a low order model of the heinrich event cycle if heinrich events result from free oscillations in the size and basal melting conditions of the laurentide ice sheet a rigorous quantitative analysis of ice sheet physics should describe their dynamics to explore this possibility i exploit two characteristic timescales which arise from ice sheet physics to construct a relaxation oscillator model of the north atlantic s heinrich events the numerical implementation of this model confirms the notion that the periodicity of heinrich events approximately 7 000 years is determined by the gross properties of a steady glacial climate e g an annual average sea level temperature of 10 c and an adiabatic lapse rate

the mutual interaction between continental scale ice sheets and atmospheric stationary waves the great continental ice sheets of the pleistocene represented significant obstacles to the northern hemisphere midlatitude westerlies they must therefore have forced large changes in the atmospheric circulation and consequently also in the patterns of accumulation and melting over the ice sheets themselves a simplified threedimensional coupled ice sheet stationary wave model is developed in order to understand the ice sheet s response to the circulation changes that it induces consistent with ice age climate simulations the ice sheet topography induces an anticyclonic circulation over the ice sheet causing a slight warming over the western slopes and a stronger cooling over the remainder the modeled feedbacks significantly affect the ice sheet configuration with the most important influences being the patterns of summer temperature and the topographically induced precipitation field the time evolution of the ice sheet is also changed by the atmospheric feedbacks and the results suggest the possibility of multiple equilibrium solutions

depleted mantle source for the ulungur river a type granites from north xinjiang china geochemistry and nd sr isotopic evidence and implications for phanerozoic crustal growth abstract the ulungur river granites from north xinjiang china display two petrological trends 1 a peralkaline trend composed of alkali amphibole and alkali pyroxene bearing granites and dykes and 2 an aluminous trend represented by alkali granites monzogranites and aluminous granite dykes within the peralkaline granites rb sr whole rock isochron dating yielded an age of 300 ma for the emplacement of the peralkaline granites and 270 ma for the aluminous granites the ulungur river granites share all the geochemical features common to a type granites of the world the ulungur river granites are post orogenic and emplaced in a post collisional extensional environment the peralkaline and aluminous rocks have indistinguishable nd isotopic compositions they have unusually positive and relatively uniform nd t values ranging from 5 1 to 6 7 for the peralkaline rocks at 300 ma and from 5 5 to 5 9 for the aluminous rocks at 270 ma the initial 87sr 86sr ratios cannot be determined precisely by the whole rock isochron technique mainly because most samples are highly radiogenic and have high rb sr ratios in addition the large variation in sr isotopic compositions can partly be explained by crustal contamination during magma differentiation the nd isotopic data clearly indicate that both the peralkaline and aluminous rocks were derived from a long lived depleted mantle reservoir the formation of the ulungur river a type granites is hypothesized as follows 1 a depleted mantle was first metasomatised then subjected to partial melting leading to the generation of basic magmas now represented by the kalatongke mafic ultramafic complex in the ulungur river area 2 the basic magmas have undergone high degrees of fractional crystallisation with separation of biotite hornblende plagioclase potassium feldspar and other accessory phases such as apatite and ilmenite with the residual magmas forming the peralkaline granites during the magmatic differentiation the liquids were probably contaminated by crustal materials as evidenced by nb depletion and nd sr isotopic systematics on the other hand the formation of aluminous granites involved a replenishment of new mantle derived magmas into the differentiating magma chamber and followed by fractional crystallisation with retention of hydrous phases hornblende and biotite the petrological evolution of the ulungur river granites is thus from peralkaline to calc alkaline and not from calc alkaline to peralkaline as more often observed elsewhere the ulungur river granites in common with the voluminous granitoids occurring in the central asian orogenic belt represent a significant juvenile addition of the continental crust in the phanerozoic

modeling precipitation over ice sheets an assessment using greenland the interaction between ice sheets and the rest of the climate system at long time scales is not well understood and studies of the ice ages typically employ simplified parameterizations of the climate forcing on an ice sheet it is important therefore to understand how an ice sheet responds to climate forcing and whether the reduced approaches used in modeling studies are capable of providing robust and realistic answers this work focuses on the accumulation distribution and in particular considers what features of the accumulation pattern are necessary to model the steady state response of an ice sheet we examine the response of a model of the greenland ice sheet to a variety of accumulation distributions both observational datasets and simplified parameterizations the predicted shape of the ice sheet is found to be quite insensitive to changes in the accumulation the model only differs significantly from the observed ice sheet for a spatially uniform accumulation rate and the most important factor for the successful simulation of the ice sheet s shape is that the accumulation decreases with height according to the ability of the atmosphere to hold moisture however the internal ice dynamics strongly reflects the influence of the atmospheric circulation on the accumulation distribution

re evaluation of paleo accumulation parameterization over northern hemisphere ice sheets during the ice age examined with a high resolution agcm and a 3 d ice sheet model

the thermomechanical response of the greenland ice sheet to various climate scenarios using a three dimensional numerical model for land based ice sheets in the shallow ice approximation simulations are performed to determine the velocity and temperature distributions within the greenland ice sheet through time for various climate scenarios the ice is treated as a rheologically nonlinear heat conducting viscous fluid and the substrate is a heat conducting rigid solid this system is fed from above by prescribing as the climatic inputs the atmospheric temperature and the accumulation ablation rate functions at the free surface and from below by the geothermal heat we present the governing equations in the shallow ice approximation discuss the parameterizations used in the descriptions of the ice surface temperature and accumulation ablation functions briefly state how the complicated initial boundary value problem is numerically solved and how the input data that are available from measurements are implemented results of preliminary calculations disclose how the model performs and delimit its validity we study the role played by basal sliding and make clear that sliding should be accounted for where ever the basal ice is temperate and that the frictional heat generated in this sliding is thermomechanically significant we also study the reaction of the greenland ice sheet to various climate scenarios and make clear that today s thermal regime depends significantly upon the prior climate history moreover the thermomechanical properties of the ice are equally significant as is the thermal interaction of the ice sheet with the rockbed beneath

