The last interglacial in northernmost Sweden

Quaternary International, Vol. 10-12, pp. 173-181, 1991. 1040-6182/91 $0.00 + .50 Printed in Great Britain. All rights reserved. (~) 1992 INQUA/Pergamon Press Ltd

T H E L A S T I N T E R G L A C I A L I N N O R T H E R N M O S T S W E D E N

A n n - M a r i e R o b e r t s s o n Geological Survey o f Sweden, Box 670, 751 28 Uppsala, Sweden

An outline is given of the present knowledge about the last interglacial in northernmost Sweden. Until now only one site, Leve~iniemi, has been found with deposits covering the whole interglacial cycle. The Leve~iniemi sediments have been thoroughly investigated by means of different biostratigraphical methods including micro- and macrofossils. Two other localities with Eemian deposits, K~itkij~irvet and Seitevare, are also described and discussed. At K~itkij~irvet the main part of the interglacial is represented, while at Seitevare only the later part is present. The climate during the last interglacial was more favourable than during the Holocene in the same area. Plant and insect remains suggest a more continental (higher summer temperatures) character to the climate. At K~itkij~irvet, open vegetation prevailed during the interglacial, but the forest limit was possibly located at a higher altitude than today. Coniferous forests including pine and spruce were growing closer to the site during part of the interglacial. Correlations with northern Norway and Finland are discussed.

30* INTRODUCTION

The present knowledge about the palaeoenvironment in northern Sweden during the Eemian Inter- glacial is still limited. Up to now most information has. been obtained f rom one important site, namely Leve~iniemi in northern Lapland (Lundqvist, 1971). There, a till-covered basin was found, containing freshwater gyttja and peat covering a whole interglacial cycle. Since then Leve/iniemi, has been the only site in Sweden where a more or less complete interglacial sequence has been documented.

A large amount of new information about the Pleistocene stratigraphy in the nor thernmost latitudes of Scandinavia, called Nordkalot ten, was gained in connection with the Nordkalot t project (Nordkalot t Project , 1986a,b; Hirvas et al., 1988). This was a joint venture between the Geological Surveys of Norway, Sweden and Finland, and comprised different aspects of the geology for the three countries north of 66 ° latitude (Hirvas et al., 1988).

The Pleistocene stratigraphy in nor thernmost Sweden has been examined and documented in about 700 machine-made pits and more than 100 boreholes. Some 150 of these sites were presented in the Nord- kalott maps (Nordkalot t Project , 1986a). Two sites, K~itkij~rvet and Seitevare (Fig. 1), were found to include interglacial deposits. The Quaternary deposits often displayed two or more different till beds some- times intercalated by water-laid fine-grained sediments. The till beds represent different Weichselian stadials and ice-flows with till-covered, more or less organic bearing sediments, that proved to represent two (Early) Weichselian interstadials. The environmental conditions and their development during the interstadials of the latest glaciation have been reconstructed (Lagerb~ick, 1988a,b; Lagerback and Robertsson, 1988).

10 ° 20*

70*

68*_

66*

SWEDEN 64*

NORWAY

30*

\ 20*

0 300 km

FIG. 1. Location map with the interglacial sites described. K = K~itkij/irvet, L = Leve~iniemi and S = Seitevare.

PRE-WEICHSELIAN DEPOSITS

In the Swedish part of the Nordkalot t area 18 localities with till beds interpreted as pre-Weichselian have been observed and documented (Nordkalott Project, 1986a). The older till is resting on chemically weathered bedrock at Vahtanvaara , ca. 20 km NNE of Levehniemi. Fabric analyses of the older till indicate

173

174 A.-M. Robertsson

that westerly or northerly ice flows dominated during the deposition of the till beds, but the observations are too few to allow any conclusions about regional patterns (Fig. 2, Hirvas et al., 1988; Nordkalott Project, 1986b).

The site at Seitevare (Fig. 1) contains an organic layer suggested to have been deposited during the Eemian Interglacial. Below the thin peat layer a till bed was observed (Robertsson and Rodhe, 1988). It was a gravelly-sandy greyish till, resting on unweathered granitic bedrock, on which no striations were found. The till is suggested to have been transported from the northwest, but other directions cannot be excluded. The petrographical composition of this oldest till is not very different from the younger till beds occurring above the interglacial sequence (Robertsson and Rodhe, 1988, Fig. 3). Pre-Weichselian till beds have also been identified at sites in northernmost Finland and Norway (Nordkalott Project, 1986a; Olsen, 1988, 1989).

