subgrain boundaries in antarctic ice quantified by x ray laue diffraction ice in polar ice sheets undergoes deformation during its flow towards the coast deformation and recrystallization microstructures such as subgrain boundaries can be observed and recorded using high resolution light microscopy of sublimation edged sample surfaces microstructure mapping subgrain boundaries observed by microstructure mapping reveal characteristic shapes and arrangements as these arrangements are related to the basal plane orientation full crystallographic orientation measurements are needed for further characterization and interpretation of the subgrain boundary types x ray laue diffraction measurements validate the sensitivity of different boundary types with sublimation used by microstructure mapping for the classification x ray laue diffraction provides misorientation values of all four crystal axes line scans across a subgrain boundary pre located by microstructure mapping can determine the rotation axis and angle together with the orientation of the subgrain boundary this yields information on the dislocation types tilt and twist boundaries composed of dislocations lying in the basal plane and tilt boundaries composed of nonbasal dislocations were found a statistical analysis shows that nonbasal dislocations play a significant role in the formation of all subgrain boundaries

ice microstructure and fabric an up to date approach for measuring textures automatic c axes analyzers have been developed over the past few years leading to a large improvement in the data available for analysis of ice crystal texture such an increase in the quality and quantity of data allows for stricter statistical estimates the current textural parameters i e fabric crystallographic orientations and microstructure grain boundary networks are presented these parameters define the state of the polycrystal and give information about the deformation undergone by the ice to reflect the findings from automatic measurements some parameter definitions are updated and new parameters are proposed moreover a matlab toolbox has been developed to extract all the textural parameters this toolbox which can be downloaded online is briefly described

the characteristics of movable fluid in the triassic lacustrine tight oil reservoir a case study of the chang 7 member of xin anbian block ordos basin china abstract the micropore throat structure of a tight oil reservoir is complex and its movable fluid distribution differs from that of a conventional reservoir by using nuclear magnetic resonance nmr and centrifugation the movable fluid distribution in the chang 7 tight reservoir in the xin anbian block of the ordos basin was quantitatively evaluated combined with the results of the physical property and high pressure mercury intrusion experiments the effects of the porosity permeability pore size distribution and micropore throat structure on the movable fluid distribution in the chang 7 tight reservoir were analyzed the nmr results show that the t2 spectral morphology of the chang 7 tight oil reservoir in the xin anbian block can be characterized by three main patterns with different amplitudes of front and post peaks the front peak corresponds to the scale of small pores and the post peak corresponds to the scale of large pores based on the three t2 spectrum patterns the chang 7 tight reservoir can be divided into three types type 1 reservoir with more small pores and fewer large pores type 2 reservoir with similar amounts of small and large pores and type 3 reservoir with fewer small pores and more large pores thus the characteristics of these three types of reservoirs can be determined the movable fluid parameters were calculated by nmr and centrifugation the movable fluid saturation of the type 1 reservoir is low with an average of 20 38 and the average movable fluid porosity is 1 47 the type 2 and type 3 reservoirs have higher movable fluid contents of 40 79 and 47 04 respectively the average movable fluid porosities of the type 2 and type 3 reservoirs are 3 37 and 3 48 respectively the greater number of small pores and smaller number of large pores in the type 1 reservoir result in weak correlations of the movable fluid with the physical properties and pore throat structure parameters the movable fluid saturation is negatively correlated with the maximum pore radius mainstream throat radius and maximum mercury saturation and is strongly positively correlated only with the porosity of the large pores as the number of large pores increases the movable fluid saturation strongly correlates with the property parameters pore size distribution and pore throat structure parameters in the type 2 and type 3 reservoirs the movable fluid saturations are positively affected by the maximum pore radius median radius mainstream throat radius and effective porosity but are strongly negatively correlated only with the radius mean value these results indicate that the fluid mobility of these two types of reservoirs can be promoted by the pore throat system to obtain favorable seepage characteristics

