precise analysis of icesat altimetry data and assessment of the hydrostatic equilibrium for subglacial lake vostok east antarctica summary based on the ice cloud and land elevation satellite icesat laser altimetry data the hydrostatic equilibrium he condition for the subglacial lake vostok east antarctica is evaluated a digital elevation model dem of the ice surface is derived by a regional crossover adjustment the analysis of the dem and its comparison with gps derived ice surface elevations and an ice surface dem based on radar altimetry data reveal an overall accuracy of better than 0 7 m for the lake area the dem is combined with an ice thickness model and a regional geoid model to determine the deviation of the local ice surface height from he for large parts of the lake the ice sheet fulfils the he our results reveal a strong positive deviation of about 10 m along the lake shoreline in addition positive deviations are found in the northern part of the lake which coincide with ice rumples detected by radio echo sounding in the southern part of the lake we find a linear negative deviation 4 0 m which coincides with the convoy route from vostok station to mirny base in addition to the dem relative biases for the icesat laser operational periods are determined in the regional crossover adjustment

response to comment by t scambos and c shuman 2016 on mass gains of the antarctic ice sheet exceed losses by h j zwally and others 2015

the changes in isotope composition and accumulation of snow at vostok station east antarctica over the past 200 years high resolution records of isotope composition dd and accumulation of snow have been obtained from 10 12 m deep snow pits dug in the vicinity of vostok station during the 1979 80 and 1999 2000 antarctic field seasons we employ meteorological balloon sounding and snow stake data to interpret the isotope record in terms of past temperature changes our reconstruction suggests that snow accumulation rate and the near surface air temperature at vostok have varied during the past 200 years between 15 and 30 kg m 2 a 1 and between 56 and 558c respectively with a slight general tendency to increase from the past to the present both parameters reveal a 50 year periodicity that correlates with the pacific decadal oscillation index implying a climatic teleconnection between central antarctica and the tropical pacific

uncertainties in antarctic sea ice thickness retrieval from icesat a sensitivity study was carried out for the lowest level elevation method to retrieve total sea ice snow freeboard from ice cloud and land elevation satellite icesat elevation measurements in the weddell sea antarctica varying the percentage p of elevations used to approximate the instantaneous sea surface height can cause widespread changes of a few to 10 cm in the total freeboard obtained other input parameters have a smaller influence on the overall mean total freeboard but can cause large regional differences these results together with published icesat elevation precision and accuracy suggest that three times the mean per gridcell single laser shot error budget can be used as an estimate for freeboard uncertainty theoretical relative ice thickness uncertainty ranges between 20 and 80 for typical freeboard and snow properties ice thickness is computed from total freeboard using advanced microwave scanning radiometer for earth observing system amsr e snow depth data average ice thickness for the weddell sea is 1 73 0 38 m for icesat measurements from 2004 to 2006 in agreement with previous work the mean uncertainty is 0 72 0 09 m our comparison with data of an alternative approach which assumes that sea ice freeboard is zero and that total freeboard equals snow depth reveals an average sea ice thickness difference of 0 77 m

revisiting grace antarctic ice mass trends and accelerations considering autocorrelation previous grace derived ice mass trends and accelerations have almost entirely been based on an assumption that the residuals to a regression model including also semi annual annual and tidal aliasing terms are not serially correlated we consider ice mass change time series for antarctica and show that significant autocorrelation is in fact present we examine power law and autoregressive models and compare them to those that assume white uncorrelated noise the data do not let us separate autoregressive and power law models but both indicate that white noise uncertainties need to be scaled up by a factor of up to 4 for accelerations and 6 for linear rates depending on length of observations and location for the whole of antarctica east antarctica and west antarctica the scale factors are 1 5 1 5 and 2 2 respectively for the trends and for the accelerations 1 5 1 5 and 2 1 substantially lower scale factors are required for offshore time series suggesting much of the time correlation is related to continental mass changes despite the higher uncertainties we find significant 2 sigma accelerations over much of west antarctica overall increasing mass loss and dronning maud land increasing mass gain as well as a marginally significant acceleration for the ice sheet as a whole increasing mass loss

strike slip orogens an explanation for the hercynides the nature of oblique plate interactions is not yet well understood the oblique interaction of pacific and n american plates has resulted in large scale migration rotation and accretion of microplates intervening basins are ephemeral individually filled and deformed oblique motion results in zones of both compression and extension the latter may be sufficient to generate new oceanic crust and the former to obduct it both subduction and collision are restricted in time and space the resulting orogen is extraordinarily complex in detail the far more complicated hercynides are the result of dextral interaction between continental plates europe and africa the lateral variability of the orogen the major and long lived strike slip faults the bimodal volcanism and related ore deposits of the european trough the lizard ophiolite and its melanges the nw spanish ophiolites and the iberian pyrite belt are all explained using an oblique collision model in particular the generation deformation and obduction of ophiolites in oblique slip mobile zones is discussed

time variable gravity observations of ice sheet mass balance precision and limitations of the grace satellite data time variable gravity data from the gravity recovery and climate experiment grace mission have been available since 2002 to estimate the mass balance of the greenland and antarctic ice sheets we analyze current progress and uncertainties in grace estimates of ice sheet mass balance we discuss the impacts of errors associated with spherical harmonic truncation spatial averaging temporal sampling and leakage from other time dependent signals e g glacial isostatic adjustment gia the largest sources of error for antarctica are the gia correction the omission of l 1 terms nontidal changes in ocean mass and measurement errors for greenland the errors come mostly from the uncertainty in the scaling factor using release 5 0 rl05 grace fields for january 2003 through november 2012 we find a mass change of 258 41 gt yr for greenland with an acceleration of 31 6 gt yr2 and a loss that migrated clockwise around the ice sheet margin to progressively affect the entire periphery for antarctica we report changes of 83 49 and 147 80 gt yr for two gia models with an acceleration of 12 9 gt yr2 and a dominance from the southeast pacific sector of west antarctica and the antarctic peninsula

