relating the occurrence of crevasses to surface strain rates the presence of crevasses on the surface of ice masses indicates that a fracture criterion has been met understanding how crevasses form will provide information about the stress and strain rate fields in the ice this study derives a relationship between measurements of strain rate and observations of crevassing on the surface of ice masses a literature search yielded 17 polar and alpine locations where strain rates had been measured and crevassing recorded by plotting strain rates converted to stresses using a creep law using axes representing the surface parallel principal stresses failure envelopes were derived by enclosing measurements where surface crevassing was absent the derived failure envelopes were found to conform well to theoretical ones predicted by the coulomb and the maximum octahedral shear stress von mises theories of failure the derived failure envelopes were scaled by the tensile strength which was found to vary from 90 to 320 kpa there was no systematic variation of tensile strength with either temperature at 10 m depth or the method used to locate the crevasses the observed variation in tensile strength could result from variations in ice properties e g crystal size impurity content or density or could be related to uncertainty in the constitutive relation creep flow and fracture share a very similar temperature dependence suggesting similar crystal scale processes are responsible for both the observed relationship will provide a supplementary tool with which to verify and test models of ice dynamics against remotely sensed imagery the study also indicates that a temperature rise of a few degrees throughout the ice column will not result directly in any increase in calving rates from the large antarctic ice shelves such as the filchner ronne or ross ice shelves

grain size trends associated with net sediment transport patterns an example from the belgian continental shelf grain size trends in relation to net sediment transport pathways are examined using some commonly used grain size parameters the results of the trend analysis for 15 types of the trends are compared with a known net sediment transport pathways established on the basis of the sandbank hydro and sediment dynamics i e general water and sediment movement patterns and bedform asymmetry it is shown that 1 the results for grain size trends associated with a worsening in sorting along the transport pathways have little similarity to transport pathways 2 the results for grain size trends associated with an improvement in sorting along the transport pathways have relatively high degree of similarity to the identified pathways and 3 the residual pattern derived on the basis of a combined grain size trend used elsewhere gao and collins 1992 appears to be most similar and suitable for defining transport pathways

sand waves and sediment transport around the end of a tidal sand bank two detailed surveys have been made of the north western end of the haisborough sand off the norfolk coast using echosounder 3 5 khz reflection profiler and side scan sonar asymmetrical sand waves indicate north westerly sand movement on the southern side and south easterly sand movement on the steeper northern side of the bank secondary superimposed megaripples which are probably better indicators of sediment movement give evidence of a cross bank component between the north westerly and south easterly facing sand waves on the tip of the bank there is a zone of symmetrical sand waves these are usually taken to indicate zero net transport but in this case the oblique orientation of megaripples in their troughs indicates transport parallel to the sand wave crests this suggests the route by which sand travels around the end of the bank to form a roughly closed circulation sediment textural parameters support the notion that sand is winnowed from the foot of the bank on both sides and is transported to the middle with an overall net transport from the south to the north analysis of charts dating back to 1886 shows that the bank is stable within the error limits of position fixing though that could allow more than 0 25 km shift to the north east in 100 years to pass undetected a box model is drawn up for the estimated sediment fluxes around the end of the bank and implications for residence times and circulation rates are drawn from it

recent sea level contributions of the antarctic and greenland ice sheets after a century of polar exploration the past decade of satellite measurements has painted an altogether new picture of how earth s ice sheets are changing as global temperatures have risen so have rates of snowfall ice melting and glacier flow although the balance between these opposing processes has varied considerably on a regional scale data show that antarctica and greenland are each losing mass overall our best estimate of their combined imbalance is about 125 gigatons per year of ice enough to raise sea level by 0 35 millimeters per year this is only a modest contribution to the present rate of sea level rise of 3 0 millimeters per year however much of the loss from antarctica and greenland is the result of the flow of ice to the ocean from ice streams and glaciers which has accelerated over the past decade in both continents there are suspected triggers for the accelerated ice discharge surface and ocean warming respectively and over the course of the 21st century these processes could rapidly counteract the snowfall gains predicted by present coupled climate models

morphology and maintenance of steep dunes near dune asymmetry transitional areas on the shallow shelf beibu gulf northwest south china sea abstract symmetrical dunes can be found in areas where dune asymmetry changes sense but in areas where dunes are not expected to be moving the development and maintenance of symmetrical dunes are poorly understood in the southeastern beibu gulf we have identified five typical dune asymmetry transitions at the depths of shallower than 50 m basing on the latest bathymetric data those regions are separated into two classes according to their different shapes and locations sand transport directions are deduced by dune migrations between surveys simulated regional currents and sand size trends the two classes of the regions are found to experience opposing net sand transports from the north and south around the asymmetry transitions the dunes have impressive steep sections relatively symmetrical shapes and almost straight crests they have a general height length h l relation h 0 0575 l1 0836 r2 0 834 which is steeper than dunes in other seas some of the dunes have extraordinary large heights exceeding a quarter of water depth which is attributed to the preserved sand body bidirectional sand accretion erosion in troughs and absence of significant erosion on crests the steepness and straight crests of dunes adapt to the nearly symmetrical or weak asymmetrical reversing currents which induce flow separations and reversals on both flanks of the dunes it also suggests that dunes develop differently under sufficient and limited sand supply dunes will gradually grow in size when being continually supplied with sand however if sand supply is restricted dunes may separate or turn into more isolated individuals to sustain the h l relation the h l relation of dunes can be maintained during either growth or separation overall the nearly symmetrical or weak asymmetrical reversing currents maintain the steep shapes of dunes while variations in sand supply affect their sizes to work out the detailed transports of sediment future observations of flow structures and related models of such two sided steep dunes are still required

shallow shelf approximation as a sliding law in a thermomechanically coupled ice sheet model 1 the shallow shelf approximation a balance of membrane stresses for ice flow is an effective sliding law for ice sheet modeling our use of it as a sliding law becomes a standard model for ice stream flow when the sliding velocity is large 100 m a 1 and faster following schoof 2006a we describe the basal resistance as plastic till for which the yield stress is given by a mohr coulomb formula pore water pressure is related to basal melt rate the velocity field used in the mass continuity and conservation of energy equations is an average of velocities from the shallow shelf approximation and the nonsliding shallow ice approximation using this scheme our model has realistic time dependent ice streams which exhibit the range of surface velocities seen in actual ice streams we demonstrate the model at high spatial resolution 5 km grid over multiple millenia using its implementation in the parallel ice sheet model numerical experiments show that the entire scheme is stable with respect to many parameter changes some experiments reveal significant ice stream variability in a hypothetical steady climate with characteristic cycles on the order of 1000 years we believe this is the first practical whole ice sheet model with a unified treatment of vertical shear stresses and membrane stresses it is capable of high resolution thermomechanically coupled multimillenia simulations of ice sheets containing ice streams

