surface and bed topography of trapridge glacier yukon territory canada digital elevation models and derived hydraulic geometry measurements of ice thickness and surface elevation are prerequisite to many glaci ological investigations a variety of techniques has been develo ped for inter pret ation of these data including means of constructing regularly gridded digital eleva tion models dem s for use in numcrical studies here we present a simple yet stati sti cally sound method for processing ice pe netr ating radar data and descr ibe a tech nique for interpol ating these data onto a regular grid dems generatcd fort rapridge glacier yukon territory canada are used to derive geometric quantities lhal give pre liminary insights into the underlying basin scale hydrol ogical syste m this simple geo metric analysis suggests that at low water press ures a dendrilic drainage netw ork exists that evolves into a uniaxial morphology as water pressure approaches notation these pre dictions are compared to hydraulic connection probabilities based on bore hole drilling

greenland flow variability from ice sheet wide velocity mapping using radarsat synthetic aperture radar data we have mapped the flow velocity over much of the greenland ice sheet for the winters of 2000 01 and 2005 06 these maps provide a detailed view of the ice sheet flow including that of the hundreds of glaciers draining the interior the focused patterns of flow at the coast suggest a strong influence of bedrock topography differences between our two maps confirm numerous early observations of accelerated outlet glacier flow as well as revealing previously unrecognized changes the overall pattern is one of speed up accompanied by terminus retreat but there are also several instances of surge behavior and a few cases of glacier slowdown comprehensive mappings such as these at regular intervals provide an important new observational capability for understanding ice sheet variability

pb isotopic evidence for an early enriched crust on mars abstract martian meteorite nwa 7533 is a regolith breccia that compositionally resembles the martian surface measured by orbiters and landers nwa 7533 contains monzonitic clasts that have zircon with u pb ages of 4 428 ga the pb isotopic compositions of plagioclase and alkali feldspars as well as u pb isotopic compositions of chlorapatite in the monzonitic clasts of nwa 7533 have been measured by secondary ion mass spectrometry sims the u pb isotopic compositions measured from the chlorapatite in nwa 7533 yield an age of 1 357 81 ga 2 the least radiogenic pb isotopic compositions measured in plagioclase and k feldspar lie within error of the 4 428 ga geochron these data indicate that the monzonitic clasts in nwa 7533 are a product of a differentiation history that includes residence in a reservoir that formed prior to 4 428 ga with a value 238 u 204 pb of at least 13 4 1 7 2 and a value 232 th 238 u of 4 3 this value is more than three times higher than any other documented martian reservoir these results indicate either the martian mantle is significantly more heterogeneous than previously thought value of 1 14 vs 1 5 and or the monzonitic clasts formed by the melting of martian crust with a value of at least 13 4 therefore nwa 7533 may contain the first isotopic evidence for an enriched differentiated crust on mars

widespread volcanism in the greenland north atlantic region explained by the iceland plume in the classical concept a hotspot track is a line of volcanics formed as a plate moves over a stationary mantle plume defying this concept intraplate volcanism in greenland and the north atlantic region occurred simultaneously over a wide area particularly around 60 million years ago showing no resemblance to a hotspot track here we show that most of this volcanism can nonetheless be explained solely by the iceland plume interacting with seafloor spreading ridges global mantle flow and a lithosphere the outermost rigid layer of the earth with strongly variable thickness an east west corridor of thinned lithosphere across central greenland as inferred from new highly resolved tomographic images could have formed as greenland moved westward over the iceland plume between 90 and 60 million years ago our numerical geodynamic model demonstrates how plume material may have accumulated in this corridor and in areas east and west of greenland simultaneous plume related volcanic activities starting about 62 million years ago on either side of greenland could occur where and when the lithosphere was thin enough due to continental rifting and seafloor spreading possibly long after the plume reached the base of the lithosphere volcanism across the north atlantic region 62 million years ago is consistent with an iceland plume source despite the absence of a classic hotspot track suggest tomographic images and geodynamic models

a continuum mechanical formulation for shallow polythermal ice sheets this paper is concerned with a new theoretical approach to model grounded ice sheets in three dimensions these are considered as polythermal i e there will be regions with temperatures below the

bed topography and subglacial landforms in the onset region of the northeast greenland ice stream the northeast greenland ice stream negis is an important dynamic component for the total mass balance of the greenland ice sheet as it reaches up to the central divide and drains 12 of the ice sheet the geometric boundary conditions and in particular the nature of the subglacial bed of the negis are essential to understand its ice flow dynamics we present a record of more than 8000 km of radar survey lines of multi channel ultra wideband radio echo sounding data covering an area of 24 000 km2 centered on the drill site for the east greenland ice core project egrip in the upper part of the negis catchment our data yield a new detailed model of ice thickness distribution and basal topography in the region the enhanced resolution of our bed topography model shows features which we interpret to be caused by erosional activity potentially over several glacial interglacial cycles off nadir reflections from the ice bed interface in the center of the ice stream indicate a streamlined bed with elongated subglacial landforms our new bed topography model will help to improve the basal boundary conditions of negis prescribed for ice flow models and thus foster an improved understanding of the ice dynamic setting

crustal structure of iceland and greenland from receiver function studies 1 we used data from both permanent and temporary seismic networks on iceland and greenland to investigate the crustal thickness by partly reinterpreting earlier data p receiver functions and adding s receiver functions to better constrain the results we obtained good conversions from the moho and also crustal multiples in both iceland and greenland the central ice covered part of greenland has an average crustal thickness of 40 km typical for a craton at the edges of greenland the crustal thickness decreases to 30 40 km on the east coast of greenland where the track of the iceland plume is thought to have affected the lithosphere the crustal thickness is only 24 26 km in contrast to previous studies we find that the crustal thickness in the east and the northwest coastal regions of iceland is more than 40 km similar to beneath the active volcanic region in the southwest region of iceland and along the mid ocean ridge the crustal thickness is only 25 km or less also in contrast to earlier receiver function interpretations which deduced a broad crust mantle transition zone for iceland we find indications for a normal sharper moho beneath a number of sites

crust and uppermost mantle structure of greenland and the northwest atlantic from rayleigh wave group velocity tomography the greenland landmass preserves 4 billion years of tectonic history but much of the continent is inaccessible to geological study due to the extensive inland ice cap we map out for the first time the 3 d crustal structure of greenland and the nw atlantic ocean using rayleigh wave anisotropic group velocity tomography in the period range 10 80 s from regional earthquakes and the ongoing glatis glisn seismograph networks 1 d inversion gives a pseudo 3 d model of shear wave velocity structure to depths of 100 km with a horizontal resolution of 200 km crustal thickness across mainland greenland ranges from 25 km to over 50 km and the velocity structure shows considerable heterogeneity the large sedimentary basins on the continental shelf are clearly visible as low velocities in the upper 5 15 km within the upper continental basement velocities are systematically lower in northern greenland than in the south and exhibit a broadly nw se trend the thinning of the crust at the continental margins is also clearly imaged upper mantle velocities show a clear distinction between typical fast cratonic lithosphere vs 4 6 km s 1 beneath greenland and its ne margin and anomalously slow oceanic mantle vs 4 3 4 4 km s 1 beneath the nw atlantic we do not observe any sign of pervasive lithospheric modification across greenland in the regions associated with the presumed iceland hotspot track though the average crustal velocity in this region is higher than that of areas to the north and south crustal anisotropy beneath greenland is strong and complex likely reflecting numerous episodes of tectonic deformation beneath the north atlantic and baffin bay the dominant anisotropy directions are perpendicular to the active and extinct spreading centres anisotropy in the subcontinental lithosphere is weaker than that of the crust but still significant consistent with cratonic lithosphere worldwide

