seismic and gpr surveys of mullins glacier mcmurdo dry valleys antarctica ice thickness internal structure and implications for surface ridge formation we present results from ground penetrating radar gpr and seismic surveys for the mullins valley debris covered glacier mullins glacier antarctica that yield local ice thickness estimates of 80 110 m in upper mullins valley and 150 m in upper beacon valley englacial debris in upper mullins glacier occurs as scattered cobbles and as discrete layers one extensive englacial debris layer which appears as a coherent reflector dipping 40 458 up valley intersects the ground surface within an 8m high ice cored ridge the largest of several ridges that mark the glacier surface field excavations reveal that this englacial interface consists of multiple debris bands that can be directly correlated with ridge microtopography englacial debris layers most probably originate as concentrated rockfall in ice accumulation zones and or as surface lags that form as dirty ice sublimes during periods of negative mass balance a similar pattern of surface ridges on friedman glacier 2 5 km west of mullins glacier suggests regional environmental changes are involved in ridge formation these observations carry implications for evaluating debris entrainment and surface ridge formation mechanisms in cold based debris covered glaciers and provide a glaciological framework for evaluating and interpreting paleoclimate records from mullins glacier

layered and laterally constrained 2d inversion of resistivity data in a sedimentary environment quasi layered models often can represent the actual geology more accurately than smooth minimum structure models we present a 2d inversion scheme with lateral constraints and sharp boundaries lci for continuous resistivity data all data and models are inverted as one system producing layered solutions with laterally smooth transitions the models are regularized through lateral constraints that tie interface depths or thicknesses and resistivities of adjacent layers a priori information used to resolve ambiguities and to add for example geological information can be added at any point of the profile and migrates through the lateral constraints to parameters at adjacent sites similarly information from areas with well resolved parameters migrates through the constraints to help resolve areas with poorly constrained parameters the estimated model is complemented by a full sensitivity analysis of the model parameters supporting quantitative evaluation of the inversion result a simple synthetic model proves the need for a quasilayered 2d inversion when compared with a traditional 2d minimum structure inversion a 2d minimum structure inversion produces models with spatially smooth resistivity transitions making identification of layer boundaries difficult a continuous vertical electrical sounding field example from sweden with a depression in the depth to bedrock supports the conclusions drawn from the synthetic example a till layer on top of the bedrock hidden in the traditional inversion result is identified using the 2d lci scheme furthermore the depth to the bedrock surface is easily identified for most of the profile with the 2d lci model which is not the case with the model from the traditional minimumstructure inversion

the formation of ca cl rich groundwaters in the dry valleys of antarctica field measurements and modeling of reactive transport abstract ca cl rich brines have been found in shallow subsurface flows groundwater systems lakes and ponds throughout the dry valleys of antarctica the apparent abundance of ca cl rich waters near the surface is unusual compared to global surface water compositions and a number of theories have been proposed to explain the genesis of these brines we show that an ice cemented soil developing on fluvial sediment in taylor valley also contains ca cl rich brine the distribution of soluble ions exchangeable cations and stable isotopes down to 2 1 m depth in the soil suggests that cacl2 was formed by cation exchange reactions during downward reactive transport of na cl rich brine from the soil surface to explore the implications of exchange reactions for the formation of ca cl rich brine ca na and ca mg exchange properties were measured in 1 mm 0 1 m and 4 75 m solutions low temperature reactions and brine transport were modeled in phreeqc by incorporating frezchem pitzer parameters and solubility products into phreeqc modeling shows that by freezing soils in equilibrium with dry valley surface waters a strong ca mg enrichment of the soil solution is caused by the exchange of aqueous na with exchangeable ca2 and mg2 ca mg enrichment also occurs as na cl rich brine from the soil surface advects into ice cemented soil by modeling this process in the borehole soil trends in ion distributions with depth can be predicted brine compositions from cation exchange reactions are consistent with ca cl rich brine compositions in the dry valleys although additional water rock interaction is proposed to account for the low mg2 concentrations in don juan pond furthermore the amount of cacl2 that can be produced by exchange reactions is consistent with estimated amounts of cacl2 in groundwaters beneath don juan pond this suggests that cation exchange reactions can explain the ca cl rich composition of the enigmatic don juan pond and other brines in the dry valleys

dynamics of stick slip motion whillans ice stream antarctica article i nfo the stick slip motion and associated seismic emissions of whillans ice stream wis west antarctica are two of the many recent observations of unexpected ice sheet behavior that are challenging traditional models of rapid glacier motion here we find that the wis slip events repeatedly nucleate from a sticky spot located in the middle of the ice stream acting similar to an asperity in traditional models of earthquake physics this region shows less motion than surrounding areas during the inter slip periods thus concentrating stress and producing a pulse of seismic energy at the onset of slip the propagating rupture breaks through an additional asperity in the northern part of the ice stream producing another pulse of seismic energy 6 12 min after initiation both asperities are regions of higher hydraulic potential than surrounding regions suggesting they may have greater bed friction due to reduced water lubrication tidal pacing of the stress accumulation combined with fault healing controls the applied stress at failure with higher stress giving faster propagation of the rupture front and higher slip velocities these differences are reflected in the timing of the teleseismic arrivals our results highlight both the great sensitivity of large ice streams to small changes in external forcing and the importance of limited regions of the subglacial bed in controlling their motion as well as providing insights to the mechanics of repeating earthquakes

soluble salt accumulations in taylor valley antarctica implications for paleolakes and ross sea ice sheet dynamics 1 soluble salt accumulations in soils of taylor valley antarctica provide a history of paleolakes and the advance of the ross sea ice sheet rsis in western taylor valley soluble salt accumulations are relatively high and are composed primarily of na ca2 cl and so42 in eastern taylor valley soluble salt accumulations are much lower and are composed primarily of na and hco3 na hco3 rich compositions in eastern taylor valley are formed through leaching calcite dissolution and cation exchange reactions and appear to influence the chemistry of nearby streams and lakes the data presented here support hypotheses that a lobe of the rsis expanded into eastern taylor valley and dammed proglacial paleolakes however in contrast to previous studies our findings indicate that the rsis advanced deeper into taylor valley and that paleolakes were less extensive by comparing soluble salt distributions across taylor valley we conclude that a lobe of the rsis filled all of eastern taylor valley and dammed paleolakes in western taylor valley up to approximately 300 m elevation following ice retreat smaller paleolakes formed in both western and eastern taylor valley up to about 120 m elevation with prominent still stands controlled by the elevation of major valley thresholds at higher elevations soluble salt accumulations are consistent with older soils that have not been affected by the most recent rsis advance