residual sediment transport paths on a tidal sand bank a comparison between the modified mclaren model and bedform analysis abstract in order to evaluate the validity of the modified mclaren model for sand transport on a tidal sand bank 42 grab samples taken from the northern part of the kwintebank were decalcified sieved and analyzed for statistical parameters the areal distribution of mean grain size standard deviation and skewness were defined and used to determine the residual transport directions the grain size trends taken into consideration were that the sediment becomes finer better sorted and more negatively skewed along the transport pathways f b and the sediment becomes coarser better sorted and more positively skewed along the transport pathways c b these were compared with the transport paths inferred from bedform asymmetries obtained by side scan sonar records there were major differences between the sediment transport paths resulting from the two techniques when the f b trend and small dune asymmetries are compared only 5 out of 24 cases showed agreement 30 whereas in 9 cases the trends were 180 30 different there was even less agreement between the c b trend and the asymmetry of the small dunes only 1 out of 24 cases agreed 30 and almost half of the cases were 90 30 different the sediment transport directions deduced from the morphological analysis of the large dunes all point in the direction of the peak flood current the large dunes seem to be more suitable indicators of the sediment transport paths than the small ones however they conflict with the vectors resulting from the f b trend only one out of 20 cases agrees 30 while in 13 cases the vectors are 180 30 different and in three cases 90 30 neither do the transport directions from the c b trend agree with those deduced from the large dunes only one case agrees 30 while in four out of 20 cases the vectors point in opposite directions in four cases the vectors are 90 30 different the asymmetric cross section of the sand bank suggests sediment transport in a northwest direction it does not seem probable however that sediment transport occurs by migration of the whole sand bank the transport directions inferred from the bank morphology are quite different from the f b trend in 12 out of 42 cases the vectors agree 30 in four cases the vectors point in opposite directions and in 15 cases they are 90 30 different there is slightly more agreement between the c b trend and the transport directions deduced from the sand bank 16 out of 42 cases agree 30 in nine cases the vectors point in opposite directions and three cases are 90 30 different from this study it seems that the residual sediment transport directions can best be deduced from the morphological characteristics of the large dunes the departures between the results of the sediment trend analysis according to the modified mclaren model and the morphology of the subaqueous bedforms are partly explained by the location of the study area near the edge of the sand bank probably better results of the sediment trend analysis would be obtained if samples over the whole length of the sand bank were analyzed and compared perhaps a smaller spacing between the samples would improve the results as well and maybe the samples should not have been decalcified possibly the different transport patterns are attributed to different sediment transport processes which makes a comparison virtually impossible

potential gain and loss of sand by some sand banks in the southern bight of the north sea abstract a detailed echo sounding and side scan sonar survey has shown that sand banks in the southern bight of the north sea have rounded heads in the approximate upstream direction and tapered tails in the approximate downstream direction of net regional sand transport thus the plan view of sand banks can be used as an indicator of the approximate direction of net sand transport sand wave aprons occur in front of the heads and downstream of the tails of the sand banks the asymmetry of the sand waves implies that sand is being added to sand wave aprons at the heads and to the upstream flanks of these sand banks but is shed from the downstream sand wave aprons there is no evidence for a closed sand circulation on the banks

melting of floating ice and sea level rise 1 contrary to popular belief the melting of floating ice in the form of ice shelves icebergs and sea ice may have a non zero impact on sea level this is because the melting process cools and dilutes the oceans on average and unless these opposing effects exactly balance each other there will be a net change in the ocean density we discuss how these subtle effects can be quantified and put bounds on the potential sea level rise associated with melting of the ice masses that are currently afloat in the world s oceans

northern larsen ice shelf antarctica further retreat after collapse changes of larsen ice shelf antarctica and the surrounding glaciers after its collapse in 1995 were investigated using satellite radar imagery with emphasis onchanges in the glaciers which previously nourished the ice shelf north of seal nunataks the large glaciers retreated several kilometres inland of the previousgrounding line the velocity field of drygalski glacier the largest glacier in this area was mapped by means of interferograms derived from pairs of ers syntheticaperture radar images from 1995 and 1999 the main part of the glacier showed significant acceleration of flow over these 4 years with an increase of veocity upto three fold at the terminus similar accelerations were observed by means of interferometry on several other grounded glaciers suggesting that the removal ofice shelves could lead to an effect on eustatic sea level for larsen b the northernmost surviving part of larsen ice shelf the retreat of the ice front to october2000 is documented

use of swath bathymetry in the investigation of sand dune geometry and migration around a near shore banner tidal sandbank banner tidal sandbanks in the bristol channel have been repeatedly surveyed with a multibeam sonar to study the geometry and migration patterns of superimposed dunes the data presented in this paper constitute one of the first studies concerned with sediment transport around a banner sandbank helwick sands in the bristol channel using repeated swathbathymetry the data reveal that the dunes maintain their shapes over a period of 11 months and that they migrate in opposite directions on the alternate sides of the bank curiously dunes connect over the crest of the bank despite opposing sediment transport directions on the flanks dune height increases with water depth as found in similar environments we suggest how the morphology of the dunes results from the complex interaction between surface waves and tidal currents that occurs within the proximity of the headland 1995 furthermore repeated full bathymetric coverage of sub tidal sedimentary structures using swath sonar technology will provide information of their dynamics and threedimensional characteristics which are of interest for the understanding of coastal and shelf sedimentary transport this paper describes the morphology and dynamical characteristics of sand dunes at the eastern end of the helwick sandbank lying near a headland in the bristol channel fig 1 the analysis is based on two sets of swathbathymetry data collected on 26 september 2001 and 20 august 2002 the main objective of this paper is to demonstrate the potential for tracking dune migration and dune geometry changes from repeated bathymetric multibeam surveys this paper is therefore intended to a describe the morphological characteristics of the dunes and their spatiotemporal evolution b estimate dune migration rate and from that infer sediment transport flux and c relate these data to the work elsewhere of sediment transport around sandbanks and their maintenance in describing the sand dunes in this paper we follow the terminology developed by ashley 1990

mechanisms underlying the regional morphological differences between the northern and southern radial sand ridges along the jiangsu coast china abstract radial sand ridges rsrs spread in a fan shaped pattern over the seabed of the southern yellow sea along the jiangsu coast china with pronounced differences between the northern and southern channel shoals indicated as ncs and scs respectively a depth averaged nested numerical model is employed to analyze the tidal hydrodynamics and sediment dynamics of the rsrs model results show considerable regional differences in terms of tidal current vector ellipses eight shaped ellipses are typical in the ncs where rotational tidal waves dominate while the scs are characterized by egg shaped ellipses because of the dominance of progressive tidal waves sand ridges tend to be elongated and straight in the ncs while short and dincontinuous in the scs numerical lagrange particles released near the ridges in the ncs tend to travel across the crest of the ridges rather than move around them as observed in the scs consistently the patterns of sediment convergence and divergence indicate that suspended sediments converge over the whole xiaoyinsha ridge located in the ncs while converge to the two sides of the hetunsha ridge located in the scs model results also highlight the importance of initial sediment compositions in determining patterns of the net sediment fluxes the suspended sediment concentration ssc time series during one tidal cycle may show single or double peaks depending on the grain size and the related lag effect bed load transport is much smaller than suspended load so sediment compositions should be carefully considered when simulating the ssc field and the hydrodynamic of the morphological evolution of the rsrs at the regional scale overall this study suggests that the ncs and scs may be treated as two relatively independent geomorphological systems characterized by different tidal flows and sediment compositions

