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

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

apatite for destruction isotopic and geochemical analyses of bioapatites and sediments from the upper devonian escuminac formation miguasha quebec abstract the paleoenvironmental context and diagenetic history of the middle frasnian escuminac formation were investigated through geochemical rare earth elements and isotopic rb sr nd sm analyses of fossil fish material and sediments samples of sediments and bioapatite were collected from the base to the top of the escuminac formation strontium isotope ratios 87 sr 86 sr for bioapatites vary from 0 70804 to 0 70845 and overlap with frasnian seawater sr isotope compositions though most of the biogenic apatites analyzed show a variable drift from marine values toward a more radiogenic continental signature this trend is possibly the result of post mortem sr exchange between fossils and a fluid isotopically distinct from the waters in which they developed all bioapatites are heavily enriched in rare earth elements ree and display a shale normalized pattern that shows heavy ree hree depletion and a slight middle ree mree enrichment the e nd t values of the sediments range from 4 8 to 6 4 and are consistent with a sedimentary provenance derived from an appalachian source for the escuminac formation in contrast more radiogenic e nd t values ranging from 2 6 to 4 6 were obtained for the bioapatites the discrepancy between sediments and fossil material e nd t implies the presence of a second diagenetic nd reservoir for the escuminac bioapatites we propose that this reservoir reflects in part seawater the similarity between the nd isotope compositions of the escuminac fossils and late devonian conodonts from poland argues for a close connection between waters of the devonian rheic ocean and the escuminac formation collector basin

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

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

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

tracking mammoths and mastodons reconstruction of migratory behavior using strontium isotope ratios variations in the strontium isotope ratio 87 sr 86 sr of tooth enamel are used to examine the migration patterns of late pleistocene mammoths and mastodons from florida an animal s 87 sr 86 sr ratio tracks the ratios of its environment which vary with differences in bedrock and soil consequently the environmentally controlled differences in 87 sr 86 sr ratio recorded in mineralized tissue such as tooth enamel may be used to reconstruct the movement patterns of an individual we map variations in local 87 sr 86 sr ratios across modern florida and georgia through analysis of rodent teeth plants and surface water then use this map to interpret the movement patterns of extinct mammals mastodons from northern and central florida have higher 87 sr 86 sr ratios than both modern environmental samples from florida and fossils from nonmigratory species suggesting that mastodons migrated north into georgia mammoths display ratios similar to those of environmental samples and resident species suggesting that they did not migrate outside florida

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

effects of diagenesis on strontium carbon nitrogen and oxygen concentration and isotopic composition of bone abstract paleodietary analysis based on variations in the trace element and stable isotopic composition of inorganic and organic phases in fossil bone depends on the assumption that measured values reflect in vivo values to test for postmortem alteration we measured 87 sr 86 sr 13 c 12 c 18 o 16 o and 15 n 14 n ratios and sr concentrations in modern and prehistoric 610 to 5470 yr old bones of animals with marine or terrestrial diets from greenland bones from modern terrestrial feeders have substantially lower sr concentrations and more radiogenic 87 sr 86 sr ratios than those from modern marine feeders this contrast was not preserved in the prehistoric samples which showed almost complete overlap for both sr concentration and isotopic composition in bones from the two types of animals leaching experiments x ray diffraction analysis and infrared spectroscopy indicate that alteration of the sr concentration and isotopic composition in prehistoric bone probably results from nearly complete exchange with groundwater oxygen isotope ratios in fossil apatite carbonate also failed to preserve the original discrimination between modern terrestrial and marine feeders the c isotope ratio of apatite carbonate did not discriminate between animals with marine or terrestrial diets in the modern samples even so the ranges of apatite 13 c values in prehistoric bone are more scattered than in modern samples for both groups suggesting alteration had occurred 13 c and 15 n values of collagen in modern bone are distinctly different for the two feeding types and this distinction is preserved in most of the prehistoric samples our results suggest that postmortem alteration of dietary tracers in the inorganic phases of bone may be a problem at all archaeological sites and must be evaluated in each case while collagen analyzed in this study was resistant to alteration evaluation of the possibility of diagenetic alteration of its isotopic composition in bones from other contexts is also warranted

