physical properties of the wais divide ice core the wais west antarctic ice sheet divide deep ice core was recently completed to a total depth of 3405 m ending 50 m above the bed investigation of the visual stratigraphy and grain characteristics indicates that the ice column at the drilling location is undisturbed by any large scale overturning or discontinuity the climate record developed from this core is therefore likely to be continuous and robust measured grain growth rates recrystallization characteristics and grain size response at climate transitions fit within current understanding significant impurity control on grain size is indicated from correlation analysis between impurity loading and grain size bubble number densities and bubble sizes and shapes are presented through the full extent of the bubbly ice where bubble elongation is observed the direction of elongation is preferentially parallel to the trace of the basal 0001 plane preferred crystallographic orientation of grains is present in the shallowest samples measured and increases with depth progressing to a vertical girdle pattern that tightens to a vertical single maximum fabric this single maximum fabric switches into multiple maxima as the grain size increases rapidly in the deepest warmest ice a strong dependence of the fabric on the impurity mediated grain size is apparent in the deepest samples

an overview of seasonal snow metamorphism the grains in seasonal snow undergo rapid and radical transformations in size shape and cohesion these grain characteristics affect all of the basic properties of snow snow is characterized as either wet or dry depending on the presence of liquid water wet snow is markedly different at low and high liquid contents dry snow is characterized as either an equilibrium form or a kinetic growth form that is it is either well rounded or faceted of course many snow grains display either transitional features between two of these categories or features which arise from other processes snow is classified depending on the dominant processes of its metamorphism

modelling the liquid water vein system within polar ice sheets as a potential microbial habitat based on the fundamental and distinctive physical properties of polycrystalline ice ih the chemical and temperature profiles within the polar ice sheets and the observed selective partitioning of bacteria into liquid water filled veins in the ice we consider the possibility that microbial life could survive and be sustained within glacial systems here we present a set of modelled vertical profiles of vein diameter vein chemical concentration and vein water volume variability across a range of polar ice sheets using their ice core chemical profiles a sensitivity analysis of veinsinice1 0 the numerical model used in this study shows that the ice grain size and the local borehole temperature are the most significant factors that influence the intergranular liquid vein size and the amount of freeze concentrated impurities partitioned into the veins respectively model results estimate the concentration and characteristics of the chemical broth in the veins to be a potential extremophilic microbial medium the vein sizes are estimated to vary between 0 3 m to 8 m across the vertical length of many polar ice sheets and they may contain up to 2 l of liquid water per litre of solid ice the results suggest that these veins in polar ice sheets could accommodate populations of psychrophilic and hyperacidophilic ultra small bacteria and in some regions even support the habitation of unicellular eukaryotes this highlights the importance of understanding the potential impact of englacial microbial metabolism on polar ice core chemical profiles and provides a model for similar extreme habitats elsewhere in the universe

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

diffusion of isotopes in the annual layers of ice sheets the annual layering in the greenland and antarctic ice sheets revealed by the 18 o record becomes less distinct with depth because of self diffusion however the calculated diffusion rates are too slow to explain the observations it is suggested that the presence of veins of liquid water increases the effective diffusion constant by a factor of about 20

isotopic diffusion in cold snow and firn molecular mixing in cold snow and firn is ordinarily controlled by vapor diffusion diffusion through or along the ice matrix is slow whereas diffusion from the air to within separate grains is comparatively rapid thus mixing occurs in the vapor phase and exchange with the local ice involves all the water molecules a quantification of these concepts is applied to the diffusion of oxygen isotopic depth profiles in southern greenland and at dome c antarctica it successfully describes the smoothing of measured profiles the smoothing rate is strongly dependent on temperature and density of the firn