coupling of ice shelf melting and buttressing is a key process in ice sheets dynamics 1 increase in ice shelf melting is generally presumed to have triggered recent coastal ice sheet thinning using a full stokes finite element model which includes a proper description of the grounding line dynamics we investigate the impact of melting below ice shelves we argue that the influence of ice shelf melting on the ice sheet dynamics induces a complex response and the first naive view that melting inevitably leads to loss of grounded ice is erroneous we demonstrate that melting acts directly on the magnitude of the buttressing force by modifying both the area experiencing lateral resistance and the ice shelf velocity indicating that the decrease of back stress imposed by the ice shelf is the prevailing cause of inland dynamical thinning we further show that feedback from melting and buttressing forces can lead to nontrivial results as an increase in the average melt rate may lead to inland ice thickening and grounding line advance

ice shelf disintegration by plate bending and hydro fracture satellite observations and model results of the 2008 wilkins ice shelf break ups abstract satellite remote sensing observations of three break up events in 2008 for the wilkins ice shelf 28 february to 6 march 27 may to 31 may and 28 june to mid july provide unprecedented detail of ice shelf calving during rapid break up the observations reveal that the wilkins break ups occur through a distinctive type of shelf calving which we term disintegration as well as more typical rifting and calving here we focus on the disintegration process which is characterized by repeated rapid fracturing that creates narrow ice edge parallel blocks with subsequent block toppling and fragmentation forming an expanding iceberg and ice rubble mass we use these data to develop and test a model of floating ice plate disintegration in which ice plate bending stresses at the ice front arising from buoyancy forces can lead to runaway calving when free mobile water is available high resolution satellite images and laser altimetry of the first break up event provide details of fracture spacings ice thicknesses and plate bending profiles that agree well with our model predictions we suggest that surface or near surface meltwater is the main pre condition for disintegration and that hydro fracture is the main mechanism brine layers from near waterline brine infiltration can support a similar process but this is less effective unless regional ice stress patterns contribute to the net stress available at the crack tip for fracturing a combination of brine enhanced fracturing and changing internal net extensional stresses was the likely mechanism behind the latter two wilkins events

a theory of the ka band radar imaging mechanism of a submerged wreck and associated bed forms in the southern north sea the ka band radar imaging mechanism of the submerged wreck sand ribbon of the motor vessel m v birkenfels in the southern north sea is investigated by applying the quasi specular scattering theory and considering the capillary as well as the gravity wave ranges of the wave energy density spectrum for the imaging of wrecks and other oceanographic and meteorological phenomena at the sea surface it is assumed that quasi specular scattering becomes dominant at higher radar frequencies like ka and x band and wind speeds 7 8 m s 1 multibeam echo sounder images of the birkenfels wreck and associated sand ribbons as well as other available environmental in situ data have been analyzed the formation of sand ribbons at the sea bed and the manifestation of its radar signatures at the water surface are caused by an elliptical vortex or helical flow cell triggered by unidirectional tidal current flow interacting with the wreck the difference between simulated and measured normalized radar cross section nrcs modulation as a function of the space variable is less than 31 6 results are presented for nrcs simulations dependent on position for different effective incidence angles unidirectional current speeds wind speeds and relaxation rates the calculated current gradient or strain rate of the imaging theory has the same order of magnitude as those obtained for marine sand waves this implies that the responsible hydrodynamic interaction mechanism is able to produce radar signatures of submerged wrecks sand ribbons and make them visible at the sea surface

marine ice in larsen ice shelf 1 it is argued that larsen ice shelf contains marine ice formed by oceanic freezing and other mechanisms missing basal returns in airborne radar soundings and observations of a smooth and healed surface coincide downstream of regions where an ocean model predicts freezing visible imagery suggests that marine ice currently stabilizes larsen c ice shelf and implicates failure of marine flow bands in the 2002 larsen b ice shelf collapse ocean modeling indicates that any regime change towards the incursion of warmer modified weddell deep water into the larsen c cavity could curtail basal freezing and its stabilizing influence

enhancement factors for grounded ice and ice shelves inferred from an anisotropic ice flow model polar ice is known to be one of the most anisotropic natural materials for a given fabric the polycrystal viscous response is strongly dependent on the actual state of stress and strain rate within an ice sheet grounded ice parts and ice shelves have completely different stress regimes so one should expect completely different impacts of ice anisotropy on the flow the aim of this work is to quantify through the concept of enhancement factors the influence of ice anisotropy on the flow of grounded ice and ice shelves for this purpose a full stokes anisotropic marine ice sheet flowline model is used to compare isotropic and anisotropic diagnostic velocity fields on a fixed geometry from these full stokes results we propose a definition of enhancement factors for grounded ice and ice shelves coherent with the asymptotic models used for these regions we then estimate realistic values for the enhancement factors induced by ice anisotropy for grounded ice and ice shelves

a comparative modeling study of the brunt ice shelf stancomb wills ice tongue system east antarctica two diagnostic dynamic thermodynamic ice shelf models are applied to the brunt ice shelf stancomb wills ice tongue system located off caird coast coats land antarctica the brunt ice shelf stancomb wilis ice tongue system is characterized as a thin unbounded ice shelf with an atypical and highly heterogeneous structure in contrast to other ice shelves a composite mass of icebergs that calved at the grounding line and were then locked within fast sea ice exists between the fast moving stancomb wills ice stream and the slow moving brunt ice shelf we simulate the present flow regime of the ice shelf that results from the ice thickness distribution and the inflow at the grounding line with two different models and compare the model results with feature tracking and insar flow velocities we then incorporate two observed features a rift and a shear margin into the models with two different approaches and demonstrate the effects of variations in numerical values for the shear strength and viscosity in these zones on the simulated velocity field a major result is that both kinds of implementation of the rifts lead to similar effects on the entire velocity field while there are discrepancies in the vicinity of the rifts

tide and wave dynamics on a sand bank from the deep shelf of the western channel approaches abstract the sedimentary response of deep shelf sediments to the interplay of tides and waves based on the example of the celtic sea sand banks is described grab samples and 1500 km of multibeam and side scan data have been collected from the 300 km2 densely surveyed kaiser bank area the bank is 140 170 m deep 30 m high and 60 km long oriented perpendicular to the shelf edge and nearly parallel to the major axis of the tidal ellipse surficial sediments of the bank consist of medium to gravely biolithoclastic sands they are swept by tidal currents that reach 0 9 m s 1 m above the seabed and under the occasional influence of waves the mobile sands are commonly bedformed and rest on a highly backscattering lag the flanks below 140 m mainly exhibit transverse tidal dunes and sand ribbons whereas the top of the bank mostly displays discontinuous sand patches and wave ripples the sand patches are interpreted as the remnants of tidal bedforms reworked by waves calculations of the threshold of motion of four modal grain sizes under various conditions at the seabed show that tidal bedforms are active during spring tides sometimes associated with waves whereas only the largest annual waves may explain the observed wave ripples as in active tidal banks there is a reversal of tidal bedload transport from one bank side to the other the sedimentation rate is very low on the kaiser bank in addition there is a loss of sediment at both bank extremities and the tidal bedload cross bank transfer is very small probably controlled by shelf residual currents and long term drift of sediment resuspended by waves however the present day action of waves at the bank top is less intense than it was at lower sea levels during the holocene as evidenced by seismic studies

