twenty first century warming of a large antarctic ice shelf cavity by a redirected coastal current the redirection of warm water under the filchner ronne ice shelf during the second half of this century could cause the ice shelf base to melt at a rate 20 times higher than at present warm ocean currents are known to erode ice shelves from below but changes in currents can be forced by many different mechanisms leading to uncertain outcomes this study highlights the vulnerability to climate change of a small antarctic coastal region which has potentially severe consequences for the mass balance of a large antarctic ice shelf hellmer et al use climate modelling to show that the projected loss of sea ice in the weddell sea east of the antarctic peninsula leads to an increase in wind stress which in turn accelerates a warm ocean current far underneath the vast filchner ronne ice shelf the authors predict that the increased warmth could increase melt by a factor of 20 with possible consequences for ice stream dynamics in the east antarctic ice sheet the antarctic ice sheet loses mass at its fringes bordering the southern ocean at this boundary warm circumpolar water can override the continental slope front reaching the grounding line1 2 through submarine glacial troughs and causing high rates of melting at the deep ice shelf bases3 4 the interplay between ocean currents and continental bathymetry is therefore likely to influence future rates of ice mass loss here we show that a redirection of the coastal current into the filchner trough and underneath the filchner ronne ice shelf during the second half of the twenty first century would lead to increased movement of warm waters into the deep southern ice shelf cavity water temperatures in the cavity would increase by more than 2 degrees celsius and boost average basal melting from 0 2 metres or 82 billion tonnes per year to almost 4 metres or 1 600 billion tonnes per year our results which are based on the output of a coupled ice ocean model forced by a range of atmospheric outputs from the hadcm35 climate model suggest that the changes would be caused primarily by an increase in ocean surface stress in the southeastern weddell sea due to thinning of the formerly consolidated sea ice cover the projected ice loss at the base of the filchner ronne ice shelf represents 80 per cent of the present antarctic surface mass balance6 thus the quantification of basal mass loss under changing climate conditions is important for projections regarding the dynamics of antarctic ice streams and ice shelves and global sea level rise

rapid magma ascent recorded by water diffusion profiles in mantle olivine mechanisms and rates of magma ascent play a critical role in eruption dynamics but remain poorly constrained phenomena water dissolved in mantle minerals as hydrogen and partitioned into the magma during ascent may provide clues to quantifying magma ascent rates prior to eruption we determined the dehydration profiles in olivine crystals from peridotite mantle xenoliths within the pali aike alkali basalt from patagonia chile the results demonstrate that the amount of water stored in the uppermost mantle has likely been underestimated due to water loss during transport using experimental diffusion data for hydrogen we estimate that the xenoliths reached the surface from 60 70 km depth in several hours a surprisingly rapid rise comparable to ascent rates for kimberlite magmas

complex evolving patterns of mass loss from antarctica s largest glacier pine island glacier has contributed more to sea level rise over the past four decades than any other glacier in antarctica model projections indicate that this will continue in the future but at conflicting rates some models suggest that mass loss could dramatically increase over the next few decades resulting in a rapidly growing contribution to sea level and fast retreat of the grounding line where the grounded ice meets the ocean other models indicate more moderate losses resolving this contrasting behaviour is important for sea level rise projections here we use high resolution satellite observations of elevation change since 2010 to show that thinning rates are now highest along the slow flow margins of the glacier and that the present day amplitude and pattern of elevation change is inconsistent with fast grounding line migration and the associated rapid increase in mass loss over the next few decades instead our results support model simulations that imply only modest changes in grounding line location over that timescale we demonstrate how the pattern of thinning is evolving in complex ways both in space and time and how rates in the fast flowing central trunk have decreased by about a factor five since 2007 thinning rates of pine island glacier antarctica s largest glacier are now highest in slowly flowing regions suggesting that future changes in the grounding line may be more modest than thought according to high resolution satellite data

fast recession of a west antarctic glacier satellite radar interferometry observations of pine island glacier west antarctica reveal that the glacier hinge line position retreated 1 2 0 3 kilometers per year between 1992 and 1996 which in turn implies that the ice thinned by 3 5 0 9 meters per year the fast recession of pine island glacier predicted to be a possible trigger for the disintegration of the west antarctic ice sheet is attributed to enhanced basal melting of the glacier floating tongue by warm ocean waters

ice shelf melt response to changing winds and glacier dynamics in the amundsen sea sector antarctica it has been suggested that the coastal southern ocean subsurface may warm over the 21st century in response to strengthening and poleward shifting winds with potential adverse effects on west antarctic glaciers however using a 1 128 ocean regional model that includes ice shelf cavities we find a more complex response to changing winds in the amundsen sea simulated offshore subsurface waters get colder under strengthened and poleward shifted winds representative of the sam projected trend the buoyancy driven circulation induced by ice shelf melt transports this cold offshore anomaly onto the continental shelf leading to cooling and decreased melt below 450 m in the vicinity of ice shelf fronts ekman pumping contributes to raise the isotherms in response to changing winds this effect overwhelms the horizontal transport of colder offshore waters at intermediate depths between 200 and 450 m and therefore increases melt rates in the upper part of the ice shelf cavities which reinforces the buoyancydriven circulation and further contributes to raise the isotherms then prescribing an extreme grounding line retreat projected for 2100 the total melt rates simulated underneath thwaites and pine island are multiplied by 2 5 such increase is explained by a larger ocean ice interface exposed to cdw which is then amplified by a stronger melt induced circulation along the ice draft our main conclusions are that 1 outputs from ocean models that do not represent ice shelf cavities e g cmip5 models should not be directly used to predict the thermal forcing of future ice shelf cavities 2 coupled ocean ice sheet models with a velocity dependent melt formulation are needed for future projections of glaciers experiencing a significant grounding line retreat

rocas verdes ophiolites southernmost south america remnants of progressive stages of development of oceanic type crust in a continental margin back arc basin

spatial and temporal evolution of pine island glacier thinning 1995 2006 we use ers 2 and envisat satellite radar altimetry to examine spatial and temporal changes in the rate of thinning of the pine island glacier west antarctica during the period 1995 to 2006 we show that the pattern of thinning has both accelerated and spread inland to encompass tributaries flowing into the central trunk of the glacier within the 5 400 km 2 central trunk the average rate of volume loss quadrupled from 2 6 0 3 km 3 yr 1 in 1995 to 10 1 0 3 km 3 yr 1 in 2006 the region of lightly grounded ice at the glacier terminus is extending upstream and the changes inland are consistent with the effects of a prolonged disturbance to the ice flow such as the effects of ocean driven melting if the acceleration continues at its present rate the main trunk of pig will be afloat within some 100 years six times sooner than anticipated citation wingham d j d w wallis and a shepherd 2009 spatial and temporal evolution of pine island glacier thinning 1995 2006 geophys res lett 36 l17501 doi 10 1029 2009gl039126

kinematic constraints on glacier contributions to 21st century sea level rise on the basis of climate modeling and analogies with past conditions the potential for multimeter increases in sea level by the end of the 21st century has been proposed we consider glaciological conditions required for large sea level rise to occur by 2100 and conclude that increases in excess of 2 meters are physically untenable we find that a total sea level rise of about 2 meters by 2100 could occur under physically possible glaciological conditions but only if all variables are quickly accelerated to extremely high limits more plausible but still accelerated conditions lead to total sea level rise by 2100 of about 0 8 meter these roughly constrained scenarios provide a most likely starting point for refinements in sea level forecasts that include ice flow dynamics

