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

surface melt driven seasonal behaviour englacial and subglacial from a soft bedded temperate glacier recorded by in situ wireless probes we investigate the spatial and temporal englacial and subglacial processes associated with a temperate glacier resting on a deformable bed using the unique glacsweb wireless in situ probes embedded in the ice and the till combined with other techniques including ground penetrating radar gpr and borehole analysis during the melt season spring summer and autumn high surface melt leads to high water pressures in the englacial and subglacial environment winter is characterised by no surface melting on most days base apart from a series of positive degree days once winter begins a diurnal water pressure cycle is established in the ice and at the ice sediment interface with direct meltwater inputs from the positive degree days and a secondary slower englacial pathway with a five day lag this direct surface melt also drives water pressure changes in the till till deformation occurred throughout the year with the winter rate approximately 60 that of the melt season we were able to show the bed comprised patches of till with different strengths and were able to estimate their size relative percentage and temporal stability we show that the melt season is characterised by a high pressure distributed system and winter by a low pressure channelized system we contrast this with studies from greenland overlying rigid bedrock where the opposite was found we argue our results are typical of soft bedded glaciers with low englacial water content and suggest this type of glacier can rapidly respond to surface driven melt based on theoretical and field results we suggest that the subglacial hydrology comprises a melt season distributed system dominated by wide anastomosing broad flat channels and thin water sheets which may become more channelized in winter and more responsive to changes in meltwater inputs

statistical cyclicity of the supercontinent cycle supercontinents like pangea impose a first order control on earth s evolution as they modulate global heat loss sea level climate and biodiversity in a traditional view supercontinents form and break up in a regular perhaps periodic manner in a cycle lasting several 100 myr as reflected in the assembly times of earth s major continental aggregations columbia rodinia and pangea however modern views of the supercontinent cycle propose a more irregular evolution on the basis of an improved understanding of the precambrian geologic record here we use fully dynamic spherical mantle convection models featuring plate like behavior and continental drift to investigate supercontinent formation and breakup we further dismiss the concept of regularity but suggest a statistical cyclicity in which the supercontinent cycle may have a characteristic period imposed by mantle and lithosphere properties but this is hidden in immense fluctuations between different cycles that arise from the chaotic nature of mantle flow

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

dynamic causes of the relation between area and age of the ocean floor estimates for the area and age of the ocean floor are at odds with assumptions for mantle convection which imply that an older sea floor rather than a new one would be preferentially subducted over time previous efforts to explain these relationships have been based on geologic evidence and simple models coltice et al p 335 1 created numerical three dimensional convection models representing more realistic physical boundaries including a spherical earth the existence of continents and supercontinents over time and realistic rheologies a combination of continents and plate like behavior of the ocean floor sufficed to produce the observed relationship between plate area and plate age which explains why some old oceanic crust still remains 1 lookup doi 10 1126 science 1219120

geodynamically consistent inferences on the uniform sampling of earth s paleomagnetic inclinations abstract paleomagnetism is a key method to reconstruct the earth s paleogeography and thus essential for understanding tectonic evolution but it assumes that the earth s magnetic field structure has always averaged to a geocentric axial dipole gad the gad hypothesis may be tested using the observed inclination frequency distribution but only if continents sampled all of the earth s latitudes uniformly which is not known here we provide new insight into the uniform sampling problem by employing a suite of 3d spherical mantle convection models that feature the self consistent evolution of mantle convection plate tectonics and continental drift over timescales of 2 gyr or more our results suggest that continents unlikely sampled latitudes uniformly during the phanerozoic consistent with previous suggestions this finding is robust for a variety of geodynamic evolutions with different mantle and lithosphere structures at least in the absence of true polar wander for longer sampling durations uniform sampling typically becomes more feasible but may only be achieved with confidence after time scales of minimum 1 3 gyr this time scale depends on the structure of the mantle and lithosphere and may be shortest when upper mantle viscosity is small such that reduced resistive drag at the cratonic base allows for faster continental drift weak plates low plastic yield strength promote more dispersed continent configurations which tends to facilitate uniform sampling if these conditions are not met the uniform sampling time scale can easily exceed several billion years even the minimum estimate of 1 3 gyr challenges the validity of using the phanerozoic inclination frequency distribution to infer the past average magnetic field structure the approach could however still be applicable using the precambrian inclination record