tidal and storm influenced sedimentation from a cambrian transgressive passive margin sequence the basal cambrian clastic sediments in northwest scotland record a transgressive sequence of barrier island to tidal shelf quartz arenites eriboll sandstone lower member and pipe rock overlain by a storm dominated interval of mixed clastic carbonate sediments fucoid beds two shallowing events interrupt this sequence the lower near the middle of the pipe rock and the upper at the top of the fucoid beds which led to the deposition of the quartz arenites of the salterella grit variations in the proportion of storm deposited facies and in the thickness of tidally generated structures suggests that tidal currents progressively increased during deposition of the quartz arenites and then decreased until deposition of the mixed clastic carbonate deposits these features suggest amplification of tides as the shelf approached a resonant width during transgression further transgression produced a wide non resonant shelf where tides were frictionally damped and storm processes were dominant similar sequences of the same age in western newfoundland eastern greenland and spitsbergen may also have been produced by the shelf bordering the north american craton responding to tidal amplification followed by frictional damping

giant sandwaves in the hola glacial trough off vesteralen north norway abstract seabed data acquired from the hola glacial trough off vesteralen north norway reveal the morphology of spectacular sandwave fields and large sandwaves in great detail the sandwave fields are among the deepest known occurring in water depths of 200 260 m multibeam bathymetry and backscatter shallow seismic sediment samples and video data collected by the mareano programme and bottom current modelling results have been used to interpret morphology transport and distribution of the seabed sediments sand supply to the hola trough was initially from wave and current reworking of glacial and glaciomarine deposits during the eustatic sea level lowstand and sea level rise after the last glaciation more recently deposited bioclastic sand dominates the upper part of the sand unit which locally reaches 10 m thickness sand predominantly accumulates behind ridges where current strength decreases and in topographic lows straight crested and barchan type sandwaves up to 3 km long and 7 m high are interpreted to form by tidal and geostrophic currents with bottom current speeds reaching 0 7 0 8 m s 1 a large cluster of coral reefs occur immediately east of the barchan type sandwaves where modelling shows bottom current velocities up to c 1 m s 1 sandwaves occur in four major sandwave fields strong variation in the nature of sandwaves over short distances e g migration direction between neighbouring sandwave fields point to important local variations in current regime and sedimentary processes the present work demonstrates how local topography and interaction of several current systems may produce complex depositional patterns on a local scale detailed multibeam mapping and video inspection of open shelf troughs in other narrow shelf settings worldwide may reveal similar sandwave fields

experimental and theoretical fracture mechanics applied to antarctic ice fracture and surface crevassing recent disintegration of ice shelves on the antarctic peninsula has highlighted the need for a better understanding of ice shelf fracture processes generally in this paper we present a fracture criterion incorporating new experimental fracture data coupled with an ice shelf flow model to predict the spatial distribution of surface crevassing on the filchner ronne ice shelf we have developed experiments that have enabled us to quantify for the first time quasi stable crack growth in antarctic ice core specimens using a fracture initiation toughness kinit for which crack growth commences the tests cover a full range of near surface densities 560 871 kg m 3 10 9 75 7 m depth results indicate an apparently linear dependence of fracture toughness on porosity such that kinit 0 257 80 7 predicting a zero porosity toughness of ko 155 kpa m1 2 we have used this data to test the applicability to crevassing of a two dimensional fracture mechanics criterion for the propagation of a small sharp crack in a biaxial stress field the growth of an initial flaw into a larger crevasse which involves a purely tensile crack opening depends on the size of the flaw the magnitude of kinit and the nature of the applied stress field by incorporating the criterion into a stress map of the filchner ronne ice shelf derived from a depth integrated finite element model of the strain rate field we have been able to predict regions of potential crevassing these agree well with satellite imagery provided an initial flaw size is assumed in the range 5 50 cm

a stratigraphic investigation of the celtic sea megaridges based on seismic and core data from the irish uk sectors the celtic sea contains the world s largest continental shelf sediment ridges these megaridges were initially interpreted as tidal features formed during post glacial marine transgression but glacigenic sediments have been recovered from their flanks we examine the stratigraphy of the megaridges using new decimetric resolution geophysical data correlated to sediment cores to test hypothetical tidal vs glacial modes of formation the megaridges comprise three main units 1 a superficial fining upward drape that extends across the shelf above an unconformity underlying this drape is 2 the melville formation mfm which comprises the upper bulk of the megaridges sometimes displaying dipping internal acoustic reflections and consisting of medium to coarse sand and shell fragments characteristics consistent with either a tidal or glacifluvial origin the mfm unconformably overlies 3 the upper little sole formation ulsfm previously interpreted to be of late pliocene to early pleistocene age but here shown to correlate to late pleistocene glacigenic sediments forming a precursor topography the superficial drape is interpreted as a product of prolonged wave energy as tidal currents diminished during the final stages of post glacial marine transgression we argue that the stratigraphy constrains the age of the mfm to between 24 3 and 14 ka bp based on published dates coeval with deglaciation and a modelled period of megatidal conditions during post glacial marine transgression stratigraphically and sedimentologically the megaridges could represent preserved glacifluvial features but we suggest that they comprise post glacial tidal deposits mfm mantling a partially eroded glacial topography ulsfm the observed stratigraphy suggests that ice extended to the continental shelf edge

present stability of the larsen c ice shelf antarctic peninsula we modelled the flow of the larsen c and northernmost larsen d ice shelves antarctic peninsula using a model of continuum mechanics of ice flow and applied a fracture criterion to the simulated velocities to investigate the ice shelf s present day stability constraints come from satellite data and geophysical measurements from the 2008 09 austral summer ice shelf thickness was derived from bedmap and icesat data and the density depth relationship was inferred from our in situ seismic reflection data we obtained excellent agreements between modelled and measured ice flow velocities and inferred and observed distributions of rifts and crevasses residual discrepancies between regions of predicted fracture and observed crevasses are concentrated in zones where we assume a significant amount of marine ice and therefore altered mechanical properties in the ice column this emphasizes the importance of these zones and shows that more data are needed to understand their influence on ice shelf stability modelled flow velocities and the corresponding stress distribution indicate that the larsen c ice shelf is stable at the moment however weakening of the elongated marine ice zones could lead to acceleration of the ice shelf due to decoupling from the slower parts in the northern inlets and south of kenyon peninsula leading to a velocity distribution similar to that in the larsen b ice shelf prior to its disintegration

a simple law for ice shelf calving a major problem for ice sheet models is that no physically based law for the calving process has been established comparison across a diverse set of ice shelves demonstrates that iceberg calving increases with the along flow spreading rate of a shelf this relation suggests that frictional buttressing loss which increases spreading also leads to shelf retreat a process known to accelerate ice sheet flow and contribute to sea level rise

calving of large tabular icebergs from ice shelf rift systems 1 we used interferometric synthetic aperture radar to study the detachment process that allowed two large icebergs to calve from the ross ice shelf antarctica time series of rift geometries indicate that rift widths increased steadily whereas rift lengths increased episodically through several discrete rift tip propagation events we also conducted modeling experiments constrained by the observed rift geometry both the observations and model suggest that rift opening and thus tabular iceberg calving are largely driven by glaciological stresses stress introduced by the effect of gravity on the ice shelf rather than by stress introduced by the ocean and atmosphere e g tides and storms this style of rift propagation is expected to determine the steady background calving rate of ice shelves and thus differs significantly from styles that led to the recent disintegration of ice shelves in response to climate warming e g the larsen b ice shelf on the antarctic peninsula