use of nmr logging to obtain estimates of hydraulic conductivity in the high plains aquifer nebraska usa 1 hydraulic conductivity k is one of the most important parameters of interest in groundwater applications because it quantifies the ease with which water can flow through an aquifer material hydraulic conductivity is typically measured by conducting aquifer tests or wellbore flow wbf logging of interest in our research is the use of proton nuclear magnetic resonance nmr logging to obtain information about water filled porosity and pore space geometry the combination of which can be used to estimate k in this study we acquired a suite of advanced geophysical logs aquifer tests wbf logs and sidewall cores at the field site in lexington nebraska which is underlain by the high plains aquifer we first used two empirical equations developed for petroleum applications to predict k from nmr logging data the schlumberger doll research equation ksdr and the timur coates equation kt c with the standard empirical constants determined for consolidated materials we upscaled our nmr derived k estimates to the scale of the wbf logging k kwbf logging estimates for comparison all the upscaled kt c estimates were within an order of magnitude of kwbf logging and all of the upscaled ksdr estimates were within 2 orders of magnitude of kwbf logging we optimized the fit between the upscaled nmr derived k and kwbf logging estimates to determine a set of site specific empirical constants for the unconsolidated materials at our field site we conclude that reliable estimates of k can be obtained from nmr logging data thus providing an alternate method for obtaining estimates of k at high levels of vertical resolution

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

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

evidence of dynamic recrystallization in polar firn 1 microstructural analyses have been performed on polar firn from the european project for ice coring in antarctica drilling site in dronning maud land antarctica the results derived from images of the firn structure in microscopic resolution indicate that dynamic recrystallization is active in firn at all depths and it dominates the evolution of the microstructure when the firn density exceeds a critical value of 730 kg m3 overburden snow load 0 2 mpa at the firn ice transition density 820 kg m3 the microstructure is characterized by many small grains and bulged or irregularly shaped grain boundaries more than half of all grains show subgrain boundaries thus strain induced boundary migration is an essential feature to describe the irregular grain structure in agreement with previous studies significant grain growth has been observed with depth for the largest grains in the samples however our microscopic analysis reveals that the grain growth with depth in fact vanishes if all grains larger than 65 m in diameter are taken into account this result reflects the fact that the growth of the largest grains is counteracted by grain size reduction by shrinking and subdivision of old grains as well as production of new grains consequently previous conclusions that grain growth in polar firn is essentially analogous to normal grain growth in metallic and ceramic sinters and that the stored strain energy is small in comparison with grain boundary energy can no longer be supported additionally our observations show that the incipience of dynamic recrystallization in polar ice sheets is not as sensitive to temperature as supposed so far a discussion of the change of the mean grain size due to the measuring technique is imperative

eight glacial cycles from an antarctic ice core the antarctic vostok ice core provided compelling evidence of the nature of climate and of climate feedbacks over the past 420 000 years marine records suggest that the amplitude of climate variability was smaller before that time but such records are often poorly resolved moreover it is not possible to infer the abundance of greenhouse gases in the atmosphere from marine records here we report the recovery of a deep ice core from dome c antarctica that provides a climate record for the past 740 000 years for the four most recent glacial cycles the data agree well with the record from vostok the earlier period between 740 000 and 430 000 years ago was characterized by less pronounced warmth in interglacial periods in antarctica but a higher proportion of each cycle was spent in the warm mode the transition from glacial to interglacial conditions about 430 000 years ago termination v resembles the transition into the present interglacial period in terms of the magnitude of change in temperatures and greenhouse gases but there are significant differences in the patterns of change the interglacial stage following termination v was exceptionally long 28 000 years compared to for example the 12 000 years recorded so far in the present interglacial period given the similarities between this earlier warm period and today our results may imply that without human intervention a climate similar to the present one would extend well into the future

novel shearing apparatuses in confined flow for investigating recrystallization and fabric evolution processes in mono and polycrystalline ice