evaluation of cryo hydrologic warming as an explanation for increased ice velocities in the wet snow zone sermeq avannarleq west greenland 1 wintertime satellite derived ice surface velocities from 2001 through 2007 suggest an increase in ice velocity in the wet snow zone of southwest greenland we present a thermomechanical model to evaluate the influence of surface meltwater runoff on englacial temperatures via cryo hydrologic warming chw as a possible mechanism to explain this velocity increase at sermeq avannarleq the model incorporates chw through a previously published dual column parameterization we compare model simulations with i chw active over the entire ice thickness base case chw ii chw active only in the surface 80 m of the ice sheet surface chw and iii no chw to represent a traditional thermomechanical model the horizontal extent of chw is prescribed based on equilibrium line altitude position and thus incorporates the upstream expansion of the ablation zone over the past decade the base case chw simulations reproduce the observed increase in inland ice velocity between 2001 and 2007 reasonably well the no chw and surface chw simulations significantly underestimate observed ice surface velocities in both epochs the higher ice velocities in the base case chw simulations are attributable to both decreased basal ice viscosities associated with increased basal ice temperatures and an increase in the extent of basal sliding permitted by temperate bed conditions only the temperate bed extent predicted by the base case chw simulation is consistent with independent observations of basal sliding based on our sensitivity analysis of chw we evaluate alternative explanations for an increase in inland ice velocity and suggest chw is the most plausible mechanism

increased ice losses from antarctica detected by cryosat 2 we use 3 years of cryosat 2 radar altimeter data to develop the first comprehensive assessment of antarctic ice sheet elevation change this new data set provides near continuous 96 coverage of the entire continent extending to within 215 km of the south pole and leading to a fivefold increase in the sampling of coastal regions where the vast majority of all ice losses occur between 2010 and 2013 west antarctica east antarctica and the antarctic peninsula changed in mass by 134 27 3 36 and 23 18 gt yr 1 respectively in west antarctica signals of imbalance are present in areas that were poorly surveyed by past missions contributing additional losses that bring altimeter observations closer to estimates based on other geodetic techniques however the average rate of ice thinning in west antarctica has also continued to rise and mass losses from this sector are now 31 greater than over the period 2005 2010

observations of pronounced greenland ice sheet firn warming and implications for runoff production field measurements of shallow borehole temperatures in firn across the northern greenland ice sheet are collected during may 2013 sites first measured in 1952 1955 are revisited showing long term trends in firn temperature results indicate a pattern of substantial firn warming up to 5 7 c at midlevel elevations 1400 2500 m and little temperature change at high elevations 2500 m we find that latent heat transport into the firn due to meltwater percolation drives the observed warming modeling shows that heat is stored at depth for several years and energy delivered from consecutive melt events accumulates in the firn the observed warming is likely not yet in equilibrium with recent melt production rates but captures the progression of sites in the percolation facies toward net runoff production

variability in the surface temperature and melt extent of the greenland ice sheet from modis 1 satellite derived moderate resolution imaging spectroradiometer modis ice surface temperature ist of the greenland ice sheet shows a positive trend and two major melt events from 2000 to present ist increased by 0 55 0 44 c decade with the greatest increase 0 95 0 44 c decade found in northwestern greenland where coastal temperatures and mass loss are also increasing and outlet glaciers are accelerating ist shows the highest rates of increase during summer 1 35 0 47 c decade and winter 1 30 1 53 c decade followed by spring 0 60 0 98 c decade in contrast a decrease in ist was found in the autumn 1 49 1 20 c decade the ist trends in this work are not statistically significant with the exception of the trend in northwestern greenland major surface melt covering 80 or more of the ice sheet occurred during the 2002 and 2012 melt seasons where clear sky measurements show a maximum melt of 87 and 95 of the ice sheet surface respectively in 2002 most of the extraordinary melt was ephemeral whereas in 2012 the ice sheet not only experienced more total melt but melt was more persistent and the 2012 summer was the warmest in the modis record 6 38 3 98 c our data show that major melt events may not be particularly rare during the present period of ice sheet warming

initial in situ measurements of perennial meltwater storage in the greenland firn aquifer 1 a perennial storage of water in a firn aquifer was discovered in southeast greenland in 2011 we present the first in situ measurements of the aquifer including densities and temperatures water was present at depths between 12 and 37 m and amounted to 18 7 0 9 kg in the extracted core the water filled the firn to capacity at 35 m measurements show the aquifer temperature remained at the melting point representing a large heat reservoir within the firn using model results of liquid water extent and aquifer surface depth from radar measurements we extend our in situ measurements to the greenland ice sheet the estimated water volume is 140 20 gt representing 0 4 mm of sea level rise slr it is unknown if the aquifer temporary buffers slr or contributes to slr through drainage and or ice dynamics

the extreme melt across the greenland ice sheet in 2012 1 the discovery of the 2012 extreme melt event across almost the entire surface of the greenland ice sheet is presented data from three different satellite sensors including the oceansat 2 scatterometer the moderate resolution imaging spectroradiometer and the special sensor microwave imager sounder are combined to obtain composite melt maps representing the most complete melt conditions detectable across the ice sheet satellite observations reveal that melt occurred at or near the surface of the greenland ice sheet across 98 6 of its entire extent on 12 july 2012 including the usually cold polar areas at high altitudes like summit in the dry snow facies of the ice sheet this melt event coincided with an anomalous ridge of warm air that became stagnant over greenland as seen in melt occurrences from multiple ice core records at summit reported in the published literature such a melt event is rare with the last significant one occurring in 1889 and the next previous one around seven centuries earlier in the medieval warm period given its rarity the 2012 extreme melt across greenland provides an exceptional opportunity for new studies in broad interdisciplinary geophysical research

a mascon approach to assess ice sheet and glacier mass balances and their uncertainties from grace data the purpose of this paper is to assess the mass changes of the greenland ice sheet gris ice sheets over antarctica and land glaciers and ice caps with a global mascon method that yields monthly mass variations at 10 242 mascons input for this method are level 2 data from the gravity recovery and climate experiment grace system collected between february 2003 and june 2013 to which a number of corrections are made with glacial isostatic adjustment gia corrections from an ensemble of models based on different ice histories and rheologic earth model parameters we find for greenland a mass loss of 278 19 gt yr whereas the mass balances for the gris appear to be less sensitive to gia modeling uncertainties this is not the case with the mass balance of antarctica ice history models for antarctica were recently improved and updated historic ice height data sets and gps time series have been used to generate new gia models we investigated the effect of two new gia models for antarctica and found 92 26 gt yr which is half of what is obtained with ice 5g based gia models where the largest gia model differences occur on east antarctica the mass balance of land glaciers and ice caps currently stands at 162 10 gt yr with the help of new gia models for antarctica we assess the mass contribution to the mean sea level at 1 47 0 09 mm yr or 532 34gt yr which is roughly half of the global sea level rise signal obtained from tide gauges and satellite altimetry

