impact of bedrock surface topography on spatial distribution of quaternary sediments and on the flow pattern of late weichselian glaciers on the east european craton russian plain this paper presents the first high quality maps of the bedrock surface topography and the quaternary sediment distribution for the late weichselian glaciation area on the east european craton eec and the reasoning behind the estimated sediment distribution geographic information system gis based three dimensional model of the bedrock surface topography the current digital terrain model dtm and the ice flow pattern of the last glaciation provided the main source data for the analyses and interpretations in the current continental area of the western eec which was covered by the last scandinavian ice sheet sis the bedrock lies overwhelmingly 85 above the contemporary sea level with an average altitude of 54 m above sea level the topography of the bedrock surface is very well reflected in the current topography and an 80 m difference in the ice bed surface altitudes results in the topographic focusing of the ice flow and thus all late weichselian ice streams were accommodated to the depressions in the subglacial surface such depressions are characterised by a thin 10 m layer of sediment with one till bed just 50 m upslope from the bottom of the depressions the thickness of the sediment increases two to threefold and comprises five or more till layers indicating relief enhancement through subsequent glaciations the average calculated thickness of the quaternary deposits in the study area was 49 m and the total volume of sediments was 20 280 km3

unprecedented erosion of the upper texas coast response to accelerated sea level rise and hurricane impacts the upper texas coast is an ideal location to examine coastal response to global change over geologic and historic time here we quantify the long term sequestration for sand eroded from an island into two main sinks offshore galveston island and san luis pass tidal delta in order to compare long term and short term erosion we determine the average storm related offshore sand fl ux for the middle part of the holocene ca 5240 5040 2 cal yr b p to present to be 4200 4400 670 m 3 yr with a decrease in the offshore sand fl ux to 920 970 270 m 3 yr during the latter part of the holocene ca 2730 2610 2 cal yr b p to present the tidal delta initially formed ca 2100 1 median cal yr b p when the rate of sealevel rise slowed from 2 0 mm yr to 0 60 mm yr we calculate the sand fl ux from galveston island into san luis pass from ca 2100 1 median cal yr b p to 200 yr ago to be 4700 m 3

assessing the ability of numerical ice sheet models to simulate grounding line migration grounding line migration is a key process affecting the stability of marine ice sheets such as the west antarctic ice sheet wais grounding line motion is often included in numerical models simulating the past and future evolution of the wais however little attention has been paid to the numerical consistency of these models the aim of this paper is to assess the ability of simple versions of existing marine ice sheet models to simulate grounding line migration in particular we investigate the response of the grounding line to external forcing and the sensitivity of the models predictions to their numerics and the mechanical coupling between ice sheet and shelf from the model comparison there is no consensus on how the grounding line should react to changes in boundary conditions a crucial finding is the strong dependency of models using a fixed grid on numerical details such as the horizontal grid size this implies that we should be very wary about grounding line predictions from such models including mechanical coupling at the grounding line does not seem to change the qualitative behavior of the models this suggests that the way the grounding line is treated in marine ice sheet models dominantly determines the grounding line dynamics we find that models that employ a moving grid to explicitly track the grounding line do not share many of the deficiencies of the fixed grid models we conclude that at present no reliable model of the grounding line is available and further model development is urgently needed

stress and velocity fields in glaciers part ii sliding and basal stress distribution numerical methods are used to examine the interaction between the spatial distribution of the basal shear traction and the corresponding basal velocity for an inclined slab geometry in our improved treatment we reject the common assumption that basal velocity is a simple function of local variables in favour of a non local treatment that includes normal deviatoric stress and takes basal velocity to be an integrated response to spatially varying influences computationally one must either iterate the basal velocity with a friction parameterization that relates basal shear traction to basal velocity or alternatively prescribe the basal shear traction that results from bed decoupling and substrate deformation the average of basal shear traction over the entire bed of the ice mass is invariant under changes in sliding distribution and thus constitutes a useful reference any local relative reduction of traction leads to basal movement either sliding over the bed or moving with a deforming subglacial layer the local stress reduction is accompanied by a concentration of traction up and down glacier of the moving base growth decay and possible migration ofbasal stress concentrations may be closely related to short lived sliding events and to surges

ancient shelf ridges a potentially significant component of the transgressive systems tract case study from offshore northwest java detailed stratigraphic evaluation of three dimensional 3 d seis mic volumes calibrated with well log and core data from the mio cene section of the offshore northwest java shelf reveals the exten sive presence of preserved shelf ridge deposits these features are long linear bodies ranging from 0 3 to 2 0 km wide more than 20 km long and up to 17 m high on close inspection these features appear to be asymmetric characteristically sharp edged and thicker on one side and gradually thinning with an irregular edge on the other side possible sand waves smaller in scale are observed super imposed on these ridges and oriented oblique to the long axes of the ridges the observed shelf ridge deposits tend to be sand prone and overlie ravinement surfaces the ridges appear to be oriented parallel with the axes of broad paleoembayments associated with the structural fabric of the basin in addition to shelf ridges shelf ribbons possibly less than 5 m thick and less than 100 m wide are also imaged sand ridges are common on modern shelves but significantly less commonly recognized in the subsurface or in outcrop the features observed here represent examples of preserved ancient shelf ridges these ridges are thought to have formed as a result of erosion and subsequent reworking of sand prone deltaic and or coastal plain deposits by shelf tidal currents which became active immediately after shoreline transgression these deposits appear to have migrated across the ancient sea floor and represent an important component of the transgressive systems tract these transgressive systems tract deposits have significant exploration potential because they are commonly sand prone and tend to be encased in shelf mudstone seal facies depending on the degree to which sand is present in interridge locations these linear sand bodies can comprise potential stratigraphic traps begin page 76

an analysis of the ice sheet surface and subsurface topography above the vostok station subglacial lake central east antarctica radio echo sounding res and satellite radar altimeter data were analyzed in order to identify the surface and subsurface topography around the largest known subglacial lake at vostok station east antarctica named here as lake vostok in doing so a data set was established from which a qualitative description of the flow of ice across the lake was developed patterns of ice flow were constructed through consideration of 1 the ice sheet surface from the satellite altimeter and 2 internal layering from res it is concluded that lake vostok influences the dynamics of the overriding ice specifically although the flow of ice across the lake is dominated by the general eastward flow of the grounded ice sheet a southward velocity component caused by an ice shelf type ice flow mechanism is also present over the subglacial lake a further 67 smaller subglacial lakes exist within central regions of the antarctic ice sheet occupying 5 10 of the ice sheet base subglacial lakes may therefore exert a significant control on ice dynamics within central antarctica

a continuum mixture model of ice stream thermomechanics in the laurentide ice sheet 1 theory we employ a continuum mixture framework to incorporate ice streams in a three dimensional thermomechanical model of the laurentide ice sheet the ice mass is composed of a binary mixture of sheet ice which deforms by viscous creep and stream ice which flows by sliding and or sediment deformation at the bed dynamic and thermal evolutions are solved for each component in the mixture with coupling rules to govern transfer between flow regimes we describe two different transfer mechanisms 1 creep exchange the nourishment of ice streams by viscous creep inflow from the surrounding ice sheet and 2 bed exchange the activation growth and deactivation of ice streams perpetrated by transfers of bed area between flow constituents this paper develops the underlying mixture theory we express the governing equations for mass momentum and energy balance in a form suitable for direct incorporation in existing numerical models of ice thermomechanics a companion paper in this issue explores mixture and ice stream behavior in applications with the laurentide ice sheet

