The history of the Yoldia Sea in Northern Estonia: palaeoenvironmental conditions and climatic oscillations

(1)

Geo log i cal Quar terly, 2007, 51 (3): 295–306

The his tory of the Yoldia Sea in North ern Es to nia: palaeoenvironmental con di tions and cli ma tic os cil la tions

Atko HEINSALU and Siim VESKI

Heinsalu A. and Veski S. (2007) — The his tory of the Yoldia Sea in North ern Es to nia: palaeoenvironmental con di tions and cli ma tic os - cil la tions. Geol. Quart., 51 (3): 295–306. Warszawa.

Late gla cial and Early Ho lo cene sed i ment se quences from North ern Es to nia were in ves ti gated us ing di a tom and pol len anal y sis and the ac cel er a tor mass spec trom e try (AMS) ¹⁴C dat ing. The re sults of di a tom anal y sis in di cate that fresh wa ter con di tions pre vailed dur ing the ini tial and fi nal phases of the Yoldia Sea in the in ves ti gated area. A near-bot tom sa line wa ter cur rent that pen e trated into the Bal tic Sea Ba sin dur ing the brack ish phase of the Yoldia Sea spread into the Gulf of Fin land at ca. 11 300–11 200 cal en dar years BP. Coastal upwelling prob a bly caused mix ing of the wa ter col umn and the cir cu la tion of brack ish wa ter up to the sur face in cer tain near-shore ar eas in the Gulf of Fin land. A slight change in the pol len com po si tion may sug gest de te ri o ra tion in the cli mate and can be cor re lated to the Preboreal Os cil la tion. AMS ¹⁴C dates on aquatic plant macrofossils sug gest a res er voir ef fect more than 1000 year for the brack ish phase of the Yoldia Sea.

Atko Heinsalu and Siim Veski, In sti tute of Ge ol ogy, Tallinn Uni ver sity of Tech nol ogy, Ehitajate tee 5, 19086 Tallinn, Es to nia, e-mails:

heinsalu@gi.ee, veski@gi.ee (re ceived: No vem ber 11, 2006; ac cepted: April 3, 2007).

Key words: Es to nia, Yoldia Sea, palaeoenvironment, di a toms, pol len anal y sis.

INTRODUCTION

The his tory of the Bal tic Sea Ba sin has been an im por tant sub ject for the Qua ter nary geo log i cal stud ies around the Bal tic area and much re search has been car ried out to un der stand the pe cu liar i ties of its de vel op ment (e.g. Gudelis and Königsson, 1979; Björck, 1995). There seems to be an agree ment upon the main out lines of the Bal tic Sea evo lu tion since the last deglaciation. The late gla cial and Ho lo cene de vel op ment of the Bal tic Sea Ba sin has been sub di vided into four ma jor stages:

the Bal tic Ice Lake (fresh wa ter lake dammed above the ocean level; from deglaciation to ca. 11 550 years BP), the Yoldia Sea (par tially brack ish-wa ter ba sin; ca. 11 550–10 700 cal i brated years BP), the Ancylus Lake (fresh wa ter ba sin; ca. 10 700–9500 cal i brated years BP) and the Litorina Sea (brack - ish-wa ter ba sin; ca. 9500 cal i brated years BP to the pres ent).

The al ter na tions of the fresh and brack ish-wa ter stages were reg u lated in par tic u lar by in ter ac tions be tween the deglaciation dy nam ics, glacio-iso static land up lift and eustatic sea level changes, which af fected the lo ca tion of the thresh olds and the out let and in flow pas sages and the mag ni tude, du ra tion and di -

rec tion of the wa ter ex change be tween the At lan tic Ocean and the Bal tic Sea Ba sin.

An ex ten sively dis cussed is sue con cern ing the evo lu tion of the post-gla cial Bal tic Sea Ba sin is the palaeoenvironment of the Yoldia Sea stage, and es pe cially the ex tent and tim ing of the sa line wa ter in tru sion. The Yoldia Sea stage was named af ter the arc tic ma rine bi valve Portlandia arctica, pre vi ously called Yoldia arctica, first re corded from gla cial varved clays in the vi cin ity of Stock holm (Fries et al., 1863). The Yoldia Sea formed when the fi nal drain age of the Bal tic Ice Lake at ca. 11 550 GRIP (Green land Ice Core Pro ject) ice core years BP (Andrén et al., 2002) took place, the wa ter level dropped ap - prox i mately 25 m to the level of the At lan tic Ocean and a con - nec tion with the ocean opened across south-cen tral Swe den.

The Yoldia Sea stage ter mi nated when the glacioisostatic up lift emerged the con nect ing straits above sea level at ca. 10 700 cal - i brated years BP (Andrén et al., 2000), so that the wa ters of the Bal tic Sea Ba sin were dammed up, and the Ancylus Lake stage be gan. The Yoldia Sea stage close to the in flow area in south-cen tral Swe den and the Bal tic Sea proper has been sub di - vided into three phases with brack ish con di tions dur ing the mid dle phase (Svens son, 1989; Wasteg¯rd et al., 1995). The ini tial 200–300 years of the Yoldia Sea were char ac ter ized by

(2)

fresh wa ter con di tions due to vast amount of fresh wa ter pro - duced by melt ing ice sheet dis charg ing out from the still shal - low and nar row straits (Svens son, 1989; Strömberg, 1994;

Björck, 1999). This was suc ceeded by a phase with brack - ish-wa ter con di tions that lasted pre sum ably dur ing a short pe - riod of 60–200 years (Strömberg, 1989; Svens son, 1991;

Wasteg¯rd et al., 1995). This Early Preboreal ingression of sa - line wa ter into the Bal tic Sea Ba sin has been de tected by di a tom flora, cal car e ous ben thic fauna and symmict varved clays all the way from the west ern coast of Swe den (Schoning et al., 2001) into Lake Vänern Ba sin (Fredén, 1988), through the Närke Strait (Wasteg¯rd et al., 1998) into the Stock holm area (Brunnberg, 1995) and fur ther south and eastwards into the Bal tic Sea proper (Abelmann, 1985; Lepland et al., 1999). A pos si ble late gla cial con nec tion be tween the White and Bal tic Sea bas ins has been dis proved (Saarnisto et al., 1995). Di a tom re cords in di cate that fresh wa ter con di tions again ex isted dur ing the fi nal phase of the Yoldia Sea stage at ca. 11 100–10 700 years BP (Andrén et al., 2000).

De spite much study on the Yoldia Sea stage at the in flow area the ques tion of the sa lin ity of the Yoldia Sea in the east ern part of the Bal tic Sea Ba sin is still un solved. The bal ance be - tween sa line wa ter in tru sion from the At lan tic Ocean and the fresh wa ter dis charge from the rap idly melt ing Fennoscandian ice sheet prob a bly de ter mined how far east into the Bal tic Sea ba sin the brack ish-wa ter flow could have reached. Two ma jor al ter na tives have been sug gested. The oc cur rence in the Yoldia Sea sed i ments of an eco log i cally mixed di a tom as sem blage, fea tur ing both fresh wa ter and sa lin ity-tol er ant di a toms, was in - ter preted as a con se quence of in creased sa lin ity in the Gulf of Fin land (e.g. Alhonen, 1971). More over, based on the at tempts to cor re late the Swed ish and Finn ish clay-varve chro nol o gies, Donner (1969) and Strömberg (1990) have cal cu lated that a de - lay of some 80–90 years could be pre sumed for the in flux of sa - line wa ter from Swe den to South ern Fin land. Ac cord ing to the sec ond ap proach (Hyvärinen et al., 1992; Raukas, 1994, 1995), brack ish-wa ter wa ter did not reach the east ern part of the Bal tic Sea Ba sin and the Gulf of Fin land at all and the pres ence of ma - rine and brack ish-wa ter di a toms was ex plained by redeposition from the Eemian Inter gla cial de pos its.

