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Hydrographic thresholds in the western Baltic Sea: Late Quaternary geology and the Dana River concept
Marine Geology 176 (2001) 191±201
www.elsevier.nl/locate/margeo
Hydrographic thresholds in the western Baltic Sea: Late Quaternary geology and the Dana River concept W. Lemke a,*, J.B. Jensen b, O. Bennike b, R. Endler a, A. Witkowski c, A. Kuijpers b a
Sektion Marine Geology, Baltic Sea Research Institute (IOW), Seestr. 15, PF 30 11 61, D-18119 Rostock-WarnemuÈnde, Germany b Geological Survey of Denmark and Greenland, Thoravej 8, Dk-2400 Copenhagen NV, Denmark c Institute of Marine Sciences, University of Szczecin, Felczaka 3a, 71-412 Szczecin, Poland Received 4 August 2000; accepted 16 March 2001
Since the ®nal Weichselian deglaciation the water exchange between Kattegat and the Baltic Sea and hence the palaeogeographical development of the (western) Baltic has been controlled by a number of sills. Major thresholds are found at the southern entrance of the Great Belt and in the Darss Sill area. The easternmost of these structures is formed by Late Weichselian sands at a level of 24 m below present sea level (bsl) at the Falster±RuÈgen sand plain. A second threshold consisting of till was found within the Kadet Channel at a level of 23±24 m bsl. Thus, we exclude water exchange between Mecklenburg Bay and the Arkona Basin at levels below 24 m since the end of the Pleistocene. A high resolution sediment echosounder survey at the triple junction area of Langeland Channel, Vejsnaes Channel and Winds Grave Channel showed outcropping glacial deposits. Narrow channels incised at a level below 25 m bsl are ®lled partly with ®ne grained organic rich late- and postglacial sediments. Assuming no substantial crustal movements in this area in the Holocene no Ancylus Lake drainage at a level deeper than 25 m bsl is likely to have occurred here. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Baltic region; Palaeogeography; Ancylus Lake; Dana River; Holocene
The late- and postglacial history of the Baltic Sea is characterised by a succession of isolation and inundation stages. During the Late Weichselian the éresund functioned as the main outlet for Baltic Ice Lake freshwater discharge (Bergsten and Nordberg, 1992). A short interruption occurred in the Late Allerùd, when drainage shifted and was via the Mt. Billingen area (BjoÈrck, 1995). For the Late Weichselian, no
* Corresponding author. E-mail address: [email protected] (W. Lemke).
evidence of any brackish water ingression into the Baltic Basin can be found in the geological record. Abrupt warming at the Pleistocene/Holocene boundary caused accelerated deglaciation in Scandinavia. When the ice margin receded from the Mt. Billingen area, the ®nal drainage of the Baltic Ice Lake resulted in a 25 m water level drop. An open connection between Kattegat and the Baltic was established through the south central Swedish lowlands. However, it lasted more than 200 years before brackish waters could enter the western Gotland Basin and the duration of this brackish event is supposed to be in the order of 100± 200 years (StroÈmberg, 1989; WastegaÊrd et al., 1995). Due to the rapid isostatic uplift of Scandinavia
0025-3227/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0025-322 7(01)00152-9
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the straits to Kattegat and Skagerrak were gradually closed, which resulted in a renewed decoupling between the open sea and rising water level in the Baltic. This marks the beginning of the Ancylus Transgression at 9500 14C years bp (BjoÈrck, 1995). The higher isostatic uplift in the north caused a transgression in the southern part of the up-dammed Baltic. Kliewe and Janke (1982, 1991) suggested a maximum Ancylus Lake level at 8 m below present sea level (bsl) based on ®ndings of Ancylus ¯uviatilis shells and freshwater diatoms in borings along the northeast German coast. According to Kolp (1986) the maximum level was about 12 m bsl while BjoÈrck (1995) proposed this level to be in the range of 20 m bsl. Recent investigations place the maximum Ancylus Lake level in the south-western Baltic at 19 m bsl (Jensen et al., 1999). According to Kolp (1986) and Eronen (1988) the succeeding regression lowered the level of the Ancylus Lake by 20 m while Svensson (1991) and BjoÈrck (1995) found indications of 8±10 m lowering in south-eastern Sweden. Kessel and Raukas (1979) reported an Ancylus Lake level lowering of 25 m in Estonia. A value of 13±15 m was suggested by Alhonen (1979) for Finland and Gudelis (1979) in Lithuania. The crucial question in this context is the location of the ®nal Ancylus Lake threshold. A preliminary solution of this problem was provided by Kolp (1986) and BjoÈrck (1995), who proposed the Darss Sill being the threshold. They suggested a more or less catastrophic over¯ow of the sill between the German Darss Peninsula and the Danish island of Falster. As a consequence of this over¯ow and associated gradual erosion of the sill a river was formed which drained the Ancylus Lake via Fehmarn Belt and Great Belt into the Kattegat for a period of about 1000 years. Post (1929) named this hypothetical river Dana River. Deep channels in the western Baltic with a present water depth of 32 m (Kadet Channel) and more (Fehmarn Belt, Windsgrav Channel, Langeland Channel) were interpreted to have been parts of the Dana River. In this context not much attention was paid, however, to the present sills between the channels. So, Kolp (1986) assumed a buried north-eastward continuation of the Kadet Channel to the Arkona Basin maintaining a drainage pathway for the Ancylus
Lake at a level of 32 m bsl until the onset of Littorina transgression. Joint Danish, Swedish, German and Polish investigations carried out recently in three of the critical threshold areas found along the course of the proposed Dana River (Fig. 1) have produced new information about their Early Holocene palaeogeography.
