timing of millennial scale climate change at siple dome west antarctica during the last glacial period using atmospheric methane and the isotopic composition of o2 as correlation tools we place the dd record of ice from the siple dome west antarctica ice core on a precise common chronology with the gisp2 greenland ice core for the period from 9 to 57 ka the onset of major millennial warming events in siple dome preceded major abrupt warmings in greenland and the pattern of millennial change at siple dome was broadly similar though not identical to that previously observed for the byrd ice core also in west antarctica the addition of siple dome to the database of well dated antarctic paleoclimate records supports the case for a coherent regional pattern of millennial scale climate change in antarctica during much of the last ice age and glacial interglacial transition

subsurface crevasse formation in glaciers and ice sheets 1 crevasses form in response to tensile stresses in glaciers and ice sheets it has been widely assumed that crevasses initiate at or near the surface of the ice from starter cracks up to a few centimeters long if the tensile stress is sufficient these cracks propagate downward into the ice to form a crevasse until the weight induced lithostatic stress prevents them penetrating deeper we present ground penetrating radar data acquired on the rutford ice stream antarctica which indicate that crevasses occur at depths of several meters beneath the ice surface and were formed in areas where surface crevassing is absent the data support the hypothesis that these are examples of subsurface crevasse formation using linear elastic fracture mechanics lefm we investigate the feasibility of crevasse initiation at depth we consider the initiation of an isolated crevasse from a subsurface crack subject to a dynamic tensile stress which results from deformation associated with ice movement and a weight induced lithostatic stress the lefm approach allows us to estimate a init the minimum length a crack must be before crack propagation will occur in earlier models of crevasse formation it was assumed that the dynamic tensile stress is constant with depth we consider a more realistic scenario where the dynamic tensile stress varies with depth in such a way that the tensile strain rate remains constant we show that in this scenario crevasse initiation from centimeter scale starter cracks is feasible at depths of 10 30 m as well as at the surface at present the formulation of a reliable predictive model is limited by an incomplete knowledge of the mechanical properties of firn in previous studies the depth of buried crevasses has been used to estimate the time elapsed since ice was exposed to higher stresses and different flow regimes in the light of the results presented here those estimates may need to be reviewed

fabric and texture at siple dome antarctica preferred c axis orientations are present in the firn at siple dome west antarctica and recrystallization begins as shallow as 200 m depth in ice below 20 c based on digital analysis of c axis fabrics grain sizes and other characteristics of 52 vertical thin sections prepared in the field from the kilometer long siple dome ice core the shallowest section analyzed from 22 m shows clustering of c axes toward the vertical by 200 m depth girdle fabric and other features of recrystallized ice are evident in layers or regions separated by layers regions of typically finer grained ice lacking evidence of recrystallization ice from about 700 780m depth which was deposited during the last ice age is especially fine grained with strongly vertical c axes but deeper ice shows much larger crystals and strong evidence of recrystallization azimuthal asymmetry of some c axis fabrics trends in grain size and other indicators reveal additional information on processes and history of ice flow at siple dome

migration of the siple dome ice divide west antarctica the non linearity of the ice flow law or a local accumulation low over an ice divide can cause isochrones internal layers to be shallower under the divide relative to the flanks forming a divide bump in the internal layer pattern this divide signature is analyzed using ice flow models and inverse techniques to detect and quantify motion of the siple dome ice divide west antarctica the principal feature indicating that migration has occurred is a distinct tilt of the axis of the peaks of the warped internal layers beneath the divide the calculated migration rate is 0 05 0 50 m a 1 toward ice stream d and depends slightly on whether the divide bump is caused by the non linearity of ice flow or by a local accumulation low our calculations also suggest a strong south north accumulation gradient of 5 10 x 10 6 a 1 in a narrow zone north of the divide a consequence of divide migration is that pre holocene ice is thickest about 0 5 km south of the present divide position divide motion indicates that non steady processes possibly associated with activity of the bounding ice streams are affecting the geometry of siple dome the migration rate is sufficiently slow that the divide bump is maintained in the internal layer pattern at all observable depths this suggests that major asynchronous changes in the elevation or position of the bounding ice streams are unlikely over at least the past 10 3 10 4 years