the north american late wisconsin ice sheet and mantle viscosity from glacial rebound analyses abstract observations of sea level and crustal response to glacial loading cycles provide constraints on the mantle rheology function e and as well as on the ice load i with the latter being largely free from a priori glaciological or climate assumptions and appropriate therefore for testing any such hypotheses this paper presents new results for both continental mantle e and i for the late wisconsin ice sheet using geological evidence for relative sea level change rsl and tilting of palaeo lake shorelines complemented with loose constraints from observations of present day radial crustal displacement across north america the focus is on evidence from near or within the former maximum ice margins and the resulting earth response is representative of sub continental mantle conditions the inversion of the sea level information has limited resolution for earth rheology and simple three layer models characterized by depth averaged effective lithospheric thickness h and upper and lower mantle viscosities um and um respectively adequately describe the response function yielding parameters earth model e 6 of h 102 85 120 km um 5 1 1020 3 5 7 5 x1020 lm 1 3 1022 0 8 2 8 x1022 where the numbers in parenthesis are 95 confidence limits the details of the ice sheet with one exception are not strongly dependent on the rheological assumptions within this range the exception is the lower mantle viscosity that remains correlated with the magnitude scaling of the ice sheet a link that is largely broken by introducing constraints from glacial loading effects on the earth s rotation and dynamic flattening the difference between the continental um and the comparable estimate of 1 2 5 x1020 for ocean mantle is statistically significant shoreline gradient information from glacial lakes mcconnell agassiz algonquin and ojibway provide strong constraints on the response within the interior of the ice sheet and the resulting ice sheet model lw 6 is characterized by multiple ice domes from at least 17 18 ka onwards the resolution for earlier periods is largely constrained by global ice volume considerations the two principal domes are over southern nunavut the keewatin dome and over quebec labrador both of 3500 m thickness separated by an ice ridge some 1500 m lower than the domes the latter a requirement imposed by the gradient information over western canada ice thickness gradients east of the cordilleras are partly constrained by the shoreline data for former lake mcconnell as is the lateglacial ice thickness of the keewatin dome and indicate that any ice free corridor between the laurentian and cordilleran components is unlikely to have existed before 13ka reconstructions of the glacial lakes are consistent with the locations and timing of the observational evidence for the four major lake systems with the likely drainage routes identified the evolution of the lw 6 ice volume function expressed as equivalent sea level is characterized by a rapid decrease in ice volume from 15 to 14 5 ka corresponding to the bolling allerod period in the main from rapid ice retreat along the southern margin with further contributions from drainage through the st lawrence river valley and from ice retreat from the major northern straits and gulfs but not hudson strait where the rsl data point to late removal of ice after 10 ka the contribution of the drainage of the lakes to global sea level rise is small

formation and palaeozoic evolution of the gorny altai altai mongolia suture zone south siberia zircon u pb constraints on the igneous record abstract the altai sayan fold belt epitomises how phanerozoic island arc continental crust contributed to the growth of the palaeo eurasian continent its formation is dominated by the closure of the palaeo asian ocean pao when island arc systems and gondwana derived terranes accreted to the siberian margin relics of the pao related tectonic units and associated granitoids occur along the ophiolitic charysh terekta ulagan sayan suture ctuss between gorny altai and altai mongolia zircon la icp ms u pb dating of this igneous record constrains the multi stage geodynamic pao evolution primitive kuznetsk altai island arc crust formed at the siberian margin during the ediacaran early cambrian 525 555 ma this island arc matured during the middle late cambrian 510 ma and was consumed by pao subduction in the late cambrian early ordovician 480 490 ma forming the siberian early caledonian accretion collision belt south of the ctuss granitic magmatism occurred within gondwana derived altai mongolia during the middle late ordovician 450 470 ma palaeo kazakhstan assembly and during a silurian early devonian 400 425 ma andean type collision middle late devonian 360 395 ma granitoids were emplaced as a result of the collision of altai mongolia with siberia during final pangaea amalgamation the suture was strike slip reactivated associated magmatism 295 ma the youngest 220 255 ma sampled granitoids originated in an intra plate setting

mafic magmatic enclaves and mafic rocks associated with some granitoids of the central sierra nevada batholith california nature origin and relations with the hosts abstract the calc alkaline granitoids of the central sierra nevada batholith are associated with abundant mafic rocks these include both country rock xenoliths and mafic magmatic enclaves mme that commonly have fine grained and less commonly cumulate textures scarce composite enclaves consist of either xenoliths enclosed in mme or of mme enclosed in other mme with different grain size and texture enclaves are often enclosed in mafic aggregates and form meter size polygenic swarms mostly in the margins of normally zoned plutons enclaves may locally divert schlieren layering mafic dikes which also occur in swarms are undisturbed composite or largely hybridized in central sierra nevada with the exception of xenoliths that completely differ from the other rocks host granitoids mafic aggregates mme and some composite dikes exhibit a bulk compositional diversity and at the same time important mineralogical and geochemical including isotopic similarities mme and host granitoids display distinct major and trace element compositions however strong correlations between mme host granitoid pairs indicate interactions and parallel evolution of mme and enclosing granitoid in each pluton identical mafic mineral compositions and isotopic features are the result of these interactions and parallel evolution mafic dikes have broadly the same major and trace element compositions as the mme although variations are large between the different dikes that are at distinctly different stages of hybridization and digestion by the host granitoids the composition of the granitoids and various mafic rocks reflects three distinct stages of hybridization that occurred respectively at depth during ascent and emplacement and after emplacement the occurrence and succession of hybridization processes were tightly controlled by the physical properties of the magmas the sequential thorough or partial mixing and mingling were commonly followed by differentiation and segregation processes unusual mme that contain abundant large crystals of hornblende resulted from disruption of early cumulates at depth whereas those richer in large crystals of biotite were formed by disruption of late mafic aggregates or schlieren layerings at the level of emplacement mme and host granitoids are considered cogenetic because both are hybrid rocks that were produced by the mixing of the same two components in different proportions the felsic component was produced by partial melting of preexisting crustal materials whereas the dominant mafic component was probably derived from the upper mantle however in the lack of a clear mantle signature the origin of the mafic component remains questionable

rate dependent mechanical behavior of rough rock joints abstract the influence of shear rate on the mechanical behavior of rough rock joints prepared using a molding method was examined by testing jointed samples of four rock joint groups which were obtained from the underground caverns of the huangdao state oil reserves in china the tests were performed using a jaw 600 coupled shear flow machine under constant normal load cnl conditions shear rate and joint roughness coefficient jrc were the two primary considerations in this study the results indicate that the peak shear strength is controlled by shear rate and joint roughness shear rate has a nonlinear relationship with the peak shear strength whereas joint roughness exhibits good linearity with a high correlation coefficient 0 97 except for results under a shear rate of 24 0 mm min furthermore shear rate affects the damage incurred by the rock joints as illustrated by the acoustic emission of rock joints monitored by a pac acoustic emission system the joint surface breaks earlier with increased shear rate furthermore because of the more rapid generation of macrocracks under higher shear rate the corresponding cumulative acoustic emission ae count and ae energy are inversely proportional to the shear rate a new shear strength criterion that considers both shear rate and joint roughness was developed based on the experimental results and barton s shear strength criterion then the validity of this proposed criterion was verified with an average estimated error of 10 539 using the shear strength values of rock joints proposed by barton indicating that the new criterion is capable of providing an accurate estimate of shear strength for rough rock joints under different shear rates