application of a polythermal three dimensional ice sheet model to the greenland ice sheet response to steady state and transient climate scenarios abstract steady state and transient climate change computations are performed with the author s three dimensional polythermal ice sheet model simulation code for polythermal ice sheets for the greenland ice sheet the distinctive feature of this model is the detailed consideration of the basal temperate ice layer in which the water content and its impact on the ice viscosity are computed its transition surface to the cold ice region is accounted for by continuum mechanical jump conditions on this interface the simulations presented include steady states subject to a range of physical parameters and two different climates present and glacial conditions as well as three types of transient scenarios namely i sinusoidal milankovic period forcing ii paleoclimatic forcing from the greenland ice core project core reconstruction and iii future greenhouse warming forcing

force budget i theory and numerical methods a practical method is developed for calculating stresses and velocities at depth using field measurements of the geometry and surface velocity of glaciers to do this it is convenient to partition full stresses into lithostatic and resistive components the horizontal gradient in vertically integrated lithostatic stress is the driving stress and it describes the horizontal action of gravity the horizontal resistive stress gradients describe the reactions resistive stresses are simply related to deviatoric stresses and hence to strain rates through a constitutive relation a numerical scheme can be used to calculate stresses and velocities from surface velocities and slope and from ice thickness there is no mathematical requirement that the variations in these quantities be small

feedback between ridge and swale bathymetry and barrier island storm response and transgression abstract the shoreface of santa rosa island in northwest florida is characterized by a ridge and swale bathymetry that forces an alongshore variation in beach and dune morphology the alongshore variation in dune morphology in turn controls the modern island response to and recovery from tropical storms and hurricanes and is therefore an important control on island transgression with relative sea level rise field sampling and remote sensing are used in the present study to describe the geologic framework of santa rosa island and to elucidate on the origins of the shore attached ridge and swale bathymetry vibracores and seismic and gpr surveys were completed along 42 cross shore transects and 3 shore parallel transects to examine the structure of the 21 ridge and swale structures found along santa rosa island the shore parallel seismic surveys reveal strong near horizontal reflectors through the ridges at depths consistent with thick back barrier muds extracted from vibracores taken across and along the ridges near horizontal reflectors are identified in ground penetrating radar gpr surveys between the ridges and cuspate spits along the back barrier shoreline but are not present in the narrow sections of the island landward of the swales continuation of the seismic surveys in the back barrier also reveals near horizontal reflectors at the cuspate spits that are characterized by seagrass beds salt marsh and maritime forest consistent with the gpr survey there is an absence of horizontal reflectors between cuspate spits where the washover deposits extend to the back barrier shoreline it is argued that the ridge and swale bathymetry is a transgressive surface and the remnants of cuspate spits that are present along the back barrier shoreline in this respect the cuspate spits had to first develop along the back barrier shoreline and eventually evolve into the mud cored ridges as the island transgressed with relative sea level rise once the ridge and swale bathymetry emerged on the gulf of mexico shoreface it was able to reinforce the alongshore variation in dune height and storm response it is further argued that the cuspate spits are reinforced by the shoreface ridges through alongshore transport of sediment from adjacent washover fans although the ridge orientation suggests that the spits have migrated westward by 750 m in this respect the alongshore variation in beach and dune morphology on this island is the expression of this large scale feedback and suggests a top down model in which meso scale processes and landforms depend on the geologic context

results from the eismint model intercomparison the effects of thermomechanical coupling this paper discusses results from the second phase of the european ice sheet modelling initiative eismint it reports the intercompartison of ten operational ice sheet models and uses a series of experiments to examine the implications of thermomechanical coupling for model behaviour a schematic circular ice sheet is used in the work which investigates both steady states and the response to stepped changes in climate the major finding is that radial symmetry implied in the experimental design can under certain circumstances break down with the formation of distinct regularly spaced spokes of cold ice which extended from the interior of the ice sheet outward to the surrounding zone of basal melt these features also manifest themselves in the thickness and velocity distributions predicted by the models they appear to be a common feature to all of the models which took part in the intercomparison and may stem from interactions between ice temperature flow and surface form the exact nature of these features varies between models and their existence appears to be controlled by the overall thermal regimne of the ice sheet a second result is that there is considerable agreement between the models in their predictions of global scale response to imposed climate change

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

deformation in the vicinity of ice divides calculs numeriques aux elements finis de la variation de la vitesse horizontale avec la profondeur au voisinage d une difference de la glace symetrique isotherme et ou glissement sur un lit plat

the role of lateral drag in the dynamics of ice stream b antarctica the partitioning of resistive force between the bed and sides of ice stream b antarctica is obtained for three large areas that have been measured using repeat aerial photogrammetry problems associated with data errors and local variations in ice strength and velocity are reduced by considering the areally averaged budget of forces for each photo block results indicate that the bed under ice stream b must be very weak and unable to provide muche resistance mechanical control on this ice stream emanates almost entirely from the lateral margins

high resolution swath sonar investigation of sand ridge dune and ribbon morphology in the offshore environment of the new jersey margin abstract sand ridges on the northeast us atlantic shelf form in the near shore environment most likely in response to storm driven flows as the holocene transgression has continued the ridges have been transferred to an offshore hydrodynamic regime where currents are not constrained by the coast and storms do not influence bottom currents as frequently or as strongly here we qualitatively and quantitatively investigate the morphology of offshore sand ridges and smaller scale features in an effort to place constraints on bedform development in these deeper waters a recent high resolution swath sonar survey mapped a portion of the new jersey shelf from 20 m water depth to the shelf break 120 m imaging both sand ridges and smaller scaled dunes and ribbons in far greater detail than has been previously possible using a robust statistical analysis we find that the gross morphology of ridges height width length does not change with depth beyond 20 m water depth and changes in ridge orientation generally mirror changes in regional contour orientation hence ridges have not continued to grow since transgression has brought them into the offshore hydrodynamic regime however on the inner shelf 20 m water depth to the mid shelf shore we do find evidence in the ridge shape which has an asymmetry opposite to that seen near shore and in the complex backscatter response that some important modifications to ridges are taking place at these water depths in contrast on the mid shelf from the mid shelf shore to the franklin shore ridges tend to have higher backscatter at the crests implying that these are largely winnowed relict features lineated smaller scale 100 500 m wavelength

transient glacier response with a higher order numerical ice flow model in this paper a higher order numerical flowline model is presented which is numerically stable and fast and can cope with very small horizontal grid sizes 10 m the model is compared with the results from blatter and others 1998 on haut glacier d arolla switzerland and with the european ice sheet modelling initiative benchmarks huybrechts and others 1996 results demonstrate that the significant difference between calculated basal drag and driving stress profiles in a fixed geometry disappears when the glacier profile is allowed to react to the surface mass balance conditions and reaches a steady state dynamic experiments show that the mass transfer in higher order models occurs at a different speed in the accumulation and ablation areas and that the front position is more sensitive to migration compared to the shallow ice approximation

diamict fans in subglacial water filled cavities a new glacial environment abstract the occurrence of glaciolacustrine diamict fans on the south side of volcanic tuyas in the laugarvatn area iceland is reported these fans are composed of sheets of diamict gravel and sand and contain little or no primary volcanic material they post date the volcanic pile within the tuyas and appear to have been deposited in subglacial water filled cavities confined to the down ice side of the tuyas it is suggested that these subglacial water bodies formed as result of an enhanced geothermal heat flux associated with the tuyas the volume of glacial sediment within the fans implies that either basal sediment loads were high in this sector of the former ice sheet or that ice flow converged towards the tuyas during their formation concentrating basal sediment into the lee side cavity and fan the glaciolacustrine diamict fans add to our knowledge of the landforms and sediments associated with tuyas and may also provide evidence of ice flow convergence at thermal anomalies

