Late Weichselian and Holocene Shore Displacement History of the Baltic Sea in Finland

Late Weichselian and Holocene shore displacement history of the Baltic Sea in Finland MATTI TIKKANEN AND JUHA OKSANEN Tikkanen, Matti & Juha Oksanen (2002). Late Weichselian and Holocene shore displacement history of the Baltic Sea in Finland. Fennia 180: 1–2, pp. 9–20. Helsinki. ISSN 0015-0010. About 62 percent of Finland’s current surface area has been covered by the waters of the Baltic basin at some stage. The highest shorelines are located at a present altitude of about 220 metres above sea level in the north and 100 metres above sea level in the south-east. The nature of the Baltic Sea has alter- nated in the course of its four main postglacial stages between a freshwater lake and a brackish water basin connected to the outside ocean by narrow straits. This article provides a general overview of the principal stages in the history of the Baltic Sea and examines the regional influence of the associated shore displacement phenomena within Finland. The maps depicting the various stages have been generated digitally by GIS techniques. Following deglaciation, the freshwater Baltic Ice Lake (12,600–10,300 BP) built up against the ice margin to reach a level 25 metres above that of the ocean, with an outflow through the straits of Öresund. At this stage the only substantial land areas in Finland were in the east and south-east. Around 10,300 BP this ice lake discharged through a number of channels that opened up in central Sweden until it reached the ocean level, marking the beginning of the mildly saline Yoldia Sea stage (10,300–9500 BP). As the connecting channels rose above sea level, however, the Baltic Sea became confined once more, to form the Ancylus Lake (9500–8000 BP). During its existence the outflow channel to the ocean shifted to the Straits of Denmark and the major lake systems of central Finland became isolated from the Baltic basin. After the brief Mastogloia transition phase, a greater influx of saline water began to take place through the Straits of Denmark, marking the Litorina Sea stage (7500– 4000 BP), to be followed by a somewhat less saline stage known as the Lim- nea Sea. After a transgressive period early in the Litorina Sea stage, shoreline displacement in Finland has proceeded at a steadily declining rate. Matti Tikkanen, Department of Geography, P. O. Box 64, FIN-00014 Universi- ty of Helsinki, Finland. E-mail: [email protected] Juha Oksanen, Finnish Geodetic Institute, Department of Geoinformatics & Cartography, P. O. Box 15, FIN-02431 Masala, Finland. E-mail: juha.oksanen@ fgi.fi Introduction period of several thousand years and has corre- spondingly increased the land area of Finland. The The Baltic Sea has a long and chequered history, present area of the Baltic Sea is about 377,000 in the course of which the area nowadays occu- square kilometres, and it has a freshwater influx pied by Finland has undergone substantial chang- of some 660 cubic kilometres a year from a drain- es. The phenomenon known as land uplift, which age basin of 1.6 million square kilometres. In ad- still operates in the Baltic region and currently dition, some 475 square kilometres of saline wa- amounts to some eight millimetres per year on the ter a year flows into the Baltic through the nar- Finnish coast of the northernmost Gulf of Both- row Straits of Denmark. The total outflow of brack- nia (Kakkuri 1990; Mäkinen & Saaranen 1998), ish water is of the order of 950 cubic kilometres a has had the effect of reducing the sea area over a year (Björck 1995: 19). 10Matti Tikkanen and Juha Oksanen FENNIA 180: 1–2 (2002) Fig. 1. The Baltic Sea and the surrounding region. The Baltic Sea is the largest brackish water basin waters (Björck 1995: 20). The salinity has also var- in the world, although it is fairly shallow – its deep- ied greatly in the course of the history of the basin. est point is only 459 metres. Thresholds divide the Finland and the whole Baltic basin have been basin into a number of separate sections (Fig. 1). buried beneath the continental ice sheet on sev- The sea’s salinity varies from 0.1 percent in the north eral occasions during the Quaternary period to 0.6–0.8 percent in the central parts, and can (Taipale & Saarnisto 1991: 212). These glaciations reach as much as 1.5–2.0 percent in the deepest have been separated by more favourable climatic FENNIA 180: 1–2 (2002)Late Weichselian and Holocene shore displacement... 