What comes after the ice? Disappearing glacier and dead ice bodies and their significance for proglacial landform and lake evolution as revealed by surface, subsurface and subaquatic long-term monitoring

Deglaciation is a rapid process in many glaciated regions of the world. In the Austrian Alps, data from the Austrian Alpine Association's glacier monitoring program reveal high losses in glacier length in recent years. The ten highest glacier retreat rates for all measured glaciers in Austria in the period since 1961 were all recorded after 2003, with the four highest values after 2016. The emerging proglacial areas are affected by different gravitational, aeolian, fluvial, and lacustrine processes. In this study, we investigated the evolution of proglacial landforms and water bodies at Pasterze Glacier, Austria (47.08°N, 12.72°E) in 2003-2024 using different techniques for surface, subsurface, and subaquatic monitoring of ice bodies to understand their influence on geomorphic dynamics. We applied global navigation satellite system (GNSS) measurements mainly for glacier boundary delineation, airborne photogrammetry and derived digital elevation models for land cover classification and morphometric analyses, ground surface temperature (GST) measurements for information on the ground thermal regime, geophysical measurements (electrical resistivity tomography/ERT, seismic refraction/SR) for ground ice detection and monitoring, and bathymetric measurements for detecting lake-bottom conditions and changes at the ice-contact lake “Pasterzensee”. Pasterze Glacier receded by about 1.1km between 2003 and 2024 based on GNSS data. Ground temperature data in the proglacial region suggest unfavorable conditions for long-term survival of ground ice and permafrost. The mean annual GST in the proglacial area increased substantially between 2007/08 and 2023/24 although interannual changes of the seasonal snow cover conditions make it difficult to receive statistically significant results at sites with shorter timeseries. Geophysical measurements (69 ERT and 1 SR profiles) carried out between 2015 and 2023 combined with the photogrammetry-derived data allowed to monitor 2D and 3D changes of sediment-covered dead ice bodies. Related to this dead-ice degradation and the recession of Pasterze Glacier, the size of the proglacial lake increased from 0.005km² in 2003 to 0.460km² in 2024. Sonar campaigns in 2019-2024 (single- and multi beam echo sounders and sub-bottom profiler) revealed several sub-basins along the 1.2km long and up to 300m wide lake basin, a maximum depth of 48.2m, a mean depth of 13.4m, and a total water volume of 4 Mio. m³ (in 2019). Since 2019, the lake size has increased by a factor of 0.5. Lake volume has also increased, although analysis of more recent bathymetric data is still ongoing. We conclude that the interplay between (a) surface, subsurface and subaquatic ice melt and ice disintegration caused by present glacier-unfavorable climatic conditions, (b) lack of permafrost, (c) high rates of sediment erosion, transport and redeposition by different agents (gravitational, fluvial, glacial, aeolian), and (d) projected future climate warming will further rapidly modify the glacial-proglacial transition zone at Pasterze Glacier leading to the total vanishment of the 1,8 km² large glacier tongue within the next decades. This means that the entire valley area of what was once the largest glacier in the Eastern Alps will become ice-free. Thereby, it serves as a massive laboratory for glaciological, geomorphological and ecological studies.