Pergamon

11)4tl-6182(95)00068-9

Quaternary International, Vol. 27, pp. 111-120, 1995. Copyright © 1995 INQUA/Elsevier Science Ltd

Printed in Great Britain. All rights reserved. 104(0-6182/95 $29.00

LATEGLACIAL OF LAKE ONEGA m CONTRIBUTION TO THE HISTORY OF THE EASTERN BALTIC BASIN

Matti Saarnisto,* Tuulikki Gr6nlund* and Ilpo Ekmant *Geological Survey of Finland, 02150 Espoo, Finland

t lnstitute of Geology, Karelian Science Center, 185610 Petrozavodsk, Russia

New stratigraphy data especially diatom analyses are presented from the area north of Lake Onega relevant to the discussion on possible Lateglacial connection between the Baltic Sea and White Sea. Following the deglaciation 12,0(O-11,000 years ago Lake Onega basin was occupied by a system of ice dammed lakes which drained to the Lake Ladoga basin. When the ice retreated from the threshold leading to the White Sea 11,000 years ago the water level dropped and since that the lake was controlled by its thresholds to the White Sea and the Baltic basin. Lake Ortega has remained distinctly above the sea level and no Late Weichselian sea connection existed between the White Sea and Baltic basins. The marine diatoms frequently found in sediments are reworked older fossils as also suggested by some earlier workers. Lake Ortega drained to the White Sea basin over the Maselga threshold and also to Lake Ladoga between 11,000 and 10,000 radiocarbon years ago, and to the White Sea alone between 10,000 and 9500 years ago, and the present outlet of Lake Onega, River Svir, originated 9500 years ago.

INTRODUCTION

The greatest lakes of Europe, Lake Ladoga and Lake Onega in Russia, belong to the Baltic Sea drainage area which is separated from the White Sea drainage area by a

water divide called Maselga (Maaselk~i) (Fig. 1). Both lakes were entirely covered by the Weichselian ice sheet although the outer margin of the glacier was only less than 100 km southeast of Lake Onega. The lakes have a complicated Late Weichselian and Holocene history including great water

F I N L A N D j "

/ V

LZ

NV

R U S S I A

Vp

L. Ladoga

Gulf of Finland =S...~t

8

L,,,,,/. Thresholds .- Main water divide / ' End moraine

200 km I •

FIG. 1. Map of Russian Karelia showing main end moraines according to Ekman and Iljin (1991), low points at the Maselga (Maaselkli) water divide between the Baltic Sea and White Sea (M = Maselga, M - M = Morskaya Maselga) and between Lake Ladoga and Lake Onega (V = Vodlozero, Vieljiirvi), and new stratigraphical sites 1, 3 and 4. Endmoraines: Vp -- Vepsian, Kr = Krestets, LZ = Luga, NV = Neva, R

= Rukajtirvi, K = Kalevala.

111

112 M. Saarnisto et al.

level and size fluctuations as well as changes in drainage routes as a result of the progress of deglaciation and differential land uplift. In addition, Lake Ladoga and Lake Onega are closely linked to the Lateglacial history of the Baltic Sea and the White Sea including the possible connection between these basins.

In the present paper, the results of stratigraphical studies from three small lake basins from the vicinity of the Baltic Sea-White Sea divide north of Lake Onega will be presented. They will contribute to the Late Weichselian and Early Holocene hydrology of the Lake Onega basin as well as to the discussion on the possibility of the White Sea-Baltic Sea connection. First, however, a brief summary is presented of old, highly conflicting ideas of the extent and timing of Lateglacial water bodies along the eastern sector of the Scandinavian ice sheet.

