the role of ice thickness and bed properties on the dynamics of the enhanced flow tributaries of bailey ice stream and slessor glacier east antarctica

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

effects of birefringence within ice sheets on obliquely propagating radio waves in this paper effects of birefringence on radio waves obliquely propagating though polar ice sheets are examined to facilitate interpretations of bistatic and side looking radar data a formalism applicable for arbitrary radar configurations is developed to predict the returned power from within and beneath the ice sheets that have arbitrary alignments of ice crystals ice fabrics we applied this formalism to a range of ice fabrics found in ice cores and assessed the effects of birefringence in terms of ray path configurations ice fabrics and radar frequency predicted frequency dependence of the bed return power replicates prominent features observed at greenland ngrip ice core site results show that birefringence in ice of 1 km or more thickness with strong weak fabric can reduce the power returned from the bed 2 db or more at frequencies higher than 200 mhz 20 mhz as compared to isotropic ice this suggests that quantitative interpretation of the power returned from the bed requires careful assessment of birefringence almost everywhere over the ice sheets application of this formalism also suggests a radar frequency range usable for attenuation measurements possible effects of fabric on synthetic aperture radar processing and a feasibility of remote sensing of ice fabric

basal conditions at the grounding zone of whillans ice stream west antarctica from ice penetrating radar we present a comprehensive ice penetrating radar survey of a subglacial embayment and adjacent peninsula along the grounding zone of whillans ice stream west antarctica through basal waveform and reflectivity analysis we identify four distinct basal interfaces 1 an ice water saturated till interface inland of grounding 2 a complex interface in the grounding zone with variations in reflectivity and waveforms caused by reflections from fluting sediment deposits and crevasses 3 an interface of anomalously low reflectivity downstream of grounding in unambiguously floating areas of the embayment due to basal roughness and entrained debris and 4 a high reflectivity ice seawater interface that occurs immediately seaward of grounding at the subglacial peninsula and several kilometers seaward of grounding in the embayment occurring after basal debris and grounding zone flutes have melted off the ice bottom sediment deposition via basal debris melt out occurs in both locations the higher basal melt rate at the peninsula contributes to greater grounding line stability by enabling faster construction of a stabilizing sediment wedge in the embayment the low slopes of the ice bottom and bed prevent development of a strong thermohaline circulation leading to a lower basal melt rate and less rapid sediment deposition thus grounding lines in subglacial embayments are more likely to lack stabilizing sediment deposits and are more prone to external forcing whether from the ocean the subglacial water system or large scale ice dynamics our conclusions indicate that subglacial peninsulas and embayments should be treated differently in ice sheet ocean models if these models are to accurately simulate grounding line response to external forcing

greenland subglacial lakes detected by radar 1 subglacial lakes are an established and important component of the basal hydrological system of the antarctic ice sheets but none have been reported from greenland here we present airborne radio echo sounder res measurements that provide the first clear evidence for the existence of subglacial lakes in greenland two lakes with areas 8 and 10 km2 are found in the northwest sector of the ice sheet 40 km from the ice margin and below 757 and 809 m of ice respectively the setting of the greenland lakes differs from those of antarctic subglacial lakes being beneath relatively thin and cold ice pointing to a fundamental difference in their nature and genesis possibilities that the lakes consist of either ancient saline water in a closed system or are part of a fresh modern open hydrological system are discussed with the latter interpretation considered more likely

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

migration of the siple dome ice divide west antarctica the non linearity of the ice flow law or a local accumulation low over an ice divide can cause isochrones internal layers to be shallower under the divide relative to the flanks forming a divide bump in the internal layer pattern this divide signature is analyzed using ice flow models and inverse techniques to detect and quantify motion of the siple dome ice divide west antarctica the principal feature indicating that migration has occurred is a distinct tilt of the axis of the peaks of the warped internal layers beneath the divide the calculated migration rate is 0 05 0 50 m a 1 toward ice stream d and depends slightly on whether the divide bump is caused by the non linearity of ice flow or by a local accumulation low our calculations also suggest a strong south north accumulation gradient of 5 10 x 10 6 a 1 in a narrow zone north of the divide a consequence of divide migration is that pre holocene ice is thickest about 0 5 km south of the present divide position divide motion indicates that non steady processes possibly associated with activity of the bounding ice streams are affecting the geometry of siple dome the migration rate is sufficiently slow that the divide bump is maintained in the internal layer pattern at all observable depths this suggests that major asynchronous changes in the elevation or position of the bounding ice streams are unlikely over at least the past 10 3 10 4 years