water tracks and permafrost in taylor valley antarctica extensive and shallow groundwater connectivity in a cold desert ecosystem water tracks are zones of high soil moisture that route water downslope over the ice table in polar environments we present physical hydrological and geochemical evidence collected in taylor valley mcmurdo dry valleys antarctica which suggests that previously unexplored water tracks are a significant component of this cold desert land system and constitute the major flow path in a cryptic hydrological system geological geochemical and hydrological analyses show that the water tracks are generated by a combination of infiltration from melting snowpacks melting of pore ice at the ice table beneath the water tracks and melting of buried segregation ice formed during winter freezing the water tracks are enriched in solutes derived from chemical weathering of sediments as well as from dissolution of soil salts the water tracks empty into ice covered lakes such as lake hoare resulting in the interfingering of shallow groundwater solutions and glacier derived stream water adding complexity to the geochemical profile approximately four orders of magnitude less water is delivered to lake hoare by any given water track than is delivered by surface runoff from stream flow however the solute delivery to lake hoare by water tracks equals or may exceed the mass of solutes delivered from stream flow making water tracks significant geochemical pathways additionally solute transport is two orders of magnitude faster in water tracks than in adjacent dry or damp soil making water tracks salt superhighways in the antarctic cold desert accordingly water tracks represent a new geological pathway that distributes water energy and nutrients in antarctic dry valley cold desert soil ecosystems providing hydrological and geochemical connectivity at the hillslope scale

helicopter borne transient electromagnetics in high latitude environments an application in the mcmurdo dry valleys antarctica abstractthe mcmurdo dry valleys are a polar desert in coastal antarctica where glaciers permafrost ice covered lakes and ephemeral summer streams coexist liquid water is found at the surface only in lakes and in the temporary streams that feed them past geophysical exploration has yielded ambiguous results regarding the presence of subsurface water in 2011 we used a helicopter borne time domain electromagnetic tdem sensor to map resistivity in the subsurface across the dry valleys the airborne electromagnetic aem method excels at finding subsurface liquid water in polar deserts where water remains liquid under cold conditions if it is sufficiently saline and therefore electrically conductive over the course of 26 h of helicopter time we covered large portions of the dry valleys and vastly increased our geophysical understanding of the subsurface particularly with respect to water our data show extensive subsurface low resistivity layers approximately 150 250 m below the surface and ben

dynamics and gpr stratigraphy of a polar rock glacier on james ross island antarctic peninsula we describe field measurements ground penetrating radar gpr geodetic survey and ice core drilling to provide new information on the movement mechanism and internal structure of a polar rock glacier on james ross island antarctic peninsula we collected gpr data along longitudinal and transverse profiles the longitudinal gpr profiles identify inter bedded debris rich layers that dip up glacier similar to the thrust structures in the compression zone of a valley glacier the transverse gpr profiles indicate a syncline structure inclined towards the central part of the rock glacier resembling the transverse foliation of a valley glacier the stratigraphy of two boreholes shows that the rock glacier consists primarily of bubbly ice with thin debris rich layers an internal structure similar to the nested spoons structure common in the interior of valley glaciers these results indicate that the glacier motion is controlled by shear movement common in valley glaciers the geodetic survey confirms that flow velocities decrease towards the lower part of the rock glacier such heterogeneous movement causes longitudinal compression and forms thrusts which then create the debris rich layer by uplifting basal ice and debris pushing of the upstream ice against the downstream ice bends the surface layers forming transverse ridges on the rock glacier surface

subglacial methanogenesis a potential climatic amplifier 1 subglacial environments are a previously neglected component of the earth s global carbon cycle a reflection of the view held until recently that they are dominated by abiotic and oxic conditions here we provide a realistic assessment of the theory that the basal regions of the ice sheets that formed over north america and europe during glaciations were host to significant populations of anaerobic microorganisms including methanogens able to metabolize organic carbon sequestered during interglacials and overridden during quaternary glacials in doing so we review the current evidence for subglacial methane release during deglaciation estimate the size of the subglacial reservoir of organic carbon soc and assess the amount of soc available to subglacial microbes and the likely pathways and rates of carbon turnover we then discuss the fate of subglacial methane and the potential impact of its release on atmospheric methane concentrations we calculate that the soc equates to 418 610 pg c and includes carbon from terrestrial soils vegetation peatlands lake and marine sediments the soc that is potentially available for microbial conversion to methane is smaller than this estimate due to 1 glacial erosion 2 accumulation of recalcitrant organic carbon compounds over time 3 conversion to carbon dioxide by aerobic anaerobic respiration and 4 incomplete conversion of labile organic matter to methane we estimate that the total soc available for conversion to methane is 63 pg c our estimates of methane production potentials span a wide range because of the current uncertainty surrounding subglacial metabolic rates we believe however that there is a strong likelihood that subglacial microbes could convert 63 pg of soc to methane during a glacial cycle if this were the case release of this methane from the ice sheet margins during retreat would need to be episodic in order to significantly impact atmospheric methane concentrations our findings suggest that it may well be important to consider subglacial environments as active components of the earth s carbon cycle conclusive determination of the potential impact of subglacial methane production on atmospheric methane concentrations during deglaciation however awaits more precise determination of the ability of subglacial microbes to degrade organic carbon components and their associated rates of metabolism under in situ conditions

subsurface imaging reveals a confined aquifer beneath an ice sealed antarctic lake liquid water oases are rare under extreme cold desert conditions found in the antarctic mcmurdo dry valleys here we report geophysical results that indicate that lake vida one of the largest lakes in the region is nearly frozen and underlain by widespread cryoconcentrated brine a ground penetrating radar survey profiled 20 m into lake ice and facilitated bathymetric mapping of the upper lake basin an airborne transient electromagnetic survey revealed a low resistivity zone 30 100 m beneath the lake surface based on previous knowledge of brine chemistry and local geology we interpret this zone to be a confined aquifer situated in sediments with a porosity of 23 42 discovery of this aquifer suggests that subsurface liquid water may be more pervasive in regions of continuous permafrost than previously thought and may represent an extensive habitat for microbial populations

