on the use of the dual frequency envisat altimeter to determine snowpack properties of the antarctic ice sheet abstract the primary purpose of ice sheet altimetry is to monitor the changes in ice sheet topography which may impact on global sea level however the altimetric signal is sensitive to different properties of the snowpack and therefore can also be used to determine these properties the radar altimeter onboard the european space agency s envisat satellite provides a dual frequency dataset at ku 13 6 ghz and s band 3 2 ghz in this paper these signals are studied over the antarctic ice sheet during the 4 first years of the mission 2002 2006 in order to retrieve snowpack properties the altimeter signal can be described by 4 classical waveform parameters the 4 year time series of all these parameters are decomposed into a linear and a seasonal time component the linear component is almost constant the distribution of the mean parameters over the antarctic ice sheet shows that the altimeter signal is sensitive to small scale mm surface roughness for the first time the amplitudes and phases of the seasonal variations are characterized the s band amplitudes are greater than the ku band and the phase varies over the entire ice sheet previous studies suggested that the seasonal variations of the altitude from the altimeter are created by a decrease of the snowpack height through compaction the dual frequency observations shown here suggest that this hypothesis is too simple instead the altitude variations observed in the altimetric signal are not created by the snowpack height change but are more likely caused by the seasonal change of the snow properties which cause a different response between the s and ku bands therefore both the linear and the seasonal variations of the altimetric signal can be used to retrieve snowpack properties here we compare the dual frequency envisat signal with a model of the altimetric echo over the antarctic ice sheet the model combines a surface model with a sub surface model for both the s and ku bands the brown model brown g s 1977 the average impulse response of a rough surface and its applications ieee transactions on antennas and propagation 25 1 is used to describe the interaction of the radar wave with the snow surface the backscatter coefficient of the surface is derived using the iem method fung a k 1994 microwave scattering and emission models and their applications boston ma artech house the sub surface signal takes into account both the layering effects and the scattering caused by the homogeneous media which is composed of small snow grains the model is tested in two areas of the antarctic plateau which present very different waveform parameters the sensitivity of the radar signal to the different snowpack properties is investigated the analysis of the waveform behaviours shows that the sub surface signal can be completely masked by the small scale surface roughness signal finally the temperature and surface density effects are investigated in order to explain the seasonal variations of the altimetric signal both the temperature and the compaction rate of the snow change seasonally temperature is shown to impact on the ku band signal furthermore the compaction rate of the snow surface can explain all of the seasonal variation characteristics observed at both the s and ku bands the seasonal change of compaction rate in the snow creates a change in the waveform shape that can bias the altitude in particular the snow compaction can induce a bias in the retrieved altimetric altitude of more than 80 cm for the ku band and 1 5 m for the s band this work underlines that the altitude time series needs to be corrected for the shape of the altimetric echo over ice sheets

late quaternary halimeda bioherms and aragonitic faecal pellet dominated sediments on the carbonate platform of the western continental shelf of india abstract the fifty fathom flat is a drowned carbonate platform located on the outer continental shelf of western india it is unique because the carbonate depositional environment prevailed until early holocene in spite of its proximity to a major siliciclastic tidal embayment study of the bathymetry seismic and sonograph data together with the petrology mineralogy and geochemistry of the carbonate sediments and limestones recovered from the platform resulted in a the discovery of late quaternary halimeda bioherms and b aragonitic faecal pellet dominated sediments echograms reveal buried pinnacles between 62 and 38 m and exposed 2 to 14 m high pinnacles between 65 and 85 m on the platform these pinnacles occur as narrow to broad mounds bioherms in seismic profiles and as linear ridges and massive structures in sonographs seismic profiles also show a prominent reflector about 25 m below the sea bed identified by boreholes as a late pleistocene erosion surface the platform consists of relict carbonate sands and limestones rounded and ovoid to ellipsoidal aragonite grains are predominant in the sediments sem studies indicate elongated parallel canals and crescent to hook shaped canals corresponding to faecal pellets occur in some grains algal grains and pellets enclosing algal fragments are also present the grains in the limestones are similar to those in the sediments halimeda and crustacean faecal pellets are abundant components with minor echinoid mollusc and benthic foraminifera the limestones are characterised by abundant pores dissolution cavities relic aragonite needles meniscus cements and drusy calcite fill the pores produced by dissolution of aragonite needles these features indicate vadose diagenesis the stable isotopes of the carbonate sands and limestones are almost identical and similar to isotope values of beachrocks of the red sea the limestones ages 8400 9200 yr b p lie within the same age limits 10 400 8300 yr b p as the surficial sediments it appears that fluvial sediments largely accumulate in the inner shelf and did not hinder carbonate growth on the platform lying on the outer shelf we presume that arid climate and nutrients brought by river runoff and shelf edge currents favoured the growth of algal bioherms dominated by halimeda between 13 700 and 8300 yr b p the absence of late holocene and modern carbonates on the platform may be due to the combined influence of climatic change upwelling and neo tectonic activity at about 9000 yr b p the bioherms dominated by halimeda produced abundant aragonite muds deposit feeding crustaceans and other invertebrates subsequently excreted these muds as faecal pellets the platform thus contains faecal pellet dominated sediments

response of shallow marine carbonate facies to third order and high frequency sea level fluctuations hauptrogenstein formation northern switzerland abstract facies patterns in shallow marine carbonates of the hauptrogenstein formation of northern switzerland middle bajocian to early bathonian reflect small changes in sea level third order sea level fluctuations calculated from large scale facies patterns alternations of marl and carbonate dominated sedimentary units on the level of members and subsidence show two phases of sea level rise and fall in the late bajocian and a relative sea level rise in the early bathonian most of the sedimentary rocks were produced during phases of relative rise and highstand of sea level no periods of emergence during lowstands are preserved sea level changes of a higher order have been analyzed by facies changes at the outcrop scale thin shallowing up parasequences 0 5 6 m thick have been correlated across platform ramp and offshore sedimentary environments in each of these environments carbonates react differently to sea level fluctuations relative rises in sea level are represented by non deposition and biogenic sediments periods of stable or slightly falling sea level are dominated by oolitic sediments 1 in the offshoal environment marl layers at the base of thin shallowing up units are covered by increasingly proximal tempestites 2 in the ramp environment oblique stratified oolitic and bioclastic grainstones are bounded by marine flooding surfaces 3 on the platform marine flooding surfaces are overlain by beds deposited under moderate energy conditions coral beds oncoid beds platform tempestites which are in turn covered by high energy oblique stratified oolitic grainstones such parasequences deposited in response to small scale high frequency sea level changes can be correlated over distances of more than 100 km a model for evolution and response of carbonate facies to sea level changes is presented