weak bed control of the eastern shear margin of thwaites glacier west antarctica recent acceleration and thinning of thwaites glacier west antarctica motivates investigation of the controls upon and stability of its present ice flow pattern its eastern shear margin separates thwaites glacier from slower flowing ice and the southern tributaries of pine island glacier troughs in thwaites glacier s bed topography bound nearly all of its tributaries except along this eastern shear margin which has no clear relationship with regional bed topography along most of its length here we use airborne ice penetrating radar data from the airborne geophysical survey of the amundsen sea embayment antarctica agasea to investigate the nature of the bed across this margin radar data reveal slightly higher and rougher bed topography on the slower flowing side of the margin along with lower bed reflectivity however the change in bed reflectivity across the margin is partially explained by a change in bed roughness from these observations we infer that the position of the eastern shear margin is not strongly controlled by local bed topography or other bed properties given the potential for future increases in ice flux farther downstream the eastern shear margin may be vulnerable to migration however there is no evidence that this margin is migrating presently despite ongoing changes farther downstream

extensive winter subglacial water storage beneath the greenland ice sheet w c is a recipient of the nasa earth and space science fellowship d m s is supported by a grant from the nasa cryospheric sciences program h s is supported by grants from the nasa cryospheric sciences and sea level rise programs t t c and r e b are supported by grants from national science foundation nsf and nasa cryospheric sciences s p is supported by the natural environment research council s centre for polar observation

predicting radar attenuation within the antarctic ice sheet abstract to better understand the ability of ice penetrating radar to diagnose the subglacial environment from bed returned power we model the englacial radar attenuation of antarctic ice first we use a one dimensional thermo mechanical model to evaluate the sensitivity of the depth averaged attenuation rates to ice temperature as a function of surface accumulation rate geothermal flux and ice thickness we find that attenuation is most sensitive to variations in geothermal flux and accumulation rate when the bed temperature is close to the pressure melting point but even if geothermal flux and accumulation rate remain fixed attenuation can easily vary with ice thickness such high sensitivities show that one should not assume a uniform attenuation rate in the radar data analysis then using ensembles of modeled ice temperatures with different boundary conditions we generate multiple attenuation predictions for the antarctic ice sheet and evaluate the resulting uncertainties the largest contributor to uncertainty in these predictions is the geothermal flux this uncertainty is localized within the deeper half of the ice sheet by combining these temperature ensembles with ice core chemistry data we show that the sea salt adds little to the attenuation but the contribution from acids accounts for 29 inland to 53 coast of the total attenuation we conclude that improving radar diagnosis of the subglacial environment using bed returned power requires both 1 better data interpretation algorithms that account for attenuation variations and 2 better constraints of geothermal flux and bulk chemistry

radar characterization of the basal interface across the grounding zone of an ice rise promontory in east antarctica radar power returned from the basal interface along a 42 km long profile over an ice rise promontory and the adjacent roi baudouin ice shelf dronning maud land east antarctica is analyzed to infer spatial variations in basal reflectivity and hence the basal environment extracting basal reflectivity from basal returned power requires an englacial attenuation model we estimate attenuation in two ways 1 using a temperature dependent model with input from thermomechanical ice flow models and 2 using a radar method that linearly approximates the geometrically corrected returned power with ice thickness the two methods give different results we argue that attenuation calculated using a modeled temperature profile is more robust than the widely used radar method especially in locations where depth averaged attenuation varies spatially or where the patterns of basal reflectivity correlate with the patterns of the ice thickness

