molar gas ratios of air entrapped in ice a new tool to determine the origin of relict massive ground ice bodies in permafrost abstract the molar ratios of atmospheric gases change during dissolution in water due to differences in their relative solubilities we exploited this characteristic to develop a tool to clarify the origin of ice formations in permafrost regions extracted from ice molar gas ratios can distinguish buried glacier ice from intrasedimental ground ice formed by freezing groundwaters an extraction line was built to isolate gases from ice by melting and trapping with liquid he followed by analysis of n2 o2 ar 18oo2 and 15nn2 by continuous flow mass spectrometry the method was tested using glacier ice aufeis ice river icing and intrasedimental ground ice from sites in the canadian arctic o2 ar and n2 ar ratios clearly distinguish between atmospheric gas in glacial ice and gases from intrasedimental ground ice which are exsolved from freezing water 15nn2 and 18oo2 in glacier ice aufeis ice and intrasedimental ground ice do not show clear distinguishing trends as they are affected by various physical processes during formation such as gravitational settling excess air addition mixing with snow pack and respiration

deep air convection in the firn at a zero accumulation site central antarctica abstract ice cores provide unique archives of past atmospheres and climate but interpretation of trapped gas records and their climatic significance has been hampered by a poor knowledge of the prevalence of air convection in the firn layer on top of polar ice sheets in particular the phasing of greenhouse gases and climate from ice cores has been obscured by a discrepancy between empirical and model based estimates of the age difference between trapped gases and enclosing ice which may be due to air convection here we show that deep air convection 23 m occurs at a windy near zero accumulation rate site in central antarctica known informally as the megadunes site 80 77914 s 124 48796 e deep convection is evident in depth profiles of air withdrawn from the firn layer in the observed pattern of the nitrogen isotope ratio 15 n 14 n the argon isotope ratio 40 ar 36 ar and in the mixing ratios of the anthropogenic halocarbons methyl chloroform ch 3 ccl 3 and hfc 134a ch 2 fcf 3 transport parameters diffusivities were inferred and air was dated using measured carbon dioxide co 2 and methane ch 4 mixing ratios by comparing with the law dome atmospheric record which shows that these are the oldest firn air samples ever recovered co 2 mean age 1863 ad the low accumulation rate and the consequent intense metamorphism of the firn due to prolonged exposure to seasonal temperature cycling likely contribute to deep air convection via large grain size and vertical cracks that act as conduits for vigorous air motion the megadunes site provides a possible modern analog for the glacial conditions in the vostok dome fuji and dome c ice core records and a possible explanation for lower than expected 15 n 14 n ratios in trapped air bubbles at these times a general conclusion is that very low accumulation rate causes deep air convection via its effect on firn structural characteristics

10 be in ice at vostok antarctica during the last climatic cycle we have previously reported accelerator mass spectrometer measurements of the cosmogenic isotope 10be half life 1 5 myr in an 906 m ice core recovered at dome c antarctica1 in addition to an enhanced concentration during a period of reduced solar activity maunder minimum ad 1645 1715 2 we found substantially increased concentrations during that part of the core corresponding to the last ice age both effects have been confirmed and extended in greenland ice cores3 4 the recovery of a 2 083 m ice core at vostok antarctica5 together with an extended oxygen isotope chronology reported elsewhere in this issue6 permits an extension of 10be studies over the whole of the last climatic cycle 125 kyr we report here such measurements which show an excellent correlation with the oxygen isotope and thus presumably climate record over the whole of this period the results imply that precipitation rates in the antarctic during the last interglacial were similar to those of the holocene but were roughly halved during the last glaciation

on the cause of the asymmetric distribution of seamounts about the juan de fuca ridge ridge crest migration over a heterogeneous asthenosphere abstract the distribution of non hotspot seamounts in the northeast pacific is highly asymmetric small seamount chains and isolated edifices are numerous on the pacific plate but nearly absent on the juan de fuca plate we propose a hypothesis for the asymmetric generation of seamounts near a ridge axis in which upwelling and early melting of upper mantle heterogeneities occurin advance of a spreading centre which migrates with respect to the asthenospheric frame of reference plate motion solutions indicate that the juan de fuca ridge is migrating to the west at a ridge perpendicular rate of 20 mm a 1 which is large compared to the half spreading rate of 30 mm a 1 migration of the ridge axis will result in the initiation of upwelling in the upper mantle in advance of the spreading centre if slightly enriched lower melting temperature heterogeneities are present in the upper mantle they will intersect their solidus at a greater depth and will begin to melt earlier than the host peridotite seamount volcanism will occur preferentially on the pacific plate because of two factors firstly more asthenosphere and hence more early melt heterogeneities must ascend to supply the pacific plate which is translating more rapidly than the relatively stationary juan de fuca plate secondly the asthenosphere that is required to ascend to supply the thickening of the juan de fuca plate will have been flushed of its significant shallow level heterogeneities by the previous advance and passing of the ridge few should remain to cause volcanism on the juan de fuca plate testable physical and petrologic predictions of this model can be identified for example within a small seamount chain the age difference between individual seamounts and the crust on which they lie will decrease toward the ridge axis variation in lava chemistry along a small seamount chain which is interpreted to represent magmatism from a single large heterogeneity will provide the major discriminant of this hypothesis systematic variations should be observed with earlier formed edifices being constructed of more enriched lavas and later younger ones of more depleted lavas isotope ratios should be unaffected by varying degrees of partial melting but could display increased dilution towards the ridge axis as melting of the host mantle and magma mixing become significant

climate records covering the last deglaciation the oxygen 18 oxygen 16 ratio of molecular oxygen trapped in ice cores provides a time stratigraphic marker for transferring the absolute chronology for the greenland ice sheet project gisp ii ice core to the vostok and byrd ice cores in antarctica comparison of the climate records from these cores suggests that near the beginning of the last deglaciation warming in antarctica began approximately 3000 years before the onset of the warm bolling period in greenland atmospheric carbon dioxide and methane concentrations began to rise 2000 to 3000 years before the warming began in greenland and must have contributed to deglaciation and warming of temperate and boreal regions in the northern hemisphere

