Mid-Holocene Baltic Sea transgression along the coast of Blekinge

GFF volume 126 (2004), pp. 257–272.

Article

Mid-Holocene Baltic Sea transgression along the coast of Blekinge, SE Sweden – ancient lagoons correlated with beach ridges SHI-YONG YU1, BJÖRN E. BERGLUND1,*, ELINOR ANDRÉN2 and PER SANDGREN1 Yu, S.-Y., Berglund, B.E., Andrén, E. & Sandgren, P., 2004: Mid-Holocene Baltic Sea transgression along the coast of Blekinge, SE Sweden – ancient lagoons correlated with beach ridges. GFF, Vol. 126 (Pt. 3, September), pp. 257–272. Stockholm. ISSN 1103-5897.

Abstract: The mid-Holocene Littorina transgression in southern Scandinavia is well documented. Multiple-stratigraphic sequences in ancient Littorina lagoons in the coastal area of Blekinge, SE Sweden reveal a maximum relative sea level of 7–8 m above present sea level between 8000–6000 cal. BP. Evidence for at least two transgression waves is found within this period. In this study these are documented in one modern lake and correlated with an ancient beach-lagoon stratigraphy. Furthermore, two younger transgressions are documented at one site, altogether establishing a firm transgression chronology for the time span 8000–4000 cal. BP (sea level 5–8 m a.s.l.) as a basis for understanding the dynamics of Baltic sea-level changes. Neolithic cultural layers are correlated to regression periods, indicating more favorable conditions for beach settlement between stormy transgression periods. Keywords: Baltic sea level, beach ridges, mid-Holocene storminess, Littorina transgression, Neolithic coastal cultures GeoBiosphere Science Centre, Department of Geology/Quaternary Sciences, Lund University, Sölvegatan 12, SE-223 62 Lund, Sweden; [email protected] 2 Department of Earth Sciences, Palaeobiology, Uppsala University, Villavägen 16, SE–752 36 Uppsala, Sweden; [email protected] * Corresponding author; [email protected] Manuscript received 9 August 2003. Revised manuscript accepted 29 June 2004. 1

Introduction

The pronounced decay of continental ice sheets following the Last Glacial Maximum led to a rapid 120 m rise in global sea level by 7000 cal. BP (Hanebuth et al. 2000; Yokoyama et al. 2000; Lambeck & Chappell 2001; Lambeck et al. 2002). Simultaneously, the rate of crustal rebound along the margin of the previously glaciated area in Scandinavia began to slow down from the middle Holocene (Gudelis & Königsson 1979; Björck 1995). Therefore, the Littorina transgression along the Baltic coast, peaking between 8000–4000 cal. BP, is to a great extent supposed to be a manifestation of the ice-volume-equivalent sealevel rise (Fairbanks 1989; Chappell & Polach 1991; Bard et al. 1996), most probably triggered by the partial collapse of the Antarctic ice sheets (Ingólfsson & Hjort 1999; Hjort et al. 2004). Increasing evidence from elsewhere of the Baltic and Kattegat coast (e.g. Mörner 1969; Digerfeldt 1975; Liljegren 1982; Christensen 1995; Clemmensen et al. 2001) generally supports a multiple transgression pattern as proposed by Berglund (1964, 1971). However, this pattern has been questioned by Finnish colleagues with experience from the Gulf of Finland, where the isostatic land uplift has a greater influence on the relative sea level (Eronen 1974; Hyvärinen 1991; Miettinen 2002). Our strategy when studying the Littorina transgression in SE Sweden has been to correlate multiple-stratigraphic sequences from lagoons

with distinct overflow thresholds and with the stratigraphy at beach ridges in close contact with ancient lagoons, aiming to address the transgression pattern and its causes. In this paper, we present new results from Lake Färsksjön, a former Littorina lagoon in central Blekinge, SE Sweden and compare these with a reinterpretation of old studies of a beach ridge-lagoon sequence at Siretorp in western Blekinge (Berglund 1971). Three other beach ridge sites also are briefly discussed. Finally, a comparison is made with other regions along a W–E Baltic transect. This paper is part of a wider study of coastal sediments in Blekinge, addressing the sedimentary response to oceanographic and climatic changes during the Holocene.

