Palaeoenvironment during Preboreal-Boreal in Närke, south central Sweden

QuaternaryInternational,Vol. 27, pp. 103-109, 1995. Copyright © 1995 lNQUA/ElsevierScience Ltd Printed in Great Britain. All fights reserved. 1040--6182/95 $29.00

Pergamon

1040--61s2(~)00067-0

PALAEOENVIRONMENT DURING PREBOREAL-BOREAL IN NARKE, SOUTH CENTRAL SWEDEN Ann-Marie Robertsson

Geological Survey of Sweden, Box 670, S-751 28 Uppsala, Sweden Four sites have been investigated to illustrate the shore displacement and vegetation history during the Yoldia Sea stage in an area east of the Nlrke Strait. Diatom analysis shows that two of the localities at 165-170 m and 147 m altitude were located above the highest shore line formed during the Yoldia Sea stage, and that only freshwater diatom assemblages were registered in the sediments deposited during the Preboreal and Boreal chronozones. Acidification of the water seems to have taken place rather early according to the composition of the diatom flora. The basin Ladfallet at 142 m was situated slightly above or at the Yoldia limit, and might have been temporarily influenced by the Yoldia Sea as the diatom flora contains the slightly brackish littoral taxon Mastogloia smithi v. lacustris Grunow. The basin Skelnabltckakarret at 127 m was isolated from a hay of the Yoldia Sea, which is clearly reflected by the brackish marine diatom flora. The composition of the pollen flora illustrates that the isolation occurred during the Betula-Empetrum pollen assemblage zone. Radiocarbon datings of the sediments deposited at the isolation gave too high ages, but the spread of Corylus and Alnus were dated approximately 9400 and 8500 BP, respectively. The pollen flora at the four sites studied indicates a vegetational succession from open herb and shrub communities with Betula nana and Erapetrum followed by a phase of open birch forests with Juniperus before the immigration of Pinus, Corylus and Alnus.

BACKGROUND

Pollen Spectra and Radiocarbon Datings

Sedimentary sequences in four basins located between 170 and 127 m above present sea level (a.s.1.) have been studied to illustrate shore displacement during the Yoldia Sea stage east of the NArke Strait in south central Sweden (Fig. I(A) and (B)). The diatom flora at the fen Skelnab~ickak~'ret has been described and discussed earlier, as well as the background of the diatom studies (Robertsson, 1990). To date the isolation levels pollen analyses have been carried out, as the radiocarbon datings gave very uncertain and shifting results. The vegetational history of the areas surrounding the four basins show a good correlation when compared. Standard methods of preparations and analyses of pollen and diatoms have been used (Faegri and Iversen, 1975; Miller, 1964). The basic sums for calculations usually were around 300 diatom frustules and 500 pollen grains of trees, shrubs and herbs at each level analysed. The pollen and diatom assemblages are presented in summary diagrams and separate curves for selected taxa (Figs 2-5). The radiocarbon datings have been carried out on bulk samples and are given in years BP (before 1950) uncorrected.

Diatom Assemblages

RESULTS

Majorsmossen The sedimentary basin is now an open mire covered by

Carex and Sphagnum vegetation and situated 165-170 m a.s.1. The lithostratigraphy is: 0-485 cm peat (Sphagnum, Eriophorum, Carex) 485-594 Phragmites peat 594--610 coarse detritus gyttja 610--619 fine deritus gyttja 619-625 clay gyttja-gyttja clay 625silt

Three local pollen assemblage zones (PAZ) were distinguished (Fig. 2). The silt and gyttja clay-clay gyttja up to 619 cm depth contain very few pollen grains, and no qualitative pollen analyses could be carried out. At 616 cm the pollen flora is dominated by Betula pubescens-type, Betula nana-type and Empetrum, and this spectrum is assigned the Betula-Empetrum PAZ. The Betula-Juniperus PAZ illustrates the transition between the open birch woods and the more closed forest occurring in the beginning of the Holocene. The spread of Pinus and Corylus in the area is reflected in the uppermost Pinus-Corylus PAZ, when pollen of shrubs decrease. Three radiocarbon datings of samples between 619 and 594 cm gave 9710 ± 195 BP (ST10766) for the fine detritus gyttja deposited before the spread of Corylus and 9435 ± 100 (ST10767) for the immigration of Corylus. The uppermost age obviously is much too young, 8180 ± 310 BP (ST10768), as Alnus was not present in the pollen flora. The immigration of Alnus probably took place approximately 8500 BP (see below).

