borehole imagery of meteoric and marine ice layers in the amery ice shelf east antarctica a real time video camera probe was deployed in a hot water drilled borehole through the amery ice shelf east antarctica where a total ice thickness of 480 m included at least 200 m of basal marine ice down looking and side looking digital video footage showed a striking transition from white bubbly meteoric ice above to dark marine ice below but the transition was neither microscopically sharp nor flat indicating the uneven nature at centimetre scale of the ice shelf base upstream where the marine ice first started to accrete marine ice features were imaged including platelet structures cell inclusions entrained particles and the interface with sea water at the base the cells are assumed to be entrained sea water and were present throughout the lower 100 150m of the marine ice column becoming larger and more prevalent as the lower surface was approached until near the base they became channels large enough that the camera field of view could not contain them platelets in the marine ice at depth appeared to be as large as 1 2 cm in diameter particles were visible in the borehole meltwater probably marine and mineral particles liberated by the drill but their distribution varied with depth

fast recession of a west antarctic glacier satellite radar interferometry observations of pine island glacier west antarctica reveal that the glacier hinge line position retreated 1 2 0 3 kilometers per year between 1992 and 1996 which in turn implies that the ice thinned by 3 5 0 9 meters per year the fast recession of pine island glacier predicted to be a possible trigger for the disintegration of the west antarctic ice sheet is attributed to enhanced basal melting of the glacier floating tongue by warm ocean waters

positive mass balance of the ross ice streams west antarctica we have used ice flow velocity measurements from synthetic aperture radar to reassess the mass balance of the ross ice streams west antarctica we find strong evidence for ice sheet growth 26 8 gigatons per year in contrast to earlier estimates indicating a mass deficit 20 9 gigatons per year average thickening is equal to 25 of the accumulation rate with most of this growth occurring on ice stream c whillans ice stream which was thought to have a significantly negative mass balance is close to balance reflecting its continuing slowdown the overall positive mass balance may signal an end to the holocene retreat of these ice streams

finite amplitude bars in mixed bedrock alluvial channels we present a nonlinear asymptotic theory of fully developed flow and bed topography in a wide channel of constant curvature to describe finite amplitude perturbations of bottom topography subject to an inerodible bedrock layer the flow field is evaluated at the leading order of approximation as a slowly varying sequence of locally uniform flows slightly perturbed by a weak curvature induced secondary flow using the constraint of constant fluid discharge and sediment flux we calculate an analytical solution for the cross sectional profile of flow depth and bed topography and we determine the average slope in the bend necessary to transport the sediment supplied from a straight alluvial upstream reach both fully alluvial bends and bends with partial bedrock exposure are shown to require a larger average slope than a straight upstream reach the relative slope increase is much larger for mixed bedrock alluvial bends curvature and sediment supply are shown to have a strong effect on the characteristics of the point bars in mixed bedrock alluvial channels higher curvature bends produce bars of larger amplitude and more bedrock exposure through the cross section and increasing the sediment supply leads to taller and wider point bars differences in the relative roughness of sediment and bedrock have a smaller secondary effect on point bar characteristics our analytical approach can potentially be extended to the case of arbitrary yet slowly varying curvature and should ultimately lead to an improved understanding of the formation of meanders in bedrock channels

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

application of tandem accelerator mass spectrometer dating to late pleistocene holocene sediments of the east antarctic continental shelf abstract glacial recession from the antarctic continental shelf is recorded by glacial marine diamictons sands and overlying siliceous oozes in order to clarify the chronology for this sequence use was made of the university of arizona tandem accelerator mass spectrometer tams for 14 c dating small samples of benthic and planktonic foraminifera were selectively removed from diamictons graded sands and surface sediments which were recovered from the wilkes land continental shelf and slope east antarctica organic carbon was also utilized as a source for tams dating of the siliceous oozes and muds uncorrected ages varied from 14 260 140 to 3230 200 yr b p carbon fixed by phytoplankton and foraminifera is strongly influenced by old glacial derived co 2 thus reservoir corrections of up to 5500 yr are needed for the 14 c dates iceberg turbation reworks foraminifera so that dates from resulting deposits diamictons are interpreted as maximum ages the consistency of corrected ages from the shelf along with the sedimentologic interpretation suggests a rather recent recession perhaps mid holocene for this portion of the east antarctic ice sheet further application of the tams method should help clarify other problems concerning the late quaternary glacial history of antarctica

terrestrial sea ice morphology considerations for europa abstract the galileo mission has returned the first high resolution 21 m pixel images of the surface of europa these images reveal structures with morphologies reminiscent of those seen on terrestrial sea ice although it is premature to make one to one analogies between sea ice and europa s surface a review of the types of surface features commonly formed on earth and of various sea ice processes can provide insight into the complex geology of europa for example deformation of terrestrial sea ice results from winds tides and currents and from thermally induced stresses the resulting features include fractures ranging in width from millimeters to kilometers pressure ridges shear ridges and rafted ice potential agents of deformation on europa are more likely to be limited to tidal flexing and possibly convection but could produce similar features and perhaps account for the ridges and fractures seen in many areas subtle differences in albedo and color in terrestrial sea ice result from differences in ice thickness and grain size attributed to factors such as the rate of ice crystal growth water turbulence age of the ice and deformation similar factors could account for differences observed in the bright icy plains of europa moreover salts in both the solid form and as brine vary in concentration and composition as a function of space and time on earth leading to differences in density and the strength of ice sheets salts are also suspected in the europan ice and could lead to similar differences enhancing the creation of topographic relief from density contrasts and the formation of fractures from brittle failure of the ice differences in the environments between europa and terrestrial sea ice in terms of parameters such as temperature gravity time and ice compositions suggest caution in drawing direct analogies future work by the planetary and sea ice communities must include understanding the terrestrial processes sufficiently for extrapolation to europa

