palaeotemperatures still exist in the greenland ice sheet the temperature distribution through the greenland ice sheet at the dye 3 borehole is a record of the past climatic changes in the arctic the numerical model of the temperature distribution now presented reproduces the observed temperature distribution within 0 03 k and shows that the basal ice is still cooled 5 k by the cold ice age climate the results suggest a mean ice age temperature of 32 2 c which is 12 k colder than the present temperature and a precipitation rate 50 25 of the present rate calculations of a more detailed temperature history through the present inter glacial period reveal evidence of the ad 1920 50 maximum the little ice age and the atlantic period

variations in melt layer frequency in the gisp2 ice core implications for holocene summer temperatures in central greenland the rare melt features in the gisp2 central greenland deep ice core have decreased in frequency over the most recent 7000 years calibration of this change in melt frequency against modern spatial variation of melt frequency and temperature in central greenland and against modern temporal variability of temperatures in central greenland indicates that mean mid summer temperatures have cooled over the most recent 7000 years probably by slightly more than l c if variability of summer temperatures has not changed comparison to grip isotopic records from central greenland and to the melt record from the agassiz ice cap arctic canada suggests some seasonal and regional coherence for this cooling signal as well as for a cold event about 8000 8500 bp

the effects of selective sorting on factor analyses of heavy mineral assemblages abstract an investigation has been undertaken of the effects of selective grain sorting on the ability of factor analyses to identify source end members within heavy mineral assemblages q mode factor analysis of samples randomly collected from the beach at otter rock oregon yielded two factors one is dominated by augite hornblende and hypersthene and the second consists of garnet plus zircon these factors correspond closely to selective sorting patterns of minerals on this beach determined from a series of samples along a beach profile collected during a period of beach face erosion factor analysis of those profile samples showed that the garnet and zircon are associated with the high density opaques which tend to concentrate at the landward limit of wave run up while the lower densit augite etc are transported to the outer surf zone factor analyses were also performed on a synthetic data set which simulates the along coast mixing of mineral assemblages from four spatially distinct sources analyses involved pure mixtures of source end members mixtures where random errors were introduced to simulate counting uncertainties and mixtures of the end members where systematic changes were added to simulate the effects of local grain sorting the results indicate that sorting can significantly affect the results from factor analyses with new factors introduced and with random irregularities produced in the spatial patterns of the factors

climate correlations between greenland and antarctica during the past 100 000 years the ice cores recovered from central greenland by the grip1 2 and gisp23 projects record 22 interstadial warm events during the part of the last glaciation spanning 20 105 kyr before present the ice core from vostok east antarctica records nine interstadials during this period4 5 here we explore links between greenland and antarctic climate during the last glaciation using a high resolution chronology derived by correlating oxygen isotope data for trapped o2 in the gisp2 and vostok cores we find that interstadials occurred in east antarctica whenever those in greenland lasted longer than 2 000 years our results suggest that partial deglaciation and changes in ocean circulation are partly responsible for the climate teleconnection between greenland and antarctica ice older than 115 kyr in the gisp2 core shows rapid variations in the 18o of o2 that have no counterpart in the vostok record the age depth relationship and thus the climate record in this part of the gisp2 core appears to be significantly disturbed

stable isotope air temperature relationships in ice cores from dolleman island and the palmer land plateau antarctic peninsula whilst stable isotope analysis of ice cores yields the best quantitative evidence for past climate there remains considerable uncertainty about the detailed relationship between the isotopic composition and air temperature analysis of two ice cores from the antarctic peninsula a 47 2 m core from the palmer land plateau 74 oi s 70 38 w and a 32 m core from dolleman island 70035 2 s 60055 5 w has shown that an oxygen isotope temperature relationship exists at a resolution of inter annual variations during the period 1938 86 all the major regional temperature anomalies known from climatic records at several stations are visible in the isotope profiles including the overall temperature increase between 1960 and 1980 an isotope temperature gradient of 0 5 o 6 c is indicated for the climatic interpretation of isotopic fluctu

deformation in the vicinity of ice divides calculs numeriques aux elements finis de la variation de la vitesse horizontale avec la profondeur au voisinage d une difference de la glace symetrique isotherme et ou glissement sur un lit plat

