ice deformed in compression and simple shear control of temperature and initial fabric layered and polycrystalline ice was experimentally deformed in general shear involving axial compression strain magnitude 0 5 17 and simple shear strain magnitude 0 1 1 4 as the temperature is increased from 20 c to 2 c there is at least a twofold enhancement in octahedral shear strain rate which coincides with the onset of extensive dynamic recrystallization and a change in grain size distribution at 15 c between 150c and 10 c the c axis preferred orientation rapidly evolves with the initiation of two maxima fabrics in shear zones from 10 c to 2 c there is progressive evolution of a final c axis pattern that is asymmetric with respect to the direction of shortening with a strong maximum at 5 to the pole of the shear zone a sense of asymmetry in the direction of the shear and a secondary maximum inclined at 45 to the plane of shearing an initial c axis preferred orientation plays a critical role in the initial mechanical evolution in contrast to established ideas a strong alignment of basal planes parallel to the plane of easy glide inhibited deformation and there was an increased component of strain hardening until recrystallization processes become dominant

quantification of the microstructural evolution of polycrystalline fabrics using fame application to in situ deformation of ice abstract in geology glaciology and material science new technological advances result in an ever increasing amount of data and datasets in particular when in situ experiments are conducted rapid rigorous and reliable statistical treatment is needed to allow researchers to access these large datasets for further analysis here we present fame fabric analyser based microstructure evaluation a suite of matlab scripts that utilize the matlab open source toolboxes mtex and polylx optional for rapid quantification of thin section data the data has been collected using an automated fabric analyser at a spatial resolution of 5 m pixel from the dataset grain maps are reconstructed and grain and grain boundary statistics are determined the same scripts calculate orientation density distribution diagrams and eigenvalues of the orientation tensor in the case of 2d in situ plane strain deformation experiments on polycrystalline heavy water d2o a matlab script based batch analysis for each deformation step was performed fame offers the possibility to generate movie files that combine optical data with c axes orientation data and grain statistics application of the presented scripts to analyse in situ deformation experiments of polycrystalline ice shows the value of rigorous quantification of the microstructural evolution data for process oriented research

microstructure and fabric development in ice lessons learned from in situ experiments and implications for understanding rock evolution abstract in this contribution we present a review of the evolution of microstructures and fabric in ice based on the review we show the potential use of ice as an analogue for rocks by considering selected examples that can be related to quartz rich rocks advances in our understanding of the plasticity of ice have come from experimental investigations that clearly show that plastic deformation of polycrystalline ice is initially produced by basal slip interaction of dislocations play an essential role for dynamic recrystallization processes involving grain nucleation and grain boundary migration during the steady state flow of ice to support this review we describe deformation in polycrystalline standard water ice and natural ice samples summarize other experiments involving bulk samples and use in situ plane strain deformation experiments to illustrate the link between microstructure and fabric evolution rheological response and dominant processes most terrestrial ice masses deform at low shear stresses by grain size insensitive creep with a stress exponent n 3 however from experimental observations it is shown that the distribution of plastic activity producing the microstructure and fabric is initially dominated by grain boundary migration during hardening primary creep followed by dynamic recrystallization during transient creep secondary creep involving new grain nucleation with further cycles of grain growth and nucleation resulting in near steady state creep tertiary creep the microstructural transitions and inferred mechanism changes are a function of local and bulk variations in strain energy i e dislocation densities with surface grain boundary energy being secondary except in the case of static annealing as there is a clear correspondence between the rheology of ice and the high temperature deformation dislocation creep regime of polycrystalline quartz we suggest that lessons learnt from ice deformation can be used to interpret polycrystalline quartz deformation different to quartz ice allows experimental investigations at close to natural strain rate and through in situ experiments offers the opportunity to study the dynamic link between microstructural development rheology and the identification of the dominant processes