properties of a marine ice layer under the amery ice shelf east antarctica the amery ice shelf east antarctica undergoes high basal melt rates near the southern limit of its grounding line where 80 of the ice melts within 240 km of becoming afloat a considerable portion of this later refreezes downstream as marine ice this produces a marine ice layer up to 200 m thick in the northwest sector of the ice shelf concentrated in a pair of longitudinal bands that extend some 200 km all the way to the calving front we drilled through the eastern marine ice band at two locations 70 km apart on the same flowline we determine an average accretion rate of marine ice of 1 1 0 2 m a 1 at a reference density of 920 kg m 3 between borehole sites and infer a similar average rate of 1 3 0 2 m a 1 upstream the deeper marine ice was permeable enough that a hydraulic connection was made whilst the drill was still 70 100 m above the ice shelf base below this marine close off depth borehole video imagery showed permeable ice with water filled cavities and individual ice platelets fused together while the upper marine ice was impermeable with small brine cell inclusions we infer that the uppermost portion of the permeable ice becomes impermeable with the passage of time and as more marine ice is accreted on the base of the shelf we estimate an average closure rate of 0 3 m a 1 between the borehole sites upstream the average closure rate is faster at 0 9 m a 1 we estimate an average porosity of the total marine ice layer of 14 20 such that the deeper ice must have even higher values high permeability implies that sea water can move relatively freely through the material and we propose that where such marine ice exists this renders deep parts of the ice shelf particularly vulnerable to changes in ocean properties

a theory for ice till interactions and sediment entrainment beneath glaciers and frozen onto or melted from the glacier base can achieve a steady state that is in balance with the rate that latent heat is transported to or from the basal interface at constant n when a gradual increase in heat flow from the glacier base causes the rate of melting to decrease h increases and continues to do so when the heat flow is great enough to produce freezing as freezing becomes more rapid and h increases further the rate of fluid supply to the glacier base reaches a maximum when the effective permeability is sufficiently reduced by the partial ice saturation in the fringe larger h can be achieved with slower freezing at the glacier base but steady states with larger h are unstable the maximum rate of fluid supply to the glacier base is greater at lower n higher temperature gradients and for sediments with higher permeabilities unsteady behavior can lead to large changes in h when there is a mismatch between the rate that latent heat can be extracted and the rate that fluid is supplied to the glacier base transient behavior driven by abrupt changes in n is characterized by rapid variations in freezing rate followed by slower adjustments to h that are limited by the timescale for the conduction of latent heat the resulting patterns of sediment deformation are expected to commonly be distributed over finite depth ranges even when shear is perfectly localized at any single instant in time

reply to comment by a w rempel et al on a quantitative framework for interpretation of basal ice facies formed by ice accretion over subglacial sediment 1 we are grateful to rempel et al 2007 hereinafter referred to as rww2007 for giving us this opportunity to discuss in greater detail the fascinating problem of developing quantitative models of subglacial freeze on and basal ice formation we find many of their comments valuable but disagree with their general negative assessment of our previous work on the subject christoffersen et al 2006 hereinafter referred to as ctcb2006 we find it particularly disappointing that rww2007 dismiss our model on the basis of misconstrued model physics rww2007 criticize our assumed treatment of the freezing process but never really give specific examples of how changing these assumptions would improve the ability of our model to realistically reproduce different facies of debris bearing basal ice here we lay out the details of our disagreement with their dismissal of our work 2 despite the claim made by rww2007 there is no sign error in equation 1b of christoffersen and tulaczyk 2003 hereinafter referred to as ct2003 if we switch the sign in this equation as rww2007 imply should be done then we would get pore water pressures higher than ice pressure at the freezing ice water interface this would result in water being driven away from the ice water interface rather than being drawn toward it as the phenomenon of frost heaving requires o neill and miller 1985 their claim that our equation is erroneous is the more surprising since it is straightforward to show that their equation for interfacial pressure rempel et al 2004 hereinafter referred to as rww2004 takes on the same form as equation 1b of ct2003 once the assumption is made that ice saturation is zero in the till underlying a freezing ice base i e no frozen fringe under this assumption the upper limit of integration in equation 3 4 of rww2004 is zl zf and the equation for interfacial pressure piw at a freezing icewater interface can be expanded to a form similar to our equation 1b ct2003

channel evolution after dam removal in a poorly sorted sediment mixture experiments and numerical model dam removal is commonly used for river restoration however there are still some uncertainties associated with dam removal mainly related to the sediment transport rates released downstream from the deposit that had previously filled the impoundment this research studies the physical response to dam removal in the antecedent deposit by answering the following questions a how does an initial channel excavated into the deposit evolve and b what is the time distribution of the material released during the early stages of the process these goals are achieved by an experimental campaign using a poorly sorted mixture of sediment in the antecedent deposit the research shows that for the given conditions of our experiments the rate at which the sediment is released depends on the height of the removed dam the water discharge and the maximum potential volume of sediment to be eroded this investigation provides new insights of the width evolution when the sediment is composed of a poorly sorted mixture this evolution is linked to the bed degradation rates channel narrows during a rapid incisional phase and subsequently widens when bed degradation rates decrease channel width changes propagate upstream as a convection like perturbation associated with a kinematic wave starting at the location of the antecedent dam these features are modeled through a new numerical model accounting for mixtures more specifically a set of equations has been derived for the variation of bed elevation channel bottom width and bed grain size distribution which when solved numerically describe the observed channel processes

debris entrainment and landform genesis during tidewater glacier surges the englacial entrainment of basal debris during surges presents an opportunity to investigate processes acting at the glacier bed the subsequent melt out of debris rich englacial structures during the quiescent phase produces geometrical ridge networks on glacier forelands that are diagnostic of surge activity we investigate the link between debris entrainment and proglacial geomorphology by analyzing basal ice englacial structures and ridge networks exposed at the margins of tunabreen a tidewater surge type glacier in svalbard the basal ice facies display clear evidence for brittle and ductile tectonic deformation resulting in overall thickening of the basal ice sequence the formation of debris poor dispersed facies ice is the result of strain induced metamorphism of meteoric ice near the bed debris rich englacial structures display a variety of characteristics and morphologies and are interpreted to represent the incorporation and elevation of subglacial till via the squeezing of till into basal crevasses and hydrofracture exploitation of thrust faults reoriented crevasse squeezes and preexisting fractures these structures are observed to melt out and form embryonic geometrical ridge networks at the base of a terrestrially grounded ice cliff ridge networks are also located at the terrestrial margins of tunabreen neighboring von postbreen and in a submarine position within tempelfjorden analysis of network characteristics allows these ridges to be linked to different formational mechanisms of their parent debris rich englacial structures this in turn provides an insight into variations in the dominant tectonic stress regimes acting across the glacier during surges