Shallow seismic pro®ling and sediment sampling were carried out using the research vessels A.v. Humboldt and Professor Albrecht Penck. The seismoacoustic equipment used in this study included a Boomer (Uniboom, 0.8±16 kHz), a subbottom pro®ler (ORE, 3.5 kHz), Datasonics and Geoacoustics CHIRP (1±10kHz), a dual frequency echosounder (DESO 25, 15 and 210 kHz) and a sediment echosounder (SES 96) developed by the University of Rostock (Wendt et al., 1998). Air gun data provided by Tom FlodeÂn (Stockholm University) were used to map the uppermost till's surface in the area between Arkona Basin and Mecklenburg Bay. Sediment cores were collected by a 6 m vibrocorer. Navigational data were provided by differential GPS and the Sercel Syledis system with an accuracy of within 10 m. A number of cores were subsampled at selected levels for radiocarbon dating and analyses of macroand microfossils, in order to obtain further information on the age of the sediments and the depositional environment. The samples were dated either by conventional 14C-dating at the Dating Laboratory of the Danish National Museum and GEUS, or by accelerator mass spectrometry (AMS) 14C-dating at the Institute of Physics and Astronomy, Aarhus University, following the method described by Heinemeier and Andersen (1983). All ages in this paper are given as uncalibrated 14C years bp (Table 1). For more detailed information on the seismic interpretations, macrofossil- and diatom analysis and radiocarbon dating we refer to earlier publications (Jensen, 1992; Jensen et al., 1997; Jensen et al., 1999; Lemke and Kuijpers, 1995).
Today water exchange between the Baltic and
W. Lemke et al. / Marine Geology 176 (2001) 191±201
193
Fig. 1. Geographical setting, bathymetry according to Seifert and Kayser (1995). The thick solid line marks the course of Dana River as proposed by Kolp (1986) and BjoÈrck (1995). Contour intervals are 4 m. The dashed lines embrace the Darss Sill in geological terms as introduced by Kolp (1965). The white boxes mark critical thresholds referred to in the text: 1 Falster±RuÈgen sand plain; 2 central part of Kadet Channel; 3 triple junction of Langeland, Vejsnaes and Winds Grave channels at the southern entrance of Langeland Belt.
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W. Lemke et al. / Marine Geology 176 (2001) 191±201
Table 1 Radiocarbon dates from investigated cores. Ages are given as uncalibrated 14C years bp (bsl below present sea level) Core
N. Lat.
E. Long.
Water depth (m)
Sample depth bsl (m)
Lab. no.
Material
FR A 6950/23 200540
54839.48 0 54843.48 0
12834.56 0 12845.96 0
17.8 21.8
22.4 24.4
K-6343 AAR-2639
200540 200540
54843.48 0 54843.48 0
12845.96 0 12845.96 0
21.8 21.8
26.8 27.7
AAR-3040 AAR-2637
222620
54839.69 0
10845.07 0
25.3
26.0
AAR-5142
222620
54839.69 0
10845.07 0
25.3
28.0
AAR-5143
Detritus gyttja Seeds of Menyanthes trifoliata, Fruits of Scirpus lacustris, Seeds of Pinus sylvestris Salix sp. Twigs Betula nana, Salix herbacea leaves, nuts, twig Well preserved twigs of land plant Populus tremula (?) bark fragment
North Sea is maintained via the Danish Straits, i.e. mainly (75%) via the Darss Sill±Fehmarn Belt, and for the remaining part via the éresund (see Fig. 1). With an area of ca. 40 £ 50 km 2 and an average water depth of 18 m Darss Sill is the critical threshold between Mecklenburg Bay/Fehmarn Belt in the west (maximum water depth 30 m) and the Arkona Basin in the east (max. water depth 50 m). Following Kolp (1965), in geological terminology the Darss Sill is restricted to a 10±12 km wide zone extending from the isle of Falster to Fischland-Darss, Germany (Fig. 1). The zone is characterised by the presence of submarine till outcrops, which belong to the Late Weichselian ice marginal zone G (ªVelgaster Staffelº) according to Richter (1937). It was formed between 14,000 and 13,000 years bp during the Late Weichselian deglaciation (Aurada, 1988; Liedtke, 1981). In nautical charts a 32 m deep NE±SW trending channel (Kadet Channel) is incised in the till surface which is found at water depths of 14±20 m. Its central part is marked by box no. 2 in Fig. 1. The seabed in the remaining part of the hydrographical threshold consists mainly of thick sands. It was called the ªFalster±RuÈgen sand plainº by Kolp (1965; box no. 1 in Fig. 1). According to the Dana River concept an at least 32 m deep valley ®lled with approximately 14±15 m of post-Ancylus, i.e. marine sands, was expected to exist here. However, only
Age ( 14C years bp)
d 13C (½)
9660^145 9810^75
2 30.0 2 24.4
12,180^100 12,700^110
2 29.4 2 28.8
9015^60
2 18.2
9160^75
2 28.7
limited information existed about structure and previous genesis of this sand plain. Another bathymetric threshold is situated at the triple junction of Windsgrav-, Vejsnaes- and Langeland channels at the southern entrance of Langeland Belt (Box 3 in Fig. 1). This threshold has a width of 1 km and the seabed consists of till. The average water depth is 23 m. According to Winn (1974), ªthe VejsnaÈs Channel continues directly into Great Belt with a threshold depth of 28 m while the Winds Grave Channel joins the VejsnaÈs Channel with a shallower depth of 25.5 m just before it joins the Great Beltº. The data forming the basis for this statement were collected in the early 70s. Therefore, a new survey with more accurate positioning and a more comprehensive sediment echosounder technology was carried out.
4.1. Falster±RuÈgen sand plain In order to provide evidence for the suggested Dana River valley to be found in the area between the Arkona Basin and Mecklenburg Bay a grid of shallow seismic lines was run in the area of the Falster±RuÈgen sand plain. Instead of the expected valley structure,
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Fig. 2. Surface of Pleistocene deposits at the Darss Sill in m bsl. Contour intervals are 2 m. Solid lines indicate the course of shallow channels.