introducing δ88 86sr analysis in archaeology a demonstration of the utility of strontium isotope fractionation in paleodietary studies abstract isotopic methods are widely used in archaeology to investigate paleodiet here we present a new method to identify trophic level in archaeological human populations and to investigate paleodiet we demonstrate that strontium isotope compositions reported as 88 86sr vary in a mass dependent manner with increasing trophic level and can elucidate paleodiet in archaeological human populations we present new mass dependent strontium isotope data from tooth enamel and bone from individuals buried during the late intermediate period c ad 1000 1300 in the large cemeteries of chiribaya alta chiribaya baja san geronimo and el yaral in the ilo and moquegua valleys of southern peru we compare these data to radiogenic strontium isotope data 87sr 86sr and light stable isotope data 15ncol and 13ccol from the same individuals to investigate geologic variability in strontium sources as well as marine food consumption among the chiribaya our results demonstrate the utility of measurements of strontium isotope fractionation as a new tool for archaeological investigation of paleodiet importantly this new technique can be used to generate paleodietary 88 86sr and paleomobility 87sr 86sr data from the same specimen minimizing destructive analyses of invaluable archaeological material and provides a new way to examine paleodiet through hydroxyapatite which is particularly important when collagen is poorly preserved

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

the last domicile of the iceman from hauslabjoch a geochemical approach using sr c and o isotopes and trace element signatures abstract we present sr c and o isotope ratios and quantitative ratios of trace element ratios in the rib and the femur of the tyrolean iceman otzi the results from the rib and the femur are compared with those obtained from pre industrial historic human skull bone fragments collected in the late 19th century from charnel houses located on different geological backgrounds throughout north and south tyrol comparison of strontium isotope data from the bone fragments locates the iceman s domicile during the last 5 to 10 years of his life to the crystalline part of the central alps statistical analysis of the trace element ratios together with oxygen isotope results suggests that the iceman probably spent most of his last years in southern elevated part of the otztal however the results from iceman only represent those from a single individual and are not a representative average of a group it cannot therefore be established with absolute certainty that the original trace element signatures were conserved especially during the first stage of mummification our conclusions should therefore be regarded as tentative but nevertheless the best possible indication his domicile and a stimulus for further research

fish fossils as paleo indicators of ichthyofauna composition and climatic change in lake malawi africa abstract numerous biological and chemical paleorecords have been used to infer paleoclimate lake level fluctuation and faunal composition from the drill cores obtained from lake malawi africa however fish fossils have never been used to examine changes in african great lake vertebrate aquatic communities nor as indicators of changing paleolimnological conditions here we present results of analyses of a lake malawi core dating back 144 ka that describe and quantify the composition and abundance of fish fossils and report on stable carbon isotopic data 13c from fish scale bone and tooth fossils we compared the fossil 13c values to 13c values from extant fish communities to determine whether carbon isotope ratios can be used as indicators of inshore versus offshore pelagic fish assemblages fossil buccal teeth pharyngeal teeth and mills vertebra and scales from the fish families cichlidae and cyprinidae occur in variable abundance throughout the core carbon isotopic ratios from numerous fish fossils throughout the core range between 7 2 and 27 5 similar to those found in contemporary lake malawi benthic and pelagic fish faunas these results are the first paleo record of fish fossils from a lake malawi sediment core and the first reported 13c values from lake malawi fish fossils this approach provides a new methodology and framework for interpreting pelagic versus inshore fish faunas lake level fluctuations and the evolution of the lake malawi fish assemblages

isotope domain mapping of 87sr 86sr biosphere variation on the isle of skye scotland abstract the potential of 87 sr 86 sr isotope composition for tracking and determining the origin of material whether it is humans animals water or wine can be fully realized only if high quality reference datasets are available against which to compare the unknowns this is currently not the case despite the rapid increase in 87 sr 86 sr based provenance studies of biosphere materials there are no well documented maps available most researchers resort to generating reference maps from a variety of geological datasets plus the analysis of sparse environmental9 samples a major reason for this lack of reference material is the investment needed to undertake systematic sampling and analysis over large areas a further problem is associated with the choice of proxy material as organisms differ in their habitat and exploitation of resources in this paper various approaches to isotope biosphere sampling are considered some of the problems are assessed the precision required for the study of human populations is discussed and a first attempt is made to condense all of the available information into an isotope package9 map the work is based on a study undertaken on the isle of skye in the inner hebrides and reduces the complexity of geological and isotope data into five main 87 sr 86 sr packages package 1 northern area 0 705 0 7092 package 2 ne coast 0 7082 0 7102 package 3 granitic rocks 0 716 0 720 package 4 metamorphic rocks 0 7092 0 7188 package 5 seawater 0 7092 0 005 2 the approach of trying to constrain the isotope composition of certain areas and or lithologies offers the possibility of extrapolating to other similar areas without having to sample at the original density

