Takeo Hondoh

Acid ions at triple junction of Antarctic ice observed by Raman scattering

We measured the Raman spectra of the junctions where three crystal grains met (triple-junctions) in polycrystalline ice from two Antarctic sites (Nansen ice and South Yamato ice) in order to obtain direct evidence that sulfuric and nitric acids are present as liquid at the triple-junctions. We found that Raman spectra of Nansen ice have a peak (1050 cm−1) of HSO4− and NO3− in sulfuric and nitric acid solutions when the measured temperatures of the ice are −8 ∼ −35°C. Thus, sulfuric and nitric acids dissociate to HSO4− and NO3− at the triple-junctions of Nansen ice. Raman spectra of South Yamato ice have a peak (980 cm−1) of SO42− in sulfuric acid solution when the measured temperatures of the ice are −8 ∼ −20°C. Thus, sulfuric acid dissociates to SO42− at the triple-junctions of South Yamato ice. The results showed that aqueous solutions of the acids exist at Antarctic ice-sheet temperatures.

Extreme fractionation of gases caused by formation of clathrate hydrates in Vostok Antarctic Ice

Atmospheric gases are trapped in ice sheets. These gases stored in air-bubbles at shallower depth are gradually transformed into clathrate hydrates below the depth where the hydrostatic pressure exceeds the dissociation pressure of the clathrate hydrates. We measured Raman spectra of air-bubbles and clathrate hydrates in Vostok Antarctic ice cores in order to determine the fractionation effects on the concentrations of gases during their transition process. The results showed variations of the N2/O2 ratios with depth. The average N2/O2 ratio in the air-bubbles increases from the atmospheric value at the beginning of the transition to 11.7 at the end. The average N2/O2 ratio for the clathrate hydrates is 2.0 at the beginning, and asymptotically approaches the atmospheric value. This fractionation is attributed to faster diffusion of O2 than N2 through the ice lattice.

The electron density distribution in ice Ih determined by single‐crystal x‐ray diffractometry

Precise structure analysis of ice Ih was carried out by single‐crystal x‐ray diffractometry. The reliability factor R was as small as 0.007 for the refined structure based on the half‐hydrogen model. Both of the O–H distances at 243 K, those oblique and parallel to the c axis, were determined to be 0.85(2) and 0.82(3) A, respectively, which are significantly shorter than those determined by the neutron diffraction method. These discrepancies were attributed to the difference between the electron and protonic density distribution along the O–H‐‐‐O bond. Fourier synthesis and difference synthesis were computed for clarifying the electron density distribution relating to the hydrogen bonding.

The crystallographic structure of the natural air-hydrate in Greenland dye-3 deep ice core

We have carried out X-ray diffraction studies on single crystals of natural air-hydrate in deep ice cores recovered at Dye-3 Greenland. Integrated intensities for 470 diffracting planes were measured by an automated four-circle diffractometer. The space group determined is cubicFd3m and the lattice constant is 17.21(3) Å. These results indicate that the crystallographic structure is the Stackelbergs structure II, in contrast to the previously anticipated structure. This finding agrees with the recent results on the synthetic air-hydrate by Davidsonet al. It was also found by difference Fourier synthesis for guest molecules that electron density in a 16-hedral cage has multiple maxima displaced from the center of the cage while that in the 12-hedron was approximately spherical.

Raman spectra of natural clathrates in deep ice cores

Abstract Raman spectra of natural clathrates at a depth of 1501 m in ice cores collected at Dye-3 (Greenland) have been measured from 10 to 3500 cm−1. The guest molecules in the clathrates were identified as O2 and N2 from their molecular vibrations. The energy shifts of the vibrational modes have been analysed by a simple model of a spherical cavity in a dielectric medium. From a comparison of calculations with observations, it is concluded that guest molecules occur only in the larger cages of the clathrates. Temperature-dependent Raman spectra have been measured from 10 to 100 cm−1. The peak position shifts to a lower Raman shift at lower temperatures. This structure is considered to arise from over-damping of the rotation modes of the guest molecules.

Air-hydrate crystals in deep ice-core samples from Vostok Station, Antarctica

Microscopic observation of air-hydrate crystals was carried out using 34 deep ice-core samples retrieved at Vostok Station, Antarctica. Samples were obtained from depths between 1050 and 2542 m, which correspond to Wisconsin/ Sangamon/Illinoian ice. It was found that the volume and number of air-hydrate varied with the climatic changes. The volume concentration of air-hydrate in the interglacial ice was about 30% larger than that in the glacial ice. In the interglacial ice, the number concentration of air-hydrate was about a half and the mean volume of air-hydrate was nearly three times larger than that in the glacial-¥e ice. The air-hydrate crystals were found to grow in the ice sheet, about 6.7 x 10cm year-I, in compensation for the disappearance of smaller ones. The volume concentration of air-hydrate was related to the total gas content by a geometrical equation with a proportional parameter et. The mean value of et below 1250 m, where no air bubbles were found, was about 0.79. This coincided with an experimentally determined value of the crystalline site occupancy of the air-hydrate in a 1500 m core obtained at Dye 3, Greenland (Hondoh and others, 1990). In the depth profile of calculated et for many samples, et in the interglacial ice was about 30% smaller than that in the glacial-age ice.

Raman spectroscopic analyses of the growth process of CO2 hydrates

CO 2 hydrate crystals with polyhedral facet were observed to grow from CO 2 solution. For both CO 2 hydrates and CO 2 solution, Raman spectroscopic analyses were carried out. Raman spectra of CO 2 and H 2 O molecules in CO 2 hydrate were different from those in CO 2 solution. We presented an attempt to estimate the density of CO 2 hydrate by the Raman intensity ratio, and found that the densities of CO 2 hydrates were considerably smaller than those used in some modeling studies

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.

In Situ Observation of CO2 Hydrate by X‐ray Diffraction

Abstract: In situ observations of CO2 hydrate growth using high‐energy X‐rays were done at high‐pressures and the growth rates of CO2 hydrate from ice particles were measured. Assuming a model of diffusion through CO2 hydrate layers, interdiffusion coefficients of CO2 and H2O molecules were calculated between 233 K and 272.5 K. The diffusion coefficient at 263 K was 7.4 × 10−16 m2/s, and the activation energy of diffusion was 0.40 eV.

Molecular dynamics studies of molecular diffusion in ice Ih

We performed molecular dynamics simulations of the diffusion of interstitial He and H2O in ice Ih and found diffusion hops for these interstitial molecules from a stable site to an adjacent site. By observing the jumps of these diffusing species, we determined the jump frequencies, the crystal orientation dependence of the diffusion coefficients, and the diffusion activation energies. Most jumps are along the c axis, because the energy barrier for diffusion along the c axis is lower than that in the a–b plane. Furthermore, the diffusion mechanism for He significantly differ from that for H2O; interstitial H2O diffused by distorting the ice lattice, whereas He atom migrated by jumping from a stable interstitial site to an adjacent site without distorting the lattice. The transverse optic mode for translational lattice vibrations of the lattice surrounding the interstitial H2O shifts to high energy in comparison with that of the pure ice Ih. This upward shift is attributed to a strong coupling between local...