sensitivity of the ice divide position in greenland to climate change model calculations of depth age relations for deep ice cores in central greenland are sensitive to stability of the ice divide position in addition the folding of layers observed in the deep ice could be instigated by divide migration changing the velocity and particle paths of ice flow we use simple steady state calculations to show that lateral divide migration of between 10 km and 50 km and elevation change of approximately 100 m is likely on glacial interglacial time scales enough to affect model dating the ice divide location appears to be most sensitive to the position of the ice sheet margins by contrast the ice divide elevation is most sensitive to the accumulation rate the temperature profile and the ice stiffness profile

abrupt increase in greenland snow accumulation at the end of the younger dryas event the warming at the end of the last glaciation was characterized by a series of abrupt returns to glacial climate the best known of which is the younger dryas event1 despite much study of the causes of this event and the mechanisms by which it ended many questions remain unresolved1 oxygen isotope data from greenland ice cores2 4 suggest that the younger dryas ended abruptly over a period of about 50 years dust concentrations2 4 in these cores show an even more rapid transition 20 years this extremely short timescale places severe constraints on the mechanisms underlying the transition but dust concentrations can reflect subtle changes in atmospheric circulation which need not be associated with a large change in climate here we present results from a new greenland ice core gisp2 showing that snow accumulation doubled rapidly from the younger dryas event to the subsequent preboreal interval possibly in one to three years we also find that the accumulation rate change from the oldest dryas to the bo11ing allerod warm period was large and abrupt the extreme rapidity of these changes in a variable that directly represents regional climate implies that the events at the end of the last glaciation may have been responses to some kind of threshold or trigger in the north atlantic climate system

characteristics of the heat balance of the greenland ice sheet for modelling les donnees du bilan thermique recoltees au cours de l egig de 1959 a 1967 sont appliquees au calcul du deplacement de la ligne d equilibre du au changement climatique

a new high precision borehole temperature logging system used at gisp2 greenland and taylor dome antarctica we describe a high precision 0 1 1 0 mk borehole temperature bt logging system developed at the united states geological survey usgs for use in remote polar regions we discuss calibration operational and data processing procedures and present an analysis of the measurement errors the system is modular to facilitate calibration procedures and field repairs by interchanging logging cables and temperature sensors measurements can be made in either shallow air filled boreholes or liquid filled holes up to 7 km deep data can be acquired in either incremental or continuous logging modes the precision of data collected by the new logging system is high enough to detect and quantify various thermal effects at the milli kelvin level to illustrate this capability we present sample data from the 3 km deep borehole at gisp2 greenland and from a 130m deep air filled hole at taylor dome antarctica the precision of the processed gisp2 continuous temperature logs is 0 25 0 34 mk while the accuracy is estimated to be 4 5 mk the effects of fluid convection and the dissipation of the thermal disturbance caused by drilling the borehole are clearly visible in the data the precision of the incremental taylor dome measurements varies from 0 11 to 0 32 mk depending on the wind strength during the experiments with this precision we found that temperature fluctuations and multi hour trends in the bt measurements correlate well with atmospheric pressure changes

the gisp2 δ18o climate record of the past 16 500 years and the role of the sun ocean and volcanoes measured 18o 16o ratios from the greenland ice sheet project 2 gisp2 ice core extending back to 16 500 cal yr b p provide a continuous record of climate change since the last glaciation high resolution annual 18o 16o results were obtained for most of the current millennium a d 818 1985 and record the medieval warm period the little ice age and a distinct 11 yr 18o 16o cycle volcanic aerosols depress central greenland annual temperature 1 5 c maximally and annual 18o 16o for about 4 yr after each major eruptive event on a bidecadal to millennial time scale the contribution of solar variability to holocene greenlandic temperature change is 0 4 c the role of thermohaline circulation change on climate problematic during the holocene is more distinct for the 16 500 10 000 cal yr b p interval analogous to 14c age calibration terminology we express time in calibrated cal yr b p a d 1950 0 cal yr b p the oldest dryas bolling older dryas allerod younger dryas sequence appears in great detail bidecadal variance in 18o 16o but not necessarily in temperature is enhanced during the last phase of lateglacial time and the younger dryas interval suggesting switches of air mass transport between jet stream branches the branched system is nearly instantaneously replaced at the beginning of the bolling and holocene at 14 670 and 11 650 cal yr b p respectively by an atmospheric circulation system in which 18o 16o and annual accumulation initially track each other closely thermodynamic considerations of the accumulation rate temperature relationship can be used to evaluate the 18 o 16o temperature relationship the gisp2 ice layer count years of major gisp2 climate transitions also support the use of coral 14c ages for age calibration