EARLIER STUDIES

The possible connection between the Lateglacial Baltic Sea and White Sea has been debated from the early days of glacial investigation in eastern Fennoscandia (see e.g. Hyv~rinen and Eronen, 1979). The development of ideas is presented in a series of palaeogeographical maps in Fig. 2. In the early palaeogeographical maps the Lateglacial Baltic Sea

(the Yoldia Sea by that time) extended to the White Sea (e.g. Ramsay, 1898). This extension of the Baltic was based, in addition to raised beaches, partly on the distribution of relict crustaceans in the area. The presence of the Baltic Ice Lake dammed above the sea level and preceding the Yoldia Sea was first recognized by Munthe (1910). The Baltic Ice Lake finally drained to the sea level when the ice retreated from the Middle Swedish end moraines and north of Salpausselka II in Finland. On palaeogeographical maps published by Ramsay (1927) and Sauramo (1929, 1934) (Fig. 2(A)) the ice margin during the Baltic Ice Lake and early Yoldia stage is shown continuing from the Salpausselk~is to Russian Karelia. On the maps by Sauramo no connection between the Baltic and White Sea is shown.

The idea of a possible connection was reactivated by Hyypp~i (1936) who suggested that the deglaciation of eastern Finland was such an early event that the Baltic Ice Lake extended from North Karelia farther north and drained to White Sea via Kuusamo and Salla in NE Finland (Fig. 2(B)). Sauramo accepted this concept (e.g. 1940, 1958). It was based on raised shorelines in eastern Finland which had previously been interpreted as shore levels of local ice dammed lakes (Tanner, 1915). The idea of the direct Lateglacial sea connection between the Baltic Sea and White Sea preceding the Baltic Ice Lake gained support during

' 6 ~ o h , . ' '

B, g 2 ~

FIG. 2. Palaeogeographical maps showing the development of ideas concerning the Lateglacial hydrology of eastern Baltic Sea-White Sea areas. (A) Early Holocene Yoldia Sea, according to Sauramo (1934, Fig. 17). (B) Late-Weichselian Baltic Ice Lake, according to Hyypp~i (1936, Fig. 7). (C) Lateglacial Yoldia Sea before the Baltic Ice Lake, according to Sauramo (1958, Fig. 133). (D) Baltic lee Lake, according

to Hyv~irinen (1975, Fig. 2).

Lateglacial of Lake Onega 113

World War II from Finnish geologists who were able to make observations in the Vodlozero (VieljErvi) threshold area between Lakes Ladoga and Onega as well as north of Lake Ortega. The bottom-most minerogenic sediments sampled in the threshold areas contained marine diatoms which supported the view that an open sea connection existed at an early stage (Mtlder, 1944). Hyypp~i (1943) named this sea the "Karelian Ice Sea" and Sauramo (1958) the "Lateglacial Yoldia Sea" (Fig. 2(C)). This postulated sea thus predated the Baltic Ice Lake and was assigned to the Aller~l Interstadial or even earlier.

Early Soviet authors were in favour of this sea connection (e.g. Markov, 1935; Zemljakov, 1936) but in later studies a more cautious view has been taken (e.g. Kvasov et al., 1970; Biske et al., 1974). The marine diatoms frequently found in Lateglacial sediments between Lakes Ladoga and Onega and north of Lake Onega are most probably reworked last interglacial fossils. These diatoms are almost certainly derived from sediments laid down when marine connection existed between the Baltic and White Seas during high sea levels of the Eemian stage. This connection is well established (e.g. Ramsay, 1898; Zemljakov, 1936; Grfinlund, 1991). The present stratigraphical material from the vicinity of the Baltic Sea-White Sea water divide north of Lake Onega was collected in order to solve the nature of the possible Lateglacial connection over the water divide.

STUDY SITES

Three small lake basins north of Lake Onega were sampled through ice in March 1991, using a 50 mm diameter piston corer. Sampling was started from basal sands and it was continued only to the lower part of organic gyttja which was considered as representing the sediment of a small lake or pond isolated from a larger water body. Three identical cores were taken from the sequence which crosses the inorganic/organic sediment contact. The water depth at coring points varied from 2 to 4 m. The cores were visually correlated and sampled for radiocarbon (Table 1), pollen (not reported here) and diatom analyses. Loss on ignition of oven dried samples was measured at +500"C.

Site No. 1. Lake Leukilampi (Ozero Leugii) at 115.3 m a.s.1, and 300 x 400 m in size is situated 2 km south of the main Maselga water divide and the village of Karelsky Maselga between Lake Onega and Lake Segozero. The water

divide is approximately 125 m a.s.1. Samples were taken close to the north shore of the lake.

