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Quaternary International, Vol. 27, pp. 121-129, 1995. Copyright © 1995 INQUA/Elsevier Science Ltd

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D I F F E R E N T P H A S E S O F T H E Y O L D I A SEA IN T H E N O R T H - W E S T E R N BALTIC P R O P E R

Stefan Wasteg~d,* Thomas Andr6n,* Gustav Sohlenius* and Per Sandgrent *Department of Quaternary Research, Stockholm University, Odengatan 63, S-113 22 Stockholm, Sweden

t Department of Quaternary Geology, Lund University, Tornaviigen 13, S-223 63 Lund, Sweden

Four sediment cores from the north-western Baltic Proper, covering the time from the Weichselian deglaciation to the present have been studied. Main interest is focused on the Yoldia stage, and especially the duration of the marine phase of this stage. The lithological compositions of the sediments and results of different analyses (magnetic, carbonate, ostracods and foraminifera) have resulted in a subdivision into five stratigraphical units (A-E, from older to younger). Unit A was deposited in a glaciolacustrine environment, probably during the fwst non-saline phase of the Yoldia stage. The first ingression of saline water is recorded in unit B. Brackish water ostracods and foraminifera occur exclusively in this unit, which represents the only saline phase of the Yoldia stage. This phase lasted for less than 120 clay varve years. Unit C was deposited in fresh water, probably during the last phase of the Yoldia stage and the Ancylus stage. The uppermost units, D and E are of Holocene age and represent different phases of the Litorina and Post-Litorina stages.

INTRODUCTION

The aim of the present project is to study sediment sequences from the north-western Baltic Proper covering the

time from the Weichselian deglaciation to the present. The main emphasis is put on laminated sediments deposited during different stages of the development of the Baltic Basin: varved glacial clay deposited during the Baltic Ice

17 ° S t o c k h o l m ..

0 50 km :ii..~. ,,J~ I , I . . "~ ' . :"

, ,.'.,

l Tro, sa

Sbdertbrn " "~..

~.:: .. Peninsula ~ . . . • . . . ' . , ~

t>

19"

9106

- \ 9 1 0 1

t

180 "v~ - t

FIG. 1. Investigated area and location of the sampling sites. Water depth given in metres.

121

122 s. Wasteg~rd et al.

Lake and the Yoldia stages, and laminated gyttja clay and clay gyttja deposited at the postglacial climatic optimum during the Litorina stage and in the deepest parts of the present Baltic Proper.

Two time scales, based on radiocarbon datings and clay varve chronology are used in this paper. Radiocarbon datings are presented as uncalibrated 14C years BP and clay varve datings as clay varve years BP in the Swedish Time Scale (e.g. Cato, 1985). Different attempts to correlate between the varve chronology and the 14C time scale suggest that the varve chronology contains ca. 500 more years than the radiocarbon time scale at the end of the Younger Dryas (e.g. Bjtrck et al., 1992).

In this paper we present results of analyses carried out on four cores: 9101, 9103, 9105 and 9106 (Fig. 1). Special interest is focused on the extension of the Yoldia stage in time and space and the duration of the marine phase of this stage. The Yoldia stage, defined as the period when the Baltic Basin was in contact with the sea, has recently been dated to ca. 10,300-9600 Jac years BP (Svensson, 1989, 1991). This stage began after the final drainage of the Baltic Ice Lake and ended with the transition to the freshwater Ancylus stage due to the uplift of the thresholds relative to the sea. Svensson (1989) has divided the Yoldia stage into three sub-stages, with brackish conditions during the middle stage. This brackish period corresponds most likely to the findings of the marine bivalve Portlandia (Yoldia) arctica (Gray) in a reddish glacial varved clay in the Stockholm area (e.g. Erdmann, 1868; De Geer 1940; Brunnberg and Possnert, 1992). The duration of this period has been estimated to ca. 200 years (Strtmberg, 1989).

This brackish period during the Yoldia stage has also been identified by the diatom floras which have been reported from different parts of the Baltic Basin and adjacent land areas (e.g. Thomasson, 1927; Florin, 1977; Gudelis, 1979; Paabo, 1985; Svensson, 1989, 1991).