modelling circumpolar deep water intrusions on the amundsen sea continental shelf antarctica results are presented from an isopycnic coordinate model of ocean circulation in the amundsen sea focusing on the delivery of circumpolar deep water cdw to the inner continental shelf around pine island bay the warmest waters to reach this region are channeled through a submarine trough accessed via bathymetric irregularities along the shelf break temporal variability in the influx of cdw is related to regional wind forcing easterly winds over the shelf edge change to westerlies when the amundsen sea low migrates west and south in winter spring this drives seasonal on shelf flow while inter annual changes in the wind forcing lead to inflow variability on a decadal timescale a modelled period of warming following low cdw influx in the late 1980 s and early 1990 s coincides with a period of observed thinning and acceleration of pine island glacier

petrochemistry and recrystallization history of granulite xenoliths from the pali aike volcanic field chile mafic granulite facies xenoliths found in the pleistocene recent alkali basalts of southern patagonia south america have an unfoliated granoblastic matrix with the mineral assemblage clinopyroxene orthopyroxene plagioclase or olivine or titanomagnetite wherever olivine and plagioclase are in close proximity to one another they are separated by symplectic intergrowths of pyroxenes and spinel two pyroxene geothermometry indicates that the granoblastic matrix phases equilibrated at approximately 900 degrees c the pyroxene spinel symplectites formed at 860 degrees c at which temperature the reaction boundary of olivine plagioclase to produce pyroxenes spinel occurs at 5 7 kbar the xenoliths are interpreted as fragments of a gabbroic pluton which exists at a depth of approximately 20 km in the crust this is consistent with suggestions that mafic igneous rocks comprise a large portion of the lower continental crust capture of the xenoliths in the host basalt must have resulted in extensive heating but the petrologic evidence of this event is restricted to heterogeneously distributed high temperature oxidation effects modified journal abstract

ice shelf collapse from subsurface warming as a trigger for heinrich events episodic iceberg discharge events from the hudson strait ice stream hsis of the laurentide ice sheet referred to as heinrich events are commonly attributed to internal ice sheet instabilities but their systematic occurrence at the culmination of a large reduction in the atlantic meridional overturning circulation amoc indicates a climate control we report mg ca data on benthic foraminifera from an intermediate depth site in the northwest atlantic and results from a climate model simulation that reveal basin wide subsurface warming at the same time as large reductions in the amoc with temperature increasing by approximately 2 c over a 1 2 kyr interval prior to a heinrich event in simulations with an ocean model coupled to a thermodynamically active ice shelf the increase in subsurface temperature increases basal melt rate under an ice shelf fronting the hsis by a factor of approximately 6 by analogy with recent observations in antarctica the resulting ice shelf loss and attendant hsis acceleration would produce a heinrich event

sensitivity of 21st century sea level to ocean induced thinning of pine island glacier antarctica 1 pine island glacier pig antarctica is rapidly losing mass supporting arguments that it may play a major role in 21st century sea level rise yet this glacier s quantitative contribution to sea level based on theoretical and computational models is poorly known we have developed a basin scale glaciological model to examine the sensitivity of pig to a range of environmental forcings while oceanic melt likely played the leading role in recent thinning and retreat we find that the particular grounding line geometry with an extended ice plain in the 1990s made it susceptible to such forcing our model further indicates that while the rate of grounding line retreat should diminish soon the glacier s mass loss may continue at rates similar to or moderately elevated from the present while substantial our model derived maximum rate of 2 7 cm century is considerably smaller than previous heuristically derived bounds on the sea level contribution

coronas and high p veins in metagabbros of the kohistan island arc northern pakistan evidence for crustal thickening during cooling abstract the e w trending kohistan terrane in the nw himalaya is a sandwich of a magmatic arc between the collided karakoram asian and indian plates the southern part of the kohistan arc is principally made up of amphibolites derived from volcanic and plutonic rocks of early cretaceous age gabbroic relics in the amphibolites display calc alkaline character and their mineralogy is similar to low p plutonic rocks reported from modern and ancient island arcs the largest of these relics occurring along the southern margin of the amphibolite belt near khwaza khela is subcircular in outline and is about 1 km across it consists of cumulate gabbros and related rocks displaying a record of cooling and crustal thickening primary olivine and anorthite reacted to produce coronas consisting of two pyroxenes mg fe2 al spinel tschermakitic hornblende at about 800 c 5 5 7 5 kbar this thermotectonic event is of regional extent and may be related to the overthrusting of the karakoram plate onto the kohistan arc some 85 ma ago or even earlier later the gabbros were locally traversed by veins containing high p assemblages garnet kyanite zoisite paragonite oligoclase calcite scapolite and quartz chlorite corundum diopside formed in the range 510 600 c and 10 12 kbar these suggest further thickening and cooling of the crust before its uplift during the tertiary this paper presents microprobe data on the minerals and discusses the tectonic implications of the coronitic and vein assemblages in the gabbros

grounding line migration from 1992 to 2011 on petermann glacier north west greenland we use satellite radar interferometry to investigate changes in the location of the petermann glacier grounding line between 1992 and 2011 the grounding line location was identified in 17 quadruple difference interferograms produced from european remote sensing ers 1 2 data the most extensive time series assembled at any ice stream to date there is close agreement 20 6 cm between vertical displacement of the floating ice shelf and relative tide amplitudes simulated by the arctic ocean dynamics based tide model 5 aodtm 5 arctic tide model over the 19 a period the groundling line position varied by 470 m on average with a maximum range of 7 0 km observed on the north east margin of the ice stream although the mean range 2 8 km and variability 320 m of the grounding line position is considerably lower if the unusually variable north east sector is not considered our observations demonstrate that large isolated movements cannot be precluded thus sparse temporal records should be analysed with care the grounding line migration observed on petermann glacier is not significantly correlated with time r 2 0 22 despite reported ice shelf thinning and episodes of large iceberg calving which suggests that unlike other ice streams on the south west margin of the greenland ice sheet petermann glacier is dynamically stable

recent antarctic ice mass loss from radar interferometry and regional climate modelling observed estimates of ice losses in antarctica combined with regional modelling of ice accumulation in the interior suggest that east antarctica is close to a balanced mass budget but large losses of ice occur in the narrow outlet channels of west antarctic glaciers and at the northern tip of the antarctic peninsula

simulation of subice shelf melt rates in a general circulation model velocity dependent transfer and the role of friction two parameterizations of turbulent boundary layer processes at the interface between an ice shelf and the ocean beneath are investigated in terms of their impact on simulated melt rates and feedbacks the parameterizations differ in the transfer coefficients for heat and freshwater fluxes in their simplest form they are assumed constant and hence are independent of the velocity of ocean currents at the ice shelf base an augmented melt rate parameterization accounts for frictional turbulence via transfer coefficients that do depend on boundary layer current velocities via a drag law in simulations with both parameterizations for idealized as well as realistic cavity geometries under pine island ice shelf west antarctica significant differences in melt rate patterns between the velocity independent and velocity dependent formulations are found while patterns are strongly correlated to those of thermal forcing for velocity independent transfer coefficients melting in the case of velocity dependent coefficients is collocated with regions of high boundary layer currents in particular where rapid plume outflow occurs both positive and negative feedbacks between melt rates boundary layer temperature velocities and buoyancy fluxes are identified melt rates are found to increase with increasing drag coefficient inline image in agreement with plume model simulations but optimal values of cd inferred from plume models are not easily transferable uncertainties therefore remain both regarding simulated melt rate spatial distributions and magnitudes