Regional setting and site description

The Blekinge coast is a low-lying peneplain exposed southward to the Baltic Sea (Fig. 1A). The bedrock is Precambrian granitic gneiss, which is tectonically deformed mainly along N–S fault lines. Tectonic valleys between bedrock ridges have favored the development of lake basins with protecting sills. Previous investigations reveal that basins below 8 m a.s.l. were flooded during the middle Holocene (Berglund 1971; Liljegren 1982) and then isolated due to the continued land uplift. Such basins, particu-

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Yu et al.: Mid-Holocene Baltic Sea transgression along the coast of Blekinge

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Fig. 1. A. Survey map of Blekinge, SE Sweden. Sites mentioned in text are indicated with solid dots. B. Topographic map showing the relief of the Siretorp area. Hatched areas denote the distribution of Neolithic settlements (from Bagge & Kjellmark 1939). The Littorina beach ridge is situated ca. 400 m north of the Sandviken Bay. C. Topographic map of Lake Färsksjön and surrounding areas. Arrow shows the outflow threshold. The position of the master core sites and stratigraphic transects shown in Figs. 2 and 8 are indicated.

larly those with bedrock thresholds along the Blekinge coast, can provide powerful constraints on regional sea-level history (Yu et al. 2003a). Continuous sediment sequences also may be recovered from those basins. Moreover, the Blekinge coast is almost parallel to the Littorina isobase, which means that the entire area is affected by an identical isostatic component (Ekman 1996). This also may facilitate a comparison of the transgression pattern with other regions along the same isobase (e.g. from Sjæl-land via Blekinge to the Leningrad area). The Littorina beach at Siretorp (14°36ʼE, 56°01ʼN) is situated on the coastal plain of the Lister Peninsula in SW Blekinge (Fig. 1B), where beach ridges have been formed between drumlin hills during the Littorina transgression. In most cases, the beach ridges have a W–E direction and are exposed towards the sea to the south. The Siretorp beach ridge has blocked a valley depression to the north with a sandy till threshold ca. 5 m a.s.l., now

a wooded peatland partly surrounded by cultivated fields. The beach ridge area has nowadays been exploited by a summer village. Extensive archaeological excavations were performed in the 1930ʼs, together with reconnaissance mapping of Neolithic settlements (Bagge & Kjellmark 1939; Fig. 1B). The S–N trending transect revealed an interesting stratigraphy of the beach ridge, as well as the peatland in the basin (Sandegren 1939). With this background, new stratigraphic studies were conducted in 1968 (Berglund 1971) to reveal the transgression strati-graphy in greater detail in the former lagoon. Besides this, an archaeological study was made in 1971 (Berglund & Welinder 1972) to correlate culture layers with the peat/gyttja sequence at the shore of the former lagoon. Here, we summarize the results of old analyses (pollen, diatoms, 14C, δ13C and organic content) from the master core taken in 1968.

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The basin of Lake Färsksjön (14°59ʼ30”E, 56°10ʼ30”N) developed in a nearly N–S trending tectonic valley with a bedrock outflow threshold leveled to 7.2 m a.s.l. (Fig. 1C). The lake has been drained once by the local farmers in the 19th century through blasting the outflow bedrock threshold. The anthropogenic drainage led to a significant drop of the lake level (today at ca. 4.5 m a.s.l.), therefore, several peat islands were formed (Fig. 1C).