competition between grain growth and grain size reduction in polar ice static or normal grain growth i e grain boundary migration driven solely by grain boundary energy is considered to be an important process in polar ice many ice core studies report a continual increase in average grain size with depth in the upper hundreds of metres of ice sheets while at deeper levels grain size appears to reach a steady state as a consequence of a balance between grain growth and grain size reduction by dynamic recrystallization the growth factor k in the normal grain growth law is important for any process where grain growth plays a role and it is normally assumed to be a temperature dependent material property here we show using numerical simulations with the program elle that the factor k also incorporates the effect of the microstructure on grain growth for example a change in grain size distribution from normal to log normal in a thin section is found to correspond to an increase in k by a factor of 3 5

multiscale modeling of ice deformation behavior understanding the flow of ice in glaciers and polar ice sheets is of increasing relevance in a time of potentially significant climate change the flow of ice has hitherto received relatively little attention from the structural geological community this paper aims to provide an overview of methods and results of ice deformation modeling from the single crystal to the polycrystal scale and beyond to the scale of polar ice sheets all through these scales various models have been developed to understand describe and predict the processes that operate during deformation of ice with the aim to correctly represent ice rheology and self induced anisotropy most of the modeling tools presented in this paper originate from the material science community and are currently used and further developed for other materials and environments we will show that this community has deeply integrated ice as a very useful model material to develop and validate approaches in conditions of a highly anisotropic behavior this review by no means exhaustive aims at providing an overview of methods at different scales and levels of complexity

a model of crystal size evolution in polar ice masses in the deep ice cores drilled at the grip ngrip and gisp2 sites in greenland and at byrd station and the summit of law dome in antarctica the mean crystal size increases with depth in the shallow subsurface and reaches steady values at intermediate depth this behaviour has been attributed to the competition between grain boundary migration driven crystal growth and crystal polygonization but the effects of changing crystal dislocation density and non equiaxed crystal shape in this competition are uncertain we study these effects with a simple model it describes how the mean height and width of crystals evolve as they flatten under vertical compression and as crystal growth and polygonization compete the polygonization rate is assumed to be proportional to the mean dislocation density across crystals migration recrystallization which can affect crystal growth via strain induced grain boundary migration but whose impact on the mean crystal size is difficult to quantify for ice at present is not accounted for when applied to the five ice core sites the model simulates the observed crystal size profiles well down to the bottom of their steady regions although the match for law dome is less satisfactory polygonization rate factors retrieved for the sites range from 10 5 to 10 2 a 1 we conclude that since crystal size and dislocation density evolve in a strongly coupled manner consistent modelling requires multiple differential equations to track both of these variables future ice core analysis should also determine crystal size in all three principal directions

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

dynamics of crystal formation in the greenland northgrip ice core the north greenland icecore project northgrip ice core provides palaeoclimatic information back to about 120 kyr bp the size distributions of ice crystals in the upper 880 m of the northgrip ice core which cover a time span of approximately 5300 years have been obtained previously the distributions evolve towards a universal curve indicating a common underlying physical process in the formation of crystals we identify this process as an interplay between fragmentation of the crystals and diffusion of their grain boundaries the process is described by a two parameter differential equation to which we obtain the exact solution the solution is in excellent agreement with the measured distributions

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

determination of pore size distribution in sedimentary rocks by proton nuclear magnetic resonance abstract the distribution of pore sizes in sedimentary rocks can be determined from laboratory measurements of proton nuclear magnetic resonance nmr longitudinal relaxation t 1 in water saturated rocks the details of the longitudinal relaxation curve are converted into a t 1 distribution curve by approximating the data with a sum of single exponential decays the non linear least squares minimization procedure includes a regularization term that reduces excursions in the computed density function thus producing smooth and robust results the t 1 distribution curve is scaled to pore size distributions by a phenomenological parameter that describes the strength of the proton interaction between the fluid and the pore wall this parameter is relatively constant in sandstones but is an order of magnitude less with greater variability in carbonates there is excellent qualitative and quantitative agreement between pore size distributions determined by nmr and independently determined information on pore sizes for a variety of sedimentary rocks the laboratory derived t 1 distributions are used to estimate the irreducible water saturation in sandstones by identifying the fraction of pores less than 4 m in diameter