Stratigraphy: 500-546 cm gyttja, loss on ignition 11.2-20.7% 546-592 gyttja clay, loss on ignition 1.8-5.5% 592--690 laminated sand, base not seen. Site No. 2. Unnamed lake at 104.0 m 5.5 km west of the

main Maselga water divide which is situated at the Lake Onega-White Sea kanal at 110-115 m a.s.1, south of the village of Morskaya Maselga. The lake measures 100 × 500 m and the samples were taken close to its north-west shore.

Stratigraphy: 700-716 cm gyttja, loss on ignition 7.4-16.1% 716-732 gyttja clay, loss on ignition 3.3-5.4% 732-750 clay, loss on ignition 0.9-2.4% 750-770 clay gyttja, loss on ignition 1.4-3.3% 770-790 sand, base not seen, loss on ignition

0.1-0.3%. Site No. 3. Unnamed lake at 154.2 m a.s.1. 2 km SW of

Lake Upper Volozero. The lake is surrounded by mires, and it measures 200 x 250 m. The samples were taken closer to the west shore. The site was chosen as stratigraphically representative outside the Neva-stage end moraine zone and well above the main Maselga water divide, and the highest shoreline.

Stratigraphy: 500-520 cm 520-544 544-560 560-570

gyttja, loss on ignition 7.7-45.0% gyttja clay, loss on ignition 3.6-5.3% gyttja, loss on ignition 10.2-40.2% sand, base not seen, loss on ignition 0.9%.

Site Nos 1 and 2 are close to the lowest points, and a little below, the Baltic Sea-White Sea water divide. The other critical thresholds are between Lake Onega and Lake Ladoga: the thresholds north of Lake Vediozero (Vieljarvi) (Fig. 1) at approximately 115 m a.s.l, and River Svir, the present outlet of Lake Onega at 32 m a.s.1.

COMMENT ON L I T H O S T R A T I G R A P H Y

In Site No. 2 near Morskaya Maselga and Site No. 3 near Lake Volozero a layer of less organic sediment, in fact almost pure silt and clay is present in the lower part of the organic gyttja or clay gyttja, which rests on bottom sand. In Site 2 this minerogenic layer was deposited between 10,830 and 10,180 radiocarbon years ago and in Site 3 its end is older than 10,060 and beginning older than 10,190 but

TABLE 1. Radiocarbon ages of the samples from the study sites

Lab. No. Material Depth 14C age ~13C Site (cm) (yr BP) (%,)

1. Lake Leuldlampi Su-2132 Gyttja clay 545.5-547.5 8790 ± 80 -30.0 Su-2133 Gyttja clay 552-554 9360 ± 80 -30.9

2. Unnamed lake Su-2137 Gyttja 710-713 10,250 ± 100 -26.4 Su-2138 Gyttja 713-716 10,180 ± 100 -27.0 Su-2139 Gyttja clay 749-752 10,830 ± 110 -26.5

3. Unnamed lake Su-2134 Gyttja 516-519 10,060 ± 100 -29.2 Su-2135 Gyttja clay 544-547 10,190 ± 90 -30.1 Su-2136 Gyttja 557-560 10,530 ± 100 -28.4

4. Harjunsuo, Vielj~irvi Su-2418 Clay gyttja 5125-517.5 10,040 ± 80 -32.5 Su-2417 Sandy gyttja 517.5-522.5 10,020 ± 80 -32.3

114 M. Saarnisto et al.

younger than 10,530 years. These dates suggest that the minerogenic layers represent the Younger Dryas Stadial. Especially in Site 3 the changes in lithostratigraphy are clear from highly organic gyttja with loss on ignition up to 40%, to clay gyttja with loss on ignition of only 3-5%. In the upper part of the Younger Dryas sequence more organic material is present in both lakes. Both the lower and upper boundaries of the sequence are distinct in both sites. The studied sequences in both sites represent the time period from about 11,000 years ago to a little less than 10,000 years ago. Site 2 near Morskaya Maselga was a bayment of larger body of water whereas Site 3 was a small lake. Therefore the organic content of their sediments is different.