metrology and traceability of u pb isotope dilution geochronology earthtime tracer calibration part i abstract mixed 235u 233u 205pb 202pb tracers for u pb isotope dilution isotope ratio mass spectrometry have been prepared under the auspices of the earthtime initiative the methods and results for the preparation and calibration of the u pb ratio and isotopic abundances are given and the various sources of uncertainty are discussed and quantified the accuracy of the earthtime u pb tracer isotopic composition can be traced back to si units via a series of assay and isotopic composition reference materials combined with the experiments described herein the parameters used in calculating u pb ratios and inferentially u pb dates have correlated uncertainties that result in a total uncertainty contribution to 206pb 238u dates of

decline of giant impacts on mars by 4 48 billion years ago and an early opportunity for habitability the timing of the wane in heavy meteorite bombardment of the inner planets is debated its timing determines the onset of crustal conditions consistently below the thermal and shock pressure limits for microbiota survival and so bounds the occurrence of conditions that allow planets to be habitable here we determine this timing for mars by examining the metamorphic histories of the oldest known martian minerals 4 476 4 429 gyr old zircon and baddeleyite grains in meteorites derived from the southern highlands we use electron microscopy and atom probe tomography to show that none of these grains were exposed to the life limiting shock pressure of 78 gpa 97 of the grains exhibit weak to no shock metamorphic features and no thermal overprints from shock induced melting by contrast about 80 of the studied grains from bombarded crust on earth and the moon show such features the giant impact proposed to have created mars hemispheric dichotomy must therefore have taken place more than 4 48 gyr ago with no later cataclysmic bombardments considering thermal habitability models we conclude that portions of mars crust reached habitable pressures and temperatures by 4 2 gyr ago the onset of the martian wet period about 0 5 gyr earlier than the earliest known record of life on earth early abiogenesis by 4 2 gyr ago is now tenable for both planets the oldest known minerals from mars have no strong shock features indicating early cessation of giant impacts there according to microanalysis of zircon and baddeleyite grains in meteorites

dynamical processes involved in the retreat of marine ice sheets marine ice sheets with mechanics described by the shallow ice approximation by definition do not couple mechanically with the shelf such ice sheets are known to have neutral equilibria we consider the implications of this for their dynamics and in particular for mechanisms which promote marine ice sheet retreat the removal of ice shelf buttressing leading to enhanced flow in grounded ice is discounted as a significant influence on mechanical grounds sea level rise leading to reduced effective pressures under ice streams is shown to be a feasible mechanism for producing postglacial west antarctic ice sheet retreat but is inconsistent with borehole evidence warming thins the ice sheet by reducing the average viscosity but does not lead to grounding line retreat internal oscillations either specified or generated via a macayeal payne thermal mechanism promote migration this is a noise induced drift phenomenon stemming from the neutral equilibrium property of marine ice sheets this migration occurs at quite slow rates but these are sufficiently large to have possibly played a role in the dynamics of the west antarctic ice sheet after the glacial maximum numerical experiments suggest that it is generally true that while significant changes in thickness can be caused by spatially uniform changes spatial variability coupled with dynamical variability is needed to cause margin movement

lithospheric composition and thermal structure of the arabian shield in jordan abstract in this paper a unique set of samples from the uppermost crust down to the lithospheric mantle of jordan is analyzed for composition and petrophysical properties density thermal conductivity radiogenic heat production these data covering a vertical section of almost 65 km are used in conjunction with surface heat flow to generate a detailed and comprehensive lithospheric thermal model that reflects the conditions of the arabian shield as prior to the post oligocene onset of lithosphere thinning and voluminous basaltic volcanism the pre miocene model geotherms based on conductive surface heat flows of 55 and 60 mw m 2 a meet the range of lithosphere asthenosphere boundary depths of 110 160 km known from seismology b conform to results of thermomechanical models on the origin of the dead sea basin that started in miocene time and c are consistent with typical xenolith derived geotherms for terranes of similar age and lithospheric thickness moho temperatures at depths between 35 and 40 km of the as in pre miocene times were most likely in the order of 530 650 c with mantle heat flows averaging between 24 and 29 mw m 2 results contradict former views of the late proterozoic early cambrian stabilized as being an anomalously cold terrane a cold thermal structure inferred from previously measured low surface heat flows generally 45 mw m 2 is inconsistent with the thickness composition and petrophysical properties of the stable lithosphere of the shield

melting at the base of the greenland ice sheet explained by iceland hotspot history basal melting is widespread in the north central greenland ice sheet geophysical data and numerical modelling suggest a geothermal anomaly in this region resulting from the earlier passage of greenland over the iceland mantle plume

application of a polythermal three dimensional ice sheet model to the greenland ice sheet response to steady state and transient climate scenarios abstract steady state and transient climate change computations are performed with the author s three dimensional polythermal ice sheet model simulation code for polythermal ice sheets for the greenland ice sheet the distinctive feature of this model is the detailed consideration of the basal temperate ice layer in which the water content and its impact on the ice viscosity are computed its transition surface to the cold ice region is accounted for by continuum mechanical jump conditions on this interface the simulations presented include steady states subject to a range of physical parameters and two different climates present and glacial conditions as well as three types of transient scenarios namely i sinusoidal milankovic period forcing ii paleoclimatic forcing from the greenland ice core project core reconstruction and iii future greenhouse warming forcing

recovery of subglacial water extent from greenland radar survey data subglacial water threatens the stability of a deep ice sheet the greenland ice sheet has been the subject of extensive radar surveys under nasa s parca program and others yet the extent of basal water has not previously been explored in detail this paper provides a method for recovering basal radar reflection characteristics that yield robust discrimination of subglacial water using parca data by narrowing the spread of measured echo intensities sources of validation are provided and they support the procedure and its results the method can be extended to the entire greenland radar dataset it is not dependent on details of radar characteristics but relies on sufficient penetration performance high fidelity data acquisition and appropriate treatment of signals and interface statistics

mineralogical constraints on the thermal history of martian regolith breccia northwest africa 8114 abstract martian meteorite northwest africa nwa 8114 a paired stone to nwa 7034 provides an opportunity to examine the thermal history of a martian regolith and study near surface processes and ancient environmental conditions near an impact crater on mars our study reports petrographic and alteration textures and focuses on pyroxene and iron oxide grains some of the pyroxene clasts show exsolution lamellae indicating a high temperature magmatic origin and slow cooling however transmission electron microscopy reveals that other predominantly pyroxene clasts are porous and have partially re crystallised to form magnetite and a k bearing feldspathic glassy material together with relict pyroxene this breakdown event was associated with oxidation with up to 25 fe3 fe in the relict pyroxene measured using fe k xanes by comparison with previous studies this breakdown and oxidation of pyroxene is most likely to be a result of impact shock heating being held at a temperature above 700 c for at least 7 days in an oxidising regolith environment we report an approximate 40ar 39ar maximum age of 1 13 1 25 ga for an individual separated augite clast the disturbed nature of the spectra precludes precise age determination in section this clast is porous and contains iron oxide grains this shows that it has undergone the high temperature partial breakdown seen in other relict pyroxene clasts and has up to 25 fe3 fe we infer that the age corresponds to the impact shock heating event that led to the high temperature breakdown of many of the pyroxenes after consolidation of the impact ejecta blanket high temperatures above 700 c may have been maintained for long enough to remobilise and congruently partially melt some of the alkali feldspar clasts to produce the feldspar veins and aureoles that crosscut and in some cases surround the oxidised pyroxene however the veins could alternatively be the result of a hydrothermal event in the impact regolith a simple fourier cooling model suggests that a regolith of at least five metres depth would be sufficient to maintain temperatures associated with the pyroxene breakdown for over seven days low temperature hydrous alteration took place forming goethite identified via xrd xanes and ftir comparing with previous studies the goethite is likely to be terrestrial alteration pseudomorphing martian pyrite