larsen ice shelf has progressively thinned the retreat and collapse of antarctic peninsula ice shelves in tandem with a regional atmospheric warming has fueled speculation as to how these events may be related satellite radar altimeter measurements show that between 1992 and 2001 the larsen ice shelf lowered by up to 0 27 0 11 meters per year the lowering is explained by increased summer meltwater and the loss of basal ice through melting enhanced ocean driven melting may provide a simple link between regional climate warming and the successive disintegration of sections of the larsen ice shelf

subsurface crevasse formation in glaciers and ice sheets 1 crevasses form in response to tensile stresses in glaciers and ice sheets it has been widely assumed that crevasses initiate at or near the surface of the ice from starter cracks up to a few centimeters long if the tensile stress is sufficient these cracks propagate downward into the ice to form a crevasse until the weight induced lithostatic stress prevents them penetrating deeper we present ground penetrating radar data acquired on the rutford ice stream antarctica which indicate that crevasses occur at depths of several meters beneath the ice surface and were formed in areas where surface crevassing is absent the data support the hypothesis that these are examples of subsurface crevasse formation using linear elastic fracture mechanics lefm we investigate the feasibility of crevasse initiation at depth we consider the initiation of an isolated crevasse from a subsurface crack subject to a dynamic tensile stress which results from deformation associated with ice movement and a weight induced lithostatic stress the lefm approach allows us to estimate a init the minimum length a crack must be before crack propagation will occur in earlier models of crevasse formation it was assumed that the dynamic tensile stress is constant with depth we consider a more realistic scenario where the dynamic tensile stress varies with depth in such a way that the tensile strain rate remains constant we show that in this scenario crevasse initiation from centimeter scale starter cracks is feasible at depths of 10 30 m as well as at the surface at present the formulation of a reliable predictive model is limited by an incomplete knowledge of the mechanical properties of firn in previous studies the depth of buried crevasses has been used to estimate the time elapsed since ice was exposed to higher stresses and different flow regimes in the light of the results presented here those estimates may need to be reviewed

spatial heterogeneities in a bioclastic shoal outcrop reservoir analogue architectural cyclical and diagenetic controls on lateral pore distribution abstract the distribution of pores in carbonate rocks is controlled by the synergy of different factors such as the initial sediment environment processes and diagenesis prediction of the texture related pore distribution to evaluate reservoir quality is still a challenging task in geothermal and hydrocarbon exploration in order to degrease these uncertainties 3d facies models are produced on the basis of facies types and their relationship to petrophysical properties however several studies have shown that these relationships are often rather controlled by diagenesis that is often described as a sequence for an entire reservoir two outcrops as part of the same bioclastic shoal reservoir analogue present two different architectural styles i layer cake architecture and ii high angle foreset architecture in order to identify petrophysical trends and related controlling factors both outcrops were mapped and studied in detail based on the investigation of 41 thin sections microfacies types were defined and diagenesis was analysed in addition permeability porosity was measured and pores were digitally characterized although both outcrop successions studied are associated with the same reservoir interval varied architecture leads to fundamental differences in the porosity permeability distribution petrophysical trends in the layer cake architecture are mainly controlled by cyclicity and channel features compared to layer cake architecture trends in the high angle foreset architecture are dominantly controlled by the depositional environment both outcrops show differences in their diagenesis while layer cake architecture suffered the entire sequence large parts of the high angle foreset architecture were not influenced by late burial and telogenetic processes as a consequence primary porosity is persevered within the high angle foreset architecture in addition results of this investigation indicate that primary porosity has a bigger influence on good permeability values compared to telogenetic associated vug enhancement this investigation therefore demonstrates that individual carbonate reservoirs can suffer different diagenetic pathways moreover the study highlights the potential of outcrop studies to improve the principal understanding of fundamental trends within potential subsurface reservoirs

sedimentology of an early silurian carbonate ramp the manitoulin formation southern ontario the manitoulin formation is a pervasively dolomitized lower silurian carbonate unit that was deposited in the michigan basin and locally in the appalachian basin the formation reaches a maximum thickness of 11 1 m in southern ontario and can be subdivided into eight facies and four regionally correlatable facies assemblages owing to the relatively continuous transition of shallow to deeper water facies from the northern to southern portions of the study area the manitoulin formation is interpreted as having formed on a carbonate ramp with a southerly component of dip our study suggests that the algonquin arch which is transected by the outcrop belt did not significantly influence deposition or separate the michigan and appalachian basins depending on its location on the ramp the manitoulin formation shows evidence of varying degrees of episodic storm events alternating with fair weather processes such as wave shoaling sediment reworking and bioturbation apart from bioturbation these fair weathe

high energy carbonate sand accumulation the quicksands southwest florida keys abstract high resolution seismic reflection profiles of the quicksands located along a broad ridge on the platform shelf west of key west florida indicate a significant deposit of non oolitic carbonate sand occurs in a belt 47 km long by 28 km wide the surface of the belt is ornamented by large 5 m migrating tidal bars oriented in a north south direction on which sand waves oriented in an east west direction are superimposed some of the sand waves are awash at low tide the sand waves are formed by strong reversing tidal currents flowing between the gulf of mexico and the straits of florida the waves migrate directly over pleistocene bedrock to the east but the deposit thickens to the west and sand waves there overlie non oolitic holocene accumulations as thick as 12 m westward ipping accretionary bedding indicates that net migration of the sands is to the west despite north south movement of tidal currents the westward edge of the accumulation has accreted over deeper muddier deposits although tidal currents and resultant bedforms appear identical to those of active ooid deposits in the bahamas and elsewhere no oolitically coated grains were found in this study thin section analyses show the principal component average 48 of the sands is fragmented plates of species of the green alga halimeda followed by particulate coral average 17 which increases off the flanks of the main sand body short vibracores confirm the presence of cross bedding