steep reverse bed slope at the grounding line of the weddell sea sector in west antarctica the bed of the west antarctic ice sheet is in places more than 1 5 km below sea level1 2 it has been suggested that a positive ice loss feedback may occur when an ice sheet s grounding line retreats across a deepening bed1 2 3 applied to the west antarctic ice sheet this process could potentially raise global sea level4 by more than 3 m hitherto attention has focussed on changes at the siple coast5 6 7 and amundsen sea embayment8 9 10 sectors of west antarctica here we present radio echo sounding information from the ice sheet s third sector the weddell sea embayment that reveals a large subglacial basin immediately upstream of the grounding line the reverse bed slope is steep with about 400 m of decline over 40 km the basin floor is smooth and flat with little small scale topography that would delay retreat indicating that it has been covered with marine sediment5 11 and was previously deglaciated upstream of the basin well defined glacially carved fjords with bars at their mouths testify to the position of a former ice margin about 200 km inland from the present margin evidence so far suggests that the weddell sea sector of the west antarctic ice sheet has been stable but in the light of our data we propose that the region could be near a physical threshold of substantial change

mantle diversity beneath the colombian andes northern volcanic zone constraints from sr and nd isotopes in order to provide mantle and crustal constraints during the evolution of the colombian andes sr and nd isotopic studies were performed in xenoliths from the mercaderes region northern volcanic zone colombia xenoliths are found in the granatifera tuff a deposit of cenozoic age in which mantle and crustal derived xenoliths are present in bombs and fragments of andesites and lamprophyres compositions garnet bearing xenoliths are the most abundant mantle derived rocks but websterites garnet free xenoliths and spinel bearing peridotites are also present in minor amounts amphibolites pyroxenites granulites and gneisses represent the lower crustal xenolith assemblage isotopic signatures for the mantle xenoliths together with field petrographic mineral and whole rock chemistry and pressure temperature estimates suggest three main sources for these mantle xenoliths garnet free websterite xenoliths derived from a source region with low p and t 16 kbar 1065 c and morb isotopic signature 87sr 86sr ratio of 0 7030 and 143nd 144nd ratio of 0 5129 garnet bearing peridotite and websterite xenoliths derived from two different sources in the mantle i a source with intermediate p and t 29 35 kbar 1250 1295 c conditions similar to that of sub oceanic geotherm with an oib isotopic signature 87sr 86sr ratio of 0 7043 and 143nd 144nd ratio of 0 5129 and ii another source with p and t conditions similar to those of a sub continental geotherm 38 kbar 1140 1175 c and oib isotopic characteristics 87sr 86sr ratio 0 7041 and 143nd 144nd ratio 0 5135

scapolite phase equilibria and carbon isotopes constraints on the nature and distribution of co2 in the lower continental crust abstract recent applications of scapolite phase equilibria and carbon isotopes are reviewed in order to evaluate the role played by co2 in the genesis of granulites in the lowermost crust scapolite occurs mainly in mafic granulites from exposed high grade terranes most of which have had a prior upper crustal history before equilibration at 9 10 kbar and in high pressure 11 15 kbar xenoliths and orthogneisses which likely represent mantle derived material that has not interacted with supracrustal lithologies calculated equilibrium co2 activities are sufficiently high 0 7 to be consistent with the presence of a co2 rich fluid phase accounting for the low h2o activities in scapolite bearing granulites and some xenoliths co2 activities less than ca 0 5 or the absence of scapolite are consistent with fluid absent metamorphism the measured isotopic composition of carbon 13cpdb in scapolite is only a tight constraint on the source of carbon in scapolite for mafic xenoliths and high pressure orthogneisses carbon isotope values for these lithologies 7 to 2 overlap the range of values for co2 dissolved in or exsolved from basalts and are consistent with a magmatic origin for the co2 the carbon isotope composition of scapolite in granulites from exposed terranes 10 to 4 is more equivocal and may instead be related to pregranulite facies carbonate present in the rock minimum values of volumetric fluid rock calculated to account for the modal abundance and composition of scapolite in granulites on the order of 0 001 to 0 1 are much lower than those required for the desiccation of amphibolites to granulites 0 1 to 0 5 thus scapolite should form in response to infiltration of such large volumes of co2 if p t and bulk compositional constraints are met however only seven of twenty granulite occurrences contain primary carbonate bearing scapolite in mafic granulites it appears that co2 occurs in the lower crust only locally in a co2 rich fluid phase or dissolved in mafic melts and not on a pervasive scale or at the high volumetric fluid rock required the model of carbonic metamorphism

investigations of an ice plain in the mouth of pine island glacier antarctica we present newly acquired airborne radar data showing ice thickness and surface elevation for pine island glacier antarctica these data when combined with earlier measurements suggest the presence of a lightly grounded area immediately above the grounding line of pine island glacier we identify this region as a ice plain it lies close to the centre line of the glacier has an elevation above buoyancy of 28 km the upstream edge of the ice plain is defined by a coupling line the configuration of the ice plain implies that nearby thinning of the ice stream would result in substantial grounding line retreat we suggest that the grounding line retreat of pine island glacier observed between 1992 and 1996 probably commenced sometime after 1981

ocean forced ice shelf thinning in a synchronously coupled ice ocean model the first fully synchronous coupled ice shelf ocean model with a fixed grounding line and imposed upstream ice velocity has been developed using the mitgcm massachusetts institute of technology general circulation model unlike previous asynchronous approaches to coupled modeling our approach is fully conservative of heat salt and mass synchronous coupling is achieved by continuously updating the ice shelf thickness on the ocean time step by simulating an idealized warm water ice shelf we show how raising the pycnocline leads to a reduction in both ice shelf mass and back stress and hence buttressing coupled runs show the formation of a western boundary channel in the ice shelf base due to increased melting on the western boundary due to coriolis enhanced flow eastern boundary ice thickening is also observed this is not the case when using a simple depth dependent parameterized melt as the ice shelf has relatively thinner sides and a thicker central bulge for a given ice shelf mass ice shelf geometry arising from the parameterized melt rate tends to underestimate backstress and therefore buttressing for a given ice shelf mass due to a thinner ice shelf at the boundaries when compared to coupled model simulations

basal shear stress of the ross ice streams from control method inversions 1 we used control method inversions to determine the basal shear stress beneath the ross ice streams where new high resolution velocity data sets have recently become available the inversion algorithm was adapted from an earlier viscous bed algorithm to allow solution for the basal shear stress corresponding to a weak plastic bed we performed several experiments using synthetic data to determine the quality of the inversions these experiments indicate that with high quality surface elevation data e g errors 5 m the inversions are relatively robust with respect to errors in ice flow velocity and bed topography the inversions are consistent with seismic and borehole observations and indicate that the ross ice streams lie atop a bed that is nearly everywhere weak in contrast the tributaries feeding these ice streams overlie alternating patches of strong and weak bed