englacial phase changes and intergranular flow above subglacial lakes glaciers are permeated by liquid veins that line the triple junctions between ice grains and act as conduits for material transport the density difference between water and ice produces a hydraulic gradient that drives liquid flow down towards the glacier bed we investigate how variations in this transport rate are manifest in the lower regions of a glacier that floats on a subglacial reservoir when the temperature rises towards the glacier bed the associated permeability increase leads to more rapid fluid transport internal melting supplies the changing flow a reduction in hydraulic gradient results from surface energy effects and causes a decreasing transport rate near the base hence freezing occurs within the polycrystalline ice for representative values of the heat flux soluble impurity loading and grain size the downward flux peaks at approximately 1 100 mm a 1 several tens of centimeters above the glacier bed beneath this level the effects of surface energy control the fluid transport and cause the flow to reverse directions pulling liquid upwards into the polycrystalline ice so that the flow does not introduce glacial material into the underlying reservoir

the supply and accumulation of silica in the marine environment rivers and submarine hydrothermal emanations supply 6 1 1014g sio2 yr to the marine environment approximately two thirds of the silica supplied to the marine environment can be accounted for in continental margin and deep sea deposits siliceous deep sea sediments located beneath the antarctic polar front convergence account for over a fourth 1 6 1014g sio2 yr of the silica supplied to the oceans deep sea sediment accumulation rates beneath the polar front are highest in the south atlantic with values as large as 53cm kyr during the last 18 000 yr siliceous sediments in the bering sea sea of okhotsk and subarctic north pacific accumulate 0 6 1014g sio2 yr or 10 of the dissolved silica input to the oceans the accumulation of biogenic silica in estuarine deposits removes a maximum of 0 8 1014g sio2 yr although estuarine silica versus salinity plots indicate extensive removal of riverine silica from surface waters the removal rates must be considered as maximum values because of dissolution of siliceous material in estuarine sediments and bottom waters siliceous sediments from continental margin upwelling areas e g gulf of california walvis bay or peru chile coast have the highest biogenic silica accumulation rates in the marine environment 69 g sio2 cm2 kyr despite the rapid accumulation of biogenic silica upwelling areas account for less than 5 of the silica supplied to the marine environment because they are confined laterally to such small areas

a quantitative framework for interpretation of basal ice facies formed by ice accretion over subglacial sediment 1 we have constructed a numerical model of basal ice formation for glacier ice in contact with subglacial sediment the model predicts four different ice facies whose formation is controlled by availability of subglacial water to satisfy the basal freeze on rate clean or clotted facies may result from congelation or frazil ice growth occurring when supercooled meltwater separates ice base from substrate or if a groundwater source can supply water to the ice base at a velocity equal to the freezing rate laminated facies develops when the supply of subglacial water is sufficiently constrained for cryostatic suction to raise the subglacial effective stress above a threshold for intrusion of ice into sediment by regelation debris laminas 1 mm are entrained by short periodic regelation events of a few hours separated by longer periods days to months of congelation further meltwater limitation produces a massive dirty ice facies due to stacking of debris laminas the model predicts growth of solid dirty ice facies when the bed is meltwater depleted with fast freezing 5 mm yr 1 causing enhanced erosion 30 mm yr 1 we find that basal ice facies and sediment entrainment are controlled mainly by the ratio of freezing rate to water supply rate the predicted ice facies compare favorably with borehole camera imagery of the basal ice layer in kamb ice stream west antarctica facies variability in this layer suggests complex hydrologic history for the west antarctic ice sheet with significant changes occurring over a period of several thousand years

subglacial conditions during and after stoppage of an antarctic ice stream is reactivation imminent borehole observations from the base of the west antarctic ice sheet wais reveal the presence of a 10 to 15 m thick accretionary basal ice layer in the upstream area of kamb ice stream kis this ice layer has formed over a time of several thousand years by freeze on of subglacial water to the ice base and has recorded during this time basal conditions upstream of its current location analysis of samples and videos sequences from boreholes drilled to the bottom of kis confirms that kis stoppage was due to basal freeze on and that relubrication of the ice stream is well underway these results further suggest that ice stream cyclicity may be shorter than expected 1000s of years and that a restart of kis may be imminent within decades to centuries

response of subglacial sediments to basal freeze on 1 theory and comparison to observations from beneath the west antarctic ice sheet 1 we have constructed a high resolution numerical model of heat water and solute flows in sub ice stream till subjected to basal freeze on the model builds on quantitative treatments of frost heave in permafrost soils the full version of the clapeyron equation is used hence ice water phase transition depends on water pressure osmotic pressure and surface tension the two latter effects can lead to supercooling of the ice base this supercooling in turn induces hydraulic gradients that drive upward flow of pore water which feeds the growth of segregation ice onto the freezing interface this interface may progress into the till and form ice lenses if supercooling is sufficiently strong hence the ice segregation process develops a stratified basal ice layer in our model a high basal temperature gradient 0 054 c m 1 triggers ice stream stoppage and the loss of basal shear heating leads to relatively high basal freeze on rates 3 5 mm a 1 in response the subglacial till experiences comparatively rapid consolidation till porosity can decrease from 40 to 120 kpa approximately within one century basal supercooling arising from redistribution of solutes and ice water interfacial effects amounts to ca 0 35 c below the pressure melting point fine grained till is in our model associated with widely spaced thick ice lenses coarse grained till yields thinner ice lenses that are more closely spaced our model results compare favorably although not in all details with available observational evidence from borehole studies of west antarctic ice streams

geomorphology under ice streams moving from form to process ice streams are integral components of an ice sheet s mass balance and directly impact on sea level their flow is governed by processes at the ice bed interface which create landforms that in turn modulate ice stream dynamics through their influence on bed topography and basal shear stresses thus ice stream geomorphology is critical to understanding and modelling ice streams and ice sheet dynamics this paper reviews developments in our understanding of ice stream geomorphology from an historical perspective with a focus on the extent to which studies of modern and palaeo ice streams have converged to take us from a position of near complete ignorance to a detailed understanding of their bed morphology during the 1970s and 1980s our knowledge was limited and largely gleaned from geophysical investigations of modern ice stream beds in antarctica very few palaeo ice streams had been identified with any confidence during the 1990s however glacial geomorphologists began to recognise their distinctive geomorphology which included distinct patterns of highly elongated mega scale glacial lineations ice stream shear margin moraines and major sedimentary depocentres however studying relict features could say little about the time scales over which this geomorphology evolved and under what glaciological conditions this began to be addressed in the early 2000s through continued efforts to scrutinise modern ice stream beds at higher resolution but our current understanding of how landforms relate to processes remains subject to large uncertainties particularly in relation to the mechanisms and time scales of sediment erosion transport and deposition and how these lead to the growth and decay of subglacial bedforms this represents the next key challenge and will require even closer cooperation between glaciology glacial geomorphology sedimentology and numerical modelling together with more sophisticated methods to quantify and analyse the anticipated growth of geomorphological data from beneath active ice streams

