Boulder-dominated periglacial and related landforms as palaeoclimatic and morphodynamic indicators in Breheimen, South Norway

Boulder-dominated periglacial, paraglacial and related landforms are important elements which can help to improve our knowledge about past climatic conditions and morphodynamic processes. As the formation and stabilization of these landforms can be associated to cold or transitioning climatic conditions from cold to warm, putting them on a solid temporal basis is vital to connect their evolution to changing climatic conditions throughout the Holocene. In this study, Schmidt-hammer exposure-age dating (SHD) was performed at different landforms including sorted polygons, rock-slope failure deposits and a blockfield in and around Breheimen, South Norway. By obtaining an old and a young control point, it is possible to calculate a calibration curve, from which the respective landform ages were estimated. The SHD age estimates ranged from 8.02 ± 0.72 to 3.45 ± 0.70 ka showing their relict character. The sorted polygon ages of 6.55 ± 0.68 and 4.76 ± 0.63 ka point to a stabilization within and towards the Holocene Thermal Maximum (HTM; ~8.0–5.0 ka). Whereas the ages of the investigated rock-slope failures from 8.02 ± 0.72 to 3.45 ± 0.70 ka can be divided in two groups. The first group consists of two rock-slope failures with overlapping ages with a mean age of ~7.6 ka. This timing can be related to the onset of the HTM characterized by warmer temperatures possibly leading to slope weakening due to a variety of factors, such as permafrost degradation and increasing cleft-water pressure. Ages of the second group, with three rock-slope failures, cluster around ~3.7 ka, shortly after a cold climatic period between 4.75–3.85 ka. Therefore, we assume that the occurrences of these rock-slope failures could have been climatically induced by warmer temperatures. The blockfield age of 5.24 ± 0.79 ka is significantly younger than other dated blockfields in South Norway and indicates longer activity of the boulders at the blockfield surface. Surface exposure ages from boulder-dominated landforms stress that these landforms can be valuable elements in improving our knowledge about landform evolution and palaeoclimatic fluctuations within the Holocene in South Norway.