calcified microbial reefs in cambrian series 2 north china platform implications for the evolution of cambrian calcified microbes abstract this study focuses on the microbial reefs of the zhushadong formation cambrian series 2 in shandong province china in order to understand the evolution of calcified microbes in the north china platform during the cambrian series 2 and 3 the microbial reefs occur in a thin unit ca 3 m thick over an area of 1 km 2 they consist of three types of thrombolite based on their mesostructures rimmed grainstone patch and dendritic the thrombolites mainly occur in various coarse grained carbonate facies including crudely stratified oolitic grainstone stromatolitic grainstone and disorganized limestone conglomerate calcified microbes in the thrombolites include epiphyton kordephyton a tubiform microbe bija tarthinia renalcis amgaina and razumovskia the zhushadong thrombolites were formed within a grainstone shoal and experienced repeated burial and exposure the rimmed thrombolite and grainstone patch thrombolite experienced abundant input of carbonate grains forming grainstone patches in contrast the dendritic thrombolite formed solely by calcification of microbes that mainly include epiphyton tarthinia and the tubiform microbe the outer crusts of the rimmed thrombolite were formed by amgaina under high energy conditions the diverse calcified microbes of the zhushadong formation form the earliest assemblage of their type in the north china platform their descendants mostly epiphyton subsequently thrived forming a ca 180 m thick microbialite oolite dominated succession during the cambrian series 3 zhangxia formation although the reefs in the zhushadong formation are much smaller than those of the overlying zhangxia formation their calcified microbes are more diverse this most likely reflects changes in depositional environments e g abundant siliciclastic input and tidal effects vs those of a stable carbonate platform and or global changes within reef environments e g end cambrian series 2 extinction of archaeocyaths and calcified microbes a decrease in diversity of calcified microbes in the north china platform where archaeocyaths were absent may help to account for evolutionary trends in calcified microbes that occurred independently of archaeocyath influence

strain rate dependence for evolution of steady state grain sizes insights from high strain experiments on ice abstract understanding of the microstructural evolution and equilibrium grain size development during steady state tertiary flow is essential in order to improve our knowledge of ice and rock deformation this contribution presents results from in situ transmitted light deformation experiments of natural glacier ice with the development of the microstructure in a tertiary flow regime we conducted one relative slower 1 10 6 1 s and two relative faster strain rate 2 10 6 1 s pure shear experiments at 10 c up to a shortening of 57 microstructure development was followed by time lapse observations and two new microstructure based indicators the seeding rate and the microstructure activity were introduced to evaluate whether a steady state was reached these indicate that a steady state was only reached in the two faster strain rate experiments in contrast during a slow deformation there is insufficient seeding of new grains to enable continuous recovery and there is a bimodal grain size distribution these results are explained by stress concentrations within grains particularly where basal planes are in unfavourable orientations for basal slip hard glide orientation coinciding with the development of a bimodal grain size distribution in the case of bimodal grain size distributions the use of a stabilised mean grain size as a stress piezometer as a criterion for steady state should be handled with caution

late jurassic epiphyton like cyanobacteria indicators of long term episodic variation in marine bioinduced microbial calcification abstract epiphytaceans occur in late jurassic shallow marine reef limestones at several localities in the carpathian mountains of romania these calcified dendritic microfossils are well preserved in sparry calcite as sub millimetric radial clusters of narrow well defined filaments 10 30 m in diameter that show dichotomous branching and consist of dense dark micrite locally with tubiform structure epiphytaceans are widely interpreted as photosynthetic algae or bacteria but their precise affinities remain elusive and the group may be heterogeneous these late jurassic examples most closely resemble cambrian tubomorphophyton and late devonian paraepiphyton we interpret them to be calcified cyanobacterial sheaths post devonian records of epiphytaceans are extremely scarce the kimmeridgian tithonian specimens reported here represent one of the youngest known occurrences of epiphytaceans their highly sporadic geological distribution resembles that of marine calcified cyanobacteria which show phanerozoic abundance peaks in the early paleozoic late devonian mississippian and late jurassic early cretaceous we propose that kimmeridgian tithonian epiphytacean cyanobacteria reflect environmental conditions that favored bioinduced calcification in particular elevated seawater carbonate saturation state

the localization of strain and c axis evolution in anisotropic ice using a series of combined compression simple shear experiments it has been possible to investigate strain distributions and crystal orientation fabrics related to varying layer orientation in ice a variety of flattening strains accompanied by layer buckling simple shear and the development of a lenticular layering are produced in anisotropic ice masses in samples where the creep curve has only just reached a minimum strain rate the c axis preferred orientation is similar to that in the starting material with specific c axis concentrations affected by the extent of preserved host grains at shear strains where 1 it was found that the c axis preferred orientations were highly variable depending on the magnitude of strain strain distribution and upon the modification and degree of rotation of initial c axis preferred orientation however once recrystallization dominates in high strain zones 1 there is a rapid development of an asymmetric two maxima fabric with little evidence of any contributions from inherited fabric elements the final c axis pattern is asymmetric with respect to the direction of shortening with a strong maximum at 80 to the shear zone with a sense of asymmetry in the direction of the shear and a secondary maximum lying at 50 to the plane of shearing

