the instability theory of drumlin formation and its explanation of their varied composition and internal structure despite their importance in understanding glaciological processes and constraining large scale flow patterns in palaeo glaciology there is little consensus as to how drumlins are formed attempts to solve the drumlin problem often fail to address how they are created from an initially flat surface in the absence of obvious cores or obstacles this is a key strength of the instability theory which has been described in a suite of physically based mathematical models and proposes that the coupled flow of ice and till causes spontaneous formation of relief in the till surface encouragingly model predictions of bedform height and length are consistent with observations and furthermore the theory has been applied to a range of subglacial bedforms and not just drumlins however it has yet to confront the myriad observations relating to the composition and internal structure of drumlins and this could be seen as a major deficiency this paper is a first attempt to assess whether the instability theory is compatible with the incredible diversity of sediments and structures found within drumlins we summarise the underlying principles of the theory and then describe and attempt to explain the main types of drumlin composition e g bedrock till glaciofluvial sediments and combinations thereof contrary to a view which suggests that the presence of some sedimentary sequences e g horizontally stratified cores is inconsistent with the theory we suggest that one would actually expect a diverse range of constituents depending on the inheritance of sediments that pre date drumlin formation the duration and variability of ice flow and the balance between erosion and deposition till continuity at the ice bed interface we conclude that the instability theory is compatible with and potentially strengthened by what is known about drumlin composition and as such offers the most complete and promising solution to the drumlin problem to date

a generalized additive model for the spatial distribution of stable isotopic composition in antarctic surface snow abstract using the most recent compilation of surface antarctic snow isotopic composition data and a 1 km resolution digital elevation model dem a generalized additive model gam was used to develop a new gridded map of stable isotopic composition in antarctic surface snow the model explained 93 9 of the observed variance in the 18 o data 92 7 of the observed d variance and 66 6 of the observed deuterium excess the performance of the model was evaluated by applying different quantitative approaches to the residuals from a cross validation test this modeling approach is a useful tool for interpolating local surface snow isotopic composition over antarctica both in terms of the low estimated errors and the possibility of understanding the processes that control stable isotopic composition in antarctic surface snow through the response curves of each independent variable we also presented and interpreted the resulting spatial variation patterns of 18 o d and deuterium excess to better use in many hydrological and paleoclimate applications

nucleation and seismic tremor associated with the glacial earthquakes of whillans ice stream antarctica 1 the ability to monitor transient motion along faults is critical to improving our ability to understand many natural phenomena such as landslides and earthquakes here we usedata from a gps and seismometer network that were deployed to monitor the regularly repeating glacial earthquakes of whillans ice stream west antarctica to show that a unique pattern of precursory slip precedes complete rupture along the bed of the ice stream additionally we show that rupture can be independently tracked by increased levels of microseismic activity including harmonic tremor that are coincident with the onset of slip at any location thus providing a remote means of monitoring stress and rupture propagation during the glacial earthquakes

spatial variability of surface mass balance along a traverse route from zhongshan station to dome a antarctica stakes at 2 km intervals were installed in january 1997 and remeasured in february 1998 january 1999 january 2005 and during the 2007 08 austral summer along a 1248 km traverse route from zhongshan station to dome a east antarctica based on topographical parameters meteoro logical features and the records of 650 stakes and six stake arrays the route is divided into five zones we find that the snow accumulation rate decreases with increasing altitude as one progresses inland except in the zone 800 1128 km from the coast where the average annual accumulation rate is higher than in the zone 524 800 km from the coast the dome a zone 1128 1248 km has the lowest accumulation rate 35 kg m 2 a 1 2005 08 due to having the highest elevation and being furthest from the coast the surface mass balance in the region 202 1128 km from the coast exhibits no temporal change from 1999 2005 to 2005 08 but there is a change in the accumulation distribution the zone from 202 to 524 km shows a decrease in surface mass balance from 84 kg m 2 a 1 in 1999 2005 to 67 kg m 2 a 1 in 2005 08 while the zone between 800 and 1128 km shows an increase from 67 kg m 2 a 1 in 1999 2005 to 75 kg m 2 a 1 in 2005 08

understanding the 17o excess glacial interglacial variations in vostok precipitation combined measurements of delta o 18 delta o 17 and delta d in ice cores leading to d excess and o 17 excess are expected to provide new constraints on the water cycle and past climates we explore different processes both in the source regions and during the poleward transport that could explain the o 17 excess increase by 20 per meg observed from the last glacial maximum lgm to early holocene eh at the vostok station using a single column model over tropical and subtropical oceans we show that the relative humidity at the surface is the main factor controlling o 17 excess in source regions then using a rayleigh type model we show that the o 17 excess signal from the source region is preserved in the polar snowfall contrary to d excess evaporative recharge over mid and high latitudes and delta o 18 seasonality in polar regions can also affect the vostok o 17 excess but cannot account for most of the 20 per meg deglacial increase from lgm to eh on the other hand a decrease of the relative humidity at the surface rh s by 8 to 22 would explain the observed change in o 17 excess such a change would not necessarily be incompatible with a nearly unchanged boundary layer relative humidity if the surface thermodynamic disequilibrium decreased by 4 degrees c such a change in rh s would affect source and polar temperatures reconstructions from delta o 18 and d excess measurements strengthening the interest of o 17 excess measurements to better constrain such changes