Very little attention has been paid to the ostracod and foraminifera faunas of the different Baltic stages. Recent faunas are fairly well described (e.g. Rosenfeld, 1977; Hermelin, 1987) but only incomplete records of ostracods and foraminifera from older Baltic stages exist (e.g. Munthe, 1896; Ltnnberg , 1908). A record of foraminifera and ostracods in Yoldia clay from Duvbo, ca. 10 km northwest of Stockholm (Munthe, 1896), has generally been overlooked in the literature. Munthe described two species of foraminifera, Polystomella striato-punctata and Polystomella subnodosa and one ostracod species, Cytheropteron montrosiense. The recorded specimens are not illustrated, but it is likely that the Polystomella species are synonymous with Elphidium excavatum, indicating brackish conditions when these sediments were deposited.

In the present study foraminifera and ostracod analyses have been carried out by Stefan Wasteggtrd, and carbonate and mineral magnetic analyses by Gustav Sohlenius. Thomas Andrtn acted as project co-ordinator supervising the field work and he was also responsible for the clay varve measurements. The mineral magnetic measurements were carried out at the Department of Quaternary Geology in Lund

under the supervision of Per Sandgren. All four authors contributed to the interpretation of the results anddiscussion.

INVESTIGATED AREA

The investigated area is located between 58°00 , and 59°20 , N, 17°00 , and 20°00" E. The water depths at the sampling sites range from 73 to 111 m. The sampling sites are shown in Fig. 1.

GENERAL LITHOSTRATIGRAPHY

A general l i thostratigraphy for the area (Fig. 2), related to the different stages of the Baltic Basin was established by Ignatius et al. (1981). As sedimentation in the Baltic Basin is time transgressive along a north-south traverse the presented stratigraphy is valid only for the investigated area. Stratigraphically the sediments can be separated into three units. Till, proximal and distal clay varves and finally homogeneous clay, representing the lowermost unit in Fig. 2, is supposed to have been deposited during the Baltic Ice Lake stage and the older parts of the Yoldia stage.

During the younger parts of the Yoldia stage and the Ancylus stage a transition clay was deposited. This clay consists of a grey to bluish grey homogeneous clay often with black sulphide layers in its lower parts. In fact it still exhibits some characteristics of a glacial sediment although it is postglacial (Ignatius et al., 1981).

The uppermost lithostratigraphic unit was deposited during the Litorina and Post-Litorina stages. It consists of a lower gyttja clay and an upper clay gyttja. The lower and upper parts are laminated, separated by a sequence of homogeneous gyttja clay. The lower laminated gyttja clay was deposited during the postglacial climatic optimum. Its lower boundary constitutes in most cases a distinct acoustic reflector easily identifiable in the echo-sounding profiles.

POSTGLACIAL

MUD

TRANSITION

CLAY

GLACIAL

CLAY & SILT

Clay gyttja laminated

Gyttja clay homogeneous

Gyttja clay laminated

Clay homogeneous,

sulphide stained

Clay, homogeneous

Clay thin, distal varves

PmJdmal varves

Till or stratified drift

PRESENT BALTIC

LITORINA SEA

ANCYLUS LAKE

YOLDIA SEA

BALTIC ICE LAKE

FIG. 2. Generalised lithostratigraphy for the north-western Baltic Proper (after Ignatius et al., 1981).

Different Phases of the Yoldia Sea 123

METHODS

Positioning The Decca Navigator, North Baltic chain (4B), was used

in this investigation parallel with a GPS-navigator. Within the investigated area the positional accuracy can be assumed to be within _+ 50 m.

Sampling Technique Different sampling sites were chosen after detailed

studies of mud-penetrating echo-sounding profiles. The cores were retrieved with a 6 m long Kullenberg piston corer.

Clay Varve Measurements The plastic tubes from the Kullenberg sampler were

opened in the laboratory. One half was used for the mineral magnetic measurements and chemical analyses and the other half for varve counting and measurement of varve thicknesses.

The varve series have been drawn as graphs according to De Geer's (1940) method with years equally spaced on the X-axis and varve thickness on the Y-axis, but on the scale 1:5.