gas age ice age differences and the chronology of the vostok ice core 0 100 ka 1 gas is trapped in polar ice at depths of 50 120 m and is therefore significantly younger than the ice in which it is embedded the age difference is not well constrained for slowly accumulating ice on the east antarctic plateau introducing a significant uncertainty into chronologies of the oldest deep ice cores vostok dome fuji and dome c we recorrelate the gas records of vostok and greenland ice sheet project 2 gisp2 cores in part on the basis of new ch4 data and use these records to construct six vostok chronologies that use different assumptions to calculate gas age ice age differences we then evaluate these chronologies by comparing times of climate events at vostok with correlative events in the well dated byrd ice core west antarctica from this evaluation we identify two leading chronologies for the vostok core that are based on recent models of firn temperature firn densification and thinning of upstream ice one chronology involves calculating gas age ice age differences from these models the second new approach involves calculating ice depths in the core that are contemporaneous with depths in the same ice core whose gas ages are well constrained this latter approach circumvents problems associated with highly uncertain accumulation rates in the vostok core the uncertainty in vostok chronologies derived by correlating into the gisp2 gas record remains about 1 kyr and high precision correlations continue to be difficult

δ15n of n2 in air trapped in polar ice a tracer of gas transport in the firn and a possible constraint on ice age gas age differences with respect to gas transport the fim near the surface of an ice sheet can be divided into three zones the uppermost is a convective zone located just below the surface of the ice sheet in which the air is rapidly flushed by convective exchange with the overlying atmosphere below the convective zone there is a diffusive air column in which diffusion is rapid but there is no convection between the bottom of the diffusive air column and the bubble close off region there may be a nondiffusive zone in which diffusion is so slow that negligible gas transport occurs the diffusive air column is characterized by progressive enrichment with depth of 15n in n2 and heavy isotopes of gases in general as predicted using the barometric equation in this paper we present data on the 15n of n2 in recently trapped air samples from 12 ice cores along with numerous downcore samples from byrd vostok and dome c bubble close off depths for these cores calculated from a densification model ranged from 51 to 114 meters below the surface mbs we used these data and the barometric equation to calculate the thickness of the diffusive air column and found that it comprised 46 to 93 of the total firn thickness at our study sites paleo close off depths calculated from the densification model for glacial sections of byrd vostok and dome c are 15 25 m deeper than close off depths today diffusive column heights calculated from 15n varied in a more complex manner the diffusive column height at byrd appears to have decreased from 74 m during the last glacial period to 50 m during the holocene at vostok and dome c the diffusive column height calculated from 15n increase from about 65 m during the last glacial period to about 80 m in the holocene we use records of surface temperature and co2 at byrd and vostok along with their respective chronologies to constrain the ice age gas age difference age throughout the section of the vostok ice core corresponding to the last glacial termination in principle age values calculated from these data can be used to discriminate whether gas in the fim mixes to the bubble close off depth or to a depth equivalent to the diffusive column height in practice however uncertainties in the chronology of byrd and vostok are too great to allow us to distinguish between these two possibilities one can only say that at the time of the last termination age for vostok was between 3 and 10 5 kyr previous estimates fall within this range

evidence of microbial consortia metabolizing within a low latitude mountain glacier carbon dioxide co 2 nitrous oxide n 2 o and methane ch 4 concentrations of ice samples from a south american mountain glacier are larger than those in coeval polar ice cores of the six possible mechanisms that may cause the composition of air measured in ice cores to depart from that of the overlying troposphere gravitational fractionation thermal fractionation gases trapped in refrozen meltwater atmospheric heterogeneity abiotic chemical reactions and biologically mediated chemical reactions only metabolism of a consortium of microorganisms within the ice can explain our data

convective mixing of air in firn at four polar sites air withdrawn from the firn at four polar sites dome fuji h72 and ym85 antarctica and north grip greenland was measured for 15 no f n2 and 18 oo f o2 to test for the presence of convective air mixing in the top part of the firn known as the convective zone understanding the convective zone and its possible relationship to surface conditions is important for constructing accurate ice core greenhouse gas chronologies and their phasing with respect to climate change the thickness of the convective zone was inferred from a regression line with barometric slope of the data in the deep firn it is less than a few meters at h72 and ngrip whereas a substantial convective zone is found at dome fuji 8 6 2 6 m and ym85 14 0 1 8 m by matching the outputs of a diffusion model to the data effective eddy diffusivities required to mix the firn air are found at the surface of dome fuji and ym85 these are found to be several times greater than the molecular diffusivity in free air the crossover from dominance of convection to molecular diffusion takes place at 7 2 11 2 and 0 5 0 5 m at dome fuji ym85 and ngrip respectively these depths can be used as an alternative definition of the convective zone thickness the firn permeability at dome fuji is expected to be high because of intense firn metamorphism due to the low accumulation rate and large seasonal air temperature variation at the site the firn layers in the top several meters are exposed to strong temperature gradients for several decades leading to large firn grains and depth hoar that enhance permeability the thick convective zone at ym85 is unexpected because the temperature accumulation rate and near surface density are comparable to ngrip the strong katabatic wind at ym85 is probably responsible for creating the deep convection the largest convective zone found in this study is still only half of the current inconsistency implied from the deep ice core gas isotopes and firn densification models 2006 elsevier b v all rights reserved