tides and the flow of rutford ice stream west antarctica 1 surface speeds of rutford ice stream west antarctica are known to vary by around 10 20 depending on location with a fortnightly periodicity corresponding to a springneap tidal cycle the reasons for these periodic variations in flow are unclear here the possible role of tidal stress transmission upstream of the grounding line in affecting rates of basal motion is investigated it is found that nonlinear rheological effects within the till when coupled with transmission of tidal stresses within the ice that are linearly related to tidal amplitude can give rise to the type of periodic oscillations in flow observed this nonlinear interaction between tidal forcing and till deformation increases the mean ice flux across the grounding line by a few percent above what might be expected in the absence of tidal forcing periodic velocity fluctuations of this type have not been observed on other ice streams however modeling suggests that this may be due to lack of data and that such flow variations are likely to be common features of active ice streams draining into the ronne ice shelf as well as of other ice streams subjected to similar tidal forcing

subcritical crack propagation as a mechanism of crevasse formation and iceberg calving recent investigations of crevassing on alpine glaciers and ice shelves have been based on linear elastic fracture mechanics lefm however lefm is unable to explain some aspects of crevasse formation such as the initiation of crevasse propagation from crystal scale mm microcracks the slow propagation of large fractures in ice shelves and the acceleration of crevasse opening before breaking of the ice terminus here another mechanism to account for these observations is proposed subcritical crevassing subcritical crack growth documented in many materials though not yet explored in ice is characterized by a crack velocity that scales as a power of the tensile stress intensity factor but is much less than that associated with critical crack propagation this mechanism allows crevasse propagation from mm scale microcracks at velocities much lower than body wave speeds and explains crevasse opening accelerations in a natural way subcritical crevassing is theoretically explored for several simplified situations but is limited by a lack of available data on crevasse evolution

mechanisms for linear sandbank formation and maintenance in relation to dynamical oceanographic observations abstract theories associated with flow in the vicinity of headlands and sandbanks together with proposed mechanisms for sandbank formation and maintenance are reviewed to exemplify water and sand movement around a sandbank on a tidally dominated shelf tidal currents in the vicinity of a headland associated linear sandbank are described these data are integrated with sedimentological information on bedforms and grain size distributions the sandbank under detailed investigation the scarweather sands is located along the northern coastline of the bristol channel uk which is an area susceptible to high energy inputs the maximum spring tidal range here is around 10m and a 6000km fetch is available for storm wave approach from the atlantic ocean but the area is 460km from the shelf break detailed observations are presented from 11 long term current meter stations located within 10km of the sandbank currents are described in terms of residual water circulation shallow water harmonics and tidal ellipse characteristics bedload transport paths are inferred from the distribution of bedforms grain size and m2 m4 tidal current interactions transport vectors are predicted from near bed current meter observations combined with empirically derived formulae it is demonstrated that water circulation and sand transport paths form part of a clockwise eddy system around the sandbank mechanisms controlling the location and the characteristics of this particular sandbank are shown to be explicable in terms of a combination of models for sandbank formation and maintenance the results of the present study are thought to be applicable directly to headland associated sandbanks elsewhere it is shown on the basis of theoretical considerations that the pattern of residual water movement and possibly sand transport around this sandbank could be extended to continental shelves on a global basis

experimental fracture and mechanical properties of antarctic ice preliminary results an experimental study of the fracture mechanics and rheology of ice from the ronne ice shelf antarctica is currently being undertaken the apparent critical stress intensity factor or apparent fracture toughness k q for crack propagation has been measured using a three point bend method for inducing crack growth perpendicular to the axis of cylindrical ice core specimens tensile crack nucleation under applied uniaxial compressive stress has also been evaluated both methods have allowed a profile of ice elastic and fracture properties with depth through the ice shelf to be constructed preliminary results indicate that the measured elastic modulus increases with depth through the firn and upper meteoric ice before reaching a constant value in the deeper dense meteoric and basal marine ice the resistance to fracture as measured by changes in apparent fracture toughness and crack nucleation stress increases with depth right through the firn and meteoric ice layers a simple fracture mechanics model applied to the ronne ice shelf indicates that crevasses form from small surface cracks less than 40 cm deep which quickly grow to depths of 40 60m and then remain stable

roles of marine ice rheology and fracture in the flow and stability of the brunt stancomb wills ice shelf marine ice sometimes as part of an ice melange significantly affects ice shelf flow and ice fracture the highly heterogeneous structure of the brunt stancomb wills ice shelf bsw system in the east weddell sea offers a rare setting for uncovering the difference in rheology between meteoric and marine ice here we use data assimilation to infer the rheology of the brunt stancomb wills ice shelf by an inverse control method that combines interferometric synthetic aperture radar measurements with numerical modeling we then apply the inferred rheology to support the hypothesis attributing the observed 1970s ice shelf flow acceleration to a change in the stiffness of the ice melange area connecting brunt proper with stancomb wills and to examine the consequences of frontal rift propagation we conclude that while the brunt stancomb wills system is currently not susceptible to extreme fragmentation similar to that of the larsen b ice shelf in 2002 our inverse and forward modeling results emphasize its vulnerability to destabilization by relatively rapid changes in the ice melange properties resulting from the interaction of its marine ice component with ocean water or by the further propagation of a frontal rift copyright 2009 by the american geophysical union

sandbanks sandwaves and megaripples on spitsbergenbanken barents sea abstract recently acquired multibeam echosounder data from the shallowest part 26 53 m depth of spitsbergenbanken in the western barents sea reveal a variety of bedforms including megaripples sandwaves and sandbanks the bedforms exhibit varying degrees of superimposition and differ in their age of formation and present depositional regime being either active or moribund these are the first observations of co occurring current induced bedforms in the western barents sea and provide evidence of a high energy environment in the study area the bedforms indicate both sediment erosion and transport and confirm that there is enough sand available in this area to maintain them such conditions are not known to be common in the western barents sea and reflect the unique oceanographic and benthic environment of spitsbergenbanken

environmental setting morphology and volumetric evolution of the middelkerke bank southern north sea abstract the middelkerke bank a tidal sandbank located in the southern north sea on the belgian continental platform has been the object of a multidisciplinary and international research project the morphological and hydrodynamical setting of the study area is presented a complete side scan sonar coverage of the bank allowed a detailed analysis of the morphology of the bank and of its superimposed bedforms large dunes cover the flanks and summit of the middelkerke bank they have a constant orientation a height ranging from 0 5 to 5 m a wavelength from 75 to 150 m and in most cases an asymmetrical profile the slopes of both lee and stoss flanks are very low small and medium dune fields occur on the entire bank and in the adjacent swales they are 2 d and present a wavelength ranging from 1 to 15 m flood and ebb oriented small dunes cover respectively the western and eastern flank of the bank bathymetric surveying of the middelkerke bank was carried out to assess the importance of volumetric variations of the bank mass caused by seasonal factors results show that changes in the bank volume can be quite important the bank s volume decreases in periods of heavy weather after which the process of sand uppiling acting during long periods of fair weather conditions causes the bank to restore itself volumetric variations of the total bank volume in a winter summer period can range along certain sections of the bank from 21 to 26 despite these large seasonal volumetric variations the bank shows a long term dynamic equilibrium as proven by the obtained evolution trend

