internal layer tracing and age depth accumulation relationships for the northern greenland ice sheet clues to previous ice sheet structure and long term glaciological processes are preserved in the internal layering configuration of the greenland ice sheet information about these internal layers has been retrieved over many parts of the ice sheet with the university of kansas ice penetrating radar we report on the coherence of these layers over very large distances describe a method of tracing these layers along thousands of kilometers of flight line and do so for one flight during the 1999 program for arctic regional climate assessment parca aircraft campaign we determine the ages of these layers based on information at the grip ice core site and extend these ages along the flight line to camp century where they are compared to modeled derived age estimates these ages agree with each other to between 2 and 15 differences that can be substantially reduced with minor changes to the model parameters accumulation rate and shear layer depth finally we are able to derive estimates of accumulation rates along the flight line by fitting the age depth data from layer tracing to a dansgaard johnsen model with a minimization technique providing estimates that match recent accumulation patterns within a few centimeters per year

greenland flow variability from ice sheet wide velocity mapping using radarsat synthetic aperture radar data we have mapped the flow velocity over much of the greenland ice sheet for the winters of 2000 01 and 2005 06 these maps provide a detailed view of the ice sheet flow including that of the hundreds of glaciers draining the interior the focused patterns of flow at the coast suggest a strong influence of bedrock topography differences between our two maps confirm numerous early observations of accelerated outlet glacier flow as well as revealing previously unrecognized changes the overall pattern is one of speed up accompanied by terminus retreat but there are also several instances of surge behavior and a few cases of glacier slowdown comprehensive mappings such as these at regular intervals provide an important new observational capability for understanding ice sheet variability

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

switch off of a major enhanced ice flow unit in east antarctica 1 the east antarctic ice sheet eais is the largest reservoir of ice on the planet by an order of magnitude compared with the west antarctic ice sheet wais it is traditionally considered to be relatively stable with only minor adjustments to its configuration over glacial interglacial cycles here we present the results of a radio echo sounding survey from coats land east antarctica which suggests that parts of the eais outlet drainage system may have changed significantly since the last glacial maximum we have identified an enhanced flow unit from buckled internal layering and smooth bed morphology that is no longer active we believe this feature to have shut down at some point since the last glacial maximum 20 ka bp

on the necessity of applying a rotation to instantaneous velocity measurements in river flows in studies on river channel flow turbulence it is often the case that the measured mean vertical velocity is different from zero indicating that the frame of reference of the current meter is not parallel to the flow streamline this situation affects the estimate of reynolds shear stress in the streamwise and vertical planes and consequently the analysis of the flow turbulent structure one way to solve this problem is to correct data by applying a rotation and this is reviewed in the first part of the paper however in fluvial geomorphology the studied flow is often complex and streamlines may exhibit significant changes from one point of measurement to the other in this context applying a rotation complicates the situation more than it simplifies it the second part of this paper examines the question of velocity data correction in complex flows using a field example of the turbulent boundary layer over a very rough gravel bed and a laboratory example taken from flow at a river channel confluence in both cases velocity vectors are spatially variable in the first case errors in the reynolds shear stress estimates are relatively low ranging from 13 to 7 per cent deg while in the second case they are much larger 200 to 164 per cent deg the significance of these errors on the interpretation of turbulence statistics in river channel flows is discussed we propose that corrections should be applied in all clear cases of sensor misalignment and when the frame of reference changes spatially and temporally however no corrections should be used where different flow velocity vector orientations not sensor misalignment are responsible for the mean vertical velocity differing from zero

basal conditions and ice dynamics inferred from radar derived internal stratigraphy of the northeast greenland ice stream we analyze the internal stratigraphy in radio echo sounding data of the northeast greenland ice stream to infer past and present ice dynamics in the upper reaches of the ice stream we propose that shear margin steady state folds in internal reflecting horizons irhs form due to the influence of ice flow over spatially varying basal lubrication irhs are generally lower in the ice stream than outside likely because of greater basal melting in the ice stream from enhanced geothermal flux and heat of sliding strain rate modeling of irhs deposited during the holocene indicates no recent major changes in ice stream vigor or extent in this region downstream of our survey irhs are disrupted as the ice flows into a prominent overdeepening when combined with additional data from other studies these data suggest that upstream portions of the ice stream are controlled by variations in basal lubrication whereas downstream portions are confined by basal topography

salt tectonics in the atlantic margin of morocco the offshore basins of morocco s atlantic margin represent the westernmost exploration frontier area in north africa figure 1 whereas the presence of an active petroleum system is very well established onshore e g essaouira basin only promising oil shows and one minor oil accumulation have been found to date offshore e g tarfaya basin however all previous exploration effort concentrated only on the shelfal region and therefore it should not be very surprising that several blocks were licensed in the last few years covering large offshore acreage in water depths to almost 3000 m figure 1 as the reconnaissance licenses for these permits are in the process of being converted to exploration licenses a shift from regional scale 2 d seismic reflection data acquisition to prospect scale 2 d and even 3 d is under way figure 1 index map of the atlantic margin of morocco showing location of a regional cross section figure 2 and simplified salt tectonics map figure 3 an active petroleum system is not proven in the deepwater region although a broad world class salt basin stretches along the coastline creating spectacular structural traps the first deepwater well might be spudded as early as next year to test one of these salt related structures a potential target zone within the deepwater salt basin is at the westernmost leading edge of the salt deformational front where a very well developed toe thrust zone formed during the mid tertiary the salt tectonics of offshore morocco may provide some very specific analogs for exploration efforts in other salt basins of the world perhaps the easiest way to understand the scale of the salt basin offshore morocco is to compile regional seismic transects figure 2 shows a line drawing of a regional composite seismic profile across the ras tafelney and safi haute mer permits figure 1 note that the atlantic passive margin of

