Fabric, texture, and bubble characteristics of the million-year old Allan Hills blue ice core ALHIC1901

Ice cores from the Allan Hills blue ice area in Antarctica have been dated to several million years of age. However, the stratigraphy of these cores is often disturbed, and age reversals are common, hampering the interpretation of the derived climate records. To better understand the physical processes affecting the ice, we here use a variety of microstructural methods to investigate the fabric, texture and bubble characteristics of four depth regimes in the ALHIC1901 core. We find single maximum CPOs with an occasional second maximum; stripes of differently orientated crystals are common. We further observe small, potentially elongated, grains containing indications of dynamic recrystallisation. 2D and 3D bubble data show strongly elongated bubbles and very low bubble number densities compared to similar depths of deep ice cores. We interpret these findings as indications of simple shear, including kink and z-folds at the centimeter scale and potential alterations to the original air bubbles and, thus, their gas content. Extended studies of Allan Hills ice cores using continuous sections are needed to evaluate the spatial scale of the folding and its impact on the climate record.