightly wider average 5 m than in mount simon grains average measurable aperture width 4 m quartz grains in the frio have a variety of fracture morphologies including wedge shaped apertures intense comminution at grain contacts and grains with exploded fabrics apparent fracture apertures in mount simon grains are thinner and transect grains as straight uniform traces differences in fracture intensity and pattern are attributed to the fact that cementation in the frio began at greater depth than cementation in the mount simon combining information on the degree of brittle deformation and the amount of quartz cement localized within microfractures here referred to as cf or volume of intragranular cement allows calculation of the amount that brittle deformation influences compaction in the frio formation 0 12 to 8 37 of porosity loss due to compaction can be attributed to brittle deformation whereas the values for the mount simon formation lie between 0 25 to 2 16 the larger values within both formations are affiliated with deeper samples in which a majority of grains manifest fracturing

porosity loss fluid flow and mass transfer in limestone reservoirs application to the upper jurassic smackover formation mississippi ooid grainstones of the upper jurassic smack over formation are buried to a depth of over 6 km and exposed to temperatures in excess of 200 c at black creek field mississippi combined effects of mechanical compaction intergranular pressure solution and cementation have reduced intergranular porosity of these ooid grainstones to 0 indicating that porosity reduction has gone to completion modal analysis of 24 samples lacking preburial cements indicates that from the original 40 porosity 13 porosity units range 4 to 21 were lost by mechanical compaction 15 porosity units range 8 to 23 were reduced by intergranular pressure solution and 12 porosity units range 1 to 26 were destroyed by cementation intergranular pressure solution caused an average of 28 range 15 to 51 vertical shortening in smackover ooid grainstones under ideal conditions the 28 vertical shortening will generate enough calcium carbonate to precipitate 10 calcite cement this is close to the measured volume of cements in smackover grainstones 12 suggesting that intergranular pressure solution provided most of the calcite cement present no external sources of calcium carbonate are required fine grained samples that experienced high degrees of intergranular pressure solution and contain only small amounts of cement occur at the top of the reservoir whereas coarse grained samples with abundant cement and low degrees of pressure solution occur in the middle and basal parts of the reservoir suggesting that the fine grained intervals acted as sources and the coarse grained intervals as sinks for calcium carbonate mass transfer of pressure solution generated calcium carbonate from the top of the unit to precipitation sites in the middle and basal parts of the reservoir could have occurred by a non rayleigh type convection cell due to calcite s reverse solubility with respect to temperature the cooling upward moving limb of the convection cell would become progressively more undersaturated and hence able to transport more dissolved calcium carbonate released by intergranular pressure solution in the upper portion of the reservoir pore fluids descending in the downward moving limb of the cell would become progressively more supersaturated and calcium carbonate would tend to precipitate as cement in the middle and basal parts of the reservoir as fluids become progressively hotter

geological setting and paleomagnetism of the eocene red beds of laguna brava formation quebrada santo domingo northwestern argentina abstract the red bed succession cropping out in the quebrada santo domingo in northwestern argentina had been for long considered as upper triassic lower jurassic in age based on weak radiometric and paleontological evidence preliminary paleomagnetic data confirmed the age and opened questions about the nature of fossil footprints with avian features discovered in the section recently the stratigraphic scheme was reviewed with the identification of previously unrecognized discontinuities and a radiometric dating obtained in a tuff indicated an eocene age for the laguna brava formation and the fossil bird footprints much younger than the previously assigned we present a detailed paleomagnetic study interpreted within a regional tectonic and stratigraphic framework looking for an explanation for the misinterpretation of the preliminary paleomagnetic data the characteristic remanent magnetizations pass a tilt test and a reversal test the main magnetic carrier is interpreted to be low ti titanomagnetites and to a lesser extent hematite the characteristic remanent magnetization would be essentially detrital the obtained paleomagnetic pole pp for the laguna brava formation has the following geographic coordinates and statistical parameters n 29 lon 184 5 e lat 75 0 s a 95 5 6 and k 23 7 when this pp is compared with another one with similar age obtained in an undeformed area a declination anomaly is recognized this anomaly can be interpreted as laguna brava formation belonging to a structural block that rotated about 16 clockwise along a vertical axis after about 34 ma this block rotation is consistent with the regional tectonic framework and would have caused the fortuitous coincidence of the pp with early jurassic poles according to the interpreted magnetostratigraphic correlation the laguna brava formation would have been deposited during the late eocene with a mean sedimentation rate of about 1 4 cm per thousand years probably in relation to the onset of the andean deformation

reservoir architecture and diagenesis in downthrown fault block plays the lowlander prospect of block 14 20b witch ground graben outer moray firth uk north sea

tectono thermal evolution of cambrian ordovician source rocks and implications for hydrocarbon generation in the eastern tarim basin nw china abstract the tectono thermal evolution is an important control on petroliferous basin development however this evolution is difficult to reconstruct in sedimentary basins subjected to multiple tectono thermal events the tarim basin is the largest petroliferous basin in northwestern china and hosts a complex oil gas system comprising several sets of source reservoir and cap rocks that have experienced multiple tectonic thermal and sedimentary events large oil and gas reserves have been discovered in the central and northern tarim basin during the last 30 years but none in the eastern part of the basin this is generally attributed to the high thermal maturity reached by source rocks during the late ordovician and subsequent hydrocarbon loss therefore reconstructing the cambrian ordovician tectono thermal evolution is critical in facilitating successful oil and gas exploration in this region we compiled equivalent vitrinite reflectance data n 178 of source rocks from seven wells in the tarim basin which for cambrian ordovician source rocks are 1 3 beyond the hydrocarbon generation threshold and in the main gas and dry gas generation stages the cambrian ordovician source rocks experienced rapid burial and heating during the caledonian period entered the hydrocarbon generation stage during the middle late ordovician and reached peak hydrocarbon generation at the end caledonian period they might have experienced secondary hydrocarbon generation after the jurassic homogenization temperatures of fluid inclusions n 616 from wells gc4 lx1 md1 and yd2 as well as the modeled thermal evolution reveal a main hydrocarbon generation event during the caledonian period and a secondary event in the yanshanian and himalayan period in other wells lx1 md1 and yd2 hercynian and indosinian uplift and exhumation in the eastern tarim basin controlled the main structures and faulting created favorable conditions for secondary hydrocarbon generation and migration at this time since the yanshanian and himalayan the tarim basin has experienced further sedimentation and heating and the tectono thermal evolution might have been related to the formation of the tibetan plateau in summary our reconstruction of the tectono thermal evolution of cambrian ordovician strata and the hydrocarbon generation history of the tarim basin suggests that the eastern tarim basin may also have hydrocarbon generation potential and can guide future oil and gas exploration