past and future grounding line retreat of the west antarctic ice sheet the history of deglaciation of the west antarctic ice sheet wais gives clues about its future southward grounding line migration was dated past three locations in the ross sea embayment results indicate that most recession occurred during the middle to late holocene in the absence of substantial sea level or climate forcing current grounding line retreat may reflect ongoing ice recession that has been under way since the early holocene if so the wais could continue to retreat even in the absence of further external forcing

frequency wavenumber velocity spectra taylor s hypothesis and length scales in a natural gravel bed river 1 macroscale turbulent coherent flow structures in a natural fast flowing river were examined with a combination of a novel 2 mhz acoustic doppler beam adb and a maximum likelihood estimator mle to characterize the streamwise horizontal length scales and persistence of coherent flow structures by measuring the frequency f streamwise wavenumber ks energy density velocity spectrum e f ks for the first time in natural rivers the adb was deployed under a range of froude numbers 0 1 0 6 at high reynolds numbers 106 based on depth and velocity conditions within a gravel bed reach of the kootenai river idaho the mle employed on the adb data increased our ability to describe river motions with relatively long 10 m length scales in 1 m water depths the e f ks spectra fell along a ridge described by v f ks where vis the mean velocity over depth consistent with taylor s hypothesis new consistent length scale measures are defined based on averaged wavelengths of the low frequencye f ks and coherence spectra energetic 50 of the total spectral energy low frequency f 1 m s lmwere found to be significantly longer than their corresponding coherence lengths suggesting that the turbulent structures evolve rapidly under these conditions this is attributed to the stretching and concomitant deformation of preexisting macroturbulent motions by the ubiquitous bathymetry induced spatial flow accelerations present in a natural gravel bed river

anomalous seafloor mounds in the northern natal valley southwest indian ocean implications for the east african rift system abstract the natal valley southwest indian ocean has a complicated and protracted opening history as has the surrounding southwest indian ocean recently collected multibeam swath bathymetry and 3 5 khz seismic data from the natal valley reveal anomalous seafloor mounds in the northern natal valley the significance of these domes as recorders of the geological history of the natal valley and se african margin has been overlooked with little attempt made to identify their origin evolution or tectonic significance this paper aims to describe these features from a morphological perspective and to use their occurrence as a means to better understand the geological and oceanographic evolution of this basin the seafloor mounds are distinct in both shallow seismic and morphological character from the surrounding seafloor of the natal valley between 25 km and 31 km long and 16 km and 18 km wide these features rise some 400 m above the sedimentary deposits that have filled in the natal valley such macro scale features have not previously been described from the natal valley or from other passive margins globally they are not the result of bottom water circulation salt tectonics rather igneous activity is favoured as the origin for these anomalous seafloor features we propose a hypothesis that the anomalous seafloor mounds observed in the natal valley are related to igneous activity associated with the ears the complicated opening history and antecedent geology coupled with the southward propagation of the east african rift system creates a unique setting where continental rift associated features have been developed in a marine setting

recovery of subglacial water extent from greenland radar survey data subglacial water threatens the stability of a deep ice sheet the greenland ice sheet has been the subject of extensive radar surveys under nasa s parca program and others yet the extent of basal water has not previously been explored in detail this paper provides a method for recovering basal radar reflection characteristics that yield robust discrimination of subglacial water using parca data by narrowing the spread of measured echo intensities sources of validation are provided and they support the procedure and its results the method can be extended to the entire greenland radar dataset it is not dependent on details of radar characteristics but relies on sufficient penetration performance high fidelity data acquisition and appropriate treatment of signals and interface statistics

effects of turbulence on the transport of individual particles as bedload in a gravel bed river this study investigates the association between mean and turbulent flow variables and the movement of individual particles in a gravel bed river the experimental design implemented in the eaton north river quebec canada is based on the simultaneous observations at a high temporal resolution of both particle movements as bedload using an underwater video camera and of the streamwise and vertical flow velocity components using a vertical array of three electromagnetic current meters ecms the frequency and distance of displacement of particles larger than 20 mm that were sliding or rolling on the bed were measured from a 10 minutes long film mean and turbulent flow properties obtained for periods without sediment transport are compared to those when particles were sliding and rolling when particles are sliding weak differences are present for the mean streamwise velocity and normal vertical stresses instantaneous reynolds shear stresses are significantly lower for sliding events which was not expected but could be explained by the important dominance of quadrant 3 events inward interactions when particles are rolling only the vertical normal stresses show a weak difference from those observed in the absence of transport but they tend to occur when quadrant 2 ejections dominate the flow field for both sliding and rolling particles vertical and or streamwise fluid accelerations show high magnitude values when compared to periods without transport for sliding particles streamwise acceleration is mostly negative and combines most of the time with a positive vertical acceleration for rolling particles streamwise and vertical acceleration are predominantly of opposite sign these results suggest that fluid acceleration or deceleration in the streamwise and vertical directions is affecting the pressure field around particles in this study fluid acceleration seems to play a more important role than reynolds shear stress or normal stresses for bedload movements copyright 2010 john wiley sons ltd