three new upper cambrian stages for north america the existing upper cambrian stages for north america are obsolete three new stratotype defined stages are equivalent to and replace three successive biomeres marjuman stage replaces marjumiid biomere lower boundary at base ptychagnostus atavis zone steptoean stage replaces pterocephaliid biomere lower boundary at base aphelaspis zone sunwaptan stage replaces ptychaspid biomere lower boundary at base irvingella major subzone the base of the overlying ibexian series is placed at the base of the eurekia apopsis subzone

plate tectonics and the evolution of the british isles this paper is an essay on the rationalism and difficulties of the geological corollaries of plate tectonics with illustrations particularly from the geological history of the british isles there is special emphasis on the mechanics of continental lithosphere in extension when basins are formed and in convergence whereby continental lithosphere thinned in extension is shortened and restacked to form orogenic belts an upper lithosphere stress concentrating elastic lid suffers extremely complex inhomogeneous deformations above a simpler viscoelastic lower lithosphere whose deformation drives strain and displacement in the elastic lid the very nature of plate tectonics as relative motion among a multi plate mosaic of plates provides a rational semi quantitative framework for large scale geological complexity in space and time the caledonian and hercynian history of the british isles may be explained semi quantitatively in terms of plate boundary evolution but it is not possible to define unequivocal solutions the rationalism of plate tectonics is exhibited on a smaller more direct field scale by many of the complexities of ophiolite complexes such as ballantrae scotland that can be explained as the effects of the evolution of ridge ridge transforms and by diachronous deformation and sedimentation in subduction accretion prisms such as the southern uplands deformational and sedimentary patterns in nw europe from triassic times to the present day resulted from a complex interaction of extensional events related to diachronous atlantic opening and compressional events related to sequential alpine shortening phases those widespread events are typical of intracontinental plate boundary systems and create great difficulties for the geologist wishing to decipher relative plate motion from geological sequences because huge areas much larger than the british isles must be analysed and synthesized before much sense can be extracted further difficulties arise from the palinspastic and preservation problems of convergent boundaries orogenic shortening telescopes and partially subducts facies belts most orogenic belts were assembled as a collage of displaced and rotated terrain elements that were subsequently flattened elongated and straightened to an apparent but deceiving simplicity therefore while plate tectonics provides a rational geological framework against which for example british basin evolution can be tested and explained we cannot yet use the geological effects of plate boundary evolution to reconstruct relative plate motion quantitatively

influence of plate tectonic mode on the coupled thermochemical evolution of earth s mantle and core we investigate the influence of tectonic mode on the thermochemical evolution of simulated mantle convection coupled to a parameterized core cooling model the tectonic mode is controlled by varying the friction coefficient for brittle behavior producing the three tectonic modes mobile lid plate tectonics stagnant lid and episodic lid the resulting compositional structure of the deep mantle is strongly dependent on tectonic mode with episodic lid resulting in a thick layer of subducted basalt in the deep mantle whereas mobile lid produces only isolated piles and stagnant lid no basaltic layering the tectonic mode is established early on with subduction initiating at around 60 myr from the initial state in mobile and episodic cases triggered by the arrival of plumes at the base of the lithosphere crustal production assists subduction initiation increasing the critical friction coefficient the tectonic mode has a strong effect on core evolution via its influence on deep mantle structure episodic cases in which a thick layer of basalt builds up experience less core heat flow and cooling and a failed geodynamo thus a continuous mobile lid mode existing from early times matches earth s mantle structure and core evolution better than an episodic mode characterized by large scale flushing overturn events

origin of the rheic ocean rifting along a neoproterozoic suture the rheic ocean is widely believed to have formed in the late cambrian early ordovician as a result of the drift of peri gondwanan terranes such as avalonia and carolina from the northern margin of gondwana and to have been consumed in the devonian carboniferous by continent continent collision during the formation of pangea other peri gondwanan terranes e g armorica ossa morena northwest iberia saxo thuringia moldanubia remained along the gondwanan margin at the time of rheic ocean formation differences in the neoproterozoic histories of these peri gondwanan terranes suggest the location of the rheic ocean rift may have been inherited from neoproterozoic lithospheric structures formed by the accretion and dispersal of peri gondwanan terranes along the northern gondwanan margin prior to rheic ocean opening avalonia and carolina have sm nd isotopic characteristics indicative of recycling of a juvenile ca 1 ga source and they were accreted to the northern gondwanan margin prior to voluminous late neoproterozoic arc magmatism in contrast sm nd isotopic characteristics of most other peri gondwanan terranes closely match those of eburnian basement suggesting they reflect recycling of ancient 2 ga west african crust the basements of terranes initially rifted from gondwana to form the rheic ocean were those that had previously accreted during neoproterozoic orogenesis suggesting the rift was located near the suture between the accreted terranes and cratonic northern gondwana opening of the rheic ocean coincided with the onset of subduction beneath the laurentian margin in its predecessor the iapetus ocean suggesting geodynamic linkages between the destruction of the iapetus ocean and the creation of the rheic ocean

