Air Clathrate Research Articles

New Estimates of Ice and Oxygen Fluxes Across the Entire Lid of Lake Vostok From Observations of Englacial Radio Wave Attenuation

AbstractOver deep Antarctic subglacial lakes, spatially varying ice thickness and the pressure‐dependent melting point of ice result in distinct areas of melting and freeze‐on at the ice‐water interface, that is, at the lake lid. These ice mass fluxes drive lake circulation and, because basal Antarctic ice contains air clathrate, affect the input of oxygen to the lake, with implications for subglacial life. Inferences of rates of melting and freeze‐on, that is, accretion rates, from radar layer tracking and geodesy are limited in spatial coverage and resolution. Here we develop a new method to estimate accretion rate, and the resulting oxygen input at a lake lid, using airborne radar data over Lake Vostok together with ice‐temperature and chemistry data from the Vostok ice core. Because the lake lid is a coherent reflector of known reflectivity (at our radar frequency), we can infer depth‐averaged radio wave attenuation in the ice, with a spatial resolution of ~1 km along flight lines. Spatial variation in attenuation depends mostly on variation in ice temperature near the lid, which in turn varies strongly with ice mass flux at the lid. We model ice temperature versus depth with ice mass flux as a parameter, thus linking that flux to observed depth‐averaged attenuation. The resulting map of melt and accretion rates independently reproduces features known from earlier studies but now covers the entire lid. We find that freeze‐on is dominant when integrated over the lid, with an ice imbalance of 0.05 to 0.07 km3/year, which is robust against uncertainties.

Effect of thermal history on lattice expansion and guest distribution of tetrahydrofuran clathrate hydrate with air molecules

Air hydrates can form under the high-pressure and low-temperature conditions found in deep ice sheets of the Arctic and Antarctic regions. These hydrates can play a major role in the analysis of data gathered in these regions. Through this Letter, we investigate on thermal expansivity and guest-molecule dynamics of THF+air clathrate hydrate provided by a neutron scattering experiment and address two noteworthy features. First is the effect of thermal history on host water-lattice expansion and related gas-inclusion effect on thermal expansion coefficients. Second is the changes in guest-molecule distribution during the degassing process caused by thermal stimulation.

Phase equilibrium condition measurements in nitrogen and air clathrate hydrate forming systems at temperatures below freezing point of water

Contained in this paper are the three phase equilibrium conditions of the (ice+clathrate hydrate+guest-rich) vapour in the (nitrogen+water) and the modelled (air+water) systems at temperatures below the freezing point of water. The precise determination of the equilibrium conditions in those systems are of importance for the analysis of the past climate change using the cored samples from the ice sheets at Antarctica and Greenland because the air hydrates keep the ancient climate signals. The mole ratio of the modelled air composed of nitrogen and oxygen is 0.790:0.210. The equilibrium conditions were measured by the batch, isochoric procedure. The temperature range of the measurements in the nitrogen hydrate forming system is (244.05<T<266.55)K and the corresponding equilibrium pressure range is (7.151<p<12.613)MPa. The temperature range of the measurements in the modelled air hydrate forming system is (242.55<T<267.85)K, and the corresponding equilibrium pressure range is (6.294<p<12.144)MPa. The data obtained quantitatively compared with the previously reported data.

On the Abundances of Noble and Biologically Relevant Gases in Lake Vostok, Antarctica

Motivated by the possibility of comparing theoretical predictions of Lake Vostok's composition with future in situ measurements, we investigated the composition of clathrates that are expected to form in this environment from the air supplied to the lake by melting ice. To establish the best possible correlation between the lake water composition and that of air clathrates formed in situ, we used a statistical thermodynamic model based on the description of the guest-clathrate interaction by a spherically averaged Kihara potential with a nominal set of potential parameters. We determined the fugacities of the different volatiles present in the lake by defining a "pseudo" pure substance dissolved in water owning the average properties of the mixture and by using the Redlich-Kwong equation of state to mimic its thermodynamic behavior. Irrespective of the clathrate structure considered in our model, we found that xenon and krypton are strongly impoverished in the lake water (a ratio in the 0.04-0.1 range for xenon and a ratio in the ≈ 0.15-0.3 range for krypton) compared to their atmospheric abundances. Argon and methane were also found to be depleted in the Lake Vostok water by factors in the 0.5-0.95 and 0.3-0.5 ranges, respectively, compared to their atmospheric abundances. On the other hand, the carbon dioxide abundance was found to be substantially enriched in the lake water compared to its atmospheric abundance (by a factor in the 1.6-5 range at 200 residence times). The comparison of our predictions of the CO2 and CH4 mole fractions in Lake Vostok with future in situ measurements will allow disentangling between the possible supply sources.