Few observations exist of pre-Weichselian till beds and ice flow directions in northernmost Sweden (Fig. 2), and the rest of the Nordkalott area. This supports the evidence for a very active early Weichselian glaciation (Hirvas et al., 1988; Lagerback, 1988a; Lagerback and Robertsson, 1988; Olsen, 1988).

SEQUENCES CORRELATED WITH THE EEMIAN

Leveiiniemi The site (Latitude 67”38’N, Longitude 21”01’E, Fig.

1) is presently located in the northern boreal coniferous forest region, with pine, spruce and birch as the dominant trees. The present day mean July temperature is 13°C.

The stratigraphy includes the following main units from the surface downwards (Lundqvist, 1971; Garcia Ambrosiani, 1990): 1.0-1.5 m peat, Holocene; 3.0-4.0 m till, upper bed, transported from southwest; 0.2-0.5 m sediments, minerogenic fine-grained, glaciofluvial or glaciolacustrine; 3.0-4.0 m till, lower bed, transported from northwest; 0.5-1.5 m organic sequence: peat and lacustrine gyttja sediments; and bedrock, weathered.

The Weichselian till beds and the intercalated interstadial sediments, above the interglacial sequence, were studied by Garcia Ambrosiani (1990), who corre- lates the intertill glaciofluviaYlacustrine sediments with an Early Weichselian interstadial, most probably the Perapohjola (Brorup).

The interglacial organic layers correlated to the Eemian were investigated by means of macrofossils, plant remains and insects, and microfossils, pollen and diatoms (Tralau, Lindroth and Coope, Robertsson and

9 SO

FIG. 2. Pre-Weichselian ice-flow directions in the Nordkalott area (Nordkalott Project, 1988b, map sheet 5:4).

The Last Interglacial in Nor thernmost Sweden 175

Miller in Lundqvist (1971) as Appendices 2, 3, 4 and 5). Identified carpological remains (mainly seeds and fruits) provided the following results (Tralau in Lund- qvist, 1971, p. 68):

(1) Some of the species occurred far beyond their present northern range limits, e.g. Potamogeton friesii, Carex pseudocyperus and Calla palustris;

(2) plants with a modern southeastern and southern distribution, such as Lysimachia thyrsiflora were present;

(3) the general climatic conditions under which the fossil flora grew were more favourable than today at the same latitude;

(4) the fossil flora seems to have its modern equivalent in the Baltic coastal areas of northern Sweden and Finland (Fig. 3); and

(5) the aquatic species found indicate a eutrophic lake with alkaline water (Potarnogeton friesii, P. spp., Myriophyllum spicatum, M. alterniflorum).

Identified beetles gave the following information on the Eemian climatic conditions (Lindroth and Coope, in Lundqvist, 1971):

(1) The majority of the species observed are foreign to the area today;

(2) the climate during the interglacial optimum was more temperate than during the Holocene optimum and had a pronounced continental character with warmer summers; and

(3) the 14 species identified can only be found

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Beetles:

Present occutence of the 14 species identif ied in the Leve~lnlerni semplas

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Macrofossll plant remains:

/ . ~ Phytogeographlcal modern f - - - - equivalent to the fossil

~ - ~ aquatic flora

FIG. 3. The modern equivalent areas in Fennoscandia to the beetle fauna and aquatic flora identified in the Leve~iniemi interglacial

deposits.

together today in inner central Finland (Fig. 3), where the mean July temperature of today is 15°C or 2 ° higher than at present for Leve~iniemi.

The pollen-analytical results can be summarized as follows (Robertsson, in Lundqvist, 1971):

(1) Six pollen zones a-f, were identified, reflecting a whole interglacial cycle (Fig. 4): Zone a; the initial phase with herbs, shrubs, birch, pine; Zone b, c; the climatic optimum with birch, pine, alder and possibly some hazel; Zone c, d; soil degeneration (podsoliza- tion), start of climatic deterioration, immigration of spruce; Zone e, f; final phase with increasing erosion and solifluction, birch and pine predominate.

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FIG. 4. Arboreal pollen diagram from Leve~iniemi, section C Robertsson, in Lundqvist , 1971, Fig. 4:1).

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(2) The forests were dominated by birch, pine, alder and spruce. During the climatic optimum some stands of hazel could have existed in sheltered positions, indicating more favourable conditions than during the Holocene optimum in the same area.

(3) A rich aquatic flora is represented in the pollen flora with Potamogeton, Typha-Sparganium, Nuphar, Nymphaea and Myriophyllum. Leaf tips of Ceratophyl- lum were also identified.