Three diatom spectra at 626, 616 and 595 cm sediment show a freshwater flora dominated by the acidophilous taxa Anomoeoneis brachysira vat. zeUensis (Grunow) Krammer andA. vitrea (Grunow) Ross, living in water with a pH below 7, and often with optimal conditions around 5 (Cholnoky, 1968). The genera Eunotia, Frustulia, Cymbella and Pinnularia also occur with several acidophilous taxa. The water of the basin became acidic rather early, and the pH probably sank below 6 already during the Preboreal. The basin was never connected with the Baltic basin after the Late Weichselian deglaciation.

103

104

A.-M. Robertsson

FIG. I(A). Paleogeographicalmap of the Yoldia Sea stage in south central Sweden with areas above the Yoldia shore line shaded (modified after Florin, 1977, Fig. 1). The Narke province is marked by the broken line. Yoldia diatom floras earlier investigated and discussed are marked with crosses, e.g. Kilsbergen (Florin, 1944), map area Link6ping NO, north of Lake Roxen (Fromm, 1976), the Rejmyre area (Persson, 1979) and Kolm~den (Florin, 1977).

FIG. I(B). Sites investigated in the area covered by the Quaternary map Finsp~ngNV (Bj6mbom, 1989).

Ostra TorrtaUemossen The site consists o f an open oligotrophic fen covered by Sphagnum spp. and Phragmites. Parts o f the mire are developed as a bog where Pinus and ericaceous shrubs are the main components o f the present flora. The threshold of the basin has been l e v e l l e d to 147 m a.s.l. The lithostratigraphy is: 0--300 cm peat (Carex, Sphagnum, Phragmites) 300-320 gyttja 320-335 clay gyttja

335-342.5 342.5-

gyttja clay silt

Pollen spectra and radiocarbon datings Seven pollen spectra are presented in Fig. 3 and three P A Z have been separated. The l o w e r m o s t spectrum, distinguished as the Empetrum-Betula PAZ, is dominated by Empetrum and Betula pubescens-type, but also Betula nanatype and Salix occur rather frequently. In the Corylus-Pinus P A Z the i m m i g r a t i o n o f Corylus is reflected and Pinus

Palaeoenvironment during Preboreal-Boreal in Niirke, South Central Sweden

105

POLLENFLORA

i o

-!

:':

i

O

DIATOMFLORA

Corylus

[ ' - - " ] Herbs

i

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620-

//

mn¢~e~t ~o%

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fine detritus Oytt~

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.

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.610 &

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.620

sE~

PLModlo

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FIG. 2. Pollen and diatom flora in the Preboreal sediments from Majorsmossen.

OSTRATORRTALLEMOSSEN(threshold147m) POLLENFLORA

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i

_

p~u¢

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o Corytus

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.

,

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u

DIATOMFLORA

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Shrubs

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acidophilous

/ / k~ltnerem / t/ "lkaliphllOu$

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FIG. 3, Pollen and diatom flora in the Late Preboreal-Boreal sediments from Osla'aTorrt~lemossen.

tt.Mod~

106

A.-M. Robertsson

increases, and pollen of shrubs and herbs decrease. Above 320 cm the spread of Alnus is registered in the gyttja. Six radiocarbon datings have been carried out on gyttja samples between 335 and 295 cm. The lowermost dating 8865 _ 160 BP (ST10769) marks the internal after the immigration of Corylus but before the spread of Alnus, which has been dated to occur after 8770_ 175 BP (ST10770) but earlier than 8450 _+110 BP (ST10771). The uppermost three datings represent sedimentation after the immigration of Alnus, 8340 _ 95 BP (ST13010), 8140 _+ 85 BP (ST10772) and 8030 _+ 90 BP (ST13001).