regional reconstruction of subglacial hydrology and glaciodynamic behaviour along the southern margin of the cordilleran ice sheet in british columbia canada and northern washington state usa abstract subglacial landsystems in and around okanagan valley british columbia canada are investigated in order to evaluate landscape development subglacial hydrology and cordilleran ice sheet dynamics along its southern margin major landscape elements include drumlin swarms and tunnel valleys drumlins are composed of bedrock diamicton and glaciofluvial sediments their form truncates the substrate tunnel valleys of various scales km to 100s km length incised into bedrock and sediment exhibit convex longitudinal profiles and truncate drumlin swarms okanagan valley is the largest tunnel valley in the area and is eroded 300 m below sea level over 600 m of late wisconsin age sediments consisting of a fining up sequence of cobble gravel sand and silt fill okanagan valley landform substrate relationships landform associations and sedimentary sequences are incompatible with prevailing explanations of landsystem development centred mainly on deforming beds they are best explained by meltwater erosion and deposition during ice sheet underbursts during the late wisconsin glaciation okanagan valley functioned as part of a subglacial lake spanning multiple connected valleys few 100s km of southern british columbia subglacial lake development started either as glaciers advanced over a pre existing sub aerial lake catch lake or by incremental production and storage of basal meltwater high geothermal heat flux geothermal springs and or subglacial volcanic eruptions contributed to ice melt and may have triggered along with priming from supraglacial lakes subglacial lake drainage during the underburst s sheetflows eroded drumlins in corridors and channelized flows eroded tunnel valleys progressive flow channelization focused flows toward major bedrock valleys in okanagan valley most of the pre glacial and early glacial sediment fill was removed a fining up sequence of boulder gravel and sand was deposited during waning stages of the underburst s and bedrock drumlins in okanagan valley were enhanced or wholly formed by this underburst s subglacial lake development and drainage had an impact on ice sheet geometry and ice volumes the prevailing conceptual model for growth and decay of the cis suggests significantly thicker ice in valleys compared to plateaus subglacial lake development created a reversal of this ice sheet geometry where grounded ice on plateaus thickened while floating valley ice remained thinner due to melting and enhanced sliding with significant transfer of ice toward the ice sheet margin subglacial lake drainage may have hastened deglaciation by melting ice lowering ice surface elevations and causing lid fracture this paper highlights the importance of ice sheet hydrology its control on ice flow dynamics distribution and volume in continental ice masses

change in morphology and modern sediment thickness on the inner continental shelf offshore of fire island new york between 2011 and 2014 analysis of hurricane impact abstract seafloor mapping investigations conducted on the lower shoreface and inner continental shelf offshore of fire island new york in 2011 and 2014 the period encompassing the impacts of hurricanes irene and sandy provide an unprecedented perspective regarding regional inner continental shelf sediment dynamics during large storm events analyses of these studies demonstrate that storm induced erosion and sediment transport occurred throughout the study area in water depths up to 30 m acoustic backscatter patterns were observed to move from 1 m to 450 m with a mean of 20 m and movement tended to decrease with increasing water depth these patterns indicate that both of the primary inner continental shelf sedimentary features in the study area linear sorted bedforms offshore of eastern fire island and shoreface attached sand ridges offshore of central and western fire island migrated alongshore to the southwest the migration of the sorted bedforms represents the modification of an active ravinement surface and is thought to have liberated a significant volume of sediment comparison of isopach maps of sediment thickness show that the volume of modern sediment composing the lower shoreface and shoreface attached sand ridges decreased by 2 8 10 6 m 3 across the 73 km 2 of common seafloor mapped in both surveys however a similar analysis for the relatively calmer 15 yr period prior to 2011 revealed significant accretion this allows speculation that the shoreface attached sand ridges are maintained over decadal timescales via sediment supplied through erosion of pleistocene outwash and lower holocene transgressive channel fill deposits exposed on the inner continental shelf but that the sand ridges also periodically erode and move to the southwest during large storm events analyses show that significant storm induced erosion and sediment transport occurs far seaward of the 5 to 9 m depth of closure assumed for fire island where it is thought that an onshore directed sediment flux from the inner continental shelf to the littoral system is required to balance the coastal sediment budget it is also thought that the morphology of the shoreface attached sand ridges controls the persistent shape of the adjacent shoreline through modification of incident waves thus we suggest that the sediment dynamics of the inner continental shelf and both storm induced and anthropogenic modification of the field of shoreface attached sand ridges be considered in future coastal resiliency planning

controls on coastal dune morphology shoreline erosion and barrier island response to extreme storms abstract the response of a barrier island to an extreme storm depends in part on the surge elevation relative to the height and extent of the foredunes which can exhibit considerable variability alongshore while it is recognized that alongshore variations in dune height and width direct barrier island response to storm surge the underlying causes of the alongshore variation remain poorly understood this study examines the alongshore variation in dune morphology along a 11 km stretch of santa rosa island in northwest florida and relates the variation in morphology to the response of the island during hurricane ivan and historic and storm related rates of shoreline erosion the morphology of the foredune and backbarrier dunes was characterized before and after hurricane ivan using empirical orthogonal function eof analysis and related through canonical correlation analysis cca the height and extent of the foredune and the presence and relative location of the backbarrier dunes varied alongshore at discrete length scales of 750 1450 and 4550 m that are statistically significant at the 95 confidence level cospectral analysis suggests that the variation in dune morphology is correlated with transverse ridges on the inner shelf the backbarrier cuspate headlands and the historical and storm related trends in shoreline change sections of the coast with little to no dune development before hurricane ivan were observed in the narrowest portions of the island between headlands west of the transverse ridges overwash penetration tended to be larger in these areas and island breaching was common leaving the surface close to the watertable and covered by a lag of shell and gravel in contrast large foredunes and the backbarrier dunes were observed at the widest sections of the island the cuspate headlands and at crest of the transverse ridges due to the large dunes and the presence of the backbarrier dunes these areas experienced less overwash penetration and most of the sediment from the beachface and dunes was deposited within the upper shoreface it is argued that this sediment is returned to the beachface through nearshore bar migration following the storm and that the areas with larger foredunes and backbarrier dunes have smaller rates of historical shoreline erosion compared to areas with smaller dunes and greater transfer of sediment to the washover terrace since the recovery of the dunes will vary depending on the availability of sediment from the washover and beachface it is further argued that the alongshore pattern of dune morphology and the response of the island to the next extreme storm is forced by the transverse ridges and island width through alongshore variations in storm surge and overwash gradients respectively these findings may be particularly important for coastal managers involved in the repair and rebuilding of coastal infrastructure that was damaged or destroyed during hurricane ivan

