Processes and feedbacks associated with iceberg calving and subaqueous terminus morphology, Tasman/Haupapa Glacier, New Zealand.
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
Global deglaciation is increasing the number of glaciers that are forming or have formed proglacial lake. Deglaciation is also changing the size of proglacial lakes in temperate settings, resulting in more glaciers losing mass via iceberg calving. In order to better estimate future ice volume change, we need to understand processes driving calving in freshwater environments. Calving is an important process influencing the mass balance of a glacier, as the calving process is a cause of high rates of ablation in comparison to melt out rates alone. The calving rate and magnitude of iceberg calving can be influences by the presence of subaqueous ice ramps. Some calving glaciers form subaqueous ice ramps or ‘ice feet’ due in part to differences in the rates of subaerial and subaqueous calving. Spatial and temporal variation of these subaqueous features indicate that other processes also exert control. Photographic and satellite imagery was combined with bathymetric and limnological surveys from Tasman Glacier (2013-2018) to explore the processes that drive spatial and temporal variability in iceberg calving and subaqueous terminus morphology, the relationships between near-terminus surface velocity, iceberg calving and subaqueous morphology;
A large number of calving events in spring and summer are associated with an increase in surface velocity and lake level fluctuations. One significant calving event over a 10-day period had a surface velocity of 0.39 md-1 ±0.04 md-1. This was followed by a 58.97% decrease over a 20-day interval post calving event. Lake level increased on the day of the calving. Lake levels increased from 7.17.84 m ± 0.02 m to 719.93 m ±0.10 m over a 5-day periof. Increases in surface velocity and lake levels did not always result in major calving activity. Velocity or lake level increase may not always be an indication that a large calving event is imminent. The lack of large calving events after some surface velocity increases indicates that large calving events 1 are likely a result of multiple processes. Velocity and calving have a strong relationship with subaqueous morphology. Velocity varies spatially across the terminus with lower velocities on the eastern side of the terminus. The slower velocity combined with the increase in lateral drag at the margins of the glacier means that calving events are less frequent resulting in a more stable setting. Faster surface velocities are found in the central region of the terminus, result in a higher longitudinal strain rate and therefore higher calving rate.
A higher aerial calving rate than subaqueous calving rate leads to the formation of subaqueous ice ramps which are subject to subaqueous melt and buoyant calving processes. In order to prevent floatation, the terminus needed to exceed 23 m ± 5 m. The central ice cliff section does not meet HF, but the terminus is unlikely to be buoyant due to cliff height exceeding the floatation thickness at each margin. As such, the dominant form of subaqueous calving is driven by buoyant calving rather than from the disintegration of a floating ice tongue.
The distribution of ramp lengths from 2013 – 2018 are commonly between 40 – 120 m in length (n=170) with the next largest group (n=12) between 120 – 200 m, only three ramps have been identified as greater than 200 m over the five-year survey time. From 2013-2018 a limited number of ice ramps exceeded 200 m in length, suggesting that ice ramps greater that 200 m are highly prone to buoyant calving, given that no ice ramp has exceeded 380 m during the 6-year survey period.
Findings to date indicate the primary driver of subaqueous ice ramp formation is the faster aerial calving rate over subaqueous calving. Spatial and temporal variation in velocity, lake limnology and subglacial hydrology also drive the formation and calving of subaqueous morphology but on a lesser scale than that of aerial calving.