The lithostratigraphy of Lake Leukilampi, Site 1, indicates a lowering water level from a large water body influenced by glacial meltwater (laminated sand and silts) to a closed small lake before 9360 years ago.

DIATOM STRATIGRAPHIES

The organic matter was removed for diatom analysis by bleaching in H202 for 24 hr at 500C followed by several washings, and the mineral matter by repeated suspension and decantation. Whenever possible, at least 500 diatom valves were identified in each subsample (Gr6nlund, 1994).

Site No. 1. Lake Leukilampi Diatoms were studied from 19 sampling depths between

500 and 680 cm (Fig. 3). The abundance of diatoms was low at all depths except the two uppermost ones, where it was high, and 560 and 600 cm, where there were over 200 diatom specimens per slide. A total of 80 taxa of 29 diatom genera were named from the sequence.

Both freshwater and saline water diatoms were met with in the laminated sand at the base of the sequence. The latter include Paralia sulcata (Ehrenberg) Cleve, P. sulcata var. sibirica Grunow, P. sulcata var. crenulata (Grunow) Frenguelli. According to Loseva (1990), P. sulcata var. siberica is more coarsely built than the nominate variety, with a broad edge that appears dark in LM, and P. sulcata var. crenulata has the mantle external specific thickenings that make the valve outline appear wavy. Thalassiosira sp., Hemiaulus sp. and Stephanopyxis sp. were also found. Fragments of diatoms probably deriving from saline water species were also encountered but could not be named. Dictyocha fibula Ehrenberg, a saline water silicoflagellate, was also found.

In the very uppermost part of the fine sand (600 cm) the abundance of diatoms is distinctly higher (216 taxa per slide) than elsewhere in the sand sequence. The majority of the species are freshwater diatoms, with alkalibiontic Epithemia adnata (Kiitzing) Rabenhorst clearly dominant (68%). Other species are Cocconeis placentula Ehrenberg, C. placentula var. euglypta (Ehrenberg) Grunow, Amphora libyca Ehrenberg, Stauroneis phoenicenteron (Nitzsch) Ehrenberg and Eunotia spp. (8.8%). P. sulcata, Hyalodiscus scoticus (Kiitzing) Grunow and Stephanopyxis turris (Greville) Ralfs, and fragments of Stephanopyxis and Hemiaulus( ?) are saline water species.

There are only a few diatom specimens in the gyttja clay excluding the depth of 560 cm, from where 396 diatoms were named, all of them freshwater species. Ellerbeckia arenaria (Moore) Crawford is dominant at this depth (83%). E. arenaria is aerophile in biotype but also grows in the littoral of different water systems, on sandy beaches in particular. It is typical in Lateglacial sediments (cf. Alhonen, 1968). It belongs also to the Ancylus diatom assemblage that grew in the Holocene Ancylus Lake of the Baltic Sea (e.g. Alhonen, 1979). The E. arenaria species found were rather worn. Of the other species, Gyrosigma attenuatum (Kiitzing) Rabenhorst, Navicula diluviana Krasske, N. farta Hustedt and Achnanthes lanceolata var. elliptica Cleve were the most abundant.

The upper part of the gyttja is characterized by alkaliphilous indifferent diatom flora, in which Fragilaria and Achnanthes genera are well represented. F. construens (Ehrenberg) Grunow, F. construens var. binodis (Ehrenberg) Grunow, F. construens var. venter (Ehrenberg) Grunow and Achnanthes levanderi Hustedt are the dominant species. Navicula schoenfeldii Hustedt, N. laterostrata Hustedt and N. vitabunda Hustedt are also quite common.

Site No. 2. Unnamed Lake Diatoms were studied from 12 sampling depths (700-

785 cm) (Fig. 4). They were found to be fairly abundant at all depths excluding the two lowermost ones. Altogether 100 taxa of 33 genera were named (Gr6nlund, 1994).