diagenetic processes influencing porosity in sandstones from the triassic buntsandstein of the iberian range spain abstract sandstones of the triassic buntsandstein of spain were studied in an attempt to quantify effects of diagenetic processes such as compaction cementation and leaching on reservoir properties samples were taken from shell well siguenza 44 3 some 110 km northeast of madrid drilled on the siguenza high in the iberian range the sandstones are arkoses to sublitharenites of a typical triassic redbed facies characteristic of the whole region the present day depth of the base buntsandstein is 538 m whilst a maximum burial depth of ca 2600 m was estimated from vitrinite reflectance 0 94 to 1 14 kaolinite temperature 76 c and illite crystallinity kuebler index 4 9 from an estimated average initial porosity of 38 porosity was reduced to the present value of 13 the strongest process to reduce porosity was found to be mechanical compaction causing a loss of 15 followed by chemical compaction pressure solution with a loss of about 6 the cement 11 4 r v apparently reduced the initial porosity by about only 5 early carbonate and sulphate cements are interpreted to have stabili sed the rock framework working against mechanical compaction and pressure solution this is supported by an increase in sutured grain contacts where early cements are absent or rare the later quartz cement interpreted to be largely the product from pressure solution has reduced porosity in a ratio of 0 5 1 this suggests that compaction continued after the formation of the late quartz cement although leaching was observed especially in feldspar grains clay and k feldspar neomorphosis is interpreted to have counteracted the formation of substantial secondary porosity permeability in the buntsandstein is generally low average 17 4 md and has been strongly affected by the alteration of feldspars to clay causing a wide range of permeability from 0 11 to 73 0 md

an inventory of active subglacial lakes in antarctica detected by icesat 2003 2008 through the detection of surface deformation in response to water movement recent satellite studies have demonstrated the existence of subglacial lakes in antarctica that fill and drain on timescales of months to years these studies however were confined to specific regions of the ice sheet here we present the first comprehensive study of these active lakes for the antarctic ice sheet north of 868 s based on 4 5 years 2003 08 of nasa s ice cloud and land elevation satellite icesat laser altimeter data our analysis has detected 124 lakes that were active during this period and we estimate volume changes for each lake the icesat detected lakes are prevalent in coastal antarctica and are present under most of the largest ice stream catchments lakes sometimes appear to transfer water from one to another but also often exchange water with distributed sources undetectable by icesat suggesting that the lakes may provide water to or withdraw water from the hydrologic systems that lubricate glacier flow thus these reservoirs may contribute pulses of water to produce rapid temporal changes in glacier speeds but also may withdraw water at other times to slow flow

understanding controls on rapid ice stream retreat during the last deglaciation of marguerite bay antarctica using a numerical model using a one dimensional numerical model of ice stream flow with robust grounding line dynamics we explore controls on paleo ice stream retreat in marguerite bay antarctica during the last deglaciation landforms on the continental shelf constrain the numerical model and suggest retreat was rapid but punctuated by a series of slowdowns we investigate the sensitivity of ice stream retreat to changes in subglacial and lateral topography and to forcing processes including sea level rise enhanced melting beneath an ice shelf atmospheric warming and ice shelf debuttressing our experiments consistently reproduce punctuated retreat on a bed that deepens inland with retreat rate slowdowns controlled by narrowings in the topography sensitivity experiments indicate that the magnitudes of change required for individual forcing mechanisms to initiate retreat are unrealistically high but that thresholds are reduced when processes act in combination the ice stream is however most sensitive to ocean warming and associated ice shelf melting and retreat was most likely in response to external forcing that endured throughout the period of retreat rather than to a single triggering event timescales of retreat are further controlled by the delivery of ice from upstream of the grounding line due to the influence of topography modeled retreat patterns are insensitive to the temporal pattern of forcing evolution we therefore suggest that despite regionally similar forcing mechanisms landscape controls significant contrasts in retreat behavior between adjacent but topographically distinct catchments patterns of ice stream retreat in the past present and future should therefore be expected to vary significantly

controls on last glacial maximum ice extent in the weddell sea embayment antarctica the weddell sea sector of the antarctic ice sheet is hypothesized to have made a significant contribution to sea level rise since the last glacial maximum using a numerical flowline model we investigate the controls on grounding line motion across the eastern weddell sea and compare our results with field data relating to past ice extent specifically we investigate the influence of changes in ice temperature accumulation sea level ice shelf basal melt and ice shelf buttressing on the dynamics of the foundation ice stream we find that ice shelf basal melt plays an important role in controlling grounding line advance while a reduction in ice shelf buttressing is found to be necessary for grounding line retreat there are two stable positions for the grounding line under glacial conditions at the northern margin of berkner island and at the continental shelf break global mean sea level contributions associated with these two scenarios are 50 mm and 130 mm respectively comparing model results with field evidence from the pensacola mountains and the shackleton range we find it unlikely that ice was grounded at the continental shelf break for a prolonged period during the last glacial cycle however we cannot rule out a brief advance to this position or a scenario in which the grounding line retreated behind present during deglaciation and has since re advanced better constraints on past ice sheet and ice shelf geometry ocean temperature and ocean circulation are needed to reconstruct more robustly past behavior of the foundation ice stream

