geomorphic thresholds and complex response of fluvial systems some implications for sequence stratigraphy first generation sequence stratigraphic models have dealt in a very rudimentary fashion with the response of fluvial systems to eustasy a major element of presently accepted models is that rivers incise when sea level falls and aggrade during the ensuing rise well established geomorphic principles state that fluvial systems are complex process response systems that can adjust to internal and external changes in other ways besides incision and aggradation by modifying their stream patterns and channel geometries application of geomorphic principles to sequence stratigraphic models results in the following observations during eustatic lowstands rivers may adjust to lowered base levels and changes in slope by modifying channel patterns therefore not all lowstands produce type 1 sequence boundaries type 1 sequence boundaries characterized by fluvial valley incision are more likely to develop when sea level drops below the shelf slope break resulting in topological relief near the strandline in which headwardly eroding knickpoints form rates of eustatic change although rapid in a geological context are sufficiently low that geomorphic systems can easily maintain their equilibrium during eustatic changes and migrate back and forth across the shelf without major modifications finally und r conditions of relatively static sea level sequences and parasequences of the same scale in time and space as those which form during eustatic change can be deposited as the result of purely intrinsic causes and responses of a fluvial system in general eustasy controls the location of deposition and erosion but the resultant stratal geometry is controlled by sediment supply and processes acting on the sediments as the shoreline moves across the shelf in response to eustasy

interaction between aeolian fluvial and playa environments in the permian upper rotliegend group uk southern north sea the permian upper rotliegend group in offshore uk quadrants 42 43 47 and 48 comprises a sequence of mixed aeolian fluvial playa deposits these deposits are up to 300 m thick and contain a record of the interaction between desert fluvial systems and adjacent aeolian and playa environments the relative dominance of water vs wind transport and deposition in this stratigraphic package was a function of fluctuations in the discharge of ephemeral fluvial systems and changes in water table playa level driven by a combination of climatic change and syndepositional tectonics the rotliegend sedimentary record is punctuated by numerous surfaces recording erosion by wind and water the origin of these surfaces is mostly climatic with periods of increased runoff resulting in fluvial incision especially near active faults during periods of reduced runoff wind erosion of fluvial deposits occurred with fluvially derived sand being reworked into expanding aeolian dune fields wind erosion also occurred as a rising water table isolated dunes from their sediment supply resulting in deflation of dunes down to the water table these surfaces formed in a basin that was subsiding thus even in a background of overall increasing accommodation space climatically driven falls in the water table allowed for periods of erosion the occurrence of significant erosion especially near syndepositional fault zones resulted in a sedimentary record that shows pronounced lateral as well as vertical facies variations

fracture of antarctic shelf ice 1 we have investigated the fracture of antarctic shelf ice core using two fracture mechanics test methods the chevron notched short rod specimen loaded in tension and the chevron notched round bar specimen loaded in three point bending these tests have been used to measure the fracture initiation toughness kinit at which crack growth starts on samples taken through the entire thickness of the ronne ice shelf from low density firn through consolidated meteoric ice to basal marine ice the fracture data are presented together with depth profiles of relevant physical and mechanical properties derived from the test specimens temperature density elastic modulus and grain size it is found that the trend in measured fracture toughness closely reflects changes in ice density and elastic modulus we augment the experimental study by presenting a fracture mechanics analysis of ice shelf surface and basal crevassing which directly incorporates our measurements for the examined ice shelf profiles basal crevasses are found to be inherently unstable unless an external restraining force is imposed which has important implications for overall ice shelf stability on the other hand surface crevassing is shown to be innately stable at depth our fracture mechanics model is used to predict local ice shelf back stresses in the vicinity of basal crevassing and is validated directly against field observations of crevasse penetration on the ronne ice shelf