Methods Field work

In the summer of 2001, a coring was conducted in the center of Lake Färsksjön using a standard Russian peat sampler. Water depth at the coring place was 1.4 m. Lithology of the core was described during fieldwork (Table 1). The separate 1 m core segments were subsequently spliced to a master core in the laboratory. All analyses are based on this master core. In the autumn of 2002, a complementary stratigraphic investigation was performed by coring along a NW–SE transect on the largest peat island (Fig. 1C). At Siretorp, a standard Hiller peat sampler (inner diameter 35 mm, length 1 m) was used besides complementary digging of open sections. Elevation of thresholds and transects was leveled using Wild Kern Level equipment.

Mineral magnetic and loss-on-ignition (LOI) measurements

Samples for mineral magnetic measurements were taken along the master core of Färsksjön by contiguously pushing the 7 cm3 polystyrene pots into the cleaned sediment surface, representing ca. 2 cm intervals. All the measurements were performed in the Mineral Magnetic Laboratory of Lund University, following the procedures described by Sandgren et al. (1999). After the magnetic measurements, samples were dried at 50°C and 105°C, respectively, and then combusted at 550°C. The dry weight of samples at 50°C was used to calculate mass specific magnetic parameters. The percentage weight loss from 105°C to 550°C was expressed as LOI.

Diatom analyses

Small amounts of fresh sediment were prepared for diatom analysis according to the method described by Battarbee (1986). The enriched diatom samples were dried onto coverslips and mounted in Naphrax™. Quantitative analyses were carried out with a light microscope at a magnification of I1000. The counting convention of Schrader & Gersonde (1978) was used, and 300 to 400 diatom valves, excluding Fragilaria spp. sensu Hustedt,

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were counted at each level. The genus Fragilaria sensu Hustedt has been divided into several different genera (Williams & Round 1987). In this paper Fragilaria spp. actually consists of several different genera e.g. Staurosira, Staurosirella, Pseudostaurosira, Opephora and Fragilaria. Some very small brackish Fragilaria taxa, occurring together in a similar environment, have been clumped together as Fragilaria elliptica aggregate, which consists of Fragilaria elliptica, Staurosira elliptica, Staurosira punctiformis, Staurosira sopotensis, Pseudo-staurosira zeilleri and Opephora krumbeinii. Diatom floras and papers used for identification and as sources of ecological information were Håkansson (1974, 1978), Stabell (1985), Denys (1990), Witkowski et al. (2000), and the remainder were listed in Andrén (1999). The diatoms were grouped with respect to salinity tolerance according to the Baltic Sea intercalibration guides published by Snoeijs (1993), Snoeijs & Vilbaste (1994), Snoeijs & Potapova (1995), Snoeijs & Kasperoviciene (1996) and Snoeijs & Balashova (1998).

Pollen and dinoflagellate analyses

Samples for pollen and dinoflagellate analyses were processed following the guidelines of Berglund and Jasiewiczowa (1986). Dinoflagellate cysts were enumerated alongside pollen counts. Identification of dinoflagellate cysts follows the morphology depicted by Rochon et al. (1999). More than 700 palynomorphs were counted at each level. The abundance of species is expressed as relative to a sum of at least 500 terrestrial pollen grains. Zonation of the pollen diagram is based on terrestrial plants by applying numerical treatments in the Tilia computer program (Grimm 1988).

Macrofossil analyses

Macrofossils and macroscopic charcoal were extracted from contiguous slices representing 1 cm interval in the Färsksjön core. Subsamples of 50 mL bulk sediments were taken and then rinsed using a jet of water through a 200 μm sieve. The residue was sorted under a dissecting microscope, and all identifiable remains were picked out and then stored in plastic vials. Macrofossils were identified to the lowest possible taxonomic level with the aid of published keys (Martin & Barkley 1961; Tomlinson 1985) and modern reference collections at the Department of Geology, Lund University. All macrofossils were counted, and their frequencies are presented as concentration per unit volume. After identification and when a sufficient amount was available, terrigenous plant macrofossils from certain levels were submitted for radiocarbon dating.

