high resolution study of layering within the percolation and soaked facies of the greenland ice sheet within the percolation and soaked facies of the greenland ice sheet the relationship between radar derived internal reflection horizons and the layered structure of the firn column is unclear we conducted two small scale ground penetrating radar gpr surveys in conjunction with 10 m firn cores that we collected within the percolation and soaked facies of the greenland ice sheet the two surveys were separated by a distance of 50 km and 340 m of elevation leading to 40 days of difference in the duration of average annual melt at the higher site 1997 m a s l which receives less melt we found that internal reflection horizons identified in gpr data were largely laterally continuous over the grid however stratigraphic layers identified in cores could not be traced between cores over any distance from 1 5 to 14 0 m thus we found no correlation between firn core stratigraphy observed directly and radar derived internal reflection horizons at the lower site 1660 m a s l which receives more melt we found massive ice layers 0 5 m thick and stratigraphic boundaries that span 15 m horizontally some ice layers and stratigraphic boundaries correlate well with internal reflection horizons that are laterally continuous over the area of the radar grid internal reflection horizons identified at 1997 m a s l are likely annual isochrones but the reflection horizons identified at 1660 m a s l are likely multi annual features we find that mapping accumulation rates over long distances by tying core stratigraphy to radar horizons may lead to ambiguous results because 1 there is no stratigraphic correlation between firn cores at the 1997 m location and 2 the reflection horizons at the 1660 m location are multi annual features

detection of snowmelt regions on the greenland ice sheet using diurnal backscatter change snowmelt regions on greenland ice are mapped daily with the seawinds wideswath ku band 13 4 ghz scatterometer on the quikscat satellite the approach exploits the high temporal resolution of seawinds quikscat data for the melt mapping using diurnal backscatter change independent of the absolute calibration the results reveal several pronounced melting and refreezing events and effects of topography are evident in the melt patterns the spatial resolution is sufficient to identify melt features on the sukker toppen iskappe west of the main ice sheet an anomalous warming event caused by downward mixing of warm air is detected in late september 1999 over the west flank of the southern greenland ice sheet time series images of melt regions are presented over the period from summer to the fall freeze up the satellite observations are verified with in situ measurements from the greenland climate network stations

georadar derived estimates of firn density in the percolation zone western greenland ice sheet 1 greater understanding of variations in firn densification is needed to distinguish between dynamic and melt driven elevation changes on the greenland ice sheet this is especially true in greenland s percolation zone where firn density profiles are poorly documented because few ice cores are extracted in regions with surface melt we used georadar to investigate firn density variations with depth along a 70 km transect through a portion of the accumulation area in western greenland that partially melts we estimated electromagnetic wave velocity by inverting reflection traveltimes picked from common midpoint gathers we followed a procedure designed to find the simplest velocity versus depth model that describes the data within estimated uncertainty on the basis of the velocities we estimated 13 depth density profiles of the upper 80 m using a petrophysical model based on the complex refractive index method equation at the highest elevation site our density profile is consistent with nearby core data acquired in the same year our profiles at the six highest elevation sites match an empirically based densification model for dry firn indicating relatively minor amounts of water infiltration and densification by melt and refreeze in this higher region of the percolation zone at the four lowest elevation sites our profiles reach ice densities at substantially shallower depths implying considerable meltwater infiltration and ice layer development in this lower region of the percolation zone the separation between these two regions is 8 km and spans 60 m of elevation which suggests that the balance between dry firn and melt induced densification processes is sensitive to minor changes in melt

accumulation over the greenland ice sheet from historical and recent records water accumulation defined as precipitation minus evaporation was estimated over all of greenland as part of a program to understand changes in ice sheet mass and elevation over 360 historical and recent point accumulation estimates on the greenland ice sheet were evaluated and 276 estimates that were judged to be high quality were used to develop the accumulation map the data set includes 99 points developed as part of four investigations of the past 5 15 years these are judged to have the greatest accuracy using kriging the average accumulation over greenland is estimated to be 30 g cm 2 yr 1 for the interior part of the ice sheet above 1800 m elevation where most of the data were acquired the average accumulation is also estimated to be 30 g cm 2 yr 1 there are still many areas on the ice sheet including northwest southeast and southern greenland where accumulation is highly uncertain exceeding the mean ice sheet uncertainty at a point of 20 25 in these regions further sampling will be required to reduce uncertainty in both regional and ice sheet wide accumulation