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

mantle convection with longest wavelength thermal heterogeneity in a 3 d spherical model degree one or two 1 the formation of longest wavelength mantle convection in the sluggish lid regime is investigated using a three dimensional spherical model the bottom rayleigh number is fixed at 107 considering temperature dependent rheology degree one dominant thermal convection occurs for both purely basal heating and mixed i e basal and internal heating modes for the purely basal heating mode degree one convection occurs when the viscosity contrast due to temperature dependent rheology is 103 104 in both boussinesq and extended boussinesq fluids however with extended boussinesq fluid degree one convection may only occur in the basal heating mode in the mixed heating mode degree one convection shifts to one with high degree modes presumably because of enhanced viscous dissipation in the highly viscous lid over up downwelling plumes the geophysically relevant degree two convection with sheet like downwellings is not observed in this study the inclusion of visco plastic rheology in the top part of the mantle breaks down degree one convection

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

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

testing the tracer ratio method for modeling active compositional fields in mantle convection simulations 1 tracer methods are attractive for modeling compositional fields because they offer the potential of zero numerical diffusion composition is typically taken to be proportional to the absolute local concentration of tracers but an increasingly popular method is to have dense and regular tracers with composition being equal to the local fraction of dense tracers this paper tests this ratio method using established benchmarks and by comparing the performance of the two tracer methods and grid based methods for simulating the long term evolution of a convecting mantle with a thick dense stable layer for this scenario the ratio method is found to have several advantages giving sharp stable long term layering with no tracer settling minimal statistical noise and low entrainment even with only 5 tracers per cell the method is equally applicable to finite volume and finite element treatments of the underlying flow entrainment in grid based advection methods is heavily dependent on resolution and numerical details and is reduced 1 order of magnitude by the filter proposed by a lenardic numerical determination of physically correct entrainment rates remains a challenging problem comparing tracer and grid based methods the spatial pattern of the thermal and chemical fields appear to be converging on the finest grids however the estimated entrainment differs significantly

does the golden spike still glitter a modus operandi for precambrian stratigraphy is suggested which combines the geochronometrical scale and late proterozoic chronostratigraphy the development of phanerozoic stratigraphical classification is followed from the pioneering days of murchison through the growing understanding of fossils in relation to time and the later distinction of rock and time rock to the advent of the golden spike criticisms of chronostratigraphy global standard stratigraphy are assessed and its value upheld stratigraphical procedures such as those of event stratigraphy are useful in particular situations but correlative methods in the phanerozoic are dominated by biostratigraphy examples are taken of its use and potential in north american cordilleran palaeozoic stratigraphy in the triassic system as a whole and in the caledonides of britain and ireland

strain rate enhancement at dye 3 greenland ice at depth in ice sheets can be softer in bed parallel shear than glen s flow law predicts for example at dye 3 greenland enhancement factors of 3 4 are needed in order to explain the rate of borehole tilting previous authors have identified crystal fabric as the dominant contributor but the role of impurities and crystal size is still incompletely resolved here we use two formulations of anisotropic flow laws for ice azuma s and sachs models to account for the effects of anisotropy and show that the measured anisotropy of the ice at dye 3 cannot explain all the detailed variations in the measured strain rates the jump in enhancement across the holocene wisconsin boundary is larger than expected from the measured fabrics alone dust and soluble ion concentration divided by crystal size correlates well with the residual enhancement indicating that most of the excess deformation may be due to impurities or crystal size while the major features of the deformation at dye 3 are explained by anisotropy and temperature results also suggest that further research into the role of impurities and crystal size is warranted