a shallow subbottom re¯ector was identi®ed over nearly the entire area. Usually, this re¯ector is located some decimetres below the sea bottom. Locally, the overlying bed thins out completely so that the re¯ector forms the seabed surface. Vibrocoring con®rmed that
the seismic re¯ector marks the boundary between two different lithotypes. Lithologically, the upper bed consists of ®ne to medium sand with marine shells. Another ®ne sand below the boundary is rich in dispersed carbonate and
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W. Lemke et al. / Marine Geology 176 (2001) 191±201
does not contain any marine fossils. Fine humus particles are common. Shallow channels partly ®lled by organic detritus are incised in this lower sand body with a maximum depth of 23 m bsl. The minimum age of the non-marine sand was determined by dating organic in®llings within the shallow channels. A detritus gyttja was dated to 9660 ^ 145 years bp by conventional radiocarbon dating. Plant remains (Menyanthes trifoliata, Scirpus lacustris, Pinus sylvestris) within such a layer gave an age of 9810 ^ 75 years bp by using the AMS technique. Within the ®ne sand itself two dates of remains of Betula nana and Salix polaris yielded ages of 12,180 ^ 100 and 12,700 ^ 110 years bp, respectively. Together with ®ndings in the Arkona Basin and Mecklenburg Bay (Jensen et al., 1997; Jensen et al., 1999) these dates con®rm a late glacial age of the sand below the fragmentary thin marine sediment cover. It could be shown, that the sand belongs to a late glacial deltaic outbuilding system, fed by meltwater from glacial valleys in the southwest (Jensen et al., 1997; Lemke, 1998; Lemke et al., 1999). A map of the late glacial sand's upper boundary (Fig. 2) clearly shows a Pleistocene threshold instead of the expected 32 m deep river valley east of Kadet Channel. As described above, the deepest incisions in the Pleistocene sand do not go beyond 23 m bsl. Actually, Boreal deposits were found in these channel structures. They consist, however, of calcareous gyttja re¯ecting a quiet lacustrine or paludal rather than a ¯uvial depositional environment. Similar Boreal deposits of local lake, mire or swamp origin are found at several places in the Darss Sill area and were dated by radiocarbon and pollen analysis (Bennike et al., 1998). 4.2. Kadet Channel The Kadet Channel is incised into the Darss Sill and has a present maximum water depth of 32 m bsl. Considering the 20 m depth contour as shown on in nautical charts, it appears to be an elongated valley with a length of about 35 km and a width of ca. 5 km. Based on available information Kolp (1965) interpreted it as a large glacial valley (ªUrstromtalº).
197
A detailed re-examination of existing bathymetric data proved the existence of a sill within the Kadet Channel at a level of only 23±24 m bsl (box 2 in Fig. 1, Fig. 3). Provided the Kadet Channel is a glacial valley with the typical U-shaped morphology, the threshold within it must be younger than the ®nal deglaciation. If the Kadet Channel was part of Dana River, the threshold must have been formed even after the Ancylus Lake drainage, i.e. it should be younger than 9200 14C years bp. Boomer sections across the threshold indicate the uniform presence of well-consolidated deposits (Fig. 3). Core data and diver observations prove that they consist of till and associated lag deposits. Thus, a glacial origin of the threshold is very likely. This indicates that the Kadet Channel with its complicated bathymetric structure might be a succession of kettle holes rather than a large glacial valley. Moreover, we have to conclude that water exchange via Kadet Channel at levels deeper than 24 m bsl has not occurred since the Late Weichselian. 4.3. Langeland Belt In June 1999, a high resolution sediment echosounder survey was made at the southern entrance of Great Belt using r/v A.v. Humboldt. The triple junction area of Langeland Channel, Vejsnaes Channel and Winds Grave Channel is characterised by outcropping glacial deposits with a maximum water depth of 25 m. Deeper channels incised in the till are partly ®lled with ®ne grained organic rich late- and postglacial sediments, resulting in a present water depth of less than 25 m (Fig. 4). Vibrocore 222620 taken in the channel system close to the triple junction contains plant remains which were dated at 9015 ^ 60 years bp for the level of 26 m bsl and 9160 ^ 75 years bp for the level of 28 m bsl. When examining the channel topography having been stripped for its postglacial sediment in®ll there are still places, where water depth is less than 30 m. With the 25 m depth contour of glacial deposits as a reference basis, the average width of the channel system is about 280 m. For further reconstruction, we applied
Fig. 3. Detail of the Kadet Channel bathymetry with boomer sections across the sill (23±24 m bsl) found within the Kadet Channel. Stars mark sites sampled by divers or box corer.
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the following calculations: Based on an average channel width of 280 m as found for the 25 m glacial contour level and an associated average depth of 5 m, the resulting average cross section amounts to 700 m 2. Considering a length of about 10 km of the channels in the surveyed area the available volume for water exchange in the 25±30 m layer is 7 £ 10 6 m 3. When assuming an extraordinary high current velocity of 8 m/s which is one of the highest velocities ever measured in river channels (Leopold et al., 1964), 1.8 £ 10 11 m 3/year may pass the area. The present yearly fresh water surplus and discharge volume of the Baltic Sea area is 4.8 £ 1011 m 3. Comparing the climatic situation in the Boreal chronozone with recent conditions, a substantial reduction of the discharge is unlikely. Therefore, in order to allow such a discharge rate, which thus exceeds the maximum rate calculated for the 25±30 m layer by nearly a factor 3, a hypothetical current velocity of about 20 m/s would be required, which is far from realistic. When calculating an Ancylus Lake maximum highstand of 19 m bsl and a drop by 5±24 m bsl a critical section exists between southern Langeland and Lolland. Here, the maximum water depth is 32 m, the maximum width at 19 m bsl amounts to 3200 m giving an cross-section area of about 11,250 m 2. Considering a channel length of 1 m with this area 3.55 £ 10 11 m 3 water/year could pass it at a current speed of 1 m/s, i.e. a current speed of 1.35 m/s is needed to drain the yearly freshwater surplus. In order to lower the water level in the Baltic Basin from 19 to 24 m bsl an average yearly current velocity of 5.3 m/s would be necessary which is still very high, but not completely unrealistic.
