MICROSCOPIC OBSERVATIONS OF THE AIR HYDRATE-BUBBLE. TRANSFORMATION PROCESS IN GLACIER ICE

Des examens microscopiques des inclusions d'hydrates d'air ont été faits sur des échantillons provenant de forages profonds a Dye-3 et Camp-Century, Groënland et Byrd Station en Antarctique. Les plus faibles profondeurs pour lesquelles les hydrates d'air sont observés à Dye-3 Camp Century et Byrd Station correspondent respectivement à 1092 m, 1099 m et 727 m. Pour les forages à Dye-3 et Camp Century, les profondeurs observées pour l'apparition des hydrates d'air sont en accord avec les calculs de Miller (1). Pour le forage à Byrd Station cette apparition a lieu environ 100 m moins profond que prévu par les calculs de Miller. Cette différence apparente peut être attribuée au flux ascendant de glace qui provient d'environ 5 km en amont de Byrd Station. Les observations de joints de grains et de phase et les expériences de déformation révèlent que l'énergie de joint de phases est supérieure à celle de joint de grains et que le processus de transformation de l'hydrate d'air en bulle est clairement lié à un mécanisme de nucléation induite par déformation. Ces résultats suggèrent que le processus de transformation hydrate d'air/bulle est étroitement contrôlé par un processus de restauration se produisant à la fois in situ et postérieurement à l'extraction.Microscopie examinations for air hydrate inclusions were made on specimens of the Dye-3 and Camp Century, Greenland and Byrd Station, Antarctica deep ice cores. The shallowest depths at which air hydrates are observed in the Dye-3, Camp Century and Byrd Station cores are at 1092 m, 1099 m and 727 m depths respectively. For the Dye-3 and Camp Century cores, the observed depths for air hydrate appearance agree with Miller's calculation [1]. For the Byrd Station core, the observed depth for the appearance is about 100 m shallower than the calculation result by Miller. This apparent difference at Byrd Station may be attributed to the general upward ice flow trajectory which begins about 5 km upstream from the Byrd Station location. The phase/grain boundary observations and deformation experiments revealed that phase boundary energy is much higher than grain boundary energy and that the transformation process from air hydrate to bubble is clearly related to the strain-induced nucleation process. These findings suggest that the air hydrate/bubble transformation process is strongly controlled by both in situ and post ice core recovery nucleation activation process

Variations of the CO2 concentration of occluded air and of anions and dust in polar ice cores

Analysis of impurities entrapped in natural ice is the most promising method for reconstructing the history of atmospheric composition before the period of direct measurement and offers the possibility of extending the record to at least 100,000 years B.P. We report here the present state of work in this field, with special emphasis on atmospheric CO2 concentration. After discussing the mechanism by which atmospheric gases are entrapped in ice, we report CO2 concentrations in ice core samples, up to 100,000 years old, from deep drilling projects in Greenland and the Antarctic. Results from ice deposited during the last 2000 years allow us to estimate the preindustrial atmospheric CO2 level, an important boundary condition for modelling the anthropogenic CO2 increase. Using older samples from a deep ice core drilled at Dye 3, Greenland, we show that the CO2 concentration was 180 to 200 ppmv at the end of the Wisconsin and increased during the transition to the Holocene to values in the 260 to 300 ppmv range. Detailed CO2 measurements on sections of the Wisconsin part of the Dye 3 core which, based on δ18O, were deposited during times of significant climatic variation, show that the δ18O variations were accompanied by simultaneous correlated rapid CO2 variations. Other parameters, including micro-particle concentration and Cl−, NO− 3 and SO2− 4 concentrations also showed significant variations which correlate with the measured δ18O shifts

Late glacial climate history from ice cores

Ice cores contain information on climatic variations and their causes. Recent results obtained on the new deep ice core drilled in 1981 at Dye 3, South Greenland, in the frame of the US-Danish-Swiss Greenland Ice Sheet Program, are: - Comparison of the θ18O variations in the Greenland ice cores with those in European lake carbonate exhibits strong similarities and provides time marks (13,000, 11,000, 10,000 B.P.) for the Late Glacial section of the ice cores; - CO2 concentration measurements in the occluded air indicate low (180–200 ppm) CO2 concentration 30,000 to 15,000 B.P. and an increase to ca. 300 ppm around 13,000 B.P. The CO2 increase might reflect a change in the ocean circulation at the end of the last glaciation and could have contributed to the establishment of the Holocene environmental conditions; - 10Be concentration measurements on samples covering the last 50,000 years show a correlation with θ18O, low θ18O values corresponding to high 10Be concentrations (atoms per g of ice). It is likely that this correlation primarily reflects changes in the rate of precipitation in the northern hemisphere. Based on the ice core information, climatic events during the Glacial-Postglacial transition are discussed