Time-syn chro nous marker ho ri zons are an im por tant tool for long dis tance geo chron ol ogi cal cor re la tion be tween dif fer - ent geo log i cal ar chives. Geo log i cally short-lived events, which have left some traces in the sed i men tary re cords, such as vol ca - nic erup tions, earth quakes, me te or ite im pacts, cli ma tic events and sea level changes, are com par a tively rare and may be em - ployed for cor re la tion pur poses (Whittaker et al., 1991). The on set of the Ho lo cene is re garded as a rapid global warm ing of the cli mate, how ever, proxy re cords from around the North At - lan tic re gion pro vide ev i dence of a short-lived cli ma tic de te ri o - ra tion very early in the Ho lo cene, an ep i sode that has been termed the Preboreal os cil la tion (e.g. Björck et al., 1996; Fisher et al., 2002; Van der Plicht et al., 2004). Sed i ment re cords from the Nordic coun tries dis play the Preboreal os cil la tion in the form of a de crease in pol len con cen tra tion in com bi na tion with de creased tree-pol len and in creased herb pol len fre quen cies and a de cline in car bon con tent (Björck et al., 1997). This cli - ma tic re ver sal is also re flected as a mark edly colder event in the ox y gen iso tope sig nal from the GRIP ice core cen tred on 11

200 GRIP ice core years BP (Walker et al., 1999). The Preboreal Os cil la tion started ca. 300 years af ter the ter mi na tion of the Youn ger Dryas. The tim ing of the Early Preboreal cool - ing co in cides with the brack ish phase of the Yoldia Sea (Björck et al., 1996).

The aim of the pres ent work is to doc u ment the palaeosalinity re cords of the Yoldia Sea stage in North ern Es to - nia, to de ter mine the dis tri bu tion and dy nam ics of the ingression of sa line wa ter into the east ern part of the Bal tic Sea Ba sin and to test, whether that short brack ish phase of the Yoldia Sea can be used as a syn chro nous time marker for com - par a tive stud ies in the re gion. For this pur pose sev eral sed i ment se quences in North ern Es to nia were ana lysed in de tail by means of di a tom and pol len anal y sis and AMS ¹⁴C dat ing. For the palaeoenvironmental re con struc tion of the Yoldia Sea our data was com pared with prox ies from the in flow area in south-cen tral Swe den as well as with off shore sed i ment re cords from the Bal tic Sea proper and the Gulf of Fin land.

STUDY AREA AND PREVIOUS INVESTIGATIONS

The study area (Fig. 1) is sit u ated in North ern Es to nia, around Tallinn. An out stand ing es carp ment with the height up to 20–30 m, the North Es to nian klint, di vides the area into the foreklint low land and a lime stone pla teau on the klint. In the basal part of the klint Cam brian terrigenous rocks crop out, whereas the crest of the scarp con sists of Or do vi cian lime stone

296 Atko Heinsalu and Siim Veski

Fig. 1. The Bal tic re gion with geo graphic names men tioned in the text

A — study area, B — lo ca tion of the sites in ves ti gated; SK — Skagerrak, KA — Kattegat, LVN — Lake Vänern, LVT — Lake Vättern, LD — Landsort Deep, SI — Suursaari In land

(3)

and do lo mite. The thick ness of Qua ter nary de pos its along the lime stone pla teau is nor mally rather thin. In the klint bays, which are of ten as so ci ated with an cient bur ied val leys, the Bal - tic Sea sed i ments can be of sig nif i cant thick ness. Coastal for - ma tions of the Yoldia Sea are frag mented and scat tered in the re gion. The pres ent land up lift ob served in North ern Es to nia is 1 mm y^–1. Glacio-iso static land up lift was fairly rapid dur ing the Youn ger Dryas/Preboreal tran si tion, ap prox i mately 3 cm y^–1 (Kessel and Miidel, 1973). The ex trap o lated shore level for the Tallinn area prior to the fi nal drain age of the Bal tic Ice Lake is 75 m a.s.l. (Kessel, 1961). Dur ing the drain age the wa ter level of the Bal tic Ice Lake dropped to around 50 m a.s.l., and the re gres sion of the shore-line con tin ued dur ing the Yol - dia Sea at least to 28 m a.s.l. (Saarse et al., 1997). The cul mi na - tion of the Ancylus Lake trans gres sion reached an al ti tude of 34 m a.s.l. (Veski, 1998) in the area, partly in un dat ing the Yoldia Sea shore line. De spite the to pog ra phy in North ern Es to - nia is gen er ally flat, the area around Tallinn is rel a tively rich in small lake and mire bas ins, which were iso lated from the Bal tic Sea Ba sin dur ing the Yoldia Sea stage.

Kessel and Raukas (1979) and Saarse et al. (1997) have ear lier in ves ti gated the Ho lo cene shore line dis place ment in the Tallinn area. Veg e ta tion de vel op ment of many bas ins were stud ied and sev eral con ven tional¹⁴C dates from sed i ment se - quences rep re sent ing iso la tion from the Yoldia Sea stage of the Bal tic Sea Ba sin were ob tained in the area (Poska, 1994;

Kihno, 1996; Veski, 1998; Saarse et al., 1999). Un for tu nately, still few di a tom re cords are avail able from the Yoldia Sea sed i - ments in Es to nia. Eight in ves ti gated deep bore holes are lo cated on the foreklint low land that re flect the Yoldia Sea depositional set ting, with the wa ter depth be tween 25 and 70 m (Lepland et al., 1995), show a low abun dance of di a toms and a mixed ma - rine, brack ish-wa ter and fresh wa ter lit to ral as sem blage (Kessel and Pun ning, 1969; Kessel and Pork, 1971). How ever, high tur bid ity and low trans par ency of the Yoldia Sea sur face wa ter pre clude pos si ble re pro duc tion of ben thic di a toms in such a deep en vi ron ment. There fore most prob a bly these di a toms are trans ported from the coastal area or even par tially re worked from the older de pos its, and for that rea son these re cords must be used with cau tion for palaeosalinity re con struc tion pur - poses. The la goonal de pos its of the Sájamäe Ba sin, that con tain some poorly pre served frag ments of lit to ral brack ish-wa ter di a - toms, were cor re lated with the Yoldia Sea (Kessel and Pun - ning, 1969). Re cently a new site, the Tondi Mire, was stud ied and a slight in flu ence of brack ish wa ter in the lit to ral en vi ron - ment dur ing the Yoldia Sea has been sug gested (Lepland et al., 1995; Kimmel et al., 1996). Pre-iso la tion sed i ments of Lake Kahala that had ac cu mu lated dur ing the Youn ger Dryas and the Early Preboreal are char ac ter ized by large-lake fresh wa ter di a - tom as sem blage (Saarse et al., 1999).

METHODS

Sed i ment sam ples were cored with a 1 m long Rus sian peat sam pler with a di am e ter of 10 cm. At the sites where sam ples for AMS ¹⁴C dat ing were col lected we ob tained up to 10 par al - lel cores from the iso la tion sed i ment se quence.

Sam ples for di a tom anal y ses were heated in 30% H2O2 to get rid of or ganic ma te rial and there af ter fine and coarse min - eral par ti cles were re moved by re peated decantation. If pos si - ble, in each sam ple ca. 400–500 non-Fragilaria spp. valves were counted. Be cause of mass oc cur rence, spe cies of the ge - nus Fragilaria are ex cluded from the to tal in the per cent age cal cu la tions. The di a toms were grouped into brack ish-wa ter, large-lake, la goonal and small-lake taxa ac cord ing to their ecol - ogy and sa lin ity pref er ences, and into plank tonic and lit to ral taxa in ac cor dance with their liv ing hab i tats.

The pol len sam ples were pre pared by the stan dard aceto - lysis method (Berglund and Ralska-Jasiewiczowa, 1986).