The concept of a Dana River draining the Ancylus Lake via Darss Sill, Fehmarn Belt and Great Belt at a level of 32 m bsl as proposed by Kolp (1986) is in obvious contradiction to the results presented here. As all the threshold areas are situated close to the isostatic zero-line (Kolp, 1986; Winn, 1974) it
199
may be assumed that seabed ¯uctuations have been negligible since the last deglaciation here. Since the beginning of the Holocene at least two different thresholds have prevented water exchange at depths below 24 m bsl between the Arkona Basin and Mecklenburg Bay. The sedimentary in®ll of the shallow channels incised in the Pleistocene sands of the Falster±RuÈgen sand plain re¯ects a depositional environment characterised by local lakes, bogs and swamps after regression of the Ancylus Lake at about 9200 years bp (Bennike et al., 1998). According to BjoÈrck (1995), the Ancylus Lake became level with the sea after the regression. Actually, data from Tromper Wiek, northeast of RuÈgen island, indicate a postregressional level of the Ancylus Lake at about 32 m bsl (Lemke et al., 1998). This implies necessarily a connection between the Kattegat and the Ancylus Lake. As the Pleistocene thresholds allow a maximum of 5 m drainage down to a level of 24 m bsl via the Darss Sill, it has to be looked for the outlet in another place. Without such a connection in a different locality, a new transgression in the Darss Sill area would have occurred. However, there is no indication for this. Furthermore, if the Dana River would have drained the Ancylus Lake via the Kadet Channel for a time span of several hundreds of years, a prograding system was likely to have developed in the southwestern Kadet Channel exit. Interpretations of seismic data from this area show no indications of such a prograding system. According to our data, Mecklenburg Bay was separated from the Ancylus Lake east of Darss Sill until the Littorina transgression which inundated Darss Sill between 7000 and 7500 years bp (Jensen et al., 1996; Lemke et al., 1997). If Darss Sill is ruled out as drainage area for the Ancylus Lake, only éresund or the Lake VaÈnern area are left, though this is in contradiction to most current publications by Scandinavian authors (e.g. BjoÈrck, 1995). On the other hand, assuming an initial Littorina transgression via éresund at 8200 years bp as proposed by BjoÈrck (1995) implies that at the end of
Fig. 4. Channel con®guration south of Langeland island; white dashed lines echosounder sections; white solid lines channel width at 30 m bsl; black dashed line 30 m water depth; black solid lines 25 m water depth.
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the Boreal chronozone the sill depth must have been lower here than in the Darss Sill/Langeland areas. In this the éresund has to be regarded as possible drainage pathway of the Ancylus Lake. However, the present depth difference between 7 m bsl in the éresund and 23 m bsl at the Darss Sill cannot be explained then by differential glacioisostatic uplift as measured today for these two areas. According to Kolp (1986) and Striggow and Till (1987) the difference in uplift rate between the éresund and Darss Sill is about 1 mm/year. Using this as a constant for the Holocene back to 9000 14 C years bp (10,240 cal. years bp according to Stuiver et al., 1998), the Darss Sill would still be 5±6 m below the elevation of the éresund treshold. Therefore, if regarding the éresund as a possible drainage pathway for the Ancylus Lake and gateway for the initial Littorina transgression, we have to invoke additional uplift in course of the Holocene. In this context the ®nding of extraordinary high late glacial uplift rates at Kullen Peninsula compared to the surrounding area (Sandgren et al., 1999) gives rise to speculations about local or regional deviations from the general large scale glacio-isostatic pattern. Further investigations are needed to tackle this problem. After the Ancylus Lake regression, Mecklenburg Bay remained an isolated lake which probably drained via the Great Belt into Kattegat. The dimensions of the channel system south of Langeland correspond well with such a regional drainage pattern. The results presented do exclude the possible existence of an incised Dana River draining the Ancylus Lake at a level deeper than 24 m bsl. This leaves, however, further questions to be answered: ² Where was the active drainage pathway in the time span from the Ancylus Lake regression to the Littorina transgression? ² Where and when was the ®rst major in¯ow of brackish waters into the Baltic Basin leading to the Littorina transgression?
We thank the crews of the research vessels A.v. Humboldt and Professor Albrecht Penck for excellent assistance. AMS radiocarbon dating was performed
by the University of Aarhus, under the supervision of J. Heinemeier. Air gun data for mapping the till surface in the Falster±RuÈgen area were provided by Tom FlodeÂn (University of Stockholm). Parts of the study were made in the frame of a mapping programme supported by the Federal Maritime and Hydrographic Agency of Germany (ªBSHº). In this context we are especially grateful to Dr. K. Figge who has co-ordinated the mapping programme. The journal's referees made several useful comments that have improved the content of this paper.
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W. Lemke et al. / Marine Geology 176 (2001) 191±201 Quaternary History of the Baltic. Acta Universitatis Upsaliensis, Uppsala, pp. 127±146. Kliewe, H., Janke, W., 1982. Der holozaÈne Wasserspiegelanstieg der Ostsee im nordoÈstlichen KuÈstengebiet der DDR. Petermanns Geographische Mitteilungen 126, 65±74. Kliewe, H., Janke, W., 1991. HolozaÈner KuÈstenausgleich im suÈdlichen Ostseegebiet bei besonderer BeruÈcksichtigung der BoddenausgleichskuÈste Vorpommerns. Petermanns Geographische Mitteilungen 135, 1±14. Kolp, O., 1965. PalaÈogeographische Ergebnisse der Kartierung des Meeresgrundes der westlichen Ostsee zwischen Fehmarn und Arkona. BeitraÈge zur Meereskunde 12±14, 1±58. Kolp, O., 1986. Entwicklungsphasen des Ancylus-Sees. Petermanns Geographische Mitteilungen 130, 79±94. Lemke, W., 1998. Sedimentation und palaÈogeographische Entwicklung im westlichen Ostseeraum (Mecklenburger Bucht bis Arkonabecken) vom Ende der Weichselvereisung bis zur Litorinatransgression. Meereswissenschaftliche Berichte, 31. Institut fuÈr Ostseeforschung, Rostock-WarnemuÈnde, 156 pp. Lemke, W., Kuijpers, A., 1995. Early Holocene paleogeography of the Darss Sill. Prace PanÂstwowego Instytutu Geologicznego, CXLIX, pp. 109±116. Lemke, W., Jensen, J.B., Bennike, O., Witkowski, A., 1997. Sequence stratigraphy of Late Pleistocene and Holocene deposits in the Mecklenburg Bay, south-western Baltic Sea. In: Cato, I., Klingberg, F. (Eds.), Fourth Marine Geological Conference Ð The Baltic. Sveriges Geologiska UndersoÈkning, Uppsala, Ser. ca 86, pp. 117±122. Lemke, W., Endler, R., Tauber, F., Jensen, J.B., Bennike, O., 1998. Late- and postglacial sedimentation in the Tromper Wiek northeast of RuÈgen. Meyniana 50, 155±173. Lemke, W., Jensen, J.B., Bennike, O., Witkowski, A., Kuijpers, A., 1999. No indication of a deeply incised Dana River between Arkona Basin and Mecklenburg Bay. Baltica 12, 66±70.