common millennial scale variability of antarctic and southern ocean temperatures during the past 5000 years reconstructed from the epica dome c ice core measurements of the two water stable isotopes dd and d18o along epica european project for ice coring in antarctica dome c ice core are combined with simple isotopic modelling distillation models to reconstruct the variability of both the site temperature east antarctica and the moisture source temperature nowadays probably the subantarctic indian ocean we discuss the difference between the reconstructed site and source temperature pro les with respect to the initial isotopic data we show that i the early holocene optimum appears rst in antarctica and 800 years later in the southern ocean and ii during the last 5000 years the site and source temperatures co vary at the centennial timescale an 833 year periodicity is observed only on deuterium and site temperature and therefore probably of local origin

in search of ice stream sticky spots the form drag of large bedrock bumps sticking into the base of an ice stream can produce effective sticky spots supporting large basal shear stress bedrock regions surrounded by lubricating till at the same topographic level can cause sticky spots but tend to collect lubricating water and thus are unlikely to support a shear stress of more than a few tenths of a bar unless they contain abundant large bumps raised regions on the ice air surface also can cause moderate increases in the shear stress supported on the bed beneath surveys of large scale bedrock roughness strain grids across the margins of ice surface highs and possibly water pressure measurements in regions of thin or zero till would help identify and characterize sticky spots

the periodic topography of ice stream beds insights from the fourier spectra of mega scale glacial lineations ice stream bed topography contains key evidence for the ways ice streams interact with and are potentially controlled by their beds here we present the first application of two dimensional fourier analysis to 22 marine and terrestrial topographies from 5 regions in antarctica and canada with and without mega scale glacial lineations msgls we find that the topography of msgl rich ice stream sedimentary beds is characterized by multiple periodic wavelengths between 300 and 1200 m and amplitudes from decimeters to a few meters this periodic topography is consistent with the idea that instability is a key element to the formation of msgl bedforms dominant wavelengths vary among locations and on one paleo ice stream bed increase along the direction of ice flow by 1 7 0 52 km 1 we suggest that these changes are likely to reflect pattern evolution via downstream wavelength coarsening even under potentially steady ice stream geometry and flow conditions the amplitude of msgls is smaller than that of other fluvial and glacial topographies but within the same order of magnitude however msgls are a striking component of ice stream beds because the topographic amplitude of features not aligned with ice flow is reduced by an order of magnitude relative to those oriented with the flow direction this study represents the first attempt to automatically derive the spectral signatures of msgls it highlights the plausibility of identifying these landform assemblages using automated techniques and provides a benchmark for numerical models of ice stream flow and subglacial landscape evolution

tidally controlled stick slip discharge of a west antarctic ice a major west antarctic ice stream discharges by sudden and brief periods of very rapid motion paced by oceanic tidal oscillations of about 1 meter acceleration to speeds greater than 1 meter per hour and deceleration back to a stationary state occur in minutes or less slip propagates at approximately 88 meters per second suggestive of a shear wave traveling within the subglacial till a model of an episodically slipping friction locked fault reproduces the observed quasi periodic event timing demonstrating an ice stream s ability to change speed rapidly and its extreme sensitivity to subglacial conditions and variations in sea level

aegean sea water masses during the early stages of the eastern mediterranean climatic transient 1988 90 abstract the aegean water masses and circulation structure are studied via two large scale surveys performed during the late winters of 1988 and 1990 by the r v yakov gakkel of the former soviet union the analysis of these data sheds light on the mechanisms of water mass formation in the aegean sea that triggered the outflow of cretan deep water cdw from the cretan sea into the abyssal basins of the eastern mediterranean sea the so called eastern mediterranean transient it is found that the central aegean basin is the site of the formation of aegean intermediate water which slides southward and depending on their density renews either the intermediate or the deep water of the cretan sea during the winter of 1988 the cretan sea waters were renewed mainly at intermediate levels while during the winter of 1990 it was mainly the volume of cdw that increased this aegean water mass redistribution and formation process in 1990 differed from that in 1988 in two major aspects i during the winter of

spatial distribution of 17o excess in surface snow along a traverse from zhongshan station to dome a east antarctica the influence of temperature on the triple isotopic composition of oxygen in water is still an open question and limits the interpretation of water isotopic profiles in antarctic ice cores the main limitation arises from the lack of 17o excess measurements in surface snow and especially for remote regions characterized by low temperature and accumulation rate in this study we present new 17o excess measurements of surface snow along an east antarctic traverse from the coastal zhongshan station to the highest point of the antarctic ice sheet at dome a the 17o excess data significantly decrease inland with a latitudinal gradient of 1 33 0 41 per meg degree an altitudinal gradient of view the mathml source and a temperature gradient of view the mathml source theoretical calculations performed using a rayleigh model attribute this inland decrease to kinetic isotopic fractionation occurring during condensation from vapor to ice under supersaturation conditions at low temperatures however large heterogeneity of 17o excess in antarctic precipitation cannot only be explained by temperature at condensation and or influences of relative humidity in the moisture source region

rapid erosion drumlin formation and changing hydrology beneath an antarctic ice stream what happens beneath a glacier affects the way it flows and the landforms left behind when it retreats direct observations from beneath glaciers are however rare and the subglacial environment remains poorly understood we present new repeat observations from west antarctica that show active processes beneath a modern glacier which can normally only be postulated from the geological record we interpret erosion at a rate of 1 m a 1 beneath a fast flowing ice stream followed by cessation of erosion and the formation of a drumlin from mobilized sediment we also interpret both mobilization and increased compaction of basal sediment with associated hydrological changes within the glacier bed all these changes occurred on time scales of a few years or less this variability suggests that an ice stream can reorganize its bed rapidly and that present models of ice dynamics may not simulate all the relevant subglacial processes