the effect of ductile lithic sand grains and quartz cement on porosity and permeability in oligocene and lower miocene clastics south china sea prediction of reservoir quality ductile clay rich sand grains control porosity and permeability in the fluvio deltaic oligocene and lower miocene sandstones of the south china sea ductile grains account for between approximately 5 and 50 of the original sand grain population there is a pronounced loss of porosity with increasing burial depth in the basin at depths of less than 3000 m this is due solely to ductile grain compaction where the rate of porosity loss with depth increases with increasing ductile grain content at depths greater than 3000 m the steep porosity loss with depth is due to combined ductile grain compaction and quartz cementation the amount of quartz cement increases with increasing burial depth however cleaner sandstones tend to have greater amounts of quartz cement at any given depth below 3000 m this leads to convergence of porosity evolution for the clean and ductile rich sandstones below 3000 m there is a rapid loss of permeability with decreasing porosity because compaction of ductile grains smears them between rigid quartzose grains leading to blocked pore throats a consequence of this process is that the lowest permeabilities are found in sandstones with the highest ductile grain contents quartz cement does not have a clear and discernible control on permeability the pronounced loss of porosity with increasing depth and permeability with decreasing porosity leads to low permeability at relatively shallow burial depths reservoir quality is thus controlled by the nature of the primary sand and depth of burial the sediment supply system led to systematic changes in ductile grain content across the basin with ductile content increasing into the more distal part of the sediment system the consequence is that depth of economic basement in terms of porosity or permeability can be predicted as a function of ductile grain content and burial depth for prospects across the basin

sea level as a stabilizing factor for marine ice sheet grounding lines climate change could potentially destabilize marine ice sheets which would affect projections of future sea level rise1 2 3 4 specifically an instability mechanism5 6 7 8 has been predicted for marine ice sheets such as the west antarctic ice sheet that rest on reversed bed slopes whereby ice sheet thinning or rising sea level leads to irreversible retreat of the grounding line however existing analyses of this instability mechanism have not accounted for deformational and gravitational effects that lead to a sea level fall at the margin of a rapidly shrinking ice sheet9 10 11 here we present a suite of predictions of gravitationally self consistent sea level change following grounding line migration our predictions vary the initial ice sheet size and also consider the contribution to sea level change from various subregions of the simulated ice sheet using these results we revisit a canonical analysis of marine ice sheet stability5 and demonstrate that gravity and deformation induced sea level changes local to the grounding line contribute a stabilizing influence on ice sheets grounded on reversed bed slopes we conclude that accurate treatments of sea level change should be incorporated into analyses of past and future marine ice sheet dynamics

ice sheet grounding line dynamics steady states stability and hysteresis 1 the ice sheet ice shelf transition zone plays an important role in controlling marine ice sheet dynamics as it determines the rate at which ice flows out of the grounded part of the ice sheet together with accumulation this outflow is the main control on the mass balance of the grounded sheet in this paper we verify the results of a boundary layer theory for ice flux in the transition zone against numerical solutions that are able to resolve the transition zone very close agreement is obtained and grid refinement in the transition zone is identified as a critical component in obtaining reliable numerical results the boundary layer theory confirms that ice flux through the grounding line in a two dimensional sheet shelf system increases sharply with ice thickness at the grounding line this result is then applied to the large scale dynamics of a marine ice sheet our principal results are that 1 marine ice sheets do not exhibit neutral equilibrium but have well defined discrete equilibrium profiles 2 steady grounding lines cannot be stable on reverse bed slopes and 3 marine ice sheets with overdeepened beds can undergo hysteresis under variations in sea level accumulation rate basal slipperiness and ice viscosity this hysteretic behavior can in principle explain the retreat of the west antarctic ice sheet following the last glacial maximum and may play a role in the dynamics of heinrich events

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

dynamics of the last glacial maximum antarctic ice sheet and its response to ocean forcing retreat of the last glacial maximum lgm antarctic ice sheet is thought to have been initiated by changes in ocean heat and eustatic sea level propagated from the northern hemisphere nh as northern ice sheets melted under rising atmospheric temperatures the extent to which spatial variability in ice dynamics may have modulated the resultant pattern and timing of decay of the antarctic ice sheet has so far received little attention however despite the growing recognition that dynamic effects account for a sizeable proportion of mass balance changes observed in modern ice sheets here we use a 5 km resolution whole continent numerical ice sheet model to assess whether differences in the mechanisms governing ice sheet flow could account for discrepancies between geochronological studies in different parts of the continent we first simulate the geometry and flow characteristics of an equilibrium lgm ice sheet using pan antarctic terrestrial and marine geological data for constraint then perturb the system with sea level and ocean heat flux increases to investigate ice sheet vulnerability our results identify that fast flowing glaciers in the eastern weddell sea the amundsen sea central ross sea and in the amery trough respond most rapidly to ocean forcings in agreement with empirical data most significantly we find that although ocean warming and sea level rise bring about mainly localized glacier acceleration concomitant drawdown of ice from neighboring areas leads to widespread thinning of entire glacier catchments a discovery that has important ramifications for the dynamic changes presently being observed in modern ice sheets

