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

Abstract

ABSTRACT. 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-a;e ice. The air-hydrate crystals were found to grow in the ice sheet, about 6.7 x 10-1 cm 3 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 a . The mean value of a 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 a for many samples, a in the interglacial ice was about 30% smaller than that in the glacial-age ice.

Air bubble to clathrate hydrate transformation in polar ice sheets: A reconsideration based on the new data from Dome Fuji ice core

“…They include the number concentration of air inclusions N, their mean equivalent-sphere radius r, the number fraction of air inclusions located on grain boundaries N gb /N, the number fraction of hydrates of different shapes N s /N and the spatial variability of the inclusion number and volume concentrations expressed by relative standard deviations of these characteristics in individual cells, i.e., dN i /N and dv i /v, respectively. We express the volume of inclusions by volume of occluded air reduced to standard temperature and pressure [Uchida et al, 1994;Lipenkov, 2000], thus considering the difference in gas density between coexisting bubbles and hydrates.…”

“…These changes were first reported for air bubbles [Barkov and Lipenkov, 1984] and then discovered in the air-hydrate records from a number of deep polar ice cores [Uchida et al, 1994;Pauer et al, 1999;Narita et al, 1999;Lipenkov, 2000]. Also, Lipenkov et al [1999Lipenkov et al [ , 2000 showed that the size and abundance of air bubbles in polar ice depend on the temperature and accumulation rate prevailing during the snow-ice transformation.…”

Diffusion-controlled metabolism for long-term survival of single isolated microorganisms trapped within ice crystals

“…For depths shallower than the air-hydrate phase boundary, the equilibrium molar fraction, C e , of gas in the ice lattice is proportional to pressure, as given in SI Table 2. For greater depths, C e is approximately constant and equal to the concentration at nucleation (37).…”

“…Diffusion of gases dissolved in the ice lattice has been experimentally observed in a number of situations, including: diffusion of gas from small bubbles to larger bubbles, diffusion from bubbles to air-hydrate crystals, and diffusion from small hydrate crystals to large ones (37). Further, each of these observed processes can be understood as the result of a slowly diffusing gas seeking the state of lowest free energy.…”

“…Further, each of these observed processes can be understood as the result of a slowly diffusing gas seeking the state of lowest free energy. From measurements of relative sizes of air-hydrate crystals at typical separations of Ϸ1 mm and at depths 1,350 to 2,040 m, Uchida et al (37) obtained an average value of the product of the diffusion rate, D, and the equilibrium molar fraction, C e , as DC e Ϸ 10 Ϫ19 m 2 ⅐s Ϫ1 at a temperature of Ϫ40°C. This is consistent with the values from SI Tables 1 and 2 for oxygen and nitrogen and implies a gas mobility of several tens of centimeters per 10 5 yr.…”