gas records from the west greenland ice margin covering the last glacial termination a horizontal ice core abstract certain sites along ice sheet margins provide an easily accessible and almost unlimited supply of ancient ice at the surface measurements of gases in trapped air from ice outcropping at pakitsoq west greenland demonstrate that ancient air is mostly well preserved no alterations in 18 o atm and 15 n of n 2 are apparent and alterations in methane are found in only a few ice sections using measurements of these gases we have unambiguously identified a stratigraphic section containing ice from the end of last glacial period as well as bolling allerod younger dryas and preboreal intervals extensive sections of ice from the holocene and most ages within the last glacial period are probably also present very accurate dating has been possible in the ice section containing the younger dryas preboreal abrupt climate transition signal the ice at pakitsoq is folded and non uniformly thinned with many cross cutting bands of bubble free ice and dust the cross cutting features are associated with anomalies in both the gas and the ice records with careful sampling to avoid these the ice at pakitsoq is suitable for recovery of large volume samples of the ancient atmosphere for analysis of trace constituents such as 14 ch 4

delta15n measurements as a calibration tool for the paleothermometer and gas ice age differences a case study for the 8200 b p event on grip ice isotopic composition measurement on atmospheric nitrogen 15n trapped in ice cores is established as a gravitational enrichment indicator recently it was discovered that thermal diffusion has a significant influence on this parameter during periods of large temperature fluctuations severinghaus et al 1998 here we present results for the 8200 b p event the largest temperature excursion during the holocene which underpin this view and extend it to medium range temperature changes the thermal diffusion component is about one third of the gravitational enrichment for this event the shortness of this cooling event does not significantly affect the gravitational enrichment as documented by rather constant close off depths therefore the 15n residuals can be directly used to estimate temperature changes hence calibrating the paleothermometer since our 15n record can resolve temperature variations as short as a few decades it complements the calibration technique of borehole temperature measurements for which the resolution decreases rapidly with increasing age we obtained a relation 18oice temperature of 0 30 0 11 k which agrees nicely with the temporal dependence for cold periods about 0 30 k and is significantly lower than today s greenland spatial relation 0 67 k this corresponds to a range of the surface temperature shift for the 8200 b p event in central greenland between 5 4 and 11 7 k with a best estimate of 7 4 k by matching the 15n variations with 18oice variations we estimate the mean gas ice age difference to be 209 15 years which is in good agreement with model estimates

observing and modeling the influence of layering on bubble trapping in polar firn interpretation of ice core trace gas records depends on an accurate understanding of the processes that smooth the atmospheric signal in the firn much work has been done to understand the processes affecting air transport in the open pores of the firn but a paucity of data from air trapped in bubbles in the firn ice transition region has limited the ability to constrain the effect of bubble closure processes here we present high resolution measurements of firn density methane concentrations nitrogen isotopes and total air content that show layering in the firn ice transition region at the west antarctic ice sheet wais divide ice core site using the notion that bubble trapping is a stochastic process we derive a new parameterization for closed porosity that incorporates the effects of layering in a steady state firn modeling approach we include the process of bubble trapping into an open porosity firn air transport model and obtain a good fit to the firn core data we find that layering broadens the depth range over which bubbles are trapped widens the modeled gas age distribution of air in closed bubbles reduces the mean gas age of air in closed bubbles and introduces stratigraphic irregularities in the gas age scale that have a peak to peak variability of 10 years at wais divide for a more complete understanding of gas occlusion and its impact on ice core records we suggest that this experiment be repeated at sites climatically different from wais divide for example on the east antarctic plateau

synchronising edml and northgrip ice cores using δ18o of atmospheric oxygen δ18oatm and ch4 measurements over mis5 80 123 kyr abstract water isotope records from the epica dronning maud land edml and the northgrip ice cores have revealed a one to one coupling between antarctic isotope maxima aim and greenland dansgaard oeschger do events back to 50 kyr in order to explore if this north south coupling is persistent over marine isotopic stage 5 mis 5 a common timescale must first be constructed here we present new records of 18o of o2 18oatm and methane ch4 measured in the air trapped in ice from the edml 68 147 kyr and northgrip 70 123 kyr ice cores we demonstrate that through the period of interest ch4 records alone are not sufficient to construct a common gas timescale between the two cores millennial scale variations of 18oatm are evidenced over mis 5 both on the antarctic and greenland ice cores and are coupled to ch4 profiles to synchronise the northgrip and edml records they are shown to be a precious tool for ice core synchronisation with this new dating strategy we produce the first continuous and accurate sequence of the north south climatic dynamics on a common ice timescale for the last glacial inception and the first do events of mis 5 reducing relative dating uncertainties to an accuracy of a few centuries at the onset of do events 24 to 20 this edml northgrip synchronisation provides new firm evidence that the bipolar seesaw is a pervasive pattern from the beginning of the glacial period the relationship between antarctic warming amplitudes and their concurrent greenland stadial duration highlights the particularity of do event 21 and its antarctic counterpart our results suggest a smaller southern ocean warming rate for this long do event compared to do events of mis 3

densification of layered firn of the ice sheet at neem greenland densification of firn at the north greenland eemian ice drilling neem camp is investigated using density surrogates dielectric permittivities v and h at microwave frequencies with electrical fields in the vertical and horizontal planes respectively dielectric anisotropy v h is then examined as a surrogate for the anisotropic geometry of firn its size fluctuations and mutual correlations are investigated in samples taken at depths from the surface to 90m the initial of 0 06appears within the uppermost 0 2 m after that decreases rapidly until 21 26 m depth below this decreases slowly layers with more ions of fluorine chlorine and some cations deposited between the autumn and the subsequent summer deform preferentially during all these stages this layered deformation is explained partly by the textural effects initially formed by the seasonal variation of metamorphism and partly by ions such as fluorine chlorine and ammonium which are known to modulate dislocation movement in the ice crystal lattice insolation sensitive microstructure appears to be preserved all the way to the pore close off within layers of the summer to autumn metamorphism like previous authors we hypothesize that calcium is not the active agent in the reported deformation calcium correlations