Diatom assemblages Five diatom spectra in the silt, clay gyttja and gyttja expose a freshwater flora dominated by acidophilous and pH-indifferent taxa. The genera Eunotia, Anomoeoneis, Pinnularia and Achnanthes are most common and the composition of the flora is in general the same as in the sediments from Majorsmossen. Acidophilous taxa constitute approximately 50% of the total flora in the upper part of the gyttja layer. The basin was obviously not connected with the Baltic during the Yoldia Sea stage.

Ladfallet The site is an open mire with a vegetation dominated by

Carex and Sphagnum spp. The threshold of the basin has been levelled to 0-390 cm 390--471 471-475 475-479 479-

142 m a.s.1. The stratigraphy is: peat (Sphagnum, Eriophorum, Carex) gyttja clay gyttja silt clay

Pollen spectra and radiocarbon datings The pollen spectrum in the clay gyttja at 474 cm has been assigned the Betula-Empetrum PAZ, as Betula pubescenstype reach 50% and Empetrum pollen is represented by 5%. Herb pollen includes Cyperaceae, Gramineae, Rumex, Artemisia and Filipendula. At 470 cm Betula has a maximum of 68% and pollen of shrubs are represented by

Juniperus, Betula nana-type and Salix. The Pinus-Corylus PAZ reflects the increase of Pinus and the immigration of Corylus. The vegetation was still open and competition of light and space not so intense since Hippophae shrubs were growing around the basin. The Alnus-Pinus PAZ reflects the spread of Alnus and a closer forest vegetation. Three radiocarbon datings were performed on bulk samples of clay gyttja and gyttja between 475 and 455 cm. All ages are too old according to the composition of the pollen flora, 11,530 _+ 400 ( S T I l l 2 1 ) , 10,395 + 240 (STl1122) and 10,235 _+ 115 (ST11123).

Diatom spectra Ten samples were analysed for their content of diatoms, but only four levels contained enough frustules for quantitative analyses. The diatom flora is poor in taxa and all spectra are dominated by Navicula diluviana Krasske, which is described from alkaline Late Pleistocene sediments (Florin, 1970) and was observed by H~tkansson in slightly brackish littoral deposits accumulated at or after the isolation from the Yoldia Sea (Svensson, 1989, p. 122). The taxon is frequent in sediments accumulated in a shallow bay after the isolation from the Yoldia Sea in northern Estonia (Lepland et al., 1995). N. diluviana has been described from freshwater floras in Byelorussia, where it was very common

LADFN..LET ( ~ e h e ~ 142 rn) P(X.I.IEN FLORA 0

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440.

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FIG. 4. Pollen and diatom flora in the Preboreal and Boreal sediments from Ladfallet.

107

Palaeoenvironment during Preboreal-Boreal in Niirke, South Central Sweden

and even has mass occurrence in some basins and is a main constituent of the Late Pleistocene and Preboreal sediments (Khursevich and Loginova, 1980). The deposition of clay gyttja at LadfaUet probably took place when the shore line of the Yoldia Sea was very close to the basin as the halophilous diatom Mastogloia smithi var. lacustris Grunow reaches 20% of the flora (Fig. 4). The occurrence of M. smithi var. lacustris could also indicate alkaline conditions with the presence of unleached soils around the basin. Anomoeoneis brachysira vat. zellensis and A. vitrea are other taxa noted at the same level, and together they constitute 23%, probably indicating inflow of acidic water into the basin. At 450 cm the diatom flora consists of 85% Eunotia, Pinnularia and Anomoeoneis illustrating a shallow lake with a pH markedly below 7 (Cholnoky, 1968). Skelnabi~ckakarret The basin is a mire covered by Pinus, ericaceous shrubs and Sphagnum. The threshold has been levelled to 127 m above sea level. The stratigraphy is: 0-300 cm peat (Sphagnum, Carex) 300-340 gyttja 340-345 clay gyttja 345-350 gyttja clay 350clay Pollen spectra and radiocarbon datings Nine samples were analysed between 360 and 320 cm representing three local PAZ (Fig. 5). The clay and gyttja clay contain a pollen flora dominated by Betula (Betula pubescens- and nana-type) and Empetrum, which represent the Betula nana - - Empetrum PAZ. The pollen frequencies were low indicating a sparse open shrub and herb vegetation on the islands recently exposed after the deglaciation of the area (Fig. I(B)). The herb pollen noted includes Artemisia,

