Pore Close-off Research Articles

Ice-Core Micro-CT Image Segmentation with Dual Stream Spectrum Deconvolution Neural Network and Gaussian Mixture Model

Polar ice sheets, or ice cores, are among the most well-known natural archives that might provide crucial historical details about our planet's previous environment. An important factor in establishing the fundamental characteristics of ice, likes pore close-off, albedo, melt events, is the ice-core microstructure. To engulf these complications Ice-Core Micro-CT Image Segmentation with Dual Stream Spectrum Deconvolution Neural Network and Gaussian Mixture Model (ICMCTS-WSOA-DSSDNN-GMM)is proposed. Initially, micro scale CT images are collect from Alfred Wegener Institute ice-core storage as input. Then,data’s are given to pre-processing. In pre-processing, it enhance image brightness, remove salt pepper noise, crop outer ring (carbon fiber casing) to have only ice particles in image utilizing Federated Neural Collaborative Filtering (FNCF).The pre-processing output is given to segmentation. Here, micro scale CT image is utilized for segmenting high-resolution scans using Gaussian Mixture Model (GMM). After that, the segmented images are given to Dual Stream Spectrum Deconvolution Neural Network optimized with Water Strider Optimization Algorithm for classifying micro scale CT images as sintered snow, compacted firn and bubbly ice. The proposed ICMCTS-WSOA-DSSDNN-GMM method is executed in python. The performance of ICMCTS-WSOA-DSSDNN-GMM approach attains 16.24%, 17.90% and 27.7% high accuracy, 14.04%, 25.51% and 19.31% higher precision and 14.36%, 12.65%, 14.51% higher recall analyzed with existing techniques likes Ice-Core Micro-CT Image Segmentation With Deep Learning and Gaussian Mixture Method (ICMCTS-U-net-GMM), Computer Aided Detection of COVID 19 from CT Images Depend on Gaussian Mixture Method with Kernel Support Vector Machines Classifer (ICMCTS-KNN-GMM) and the Gmmseg: Gaussian mixture based generative semantic segmentation methods(ICMCTS-FCN-GMM) respectively.

Reconstructing atmospheric H2 over the past century from bi-polar firn air records

Abstract. Historical atmospheric H2 levels were reconstructed using firn air measurements from two sites in Greenland (NEEM and Summit) and two sites in Antarctica (South Pole and Megadunes). A joint reconstruction based on the two Antarctic sites yields H2 levels monotonically increasing from about 330 ppb in 1900 to 550 ppb in the late 1990s, leveling off thereafter. These results are similar to individual reconstructions published previously (Patterson et al., 2020, 2021). Interpretation of the Greenland firn air measurements is complicated by challenges in modeling enrichment induced by pore close-off at these sites. We used observations of neon enrichment at NEEM and Summit to tune the parameterization of enrichment induced by pore close-off in our firn air model. The joint reconstruction from the Greenland data shows H2 levels rising 30 % between 1950 and the late 1980s, reaching a maximum of 530 ppb. After 1990, reconstructed atmospheric H2 levels over Greenland are roughly constant, with a small decline of 3 % over the next 25 years. The reconstruction shows good agreement with the available flask measurements of H2 at high northern latitudes.

Pure rotational Raman spectroscopy applied to N2/O2 analysis of air bubbles in polar firn

AbstractEarlier gas measurements of firn air (atmosphere in open pore channels) at polar sites have revealed the occurrence of gas fractionation phenomena during bubble close-off, in addition to well-known thermal and gravitational gas separation. Nevertheless, because of difficulties posed by measurement, little is known about the distribution of air constituents in already closed pores (bubbles) in firn. Herein, we describe the application of high-sensitivity pure rotational Raman spectroscopy, combined with sample immersion in the fluorocarbon-based inert fluid for removing the optical disturbance by diffused reflection. That application efficiently elicits information about nitrogen and oxygen composition ratios (N2/O2 or O2/N2) for each air bubble in firn. The developed methodology presents important implications for elucidating how gas records are formed and modified in the course of pore close-off in polar firn.

