fracture propagation to the base of the greenland ice sheet during supraglacial lake drainage surface meltwater that reaches the base of an ice sheet creates a mechanism for the rapid response of ice flow to climate change the process whereby such a pathway is created through thick cold ice has not however been previously observed we describe the rapid 2 hours drainage of a large supraglacial lake down 980 meters through to the bed of the greenland ice sheet initiated by water driven fracture propagation evolving into moulin flow drainage coincided with increased seismicity transient acceleration ice sheet uplift and horizontal displacement subsidence and deceleration occurred over the subsequent 24 hours the short lived dynamic response suggests that an efficient drainage system dispersed the meltwater subglacially the integrated effect of multiple lake drainages could explain the observed net regional summer ice speedup

greenland subglacial lakes detected by radar 1 subglacial lakes are an established and important component of the basal hydrological system of the antarctic ice sheets but none have been reported from greenland here we present airborne radio echo sounder res measurements that provide the first clear evidence for the existence of subglacial lakes in greenland two lakes with areas 8 and 10 km2 are found in the northwest sector of the ice sheet 40 km from the ice margin and below 757 and 809 m of ice respectively the setting of the greenland lakes differs from those of antarctic subglacial lakes being beneath relatively thin and cold ice pointing to a fundamental difference in their nature and genesis possibilities that the lakes consist of either ancient saline water in a closed system or are part of a fresh modern open hydrological system are discussed with the latter interpretation considered more likely

oxygen isotope studies of carbonaceous chondrites abstract the carbonaceous chondrites display the widest range of oxygen isotopic composition of any meteorite group as a consequence of the interaction of primordial isotopic reservoirs in the solar nebula these isotopic variations can be used to identify the reservoirs and to determine conditions and loci of their interactions we present a comprehensive set of whole rock analyses of cv co ck cm cr ch and ci chondrites as well as selected components of some of these meteorites a simple model is developed which describes the isotopic behavior during parent body aqueous alteration processes the process of thermal dehydration also produces a recognizable effect in the oxygen isotopic composition

tidal flexure at ice shelf margins hinge zones accommodate the differential tidal movement between floating ice shelves which move with the sea tide and grounded ice sheets which do not observations on rutford ice stream and ronne ice shelf antarctica using the kinematic method of global positioning system have yielded the first continuous tidal displacement profiles from an ice sheet hinge zone the form of these tidal displacement profiles indicates that the flexure can be fit to an elastic beam model using a parameter space optimization technique the model matches the observed tidal deflection profile to within 5 cm which is similar to the observational uncertainty the same model and parameter fitting technique is then successfully applied to various other published and unpublished tidal displacement data from doake ice rumples bach ice shelf and ekstrom ice shelf antarctica and jakobshavns glacier greenland i conclude that the elastic beam model with a single value of the elastic modulus 0 88 0 35 gpa adequately describes almost all the data and so can usefully be applied elsewhere earlier studies of tidal dissipation in ice shelf hinge zones based on a viscous or transient creep rheology showed that perhaps 30 of global tidal energy dissipation could be occurring in ice shelf hinge zones this study suggests however that an elastic rheology may be equally appropriate in which case this estimation of the tidal energy dissipation is excessively high

seasonal evolution of water contributions to discharge from a greenland outlet glacier insight from a new isotope mixing model the greenland ice sheet gris subglacial hydrological system may undergo a seasonal evolution with significant geophysical and biogeochemical implications we present results from a new isotope mixing model to quantify the relative contributions of surface snow glacial ice and delayed flow to the bulk meltwater discharge from a small 5k m 2 land terminating gris outlet glacier during melt onset may and at peak melt july we use radioactive 222 rn and stable isotopes 18 o deuterium to differentiate the water source contributions atmospherically derived 7 be further constrains meltwater transit time from the glacier surface to the ice margin we show that 1 222 rn is a promising tracer for glacial waters stored at the bed and 2 a quantitative chemical mixing model can be constructed by combining 222 rn and the stable water isotopes applying this model to the bulk subglacial outflow from our study area we find a constant delayed flow stored component from melt onset through peak melt this component is diluted first by snowmelt and then by increasing glacial ice melt as the season progresses results from this pilot study are consistent with the hypothesis that subglacial drainage beneath land terminating sections of the gris undergoes a seasonal evolution from a distributed to a channelized system

preliminary firn densification model with 38 site dataset firn densification modeling based on hot isostatic pressing with power law creep is investigated using depth density data from 38 sites that collectively have mean annual temperatures ranging from 216 to 256 k and accumulation rates ranging from 0 022 to 1 2 m w e a 1 we use an inverse technique to obtain free parameters in a simple physical model for different stages of time dependent firn densification our model works as well as or slightly better than previous models interpolating within the data range but extrapolating would require additional physics

greenland supraglacial lake drainages triggered by hydrologically induced basal slip a dense network of gps observations shows that rapid lake drainage events on the western greenland ice sheet are preceded by period of ice sheet uplift and or enhanced basal slip surface lakes are common during the warm season on the greenland ice sheet especially in middle to lower elevations these lakes can drain rapidly often preceded by a hydro fracturing event the mechanism behind the hydro fracturing and thus the connection between the lakes and the broader greenland hydrological system has been unclear now laura stevens and colleagues use a dense network of gps observations to show that rapid lake drainage events at their study site in western greenland were preceded by uplift events likely from water entering the basal system from neighboring moulins vertical conduits connecting the surface and bedrock the uplift created sufficient stress in the ice sheet to open the hydro fractures thus allowing the rapid lake drainage although speculative the authors suggest that the comparative lack of crevassing in upper elevations might limit the geographical spread of the uplift fracturing mechanism water driven fracture propagation beneath supraglacial lakes rapidly transports large volumes of surface meltwater to the base of the greenland ice sheet1 these drainage events drive transient ice sheet acceleration1 2 3 and establish conduits for additional surface to bed meltwater transport for the remainder of the melt season1 4 5 6 although it is well established that cracks must remain water filled to propagate to the bed7 8 9 the precise mechanisms that initiate hydro fracture events beneath lakes are unknown here we show that for a lake on the western greenland ice sheet drainage events are preceded by a 6 12 hour period of ice sheet uplift and or enhanced basal slip our observations from a dense global positioning system gps network allow us to determine the distribution of meltwater at the ice sheet bed before during and after three rapid drainages in 2011 2013 each of which generates tensile stresses that promote hydro fracture beneath the lake we hypothesize that these precursors are associated with the introduction of meltwater to the bed through neighbouring moulin systems vertical conduits connecting the surface and base of the ice sheet our results imply that as lakes form in less crevassed interior regions of the ice sheet10 11 12 13 14 where water at the bed is currently less pervasive5 14 15 16 the creation of new surface to bed conduits caused by lake draining hydro fractures may be limited

modeling supraglacial water routing and lake filling on the greenland ice sheet 1 during more intense melt years supraglacial lakes on the greenland ice sheet are found to fill more rapidly and to drain earlier in the melt season they also develop at higher latitudes and elevations since the rapid drainage of lakes has been shown to enhance basal sliding and since the lake volume can play an important role in determining if and when it drains understanding and modeling the processes which control lake filling rates forms a key development in successfully modeling the possible impacts of lake drainage events on ice motion we have developed a surface water routing and lake filling model and applied it to a 100 km2area of the paakitsoq region west greenland the model takes the time series of calculated runoff melt minus refreezing over the area and calculates flow paths and water velocities over the snow ice covered surface routing the water into topographic depressions which can fill to form lakes runoff is calculated from a distributed surface energy balance model coupled to a subsurface model which calculates changes in temperature density and water content in the snow firn and upper ice layers and hence refreezing and therefore net runoff the model is calibrated against field measurements of a filling lake in our study area during june 2011 and can be used to calculate the filling rate of the instrumented lake with a high degree of accuracy the filling rate of the instrumented modeled lake depends on melt and routing from the immediate lake catchment and from overflowing lakes in upstream catchments

theoretical contemporary observational and palaeo perspectives on ice sheet hydrology processes and products publisher elsevier articletitle theoretical contemporary observational and palaeo perspectives on ice sheet hydrology processes and products journaltitle earth science reviews articlelink http dx doi org 10 1016 j earscirev 2016 01 010 content type article copyright copyright 2016 elsevier b v all rights reserved

over winter persistence of supraglacial lakes on the greenland ice sheet results and insights from a new model we present a newly developed 1 d numerical energy balance and phase transition supraglacial lake model glacierlake glacierlake incorporates snowfall in situ snow and ice melt incoming water from the surrounding catchment ice lid formation basal freeze up and thermal stratification snow cover and temperature are varied to test lake development through winter and the maximum lid thickness is recorded average wintertime temperatures of 2 to 30 circ rm c and total snowfall of 0 to 3 45 m lead to a range of the maximum lid thickness from 1 2 to 2 8 m after sim 250 days with snow cover exerting the dominant control an initial ice temperature of 15 circ rm c with simulated advection of cold ice from upstream results in 0 6 m of basal freeze up this suggests that lakes with water depths above 1 3 to 3 4 m dependent on winter snowfall and temperature upon lid formation will persist through winter these buried lakes can provide a sizeable water store at the start of the melt season expedite future lake formation and warm underlying ice even in winter