dilatant till facilitates ice stream flow in northeast greenland abstract we present radio echo sounding res global positioning system gps and active source seismic data across the central portion of the northeast greenland ice stream negis negis widens downglacier from a small region of high geothermal flux near the ice divide our data reveal high porosity 40 water saturated till lubricating the ice stream ice accelerates and thins as it flows into negis producing marginal troughs in surface topography these troughs create steep gradients in the subglacial hydropotential that generate parallel sticky and slippery bands beneath the shear margins the low porosity sticky sediment bands limit ice entrainment across the margins and thus restrict further widening producing the long narrow and relatively stable ice stream however the observed relations among surface elevation basal water routing broad sedimentary drape and till dilatancy suggest that rapid shifts in ice dynamics are possible including rapid transmission of ocean forcing inland the source and routing of the subglacial till are unclear but our data help constrain hypotheses

pitfalls in radar diagnosis of ice sheet bed conditions lessons from englacial attenuation models 1 radar power returned from ice sheet beds has been widely accepted as an indicator of bed conditions however the bed returned power also depends on englacial attenuation which is primarily a function of ice temperature here using a one dimensional attenuation model it is demonstrated that in most cases variations in bed returned power are dominated by variations in englacial attenuation rather than bed reflectivity both accumulation rate and geothermal flux anomalies can interfere with the interpretation with the consequence analytical radar algorithms that have been widely accepted likely yield false delineations of wet dry beds more careful consideration is needed when diagnosing bed conditions spatial patterns of shallow englacial radar reflectors can be used as a proxy for accumulation rates which affect ice temperature and thus returned power i argue that it is necessary to simultaneously interpret the returned power and englacial reflector patterns to improve the bed diagnosis

evidence of a hydrological connection between the ice divide and ice sheet margin in the aurora subglacial basin east antarctica subglacial hydrology in east antarctica is poorly understood yet may be critical to the manner in which ice flows data from a new regional airborne geophysical survey icecap have transformed our understanding of the topography and glaciology associated with the 287 000 km2 aurora subglacial basin in east antarctica using these data in conjunction with numerical ice sheet modeling we present a suite of analyses that demonstrate the potential of the 1000 km long basin as a route for subglacial water drainage from the ice sheet interior to the ice sheet margin we present results from our analysis of basal topography bed roughness and radar power reflectance and from our modeling of ice sheet flow and basal ice temperatures although no clear cut subglacial lakes are found within the aurora basin itself dozens of lake like reflectors are observed that in conjunction with other results reported here support the hypothesis that the basin acts as a pathway allowing discharge from subglacial lakes near the dome c ice divide to reach the coast via the totten glacier

using radar sounding data to identify the distribution and sources of subglacial water application to dome c east antarctica basal radar reflectivity is the most important measurement for the detection of subglacial water however dielectric loss in the overlying ice column complicates the determination of basal reflectivity dielectric attenuation is a function of ice temperature and impurity concentration temperature distribution is a function of climate history basal heat flow and vertical strain rate all of which can be partially inferred from the structure of dated internal layers using 11 dated layers isotope records from dome c east antarctica and a model of the spatial variation of geothermal flux we calculate the vertical strain rate and accumulation rate history allowing identification of areas where the basal melt rate exceeds 1 5 mm a 1 the accumulation rate history and vertical strain rates are then used as inputs for a transient temperature model the model outputs for the present day temperature distribution are then combined with depth dependent ionic concentrations to model dielectric loss and infer basal reflectivity the resulting reflection coefficients are consistent 5 db across a variety of subglacial water bodies we also identify a high reflectivity 15 db in concordia trench and along suspected subglacial water flow routes in vincennes basin highland areas tend to have highly variable reflection coefficients near 30 db consistent with an ice bedrock interface this combined model also identifies three areas of enhanced basal melting along concordia ridge concordia subglacial lake and vincennes basin melt at concordia subglacial lake exceeds 5 mm a 1 the inferred basal melt at these locations is not possible without enhanced geothermal flux we demonstrate how radar sounding data can provide both input and verification for a self consistent model of vertical strain vertical temperature distribution and meltwater distribution