age difference between polar ice and the air trapped in its bubbles air entrapped in bubbles formed in cold ice has essentially the same composition as that of the atmosphere at the time of bubble formation the analysis of dated ice samples therefore enables the history of atmospheric composition to be investigated 1 3 the age of the entrapped air is however not the same as that of the surrounding ice because air bubbles only become isolated from the atmosphere during the transition from firn to ice typically the age of the ice at this transition is between 100 and 3 000 yr depending mainly on firn temperature and snow accumulation rate the mean age difference between ice and enclosed air as well as the age distribution width for a given sample are especially important for the investigation of the anthropogenic increase of co2 and trace gases in the atmosphere over the last centuries and for the comparison of climatic parameters recorded in the ice with parameters recorded in the bubbles for siple station antarctica this age difference and age distribution width were deduced from the bubble volume measured as a function of depth the values are 95 yr and 22 yr respectively

firn air δ15n in modern polar sites and glacial interglacial ice a model data mismatch during glacial periods in antarctica the phase lag between atmospheric composition air bubbles and temperature water isotopes can be quantified from ice cores provided that the age difference between entrapped air and the surrounding air can be correctly estimated this difference depends on the lock in depth lid when air no longer mixes with the atmosphere the lid can be estimated from firnification models or from the air isotopic composition 15n and 40ar both methods give consistent results for greenland and one coastal site in antarctica byrd new firn measurements in greenland northgrip and antarctica berkner island bas depot dome c confirm that firnification models correctly reproduce the present lid over a large range of surface conditions however a systematic mismatch is observed for the last glacial maximum lgm in east antarctic sites vostok dome c dome f questioning the model s validity here we use new 15n measurements from two coastal antarctic sites kohnen station and law dome providing depth estimates again distinct from firnification model calculations we show that this discrepancy can be resolved by revising the estimate of past accumulation rates 15n measurements can therefore help to constrain past accumulation rate and improve ice core dating

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

modeling the densification of polar firn including heat diffusion application to close off characteristics and gas isotopic fractionation for antarctica and greenland sites 1 modeling the densification of polar firn is essential to estimate variations of close off characteristics density close off depth delta age in relation with past climate parameters temperature and accumulation rates furthermore the air composition in the firn is modified by gravitational and thermal fractionation and stable isotope measurements of permanent gases like nitrogen or argon can provide information on the amplitude of these fractionations in this work we propose a new model coupling firn densification and heat diffusion in addition to the determination of the firn thickness and gas ice age differences the model allows a reconstruction of the time evolution of the temperature for different sites in antarctica vostok and greenland gisp2 and therefore the evolution of gravitational and thermal isotopic fractionations in firn under present day conditions the modeled profiles are in good agreement with the available temperature measurements in firn for sites with low accumulation rates such as vostok the results show the existence of temperature gradients in the firn column even when no rapid climatic changes occur the comparison of the modeled 15n results to measurements allows to better constrain the 18o temperature relationship used to infer the surface temperature history and for gisp2 the model validates the long term borehole based temperature

a comparison of deep antarctic ice cores and their implications for climate between 65 000 and 15 000 years ago abstract three ice cores drilled in the central part of the antarctic continent extend back to the last glacial period one from west antarctica byrd and two from east antarctica vostok and dome c this period is also partly covered by a few cores from the coastal areas in these cores climatic information is mostly derived from the isotopic profiles d or 18 o from which surface temperature and more indirectly precipitation rate can be estimated the main objective has been to compare thoroughly the three deep ice cores for the main part of the last glacial period from ca 65 000 15 000 yr b p the time scales have been examined in detail and a new 40 000 yr chronology for the dome c core adopted special emphasis is placed on the link between the concentration of 10 be and past accumulation changes and on the use of peaks in the concentration of this cosmogenic isotope as stratigraphic markers elevation changes of the ice sheet derived from gas content and isotopic data bear directly on interpretations of past temperature and precipitation rate changes

paleomagnetic directions and thermoluminescence dating from a bread oven floor sequence in lubeck germany a record of 450 years of geomagnetic secular variation 1 a record of about 450 years of geomagnetic secular variation is presented from a single archaeological site in lubeck germany where a sequence of 25 bread oven floors has been preserved in a bakery from medieval times until today the age dating of the oven floor sequence is based on historical documents 14c dating and thermoluminescence dating it confines the time interval from about 1300 to 1800 a d paleomagnetic directions have been determined from each oven floor by means of 198 oriented hand samples after alternating field as well as thermal demagnetization experiments the characteristic remanent magnetization direction was obtained using principal component analysis the mean directions of 24 oven floors are characterized by high fisherian precision parameters 146 and small 95 confidence limits 1 2 4 6 for obtaining a smooth curve of geomagnetic secular variation for lubeck a spherical spline function was fitted to the data using a bayesian approach which considers not only the obtained ages but also stratigraphic order correlation with historical magnetic records suggests that the age estimation for the upper 10 layers was too young and must date from the end of the sixteenth to the mid of the eighteenth century for the lowermost 14 layers dating is reliable and provides a secular variation curve for germany the inclination shows a minimum in the fourteenth century and then increases by more than 10 declination shows a local minimum around 1400 a d followed by a maximum in the seventeenth century this is followed by the movement of declination about 30 to western directions

the layered character of snow covers snow studies have generally ignored the layered nature of snow covers having achieved a good understanding of the properties and processes in homogeneous snow snow scientists should develop more insight into the evolution of the layers and their effects on overall snow response many of the outstanding problems in snow studies can only be solved by dealing with snow as a layered medium the various mechanisms by which layers are generated their effects and some of the outstanding research problems are described here

