PhD Thesis Defense by Miguel Angel Sánchez Moreno

Title: Microstructure mapping and ice rheology in ice core sciences.

Abstract: This dissertation develops and applies a new methodology for continuous, high-resolution mapping of polar ice core microstructure at the microscale (5 μm/px), bridging the gap between traditional visual stratigraphy and point-scale microstructure measurements. The integrated system combines an automated microtome for reproducible sample preparation, controlled sublimation to enhance grain boundary visibility, and an Extra Large Area Scanning Microscope (xLASM) for rapid, tile-based image acquisition over meter-scale sections. The resulting image data are processed using automated routines to extract quantitative metrics of grain size, shape, and grain boundary characteristics.

The method is applied to a 16-meter section of the North Greenland Eemian Ice Drilling (NEEM) ice core, covering the Dansgaard–Oeschger event GS-20. The microstructure data are interpreted in combination with borehole deformation rates and impurity records. The analysis reveals that impurity-rich stadial layers contain finer and more elongated grains and show elevated shear strain rates, despite the presence of strong crystallographic preferred orientation (CPO). These findings suggest that impurity-driven grain size reduction enhances local deformation, independent of CPO evolution.

Overall, the methodology provides a scalable and reproducible approach to microstructure analysis in deep ice cores and contributes new constraints for the integration of grain-size-sensitive processes into large-scale ice flow models.

Committee: Associate Professor Aslak Grinsted (Chair), Pofessor Martyn Drury (Utrecht University) and Professor Paul Bons (Tübingen University)

Supervisors: Professor Dorthe Dahl-Jensen (Primary), André Lamott (Schäfter + Kirchhoff GmbH)