accelerated ice discharge from the antarctic peninsula following the collapse of larsen b ice shelf acceleration exceeds 27 km 3 per year and ice is thinning at rates of tens of meters per year we attribute this abrupt evolution of the glaciers to the removal of the buttressing ice shelf the magnitude of the glacier changes illustrates the importance of ice shelves on ice sheet mass balance and contribution to sea level change index terms 1827 hydrology glaciology 1863 1863 hydrology snow and ice 1827 3349 meteorology and atmospheric dynamics polar meteorology 6924 radio science interferometry 9310 information related to geographic region antarctica citation rignot e g casassa p gogineni w krabill a rivera and r thomas 2004 accelerated ice discharge from the antarctic peninsula following the collapse of larsen b ice shelf geophys res lett 31 l18401 doi 10 1029 2004gl020697

causes of pre collapse changes of the larsen b ice shelf numerical modelling and assimilation of satellite observations satellite observations revealed that beside a rapid thinning the larsen b ice shelf lbis was undergoing a significant acceleration before its collapse in 2002 this paper investigates the ice shelf acceleration between 1995 and 1999 using a combination of data assimilation and numerical modelling based on a flow model adjusted to the 1995 insar velocities perturbation experiments are performed such as ice front retreat thinning increase in tributary flow and rheological weakening furthermore an inversion for ice shelf rheology and tributary flow velocity is performed for both the 1995 and the 1999 insar velocities the perturbation experiments together with the inversion strongly suggest that the acceleration cannot solely be explained by the retreat of the ice shelf front but relies on a further significant rheological weakening of the already weak shear zones within the lbis minor tributary acceleration is found to be an effect rather than a cause of the ice shelf acceleration furthermore the observed acceleration cannot be explained by the observed recent thinning we conclude that for smaller ice shelves such as the lbis such weak shear margins play a crucial role in controlling their dynamics and are the key to understand changes in the future finally we compare the dynamic thinning likely to be associated with the observed acceleration with the observed thinning for the ice shelf as a whole this thinning accounts for 20 of the observed value which implies that factors such as enhanced basal melt were the primary cause of the observed thinning

recent dramatic thinning of largest west antarctic ice stream triggered by oceans a growing body of observational data suggests that pine island glacier pig is changing on decadal or shorter timescales these changes may have far reaching consequences for the future of the west antarctic ice sheet wais and global sea levels because of pig s role as the ice sheet s primary drainage portal we test the hypothesis that these changes are triggered by the adjoining ocean specifically we employ an advanced numerical ice flow model to simulate the effects of perturbations at the grounding line on pig s dynamics the speed at which these changes are propagated upstream implies a tight coupling between ice sheet interior and surrounding ocean

on the formation of sand banks of finite extent we consider a model where the fluid depth depends on both horizontal coordinates quasi steady depth uniform non divergent fluid flow is governed by inertial pressure and bottom frictional forces sand transport is proportional to the cube of the instantaneous current but augmented by a down slope component and by wind wave action and sand is conserved it is found that low parallel banks grow fastest so that in an extensive spatially uniform sea previous calculations for linear banks are appropriate the inclination of banks to the tidal currents can be interpreted in terms of similarly inclined deposition bands resulting from vorticity generation and advection in flow over a small isolated hump a small bump can evolve to an equilibrium bank typically after an initial rapid extension across the tidal currents provided that sand is sufficiently restricted and particularly if some wind wave action prevents growth up to the sea surface sand banks are likely to be in a late stage of evolution when the main change is a slow lengthening as the net current and transport along the bank side slows and turns around the bank end with net deposition the equilibrium is apparently stable except when there is an overall bed slope in the direction of the tidal currents or when sand is abundant

bedmap a new ice thickness and subglacial topographic model of antarctica measurements of ice thickness on the antarctic ice sheet collected during surveys undertaken over the past 50 years have been brought together into a single database from these data a seamless suite of digital topographic models have been compiled for antarctica and its surrounding ocean this includes grids of ice sheet thickness over the grounded ice sheet and ice shelves water column thickness beneath the floating ice shelves bed elevation beneath the grounded ice sheet and bathymetry to 60 s including the sub ice shelf cavities these grids are consistent with a recent high resolution surface elevation model of antarctica while the digital models have a nominal spatial resolution of 5 km such high resolution is justified by the original data density only over a few parts of the ice sheet the suite does however provide an unparalleled vision of the geosphere beneath the ice sheet and a more reliable basis for ice sheet modeling than earlier maps the total volume of the antarctic ice sheet calculated from the bedmap grid is 25 4 million km3 and the total sea level equivalent derived from the amount of ice contained within the grounded ice sheet is 57 m comprising 52 m from the east antarctic ice sheet and 5 m from the west antarctic ice sheet slightly less than earlier estimates the gridded data sets can be obtained from the authors

tidal flexure of jakobshavns glacier west greenland jakobshavns glacier a floating outlet glacier on the west greenland coast was surveyed during july 1976 the vertical displacements of targets along two profiles perpendicular to the fjord wall bounding the north margin of the glacier were analyzed to determine the effect of flexure caused by tidal oscillations within the fjord an analysis based on the assumption that vertical displacements of the glacier reflected pure elastic bending yielded the conclusion that the effective thickness of the ice i e the thickness which remained unaffected by surface and basal cracking and which behaved as a continuum was 160 m 2 6 km upglacier from the calving front and 110 m 0 6 km from the calving front an analysis based on the more realistic assumption that observed bending reflected elastic and viscoplastic deformation yielded the conclusion that the average effective thickness of the ice was 316 74 m 40 of the estimated 800 m total thickness 2 6 km from the calving front and 160 48 m 21 of the estimated 750 m total 0 6 km from the calving front a constitutive relationship appropriate for hard glide during flexure was used

role of small ice shelves in sea level rise 1 recent observations show marked acceleration and thinning of tributary ice streams following shrinkage or loss of a few of the many small ice shelves that ring antarctica and greenland our model experiments which adopt a simplified geometry show that calving from or partial disintegration of a small ice shelf has more effect on tributary ice streams than does loss of a berg of equal length from a larger ice shelf our results which are consistent with the recent observations indicate that even small ice shelves can provide important restraint on tributary flow from ice sheets implying that characterization and modeling of the full range of ice shelves will be required for accurate projection of ice sheet changes and their influence on sea level

large scale sediment bodies and superimposed bedforms on the continental shelf close to the strait of gibraltar interplay of complex oceanographic conditions and physiographic constraints a previously unknown field of large scale sedimentary bodies has been mapped and studied on the continental shelf off the cape trafalgar near the strait of gibraltar with particular emphasis on the relationship between large scale sediment bodies and the superimposed bedforms this study is based on a grid of 975 km of high resolution seismic profiles collected at water depths ranging between 15 and 60 m high variability of large scale sedimentary bodies is attributed to the complex interaction of hydrodynamic agents the most prominent sedimentary features are sand banks and ridges that indicate long term southwest and southward directed sediment transport patterns possibly due to the interplay of two dominant current systems flowing southward and westward these sediment bodies evolve laterally to distinct external geometries such as sand shoals in shallow water and sand sheets in the vicinity of larger sand banks that indicate moderate current velocities in addition pre existing physiography is considered to play a role in the generation of certain sediment bodies developed over inclined surfaces or confined laterally by elevations relationships between superimposed bedforms mostly very large dunes and underlying sediment bodies vary across the study area most superimposed bedforms occur over the complex mosaic of sediment banks and sheets suggesting the interaction of several high energy currents with different directions such as tidal and or wind driven currents copyright 2010 john wiley sons ltd

