antarctic ice sheet melting in the southeast pacific the first oceanographic measurements across a deep channel beneath the calving front of pine island glacier reveal a sub ice circulation driven by basal melting of 10 12 m yr 1 a salt box model described here gives a melt rate similar to that of ice balance and numerical models 5 50 times higher than averages for the george vi and ross ice shelves melting is fueled by relatively warm circumpolar deep water that floods the deep floor of the amundsen and bellingshausen sea continental shelves reaching the deep draft of this floating glacier a revised melt rate for ice shelves in the southeast pacific sector raises circumpolar ice shelf melting to 756 gt yr 1 given prior estimates of surface accumulation and iceberg calving this suggests that the antarctic ice sheet is currently losing mass to the ocean

change of methane production pathway with sediment depth in a lake on the tibetan plateau abstract methane production is the terminal step in the degradation of organic matter in most anoxic lake sediments it has been suggested that ch 4 production in lake sediments is dominated by hydrogenotrophic methanogenesis especially in deeper layers we investigated the vertical sediment profile from the surface to 50 cm depth in the sediment of bangong co an oligotrophic high altitude lake on the tibetan plateau we measured ch 4 production stable carbon isotopefractionation and the archaeal community structure we found that the methane production rates the acetate concentrations and the numbers of bacteria and archaea strongly decreased with sediment depth the enrichment factor e for hydrogenotrophic methanogenesis also decreased with depth while 13 c of acetate stayed fairly constant the contribution of hydrogenotrophic methanogenesis to total ch 4 production increased with depth from 36 to 100 analysis of terminal restriction fragment polymorphism t rflp of archaeal 16s rrna genes showed that the relative abundance of aceticlastic methanosaetaceae methanogens also decreased with depth disappearing completely at 50 cm depth our study firstly showed that the methanogenic pathway and the methanogenic archaeal community systematically changed with sediment depth in a high altitude lake probably controlled by the availability of easily degradable organic matter

tide model accuracy in the amundsen sea antarctica from radar interferometry observations of ice shelf motion 1 this study assesses the accuracy of tide model predictions in the amundsen sea sector of west antarctica tide model accuracy in this region is poorly constrained yet tide models contribute to simulations of ocean heat transfer and to the removal of tidal signals from satellite observations of ice shelves we use two satellite based interferometric synthetic aperture radar insar methods to measure the tidal motion of the dotson ice shelf at multiple epochs a single difference technique that measures tidal displacement and a double difference technique that measures changes in tidal displacement we use these observations to evaluate predictions from three tide models tpxo7 1 cats2008a opt and fes2004 all three models perform comparably well exhibiting root mean square deviations from the observations of 9 cm single difference technique and 10 cm double difference technique care should be taken in generalizing these error statistics because 1 the dotson ice shelf experiences relatively small semidiurnal tides and 2 our observations are not sensitive to all tidal constituents an error analysis of our insar based methods indicates measurement errors of 7 and 4 cm for the single and double difference techniques respectively a model based correction for the effect of fluctuations in atmospheric pressure yields an 6 improvement in the agreement between tide model predictions and observations this study suggests that tide model accuracy in the amundsen sea is comparable to other antarctic regions where tide models are better constrained these methods can be used to evaluate tide models in other remote antarctic waters

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

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

warm ocean is eroding west antarctic ice sheet 1 satellite radar measurements show that ice shelves in pine island bay have thinned by up to 5 5 m yr 1 over the past decade the pattern of shelf thinning mirrors that of their grounded tributaries the pine island thwaites and smith glaciers and ocean currents on average 0 5 c warmer than freezing appear to be the source the synchronised imbalance of the inland glaciers is the result of reduced lateral and basal tractions at their termini and the drawdown of grounded ice shows that antarctica is more sensitive to changing climates than was previously considered

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

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

observations of ice sheet motion in greenland using satellite radar interferometry we present interferometric observations of ice sheet motion in western greenland based on pairs of ers 1 synthetic aperture radar sar images these observations provide the first detailed regional view of ice motion associated with dynamically supported topography near the margin of an ice sheet the interferograms of this area are much more complicated than other interferograms of ice sheets presented to date we devote the largest part of this paper to explaining the source of the complexity in these interferograms a synthetic interferogram based on a simple model helps to illustrate the effects of different components of the ice velocity field in interferometric data and suggests a method for estimating the large scale ice velocity field from such data