greenland snow accumulation estimates from satellite radar scatterometer data data collected by the c band ers 2 wind scatterometer escat the ku band adeos 1 nasa scatterometer nscat and the ku band seawinds on quikscat qscat satellite instruments are used to illustrate spatiotemporal variability in snow accumulation on the greenland ice sheet microwave radar backscatter images of greenland are derived using the scatterometer image reconstruction sir method at 3 day intervals over the periods 1991 1998 and 1996 1997 for escat and nscat respectively the backscatter coefficient normalized to 40 incidence a and gradient in backscatter b in the range 20 60 are compared with historical snow accumulation data and recent measurements made in the program for arctic regional climate assessment parca shallow snow pits empirical relationships derived from these comparisons reveal different exponential relationships between c and ku band a values and dry snow zone mean annual accumulation q frequency difference images between overlapping scatterometer images suggest that c band data are more sensitive to snow layering and buried inhomogeneities whereas ku band data are more sensitive to volume scattering from recently accumulated snow direct comparisons between nscat b values and in situ q measurements show a linear relationship between ln q and b with a negative rank correlation of r 0 8 the root mean square residual in fitting regression line equation ln q 3 08 17 83b to the data is 0 05 m snow water equivalent this simple ku band empirical relationship is exploited to investigate decadal changes in dry snow zone accumulation between seasat 1978 and nscat 1996 additional comparisons between nscat and recent qscat 1999 data reveal significant upslope shifts in the dry snow line along the southwestern flank of the ice sheet recent acceleration in the increase in intensity of scattering is observed in the percolation zone suggesting increased melting between 2000 and 3000 m elevation in the southern half of the ice sheet

spatial and temporal variability in the snowpack of a high arctic ice cap implications for mass change measurements interpretation of ice mass elevation changes observed by satellite altimetry demands quantification of the proportion of elevation change which is attributable to variations in firn densification detailed stratigraphic logging of snowpack structure and density was carried out at 1k m intervals along a 47 km transect on devon ice cap canada in spring pre melt and autumn during after melt 2004 and 2006 to characterize seasonal snowpack variability across the full range of snow facies simultaneous meteorological measurements were gathered spring pre melt snowpacks show low variability over large spatial scales with low magnitude changes in density the end of summer autumn density profiles show high variability in both 2004 and 2006 with vastly different melt regimes generating dissimilar patterns of ice layer formation over the two melt seasons dye tracing experiments from spring to autumn 2006 reveal that vertical and horizontal distribution of meltwater flow within and below the annual snowpack is strongly affected by the pre existing often subtle stratigraphic interfaces in the snowpack rather than its bulk properties strong interannual variability suggests that using a simple relationship between air temperature elevation and snowpack densification to derive mass change from measurements of elevation change across high arctic ice caps may be misguided melt timing and duration are important extrinsic factors governing snowpack densification and ice layer formation in summer rather than averaged air temperatures

soil moisture fluctuations recorded in saharan dust deposits on lanzarote canary islands over the last 180 ka abstract aeolian sediments trapped in volcanically dammed valleys on lanzarote canary islands were investigated in order to reveal environmental changes over the last 180 ka clay content and frequency dependent magnetic susceptibility were used as proxies for pedogenesis and palaeo soil moisture during the last 180 ka these proxies showed a general pattern of enhanced soil moisture during glacials and stadials and more arid conditions during interglacials and interstadials comparisons of these results with proxies from regional palaeoclimate studies identified a positive correlation with proxies of trade wind strength off northwest africa and inverse correlations with both sea surface temperatures in the northeast atlantic and the extent of mediterranean vegetation possible causes for the observed pattern include a glacial enhancement of precipitation from westerly cyclones a change in relative humidity due to fluctuating air temperatures and an occasional influence of the african summer monsoon although it is not yet possible to clearly differentiate among these factors it is clear that the first two factors must have been primarily dominant these results represent the first quasi continuous terrestrial data testifying to environmental changes in the northwest african coastal area for the last 180 ka and complement the abundant data derived from marine cores of the region high latitude dynamics had a major influence in this area and were intermediated by north atlantic sea surface temperatures a possible negative correlation can also be observed with the orbital obliquity cycle with a 10 ka time lag which is similar to the lag recorded from north atlantic sea surface temperatures