long term climatic changes indicated by crystal growth in polar ice the reconstruction of palaeotemperatures from polar ice samples is essentially based on the isotopic composition of the ice in this paper we propose a new and independent way to obtain such data by using crystal size change profiles from central antarctica dome c and vostok ice cores data we suggest that crystal growth rate is mainly driven by a built in memory9 of the surface temperature conditions at the time of deposition a semi empirical model of crystal grain growth is proposed leading to last glacial maximum holocene temperature change estimates in good agreement with isotope interpretations however the possible palaeocli matic application of this model suffers some limitations connected in particular with in situ strain conditions

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

the crossover stress anisotropy and the ice flow law at siple dome west antarctica we used observations and modeling of siple dome west antarctica a ridge ice divide to infer the importance of linear deformation mechanisms in ice sheet flow we determined the crossover stress a threshold value of the effective deviatoric stress below which linear flow mechanisms dominate over nonlinear flow mechanisms by combining measurements of ice properties with in situ deformation rate measurements and a finite element ice flow model that accounts for the effects of viscous anisotropy induced by preferred crystal orientation fabric we found that a crossover stress of 0 18 bar produces the best match between predicted and observed deformation rates for siple dome this means that including a linear term in the flow law is necessary but generally the flow is still dominated by the nonlinear glen n 3 term the pattern of flow near the divide at siple dome is also strongly affected by crystal fabric measurements of sonic velocity which is a proxy for vertically oriented crystal fabric suggest that a bed parallel shear band exists several hundred meters above the bed within the ice age ice

the microstructure of meteoric ice from vostok antarctica the 3623 m long 5g core collected at vostok station antarctica contains alternating layers of meteoric ice with two distinctly different microstructures in this paper we present the microstructure and impurity content of a number of specimens ranging in depth from 97 to 3416 m describe in detail the characteristics of the different layers and propose a mechanism for their microstructural development digital image analysis ion chromatography scanning electron micro scopy and energy dispersive x ray spectroscopy were used to measure texture and the location and type of impurities electron backscatter diffraction was used to determine crystal orientation the ice associated with interglacial periods is characterized by relatively coarse grains and a strong preferred orientation of the c axes in a plane encompassing the coring direction producing a vertical girdle fabric in contrast ice from glacial periods is characterized by a much smaller grain size and a strong single maximum fabric where the c axes are clustered around the vertical calcium is uniquely present in the grain boundaries of the fine grained glacial layers and its effect on grain boundary mobility and the misorientation dependence of mobility can explain the development of the discontinuous micro structure seen in glacial ice at vostok station

a time marker at 17 5 kyr bp detected throughout west antarctica deep radar soundings as part of the international trans antarctic scientific expedition us itase traverses in west antarctica have revealed a bright internal reflector that we have imaged throughout widespread locations across the ice sheet the layer is seen in traverses emanating from byrd station in four directions and has been traced continuously for distances of 535 km toward the weddell sea drainage 500 km toward south pole 150 km toward the executive committee range and 160 km toward kamb ice stream former ice stream c the approximate area encompassed by the layer identified in these studies is 250 000 km 2 if the layer identification can also be extended to siple dome where we have additional radar soundings jacobel and others 2000 the approximate area covered would increase by 50 in many locations echo strength from the layer rivals the bed echo in amplitude even though it generally lies at a depth greater than half the ice thickness at byrd station where the layer depth is 1260 m an age of 17 5 kyr bp has been assigned based on the blunier and brook 2001 chronology hammer and others 1997 note that the acidity at this depth is 20 times the amplitude of any other part of the core the depiction of this strong and widespread dated isochrone provides a unique time marker for much of the ice in west antarctica we apply a layer tracing technique to infer the depth time scale at the inland west antarctic ice sheet divide and use this in a simple model to estimate the average accumulation rate