grounding line retreat of the west antarctic ice sheet from inner pine island bay ice loss from the marine based potentially unstable west antarctic ice sheet wais contributes to current sea level rise and may raise sea level by 3 3 m or even 5 m in the future over the past few decades glaciers draining the wais into the amundsen sea embayment ase have shown accelerated ice flow rapid thinning and fast retreat of the grounding line gl however the long term context of this ice loss is poorly constrained limiting our ability to accurately predict future wais behavior here we present a new chronology for wais retreat from the inner continental shelf of the eastern ase based on radiocarbon dates from three marine sediment cores the ages document a retreat of the gl to within 100 km of its modern position before ca 10 000 calibrated cal yr b p this early deglaciation is consistent with ages for gl retreat from the western ase our new data demonstrate that in contrast to the ross sea wais retreat from the ase shelf was largely complete by the start of the holocene our results further suggest either slow gl retreat from the inner ase shelf throughout the holocene or that any episodes of fast gl retreat must have been short lived thus today s rapid retreat may be exceptional during the holocene and may originate in recent changes in regional climate ocean circulation or ice sheet dynamics

constraining past accumulation in the central pine island glacier basin west antarctica using radio echo sounding the potential for future dynamical instability of pine island glacier west antarctica has been addressed in a number of studies but information on its past remains limited in this study we use airborne radio echo sounding res data acquired over pine island glacier to investigate past variations in accumulation pattern in the dataset a distinctive pattern of layers was identified in the central part of the glacier basin we use these layers as chronological identifiers in order to construct elevation maps of the internal stratigraphy the observed internal layer stratigraphy is then compared to calculated stratigraphy from a three dimensional ice flow model that has been forced with different accumulation scenarios the model results indicate that the accumulation pattern is likely to have changed at least twice since the deposition of the deepest identified layer additional res data linked to the byrd ice core provide an approximate timescale this timescale suggests that the layers were deposited at the beginning of or during the holocene period thus the widespread changes occurring in the coastal extent of thewest antarctic ice sheet at the end of the last glacial period could have been accompanied by changes in accumulation pattern

numerical modeling of ocean ice interactions under pine island bay s ice shelf 1 a two dimensional numerical model is used to simulate the dynamics of buoyant meltwater rich plumes flowing beneath the ice shelf occupying much of pine island bay west antarctica recent studies have shown that this ice shelf along with all others fringing the amundsen sea is thinning rapidly in the model both the coriolis effect and subshelf topography are important in controlling plume dynamics and the spatial distribution of ice melt melt is concentrated in a narrow zone within 20 km of the grounding line where steep subshelf slopes and access to warm ambient water allow melt rates to exceed 100 m yr 1 the plume generated by entrainment of ambient water into the meltwater in these areas is guided by the topography of the ice shelf underside and exits the ice shelf at three distinct outflow locations melt rates generated along the course of the plume are higher approximately 2 5 than rates elsewhere the model suggests that the observed ice shelf thinning rates could have resulted from a hypothetical instantaneous 0 25 c warming of the ambient water entrained by the plume a context for this value is provided by the 40 year warming trend documented by jacobs et al 2002 for circumpolar deep water in the nearby ross sea

remote sensing of glacier and ice sheet grounding lines a review abstract the precise positioning and long term monitoring of the grounding line forming the boundary between grounded and floating ice of marine ice sheets and tidewater glaciers is critical for assessing ice sheet glacier stability ice sheet glacier mass balance calculations and numerical ice modelling however mapping of the grounding line is a challenging task since it is a subglacial and transient feature remote sensing techniques do not map the grounding line directly but locate surface features in the grounding zone that serve as proxies for the true grounding line position the large variety of methods products and publications as well as an inconsistent use of terminology additionally complicate the topic we present the first detailed review of existing remote sensing techniques and data for grounding line mapping benefits and limitations of the different techniques and data sources are discussed and illustrated with examples we identify a present tradeoff between accuracy and both spatial coverage and temporal resolution of the derived grounding line products rather new methods like sentinel 1 sar synthetic aperture radar differential range offset tracking drot and pseudo crossover radar altimetry pcra offer the potential for recent measurements and to fill gaps in some areas nevertheless there is a high demand of new spaceborne radar sensors that provide time series of very short repeat pass data to enable regular grounding line monitoring with most accurate differential sar interferometry dinsar on a continental scale

rheological nonlinearity and flow instability in the deforming bed mechanism of ice stream motion contrary to what has recently been assumed in modeling the proposed deforming bed mechanism for the rapid motion of antarctic ice streams the rheology of water saturated till is probably highly nonlinear according to information from soil mechanics and preliminary experiments on till from the base of ice stream b the equivalent flow law exponent n is probably as high as 100 and the nonlinearities of the shear stress and effective pressure dependences are closely linked the high nonlinearity has important consequences for the deforming bed mechanism a flow system operating by this mechanism can be unstable as a result of feedback from the generation of basal water by shear heating of basal till the short term feedback effect is analyzed for a perturbation in a model ice stream in which the basal meltwater is transported through a distributed system of narrow gap conduits at the ice till interface although the analysis is approximate and some of the system parameters are poorly known the results suggest that the deforming bed mechanism is unstable for n 20 the apparent lack of such an instability in the currently active ice streams implies that their motion is controlled not by the deforming bed mechanism but by some other as yet unidentified mechanism

investigation of land ice ocean interaction with a fully coupled ice ocean model 1 model description and behavior 1 antarctic ice shelves interact closely with the ocean cavities beneath them with ice shelf geometry influencing ocean cavity circulation and heat from the ocean driving changes in the ice shelves as well as the grounded ice streams that feed them we present a new coupled model of an ice stream ice shelf ocean system that is used to study this interaction the model is capable of representing a moving grounding line and dynamically responding ocean circulation within the ice shelf cavity idealized experiments designed to investigate the response of the coupled system to instantaneous increases in ocean temperature show ice ocean system responses on multiple timescales melt rates and ice shelf basal slopes near the grounding line adjust in 1 2 years and downstream advection of the resulting ice shelf thinning takes place on decadal timescales retreat of the grounding line and adjustment of grounded ice takes place on a much longer timescale and the system takes several centuries to reach a new steady state during this slow retreat and in the absence of either an upward or downward sloping bed or long term trends in ocean heat content the ice shelf and melt rates maintain a characteristic pattern relative to the grounding line

ice sheet grounding line dynamics steady states stability and hysteresis 1 the ice sheet ice shelf transition zone plays an important role in controlling marine ice sheet dynamics as it determines the rate at which ice flows out of the grounded part of the ice sheet together with accumulation this outflow is the main control on the mass balance of the grounded sheet in this paper we verify the results of a boundary layer theory for ice flux in the transition zone against numerical solutions that are able to resolve the transition zone very close agreement is obtained and grid refinement in the transition zone is identified as a critical component in obtaining reliable numerical results the boundary layer theory confirms that ice flux through the grounding line in a two dimensional sheet shelf system increases sharply with ice thickness at the grounding line this result is then applied to the large scale dynamics of a marine ice sheet our principal results are that 1 marine ice sheets do not exhibit neutral equilibrium but have well defined discrete equilibrium profiles 2 steady grounding lines cannot be stable on reverse bed slopes and 3 marine ice sheets with overdeepened beds can undergo hysteresis under variations in sea level accumulation rate basal slipperiness and ice viscosity this hysteretic behavior can in principle explain the retreat of the west antarctic ice sheet following the last glacial maximum and may play a role in the dynamics of heinrich events