ocean tides in the weddell sea new observations on the filchner ronne and larsen c ice shelves and model validation 1 ocean tides under the large weddell sea ice shelves are among the least well observed on earth here we present new spatially extensive observations of the vertical tidal motion of the filchner ronne and larsen c ice shelves using global positioning system gps data spanning a few weeks to years we pay particular attention to the major tidal constituents m2 s2 o1 k1 as well as important grace aliasing periods k2 and s1 we compare the estimated constituents with recent global and regional tide models and find that no single model is the most accurate across all constituents or ice shelves the root sum square errors are 7 8 cm cats2008a and tpxo7 2 and 11 12 cm got4 7 and fes2004 with the energetic m2 rmse 4 8 cm and s2 4 5 cm generally dominating these statistics the fes2004 k1 is particularly inaccurate near the larsen c ice shelf with errors approaching 20 cm meaning that grace release 4 estimates of mass change in the northern antarctic peninsula will be biased we find tidal energy at 3 4 5 6 and weakly at 7 cycles per day at all of our sites the largest amplitudes within these bands are at m4 mo3 and sp3 and approach 30 mm although significant spatial variations exist we show that they generally do not appear to originate in areas of reduced water column in ice shelf grounding zones comparing model estimates with our m4 ms4 and mn4 values shows that models do not accurately represent these terms

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

sediment carbon fate in phreatic karst part 1 conceptual model development abstract recent research has paid increased attention to quantifying the fate of carbon pools within fluvial networks but few if any studies consider the fate of sediment organic carbon in fluviokarst systems despite that karst landscapes cover 12 of the earth s land surface the authors develop a conceptual model of sediment carbon fate in karst terrain with specific emphasis upon phreatic karst conduits i e those located below the groundwater table that have the potential to trap surface derived sediment and turnover carbon to assist with their conceptual model development the authors study a phreatic system and apply a mixture of methods traditional and novel to karst studies including electrical resistivity imaging well drilling instantaneous velocimetry dye tracing stage recording discrete and continuous sediment and water quality sampling and elemental and stable carbon isotope fingerprinting results show that the sediment transport carrying capacity of the phreatic karst water is orders of magnitude less than surface streams during storm activated periods promoting deposition of fine sediments in the phreatic karst however the sediment transport carrying capacity is sustained long after the hydrologic event has ended leading to sediment resuspension and prolonged transport the surficial fine grained laminae occurs in the subsurface karst system but unlike surface streams the light limited conditions of the subsurface karst promotes constant heterotrophy leading to carbon turnover the coupling of the hydrological processes leads to a conceptual model that frames phreatic karst as a biologically active conveyor of sediment carbon that recharges degraded organic carbon back to surface streams for example fluvial sediment is estimated to lose 30 of its organic carbon by mass during a one year temporary residence within the phreatic karst it is recommended that scientists consider karst pathways when attempting to estimate organic matter stocks and carbon transformation in fluvial networks

ice shelf elevation changes due to atmospheric pressure variations the inverse barometer effect ibe is the isostatic response of ocean surface height to changes in atmospheric pressure p air at a rate of about 1 cm hpa 1 the ibe is a significant contributor to variability of ice shelf surface elevation ice as we demonstrate with simultaneous global positioning system measurements of ice and local measurements of p air from the amery brunt and ross ice shelves antarctica we find that an ibe correction is justified for frequencies covering the weather band 0 03 0 5 cpd cycles per day the ibe correction reduces the standard deviation of the weather band signal of ice from 9 cm to 3 cm with this correction the largest remaining high frequency error signal in ice is the inaccuracy of the present generation of antarctic tide models estimated to be of order 10 cm for most of antarctica

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

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

velocity map of the thwaites glacier catchment west antarctica the two dimensional surface velocity of thwaites glacier west antarctica was mapped with 23 ascending and 22 descending orbit european remote sensing satellite synthetic aperture radar ers sar interferograms time range 1995 2000 the velocity map covers 175 500 square km from the amundsen coast to the southern turning point of the satellite orbit and comprises over 80 of the thwaites catchment