a comparison of supraglacial lake observations derived from modis imagery at the western margin of the greenland ice sheet supraglacial lakes sgls affect the dynamics of the greenland ice sheet by storing runoff and draining episodically we investigate the evolution of sgls as reported in three datasets each based on automated classification of satellite imagery although the datasets span the period 2001 10 there are differences in temporal sampling and only the years 2005 07 are common by subsampling the most populous dataset we recommend a sampling frequency of one image per 6 5 days in order to minimize uncertainty associated with poor temporal sampling when compared with manual classification of satellite imagery all three datasets are found to omit a sizeable 29 48 and 41 fraction of lakes and are estimated to document the average size of sgls to within 0 78 0 48 and 0 95km2 we combine the datasets using a hierarchical scheme producing a single optimized dataset this combined record reports up to 67 more lakes than a single dataset during 2005 07 the rate of sgl growth tends to follow the rate at which runoff increases in each year in 2007 lakes drain earlier than in 2005 and 2006 and remain absent despite continued runoff this suggests that lakes continue to act as open surface bed conduits following drainage

modeling subglacial water routing at paakitsoq w greenland 1 the functioning of the greenland ice sheet s subglacial drainage system and its effect on ice dynamics have been inferred largely from hypothetical hydrology dynamics models and from analysis of satellite data and in situ gps measurements despite this there is still uncertainty about how the surface hydrology interacts with the subglacial drainage system and affects basal water pressures and ice flow especially over annual time scales to address this we developed a high spatial 100 m and temporal 1 h resolution distributed physically based subglacial hydrological model and applied it to the paakitsoq region western greenland the model is driven with moulin input hydrographs calculated by a surface routing and lake filling draining model forced ultimately with distributed hourly runoff calculated by a surface mass balance model key outputs from the model are spatially and temporally varying subglacial water pressures and proglacial stream hydrographs early in the melt season short spikes in water pressure lasting less than a day occur as a result of lake drainage events during midsummer there are sustained periods of high water pressure lasting days to weeks even at times when the subglacial system is inferred to be predominantly efficient later in the summer large diurnal fluctuations in water pressure occur with peaks regularly exceeding ice overburden pressure superimposed on a gradually declining trend these phenomena support the results of previous hypothetical modeling efforts and inferences drawn from gps measurements

controls on rapid supraglacial lake drainage in west greenland an exploratory data analysis approach the controls on rapid surface lake drainage on the greenland ice sheet gris remain uncertain making it challenging to incorporate lake drainage into models of gris hydrology and so to determine the ice dynamic impact of meltwater reaching the ice sheet bed here we first use a lake area and volume tracking algorithm to identify rapidly draining lakes within west greenland during summer 2014 second we derive hydrological morphological glaciological and surface mass balance data for various factors that may influence rapid lake drainage third these factors are used within exploratory data analysis to examine existing hypotheses for rapid lake drainage this involves testing for statistical differences between the rapidly and non rapidly draining lake types as well as examining associations between lake size and the potential controlling factors this study shows that the two lake types are statistically indistinguishable for almost all factors investigated except lake area thus we are unable to recommend an empirically supported deterministic alternative to the fracture area threshold parameter for modelling rapid lake drainage within existing surface hydrology models of the gris however if improved remotely sensed datasets e g ice velocity maps climate model outputs were included in future research it may be possible to detect the causes of rapid drainage

accretionary rims on inclusions in the allende meteorite abstract many inclusions in allende particularly those with irregular shapes are surrounded by a sequence of thin layers which differ from one another in texture mineralogy and mineral chemistry the layer underlying all others contains either ia pyroxene needles olivine clumps of hedenbergite and andradite ib olivine doughnuts or ic rectangular olivine crystals the next layer outward ii contains tiny

seasonal evolution of supra glacial lakes on the greenland ice sheet abstract a survey of supra glacial lakes on the western margin of the greenland ice sheet reveals a seasonally driven hydrological system culminating in widespread lake drainage in late summer we used satellite imagery to study the evolution of 292 lakes across two sites totalling 22 000 km 2 in area during 2001 the lakes combined area increased to 75 5 km 2 by the beginning of july over the following 25 days an area totalling 36 3 5 km 2 drained from 216 lakes at one study site we used meteorological data and a positive degree day model to calculate the volume of water generated by melting in the lake catchments based on this estimate the mean depth of filling lakes surveyed rose from 1 5 0 7 m on 7th july to 3 9 1 1 m on 1st august in agreement with a value for one lake of 4 4 0 9 m we have derived from airborne altimetry during this 25 day period we estimate that 38 18 10 7 m 3 of water drained from the surface at this site and that there was an average water flux of 1 3 0 3 m 3 s 1 passing through each lake that drained completely

ice melting and downward transport of meltwater by two phase flow in europa s ice shell with its young surface very few impact craters and the abundance of tectonic and cryovolcanic features europa has likely been subjected to relatively recent endogenic activity morphological analyses of chaos terrains and double ridges suggest the presence of liquid water within the ice shell a few kilometers below the surface which may result from enhanced tidal heating a major issue concerns the thermal gravitational stability of these water reservoirs here we investigate the conditions under which water can be generated and transported through europa s ice shell we address particularly the downward two phase flow by solving the equations for a two phase mixture of water ice and liquid water in one dimensional geometry in the case of purely temperate ice we show that water is transported downward very efficiently in the form of successive porosity waves the time needed to transport the water from the subsurface region to the underlying ocean varies between 1 and 100 kyr depending mostly on the ice permeability we further show that water produced in the head of tidally heated hot plumes never accumulates at shallow depths and is rapidly extracted from the ice shell within less than a few hundred kiloyears our calculations indicate that liquid water will be largely absent in the near subsurface with the possible exception of cold conductive regions subjected to strong tidal friction recently active double ridges subjected to large tidally driven strike slip motions are perhaps the most likely candidates for the detection of transient water lenses at shallow depths on europa

seasonal evolution of supraglacial lakes on an east antarctic outlet glacier supraglacial lakes are known to influence ice melt and ice flow on the greenland ice sheet and potentially cause ice shelf disintegration on the antarctic peninsula in east antarctica however our understanding of their behavior and impact is more limited using 150 optical satellite images and meteorological records from 2000 to 2013 we provide the first multiyear analysis of lake evolution on langhovde glacier dronning maud land 69 11 s 39 32 e we mapped 7990 lakes and 855 surface channels up to 18 1 km inland 670 m above sea level from the grounding line and document three pathways of lake demise i refreezing ii drainage to the englacial subglacial environment on the floating ice and iii overflow into surface channels on both the floating and grounded ice the parallels between these mechanisms and those observed on greenland and the antarctic peninsula suggest that lakes may similarly affect rates and patterns of ice melt ice flow and ice shelf disintegration in east antarctica

measurements and numerical simulation of fabric evolution along the talos dome ice core antarctica we present measurements of fabrics and microstructures made along the talos dome ice core a core drilled in east antarctica in the framework of the taldice project fabric and average grain size data are analyzed regarding changes in climatic conditions in particular the fabric strength increases sharply going downward from holocene to wisconsin ice following durand et al 2007 this change is associated with a positive feedback between variations in ice viscosity due to variations in dust content and the impact of a shear stress component increasing with depth a viscoplastic self consistent modeling approach is used to simulate the fabric evolution for a perfect dome configuration the discrepancies between the measured and the simulated fabrics highlight the depth ranges where shear strongly affects the fabric strengthening finally the grain size and fabric analyses show the occurrence of dynamic recrystallization mechanisms continuous and discontinuous along the core

the local insolation signature of air content in antarctic ice a new step toward an absolute dating of ice records an accurate chronology of ice cores is still needed for interpreting the paleoclimatic record and to understand the relation between insolation and climate a new domain of research in this area has been stimulated by the work of m bender linking the record of n2 o2 ratio in the air trapped in the ice with the local insolation here we investigate the potential of the air content of polar ice v as another and complementary ice proxy of local insolation we propose that the long term changes in air content recorded in ice from the high antarctic plateau is dominantly imprinted by the local summer insolation the v measurements covering the last 440 000 yr and obtained along the epica dc edc ice core are presented 86 of the variance observed in the v record can be explained neither by air pressure nor by temperature changes and then should reflect properties influencing the porosity at close off other than temperature a wavelet analysis indicates a dominant obliquity period around 41 ka over the last 440 ka we propose a mechanism which can account for the observed anti correlation between local insolation and v and explain how the local insolation via the integrated summer insolation can affect the near surface snow structure and consequently the porosity at close off the v and the integrated summer insolation changes show high coherency over all the record and a variable phase relationship with a maximum phase difference of about 4000 yr around 200 ka ago the phase difference may reflect the difference between the edc2 chronology based on an inverse dating method and an accurately dated insolation curve our work by proposing a new and complementary ice proxy for local insolation should eventually contribute to the establishment of an absolute dating of the ice paleo record

evolution of the subglacial hydrologic system beneath the rapidly decaying cordilleran ice sheet caused by ice dammed lake drainage implications for meltwater induced ice acceleration abstract a positive correlation between ice dammed lake drainage and ice acceleration at antarctic ice sheets ais and land terminating sections of the greenland ice sheet gris has been implicated in enhanced ice sheet decay however the paucity of direct measurements at the ice sheet bed restricts our understanding of subglacial drainage system evolution in response to transient water inputs we present evidence that two meltwater corridors on the former bed of the thin 600 m at last glacial maximum over the interior plateaus of british columbia and rapidly decaying cordilleran ice sheet cis were generated subglacially in response to the drainage of an ice dammed lake and operated as canals tunnel channels geomorphological ground penetrating radar gpr and electrical resistivity tomography ert data reveal a simple event sequence that includes initial propagation of a broad at least 2 5 km wide floodwave inefficient drainage from an ice dammed lake over relatively short 3 24 km zones at the corridor heads that collapsed into efficient canals large up to 0 25 2 5 km wide channels incised down into the sediment bed and up into the ice downglacier canal formation on the southern fraser plateau involved synchronous along the full canal length system development including elements of headward erosion and plunge pool formation our data suggest that ice dammed lake drainage beneath a rapidly decaying thin ice mass that has an efficient antecedent drainage network is not conducive to large scale ice acceleration these data may aid better assessment of the role of ice dammed lake drainage on the dynamics of former as well as contemporary ice sheets