space and time variation of δ18o and δd in antarctic precipitation revisited 1 to better understand the variations over time of precipitation water isotopes measured in polar ice cores we have developed an intermediate complexity model icm of atmospheric water vapor transport and associated isotopic distillation the model builds directly from earlier work by d fisher and m hendricks and is calibrated against the measured modern spatial distribution of d and deuterium excess d model improvements include a correction to the equation governing advective transport which solves a major puzzle arising from the earlier work the new model shows isotopic data to be consistent with dominance of eddy diffusive transport at high latitudes model experiments are used to show that a wide variety of climate changes in subtropical source regions can affect antarctic d and that such d changes are consistently anticorrelated with changes of d magnitudes of d d are 1 to 5 and are much higher at the ice sheet margin than in the interior of the continent changes in the relative dominance of diffusive and advective transport are also shown to cause anticorrelated changes of similar magnitude isotopic sensitivities to temperature change d t are shown to also vary spatially with low values in marginal zones and higher values inland effects of ocean surface relative humidity on antarctic d are shown to be nearly uniform across the continent in contrast to earlier results given by petit et al 1991 but quantitatively small the method used by vimeux et al 2001 for folding humidity effects into source temperature effects is supported we draw attention to a pervasive misapplication of marine composition corrections to antarctic d records arguments are made in support of simple interpretations of calibrated ice core isotopic time series in terms of temperature changes despite the potential complexities in the system

onset of deglacial warming in west antarctica driven by local orbital forcing an annually resolved ice core record from west antarctica indicates that warming driven by local insolation resulting from sea ice decline began in that region about 2 000 years before warming in east antarctica reconciling two alternative explanations for deglacial warming in the southern hemisphere

recent climate and ice sheet changes in west antarctica compared with the past 2 000 years west antarctic temperature and pressure observations begin onlyin 1957 and reliable satellite observations of antarctic sea icedate to 1979 comprehensive observations of glacier dynamicsin the most rapidly changing areas were initiated in the 1990s borehole temperature data from the wais although confirmingthe recent rapid rise in temperature do not resolve decadal scalevariability in the past

evolution of fractures and alteration in the sierrita esperanza hydrothermal system pima county arizona the sierrita esperanza porphyry copper complex is central to a large fossil hydrothermal system located in the southeastern part of the sierrita mountains arizona abundant closely spaced fractures exert a profound control of sulfide minerals and alteration in the mineralized center of the system the extent and limits of the hydrothermal system in peripheral areas have been defined in this study by determination of the distribution and abundance of the altered and mineralized fractures mapping of these elements to their limit of occurrence has resulted in recognition of an area of about 60 km 2 where fractures related to porphyry copper mineralization occur and a contiguous area of about 40 km 2 where slightly older altered fractures related to the first stages of laramide igneous activity in the region are present within the orebody distinctive fracture types as recognized by fracture style and alteration assemblages occur in a consistent paragenetic succession this same succession has been recognized in the areas peripheral to the pits but with variations for each alteration type as influenced by wall rock mineralogies the establishment of a sequence of vein alteration commencing with quartz orthoclase alteration passing through precipitation of quartz and sulfides and ending with formation of epidote orthoclase chlorite has allowed separation of times of fracture formation in the orebody periphery fracture density fracture length sample area maps have been prepared from data collected at 1 310 stations in the study area the data were processed and transformed into maps of moving average contours of fracture abundance for different times of fracture formation as interpreted from the alteration paragenesis the distinctive differences in alteration types in crosscutting veins reveal the episodic nature of fracturing and the fracture density maps show shrinking of fractured rock volumes toward the central porphyry mass with successive fracture events evaluation of the fracture area in a 1 km thick slab of the 40 km 2 area at the time of precipitation of quartz and sulfide minerals in veins indicates that 4 x 10 5 km 2 of crack area existed a slightly smaller value of 6 x 10 4 km 2 per km thick slab existed at the time of the earliest quartz orthoclase alteration