Table 1. Sediment stratigraphy of Färsksjön, Core 1. Depth below Sediment lake level (cm) units

Sediment description

250–388 388–395 395–410 410–501

9 8 7 6

501–509 509–627 627–631 631–646 646–650

5 4 3 2 1

Dark green-brown fine-detritus gyttja; very homogeneous Coarse-detritus algal gyttja; slightly laminated Slightly light green-brown fine-detritus gyttja; upper boundary gradual (>1 cm) Grey-green slightly laminated fine-detritus gyttja; 443–450 cm and 490–501 cm: brownish bands; 455–457 cm and 466–467 cm: dark bands of algal gyttja Dark-green fine-detritus algal gyttja Dark-brown fine-detritus gyttja; very homogeneous; upper boundary rather sharp (100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100

–27.3 –23.4 –21.8 –21.0 –21.0 –21.8 –21.8 –21.8 –27.6 –23.1 –30.1 –21.9 –26.5 –26.5

3820±65 3950±60 4270±60 4470±80 5040±65 5030±80 5290±65 5430±65 5700±70 6000±70 5930±70 6090±70 6220±70 6640±70 6820±75

4180 (4340–4110) 4420 (4490–4290) 4840 (4910–4700) 5190 (5250–4960) 5830 (5880–5690) 5830 (5880–5670) 6100 (6170–5970) 6230 (6280–6100) 6460 (6600–6410) 6820 (6930–6750) 6750 (6850–6670) 6910 (7090–6830) 7160 (7220–7010) 7540 (7580–7450) 7670 (7720–7590)

Archaeological series (S. Håkansson 1972) Lu-563 Layer 7 Charcoal Lu-577 Layer 5 Charcoal

>100 >100

-

4360±75 4610±65

4870 (5040–4840) 5320 (5470–5070)

Olsäng (S. Håkansson 1979) Lu-1461 BP 27 Lu-1462 BP 23

>100 >100

-

5690±70 9150±90

6460 (6570–6400) 10 240 (10410–10190)

Siretorp, Core 2 Geological series (S. Håkansson 1970) Lu-313 46.0 Lu-312 51.0 Lu-311 58.0 Lu-310 69.0 Lu-309 75.0 Lu-308 84.0 Lu-367 88.0 Lu-366 92.0 Lu-307 95.0 Lu-306 98.0 Lu-305 109.0 Lu-304 114.0 Lu-303 127.0 Lu-302 134.5 Lu-269‡ 160.0

Bulk sediments Bulk sediments Bulk sediments Bulk sediments Bulk sediments Bulk sediments Bulk sediments Bulk sediments Bulk sediments Bulk sediments Bulk sediments Bulk sediments Bulk sediments Bulk sediments Quercus charcoal

Peat Sphagnum peat

† Dates rejected as anomalously old; ‡ Date from Unit 1 in Siretorp Core 1.

1σ calibrated age range (yr BP)

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All dates reported here are calibrated radiocarbon years (cal. BP) using the OxCal program. Age models have been constructed for the master cores of the two sites Färsksjön and Siretorp, also considering stratigraphic boundaries. In the Siretorp site, a hiatus between units 5 and 6 is supported by the dating sequence. We have also considered the error of humus contamination mentioned above. We are referring to dates on charcoal from culture layers at Siretorp (Berglund & Welinder 1972), although such are often problematic.

Results and interpretation from Lake Färsksjön Stratigraphy and chronology

The stratigraphic transect at the eastern shore (Fig. 2) reveals a consistent sequence between coring points 2 and 5, easily correlated with the master core described in Table 1. The clay and gyttja clay of units 1 and 2 in the bottom were deposited in the Baltic Ice Lake (Björck 1995). Unit 3 is a thin transitional layer deposited just after the drainage of the Baltic Ice Lake, whereas unit 4 is limnic fine-detritus gyttja deposited in an Early Holocene small lake of Färsksjön. The thin algal gyttja (unit 5) indicates the transition to the brackish fine-detritus gyttja of the Littorina Sea (unit 6), in which the dark bands probably indicate increased algae content (cf. LOI curve of Fig. 4). Units 7–8 are thin gyttja layers deposited at the end of the transgression. Finally the limnic finedetritus gyttja of unit 9 was deposited during the Late Holocene lake phase. Seven AMS radiocarbon dates evenly covering the transgression layer were obtained (Table 2). In contrast to bulk sediment samples (LuA-5430, LuA-5431) in the upper half of the transgression layer, the dates of terrigenous macrofossils (LuA-5283, LuA-5284) are anomalously old, probably due to the introduction of reworked materials. An age-depth model for the transgression layer was constructed on the basis of a combination of bulk sediment and macrofossil AMS dates (Fig. 3).