two dimensional electrical stratigraphy of the siple dome antarctica ice core a two dimensional array of electrical conductivity measurements ecm has been used to image the acidity of the siple dome antarctica ice core in a vertical plane annual layering and possible stratigraphic discontinuities are apparent a brief disruption to the chemical stratigraphy is detected at 680 m below 800 m weaker layering occurs and is interpreted as the result of post depositional migration of chemical species this technique provides a way to observe the horizontal continuity of chemical layers in an ice core and identify some types of flow irregularity

mapping c axis fabrics to study physical processes in ice mapping the spatial distribution of c axis orientations in ice thin sections is not much more difficult than preparing c axis scatter plots but can reveal additional information about processes responsible for the observed fabric and texture of the ice distributions of angles between c axes of neighboring grains from the byrd station west antarctica ice core suggest that polygonization causes average grain size to stabilize below 400 m depth

textures and fabrics in the grip ice core a comprehensive study of textures and fabrics has been carried out on the greenland ice core project grip ice core crystal sizes and c axis orientations have been measured on thin sections with conventional techniques yielding new information on the growth rotation and recrystallization of ice crystals in the greenland ice sheet normal grain growth is found to persist to a depth of 700 m in the core where the onset of polygonization due to increasing strain prevents a further increase in grain size in the holocene ice smaller crystals are observed in the wisconsin ice larger crystals are found in the eemian ice and the crystal size is found to vary with climatic parameters in these periods this dependence which probably results from variable impurity content in the ice persists to a depth of 2930 m coarse grained ice probably resulting from rapid growth of crystals at comparatively high temperatures is found in the lowest 100 m of the core the data on c axis orientations reveal a steady evolution of the fabric from random near the surface to a strong single maximum in the lower part of the ice sheet a significant strengthening is not observed at the holocene wisconsin transition the fabric development indicates that vertical compression at the ice divide is the main mode of deformation down to a depth of 2850 m the evolution toward a single maximum fabric hardens the ice against vertical compression but softens it against simple shear evidence of simple shear deformation is clearly observed between 2850 m and 2950 m depth stretched fabrics in coarse grained ice in the lowest 100 m could be due to tensional stresses this ice is unlikely to be undergoing any significant horizontal deformation at the present time

the distribution and timing of tephra deposition at siple dome antarctica possible climatic and rheologic implications approximately 300 volcanic ash and dust layers were observed in the siple dome antarctica ice core most of this tephra deposited between 700 and 800 m depth consisted primarily of glass shards with varying amounts of crystalline material and groundmass fragments the pattern of distribution of tephra fallout closely replicates that found in the byrd ice core indicative of contemporaneous deposition at both locations peak fallout occurred approximately 19 500 years ago based on methane tie points in the siple dome and greenland ice sheet project 2 gisp2 ice cores mount berlin was identified as a potential source of tephra although other volcanoes in west and east antarctica appear to have contributed ash and dust ice between 697 and 730 m in which fine grained tephra is concentrated has undergone enhanced thinning compared to ice with a similar concentration of tephra deposited contemporaneously between 1300 and 1540m at byrd it is speculated that this thinning has occurred in response to dynamic interaction between ice at siple dome and the two ice streams flanking it a dramatic change to a shear fabric appears to be directly related to the higher concentration of volcanic particles in the ice between 700 and 800 m

grounding line basal melt rates determined using radar derived internal stratigraphy we use ice penetrating radar data across grounding lines of siple dome and roosevelt island antarctica to measure the spatial pattern magnitude and duration of sub ice shelf melting at these locations stratigraphic layers across the grounding line show in places a large amplitude downwarp at or slightly downstream of the grounding line due to sub ice shelf basal melting localized downwarping indicates that melting is transient melt rates or the grounding line position have changed within a few hundred years in order to produce the observed stratigraphy elsewhere no melt related stratigraphic signature is preserved in part heterogeneity in the amount of sub ice shelf melt is due to regional circulation patterns in the sub shelf cavity but local on the order of tens of kilometers heterogeneity in the melt pattern may reflect small differences in the shape of the ice shelf base at the grounding line we find that all of the grounding lines crossed have been in place for at most 400 years