a diverse calcified cyanobacteria assemblage in the latest ediacaran abstract a three dimensionally preserved phosphatized cyanobacterial fossil assemblage tentatively identified as a distinct calcified cyanobacterial combination of oscillatorialeans and rivulariacean is recognized in the latest ediacaran in the southern shaanxi province china based on the morphological and preservation modes oscillatorialean cyanobacteria herein contain 3 genera which are girvanella obruchevella and subtifloria with thin walled tubes tangled coiled or aligned a kind of fan shaped fossils composed of filaments with tapered trichomes and pseudo branches cambricodium is clarified as calcified rivulariacean cyanobacteria prosperous calcified cyanobacteria co occurring with the earliest mineralized skeletal tubular fossils cloudina and sinotubulites in addition to contributing new morphological and preservation information the discovery of these cyanobacteria perhaps strengthens the case for prosperity in cyanobacterial calcification in the terminal neoproterozoic

effect of sample and grain size on the compressive strength of ice the effect of sample and grain size on the uni axial compressive strength of polycrystalline ice has been investigated at 10 c at a strain rate of 5 5 x 10 4 s 1 the results show a that the sample size must be 12 or more times greater than the grain size for it to have no effect on the strength and b that there is no significant dependence of compressive strength on grain size within the grain size range of 0 6 to 2 0 mm

strength and consistency of artificial clay carbonate mixtures simulation of natural sediments abstract marl a common rock within carbonate sequences in israel occurs as an intimate mixture of varying proportions of clay and carbonate the diagenetic process involved in the formation of the rock is controlled by the physical and chemical properties of the material as it progresses from sedimentation through diagenesis to lithification to provide a better understanding of the formation of these rocks natural sediments were simulated in the laboratory this was done by mixing varying proportions of upper cretaceous smectites bentonite and montmorillonite clay and jurassic kaolinite from makhtesh ramon with eocene chalk from the northern negev the strength and consistency of these mixtures was then determined the relationship of plasticity index to liquid limit with a correlation coefficient of 0 945 0 982 shows that the bentonite mixtures are of medium to high compressibility in comparison to the kaolinite mixtures which have a low plasticity the montmorillonite mixtures have a medium plasticity free swell of the bentonite and montmorillonite mixtures is in general twice that of the kaolinite mixtures and decreases in all cases as the carbonate content increases the effective range in friction angle under drained conditions similar to those occurring in the natural environment and saturation of up to 80 is generally consistent with increasing carbonate being only slightly higher in the montmorillonite mixtures cohesion is higher in the bentonite mixtures marls containing bentonite or montmorillonite undergo relatively higher volume changes than kaolinite muds they also deform plastically being more sensitive to changes in natural water content generally kaolinite muds are more stable and less deformed during lithification

incremental pluton emplacement during inclined transpression abstract multidisciplinary investigations suggest that many igneous plutons were assembled incrementally through emplacement of discrete magma batches internal pluton contacts between different units can be remarkably cryptic and or complex in nature this presents a major challenge in understanding both how repeated intrusions were emplaced relative to each other and in their relationships with regional deformation here we present the results of a multidisciplinary study from a neoarchean pluton exposed in the yilgarn craton of western australia our new structural geophysical geochemical and geochronology datasets indicate that the pluton was emplaced incrementally along an active inclined transpressional shear zone structural relationships indicate that magmatic to high temperature solid state fabrics locally postdate a mid to upper greenschist tectonic fabric in an adjacent older unit we consider these relationships as diagnostic of incremental emplacement indicating that the shear zone recorded several cycles of strain softening and hardening in relation to the syntectonic emplacement of discrete magma pulses followed by syndeformational cooling down to upper greenschist facies conditions our data also suggest that pluton growth occurred through over accretion of progressively more evolved magma pulses

phosphatized epiphyton from the terminal neoproterozoic and its significance abstract the fossil record of calcified cyanobacteria in the precambrian is scarce especially for fossils with dichotomous filaments epiphyton which have complex branching structures like cyanobacteria and are usually regarded as calcimicrobe with uncertain affinity and are widely considered to appear at the beginning of the cambrian exceptional phosphatized three dimensional epiphyton are reported from the gaojiashan member dengying formation of the terminal neoproterozoic lijiagou section southern shaanxi province of china based on data from scanning electron microscopy and x ray tomographic microscopy two preservational modes of epiphyton namely tubular and solid branches and their corresponding phosphatic processes are proposed branching forms of the thallus sizes and outer surface ornamentations of the filaments are characteristic of phanerozoic epiphyton and has allowed us to give a possible interpretation of this genus and to recognize its similarity to cyanobacteria the finding enhances our understanding of the diversity of cyanobacteria in the terminal neoproterozoic give us an opportunity to reassess the theories involving the coevolution of calcification of microbes and metazoans