More than 1000 ar bo real pol len (AP) grains were counted at each level. The pol len per cent ages were cal cu lated as ar bo real pol len + non-ar bo real pol len (NAP) = 100% (pol len sum). Pol - len con cen tra tion was es ti mated by add ing Lycopodium spores to a known vol ume of sed i ment (Stockmarr, 1971).

The or ganic (OM) and car bon ate con tent (CaCO3) were de ter mined by loss-on-ig ni tion at 550 and 825°C re spec - tively. The ig ni tion res i due was es ti mated as the terrigenous frac tion (TER).

Subsamples for AMS ¹⁴C dat ing, which com prised 3–5 cm slices of sed i ment, were sieved through a 0.5 mm mesh. Ex - tracted macrofossils were iden ti fied un der a bin oc u lar mi cro - scope. De spite of large sam ple vol umes ter res trial macrofossils were not de tected. There fore seeds of aquatic macrophytes were se lected for ra dio car bon dat ing. Af ter iden ti fi ca tion, macrofossil sam ples were im me di ately dried in an alu minium foil pack age at 50°C over night, and were stored in steri lized glass bot tles to pre vent up take of mod ern CO2 or ef fects of bac - te ria. Pre-treat ment of macrofossils fol lowed the stan dard hot acid-al ka line-acid tech nique (Possnert, 1990) and mea sure - ments were car ried out us ing the tan dem ac cel er a tor tech nique at Uppsala Uni ver sity.

The al ti tudes of iso la tion thresh olds of bas ins were lev elled and tied to the near est benchmarks or es ti mated from top o - graphic maps. The re con struc tion of the shore dis place ment curve is based on the thresh old al ti tudes of iso lated bas ins. Iso - la tion al ti tudes of the bas ins are pro jected to the Sájamäe–Rae isobase for the Yoldia Sea run ning from north-east to south- west in North ern Es to nia (Fig. 1). The earth has emerged a lit tle bit faster in the west than in the east. For com pen sa tion of the dif fer en tial up lift heights of the ba sin thresh olds on the both side of the isobase are cor rected.

RESULTS AND INTERPRETATION

PALAEOENVIRONMENTAL RECORDS

In Lake Rummu basal sandy silt and ho mo ge neous clay at 580–505 cm core-depth con tain large-lake di a toms, e.g.

plank tonic Aulacoseira islandica (O. Müller) Simonsen, lit to - ral Diploneis domblittensis (Grunow) Cleve, Gyrosigma attenuatum (Kützing) Rabenhorst and G. spencerii (W.

Smith) Cleve (Fig. 2), in di cat ing a rather deep fresh wa ter en - vi ron ment. This sed i ment unit is cor re lated with the ini tial phase of the Yoldia Sea. The iso la tion of the ba sin is re corded

The history of the Yoldia Sea in Northern Estonia: palaeoenvironmental conditions and climatic oscillations 297

(4)

at the core-depth of 505 cm. The num ber of epipsammic Fragilaria spp., Am phora pediculus (Kützing) Grunow, Nitzschia perminuta (Grunow) Peragallo and Epithemia spp.

in creases, whereas large-lake di a toms de cline. A coastal lake with a shal low wa ter depth and silty bot tom prob a bly de vel - oped in the ba sin. In the gyttja with cal car e ous interlayers di a - tom as sem blage, such as Denticula kuetzingii Grunow and Cymbella diluviana (Krasske) Flo rin sug gests a small shal low hard-wa ter lake.

In the Pääsküla Mire core an abrupt change from large-lake di a tom as sem blage to the brack ish-wa ter taxa, such as Diploneis smithii (Brébisson) Cleve, Tryblionella navicularis (Brébisson) Ralfs and T. punctata W. Smith takes place at 445 cm (Fig. 3).

This shift in the di a tom com po si tion cor re lates with the spread of sa line wa ter into the coastal area of North ern Es to nia. The ex - pan sion of Fragilaria spp., dis ap pear ance of brack ish-wa ter di a - toms and pre dom i nance of small-lake taxa at the bound ary be - tween the clay and the silt re flects the iso la tion level.

Fig. 2. Di a tom di a gram for the sed i ment core from Rummu Lake

PLL — plank tonic large-lake di a toms, LLL — lit to ral large-lake di a toms, IND — in dif fer ent fresh wa ter di a toms, LAG — la goonal fresh wa ter di a toms, SL — small-lake di a toms, OM — or ganic mat ter, TER — terrigenous mat ter

Fig. 3. Di a tom di a gram for the sed i ment core from the Pääsküla Mire LBW — lit to ral brack ish-wa ter di a toms, for other ex pla na tions Fig ure 2

(5)

In the Rae Mire the lower part of clayey silt con tains mostly of large-lake di a toms (Fig. 4), the fre quency of plank tonic di a - toms con stantly de creases and the share of lit to ral spe cies si - mul ta neously in creases. This sug gests that the re gres sion of the ini tial fresh wa ter Yoldia Sea led to a con tin u ous drop of the wa ter level in the ba sin. A dis tinct change in the fos sil di a tom as sem blage is re corded at the core-depth of 685 cm. An in - crease of Fragilaria spp., ex pan sion of la goonal epiphytic Epithemia spe cies and epipsammic di a tom Am phora pediculus, to gether with a rise in slightly brack ish-wa ter spe - cies in fer that a shel tered la goon, which was con nected with the brack ish Yoldia Sea by shal low straits de vel oped in the Rae Ba sin. The fi nal iso la tion of the Rae Ba sin is re corded in the

tran si tion be tween the silt and the lake lime and is char ac ter ized by the pre dom i nance of a di a tom com po si tion, which re sem - bles a shal low hard-wa ter lake.

PALAEOCLIMATIC RECORDS

The basal sandy and clayey silt in Lake Rummu at 580–510 cm is char ac ter ized by a Youn ger Dryas pol len flora with re de pos ited AP pol len grains such as Alnus, Picea, Corylus, Ulmus, Quercus and Carpinus (Fig. 5). The pol len val ues for Pinus and Betula are rather uni form, the lat ter is mostly rep re sented as Betula nana-type. Of the other lo cally

Fig. 4. Di a tom di a gram for the sed i ment core from the Rae Mire For other ex pla na tions see Fig ures 2 and 3

Fig. 5. Pol len di a gram from Lake Rummu

PBO cor re sponds to the Preboreal os cil la tion; QM — quercetum mixtum; AP — ar bo real pol len; NAP — non-ar bo real pol len;

for other ex pla na tions see Fig ure 2

(6)

pro duced AP Juniperus and Salix are well rep re sented. The share of NAP is around 30%. The val ues of or ganic frac tion are very low in the pre-iso la tion sed i ments (Fig. 2). At 510 cm Betula nana-type and Juniperus de cline, so do the NAP, Ar te - mi sia and Chenopodiaceae. This shift in di cates the on set of the Ho lo cene warm ing. From 500 cm the NAP curve abruptly rises, there is a rise also in Salix, Juniperus, Betula nana-type, Ar te mi sia, Chenopodiaceae and Ericaceae (Empetrum nigrum) cul mi nat ing at 475 cm. A de cline in Filipendula oc curs at the same level. These changes may sug gest a cool ing of the cli - mate. There is also a slight in crease in terrigenous frac tion at 475 cm, oth er wise the con tent of terrigenous mat ter is sta ble and de creas ing in the sed i ment in ter val 500–475 cm. From 475 cm and up wards the AP/NAP ra tio changes again in fa vour of the first and pre sum ably in di cates veg e ta tion re sponse to the ter mi na tion of cli ma tic cool ing event.

The pol len re cord of the Pääsküla Mire shows a higher per - cent age of herbs from 445–435 cm (Fig. 6). Con tem po ra ne ous peaks of Juniperus, Betula nana-type and Ericaceae oc cur at the same level. The pol len ev i dence may be in ter preted as a slight cool ing. In crease of pol len taxa in di cat ing de te ri o ra tion of the cli mate oc curs si mul ta neously with the ap pear ance of brack ish-wa ter di a toms in the sed i ment se quence.