atmospheric effects in interferometric synthetic aperture radar surface deformation and topographic maps interferogram images derived from repeat pass spaceborne synthetic aperture radar systems exhibit artifacts due to the time and space variations of atmospheric water vapor other tropospheric variations such as pressure and temperature also induce distortions but the effects are smaller in magnitude and more evenly distributed throughout the interferogram than the wet troposphere term spatial and temporal changes of 20 in relative humidity lead to 10 cm errors in deformation products and perhaps 100 m of error in derived topographic maps for those pass pairs with unfavorable baseline geometries in wet regions such as hawaii these are by far the dominant errors in the spaceborne imaging radar c and x band synthetic aperature radar sir c x sar interferometric products the unknown time delay from tropospheric distortion is independent of frequency and thus multiwavelength measurements such as those commonly used to correct radar altimeter and global positioning system gps ionospheric biases cannot be used to rectify the error in the topographic case the errors may be mitigated by choosing interferometric pairs with relatively long baselines as the error amplitude is inversely proportional to the perpendicular component of the interferometer baseline for the sir c x sar hawaii data we found that the best longest baseline pair produced a map supporting 100 m contouring whereas the poorest baseline choice yielded an extremely noisy topographic map even at this coarse contour interval in the case of deformation map errors the result is either independent of baseline parameters or else very nearly so here the only solution is averaging of independent interferograms so in order to create accurate deformation products in wet regions many multiple passes may be required rules for designing optimal data acquisition and processing sequences for interferometric analyses in nondesert parts of the world are 1 to use the longest radar wavelengths possible within ionospheric scintillation and faraday rotation limits 2 for topography maximize interferometer baseline within decorrelation limits and 3 for surface deformation use multiple observations and average the derived products following the above recipe yields accuracies of 10 m for digital elevation models and 1 cm for deformation maps even in very wet regions such as hawaii

ice velocity determined using conventional and multiple aperture insar abstract we combine conventional interferometric synthetic aperture radar insar and multiple aperture insar mai to determine the ice surface velocity on the langjokull and hofsjokull ice caps in iceland in 1994 this approach allows the velocity of the 20 principal ice cap outlet glaciers to be fully resolved we show that mai leads to displacement estimates of finer resolution 15 versus 150 m and superior precision 5 versus 15 cm to those afforded by the alternative technique of speckle tracking using sar data acquired in ascending and descending orbits we show that ice flows within 15 of the direction of maximum surface slope across 66 of the ice caps it is therefore possible to determine ice displacement over the majority of the ice caps using a single sar image pair thereby reducing errors associated with temporal fluctuations in ice flow

ice sheet motion and topography from radar interferometry both topography and motion information are present in repeat pass ers 1 interferograms over ice sheets the authors demonstrate that the topography is separable from the surface displacement field when a sequence of radar images are available if the velocity field is constant over the time span of observation the topography can be derived from differential interferograms formed from sequential observations with this measurement a pure displacement field can then be obtained by removal of the topographic contribution to the interferometric phase at each pixel further they discuss how the vertical and horizontal components of displacement affect the interferometrically derived motion field they illustrate their approach with four successive 3 day repeat ers 1 images of a flow feature in northeastern greenland

methanogenic pathways 13c isotope fractionation and archaeal community composition in lake sediments and wetland soils on the tibetan plateau 1 lakes are a significant source of atmospheric ch4 and play an important role in the global carbon cycle little information on methane production in high altitude lakes is available we determined the methanogenic archaeal community composition the methanogenic pathways and carbon isotope fractionation in lake sediments and wetland soils on the tibetan plateau at about 4000 m above sea level asl we measured ch4 production rates and the 13c of co2 ch4 and acetate in the presence and absence of methyl fluoride an inhibitor of acetotrophic methanogenesis in sediments of five lakes awong co bangong co gongzhu co daze co and ranwu and wetland soils adjacent to bangong co and ranwu methane in bangong co sediment and in the wetland soil near ranwu was mainly produced by acetotrophic methanogenesis whereas methane in the sediments of the two saline lakes awong co and gongzhu co was mainly generated by hydrogenotrophic methanogensis chemolithotrophic acetogenesis and methanol dependent methanogenesis may also have played a role the stable carbon isotope fractionation during ch4 production from co2 was relatively large average e 78 the methanogenic communities were similar to those found in lowland lake sediments but those of saline and nonsaline tibetan lakes differed hydrogenotrophic methanogens were dominant in lake sediments while acetotrophic methanogens were dominant in wetland soils our results revealed diversity in the methanogenic communities and their methanogenic pathways and indicated that they are affected by sediment characteristics such as salinity however the ch4 production rates ranging from 6 to 122 nmol day 1 g dry weight 1 showed no relationship to environmental characteristics and were not limited by microbial abundance

