spatial variation of radar derived basal conditions on kamb ice stream west antarctica radar profiles of bed echo intensity can survey conditions at the ice bed interface and test for the presence or absence of water however extracting information about basal conditions from bed echo intensities requires an estimate of the attenuation loss through the ice we used the relationship between bed echo intensities from constant offset radar data and ice thickness to estimate depth averaged attenuation rates at several locations on and near kamb ice stream kis west antarctica we found values varying from 29 db km 1 at siple dome to 15 db km 1 in the main trunk region of kis in agreement with a previous measurement and models using these attenuation rate values we calculated the relative bed reflectivity throughout our kis surveys and found that most of the bed in the trunk has high basal reflectivities similar to those obtained in the location of boreholes that found water at the bed areas of lower bed reflectivity are limited to the sticky spot where a borehole found a dry bed and along the margins of kis these results support previous hypotheses that the basal conditions at locations like the sticky spot on kis control its stagnation and possible reactivation

the shape of change an eof approach to identifying sources of transient thickness change in an ice shelf ross ice shelf ris is known to experience transient thickness change due to changes in the flow of its tributary ice streams and glaciers and this may complicate identification of external climate forced signals in contemporary observations of ice shelf thinning and thickening flux changes at the lateral boundaries produce both instantaneous and longer timescale adjustments in the coupled velocity and thickness fields here we adapt a statistical approach to output from a numerical model of ice shelf flow to identify characteristic patterns spatial response surfaces associated with stepped and cyclic perturbations to boundary fluxes once known characteristic patterns identified in observational data may be attributed to specific sources an example involving discharge of byrd glacier into ris is described we find that spatial response surfaces for thickness and velocity generated in individual flow model experiments appear to be independent of flux perturbation shape and magnitude additionally recent acceleration of byrd glacier is apparent in icesat detected change in ris thickness

variability in the mass flux of the ross ice streams west antarctica over the last millennium we synthesize previously published remote sensing observations radar data and model output to obtain a 1000 year ice flow history for the siple coast ice stream system in west antarctica to investigate the timing and magnitude of changes in mass flux the synthesized history shows significant short term variability in ice stream shear margin and grounding line position due to internal variability of the coupled system the chronology highlights the interplay between adjacent ice streams which implies that the behavior of any individual ice stream should not be examined in isolation furthermore individual events cannot be fully interpreted without an understanding of the broad scale long term variability in the ice sheet in the context of this millennium scale history we interpret the relatively recent stagnation of kamb ice stream kis as just one stage in the thermodynamic cycle of an ice stream in this region the changes in mass balance that result from the kis stagnation may thus be viewed as century scale noise relative to the longer term trend understanding and characterizing this noise is a necessary step before accurate model based predictions of ice sheet mass balance for the next century can be made

palaeoglaciology of the alexander island ice cap western antarctic peninsula reconstructed from marine geophysical and core data abstract the glacial history of the continental shelf northwest of alexander island is not well known due mainly to a lack of targeted marine data on antarctica s palaeo ice sheets in their inter ice stream areas recently it has been argued that the region was ice free at the last glacial maximum lgm and thus a potential site for glacial refugia in this paper multibeam swath bathymetry sub bottom profiles and sediment cores are used to map the alexander island sector of the antarctic peninsula margin in order to reconstruct the shelf s palaeoglaciology sea floor bedforms provide evidence that an independent ice cap persisted on alexander island through the lgm and deglaciation we show that this ice cap drained via two major previously undescribed tidewater outlets rothschild and charcot glaciers sourced from an ice dome centred over the west of the island and near shore areas the glaciers grounded along deep fjord like cross shelf troughs to within at least 10 20 km of the shelf edge and probably reached the shelf break only one small outer shelf zone appears to have remained free of ice throughout an otherwise extensive lgm during retreat grounding line geomorphology indicates periodic stabilisation of charcot glacier on the mid shelf after 13 500 cal yrs bp while rothschild glacier retreated across its mid shelf by 14 450 cal yrs bp the timing of these events is in phase with retreat in nearby marguerite trough and we take this as evidence of a common history and forcing with the antarctic peninsula ice sheet the fine details of ice flow documented by our new reconstruction highlight the importance of capturing complex ice flow patterns in models e g in inter stream areas for understanding how region specific parts of antarctica may change in the future moreover the reconstruction shows that glacial refugia if present cannot have been extensive on the alexander island shelf at the lgm as indicated by previous biological studies instead we argue that any ice free refugia were probably restricted to isolated outer shelf pockets that opened closed or were maintained through diachronous ice sheet advance and retreat

a decade of progress in observing and modelling antarctic subglacial water systems in the decade since the discovery of active antarctic subglacial water systems by detection of subtle surface displacements much progress has been made in our understanding of these dynamic systems here we present some of the key results of observations derived from icesat laser altimetry cryosat 2 radar altimetry operation icebridge airborne laser altimetry satellite image differencing and ground based continuous global positioning system gps experiments deployed in hydrologically active regions these observations provide us with an increased understanding of various lake systems in antarctica whillans mercer ice streams crane glacier recovery ice stream byrd glacier and eastern wilkes land in several cases subglacial water systems are shown to control ice flux through the glacier system for some lake systems we have been able to construct more than a decade of continuous lake activity revealing internal variability on time scales ranging from days to years this variability indicates that continuous accurate time series of altimetry data are critical to understanding these systems on whillans ice stream our results from a 5 year continuous gps record demonstrate that subglacial lake flood events significantly change the regional ice dynamics we also show how models for subglacial water flow have evolved since the availability of observations of lake volume change from regional scale models of water routeing to process models of channels carved into the subglacial sediment instead of the overlying ice we show that progress in understanding the processes governing lake drainage now allows us to create simulated lake volume time series that reproduce time series from satellite observations this transformational decade in antarctic subglacial water research has moved us significantly closer to understanding the processes of water transfer sufficiently for inclusion in continental scale ice sheet models

ice flow velocities on rutford ice stream west antarctica are stable over decadal timescales surface ice flow velocities measured at stakes on rutford ice stream west antarctica covering a period of similar to 25 years are analysed for evidence of temporal variations in flow no indications of significant long term changes in flow are found earlier observations have shown significant tidally related variations in flow speed we conclude that temporal variability on rutford ice stream west antarctica appears limited to tidal periods of days and weeks and weaker interannual variation possibly related to long period tides or seasonal effects while long term decadal changes in flow speed are either absent or smaller than similar to 0 1 a 1

