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

Firn microstructure properties from six different sites in Greenland and Antarctica are investigated by means of X-ray microcomputer tomography. The optical effective radius is calculated from the specific surface area (SSA) and used as a measure of grain size. It is shown that the recently introduced spherical approximation of firn grains using the effective radius R_eff is representative of grain size in the microwave frequency region. The measured profiles show the well-known increase of grain size with depth at all sites, where the increase is largest at near-surface depths. A large variability in grain size on the decimeter-to-centimeter scale as a result of different grain properties of single layers is superimposed on the overall trend at each site. A simple empirical parameterization of grain-size evolution is developed which allows the rapid grain growth in the uppermost layers of the firn to be predicted. The growth is driven by strong seasonal and diurnal temperature gradients. The model can be used to simulate grain-size profiles required by models of firn/microwave interaction (e.g. for retrieval of accumulation rates from satellite microwave sensors) in a more realistic fashion.

Polar ice cores provide a wealth of information on past climate through a well stratified archive of past snow, firn and ice as well as entrapped atmosphere. Size, shape and number of air bubbles and the air content in ice cores are believed to preserve the pore-space structure at the time of firn-ice transition and therefore provide information on temperature or accumulation (Spencer et al., 2006) or even local insolation (Raynaud et al., 2007). In case of the air content, it may be influenced by temperature, accumulation and pressure (ice sheet elevation) on a longer time-scale or on shorter time-scale by imprinted structural modifications with changing surface snow properties or changing firnification being potential candidates.For low accumulation sites like Vostok and EDC, changes in air content linked to stratigraphic heterogeneity has been found to be non existent or comparable with the uncertainty of the measurement. On high accumulation sites seasonal variations in air content of up to 25 % have been found and interpreted as sealing effect of summer snow layers by winter snow layers (Martinerie, 1992).For the case of DML we revealed a possible link between the air content and the stratigraphic heterogeneity of ice on the B 34 ice core close to EDML from the depth interval 193.0 to 194.0 m by combining total air measurements with high-resolution line-scans of ice sections.

Two methods are described to obtain grain (crystal) size from images of firn and ice samples taken in microscopic resolution. Microstructure mapping provides a digital mosaic image of a firn/ice section assembled from single microphotographs (resolution 3.25 µm/pixel). Alternatively, the large area scan macroscope (LASM; Schäfter and Kirchhoff, Hamburg, Germany) scans a section as one single image. The lower resolution (5 µm/pixel) is compensated by the much faster working speed (1 minute versus 1 hour for mapping). The preparation of the samples is identical. Grain size is derived from the network of grain boundary grooves emerging at a carefully microtomed surface.First results of a 110 m long shallow ice core drilled in Dronning Maud Land, Antarctica, are presented. Average grain sizes were determined over 1 m long intervals every 10 m using the faster LASM. For comparison additional grain sizes were derived from the mosaic images. The main result obtained so far is that average grain size depends on resolution, i.e. on the cut-off grain size taken for the averages. This concerns particularly the average grain sizes below about 30 m depth. If the average grain size is determined from the 100 or 500 largest grains in a section (as sometimes done in the past) there is significant grain growth. However, grain size hardly increases with depth or even vanishes if all grain sizes larger than 50 m are included in the averages. A first answer to explain this discrepancy comes from the grain size distributions. They show a clear shift to more smaller grains at depths close and across the firn-ice transition which points to formation of new grains and dynamic recrystallization. The question how to derive average grain size from high resolution images and how to interpret these results is discussed.

The classical view of the microstructure evolution in polar is: normal grain growth, i.e. grain growth driven by grain boundary surface energy, predominates in the upper part of an ice sheet; in the middle part grain division by polygonization, which explains the stop of grain growth in some ice cores (e.g. Byrd or GRIP); in the bottom part, which is characterized by high temperature and shearing, grain boundary migration recrystallization (GBMR) leads to the large grain sizes observed close to bedrock. New results discussed here question this scheme as too simplistic.High resolution imaging methods (e.g. microstructure mapping) reveal microstructural features characteristic of dynamic recrystallization in firn, shallow and deep ice: e.g. different types of subgrain boundaries, irregularly shaped and bulging grain boundaries, formation of new grains, and slip bands. In contrast to previous results which report a linear increase in grain size in firn and the upper hundreds of meters of ice sheets, we find no significant growth in firn and across the firn-ice transition. Our results suggest that dynamic recrystallization is active in firn and ice at any depth and that in the EPICA Dronning Maud Land (EDML) core dynamic recrystallization is the predominant recrystallization regime from the firn-ice transition to bedrock.