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

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

tidal motion ice velocity and melt rate of petermann gletscher greenland measured from radar interferometry over a floating glacier ice tongue or an ice shelf the glacier motion measured by a single repeat pass radar interferogram is difficult to analyze because the long term steady motion of the ice is intermixed with its cyclic downward motion induced by tidal forcing multiple interferograms and a quadruple difference technique are necessary to separate the tidal signal from the long term steady motion of the ice an example of application of this technique is given here using ers 1 radar images of petermann gletscher a major outlet glacier of northern greenland tidal displacements are measured with 5 mm statistical noise the long term ice velocity is measured with a precision of 1 m a 1 the inferred tidal displacements agree well with model predictions from a fixed elastic beam with an elastic damping factor of 0 47 0 01 km 1 the hinge line is mapped with a precision of 20 80 m combining the interferometric ice velocities with ice thickness data the glacier ice discharge is calculated at and below the hinge line at the hinge line the ice flux is 12 1 1 km3 a 1 at the ice front calf ice production is only 0 59 km3 a 1 meaning that 95 of the ice that crosses the grounding line melts before it reaches the calving front assuming steady state conditions the melt rate of the glacier tongue averages 12 1 m a 1 with peak values exceeding 20 m a 1 near the hinge line this high melt rate cannot be accommodated by surface ablation alone only about 2 3 m a 1 and is attributed to pronounced basal melting of the ice tongue basal melting often assumed to be negligible in greenland is the dominant process of mass release from the floating section of petermann gletscher

wetland evolution in the qinghai lake area china in response to hydrodynamic and eolian processes during the past 1100 years abstract the daotanghe riverine wetland in close proximity to the qinghai lake was investigated to demonstrate the interrelationships between qinghai lake hydrodynamic processes eolian mobility and ecological conditions during the past 1100 years in response to climate change we used ostracod assemblages from various sites east of qinghai lake and from the sediment core qw15 of daotanghe pond and combined them with grain size and geochemical data from the same core the statistical extraction of grain size endmembers em revealed three different transportation processes responsible for pond related fluvio lacustrine pure fluvial and eolian deposits identified seasonal effects eolian mobility phase and timing of ice cover are possible tracers for the competing influence between the asian summer monsoon and the westerlies in the daotanghe wetland and surrounding area our results show that ostracod associations and sediment properties are evidence of a fluvio lacustrine fresh water environment without ingression of qinghai lake into the wetland hydrodynamic variations coupled with phases of eolian input indicate highly variable water budgets in response to climate induced effective moisture supply the medieval warm period mwp until about 1270 ce displays generally moist and warm climate conditions with minor fluctuations likely in response to variations in summer monsoon intensity the three partite period of the little ice age lia shows hydrologically unstable conditions between 1350 and 1530 ce with remarkably colder periods assigned to a prolonged seasonal ice cover pond desiccation and replacement by fluvial deposits occurred between 1530 and 1750 ce superimposed by eolian deposits the phase 1730 1900 ce is recorded by the re occurrence of a pond environment with reduced eolian input principal component analysis pca on ostracod abundances shows similar trends all three phases of the lia developed during a weak summer monsoon influence favoring westerly derived climate conditions until ca 1850 ce in accordance with records from the adjacent regions seasonal freezing periods in excess of the average time of frozen water bodies also occurred in periods of the well known grand solar minima and indicate stronger seasonality possibly independent from variations in summer monsoon strength but with links to global northern hemispheric climate

distribution of bacterial and archaeal ether lipids in soils and surface sediments of tibetan lakes implications for gdgt based proxies in saline high mountain lakes bacterial and archaeal lipids such as glycerol dialkyl glycerol tetraethers gdgts and dialkyl glycerol diethers are increasingly used as proxies for specific environmental parameters such as air temperature and soil ph in lacustrine environments little is known however about the distribution and applicability of bacterial and archaeal lipids on the tibetan plateau we investigated nine different watersheds across the plateau by way of sediments from lakes and rivers as well as the surrounding soils our transect study included a salinity gradient and focused on saline lakes which are rarely examined we analyzed archaeal isoprenoid i and bacterial branched b gdgts as well as archaeol to trace their sources and environmental factors influencing their distributions we could show that igdgts were produced in situ and bgdgts were primarily soil derived although we could not exclude in situ production of bgdgts in the lakes the most important environmental variables correlating with gdgt distributions were temperature and salinity bacterial gdgt distributions correlated mainly with salinity while archaeal lipid distributions correlated with temperature based on the correlation of methylation mbt and cyclisation cbt indices of bgdgts with ph and mean annual air temperature maat we established local calibrations for the tibetan lakes tex86 could also be applied to reconstruct temperature which was strongly biased towards measured summer lake water temperature indicating enhanced production of igdgts in the summer months existing proxies show therefore potential for palaeoclimate reconstruction on the tibetan plateau if local calibrations are applied