metamorphism of stratified firn at dome fuji antarctica a mechanism for local insolation modulation of gas transport conditions during bubble close off 1 the evolution of the structure of a 112 59 m long firn core recovered at dome fuji east antarctica was investigated in order to improve understanding of firn densification and bubble formation processes which are important for interpreting local insolation proxies used for astronomical dating of deep ice cores using selected samples we measured physical properties including 1 the relative dielectric permittivities in both the vertical and horizontal planes 2 the bulk density at a resolution of millimeters 3 the three dimensional geometric structure of pore space and 4 crystal orientation fabrics we found that the firn at dome fuji contains horizontal strata with thicknesses of several centimeters near the surface of the ice sheet these strata are characterized by contrasting bulk density earlier field studies suggest that summer insolation causes densification of surface firn down to 30 m density maxima exhibited a clear positive correlation with the strength of structural anisotropy and c axis clustering around the vertical in contrast the correlation is negative in deeper firn confirming previous findings that initially less dense firn became denser than initially dense firn in addition numerous examples of textures indicating that deformation preferentially occurred in weaker layers were found moreover the initially dense firn layers were more permeable for air near the bottom of firn we propose a model linking firn properties with conditions for the gas transport processes near the bottom of firn the model explains how stronger insolation can lead to bulk ice with a lower o2 n2 ratio and smaller total gas content

matrix mineralogy of the lance co3 carbonaceous chondrite a transmission electron microscope study the lance co3 carbonaceous chondrite cc is less altered than the ci and cm chondrites and so provides a view of the mineralogy and textures resulting from the earliest stages of aqueous alteration of ccs matrix olivine in lance has been partly altered to fine grained fe bearing serpentine and poorly crystalline fe3 oxide a process that required both hydration and oxidation serpentine occurs as discrete packets separated from the olivine surfaces by the fe3 oxide the fe released during the dissolution of olivine was partly incorporated into the serpentine the remainder was oxidized to form fe3 oxide matrix metal was also altered to produce fe oxides leaving the residual metal enriched in ni olivine grains in lance matrix contain channels along their 100 and 001 directions the formation and convergence of such channels resulted in a grain size reduction of the olivine the alteration was pervasive but incomplete suggesting a limited availability of fluid a brief study of two other co chondrites kainsaz and warrenton shows that these meteorites do not contain phyllosilicates in their matrices although both contain fe3 oxide between olivine grains prior to its alteration lance probably resembled kainsaz an unaltered co3 chondrite the alteration assemblage in lance is only slightly different from that in mokoia and essentially the same as that in c3 xenoliths from murchison alteration products in lance show greater similarities to ci than to cm chondrites

exploring the ingredients required to successfully model the placement generation and evolution of ice streams in the british irish ice sheet abstract ice stream evolution is a major uncertainty in projections of the future of the greenland and antarctic ice sheets accurate simulation of ice stream evolution requires an understanding of a number of ingredients that control the location and behaviour of ice stream flow here we test the influence of geothermal heat flux grid resolution and bed hydrology on simulated ice streaming the palaeo record provides snapshots of ice stream evolution with a particularly well constrained ice sheet being the british irish ice sheet biis we implement a new basal sliding scheme coupled with thermo mechanics into the bisicles ice sheet model to simulate the evolution of the biis ice streams we find that the simulated location and spacing of ice streams matches well with the empirical reconstructions of ice stream flow in terms of position and direction when simple bed hydrology is included we show that the new basal sliding scheme allows the accurate simulation for the majority of biis ice streams the extensive empirical record of the biis has allowed the testing of model inputs and has helped demonstrate the skill of the ice sheet model in simulating the evolution of the location spacing and migration of ice streams through millennia simulated ice streams also prompt new empirical mapping of features indicative of streaming in the north channel region ice sheet model development has allowed accurate simulation of the palaeo record and allows for improved modelling of future ice stream behaviour

drainage from water filled crevasses along the margins of jakobshavn isbrae a potential catalyst for catchment expansion 1 saturated crevasses occur in local depressions within the shear margins of jakobshavn isbrae at inflections in the ice stream s flow direction spatio temporal variability of seven distinctive saturated crevasse groups was examined during the 2007 melt season the area of saturated crevasses reached its maximum extent 1 8 km2 in early july and remained largely constant until early august filling rates are correlated with regional melt production while drainage rates are highly correlated with areal extent estimates on potential drainage volume from the largest crevasse system are 9 23 10 3 km3 2 15 10 8 km3 and 4 92 10 2 km3 3 58 10 8 km3 respectively over a 16 day interval and are more than required for a distributed basal hydrologic system across this area to temporarily flood bedrock obstacles believed to control basal sliding future drainage events likely extending farther inland with warming could result in enhanced lateral mass discharge into the ice stream with implications for the dynamic evolution of the entire basin

accretionary dust mantles in cm chondrites evidence for solar nebula processes abstract the texture and modal composition of fourteen carbonaceous chondrites of the cm group were studied in detail to unravel the origin of their various components and the evolution of their parent body ies the analysis and mapping of complete thin sections by scanning electron and optical microscopy revealed textural units in most samples in which all coarse grained primordial chondritic components chondrules chondrule fragments refractory inclusions particles rich in hydrous silicates pcp and various mineral fragments are coated by fine grained serpentine and tochilinite rich dust the dust mantled particles have spheroidal shapes and form a special lithology in which they are densely aggregated and compacted without any interstitial clastic material the dust mantles termed accretionary dust mantles and their host lithology called primary accretionary rock are interpreted as products of accretion processes in the solar nebula an origin by processes on the parent body is excluded for textural and mineralogical reasons most cm chondrites are affected by secondary impact induced brecciation of variable intensity except for yamato 791198 which is composed entirely of a primary accretionary rock obviously unaltered by secondary parent body processes such as aqueous alteration or brecciation the cm chondrites y74662 haripura cold bokkeveld kivesvaara mighei pollen nogoya y793321 alh 83100 murray and murchison consist of clasts of primary accretionary rock embedded in a fine grained clastic matrix derived from primary rock material the volume percentage of the clastic matrix ranges from 5 y74662 to 76 murchison bells and essebi are brecciated and affected by aqueous alteration to an extent that fragments of primary rock are not preserved the evaluation of literature data indicates that solar wind implanted noble gases are absent in the primary accretionary rock component of cm chondrites they are only present in brecciated samples it must be concluded from the relation between the content of trapped solar wind and degree of brecciation that 1 the solar noble gases are residing exclusively in the clastic matrix and 2 some cm chondrites are regolith breccias and others are fragmental or monomict breccias both types were formed on the parent body although the latter were never exposed to the solar wind based on the results of textural chemical and mineralogical investigations we conclude that a major portion of the water bearing phases in cm chondrites were produced by the hydration of refractory phases prior to the formation of dust mantles and prior to the formation of the recent cm parent body this process requires special physicochemical conditions which allow the crystallization of oh bearing silicates e g higher density of the nebula gas or special precursor planetesimals of the cm chondrite parent body

molecular characterization of dissolved organic matter associated with the greenland ice sheet abstract subsurface microbial oxidation of overridden soils and vegetation beneath glaciers and ice sheets may affect global carbon budgets on glacial interglacial timescales the likelihood and magnitude of this process depends on the chemical nature and reactivity of the subglacial organic carbon stores we examined the composition of carbon pools associated with different regions of the greenland ice sheet subglacial supraglacial proglacial in order to elucidate the type of dissolved organic matter dom present in the subglacial discharge over a melt season electrospray ionization esi fourier transform ion cyclotron resonance ft icr mass spectrometry coupled to multivariate statistics permitted unprecedented molecular level characterization of this material and revealed that carbon pools associated with discrete glacial regions are comprised of different compound classes specifically a larger proportion of protein like compounds were observed in the supraglacial samples and in the early melt season spring subglacial discharge in contrast the late melt season summer subglacial discharge contained a greater fraction of lignin like and other material presumably derived from underlying vegetation and soil these results suggest 1 that the majority of supraglacial dom originates from autochthonous microbial processes on the ice sheet surface 2 that the subglacial dom contains allochthonous carbon derived from overridden soils and vegetation as well as autochthonous carbon derived from in situ microbial metabolism and 3 that the relative contribution of allochthonous and autochthonous material in subglacial discharge varies during the melt season these conclusions are consistent with the hypothesis that given sufficient time e g overwinter storage resident subglacial microbial communities may oxidize terrestrial material beneath the greenland ice sheet