across shelf benthic transports on the inner shelf of the middle atlantic bight during the halloween storm of 1991 instrumented bottom boundary layer tripods were deployed on the inner shelf at depths of 13 m and 8 m off the u s army corps of engineers field research facility at duck north carolina usa over a two week period that included the severe and prolonged halloween storm of late october 1991 the storm persisted for five days and generated waves with heights and periods of up to 6 m and 22 s although the instrumentation was destroyed current profile and suspended sediment concentration profile data were recovered from the 13 m site wind driven mean along shelf currents at 1 24 m above the bed attained speeds of nearly 0 5 m s 1 across shelf flows primarily seaward directed had speeds varying from 0 05 to 0 15 m s 1 these seaward flows intensified in association with groups of high waves total mean current and skin friction bed shear stresses were increased relative to moderate energy values by more than an order of magnitude notably the highest shear stresses occurred in association with high long period swell during the late phase of the storm after winds had turned offshore creating shoreward mean flows near the bed suspended sediment concentrations exceeded 1 g 1 1 throughout the lower meter of the water column and at elevations well above the top of the wave boundary layer net across shelf suspended sediment fluxes were strongly dominated by mean flows cospectral analyses show that important but secondary roles were also played by infragravity oscillations and by wave orbital velocities

relative rates of optical maturation of regolith on mercury and the moon 1 quantifying relative differences in regolith optical maturation rates is critical to interpreting multispectral reflectance measurements of airless bodies in this study normalized reflectance measurements of crater ejecta blankets and rays are used as indicators of the relative state of regolith maturation on mercury and the moon as well as for a comparison of surface reflectance characterization of craters with high reflectance ejecta from lunar reconnaissance orbiter camera and mercury dual imaging system orbital mosaics indicates that the optical maturation rate is up to 4 times faster on mercury than on the moon observations also show that there are fewer immature craters per unit area on mercury than on the moon and suggest a younger average age for mercurian rayed craters than for lunar rayed craters a comparison of crustal reflectance of immature material yields a ratio of 1 9 0 4 for the average photometrically normalized reflectance of lunar material to that of mercurian material the difference in reflectance is attributed primarily to differences in composition between the lunar highlands and average surface material on mercury the new observations of immature craters per unit area and surface reflectance of immature materials are consistent with previous proposals that regolith maturation rates are faster on mercury than the moon and but here we quantify the relative rate from empirical observations for the first time

holocene evolution of the east texas coast and inner continental shelf along strike variability in coastal retreat rates abstract the low gradient east texas coast and inner continental shelf from sabine lake at the texas louisiana border to the western end of galveston island experienced extreme along strike variations in rates of holocene coastal retreat around 7 7 ka a barrier shoreline was located approximately 55 km offshore toward the western end of galveston island the shoreline retreated 55 km occupying a position on the lagoon side of the island by about 5 3 ka toward the texas louisiana border the shoreline retreated more gradually occupying a position seaward of sabine bank by 5 3 ka between 4 7 ka and 2 8 ka the shoreline at sabine bank retreated 30 km while galveston island prograded seaward bolivar peninsula began to accrete around 1 5 ka heald and sabine banks located on the inner continental shelf above terraced fluvial deposits of the trinity sabine incised valley are the only preserved remnants of these former shoreline positions fluctuating rates of sea level rise were not the forcing mechanism behind episodes of rapid shoreline retreat because these events were localized rather along strike variations in the rate of transgression were caused by the variable inner shelf gradients which increase towards the west and the orientation of the sabine incised valley and associated terraced fluvial deposits which trend northeast southwest parallel to shore as shorelines retreated over fluvial deposits these served as local sand sources that enabled barrier islands to persist figure 1 map of the east texas inner shelf showing locations of sabine heald and shepard banks paleotopographic map of the pleistocene surface thin contours siringan 1993 delineates the trinity sabine incised valley which formed during the last lowstand of sea level thomas and anderson 1994 bathymetric contours are shown as thick lines with bold numbers dots are core locations end page 405 offshore out of equilibrium with sea level once these sand sources became depleted and or sea level reached some critical threshold barrier shorelines became stranded offshore as banks and new shorelines formed landward the geologic setting of coastal areas specifically antecedent topography plays a primary role in controlling coastal evolution to accurately forecast long term centennial to millennial coastal evolution it is essential that impacts associated with variations in the underlying geology of coastal areas be incorporated into coastal forecasting models

statistical modeling of a former arctic ocean ice shelf complex using antarctic analogies 1 geophysical mapping and coring of the central arctic ocean seafloor provide evidence for repeated occurrences of ice sheet ice shelf complexes during previous glacial periods several ridges and bathymetric highs shallower than present water depths of 1000m show signs of erosion from deep drafting armadas of icebergs which originated from thick outlet glaciers and ice shelves mapped glacigenic landforms and dates of cored sediments suggest that the largest ice shelf complex was confined to the amerasian sector of the arctic ocean during marine isotope stage mis 6 however the spatial extent of ice shelves can not be well reconstructed from occasional groundings on bathymetric highs therefore we apply a statistical approach to provide independent support for an extensive mis 6 ice shelf complex which previously was inferred only from interpretation of geophysical and geological data specifically we assess whether this ice shelf complex comprises a likely source of the deep draft icebergs responsible for the mapped scour marks the statistical modeling is based on exploiting relations between contemporary antarctic ice shelves and their local physical environments and the assumption that arctic ocean mis6 ice shelves scale similarly analyzing ice thickness data along the calving front of contemporary ice shelves a peak over threshold method is applied to determine sources of deep drafting icebergs in the arctic ocean mis6 ice shelf complex this approach is novel to modeling arctic paleoglacial configurations predicted extreme calving front drafts match observed deep draft iceberg scours if the ice shelf complex is sufficiently large

ice sheet modelling at different spatial resolutions focus on the grounding zone a high resolution time dependent two dimensional flowline model was developed which takes into account all relevant stresses i e both shear and longitudinal stresses and which is solved on a fixed finite difference grid special attention was paid to the computational efficiency a comparison is made with a similar model that solves the velocity field according to the shallow ice approximation both models were applied to an east antarctic flowline for which the data on surface and bedrock elevation were sampled at different spatial resolutions the numerical analysis deals with the importance of longitudinal stresses at the ice sheet ice shelf interface at different grid sizes and the importance of basal drag versus driving stress in basal sliding laws at a grid resolution of 5 to 10 km the force balance indicates that all stress components are of equal importance in an area that is larger than the nominal grid size when sliding becomes dominant this transition zone considerably widens furthermore the longitudinal stress deviator is found to vary considerably with depth due to the thermomechanical coupling fpattyn pmg nipr ac jp

offshore transport of sediment during cyclonic storms hurricane ike 2008 texas gulf coast usa extreme storms can have a large impact on coastal sediment budgets and the character of strata preserved in the geologic record here we investigate the impact of a cyclonic storm surge flood and ebb on sediment transport in a microtidal beach barrier shelf system hurricane ike made landfall on the texas united states coast on 13 september 2008 the accompanying storm surge flooded galveston bay with up to 5 m of water above sea level the surge flood and ebb preferentially flowed over a low elevation bay fronting spit known as the bolivar peninsula destroying buildings and eroding sediments surge waters also flowed through bolivar roads tidal inlet the main passageway through the barrier system that separates the gulf and the bay bathymetry chirp data and samples were collected in bolivar roads tidal inlet 9 to 10 d after the storm and we compare them here to data collected 4 mo prior additional data were collected offshore of bolivar peninsula in october 2008 our results document the dominance of the storm surge ebb in forcing sediment transport through the inlet which is not considered in models of beach barrier evolution shoreface sands appear to have been incised by the storm and advected with beach barrier sediments sufficiently offshore by the storm surge ebb that they cannot be reincorporated indicating a significant loss to the barrier system9s sediment budget as a result of a single storm