p and s deep velocity structure of the hellenic area obtained by robust nonlinear inversion of travel times we derived a new model for the p and s velocity structure of the lithosphere in the subduction area of the hellenic arc and the aegean sea from the inversion of travel times of local events the inversion technique applied is nonlinear since three dimensional ray tracing is incorporated at the same time an appropriate preconditioning of the final linearized system is used in order to reduce ray density effects on the results the p s coherency is controlled by an additional damping of the vp vs ratio the study focuses mainly on the structure of the hellenic subduction in the southern aegean region interesting features and details of the subducted slab can be recognized in the final tomographic images on the western part the shallower section of the slab is dipping at a very small angle 10 this changes to approximately 25 for the deeper part of the subduction resulting in a prominent kink at a depth of about 70 km which is in accordance with the general characteristics of the associated benioff zone the smaller eastern part of the slab is not resolved with the same detail but a clearly steeper subduction zone can be recognized moreover detailed information about the crustal thickness variations are inferred from the velocity structure and correlate very well with the existing results from refraction experiments finally the results confirm the recent suggestion concerning the existence of a low velocity crustal layer at shallow depths 10 15 km in the accretionary prism of the alpine belt

a semiautomated multilayer picking algorithm for ice sheet radar echograms applied to ground based near surface data snow accumulation over an ice sheet is the sole mass input making it a primary measurement for understanding the past present and future mass balance near surface frequency modulated continuous wave fmcw radars image isochronous firn layers recording accumulation histories the semiautomated multilayer picking algorithm sampa was designed and developed to trace annual accumulation layers in polar firn from both airborne and ground based radars the sampa algorithm is based on the radon transform rt computed by blocks and angular orientations over a radar echogram for each echogram s block the rt maps firn segmented layer features into peaks which are picked using amplitude and width threshold parameters of peaks a backward rt is then computed for each corresponding block mapping the peaks back into picked segmented layers the segmented layers are then connected and smoothed to achieve a final layer pick across the echogram once input parameters are trained sampa operates autonomously and can process hundreds of kilometers of radar data picking more than 40 layers sampa final pick results and layer numbering still require a cursory manual adjustment to correct noncontinuous picks which are likely not annual and to correct for inconsistency in layer numbering despite the manual effort to train and check sampa results it is an efficient tool for picking multiple accumulation layers in polar firn reducing time over manual digitizing efforts the trackability of good detected layers is greater than 90

a microbial ecosystem beneath the west antarctic ice sheet liquid water has been known to occur beneath the antarctic ice sheet for more than 40 years but only recently have these subglacial aqueous environments been recognized as microbial ecosystems that may influence biogeochemical transformations on a global scale here we present the first geomicrobiological description of water and surficial sediments obtained from direct sampling of a subglacial antarctic lake subglacial lake whillans slw lies beneath approximately 800 m of ice on the lower portion of the whillans ice stream wis in west antarctica and is part of an extensive and evolving subglacial drainage network the water column of slw contained metabolically active microorganisms and was derived primarily from glacial ice melt with solute sources from lithogenic weathering and a minor seawater component heterotrophic and autotrophic production data together with small subunit ribosomal rna gene sequencing and biogeochemical data indicate that slw is a chemosynthetically driven ecosystem inhabited by a diverse assemblage of bacteria and archaea our results confirm that aquatic environments beneath the antarctic ice sheet support viable microbial ecosystems corroborating previous reports suggesting that they contain globally relevant pools of carbon and microbes that can mobilize elements from the lithosphere and influence southern ocean geochemical and biological systems

the role of seawater freezing in the formation of subsurface brines abstract several mechanisms evaporation water rock interaction ultra filtration have been suggested to explain the evolution of ubiquitous ca chloride subsurface brines in the present paper the freezing of seawater in polar regions and in even wider areas during glacial periods is proposed as an additional possible path of brine formation four detailed seawater freezing experiments to 14 c resulting in a concentration factor of about 5 were carried out and na k ca mg sr cl so4 and br were analysed in the residual brines and in the ice br and sr whose behavior during the freezing of seawater is reported here for the first time show a conservative behavior throughout the studied temperature range our data and earlier literature show that the high salinities which are common in subsurface brines 300 g l may be obtained by the removal of h2o as ice in the primary glacial environment the decrease in the na cl ratio is caused by the crystallization of mirabilite na2so4 10h2o supplemented by hydrohalite nacl 2h2o sulfate is removed both in mirabilite and by bacterial reduction the brine then migrates to the subsurface heats up under the local geothermal gradient and interacts with the adjacent rocks at this stage it may be diluted by meteoric waters its mg ca ratio decreases dolomitization and chloritization the so 4 cl ratio varies according to the local gypsum anhydrite equilibrium conditions and the ca so 4 hco 3 ratio increases as a result of dolomitization or chloritization the interaction with rocks in the subsurface may affect both the original 87 sr 86 sr and the 18 o 16 o ratios of the brine although several of the processes which lead to the formation of ca chloride brines are common for both the evaporative and the freezing models the na br cl relationship in a given brine can be used to discriminate between the two modes of brine evolution several subsurface brines from the canadian shield and one brine from finland are used as examples of the seawater freezing model and an explanation is proposed for the necessary mass production of brines in glacial environments

the hellenic subduction zone a tomographic image and its geodynamic implications new tomographic images of the hellenic subduction zone demonstrate slab penetration in the aegean upper mantle to depths of at least 600 km beneath greece the lower part of the slab appears to be detached at a depth of about 200 km whereas it still seems to be unruptured beneath the southern aegean schematically we derive minimum time estimates for the duration of the hellenic subduction zone that range from 26 to 40 ma this is considerably longer than earlier estimates which vary between 5 and about 13 ma