holocene sediment records from the continental shelf of mac robertson land east antarctica geochemical records are presented for five sediment cores from basins on the continental shelf of mac robertson land east antarctica the cores contain 2 4 m thick sequences of hemipelagic siliceous mud and ooze smo deposited under seasonally open marine conditions the inner and middle shelf smo sequences are massive dark olive green material whereas the outer shelf smo sequences are dark olive material interspersed with light olive green layers 1 10 cm thick the biogenic material is dominated by marine diatoms including fragilariopsis curta fragilariopsis cylindrus and chaetoceros spp in the dark colored smo and corethron criophilum in the light colored layers radiocarbon dates suggest that the cores provide continuous accumulation records extending from 1 kyr before present b p back as far as 4 15 kyr b p with estimated accumulation rates of 0 07 5 mm yr 1 the three core records from the middle and outer shelf suggest six episodes of increased accumulation of biogenic material at 5 5 kyr b p all three cores 1 2 and 6 2 kyr b p two of the three cores and 3 8 and 10 8 kyr b p one core most of which coincide with corethron layers we interpret these features as the result of enhanced diatom production over the outer shelf possibly related to climatic warm periods the absence of such features in the inner shelf core records is thought to reflect a relatively constant level of seasonal diatom production in adjacent waters maintained by a coastal polynya

thermodynamics of basal freeze on predicting basal and subglacial signatures of stopped ice streams and interstream ridges we have constructed a numerical model that simulates the response of subglacial sediments to basal freeze on the model is set up to emulate the basal zone of drilling sites in the ross sea sector of thewest antarctic ice sheet we treatbasal freeze on atanice sedimentinterfaceasathermodynamicprocessthatcouplestheflowofwater heat and solutes inunfrozensubglacial sedimentsunderlyinga freezingicebase thecouplingof these flows occurs through the clapeyron equation which specifies the dependence of the basal freezing melting temperature on ice pressure water pressure solute concentration and surface tension effects thermally driven water flow is induced when an ice base becomes supercooled below the pressure melting point because ice water surface tension inhibits ice growth in small pore spaces of fine grained subglacial sediments our model results show that basal freeze on is capable of inducing considerable changes in the basal zone of both ice streams and interstream ridges these changes are associated with specific signatures that compare with borehole observations and geophysical surveys water pres sure levels are reduced and thick layers of debris laden basal ice develop these basal ice layers andunderlying sedimentscontain a distinct isotopic signal the predicted stable iso tope ratios reflect rayleigh type isotopic fractionation whose significance increases with increasing freezing rates supercooling of the ice base induces also measurable changes in the ice temperature profile of the glacier till porosity represents another quantity whose evolution is influenced strongly by basal freeze on in particular measurements of vertical porositydistributionbeneathstoppedicestreamscouldbeusedtoback calculatethetiming of the onsetof basal freezing our modelresults show thatthebasal zoneof ice streams and interstream ridges responds sensitively to changes inbasal melting freezing rates this sen sitivity may allow reconstruction of past conditions beneath ice streams and interstream ridges from measurements made on basal ice samples and subglacial sediment samples our model resultsalso indicate thatmeltwater from fast flowing ice streams may be driven towardsthe freezing icebase of interstream ridges

a borehole camera system for imaging the deep interior of ice sheets the design and first deployment is described for the jet propulsion laboratory california institute oftechnology ice borehole camera system for acquisition of down looking and side looking images in a borehole made by a hot water drill the objective of the system is to acquire images in support of studies of the basal dynamics and thermodynamics of west antarctic ice streams a few sample images obtained during the 2000 01 antarctic field season are shown from the basal layers of ice stream c

bed load at low shields stress on arbitrarily sloping beds alternative entrainment formulation 1 according to the bagnold hypothesis for equilibrium bed load transport a necessary constraint for the maintenance of equilibrium bed load transport is that the fluid shear stress at the bed must be reduced to the critical or threshold value associated with incipient motion of grains it was shown in a companion paper seminara et al 2002 however that the bagnold hypothesis breaks down when applied to equilibrium bed load transport on beds with transverse slopes above a relatively modest value that is well below the angle of repose an investigation of this failure resulted in a demonstration of its lack of validity even for nearly horizontal beds the constraint is here replaced with an entrainment formulation according to which a dynamic equilibrium is maintained by a balance between entrainment of bed grains into the bed load layer and deposition of bed load grains onto the bed the entrainment function is formulated so that the entrainment rate is an increasing function of the excess of the fluid shear stress at the bed over the threshold value the formulation is implemented with the aid of a unique set of laboratory data that characterizes equilibrium bed load transport at relatively low shear stresses for streamwise angles of bed inclination varying from nearly 0 to 22 the formulation is shown to provide a description of bed load transport on nearly horizontal beds that fits the data as well as that resulting from the bagnold constraint the entrainment formulation has the added advantage of not requiring the unrealistically high dynamic coefficient of coulomb friction resulting from the bagnold constraint finally the entrainment formulation provides reasonable and consistent results on finite streamwise and transverse bed slopes even those at which the bagnold formulation breaks down completely