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

ice stream stability on a reverse bed slope marine ice streams whose beds deepen inland are thought to be inherently unstable numerical modelling of the maguerite bay ice stream retreat in west antarctica since the last glacial maximum suggests that an ice stream can stabilize on an inland sloping bed owing to increased lateral drag where the ice stream narrows marine based ice streams whose beds deepen inland are thought to be inherently unstable1 2 3 this instability is of particular concern because significant portions of the marine based west antarctic and greenland ice sheets are losing mass and their retreat could contribute significantly to future sea level rise4 5 6 7 however the present understanding of ice stream stability is limited by observational records that are too short to resolve multi decadal to millennial scale behaviour or to validate numerical models8 here we present a dynamic numerical simulation of antarctic ice stream retreat since the last glacial maximum lgm constrained by geophysical data whose behaviour is consistent with the geomorphological record we find that retreat of marguerite bay ice stream following the lgm was highly nonlinear and was interrupted by stabilizations on a reverse sloping bed where theory predicts rapid unstable retreat we demonstrate that these transient stabilizations were caused by enhanced lateral drag as the ice stream narrowed we conclude that as well as bed topography ice stream width and long term retreat history are crucial for understanding decadal to centennial scale ice stream behaviour and marine ice sheet vulnerability

a data constrained large ensemble analysis of antarctic evolution since the eemian abstract reconstructing the historical behaviour of the antarctic ice sheet is essential for understanding and predicting climatic interactions ice sheet sensitivities and sea level contributions furthermore to interpret such reconstructions with any degree of confidence meaningful uncertainty estimates are required working toward this goal this article presents results from a large ensemble data constrained study of antarctic evolution over the last glacial cycle ice sheet chronologies have been generated using a 3 d glacial systems model that includes sheet stream shelf flow a parametrized basal drag coefficient accounting for sub grid topographic roughness sediment likelihood and systematic model to observation thickness misfits a sub grid grounding line flux parametrization a visco elastic bedrock response component parametrized climate forcing separate shelf and tidewater calving treatments and a physically motivated empirically derived sub shelf melt component there are 31 ensemble parameters to capture uncertainties in the glacial cycle climate mass balance processes and ice dynamics once generated the ensemble of model runs is constrained using a suite of observational data including constraints on relative sea level past ice thickness and grounding line retreat data points as well as present day ice volumes and configuration and an evaluation methodology that produces a misfit to observations score for each run assuming variants of a gaussian error model the scores are used to generate probability distributions for the past evolution of the ice sheet in our ensemble based analysis the last glacial maximum lgm of antarctic ice volume occurs at 25 24 ka the lgm ice volume excess relative to present day is likely between 5 6 and 14 3 m equivalent sea level mesl and with less confidence 10 mesl there is little change in the grounded ice volume from 24 ka until 17 16 ka at which time widespread deglaciation commences the ross and weddell sea sectors are the largest sources of ice mass loss the major period of grounding line retreat in the weddell sea sector resulting in the present day ronne filchner shelf system occurs after 10 ka the grounding line in the ross sea sector start its major retreat phase after 12 ka there is no significant contribution during the eemian the modelled antarctic ice sheet has a minimal ice volume at 114 ka contributing 2 mesl relative to present day the mass loss is principally due to a major retreat of the grounding line in the siple coast sector of the ross sea we present standard deviation plots of the ensemble results for present day and at the lgm these plots highlight geographic areas that would most benefit from additional constraint and therefore offer priorities and guidance for future field campaigns

marine geological constraints for the grounding line position of the antarctic ice sheet on the southern weddell sea shelf at the last glacial maximum abstract the history of grounded ice sheet extent on the southern weddell sea shelf during the last glacial maximum lgm and the timing of post lgm ice sheet retreat are poorly constrained several glaciological models reconstructed widespread grounding and major thickening of the antarctic ice sheet in the weddell sea sector at the lgm in contrast recently published onshore data and modelling results concluded only very limited lgm thickening of glaciers and ice streams feeding into the modern filchner and ronne ice shelves these studies concluded that during the lgm ice shelves rather than grounded ice covered the filchner and ronne troughs two deep palaeo ice stream troughs eroded into the southern weddell sea shelf here we review previously published and unpublished marine geophysical and geological data from the southern weddell sea shelf the stratigraphy and geometry of reflectors in acoustic sub bottom profiles are similar to those from other west antarctic palaeo ice stream troughs where grounded ice had advanced to the shelf break at the lgm numerous cores from the southern weddell sea shelf recovered sequences with properties typical for subglacially deposited tills or subglacially compacted sediments these data sets give evidence that grounded ice had advanced across the shelf during the past thereby grounding in even the deepest parts of the filchner and ronne troughs radiocarbon dates from glaciomarine sediments overlying the subglacial deposits are limited but indicate that the ice grounding occurred at the lgm and that ice retreat started before 15 1 corrected 14c kyrs before present bp on the outer shelf and before 7 7 corrected 14c kyrs bp on the inner shelf which is broadly synchronous with ice retreat in other antarctic sectors the apparent mismatch between the ice sheet reconstructions from marine and terrestrial data can be attributed to ice streams with very low surface profiles similar to those of ice plains that had advanced through filchner trough and ronne trough at the lgm considering the global sea level low stand of 130 m below present a low surface slope of the expanded lgm ice sheet in the southern weddell sea can reconcile grounding line advance to the shelf break with limited thickening of glaciers and ice streams in the hinterland this scenario implies that ice sheet growth in the weddell sea sector during the lgm and ice sheet drawdown throughout the last deglaciation could only have made minor contributions to the major global sea level fluctuations during these times