stratigraphy and structure of the schoonover sequence northeastern nevada implications for paleozoic plate margin tectonics radiolarian biostratigraphy and detailed geologic mapping have been used to resolve the complex structure and stratigraphy of part of the golconda allochthon in the independence mountains nevada here the schoonover sequence is latest devonian to early permian in age spanning the time interval from the emplacement of the roberts mountains allochthon onto the shelf in the earliest mississippian antler orogeny to the inception of the sonoma orogeny the history of the schoonover basin is tied to that of the adjacent shelf in nevada and provides important insights into the upper paleozoic paleogeographic framework of the continental margin in the schoonover sequence latest devonian famennian chert overlies basaltic and andesitic greenstone and is in turn overlain by early mississippian kinderhookian chert interbedded with tuff and volcaniclastic rocks derived from an island arc the kinderhookian volcaniclastic rocks grade upward with no obvious depositional break into siliciclastic turbidites and pebbly mudstones that contain debris derived primarily from erosion of the antler orogenic belt although in some beds volcanic rock fragments and greenschist grade metasedimentary clasts are also common the age of underlying kinderhookian and interbedded and overlying osagian to meramecian chert sequences indicates that the siliciclastic turbidites are synchronous with the early deposits of the autochthonous foredeep basin of the antler orogenic belt the dual or composite source terranes represented by the schoonover siliciclastic rocks place the basin between an arc and the antler orogen on the edge of the shelf meramecian age basalt flows in the schoonover are coeval with subsidence and basaltic volcanism in autochthonous shelf sequences in northern nevada suggesting that a rifting event may have occasioned the end of antler age compression in nevada the onset of limestone turbidite deposition in the schoonover corresponds to the reestablishment of carbonate shelf conditions on the continental margin in latest mississippian earliest pennsylvanian time the late devonian earliest mississippian part of the schoonover depositional history has not yet been documented elsewhere in the golconda allochthon but late mississippian to permian rocks in the schoonover sequence are analogous to those of the havallah sequence suggesting deposition in the same basin thrust faults in the schoonover repeat the late devonian to permian section detaching the basinal sequence from its depositional basement thrust faults extend along strike for at least 10 km but thrust plates are the stratigraphic relations documented in the schoonover sequence are compatible with a back arc thrusting model for the formation of both the roberts mountains and golconda allochthons but they are more difficult to reconcile with models that interpret the allochthons as accretionary prisms developed in front of farther traveled arcs that collided with a passive margin in a broader context the stratigraphy of upper paleozoic allochthonous rocks in nevada records short lived episodes of crustal shortening along the continental margin separated by longer episodes of extensional and or transcurrent tectonics this data suggest that the western united states was a southwest pacific style active margin at least as far back as the devonian

slab rollback instability and supercontinent dispersal supercontinents coalesce over subduction zone complexes and their subsequent dispersal is usually attributed to heating and upwelling of continent insulated mantle this dispersal mechanism however requires considerable mantle internal heating alternatively the supercontinent configuration may be mechanically unstable and disperse regardless of heating mode in particular increased drag on plates or subducting slabs e g by accumulating continents causes them to slow down and trenches to rollback once subcontinental slabs are slightly separated resistance to their descent increases inducing further trench migration slabs thus undergo a rollback instability which disperses supercontinents a simple theoretical model illustrates this instability and shows that there are two equilibrium states one unstable supercontinent state where slabs are conjoined and one stable state where slabs are widely separated slab rollback from the unstable to stable states occurs at typical slow tectonic speeds and over a period commensurate with the age of ocean basins and the wilson cycle

subdivision of precambrian time recommendations and suggestions by the subcommission on precambrian stratigraphy abstract the subcommission on precambrian stratigraphy consisting at the time of its last formal meeting in 1982 of 12 titular members and 30 correspondents has been actively working toward an acceptable subdivision of precambrian time for more than a decade comprehensive time rock charts have been prepared for major precambrian shields and this composite record has been analysed for patterns and events common to several or to all a geochronologic subdivision scheme has evolved that reflects these patterns of major events in earth history while still recognizing that those events are typically varied in intensity and timing from one region to another time unit boundaries have been selected so as to enclose or delimit principal cycles of sedimentation orogeny and magmatism but the boundaries are defined in years without specific reference to any bodies of rock several recommendations developed sequentially at formal meetings held at intervals of 2 4 years and announced in geologic journals with calls for comment and criticism are proposed 1 1 the recommendation to establish two major time units of eon rank proterozoic and archaean archean separated by a time boundary at 2500 ma is reaffirmed 2 2 the provisional recommendation to subdivide the proterozoic eon into three units of era rank provisionally labelled proterozoic i ii and iii with time boundaries at 2500 1600 900 and about 570 ma respectively is confirmed and 3 3 further subdivision of the proterozoic into eight time units of period rank as yet unnamed with time boundaries at 2500 2100 1800 1600 1400 1200 900 700 and about 570 ma respectively is provisionally recommended the nature of the subcommission s deliberations and the guidelines developed are described herein comment and criticism both on the procedure followed and on the recommendations themselves are invited