warming glacier melt and surface energy budget from weather station observations in the melville bay region of northwest greenland the glaciers in the melville bay region of northwest greenland have shown a mean retreat since the earliest observations at the beginning of the 20th century the largest steenstrup gletscher has retreated 20 km partly during the period of atmospheric cooling 1940 80 melville bay air temperature observations starting in 1981 indicate a regional change of 0 208 ca 1 this exceeds the warming on the east coast of greenland confirming the west coast to be a region of relatively large change also in a global perspective the largest temperature increase is observed in the winter months 0 3 0 48 ca 1 results from a 4 year 2004 08 net ablation record obtained by an automatic weather station aws near the calving front of steenstrup gletscher show an ablation rate that is relatively low for a low elevation position on the greenland ice sheet 2 4 m ice equivalent per year a first order estimate from positive degree day totals suggests that net ablation has roughly doubled since the 1980s a surface energy and mass balance model is applied to the steenstrup aws data to quantify the energy flux contributions to surface melt solar radiation is the main source for melt energy but due to shortwave radiation penetration into the ice only one third of the melt takes place at the glacier surface nearly two thirds occurs within the upper ice layers

rapid loss of firn pore space accelerates 21st century greenland mass loss 1 mass loss from the two major ice sheets and their contribution to global sea level rise is accelerating in antarctica mass loss is dominated by increased flow velocities of outlet glaciers following the thinning or disintegration of coastal ice shelves into which they flow in contrast 55 of post 1992 greenland ice sheet gris mass loss is accounted for by surface processes notably increased meltwater runoff a subtle process in the surface mass balance of the gris is the retention and refreezing of meltwater currently preventing 40 of the meltwater to reach the ocean here we force a high resolution atmosphere snow model with a mid range warming scenario rcp4 5 1970 2100 to show that rapid loss of firn pore space by 50 at the end of the 21st century quickly reduces this refreezing buffer as a result gris surface mass loss accelerates throughout the 21st century and its contribution to global sea level rise increases to 1 7 0 5 mm yr 1 more than four times the current value

firn cold content evolution at nine sites on the greenland ice sheet between 1998 and 2017 current sea level rise partly stems from increased surface melting and meltwater runoff from the greenland ice sheet multi year snow also known as firn covers about 80 of the ice sheet and retains part of the surface meltwater since the firn cold content integrates its physical and thermal characteristics it is a valuable tool for determining the meltwater retention potential of firn we use gap filled climatological data from nine automatic weather stations in the ice sheet accumulation area to drive a surface energy budget and firn model validated against firn density and temperature observations over the 1998 2017 period our results show a stable top 20 m firn cold content cc20 at most sites only at the lower elevation dye 2 site did cc20 decrease by 24 in 2012 before recovering to its original value by 2017 heat conduction towards the surface is the main process feeding cc20 at all nine sites while cc20 reduction occurs through low cold content fresh snow addition at the surface during snowfall and latent heat release when meltwater refreezes our simulations suggest that firn densification while reducing pore space for meltwater retention increases the firn cold content enhances near surface meltwater refreezing and potentially sets favourable conditions for ice slab formation

thermal tracking of meltwater retention in greenland s accumulation area 1 poorly understood processes controlling retention of meltwater in snow and firn have important implications for greenland ice sheet s mass balance and flow dynamics here we present results from a 3 year 2007 2009 field campaign studying firn thermal profiles and density structure along an 85 km transect of the percolation zone of west greenland we installed one or two thermistor strings at 14 study sites each string having 32 sensors spaced between 0 and 10 m depth data from our network of over 500 sensors were collected at 15 60 min intervals for 1 2 years thereby recording the thermal signature of meltwater infiltration and refreezing during annual melt cycles we document three types of heating of firn related to different mechanisms of meltwater motion and freezing including heterogeneous breakthrough events wetting front advance and year round heating from freezing of residual deep pore water vertically infiltrating meltwater commonly penetrates through cold firn accumulated over decades even where ice layers are present at the previous summer surface and where ice layer thickness exceeds several decimeters the offset between the mean annual air temperature and the 10 m firn temperature reveals the elevation dependency of meltwater retention along our transect the firn is 10 c warmer than the mean annual air temperature at the region where meltwater runoff initiates during 2007 2009 runoff was limited to elevations lower than 1500 m with no sharp runoff limit rather the ratio of retention to runoff transitioned from all retention to all runoff across a 20 km wide zone

a reconciled estimate of ice sheet mass balance we combined an ensemble of satellite altimetry interferometry and gravimetry data sets using common geographical regions time intervals and models of surface mass balance and glacial isostatic adjustment to estimate the mass balance of earth s polar ice sheets we find that there is good agreement between different satellite methods especially in greenland and west antarctica and that combining satellite data sets leads to greater certainty between 1992 and 2011 the ice sheets of greenland east antarctica west antarctica and the antarctic peninsula changed in mass by 142 49 14 43 65 26 and 20 14 gigatonnes year 1 respectively since 1992 the polar ice sheets have contributed on average 0 59 0 20 millimeter year 1 to the rate of global sea level rise

an improved mass budget for the greenland ice sheet extensive ice thickness surveys by nasa s operation icebridge enable over a decade of ice discharge measurements at high precision for the majority of greenland s marine terminating outlet glaciers prompting a reassessment of the temporal and spatial distribution of glacier change annualmeasurements for 178 outlet glaciers reveal that despite widespread acceleration only 15 glaciers accounted for 77 of the 739 29 gt of ice lost due to acceleration since 2000 and four accounted for 50 among the top sources of loss are several glaciers that have received little scientific attention the relative contribution of ice discharge to total loss decreased from 58 before 2005 to 32 between 2009 and 2012 as such 84 of the increase in mass loss after 2009 was due to increased surface runoff these observations support recentmodel projections that surface mass balance rather than ice dynamics will dominate the ice sheet s contribution to 21st century sea level rise

