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

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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 pre- served 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 com- bined with palynological results from sediments not earlier recovered and an independent OSL/AMS 14C chronology allow us here to define the Late Quaternary climate-stratigra- phy 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 ten- tatively 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 dur- ing 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.
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