Insights into the seasonal dynamics of the lake-terminating glacier Fjallsjökull, South-East Iceland, inferred using ultra-high resolution repeat UAV imagery

Abstract

Proglacial lakes are becoming ubiquitous at the termini of many glaciers worldwide, leading to increased glacier mass loss and terminus retreat due to the influence of these proglacial lakes on ice dynamics. However, despite the highly dynamic nature and relative insensitivity to climate of many lake-terminating glaciers, an understanding of the key processes forcing their behaviour is lacking. As a result, it is difficult at present to accurately assess and predict how these glaciers may respond in the future. A novel method to address this difficulty, however, is through the use of repeat uncrewed aerial vehicle (UAV) imagery, which can provide high to ultra-high resolution (cm-dm scale) imagery of the ice surface at varying spatial and temporal scales, depending on the needs of the study, although its use as a tool for investigating the dynamics of lake-terminating glaciers is so far limited. This research utilised ultra-high resolution repeat UAV imagery to provide insights into the changing dynamics of Fjallsjökull, a large lake-terminating glacier in southeast Iceland, across the 2019 and 2021 summer melt seasons. The findings indicate that the overall dynamics of the glacier are controlled by the ~120 m deep bedrock channel under the study region, which is causing the glacier to flow faster as it enters deeper water, leading to increased ice acceleration, thinning and retreat, with the glacier being decoupled from the local climate as a result. Such a close correspondence between ice velocity and surface thinning suggests the implementation of the positive feedback mechanism dynamic thinning in this region of Fjallsjökull, with such heightened rates of surface thinning and frontal retreat likely to continue in the future until the glacier recedes out of the bedrock channel into shallower water. Within this overall pattern, however, more localised, short-term changes in glacier dynamics were also observed, with these likely being forced primarily by subaqueous melting at the waterline, rather than the specific bedrock topography. Finally, supraglacial lake drainage may also be important for forcing sub-daily (e.g. hourly) increases in velocity, although further work is required to quantify its influence more accurately. As a result, these findings clearly indicate the complex nature of the calving process, as well as the dynamics of calving glaciers in general, highlighting the need for continued monitoring of lake-terminating glaciers at varying spatial and temporal scales in order to better understand and predict how they may respond in future.

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Identifiers

ORCID for Nathaniel Baurley: orcid.org/0000-0002-0444-8721

ORCID for Jane Hart: orcid.org/0000-0002-2348-3944

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