long melt seasons on ice shelves of the antarctic peninsula an analysis using satellite based microwave emission measurements we have examined the record of melt season duration on the antarctic peninsula using two techniques for detecting the presence of a melt signal in microwave emission time series covering the period 1978 2000 we have obtained similar estimates of melt season length using the cross polarized gradient ratio xpgr technique and calibrations previously applied in greenland and a technique which detects the jump in emission caused by melt without using a sensor and frequency dependent threshold value the close correspondence between results from the two techniques on peninsula ice shelves suggests that the xpgr analysis can be used over the length of the time series the results show that the long melt seasons of 1992 93 and several later years were exceptional occurrences on the northern parts of the larsen ice shelf these melt seasons were followed by disintegration events supporting a possible cause and effect relationship

multi year monitoring of rift propagation on the amery ice shelf east antarctica we use satellite imagery from four sensors multi angle imaging spectroradiometer misr enhanced thematic mapper etm and radarsat and ers synthetic aperture radar sar to monitor the lengths of two rifts on the amery ice shelf from 1996 to 2004 we find that the rifts have each been propagating at a steady annual rate for the past 5 years superimposed on this steady rate is a seasonal signal where propagation rates are significantly higher in the summer period i e september april than in the winter period i e april september possible causes of this summer winter effect are changing properties of the ice melange which fills the rifts and seasonal changes in ocean circulation beneath the ice shelf

dual frequency altimeter signal from envisat on the amery ice shelf in antarctica radar altimeter measurements are sensitive to dielectric and penetration properties of the sensed medium snow such that the spacecraft s altitude can be biased since 2002 relatively low frequency radar measurements over the amery ice shelf east antarctica have been acquired using the envisat dual frequency altimeter at s 3 2 ghz and ku 13 6 ghz bands which penetrate a few meters into the firn the altimeter signal is however modified in summer by the presence of snowfilled crevasses indeed the specularity of the snow surfaces in summer makes the altimetric signal sensitive mostly to nadir echoes that increases the ratio between the crevasse signal and the surrounding ice shelf signal at nadir crevasses are distinguished by differences in backscattering behavior compared with the surrounding ice shelf signal crevasses are characterized by a strong backscatter coefficient at ku band and anomalies in the s band altitude estimation these two characteristics make snowfilled crevasses detectable by the dual frequency altimeter of envisat we first retrieve the geometric properties of the crevasses using a hyperbolic shape function created by strong crevasse backscatter in the ku waveform measurements from this retrieved crevasse signal and further waveform analysis we assess the properties of the snow surface and its sub surface the crevasse due to its small size compared to the altimeter footprint is found to be an excellent target to study snow properties of the ice shelf the anomalies in the s band altitude measurements over crevasses can then be explained by the presence of a double echo in the s band waveforms this echo is attributed to a reflection at the base of the snowbridge where we see evidence of sub surface echos in the individual altimeter waveforms based on this observation a methodology is developed to estimate the thickness of the snowbridge we calculate the penetration depths in the summer snow surface of the amery at ku band that is found to be around 6 m

west antarctic ice streams draining into the ross ice shelf configuration and mass balance extensive airborne radar soundings closely tied to accurately located ground stations made over a portion of the west antarctic inland ice sheet and the ross ice shelf have enabled us to map the boundaries of the ice streams and their flow bands on the ross ice shelf the surfaces of the active ice streams a and b are heavily crevassed but there are no visible crevasses on ice stream c however the existence of numerous crevasses at a depth of about 35 m implies that ice stream c ceased to be active about 250 years ago there is a complex zone at the head of ice stream b that suggests that ice stream b is currently widening and advancing toward the interior of the ice sheet the grounding lines between the inland ice and the ice shelf were mapped from the surface elevations using the requirement of hydrostatic balance and the character of the radar echoes the inland ice is more extensive than previously mapped and includes a large weakly grounded area downstream of ice streams a and b where the surface elevations are barely above hydrostatic equilibrium crary ice rise extends 70 km upstream of the previously mapped position to relate surface elevation data from satellite observations to sea level three geoidal models were tested we conclude that the gem 10c model is the best of the three for this section of antarctica a new map of surface elevation shows deep marginal troughs and longitudinal ridges along ice streams a and b but not on ice stream c whose surface contours show signs of stagnation ice stream c shows a strongly positive and ice stream b a significantly negative net balance rate ice streams a and d also show a significantly negative net flux whereas ice streams e and f do not the overall net balance of this part of the west antarctic inland ice is suggestively negative 23 15 km3 yr 1 from the measured flux into the ross ice shelf and previous measurements we calculate an average basal melt rate from beneath the ice shelf of 0 12 0 03 m yr 1

geostatistical facies modeling trends for oolitic tidal sand shoals to assess prospective modeling trends for oolitic tidal sand shoals and explore potential patterns of reservoir heterogeneity this study examines quantifies and models the cycle scale architecture of the holocene mobile oolitic tidal sand shoal complex at schooner cays bahamas process based stratigraphic trends are captured in quantitative geocellular models of the shoal from analyses of satellite imagery two dimensional high frequency seismic chirp data and sediment cores data show that longitudinal tidal sand ridges extend up to 8 km 5 mi along depositional dip gradually transforming bankward into channel bound compound barforms consisting of linear parabolic and shoulder bars these bars terminate into a laterally extensive 10 km 6 mi strike elongate sand sheet each bar type includes distinct internal architecture grain size and sorting related to feedbacks among hydrodynamics geomorphology and sedimentology building on these data and concepts from the holocene accumulations this study demonstrates a methodology for quantifying and validating probabilistic stratigraphic trends prior to their inclusion in stochastic based facies modeling algorithms inclusion of statistically robust facies probability volumes during truncated gaussian simulation generated ordered and geologically accurate facies distributions relative to bar crest centerlines water depth and geomorphic position petrophysical models that incorporate facies specific porosity permeability and water saturation functions display pronounced cycle scale heterogeneity that could provide insights into variable production rates and poor sweep efficiency commonly encountered during development of analogous oolitic reservoirs

mapping of the topography of continental ice by inversion of satellite altimeter data satellite altimeter data over ice sheets provide the best tool for mapping their topography and its possible climatic variations however these data are affected by measurement errors orbit errors and slope errors we develop here a three step inversion technique which accommodates the a priori information on the expected topography and correctly handles and propagates the data errors it estimates first a large scale reference surface then maps the residuals related to undulations and finally iteratively corrects the slope error the method is tested on overlapping small fragments of the antarctic ice sheet using a sub set of seasat data finally a topographic map of terre adelie is produced over areas of small slopes the a posteriori error should be of the order of 0 4 m using ers i data it is therefore expected that climatic variations in the ice sheet topography since the introduction of seasat will be observable