application of nmr logging to reservoir characterization of low resistivity sands in the gulf of mexico1 nuclear magnetic resonance nmr technology from laboratory data and well logs is rapidly gaining acceptance as a petrophysical tool for evaluating reservoir quality the evaluation of laboratory nmr data as a core analysis technique shows that nmr relaxation time distributions match independent estimates of pore size distributions these other pore size estimates come from grain size analyses pore throat distributions from mercury porosimetry thin section petrography and permeability values irreducible water saturation estimates from nmr based pore size distributions also correspond well with independent measures of irreducible water from special core flood tests in this study we look at the relaxation time distributions extracted from laboratory and downhole nmr measurements to determine pore geometry and volumetrics within a reservoir this understanding leads to a better overall reservoir characterization both core and log nmr data are used to demonstrate that there is a direct connection between nmr relaxation time distributions and the fundamental property of pore size in sandstones this description of pore size distributions allows us to estimate permeability and irreducible water saturation additionally the nmr logs accurately characterize the nmr core lab measurements and provide important information that enhances the description of low resistivity pay

seasonal variability in ice crystal properties at northgrip a case study around 301 m depth the aim of this case study is to quantify the seasonal variability in crystal properties and to discuss the reason for the variability a continuous 1 10 m long vertical thin section profile covering approximately five annual cycles has been obtained from the northgrip ice core at around 301 m depth the crystal outline and the c axis orientation of more than 13k crystals in the profile have been measured on a new australian automated ice crystal analyzer in 2 5 cm resolution we observe a strong seasonal variability in crystal areas of more than 30 deviation from the average value of 6 7 mm2 each year a band of smaller crystals is observed in ice deposited during springtime the area distribution function is found to be close to a log normal distribution the crystal areas are compared to the concentration of chemical impurities in the ice at a 5 cm resolution the best correlation is found with the concentration of ca2 our results show no seasonal variability of the average c axis orientation of ice crystals

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

high resolution record of northern hemisphere climate extending into the last interglacial period high resolution record of northern hemisphere climate extending into the last interglacial period

dynamic recrystallisation of ice aggregates during co axial viscoplastic deformation a numerical approach results of numerical simulations of co axial deformation of pure ice up to high strain combining full field modelling with recrystallisation are presented grain size and lattice preferred orientation analysis and comparisons between simulations at different strain rates show how recrystallisation has a major effect on the microstructure developing larger and equi dimensional grains but a relatively minor effect on the development of a preferred orientation of c axes although c axis distributions do not vary much recrystallisation appears to have a distinct effect on the relative activities of slip systems activating the pyramidal slip system and affecting the distribution of a axes the simulations reveal that the survival probability of individual grains is strongly related to the initial grain size but only weakly dependent on hard or soft orientations with respect to the flow field dynamic recrystallisation reduces initial hardening which is followed by a steady state characteristic of pure shear deformation

the palynological response to the toarcian oceanic anoxic event early jurassic at peniche lusitanian basin western portugal abstract the uppermost pliensbachian and lower toarcian lower jurassic succession exposed at peniche in the central part of the lusitanian basin western portugal was examined for palynomorphs the 45 samples span the emaciaticeras emaciatum dactylioceras polymorphum and hildaites levisoni ammonite biozones abs and the succession includes the global boundary stratotype section and point gssp for the toarcian stage and the toarcian oceanic anoxic event t oae a low diversity dinoflagellate cyst flora typical of the sub boreal realm was recovered from the emaciaticeras emaciatum and dactylioceras polymorphum abs the dominant element is the cold water species luehndea spinosa which is an index for the pliensbachian to earliest toarcian and is thought to have migrated from the more northerly boreal realm prior to the t oae dinoflagellates thrived in the lusitanian basin except during a brief warm period in the earliest toarcian despite the latter the recovery from this event was relatively rapid and was characterised by a return to relatively cool temperatures the hildaites levisoni ab at peniche represents the t oae and the overlying strata and is characterised by a profound reduction in dinoflagellate cyst relative abundances this dinoflagellate cyst blackout and the associated rise of prasinophytes reflects significant environmental stress such as marine anoxia elevated temperatures and reduced salinity with the former two probably being most important the low proportions of dinoflagellate cysts following the t oae indicates a protracted recovery phase from the bottom and water column anoxia developed throughout the lusitanian basin