analysis techniques for coherent airborne radar sounding application to west antarctic ice streams 1 analysis of coherent radar sounding echoes from polar ice sheets can provide information suitable for classifying the subglacial environment echoes from a general interface consist of both specularly reflected and diffusely scattered contributions specular reflection results from smooth uniform interfaces whereas diffuse scattering results from rough nonuniform interfaces and inhomogeneous media this article discusses how these phenomena are important to the acquisition and analysis of coherent radar sounding data reflection results are presented from airborne surveys conducted in 1987 over the downstream portions of whillans ice stream and ice stream c west antarctica additionally reflection and scattering analyses along with new results are presented for repeat profiles flown in 2001 over ice stream c analysis methods include using echo amplitudes to compute reflection coefficients which are used for inferring the dielectric properties of the subglacial material echo phase analysis provides the locations of dominant scattering centers which relate to reflection or scattering from the interface as well as provide interface roughness estimates comparison of low and high resolution imaging obtained from synthetic aperture radar techniques indicates a reflecting and or scattering interface combining the results from these independent analyses provides classification of the subglacial environment classified regions include smooth seawater smooth saturated sediments accreted marine ice rough bottom crevasses mixed conditions with partial subglacial water and dry frozen conditions

post stagnation behavior in the upstream regions of ice stream c west antarctica the region where two active tributaries feed into the now stagnant ice stream c isc west antarctica is thickening in this region we observe a correlation between faster ice flow the tributaries and elevated topography we conclude that stagnation of isc resulted in compression and thickening along the tributaries eventually forming a bulge on the ice sheet surface modern hydraulic potential gradients would divert basal meltwater from isc to ice stream b isb these gradients are primarily controlled by the bulge topography and so likely formed subsequent to trunk stagnation as such we argue against water piracy as being the cause for isc s stagnation kinematic wave theory suggests that thickness perturbations propagate downstream over time but that kinematic wave speed decreases near the stagnant trunk this and modest diffusion rates combine to trap most of the tributary fed ice in the bulge region using interferometric synthetic aperture radar velocity measurements we observe that half of the ice within isc s southern tributary flows into isb that flow pattern and other observations of non steady flow in the region likely result from stagnation induced thickening along upper isc combined with a longer period of thinning on upper isb if current trends in thickness change continue more ice from upper isc will be diverted to isb

seismic measurements reveal a saturated porous layer beneath an active antarctic ice stream seismic reflection studies recently conducted on ice stream b part of the marine ice sheet of west antarctica show a metres thick layer immediately beneath the ice in which both compressional p and shear s wave speeds are very low these low wave speeds imply that the material in the layer is highly porous and is saturated with water at a high pore pressure from this and from arguments presented in an accompanying paper1 to the effect that the layer would be too weak to support the shear stress exerted by the overlying ice we conclude that the layer is deforming and that the ice stream probably moves principally by such deformation

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

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

tidal forcing of basal seismicity of ice stream c west antarctica observed far inland the seismicity rate beneath the downglacier 85 km of ice stream c west antarctica is modulated by the tide the tide beneath the ross ice shelf modifies the force balance of the ice stream basal environment enough to change the rate of basal microearthquake generation by an order of magnitude this tidal forcing travels up the ice stream as an attenuating wave at approximately 1 6m s 1 and is detectable 85km from the grounding line we successfully model this behavior as an elastic ice stream underlain by a viscous substrate of viscosity and thickness hb and calculate that the substrate has an apparent stiffness hb of o 108 pa s m 1 this finding suggests that the conditions of the till layer at the bed of ice stream c are similar to those of ice stream b and that the reason for the recent stagnation of ice stream c is other than loss of till we further find that the ice stream at the grounding line is more strongly affected by ice shelf processes than by the basal shear stress

tidally driven stick slip motion in the mouth of whillans ice stream antarctica we show that the ice plain in the mouth of whillans ice stream formerly ice stream b antarctica moves by stick slip motion during a spring tide period rapid motions regularly occur near high tide and during falling tide this correlation is weaker during a neap tide period when the tidal magnitudes are less precise timing of these motion events suggests that they propagate through the region with a mean velocity of 88 ms 1 we hypothesize that this speed is associated with the propagation of shear waves through a wet subglacial till motion events are also seen on more smoothly flowing floating ice event delays are very short between grounded and floating stations suggesting the events propagate through the ice shelf as an elastic wave we further hypothesize the events are caused by the interaction of a sticky bed the accumulation of stored elastic strain through the compression of ice by upstream inflow and tidal forcing motion events seem to be triggered either by reduction of vertical normal stresses at high tide or by the increase of shear stresses from sub shelf ocean currents during falling tide event magnitudes are not related to the length of the preceding quiescent period suggesting significant viscous dissipation within the till

in search of ice stream sticky spots the form drag of large bedrock bumps sticking into the base of an ice stream can produce effective sticky spots supporting large basal shear stress bedrock regions surrounded by lubricating till at the same topographic level can cause sticky spots but tend to collect lubricating water and thus are unlikely to support a shear stress of more than a few tenths of a bar unless they contain abundant large bumps raised regions on the ice air surface also can cause moderate increases in the shear stress supported on the bed beneath surveys of large scale bedrock roughness strain grids across the margins of ice surface highs and possibly water pressure measurements in regions of thin or zero till would help identify and characterize sticky spots

recent changes in the flow of the ross ice shelf west antarctica abstract comparison of surface velocities measured during the ross ice shelf geophysical and glaciological survey riggs 1973 to 1978 and velocities measured via feature tracking between two moderate resolution imaging spectroradiometer modis mosaics compiled from 2003 4 and 2008 9 images reveals widespread slowing and minor areas of acceleration in the ross ice shelf ris over the approximately 30 year interval the largest changes 13 ma 2 occur near the whillans and mercer ice streams grounding line in the southernmost part of the ice shelf speed has increased over the interval up to 5 ma 2 between the macayeal ice stream grounding line and the shelf front and along the eastern shelf front changes in ice thickness computed using icesat laser altimetry are used together with a well tested model of the ice shelf to investigate underlying causes of change in the flow of the ice shelf over time the observed transients represent a combination of recent forcings and ongoing response to ice stream discharge variations over the past millennium while evidence of older events may be present the modern signal is dominated by shorter time scale events including the stagnation of kamb ice stream about 160 years ago recent changes in basal drag on the whillans ice stream ice plain and perhaps iceberg calving details in embayment geometry for example the shallow sea floor below crary ice rise modulate the spatial pattern of ice shelf response to boundary condition perturbations