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

systems analysis of complex glaciological processes and application to calving of amery ice shelf east antarctica calving is a complex process subject to several cooperating atmospheric oceanographic and glaciological forcings that vary in space and time and whose relative effects are challenging to separate statistical systems analysis is commonly used in engineering and economics to extricate complex force response relationships here we apply systems analysis to the amery rift system east antarctica we develop a scalable system model driven by a coarsely sampled dataset characteristic of glaciological observations in remote locations and validate it using rift lengths observed in 2000 06 and 2012 in this initial demonstration we forecast a detachment date of 2019 5 years for the large tabular iceberg colloquially known as the loose tooth for which relative humidity surprisingly emerges as the best statistical predictor racmo2 climate modelling reveals that relative humidity correlates best with surface albedo and snowmelt both of which are intimately linked to firn compaction and ice shelf temperature and flow we postulate that relative humidity can therefore serve as a proxy for internal stress a known key control of loose tooth calving although no physical causality is implied in systems analysis postulates such as this can aid in setting priorities in studies of complex glaciological processes

acceleration and spatial rheology of larsen c ice shelf antarctic peninsula the disintegration of several antarctic peninsula ice shelves has focused attention on the state of the larsen c ice shelf here we use satellite observations to map ice shelf speed from the years 2000 2006 and 2008 and apply inverse modeling to examine the spatial pattern of ice shelf stiffness results show that the northern half of the ice shelf has been accelerating since 2000 speeding up by 15 between 2000 and 2006 alone the distribution of ice stiffness exhibits large spatial variations that we link to tributary glacier flow and fractures our results reveal that ice down flow from promontories is consistently softer with the exception of churchill peninsula where we infer a stabilizing role for marine ice we conclude that although larsen c is not facing imminent collapse it is undergoing significant change in the form of flow acceleration that is spatially related to thinning and fracture copyright 2011 by the american geophysical union

thirty years of elevation change on antarctic peninsula ice shelves from multimission satellite radar altimetry 1 we use data acquired between 1978 and 2008 by four satellite radar altimeter missions seasat ers 1 ers 2 and envisat to determine multidecadal elevation change rates dhi dt for six major antarctic peninsula ap ice shelves in areas covered by the seasat orbit to 72 16 s regional averaged 30 year trends were negative surface lowering with rates between 0 03 and 0 16 m a 1 surface lowering preceded the start of near continuous radar altimeter operations that began with ers 1 in 1992 the average rate of lowering for the first 14 years of the period was typically smaller than the 30 year average the exception was the southern wilkins ice shelf which experienced negligible lowering between 2000 and 2008 when a series of large calving events began analyses of the continuous ers envisat time series to 81 5 for 1992 2008 reveal a period of strong negative dhi dt on most ice shelves between 1992 and 1995 based on prior studies of regional atmospheric and oceanic conditions we hypothesize that the observed elevation changes on larsen c ice shelf are driven primarily by firn compaction while the western ap ice shelves are responding to changes in both surface mass balance and basal melt rates our time series also show that large changes in dhi dt can occur on interannual time scales reinforcing the importance of long time series altimetry to separate long term trends associated with climate change from interannual to interdecadal natural variability

basal crevasses and associated surface crevassing on the larsen c ice shelf antarctica and their role in ice shelf instability we identify a series of basal crevasses along a 31 km transect across the northern sector of the larsen c ice shelf antarctica using in situ ground penetrating radar the basal crevasses propagate from a region of multiple shallow basal fractures to form widely spaced 0 5 2 0 km but deeply incised 70 134 m features surface troughs observed in visible imagery exist above the basal crevasses as the ice vertically shears to reach hydrostatic equilibrium while widespread surface crevassing occurs along the crests and on the flanks of the undulations primarily aligned with the topography we suggest based on the location of the surface crevasses and the along flow evolution of the basal crevasses that the former are induced by a bending stress created by gradients in hydrostatic forces using a linear elastic fracture mechanics model we investigate the sensitivity of basal crevasse propagation to observed trends of ice shelf thinning and acceleration basal crevasses are large scale structural weaknesses that can both control meltwater ponding and induce surface crevassing together these features may represent an important mechanism in both past and future ice shelf disintegration events on the antarctic peninsula

