grounding line movement and ice shelf buttressing in marine ice sheets 1 understanding the dynamics of marine ice sheets is integral to studying the evolution of the antarctic ice sheet in both the short and long terms an important component of the dynamics grounding line migration has proved difficult to represent in numerical models and most successful attempts have made use of techniques that are only readily applicable to flow line models however to capture the stress transmission involved in another important component the buttressing of a marine ice sheet by its ice shelf the transverse direction must also be resolved we introduce a model that solves the time dependent shelfy stream equations and makes use of mesh adaption techniques to overcome the difficulties typically associated with the numerics of grounding line migration we compare the model output with a recent benchmark for flow line models and show that our model yields an accurate solution while using far less resources than would be required without mesh adaption we also show that the mesh adapting techniques extend to two horizontal dimensions experiments are carried out to determine how both ice shelf buttressing and ice rises affect the marine instability predicted for an ice sheet on a foredeepened bed we find that buttressing is not always sufficient to stabilize such a sheet but collapse of the grounded portion is still greatly delayed we also find that the effect of an ice rise is similar to that of narrowing the ice shelf

volume loss from antarctic ice shelves is accelerating the floating ice shelves surrounding the antarctic ice sheet restrain the grounded ice sheet flow thinning of an ice shelf reduces this effect leading to an increase in ice discharge to the ocean using 18 years of continuous satellite radar altimeter observations we have computed decadal scale changes in ice shelf thickness around the antarctic continent overall average ice shelf volume change accelerated from negligible loss at 25 64 cubic kilometers per year for 1994 2003 to rapid loss of 310 74 cubic kilometers per year for 2003 2012 west antarctic losses increased by 70 in the past decade and earlier volume gain by east antarctic ice shelves ceased in the amundsen and bellingshausen regions some ice shelves have lost up to 18 of their thickness in less than two decades

sustained increase in ice discharge from the amundsen sea embayment west antarctica from 1973 to 2013 we combine measurements of ice velocity from landsat feature tracking and satellite radar interferometry and ice thickness from existing compilations to document 41 years of mass flux from the amundsen sea embayment ase of west antarctica the total ice discharge has increased by 77 since 1973 half of the increase occurred between 2003 and 2009 grounding line ice speeds of pine island glacier stabilized between 2009 and 2013 following a decade of rapid acceleration but that acceleration reached far inland and occurred at a rate faster than predicted by advective processes flow speeds across thwaites glacier increased rapidly after 2006 following a decade of near stability leading to a 33 increase in flux between 2006 and 2013 haynes smith pope and kohler glaciers all accelerated during the entire study period the sustained increase in ice discharge is a possible indicator of the development of a marine ice sheet instability in this part of antarctica key points sustained ase mass flux increase 77 since 1973 thwaites accelerated by 33 during the last 6 years speed changes are pervasive and rapid major implications for ice flow modeling 2014 american geophysical union all rights reserved

modeling the instantaneous response of glaciers after the collapse of the larsen b ice shelf following the disintegration of the larsen b ice shelf antarctic peninsula in 2002 regular surveillance of its 20 tributary glaciers has revealed a response which is varied and complex in both space and time the major outlets have accelerated and thinned smaller glaciers have shown little or no change and glaciers flowing into the remnant scar inlet ice shelf have responded with delay in this study we present the first area wide numerical analysis of glacier dynamics before and immediately after the collapse of the ice shelf combining new datasets and a state of the art numerical ice flow model we simulate the loss of buttressing at the grounding line and find a good qualitative agreement between modelled changes in glacier flow and observations through this study we seek to improve confidence in our numerical models and their ability to capture the complex mechanical coupling between floating ice shelves and grounded ice