a new 27 ky high resolution east antarctic climate record the ice core recently drilled at the dome concordia site on the east antarctic plateau provides a new high resolution isotope record covering part of the last glacial the last transition and the holocene the two step shape of the deglaciation is remarkably similar for all the ice cores now available on the east antarctic plateau the first warming trend ends about 14000 years ago and is followed by the well marked antarctic cold reversal acr with a secondary peak common to all records during the deglaciation there are more similarities between the near coastal site of taylor dome and inland east antarctica than between taylor dome and central greenland however the results for epica do appear to confirm the taylor dome timescale after about 14 ka showing cooling into the acr roughly in phase between greenland and antarctica while the overall deglacial pattern is asynchronous this suggests that the now classical picture of a temperature seesaw between antarctica and greenland may be too simplistic

a 420 000 year deuterium excess record from east antarctica information on past changes in the origin of precipitation at vostok here we present and analyze the complete vostok ice cores deuterium excess record which spans the last four climatic cycles back to 420 000 years b p to extract paleoclimate information from this record we use isotopic modeling showing that changes in deuterium excess d d 8 18o of vostok precipitation reflect changes in the average temperature of oceanic moisture sources over the last 250 000 years the deuterium excess is strongly anticorrelated with changes in earth s obliquity this reflects changes in the relative contribution of low and high latitudes to the vostok precipitation resulting from changes in the latitudinal annual mean insolation gradient governed by the obliquity however this modulation by obliquity is not observed prior to 250 000 years b p we attribute this difference to the ice flow the deeper ice under vostok station having accumulated in a location upstream vostok receiving precipitation from different oceanic origins despite this difference between the earlier and the later portions of the record the deuterium excess changes during all the glacial inceptions are similar in amplitude and timing relative to the deuterium decrease the glacial inceptions are characterized by high deuterium excess indicating the significant role of the tropics in supplying moisture to the already cold poles

variable deceleration of whillans ice stream west antarctica the whillans ice stream ice plain wip has been slowing since at least 1963 prior constraints on this slowdown were consistent with a constant long term deceleration rate new observations of ice velocity from 11 continuous and 3 seasonal gps sites indicate the deceleration rate varies through time including on interannual time scales between 2009 and 2012 wip decelerated at a rate 6 1 to 10 9 2 m yr2 that was double the multidecadal average 3 0 to 5 6 2 m yr2 to identify the causes of slowdown we used new and prior velocity estimates to constrain longitudinal and transverse force budget models as well as a higher order inverse model all model results support the conclusion that the observed deceleration of wip is caused by an increase in basal resistance to motion at a rate of 10 to 40 pa yr subglacial processes that may be responsible for strengthening the ice stream bed include basal freeze on changes in subglacial hydrology or increases in the area of resistant basal substrate through differential erosion the observed variability in wip deceleration rate suggests that dynamics in subglacial hydrology plausibly driven by basal freeze on and or activity of subglacial lakes plays a key role in modulating basal resistance to ice motion in the region

seasonal variations of 17 o excess and d excess in snow precipitation at vostok station east antarctica the use of water isotopes in polar regions is essential for reconstructing past climate over glacial interglacial cycles in addition to d do rd 18 o linearly related to condensation temperature the second order parameters d excess and 17 o excess provide important information on the climatic conditions of the source of precipitations in order to best interpret the glacial interglacial records of d excess and 17 o excess in polar ice cores it is important to document their present variability especially in remote and cold regions of east antarctica indeed the current climatic conditions encountered in these regions provide a good analogy with glacial climatic conditions in a large part of antarctica here we present the first seasonal variations of 17 o excess and d excess at vostok station on an event basis i e samples were collected immediately after each precipitation event over 1 year these records show strong correlation between 17 o excess and d 18 o over the course of the year with an amplitude 40 per meg 10 3 in the 17 o excess seasonal cycle and strong anticorrelation between d excess and d 18 o with d excess variations up to 20 the d excess and 17 o excess variations can be explained by the influence of kinetic fractionation at very low temperatures the comparison with simple isotopic models confirms this explanation but cannot explain the link between 17 o excess d excess and temperature without 1 a particular relationship between condensation and surface temperature and or 2 seasonal changes in the climatic conditions of the source regions

size shape and spatial arrangement of mega scale glacial lineations from a large and diverse dataset mega scale glacial lineations msgls are a characteristic landform on ice stream beds solving the puzzle of their formation is key to understanding how ice interacts with its bed and how this in turn influences the dynamics of ice streams however a comprehensive and detailed characterization of this landform s size shape and spatial arrangement which might serve to test and refine formational theories is largely lacking this paper presents a detailed morphometric analysis and comparison of 4043 msgls from eight palaeo ice stream settings three offshore norway and antarctica four onshore canada and one from under a modern ice stream in west antarctica the length of msgls is lower than previously suggested mode 1000 2000 m median 2892 m and they initiate and terminate at various locations on an ice stream bed their spatial arrangement reveals a pattern that is characterized by an exceptional parallel conformity 80 of all mapped msgls have an azimuth within 5 from the mean values and a fairly constant lateral spacing mode 200 300 m median 330 m which we interpret as an indication that msgls are a spatially self organized phenomenon results show that size shape and spatial arrangement of msgls are consistent both within and also generally between different ice stream beds we suggest this results from a common mechanism of formation which is largely insensitive to local factors although the elongation of msgls mode 6 8 median 12 2 is typically higher than features described as drumlins these values and those of their width mode 100 200 m median 268 m overlap which suggests the two landforms are part of a morphological continuum and may share a similar origin we compare their morphometry with explicit predictions made by the groove ploughing and rilling instability theories of msgl formation although the latter was most compatible neither is fully supported by observations 2014 the authors earth surface processes and landforms published by john wiley sons ltd