doppler based discrimination of radar sounder target scattering properties a case study of subsurface water geometry in europa s ice shell abstract radar sounding reflectometry is a powerful method of inferring subsurface geophysical properties of planetary bodies the upcoming europa clipper mission will employ a dual frequency radar sounder to image and characterize europa s subglacial environment in terrestrial subglacial radar sounding characteristic doppler signatures of radar returns have been used to discriminate between different subglacial features peters et al 2007 we propose that analogous techniques can be used in conjunction with traditional reflectometry methods to discriminate between different proposed models of subglacial and englacial water distribution within europa s ice shell we introduce the synthetic aperture radar sar processing considerations that are relevant for such a technique propose a simple hypothesis test for doppler based subglacial target discrimination and briefly discuss the implications and applicability of such a technique for planetary radar sounding

radar attenuation and temperature within the greenland ice sheet 2015 american geophysical union all rights reserved the flow of ice is temperature dependent but direct measurements of englacial temperature are sparse the dielectric attenuation of radio waves through ice is also temperature dependent and radar sounding of ice sheets is sensitive to this attenuation here we estimate depth averaged radar attenuation rates within the greenland ice sheet from airborne radar sounding data and its associated radiostratigraphy using existing empirical relationships between temperature chemistry and radar attenuation we then infer the depth averaged englacial temperature the dated radiostratigraphy permits a correction for the confounding effect of spatially varying ice chemistry where radar transects intersect boreholes radar inferred temperature is consistently higher than that measured directly we attribute this discrepancy to the poorly recognized frequency dependence of the radar attenuation rate and correct for this effect empirically resulting in a robust relationship between radar inferred and borehole measured depth averaged temperature radar inferred englacial temperature is often lower than modern surface temperature and that of a steady state ice sheet model particularly in southern greenland this pattern suggests that past changes in surface boundary conditions temperature and accumulation rate affect the ice sheet s present temperature structure over a much larger area than previously recognized this radar inferred temperature structure provides a new constraint for thermomechanical models of the greenland ice sheet

antarctic subglacial conditions inferred from a hybrid ice sheet ice stream model abstract subglacial conditions of large polar ice sheets remain poorly understood despite recent advances in satellite observation major uncertainties related to basal conditions such as the temperature field are due to an insufficient knowledge of geothermal heat flow here a hybrid method is presented that combines numerical modeling of the ice sheet thermodynamics with a priori information using a simple assimilation technique additional data are essentially vertical temperature profiles measured in the ice sheet at selected spots as well as the distribution of subglacial lakes in this way geothermal heat flow datasets are improved to yield calculated temperatures in accord with observations in areas where information is available results of the sensitivity experiments show that 55 of the grounded part of the antarctic ice sheet is at pressure melting point calculated basal melt rates are approximately 65 gt year 1 which is 3 of the total surface accumulation although these sensitivity experiments exhibit small variations in basal melt rates the impact on the ice age of basal layers is quite important in areas that are characterized by relatively low melt rates this may lead to standard deviations up to 50 in a model run over a period of 10 6 years subglacial water flow is concentrated underneath the large outlet glaciers of antarctica the larger subglacial lakes are perched at the head of these subglacial water systems an exception however is the lake system of the recovery ice stream catchment that receives a substantial amount of subglacial melt water from the upstream catchment

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

identification and control of subglacial water networks under dome a antarctica 1 subglacial water in continental antarctica forms by melting of basal ice due to geothermal or frictional heating subglacial networks transport the water from melting areas and can facilitate sliding by the ice sheet over its bed subglacial water flow is driven mainly by gradients in overburden pressure and bed elevation we identify small median 850 m water bodies within the gamburtsev subglacial mountains in east antarctica organized into long 20 103 km coherent drainage networks using a dense 5 km grid of airborne radar data the individual water bodies are smaller on average than the water bodies contained in existing inventories of antarctic subglacial water and most are smaller than the mean ice thickness of 2 5 km reflecting a focusing of basal water by rugged topography the water system in the gamburtsev subglacial mountains reoccupies a system of alpine overdeepenings created by valley glaciers in the early growth phase of the east antarctic ice sheet the networks follow valley floors either uphill or downhill depending on the gradient of the ice sheet surface in cases where the networks follow valley floors uphill they terminate in or near plumes of freeze on ice indicating source to sink transport within the basal hydrologic system because the ice surface determines drainage direction within the bed constrained network the system is bed routed but surface directed along flow variability in the structure of the freeze on plumes suggests variability in the networks on long 10s of ka timescales possibly indicating changes in the basal thermal state