dating ice flow change near the flow divide at roosevelt island antarctica by using a thermomechanical model to predict radar stratigraphy 1 radar detected internal layering contained in some ice divides shows upwarped arches termed raymond bumps the distribution of their amplitude with height can date the onset of divide flow reflecting changes in the basin structure of the ice sheet the distribution depends on rheology surface geometry accumulation rate and temperature conway et al 1999 used an isothermal parameterized ice flow model to estimate a date of 3200 years b p with no error estimate for the onset of divide flow in roosevelt island ross ice shelf which they associated with grounding line retreat of the west antarctic ice sheet no other retreat dating exists for an area of the ross ice shelf distant from geological exposures employing a full thermomechanically coupled transient model we use a direct search to determine which ice rheology best fits the observed bump distribution and surface profile estimating the sensitivity of the dating to model parameters and exploring possible reasons for the observed asymmetry of the surface profile our main results are as follows 1 a best estimate of the date of grounding line retreat near roosevelt island is 3000 years b p bounded by 2300 and 4200 years b p 2 standard rheology power n 3 can only match the observed bump distribution for unrealistic isothermal models 3 for thermomechanically coupled models a high power n 4 in glen s law is required 4 low power rheologies and wind scouring cannot produce correct bump amplitude distributions 5 asymmetric accumulation cannot explain the observed asymmetry of the surface profile 6 modeling isochrones in flanking regions also indicates changes in flow around 3000 years b p

three dimensional flow influences on radar layer stratigraphy variations in the depth of radar detectable englacial layers isochrones are commonly used to assess past variability in accumulation rates but little is known about the effect of internal and basal flow variations on isochrone deflections e g bumps troughs in this paper we show how the isochrones are affected by such variation using a three dimensional flow model to investigate changes in the flow mode and in increased basal melting we also investigate how transverse flows with lateral velocity gradients affect the development of isochrones we use the model to visualize how such variations will be seen in radar lines which cross the flow direction we show that in the presence of lateral gradients in the flow field we can produce bumps and troughs when viewed along transects perpendicular to the flow the model results show that the influences of flow convergence melting and changes in flow mode when coupled together affect isochrones over the whole depth of the ice sheet finally changes in the near surface layers cannot be solely attributed to spatial variation in the accumulation rate there can also be a strong signal from changes in the flow mode

coherent radar ice thickness measurements over the greenland ice sheet we developed two 150 mhz coherent radar depth sounders for ice thickness measurements over the greenland ice sheet we developed one of these using connectorized components and the other using radio frequency integrated circuits rfics both systems are designed to use pulse compression techniques and coherent integration to obtain the high sensitivity required to measure the thickness of more than 4 km of cold ice we used these systems to collect radar data over the interior and margins of the ice sheet and several outlet glaciers we operated both radar systems on the nasa p 3b aircraft equipped with gps receivers radar data are tagged with gps derived location information and are collected in conjunction with laser altimeter measurements we have reduced all data collected since 1993 and derived ice thickness along all flight lines flown in support of program for regional climate assessment parca investigations and the north greenland ice core project radar echograms and derived ice thickness data are placed on a server at the university of kansas http tornado rsl ukans edu greenlanddata htm for easy access by the scientific community we obtained good ice thickness information with an accuracy of 10 m over 90 of the flight lines flown as a part of the parca initiative in this paper we provide a brief description of the system along with samples of data over the interior along the 2000 m contour line in the south and from a few selected outlet glaciers

on the effects of anisotropic rheology on ice flow internal structure and the age depth relationship at ice divides we use numerical modeling with a full system stokes solver to elucidate the effects of nonlinear rheology and strain induced anisotropy on ice flow at ice divides we find that anisotropic rheology profoundly affects the shape of both isochrone layering and surface topography anisotropic effects cause the formation of a downward curving fold i e a syncline in isochrones in the lower central area beneath the ice divide when the resulting syncline is superimposed on the well known raymond anticline a double peaked raymond bump is formed furthermore to each side of the raymond bump flanking synclines are formed in addition anisotropic effects are found to give rise to a subtle concavity in the surface profile to both sides of the summit the lower center syncline the flanking synclines and the near summit surface concavity have all previously been observed in nature but hitherto no explanation for the genesis of these features has been given we compare modeling results with radiograms collected from fuchs ice piedmont and kealey ice rise antarctica good overall agreement is found in particular we are able to reproduce all observed qualitative features of surface geometry and internal layering by including and only by including the effects of induced nonlinear rheological anisotropy on flow rheological anisotropy has the potential to profoundly affect the age distribution with depth and caution must be exercised when estimating age of ice from ice cores with an isotropic model the occurrence of linear features parallel to the ridge of ice divides often seen in satellite imagery is indicative of long term stability rather than signs of ongoing ice divide migration as previously suggested such ice divides are ideal locations for extracting ice cores

information on grain sizes in gravel bed rivers by automated image analysis abstract the time required in the field to characterize textural variation over gravel surfaces can be reduced by taking vertical photographs for subsequent image analysis we present modified edge detection algorithms which combine edge seeding with an image porosity concept and partial watershed segmentation the methods allow quick reliable and operator independent size analysis from a wide range of vertical bed surface images they are tested using 24 naturally lit images of an exposed river bed with mixed lithologies and partial burial of gravel by sand grain size percentiles derived by automated image analysis correlate closely with those from manual image analysis with only small and consistent degrees of bias they also correlate well with percentiles from field measurements with substantial bias which however is consistent so that it can be corrected for leaving a residual scatter of 0 25 where log2 mm over a wide range of bed conditions the bias depends somewhat on sand cover and the biggest residual discrepancies are for tail percentiles