The 2 metre sec tion of sed i ment in ves ti gated from Rae Mire shows a tran si tion from Youn ger Dryas to Early Ho lo - cene (Fig. 7). There seems to be a grad ual warm ing from 730–700 cm, with in creas ing to tal pol len con cen tra tion (TPC) and AP, at the same time, de creas ing per cent ages of Betula nana-type, Ar te mi sia, Chenopodiaceae and herbs in gen eral.

There are two de creases in TPC at 680 and 650 cm re spec - tively, the lat ter be ing more pro nounced. As so ci ated with those de creases in TPC are peaks of Betula nana-type, dwarf shrubs (Ericaceae: Empetrum nigrum), NAP in gen eral, Ar te - mi sia, Chenopodiaceae, Selaginella and Rubus chamaemo - rus. The doc u mented pol len pat tern again sug gests a cool ing of the cli mate. Above 650 cm, to gether with the change in li - thol ogy from silt to lake lime the TPC and the val ues of AP rise re mark ably, the val ues of NAP, dwarf shrubs and Betula nana-type de crease. The pol len ev i dence shows a dis tinct warm ing of the cli mate.

RADIOCARBON DATES

The re sults of the AMS ¹⁴C dat ing are sum ma rized in Ta - ble 1. The up per most sam ple (Ua–15322) from the post- isola -

Fig. 6. Pol len di a gram from the Pääsküla Mire

PBO cor re sponds to the Preboreal os cil la tion; for other ex pla na tions see Fig ure 3

Fig. 7. Pol len di a gram from the Rae Mire

PBO cor re sponds to the Preboreal Os cil la tion; for other ex pla na tions see Fig ure 4

(7)

tional sed i ment of Pääsküla Mire yielded an age that ei ther in - fers to the hi a tus in the sed i ment re cord. Mea sure ments from the post-isolational sed i ments of Pääsküla and Rae bas ins (Ua–15321, Ua–15328, Ua–15327) are ob vi ously older by sev eral hun dred years than the age ob tained ac cord ing to the iso la tion al ti tude and pol len stra tig ra phy, and might be re lated to the hard-wa ter ef fect. Both the rel a tively high val ues of CaCO3 frac tion and the di a tom as sem blage tol er ant of car bon - ate-rich wa ter (Fig. 4) re veals that a lake in the Rae Ba sin was fed by hard ground wa ter. In ad di tion, six mea sure ments on the float ing-leaved aquatic macrophyte seeds from the sed i ments cor re spond ing to the brack ish-wa ter phase of the Yoldia Sea yielded in con sis tent and too old ages. The high age of ra dio car - bon dates ob tained from these pre-iso la tion sed i ments might be at trib uted to a con sid er able ma rine res er voir ef fect in the Bal tic Sea Ba sin dur ing the Early Ho lo cene.

SHORELINE DISPLACEMENT

Dur ing the fi nal drain age of the Bal tic Ice Lake wa ter level dropped by around 25 m to the level of 50 m a.s.l. in North ern Es to nia. Due to in ten sive land up lift the shore line dis place ment was re gres sive through out the Yoldia Sea stage (Fig. 8). Af ter the re treat of the Yoldia Sea, coastal de pres sions of the emerg - ing for mer sea bot tom were oc cu pied by fens. The sandy in fer - tile near-shore sed i ments were a suit able sub strate for pine for - est im mi gra tion. The trans gres sion of the Ancylus Lake led once more to the in un da tion of the area. The lay ers of peat and bur ied pine stumps un der ly ing the transgressive Ancylus Lake de pos its al low us to glance at the ex tent of the Yoldia Sea lowstand. A bur ied peat in Lake Ülemiste at an al ti tude of 25.5 m a.s.l. and a piece of wood in Lake Maardu have been dated to 9500 ¹⁴C BP (Saarse et al., 1997). A thin peat layer in Lake Maardu at an al ti tude of 24 m a.s.l. is the low est layer of bur ied or ganic mat ter ob served in the area of the Tallinn and prob a bly is re lated to the low est shore-level of the Yoldia Sea, in di cat ing that the to tal am pli tude of re gres sion for the Yoldia Sea in the study area amounted to some 25 m (Fig. 8).

DISCUSSION

PALAEOENVIRONMENTAL CONDITIONS FOR THE YOLDIA SEA

The fi nal drain age of the Bal tic Ice Lake took place ca. 35 years be fore the Youn ger Dryas/Preboreal tran si tion (Andrén et al., 2002) and the con se quent rapid warm ing which is dated to ca. 11 550 GRIP ice core years BP (Björck et al., 1998). The varved clays that ac cu mu lated in the Bal tic Sea proper (Abelmann, 1985; Andrén et al., 2000) dur ing the ini tial phase of the Yoldia Sea are al most bar ren of di a toms. The small num - ber of plank tonic large-lake di a tom Aulacoseira islandica prob a bly sug gests that the off shore ar eas were still oc cu pied by fresh wa ter. Di a tom pro duc tiv ity was low dur ing the be gin ning of the Yoldia Sea prob a bly be cause of high tur bid ity and low trans par ency from silt and clay in tro duced by melt-wa ter from the re treat ing ice sheet. Di a tom scar city can also be at trib uted to the very high sed i men ta tion rates caus ing a pro nounced di lu - tion ef fect in the sed i ment. Sev eral pre-iso la tion sed i ment re - cords in North ern Es to nia (Ta ble 2; Heinsalu, 2000) in di cate fresh wa ter con di tion in the lit to ral ar eas along the south ern coast of the Gulf of Fin land dur ing the ini tial 250 years of the Yoldia Sea (Fig. 8). In very shal low lit to ral zone, whereas light pen e trated down to the bot tom, con di tions for pho to syn the sis of large-lake periphytic di a toms ex isted.

The on set of the brack ish phase of the Yoldia Sea can be placed at 11 310 cal en dar years BP (Björck, 1999). Wasteg¯rd et al. (1998) clearly dem on strated that the main sa line wa ter ingression into the Bal tic Sea Ba sin dur ing the Yoldia Sea fol - lowed the deglaciation of the Närke Strait. As the brack ish Yoldia Sea phase al most co in cides with the Preboreal os cil la - tion, Björck et al. (1996) im plied that this cool ing event prob a - bly trig gered a tem po rary en try of sa line wa ter into the Bal tic Sea Ba sin in con nec tion with di min ished melt wa ter dis charge from the ice sheet and cor re spond ingly de creased fresh wa ter trans port from the Bal tic Sea Ba sin. The cli ma tic cool ing dur - ing the Preboreal os cil la tion and pos si ble re or ga ni za tion of the ocean/atmophere cir cu la tion in the North At lan tic may have a

T a b l e 1 AMS ¹⁴C dates on plant macrofossil from North ern Es to nia

Site Depth

[cm] En vi ron ment Ma te rial dated

14C age BP (±1ó)

d¹³C ‰

PDB Lab code

Pääsküla Mire 420–425 post-isolational seeds: Schoenoplectus lacustris 8 630±100 –24.5 Ua–15322 Pääsküla Mire 432–437 post-isolational seeds: Potamogeton natans 10 605±95 –12.3 Ua–15321 Pääsküla Mire 437–441 pre-isolational seeds: Potamogeton natans 11 360±115 –15.8 Ua–15320 Pääsküla Mire 441–445 pre-isolational seeds: Potamogeton spp. 11 420±95 –13.9 Ua–15319 Rae Mire 643–648 post-isolational seeds: Potamogeton spp. 10 305±110 –9.0 Ua–15328 Rae Mire 650–653 post-isolational seeds: Potamogeton spp. 10 485±105 –10.9 Ua–15327

Rae Mire 658–664 la goon seeds: Potamogeton spp. 11 060±125 –10.2 Ua–15326

Rae Mire 670–675 la goon seeds: Potamogeton spp. 11 070±115 –11.7 Ua–15325

(8)

large im pact for the gen er a tion of stron ger west erly winds and strength ened storm tracks. The pres ent wa ter ex change be - tween the Kattegat and the Bal tic Sea is re - stricted by the shal low Dan ish Sounds.