ice motion over lake vostok antarctica constraints on inferences regarding the accreted ice ice motion over lake vostok antarctica is measured using repeat pass synthetic aperture radar sar interferometry the coverage of the lake and the components of the vector field are resolved using 10 overlapping data takes from ascending and descending look directions seventy day temporal baselines provide the sensitivity required to observe the range of ice motion 0 6 m a 1 over the lake and the adjacent ice sheet it is remarkable that the scattering field remained coherent over these time separations this is critical for interferometric analysis and can be attributed to the low surface accumulation and low air temperature at this elevation the regional flow of the ice sheet around lake vostok is from west to east perpendicular to the surface elevation contours as the ice flows past the grounding line a southward component of motion develops that is correlated with the north south surface slope along the length of the lake the surface velocity increases slowly from the northern tip of the lake and then more rapidly south of 77 s at vostok station the ice motion is 4 2 m a 1 across the lake and away from boundary effects the down lake flow pattern takes on a parabolic profile with maximum velocity close to the center line of the lake the overall influence of the subglacial lake is the addition of a down lake motion component to the prevailing west east motion of the ice sheet as a result we estimate 10 of the mass flowing onto the lake is diverted south reconstructions based on the vostok ice core indicate that the ice was grounded up glacier from the core site approximately 5000 years ago this suggests a minimum freezing rate of 40 mm a 1 for the subglacial accretion ice 10 times greater than that inferred from thermodynamic modeling of the upper 2 km of the ice core

variations in shoreface progradation and ravinement along the texas coast gulf of mexico shoreface architecture evolution mid holocene to present and depths of transgressive ravinement were examined from sabine pass at the texas louisiana border to south padre island near the texas mexico border using 30 shoreface transects shoreface transects extend out to 16 m water depth each created from an echo sounding profile and on average seven sediment cores the shoreface is composed of three broad sedimentological facies the upper shoreface composed almost entirely of sand the proximal lower shoreface composed of sand with thickly to medium bedded 50 10 cm mud and the distal lower shoreface composed dominantly of mud with medium to thinly bedded 20 3 cm sand shoreface architecture and evolution is extremely variable along the texas coast shoreface gradients increase from 2 25 m km 1 in east texas to 3 50 m km 1 in south texas shoreface sands coarsen towards south texas east and south texas shoreface deposits are thin and retrograding whereas central texas shoreface deposits are thicker and prograding central texas is characterized by stacked shoreface successions whereas in east texas lower shoreface sands are preserved only in offshore banks preservation of shoreface deposits is low in south texas although eustatic fluctuations and accommodation space have a strong impact on overall mid holocene to present shoreface evolution and preservation potential along strike variations in sediment supply and wave energy are the main factors controlling shoreface architecture the transgressive ravinement surface varies from 6 to 15 m along the texas coast

modern shoreface and inner shelf storm deposits off the east texas coast gulf of mexico abstract the modern shoreface and inner shelf sediments off the east texas coast on the gulf of mexico were examined using sediment cores and high resolution seismic data low sediment input and low effective accommodation space resulted in the overall thinness of the modern marine sediment cover in the region erosion on the inner shelf during historic 100 yr times produced patches of sand in a predominantly mud dominated environment storm beds are scarce on the inner shelf the shoreface succession off galveston island is characterized by offshore directed crosscutting storm channels of varying sizes the existence of these channels and the relatively high frequency of storms and hurricanes that have affected the region during historical imes are inconsistent with the paucity of storm deposits on the inner shelf the offshore directed channelized flows may have been prevented from going beyond the shoreface by strong along shelf storm currents that impinged on the shoreface in contrast to the storm record on the east texas shelf the central texas shelf shows a greater frequency of and more laterally persistent storm sand beds due to greater effective accommodation space and higher sediment supply there

shoreface ravinement evolution tracked by repeat geophysical surveys following hurricane ike bolivar peninsula texas 2008 2013 we have investigated the impact of hurricane ike on the shoreface of the bolivar peninsula texas using three nearsurface marine geophysical surveys during a five year span following the storm which made landfall in september of 2008 multibeam bathymetry sidescan backscatter and chirp subbottom reflection data were collected during each survey the first survey in november 2008 discovered the presence of an erosional landward facing scarp across a broad of the barrier spit shoreface at approximately 3 5 m water depth the erosion incised into a shallow reflection horizon interpreted as the holocene shoreface ravinement exposing lag deposits that were truncated at the scarp and forming a thin high backscatter anomaly up to 1 0 m of material was excavated shoreward of the scarp including up to 0 5 m of shoreface sand above the shoreface ravinement and another 0 5 m of indeterminate

sensitivity of a greenland ice sheet model to ice flow and ablation parameters consequences for the evolution through the last climatic cycle sensitivity experiments are conducted to test the influence of poorly known model parameters on the simulation of the greenland ice sheet by means of a three dimensional numerical model including the mechanical and thermal processes within the ice two types of experiments are performed steady state climatic conditions and simulations over the last climatic cycle with a climatic forcing derived from the grip record the experiments show that the maximum altitude of the ice sheet depends on the ice flow parameters deformation and sliding law coefficients geothermal flux and that it is low when the ice flow is fast on the other hand the maximum altitude is not sensitive to the ablation strength and consequently during the climatic cycle it is driven by changes in accumulation rate the ice sheet extension shows the opposite sensitivity it is barely affected by ice flow velocity and the ice covered area is smaller for large ablation coefficients for colder climates when there is no ablation the ice sheet extension depends on the sea level an interesting result is that the variations with time of the altitude at the ice divide summit do not depend on the parameters we tested the present modelled ice sheets resulting from the climatic cycle experiments are compared with the present measured ice sheet in order to find the set of parameters that gives the best fit between modelled and measured geometry it seems that compared to the parameter set most commonly used higher ablation rate coefficents must be used

surveying batavia s graveyard geophysical controlled experiments and subsurface imaging of archaeological sites on an indian ocean coral island abstractwe conducted geophysical surveys on beacon island in the houtman abrolhos archipelago offshore western australia to investigate areas of archaeological interest related to the 1629 batavia shipwreck mutiny and massacre we used three complementary near surface geophysical survey techniques total magnetic intensity electromagnetic induction mapping and ground penetrating radar to identify anomalous target zones for archaeological excavation interpreting near surface geophysical anomalies is often complex and nonunique although it can be significantly improved by achieving a better understanding of site specific factors including background conditions natural variability detectability limits and the geophysical response to and spatial resolution of buried targets these factors were not well understood for beacon island nor indeed for the australian coastal environment we have evaluated the results of controlled experiments in which we bury known targets at representative depths and a