seismological and gps evidence for the aegean anatolia interaction seismological and gps data are used in order to examine the seismotectonic setting and the plate motions in the eastern mediterranean the main objective is to reevaluate the recently proposed idea of a single anatolia aegean microplate which performs a rigid body rotation about an euler pole located in northern africa both data sets show a more complicated setting south of the north anatolia fault naf the plate motion of anatolia relative to europe can be adequately described by a simple rigid body rotation with a dominant east west component and an average velocity of 22mm yr however both types of data show a significant increase of the deformation velocities in the aegean area 30 35mm yr and exhibit a much larger n s component up to 20mm yr than the one predicted from rigid body rotation this deformation pattern is more consistent with models which consider the aegean as a separate microplate

dynamics and mass balance of taylor glacier antarctica 2 force balance and longitudinal coupling 1 taylor glacier antarctica exemplifies an ice sheet outlet that flows through a region of rugged topography and dry climate in contrast to other well studied outlets taylor glacier moves very slowly despite a thickness of order 1 km and driving stresses averaging 1 5 bars here we analyze new measurements of glacier geometry and surface velocity to elucidate flow dynamics of taylor glacier force balance and basal temperatures are calculated at six locations along the glacier s length using an algorithm developed for this study the effects of stress gradient coupling on longitudinal flow variations are also examined we ask whether kamb and echelmeyer s 1986 linearized theory adequately describes the observed response of flow to large amplitude variations in driving stress the force balance calculations indicate that no basal motion is needed to explain the observed flow of taylor glacier inferred basal temperatures are within a few degrees of the melting point in regions of kilometer thick ice and well below the melting point elsewhere deformation of subfreezing ice largely controls the flow of taylor glacier basal drags are mostly in the range 0 9 to 1 2 bars and lateral drags are in the range 0 2 to 0 5 bar stress gradient coupling strongly reduces the variability of velocities along the glacier the velocity variations can be described as the convolution of a forcing function with a spatial filter as kamb and echelmeyer suggested but the form of the forcing function differs from the theoretical relation derived for small amplitude perturbations the power on driving stress is one not three

peculiarities of surface wave propagation across central eurasia abstract the recent installation of six broadband digital iris ida seismic stations in the ussr has provided new opportunities for studying surface wave propagation across eurasia group velocities of fundamental rayleigh and love modes between epicenters and these stations were determined for 35 events that occurred since april 1989 to the middle of july 1990 near eurasia differential phase velocities were found for the same arrivals along paths between several pairs of stations group and phase velocities were obtained in the period range from 15 to 300 sec frequency time polarization analysis was used for studying polarization properties of surface waves in some cases significant anomalies in the particle motion for periods up to 100 sec were observed they are attributed to surface wave refraction and scattering due to lateral inhomogeneities at the boundaries and inside the eurasia continent

tomography of the crust and upper mantle in southeast europe compressional velocity structure of the crust and the upper mantle in southeastern europe broader aegean area is studied by inverting residuals of the first p arrivals from earthquakes in this region 16 e 31 e 34 n 43 n the data used are from regional events recorded by the permanent network of stations during the period 1971 1987 enriched with data from experiments with portable seismographs in four regions of this broad area this study confirms the strong variations of crustal thickness in this area as well as the subduction of the eastern mediterranean lithosphere under the southern aegean and gives further detailed information on the crustal and upper mantle structure of the area important new information is the existence of a low velocity crustal layer in western greece and albania and that the velocity anomaly in the mantle under the southern aegean extends much farther and deeper to the northeast than the benioff zone of the intermediate depth earthquakes indicates furthermore evidence is presented about the possible existence of older subduction in the northern aegean and about the influence of the tectonic regime on the velocity field

rayleigh wave group velocity tomography in the aegean area data from a large scale experiment which took place in greece during the period january july 1997 have been used to investigate the structure of the aegean area using surface waves during this experiment 30 seismic broadband instruments were deployed throughout the whole greek area additional data during the period 1996 2000 from other temporary networks have been included in the dataset one hundred eighty five events with magnitudes 4 0vmwv5 5 recorded by these stations have been collected and processed the individual dispersion curves of the group velocity of rayleigh waves for each source station path have been calculated producing more than 700 paths covering the studied region these curves have been used to determine rayleigh group velocity maps using a 2d tomography method on the basis of a regionalization of the dispersion measurements local averaged dispersion curves have been obtained and non linearly inverted to obtain models of shear wave velocity versus depth since the dispersion curves in the period range 5 svtv30 s are mostly affected by the crustal structure the model velocities are estimated down to a depth of approximately 35 45 km the results from the non linear hedhehog inversion as applied to a few local dispersion curves show a crustal thickness of approximately 32 km for the northern aegean sea and a relatively thin crust of approximately 22 24 km for the southern aegean sea d 2002 elsevier science b v all rights reserved

shear wave anisotropy in the upper mantle beneath the aegean related to internal deformation seismic anisotropy deduced from sks splitting measured at 25 stations installed in the aegean does not show a homogeneous pattern it is not restricted to the north anatolian fault but is distributed over a region several hundreds kilometers wide little anisotropy is observed in continental greece or along the hellenic arc however significant anisotropy is observed in the north aegean sea large values of delay times suggest that anisotropy is due to a long path within the upper mantle and to strong intrinsic anisotropy our results both in fast polarization directions and in values of delay time do not support the idea that anisotropy is associated with inherited tectonic fabric nor are they consistent with the present day aegean motion relative to an absolute frame in contrast the direction of fast polarization and the magnitude of delay times correlate well with the present day strain rate observed at the surface deduced from both geodetic measurements and seismicity this anisotropy is not horizontally restricted to major surface faults but is spread over a wide region

halogen geochemistry of the mcmurdo dry valleys lakes antarctica clues to the origin of solutes and lake evolution abstract we have determined the halogen and boron concentrations in the ice covered lakes of taylor valley antarctica to better establish the sources of salts and evolutionary histories of these unusual water bodies in addition we report on a series of 129 i measurements that were compared with previous 36 cl data that also help constrain the source of solutes and histories of the lakes the new data when put into context of previous work on these systems over the past forty years allow us to make the following conclusions the primary source of solutes to lake hoare the youngest of the lakes is the dissolution of marine aerosols and aeolian salts and the chemical weathering of dust on the glaciers the geochemistry of lake fryxell the brackish water lake is primarily dominated by the diffusion from a halite saturated brine at the sediment water interface and the recent infilling of the lake by glacier meltwater these waters have chemical weathering and marine aerosols components lake bonney has two distinct lobes whose hypersaline hypolimnia have different chemistries both of the lobes are remnants of ancient marine waters that have been modified by the input of weathering products this lake has also been modified by periods of cryogenic concentration when solutes have been lost via mineral precipitation thus the geochemistry of lake bonney owes its unusual geochemistry in part to variations in the climate in the taylor valley over at least the past 300kyr the 129 i data from the taylor valley are similar to those from fracture fluids in crystalline rocks from the northern hemisphere