Foraminifera and Ostracod Analyses Samples for foraminifera and ostracod analyses were sub-

sampled from the clay varve measured half of the cores, with sampling intervals ranging between 5 and 35 cm. The samples (30 to 75 g dry weight) were prepared in accordance with Meldgaard and Knudsen (1979). The size fraction 0.1-1.0 mm was examined for the occurrence of calcareous fossils. The entire content in each sub-sample was counted and the number of specimens (foraminifera) and valves (ostracods) per 100 g of dry weight was calculated.

Magnetic and Calcium Carbonate Analyses All cores have been sampled for mineral magnetic

analyses. The sediment sequences have been sub-sampled continuously in 2 cm intervals. In the upper part of core 9106, however, the sampling interval was 10 cm.

Initially the low-frequency magnetic susceptibility was measured on all sub-samples using an air-cored Barington susceptibility bridge. The sub-samples were then artificially magnetised using a pulse magnetic charger in a field of 1 Tesla (T) and the induced remanence, SIRM (Saturation Isothermal Remanent Magnetisation), was measured on a Minispin fluxgate spinner magnetometer. Thereafter the samples were magnetised in a low negative magnetic field of 0.1 T (IRM_10omr) and the remanence remeasured on the spinner magnetometer. Based on the induced remanences the S-ratio was calculated as S = IRM_10om~/SIRM.

All magnetic measurements were carried out on wet samples. On some samples from core 9101 susceptibility was measured after drying at room temperature as well.

Susceptibility is a measure of the ease with which a material can be magnetised. Most natural materials have susceptibilities proportional to their magnetite (Fe304) content but authigenically formed ~on sulphides, such as greigite (FeaS4) and pyrrhotite (Fe7S8), can also contribute to

susceptibility (Thompson and Oldfield, 1986; Anderson and Rippey, 1988). Iron sulphides are formed during reducing conditions predominantly in a marine environment (Berner, 1971).

SIRM is mainly a measure of the magnetite concentration but is also influenced by magnetic grain sizes. Small magnetic grains have a higher remanence than coarser ones. Authigenically formed iron sulphides also contribute to SIRM (Thompson and Oldfield, 1986).

The S-ratio (Stober and Thompson, 1979) ranging from +1 to -1, is a measure of the relative proportions of ferrimagnetic to antiferromagnetic minerals. Ferrimagnetic minerals (mainly magnetite) are saturated at lower field strengths than antiferromagnetic minerals (mainly hematite). Low negative S-ratios, below -0.8, indicate a magnetic assemblage entirely consisting of ferrimagnets.

The water content has been calculated as per cent water in wet samples. Quantitative analyses of the CaCO 3 content have been carried out with a Passons apparatus (Talme and Alm6n, 1975).

Isotope Analysis Analysis of stable isotopes was performed on three samples

of the ostracod Cytheropteron montrosiense from the core 9106. The analyses include ~13C and 61sO versus PDB.

RESULTS

The sediments of the four cores have been subdivided into five stratigraphical units (A-E, from older to younger) based upon the lithological composition and the results of the different analyses. Core correlation is based on the concentration of magnetic minerals as reflected by the SIRM parameter (Fig. 3).

The clay varves in all cores have been measured. The varves were generally well developed and easily measured in unit A. In units B and C1, however, the varve boundaries were diffuse. Although the cores can be correlated by means of their magnetic properties it has not yet been possible to make any reliable clay varve correlations (Andr6n and Sohlenius, this volume).

Samples from all the described units were examined for their content of calcareous fossils. Foraminifera and ostracods were found exclusively within unit B. No findings of the mollusc Portlandia artica were made as in the reddish varved clay in the Stockholm area (e.g. Erdmann, 1868; De Geer 1940; Brunnberg and Possnert, 1992).

Unit A The lowermost unit consists of a grey to brown varved

clay (Fig. 4). The unit can be identified in all cores and the maximum penetrated thickness exceeds 2.5 m in core 9105 (Fig. 3). SIRM values are very stable, around 10 mAmZkg -1, with very little scatter indicating the homogeneous character of this varved clay (Fig. 3). The CaCO 3 content does not exceed 5% (Figs 5-7). No calcareous fossils were found. The longest varved sequence of this unit was measured in core 9105 and contained ca. 250 clay varves.