late quaternary foraminiferal assemblages from western ross sea antarctica in relation to the main glacial and marine lithofacies abstract investigations on foraminifers from upper pleistocene holocene sediments were carried out on twelve cores from the western ross sea continental margin drygalski joides north victoria land basins as part of a progetto nazionale di ricerche in antartide p n r a multidisciplinary project data on the foraminiferal frequency species diversity tests abundance and their state of preservation were presented as a synthesis of 404 core samples to establish their relationships with the main glacial and marine lithofacies of this area a total of 126 benthic species pertaining to 73 genera have been identified just few taxa such as cibicides spp globocassidulina spp trifarina angulosa and miliammina spp being the most ubiquitous and in some cases the dominant species of these paleoenvironments two variants of neogloboquadrina pachyderma including thin and thick shelled forms have been recovered we propose to use these results to provide the degree of glacial control during the last glacial maximum and the following holocene retreat of the ice sheets high test fragmentation low diversity and density tests reflect higher glacial influence of the ice sheet in the drygalski basin whereas the decreasing percentage of fragmentation and a relative increase of density and diversity in drygalski joides and north victoria land basins indicate the paleoenvironmental passage from the ice sheet to the ice shelf condition the ice shelf retreat is well evidenced in the joides basin by a succession of levels barren of foraminifers alternating with high density levels rich in t angulosa followed by a total disappearance of the calcareous foraminifers open marine settings indicative of lower glacial influence and increased corrosiveness of the water masses is testified by the miliammina foraminiferal assemblage during the holocene in drygalski and joides basins cores on the contrary rich and abundant benthic and planktonic assemblages characterize the holocene paleoenvironment of the north victoria land area indicating that the water masses were less corrosive with respect to the other areas in addition to the glacial reworking of the tests and the dissolution due to the corrosive water mass conditions the volcaniclastic sediments recovered in the north victoria land basin cores also affected the condition of test preservation in volcaniclastic sediments older than about 20 ka bp the foraminifers concentration tends to zero and when present their tests are highly damaged or completely broken

modeling the subglacial hydrology of the james lobe of the laurentide ice sheet to investigate the drainage conditions that might be expected to develop beneath soft bedded ice sheets we modeled the subglacial hydrology of the james lobe of the laurentide ice sheet from hudson bay to the missouri river simulations suggest the james lobe had little effect on regional groundwater flow because the poorly conductive upper cretaceous shale that occupies the upper layer of the bedrock would have functioned as a regional aquitard this implies that general northward groundwater flow out of the williston basin has likely persisted throughout the quaternary moreover the simulations indicate that the regional aquifer system could not have drained even the minimum amount of basal meltwater that might have been produced from at the glacier bed therefore excess drainage must have occurred by some sort of channelized drainage network at the ice till interface using a regional groundwater model to determine the hydraulic conductivity for an equivalent porous medium in a 1 m thick zone between the ice and underlying sediment and assuming conduit dimensions from previous theoretical work we use a theoretical karst aquifer analog as a heuristic approach to estimate the spacing of subglacial conduits that would have been required at the ice till interface to evacuate the minimum water flux results suggest that for conduits assumed to be on the order of a tenth of a meter deep and up to a meter wide inter conduit spacing must be on the order of tens hundreds of meters apart to maintain basal water pressures below the ice overburden pressure while evacuating the hypothesized minimum meltwater flux r 2007 elsevier ltd all rights reserved

bed load transport equation on arbitrarily sloping beds this work presents a simple tool to evaluate bed load transport intensity and direction on arbitrarily sloping beds with local longitudinal and transversal inclinations up to 25 and in the case of uniform sediments and low values of the applied shields stress the tool is composed of a set of equations which fit the results obtained via an iterative procedure by the semiempirical model recently proposed in 2003 by parker seminara and solari the tool provides a fully nonlinear description of bed load transport which overcomes the limitations of linear formulations developed in the case of negligible local bed inclinations the proposed tool can be easily implemented in any morphodynamic model to describe the evolution of the bottom topography and to capture the dynamics of relevant sloping beds

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

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

high geothermal heat flow basal melt and the origin of rapid ice flow in central greenland age depth relations from internal layering reveal a large region of rapid basal melting in greenland melt is localized at the onset of rapid ice flow in the large ice stream that drains north off the summit dome and other areas in the northeast quadrant of the ice sheet locally high melt rates indicate geothermal fluxes 15 to 30 times continental background the southern limit of melt coincides with magnetic anomalies and topography that suggest a volcanic origin

ice stream sticky spots a review of their identification and influence beneath contemporary and palaeo ice streams rapidly flowing ice streams are the arterial drainage routes in continental ice sheets and exert a major influence on ice sheet mass balance recent observations have revealed that ice stream flow exhibits considerable variability with relatively rapid changes taking place in speed and direction this spatial and temporal variability is intimately linked to the conditions at the base of the ice streams and the distribution of localised patches of basal friction known as sticky spots in this paper we provide a detailed review of sticky spot observations from both contemporary and palaeo ice stream beds in order to better understand their nature and influence observations and theoretical considerations reveal four primary causes of stickiness i bedrock bumps ii till free areas iii areas of strong well drained till and iv freeze on of subglacial meltwater these may act together in one location or in isolation and a progressive increase in their distribution could lead to ice stream shut down bedrock bumps are influential under active ice streams where they provide form drag and can create thinner ice which increases the likelihood of basal freeze on increased bed roughness may prevent the lateral migration of some ice streams but bedrock bumps are unlikely to cause ice stream shut down because over long time scales ice stream erosion might be expected to reduce their amplitude the influence of till free areas beneath an ice stream will depend critically on the amount of water that might be drawn out of the surrounding till to lubricate such areas they are likely to be most important in ice stream onset zones but their identification has proved difficult beneath active ice streams if an ice stream operates solely by till deformation it is conceivable that a progressive increase in the exposure of till free areas could lead to shut down through a process of sediment exhaustion areas of strong well drained till have been identified beneath both active and ancient ice streams and are most likely to result from the reorganisation of subglacial meltwater the collapse of an inefficient cannalised system to a more efficient channelised system can occur rapidly and this mechanism has been hypothesised as a candidate for ice stream shut down in both contemporary and palaeo settings basal freeze on has also been observed and inferred from beneath modern and palaeo ice streams and a reduction in basal meltwater supply coupled with ice stream drawdown and the advection of cold ice increases the likelihood of switching off an ice stream a paucity of data from ice stream sticky spots limits a better understanding of their nature distribution and evolution beneath ice streams future technological advances are likely to improve the resolution of the data collected from the beds of modern ice streams but well preserved palaeo ice stream beds also hold potential for investigating their influence on ice stream flow and we present simple landsystems models to aid their identification such data will considerably enhance the basal boundary condition in ice stream models which will ultimately refine our predictions of the response of contemporary ice sheets to future changes in climate

the ross sea s contribution to eustatic sea level during meltwater pulse 1a abstract radiocarbon dates obtained from ross sea sediments combined with constraints on maximum ice extent based on sedimentological data and ice sheet modeling results indicate that the ross embayment antarctica is unlikely to have been a substantial contributor to meltwater pulse mwp 1a at 12 5 14c ka the ice sheet in the ross embayment during the last glacial maximum is estimated to have contained ice volume equivalent to 3 6 m eustatic sea level rise and does not show evidence of rapid retreat grounded ice advance occurred during initial late pleistocene eustatic rise and ice sheet retreat began coeval with or subsequent to mwp 1a chronological and sedimentological data from the ross embayment indicate that deglaciation in this region has occurred throughout the holocene and the maximum possible contribution of eustatic rise from the ross embayment is

