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

radar signatures beneath a surface topographic lineation near the outlet of kamb ice stream and engelhardt ice ridge west antarctica visible and infrared satellite images reveal numerous lineations on the siple coast region of west antarctica we used 5 mhz ice penetrating radar to probe the interior and the bed of the ice sheet beneath a lineation at the boundary between engelhardt ice ridge and flat ice terrain to the south of the kamb ice stream kis outlet results show curved reflectors that emerge from the bed beneath 600 m thick ice the tops of the reflectors extend about 100 m into the ice above the bed where they become almost horizontal apparent reflectivity of the horizontal section is about 20 db less than that of the bed we conclude that the likely cause of such strong reflection is sea water that was accreted into basal crevasses when the flat ice terrain was floating internal layers are warped downward just downslope from the basal reflectors it is thought that the downwarping was caused by localized basal melting in the past the spatial pattern of downwarping suggests that localized basal melting was stronger on the north side than on the south side of kis apparently ice ocean interactions on the two sides of kis were different

variations in basal conditions on rutford ice stream west antarctica seismic reflection data from two lines on rutford ice stream are presented and are compared with data already published from a third line on the ice stream the amplitude and phase of the reflections have been used to investigate the properties of the sub ice material multiple reflections on long record length data allowed calibration of the reflection coefficient at the ice bed interface and determination of the acoustic impedance of the bed material the characteristics of the bed material vary both along and across the ice stream the average acoustic impedance of the bed material across the glacier at the upstream line is 3 88 x 106 kg m 2s 1 this decreases to 3 19 x 106kg m 2s 1 52 km further downstream these values are within the range which is typical of soft sediments using acoustic impedance as an indicator of subglacial porosity some areas of the ice stream bed are interpreted as dilatant water saturated sediments undergoing pervasive deformation in other areas the bed is not deforming and basal sliding may be a more important process the proportion of the ice stream width over which bed deformation occurs increases downstream

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

modis based mosaic of antarctica moa data sets continent wide surface morphology and snow grain size abstract we present digital image mosaics of surface morphology and optical snow grain size for the antarctic continent and surrounding islands assembled from 260 moderate resolution imaging spectroradiometer modis images the products are derived from modis band 1 red light 647 nm and band 2 infrared 857 nm orbit swath data acquired during the 2003 2004 austral summer multiple images of all areas are combined in a data cumulation scheme to improve spatial resolution and increase radiometric content of the mosaics the component images were de striped geo registered and re sampled to the projection grid using the modis swath to grid toolbox ms2gt software a 125 m ground equivalent polar stereographic projection was used identical to previous radar image mosaics and similar to several other continent wide data sets geo location accuracy of the final mosaics is better than 125 m swath acquisition times were limited to the range 05 00 13 30 ut to provide broad but uniform mosaic illumination regions of cloud cover blowing snow and intense forward scattering were masked prior to compositing the moa snow grain size data set image provides the first continent wide semi quantitative map of mean summer snow firn and blue ice grain size optical grain size radius was determined using a normalized difference index derived from modis band 1 and band 2 rad values atmospheric correction adjustments for directional reflectance variation and snow grain size estimation were accomplished by a look up table generated by a combination of published models of snow spectral reflectance and atmosphere radiative transfer validation of the snow grain size image is provided by in situ spectra of snow covered sea ice made in october 2003 we use the new data sets to determine major ice shelf areas and shelf edge coastline and ice grounding line vector files and determine areas of selected blue ice regions these data sets are available for download at http nsidc org data moa

characterizing englacial drainage in the ablation zone of the greenland ice sheet rapid local drainage of surface meltwater to the base of the greenland ice sheet is thought to result in surface velocity variations as far inland as the equilibrium zone zwally and others 2002 ice penetrating radar surveys throughout this region allow us to characterize englacial drainage features that appear as vertically stacked diffraction hyperbolae in common offset profiles these data are used with a radar simulation model which allows for variations in geometry penetration depth and infill material to understand the characteristics of these hyperbolae and the likelihood that they are produced by moulins we find only a moderate correlation between the locations of these possible moulins and supraglacial lakes indicating that many lakes drain over the surface of the ice sheet or do not contain sufficient water to reach the bed through moulin formation we find a strong correlation between moulin location in the ablation region and elevated along flow tension due to flow over rough bedrock which generates surface crevassing and provides an entry point for meltwater although theory suggests that moulins may form anywhere on the ice sheet given sufficient meltwater input our data suggest that they are far more common in the ablation zone than near or inland from the equilibrium line