The lowermost fine sand contained mainly freshwater diatom such as Aulacoseira lirata (Ehrenberg) Ross, Cymbella aspera (Ehrenberg) Peragalli, Eunotia robusta Ralfs, Stauroneis phoenicenteron and Surirella robusta Ehrenberg. Saline water species include Paralia sulcata, P. sulcata var. sibirica and Stephanopyxis turris. Halophile Surirella turgida Smith and Anomoeoneis sphaerophora (Ehrenberg) Pfitzer are also present. The diatom flora of the gyttja clay/clay/gyttja clay deposit midway in the sequence is dominated by Ellerbeckia arenaria (34-77%). Other freshwater diatoms are Cymbella aspera, Stauroneis phoenicenteron and Pinnularia species. Aerophile Hantzschia amphioxys (Ehrenberg) Grunow and Pinnularia borealis Ehrenberg are at their most abundant in the clay in the middle of the deposit. Diatoms indicating saline water are fairly common. These include Paralia sulcata, P. sulcata var. sibirica, Podosira stelligera (Bailey) Mann, Melosira nummuloides Agardh, Melosira moniliformis (MUller) Agardh, Diploneis bombus (Ehrenberg) Cleve, Grammatophora oceanica, Stephanopyxis turris and Hyalodiscus scoticus. In addition, fragments of diatoms were encountered of which some probably belong to species of the genus Hemialus.

Species of Fragilaria, most commonly F. construens, F. construens var. renter and F. pinnata, are dominant in the gyttja in the upper part of the sequence. Achnanthes species, such as A. minutissima Kiltzing, A. levanderi and A. lanceolata (Brebisson) Grunow are also abundant.

Site No. 3. Unnamed Lake Diatoms were studied from 10 sampling depths (500-

Lateglacial of Lake Onega l 15

115.3 m a.s,l.

5.0-

DIATOMS OLIGOHALOBOUS

I

I ~ Q

- 114 x <

and I

POOR IN DIATOMS

5.5

POOR IN DIATOMS

6.0

6.5

POOR IN DIATOMS

6.9 % 20 20 4(} 60 80 1010 20 40 60 80 20 41310 20 40 513 20 40 6010101010101010

Anal. T GrOnlund

FIG. 3. Diatom diagram of Site No. 1, Lake Leuldlampi. Selected species. Sediment symbols - - see Fig. 4.

565 cm) (Fig. 5). Apart from the lower part of the gyttja clay and the uppermost gyttja, the abundance of diatoms was low. Altogether 85 taxa of 27 genera were named (Gr6nlund, 1994).

A few freshwater and saline water diatoms were found in the sand. The lowermost gyttja contains only a few diatoms, including aerophile Aulacoseira epidendron (Ehrenberg) Crawford, Hantzschia amphioxys and Pinnularia borealis. Paralia sulcata and var. sibirica were also found. The freshwater diatoms of the genera Pinnularia and Navicula are abundant in the gyttja clay deposit. The assemblage includes Pinnularia viridis (Nitzsch) Ehrenberg, P. gentilis (Donkin) Cleve, Navicula bacillum Ehrenberg, N.

amphibola Cleve, N. gastrum (Ehrenberg) Ktitzing, Stauroneis phoenicenteron and Stauroneis acuta Smith. Aerophile Pinnularia borealis, Hantzschia amphioxys and Aulacoseira epidendron are also fairly common (4.5-10.3%). Marine diatoms, such as Hyalodiscus scoticus, Paralia sulcata, P. Sulcata var. sibirica, Grarnmatophora oceanica Ehrenberg, Campylodiscus echeneis Ehrenberg and fragments of Stephanopyxis sp., are also encountered.

The uppermost gyttja contains only freshwater diatoms. Fragilaria construens var. venter is clearly dominant. Other species present are Fragilaria virescens Ralfs, F. construens, Cymbella gauemannii Meister, and at the uppermost sampling depth, acidophilous Aulacoseira

116 M. Saamis to etal.

104m a.s.I.

z $ 8 I-- t~

7.5 1 0 8 3 0 + 1 1 0 - - ~

7.9 %

DIATOMS OLIGOHALOBOUS

<

< Z w rr

w < ,,e

5

i= . J 1

20 20

I I I --7--

I

~ ^ . ~ , ~

i ~ D < J

I I • I I I III I

!