allan hills 88019 an antarctic h chondrite with a very long terrestrial age we have measured the concentrations of the cosmogenic radionuclides loge 26al and 36cl half lives 1 51 ma 716 ka and 300 ka respectively in two different laboratories by accelerator mass spec trometry ams techniques as well as concentrations and isotopic compositions of stable he ne and ar in the antarctic h chondrite allan hills alh 88019 in addition nuclear track densities were measured from these results it is concluded that the meteoroid alh 880 19 had a preatmospheric radius of 20 5 cm and a shielding depth for the analyzed samples of between 4 and 8 cm using calculated and experimentally deter mined production rates of cosmogenic nuclides an exposure age of 40 ma is obtained from cosmogenic 21ne and 38ar the extremely low concentrations of radionuclides are explained by a very long terrestrial age for this meteorite of 2 f 0 4 ma a similarly long terrestrial age was found so far only for the antarctic l chon drite lewis cliff lew 86360 such long ages establish one boundary condition for the history of mete orites in antarctica

annual down glacier drainage of lakes and water filled crevasses at helheim glacier southeast greenland supraglacial lake drainage events are common on the greenland ice sheet observations on the west coast typically show an up glacier progression of drainage as the annual melt extent spreads inland we use a suite of remote sensing and modelling techniques in order to study a series of lakes and water filled crevasses within 20km of the terminus of helheim glacier south east greenland automatic classification of surface water areas shows a down glacier progression of drainage which occurs in the majority of years between 2007 and 2014 we demonstrate that a linear elastic fracture mechanics model can reliably predict the drainage of the uppermost supraglacial lake in the system but cannot explain the pattern of filling and draining observed in areas of surface water downstream we propose that the water levels in crevasses downstream of the supraglacial lake can be explained by a transient high pressure wave passing through the subglacial system following the lake drainage we support this hypothesis with analysis of the subglacial hydrological conditions which can explain both the position and interannual variation in filling order of these crevasses similar behaviour has been observed in association with jokulhaups surging glaciers and antarctic subglacial lakes but has not previously been observed on major outlets of the greenland ice sheet our results suggest that the behaviour of near terminus surface water may differ considerably from that of inland supraglacial lakes with the potential for basal water pressures to influence the presence of surface water in crevasses close to the terminus of tidewater glaciers

quantifying supraglacial meltwater pathways in the paakitsoq region west greenland increased summer ice velocities on the greenland ice sheet are driven by meltwater input to the subglacial environment however spatial patterns of surface input and partitioning of meltwater between different pathways to the base remain poorly understood to further our understanding of surface drainage we apply a supraglacial hydrology model to the paakitsoq region west greenland for three contrasting melt seasons during an average melt season crevasses drain 47 of surface runoff lake hydrofracture drains 3 during the hydrofracturing events themselves while the subsequent surface to bed connections drain 21 and moulins outside of lake basins drain 15 lake hydrofracture forms the primary drainage pathway at higher elevations above 850 m while crevasses drain a significant proportion of meltwater at lower elevations during the two higher intensity melt seasons model results show an increase 5 and 6 of total surface runoff in the proportion of runoff drained above 1300 m relative to the melt season of average intensity the potential for interannual changes in meltwater partitioning could have implications for how the dynamics of the ice sheet respond to ongoing changes in meltwater production

transmission electron microscopy of subsolidus oxidation and weathering of olivine olivine crystals in basaltic andesites which crop out in the abert rim south central oregon have been studied by high resolution and analytical transmission electron microscopy the observations reveal three distinct assemblages of alteration products that seem to correspond to three episodes of olivine oxidation the olivine crystals contain rare dense arrays of coherently intergrown ti free magnetite and inclusions of a phase inferred to be amorphous silica we interpret this first assemblage to be the product of an early subsolidus oxidation event in the lava the second olivine alteration assemblage contains complex ordered intergrowths on 001 of forsterite rich olivine and laihunite distorted olivine structure with fe3 charge balanced by vacancies based on experimental results for laihunite synthesis kondoh et al 1985 these intergrowths probably formed by olivine oxidation between 400 and 800 c the third episode of alteration involves the destruction of olivine by low temperature hydrothermal alteration and weathering elongate etch pits and channels in the margins of fresh olivine crystals contain semi oriented bands of smectite olivine weathers to smectite and hematite and subsequently to arrays of oriented hematite crystals the textures resemble those reported by eggleton 1984 and smith et al 1987 we find no evidence for a metastable phase intermediate between olivine and smectite m eggleton 1984 the presence of laihunite exerts a strong control on the geometry of olivine weathering single laihunite layers and laihunite forsteritic olivine intergrowths increase the resistance of crystals to weathering preferential development of channels between laihunite layers occurs where growth of laihunite produced compositional variations in olivine rather than where coherency strain is associated with laihunite olivine interfaces

constraints on the lake volume required for hydro fracture through ice sheets 1 water filled cracks are an effective mechanism to drive hydro fractures through thick ice sheets crack geometry is therefore critical in assessing whether a supraglacial lake contains a sufficient volume of water to keep a crack water filled until it reaches the bed in this study we investigate fracture propagation using a linear elastic fracture mechanics model to calculate the dimensions of water filled cracks beneath supraglacial lakes we find that the cross sectional area of water filled cracks increases non linearly with ice sheet thickness using these results we place volumetric constraints on the amount of water necessary to drive cracks through 1 km of sub freezing ice for ice sheet regions under little tension lakes larger than 0 25 0 80 km in diameter contain sufficient water to rapidly drive hydro fractures through 1 1 5 km of subfreezing ice this represents 98 of the meltwater volume held in supraglacial lakes in the central western margin of the greenland ice sheet

seasonal speedup of a greenland marine terminating outlet glacier forced by surface melt induced changes in subglacial hydrology we present subdaily ice flow measurements at four gps sites between 36 and 72 km from the margin of a marine terminating greenland outlet glacier spanning the 2009 melt season our data show that 35 km from the margin seasonal and shorter time scale ice flow variations are controlled by surface melt induced changes in subglacial hydrology following the onset of melting at each site ice motion increased above background for up to 2 months with resultant up glacier migration of both the onset and peak of acceleration later in our survey ice flow at all sites decreased to below background multiple 1 to 15 day speedups increased ice motion by up to 40 above background these events were typically accompanied by uplift and coincided with enhanced surface melt or lake drainage our results indicate that the subglacial drainage system evolved through the season with efficient drainage extending to at least 48 km inland during the melt season while we can explain our observations with reference to evolution of the glacier drainage system the net effect of the summer speed variations on annual motion is small 1 this in part is because the speedups are compensated for by slowdowns beneath background associated with the establishment of an efficient subglacial drainage system in addition the speedups are less pronounced in comparison to land terminating systems our results reveal similarities between the inland ice flow response of greenland marine and land terminating outlet glaciers

long term changes in the snow firn pack stratigraphy on urumqi glacier no 1 eastern tien shan china we report on changes to the snowpack of urumqi glacier no 1 eastern tien shan china resulting from climate change between 1961 and 2005 data from more than 120 snow pits were used to investigate temporal changes in the snow stratigraphy for three periods 1961 62 1980 83 and 2002 05 both the thickness and structure of the snow were found to be significantly altered by climatic warming the proportion of coarse grained firn which is usually formed by infiltration water was found to have increased from 40 to 65 over the period 1961 2005 and the distribution of the glacier zones also changed significantly with all boundaries moving up slope the recrystallization infiltration zone disappeared in 1989 the mean ablation rate over the melt season increased by 26 from 1962 to 1980 at the west branch site and by 75 from 1980 to 2004 at the east branch site

remote sounding of greenland supraglacial melt lakes implications for subglacial hydraulics a supraglacial lake depth retrieval function is developed based on the correspondence between moderate resolution imaging spectroradiometer modis reflectance and water depth measured during raft surveys individual lake depth area and volume statistics including short term temporal changes for greenland s southwestern ablation region were compiled for 2000 05 the maximum area of an individual lake was found to be 8 9 km 2 the maximum volume 53 0 10 6 m 3 and the maximum depth 12 2 m sampling over 0 0625 km 2 pixel areas the total lake volume reaches 1 km 3 in this region by july each year the importance of melt lake reservoirs to greenland ice sheet flow may be a feedback between abrupt lake drainage events and ice dynamics lake outburst volumes up to 31 5 10 6 m 3 d 1 are capable of providing sufficient water via moulins to hydraulically pressurize the subglacial environment since the overburden pressure at the base of a flooded moulin is greater than that provided by ice lake outburst events seem capable of exerting sufficient upward force to lift the ice sheet locally if water flow in the subglacial environment is constrained laterally considering a moulin with a 10 m 2 cross sectional area basal pressurization can be maintained over lake outburst

a pressure sintering model for the densification of polar firn and glacier ice on presente les predictions de la theorie de frittage pression pour la glace pure et on les compare avec les mesures d indlandsis au groenland et en antarctique

a model for subglacial flooding through a preexisting hydrological network during the rapid drainage of supraglacial lakes increasingly large numbers of supraglacial lakes form and drain every summer on the greenland ice sheet some of these lakes drain rapidly within the timescale of a few hours and the vertical discharge of water during these events may find a preexisting film of water potentially within a distributed drainage system of linked cavities here we present a model for subglacial flooding applied specifically to such circumstances one of many interesting results we find is that water flows in from the far field prior to the arrival of flooding we systematically evaluate the effect of initial ice bed opening on the degree of perturbation to the subglacial system of particular importance we find that floods propagate much faster and vertical displacements are much greater for larger openings for 10 and 1 cm of initial opening for example floods travel about 68 and 50 farther than in the fully coupled ice bed scenario after 2 h of drainage respectively for the same choices of initial opening the elastostatic displacement at the injection point is about 1 39 and 1 26 times that of the fully coupled scenario using the framework with a preexisting water film results in avoiding the pressure singularity that is inherent to classical hydrofracture models thus opening an avenue for integrating the likes of our model within continuum subglacial hydrological models furthermore we foresee that the theory presented can be used to potentially infer subglacial hydrological conditions from surface observables

