Darss Sill as a biological border in the fossil record of the Baltic Sea: evidence from diatoms

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Abstract

Biostratigraphical and palaeoecological analyses of cores along a transect from Femer Belt to the Arkona Basin reveal that North Sea waters began to enter the western Baltic Sea between 8600 and 8400 calibrated years BP. Studies of diatoms indicate that Mecklenburg Bay was characterised by slightly brackish-water conditions between 8400 and 8000 cal. years BP. At around 8000 cal. years BP increasing salinity is indicated by a strong dominance of the diatoms Paralia sulcata and Dimeregramma minor. Some centuries later another diatom assemblage appeared and became dominant in Mecklenburg Bay. This assemblage includes Hyalinella lateripunctata and Pravifusus hyalinus species typical of shallow water areas along the Atlantic coast today. At this time the first marine molluscs made their appearance. The oldest shell of a marine mollusc found in our material is dated to 7600 cal. years BP. The associated assemblage that includes adult specimens of the gastropod Aporrhais pespelicani indicates higher salinities than today.

During the Littorina Sea stage a marine diatom flora with P. sulcata, Catenula adhaerens and D. minor crossed the Darss Sill and became widely distributed in the Arkona Basin, Pomeranian Bay and the Baltic Sea proper. In contrast, taxa indicative of the Hyalinella lateripunctata/P. hyalinus assemblage are only found west of the Darss Sill in Femer Belt and Mecklenburg Bay. Apparently, the Darss Sill threshold has been acting as an important salinity border from around 7800 cal. years BP until today.

Introduction

The Baltic Sea Basin during the Late Glacial and Early Holocene was subject to transformations, which were related to isostatic rebound and eustatic sea level rise (Svensson, 1991; Björck, 1995; Emeis et al., 2002). The early stage of the Baltic Sea history, the Baltic Ice Lake, began after the recession of the Fennoscandian Ice Sheet at ca 18 000 cal. years BP. The Baltic Ice Lake existed until ca 11 500 cal. years BP. After that time, the Baltic Sea Basin was connected with the North Sea through south-central Sweden. As a result the weakly saline Yoldia Sea originated. It terminated at ca 10 700 cal. years BP when the isostatic rebound of Scandinavia interrupted the connection to the North Sea. The following freshwater basin is called the Ancylus Lake. Finally, further eustatic sea level rise resulted in a new transgression in the southern Baltic.

The initial saline water inflows to the Baltic Sea Basin in the Early Holocene are assigned to the Mastogloia stage or the Pre-Littorina Sea stage. The pathway of the Holocene transgression has been debated, but the Great Belt is considered the most important (e.g. Krog (1965), Krog (1973); Winn, 1974; Winterhalter et al., 1981; Eronen, 1988; Emeis et al., 2002). Some doubts were raised recently from studies of diatom floras in sediment cores from the Baltic Sea proper. Saline water diatom floras are recorded in sediment dated to between 8900 and 10 200 cal. years BP from the Gotland Deep (e.g. Sohlenius et al (1996), Sohlenius et al (2001); Kowalczyk et al., 1999; Andrén et al., 2000a), the Gdańsk Basin (Witkowski, 1994) and the Bornholm Basin (Zachowicz, 1995; Witkowski and Miller, 1999; Andrén et al., 2000b). Recently, Berglund et al. (2001) dated the beginning of the brackish-water phase in Blekinge (southern Sweden) to the time span between ca 10 200 and 9200 cal. years BP. According to these authors, the transitional stage, the Mastogloia stage lasted until ca 8300 cal. years BP.

The marine transgression in Mecklenburg Bay and Kiel Bight has been subject to several studies (e.g. Winn and Averdieck, 1984; Kępińska et al., 1988b; Eronen et al., 1990; Novak, 2002). Eronen et al. (1990) dated the first indicators of rising salinity, as determined from diatom floras, in the Mecklenburg Bay to between 9500 and 8900 cal. years BP. Winn and Averdieck (1984) dated the first marine inflows in the Kiel Bight to the early part of the Atlantic chronozone. However, results from the present study based on diatom analyses of dated cores indicate that the first brackish-water inflows occurred between ca 8900 and 8400 cal. years BP.