Horizontal ice flow impacts the firn structure of Greenland's percolation zone

Abstract. One-dimensional simulations of firn evolution neglect horizontal advection from ice flow, which transports the firn column across climate gradients as it is buried by accumulation. Using a suite of model runs, we demonstrate the impacts of horizontal advection on the development of firn density, temperature, and the stratigraphy of melt features through the Greenland ice sheet percolation zone. The simulations isolate processes in synthetic runs and investigate four specific transects and an ice core site. Relative to one-dimensional simulations, the horizontal advection process tends to increase the pore close-off depth, reduce the heat content, and decrease the frequency of melt features with depth by emplacing firn sourced from higher locations under increasingly warm and melt-affected surface conditions. Preservation of the advected pore space and cold content is strongly dependent upon the depth of meltwater infiltration. Horizontal ice flow interacts with topography, climate gradients, and meltwater infiltration to influence the evolution of the firn column structure; the interaction between these variables modulates the impact of horizontal advection on firn at locations around Greenland. Pore close-off and firn temperature are mainly impacted in the lowermost 20–30 km of the percolation zone, which may be relevant to migration of the lower percolation zone. Relatively high in the percolation zone, however, the stratigraphy of melt features can have an advection-derived component that should not be conflated with changing climate.

Layering of surface snow and firn at Kohnen Station, Antarctica: Noise or seasonal signal?

The density of firn is an important property for monitoring and modeling the ice sheet as well as to model the pore close-off and thus to interpret ice core-based greenhouse gas records. One feature, which is still in debate, is the potential existence of an annual cycle of firn density in low-accumulation regions. Several studies describe or assume seasonally successive density layers, horizontally evenly distributed, as seen in radar data. On the other hand, high-resolution density measurements on firn cores in Antarctica and Greenland showed no clear seasonal cycle in the top few meters. A major caveat of most existing snow-pit and firn-core based studies is that they represent one vertical profile from a laterally heterogeneous density field. To overcome this, we created an extensive dataset of horizontal and vertical density data at Kohnen Station, Dronning Maud Land on the East Antarctic Plateau. We drilled and analyzed three 90 m long firn cores as well as 160 one meter long vertical profiles from two elongated snow trenches to obtain a two dimensional view of the density variations. The analysis of the 45 m wide and 1 m deep density fields reveals a seasonal cycle in density. However, the seasonality is overprinted by strong stratigraphic noise, making it invisible when analyzing single firn cores. Our density dataset extends the view from the local ice-core perspective to a hundred meter scale and thus supports linking spatially integrating methods such as radar and seismic studies to ice and firn cores.

Gas diffusivity and permeability through the firn column at Summit, Greenland: measurements and comparison to microstructural properties

Abstract. The physical structure of polar firn plays a key role in the mechanisms by which glaciers and ice sheets preserve a natural archive of past atmospheric composition. This study presents the first measurements of gas diffusivity and permeability along with microstructural information measured from the near-surface firn through the firn column to pore close-off. Both fine- and coarse-grained firn from Summit, Greenland are included in this study to investigate the variability in firn caused by seasonal and storm-event layering. Our measurements reveal that the porosity of firn (derived from density) is insufficient to describe the full profiles of diffusivity and permeability, particularly at porosity values above 0.5. Thus, even a model that could perfectly predict the density profile would be insufficient for application to issues involving gas transport. The measured diffusivity profile presented here is compared to two diffusivity profiles modeled from firn air measurements from Summit. Because of differences in scale and in firn processes between the true field situation, firn modeling, and laboratory measurements, the results follow a similar overall pattern but do not align; our results constitute a lower bound on diffusive transport. In comparing our measurements of both diffusivity and permeability to previous parameterizations from numerical 3-D lattice-Boltzmann modeling, it is evident that the previous relationships to porosity are likely site-specific. We present parameterizations relating diffusivity and permeability to porosity as a possible tool, though use of direct measurements would be far more accurate when feasible. The relationships between gas transport properties and microstructural properties are characterized and compared to existing relationships for general porous media, specifically the Katz–Thompson (KT), Kozeny–Carman (KC), and Archie's law approximations. While those approximations can capture the general trend of gas transport relationships, they result in high errors for individual samples and fail to fully describe firn variability, particularly the differences between coarse- and fine-grained firn. We present a direct power law relationship between permeability and gas diffusivity based on our co-located measurements; further research will indicate if this type of relationship is site-specific. This set of measurements and relationships contributes a unique starting point for future investigations in developing more physically based models of firn gas transport.

