Event stratigraphy for the Last Glacial-Holocene transition in eastern middle Sweden

Sammanfattning: This thesis outlines an event stratigraphy for the Last Glacial-Holocene transition in eastern middle Sweden. Events are short-lived occurrences that have left traces in the geological record and which, therefore, may be used for correlation and inter-regional comparisons. The event stratigraphy for eastern middle Sweden comprises ten events and covers the time-span from 12,900 to 10,200 calendar years BP. The local event stratigraphy has been linked to the GReenland Ice-core Project (GRIP) isotopic event stratigraphy by different correlations.Lateglacial and early Holocene sediment sequences from nine lakes in Västergötland, Östergötland and Närke, two bogs in Östergötland and two cores from the northwestern Baltic proper have been studied. The parameters analysed were: pollen, plant macrofossils, marine calcareous fossils, diatoms, microtephra, CaCO3 and organic carbon. Radiocarbon and clay varve datings were also used in order to detect and date regional events.It is concluded that a park tundra with tree birch existed in southern Östergötland during the end of the Allerød warm period (12,900-12,650 calendar years BP). The Allerød -Younger Dryas pollen zone boundary has been defined in an 800 year local varve chronology from southeastern Sweden. The pollen zone boundary coincides with an apparent drop in ages from 11,000 14C years BP to 10,600-10,800 14C years BP. The park tundra was replaced by a tundra vegetation of herbs and dwarf shrubs at the Allerød -Younger Dryas transition.The discovery of the mid Younger Dryas Vedde Ash at two sites shows that this tephra can be found more than 2300 km from the eruption centre of the Katla volcano on Iceland. This marker can be used to correlate Lateglacial terrestrial sequences in south Sweden with other archives, such as the Greenland ice-cores and marine cores from the north Atlantic with a high level of precision.It has been possible to correlate the mean varve thickness of the varved clays in the northwestern Baltic proper with the 18O record from the GRIP ice-core. The Younger Dryas-Preboral climatic shift, defined by a distinct pollen signal and a marked increase in varve thickness in the Baltic basin, is correlated with the rapid increase in 18O in the GRIP record at the Younger Dryas-Preboreal transition. This gives an age of c. 10,650 varve years BP corresponding to 11,525 GRIP years BP for this climatic transition. Furthermore, a climate deterioration, lasting from 10,440 to 10,320 varve years BP, corresponding to 11,315-11,195 GRIP years BP, correlated with the Preboreal oscillation, was recorded in the pollen spectra and varve data. This age discrepancy together with the age discrepancy of the Younger Dryas-Preboreal boundary suggests that as much as 875 years are missing from the Swedish Time Scale in the part younger than c. 10,300 clay varve years BP.The results of pollen analysis and measurements of organic carbon content from two lacustrine sites show that at least one, or possibly two, cooling events followed shortly after the general early Preboreal warming.Investigations in Närke and Västergötland indicate that at least one short saline ingression occurred into the Baltic Sea basin prior to the main brackish phase of the Yoldia Sea stage. After the deglaciation of the Närke strait a cold-water benthic marine fauna entered the Baltic basin through the Närke strait during a regional Betula-Empetrum-Hippophaë pollen zone, which in southern Sweden is correlated with the early Preboreal. Pollen analysis and radiocarbon dating suggest that clay varve deposition between 10,735 and 10,430 varve years BP occurred during the early Holocene at c. 11,500 and 11,300 calendar years BP.A previously unrecorded rhyolitic tephra horizon has been detected in early Holocene sediments at one site. The concentration of shards is very low and the geochemistry is not yet fully established. The age of this tephra horizon has been estimated to c. 9000 14C years BP corresponding to c. 10,200 calendar year BP.