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

87sr 86sr and sr ca investigation of jurassic mollusks from scotland implications for paleosalinities and the sr ca ratio of seawater measurements of 87 sr 86 sr sr ca 18 o and 13 c were performed on molluskan fos sils from the great estuarine group scot land to deduce paleosalinity patterns and hydrodynamic environments in this classic jurassic marginal marine succession fos sils are found as winnowed accumulations in shell beds that bear low diversity assem blages consistent with deposition in a la goonal paleoenvironment previous studies have concluded that estuarine mixing of seawater and freshwater took place based on correlated trends in 18 o and 13 c most notably in the mussel praemytilus strathair densis from the kildonnan member this interpretation is shown to be incorrect based on 87 sr o 13 c and the sr ca ratio this is consistent with a lagoonal hydrology controlled more by seasonal changes in evaporation precipitation and flooding than by direct inputs of seawater or river ine water transported over large distances lagoons containing the oyster praeexogyra in the duntulm formation have similar in ferred o and c isotope characteristics but based on their 87 sr 86 sr ratios they have a higher proportion of seawater than the praemytilus lagoons of the kildonnan member the data thus far suggest that many of the lagoonal environments in the great es tuarine group were nearly hydrologically closed at least over the interval represent ing the molluskan invasion of these habi tats rendering o and c isotopes unreliable proxies of paleosalinity because strontium isotopes are not directly affected by evap oration of the lagoons knowledge of marine and freshwater 87 sr 86 sr ratios along with estimates of sr concentration are used to calculate the proportion of seawater in the brackish water mixture unio is nearly al ways the most radiogenic taxon compared to other genera in closely adjacent sedimen tary strata confirming its freshwater affin ity the purported marine brackish bivalve tancredia yields a low paleosalinity of 4 only slightly more saline than unio and praemytilus at 1 3 the oyster praeexo gyra hebridica yields paleosalinities between 2 and 26 attesting to the wide salinity tolerance of this species thus far only neomiodon from the dun tulm and staffin bay formations show co variant relationships between 87 sr 86 sr and ca sr ratio that can be reconciled with a dynamic response to seawater freshwater mixing the slopes of these trends converge at 0 7068 which is close to the early cal lovian seawater composition of 0 7069 us ing values of dsr from modern aragonite se creting mollusks the sr ca ratio of middle jurassic seawater is constrained from the best correlated mixing line to be 5 4 0 6 millimoles mole in contrast sr ca ratios in the great estuarine group lagoons and continental watershed range from 5 4 to 19 0 these are much higher than the av erage value of 2 3 for modern rivers but typical of sr ca ratios in rivers draining arid climate regions the semirestricted hy drodynamic regime inferred from the high seawater like 18 o values and high but uni form sr ca ratios in many of the great es tuarine group mollusks is consistent with a seasonally dry mediterranean style climate for the middle jurassic of britain

calcium isotopes in archaeological bones and their relationship to dairy consumption abstract the calcium isotope ratios 44 42 ca of bones from humans and fauna from three archaeological sites taforalt abu hureyra and danebury are evaluated in order to assess whether calcium isotope ratios of bones can be used to detect dairy consumption by adult humans at each site the fauna 44 42 ca is the same regardless of species while the humans have lower 44 42 ca than the local animals by 0 24 0 41 site means however we cannot ascribe this difference to dairy consumption given this human faunal difference also occurs in epipalaeolithic and mesolithic adult humans where dairy consumption is unlikely rather this difference appears to be a result of differences in metabolic processes or other aspects of diet between humans and fauna minimal isotopic change in sequential acid leaches of bone powders and consideration of the high calcium concentration in bone suggest that bone calcium isotope ratios are not substantially affected by diagenetic change