Only ma jor in flows trans port sub stan tial amounts of highly sa line wa ter into the deep bas ins of the Bal tic Sea (Matthäus and Franck, 1992). These rare in flows oc - cur in ter mit tently af ter pe ri ods of per sis - tent strong west erly winds (Krauss and Brügge, 1991) and are pos si bly reg u lated by the North At lan tic os cil la tion (Hän - ninen et al., 2000).

An in flow of dense brack ish bot tom wa - ter in the Stock holm area is reg is tered by fos sil finds of the arc tic ma rine mol lusc Portlandia arctica, ben thic foraminifera and ostracods, as well as by the tran si tion from grey diatactic varved clay to sym - mictic clay varves with red dish col our and FeS stain ing (Brunnberg, 1995; Wasteg¯rd and Schoning, 1997). Ben thic cal car e ous fauna in the north west ern Bal tic Sea proper

Fig. 8. Syn the sis of the early post-gla cial his tory of the Bal tic Sea and palaeoenvironmental and cli ma tic con di tions in the stud ied area, the Gulf of Fin land and the Bal tic Sea proper

The rect an gles on the shore dis place ment curve show the iso la tion al ti tudes of the bas ins in ves ti gated; the filled rect an gles cor re spond to a freshwa ter en - vi ron ment and the opened rect an gles in di cate brack ish-wa ter con di tions; the cal en dar years time-scale fol lows the event stra tig ra phy for the Early Ho lo - cene in south-cen tral Swe den de fined by Björck (1999)

T a b l e 2 Lo ca tion of the in ves ti gated bas ins (Fig. 1) in North ern Es to nia with in di ca tion of thresh old al ti tudes be fore and af ter the cor rec tion for dif fer ence in land up lift and cor re spond ing pre-isolational en vi ron men tal con di tions by means of di a tom anal y sis

Site Lat i tude

N Lon gi tude E

Thresh old al ti tude [m a.s.l.]

Cor rected al ti tude [m a.s.l.]

Pre-isolational en vi ron ment

Männi Bog 59°17’00’’ 24°54’30’’ 43 48 fresh wa ter

Vaida Mire 59°18’30’’ 24°57’15’’ 41 45.5 fresh wa ter

Lake Limu 59°20’20’’ 24°59’50’’ 39 43.5 fresh wa ter

Lake Järveotsa 59°05’45’’ 24°09’15’’ 41 43 fresh wa ter

Lake Rummu 59°27’10’’ 25°17’10’’ 37 42 fresh wa ter

Maardu Mire 59°24’45’’ 25°02’20’’ 37.5 40.5 fresh wa ter

Lake Kahala 59°29’30’’ 25°32’00’’ 33.5 40 fresh wa ter

Vandjala Mire 59°26’20’’ 25°03’00’’ 37.5 39 fresh wa ter

Tondi site 59°26’30’’ 24°51’40’’ 38.5 38 brack ish

Rae Mire 59°24’15’’ 24°51’45’’ 37.5 37.5 brack ish

Sájamäe site 59°25’45’’ 24°50’10’’ 37.5 37.5 brack ish

Saha Mire 59°25’45’’ 24°59’15’’ 34 35 brack ish

Pääsküla Mire 59°21’15’’ 24°40’00’’ 34 33.5 brack ish

Lake Maardu 59°26’30’’ 24°59’30’’ 32 33 fresh wa ter

Lake Ülemiste 59°23’30’’ 24°46’15’’ 31.5 31 fresh wa ter

(9)

in fer bot tom wa ter sa lin ity around 10‰ for the brack ish phase of the Yoldia Sea (Schoning et al., 2001). A peak of the plank tonic di a tom Thalassiosira baltica (Grunow) Ostenfeld is well rep li - cated in sev eral sed i ment cores from the Bal tic Sea proper and can be con sid ered to be a sig nif i cant ev i dence that the sa line wa - ter pulse was strong enough to break the wa ter col umn strat i fi ca - tion. The high abun dance of T. baltica is doc u mented in a thin sed i ment se quence with black bands and lenses of amor phous Fe-mono sulphides (e.g. Sohlenius et al., 1996; Lepland et al., 1999). The for ma tion of FeS pre cip i ta tion can in di cate the pres - ence of sa line and anoxic bot tom wa ter (Huckriede et al., 1996).

T. baltica pre fers sur face wa ter with low sa lin ity (Snoeijs, 1993).

In ad di tion, T. baltica in hab its sea ice in the mod ern Bal tic Sea (Ikävalko and Thom sen, 1997). This ev i dence in di cate a cli ma tic cool ing and sup ports si mul ta ne ity of the brack ish Yoldia Sea phase and the Preboreal os cil la tion. Com bi na tion of the strati - graphic ev i dence sug gests that a slightly brack ish sur face wa ter layer above more sa line wa ter col umn dis trib uted in the west ern part of the Bal tic Sea Ba sin dur ing the brack ish phase of the Yoldia Sea (Fig. 9). Sur face wa ter sa lin ity fa voured co ag u la tion of clay par ti cles, trans par ency of the wa ter col umn in creased and suf fi cient light con di tion for bloom ing of plank tonic di a toms was es tab lished.

Al to gether five lo cal i ties in North ern Es to nia show abun - dant brack ish-wa ter di a tom as sem blages and in di cate that brack ish wa ter reached the Gulf of Fin land dur ing the Yoldia Sea stage (Heinsalu, 2000). Two sed i ment re cords, in Pääsküla and Saha bas ins, re veal tran si tion from the ini tial fresh wa ter Yoldia Sea phase to the brack ish phase. Large-lake di a toms al - most dis ap peared and were re placed by abun dant lit to ral brack -

ish-wa ter taxa, e.g. Tryblionella navicularis, T. punctata, Diploneis smithii, D. interrupta and Mastogloia braunii. Ac - cord ing to the di a tom as sem blages the sa lin ity dur ing the brack ish phase of the Yoldia Sea in the lit to ral ar eas of the Gulf of Fin land was lower than that of to day. In ad di tion, lit to ral sed - i ment se quences of the Yoldia Sea with sim i lar brack ish-wa ter di a tom as sem blages are doc u mented south of the Salpausselkä end mo raine zone in Fin land (e.g. Valovirta, 1965; Tynni, 1966), in the vi cin ity of Vyborg, Rus sian Karelia (Hyyppä, 1937) and on the Karelian Isth mus (Saarnisto et al., 1999). The mix ing of the wa ter col umn and cir cu la tion of brack ish wa ter up to the sur face in cer tain ar eas along the coast of the Gulf of Fin land (Fig. 9) was prob a bly trig gered by coastal upwelling gen er ated by per sis tent strong winds blow ing off shore or along the shore in the op po site di rec tion to the per ma nent sur face cur - rent, trans port ing the sur face wa ter away and lift ing sa line wa - ter from the deeper lay ers. Upwelling at the coasts of the mod - ern Bal tic Sea oc curs fre quently (e.g. Kahru et al., 1995;

Alenius et al., 1998).

In con trast with the other sub-bas ins of the Bal tic Sea, the Gulf of Fin land is a di rect con tin u a tion of the Bal tic Sea proper with out any no ta ble sill. Even if the cas cade of sub-bas ins is a se ri ous ob sta cle for the spread of sa line wa ter into the mar ginal down stream bas ins, a ma jor sa line in flow through the Dan ish Straits in win ter 1993 was de tected in the Gulf of Fin land al - ready some 18 months af ter the pulse (Alenius et al., 1998).