electrical conductivity measurements from the gisp2 and grip greenland ice cores the direct current electrical conductivity of glacial ice depends on its acidity1 3 and can also indicate changes in climate as ice formed in cold dusty periods has a high concentration of alkaline dust1 4 5 which significantly reduces the conductivity6 7 compared to warmer less dusty periods here we present electrical conductivity records for the greenland ice sheet project 2 gisp2 and greenland ice core project grip ice cores drilled 28 km apart to enable direct comparison of the results the upper parts of both records are consistent with previous evidence from other greenland cores4 8 12 for a stable greenland climate during the holocene and a series of warm events punctuating the last glacial period however there is a significant discrepancy between the two records in the bottom 10 of the cores calling into question recent reports of climate variability in the last interglacial4 8 and the penultimate glaciation8 at this stage it is too early to say what exactly is causing the discrepancy although ice flow may have introduced some discontinuities into the records further work will be necessary to establish how much climatic information it will eventually be possible to extract from the lower parts of the two cores

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

isotope provinces mechanisms of generation and sources of the continental crust in the central asian mobile belt geological and isotopic evidence abstract the available geological geochronological and isotopic data on the felsic magmatic and related rocks from south siberia transbaikalia and mongolia are summarized to improve our understanding of the mechanisms and processes of the phanerozoic crustal growth in the central asian mobile belt camb the following isotope provinces have been recognised precambrian t dm 3 3 2 9 and 2 5 0 9 ga at the microcontinental blocks caledonian t dm 1 1 0 55 ga hercynian t dm 0 8 0 5 ma and indosinian t dm 0 3 ga that coincide with coeval tectonic zones and formed at 570 475 420 320 and 310 220 ma continental crust of the microcontinents is underlain by or intermixed with juvenile crust as evidenced by its isotopic heterogeneity the continental crust of the caledonian hercynian and indosinian provinces is isotopically homogeneous and was produced from respective juvenile sources with addition of old crustal material in the island arcs or active continental margin environments the crustal growth in the camb had episodic character and important crust forming events took place in the phanerozoic formation of the camb was connected with break up of the rodinia supercontinent in consequence of creation of the south pacific hot superplume intraplate magmatism preceding and accompanying permanently other magmatic activity in the camb was caused by influence of the long term south pacific plume or the asian plume damping since the devonian

hf isotopes in zircon reveal contrasting sources and crystallization histories for alkaline to peralkaline granites of temora southeastern australia peralkaline granites exhibit the hallmark features of a type igneous rocks but strongly differentiated chemistry and intense hydrothermal alteration camouflage their ultimate origins we present the first in situ hf isotope data from zircons of peralkaline granites aimed at clarifying the protoliths of these plutons and their genetic relationship to associated metaluminous weakly peraluminous granites this study used rocks of the devonian narraburra complex in southeastern australia and found that correlations between hf isotopes and trace element ratios reveal fundamentally different origins for the nonperalkaline and peralkaline granites the latter have a depleted mantle like ancestry whereas a weakly peraluminous rock formed from melts of older arc crust that were modified by interaction with juvenile probably alkaline magmas juxtaposition of crust and mantle derived magmas reflects the high heat flow and lithosphere scale faults associated with continental extension and explains the diversity of a type granites

fractal analysis of shear joint roughness the morphology of mortar replicas of two natural joints a granite and a schist made from the silicone rubber casts was characterized using a laser sensor profilograph to create three dimensional topographic maps before and after each shear test three successive direct constant normal load shear tests under three normal stress levels have been carried out on the joint replicas n 0 4 0 8 1 2mpa and n 0 4 0 8 1 4mpa for the schist and the granite joint replicas respectively based on geostatistical methods the variation in surface roughness in the course of shear tests is described using the fractal dimension d computation a roughness parameter srs and a degradation parameter dw were also defined and computed srs and dw quantify respectively the degree of joint surfaces roughness and the degree of degradation of sheared joint surfaces the variation of d as a function of n have been compared to the variation of srs as a function of n in the course of shear tests results indicate a good correlation between d and srs

precipitation over greenland retrieved by a dynamic method and its relation to cyclonic activity abstract in order to calculate the vertical motion over some high mountain regions such as greenland an equation without the quasigeostrophic approximation in coordinates has been developed a dynamic method for retrieving precipitation over greenland is based on this equation the retrieved annual mean precipitation distribution for 1987 and 1988 is in very good agreement with the observed annual accumulation pattern over the greenland ice sheet the major weather system producing precipitation over greenland is the frontal cyclone based on the precipitation characteristics greenland can be divided into five subregions precipitation over the north coastal and central interior regions primarily occurs in summer for the three other subregions if the composite monthly mean sea level pressure charts for high and low monthly precipitation amounts are constructed a clear relationship between precipitation and cyclonic activity emerges if a mean cyclone exists in the labrador sea heavy precipita

orographic precipitation and the relief of mountain ranges 1 the influence of climate on the relief of mountain ranges has long been supposed but has received little quantitative investigation the simplified system of a longitudinal river profile in a bedrock channel coupled to a physically based model of orographic precipitation is used to explore how orographically influenced patterns of precipitation affect the relief of fluvially sculpted mountain ranges it is shown that the impact on the orography depends on the nature of the precipitation regime significant changes in relief also occur for imposed temperature changes of realistic magnitude although the ability of the relief to respond to any given change depends sensitively on its response time and the timescale of the forcing

atmospheric circulation around the greenland crest the greenland ice sheet program 2 gisp2 required meteorological support for the layout of the field camp ice core site landing strip snow sampling sites and air sampling sites an automatic weather station installed in may 1987 provided the initial data for the support of several components of the program using the 2 year wind speed and direction distributions the automatic weather station aws network was expanded to six sites with four sites located approximately 100 km from the highest point in greenland in the cardinal directions and sites located at gisp2 and at the air sampling site the wind direction and speed wind roses for august and september 1994 show that there is considerable variation in the wind field around the greenland crest the constancy ratio of the vector wind speed to the scalar wind speed of the monthly wind vectors was lowest at the southern site and increased as one moved clockwise around the crest to the eastern site vorticity and divergence were determined from triangles formed by three aws sites the mean horizontal divergence for august and september 1994 was 1 4 x 10 5 s 1 and the vorticity was 3 7 x 10 5 s 1 apparently the flow around the greenland crest is most influenced by topography when the 500 hpa contours indicate a pressure ridge over central greenland the surface winds showed anticyclonic flow positive horizontal divergence and negative vertical vorticity in the mean this configuration indicates the presence of an inversion wind especially at the sites located on the slopes to the west and east of the crest