studies of the grounding line location on ice streams d and e antarctica landsat thematic mapper tm images were used prior to the 1991 92 field season to infer the position of the grounding line at the mouths of ice streams d and e west antarctica our field plan for mass balance studies was based on this determination and thus the imagery played a central role in both the scientific and logistics planning a radar profile along the flow direction was made across the inferred grounding line at one location and ice thickness measurements together with surface surveying enable us to compare the hydrostatic surface and the actual topography to determine the point at which the ice becomes grounded the profile transits from floating to grounded ice at the same location as the grounding line inferred from the imagery changes in the radar echo strength also occur at this location giving further support to this interpretation tilt studies of the ice flexure caused by tidal variations at locations on either side of this grounding point give additional evidence that grounding is occurring close by the combination of these three measurements thus confirms the grounding line location derived from the satellite imagery and supports the use of this technique to determine grounding

high altitude cusp flow dependence on imf orientation a 3 year cluster statistical study 1 we report on the statistical properties of the plasma flows measured by the cluster spacecraft in the high altitude cusp region of the northern hemisphere as a function of the interplanetary magnetic field imf orientation with selected clock angle intervals the technique uses a magnetic field model taking into account the actual solar wind conditions and level of geomagnetic activity in order to model the magnetopause and cusp displacements as a function of these conditions the distributions of the magnetic field vector show a clear consistency with the imf clock angle intervals chosen and demonstrate that the technique used here fixes the positions of the cusp boundaries adequately the antisunward convection observed in the exterior cusp suggests that this region is statistically quite convective under southward imf while for northward imf the region appears more stagnant the presence of large parallel downward flows at the equatorward edge of the cusp shows that plasma penetration occurs preferentially at the dayside low latitude magnetopause for southward imf conditions in contrast under northward imf the results are suggestive of plasma penetration from the poleward edge of the cusp combined with a substantial sunward convection but no flows are observed at all at the dayside boundary with the plasma sheet the comparison of the measured flow speed with the alfven speed suggests that the magnetosheath adjacent to the external boundary is more sub alfvenic even for high magnetic latitudes under northward imf than under southward imf this result is consistent with the preference for the plasma depletion layer to develop under such conditions the transverse plasma convection in the exterior cusp appears to be controlled by the imf by component as well for dawnward duskward imf orientations the convection is preferentially directed toward dusk dawn these results are interpreted as strong arguments in favor of the cusp being structured at large scales by the occurrence of magnetic reconnection at the high latitude magnetopause for northward imf and at the low latitude magnetopause for southward imf

the morphology of ice streams a b and c west antarctica and their environs airborne radar soundings of the ice sheet surface made in 1984 1985 together with elevations measured by oversnow traverses between 1957 and 1964 have been used to produce a surface elevation map of ice streams a b and c and much of the region around them the flight positions and elevations were tied to many ground control stations whose positions in three dimensions were fixed by satellite tracking ice thicknesses were also measured along the flight lines but have not yet been mapped the surfaces of active ice streams a and b exhibit a longitudinal ridgetrough topography of uncertain origin prominent surface valleys are associated with most but not all of their marginal shear zones there is a deep subglacial trough beneath the grid northeastern side of ice stream a that connects to the subglacial trough beneath reedy glacier the boundary between ice streams bl and b2 the two tributaries of ice stream b persists as a suture zone for 250 km downstream from their merger ice stream bl overlies a relatively deep subglacial trough that continued under the corresponding part of ice stream b until it disappears close to the grounding line between ice streams bl and b2 there is a complex zone containing several regions of undisturbed ice separated by bands of disturbed ice which suggest to us that rafts of ice are being incorporated into the ice streams the previously reported negative mass balance for the drainage system of ice stream b may arise from the expansion of the ice stream into the interior of the west antarctic ice sheet crevasse bands at some places on ridge ab together with high glacial velocities reported elsewhere also suggest this expansion close to the ross ice shelf ice stream b widens into a nearly flat area where the mean surface slope is only 3 5 10 4 and the surface elevations are only a few tens of meters above hydrostatic equilibrium inactive ice stream c differs from active ice streams a and b in surficial and basal characteristics no elongated ridges and troughs are observed instead the ice stream surface exhibits several terraces including some maxima and minima in elevation radar sounding reveals areas where basal echoes are strong and steady indicating subglacial water alternating with areas of weaker echoes with short fading lengths calculations indicate that the strong reflections require a water layer at least several centimeters to several meters thick depending on the electrical conductivity of the water due to reversals in the surface slope the hydrologic potential gradient changes sign several times along ice stream c which implies that subglacial water should collect in some locations

west antarctic ice streams draining into the ross ice shelf configuration and mass balance extensive airborne radar soundings closely tied to accurately located ground stations made over a portion of the west antarctic inland ice sheet and the ross ice shelf have enabled us to map the boundaries of the ice streams and their flow bands on the ross ice shelf the surfaces of the active ice streams a and b are heavily crevassed but there are no visible crevasses on ice stream c however the existence of numerous crevasses at a depth of about 35 m implies that ice stream c ceased to be active about 250 years ago there is a complex zone at the head of ice stream b that suggests that ice stream b is currently widening and advancing toward the interior of the ice sheet the grounding lines between the inland ice and the ice shelf were mapped from the surface elevations using the requirement of hydrostatic balance and the character of the radar echoes the inland ice is more extensive than previously mapped and includes a large weakly grounded area downstream of ice streams a and b where the surface elevations are barely above hydrostatic equilibrium crary ice rise extends 70 km upstream of the previously mapped position to relate surface elevation data from satellite observations to sea level three geoidal models were tested we conclude that the gem 10c model is the best of the three for this section of antarctica a new map of surface elevation shows deep marginal troughs and longitudinal ridges along ice streams a and b but not on ice stream c whose surface contours show signs of stagnation ice stream c shows a strongly positive and ice stream b a significantly negative net balance rate ice streams a and d also show a significantly negative net flux whereas ice streams e and f do not the overall net balance of this part of the west antarctic inland ice is suggestively negative 23 15 km3 yr 1 from the measured flux into the ross ice shelf and previous measurements we calculate an average basal melt rate from beneath the ice shelf of 0 12 0 03 m yr 1