POOR IN DIATOMS

-11 1 26 6b 80 , oo i o Ioioio 2b

FIG. 4. Dia tom d iagram of Site No. 2. Selected species.

POLY- and MESOHALOBOUS

< _o

rr--~

80 1001010101010101010101010

Anal. T. GrOnlund

perglabra (Oestrup) Haworth, Pinnularia interrupta Smith and Frustulia rhomboides (Ehrenberg) De Toni.

COMPARISON OF DIATOM STRATIGRAPHY

The paucity of diatoms is conspicuous, particularly in the lower sandy parts of the sequences. The clay-bearing material in the middle of the sequences contains variable amounts of diatoms. The diatom flora in all three sites is dominated by freshwater species. Common to the sequences at Site No. 1, Lake Leukilampi and Site No. 2 is the distinct dominance of Ellerbeckia arenaria which with some other less common species suggest a large lake environment before the beginning of a small lake development as indicated by abundant Fragilaria flora. At two sites studied by M61der (1944) at Vieljarvi near the Lake Onega-Lake Ladoga threshold (Fig. 1), about 200 km southwest of Leukilampi and Site No. 2 there are also distinct maxima of E. arenaria (up to 96%). E. arenaria is distinctly less common in the gyttja clay sequence of Site No. 3, which is characterized by Pinnularia species and some euterrestrial species instead. In addition, the Leukilampi sequence exhibits an ephemeral maximum of Epithemia adnata.

All three sequences contain marine species. In the Leukilampi sequence, saline water diatoms were found only

in sand in the lower part of the sequence, whereas in Site No. 2 and Site No. 3 sequences the same assemblage is present in the lowermost sand and the overlying clay deposit. This marine assemblage probably contains species different in age. For example, Paralia sulcata is a species that has existed from the Palaeogene to the present (Loseva, 1990). It was common in the Eemian interglacial basin of the Baltic Sea but is rare in the modem Baltic Sea. P. sulcata var. sibirica and var. crenulata are characteristic of Upper Cretaceous and Palaeogene sediments and have been redeposited in younger deposits (Loseva, 1990). It is possible that the other unidentified fragments also derive from Tertiary deposits. Paralia sulcata, Hyalodiscus scoticus, Diploneis bombus Ehrenberg, Podosira stelligera, Grammatophora oceanica, Campylodiscus echeneis, C. clypeus, Melosira moniliformis, M. numrauloides, Stephanopyxis sp., S. turris and Thalassiosira sp. are all interglacial species, probably of the Eemian Baltic Sea (cf. Gr6nlund, 1991). The silicoflagellates found in the Leukilampi sequence is also common in deposits of the Eemian Baltic Sea.

The marine diatoms do not show any succession, but in subsamples where they are abundant, several species occur. This supports the idea of redeposition, as well as the discovery of more or less the same main species in all three

Lateglacial of Lake Onega 117

DIATOMS OLIGOHALOBOUS

>- x

154.2 m ~

T ~ t,,l "J -

5.0-

10190+90

10530+101]

5.5

P ~ Y - and M E S O ~ A L ~ S

g ~ w

~m¢O~Nm

O O J ~ -

X

Z 0 co <

i ° o. _,,, z o.: n-

< . ::)

< E

POOR IN DIATOMS

POOR IN DIATOMS

40 % 20 4050 2010 20 601010 20 2010 20 40 60 80 20 4010101010101010101010

FIG. 5. Diatom diagram of Site No. 3. Selected species. Sediment symbols - - see Fig. 4.

sites including Site No. 3, which is situated well above all possible Lateglacial water bodies in the Onega basin.