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

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

assimilation of surface velocities acquired between 1996 and 2010 to constrain the form of the basal friction law under pine island glacier in ice sheet models slip conditions at the base between the ice and the bed are parameterized by a friction law the most common relation has two poorly constrained parameters c and m the basal slipperiness coefficient c depends on local unobserved quantities and is routinely inferred using inverse methods while model results have shown that transient responses to external forcing are highly sensitive to the stress exponent m no consensus value has emerged with values commonly used ranging from 1 to depending on the slip processes by assimilation of pine island glacier surface velocities from 1996 to 2010 we show that observed accelerations are best reproduced with m 5 we conclude that basal motion in much of the fast flowing region is governed by plastic deformation of the underlying sediments this implies that the glacier bed in this area can not deliver resistive stresses higher than today making the drainage basin potentially more sensitive to dynamical perturbations than predicted with models using standard values m 1 or 3

the origin of the juch uypampa cave mummies strontium isotope analysis of archaeological human remains from bolivia previous analyses of strontium isotopes from human bone and teeth have identified diverse patterns of residential mobility in the south central andes during the middle horizon ad 500e1100 during this time the large polity of tiwanaku exerted great influence over what are now southern peru northern chile northwestern argentina and bolivia recently five naturallymummified individuals were discovered in the cave of juch uypampa in the pulacayo region of southern bolivia although these individuals were buried with a number of fine tiwanaku style grave goods as well as non tiwanaku items the burial site is isolated and does not conform to the pattern of large tiwanaku affiliated cemeteries and residential sites outside of the lake titicaca basin strontium isotope analysis was performed on enamel from two adult men and bone from a third adult male in order to test the hypotheses that one or more of the males was from either the tiwanaku heartland in the lake titicaca basin the chilean oasis of san pedro de atacama which contains a series of cemeteries with tiwanaku style grave goods or the local area in which they were buried results show that two individuals likely spent their childhood in the local area where they were interred while the third man probably spent at least the last twenty years of his life in that region before being buried there this raises interesting questions regarding the nature of tiwanaku influence in southern bolivia and the relationship between the juch uypampa mummies and the tiwanaku polity 2005 elsevier ltd all rights reserved

oxygen and strontium isotopes as provenance indicators of fish at archaeological sites the case study of sagalassos sw turkey abstract in this paper we investigate the potential use of oxygen and strontium isotope ratios 18 o p and 87 sr 86 sr measured in archaeological fish enamel as provenance indicators 18 o p and 87 sr 86 sr were measured in a suite of archaeological carp remains recovered from the anatolian townsite of sagalassos dated to the early byzantine period ad 450 650 and compared to that of modern fish river and lake waters from the anatolian region we used sequential leaches in weak acetic acid to remove diagenetic sr from fossil tooth enamel monitoring the effectiveness of this approach by measuring the sr ca ratios of the leachates via an isotope dilution thermal ionization mass spectrometry method id tims 18 o p values mostly excluded a riverine origin 87 sr 86 sr ratios of one fish overlapped with the 87 sr 86 sr signatures of two lakes in the anatolian region and at least one lake golcuk could be removed as a candidate owing to a very distinctive 87 sr 86 sr signature not found in any of the fish remains most of the tooth samples analyzed could not be assigned a precise geographical origin since the 87 sr 86 sr ratios measured in enamel did not match that of any of the local lakes selected as potential origin this result suggests that carp may have originated from lakes that have not yet been sampled although this conclusion is not supported by other archaeological evidence alternatively the lack of correspondence between lakes and fish sr isotope ratios highlights several possible sources of uncertainties including spatial heterogeneity in 87 sr 86 sr ratio within a lake the contribution of dietary strontium to the 87 sr 86 sr ratio of fish tooth enamel and post mortem alteration of the tooth sr isotope signal during fossilization in spite of the high precision of the strontium isotope analyses and the wide range of variation in the surface waters of the anatolian lakes and rivers this method may remain limited to distinguishing between lakes situated in regions of bedrock of very distinct age and geology until these sources of uncertainty are more fully investigated