altimetric observations of surface characteristics of the antarctic ice sheet the aim of this paper is to investigate the geophysical characteristics of the antarctic ice sheet using radar altimetric observations to do this we use an altimetric waveform simulator in situ observations ers 1 european remote sensing satellite data and spot satellite pour l observation de la terre images the small scale study takes place at dome c terre adelie which is a relatively flat region with gentle undulations and low wind speed despite this the altimetric waveform parameters height energy leading edge and trailing edge are highly noisy the effect of undulations on the waveform parameters is found to be dominant the combination of a subsurface signal and a rough surface produces a linear effect on the altimetric backscattering or on the trailing edge of the waveform but a strongly non linear effect on the leading edge of the waveform or height estimation as a consequence the height measurement is very sensitive to the altimeter technical or orbital characteristics and is not reproducible from one mission to another observations show sastrugi fields that enhance the leading edge and affect the whole waveform observed local backscattering changes probably due to local variations in surface microroughness enhance the backscattered energy and may artificially create a topographic signal the continental scale study shows coherent patterns even if both surface and subsurface components affect the altimetric observation the large scale signal is mostly controlled by surface backscattering variations the surface or near subsurface characteristics of the snowpack may then be reached by altimetric observations

velocity and strain rates derived from insar analysis over the amery ice shelf east antarctica we use displacements derived from matching complex synthetic aperture radar data using maximum coherence to generate a dense network of velocity estimates over the amery ice shelf from these velocities we generate the horizontal strain rate components and resolve them with respect to the local flow direction we present the spatial distributions of velocity and transverse shear strain rate and use them to investigate features of the flow regime for the shelf from the southern end of the shelf velocity decreases from a high of about 800ma 1 to around 300 ma 1 and then increases to a maximum of about 1350ma 1 at the centre of the front strain rates vary systematically across and along the shelf the pattern of the transverse shear strain rate clearly identifies the shear margins where values exceed 0 1 a 1 in the southern section of the shelf the pattern also shows longitudinal bands of enhanced shear strain rate containing ice with a strong preferred crystal fabric that was advected from shear margins upstream in the northern section of the shelf significant values of longitudinal and traverse stresses lead to enhanced shear deformation through their effect on the octahedral shear stress term

introduction to this special section near surface modeling and imaging where and what is the near surface according to butler 2005 the majority of near surface surveys are shallower than 30 m occasionally down to 100 m and very rarely considered for depths beyond 100 m in saudi arabia the majority of upholes are of 100 m depth and models of the near surface are deeper than 100 m in order to model to the seismic datum in metalliferous mining areas low grade metamorphosed sediments or igneous rocks can constitute the near surface for mining engineering but are not considered near surface in the search for hydrocarbons for those studying the earth s crust and who have a different perspective sharma 1978 finds velocity distributions show a well defined low velocity layer at 100 150 km depth in those studies the whole of the sedimentary section would be included in the near surface

different ers altimeter measurements between ascending and descending tracks caused by wind induced features over ice sheets the ers1 satellite provides a high resolution radar altimeter measurement up to 81 5 latitude for polar cap studies here we examine one particular effect of the interaction between the radar wave and the snowpack surface on height measurements measurements from ascending and descending tracks show systematic differences as large as 1 m such an artifact to be removed if it can be estimated the difference appears to be related to the way the altimeter perceives wind induced sculpting of the snowpack differences in the volume echo weight between ascending and descending passes appear to be the explanation seasat scatterometer allowed us to estimate independently the signal that could be inferred by wind features our results yield a map of the effect of wind on radar altimetric measurement finally we consider the possibility of long term variations in altimeter measurements caused by climatic changes

snow facies over ice sheets derived from envisat active and passive observations this paper aims to separate different snow regions over the terrestrial ice sheets based on their measured microwave signatures it takes advantage of coregistered data from passive and active sensors on the environmental satellite envisat to directly derive a snow facies indicator in a point by point basis this paper represents the first attempt of this kind in exploiting nadir viewing and dual frequency data from both altimeter and radiometer sensors the approach is based on a clustering method such representation of the data by means of fewer clusters necessarily loses fine details but achieves simplification in geographical representation and eases the description of the condition of the ice sheets in 2004 our approach broadens the description of the snow pack by taking into account characteristics such as surface roughness grain size stratification and snowmelt effects whereas the latter has often solely been considered in most previous work such partition of the ice sheets might help to better understand relationships between microwave signatures and snow morphology it could also be useful for estimating elevation uncertainty in altimeter data which in turn is essential to correctly interpret the significance of the rates of elevation change in a changing climate and to convert elevation change to snow mass change

regional quaternary submarine geomorphology in the florida keys high quality seismic reflection profiles fill a major gap in geophysical data along the south florida shelf allowing updated interpretations of the history of the quaternary coral reef system incorporation of the new and existing data sets provides the basis for detailed color maps of the pleistocene surface and thickness of overlying holocene accretions the maps cover the florida keys to a margin wide upper slope terrace 30 to 40 m deep and extend from the elbow reef north key largo to rebecca shoal gulf of mexico the data indicate that pleistocene bedrock is several meters deeper to the southwest than to the north east yet in general holocene sediments are 3 to 4 m thick shelf wide the pleistocene map demonstrates the significance of a westward dipping bedrock surface to holocene flooding history and coral reef evolution seismic facies show evidence for two possible holocene stillstands aerial photographs provide information on the seabed surface much of which is below seismic resolution the photographs define a prominent regional nearshore rock ledge that extends 2 5 km seaward from the keys9 shoreline they show that bands of rock ridges exist along the outer shelf and on the upper slope terrace the photographs also reveal four tracts of outlier reefs on the terrace one more than had been documented seismically seismic and photographic data indicate the tracts are 200 km long nearly four times longer than previously thought new interpretations provide insights into a youngest possible terrace age ca 175 ka and the likelihood that precise ages of oxygen isotope substage 5e ooid tidal bar and coral reef components may differ the tidal bar reef complex forms the florida keys