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

the role of mobile belts for the longevity of deep cratonic lithosphere the crumple zone model many archean cratons are surrounded by proterozoic mobile belts that have experienced episodes of tectonic re activation over their lifetimes this suggests that mobile belt lithosphere may be associated with long lived inherited weakness it is proposed that the proximity of this weakness can increase the longevity of deep archean lithosphere by buffering archean cratons from mantle derived stresses the physical plausibility of this idea is explored through numerical simulations of mantle convection that include continents and allow for material rheologies that model the combined brittle and ductile behavior of the lithosphere within the simulations the longevity of deep cratonic lithosphere does increase if it is buffered by mobile belts that can fail at relatively low stress levels

a model for the crustal evolution of southern scotland in the late silurian southern scotland was a mature accretionary active margin of sumatra type on the southeastern edge of north america while england was on a passive margin on the opposite side of the iapetus ocean southward subduction below england during the middle ordovician produced a magmatic arc which now lies close to the iapetus suture and thus argues for a missing forearc terrane tectonic erosion or strike slip removal may be responsible for some of this missing terrane to the north of the lake district but we now suggest that underthrusting below southern scotland may also be an important cause if the ordovician forearc terrane of northern england is now below the southern uplands we would have an explanation for the evidence for similar lower crustal foundations on either side of the surface trace of the iapetus suture velocity structure from the lispb study sr u pb isotopic signatures and geophysical attributes of the c 400 ma granites the under thrusting model still leaves room for considerable modification of both margins by large strike slip faults seismic refraction profiles of modern accretionary active margins sumatra sw japan allow us to propose a likely velocity profile for late silurian southern scotland we choose a mesozoic forearc truncated by strike slip faulting the queen charlotte islands margin of the ne pacific as an analogue for the late silurian southern iapetus margin following previous suggestions of highly oblique and slow convergence during the final stages of closure of iapetus we combine the two profiles by under thrusting the southern margin to produce a model which involves minimal shortening in the collision zone the crustal profile resulting from such a collision would be very similar to the modern southern scotland profile both in terms of the scale of the tectono stratigraphic terranes involved and of the resulting crustal velocity structure the cessation of accretion after the mid wenlock may be related to impingement of the english continental crust against the scottish trench subsequent uplift of the southern uplands may be connected with the appearance of coarse detritus in the lake district during the silurian

supercontinent cycles disrupted by strong mantle plumes a theoretical basis for the regularity of supercontinent cycles is lacking here we show that periodic supercontinent cycles are unlikely if thermal instabilities originating at the core mantle boundary are of sufficient strength we couple multiple mobile continents with vigorous mantle convection in a spherical geometry regular supercontinent cycles lasting 400 50 m y occur in idealized models with three continents and a mantle heated purely from within by radioactive decay in a model incorporating six continents and strong mantle plumes this regularity is broken and supercontinents form only sporadically our results suggest that periodic supercontinent cycles are unlikely to occur in realistic earth models

global tectonics and eustasy for the past 2 billion years abstract continental freeboard and eustasy as gauged by the relative position of the world shelf break with respect to sea level have varied by 250 m from today s ice free shelf break depth of 200 m during the past 600 ma assuming constant or uniformly accreting continental crust and ocean water volume in an ice free world sea level fluctuations can be attributed to variation in the world ocean basin volume caused by changes in either its area or its depth relative to the world shelf break an increase in volume and lowering of sea level occur as 1 the world ocean floor ages cools and subsides 2 accreting continents collide thicken and decrease in area and 3 poorly conductive continental platforms become thermally elevated due to a size induced stasis over the mantle conversely a decrease in the age of the world ocean floor attenuation of continental crust during rifting and an increase in continent number and mobility will reduce the world ocean basin volume and raise sea level theoretical sea level calculated from these principles correlates well with calibrated first order cycles of eustatic sea level change for the phanerozoic the record closely fits a simple model of retardation and acceleration of terrestrial heat loss during alternating periods of supercontinent accretion and fragmentation calibrated to sea level highstands successive first order marine transgressions and orogenic pangea regressions characterize a self sustaining 440 ma plate tectonic cycle for the late precambrian and phanerozoic the cycle can be recognized as far back as 2 ga from the tectonic evidence of continental collision and rifting recorded in global orogenic peaks and mafic dike swarms and may be related to major episodes of glaciation and evolutional biogenesis

