February 20 – Licentiate Presentation

The deglaciation of southern central Sweden reflected in the seismic and sedimentary stratigraphy of southern Lake Vättern

by Henrik Swärd

Time and date: 13.00, 20th February
Place: Ahlmannsalen, Geohuset (link to the house plan)

The withdrawal of the Fennoscandian ice sheet resulted in an excess of meltwater and rapid isostatic uplift in southern Scandinavia which led to the formation and continuous growth of the proglacial Baltic Ice Lake. An increasing altitudinal difference between the North Atlantic and the Baltic Ice Lake was suddenly erased in the end of Younger Dryas when the Baltic Ice Lake finally drained into the sea and dropped 25 m within 1–2 years.  Lake Vättern in central southern Sweden is situated right on the edge of this dramatic event constituting the westernmost outpost of the Baltic Ice Lake before the final drainage. Furthermore, its alignment with an old graben structure makes it unusually deep resulting in a very potent sediment trap during the deglaciation.  Its exceptionally thick sediment archive (>300 m) holds imprints of paleoenvironmental changes after, during and possibly before the late Weichselian glaciation. Terrestrial ice recession data to the east and particularly to the west of Lake Vättern are abundant but corresponding data from Lake Vättern itself are rare.

Here we present the results from a 2012 drilling campaign in southern Lake Vättern where a composite sediment core down to 74 m below lake floor was recovered. The sediment core was analyzed to obtain physical properties, chemical properties of the sediments and chemical speciation of the pore waters. Three lithostratigraphic units were identified on the basis of these analyses and supporting seismic reflection data. These units correspond to the Late Pleistocene to Holocene evolution of Lake Vättern: A lowermost glacial clay unit (U3) accumulated during a pro-glacial Lake Vättern stage, a middle unit of post-glacial clay (U2) deposited during the Yoldia Sea stage and finally an uppermost gyttjaclay (U1) that has been deposited since Lake Vättern became isolated from the Yoldia Sea. The transition between U3 and U2 marks the widely recognized final drainage of the Baltic Ice Lake. At 54 meter below lake floor (U3) glaciotectonized sediments indicate ice grounding attributed to the ice readvance that formed the Levene ice marginal line. Furthermore, at 33 meter below lake floor (U3) porewater chlorinity data highlight a marine incursion into the Vättern basin that is likely connected to a previously speculated drainage of the Baltic Ice Lake near the Alleröd-Younger Dryas transition.      

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