radar evidence of ponded subglacial water in greenland the thermal state at the bed of a large ice sheet is a critical boundary condition governing its future evolution radar surveys provide an opportunity for direct but remote observation of the ice sheet bed and therefore offer a means of constraining numerical ice sheet models at the ice bed interface here we have processed results of radar surveys of the greenland ice sheet undertaken by the program for arctic regional climate assessment parca between 1999 and 2003 to explore this opportunity we consider the robustness of the measurements in the context of uncertain dielectric losses in the ice sheet concluding that the observed radar signal characteristics reflect the character of the bed itself rather than that of uncertain englacial absorption however the identification of thaw is restricted to areas where subglacial water has sufficient depth to influence the radar reflection we derive a map of inferred areas of subglacial thaw and compare our results with other studies predicting regions with temperate bed we show that in many areas the radar inferences of ponded water lie within areas predicted to be thawed by modelling and radiostratigraphy there is clear disagreement in certain areas suggesting the presence of high geothermal flux anomalies

electromagnetic ice absorption rate at dome c antarctica radio echo sounding res is a radar technique widely employed in antarctica and greenland to define bedrock topography but over the last decade it has also played an important role in subglacial lake exploration and hydrogeological studies at the bedrock ice interface in recent studies bedrock characterization has been improved through analysis of radar power echoes to evaluate the electromagnetic em properties of the interface and allow the distinction between wet and dry interfaces the res received signal power depends on ice absorption and bedrock reflectivity which is closely linked to the specific physical condition of the bedrock in this paper an evaluation of em ice absorption was conducted starting from res measurements collected over subglacial lakes in antarctica the idea was to calculate ice absorption starting from the radar equation in the case of subglacial lakes where the em reflectivity value is considered a known constant these values were compared with those obtained from analysis of ice core dielectric profiles from epica ice core drilling data our analysis reveals that the ice absorption rate calculated from res measurements has an average value of 7 2dbkm 1 and it appears constant independent of the subglacial lake depth in different zones of the dome c area

recovery lakes east antarctica radar assessment of sub glacial water extent 1 a fast flowing tributary of recovery ice stream penetrates more than 500 km into the interior of east antarctica recent satellite based studies found surface features in the onset area of this tributary that indicate a significant subglacial hydraulic system including four large smooth basins the typical surface expression of large subglacial lakes as well as eleven smaller areas over which ice sheet surface elevations rapidly changed by discharge filling of subglacial water here we present the first ice penetrating radar evidence of subglacial conditions in this area we identified a distinct ice water interface only over a limited area within the boundaries of the investigated large smooth basins previously hypothesized to be water filled lakes the radar characteristics in some areas are similar to those of a drained lake indicating that parts of the bed are wet but not a typical lake we also find evidence for discrete water bodies outside of the lake boundaries the lines of evidence indicate that the northern most two recovery lakes have recently drained

evidence for variable grounding zone and shear margin basal conditions across thwaites glacier west antarctica abstractdefinitive interpretation of ice sheet basal conditions from radar sounding data beneath outlet glacier grounding zones and shear margins can be problematic due to poorly constrained and spatially variable englacial attenuation rates and losses from propagation through a rough ice surface to correct for spatially variable attenuation rates we developed a novel radar analysis approach that provided improved empirical attenuation correction by fitting linearly variable attenuation rates along radar sounding profiles from the ice sheet interior to the grounding zone we also corrected for ice surface propagation losses by using surface echo amplitude distributions to constrain the loss of coherent power for surface reflections and two way propagation through a rough ice surface by applying this approach to airborne radar sounding observations of the thwaites glacier catchment in west antarctica we produced relative reflectivity profiles which show grounding zone basal conditions varying across t