tracer transport in the greenland ice sheet three dimensional isotopic stratigraphy abstract polar ice cores can provide both a record of climate history and a sharp test of the performance of numerical ice dynamics models the stratigraphic structure of an ice sheet is an expression of its full depositional and dynamic history and thus presents a greater challenge to computer models than merely matching the contemporary ice thickness and areal extent we describe a coupled model of ice and tracer dynamics that is realized by adding a semi lagrangian tracer transport scheme to a conventional thermomechanical ice dynamics model model skill is demonstrated by using ice core data from the grip site near summit greenland to successfully predict the isotopic stratigraphy of ice cores at other deep drilling sites the success of this effort indicates that when compensated for the effects of ice flow and elevation all the deep cores relate a coherent glacial history over the past 120 000 years according to the simulation results the oldest greenland ice lies beneath the grip gisp2 and northgrip sites although comparably old ice may also be found in north greenland and east central greenland

ice flow direction change in interior west antarctica upstream of byrd station west antarctica ice penetrating radar data reveal a distinctive fold structure within the ice in which isochronous layers are unusually deep the fold has an axis more than 50 kilometers long which is aligned up to 45 to the ice flow direction although explanations for the fold s formation under the present flow are problematic it can be explained if flow was parallel to the fold axis 1500 years ago this flow change may be associated with ice stream alterations nearer the margin if this is true central west antarctica may respond to future alterations more than previously thought

a continuity index for assessing ice sheet dynamics from radar sounded internal layers radio echo sounding res of polar ice sheets reveals extensive internal layering the degree of continuity of internal layering holds critical information about the ice flow field but previous analyses of this parameter have been limited to qualitative classifications here we present a new quantitative method for analyzing internal layer continuity named the continuity index when applied to data from pine island glacier west antarctica the new method clearly identifies a continuum of discontinuity of internal layers that corresponds with the current ice velocity field the analysis provides further support that the main trunk and tributaries are unlikely to have undergone substantial migration since the deposition of the internal layering significantly our new method for analyzing internal layers is readily transferable across res datasets offering promise for data led assessments of past and present flow dynamics across large areas of antarctica and greenland

isochronous information in a greenland ice sheet radio echo sounding data set the evaluation of ice sheet models is one of the pressing problems in the study of ice sheet dynamics here we examine the question of how much isochronous information is contained within the publicly available center for remote sensing of ice sheets cresis greenland airborne radio echo soundings data set we identify regions containing isochronous reflectors using automatic radio echo sounding processing aresp algorithms we find that isochronous reflectors are present within 36 of the cresis radio echo sounding englacial data by location and 41 by total number of data between 1000 and 3000 m in depth isochronous reflectors are present along more than 50 of the data set flight path lower volumes of cold glacial period ice also correspond with more isochronous reflectors we find good agreement between aresp and continuity index results providing confidence in these findings ice structure data sets based on data identified here will be of use in evaluating ice sheet simulations and the assessment of past rates of snow accumulation

increasing rates of ice mass loss from the greenland and antarctic ice sheets revealed by grace 1 we use monthly measurements of time variable gravity from the grace gravity recovery and climate experiment satellite gravity mission to determine the ice mass loss for the greenland and antarctic ice sheets during the period between april 2002 and february 2009 we find that during this time period the mass loss of the ice sheets is not a constant but accelerating with time i e that the grace observations are better represented by a quadratic trend than by a linear one implying that the ice sheets contribution to sea level becomes larger with time in greenland the mass loss increased from 137 gt yr in 2002 2003 to 286 gt yr in 2007 2009 i e an acceleration of 30 11 gt yr2 in 2002 2009 in antarctica the mass loss increased from 104 gt yr in 2002 2006 to 246 gt yr in 2006 2009 i e an acceleration of 26 14 gt yr2 in 2002 2009 the observed acceleration in ice sheet mass loss helps reconcile grace ice mass estimates obtained for different time periods

resolving the internal and basal geometry of ice masses using imaging phase sensitive radar the phase sensitive radio echo sounder pres is a powerful new instrument that can measure the depth of internal layers and the glacier bed to millimetre accuracy we use a stationary 16 antenna pres array on store glacier in west greenland to measure the three dimensional orientation of dipping internal reflectors extending the capabilities of pres beyond conventional depth sounding this novel technique portrays the effectiveness of pres in deriving the orientation of dipping internal layers that may complement profiles obtained through other geophysical surveying methods deriving ice vertical strain rates from changes in layer depth as measured by a sequence of pres observations assumes that the internal reflections come from vertically beneath the antenna by revealing the orientation of internal reflectors and the potential deviation from nadir of their associated reflections the use of an antenna array can correct this assumption while the array configuration was able to resolve the geometry of englacial layers the same configuration could not be used to accurately image the glacier bed here we use simulations of the performance of different array geometries to identify configurations that can be tailored to study different types of basal geometry for future deployments

macroturbulent structure of open channel flow over gravel beds the turbulent structure of open channel flow over a mobile gravel bed was investigated in an 8 m long 0 3 m wide and 0 3 m deep tilting flume a flow visualization technique was used and complemented by measurements of flow velocity fluctuations near the bed the experiments reveal that turbulent flow consists of a sequence of large scale eddies with a vertical size close to the flow depth an average length equal to four to five depths and a width of about two depths the downstream motion of these eddies causes quasiperiodic fluctuations of the local flow velocity components the development of longitudinal troughs and ridges on the mobile bed and preferential transport of bed particles along troughs are related to the effect of the eddies the experimental results indicate that the depth scale eddies are an important turbulence mechanism contributing to sediment transport