11 periods, during which the Baltic basin has been their influence on the pattern of shore displace- occupied by water and land uplift has caused pro- ment in Finland. The technique employed for con- gressively greater areas of dry land to emerge on structing the palaeogeographical maps of Finland its coasts, in the same manner as during this last is based on a digital elevation model (DEM) for postglacial period. During the Eemian interglacial, the entire country. The model can be intersected about 130,000–115,000 BP, the basin contained by different surfaces in accordance with the wa- what is referred to as the Eemian Sea. Its waters ter level that prevailed in the Baltic at each stage. were evidently much more saline than those of The contour-based DEM was constructed by the the Baltic Sea nowadays, as deduced from dia- National Land Survey, and it has horizontal reso- toms recovered from sediments dated to that pe- lution of 200 metres. riod. It is possible that there was a connection from the Baltic basin to the Arctic Ocean across Karelia at the beginning of this Eemian Sea stage, The Baltic Ice Lake although it was closed later due to the effects of (12,600–10,300 BP) land uplift (Björck & Svensson 1994). The ice sheet associated with the Weichselian The ice margin began to retreat from the south- glaciation, which followed the Eemian intergla- ernmost parts of the Baltic basin around 13,500– cial, reached its maximum extent about 18,000– 13,000 BP, leading to the creation of the first post- 20,000 BP. It was more than three kilometres thick glacial water bodies in the region (Björck 1995: in the area of Finland, so that it depressed the 21). Deglaciation then proceeded very rapidly, so earth’s crust to such a degree that a considerable that the margin was situated close to the south part of the country’s present area lay beneath the coast of Finland by about 12,000 BP (Niemelä waters of the Baltic basin immediately after de- 1971; Saarnisto & Salonen 1995: 10). By that glaciation. As the burden of the overlying ice was stage the large volumes of meltwater from the ice released, the crust began to rise rapidly, howev- sheet were able to discharge from the Baltic Ice er, so that it is estimated that the total rise up to Lake into the ocean via the Strait of Öresund the present time has been 600–700 metres on the (Agrell 1976; Björck 1979; Björck 1995) (Fig. 2A). northern coast of the Gulf of Bothnia, 400–500 Before the ice margin had reached the point metres in the middle part of Finland and in cen- now marked by the First Salpausselkä moraines, tral Lapland, and around 300 metres on the coast however, the bedrock threshold in the Strait of of the Gulf of Finland and in northern Lapland Öresund had risen above the ocean level and the (Mörner 1980). The majority of this rise neverthe- waters of the Baltic basin had begun to be less took place as the ice was melting, before the dammed up to form a freshwater ice lake. Then, ground surface became exposed. around 11,200 BP, a (possibly subglacial) connec- Four main stages can be recognized in the his- tion to the west opened up north of Mount Bil- tory of the Baltic basin since the last glaciation, lingen in central Sweden. This remained function- influenced by a complex interaction between de- al for some 400 years, and allowed the water level glaciation, regional differences in land uplift, and in the Baltic to return to that of the outside ocean, eustatic changes in sea level (Tikkanen et al. presumably implying a decline of 5–10 metres 1999). The connection with the outside ocean has (Björck 1995). This has been referred to as the g- been located variably in the Straits of Denmark stage, derived from a now obsolete system of la- or central Sweden. There is a long tradition of re- belling shore levels in the Baltic with letters from search into the history of the Baltic basin and a to p (Eronen 1990). shore displacement in Finland and the other The climate became cooler once more during countries of the Baltic region (see Eronen 1983; the Younger Dryas period, causing the ice margin Björck 1995; Heinsalu et al. 2000), and the re- to advance. With the closure of the connection gional changes in shore displacement during the with the ocean around 10,800 BP, the next 600 various stages can nowadays be traced fairly ac- years proved to be a transgressive period on the curately by combining the vast amount of data shores of the southern parts of the Baltic Ice Lake, available with altitude data using GIS techniques its water level gradually rising to about 25 metres (Tikkanen & Oksanen 1999). above that of the ocean and the outflow channel This article will provide a brief overview of the shifting to the Strait of Öresund once again (Björck main stages in the history of the Baltic Sea and 1995).

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