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

ice dynamic conditions of shirase glacier antarctica inferred from ers sar interferometry the surface velocity field of shirase glacier a fast flowing east antarctic outlet glacier is determined from ers synthetic aperture radar sar images by means of speckle tracking using phase correlation a technique which matches small image kernels of two complex sar images by maximization of the local coherence velocity estimates are used to calculate surface strain rates which are then used to calculate the large scale vertically integrated force balance and to determine the major stress components resisting the driving stress for the whole glacier system the driving stress is largely balanced by the basal drag but with contributions from lateral drag up to 15 of the driving stress at the grounding line longitudinal stress gradients have only local importance to the balance of forces limited to an area of a few square kilometers near the grounding line where they resist the driving stress in the grounded part of the glacier 90 of the total ice velocity is due to basal sliding comparison with a balance flux distribution of the antarctic ice sheet suggests that the glacier in the downstream part of the shirase drainage basin is close to equilibrium showing a slight negative imbalance

factors controlling 13c contents of sedimentary organic compounds principles and evidence the carbon isotopic composition of any naturally synthesized organic compound depends on 1 the carbon source utilized 2 isotope effects associated with assimilation of carbon by the producing organism 3 isotope effects associated with metabolism and biosynthesis and 4 cellular carbon budgets these factors are reviewed and quantitative considerations summarized particularly with regard to active and passive modes of carbon assimilation by phytoplankton and the existence of discernible regularities in isotope effects associated with lipid biosynthesis it is concluded that n alkyl lipids are in general depleted in 13c by about 1 5 relative to polyisoprenoid lipids produced by the same organism the effects of biological reworking on isotopic compositions of organic carbon are examined and it is suggested that enrichment of 13c in sedimentary organic carbon may result from loss of ch4 from zooplanktonic gut communities isotopic methods for estimation of ancient co2 levels are considered and a hyperbolic form favored for the relationship between concentrations of dissolved co2 and isotopic fractionation

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

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

ice shelf tidal motion derived from ers altimetry 1 ocean tides beneath floating ice shelves can affect heat and water circulations within the subshelf cavity which may influence patterns of basal melting accurate tidal predictions are essential to eliminate vertical tide motions when mapping the flow speeds and thickness changes of floating ice global ocean tide solutions are typically accurate to 2 3 cm in amplitude however their coarse resolution limits their utility in coastal regions moreover the absence of satellite and extensive gauge station data in the polar regions has resulted in significant differences between recent tide model solutions around the antarctic continent we use 9 years of high resolution european remote sensing ers satellite radar altimeter range measurements and a modified form of the orthotide method to generate tidal solutions for a series of ice shelves around the antarctic peninsula we compare our tidal solution at george vi ice shelf on the western coast of the antarctic peninsula to direct measurements from british antarctic survey tide gauge records and contrast the results with solutions from the fes99 and kantha2 0 global and the cats01 02 antarctic ocean tide models the average root mean square vector difference between the tide gauge and tide model predictions of amplitude and phase was 10 9 17 1 4 2 and 15 4 cm for the ers ice shelf kantha2 0 cats01 02 and fes99 models respectively tidal predictions derived from the ers ice shelf model are closer to the tide gauge recordings than those of the global models and the inclusion of ers altimeter data may lead to improved global model performance

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

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

mapping ice shelf flow with interferometric synthetic aperture radar stacking interferometric synthetic aperture radar insar observations of ice shelf flow contain ocean tide and atmospheric pressure signals a model based correction can be applied but this method is limited by its dependency upon model accuracy which in remote regions can be uncertain here we describe a method to determine two dimensional ice shelf flow vectors independently of model predictions of tide and atmospheric pressure by stacking conventional and multiple aperture insar mai observations of the dotson ice shelf west antarctica in this way we synthesize a longer observation period which enhances long period flow displacement signals relative to rapidly varying tide and atmospheric pressure signals and noise we estimate the error associated with each component of the velocity field to be 22ma 1 which could be further reduced if more images were available to stack with the upcoming launch of several satellite missions offering the prospect of regular short repeat sar acquisitions this study demonstrates that stacking can improve estimates of ice shelf flow velocity