greenland ice sheet volume sensitivity to basal surface and initial conditions derived from an adjoint model we extend the application of control methods to a comprehensive three dimensional thermomechanical ice sheet model sicopolis simulation code for polythermal ice sheets lagrange multipliers i e sensitivities are computed with an exact efficient adjoint model that has been generated from sicopolis by rigorous application of automatic differentiation the case study uses the adjoint model to determine the sensitivity of the total greenland ice volume to various control variables over a 100 year period the control space has of the order 1 2 10 6 elements consisting of spatial fields of basal flow parameters surface and basal forcings and initial conditions reliability of the adjoint model was tested through finite difference perturbation calculations for various control variables and perturbation regions ascertaining quantitative inferences of the adjoint model as well as confirming qualitative aspects of ice sheet sensitivities e g expected regional variations we detect regions where model sensitivities are seemingly unexpected or counter intuitive albeit real in the sense of actual model behavior an example is inferred regions where sensitivities of ice sheet volume to basal sliding coefficient are positive i e where a local increase in basal sliding parameter increases the ice sheet volume similarly positive generally negative ice temperature sensitivities in certain parts of the ice sheet are found the detection of which seems highly unlikely if only conventional perturbation experiments had been used the object of this paper is largely a proof of concept available adjoint code generation tools now open up a variety of novel model applications notably with regard to sensitivity and uncertainty analyses and ice sheet state estimation or data assimilation

widespread persistent thickening of the east antarctic ice sheet by freezing from the base an international polar year aerogeophysical investigation of the high interior of east antarctica reveals widespread freeze on that drives substantial mass redistribution at the bottom of the ice sheet although the surface accumulation of snow remains the primary mechanism for ice sheet growth beneath dome a 24 of the base by area is frozen on ice in some places up to half of the ice thickness has been added from below these ice packages result from the conductive cooling of water ponded near the gamburtsev subglacial mountain ridges and the supercooling of water forced up steep valley walls persistent freeze on thickens the ice column alters basal ice rheology and fabric and upwarps the overlying ice sheet including the oldest atmospheric climate archive and drives flow behavior not captured in present models

is the critical shields stress for incipient sediment motion dependent on channel bed slope data from laboratory flumes and natural streams show that the critical shields stress for initial sediment motion increases with channel slope which indicates that particles of the same size are more stable on steeper slopes this observation is contrary to standard models that predict reduced stability with increasing slope due to the added downstream gravitational force processes that might explain this discrepancy are explored using a simple force balance model including increased drag from channel walls and bed morphology variable friction angles grain emergence flow aeration and changes to the local flow velocity and turbulent fluctuations surprisingly increased drag due to changes in bed morphology does not appear to be the cause of the slope dependency because both the magnitude and trend of the critical shields stress are similar for flume experiments and natural streams and significant variations in bed morphology in flumes is unlikely instead grain emergence and changes in local flow velocity and turbulent fluctuations seem to be responsible for the slope dependency due to the coincident increase in the ratio of bed roughness scale to flow depth i e relative roughness a model for the local velocity within the grain roughness layer is proposed based on a 1 d eddy viscosity with wake mixing in addition the magnitude of near bed turbulent fluctuations is shown to depend on the depth averaged flow velocity and the relative roughness extension of the model to mixed grain sizes indicates that the coarser fraction becomes increasingly difficult to transport on steeper slopes

automated processing to derive dip angles of englacial radar reflectors in ice sheets we present a novel automated processing method for obtaining layer dip from radio echo sounding res data the method is robust easily applicable and can be used to process large several terabytes ground and airborne res datasets using modest computing resources we give test results from the application of the method to two antarctic datasets the fletcher promontory ground based radar dataset and the wilkes subglacial basin airborne radar dataset the automated res processing aresp method comprises the basic steps 1 res noise reduction 2 radar layer identification 3 isolation of individual layer objects 4 measurement of orientation and other object properties 5 elimination of noise in the orientation data and 6 collation of the valid dip information the apparent dip datasets produced by the method will aid glaciologists seeking to understand ice flow dynamics in greenland and antarctica aresp could enable a shift from selective regional case studies to ice sheet scale studies

salt tectonics in the cap boujdour area aaiun basin nw africa seismic reflection data indicate the moroccan salt basin extends to the cap boujdour area in the aaiun basin two salt diapir structures have been identified and several areas of collapsed strata indicate probable salt removal at the shelf edge the presence of salt in this area correlates to the conjugate southern george s bank basin and the baltimore canyon area and it is suggested that the salt extends southward from the known salt diapir province in the george s bank basin southward to the great stone dome the paucity of salt diapirs is attributed to the thick carbonate and anhydrite sequence which was deposited soon after salt deposition that inhibited halokinesis the presence of salt along this large segment of the atlantic margin should increase its hydrocarbon potential with traps created around salt diapirs and provision of migration pathways from deep potential source rocks in the early cretaceous and jurassic strata to shallower levels

