Densité de la glace et Étude des gaz en profondeur dans un glacier antarctique

Abstract

Different measurements have been performed on ice samples taken from a drilling about 100 meters deep in the coastal region of Term Adelie (Antarctica); the mean temperature in the hole was approximately -10°C. The results show that the pressure of gaz inclusions and the density of the ice incream with the depth. The values of the density increase from 0.880 g/cm* to 0.918 g/cm8 while the total volume of air decreases from 10 cma/100 g of ice for the samples taken from the surface to 6 cms/lOO g at a depth of about 80 meters. At a greater depth, in the section of the drilling containing morainic debris, values of approximately 4 cma/lOO g have been found. Lastly, in the blue ice which exists in contact with rock sediments, the volume of air becomes practically nil; this observation confirms that the blue ice has been formed through freezing. The gaz inclusions that are generally spherical show a definite lengthening in the samples taken from a depth of approximately 30 to 60 meters. This is in accordance with the appearance in the drilling of ice from a far-off origin characterized by its deuterium content. The total volume of gaz depends on the place where the ice was formed and the shape of the inclusions is linked to deformations due to ice flow phenomena.The concentrations of carbonic anhydride and argon found in the gaz contained within the ice approach the concentrations found in atmospheric air. The steady variations of about 1 To observed for oxygen proportions (decreasing with the depth) and nitrogen proportions (increasing with the depth) suggest that the composition of the gaz contained within the ice could depend on the conditions prevalent during the formation of this ice, even in the absence of fusion, through adsorption phenomena. Knowledge of the fundamental processes which take part in the formation of gaz inclusions is thus necessary in order to undertake the study of the ancient atmosphere through the gaz contained within the ice. naissance des processus fondamentaux pouvant partir des gaz contenus dans la glace.

Gas inclusions in the Antarctic ice sheet and their glaciological significance

“…However, the air-free nature of these dirty cores constitutes very good evidence that this bottom ice originated from the refreezing of meltwater formed at the base of the ice sheet, that this refreezing occurred sufficiently slowly to allow for practically total rejection of any dissolved air, and that the rock debris, ranging in size from clay to cobbles, was incorporated at the same time. Lorius et al [1968] have observed air-free ice at the bottom of the ice sheet in East Antarctica which they also have attributed to freezing of water at the glacier bed. The fact that water entered the drill hole soon after the bottom ice was penetrated would tend to indicate that bottom melting rather than bottom freezing is currently occurring in the immediate vicinity of the drill hole at Byrd Station.…”

“…These bubbles--in reality air-filled cavities--undergo changes in size, shape, and pressure in response to stresses associated with increasing depth of burial and so can provide very useful information on englacial deformation. Additionally, analysis of the inclosed gases can be used to investigate the chemical composition of ancient atmospheres [Scholander et al, 1956;Langway, 1958;Matsuo and Miyake, 1966;Lorius et al, 1968]. Currently, techniques are being perfected to extract large quantities of air for carbondating ice at various levels in glaciers and ice sheets [Langway et al, 1965;Oeschger et al, 1970].…”

Climatic and insolation control on the high-resolution total air content in the NGRIP ice core

“…The total air content (TAC) in ice cores from polar regions is one of the many parameters that inform us about past environmental conditions. TAC was initially developed to pro-vide robust information about the past surface elevation of ice sheets (Lorius et al, 1968;Raynaud and Lorius, 1973), due to its pressure and, thus, altitude dependence. However, consecutive studies showed that the densification and bubble close-off processes have an even larger influence on the pore volume enclosed in polar ice and, thus, on TAC.…”

Climatic and insolation control on the high-resolution total air content in the NGRIP ice core

“…TAC was initially developed to provide robust information about the past surface elevation of ice sheets (Lorius et al, 1968;Raynaud and Lorius, 1973), due to its pressure and, thus, altitude dependence. However, consecutive studies showed that the densification and bubble close-off processes have an even larger influence on the pore volume enclosed in polar ice and, thus, on TAC.…”