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

bacterial methane in the atzbach schwanenstadt gas field upper austrian molasse basin part ii retracing gas generation and filling history by mass balancing of organic carbon conversion applying hydrogeochemical modelling abstract in the austrian molasse basin bacterial methane gas accumulations occur in upper oligocene to early miocene deepwater clastic sediments gas is produced from the upper puchkirchen formation aquitanian in the atzbach schwanenstadt gas field the diagenetic pathway of the reservoir cements was controlled by the conversion of metabolizable organic carbon incorporated in the fine grained sediments these pathways have been retraced by hydrogeochemical modelling the redox conversion of metabolizable organic carbon this approach unravels the relative timing of the coupled processes gas generation and precipitation dissolution of cements the labile organic material was decomposed by fermentation and subsequent co 2 reduction co 2 released via these reactions was nearly completely fixed as carbonate cement methane generated during early diagenesis 10 s to max 100 m sediment depth was dissolved in pore water until saturation after exsolution as a free gas phase methane could have been fixed as hydrate within pore space due to the prevailing paleoceanographic conditions sediment water interface 1000 mbsl 4 c the temperature increase in consequence of the rapid basin subsidence and high sedimentation rates led to the decomposition of the hydrates and to charging of the reservoir by methane still during the aquitanian a further consequence of hydrate decomposition was dilution of pore water salinity according to the results the presented approach offers a a tool to retrace the bacterial methane potential by analysis of diagenetic cement as a quantitative indicator and b to predict the interaction between porosity development and reservoir charging by methane additionally c gas field water salinity and hydrochemistry may be applied in similar architectural elements of deepwater channels as tracers for fossil gas hydrate formation as low chloride concentrations are result of sediment compaction and dilution by pure h 2 o due to gas hydrate dissociation

jakobshavns glacier drainage basin a balance assessment maximum and minimum estimates are made of the drainage basin feeding the jakobshavns glacier by using surface elevation maps derived from seasat altimetry benson s 1962 net balance measurements are used to calculate the balance flow within the basin comparisons of the balance flux at the terminus with estimates of actual flux suggest the basin is in overall equilibrium or slightly thickening this agrees with measurements along the nearby egig traverse balance velocities accelerate rapidly within 100 km of the coast farther upstream balance velocities are consistent with both measured velocities along the egig traverse and calculated deformation velocities it is estimated that jakobshavns glacier discharges between 4 8 and 7 6 of the annual net balance over greenland and drains between 3 7 and 5 8 of the ice sheet area

radar absorption due to impurities in antarctic ice measurements of radar echo strength have been carried out on ronne and george vi ice shelves antarctica the differences in ice column absorption between the two sites cannot be resolved by considering ice temperatures alone we present an absorption model that resolves the discrepancy by considering both the ice temperature and the ice impurity concentrations in antarctic coastal sites the concentrations of acid and sea salt impurities both of which affect absorption have a marked spatial variability and can dominate the absorption budget using the absorption model impurity levels in the ice column at the george vi site are deduced from the radar strength measurements and found to be typical of levels measured in antarctic ice at similar elevations and distances from the sea in particular the measurements suggest that the concentrations of biogenically derived acids are much lower than the unusually high levels found on the east coast of the antarctic peninsula

macro scale bed roughness of the siple coast ice streams in west antarctica subglacial bed conditions are key to understanding ice stream behaviour and evolution with bed roughness re ecting substrate composition and ow resistance here we present an analysis of bed roughness in the siple coast region of west antarctica from airborne radio echo sounding data the ice streams are associated generally with low bed roughness values which decrease downstream the bed of the slow owing ice stream c 10 000 km2 is also characterized by being smooth at all scales wavelengths ranging from 5 km to in excess of 40 km furthermore the bed is smooth either side of ice stream c this suggests the location of the ice stream is controlled by internal ice sheet dynamics rather than by bed morphology if the ice stream were encouraged to migrate laterally when active there would be little resistance offered by the subglacial morphology other inter ice stream regions are rough however indicating a subglacial topographic in uence on ice stream position bed roughness increases up ow of ice streams which unless the bed is modi ed may limit the inland migration of these systems copyright 2004 john wiley sons ltd

till beneath ice stream b 3 till deformation evidence and implications most of the velocity of ice stream b near the upstream b camp upb west antarctica appears to arise from deformation of a seismically detected subglacial till layer that averages 6 m thick available evidence indicates that the entire thickness of this till layer is deforming and is eroding subjacent bedrock into flutes parallel to ice flow and hundreds of meters across the resulting till flux beneath upb is equivalent to an average erosion rate of about 0 4 mm yr 1 in the catchment area and suggests that till deltas tens of kilometers long have been deposited at the grounding line during the holocene such deltas should be characterized by partial ice till decoupling across a water film and by a small ice air surface slope they may have been discovered by recent geophysical work

remote sensing of glacier and ice sheet grounding lines a review abstract the precise positioning and long term monitoring of the grounding line forming the boundary between grounded and floating ice of marine ice sheets and tidewater glaciers is critical for assessing ice sheet glacier stability ice sheet glacier mass balance calculations and numerical ice modelling however mapping of the grounding line is a challenging task since it is a subglacial and transient feature remote sensing techniques do not map the grounding line directly but locate surface features in the grounding zone that serve as proxies for the true grounding line position the large variety of methods products and publications as well as an inconsistent use of terminology additionally complicate the topic we present the first detailed review of existing remote sensing techniques and data for grounding line mapping benefits and limitations of the different techniques and data sources are discussed and illustrated with examples we identify a present tradeoff between accuracy and both spatial coverage and temporal resolution of the derived grounding line products rather new methods like sentinel 1 sar synthetic aperture radar differential range offset tracking drot and pseudo crossover radar altimetry pcra offer the potential for recent measurements and to fill gaps in some areas nevertheless there is a high demand of new spaceborne radar sensors that provide time series of very short repeat pass data to enable regular grounding line monitoring with most accurate differential sar interferometry dinsar on a continental scale

