an isotopic model for basal freeze on associated with subglacial upward flow of pore water 1 subglacial freezing in polar glaciers can have a significant dynamical effect recent studies have shown that freezing of pore water flowing upward through subglacial fine grained sediments at the freezing interface and progression of this freezing front downward are responsible for fast ice flow stoppage in ice streams the upward pore water flow leads to the formation of debris bearing basal ice layers a model for stable isotope composition both in d and 18o is developed for predicting the isotopic composition of the ice segregated by such a mechanism the development of this isotopic model for water films present along the grains of the subglacial sediment predicts the absence of apparent fractionation for the ice formed this prediction is tested against two east antarctic outlet glaciers by studying the d 18o relationships in the basal ice layers of these glaciers

strain induced phase changes within cold basal ice from taylor glacier antarctica indicated by textural and gas analyses this paper reports detailed textural and gas measurements conducted in cold basal ice 178c from the margin of taylor glacier an outlet glacier of the east antarctic ice sheet the analyzed samples were retrieved from a basal ice sequence excavated at the end of a subglacial tunnel dug near the glacier snout the basal sequence exhibits two contrasting ice facies defined as the englacial and stratified facies on the one hand analysis of ice crystal textures from the basal ice sequence provides evidence for localized ductile deformation especially within the stratified facies where significant dynamic recrystallization was detected on the other hand high resolution gas analyses reveal that strong changes in gas composition occurred at the structural interfaces of the stratified facies these gas composition changes are typical of melting refreezing processes but are not associated with any significant loss of gas volume given the specific subglacial thermal conditions at the margin of taylor glacier we interpret this phenomenon as resulting from microscopic phase changes involving selective gas redistribution through the pre melt phase it is argued that such processes may play an important role in the post genetic geochemical evolution of cold debris laden ice and may be enhanced through intense strain conditions

relation between neighbouring grains in the upper part of the northgrip ice core implications for rotation recrystallization abstract we propose a new method to investigate the relationships between neighbouring crystals and apply it to the textures measured along the upper 900 m of the northgrip ice core this method shows unexpected correlations between neighbours in the so called normal grain growth regime questioning the classical view on the onset of rotation recrystallization in ice sheets moreover the fractionation rate associated to the rotation recrystallization appears constant through time finally grains with low angle boundaries do not present a special distribution pattern of their c axes this suggests that rotation recrystallization is an isotropic process not affected by the direction of the macroscopic strain

south atlantic and benthic foraminifer δ13c deviations implications for reconstructing the late quaternary deep water circulation abstract benthic foraminifer species cibicidoides wuellerstorfi and related genera are assumed to secrete calcite very close to the carbon isotope values of the ambient bottom water co2 recently attention has been focused on substantial productivity linked 13c depletions to examine further the productivity effect on benthic 13c deviations we present data from the south atlantic between 15 and 35 s including water samples from 10 hydrographic stations and related surface sediment measurements on c wuellerstorfi we compare open ocean data with observations in the namibia upwelling area as a result 13c co2 values as well as phosphate concentrations in water samples of the upwelling realm differ significantly from those of the open ocean realm at least in the upper and mid depth water masses sacw aaiw ucdw however deviations from the redfield fractionation caused by air sea fractionation remain constant within each water mass which means that the carbon isotope changes toward upwelling areas are exclusively determined by biological cycling in addition to lower 13c co2 values in upwelling areas a depletion in the 13c of epibenthic foraminifer calcite is observed which is most likely explained by the decay of organic matter reducing the 13c 12c ratio in the pore water and influencing the carbon isotopic composition of the c wuellerstorfi shells of highly productive areas the paleoceanographic implication of this effect for reconstructing the late quaternary deep water circulation is discussed using carbon isotope records of several sediment cores within and outside the namibia upwelling area