formation of mega scale glacial lineations observed beneath a west antarctic ice stream most discharge from large ice sheets takes place through fast flowing ice streams and their speed is strongly modulated by interactions between the ice and the underlying sediments seismic surveys and investigations through boreholes have revealed a spatial association between fast ice flow and saturated deformable sediments nevertheless our knowledge of the morphology of the interface between ice and sediments is still limited resulting in only rudimentary understanding of the basal boundary conditions beneath ice streams and the generation of subglacial bedforms here we present radar data from the bed of a west antarctic ice stream that reveal the presence of mega scale glacial lineations we combine these data with previously published seismic data and show that these lineations develop in areas of dilatant deforming till and are part of a dynamic sedimentary system that undergoes significant change by erosion and deposition on decadal timescales we find that the mega scale glacial lineations are indistinguishable from those found on beds of palaeo ice streams providing conclusive evidence for the hypothesis that highly elongate bedforms are a characteristic of fast flow regions in ice sheets

an observation based method for reconstructing ocean surface changes using a 340 000 year deuterium excess record from the dome fuji ice core antarctica 1 an extended deuterium excess d or d excess record covering the last three glacial interglacial cycles ca 340 000 years was obtained from the dome fuji ice core antarctica in order to interpret the antarctic d record as a climatic history of vapor source regions the record should be corrected for the antarctic site temperature effect we propose an observation based method for correcting the d record that method uses a regression curve of the observed spatial relationship between d and d in antarctica on the assumption that the present slope has not changed unlike the previously proposed model based method our correction method results in considerable change in the d profile of the dome fuji ice core the corrected d record anticorrelates with a mean ocean 18o record r2 0 74 and correlates with a sst record r2 0 58 throughout the observed period these results emphasize the significance of our empirical correction method

seismic and geodetic evidence for grounding line control of whillans ice stream stick slip events the tidally modulated stick slip events of whillans ice stream in west antarctica produce seismic energy from three locations near the grounding line using ice velocity records obtained by combining time series from colocated broadband seismometers and gps receivers installed on the ice stream during the 2010 2011 and 2011 2012 austral summers along with far field seismic recordings of elastic waves we locate regions of high rupture velocity and stress drop these regions which are analogous to asperities in traditional seismic fault studies are areas of elevated friction at the base of the ice stream slip events consistently initiate at one of two locations near the center of the ice stream where events associated with the ross sea high tide originate or a grounding line spot where events associated with the ross sea low tide initiate as well as occasional high tide events following a skipped low tide event the grounding line site but not the central site produces rayleigh waves observable up to 1000 km away through fast expansion of the slip area grounding line initiation events also show strong directivity in the downstream direction indicating initial rupture propagation at 1 5 km s compared to an average of 0 150 km s for the entire slip event following slip initiation additional seismic energy is produced from two sources located near the grounding line first at the downstream end of subglacial lake engelhardt and second toward the farthest downstream extent of the ice stream this evidence suggests that the stronger higher friction material along the grounding line controls motion throughout the stick slip region

significant groundwater contribution to antarctic ice streams hydrologic budget satellite observations have revealed active hydrologic systems beneath antarctic ice streams but sources and sinks of water within these systems are uncertain here we use numerical simulations of ice streams to estimate the generation flux and budget of water beneath five ice streams on the siple coast we estimate that 47 of the total hydrologic input 0 98 km3 yr 1 to whillans wis mercer mis and kamb kis ice streams comes from the ice sheet interior and that only 8 forms by local basal melting the remaining 45 comes from a groundwater reservoir an overlooked source in which depletion significantly exceeds recharge of the total input to bindschadler bis and macayeal macis ice streams 0 56 km3 yr 1 72 comes from the interior 19 from groundwater and 9 from local melting this contrasting hydrologic setting modulates the ice streams flow and has important implications for the search for life in subglacial lakes

interannual variation of water isotopologues at vostok indicates a contribution from stratospheric water vapor combined measurements of water isotopologues of a snow pit at vostok over the past 60 y reveal a unique signature that cannot be explained only by climatic features as usually done comparisons of the data using a general circulation model and a simpler isotopic distillation model reveal a stratospheric signature in the 17o excess record at vostok our data and theoretical considerations indicate that mass independent fractionation imprints the isotopic signature of stratospheric water vapor which may allow for a distinction between stratospheric and tropospheric influences at remote east antarctic sites

geomorphology under ice streams moving from form to process ice streams are integral components of an ice sheet s mass balance and directly impact on sea level their flow is governed by processes at the ice bed interface which create landforms that in turn modulate ice stream dynamics through their influence on bed topography and basal shear stresses thus ice stream geomorphology is critical to understanding and modelling ice streams and ice sheet dynamics this paper reviews developments in our understanding of ice stream geomorphology from an historical perspective with a focus on the extent to which studies of modern and palaeo ice streams have converged to take us from a position of near complete ignorance to a detailed understanding of their bed morphology during the 1970s and 1980s our knowledge was limited and largely gleaned from geophysical investigations of modern ice stream beds in antarctica very few palaeo ice streams had been identified with any confidence during the 1990s however glacial geomorphologists began to recognise their distinctive geomorphology which included distinct patterns of highly elongated mega scale glacial lineations ice stream shear margin moraines and major sedimentary depocentres however studying relict features could say little about the time scales over which this geomorphology evolved and under what glaciological conditions this began to be addressed in the early 2000s through continued efforts to scrutinise modern ice stream beds at higher resolution but our current understanding of how landforms relate to processes remains subject to large uncertainties particularly in relation to the mechanisms and time scales of sediment erosion transport and deposition and how these lead to the growth and decay of subglacial bedforms this represents the next key challenge and will require even closer cooperation between glaciology glacial geomorphology sedimentology and numerical modelling together with more sophisticated methods to quantify and analyse the anticipated growth of geomorphological data from beneath active ice streams