the δ18o of atmospheric o2 from air inclusions in the vostok ice core timing of co2 and ice volume changes during the penultimate deglaciation one important question about the co2 climate connection is how increasing atmospheric pco2 levels affect climate and thereby the mass balance of continental ice sheets a record of atmospheric co2 variations over the last 160 000 years has recently been constructed by analyzing the trapped gas in the vostok ice core barnola et al 1987 the relationship between changes in atmospheric co2 and the size of the continental ice sheets has been difficult to ascertain because the co2 record is obtained from ice cores while the ice volume record has been constructed from the stable isotopic composition of biogenic caco3 in deep sea sediment cores in order to compare these two records in a more precise manner we present a record of the isotopic composition 18o of atmospheric o2 trapped in the vostok ice core and propose that it may be considered a proxy for the 18o of seawater and hence ice volume having a record of atmospheric co2 along with a continental ice volume proxy in trapped air in the same ice core allows us to compare the timing of changes in these two parameters with little uncertainty in the relative ages of important events our results suggest that during the penultimate glacial termination atmospheric co2 began to increase at least 3 kyr before the initial introduction of meltwater to the oceans possible errors in the relative age depth curve of the vostok ice core and uncertainties in the influence of biological and hydrologic processes on the 18o of atmospheric o2 introduce some uncertainty into our conclusions however our results are in general agreement with the observed phase relationship between atmospheric co2 and ice volume imbrie et al 1984 imbrie et al 1989 inferred from changes in the 13c difference between contemporaneous planktonic and benthic forams deposited in deep sea cores and the specmap ice volume record

a method for precise measurement of argon 40 36 and krypton argon ratios in trapped air in polar ice with applications to past firn thickness and abrupt climate change in greenland and at siple dome antarctica we describe a method for measuring the 40ar 36ar ratio and the 84kr 36ar ratio in air from bubbles trapped in ice cores these ratios can provide constraints on the past thickness of the firn layer at the ice core site and on the magnitude of past rapid temperature variations when combined with measured 15n 14n both variables contribute to paleoclimatic studies and ultimately to the understanding of the controls on earth s climate the overall precision of the 40ar 36ar method 1 standard error of the mean is 0 012 for a sample analyzed in duplicate corresponding to 0 6 m in reconstructed firn thickness we use conventional dynamic isotope ratio mass spectrometry with minor modifications and special gas handling techniques designed to avoid fractionation about 100 g of ice is used for a duplicate pair of analyses an example of the technique applied to the gisp2 ice core yields an estimate of 11 3k of abrupt warming at the end of the last glacial period 15 000 years ago the krypton argon ratio can provide a diagnostic of argon leakage out of the bubbles which may happen naturally during bubble close off or artifactually if samples are warmed near the freezing point during core retrieval or storage argon leakage may fractionate the remaining 40ar 36ar ratio by 0 007 per change in 84kr 36ar introducing a possible bias in reconstructed firn thickness of about 2 m if thermal diffusion is not accounted for or 6 m if thermal diffusion effects are quantified with measured 15n 14n reproducibility of 84kr 36ar measured in air is about 0 2 1 standard error of the mean but is about 1 for ice core samples ice core samples are systematically enriched in 84kr 36ar relative to atmosphere by 5 probably reflecting preferential size dependent exclusion of the smaller argon atom during bubble entrapment recent results from the siple dome ice core reveal two climate events during the last deglaciation including an 18 m reduction in firn thickness associated with an abrupt warming at sometime between 18 and 22 kyr bp and a partial or total removal of the firn during an ablation event at 15 3 kyr bp

synchronous climate changes in antarctica and the north atlantic central greenland ice cores provide evidence of abrupt changes in climate over the past 100 000 years many of these changes have also been identified in sedimentary and geochemical signatures in deep sea sediment cores from the north atlantic confirming the link between millennial scale climate variability and ocean thermohaline circulation it is shown here that two of the most prominent north atlantic events the rapid warming that marks the end of the last glacial period and the bolling allerod younger dryas oscillation are also recorded in an ice core from taylor dome in the western ross sea sector of antarctica this result contrasts with evidence from ice cores in other regions of antarctica which show an asynchronous response between the northern and southern hemispheres

greenland temperature response to climate forcing during the last deglaciation greenland ice core water isotopic composition 18 o provides detailed evidence for abrupt climate changes but is by itself insufficient for quantitative reconstruction of past temperatures and their spatial patterns we investigate greenland temperature evolution during the last deglaciation using independent reconstructions from three ice cores and simulations with a coupled ocean atmosphere climate model contrary to the traditional 18 o interpretation the younger dryas period was 4 5 2 c warmer than the oldest dryas due to increased carbon dioxide forcing and summer insolation the magnitude of abrupt temperature changes is larger in central greenland 9 to 14 c than in the northwest 5 to 9 c fingerprinting a north atlantic origin simulated changes in temperature seasonality closely track changes in the atlantic overturning strength and support the hypothesis that abrupt climate change is mostly a winter phenomenon

age scale of the air in the summit ice implication for glacial interglacial temperature change the air occluded in ice sheets and glaciers has in general a younger age defined as the time after its isolation from the atmosphere than the surrounding ice matrix because snow is first transformed into open porous firn in which the air can exchange with the atmosphere only at a certain depth firn ice transition the pores are pinched off and the air is definitely isolated from the atmosphere the firn ice transition depth is at around 70 m under present climatic conditions at summit central greenland the air at this depth is roughly 10 years old due to diffusive mixing whereas the ice is about 220 years old this results in an age difference between the air and the ice of 210 years this difference depends on temperature and accumulation rate and did thus not remain constant during the past we used a dynamic firn densification model to calculate the firn ice transition depth and the age of the ice at this depth and an air diffusion model to determine the age of the air at the transition past temperatures and accumulation rates have been deduced from the 18o record using time independent functions we present the results of model calculations of two paleotemperature scenarios yielding a record of the age difference between the air and the ice for the greenland ice core project grip and the greenland ice sheet project two gisp2 ice cores for the last 100 000 years during the holocene the age difference stayed rather stable around 200 years while it reached values up to 1400 years during the last glaciation for the colder scenario the model results are compared with age differences obtained independently by matching corresponding climate events in the methane and 18o records assuming a very small phase lag between variations in the greenland surface temperature and the atmospheric methane the past firn ice transition depths are compared with diffusive column heights obtained from 15n of n2 measurements the results of this study corroborate the large temperature change of 20 to 25 k from the coldest glacial to holocene climate found by evaluating borehole temperature profiles