dynamic implications of discontinuous recrystallization in cold basal ice taylor glacier antarctica crystallographic investigations have been conducted of cold 17 c debris bearing ice from the base of an antarctic outlet glacier taylor glacier the 4 m thick sequence studied has been retrieved from a 20 m long tunnel dug from the glacier snout and has been analyzed with an automatic ice fabric analyzer aifa the top and bottom of the sequence consists of clean meteoric ice englacial facies whereas alternating debris rich and clean bubbly ice layers are found in the middle part stratified facies ice from the englacial facies displays a polygonal texture and a strong c axis clustering toward the vertical denoting recrystallization through subgrain rotation sgr in contrast clean ice from the stratified facies shows sgr fabrics which are delimited at the contact with debris rich layers by large interlocking grains organized in ribbons these two distinct textures within the stratified facies are associated with looser c axis patterns at the scale of single thin sections which is interpreted as resulting from migration recrystallization mr the change from sgr to mr trends marks a clear increase in grain boundary and nucleation kinetics hence the term discontinuous recrystallization and may be associated with strain localization at rheological interfaces during basal ice genesis analogies with bottom ice from deep polar ice sheets where temperature is commonly higher than at the studied site are highlighted two recrystallization scenarios are proposed accounting for the development of both types of fabrics it is shown that by controlling the repartition of stress and strain energy within basal ice the rheology of debris bearing ice layers plays a decisive role in recrystallization dynamics at structural interfaces we also demonstrate how the same recrystallization regimes may occur in cold glaciers and temperate ice sheets provided that strain accumulation has been high enough in the former this challenges the common belief that migration fabrics observed in bottom ice from deep ice sheets are exclusive to warm stagnant annealed ice copyright 2008 by the american geophysical union

direct observation of basal sliding and deformation of basal drift at sub freezing temperatures in a tunnel at the base of sub polar urumqi glacier no 1 china three new mechanisms of glacier flow at sub freezing temperatures have been observed taken individually or in combination these modes of flow can account for nearly all 60 80 of the overall glacier motion and yet they act only within the lowermost 1 2 of the effective glacier thickness these mechanisms are 1 enhanced deformation of the frozen and ice laden subglacial drift 2 motion across discrete shear planes or shear bands within the frozen drift or at the ice drift interface and 3 basal sliding at an ice rock interface at a temperature of nearly 5 c the ice laden drift has an effective viscosity of more than one hundred times less than that measured in the overlying ice thus allowing very rapid shear deformation the observed rate or basal sliding at the ice rock interface agrees favorably with that predicted by the recent work of shreve 1984 if proper account is taken of the measured surface roughness and reduced ice viscosity

instruments and methods diamond wire saw cutting technique for investigating textures and fabrics of debris laden ice and brittle ice presents an original method for sampling special ice types like debris rich ice or brittle ice that cannot be prepared using the standard band saw microtome procedure the general working principles of the diamond wire sawing technique are explained and special adaptations for ice cutting thick sectioning and thin sectioning are discussed author

a flow law for anisotropic ice and its application to ice sheets a new formulation of the flow law of polycrystalline ice was derived for an rbitrary c axis orientation fabric pattern the rate factor of the flow law is given by a geometrical factor that can be calculated with c axis fabric data and stress condition a theoretical calculation combining this flow law with a lattice rotation model by intracrystalline slip in the constituent grains predicts the development of characteristic fabric patterns observed in many laboratory experiments and in the field according to this flow model of polycrystalline ice if significant recrystallization does not occur uniaxial compression makes the ice body harder with increasing strain whereas simple shear makes the ice body softer with strain this may significantly affect ice sheet dynamics

the shear strength characteristics of frozen coarse granular debris the effect of ice content and normal load on the shear strength characteristics of a frozen coarse granular debris was investigated 31 shear tests were carried out in a modified shearbox allowing a sample temperature of 1 0 0 2 c and a load rate of 9 63 10 4 cm min the tests showed that as the ice content of the frozen debris was increased from 0 under saturated to 25 saturated sample shear strength was markedly increased in contrast sample shear strength was reduced as ice content was increased from 25 saturated to 100 supersaturated the changes in shear strength with increasing ice content were attributed directly to changes in internal friction and the cohesive effects of the pore ice the shear tests also indicate that shear strength increases with increasing normal load up to a critical limit above this limit dilatancy is suppressed causing the shear strength to decrease or remain relatively constant with increased normal load the stress strain curves of the 31 tests indicated that samples with higher ice contents tended to reach peak strength p with less displacement during shear moreover the difference between p and r residual strength was lowest for pure polycrystalline ice and highest for ice saturated samples the mohr coulomb failure envelopes displayed very distinctive parabolic curvilinearity the degree of curvature is thought to be a function of ice creep at low normal loads and particle fracture and crushing at high normal loads