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

response of the east antarctica ice sheet to sea level rise a nonsteady ice flow model that explicitly includes longitudinal stresses is used to study the effects of marginal changes on the interior portions of an ice sheet sea level rise is found to cause a wave of thinning to propagate upglacier and this wave is slowed diffused and damped at the ice divide at dome c in central east antarctica post wisconsinan sea level rise is modeled as having caused 110 m thinning with response now 70 complete if accumulation rates increased by 10 at the same time then calculated net thinning has been only 75 m ice sheet response to changes in sea level and accumulation rate is relatively rapid response to temperature changes is an order of magnitude slower response times are shorter if the marginal position of an ice sheet is controlled

temperature accumulation and ice sheet elevation in central greenland through the last deglacial transition we present a combined heat and ice flow model constrained by measurements of temperature in the greenland ice sheet project 2 gisp2 borehole and by the gisp2 18o record and depth age scale which determines a history of temperature accumulation rate and ice sheet elevation for the past 50 000 years in central greenland important results are that the temperature increase from average glacial to holocene conditions was large approximately 15 c with a 20 c warming from late glacial to holocene that the average accumulation rate during the last glacial maximum between 15 and 30 kyr b p was 5 5 to 7 cm yr 1 approximately 25 of the modern accumulation rate that long term 500 1000 years averaged accumulation rate and temperature have been inversely correlated during the most recent 7 millennia of the holocene and that the greenland ice sheet probably thickened during the deglacial transition the inverse correlation of accumulation rate and temperature in the mid and late holocene suggests that the greenland ice sheet is more prone to volume reduction in a warmed climate than previously thought and demonstrates that accumulation rate is not a reliable proxy for temperature the elevation history of the ice sheet is poorly constrained by the model and independent evidence is needed we also present a simple estimate of the response time for thinning of the interior region of an ice sheet due to retreat of its margins this was approximately 1900 years for central greenland during deglaciation

the δ18o record along the greenland ice core project deep ice core and the problem of possible eemian climatic instability over 70 000 samples from the 3029 m long greenland ice core project grip ice core drilled on the top of the greenland ice sheet summit have been analyzed for 8o a highly detailed and continuous 8o profile has thus been obtained and is discussed in terms of past temperatures in greenland we also discuss a three core stacked annual 8o profile for the past 917 years the short term 50 years variability of the annual 8o signal is found to be 1 in the holocene and estimates for the coldest parts of the last glacial are 3 or higher these data also provide insights into possible disturbances of the stratigraphic layering in the core which seems to be sound down to the onset of the eemian spectral analysis of highly detailed sequences of the profile helps determine the smoothing of the 8o signal which for the holocene ice is found to be considerably stronger than expected we suggest this is due to a process involving diffusion of water molecules along crystal boundaries in the recrystallizing ice matrix deconvolution techniques were employed for restoring with great confidence the highly attenuated annual 8o signal in the holocene we confirm earlier findings of dramatic temperature changes in greenland during the last glacial cycle abrupt and strong climatic shifts are also found within the eem sangamon interglaciation which is normally recorded as a period of warm and stable climate in lower latitudes the stratigraphic continuity of the eemian layers is consequently discussed in section 3 of this paper in terms of all pertinent data which we are not able to reconcile