relations between geomorphic form and sedimentologic stratigraphic variability holocene ooid sand shoal lily bank bahamas although the linkages among surface sediments geomorphic forms and hydrodynamics in holocene ooid tidal sand shoals have been evaluated recently how these factors are reflected in the geomorphic evolution and stratigraphic record of shoals is less constrained yet such understanding is essential to developing meaningful predictive conceptual models of three dimensional architecture of ancient reservoir analogs integrating remote sensing imagery high frequency seismic data and core characterization from lily bank a modern tidally dominated bahamian ooid shoal in which sedimentologic processes are well documented reveals the stratigraphic record of geomorphic change an irregular gently dipping rocky surface interpreted as the top pleistocene with no pronounced topographic high underlies the holocene oolitic succession a 6 m 20 ft thick poorly sorted gravelly muddy sand with few ooids overlies this basal surface this lower interval is overlain by sand with an upward increase in proportion of ooids sorting and grain size the uppermost unit present only under active bar forms is well sorted oolitic medium sand with accretionary foresets sediments vary stratigraphically and geomorphically the lower unit is finer and less well sorted than the upper units and in the oolitic upper unit sediment size and sorting on bar crests are distinct from bar flanks collectively these results suggest that a marked antecedent bump is not necessary for occurrence of ooid shoals and that the stratigraphic record of analogous ooid shoal systems may preserve clues of geomorphic position as well as geobody size and orientation

carbonate ramp depositional systems abstract the classification tectonic settings stratigraphy and early diagenesis of carbonate ramp systems are reviewed carbonate ramps are common in all geological periods but were dominant at times when reef constructing organisms were absent or inhibited ramps can be subdivided into inner mid and outer ramp environments the mid ramp zone extends from fair weather wave base to normal storm wave base although the water depths which these boundaries represent vary an additional outer slope environment occurs on distally steepened ramps as with siliciclastic shelves a range of wave storm and tide dominated ramps can be recognised and this forms the most convenient basis for ramp classification the carbonate productivity profile of ramps differs from that of rimmed shelves with the inner ramp showing lower production rates than comparable shallow water facies on rimmed shelves the zone of greatest organic carbonate sediment production appears to have shifted from the mid ramp to the inner ramp since the late jurassic carbonate ramps occur in most types of sedimentary basin but are best developed where subsidence is flexural and gradients are slight over large areas as in foreland and cratonic interior basins and along passive margins the featureless depositional profiles of many ramps means that sequence geometries are best observed on regional seismic lines some show low angle sigmoidal or shingled clinoforms suggesting that ramps may seldom be homoclinal but possess subtle slope geometries which reflect depositional environments because of their low angle slopes ramps respond differently to rimmed shelves during relative sea level changes although results seem to be strongly dependent on the rate of relative sea level change during a minor fall shallow ramp facies belts will simply shift basinwards in a forced regression in contrast the whole surface of a steep sloped flat topped rimmed shelf may be exposed so that sediment production ceases or is drastically reduced during a major fall shallow ramp bounded basins may empty completely conversely ramps also flood gradually whereas rimmed shelves do so more rapidly homoclinal ramps develop no resedimented lowstand deposits rimmed shelves and distally steepened ramps in contrast may develop lowstand talus or turbiditic wedges distally steepened ramps may behave more like homoclinal ramps during minor base level falls and like rimmed shelves during major base level falls many ramps consist of layered successions of several ramp sequences stacked one upon the other ramp stacks of this sort may show gross vertical accretion but individual ramp sequences seldom appear to develop in a keep up style apart from minor organic buildups as with many rimmed shelves steepening of the outer ramp margin due to tectonism slope inheritance or differential sedimentation may promote the development of a distally steepened ramp or rimmed shelf a wide variety of organisms have constructed buildups in mid and outer ramp environments isolated buildups may seed early in ramp development accrete to wave base or sea level and continue growth by stacking through successive ramp sequences so that depositional and diagenetic features within them are in concert with those of the shallow ramp the location of isolated buildups on ramps is governed by tectonism halokinesis antecedent topography or by the subtle slope geometry of the previous ramp sequence diagenesis on ramps shows some major variations compared with diagenesis on steep sloped flat topped carbonate platforms ramp bounded basins may form prolific petroleum sourcing and reservoiring systems and offer a range of subtle stratigraphic play types and lateral facies variations which determine reservoir quality and distribution isolated buildups in the mid and outer ramp environments represent one of the commonest petroleum reservoirs in ramp systems and tend to have their foundations in transgressive systems tracts grainstone and packstone reservoirs are widespread and range from shoreline carbonate sandbodies to major detached shoal complexes or shoals over offshore highs grainstone sandbodies occur in both highstand systems tracts and in prograding lowstand wedges

development of small carbonate banks on the south florida platform margin response to sea level and climate change abstract geophysical and coring data from the dry tortugas tortugas bank and riley s hump on the southwest florida margin reveal the stratigraphic framework and growth history of these carbonate banks the holocene reefs of the dry tortugas and tortugas bank are approximately 14 and 10 m thick respectively and are situated upon pleistocene reefal edifices tortugas bank consists of the oldest holocene corals in the florida keys with earliest coral recruitment occurring at 9 6 cal ka growth curves for the tortugas bank reveal slow growth

envisat radar altimeter measurements over continental surfaces and ice caps using the ice 2 retracking algorithm abstract this study presents the first results obtained with the envisat satellite radar altimeter ra 2 over lands and ice caps as radar altimetry was firstly devoted to ocean study this paper starts with a detailed description of the radar altimetry principle over continental surfaces a general formulation of the radar altimeter echoes according to different surface types is given and the retracking procedure ice 2 specially implemented on the envisat satellite ground segment to provide output parameters over continents is clearly presented the envisat ra 2 datasets processed with the ice 2 procedure are then mapped given an overall description of the output parameters over various surfaces the behaviour of these envisat altimetric parameters are analysed and related to main land surface characteristics such as surface volume echo specularity roughness apart from few unexpected signals which will need to be further investigated the new envisat dataset confirms the potential of inland altimetry and provides a new opportunity to monitor continental surfaces the ku band measurements confirm previous results recently obtained with the altimeters on board ers or topex poseidon and offer a temporal continuity the s band altimeter measurements are in accordance with theoretical expected ones and confirm previous topex poseidon observations in c band the s band and the use of the dual frequency thus bring new capabilities for altimetric applications with important deliveries for temporal evolution survey for instance s band will be used over continents to better detect wetlands located under vegetated areas or to better quantify wave penetration in terrestrial snowpack to accurately retrieve snow depth evolution over ice sheets the dual frequency s and ku band waveforms parameters will provide a more accurate height estimation as it will be possible to quantify the error on height measurements induced by snowpack structure change over time this will then provide a more confident survey of the ice sheets surface topography evolution the combination and complementarities with the other altimetry missions such as ers topex poseidon and jason look already promising and offer a 10 year span dataset for continental altimetry applications