the lithosphere asthenosphere boundary in the north west atlantic region abstract a detailed knowledge of the thickness of the lithosphere in the north atlantic is an important parameter for understanding plate tectonics in that region we achieve this goal with as yet unprecedented detail using the seismic technique of s receiver functions clear positive signals from the crust mantle boundary and negative signals from a mantle discontinuity beneath greenland iceland and jan mayen are observed according to seismological practice we call the negative phase the lithosphere asthenosphere boundary lab the seismic lithosphere under most of the iceland and large parts of central greenland is about 80 km thick this depth in iceland is in disagreement with estimates of the thickness of the elastic lithosphere 10 20 km found from postglacial rebound data in the region of flood basalts in eastern greenland which overlies the proposed iceland plume track the lithosphere is only 70 km thick about 10 km less than in iceland which is located directly above the proposed plume at the western greenland coast the lithosphere thickens to 100 120 km with no indication of the iceland plume track identified below jan mayen the lithospheric thickness varies between 40 and 60 km

evidence for extremely rapid magma ocean crystallization and crust formation on mars the formation of a primordial crust is a critical step in the evolution of terrestrial planets but the timing of this process is poorly understood the mineral zircon is a powerful tool for constraining crust formation because it can be accurately dated with the uranium to lead u pb isotopic decay system and is resistant to subsequent alteration moreover given the high concentration of hafnium in zircon the lutetium to hafnium 176lu 176hf isotopic decay system can be used to determine the nature and formation timescale of its source reservoir1 3 ancient igneous zircons with crystallization ages of around 4 430 million years myr have been reported in martian meteorites that are believed to represent regolith breccias from the southern highlands of mars4 5 these zircons are present in evolved lithologies interpreted to reflect re melted primary martian crust 4 thereby potentially providing insight into early crustal evolution on mars here we report concomitant high precision u pb ages and hf isotope compositions of ancient zircons from the nwa 7034 martian regolith breccia seven zircons with mostly concordant u pb ages define 207pb 206pb dates ranging from 4 476 3 0 9 myr ago to 4 429 7 1 0 myr ago including the oldest directly dated material from mars all zircons record unradiogenic initial hf isotope compositions inherited from an enriched andesitic like crust extracted from a primitive mantle no later than 4 547 myr ago thus a primordial crust existed on mars by this time and survived for around 100 myr before it was reworked possibly by impacts4 5 to produce magmas from which the zircons crystallized given that formation of a stable primordial crust is the end product of planetary differentiation our data require that the accretion core formation and magma ocean crystallization on mars were completed less than 20 myr after the formation of the solar system these timescales support models that suggest extremely rapid magma ocean crystallization leading to a gravitationally unstable stratified mantle which subsequently overturns resulting in decompression melting of rising cumulates and production of a primordial basaltic to andesitic crust6 7

eight glacial cycles from an antarctic ice core the antarctic vostok ice core provided compelling evidence of the nature of climate and of climate feedbacks over the past 420 000 years marine records suggest that the amplitude of climate variability was smaller before that time but such records are often poorly resolved moreover it is not possible to infer the abundance of greenhouse gases in the atmosphere from marine records here we report the recovery of a deep ice core from dome c antarctica that provides a climate record for the past 740 000 years for the four most recent glacial cycles the data agree well with the record from vostok the earlier period between 740 000 and 430 000 years ago was characterized by less pronounced warmth in interglacial periods in antarctica but a higher proportion of each cycle was spent in the warm mode the transition from glacial to interglacial conditions about 430 000 years ago termination v resembles the transition into the present interglacial period in terms of the magnitude of change in temperatures and greenhouse gases but there are significant differences in the patterns of change the interglacial stage following termination v was exceptionally long 28 000 years compared to for example the 12 000 years recorded so far in the present interglacial period given the similarities between this earlier warm period and today our results may imply that without human intervention a climate similar to the present one would extend well into the future

greenland ice sheet higher nonlinearity of ice flow significantly reduces estimated basal motion in times of warming in polar regions the prediction of ice sheet discharge is of utmost importance to society because of its impact on sea level rise in simulations the flow rate of ice is usually implemented as proportional to the differential stress to the power of the exponent n 3 this exponent influences the softness of the modeled ice as higher values would produce faster flow under equal stress we show that the stress exponent which best fits the observed state of the greenland ice sheet equals n 4 our results which are not dependent on a possible basal sliding component of flow indicate that most of the interior northern ice sheet is currently frozen to bedrock except for the large ice streams and marginal ice ice in the polar ice sheets flows towards the oceans under its own weight knowing how fast the ice flows is of crucial importance to predict future sea level rise the flow has two components 1 internal shearing flow of ice and 2 basal motion which is sliding along the base of ice sheets especially when the ice melts at this base to determine the first component we need to know how soft the ice is by considering the flow velocities at the surface of the northern greenland ice sheet and calculating the stresses that cause the flow we determined that the ice is effectively softer than is usually assumed previous studies indicated that the base of the ice is thawed in large parts up to about 50 of the greenland ice sheet our study shows that that is probably overestimated because these studies assumed ice to be harder than it actually is our new assessment reduces the area with basal motion and thus melting to about 6 13 in the greenland study area

petrology on mars petrologic investigations of martian rocks have been accomplished by mineralogical geochemical and textural analyses from mars rovers with geologic context provided by orbiters and by laboratory analyses of martian meteorites igneous rocks are primarily lavas and volcaniclastic rocks of basaltic composition and ultramafic cumulates alkaline rocks are common in ancient terranes and tholeiitic rocks occur in younger terranes suggesting global magmatic evolution relatively uncommon feldspathic rocks represent the ultimate fractionation products and granitic rocks are unknown sedimentary rocks are of both clastic mudstone sandstone conglomerate all containing significant igneous detritus and chemical evaporitic sulfate and less common carbonate origin high silica sediments formed by hydrothermal activity sediments on mars formed from different protoliths and were weathered under different environmental conditions from terrestrial sediments metamorphic rocks have only been inferred from orbital remote sensing measurements metabasalt and serpentinite have mineral assemblages consistent with those predicted from low pressure phase equilibria and likely formed in geothermal systems shock effects are common in martian meteorites and impact breccias are probably widespread in the planet s crustal rocks the martian rock cycle during early periods was similar in many respects to that of earth however without plate tectonics mars did not experience the thermal metamorphism and flux melting associated with subduction nor deposition in subsided basins and rapid erosion resulting from tectonic uplift the rock cycle during more recent time has been truncated by desiccation of the planet s surface and a lower geothermal gradient in its interior the petrology of mars is intriguingly different from earth but the tried and true methods of petrography and geochemistry are clearly translatable to another world