physical conditions at the base of a fast moving antarctic ice stream boreholes drilled to the bottom of ice stream b in the west antarctic ice sheet reveal that the base of the ice stream is at the melting point and the basal water pressure is within about 1 6 bars of the ice overburden pressure these conditions allow the rapid ice streaming motion to occur by basal sliding or by shear deformation of unconsolidated sediments that underlie the ice in a layer at least 2 meters thick the mechanics of ice streaming plays a role in the response of the ice sheet to climatic change

tidally controlled stick slip discharge of a west antarctic ice a major west antarctic ice stream discharges by sudden and brief periods of very rapid motion paced by oceanic tidal oscillations of about 1 meter acceleration to speeds greater than 1 meter per hour and deceleration back to a stationary state occur in minutes or less slip propagates at approximately 88 meters per second suggestive of a shear wave traveling within the subglacial till a model of an episodically slipping friction locked fault reproduces the observed quasi periodic event timing demonstrating an ice stream s ability to change speed rapidly and its extreme sensitivity to subglacial conditions and variations in sea level

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

an improved coherent radar depth sounder the university of kansas developed a coherent radar depth sounder during the 1980s this system was originally developed for glacial ice thickness measurements in the antarctic during the field tests in the antarctic and greenland we found the system performance to be less than optimum the field tests in greenland were performed in 1993 as a part of the nasa program for arctic climate assessment parca we redesigned and rebuilt this system to improve the performance the radar uses pulse compression and coherent signal processing to obtain high sensitivity and fine along track resolution it operates at a center frequency of 150 mhz with a radio frequency bandwidth of about 17 mhz which gives a range resolution of about 5 m in ice we have been operating it from a nasa p 3 aircraft for collecting ice thickness data in conjunction with laser surface elevation measurements over the greenland ice sheet during the last 4 years we have demonstrated that this radar can measure the thickness of more than 3 km of cold ice and can obtain ice thickness information over outlet glaciers and ice margins in this paper we provide a brief survey of radar sounding of glacial ice followed by a description of the system and subsystem design and performance we also show sample results from the field experiments over the greenland ice sheet and its outlet glacicrs

investigations of an ice plain in the mouth of pine island glacier antarctica we present newly acquired airborne radar data showing ice thickness and surface elevation for pine island glacier antarctica these data when combined with earlier measurements suggest the presence of a lightly grounded area immediately above the grounding line of pine island glacier we identify this region as a ice plain it lies close to the centre line of the glacier has an elevation above buoyancy of 28 km the upstream edge of the ice plain is defined by a coupling line the configuration of the ice plain implies that nearby thinning of the ice stream would result in substantial grounding line retreat we suggest that the grounding line retreat of pine island glacier observed between 1992 and 1996 probably commenced sometime after 1981

