Air bubbles and air-hydrate crystals in the Vostok ice core

Source: OAI

ABSTRACT International Symposium on Physics of Ice Core Records. Shikotsukohan, Hokkaido, Japan, September 14-17, 1998. The geometrical properties of air-bubble and air-hydrate ensembles in the 3310-m deep Vostok core and in other ice cores were studied. The principle results are the following: 1) the size and abundance of air bubbles in polar ice depend on the temperature and accumulation rate prevailing over the time of the snow-ice transformation, 2) the climate signal imposed on the bubble properties at pore close-off is only slightly modified in the course of the bubble-hydrate transition (500–1250 m at present time at Vostok) and in the first, transient, phase of air-hydrate crystal growth (1150–1500 m); as a consequence, the last four glacial-interglacial cycles are resolved in variations of the number and size of air inclusions along the Vostok ice core, and 3) the air-bubble and air-hydrate records from polar ice cores can provide an independent experimental constraint on the temperature-accumulation relations in the past.

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    ABSTRACT: To 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.
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    ABSTRACT: Air bubbles in ice cores play an essential role in climate research, not only because they contain samples of the palaeoatmosphere, but also because their shape, size and distribution provide information about the past firn structure and the embedding of climate records into deep ice cores. In this context, we present profiles of average bubble size and bubble number for the entire EDML (Antarctica) core and the top 600 m of the EDC (Antarctica) core, and distributions of bubble sizes from selected depths. The data are generated with an image-processing framework which automatically extracts position, orientation, size and shape of an elliptical approximation of each bubble from thick-section micrographs, without user interaction. The presented software framework allows for registration of overlapping photomicrographs to yield accurate locations of bubble-like features. A comparison is made between the bubble parameterizations in the EDML and EDC cores and data published on the Vostok (Antarctica) ice core. The porosity at the firn/ice transition is inferred to lie between 8.62% and 10.48% for the EDC core and between 10.56% and 12.61% for the EDML core.
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    ABSTRACT: To help characterize the albedo of “sea glaciers” on Snowball Earth, a study of the migration rates of air bubbles in freshwater ice under a temperature gradient was carried out in the laboratory. The migration rates of air bubbles in both natural glacier ice and laboratory-grown ice were measured for temperatures between −36°C and −4°C and for bubble diameters of 23–2000 μm. The glacier ice was sampled from a depth near close-off (74 m) in the JEMS2 ice core from Summit, Greenland. Migration rates were measured by positioning thick sections of ice on a temperature gradient stage mounted on a microscope inside a freezer laboratory. The maximum and minimum migration rates were 5.45 μm h−1 (K cm−1)−1 at −4°C and 0.03 μm h−1 (K cm−1)−1 at −36°C. Besides a strong dependence on temperature, migration rates were found to be proportional to bubble size. We think that this is due to the internal air pressure within the bubbles, which may correlate with time since close-off and therefore with bubble size. Migration rates show no significant dependence on bubble shape. Estimates of migration rates computed as a function of bubble depth within sea glaciers indicate that the rates would be low relative to the predicted sublimation rates, such that the ice surface would not lose its air bubbles to net downward migration. It is therefore unlikely that air bubble migration could outrun the advancing sublimation front, transforming glacial ice to a nearly bubble-free ice type, analogous to low-albedo marine ice.
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