the relationship between sticky spots and radar reflectivity beneath an active west antarctic ice stream isolated areas of high basal drag or sticky spots are important and poorly understood features in the force balance and dynamics of west antarctic ice streams characterizing sticky spots formed by thin or drying subglacial till using ice penetrating radar is theoretically possible as high radar bed returned power brp is commonly related to an abundance of free water at the ice bed interface provided losses from englacial attenuation can be estimated in this study we use airborne radar data collected over evans ice stream to extract brp profiles and test the sensitivity of brp to the adopted englacial attenuation correction we analyse 11 20 km profiles in four fast flow areas where sticky spots have been inferred to exist on the basis of model and surface data inversions in the majority of profiles we note that the increase in basal drag is accompanied by a decrease in brp and suggest that this is evidence both for the presence of a sticky spot in those locations and that local variations in subglacial hydrology are responsible for their existence a comparison is made between empirical and numerical modelling approaches for deriving englacial attenuation and our findings generally support previous studies that advocate a modelling approach

inference of accumulation rate patterns from deep layers in glaciers and ice sheets the spatial pattern of accumulation rate can be inferred from internal layers in glaciers and ice sheets non dimensional analysis determines where finite strain can be neglected shallow layer approximation or approximated with a local one dimensional flow model local layer approxima tion and where gradients in strain rate along particle paths must be included deep layers we develop a general geophysical inverse procedure to infer the spatial pattern of accumulation rate along a steady state flowband using measured topography of the ice sheet surface bed and a deep layer a variety of thermomechanical ice flow models can be used in the forward problem to calculate surface topography and ice velocity which are used to calculate particle paths and internal layer shapes an objective tolerance criterion prevents over fitting the data after making site specific simplifications in the thermomechanical flow algorithm we find the accumulation rate along a flowband through taylor mouth a flank site on taylor dome antarctica using a layer at approximately 100 m depth or 20 of the ice thickness accumulation rate correlates with ice surface curvature at this site gradients along flow paths critically impact inference of both the accumulation pattern and the depth age relation in a 100 m core

differentiating between gravitational and tectonic faults by means of geomorphological mapping trenching and geophysical surveys the case of the zenzano fault iberian chain n spain abstract the zenzano fault scarp has been developed on the top of a 475 m high erosional escarpment underlain by an upper brittle sequence and a lower halite bearing evaporitic formation the 1850 m long fault with a maximum displacement of 45 m and parallel to the escarpment has offset the back of a dip slope generating an antislope scarp with prominent triangular facets up to 38 m high and obstructing consequent transverse drainages defeated streams both gravitational and tectonic interpretations were previously proposed for this active surface rupture the fault scarp has been investigated by detailed geological and geomorphological mapping geophysical surveys gpr and ert and two trenches excavated across the fault in disrupted drainages the geophysical profiles clearly imaged the fault helping to site the trenches and roughly estimate the thickness of the deposits accumulated in the downthrown block although the maximum displacement to length ratio dmax l of the fault fall within the range of tectonic faults of similar length according to the world database the geological and geomorphological context together with several parameters reveal that this structure corresponds to a gravitational failure 1 anomalously high vertical displacement per event 1 4 m 2 high apparent vertical slip rate 0 6 mm year and 3 short recurrence interval of faulting events the negligible horizontal displacement component on the dip slip fault as well as the presence of very large caprock collapse sinkholes and saline springs strongly suggest that interstratal karstification of the halite bearing evaporites has played a primary role in the development of the gravitational failure the formation of this morphostructure has been favoured by pre existing tectonic faults erosional unloading related to the removal of an overburden ca 800 m thick and enhanced groundwater recharge due to the blockage of the drainage this work illustrates that the proposed multidisciplinary approach allows the identification of features and calculation of parameters that may help to elucidate whether an active failure has a gravitational or a tectonic origin

sinkholes collapse structures and large landslides in an active salt dome submerged by a reservoir the unique case of the ambal ridge in the karun river zagros mountains iran abstract ambal ridge covering 4 km2 is a salt pillow of gachsaran formation with significant salt exposures in direct contact with the karun river zagros mountains the highly cavernous salt dome is currently being flooded by the gotvand reservoir second largest in iran geomorphic evidence including the sharp deflection of the karun river and defeated streams indicate that ambal is an active halokinetic structure probably driven by erosional unloading around 30 of the salt dome is affected by large landslides up to ca 50 106 m3 in volume slope oversteepening related to fluvial erosion and halokinetic rise seems to be the main controlling factor a total of 693 sinkholes have been inventoried 170 sinkholes km2 for which a scaling relationship has been produced the depressions occur preferentially along a belt with a high degree of clustering this spatial distribution is controlled by the proximity to the river slope gradient and halite content in the bedrock a large compound depression whose bottom lies below the normal maximum level of the reservoir will likely be flooded by water table rise forming a lake the impoundment of the reservoir has induced peculiar collapse structures 220 280 m across expressed by systems of arcuate fissures and scarps rapid subsurface salt dissolution is expected to generate and reactivate a large number of sinkholes and may reactivate landslides with a significant vertical component due to lack of basal support