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

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

astronomical forcing and sedimentary noise modeling of lake level changes in the paleogene dongpu depression of north china abstract challenges in interpreting continental sequence stratigraphy and typically uncertain geochronology hinder the understanding of paleolake evolution and hydrocarbon exploration in terrestrial basins the dongpu depression in north china is a lacustrine basin with abundant hydrocarbon resources an accurate chronology for the paleogene stratigraphy in the dongpu depression is lacking and the mechanism of lake level variation remains unclear in this study we utilize high resolution gamma ray logs to conduct a cyclostratigraphic analysis of the shahejie formation which is a paleogene succession in the dongpu depression characterized by sandstones and dark mudstones interbedded with thin salt rocks time series analysis reveals evidence for 405 kyr eccentricity cycles in the gamma ray series which is supported by statistical modeling of optimal sedimentation rates tuning of the gamma ray data to this 405 kyr eccentricity cyclicity enables a 14 7 myr long astronomical time scale to be constructed this astrochronology is anchored to the astronomical age of the dongying formation shahejie formation boundary 28 86 ma in the bohai bay basin thus providing an absolute timescale for the studied interval that extends from 28 86 ma to 43 59 ma using this anchored astrochronology we show that a recently established sedimentary noise model for inferring sea level change in marginal marine settings can be used to similarly infer lake level fluctuations in terrestrial basins in particular sedimentary noise modeling of the tuned gamma ray series reveals high resolution changes in sedimentary noise that are indicative of lake level variations linked to million year scale i e 4 8 myr 2 4 myr and 1 2 myr astronomical forcing these inferred changes in lake level are supported by previously published sequence stratigraphic interpretations moreover an evolutionary correlation coefficient ecoco analysis of the gamma ray series also indicates recurrent distortions in sedimentation that may be linked to lake level changes this study provides new methods for the assessment of paleolake level variations as well as insights into the connection between astronomical forcing and lake evolution across long timescales in terrestrial basins

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

sequential stagnation of kamb ice stream west antarctica 1 ice penetrating radar data confirm the presence of two relict northern margins of kamb ice stream kis west antarctica in an area called the duckfoot between the two relict margins deep layers are preserved and buried crevasses remain at a constant depth until roughly 1 5 km from the innermost youngest margin suggesting that this outer portion of the ice stream stagnated suddenly 10 years stagnation was likely accomplished through changes in basal conditions i e reduced lubrication beneath this outer portion of the ice stream and resulted in a narrowing of the ice stream trunk by 27 and a reduction in flow speed 200 years prior to complete stagnation flowstripe deformation as a result of duckfoot stagnation provides an estimate of kis paleo velocity of 210 m a after duckfoot stagnation

subglacial seismic reflection strategies when source amplitude and medium attenuation are poorly known seismic reflection techniques are a powerful way to probe physical properties of subglacial strata inversion of seismic data for physical properties may be hampered however by lack of knowledge of the source amplitude as well as lack of knowledge of the compressional and shear attenuation in the ice new methods are described to measure the source signature that require no a priori knowledge of the ice attenuation profile another new method is described to obtain the angular dependence of the subglacial bed reflection coefficient that is relatively insensitive to knowledge of the ice attenuation finally a correction is provided to a long standing error in the literature regarding measurement of the bed normal incidence reflection coefficient

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

mapping the grounding zone of the ross ice shelf antarctica using icesat laser altimetry we use laser altimetry from the ice cloud and land elevation satellite icesat to map the grounding zone gz of the ross ice shelf antarctica at 491 locations where icesat tracks cross the grounding line gl ice flexure in the gz occurs as the ice shelf responds to short term sea level changes due primarily to tides icesat repeat track analysis can be used to detect this region of flexure since each repeated pass is acquired at a different tidal phase the technique provides estimates for both the landward limit of flexure and the point where the ice becomes hydrostatically balanced we find that the icesat derived landward limits of tidal flexure are in many places offset by several km and up to 60 km from the gl mapped previously using other satellite methods we discuss the reasons why different mapping methods lead to different gl estimates including instrument limitations variability in the surface topographic structure of the gz and the presence of ice plains we conclude that reliable and accurate mapping of the gl is most likely to be achieved when based on synthesis of several satellite datasets

response of a landfill drainage pipe buried in a trench the structural response of drainage pipes used in landfill leachate collection systems depends on the properties of both the pipe and the backfill soil as well as the type of backfill configuration the implications of using different backfill configurations for the structural performance of the pipe are discussed for laterally extensive blankets mounds and trenches the objective of this paper is to quantify the structural performance of a landfill pipe when buried in a trench unlike in the trenches for many gravity flow pipes where only a proportion of the vertical overburden stresses may reach the pipe because of conventional trench arching it is shown the stresses in a landfill trench may be larger than the overburden stresses this occurs when the backfill in the trench is stiffer than the surrounding material denoted as negative trench arching results from a large scale laboratory test and numerical analysis of a pipe placed in a gravel trench cut into soft clay are presented the configurat