satellite radar interferometry for monitoring ice sheet motion application to an antarctic ice stream satellite radar interferometry sri provides a sensitive means of monitoring the flow velocities and grounding line positions of ice streams which are indicators of response of the ice sheets to climatic change or internal instability the detection limit is about 1 5 millimeters for vertical motions and about 4 millimeters for horizontal motions in the radar beam direction the grounding line detected by tidal motions where the ice goes afloat can be mapped at a resolution of 0 5 kilometer the sri velocities and grounding line of the rutford ice stream antarctica agree fairly well with earlier ground based data the combined use of sri and other satellite methods is expected to provide data that will enhance the understanding of ice stream mechanics and help make possible the prediction of ice sheet behavior

deeply incised submarine glacial valleys beneath the greenland ice sheet the bed topography beneath the greenland ice sheet controls the flow of ice and its discharge into the ocean a combination of sparse radar soundings of ice thickness and high resolution ice motion data suggest that many submarine ice covered valleys extend significantly deeper below sea level and farther inland than thought

representation of sharp rifts and faults mechanics in modeling ice shelf flow dynamics application to brunt stancomb wills ice shelf antarctica ice shelves play a major role in buttressing ice sheet flow into the ocean hence the importance of accurate numerical modeling of their stress regime commonly used ice flow models assume a continuous medium and are therefore complicated by the presence of rupture features crevasses rifts and faults that significantly affect the overall flow patterns here we apply contact mechanics and penalty methods to develop a new ice shelf flow model that captures the impact of rifts and faults on the rheology and stress distribution of ice shelves the model achieves a best fit solution to satellite observations of ice shelf velocities to infer the following 1 a spatial distribution of contact and friction points along detected faults and rifts 2 a more realistic spatial pattern of ice shelf rheology and 3 a better representation of the stress balance in the immediate vicinity of faults and rifts thus applying the model to the brunt stancomb wills ice shelf antarctica we quantify the state of friction inside faults and the opening rates of rifts and obtain an ice shelf rheology that remains relatively constant everywhere else on the ice shelf we further demonstrate that better stress representation has widespread application in examining aspects affecting ice shelf structure and dynamics including the extent of ice melange in rifts and the change in fracture configurations all are major applications for better insight into the important question of ice shelf stability

a spatially adjusted elevation model in dronning maud land antarctica based on differential sar interferometry in this paper a new digital elevation model dem is derived for the ice sheet in western dronning maud land antarctica it is based on differential interferometric synthetic aperture radar sar from the european remote sensing 1 2 ers 1 2 satellites in combination with icesat s geoscience laser altimeter system glas a dem mosaic is compiled out of 116 scenes from the ers 1 ice phase in 1994 and the ers 1 2 tandem mission between 1996 and 1997 with the glas data acquired in 2003 that served as ground control using three different sar processors uncertainties in phase stability and baseline model resulting in height errors of up to 20 m are exemplified atmospheric influences at the same order of magnitude are demonstrated and corresponding scenes are excluded for validation of the dem mosaic covering an area of about 130 000 km2 on a 50 m grid independent icesat heights 2004 2007 ground based kinematic gps 2005 and airborne laser scanner data als 2007 are used excluding small areas with low phase coherence the dem differs in mean and standard deviation by 0 5 10 1 1 1 6 4 and 3 1 4 0 m from icesat gps and als respectively the excluded data points may deviate by more than 50 m in order to suppress the spatially variable noise below a 5 m threshold 18 of the dem area is selectively averaged to a final product at varying horizontal spatial resolution apart from mountainous areas the new dem outperforms other currently available dems and may serve as a benchmark for future elevation models such as from the tandem x mission to spatially monitor ice sheet elevation