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

rate controlling processes in the creep of polar ice influence of grain boundary migration associated with recrystallization abstract information on the deformation modes and recrystallization processes in polar ice sheets was obtained thanks to the analysis of the ice structure along two deep ice cores from greenland and antarctica it is shown that the deformation of polar ice at low stresses is produced by intracrystalline slip accommodated by grain boundary migration gbm a deformation model based on an equilibrium between work hardening and recovery processes has been developed the decrease in dislocation density is due to both gbm associated with grain growth and the formation of boundaries by recrystallization the value of the dislocation density along the byrd and grip ice cores obtained thanks to the model is about 1 10 11 m 2 this value is in agreement with data from synchrotron x ray diffraction on samples taken along the core this deformation model can account for the deformation of polar ice at low stresses it is shown that the flow law with a stress exponent lower than 2 can be related to the efficiency of gbm as a recovery process

influence of carbonate content of marls used in dams geotechnical and statistical characterization abstract the relationships between geotechnical parameters identified from tests of slake durability swelling behavior soil dispersion and consolidation using samples of marls utilized to build impervious cores of spanish dams were analyzed the analyses have determined the incidence of the processes of internal erosion and piping self stability strength durability behavior during dry wet cycles and their variation with the carbonate content of marls a total of 118 samples were selected from marl quarries used for the construction of three impervious cores of dams the carbonate content of the samples varied from less than 10 to more than 70 the type and amount of clay also varied the results show that the carbonate content alters the short term to medium term cohesion of soils by the effect of possible cementation and break up the medium low correlation indexes found in this work although acceptable are due to the complex mechanics of the durability and dispersibility phenomenons

structures and fabrics in glacial ice a review abstract glaciers ice sheets and ice caps represent tectonic systems driven by gravity their movement can be studied in real time and the rheological properties and strength of ice determined from laboratory experiments and field measurements all glacial ice has primary stratification exhibited by variations in grain size bubble content and debris content as it deforms with deformation dominated by plastic flow and recrystallization accompanied locally by fracture under tension a suite of structures develops that reflects the primary fabric of the ice and the anisotropy that develops as a result of cumulative deformation initial variations in solid impurity content and strain dependent anisotropy as a result of a crystallographic fabric give rise to effective viscosity increases or decreases compared to isotropic polycrystalline ice of about a factor of ten foliation develops from inherited mostly stratification or introduced mostly ice veins or fracture traces fabric elements and from dynamic recrystallization it is largely dependent on the accumulated strain which is highest at the base and near the margins of glaciers ice sheets and ice streams folds develop largely passively due to initial amplification of irregularities in the primary stratification to variations in flow with time or to inhomogeneous flow associated with shear zones and ductile accommodation around open fractures buckle folds and boudinage mostly on a small scale occur where viscosity contrast is large mostly in basal ice thrusting and wrench faulting are documented in surging glaciers but theoretically most unlikely and rare or absent elsewhere many structures interpreted as faults are not due to shear failure but rather result from shear displacements during opening and closing of tensile fractures

grain growth in polar ice i theory many observations regarding grain growth in ice sheets are glaciologically interesting but imperfectly understood here we develop the theory of grain growth in ice that is not deforming rapidly and in the succeeding paper we use this theory to explain observations from glacial ice in the absence of significant strain energy the driving force for grain growth arises from grain boundary curvature grain growth is slowed by the interaction of grain boundaries with extrinsic materials microparticles bubbles and dissolved impurities if the driving force for growth is not large enough to cause boundaries to separate from an extrinsic material then the grain boundary velocity is determined by the velocity characteristic of the extrinsic material low velocity regime if the driving force is large enough to cause separation then boundaries migrate more rapidly than the extrinsic material high velocity regime but the net driving force is reduced through transient pinning by the extrinsic material polar ice is typically in the low velocity regime relative to dissolved impurities and the high velocity regime relative to microparticles and bubbles cross sectional area of grains is predicted to increase linearly with time under most but not all circumstances