Polar snow and firn represents the result of snow metamorphism at very long time scales of many tens of years and in a broad variability of climate conditions. At polar ice sheets the diverse snow types of firn layers and the long term behavior of their microstructure and density with depth and at different sites can be studied. We present microstructure properties of polar snow and firn obtained by X-Ray Micro-Computer-Tomography from various sites in Greenland and Antarctica with different annual mean temperatures and accumulation rates. We will show and discuss the evolution of three-dimensional microstructural features such as coordination number, shape factor or specific surface area with depth and time at the different sites. In order to parameterize the evolution of microstructure with time we map out universal characteristics and regional distinctions of the polar snow- and firn microstructure.

The high resolution density of 16 firn and shallow ice cores from Greenland and Antarctica was determined by applying the gamma-ray absorption method. Profiles of mean density and density fluctuations (i. e. deviations from the mean) are presented and discussed.The profiles of mean density show the patterns well known from the numerous firn cores studied so far. However, the density fluctuations appear to behave anomalous insofar, as for a layered medium we expect a direct correlation of the density fluctuations with the increase in density. That means that the deviation from the mean decreases steadily with increasing density, until the density of bubbly ice is reached. But in contrast to the expected steady decrease we observe a relatively fast decrease of the amplitudes in density fluctuations close to the surface. They assume a local minimum for densities between 600 and 650 kg/m^3 (in depths of 20 to 50 meter, depending on accumulation rate and annual mean temperature) but then increase again. Local maxima are obtained for densities between 750 and 850 kg/m^3, leading to increased amplitudes throughout the firn-ice transition. Below the fluctuations decrease again, approaching the values for bubbly ice.Since the investigated cores reflect different climate conditions with accumulation rates varying from 30 mm water equivalent per year to >1500 mm water equivalent per year and annual mean temperatures from -20°C to -55°C we assume, that the behaviour of the density fluctuations observed in our cores is a fundamental property of polar firn, not discussed much in literature due to the lack of high resolution density profiles. The possible origin and implications of this phenomenon, e. g. for modelling the densification of polar firn or the air enclosure processes, are discussed.

Polar firn forms a porous and permeable medium. We take 3D micro structural properties obtained by X-ray micro Computer- Tomography to model the measured permeability. The empirical relationship fairly well simulates low permeable layers and the stratigraphic induced variability of the measured profile, but fails for high permeable layers. By applying a sequence of morphological opening the connectivity function is obtained. High permeable layers are characterized by large well connected pores. The occurrence of these specific layers can be linked to the time, the firn was exposed to near-surface temperature gradient metamorphism, convection and coarsening.

It is of fundamental importance for ice core interpretations to understand how near surface properties of firn microstructure are linked to the process of air entrapment at the firn-ice transition in 60 to 100 m depth. There is still an open debate with the consensus that temperature and accumulation rate are not sufficient to parameterize the pore close-off density/porosity especially its change over time at one specific site. There are suggestions that some anisotropic features (extended connections via vertical pores) the layering (increase of mean air content due to the sealing effect of dense layers) or differences in grain size (sintering of larger grains increases enclosed air volume) might affect the close-off density respective air content however without much experimental evidence. In this talk we will give experimental arguments for the layering hypothesis. The measurements on microstructure at various polar sites were done with the means of three different methods including 3D-X-ray micro-computer tomography (µCT), 2D- microstructure mapping (µSM) and 1D-absorption of gamma radiation (AGM). The MiCT measurements provide the first data set of three dimensional parameters on dry polar firn including the coordination number, the structural anisotropy, neck radii, grain clustering and open/closed porosity determinations. The investigations at the firn-ice transition reveal surprisingly large porosity variations on the cm-scale (large layering). However the estimates of open/closed porosity suggest constant critical porosities for firn microstructure (independent of grain size). Even no evidence for significant anisotropies at the firn-ice transition has been detected. The microstructure observations at the firn-ice transition are compared with results from a percolation model based on a regular bcc lattice which has taken the layered structure into account. The agreement between model results and observations supports the layering hypothesis and show that th