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

numerical modelling of ice sheet dynamics across the vostok subglacial lake central east antarctica a numerical model of the ice sheet ice shelf transformation was used to investigate ice sheet dynamics across the large subglacial lake beneath vostok station central east antarctica european remote sensing satellite ers 1 altimetry of the ice surface and 60 mhz radio echo sounding res of the ice sheet base and internal ice sheet layering were used to develop a conceptual flowline across the ice sheet which the model used as input the model calculates horizontal and vertical velocities and stresses from which particle flow paths can be obtained and the ice sheet temperature distribution an inverse approach to modelling was adopted where particle flow paths were forced to match those identified from internal res layering results show that ice dynamics across the inflow grounding line are similar to an ice sheet ice shelf transition model particle flow paths match internal res layering when ice is a taken away from the ice base across the first 2 km of the flowline over the lake and b added to the base across the remainder of the lake we contend that the process causing this transfer of ice is likely to be melting of ice and freezing of water at the ice water interface other explanations such as enhanced rates of accumulation over the grounding line of three dimensional convergent divergent flow of ice are inconsistent with available measurements such melting and refreezing would be responsible for circulation and mixing of at least the surface layers of the lake water our model suggests that several tens of metres of refrozen basal ice would accrete from lake water to the ice sheet before the ice regrounds

the spatial and temporal variability of sand bar morphology the spatial and temporal variability of nearshore sand bar morphology is quantified using a unique data set spanning 2 years the data consist of daily time exposure images of incident wave breaking on an open coast sandy beach which may be used to infer bar morphology lippmann and holman 1989 the morphology in each image is classified into an eight state morphologic scheme in which bars are uniquely defined by four independent criteria the most frequently observed morphologies are the longshore periodic rhythmic bars observed in 68 of the data linear bars occur under highest wave conditions h s 1 78 m and are unstable mean residence time 2 days shore attached rhythmic bars are the most stable mean residence time 11 days and generally form 5 16 days following peak wave events non rhythmic three dimensional bars are very transient mean residence time 3 days eighty seven percent of transitions to lower bar types defined in text occurred one state at a time supporting our selection of the ordering of states and suggesting the suitability of a sequential morphology model transitions to higher states occurred under rising wave energy and were evenly spread among the possible higher states with more substantial changes in morphology resulting from larger wave increases this suggests that up state erosional transitions based on offshore bar migration are better described by an equilibrium model where response is better correlated with incident wave energy than with preceding morphological state time exposure images were also digitized to yield quantitative estimates of bar crest location as a function of longshore distance principal component analysis was used to decompose bar position into two dimensional linear and three dimensional longshore variable components cross shore linear bar position ranges 50 m about the 2 year mean 27 m standard deviation and dominates bar variability 74 6 three dimensional bar structure accounts for 14 of the variance 12 m standard deviation changes in incident wave height precede cross shore bar migration by less than 1 day changes in longshore variability are inversely correlated to changing wave conditions with bar morphology becoming linear rapidly during storms on time scales of less than 1 day evolution to significantly three dimensional structure typically occurs over 5 7 days following peak wave events

a new three dimensional higher order thermomechanical ice sheet model basic sensitivity ice stream development and ice flow across subglacial lakes 1 a new three dimensional thermomechanically coupled ice sheet model is developed contrary to the majority of three dimensional ice sheet models shallow ice approximation higher order stress gradients longitudinal and transverse are accounted for in the force balance equations the horizontal velocity field is determined from the force balance equations in their derivative form elliptical equation using the method presented by pattyn 2000 2002a the model is solved on a regular grid in the horizontal and an irregular grid in the vertical and is numerically stable basic experiments include the european ice sheet modeling initiative eismint benchmarks for largescale ice sheet models and a comparison with the saito blatter ice sheet model including higher order stress gradients saito et al 2003 detailed calculations of ice flow over three dimensional bedrock perturbations showed the validity of the higher order solution the model is capable of simulating the evolution of an ice stream within the ice sheet and shows important aspects of observed ice stream features such as the surface flattening and the importance of side drag the simulation of the ice flow over a subglacial lake results in a flattening of the surface a local velocity increase over the lake and a deviation of the ice flow from the main flow direction features which are also observed at lake vostok antarctica index terms 1827 hydrology glaciology 1863 1863 hydrology snow and ice 1827 3230 mathematical geophysics numerical solutions keywords higher order ice sheet model thermomechanical ice stream subglacial lake citation pattyn f a new three dimensional higher order thermomechanical ice sheet model basic sensitivity ice stream development and ice flow across subglacial lakes j geophys res 108 b8 2382 doi 10 1029 2002jb002329 2003

across shelf sediment transport interactions between suspended sediment and bed sediment 1 we use a two dimensional time dependent sediment transport model to quantify across shelf transport deposition and sorting during wave driven resuspension events characteristic of those that dominate sediment transport on many continental shelves decreases in wave orbital velocities as water depth increases and the resulting cross shelf gradient in bed shear stress favor a net offshore transport of sediment on wide flat shelves slopes 0 1 percnt these gradients are low and the depth to which the seabed is reworked depends mainly on bottom shear stress and local sediment availability on narrow steep shelves slopes 0 5 percnt however the gradient in bottom stress generates significant cross shelf suspended sediment flux gradients that create regions of net erosion and deposition while the magnitude of waves generally determines the water depth to which sediment can be resuspended erosional and depositional patterns on narrow shelves are sensitive to cross shelf gradients in wave energy nonlocal sediment availability and the direction and magnitude of the cross shelf current during energetic waves cross shelf divergence of suspended sediment flux can create a coarsened erosional area on the inner shelf that abuts a region of fine grained sediment deposition on the mid to outer shelf if currents are strongly shoreward however flux divergence leads to erosion over the entire shelf

velocity and stress fields in grounded glaciers a simple algorithm for including deviatoric stress gradients a new and efficient algorithm for computing the three dimensional stress and velocity fields in grounded glaciers includes the role of deviatoric stress gradients a consistent approximation of first order in the aspect of ratio of the ice mass gives a set of eight field equations for the five stress and three velocity components and the corresponding boundary conditions a coordinate transformation mapping the local ice thickness on to unity and approximating the derivatives in the horizontal direction by centred finite differences yields five ordinary differential and three algebraic equations this allows use of the method of lines starting the integration with prescribed stress and velocity components at the base and a simple iteration procedure converges rapidly the algorithm can lie used for a wide range of stress strain rate relations as long as strain only depends on deviatoric and shear stresses and on temperature sensitivity tests using synthetic and realistic ice geometries show the relevance of normal deviatoric stresses in the solutions for the velocity components even for ice sheets stress and velocity fields may deviate substantially from the widely used shallow ice approximation

storm driven variability of the beach nearshore profile at duck north carolina usa 1981 1991 the us army corps of engineers field research facility frf at duck north carolina has collected approximately biweekly beach nearshore profile data to 8 m depth and associated wave data since 1981 sediment budget analysis was used to examine the medium scale years to a decade variability of the beach nearshore profile from 1981 to 1991 significant changes occurred during four groups of energetic storm events during february march of 1983 1987 1989 and december 1989 each group was comprised of at least two storms within a period of less than 39 days both with hmo 4 m during each storm group offshore sediment movement caused a distinct outer bar to migrate offshore and grow in size resulting in an abrupt increase in the volume of sediment on the upper shoreface the net profile changes were much larger than the changes due to single storms and the cumulative effect of the storms can be considered as one event during these events the first storm appears to have a destabilizing effect on the profile which has insufficient time to recover before the second and subsequent storm s as a result several storms in quick succession are able to have a large impact on the morphology the intervening periods between the groups of storm events termed fairweather conditions lasted up to 4 years they are characterized by slow but steady sediment redistribution averaging 33 m3 m 1 year 1 from the upper shoreface 5 m depth toward the shore while the total sediment volume was effectively constant the onshore feed of sediment was not significantly affected by individual storms during the fairweather conditions these two processes of 1 morphologic change during groups of storm events and 2 the steady onshore feed of sediments from the shoreface during fairweather conditions appear to play an important role on medium and long term profile evolution at least at duck

time step limits for stable solutions of the ice sheet equation various spatial discretizations for the ice sheet are compared for accuracy against analytical solutions in one and two dimensions the computational efficiency of various iterated and non iterated marching schemes is compared the stability properties of different marching schemes with and without iterations on the non linear equations are compared newton raphson techniques permit the largest time steps a new technique which is based on the fact that the dynamics of unstable iterated maps contain information about where the unstable root lies is shown to improve substantially the performance of picard iteration at a negligible computational cost