crystal size variations in eemian age ice from the grip ice core central greenland continuous measurements of ice crystal size have been carried out on an 80 m sequence between 2790 and 2870 m depth in the grip ice core from central greenland the ice in this interval is at present considered to originate from the eemian interglacial period the record reveals that the crystal size in ice older than 100 000 yr is highly dependent on climatic conditions at the time of snowfall this dependence shows up as a strong correlation between delta 18 o values and crystal size throughout the eemian as well as a negative correlation between crystal size and several soluble and insoluble impurities although high resolution impurity records are available from selected parts of the eemian ice the study is not conclusive on which impurities are most effective in slowing grain growth it is shown that the normal grain growth process commonly observed in the upper few hundred metres of polar ice sheets does not yield grain sizes compatible with observed ones at this depth in the ice sheet even in those parts of the eemian ice where impurity drag effects are not present polygonization of crystals within the ice sheet and the nucleation and rapid growth of new grains at relatively high temperatures in the lowest part probably play an important role in producing the observed grain size variations the relevance of possible flow disturbances of the grip eemian climatic record for the results presented is discussed briefly

the u series ages of carnotites and implications regarding their formation the u series nuclides 230th 234u 238u 227ac 231pa and 210pb were analyzed in up to 75 carnotites from a group of trenches in southwestern israel though 230th234u had the large range of 0 14 to 0 99 its spatial distribution was not random and the ratio never exceeded unity 231pa230th was at least 91 concordant with 230th234u in four of five cases these facts together with the details of the 230th234u distribution especially among ten samples from one small block of rock indicate that 230th234u ages of carnotites are at least a reasonable approximation of true ages the unexpectedly widespread deficiencies in both 227ac and 210pb indicate that although carnotites are nearly closed to the migration of uranium they are open to those of actinium and to lead radon or radium a model of carnotite precipitation is presented which attributes the large range in 230th234u to a continual shift in the site of carnotite precipitation in keeping with a continual change in the geometry of joints this model may apply to other minerals as well

ice dynamics and thermal regime of taylor glacier south victoria land antarctica les donnees glaciologiques recueillies sur le taylor glacier ont servi a reconstituer les principaux traits de la dynamique et du regime thermique du glacier les etudes du bilan de masse montrent que le glacier est proche de l equilibre les conditions thermiques de la glace basale sur une grande partie de la zone d ablation ont ete calculees a partir d estimation du flux de chaleur geothermique et de mesures des temperatures de la glace pres de la surface des vitesses et de l epaisseur de la glace on a trouve que jusqu a 50 de la zone d ablation la glace basale pouvait etre fondante

optical microstructure of partly recrystallized calcite in some naturally deformed marbles abstract deformed marbles from the mt lofty ranges south australia have microstructures characterized by deformed relics of calcite interspersed with much finer grained recrystallized calcite and with minor quartz and diopside the deformed calcite relics contain numerous subgrains and show abundant evidence of strain induced migration of twin and grain boundaries new calcite grains have nucleated and grown preferentially along twins especially at twin intersections at grain and kink like boundaries and apparently randomly inside deformed relics optical microstructural evidence suggests that dynamic syntectonic recrystallization has taken place along with some static recrystallization probable mechanisms for the initiation of recrystallization include bulge nucleation and subgrain nucleation

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

discontinuous grain growth in recrystallised vein quartz implications for grain boundary structure grain boundary mobility crystallographic preferred orientation and stress history abstract grain growth is a process driven by the reduction of interfacial free energy leading to an increase in grain size and a reduction of the number of grains in a given volume of a single phase polycrystalline material discontinuous grain growth dcgg as opposed to normal grain growth ngg involves just a few grains blasts growing at the expense of a matrix that itself undergoes much slower modification in grain size hence producing a bimodal grain size distribution as a transient state prior to impingement of the blasts such a transient state has been found in a quartz layer in metamorphic rocks of the sesia zone western alps there large strain free quartz grains occur that reveal concave outward grain boundaries against a matrix of small grains with a pronounced crystallographic and a weak dimensional preferred orientation the misorientation of the blasts with respect to the matrix grains systematically exceeds 30 this finding suggests that the mobility of high angle grain boundaries in a natural quartzite depends on the degree of their misorientation among other factors as known for metals and alloys the width of the grain boundaries must be sufficiently small to allow for this effect dcgg is promoted by a pronounced crystallographic preferred orientation cpo that leaves only a few grain boundaries with sufficient misorientation to be mobile in contrast to ngg dcgg wipes out any pre existing cpo and information on activated glide systems or kinematics during the preceding stage of flow is lost after dcgg has gone to completion a complementary new cpo could even develop that could be difficult to interpret with respect to tectonic history interfacial free energy driven grain growth reflects a period of low stress annealing and thus constitutes a first order signal