Formation of air clathrate hydrates in polar ice sheets: heterogeneous nucleation induced by micro-inclusions

AbstractTo investigate factors influencing nucleation of air clathrate hydrates in polar ice sheets, we have performed high-resolution mapping of the distributions of soluble impurities, air bubbles and air- hydrate crystals versus depth in the Dome Fuji Antarctic ice. Significant correlation observed between the concentrations of air inclusions and impurities in ice along with frequent occurrence of impurities inside hydrate crystals suggest that micro-inclusions promote hydrate nucleation in the ice matrix. Our observations also show that the diffusive macroscopic-scale redistribution of air constituents in ice in the bubble-hydrate transition zone is controlled by the original sedimentary layering of soluble impurities acting as nucleation helpers. The results of this study are important for the correct interpretation of high-resolution gas analyses of ice cores and for better understanding the global bubble-to-hydrate transformation process in polar ice sheets.

Raman Spectroscopy of Nitrogen Clathrate Hydrates

Nitrogen hydrate samples were synthesized using liquid nitrogen and powder ice at 16 MPa and 253 K. Confocal laser Raman spectroscopy was used to investigate the characteristics of nitrogen clathrate hydrates. The results show that the Raman peaks of NN and OH stretching vibration in nitrogen hydrates are observed at 2322.4 and 3092.1 cm1, respectively, which are very similar to those in natural air clathrate hydrates. For comparison, we measured the Raman peaks of NN stretching vibration both in liquid nitrogen and nitrogen molecules saturated water, which appear at 2326.6 and 2325.0 cm1, respectively. The Raman spectroscopic observations on the dissociation process suggest that nitrogen molecules occupy both the large and small cages in nitrogen clathrate hydrates. However, only one Raman peak is observed for NN stretching vibration because the difference of the environment of nitrogen molecules between large and small cages is too small to be differentiated by Raman spectroscopy.

Observation of low-temperature object by phase-contrast x-ray imaging: Nondestructive imaging of air clathrate hydrates at 233K

A cryochamber and a liquid cell that are designed for nondestructive three dimensional observations and arranged in a two-crystal x-ray interferometer expand the use of phase-contrast x-ray imaging that could only be performed at room temperature in previous studies to a new temperature range of 190K to room temperature. The methyl acetate in the liquid cell prevents undesirable sample outline contrasts and enables internal observations. Both a nondestructive observation and a highly accurate absolute density of the materials under low-temperature conditions can be obtained with a single measurement using this new technique. A three dimensional x-ray computed tomography (x-ray CT) of the air clathrate hydrate in the hexagonal ice drilled from Dome Fuji in Antarctica is shown, and the density of the air hydrate is estimated to be 0.937(3)g∕cm3 at 233K.

Microstructure mapping: a new method for imaging deformation-induced microstructural features of ice on the grain scale

AbstractThis work presents a method of mapping deformation-related sublimation patterns, formed on the surface of ice specimens, at microscopic resolution (3–4 μm pixel−1). The method is based on the systematic sublimation of a microtomed piece of ice, prepared either as a thick or a thin section. The mapping system consists of an optical microscope, a CCD video camera and a computer-controlled xy-stage. About 1500 images are needed to build a high-resolution mosaic map of a 4.5 × 9 cm section. Mosaics and single images are used to derive a variety of statistical data about air inclusions (air bubbles and air clathrate hydrates), texture (grain size, shape and orientation) and deformation-related features (subgrain boundaries, slip bands, subgrain islands and loops, pinned and bulged grain boundaries). The most common sublimation patterns are described, and their relevance for the deformation of polar ice is briefly discussed.