The pres ent gen eral cir cu la tion pat tern in the Gulf of Fin land is gov erned by den sity dif fer ences and the Coriolis ef fect lead ing to the mean cy clonic (anti-clock wise) wa ter cir cu la tion (Mälkki and Tamsalu, 1985) and we may as sume that sim i lar ba sic hy dro dy namic pro cesses played a sig nif i - cant role in the past. Thus the pen e tra tion of salt wa ter dur ing the Yoldia Sea took prob a bly place mainly along the south coast of the Gulf of Fin - land and the sa line wa ter pulse would flow as a near-bot tom cur rent, whereas at the same time the out flow of melt wa ter flux dis charg ing from the ice sheet oc curred along the north coast. So far, no ev i dence of the brack ish sur face wa ter layer has been re vealed in the off shore set tings of the Gulf of Fin land dur ing the Yoldia Sea. In stead, a strat i fied wa ter col umn with brack ish wa ter at the bot tom and tur bid fresh wa ter in the sur face most likely ex isted at the en trance of the Gulf of Fin - land (Heinsalu et al., 2000a). Per haps the per ma - nent halocline was deep enough and sup pressed ef fi cient ver ti cal mix ing of the wa ter col umn up to the sur face.

The end of the brack ish phase of the Yoldia Sea is dated to 10 315 varved clay years BP, which cor re sponds to an age of 11 190 GRIP ice core years BP (Björck, 1999). Di a tom as sem blages from the lit to ral pre-iso la tion sed i ment se quences of Maardu and Ülemiste lakes in di cate fresh wa ter con di tions dur ing the fi nal phase of the Yoldia Sea.

Sim i lar en vi ron men tal con di tions have been reg is - tered on the Suursaari Is land in the Gulf of Fin land (Heinsalu et al., 2000b). The off shore sed i ments from the Bal tic Sea proper (Andrén et al., 2002)

Fig. 9. Palaeo ge ogra phy and palaeohydrology of the brack ish Yoldia Sea phase around 11 300 years BP

(10)

and the Gulf of Fin land (Heinsalu et al., 2000a) dis play also fresh wa ter con di tions for the sur face wa ters and a very low ab so - lute abun dance of di a toms. The abrupt de cline and dis ap pear - ance of Thalassiosira baltica sug gests a rapid fresh en ing of the sur face wa ters in the Bal tic Sea Ba sin (Lepland et al., 1999) pre - sum ably due to emerg ing and nar row ing sills (Björck, 1995) as well as a con se quence of in creased melt wa ter dis charge fol low - ing the Early Preboreal cool ing (Björck et al., 1997).

TIMING OF THE BRACKISH YOLDIA SEA PHASE IN THE EASTERN BALTIC SEA

AMS ¹⁴C dates on aquatic plant seeds from the sed i ments of North ern Es to nia in di cate that the res er voir age for the Yoldia Sea was con sid er ably higher, 1000–1500 years, than the es ti - mate for mod ern ma rine and brack ish-wa ter res er voir ef fect ca.

400 years (Stuvier et al., 1998). A res er voir ef fect of more than 1000 years has also been de tected by AMS ¹⁴C dates of ma rine ben thic fauna from varved clays in Swe den (Wasteg¯rd and Schoning, 1997; Björck et al., 2001). The con sid er able vari - abil ity in at mo spheric ¹⁴C con tent dur ing the Early Preboreal (Björck et al., 1996) must be taken into ac count. Björck et al.

(2003) have sug gested that sim i larly high ma rine res er voir ages dur ing the Youn ger Dryas cold pe riod in the Nor we gian Sea have been caused by chang ing at mo spheric ¹⁴C con tent.

The prob lems as so ci ated with the es tab lish ment of the Yoldia Sea ra dio car bon chro nol ogy for the east ern Bal tic Sea Ba sin can be solved by cor re lat ing the en vi ron men tal and cli - ma tic changes dur ing the Yoldia Sea with al ter na tive chro nol o - gies. Gla cial varved clays with their light silty sum mer and dark clayey win ter layer form ing within one year de pos ited in front of the re ced ing ice mar gin in south-cen tral Swe den dur ing the Yoldia Sea stage. Mea sure ment and cor re la tion of the varve se - quences has al lowed Swed ish ge ol o gist to pro duce the Swed ish Time Scale (STS), a con tin u ous cal en dar year re cord cov er ing the last 13 200 varve years BP (e.g. Wohlfarth et al., 1995).

How ever, the com par i son of dendrochronological and GRIP ice core re cords with the STS shows that 800–900 years are miss ing from the Ho lo cene part of the Swed ish Time Scale (e.g. Wohlfarth et al., 1997). The Youn ger Dryas/Preboreal cli - mate shift and cli ma tic de te ri o ra tion as so ci ated with the Preboreal os cil la tion and re corded in the pol len spec tra and varve thick ness proxy in the north west ern Bal tic Sea proper made it pos si ble to link the STS with chro nol ogy from the GRIP ice core (Andrén et al., 1999). The Preboreal os cil la tion oc curred in be tween 11 315 and 11 195 GRIP ice core years BP (Björck, 1999) and is iden ti fied in nu mer ous pol len re cords in North ern Eu rope (Björck et al., 1997; Andrén et al., 1999).

On the ba sis of pol len stra tig ra phy of four cores from North - ern Es to nia the floristic re sponse to the Preboreal os cil la tion was fairly dis tinct, de spite the area was only re cently emerged. There is a de crease in TPC in com bi na tion with some in crease in herb pol len (Ar te mi sia, Chenopodiaceae, Rumex acetosa/ace - tosella-type, Rubus chamaemorus and spores of Selaginella), Betula nana-type, Juniperus and Salix and also in the dwarf

shrubs. A de cline in the thermophilous Filipendula oc curs at the same time. The pol len com po si tion sug gests a de te ri o ra tion of the cli mate at that time. In Pääsküla, Rae and Saha bas ins (Kihno, 1996) these changes in the veg e ta tion oc cur si mul ta - neously with the ap pear ance of lit to ral brack ish-wa ter di a toms in pre-iso la tion sed i ments. In the Rummu Ba sin, which was iso - lated prior to the spread of brack ish wa ter to the coast of North - ern Es to nia, the changes in veg e ta tion dis play ing cli ma tic de te ri - o ra tion are ob served af ter the iso la tion of the lake.

The co in ci dence of the prox ies show ing cli ma tic cool ing dur ing the Early Preboreal with those in di cat ing brack ish-wa - ter con di tions dur ing the Yoldia Sea in North ern Es to nia is good. More over, shore line dis place ment curve for the Tallinn area in di cates that brack ish-wa ter con di tions per sisted for a pe riod about 100 years (Fig. 8). Con se quently, that ev i dence leaves no room for the as sump tion that a de lay of some 80–90 years oc curred in sa line wa ter pen e tra tion from east ern cen tral Swe den to South ern Fin land (Donner, 1969;

Strömberg, 1990). It seems fairly rea son able to con clude that the brack ish-wa ter flow from the in flow area reached the Gulf of Fin land within a very short time.

CONCLUSIONS

1. The his tory of the Yoldia Sea stage in North ern Es to nia can be di vided into three phases. The ini tial and fi nal phases were char ac ter ized by fresh wa ter con di tions, while brack - ish-wa ter con di tions pre vailed for a short pe riod in be tween.

The re sults of new di a tom stud ies cor re late with the no tion of a well-con strained de vel op ment of the Yoldia Sea ex tend ing from the in flow area in south-cen tral Swe den into the Bal tic Sea proper, but they do not sup port the hy poth e sis that the sa - line wa ter failed to reach the Gulf of Fin land.

2. The lit to ral di a tom as sem blages in North ern Es to nia in di - cate that brack ish wa ter reached the Gulf of Fin land dur ing the Yoldia Sea stage. Coastal upwelling caused mix ing of the wa ter col umn and the cir cu la tion of brack ish wa ter up to the sur face in cer tain near-shore ar eas in the Gulf of Fin land. Sa lin ity was prob a bly lower in these coastal ar eas than it is to day.