S I ( E L ~ E T

~

Thalictrum and Rumex. The Betula-Juniperus PAZ (345-330 cm) represents an open light demanding (heliophilous) vegetation where Hippophae was a component of the flora. The Corylus-Pinus PAZ reflects the spread of Corylus and increase of Pinus into the area. Two datings were carried out giving the ages 10735 + 275 BP (STl1124) and 9735 + 200 BP (STll125). The composition of the pollen flora confirm that the dates represent too high ages, as pollen of Corylus is present reflecting the vegetation existing around 9500 BP and later. Diatom spectra The results of the diatom studies have been presented and discussed earlier (Robertsson, 1990). The clay was deposited in a brackish marine littoral environment as the diatom flora is composed mainly of Nitzschia navicularis (Br6bisson) Grunow, N. punctata (W. Smith) Grnnow, N. obtusa W. Smith, Diploneis smithff (Br6bisson) Cleve (including var. pumila (Grunow) Hustedt). These taxa have been reported from sediments supposed to have been deposited in the Yoldia Sea in central Sweden (Florin, 1944, 1977; Fromm, 1976; Persson, 1979). However, further to the southeast in eastern Sm~and and on the island of Gotland the Yoldia Sea sediments contain a brackish water Mastogloia flora (Svensson, 1989) dated ca. 10,000- ca. 9600 BP. CONCLUSIONS The local pollen assemblage zones and regional vegetation history of the area studied and the sedimentary environments according to the diatom assemblages are presented in Fig. 6. The sites Majorsmossen at 165-170 m and Ostra Torrtallemossen at 147 m are located above the highest shore line, which in the study area was formed during

127 m)

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t>" FIG. 5. Pollen and diatom flora in the Preboreal and Early Boreal sediments from S k e l n a b ~ c k ~ t .

108

A.-M. Robertsson

REGIONAL VEGETATION

cloeed formlt$ with

klijorsmossen 165-170 m a,s.l.

Local PAZ

D e ~ l mwiror~nent

pine. alder and birch pine, birch ancl hazel fore~s

Pmu$ Co~us

oligowoph¢ like~

open birch forays with iuolPer$ a n d

Bstuli -

low pH, she P,o w water,

tea bu~thom

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(sParse vegstmional cover)

Juniperus Bstula Empetrum

baein a b o v e hiQhest Yoldia Emit

Ottra Tot rlalemo$$en 147 m a.~L

Ladfallit 142 m a.s.I

Lokal PAZ

Depo~ionat environment

Local PAZ

Pinu$ Alnus

oligot rophic like, low pit.

Pinu$ Alnus

Cory~u$ Pinu$ Erngetrum Bstuli

shallow water, basin above highest Yoldia limit

Skelnab~ckakJirrst 127 m &sJ

OegolitKmal environmemt

lake

Local PAZ

Betuli -

lemporarily connected with the

Empstfum

Yoldia S e a ?

alkaline fre#nwater

Bom=t - 9 0 0 0 BP,

PiNU$ Corylus

Pinus Corylus

CHRONOZONES

Oepo~tional erwironmemt

lake

Late - 95oo B.P.-- P r e b o r e a l -

Bet u l i Jun~lrus Betuli nar~ Empstrum

Early isolation bay of the marine-brackish Yoldia Sea

FIG. 6. Local pollen assemblagezones and summary of regional vegetationhistory, and the depositionalenvironmentsin the basins studied according to the diatom assemblagesidentified.