distribution of seismicity across strike slip faults in california 1 the distribution of seismicity about strike slip faults provides measurements of fault roughness and damage zone width in california seismicity decays with distance from strike slip faults according to a power law 1 x2 d2 2 this scaling relation holds out to a fault normal distance x of 3 6 km and is compatible with a rough fault loading model in which the inner scale d measures the half width of a volumetric damage zone and the roll off rate is governed by stress variations due to fault roughness according to dieterich and smith s 2 d simulations approximates the fractal dimension of along strike roughness near fault seismicity is more localized on faults in northern california nocal d 60 20 m 1 65 05 than in southern california socal d 220 40 m 1 16 05 the parkfield region has a damage zone half width d 120 30 m consistent with the safod drilling estimate its high roll off rate 2 30 25 indicates a relatively flat roughness spectrum k 1 versus k 2 for nocal k 3 for socal our damage zone widths the first direct estimates averaged over the seismogenic layer can be interpreted in terms of an across strike fault core multiplicity that is 1 in nocal 2 at parkfield and 3 in socal the localization of seismicity near individual faults correlates with cumulative offset seismic productivity and aseismic slip consistent with a model in which faults originate as branched networks with broad multicore damage zones and evolve toward more localized lineated features with low fault core multiplicity thinner damage zones and less seismic coupling our results suggest how earthquake triggering statistics might be modified by the presence of faults

crystalline texture of the 2083 m ice core at vostok station antarctica crystalline texture and c axis orien tation of the 2083 m ice core at vostok station covering more than 150 kyear reveal the existence of strong anisotropies changes in crystal size with depth are compatible with the growth of grains driven by the free energy of grain boundaries a smaller growth rate appears to be associated with cold periods a gradual increase in the horizontal elongation of grains was observed between 350 and 680 m but the mean value of the coefficient of the linear dimensional orientation of grains does not cha nge below 700 m the e axis orientation of ice grains tends to orientate perpendicular to the direction of the elongation of grains forming a vertical girdle patt ern this charac teristic fabric has been interpreted as resulting from the gradual rotation of grains by basal glide under uniaxial longit udi nal tension the rotation of grains was calcu lated with respect to the total strain simulating the form ation of the girdle fabric pattern the fabric enhance ment factor was calculated at various depths it appears that vostok ice hardens gradua lly

the microstructure of polar ice part ii state of the art an important feature of natural ice in addition to the obvious relevance of glaciers and ice sheets for climate related issues is its ability to creep on geological time scales and low deviatoric stresses at temperatures very close to its melting point without losing its polycrystalline character this fact together with its strong mechanical anisotropy and other notable properties makes natural ice an interesting model material for studying the high temperature creep and recrystallization of rocks in earth s interior after having reviewed the major contributions of deep ice coring to the research on natural ice microstructures in part i of this work faria et al 2014 here in part ii we present an up todate view of the modern understanding of natural ice microstructures and the deformation processes that may produce them in particular we analyze a large body of evidence that reveals fundamental flaws in the widely accepted tripartite paradigm of polar ice microstructure also known as the three stage model cf part i these results prove that grain growth in ice sheets is dynamic in the sense that it occurs during deformation and is markedly affected by the stored strain energy as well as by air inclusions and other impurities the strong plastic anisotropy of the ice lattice gives rise to high internal stresses and concentrated strain heterogeneities in the polycrystal which demand large amounts of strain accommodation from the microstructural analyses of ice cores we conclude that the formation of many and diverse subgrain boundaries and the splitting of grains by rotation recrystallization are the most fundamental mechanisms of dynamic recovery and strain accommodation in polar ice additionally in fine grained high impurity ice layers e g cloudy bands strain may sometimes be accommodated by diffusional flow at low temperatures and stresses or microscopic grain boundary sliding via microshear in anisotropic ice sheared at high temperatures grain boundaries bulged by migration recrystallization and subgrain boundaries are endemic and very frequent at almost all depths in ice sheets evidence of nucleation of new grains is also observed at various depths provided that the local concentration of strain energy is high enough which is not seldom the case as a substitute for the tripartite paradigm we propose a novel dynamic recrystallization diagram in the three dimensional state space of strain rate temperature and mean grain size which summarizes the various competing recrystallization processes that contribute to the evolution of the polar ice microstructure dedicated to sepp kipfstuhl on occasion of his 60th birthday