In situ observation of the transformation from air bubbles to air clathrate hydrate crystals using a Mizuho ice core

The transformation from air bubbles to air clathrate hydrate crystals (air-hydrates) in polar ice sheets has been studied using a natural ice-core drilled at Mizuho Station, East Antarctica. In situ observations using an optical microscope were done under hydrostatic pressures of 14.7 – 22.5 MPa at 258 – 268 K. The air-hydrate nucleated only at the interface between an air bubble and ice from 20 to 330 h after the pressurization. The nucleation ratio, which we defined as the number of air bubbles containing hydrate 330 h after pressurization divided by the initial number of air bubbles in a given volume, depended on the excess pressure above the equilibrium pressure at a given temperature. Of two ice samples studied, this nucleation ratio was higher in the sample with greater chemical impurity content. Based on the experimental results, we discuss the nucleation mechanism of air-hydrate caused by chemical impurity in polar ice sheets.

Variation in N2/O2 ratio of occluded air in Dome Fuji antarctic ice

Ancient atmospheric gases are trapped in polar ice sheets. The gas molecules are stored in air bubbles at shallow depth and are incorporated into clathrate hydrates below a depth at which the hydrostatic pressure becomes greater than the formation pressure of the air clathrate hydrate. Significant gas fractionation has been found from measurements of the depth profile of the N2/O2 composition ratios in clathrate hydrates and air bubbles of Vostok antarctic ice. To investigate the effect of the ice condition on the fractionation process, we measured the N2/O2 ratios in clathrate hydrates and air bubbles from Dome Fuji antarctic ice using Raman spectroscopy. The results showed that the N2/O2 ratios in the clathrate hydrates of the Dome Fuji ice are slightly lower than those of the Vostok ice, although the tendency of the variation of the N2/O2 ratio with depth is similar. The difference in the N2/O2 ratio between the Dome Fuji ice and the Vostok ice for the transition zone is attributed to the difference of the ice temperature and the snow accumulation rate. On the other hand, it is concluded that the difference in the bubble‐free ice zone was caused by gas loss from the ice core after coring. The N2/O2 ratio of clathrate hydrate increases after coring because of higher diffusion rate and lower dissociation pressure of O2 than of N2. Our data suggest that the effect of gas loss in the Dome Fuji ice is relatively small, and so the gas composition in the Dome Fuji ice can be a precise paleoenvironmental indicator.

Kinetics of air–hydrate nucleation in polar ice sheets

Nucleation of air clathrate hydrates in air bubbles and diffusive air-mass exchange between coexisting ensembles of bubbles and hydrate crystals are the major interrelated processes that determine the phase change in the air–ice system in polar ice. In continuation of Salamatin et al. (J. Crystal Growth 193 (1998) 197; Ann. Glaciol. 29 (1999) 191) where the post-nucleation conversion of single air bubbles to hydrates was considered, we present here a statistical description for transformation of air bubbles to air clathrate hydrates based on the general theory of evolution of these two ensembles, including the gas fractionation effects. The model is fit to data on ice cores from central Antarctica, and then compared to other ice-core data. The focus is on the rate of clathrate–hydrate nucleation, which is determined to be the product of the inverse relative bubble size raised to the power λ≈5.8 with the relative supersaturation to the power β≈2. The clathration-rate constant is k 0≈3.2–4.5×10 −6 yr −1 at 220 K. The N 2- and O 2-permeation coefficients in ice, at 220 K, are inferred to be D N 2 0≈1.8–2.5×10 −8 mm 2 yr −1 and D O 2 0≈5.4–7.5×10 −8 mm 2 yr −1, respectively. Comparison of observations to simulations of bubble-to-hydrate transformation in Greenland ice sheet gave estimates for activation energies of hydrate formation and air diffusion of Q J≈70 kJ mol −1 and Q d ≈50 kJ mol −1, respectively.

Lattice Constants and Thermal Expansion Coefficient of Air Clathrate Hydrate in Deep Ice Cores from Vostok, Antarctica

We measured the temperature-dependent lattice constant of air hydrates in deep ice cores from Vostok, Antarctica, using single-crystal X-ray diffraction. Thus the thermal expansion coefficient was determined, the first for a structure II clathrate hydrate containing small guest molecules. Despite their high dissociation pressures, measurements were done at atmospheric pressure because the air hydrates had stabilized during their original residence at high pressure. At 200 K, the lattice constant and thermal expansion coefficient were 17.20 Å and 44 (±9) × 10-6 K-1, respectively. This thermal expansion coefficient is approximately equal to that from tetrahydrofuran hydrate, another structure II hydrate, but the lattice constant is 0.3% smaller. Crystal structure refinement determined that the total cage occupancy by air molecules was 0.9.

