Bathymetric and terminus evolution as determined by remote-sensing techniques: Tasman Glacier, New Zealand

Abstract

Global glacier recession is increasing the number of glaciers that terminate in proglacial lakes, yet knowledge about the processes that drive ice-berg calving are still poorly understood. This knowledge-gap is in part due to the challenge of obtaining good data sets in a highly dynamic and dangerous environment. We are using emerging remote technologies, in the form of a remote controlled jet boat to survey bathymetry, and Structure from Motion (SfM) to characterise terminus morphology, to better understand relationships between lake growth and terminus evolution. Comparison of results between the jet boat mounted dual-frequency Garmin fish-finder with an Odom Echotrac DF3200 MKII with 200/38 kHz dual-frequency transducer, showed that after a sound velocity adjustment, the remote survey obtained depth data within ± 1 m of the higher grade survey equipment. A maximum water depth of 240 m was recorded 250 m away from the terminus, and the sub-aerial cliff height ranged from 5-44 m. However in some regions, water depth was 180 m only 20 m away from the calving face. Here the sub-aerial cliff height was around 20-25 m, meaning portions of the terminus were, or very close to, buoyancy. A sub-aqueous ice-ramp, 30 m below the water, extended out into the lake from the terminus for approximately 80 m in the central-eastern region. Despite a lake expansion of nearly 1 km since the survey of Dykes et al. (2011), the spatial similarity of the sub-aqueous ramp may indicate that other processes, for example, subglacial hydrology, have influence on the evolving terminus morphology.