International Geologiical Congress - Oslo 2008

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CGC-09 Glacial-interglacial vegetation dynamics

 

Weichselian vegetation dynamics in the central area of the Scandinavian glaciations recorded in a long sediment sequence from Sokli (N-Finland)

 

Karin Helmens, Stockholm University (Sweden)
Heikki Seppa, University of Helsinki (Finland)
Hanneke Bos, Vrije Universiteit Amsterdam (Netherlands)
Stefan Engels, Vrije Universiteit Amsterdam (Netherlands)
Minna Valiranta, University of Helsinki (Finland)
John Birks, University of Bergen (Norway)
 

 

The long and nearly continuous Late Quaternary sediment sequence recovered at Sokli in the northern boreal forest of Finnish Lapland provides a unique record of environmental and climatic changes for a region that is situated in the central area of the Scandinavian glaciations. An independent chronology based on optically stimulated luminescence and radiocarbon dating shows that Sokli was glaciated, and till beds were laid down in the small sedimentary basin, during Marine Isotope Stages (MIS) 5b, 4 and late 3/2.

Lacustrine conditions prevailed during the ice-free intervals dated to MIS 5e (Eemian Interglacial), late MIS 5d-5c (∼ 110-93 ka), MIS 5a (around 80 ka) and early MIS 3 (around 50 ka). Following a low-resolution pollen analysis of the entire series of lacustrine intercalations in the Sokli sequence, the lacustrine sediments of Weichselian age are presently subjected to a high-resolution, multi-proxy analysis. Pollen and macro-fossil evidence from the early MIS 3 deposit shows shrub tundra vegetation remarkably similar to the modern tundra in northern Fennoscandia. The reconstructed low-arctic tundra with a proximity of the range limits of Betula and Pinus contrasts with the inferred vegetation during MIS 3 in central Europe of a non-forested biome, mostly a dwarf-shrub tundra, steppe tundra/warm steppe or grassland.

There is no evidence for the local presence of coniferous trees despite the fact that the aquatic biota in the lacustrine sediments (chironomids, diatoms, aquatic plants, bryozoa) consistently suggests present-day mean July temperatures. Evidence from diatom analysis and digital elevation data indicate that the ice margin most probably remained close to the Sokli site during the early MIS 3 deglaciation event. A gradual development from open vegetation with grasses, Artemisia and Caryophyllaceae towards a pine-birch forest and back to tundra is reconstructed in the Sokli pollen and macro-fossil record for MIS 5d-c. The inferred vegetation, which developed in a floodplain environment that persisted in the Sokli Basin throughout MIS 5d-c, is compared with quantitative climate inferences based on chironomids that are presently in progress.

 

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