assemblages of calcareous algae near the precambrian cambrian boundary in siberia and mongolia calcareous algae make their first appearance near the base of the tommotian the oldest confirmed occurrences are of gemma obruchevella and renalcis in the nemakit daldyn horizon and late yudomian of siberia a little higher in the sequence in the aldanocyathus sunnaginicus zone of the earliest tommotian angulocellularia bajanophyton batenevia botomaella girvanella hedstroemia korilophyton proaulopora subtifloria and tarthinia appear immediately above this in the dokidocyathus regularis zone of the middle tommotian chabakovia appears with the epiphytacean genera epiphyton gordonophyton and tubomorphophyton kordephyton appears in the dokidocyathus lenaicus zone of the late tommotian and razumovskia in the early atdabanian these fossils constitute three calcareous algae assemblages caa 1 2 3 characteristic of the nemakit daldyn horizon late yudomian early tommotian and middle tommotian atdabanian respectively most of the genera are long ranging but their rapid appearance and diversification near the base of the tommotian represents an important event in the history of calcareous algae and augments faunal data concerning evolution and biostratigraphy near the precambrian cambrian boundary modified journal abstract

mesoproterozoic carbon dioxide levels inferred from calcified cyanobacteria filamentous and shrub like carbonate fabrics produced by in vivo cyanobacterial sheath calcification in stromatolites of the ca 1200 ma society cliffs formation baffin and bylot islands arctic canada are 400 m y older than previously reported examples in vivo sheath calcification is promoted by carbon dioxide concentrating mechanisms ccms and is a direct ecophysiological link to atmospheric co 2 concentration ccms are induced in present day cyanobacteria under experimental conditions when pco 2 is below 0 36 10 times present atmospheric level pal society cliffs calcified cyanobacteria consequently imply pco 2 levels of 2 of 7 10 pal in the late mesoproterozoic combined petro graphic experimental and modeling results therefore suggest that mesoproterozoic pco 2 concentrations were not substantially different from phanerozoic values and were significantly less than previous estimates of up to 200 pal assuming 10 lower solar luminosity in the late mesoproterozoic pco 2 levels of 10 pal or less require the presence of additional greenhouse gases for maintenance of an ice free earth at 10 pal pco 2 methane concentrations of 100 200 ppm would have been sufficient to sustain warm earth surface conditions the low atmospheric oxygen and limited marine sulfate concentrations required to sustain atmospheric methane provide additional support for sulfur isotope models that suggest protracted oxygenation of earth9s proterozoic biosphere

on shearing magmatism and regional deformation in neoarchean granite greenstone systems insights from the yilgarn craton abstract the structure of the neoarchean yilgarn craton is dominated by craton scale high strain zones mostly associated with highly deformed elongate granitic bodies and transposed greenstone belts these shear zones developed during widespread and prolonged magmatic activity that led to a nearly complete reworking of the felsic continental crust the spatial temporal and genetic relationships between such a voluminous and protracted event of crustal reworking and the development of the craton scale shear zone network are unclear here we combine new structural geophysical and geochemical data to investigate the relationship between crustal scale shear zones and large syntectonic plutons in the yilgarn craton of western australia we propose that archean granite greenstone systems may have commonly evolved through the interaction of three fundamental geological processes i emplacement of large scale syntectonic plutons ii activity of crustal scale shear zones iii pervasive largely syn metamorphic polyphase deformation in greenstone belts adjacent to syntectonic plutons we propose that the concept of archean regional deformation event need to be reassessed a regional event is probably pluton or batholith size and the structural metamorphic evolution of adjacent greenstone belts might have proceeded quite independently and potentially in a time transgressive way if those belts were not spatially related to the same syntectonic pluton