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

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

ephemeral grounding as a signal of ice shelf change interferometric synthetic aperture radar insar observations of ice shelf tidal deformation reveal the wide transition between grounded and floating ice as well as local areas where the ice shelf is only grounded at low tide a condition that we call ephemeral grounding ephemeral grounding creates a subtle local disturbance on the vertical motion field of the ice shelf surface in response to changes in occanic tide which is detected with millimetric precision using insar these ice shelf features are however not expected to produce a noticeable disturbance on the ice shelf velocity field to illustrate the influence of ephemeral grounding on ice shelf creep flow we use a finite element model in which ephemeral grounding is incorporated through a variable basal friction coefficient the results show that while ice rises permanently grounded areas have a pronounced influence on the ice shelf velocity field areas of ephemeral grounding have a vanishingly small influence what is thus of most interest is the capacity for observations of ephemeral grounding to reveal subtle changes in ice shelf thickness over time we discuss an example in the thwaites glacier area west antarctica where multi year data show how ice rises become ephemeral grounding and subsequently disappear this result is consistent with the grounding line retreat and ice thinning of thwaites glacier

glacier acceleration and thinning after ice shelf collapse in the larsen b embayment antarctica 1 ice velocities derived from five landsat 7 images acquired between january 2000 and february 2003 show a two to six fold increase in centerline speed of four glaciers flowing into the now collapsed section of the larsen b ice shelf satellite laser altimetry from icesat indicates the surface of hektoria glacier lowered by up to 38 6 m in a six month period beginning one year after the break up in march 2002 smaller elevation losses are observed for crane and jorum glaciers over a later 5 month period two glaciers south of the collapse area flask and leppard show little change in speed or elevation seasonal variations in speed preceding the large post collapse velocity increases suggest that both summer melt percolation and changes in the stress field due to shelf removal play a major role in glacier dynamics

differential subduction and exhumation of crustal slices in the sulu hp uhp metamorphic terrane insights from mineral inclusions trace elements u pb and lu hf isotope analyses of zircon in orthogneiss based on new evidence the sulu orogen is divided from south east to north west into high pressure hp crustal slice i and ultrahigh pressure uhp crustal slices ii and iii a combined set of mineral inclusions cathodoluminescence images u pb shrimp dating and in situ trace element and lu hf isotope analyses was obtained on zircon from orthogneisses of the different slices zircon grains typically have three distinct domains that formed during crystallization of the magmatic protolith hp or uhp metamorphism and late amphibolite facies retrogression respectively i oscillatory zoned cores with low pressure lp mineral inclusions and th u 0 38 ii high luminescent mantles th u 0 10 with hp mineral inclusions of qtz grt arg phe ap for slice i zircon and coe grt phe kfs ap for both slices ii and iii zircon iii low luminescent rims with lp mineral inclusions and th u 0 08 zircon u pb shrimp analyses of inherited cores point to protolith ages of 785 770 ma in all seven orthogneisses the ages recorded for uhp metamorphism and subsequent retrogression in slice ii zircon c 228 and c 215 ma respectively are significantly older than those of slice iii zircon c 218 and c 202 ma respectively while slice i zircon recorded even older ages for hp metamorphism and subsequent retrogression c 245 and c 231 ma respectively moreover ar ar biotite ages from six paragneisses interpreted as dating amphibolite facies retrogression gradually decrease from hp slice i c 232 ma to uhp slice ii c 215 ma and uhp slice iii c 203 ma the combined data set suggests decreasing ages for hp or uhp metamorphism and late retrogression in the sulu orogen from south east to north west thus the hp uhp units are interpreted to represent three crustal slices which underwent different subduction and exhumation histories slice i was detached from the continental lithosphere at 55 65 km depth and subsequently exhumed while subduction of the underlying slice ii continued to 100 120 km depth uhp before detachment and exhumation slice iii experienced a similar geodynamic evolution as slice ii however both uhp metamorphism and subsequent exhumation took place c 10 myr later magmatic zircon cores from two types of orthogneiss in uhp slices ii and iii show similar mid neoproterozoic crystallization ages but have contrasting hf isotope compositions ehf 785 2 7 to 2 2 and 17 3 to 11 1 respectively suggesting their formation from distinct crustal units mesoproterozoic and paleoproterozoic to archean respectively during the breakup of rodinia the uhp and the retrograde zircon domains are characterized by lower th u and 176 lu 177 hf but higher 176 hf 177 hf t than the neoproterozoic igneous cores the similarity between uhp and retrograde domains indicates that late retrogression did not significantly modify chemical and isotopic composition of the uhp metamorphic system