basal crevasses on the larsen c ice shelf antarctica implications for meltwater ponding and hydrofracture a key mechanism for the rapid collapse of both the larsen a and b ice shelves was meltwater driven crevasse propagation basal crevasses large scale structural features within ice shelves may have contributed to this mechanism in three important ways i the shelf surface deforms due to modified buoyancy and gravitational forces above the basal crevasse creating 10 m deep compressional surface depressions where meltwater can collect ii bending stresses from the modified shape drive surface crevassing with crevasses reaching 40 m in width on the flanks of the basal crevasse induced trough and iii the ice thickness is substantially reduced thereby minimizing the propagation distance before a full thickness rift is created we examine a basal crevasse 4 5 km in length 230 m in height and the corresponding surface features in the cabinet inlet sector of the larsen c ice shelf using a combination of high resolution 0 5 m satellite imagery kinematic gps and in situ ground penetrating radar we discuss how basal crevasses may have contributed to the breakup of the larsen b ice shelf by directly controlling the location of meltwater ponding and highlight the presence of similar features on the amery and getz ice shelves with high resolution imagery 2012 american geophysical union all rights reserved

radarsat 1 antarctic mapping project change detection and surface velocity campaign theradarsat 1antarcticmappingproject ramp isacollaboration betweennasaandthecanadianspaceagencytomapantarcticausingsyntheticaperture radar sar the first antarctic mapping mission amm 1 wassuccessfullycompleted in october 1997 data from the acquisition phase of the 1997 campaign have been used to achievetheprimarygoalofproducingthefirsthigh resolutionsarimagemapofantarctica the modified antarctic mapping mission mamm occurredduring thefallof 2000 the acquisition strategy concentrated on collecting highest resolution radarsat 1 data of antarctica s fast glaciers for change detection and feature retracking estimates of surface velocity additionally extensive data were acquired for interferometric analysis over the entire viewable region which extends north of 80 1 s latitude this paper summarizes the goalsand strategybehind mamm it goes onto discuss ice sheet marginchanges observed on several ice shelvesaroundthecontinent margin changesare documentedbycomparing amm 1and mamm datawith earlier datasets including european remote sensing satel lite 1sar imagery landsat imagery the antarctic digital database version1 coastline and declassified intelligence satellite photography analysis reveals a complex pattern of ice margin advance and retreat without indicating any systematic behavior in ice sheet extentabouttheice sheetperimeter

marine ice formation in a suture zone on the larsen c ice shelf and its influence on ice shelf dynamics 1 antarctic ice shelves are fed primarily by the glaciers flowing into them downstream of promontories separating these glaciers supercooled water can rise and freeze into suture zones leading to the accretion of marine ice marine ice bodies have been found in several antarctic ice shelves but little is known about their detailed geometry rate of accretion or influence on ice dynamics in this study we investigate marine ice in a suture zone downstream of the joerg peninsula in the southern part of the larsen c ice shelf antarctic peninsula we present ground penetrating radar data from which we infer the base of the meteoric ice and in combination with gps data and assuming hydrostatic equilibrium estimate marine ice thickness within a suture zone we show that the joerg peninsula suture zone contains marine ice layer which is increasing in thickness along flow from 140 m to 180 m over 20 km implying an average basal accretion rate of 0 5 m a 1 in our study area we examined the impact of this inferred marine ice on ice shelf dynamics by modeling the suture zone within an ice flow model the results which replicate observed surface velocities and strain rates show that the warmer and thus softer ice of the suture zone serves to channel shear deformation this enables decoupling of neighboring flow units with different flow velocities while maintaining the structural integrity of the ice shelf

numerical modelling and data assimilation of the larsen b ice shelf antarctic peninsula in this study the flow and rheology of pre collapse larsen b ice shelf are investigated by using a combination of flow modelling and data assimilation observed shelf velocities from satellite interferometry are used to constrain an ice shelf model by using a data assimilation technique based on the control method in particular the ice rheology field and the velocities at the inland shelf boundary are simultaneously optimized to get a modelled flow and stress field that is consistent with the observed flow the application to the larsen b ice shelf shows that a strong weakening of the ice in the shear zones mostly along the margins is necessary to fit the observed shelf flow this pattern of bands with weak ice is a very robust feature of the inversion whereas the ice rheology within the main shelf body is found to be not well constrained this suggests that these weak zones play a major role in the control of the flow of the larsen b ice shelf and may be the key to understanding the observed pre collapse thinning and acceleration of larsen b regarding the sensitivity of the stress field to rheology the consistency of the model with the observed flow seems crucial for any further analysis such as the application of fracture mechanics or perturbation model experiments