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Leopold, L.B., Wolman, M.G., Miller, J.P., 1964. Fluvial Process in Geomorphology. Freeman, San Francisco, CA, 522 pp. Liedtke, H., 1981. Die nordischen Vereisungen in Mitteleuropa. Forschungen zur Deutschen Landeskunde, 204, 307 pp. Post, V.L., 1929. Svea, GoÈta och Dana aÈlvar. Ymer 49, 1±33. Richter, K., 1937. Die Eiszeit in Norddeutschland. Deutscher Boden 4, 176. Sandgren, P., Snowball, I.F., Risberg, J., Hammarlund, D., 1999. Stratigraphic evidence of a high marine shore-line during the Late Weichselian deglaciation on the Kullen Peninsula, southern Sweden. J. Quat. Sci. 14, 223±237. Seifert, T., Kayser, B., 1995. A high resolution spherical grid topography of the Baltic Sea. Meereswissenschaftliche Berichte. Institut fuÈr Ostseeforschung, Rostock-WarnemuÈnde 9, 73±88. Striggow, K., Till, K.H., 1987. EinhundertjaÈhrige Pegelregistrierungen des suÈdwestlichen Ostseeraumes. Zeitschrift fuÈr geologische Wissenschaften 15, 225±241. StroÈmberg, B., 1989. Late Weichselian deglaciation and clay-varve chronology in east-central Sweden. Sveriges Geologiska UndersoÈkning, Uppsala, ca 73, 70 pp. Stuiver, M., Reimer, P.J., Bard, E., Beck, J.W., Burr, G.S., Hughen, K.A., Kromer, B., McCormack, F.G., Plicht, J.v.d., Spurk, M., 1998. INTCAL98 Radiocarbon Age Calibration, 24,000± 0 cal. bp. Radiocarbon 40, 1041±1083. Svensson, N.-O., 1991. Late Weichselian and Early Holocene shore displacement in the Central Baltic Sea. Quat. Int. 9, 7±26. WastegaÊrd, S., AndreÂn, T., Sohlenius, G., Sandgren, P., 1995. Different phases of the Yoldia Sea in the Northwestern Baltic proper. Quat. Int. 27, 121±129. Wendt, G., Heinitz, W.-D., Wunderlich, J., Ewert, J., Endler, R., 1998. Schluûbericht zum BMBF-Vorhaben ASS (Adaptive seismoakustische Systeme fuÈr die Ostsee). Unpublished report, University of Rostock, Baltic Sea Research Institute, 240 pp. Winn, K., 1974. Present and postglacial sedimentation in the Great Belt Channel (western Baltic). Meyniana 26, 63±101.
www.elsevier.nl/locate/margeo
Hydrographic thresholds in the western Baltic Sea: Late Quaternary geology and the Dana River concept W. Lemke a,*, J.B. Jensen b, O. Bennike b, R. Endler a, A. Witkowski c, A. Kuijpers b a
Sektion Marine Geology, Baltic Sea Research Institute (IOW), Seestr. 15, PF 30 11 61, D-18119 Rostock-WarnemuÈnde, Germany b Geological Survey of Denmark and Greenland, Thoravej 8, Dk-2400 Copenhagen NV, Denmark c Institute of Marine Sciences, University of Szczecin, Felczaka 3a, 71-412 Szczecin, Poland Received 4 August 2000; accepted 16 March 2001
Since the ®nal Weichselian deglaciation the water exchange between Kattegat and the Baltic Sea and hence the palaeogeographical development of the (western) Baltic has been controlled by a number of sills. Major thresholds are found at the southern entrance of the Great Belt and in the Darss Sill area. The easternmost of these structures is formed by Late Weichselian sands at a level of 24 m below present sea level (bsl) at the Falster±RuÈgen sand plain. A second threshold consisting of till was found within the Kadet Channel at a level of 23±24 m bsl. Thus, we exclude water exchange between Mecklenburg Bay and the Arkona Basin at levels below 24 m since the end of the Pleistocene. A high resolution sediment echosounder survey at the triple junction area of Langeland Channel, Vejsnaes Channel and Winds Grave Channel showed outcropping glacial deposits. Narrow channels incised at a level below 25 m bsl are ®lled partly with ®ne grained organic rich late- and postglacial sediments. Assuming no substantial crustal movements in this area in the Holocene no Ancylus Lake drainage at a level deeper than 25 m bsl is likely to have occurred here. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Baltic region; Palaeogeography; Ancylus Lake; Dana River; Holocene
The late- and postglacial history of the Baltic Sea is characterised by a succession of isolation and inundation stages. During the Late Weichselian the éresund functioned as the main outlet for Baltic Ice Lake freshwater discharge (Bergsten and Nordberg, 1992). A short interruption occurred in the Late Allerùd, when drainage shifted and was via the Mt. Billingen area (BjoÈrck, 1995). For the Late Weichselian, no
* Corresponding author. E-mail address: [email protected] (W. Lemke).