retention of greenland runoff by refreezing implications for projected future sea level change melting produced at the surface of subfreezing permeable firn does not freely percolate but instead refreezes near its point of origin raising the firn temperature and decreasing the pore space if sufficient meltwater is introduced the firn will warm to 0 c and subsequent water will fill the remaining pore space without freezing only after the residual water content of the firn is exceeded will water flow through the firn and possibly escape as runoff this process prevents summer melt on high arctic glaciers and ice caps from escaping in its entirety and it must be accounted for in modeling glacier runoff on the basis of surface energy balance a model is presented here which describes in a simple way the transient process of infiltration refreezing and runoff in a future warming climate the model is applied to greenland for which predictions of runoff induced sea level rise that do not consider the refreezing process could be as much as 5 0 cm too high over 150 years

ice flow velocities over vostok subglacial lake east antarctica determined by 10 years of gnss observations repeated global navigation satellite systems gnss observations were carried out at 50 surface markers in the vostok subglacial lake east antarctica region between 2001 and 2011 the horizontal ice flow velocity vectors were derived with accuracies of 1 cma and 0 58 representing the first reliable information on ice flow kinematics in the northern part of the lake within the lake area ice flow velocities do not exceed 2ma the ice flow azimuth is southeast in the southern part of the lake and turns gradually to east northeast in the northern part in the northern part as the ice flow enters the lake at the western shore the velocity decreases towards the central lake axis then increases slightly past the central axis in the southern part a continued acceleration is observed from the central lake axis across the downstream grounding line based on the observed flow velocity vectors and ice thickness data mean surface accumulation rates are inferred for four surface segments between ridge b and vostok subglacial lake and show a steady increase towards the north

antarctica greenland and gulf of alaska land ice evolution from an iterated grace global mascon solution we have determined the ice mass evolution of the antarctica and greenland ice sheets ais and gis and gulf of alaska goa glaciers from a new grace global solution of equal area surface mass concentration parcels mascons in equivalent height of water the mascons were estimated directly from the reduction of the inter satellite k band range rate kbrr observations taking into account the full noise covariance and formally iterating the solution the new solution increases signal recovery while reducing the grace kbrr observation residuals the mascons were estimated with 10 day and 1 arc degree equal area sampling applying anisotropic constraints an ensemble empirical mode decomposition adaptive filter was applied to the mascon time series to compute annual mass balances the details and causes of the spatial and temporal variability of the land ice regions studied are discussed the estimated mass trend over the total gis ais and goa glaciers for the time period 1 december 2003 to 1 december 2010 is 380 plus or minus 31 gt a exp 1 equivalent to 1 05 plus or minus 0 09 mma exp 1 sea level rise over the same time period we estimate the mass acceleration to be 41 plus or minus 27 gt a exp 2 equivalent to a 0 11 plus or minus 0 08 mm a exp 2 rate of change in sea level the trends and accelerations are dependent on significant seasonal and annual balance anomalies

early devonian pre acadian magnetization directions in lower old red sandstone of south wales uk summary two components of magnetization have been resolved from the lower old red sandstone of south wales all 39 sampled sites lie north of the variscan front and all but three are largely unaffected by variscan folding thirteen sites yield a high unblocking temperature magnetization component which pre dates acadian mid devonian folding the site mean direction for this component dec 232 2 inc 31 9 95 8 5 yields a pole position lat 7 3 s long 306 7 e which can be assigned an early devonian age a lower unblocking temperature component is present at 38 of the sampled sites this component post dates both the acadian and variscan folding has reversed polarity and a mean direction dec 193 8 inc 7 2 95 2 6 which corresponds to a pole position lat 40 4 s long 338 0 e consistent with a latest carboniferous or early permian age the early devonian pole position confirms the existence of a siluro devonian inflexion in the apparent polar wander path apwp for southern britain which coincides with an inflexion in the apwp for scotland britain north of the iapetus suture indicating the closure of the iapetus ocean by siluro devonian time the early devonian palaeolatitude 17 s 5 is consistent with an early devonian configuration in which gondwana laurentia baltica and avalonia were part of a single supercontinent

thermal tracing of retained meltwater in the lower accumulation area of the southwestern greenland ice sheet we present in situ firn temperatures from the extreme 2012 melt season in the southwestern lower accumulation area of the greenland ice sheet the upper 2 5 m of snow and firn was temperate during the melt season when vertical meltwater percolation was inefficient due to a 5 5 m thick ice layer underlying the temperate firn meltwater percolation and refreezing beneath 2 5 m depth only occurred after the melt season deviations from temperatures predicted by pure conductivity suggest that meltwater refroze in discrete bands at depths of 2 0 2 5 5 0 6 0 and 8 0 9 0 m while we find no indication of meltwater percolation below 9 m depth or complete filling of pore volume above firn at 10 and 15 m depth was respectively 4 2 4 5 c and 1 7 c higher than in a conductivity only simulation even though meltwater percolation in 2012 was inefficient firn between 2 and 15 m depth the following winter was on average 4 7 c warmer due to meltwater refreezing our observations also suggest that the 2012 firn conditions were preconditioned by two warm summers and ice layer formation in 2010 and 2011 overall firn temperatures during the years 2009 13 increased by 0 6 c

snow crystal growth with varying surface temperatures and radiation penetration the temperature field is derived for a sinusoidally varying surface temperature with varying solar radiation penetration the growth rates of snow crystals are calculated to explain the rapidly growing layers of faceted crystals i e depth hoar that form just below the surface at high altitudes and in polar snow the solutions also show that a layer of wet snow can exist just below the surface even on days when the surface temperature remains sub freezing

determination of timing and location of water movement and ice layer formation by temperature measurements in sub freezing snow measurements of temperature in snow along a vertical profile during the onset of spring melting are used to calculate spatial and temporal temperature gradients and terms of the conduction equation with an internal energy production term are calculated heat transfer information is combined with stratigraphic observations made during melting and allow detailed determination of the timing and location of heterogeneous water movement and of refreezing internal energy production is interpreted as latent heat of refreezing of percolated meltwater times and locations of flow and refreezing of meltwater are calculated and compared to stratigraphic observations of layering and changes in density and liquid water content sequences of melt piping layering and refreezing seen in calculations and in stratigraphy demonstrate retarded flow at fine to coarse transitions flow along such transitions and refreezing at the transitions to form ice layers downslope flow is also observed in the absence of an impermeable horizon to redirect flow from the vertical

glaciological investigations in the crete area central greenland a search for a new deep drilling site the results of the 1984 85 post gisp campaigns in central greenland are presented eight ice cores were obtained some spanning up to 360 years we present i geographical positions and elevations at the drill sites