climatic and tectonic controls on fluvial incision and aggradation in the spanish pyrenees the influences of tectonic and climatic changes on upland river systems are investigated using data from plio pleistocene terraces of the rio cinca river in the southern pyrenees this river runs transverse to the main thrust fault structures and is a major conduit for sediment delivery to the ebro basin detailed field mapping combined with long profile and palaeohydraulic reconstructions yields a comprehensive picture of changes in palaeoriver character during the plio pleistocene as the area is over 150 km from the basin outlet in the mediterranean sea changes in base level are unlikely to have influenced terrace development although tectonic activity has exerted a strong control on the position of the river the main period of thrust propagation pre dates the terraces and activity has waned from the pliocene through to the present it is concluded that the main control on incision in this area is climate through its influence on sediment supply rivers which are starved of sediment by climate change will have the power to incise whereas aggradational phases are linked to periods of increased sediment flux

seismicity within a propagating ice shelf rift the relationship between icequake locations and ice shelf structure iceberg calving is a dominant mass loss mechanism for antarctic ice shelves second only to basal melting an important process involved in calving is the initiation and propagation of through penetrating fractures called rifts however the mechanisms controlling rift propagation remain poorly understood to investigate the mechanics of ice shelf rifting we analyzed seismicity associated with a propagating rift tip on the amery ice shelf using data collected during the austral summers of 2004 2007 we apply a suite of passive seismological techniques including icequake locations back projection and moment tensor inversion we confirm previous results that show ice shelf rifting is characterized by periods of relative quiescence punctuated by swarms of intense seismicity of 1 to 3 h even during periods of quiescence we find significant deformation around the rift tip moment tensors calculated for a subset of the largest icequakes mw 2 0 located near the rift tip show steeply dipping fault planes horizontal or shallowly plunging stress orientations and often have a significant volumetric component they also reveal that much of the observed seismicity is limited to the upper 50 m of the ice shelf this suggests a complex system of deformation that involves the propagating rift the region behind the rift tip and a system of rift transverse crevasses small scale variations in the mechanical structure of the ice shelf especially rift transverse crevasses and accreted marine ice play an important role in modulating the rate and location of seismicity associated with the propagating ice shelf rifts

structural and climatic controls on facies distribution in a mixed fluvial and aeolian reservoir the triassic sherwood sandstone in the irish sea

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

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

impact of tide topography interactions on basal melting of larsen c ice shelf antarctica 1 basal melting of ice shelves around antarctica contributes to formation of antarctic bottom water and can affect global sea level by altering the offshore flow of grounded ice streams and glaciers tides influence ice shelf basal melt rate wb by contributing to ocean mixing and mean circulation as well as thermohaline exchanges with the ice shelf we use a three dimensional ocean model thermodynamically coupled to a nonevolving ice shelf to investigate the relationship between topography tides andwb for larsen c ice shelf lcis in the northwestern weddell sea antarctica using our best estimates of ice shelf thickness and seabed topography we find that the largest modeled lcis melt rates occur in the northeast where our model predicts strong diurnal tidal currents 0 4 m s 1 this distribution is significantly different from models with no tidal forcing which predict largest melt rates along the deep grounding lines we compare several model runs to explore melt rate sensitivity to geometry initial ocean potential temperature 0 thermodynamic parameterizations of heat and freshwater ice ocean exchange and tidal forcing the resulting range of lcis averagedwb is 0 11 0 44 m a 1 the spatial distribution of wb is very sensitive to model geometry and thermodynamic parameterization while the overall magnitude of wb is influenced by 0 these sensitivities in wbpredictions reinforce a need for high resolution maps of ice draft and sub ice shelf seabed topography together with ocean temperature measurements at the ice shelf front to improve representation of ice shelves in coupled climate system models

antarctic ice rise formation evolution and stability antarctic ice rises originate from the contact between ice shelves and one of the numerous topographic highs emerging from the edge of the continental shelf while investigations of the raymond effect indicate their millennial scale stability little is known about their formation and their role in ice shelf stability here we present for the first time the simulation of an ice rise using the bisicles model the numerical results successfully reproduce several field observable features such as the substantial thinning downstream of the ice rise and the successive formation of a promontory and ice rise with stable radial ice flow center showing that ice rises are formed during the ice sheet deglaciation we quantify the ice rise buttressing effect found to be mostly transient delaying grounding line retreat significantly but resulting in comparable steady state positions we demonstrate that ice rises are key in controlling simulations of antarctic deglaciation