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

an active subglacial water system in west antarctica mapped from space satellite laser altimeter elevation profiles from 2003 to 2006 collected over the lower parts of whillans and mercer ice streams west antarctica reveal 14 regions of temporally varying elevation which we interpret as the surface expression of subglacial water movement vertical motion and spatial extent of two of the largest regions are confirmed by satellite image differencing a major previously unknown subglacial lake near the grounding line of whillans ice stream is observed to drain 2 0 cubic kilometers of water into the ocean over 3 years while elsewhere a similar volume of water is being stored subglacially these observations reveal a wide spread dynamic subglacial water system that may exert an important control on ice flow and mass balance

changes in the west antarctic ice sheet since 1963 from declassified satellite photography comparison of declassified satellite photography taken in 1963 with more recent satellite imagery reveals that large changes have occurred in the region where an active ice stream enters the ross ice shelf ice stream b has widened by 4 kilometers at a rate much faster than suggested by models and has decreased in speed by 50 percent the ice ridge between ice streams b and c has eroded 14 kilometers these changes along with changes in the crevassing around crary ice rise imply that this region s velocity field shifted during this century

changes in west antarctic ice stream velocities observation and analysis 1 we have produced a map of velocity covering much of the siple coast ice streams the map confirms earlier estimates of deceleration on whillans ice stream comparison with bed elevation data indicates that subglacial topography and the location of consolidated sediment play a strong role in determining the location of the tributaries feeding the ice streams force balance estimates based on these data indicate that the tributaries have beds nearly an order of magnitude stronger than those beneath many of the ice streams we have used a theoretical analysis to examine the controls on fast flow this analysis suggests that ice plains very wide ice streams are inherently unstable this instability may be responsible for the current deceleration on the ice plain of whillans ice stream and the shutdown of ice stream c 150 years ago thinning induced reductions in driving stress may also explain some of the observed deceleration particularly in upstream areas the active portions of ice stream c coincide well with the areas where we estimate that melt should be taking place current topography and inferences of large thickening following a shutdown suggest the upstream migration of a stagnation front that initiated at the ice plain uncertainty remains about the basal conditions on ice stream d while the basal resistance on ice stream e is large enough to ensure basal melting

switch of flow direction in an antarctic ice stream fast flowing ice streams transport ice from the interior of west antarctica to the ocean and fluctuations in their activity control the mass balance of the ice sheet the mass balance of the ross sea sector of the west antarctic ice sheet is now positive that is it is growing mainly because one of the ice streams ice stream c slowed down about 150 years ago1 here we present evidence from both surface measurements and remote sensing that demonstrates the highly dynamic nature of the ross drainage system we show that the flow in an area that once discharged into ice stream c has changed direction now draining into the whillans ice stream formerly ice stream b this switch in flow direction is a result of continuing thinning of the whillans ice stream and recent thickening of ice stream c further abrupt reorganization of the activity and configuration of the ice streams over short timescales is to be expected in the future as the surface topography of the ice sheet responds to the combined effects of internal dynamics and long term climate change we suggest that caution is needed when using observations of short term mass changes to draw conclusions about the large scale mass balance of the ice sheet

bacterial methane in the atzbach schwanenstadt gas field upper austrian molasse basin part i geology abstract bacterial methane gas accumulations occur in upper oligocene to early miocene clastic deepwater sediments in the austrian molasse basin methane gas is produced from the upper puchkirchen fm aquitanian in the atzbach schwanenstadt gas field which is one of the largest gas fields in this basin the bacterial gas is composed of 97 vol methane 13 c 62 d 200 connate pore water below the gas water contact is characterized by low salinity ccl ca 0 25 mol kgw or 8900 mg kgw the top of the main reservoir zone a4 is approximately 1600 m subsurface thickness of reservoir intervals in the a4 sands the main reservoir is variable and ranges from 8 to 30 m with decreasing thickness to the north the porosity of the sand ranges from 13 to 20 permeability from 6 md to 70 md the reservoir temperature is 63 c reservoir rocks of this field wedge out to the north and represent dam break through splay sediment this reservoir architecture enabled early gas accumulation in a stratigraphic trap the source rocks and reservoir rocks are intimately connected claystone layers intercalate and seal the reservoir sediments these fine grained sediments with potential source rock characteristics contain immature organic matter of kerogen types ii iii the toc contents of these fine grained sediments vary from 0 34 to 0 94 wt the cement inventory of the source and reservoir rocks is characteristic for diagenetic pathways of organic rich sediments pyrite is preserved in the sediments as the first precipitation product due to rapidly developing anoxic pore water conditions a following dolomite precipitation dissolution phase was replaced by precipitation of calcite ankerite and siderite a last cement phase is chlorite the about 700 m thick upper puchkirchen fm was deposited during a time interval of about 2 6 ma aquitanian in a deep sea fan environment water depth 1000 m bottom water temperature of about 4 c free gas phases may have been trapped as gas hydrate due to basin subsidence and high sedimentation rates hydrates have been decomposed below the base of the gas hydrate stability zone 200 40 mbsf still during the aquitanian and thus still during or shortly after deposition of the upper puchkirchen fm

position and variability of complex structures in the central east antarctic ice sheet although the flow of the east antarctic ice sheet is well constrained from surface measurements and altimetry our knowledge of the dynamic processes within the ice sheet remains limited recent high resolution radar data from the gamburtsev subglacial mountains in central east antarctica reveal a series of anomalous englacial reflectors in the lower half of the ice column that cannot be explained by conventional ice flow expanding on previous analyses we describe the geometrical and morphological features of 12 of these anomalous reflectors our description reveals a previously unacknowledged diversity in size geometry and internal structure of these reflectors we are able to identify four distinct morphological features 1 fingers 2 inclusions 3 sheets and 4 folds the fingers and inclusions probably form by shear instabilities at the boundary between the reflectors and the surrounding meteoric ice the sheets highlight that basal ice can be uplifted off of the bed and above surrounding meteoric ice and the folds may have formed in local regions of converging flow associated with subglacial topography the study provides key insights into the rheology stress and deformational regimes deep within the central east antarctic ice sheet