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

measurement of interseismic strain accumulation across the north anatolian fault by satellite radar interferometry in recent years interseismic crustal velocities and strains have been determined for a number of tectonically active areas through repeated measurements using the global positioning system the terrain in such areas is often remote and difficult and the density of gps measurements relatively sparse in principle satellite radar interferometry can be used to make millimetric precision measurements of surface displacement over large surface areas in practice the small crustal deformation signal is dominated over short time intervals by errors due to atmospheric topographic and orbital effects here we show that these effects can be overcome by stacking multiple interferograms after screening for atmospheric anomalies effectively creating a new interferogram that covers a longer time interval in this way we have isolated a 70 km wide region of crustal deformation across the eastern end of the north anatolian fault turkey the distribution of deformation is consistent with slip of 17 32 mm yr below 5 33 km on the extension of the surface fault at depth if the gps determined slip rate of 24 1 mm yr is accepted the locking depth is constrained to 18 6 km

loma prieta aftershock relocation with s p travel times effects of 3 d structure and true error estimates abstract aftershocks of the 18 october 1989 loma prieta california earthquake are located using s p arrival time measurements from stations of the passcal aftershock deployment we demonstrate the effectiveness of using s p arrival time data in locating earthquakes recorded by a sparse three component network events are located using the program quake3d nelson and vidale 1990 with both 2 d and 3 d velocity models that have been developed independently for this region the dense coverage of the area around the loma prieta rupture zone by instruments of the california network calnet has allowed the u s geological survey usgs to find p wave earthquake locations for both velocity models which we compare with our solutions we also perform synthetic calculations to estimate realistic location errors resulting from uncertainties in both the 3 d velocity structure and the timing of arrivals these calculations provide a comparison of location accuracies obtained using s p arrival times s and p arrival times and p times alone we estimate average absolute errors in epicentral location and in depth for the loma prieta aftershocks to be just under 2 km and 1 km respectively using s p phase data and the sparse passcal instrument coverage the synthetic tests show that these errors are much smaller than those predicted using p wave data alone and are nearly the same as those predicted using s and p phase data separately this suggests that future aftershock recording deployments with sparse networks of three component data can retrieve accurate event locations even if absolute timing is problematic we find moderate differences between our locations and those determined by the usgs from a larger network of stations however common characteristics in both seismicity patterns are apparent neither set of locations yields earthquake patterns that can be easily interpreted in terms of simple faulting geometries the absence of a simple pattern in both sets of earthquake locations indicates that this complexity is not the result of earthquake mislocation but is a genuine feature of the seismicity a deep southwesterly dipping plane and a near vertical fault extending from the surface to at least 7 km depth beneath the surface trace of the san andreas fault are imaged by both sets of earthquake locations although earthquake locations indicate the existence of many more fault segments the complexity of this region requires that a definitive picture of the faulting geometry will have to await improvement in our knowledge of the p and s wave velocity structures

the accuracy of digital elevation models of the antarctic continent abstract the accuracy of two widely used digital elevation models of antarctica was assessed using data from the geoscience laser altimeter system onboard icesat the digital elevation models were derived from satellite radar altimeter and terrestrial data sets the first termed jlb97 was produced predominantly from ers 1 data while the second termed rampv2 included other sources of data in areas of high relief and poor coverage by ers 1 the accuracy of the models was examined as a function of surface slope and original data source large errors in excess of 100 m were ubiquitous in both models in areas where terrestrially derived elevation data had been used but were more extensive in rampv2 elsewhere the systematic error bias was found to be a monotonic function of slope for jlb97 with a more complex less predictable bias for rampv2 the magnitude of the global slope dependent bias ranged from less than a metre to slightly over 10 m but with much larger regional deviations the random error ranged from about 1 m to over 100 m depending on the dem and slope the random error was consistently over a factor two larger for rampv2 compared to jlb97

ice shelf dynamics near the front of the filchner ronne ice shelf antarctica revealed by sar interferometry fifteen synthetic aperture radar sar images of the ronne ice shelf also referred to as the filchner ronne ice shelf antarctica obtained by the european remote sensing satellites ers 1 and 2 are used to study ice shelf dynamics near two ends of the iceberg calving front interferograms constructed from these sar images are used to resolve the ice shelf displacement along several directions in response to both ocean tide and long term creep flow tidal motion is separated from creep flow using differential interferometry i e two or more interferograms in which fringe patterns common to all are predominantly associated with creep flow creep flow velocities thus determined compare well with prior ice shelf velocity surveys using these data we studied the influence of large scale rifts ice rises and coastal separation on the ice shelf flow many of the large rifts that appear to form the boundaries where tabular icebergs may eventually detach from the ice shelf are filled with a melange of sea ice ice shelf debris and wind blown snow the interferograms show that this melange tends to deform coherently in response to the ice shelf flow and has sufficient strength to trap large tabular ice shelf fragments for several decades before the fragments eventually become icebergs in many instances the motion of the tabular fragments is a rigid body rotation about a vertical axis that is driven by velocity shear within the melange the mechanical role of the rift filling melange may be to bind tabular ice shelf fragments to the main ice shelf before they calve this suggests two possible mechanisms by which climate could influence tabular iceberg calving first spatial gradients in oceanic and atmospheric temperature may determine where the melange melts and thus the location of the iceberg calving margin second melting or weakening of ice melange as a consequence of climate change could trigger a sudden or widespread release of tabular icebergs and lead to rapid ice shelf disintegration