petrogenetic interpretation of granitoid rock series using multicationic parameters abstract granitoid rock compositions from a range of tectonic environments are plotted on a multicationic diagram devised by de la roche and his coworkers this shows that there is a systematic change through an orogenic cycle which leads progressively to the ultimate development of alkaline magmas possible source materials and mechanisms of magma generation are considered from analysis of mineral compositional vectors these suggest that most granitoid series result from a two stage process first fractional crystallisation of clinopyroxene olivine and calcic plagioclase from a basic source with tholeiitic affinities produces a magma of intermediate composition this magma then undergoes periodic mixing with a felsic magma followed by in situ fractionation to generate individual intrusions within granitoid series

comparison of oxygen isotope records from the gisp2 and grip greenland ice cores recent results1 2 from the greenland ice core project grip summit ice core suggest that the climate in greenland has been remarkably stable during the holocene but was extremely unstable for the time period represented by the rest of the core spanning the last two glaciations and the intervening eemian inter glacial here we present the complete oxygen isotope record for the greenland ice sheet project 2 gisp2 core drilled 28 km west of the grip core we observe large rapid climate fluctuations throughout the last glacial period which closely match those reported for the grip core however in the bottom 10 of the cores spanning the eemian interglacial and the previous glacia tion there are significant differences between the two records it is possible that ice flow may have altered the chronological sequences of the stratigraphy for the bottom part of one or both of the cores considerable further work will be necessary to evaluate the likelihood of this and the extent to which it will still be possible to extract meaningful climate information from the lowest sections of the cores

paleozoic crustal growth and metallogeny of central asia evidence from magmatic hydrothermal ore systems of central kazakhstan abstract the ore deposit spectrum of central kazakhstan reflects a range of lithospheric magmatism controlled by the polycyclic geodynamic evolution of an active continental margin which develops from back arc oceanic settings volcanic hosted massive sulphide cu au ore deposits to subduction controlled calcalkaline magmatism copper porphyries with subsequent stages of crust differentiation of low degree partial melting and extended intramagmatic fractionation molybdenum porphyries on to continental rifting peralkaline ree zr nb rich systems mesothermal gold deposits mark transcrustal shear zone controlled fluid conduits formed during arc slicing and thermal relaxation in a post orogenic setting the ore deposits characterize successive and spatially overlapping stages of crustal growth and consolidation the general subduction setting of this range of ore deposits is reflected by calc alkaline trends in the associated subalkaline high k felsic rocks with a wide range in silica contents the predominant metal spectrum of au cu and mo indicates relatively oxidized magmatic hydrothermal systems in accordance with the metaluminous i type nature of their igneous host rocks i e oxidized igneous precursor material despite of presumably highly variable ratios of mantle crust components in individual magmatic systems gabbros to leucogranites most rocks have very similar positive initial e nd values of 0 to 5 5 and depleted mantle model ages in the range of 500 800 ma the ubiquitous relatively young mantle extraction age is likely to characterize the lower crust of central kazakhstan this basement is interpreted as back arc oceanic crust that formed behind oceanwards drifting continental margin slivers during the initial stages of the active continental margin evolution of the late precambrian angara baltica supercontinent the early back arc oceanic crust was transformed into the present lower crust during the paleozoic orogenic evolution in which the back arc crust was buried by magmatic arc and sedimentary material and recurrently affected by high grade metamorphism basaltic melt injection underplating and granitic melt extraction anorogenic rift related permian peralkaline riebeckite granites with ree zr nb mineralization have very high positive e nd of 5 to 8 which is probably inherited from young subcrustal lithosphere at the mantle crust boundary that was affected by partial remelting due to asthenospheric upwelling during rifting

quantitative three dimensional description of a rough surface and parameter evolution with shearing note roches reference lmr article 2002 005view record in web of science record created on 2006 11 09 modified on 2016 08 08

accretionary growth and crust formation in the central asian orogenic belt and comparison with the arabian nubian shield kroner a windley b f badarch g tomurtogoo o hegner e jahn b m gruschka s khain e v demoux a and wingate m t d 2007 accretionary growth and crust formation in the central asian orogenic belt and comparison with the arabian nubian shield in hatcher r d jr carlson m p mcbride j h and martinez catalan j r eds 4 d framework of continental crust geological society of america memoir 200 p 181 209 doi 10 1130 2007 1200 11 for permission to copy contact editing geosociety org 2007 the geological society of america all rights reserved kroner kroener mail uni mainz de windley brian windley btinternet com tomurtogoo igmr magicnet mn hegner hegner lmu de jahn jahn earth sinica edu tw khain khain ginras ru demoux demoux uni mainz de wingate mwingate tsrc uwa edu au

mass balance and surface movement of the greenland ice sheet at summit central greenland during the grip deep drilling in central greenland the ice sheet topography and surface movement at summit has been mapped with gps measurements of the surface velocity are presented for a strain net consisting of 13 poles at distances of 25 60 km from the grip site some results are the grip site is located approximately 2 km nw of the topographic summit the surface velocity at the gisp 2 site is 1 7 m yr in the w direction the present mass balance at summit is calculated to be 0 03 0 04 m yr i e close to steady state this result is the best now available for summit a small thinning rate might be a transient response of the greenland ice sheet due to the temperature increase at the wisconsin holocene transition