gases in ice cores air trapped in glacial ice offers a means of reconstructing variations in the concentrations of atmospheric gases over time scales ranging from anthropogenic last 200 yr to glacial interglacial hundreds of thousands of years in this paper we review the glaciological processes by which air is trapped in the ice and discuss processes that fractionate gases in ice cores relative to the contemporaneous atmosphere we then summarize concentration time records for co2 and ch4 over the last 200 yr finally we summarize concentration time records for co2 and ch4 during the last two glacial interglacial cycles and their relation to records of global climate change

thermal fractionation of air in polar firn by seasonal temperature gradients 1 air withdrawn from the top 5 15 m of the polar snowpack firn shows anomalous enrichment of heavy gases during summer including inert gases following earlier work we ascribe this to thermal diffusion the tendency of a gas mixture to separate in a temperature gradient with heavier molecules migrating toward colder regions summer warmth creates a temperature gradient in the top few meters of the firn due to the thermal inertia of the underlying firn and causes gas fractionation by thermal diffusion here we explore and quantify this process further in order to 1 correct for bias caused by thermal diffusion in firn air and ice core air isotope records 2 help calibrate a new technique for measuring temperature change in ice core gas records based on thermal diffusion severinghaus et al 1998 and 3 address whether air in polar snow convects during winter and if so whether it creates a rectification of seasonality that could bias the ice core record we sampled air at 2 m depth intervals from the top 15 m of the firn at two antarctic sites siple dome and south pole including a winter sampling at the pole we analyzed 15n 14n 40ar 36ar 40ar 38ar 18o 16o of o2 o2 n2 84kr 36ar and 132xe 36ar the results show the expected pattern of fractionation and match a gas diffusion model based on first principles to within 30 although absolute values of thermal diffusion sensitivities cannot be determined from the data with precision relative values of different gas pairs may at siple dome 40ar 4 is 66 2 as sensitive to thermal diffusion as 15n in agreement with laboratory calibration 18o 2 is 83 3 and 84kr 48 is 33 3 as sensitive as 15n the corresponding figures for summer south pole are 64 2 81 3 and 34 3 accounting for atmospheric change the figure for o2 n2 4 is 90 3 at siple dome winter south pole shows a strong depletion of heavy gases as expected however the data do not fit the model well in the deeper part of the profile and yield a systematic drift with depth in relative thermal diffusion sensitivities except for kr constant at 34 4 suggesting the action of some other process that is not currently understood no evidence for wintertime convection or a rectifier effect is seen

estimation of temperature change and of gas age ice age difference 108 kyr b p at vostok antarctica air trapped in ice core bubbles provides our primary source of information about past atmospheres air isotopic composition 15n 14n and 40ar 36ar permits an estimate of the temperature shifts associated with abrupt climate changes because of isotope fractionation occurring in response to temperature gradients in the snow layer on top of polar ice sheets a rapid surface temperature change modifies temporarily the firn temperature gradient which causes a detectable anomaly in the isotopic composition of nitrogen and argon the location of this anomaly in depth characterizes the gas age ice age difference age during an abrupt event by correlation with the d or 18o anomaly in the ice we focus this study on the marine isotope stage 5d 5c transition 108 kyr b p a climate warming which was one of the most abrupt events in the vostok antarctica ice isotopic record petit et al 1999 a steplike decrease in 15n and 40ar 4 from 0 49 to 0 47 possibly a gravitational signal due to a change in firn thickness is preceded by a small but detectable 15n peak possibly a thermal diffusion signal we obtain an estimate of 5350 300 yr for age close to the model estimate of 5000 years obtained using the vostok glaciological timescale our results also suggest that the use of the present day spatial isotope temperature relationship slightly underestimates but by no more than 20 15 the vostok temperature change from present day at that time which is in contrast to the temperature estimate based on borehole temperature measurements in vostok which suggests that antarctic temperature changes are underestimated by up to 50 salamatin et al 1998

vostok ice core provides 160 000 year record of atmospheric co2 direct evidence of past atmospheric co2 changes has been extended to the past 160 000 years from the vostok ice core these changes are most notably an inherent phenomenon of change between glacial and interglacial periods besides this major 100 000 year cycle the co2 record seems to exhibit a cyclic change with a period of some 21 000 years

abrupt climate change around 22 ka on the siple coast of antarctica a new ice core from siple dome antarctica suggests the surface temperature increased by 6 c in just several decades at approximately 22 ka bp this abrupt change did not occur 500 km away in the byrd ice core or in climate proxy records in the siple dome core indicative of the mid latitude pacific this demonstrates there was significant spatial heterogeneity in the response of the antarctic climate during the last deglaciation and draws attention to unexplained mechanisms of abrupt climate change in antarctica

history of the greenland ice sheet paleoclimatic insights abstract paleoclimatic records show that the greenland ice sheet consistently has lost mass in response to warming and grown in response to cooling such changes have occurred even at times of slow or zero sea level change so changing sea level cannot have been the cause of at least some of the ice sheet changes in contrast there are no documented major ice sheet changes that occurred independent of temperature changes moreover snowfall has increased when the climate warmed but the ice sheet lost mass nonetheless increased accumulation in the ice sheet s center has not been sufficient to counteract increased melting and flow near the edges most documented forcings and ice sheet responses spanned periods of several thousand years but limited data also show rapid response to rapid forcings in particular regions near the ice margin have responded within decades however major changes of central regions of the ice sheet are thought to require centuries to millennia the paleoclimatic record does not yet strongly constrain how rapidly a major shrinkage or nearly complete loss of the ice sheet could occur the evidence suggests nearly total ice sheet loss may result from warming of more than a few degrees above mean 20th century values but this threshold is poorly defined perhaps as little as 2 c or more than 7 c paleoclimatic records are sufficiently sketchy that the ice sheet may have grown temporarily in response to warming or changes may have been induced by factors other than temperature without having been recorded