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

correlation of marine and continental glacial and interglacial events arctic ocean and banks island correlation of marine and continental glacial and interglacial events arctic ocean and banks island

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

rapid collapse of northern larsen ice shelf antarctica in january 1995 4200 square kilometers of the northern larsen ice shelf antarctic peninsula broke away radar images from the ers 1 satellite complemented by field observations showed that the two northernmost sections of the ice shelf fractured and disintegrated almost completely within a few days this breakup followed a period of steady retreat that coincided with a regional trend of atmospheric warming the observations imply that after an ice shelf retreats beyond a critical limit it may collapse rapidly as a result of perturbated mass balance

fluvial aeolian interactions part ii ancient systems an understanding of fluvial aeolian deposition derived from modern case examples in a previous study is applied to the permian cutler formation and cedar mesa sandstone on the colorado plateau these formations supply an excellent three dimensional exposure of intertonguing fluvial and aeolian strata four distinct facies associations form the bulk of the cutler formation and cedar mesa sandstone 1 aeolian dune deposits 2 wet interdune deposits 3 fluvial channel deposits and 4 overbank interdune deposits in addition two distinctive types of erosion surfaces are found within the cutler formation and cedar mesa sandstone pebble to granule rich erosion surfaces aeolian deflation surfaces and flood surfaces fluvial and aeolian intertonguing result in extensive tabular sheets of aeolian sandstone separated by flood surfaces and overbank interdune deposits fluvial channels are associated with the deposits overlying flood surfaces and are incised into the underlying aeolian sandstones overbank interdune deposits and wet interdune deposits cover flood surfaces and intertongue with overlying aeolian sandstones the primary characteristics of ancient fluvial aeolian deposition are overbank interdune deposits and pronounced extensive erosion surfaces flood surfaces which are parallel to underlying fluvial sandstones and thus trend parallel to the palaeoslope and palaeohydrological gradient

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

a structural glaciological analysis of the 2002 larsen b ice shelf collapse this study provides a detailed structural glaciological analysis of changes in surface structures on the larsen b ice shelf on the antarctic peninsula prior to its collapse in february march 2002 mapped features include the ice shelf front rifts crevasses longitudinal linear surface structures and meltwater features we define domains on the ice shelf related to glacier source areas and demonstrate that prior to collapse the central larsen b ice shelf consisted of four sutured flow units fed by crane jorum punchbowl and hektoria green evans glaciers between these flow units were suture zones of thinner ice where the feeder glaciers merged prior to collapse large open rift systems were present offshore of foyn point and cape disappointment these rifts became more pronounced in the years preceding break up and ice blocks in the rifts rotated because of the strong lateral shear in this zone velocity mapping of the suture zones indicates that the major rifts were not present more than about 20 years ago we suggest that the ice shelf was preconditioned to collapse by partial rupturing of the sutures between flow units this we believe was the result of ice shelf front retreat during 1998 2000 reducing the lateral resistive stress on the upstream parts of the shelf and glacier flow units ice shelf thinning and pre shelf break up glacier acceleration

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

the air content of larsen ice shelf 1 the air content of glacial firn determines the effect and attribution of observed changes in ice surface elevation but is currently measurable only using labor intensive ground based techniques here a novel method is presented for using radar sounding measurements to decompose the total thickness of floating ice shelves into thicknesses of solid ice and firn air or firn water the method is applied to a 1997 98 airborne survey of larsen ice shelf revealing large spatial gradients in air content that are consistent with existing measurements and local meteorology the gradients appear to be governed by meltwater induced firn densification we find sufficient air in larsen c ice shelf for increased densification to account for its previously observed surface lowering and the rate of lowering superficially agrees with published trends in melting this does not preclude a contribution to the lowering from oceanic basal melting but a modern repeat of the survey could conclusively distinguish atmosphere led from ocean led change the technique also holds promise for the calibration of firn density models derivation of ice thickness from surface elevation measurements and calculation of the sea level contribution of changes in grounded ice discharge

