Novel insights into the microstructure and crystal-preferred orientation of million-year-old Allan Hills ice

Ice cores from the Allan Hills (AH) Blue Ice Area, Antarctica, are up to 6 million years of age, providing novel snapshots in time reaching back into the Miocene. However, AH ice core records are often discontinuous, probably caused by a complicated flow behaviour and, so far unknown, history. Deriving a better understanding of the past and current deformation via ice crystal orientation and microstructure analysis will help interpret these precious ice samples. We, therefore, apply a cascade of structural glaciology methods focusing on four depth regimes around identified age reversals from the 159 m long AH1901 core. Visible features in this, and other AH cores, are large, strongly elongated bubbles. We thus analyse the 2D shape preferred orientation (SPO) of almost 20,000 air bubbles within polished AH samples using established optical mapping methods. Bubble elongation (aspect ratio) is up to 3 times larger than in, e.g., the WAIS divide ice core and is comparably consistent throughout all samples, implying a critical ice-strain rate for a significant time. Similar results were derived via 3D micro-CT investigations. High-resolution grain boundary network analyses via Large Area Scanning Macroscope (LASM) reveal comparably large, bulging crystals with amoeboid shapes, indicating strong recrystallisation. Mean crystal sizes in horizontally (to the core axis) orientated samples are roughly 2-5 times larger than in vertically oriented crystals indicating highly elongated crystal shapes. Finally, we investigated the crystal-preferred orientation (CPO or fabric) within polished thin sections (300 μm) with an automated fabric analyser (G50). Preliminary data show broad single maxima CPOs with several deviating crystals. Closer investigations identify these diverging crystals as bands of crystals with a different orientation intruding the matrix of similar-orientated crystals. Comparable observations were made in the NEEM core (tilted lattice bands), and they could indicate highly localised shear zones. Further investigations on additional samples will help characterise the flow history of AH ice.