hf isotopic compositions of the standard zircons and baddeleyites used in u pb geochronology abstract zircon and baddeleyite u pb geochronological dating is widely used in solid earth sciences and the advent of rapid in situ methods of analysis such sims and icp ms has re emphasized the importance of having uniform standards recently it has been shown that hf isotopic data can provide important information on these minerals since they contain high concentrations of hf but have low lu hf ratios which results in negligible age correction however the complex internal structures that result from multiple thermal events such as inherited cores and metamorphic overgrowths require that the hf isotopic data be measured with high spatial resolution however the isobaric interferences of 176 yb and 176 lu on 176 hf hamper the precise determination of the 176 hf 177 hf ratio during in situ laser ablation mc icpms analysis it is shown here that mass biases of yb yb and hf hf change with time during analyses and behave differently for solutions and solid material therefore it is suggested that the mean yb value of the individual spot be used to obtain the precise isotopic composition for in situ zircon and baddeleyite hf isotopic analyses for low yb hf 176 yb 177 hf yb value have little effect on the accuracy of the hf isotopic composition however the appropriate yb isotopic abundance is necessary for high yb hf 176 yb 177 hf 0 001 zircons and baddeleyites since the interference of 176 yb on 176 hf is significant using the mean yb value of the individual spot and newly published yb isotopic abundance data six standard zircons and two standard baddeleyites have been investigated using a neptune mc icpms with 193 nm laser for zircons the obtained 176 hf 177 hf ratios are 0 282307 31 2sd for 91500 0 282680 31 2sd for temora 0 281729 21 2sd for cz3 282177 17 2sd for cn92 1 0 282983 17 2sd for fm0411 and 281234 11 2sd for phalaborwa the baddeleyites from phalaborwa and sk10 2 have 176 hf 177 hf ratios of 0 281238 12 and 0 282738 13 2sd these results agree well with the values obtained by the solution method and indicate that these standards have different hf isotopic compositions in which the extremely low 176 lu 177 hf and 176 yb 177 hf values of cz3 zircon and phalaborwa baddeleyite make them excellent standards for machine calibration during in situ zircon hf isotopic measurement with the other standards being more suitable for the development of the correction method

influence of asperity degradation on the mechanical behavior of rough rock joints under cyclic shear loading a cyclic shear testing system was established to investigate the mechanical behavior of rough rock joints under cyclic loading conditions laboratory cyclic shear tests were conducted for two joint types of hwangdeung granite and yeosan marble saw cut and split tensile joints prior to test the roughness of each specimen was characterized by measuring the surface topography using a laser profilometer several important aspects of cyclic joint behavior such as high peak shear strength and non linear dilation in the first loading cycle different frictional resistance for the reversed shear loading direction and anisotropic shear behavior and its dependence on the normal stress level were identified from the cyclic shear test results these features and their variations in the subsequent loading cycles are mainly due to the effect of second order asperities and strength of rock material it was also observed from experimental results that degradation of asperities under cyclic shear loading also followed the exponential degradation laws for asperity angle and the mechanism for asperity degradation would be different depending upon the shearing direction and the type of asperities based on the experimental results an elasto plastic constitutive model which can consider the degradation of second order asperities was proposed numerical simulations for the monotonic and cyclic shear loading indicated agreement with the laboratory test results

point measurements of mass balance of the greenland ice sheet using precision vertical global positioning system gps surveys rates of ice sheet thickening or thinning in greenland are measured using the coffee can technique this entails computing the difference in the vertical velocity of markers anchored in firn and the long term rate of snow accumulation the velocities are obtained from repeat surveys using the global positioning system gps with corrections for densification and along slope flow this difference is the local mass balance for two sites in western greenland camp century and a site just south of the egig line crawford point results show ice sheet thinning a third west greenland site inland from upernavik is close to balance two sites dye 2 in western greenland and summit are thickening slowly

sensitivity of a greenland ice sheet model to ice flow and ablation parameters consequences for the evolution through the last climatic cycle sensitivity experiments are conducted to test the influence of poorly known model parameters on the simulation of the greenland ice sheet by means of a three dimensional numerical model including the mechanical and thermal processes within the ice two types of experiments are performed steady state climatic conditions and simulations over the last climatic cycle with a climatic forcing derived from the grip record the experiments show that the maximum altitude of the ice sheet depends on the ice flow parameters deformation and sliding law coefficients geothermal flux and that it is low when the ice flow is fast on the other hand the maximum altitude is not sensitive to the ablation strength and consequently during the climatic cycle it is driven by changes in accumulation rate the ice sheet extension shows the opposite sensitivity it is barely affected by ice flow velocity and the ice covered area is smaller for large ablation coefficients for colder climates when there is no ablation the ice sheet extension depends on the sea level an interesting result is that the variations with time of the altitude at the ice divide summit do not depend on the parameters we tested the present modelled ice sheets resulting from the climatic cycle experiments are compared with the present measured ice sheet in order to find the set of parameters that gives the best fit between modelled and measured geometry it seems that compared to the parameter set most commonly used higher ablation rate coefficents must be used

steady state characteristics of the greenland ice sheet under different climates the greenland ice sheet is modelled to simulate its extent and volume in warmer climates and also to find out whether the ice sheet would re form on theice free bedrock under present climatic conditions the ice sheet model is a three dimensional thermo mechanical model with a fine resolution grid thebedrock surface beneath the ice sheet was mapped using radio echo sounding measurements by the electromagnetic institute copenhagen the modelexperiments show that increased temperature will result in ice margin retreat but the ice sheet is relatively stable it takes a rather high temperature rise of atleast 6ic for the ice sheet to disappear completely which indicates that the ice sheet probably survived the last interglacial also it appears that the greenlandice sheet is not a mere relict ice mass from a previously colder climate but that the ice sheet will still re form on the bare bedrock under the present or evenslightly warmer climatic conditions

a continuous simulation of global ice volume over the past 1 million years with 3 d ice sheet models sea level records show large glacial interglacial changes over the past million years which on these time scales are related to changes of ice volume on land during the pleistocene sea level changes induced by ice volume are largely caused by the waxing and waning of the large ice sheets in the northern hemisphere however the individual contributions of ice in the northern and southern hemisphere are poorly constrained in this study for the first time a fully coupled system of four 3 d ice sheet models is used simulating glaciations on eurasia north america greenland and antarctica the ice sheet models use a combination of the shallow ice and shelf approximations to determine sheet shelf and sliding velocities the framework consists of an inverse forward modelling approach to derive a self consistent record of temperature and ice volume from deep sea benthic 18o data over the past 1 million years a proxy for ice volume and temperature it is shown that for both eustatic sea level and sea water 18o changes the eurasian and north american ice sheets are responsible for the largest part of the variability the combined contribution of the antarctic and greenland ice sheets is about 10 for sea level and about 20 for sea water 18o during glacial maxima however changes in interglacials are mainly caused by melt of the greenland and antarctic ice sheets with an average time lag of 4 kyr between melt and temperature furthermore we have tested the separate response to changes in temperature and sea level for each ice sheet indicating that ice volume can be significantly influenced by changes in eustatic sea level alone hence showing the importance of a simultaneous simulation of all four ice sheets this paper describes the first complete simulation of global ice volume variations over the late pleistocene with the possibility to model changes above and below present day ice volume constrained by observations of benthic 18o proxy data