ice temperature and high geothermal flux at siple dome west antarctica from borehole measurements a verlical temperature profile through the west antarctic ice sheet wais at the summit of siple dome reveals an elevated geothermal flux this could be the root cause for the existence of a dynamic ice stream system in the wais siple dome is still frozen on its bed but adjacent ice streams have bed temperatures at the pressure melting point of ice although present day temperature increases due to climatic change do not have an immediate effect on the basal conditions that control the velocity of the ice indirect ellbcts like a rapid disintegration of the floating ice shelves or additional melt water input at the surface could give rise to speed up of the ice streams with an ensuing rise in sea level ongoing melt at the base of the ice and changes at the margins will allow continued rapid flow of the ice streams with a possibility of disintegration within a relatively short period of time of at least part of the wais

ice fabrics study in the upper 1500 m of the dome c east antarctica deep ice core abstract a study of c axis orientations in the upper 1500 m of the dome c deep ice core has been carried out using an automatic ice fabric analyzer aifa 29 vertical and a few horizontal thin sections from different depths in the core have been analyzed presentation of the fabric data and resulting parameters is in accordance with new standards developed for automatic ice fabric analyzers the fabric diagrams display a near random c axis orientation distribution in the uppermost parts of the ice sheet a tendency of c axis rotation towards a broad single maximum fabric is observed in the lowest part of the studied interval the fabric development at dome c thus appears typical for ice sheet summit and dome sites the fabric development at dome c is compared with the fabric evolution of the dome f and grip ice cores a nearest neighbor correlation analysis reveals little evidence for polygonization while microscopic observations show that subgrain boundaries are present in half of the grains in any depth

on a threefold classification of extratropical cyclogenesis a recently proposed extension to the twofold extratropical cyclogenesis classification scheme of petterssen and smebye is discussed a third class of extratropical cyclone type c is described in which initial development is controlled by a pre existing upper level potential vorticity pv anomaly in its early stages such a system is indistinguishable from the classical type b cyclone of the petterssen and smebye scheme however subsequent development cannot be understood in terms of a co operative interaction of the upper level feature with a low level baroclinic zone rather the cyclogenetic dynamics of type c systems are dominated by the action of strong midlevel latent heating such heating can generate important anomalies of pv that act to suppress the formation of a low level thermal anomaly and that interact destructively with the pre existing upper level feature candidate type c events are identified using recently developed height attributable quasi geostrophic vertical motion diagnostics the application of one such diagnostic to a climatology of instantaneous cyclonic features suggests that type c events may occur with reasonable frequency the generic behaviour of system types in the proposed threefold classification scheme is compared with the actual dynamics of some cyclones from the fronts and atlantic storm track experiment fastex the analysis is based on piecewise pv inversions and numerical simulations this approach is able to provide a good description of a case of standard type b development consistent with the qualitative description of petterssen and smebye and with the quasi geostrophic diagnostics within the same framework two systems are discussed whose behaviours do not fit the simple a b classification but are consistent with the proposed type c mechanism copyright 2003 royal meteorological society

the effect of particles on creep rate and microstructures of granular ice the microstructures of particle free granular freshwater ice and ice containing 1 wt of 50 10 mm uniformly distributed particles were investigated before and after compressive creep to 10 strain with stresses of 1 45 mpa at 10 c and 0 4 mpa at 5 c creep rates of particle containing ice were always higher than those of particle free ice for an initial stress of 1 45 mpa at 10 c dynamic recrystallization occurred with new grains nucleating and growing along grain boundaries for both sets of specimens and the ice with particles showed a higher nucleation rate under creep with an initial stress of 0 4 mpa at 5 c dynamic recrystallization also occurred by the nucleation and growth of new grains along the grain boundaries for ice containing particles but recrystallization in the particle free ice occurred through grain boundary migration

fabrics in polar ice sheets development and prediction fabrics in polar ice sheets provide a record of deformational history and control the viscosity of ice during further deformation they affect geophysical sensing of ice sheets and provide an accessible analogue to fabric development during deformation of other geological and engineering materials a new synthesis of experimental and theoretical results shows that c axis fabrics are quantitatively related to cumulative strain and stress state in ice sheets for the full range of likely flow patterns basal shear divergent flow and parallel flow cause c axes to rotate toward the vertical axis whereas convergent flow causes c axes to rotate toward a vertical plane transverse to flow

