Ice-Free Intervals Continuing Into Marine Isotope Stage 3 at Sokli in the Central Area of the Fennoscandian Glaciations

Bulletin of the Geological Society of Finland, Vol. 79, 2007, pp. 17–39

Ice-free intervals continuing into Marine Isotope Stage 3 at Sokli in the central area of the Fennoscandian glaciations

Karin F. Helmens1)*, Peter W. Johansson2), Matti E. Räsänen3), Helena Alexanderson1) and Kari O. Eskola4) 1) Department of Physical Geography and Quaternary Geology, Stockholm University, 106 91 Stockholm, Sweden 2) Geological Survey of Finland, P.O. Box 77, FI-96101 Rovaniemi, Finland 3) Department of Geology, University of Turku, FI-20500 Turku, Finland 4) Dating Laboratory, University of Helsinki, P.O. Box 64, 00014 Helsinki, Finland

Abstract An unusually long and continuous Late Quaternary sedimentary sequence has been preserved in a sedimentary basin formed in the Sokli Carbonatite Massif in eastern-central Finnish Lapland. A nearly complete sediment recovery from the central Sokli basin combined with palynological results from sediments not earlier recovered and an independent OSL/AMS 14C chronology allow us here to define the Late Quaternary climate-stratigraphy at Sokli and describe in detail the environmental record. Three interstadial intervals of Weichselian age are distinguished that correlate with MIS 5c, 5a and part of MIS 3 in the marine oxygen-isotope record. The interstadials of MIS 5c and 3 age are here defined as the Sokli and Tulppio Interstadials, respectively. The MIS 5a interstadial is correlated with the Maaselkä/Peräpohjola Interstadials of Finnish Lapland, which previously have been tentatively assigned a MIS 5c age. Till beds in the Sokli sequence (deposited during stadials 3– 1) correlate to MIS 5b, 4 and 3/2, respectively. Depositional environments and vegetational changes during the ice-free intervals at Sokli are discussed. The Sokli sedimentary sequence indicates significantly less extensive and more variable ice-cover over Finnish Lapland during the Weichselian than has been earlier suggested based on the long-distance correlation of litho- and bio-stratigraphic fragmentary evidence.

Key words: glacial geology, sediments, stratigraphy, pollen analysis, paleobotany, climate change, interstadial environment, interglacial environment, chronology, Pleistocene, Sokli, Lapland Province, Finland

*Corresponding author email: [email protected] 1. Introduction

The Sokli sequence represents a for Fennoscandia un- 130 ka (103 years). The sediments have been preserved usually long and continuous sedimentary sequence in a small sedimentary basin formed in the Sokli Car- that spans over the Last Interglacial-Glacial cycle and bonatite Massif in eastern-central Finnish Lapland. the Present Interglacial, i.e. representing the last ca. The Sokli basin was situated under the easternmost 18 K. F. Helmens,P.W.Johansson, M.E.Räsänen, H.Alexanderson and K.O.Eskola

LGM 15oo30 15oo30 SOKLI A r c t i c C i r c l e

FINLAND SWEDEN RUSSIA

NORWAY

LGM

Ocean ctic Ar

NORWAY

MIS 3(~50 ka) 1a ice limit Kola LAPLAND2 Peninsula limits Mountains Sokli ice limit eskers B RUSSIA 5c/a 1b ice MIS Moraine5,6 Arctic Circle iki 5d e Fennoscandian d Ve i v i 3 d MIS free t ice i ka) Pudasjärvi m oraine i e M ~34 l White Sea c 3( i End e MIS c limit i ice Gulf of 7 5b 4

SWEDEN S

MIS I Bothnia M

M 100 km G L 4 FINLAND

Fig. 1. Mapofnorthern Fennoscandia with the Sokli basin in eastern-central Finnish Lapland. In addition to the Sokli basin, which is the type locality forthe Sokli and Tu lppio Interstadials,type localities in northern Finland and Swe- den areshown: 1a =Paloseljänoja, 1b =Tepsankumpu (typelocalities forTepsankumpu Interglacial); 2=Maaselkä Interstadial;3=Permantokoski (type locality forPeräpohjola Interstadial); 4=Oulainen Interstadial of Swedish Norrbotten; 5=Onttovaara, Onttoharjut, Takanenmännikkö, Riipiharju (type localities forthe older interstadial); 6=Tärendö Interstadial. 7indicates the Iijoki site (Ukkonen et al, 2000).The ice-limits of the Fennoscandian Ice- Sheet during MIS 4and MIS 2(LGM) aretaken from (Svendsen et al., 2004).Based on the data discussedinthe present paper,possibleice-limits forMIS 5d, 5b and partofMIS 3are indicated. Permanent ice-coverduring MIS 5c and 5a was most probablyrestricted to the Fennoscandian mountains (Lundqvist, 1992).The LGM ice-divide is according to Siegertetal. (1999). Ice-free intervals continuing into Marine Isotope Stage 3 at Sokli in the central area... 19 part of the ice-divide zone of the Fennoscandian Ice- noscandian Ice-Sheet. Further northwards, in Fen- Sheet during the Late Weichselian Glaciation Maxi- noscandia, the record becomes highly fragmented, mum (LGM at 20 ka; Fig. 1). The sequence consists and in the absence of absolute chronologies, the age of fossil-rich lacustrine sediments that alternate with assignments to the stratigraphic fragments that are a series of glacial-deglacial sequences consisting of till found here are based on long-distance correlation of overlain by ice-marginal fluvial sediment. A fluvial bio-stratigraphic data with the northwest European deposit with a large contribution of slope material oc- mainland stratigraphy (e.g. Donner, 1996). curs intercalated near the base of the sequence. High-resolution oxygen-isotope studies of ice cores A first study of the Sokli sedimentary sequence by from Greenland have revealed a higher climate vari- Helmens et al. (2000) had allowed the identification ability for the Weichselian than indicated by the ma- of a series of warm and cold stage intervals. The strati- rine oxygen-isotope record (e.g. Johnsen et al., 2001). graphic sequence presented in Helmens et al. (2000), The predominantly westerly atmospheric circulation however, was in part composite as a result of incom- over western Europe makes it a key area to study the plete borehole recoveries. It is based on a correlation frequent large and abrupt climate changes (referred of litho-/bio-stratigraphic evidence from the central to as Dansgaard-Oeschger (D/O) events) recognized part of the basin, i.e. where the sedimentary sequence in the Greenland records. High climate variability is thickest and most differentiated, with that from for the Weichselian has been inferred from long pol- the basin’s margin. Moreover, with the exception of len records in the Mediterranean region (Allen et al., bracketing, preliminary luminescence dates for the 1999). Yet, northwards, the generally discontinuous Eemian Interglacial, age control was mostly based on nature of sedimentation and repeated erosion com- stratigraphic correlation: the sequence shows for both bined with poor dating control and a scarcity of high- the warm and cold stages of the Weichselian progres- resolution sequences hamper the recognisation of the sively colder conditions as the period evolves. This is D/O climate variability in the continental record. In also registered in the marine oxygen-isotope record addition, the highly fragmented nature of the strati- and the climate/chrono-stratigraphic scheme of the graphic record in Fennoscandia means that still many northwest European mainland. As such, a marine uncertainties exist in the Weichselian glaciation histo- oxygen-isotope stage (MIS) 3 age was tentatively as- ry that has been reconstructed for the Fennoscandian signed to the youngest interstadial interval with ice- Ice-Sheet (e.g. Mangerud, 1991; Saarnisto & Lunk- free conditions at Sokli. ka, 2004). In western Europe, sedimentary sequences that Here we present a new borehole that represents a provide a continuous environmental record for the nearly continuous sediment recovery from the central Last Interglacial-Glacial cycle are rare. Long contin- Sokli basin. The lithological data and newly obtained uous palynological records have been retrieved from palynological data from the sediments that were not southern sites in France (e.g. Guiot et al., 1989). The earlier recovered from the central basin allow us now Late Quaternary climate-stratigraphy of the north- to define the Late Quaternary climate-stratigraphic west European mainland is composite and based on sequence at Sokli and describe in more detail the en- correlation of bio-stratigraphic evidence (e.g. Zagw- vironmental record. The data presented here confirms ijn, 1985; Andersen & Mangerud, 1989). The only the composite stratigraphy as presented in Helmens well-documented site with interstadial deposits of et al. (2000). In addition, an independent absolute Middle Weichselian age (MIS 3) found in sequence chronology is given for the Weichselian sediments, with Early Weichselian Interstadial and Eemian Inter- based on optically stimulated luminescence (OSL) glacial sediments (MIS 5a, c and e, respectively) is the dating, which supports the earlier made stratigraph- Oerel site in northern Germany (Behre, 1989), which ic correlation with the northwest European mainland is situated just south of the LGM limit of the Fen- and the deep sea. We will discuss our results in the 20 K. F. Helmens, P. W. Johansson, M.E. Räsänen, H. Alexanderson and K.O. Eskola context of the existing stratigraphic schemes of Finn- central Finnish Lapland with clear evidence for two ish and Swedish Lapland and the Weichselian glacial or more warm stage deposits preserved in sequence. history of northeast Fennoscandia. The unique preservation at Sokli of 4 nearly complete Furthermore, this paper forms a stratigraphic ba- warm stage deposits predating the Holocene, inter- sis for detailed studies of the Sokli sequence that are layered with mostly till, can therefore not solely be as- presently in progress. These include high-resolution, cribed to specific conditions at the base of the Fenno- multi-proxy analyses (sediment characteristics, pol- scandian Ice-Sheet. len, macro-botanical remains, chironomids, diatoms) An important additional factor in the preservation of the thick and highly differentiated non-glacial sed- of the Sokli sediments is most probably the atypi- iment intercalations. Modern plant/animal-climate cal local bedrock conditions. The Sokli basin (cover- transfer functions will be applied to the fossil data in ing a surface area of ca. 2 km2) is formed in the cen- order to obtain quantitative climate records for a se- tral part of the Sokli Carbonatite Massif. This mas- ries of time intervals of Late Quaternary age. This will sif represents a Palaeozoic (ca. 360 Ma old) carbon- provide a unique possibility to study in detail Late ate-rich magma intrusion of only very limited extent Quaternary climate variability in the northern Atlan- (diameter ca. 6 km) into the surrounding crystalline tic region based on a terrestrial sediment archive. rocks of the Precambian Shield (Vartiainen, 1980). Either a deep weathering depression, or a depression 2. Study area carved out in the relatively soft, deeply weathered and fractured rocks of the central Carbonatite Massif dur- Despite of the fact that during the Weichselian the ing Saalian glaciation (MIS 6, i.e. when the ice-divide Sokli area was repeatedly glaciated by the Fennoscan- zone was situated over northernmost Lapland (Hir- dian Ice-Sheet, till beds and ice-marginal fluvial sed- vas, 1991)), has offered the additional protection for iments deposited during glacial events, as well as la- the sediments again erosion. custrine and fluvial sediments that were laid down The Sokli mire and riverlet at 220 m elevation in the Sokli basin during ice-free intervals, were not (a.s.l.) are surrounded by gently sloping forested eroded. This has allowed the preservation of a sedi- flanks of hills reaching elevations of around 300 – ment sequence that represents 5 major climate cycles 350 m (see map in Helmens et al., 2000). Zonal vege- of Late Quaternary age (described below). tation consists of northern boreal forest with pine (Pi- The Sokli basin (lat. 67°48’N, long. 29°18’E; nus sylvestris), birch (Betula pubescens and Betula pen- Fig. 1) is situated in a zone that stretches over cen- dula) and spruce (Picea abies). The present climate is tral Swedish and Finnish Lapland and onto the Kola cold temperate with a mean July temperature of 13°C Peninsula in Russia, in which a whole array of sedi- and a mean February temperature of -14°C. Annu- ments and landforms predating the LGM is found. al precipitation is between 500 and 550 mm (At- Their preservation is ascribed to limited glacial ero- las of Finland, 1992). At ca. 100 km north of Sokli, sion due to low ice velocities under the LGM ice- the boreal forest gives way to pine forest and then to divide zone of the Fennoscandian Ice-Sheet and/or sub-arctic birch forest. The forest limit at ca. 300 km frozen bed conditions (Kleman et al., 1999; Boul- north of Sokli is formed by the polycormic mountain ton et al., 2001). Central Finnish Lapland holds a birch Betula pubescens subsp. tortuosa. The vegetation distinct concentration of findings of sub-till organic of the tundra region beyond the forest limit is mainly beds (Hirvas, 1991). A close examination of the latter a dwarf-shrub tundra dominated by dwarf birch (Bet- data, however, shows that most organic beds are trun- ula nana) and Ericales (Ahti et al., 1968). cated and are only a few centimetres or decimetres thick, and many occur in a secondary position. Out- side the Sokli basin, there is not any site recorded in Ice-free intervals continuing into Marine Isotope Stage 3 at Sokli in the central area... 21