124 S. Wasteg~d et al.

3OO

4 O O

C o r e 9 1 0 1

- 8 3 m

0

100

2OO

500

A

Core 9103 - 1 1 0 m

Core 9103 - 111 m

E

600 . . . . I . . . . I . . . . [ . . . . i i . . . . I . . . . I . . . . l . . . . l 0,1 1 10 100 1000 0,1 1 10 100 1000 0,1 1 10 100 1000

C o r e 9 1 0 6

- 7 3 m

0,1 1 10 100 1000

S I R M ( m A r n = k g . )

FIG. 3. Core zonation based on the SIRM parameter, cores 9101,9103, 9105 and 9106. Stratigraphical units are shown for each core. Dotted lines indicate uncertain unit boundaries. Water depths in metres are given under each core label.

E

8 15

I ¢-

Core 9105 ~P

/ ¢ z e / f 0

100

200

a00

E

4 )

1= ¢)

"[2 (D u~

0

oyl~ e,b/

c ~ ! =ulpa~ =Iraqi

U ~ l n h H

dltltum R

c ~

• ! E o

C

B

1' i'

J l A

0oo o ~, # # ,~,,~,.*~,~,~,o., ,~,,,.~#.~ _~

FIG. 4. Lithology, water content, magnetic properties and stratigraphical units for core 9105.

Different Phases of the Yoldia Sea 125

Core 9101 #~.~,,,#' ~'#"

oo° / liZlo ,,

I I I I I C l

4OO

i i " i B ~o 5oo

.} 550 , , , A

0 250 500 0 20 40 0 4 8 12 Number of valves (os~acodsyspecimens (foraminifera) per 100 g dry weight

FIG. 5. Diagram showing the content of calcareous fossils and CaCO 3 concentration in core 9101.

Core 9105 :_,,,~ / . / /

,o+ "+ I .o<>'+ -~" / ~ " .~ # i u.,, 150

E 200

I I

==

250

,,Q e,-

Q

350, 0 250 500

!i 7;:::>

I I I

0 2 0 0 2 0 4 0

1 1 .

! \

r i i /

O 4 8 12 Number of valves (ostracods)/speclmens (foraminifera) per I00 g dry weight

c I

B

A

FIG. 6. Diagram showing the content of calcareous fossils and CaCO3 concentration in core 9105.

126 s . W a s t e g ~ r d et al.

Core 9106

/ / / // / O~ 00'~ UNIT

I I

E

-.,i o~ e -

• -~ 450

t "

a

I I 400

C/D?

B

A 5 0 0 I i , I I

0 1000 0 150 300 0 4 8 12 Number of valves (ostracods)/specirnens (formninifera) per 100 g dry weight

FIG. 7. Diagram showing the content of calcareous fossils and CaCO3 concentration in core 9106.

Unit B The sediment of this unit, which was identified in all

cores, consists of a diffusely varved FeS-stained silty clay (Fig. 4). The thickness ranges between 40 and 150 cm (Fig. 3). The unit is characterized by higher SIRM and susceptibility values with a number of high pronounced peaks and a drop in the S-ratio and the water content (Fig. 4). In core 9101 susceptibility values display 10-20% lower values after drying. A possible explanation for this could be the oxidation of ferrimagnetic iron sulphides. The S-ratio ranges from -0.7 to -0.8 indicating that ferrimagnetic minerals more or less dominate the mineral magnetic assemblage.

Brackish water ostracods and foraminifera occur exclusively in this unit (Figs 5-7) The highest number of specimens is recorded in the middle part of the unit. The ostracod Cytheropteron rnontrosiense Brady, Crosskey and Robertson was abundant in unit B in all cores and the foraminifer Elphidium excavatum (Terquem) was found in low numbers. A second ostracod species, Paracyprideis fennica (Hirschmann), was found in low numbers in core 9105. The ostracod and foraminifera specimens were generally well preserved with no indication of reworking or dissolution. The absence of calcareous fossils in the other units deposited in brackish water (E and perhaps D), is probably due to post mortem dissolution of the calcareous tests, since the present Baltic Sea is undersaturated with respect to calcium carbonate (Jarke, 1961).

