Shinji Mae

Nature of radio echo layering in the Antarctic ice sheet detected by a two-frequency experiment

A two-frequency radio echo sounding experiment was carried out at Dome Fuji, the second highest dome in East Antarctica, and along a 1150-km-long traverse line from the dome to the coast. The goal was to determine the dominant causes of the radio echo internal reflections and to investigate their possible changes with depth ranges and regions. From the two-frequency (60 MHz and 179 MHz) radio echo responses at various sites, we distinguished four zones. Each of the zones is characterized by a dominant cause of radio echo internal reflection as follows. In the “PD” zone, changes in dielectric permittivity are mainly due to density fluctuations; in the “PCOF” zone, changes in dielectric permittivity are mainly due to changes in crystal-orientation fabrics; and in the “CA” zone, changes in electrical conductivity are mainly due to changes in acidity induced by past volcanic eruptions. In each of these three zones, the changes occur commonly along isochrones. In addition, a basal echo-free zone, the fourth zone, was found to appear always below the PCOF zone. These four zones and their distribution suggested variations of the physical conditions within the ice sheet.

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.

EFFECT OF TEMPERATURE ON DIELECTRIC PROPERTIES OF ICE IN THE RANGE 5-39 GHZ

The relative complex dielectric permittivity, e*=e′−ie″, of ice has been measured in the frequency range 5–39 GHz and in the temperature range 190–265 K. The cavity resonator method at 5 and 10 GHz and the open resonator method at 33 and 39 GHz were used to determine the low dielectric loss of ice. The real part of permittivity e′ was independent of the frequency. The temperature dependence of e′ was observed and is discussed in terms of contributions from anharmonic effects to infrared polarizability. The e″ obtained bridges the gap of previous results between 200 and 258 K. We discuss the frequency and temperature dependence of the effect of the infrared absorption band on e″. The e″ variation with frequency increased as the temperature decreased at 5–39 GHz. It is possible that absorption takes place at frequencies below the infrared region.

Precise measurement of dielectric anisotropy in ice Ih at 39 GHz

The dielectric permittivities parallel and perpendicular to the c axis (optic axis) of ice Ih were measured using an open resonator at 39 GHz in the temperature range 194–262 K. The dielectric anisotropy in ice at microwave frequencies is important for understanding remote sensing data in polar regions, obtained by ice radar and satellite-born microwave radar and radiometer. The measured samples were natural single-crystal ice collected from Mendenhall Glacier, Alaska. A very precise measurement was achieved by detecting two resonant peaks, one from the ordinary component and the other from the extraordinary component, simultaneously, from one sample. The real part of dielectric anisotropy, Δe′=e∥c′−e⊥c′, at 39 GHz was 0.0339±0.0007 (1.07%±0.02%) at 252 K and slightly depended on temperature. Reference measurements at 1 MHz using parallel plate electrodes were also carried out. The measured dielectric anisotropy at microwave frequencies agrees very well with the value at 1 MHz. The absolute values of e∥c′...

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.

Plastic Yielding in Ice Single Crystals

Stress-strain relations in single crystals were investigated using ice cylinders in tension. The cylinders were deformed at strain rates varying from 8×10-6 min-1 to 4×10-4 min-1 and at temperatures between -15 and -40°C. The stress-strain curves obtained showed large yield drops and the amount of the drop increased with an increase in the deformation rate or with decreasing temperature. The stress-strain curves of ice single crystals from those of materials such as LiF, Ge, InSb etc., which also show large yield drops in that the slope of initial, linear portion of the curves varies with the strain rate and with temperature. Another characteristic feature of the curves is that past the yield point the curves do not show minimums. This indicates that ice crystals do not work-harden. The maximum stress τmax, and the slope of the initial, linear portion M=(dτ/de)i are taken as characteristic parameters of the curves. Their dependence on temperature and strain rate are given as follows: M=(M0+D\dote exp (E1/RT) and τmax∝\dote1/m exp (E2/RT) where m=1.53 and E1=8.4 Kcal/mole and E2=10.4 Kcal/mole Values for the activation energy E2 and m indicate that the results are in good agreement with those expected from interpretations of creep experiments based on Johnstons theory of movement and multiplication of dislocations. Specific features of the stress-strain curves are also discussed in terms of a special characteristic of ice crystals.

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.