GFF volume 129 (2007), pp. 277–285. Article New insights on the Yoldia Sea low stand in the Blekinge archipelago, southern Baltic Sea THOMAS ANDRÉN 1 , ELINOR ANDRÉN 2 , BJÖRN E. BERGLUND 3 and SHI-YONG YU 4 Andrén, T., Andrén, E., Berglund, B.E. & Yu, S.-Y., 2007: New insights on the Yoldia Sea low stand in the Blekinge archipelago, southern Baltic Sea. GFF, Vol. 129 (Pt. 4, December), pp. 277–285. Stockholm. ISSN 1103-5897. Abstract: One sediment core from the Järnavik bay in Blekinge archipelago has been investigated for its content of pollen and diatoms and its chemical properties. Two levels were also dated by radiocarbon. Based on the results the sediment sequence analysed has been divided into three environmental units largely corresponding to the lithology of the sequence. A lowermost unit consisting of weakly varved and homogeneous clay was deposited during the end of the brackish phase of the Yoldia Sea at a moderate water depth. On top of this unit a gyttja-clay unit was deposited. The onset of the deposition of this unit has been dated to c. 11 100 cal. yrs. BP. An increasing organic production and increased terrestrial influ- ence is recorded in the chemical data and a very shallow water depth is indicated in the pollen and diatom flora. These results point to conditions in a bay probably isolated from the Yoldia Sea. A local tentative shore displacement curve have been constructed and it is proposed that this unit represents the low stand at c. –18 m during the Yoldia Sea stage in this part of the Baltic Sea basin. The uppermost unit consists of homogeneous clay with a low content of organic carbon. An increasing water depth is indicated by the composition of both pollen and diatoms. The diatom flora also displays an increase in freshwater species. This environmental change was probably the result of a transgression in the beginning of the Ancylus Lake stage. Keywords: Baltic Sea, Blekinge archipelago, pollen, diatoms, submarine pine stumps, Yoldia Sea, Yol- dia regression, Ancylus Lake, Ancylus transgression, local Blekinge shore displacement curve. 1 Department of Geology and Geochemistry, Stockholm University, Sweden; thomas.andren@geo.su.se 2 School of Life Sciences, Södertörn University College, Sweden; elinor.andren@sh.se 3 Department of Quaternary Geology, Lund University, Sweden; bjorn.berglund@geol.lu.se 4 Large Lakes Observatory, University of Minnesota Duluth, USA; syu@umn.edu Revised manuscript received 20 September 2007. Revised manuscript accepted 5 November 2007. Introduction The Yoldia Sea stage of the Baltic Sea was named after the arctic bivalve Yoldia arctica (nowadays Portlandia arctica) and is traditionally defined as starting when the Baltic Ice Lake was drained and lowered 25 m down to the level of the North Atlantic (Munthe 1910). This drainage event has been dated to 11 560±10 cal. yrs. BP in the Swedish clay-varve chronology (Andrén et al. 2002) and Saarnisto & Saarinen (2001) concluded that it more or less coincided with the ice margins retreat from the Salpausselkä II end moraines. They dated this to 11 590±100 cal. yrs. BP by varve chronological and palaeomagnetic methods. Similar ages for the drainage of the Baltic Ice Lake have also been presented by Brunnberg (1995) and Strömberg (1994) based on clay-varve chronological investigations from the Södertörn and Västergöt- land areas respectively. There are also several investigations indicating radiocarbon ages between 10 300 and 10 000 years BP corresponding approximately to 12 200 and 11 200 cal. yrs. BP (e.g. Svensson 1989; Bodén et al. 1997). As the drainage occurred during the so-called 10 000-year radiocarbon plateau these datings are uncertain but they seem to indicate similar age as achieved by other methods. It is thus reasonable to believe that the final drainage of the Baltic Ice Lake, and hence the onset of the Yoldia Sea stage, took place between 11 600 and 11 550 cal. yrs. BP. The following c. 250 years was characterised by freshwater conditions in the Baltic Sea basin probably best explained by two facts. Firstly, due to the Preboreal warming, huge amounts of melt water was produced by the ice sheet, and secondly, the narrowness of the straits north of Mt. Billingen that the outflow- ing melt water had to pass (Strömberg 1992) probably hindered the marine water to enter the Baltic Sea basin. Brackish water conditions started at c. 11 300 cal. yrs. BP in the western part of the central Baltic Sea where brackish wa- ter ostracodes and foraminifera are recorded (Wastegård et al. 1995). Björck et al. (2001) report a similar age for the first ma- rine ingression from Västergötland and Närke area in southern Sweden recorded as findings of fossil ostracodes and forminifera together with shells of Portlandia arctica in symmict varved glacial clay of Yoldia Sea age. Some 100 years later brackish-