the Yoldia Sea stage (Agrell, 1979). The diatom flora in the sediments from these two basins shows a c o m p o s i t i o n d o m i n a t e d b y a c i d o p h i l o u s and p H - i n d i f f e r e n t taxa indicating that the p H dropped below 7 rather early after deglaciation. The rapid acidification of the water could have been caused by the spread of Empetrum heaths, where an acid humus l a y e r was formed. The b e d r o c k o f the area consists o f o l d e r granites, m e t a v o l c a n i c s and C a m b r i a n sandstones ( W i k s t r t m , 1989), which have resulted in s a n d y - s i l t y tills o f acidic character. The e x p a n s i o n o f Empetrum heaths was c o m m o n in large areas of northwestern Europe at the transition from the Pleistocene open pioneer vegetation to birch forests (Berglund, 1966, 1976; Florin, 1969). The highest shore line in the study area was estimated to about 140 m a.s.l. ( B j t m b o m , 1989), and it has been determined as the upper limit o f wave-washing to about 143 m a.s.l., 20 k m to the east of the area (Lindtn, in press). The vegetation which i m m i g r a t e d on the islands emerging after deglaciation consisted o f herbs, some of them representing Late G l a c i a l flora elements e.g. Artemisia, Thalictrum and Rumex. S i m i l a r plant c o m m u n i t i e s have been described from the archipelago of Kolmfirden (Florin, 1977) situated 7 0 - 8 0 k m to the east (Fig. I(A)). Juniperus seems to have been an important component of the flora during the transition from open herb and shrub tundra to forest vegetation. A Betula-Juniperus P A Z has been identified on Mt. Billingen to the south (Digerfeldt, 1977; Hedin, 1985; B j t r c k and Digerfeldt, 1986) and in Kolmfirden to the east (Florin, 1977). The basin Ladfallet with a threshold at 142 m a.s.1, was probably situated very close to or slightly above the Yoldia sea shore line according to the d i a t o m flora. The alkaliphilous freshwater taxon Navicula diluviana has a mass occurrence and must have been living in the basin as the diatoms are very well preserved and appear as whole frustules. The presence o f Mastogloia smithi var. lacustris indicates that the basin could t e m p o r a r i l y have been influenced by slightly b r a c k i s h water. A n alternative possibility is that the presence o f unleached soils supported the presence of alkaliphilous and halophilous diatom taxa, The high frequencies o f acidophilous Anomoeoneis taxa possibly reflect inflow o f acidic water from the surroundings o f the basin. Mastogloia smithi var. lacustris occurs in the Betula-Empetrum P A Z which confirms that the clay gyttja

was deposited during the Early Preboreal and the Yoldia Sea stage, and that the radiocarbon datings gave too high ages. The basin Skelnab~ckakiirret was situated b e l o w the highest shore limit and deposition took place in a shallow bay o f the Y o l d i a Sea. The d i a t o m s reflect a littoral environment as epiphytic and benthic taxa but no plankton were registered. The results o f the pollen analyses confirm that the isolation o f the basin took p l a c e during the Betula-Empetrum P A Z in the Early Preboreal chronozone. ACKNOWLEDGEMENTS H/ikan Modig, Geological Survey of Sweden, carried out diatom analyses of material from two of the sites. Valuable comments on the manuscriptwere given by Urve Miller, Jan Risberg, Anders Lindtn and Lars Brunnberg. The investigations were financially supported by the Documentation Programme at the Geological Survey of Sweden. The radiocarbon datings were made at the Laboratory of Isotope Geology, Museum of Natural History in Stockholm. The drawings were made by Barbro Landerborg, and the English language was checked by Francis Markey. I am very grateful to all persons involved. REFERENCES Agrell, H. (1979). The Quaternary history of the Baltic, Sweden. In: Gudelis, V. and KOnigsson,L.-K. (ads), The Quaternaryof the Baltic, pp. 219-239. Acta Universitatis Upsaliensis, Symposia Universitatis Upsaliensis Annum QuingentesimusCelebrantis: 1. Berglund, B.E. (1966). Late-Quaternary vegetation in eastern Blekinge, south-eastern Sweden. A pollen-analytical study. I. Late-Glacial time. OperaBotanica, 12(1), 1-180. Berglnnd, B.E. (1976). The deglaciation of southern Sweden. Presentation of a research project and a tentative radiocarbon chronology. In cooperation with L. Bjelm, S. Bjarck, G. Digeffeldt, S. H/tkansson,A. Hilldtn, G. Knutsson, E. Lagerlund and B. Ringberg. University of Lund, Department of Quaternary Geology, Report 10, pp. 1~7. Bj6rck, S. and Digerfeldt, G. (1986). Late Weichselian-Early Holocene shore displacement west of Mt Billingen, within the Middle Swedish end moraine zone. Boreas, 15, 1-18. Bjfrnbom, S. (1989). Beskrivning till jordartskartan Finsp~ing NV. Description to the Quaternary map FinspfingNV. Sveriges geologiska unders#kning, Ae 92, pp. 1-45. Cholnoky, B.J. (1968). Die OkologiederDiatomeen,pp. 1--699.Verlag von J. Cramer. Digerfeldt, G. (1977). The Flandrian development of Lake Flarken. Regional vegetation history and palaeolimnology.University of Lurid, Department of Quaternary Geology, Report 13, pp. 1-101. Faegfi, K. and Iversen, J. (1975). Textbookof PollenAnalysis, pp. 1-295. Munksgaard, Copenhagen. Florin, M.-B. (1944). En sensubarktisk transgression i trakten av stdra Kilsbergen enligt diatomesuccessionen i omrAdets h6gre bel~igna fornsjtlagerfOljder. Geologiska F#reningens i Stockholm Fi~rhandlingar, 66, 417-448. Florin, M.-B. (1969). Late-Glacial and Pre-Boreal vegetation in central Sweden. SvenskBotanisk Tidskrift, 63, 143-187.