importance of seasonal and annual layers in controlling backscatter to radar altimeters across the percolation zone of an ice sheet radar altimeters are one of the main tools for measuring elevation changes across the antarctic and greenland ice sheets and larger ice caps a ground based radar was deployed in autumn 2004 and spring 2006 in the percolation zone of the greenland ice sheet this radar is a high bandwidth system operating in the ku band the same frequency as several satellite altimeters measurements were made over an elevation range of 1795 to 2350 m along with snow pit and shallow core studies these measurements demonstrate the spatial and temporal variations in the backscatter relative strengths of surface and volume reflections change dramatically between spring and autumn and there is also high spatial variability across the percolation zone the extent of percolation will affect elevation estimates made by radar altimeters

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

a ground based radar backscatter investigation in the percolation zone of the greenland ice sheet abstract satellite radar altimeters and scatterometers deployed over ice sheets experience backscatter from the surface and from within the snowpack termed surface and volume backscatter respectively in order to assess the errors in satellite altimeter measurements it is vital to know where the return is originating from in the snowpack this return can vary spatially and temporally seasonal variations in the volume backscatter can be a major complicating factor in the radar return from the percolation zone ground based step frequency radar was deployed in the percolation zone of the greenland ice sheet at 1945 m elevation 69 51n 47 15w previous measurements in this area made by scientists from the byrd polar research centre and the university of kansas undertaken prior to summer melt events have shown the strongest backscatter from ice features at around 1 m depth buried beneath the previous end of summer surface in autumn 2004 radar measurements in the ku band with bandwidths of 1 and 8 ghz were made alongside detailed stratigraphic observations within a 1 km 2 site the radar results revealed no continuous reflecting horizons in the upper 3 5 m of the firn shallow cores and snowpits also indicated that there were no spatially continuous stratigraphic horizons across the study site an average electromagnetic wave velocity of 2 11 0 05 10 8 m s 1 was determined for the upper metre of the firn surface and volume backscatter at vertical incidence were calculated using a standard model the contribution of the surface backscatter to the total backscatter was on average 6 db higher than that of the volume backscatter however at the higher 8 ghz bandwidth the strongest return frequently originated not from the surface but from within the upper 30 cm of the snowpack most probably from thin ice layers at 1 ghz bandwidth these ice layers were not always resolved their return merged with the surface return causing it to broaden with the peak and leading edge moving down modelling using density and thickness measurements from shallow cores and snowpits showed that the backscatter from these shallow thin ice layers could be stronger than the surface return owing to constructive interference from the top and base of the layers

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

snow facies over ice sheets derived from envisat active and passive observations this paper aims to separate different snow regions over the terrestrial ice sheets based on their measured microwave signatures it takes advantage of coregistered data from passive and active sensors on the environmental satellite envisat to directly derive a snow facies indicator in a point by point basis this paper represents the first attempt of this kind in exploiting nadir viewing and dual frequency data from both altimeter and radiometer sensors the approach is based on a clustering method such representation of the data by means of fewer clusters necessarily loses fine details but achieves simplification in geographical representation and eases the description of the condition of the ice sheets in 2004 our approach broadens the description of the snow pack by taking into account characteristics such as surface roughness grain size stratification and snowmelt effects whereas the latter has often solely been considered in most previous work such partition of the ice sheets might help to better understand relationships between microwave signatures and snow morphology it could also be useful for estimating elevation uncertainty in altimeter data which in turn is essential to correctly interpret the significance of the rates of elevation change in a changing climate and to convert elevation change to snow mass change