Fig. 2. Comparison of the new boreholes Sokli A- and B-series with earlier obtained borehole lithologies from the Sokli basin. Boreholes A- and B-series and boreholes 900/901 were taken within a few meter distance from each other. Borehole 52 was taken ca. ten metre to the NE and boreholes 902/905 (from the margin of the basin) ca. thirty metre to the SW of the central cores. For location of boreholes 900/901 and 902/905 see map in Helmens et al. (2000). The Late Quaternary climate-stratigraphic units indicated at the right side of the figure are discussed in the text. 22 K. F. Helmens, P. W. Johansson, M.E. Räsänen, H. Alexanderson and K.O. Eskola

SOKLI B-series (central basin) n (220 m a.s.l., 67°48’N, 29°18’E) MIS) S( ka Europea OI climate- NW ka depth age, in ) mainland Lithofacies Marine (m Depositional Sokli climate/chrono- Core OSL unit Lithology stratigraphy ()age 0 Lithofacies environment stratigraphy

1 U P mire Present Holocene Interglacial 1 2 S/T lacustrine glacio-fluvial 3 Gm ()12 Sm(ng) R D(S)mm Middle-Late 4 Stadial 1 Gm glacial Weichselian 2/3

Q D(S)mm 5

SSim 6 P glacio-lacustrine Tulppio Middle SiCl 3 Interstadial Weichselian 7 Sm glacio-fluvial O GSm 8 48±16 () D(G)m 60 N Gy(S)m glacial Stadial 2 4 9 M D(S)m 74±26 Sm (gelifluction) L Gyl Sil 10 SSil lacustrine Maaselkä 5a K Interstadial (80) Sil 80±26 11 J GSm(ng) glacio-fluvial I SGm CoGm 12 glacial Stadial 3 5b (90) H 13 CoGm Dm G Gy(S)l lacustrine 14 94±16 GSm(ng) fluvial/ F P/Si gelifluction D(S)m Gy(S)l 15 SGm Early Lithofacies codes & log symbols 94±19 E Sokli 5c Gyl Interstadial Weichselian (100) Grain size lacustrine CoG Cobbles and gravels 16 G Gravel D Gyl SG Sandy gravel GS Gravelly sand 17 SiSm S Sand lacustrine/ SiS Silty sand C fluvial SSi Sandy silt 18 SiSm Si Silt CSil CSi Clayey silt Sim SiC Silty clay D(S)m/ 19 GSm Stadial 4 5d P/SG (110) D(G,..)Diamicton (gravelly...) GSm(ng) fluvial/ B Structures & content 20 gelifluction Gy gyttja

diatom gyttja GSm 21 P peat Gym cm clast-supported, massive

22 mm matrix-supported,massive m massive

l laminated (more distinct ) Gym organic laminations 23 Tepsan- (ng) normal grading 5e organic content lacustrine kumpu Eemian A Interglacial (125) plant fragments 24 wood fragments cryoturbation Other symbols 25 warm stage Gym cold stage core gap 26 Clay Silt Sand Gravel

Fig. 3. Simplified lithological column of the Sokli B-series borehole from the central Sokli basin. A first interpretation in terms of former depositional environments is indicated directly to the right, and the OSL dates obtained on the borehole are indicated to the left. The Late Quaternary climate-stratigraphic sequence at Sokli and its correlation with the northwest European mainland climate/chrono-stratigraphy and marine oxygen isotope stratigraphy as indicated at the right side of the figure are discussed in the text. Ice-free intervals continuing into Marine Isotope Stage 3 at Sokli in the central area... 23