constraining ancient magmatic evolution on mars using crystal chemistry of detrital igneous minerals in the sedimentary bradbury group gale crater mars

a model of variations in the north atlantic oscillation 1 the major source of interannual variability in the atmospheric circulation over the north atlantic ocean is the north atlantic oscillation nao which is associated with the strength of the winter surface westerly winds over the ocean during the last century the nao has shown pronounced multi annual and multi decadal variations fluctuations that have been accompanied by changes in the marine and terrestrial ecosystems of the north atlantic region this paper presents a simple model which reproduces the major trends in the nao index from 1866 to 2004 the model is based on climatic resonators that are forced by the global mean temperature and the southern oscillation of the pacific ocean so the ocean component of the model shows similar variations to time series from the north atlantic

the configuration sensitivity and rapid retreat of the late weichselian icelandic ice sheet abstract the fragmentary glacial geological record across the icelandic continental shelf has hampered reconstruction of the volume extent and chronology of the late weichselian ice sheet particularly in key offshore zones marine geophysical data collected over the last two decades reveal that the ice sheet likely attained a continental shelf break position in all sectors during the last glacial maximum though its precise timing and configuration remains largely unknown within this context we review the available empirical evidence and use a well constrained three dimensional thermomechanical model to investigate the drivers of an extensive late weichselian icelandic ice sheet its sensitivity to environmental forcing and phases of deglaciation our reconstruction attains the continental shelf break across all sectors with a total ice volume of 5 96 105 km3 with high precipitation rates being critical to forcing extensive ice sheet flow offshore due to its location astride an active mantle plume a relatively fast and dynamic ice sheet with a low aspect ratio is maintained our results reveal that once initial ice sheet retreat was triggered through climate warming at 21 8 ka bp marine deglaciation was rapid and accomplished in all sectors within c 5 ka at a mean rate of 71 gt of mass loss per year this rate of ice wastage is comparable to contemporary rates observed for the west antarctic ice sheet the ice sheet subsequently stabilised on shallow pinning points across the near shelf for two millennia but abrupt atmospheric warming during the bolling interstadial forced a second dramatic collapse of the ice sheet onshore with a net wastage of 221 gt a 1 over 750 years analogous to contemporary greenland rates of mass loss geothermal conditions impart a significant control on the ice sheet s transient response particularly during phases of rapid retreat insights from this study suggests that large sectors of contemporary ice sheets overlying geothermally active regions such as siple coast antarctica and ne greenland have the potential to experience rapid phases of mass loss and deglaciation once initial retreat is initiated

north atlantic warming and the retreat of greenland s outlet glaciers mass loss from the greenland ice sheet quadrupled over the past two decades contributing a quarter of the observed global sea level rise increased submarine melting is thought to have triggered the retreat of greenland s outlet glaciers which is partly responsible for the ice loss however the chain of events and physical processes remain elusive recent evidence suggests that an anomalous inflow of subtropical waters driven by atmospheric changes multidecadal natural ocean variability and a long term increase in the north atlantic s upper ocean heat content since the 1950s all contributed to a warming of the subpolar north atlantic this led in conjunction with increased runoff to enhanced submarine glacier melting future climate projections raise the potential for continued increases in warming and ice mass loss with implications for sea level and climate

oxygen isotope and palaeotemperature records from six greenland ice core stations camp century dye 3 grip gisp2 renland and northgrip oxygen isotope variations spanning the last glacial cycle and the holocene derived from ice core records for six sites in greenland camp century dye 3 grip gisp2 renland and northgrip show strong similarities this suggests that the dominant influence on oxygen isotope variations reflected in the ice sheet records was regional climatic change differences in detail between the records probably reflect the effects of basal deformation in the ice as well as geographical gradients in atmospheric isotope ratios palaeotemperature estimates have been obtained from the records using three approaches i inferences based on the measured relationship between mean annual 18o of snow and of mean annual surface temperature over greenland ii modelled inversion of the borehole temperature profile constrained either by the dated isotopic profile or iii by using monte carlo simulation techniques the third of these approaches was adopted to reconstruct holocene temperature variations for the dye 3 and grip temperature profiles which yields remarkably compatible results a new record of holocene isotope variations obtained from the northgrip ice core matches the grip short term isotope record and also shows similar long term trends to the dye 3 and grip inverted temperature data the northgrip isotope record reflects i a generally stronger isotopic signal than is found in the grip record ii several short lived temperature fluctuations during the first 1500 yr of the holocene iii a marked cold event at ca 8 2 ka the 8 2 ka event iv optimum temperatures for the holocene between ca 8 6 and 4 3 ka a signal that is 0 6 stronger than for the grip profile v a clear signal for the little ice age and vi a clear signal of climate warming during the last century these data suggest that the northgrip stable isotope record responded in a sensitive manner to temperature fluctuations during the holocene copyright 2001 john wiley sons ltd

predicting the geothermal heat flux in greenland a machine learning approach geothermal heat flux ghf is a crucial boundary condition for making accurate predictions of ice sheet mass loss yet it is poorly known in greenland due to inaccessibility of the bedrock here we use a machine learning algorithm on a large collection of relevant geologic features and global ghf measurements and produce a ghf map in greenland which we argue is within 15 accuracy the main features of our predicted ghf map include a large region with high ghf in central north greenland surrounding the northgrip ice core site and hotspots in the jakobshavn isbrae catchment upstream of petermann gletscher and near the terminus of nioghalvfjerdsfjorden glacier our model also captures the trajectory of greenland movement over the icelandic plume by predicting a stripe of elevated ghf in central east greenland finally we show that our model can produce substantially more accurate predictions if additional measurements of ghf in greenland are provided

deformation warming and softening of greenland s ice by refreezing meltwater when basal meltwater refreezes the resulting warm ice can influence the flow dynamics of the ice sheet above an analysis of airborne gravity and radar data identifies extensive basal ice units across the northern greenland ice sheet that coincide with areas of deformed ice and fast ice flow

mars in the aftermath of a colossal impact abstract the abundance of highly siderophile elements hses inferred for mars mantle from martian meteorites implies a late veneer lv mass addition of 0 8 wt with broadly chondritic composition late accretion to mars by a differentiated ceres sized 1000 km diameter object can account for part of the requisite lv mass and geochronological constraints suggests that this must have occurred no later than ca 4480 ma here we analyze the outcome of the hypothetical lv giant impact to mars with smoothed particle hydrodynamics simulations together with analytical theory results show that in general about 50 of the impactor s metallic core shatters into 10 m fragments that subsequently fragment into sub mm metallic hail at re accretion this returns a promising delivery of hses into martian mantle compared to either a head on and hit and run collision in both cases 10 myr a dense pre noachian co2 atmosphere should lower the escape efficiency of hydrogen by ir emission a more detailed hydrodynamic atmospheric model of this early hydrogen atmosphere is warranted to examine its effect on pre noachian mars