recent atmospheric warming and retreat of ice shelves on the antarctic peninsula in 1978 mercer1 discussed the probable effects of climate warming on the antarctic ice sheet predicting that one sign of a warming trend in this region would be the retreat of ice shelves on the antarctic peninsula analyses of 50 year meteorological records have since revealed atmospheric warming on the antarctic peninsula2 3 and a number of ice shelves have retreated4 8 here we present time series of observations of the areal extent of nine ice shelves on the antarctic peninsula showing that five northerly ones have retreated dramatically in the past fifty years while those further south show no clear trend comparison with airtemperature data shows that the pattern and magnitude of ice shelf retreat is consistent with the existence of an abrupt thermal limit on iceshelf viability the isotherm associated with this limit having been driven south by the atmospheric warming ice shelves therefore appear to be sensitive indicators of climate change

postseismic mantle relaxation in the central nevada seismic belt holocene acceleration of deformation and postseismic relaxation are two hypotheses to explain the present day deformation in the central nevada seismic belt cnsb discriminating between these two mechanisms is critical for understanding the dynamics and seismic potential of the basin and range province interferometric synthetic aperture radar detected a broad area of uplift 2 to 3 millimeters per year that can be explained by postseismic mantle relaxation after a sequence of large crustal earthquakes from 1915 to 1954 the results lead to a broad agreement between geologic and geodetic strain indicators and support a model of a rigid basin and range between the cnsb and the wasatch fault

improving antarctic tide models by assimilation of icesat laser altimetry over ice shelves 1 assimilation of laser altimeter data from the ice cloud and land elevation satellite icesat significantly improves the accuracy of ocean tide models for the ross ice shelf ris for the most energetic tidal harmonic k1 assimilation reduces the root mean square error rmse between the model and a set of 16 independent tide records on and near the ris from 6 0 to 2 8 cm and the combined rmse for the six most energetic tidal harmonics from 7 7 to 5 4 cm when only the six most recent and highest quality tide records are considered the combined rmse is 4 8 cm this value is close to the uncertainty expected from tidal analyses of the short 1 2 month validation records indicating that assessing further improvements in tide model accuracy will require development of a higher quality validation data set

inland thinning of the amundsen sea sector west antarctica 1 together with the pine island glacier pig the thwaites tg and smith sg glaciers are the principal drainage systems of the amundsen sea as sector of western antarctica here we use satellite radar altimetry and interferometry to show that a rapid thinning of ice has occurred within the fastest flowing sections of all as outlet glaciers the pattern of thinning extends to distances greater than 150 km inland between 1991 and 2001 the tg and sg thinned by more than 25 and 45 m at their grounding lines and a total of 154 16 km 3 of ice or 0 43 mm of eustatic sea level rise was lost from the as sector glaciers to the ocean we show that the thickness changes may have caused the pig tg and sg to retreat inland by over 8 4 and 7 km respectively in line with independent estimates of grounding line migration

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

estimation of ice sheet motion using satellite radar interferometry method and error analysis with application to humboldt glacier greenland satellite radar interferometry provides glaciologists with an important new tool for determining the motion and topography of large ice sheets we examine the sources of error in interferometrically derived ice motion measurements including those errors due to inaccurate estimates of the interferometric baseline several simulations are used to assess baseline accuracy in terms of tie point error and the number and distribution of tie points these results give insight into how best to select tie points and also demonstrate the level of accuracy that can be achieved examination of two representative cases likely to occur in mapping ice sheet motion leads to the conclusion that with adequate tie point information ice velocity can be measured accurately to within a few meters per year a method to correct horizontal velocity estimates for the effect of vertical displacement using surface slopes is also developed finally we estimate the single component velocity field for an area on humboldt glacier northern greenland using interferograms formed from ers i sar images we estimate that these velocity measurements are accurate to within 2 3 m year 1

