Holocene lake-level changes in the Siljan Lake District – Towards validation of von Post’s drainage scenario

Sammanfattning: The Dalarna province in south-central Sweden shows a great variety of geological evidence from the Late Quaternary. A peculiar deglaciation pattern, along with unique hydrological conditions, has left a lot of space for speculations over the palaeoenvironmental conditions during the early Holocene. Lake Siljan is in the southern perimeter of the Siljan impact structure, the most prominent in Europe. At deglaciation, c. 10.6 kyr ago, the retreating ice margin was followed by the inundation of the Ancylus Lake, forming the highest shoreline at c. 205 m a.s.l. in the eastern part of the Siljan basin. However, due to fast glacio-isostatic rise and consequent shore regression, the Siljan basin got isolated from the Baltic and formed the ´ancient Lake Siljan,´ probably as early as c. 10 kyr ago. In 1934, the Swedish geologist Lennart von Post published a fascinating study, proposing a scenario of catastrophic drainage of ancient Lake Siljan, at which the outflow through the Åkerö channel moved northwards, forming the present outlet of River Österdalälven at the town of Leksand. This drainage event led to a lake-level drop of about 6 m. At the same time, also according to von Post (1934), a basin at Heden, now a peat bog located 2 km south of Leksand, that before the drainage was part of Lake Siljan, became isolated. The present study is an evaluation of von Post’s scenario by employing both traditional and modern geological techniques to sediment records from two coring locations, at Åkerö and Heden. The main findings suggest that during a stochastic high lake level stand, areas around the lake became submerged, and erosion processes were initiated that led to the opening of the new outlet at Leksand and the lake drained at some point close to 8.8 cal kyr BP. The aftermath of this drainage rerouting found the previous outlet channel at Åkerö abandoned, and the basin in Heden isolated. The drainage scenario, as suggested by von Post (1934), is thus confirmed but took place close to 2000 years earlier than was initially proposed, based on pollen zonation and before the development of accurate chronological dating techniques.