evidence for subglacial ponding across taylor glacier dry valleys antarctica hubbard alun lawson w anderson b hubbard b p 2004 evidence for subglacial ponding across taylor glacier dry valleys antarctica annals of glaciology 39 1 79 84 rae2008

ice crystal properties of amber ice and strain enhancement at the base of cold antarctic glaciers to improve our understanding of the deformation properties of cold based polar glaciers we examine here some of the factors leading to the localization of strain within the amber ice facies we present a crystallographic case study of amber ice a fine grained bubbly ice containing a relatively high impurity content sampled at the base of two antarctic glaciers the crystal fabrics and textures of amber ice were computed by application of a recently developed automated method to date it was tedious and awkward to determine amber ice facies accurately because of the sub millimetric crystal size and relatively high debris content of this facies the authomatic analytical method applied in this study allows not only for improving analytical accuracy in this task but also for considerably reducing the time of analysis our investigations reveal highly homogeneous crystallographic properties for the studied amber ice the ice crystals are mainly polygonal equant and sub millimetric and show a strong lattice preferred orientation these properties beside the relatively high impurity content are likely to exert a major control on strain enhancement in amber ice when this facies is present at the base of cold glaciers

direct shear tests of materials from a cold glacier implications for landform development abstract the mechanical properties of materials at the ice bed interface exert a major control on glacier behaviour and define the nature and rate of subglacial erosion this paper presents new data on the strength and behaviour of basal ice and substrate of the suess glacier south victoria land antarctica which is a small alpine dry based glacier that has a basal temperature of 17 c a tunnel excavated in the glacier revealed a substrate composed of frozen sand and gravel and a basal zone that was 3 8 m thick from bottom to top the basal zone was composed of 1 8 m of stratified complexly deformed layers of ice and debris laden ice that was overlain by a 0 9 m thick layer of frozen sediment and a 0 8 m thick layer of discoloured ice that lies immediately beneath clean glacier ice direct shear tests were performed in the field on 36 samples using a strain rate of 0 85 mm h and on replicate samples in a laboratory at strain rates of 0 08 and 0 01 mm h the experiments performed in the field show that the average peak shear strength of substrate samples was 2 53 mpa which is almost twice as strong as the average value for basal ice 1 28 mpa and the glacier ice samples 1 39 mpa the laboratory experiments show that the behaviour and peak strength of samples sheared at the higher strain rates are considerably greater than the peak and residual strengths measured during the lower strain rate tests the direct shear tests suggest that the glacier substrate is unlikely to deform at the current temperature

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

grain boundary migration microstructures in a naturally deformed quartzite abstract in this study several grain scale microstructures are presented that are thought to demonstrate the migration direction of once mobile grain boundaries in a naturally deformed quartzite an analysis is presented of the sense of migration of the boundaries and the characteristics of the patterns of relative grain growth and shrinkage grain boundary migration seems to be correlated with the relative crystallographic orientations of neighbouring grains for the quartz quartz grain boundaries and the pattern of preferred grain growth is roughly symmetrical about the mica foliation plane

dislocation creep regimes in quartz aggregates abstract using optical and tem microscopy we have determined that three regimes of dislocation creep occur in experimentally deformed quartz aggregates depending on the relative rates of grain boundary migration dislocation climb and dislocation production within each regime a distinctive microstructure is produced due primarily to the operation of different mechanisms of dynamic recrystallization at lower temperatures and faster strain rates the rate of dislocation production is too great for diffusion controlled dislocation climb to be an effective recovery mechanism in this regime recovery is accommodated by strain induced grain boundary migration recrystallization with an increase in temperature or decrease in strain rate the rate of dislocation climb becomes sufficiently rapid to accommodate recovery in this regime dynamic recrystallization occurs by progressive subgrain rotation with a further increase in temperature or decrease in strain rate dislocation climb remains sufficiently rapid to accommodate recovery however in this regime grain boundary migration is rapid thus recrystallization occurs by both grain boundary migration and progressive subgrain rotation the identification of the three regimes of dislocation creep may have important implications for the determination of flow law parameters and the calibration of recrystallized grain size piezometers in addition the identification of a particular dislocation creep regime could be useful in helping to constrain the conditions at which a given natural deformation has occurred