the early differentiation of mars inferred from hf w chronometry abstract mars probably accreted within the first 10 million years of solar system formation and likely underwent magma ocean crystallization and crust formation soon thereafter to assess the nature and timescales of these large scale mantle differentiation processes we applied the short lived 182 hf 182 w and 146 sm 142 nd chronometers to a comprehensive suite of martian meteorites including several shergottites augite basalt nwa 8159 orthopyroxenite alh 84001 and polymict breccia nwa 7034 compared to previous studies the 182 w data are significantly more precise and have been obtained for a more diverse suite of martian meteorites ranging from samples from highly depleted to highly enriched mantle and crustal sources our results show that martian meteorites exhibit widespread 182 w 184 w variations that are broadly correlated with 142 nd 144 nd implying that silicate differentiation and not core formation is the main cause of the observed 182 w 184 w differences the combined 182 w 142 nd systematics are best explained by magma ocean crystallization on mars within 20 25 million years after solar system formation followed by crust formation 15 million years later these ages are indistinguishable from the i pu xe age for the formation of mars atmosphere indicating that the major differentiation of mars into mantle crust and atmosphere occurred between 20 and 40 million years after solar system formation and hence earlier than previously inferred based on sm nd chronometry alone

subglacial thermal balance permits ongoing grounding line retreat along the siple coast of west antarctica changes in the discharge of west antarctic ice streams are of potential concern with respect to global sea level the six relatively thin fast flowing ross ice streams are of interest as low slope end members among antarctic ice streams extensive research has demonstrated that these rivers of ice have a history of relatively high frequency asynchronous discharge variations with evolving lateral boundaries amidst this variability a 1300 km grounding line retreat has occurred since the last glacial maximum numerical studies of ice stream d parizek and others 2002 indicate that a proposed thermal regulation mechanism clarke and marshall 1998 hulbe and macayeal 1999 tulaczyk and others 2000a b which could buffer the west antarctic ice sheet against complete collapse may be over ridden by latent heat transport within melt water from beneath inland ice extending these studies to ice stream a whillans ice stream and ice stream c suggests that further grounding line retreat contributing to sea level rise is possible thermodynamically however the efficiency of basal water distribution may be a constraint on the system because local thermal deficits promote basal freeze on especially on topographic highs observed short term variability is likely to persist

a thick crustal block revealed by reconstructions of early mars highlands the global scale crustal structure of mars is shaped by impact basins volcanic provinces and a hemispheric dichotomy with a thin crust beneath the northern lowlands and a thick crust beneath the southern highlands the southern highlands are commonly treated as a coherent terrain of ancient crust with a common origin and shared geologic history plausibly originating from a giant impact s or a hemispheric scale mantle upwelling previous studies have quantified the contribution of volcanism to this crustal structure however the influence of large impacts remains unclear here we present reconstructions of the past crustal thickness of mars about 4 2 gyr ago where the four largest impact basins hellas argyre isidis and utopia are removed assuming mass conservation as well as the main volcanic provinces of tharsis and elysium our reconstruction shows more subdued crustal thickness variations than at present although the crustal dichotomy persists however our reconstruction reveals a region of discontinuous patches of thick crust in the southern highlands associated with magnetic and geochemical anomalies this region corresponding to terra cimmeria sirenum is interpreted as a discrete crustal block our findings suggest that the southern highlands are composed of several crustal blocks with different geological histories such a complex architecture of the southern highlands is not explained by existing scenarios for crustal formation and evolution a discrete block of thick ancient crust revealed by a crustal reconstruction suggests a complex geologic history for the southern highlands of mars

in situ evidence for continental crust on early mars understanding of the geologic evolution of mars has been greatly improved by recent orbital 1 3 in situ 4 5 and meteorite 6 8 data but insights into the earliest period of martian magmatism 4 1 to 3 7 billion years ago remain scarce 9 the landing site of nasa s curiosity rover gale crater which formed 3 61 billion years ago 10 within older terrain 11 provides a window into this earliest igneous history along its traverse curiosity has discovered light toned rocks that contrast with basaltic samples found in younger regions 12 here we present geochemical data and images of 22 specimens analysed by curiosity that demonstrate that these light toned materials are feldspar rich magmatic rocks the rocks belong to two distinct geochemical types alkaline compositions containing up to 67 wt sio2 and 14 wt total alkalis na2o k2o with fine grained to porphyritic textures on the one hand and coarser grained textures consistent with quartz diorite and granodiorite on the other hand our analysis reveals unexpected magmatic diversity and the widespread presence of silica and feldspar rich materials in the vicinity of the landing site at gale crater combined with the identification of feldspar rich rocks elsewhere 9 13 14 and the low average density of the crust in the martian southern hemisphere 15 we conclude that silica rich magmatic rocks may constitute a significant fraction of ancient martian crust and may be analogous to the earliest continental crust on earth

accretion timescale and impact history of mars deduced from the isotopic systematics of martian meteorites abstract high precision sm nd isotopic analyses have been completed on a suite of 11 martian basaltic meteorites in order to better constrain the age of silicate differentiation on mars associated with the formation of their mantle sources these data are used to evaluate the merits and disadvantages of various mathematical approaches that have been employed in previous work on this topic ages determined from the sm nd isotopic systematics of individual samples are strongly dependent on the assumed nd isotopic composition of the bulk planet this assumption is problematic given differences observed between the nd isotopic composition of earth and chondritic meteorites and the fact that these materials are both commonly used to represent bulk planetary nd isotopic compositions ages determined from the slope of 146sm 142nd whole rock isochrons are not dependent on the assumed 142nd 144nd ratio of the planet but require the sample suite to be derived from complementary contemporaneously formed reservoirs in this work we present a mathematical expression that defines the age of formation of the source regions of such a suite of samples that is based solely on the slope of a 143nd 142nd whole rock isochron and is also independent of any a priori assumptions regarding the bulk isotopic composition of the planet this expression is also applicable to mineral isochrons and has been used to successfully calculate 143nd 142nd model crystallization ages of early refractory solids as well as lunar samples this permits ages to be obtained using only nd isotopic measurements without the need for 147sm 144nd isotope dilution determinations when used in conjunction with high precision nd isotopic measurements completed on martian meteorites this expression yields an age of formation of the martian basaltic meteorite source regions of 4504 6 ma because the sm nd model ages for the formation of martian source regions are commonly interpreted to record the age at which large scale mantle reservoirs formed during planetary differentiation associated with magma ocean solidification the age determined here implies that magma ocean solidification occurred several tens of millions of years after the beginning of the solar system recent thermal models however suggest that mars sized bodies cool rapidly in less than 5 ma after accretion ceases even in the presence of a thick atmosphere assuming these models are correct an extended period of accretion is necessary to provide a mechanism to keep portions of the martian mantle partially molten until 4504 ma late accretional heating of mars could either be associated with protracted accretion occurring at a quasi steady state or alternatively be associated with a late giant impact if this scenario is correct then accretion of mars sized bodies takes up to 60 ma and is likely to be contemporaneous with the core formation and possibly the onset of silicate differentiation this further challenges the concept that isotopic equilibrium is attained during primordial evolution of planets and may help to account for geochemical evidence implying addition of material into planetary interiors after core formation was completed

weakened lithosphere beneath greenland inferred from effective elastic thickness a hot spot effect the effective elastic thickness t e of the lithosphere provides geophysical information about long term flexural strength and can be used to constrain thermorheological properties of the lithosph

