Continuous and episodic sedimentation in western Norwegian fjord lakes. A Holocene climatic perspective
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A Holocene chronology of hazardous events has been constructed in parallel with a new record of glacier variability from inner Nordfjord in western Norway, based on the analysis of seismic profiles and sediment cores from the lakes Oldevatnet (8 km2), Nerfloen (1 km2) and Oppstrynsvatnet (30 km2). In paper I the frequency of episodic sedimentation events over the last 7300 years was investigated in a sediment core retrieved from Oldevatnet in inner Nordfjord, western Norway. Our data suggest that the event record is dominated by snow-avalanches, whereas inferred floods and density currents are too infrequent to be of any palaeoclimatic significance. Altogether forty-seven snow-avalanche events are recorded over the investigated interval. Periods of enhanced snow-avalanche activity are recorded at 5500– 5400, 5000–4900, 1200–1100 cal. yr BP and during the ‘Little Ice Age’ (LIA) glacier maximum (c. 400–100 cal. yr BP). Periods without any large snow-avalanches entering the lake are seen between 7100-5700, 4200-3700, 3200-2800, and 1400-1300 cal. yr BP. A compilation of snow-avalanche records from western Norway reflect an increasing trend through the Holocene, similar to what is seen in records of other types of extreme events such as floods. It seems likely that regional changes in winter precipitation are crucial for the fluctuations observed in snow-avalanche activity, although local effects may give rise to site-specific responses. Around Oldevatnet, the glacier expansion during the LIA probably served to increase the local snow-avalanche activity as the glacier fronts expanded into the steep slopes surrounding the lake. In paper II a Principal Component Analysis (PCA) was applied to a suite of different sedimentary parameters with the purpose of reconstructing past glacier activity in the 440 km2 upstream catchment of lake Nerfloen in Stryn, western Norway. The PCA reveals a strong signal contained in the sediment record, and is able to express 76% of the total variability of the fifteen investigated parameters through the first principal component. Changes in grain-size distribution is seen as the main driver of the sedimentary signal, and a comparison with known glacier fluctuations in the area indicate that it is closely connected with glacier extent in the upstream catchment. This interpretation was supported by measurements of bulk magnetic susceptibility (χBulk) of glacial and non-glacial sediment samples from the catchment, which for relevant grain-size fractions could be directly correlated with χBulk values in the core. The ratio between χBulk measured at temperatures of 77K and 293K show a high sensitivity to changes between sedimentation regimes dominated by glacial and non-glacial sedimentation in the lake. By combining the 77K/293K-ratio with the PCA, periods of significant glacier retreat can be robustly determined. In the ~8000 year long record, only minor glacial input is indicated between 6700-5700 cal. yr BP, probably reflecting a situation when most glaciers in the catchment had melted away, whereas the highest glacier activity is observed around 600 and 200 cal. yr BP. During the local Neoglacial interval (after ~4200 cal. yr BP) five individual periods of significant glacier retreat are identified at ~3400, 3000-2700, 2100-2000, 1700-1500 and ~900 cal. yr BP. Paper III deals with the Storegga tsunami, which has evidently caused the most significant episodic sedimentation event recorded in Nerfloen and adjacent Oppstrynsvatnet (29 m a.s.l.). Triggered by the ~3500 km3 Storegga Slide that occurred off the coast of Norway at around 8100 cal. yr BP, this catastrophic tsunami inundated coastal areas around the North Atlantic, and numerical simulations have suggested that the tsunami wave propagated into the fjords of western Norway with a considerable amplification towards the fjord heads. This is, however, the first time that actual geologic evidence of the tsunami has been discovered at the head of a major fjord in western Norway, and the first finding of Storegga tsunami deposits within the Nordfjord system. The impact of the tsunami can be seen as a distinct erosive boundary in Nerfloen and the shallow parts of Oppstrynsvatnet, whereas in the main Oppstrynsvatnet basin an up to 2.5 m thick ‘rapidly deposited layer’ (RDL) has been deposited across an area of 5 km2. In the sediment core from Nerfloen, the event is reflected by a 15 cm thick turbidite deposit overlain by a 5 cm thick unit consisting of terrestrial debris. Although the exact run-up of the Storegga tsunami in Stryn cannot be determined from our data, the magnitude of the observed erosion and deposition related to the event suggest that the impact involved massive amounts of energy. By verifying to some extent the numerical simulations that has been performed of the Storegga tsunami, these results underline how vulnerable fjord areas are to tsunamis generated in the open ocean.
Has partsPaper I: Vasskog, K., Nesje, A., Støren, E. N., Waldmann, N., Chapron, E., and Ariztegui, D., 2011, A Holocene record of snow-avalanche and flood activity reconstructed from a lacustrine sedimentary sequence in Oldevatnet, western Norway: The Holocene 21(4): 597-614, June 2011. Full text not available in BORA due to publisher restrictions. The article is available at: http://dx.doi.org/10.1177/0959683610391316
Paper II: Vasskog, K., Paasche, Ø., Nesje, A., Boyle, J. F. and Birks, H.J.B., Building continuous glacier reconstructions based on the integrated sedimentary response of large catchments. Full text not available in BORA.
Paper III: Vasskog, K., Waldmann, N., Nesje, A., Chapron, E. and Ariztegui, D., New insight into the 8100 cal yr BP catastrophic Storegga tsunami event from western Norway. Full text not available in BORA.