The species found in unit B indicate an extreme environment with a very cold, brackish bottom water with a high sediment load. Cytheropteron montrosiense is frequently found in Pleistocene deposits in north-western Europe (references in Whatley and Masson, 1979). Today it inhabits shallow waters in high latitudes (Cronin et al., 1991). Elphidium excavatum is widely distributed in Pleistocene and Holocene deposits in Europe and North America. Living specimens have been recorded in the Baltic Proper, as far north as the archipelago south-east of Trosa (Ankar and Elmgren, 1976) (Fig. 1). The specimens occurring in zone B resemble the mainly boreal f. selseyensis (Heron-Allen and Earland), but have larger tests (in average 0.5-0.7 mm) and a slightly different morphology. The low-salinity ostracod Paracyprideis fennica is one of the most abundant ostracods in the present Baltic Sea. It is endemic in the Baltic Sea and has been considered an arctic relict (e.g. J~irvekiilg, 1973). The specimens found in core 9106 are the first fossil records of this species. Fragments of the brackish water diatom Rhabdonema arcuatum v. robusta (Grun.) Hustedt (J. Risberg, pers. commun., 1992) were also found.

The CaCO3 content increases and reaches a peak of ca. 10% in the middle of unit B (Figs 5-7). Analyses of stable isotopes gave between -5.78 and -6.05%0 for 513C and between -9.00 and -9.25%0 for 5180, indicating a very high fresh water influence. The analyses were performed on three samples of Cytheropteron montrosiense from core 9106.

Different Phases of the Yoldia Sea 127

Fossils indicating brackish conditions have been recorded in a sequence of ca. 70 clay varves in core 9101 and ca. 60 varves in core 9106. The mineral magnetic properties of unit B in core 9106, however, indicate that the brackish influence may have lasted as long as ca. 120 clay varve years (Fig. 8).

Unit C This unit was recorded in two cores, 9101 and 9105, with

a maximum thickness of ca. 150 cm in core 9101 (Fig. 3). Sediments representing unit C or D are also present in core 9106. The magnetic signature, however, does not allow a direct correlation with cores 9101 and 9105.

The lower boundary of unit C is not distinct. The lower part (CO consists of a thin-varved (1-2 mm) greyish clay which gradually merges into a black sulphide-stained non- varved clay (C2). Susceptibility and SIRM values are more or less stable, showing a weak decreasing trend (Fig. 4). the S-ratio however, increases upwards. Unit C, as well as units D and E, contains no measurable amounts of CaCO 3 and no calcareous fossils.

Unit D This unit, recorded in at least two cores, 9101 and 9105,

and probably also in core 9106, consists of a grey sulphide- stained clay (Figs 3 and 4). The thickness of the unit is ca. 50 cm. SIRM, susceptibility and S-ratio exhibit some pronounced peaks but the general trend is decreasing. The boundaries of the unit are diffuse. Preliminary diatom analyses show that this unit, as well as unit E, was deposited in brackish water (J. Risberg, pers. commun., 1993).

Unit E The uppermost unit, which has been found in all four

cores, consists of a partly laminated gyttja clay and clay (Figs 3 and 4). The maximum thickness is 340 cm recorded in core 9106. The lowest susceptibility and SIRM values are recorded in this unit. The S-ratio fluctuates around -0.8. The lower boundary of the laminated sequence has been AMS- dated (bulk sediments) to 6580 ± 80 ~4C years BP (Ua-3071) in core 9105 (-71 cm) and to 6890 ± 75 t4C years BP (Ua- 3379) in core 9106 (-286 cm).

DISCUSSION AND CONCLUSIONS

The investigated cores show good correlation based upon the lithostratigraphy and the magnetic properties. This indicates that the sediments have a similar composition in this part of the north-western Baltic Proper during the Late and Post Glacial Baltic stages. The variation in thickness of the correlated units indicates that the rate of sedimentation has varied between the sampling sites.