a deglacial model for antarctica geological constraints and glaciological modelling as a basis for a new model of antarctic glacial isostatic adjustment abstract we present a new reconstruction of the antarctic ice sheets between 20 ka bp and the present day our reconstruction is derived using a numerical model to generate a physically consistent ice surface across the whole of the continent we define the extent of the ice sheet at five time slices 20 15 10 5 and 0 ka bp assuming an equilibrium state for the 20 ka bp time slice and a transient state for the deglacial time slices the evolution of the ice sheet within the numerical model is driven by variations in temperature accumulation rate and relative sea level in order to reconstruct the concave profile of the ice sheet in marine grounded regions such as the ross and weddell seas we force our model to develop channels of faster flow by defining greater basal sliding along the trajectory of former ice streams we find a strong dependence upon the basal sliding parameters and also the position of the grounding line we use an extensive data base of geological and glaciological data to constrain our ice sheet reconstruction grounding line extent is prescribed from marine geological data and we test ice sheet thickness against onshore geological data at 62 sites of the five time slices considered our 20 ka bp reconstruction is the best constrained by data and has an rms misfit of 147 6 m when compared to observations of ice thickness change between 20 ka bp and the present day across all time slices there are large regions of the ice sheet which are poorly constrained especially after 20 ka bp we estimate the spatial distribution of uncertainty in our ice sheet reconstruction and note that the solutions are least reliable in regions of complex topography we predict that the antarctic ice sheets contributed 9 1 5 m of eustatic sea level to the global ocean between 20 ka bp and the present and our reconstruction with minimum misfit contributes 8 m eustatic sea level during this period these values which we argue are an upper bound are lower than many previous estimates the reconstructed pattern of ice unloading can serve as a new input for glacial isostatic models

reconstruction of changes in the weddell sea sector of the antarctic ice sheet since the last glacial maximum the weddell sea sector is one of the main formation sites for antarctic bottom water and an outlet for about one fifth of antarctica s continental ice volume over the last few decades studies on glacial geological records in this sector have provided conflicting reconstructions of changes in ice sheet extent and ice sheet thickness since the last glacial maximum lgm at ca 23 19 calibrated kiloyears before present cal ka bp terrestrial geomorphological records and exposure ages obtained from rocks in the hinterland of the weddell sea ice sheet thickness constraints from ice cores and some radiocarbon dates on offshore sediments were interpreted to indicate no significant ice thickening and locally restricted grounding line advance at the lgm other marine geological and geophysical studies concluded that subglacial bedforms mapped on the weddell sea continental shelf subglacial deposits and sediments over compacted by overriding ice recovered in cores and the few available radiocarbon ages from marine sediments are consistent with major ice sheet advance at the lgm reflecting the geological interpretations different ice sheet models have reconstructed conflicting lgm ice sheet configurations for the weddell sea sector consequently the estimated contributions of ice sheet build up in the weddell sea sector to the lgm sea level low stand of 130 m vary considerably in this paper we summarise and review the geological records of past ice sheet margins and past ice sheet elevations in the weddell sea sector we compile marine and terrestrial chronological data constraining former ice sheet size thereby highlighting different levels of certainty and present two alternative scenarios of the lgm ice sheet configuration including time slice reconstructions for post lgm grounding line retreat moreover we discuss consistencies and possible reasons for inconsistencies between the various reconstructions and propose objectives for future research the aim of our study is to provide two alternative interpretations of glacial geological datasets on antarctic ice sheet history for the weddell sea sector which can be utilised to test and improve numerical ice sheet models

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

simple energy balance model resolving the seasons and the continents application to the astronomical theory of the ice ages a one level seasonal energy balance climate model which has explicit two dimensional land sea geography is introduced and its properties analyzed the model distinguishes land from sea surface strictly by the local thermal inertia employed the transport is governed by a smooth latitude dependent diffusion mechanism the model s seasonal cycle is solved for with and without ice feedback in each case good agreement with real data is accomplished with minimal tuning of the adjustable parameters the model sensitivity for small solar constant changes is in agreement with that of other accepted models however when the solar constant is lower a few percent the ice cap grows discontinuously from covering greenland and vicinity to a size three times larger and mostly into northern canada a similar discontinuous growth occurs when the orbital elements are changed to favor cool summers in the northern hemisphere this discontinuous sensitivity is discussed in the context of the milankovitch theory of the ice ages and the associated branch structure is shown to be analogous to the so called small ice cap instability known from simpler models although the discontinuity in sensitivity is somewhat conjectural both linear and nonlinear models have their largest sensitivity to orbital parameter changes in the northern portions of the large continents in the western hemisphere this also corresponds to the ice sheet boundary suggesting that the land sea configuration is crucial to large glacier growth irrespective of moisture budgets other experiments and implications are also discussed the remarkable paper by hays et al 1976 hereafter his presented compelling empirical evidence that continental ice sheet volume changes and sea surface temperature variations in the last 700 000 years are correlated with the changes in the earth s orbital elements theoretical arguments for such a connection date back to the last century but the most detailed and convincing arguments are due to milankovitch 1941 and the theory usually is associated with his name if the hypothesis is correct the theory and observations provide the cleanest experiment to determine the sensitivity of the earth s climate to truly external influences such a measure of climate sensitivity is of obvious importance considering the recent interest in estimating the change in climate due to other forcings such as the enhanced greenhouse effect due to increased levels of co2 in the atmosphere e g ramanathan 1981 hansen et al 1981 manabe and stouffer 1980 in effect climate models might be tested or calibrated by this natural experiment climate modeling has progressed dramatically in the last decade with the construction and testing of several general circulation models gcm s although some interesting ice age experiments have been done with these models gates 1976 manabe and hahn 1977 they still include only crude parameterizations of the slower components such as the oceans these numerical models are the ultimate tool that can be brought to bear upon the climate problem however they are almost as complicated as the climate itself hence experiments must be carefully designed and the interpretation is often difficult due to the expense of obtaining definitively long runs in parallel with gcm construction has been the development of x permanently at applied research corporation landover maryland 20785