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

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

thermal infrared spectroscopy and partial least squares regression to determine mineral modes of granitoid rocks 1 in this paper we present an approach to extracting mineralogic information from thermal infrared tir spectra that is not based on an input library of pure mineral spectra nor tries to extract spectral end members from the data instead existing modal mineralogy for a number of samples are used to build a partial least squares regression plsr model that links the mineralogy of the samples to their respective tir spectral signatures the resulting plsr models can be applied to a larger group of samples for which the mineralogic composition can be estimated from the tir spectra alone thermal infrared reflectance spectra were recorded from 1330 625 cm 1 7 5 to 16 0 m the method is tested on igneous rocks from a porphyry copper deposit in yerington nevada as a reference modal mineralogic composition was determined with traditional polarization microscopy on thin sections partial least squares regression models were developed to link the thermal infrared spectra to the thin section determined mineral modes of alkali feldspar plagioclase and quartz as well as the average plagioclase composition information results indicate that rock samples can be classified successfully in a quartz alkali feldspar plagioclase diagram based on thermal infrared spectroscopy and partial least squares regression modeling estimated errors for the mineralogic composition model results were found to be smaller or equal to traditional methods with errors of 5 1 absolute for alkali feldspar 8 5 absolute for plagioclase and 6 9 absolute for quartz the regression model for plagioclase composition predicted with estimated errors of 7 8 mol anorthite

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

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

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

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

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

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

calibration of a conodont apatite based ordovician 87sr 86sr curve to biostratigraphy and geochronology implications for stratigraphic resolution the ordovician 87 sr 86 sr isotope seawater curve is well established and shows a decreasing trend until the mid katian however uncertainties in calibration of this curve to biostratigraphy and geochronology have made it diffi cult to determine how the rates of 87 sr 86 sr decrease may have varied which has implications for both the stratigraphic resolution possible using sr isotope stratigraphy and efforts to model the effects of ordovician geologic events we measured 87 sr 86 sr in conodont apatite in north american ordovician sections that are well studied for conodont biostratigraphy primarily in nevada oklahoma the appalachian region and ohio valley our results indicate that conodont apatite may provide an accurate medium for sr isotope stratigraphy and strengthen previous reports that point toward a signifi cant increase in the rate of fall in seawater 87 sr 86 sr during the middle ordovician darriwilian stage our 87 sr 86 sr results suggest that sr isotope stratigraphy will be most useful as a high resolution tool for global correlation in the mid darriwilian to mid sandbian when the maximum rate of fall in sr

a model of fossil tooth and bone diagenesis implications for paleodiet reconstruction from stable isotopes abstract diagenesis of structural carbonate in biogenic apatite in the postmortem environment is modeled as a process of water mineral interaction both closed and open system model calculations suggest that 13 c of structural carbonate in biogenic apatite is much more resistant to diagenetic modification than 18 o apatite structural carbonate retains an original 13 c signal in closed system diagenesis for open systems 13 c of biogenic apatite with very low porosity such as tooth enamel can survive most diagenesis and retain a primary isotopic signature on the hand 18 o of structural carbonate in biogenic apatite may not be a reliable indicator of the original isotopic ratios except in a diagenetic system with low temperature and very low water biomineral ratio application of the model to teeth and carbonate cement in sediments from the badlands south dakota illustrates that 13 c values of enamel samples preserve the original signal whereas 13 c values of dentine represent varying extents of diagenesis 18 o values of fossil teeth may record an early or late part of their water biomineral interaction history during diagenesis

sr isotopic composition of hydroxyapatite from recent and fossil salmon the record of lifetime migration and diagenesis by comparing the sr isotopic composition of migratory fossil salmon which lived in the ocean but died in continental regions to the well established marine sr isotopic record the age of the continental deposit could be determined with high accuracy this approach to marine continental correlation and dating requires 1 that marine resident salmon bear a marine s7sr 86sr value in their bones or teeth and 2 that the original s7sr 86sr value of fossils is not overprinted by diagenesis the vertebrae of modern hatchery reared salmon exhibit sr isotopic variations indicative of freshwater to marine migration during bone growth modern marine s7sr s6sr values were preserved in growth layers formed later in life marine phase growth layers in the bones and teeth of the late miocene migratory salmon oncorhynchus rastrosus were subjected to stepwise selective leaching to separate biogenic hydroxyapatite from diagenetic calcium carbonate and recrystallized hydroxyapatite although the procedure yielded leachates with sr ca and ca p values characteristic of apatite the leachates had 87sr s6sr values consistently less radiogenic than values for late miocene seawater 0 7087 the fossils were substantially contaminated by sr from the hosting clastic sediments specimens in continental deposits differed in 87sr 86sr value from host sediments by 0 0002 to 0 0200 supporting the conclusion that these salmon were migrants from marine waters however because the original sr isotopic composition of fossil bones and teeth cannot be determined with confidence archaeological paleobiological and stratigraphic applications of this technique may be limited