flow dynamics and till genesis associated with a marine based antarctic palaeo ice stream geophysical and geological data indicate that during the last glacial cycle a palaeo ice stream drained the antarctic peninsula ice sheet apis through marguerite bay to the edge of the continental shelf via a bathymetric trough marguerite trough mega scale glacial lineations msgl in outer marguerite trough are formed in an acoustically transparent sediment unit consisting of a soft deformation till the association of msgl with deformation till indicates that bed deformation occurred beneath the ice stream and the thickness of the subglacial deforming layer was on the order of several metres msgl within outer marguerite trough are related to a pre existing thick sequence of soft sediments on the outer shelf and are inferred to have formed by a combination of subglacial sediment deformation by attenuation from a point source and groove ploughing morphologically the msgl differ in several respects from those documented elsewhere and this variation suggests a polygenetic origin for this subglacial landform an initial advance of the apis across the marguerite bay continental shelf at the lgm deposited a stiff subglacial till following this advance the ice margin within marguerite trough retreated 70 100 km before stabilising and depositing a grounding zone wedge an ice stream then developed in the trough and extended to the shelf edge forming a soft deformation till development of the ice stream may have been a glacio dynamic response to regional deglaciation subsequent retreat of the marguerite trough ice stream was underway by 13 490 uncorrected 14c years before present based on the geophysical and core data ice stream retreat appears to have been rapid and it contrasts with reconstructions of palaeo ice streams from other locations in antarctica this implies marked regional variations in antarctic ice stream dynamics during late quaternary deglaciation

ice stream sticky spots a review of their identification and influence beneath contemporary and palaeo ice streams rapidly flowing ice streams are the arterial drainage routes in continental ice sheets and exert a major influence on ice sheet mass balance recent observations have revealed that ice stream flow exhibits considerable variability with relatively rapid changes taking place in speed and direction this spatial and temporal variability is intimately linked to the conditions at the base of the ice streams and the distribution of localised patches of basal friction known as sticky spots in this paper we provide a detailed review of sticky spot observations from both contemporary and palaeo ice stream beds in order to better understand their nature and influence observations and theoretical considerations reveal four primary causes of stickiness i bedrock bumps ii till free areas iii areas of strong well drained till and iv freeze on of subglacial meltwater these may act together in one location or in isolation and a progressive increase in their distribution could lead to ice stream shut down bedrock bumps are influential under active ice streams where they provide form drag and can create thinner ice which increases the likelihood of basal freeze on increased bed roughness may prevent the lateral migration of some ice streams but bedrock bumps are unlikely to cause ice stream shut down because over long time scales ice stream erosion might be expected to reduce their amplitude the influence of till free areas beneath an ice stream will depend critically on the amount of water that might be drawn out of the surrounding till to lubricate such areas they are likely to be most important in ice stream onset zones but their identification has proved difficult beneath active ice streams if an ice stream operates solely by till deformation it is conceivable that a progressive increase in the exposure of till free areas could lead to shut down through a process of sediment exhaustion areas of strong well drained till have been identified beneath both active and ancient ice streams and are most likely to result from the reorganisation of subglacial meltwater the collapse of an inefficient cannalised system to a more efficient channelised system can occur rapidly and this mechanism has been hypothesised as a candidate for ice stream shut down in both contemporary and palaeo settings basal freeze on has also been observed and inferred from beneath modern and palaeo ice streams and a reduction in basal meltwater supply coupled with ice stream drawdown and the advection of cold ice increases the likelihood of switching off an ice stream a paucity of data from ice stream sticky spots limits a better understanding of their nature distribution and evolution beneath ice streams future technological advances are likely to improve the resolution of the data collected from the beds of modern ice streams but well preserved palaeo ice stream beds also hold potential for investigating their influence on ice stream flow and we present simple landsystems models to aid their identification such data will considerably enhance the basal boundary condition in ice stream models which will ultimately refine our predictions of the response of contemporary ice sheets to future changes in climate

episodic ice velocity fluctuations triggered by a subglacial flood in west antarctica height change anomalies in satellite altimeter data have been interpreted as the surface expressions of basal water moving into and out of subglacial lakes these signals have been mapped throughout antarctica on timescales of months to years but only broad connections have been made between active lakes and ice dynamics we present the first high frequency observations of ice velocity evolution due to a cascading subglacial lake drainage event collected over 5 years 2010 2015 using global positioning system data on whillans and mercer ice streams west antarctica we observed three episodic ice velocity changes over 2 years where flow speed increased by up to 4 as well as an 11 month disruption of the tidally modulated stick slip cycle that dominates regional ice motion our observations reveal that basal conditions of an antarctic ice stream can rapidly evolve and drive a dynamic ice response on subannual timescales which can bias observations used to infer long term ice sheet changes