trends and connections across the antarctic cryosphere satellite observations have transformed our understanding of the antarctic cryosphere the continent holds the vast majority of earth s fresh water and blankets swathes of the southern hemisphere in ice reductions in the thickness and extent of floating ice shelves have disturbed inland ice triggering retreat acceleration and drawdown of marine terminating glaciers the waxing and waning of antarctic sea ice is one of earth s greatest seasonal habitat changes and although the maximum extent of the sea ice has increased modestly since the 1970s inter annual variability is high and there is evidence of longer term decline in its extent this paper discusses how antarctic ice has changed over recent decades and how these changes have been recorded in satellite observations

timing of recent accelerations of pine island glacier antarctica 1 we have used interferometric synthetic aperture radar insar data and sequential landsat imagery to identify and temporally constrain two acceleration events on pine island glacier pig these two events are separated by a period of at least seven years 1987 1994 the change in discharge between two flux gates indicates that the majority of the increase in discharge associated with the second acceleration originates well inland 80 km from the grounding line an analysis indicates that changes in driving stress consistent with observed thinning rates are sufficient in magnitude to explain much of the acceleration

thwaites glacier grounding line retreat influence of width and buttressing parameterizations major ice loss has recently been observed along coastal outlet glaciers of the west antarctic ice sheet mainly due to increased melting below the ice shelves however the behavior of this marine ice sheet is poorly understood leading to significant shortcomings in ice sheet models attempting to predict future sea level rise the stability of a marine ice sheet is controlled by the dynamics at the grounding line the boundary between the grounded ice stream and the floating ice shelf one of the largest contributors to current sea level rise is the fast flowing thwaites glacier which flows into the amundsen sea here we use an ice stream ice shelf model and perform a number of experiments along a central flowline to analyze the sensitivity of its grounding line on centennial timescales in the absence of width and buttressing effects we find that the grounding line retreats by 300 km in 200 years from the present day rate of 1 5 km a 1 with variable glacier width implemented in the model flow convergence slows the retreat of thwaites grounding line at 0 3 1 2 km a 1 the parameterization of ice shelf buttressing according to different observed scenarios further reduces the glacier retreat and can even lead to a slight advance in the most buttressed case

subglacial bathymetry and sediment layer distribution beneath the pine island glacier ice shelf west antarctica modeled using aerogravity and autonomous underwater vehicle data

modeling ice dynamic contributions to sea level rise from the antarctic peninsula the future ice dynamical contribution to sea level rise slr from 199 ice shelf nourishing drainage basins of the antarctic peninsula ice sheet is simulated using the british antarctic survey antarctic peninsula ice sheet model simulations of the grounded ice sheet include response to ice shelf collapse estimated by tracking thermal ice shelf viability limits in 14 intergovernmental panel on climate change global climate models ensemble temperature projections grounding line retreat in response to ice shelf collapse is parameterized with a new multivariate linear regression model utilizing a range of glaciological and geometric predictor variables multimodel means project slr up to 9 4 mm sea level equivalent sle by 2200 and up to 19 mm sle by 2300 rates of slr from individual drainage basins throughout the peninsula are similar to 2100 yet diverge between 2100 and 2300 due to individual basin characteristics major contributors to slr are the outlet glaciers feeding southern george vi ice shelf accounting for 75 of total slr in some model runs ice sheet thinning induced by ice shelf removal is large up to 500 m especially in palmer land in the southern antarctic peninsula and may propagate as far as 135 km inland these results emphasize the importance of the ice dynamical contribution to future sea level of the apis on decadal to centennial timescales

terminus driven retreat of a major southwest greenland tidewater glacier during the early 19th century insights from glacier reconstructions and numerical modelling tidewater glaciers in greenland experienced widespread retreat during the last century information on their behaviour prior to this is often poorly constrained due to lack of observations while determining the drivers prior to instrumental records is also problematic here we present a record of the dynamics of kangiata nunaata sermia kns southwest greenland from its little ice age maximum liamax to 1859 the period before continuous air temperature observations began at nuuk in 1866 using glacial geomorphology historical accounts photographs and gis analyses we provide evidence kns was at its liamax by 1761 had retreated by 5 km by 1808 and a further 7 km by 1859 this predates retreat at jakobshavn isbrae by 43 113 years demonstrating the asynchroneity of tidewater glacier terminus response following the lia we use a one dimensional flowband model to determine the relative sensitivity of kns to atmospheric and oceanic climate forcing results demonstrate that terminus forcing rather than surface mass balance drove the retreat modelled glacier sensitivity to submarine melt rates is also insufficient to explain the retreat observed however moderate increases in crevasse water depth driving an increase in calving are capable of causing terminus retreat of the observed magnitude and timing

subglacial melt channels and fracture in the floating part of pine island glacier antarctica 1 a dense grid of ice penetrating radar sections acquired over pine island glacier west antarctica has revealed a network of sinuous subglacial channels typically 500 m to 3 km wide and up to 200 m high in the ice shelf base these subglacial channels develop while the ice is floating and result from melting at the base of the ice shelf above the apex of most channels the radar shows isolated reflections from within the ice shelf comparison of the radar data with acoustic data obtained using an autonomous submersible confirms that these echoes arise from open basal crevasses 50 100 m wide aligned with the subglacial channels and penetrating up to 1 3 of the ice thickness analogous sets of surface crevasses appear on the ridges between the basal channels we suggest that both sets of crevasses were formed during the melting of the subglacial channels as a response to vertical flexing of the ice shelf toward the hydrostatic condition finite element modeling of stresses produced after the formation of idealized basal channels indicates that the stresses generated have the correct pattern and if the channels were formed sufficiently rapidly would have sufficient magnitude to explain the formation of the observed basal and surface crevasse sets we conclude that ice shelf basal melting plays a role in determining patterns of surface and basal crevassing increased delivery of warm ocean water into the sub ice shelf cavity may therefore cause not only thinning but also structural weakening of the ice shelf perhaps as a prelude to eventual collapse

rapid subglacial erosion beneath pine island glacier west antarctica we present measurements of ice thickness gravimetry and surface elevation on pine island glacier west antarctica separated by a period of 49 years at one station on the main trunk of the glacier we measured a surface elevation lowering with no significant change in ice thickness we interpret these as indicating subglacial erosion of 31 8 13 4 m at this location at a mean rate over the measurement period of 0 6 0 3 m a 1 and suggest that a current erosion rate of 1 m a 1 is possible our results emphasize that locally basal processes can have a significant effect on ice sheet changes particularly where fast flowing ice has an easily erodible bed