a diverse benthic assemblage 100 km from open water under the amery ice shelf antarctica 1 a hot water drill was used to penetrate 480 m of ice to reveal a diverse benthic assemblage dominated by suspension feeding invertebrates under the amery ice shelf east antarctica at a location 100 km from open water and at a depth of 775 m below sea level 840 m below the ice shelf surface this is the first record of a benthic assemblage of this type found at this distance under an ice shelf the few previous reports of life under ice shelves describe assemblages with very different trophic strategies e g sparse assemblages of mobile scavengers or chemotrophs or are in circumstances in which in situ photosynthesis at tide cracks or through the ice cannot be ruled out as a potential source of primary production the physical characteristics of the amery ice shelf and the feeding strategies represented together indicate that the only likely source of primary production to sustain the benthic assemblage is material advected from open water this suggestion is supported by observed current speeds in the vicinity and reported rates of particle settling the observation under an ice shelf of a benthic assemblage that is very similar to those found elsewhere in antarctica in locations dominated by annual sea ice or at depths below the photic zone has implications for the interpretation of sediment paleorecords to represent the history of ice shelf advance and retreat without observations of this living assemblage in situ the remnants of its component species in the sediment record such as sponge spicules echinoderm ossicles and bryozoan fragments could be interpreted reasonably but erroneously to represent open water conditions

kamb ice stream flow history and surge potential a basal zone tens of meters thick of debris laden ice was observed in kamb ice stream west antarctica using a video camera lowered into boreholes made by hot water drilling the debris content varies sometimes abruptly forming a sequence of layers that reflect the complex history of fast ice flow and bed interaction in most parts the concentration of debris is low a few percent by weight with particles often mud clots dispersed in a matrix of clear ice the nature of the debris distribution can be interpreted in terms of specific time intervals in the history of fast motion of kamb ice stream including processes leading up to the termination of its streaming behavior and possible reactivation

sediment transport on arbitrary slopes simplified model the paper presents a study on the influence of gravity on the incipient motion and the bed load transport of sediment the computation of critical bed shear stress is revisited considering the balance of forces hydrodynamic forces and submerged self weight acting on a solitary sediment particle lying on an arbitrary sloping bed modified effective bed shear stress and the corresponding critical bed shear stress which are defined to assess the incipient motion of sediment in the direction of resultant force are applied for the estimation of bed load transport rate in the direction of resultant force the sediment transport induced by the gravitational force which is oblique to the direction of the drag force induced by flow is incorporated into the bed load transport equation this modified model provides a reasonable prediction of the critical bed shear stress and the bed load transport rate the model is validated by experimental data it can be applied to steep slopes and can also avoid the problem of singularity that arises in numerically calculation of sediment transport rate additionally the vectorial transport rate obtained in the model calculation can be implemented in a numerical simulation of channel bed evolution

ice stream subglacial access for ice sheet history and fast ice flow the beamish project on rutford ice stream west antarctica and initial results on basal conditions

a non destructive method for measuring the salinity and solid fraction of growing sea ice in situ we describe an instrument developed to make in situ measurements of salinity and solid fraction profiles in growing sea ice the vertical resolution of the measurements is up to a few millimeters with a temporal resolution of up to fractions of a second the technique is based on impedance measurements between platinum wires around which sea ice grows data obtained using this instrument in laboratory experiments are in good agreement with theoretical predictions in a field test in the arctic the bulk salinity of growing sea ice has been measured in situ throughout the whole depth of the ice layer the data are compared with bulk salinities obtained from ice cores and confirm the general understanding that the bulk salinity in ice core studies is significantly underestimated in the lower parts of the cores the approach can also be used in other glaciological applications and for general studies of two phase two component porous media

preservation efficiencies and accumulation rates for biogenic silica and organic c n and p in high latitude sediments the ross sea rates of biogenic sediment accumulation biogenic silica and organic c n and p and pore water flux have been established for a variety of depositional environments in the ross sea on the basis of 14c measurements in kasten cores sediment accumulation rates ranged from 250 cm kyr 1 in the coastal basin of granite harbor to 1 2 cm kyr 1 in the shelf and slope environments of the northern and eastern ross sea burial of biogenic material was most rapid in the southwestern ross sea where biogenic silica accumulation rates ranged from 2 to 31 g cm 2 kyr 1 and organic carbon accumulation rates ranged from 0 05 to 1 4 g cm 2 kyr 1 in the northern and eastern ross sea biogenic silica and organic carbon accumulation rates typically equaled 0 02 0 03 g sio2 cm 2 kyr 1 and 0 002 0 004 g c cm 2 kyr 1 flux core measurements were used to estimate seabed regeneration rates for biogenic silica organic carbon and phosphorus pore water fluxes in general showed much less variability across the ross sea than did the biogenic accumulation rates pore water silicate fluxes in the study area ranged from 0 6 to 5 3 g sio2 cm 2 kyr 1 whereas carbon fluxes ranged from 0 1 to 1 2 g c cm 2 kyr 1 and phosphate fluxes varied from 0 006 to 0 012 g p cm 2 kyr 1 seabed preservation efficiencies were calculated for biogenic silica organic carbon and phosphorus by combining the solid phase and pore water data the seabed preservation efficiencies for biogenic silica 1 86 were greater than for organic carbon 1 71 at all nine stations examined the preferential preservation of biogenic silica relative to organic carbon also was apparent in ross sea sediments because the biogenic silica organic carbon ratio in the material buried in the seabed generally was 2 times greater than the ratio in sediment particles arriving at the sediment water interface sediment accumulation rate correlated strongly with both the biogenic silica and the organic carbon preservation efficiency data p preservation efficiencies remained relatively high 24 65 even when the accumulation rates were low 1 2 cm kyr 1 because of the near zero phosphate fluxes out of the seabed of the total p in the seabed 0 04 0 09 wt approximately 25 existed in the form of organic p the total amount of biogenic silica accumulating on the ross sea shelf is 2 3 1012 g sio2 yr 1 which is approximately an order of magnitude less than the ledford hoffman et al 1986 estimate that was based on 210pb chronologies biogenic silica accumulation rates in the southern central and western ross sea increased during the mid and late holocene reaching their maximum values during the past 500 to 1000 years

local high slope effects on sediment transport and fluvial bed form dynamics 1 we investigate the morphodynamic evolution of an arbitrarily tilted coarse mobile bed under the action of a free surface water flow the analysis is based on the experimental observations of the evolution of a mobile bed from its initial laterally tilted configuration to the final laterally flat state due to the presence of a lateral component of bed load transport measurements of the bed topography at different times allow one to reconstruct the flattening process the experimental observations are interpreted by means of a three dimensional numerical hydromorphodynamic model employing different relationships linear and nonlinear to evaluate bed load intensity and direction the analysis shows that the inclusion of nonlinear gravitational effects in the description of the bed load leads to a more satisfactory description of the bed evolution this finding opens important issues on how the inclusion of the nonlinear gravitational effects on the bed load can affect the morphodynamic evolution of complex bed topography occurring in natural rivers to investigate this point an application to the evolution of river bars is presented