a first detailed look at the greenland lithosphere and upper mantle using rayleigh wave tomography we present the results of a surface wave study carried out across greenland as part of the glatis greenland lithosphere analysed teleseismically on the ice sheet project rayleigh wave phase velocity dispersion curves were estimated for 45 two station paths across greenland using data from large teleseismic earthquakes the individual dispersion curves show characteristics broadly consistent with those of continental shields worldwide but with significant differences across the greenland landmass reliable phase velocity measurements were made over a period range of 25 160 s providing constraint on mantle structure to a depth of 300 km an isotropic tomographic inversion was used to combine the phase velocity information from the dispersion curves in order to calculate phase velocity maps for greenland at several different periods the greatest lateral variation in phase velocity is observed at intermediate periods 50 80 s where a high velocity anomaly is resolved beneath central southwestern greenland and a low velocity anomaly is resolved beneath southeastern greenland the results of the phase velocity inversion were used to construct localized dispersion curves for node points along two parallel north south profiles in southern greenland these curves were inverted to obtain models of shear wave velocity structure as a function of depth again with the assumption of isotropic structure a similar inversion was carried out for two two station dispersion curves in northern greenland where the resolution of the phase velocity maps is relatively low the models show a high velocity lid structure overlying a zone of lower velocity beneath which the velocity gradually increases with depth the lid structure is interpreted as the continental lithosphere within the lithosphere the shear wave velocity is 4 12 per cent above global reference models with the highest velocities beneath central southwestern greenland however the assumption of isotropic structure means that the maximum velocity perturbation may be overestimated by a few per cent the lithospheric thickness varies from 100 km close to the southeast coast of greenland to 180 km beneath central southern greenland

the internal structure and geodynamics of mars inferred from a 4 2 gyr zircon record combining u pb ages with lu hf data in zircon provides insights into the magmatic history of rocky planets the northwest africa nwa 7034 7533 meteorites are samples of the southern highlands of mars containing zircon with ages as old as 4476 3 0 9 ma interpreted to reflect reworking of the primordial martian crust by impacts we extracted a statistically significant zircon population n 57 from nwa 7533 that defines a temporal record spanning 4 2 gyr ancient zircons record ages from 4485 5 2 2 ma to 4331 0 1 4 ma defining a bimodal distribution with groupings at 4474 10 ma and 4442 17 ma we interpret these to represent intense bombardment episodes at the planet s surface possibly triggered by the early migration of gas giant planets the unradiogenic initial hf isotope composition of these zircons establishes that mars s igneous activity prior to 4 3 ga was limited to impact related reworking of a chemically enriched primordial crust a group of younger detrital zircons record ages from 1548 0 8 8 ma to 299 5 0 6 ma the only plausible sources for these grains are the temporally associated elysium and tharsis volcanic provinces that are the expressions of deep seated mantle plumes the chondritic like hf isotope compositions of these zircons require the existence of a primitive and convecting mantle reservoir indicating that mars has been in a stagnant lid tectonic regime for most of its history our results imply that zircon is ubiquitous on the martian surface providing a faithful record of the planet s magmatic history

the treatment of meltwater retention in mass balance parameterisations of the greenland ice sheet retention of melt water runoff by percolation and or refreezing in the snowpack cannot be neglected when studying the surface mass balance of the greenlandice sheet in this paper we make a detailed comparison of several treatments proposed in the literature to account for this process in large scale mass balanceparameterizations the melt on the greenland ice sheet is calculated with a revised degree day model using updated datasets of surface elevation and precipitationrate on a 5 km grid crucial model parameters are recalibrated by comparing mass balance characteristics with available observations on a regional basis wediscuss the role of meltwater retention in the light of the overall mass balance of the greenland ice sheet and its sensitivity to climatic change and displaypatterns of effective retention fractions for the various methods as a main conclusion it appears that overall results are quite similar for the various models butthat meltwater retention has a large spatial variation not described by the simple treatments using the most comprehensive retention model the sensitivity of therunoff is found to be 0 35 mm deg c of sea level change per year we also present a new map of the different zones facies that characterize the accumulationarea of the greenland ice sheet that is useful for interpreting field data and calibrating satellite observations

high geothermal heat flow basal melt and the origin of rapid ice flow in central greenland age depth relations from internal layering reveal a large region of rapid basal melting in greenland melt is localized at the onset of rapid ice flow in the large ice stream that drains north off the summit dome and other areas in the northeast quadrant of the ice sheet locally high melt rates indicate geothermal fluxes 15 to 30 times continental background the southern limit of melt coincides with magnetic anomalies and topography that suggest a volcanic origin

radar evidence of ponded subglacial water in greenland the thermal state at the bed of a large ice sheet is a critical boundary condition governing its future evolution radar surveys provide an opportunity for direct but remote observation of the ice sheet bed and therefore offer a means of constraining numerical ice sheet models at the ice bed interface here we have processed results of radar surveys of the greenland ice sheet undertaken by the program for arctic regional climate assessment parca between 1999 and 2003 to explore this opportunity we consider the robustness of the measurements in the context of uncertain dielectric losses in the ice sheet concluding that the observed radar signal characteristics reflect the character of the bed itself rather than that of uncertain englacial absorption however the identification of thaw is restricted to areas where subglacial water has sufficient depth to influence the radar reflection we derive a map of inferred areas of subglacial thaw and compare our results with other studies predicting regions with temperate bed we show that in many areas the radar inferences of ponded water lie within areas predicted to be thawed by modelling and radiostratigraphy there is clear disagreement in certain areas suggesting the presence of high geothermal flux anomalies

a synthesis of the basal thermal state of the greenland ice sheet the basal thermal state of an ice sheet frozen or thawed is an important control upon its evolution dynamics and response to external forcings however this state can only be observed directly within sparse boreholes or inferred conclusively from the presence of subglacial lakes here we synthesize spatially extensive inferences of the basal thermal state of the greenland ice sheet to better constrain this state existing inferences include outputs from the eight thermomechanical ice flow models included in the searise effort new remote sensing inferences of the basal thermal state are derived from holocene radiostratigraphy modern surface velocity and modis imagery both thermomechanical modeling and remote inferences generally agree that the northeast greenland ice stream and large portions of the southwestern ice drainage systems are thawed at the bed whereas the bed beneath the central ice divides particularly their west facing slopes is frozen elsewhere there is poor agreement regarding the basal thermal state both models and remote inferences rarely represent the borehole observed basal thermal state accurately near northgrip and dye 3 this synthesis identifies a large portion of the greenland ice sheet about one third by area where additional observations would most improve knowledge of its overall basal thermal state

insights into the chemical diversity of the martian mantle from the pb isotope systematics of shergottite northwest africa 8159 abstract shergottite northwest africa nwa 8159 is a basaltic rock derived from a mantle source with chemical characteristics that are unique in the martian meteorite suite to further investigate this source reservoir the pb isotope compositions of plagioclase maskelynite pyroxene phosphates and shock melt glass in nwa 8159 have been measured in situ by secondary ion mass spectrometry sims due to the limited spread in pb isotope data these pb isotope compositions have been used to calculate an imprecise pb pb isochron age of 3 4 2 1 ga 2 which is broadly consistent with the crystallization age of 2 37 0 25 ga determined previously by 147sm 143nd the lack of radiogenic in growth within individual minerals since 2 4 ga means that this sample is depleted in u which is in agreement with nwa 8159 s positive initial e143nd an initial pb composition was calculated using an x y weighted average of the least radiogenic pb isotope population measured in the sample this initial pb composition is not consistent with the model for pb growth in the shergottite mantle at 2 4 ga this composition is however consistent with the model for the nakhla chassigny mantle using the latter model a source 238u 204pb of 2 6 0 6 has been calculated this value is in contrast with the other depleted shergottites 1 4 1 5 and falls significantly off the array of source e143nd vs defined by the rest of the martian meteorite suite and thus necessitates a differentiation history distinct from the other martian meteorites sequestering pb in sulphides during differentiation is the only mechanism to fractionate u from pb and create a low reservoir consequently the relatively high value of the source of nwa 8159 is in contrast with the positive initial e143nd and indicates that its mantle source region likely lacked significant sulphur this is consistent with the lack of sulphides in the sample itself and could have played a role in its complicated oxidation history