Free or hydrate-bound gas in the seafloor has been of scientific, ecologic and economic interest for many years because it predominantly contains high concentrations of low-molecular-weight hydrocarbons. A prerequisite of accurate quantifications of gases in sediments is to preserve pressure and temperature close to the in situ conditions during recovery. Here we introduce two new sediment coring devices that allow for the recovery of near-surface gas- and gas-hydrate-bearing sediments and subsequent investigations using several different techniques such as visualisation by computerized tomography, quantitative degassing, and sediment and porewater analyses. The first coring tool, the Multiple Autoclave Corer (MAC), resembles a standard multiple corer in terms of applications, size and core length of about 55 cm. The second tool, the Dynamic Autoclave Piston Corer (DAPC), is similar to a piston corer in application and size and enables one to take cores of up to 2.5 m length. Both focus on the investigation of near-surface sediments, which are most strongly affected by changes in bottom-water temperature and hydrostatic pressure, which in turn influence continental slope stability. Some results from recent offshore applications show the potential of these tools.

The firnification process of dry polar firn is of central interest in the context of refined interpretations of climate proxi parameters in deep polar ice cores. Especially the dating of enclosed air relies on realistic densification models. However, recent models follow mainly an empirical approach and do not account for the observed strong firn stratification which could generate significant errors under different climate conditions. More physically motivated models seem to be needed on the basis of comprehensive descriptions of microstructure. In this study we combine two experimental methods to investigate the microstructure of polar firn: The three dimensional structure of the ice and pore phases has been measured with the means of X-ray micro-computer tomography (µCT). This method offers a large set of parametrizations for the description of layered firn. Our approach includes different definitions of ice and pore clusters, neck radii , intercept lengths, distributions of surface normals, surface density, anisotropy ratios, tortuousity, Euler number and connectivity functions. Surface sections of cut 3D-sample are additionally analysed with the means of the 2D-microstructure mapping method (µSM). The µSM under microscope and in reflected light records grain boundaries and pore structures with high resolution. From µSM the classical grain and pore size distributions, two dimensional derivations of neck radii and shape factors are estimated. All measurements were carried out on shallow firn cores during the field campaign 2005/2006 at the EPICA (European Project of Ice Coring in Antarctica) DML-drill site, Antarctica. We show that the number of bonds per cluster in samples near the surface is highly correlated to their respective density. Accordingly, the neck radii are anticorrelated to the density. Maxima of pore size and anisotropy are observed at 2 to 3 meter depth where the metamorphism is dominated by temperature gradients. To highlight the methodological

The firnificaton of dry polar firn has been investigated with the means of X-ray micro-computer tomography (CT). The measurements were carried out on shallow firn cores and snow pit samples during the field campaign 2005/2006 at the EPICA (European Project of Ice Coring in Antarctica) DML-drill site, Antarctica. The CT-devices have been successfully tested in the field and offered us a novel method to study the fragile three dimensional structures of deposited snow and firn. Several events of dune formation have been observed during the summer campaign. The dunes are well preserved in the deeper snow with the consequence of highly stratified firn. In this presentation we give a detailed description of stratigraphy with the focus on the evolution of pore/ice cluster sizes and intergranular bonding. We show that the number of bonds per cluster in samples near the surface is highly correlated to their respective density. Accordingly, the neck radii are anticorrelated to the density. Maxima of pore size and anisotropy are observed at 2 to 3 meter depth where the metamorphism is dominated by temperature gradients. To highlight the methodological aspect we present different size parametrisations and discuss the three dimensional CT- approach in relation to the standard two dimensional techniques.