The composition of the diatom flora (nine diatom zones were identified) gave information about the aquatic changes in the basin during the Eemian (Miller, in Lundqvist, 1971). The main conclusions drawn on the basis of the diatom analytical results were: (Fig. 5)

(1) The basin was a small, shallow freshwater lake; (2) eutrophic conditions with alkaline, slightly

brackish, transparent water rich in mineral salts, prevailed during the beginning of the interglacial;

(3) after the climatic optimum a poor diatom flora indicates drainage of the basin (lowering of the water table) and accumulation of drift sediments;

(4) acid water with oligotrophic-dystrophic conditions during peat formation;

(5) the terrestrial phase exposes acidic conditions and a diatom flora of low diversity; and

(6) the interglacial ends with a moister phase when an admixture of acid and alkaline diatoms occur (barren soils, erosion ?).

In total the following main conclusions were drawn (Lundqvist, 1971, pp. 22-24): Summer temperatures at Leve~iniemi were probably 1-2°C higher than today, but the mean annual temperature was possibly as much as 4 ° higher than today in the same area.

L E V E ~ N I E H I : I n t e r g t a c i a t C

8. FALKENSTR~H o.U. HILLER lg70

FIG. 5. Diagram showing the ecological development of the diatom flora at section C, Leve~iniemi. Stratigraphical legend see Fig. 4

(from Miller, in Lundqvist, 1971, Fig. 5:3b).

A continental climate (= warm summers) is indicated by the occurrence of southeastern plant and beetle taxa.

Kiitkiji~rvet The site (Latitude 68°47'N, Longitude 20°48'E, Fig.

1) is located in northwestern Lapland close to a cluster

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FIG. 6. Pollen diagram from K~itkij/irvet. Herbaceous pollen and spores identified are listed in Table 1.

The Last Interglacial in Northernmost Sweden 177

of small pingos on a treeless mountain plateau, about 750 m above sea level (Lagerb/ick and Rodhe, 1985). Today the area is covered by mountain heath vegetation including Empetrum hermaphroditum, Vaccinium myrtillus, Festuca ovina, Calamagrostis lapponica and Betula nana. Along the lake shores and rivers, Salix spp. and Carex spp. occur in plant communities characterized as dry or wet fens (von Sydow, 1983, Andersson et al., 1985). Fens with hummocky vegetation of dwarf shrubs and Sphagnum spp. are also present.

The investigation area is situated 150-200 m above the present tree limit. Mountain birch (Betula tortuosa) is the only tree species growing in the closest forest 9- 10 km to the northeast of K~itkij/irvet. Pine and spruce forests are over 100 km away today. The sequence, sampled by drilling, includes the following stratigraphy (R. Lagerb/ick, pers. commun.): 0-1.50 m: peat, silt, sand (Holocene); 1.50-4.50 m: till, silty-sandy; sediment series: 4.50-5.20 m: silty sediments, disturbed; 5.20-9.55 m: silty sediments, varved, low organic content; 9.55-9.95 m: clay-clayey silt; 9.95-10.20 m: silt with moss remains, increased organic content; 10.20-10.85 m: silt, varved, downwards decreasing organic content; 10.85-11.90 m: silty-sandy sediments, no visible organic remains; 11.90-12.35 m: till, silty, including some organic material; and 12.35-29.90 m: mainly till (not sampled).

A drilling in one of the pingos revealed a stratigraphy containing ice with thin minerogenic layers in the upper part below a cover of 5.5 m till, silt and sand. Downwards more or less pure ice occurs to a depth of at least 15 m below the surface (Lagerb/ick and Rodhe, 1985, Fig. 4). Based on the composition of the pollen flora the sediment sequence (6.5-11.9 m) can be divided into three main pollen assemblage zones (Fig. 6).

The pollen of herbs and spores identified are listed in Table 1. Pollen analysis at 12.35 m indicates that the till contains redeposited pollen.

NAP-Betula-Alnus-Picea (11.85-10.00 m) Pollen of trees, including Picea compose approxi-

mately 50% of the total pollen flora, while herb pollen are frequent along with shrubs. Betula spp., both tree birch and dwarf birch, dominate the tree pollen flora. Alnus (viridis-type) occurs at more than 10%. Herbs are represented by graminids, Cyperaceae and Artemi- sia. Ericales are also registered together with Salix.

NAP-Pinus-Picea (10.00--9.90 m) Pinus pollen increases, Picea is represented at about

3%. Shrubs and herbs constitute about 50% of the total flora.