evolution of supraglacial lakes on the larsen b ice shelf in the decades before it collapsed the larsen b ice shelf collapsed in 2002 losing an area twice the size of greater london to the sea 3 000 km 2 in an event associated with widespread supraglacial lake drainage here we use optical and radar satellite imagery to investigate the evolution of the ice shelf s lakes in the decades preceding collapse we find 1 that lakes spread southward in the preceding decades at a rate commensurate with meltwater saturation of the shelf surface 2 no trend in lake size suggesting an active supraglacial drainage network which evacuated excess water off the shelf and 3 lakes mostly refreeze in winter but the few lakes that do drain are associated with ice breakup 2 4 years later given the relative scale of lake drainage and shelf breakup however it is not clear from our data whether lake drainage is more likely a cause or an effect of ice shelf collapse

surface structure and stability of the larsen c ice shelf antarctic peninsula a structural glaciological description and analysis of surface morphological features of the larsen c ice shelf antarctic peninsula is derived from satellite images spanning the period 1963 2007 the data are evaluated in two time ranges a comparison of a 1963 satellite image photomosaic with a modern digital mosaic compiled using 2003 04 austral summer data and an image series since 2003 showing recent evolution of the shelf we map the ice shelf edge rift swarms crevasses and crevasse traces and linear longitudinal structures called flow stripes or streak lines the latter are observed to be continuous over distances of up to 200 km from the grounding line to the ice shelf edge with little evidence of changes in pattern over that distance integrated velocity measurements along a flowline indicate that the shelf has been stable for 560 years in the mid shelf area linear longitudinal features may be grouped into 12 units each related to one or a small group of outlet feeder glaciers to the shelf we observe that the boundaries between these flow units often mark rift terminations the boundary zones originate upstream at capes islands or other suture areas between outlet glaciers in agreement with previous work our findings imply that rift terminations within such suture zones indicate that they contain anomalously soft ice we thus suggest that suture zones within the larsen c ice shelf and perhaps within ice shelves more generally may act to stabilize them by reducing regional stress intensities and thus rates of rift lengthening

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

glacier motion estimation using sar offset tracking procedures two image to image patch offset techniques for estimating feature motion between satellite synthetic aperture radar sar images are discussed intensity tracking based on patch intensity cross correlation optimization and coherence tracking based on patch coherence optimization are used to estimate the movement of glacier surfaces between two sar images in both slant range and azimuth direction the accuracy and application range of the two methods are examined in the case of the surge of monacobreen in northern svalbard between 1992 and 1996 offset tracking procedures of sar images are an alternative to differential sar interferometry for the estimation of glacier motion when differential sar interferometry is limited by loss of coherence i e in the case of rapid and incoherent flow and of large acquisition time intervals between the two sar images in addition an offset tracking procedure in the azimuth direction may be combined with differential sar interferometry in the slant range direction in order to retrieve a two dimensional displacement map when sar data of only one orbit configuration are available

structural and environmental controls on antarctic ice shelf rift propagation inferred from satellite monitoring 1 iceberg calving from ice shelves accounts for nearly half of the mass loss from the antarctic ice sheet yet our understanding of this process is limited the precursor to iceberg calving is large through cutting fractures called rifts that can propagate for decades after they have initiated until they become iceberg detachment boundaries to improve our knowledge of rift propagation we monitored the lengths of 78 rifts in 13 antarctic ice shelves using satellite imagery from the moderate resolution imaging spectroradiometer and multiangle imaging spectroradiometer between 2002 and 2012 this data set allowed us to monitor trends in rift propagation over the past decade and test if variation in trends is controlled by variable environmental forcings we found that 43 of the 78 rifts were dormant i e propagated less than 500 m over the observational interval we found only seven rifts propagated continuously throughout the decade an additional eight rifts propagated for at least 2 years prior to arresting and remaining dormant for the rest of the decade and 13 rifts exhibited isolated sudden bursts of propagation after 2 or more years of dormancy twelve of the fifteen active rifts were initiated at the ice shelf fronts suggesting that front initiated rifts are more active than across flow rifts although we did not find a link between the observed variability in rift propagation rate and changes in atmospheric temperature or sea ice concentration correlated with we did find a statistically significant correlation between the arrival of tsunamis and propagation of front initiated rifts in eight ice shelves this suggests a connection between ice shelf rift propagation and mechanical ocean interaction that needs to be better understood