The above mentioned assemblage, which is interpreted as interglacial, was also found in several sedimentary deposits studied by MSlder (1944) in the areas of S~i~im~ij~irvi, Nuosj~rvi, Sotj~'vi, Vielj~_,'vi and Yl~ijoki near the threshold between Lake Onega and Ladoga (Fig. 1). M/51der interpreted the species as having belonged to the flora of the Karelian Ice Sea, which, according to contemporary understanding, preceded the Baltic Ice Lake and connected the Baltic Sea to the Arctic Ocean. Among the few diatoms present in the Ladoga and Onega deposits of the Younger Dryas, Davidova (1969) found some that could be interpreted as belonging to the assemblage of the Eemian Baltic Sea. The same species were also found in the younger Holocene deposits of those lakes. According to Davidova, these species were transported by rivers from the surroundings of the lakes.

The gyttja in surficial parts of all sequences has an alkaliphilous freshwater diatom flora with Achnanthes and Fragilaria genera well represented. It is typical of the lacustrine evolutionary phase of the Early Postglacial period that the phase begins with Fragilaria species. In Site No. 3 the alkaline initial phase was brief, and the diatom flora soon changed in response to the acidophilous indifferent conditions.

DEGLACIATION AND LATEGLACIAL HYDROLOGY OF THE ONEGA BASIN

Lake Onega was deglaciated when the ice retreated from

the Luga end moraine to the Neva stage and further to the Rugozero (Rukaj~vi) stage indicated by prominent end moraines (Fig. 1), which are conventionally dated to 13,000, 12,000 and 11,000 years ago, respectively (e.g. Ekman and Iljin, 1991). The position of the ice margin in the southern Onega basin during the Neva stage is not exactly known. It may well have extended farther south than indicated on the map. This is supported by discoveries of identical and correlative varved clay sequences in the bottom of the Onega basin both inside and outside the inferred ice margin. Cores sampled in 1992 from southern, middle and northern basin contained approximately 1000 varves resting on top of till. This indicates that the deglaciation of the Onega basin took place within a thousand years, as also suggested by Markov (1935) on the basis of exposed varved clay sequences.

Palaeomagnetic measurements from varved bottom clays of Lake Onega (T. Saarinen, pers. commun., August, 1993) can be correlated with the varved clay sequence at Helyl~, north shore of Lake Ladoga (Bakhmutov and Zagniy, 1990) and according to this correlation they represent the time period 12,500-11,500 years ago, which is a reasonable age estimation for the deglaciation of the Onega basin. These dates are based on the old Finnish varve chronology and its old correlation to the Swedish varve chronology (Sauramo, 1929). In conventional radiocarbon years this age interval would be approximately 500 years younger, i.e. 12,000-11,000 years ago (e.g. Saamisto, 1991).

The exposed varved clays on the shores of northern Lake Onega are covered by sand which may indicate the lowering of the level of the Onega ice lake due to the deglaciation of

118 M Saarnisto et al.

FIG. 6. Diatoms and fragments from the sites studied. 1, 2 and 4 - - fragments of unknown diatoms. 3 - - S tephanopyx i s turns . 5 - - S t ephanopyx i s sp. 6 - - Paral ia su lcata var. sibirica. 7 - - Paral ia su lcata vat. crenulata . 8 - - Diploneis bombus . 9, 10 - - Paral ia sulcata.

11 - - Au lacose i ra ep idendron . 12 - - E l lerbeck ia arenaria. Light micrographs, scale bar = 10 I.trn.

Lateglacial of Lake Onega 119

the northern thresholds leading to White Sea. During the deglaciation the Onega ice lake drained first to the south via the Vytegra and Ojat river valleys and, later via the lower course of the Svir river valley to the Baltic Ice Lake which occupied the Lake Ladoga basin (Kvasov, 1979).

When the ice margin retreated from the Vepsian and Krestets end moraines to the Onega basin an ice lake was formed in the Vodla River area east of the Onega basin. Laminated silts and clays are exposed on a 23 m high river bank near the town of Pudozh suggesting a rather long ice lake period. Bakhmutov and Zagniy (1990) estimated that this laminated sequence should cover 3000 years between 16,000 and 13,000 years ago, which is perhaps, however, too high a figure.