The Darss Sill plays an important role in the development of the hydrographical conditions in the present Baltic Sea. The region plays a crucial role in the water exchange between the Baltic Sea proper, the Kattegat and thus the North Sea (Lemke et al., 1994; Matthäus, 1996). The present salinity gradient exists between waters with salinities exceeding 20‰ (Kattegat) and those below 10‰ in the Baltic Proper.

Snoeijs (1999b) published the most comprehensive data on the salinity gradient impact on the diversity of macroalgae in the Baltic Sea. As indicated by Snoeijs (1999b) the number of macroalgae decreases from 223 in Kattegat to ca 170 taxa in the Öresund and Belt Seas. In the western Baltic Sea (ca 8–12 PSU) only ca 100 species of macroalgae occur, while in the Baltic Proper (ca 6–7 PSU) their number is reduced to 70 (cf. also Nielsen et al., 1995). Kell (1973) studied the distribution of phytoplankton including diatoms. He has shown that the Darss Sill is an important biological limit in the distribution of polyhalobous phytoplanktonic organisms. Snoeijs (1993), Snoeijs (1999a), Snoeijs and Vilbaste (1994), Snoeijs and Potapova (1995), Snoeijs and Kasperoviviene (1996) and Snoeijs and Balashova (1998) documented changes in the diatom flora along a transect from the Kattegat to the Bothnian Bay.

The development of the salinity barrier based on fossil diatom flora has not previously been studied in the Baltic Sea Basin. In the present paper, we show that diatoms may be a useful tool in reconstructing the development and the position of the salinity barrier. Reconstructions of environmental changes applying diatom floras at oceanic scale (e.g. Sancetta, 1982; Koç Karpuz and Schrader, 1990; Koç Karpuz and Jansen, 1992; Gersonde and Zielinski, 1997) or in smaller water bodies like the Baltic Sea are based on planktonic diatoms (e.g. Abelmann, 1985; Witkowski, 1994; Westman and Sohlenius, 1999; Andrén et al., 2000b; Sohlenius et al., 2001). The shallow water deposits studied here are rich in benthic diatoms.

Section snippets

Material and methods

The material comes from thirty 10 cm in diameter sediment cores that are up to 6 m long. The cores were subsampled at 5–20 cm intervals depending on the lithology. A total of ca 700 samples were analysed. Subsamples of 1–2 g of the sediment were used to prepare microscopic slides. A part of the samples was treated with 10% HCl to remove calcium carbonate, washed several times with distilled water, and boiled in concentrated H2SO4 with small amounts of KNO3 added at ca 15 min intervals in order to

Results

Although, the results of the former publications (Jensen et al (1997), Jensen et al (1999); Lemke et al (1999), Lemke et al (2001)) dealt with the diatom analyses of the Late Glacial and Early Holocene freshwater stages, no results on the saline water diatoms from this material were so far published. A total of ca 320 taxa were identified in the material. The diatom frustules show a fairly variable state of preservation. In general, the best preserved frustules are found in lacustrine and

Discussion

Diatom analyses of sediment cores sampled along a transect from the Femer Belt to the Arkona Basin have proven to be a very useful tool in reconstructing the geological development of the western Baltic Sea. The following scenario of events is proposed for the Early to Mid-Holocene in the study area as a response to eustatic sea level rise. Studies of the diatom flora and the distribution of the diatom ecological groups enable us to distinguish between three phases of environmental changes from

Concluding remarks

Diatoms are sensitive indicators of salinity changes in the south-western Baltic Sea, which is a transitional area between the North Sea and the Baltic Sea. During the course of the Holocene, the south-western Baltic Sea was subject to environmental changes from shallow water lacustrine through littoral/lagoonal brackish-water to fully marine. The marine transgression into the Baltic Sea took place in several pulses characterised by increasing salinity.

In the sedimentary sequence a distinct

Acknowledgements

Part of the material presented in this paper was analysed within a framework of the Alexander von Humboldt Foundation fellowship. The authors are grateful to the Alexander von Humboldt Foundation for granting a scholarship to A.W. The contribution of B.K.-J. was supported by the State Committee on Scientific Research (KBN) in Warsaw (Grant No. 6PO4 043 14). The authors are grateful to the reviewers for their helpful comments, which distinctly improved the manuscript.

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