3. Materials and methods et al. (2000) distinguished a sequence of pollen assemblage zones, we here introduce a numbering in Boreholes 900/901 and 902/905 in Helmens et al. the pollen zonation (pollen zones I–VI), that follows (2000) represented the most continuous sediment re- the climate-stratigraphic sequence described below. coveries from the central and marginal parts of the A formal description of Biozones will be presented Sokli basin, respectively, that had been obtained so upon completion of the high-resolution pollen anal- far (Fig. 2). Earlier information on the sedimenta- yses that are presently in progress. The pollen data ry sequence, which was in part based on drilling car- corresponding to pollen zones II, IIIa–b, and IVa are ried out in connection with carbonatite prospecting, presented here for the first time. We applied a sim- had been rather fragmented (Ilvonen, 1973a,b; For- ilar sample preparation, microfossil analysis, pollen sström, 1990; Johansson & Räsänen, 1994; Räsänen sum, construction of pollen diagram (e.g. calcula- & Johansson, unpublished data). In the lithological tions of microfossil percentages) and their interpreta- column for the central basin presented in Helmens et tion (which uses modern vegetation-pollen relation- al. (2000), however, a major coring gap separated the ships), as outlined in Helmens et al. (2000). Eemian Interglacial sediments from the first Weichse- The optically stimulated luminescence (OSL) lian interstadial bed (boreholes 901 resp. 900 in Fig. measurements that have provided the ages indicat- 2). This coring gap was mostly closed in the winter ed to the left of the lithological column of Sokli B-se- of 2002 with the coring of boreholes Sokli A- and B- ries core in Figure 3 were made at the Dating labora- series, which were taken at distances of 1–2 m apart tory of the University of Helsinki. The OSL datings from boreholes 900/901. were made on quartz using a SAR protocol (Mur- The Sokli B-series borehole has only minor gaps ray & Wintle, 2000). Sampling was focused on wa- corresponding with the coarsest sediment layers. Its ter transported sediments, e.g. glacio-fluvial or fluvial borehole lithology is shown in more detail in Figure sediments, as these are probably most well-bleached. 3. The sequence of lithofacies units A–U indicated The ages were corrected for water content. A saturat- to the left of the lithological column is distinguished ed water content was assumed to represent a good ap- here to facilitate the description and interpretation proximation of the average water content since time of the borehole’s lithology (see below); formal defini- of deposition, because the Sokli site is situated in a tion of stratigraphic units in the Sokli sedimentary se- depression where today is a peatland, and in the past quence will be presented in a forthcoming paper. Al- repeatedly a lake, and the sampled sediments were though the thickness of the surface peat deposit was all taken below the average groundwater table. The recorded at the coring site, the Holocene sequence rather large error limits on the final dates are main- was not recovered in the coring operation. The coring ly due to the luminescence properties of the analysed was carried out by the Geological Survey of Finland. sediment and the sampling technique (i.e. coring) Use was made of a hydraulic piston corer (GM 100) which resulted in small sample sizes and uncertainties designed by Mr. Seppo Putkinen. Cores were taken in dose-rate determinations. Nevertheless, the dates into a dark coloured PVC tube, 2 m long and 4 cm in are in sequence and can be separated into three age diameter, which was driven by vibration into the sed- groups, which agree with the relative chronology as iment while inserted in a steel tube. The surface peat derived from stratigraphy (see below). deposit in the central basin has recently been collect- AMS 14C ages of > 42,000 yr BP and 42,450 ± ed by hand-coring (borehole 910 in Fig. 2). 3,570 yr BP have been previously obtained from the A simplified pollen diagram with curves of select- uppermost parts of lithofacies units L respectively P ed taxa which summarizes the palynological analy- (Fig. 3) in the parallel borehole 900 (Fig. 4; see be- ses carried out by Helmens et al. (2000) and in the low). Both wood samples (Table 1 in Helmens et al., present study is given in Figure 4. Whereas Helmens 2000) underwent a thorough chemical pre-treatment 24 K. F. Helmens, P. W. Johansson, M.E. Räsänen, H. Alexanderson and K.O. Eskola