the distribution of basal water between antarctic subglacial lakes from radar sounding antarctica s subglacial lakes have two end member geophysical expressions as hydraulically flat radar reflective regions highlighted in ice surface topography and radar sounding profiles definit

hydration diapirism a climate related initiation of evaporite mounds in two continental neogene basins of central spain abstrlld wllile rrany halokinetic mode conslder rhat a thlck overburdea is necessary bcfore jiapni i l develops other ludie how ttnu all migllt hegin ti llltwe soca flef x curs with no lverburden on iop of lhem the iritum rus suggcsung thc eaely hulging of lhe evaporitcs f the l ow r unil forming 3 p t currlief oerorc and during the deposirion of thc lntermediate unit rely on certam cuaructer isl cs ollhe palaeokarcts developcd on the roo uf thc wwer unir as wehas in the nanre dispo ilion thiekne s and yn edll1eljl iry prccesses of ihe intermediale 1jnil id f il11 tell ion gashcs and collapse structures develop withln lhe ev r ritc mound and these dcformatiol teanuv mighl be related 1 lhelt upliit histor thi agent of iniriarton suggested in this papa for the evapmite mounds hydraticn diapirism or a volume increase related to the hydrauon of anhy mile to gyp ulli ha not been l reviou ly recognize j in lhe literature a series 01 sedimentologi cal and palaeontclogical argumcnts flll1h ili g st hat thi hemi a rc cu was trii clell by a chrnatic changc at the boundary betwcen the luwer and the inrc rnediate units

gypsum scarps and asymmetric fluvial valleys in evaporitic terrains the role of river migration landslides karstification and lithology ebro river ne spain abstract most of the spanish fluvial systems excavated in tertiary evaporitic gypsum formations show asymmetric valleys characterized by a stepped sequence of fluvial terraces on one valley flank and kilometric long and 100 m high prominent river scarp on the opposite side of the valley scarp undermining by the continuous preferential lateral migration of the river channel toward the valley margin leads to vertical to overhanging unstable slopes affected by a large number of slope failures that become the main geological hazard for villages located at the toe of the scarps detailed mapping of the gypsum scarps along the ebro and huerva rivers gypsum scarps demonstrates that landslides and lateral spreading processes are predominant when claystones crop out at the base of the scarp while rockfalls and topples become the dominant movement in those reaches where the rock mass is mainly constituted by evaporites the dissolution of gypsum nodules seasonal swelling and shrinking and dispersion processes contribute to a decrease in the mechanical strength of claystones the existence of dissolution enlarged joints sinkholes and severely damaged buildings at the toe of the scarp from karstic subsidence demonstrates that the interstratal karstification of evaporites becomes a triggering factor in the instability of the rock mass the genesis of asymmetric valleys and river gypsum scarps in the study area seem to be caused by the random migration of the river channel in the absence of lateral tilting related to tectonics or dissolution induced subsidence once the scarp is developed its preservation depends on the physicochemical properties of the substratum the ratio between bedrock erosion and river incision rates and climatic conditions that favour runoff erosion versus dissolution

