Physical and structural properties of the Greenland Ice Sheet Project 2 ice core: A review

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Substantial data sets have been collected on the relaxation characteristics, density, grain size, c axis fabrics, and ultrasonic velocities of the Greenland Ice Sheet Project 2 (GISP2) core to its contact with bedrock at 3053.4 m. Changes in all these properties paralleled closely those found in cores from Byrd Station, Antarctica, and Dye 3, Greenland. Density increased progressively with depth to a maximum of 0.921 Mg/m3 at about 1400 m, at which depth the ice became bubble free. Below about 2000 m, in situ densities began to decrease in response to increasing ice sheet temperatures. Since drilling, much of the ice core has undergone significant volume expansion (relaxation) due to microcracking and the exsolving of enclathratized gases, especially in the brittle ice zone between 650 and 1400 m. Grain size increased linearly to about 1000 m, thereafter remaining fairly constant until the Younger Dryas event at 1678 m where a twofold to threefold decrease in grain size occurred. These grain size changes were accompanied by a progressive clustering of crystal c axes toward the vertical, including a small increase in c axis concentration across the Younger Dryas/Holocene boundary. Increased dust levels in the Wisconsin ice have contributed to the maintenance of a fine‐grained texture which, with its strong vertical c axis fabric, persisted to nearly 3000 m. However, beginning at about 2800 m, layers of coarse‐grained ice intermixed with the much finer‐grained matrix ice are observed. Below 3000 m the ice became very coarse grained. This change, attributed to annealing recrystallization at elevated temperatures in the ice sheet, was accompanied by a dispersed or ring‐like redistribution of the c axes about the vertical. Ultrasonic measurements of vertical and horizontal P wave velocities made at 10‐m intervals along the entire length of the GISP2 core fully confirmed the results of the crystallo‐optical observations. A return to fine‐grained ice coincided with the first appearance of brown, silty ice 13 m above bedrock. Bedrock material consisted of 48 cm of till, including boulders and cobbles, overlying gray biotite granite comprising the true bedrock. There is evidence that disturbed structure in the GISP2 cores begins little more than 70% of the way through the ice sheet. This disturbance increases with depth until it becomes large enough to cast suspicion on features lasting centuries or more in the bottom 10% of the ice sheet.