De-icing processes in glacier forelands: generic or site-specific? Answers and challenges from field observations

Global glacier retreat has resulted in an unprecedented loss of glacier mass on the one hand and large volumes of debris previously covered by, or covering, ice to be released on the other hand. While active geomorphological processes such as the formation of distinct moraines and other diagnostic glacial landforms may have ceased in many forelands due to recent atmospheric warming (e.g. Rettig et al., 2023), it remains unclear whether this implies that we as a community lose the ability to identify the location and duration of retreat of ice masses in their final stages of demise in palaeo-settings beyond the ‘universally-accepted’ sets of sediment-landform associations we have come to associate with active glacier processes (e.g. moraines). It is therefore imperative to take a closer look at de-icing processes systematically. This contribution is a first attempt of synthesising over 20 years of field observations from a wide range of Arctic and Alpine glacier forelands to provide possible answers and highlight remaining challenges.

De-icing processes comprise both ‘regular’ depositional processes such as passive debris release by meltout (dumping) from actively-retreating ice and a whole host of secondary release processes of debris initially deposited on top of, in and around stagnant and dead ice bodies near former ice margins. The processes of ice burial of a formerly-coherent body of active glacier ice vary from site to site due to their dependence on climatic, glaciological and topographic boundary conditions, but the physical processes of what happens to the debris appear to have a large number of similarities and have been observed in several forelands in Svalbard, Sweden, Norway, the Russian Altai, the European Alps and the Southern Alps of New Zealand.

Following meltout and debris redistribution, any vaguely diagnostic glacial landform signature disappears, making most modern forelands of the last few years to decades difficult to use as palaeo-environmental tools. This is also due to uninterrupted retreat and thus little clear geomorphological evidence being preserved that allows any reconstruction of glacier extent, for example, but also due to our reliance on these clear diagnostic landforms. However, zooming into the usually-resulting chaotic sediment cover itself, a mixture of diamictic and sorted sediments displays what appear to be diagnostic criteria, all indicative of a loss of ice support.

While preservation of these subtle indicators is the biggest challenge and found to be dependent on the hydrogeological conditions of the foreland in question, the possibility of being able to identify diagnostic criteria of de-icing processes in palaeo-settings is exciting. Further work is ongoing to test this possibility with the aim of extending the areal coverage and timeframe over which the very final phases of Quaternary glaciations could be reconstructed and dated.

 

Reference

Rettig, L., Lukas, S. and Huss, M., 2023. Implications of a rapidly thinning ice margin for annual moraine formation at Gornergletscher, Switzerland. Quaternary Science Reviews, 308: 108085.