sand drift surfaces a neglected type of bounding surface sand drift surfaces are sharp generally erosional surfaces separating waterlaid deposits from overlying eolian sand sheet deposits they form in many modern playa lake ephemeral stream and alluvial fan environments but have rarely been reported from ancient sequences sand drift surfaces are an important type of bounding surface one which marks the transition from subaqueous to eolian depositional processes thereby providing valuable information for environmental reconstructions architectural element analysis and short and long term climate changes because of their nature sand drift surfaces cannot be described by existing bounding surface classifications to overcome some of the classification problems we suggest a simple nonhierarchical terminology

bounding surfaces in a mixed aeolian fluvial system rotliegend wessex basin sw uk the dawlish sandstone formation is a late permian succession of mixed aeolian and fluvial deposits in the wessex basin sw england it is used to illustrate two contrasting types of fluvial aeolian bounding surface planar and incised planar bounding surfaces separate tabular bodies of fluvial conglomerate and aeolian dune sandstone they were produced primarily by wind scour to groundwater table with the later emplacement of conglomerates resulting in local fluvial erosion of cemented aeolian dune sandstones incised bounding surfaces were produced by fluvial downcutting the erosive relief was infilled with mixed aeolian fluvial deposits the dawlish sandstone formation may provide the first outcrop example of these incised valley fills which have recently been identified as a major component of the subsurface rotliegend in the southern north sea basin the potential variability of aeolian fluvial sedimentary architecture has important implications for well to well correlation and reservoir modelling

fracture propagation and stability of ice shelves governed by ice shelf heterogeneity tabular iceberg calving and ice shelf retreat occurs after full thickness fractures known as rifts propagate across an ice shelf a quickly evolving rift signals a threat to the stability of larsen c the antarctic peninsula s largest ice shelf here we reveal the influence of ice shelf heterogeneity on the growth of this rift with implications that challenge existing notions of ice shelf stability most of the rift extension has occurred in bursts after overcoming the resistance of suture zones that bind together neighboring glacier inflows we model the stresses in the ice shelf to determine potential rift trajectories calving perturbations to ice flow will likely reach the grounding line the stability of larsen c may hinge on a single suture zone that stabilizes numerous upstream rifts elevated fracture toughness of suture zones may be the most important property that allows ice shelves to modulate antarctica s contribution to sea level rise

the structure and effect of suture zones in the larsen c ice shelf antarctica ice shelf fractures frequently terminate where they encounter suture zones regions of material heterogeneity that form between meteoric inflows in ice shelves this heterogeneity can consist of marine ice meteoric ice with modified rheological properties or the presence of fractures here we use radar observations on the larsen c ice shelf antarctica to investigate i the termination of a 25km long rift in the churchill peninsula suture zone which was found to contain similar to 60m of accreted marine ice and ii the along flow evolution of a suture zone originating at cole peninsula we determine a steady state field of basal melting freezing rates and apply it to a flowline model to delineate the along flow evolution of layers within the ice shelf the thickening surface wedge of locally accumulated meteoric ice which likely has limited lateral variation in its mechanical properties accounts for similar to 60 of the total ice thickness near the calving front thus we infer that the lower similar to 40 of the ice column and the material heterogeneities present there are responsible for resisting fracture propagation and thereby delaying tabular calving events as demonstrated in the 40year time series leading up to the 2004 2005 calving event for larsen c this likely represents a highly sensitive aspect of ice shelf stability as changes in the oceanic forcing may lead to the loss of this heterogeneity