the role of salt in fold and thrust belts davis d m and engelder t 1985 the role of salt in fold and thrust belts in n l carter and s uycda editors collision tectonics deformation of continental lithosphere tectonophysics 119 67 88 the style of deformation in thin skinned fold and thrust belts is critically dependent upon the resistance to sliding along the detachment between the mass of deforming sediments and the underlying rocks evaporites can provide an extremely weak horizon within which a basal detachment can form and along which only a relatively small shear traction can be supported fold and thrust belts that form atop a salt layer such as the appalachian plateau the franklin mountains in northwestern canada and the jura of the alps among others share several readily observable characteristics as predicted by a simple mechanical model for fold and thrust belts a detachment in salt permits a thrust belt to have an extremely narrow cross sectional taper in addition predicted orientations of the principal stress axes over a salt dtcollement are consistent with the commonly observed lack of a consistently dominant vergence direction of structures within the thrust belt other common attributes of salt basal thin skinned deformation include the presence of several widely but regularly spaced folds and abrupt changes in deformational style at the edge of the salt basin

satellite derived surface type and melt area of the greenland ice sheet using modis data from 2000 to 2005 a new surface classification algorithm for monitoring snow and ice masses based on data from the moderate resolution imaging spectroradiometer modis is presented the algorithm is applied to the greenland ice sheet for the period 2000 05 and exploits the spectral variability of ice and snow reflectance to determine the surface classes dry snow wet snow and glacier ice the result is a monthly glacier surface type gst product on a 1 km resolution grid the gst product is based on a grouped criteria technique with spectral thresholds and normalized indices for the classification on a pixel by pixel basis the gst shows the changing surface classes revealing the impact of climate variations on the greenland ice sheet over time the area of wet snow and glacier ice is combined into the glacier melt area gma product the gma is analyzed in relation to the different surface classes in the gst product the results are validated with data from weather stations and similar types of satellite derived products the validation shows that the automated algorithm successfully distinguishes between the different surface types implying that the product is a promising indicator of climate change impact on the greenland ice sheet

investigation of surface melting and dynamic thinning on jakobshavn isbrae greenland jakobshavn isbrae is the most active glacier in greenland with an annual discharge of about 30 km 3 of ice and it is one of the few recently surveyed glaciers to thicken between 1993 and 1998 despite locally warm summers repeated airborne laser altimeter surveys along a 120 km profile in the glacier basin show slow sporadic thickening between 1991 and 1997 suggesting a small positive mass balance but since 1997 there has been sustained thinning of several m a 1 within 20 km of the ice front with lower rates of thinning further inland here we use weather station data from the coast and the ice sheet to estimate the effects on surface elevation of interannual variability in snowfall and surface melt rates and thus to infer the temporal and spatial patterns of dynamic thinning these show the glacier to have been close to balance before 1997 followed by a sudden transition to rapid thinning initially confined to the lower reaches of the glacier below about 500 m elevation but progressively spreading inland until between 1999 and 2001 thinning predominated over the entire surveyed region up to 2000 m elevation if this continues the glacier calving front and probably its grounding line will retreat substantially in the very near future

mapping of ice layer extent and snow accumulation in the percolation zone of the greenland ice sheet 1 the greenland ice sheet underwent record extensive melt in 2002 and prolonged melt in 2003 the severe melting created a significant and extensive ice layer over the greenland ice sheet an innovative approach is developed to detect the ice layer formation using data acquired by the seawinds scatterometer on the quikscat satellite quikscat backscatter together with in situ data from automatic weather stations of the greenland climate network are used to map the extent of ice layer formation the results reveal areas of extensive ice layer formed by the 2002 melt which is consistent with the maximum melt extent in 2002 moreover during freezing seasons quikscat data show a linear decrease in backscatter in decibels or db that is related to the amount of snow accumulation in the ice layer formation region this snow accumulation signature is caused by the attenuation of radar waves in the snow layer accumulating since the last major melt event whose thickness appears as an exponential function in relation to the backscatter signature we use the greenland climate network data to calibrate the quikscat accumulation rate in order to estimate and map snow accumulation quikscat results capture the extreme snowfall in mid april 2003 which deposited more than 0 5 m of snow in a day as measured by the automated weather station at the nasa south east site large scale quikscat results show an anomalous increase of snow accumulation over the southeast region of greenland during the 2002 2003 freezing season