Impact of physical properties and accumulation rate on pore close-off in layered firn

Abstract. Investigations into the physical characteristics of deep firn near the lock-in zone through pore close-off are needed to improve understanding of ice core records of past atmospheric composition. Specifically, the permeability and microstructure profiles of the firn through the diffusive column influence the entrapment of air into bubbles and thus the ice age–gas age difference. The purpose of this study is to examine the nature of pore closure processes at two polar sites with very different local temperatures and accumulation rates. Density, permeability, and microstructure measurements were made on firn cores from the West Antarctic Ice Sheet (WAIS) Divide, a site that has moderate accumulation rates with a seasonal climate archive, and Megadunes in East Antarctica, a site that is a natural laboratory for accumulation rate effects in the cold low-accumulation desert. We found that the open pore structure plays a more important role than density in predicting gas transport properties, throughout the porous firn matrix. For firn below 50 m depth at both WAIS Divide and Megadunes, finer-grained layers experience close-off shallower in the firn column than do coarser-grained layers, regardless of which grain size layer is the denser layer at depth. Pore close-off occurs at a critical open porosity that is accumulation rate dependent. Defining pore close-off at a critical open porosity for a given accumulation rate as opposed to a critical total porosity accounts for the pore space available for gas transport. Below the critical open porosity, the firn becomes impermeable despite having small amounts of interconnected pore space. The low-accumulation sites, with generally coarse grains, close off at lower open porosities (~<10%) than the open porosity (~>10%) of high-accumulation sites that have generally finer grains. The microstructure and permeability even near the bottom of the firn column are relic indicators of the nature of accumulation when that firn was at the surface. The physical structure and layering are the primary controlling factors on pore close-off. In contrast to current assumptions for polar firn, the depth and length of the lock-in zone is primarily dependent upon accumulation rate and microstructural variability due to differences in grain size and pore structure, rather than the density variability of the layers.

Densification of layered firn of the ice sheet at NEEM, Greenland

AbstractDensification of firn at the North Greenland Eemian Ice Drilling (NEEM) camp is investigated using density surrogates: dielectric permittivities ∊v and ∊h at microwave frequencies with electrical fields in the vertical and horizontal planes, respectively. Dielectric anisotropy Δ∊ (= ∊v − ∊h) is then examined as a surrogate for the anisotropic geometry of firn. Its size, fluctuations and mutual correlations are investigated in samples taken at depths from the surface to ~90 m. The initial Δ∊ of ~0.06 appears within the uppermost 0.2 m. After that, Δ∊ decreases rapidly until 21–26 m depth. Below this, Δɛ decreases slowly. Layers with more ions of fluorine, chlorine and some cations deposited between the autumn and the subsequent summer deform preferentially during all these stages. This layered deformation is explained partly by the textural effects initially formed by the seasonal variation of metamorphism, and partly by ions such as fluorine, chlorine and ammonium, which are known to modulate dislocation movement in the ice crystal lattice. Insolation-sensitive microstructure appears to be preserved all the way to the pore close-off, within layers of the summer-to-autumn metamorphism. Like previous authors, we hypothesize that calcium is not the active agent in the reported deformation– calcium correlations.

Microstructural evolution of fine-grained layers through the firn column at Summit, Greenland

AbstractWe present a microstructural characterization of fine-grained layers from the top 90 m of firn from Summit, Greenland, performed using a combination of scanning electron microscopy techniques including secondary electron imaging, energy-dispersive spectroscopy and electron backscattered patterns, and X-ray microcomputed tomography. The impurities in the firn, both soluble impurities and dust particles, were found largely in the grain interiors. Both c- and a-axis pole figures do not show strong evidence of a preferred orientation of the grains even at the bottom of the firn column. The firn structure became increasingly anisotropic with vertical alignment in the top 3 m, probably due to vapor transport associated with dry-snow metamorphism. The anisotropy decreases below this level until at ∼50 m the average firn structure is close to isotropic. In the near surface, the level of anisotropy is weaker than at Hercules Dome, Antarctica, confirming that differences in accumulation rates and temperatures leave enduring evidence in the structure of the firn. The fraction of closed-off pores is relatively low until ∼65 m; below that it rises through the end of our sampling at 90 m. Our microstructure measurements on the microscale are consistent with in situ firn-air sampling measurements on a decimeter scale, both indicating the existence of the lock-in zone starting near 69 m depth, and pore close-off at 81 m at this site.