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

tropical forcing of circumpolar deep water inflow and outlet glacier thinning in the amundsen sea embayment west antarctica outlet glaciers draining the antarctic ice sheet into the amundsen sea embayment ase have accelerated in recent decades most likely as a result of increased melting of their ice shelf termini by warm circumpolar deep water cdw an ocean model forced with climate reanalysis data shows that beginning in the early 1990s an increase in westerly wind stress near the continental shelf edge drove an increase in cdw inflow onto the shelf the change in local wind stress occurred predominantly in fall and early winter associated with anomalous high sea level pressure slp to the north of the ase and an increase in sea surface temperature sst in the central tropical pacific the slp change is associated with geopotential height anomalies in the middle and upper troposphere characteristic of a stationary rossby wave response to tropical sst forcing rather than with changes in the zonally symmetric circulation tropical pacific warming similar to that of the 1990s occurred in the 1940s and thus is a candidate for initiating the current period of ase glacier retreat

rapid bottom melting widespread near antarctic ice sheet grounding lines as continental ice from antarctica reaches the grounding line and begins to float its underside melts into the ocean results obtained with satellite radar interferometry reveal that bottom melt rates experienced by large outlet glaciers near their grounding lines are far higher than generally assumed the melting rate is positively correlated with thermal forcing increasing by 1 meter per year for each 0 1 c rise in ocean temperature where deep water has direct access to grounding lines glaciers and ice shelves are vulnerable to ongoing increases in ocean temperature

observations beneath pine island glacier in west antarctica and implications for its retreat thinning ice in west antarctica is currently contributing about 10 of the observed rise in global sea level observations obtained from an autonomous underwater vehicle operating beneath pine island glacier west antarctica reveal that the glacier was recently grounded on a transverse ridge in the sea floor but now warm sea water flows through the widening gap above the ridge thinning ice in west antarctica resulting from acceleration in the flow of outlet glaciers is at present contributing about 10 of the observed rise in global sea level1 pine island glacier in particular has shown nearly continuous acceleration2 3 and thinning4 5 throughout the short observational record the floating ice shelf that forms where the glacier reaches the coast has been thinning rapidly6 driven by changes in ocean heat transport beneath it as a result the line that separates grounded and floating ice has retreated inland7 these events have been postulated as the cause for the inland thinning and acceleration8 9 here we report evidence gathered by an autonomous underwater vehicle operating beneath the ice shelf that pine island glacier was recently grounded on a transverse ridge in the sea floor warm sea water now flows through a widening gap above the submarine ridge rapidly melting the thick ice of the newly formed upstream half of the ice shelf the present evolution of pine island glacier is thus part of a longer term trend that has moved the downstream limit of grounded ice inland by 30 km into water that is 300 m deeper than over the ridge crest the pace and ultimate extent of such potentially unstable retreat10 are central to the debate over the possibility of widespread ice sheet collapse triggered by climate change11 12

grounding line variability and subglacial lake drainage on pine island glacier antarctica we produced a 6 year time series of differential tidal displacement for pine island ice shelf antarctica using speckle tracking methods applied to fine resolution terrasar x data these results reveal that the main grounding line has maintained a relatively steady position over the last 6 years following the speedup that terminated in 2009 in the middle of the shelf there are grounded spots that migrate downstream over the 6 year record examination of high resolution digital elevation models reveals that these grounded spots form where deep keels thickness anomalies advect over an approximately flow parallel bathymetric high maintaining intermittent contact with the bed these data sets also reveal several subsidence and uplift events associated with subglacial lake drainages in the fast flowing region above the grounding line although these drainages approximately double the rate of subglacial water flow over periods of a few weeks they have no discernible effect on horizontal flow speed

antarctic grounding line mapping from differential satellite radar interferometry 1 the delineation of an ice sheet grounding line i e the transition boundary where ice detaches from the bed and becomes afloat in the ocean is critical to ice sheet mass budget calculations numerical modeling of ice sheet dynamics ice ocean interactions oceanic tides and subglacial environments here we present 15 years of comprehensive high resolution mapping of grounding lines in antarctica using differential satellite synthetic aperture radar interferometry dinsar data from the earth remote sensing satellites 1 2 ers 1 2 radarsat 1 and 2 and the advanced land observing system alos palsar for years 1994 to 2009 dinsar directly measures the vertical motion of floating ice shelves in response to tidal oceanic forcing with millimeter precision at a sample spacing better than 50 m simultaneously over areas several 100 km wide in contrast with earlier methods that detect abrupt changes in surface slope in satellite visible imagery or altimetry data on stagnant and slow moving areas we find that breaks in surface slope are reliable indicators of grounding lines but on most fast moving glaciers and ice streams our dinsar results reveal that prior mappings have positioning errors ranging from a few km to over 100 km a better agreement is found with icesat s data also based on measurements of vertical motion but with a detection noise one order of magnitude larger than with dinsar overall the dinsar mapping of antarctic grounding lines completely redefines the coastline of antarctica

inland thinning of pine island glacier west antarctica the pine island glacier pig transports 69 cubic kilometers of ice each year from 10 of the west antarctic ice sheet wais it is possible that a retreat of the pig may accelerate ice discharge from the wais interior satellite altimetry and interferometry show that the grounded pig thinned by up to 1 6 meters per year between 1992 and 1999 affecting 150 kilometers of the inland glacier the thinning cannot be explained by short term variability in accumulation and must result from glacier dynamics

oceanographic observations at the shelf break of the amundsen sea antarctica the part of the west antarctic ice sheet that drains into the amundsen sea is currently thinning at such a rate that it contributes nearly 10 of the observed rise in global mean sea level acceleration of the outlet glaciers appears to be caused by thinning at their downstream ends where the ice goes afloat indicating that the changes are probably being forced from the ocean observations made since the mid 1990s on the amundsen sea continental shelf have revealed that the deep troughs carved by previous glacial advances are flooded by almost unmodified circumpolar deep water cdw with temperatures around 3 4 c above the freezing point and that this water mass drives rapid melting of the floating ice here we report observations of water properties and currents made in the region where one of those troughs reaches the continental shelf edge we estimate the absolute circulation within the trough from a combination of detided acoustic doppler current profiler data and geostrophic shear derived from conductivity temperature depth sections the shelf edge region is characterised by a landward deepening of the pycnocline separating cdw from the overlying colder and fresher surface waters this feature the so called antarctic slope front asf is almost circumpolar in extent and is typically a full depth feature the pycnocline intersecting the seabed over the upper continental slope however the asf is weaker in the amundsen sea where it is rarely a full depth feature geostrophic shear associated with the asf leads to a weakening of the associated westward current with depth and an eastward undercurrent of varying strength has been reported at other locations at the time of our amundsen sea observations the westward surface flow was weak giving rise to a strong eastward undercurrent flowing along the continental shelf edge and upper slope at the upstream western side of the trough the undercurrent turns south driving a net on shelf flow of cdw in the western part of the trough and leaving a weakened shelf edge flow that re establishes itself on the downstream eastern side of the trough an analogous feature was captured albeit crudely in an earlier coarse resolution model of the circulation on the amundsen sea shelf and variability in its strength associated with variability in the surface wind stress was the main cause of variations in the heat content of the waters on the inner continental shelf our observations thus lend support to the earlier hypothesis that changes in atmospheric forcing over the continental shelf edge could be the ultimate driver of changes in the west antarctic ice sheet outlet glaciers that drain into the amundsen sea

