In a new study published in The Holocene, Kristian Vasskog and colleagues from the Bjerknes Centre, the Department of Earth Science (UoB) and the universities in Orléans and Geneva, investigate how the frequency of snow-avalanches has changes during the last 7000 years. Their results show that the farmers living in Olden during the ‘little ice age’ (AD 1650-1920) probably experienced the highest avalanche activity seen in more than 7000 years.
Lake sediments
The study is based on analyses of lake sediments and seismic profiles from Lake Oldevatnet (Figure 1). When a ‘dirty’ snow-avalanche hits the lake, any entrained material is distributed over a large area by floating snow or ice (Figure 1 B), creating distinct layers of drop stones and terrestrial organic debris in the lake sediments (Figure 2). The age of the sediment may be determined by radiocarbon dating, allowing the scientists to build a chronology of past snow-avalanche activity spanning most of the Holocene.
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| Figure 1 (A) Oldevatnet seen from the North (Photo: Atle Nesje). The avalanche area at Beinnes is shaded white. Red dots mark the core locations. (B) Picture taken after a snow-avalanche at Beinnes (Photo: Fjordingen). (C) Seismic profile between the core locations (marked with a black line in (A)). Seismic reflectors associated with particularly marked avalanche layers are marked with blue (R1-R3 and Ra-Rb). |
Snow-avalanche and climate
From historical data it is fairly well-known which meteorological factors that influence the risk of snow-avalanches, such as extreme snow-fall events or periods with strong winds that accumulate snow in steep leeside slopes. It is not straightforward to relate such extreme weather events to climate changes occurring over scales of hundreds- to thousands of years. However, by compiling several datasets of Holocene snow-avalanche activity in western Norway, a common pattern becomes apparent: all datasets reflect a very low snow-avalanche activity during the warm period in the early-middle Holocene (around 6000 years before present) and an increasing activity towards the present. The scientists suggest that this could be due to climatic factors working over long time-scales, but they stress that this is a field that requires further study.
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| Figure 2 (A) Snow-avalanches recorded in historical documents from inner Nordfjord (from Grove 1972) compared to (B) results from the study in Oldevatnet that spans the last 7300 years. (C) The upper 50 cm of the core from Oldevatnet covers roughly the same time interval as the historical data in (A), i.e. the last 500 years. Avalanche layers can be distinguished both visually (as dark layers) and in geochemical parameters (white graph). |
Local factors
The scientists also underline that local factors may play a significant role in the observed avalanche activity. For example, most snow-avalanches in Olden are triggered along the margins of local glaciers. During the ‘little ice age’ these glaciers grew closer to the steep valley sides around Oldevatnet, something that may explain why the increase in snow-avalanche activity observed for Olden during this period was higher relative to other areas in western Norway. The scientists hope that continued research in the same region will enable them to separate local and regional factors and thereby increase our understanding of the relationship between long-term climate change and avalanche activity.
Reference:
Vasskog, K., Nesje, A., Støren, E. N., Waldmann, N., Chapron, E., and Ariztegui, D. A Holocene record of snow-avalanche and flood activity reconstructed from a lacustrine sedimentary sequence in Oldevatnet, western Norway. The Holocene, doi:10.1177/0959683610391316