solar heating rates and temperature profiles in antarctic snow and ice observations of temperature maxima at about 10 cm depth in cold antarctic snow during summer have previously been explained by proposing that solar heating is distributed with depth whereas thermal infrared cooling is localized at the surface the solid state greenhouse an increase in temperature from the surface to 10 cm depth t 4 k found by rusin 1961 on the antarctic plateau was successfully reproduced by schlatter 1972 in a combined radiative transfer and heat transfer model

greenland ice sheet contribution to sea level rise buffered by meltwater storage in firn observations of firn structure and meltwater retention on the greenland ice sheet s percolation zone a region of the ice sheet that is perennially covered by snow and firn quantify the capacity of the firn to store future surface meltwater and to delay expansion of the area contributing to sea level rise the extent and the fate of surface melt on the greenland ice sheet are important factors contributing to sea level change the pore spaces in firn a type of partly compacted snow can be filled by meltwater and thus act as a temporary reservoir here joel harper et al show that the greenland ice sheet parts of which are permanently covered by snow and firn can buffer several hundred gigatonnes of meltwater for several decades although this buffering capacity is only temporary and cannot be easily regained it has the potential to delay the pace of near term sea level rise surface melt on the greenland ice sheet has shown increasing trends in areal extent and duration since the beginning of the satellite era1 2 3 records for melt were broken in 20054 20075 20106 and 20127 much of the increased surface melt is occurring in the percolation zone a region of the accumulation area that is perennially covered by snow and firn partly compacted snow the fate of melt water in the percolation zone is poorly constrained some may travel away from its point of origin and eventually influence the ice sheet s flow dynamics and mass balance and the global sea level whereas some may simply infiltrate into cold snow or firn and refreeze with none of these effects here we quantify the existing water storage capacity of the percolation zone of the greenland ice sheet and show the potential for hundreds of gigatonnes of meltwater storage we collected in situ observations of firn structure and meltwater retention along a roughly 85 kilometre long transect of the melting accumulation area our data show that repeated infiltration events in which melt water penetrates deeply more than 10 metres eventually fill all pore space with water as future surface melt intensifies under arctic warming a fraction of melt water that would otherwise contribute to sea level rise will fill existing pore space of the percolation zone we estimate the lower and upper bounds of this storage sink to be 322 44 gigatonnes and gigatonnes respectively furthermore we find that decades are required to fill this pore space under a range of plausible future climate conditions hence routing of surface melt water into filling the pore space of the firn column will delay expansion of the area contributing to sea level rise although once the pore space is filled it cannot quickly be regenerated

estimation of icesat intercampaign elevation biases from comparison of lidar data in east antarctica 1 the ice cloud and land elevation satellite icesat laser campaign elevation biases are estimated from an analysis of icesat and airborne land vegetation and ice sensor lidar intrasensor and intersensor elevation differences from two regions of the antarctic ice sheet that experienced minimal surface elevation change elevation observations are corrected for a combination of accumulation melting and firn densification processes and glacial isostatic adjustment icesat elevations are adjusted for the saturation and gaussian centroid corrections relative to laser campaign l3i biases are found to be less than 8 cm except for campaign l2e at 14 72 cm corresponding to the time of a significant accumulation anomaly in east antarctica the intercampaign bias trend estimated from intersensor elevation differences computed over campaigns l2a to l2f september 2003 to october 2009 excluding l2e is 1 04 0 48 cm yr the intercampaign bias trend represents a correction to icesat derived antarctica mass balance of 117 53 gt yr

variation of accumulation rates over the last eight centuries on the east antarctic plateau derived from volcanic signals in ice cores 1 volcanic signatures in ice core records provide an excellent means to date the cores and obtain information about accumulation rates from several ice cores it is thus possible to extract a spatio temporal accumulation pattern we show records of electrical conductivity and sulfur from 13 firn cores from the norwegian usa scientific traverse during the international polar year 2007 2009 ipy through east antarctica major volcanic eruptions are identified and used to assess century scale accumulation changes the largest changes seem to occur in the most recent decades with accumulation over the period 1963 2007 08 being up to 25 different from the long term record there is no clear overall trend some sites show an increase in accumulation over the period 1963 to present while others show a decrease almost all of the sites above 3200 m above sea level asl suggest a decrease these sites also show a significantly lower accumulation value than large scale assessments both for the period 1963 to present and for the long term mean at the respective drill sites the spatial accumulation distribution is influenced mainly by elevation and distance to the ocean continentality as expected ground penetrating radar data around the drill sites show a spatial variability within 10 20 over several tens of kilometers indicating that our drill sites are well representative for the area around them our results are important for large scale assessments of antarctic mass balance and model validation

recent snowfall anomalies in dronning maud land east antarctica in a historical and future climate perspective 1 enhanced snowfall on the east antarctic ice sheet is projected to significantly mitigate 21st century global sea level rise in recent years 2009 and 2011 regionally extreme snowfall anomalies in dronning maud land in the atlantic sector of east antarctica have been observed it has been unclear however whether these anomalies can be ascribed to natural decadal variability or whether they could signal the beginning of a long term increase of snowfall here we use output of a regional atmospheric climate model evaluated with available firn core records and gravimetry observations and show that such episodes had not been seen previously in the satellite climate data era 1979 comparisons with historical data that originate from firn cores one with records extending back to the 18th century confirm that accumulation anomalies of this scale have not occurred in the past 60 years although comparable anomalies are found further back in time we examined several regional climate model projections describing various warming scenarios into the 21st century anomalies with magnitudes similar to the recently observed ones were not present in the model output for the current climate but were found increasingly probable toward the end of the 21st century

regional acceleration in ice mass loss from greenland and antarctica using grace time variable gravity data we use gravity recovery and climate experiment grace monthly gravity fields to determine the regional acceleration in ice mass loss in greenland and antarctica for 2003 2013 we find that the total mass loss is controlled by only a few regions in greenland the southeast and northwest generate 70 of the loss 280 58 gt yr mostly from ice dynamics the southwest accounts for 54 of the total acceleration in loss 25 4 1 2 gt yr 2 from a decrease in surface mass balance smb followed by the northwest 34 and we find no significant acceleration in the northeast in antarctica the amundsen sea as sector and the antarctic peninsula account for 64 and 17 respectively of the total loss 180 10 gt yr mainly from ice dynamics the as sector contributes most of the acceleration in loss 11 4 gt yr 2 and queen maud land east antarctica is the only sector with a significant mass gain due to a local increase in smb 63 5 gt yr