analysis of synthetic aperture radar data collected over the southwestern greenland ice sheet analyses of the first aircraft multi frequency polarimetric synthetic aperture radar sar data acquired over the southwestern greenland ice sheet are presented data were collected on 31 august 1989 by the jet propulsion laboratory sar using the nasa dc 8 aircraft along with curvilinear patterns associated with large scale morphologic features such as crevasses lakes and streams frequency and polarization dependencies are observed in the p l and c band image products

snow and ice facies variability and ice layer formation on canadian arctic ice caps 1999 2005 1 time series of enhanced resolution data from the seawinds scatterometer aboard quikscat were used to map the distribution of snow and ice surface facies and ice layer formation in the percolation zone on ice caps in the queen elizabeth islands during the period 1999 2005 iterative self organizing data analysis classification of the mean postfreeze up biweekly average 0 signal for the 7 year period resulted in the delineation of four snow and ice surface facies interpreted as the percolation saturation superimposed ice and glacier ice zones analysis with national centers for environmental prediction national center for atmospheric research reanalysis reveals that changes in geopotential height in the troposphere 700 500 and 300 hpa and air temperature 700 hpa level are positively negatively correlated with area changes in the glacier ice percolation and saturation zones and changes in facies boundary elevation the change in biweekly averaged backscatter following the freeze up periods between successive autumns was used to map changes in the distribution of ice layers formed by meltwater percolation and refreezing in the snowpack within the percolation zone strongly positive air temperature anomalies at the 700 hpa level in 2001 and 2005 are consistent with extensive ice layer formation in the percolation zones of all ice caps such large interannual changes in ice layer formation are likely associated with large changes to the density profile of the snowpack and may be associated with surface elevation changes that are unrelated to changes in surface mass balance

growth of the southern greenland ice sheet the principal conclusion of c h davis et al 1 1 that the elevation of the southern greenland ice sheet increased at a smaller rate than the 23 cm year we previously reported 2 2 3 3 is in qualitative agreement with our new analysis their statement however that surface

a contribution to the understanding of late pleistocene dune sand paleosol sequences in fuerteventura canary islands abstract this paper describes dune sand paleosol sequences of four pits in northern fuerteventura canary islands the elaborated stratigraphy is reinforced with luminescence dating to provide a first chronological estimation apart from a holocene colluvial layer the sequence spans the period from ca 50 ka to ca 280 ka paleosols were formed during glacial times and point to a standstill in sand supply the isotopic composition of terrestrial gastropod shells retrieved from soil horizons reflects fluctuations in humidity conditions during different edaphic phases because eolian sands were deposited during glacial times as well it is inferred that soil development was simply caused by a decrease in sand supply independent of climate change our geomorphic geochronological and isotopic results are discussed considering different perspectives of dune sand paleosol intercalation and more broadly soil forming conditions in general

changes in greenland ice sheet elevation attributed primarily to snow accumulation variability the response of grounded ice sheets to a changing climate critically influences possible future changes in sea level recent satellite surveys over southern greenland show little overall elevation change at higher elevations but large spatial variability using satellite studies alone it is not possible to determine the geophysical processes responsible for the observed elevation changes and to decide if recent rates of change exceed the natural variability here we derive changes in ice sheet elevation in southern greenland for the years 1978 88 using a physically based model of firn densification and records of annual snow accumulation reconstructed from 12 ice cores at high elevation our patterns of accumulation driven elevation change agree closely with contemporaneous satellite measurements of ice sheet elevation change and we therefore attribute the changes observed in 1978 88 to variability in snow accumulation similar analyses of longer ice core records show that in this decade the greenland ice sheet exhibited typical variability at high elevations well within the long term natural variability our results indicate that a better understanding of ice sheet mass changes will require long term measurements of both surface elevation and snow accumulation

greenland ice sheet increased coastal thinning repeated laser altimeter surveys and modelled snowfall summer melt show average ice loss from greenland between 1997 and 2003 was 80 12 km3 yr 1 compared to about 60 km3 yr 1 for 1993 4 1998 9 half of the increase was from higher summer melting with the rest caused by velocities of some glaciers exceeding those needed to balance upstream snow accumulation velocities of one large glacier almost doubled between 1997 and 2003 resulting in net loss from its drainage basin by about 20 km3 of ice between 2002 and 2003 copyright 2004 by the american geophysical union