Air is enclosed during the firn-ice transition and stored in polar ice, forming a reliable archive for ancient atmospheric air. The complex enclosure mechanism determines the age difference between ice and enclosed air, but also the air content. The firn-ice transitions at two sites in Antarctica (EDC and EDML) and one in North Greenland (B26) have been investigated by using X-ray micro-computer-tomography (µCT) on a series of samples from the transition zone. The study focuses on the small-scale density/porosity variations across the firn-ice transition, the three dimensional structure of the pore space and especially the partition between open and closed pores. The observations are compared with results from a percolation model based on a regular bcc lattice. The agreement between model results and observations for the relatively small sample sizes are surprisingly good and allow to extend important results to a larger spatial extent. This allows especially to quantitatively estimate the connectivity through thin layers with larger densities. Small permeability is still possible through layers of ice with a density of up to 850 kg m-3 if the layer thickness is only a few millimeters. Model calculations show that the extend of the zone of air trapping and especially the final air content of the ice depend not only on the amplitude of porosity, respectively density variations with depth but also on the frequency of such fluctuations. We conclude that the mean porosity at the firn-ice transition (lock-in zone where vertical molecular diffusion stops) depends primarily on stratigraphic features and not or not so much on parameters at the surface as temperature or grain size as previously assumed. The strong influence of stratigraphy is supported by model results of the percolation approach. The air content and the extent of the lock-in zone for the three drilling sites are calculated based on such simulations.

Various processes on atomic scale (e.g. dislocation climb and glide, diffusionalprocesses etc.) are often competing during flow and deformation of ice sheets. Asthese processes are acting on the atomic scale, they are difficult to observe directlyin deformed polycrystalline ice. However, they leave behind certain structures on themicroscopic scale indicating deformation mechanisms.The microstructure mapping method enables detailed observation and recording ofmany kinds of microstructures (Fig.1) such as grain boundaries, sub-grainboundaries, slip lines and any kind of visible inclusions as air bubbles, air hydrates orparticles (tephra, dust or salt). Data on grain size, sub-grain boundary occurrence,grain elongation and grain shape have been obtained with this method.Mean grain size is increasing with depth mainly in the lower 1500 m. Mean sub-grainboundary density varies between ca. 1 to 2 mm-1 along the length of the core, butdoes not show a distinct depth region of increased sub-grain boundary formation.Mean grain aspect ratio reveals values between ca. 1.7 to 2, though apart from adistinct maximum of ca. 2.3 at 2400 m depth it does not show significant trends.

Polares Eis speichert Informationen über das Klima vergangener Zeit. Vonbesonderem Interesse sind eingeschlossene Luftbläschen, die Rückschlüsseauf Änderungen der Atmosphärenzusammensetzung zulassen. Um jedoch diegespeicherten Klimasignale interpretieren zu können, ist es notwendig,den Entstehungsprozess des Archivs und damit die Sinterung des Schneeszu Eis und den Einschlussmechanismus der Luft im Eis zu klären. In demVortrag werden erste Ergebnisse von Strukturuntersuchungen vorgestellt,die im Rahmen des Antarktischen Tiefbohrprojektes EPICA in der Saison2005/2006 mittels portabler Röntgen-Mikro-Computertomografen im Feldgewonnen wurden. Neben der Strukturbeschreibung im frühen Stadium derSinterung liegt der Schwerpunkt der Arbeiten bei der Ableitung vonkritischen Porositäten für den Lufteinschluss und den Einfluss derSchichtung innerhalb der Antarktischen Schneedecke. Dafür werden Datenaus der Bildanalyse mit einem vereinfachten Perkolationsansatz gekoppelt.

The firnificaton of dry polar snow has been investigated with the means of X-ray micro-computer tomography (CT). The measurements were carried out on shallow firn cores and snow pit samples during the field campaign 2005/2006 at the EPICA (European Project of Ice Coring in Antarctica) DML-drill site, Antarctica. The CT-devices have been successfully tested in the field and offered us a novel method to study the fragile three dimensional structures of deposited snow and firn.In this presentation we focus on the evolution of pore/ice cluster sizes and on the development of intergranular bonding. The parameters are derived from volume image analysis of the three-dimensional CT-reconstructions. We show that the number of bonds per cluster in samples near the surface is highly correlated to their respective density. Accordingly, the neck radii are anticorrelated to the density. Maxima of pore size and anisotropy are observed at depths where the metamorphism is dominated by temperature gradients. The relevance of experimental findings is discussed in relation to recent firnification models.