height changes over subglacial lake vostok east antarctica insights from gnss observations height changes of the ice surface above subglacial lake vostok east antarctica reflect the integral effect of different processes within the subglacial environment and the ice sheet repeated gnss global navigation satellite systems observations on 56 surface markers in the lake vostok region spanning 11 years and continuous gnss observations at vostok station over 5 years are used to determine the vertical firn particle movement vertical marker velocities are derived with an accuracy of 1 cm yr or better repeated measurements of surface height profiles around vostok station using kinematic gnss observations on a snowmobile allow the quantification of surface height changes at 308 crossover points the height change rate was determined at 1 5 mm yr thus indicating a stable ice surface height over the last decade it is concluded that both the local mass balance of the ice and the lake level of the entire lake have been stable throughout the observation period the continuous gnss observations demonstrate that the particle heights vary linearly with time nonlinear height changes do not exceed 1 cm at vostok station and constrain the magnitude of spatiotemporal lake level variations icesat laser altimetry data confirm that the amplitude of the surface deformations over the lake is restricted to a few centimeters assuming the ice sheet to be in steady state over the entire lake estimates for the surface accumulation on basal accretion melt rates and on flux divergence are derived

high resolution ground based gps measurements show intercampaign bias in icesat elevation data near summit greenland the geoscience laser altimeter system glas aboard the national aeronautics and space administration s ice cloud and land elevation satellite icesat collected data from early 2003 to late 2009 with the specific goal of measuring ice surface elevation changes while the precision of glas instrumentation has been studied over its intended target ice its accuracy has only been robustly estimated using independent terrestrial nonlaser methods over salt flats here we perform repeat high precision global positioning system gps surveys under four passes of icesat track 0412 campaigns l3i l3j l2d and l2e to compare directly glas elevation data footprints to a coincident gps ground truth near summit greenland analysis and comparison of glas data with gps data show a campaign dependent elevation bias ranging from 0 112 0 030 m l3j to 0 121 0 071 m l2e although uncorrected reflectance values and field observations both indicate that forward scattering of the laser signal through the atmosphere accounts for the anomalously negative l3j bias the biases of all campaigns studied are within the instrument s goal accuracy of 0 15 m however our analysis shows a campaign dependence in the bias which may propagate through estimates of mass balance the error introduced from intercampaign biases illustrates the importance of long term independent validation experiments of satellite altimetry data over ice sheets

palaeozoic palaeomagnetic studies in the welsh basin recent advances in the last two years new palaeomagnetic data fro m wale s have resulted i n radical revision of the ordovician palaeogeography of eastern avalonia part of the southern margin of the iapetus ocean combined with palaeozoic palaeomagnetic data from laurentia and gondwana these data suggest that eastern avalonia was a peri gondwanide high latitude continental fragment during at least part of ordovician time with a palaeolatitude of about 62 s and 51 s in arenig and llanvirn time respectively this implies a latitudinal width of the early ordovician iapetus ocean between eastern avalonia and laurentia of at least 30 geological evidence for the proximity of eastern avalonia and laurentia suggests that the intervening iapetus ocean closed during silurian time from late llandovery to early ludlow recent palaeolatitude data from the iapetus bordering continents are consistent wit h closure b y middle to late silurian time new pre acadian early devonian palaeomagnetic data from the old red sandstone places the welsh basin at about 17 s consistent with a palaeogeography in which laurentia baltica avalonia armorica and possibly gondwana wer e part o f a singl e supercontinent pervasiv e lat e carboniferous early permia n remagnetization affect s the welsh basin the remagnetization i s probably associated with fluids emanating from th e variscan thrust front w e do not observe remagnetization associate d with acadian orogen y and the remagnetizations which have been studied in more detail in north america appear to be a unique feature of the variscan hercynian alleghenian orogeny

changes in accumulation area firn stratigraphy and meltwater flow during a period of climate warming devon ice cap nunavut canada 1 to document climate driven changes in firn stratigraphy and their implications for meltwater flow patterns within firn on the devon ice cap nunavut during the 21st century summer warming 500 mhz ground penetrating radar gpr surveys were conducted along a 40 km transect in each spring from 2007 to 2012 these linear gpr surveys were supplemented by four 190 m by 100 m gpr grid surveys and 36 firn cores increased meltwater percolation and infiltration ice formation associated with high surface melt rates since 2005 modified the firn stratigraphy substantially over a horizontal distance of nearly 30 km the most dramatic change involved the growth of a thick ice layer within the firn body this layer grew primarily by upward accretion over an initial widespread ice layer formed during summer 2005 it thickened by between 0 5 and 4 5 m over the study period and filled much of the pore volume in the upper part of the firn reducing vertical percolation of meltwater into deeper sections of the firn and thus the water storage potential of much of the firn reservoir heterogeneous percolation of surface meltwater promoted by rolling topography played an important role in meltwater infiltration and drainage encouraging lateral flow at the tops of small hills and ponding and refreezing of meltwater beneath surface depressions

observational evidence on the stability of the hydro glaciological regime of subglacial lake vostok 1 combined geodetic geophysical and glaciological in situ measurements are interpreted regarding surface height changes over subglacial lake vostok and the local mass balance of the ice sheet at vostok station repeated gps observations spanning 5 years and long term surface accumulation data show that the height of the lake surface has not changed over the observation period the application of the mass conservation equation to purely observational data yields an ice mass balance for vostok station close to equilibrium

antarctic contribution to sea level rise observed by grace with improved gia correction antarctic volume changes during the past 21 thousand years are smaller than previously thought and here we construct an ice sheet history that drives a forward model prediction of the glacial isostatic adjustment gia gravity signal the new model in turn should give predictions that are constrained with recent uplift data the impact of the gia signal on a gravity recovery and climate experiment grace antarctic mass balance estimate depends on the specific grace analysis method used for the method described in this paper the gia contribution to the apparent surface mass change is re evaluated to be 55 13 gt yr by considering a revised ice history model and a parameter search for vertical motion predictions that best fit the gps observations at 18 high quality stations although the gia model spans a range of possible earth rheological structure values the data are not yet sufficient for solving for a preferred value of upper and lower mantle viscosity nor for a preferred lithospheric thickness grace monthly solutions from the center for space research release 04 csr rl04 release time series from january 2003 to the beginning of january 2012 uncorrected for gia yield an ice mass rate of 2 9 29 gt yr the new gia correction increases the solved for ice mass imbalance of antarctica to 57 34 gt yr the revised gia correction is smaller than past grace estimates by about 50 to 90 gt yr the new upper bound to the sea level rise from the antarctic ice sheet averaged over the time span 2003 0 2012 0 is about 0 16 0 09 mm yr