integrated ediacaran sinian chronostratigraphy of south china abstract the ediacaran period was a critical time in the history of life and earth understanding the complex interactions between geological and biological events during this interval requires high resolution chronostratigraphy thanks to its uniquely favorable palaeogeographic setting south china is an ideal region for constructing a framework for global chronostratigraphic correlation of ediacaran strata here continuous ediacaran successions contain both siliciclastic and carbonate rocks that range from shallow to deep marine and contain rich fossil assemblages showing a variety of preservation styles as part of the present study 62 reference sections distributed throughout the yangtze platform were investigated analyses of the sedimentary facies and sequence stratigraphy of more than 40 sections revealed four major sequences in the shallow marine facies three major sequence boundaries occur respectively in the middle and upper parts of the doushantuo formation and in the middle part of the dengying formation subdivision and correlation of the ediacaran successions of the yangtze platform based on sequence stratigraphy is congruent with the carbon isotope chemostratigraphy of 12 reference sections 1075 samples the sequence boundary sb2 in the middle part of the doushantuo formation is suggested here as the lower xiadongian upper yangtzean series boundary of the ediacaran which coincides with several globally recognizable events namely 1 the end of the neoproterozoic glaciations 2 the first appearance of animals 3 the sudden increase in the diversity of acanthomorphic acritarchs and 4 a negative 13 c excursion the ediacaran system of south china may be subdivided into five stages furthermore the criteria for defining these stages may be applicable throughout the world also the composite carbon isotope profile of the ediacaran of south china can be used as a global reference for chronostratigraphic subdivision and correlation in addition to the globally recognizable negative 13 c excursion in the cap carbonate at the base of the ediacaran the negative 13 c excursion in the uppermost doushantuo wonoka shuram interval represents another globally correlatable major excursion the positive 13 c excursion immediately following the uppermost doushantuo excursion and ending at 548 ma is also useful for global correlation current ediacaran chronostratigraphy and global correlations indicate that the upper doushantuo wonoka shuram negative 13 c excursion ranges from ca 560 ma to 551 ma and therefore coincides with the evolutionary renovation of life feedback between evolutionary innovation and geochemical cycling was the key mechanism for this unique and most strongly negative 13 c excursion in earth history our chronostratigraphic correlations also imply that the earliest known metazoan biotas namely the weng an and characteristic ediacaran biotas do not constitute two temporally separate evolutionary lineages as proposed previously but represent two parallel evolutionary pathways preceding the cambrian explosion of animal life

design of filters for clay cores of dams the clay core of the balderhead dam suffered cracking and internal erosion in 1967 segregation of the eroded debris was observed and the filter failed to retain the finer particles that reached it this observation has lead to the development of a method of filter design for clays in flocculating conditions based on the retention of clay flocs which are the smallest particles which can arise during erosion such a filter will operate irrespective of cracking and the amount of segregation of the eroded debris factors controlling floc size are described and an experimental relationship between permeability of a filter and the size of particle which will pass through it is presented it is suggested that the retention characterists of a filter can be described more generally by its permeability than by its grading

advanced microstructural characterization of four east antarctic firn ice cores the microstructures and microchemistry of four us international trans antarctic scientific expedition ice cores were examined at three depths 30 60 90m each using scanning electron microscopy including electron backscattered patterns and energy dispersive spectroscopy eds in order to assess the relationship between chemical and physical properties the physical characteristics grain size porosity density internal surface volume and crystallographic orientation at the four sites were inhomogeneous as expected on the basis of differences in the moisture content of deposited snow and the accumulation rate at their respective locations evidence of shallow subgrain boundary formation and trends in internal surface volume having implications for the study of firn densification and ice sheet modeling were also found chemical characterization revealed that site specific variations in particulate concentration and source could accurately be determined using eds analysis it was also found that the combination of elements predominant within the sample controls the morphology and microstructural location of the impurities

dynamic recrystallization and fabric development during the simple shear deformation of ice abstract in situ observations of polycrystalline ice deformed in simple shear between 10 and 1 c are presented this study illustrates the processes responsible for the deformation the development of a preferred crystallographic orientation and the formation of a preferred dimensional orientation intracrystalline glide on the basal plane accompanying grain rotations and dynamic recrystallization helps to accommodate the large intragranular strains these are the most important mechanisms for crystallographic reorientation and produce a stable fabric that favours glide on the basal plane localized kinks developed in grains unfavourably oriented for easy glide are unstable and are overprinted by dynamic recrystallization dynamic recrystallization is a strain softening process with nucleation occurring in the form of equiaxed grains that grow subparallel to pre existing grain anisotropies and become elongate during deformation plots of grain axial ratio against orientation r indicate a weak shape fabric which does not correspond to the theoretical foliation and elongation for the appropriate increment of shear strain we argue that estimates of the strain magnitude made from orientation of elongate grains are unreliable in high temperature shear zones these results are applicable to both geological and glacial shear environments