limits to future expansion of surface melt enhanced ice flow into the interior of western greenland moulins are important conduits for surface meltwater to reach the bed of the greenland ice sheet it has been proposed that in a warming climate newly formed moulins associated with the inland migration of supraglacial lakes could introduce surface melt to new regions of the bed introducing or enhancing sliding there by examining surface strain rates we found that the upper limit to where crevasses and therefore moulins are likely to form is 1600m this is also roughly the elevation above which lakes do not drain completely thus meltwater above this elevation will largely flow tens of kilometers through surface streams into existing moulins downstream furthermore results from a thermal ice sheet model indicate that the 1600m crevassing limit is well below the wet frozen basal transition 2000m together these data sets suggest that new supraglacial lakes will have a limited effect on the inland expansion of melt induced seasonal acceleration key points greenland ice sheet meltwater volumes are increasing notably at high elevations low strain rates limit the likelihood of moulin formation at high elevations high elevation meltwater will reach an already wet bed at lower elevations

co2 condensation is a serious limit to the deglaciation of earth like planets abstract it is widely believed that the carbonate silicate cycle is the main agent through volcanism to trigger deglaciations by co2 greenhouse warming on earth and on earth like planets when they get in a frozen state here we use a 3d global climate model to simulate the ability of planets initially completely frozen to escape from glaciation episodes by accumulating enough gaseous co2 the model includes co2 condensation and sublimation processes and the water cycle we find that planets with earth like characteristics size mass obliquity rotation rate etc orbiting a sun like star may never be able to escape from a glaciation era if their orbital distance is greater than 1 27 astronomical units flux 847 w m 2 or 62 of the solar constant because co2 would condense at the poles here the cold traps forming permanent co2 ice caps this limits the amount of co2 in the atmosphere and thus its greenhouse effect furthermore our results indicate that for 1 high rotation rates p rot 24 h 2 low obliquity obliquity 0 6 this critical limit could occur at a significantly lower equivalent distance or higher insolation for each possible configuration we show that the amount of co2 that can be trapped in the polar caps depends on the efficiency of co2 ice to flow laterally as well as its gravitational stability relative to subsurface water ice we find that a frozen earth like planet located at 1 30 au of a sun like star could store as much as 1 5 4 5 and 15 bars of dry ice at the poles for internal heat fluxes of 100 30 and 10 mw m 2 respectively but these amounts are in fact lower limits for planets with a significant water ice cover we show that co2 ice deposits should be gravitationally unstable they get buried beneath the water ice cover in geologically short timescales of 104 yrs mainly controlled by the viscosity of water ice co2 would be permanently sequestered underneath the water ice cover in the form of co2 liquids co2 clathrate hydrates and or dissolved in subglacial water reservoirs if any this would considerably increase the amount of co2 trapped and further reduce the probability of deglaciation

bubbly ice densification in ice sheets i theory dry snow on the surface of polar ice ice sheets is first densified and metamorphosed to produce firn bubbly ice is the next stage of the transformation process which takes place below the depth of pore closure this stage extends to the transition zone where due to high pressures and low temperatures air trapped in bubbles and ice begins to form the mixed air clathrate hydrates while the gas phase progressively disappears here we develop a model of bubbly ice rheology and ice sheet dynamics taking into account glacier ice compressibility the interaction between hydrostatic compression of air bubbles deviatoric uniaxial compressive deformation of the ice matrix and global deformations of the glacier body is considered the ice matrix pressure and the absolute load pressure are distinguished similarity theory and scale analysis are used to examine the resultant mathematical model of bubbly ice densification the initial rate of bubble compression in ice sheets appears to be relatively high so that the pressure density relaxation process takes place only 150 200 m in depth below pore close off to reach its asymptotic phase wherein the minimal drop between bubble and ice pressures is governed by the rate of loading ice accumulation this makes it possible to consider densification under stationary present day conditions of ice formation as a special case of primary interest the computational tests performed with the model indicate that both ice porosity and bubble pressure profiles in ice sheets are sensitive to variations of the rheological parameters of pure ice however only the bubble pressure profile distinguishes between the rheological properties at low and high stresses the porosity profile at the asymptotic phase is mostly determined by the air content in the ice in the companion paper lipenkov and others 1997 we apply the model to experimental data from polar ice cores and deduce through an inverse procedure the rheological properties of pure ice as well as the mean air content in holocene and glacial ice sediments at vostok station antarctica

on the potential of coupling air content and o2 n2 from trapped airfor establishing an ice core chronology tuned on local insolation the potential to access an ice core chronology tuned on local insolation has been demonstrated for the first time in the work of m bender 2002 linking the variations of o2 n2 ratios in the air trapped in the vostok ice with local 78 s summertime insolation more recently it has been shown that the long term changes in air content v recorded in ice from the high antarctic plateau are also dominantly imprinted by the local summer insolation raynaud et al 2007 this paper presents a new v record from vostok which is compared with the published vostok o2 n2 record for the same period of time 160 390 ka bp by using the same spectral analysis methods the spectral differences between the two properties and the possible mechanisms linking them with insolation through the surface snow structure and the close off processes are discussed the main result of our study is that the two experimentally independent local insolation proxies lead to absolute orbital timescales which agree together within less than 1 ka on average this result strongly adds credibility to the total content and the o2 to n2 ratio of the air trapped in ice as reliable and complementary tools for accurate dating of existing and future deep ice cores

seasonal variations in greenland ice sheet motion inland extent and behaviour at higher elevations abstract we present global positioning system observations that capture the full inland extent of ice motion variations in 2009 along a transect in the west greenland ice sheet margin in situ measurements of air temperature and surface ablation and satellite monitoring of ice surface albedo and supraglacial lake drainage are used to investigate hydrological controls on ice velocity changes we find a strong positive correlation between rates of annual ablation and changes in annual ice motion along the transect with sites nearest the ice sheet margin experiencing greater annual variations in ice motion 15 18 than those above 1000 m elevation 3 8 patterns in the timing and rate of meltwater delivery to the ice bed interface provide key controls on the magnitude of hydrologically forced velocity variations at each site in the lower ablation zone the overall contribution of variations in ice motion to annual flow rates is limited by evolution in the structure of the subglacial drainage system at sites in the upper ablation zone a shorter period of summer melting and delayed establishment of a hydraulic connection between the ice sheet surface and its bed limit the timeframe for velocity variations to occur our data suggest that land terminating sections of the greenland ice sheet will experience increased dynamic mass loss in a warmer climate as the behaviour that we observe in the lower ablation zone propagates further inland findings from this study provide a conceptual framework to understand the impact of hydrologically forced velocity variations on the future mass balance of land terminating sections of the greenland ice sheet

evolution of supraglacial lakes on the larsen b ice shelf in the decades before it collapsed the larsen b ice shelf collapsed in 2002 losing an area twice the size of greater london to the sea 3 000 km 2 in an event associated with widespread supraglacial lake drainage here we use optical and radar satellite imagery to investigate the evolution of the ice shelf s lakes in the decades preceding collapse we find 1 that lakes spread southward in the preceding decades at a rate commensurate with meltwater saturation of the shelf surface 2 no trend in lake size suggesting an active supraglacial drainage network which evacuated excess water off the shelf and 3 lakes mostly refreeze in winter but the few lakes that do drain are associated with ice breakup 2 4 years later given the relative scale of lake drainage and shelf breakup however it is not clear from our data whether lake drainage is more likely a cause or an effect of ice shelf collapse

water generation and transport below europa s strike slip faults jupiter s moon europa has a very young surface with the abundance of unique terrains that indicate recent endogenic activity morphological models as well as spectral observations suggest that it might possess shallow lenses of liquid water within its outer ice shell here we investigate the generation and possible accumulation of liquid water below the tidally activated strike slip faults using a numerical model of two phase ice water mixture in two dimensional cartesian geometry our results suggest that generation of shallow partially molten regions underneath europa s active strike slip faults is possible but their lifetime is constrained by the formation of rayleigh taylor instabilities due to the negative buoyancy of the melt once formed typically within a few million years these instabilities efficiently transport the meltwater through the shell consequently the maximum water content in the partially molten regions never exceeds 10 which challenges their possible detection by future exploration mission