antarctic ice rises and rumples their properties and significance for ice sheet dynamics and evolution locally grounded features in ice shelves called ice rises and rumples play a key role buttressing discharge from the antarctic ice sheet and regulating its contribution to sea level ice rises typically rise several hundreds of meters above the surrounding ice shelf shelf flow is diverted around them on the other hand shelf ice flows across ice rumples which typically rise only a few tens of meters above the ice shelf ice rises contain rich histories of deglaciation and climate that extend back over timescales ranging from a few millennia to beyond the last glacial maximum numerical model results have shown that the buttressing effects of ice rises and rumples are significant but details of processes and how they evolve remain poorly understood fundamental information about the conditions and processes that cause transitions between floating ice shelves ice rises and ice rumples is needed in order to assess their impact on ice sheet behavior targeted high resolution observational data are needed to evaluate and improve prognostic numerical models and parameterizations of the effects of small scale pinning points on grounding zone dynamics

uplift rates from a new high density gps network in palmer land indicate significant late holocene ice loss in the southwestern weddell sea the measurement of ongoing ice mass loss and associated melt water contribution to sea level change from regions such as west antarctica is dependent on a combination of remote sensing methods a key method the measurement of changes in earth s gravity via the grace satellite mission requires a potentially large correction to account for the isostatic response of the solid earth to ice load changes since the last glacial maximum in this study we combine glacial isostatic adjustment modelling with a new gps dataset of solid earth deformation for the southern antarctic peninsula to test the current understanding of ice history in this region a sufficiently complete history of past ice load change is required for glacial isostatic adjustment models to accurately predict the spatial variation of ongoing solid earth deformation once the independently constrained effects of present day ice mass loss have been accounted for comparisons between the gps data and glacial isostatic adjustment model predictions reveal a substantial misfit the misfit is localized on the southwestern weddell sea where current ice models under predict uplift rates by approximately 2 mm yr 1 this under prediction suggests that either the retreat of the ice sheet grounding line in this region occurred significantly later in the holocene than currently assumed or that the region previously hosted more ice than currently assumed this finding demonstrates the need for further fieldwork to obtain direct constraints on the timing of holocene grounding line retreat in the southwestern weddell sea and that grace estimates of ice sheet mass balance will be unreliable in this region until this is resolved

calcite cemented concretions in cretaceous sandstone wyoming and utah u s a abstract spheroidal cannon ball calcite cemented concretions some of gigantic size were studied from three cretaceous shelf sandstone units ferron frontier and second frontier the concretions have diameters between 2 cm and 6 m those larger than 40 cm in diameter in the ferron and frontier have septarian structure rare concretions in the second frontier with cone in cone also have septaria the pattern of carbon and oxygen isotopic data and intergranular volume igv data across concretions indicate that concretion growth was complex not all concretions grew progressively from their centers toward their edges in many concretions displacive fascicular optic high mg calcite foc precipitated simultaneously throughout the concretions pervasive growth pattern although the earliest cementation was concentrated in and near their centers remaining pores were subsequently filled by poikilotopic calcite of lower mg content displacive foc cement in the bodies of concretions and in cone in cone structure and the large igv values indicate that these carbonate phases formed at burial depths no greater than tens of meters igv values indicate that spar cement continued to depths of at least 50 m and possibly deeper the association of organic matter and microbial structures or microbial byproducts with displacive foc and cone in cone calcite raises the possibility that biogenic processes were responsible for the apparently rapid growth of the displacive calcite the oxygen isotope composition of pore waters was fairly uniform in the range of 18o of 4 to 6 smow during 80 of concretion growth but became even more depleted in 18o during the last stage of growth and during septarian fracture filling ranging into values as depleted as 14 smow 13c in calcite is negative 4 to 18 v pdb but variable reflecting a range of microbial processes under both oxidizing and reducing conditions earlier calcites fall mostly in the range of 10 to 16 v pdb relatively later calcite in veins and on concretion margins is more enriched in 13c fe and mn concentrations in calcites have a rough positive correlation with higher 13c values the concentration of mg derived from seawater and altered volcanic rock fragments covaries positively with the degree of meteoric influence lower mg contents correlate positively with relatively depleted 18o values 87sr 86sr values are typical of sr derived from coeval seawater or marine skeletal debris growth of concretions 4 to 6 m in diameter apparently in 5 my requires some spatial localization of growth sites by a self organization process and probably a large supply of carbonate shells in the original sediment elongation of larger concretions in the plane of bedding suggests that advective rather than diffusive supply of cement components dominated late in concretion growth history concretions formed close to the tops of parasequence boundaries