water isotope diffusion rates from the northgrip ice core for the last 16 000 years glaciological and paleoclimatic implications abstract a high resolution 0 05 m water isotopic record o 18 is available from the northgrip ice core in this study we look into the water isotope diffusion history as estimated by the spectral characteristics of the o 18 time series covering the last 16 000 years the diffusion of water vapor in the porous medium of the firn pack attenuates the initial isotopic signal predominantly having an impact on the high frequency components of the power spectrum higher temperatures induce higher rates of smoothing and thus the signal can be used as a firn paleothermometer we use a water isotope diffusion model coupled to a steady state densification model in order to infer the temperature signal from the site assuming the accumulation and strain rate history as estimated using the gicc05 layer counted chronology and a dansgaard johnsen ice flow model the temperature reconstruction accurately captures the timing and magnitude of the bolling allerod and younger dryas transitions a holocene climatic optimum is seen between 7 and 9 ky b2k with an apparent cooling trend thereafter our temperature estimate for the holocene climatic optimum points to a necessary adjustment of the ice thinning function indicating that the ice flow model overestimates past accumulation rates by about 10 at 8 ky b2k this result is supported by recent gas isotopic fractionation studies proposing a similar reduction for glacial conditions finally the record presents a climatic variability over the holocene spanning millennial and centennial scales with a profound cooling occurring at approximately 4000 years b2k the new reconstruction technique is able to provide past temperature estimates by overcoming the issues apparent in the use of the classical o 18 slope method it can at the same time resolve temperature signals at low and high frequencies

on the impact of impurities on the densification of polar firn abstract understanding polar firn densification is crucial for reconstructing the age of greenhouse gas concentrations extracted from ice cores and for the interpretation of air in ice as a dating tool or as a climate proxy firn densification is generally modeled as a steady burial and sintering process of defined layers where the structure of the layering is maintained along the whole firn and ice column however available high resolution density data as well as firn air samples question this picture and point to a lack of understanding of firn densification based on analysis of high resolution density and calcium concentration records from antarctic and greenland ice cores we show for the first time that also impurities may have a significant impact on the densification analysis of firn cores shows a correlation between density and the calcium ion ca concentration and this correlation increases with depth the existence of this relationship is independent of the local climatic conditions at the core sites analyzed the strong positive correlation between the density and the logarithm of ca concentration indicates that impurities induce softening and lead to faster densification over a wide range of concentrations in one core the impurity effect manifests itself so strongly that the density develops a seasonal cycle closely following the seasonal cycle of ca our results clearly show that the structure of the firn layering changes with depth and suggest that the increased variability in density observed in deep firn recently described as a universal feature of polar firn may arise from the influence of ca and or other impurities the impurity effect is likely to have direct implications on our understanding of glacial firn densification and on glacial gas age estimates

timing of millennial scale climate change in antarctica and greenland during the last glacial period a precise relative chronology for greenland and west antarctic paleotemperature is extended to 90 000 years ago based on correlation of atmospheric methane records from the greenland ice sheet project 2 and byrd ice cores over this period the onset of seven major millennial scale warmings in antarctica preceded the onset of greenland warmings by 1500 to 3000 years in general antarctic temperatures increased gradually while greenland temperatures were decreasing or constant and the termination of antarctic warming was apparently coincident with the onset of rapid warming in greenland this pattern provides further evidence for the operation of a bipolar see saw in air temperatures and an oceanic teleconnection between the hemispheres on millennial time scales

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

water isotope diffusion in the wais divide ice core during the holocene and last glacial we use a high resolution water isotope record from the west antarctic ice sheet divide ice core wdc to evaluate the effects of water isotope diffusion for the last 29 ka b p using spectral analysis of the data we determine diffusion lengths in depth and time domains the diffusion length quantifies the mean cumulative diffusive displacement of water molecules relative to their original location at time of deposition we simulate the observed signal with models and find that our understanding of processes and conditions in the ice sheet is incomplete with the effects of ice deformational thinning removed portions of the holocene record show total diffusion lengths smaller than predicted for a lower limit case of diffusion through a single ice crystal such reduced diffusion is probably due to structural features such as crusts and tortuous porosity that inhibit vapor transport in the firn in the late glacial portion of the record diffusion lengths double between 19 5 and 17 ka b p known dependencies of diffusion on climatic variables do not account for this enhancement in models and we hypothesize that it could arise from thermal gradients in the firn column impurity driven enhancement of solid ice diffusion or changes in firn grain properties that alter vapor access to open pores despite model uncertainties the wdc diffusion length chronology will be an essential input to future studies of high frequency variability in the water isotope climate record as it allows for the effects of diffusion to be removed

