Relaxation of ice in Deep Drill Cores from Antarctica

Cores obtained to a depth of 2164 meters in the Antarctic ice sheet at Byrd station have undergone considerable relaxation since they were drilled. This relaxation, manifested as a density decrease of the ice with time, could still be detected more than 16 months after the cores had been pulled to the surface. The greatest measurable relaxation, of the order 0.6% expansion, occurred in cores from around 800 meters' depth. Ice from 400–900 meters proved to be much more brittle than deeper ice and was characterized by an abundance of highly compressed air bubbles. These bubbles had disappeared completely by 1100 meters, most probably as a result of the diffusion of gas molecules into the ice lattice, and this, together with the formation below 1200 meters of an oriented crystal fabric (characterized by a strong vertical orientation of crystallographic c axes) is believed to be responsible for the greatly increased ductility of the deeper cores. Relaxation of ice from the brittle zone occurs primarily as a result of expansion of pressurized air bubbles. In deeper, bubble-free ice, however, relaxation can be attributed to the creation of new space resulting from the formation of cleavage cracks, plate-like voids and cavities, especially the latter, which become filled with gas derived from the air originally dissolved under pressure in the ice sheet.