evidence of any brackish water ingression into the Baltic Basin can be found in the geological record. Abrupt warming at the Pleistocene/Holocene boundary caused accelerated deglaciation in Scandinavia. When the ice margin receded from the Mt. Billingen area, the ®nal drainage of the Baltic Ice Lake resulted in a 25 m water level drop. An open connection between Kattegat and the Baltic was established through the south central Swedish lowlands. However, it lasted more than 200 years before brackish waters could enter the western Gotland Basin and the duration of this brackish event is supposed to be in the order of 100± 200 years (StroÈmberg, 1989; WastegaÊrd et al., 1995). Due to the rapid isostatic uplift of Scandinavia
0025-3227/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0025-322 7(01)00152-9
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the straits to Kattegat and Skagerrak were gradually closed, which resulted in a renewed decoupling between the open sea and rising water level in the Baltic. This marks the beginning of the Ancylus Transgression at 9500 14C years bp (BjoÈrck, 1995). The higher isostatic uplift in the north caused a transgression in the southern part of the up-dammed Baltic. Kliewe and Janke (1982, 1991) suggested a maximum Ancylus Lake level at 8 m below present sea level (bsl) based on ®ndings of Ancylus ¯uviatilis shells and freshwater diatoms in borings along the northeast German coast. According to Kolp (1986) the maximum level was about 12 m bsl while BjoÈrck (1995) proposed this level to be in the range of 20 m bsl. Recent investigations place the maximum Ancylus Lake level in the south-western Baltic at 19 m bsl (Jensen et al., 1999). According to Kolp (1986) and Eronen (1988) the succeeding regression lowered the level of the Ancylus Lake by 20 m while Svensson (1991) and BjoÈrck (1995) found indications of 8±10 m lowering in south-eastern Sweden. Kessel and Raukas (1979) reported an Ancylus Lake level lowering of 25 m in Estonia. A value of 13±15 m was suggested by Alhonen (1979) for Finland and Gudelis (1979) in Lithuania. The crucial question in this context is the location of the ®nal Ancylus Lake threshold. A preliminary solution of this problem was provided by Kolp (1986) and BjoÈrck (1995), who proposed the Darss Sill being the threshold. They suggested a more or less catastrophic over¯ow of the sill between the German Darss Peninsula and the Danish island of Falster. As a consequence of this over¯ow and associated gradual erosion of the sill a river was formed which drained the Ancylus Lake via Fehmarn Belt and Great Belt into the Kattegat for a period of about 1000 years. Post (1929) named this hypothetical river Dana River. Deep channels in the western Baltic with a present water depth of 32 m (Kadet Channel) and more (Fehmarn Belt, Windsgrav Channel, Langeland Channel) were interpreted to have been parts of the Dana River. In this context not much attention was paid, however, to the present sills between the channels. So, Kolp (1986) assumed a buried north-eastward continuation of the Kadet Channel to the Arkona Basin maintaining a drainage pathway for the Ancylus
Lake at a level of 32 m bsl until the onset of Littorina transgression. Joint Danish, Swedish, German and Polish investigations carried out recently in three of the critical threshold areas found along the course of the proposed Dana River (Fig. 1) have produced new information about their Early Holocene palaeogeography.
Shallow seismic pro®ling and sediment sampling were carried out using the research vessels A.v. Humboldt and Professor Albrecht Penck. The seismoacoustic equipment used in this study included a Boomer (Uniboom, 0.8±16 kHz), a subbottom pro®ler (ORE, 3.5 kHz), Datasonics and Geoacoustics CHIRP (1±10kHz), a dual frequency echosounder (DESO 25, 15 and 210 kHz) and a sediment echosounder (SES 96) developed by the University of Rostock (Wendt et al., 1998). Air gun data provided by Tom FlodeÂn (Stockholm University) were used to map the uppermost till's surface in the area between Arkona Basin and Mecklenburg Bay. Sediment cores were collected by a 6 m vibrocorer. Navigational data were provided by differential GPS and the Sercel Syledis system with an accuracy of within 10 m. A number of cores were subsampled at selected levels for radiocarbon dating and analyses of macroand microfossils, in order to obtain further information on the age of the sediments and the depositional environment. The samples were dated either by conventional 14C-dating at the Dating Laboratory of the Danish National Museum and GEUS, or by accelerator mass spectrometry (AMS) 14C-dating at the Institute of Physics and Astronomy, Aarhus University, following the method described by Heinemeier and Andersen (1983). All ages in this paper are given as uncalibrated 14C years bp (Table 1). For more detailed information on the seismic interpretations, macrofossil- and diatom analysis and radiocarbon dating we refer to earlier publications (Jensen, 1992; Jensen et al., 1997; Jensen et al., 1999; Lemke and Kuijpers, 1995).
Today water exchange between the Baltic and
W. Lemke et al. / Marine Geology 176 (2001) 191±201
193
Fig. 1. Geographical setting, bathymetry according to Seifert and Kayser (1995). The thick solid line marks the course of Dana River as proposed by Kolp (1986) and BjoÈrck (1995). Contour intervals are 4 m. The dashed lines embrace the Darss Sill in geological terms as introduced by Kolp (1965). The white boxes mark critical thresholds referred to in the text: 1 Falster±RuÈgen sand plain; 2 central part of Kadet Channel; 3 triple junction of Langeland, Vejsnaes and Winds Grave channels at the southern entrance of Langeland Belt.
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Table 1 Radiocarbon dates from investigated cores. Ages are given as uncalibrated 14C years bp (bsl below present sea level) Core
N. Lat.
E. Long.
Water depth (m)
Sample depth bsl (m)
Lab. no.
Material
FR A 6950/23 200540
54839.48 0 54843.48 0
12834.56 0 12845.96 0
17.8 21.8
22.4 24.4
K-6343 AAR-2639
200540 200540
54843.48 0 54843.48 0
12845.96 0 12845.96 0
21.8 21.8
26.8 27.7
AAR-3040 AAR-2637
222620
54839.69 0
10845.07 0
25.3
26.0
AAR-5142
222620
54839.69 0
10845.07 0
25.3
28.0
AAR-5143
Detritus gyttja Seeds of Menyanthes trifoliata, Fruits of Scirpus lacustris, Seeds of Pinus sylvestris Salix sp. Twigs Betula nana, Salix herbacea leaves, nuts, twig Well preserved twigs of land plant Populus tremula (?) bark fragment
North Sea is maintained via the Danish Straits, i.e. mainly (75%) via the Darss Sill±Fehmarn Belt, and for the remaining part via the éresund (see Fig. 1). With an area of ca. 40 £ 50 km 2 and an average water depth of 18 m Darss Sill is the critical threshold between Mecklenburg Bay/Fehmarn Belt in the west (maximum water depth 30 m) and the Arkona Basin in the east (max. water depth 50 m). Following Kolp (1965), in geological terminology the Darss Sill is restricted to a 10±12 km wide zone extending from the isle of Falster to Fischland-Darss, Germany (Fig. 1). The zone is characterised by the presence of submarine till outcrops, which belong to the Late Weichselian ice marginal zone G (ªVelgaster Staffelº) according to Richter (1937). It was formed between 14,000 and 13,000 years bp during the Late Weichselian deglaciation (Aurada, 1988; Liedtke, 1981). In nautical charts a 32 m deep NE±SW trending channel (Kadet Channel) is incised in the till surface which is found at water depths of 14±20 m. Its central part is marked by box no. 2 in Fig. 1. The seabed in the remaining part of the hydrographical threshold consists mainly of thick sands. It was called the ªFalster±RuÈgen sand plainº by Kolp (1965; box no. 1 in Fig. 1). According to the Dana River concept an at least 32 m deep valley ®lled with approximately 14±15 m of post-Ancylus, i.e. marine sands, was expected to exist here. However, only
Age ( 14C years bp)
d 13C (½)
9660^145 9810^75
2 30.0 2 24.4
12,180^100 12,700^110
2 29.4 2 28.8
9015^60
2 18.2
9160^75
2 28.7
limited information existed about structure and previous genesis of this sand plain. Another bathymetric threshold is situated at the triple junction of Windsgrav-, Vejsnaes- and Langeland channels at the southern entrance of Langeland Belt (Box 3 in Fig. 1). This threshold has a width of 1 km and the seabed consists of till. The average water depth is 23 m. According to Winn (1974), ªthe VejsnaÈs Channel continues directly into Great Belt with a threshold depth of 28 m while the Winds Grave Channel joins the VejsnaÈs Channel with a shallower depth of 25.5 m just before it joins the Great Beltº. The data forming the basis for this statement were collected in the early 70s. Therefore, a new survey with more accurate positioning and a more comprehensive sediment echosounder technology was carried out.