Fig. 4. Late Quaternary pollen stratigraphy at Sokli. The simplified pollen diagram summarizes the palynological results obtained in Helmens et al. (2000) and the present study (borehole Sokli A-series). The Late Quaternary climate-stratigraphic sequence at Sokli and its correlation with the northwest European mainland climate/chrono- stratigraphy and marine oxygen isotope stratigraphy are discussed in the text. Ice-free intervals continuing into Marine Isotope Stage 3 at Sokli in the central area... 25 26 K. F. Helmens, P. W. Johansson, M.E. Räsänen, H. Alexanderson and K.O. Eskola to remove possible contaminants. Therefore, the date ropean mainland and marine stratigraphies is sup- for the youngest interstadial sediments was accepted ported by the independent absolute chronology for as a finite age, although it is close to the limit of the the Sokli sequence (Figs. 3 and 4). The lithological radiocarbon dating method (Helmens et al., 2000). and palynological evidence, which allow us to recon- The latter date agrees, within error limits, with the struct the environmental conditions for each climate- OSL date obtained from the same stratigrapic unit stratigraphic unit at Sokli, will be discussed in an in- (Fig. 3). tegrated way, as former changes in depositional environments seem generally to have been closely related 4. Late Quaternary climate-stratig- to changes in vegetation and climate. The discussion raphy and environmental record at makes use of the lithological data presented in Fig- Sokli ure 3 (lithological units A–U) and in part Figure 4, and the palynological data as summarized in Figure The lithological, palynological and absolute chrono- 4 (pollen zones I–VI). For the earliest part of the se- logical data now available from the Sokli basin allows quence (i.e. transition from the Late Saalian to Eemi- us here to distinguish a sequence of Late Quaternary an) and the latest part (Late Weichselian-Holocene climate-stratigraphic units and discuss the environ- transition), reference is made to previously published mental record in more detail than presented in Hel- borehole lithologies. mens et al. (2000). In addition to a correlation with, or reference to, Where possible, based on bio-stratigraphic bio- and litho-stratigraphic data from central and evidence, warm stage units in the Sokli sequence are southern Lapland, and central Ostrobothnia further to correlated with previously named interglacials/inter- the south, the Sokli record will be compared with mor- stadials in central Finnish Lapland. Palynological/ pho-stratigraphic evidence from eastern central Lap- lithological sequences that could correlate with the land. The latter stratigraphy includes a series of sizeable oldest and youngest Weichselian interstadial deposits cross-cutting eskers, several of them till-covered (Fig. 5; at Sokli have not been previously described, and these Johansson, 1995; Johansson & Kujansuu, 1995). interstadials are here defined for the first time. Al- though the number of till beds in the Sokli basin con- 4.1. The Last Interglacial (MIS 5e): forms to the Weichselian till stratigraphy of eastern the Tepsankumpu Interglacial central Lapland (Johansson, 1995), small sample sizes due to our sampling technique (i.e. coring) precludes Drilling carried out by Räsänen & Johansson (un- a detailed study of the glacial deposits in the Sokli ba- published data) indicates for the depth interval 23– sin, and this hampers a direct collection with the till 29 m in the central Sokli basin a series of upward beds found exposed in the surrounding region. Fur- coarsening, parallel and cross-bedded sands to gravel thermore, the oldest cold stage interval of Weichse- on top of diamicton (core 52 in Fig. 2). These sands lian age at Sokli was not characterized by glaciation, and diamicton, which underlie the Last Interglacial but instead is represented by fluvial/lacustrine depos- bed, rest on weathered bedrock at a depth of ca. 30 m its with an arctic pollen content. Therefore, we here according to drilling information presented in Ilvo- introduce a sequence of cold stage intervals for the nen (1973b). These oldest sediments in the Sokli se- Weichselian at Sokli, which we number from young- quence most probably represent the end phase of the est to oldest as stadials 1–4. Saalian Glaciation (MIS 6) and the subsequent tran- In the description of the climate-stratigraphy be- sition towards the Last Interglacial. low, units are grouped into 5 major climate cycles of A distinct interglacial pollen sequence is present in Late Quaternary age, each cycle starting with cold a diatom gyttja bed that as a marker horizon stretch- stage conditions. Correlation with the northwest Eu- es through the Sokli basin near the base of the uncon- Ice-free intervals continuing into Marine Isotope Stage 3 at Sokli in the central area... 27 solidated sediment infill. The interglacial pollen flo- ty Larix sibirica becomes an important component of ra was first noted by Ilvonen (1973a) and the pollen the Russian northern taiga. Sedimentation of diatom sequence was studied in detail by Forsström (1990). gyttja continued during the establishment of boreal The sediments that were analysed in Forsström forest, although near the end of this phase it was re- (1990), however, had been collected with auger from peatedly interrupted by the deposition of sandy and the marginal part of the basin. They not only showed gravelly sediment. An abundance of algae, relatively evidence for a hiatus in the upper part of the intergla- high pollen percentages for the aquatic plant Myrio- cial sequence, but also provided little detailed infor- phyllum, together with a near absence of wetland ele- mation on the sedimentary sequence. ��Helmens et al. ments, indicate that throughout the deposition of di- (2000) tried to collect samples from the diatom gyttja atom gyttja, the Sokli basin was occupied by a rela- bed in the central part of the basin, but at a depth of tivelly deep lake that was surrounded by only a limit- ca. 20 m only reached its upper part (borehole 901 in ed zone of wetland vegetation. Figs. 2 and 4). The early part of the interglacial vege- A marked cooling in climate indicated by a re- tation development was therefore studied in sediment versed, southerly shift in the northern forest limit is cored from the basin’s margin (boreholes 902/905 in registered by the pollen found in sandy gyttja overly- Figs. 2 and 4). The bed was found highly compact- ing the diatom gyttja deposit. Pollen zone Ie suggests ed during the coring operation. The diatom gyttja de- the presence of birch forest with larch and alder. At posit has a thickness of up to several meters (Ilvonen, present, stands of Larix sibirica, Betula pubescens sub- 1973b) and was for an important part collected from sp. tortuosa (mountain birch) and Alnus (Ducheckia) the central basin with improved coring equipment in fruticosa are found near the forest limit in the Russian 2002 (unit A in Fig. 3). Polar Urals (Igoshina, 1963; Flora of northeast Euro- The pollen assemblage in stratified sands direct- pean Russia, 1976). ly underlying the diatom gyttja bed (pollen zone Ia A minimum OSL age of ca. 95 ka (Fig. 3) com- in Fig. 4), which includes high pollen percentages for bined with its stratigraphic position indicate that the tree birch (Betula pubescens/pendula) and dwarf birch interglacial diatom gyttja bed and associated sandy (Betula nana), indicates a regional vegetation that sediment found near the base of the Sokli sequence probably resembled the present vegetation near the most probably represent the northwest European sub-arctic forest limit in Finnish Lapland, i.e. where mainland Eemian Interglacial or MIS 5e dated in the open birch forest passes into shrub tundra. As the marine record between ca. 125 and 110 ka (Martin- sands turn to sandy gyttja and then to massive diatom son et al., 1987). The Last Interglacial has been de- gyttja, the pollen sequence reveals the start of a warm- fined as the Tepsankumpu Interglacial in Finnish La- ing trend that allowed pine forest to gradually develop pland (Hirvas, 1991). The type localities are Tepsan- on the hill slopes surrounding the Sokli basin (zone kumpu and Paloseljänoja (Fig. 1), where the lower Ib). The next phase in forest development shows the respectively upper parts of the interglacial sequence establishment of northern boreal forest, with first have been preserved. The pollen sequence shown at the immigration of spruce (zone Ic) and then that of the latter sites (Saarnisto et al., 1999) is very similar larch (zone Id). The boreal forest with larch seems to to the one described above at Sokli. have had a more open character than the present boreal forest in the Sokli area, allowing Juniperus pollen 4.2. The first stadial-interstadial sequence of to significantly contribute to the NAP. Although the Early Weichselian age (MIS 5d–5c): stadial 4 last interglacial climate had become at least as warm and the Sokli Interstadial as that of today, it might have had a more continental character. The present range of larch starts some 500 A thick sequence of silts and sands was found by Fors km east of Sokli, where with increasing continentali- ström (1990) overlying the interglacial gyttja depos- 28 K. F. Helmens, P. W. Johansson, M.E. Räsänen, H. Alexanderson and K.O. Eskola it along the basin’s margin. These sediments were in- ty of well-preserved moss remains, probably represent terpreted to have a possible glacial origin and their pieces of the eroded surface mat. The ca. 2 m thick pollen content as re-deposited, representing the up- silt/sand sequence that overlies the gravelly deposit per parts of the underlying interglacial bed. The silt/ shows at several depth intervals a distinct, mostly par- sand sequence revealed in its lower part an open tun- allel layering (unit C; see also borehole A-series in Fig. dra pollen assemblage, whereas in the upper part, 4). The silts and sands are interpreted to have been where the silts turn slightly more organic, a birch for- deposited in a more quiet fluvial environment than est assemblage was found. A pollen assemblage sim- recorded in the underlying gravels, or in a shallow ilar to the latter birch forest assemblage was subse- lake environment (late part of stadial 4). Distortion quently encountered by Helmens et al. (2000) in a structures at the top of the first recorded stratified bed thick gyttja horizon in the central basin (borehole might represent disturbance under the influence of 900 in Fig. 3). As also in the central part of the basin cryoturbation. Distinct arctic conditions at the time the gyttja horizon with the birch forest pollen assem- of deposition of the silt/sand sequence are reflected by blage seemed to be separated from the interglacial bed its pollen content (pollen zone II in Fig. 4), which is by a thick sequence of mostly sandy sediment, Hel- very similar as described by Forsström (1990). Open mens et al. (2000) concluded that the sequence out- tundra vegetation with the herbs Artemisia, Caryo- lined in Forsström (1990) mostly probably occurred phyllaceae, Chenopodiaceae, Compositae and grasses in situ, recording a stadial-interstadial sequence. The (Gramineae), as well as shrubs of Betula nana and Sa- sandy sediments had not been cored by Helmens et lix, characterized the regional vegetation in the Sok- al. (2000), but their occurrence in the central basin li area. The representation of the algae Botryococcus in was inferred from earlier obtained borehole informa- the microfossil assemblage points to aquatic condi- tion (Räsänen & Johansson, unpublished data). The tions at the coring site, bordered/surrounded by wet- major coring gap of ca. 5 m that separated the inter- land vegetation with Equisetum. glacial bed from the first interstadial bed in the cen- As the silt/sand sequence becomes more organic at tral basin (i.e. between boreholes 900 and 901, Fig. a depth of ca. 17 m, the pollen record indicates the 2) was mostly closed in 2002 with the coring of bore- start of a warming trend that continues into the over- holes Sokli B- and A-series (Figs. 2–4). The lithologi- lying, over 2 m thick laminated gyttja bed (early part cal and palynological data presented here confirm the of the Sokli Interstadial). Pollen zone IIIa in the or- stratigraphy as presented by Helmens et al. (2000) and ganic silts/sands represents a dramatic decrease in pol- provide detailed information on the first stadial-inter- len percentages of Artemisia. Increased values for tree stadial sequence following MIS 5e. Sediments with a birch (Betula pubescens/pendula) suggest the nearby pollen sequence similar to the oldest interstadial sed- presence of the sub-arctic birch forest limit. The local iments at Sokli have not been earlier described from vegetation near the coring site was enriched with Po- Finnish Lapland, and the first interstadial interval of tentilla, Cyperaceae, Sphagnum and fungi. Tree birch Weichselian age is here defined as the Sokli Interstadi- pollen percentages continue to rise in the lower part al. The first cold stage interval is numbered stadial 4. of the overlying gyttja deposit, and the pollen assem- The early part of stadial 4 is represented by a gravelly blage defined here (zone IIIb) resembles the one re- deposit showing distinct upward-fining trends (unit corded near the end of the Tepsankumpu Intergla- B in Fig. 3). The deposit has a most probably fluvi- cial (zone Ie): birch forest had returned to the Sok- al origin, although the angularity of the gravel frag- li area, in which alder (Alnus) was well-represented, ments suggests a large contribution of slope material and juniper (Juniperus) and fern stands (Polypodiace- that was possibly transported down-slope under the ae) formed important elements in the under-storey. influence of gelifluction. Slabs of organic matter that Pollen zone IIIc in the upper part of the gyttja depos- occur in sandy sediment, and which consist of a varie- it, which is characterized by a distinct increase in Pi- Ice-free intervals continuing into Marine Isotope Stage 3 at Sokli in the central area... 29 nus pollen and a further decrease in dwarf birch (Bet- slopes formed an important contribution, the pollen ula nana) pollen, registers the subsequent approach of record registers a reversed, southerly shift in the birch the pine forest limit, and the recorded pine pollen val- forest limit towards the vicinity of the Sokli basin, ues suggest that pine trees formed part of the regional marked by strongly declining values of pollen of Al- vegetation at Sokli forming a birch-pine forest. Alder, nus and Juniperus and spores of Polypodiaceae, and a juniper and ferns were still well-represented and also distinct increase in pollen values for Betula nana, Sa- larch formed part of the forest vegetation. Since larch lix, Ericales, and Artemisia (zone IIId). As during this is an extremely poor pollen producer (e.g. Ritchie, cooling event also the pine forest limit can be expect- 1974), it is possible that stands of larch were already ed to have shifted to a more southerly position, the present during the preceding interval with birch for- still high percentages of Pinus pollen in zone IIId are est vegetation, as they had been at the end of the Tep- most probably due to an over-representation of long- sankumpu Interglacial. Relatively high NAP values distance transported pine pollen as the vegetation at shown in the birch-pine forest pollen assemblage are Sokli obtained a more open character (Aario, 1940; related to an over-representation of pollen from sedg- Prentice, 1978; Hicks, 1994). Short-term, laminated es (Cyperaceae) from the local wetland vegetation, as sandy gyttja deposition at the end of the Sokli Inter- findings of fungal remains of Gaeumannomyces indi- stadial (unit G) was most probably related to a later- cate the nearby presence of Carex stands (Van Geel et al shift in the local riverlet, as a distinct rise in NAP al., 1983). The climate during the Sokli Interstadial values indicates further cooling to arctic conditions had become nearly as warm as today, as the northern (zone IIIe). limit of pine is presently situated only some 200 km Two OSL ages of 94 ± 19 and 94 ± 16 ka for the north of Sokli. Sokli Interstadial deposit, combined with stratig- The thick gyttja deposit of the early part of the raphy, indicate that the first stadial-interstadial se- Sokli Interstadial is in its lowermost part rythmical- quence recorded at Sokli following MIS 5e most ly interlayered with very thin silt laminea, and if the probably correlates to the northwest European main- laminations have an annual origin (related to spring land Herning Stadial and Brørup Interstadial or MIS snow-melt), then some 900 years of sedimentation is 5d and 5c dated in the marine record at around 110 recorded by unit D (Fig. 3). The gyttja bed becomes and 100 ka, respectively (Figs. 3 and 4). in its remaining part increasingly interlayered with A minor coring gap still separates the Tepsan- sand (and in the uppermost part with some gravel) kumpu Interglacial sediments from the stadial 4 de- and wood (unit E). As the sand/wood deposition oc- posit in the central Sokli basin (see unit B in Fig. 3; curred while the climate became progressively warm- Fig. 2). The sediment, which was lost in the coring er, it was most probably related with a change in local operation, most probably consisted of loose sand or sedimentary conditions. The lake in which the Sok- gravel. Coring information from the marginal part of li Interstadial gyttja accumulated might have formed the basin reveals unit B directly overlying the inter- part of a floodplain and at intervals of high water be- glacial diatom gyttja bed (borehole 905 in Fig. 2). came flooded by the adjacent river, or the coarse- The Tepsankumpu Interglacial, stadial 4 and the Sok- grained intercalations were due to lateral change or li Interstadial can be found as a unit both in the cen- prograding of sand and gravel deposition. tral and marginal parts of the Sokli basin. The occur- The deposition of sands and angular gravel starts rence of the younger interstadial deposits (described to predominate from a depth of ca. 14.5 m upwards, below) is restricted to the deepest central basin. and once again large slabs of organic matter similar as encountered in unit B were deposited at the coring site (unit F). Parallel to this change towards fluvial deposition in which sediment from the surrounding 30 K. F. Helmens, P. W. Johansson, M.E. Räsänen, H. Alexanderson and K.O. Eskola