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

comment on relative sea level change in western istria croatia during the last millennium by sanja faivre eric fouache matthieu ghilardi fabrizio antonioli stefano furlani and vladimir kovacic quaternary international 232 2012

subglacial hydrological connectivity within the byrd glacier catchment east antarctica ice cloud and land elevation satellite icesat repeat track laser altimetry has identified 17 sites within the byrd glacier catchment east antarctica where rapid ice surface height changes have occurred which have been interpreted as evidence for active subglacial lakes here we present evidence from a new radio echo sounding res survey at 11 of these locations to understand the bed conditions associated with the proposed hydrological activity at none of the sites examined did we find evidence in support of substantial pooled basal water in the majority of cases along track res bed reflection amplitudes either side of the locations of surface height change are indistinguishable from those within the features these results indicate that in most cases hypothesized active lakes are not discrete radar targets and are therefore much smaller than the areas of surface height change in addition we have identified three new relatively large subglacial lakes upstream of the region where most active subglacial lakes are found in an area where the hydraulic gradient is significantly lower our results suggest that substantial and long lasting basal water storage in the byrd glacier catchment occurs only under low hydraulic gradients while coast proximal sites of hydraulic activity likely involve small or temporary accumulations of basal water

radar detection of accreted ice over lake vostok antarctica abstract airborne radar data collected over lake vostok east antarctica show a reflection close to the vostok ice core below the depth of the meteoric accreted ice boundary maib and near the boundary between impurity rich 1 and impurity poor 2 accreted ice a12b the spatial pattern of detection of this reflection is aligned with ice flow over the southern half of the lake ice core data from the accreted ice show large variations in the number density of inclusions soluble chloride ion concentration cl charge balance derived acidity h cb and crystal orientation fabric here we investigate the existing hypothesis that this reflection is due to the maib and seek possible reflection causes that explain the radar derived reflectivity 51 8 db the likely maximum reflectivity due to an insoluble mineral inclusion contrast is 125 db if none of the cl or h ions are trapped within inclusions then ice conductivity models predict a reflectivity of 50 db near the maib and 58 db near the a12b both primarily due to large h cb changes if some of those ions are trapped within inclusions then those values are smaller fabric contrasts at either boundary can produce significant reflectivities 62 db these results indicate that the mineral inclusion contrast near the maib does not cause the observed reflection because of uncertainty in the in situ location of the soluble impurities and because impurity rich accreted ice 1 is believed to only originate in the small bay in the southwestern corner of the lake yet the reflection is observed over a much wider area we argue that a fabric contrast at the a12b is the probable reflection cause both predicted spatial variations in accretion rates near the upstream shoreline or lower ice temperatures along flowbands that originate at bedrock ridges near the upstream shoreline may explain why the reflection is not detected everywhere that accretion is predicted

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

assessment of electromagnetic absorption of ice from ice core measurements ice core drillings have been performed in various zones in antarctica and greenland to obtain climatological information study ice properties or analyze air and dust encapsulated in the ice during the quaternary period during these procedures a set of measurements to characterize the ice and to evaluate its physical and chemical properties are usually performed in situ in particular using known temperature and dielectric profiles dep measurements it is possible to evaluate the ice electromagnetic power absorption profile valid at the drilling site in the last decades bedrock characterization through radio echo sounding surveys has been improved by the analysis of the power of radar echoes this way analysis of the electromagnetic properties of bedrock interfaces makes it possible to assess the physical characteristics and to distinguish between wet and dry conditions power variation of the received echoes also depends on ice absorption and on bedrock reflectivity due to specific physical conditions of the ice in this paper the propagation of electromagnetic waves through the ice sheet is examined and in particular a new method for establishing the electromagnetic absorption profile for ice from core drilling measurements is proposed and discussed variation in the ice absorption is deduced starting from the analysis of ice core data from the european project for ice coring in antarctica epica at the concordia station antarctica and from the greenland ice core project grip site greenland this direct method of measurement is proposed with the aim of defining common characteristics of the ice absorption rate that are valid both in antarctica and in greenland

deformation associated with gypsum karst in the salt river escarpment northeastern alberta an escarpment formed by the carbonate keg river and evaporitic chinchaga formations of middle devonian age extends southwards from the slave river 30 km west of fort smith northwest territories newly described folds in the bedrock are due to hydration of the anhydrite to gypsum in the chinchaga formation underlying the escarpment local groundwater flow has also dissolved subsurface cavities in the chinchaga formation as these openings grew the carbonates of the keg river formation subsided along stepped normal faults depressions within the fault blocks occurred where subsidence was concentrated along joints