3. The pol len stra tig ra phy of the cores from North ern Es - to nia points to a fairly dis tinct floristic re sponse to the Preboreal os cil la tion that oc curred si mul ta neously with the ap pear ance of lit to ral brack ish-wa ter di a toms. Thus the short-lived brack ish phase of the Yoldia Sea ex isted ca.

11.300–11.200 cal en dar years BP. AMS dates ob tained for aquatic macrofossils sug gest res er voir ages of 700–1500 years for the brack ish Yoldia Sea phase.

Ac knowl edge ments. A. Mankki per formed the pol len anal y ses of the Pääsküla Mire sed i ment se quence. We thank Dr. S. Wasteg¯rd, Prof. L. Marks and Dr. P. Johansson for con - struc tive com ments on an ear lier ver sion of the manu script.

This re search was funded by the Nessling Foun da tion, Es to - nian Sci ence Foun da tion (grants 6736,7029) and Es to nian Min is try of Ed u ca tion (re search programme SF0332710s06).

(11)

REFERENCES

ABELMANN A. (1985) — Palökologische und ökostratigraphische Untersuchungen von Diatomeenassoziationen an holozänen Sedimenten der zentralen Ostsee. Ber. – Rep., Geol.-Paläontol. Inst.

der Univ. Kiel, 9: 1–200.

ALENIUS P., MYRBERG K. and NEKRASOV A. (1998) — Phys i cal ocean og ra phy of the Gulf of Fin land: a re view. Bo real En vi ron. Res., 3: 97–125.

ALHONEN P. (1971) — The stages of the Bal tic Sea as in di cated by the di - a tom stra tig ra phy. Acta Bot. Fenn., 92: 1–18.

ANDRÉN E., ANDRÉN T. and KUNZENDORF H. (2000) — Ho lo cene his tory of the Bal tic Sea as a back ground for as sess ing re cords of hu - man im pact in the sed i ments of the Gotland Ba sin. Ho lo cene, 10:

687–702.

ANDRÉN T., BJÖRCK J. and JOHNSEN S. (1999) — Cor re la tion of Swed ish gla cial varves with the Green land (GRIP) ox y gen iso tope re - cord. J. Quat. Sc., 14: 361–371.

ANDRÉN T., LINDEBERG G. and ANDRÉN E. (2002) — Ev i dence of the fi nal drain age of the Bal tic Ice Lake and the brack ish phase of the Yoldia Sea in gla cial varves from the Bal tic Sea. Boreas, 31: 226–238.

BERGLUND B. E. and RALSKA-JASIEWICZOWA M. (1986) — Pol len anal y sis and pol len di a grams. In: Hand book of Ho lo cene Palaeo ec ol - ogy and Palaeohydrology (ed. B. E. Berglund): 455–484. John Wiley and Sons Ltd. Chichester.

BJÖRCK S. (1995) — A re view of the his tory of the Bal tic Sea, 13.0–8.0 ka BP. Quat. Int., 27: 19–40.

BJÖRCK J. (1999) — Event stra tig ra phy for the Last Gla cial-Ho lo cene tran si tion in east ern mid dle Swe den. Quaternaria A, 6: 1–48.

BJÖRCK S., KOÇ N. and SKOG G. (2003) — Con sis tently large ma rine res er voir ages in the Nor we gian Sea dur ing the Last Deglaciation.

Quat. Sc. Rev., 22: 429–435.

BJÖRCK S., KROMER B., JOHNSEN S., BENNIKE O., HAMMARLUND D., LEMDAHL G., POSSNERT G., RASMUSSEN T. L., WOHLFARTH B., HAMMER C. U. and SPURK M. (1996) — Syn chro nized ter res trial-at mo spheric deglacial re cords around the North At lan tic. Sci ence, 274: 1155–1160.

BJÖRCK J., POSSNERT G. and SCHONING K. (2001) — Early Ho lo - cene deglaciation chro nol ogy in Västergötland and Närke, south ern Swe den — biostratigraphy, clay varve, ¹⁴C and cal en dar year chro nol - ogy. Quat. Sc. Rev., 20: 1309–1326.

BJÖRCK S., RUNDGREN M., INGÓLFSSON Ó. and FUNDER S. (1997)

— The Preboreal os cil la tion around the Nordic Seas: ter res trial and lac us trine re sponses. J. Quat. Sc., 12: 455–465.

BJÖRCK S., WALKER M. J. C., CWYNAR L. C., JOHNSEN S., KNUDSEN K.-L., LOWE J. J., WOHLFARTH B. and In ti mate Mem - bers (1998) — An event stra tig ra phy for the Last Ter mi na tion in the North At lan tic re gion based on the Green land Ice-core re cord: a pro - posal by the INTIMATE group. J. Quat. Sc., 13: 283–292.

BRUNNBERG L. (1995) — Clay-varve chro nol ogy and deglaciation dur - ing the Youn ger Dryas and Pre-bo real in the east ern most part of the Mid dle Swed ish Ice Mar ginal Zone. Quaternaria A, 2: 1–94.

DONNER J. J. (1969) — Land/sea level changes in south ern Fin land dur - ing the for ma tion of the Salpausselkä endmoraines. Bull. Geol. Soc.

Finl., 41: 135–150.

FISHER T. G., SMITH D. G. and ANDREWS J. T. (2002) — Preboreal os - cil la tion caused by a gla cial Lake Agassiz flood. Quat. Sc. Rev., 21:

873–878.

FREDÉN C. (1988) — Ma rine life and deglaciation chro nol ogy of the Vänern ba sin south west ern Swe den. Sver. Geol. Unders., Ca, 71:

1–80.

FRIES J. O., WAHLQVIST A. H. and TÖRNEBOHM A. E. (1863) — NDgra ord till upplysning om bladet “Stock holm”. Sver. Geol.

Unders., Aa, 6: 1–75.

GUDELIS V. and KÖNIGSSON L.-K. eds. (1979) — The Qua ter nary his - tory of the Bal tic. Acta Univ. Ups. Symp. Univ. Ups. Ann. Quing. Cel., 1: 1-279.

HÄNNINEN J., VUORINEN I. and HJELT P. (2000) — Cli ma tic fac tors in the At lan tic con trol the ocean o graphic and eco log i cal changes in the Bal tic Sea. Limnol. Oceanogr., 45: 703–710.

HEINSALU A. (2000) — Di a tom stra tig ra phy and palaeoenvironment of the Yoldia Sea in north ern Es to nia. Proc. Es to nian Acad. Sc. Geol., 49:

218–243.

HEINSALU A., KOHONEN T. and WINTERHALTER B. (2000a) — Early post-gla cial en vi ron men tal changes in the west ern Gulf of Fin - land based on di a tom and lithostratigraphy of the sed i ment core B–51.

Baltica, 13: 51–60.

HEINSALU A., VESKI S. and VASSILJEV J. (2000b) — Palaeoenvironment and shore line dis place ment on Suursaari Is land, Gulf of Fin land. Bull. Geol. Soc. Finl., 71: 21–46.

HUCKRIEDE H., CLASEN S. and MEISCHNER D. (1996) — Hy dro - graphic and cli ma tic changes re corded in Ho lo cene sed i ments of the Cen tral Bal tic Sea. Baltica, 9: 76–91.

HYVÄRINEN H., RAUKAS A. and KESSEL H. (1992) — Yoldia and Echeneis Seas (in Rus sian with Eng lish sum mary). In: Ge ol ogy of the Gulf of Fin land (eds. A. Raukas and H. Hyvärinen): 276–282. Es to - nian Acad. Sc. Tallinn.

HYYPPÄ E. (1937) — Post-gla cial changes of shore-line in South Fin land.

Bull. Comm. Géol. Finl., 120: 1–225.

IKÄVALKO J. and THOMSEN H. A. (1997) — The Bal tic Sea ice biota (March 1994): a study of the Protistan com mu nity. Eur. J. Protistol., 33: 229–243.

KAHRU M., HDKANSSON B. and RUD O. (1995) — Dis tri bu tions of the sea-sur face tem per a ture fronts in the Bal tic Sea as de rived from sat el - lite im ag ery. Cont. Shelf Res., 15: 663–679.