ice sheet elevation change in west antarctica from ka band satellite radar altimetry satellite altimetry has been used to track changes in ice sheet elevation using a series of ku band radars in orbit since the late 1970s here we produce an assessment of higher frequency ka band satellite radar altimetry for the same purpose using saral altika measurements recorded over west antarctica altika elevations are 3 8 0 5 and 2 5 0 1 m higher than those determined from airborne laser altimetry and cryosat 2 respectively likely due to the instruments coarser footprint in the sloping coastal margins however altika rates of elevation change computed between 2013 and 2019 are within 0 6 2 4 and 0 1 0 1 cm year of airborne laser and cryosat 2 respectively indicating that trends in radar penetration are negligible the fast flowing trunks of the pine island and thwaites glaciers thinned by 117 10 and 100 20 cm year respectively amounting to a 9 reduction and a 43 increase relative to the 2000s

controls on ers altimeter measurements over ice sheets footprint scale topography backscatter fluctuations and the dependence of microwave penetration depth on satellite orientation we consider the reliability of radar altimeter measurements of ice sheet elevation and snowpack properties in the presence of surface undulations we demonstrate that over ice sheets the common practice of averaging echoes by aligning the first return from the surface at the origin can result in a redistribution of power to later times in the average echo mimicking the effects of microwave penetration into the snowpack algorithms that assume the topography affects the radar echo shape in the same way that waves affect altimeter echoes over the ocean will therefore lead to biased estimates of elevation this assumption will also cause errors in the retrieval of echo shape parameters intended to quantify the penetration of the microwave pulse into the snowpack using numerical simulations we estimate the errors in retrievals of extinction coefficient surface backscatter and volume backscatter for various undulating topographies in the flatter portions of the antarctic plateau useful estimates of these parameters may be recovered by averaging altimeter echoes recorded by the european remote sensing satellite ers 1 by numerical deconvolution of the average echoes we resolve the depths in the snowpack at which temporal changes and satellite travel direction effects occur both of which have the potential to corrupt measurements of ice sheet elevation change the temporal changes are isolated in the surface backscatter cross section while directional effects are confined to the extinction coefficient and are stable from year to year this allows the removal of the directional effect from measurement of ice sheet elevation change

geomorphic and sedimentologic heterogeneity along a holocene shelf margin caicos platform abstract reef rimmed margins on isolated carbonate platforms are heterogeneous and studying holocene platforms provides a valuable means for characterizing the spectrum of facies variability and understanding the processes behind facies changes in this study insights from petrographic data granulometric measurements bottom observations and remote sensing and subbottom profiler data from the northwestern part of caicos platform make this one of the more intensely studied shallow marine carbonate systems in the caribbean collectively analyses of the thicknesses and heterogeneity of shallow water holocene carbonates and the platform margin reveal details of the considerable geomorphic variability present on this part of the margin on which the orientation changes markedly in this area the shelf margin arcs from northwest to north to northeast facing accompanied by changes in morphology as well as the distribution thickness and type of sediments the nw facing margin has discontinuous reefs near the margin flanked platformward by coarse sands in poorly developed aprons that transition to a deeper rocky bottom interpreted as pleistocene with patches of sand to fine gravel sized sediment locally lobate to arcuate shoals with medium sands 500 m mean size form a reef apron that is continuous along strike and over 1000 m wide because the interpreted top pleistocene surface is gently dipping seaward holocene sediment thicknesses vary with changes in bathymetry and generally thin away from the reefs sediments in most areas are between 1 5 and 3 m thick but can reach over 6 m locally although the interpretation of the base of the holocene can be ambiguous under shelf margin reefs subbottom profiles illustrate that backreef shelf patch reefs do not necessarily nucleate on pleistocene bedrock highs facies trends are interpreted to reflect a combination of physical oceanographic sedimentologic and biologic processes acting within the framework of pleistocene bedrock configuration the ne facing margin with the continuous reef and expansive sand apron is interpreted to be more wave dominated influenced by swells from the open atlantic and by locally derived wind waves comparison with bahamian platforms suggests that this trend is not unique margins facing open atlantic swells from the north northeast have the best developed reefs not east facing windward margins in contrast the beaches and tidal deltas on the nw facing margin suggest tidal currents and longshore transport are more pronounced here consistent with the nature of geomorphic changes through time the enhanced tidal influence reflects the inlets open to the open bank interior a factor which also may discourage growth of reefs on this part of the margin nonetheless other bahamian platforms with fewer tidal passes have comparable discontinuous reefs suggesting that island continuity is not the most influential parameter on leeward reef development collectively these results illustrate the nature scale and causes of along strike heterogeneity and provide a conceptual model for ancient carbonate platform margins some of which may be equally complex in their facies architecture

asiras airborne radar resolves internal annual layers in the dry snow zone of greenland 1 we present data from the european space agency s airborne sar interferometric radar altimeter system asiras flown during the cryovex 2004 field calibration validation campaign and new high resolution depth profiles of snow density measured in the field by neutron scattering we combine these data to calculate the depth of internal reflecting horizons in the asiras data the high resolution density data allow us to identify annual layers in the snow density profile and correlate their peaks with the reflecting horizons we use the thickness of the annual layers combined with the density profile to determine the spatial and temporal pattern of snow accumulation along the radar track for a period of 6 years from 1995 2002 our mean annual accumulation rate is 0 47 0 09 ma 1 water equivalent in agreement with the value determined from a core taken in this location in 1992 similarly our inter annual variability shows the same trends as recent model estimates over the entire ice sheet because asiras was designed to mimic as closely as possible the characteristics of the sar interferometric radar altimeter siral the principal payload of cryosat the detection of internal layering with asiras illuminates the possibility of detecting internal layers with a space borne radar and thus the possible application of this technique to the dry snow zones of antarctica greenland and smaller ice bodies