The most complete sediment sequences have been obtained in cores 9101 and 9105. Units C and D are missing in core 9103, indicating a change in the sedimentary conditions, e.g. a transition from accumulation to erosion (H/tkansson and Jansson, 1983). The correlation of core 9106 with the other cores, using magnetic parameters, is difficult. This could be explained by the different sampling interval in

the upper part, and the fact that unit C may be incomplete in this core.

The lowermost varved clay, unit A, shows very stable magnetic values and no records of brackish fossils. This unit corresponds most likely to the grey and brown valved clay units on the Stiderttm Peninsula (e.g. Brunnberg, 1990). However, abrupt changes in the colour, from a dark greyish brown to a dark grey clay, and increases in varve thickness observed on the S/Sderttrn Peninsula and further westwards have not been recorded in unit A in the present study (cf. Andrtn and Sohlenius, this volume). The colour changes in the clay sequences on the SOderttrn Peninsula have been interpreted as being caused by environmental changes in the Baltic basin due to the drainage of the Baltic Ice Lake at ca. 10,740 clay varve years BP (e.g. Brunnberg, 1990). These changes have not been recorded in unit A, and it is suggested that this unit was deposited after the drainage of the Baltic Ice Lake and thus belongs to the first non-saline phase of the Yoldia stage. This is also supported by the fact that no more than 250 varves were recorded in any core before the first marine ingression in unit B. This ingression has been dated to ca. 10,430 clay varve years BP in the Stockholm area (Brunnberg, 1990).

The ostracod and foraminifera fauna recorded in unit B show that the unit was deposited in coM, brackish water with a high sediment load. The very sharp lower boundary of this unit, best recorded in cores 9103, 9105 and 9106, implies that the first marine ingression was instantaneous and probably synchronous in the north-western Baltic Proper. The high magnetic values in the unit are partly an effect of a high silt content, but the FeS-staining may also contribute to the high susceptibility and SIRM values. The unit most probably corresponds to the brackish phase of the Yoldia stage, dated to ca. 10,000-9900 14C years BP (Svensson, 1989, 1991). According to varve counts, unit B represents no more than 120 varve years in the north-western Baltic Proper.

The content of brackish fossils and the changes in the mineral magnetic parameters are significant in the varved sequences of the cores 9101 and 9106. However, it has not yet been possible to use this saline ingression as a marker in the traditional clay varve correlations. This means that there is no correlation between the clay varve diagrams in Fig. 8. This may be explained by the fact that the sampling stations are located too far apart (34 kin) and that local variations in the sedimentation to a great extent have influenced the varve thickness.

The sudden change to brackish conditions at ca. 10,000 t4C years BP is also recorded at Mt BiUingen (BjSrck and Digerfeldt, 1986), in the varved clay in the Stockholm area (e.g. Brunnberg and Possnert, 1992) and in the central and southern parts of the Baltic (e.g. Svensson, 1989; Bjtrck et al., 1990). Svensson also recorded high susceptibility values in sediments deposited during this phase (Svensson, 1989, p. 77).

The high calcium carbonate concentration in unit B may be due to precipitation in connection with sulphate reduction (e.g. Presley and Kaplan, 1968). Similar high concentrations of calcium carbonate in Yoldia clay have also been described by Brunnberg and Possnert (1992).

No signs of brackish influence can be seen in unit C. The

128

250

S. Wasteg~d et al.

/

0

9101

250 200 150 100 50

/

o

9 1 0 6

Magnetic properties of unit B Marine/brackish fossils

FIG. 8. Clay varve diagrams for cores 9101 and 9106. See text for explanation, Magnetic properties of unit B and occurrence of marine/brackish foraminifera/ostracods are indicated.

lowermost part, sub-unit C~, is partly varved, but the varves are thin and diffuse. The black, sulphide-stained non-varved clay of sub-unit C2, is generally assigned to the Ancylus stage (e.g. Winterhalter, 1992).