accelerated west antarctic ice mass loss continues to outpace east antarctic gains a b s t r a c t article history while multiple data sources have confirmed that antarctica is losing ice at an accelerating rate different measurement techniques estimate the details of its geographically highly variable mass balance with different levels of accuracy spatio temporal resolution and coverage some scope remains for methodological improvements using a single data type in this study we report our progress in increasing the accuracy and spatial resolution of time variable gravimetry from the gravity recovery and climate experiment grace we determine the geographic pattern of ice mass change in antarctica between january 2003 and june 2014 accounting for glacio isostatic adjustment gia using the ij05 r2 model expressing the unknown signal in a sparse slepian basis constructed by optimization to prevent leakage out of the regions of interest we use robust signal processing and statistical estimation methods applying those to the latest time series of monthly grace solutions we map antarctica s mass loss in space and time as well as can be recovered from satellite gravity alone ignoring gia model uncertainty over the period 2003 2014 west antarctica has been losing ice mass at a rate of 121 8g t yr and has experienced large acceleration of ice mass losses along the amundsen sea coast of 18 5g t yr 2 doubling the mass loss rate in the past six years the antarctic peninsula shows slightly accelerating ice mass loss with larger accelerated losses in the southern half of the peninsula ice mass gains due to snowfall in dronning maud land have continued to add about half the amount of west antarctica s loss back onto the continent over the last decade we estimate the overall mass losses from antarctica since january 2003 at 92 10 gt yr

the thermal conductivity of seasonal snow twenty seven studies on the thermal conductivity of snow k eff have been published since 1886 combined they comprise 354 values of k eff and have been used to derive over 13 regression equation and predicting k eff vs density due to large and largely undocumented differences in measurement methods and accuracy sample temperature and snow type it is not possible to know what part of the variability in this data set is the result of snow microstructure we present a new data set containing 488 measurements for which the temperature type and measurement accuracy are known a quadratic equation where is in g cm 3 and k eff is in w m 1 k 1 can be fit to the new data r 2 0 79 a logarithmic expression can also be used the first regression is better when estimating values beyond the limits of the data the second when estimating values for low density snow within the data set snow types resulting from kinetic growth show density independent behavior rounded grain and wind blown snow show strong density dependence the new data set has a higher mean value of density but a lower mean value of thermal conductivity than the old set this shift is attributed to differences in snow types and sample temperatures in the sets using both data sets we show that there are well defined limits to the geometric configurations that natural seasonal snow can take

deflection of magnetic remanence during progressive cleavage development in the pembrokeshire old red sandstone summary this study examines the effects of kinking and cleavage formation on the palaeomagnetic remanence carried by folded red quartz sandstones of lower devonian age from southwest wales these folds contain spaced zones of cleavage which have evolved from single kink bands by reorientation of micas and pressure solution of quartz in many of these cleavage zones the deflected bedding laminations are still preserved a comparison of the remanence components from within the kink bands with those outside the zones demonstrates a significant difference in orientation with a correlation in magnitude and direction between the deflection of the remanence and the amount of kinking similar comparisons of the respective aniostropy of susceptibility ellipsoids show that the principal axis lies in the bedding plane and is deflected by the same amount or slightly more than the bedding within the kink bands in contrast the degree of remanence deflection is systematically less than the relative dip of the kink we test theories on remanence reorientation such as grain rotation or passive marker behaviour and oversteepening due to pressure solution using our data the latter model implies that the dipping laminations in kink bands appear steeper than their true kink deflection because of material volume loss from surfaces parallel to the cleavage planes our interpretation is that 1 the remanence is syn folding and has subsequently been rotated within the kink bands mostly like a passive marker 2 oversteepening due to pressure solution provides only a small contribution to the excess of the kink dip over the remanence deflection specimens from more penetratively deformed zones where kinks have been completely overprinted by cleavage show that remanence still preserves the deflection inherited from the original kink although no trace of deflected laminations remains this highlights the dangers of applying the palaeomagnetic technique in penetratively strained material even at very low strain levels

palaeomagnetic results from the lower devonian llandstadwell formation dyfed wales discussion discussion sur l article de carola stearns et rob van der voo paru dans tectonophysics vol 143 no 4 p 335 336

constraining the mass balance of east antarctica we investigate the mass balance of east antarctica for the period 2003 2013 using a bayesian statistical framework we combine satellite altimetry gravimetry and gps with prior assumptions characterizing the underlying geophysical processes we run three experiments based on two different assumptions to study possible solutions to the mass balance we solve for trends in surface mass balance ice dynamics and glacial isostatic adjustment the first assumption assigns low probability to ice dynamic mass loss in regions of slow flow giving a mean dynamic trend of 17 10 gt yr 1 and a total mass imbalance of 57 20 gt yr 1 the second assumption considers a long term dynamic thickening hypothesis and an a priori solution for surface mass balance from a regional climate model the latter results in estimates 3 to 5 times larger for the ice dynamic trends but similar total mass imbalance in both cases gains in east antarctica are smaller than losses in west antarctica

palaeomagnetic studies of fold development and propagation in the pembrokeshire old red sandstone abstract the geometry of two folds in lower devonian old red sandstone in pembrokeshire deformed in the hercynian has been defined at intermediate stages during their evolution by this preliminary palaeomagnetic study this is possible because remagnetization appears to have occurred in discrete episodes during deformation and the shape can be reconstructed by restoring the fold limbs to an angle at which the remagnetization directions coincide the plunge of the folds is shown to vary considerably during evolution one fold having an early easterly plunge which changed to the present westerly plunge the other fold having an early plunge which was more steeply to the west than its present attitude clockwise rotation of the whole of southern pembrokeshire by 40 towards the end of or since the hercynian deformation is indicated by the palaeomagnetic data and a structural model for this is proposed