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

geochemical characterization of a marine sediment core from the joides basin ross sea antarctica abstract the chemical fractionation of ten metals al ti cr fe cd cu mn pb zn and ni is reported for a marine sediment collected in the joides basin ross sea antarctica to evaluate their mobility and availability the bcr sequential extraction procedure was applied on 11 sections of the sediment the analyses performed both by icp oes and gf aas highlighted that most of the elements were in the residual phase showing a strong binding with the matrix and a low availability the bcr sequential procedure allowed us to have an insight into the chemical properties of the sediment and provided useful information about the redox state of the sediment itself and important indication on the availability and partitioning of different metals a chemometric treatment was applied on the dataset and it showed a separation between the superficial and the deep sections of the core the first being more influenced by the presence of mn and the second by an increase in al ti fe cr and pb concentrations

late pleistocene holocene sedimentary fluxes of organic carbon and biogenic silica in the northwestern ross sea antarctica eight sediment gravity cores collected from thejoides and drygalski basins were analysed in order to understand late pleistocene holocene biogenic dux changes in the ross sea driven by paleoenvironmental changes core lithologies and magnetic susceptibility depth profiles were used for core logging and stratigraphic corre lation nineteen ams radiocarbon dates of bulk organic matter were used to set chrono logical constraints and calculate sediment accumulation rates these rates which vary from 1 4 38 cm ka i were used to obtain the burial f1uxes of biogenic components the highest f uxes occur in the deepest parts o the basins toc 0 05 0 2 g cm 2 ka i biogenic silica 1 5 5 g cm 2 ka i whereas topographic highs show the lowest values toc 0 010 1 g cm 2 ka i biogenic silica 0 1 1 4 g cm 2 ka i dramatic changes in both physical properties and fluxes record the establishment of open marine sedimentation conditions which occurred first in thejoides basin and then with a delay of ca 6000 years in the drygalski basin both toc and biogenic silica ouxes increase through the holocene though slightly difterently the high f1uxes of both lobe and biogenic ba suggest that sedi ment accumulation at basin sites is strongly influenced by lateral transport

basal ice sequences in antarctic ice stream exposure of past hydrologic conditions and a principal mode of sediment transfer 1 the brightness distribution of sequentially extracted borehole camera imagery shows two distinct sequences of basal ice in kamb ice stream the upper sequence 7 3 m comprises clear ice and layers with dispersed and stratified debris the lower sequence 8 2 m consists of accretion ice with alternating layers of stratified and solid debris the two sequences have volumetric debris contents of about 5 and 20 respectively we infer that the upper sequence formed in a tributary where subglacial meltwater was abundant and that the lower sequence formed on the siple coast plain where basal freezing currently dominates the basal thermal regime the basal ice layer contains the equivalent of 2 1 0 4 m of frozen sediment the high volume of sediment is a result of debris rich layers interpreted to have formed by accretion in the stagnant phase of ice stream on off cycles fast ice streaming punctuated by entrainment of debris during stagnation episodes is consistent with inferred past flow of adjacent ice streams and geologic features in the ross sea our results show that sediment wedges near grounding lines may form by melt out of basal ice debris this is different from previous studies where it was assumed that sediment transfer occurs through sediment deformation within layers of subglacial till cumulative freezing in recurring ice stream cycles shows that entrainment of sediment by basal freeze on is an important erosion mechanism and that high sediment fluxes can occur when ice streams override coulomb plastic substrates

sedimentation on the ross sea continental shelf antarctica abstract glacial marine sedimentation has prevailed throughout most of the early glacial history of the ross sea region however since late miocene to early pliocene time the west antarctic ice sheet has advanced onto the continental shelf perhaps several times glacial troughs of the western ross sea may also have formed at this time these glacial troughs border outlet glaciers of the east antarctic ice sheet a widespread lodgement till facies was deposited during this glacial advance holocene retreat of this expanded ice sheet left no conspicuous meltwater record so decoupling of the ice sheet from the seafloor rather than rapid melting is implied modern glacial sediment is far less prevalent than biogenic sediment and reworked glacial sediment except in coastal areas adjacent to small ice shelves and ice tongues little or no debris is being transported beyond the calving line of the ross ice shelf siliceous mud and ooze occurs in the western ross sea where surface circulation is most vigorous and warm deep waters up well this siliceous sediment along with terrigenous silt and clay blankets the seafloor below approximately 300 m water depth most of the ross sea continental shelf is deeper than this surface sediments on shallow banks and coastal platforms have clearly been reworked by marine currents the most vigorous bottom current activity occurs in those areas where geostrophic currents impinge onto the shelf debris flows and turbidity currents are also important in eroding and redistributing glacial sediments on the shelf terrigenous silt and clay are more abundant in surface sediment than basal till the source of this silt and clay is problematic the most plausible sources being onshore transport of suspended sediment by impinging currents and subglacial meltwater input

barium accumulation in the atlantic sector of the southern ocean results from 190 000 year records extensive investigations of sedimentary barium were performed in the southern south atlantic in order to assess the reliability of the barium signal in antarctic sediments as a proxy for paleoproductivity maximum accumulation rates of excess barium were calculated for the antarctic zone south of the polar front where silica accumulates at high rates the correspondence between barium and opal supports the applicability of barium as a proxy for productivity within the antarctic zone north of today s average sea ice maximum interglacial vertical rain rates of excess barium are high with a maximum occurring during the last deglaciation and early holocene and during oxygen isotope chronozone 5 5 during these periods the maximum silica accumulation was supposedly located south of the polar front glacial paleoproductivity instead was low within the antarctic zone north of the polar front significantly higher barium accumulation occurs during glacial times the vertical rain rates however are as high as in the glacial antarctic zone therefore there was no evidence for an increased productivity in the glacial southern ocean