california foreshock sequences suggest aseismic triggering process 1 foreshocks are one of the few well documented precursors to large earthquakes therefore understanding their nature is very important for earthquake prediction and hazard mitigation however the triggering role of foreshocks is not yet clear it is possible that foreshocks are a self triggering cascade of events that simply happen to trigger an unusually large aftershock alternatively foreshocks might originate from an external aseismic process that ultimately triggers the mainshock in the former case the foreshocks will have limited utility for forecasting the latter case has been observed for several individual large earthquakes however it remains unclear how common it is and how to distinguish foreshock sequences from other seismicity clusters that do not lead to large earthquakes here we analyze foreshocks of three m 7 mainshocks in southern california these foreshock sequences appear similar to earthquake swarms in that they do not start with their largest events and they exhibit spatial migration of seismicity analysis of source spectra shows that all three foreshock sequences feature lower average stress drops and depletion of high frequency energy compared with the aftershocks of their corresponding mainshocks using a longer term stress drop catalog we find that the average stress drop of the landers and hector mine foreshock sequences is comparable to nearby swarms our observations suggest that these foreshock sequences are manifestations of aseismic transients occurring close to the mainshock hypocenters possibly related to localized fault zone complexity which have promoted the occurrence of both the foreshocks and the eventual mainshock

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

a synchronized dating of three greenland ice cores throughout the holocene as part of the effort to create the new greenland ice core chronology 2005 gicc05 a synchronized stratigraphical timescale for the holocene parts of the dye 3 grip and ngrip ice cores is made by using volcanic reference horizons in electri cal conductivity measurements to match the cores the main annual layer counting is carried out on the most suited records only exploit ing that the three ice cores have been drilled at locations with different climatic con ditions and differences in ice flow however supplemental counting on data from all cores has been performed between each set of reference horizons in order to verify the valid ity of the match after the verification the main dating is transferred to all records us ing the volcanic reference horizons as tie points an assessment of the mean annual layer thickness in each core section confirms that the new synchronized dating is consistent for all three cores the data used for the main annual layer counting of the past 7900 years are the dye 3 grip and ngrip stable isotope records as the high accumulation rate at the dye 3 drill site makes the seasonal cycle in the dye 3 stable isotopes very resistant to firn diffusion an effort has been made to extend the dye 3 holocene record the new syn chronized dating relies heavily on this record of 75 000 stable isotope samples the dat ing of the early holocene consists of an already established part of gicc05 for grip and ngrip which has now been transferred to the dye 3 core gicc05 dates the younger dryas termination as defined from deuterium excess to 11 703 b2k 130 years earlier than the previous grip dating