antarctic surface melting dynamics enhanced perspectives from radar scatterometer data 1 antarctic ice sheet surface melting can regionally influence ice shelf stability mass balance and glacier dynamics in addition to modulating near surface physical and chemical properties over wide areas here we investigate variability in surface melting from 1999 to 2009 using radar backscatter time series from the seawinds scatterometer aboard the quikscat satellite these daily continent wide observations are explored in concert with in situ meteorological records to validate a threshold based melt detection method radar backscatter decreases during melting are significantly correlated with in situ positive degree days as well as meltwater production determined from energy balance modeling at neumayer station east antarctica these results support the use of scatterometer data as a diagnostic indicator of melt intensity i e the relative liquid water production during melting greater spatial and temporal melting detected relative to previous passive microwave based studies is attributed to a higher sensitivity of the scatterometer instrument continental melt intensity variability can be explained in part by the dynamics of the southern annular mode and the southern oscillation index and extreme melting events across the ross ice shelf region may be associated with el nino conditions furthermore we find that the antarctic peninsula accounts for only 20 of antarctic melt extent but greater than 50 of the total antarctic melt intensity over most areas annual melt duration and intensity are proportional however regional and localized distinctions exist where the melt intensity metric provides greater insight into melting dynamics than previously obtainable with other remote sensing techniques

the melt anomaly of 2002 on the greenland ice sheet from active and passive microwave satellite observations 1 active and passive microwave satellite data are used to map snowmelt extent and duration on the greenland ice sheet the passive microwave pm data reveal the extreme melt extent of 690 000 km2 in 2002 as compared with an average extent of 455 000 km2 from 1979 2003 a statistical analysis of the melt time series affirms an increased melt extent earlier in the melt season the quikscat qscat or am analysis for 1999 to 2004 confirms the extreme melt situation in 2002 qscat data show a significant increase in melt season length over several areas in 2003 ssm i and qscat melt data reveal that the later detects melt earlier and is more sensitive to surface melt qscat detects an earlier stage of melt and ssm i detects a later stage of melt the ssm i xgpr melt extent is approximately confined to the qscat melt areas experiencing 2 weeks or more of melting time

changes in pleistocene gastropod faunas on fuerteventura canary islands and implications on shifting palaeoenvironmental conditions abstract malacological studies on middle to late pleistocene aeolianite palaeosurface sequences have been approached in order to detect environmental changes and system shifts on the eastern canary islands our results reveal that over the last approximately 360 ka gastropod associations changed fundamentally allowing the derivation of eight different biozones the majority of these malacozones and particular species were limited to specific stratigraphic positions and periods we therefore assume that quaternary gastropods on fuerteventura may offer the opportunity to correlate lithostratigraphic units on the basis of key species furthermore environmental conditions could be derived from these associations pointing to colder semi arid conditions related to the obelus zone around 360 to 340 30 ka a subsequent temperature rise around 340 30 ka leading to moderate and dryer conditions for the cochlicella zone a strong environmental change around 300 30 ka with frequent ephemeral rainfalls for the pomatias zone and an increase in temperature and aridity for late pleistocene deposits linked to the rumina zone we assume that shifts in malacozonation were triggered by changing climate conditions furthermore the alternation between sand layers and palaeosurfaces is not continuously in conformity with malacozonation we therefore assume these stratigraphic changes to be not primarily related to climatic forcing but instead biased by an overlap of multiple factors as palaeosurfaces indicate favourable conditions increased gastropod biodiversities and distinct shrub vegetation we assume that these were caused by increased edaphic moisture due to a better water holding capacity in silty palaeosurfaces rather than by more humid climate conditions