combined measurements of 17oexcess and d excess in african monsoon precipitation implications for evaluating convective parameterizations water stable isotopes d18o dd are useful tools to depict and to understand the atmospheric water cycle in tropical regions they record the variations of convective activity and their implementation in convection schemes brings constraints on our understanding and parameterization of this phenomena here we present for the first time measurements of a new isotopic marker of the hydrological cycle 17oexcess resulting from the combination of d17o and d18o of water in convective regions on two different time scales i during the african monsoon onset and intra seasonal variability banizoumbou 2006 and ii during the squall line of the 11th of august 2006 niamey 17oexcess responds to the monsoon onset by a 30 per meg increase as well as to different convective processes in squall lines by 20 per meg variations these variations parallel those of d excess at first order and display significant correlation with relative humidity in the lower troposphere still higher correlation coefficients are observed between d excess and relative humidity than between 17oexcess and relative humidity suggesting a higher influence of relative humidity on d excess than on 17oexcess using a simple reevaporation model and a more sophisticated 2d model of a squall line we show that reevaporation is the process explaining the increase of d excess and 17oexcess with relative humidity for these two studies we also show that the combination of 17oexcess and d excess is a powerful tool to constrain the representation of isotopic processes during rain reevaporation in turn a good representation of such processes enables to use water isotopes to evaluate convective parameterization in atmospheric models 2010 elsevier b v

asymmetries in the moisture origin of antarctic precipitation the seasonality of moisture sources for precipitation in antarctica is studied with a lagrangian moisture source diagnostic moisture origin for precipitation in antarctica has strongly asymmetric properties which are related to the antarctic topography seasonal sea ice coverage and the land ocean contrasts in the mid latitudes of the southern hemisphere the highest altitudes of the east antarctic ice shield where major ice cores have been drilled have mean source latitudes of 45 40 s year round this finding contrasts to results from previous lagrangian studies which detected a more southerly moisture origin due to too short trajectories now results from lagrangian moisture source diagnostics are consistent with findings from general circulation models with tagged tracers thus both approaches can serve as a common benchmark for the interpretation of moisture source indicators based on stable isotopes such as deuterium excess in antarctic ice cores

new insights into southern hemisphere temperature changes from vostok ice cores using deuterium excess correction abstract the combination of both vostok ice core deuterium and deuterium excess histories over the last four climate cycles back to 420 ka bp offers a unique opportunity for reconstructions of vapor source region and local temperature variations tsource and tsite respectively relative to their modern values our study is based on an inversion of ice isotopic composition using a rayleigh based isotopic model which estimates the dependence of vostok precipitation isotopic composition on different climate controls moisture source temperature local deposition temperature and ocean isotopic composition both reconstructed local and source temperatures show no substantial effects from this correction in the sense that none of the rapid or large variations in raw deuterium and deuterium excess profiles can be entirely attributed to a second order climate control our discussion is focused on moisture source temperature and meridional temperature gradient history tsource tsite first using sea surface temperature time series we confirm that moisture source temperature signal recorded in vostok deuterium excess over the last 150 ka fully reflects the obliquity time varying relative contribution of low and high ocean latitudes to vostok precipitation second comparisons of source to site meridional temperature gradient with the vostok sodium record over 420 ka suggest that moisture transport and sea salt transport to the ice sheet are controlled by processes in different latitudes the sea salt being more of a high latitude signal

understanding the climatic signal in the water stable isotope records from the neem shallow firn ice cores in northwest greenland 1 samples of precipitation and atmospheric water vapor were collected together with shallow firn ice cores as part of the new deep drilling project in northwest greenland the neem project these samples were analyzed for their isotope composition to understand the processes affecting the climatic signal archived in the water stable isotope records from the neem deep ice core the dominant moisture source for the snow deposited at the neem site may be originating as far south as 35 n from the western part of the atlantic ocean the surface atmospheric water vapor appears in isotopic equilibrium with the snow surface indicating a large water exchange between the atmosphere and snowpack the interannual variability of neem shallow firn ice cores stable isotope data covering the last 40 years shows an unexpectedly weak nao signal regional to global atmospheric models simulate a dominant summer precipitation in the neem area suggesting that the intermittency of modern winter precipitation is responsible for the lack of a strong nao imprint the interannual variability of neem isotope data however shows a strong correlation with interannual variations of baffin bay sea ice cover a relationship consistent with air mass trajectories neem deep ice core isotopic records may therefore provide detailed information on past baffin bay sea ice extent neem stable water isotope content increasing trend points to a local warming trend of 3 0 c over the last 40 years

record of δ18o and 17o excess in ice from vostok antarctica during the last 150 000 years 1 we measured 17 o and 18 o in recent antarctic snow and down the vostok ice core and calculated the excess of 17 o with respect to vsmow the magnitude of the 17 o excess in the holocene and the last interglacial is 45 per meg and it remains constant in a transect from the coast to the continental interior analysis of the transect data shows that the 17 o excess is not sensitive to temperature variations over the continent there are significant shifts in 70 excess from low values in glacial to high values in interglacial times the observed shifts suggest higher normalized relative humidity and or wind speeds over the source oceanic regions in glacial times

tidal pacing skipped slips and the slowdown of whillans ice stream antarctica we summarize new observations of the deceleration and stick slip motion of whillans ice stream wis antarctica we refine the location of the large sticky spots that resist motion between slip events the locations of which are controlled by the patterns of subglacial water flow our examination of the long term velocity time series for the ice stream reveals that the decadal scale deceleration is not occurring at a steady rate but varies at the sub decadal timescale this unsteady deceleration modulates the temporal evolution of a broad 50 km across surface elevation bulge forming at the junction between the relatively narrow upstream portion of the ice stream and broad ice plain that constitutes the downstream end of wis comparison of observations from april 2003 and november 2010 reveals significant changes in the tidally modulated stick slip cycle that regulates motion on the ice plain we observe that the timing of slip events has become less regular in response to decreased flow speed in the upstream portions of the ice stream the decreased regularity of slip events has reduced the release of stored elastic strain during slip events increasing the rate of deceleration