using airborne ku band altimeter waveforms to investigate winter accumulation and glacier facies on austfonna svalbard winter balance is an important metric for assessing the change on glaciers and ice caps yet measuring it using ground based techniques can be challenging we use the european space agency prototype airborne sar interferometric radar altimeter system asiras to extract snow depths from the received altimeter waveforms over austfonna ice cap svalbard additionally we attempt to distinguish the long term firn area from other glacier facies we validate our results using snow depth and glacier facies characterizations determined from ground based radar profiles snow pits and a multi look satellite synthetic aperture radar image we show that the depth of the winter snowpack can be extracted from the altimeter data over most of the accumulation zone comprising wet snow zone and a superimposed ice zone the method struggles at lower elevations where internal reflections within the winter snowpack are strong and the winter snow depth is less than 1m we use the abruptness of the reflection from the last summer surface lss to attempt to distinguish glacier facies while there is a general correlation between lss abruptness and glacier facies we do not find a relationship that warrants a distinct classification based on asiras waveforms alone

coral reef complexes at an atypical windward platform margin late quaternary southeast florida major coral reef complexes rim many modern and ancient carbonate platforms their role in margin evolution is not fully understood particularly when they border a margin atypical of the classic model classic windward margins are steeply inclined the windward margin of southeast florida is distinct with a very low gradient slope and a shelf edge ringed with 30 m high quaternary outlier reefs on a shallow upper slope terrace a newly developed synthesis of temporally well constrained geologic events is used with surface and subsurface seismic reflection contours to construct morphogenetic models of four discontinuous reef complex sequences the models show uneven subsurface topography upward and landward buildups and a previously unreported rapid holocene pro gradation the terms backstepped reef complex margin backfilled prograded margin and coalesced reef complex margin are proposed for sections exhibiting suitable signatures in the stratigraphic record the models have significant implications for interpretation of ancient analogues the florida record chronicles four kinds of geologic events 1 thirteen transgressions high enough for marine deposition occurred between ca 325 ka and the present six gave rise to stratigraphically successive coral reef complexes between ca 185 and ca 77 8 ka the seventh reef ecosystem is holocene 2 two primary coral reef architectures built the outer shelf and margin producing respective ridge and swale and reef and trough geometries of very different scales 3 massive outlier reefs developed on an upper slope terrace between ca 106 5 and ca 80 ka and are inferred to contain corals that would date to highstands at ca 140 and 125 ka 4 sea level remained below elevation of the shelf between ca 77 8 and ca 9 6 ka

sea level history of the past two interglacial periods new evidence from u series dating of reef corals from south florida abstract as a future warm climate analog much attention has been directed to studies of the last interglacial period or marine isotope substage mis 5 5 which occurred 120 000 years ago nevertheless there are still uncertainties with respect to its duration warmth and magnitude of sea level rise here we present new data from tectonically stable peninsular florida and the florida keys that provide estimates of the timing and magnitude of sea level rise during the last interglacial period the last interglacial high sea stand in south florida is recorded by the key largo limestone a fossil reef complex and the miami limestone an oolitic marine sediment thirty five new high precision uranium series ages of fossil corals from the key largo limestone indicate that sea level was significantly above present for at least 9000 years during the last interglacial period and possibly longer ooids from the miami limestone show open system histories with respect to u series dating but show a clear linear trend toward an age of 120 ka correlating this unit with the last interglacial corals of the key largo limestone older fossil reefs at three localities in the florida keys have ages of 200 ka and probably correlate to mis 7 these reefs imply sea level near or slightly above present during the penultimate interglacial period elevation measurements of both the key largo limestone and the miami limestone indicate that local relative sea level was at least 6 6 m and possibly as much as 8 3 m higher than present during the last interglacial period

sediments of the dry tortugas south florida usa facies distribution on a ramp like isolated carbonate platform abstract four sedimentary facies may be delineated based on quantitative analysis of texture and composition of modern surface sediments on the dry tortugas carbonate platform these include 1 mollusk halimeda wackestone 2 mollusk packstone to grainstone 3 coralgal halimeda grainstone and 4 coralgal grainstone even though the tortugas platform is characterized by an open circulation due to deep broad marginal channels and a lack of a continuous surface breaking marginal reef facies are not distributed at random and show bathymetrical zonation also facies appear to cover wide belts rather than forming a mosaic mollusk halimeda wackestone occurs in protected platform interior areas ca 10 18 m deep mollusk packstone to grainstone occurs in more open platform interior settings adjacent to channels and in deeper outer reef areas of 14 25 m water depth coralgal halimeda grainstone is found on shallow marginal shoals 1 11 m deep and coralgal grainstone on the somewhat deeper 3 16 m seaward edges of these shoals however there is bathymetrical overlap of facies in intermediate depths of ca 5 17 m this limitation has implications for the interpretation of the fossil record because changes in water depth are commonly thought to be reflected in facies changes e g in sequence stratigraphy comparison with previous sediment studies of the 1930s 1960s and 1970s in the area exhibit a decrease in coral fragments and increases in coralline algal and mollusk shell fragments these observations might be a result of environmental changes such as coral decline and die outs during temperature events disease and the increase in macroalgae due to the ecological extinction of the echinoid diadema the results suggest that more long term studies are needed that further explore the influence of environmental change on reef sediment composition dry tortugas surface sediments consist of lower portions of halimeda plates and mollusk shell fragments and higher percentages of coralline algal fragments as compared to sediments of the adjacent florida reef tract possibly as a consequence of differences in platform morphology and exposure to waves and currents

using the temporal variability of satellite radar altimetric observations to map surface properties of the antarctic ice sheet the problem of measuring surface height and snowpack characteristics from satellite radar altimeter echoes is investigated in this paper we perform an analysis of the ers1 altimeter dataset acquired during a 3 day repeat orbit the analysis reveals that there are temporal variations in shapes of the radar altimeter echo and that these variations are linked to meteorological phenomena the time and space scales over which these variations apply are a few to tens of days and a few hundred kilometres respectively this phenomenon if not accounted for can create error in the height measurement a numerical echo model is used to recover snowpack characteristics by taking advantage of the temporal variations of the radar echoes a map of penetration depth of the radar waves in the ku band over the antarctic continent is obtained and suggests that grain size produces the dominant effect on radar extinction in the snowpack at this frequency finally a procedure is proposed to correct the height measurement within the context of ice sheet mass balance survey