an accumulation map for the greenland dry snow facies derived from spaceborne radar 1 an important component of ice sheet mass balance calculations is the snow accumulation rate traditionally determined across the ice sheet from interpolated ice core data we derive an accumulation rate map within the greenland dry snow facies using c band radar backscatter and the mean annual temperature values for the backscatter were obtained from the ers 1 synthetic aperture radar mosaic acquired during september november 1992 an empirical equation based on elevation and latitude is used to determine the mean annual temperature we examine the influence of accumulation rate and mean annual temperature on c band 5 3 ghz radar backscatter using a forward model which incorporates snow metamorphosis and radar backscatter components our model is run over a range of accumulation and temperature conditions consistent with the greenland dry snow facies on the basis of our modeled results a look up table is generated which uniquely maps the measured radar backscatter and mean annual temperature to accumulation rate our results compare favorably 15 4 average difference with 124 in situ accumulation rate measurements falling within our study area we attribute some of the differences to 1 unreliable in situ data points which were based on a single year s accumulation and 2 a systematic underestimate of accumulation rate by our forward model in the low accumulation regions associated with the northern portion of our map with questionable data points and those above 77 n removed the average differences were less than 10 3 as determined from 73 ice cores

radar response of firn exposed to seasonal percolation validation using cores and fdtd modeling we use ground penetrating radars gprs firn cores and electromagnetic finite difference time domain fdtd numerical modeling to characterize the gpr response to a frozen high arctic firn pack as a result of extensive summertime percolation the firn pack comprises a high fraction of ice layers lenses and vertical glands we show that the gpr response on the firn pack mainly depends on the following 1 the thickness of the ice layers 2 the distance between layers 3 the layer roughness and 4 the presence or absence of elliptical ice lenses using 3 d fdtd modeling we show that the gpr is not sensitive to typical ice glands which implies that the gpr underestimates firn heterogeneity such that firn stratigraphy in percolation and wet snow zones could be incorrectly interpreted as being better preserved than it actually is we find that thin ice layers 0 05 m or multiple thin ice layers give a strong response thicker ice layers typically give a weaker backscatter per unit area mainly due to the lack of interference of the reflections from the upper and lower interfaces but are due to their continuity easily trackable ice layers with a thickness comparable to the gpr wavelength give 180deg phase shifted upper and lower reflections and are in general separated by a band of low gpr response due to the lack of permittivity contrast within the ice layers despite the ice lenses relatively short horizontal correlation length as inferred from cores bands of high amplitude clutter caused by these features can be traced over several kilometers in gpr profiles

variations of near surface firn density in the lower accumulation area of the greenland ice sheet pâkitsoq west greenland firn density variations have been studied in the lower accumulation area of the greenland ice sheet 1440 1620 m a s l near p kitsoq west greenland the main control on density in the near surface firn layer of 5 10 m thickness is the formation of ice layers by the refreezing of meltwater that reaches depths of 2 4 m below the surface the density variations are described by the ratio of annual surface melt m to the annual accumulation c the ratio m c is about 0 6 at the run off limit at about 1400 m a s l in the study area where refreezing of meltwater transforms snow into impermeable ice the mean density of near surface firn decreases with elevation reflecting a decrease in melt with elevation there is a surprising decrease in firn density at depths of more than about 4 m below the 1991 summer surface which reflects lower melt rates and or higher accumulation in the early 1980s and late 1970s when this firn was passing through the surface layer the formation of such low density firn may have partially contributed to the 1978 85 thickening of the ice sheet observed by satellite radar altimetry near surface firn density is therefore very sensitive to climate change and might be an attractive target for climate monitoring

melt season duration and ice layer formation on the greenland ice sheet 2000 2004 1 time series of enhanced resolution quikscat images were used to map the extent and duration of surface melt and the distribution of ice layer formation on the greenland ice sheet in the period 2000 2004 apart from the bare ice and the mixed pixels 10 along the ice sheet margin where melt cannot be determined reliably annual melt duration and melt anomaly maps were produced across the ice sheet 90 for each summer over the 5 year period the mean melt duration for the ice sheet ranged from 14 3 to 20 5 d the proportion of the ice sheet that experienced melting in a given year ranged from 44 2 to 79 2 extensive melt in 2002 was caused by a single melt event of a few days duration this event was associated with the intrusion of a ridge of high pressure from the north atlantic ocean that brought warm air onto the ice sheet the change in biweekly averaged backscatter between the freeze up periods in the current and previous falls was used to identify changes in the distribution of ice layers in snow between successive melt seasons the year 2002 had the maximum melt extent and duration and ice layer formation extended to higher elevations than in other years interannual changes in the distribution of ice layer formation may be associated with changes in surface height that are not necessarily indicative of mass balance changes mapping of ice layer formation is thus potentially useful for interpreting both real and apparent height changes measured by altimetry