evidence for subglacial ponding across taylor glacier dry valleys antarctica hubbard alun lawson w anderson b hubbard b p 2004 evidence for subglacial ponding across taylor glacier dry valleys antarctica annals of glaciology 39 1 79 84 rae2008

cold based debris covered glaciers evaluating their potential as climate archives through studies of ground penetrating radar and surface morphology we describe the morphology and internal structure of mullins and friedman glaciers two cold based debris covered alpine glaciers that occur in neighboring valleys in the mcmurdo dry valleys antarctica both glaciers are overlain by a single dry supraglacial debris layer 8 75 cm thick each mantling debris layer is marked with near identical patterns of arcuate ridges and steps as well as corresponding changes in bulk grain size meter scale surface topography and thermal contraction crack polygons results from 24 km of ground penetrating radar data show that the ice within the uppermost 1 2 km of mullins and friedman glaciers is essentially free of englacial debris 1 by volume but thereafter is interspersed with bands of englacial debris each 3 m thick and dipping up glacier that intersect the ice surface at all major surface ridges and steps the similarity in number and pattern of englacial debris bands and corresponding surface ridges and steps across both glaciers along with model results and observations that call for negligible basal entrainment is best explained by episodic environmental change at valley headwalls our working hypothesis is that layers of englacial debris originate as supraglacial lags that form in ice accumulation areas during times of reduced net ice accumulation following renewed net ice accumulation these lags are subsequently buried by snow and ice flow englacially and intersect the ice surface to impart distinctive changes in the texture of supraglacial debris and topographic relief the implication is that the englacial structure and surface morphology of these cold based debris covered glaciers preserves a consistent record of climate and environmental change

quantification of modeling errors in airborne tem caused by inaccurate system description beingabletorecoveraccurateandquantitativedescriptionsof the subsurface electrical conductivity structure from airborne electromagneticdataisbecomingmoreandmorecrucialinmany applications such as hydrogeophysical and environmental mapping but also for mining exploration the effect on the inverted models of inaccurate system description in the 1d forward modeling of helicopter time domain electromagnetic tem data was studied the most important system parameters needed for accurate description of the subsurface conductivity were quantified using a nominal airborne tem system and three different reference models to ensure the generality of the conclusions by calculating forward responses the effect of changing the system transfer function of the nominal airbornetem system was studied in detail the data were inverted and the consequences of inaccuratemodelingofthesystemtransferfunctionwerestudiedin themodelspace errorsinthedescriptionofthetransferfunction influence the inverted model differently the low pass filters current turn off and receiver transmitter rx tx timing issues primarily influenced the early time gates the waveform repetition gate integration altitude and geometry mainly influenced thelatetimegates depthofinvestigationishighlymodeldependent but in general the early times hold information on the shallower parts of the model and the late times hold information on thedeeperpartsofthemodel amplitude gain andcurrentvariations affect the entire sounding and therefore the entire model the results showed that all of these parameters should be measured and modeled accurately during inversion of airbornetem data if not the output model can differ quite dramatically from the true model layer boundaries can be inaccurate by tens of meters and layer resistivities by as much as an order of magnitude in the worst cases the measured data simply cannot be fittedwithinnoiselevel

understanding the hydrogeology of the venice lagoon subsurface with airborne electromagnetics abstract the occurrence of alternating dry wet conditions in transitional environments such as wetlands deltas and lagoons usually challenges the use of traditional direct and geophysical surveys for comprehensive hydrogeologic investigations moreover significant mixing between continental fresh groundwater and marine salty surface waters generally takes place in these flat coastal areas airborne electromagnetics aem is a promising tool in this respect as it provides in a fast and cost effective manner large scale distribution of bulk electrical conductivities that can be used profitably to develop hydrogeologic models the results of a skytem aem survey in the venice lagoon italy show the capability of this technique to significantly improve the knowledge of the hydrogeologic setting of the lagoon and nearby coastland subsurface irrespective of the different features characterizing the area the environment consists of salt marshes mud flats shallows tidal channels islands together with reclaimed farmlands crossed by natural watercourses and drainage channel networks in particular the aem shows i the presence of fresh water with resistivity larger than 20 m underneath the central part of the lagoon at depths from 10 to 25 m below m s l ii the interface between different relevant stratigraphic units e g the clayey layer bounding the holocene pleistocene sedimentation and iii the occurrence of areas with possible submarine fresh groundwater discharge moreover the source and inland extent of the saltwater contamination in the shallow coastal aquifers along the southern margin of the lagoon are clearly revealed aem data were complemented with very high resolution seismic vhrs acquisitions the integrated analysis of the two data sets allows distinguishing between lithostratigraphic heterogeneity and variability of the subsurface fluids

the main characteristics of the lithosphere asthenosphere system in italy and surrounding regions we present a summary of the available information on rayleigh wave dispersion data for the italian area the dispersion curves can be grouped into two main families according to their shape one with an almost linear trend in the period range from 40 to 150 sec the other with a relatively flat portion in the period range from 40 to 80 sec the inversion of these data shows that there are strong lateral variation in the lithosphere system a very rapid transition in the lithospheric thickness from about 130 km to about 30 km as well as in the s wave velocity of the lithosphere and asthenosphere