steady thermomechanical flow along two dimensional flow lines in large grounded ice sheets a thermomechanical flow model has been developed in order to perform simultaneous calculations of the surface elevation and the distribution of stresses velocities and temperatures along two dimensional flow lines on large grounded ice sheets in order to solve the complex system of full thermomechanical coupled equations the following approximations have been made a coordinate scaling giving a lead order reduced model and an iteration procedure which decouples the energy balance equation from the rest of the equations glen s flow law is used as the constitutive relationship between stresses and deformation in the model and it is found that the longitudinal deviatoric stresses have a significant role in the lead order equations at the ice divide the solution shows that the velocity stress and temperature distributions change rapidly here the basal temperature increases creating a hot spot the surface strain rates increase 50 and the horizontal velocity profile is concave near the base with an inflection point away from the ice divide basal temperature increases and in the example presented here the basal temperature reaches the pressure melting point after 2 3 of the lateral extent and basal sliding begins to influence the flow in the ablation region it is seen that the heat produced by internal deformation is of the same order of magnitude as the geothermal heat flux from the bedrock

impacts of hurricane ike on the beaches of the bolivar peninsula tx usa abstract hurricane ike caused substantial beach erosion along the coast of the bolivar peninsula tx much of the erosion was caused by the offshore ebb flow of the ca 5 m storm surge that formed spatially discrete scour features using aerial photography and repeat lidar data we identify five types of scour features and describe the alongshore distribution in four flow environments type 1 scours are relatively small and compact features associated mainly with flow off a wide vegetated grasses shrubs trees surface across a wide beach type 2 scours are large and branching forms associated mainly with flow that was channeled by streets or gaps between structures type 3 scours are large and blocky features associated with flow off a marsh surface across a highway which removed almost all beach sands from the surface type 4 scours are elongated shore perpendicular channels associated with the same flow characteristics as type 3 scours type 5 scours are elongated shore perpendicular features sometimes branching associated with flow through gaps in a destroyed shore protection structure repeat imagery indicates that many of the features persisted for at least seven months recent aerial photography indicates that aspects of some features remained evident more than three years after ike s landfall

morphological barrier island changes and recovery of dunes after hurricane dennis st george island florida abstract during the summer of 2005 hurricane dennis overwashed the eastern portion of st george island part of the northwest barrier island chain located along the florida panhandle in this paper lidar based morphological changes of the barrier island are analyzed along with the short term post storm recovery of secondary dunes results show that overwash from the storm surge removed nearly the entire foredune complex and the initial breaching probably occurred where the complex was either low or discontinuous in these locations beach widening was less in contrast approximately 10 m of beach widening occurred where foredunes were higher and continuous implying that more sediment was available for seaward transport during storm conditions the secondary dunes recovered at an average linear rate of 3 4 cm per month in the presence of vegetation although monthly averages varied from 1 5 to 1 4 m3 m and total volume changes varied from 17 9 to 16 4 m3 m for the duration of the study furthermore vegetation deterred dune migration thus favoring dune growth and reducing erosion due to wind in contrast the absence of vegetation inhibited dune growth insignificant changes in elevation occurred in areas of storm debris or lag deposit finally distributions of topographic gradients and curvature calculated numerically from pre and post storm lidar data are introduced as a potential tool in determining the relative post storm recovery of the dune field

recent and modern marine erosion on the new jersey outer shelf abstract recent chirp seismic reflection data combined with multibeam bathymetry backscatter and analysis of grab samples and short cores provide evidence of significant recent erosion on the outer new jersey shelf the timing of erosion is constrained by two factors 1 truncation at the seafloor of what is interpreted to be the transgressive ravinement surface at the base of the surficial sand sheet and 2 truncation of apparently moribund sand ridges along erosional swales oriented parallel to the primary direction of modern bottom flow and oblique to the strike of the sand ridges these observations place the erosion in a marine setting post dating the passage of the shoreface ravinement and the evolution of sand ridges that form initially in the near shore environment also truncated by marine erosion are shallowly buried fluvial channel systems formed during the last glacial maximum and filled during the transgression and a regional reflector r that is 40 kyr depths of erosion range from a few meters to 10 m the seafloor within eroded areas is often marked by ribbon morphology seen primarily in the backscatter data as areas of alternating high and low backscatter elongated in the direction of primary bottom flow ribbons are more occasionally observed in the bathymetry where observed crests exhibit low backscatter and troughs exhibit high backscatter sampling reveals that the high backscatter areas of the ribbons consist of a trimodal admixture of mud sand and shell hash with a bimodal distribution of abraded and unabraded sand grains and microfauna the shell hash is interpreted to be an erosional lag while the muds and unabraded grains are in this non depositional environment evidence of recent erosion at the seafloor of previously undisturbed strata the lower backscatter areas of the ribbon morphology were found to be a well sorted medium sand unit only a few 10 s of cm thick overlying the shelly muddy sandy material concentrations of well rounded gravels and cobbles were also found in eroded areas with very high backscatter and at least one of these appears to be derived from the base of an eroded fluvial channel seafloor reworking over the transgressive evolution of the shelf appears to have switched from sand ridge evolution which is documented to 40 m water depth to more strictly erosional modification at greater water depths we suggest that this change may be related to the reduction with water depth in the effectiveness of sediment resuspension by waves resuspension is a critical factor in the grain size sorting during transport by bottom currents over large bedforms like sand ridges otherwise we speculate displacement of sand by unidirectional currents will erode the seafloor

a 3 d model for the antarctic ice sheet a sensitivity study on the glacial interglacial contrast on the longer climatic time scales changes in the elevation and extent of the antarctic ice sheet have an important role in modulating global atmospheric and oceanographic processes and contribute significantly to world wide sea levels in this paper a 3 d time dependent thermomechanical model for the entire ice sheet is presented that is subsequently used to examine the effects of glacial interglacial shifts in environmental boundary conditions on its geometry the model takes into account a coupled ice shelf grounding line dynamics basal sliding and isostatic bed adjustment and considers the fully coupled velocity and temperature fields ice flow is calculated on a fine mesh 40 km horizontal grid size and 10 layers in the vertical for grounded and floating ice and a stress transition zone in between at the grounding line where all stress components contribute in the effective stress in the flow law there is free interaction between ice sheet and ice shelf so that the entire geometry is internally generated a simulation of the present ice sheet reveals that the model is able to yield realistic results a series of sensitivity experiments are then performed in which lower temperatures reduced accumulation rates and lower global sea level stands are imposed either singly or in combination by comparing results of pairs of experiments the effects of each of these environmental changes can be determined in agreement with glacial geological evidence we found that the most pronounced changes show up in the west antarctic ice sheet configuration they appear to be essentially controlled by variations in eustatic sea level whereas typical glacial interglacial changes in temperature and ice deposition rates tend to balance one another these findings support the hypothesis that the antarctic ice sheet basically follows glacial episodes in the northern hemisphere by means of sea level teleconnections grounding occurs more readily in the weddell sea than in the ross sea and long time scales appear to be involved it may take up to 30 40000 years for these continental shelf areas to become completely grounded after an initial stepwise perturbation in boundary conditions according to these reconstructions a steady state antarctic ice sheet may contribute some 16 m to global sea level lowering at maximum glaciation