the galilean satellites nasa s galileo mission to jupiter and improved earth based observing capabilities have allowed major advances in our understanding of jupiter s moons io europa ganymede and callisto over the past few years particularly exciting findings include the evidence for internal liquid water oceans in callisto and europa detection of a strong intrinsic magnetic field within ganymede discovery of high temperature silicate volcanism on io discovery of tenuous oxygen atmospheres at europa and ganymede and a tenuous carbon dioxide atmosphere at callisto and detection of condensed oxygen on ganymede modeling of landforms seen at resolutions up to 100 times as high as those of voyager supports the suggestion that tidal heating has played an important role for io and europa

basal ice facies a review and unifying approach abstract over the past 30 years numerous basal ice facies have been identified named and classified however the resulting facies descriptions and names are inconsistent and no single scheme encompasses all of the different ice types that exist at different ice masses in this paper we review and critique existing basal ice facies names descriptions and classification schemes and propose a new non genetic approach that has the capacity to name and describe all basal ice types we define six fundamental basal cryofacies and a further 12 composite cryofacies which can all be defined on the basis of a cursory evaluation of debris disposition and concentration more detailed cryofacies description is based on characterizing three sets of ice properties that can also be estimated visually in the field a the thickness of the basal ice facies and its constituent sub layers b the concentration and texture of debris included within any or all of those layers and c the concentration and size of bubbles included within any or all of those layers we also propose a shorthand method for the presentation of this descriptive information here codes for the layer thicknesses are presented as standard text and codes for the included debris characteristics and included bubble characteristics are presented as superscripts and subscripts respectively sub layers are characterized similarly but within a nested sequence of brackets we evaluate the effectiveness of these two schemes by using them to rename and reclassify several existing basal ice facies results indicate that the schemes are robust and that they provide a coherent non genetic framework for the effective naming and description of basal cryofacies

palaeoproductivity in the ross sea antarctica during the last 15 kyr bp and its link with ice core temperature proxies a detailed study of organic carbon content obtained from two sediment cores collected in the joides basin western ross sea antarctica was carried out the variations observed during the last deglaciation and the holocene were compared to the high resolution climatic records epica dc and taylor dome preserved in the ice the importance of the carbon content as a proxy for palaeoclimatic and palaeoenvironmental changes was investigated a dramatic decrease in the ross sea palaeoproductivity was observed during the antarctic cold reversal 12 5 14 kyr bp another decrease in total organic carbon in the second half of the holocene after 5 6 kyr bp confirms the climate worsening observed in previous studies

comment on a quantitative framework for interpretation of basal ice facies formed by ice accretion over subglacial sediment by poul christoffersen et al 1 frost heave is the term given to the deformation at the earth s surface that is caused by the formation and growth of segregated ice lenses in permafrost and seasonally frozen ground efforts to understand and predict the characteristics of solidification in porous media led o neill and miller 1985 to produce the first comprehensive and tractable model for the thicknesses spacings and growth rates of sediment free ice lenses a central feature of the model involved an assessment of the balance of forces within a frozen fringe of partially ice saturated sediments beneath the lowest segregated ice interface when o neill and miller 1985 formulated their rigid ice model it was not yet clear how exactly to calculate the net force transmitted by sediment particles to the ice as a consequence the rigid ice model is only approximately correct in its treatment of the force balance that describes how lenses grow and an ad hoc formulation was needed to prescribe when new lenses would form always somewhere within the frozen fringe accurate predictions could nevertheless be obtained by tuning the model to reproduce the results of available empirical studies we now understand that the forces transmitted by the sediment particles to the ice are a consequence of interfacial premelting which is a basic aspect of phase behavior that ice shares in common with all other classes of solids see e g wettlaufer and worster

bed load at low shields stress on arbitrarily sloping beds failure of the bagnold hypothesis 1 the bagnold hypothesis has been a major tool in the development of mechanistic models of bed load transport according to the hypothesis a necessary constraint for the maintenance of equilibrium bed load transport is that the fluid shear stress at the bed must be reduced to the critical or threshold value associated with incipient motion of grains the constraint determines a functional form for the areal concentration of bed load grains as a function of flow parameters it is shown here however that the bagnold hypothesis breaks down when applied to equilibrium bed load transport on beds with transverse slopes above a relatively modest value that is well below the angle of repose in particular under such conditions it is shown that no areal concentration no matter how large is sufficient to reduce the fluid shear stress at the bed to the critical value this failure motivates the abandonment of the bagnold constraint even for nearly horizontal beds the framework presented here however provides a natural basis for an entrainment based model of sediment transport that neither satisfies nor suffers from the drawbacks of the bagnold constraint

bedform topography and basal conditions beneath a fast flowing west antarctic ice stream a grid of seismic reflection lines has been used to image basal topography and infer basal conditions and flow processes beneath similar to 140 km 2 of rutford ice stream west antarctica the subglacial topography in this region consists of two troughs flanking a central high and the bed is composed of water saturated sediments the two troughs are filled with deforming sediment whereas the bed in the central region appears to undergo a transition from largely deforming conditions upstream to basal sliding downstream the deforming bed is very flat along flow but undulates across flow sliding areas show rougher bed topography cross stream bed topography is characterised by streamlined mounds of deforming sediment aligned in the ice flow direction these bedforms occur superimposed on the bed in regions of both basal sliding and sediment deformation in places they form finger like mounds of material which extend into the sliding region further downstream mean bedform height is 22 m mean width is 267 m and many of them extend for at least 1 2 km along flow we interpret most of these bedforms as drumlins and one as a mega scale glacial lineation the juxtaposition of different basal conditions is consistent with models proposed from terrestrial studies in which the glacier bed is a mosaic of stable and deforming bed areas variable both spatially and temporally any theory of subglacial sediment rheology must also be able to account for our conclusion that at any given time pervasive deformation extends at least a few metres into the bed and can persist over a considerable area many km 2 bedform geometry and basal conditions concur with interpretations of former ice streams with evidence for increasing elongation ratio with distance downstream however those studies also identified bedrock cropping out at the ice bed interface for which there is no evidence on rutford ice stream c 2008 elsevier ltd all rights reserved