subsurface investigation of a rock glacier using ground penetrating radar implications for locating stored water on mars 1 the discovery of rock glacier like features on mars suggests the presence of flowing or once flowing ice rock mixtures these landforms which include lobate debris aprons concentric crater fill and lineated valley fill hold significant promise as reservoirs of stored water ice that could be used as fuel sources for human exploration of mars and provide a frozen record of the climatic history of the planet to understand the deformation and distribution of ice within these landforms fundamental descriptions of their internal structure and development are required to this end a ground penetrating radar investigation was initiated using rock glaciers in the san juan mountains of colorado as surrogates for similar martian landforms results obtained from one of these rock glaciers show that the interior of the landform is composed of a layered permafrost matrix of ice sediment and ice lenses that comprise thicker depositional units formed through high magnitude debris falls folds in the uppermost layers correspond to the surface expression of ridges and furrows suggesting that compressive stresses originating in the accumulation zone are transmitted downslope through the rock glacier rock glacier features on mars may also consist of layered permafrost which would suggest a history of development involving seasonal frost accumulation and or water influx from below in terms of water storage within martian analogs consideration must include the possibility that some water ice may be stored in relatively pure form within lenses and vein networks such as observed in the surrogate rock glacier of this study

airborne electromagnetic imaging of discontinuous permafrost 1 the evolution of permafrost in cold regions is inextricably connected to hydrogeologic processes climate and ecosystems permafrost thawing has been linked to changes in wetland and lake areas alteration of the groundwater contribution to streamflow carbon release and increased fire frequency but detailed knowledge about the dynamic state of permafrost in relation to surface and groundwater systems remains an enigma here we present the results of a pioneering 1 800 line kilometer airborne electromagnetic survey that shows sediments deposited over the past 4 million years and the configuration of permafrost to depths of 100 meters in the yukon flats area near fort yukon alaska the yukon flats is near the boundary between continuous permafrost to the north and discontinuous permafrost to the south making it an important location for examining permafrost dynamics our results not only provide a detailed snapshot of the present day configuration of permafrost but they also expose previously unseen details about potential surface groundwater connections and the thermal legacy of surface water features that has been recorded in the permafrost over the past 1 000 years this work will be a critical baseline for future permafrost studies aimed at exploring the connections between hydrogeologic climatic and ecological processes and has significant implications for the stewardship of arctic environments

stratigraphic profiling with ground penetrating radar in permafrost a review of possible analogs for mars 1 we review our past and ongoing use of ground penetrating radar to investigate permafrost in alaska and in the dry valleys of antarctica the results may be relevant to radar efforts on mars because of arid conditions and the presence of ice the pulses were centered at 50 100 and 400 mhz we interpret profiles from two sites in the eastern taylor valley to show glaciolacustrine and glaciofluvial stratigraphy the maximum depth of stratigraphy profiled there was about 33 m near fairbanks alaska the depth of penetration at 50 mhz was near 80 m in marginally frozen and stratified alluvial sands at the fairbanks sites supplementary drilling was required to differentiate between reflections from conductive bedrock a graphitic schist and those from the water table at depths of 20 25 m at a site on the north slope of alaska we profiled present and remnant freezing fronts in an alluvial floodplain the relative permittivity at most sites ranged between about 4 and 5 5 which is consistent with dry conditions the mineralogy and low ice content weak interface reflectivity or the lack of further interfaces may have limited the interpretation of maximum penetration where no water table was present because signal absorption should have been low and scarce diffractions imply that scattering was weak the interface reflectivities beneath taylor valley may be a function of only density contrasts since free water and possibly ice is absent

quasi 3d modeling of airborne tem data by spatially constrained inversion we present a new methodology spatially constrained inversion sci that produces quasi 3d conductivity modeling of electromagnetic em data using a 1d forward solution spatial constraints are set between the model parameters of nearest neighboring soundings data sets models and spatial constraints are inverted as one system the constraints are built using delaunay triangulation which ensures automatic adaptation to data density variations model parameter information migrates horizontally through spatial constraints increasing the resolution of layers that would be poorly resolved locally sci produces laterally smooth results with sharp layer boundaries that respect the 3d geological variations of sedimentary settings sci also suppresses the elongated artifacts commonly seen in interpretation results of profile oriented data sets in this study sci is applied to airborne time domain em data but it can also be implemented with other ground based or airborne data types

seismic velocity constraints in the thessaloniki and chalkidiki areas northern greece from a 3 d tomographic study abstract three dimensional tomographic inversion of p wave travel time data is used to investigate the seismic velocity structure of the crust in thessaloniki and chalkidiki northern greece local earthquakes recorded by two networks operating in the area are used as natural seismic sources two different target volumes defined on the surface by 39 50 41 50 n and 21 25 24 20 e and 40 10 41 00 n and 22 45 23 50 e are investigated the first dataset is recorded by 12 stations and the second by 29 the size of the blocks used to parameterize the areas is 10 10 km and 3 3 km in the horizontal respectively with varying depth layering the major seismic velocity anomalies within the crust obtained by the tomographic inversion are resolved with a horizontal spatial resolution of about 20 and 7 km for the first and second target volume respectively our particular interest is to illuminate velocity anomalies and more detailed characteristics of the two main neogene quaternary basins in the area vardar axios and struma strymon these basins are identified as low velocity features overlying relatively higher p wave velocity structures in the lower crust the complex mygdonian area reveals a similar pattern of low velocity basin overlying higher velocity basement overall the velocity patterns correlate well with the location and strike of the main geological and tectonic units in the area as well as the basic assumptions on basin development this highlights the utility of local tomography to illuminate structural tectonic and rheological properties within the crust

investigation of the age and migration of reversing dunes in antarctica using gpr and osl with implications for gpr on mars gpr provides high resolution images of aeolian strata in frozen sand in the mcmurdo dry valleys of antarctica the results have positive implications for potential gpr surveys of aeolian strata on mars within the lower victoria valley seasonal changes in climate and a topographically constrained wind regime result in significant wind reversals as a consequence dunes show reversing crest lines and flattened dune crests ground penetrating radar gpr surveys of the dunes reveal sets of cross strata and low angle bounding surfaces produced by reversing winds summer sand transport appears to be dominant and this is attributed to the seasonal increase in solar radiation solar radiation which heats the valley floor melts ice cements making sand available for transport at the same time solar heating of the valley floor generates easterly winds that transport the sand contributing to the resultant westward dune migration the location of the dune field along the northern edge of the lower victoria valley provides some shelter from the powerful foehn and katabatic winds that sweep down the valley topographic steering of the winds along the valley and drag against the valley wall has probably aided the formation migration and preservation of the dune field optically stimulated luminescence osl ages from dune deposits range from 0 to 1 3 kyr showing that the dune field has been present for at least 1000 yr the osl ages are used to calculate end point migration rates of 0 05 to 1 3 m yr which are lower than migration rates reported from recent surveys of the packard dunes and lower than similar sized dunes in low latitude deserts the relatively low rates of migration are attributed to a combination of dune crest reversal under a bimodal wind regime and ice cement that reduces dune deflation and restricts sand entrainment