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

airborne radar evidence for tributary flow switching in institute ice stream west antarctica implications for ice sheet configuration and dynamics despite the importance of ice streaming to the evaluation of west antarctic ice sheet wais stability we know little about mid to long term dynamic changes within the institute ice stream iis catchment here we use airborne radio echo sounding to investigate the subglacial topography internal stratigraphy and holocene flow regime of the upper iis catchment near the ellsworth mountains internal layer buckling within three discrete topographically confined tributaries through ellsworth independence and horseshoe valley troughs provides evidence for former enhanced ice sheet flow we suggest that enhanced ice flow through independence and ellsworth troughs during the mid to late holocene was the source of ice streaming over the region now occupied by the slow flowing bungenstock ice rise although buckled layers also exist within the slow flowing ice of horseshoe valley trough a thicker sequence of surface conformable layers in the upper ice column suggests slowdown more than 4000 years ago so we do not attribute enhanced flow switch off here to the late holocene ice flow reorganization intensely buckled englacial layers within horseshoe valley and independence troughs cannot be accounted for under present day flow speeds the dynamic nature of ice flow in iis and its tributaries suggests that recent ice stream switching and mass changes in the siple coast and amundsen sea sectors are not unique to these sectors and that they may have been regular during the holocene and may characterize the decline of the wais

reconstructing the last glacial maximum ice sheet in the weddell sea embayment antarctica using numerical modelling constrained by field evidence abstract the weddell sea embayment wse sector of the antarctic ice sheet has been suggested as a potential source for a period of rapid sea level rise meltwater pulse 1a a 20 m rise in 500 years previous modelling attempts have predicted an extensive grounding line advance in the wse to the continental shelf break leading to a large equivalent sea level contribution for the sector a range of recent field evidence suggests that the ice sheet elevation change in the wse at the last glacial maximum lgm is less than previously thought this paper describes and discusses an ice flow modelling derived reconstruction of the lgm ice sheet in the wse constrained by the recent field evidence the ice flow model reconstructions suggest that an ice sheet consistent with the field evidence does not support grounding line advance to the continental shelf break a range of modelled ice sheet surfaces are instead produced with different grounding line locations derived from a novel grounding line advance scheme the ice sheet reconstructions which best fit the field constraints lead to a range of equivalent eustatic sea level estimates between approximately 1 4 and 3 m for this sector this paper describes the modelling procedure in detail considers the assumptions and limitations associated with the modelling approach and how the uncertainty may impact on the eustatic sea level equivalent results for the wse

deglacial history of the west antarctic ice sheet in the weddell sea embayment constraints on past ice volume change the retreat history of the west antarctic ice sheet wais since the last glacial maximum is important for understanding the process of rapid deglaciation constraining models that seek to predict the future trajectory of the ice sheet and for estimating rates of sea level change here we report new glacial geologic data from the southwestern weddell sea embayment that demonstrate that this part of the wais was thinner than previously suggested and that there was progressive thinning of the ice sheet by 230 480 m since ca 15 ka we use geomorphological data and a numerical ice sheet model to reconstruct the ice sheet in the weddell sea at the last glacial maximum the volume of this ice would have added between 1 4 and 2 0 m to postglacial sea level rise and would not have been sufficient to contribute significantly to meltwater pulse 1a a rapid rise in sea level 14 200 yr ago

a 3 d coupled ice sheet sea level model applied to antarctica through the last 40 ky abstract we present results from a three dimensional ice sheet shelf model of antarctica coupled to a gravitationally self consistent global sea level model that incorporates maxwell viscoelastic deformation of the solid earth the coupled model captures complex post glacial changes in sea level associated with the gravitational deformational and rotational effects of the evolving surface mass ice plus ocean load over the global ocean including at the grounding lines of marine based ice the simulations are initiated at 40 ka and we focus on ice distributions and sea levels from the last glacial maximum to present our results extend and confirm the key conclusions of earlier work using a simplified 1 d ice sheet model by demonstrating that the sea level coupling has a significant stabilizing influence on marine ice sheet grounding lines the feedback of sea level changes into the ice sheet model acts to slow down the retreat and advance of the grounding line relative to simulations in which the full coupling is not incorporated differences in ice thickness between these simulations can reach 1 km close to the grounding line finally we perform preliminary comparisons of our results to relative sea level rsl histories and gps derived present day uplift rates at sites near the margins of antarctica we find that the coupling improves fits to uplift rates in several regions and that the rsl predictions of the coupled model yield a fit to the observations that is comparable to recent uncoupled simulations in which the underlying earth model was varied to obtain a best fit to the rsl histories

on postglacial sea level summary an exact method is presented for calculating the changes in sea level that occur when ice and water masses are rearranged on the surface of elastic and viscoelastic non rotating earth models the method is used to calculate the instantaneous elastic and delayed vi scoelastic sea level changes following the partial melting of late quaternary ice sheets we find that there can be large errors in the usual assumption that changes in sea level are uniform over the ocean basins if a quantity of ice equivalent to a uniform 100 m rise in sea level melts from the laurentide and fennoscandian ice sheets then in the south pacific the instantaneous rise in sea level can be as large as 120m in the north atlantic the instantaneous rise is always less than 100 m there is a zone in the north atlantic with almost no sea level change and near greenland and norway the sea level falls rather than rises by over 100 m one thousand years after the melting a forebulge migrating towards the ice loads causes water to flow from the south pacific into the north pacific suggesting that raised beaches should occur in the south pacific the gravitational attraction of an ice mass upon a nearby ocean tends to hold sea level high in the vicinity of the ice this extra load near the ice may have a significant influence on postglacial isostatic adjustment

vertical effective stress and temperature as controls of quartz cementation in sandstones evidence from north sea fulmar and gulf of mexico wilcox sandstones abstract we present quantitative petrographic data high spatial resolution oxygen isotope analyses of quartz cement basin modelling and a kinetic model for quartz precipitation for two paleocene eocene wilcox group sandstones from texas and two jurassic fulmar formation sandstones from the central north sea at each location one sandstone has been buried to ca 145 c and one to ca 185 c a key difference between the wilcox and fulmar burial histories is that the wilcox sandstones are currently at higher vertical effective stresses and from basin modelling studies have been subjected to generally higher vertical effective stresses through their burial history the amounts of quartz cement in the wilcox sandstones are between 12 and 18 and between 2 and 6 in the fulmar sandstones high spatial resolution oxygen isotope data obtained from the quartz cements suggest temperature ranges for quartz precipitation from 60 to 80 c to values approaching maximum burial temperature factors such as grain coatings or the timing of petroleum emplacement cannot explain the differences in the amounts of quartz cement petrographic data show that most of the silica for quartz cement can be derived from intergranular pressure dissolution although the sample set is small we interpret the results to suggest that the differences in quartz cementation in fulmar and wilcox sandstones can be explained better by differences in their vertical effective stress history than their temperature history in this case the supply of silica rather than the precipitation of quartz becomes an important control on the rate and extent of cementation

