The paper investigates the dynamics and age of the Triangle du Tacul (TDT) ice apron, a massive ice volume located on a steep high-mountain rock face on the French side of the Mont Blanc massif at an altitude of almost 3640 m a.s.l. In 2018 and 2019, three 60 cm long ice cores were drilled down to bedrock (i.e. the rock face) in the TDT ice apron.
Textural analyses (microstructure and preferred lattice orientation, LPO) were carried out on one core. The two remaining cores were used for radiocarbon dating of the particulate organic carbon fraction (three samples in total). Microstructure and LPO do not vary significantly along the axis of the ice core. Irregularly shaped grains were observed throughout the core, associated with strain-induced grain boundary migration and a strong single LPO maximum. The measurements indicate that the ice at the TDT deforms under a simple low strain rate shear regime, with the shear plane parallel to the surface slope of the ice apron. Dynamic recrystallisation emerges as the main mechanism for grain growth.
Micro-radiocarbon dating shows that the TDT ice gets older with increasing depth perpendicular to the ice surface. We have observed ice ages older than 600 years BP and older than 3000 years at the base of the lowest 30 cm.