asperity degradation characteristics of soft rock like fractures under shearing based on acoustic emission monitoring abstract shear failure of rock masses along pre existing discontinuities is one of the predominant failure modes of rock slopes and underground tunnels the monitoring and prediction of the impending shear failure is of great significance to ensure the stability of the rock structures and the safety of the workers in this study direct shear tests under normal stress ranging from 0 5 to 10 mpa are conducted on rock fractures analogous to soft rock discontinuities which are obtained by artificial splitting during which the ae parameters are monitored test results show that the ae parameters hit energy and events increase with the shear stress and peak at or near the peak shear strength after which the aes gradually decay with the decrease of shear stress the number of ae events first increases and then decreases with increasing normal stress which may be associated with the ductile deformation of the porous structure of cement mortar under higher normal stress the degradation of asperities on the fracture surface can be inferred from the accumulative ae hits and events which are characterized by s shapes and can be divided into slow growth fast growth and slow growth stages conceptual and mathematical asperity damage models are proposed respectively based on the temporal characteristics of ae events and the curve fitting which can be used to predict the asperity damage for a given shear stress curve the ae b value fluctuates and remains high in the shear process there is no strong correlation between the shear stress and the b value which indicates that the b value may not be an effective index to predict the quasi static shear failure of jointed rock masses possessing similar properties like cement mortar results of this study will not only contribute to a better understanding of the asperity degradation characteristics but also provide valuable knowledge for ae monitoring applications in jointed rock masses

evaluating damage during shear tests of rock joints using acoustic emissions abstract the acoustic emission ae signals generated during direct shear test were evaluated on different types of joints rock rock rock concrete and concrete concrete several boreholes were cored from a dam body rock mass and interface between dam and rock mass and the samples were prepared and tested under direct shear test a laser profilometer scanner was used for scanning the joint surfaces in order to assess surface roughness by correlating the ae signals with the shear graphs one can predict the starting point of shearing during direct shear test count and energy parameters were analyzed in two different methods to monitor the shear behavior of the joints a graph of the count and energy rates and a graph of cumulative count and energy four separated periods were observed for bonded and non bonded joints linear pre peak period non linear pre peak period post peak period and residual period this study showed that ae has enough accuracy to monitor the shear behavior of the joints and it can be used in site confidently

petrogenesis and tectonic setting of the huhetaoergai granitic pluton in the northern ya gan fault zone northern alxa china constraints from whole rock geochemistry zircon u pb ages and hf isotope compositions this study presents a new data set that contains zircon u pb ages whole rock geochemistry and hf isotopes of the huhetaoergai granitic pluton in the northern ya gan fault zone northern alxa chi

a search in north greenland for a new ice core drill site a new deep ice core drilling site has been identified in north greenland at 75 12 n 42 30 w 316 km north northwest nnw of the grip drill site on the summit of the ice sheet the ice thickness here is 3085 m the surface elevation is 2919 m the north grip ngrip site is identified so that ice of eemian age 115 130 ka bp calendar years before present is located as far above bedrock as possible and so the thickness of the eemian layer is as great as possible an ice flow model similar to the one used to date the grip ice core is used to simulate the flow along the nnw trending ice ridge surface and bedrock elevations surface accumulation rate distribution and ratio echo sounding along the ridge have been used as model input the surface accumulation rate drops from 0 23 m ice equivalent year 1 at grip to 0 19 m ice equivalent year 1 50 km from grip over the following 300 km the accumulation is relatively constant before is starts decreasing again further north ice thickness up to 3250 m bring the temperature of the basal ice up to the pressure melting point 100 250 km from grip the ngrip site is located 316 km from grip in a region where the bedrock is smooth and the accumulation rate is 0 19 m ice equivalent year 1 the modeled basal ice here has always been a few degrees below the pressure melting point internal radio echo sounding horizons can be traced between the grip and ngrip sites allowing us to date the ice down to 2300 m depth 52 ka bp an ice flow model predicts that the eemian age ice will be located in the depth range 2710 2800 m which is 285 m above the bedrock this is 120 m further above the bedrock and the thickness of the eemian layer of ice is 20 m thicker than at the grip ice core site

magma mixing in granitic rocks of the central sierra nevada california abstract the el capitan alaskite exposed in the north american wall yosemite national park was intruded by two sets of mafic dikes that interacted thermally and chemically with the host alaskite comparisons of petrographic and compositional data for these dikes and alaskite with published data for sierra nevada plutons lead us to suggest that mafic magmas were important in the generation of the sierra nevada batholith specifically we conclude that 1 intrusion of mafic magmas in the lower crust caused partial melting and generation of alaskite rhyolitic magmas 2 interaction between the mafic and felsic magmas lead to the observed linear variation diagrams for major elements 3 most mafic inclusions in sierra nevada plutons represent chilled pillows of mafic magmas related by fractional crystallization and granitoid assimilation that dissolve into their felsic host and contaminate it to intermediate granodioritic compositions 4 vesiculation of hydrous mafic magma upon chilling may allow buoyant mafic inclusions and their disaggregation products to collect beneath a pluton s domed ceiling causing the zoning mafic margins to felsic core that these plutons exhibit

terminating the 100 kyr ice age cycle we report a simulation of the most recent 100 000 year glaciationdeglaciation cycle of the late pleistocene ice age a simulation that delivers an ice sheet chronology that is in close accord with that inferred from the geological record our analyses are performed with a reduced model of the climate system that incorporates significant improvements to the representation of both climate forcing and mass balance response in a previously described theory based upon a coupled one level energy balance model ebm and vertically integrated ice sheet model ism the theory fully incorporates the influences of orbital insolation forcing glacial isostatic adjustment and variations in the atmospheric concentrations of greenhouse gases it correctly predicts the main geographical regions of the northern hemisphere that were glaciated at last glacial maximum 21 000 years ago as well as the abrupt termination of the glacial epoch that occurred subsequently the latter feature of the ice age cycle is obtained without the need to incorporate unconstrained and therefore controversial physical processes into the model a limitation of all previous attempts to understand this global scale climate cycle our analyses suggest that the radiative impact on surface glaciation due to the changing atmospheric concentration of co2 is critical to the ability of the model to deliver a synthetic history of glaciation and deglaciation that is in accord with inferences based upon surface geological and geomorphological evidence with the incorporation of this influence model predicted ice thickness distributions at last glacial maximum lgm are very similar to those of the recently described ice 4g reconstruction that was based upon the inversion of postglacial relative sea level histories

