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

Abstract

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.

Abstract (Swedish)

Landskapet Dalarna i södra centrala Sverige uppvisar en stor variation av kvartärgeologiska sediment och landformer, bildade från deglaciationen i tidig Holocen och senare. Ett speciellt deglaciationsmönster, tillsammans med unika hydrologiska förhållanden, har lämnat utrymme för spekulationer över paleo-miljömässiga förhållandena under det tidiga Holocen. Siljanbassängen, belägen i det södra randområdet av Europas största impakt-stuktur började bli isfri för c. 10.6 tusen år sedan. Härvid följde Ancylussjön, ett av Baltiska sänkans havs/ sjöstadier, den tillbakasmältande isranden in i Siljanbassängen och utbildade i dennas sydöstra del en högsta kustlinje på c. 205 m.ö.h. Emellertid, på grund av glaci-isostatisk grundad, snabb landhöjning och därmed orsakad strandlinjeförskjutning, kom Siljanbasängen att tidigt bli isolerad från Ancylussjön, troligen redan för 10 tusen år sedan, varvid ´Siljanfornsjön´ bildades. 1934 publicerade den svenska geologen Lennart von Post ett fascinerande scenario som föreslog att Siljanfornsjön, med en högre vattennivå än nuvarande Siljan, erfor en katastrofal, snabb dränering, vid vilken det dåvarande utflödet genom Åkerökanalen förflyttade sig norrut och utbildade det nuvarande utloppet för Österdalälven vid Leksand. Denna dräneringshändelse ledde till att Siljan sänktes c. 6 meter. Samtidigt, också enligt von Post (1934), isolerades en bassäng vid Heden, idag en torvmosse belägen c. 2 km söder Leksand, men vid dräneringstillfället en vik av Siljanbassängen. Föreliggande studie är en utvärdering av von Post's dränerings-scenario, genomfört genom att använda traditionella kvartärgeologisk undersökningsmetoder, men framför allt genom att använda modern dateringsmetodik, på två sedimentkärnor uppborrade på samma platser som von Post´s lokaler vid Åkerö och Heden. De viktigaste resultaten indikerar att ett onormalt högvattenstånd i Siljan för c. 8.8 tusen år sedan gjorde att sjön fann ett nytt utloppsområde och att snabba erosionsprocesser skapade ett utlopp med en lägre pasströskel än för det tidigare Åkerö-utloppet. Följden blev att den tidigare Åkerökanalen torrlades och att sjöbassängen vid Heden isolerades från Siljan. Det dränerings-scenario som föreslogs av von Post (1934) har i föreliggande studie mer eller mindre bekräftats, men dräneringshändelsen ägde rum bortåt 2000 år tidigare än vad som föreslogs av von Post baserat på pollenzonering och en kronologi pre-14C.

Popular Abstract

The early-Holocene is a period of revelations and unrest in previously glaciated areas of the planet. Glacial retreat exposed back to light redesigned topographies and initiated a new era characterized by meltwater transformations. The early Holocene climate, postglacial rebound on land after the removal of heavy load of ice, and local and eustatic changes in sea-levels acted in concert to produce a profoundly altered landscape where life was making a fresh start.

The Siljan Lake District is formed in the Siljan impact structure, the eroded remnant of the most prominent impact crater in Europe, in the Dalarna province of south-central Sweden. After nonuniform deglaciation at around 10.6 kyr ago, the Baltic stage of the transgressive Ancylus Lake inundated the area. The highest shoreline was formed at 205 m a.s.l. but relict beach ridges can be found up to 220 m a.s.l. due to deferential isostatic uplift. Lake Siljan got isolated from the regressive Ancylus Lake because of the post-glacial isostatic rebound. The threshold of isolation in the southernmost side of the Lake District was 168.5 m a.s.l. and was reached at approximately 10 kyr BP. Nowadays, Lake Siljan is around 162 m a.s.l. but intense precipitation and snowmelt can lead to significantly higher lake levels.

In 1934, the Swedish geologist Lennart von Post postulated the scenario that Lake Siljan drained catastrophically and dropped to the present lake-level after the opening of a new outlet channel in the mid-Holocene. The old drainage channel at Åkerö (Fig. 1A) got abandoned while areas above 162.5 m, like Heden (Fig. 1B), were cut-off from the lake and gradually dried-up.

The drainage scenario was tested from two sites, Åkerö and Heden (Fig. 1). After the application of traditional and modern geological surveying techniques, the findings strengthen the view that an abrupt event caused the abandonment of the Åkerö channel and led to the separation of Heden from Lake Siljan. The results extend previous findings concerning the timing of the event and are in line with the notion that a higher water level stand preceded the catastrophic drainage.

• author: Paradeisis-Stathis, Savvas
• supervisor: Per Möller; Dan Hammarlund; Karl Ljung
• organization: Department of Geology
• course: GEOR02 20201
• year: 2020
• type: H2 - Master's Degree (Two Years)
• subject: Earth and Environmental Sciences
• keywords: Lake Siljan, Dalarna, basin isolation, catastrophic drainage, early Holocene, lake sediments, XRF scanning, C/N ratio, magnetic susceptibility, pollen, Ancylus Lake
• publication/series: Dissertations in Geology at Lund University
• report number: 600
• language: English
• id: 9022892
• date added to LUP: 2020-06-26 15:53:49
• date last changed: 2020-06-26 16:25:34