Fig. 3. Age–depth model for Lake Färsksjön. Calibrated dates are plotted with 2σ standard deviation. Samples LuA-5283 and LuA-5284 are considered too old and not included in the model.

Mineral magnetic and LOI stratigraphy

Relatively high magnetic concentrations, as reflected by χ and SIRM, characterize the fine-detritus gyttja in the lowermost part of the sequence (stratigraphic unit 4),

Fig. 4. Mineral magnetic and LOI stratigraphy of Lake Färsksjön. Two transgression phases are identified and indicated with shaded bands.

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whereas the overlying sediment in general displays lower values (Fig. 4). The low S-ratios ca. –0.6 indicate that the magnetic mineralogy is dominated by magnetite. ARM is a parameter that generally is interpreted to be sensitive to the presence of finer magnetic grains. Assuming no significant contribution of superparamagnetics, χARM/SIRM values exceeding ca. 100I10–5 Am–1 indicate the presence of stable single domain (SSD) magnetite grains with a diameter between 0.03 and 0.05 µm (Maher 1988). Such grains typically characterize stratigraphic unit 6. A finegrained magnetic assemblage in this unit is further supported by (B0)CR values around 20 mT, while (B0)CR values between 40 and 50 mT in unit 4 is indicative of mixture of grain sizes. From this more general picture some deviating but important features can be noted. Clear peaks in magnetic concentrations associated with peaks in χARM/SIRM ratios and very low S-ratios (ca. –0.9) occur around 510 and 390 cm. These peaks are assumed to be a response to the increased erosion of the threshold, bringing fine-grained magnetic materials into the basin at the initial transgression at 7600 cal. BP respectively the regression that finally isolated the basin at 5550 cal. BP. Another two prominent peaks in ARM and SIRM can be noted around 445 and 460 cm. These peaks are also coupled to increased χARM/SIRM values and similar low S-ratios as found in combination with the above mentioned transgression/isolation peak. Also in this case the peaks are interpreted as a disturbance in the sedimentation as a result of mobilizing the sediment in the catchment and/or erosion of the threshold, due to changes in water level. These two peaks embrace a short period with a lower and more stable water level as reflected in the return to lower concentrations in between. The low magnetic concentration values during transgression phases I and II cannot be ascribed to the reductive dissolution of the ferromagnetic grains. This is mirrored in the S-ratio as a change towards higher (less negative) values. The variations in organic content as represented by the LOI coincide with the general litho- and mineral magnetic stratigraphy. The main conclusion is that unit 6 represents a complex transgression with two peaks, 7600–6800 cal. BP and 6400–5600 cal. BP respectively, as indicated in Fig. 4.