climate change and forest fires synergistically drive widespread melt events of the greenland ice sheet in july 2012 over 97 of the greenland ice sheet experienced surface melt the first widespread melt during the era of satellite remote sensing analysis of six greenland shallow firn cores from the dry snow region confirms that the most recent prior widespread melt occurred in 1889 a firn core from the center of the ice sheet demonstrated that exceptionally warm temperatures combined with black carbon sediments from northern hemisphere forest fires reduced albedo below a critical threshold in the dry snow region and caused the melting events in both 1889 and 2012 we use these data to project the frequency of widespread melt into the year 2100 since arctic temperatures and the frequency of forest fires are both expected to rise with climate change our results suggest that widespread melt events on the greenland ice sheet may begin to occur almost annually by the end of century these events are likely to alter the surface mass balance of the ice sheet leaving the surface susceptible to further melting

paleomagnetic results from the lower devonian llandstadwell formation dyfed wales abstract a paleomagnetic investigation of the lower devonian llandstadwell formation in southwest wales has yielded a characteristic direction declination 196 inclination 5 that passes a fold test comparison of the corresponding virtual geomagnetic pole 334 e 39 s to previously published poles for great britain indicates that the magnetization is secondary and likely to be of carboniferous age no evidence for post hercynian rotation of pembrokeshire is indicated by our data

palaeomagnetism of the early devonian lower old red sandstones of south wales implications to variscan overprinting and differential regional rotations component structures of magnetic remanence in the old red sandstones of south wales are related to tectonic history by detailed study of three contrasting areas 1 remote from 2 bordering and 3 within the variscan orogenic belt a uniform southerly shallow directed component dominates remanence in each sector in area 1 where it is considered most representative of the regional field this magnetisation has a mean di 191 1 8 0 151 samples in situ 95 2 equivalent to a pole position at 341 1 e 41 2 s fold tests show that it was acquired at an intermediate stage of orogenesis and during the carboniferous permian reversed superchron it is interpreted as a pervasive magnetic overprint imparted by northward fluid migration consequent upon loading of the orogen to the south remote from the orogenic front a higher blocking temperature dual polarity remanence of probable lower devonian age has sporadically survived di 246 38 27 samples tilt adjusted 95 7 2 corresponding to a pole position at 297 e 3 n this remanence is entirely replaced close to the frontal zone of the orogen within the orogen low blocking temperature components are more prominent and chemical alteration obscures the highest blocking temperature portion of the remanence spectrum this region was deformed into upright folds before the magnetising event following remanence acquisition continuing deformation produced asymmetry in fold structure producing a plunge and steepening of the southern limbs structures in the south were already more deformed than those in the north at the time of magnetisation implying that the causative fluid front moved northwards with time the palaeomagnetic vectors are rotated clockwise to the west in south wales and in part record the effects of regional dextral transpression the largest rotation however is observed within the same thrust block and apparent differential rotation here is probably a consequence of strain

mass gains of the antarctic ice sheet exceed losses during 2003 to 2008 the mass gain of the antarctic ice sheet from snow accumulation exceeded the mass loss from ice discharge by 49 gt yr 2 5 of input as derived from icesat laser measurements of elevation change the net gain 86 gt yr over the west antarctic wa and east antarctic ice sheets wa and ea is essentially unchanged from revised results for 1992 to 2001 from ers radar altimetry imbalances in individual drainage systems ds are large 68 to 103 of input as are temporal changes 39 to 44 the recent 90 gt yr loss from three ds pine island thwaites smith and marie bryd coast of wa exceeds the earlier 61 gt yr loss consistent with reports of accelerating ice flow and dynamic thinning similarly the recent 24 gt yr loss from three ds in the antarctic peninsula ap is consistent with glacier accelerations following breakup of the larsen b and other ice shelves in contrast net increases in the five other ds of wa and ap and three of the 16 ds in east antarctica ea exceed the increased losses alternate interpretations of the mass changes driven by accumulation variations are given using results from atmospheric model re analysis and a parameterization based on 5 change in accumulation per degree of observed surface temperature change a slow increase in snowfall with climate warming consistent with model predictions may be offsetting increased dynamic losses

icesat measurements of sea ice freeboard and estimates of sea ice thickness in the weddell sea 1 sea ice freeboard heights in the weddell sea of antarctica are derived from the ice cloud and land elevation satellite icesat laser altimeter measurements which have a unique range precision to flat surfaces of 2 cm within 70 m footprints spaced at 172 m along track although elevations of flat surfaces can be obtained to an accuracy of 10 cm 1s per footprint direct determination of freeboard heights is precluded by errors in knowledge of the geoid and temporal variability of the ocean surface therefore freeboards are determined relative to an ocean reference level detected over areas of open water and very thin ice within the sea ice pack using an along track filtering method the open water thin ice selections show good agreement in the combined analysis of icesat segments and envisat synthetic aperture radar sar imagery the average residual between the icesat measured ocean level and the egm96 geoid is 1 4 m estimates of snow depth on the sea ice from amsr e passive microwave data along with nominal densities of snow water and sea ice are used to estimate sea ice thickness four periods of icesat data in may june fall and october november late winter of 2004 and 2005 between longitudes 298e and 360e are analyzed in the fall the mean freeboards are 0 28 m in 2004 and 0 29 m in 2005 and the mean thicknesses are 1 33 m in 2004 and 1 52 m in 2005 in late winter the freeboards grew to 0 37 m in 2004 and 0 35 in 2005 and the thicknesses grew to 2 23 m in 2004 and 2 31 m in 2005 the interannual differences in freeboard are small and the larger interannual change in estimated thickness mainly represents differences in the snow depth estimates seasonal changes in the spatial patterns of freeboard and thickness over the 4 months correlate with the expected circulation of sea ice in the weddell sea as indicated by sea ice velocity fields

improved representation of east antarctic surface mass balance in a regional atmospheric climate model this study evaluates the impact of a recent upgrade in the physics package of the regional atmospheric climate model racmo2 on the simulated surface mass balance smb of the antarctic ice sheet the modelled smb increases in particular over the grounded ice sheet of east antarctica 44 gt a 1 with a small change in west antarctica this mainly results from an increase in precipitation which is explained by changes in the cloud microphysics including a new parameter ization for ice cloud supersaturation and changes in large scale circulation patterns which alter topographically forced precipitation the spatial changes in smb are evaluated using 3234 in situ smb observations and ice balance velocities and the temporal variability using grace satellite retrievals the in situ observations and balance velocities show a clear improvement of the spatial representation of the smb in the interior of east antarctica which has become considerably wetter no improvements are seen for west antarctica and the coastal regions a comparison of model smb temporal variability with grace satellite retrievals shows no significant change in performance