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Kristian Vasskog, University of Bergen, Department of Earth Science (Norway)
Eivind Nagel Støren, University of Bergen, Department of Earth Science (Norway)
Atle Nesje, University of Bergen, Department of Earth Science (Norway)
Daniel Ariztegui, University of Geneva (Switzerland)
Emmanuel Chapron, CNRS/Université d'Orléans (France)
Nicolas Waldmann, University of Geneva (Switzerland)
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January 29, 2008 a snow-avalanche hit the main road through the Oldedalen valley, leaving the inhabitants isolated for many hours. This is not a new phenomenon in this area, however; geohazards such as snow-avalanches, floods and debris flows have been reported since the beginning of recorded history. Particularly during the 'Little Ice Age' numerous events were documented in tax records when farmers applied for reduced taxes following destruction of farms and farmland. Was the 'Little Ice Age' a unique period in terms of such hazardous events in this area? And what can we expect in a warmer, more humid future climate? To investigate this, we used lake sediments from Oldevatnet to go beyond historical records and piece together a history of floods and colluvial events for the last 7300 years. Oldevatnet is a large (ca. 8 km2) lake running the length of the Oldedalen valley, with approximately 35 major snow-avalanche and debris flow trajectories surrounding it, leading down towards the lake itself. Minerogenic debris and organic material from major events in these areas will be deposited on the lake floor, creating layers that are more-or-less easily distinguished from the background sedimentation. An important goal of this project is to distinguish what kind of processes that have deposited the different layers, using methods like grain size analysis and other sedimentological parameters. This is important because we want to know what kind of geohazards that have been dominating during times of different climate regimes, and how their frequency have changed in relation to the prevailing climate. Indirectly this will also indicate changes in the frequency of local extreme weather events, as the most important triggering factor of avalanches and debris flows in western Norway are short (∼1 day) periods of extreme precipitation. Here we present data from three sediment cores that have been retrieved from different parts of Oldevatnet. The first core, named OLP-105, was collected from the middle of the lake, as far away as possible from any avalanche areas. Being the most undisturbed of the cores, it has still recorded nearly 60 events in the form of brown, organic layers rich in terrestrial plant macrofossils. There are, however, no coarse grained minerogenic layers in core OLP-105, leading us to postulate that no erosion has taken place during the recorded events. This makes the core well-suited for reconstruction of climate/glacier variability, using standard proxy methods like loss-on-ignition, magnetic susceptibility and dry bulk density.
The other two cores, OLP-305 and OLP-307, have been retrieved closer to colluvial fans, and hence display a larger number of events. Based on data from all three cores, changes in the frequency and type of events have been compared to overall climate changes for the last 7300 years. The data has also been compared with results from a similar study in Jotunheimen, an area featuring a more continental climate.
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