4.3. The second stadial-interstadial sequence of by the tundra vegetation, and/or its open character, Early Weichselian age (MIS 5b–5a): stadial 3 allowed long-distance transported pine (Pinus) pol- and the Maaselkä Interstadial len to make a significant contribution to the sum of The cooling that started during the late part of the pollen zone IVa (Fig. 4). The algae Pediastrum, Botry- Sokli Interstadial eventually led to the glaciation of ococcus and Spirogyra are well-represented in the mi- the Sokli area by the Fennoscandian Ice-Sheet. The crofossil assemblage and indicate local aquatic con- glaciation is recorded by the deposition of over 2 ditions. Subsequently, a shift in the sub-arctic birch m thick diamict deposit (Fig. 2). The diamicton is forest limit to the vicinity of the Sokli area is regis- in its lower part clast supported (unit H in Fig. 3), tered by decreasing values of Pinus pollen and an in- whereas a sandy matrix dominates in its weakly strat- crease in tree birch (Betula pubescens/pendula) pollen ified upper part (unit I), and the deposit is interpret- (zone IVb). Although aquatic conditions continued ed as representing basal till possibly overlain by abla- to prevail at the coring-site, the microfossil record tion till. The clasts become distinctly more rounded now reveals well-developed wetland vegetation with- and sorted at the base of unit J, and the upward-fin- in the Sokli Basin consisting of Cyperaceae, Equise- ing sediment sequence represented by this unit is in- tum, bryophytes and fungi. A forest limit vegetation terpreted as the subsequent deglaciation phase with most probably continued to characterize the regional ice-marginal fluvial sedimentation. Unit H was only vegetation in the Sokli area during the deposition of partly collected during the most recent coring opera- the gyttja bed (zone IVc), as the fungus Gaeumanno- tion in 2002 (Fig. 3), as the equipment broke down myces indicates that the high Cyperaceae values in the on the coarse basal till. gyttja, which occur at the expense of other NAP ele- The sediments continue to fine up to laminated ments, are derived from nearby Carex stands. The re- sands and silts (unit K) and then to laminated sandy turn of more open vegetation seems to be registered gyttja (unit L) in the overlying sediment sequence, during the latest phase of gyttja deposition by an in- which in borehole 900 attains a thickness of 2,5 m crease of long-distance transported pine pollen. The (Figs. 2 and 4). A thin bed of stratified silts interlay- latter assemblage (zone IVd) has been described based ered with wood found directly overlying the glacio- on the pollen from a thin organic bed, that consists fluvial deposit might have a fluvial origin, but the of laminated gyttja turning to sands interbedded with laminated sands and silts on top, which contain abun- organic silts, and that was found interlayered in the dant macrofossil remains of Equisetum sp, were most overlying, second diamict deposit in borehole 900 probably deposited in a shallow lake environment. (Fig. 2). The thin organic bed has been interpreted The silts directly underlying the sandy gyttja horizon by Helmens et al. (2000) as representing part of the show a similar lamination as unit D near the base of gyttja deposited previous to the diamicton, but which the Sokli Interstadial gyttja deposit, with slightly or- became dislocated during the subsequent glaciation. ganic silts being rythmically interlayered with very The laminated sandy gyttja deposit in borehole Sok- thin silt to fine sand laminea. The contact of the gyt- li B-series (Fig. 2) becomes increasingly interlayered tja horizon with the overlying, second diamicton at with sand and angular gravel before it turns to organ- a depth of ca. 9 to 9.5 m seems gradual in borehole ic sands containing organic lamineae at the base of Sokli B-series (Fig. 3), but is abrupt in borehole 900 the second diamict bed. (Figs. 2 and 4). OSL ages of 80 ± 26 and 74 ± 26 ka for the low- Directly following the deglaciation of the area, er respectively upper part of the interstadial depos- open tundra vegetation prevailed with Betula nana, it, combined with stratigraphy, indicate that the sec- Salix, Ericales, Artemisia, Chenopodiaceae and grass- ond stadial-interstadial sequence of Weichselian age es (Gramineae), as well as lycopods (Lycopodiaceae) recorded at Sokli most probably correlates to the and ferns (Polypodiaceae). A low pollen production northwest European mainland Rederstall Stadial and Ice-free intervals continuing into Marine Isotope Stage 3 at Sokli in the central area... 31