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

shear heating and weakening of the margins of west antarctic ice streams ice streams are fast flowing bands of ice separated from stagnant ridges by shear margins the mechanisms controlling the location of the margins remain unclear we use published ice deformation data and a simple one dimensional thermal model to show that west antarctic ice stream margins have temperate ice over a substantial fraction of their thickness a condition that may control their width the model predicts a triple valued relation between the thickness averaged lateral shear stress and the lateral shear strain rate observed strain rates at the margins imply that they support slightly less lateral shear stress than adjacent ice within the stream this requires enhanced basal resistance near the margin we suggest in agreement with the limited observations the presence of a channelized drainage system at the margin that reduces the pore fluid pressure at the ice till interface thus increasing the shear stress acting on the yielding coulomb plastic bed

a framework for attenuation method selection evaluated with ice penetrating radar data at south pole lake all radar power interpretations require a correction for attenuative losses moreover radar attenuation is a proxy for ice column properties such as temperature and chemistry prior studies use either paired thermodynamic and conductivity models or the radar data themselves to calculate attenuation but there is no standard method to do so and before now there has been no robust methodological comparison here we develop a framework meant to guide the implementation of empirical attenuation methods based on survey design and regional glaciological conditions we divide the methods into the three main groups 1 those that infer attenuation from a single reflector across many traces 2 those that infer attenuation from multiple reflectors within one trace and 3 those that infer attenuation by contrasting the measured power from primary and secondary reflections to assess our framework we introduce a new ground based radar survey from south pole lake comparing selected empirical methods to the expected attenuation from a temperature and chemistry dependent arrhenius model based on the small surveyed area lack of a sufficient calibration surface and low reflector relief the attenuation methods that use multiple reflectors are most suitable at south pole lake

a millennium of variable ice flow recorded by the ross ice shelf antarctica an enhanced composite advanced very high resolution radiometer avhrr image is used to map flow stripes and rifts across the ross ice shelf antarctica the patterns of these flow related features reveal a history of discharge variations from the ice streams feeding the eastern part of the shelf the most profound variations are visible in the track of rifts downstream of crary ice rise flow stripe bends to the west of this ice rise and adjacent to steershead ice rise and changes in the northern margin of ice stream b the track of rifts downstream of crary ice rise indicates that the ice rise has existed for at least 700 years the character of this track changes about 350 km downstream indicating a rearrangement of flow patterns about 550 years ago the large bulge in the flow stripes to the west of crary ice rise is shown in detail with bent flow stripes extending for several hundred kilometers along flow this feature formed from the south possibly due to a change in the discharge of ice stream a the avhrr image documents a complex history associated with the shutdown of ice stream c with changes in the margins of ice stream c and the northern margin of ice stream b and the grounding of steershead ice rise with an associated bending and truncation of flow stripes landsat imagery shows a region that appears to be actively extending just downstream of the ice rise as the shelf continues to respond to recent changes in ice stream discharge we present a four stage flow history which accounts for the features preserved in the ice shelf

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

characteristics of the sticky spot of kamb ice stream west antarctica amplitude analysis of reflection seismic data reveals the presence of highly variable bed conditions under the main sticky spot and adjacent regions of the kamb ice stream kis formerly ice stream c the sticky spot which is a zone of bed that imparts high basal resistance to ice flow is situated on a local topographic high composed of consolidated sediments or sedimentary rock any meltwater draining from upglacier along the base of the ice is routed around the sticky spot the ice over the sticky spot includes in at least some places a seismically detectable basal layer containing a low concentration of debris which locally thickens to 40 m over a topographic low in the bed the ice contact basal material ranges from dilated and highly porous to more compacted and stiff and perhaps locally frozen the softer material is preferentially in topographic lows but there is not a one to one correspondence between basal character and basal topography we speculate that the 40 m thick frozen on debris layer formed by glaciohydraulic supercooling of lake drainage events along a basal channel during the former active phase of the ice stream we also speculate that loss of lubricating water perhaps from piracy upstream contributed to the slowdown of the ice stream with drag from the sticky spot playing an important role and with the basal heterogeneity greatly increasing after the slowdown of the ice stream

ice dynamics at the mouth of ice stream b antarctica field data collected at the mouth of ice stream b show that the flow dynamics of this region are distinctly different than either the major portion of the ice stream upstream or the ice shelf downstream surface slopes in this region are as low as ice shelf surface slopes yet with the exception of patches of ice which may be floating the ice is grounded basal shear stress is negligible the resistance to flow being partitioned between shear at the sides and longitudinal gradients of longitudinal and transverse stress the surface is generally crevasse free features similar to ice rises are observed upstream of the grounding line their origin is uncertain but they move at velocities comparable to the surrounding ice the flow is laterally extensive and longitudinally compressive but there are large local variations of the strain rate from the regional trends the boundary between the two major tributaries to ice stream b followed with the radar is characterized by a band of strain rates much smaller than average detailed measurements at the downstream b network highlight this local variability of strain rates but confirm that there is a strong correlation between surface topography and strain rates the strain rates indicate that the undulating topography is locally generated the lower elevation ice is thicker and moves faster a velocity profile across the crevassed northern margin shows that the decrease of velocity toward the edge is nearly linear a calculation of ice stream discharge at this location agrees closely with two rather rough estimates of balance flux and is considerably larger than a third estimate the discharge of ice stream b does not appear to be significantly out of balance with published estimates of total ice accumulation within the present catchment basin

subglacial to proglacial depositional environments in an ordovician glacial tunnel valley alnif morocco abstract this paper presents the sedimentary analysis of an exceptional ordovician glacial tunnel valley in the eastern part of the anti atlas the valley infill comprises two major glacial erosion surfaces striated pavements each overlain by a fining upward glacial unit these units are composed of five distinct facies associations recording the evolution from subglacial to proglacial environments and an additional sixth facies association overtopping the tunnel valley infill and associated with post glacial environments the tunnel valley infill also records a transitional environment between the subglacial and proglacial settings which is compared with the antarctic ice sheet margin these three environments are defined by the position of the grounding line and the coupling line the new proposed depositional model also differs from usual ordovician depositional models in which the main tunnel valley infill is interpreted as essentially proglacial outwash deposits in a range of glaciomarine to glaciofluvial environments overall a substantial part of the valley infill 50 of volume was deposited in a subglacial setting the sedimentary bodies could form potentially thick and laterally extended although these were limited by the shape and extent of the subglacial accommodation space finally the sedimentary record when compared with regional analogues also provides information for the palaeogeographic reconstruction of the ordovician ice sheet in this region

