Surge initiation at the terminus of Borebreen (Svalbard): Drivers and impact on calving

Approximately 21% of Svalbards glaciers are classified as surge-type and undergo cyclical changes in ice velocity between quiescent (slow) and active (fast) phases. Whilst it is generally understood that processes at the glacier bed drive surge initiation, the physical mechanisms translating basal sliding to ice flow variability and cyclicity remain open questions. Recent mapping of glacier velocities across Svalbard has identified an acceleration in ice flow at the Borebreen tidewater glacier which terminates on the northwestern side of Isfjorden. Before 2018, average summer velocities at Borebreen were ~0.6 m/d but more than doubled to 2.4 m/d by 2023. Borebreen last surged ~100 years ago, hence the acceleration in ice velocity suggests it is the result of the glacier transitioning to an active surge. In this contribution, we will discuss results from a summer field campaign to Borebreen in August 2023. Using a multi-sensor network of seismic arrays, Terrestrial Laser Scanners (TLS), and drones, we characterise present day surge dynamics and use the data to understand the drivers. In addition, optical imagery from the PlanetScope constellation and Synthetic Aperture Radar (SAR) data from Sentinel-1 are used to determine surface conditions (e.g. surface melt patterns, crevasses, proglacial turbid plumes) and quantify ice velocities. Here, we will report on the following: 1) basal processes (e.g. stick-slip events, icequakes) under Borebreen; 2) calving processes at the over-steepened ice cliff of the surge front; 3) ice velocity extracted from drone photogrammetry, Planetscope imagery and Sentinel-1 SAR scenes; and 4) surface conditions (e.g. crevassing, surface melt) over the course of the ablation season and its relationship with ice dynamics. We find that the surge initiated at the glacier terminus and has been propagating upglacier. The glacier speed doubles each spring in response to elevated air temperatures which leads to surface melting and the delivery of meltwater to the glacier bed. Furthermore, we identify clusters of seismicity at the glacier bed, far from the terminus, which appear to indicate sliding. Our results push forward our understanding of the processes that initiate and sustain glacier surges and glacier instabilities in general.