growth patterns of the cambrian microbialite phototropism and speciation of epiphyton abstract microbes started constructing shallow marine stromatolitic bioherms in the archean but they transferred their role as a major buildup maker to metazoans in phanerozoic microbial buildups often recovered their predominance in the carbonate platform when reefal metazoan communities collapsed epiphyton an extinct taxon of calcified microbe that possessed branching filamentous trichomes was an important reef builder in the shallow marine carbonate platform during middle cambrian aftermath of an extinction of archaeocyath sponges which were major reef building sessile organisms in the early cambrian here we present direct evidence of phototropism of epiphyton found from fossilized behavior in micro and macro structures of meter scale microbial bioherms of the zhangxia formation middle cambrian north china platform shandong province china the bioherms consist of stacked growth layers with the inner and outer divisions divided by distinct boundary the inner division of growth layers of the epiphyton bioherm is dominated by dense uniform bush shaped epiphyton thalli whereas the curved outer division has layered texture normal to the surface comprised of elongated and chambered thalli it suggests that photosynthetic epiphyton reacted actively to the spatial changes in intensity of sunlight controlled by angle of illumination on the curved growth surface of the bioherm the inner and the outer divisions comprise different morpho types of epiphyton the spatial distributions of different morpho types in variously illuminated divisions of epiphyton might have caused further speciation of epiphyton

dynamics of ice mass deformation linking processes to rheology texture and microstructure 1 prediction of glacier and polar ice sheet dynamics is a major challenge especially in view of changing climate the flow behavior of an ice mass is fundamentally linked to processes at the grain and subgrain scale however our understanding of ice rheology and microstructure evolution based on conventional deformation experiments where samples are analyzed before and after deformation remains incomplete to close this gap we combine deformation experiments with in situ neutron diffraction textural and grain analysis that allows continuous monitoring of the evolution of rheology texture and microstructure we prepared ice samples from deuterium water as hydrogen in water ice has a high incoherent neutron scattering rendering it unsuitable for neutron diffraction analysis we report experimental results from deformation of initially randomly oriented polycrystalline ice at three different constant strain rates results show a dynamic system where steady state rheology is not necessarily coupled to microstructural and textural stability textures change from a weak single central c axis maxima to a strong girdle distribution at 35 to the compression axis attributed to dominance of basal slip followed by basal combined with pyramidal slip dislocation related hardening accompanies this switch and is followed by weakening due to new grain nucleation and grain boundary migration with decreasing strain rate grain boundary migration becomes increasingly dominant and texture more pronounced our observations highlight the link between the dynamics of processes competition and rheological and textural behavior this link needs to be taken into account to improve ice mass deformation modeling critical for climate change predictions

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

a new approach to crystallographic orientation measurement for apatite fission track analysis effects of crystal morphology and implications for automation apatite fission track analysis and in particular computer assisted fission track recognition require the determination of crystallographic orientation crystal morphology cracks fractures and inclusions the selection of a crystal for fission track analysis has largely been based on its surface and track etching characteristics that in conventional fission track dating is performed by the analyst this requires manual scanning of the entire mount for suitable grains which is a very time intensive process during manual fission track counting therefore a computer assisted grain characterisation system is of interest to allow for an automated pre selection of suitable crystals with the use of a fully automated fabric analyser c axis orientations can be measured inside an apatite crystal for each pixel with a spatial resolution of 5 m the results are represented in geometric quality and retardation maps consisting of the c axis orientations for each pixel and two quality values evaluating each orientation these maps allow the determination of the crystallography of the apatite grain as well as its outline in addition most of the crystal morphologies fractures and impurities which can influence automated fission track recognition techniques based on image analysis are determinable using the quality maps on the other hand the method is not sensitive to smaller crystal cracks high fission track densities or coating the quantification of the crystal geometric properties by this method is a step forward to develop a fully automated fission track analysis process

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

calcified algae as sediment contributors to early paleozoic limestones evidence from deep water sediments of the cow head group western newfoundland abstract calcified algae are important elements of deep water carbonate sediments in the cow head group large algal boundstone boulders in debris flow conglomerates consist of girvanella sheets and arborescent clusters of epiphyton silt and sand sized particles in fine grained thin bedded slope limestones include abundant girvanella as single tubules rafts of intertwined tubules oncolites and intraclasts integration of petrographic information from the boulders and fine grained limestones indicates that many micritic peloids of silt to very fine sand size the single most abundant particle type in the cow head group are a combination of comminuted girvanella tubules and broken epiphyton rods much of the structure grumeleuse in intraclasts is also diagenetically modified calcified algae these findings suggest that many peloids and intraclasts in early to middle paleozoic carbonates in both shallow and deep water may have been derived from calcified algae calcified algae may have been just as important in producing carbonate sediment during this time as benthic calcified algae are in the modern ocean the difficulty in recognizing the algal contribution in most ancient sediments is probably caused by the simple microfabric of the resultant grains filling and obliteration of tubules with micrite cement or micritization