ocean excited plate waves in the ross and pine island glacier ice shelves ice shelves play an important role in buttressing land ice from reaching the sea thus restraining the rate of grounded ice loss long period gravity wave impacts excite vibrations in ice shelves that can expand pre existing fractures and trigger iceberg calving to investigate the spatial amplitude variability and propagation characteristics of these vibrations a 34 station broadband seismic array was deployed on the ross ice shelf ris from november 2014 to november 2016 two types of ice shelf plate waves were identified with beamforming flexural gravity waves and extensional lamb waves below 20 mhz flexural gravity waves dominate coherent signals across the array and propagate landward from the ice front at close to shallow water gravity wave speeds 70 m s 1 in the 20 100 mhz band extensional lamb waves dominate and propagate at phase speeds 3 km s 1 flexural gravity and extensional lamb waves were also observed by a 5 station broadband seismic array deployed on the pine island glacier pig ice shelf from january 2012 to december 2013 with flexural wave energy also detected at the pig in the 20 100 mhz band considering the ubiquitous presence of storm activity in the southern ocean and the similar observations at both the ris and the pig ice shelves it is likely that most if not all west antarctic ice shelves are subjected to similar gravity wave excitation

damage accelerates ice shelf instability and mass loss in amundsen sea embayment pine island glacier and thwaites glacier in the amundsen sea embayment are among the fastest changing outlet glaciers in west antarctica with large consequences for global sea level yet assessing how much and how fast both glaciers will weaken if these changes continue remains a major uncertainty as many of the processes that control their ice shelf weakening and grounding line retreat are not well understood here we combine multisource satellite imagery with modeling to uncover the rapid development of damage areas in the shear zones of pine island and thwaites ice shelves these damage areas consist of highly crevassed areas and open fractures and are first signs that the shear zones of both ice shelves have structurally weakened over the past decade idealized model results reveal moreover that the damage initiates a feedback process where initial ice shelf weakening triggers the development of damage in their shear zones which results in further speedup shearing and weakening hence promoting additional damage development this damage feedback potentially preconditions these ice shelves for disintegration and enhances grounding line retreat the results of this study suggest that damage feedback processes are key to future ice shelf stability grounding line retreat and sea level contributions from antarctica moreover they underline the need for incorporating these feedback processes which are currently not accounted for in most ice sheet models to improve sea level rise projections

lower palaeogene stratigraphy of the northern north sea abstract the lower palaeogene stratigraphy of the northern north sea is described using wireline log and biostratigraphic data at group and formation level a close similarity can be shown with the stratigraphy defined in the outer moray firth area of the central north sea a two fold subdivision into the montrose and moray groups is recognized the montrose group consists of the maureen and lista formations while the succeeding moray group contains the sele balder dornoch and beauly formations the maureen and lista formations are dominantly argillaceous units that contain poorly defined sandstones the most laterally extensive of these is the heimdal member developed within the lista formation the dornoch and beauly formations are shelfal sandstone units that pass eastwards and northwards into the age equivalent dominantly argillaceous sele and balder formations the sele formation contains the hermod member while the balder formation contains the odin member these are massively developed sandstone units of very limited extent the lower palaeogene succession contains a number of high gamma log markers that are often associated with influxes of microfossils and microflora these are interpreted as condensed claystone horizons representing pauses in marine sedimentation during episodes of maximum transgression they can be correlated with similar marker horizons described from the outer moray firth area giving further confidence that the stratigraphy forms a regional framework that can be applied over the whole north sea area

interannual changes of the floating ice shelf of petermann gletscher north greenland from 2000 to 2012 petermann gletscher northwest greenland drains 4 of the greenland ice sheet into nares strait its floating ice shelf retreated from 81 to 48 km in length during two large calving events in 2010 and 2012 we document changes in the three dimensional ice shelf structure from 2000 to 2012 using repeated tracks of airborne laser altimetry and ice radio echo sounding icesat laser altimetry and modis visible imagery the recent ice shelf velocity measured by tracking surface features between flights in 2010 and 2011 is 1 25 km a 1 15 30 faster than estimates made before 2010 the steady state along flow ice divergence represents 6 3 gt a 1 mass loss through basal melting 5 gt a 1 and surface melting and sublimation 1 0 gt a 1 airborne laser altimeter data reveal thinning both along a thin central channel and on the thicker ambient ice shelf from 2007 to 2010 the ice shelf thinned by 5 m a 1 which represents a non steady mass loss of 4 1 gt a 1 we suggest that thinning in the basal channels structurally weakened the ice shelf and may have played a role in the recent calving events

sensitivity of pine island glacier west antarctica to changes in ice shelf and basal conditions a model study we use a finite element model of coupled ice stream ice shelf flow to study the sensitivity of pine island glacier west antarctica to changes in ice shelf and basal conditions by tuning a softening coefficient of the ice along the glacier margins and a basal friction coefficient controlling the distribution of basal shear stress underneath the ice stream we are able to match model velocity to that observed with interferometric synthetic aperture radar insar we use the model to investigate the effect of small perturbations on ice flow we find that a 5 5 13 reduction in our initial ice shelf area increases the glacier velocity by 3 5 10 at the grounding line the removal of the entire ice shelf increases the grounding line velocity by 70 the changes in velocity associated with ice shelf reduction are felt several tens of km inland alternatively a 5 reduction in basal shear stress increases the glacier velocity by 13 at the grounding line by contrast softening of the glacier side margins would have to be increased a lot more to produce a comparable change in ice velocity hence both the ice shelf buttressing and the basal shear stress contribute significant resistance to the flow of pine island glacier

fractionation and incipient self granulitization during deep crust emplacement of lower ordovician valle fertil batholith at the gondwana active margin of south america abstract large granite batholiths were emplaced at the gondwana active margin during lower ordovician in south america these have contributed to crustal growth by net addition of silicic rocks to the continental crust new u pb shrimp zircon age determinations together with thermobarometric and geochemical data yield that batholith magma intrusion is the responsible of heating and self granulitization of early gabbro pulses partially molten granulitic gabbros which appear as either early intruded into the metasedimentary host or as large inclusions within the batholith forming qtz diorites contain opx bearing trondhjemitic leucosomes surrounding hbl opx pl mafic mesosomes forming typical agmatitic structures hornblende plagioclase equilibria applied to mineral pairs of granoblastic aggregates in textural equilibrium of metagabbro mesosomes yield temperatures in the range 850 910 c for core to core pairs and in the range 1000 1075 c for rim to rim pairs at pressures of about 0 7 gpa shrimp zircon age revealed that the whole batholith was emplaced over a narrow time interval of 20 ma from 465 to 485 ma with most ages clustered at about 470 ma the age of metagabbros is 473 7 ma for older zircons and 454 4 for younger zircons these ages are almost coincident within error with the age of host migmatites 477 5 ma and those of batholith intrusion of 476 9 ma and 475 3 ma for qtz diorites and 475 5 ma for granites zircon overgrowths of these intrusive rocks yield sages clustered around 450 ma revealing a protracted thermal history more complex than previously believed the geochemical study reveals that qtz diorites tonalites and granodiorites form a continuous trend produced by magmatic fractionation from a parental dioritic magma a weak adakitic tendency with sr y 15 in several samples implies the presence of grt in the source or magma chamber at a minimum pressure of 1 0 gpa higher than the depth of emplacement at 0 7 gpa the high temperature of magma emplacement which induced the incipient self granulitization of early magmatic pulses together with the cotectic like fractionation linking coeval qtz diorites tonalites and granodiorites is compatible with fractionation at the lower crust of a deep generated infracrustal sublithospheric intermediate magma

