Periglacial and paraglacial processes control rockwall erosion across spatial and temporal scales

Abstract

Periglacial and paraglacial processes drive rockwall erosion in alpine environments. The frequency and magnitude of periglacial and paraglacial erosion will vary in space and time due to topo-climatic effects and climatic changes. We reconstructed glacier retreat since the Last Glacial Maximum (LGM), modelled permafrost distribution and frost cracking activity using measured and reconstructed rock surface temperatures, quantified recent rockwall erosion using terrestrial lasercanning and paleo rockwall erosion by applying geophysical investigations on rockfall deposits. We conducted all measurements on north-facing rockwalls along an elevational gradient ranging from 2500 to 3200 m and adjacent talus slopes in the Hungerli Valley, Swiss Alps. In this study, we analyse periglacial and paraglacial control on rockwall erosion (i) across space along an elevational gradient, (ii) through time in the Holocene and (iii) compare our results to compiled rockwall erosion rates of the European Alps. (i) Glacier reconstruction revealed that rockwalls became ice-free since LGM with recent glacier retreat affecting the cirque area between 2900 and 3200 m. Permafrost is present in rockwalls ranging from 2800 to 3200 m with elevation-dependent decreasing rock temperatures. Frost cracking magnitude increases with elevation until 3000 m followed by a slight decrease in magnitude at 3150 m. Recent erosion rates averaged 0.02 – 0.08 mm a-1 but were much higher (1.42 to 2.04 mm a-1) at elevation bands between 2900 to 3100 m where the magnitude of paraglacial and periglacial processes is highest. Therefore, recent rockwall erosion patterns follow elevation-dependent climate trajectories (Draebing et al., 2022). (ii) We found that rockwalls which have been free of glacier ice since ~10 ka experienced erosion rates two orders of magnitude higher (1.2 -1.4 mm a-1 averaged over the Holocene) than the averaged recent erosion rates. Our modelling suggests this relates to periods of higher intensities of frost cracking and cycles of permafrost aggradation and degradation in the Holocene, relative to today (Draebing et al., 2024). (iii) Compiled erosion rates of the European Alps show an elevation-dependent increase of recent erosion rates (Draebing et al., 2022) and Holocene -averaged rates that exceed recent erosion rates (Draebing et al., 2024). In summary, periglacial and paraglacial processes control spatial and temporal variation of rockwall erosion in alpine environments. Topo-climatic effects resulted in the  elevation dependency of periglacial and paraglacial processes while climatic changes during the Holocene resulted in elevational shifts. Consequently, ongoing climate change will move periglacial and paraglacial rockwall erosion up to higher elevation.   Draebing, D., Mayer, T., Jacobs, B., and McColl, S. T.: Alpine rockwall erosion patterns follow elevation-dependent climate trajectories, Communications Earth & Environment, 3, 21, https://doi.org/10.1038/s43247-022-00348-2, 2022. Draebing, D., Mayer, T., Jacobs, B., Binnie, S.A., Dühnforth, M. and McColl, S. T.: Holocene warming of alpine rockwalls decreased rockwall erosion rates, Earth and Planetary Science Letters, 626, 118496, https://doi.org/10.1016/j.epsl.2023.118496, 2024.