Odderade Interstadial or MIS 5b and 5a dated in the An Odderade age (MIS 5a) for the Peräpohjola In- marine record at around 90 and 80 ka, respectively terstadial, as suggested through correlation with the (Figs. 3 and 4). The interstadial is correlated with the Sokli record, has been first suggested by Forsström Maaselkä Interstadial as defined by Hirvas (1991) for (1988). Forsström (1988) argues that the differenc- central Lapland (see below). The cold stage interval is es in vegetation reflected in the pollen records from numbered stadial 3. the Peräpohjola sites in southern Lapland and the pollen record that was later obtained from the Oulain- en site to the south in central Ostrobothnia (Fig. 1; 4.3.1. Correlation with the composite stratigraphic schemes of northern Finland Forsström, 1982) are too great to be explained in terms of local or regional factors. The pollen record The second Early Weichselian interstadial at Sokli at the Oulainen site reveals a distinct vegetation de- has been correlated by Helmens et al. (2000) with velopment of birch forest to pine forest and back to the Maaselkä Interstadial of central Lapland (Hirvas, birch forest, and the interstadial interval has been de- 1991), which is thought to correlate to the earlier defined as the Oulainen Interstadial and correlated with fined Peräpohjola Interstadial of southern Lapland the Brørup Interstadial (MIS 5c; Donner et al., 1986; (Korpela, 1969). The pollen sequence at Maaselkä Donner, 1988). According to Forsström (1988), the (Fig. 1) is recorded in sands and gyttja overlying till. long phase with pine forest during the Oulainen In- A dominance of birch pollen combined with a high terstadial does not conform to the small amounts of NAP is interpreted as representing a birch forest veg- pine pollen in the birch forest pollen assemblage of the etation in the vicinity of the northern forest limit Peräpohjola sediments found only a few hundred kil- (Donner, 1995). A more open vegetation both at the ometers to the north. Therefore, it was suggested that beginning and the end of the interstadial is suggested the Oulainen and Peräpohjola Interstadials do not rep- by a slight increase in long-distance transported pine resent the same time interval, but the Oulainen sed- pollen. At Permantokoski (Fig. 1), the most represen- iments with the pine forest pollen assemblage would tive site and later defined as the type locality for the correlate to the Brørup and the Peräpohjola sediments Peräpohjola Interstadial (Donner et al., 1986), a sev- holding a sub-arctic birch forest assemblage correlate eral meters thick sand deposit overlain by an up to ca. to the slightly colder Odderade Interstadial. 0.5 m thick peat horizon occurs interlayered in-be- In later publications, however, Forsström (1991) tween two till beds. The pollen sequence in the fine- considers the possibility that the deposits in north- grained sediment intercalation at this more souther- ern Finland that have been interpreted as represent- ly located site, which is monotonously dominated by ing an interstadial interval may instead represent the birch, is interpreted to represent birch forest. A peak later stages of the Eemian Interglacial, but were re- in long-distance transported pine pollen is recorded deposited in the course of Late Weichselian deglacia- at the bottom of the pollen sequence. tion. Variations in fabric of associated till beds are ex- The Peräpohjola and Maaselkä Interstadials have plained in the paper by changes in ice-flow direction been originally correlated with the northwest Euro- during deglaciation. Forsström (1991) suggests that pean mainland Brørup Interstadial (MIS 5c; Korpela, major part of Finland was glaciated throughout the 1969; Donner et al., 1986; Hirvas, 1991), and the gla- Weichselian. ciation that is recorded by the underlying till bed with The data from the Sokli basin now supports the in- its NW fabric (Lower Till unit and Till bed III in the terpretation made by Forsström (1988). The first in- till stratigraphies of southern (Korpela, 1969) respec- terstadial of Early Weichselian age (Brørup Intersta- tively central Lapland (Hirvas, 1991)) was thought to dial/MIS 5c) in the Sokli basin, which in the present have occurred during the first cold stage interval of the paper is defined as the Sokli Interstadial, represents a Weichselian (Herning Stadial/MIS 5d). phase with birch-pine forest in central Lapland. This 32 K. F. Helmens, P. W. Johansson, M.E. Räsänen, H. Alexanderson and K.O. Eskola would conform to a time interval with pine forest In the Sokli region in eastern-central Lapland, the at the most southern located Oulainen site in cen- so called ‘Oldest Till unit’ (with NW fabric) has been tral Ostrobothnia (Oulainen Interstadial). The sec- correlated by Johansson (1995) with Till bed III of ond interstadial of Early Weichselian age (Odderade Hirvas (1991). Although some of the oldest eskers Interstadial/MIS 5a) at Sokli most probably corre- that are distinguished in the region might have been lates to the Maaselkä/Peräpohjola Interstadials, show- deposited at the closing phase of the Saalian Glacia- ing the development of birch forest limit vegetation tion (Kujansuu & Eriksson, 1995), most are thought in central Lapland (Sokli and Maaselkä) and birch to have developed following the glaciation that de- forest at the Peräpohjola sites in southern Lapland. posited the Oldest Till unit (Johansson, 1995; Jo- The Maaselkä/Peräpohjola Interstadial represents an hansson & Kujansuu, 1995). These till-covered es- ice-free interval following glaciation of Lapland (gla- kers defined as esker system C (Fig. 5) are composed ciation during stadial 3 in the Sokli basin; Till Bed III of massive remnants of formerly coherent esker ridges and the Lower Till unit in central and southern Lap- showing a NW-SE orientation. Also their compact- land, respectively; Rederstall Stadial/MIS 5b). ed glacio-fluvial sediment reveals a clear reworking by glacial flow.

Fig. 5. Cross-cutting esker Norway systems in eastern-central

Russia Finnish Lapland (Johansson, 1995; Johansson & Kujansuu, Sweden 1995). The till-covered eskers of systems C and B pre- date the Late Weichselian FINLAND Glaciation Maximum (LGM) Nuortti of the Fennoscandian Ice- Sheet, and are in the present paper correlated with the SOKLI Maaselkä and Tulppio Inter- stadials of Early respectively Middle Weichselian age. TULPPIO

67o MARTTI

Kemijoki 20 km

SAVUKOSKI o 29

esker systems: C B A Ice-free intervals continuing into Marine Isotope Stage 3 at Sokli in the central area... 33

