Linking crystallographic orientation and ice stream dynamics: evidence from the EastGRIP ice core

Abstract. A better understanding of glacial ice flow and how it is influenced by internal deformation is required to improve the projections of future sea level rise under a warming climate. Large ice streams, the main contributors to solid-ice discharge to the ocean, still require more observational data to be represented sufficiently in numerical ice sheet models. The East Greenland Ice-core Project (EastGRIP) successfully drilled the first continuous deep-ice core through an active ice stream, the Northeast Greenland Ice Stream (NEGIS), focusing on investigating the dynamical processes that lead to its exceptionally high velocity. Here, we show crystallographic preferred orientation (CPO) data at a 5–15 m depth resolution throughout the 2663 m core, down to bedrock, to determine the deformation regimes in this ice stream, complemented by grain size and borehole temperature profiles for context. A broad single-maximum CPO pattern is present in the upper 200 m caused by overlying snow and ice layers. Below, a crossed-girdle CPO is observed for the first time in a deep-ice core and we discuss possible formation mechanisms. Between 500 and 1230 m in depth, we observe a vertical-girdle CPO indicative of along-flow extensional deformation. A complementary simple-shear component could explain the CPO between 1230 and 2500 m, a vertical girdle with horizontal maxima of varying strength. Close to bedrock, large amoeboid-shaped grains and a multi-maxima CPO indicate migration recrystallisation due to high temperatures close to the pressure melting point. Complementary conductivity data further suggest an undisturbed stratigraphy until at least 104 ka b2k (thousands of years before 2000 CE), and microstructural data suggest even older ice from the Eemian. A comparison with other deep-ice cores from Greenland and Antarctica shows the uniquely fast development of CPOs at shallow depths in the EastGRIP ice core due to its location in an area of high strain rates, while the grain size profile with depth remains similar to less dynamic sites, confirming that it is mainly governed by the varying purity of ice deposited under varying climatic conditions. We further show that the overall plug flow of NEGIS is characterised by many small-scale variations, which remain to be considered in ice flow models.