superplasticity owing to grain growth in polar ices on considere le grossissement des grains de la glace depuis la surface jusqu a plusieurs centaines de metres de profondeur l augmentation de la taille avec leur ge est due a l energie libre des joints de grains cette migration continue des joints de grains semble etre un processus d accommodation beaucoup plus efficace que les processus de diffusion pour le glissement au joints de grains de faibles viscosites devraient resulter de cette deformation superplastique localement heterogene

conditions for bubble elongation in cold ice sheet ice highly elongated bubbles are sometimes observed in ice sheet ice elongation is favored by rapid ice deformation and opposed by diffusive processes we use simple models to show that vapor transport dominates diffusion except possibly very close to the melting point and that latent heat effects are insignificant elongation is favored by larger bubbles at pore close off but is nearly independent of bubble compression below close off the simple presence of highly elongated bubbles indicates only that a critical ice strain rate has been exceeded for significant time and provides no information on possible disruption of stratigraphic continuity by ice deformation

rheology microstructure and crystallographic preferred orientation of matrix containing a dispersed second phase insight from experimentally deformed ice abstract we utilize in situ neutron diffraction to continuously track the average grain size and crystal preferred orientation cpo development in ice during uniaxial compression of two phase and pure ice samples two phase samples are composed of ice matrix and 20 vol of second phases of two types 1 rheologically soft platy graphite and 2 rigid rhomb shaped calcite the samples were tested at 10 c below the ice melting point ambient pressures and two strain rates 1 10 5 and 2 5 10 6 s 1 to 10 and 20 strain the final cpo in the ice matrix where second phases are present is significantly weaker and ice grain size is smaller than in an ice only sample the microstructural and rheological data point to dislocation creep as the dominant deformation regime the evolution and final strength of the cpo in ice depend on the efficiency of the recrystallization processes namely grain boundary migration and nucleation these processes are markedly influenced by the strength shape and grain size of the second phase in addition cpo development in ice is further accentuated by strain partitioning into the soft second phase and the transfer of stress onto the rigid second phase

effect of strain rate cycling on microstructures and crystallographic preferred orientation during high temperature creep strain rate histories and strain magnitude are two crucial factors governing the evolution of dynamic recrystallized grain size and crystallographic preferred orientation cpo in rocks and ice masses to understand the effect of cyclic variations in strain rate or non steady state deformation we conducted two dimensional coaxial plane strain experiments with time lapse observations from a fabric analyzer there is a continuous reequilibration of microstructure and cpo development associated with constant and oscillating strain rate cycles these can be correlated with c axis small circle distributions diagnostic of dynamic recrystallization involving new grain nucleation and grain boundary migration gbm as observed in the high temperature deformation of ice and quartz inhomogeneous stress distribution can lead to grain size reduction for relatively slower strain rates and gbm for relatively faster rates within a long interval cycling event a behavior that contradicts the classic view for dynamic recrystallization processes where there is a rapid short term cycling of strain rate gbm is hampered and nucleation dominates accompanied by a marked reduction of grain size and no new cpo development during such short term cycling gbm and crystallographic change is impeded not by impurities but by an inability of newly nucleated grains to grow

stromatolitic bioherms and microphytolites from the late proterozoic draken conglomerate formation spitsbergen abstract the late proterozoic draken conglomerate formation akademikerbreen group in eastern spitsbergen displays several prominent stromatolitic bioherms in its lowermost beds the dolomitic bioherms are comprised principally of columnar stromatolites cf gymnosolenoides but include cross bedded microphytolitic grainstones the excellent exposures of the bioherms and microphytolitic grainstones along the margin of petrovbreen glacier and the exquisite preservation of structural and microscopic details permit the documentation and interpretation of these sedimentary and biological associations locally favorable ecologic circumstance or chance colonization initiated stromatolitic mat development which then became self perpetuating to form bioherms until rapid siliciclastic influx overwhelmed the mat building populations intermediate energy conditions in concert with microbial activity and inorganic processes generated the microphytolitic grainstones the interpretation of the biological and sedimentological interactions within these biohermal carbonates microphytolitic grainstones and siliciclastic rocks affords new insights into proterozoic paleoenvironments