tracer transport in an isochronal ice sheet model the full history of ice sheet and climate interactions is recorded in the vertical profiles of geochemical tracers in polar ice sheets numerical simulations of these archives promise great advances both in the interpretation of these reconstructions and the validation of the models themselves however fundamental mathematical shortcomings of existing models subject tracers to spurious diffusion thwarting straightforward solutions here i propose a new vertical discretization for ice sheet models that eliminates numerical diffusion entirely vertical motion through the model mesh is avoided by mimicking the real world flow of ice as a thinning of underlying layers a new layer is added to the surface at equidistant time intervals isochronally thus identifying each layer uniquely by its time of deposition and age this new approach is implemented for a two dimensional section through the summit of the greenland ice sheet the ability to directly compare simulations of vertical ice cores with reconstructed data is used to find optimal model parameters from a large ensemble of simulations it is shown that because this tuning method uses information from all times included in the ice core it constrains ice sheet sensitivity more robustly than a realistic reproduction of the modern ice sheet surface

mesozoic sequence of fuerteventura canary islands witness of early jurassic sea floor spreading in the central atlantic the fuerteventura jurassic sedimentary succession consists of oceanic and clastic deposits the latter derived from the southwestern moroccan continental margin normal mid oceanic ridge basalt n morb flows and breccias are found at the base of the sequence and witness sea floor spreading events in the central atlantic these basalts were extruded in a postrift environment post late pliensbachian we propose a toarcian age for the atlantic oceanic floor in this region on the basis of the presence higher up in the sequence of the bositra buchi filament microfacies aalenian bajocian and of clastic deposits reflecting tectono eustatic events e g late toarcian to mid callovian erosion of the rift shoulder the s 1 sea floor oceanic magnetic anomaly west of fuerteventura is therefore at least toarcian in age the remaining sequence records atlantic tethyan basinal facies e g callovian oxfordian red clays aptian albian black shales alternating with clastic deposits e g kimmeridgian berriasian periplatform calciturbidites and a lower cretaceous deep sea fan system the fuerteventura n morb outcrops represent the only early jurassic oceanic basement described so far in the central atlantic they are covered by a 1600 m nearly continuous sedimentary sequence which extends to upper cretaceous facies

reach scale variability of turbulent flow characteristics in a gravel bed river in gravel bed rivers bed roughness is often controlled by the spatial distribution of pebble clusters and large protuberant clasts the turbulent flow structure around these roughness elements has been studied extensively as opposed to the effects of the spatial variability of the bed roughness on the structure of the flow at the reach scale in this paper we examine the effects of roughness elements on the spatial variability of the mean flow velocity profiles and of the properties of turbulent flow structures at the reach scale the experiment was carried out in two reaches of 25 m long in the eaton north river quebec we produced detailed topographic maps of the river reaches to obtain the spatial distribution of the main roughness elements the flow is characterized with velocity time series measured using electromagnetic current meters mounted on a vertical wading rod we collected 65 velocity profiles from which we derived turbulent flow characteristics results showed that in spite of the complex configuration of the bed velocity profiles are predominantly log linear mean and turbulent flow characteristics are only affected at a local scale and over short distances by the presence of roughness elements at the reach scale the distribution of individual roughness elements does not greatly control the spatial variability of turbulent flow properties

truncation and translation of appalachian promontories mid paleozoic strike slip tectonics and basin initiation accreted terranes of the appalachian iapetan and peri gondwanan realms display structural trends that are mainly concordant with promontories and embayments in the laurentian margin indicating that during accretion the shape of the continental margin acted as a template around which accreted terranes were molded in north carolina and new foundland post accretion transcurrent motion appears to be recorded by displaced outboard portions of promontories no longer concordant with those in laurentia a bend in structural trends confined to the peri gondwanan realm at the north carolina south carolina state line is interpreted to represent the dextrally displaced outboard portion of the virginia promontory in newfoundland the hermitage flexure is interpreted as a dextrally displaced laurentian promontory that originated along strike to the northeast in both places promontories were truncated and dextrally translated for 220 250 km by a devonian mississippian orogen parallel transcurrent system which may well have extended for the length of the eastern laurentian margin south of the nearby st lawrence promontory extreme thinning of appalachian crust beneath the maritimes basin is consistent with extension at a releasing stepover estimates of extension are consistent with those obtained from promontory offsets

radiostratigraphy and age structure of the greenland ice sheet 2015 the authors several decades of ice penetrating radar surveys of the greenland and antarctic ice sheets have observed numerous widespread internal reflections analysis of this radiostratigraphy has produced valuable insights into ice sheet dynamics and motivates additional mapping of these reflections here we present a comprehensive deep radiostratigraphy of the greenland ice sheet from airborne deep ice penetrating radar data collected over greenland by the university of kansas between 1993 and 2013 to map this radiostratigraphy efficiently we developed new techniques for predicting reflection slope from the phase recorded by coherent radars when integrated along track these slope fields predict the radiostratigraphy and simplify semiautomatic reflection tracing core intersecting reflections were dated using synchronized depth age relationships for six deep ice cores additional reflections were dated by matching reflections between transects and by extending reflection inferred depth age relationships using the local effective vertical strain rate the oldest reflections dating to the eemian period are found mostly in the northern part of the ice sheet within the onset regions of several fast flowing outlet glaciers and ice streams reflections typically do not conform to the bed topography disrupted radiostratigraphy is also observed in a region north of the northeast greenland ice stream that is not presently flowing rapidly dated reflections are used to generate a gridded age volume for most of the ice sheet and also to determine the depths of key climate transitions that were not observed directly this radiostratigraphy provides a new constraint on the dynamics and history of the greenland ice sheet