ross ice shelf antarctica in situ radio frequency attenuation we have measured the in situ average electric field attenuation length hli for radio frequency signals broadcast vertically through the ross ice shelf antarctica we chose a location moore embayment south of minna bluff known for its high reflectivity at the ice sea interface we confirmed specular reflection and used the return pulses to measure the average attenuation length from 75 1250 mhz over the round trip distance of 1155 m we find hl i to vary from 500 m at 75 mhz to 300 m at 1250 mhz with an experimental uncertainty of 55 15 m

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

a sequential data assimilation approach for the joint reconstruction of mantle convection and surface tectonics with the progress of mantle convection modelling over the last decade it now becomes possible to solve for the dynamics of the interior flow and the surface tectonics to first order we show here that tectonic data like surface kinematics and seafloor age distribution and mantle convection models with plate like behaviour can in principle be combined to reconstruct mantle convection we present a sequential data assimilation method based on suboptimal schemes derived from the kalman filter where surface velocities and seafloor age maps are not used as boundary conditions for the flow but as data to assimilate two stages a forecast followed by an analysis are repeated sequentially to take into account data observed at different times whenever observations are available an analysis infers the most probable state of the mantle at this time considering a prior guess supplied by the forecast and the new observations at hand using the classical best linear unbiased estimate between two observation times the evolution of the mantle is governed by the forward model of mantle convection this method is applied to synthetic 2 d spherical annulus mantle cases to evaluate its efficiency we compare the reference evolutions to the estimations obtained by data assimilation two parameters control the behaviour of the scheme the time between two analyses and the amplitude of noise in the synthetic observations our technique proves to be efficient in retrieving temperature field evolutions provided the time between two analyses is 10 myr if the amplitude of the a priori error on the observations is large 30 per cent our method provides a better estimate of surface tectonics than the observations taking advantage of the information within the physics of convection

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

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

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

nature of radio echo layering in the antarctic ice sheet detected by a two frequency experiment a two frequency radio echo sounding experiment was carried out at dome fuji the second highest dome in east antarctica and along a 1150 km long traverse line from the dome to the coast the goal was to determine the dominant causes of the radio echo internal reflections and to investigate their possible changes with depth ranges and regions from the two frequency 60 mhz and 179 mhz radio echo responses at various sites we distinguished four zones each of the zones is characterized by a dominant cause of radio echo internal reflection as follows in the pd zone changes in dielectric permittivity are mainly due to density fluctuations in the pcof zone changes in dielectric permittivity are mainly due to changes in crystal orientation fabrics and in the ca zone changes in electrical conductivity are mainly due to changes in acidity induced by past volcanic eruptions in each of these three zones the changes occur commonly along isochrones in addition a basal echo free zone the fourth zone was found to appear always below the pcof zone these four zones and their distribution suggested variations of the physical conditions within the ice sheet

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

the impact of rayleigh number on assessing the significance of supercontinent insulation several processes unfold during the supercontinent cycle more than one of which might result in an elevation in subcontinental mantle temperatures thus multiple interpretations of the concept of continental insulation exist although a consensus seems to have formed that subcontinental mantle upwellings appear below large continents extensively ringed by subduction zones there are differing views on what role continental insulation plays in the production of elevated mantle temperatures here we investigate how the heating mode of the mantle can change the influence of the thermal blanket effect we present 2 d and 3 d cartesian geometry mantle convection simulations with thermally and mechanically distinct oceanic and continental plates the evolution of mantle thermal structure is examined after continental accretion at subduction zones e g the formation of pangea for a variety of different mantle heating modes our results show that in low rayleigh number models the impact of the role of continental insulation on subcontinental temperatures increases when compared to models with higher convective vigor broad hot upper mantle features generated in low rayleigh number models due in part to the thermal blanket effect are absent at higher rayleigh numbers we find that subcontinental heating in a high rayleigh number flow occurs almost entirely as a consequence of the influence of subduction initiation at the continental margin rather than the influence of continental insulation in our models featuring earth like convective vigor we find that it is difficult to obtain subcontinental temperatures in significant excess of suboceanic temperatures over timescales relevant to supercontinent aggregation