The magnetic curves of unit C correlate quite well between cores 9101 and 9105, sampled at a distance of 28 km. This implies that the changes in magnetic values have

a regional significance and reflect water level changes in the Ancylus Lake and possibly the youngest phase of the Yoldia stage. The increase in susceptibility and SIRM values documented in the middle of sub-unit Cl (Fig. 3) could be related to the rapid Ancylus regression at ca. 9200 laC years BP (cf. Svensson, 1989, Sandgren e t aL, 1990).

Diatom content and the results of mineral magnetic

t I I I , , o

1 0 0

A E

2oo

0 ro "6

. 0 1 -

E3

5OO

POSTGLACIAL Gy~= ¢~y PRESENT BALTIC P'~ E MUD Jeminat=d

TRANSITION c ~ CLAY Clay D LITORINA

sulphide =tined ¢~ywith ,, ANCYLUS /

t ~ e . u ~ C vivei LATE YOLDIA

c~ay wire B SALINE YOLDIA d.tu~ v=v= f

GLACIAL

CLAY & SILT c~y ' ' A EARLY YOLDIA

vwved I !

I

, , , , BALTIC ICE LAKE

FIG. 9. Preliminary interpretation of the different statigraphical units together with lithology and SIRM curve of core 9105. Lithoiogical units refer to the general lithostratigraphy for the area (Ignatius et al., 1981).

Different Phases of the Yoldia Sea 129

analyses sugges t that brackish condi t ions are r ecorded in

units D and E.

Sediments deposi ted during brackish condit ions, units B,

D and E, have l ower S-rat ios than units depos i ted dur ing

fresh condi t ions . This m a y be due to a h ighe r conten t o f

authigenical ly fo rmed fer r imagnet ic iron sulphides in these

units, The format ion o f iron sulphides during the brackish

s tages m a y h a v e b e e n g o v e r n e d by the d e v e l o p m e n t o f

halocl ines resul t ing in stagnant bo t tom condit ions. The fact

that saline water conta ins more sulphate than fresh water

may be another reason for the format ion o f iron sulphides

dur ing the brackish stages.

A p r e l i m i n a r y in t e rp re t a t i on o f the d i f f e r en t

strat igraphical units is summar ized be low and shown in Fig.

9 toge ther wi th l i tho logy and S I R M curve o f core 9105.

L i t h o l o g i c a l uni ts in F ig . 9 re fe r to the gene ra l l i tho-

stratigraphy for the area (Ignatius et al., 1981).

Uni t A was deposi ted in a glaciolacustr ine envi ronment ,

p robab ly dur ing the first non- sa l ine phase o f the Y o l d i a

stage.

Uni t B was depos i ted in brackish water during the saline

phase o f the Yold ia stage, This phase lasted for less than 120

varve years in the nor th-western Balt ic Proper.

Uni t C represents the last phase o f the Yold ia stage and

the Ancy lus stage.

The two uppermos t units, D and E, probably represent

different phases o f the Li tor ina and Post -Li tor ina stages.

ACKNOWLEDGEMENTS

We thank the crew at the RN Strombus. We are also grateful to Jan Lundqvist for critically reading the manuscript and to David N. Penney, Ju-hus for help with ostracod identification. Radiocarbon analyses (AMS) were performed at The Svedberg Laboratory at Uppsala University, Sweden. Analyses of stable isotopes were carried out at the GMS-laboratory at the University of Bergen, Norway.

The investigation was financially supported by the Swedish Natural Science Research Council (NFR) and by Stockholm University. This project is a contribution to IGCP 253 (Termination of the Pleistocene).

REFERENCES

Anderson, N.J. and Rippey, B. (1988). Diagenesis of magnetic minerals in the recent sediments of a eutrophic lake. Limnology and Oceanography, 33, 1476-1492.

Andrtn, T. and Sohlenius, G. (1995). Late Quaternary development of the north-western Baltic Proper-- results from the clay-varve investigation. Quaternary International, (this volume).

Ankar, S. and Elmgren, R. (1978). The Benthic Macro- and Meiofauna of the Askti-Landsort Area (Northern Baltic Proper). Contributions from the Aski~ Laboratory, 11, 115 pp.

Bemer, R.A. (1971). Principles of Chemical Sedimentology. McGraw-Hill, 240 pp.

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