TABLE 1. Herbaceous pollen and spores identified in the K~itkij~irvet samples

Range of percentage values

POLLEN Gramineae 0.6-21.0 Cyperaceae 4.0-27.5 Artemisia 0.6-5.6 Ranunculaceae 0.2-4.8 Caryophyllaceae including Cerastium- and Sagina-type 0.3-4.8 Saxifraga oppositifolia-type 0.4-4.0 Thalictrum 0.2-2.2 Chenopodiaceae 0.2-2.1 Compositae tubuliflorae including Serratula- and Saussurea-type 0.2-2.0 Rumex - - Oxyria-type 0.2-1.6 Rosaceae 0.2-0.9 Papaveraceae 0.2-0.9 Umbelliferae 0.2-0.7 Compositae liguliflorae 0.2-0.6 Polygonum viviparum-type 0.3-0.6 Brassicaceae 0.2-0.6 cf. Dryas 0.2-0.6 Rubiaceae 0.2-0.5 Linnea 0.2 Gentiana 0.2

SPORES Polypodiaceae 1.1-20.6 Lycopodiaceae 1.0-9.3 Sphagnum 0.7-7.9 Lycopodium selago 0.2-2.9 Selaginella selaginelloides 0.7-2.2 Botrychium 0.2-1.0

A few colonies of the green algae Pediastrum and Botryococcus were also noted. The taxa are not separated in Fig. 6, except for Artemisia. The percentages were calculated on the

total number of terrestrial pollen (trees + shrubs + dwarf shrubs + herbs) counted at each level. The values of spore taxa are based on the pollen sum + spores.


NAP-Betu la (9.90-6.40 m) Pollen of herbs and shrubs dominate. Picea dis-

appears and Alnus decreases. Ericales and Artemisia are represented with up to about 5% each.

The vegetational development in the K/itkij~irvet sediment sequence probably reflects the greater part of the Eemian interglacial cycle. The area was mainly covered by a shrub and herb vegetation. Dwarf shrubs like Betula nana and Salix spp. occurred frequently along with ericaceous taxa. Birch trees may have been present. During the climatic optimum a closer forest limit than today is reflected in the pollen flora by the appearance of spruce and pine pollen which were, however, probably long distance transported. Spruce pollen is not spread over far distances in the same way as pine pollen. This suggests that some smaller stands of spruce may have grown near the site during the interglacial. Pollen identified as Alnus viridis-type probably represents Alnus viridis which today grows in the mountain areas of south central Europe (Fig. 7). The pollen grains of Alnus viridis have less conspicuous arcus, less protruding aspides and thinner exine than A. glutinosa and A. incana. According to these character- istic features and the small size, the A. viridis-type can easily be distinguished from other Alnus pollen (Erdt- man, 1954, p. 70).

Three modern surface moss samples from K/itki- j/irvet were analyzed for their pollen content. Tree pollen constitutes around 40% of the total flora. Pinus reaches maximum 25%, Betula 17% and Picea 1%. Betula nana-type occurs with over 15%, and among herbs Cyperaceae predominates.

Seitevare The site (Latitude 66°58'N, Longitude 18°35'E, Fig.

1) is located in the boreal coniferous zone of central Swedish Lapland, just east of the mountain ranges at about 450 m above sea level. The interglacial organic deposit consists of a thin, very compressed peaty layer only 10 cm thick, covered by a gravelly-sandy diamic- ton, glacially disturbed sediments, and two different till beds (Robertsson and Rodhe, 1988, Fig. 3).

The peat and overlying silty sediments contain a pollen flora that is believed to represent the later part of the Eemian Interglacial. The tree pollen spectra include pine, birch, spruce,, alder and small amounts of hazel (Fig. 8). The vegetation is thought to have been an open coniferous forest, later replaced by birch. The diatom flora identified in the organic layer shows oligotrophic-dystrophic conditions indicative of peaty water in a small shallow basin. The pH-requirements of the diatoms indicate an acid water body (pH around 5).

FIG. 7. Present distribution of Alnus viridis and circumpolar species of Alnus (Hulth6n and Fries, 1986).

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EARLY WEICHSELIAN DEPOSITS

The first Early Weichselian glacier deposited a rather thick till bed, which can be found over large parts of the Nordkalott area (Hirvas et al., 1988). The ice was very active and also left prominent geomorphological features such as eskers and drumlins with a NW-SE direction, illustrating the main ice flow direction during this stadial.

Kettle holes situated along these old eskers in Norrbotten have shown to be important sites for the reconstruction of the Weichselian stratigraphy. In some kettle holes two organic beds separated by a till layer have been documented. Two (Early) Weichselian interstadials are represented and tentatively correlated with Br6rup and Odderade further to the south in NW Europe (Lagerb/ick and Robertsson, 1988). These stratigraphic records are very important as references when reconstructing the palaeoenvironment during the preceding Eemian Interglacial.