impact of bedrock description on modeling ice sheet dynamics recent glaciological surveys have revealed a significant increase of ice discharge from polar ice caps into the ocean in parallel ice flow models have been greatly improved to better reproduce current changes and forecast the future behavior of ice sheets for these models surface topography and bedrock elevation are crucial input parameters that largely control the dynamics and the ensuing overall mass balance of the ice sheet for obvious reasons of inaccessibility only sparse and uneven bedrock elevation data is available this raw data is processed to produce digital elevation models dems on a regular 5 km grid these dems are used to constrain the basal boundary conditions of all ice sheet models here by using a full stokes finite element code we examine the sensitivity of an ice flow model to the accuracy of the bedrock description in the context of short term ice sheet forecast we show that in coastal regions the bedrock elevation should be known at a resolution of the order of one kilometer conversely a crude description of the bedrock in the interior of the continent does not affect modeling of the ice outflow into the ocean these findings clearly indicate that coastal regions should be prioritized during future geophysical surveys they also indicate that a paradigm shift is required to change the current design of dems describing the bedrock below the ice sheets they must give users the opportunity to incorporate high resolution bedrock elevation data in regions of interest

the control of an uncharted pinning point on the flow of an antarctic ice shelf antarctic ice shelves are buttressed by numerous pinning points attaching to the otherwise freely floating ice from below some of these kilometric scale grounded features are unresolved in antarctic wide datasets of ice thickness and bathymetry hampering ice flow models to fully capture dynamics at the grounding line and upstream we investigate the role of an 8 7 km 2 pinning point at the front of the roi baudouin ice shelf east antarctica using ers interferometry and alos palsar speckle tracking we derive on a 125 m grid spacing surface velocities deviating by 5 2 4 5 m a 1 from 37 on site global navigation satellite systems derived velocities we find no evidence for ice flow changes on decadal time scales and we show that ice on the pinning point virtually stagnates deviating the ice stream and causing enhanced horizontal shearing upstream using the bisicles ice flow model we invert for basal friction and ice rigidity with three input scenarios of ice velocity and geometry we show that inversion results are the most sensitive to the presence absence of the pinning point in the bathymetry surface velocities at the pinning point are of secondary importance undersampling of pinning points results in erroneous ice shelf properties in models initialised by control methods this may impact prognostic modelling for ice sheet evolution in the case of unpinning

amphibole asbestos soil contamination in the u s a a matter of definition recent concerns surrounding asbestos exposure have extended from occupational settings into natural settings these developments have caused us to examine the distribution of amphiboles which are potential asbestiform minerals within the soils of the u s a evaluation of mineralogical data from selected sand and or silt fraction of soils from the usda nrcs national cooperative soil survey database shows that soils in all states except for rhode island contain amphiboles in 41 of the 50 states 10 or more sampled pedons contain amphiboles overall 4396 pedons out of the 34 326 pedons about 13 sampled in the u s a contained amphiboles pedons containing amphiboles ranged from less than 1 to 49 of the pedons among all states while amphibole asbestos deposits occur in mafic and ultramafic provinces soil amphiboles occur evenly distributed across the u s a the majority of the amphiboles found in the soils would likely not meet the mineralogical definition of asbestos i e they would not have been derived from asbestiform amphiboles however the majority would likely meet a commonly used regulatory definition of a fiber i e are over 5 m in length with a greater than 3 to 1 aspect ratio based on the regulatory definition 13 of soil pedons and 5 of soil horizons in the u s a are naturally contaminated

