Ground thermal regime and periglacial slope processes in Norway and Iceland

Abstract

The ground thermal regime substantially influences geomorphological processes operating in periglacial environments, although the connection may not be straightforward. Recent studies provide more evidence for enhanced rockfall activity and accelerating rockslides in terrain underlain by thawing permafrost. In contrast, there is an ongoing debate on the periglacial imprint for long-term landscape evolution. Furthermore, the concept of "frost cracking window", which refers to an optimal ground temperature range for frost weathering, was established over two decades ago to describe in a simple way the connection between frost weathering and ground temperature. Therefore, knowledge of the ground thermal regime is essential in the geomorphological context on both short- and long-term scales. This dissertation employs permafrost modelling, modelling of frost weathering, space-borne and ground-based remote sensing approaches as the main methods to investigate various concepts within cold-region geomorphology. Ground temperature is modelled using a one-dimensional heat flow model for Iceland and a two-dimensional heat flow model for rock walls in Norway. This modelling showed that Iceland’s shallow and warm permafrost and Norway’s rock wall permafrost are susceptible to the atmospheric warming that has been lasting since the 1980s. The number of cells with simulated permafrost in Iceland decreased by approximately 40 % between the 1980s and 2010–2016. The average warming of Norway’s rock walls has been 0.2 °C per decade at 20 m depth since the 1980s. Recent permafrost thawing in Norway and Iceland may have consequences for slope stability, rockslide and rock glacier dynamics, and lead to the disappearance of palsas. Sites with enhanced frost weathering were identified using the two-dimensional modelling of frost cracking performed for steep rock walls in Jotunheimen, southern Norway. Such sites are typically found between the rock wall and melting ice sheet or glaciers, as well as where the snow depth changes abruptly, resulting in large thermal gradients. Therefore, bedrock may be gradually weakened by segregation ice weathering during deglaciation, in addition to the effects of glacial debuttressing. The Younger Dryas climate was suitable for intense segregation ice weathering in the coastal areas of Norway and Iceland, according to the frost cracking modelling conducted using a one-dimensional approach. This implies that high rates of segregation ice weathering may have contributed to the formation of talus-derived rock glaciers. The enhanced segregation ice weathering at that time also compares favourably to previous studies on the timing of rockfall accumulations in Norway. Furthermore, short-term and long-term rock wall retreat rates are estimated in the Kjelen rock wall, southern Norway, using point cloud differencing and sediment volume estimation for a nearby ice-cored moraine. The average rate of rock wall retreat since deglaciation was only two times greater than the current average rate for the study period, which was limited though to only one year. Most of the detected rockfalls occurred during summer, and most of the material was lost from frequently shaded rock wall sections. In addition, slow mass movements are measured using satellite radar interferometry for the Juvflye hillslope, southern Norway. The observed displacements are related to the solifluction processes, as demonstrated by the high correlation between the temporal variations in displacement and the thaw depth from borehole temperatures. The distribution of active solifluction areas was primarily governed by ground temperature and a vegetation index from multispectral satellite images, with increasing activity for the lowest ground temperatures and sparsely vegetated areas. The presence of ground ice has a profound impact on geomorphological processes, which slowly contribute to the long-term landscape denudation in Norway and Iceland. However, the contribution of periglacial processes to the development of the high-elevation, low-relief surfaces in southern Norway remains highly uncertain.