bedmap a new ice thickness and subglacial topographic model of antarctica measurements of ice thickness on the antarctic ice sheet collected during surveys undertaken over the past 50 years have been brought together into a single database from these data a seamless suite of digital topographic models have been compiled for antarctica and its surrounding ocean this includes grids of ice sheet thickness over the grounded ice sheet and ice shelves water column thickness beneath the floating ice shelves bed elevation beneath the grounded ice sheet and bathymetry to 60 s including the sub ice shelf cavities these grids are consistent with a recent high resolution surface elevation model of antarctica while the digital models have a nominal spatial resolution of 5 km such high resolution is justified by the original data density only over a few parts of the ice sheet the suite does however provide an unparalleled vision of the geosphere beneath the ice sheet and a more reliable basis for ice sheet modeling than earlier maps the total volume of the antarctic ice sheet calculated from the bedmap grid is 25 4 million km3 and the total sea level equivalent derived from the amount of ice contained within the grounded ice sheet is 57 m comprising 52 m from the east antarctic ice sheet and 5 m from the west antarctic ice sheet slightly less than earlier estimates the gridded data sets can be obtained from the authors

observed velocity fluctuations on a major antarctic ice stream most of the antarctic ice sheet is drained by systems comprising an area of slow sheet flow which converges into a much faster valley glacier or ice stream on reaching the coast this ice then either enters an ice shelf confined by embayments and ice rises or calves off into the ocean the higher speed of ice streams enables them to respond to changes in their environment faster than the slower ice sheet in the west antarctic where the ice sheet moves across bedrock well below sea level the ice streams have the potential to rapidly increase or decrease the ice discharge into the sea1 the timescale for these changes has been placed at a few hundred years if this is true then changes should be detectable by careful measurements on a decadal timescale by comparing recent ice velocity measurements with those collected ten years ago we establish that the ice in the mouth of ice stream b has decelerated by 20 we discuss the possible causes of this deceleration on the basis of our knowledge of the current regional dynamics and the possible ramifications of this deceleration on future ice stream behaviour

bed reflectivity beneath inactive ice streams in west antarctica radio echo sounding res techniques are used to examine spatial changes inbed reflectivityacrossrelictice streams inwest antarctica measurementsfrom adjacent interstream ridgesare usedto correctthe measuredpower returned fromthebed for attenuation and losses due to geometric spreading scattering and absorption res measurements near boreholes drilled on ice stream c isc indicate high coefficients of bed reflectivity r40 1 in locations where the bed wasthawed and boreholes connected to thebasalwatersystem andlow reflectivitycoefficients r50 02 atlocationsthat were frozen and not connected intermediate values of bed reflectivity were measured at locations where the connection to the basal water systemwas weak measurements across four relictmargins show thatbedreflectivity usuallyjumpsfromlow tohighvaluesseveral kilometers inside the outermost buried crevasses we interpret this to be a transition from frozento thawedbasal conditions anddiscuss implications of these observations

tidal pacing skipped slips and the slowdown of whillans ice stream antarctica we summarize new observations of the deceleration and stick slip motion of whillans ice stream wis antarctica we refine the location of the large sticky spots that resist motion between slip events the locations of which are controlled by the patterns of subglacial water flow our examination of the long term velocity time series for the ice stream reveals that the decadal scale deceleration is not occurring at a steady rate but varies at the sub decadal timescale this unsteady deceleration modulates the temporal evolution of a broad 50 km across surface elevation bulge forming at the junction between the relatively narrow upstream portion of the ice stream and broad ice plain that constitutes the downstream end of wis comparison of observations from april 2003 and november 2010 reveals significant changes in the tidally modulated stick slip cycle that regulates motion on the ice plain we observe that the timing of slip events has become less regular in response to decreased flow speed in the upstream portions of the ice stream the decreased regularity of slip events has reduced the release of stored elastic strain during slip events increasing the rate of deceleration

credible occurrence probabilities for extreme geophysical events earthquakes volcanic eruptions magnetic storms 1 statistical analysis is made of rare extreme geophysical events recorded in historical data counting the number of events k with sizes that exceed chosen thresholds during specific durations of time under transformations that stabilize data and model parameter variances the most likely poisson event occurrence rate k applies for frequentist inference and also for bayesian inference with a jeffreys prior that ensures posterior invariance under changes of variables frequentist confidence intervals and bayesian jeffreys credibility intervals are approximately the same and easy to calculate 1 k z 2 2 k z 2 2 where z is a parameter that specifies the width z 1 z 2 corresponding to 1 68 3 2 95 4 if only a few events have been observed as is usually the case for extreme events then these error bar intervals might be considered to be relatively wide from historical records we estimate most likely long term occurrence rates 10 yr occurrence probabilities and intervals of frequentist confidence and bayesian credibility for large earthquakes explosive volcanic eruptions and magnetic storms

spatial variation of englacial radar attenuation modeling approach and application to the vostok flowline 1 constraining the spatial variation of englacial radar attenuation is critical for accurate inference of the spatial variation of the englacial and basal properties of ice sheets from radar returned power here we evaluate attenuation models that account for spatial variations in ice temperature and chemistry and test them along the flowline that passes through the vostok ice core site antarctica the simplest model often used but rarely valid assumes a uniform attenuation rate everywhere along the flowline so that total attenuation is proportional to ice thickness the next simplest model uses spatially varying temperatures predicted by an ice flow model and assumes uniform chemistry additional models account for spatially varying chemistry using englacial stratigraphy we find that the roundtrip attenuation to the bed can easily differ by 10 db or more between the uniform attenuation rate model and models that account for variable ice temperature such differences are sufficient to confound the delineation of dry and wet beds also including spatial variations in chemistry produces smaller differences 10 db but the magnitude of these differences depends on the relative importance of dry and wet deposition of impurities in the past accounting for dry deposited impurities requires ice flow modeling and results in larger differences from all other models which assume uniform chemistry or wet deposition only these results indicate that modeling the spatial variation of attenuation requires a spatially varying temperature model in order to infer bed conditions from bed returned power accurately and that both ice core data and radar stratigraphy are also strongly desirable