Micro-Raman analysis of synthetic air clathrates

Raman spectroscopic measurements on synthetic air clathrates, prepared at different pressures and temperatures, are presented. The gas fractionation for clathrates formed at -2 and -20 degrees C is determined from the integrated vibron mode intensities of nitrogen and oxygen molecules. An increasing fractionation in favour of oxygen with increasing pressure is found. An asymmetric peak shape for the O-2 vibron mode was observed in air clathrates prepared at 300 bar and -2 degrees C. The asymmetry of the N-2 vibron mode in the same sample is much less pronounced.

Raman spectroscopic and statistical studies on natural clathrates from the Greenland Ice Core Project ice core, and neutron diffraction studies on synthetic nitrogen clathrates

We present the results of Raman spectroscopic experiments on air clathrates in the GReenland Ice Core Project (GRIP) deep ice core, which differ markedly from previous measurements on the Dye 3 ice core. The N2/O2 ratio we observe is much closer to the atmospheric value. This has new implications for the interpretation of gas distributions in ice sheets. Raman spectroscopic scans to determine the N2/O2 ratios on different planes through a clathrate, in which the two axes of the scans are perpendicular to each other, give no indication of fractionation effects on the N2/O2 concentrations within a clathrate specimen. The frequency shift of the N2 and O2 peaks due to decomposition of a clathrate to an air bubble is shown qualitatively. From their peak integrals there is no indication of different retransformation rates to air bubbles between the oxygen and the nitrogen contents of clathrates. In air bubbles resulting from clathrate decomposition, the N2/O2 ratio shows similar values to those observed in clathrates and present atmospheric values. Statistical studies on the size, shape, and number concentration of clathrates are intended to give an estimate of the total amount of gas occluded in the clathrates. We present preliminary results obtained from 27 samples in a depth range between 1100 and 3000 m. The first neutron powder diffraction experiments reveal an overall degree of filling of 96.4% for a clathrate at a pressure of 449 bars and the existence of a type I phase at 1311 bars with an overall degree of filling of 107.5%.

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).

Raman spectroscopic study on the nitrogen/oxygen ratio in natural ice clathrates in the GRIP Ice Core

We have carried out Raman spectroscopic experiments on air clathrates in the GReenland Ice Core Project (GRIP) deep ice core. We present new N2/O2 measurements that markedly differ from previous measurements of the Dye‐3 ice core: the N2/O2 ratio we observe is much closer to atmospheric. This has new implications for the interpretation of gas distributions in ice sheets and the reconstruction of past atmospheric conditions.

Carbon dioxide concentration in bubbles of natural cold ice

The bubble pressure in an ice sheet is increasing with depth, due to the hydrostatic pressure of the surrounding ice. Below a certain depth, which depends on the ice temperature, bubbles are shrinking faster than expected, due to formation of air clathrates. After recovery and decompression of ice cores from below this depth, new bubbles start to form again. Within 1 year most of the air is collected again in newly formed bubbles. We measured by means of an infrared laser spectrometer the CO/sub 2/ concentration in gas extracted by mechanically crushing small ice samples at -20/sup 0/C. We analyzed samples from a depth of 1600 m below surface from the recently drilled Dye 3 core (South Greenland), a few days, a few weeks, and a few months after recovery. This allows us to investigate the influence of clathrate formation and incomplete back-diffusion to the measured CO/sub 2/ concentration. The formation of CO/sub 2/ clathrates and the back-diffusion process is certainly influenced by the solubility of CO/sub 2/ in the ice structure. Earlier measurements published by our laboratory overestimated this solubility. New measurements of the CO/sub 2/ concentration in laboratory grown single crystals allow a better estimate of the solubilitymore » of CO/sub 2/ in ice. The seasonal variations of the CO/sub 2/ concentration measured on ice samples from the Dye 3 core allow us to estimate an upper limit for the diffusion constant of CO/sub 2/ in natural polycrystalline ice at -20/sup 0/C. 6 figures.« less