Deformation of grain boundaries in polar ice

– The ice microstructure (grain boundaries) is a key feature used to study ice evolution and to investigate past climatic changes. We studied a deep ice core, in Dome Concordia, Antarctica, which records past mechanical deformations. We measured a “texture tensor” which characterizes the pattern geometry and reveals local heterogeneities of deformation along the core. These results question key assumptions of the current models used for dating. Motivations. – Polar ice cores are the focus of many investigations because they record the history of climatic changes. Owing to snow accumulation, snow to ice transformation and slow ice sheet flow (∼ 10s), a journey down to the deep layers of the ice sheet is a journey back to several hundred of thousands of years into the past [1]. A crucial step of paleoclimatic studies from ice cores is dating. In Antarctica, counting annual layers is impossible [2]: absolute dating is only possible for the very top of the ice cores where ice layers containing volcanic impurities can be related to historical volcanic eruptions. Below, dating relies on ice sheet flow models of the evolution of ice layer thinning with depth [2]. Such models are loosely constrained by the identification of large climatic transitions. For the sake of simplicity, these models assume a smooth and monotonous increase of the thinning with depth, hence ignore any possible localization of the deformation [2, 3]. In this letter, we question this essential assumption. We present a method to extract geometrical information (such as thinning, shear, localization of the deformation) from pictures of a cellular pattern using local spatial averages of the “texture tensor” [4]. We apply this analyzis to the grain boundaries (the so-called “microstructure”) of ice samples from a deep ice core. Samples. – Dome Concordia, Antarctica (75 06’ 04” S, 123 20’ 52” E, elevation 3233 m a.s.l) is at the summit of an Antarctic ice dome. It has been chosen because it is usually assumed that the ice flow is axisymmetric around the vertical (z) axis, and isotropic within the