paleofluvial and subglacial channel networks beneath humboldt glacier greenland the identification of subglacial drainage systems can inform our understanding of past and present hydrological processes landscape evolution and ice dynamics here we present evidence from satellite imagery digital elevation models and radio echo sounding data for a series of channelized networks with contrasting paleofluvial and subglacial origins beneath humboldt glacier northern greenland a 250 km long dendritic paleofluvial channel network beneath the northern portion of humboldt is interpreted as a preglacial feature roughly linear channels beneath the southern portion of humboldt which display a similar distribution to tunnel valleys found on the beds of former ice sheets are likely to have been eroded by subglacial meltwater routed along the ice sheet bed we suggest that basal meltwater is actively being routed down both the paleofluvial and subglacially formed channel networks to the coast inheritance of the preglacial channel network may have influenced the present day location and dynamics of humboldt glacier and enhanced selective erosion at its down glacier end

geologic history of martian regolith breccia northwest africa 7034 evidence for hydrothermal activity and lithologic diversity in the martian crust the timing and mode of deposition for martian regolith breccia northwest africa nwa 7034 were determined by combining petrography shape analysis and thermochronology nwa 7034 is composed of igneous impact and brecciated clasts within a thermally annealed submicron matrix of pulverized crustal rocks and devitrified impact volcanic glass the brecciated clasts are likely lithified portions of martian regolith with some evidence of past hydrothermal activity represented lithologies are primarily ancient crustal materials with crystallization ages as old as 4 4 ga one ancient zircon was hosted by an alkali rich basalt clast confirming that alkalic volcanism occurred on mars very early nwa 7034 is composed of fragmented particles that do not exhibit evidence of having undergone bed load transport by wind or water the clast size distribution is similar to terrestrial pyroclastic deposits we infer that the clasts were deposited by atmospheric rainout subsequent to a pyroclastic eruption s and or impact event s although the ancient ages of igneous components favor mobilization by impact s despite ancient components the breccia has undergone a single pervasive thermal event at 500 800 c evident by groundmass texture and concordance of 1 5 ga dates for bulk rock k ar u pb in apatite and u pb in metamict zircons the 1 5 ga age is likely a thermal event that coincides with rainout breccia lithification we infer that the episodic process of regolith lithification dominated sedimentary processes during the amazonian epoch the absence of pre amazonian high temperature metamorphic events recorded in ancient zircons indicates source domains of static southern highland crust punctuated by episodic impact modification

evidence for a low bulk crustal density for mars from gravity and topography knowledge of the average density of the crust of a planet is important in determining its interior structure the combination of high resolution gravity and topography data has yielded a low density for the moon s crust yet for other terrestrial planets the resolution of the gravity field models has hampered reasonable estimates by using well chosen constraints derived from topography during gravity field model determination using satellite tracking data we show that we can robustly and independently determine the average bulk crustal density directly from the tracking data using the admittance between topography and imperfect gravity we find a low average bulk crustal density for mars 2582 209 kg m 3 this bulk crustal density is lower than that assumed until now densities for volcanic complexes are higher consistent with earlier estimates implying large lateral variations in crustal density in addition we find indications that the crustal density increases with depth

large arctic temperature change at the wisconsin holocene glacial transition analysis of borehole temperature and greenland ice sheet project ii ice core isotopic composition reveals that the warming from average glacial conditions to the holocene in central greenland was large approximately 15 c this is at least three times the coincident temperature change in the tropics and mid latitudes the coldest periods of the last glacial were probably 21 c colder than at present over the greenland ice sheet

what controls thickness of sediments and lithospheric deformation at a pull apart basin we present a simplified three dimensional thermomechanical model of a pull apart basin formed at an overstepping of an active continental transform fault the modeling shows that for a given strike slip displacement and friction on the faults the major parameter that controls basin length thickness of sediments and deformation pattern beneath the basin is the thickness of the brittle layer the unusually large length and sediment thickness of the dead sea basin the classical pull apart basin associated with the dead sea transform can be explained by 100 km of strike slip motion and a thick 20 22 km up to 27 km locally brittle part of the cold lithosphere beneath the basin the thinner sedimentary cover in the gulf of aqaba basin located at the southernmost part of the dead sea transform close to the red sea rift is probably due to a thinner brittle part 15 km of the warmer lithosphere the modeling also suggests no more than 3 km of moho uplift beneath narrow 10 15 km pull apart basins formed in cold lithosphere such as the dead sea basin we also infer that a pull apart basin may only form if a several kilometer thick ductile detachment zone exists between the brittle crust and upper mantle modeling shows that this would not be the case for the dead sea basin if the surface heat flow there were indeed as low as 40 mw m 2 as previously reported we consider this result as an indication that the surface heat flow at the dead sea might have been underestimated

timing and duration of volcanism in the north atlantic igneous province implications for geodynamics and links to the iceland hotspot abstract we combine new and published 40 ar 39 ar age determinations from incremental heating experiments on whole rocks and mineral separates to assess the timing duration and distribution of volcanic activity during construction of the north atlantic igneous province we use these ages together with volume estimates of erupted magmas and their cumulates to calculate melt production rates for the early tertiary flood basalts of east greenland and the faeroes islands the lavas lie at opposite ends of the greenland iceland faeroes ridge the postulated iceland hotspot track and record volcanic activity leading up to during and following continental breakup between greenland and europe dominantly basaltic but also alkalic and picritic magmas were erupted from beneath thick continental lithosphere simultaneously at 61 ma from baffin island to the western and eastern margins of greenland to the faeroe islands and the western british isles a roughly circular area 2000 km in diameter volcanic activity was increasingly intermittent by 57 56 ma but at 56 1 0 5 ma the average melt production rate increased by more than an order of magnitude over previous levels from 3000 km 3 km of rift my coinciding with continental rifting and shallow decompressional melting of the mantle that fed subaerial volcanic activity along most of the eastern margin of greenland and its complement on the northwestern european margin offshore abnormally thick oceanic crust was produced along the line of plate separation the upper part of this crust comprises seismically imaged seaward dipping subaerially erupted lavas by 50 ma eruption rates had diminished drastically and volcanic activity had narrowed to a much restricted portion of the east greenland margin at the western end of the greenland iceland ridge that subsequently connected in an age progressive trend to present hotspot activity in southeastern iceland the startup mantle plume head and tail model with moderate excess temperature t 100 c and active upwelling best explains the time space and compositional aspects of volcanism in this province

a new present day temperature parameterization for greenland near surface air temperature 2 m over the greenland ice sheet gris is parameterized using data from automatic weather stations located on land and on the ice sheet the parameterization is expressed in terms of mean annual temperatures and mean july temperatures both depending linearly on altitude latitude and longitude the temperature parameterization is compared to a previous study and is shown to be in better agreement with observations the temperature parameterization is tested in a positive degree day model to simulate the present 1996 2006 mean melt area extent of the gris the model accounts for firn warming rainfall and refreezing of meltwater with different degree day factors for ice and snow under warm and cold climate conditions the simulated melt area extent is found to have reasonable agreement with satellite derived observations