4.4. The stadial-interstadial sequence of Middle have not been earlier described from Finnish Lap- Weichselian age (MIS 4–3): stadial 2 and the land, and the interstadial is here defined as the Tulp- Tulppio Interstadial pio Interstadial (Tulppio is the nearest hamlet to the Lithological units M–O (Fig. 3) represent the sec- Sokli basin; Fig. 5). The preceding cold stage inter- ond phase of glaciation of the Sokli area during the val is numbered stadial 2. An OSL date of 48 ± 16 ka Weichselian and the subsequent deglaciation with for the deglacial sediments and an AMS 14C date of proglacial fluvial sedimentation. The till bed has a re- 42,450 ± 3,570 yr BP (corresponding approximate- corded thickness of about 1 m. Palynological analysis ly to 46 ka calendar years) for the uppermost part of has revealed that the organic sediment that is mixed- the silt-clay sequence, combined with stratigraphy, in with the glacio-fluvial sediment in the upper part strongly suggest that stadial 2 and the Tulppio Inter- of unit O has its orgin in the interglacial diatom gyt- stadial correlate to MIS 4 and part of MIS 3 dated in tja bed near the base of the Sokli sequence (Helmens the marine record at around 70 and between 60–28 et al., 2000). The interglacial gyttja bed has the most ka, respectively (Figs. 3 and 4). widespread occurrence of the older organic sediments The large error limits on both the luminescence within the Sokli basin, and therefore is most likely to and radiocarbon date, however, hamper a precise cor- be re-accounted for as re-deposited organic debris. relation of the Tulppio Interstadial with the Middle The glacio-fluvial deposit, which in its upper part Weichselian interstadial sequence in the northwest becomes increasingly parallel layered, is overlain by European mainland climate/chrono-stratigraphy or an up to ca. 2 m thick minerogenic, fining upward the D/O climate sequence distinguished for the Mid- and then coarsening upward, laminated silt-clay se- dle Weichselian in the high-resolution Greenland ice quence (unit P). The silts and clays most probably record. A first comparison suggests that the Tulppio accumulated in a glacio-lacustrine environment, and Interstadial correlates to Interstadial 14 dated at ca. 50 the sediment sequence is interpreted as reflecting a re- ka in the Greenland oxygen-isotope record (Johnsen treating and subsequent approaching ice-front. Wood et al., 2001), as the first warm interstadial (Interstadi- fragments and what seems to be a dropstone in the al 16) following the very cold MIS 4 might not have uppermost part of the glacio-lacustrine deposit sug- lasted long enough for the Sokli area to get deglaci- gest overriding of the vegetation by the approaching ated and for a shrub tundra vegetation to develop on ice-front and subsequent deposition of the plant frag- the deglaciated land. A series of AMS radiocarbon ments in the lake. The contact with the overlying ba- datings of plant macrofossils from the Tulppio silt- sal till of the third glaciation (unit Q) seems gradual. clay sequence are presently in progress, and the sedi- Palynological evidence indicates that during the ments are also being analysed for the possible occur- interstadial interval, the higher, deglaciated land be- rence of tephra to be correlated with the north Atlan- came covered with shrub tundra (pollen zone V in tic marine tephrochronoloy for MIS 5–2 (Wastegård Fig. 4). The shrub tundra vegetation was dominated & Rasmussen, 2001; Rasmussen et al., 2003). by Ericales and was charcaterized by a highly diverse Although sediments with a sedimentary/pollen se- herb flora with a variety of arctic-alpine and heliophi- quence that could correlate to glacio-lacustrine envi- lous, pioneer elements including Caryophyllaceae, ronmental conditions in the Sokli basin and a shrub Rumex-Oxyria, Artemisia, Chenopodiaceae and Poly- tundra vegetation in the Sokli area, predating the gonum viviparum, as well as abundant lycopods and LGM, have not been described from northern Fin- ferns. The lake contained the algae Botryococcus and land, a second phase of glaciation/deglaciation of a was surrounded by a fringe of mire vegetation with proposed Early or Middle Weichselian age is distin- Equisetum, Sphagnum and fungi. guished in the till and morpho-stratigraphy of east- Sediments with a sedimentary/pollen sequence ern-central Lapland (Johansson, 1995; Johansson & similar to the interstadial sediments outlined above Kujansuu, 1995). The deglaciation phase is indicated 34 K. F. Helmens, P. W. Johansson, M.E. Räsänen, H. Alexanderson and K.O. Eskola by the till-covered eskers of system B, which consist ciation, and the Youngest Till unit and eskers without of generally continuous ridges with N/NNE-S/SSW till cover (esker system A) with the last deglaciation orientation (Fig. 5). These eskers occur in the same phase. The eskers of system A (Fig. 5) are tens of me- region, i.e. including Sokli, as where the so called ters high and the most fresh looking with steep flanks ‘Old Northern Till unit’ with its northern till fabric and sharp ridges (Johansson & Kujansuu, 1995). is found. It is possible that during the Middle Weich- They run in a fan-like pattern from the ice-divide selian, the western part of central Finnish Lapland re- area to SE, S and to NE/NNE, and continue all the mained glaciated, as also is suggested by the ice-mar- way to the marginal formations of the Younger Dr- ginal lacustrine conditions at Sokli during the Tulp- yas. Their orientation corresponds to the till fabric of pio Interstadial. the Youngest Till unit. In the till stratigraphy of central Lapland (Hirvas, 1991), Till Bed II is thought to 4.5. Stadial 1 of Middle/Late Weichselian age represent the main Weichselian glaciation. Depend- and the Holocene (MIS 3–1) ing on the situation to the ice-divide, Till Bed II has a WNW-WSW fabric. The youngest Till Bed I is asso- Stadial 1 represents the last phase of glaciation of the ciated with the melting phase of the last glaciation. Sokli area by the Fennoscandian Ice-Sheet. The corresponding till bed reaches a thickness of about 2 m 5. Stratigraphic correlation with in the central basin (units Q–R in Fig. 3). Permanent Swedish Lapland and Weichselian ice-cover started most probably during the later part ice-limits of MIS 3 and lasted into the Early Holocene. The deglaciation of eastern-central Finnish Lapland is dat- The recognition of only one interstadial interval of ed at ca. 10,600 (calender) years ago (Johansson and Weichselian age in the stratigraphy of Finnish Lapland Kujansuu, 2005). (Korpela, 1969; Hirvas, 1991) has for long formed a The sediment sequence that overlies the uppermost major problem concerning stratigraphic correlation till bed in the Sokli basin, which includes glacio-flu- with Swedish Lapland. In the Late Quaternary strati- vial gravels and sands (unit S) fining up to lacustrine graphic scheme of Swedish Lapland, i.e. a region sit- sands and silts (containing abundant macrofossil re- uated closer to the center(s) of ice-sheet build-up and mains of Equisetum sp.; unit T), which then grade final decay in the Fennoscandian mountain chain into the surface peat deposit (unit U), have been de- (Fig. 1), two deglaciation phases are distinguished for scribed for the basin’s margin (borehole 902 in Figure the Weichselian predating the LGM (Lagerbäck & 2). The pollen sequence in the peat registers the grad- Robertsson, 1988). The ice-free intervals are reflected ual establishment of the present-day northern bore- by the sediments and their pollen that have accumu- al forest in the Sokli area and of the modern Carex lated in kettle holes formed in a series of till-covered fen in the Sokli basin (pollen zone VIb in Fig. 4). NW-SE directed eskers in northeastern Norrbotten. The pollen assemblage in sands directly underlying Pollen from two organic-bearing deposits, separat- the surface peat deposit, with high pollen percentag- ed from each other by till, reveal similar interstadi- es for tree birch and dwarf birch, reflects a forest limit al vegetation developments, including open herb and vegetation (zone VIa). An aquatic depositional envi- shrub tundra vegetation being interrupted by an in- ronment is indicated by the presence of the algae Zyg- terval during which patches of tree birch might have nema, and the surrounded wetland vegetation includ- been present in the area forming open park tundra. A ed Cyperaceae, Umbelliferae and Cruciferae. more continental and drier climate is inferred for the Johansson (1995) correlates the Younger Till unit younger interstadial. of eastern-central Lapland (with W fabric) with the The pollen record for the older interstadial interval end phase of the main Middle/Late Weichselian gla- is composite and based on combining data from the Ice-free intervals continuing into Marine Isotope Stage 3 at Sokli in the central area... 35