the microstructure of polar ice part i highlights from ice core research polar ice sheets play a fundamental role in earth s climate system by interacting actively and passively with the environment active interactions include the creeping flow of ice and its effects on polar geomorphology global sea level ocean and atmospheric circulation and so on passive interactions are mainly established by the formation of climate records within the ice in form of air bubbles dust particles salt microinclusions and other derivatives of airborne impurities buried by recurrent snowfalls for a half century scientists have been drilling deep ice cores in antarctica and greenland for studying such records which can go back to around a million years experience shows however that the ice sheet flow generally disrupts the stratigraphy of the bottom part of deep ice cores destroying the integrity of the oldest records for all these reasons glaciologists have been studying the microstructure of polar ice cores for decades in order to understand the genesis and fate of ice core climate records as well as to learn more about the physical properties of polar ice aiming at better climate record interpretations and ever more precise models of ice sheet dynamics in this part i we review the main difficulties and advances in deep ice core drilling in antarctica and greenland together with the major contributions of deep ice coring to the research on natural ice microstructures in particular we discuss in detail the microstructural findings from camp century byrd dye 3 grip gisp2 northgrip vostok dome c edml and dome fuji besides commenting also on the earlier results of some pioneering ventures like the jungfraujoch expedition and the norwegian british swedish antarctic expedition among others in the companion part ii of this work faria et al 2014 the review proceeds with a survey of the state of the art understanding of natural ice microstructures and some exciting prospects in this field of research

a numerical study of anisotropic low reynolds number free surface flow for ice sheet modeling few ice sheet flow models have been developed that solve the complete set of mechanical equations until now these models were limited to isotropic conditions we present here a two dimensional finite difference method capable of solving the equations for the steady flow of a viscous incompressible anisotropic fluid with a free surface under isothermal conditions it is not a standard method especially with respect to the time discretization of the numerical scheme and converges for very low reynolds numbers this method is applied here to the planar flow of anisotropic ice over flat or irregular bedrock with no slip boundary conditions at the ice bedrock interface the results are presented here for newtonian behavior in the vicinity of an ice divide the ice is assumed to be isotropic at the ice sheet surface with continuous and prescribed development of anisotropy with increasing depth going from isotropic to anisotropic situations our results indicate that the free surface becomes flatter and the shear strain rates larger and more concentrated near the bedrock the flow is less sensitive to variations of the bedrock topography in the anisotropic case than in the isotropic case furthermore a new phenomenon appears in the anisotropic case the partial stagnation of ice in the holes of the bedrock these effects have significant consequences when dating the ice the isochrones obtained in the anisotropic case are flatter and the anisotropic ice is more than 10 younger above the bumps and more than 100 older within the holes than for the isotropic ice

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

integrated petrophysical evaluation of the lower middle bakken member in the viewfield pool southeastern saskatchewan canada abstract the viewfield pool of the lower middle bakken member the largest light oil pool in saskatchewan canada has been described as an unconventional resource play reservoir evaluation in this pool has been challenging because of low permeability complex lithology and reservoir heterogeneity to address the difficulties this study employed an integrated approach to derive reservoir parameters and provided a detailed petrophysical characterization of the viewfield middle bakken play which led to a division of unit a the lower middle bakken member into two subunits a1 for the lower interval and a2 for the upper interval petrophysical models were developed based on thorough analyses of core measurements conventional well logs and available advanced nuclear magnetic resonance nmr well log data the proposed models were applied to determine reservoir properties including shale content porosity permeability and water saturation for the two subunits in 163 vertical wells among which three have nmr logs the results reveal that subunits a1 and a2 differ significantly in petrophysical properties and reservoir quality a2 the very fine grained dolomitic sandstone subunit has larger pore size higher porosity and permeability lower shale content and water saturation and thus represents the preferred interval for oil production across the viewfield bakken pool area and has greater resource potential than the siltstone subunit a1 the mapped petrophysical properties show spatial trends that align well with the previously recognized regional geological features favourable potential high productivity zones can be outlined by the areas of intersection of all essential reservoir parameters above defined threshold values