impurity influence on normal grain growth in the gisp2 ice core greenland intercept analysis of approximately bi yearly vertical thin sections from the upper part of the gisp2 ice core central greenland shows that grain size ranges increase with increasing age this demonstrates that something in the ice affects grain growth rates and that grain size cannot be used directly in paleothermometry as has been proposed correlation of grain growth rates to chemical and isotopic data indicates slower growth in ice with higher impurity concentrations and especially slow growth in forest fire layers containing abundant ammonium however the impurity grain growth relations are quite noisy little correlation is found between growth rate and isotopic composition of ice

evidence for a recently abandoned shear margin adjacent to ice stream b2 antarctica from ice penetrating radar measurements satellite imagery of the area between ice streams b1 and b2 antarctica shows a lineation on the surface of the ice sheet of uncertain origin ice motion in the area 2 m yr 1 is 2 orders of magnitude slower than that of the surrounding streams and shows no significant variation that could explain the feature a low power high resolution radar system was used to image the upper 80 m of the ice sheet between the two ice streams the survey shows that the lineation is associated with what is likely an abandoned shear margin the radar data show that a set of chaotic diffractors lies buried beneath two thirds of the area while the remaining one third is undisturbed to 80 m depth the chaotic ice ends abruptly along a boundary that is parallel to but offset 2 5 km from the surface lineation also isolated linear diffractors are commonly observed in the otherwise undisturbed ice immediately adjacent to the boundary the depth location orientation and curved form of the diffractors strongly suggest they are the tops of crevasses that were active at the time the chaotic ice was being strained and that they were formed by left lateral shear this is the same sense of shear that is presently active in the b2 margin 6 km away the depth to the chaotic diffractors suggests that the shear margin abandoned its prior position 190 years b p the burial depth decreases toward the b2 margin and suggests a migration rate of 100 m yr 1 in addition a separate high power radar system was used to image the entire thickness of the ice sheet 1 km in the same area these data show numerous linear diffractors near the base of the ice sheet it is very likely that at least some of these diffractors are entrained morainal debris others may be bottom crevasses or zones of wet reflective ice that developed in the high strain environment of the now abandoned shear margin

physical properties crystalline textures and c axis fabrics of the siple dome antarctica ice core the quality of the ice core from siple dome west antarctica varied widely with significant fracturing below 400 m bubbly ice persisted to the ice rock interface at 1004 m and constituted the brittle zone the core has undergone minimal relaxation and has remained brittle and prone to fracturing more than 5 years after it was drilled this behavior is attributed to unrelieved stresses from kamb and bindschadler ice streams former ice streams c and d flanking the dome melt layers were identified sporadically throughout the core as were inclined layers tilted at angles that occasionally exceeded 10 structurally the ice was characterized by extensive recrystallization including grain size changes from 0 074 cm 2 at 59 m to 50 cm 2 at 992 m and major transitions in c axis fabrics unusual fabrics included vertical c axis clusters superimposed on vertical girdles that may reflect vertical compression acting in conjunction with horizontal tension the sudden appearance of a shear type fabric at 700 800 m appears closely linked to the occurrence of abundant tephra particles embedded in the ice the occurrence of dispersed sediment in the bottom 2 m is attributed to freeze on of basal meltwater

dating the siple dome antarctica ice core by manual and computer interpretation of annual layering the holocene portion of the siple dome antarctica ice core was dated by interpreting the electrical visual and chemical properties of the core the data were interpreted manuallyandwith acomputeralgorithm the algorithm interpretation was adjusted to be consistent with atmospheric methane stratigraphic ties to the gisp2 greenland ice sheet project 2 ice core 10 be stratigraphic ties to the dendrochronology 14 c recordandthedatedvolcanic stratigraphy thealgorithm interpretation ismorecon sistent andbetter quantifiedthanthe tedious and subjective manual interpretation