4.1. Falster±RuÈgen sand plain In order to provide evidence for the suggested Dana River valley to be found in the area between the Arkona Basin and Mecklenburg Bay a grid of shallow seismic lines was run in the area of the Falster±RuÈgen sand plain. Instead of the expected valley structure,
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195
Fig. 2. Surface of Pleistocene deposits at the Darss Sill in m bsl. Contour intervals are 2 m. Solid lines indicate the course of shallow channels.
a shallow subbottom re¯ector was identi®ed over nearly the entire area. Usually, this re¯ector is located some decimetres below the sea bottom. Locally, the overlying bed thins out completely so that the re¯ector forms the seabed surface. Vibrocoring con®rmed that
the seismic re¯ector marks the boundary between two different lithotypes. Lithologically, the upper bed consists of ®ne to medium sand with marine shells. Another ®ne sand below the boundary is rich in dispersed carbonate and
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W. Lemke et al. / Marine Geology 176 (2001) 191±201
does not contain any marine fossils. Fine humus particles are common. Shallow channels partly ®lled by organic detritus are incised in this lower sand body with a maximum depth of 23 m bsl. The minimum age of the non-marine sand was determined by dating organic in®llings within the shallow channels. A detritus gyttja was dated to 9660 ^ 145 years bp by conventional radiocarbon dating. Plant remains (Menyanthes trifoliata, Scirpus lacustris, Pinus sylvestris) within such a layer gave an age of 9810 ^ 75 years bp by using the AMS technique. Within the ®ne sand itself two dates of remains of Betula nana and Salix polaris yielded ages of 12,180 ^ 100 and 12,700 ^ 110 years bp, respectively. Together with ®ndings in the Arkona Basin and Mecklenburg Bay (Jensen et al., 1997; Jensen et al., 1999) these dates con®rm a late glacial age of the sand below the fragmentary thin marine sediment cover. It could be shown, that the sand belongs to a late glacial deltaic outbuilding system, fed by meltwater from glacial valleys in the southwest (Jensen et al., 1997; Lemke, 1998; Lemke et al., 1999). A map of the late glacial sand's upper boundary (Fig. 2) clearly shows a Pleistocene threshold instead of the expected 32 m deep river valley east of Kadet Channel. As described above, the deepest incisions in the Pleistocene sand do not go beyond 23 m bsl. Actually, Boreal deposits were found in these channel structures. They consist, however, of calcareous gyttja re¯ecting a quiet lacustrine or paludal rather than a ¯uvial depositional environment. Similar Boreal deposits of local lake, mire or swamp origin are found at several places in the Darss Sill area and were dated by radiocarbon and pollen analysis (Bennike et al., 1998). 4.2. Kadet Channel The Kadet Channel is incised into the Darss Sill and has a present maximum water depth of 32 m bsl. Considering the 20 m depth contour as shown on in nautical charts, it appears to be an elongated valley with a length of about 35 km and a width of ca. 5 km. Based on available information Kolp (1965) interpreted it as a large glacial valley (ªUrstromtalº).
197
A detailed re-examination of existing bathymetric data proved the existence of a sill within the Kadet Channel at a level of only 23±24 m bsl (box 2 in Fig. 1, Fig. 3). Provided the Kadet Channel is a glacial valley with the typical U-shaped morphology, the threshold within it must be younger than the ®nal deglaciation. If the Kadet Channel was part of Dana River, the threshold must have been formed even after the Ancylus Lake drainage, i.e. it should be younger than 9200 14C years bp. Boomer sections across the threshold indicate the uniform presence of well-consolidated deposits (Fig. 3). Core data and diver observations prove that they consist of till and associated lag deposits. Thus, a glacial origin of the threshold is very likely. This indicates that the Kadet Channel with its complicated bathymetric structure might be a succession of kettle holes rather than a large glacial valley. Moreover, we have to conclude that water exchange via Kadet Channel at levels deeper than 24 m bsl has not occurred since the Late Weichselian. 4.3. Langeland Belt In June 1999, a high resolution sediment echosounder survey was made at the southern entrance of Great Belt using r/v A.v. Humboldt. The triple junction area of Langeland Channel, Vejsnaes Channel and Winds Grave Channel is characterised by outcropping glacial deposits with a maximum water depth of 25 m. Deeper channels incised in the till are partly ®lled with ®ne grained organic rich late- and postglacial sediments, resulting in a present water depth of less than 25 m (Fig. 4). Vibrocore 222620 taken in the channel system close to the triple junction contains plant remains which were dated at 9015 ^ 60 years bp for the level of 26 m bsl and 9160 ^ 75 years bp for the level of 28 m bsl. When examining the channel topography having been stripped for its postglacial sediment in®ll there are still places, where water depth is less than 30 m. With the 25 m depth contour of glacial deposits as a reference basis, the average width of the channel system is about 280 m. For further reconstruction, we applied
Fig. 3. Detail of the Kadet Channel bathymetry with boomer sections across the sill (23±24 m bsl) found within the Kadet Channel. Stars mark sites sampled by divers or box corer.