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

high mantle temperatures following rifting caused by continental insulation the continents are thought to insulate and heat the underlying mantle geochemical analyses of lava samples formed at a mid ocean ridge in the atlantic ocean immediately after continental breakup show that the mantle was up to 150 c hotter than today and took about 70 million years to cool the distribution of continents is thought to influence the temperature of the underlying mantle over geological timescales insulation effects generate a build up of heat that may cause increased magmatism1 2 3 4 such as flood basalt volcanism5 6 and ultimately rift the continents causing them to break apart and new ocean basins to form here we use analyses of the major element geochemistry of lava samples collected from oceanic crust in the atlantic and pacific oceans to quantify the effect of continental insulation the lavas formed at mid ocean ridges following continental rifting and break up and preserve a record of upper mantle temperatures over the past 170 myr we find that samples from the pacific ocean formed more than 2 000 km from the nearest continental margin do not record raised mantle temperatures in contrast samples from the atlantic ocean that formed close to the margin of the rifted continent reveal an upper mantle temperature immediately after continental rifting that was up to 150 c higher than the present day average mantle temperatures remained high for 60 70 myr we conclude that the atlantic thermal anomaly was created by continental insulation and persisted in the mantle beneath the atlantic ocean long after the continental fragments had dispersed

the future of earth s oceans consequences of subduction initiation in the atlantic and implications for supercontinent formation subduction initiation is a cornerstone in the edifice of plate tectonics it marks the turning point of the earth s wilson cycles and ultimately the supercycles as well in this paper we explore the consequences of subduction zone invasion in the atlantic ocean following recent discoveries at the sw iberia margin we discuss a buoyancy argument based on the premise that old oceanic lithosphere is unstable for supporting large basins implying that it must be removed in subduction zones as a consequence we propose a new conceptual model in which both the pacific and the atlantic oceans close simultaneously leading to the termination of the present earth s supercycle and to the formation of a new supercontinent which we name aurica our new conceptual model also provides insights into supercontinent formation and destruction supercycles proposed for past geological times e g pangaea rodinia columbia kenorland

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

englacial and subglacial water flow at skalafellsjokull iceland derived from ground penetrating radar in situ glacsweb probe and borehole water level measurements we reconstruct englacial and subglacial drainage at skalafellsjokull iceland using ground penetrating radar gpr common offset surveys borehole studies and glacsweb probe data we find that englacial water is not stored within the glacier water content 0 0 3 instead the glacier is mostly impermeable and meltwater is able to pass quickly through the main body of the glacier via crevasses and moulins once at the glacier bed water is stored within a thin 1 m layer of debris rich basal ice 2 water content and the till the hydraulic potential mapped across the survey area indicates that when water pressures are high most of the year water flows parallel to the margin and emerges 3 km down glacier at an outlet tongue gpr data indicates that these flow pathways may have formed a series of braided channels we show that this glacier has a very low water storage capacity but an efficient englacial drainage network for transferring water to the glacier bed and therefore it has the potential to respond rapidly to changes in melt water inputs

a new approach to an absolute timescale from measurements of orbital cycles and sedimentary microrhythms when the sedimentary microrhythms of two groups of jurassic zones forming part of the chronostratigraphical scale are analysed against recent radiometric age scales there is good agreement with present day cycles resulting from the obliquity of the ecliptic sedimentary microrhythms resulting from orbital forcing can be used to establish either the relative or the true length of chronostratigraphical zones and thus provide a new approach towards an absolute timescale such a scale will be powerful in the analysis of other cycles and perturbations in the geological record