Palaeoenvironment during Preboreal-Boreal in NRrke, South Central Sweden Florin, M.-B. (1970). Late-Glacial diatoms of Kirchner marsh, southeastern Minnesota. Beiheft zur Nova Hedwigia, 31, 667-756. Florin, M.-B. (1977). Late-Glacial and Pre-boreal vegetation in southern central Sweden. II. Pollen, spore and diatom analyses. Striae, 5, 1--60. Fromm, E. (1976). Beskrivning till jordartskartan LinkOping NO. Description to the Quaternary map Link6ping NO. Sveriges geologiska undersi~kning, Ae 19, pp. 1-77. Hedin, K. (1985). Biostratigrafisk dokumentation av lagerf6ljder fr~tn Ranstad, Skaraborgs liin. Sveriges geologiska undrsiJkning, unpublished documentation report, pp. 1-13 and 9 Figs. Khursevitch, G.K. and Loginova, P. (1980). The fossil diatom flora of Byelorussia (the systematic review) (in Russian). Academy of Sciences of the Byelorussian SSR, Institute of Geochemistry and Geophysics, pp. 1-120. Lepland, A., Sakson, M. and Miller, U. (1995). Palaeoenvironmental conditions during the Yoldia Sea stage in the Tallinn area, Northern Estonia. Quaternary International (this volume).

109

Lind6n, A.G. (in press). Beskrivning till jordartskartan Finsp~mg NO. Description to the Quaternary map Finsp/mg NO. Sveriges geologiska undersOkning, Ae 118. Miller, U. (1964). Diatom floras in the Quaternary of the G~ta River Valley (western Sweden). Sveriges geologiska undersOkning, Ca 44, pp. 1-67. Persson, C. (1979). Shore displacement during Ancylus time in the Rejmyre area south central Sweden. Sveriges geologiska undersOkning, C 755, pp. 1-23. Robertsson, A.-M. (1990). The diatom flora of the Yoldia sediments in the N~ke province, south central Sweden. Beiheft zur Nova Hedwigia, 100, 255-262. Svensson, N.-O. (1989). Late Weichselian and Early Holocene shore displacement in the central Baltic, based on stratigraphical and morphological records from eastern Sm~land and Gotland, Sweden. LUNDQUA Thesis, 25, pp. 1-195. Wikstrtim, A. (1989). Berggrundskartan 9 F Finsp~ng NV. Sveriges geologiska undersOkning, Af 164, Map of pre-Quaternary rocks.