Onttovaara, Onttoharjut, Takanenmännikkö and Ri- nia (Sutinen, 1984; Fig. 1) probably mark the NE ipiharju sites (Fig. 1; Lagerbäck & Robertsson, 1988). ice-limit of the MIS 5b glaciation, i.e. when Finnish This interstadial has been correlated with the Peräpo- Lapland appear to have been entirely glaciated (see hjola Interstadial of Finnish Lapland (Korpela, 1969) above). The Pudasjärvi moraines mark the southern and the northwest European mainland Brørup Inter- limit of the Lower Till unit in the composite strati- stadial (MIS 5c). The younger interstadial has been graphic scheme of Korpela (1969; Sutinen, 1992). given the local name of Tärendö Interstadial, with the Permanent ice-cover during the interstadial intervals Riipiharju site as type locality, and correlated with the MIS 5c and 5a was most probably restricted to the Odderade Interstadial (MIS 5a). The eskers on which Fennoscandian mountains (Lundqvist, 1992). the two interstadial deposits are found occur associat- The maximum ice-extents during MIS 4 and 2 ed with the so called ‘Veiki moraine’, which as an im- (LGM at ca. 20 ka) in Figure 1 are according to Svend pressive hummocky morainic belt can be traced over sen et al. (2004). Little information exits on ice re- a distance of some 200 km parallel to the mountains treat during MIS 3. Eskers with an approximate N-S in the west (Fig. 1; e.g. Hättestrand, 1998). direction (esker system B; Johansson, 1995; Johans- Our stratigraphy in the Sokli basin now opens son & Kujansuu, 1995) and ice-marginal lacustrine the possibility for stratigraphic correlation between conditions bordered by shrub tundra vegetation at Finnish and Swedish Lapland. According to the Sok- ca. 50 ka (Tulppio Interstadial in the Sokli Basin) are li stratigraphy, the older interstadial deposit in Swed- recorded in eastern-central Finnish Lapland (Fig. 1). ish Norrbotten correlates with the Sokli Interstadial The existence of large ice-free areas in Fennoscan- and not with the Peräpohjola Interstadial. This means dia during MIS 3 has been suggested by radiocarbon that a local name should be given to the older intersta dates from Finnish fossil mammoth bones (Ukkonen dial in Sweden. Instead, it is the younger interstadial et al, 2000). The latter data includes glacially trans- defined as the Tärendö Interstadial that would corre- ported remains of mammoth molar dated at 31,970 late to the Finnish Peräpohjola Interstadial (and the ± 950 yr BP (corresponding approximately to 34 ka later defined Maaselkä Interstadial; Hirvas, 1991). It calendar years) at the Iijoki site in northern Ostro- is very implausible to correlate the Tärendö Intersta- bothnia (Fig. 1). dial with the Middle Weichselian Tulppio Intersta- A widespread occurrence of interstadial deposits dial as defined in the Sokli basin. The latter correla- of MIS 3 age in southwestern Finnish Lapland has tion would mean the seemingly unlikely situation of been recently suggested by Mäkinen (2005). A large park tundra with possible patches of tree birch close series of organic beds with mostly birch-dominated to the mountains in Sweden, and shrub tundra vege- pollen assemblages resembling those of the Peräpo- tation in an ice-marginal environment far to the east hjola and Maaselkä Interstadial deposits, found both at Sokli. Firm correlation, however, is only possible in the lower parts of Till Bed II and between Till Bed after obtaining independent absolute age control for II and the underlying glacio-fluvial formations, were the Swedish sediments. dated by 14C, TL or OSL dating. From the twenty Whereas Swedish Norrbotten appears to have been samples dated by 14C, more than half of the samples glaciated during MIS 5d, as indicated by the eskers gave infinite ages. Finite ages varied from 43 to 53 underlying the older stadial deposit, the Sokli area ka, i.e. at the limit of the method’s range of accuracy, to the east was during this time interval covered with and as the results were obtained from bulk sampled open tundra vegetation (stadial 4 in the Sokli basin). which can easily be contaminated with for instance This would place the maximum extent of the MIS plant roots (with even tiny amounts resulting in con- 5d glaciation in western Finnish Lapland (Fig. 1). siderable too young ages), these dates alone, howev- Pronounced till-covered ice-marginal landforms in er, can not be considered reliable. Organic beds meas- the Pudasjärvi area of Finnish Northern Ostroboth- ured by TL were mostly dated between 37 – 55 ka, 36 K. F. Helmens, P. W. Johansson, M.E. Räsänen, H. Alexanderson and K.O. Eskola whereas OSL dating resulted in a much larger range solute chronology for the sediment sequence are sum- of ages from 41 – 117 ka. TL and OSL dating on the marized in Figure 6a. The recorded ice-free intervals same sediments (i.e. the Kauvonkangas site) gave ages at Sokli during the Weichselian have been marked as of 55 ka (depth 5.5 m) and 57 – 66 ka (2.5 – 3.0 m), hatched intervals in the stacked marine oxygen iso- respectively. At this point we consider it unclear whet tope record indicated directly to the left (Martinson her the differences between the TL and OSL dating et al., 1987). The latter record is a proxy for global results are due to methodological differences or are ice volumes and deep-sea temperature (Shackleton, an indicator of a real age difference between the dat- 1987). Our correlation with the till and esker-stratig- ed samples. Therefore, we consider the apparent MIS raphy of eastern-central Lapland (Johansson, 1995; 3 ages obtained on the interstadial beds of southwest- Johansson & Kujansuu, 1995) is given to the right ern Finnish Lapland as still preliminary until more in Figure 6a. information on the quality of the dates (e.g. assumed The results obtained in Helmens et al. (2000) and sample water content) have been presented. the present study indicate that the non-glacial sed- Extensive ice-front retreat in the eastern sector of iment layers in the Sokli sequence each have a cha- the Fennoscandian Ice-Sheet during MIS 3 has been rateristic lithological and pollen content. The warm recently suggested by Sarala (2005). Based on a com- stage deposits, and the cold stage deposit correspond- prehensive study of the Weichselian stratigraphy, geo ing to the first cold interval of Weichselian age, repre- morphology and glacial dynamics in southern Finn- sent individual and successive developments both in ish Lapland, the latter paper presents two alternative terms of depositional and vegetational change. This interpretations regarding the Weichselian glacial his- indicates that these sediments are not the result of tory of Finnish Lapland. Model 1, which is in agree- re-deposition, but occur in situ, which is a very im- ment with our interpretations, includes a MIS 5b portant conclusion to be drawn for a site which was glacial margin at the Pudasjärvi moraines and a MIS repeadedly glaciated during the Weichselian. The 4 ice extent into Russia. However, we additionally lithological and palynological data allow us to define recognize a distinct phase of ice-front retreat during a sequence of Late Quaternary climate-stratigraph- MIS 3 that left southern and central Finland ice-free. ic units for the Sokli basin, and this data combined Their Model 2 considers the Pudasjärvi moraines and with the independent OSL/AMS14C chronology ob- other ice-marginal formations in western-central La- tained for the Sokli sediments, allow a correlation pland as representing different stages in ice-front re- with the well-dated oxygen-isotope climate-stratigra- treat during MIS 3 that left major part of Finland phy of the deep-sea. The Sokli sequence shows both ice-free. We consider Model 1 with in addition ice- for the cold and warm stages of the Weichselian pro- front retreat during MIS 3 as the most plausible mod- gressively colder conditions as the period evolves, in el in the availability of the present data, including the agreement with the deep-sea record. bio-stratigraphic evidence, but conclude that abso- Climate has been the main basis for stratigraphic lute age determinations are necessary for the Pudas- division of Late Quaternary deposits in Fennoscan- järvi ice-marginal formations to come to a final cor- dia, and the climate-stratigraphic sequence present- relation of data. ed here provides us with an important stratigraphic framework for the detailed studies of the Sokli sed- 6. Conclusions iments that are presently in progress. A division of the Sokli sedimentary sequence into allo-stratigraph- The Late Quaternary climate-stratigraphy at Sok- ic units, Biozones, and eventually possible chrono- li in eastern-central Finnish Lapland, the associated, stratigraphic units to be applicable to the climate re- main environmental changes both in the Sokli area gion of eastern Fennoscandia (see also Gibbard & and within the Sokli basin, and the independent ab- West, 2000), will be our next stratigraphic goal. Ice-free intervals continuing into Marine Isotope Stage 3 at Sokli in the central area... 37

Deep-sea Sokli Deep-sea Finnish Lapland (Eastern-Central Finnish Lapland)

YPE ENT NT NM IO UENCE Y AT ENVIRO SEQ VEGET KLI T. TIGRAPH SO E- A FOR ION AT Y E ASIN RE a TE-STRA REGIONAL DEPOSITIONALBASIN ka OSL REG n k OP IB IA in E i AIN cal. TIGRAPH ISOTAGES CLIMASOKL MAINSOKL M SOKLI AGE14 C TILL/ESKER-STRASOKLI TILL/CLIM AG ST IN IN IN ~ ; IN STRA

1 Present northern Present boreal forest mire Interglacial 10.5 Esker System A Interglacial 2 Youngest Till Till Bed I

permanent glaciation Younger Till Stadial 1 ice-cover Till Bed II 3

Tulppio shrub tundra ice-marginal 46 lake IS 14 Interstadial 48+16 Esker System B (Greenland) 4 Stadial 2 per. ice-cover glaciation Old (NW Europe:) northern Till Odderade Maaselkä sub-arctic lake with 74+26 Interstadial 5a Interstadial birch forest limit wetland 80+26 Esker System C 5b Stadial 3 per. ice-cover glaciation Oldest Till Maaselkä/ Brorup 5c Sokli birch-pine/ lake in 94+16 Peräpohjola Interstadial Interstadial pine-birch forest floodplain 94+19 Interstadial 5d Stadial 4 open tundra river/lake Till Bed III 5e Tepsankumpu northern lake Tepsankumpu Interglacial boreal forest Interglacial 6 A B

Fig. 6. The stacked marine oxygen isotope record for the Late Quaternary (Martinson et al., 1987), with indicated intervals of ice-free conditions (hatched intervals) in Finnish Lapland during the Weichselian (MIS 5d–2) according to A: the stratigraphy in the Sokli basin, and B: the composite stratigraphic schemes of Korpela (1969) and Hirvas (1991). Summarized are also past environmental conditions at Sokli and the till and esker-stratigraphy in the Sokli region (Johansson, 1995; Johansson & Kujansuu, 1995).

Our study of the Sokli sediments leads to a ma- central Lapland by Johansson (1995) and Johansson jor departure from earlier reconstructions regard- & Kujansuu (1995), indicating two phases of glaci- ing the glacial history of northeastern Fennoscan- ation-deglaciation during the Weichselian predating dia. Earlier studies based on long-distance correlation the LGM (Fig. 6a). of litho- and bio-stratigraphic fragmentary evidence had suggested that Finnish Lapland was ice-covered Acknowledgements for a major part of the Weichselian (Korpela, 1969; Hirvas, 1991; Fig. 6b). Deglaciation was thought We kindly thank Mr. Hannu Hirvasniemi and Mr. to have occurred during the Maaselkä/Peräpohjo- Seppo Putkinen (Geological Survey of Finland) for la Interstadial which had been tentatively correlated coring operations, Mrs. Raija-Liisa Huttunen (Uni- to MIS 5c. However, the long and near continuous versity of Oulu) for preparation of pollen samples, Sokli sequence indicates that the area which corre- and Mrs. Viena Arvola (Geological Survey of Fin- sponds to the easternmost part of the Late Weichse- land) for the drawing of Figures 1 and 3. Coring was lian ice-divide zone of the Fennoscandian Ice-Sheet financed by the Geological Survey of Finland. Thank became glaciated for the first time only during the is also due to the Arctic Centre (University of Lap- second cold stage of the Early Weichselian (MIS 5b; land, Rovaniemi, Finland) for providing the logistic Fig. 6a). Thereafter the area became ice-free twice. support to make a detailed description and multi- First during the Maaselkä Interstadial (dated to MIS proxy sampling of the Sokli A/B -series boreholes. 5a). The youngest interval of ice-retreat most proba- Dr. Matti Saarnisto, Dr. Pertti Sarala and an anon- bly occurred during an interstadial of Middle Weich- ymous reviewer are thanked for their comments on selian age (MIS 3). The stratigraphy in the Sokli basin the manuscript. conforms to the till and esker-stratigraphy of eastern- 38 K. F. Helmens, P. W. Johansson, M.E. Räsänen, H. Alexanderson and K.O. Eskola

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