chemical diagenetic constraints on the timing of cataclasis in deformed sandstone along the pine mountain overthrust eastern kentucky abstract cathodoluminescence imaging reveals that sandstones of the middle pennsylvanian lee formation and breathitt group along the pine mountain overthrust pmo contain inter and intra granular authigenic quartz in proportions that differ markedly between samples of contrasting deformational state fracture surfaces generated during fault movement provided nucleation substrates favorable for the emplacement of substantial amounts of intragranular quartz up to 11 volume percent in the lee formation relatively undeformed sandstones contain cement that is dominantly intergranular whereas sandstones from highly deformed cataclasites along the trace of the pmo contain cement that is dominantly localized within intragranular fractures therefore the chemical diagenetic histories of different samples of breathitt and lee sandstones diverged markedly as a consequence of differing deformational histories and the cementation history thus constitutes a record that is relevant to deciphering the timing of deformation the small quantities of early formed intergranular cement within the cataclasites average 3 7 volume percent show evidence of breakage and accordingly must have largely pre dated the deformation quartz cementation modeling touchstone suggests that emplacement of this pre deformational intergranular cement within these deformed rocks can be bracketed within a period from approximately 280 to 260 ma an interpretation of fault movement that shortly post dates this initial period of quartz cementation is consistent with other estimates for the timing of the alleghanian orogeny because of their protracted period of burial rocks in the vicinity of the pmo remained at temperatures amenable to continued quartz cementation until the middle cenozoic and thus most of the quartz cementation in the cataclasites and also in the surrounding undeformed sandstones post dates movement on the fault

ice flow structure and ice dynamic changes in the weddell sea sector of west antarctica from radar imaged internal layering recent studies have aroused concerns over the potential for ice draining the weddell sea sector of west antarctica to figure more prominently in sea level contributions should buttressing from the filchner ronne ice shelf diminish to improve understanding of how ice stream dynamics there evolved through the holocene we interrogate radio echo sounding res data from across the catchments of institute and moller ice streams iis and mis focusing especially on the use of internal layering to investigate ice flow change as an important component of this work we investigate the influence that the orientation of the res acquisition track with respect to ice flow exerts on internal layering and find that this influence is minimal unless a res flight track parallels ice flow we also investigate potential changes to internal layering characteristics with depth to search for important temporal transitions in ice flow regime our findings suggest that ice in northern iis draining the ellsworth subglacial highlands has retained its present ice flow configuration throughout the holocene this contrasts with less topographically constrained ice in southern iis and much of mis whose internal layering evinces spatial changes to the configuration of ice flow over the past 10 000 years our findings confirm siegert et al s 2013 inference that fast flow was diverted from bungenstock ice rise during the late holocene and suggest that this may have represented just one component of wider regional changes to ice flow occurring across the iis and mis catchments as the west antarctic ice sheet has thinned since the last glacial maximum

stability of the west antarctic ice sheet in a warming world equivalent to 0 28 to 0 56 mm yr 1 sea level rise with the rate growing over at least the past two decades 1 4 these observations intensify concerns about the ice sheet s stability that were first raised more than four decades ago 5

a one dimensional model of ice shelf ocean interaction large scale oceanic circulation beneath antarctic ice shelves is driven by the thenmohaline differences which result from mass and energy exchange at the ice ocean interface dense saline waters are drawn underneath the ice shelves and emerge cooled and diluted as plumes of ice shelf water a simple one dimensional model of this process has been developed in which the ice shelf water plume is treated as a turbulent gravity current initiated at the inland margin by a flow of fresh meltwater emerging from beneath the grounded ice subsequent evolution of the plume as it ascends along an ice shelf base of specified geometry can be simulated the model has been applied to a flow line on ronne ice shelf antarctica to explain the observed distribution and rate of basal melting and freezing calculations indicate that the present mean melt rate of 0 6 m yr 1 would increase to 2 6 m yr 1 if the underlying water were to warm by 0 6 c this would not only lead to significant thinning of the ice shelf but could also cause a profound change in ocean circulation on the open continental shelf

glacial interglacial variations in central east antarctic ice accumulation rates past rates of ice accumulation in central east antarctica are defined poorly for the last glacial period ice cores from which current estimates are based are limited in number there is one deep ice core at vostok station one at dome f and two others currently being extracted at dome c and dronning maud land and are restricted spatially they are one dimensional here spatial variations in ice accumulation rates during the last glacial are calculated from isochronous internal ice sheet layering measured by airborne ice penetrating radar data the layers are used to identify the stratigraphy around five east antarctic ice domes isochrons are dated by linking them to the vostok ice core and in one case the epica dome c ice core this chronostratigraphy allows the depth age function to be identified across the ice sheet a simple ice flow model using the depth age measurements as input is then used to determine the spatial and temporal variation in east antarctic ice accumulation over the last glacial cycle the lowest rates of ice accumulation were predicted in the full glacial period to the south of dome a and ridge b there are two possible explanations for this 1 the ice divide topographies may have caused a precipitation shadow in the ice age 2 with the expansion of the ross ice shelf the area to the south of dome a and ridge b would be at the very centre of the ice sheet and hence at a maximum distance from the ocean moisture source r 2002 elsevier science ltd all rights reserved