There follows the open discussion which took place at the IGS symposium on ‘The Edges of Glaciology’, in July 2023. The discussion was curated by Doug Benn. The time of speaking in minutes and seconds into the Panopto recording is given in bold figures. The recording itself is provided as electronic supplementary material. It has been transcribed and edited by Andrew Fowler, with much (and much-needed) assistance from the participants. Footnotes (mostly references) are editorial intrusions.

We present a new model for understanding ice cliff dynamics within a debris-covered glacier ablation zone. This simple energy-balance model incorporates a moving frame of reference, made necessary by the melt of the surrounding debris-covered ice. In so doing, this also formalises how different types of field measurements can be utilised and compared. Our predictions include showing: ice cliffs can endogenously select their own slope angles; that there should be an indifference between illuminated north- and south-facing ice cliff slopes; that ice cliffs grow steeper with thicker debris layers; that ice cliffs cannot stably exist below a certain critical debris thickness and that some modelling of ice cliffs (when not incorporating the moving frame) may incorrectly estimate ice mass losses. All of our results are produced using parametrisations from Baltoro Glacier, Karakoram.

The increase in iceberg discharge into the polar oceans highlights the importance of understanding how quickly icebergs are deteriorating and where the resulting freshwater injection is occurring. Recent advances in quantifying iceberg deterioration through combinations of modeling, remote sensing and direct in situ measurements have successfully calculated overall ablation rates, and surface and sidewall ablation; however, in situ measurements of basal melt rates have been difficult to obtain. Radar has successfully measured iceberg thickness, but repeat measurements, which would capture a change in iceberg thickness with time, have not yet been collected. Here we test the applicability of using an on-iceberg autonomous phase-sensitive radar (ApRES) to quantify basal ablation rates of a large (~800 m long) non-tabular Arctic iceberg during an intensive 2019 summer field campaign in Sermilik Fjord, southeast Greenland. We find that ApRES can be used to measure basal ablation even over a short deployment period (10 d), and also provide a lower bound on sidewall melt. This study fills a critical gap in iceberg research and pushes the limits of field instrumentation.

A surge of Mittie Glacier, a 50 km-long tidewater outlet glacier of Manson Icefield, occurred between approximately 1992 and 2007. Velocities increased slowly at first, but then increased rapidly to reach a peak of 4800 m a−1 in early 1996, the highest ever reported for a glacier in the Canadian Arctic. The surge initiated at the terminus and propagated up-glacier, with a maximum terminus advance of 7.3 km between 1994 and 1999. The surge was almost entirely restricted to the lower ~30 km of the glacier, in a region which radio-echo sounding shows to be grounded below sea level. A 3 km-wide crevasse with a 150 m opening occurred at the separation between faster moving ice downstream and slower moving ice upstream. Surge initiation appears to have been triggered by flotation of the lower terminus, caused by long-term thinning of this region during quiescence.

This paper presents geomorphological and historical records of the surge-type behaviour of Hansbreen, one of the most studied tidewater glaciers in Svalbard. The surge-type behaviour of the glacier has not been considered before due to the lack of evidence of this phenomenon. We integrate geomorphological mapping of the terrestrial and submarine forefields with historical data from the 19th and 20th centuries to reconstruct the glacier dynamics and identify the possible timing of surging. Landform assemblages are representative of the surging glacier landsystem, including crevasse-squeeze ridges (CSRs) and submarine streamlined glacial lineations. Abundant CSRs in the outer part of the terrestrial forefield were also documented in the 1980s, but most have been obliterated since then. We suggest the identified surge landsystem was produced during a surge of Hansbreen detected from photographs taken during the Austro-Hungarian expedition in 1872. Historical photogrammetric photos from the Norwegian expedition in 1918 revealed surge-diagnostic features in the glacier surface, including a folded medial moraine and a dense, complex network of crevasses. A potential next surge remains questionable in the following decades due to the low-lying accumulation area of the main stream hindering the mass build-up, but potential surges of the tributary glaciers should not be excluded.