elevation change of the southern greenland ice sheet seasat and geosat satellite altimeter measurements for the greenland ice sheet south of 72 degreesn latitude show that surface elevations above 2000 meters increased at an average rate of only 1 5 0 5 centimeters per year from 1978 to 1988 in contrast elevation changes varied regionally from 15 to 18 centimeters per year seasonally by 15 centimeters and interannually by 8 centimeters the average growth rate is too small to determine if the greenland ice sheet is undergoing a long term change due to a warmer polar climate

new precipitation and accumulation maps for greenland annual total precipitation and the annual accumulation on the greenland ice sheet are evaluated and presented in two maps the maps are based on accumulation measurements of 25 1 pits and cores obtained from the upper accumulation zone and precipitation measurements made at 35 meteorological stations in the coastal region to construct the accumulation map the annual precipitation was split into solid and liquid precipitation components annual total precipitation exceeding 2500 mm w e occurs on the southeastern tip of greenland while the minimum precipitation is estimated to occur on the northeastern slope of the ice sheet the mean annual precipitation for all of greenland is 340 mm w e the largest annual accumulation of about 1500 mm w e is found on the glaciers in the southeastern corner of greenland while the smallest accumulation is found on the northeastern slope of the ice sheet west of danmarkshavn the mean accumulation on the greenland ice sheet is estimated at 310 mm w e the regional difference in accumulation is examined with respect to the 850 hpa m bar level circulation the present surface topography is found to play an important role in determining regional accumulation on the ice sheet

seasonal and interannual variations of firn densification and ice sheet surface elevation at the greenland summit seasonal and interannual variations in surface elevation at the greenland summit are modeled using a new temperature dependent formulation of firn densification and are compared with elevations from european remote sensing satellite ers 1 2 radar altimetry the rate constant and activation energy usually set as constants in the arrhenius type relation are strongly temperature dependent based on measurements of crystal growth rates a multiplicative factor in the densification rate accounts for differences between densification and grain growth rates and is chosen to match the modeled and measured density profiles from 0 to 40 m the stronger temperature dependence produces a significant seasonal cycle in the densification rate in the upper firn much of the densification and consequent surface lowering occur within 3 months in late spring early summer followed by a build up from accumulation modeled elevation changes using automatic weather station measurements of temperature and accumulation and modeled precipitation agree well with observations the respective seasonal amplitudes are 18 and 25 cm peak to peak with minima in mid summer the minimum elevation in 1995 is driven mainly by a temporary accumulation decrease and secondarily by warmer temperatures increased compaction driven by a summer warming trend decreases the modeled elevation 1992 99 by 20 cm but accumulation increases in latter years dominate the overall 4 2 cm a 1 trend

melting of small arctic ice caps observed from ers scatterometer time series 1 time series of active microwave backscatter data from the ers wind scatterometers are used to demonstrate 1 the timing of melt onset can be observed over small ice caps as well as the major ice sheets and multi year sea ice and 2 temporally integrated backscatter reduction r is strongly correlated with total annual positive degree days pdd as measured at four automated weather stations on the surface of the greenland ice sheet the latter is particularly interesting because unlike melt onset r is a measure of melt processes integrated over an entire ablation season both methods work owing to the well known sharp decrease in radar backscatter triggered by the appearance of liquid water in a glacier snowpack melt onset and r are determined for 14 small arctic ice caps from 1992 2000 interannual and regional variability in the timing of melt onset are comparable averaging 33 9 and 38 13 days respectively regional variability in r exceeds interannual variability averaging 906 164 and 309 130 db d respectively the brevity of the derived nine year records limits their statistical testing for trend however their qualitative interpretation suggests that the timing of melt onset is more variable for arctic ice caps than for sea ice and the intensity of summer surface melting increased by about 20 in the 1990s