the fennoscandian ice sheet during the late weichselian geothermal evidence geothermal estimates of the ground surface temperatures for the last glacial cycle in northern europe have been analysed during the middle and late weichselian 55 12 kyr bp a substantial part of this area was covered by the fennoscandian ice sheet the analysis of geothermal data allows reconstructing the limits of ice sheet extent and its basal thermal state in the late weichselian ground surface temperatures outside the ice sheet were extremely low from 7 to 17 c within the ice sheet there were both thawed and frozen zones the revealed temperature pattern is generally consistent with the modern one for ground surface temperatures in greenland which makes it possible to consider these ice sheets as analogues the anomalous climatically induced surface heat flux and orbital insolation of the earth varied consistently outside the glaciation and independently within the limits of the ice sheet

highly temporally resolved response to seasonal surface melt of the zachariae and 79n outlet glaciers in northeast greenland the seasonal response to surface melting of the northeast greenland ice stream outlets zachariae and 79n is investigated using new highly temporally resolved surface velocity maps for 2016 combined with numerical modelling the seasonal speed up at 79n of 0 15km yr is suggested to be driven by a decrease in effective basal pressure induced by surface melting whereas for zachariae its 0 11km yr seasonal speed up correlates equally well with the break up of its large ice melange we investigate the influence 76km long floating tongue at 79n finding it provides little resistance and that most of it could be lost without impacting the dynamics of the area furthermore we show that reducing the slipperiness along the tongue wall interfaces produces a velocity change spatially inconsistent with the observed seasonal speed up finally we find that subglacial sticky spots such as bedrock bumps play a negligible role in the large scale response to a seasonally enhanced basal slipperiness of the region

heat flux variations beneath central greenland s ice due to anomalously thin lithosphere the thermal state of the earth s surface is usually influenced more by climate than by heating from the earth s interior numerical models show that in the oldest and thickest part of the greenland ice sheet geothermal heat flux through an anomalously thin lithosphere leads to strong regional variations in basal melting

gravity inferred crustal thickness of greenland abstract greenland s crust contains clues for understanding the dynamics of the earth s second largest ice sheet as well as the iceland hotspot over the past 100 ma however our knowledge of the sub ice crust is very limited due to the paucity of outcrops and seismic observations particularly in central greenland in order to augment these sparse constraints we modeled terrain and airborne free air gravity data which provide estimates for moho and related crustal thickness these estimates were obtained from the inversion of isostatically corrected terrain effects where the computed gravity effects of the terrain s rock water and ice components were subtracted from the terrain correlated free air anomalies the terrain correlated free air components were estimated from the correlation spectrum between the free air anomalies and the terrain s gravity effects which are assumed to reflect isostatic disturbances of the crustal terrain the results indicated an average crustal thickness of about 37 km for greenland with greater values by 5 10 km in the southern greenland archean blocks and caledonian foldbelt and comparably lower values in northern greenland the thinner crustal thickness estimates in the north are strongly correlated with the faster basal melt regions of the ice sheet and glaciers the crustal thickness map provides constraints for an improved interpretation of the iceland hotspot track in northern greenland at about 100 70 ma

three dimensional numerical models of the evolution of pull apart basins abstract pull apart basins are depressions that form as the result of crustal extension along strike slip systems where the sense of fault stepping or bending coincides with that of fault slip they are common features of strike slip systems we perform a number of numerical thermomechanical experiments to explore how the rheology of the lithosphere influences basin evolution and lithospheric structure beneath the basin our modeling shows that basin subsidence results from the competition of extension of the brittle part of the lithosphere which leads to its subsidence and of the compensating flow of the deeper ductile part of the lithosphere which pushes the extended brittle block upwards the result of this competition is the subsidence rate strain partitioning beneath the basin and crustal structures is controlled by i the thickness of the brittle layer and basin width ii the magnitude of strike slip displacement iii the rate of frictional softening of the crust and iv the viscosity of the ductile part of the lithosphere the thickness of the brittle layer and the viscosity of the underlying ductile part of the lithosphere in turn depend on temperature composition and material softening we interpret the modeling results deducing simple analytical expressions based on the brittle brick stretching bbs approach which despite its simplicity describes the structure and evolution of pull apart basins reasonably well we also demonstrate that the structure and evolution of the dead sea basin located at a left step of the dead sea transform in the middle east is consistent with a bbs type of deformation with only a minor contribution from compensational flow in the ductile part of the lithosphere finally we show that the formation of a deep narrow pull apart basin in relatively cold lithosphere as in the dead sea basin requires very low friction at major faults lower than 0 1 0 2 if this condition is not satisfied strike slip deformation does not localise and deep basins do not form

mapping basal melt under the northern greenland ice sheet ice sheets in greenland and antarctica are based on continental rock and are coupled strongly to changes in global sea level their beds may be frozen or thawed but it has not been known how much or where basal melt occurs except in special cases where a borehole has reached the bed or where airborne radar has revealed subglacial lakes and reflective ice stream beds in antarctica we have used a previously published technique to detect subglacial melt water in the general case and have here applied it to radar sounder data collected over northern and central greenland we have found extensive subglacial water along between 13 and 20 of the flight paths this paper provides maps of the measured locations and probable extent of subglacial water

a monte carlo tuned model of the flow in the northgrip area the north greenland icecore project northgrip drill site was chosen in order to obtain a good eemian record at the present depth 3001 m the eemian interstadial has yet to be seen clearly the flow in this area is poorly understood and needs further investigation after a review of specific features of the bottom topography it is believed that the geology changes along the flowline in order to investigate whether this explains the observed age depth relationship at northgrip the inverse monte carlo method has been applied to a simple model the inversion reveals that the main reason no eemian is observed is a high basal melt rate 2 7 mma 1 the melting is a consequence of a higher geothermal heat flux than the expected 51 mwm 2 of the precambrian shield from our analyses it is concluded that the geothermal heat flux at northgrip is 98 mwm 2 the high basal melt rate also gives rise to sliding at the bed in addition to these results an accumulation model has been established specifically for northgrip these results are essential for further modelling of the northgrip flow and depth age relationship

assessment of current methods of positive degree day calculation using in situ observations from glaciated regions the continued use of the positive degree day pdd method to predict ice sheet melt is generally favoured over surface energy balance methods partly due to the computational efficiency of the algorithm and the requirement of only one input variable temperature in this paper we revisit some of the assumptions governing the application of the pdd method using hourly temperature data from the gc net network we test the assumption that monthly pdd total pddm can be represented by a gaussian distribution with fixed standard deviation of monthly temperature m the results presented here show that the common assumption of fixed m does not hold and that m may be represented more accurately as a quadratic function of average monthly temperature for greenland the mean absolute error in predicting pddm using our methodology is 3 9 c day representing a significant improvement on current methods 7 8 c day when m 4 5 c over a range of glaciated settings our method reproduces pddm on average to within 1 5 8 5 c day compared to 4 4 15 7 c day when m 4 5 the improvement arises because we capture the systematic reduction in temperature variance that is observed over melting snow and ice when surface temperatures cannot warm above o c