The stratigraphical sites studied here are placed on a shoreline diagram (Fig. 7), whose baseline is perpendicular to the direction of the isobases of the present land uplift (Kakkuri, 1993) as well as to the tilt-axis of Lake Onega which runs from the outlet of Lake Svir at Voznesenyje to the mouth of River Vodla (e,g. Zemljakov, 1936). The isolation dates of Lake Leukilampi, Site 1, and Site 2 near Morskaya Maselga are also shown as well as the Maselga and Morskaya Maselga thresholds towards the White Sea and the Vieljiirvi (Vedlozero) threshold towards Lake Ladoga. A broken line demonstrates the position of the shoreline at the time of the emergence of the Maselga threshold more than 9400 years but less than 10,100 years ago. The role of the VieljErvi threshold is somewhat open. It probably served as an outlet for the Onega Ice Lake, too, and

became dry 10,000 radiocarbon years ago (see the Harjunsuo data, Table 1, Site 4). The emergence of the Maselga threshold means the opening of the present outlet, River Svir, to Lake Ladoga around 9500 years ago.

The Baltic basin was occupied by an ice dammed lake, the Baltic Ice Lake, during the deglaciation of the Onega basin, but this lake did not reach the thresholds between Lake Ladoga and Lake Onega. The Baltic Ice Lake in the Ladoga basin was only 50 m above the sea level at the northern end and less than 10 m in the south (e.g. Dolukhanov, 1979).

By 10,000 years ago, when the Maselga threshold towards the White Sea basin became dry, the water level of the White Sea basin at Belomorsk was already below 60 m and the highest shorelines at Rugozero end moraine, dated at 11,000 years ago, are at about 70 m compared to the elevation of 125 m of the Maselga threshold. This further supports the interpretation of the present diatom stratigraphies which suggest that Lake Onega was a freshwater body following the deglaciation, and the marine species found are reworked Eemian and/or older fossils.

Lake Onega drained to the White Sea basin between 11,000 and 9500 years ago. First through Morskaya Maselga and Maselga thresholds, but since somewhat more than 10,180 years ago only through the latter one. The Vielj~rvi threshold to the Ladoga basin most probably served also as an outlet of the Onega Ice Lake until 10,000 BP.

Due to the differential land uplift, the water level in the northern part of Lake Onega has fallen more than 90 m since the origin of River Svir 9500 years ago whereas

150 m

P A D A N Y MEDVEZEGORSK PETROZAVODSK a+4,

VOZNESENYJE

4, 150 m

MASELGA

L. SEGOZERO 1 .~9360 •

100 to White Sea ~ V E D L O Z E ~ 2 + 10180 (VIELJJO~VI)

MORSKAYA MASELGA

50

L. ONEGA ",.,,

L. Ladoga to -Baltic Sea

100

50

I I I I I I NW 250 km 200 150 100 50 0 SE

FIG. 7. A shoreline diagram for the Onega basin showing low thresholds to the White Sea, Maselga and Morskaya Maselga, and the Vedlozero threshold to Lake Ladoga, the new stratigraphical sites 1, 2, and 3 (see text) and the shoreline of the Ortega basin at the emergence

of the Maselga threshold and the origination of River Svir.

120 M. Saarnisto et al.

southeast of River Svir a transgression up to 15 m can be estimated.

To sum up, following the deglaciation 12,000-11,000 years ago Lake Onega basin was occupied by an ice dammed lake which first drained south to the River Volga system and then to the Lake Ladoga basin. When the ice retreated from the threshold leading to the White Sea more than 11,000 years ago the water level dropped and since that the lake was controlled by its thresholds to the White Sea basin and to the Baltic Sea basin. Lake Onega has remained distinctly above the sea level and no Late Weichselian sea connection existed between the White Sea and the Baltic basins. Lake Onega drained to the White Sea basin (and also to Lake Ladoga) between 11,000 and 10,000 years ago and the White Sea basin alone between 10,000 and 9500 years ago. The present outlet of Lake Onega, River Svir, originated 9500 years ago when the Maselga threshold emerged. By then Lake Segozero north of the Maselga divide became separated from the Onega basin.

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