variations of 17o 16o and 18o 16o in meteoric waters the variations of d 17 o and d 18 o in recent meteoric waters and in ice cores have proven to be an important tool for investigating the present and past hydrologic cycle in order to close significant information gaps in the present distribution ofd 17 o and d 18 o of meteoric water we have run precise measurements with respect to vsmow on samples distributed globally from low to high latitudes based on the new and existing data we present the global meteoric water line gmwl ford 17 o andd 18 o as ln d 17 o 1 0 528 ln d 18 o 1 0 000033 r 2 0 99999 in addition to meteoric water we carried out the first measurements of seawater from the pacific and atlantic oceans with respect to vsmow the obtained results show that the slope of the trend line ln d 17 o 1 vs ln d 18 o 1 of seawater samples is 0 528 the same as for meteoric water but the regression intercept is 5 per meg thus the positive intercept in the gmwl indicates an excess of 17 o in meteoric waters with respect to the ocean an excess or depletion of 17 o in water is defined as 17 o excess ln d 17 o 1 0 528 d 18 o 1 most meteoric water samples have positive 17 o excess of varying magnitudes with an average of 37 per meg with respect to vsmow we explain how these positive values originate from evaporation of sea water into marine air which is undersaturated in water vapor and how subsequent increase of 17 o excess occurs when atmospheric vapor condenses to form liquid and solid precipitation we also clarify the effect of excessive evaporation on 17 o excess finally based on the new results on 17 o excess of seawater we recalculated the relationship of d 17 o vs d 18 o in vapor diffusion in air as 18 adiff 1 0096

an oceanic cold reversal during the last deglaciation a detailed deuterium excess profile measured along the dome c epica european project for ice coring in antarctica core reveals the timing and strength of the sea surface temperature changes at the source regions for dome c precipitation we infer that an oceanic cold reversal took place in the southern indian ocean 800 years after the antarctic cold reversal the temperature gradient between the oceanic moisture source and antarctica is similar to the dome c sodium profile during the deglaciation illustrating the strong link between this gradient and the strength of the atmospheric circulation

vostok ice core a continuous isotope temperature record over the last climatic cycle 160 000 years a continuous deuterium profile along the 160 000 year vostok ice core antarctica is interpreted in terms of atmospheric temperature changes this climatic record is the awaited terrestrial complement of the deep sea records supporting the existence of a relation between the pleistocene climate and orbital forcing

deuterium excess in an east antarctic ice core suggests higher relative humidity at the oceanic surface during the last glacial maximum a continuous deuterium excess profile d d 8 18o was obtained from the dome c ice core spanning 32 000 yr it exhibits significant variations over this period in particular d was 4 5 lower around 18 000 yr bp than over the past 7 500 yr interpreted as reflecting higher relative humidity estimated 90 over the oceanic areas providing moisture for antarctic precipitation

glacial interglacial changes in ocean surface conditions in the southern hemisphere the stable isotope signatures of oxygen and hydrogen in the water of preserved ice and snow are both widely used to infer local temperatures of past environments a derived quantity based on these two signatures the deuterium excess 1 provides additional palaeoclimatic information2 3 4 as this parameter depends on the meteorological and oceanic characteristics of the water s source regions in particular their temperature2 3 and relative humidity4 published studies mainly focus on records from the past 40 000 years here we present a deuterium excess history obtained from ice cores from vostok east antarctica spanning the full glacial interglacial cycle of the past 150 000 years the deuterium excess record shows a strong anticorrelation with the earth s orbital obliquity 41 000 year periodicity and values are markedly higher during the cold stage 5d following the last interglacial than during the other cold stages we interpret the relationship with obliquity as resulting from changes in the latitudinal insolation gradient affecting ocean surface conditions and thus the delivery of moisture to the polar region we argue that the high 5d values relative to other cold stages are driven by relatively less moisture delivered from high latitudes and more from low latitudes the deuterium excess in antarctic precipitation thus provides long term spatially integrated information on ocean surface conditions and ocean atmosphere circulations in the southern hemisphere

snow accumulation and its moisture origin over dome argus antarctica the spatial and temporal variability of snow accumulation near dome argus antarctica is assessed using new snow pit and stake measurement data together with existing snow pit ice core and automatic weather station records snow accumulation rate shows large inter annual variations but stable multi decadal levels over the last seven centuries spatial variations in snow accumulation within the space of 50 km of dome argus are relatively small probably thanks to the smooth topography a comparison of theses accumulation observations with ecmwf reanalyses era 40 and era interim suggests ecmwf reanalysis captures the seasonal variations but underestimates the overall snow accumulation at dome argus by 50 the moisture sources for precipitation over dome argus are examined by means of a lagrangian moisture source diagnostic based on the tracing of specific humidity changes along air parcel trajectories for the period 2000 2004 using operational ecmwf analysis data dome argus mainly receives moisture from the mid latitude 46 4 s south indian ocean with a seasonal latitudinal shift of about 6 compared to other central east antarctic deep ice core sites such as dome f dome c vostok and epica dronning maud land dome argus has a more southerly moisture origin probably due to topographic influences on the moisture transport paths these results have important implications for the interpretation of future ice cores at dome argus