nd sr isotopic mapping of the chinese altai and implications for continental growth in the central asian orogenic belt article i nfo the chinese altai orogen is a typical domain of the central asian orogenic belt and in previous syntheses was divided into six tectonic units or terranes in this paper 41 new and 36 published whole rock nd and sr isotopic data for granitoid intrusions and mafic dykes within units 2 and 3 of the central altai and units 4 to 6 of the southern altai were used for isotopic mapping the age of the samples used is known from u pb zircon dating end t values range from 4 to 2 in the central altai units 2 and 3 and from 1 4 to 6 in the southern altai units 4 and 5 and up to 8 4 in unit 6 correspondingly nd model ages tdm decrease from 1 6 1 1 ga in the central altai to 1 0 0 5 ga in the southern altai except for a continental fragment in unit 4 isr is elevated in the central units decreasing to b0 705 in the southern units 5 and 6 these results demonstrate that the central altai contains widespread old crustal components in its basement whereas the southern altai comprises a higher proportion of juvenile crust when integrated with published u pb zircon ages our data show that new crust grew southwards in the chinese altai in addition our data suggest that units 2 and 3 are a single tectonic unit whereas the southern altai unit 4 which previously was regarded as a single young accretionary terrane should be subdivided into two units a younger arc and an older continental fragment the granitoids in the chinese altai can be divided into syn and post non orogenic groups the syn orogenic granitoids have ca 50 80 mantle derived component whereas the post non orogenic granitoids contain ca 70 90 the generation of syn and post non orogenic granitoids represents horizontal lateral and vertical crustal growth which contributed ca 18 28 and 7 8 respectively to the area of the chinese altai this work shows that isotopic mapping is an effective approach to elucidate terrane composition in the central asian orogenic belt and thereby better constrain palaeozoic crustal growth

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

microphysical and geometrical controls on the pattern of orographic precipitation patterns of orographic precipitation can vary significantly both in time and space and such variations must ultimately be related to mountain geometry cloud microphysics and synoptic conditions here an extension of the classic upslope model is presented which incorporates an explicit representation in the vertical dimension represents the finite growth time of hydrometeors their downwind advection by the prevailing wind and also allows for evaporation for a simple mountain geometry the authors derive an analytical solution for the precipitation rate which can be understood in terms of four nondimensional parameters the finite growth time and slanting hydrometeor trajectories give rise to some interesting possibilities a precipitation rate that maximizes at intermediate values of the horizontal wind speed localized precipitation efficiencies in excess of 100 and a reverse rain shadow with more precipitation falling on the leeward flank than on the windward flank

critical form and feedbacks in mountain belt dynamics role of rheology as a tectonic governor 1 the rheology that governs deformation within a convergent orogen also controls its topographic form or in other words the covaration of its height or thickness and its width under conditions pertaining to small orogens and for rheologies ranging from linear viscous to coulomb plastic we show that this topographic form is insensitive to the distributions of fluxes into and out of the wedge and is thus a fundamental property of the system it can therefore be thought of as a critical topographic form directly analogous to the well studied case of a coulomb plastic rheology which predicts a constant critical taper angle independent of its size the tendency of the system to evolve toward a critical topographic form can be regarded as a tectonic governor that strongly damps the response of the orogen to changes in climate or tectonic forcing scaling relationships can be derived for the variation of orogen width height and exhumation rate as a function of accretionary flux and precipitation rate this study explores how sensitive these scaling relationships are to the assumed rheology it is found that the scaling relationships vary by less than a factor of 2 across the range of typical geological rheologies from coulomb friction to power law viscous to linear viscous these scaling relationships provide a first order representation of the behavior of convergent wedges and are relatively insensitive to the underlying assumptions

surface roughness on the greenland ice sheet from airborne laser altimetry high resolution airborne laser altimetry is used to determine the small scale surface relief in central greenland and estimate the contribution from spatial noise to stratigraphic records the standard deviation of the surface roughness is 1 6 cm water equivalent corresponding to a standard deviation of annual layer thickness of 2 3 cm we this estimate agrees with an independent assessment of the spatial variability 2 5 cm we based on nine shallow ice cores the agreement suggests that the statistical nature of the surface in central greenland remains unchanged throughout the year by conducting airborne altimetry around proposed drilling sites the expected noise level in the core can be evaluated and sites selected where this level is lowest

northern hemisphere storm tracks in present day and last glacial maximum climate simulations a comparison of the european pmip models abstract extratropical weather systems are an essential feature of the midlatitude climate and global circulation at the last glacial maximum lgm the formation of regions of high transient activity referred to as storm tracks is strongly affected by the presence of large ice sheets over northern america and scandinavia and by differences in sea surface temperature sst distributions in the framework of the palaeoclimate modelling intercomparison project simulations of the lgm climate have been run with a wide range of atmospheric general circulation models agcms using the same set of boundary conditions allowing a valuable comparison between simulations of a climate very different from the present one in this study the authors focus on the storm track representation in the models and its relationship with the surface temperatures the mean flow and the precipitation storm tracks are described using transient eddy diagnostics such as mean sea level pressure variance and three dimensional

seasonal changes in ground penetrating radar signature observed at a polythermal glacier bylot island canada in recent years ground penetrating radar gpr has been increasingly used for characterization of subglacial and englacial environments at polythermal glaciers the geophysical method is able to exploit the dielectric difference between water air sediment and ice allowing delineation of subsurface hydrological thermal and structural conditions more recent gpr research has endeavoured to examine temporal change in glaciers in particular the distribution of the cold ice zone at polythermal glaciers however the exact nature of temporal change that can be identified using gpr has not been fully examined this research presents the results of three gpr surveys conducted over the course of a summer ablation season at a polythermal glacier in the canadian arctic a total of approximately 30 km of gpr profiles were collected in 2002 repeatedly covering the lower 2 km of stagnation glacier bylot island 72 58 n 78 22 w comparison between profiles indicated changes in the radar signature including increased noise appearance and disappearance of englacial reflections and signal attenuation in the latter survey further an area of chaotic returns in up glacier locations which was interpreted to be a wet temperate ice zone showed marked recession over the course of the ablation season combining all the temporal changes that were detected by gpr results indicate that a polythermal glacier may exhibit strongly seasonal changes in hydrological and thermal characteristics throughout the ice body including the drainage of 17 000 m 3 of temporarily stored intra glacial meltwater it is also proposed that the liquid water content in the temperate ice zone of polythermal glaciers can be described as a fraction of a specific retention capacity copyright 2006 john wiley sons ltd