an early cambrian plume induced subduction initiation event within the junggar ocean insights from ophiolitic melanges arc magmatism and metamorphic rocks abstract the central asian orogenic belt caob is one of the largest accretionary orogens in the world the mechanism of continental growth and tectonic evolution of the caob remain debated here we present an overview of early paleozoic ophiolitic melanges calc alkaline intrusions and metamorphic rocks in west junggar with an aim to provide constraints on the time and mechanism of subduction initiation in the junggar ocean a branch of the southern paleo asian ocean pao the early paleozoic ophiolitic melanges are composed of ultramafic mafic rocks cherts pelagic limestones basaltic breccias and tuffs the mafic rocks from these ophiolitic melanges are divided into morb like and oib like types the morb like rocks were formed in a fore arc setting but the oib like mafic rocks were formed by the intra plate magmatism related to mantle plume activities the early paleozoic intrusions are occurred as small stocks with a dominant composition of diorite trondhjemite and granodiorite these granitoids display high k calc alkaline affinities and have high and positive end t and ehf t values formed in an arc related setting the metamorphic complex is mainly composed of blueschist and amphibolite blocks with metamorphism ages ranging from 500 ma to 460 ma their protoliths are calc alkaline andesite and tholeiitic and alkaline basalts formed in an arc related and seamount setting respectively it is clearly show that the west junggar was under an intra oceanic subduction regime during the early paleozoic and the initial subduction of the southern pao might have occurred in the early cambrian based on our observations and in combination with previous work we propose the plume induced subduction initiation model for the early middle cambrian tectonic evolutionary of the junggar ocean our new model not only shed light on subduction initiation dynamics of the southern pao but also contribute to tectonic evolution of the caob

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

air content along the greenland ice core project core a record of surface climatic parameters and elevation in central greenland we present here measurements of the air content of the ice v performed along the greenland ice core project grip ice core the main features of the long term trends are 1 a decrease of 13 between the last glacial maximum lgm and the earliest part of the holocene and 2 an increase of 8 during the holocene the results are discussed in terms of changes in atmospheric pressure surface elevation and porosity at close off the v record contains a significant signal of past changes of surface elevation in qualitative agreement with ice sheet modeling simulations it suggests a thickening of central greenland during the transition from the lgm to the early holocene and a significant thinning through the holocene period it also stresses the large influence on past v variations of changes in ice porosity which are not explained by the present day spatial relationship with temperature and may reflect changes in other surface climatic parameters like precipitation seasonality or wind stress the potential role of temporal variations of atmospheric pressure patterns is also evaluated air content results in the grip ice older than 110 ka indicate values approximately in the same range as those observed during the last 40 000 years with generally higher air content corresponding to isotopically warmer ice

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

catch a falling star meteorites and old ice a model for the process of meteorite concentration in blue ice regions of the antarctic ice sheet is proposed based on data from near the allan hills and the assumptions that both meteorite influx and glacial flow have been constant the meteorite influx is calculated to be 60 x 10 6 kilogram per square kilometer per year and the age of the exposed ice to be 0 to 600 000 years varying with distance from the allan hills these results are in line with other estimates of influx rate and with measurements of the terrestrial ages of the meteorites providing support for the assumption of steady flow and meteorite influx this may be the oldest sequence of ice in stratigraphic order yet discovered and the results imply that this part of the east antarctic ice sheet has been approximately steady during this time interval

reaction of the accumulation zone portions of glaciers to climatic change the response of the accumulation regions of glaciers to changes in accumulation rate and in surface temperature is calculated by considering perturbations to the ice flow the analysis is limited to glaciers in which flow is dominated by internal shear and not by bottom sliding and for which flowlines are geographically parallel in general glaciers begin to alter thickness immediately after a change in accumulation rate but the effect of a change in surface temperature is delayed by the time for this temperature change to penetrate to depth in the glacier a warming leads to glacial thinning the amount and timing of the response is very different for different glaciers characteristic times are on the order of tens of years for mountain glaciers and tens of thousands of years for the east antarctic ice sheet as an example the theory is applied to the ice sheet near byrd station antarctica for hypothetical changes in surface accumulation rate and temperature rates and amounts of thickness change are calculated and it is found that the measured thinning is probably too fast to have been caused by climatic variation alone the effects of hypothetical climatic variations on the depth age and depth temperature relationships through the ice sheet are also calculated it is hoped that the theory will prove valuable in the interpretation of future drill hole results