high temperature condensation of iron rich olivine in the solar nebula abstract olivine is the most abundant mineral in many chondritic meteorites it is also the first major mg silicate to condense from a solar gas upon cooling the composition of olivine provides important information concerning its origin results of calculations are presented for the fe mn and ni contents of olivine condensing from a gas with solar composition but variable oxygen fugacity a basic computational method is described that takes into account the depletion of the gas in mg and si during condensation the first olivine that condenses from a solar gas has less than 10 2 mol feo independent of the oxygen fugacity of the otherwise solar gas at lower temperatures when most of the mg is condensed more feo can be incorporated in forsterite by condensation if the oxygen fugacity is sufficiently high the feo mno ratio of the first condensing olivine is above 1000 decreasing with decreasing temperature and increasing oxygen fugacity to values of around 100 which are actually observed in fayalitic olivine rims in allende the ni content of condensed olivine is below 10 7 mol even at high oxygen fugacities indicating that condensation of nio in olivine is a very unlikely process any ni in olivine in the ppm range is therefore indicative of metal olivine equilibration under increasingly oxidizing conditions h 2 o h 2 ratios of 0 01 0 1 chromite is an early phase to condense forming before forsterite isolated olivine grains in allende and olivine on the outside of chondrules have feo rich rims texture composition and morphology of these rims indicate that they formed by condensation from a gas our calculations suggest that the gas was enriched by a factor of 100 in oxygen relative to solar composition gas and that mg was almost completely condensed when rim formation occurred the sharp boundary between forsteritic and fayalitic olivine suggests an increase in oxygen fugacity and rapid formation of fayalitic olivine by condensation cr enrichment at the rim interface with the underlying forsterite probably reflects stability of chromite under oxidizing conditions conventional models of feo bearing silicate formation require oxidation of metal silicate assemblages at temperatures as low as 400 k it is shown here that diffusion at this temperature is so slow that the time needed for formation of the observed feo bearing phases would exceed the lifetime of the solar nebula other oxygen fugacity indicators reflect similar conditions for example the hercynite component of spinel in fine grained al rich inclusions indicates equilibration with a gas of similar composition as that responsible for condensation of fayalitic olivine suggesting formation of feo rich spinel by gas solid reactions mechanisms for producing oxidizing conditions in the solar nebula are briefly discussed

a decadal investigation of supraglacial lakes in west greenland using a fully automatic detection and tracking algorithm abstract the sudden drainage of supraglacial lakes has been previously observed to initiate surface to bed hydrologic connections which are capable of enhancing basal sliding in regions of the greenland ice sheet where ice thickness approaches 1 km in this study we develop a robust algorithm which automatically detects and tracks individual supraglacial lakes using visible satellite imagery to document the evolution of a population of west greenland supraglacial lakes over ten consecutive melt seasons validation tests indicate that the algorithm is highly accurate 99 0 of supraglacial lakes can be detected and tracked and 96 3 of reported lakes are true supraglacial lakes with accurate lake properties such as lake area and timing of formation and drainage investigation of the interannual evolution of supraglacial lakes in the context of annual melt intensity reveals that during more intense melt years supraglacial lakes drain more frequently and earlier in the melt season additionally the lake population extends to higher elevations during more intense melt years exposing an increased inland area of the ice sheet to sudden lake drainage events these observations suggest that increased surface meltwater production due to climate change will enhance the spatial extent and temporal frequency of lake drainage events it is unclear whether this will ultimately increase or decrease the basal sliding sensitivity of interior regions of the greenland ice sheet

moulin density controls drainage development beneath the greenland ice sheet this work was funded through a uk natural environment research council doctoral training grant lcag 133 a bowring junior research fellowship st catharine s college cambridge and a leverhulme newton trust early career fellowship all awarded to a f b i j h was supported by a marie curie fp7 career integration grant within the 7th european union framework programme

the connectivity of crystallite agglomerates in low density firn at kohnen station dronning maud land antarctica in this study the three dimensional 3 d microstructure of polar firn is investigated by means of x ray microfocus computer tomography mct basic topological properties including the euler and coordination numbers are derived from the reconstructed 3 d volume images it is shown that sample volumes of about 4 cm 3 are representative for polar firn in terms of their connectivity the connectivity function defined as the change of euler number with structure size is calculated via image processing routines it is used to split the ice phase at small bridges into single crystallite agglomerates the bond size distributions and the mean size of the agglomerates are estimated all mct measurements were carried out on the uppermost 9 m of a shallow firn core b35 drilled during the 2005 06 field campaign at kohnen station dronning maud land dml antarctica the results are compared with estimates from classical two dimensional 2 d surface section observations the 3 d approach confirms the linear relationship between coordination number and density which hitherto has only been derived from 2 d observations layers of buried snow dunes show a stronger connectivity than layers of moderate crystal size and density the formation of agglomerates made of crystallites is a common feature of polar firn in dml it is proposed that the growth of agglomerates leads to reduced critical densities for the transition between the densification regime of grain boundary sliding and plastic deformation

bubbly ice densification in ice sheets ii applications a mathematical model for simulating the densification of bubbly glacier ice is used to interpret the following experimental data from the vostok central antarctica ice core two ice porosity profiles obtained by independent methods and a bubble pressure profile obtained by direct measurements of air pressure within individual bubbles the rheological properties of pure polycrystalline ice are deduced from the solution of the inverse problem the model and the inferred ice flow law are then validated using porosity profiles from seven other ice cores drilled in antarctica and greenland in the temperature range from 55 to 20 c the following expression is adopted for the constitutive law 2e 1 2 exp q 1 t s 1 t r s where e and are the effective strain rate and stress respectively is the creep exponent taken as 3 5 r s is the gas constant and t t s is the temperature standard temperature the numerical values obtained for the linear and non linear viscosities are 1 2 9 1 3 mpayear and 2 0 051 0 019 mpa year and the apparent activation energy q is confirmed to be 60 kj mole 1 the corresponding flow law is in good agreement with results of both mechanical tests and independent estimations based on the analysis of different natural phenomena associated with glacier ice deformation when the model is constrained by the porosity and bubble pressure profiles from vostok the mean air content in holocene ice is inferred to be about 0 088 cm 3 g 1 the corresponding mean air pressure in bubbles at the end of pore closure is about 0 083 mpa whereas the atmospheric pressure at this depth level would be 0 063 mpa the influence of the climatic change on the ice porosity profile is discussed it resulted in an increased air content in ice at vostok during the last glacial maximum 0 096 cm 3 g 1

the layered evolution of fabric and microstructure of snow at point barnola central east antarctica snow fabric defined as the distribution of the c axis orientations of the ice crystals in snow is poorly known so far only one study exits that measured snow fabric based on a statistically representative technique this recent study has revealed the impact of temperature gradient metamorphism on the evolution of fabric in natural snow based on cold laboratory experiments on polar ice sheets snow properties are currently investigated regarding their strong variability in time and space notably because of their potential influence on firn processes and consequently on ice core analysis here we present measurements of fabric and microstructure of snow from point barnola east antarctica close to dome c we analyzed a snow profile from 0 to 3 m depth where temperature gradients occur the main contributions of the paper are 1 a detailed characterization of snow in the upper meters of the ice sheet especially by providing data on snow fabric and 2 the study of a fundamental snow process never observed up to now in a natural snowpack namely the role of temperature gradient metamorphism on the evolution of the snow fabric snow samples were scanned by micro tomography to measure continuous profiles of microstructural properties density specific surface area and pore thickness fabric analysis was performed using an automatic ice texture analyzer on 77 representative thin sections cut out from the samples different types of snow fabric could be identified and persist at depth snow fabric is significantly correlated with snow microstructure pointing to the simultaneous influence of temperature gradient metamorphism on both properties we propose a mechanism based on preferential grain growth to explain the fabric evolution under temperature gradients our work opens the question of how such a layered profile of fabric and microstructure evolves at depth and further influences the physical and mechanical properties of snow and firn more generally it opens the way to further studies on the influence of the snow fabric in snow processes related to anisotropic properties of ice such as grain growth mechanical response electromagnetic behavior

evolution of melt pond volume on the surface of the greenland ice sheet 1 the presence of surface meltwater on ice caps and ice sheets is an important glaciological and climatological characteristic we describe an algorithm for estimating the depth and hence volume of surface melt ponds using multispectral aster satellite imagery the method relies on reasonable assumptions about the albedo of the bottom surface of the ponds and the optical attenuation characteristics of the ponded meltwater we apply the technique to sequences of satellite imagery acquired over the western margin of the greenland ice sheet to derive changes in melt pond extent and volume during the period 2001 2004 results show large intra and interannual changes in ponded water volumes and large volumes of liquid water stored in extensive slush zones

a new technique for firn grain size measurement using sem image analysis firn microstructure is accurately characterized using images obtained from scanning electron microscopy sem visibly etched grain boundaries within images are used to create a skeleton outline of the microstructure a pixel counting utility is applied to the outline to determine grain area firn grain sizes calculated using the technique described here are compared to those calculated using the techniques of gow 1969 and gay and weiss 1999 on samples of the same material and are found to be substantially smaller the differences in grain size between the techniques are attributed to sampling deficiencies e g the inclusion of pore filler in the grain area in earlier methods the new technique offers the advantages of greater accuracy and the ability to determine individual components of the microstructure grain and pore which have important applications in ice core analyses the new method is validated by calculating activation energies of grain boundary diffusion using predicted values based on the ratio of grain size measurements between the new and existing techniques the resulting activation energy falls within the range of values previously reported for firn ice

aqueous alteration without a pronounced oxygen isotopic shift implications for the asteroidal processing of chondritic materials primitive meteorites exhibit certain features that are consistent with aqueous and thermal alteration on asteroids but o isotopic analyses show only a modest heavy isotope shift interpreted as indicating modification in the nebula to understand the isotopic effects of asteroidal alteration we take the l group ordinary chondrites weathered in antarctica as an analogue the data show that alteration is a two stage process with an initial phase producing only a negligible isotopic effect although surprising a possible explanation is found when we consider the alteration of terrestrial silicates numerous studies report pervasive development of channels a few to a few tens of nanometer wide in the incipient alteration of silicates we observe a similar texture alteration involves a restructuring of clay minerals along these narrow channels in which access of water is restricted the clay shows a topotactic relationship to the primary grain which suggests either epitaxial growth of the clay using the silicate as a substrate or inheritance of the original o structure by the clay our data suggests the latter with extensive inheritance of structural polymers by the weathering product the bulk o isotopic composition is comparatively unaffected this offers an explanation for the lack of an isotopic effect in the weathering of the l chondrites if substantial modification of chondritic materials may occur without a pronounced isotopic effect it also reconciles existing o analyses of cv chondrites with an asteroidal model of aqueous alteration