Late-Holocene climate evolution at the WAIS Divide site, West Antarctica: bubble number-density estimates

AbstractA surface cooling of ∼1.7°C occurred over the ∼two millennia prior to ∼1700 CE at the West Antarctic ice sheet (WAIS) Divide site, based on trends in observed bubble number-density of samples from the WDC06A ice core, and on an independently constructed accumulation-rate history using annual-layer dating corrected for density variations and thinning from ice flow. Density increase and grain growth in polar firn are both controlled by temperature and accumulation rate, and the integrated effects are recorded in the number-density of bubbles as the firn changes to ice. Number-density is conserved in bubbly ice following pore close-off, allowing reconstruction of either paleotemperature or paleo-accumulation rate if the other is known. A quantitative late-Holocene paleoclimate reconstruction is presented for West Antarctica using data obtained from the WAIS Divide WDC06A ice core and a steady-state bubble number-density model. The resultant temperature history agrees closely with independent reconstructions based on stable-isotopic ratios of ice. The ∼1.7°C cooling trend observed is consistent with a decrease in Antarctic summer duration from changing orbital obliquity, although it remains possible that elevation change at the site contributed part of the signal. Accumulation rate and temperature dropped together, broadly consistent with control by saturation vapor pressure.

Developing a bubble number-density paleoclimatic indicator for glacier ice

AbstractPast accumulation rate can be estimated from the measured number-density of bubbles in an ice core and the reconstructed paleotemperature, using a new technique. Density increase and grain growth in polar firn are both controlled by temperature and accumulation rate, and the integrated effects are recorded in the number-density of bubbles as the firn changes to ice. An empirical model of these processes, optimized to fit published data on recently formed bubbles, reconstructs accumulation rates using recent temperatures with an uncertainty of 41% (P < 0.05). For modern sites considered here, no statistically significant trend exists between mean annual temperature and the ratio of bubble number-density to grain number-density at the time of pore close-off; optimum modeled accumulation-rate estimates require an eventual ~2.02 ± 0.08 (P < 0.05) bubbles per close-off grain. Bubble number-density in the GRIP (Greenland) ice core is qualitatively consistent with independent estimates for a combined temperature decrease and accumulation-rate increase there during the last 5 kyr.

Evidence for the loss of snow-deposited MSA to the interstitial gaseous phase in central Antarctic firn

We have examined several MSA (methanesulfonic acid) records from the upper 200 m of the Antarcticice sheet and in particular the new Dome F profile. At all the four sites studied, concentration profilesexhibit similar patterns as a function of depth. They suggest that snow metamorphism and solid phasemigration are responsible for a marked release of gaseous MSA to interstitial firn air as well as probablyto the free atmosphere, in particular at extremely low accumulation sites. Snow acidity can also modifyMSA concentration. It is proposed that, below the upper few metres where the communication withthe free atmosphere is possible, gaseous MSA may remain in the firn layers and be entrapped later inair bubbles at pore close-off, i.e. when firn is transformed into ice. Chemical measurements on the firncore do not take into account the MSA released to the gaseous phase, but this fraction is measurable inice samples. In spite of these alterations occurring in the firn layers, relative changes of the atmosphericMSA concentration in the past are probably still there deep within the Antarctic ice sheet. However, for glacial periods, different processes have to be considered in relation to modified aerosol properties.

