Flow properties of the ice from the Greenland Ice Core Project ice core: The reason for folds?

Abstract

Long‐term deformation tests on ice from the Greenland Ice Core Project (GRIP) deep ice core show that ice from the different climate zones in the ice core has flow properties correlated with the concentrations of impurities in the sample. The deformation tests are performed by uniaxial unconfined compression at −16°C with an octahedral compression stress of 3 bars. The ice samples are compressed for ½ to 3 years until the tertiary strain rate is reached. It is believed that by the end all downhole flow conditions are forgotten and that the ice sample has settled in a state determined by the applied stress and temperature conditions. All samples are tested under the same stress and temperature conditions so the resulting deformation rates and final ice crystal size and fabrics can only differ due to varying impurity concentrations. The results show that ice from cold climatic periods with high concentrations of impurities deforms more slowly than ice from warm climatic periods in compression. When tertiary creep is reached, the crystal size is smaller in the cold ice than in the warm. The ice from warmer climatic periods with lower concentrations of impurities deforms at a factor of 2–3 times more rapidly in compression. The tertiary steady state crystal size is increased by 50% and the ice crystals have oriented more favorably for the applied compression in the warm ice, which is believed to be the reason why the strain rates are greater here than in the cold ice. In the bottom 200 m of the GRIP ice core, zones are observed with folds on the scale of 1–8 cm. An investigation of the ice layers in and around the folds shows that the layers are composed of ice from different climatic zones. The folding is believed to result from the different flow and rheological properties of the layers involved in the folding structures.