on the special rheological properties of ancient microparticle laden northern hemisphere ice as derived from bore hole and core measurements on observe des differences dans le fluage des glaces a forte concentration en microparticules wisconsin recent qui sont plus douces que celles de la glace contemporaine ou holocene on analyse les parametres suivants concentration ou microparticules 18 o taille des cristaux axe c ca na conductivite et densite dans des carottes prelevees a agassiz iles ellesmere sur l ile devon et au groenland

physical and structural properties of the greenland ice sheet project 2 ice core a review substantial data sets have been collected on the relaxation characteristics density grain size c axis fabrics and ultrasonic velocities of the greenland ice sheet project 2 gisp2 core to its contact with bedrock at 3053 4 m changes in all these properties paralleled closely those found in cores from byrd station antarctica and dye 3 greenland density increased progressively with depth to a maximum of 0 921 mg m3 at about 1400 m at which depth the ice became bubble free below about 2000 m in situ densities began to decrease in response to increasing ice sheet temperatures since drilling much of the ice core has undergone significant volume expansion relaxation due to microcracking and the exsolving of enclathratized gases especially in the brittle ice zone between 650 and 1400 m grain size increased linearly to about 1000 m thereafter remaining fairly constant until the younger dryas event at 1678 m where a twofold to threefold decrease in grain size occurred these grain size changes were accompanied by a progressive clustering of crystal c axes toward the vertical including a small increase in c axis concentration across the younger dryas holocene boundary increased dust levels in the wisconsin ice have contributed to the maintenance of a fine grained texture which with its strong vertical c axis fabric persisted to nearly 3000 m however beginning at about 2800 m layers of coarse grained ice intermixed with the much finer grained matrix ice are observed below 3000 m the ice became very coarse grained this change attributed to annealing recrystallization at elevated temperatures in the ice sheet was accompanied by a dispersed or ring like redistribution of the c axes about the vertical ultrasonic measurements of vertical and horizontal p wave velocities made at 10 m intervals along the entire length of the gisp2 core fully confirmed the results of the crystallo optical observations a return to fine grained ice coincided with the first appearance of brown silty ice 13 m above bedrock bedrock material consisted of 48 cm of till including boulders and cobbles overlying gray biotite granite comprising the true bedrock there is evidence that disturbed structure in the gisp2 cores begins little more than 70 of the way through the ice sheet this disturbance increases with depth until it becomes large enough to cast suspicion on features lasting centuries or more in the bottom 10 of the ice sheet

a model aided study of the origin and evolution of the anomalously high potential vorticity in the inner region of a rapidly deepening marine cyclone abstract high spatial and temporal resolution fields generated by a mesoscale prediction model are used to study a case of rapid marine cyclogenesis 26 mb in 12 h within the context of potential vorticity pv thinking the case which occurred on 23 february 1987 near the east coast of the united states was well simulated by the model as was verified by ship reports and satellite imagery three components of the cyclone were investigated 1 a surface thermal anomaly or surrogate pv anomaly 2 a low level diabatically produced pv anomaly in and near the frontal cloud band and 3 a dry pv anomaly of upper tropospheric and lower stratosphoric origin trajectory tracing revealed that the surface thermal anomaly was caused primarily by northward transport of warm air into the central region of the low although surface sensible heat flux also contributed to the warm anomaly a sensitivity experiment run with the flux withheld showed that it had little impact on the depth of the storm furthermore

pieces of the ancient lunar crust ages and composition of clasts in consortium breccia 67915 we discuss the composition and chronology of clasts representing three minor lithologies in consortium breccia 67915 the lithologies studied are sodic ferrogabbro pristine troctolitic anorthosite 67915 26 and granulitic troctolitic anorthosite 67915 67 these lithologies were presumed to represent samples of the ancient lunar crust the pristine troctolitic anorthosite a typical member of the pristine anorthosite suite contains little or no trapped liquid consistent with its two phase mineralogy the plagioclase separates have 39ar 40ar plateau ages of 4 10 0 06 b y and a final rise in the age pattern may represent a memory of the earlier evolution the granulitic troctolitic anorthosite which has 13 times larger ree abundances displays a continuous rise of the 40ar 39ar ratio the last 25 of the 39ar release data establish a minimum age of 3 5 b y but the 244pu nd chronology suggests an age of 4 3 b y the sodic ferrogabbro samples which appear to be members of the mg gabbronorite group of pristine rocks show large ar losses but indicate plateaus in the low temperature data suggesting a metamorphic event more recently than 260 m y ago a time that is consistent with the north ray impact event 50 m y ago