constraints on mantle viscosity structure from continental drift histories in spherical mantle convection models abstract earth s continents drift in response to the force balance between mantle flow and plate tectonics and actively change the plate mantle coupling thus the patterns of continental drift provide relevant information on the coupled evolution of surface tectonics mantle structure and dynamics here we investigate rheological controls on such evolutions and use surface tectonic patterns to derive inferences on mantle viscosity structure on earth we employ global spherical models of mantle convection featuring self consistently generated plate tectonics which are used to compute time evolving continental configurations for different mantle and lithosphere structures our results highlight the importance of the wavelength of mantle flow for continental configuration evolution too strong short wavelength components complicate the aggregation of large continental clusters while too stable very long wavelength flow tends to enforce compact supercontinent clustering without reasonable dispersal frequencies earth like continental drift with episodic collisions and dispersals thus requires a viscosity structure that supports long wavelength flow but also allows for shorter wavelength contributions such a criterion alone is a rather permissive constraint on internal structure but it can be improved by considering continental oceanic plate speed ratios and the toroidal poloidal partitioning of plate motions the best approximation of earth s recent tectonic evolution is then achieved with an intermediate lithospheric yield stress and a viscosity structure in which oceanic plates are 103 more viscous than the characteristic upper mantle which itself is 100 200 less viscous than the lowermost mantle such a structure causes continents to move on average 2 2 1 0 slower than oceanic plates consistent with estimates from present day and from plate reconstructions this does not require a low viscosity asthenosphere globally extending below continental roots however this plate speed ratio may undergo strong fluctuations on timescales of several 100 myr that may be linked to periods of enhanced continental collisions and are not yet captured by current tectonic reconstructions

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

transient slip events from near field seismic and geodetic data on a glacier fault whillans ice plain west antarctica 1 bidaily tidally modulated stick slip speed ups of the whillans ice plain wip provide insight into glacier dynamics and failure at a naturally repeating fault asperity we installed a network of continuously operating gps receivers in 2007 and deployed on ice broadband seismometers during the austral summer of 2008 on whillans ice stream wis west antarctica and recorded 26 glacier speed up events previous work during the 2004 field season suggested that these speed ups initiate as failure of an asperity on or near ice raft a that triggers rupture across the entire wip our results for 2008 locate the slip initiation farther to the south of this feature closer to the grounding line and the southernmost extent of the ross ice shelf the initiation may be controlled by a discontinuity in basal boundary conditions at the suture between two ice streams a strong correlation between the amplitude of seismic waves generated at the rupture front and the total slip achieved over the duration of the slip event 30 min suggests slip predictable behavior i e the ability to forecast the eventual slip based on the first minute of seismic radiation successive slip events propagate with different rupture speeds 100 300 m s that strongly correlate r 2 0 73 with the interevent duration in addition the amount of slip achieved during each event appears to be correlated with the rupture speed we use these observations to constrain basal shear stress to be 4 kpa by calculating conditions for basal freezing our observations yield information regarding mechanics and dynamics of ice streambeds at the scale of tens to hundreds of kilometers subglacial processes are notoriously difficult to constrain on these large scales which are relevant to the understanding of regional and continental ice motion

the radiation characteristics of an arbitrary antenna positioned on a polar ice sheet the radiation pattern of an arbitrarily shaped antenna system placed directly on the surface of a polar ice sheet a configuration often used for ice probing radars may be determined at any depth within the ice sheet using a two step process the distribution of the radiated field just beneath the surface may be determined first using the method of moments and geometric optics can then be used to calculate the effects of the exponential density gradient of the ice closed form solutions of the ray tracing equation valid at any depth within the ice sheet allow rapid calculation of the effective look angles and gain increases of any antenna mounted on the surface these solutions asymptotically approach previously determined relations which are valid only at great depths a numerical example demonstrates the use of this procedure in the design of a practical ice probing radar antenna system

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

constraining attenuation uncertainty in common midpoint radar surveys of ice sheets for common offset radar data there is no clear way to disentangle path effects from reflector characteristics so efforts to determine the physical properties at the bed using reflection amplitude are inherently limited by the constraints on englacial attenuation in this study we identify the theoretical considerations required for interpreting bistatic radar surveys and use data collected on the northeast greenland ice stream negis and kamb ice stream to compute local attenuation profiles we found that failing to correct for angle dependent controls on return power including antenna directivity the reflection coefficient and refractive focusing can bias the computed attenuation rates as much as 30 db km for reflectors at 1 km depth because the radiation characteristics are the dominant source of uncertainty in our data we recommend either a simplified survey design for the future where the antennae are decoupled from the ice surface or additional data collection to constrain the near field permittivity and its effect on the radiation pattern depth averaged attenuation rates computed using cmp methods for deep reflectors yield values 10 db km higher than attenuation rates computed using common offset techniques with the same data we attribute these anomolously high attenuation rates to additional wavenumber and therefore angle dependent interferences between sub wavelength reflectors

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