snow grain size measurements in antarctica grain size is an important but not well known characteristics of snow at the surface of antarctica in the past grain size has been reported using various methods the reliability reproducibility and inter comparability of which is not warranted in this paper we present and recommend depending on available logistical support 3 techniques of snow grain sampling and or imaging in the field as well as an ariginal digital image processing method which we have proved to provide reproducible and intercomparable measures of a snow grain size parameter the mean convex radius results from more than 500 samples and 3000 images of snow grains presented which yield a still spatially limited yet unprecendtedly wide picture of near surface snow grain size distribution from field work in antarctica in particular except at sites affected by a very particular meteorology surface grains in the interior of the ice sheet are uniformly small o 1 to 0 2 mm the climate related increase of grain size with depth through metamorphism is as expected not spatially uniform our antarctic snow grain size database will continue to grow as field investigators bring new samples images and measures of snow grain size

the influence of a poroelastic till on rapid subglacial flooding and cavity formation we develop a model of the rapid propagation of water at the contact between elastic glacial ice and a poroelastic subglacial till motivated by observations of the rapid drainage of supraglacial lakes in greenland by treating the ice as an elastic bending beam the fluid dynamics of contact with the subglacial hydrological network which is modelled as a saturated poroelastic till can be examined in detail the model describes the formation and dynamics of an axisymmetric subglacial cavity and the spread of pore pressure in response to injection of fluid a combination of numerical simulation and asymptotic analysis is used to describe these dynamics for both a rigid and a deformable porous till and for both laminar and turbulent fluid flow for constant injection rates and laminar flow the cavity is isostatic and its spread is controlled by bending of the ice and suction of pore water in the vicinity of the ice till contact for a deformable till this control can be modified generically a flexural wave that is initially trapped in advance of the contact point relaxes over time by diffusion of pore pressure ahead of the cavity while the dynamics are found to be relatively insensitive to the properties of the subglacial till during injection with a constant flux significant dependence on the till properties is manifest during the subsequent spread of a constant volume a simple hybrid turbulent laminar model is presented to account for fast injection rates of water in this case self similar turbulent propagation can initially control the spread of the cavity but there is a transition to laminar control in the vicinity of the ice till contact point as the flow slows finally the model results are compared with recent geophysical observations of the rapid drainage of supraglacial lakes in greenland the comparison provides qualitative agreement and raises suggestions for future quantitative comparison

fast draining lakes on the greenland ice sheet 1 the rapid drainage of supraglacial lakes around the ablation zone of the greenland ice sheet forms an important link between water at the surface and the ice sheet base allowing surface meltwater to reach the bed and hence increase glacial velocity the conduits formed by lake drainages may remain open during the remainder of the melt season providing a pathway for further meltwater to reach the base we investigated the drainage behavior of lakes from all regions of the greenland ice sheet for the period 2005 2009 we mapped the evolution of 2600 lakes from 3704 modis images detecting a mean of 263 drainage events per year of which 61 occurred in the south west region only 1 of lake drainages occurred in the rapidly thinning south east region our results show marked differences between the hydrology of the different regions of the ice sheet with few lake drainages occurring in the regions where the highest dynamic mass loss is occurring in the south west and north east lake drainages are common and could impact glacier dynamics in the south east they are rare and are thus unlikely to do so

supraglacial melt channel networks in the jakobshavn isbrae region during the 2007 melt season supraglacial channels are an important mechanism for surface water transport over the ablation zone of western greenland the first assessment of the spatio temporal distribution of surface melt channels and their relationship to supraglacial lakes over the jakobshavn isbrae region of western greenland was analysed using landsat enhanced thematic mapper plus panchromatic images during the 2007 melt season a total of 1188 melt channels were delineated and show an increase in the number of melt channels throughout the season reaching a peak on 9 august water filled melt channels advanced to a maximum elevation of 1647 m on 9 august and attained a minimum average slope of 0 009 on 8 july the ablation zone demonstrates two hydrologic modes where crevasse and moulin terminating channels dominate at elevations 800 m development of higher order networks is interrupted by flow divergence due to partitioning of melt water into vertical infiltration through moulins and crevasse fields prevalent at lower elevations tributary and connector networks between 800 and 1200 m in elevation are correlated with fewer lake occurrences lower surface velocities 50 m a 1 and ice flow dominated by internal deformation over basal sliding high order channels are associated with lake basins that exceed melt water storage capacity evolution of channel networks is coupled to changes in melt water production runoff and ice dynamics with implication for the englacial and subglacial environments 2013 the authors hydrological processes published by john wiley sons ltd

chondrites and the solar nebula the meteorites called chondrites may hold some of the keys to understanding the formation of the solar system in his perspective brearley reports on a recent meeting of meteoriticists who study such objects including the famous allende meteorite that fell in mexico in 1969 one of the more intense debates at the workshop centered on whether and in what ways the chondrites were altered by heat or water during or after formation of the solar system as the author discusses the result of the meeting was a much clearer understanding of the direction of future research

fracture propagation as means of rapidly transferring surface meltwater to the base of glaciers 1 propagation of water filled crevasses through glaciers is investigated based on the linear elastic fracture mechanics approach a crevasse will penetrate to the depth where the stress intensity factor at the crevasse tip equals the fracture toughness of glacier ice a crevasse subjected to inflow of water will continue to propagate downward with the propagation speed controlled primarily by the rate of water injection while the far field tensile stress and fracture toughness determine where crevasses can form once initiated the rate of water driven crevasse propagation is nearly independent of these two parameters thus rapid transfer of surface meltwater to the bed of a cold glacier requires abundant ponding at the surface to initiate and sustain full thickness fracturing before refreezing occurs

polythermal glacier hydrology a review 1 the manner by which meltwater drains through a glacier is critical to ice dynamics runoff characteristics and water quality however much of the contemporary knowledge relating to glacier hydrology has been based upon and conditioned by understanding gleaned from temperate valley glaciers globally a significant proportion of glaciers and ice sheets exhibit nontemperate thermal regimes the recent growing concern over the future response of polar glaciers and ice sheets to forecasts of a warming climate and lengthening summer melt season necessitates recognition of the hydrological processes in these nontemperate ice masses it is therefore timely to present an accessible review of the scientific progress in glacial hydrology where nontemperate conditions are dominant this review provides an appraisal of the glaciological literature from nontemperate glaciers examining supraglacial englacial and subglacial environments in sequence and their role in hydrological processes within glacierized catchments in particular the variability and complexity in glacier thermal regimes are discussed illustrating how a unified model of drainage architecture is likely to remain elusive due to structural controls on the presence of water cold ice near glacier surfaces may reduce meltwater flux into the glacier interior but observations suggest that the transient thermal layer of near surface ice holds a hydrological role as a depth limited aquifer englacial flowpaths may arise from the deep incision of supraglacial streams or the propagation of hydrofractures forms which are readily able to handle varied meltwater discharge or act as locations for water storage and result in spatially discrete delivery of water to the subglacial environment the influence of such drainage routes on seasonal meltwater release is explored with reference to summer season upwellings and winter icing formation moreover clear analogies emerge between nontemperate valley glacier and ice sheet hydrology the discussion of which indicates how persistent reassessment of our conceptualization of glacier drainage systems is required there is a clear emphasis that continued integrated endeavors focused on process glaciology at nontemperate glaciers are a scientific imperative to augmenting the existing body of research centered on ice mass hydrology

imaging of firn and bubbly ice in coaxial reflected light a new technique for the characterization of these porous media a new technique for characterizing the structure of firn and bubbly ice is presented this technique based on observation of etched sublimation surfaces in coax ial reflected light enables one to see simultaneously the pore network of the firn or bubbles in the ice and the crystal boundaries at the same time the main stages of image proces sing used to transform the initial photographs into clean binary images are described

vertical extension of the subglacial drainage system into basal crevasses water plays a first order role in basal sliding of glaciers and ice sheets and is often a key constituent of accelerated glacier motion subglacial water is known to occupy systems of cavities and conduits at the interface between ice and the underlying bed surface depending upon the history of water input and the characteristics of the substrate full understanding of the extent and configuration of basal water is lacking however because direct observation is difficult this limits our ability to simulate ice dynamics and the subsequent impacts on sea level rise realistically here we show that the subglacial hydrological system can have a large volume of water occupying basal crevasses that extend upward from the bed into the overlying ice radar and seismic imaging combined with in situ borehole measurements collected on bench glacier alaska reveal numerous water filled basal crevasses with highly transmissive connections to the bed some crevasses extend many tens of metres above the bed and together they hold a volume of water equivalent to at least a decimetre layer covering the bed our results demonstrate that the basal hydrologic system can extend high into the overlying ice mass where basal crevasses increase water storage capacity and could potentially modulate basal water pressure because basal crevasses can form under commonly observed glaciological conditions our findings have implications for interpreting and modelling subglacial hydrologic processes and related sliding accelerations of glaciers and ice sheets