influence of channelling on heating in ice sheet flows ice sheet flows can be channelled by perturbations in the basal topography or in the sliding coefficient these lead to spatial variation in the steady profile the flux and the dissipative heating this paper examines the linearized theory of heating variations showing that the map plane aspect ratio of the basal perturbation has a dominating effect on the qualitative behaviour for ribbing transverse to the direction of flow maximum heating occurs over bedrock and sliding viscosity highs when flow parallel channelling occurs maximum heating occurs over bedrock lows and sliding viscosity lows these results are used to examine symmetry breaking behaviour of numerical thermoviscous ice sheet models in terms of a dissipation driven creep instability

high resolution ice thickness mapping in south greenland airborne radar sounding is difficult in south greenland because of the presence of englacial water which prevents the signal from reaching the bed data coverage remains suboptimal for traditional methods of ice thickness and bed mapping that rely on geostatistical techniques such as kriging because important features are missing here we apply two alternative approaches of high resolution 300 m ice thickness mapping that are based on the conservation of mass to two regions of south greenland 1 qooqqup sermia and kiattuut sermiat and 2 ikertivaq these two algorithms solve optimization problems for which the conservation of mass is either enforced as a hard constraint or as a soft constraint for the first region very few measurements are available but there is no gap in ice motion data whereas for ikertivaq more ice thickness measurements are available but there are gaps in ice motion data we show that mass conservation algorithms can be used as validation tools for radar sounding we also show that it is preferable to apply mass conservation as a hard constraint rather than a soft constraint as it better preserves elongated features such as glacial valleys and ridges

the behaviour of nearshore bars on the time scale of years a conceptual model abstract soundings of the sandy dutch coast have demonstrated the presence of a multiple bar system that exhibits cyclic offshore directed migration on the time scale of years medium term a cycle comprises bar generation interannual offshore migration and decay in this paper we propose a conceptual model in which we qualitatively outline the relationships between short term hourly process knowledge waves currents sediment transport based on a 15 week data set of near bed hydrodynamics obtained in the multiple bar system of terschelling the netherlands and medium term bar behaviour the short term process knowledge is aggregated to the medium term by combining probabilistic characterisations of medium term wave conditions and sediment transports estimated from the short term field measurements essential to the conceptual model is the observed cross shore change in short term conditions that dominate medium term onshore and offshore sediment transport q on and q off respectively in the cross shore direction the conditions contributing most to q on change from predominantly breaking situations outside the bar zone to mainly non breaking conditions on the beach in contrast q off remains restricted to breaking conditions over the entire profile in the model we qualitatively describe how this cross shore change in the forcing conditions results in interannual offshore bar migration bar decay and the onset of the next cycle as such our model may serve as a first step towards a more coherent quantitative model on medium term periodic bar behaviour

reconciling the ice 6g_c reconstruction of glacial chronology with ice sheet dynamics the cases of greenland and antarctica we describe a theoretical and numerical framework that has been developed to investigate the compatibility of the ice 6g c reconstruction of the glaciation histories of the greenland and antarctic ice sheets with the latest understanding of ice physics the ice 6g c reconstruction has been produced solely on the basis of the theory of the glacial isostatic adjustment gia process and it has remained an issue as to whether such reconstructions of the time dependent thickness variations of grounded continental ice sheets were compatible with physics based ice mechanical considerations our analyses focus on the evolution over the last glacial cycle of these extent ice sheet complexes and demonstrate that the gia inferred models are entirely consistent with such considerations when uncertainties in net mass balance history are taken fully into account

lakes beneath the ice sheet the occurrence analysis and future exploration of lake vostok and other antarctic subglacial lakes abstract airborne geophysics has been used to identify more than 100 lakes beneath the ice sheets of antarctica the largest lake vostok is more than 250 km in length and 1 km deep subglacial lakes occur because the ice base is kept warm by geothermal heating and generated meltwater collects in topographic hollows for lake water to be in equilibrium with the ice sheet its roof must slope ten times more than the ice sheet surface this slope causes differential temperatures and melting freezing rates across the lake ceiling which excites water circulation the exploration of subglacial lakes has two goals to find and understand the life that may inhabit these unique environments and to measure the climate records that occur in sediments on lake floors the technological developments required for in situ measurements mean however that direct studies of subglacial lakes may take several years to happen

ephemeral sand waves in the hurricane surf zone abstract airborne bathymetric lidar observations along the florida panhandle after hurricane dennis 2005 show the first unequivocal observations of surf zone sand wave trains these are found in depths of 5m along the trough of the hurricane bar where hindcasts show strong longshore currents only during severe storms the waves extend over tens of kilometers of coast after dennis but are absent from the same area in four other datasets observed wavelength to water depth ratios are comparable to river dunes and tidal sand waves but height to depth ratios are smaller with the largest wave heights around 0 1 times the water depth the sand wave generation mechanism is hypothesized to be from wind and wave induced longshore currents which were hindcast to be large during dennis with destruction from water wave orbital velocities

the eismint benchmarks for testing ice sheet models we present a series of benchmark experiments designed for testing and comparing numerical ice sheet models following the outcome of two eismintworkshops organized to intercompare large scale ice sheet models currently in operation model benchmark experiments are described for ice sheets underfixed and moving margin conditions these address both steady state and time dependent behavior under schematic boundary conditions and with prescribedphysics a comparison was made of each model s prediction of basic geophysical variables such as ice thickness velocity and temperature consensusachieved in the model intercomparison provides reference solutions against which the accuracy and consistency of ice sheet modeling codes can be assessed

water exchange between the subglacial lake vostok and the overlying ice sheet it has now been known for several years that a 200 km long lake called lake vostok lies beneath the ice sheet on which sits vostok station in antarctica the conditions at the base of the ice sheet above this subglacial lake can provide information about the environment within the lake including the likelihood that it supports life here we present an analysis of the ice sheet structure from airborne 60 mhz radar studies which indicates that distinct zones of basal ice loss and accretion occur at the ice water interface subglacial melting and net ice loss occur in the north of the lake and across its 200 km long western margin whereas about 150 m of ice is gained by subglacial freezing in the south this indicates that significant quantities of water are exchanged between the base of the ice sheet and the lake waters which will enrich the lake with gas hydrates cause sediment deposition and encourage circulation of the lake water

seismic facies architecture and evolution of the bolivar roads tidal inlet delta complex east texas gulf coast abstract the seismic facies facies architecture and stratigraphy of tidal inlet and tidal delta deposits and their relation to the development of adjacent coastal lithosomes are examined using high resolution seismic profiles vibracores and borehole descriptions the bolivar roads tidal inlet delta complex along the east texas gulf coast was formed 3 3 ka by spit accretion across a baymouth following a rapid sea level rise 4 ka an increase in the tidal prism through time and entrenchment of the tidal inlet over the trinity river incised valley stabilized the inlet and intensified tidal processes on the tidal inlet delta complex the tidal inlet facies show channel stacking and cut and fill structures stacked clinoforms dip westward across the inlet the spit facies is characterized by oblique tangential clinoforms that build outward and deepen from the edge toward the center of the valley the flood tidal delta facies has a base that shallows abruptly bayward as the flood tidal delta facies thins bayward it interfingers with bay sediments near the inlet the flood tidal delta shows channel cut and fill with an overall channel stacking pattern on the seaward side the channels have a trough like geometry bayward the channels broaden and shallow the channels show a prograded fill pattern the ebb tidal delta facies exhibits gently inclined clinoforms prograding over a ravinement surface tidal inlet deposits are composed of sand shell and mud interbeds sand and clay interlaminae are ubiquitous in the tidal deltas and sand and shell beds are common near the inlet overall the bolivar roads tidal inlet delta complex is mud dominated as a result of high influx of fine sediment into the bay