Evidence for the loss of snow-deposited MSA to the interstitial gaseous phase in central Antarctic firn

We have examined several MSA (methanesulfonic acid) records from the upper 200 m of the Antarcticice sheet and in particular the new Dome F profile. At all the four sites studied, concentration profilesexhibit similar patterns as a function of depth. They suggest that snow metamorphism and solid phasemigration are responsible for a marked release of gaseous MSA to interstitial firn air as well as probablyto the free atmosphere, in particular at extremely low accumulation sites. Snow acidity can also modifyMSA concentration. It is proposed that, below the upper few metres where the communication withthe free atmosphere is possible, gaseous MSA may remain in the firn layers and be entrapped later inair bubbles at pore close-off, i.e. when firn is transformed into ice. Chemical measurements on the firncore do not take into account the MSA released to the gaseous phase, but this fraction is measurable inice samples. In spite of these alterations occurring in the firn layers, relative changes of the atmosphericMSA concentration in the past are probably still there deep within the Antarctic ice sheet. However, for glacial periods, different processes have to be considered in relation to modified aerosol properties.

Isotopic diffusion in polycrystalline ice

AbstractQuantitative ice-core paleoclimatology must account for post-depositional processes, such as vapor-phase diffusion in the firn. After pore close-off, diffusion continues to smooth the stable-isotope records δ18O and δD that are eventually recovered from the ice, leading to the loss of high-frequency information. Johnsen and others (1997) found much higher rates of diffusive smoothing in the Greenland Icecore Project (GRIP) Holocene ice than would be predicted by diffusion through solid ice alone, and Nye (1998) argued that transport through liquid veins might explain this apparent excess diffusion. However, the analysis of Johnsen and others (2000) indicates that the required vein dimensions may be unrealistically large. Here, we model the diffusion of stable isotopes in polycrystalline ice and show that the predictions of Nye (1998) and those of Johnsen and others (2000) actually represent two end-members in a range of potential behavior. Our model determines which of these asymptotic regimes more closely resembles the prevailing conditions and quantifies the role of pre-melted liquid in the smoothing of isotopic signals. The procedure thereby ties together the two approaches and provides a rostrum for accurate analysis of isotope records and paleotemperature reconstructions.

Conditions for bubble elongation in cold ice-sheet ice

AbstractHighly 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.

Conditions for bubble elongation in cold ice-sheet ice

AbstractHighly 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.

Stratigraphic Features of Firn as Proxy Climate Signals at the Summit Ice Cap of Ushkovsky Volcano, Kamchatka, Russia

Field observations were conducted at the summit ice cap of the Ushkovsky volcano, central Kamchatka, in the summer of 1996, in order to evaluate the potential of the ice cap for reconstruction of the past climate over Kamchatka. A 27-m-long firn-core contains an approximate 27-yr record of net balance with an average accumulation rate of 0.57 m a{sup -1} water equivalent. The temperature of the firn is -16.5{degrees}C at 10-m depth. Average annual accumulation rates are calculated by reference to dated ash layers and amount to between 0.38 and 0.88 m a{sup -1} during six fixed periods. Depth and age of pore close-off are calculated as 58 m and 66 yr at this site by empirical formulas. Comparisons of the accumulation rates with winter precipitation at lowland stations indicate that there is a positive relationship with winter precipitation on the eastern and western coasts of the peninsula. We cannot find a clear relation between the rates and annual average sea-ice extent in the Sea of Okhotsk, which implies the vapor source to the ice cap could have been the North Pacific. Melt feature percentage in the firn core displays a clear positive relationship with summer air temperature at 700 hPamore » over Kamchatka. The result obtained suggests that the ice cap has potential for the reconstruction of paleoclimate over the Kamchatka Peninsula. 15 refs., 9 figs.« less

Stratigraphic Features of Firn as Proxy Climate Signals at the Summit Ice Cap of Usnkovsky Volcano, Kamchatka, Russia

