Flow of Greenland ice as a function of climatically determined ice characteristics, flow history and conditions at depth, studied by laboratory experiments on grip ice samples

The Greenland Ice core Project (GRIP) is a multinational drilling project, coordinated by Denmark, whose initial objective was the retrieval of a complete ice-core from the highest point on the Greenland Ice Sheet. This objective was achieved in the summer of 1992, with a final core length of 3029m being retrieved. We were able to acquire samples of the core from a number of palaeoclimatological horizons. These samples have been subjected to controlled deformation under in-situ glaciological conditions of temperature and pressure, at low strain rates, to determine the flow characteristics of the ice as a function of depth. Deformation, temperature and pressure are controlled using the servo-controlled cryogenic triaxial deformation cell housed in the Ice Physics Laboratory at UCL. This equipment has been modified to allow acoustic emissions (AE) and elastic P-wave velocity measurements (Vp) to be made on the sample during deformation. Both brittle and plastic deformation processes occur during testing and AE and Vp measurements can be used to delineate both modes of deformation. Also, by studying the AE amplitude distribution, AE can be used to distinguish between large, grain size scale cracks occurring within grains and along grain boundaries and smaller intragranular cracks which form as a result of plastic processes such as dislocation pile-up and lattice bending operating within the grain. All ice tested was cored vertically from the GRIP core. Crystallographic c-axis orientation, which moves towards the vertical with depth, will therefore exert a controlling influence on the measured flow stress since ice is approximately 60 times stronger when shortened in a direction parallel to its c-axis than when compressed at 45° to the c-axis. Samples of randomly oriented laboratory grown ice have also been tested under the same experimental conditions and the results used to assess the effect of grain orientation, grain size and impurity content on deformation. The flow stress of the GRIP ice increases with depth in the vertical direction with a significant increase being seen across the Holocene-Wisconsin boundary. A major cause of this increase in strength is found to be the increasing crystallographic anisotropy that also occurs with depth, with c-axes being progressively re-oriented in the vertical direction. Ice grain size and the presence of insoluble inclusions and soluble impurities are found to have a minor affect the value of the peak stress. though it is not possible to quantify the effect of each characteristic individually. At very deep levels, below the Wisconsin, the ice is seen to be significantly weaker. This is due both to pressure melting occurring along grain boundaries (though it is doubtful that this occurs under true downhole conditions) and a decrease in crystallographic anisotropy. Under the conditions of stress existing away from the ice divide, where the ice deforms in simple shear, the Wisconsin ice will flow more readily than the Holocene ice. However at the ice divide where stress conditions correspond to vertical compression and pure shear, Wisconsin ice is much more resistant to flow than the overlying Holocene ice and the older Eemian and Saalean ice. This has significant implications for the flow of the ice sheet in the region of the ice divide and for the dating of the GRIP ice-core. The results have also been analyzed in terms of Glen's (1955) isotropic flow law and Azuma's (1995) flow law which incorporates an expression for the anisotropy. According to these flow laws the deformation rate is proportional to some power n of the applied stress. To determine the flow law parameters multilinear regression analysis has been performed on the peak stress values obtained from both laboratory grown ice and GRIP ice. For the laboratory grown ice n=2.5±0.2, the activation energy, Q=64±11kJmol-1 and the pre-exponential constant. In A0= 14.4±5.2MPa-ns-1. These values are in excellent agreement with those obtained by other workers. For the GRIP ice, in the vertical direction only, (measurements were not made in the horizontal direction due to the availability of samples) according to Glen's isotropic flow law, n=3.0±1.7, Q=175±72 and In A=46±19. According to Azuma's flow law, n=3.8±2.4, Q= 139±98kJmol-1 and In B0=27±17MPa-ns-1. In both cases the stress exponent n is in good agreement with values obtained by other workers on glacier ice. The high values obtained for Q and In A, together with the high standard deviations are due to the presence of two deformation mechanisms, basal and non- basal glide, the latter becoming increasingly more dominant as depth and anisotropy increase