Precise knowledge of the phase relationship between climate changes in the two hemispheres is a key for understanding the Earth’s climate dynamics. For the last glacial period, ice core stud- ies1,2 have revealed strong coupling of the largest millennial-scale warm events in Antarctica with the longest Dansgaard–Oeschger events in Greenland3–5 through the Atlantic meridional over- turning circulation6–8. It has been unclear, however, whether the shorter Dansgaard–Oeschger events have counterparts in the shorter and less prominent Antarctic temperature variations, and whether these events are linked by the same mechanism. Here we present a glacial climate record derived from an ice core from Dronning Maud Land, Antarctica, which represents South Atlantic climate at a resolution comparable with the Greenland ice core records. After methane synchronization with an ice core from North Greenland9, the oxygen isotope record from the Dronning Maud Land ice core shows a one-to-one coupling between all Antarctic warm events and Greenland Dansgaard–Oeschger events by the bipolar seesaw6. The amplitude of the Antarctic warm events is found to be linearly dependent on the duration of the concurrent stadial in the North, suggesting that they all result from a similar reduction in the meridional overturning circulation.

Cloudy bands probably caused by the high dust concentrations of winter snow deposited during glacial periods are characteristic stratigraphic features of Greenland ice cores. While sofar no (Vostok and Dome C) or only few (Dome Fuji) cloudy bands were reported from Antarctic ice cores the EDML ice core drilled in Dronning Maudland contains abundant cloudy bands like the GRIP, GISP2 or NGRIP ice cores. Number and intensity of cloudy bands vary with climate, the colder the more and the more intense cloudy bands are observed. Cloudy bands indicate the layering in an ice sheet and can be used to detect flow induced disturbances in an ice core. We find increased wavyness of cloudy bands below about 2050 m depth. Clear ice contains about 1-100 optically visible particles but cloudy band ice on the order of >10 000, intense cloudy bands typically 105 to 106 per cm3 and more. Microscopic observations show that grain size is much smaller in cloudy bands than in the clear ice between the cloudy bands but the distribution of micro-particles is not related to grain boundaries or triple junctions.

Polare Eisschilde als gigantisches Sinterungsexperiment der Natur, Röntgentomografische Analyse der Firn und Eisstrukturen, Bestimmung der kritischen Porosität am Perkolationsübergang von Firn zu Eis, Vorstellung eines Verfahrens zur Bestimmung der geschlossenen und offenen Porosität am Übergang

The air content in cold glacier ice depends on the pore space volume and on local air pressure and temperature at the time of pore close-off. A new method to measure the air content of glacier ice has been developed and will be presented. The measurement has been integrated into the Continuous Flow Analysis set-up which is used for chemical profiling of the EPICA ice cores. It allows the continuous determination of the air content at high depth resolution (<1cm). The air content has been measured continuously in the deep ice core from Dronning Maud Land (EDML), Antarctica from 700 m to 2564 m depth, which tentatively corresponds to approx. 10 ka to 230 ka before present. We find a mean air content of 0.0815 mlSTP/g, which corresponds to a mean firn density of 0.838 g/cm3 at pore close-off. The profile exhibits long term variations of about 10%; these are not dominated by the 100ka cycle but instead by the 42 ka insolation cycle. The aircontent profile from EDML is compared to the one from EPICA Dome-C with respect to mean value, degree and phasing of the variations. The high resolution record available from EDML exhibits strong variability of the air content even on the 1 cm scale. The air content does not follow a clear seasonality in the examples investigated. The high resolution examples with larger relative variability also show higher average air content. This suggests that the average air content may be increased by stronger small-scale density variations, as the effective pore close-off may happen at shallower depths in these cases due to formation of lids in layers with high density, which can trap the air below in areas with relatively high porosity. These density variations in turn may be connected to local surface insolation.