tide induced bedload transport pathways in a multiple sand ridge system offshore of hainan island in the beibu gulf northwest south china sea modern sand transport pathways in a multiple sand ridge system are elusive and have rarely been studied in recent years we report herein a field with four en echelon linear sand ridges offshore of hainan island in the beibu gulf and describe the distribution and morphology of these sand ridges in detail for the first time dune crest comparisons and seismic profiles are also interpreted to assess sand transport over the dunes based on a delft 3d model regional tidal currents and tide induced bedload transport in the multiple sand ridge system were simulated to provide insights into the related bedload transport paths the results show that bedload transport and residual flows are mostly directed to the south on the east side of the sand ridges and to the north on the west side and these differences coincide with dune asymmetries and migrations cross ridge transport is weak and mostly converges on the crests of sand ridges from the two flanks the bedload transport is unbalanced on either flank of the sand ridges thus leading to the asymmetry and crest kinks of the sand ridges a distinct correlation is not observed between the net cross ridge sand transport and sand ridge asymmetry in this system cross swale transport is significant between the neighboring sand ridges and helps construct the bedload transport circulation in the swales sand accretion also occurs in the swales and benefits from the southwestward bedload transport from the north end of sand ridge 3 distinct evidence has not been found for closed bedload transport circulation around sand ridges in this multiple sand ridge system although limited clockwise current vortices develop on several sand ridges additional field observations and simulations are required to characterize the suspended load transport in a multiple sand ridge system and the effects of local sand supply conditions on sand transport also need further evaluation copyright c 2018 john wiley sons ltd

morphology and sedimentation of sand bodies in the tidal shelf sea of eastern taiwan strait abstract changyun ridge located in the central eastern part of the taiwan strait consists of sand bodies the eastern changyun sand ridge 65 km long and 13 km wide a sand shoal 53 km long and 22 km wide and a sand sheet 60 km long 40 km wide and 22 m in height the eastern changyun sand ridge and the sand sheet are oriented parallel to the coastline but the sand shoal is perpendicular to the coast the eastern changyun sand ridge is oriented oblique to the narrow reciprocal tidal ellipses the sand shoal and sand sheet are normal and oblique to the broad rotary tidal ellipses respectively surface sediments of the changyun ridge are composed of very fine to coarse sands in a progressive seaward decrease in grain size the configuration and spatial distribution of these three sand bodies with corresponding decrease in tidal speed and in grain size together suggest that the sand bodies developed sequentially from the near shore to the offshore area presumably in response to westward shifting of the north flowing tidal currents sand waves occur widespreadly on the sand shoal and seldom appear on the eastern changyun sand ridge occurrences of sand waves suggest that the sand shoal is considered to be active and the sand ridge to be moribund chirp sonar profiles reveal the internal structures of the sand shoal showing a relatively flat basal reflector underlying the sand body the thickness of the sand accumulation is about 22 25 m and it is firstly determined by the depth difference between the modern sea bed and the basal reflector to better describe morphology and document dimensions of these sand bodies the presence of several short inclined reflectors dipping northeast and down lapping the basal flat reflector indicate that sands are most likely transported from south to north for the sand shoal the sand bodies of the changyun ridge rest upon a relative flat basal reflector suggesting a mainly hydrodynamically controlled origin the variations of the hydrodynamic conditions mainly tidal currents on the central eastern taiwan strait shelf seem to be the primary control on sedimentary facies shift morphological changes and sand ridge growth for the changyun ridge the changyun ridge is ongoing to receive sands from the south in a transitional development stage as long as the sand supply continues and over a longer time period the sand bodies eventually will develop into three individual linear sand ridges separated by swales oriented parallel to the peak currents

pattern of retreat and disintegration of larsen b ice shelf antarctic peninsula the retreat of the larsen b ice shelf antarctic peninsula and the collapse of its northern section are analyzed using satellite images acquired between january 1995 and may 2003 over 1 week during march 2002 after a period of steady retreat since 1995 2300 km 2 of the ice shelf broke up into many small icebergs this rapid collapse occurred at the end of an exceptionally warm summer and after a multi year period of decreasing surface net mass balance ice thinning flow acceleration and widening of rifts the ice shelf area decreased from 11 512 km 2 in january 1995 to 3463 km 2 in march 2002 and 2667 km 2 in april 2003 ers synthetic aperture radar sar images were used to identify ice shelf zones with different surface morphology which generated icebergs of different sizes and shapes the pattern of retreat and break up similar to that of larsen a in 1995 suggests that fracturing enhanced by abundant surface melt played a key role in addition the recent changes of grounded and residual floating ice north of larsen b are analyzed by means of envisat advanced synthetic aperture radar asar images up to summer 2003 showing significant loss of grounded ice upstream of those ice shelf sections which disintegrated in 1995 and 2002

large fluctuations in speed on greenland s jakobshavn isbrae glacier it is important to understand recent changes in the velocity of greenland glaciers because the mass balance of the greenland ice sheet is partly determined by the flow rates of these outlets jakobshavn isbrae is greenland s largest outlet glacier draining about 6 5 per cent of the ice sheet area and it has been surveyed repeatedly since 1991 ref 2 here we use remote sensing data to measure the velocity of jakobshavn isbrae between 1992 and 2003 we detect large variability of the velocity over time including a slowing down from 6 700 m yr 1 in 1985 to 5 700 m yr 1 in 1992 and a subsequent speeding up to 9 400 m yr 1 by 2000 and 12 600 m yr 1 in 2003 these changes are consistent with earlier evidence for thickening of the glacier in the early 1990s and rapid thinning thereafter our observations indicate that fast flowing glaciers can significantly alter ice discharge at sub decadal timescales with at least a potential to respond rapidly to a changing climate

ice sheet velocity mapping a combined interferometric and speckle tracking approach the first and second radarsat antarctic mapping missions amm 1 and 2 have now acquired interferometric synthetic aperture radar sar overmuch of the ice sheet the radarsat 24 day repeat cycle is nearly ideal for measuring slow ice motion e g 1 but application of sar interferometry is limited in faster moving areas with a 1day repeat period ers 1 2 tandem sar data are much better matched to fast motion but are not always available fortunately several authors have demonstrated the ability to measure velocity in fast moving areas by tracking sar speckle from image to image which works well even in the absence of visible features while these estimates have intrinsically lower resolution and poorer accuracy than direct phase measurements they serve well in areas where there are no data suitable for conventional interferometry this paper describes algorithms i have developed for merging interferometric and speckle tracking data from multiple swaths to form a single seamless mosaic of velocity at each point in the mosaic all the available data are combined to produce estimates of the velocity and the associated error this technique is demonstrated using radarsat data collected over lambert glacier antarctica during amm 1 and 2