echo source discrimination in single pass airborne radar sounding data from the dry valleys antarctica implications for orbital sounding of mars the interpretation of radar sounding data from mars where significant topographic relief occurs will require echo source discrimination to avoid the misinterpretation of surface echoes as arising from the subsurface this can be accomplished through the identification of all radar returns from the surface in order to positively identify subsurface echoes we have developed general techniques for this using airborne radar data from the dry valleys of antarctica these data were collected in a single pass including taylor glacier ice covered lake bonney and an ice free area of taylor valley the pulsed radar 52 5 67 5 mhz was coherently recorded our echo discrimination techniques included a radar simulator using a digital elevation model dem to predict the location and shape of surface echoes in the radar data real and simulated echo strengths were used to calculate a signal to clutter ratio this was complemented by the cross track migration of radar echoes onto the surface these migrated echoes were superimposed on the dem and imagery in order to correlate with surface features using these techniques enabled us to identify a number of echoes in the radar data as arising from the surface and to identify subsurface echoes including a continuous reflector under the main trunk of taylor glacier and multiple reflectors beneath the terminus of taylor glacier surface based radar confirms the thickness of the glacier at three crossing points the results illustrate the importance of using complementary techniques the usefulness of a dem and the limitations of single pass radar sounding data

drainage through subglacial water sheets 1 subglacial drainage plays an important role in controlling coupling between glacial ice and underlying bed here we study the flow of water in thin macroporous sheets between ice and bed previous work shows that small perturbations in depth of a nearly parallel sided water film grow unstably because these areas have enhanced viscous dissipation that leads to enhanced melting of an ice roof we argue that in the presence of bed protrusions bridging a water sheet downward motion of the ice roof can stabilize this sheet stability results when the rate of roof closure increases faster with water depth than the rate of viscous dissipation we therefore modify existing theory to include protrusions that partially support the overlying glacier differences in the pressure on protrusions relative to water pressure drive roof closure the mechanisms of both regelation and creep normal to the bed close the overlying ice roof and decrease the ice bed gap in order to account for multiple protrusion sizes along the bed for instance resulting from an assortment of various sized sediment grains we incorporate a method of partitioning overburden pressure among different protrusion size classes and the water sheet partitioning is dependent on the amount of ice protrusion contact and therefore water depth this method allows prediction of roof closure rates we then investigate stable steady sheet configurations for reasonable parameter choices and find that these steady states can occur for modest water depths at very low effective pressures as is appropriate for ice streams moreover we find that multiple steady sheet thicknesses exist raising the possibility of switches between low and high hydraulic conductivity regimes for the subglacial water system

landscape evolution of the dry valleys transantarctic mountains tectonic implications there are different views about the amount and timing of surface uplift in the transantarctic mountains and the geophysical mechanisms involved our new interpretation of the landscape evolution and tectonic history of the dry valleys area of the transantarctic mountains is based on geomorphic mapping of an area of 10 000 km2 the landforms are dated mainly by their association with volcanic ashes and glaciomarine deposits and this permits a reconstruction of the stages and timing of landscape evolution following a lowering of base level about 55 m y ago there was a phase of rapid denudation associated with planation and escarpment retreat probably under semiarid conditions eventually downcutting by rivers aided in places by glaciers graded valleys to near present sea level the main valleys were flooded by the sea in the miocene during a phase of subsidence before experiencing a final stage of modest upwarping near the coast there has been remarkably little landform change under the stable cold polar conditions of the last 15 m y it is difficult to explain the sirius group deposits which occur at high elevations in the area if they are pliocene in age overall denudation may have removed a wedge of rock with a thickness of over 4 km at the coast declining to 1 km at a point 75 km inland which is in good agreement with the results of existing apatite fission track analyses it is suggested that denudation reflects the differences in base level caused by high elevation at the time of extension due to underplating and the subsequent role of thermal uplift and flexural isostasy most crustal uplift 2 4 km is inferred to have occurred in the early cenozoic with 400 m of subsidence in the miocene followed by 300 m of uplift in the pliocene

gpr reflection profiles of clark and commonwealth glaciers dry valleys antarctica englacial horizons deeper than 100 m are absent within 100 mhz ground penetrating radar gpr surface profiles we recorded on clark and commonwealth glaciers in the antarctic dry valleys region both glaciers show continuous bottom horizons to 280 m with bottom signal to noise ratios near 30 db density horizons should fade below 50 m depth because impermeable ice occurred by 36 m folding within commonwealth glacier could preclude radar strata beneath about 80 m depth but there is no significant folding within clark glacier strong sulfate concentrations and contrasts exist in our shallow ice core however it appears that high background concentration levels and possible decreased concentration contrasts with depth placed their corresponding reflection coefficients at the limit of or below our system sensitivity by about 77 m depth further verification of this conclusion awaits processing of our deep core chemistry profiles

influence of debris rich basal ice on flow of a polar glacier at taylor glacier a cold based outlet glacier of the east antarctic ice sheet observed surface speeds in the terminus region are 20 times greater than those predicted using glen s flow law for cold 17 c thin 100 m ice rheological properties of the clean meteoric glacier ice and the underlying deformable debris rich basal ice can be inferred from surface velocity and ablation rate profiles using inverse theory here with limited data we use a two layer flowband model to examine two end member assumptions about the basal ice properties 1 uniform softness with spatially variable thickness and 2 uniform thickness with spatially variable softness we find that the basal ice contributes 85 98 to the observed surface velocity in the terminus region we also find that the basal ice layer must be 10 15 m thick and 20 40 times softer than clean holocene age glacier ice in order to match the observations because significant deformation occurs in the basal ice our inverse problem is not sensitive to variations in the softness of the meteoric ice our results suggest that despite low temperatures highly deformable basal ice may dominate flow of cold based glaciers and rheologically distinct layers should be incorporated in models of polar glacier flow