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Diatom assemblages

Altogether 16 samples covering the sequence 510 to 375 cm were analyzed. A diatom diagram (Fig. 5) was constructed showing all taxa with more than 2% at any level, excluding the mass occurring genus Fragilaria sensu Hustedt. The Fragilaria species with more than 3% at any level were plotted separately to the right in the diagram, and their relative abundance was calculated based on the sum of all counted diatoms. According to visual inspection of the diagram and a cluster analysis performed including the different Fragilaria taxa, the diatom diagram can be subdivided into three local diatom assemblage zones (LDAZs, Fig. 5). Zone D1 (515–504 cm, 7800–7600 cal. BP). – This zone constitutes two analyzed levels and is totally dominated by freshwater plankton, e.g. Cyclotella radiosa and Aulacoseira subarctica. The diatom assemblage shows that the Färsksjön basin at this time was a small lake isolated from the Baltic Sea. Zone D2 (504–406 cm, 7600–5700 cal. BP). This zone can be divided into three subzones. Subzone D2a (504–468 cm, 7600–6700 cal. BP). – A peak in the abundance of the brackish-water taxon Melosira westii and Fragilaria spp. in the initial stage of the subzone changed to an assemblage dominated by the brackish taxa Achnanthes fogedii and Achnanthes submarina further up. In the initial stage, the highest value of marine taxa reached ca. 6.5%, represented by Fallacia tenera and Fallacia cryptolyra. This subzone reflects the change from a freshwater lake to a brackish embayment connected with the Littorina Sea. Subzone D2b (468–450 cm, 6700–6300 cal. BP). – A peak in the Fragilaria spp. abundance simultaneous with a shift in dominance between A. fogedii and A. submarina separates this subzone from the surrounding zones. According to Denys (1990), mass abundances of Fragilaria seem to be coupled with a high degree of environmental instability, for example in terms of salinity, trophy or climate. In Scandinavia, Fragilaria previously have been used as isolation indicators, but Stabell (1985) points out that large abundances may occur before, during and after

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Fig. 5. Diatom percentage diagram for Lake Färsksjön showing taxa with more than 2% at one level. Fragilaria are excluded from the count, but the most common taxa are plotted separately. A summary diagram of diatoms excluding Fragilaria spp., divided into salinity requirements, is shown to the right.

Fig. 6. Pollen percentage diagram for Lake Färsksjön with selected taxa. Values of species less than 5% are exaggerated 10 times.

isolation from the sea. There is, however, no indication in the diatom assemblage of a change in salinity during this event. An interpretation that a regression occurred is favoured based on the different habitats of the shifting taxa A. fogedii and A. submarina. A. fogedii is possibly epipelic, which means living motile in the uppermost soft sediment in low energy environments, and A. submarina is episammic or epiphytic meaning living attached to sand grains or other plants. The decrease in A. fogedii could suggest a lowering of sea level that could favour the attached A. submarina. Fragilaria is considered as a pioneer and could cope with changes faster than other species. This is visible as peaks in the abundance in connection to the transgression of the lake in Subzone D2a and isolation to a lake in Zone D3. However, the dominating taxon in the two lowermost Fragilaria peaks is not uniform but alternates between Staurosirella pinnata and Fragilaria elliptica aggregate. In the topmost peak Staurosira con-

struens var. venter contribute equally with the two other taxa. Subzone D2c (450–406 cm, 6300–5700 cal. BP). – This subzone has a similar assemblage to the upper part of D2a, and is dominated by A. fogedii and A. submarina. Zone D3 (406–375 cm, 5700–5400 cal. BP). – This zone starts with the third Fragilaria peak and many brackish-freshwater taxa, indicating a change in the environment towards a less marine stage. In the next phase, there are peaks of brackish-water taxa Navicula peregrina, Navicula phyllepta and Cyclotella choctawhacheeana. In the uppermost analyzed levels, there still occur some brackish-water taxa, but the freshwater assemblage e.g. Cyclotella stelligera and Eunotia spp. start to increase. This zone shows the transition from a bay of the Littorina Sea to an almost, but not completely, isolated lake.

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Fig. 7. Plant macrofossil diagram for Lake Färsksjön with number of seeds/fruits etc. per 50 mL bulk sediments. All finds are seeds or fruits unless otherwise indicated.

Pollen assemblage zones

The pollen diagram can be subdivided into three local pollen assemblage zones (LPAZs, Fig. 6). Zone P1 (550–525 cm; 8500–8100 cal. BP). – Pollen assemblages are dominated by Pinus (ca. 35%) and Betula (ca. 30%), with Alnus, Quercus, Ulmus, Tilia and Corylus pollen present in low percentages (