measuring two dimensional movements using a single insar pair 1 we present a new method to extract along track displacements from insar data based on split beam insar processing to create forward and backward looking interferograms the phase difference between the two modified interferograms provides the along track displacement component thus from each conventional insar pair we extract two components of the displacement vector one along the line of sight the other in the along track direction we analyze the precision of the new method by comparing our solution to gps and offset derived along track displacements in interferograms of the 1999 hector mine earthquake the rms error between gps displacements and our results ranges from 5 to 8 8cm our method is consistent with along track displacements derived by pixel offsets which are limited to 12 15cm precision the theoretical precision of the new method depends on snr and coherence for a signal to noise ratio of 30 the expected precisions are 3 11cm for coherence of 0 8 0 4 respectively

observation of ocean tides below the filchner and ronne ice shelves antarctica using synthetic aperture radar interferometry comparison with tide model predictions tides near and under floating glacial ice such as ice shelves and glacier termini in fjords can influence heat transport into the subice cavity mixing of the under ice water column and the calving and subsequent drift of icebergs free surface displacement patterns associated with ocean variability below glacial ice can be observed by differencing two synthetic aperture radar sar interferograms each of which represents the combination of the displacement patterns associated with the time varying vertical motion and the time independent lateral ice flow we present the pattern of net free surface displacement for the iceberg calving regions of the ronne and filchner ice shelves in the southern weddell sea by comparing sar based displacement fields with ocean tidal models the free surface displacement variability for these regions is found to be dominated by ocean tides the inverse barometer effect i e the ocean s isostatic response to changing atmospheric pressure also contributes to the observed vertical displacement the principal value of using sar interferometry in this manner lies in the very high lateral resolution tens of meters obtained over the large region covered by each sar image small features that are not well resolved by the typical grid spacing of ocean tidal models may contribute to such processes as iceberg calving and cross frontal ventilation of the ocean cavity under the ice shelf

ice shelf changes in pine island bay antarctica 1947 2000 aerial photographs from 1947 and 1966 satellite optical imagery from 1973 and 1980 and interferometric synthetic aperture radar insar data from 1992 1996 and 2000 are employed to detect ice shelf changes in pine island bay antarctica the front position of the fast flowing central ice shelf did not migrate discernibly over the past 50 years new cracks and rifts appeared in the 1990s however that reveal a major weakening of the ice shelf at the grounding line center the ice shelf thinned 21 m in 8 years the northern slow moving ice shelf also shows signs of decay 1 its calving front is retreating at an accelerating rate and 2 the ice shelf is slowly unpinning from its bedrock anchors these changes are taking place in a region well beyond the temperature dependent limit of viability of ice shelves and hence differ from those observed along the antarctica peninsula they are likely due to a change in oceanic forcing not to a change in air temperature one possibility is that the documented intrusion of warm circumpolar deep water on the continental shelf has increased basal melting compared to that required to maintain the ice shelf in a state of mass balance and that this has triggered a general retreat of ice in this sector

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

accuracy assessment of ocean tide models around antarctica 1 accurate ocean tide models for the circum antarctic seas are required to remove unwanted signals from floating ice elevation and space borne time variable gravity measurements e g grace we present accuracy assessments for several global csr4 fes2004 fes99 got00 2 nao 99b tpxo6 2 and antarctic cada00 10 and cats02 01 ocean tide models using coastal and pelagic tide gauges gravimetric data and gps records of ice shelf surface elevation the accuracies of csr4 and nao 99b are poor in the ice shelf regions the optimum model for the entire circum antarctic seas is tpxo6 2 with a root mean square deviation of 5 7 cm 40 lower than the next best model fes2004 the main exception is the filchner ronne ice shelf where cada00 10 and cats02 01 most accurately represent observations from two sites near the rutford ice stream grounding line

topographic phase recovery from stacked ers interferometry and a low resolution digital elevation model a hybrid approach to topographic recovery from ers interferometry is developed and assessed tropospheric ionospheric artifacts imprecise orbital information and layover are key issues in recovering topography and surface deformation from repeat pass interferometry previously we developed a phase gradient approach to stacking interferograms to reduce these errors and also to reduce the short wavelength phase noise see sandwell and price 1998 and appendix a here the method is extended to use a low resolution digital elevation model to constrain long wavelength phase errors and an iteration scheme to minimize errors in the computation of phase gradient we demonstrate the topographic phase recovery on 16 m postings using 25 ers synthetic aperture radar images from an area of southern california containing 2700 m of relief on the basis of a comparison with 81 gps monuments the ers derived topography has a typical absolute accuracy of better than 10 m except in areas of layover the resulting topographic phase enables accurate two pass real time interferometry even in mountainous areas where traditional phase unwrapping schemes fail as an example we form a topography free 127 m perpendicular baseline interferogram spanning 7 5 years fringes from two major earthquakes and aseismic slip on the san andreas fault are clearly isolated