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W. Lemke et al. / Marine Geology 176 (2001) 191±201
the following calculations: Based on an average channel width of 280 m as found for the 25 m glacial contour level and an associated average depth of 5 m, the resulting average cross section amounts to 700 m 2. Considering a length of about 10 km of the channels in the surveyed area the available volume for water exchange in the 25±30 m layer is 7 £ 10 6 m 3. When assuming an extraordinary high current velocity of 8 m/s which is one of the highest velocities ever measured in river channels (Leopold et al., 1964), 1.8 £ 10 11 m 3/year may pass the area. The present yearly fresh water surplus and discharge volume of the Baltic Sea area is 4.8 £ 1011 m 3. Comparing the climatic situation in the Boreal chronozone with recent conditions, a substantial reduction of the discharge is unlikely. Therefore, in order to allow such a discharge rate, which thus exceeds the maximum rate calculated for the 25±30 m layer by nearly a factor 3, a hypothetical current velocity of about 20 m/s would be required, which is far from realistic. When calculating an Ancylus Lake maximum highstand of 19 m bsl and a drop by 5±24 m bsl a critical section exists between southern Langeland and Lolland. Here, the maximum water depth is 32 m, the maximum width at 19 m bsl amounts to 3200 m giving an cross-section area of about 11,250 m 2. Considering a channel length of 1 m with this area 3.55 £ 10 11 m 3 water/year could pass it at a current speed of 1 m/s, i.e. a current speed of 1.35 m/s is needed to drain the yearly freshwater surplus. In order to lower the water level in the Baltic Basin from 19 to 24 m bsl an average yearly current velocity of 5.3 m/s would be necessary which is still very high, but not completely unrealistic.
The concept of a Dana River draining the Ancylus Lake via Darss Sill, Fehmarn Belt and Great Belt at a level of 32 m bsl as proposed by Kolp (1986) is in obvious contradiction to the results presented here. As all the threshold areas are situated close to the isostatic zero-line (Kolp, 1986; Winn, 1974) it
199
may be assumed that seabed ¯uctuations have been negligible since the last deglaciation here. Since the beginning of the Holocene at least two different thresholds have prevented water exchange at depths below 24 m bsl between the Arkona Basin and Mecklenburg Bay. The sedimentary in®ll of the shallow channels incised in the Pleistocene sands of the Falster±RuÈgen sand plain re¯ects a depositional environment characterised by local lakes, bogs and swamps after regression of the Ancylus Lake at about 9200 years bp (Bennike et al., 1998). According to BjoÈrck (1995), the Ancylus Lake became level with the sea after the regression. Actually, data from Tromper Wiek, northeast of RuÈgen island, indicate a postregressional level of the Ancylus Lake at about 32 m bsl (Lemke et al., 1998). This implies necessarily a connection between the Kattegat and the Ancylus Lake. As the Pleistocene thresholds allow a maximum of 5 m drainage down to a level of 24 m bsl via the Darss Sill, it has to be looked for the outlet in another place. Without such a connection in a different locality, a new transgression in the Darss Sill area would have occurred. However, there is no indication for this. Furthermore, if the Dana River would have drained the Ancylus Lake via the Kadet Channel for a time span of several hundreds of years, a prograding system was likely to have developed in the southwestern Kadet Channel exit. Interpretations of seismic data from this area show no indications of such a prograding system. According to our data, Mecklenburg Bay was separated from the Ancylus Lake east of Darss Sill until the Littorina transgression which inundated Darss Sill between 7000 and 7500 years bp (Jensen et al., 1996; Lemke et al., 1997). If Darss Sill is ruled out as drainage area for the Ancylus Lake, only éresund or the Lake VaÈnern area are left, though this is in contradiction to most current publications by Scandinavian authors (e.g. BjoÈrck, 1995). On the other hand, assuming an initial Littorina transgression via éresund at 8200 years bp as proposed by BjoÈrck (1995) implies that at the end of
Fig. 4. Channel con®guration south of Langeland island; white dashed lines echosounder sections; white solid lines channel width at 30 m bsl; black dashed line 30 m water depth; black solid lines 25 m water depth.
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the Boreal chronozone the sill depth must have been lower here than in the Darss Sill/Langeland areas. In this the éresund has to be regarded as possible drainage pathway of the Ancylus Lake. However, the present depth difference between 7 m bsl in the éresund and 23 m bsl at the Darss Sill cannot be explained then by differential glacioisostatic uplift as measured today for these two areas. According to Kolp (1986) and Striggow and Till (1987) the difference in uplift rate between the éresund and Darss Sill is about 1 mm/year. Using this as a constant for the Holocene back to 9000 14 C years bp (10,240 cal. years bp according to Stuiver et al., 1998), the Darss Sill would still be 5±6 m below the elevation of the éresund treshold. Therefore, if regarding the éresund as a possible drainage pathway for the Ancylus Lake and gateway for the initial Littorina transgression, we have to invoke additional uplift in course of the Holocene. In this context the ®nding of extraordinary high late glacial uplift rates at Kullen Peninsula compared to the surrounding area (Sandgren et al., 1999) gives rise to speculations about local or regional deviations from the general large scale glacio-isostatic pattern. Further investigations are needed to tackle this problem. After the Ancylus Lake regression, Mecklenburg Bay remained an isolated lake which probably drained via the Great Belt into Kattegat. The dimensions of the channel system south of Langeland correspond well with such a regional drainage pattern. The results presented do exclude the possible existence of an incised Dana River draining the Ancylus Lake at a level deeper than 24 m bsl. This leaves, however, further questions to be answered: ² Where was the active drainage pathway in the time span from the Ancylus Lake regression to the Littorina transgression? ² Where and when was the ®rst major in¯ow of brackish waters into the Baltic Basin leading to the Littorina transgression?
We thank the crews of the research vessels A.v. Humboldt and Professor Albrecht Penck for excellent assistance. AMS radiocarbon dating was performed
by the University of Aarhus, under the supervision of J. Heinemeier. Air gun data for mapping the till surface in the Falster±RuÈgen area were provided by Tom FlodeÂn (University of Stockholm). Parts of the study were made in the frame of a mapping programme supported by the Federal Maritime and Hydrographic Agency of Germany (ªBSHº). In this context we are especially grateful to Dr. K. Figge who has co-ordinated the mapping programme. The journal's referees made several useful comments that have improved the content of this paper.
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