holocene accumulation and ice flow near the west antarctic ice sheet divide ice core site the west antarctic ice sheet divide core wdc provided a high resolution climate record from near the ross amundsen divide in central west antarctica in addition radar detected internal layers in the vicinity of the wdc site have been dated directly from the ice core to provide spatial variations in the age structure of the region using these two data sets together we first infer a high resolution holocene accumulation rate history from 9 2 thousand years of the ice core timescale and then confirm that this climate history is consistent with internal layers upstream of the core site even though the wdc was drilled only 24 kilometers from the modern ice divide advection of ice from upstream must be taken into account we evaluate histories of accumulation rate by using a flowband model to generate internal layers that we compare to observed layers results show that the centennially averaged accumulation rate was over 20 percent lower than modern at 9 2 thousand years before present b p increased by 40 percent from 9 2 to 2 3 thousand years b p and decreased by at least 10 percent over the past 2 thousand years b p to the modern values these holocene accumulation rate changes in central west antarctica are larger than changes inferred from east antarctic ice core records despite significant changes in accumulation rate throughout the holocene the regional accumulation pattern has likely remained similar to today and the ice divide position has likely remained on average within 5 kilometers of its modern position continent scale ice sheet models used for reconstructions of west antarctic ice volume should incorporate this accumulation history

firn processes and δ15n potential for a gas phase climate proxy abstract in order to quantify the sequence of events between changes in atmospheric composition and climate changes recorded in ice cores we must accurately account for the age difference between ice and gas at a given depth this gas age ice age difference depends on the age of the ice at the bottom of the firn layer where the bubbles are closed off firn densification models are used to calculate how this age difference varied in the past but have an uncertainty on the order of 1000 years for central antarctic sites here we explore the possibility that 15 n of n 2 is a gas phase proxy of climate which can be used to synchronize gas and ice records we present the 15 n record from the epica dome c edc ice core covering the last three glacial terminations and five glacial interglacial cycles between 300 and 800 ka previous studies have shown that gravitational settling enriches 15 n as a function of the diffusive column height in the firn if densification models prediction of deeper firn close off under glacial conditions is correct then we would expect heavier 15 n during glacial periods and a negative correlation with temperature instead edc 15 n is positively correlated with the ice deuterium content a proxy for temperature as previously reported at vostok dome fuji and epica dronning maud land we propose a mechanism that links accumulation rate firn permeability and convective mixing in the top meters of the firn to explain this correlation between 15 n and ice deuterium content the tightest correlation is observed over glacial terminations supporting the idea that 15 n is a property in the gas phase that records changes in surface conditions linked to deglacial warming

vostok ice core a continuous isotope temperature record over the last climatic cycle 160 000 years a continuous deuterium profile along the 160 000 year vostok ice core antarctica is interpreted in terms of atmospheric temperature changes this climatic record is the awaited terrestrial complement of the deep sea records supporting the existence of a relation between the pleistocene climate and orbital forcing

rapid late glacial atmospheric co2 changes reconstructed from the stomatal density record of fossil leaves the younger dryas stadial 11 000 10 000 yr bp was an abrupt return to a glacial climate during the termination of the last glaciation we have reconstructed atmospheric co2 concentrations from a high resolution sequence of fossil salix herbacea leaves through this climatic oscillation from krakenes western norway using the relationship between leaf stomatal density and atmospheric co2 concentration high allerod co2 values median 273 ppmv decreased rapidly during 130 200 14c years of the late allerod to ca 210 ppmv at the start of the younger dryas they then increased steadily through the younger dryas reaching typical interglacial values once more ca 275 ppmv in the holocene the rapid late allerod decrease in co2 concentration preceded the younger dryas temperature drop possibly by several decades this striking pattern of changes has not so far been recorded unambiguously in temporally coarse measurements of atmospheric co2 from ice cores our observed late glacial co2 changes have implications for global modelling of the ocean atmosphere biosphere over the last glacial interglacial transition

co 2 record in the byrd ice core 50 000 5 000 years bp the analysis of air in polar ice cores revealed 30 lower co2 values during glacial periods than during interglacial periods at present this is confirmed by results from six different ice cores two from greenland and four from antarctica1 5 in all cores the co2 change coincides with the change in the isotopic composition of the ice expressed as either the 18o or d ratio both are indicators for the mean annual surface temperature6 to investigate the relationship between atmospheric co2 concentration changes with changes in climate the atmospheric co2 concentration during and at the end of the last glaciation has to be known in detail to achieve this we have studied a great number of samples from the deep ice core from byrd station westantarctica drilled in 1968 these measurements allow us to reconstruct the atmospheric co02 concentration in the time period 50 000 15 000 yr bp in great detail