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

reexamining the cold conveyor belt abstract despite the popularity of the conveyor belt model for portraying the airflow through midlatitude cyclones questions arise as to the path of the cold conveyor belt the lower tropospheric airflow poleward of and underneath the warm front some studies beginning with carlson s analysis of the eastern u s cyclone of 5 december 1977 depict the cold conveyor belt moving westward reaching the northwest quadrant of the storm turning abruptly anticyclonically rising to jet level and departing the cyclone downstream hereafter the anticyclonic path other studies depict the cold conveyor belt reaching the northwest quadrant turning cyclonically around the low center and remaining in the lower troposphere the cyclonic path to clarify the path of the cold conveyor belt the present study reexamines carlson s analysis of the cold conveyor belt using an observational and mesoscale numerical modeling study of the 5 december 1977 cyclone this reexamination raises several previously unappreciated

dynamic recrystallization and texture development in ice as revealed by the study of deep ice cores in antarctica and greenland the preferred c axis orientation of ice from polar ice sheets develops essentially as a result of intracrystalline slip but dynamic recrystallization appears to alter the kinetics of the development of deformation textures and is at high temperature at the origin of recrystallization textures the purpose of this work is to obtain a better understanding of recrystallization processes that occur in polar ice sheets and to clarify the relationship between dynamic recrystallization and textures the study was based on two deep ice cores from greenland and antarctica the greenland ice core project grip and vostok ice cores the structure along the grip core displays normal grain growth in the first 100 m of the ice sheet and rotation recrystallization and migration recrystallization near the bottom only grain growth and rotation recrystallization appear to occur in the vostok ice core the transition between these recrystallization regimes was studied estimating for interglacial ice the evolution with depth of the dislocation density this calculation has shown the efficiency of grain boundary migration for the absorption of dislocations at vostok the highest value of the dislocation density is found at a depth of about 1000 m and the continuous decrease in the dislocation density below this depth is related to the increase of the grain boundary migration rate it is shown that the driving force required to initiate migration recrystallization is not reached in interglacial ice at vostok the observed textures were compared with those predicted by the self consistent approach recrystallization textures are interpreted by assuming that the less stressed grains i e the best oriented for basal slip are favored by the size advantage of subgrains the recrystallization textures are compared with those of other materials

grain growth in polar ice ii application grain growth observed in polar ice that is not deforming rapidly can be accounted for if concentrations and distributions of extrinsic materials microparticles bubbles and dissolved impurities are characterized fully dissolved impurities segregate to grain boundaries and slow grain growth in all cold glacial ice the high concentration of soluble impurities in wisconsinan ice from the dome c antarctica ice core and perhaps other ice cores probably causes the small grain sizes observed in that ice microparticles have little effect on grain growth in ordinary ice in ice layers that appear dirty owing to concentrations of volcanic tephra such as in the byrd station antarctica ice core or of morainal material micro particles reduce grain growth rates significantly the relatively high vapor pressure of ice allows rapid growth and high mobility of intergranular necks so grain growth in firn is limited by boundary migration rather than by neck growth bubbles formed by pore close off at the firn ice transition are less mobile than grain boundaries causing bubble boundary separation whenever geometric constraints are satisfied however such separation reduces grain growth rates by only about 10 the observed linear increase of grain area with time is thus predicted by theory but the growth rate depends on soluble impurity concentrations as well as on temperature