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

ice flow of the antarctic ice sheet we present a reference comprehensive high resolution digital mosaic of ice motion in antarctica assembled from multiple satellite interferometric synthetic aperture radar data acquired during the international polar year 2007 to 2009 the data reveal widespread patterned enhanced flow with tributary glaciers reaching hundreds to thousands of kilometers inland over the entire continent this view of ice sheet motion emphasizes the importance of basal slip dominated tributary flow over deformation dominated ice sheet flow redefines our understanding of ice sheet dynamics and has far reaching implications for the reconstruction and prediction of ice sheet evolution

basal friction of ice stream e west antarctica we use surface velocity derived from sequential landsat imagery and a control method to estimate the basal friction distribution of a major west antarctic ice stream the area averaged basal stress is approximately 1 4 10 4 pa or about 29 of the area averaged driving stress of 4 9 10 4 pa uncertainty of the derived area averaged basal stress is difficult to assess and depends primarily on spatial variation of the flow law rate factor in the constitutive law spatial variation associated with depth averaged temperature variation gives an uncertainty of approximately 10 3 pa approximately 60 of the ice stream has a basal stress magnitude less than 10 4 pa and approximately 30 has less than 10 3 pa these characteristics suggest the presence of a mechanically weak water charged subglacial till small scale sticky spots where basal friction exceeds the area averaged driving stress are scattered irregularly across the subglacial regime and comprise approximately 15 of the ice stream area sticky spots cluster in regions where landsat imagery suggests structural features in the underlying bedrock

antarctic ice rises and rumples their properties and significance for ice sheet dynamics and evolution locally grounded features in ice shelves called ice rises and rumples play a key role buttressing discharge from the antarctic ice sheet and regulating its contribution to sea level ice rises typically rise several hundreds of meters above the surrounding ice shelf shelf flow is diverted around them on the other hand shelf ice flows across ice rumples which typically rise only a few tens of meters above the ice shelf ice rises contain rich histories of deglaciation and climate that extend back over timescales ranging from a few millennia to beyond the last glacial maximum numerical model results have shown that the buttressing effects of ice rises and rumples are significant but details of processes and how they evolve remain poorly understood fundamental information about the conditions and processes that cause transitions between floating ice shelves ice rises and ice rumples is needed in order to assess their impact on ice sheet behavior targeted high resolution observational data are needed to evaluate and improve prognostic numerical models and parameterizations of the effects of small scale pinning points on grounding zone dynamics

evolution of derwael ice rise in dronning maud land antarctica over the last millennia ice rises situated in the ice shelf belt around antarctica have a spatially confined flow regime with local ice divides beneath the divides ice stratigraphy often develops arches with amplitudes that record the divide s horizontal residence time and surface elevation changes to investigate the evolution of derwael ice rise dronning maud land antarctica we combine radar and gps data from three consecutive surveys with a two dimensional full stokes thermomechanically coupled transient ice flow model we find that the surface mass balance smb is higher on the upwind and lower on the downwind slopes near the crest the smb is anomalously low and causes arches to form in the shallow stratigraphy observable by radar in deeper ice arches are consequently imprinted by both smb and ice rheology raymond effect the data show how arch amplitudes decrease as along ridge slope increases emphasizing that the lateral positioning of radar cross sections is important for the arch interpretation using the model with three rheologies isotropic with n 3 4 5 and anisotropic with n 3 we show that derwael ice rise is close to steady state but is best explained using ice anisotropy and moderate thinning our preferred albeit not unique scenario suggests that the ice divide has existed for at least 5000 years and lowered at approximately 0 03 m a 1 over the last 3400 years independent of the specific thinning scenario our modeling suggests that derwael ice rise has exhibited a local flow regime at least since the mid holocene

marine ice sheet collapse potentially under way for the thwaites glacier basin west antarctica resting atop a deep marine basin the west antarctic ice sheet has long been considered prone to instability using a numerical model we investigated the sensitivity of thwaites glacier to ocean melt and whether its unstable retreat is already under way our model reproduces observed losses when forced with ocean melt comparable to estimates simulated losses are moderate 1 mm per year of sea level rise collapse in the different simulations within the range of 200 to 900 years