Field observations were conducted at the summit ice cap of the Usnkovsky volcano, central Kamchatka, in the summer of 1996, in order to evaluate the potential of the ice cap for reconstruction of the past climate over Kamchatka. A 27-m-long firn-core contains an approximate 27-yr record of net balance with an average accumulation rate of 0.57 m a−1 water equivalent. The temperature of the firn is −16.5°C at 10-m depth. Average annual accumulation rates are calculated by reference to dated ash layers and amount to between 0.38 and 0.88 m a−1 during six fixed periods. Depth and age of pore close-off are calculated as 58 m and 66 yr at this site by empirical formulas. Comparisons of the accumulation rates with winter precipitation at lowland stations indicate that there is a positive relationship with winter precipitation on the eastern and western coasts of the peninsula. We cannot find a clear relation between the rates and annual average sea-ice extent in the Sea of Okhotsk, which implies the vapor source to the ice cap could have been the North Pacific. Melt feature percentage in the firn core displays a clear positive relationship with summer air temperature at 700 hPa over Kamchatka. The result obtained suggests that the ice cap has potential for the reconstruction of paleoclimate over the Kamchatka Peninsula.

Bubbly-ice densification in ice sheets: I. Theory

AbstractDry snow on the surface of polar ice ice sheets is first densified and metamorphosed to produce firn. Bubbly ice is the next stage of the transformation process which takes place below the depth of pore closure. This stage extends to the transition zone where, due to high pressures and low temperatures. air trapped in bubbles and ice begins to form the mixed air clathrate hydrates, while the gas phase progressively disappears. Here we develop a model of bubbly-ice rheology and ice-sheet dynamics taking into account glacier-ice compressibility. The interaction between hydrostatic compression of air bubbles, deviatoric (uniaxial) compressive deformation of the ice matrix and global deformations of the glacier body is considered. The ice-matrix pressure and the absolute-load pressure are distinguished. Similarity theory and scale analysis are used in examine the resultant mathematical model of bubbly-ice densification. The initial rate of bubble compression in ice sheets appears to be relatively high, so that the pressure (density) relaxation process takes place only 150-200 m in depth (below pore close-off) to reach its asymptotic phase, wherein the minimal drop between bubble and ice pressures is governed by the rate of loading (ice accumulation). This makes it possible to consider densification under stationary (present-day) conditions of ice formation as a special case of primary interest. The computational tests performed with the model indicate that both ice-porosity and bubble-pressure profiles in ice sheets are sensitive to variations of the rheological parameters of pure ice. However, only the bubble-pressure distinguishes between the rheological properties at low and high stresses. The porosity profile at the asymptotic phase is mostly determined by the air content in the ice. In the companion paper (Lipenkov and others, 1997), we apply the model to experimental data from polar ice cores and deduce, through an inverse procedure, the rhelogical properties of pure ice as well as the mean air content in Holocene and glacial ice sediments at vostok Station (Antarctica).

Bubbly-ice densification in ice sheets: I. Theory

AbstractDry snow on the surface of polar ice ice sheets is first densified and metamorphosed to produce firn. Bubbly ice is the next stage of the transformation process which takes place below the depth of pore closure. This stage extends to the transition zone where, due to high pressures and low temperatures. air trapped in bubbles and ice begins to form the mixed air clathrate hydrates, while the gas phase progressively disappears. Here we develop a model of bubbly-ice rheology and ice-sheet dynamics taking into account glacier-ice compressibility. The interaction between hydrostatic compression of air bubbles, deviatoric (uniaxial) compressive deformation of the ice matrix and global deformations of the glacier body is considered. The ice-matrix pressure and the absolute-load pressure are distinguished. Similarity theory and scale analysis are used in examine the resultant mathematical model of bubbly-ice densification. The initial rate of bubble compression in ice sheets appears to be relatively high, so that the pressure (density) relaxation process takes place only 150-200 m in depth (below pore close-off) to reach its asymptotic phase, wherein the minimal drop between bubble and ice pressures is governed by the rate of loading (ice accumulation). This makes it possible to consider densification under stationary (present-day) conditions of ice formation as a special case of primary interest. The computational tests performed with the model indicate that both ice-porosity and bubble-pressure profiles in ice sheets are sensitive to variations of the rheological parameters of pure ice. However, only the bubble-pressure distinguishes between the rheological properties at low and high stresses. The porosity profile at the asymptotic phase is mostly determined by the air content in the ice. In the companion paper (Lipenkov and others, 1997), we apply the model to experimental data from polar ice cores and deduce, through an inverse procedure, the rhelogical properties of pure ice as well as the mean air content in Holocene and glacial ice sediments at vostok Station (Antarctica).