three dimensional seismic attenuation structure beneath the aegean region and its tectonic implication abstract the three dimensional attenuation structure beneath the aegean sea and the surrounding regions is determined by inversion of seismic intensity data a large number of seismic intensity data have been accumulated in a uniform scale in the aegean region where the seismic activity is much higher than that of the other mediterranean regions nearly 7000 seismic intensity data from 50 earthquakes that have occurred in these regions are used to determine seismic attenuation structure and source acceleration of the earthquakes the resultant structure reveals a remarkable contrast of attenuation in the top layer depth 0 40 km low q is dominant in the aegean sea while high q is dominant in the surrounding land areas except for the southwestern turkey the low q regions correspond to areas of neogene quaternary grabens where the high seismicity of shallow earthquakes appears in the upper mantle high q zones corresponding to the subducting african plate dominate along the hellenic arc the irregular shape of high q zones might reflect the split or disintegration of the african slab some low q spots corresponding to the distribution of volcanoes exist along the volcanic arc the low q spots might correspond to diapirs causing the subduction volcanism

sharp spatially constrained inversion with applications to transient electromagnetic data time domain electromagnetic data are conveniently inverted by using smoothly varying 1d models with fixed vertical discretization the vertical smoothness of the obtained models stems from the application of occam type regularization constraints which are meant to address the ill posedness of the problem an important side effect of such regularization however is that horizontal layer boundaries can no longer be accurately reproduced as the model is required to be smooth this issue can be overcome by inverting for fewer layers with variable thicknesses nevertheless to decide on a particular and constant number of layers for the parameterization of a large survey inversion can be equally problematic here we present a focusing regularization technique to obtain the best of both methodologies the new focusing approach allows for accurate reconstruction of resistivity distributions using a fixed vertical discretization while preserving the capability to reproduce horizontal boundaries the formulation is flexible and can be coupled with traditional lateral spatial smoothness constraints in order to resolve interfaces in stratified soils with no additional hypothesis about the number of layers the method relies on minimizing the number of layers of non vanishing resistivity gradient instead of minimizing the norm of the model variation itself this approach ensures that the results are consistent with the measured data while favouring at the same time the retrieval of horizontal abrupt changes in addition the focusing regularization can also be applied in the horizontal direction in order to promote the reconstruction of lateral boundaries such as faults we present the theoretical framework of our regularization methodology and illustrate its capabilities by means of both synthetic and field data sets we further demonstrate how the concept has been integrated in our existing spatially constrained inversion formalism and show its application to large scale time domain electromagnetic data inversions

travel time tomography of the european mediterranean mantle down to 1400 km abstract the 3 d p wave velocity structure of the mantle below europe the mediterranean region and a part of asia minor is investigated this study is a considerable extension of an earlier tomographic experiment that was limited to imaging upper mantle structure only here the earth s volume under study encompasses the mantle to a depth of 1400 km and we increase the number of international seismological centre isc data for inversion by a factor of four by taking more years of observation and by including data from teleseismic events the most important departure from the earlier study is that we do not use the jeffreys bullen model as a reference model but an improved radially symmetric velocity model the pm2 model which is appropriate for the european mediterranean mantle our inversion procedure consists of two steps first the radial model pm2 is determined from the isc delay times by a nonlinear trial and error inversion of the data as opposed to the jeffreys bullen model the new reference model has a high velocity lithosphere a low velocity zone and seismic discontinuities at depths of 400 and 670 km next the isc data are corrected for effects related to the change in reference model and inverted for 3 d heterogeneity relative to the pm2 model we follow this two step approach to attain a better linearizable tomographic problem in which ray paths computed in the pm2 model provide a better approximation of the actual ray paths than those computed from the jeffreys bullen model hence the two step scheme leads to a more credible application of fermat s principle in linearizing the tomographic equations inversion results for the 3 d heterogeneity are computed for both the uncorrected isc data and for the pm2 data the data fit obtained in the two step approach is slightly better than in the inversion of isc data using the jeffreys bullen reference model a comparison of the tomographic results demonstrates that the pm2 data inversion is to be preferred to assess the spatial resolution an analysis is given of hit count patterns sampling of the mantle by ray paths and results of sensitivity tests with 3 d synthetic velocity models the spatial resolution obtained varies with position in the mantle and is studied both in map view and in cross section in the well sampled regions of the mantle the spatial resolution for larger scale structure can qualitatively be denoted as reasonable to good and at least sufficient to allow interpretation of larger scale anomalies a comparison is made of the results of this study with independent models of s velocity heterogeneity obtained in a number of investigations and with a prediction of the seismic velocity structure of the mantle computed from tectonic reconstructions of the mediterranean region in the context of this comparison interpretations of large scale positive anomalies found in the mediterranean upper mantle in terms of subducted lithosphere are given specifically addressed are subduction below southern spain below the western mediterranean and italy and below the aegean in the last region a slab anomaly is mapped down to depths of 800 km

ice thickness areal and volumetric changes of davies dome and whisky glacier james ross island antarctic peninsula in 1979 2006 this study calculates area volume and elevation changes of two glaciers on james ross island antarctica during the period 1979 2006 davies dome is a small ice cap whisky glacier is a valley glacier ground penetrating radar surveys indicate ice thickness which was used for calculations of the bed topography and volume of both glaciers maximum measured ice thicknesses of davies dome and whisky glacier are 83 2 and 157 2 m respectively between 1979 and 2006 the area of the ice cap decreased from 6 23 0 05 km2 to 4 94 0 01 km2 20 7 while the area of the valley glacier reduced from 2 69 0 02 km 2 to 2 40 0 01 km2 10 6 over the same period the volume of the ice cap and valley glacier reduced from 0 23 0 03 km3 to 0 16 0 02 km3 30 4 and from 0 27 0 02 km3 to 0 24 0 01 km3 10 6 respectively the mean surface elevation decreased by 8 5 2 8 and 10 1 2 8 m the average areal 0 048 0 011 km2 a 1 and volumetric 0 003 0 001 km3 a 1 changes are higher than the majority of other estimates from antarctic peninsula glaciers