isotopic diffusion in polar firn implications for interpretation of seasonal climate parameters in ice core records with emphasis on central greenland if it were possible to properly extract seasonal information from ice core isotopic records paleoclimate researchers could retrieve a wealth of new information concerning the nature of climate changes and the meaning of trends observed in ice core proxy records it is widely recognized however that the diffusional smoothing of the seasonal record makes a proper extraction very difficult in this paper we examine the extent to which seasonal information specifically the amplitude and shape of the seasonal cycle is irrecoverably destroyed by diffusion in the firn first we show that isotopic diffusion firn is reasonably well understood we do this by showing that a slightly modified version of the whillans and grootes 1985 theory makes a tenable a priori prediction of the decay of seasonal isotopic amplitudes with depth at the gisp2 site though a small adjustment to one parameter significantly improves the prediction further we calculate the amplitude decay at various other ice core sites and show that these predictions compare favorably with published data from south pole and locations in southern and central greenland and the antarctic peninsula we then present numerical experiments wherein synthetic ice core records are created diffused sampled reconstituted and compared to the original these show that alter diffusive mixing in the entire fini column seasonal amplitudes can be reconstructed to within about 20 error in central greenland but that all information about sub annual signals is permanently lost there furthermore most of the error in the amplitude reconstructions is due to the unknowable variations in the sub annual signal finally we explore how these results can be applied to other locations and suggest that dye 3 has a high potential for meaningful seasonal reconstructions while siple dome has no potential at all an optimal ice core site for seasonal reconstructions has a high accumulation rate and a low temperature

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

the first stages of erosion by ice sheets evidence from central europe abstract in almost all former and presently glaciated areas of the world glaciers have modified or even transformed pre glacial terrain during the many cold stages of the pleistocene in consequence the early stages in the development of glacial landscapes have been overprinted or erased by later phases of erosion the sudetes in central europe provide exceptional opportunities to examine the inception of glacial erosion evidence of a long geomorphic evolution before glaciation with the development of etch surfaces deep weathering covers and the preservation of neogene kaolinitic sediments provides a direct analogue to other lowland crystalline terrains as existed immediately prior to pleistocene glaciation large granite intrusions exist in the sudetes and its foreland that support hills that range in size from small tors to large domes these terrains experienced only thin ice cover for short periods when the scandinavian ice sheet reached its pleistocene maximum limits in marine isotope stage mis 12 elsterian at 440 430 ka and mis 8 early saalian at 250 240 ka in each of four study areas beyond at and within the limits of glaciation we have mapped glacial erosion indicators glacial landform assemblages that indicate progressive glacial modification of the pre glacial granite terrain we find that glacial erosion increases with distance from the ice margin due to greater ice thickness and longer ice cover but has had only limited impact on hills regolith was stripped by moving ice tors were demolished and blocks were entrained however indicators of more advanced glacial erosion such as lee side cliffs and glacial streamlining are absent even from granite domes that lay beneath 500 m of ice the survival of tors beneath ice cover and the removal of tor superstructure by glacial erosion are confirmed the presence of glacially modified tors in areas covered only by elsterian ice implies that the tors existed before 440 ka moreover the contrast between blockfield covered slopes found above elsterian and saalian glacial trimlines on summits of granite gabbro and basalt hills in the sudetic foreland and glacially stripped surfaces at lower elevations implies only limited regeneration of blockfields since 430 and 240 ka the first stages of roche moutonnee formation are recognised in the exhumation of tor stumps but development of lee side steps remains very limited granite domes retain a pre glacial morphology and local examples of hill asymmetry are determined by structural rather than glaciological controls development of roches moutonnees and asymmetric and streamlined hills by glacial erosion has not occurred in the sudetes despite the passage of largely warm based ice sheets 80 500 m thick that persisted for 5 20 ka