Cryoconite holes are structures characteristic of the glacierised ablation areas, formed from dark sediment deposition which melt the glacier ice leading to their cylindrical structure formation. The present study focuses on an unstudied blue ice area of East Antarctica over the Hells Gate Ice Shelf in Northern Victoria Land. Taking advantage of neural network methodology applied on drone acquisitions, an automatic detection of cryoconite holes was carried out (88% of accuracy), mapping ~3500 m2 (0.4% fractional cover) of cryoconite holes over an area of 900 000 m2. The surveyed area showed heterogeneity in cryoconite hole distribution, with some regions showing an area fraction of holes (regions exhibiting frozen ponds and ice bands) as high as 8%. Thanks to this detection, the spatially integrated radiative forcing of cryoconite holes over blue ice surface was estimated to be almost 1 Wm−2. This estimation was based on the measured albedo difference between the two surfaces, which averages 0.31. However, the radiative forcing is strongly increased if calculated at a single cryoconite hole scale, showing an average of ~200 Wm−2. In conclusion, the present research represents a first attempt of analysing cryoconite holes as factors impacting the blue ice in Northern Victoria Land, deepening our understanding of their glaciological role at the margins of the East Antarctic Ice Sheet.

Knowledge of how glaciations formed landscapes is particularly important as receding glaciers currently uncover subglacial landscapes that are prone to a series of natural hazards, but that also bring opportunities for hydropower or water resources. We present high-resolution (1 × 1 m) swath bathymetric data of a proglacial lake in front of the Rhonegletscher (Swiss Alps) that started to form in the early 2000s allowing a look into a freshly uncovered glacier bed and its characterized morphology in an overdeepened setting. The comparison of two surveys from 2015 and 2021 allows an unprecedented quantification of the accumulation and erosion processes in the central lake basin. This highly dynamic environment is characterized by iceberg calving, fluctuating outflow conduits, rapid sedimentation due to particle-laden meltwaters and dumped glacial debris. Assuming constant sediment yield, the Rhone Lake would persist for ~300 years. However, as intense glacier retreat continues in the coming decades, a chain of overdeepened lakes will be revealed that will act as long persisting sediment traps.

We review successes and challenges from five recent subglacial bedrock drilling campaigns intended to find evidence for Antarctic Ice Sheet retreat during warm periods in the geologic past. Insights into times when the polar ice sheets were smaller than present serve as guiding information for modeling efforts that aim to predict the rate and magnitude of future sea level rise that would accompany major retreat of the Antarctic Ice Sheet. One method to provide direct evidence for the timing of deglaciations and minimum extent of prior ice sheets is to extract subglacial bedrock cores for cosmogenic nuclide analysis from beneath the modern ice sheet surface. Here we summarize the lessons learned from five field seasons tasked with obtaining bedrock cores from shallow depths (<120 m beneath ice surface) across West Antarctica since 2016. We focus our findings on drilling efforts and technology and geophysical surveys with ground-penetrating radar. Shallow subglacial drilling provides a high risk, high reward means to test for past instabilities of the Antarctic Ice Sheet, and we highlight key challenges and solutions to increase the likelihood of success for future subglacial drilling efforts in polar regions.

Subglacial seismicity presents the opportunity to monitor inaccessible glacial beds at the epicentral location and time. Glaciers can be underlain by rock or till, a first order control on bed mechanics. Velocity-weakening, necessary for unstable slip, has been shown for each bed type, but is much stronger and evolves over more than an order of magnitude longer distances for till beds. Utilizing a de-stiffened double direct shear apparatus, we found conditions for instability at freezing temperatures and high slip rates for both bed types. During stick–slip stress-drops, we recorded acoustic emissions with piezoelectric transducers frozen into the ice. The two populations of event waveforms appear visually similar and overlap in their statistical features. We implemented a suite of supervised machine learning algorithms to classify the bed type of recorded waveforms and spectra, with prediction accuracy between 65–80%. The Random Forest Classifier is interpretable, showing the importance of initial oscillation peaks and higher frequency energy. Till beds have generally higher friction and resulting stress-drops, with more impulsive first arrivals and more high frequency content compared to rock emissions, but rock beds can produce many till-like events. Seismic signatures could enhance interpretation of bed conditions and mechanics from subglacial seismicity.