The two Weichselian interstadials with local names, Per~ipohjola and T~irend6 (Fig. 9), were characterized by shrub and herb tundra vegetation. Some patches of tree birches probably existed during the Per/ipohjola, the first interstadial. The T/irend6 interstadial shows a more continental climate indicated by the presence of steppe plants (Artemisia, Chenopodiaceae). Open, unstable soils prevailed due to periglacial processes (Lagerb/ick, 1988b).

CORRELATIONS WITH NORTHERN NORWAY AND FINLAND

In northern Norway waterlain till-covered sediments have been found at Vuolgamasjohka (Fig. 9) and tentatively correlated with the Eemian Interglacial (Olsen, 1988, 1989). However, the sediment sequence probably contains only the initial part of the interglacial. Redeposited interglacial fossils (foraminifera, molluscs, palynomorphs) were reported from Weich- selian deposits near Troms6 in northern Norway (Vorren et al., 1981).

Many localities have been documented to include interglacial organic sediments in Finnish Lapland (Hirvas et al., 1981; Hirvas, 1983). Most sequences represent only parts of an interglacial, and probably more than one interglacial is represented. Six pollen assemblage zones were differentiated in the so called Lapponian Interglacial (Hirvas, 1983). At least one site, Nakenavaara is interpreted as pre-Eemian (Hirvas and Eriksson, 1988). One of the most complete Eemian localities, Tepsankumpu, (Fig. 9), displays the same vegetational development as the Leve/iniemi deposits, but the end of the interglacial is not represented (Hirvas, 1983, Fig. 3).

At some of the Finnish sites pollen of Larix (larch) was identified. During the Eemian, larch spread to Finland from the east and probably also reached Sweden (ForsstrOm, 1990). At Sokli a more or less complete interglacial stratigraphy was investigated.

1

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l o

FIG. 9. Sites and provinces mentioned in the text.

However, an hiatus exists in the sequence, and redeposited sediments were discovered (Forsstr6m, 1990). The pollen assemblage zones suggest the following main vegetational phases from the beginning of the interglacial and onwards: birch forest; pine forest; pine-spruce(-alder) forest; spruce-larch forest; - - hiatus - - ; redeposited pollen assemblage zones: open birch-pine-larch forest; tundra with birch; herb and shrub tundra.

The important conclusion is that the vegetational development was similar in northernmost Finland and Sweden at lower altitudes. In the northernmost part of Sweden open vegetation prevailed in the mountain areas at higher altitudes.

SUMMARY

The following development can be outlined for the Eemian Interglacial in northernmost Sweden and Finland.

(1) According to pollen-analytical results the vegetational development was characterized by: initial open vegetation - - herbs, shrubs, birch; mixed forests - - pine, birch, alder, (hazel ?); coniferous f o r e s t s - pine, spruce, larch (in Finland); appearance of long distance pollen of QM, Corylus, Carpinus; open pine-birch forest; and tundra.

(2) Macrofossil remains (beetles, seeds, fruits) at Leve/iniemi suggest a more continental climate during the Eemian than during the Holocene. July mean temperature was probably 2°C higher, and mean annual temperature up to 4°C higher, than today in the same area during part of the Eemian Interglacial.

(3) The environmental changes within the lake basin at Leve/iniemi are reflected by the composition of the diatom flora. Initial eutrophic conditions with a high pH were later replaced by an oligotrophic-dystrophic phase. Paludification and acidification during the later part of the interglacial is reflected by decreasing pH of

The Last Interglacial in Northernmost Sweden 181

the.water . Mixed d ia tom floras finally i l lustrate increasing eros ion and inwash of m i n e r o g e n o u s mater ia l into

the basin at the end of the interglacial . (4) A t K~itkij~irvet, in the m o u n t a i n o u s area of

Sweden, open vegeta t ion prevai led dur ing the ma in par t of the interglacial . Herbs , shrubs, dwarf shrubs and birches p redomina ted . However , the coniferous forests inc luding spruce and pine grew closer to the site than today dur ing part of the interglacial . The forest limit was consequent ly s i tuated at a higher al t i tude.

A C K N O W L E D G E M E N T S

Robert Lagerb~ick, Urve Miller and Lars Rodhe kindly read the manuscript and gave valuable comments. Barbro Landerborg helped with the drawings and Charles Schweger corrected the English language. The original map for Fig. 7 was kindly put at my disposal by Bertil Nordenstam at the Department of Phanerogamic Botany, Swedish Museum of Natural History, Stockholm. To all these individuals I express my thanks.

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The last interglacial in northernmost Sweden