New aspects of deglaciation in southern Norway

Abstract

The investigation of periglacial and related landforms in South Norway is of high interest for exploring timings of deglaciation and to assess their geomorphological connectivity to palaeoclimatic changes during the Late Quaternary and the Holocene. The ice margins of the Scandinavian Ice Sheet during the Last Glacial Maximum (LGM) are fairly well known, the palaeo-ice thickness, however, which can only be estimated by modelling, remains unclear over large parts of Norway owing to rare field based evi-dences. Due to the significant influence of the former horizontal and vertical ice-sheet extent on sea-level rise, atmospheric and oceanic circulation patterns, erosive properties of glaciers and ice sheets, englacial thermal boundaries and deglaciation dynamics, it is crucial to better understand the topograph-ic features of the LGM ice sheet. Despite recent advances, there is a lack of terrestrial evidences from numerical data in South Norway. In this thesis two high-mountain regions and their surroundings in west (Dalsnibba, 1476 m a.s.l.) and east (Blåhø, 1617 m a.s.l.) South Norway were used to reconstruct palaeoclimatic conditions and deglaciation patterns. Terrestrial cosmogenic nuclides (10Be) and Schmidt-hammer exposure-age dating (SHD) have been utilized to determine the surface exposure of glacially transported boulders as well as of boulder-dominated glacial, periglacial and paraglacial landforms and bedrock outcrops. By developing calibration curves at both study sites for the first time, through young and old control points of known age, it was possible to obtain landform age estimates from Schmidt hammer R-(rebound) values. Beside age estimates, the formation and stabilization of those landforms and the involved processes have provided indications about the Late Quaternary and Holocene climate variability and its connectivity to landform development. <br /> The first deglaciation chronology for the western study area could be constructed based on ¹⁰Be sur-face exposure ages. Final local deglaciation on the summit of Dalsnibba probably started between 13.3 ± 0.6 and 12.7 ± 0.5 ka and progressed down to the valley bottom of Opplendskedalen (~1050 m a.s.l.) with an SHD age estimate of 7.47 ± 0.73 ka. Deglaciation during the Bølling–Allerød Interstadial (~14.7 – 12.9 ka) indicates that the summit was not ice-covered during the Younger Dryas (12.9 – 11.7 cal. ka BP). A glacially transported boulder in the summit area and summit bedrock ages without cos-mogenic nuclide inheritance further imply a minimum vertical ice extent of 1476 m and the presence of erosive warm-based ice. The SHD Dalsnibba results show that most landforms stabilized during the Holocene Thermal Maximum (~8.0 – 5.0 ka) and that their R-value characteristics with negative skew-ness were indicative for the reworking of boulders or continuous debris supply. The SHD ages from Dalsnibba imply that periglacial landforms in the western maritime setting sensitively reacted to Holo-cene climate variability. Rock-slope failures investigated at both study sites demonstrate that they do not necessarily occur shortly after local deglaciation as often inferred. Furthermore, most of the recorded rock-slope failures appear to have occurred during warm climatic conditions. Most likely, prevailing warm conditions led to permafrost degradation, enhanced snow melt and increasing cleftwater pres-sure contributed to slope instabilities probably resulting in rock-slope failures. <br /> The Blåhø SHD ages also suggest landform response on climate variations, though, in a different tem-poral context. Landforms above 1450 m a.s.l. largely shared overlapping ages and therefore appear to have stabilized during the Karmøy/Bremanger readvance (∼18.5 – 16.5 ka). This, however, seems to have been the last major geomorphic activity of these landforms as they were not reactivated by several, partly severe cold climate events such as the Younger Dryas. The SHD ages from landforms above 1450 m a.s.l. are in contrast to the previous deglaciation chronology which suggested cold-based ice coverage and slow thinning down to ∼1450 m a.s.l. at 15.0 ± 1.0 ¹⁰Be ka. Based on the results from this thesis, a severe periglacial climate without ice coverage since about 18 ka is proposed for the sum-mit area of Blåhø. The 10Be ages from Blåhø with 20.9 ± 0.8 ka for the erratic boulder and 46.4 ± 1.7 ka for the bedrock, which extend the previous deglaciation chronology, are discussed within the framework of the two most popular scenarios. Within the first scenario the boulder age represented the timing of deglaciation and the bedrock age showed inherited cosmogenic nuclides, suggesting the pres-ence of low erosive cold-based ice at Blåhø during the LGM. In the second scenario, the boulder age was affected by post-depositional disturbance, frost heave processes or shielding, potentially indicating ice-free conditions on Blåhø since at least 46.4 ± 1.7 ka. Analyzing the different onset of deglaciation in the study areas within a rather short west-east distance, together with the timing of deglaciation in neighboring regions, demonstrates complex deglaciation dynamics in southern Norway. Not only the timing of deglaciation was highly variable, the results also imply diverse basal ice temperatures within this relatively small area. In general, this thesis positively contributes new evidences pointing to a more complex and dynamic Scandinavian Ice Sheet throughout the last glacial cycle than previously assumed.