dynamics of the siple coast ice streams west antarctica results from a thermomechanical ice sheet model most of the ice leaving the antarctic ice sheet is discharged to the sea by ice streams an understanding of the dynamics of these ice streams is therefore crucial if we are to predict the future evolution of the ice sheet and its effect on global sea level the behavior of ice streams a to e of the siple coast west antarctica has attracted much attention with evidence of recent stagnation and numerous relict ice flow features here i present results from a three dimensional thermodynamic ice sheet model of the west antarctic ice sheet wais the model generates variability in the flow of the siple coast ice streams reminiscent of that mentioned above this is caused by competition between several preferred ice flow pathways in the area individual ice streams interact through changes in catchment area size a reduction in catchment area reduces the flow of ice entering a stream and can cause stagnation through the associated reduction in deformational heating

late quaternary glacial and deglacial history of eastern vestfirdir iceland using cosmogenic isotope 36cl exposure ages and marine cores the late glacial and deglacial history of vestfirdir nw iceland is interpreted from terrestrial field based evidence and compared with marine cores and seismic studies understanding the timing of deglaciation is complicated by a lack of dateable organic material on land uncertainties associated with interpretation of diamictons and ambiguities in radiocarbon dating new 36cl exposure ages of boulders and bedrock surfaces suggest that at least part of the coastal uplands were ice free by approximately 20 ka deglaciation of the valleys followed boulders from the crest of an end moraine in the sela river valley have a mean 36cl age of 14 6 ka 0 8 ka n 4 and the outermost moraine in the valley adjacent to kaldalonjokull has a mean 36cl age of 11 7 ka 0 9 ka n 2 tephrochronology provides a link between the terrestrial and marine sequences around vestfirdir the vedde ash ca 12 k cal yr bp is present in some offshore cores but it is absent from terrestrial sites suggesting that its distribution provides a constraint on ice extent during the younger dryas the saksunarvatn ash 10 2 0 6 k cal yr bp confirmed by geochemical analyses and radiocarbon dating is present in at least three terrestrial locations on eastern vestfirdir and in many cores from the fjords and shelf which constrains early holocene ice extent copyright 2006 john wiley sons ltd

radar reflections reveal a wet bed beneath stagnant ice stream c and a frozen bed beneath ridge bc west antarctica digital airborne radar data were collected during the 1987 88 antarctic field season in nine gridded blocks covering the downstream portions of ice stream b 6 km spacing and ice stream c 11 km spacing together with a portion of ridge bc between them an automated processing procedure was used for picking onset times of the reflected radar pulses converting travel times to distances interpolating missing data converting pressure transducer readings correcting navigational drift performing crossover analysis and zeroing remanent crossover errors interpolation between flight lines was carried out using the minimum curvature method maps of ice thickness estimated accuracy 20 m and basal reflection strength estimated accuracy 1 db were produced the ice thickness map confirms the characteristics of previous reconnaissance maps and reveals no new features the reflection strength map shows pronounced contrasts between the ice streams and ridge bc and between the two ice streams themselves we interpret the reflection strengths to mean that the bed of ice stream c as well as that of ice stream b is unfrozen that the bed of ridge bc is frozen and that the boundary between the frozen bed of ridge bc and the unfrozen bed of ice stream c lies precisely below the former shear margin of the ice stream

changes in the configuration of ice stream flow from the west antarctic ice sheet surface based ice penetrating radar profiles on the northeast flank of siple dome support the hypothesis that a curvilinear scar first observed in advanced very high resolution radiometer satellite imagery represents the margin of a formerly active ice stream the scar defines the southwestern boundary of an ice stream flowing from ice stream c to ice stream d close to where it enters the ross ice shelf our studies show that the scar coincides with a trough and upward step in surface topography approximately 5 km across underlain by a zone of disturbed internal stratigraphy revealed by the radar burial depth of the disturbed zone enables us to calculate the time of shutdown as occurring prior to approximately 1 3 ka the configuration of the ice streams draining the west antarctic ice sheet into the ross ice shelf evidently changes with time and attempts to predict the evolution of the ice sheet must incorporate this observation

changes in the ice plain of whillans ice stream west antarctica data from the mouth of the decelerating whillans ice stream wis west antarctica spanning 42 years are reviewed deceleration has continued with local areas of both thinning and thickening occurring the mean thinning rate is 0 48 0 77m a 1 no consistent overall pattern is observed ice thickens immediately upstream of crary ice rise where deceleration and divergence are strongest suggesting expanded upstream influence of the ice rise thinning is prevalent on the ross ice shelf grounding line advance at a rate of 0 3 km a 1 is detected in a few locations basal stresses vary across an ice stream transect with a zone of enhanced flow at the margin marginal shear is felt at the ice stream center mass balance values are less negative but larger errors of earlier measurements mask any possible temporal pattern comparisons of the recent flow field with flow stripes suggest wis contributes less ice to the deep subglacial channel carved by mercer ice stream and now flows straighter the general lack of geometric changes suggests that the regional velocity decrease is due to changing basal conditions

occurrence rate of extreme magnetic storms 1 we investigate the occurrence rate of magnetic storms at the earth during the first part of our study the standard and integral distribution functions of magnetic storm value minimal dst index for the period 1963 2012 are analyzed and results show that the standard and integral distribution functions have power law tails with indexes 4 4 and 3 4 respectively during the second part statistical analysis of occurrence rate of magnetic storms induced by different types of interplanetary drivers is made using omni data for the period 1976 2000 using our catalog of large scale types of solar wind streams we study storms induced by interplanetary coronal mass ejections icmes separately magnetic clouds mcs and ejecta and both types of compressed regions corotating interaction regions cirs and sheaths for these types of drivers we calculate the integral probabilities of storms with minimum dst 50 70 100 150 and 200 nt the highest probability in this interval of dst is observed for mcs with the probabilities for other drivers being 3 10 times lower extrapolation of these results to extreme storms shows that a magnetic storm as large as the carrington storm in 1859 with dst 1760 nt is observed on the earth with frequency not higher than one event during 500 years