effects of bubbles cracks and volcanic tephra on the spectral albedo of bare ice near the transantarctic mountains implications for sea glaciers on snowball earth 1 spectral albedo was measured along a 6 km transect near the allan hills in east antarctica the transect traversed the sequence from new snow through old snow firn and white ice to blue ice showing a systematic progression of decreasing albedo at all wavelengths as well as decreasing specific surface area ssa and increasing density broadband albedos under clear sky range from 0 80 for snow to 0 57 for blue ice and from 0 87 to 0 65 under cloud both air bubbles and cracks scatter sunlight their contributions to ssa were determined by microcomputed tomography on core samples of the ice although albedo is governed primarily by the ssa and secondarily by the shape of bubbles or snow grains albedo also correlates highly with porosity which as a proxy variable would be easier for ice sheet models to predict than bubble sizes albedo parameterizations are therefore developed as a function of density for three broad wavelength bands commonly used in general circulation models visible near infrared and total solar relevance to snowball earth events derives from the likelihood that sublimation of equatorward flowing sea glaciers during those events progressively exposed the same sequence of surface materials that we measured at allan hills with our short 6 km transect representing a transect across many degrees of latitude on the snowball ocean at the equator of snowball earth climate models predict thick ice or thin ice or open water depending largely on their albedo parameterizations our measured albedos appear to be within the range that favors ice hundreds of meters thick

modelling of the densification of polar firn characterization of the snow firn transition the transformation of dry snow to firn is described by the transition between densification by deformationless restacking and densification by power law creep the observed decrease with temperature of the density at the snow firn transition seems to result from the competition between grain boundary sliding and power law creep these two densification processess occur concurrenlly in snow although there are probably micro regions in which sliding alone occurs validation ofa geometrical densifi cation model developed for ceramics has been obtained from densification data from several antarctic and greenland sites and from the characterization of the structure of polar firn

modeling of subglacial hydrological development following rapid supraglacial lake drainage the rapid drainage of supraglacial lakes injects substantial volumes of water to the bed of the greenland ice sheet over short timescales the effect of these water pulses on the development of basal hydrological systems is largely unknown to address this we develop a lake drainage model incorporating both 1 a subglacial radial flux element driven by elastic hydraulic jacking and 2 downstream drainage through a linked channelized and distributed system here we present the model and examine whether substantial efficient subglacial channels can form during or following lake drainage events and their effect on the water pressure in the surrounding distributed system we force the model with field data from a lake drainage site 70 km from the terminus of russell glacier in west greenland the model outputs suggest that efficient subglacial channels do not readily form in the vicinity of the lake during rapid drainage and instead water is evacuated primarily by a transient turbulent sheet and the distributed system following lake drainage channels grow but are not large enough to reduce the water pressure in the surrounding distributed system unless preexisting channels are present throughout the domain our results have implications for the analysis of subglacial hydrological systems in regions where rapid lake drainage provides the primary mechanism for surface to bed connections

a model for turbulent hydraulic fracture and application to crack propagation at glacier beds glaciological observations of under flooding suggest that fluid induced hydraulic fracture of an ice sheet from its bed sometimes occurs quickly possibly driven by turbulently flowing water in a broad sheet flow taking the approximation of a fully turbulent flow into an elastic ice medium with small fracture toughness we derive an approximate expression for the crack tip speed opening displacement and pressure profile we accomplish this by first showing that a manning strickler channel model for resistance to turbulent flow leads to a mathematical structure somewhat similar to that for resistance to laminar flow of a power law viscous fluid we then adapt the plane strain asymptotic crack solution of desroches et al 1994 and the power law self similar solution of adachi and detournay 2002 for that case to calculate the desired quantities the speed of crack growth is shown to scale as the overpressure in excess of ice overburden to the power 7 6 inversely as ice elastic modulus to the power 2 3 and as the ratio of crack length to wall roughness scale to the power 1 6 we tentatively apply our model by choosing parameter values thought appropriate for a basal crack driven by the rapid drainage of a surface meltwater lake near the margin of the greenland ice sheet making various approximations perhaps relevant to this setting we estimate fluid inflow rate to the basal fracture and vertical and horizontal surface displacements and find order of magnitude agreement with observations by das et al 2008 associated with lake drainage finally we discuss how these preliminary estimates could be improved

inland advance of supraglacial lakes in north west greenland under recent climatic warming the inland advance of supraglacial lakes sgls towards the interior regions of the greenland ice sheet gris may have implications for the water volumes reaching the subglacial drainage system and could consequently affect long term ice sheet dynamics here we investigate changes to the areas volumes and elevation distributions of over 8000 manually delineated sgls using 44 landsat images of a 6200 km 2 sector of north west greenland over three decades 1985 2016 our results show that sgls have advanced to higher maximum 418 m and mean 299 m elevations and that there has been a near doubling of total regional sgl areas and volumes over the study period accelerating after 2000 these changes were primarily caused by an increased sgl area and volume at high 1200 m a s l elevations where sgl coverage increased by over 2750 during the study period many of the observed changes particularly the post 2000 accelerations were driven by changes to regional surface temperature anomalies this study demonstrates the past and accelerating response of the gris s hydrological system due to climatic warming indicating an urgent need to understand whether the increasingly inland sgls will be capable of hydrofracture in the future thus determining their potential implications for ice sheet dynamics

supraglacial lake spatial structure in western greenland during the 2007 ablation season 1 increases in surface melt rates along the greenland ice sheet margin have accelerated meltwater production resulting in supraglacial lakes that can drain rapidly and enhance basal sliding in this study supraglacial lakes in western greenland are mapped from multitemporal landsat panchromatic imagery during the 2007 melt season the analysis models lake spatial structure and establishes sequencing of drainage from 6 june through 9 august 2007 results indicate high spatial clustering at lower elevations between 100 and 700 m with minimum lake to lake distances of 100 500 m and lake sizes 0 5 km2 at higher elevations 700 1200 m clustering decreases with minimum distances on the order of 900 1400 m and lake sizes between 0 5 and 1 km2 lakes at elevations above 1200 m exhibit less clustering and smaller areas clustering was spatially heterogeneous within elevation bands with high clustering between ice streams and fewer lakes along shear margins spatial patterns of lake drainage throughout the 2007 melt season exhibit strong spatial coherency the magnitude of clustering reveals patterns caused by surface melt production and water routing through surface streams as well as in situ drainage cluster patterns are influenced by proximity to crevasses fields shear margins as well as topographic structure

air content paleo record in the vostok ice core antarctica a mixed record of climatic and glaciological parameters under present day climatic conditions the air content of ice shows a high sensitivity to the atmospheric pressure and hence to the elevation at the surface of the ice sheet this observation has been used to infer past ice sheet thickness variations of antarctica and greenland a high resolution air content profile more than 1000 measurements covering approximately the last 200 000 years was obtained along the 2546 m long vostok ice core three analytical techniques were used leading to consistent results which show large amplitude and rapid air content variations the vostok results support thicker thinner ice in the central part of east antarctica during warm cold periods

seismicity on the western greenland ice sheet surface fracture in the vicinity of active moulins we analyzed geophone and gps measurements collected within the ablation zone of the western greenland ice sheet during a 35 day period of the 2011 melt season to study changes in ice deformation before during and after a supraglacial lake drainage event during rapid lake drainage ice flow speeds increased to 400 of winter values and icequake activity peaked at times 7 days after drainage this seismicity developed variability over both diurnal and longer periods 10 days while coincident ice speeds fell to 150 of winter values and showed nightly peaks in spatial variability approximately 95 of all detected seismicity in the lake basin and its immediate vicinity was triggered by fracture propagation within near surface ice 330m deep that generated rayleigh waves icequakes occurring before and during drainage frequently were collocated with the down flow west end of the primary hydrofracture through which the lake drained but shifted farther west and outside the lake basin after the drainage we interpret these results to reveal vertical hydrofracture opening and local uplift during the drainage followed by enhanced seismicity and ice flow on the downstream side of the lake basin this region collocates with interferometric synthetic aperture radar measured speedup in previous years and could reflect the migration path of themeltwater supplied to the bed by the lake the diurnal seismic signal can be associated with nightly reductions in surface melt input that increase effective basal pressure and traction thereby promoting elevated strain in the surficial ice

the role of hydrologically driven ice fracture in drainage system evolution on an arctic glacier 1 observations from ellesmere island suggest that the connection between surface and subglacial drainage on a predominantly cold glacier is made abruptly by hydrologically driven propagation of fractures from the surface to the bed where ice is 150 m thick water ponded to a depth of 6 9 m within a supraglacial stream system before establishing a permanent bed connection multiple premonitory drainage events preceded the final drainage of ponded water implying that fracturing is necessary but insufficient to establish a permanent link between surface and subglacial drainage refreezing of water that penetrates the first fractures to form may reseal the connection while flow resistance within the subglacial system may delay the onset of continuous through flow a large volume of ponded water is required to enlarge fractures sufficiently by melting to maintain continuous drainage while feedbacks between subglacial hydrology and ice dynamics may assist in maintaining the connection and initiating subglacial outflow

simple relations for the close off depth and age in dry snow densification a physical model for the snow firn densification process salamatin and others 2006 and martinerie and others 1992 1994 correlation for the firn density at the pore closure are employed to perform a scale analysis and computational experiments in order to deduce simplified relations for the close off depth and ice age in quasi stationary ice formation conditions the critical snow density at which ice grain rearrangement stops is used to take into account variability of snow structures subjected to densification the results obtained are validated on a representative set of ice core data from 22 sites which covers wide ranges of present day temperatures and ice accumulation rates a simple analytical approximation for the density depth profile is proposed