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Land uplift-driven shift of the outlet of Lake Ähtärinjärvi, western Finland

Author(s): Heikki Seppä | Matti Tikkanen

Journal: Bulletin of the Geological Society of Finland
ISSN 0367-5211

Volume: 78;
Issue: 1-2;
Start page: 5;
Date: 2006;
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Keywords: paleolimnology | lakes | uplifts | lake-level changes | transgression | outlets | paleochannels | Holocene | Lake Ähtärinjärvi | Finland

Lake Ähtärinjärvi, at the northern end of the River Kokemäenjoki watercourse in northern central Finland, has been known to have an older outlet channel across the modern Suomenselkä watershed to the Bothnian Bay. The ancient connection over the watershed is indicatedby the existence of a conspicuously deep and wide palaeochannel that runs from the northern end of Lake Ähtärinjärvi to the headwaters of the River Ähtävänjoki. Radiocarbon datings of the basal sediments of the channel show that the flow in the old outlet channel ceased at about 1500 cal yr BP; the old channel therefore functioned as an outlet since the isolation of the lake at about 9900 cal yr BP to 1500 cal yr BP. Sedimentological studies of Lake Ouluvesi, located below the modern outlet of the lake system, suggest opening of the modern outlet at 3200 cal yr BP, indicating that the lake system had a 1700-year period of bifurcation before the final drying up of the old outlet channel. The reconstruction of the tilting of the lake level supports this pattern, and demonstrates further that during the early-Holocene the lake consisted of three basins located at different levels until the transgression caused by the tilting gradually brought the basins to the same level.
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