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

inference of velocity pattern from isochronous layers in firn using an inverse method this study investigates the suitability of a kinematic approach to nd the velocity eld from dated internal layer architecture in rn internal layers are isochrones and the depositional age of a layer particle is treated as a tracer the forward problem uses two dimensional steady state advection of age and conservation of mass to predict layer ar chitecture dieren t combinations of constraints on horizontal or vertical velocity properties are added d the inverse problem can be formulated as the solution of underdetermined and overdetermined systems of equations e the systems are solved using singular value decomposition allowing analysis of the singular value spectrum model d resolution e and data resolution d solutions of the inverse problem e are evaluated by comparing the velocity eld solutions with synthetic input velocity data compared to conventional accumulation estimates d the new approach takes lateral advection into account e enabling improved separation of spatial and temporal variations in accumulation d two glaciological applications are presented the determination of the migration velocity of a spa tially non stationary accumulation pattern and reconstruction of past accumulation and its stationarity over time e

time dependence of the spatial pattern of accumulation rate in east antarctica deduced from isochronic radar layers using a 3 d numerical ice flow model 1 in east antarctica surface mass balance data can only be obtained from the sparsely distributed ice cores when considering time periods greater than a few decades observations of internal layers measured by airborne ice penetrating radar in principle permit extrapolation of mass balance information from these ice cores we use radar survey lines gathered in the 1970s and a three dimensional numerical model to investigate the feasibility of such extrapolation seeking to match the calculations of englacial layer geometry with observations first we justify the use of a three dimensional model by showing that simple vertical flow models cannot explain all the observations and that horizontal advection is a significant glacial process then we examine processes that affect calculations of layer geometry finding that spatial accumulation rate patterns are extremely important while geothermal heat flux and flow mode sliding or internal deformation are of substantially less importance where the layer is from the last glacial maximum 17 5 ka we find a very good match between the spatial pattern of accumulation rates inferred from this layer and the modern spatial pattern of accumulation rates when considering deeper layers from beyond the current interglacial we find that a different spatial accumulation rate pattern must have existed in addition to the known change in accumulation rate from ice cores the glacial spatial accumulation rate pattern would have had proportionally greater accumulation at the south pole than now compared with the vostok and dome c ice cores

tracing the depth of the holocene ice in north greenland from radio echo sounding data radio echo sounding surveys over the greenland ice sheet show clear extensive internal layering and comparisons with age depth scales from deep ice cores allow for dating of the layering along the ice divide we present one of the first attempts to extend the dated layers beyond the ice core drill sites by locating the depth of the bolling allerod transition in 400 flight lines using an automated fitting method results show that the transition is located in the upper one third of the ice column in the central part of north greenland while the transition lowers towards the margin this pattern mirrors the present surface accumulation and also indicates that a substantial amount of pre holocene ice must be present in central north greenland

evidence for a large surface ablation zone in central east antarctica during the last ice age abstract internal isochronous ice sheet layers recorded by airborne ice penetrating radar were measured along an ice flowline across a large 1 km high subglacial hill in the foreground of the transantarctic mountains the layers dated through an existing stratigraphic link with the vostok ice core converge with the ice surface as ice flows over the hill without noticeable change to their separation with each other or the ice base a two dimensional ice flow model that calculates isochrons and particle flowpaths and accounts for ice flow over the hill under steady state conditions requires net ablation via sublimation over the stoss face for the predicted isochrons to match the measured internal layers satellite remote sensing data show no sign of exposed ancient ice at this site however given the lack of exposed glacial ice surface balance conditions must have changed recently from the net ablation that is predicted at this site for the last 85 000 years to accumulation

tracer transport in the greenland ice sheet constraints on ice cores and glacial history the climate history and dynamics of the greenland ice sheet are studied using a coupled model of the depositional provenance and transport of glacier ice allowing simultaneous prediction of the detailed isotopic stratigraphy of ice cores at all the major greenland sites adopting a novel method for reconstructing the age depth relationship we greatly improve the accuracy of semi lagrangian tracer tracking schemes and can readily incorporate an age dependent ice rheology the larger aim of our study is to impose new constraints on the glacial history of the greenland ice sheet leading sources of uncertainty in the climate and dynamic history are encapsulated in a small number of parameters the temperature and elevation isotopic sensitivities the glacial interglacial precipitation contrast and the effective viscosity of ice in the flow law comparing predicted and observed ice layering at ice core sites we establish plausible ranges for the key model parameters identify climate and dynamic histories that are mutually consistent and recover the past depositional elevation of ice cores to ease interpretation of their climatic records with the coupled three dimensional model of ice dynamics and provenance transport we propose a method to place all the ice core records on a common time scale and use discrepancies to adjust the reconstructed climate history analysis of simulated grip ice layering and borehole temperature profiles confirms that the grip record is sensitive to the dynamic as well as to the climatic history but not enough to strongly limit speculation on the state of the greenland ice sheet during the eemian in contrast our study indicates that the dye 3 and camp century ice cores are extremely sensitive to ice dynamics and greatly constrain eemian ice sheet reconstructions we suggest that the maximum eemian sea level contribution of the ice sheet was in the range of 3 5 4 5 m