basal conditions on rutford ice stream west antarctica from seismic observations a seismic reflection profile perpendicular to the ice flow direction has been acquired on rutford ice stream antarctica an interpretation of both the amplitude and phase of the seismic reflections from the ice bed interface has been made to investigate the properties of the sub ice material the interpretation assumes that acoustic impedance can be used to imply subglacial sediment porosity multiple reflections from the ice bed interface on a number of seismic wide angle lines allowed a calibration of the reflection coefficient at the bed this enabled the acoustic impedance of the bed material to be calculated from the seismic reflection data mean acoustic impedance in the different sections of the bed ranges from 2 70 106 kg m 2 s 1 to 4 31 106 kg m 2 s 1 almost three quarters of the ice stream bed at this site appears to be saturated deforming sediments the rest of the bed is probably also saturated sediments but they are not deforming to any significant degree ice flow could include a combination of subglacial deformation and basal sliding localized regions which support disproportionately high amounts of basal shear stress may also occur from the seismic data it is not possible to apportion relative amounts of restraint to these different processes there does not appear to be any correlation between the different sections identified on the ice stream bed with either satellite images or nearby surface velocity data a feature which is interpreted as a subglacial drumlin is seen on the seismic section

basal mechanics of ice streams insights from the stick slip motion of whillans ice stream west antarctica the downstream portion of whillans ice stream west antarctica moves primarily by stick slip motion the observation of stick slip motion suggests that the bed is governed by velocity weakening physics and that the basal physics is more unstable than suggested by laboratory studies the stick slip cycle of whilla s ice plain exhibits substantial variability in both the duration of sticky periods and in slip magnitude to understand this variability we modeled the forces acting on the ice stream during the stick phase of the stick slip cycle the ocean tides introduce changes in the rate at which stress is applied to the ice plain increased loading rates promote earlier failure and vice versa results show that the bed of whillans ice stream strengthens over time healing during the quiescent intervals in the stick slip cycle with the bed weakening during slip events the time dependent strengthening of the ice plain bed following termination of slip events indicates that the strength of the bed may vary by up to 0 35 kpa during the course of a single day copyright 2009 by the american geophysical union

the origin of present day antarctic precipitation from surface snow deuterium excess data the deuterium excess d is defined as a linear combination of the d h and 18 o 16 o ratios in natural waters we present an interpretation of deuterium excess data in surface antarctic snow using an isotopic model to derive information on the origin of present day antarctic precipitation the data come from near the coast d47 adelie land and from inland south pole 1000 km from the sea the one dimensional isotopic model belongs to the rayleigh family but fully accounts for mixed cloud processes between 0 c and 30 c modeling d in polar snowfalls meets the problem of a large sensitivity to the saturation conditions prevailing at snow formation therefore we decided to tune the saturation in the model in order to yield realistic mean d values but we use the seasonal phase of d versus which is less sensitive to the saturation conditions as an independent validation of the model predictions both inland and near the coast our model can simulate the observed phase between d and in snow with moisture of subtropical origin 40 20 s although we do not treat specifically storm precipitation near the coast this study indicates that the observed phase between d and brings up a new validation of isotopic models it supports a distant vapor source for large scale precipitation delivered to the antarctic continent

mega scale glacial lineations and cross cutting ice flow landforms landsat images reveal a previously unsuspected large scale pattern of streamlining within drift that is assumed to reflect former phases of ice flow such a glacial grain can be regarded as a landform assemblage comprised of a number of components drumlins and megaflutes form part of the pattern but in addition there are two previously undocumented ice moulded landform elements streamlined lineations of much greater proportions referred to as mega scale glacial lineations and a distinctive cross cutting topology within the grain the ice moulded landform assemblage is described and illustrated with reference to examples from canada possible modes of genesis of such landforms are discussed and their glaciological implications outlined the discovery of this pattern indicates the pervasive nature of subglacial deformation of sediment and demands a radical re interpretation of ice sheet dynamics

melting and freezing beneath the ross ice streams antarctica we have estimated temperature gradients and melt rates at the bottom of the ice streams in west antarctica measured velocities were used to include the effects of horizontal advection and strain heating in the temperature model and to determine shear heating at the bed our modeled temperatures agree well with measured temperatures from boreholes in regions of steady flow we find that ice stream tributaries and the inland ice account for about 87 of the total melt generated beneath the ross ice streams and their catchments our estimates indicate that the ice plains of whillans ice stream and ice stream c even when active have large areas subject to basal freezing confirming earlier estimates that import of water from upstream is necessary to sustain motion the relatively low melt rates on whillans ice stream are consistent with observations of deceleration over the last few decades and suggest a shutdown may take place in the future possibly within this century while there are pockets of basal freezing beneath ice streams d and e there are larger areas of basal melt that produce enough melt to more than offset the freezing which is consistent with inferences of relatively steady flow for these ice streams over the last millennium

implications of basal micro earthquakes and tremor for ice stream mechanics stick slip basal sliding and till erosion abstract the whillans ice plain wip is unique among antarctic ice streams because it moves by stick slip the conditions allowing stick slip and its importance in controlling ice dynamics remain uncertain local basal seismicity previously observed during unstable slip is a clue to the mechanism of ice stream stick slip and a window into current basal conditions but the spatial extent and importance of this basal seismicity are unknown we analyze data from a 2010 2011 ice plain wide seismic and gps network to show that basal micro seismicity correlates with large scale patterns in ice stream slip behavior basal seismicity is common where the ice moves the least between unstable slip events with small discrete basal micro earthquakes happening within 10s of km of the central stick slip nucleation area and emergent basal tremor occurring downstream of this area basal seismicity is largely absent in surrounding areas where inter slip creep rates are high the large seismically active area suggests that a frictional sliding law that can accommodate stick slip may be appropriate for ice stream beds on regional scales variability in seismic behavior over inter station distances of 1 10 km indicates heterogeneity in local bed conditions and frictional complexity wip unstable slips may nucleate when stick slip basal earthquake patches fail over a large area we present a conceptual model in which basal seismicity results from slip weakening frictional failure of over consolidated till as it is eroded and mobilized into deforming till