fractionation of gases in polar ice during bubble close off new constraints from firn air ne kr and xe observations gas ratios in air withdrawn from polar firn snowpack show systematic enrichments of ne n2 o 2 n2 and ar n2 in the firn ice transition region where bubbles are closing off air from the bubbles in polar ice is correspondingly depleted in these ratios after accounting for gravitational effects gas in the bubbles becomes fractionated during the process of bubble close off and fractionation may continue as ice cores are stored prior to analysis we present results from firn air studies at south pole and siple dome antarctica which add ne kr and xe measurements to the suite of observations ne o2 and ar appear to be preferentially excluded from the shrinking and occluding bubbles and these gases therefore accumulate in the residual firn air creating a progressive enrichment with time and depth in firn air early sealing of gases by thin horizontal impermeable layers into a nondiffusive zone or lock in zone greatly enhances this enrichment a simple model of the bubble close off fractionation and lock in zone enrichment fits the data adequately the model presumes that fractionation is caused by selective permeation of gas through the ice lattice from slightly overpressured bubbles the effect appears to be size dependent because ne o2 and ar have smaller effective molecular diameters than n2 and fractionation increases strongly with decreasing size ne is fractionated 34 2 times more than o2 in south pole firn air and reaches an enrichment of 90 in the deepest sample the large atoms kr and xe do not appear to be fractionated by this process despite the large size difference between the two gases suggesting a threshold atomic diameter of 3 6a above which the probability becomes very small that the gas will escape from the bubble these findings have implications for ice core and firn air studies that use gas ratios to infer paleotemperature chronology and past atmospheric composition

multidecadal variability of atmospheric methane 1000 1800 c e 1 we present a new high precision high resolution record of atmospheric methane from the west antarctic ice sheet wais divide ice core covering 1000 1800 c e a time period known as the late preindustrial holocene lpih the results are consistent with previous measurements from the law dome ice core the only other high resolution record of methane for this time period and confirm most of the observed variability multidecadal variability in methane concentrations throughout the lpih is weakly correlated or uncorrelated with reconstructions of temperature and precipitation from a variety of geographic regions correlations with temperature are dominated by changes in northern hemisphere high latitude temperatures between 1400 and 1600 c e during the onset of the little ice age times of war and plague when large population losses could have reduced anthropogenic emissions are coincident with short periods of decreasing global methane concentrations

evidence for two intervals of enhanced 10 be deposition in antarctic ice during the last glacial period we have previously reported concentration profiles of cosmic ray produced cosmogenic 10be in deep ice cores from dome c and vostok antarctica1 2 in both these cores we found a concentration of 10be approximately two times larger in ice from the late glacial period than in the holocene ice we interpreted this as probably resulting from a lower precipitation rate on the antarctic plateau during glacial periods compared to interglacial periods in the vostok profile there was one sample corresponding to 60 000 yr bp which gave an unusually large 10be concentration not correlated with any obvious climatic event we suggested that this sample might be reflecting increased 10be production as for example during a period of reduced solar modulation2 we have now measured a much more detailed concentration profile for 10be in the vostok core the results confirm a 10be peak lasting 1 000 2 000 years at 60 000 yr bp and show another similar peak at 35 000 yr bp we have also observed the latter peak in the dome c core we discuss possible sources for these peaks and their potential as stratigraphic markers

surface winds over the antarctic continent a review surface winds over antarctica often classified as katabatic are intimately related to the orientation and steepness of the underlying ice terrain such flows result primarily from the strong radiational cooling of the sloping ice surface which creates a downslope directed horizontal pressure gradient force it is possible to diagnose the time averaged near surface streamline patterns of cold air drainage currents using recently compiled detailed antarctic ice topography maps and appropriate estimates of the strength of the surface temperature inversion the resulting streamlines agree with available observations and illustrate the high degree of nonuniformity of gravity driven flows off the ice sheets in certain interior sections negatively buoyant air from a large catchment area drains through a restricted section of coastline such confluence zones represent regions of enhanced supplies of cold air and are shown to be related to anomalously strong katabatic winds implications for sea ice formation are discussed

a 150 000 year climatic record from antarctic ice during much of the quaternary the earth s climate has undergone drastic changes most notably successive glacial and interglacial episodes the past 150 kyr includes such a climatic cycle the last interglacial the last glacial and the present holocene interglacial a new climatic time series for this period has been obtained using 18 o data from an antarctic ice core

deglacial temperature history of west antarctica the most recent glacial to interglacial transition constitutes a remarkable natural experiment for learning how earth s climate responds to various forcings including a rise in atmospheric co2 this transition has left a direct thermal remnant in the polar ice sheets where the exceptional purity and continual accumulation of ice permit analyses not possible in other settings for antarctica the deglacial warming has previously been constrained only by the water isotopic composition in ice cores without an absolute thermometric assessment of the isotopes sensitivity to temperature to overcome this limitation we measured temperatures in a deep borehole and analyzed them together with ice core data to reconstruct the surface temperature history of west antarctica the deglacial warming was formula see text c approximately two to three times the global average in agreement with theoretical expectations for antarctic amplification of planetary temperature changes consistent with evidence from glacier retreat in southern hemisphere mountain ranges the antarctic warming was mostly completed by 15 kybp several millennia earlier than in the northern hemisphere these results constrain the role of variable oceanic heat transport between hemispheres during deglaciation and quantitatively bound the direct influence of global climate forcings on antarctic temperature although climate models perform well on average in this context some recent syntheses of deglacial climate history have underestimated antarctic warming and the models with lowest sensitivity can be discounted

a revised 10 4 c magnitude of the abrupt change in greenland temperature at the younger dryas termination using published gisp2 gas isotope data and air thermal diffusion constants abstract we revisit the portion of nature 391 1998 141 devoted to the abrupt temperature increase reconstruction at the younger dryas preboreal transition the original estimate of 5 to 10 c abrupt warming is revised to 10 4 c the gas isotope data from the original work were employed combined with recently measured precise air thermal diffusion constants geochim cosmochim acta 67 2003a 345 j phys chem 23a 2003b 4636 the new constants allow a robust interpretation of the gas isotope signal in terms of temperature change this was not possible at the time of the original work when no air constants were available three quasi independent approaches employed in this work all give the same result of a 10 c warming in several decades or less the new result provides a firm target for climate models that attempt to predict future climates

precise interpolar phasing of abrupt climate change during the last ice age a new ice core from west antarctica shows that during the last ice age abrupt northern hemisphere climate variations were followed two centuries later by a response in antarctica suggesting an oceanic propagation of the climate signal to the southern hemisphere high latitudes