size shape and dynamics of large scale turbulent flow structures in a gravel bed river in this paper we present a detailed investigation of the size scale and dynamics of macro turbulent flow structures in gravel bed rivers we used an array of seven electromagnetic current meters with high resolution in both space and time to measure the streamwise velocity fluctuations in a gravel bed river the array was deployed successively in various configurations in order to quantify the vertical lateral and longitudinal extent of the flow structures and to estimate their advecting velocities to depict the spatial and temporal properties of the flow structures we used space time velocity matrices space time correlation analysis and coherent structure detection schemes the results show that the large scale turbulent flow structures in a gravel bed river occupy the entire depth of the flow and that they are elongated and narrow the length of the structures is 3 to 5 times the flow depth while the width is between 0 5 and 1 times flow depth in spite of the high roughness of the bed these values are similar to those reported in the literature for laboratory experiments on large scale turbulent flow structures the dynamics of the large scale turbulent flow structures investigated using flow visualization highlight the interactions between the outer flow region and the near bed region our evidence suggests that large scale flow incursions trigger ejections in the near bed region that can develop into megabursts that can reach the water surface

radar internal layers from the greenland summit ice penetrating radar measurements made over the summit region of greenland show returns from internal layers which can be used to augment the interpretation of climate information from the two deep cores recently recovered from this area these reflecting surfaces believed to represent isochrones give information about the stress regime near the summit and may aid in a better calibration of the age depth scale between the two cores particularly in the lowest 10 of ice thickness where there is currently disagreement the approximate depth at which internal echoes become discontinuous corresponds with the observations of steep inclinations and overturned folds on the scale of centimeters in the core samples however the deepest internal layers which can be distinguished in the profiles place constraints on the scale and location of high angle or overturned folds

scalings for large turbulent flow structures in gravel bed rivers 1 the relations between frequency and duration of large scale turbulent flow structures in gravel bed rivers and the distributions of the time intervals between turbulent events are investigated using flow visualisation and velocity measurements the time of occurrence and the duration and amplitude of large bursting motions were estimated independently from a five minute underwater video and from high resolution velocity time series sampled at 20 hz during 38 minutes at three heights above the bed results from both experiments show that the frequency duration relations of the large scale turbulent flow structures follow a power function with exponents ranging between 1 4 and 1 95 values that are typical of bursting systems e g solar flares the frequency distributions of time intervals between two successive ejections display an exponential tail these findings provide models for simulating sequences of large scale turbulent events in a gravel bed river an important step in linking flow turbulence and sediment transport

predicting macroturbulence energy and timescales for flow over a gravel bed experimental results and scaling laws abstract the fluvial science community lacks predictive equations for macroturbulence energy and timescales which may serve useful for predicting sediment transport in rivers therefore we investigated the systematic variation of macroturbulence scales by varying hydraulic parameters in an experimental facility collecting velocimetry data and measuring macroturbulence scales with spectral analyses and testing scaling laws for macroturbulence prediction that consider inner variables outer variables and turbulence kinetic energy experiments spanned across 11 energy gradients and aspect ratio combinations including 116 testing conditions each with 30 minute instantaneous turbulence time series collected results showed velocimetry data and time average turbulence parameters supported the presence of an equilibrium outer region and coherent double layer assumed in the experimental design macroturbulence and bursting wavenumbers were constant in the outer region when scaled with the flow depth and were independent of distance above the boundary aspect ratio energy gradient froude number or shields parameter data results agreed with the derivation herein and showed macroturbulence energy scales with the streamwise turbulence kinetic energy semi theoretical equations for turbulence kinetic energy scaled macroturbulence energy reasonably well a novel equation is presented for the macroturbulence period and the equation s transferability is supported by consistency with past macroturbulence studies bursting formula and flow visualization studies novel equations are presented for macroturbulence energy and transferability is supported on theoretical and empirical grounds however open questions remain regarding a universal function for streamwise turbulence kinetic energy and the net contribution of macroturbulence to total turbulence energy results showed the macroturbulence wavenumber required a relatively long duration of data i e 17 min to stabilize results were used to formulate a time scale for minimum measurement length of turbulence data when information about macroturbulence is sought after we discuss macroturbulence linkages with sediment transport including prediction of the sediment carrying capacity via macroturbulence formula as one application to help close the gap between turbulence and fluvial sediment transport

polar radar for ice sheet measurements prism abstract we provide an overview of a project to develop and demonstrate radars for imaging the ice bed interface mapping shallow and deep internal layers and measuring ice thickness we developed an ultra wideband very high frequency vhf radar 120 to 300 mhz for simultaneously measuring ice thickness mapping internal layers at depth and imaging the ice bed interface and an ultra high frequency uhf ultra wideband radar 500 to 2000 mhz for fine resolution mapping of near surface internal layers we used these radars to collect data at summit camp greenland during july 2005 and the west antarctic ice sheet wais deep drill core site during january 2006 these results provide us with an unprecedented view of basal characteristics and internal layers we also developed an outreach program a communications system and rovers to facilitate geophysical measurements in the future

effect of flow depth and velocity on the scales of macroturbulent structures in gravel bed rivers 1 laboratory and field data show that the length of macroturbulent structures consistently scales with flow depth y scalings with average streamwise flow velocity u only exist for laboratory data our aim is to establish the relations between macroturbulent structures low speed wedges and ejections and u and y in gravel bed rivers fifteen velocity profiles covering a wide range of u while controlling for y were measured the duration time interval between the beginning and the end of an event and length distance between two successive similar events of the macroturbulent events are analyzed strong scalings of the duration and length of the low speed wedges and ejections exist for y ejection length is strongly related to u because the sampling design ensured the statistical independence between u and y these results should provide robust estimates of the scalings of macroturbulent structures in gravel bed rivers