uneven onset and pace of ice dynamical imbalance in the amundsen sea embayment west antarctica we combine measurements acquired by five satellite altimeter missions to obtain an uninterrupted record of ice sheet elevation change over the amundsen sea embayment west antarctica since 1992 using these data we examine the onset of surface lowering arising through ice dynamical imbalance and the pace at which it has propagated inland by tracking elevation changes along glacier flow lines surface lowering has spread slowest 6 km yr along the pope smith and kohler psk glaciers due to their small extent pine island glacier pig is characterized by a continuous inland spreading of surface lowering notably fast at rates of 13 to 15 km yr along tributaries draining the southeastern lobe possibly due to basal conditions or tributary geometry surface lowering on thwaites glacier thg has been episodic and has spread inland fastest 10 to 12 km yr along its central flow lines the current episodes of surface lowering started approximately 10 years before the first measurements on psk around 1990 on pig and around 2000 on thg ice dynamical imbalance across the sector has therefore been uneven during the satellite record

basal conditions for pine island and thwaites glaciers west antarctica determined using satellite and airborne data we use models constrained by remotely sensed data from pine island and thwaites glaciers west antarctica to infer basal properties that are difficult to observe directly the results indicate strong basal melting in areas upstream of the grounding lines of both glaciers where the ice flow is fast and the basal shear stress is large farther inland we find that both glaciers have mixed bed conditions with extensive areas of both bedrock and weak till in particular there are weak areas along much of pine island glacier s main trunk that could prove unstable if it retreats past the band of strong bed just above its current grounding line in agreement with earlier studies our forward ice stream model shows a strong sensitivity to small perturbations in the grounding line position these results also reveal a large sensitivity to the assumed bed sliding or deforming model with non linear sliding laws producing substantially greater dynamic response than earlier simulations that assume a linear viscous till rheology finally comparison indicates that our results using a plastic bed are compatible with the limited observational constraints and theoretical work that suggests an upper bound exists on maximum basal shear stress

fluid inclusions as petrogenetic indicators in granulite xenoliths pali aike volcanic field chile granulite xenoliths within alkali olivine basalts of the pali aike volcanic field southern chile contain the mineral assemblage orthopyroxene clinopyroxene plagioclase olivine green spinel these granulites are thought to be accidental inclusions of the lower crust incorporated in the mantle derived basalt during its rise to the surface symplectic intergrowths of pyroxene and spinel developed between olivine and plagioclase imply that the reaction olivine plagioclase al orthopyroxene al clinopyroxene spinel 1 occurred during subsolidus cooling and recrystallization of a gabbroic protolith of the granulites

cryospheric science vulnerable ice in the weddell sea of the west antarctic ice shelves those in the amundsen sea sector have given the most cause for concern ocean modelling of the weddell sea region together with a detailed survey of the ice bed morphology indicates that this region too may change soon

extensional fault systems in sedimentary basins a review of analogue model studies abstract scaled analogue models have provided graphic new insights into the progressive development of extensional fault systems this paper reviews recent results of an on going analogue modelling programme and compares the structural styles in the models with examples of natural extensional fault systems four fundamental model styles scaled to simulate deformation in brittle sedimentary rocks in the upper 10 km of the crust are reviewed extension above a basal detachment which undergoes stretching over a limited are produces an asymmetric rift graben bounded by planar faults and in which the internal deformation is accommodated by rotational domino faults these experimental results are directly comparable to structures found in intracontinental rifts extension above a basal detechment which undergoes extension under the whole model and which has no constraints at the ends produces domino style fault arrays in the pre rift sequence and listric growth faults in the syn rift sequence these results compare with highly extended rift basins and terranes such as the basin and range province usa extension above a simple listric fault produces a characteristic hangingwall roll over and crestal collapse graben system at high values of extension superposed crestal collapse grabens occur and a fan of listric growth faults is developed these structures bear striking similarities to growth faults in progradational delta systems ramp flat listric extensional fault systems using sand mica models produce a characteristic family of structures with an upper roll over crestal collapse system a ramp syncline and reverse fault fold zone and a lower roll over crestal collapse system an example of a ramp flat fault system from the gulf coast usa is presented as a comparison with the analogue model results preliminary results of three dimensional extensional model experiments are presented to illustrate the complexities in such fault systems in all of the experimental models it is found that the geometry of the underlying detachment system exerts fundamental control on structures produced in the hangingwall the analogue models illustrated in this review provide valid geometric and kinematic templates for the progressive evolution of extensional fault systems in sedimentary basins

changes in west antarctic ice stream dynamics observed with alos palsar data the advanced land observation system alos phased array synthetic aperture radar palsar is an l band frequency 1 27 ghz radar capable of continental scale interferometric observations of ice sheet motion here we show that palsar data yield excellent measurements of ice motion compared to c band 5 6 ghz radar data because of greater temporal coherence over snow and firn we compare palsar velocities from year 2006 in pine island bay west antarctica with those spanning years 1974 to 2007 between 1996 and 2007 pine island glacier sped up 42 and ungrounded over most of its ice plain smith glacier accelerated 83 and ungrounded as well their largest speed up are recorded in 2007 thwaites glacier is not accelerating but widening with time and its eastern ice shelf doubled its speed total ice discharge from these glaciers increased 30 in 12 yr and the net mass loss increased 170 from 39 15 gt yr to 105 27 gt yr longer term velocity changes suggest only a moderate loss in the 1970s as the glaciers unground into the deeper smoother beds inland the mass loss from this region will grow considerably larger in years to come copyright 2008 by the american geophysical union

reassessment of the potential sea level rise from a collapse of the west antarctic ice sheet theory has suggested that the west antarctic ice sheet may be inherently unstable recent observations lend weight to this hypothesis we reassess the potential contribution to eustatic and regional sea level from a rapid collapse of the ice sheet and find that previous assessments have substantially overestimated its likely primary contribution we obtain a value for the global eustatic sea level rise contribution of about 3 3 meters with important regional variations the maximum increase is concentrated along the pacific and atlantic seaboard of the united states where the value is about 25 greater than the global mean even for the case of a partial collapse

revised lower palaeogene lithostratigraphy for the outer moray firth north sea abstract the lower palaeogene lithostratigraphy of the outer moray firth has been revised using wireline log and biostratigraphic data a two fold subdivision into the montrose and moray groups is recognized the montrose group consists of the maureen and lista formations whereas the younger moray group contains the sele balder dornoch and beauly formations the lista formation contains the arenaceous andrew and balmoral members separated by the tuffaceous glamis member the dornoch and beauly formations are prograding shelfal sandstone units that pass basinwards into the age equivalent dominantly argillaceous sele and balder formations the sele formation contains the forties and cromarty members which are locally developed sand rich units stratigraphic analysis has led to the identification of a number of consistently developed high gamma log markers that are interpreted as condensed mudstone horizons these markers are believed to record clastic sediment starvation during periods of maximum marine transgression they often have a strong biostratigraphic fingerprint coinciding with influxes of rich concentrations of microfossils and microflora they are interpreted as the product of synchronous regional transgressive events and have been an important aid to the development of a confident lithostratigraphic framework for the lower palaeogene of the outer moray firth