creep of granular ice with and without dispersed particles the effects of silt sized particles average diameter of 50 m on the compressive creep of polycrystalline ice have been studied at stress levels from 0 1 to 1 45 mpa and temperatures of 12 c and 10 c dislocation densities during creep have been estimated using a dislocation based model of anelasticity the results indicate that at low concentrations up to 4wt particles increase the minimum creep rate power law behavior with an exponent of 3 was observed for both particle free ice and ice with 1 wt particles when the stress was 0 3 mpa in contrast linear behavior was observed when the stress was 0 3 mpa calculations show that the linear behavior is associated with a constant dislocation density and the power law behavior is associated with increasing dislocation densities with increasing stress

surface morphology and internal layer stratigraphy in the downstream end of kamb ice stream west antarctica satellite images of kamb ice stream formerly ice stream c west antarctica reveal several long curved linear features lineations oriented sub parallel to the ice flow direction we use ground based radar to characterize the internal layer stratigraphy of these lineations and the terrains that they bound some lineations are relict ice stream shear margins identified by hyperbolic diffractors near the surface interpreted to be buried crevasses and highly disturbed internal layers at depth satellite images show another set of lineations outside the relict margins that wrap around the ends of the surrounding inter ice stream ridges internal layers beneath these lineations are downwarped strongly into a syncline shape the internal stratigraphy of the terrain between these lineations and the relict margins is characterized by deep hyperbolic line diffractors our preferred hypothesis for the origin of this terrain is that it was floating sometime in the past the deep hyperbolas are interpreted to be basal crevasses and the strongly downwarped internal layers mark the position of a relict grounding line our study shows that lineations and intervening terrains have different internal layer characteristics implying different origins differentiation between these features is not possible using satellite images alone

flow law hypotheses for ice sheet modeling ice flow modeling requires a flow law relating strain rates to stresses in situ but a flow law cannot be measured directly in ice sheets microscopic processes such as dislocation glide and boundary diffusion control both the flow law for ice and the development of physical properties such as grain size and c axis fabric these microscopic processes can be inferred from observations of the physical properties and the flow law can then be estimated from the microscopic processes a review of available literature shows that this approach can be imperfectly successful interior regions of large ice sheets probably have depth varying flow law constants with the stress exponent n for power law creep less than 3 in upper regions and equal to 3 only in deep ice n probably equals 3 through most of the thickness of ice shelves and ice streams

grain scale processes folding and stratigraphic disturbance in the gisp2 ice core flow disturbances have been shown to alter stratigraphic order in the lower part of the ice sheet in central greenland vertical thin sections of the greenland ice sheet project 2 ice core show that in the lower 30 the expected c axis vertical fabric is interrupted by planes of grains stripes with c axes oriented approximately in the dip directions of the planes stripe parallel shear produces small scale folds the stripes can be explained qualitatively by a simple nucleation and growth model based on the strong anisotropy of ice crystal deformation nucleation probably is sensitive to spatially variable rates of polygonization producing spatially variable stripe densities stripes are modeled to affect the ice viscosity so variations in stripe density may contribute to viscosity contrasts that might produce larger deformational features and loss of stratigraphic order

on the effects of temperature on the strength of h2so4 doped ice single crystals previously it was shown that the reduction in peak stress of single crystal ice at 20 c due to h 2 so 4 could be described by kc 1 2 where c is the sulfuric acid concentration and k is a constant here we show that the strength reduction due to h 2 so 4 is more general and that the reduction is greater as the temperature decreases was again found to be proportional to c 1 2 at 10 c but k at 10 c was significantly less than at 20 c

the effect of particles on dynamic recrystallization and fabric development of granular ice during creep us army cold regions research and engineering laboratory 72 lyme road hanover new hampshire 03755 1290 usaabstract the mechanical behavior and microstructural evolution of laboratory prepared particle free fresh water ice and ice with 1wt 0 43vol silt sized particleswere investigated undercreepwith a stress level of 1 45mpa at 10 8c the particles were present both within the grains and along thegrain boundaries the creep rates of specimens with particles were always higher than those of particle free ice dynamic recrystallization occurred for both sets of specimens with new grains nucleatingalong grain boundaries in the early stages of creep the ice with particles showed a higher nucleationrate this resulted in a smaller average grain size for the ice with particles after a given creep strain fabric studies indicated that ice with particles showed a more random orientation of c axes after creepto 10 strain than the particle free ice