controls on platform scale patterns of surface sediments shallow holocene platforms bahamas isolated carbonate platforms occur throughout geological history and commonly exhibit considerable spatial variability to evaluate the controls on the nature of sediment accumulation across the expansive shallow platform tops this study systematically compares and contrasts patterns in surface sediments from several shallow 10 m holocene bahamian examples remote sensing data field observations petrographic characterization and quantitative grain size analyses reveal the spatial patterns of sediment accumulation on crooked acklins platform and the berry islands bank integration of these data with synoptic observations of waves tides and currents along with regional geochemical data provides a means to explore the factors that influence platform scale sedimentary patterns these data illustrate that the platform interiors of both crooked acklins platform and berry islands bank are blanketed with medium to coarse sand size sediment peloids are most common in the interior of crooked acklins platform whereas the berry islands bank includes more abundant composite grains in both areas very little mud is present with surface sediments averaging 2 mud comparison of these results with published data from little bahama bank great bahama bank and caicos platform suggest that contrary to previous interpretations the presence of open margins and or brisk winds are not necessary for the occurrence of a platform top with little mud although the muddy sediment fraction of the interior can be suspended by elevated wave energy wind generated current speeds in protected platform interiors are relatively low instead in parts of the platform interiors transport and winnowing of fines is enhanced greatly by tidal currents which carry suspended sediments off the shallow platforms even if shielded by islands beyond physical influences however regional geochemical compilations suggest that the bahamian tides supply highly supersaturated waters rich in dissolved oxygen to these platform interiors this exchange is interpreted to facilitate favourable conditions for calcium carbonate precipitation in the form of ooids marine cements and hardened peloids across vast expanses of the platform interiors such fundamental controls on holocene platform scale sediment dynamics are likely to have influenced carbonate systems through the geological record

sediment facies determination using acoustic techniques in a shallow water carbonate environment dry tortugas florida abstract two high frequency acoustic seafloor classification systems 12 and 15 khz were used in conjunction with sediment core analysis to characterize sediment facies at a study site near garden key in dry tortugas florida the acoustic system uses echo return amplitude to compute acoustic impedance that is then correlated with sampled sediment impedance values calculated from wet bulk density and compressional wave velocity several bottom provinces were identified using the 15 khz data to construct a real time map of ship tracks in colors that represent the surficial sediment facies type sediment facies over the entire study site 36 km 2 range from sandy silts to exposed limestone rock and coral reef structures color contour maps using the 12 khz data created after correlating acoustic impedance predictions with core measured sediment properties validates the initial facies pattern predictions made in real time the sediment facies patterns indicate a long term pattern of deposition of fine grained silt sized surficial sediments in an area adjacent to the emergent carbonate embankment two dimensional acoustic profiles along survey tracklines also provide cross sectional views of seafloor and subbottom stratigraphy that confirm the buildup of these fine sediments in the northwest corner of the study site a generous supply of sediment resulting from an abundance of benthic green algae halimeda sp on adjacent shallow platforms form a thick sequence of fine sandy silt at the base of the southeastern edge of the embankment and fringing reef sediment cover over the limestone bedrock thins and becomes coarse southeast of garden and bush keys suggesting the likely existence of a dominant flow around the shallow carbonate embankment that restricts export of fine sediments out of the area

migration of radar altimeter waveform data 1 wingham 1993 demonstrated that migration of radar waveforms appears to improve the images of ice sheet topography we examine whether this qualitative improvement might lead to quantitative benefits in measuring ice sheet elevation change we find instead that estimates based on migrated waveforms at cross over points are degraded relative to unmigrated waveforms lack of cross track data which would allow true 3d migration and migration noise related to the kirchoff algorithm are likely contributors to the degradation additionally lack of phase information in radar waveforms degrades the result this problem is not encountered in seismic applications of migration and is a limitation of the current generation of radar altimeter data we have not investigated whether other quantitative benefits may be derived from migrating altimeter waveforms for example improved resolution of smaller scale topography in migrated data may lead to better quantitative understanding of ice sheet dynamics

in situ measurements of snow surface roughness using a laser profiler the snow surface roughness at centimetre and millimetre scales is an important parameter related to wind transport snowdrifts snowfall snowmelt and snow grain size knowledge of the snow surface roughness is also of high interest for analyzing the signal from radar sensors such as sar altimeters and scatterometers unfortunately this parameter has seldom been measured over snow surfaces the techniques used to measure the roughness of other surfaces such as agricultural or sand soils are difficult to implement in polar regions because of the harsh climatic conditions in this paper we develop a device based on a laser profiler coupled with a gps receiver on board a snowmobile this instrumentation was tested successfully in midre lovenbreen svalbard in april 2006 it allowed us to generate profiles of 3 km sections of the snow covered glacier surface because of the motion of the snowmobile the roughness signal is mixed with the snowmobile signal we use a distance frequency analysis the empirical mode decomposition to filter the signal this method allows us to recover the snow surface structures of wavelengths between 4 and 50 cm with amplitudes of 1 mm finally the roughness parameters of snow surfaces are retrieved the snow surface roughness is found to be dependent on the scales of the observations the retrieved rms of the height distribution is found to vary between 0 5 and 9 2 mm and the correlation length is found to be between 0 6 and 46 cm this range of measurements is particularly well adapted to the analysis of ghz radar response on snow surfaces

modeled seasonal variations of firn density induced by steady state surface air temperature cycle seasonal variations of firn density in ice sheet firn layers have been attributed to variations in deposition processes or other processes within the upper firn a recent high resolution mm scale density profile measured along a 181 m core from antarctica showed small scale density variations with a clear seasonal cycle that apparently was not related to seasonal variations in deposition or known near surface processes gerland and others 1999 a recent model of surface elevation changes zwally and li submitted produced a seasonal variation in firn densification and explained the seasonal surface elevation changes observed by satellite radar altimeters in this study we apply our 1 d time dependent numerical model of firn densification that includes a temperature dependent formulation of firn densification based on laboratory measurements of grain growth the model is driven by a steady state seasonal surface temperature and a constant accumulation rate appropriate for the measured antarctic ice core the modeled seasonal variations in firn density show that the layers of snow deposited during spring to mid summer with the highest temperature history compress to the highest density and the layers deposited during later summer to autumn with the lowest temperature history compress to the lowest density the initial amplitude of the seasonal difference of about 0 13 reduces to about 0 09 in five years and asymptotically to 0 92 at depth which is consistent with the core measurements

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