possible precursory signals in damage zone foreshocks foreshocks may provide a precursory signal of an impending earthquake but their role in nucleation of the mainshock is unclear one way to further our understanding of foreshock failure mechanisms is to determine where they occur in the fault zone however earthquake locations commonly include uncertainties large enough to allow rupture on either the main fault interface or on subsidiary fractures within a surrounding damage zone here we obtain precise earthquake locations with 10 m uncertainty for foreshocks and aftershocks of an mw5 0 near prague ok usa repeating earthquakes imply that some precursory slow slip occurred before the mainshock in addition we show that foreshocks initially rupture faults and fractures throughout the 300 meter thick fault damage zone and later localize onto a narrower zone 100 m thick nearer the mainshock hypocenter focal mechanisms corroborate that foreshocks occur in the surrounding damage zone as well as on the mainshock rupture interface these results highlight that earthquake nucleation is most likely a complex feedback between frictional failure processes on the fault interface and deformation in the surrounding damaged rock not just nucleation on a single surface

subgrain boundaries and related microstructural features in edml antarctica deep ice core subgrain boundaries revealed as shallow sublimation grooves on ice sample surfaces are a direct and easily observable feature of intracrystalline deformation and recrystallization statistical data obtained from the epica dronning maud land edml deep ice core drilled in east antarctica cannot detect a depth region of increased subgrain boundary formation grain boundary morphologies show a strong influence of internal strain energy on the microstructure at all depths the data do not support the classical view of a change of dominating recrystallization regimes with depth three major types of subgrain boundaries reflecting high mechanical anisotropy are specified in combination with crystal orientation analysis

frequency dependence of delayed and instantaneous triggering on laboratory and simulated faults governed by rate state friction earthquake triggering by transient stresses is commonly observed however some aspects remain unexplained the first is the often observed delay between the triggered earthquakes and the triggering waves and the second is the unexpected effectiveness of transient stressing in the seismic frequency band previous theoretical and laboratory studies have suggested that seismic transients should have little impact on faults if the duration of the transient is smaller than the timescale for nucleation of slip we reexamine the dynamics of stress triggering during stick slip sliding on a laboratory fault and make three important observations that pertain to earthquake triggering 1 delayed triggering clock advance occurs for both bare granite surfaces and granular gouge prior to the onset of instantaneous triggering 2 triggering occurs much earlier in the stick slip cycle than expected for a simple coulomb stress threshold 3 shorter period higher stressing rate pulses are more effective at triggering than longer period pulses of the same stress amplitude we use numerical simulations to show that rate state friction can explain each of the observed features but not all three simultaneously only the ruina slip law for state evolution in which faults must slip to heal can reproduce early onset and stressing rate dependent triggering the laboratory and numerical experiments show that faults can remain relatively weak over much of the seismic cycle and that the triggered response depends on a competition between healing and weakening during triggered slip transient stressing at seismic frequencies may be more effective at triggering earthquakes than previously recognized

radiography of a normal fault system by 64 000 high precision earthquake locations the 2009 l aquila central italy case study 1 we studied the anatomy of the fault system where the 2009 l aquila earthquake mw 6 1 nucleated by means of 64 k high precision earthquake locations spanning 1 year data were analyzed by combining an automatic picking procedure for p and s waves together with cross correlation and double difference location methods reaching a completeness magnitude for the catalogue equal to 0 7 including 425 clusters of similar earthquakes the fault system is composed by two major faults the high angle l aquila fault and the listric campotosto fault both located in the first 10 km of the upper crust we detect an extraordinary degree of detail in the anatomy of the single fault segments resembling the degree of complexity observed by field geologists on fault outcrops we observe multiple antithetic and synthetic fault segments tens of meters long in both the hanging wall and footwall along with bends and cross fault intersections along the main fault and fault splays the width of the l aquila fault zone varies along strike from 0 3 km where the fault exhibits the simplest geometry and experienced peaks in the slip distribution up to 1 5 km at the fault tips with an increase in the geometrical complexity these characteristics similar to damage zone properties of natural faults underline the key role of aftershocks in fault growth and co seismic rupture propagation processes additionally we interpret the persistent nucleation of similar events at the seismicity cutoff depth as the presence of a rheological i e creeping discontinuity explaining how normal faults detach at depth