Englacial shearing in the context of old ice preservation

Old ice (>1 million years) has been discovered in blue ice areas in the Allan Hills, Antarctica, and ice cores have been retrieved from shallow boreholes less than 200 m depth (Higgins et al., 2015 and Yan et al., 2019). However, it remains unclear what ice properties, mechanisms, and conditions allow for the preservation of old ice in blue ice areas. A high-resolution distributed temperature sensing (DTS) deployment was carried out in the COLDEX 22/23 and 23/24 field seasons. A temperature sensitive fiber optic cable was deployed into five air-filled boreholes, one for two consecutive seasons. Temperature was recorded every ~25 cm along the fiber length at a temperature resolution of 10 mK. Borehole temperature profiles reveal both the seasonal temperature signal shallow in the ice column and recent climate temperature trends below the seasonal signal due to ground surface temperature diffusion. Discrete temperature anomalies, unrelated to ground surface temperature signals, were identified at multiple depths. The temperature anomalies appear to coincide with points of ice crystal c-axis transitions in the ice fabric, suggesting possible shear horizons at transitions between ice layers at these depths. 23/24 field season efforts aim to confirm the presence of transient shear heating events and elucidate the horizontal distribution of shear heating events as they relate to the presence of old ice.