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

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

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

preservation of seawater sr and nd isotopes in fossil fish teeth bad news and good news abstract we analyzed 87sr 86sr ratios in foraminifera pore fluids and fish teeth for samples ranging in age from eocene to pleistocene from four ocean drilling program sites distributed around the globe site 1090 in the cape basin of the southern ocean site 757 on the ninetyeast ridge in the indian ocean site 807 on the ontong java plateau in the western equatorial pacific and site 689 on the maud rise in the southern ocean sr isotopic ratios for dated foraminifera consistently plot on the global seawater sr isotope curve for sites 1090 757 and 807 sr isotopic values of the pore fluids are generally less radiogenic than contemporaneous seawater values as are values for fossil fish teeth in contrast pore fluid 87sr 86sr values at site 689 are more radiogenic than contemporaneous seawater and the corresponding fish teeth also record more radiogenic values thus sr isotopic values preserved in fossil fish teeth are consistently altered in the direction of the pore fluid values furthermore there is a correlation between the magnitude of the offset between the pore fluids and the seawater curve and the associated offset between the fish teeth and the seawater curve these data suggest that the hydroxyfluorapatite of the fossil fish teeth continues to recrystallize and exchange sr with its surroundings during burial and diagenesis therefore sr chemostratigraphy can be used to determine rough ages for fossil fish teeth in these cores but cannot be used to fine tune age models in contrast to the sr isotopic system our nd concentration data combined with published isotopic and rare earth element data suggest that fish teeth acquire nd during early diagenesis while they are still in direct contact with seawater the concentrations of nd acquired at this stage are extremely high relative to the concentrations in surrounding pore fluids as a result nd isotopes are not altered during burial and later diagenesis therefore fossil fish teeth from a variety of marine environments preserve a reliable and robust record of deep seawater nd isotopic compositions from the time of deposition

diagenesis and the reconstruction of paleoenvironments a method to restore original δ18o values of carbonate and phosphate from fossil tooth enamel abstract intra tooth 18 o variations within the carbonate 18 oc and phosphate 18 op components of tooth apatite were measured for miocene and pliocene hypsodont mammals from afghanistan greece and chad in order to evaluate the resistance of enamel to diagenetic alteration application of water apatite interaction models suggest that the different kinetic behaviours of the phosphate water and carbonate water systems can be used to detect subtle oxygen isotope disequilibria in fossil enamel when intra individual variations are considered selective alteration of the oxygen isotope composition from the carbonate component of afghan and greek enamels suggests inorganic isotopic exchange processes microbially induced isotopic exchange for phosphate is demonstrated for the first time in enamel samples from chad in association with extensive recrystallization in chad 18 op values were derived from partial isotopic exchange with fossil groundwater during early diagenesis mass balance calculations using average carbonate content in enamel as a proxy for recrystallization and the lowest 18 op value of dentine as a proxy for the isotopic composition of the diagenetic fluid indicate that diagenesis can alter 18 op by as much as 3 in some enamel samples this diagenetic alteration is also responsible for a decrease in intra individual variations of up to 1 in affected specimens the effects of diagenesis on 18 op values of fossil enamel are not systematic however and can only be estimated if sequential 18 op and 18 oc analyses are performed on fossil enamel and dentine reconstruction of large temporal or spatial scale paleoclimates based on 18 op analyses from mammalian teeth cannot be considered valid if enamel has been affected by bacterial activity or if the data cannot be corrected for diagenetic effects

strontium increases and crystallinity changes in taphonomic and archaeological bone bone strontium levels and hydroxyapatite crystal sizes increased with time in two exposed wildebeest skeletons the strontium increases could not be removed by sequential washing according to the method of sillen a 1986 biogenic and diagenetic srca in plio pleistocene fossils of the omo shungura formation paleobiology12 311 323 human subfossil bone recovered archaeologically also showed evidence of extensive diagenetic reworking that resulted in crystal growth and elevated strontium levels the increase in bone strontium content was independent of nonapatitic contaminating crystal phases

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