recent loss of floating ice and the consequent sea level contribution we combine new and published satellite observations and the results of a coupled ice ocean model to provide the first estimate of changes in the quantity of ice floating in the global oceans and the consequent sea level contribution rapid losses of arctic sea ice and small antarctic ice shelves are partially offset by thickening of antarctic sea ice and large antarctic ice shelves altogether 746 127 km 3 yr 1 of floating ice was lost between 1994 and 2004 a value that exceeds considerably the reduction in grounded ice over the same period although the losses are equivalent to a small 49 8 m yr 1 rise in mean sea level there may be large regional variations in the degree of ocean freshening and mixing ice shelves at the antarctic peninsula and in the amundsen sea for example have lost 481 38 km 3 yr 1

the calving constraints on inception of quaternary ice sheets abstract most numerical models that simulate the inception of quaternary ice sheets nucleate the ice sheets in high latitude highlands of the northern hemisphere these nuclei are initially plateau or island ice caps that then expand and merge with one another to become an ice sheet before merger however these ice caps often must cross continental shelves seas or embayments e g barents sea baltic sea foxe basin hudson bay where calving along the ice margins could prevent merger therefore the models must either ignore calving and treat the ice caps as if they were advancing across dry land or include a calving parameter that can be adjusted to allow a grounded or floating ice front to advance across a water body despite calving the first solution to this problem is unrealistic because when the ice caps reach shorelines their mass is insufficient to have lowered sea level enough to make the shorelines retreat at the rate of advancing ice the second solution is also unrealistic because as ice advances into water slab calving can prevent an ice shelf from forming and even if it does form combined slab and tabular calving along its advancing front can prevent the ice shelf from thickening enough to become grounded in the body of water calving mechanisms for tabular icebergs and ice slabs are presented to demonstrate that rates of ice calving alone are high enough to prevent ice caps from crossing the water bodies where quaternary ice sheets had become grounded calving rates produced by the models are compatible with the observed calving rates from alaskan tidewater glaciers and greenland outlet glaciers along both grounded and floating calving fronts in this case the only way by which highland or island ice caps could cross these water bodies would be if sea ice on the water bodies had thickened and grounded independently from the advancing ice caps this has been called the marine ice transgression hypothesis

processes involved in the propagation of rifts near hemmen ice rise ronne ice shelf antarctica interferometric radar images collected by ers 1 ers 2 and radarsat 1 are used to observe the rupture tip of rifts that propagate along hemmen ice rise on the ronne ice shelf antarctica interferograms generated in 1992 and 1997 allow for the observation of ice deformation accumulated over 9 and 24 days respectively these interferograms are combined in order to separate the continuous process of creep deformation from the more cyclic motion caused by variations in ocean tide an examination of local gradients in creep deformation reveals the pattern of ice deformation around and near the rupture tips and rifts with great precision up to 10 cm a 1 we compare the observations with a deformation model for ice and obtain the following results 1 the tidal oscillation of the ronne ice shelf only yields small deformations along the rifts and near the rupture tips 2 along the ice front the rifts and at the rupture tips vertical bending is observed which is well explained by a model of viscous deformation of ice furthermore the model indicates that the deformation pattern observed at the rupture tips is a sensitive indicator of the propagation state of the rifts i e active vs inactive 3 the viscous adjustment of ice is the dominant mode of deformation masking the deformation pattern predicted by linear elastic fracture mechanics lefm 4 yet at a spatial scale equivalent to the length of a rift the propagation rate is well predicted by lefm

melting and freezing beneath filchner ronne ice shelf antarctica 1 we use remote sensing data sets to evaluate the spatial distribution of melt beneath the filchner ronne ice shelf fris the net melt rate of 83 4 24 8 gtons yr is 2 5 5 times lower than previous glaciological estimates but is similar to existing oceanographic estimates the spatial distribution however differs significantly from standard conceptual and numerical models in which most melt occurs along the grounding lines our results suggest most grounding line melt is refrozen while the dominant ice shelf water isw source is melting near the ice shelf front probably associated with tidal action this suggests that changes in ice shelf extent can impact isw production rates in the weddell sea

fracture field for large scale ice dynamics recent observations and modeling studies emphasize the crucial role of fracture mechanics for the stability of ice shelves and thereby the evolution of ice sheets here we introduce a macroscopic fracture density field into a prognostic continuum ice flow model and compute its evolution incorporating the initiation and growth of fractures as well as their advection with two dimensional ice flow to a first approximation fracture growth is assumed to depend on the spreading rate only while fracture initiation is defined in terms of principal stresses the inferred fracture density fields compare well with observed elongate surface structures since crevasses and other deep reaching fracture structures have been shown to influence the overall ice shelf dynamics we propose the fracture density field introduced here be used as a measure for ice softening and decoupling of the ice flow in fracture weakened zones this may yield more accurate and realistic velocity patterns in prognostic simulations additionally the memory of past fracture events links the calving front to the upstream dynamics thus the fracture density field proposed here may be employed in fracture based calving parameterizations the aim of this study is to introduce the field and investigate which of the observed surface structures can be reproduced by the simplest physically motivated fracture source terms

field studies of bottom crevasses in the ross ice shelf antarctica analyse des crevasses observees par sondages radar aeriens et terrestres localisation de leurs points de formation

water and sediment movement in the vicinity of linear sandbanks the norfolk banks southern north sea geophysical surveys current meter measurements and fluorescent sand tracer experiments were undertaken in the vicinity of the broken bank the data are used to establish localised and regional patterns of water and sediment movement the results obtained confirm mechanisms proposed previously for sandbank growth with particular reference to the north sea these include the clockwise circulation of water and sand with convergence at the crestline secondary helical water circulations do not appear to be a dominant process influencing sandbank maintenance however such circulation may still be important in the initial formation of linear sandbanks and or at some stages in the bank development on a regional basis residual currents bedform asymmetries and tracer dispersion indicate an offshore sediment transport component this observation supports the hypothesis of other investigators formed on the basis of geomorphological observations and numerical model predictions concerning the movement of sand under extreme conditions moreover these transport pathways may persist under low wave and tidally dominated conditions as opposed to being restricted to storm activity

the link between climate warming and break up of ice shelves in the antarctic peninsula a review of in situ and remote sensing data covering the ice shelves of the antarctic peninsula provides a series of characteristics closely associated with rapid shelf retreat deeply embayed ice fronts calving of myriad small elongate bergs in punctuated events increasing flow speed and the presence of melt ponds on the ice shelf surface in the vicinity of the break ups as climate has warmed in the antarctic peninsula region melt season duration and the extent of ponding have increased most break up events have occurred during longer melt seasons suggesting that meltwater itself not just warming is responsible regions that show melting without pond formation are relatively unchanged melt ponds thus appear to be a robust harbinger of ice shelf retreat we use these observations to guide a model of ice shelf flow and the effects of meltwater crevasses present in a region of surface ponding will likely fill to the brim with water we hypothesize building on weertman 1973 hughes 1983 and van der veen 1998 that crevasse propagation by meltwater is the main mechanism by which ice shelves weaken and retreat a thermodynamic finite element model is used to evaluate ice flow and the strain field and simple extensions of this model are used to investigate crack propagation by meltwater the model results support the hypothesis