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

internal and basal ice changes near the grounding line derived from radio echo sounding evidence of changing basal and internal ice properties near the grounding line was derived from airborne radio echo sounder observations of the ice sheet around the sor rondane mountains antarctica from the trailing figure of the bottom echo signal the roughness of the ice bottom near the grounding line was inferred results show that the specular components of scattering begin to appear on the ice shell side of the grounding line furthermore double trip echoes were observed with at strong scattering in the shelf area and their boundary of occurrence was very close to the grounding line this is evidence of interaction between ice and sea water at the bottom of the ice shelf we also examined the occurrence of internal layered echoes in most of the area around the mountains internal echoes were observed continuously but they were not found at or close to the ice shelf the boundary between the appearance and disappearance of internal layer echoes is distinct and occurs 20 30 km inland from the grounding line these results suggest that some major change may occur in the internal ice on the inland side of the grounding line

basal mechanics of ice streams insights from the stick slip motion of whillans ice stream west antarctica the downstream portion of whillans ice stream west antarctica moves primarily by stick slip motion the observation of stick slip motion suggests that the bed is governed by velocity weakening physics and that the basal physics is more unstable than suggested by laboratory studies the stick slip cycle of whilla s ice plain exhibits substantial variability in both the duration of sticky periods and in slip magnitude to understand this variability we modeled the forces acting on the ice stream during the stick phase of the stick slip cycle the ocean tides introduce changes in the rate at which stress is applied to the ice plain increased loading rates promote earlier failure and vice versa results show that the bed of whillans ice stream strengthens over time healing during the quiescent intervals in the stick slip cycle with the bed weakening during slip events the time dependent strengthening of the ice plain bed following termination of slip events indicates that the strength of the bed may vary by up to 0 35 kpa during the course of a single day copyright 2009 by the american geophysical union

melting and freezing beneath the ross ice streams antarctica we have estimated temperature gradients and melt rates at the bottom of the ice streams in west antarctica measured velocities were used to include the effects of horizontal advection and strain heating in the temperature model and to determine shear heating at the bed our modeled temperatures agree well with measured temperatures from boreholes in regions of steady flow we find that ice stream tributaries and the inland ice account for about 87 of the total melt generated beneath the ross ice streams and their catchments our estimates indicate that the ice plains of whillans ice stream and ice stream c even when active have large areas subject to basal freezing confirming earlier estimates that import of water from upstream is necessary to sustain motion the relatively low melt rates on whillans ice stream are consistent with observations of deceleration over the last few decades and suggest a shutdown may take place in the future possibly within this century while there are pockets of basal freezing beneath ice streams d and e there are larger areas of basal melt that produce enough melt to more than offset the freezing which is consistent with inferences of relatively steady flow for these ice streams over the last millennium

discovery of till deposition at the grounding line of whillans ice stream we report on the discovery of a grounding line sedimentary wedge till delta deposited by whillans ice stream west antarctica our observation is that grounding line deposition serves to thicken the ice and stabilize the position of the grounding line the ice thickness at the grounding line is greater than that of floating ice in hydrostatic equilibrium thus the grounding line will tend to remain in the same location despite changes in sea level until sea level rises enough to overcome the excess thickness that is due to the wedge further our observation demonstrates the occurrence of rapid subglacial erosion sediment transport by distributed subglacial till deformation and grounding line sedimentation which have important implications for ice dynamics numerical modeling of ice flow and interpretation of the sedimentation record

hydrochemistry of ice stream beds evaporitic or microbial effects recently new data have been acquired on the chemical composition of waters within the glacial till beneath the kamb ice stream kis formerly ice stream c and the bindschadler ice stream bis formerly ice stream d two of the arteries of ice loss from the antarctic ice sheet ais we compare this water chemistry with basally stored subglacial waters from arctic and alpine glaciers and the only other measurements of sub ice sheet chemistry to date that emerging from beneath law dome near casey station sw antarctica during a jokulhlaup we find that the chemistries pose significant questions about the hydrology and biogeochemistry of ice sheet beds copyright 2010 john wiley sons ltd