evaluation of recent precipitation studies for greenland ice sheet the retrieval of an accurate spatial and temporal record of contemporary greenland precipitation is a uniquely challenging task because of the extreme variability in both atmospheric processes and the resulting precipitation distribution over relatively small spatial scales a comparison of precipitation data sets composed of monthly mean values from recent studies shows a convergence on the general features of the long term spatial patterns but substantial disagreement on the temporal variability both regionally and for all of greenland there is general agreement on a long term greenland average of about 35 cm yr 1 and on long term values for regional scales although differences for outlying data sets exceed 50 of the observed glaciological estimate for particular regions a fundamental problem is the inadequate topographic representation of greenland in the numerical analyses nearly all of the data sets are overly dry for high elevation areas as seen from comparisons with glaciological observations from summit the east central region of greenland is found to be particularly susceptible to the temporal discontinuities in data sets which employ operational analyses in contrast there is strong agreement among all methods on the temporal variability for the west central region over a 15 year period from the comparison it is concluded that none of the data sets is able to capture all of the regional scale features in general however the deficiencies of each data set are readily identifiable from comparison and evaluation in the context of circulation features agreement among the methods on particular regions and timescales gives increased confidence in drawing conclusions related to aspects of greenland s precipitation climatology in particular an enhanced precipitation retrieval method is found to be less susceptible to data artifacts than other methods using operational analyses in the north anomalously high precipitation is associated with cyclonic development near the fram strait for west central greenland the close agreement among methods is related to the dominant contribution of the mean circulation

simulation of vatnajokull ice cap dynamics 1 we apply a coupled model of ice sheet dynamics and subglacial hydrology to investigate the dynamics and future evolution of the vatnajokull ice cap iceland in this paper we describe a new theoretical approach to introducing longitudinal stress coupling in the ice dynamics solution and we analyze our ability to simulate the main features of vatnajokull with and without longitudinal stress effects equilibrium ice cap configurations exist for vatnajokull but under a narrow range of climatic boundary conditions equilibrium reconstructions have an average ice thickness greater than what is observed at vatnajokull consistent with our inability to capture surge dynamics in vatnajokull s outlet glaciers hydrological regulation of basal flow longitudinal stress coupling and a simple parameterization of the subglacial heat flux from vatnajokull s geothermal cauldrons all help to reduce average ice thickness in the equilibrium reconstructions but cases that reproduce the present day ice volume have an ice cap area that is 5 10 less than the actual ice cap present day reconstructions that adopt a realistic climate spin up for the period 1600 1990 provide improved fits to the modern day ice cap geometry this indicates that climatic disequilibrium also plays a significant role in dictating vatnajokull s morphology simulations for the period 1600 2300 illustrate that air temperature is the dominant control on vatnajokull s volume and area longitudinal stress coupling and hydrological coupling both increase vatnajokull s sensitivity to future warming

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

changes in continental and sea salt atmospheric loadings in central greenland during the most recent deglaciation model based estimates by fitting a very simple atmospheric impurity model to high resolution data on ice accumulation and contaminant fluxes in the gisp2 ice core we have estimated changes in the atmospheric concentrations of soluble major ions insoluble particulates and 10 be during the transition from glacial to holocene conditions for many species changes in concentration in the ice typically overestimate atmospheric changes and changes in flux to the ice typically underestimate atmospheric changes because times of increased atmospheric contaminant loading are also times of reduced snowfall the model interpolates between the flux and concentration records by explicitly allowing for wet and dry deposition processes compared to the warm preboreal that followed we estimate that the atmosphere over greenland sampled by snow accumulated during the younger dry as cold event contained on average four seven times the insoluble particulates and nearly seven times the soluble calcium derived from continental sources and about three times the sea salt but only slightly more cosmogenic 10 be

ice core analysis at site a greenland preliminary results ice core drilling at site a greenland 70 45 n 35 57 5 w 3145m a s l by the polar ice coring office recovered two cores of about 100 m each which are now in storage awaiting further analysis field studies show that visible strata in the cores record both individual storms and individual years a typical year comprises 15 storm deposits pore close off occurs between 75 and 80 m depth the grain growth rate in isothermal firn and ice is about 7 8 x 1o 3 mm 2 a l the 10m temperature is about 29 5 c and the accumulation rate over the last 200 years is about 290 mm a i ice melt events occur about once every 40 years and major events can be correlated between cores separated by 20 m

in situ u pb zircon dating using laser ablation multi ion counting icp ms high resolution in situ u pb zircon geochronology on zoned grains can obtain isotope signatures from multiple growth or thermal events we present a method using laser ablation multicollector inductively coupled plasma mass spectrometry la mc icp ms to overcome complications associated with intricately zoned zircon crystals through in situ sampling of zircon volumes as small as 12 m 25 m and 40 m in diameter by about 10 m in depth high precision u pb age of a series of zircon standard covering a wide age range of 30 to 1 065 ma was acquired using la mc icp ms the precision of measured pb u ratios in homogeneous zircon is about 2 2 resulting in routinely achieved precision of u pb ages obtained by external calibration of 1 2 or better all masses of interest can be simultaneously recorded with a multi ion counting system mic operating in static mode and the short ablation required to achieve such precision results in spatial resolution that is superior to comparable u pb zircon analyses by single collector icp ms the resulting present u pb age for five zircon reference samples and two geological samples show an excellent agreement with the previously reported id tims or shrimp data

substantial contribution to sea level rise during the last interglacial from the greenland ice sheet during the last interglacial period the eemian global sea level was at least three metres and probably more than five metres higher than at present1 2 complete melting of either the west antarctic ice sheet or the greenland ice sheet would today raise sea levels by 6 7 metres but the high sea levels during the last interglacial period have been proposed to result mainly from disintegration of the west antarctic ice sheet3 with model studies attributing only 1 2 m of sea level rise to meltwater from greenland4 5 this result was considered consistent with ice core evidence4 although earlier work had suggested a much reduced greenland ice sheet during the last interglacial period6 here we reconsider the eemian evolution of the greenland ice sheet by combining numerical modelling with insights obtained from recent central greenland ice core analyses our results suggest that the greenland ice sheet was considerably smaller and steeper during the eemian and plausibly contributed 4 5 5 m to the sea level highstand during that period we conclude that the high sea level during the last interglacial period most probably included a large contribution from greenland meltwater and therefore should not be interpreted as evidence for a significant reduction of the west antarctic ice sheet