conditions favourable for the formation of warm climate aeolian sand sheets aeolian sand sheets are areas of aeolian sand where dunes with slipfaces are generally absent sand sheets are ubiquitous to modern warm climate sand seas generally occurring marginal to dune fields although they may exist within the interior of a sand sea or independent of a dune field sand sheet deposits are recognized in ancient aeolian sequences where they may account for significant accumulations of low angle aeolian stratification we suggest that the occurrence of sand sheets instead of dunes indicates that conditions are outside the range within which dunes form or that one or more factors interfere with dune development while also favouring the accumulation of sand sheets a study of six modern sand sheets in north america located at great sand dunes gran desierto dumont algodones padre island and colorado river delta indicates that the factors favourable for sand sheet development are 1 a high water table 2 surface cementation or binding 3 periodic flooding 4 a significant coarse grained sediment population and 5 vegetation these factors are reflected in the nature of stratification and the accessory features of sand sheet accumulations within the areas of modern sand sheets as well as in their ancient counterparts in the triassic dolores and pennsylvanian permian rico formations

larsen b ice shelf rheology preceding its disintegration inferred by a control method a new complete velocity field from satellite remote sensing is combined with numerical modeling to infer the rheology of the larsen b ice shelf before its disintegration the resulting spatial distribution of the flow parameter exhibits large variability which reflects very well observed ice shelf features this variability is explained by factors including advection of colder ice from tributary glaciers bottom melting and the presence of zones of strong shear and fracture the inferred distribution is applied to simulate numerically the flow regime ofthe ice shelf and to examine its modification by the presence of open rifts and by the retreat of ice shelf front between 1996 and 2000 results demonstrate that variable rheology is essential to understanding ice shelf evolution especially the close relationship among frontal retreat fracture ice flow acceleration and the destabilization of ice shelves copyright 2007 by the american geophysical union

formulation of ice shelf dynamic boundary conditions in terms of a coulomb rheology coastal boundaries where fast flowing ice shelves shear past stagnant grounded ice are typically riven with surface crevasses seawater filled basal crevasses and tidal strand cracks here we formulate a boundary condition describing stress transmission through these fractured boundaries in terms of the coulomb law as a result of this formulation agreement between finite element simulations of the ross ice shelf flow and field observations is improved over agreement obtained with formulations which do not account for ice failure our results additionally suggest that shear stress transmitted through ice shelf boundaries is lower than previously thought

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

climatic cyclicity and accommodation space in arid to semi arid depositional systems an example from the rotliegend group of the uk southern north sea

a one dimensional model of ice shelf ocean interaction large scale oceanic circulation beneath antarctic ice shelves is driven by the thenmohaline differences which result from mass and energy exchange at the ice ocean interface dense saline waters are drawn underneath the ice shelves and emerge cooled and diluted as plumes of ice shelf water a simple one dimensional model of this process has been developed in which the ice shelf water plume is treated as a turbulent gravity current initiated at the inland margin by a flow of fresh meltwater emerging from beneath the grounded ice subsequent evolution of the plume as it ascends along an ice shelf base of specified geometry can be simulated the model has been applied to a flow line on ronne ice shelf antarctica to explain the observed distribution and rate of basal melting and freezing calculations indicate that the present mean melt rate of 0 6 m yr 1 would increase to 2 6 m yr 1 if the underlying water were to warm by 0 6 c this would not only lead to significant thinning of the ice shelf but could also cause a profound change in ocean circulation on the open continental shelf

dense waters of the weddell and scotia seas recent changes in properties and circulation the densest waters in the atlantic overturning circulation are sourced at the periphery of antarctica especially the weddell sea and flow northward via routes that involve crossing the complex bathymetry of the scotia arc recent observations of significant warming of these waters along much of the length of the atlantic have highlighted the need to identify and understand the time varying formation and export processes and the controls on their properties and flows here we review recent developments in understanding of the processes that control the changing flux of water through the main export route from the weddell sea into the scotia sea and the transformations of the waters within the scotia sea and environs we also present a synopsis of recent findings that relate to the climatic change of dense water properties within the weddell sea itself in the context of known atlantic scale changes among the most significant findings are the discovery that the warming of waters exported from the weddell sea has been accompanied by a significant freshening and that the episodic nature of the overflow into the scotia sea is markedly wind controlled and can lead to significantly enhanced abyssal stratification key areas for focusing future research effort are outlined

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