KESSEL H. (1961) — An cient coastal for ma tions of the Bal tic on the ter ri - tory of the Es to nian S.S.R. ENSV Tead (in Rus sian with Eng lish sum - mary). Akad. Geol. Inst. Uurimused, 8: 113–131.

KESSEL H. and MIIDEL A. (1973) — On the late and post-gla cial crustal move ments in Es to nia (in Rus sian with Eng lish sum mary). Proc. Acad.

Sc. Est. SSR, Chem., Geol., 22: 257–264.

KESSEL H. and PORK M. (1971) — On the biostratigraphy of bot tom sed - i ments of the Bal tic Sea in Es to nia. In: Palynological In ves ti ga tions in the Bal tic (in Rus sian). (ed. D. T. Bartosh): 98–110. Zinatne. Riga KESSEL H. and PUNNING J.-M. (1969) — Über die Verbreitung und

Stratigraphie der Sedimente des Joldiameeres in Estland (in Rus sian with Ger man sum mary). Proc. Acad. Sc. Est. SSR, Chem., Geol., 18:

154–163.

KESSEL H. and RAUKAS A. (1979) — The Qua ter nary his tory of the Bal - tic Es to nia. In: The Qua ter nary His tory of the Bal tic (eds. V. Gudelis and L.-K. Königsson): 127–146. Acta Univ. Ups. Symp. Univ. Ups.

Ann. Quing. Cel., 1: 127-146.

KIHNO K. (1996) — The Ho lo cene pol len re cord from Saha Mire and its cor re la tion with the vegetational his tory as re corded at Lake Maardu.

Pact, 51: 181–188.

KIMMEL K., RAJAMÄE R. and SAKSON M. (1996) — The Ho lo cene de vel op ment of Tondi Mire, north ern Es to nia: pol len, di a tom and chro no log i cal stud ies. Pact, 51: 85–102.

KRAUSS W. and BRÜGGE B. (1991) — Wind-pro duced wa ter ex change be tween the deep bas ins of the Bal tic Sea. J. Phys. Oceanogr., 21:

373–384.

LEPLAND A., HEINSALU A. and STEVENS R. (1999) — The pre-Littorina di a tom stra tig ra phy and sed i ment sulphidisation re cord from the west-cen tral Bal tic Sea: im pli ca tions of the wa ter col umn sa - lin ity vari a tions. GFF, 121: 57–65.

LEPLAND A., MILLER U. and SAKSON M. (1995) — Palaeoenvironmental con di tions dur ing the Bal tic Yoldia stage in the Tallinn area, north ern Es to nia. Quat. Int., 27: 83–94.

MÄLKKI P. and TAMSALU R. (1985) — Phys i cal fea tures of the Bal tic Sea. Fin. Mar. Res., 252: 1–110.

MATTHÄUS W. and FRANCK H. (1992) — Char ac ter is tics of ma jor Bal - tic in flows — a sta tis ti cal anal y sis. Cont. Shelf Res., 12: 1375–1400.

POSKA A. (1994) — Three pol len di a grams from coastal Es to nia.

Kvartärgeologiska Avdelningen, Uppsala Univ., 170: 1–40.

POSSNERT G. (1990) — Ra dio car bon dat ing by the ac cel er a tor tech nique.

Norw. Arch. Rev., 23: 30–37.

RAUKAS A. (1994) — Yoldia stage — the least clear in ter val in the Bal tic Sea his tory. Baltica, 8: 5–14.

(12)

RAUKAS A. (1995) — Evo lu tion of the Yoldia Sea in the east ern Bal tic.

Quat. Int., 27: 99–102.

SAARNISTO M., GRÖNLUND T. and EKMAN I. (1995) — Lateglacial of Lake Onega – con tri bu tion to the his tory of the east ern Bal tic ba sin.

Quat. Int., 27: 111–120.

SAARNISTO M., GRÖNLUND T. and IKONEN L. (1999) — The Yoldia Sea–Lake Ladoga connexion. Biostratigraphical ev i dence from the Karelian Isth mus. In: Dig it all: Pa pers Ded i cated to Ari Siiriäinen (ed.

M. Huurre): 117–130. Fin. Antiq. Soc. Arch.. Soc.

SAARSE L., HEINSALU A., POSKA A., VESKI S. and RAJAMÄE R.

(1999) — Palaeo ec ol ogy and hu man im pact in the vi cin ity of Lake Kahala, North Es to nia. Pact, 57: 373–403.

SAARSE L., HEINSALU A., POSKA A., VESKI S., RAJAMÄE R., HIIE S., KIHNO K. and MARTMA T. (1997) — Early Ho lo cene shore dis - place ment of the Bal tic Sea east of Tallinn (N Es to nia). Baltica, 10:

13–24.

SCHONING K., KLINGBERG F. and WASTEGDRD S. (2001) — Ma rine con di tions in cen tral Swe den dur ing the early Preboreal as in ferred from a sta ble ox y gen iso tope gra di ent. J. Quat. Sc., 16: 785–794.

SNOEIJS P. ed. (1993) — Intercalibration and dis tri bu tion of di a tom spe - cies in the Bal tic Sea, Vol ume 1. Bal tic Ma rine Biol. Publ., 16a.

Opulus Press. Uppsala.

SOHLENIUS G., STERNBECK J., ANDRÉN E. and WESTMAN P.

(1996) — Ho lo cene his tory of the Bal tic Sea as re corded in a sed i ment core from the Gotland Deep. Mar. Geol., 134: 183–201.

STOCKMARR J. (1971) — Tab lets with spores used in ab so lute pol len anal y sis. Pol len et Spores, 13: 615–621.

STRÖMBERG B. (1989) — Late Weichselian deglaciation and clay varve chro nol ogy in east-cen tral Swe den. Sver. Geol. Unders., Ca, 73: 1–39.

STRÖMBERG B. (1990) — A con nec tion be tween the clay varve chro nol - o gies in Swe den and Fin land. Ann. Acad. Sci. Fenn., A III, 154: 1–31.

STRÖMBERG B. (1994) — Youn ger Dryas deglaciation at Mt. Billingen, and clay varve dat ing of the Youn ger Dryas/Preboreal tran si tion.

Boreas, 23: 177–193.

STUIVER M., REIMER P. J., BARD E., BECK J. W., BURR G. S., HUGHEN K. A., KROMER B., McCORMAC G., Van Der PLICHT J.

and SPURK M. (1998) — INTCAL98 ra dio car bon age cal i bra tion, 24,000–0 cal BP. Ra dio car bon, 40: 1041–1083.

SVENSSON N.-O. (1989) — Late Weichselian and Early Ho lo cene shore dis place ment in the cen tral Bal tic, based on strati graphi cal and mor -

pho log i cal re cords from east ern SmDland and Gotland, Swe den.

Lundqua The sis, 25: 1–195.

SVENSSON N.-O. (1991) — Late Weichselian and Early Ho lo cene shore dis place ment in the cen tral Bal tic Sea. Quat. Int., 9: 7–26.

TYNNI R. (1966) — Über spät- und postglaziale Uferverschiebung in der Gegend von Askola, Südfinnland. Bull. Comm. Géol. Finl., 223: 1–97.

VALOVIRTA V. (1965) — Zur spätquartären Entwicklung Südost-Finnlands. Bull. Comm. Géol. Finl., 220: 1–101.

VAN DER PLICHT J., VAN GEEL B., BOHNCKE S. J. P., BOS J. A. A., BLAAUW M., SPERANZA A. O. M., MUSCHELER R. and BJÖRCK S. (2004) — The Preboreal cli mate re ver sal and a sub se - quent so lar-forced cli mate shift. J. Quat. Sc., 19: 263–269.

VESKI S. (1998) — Veg e ta tion his tory, hu man im pact and palaeo ge ogra - phy of West Es to nia. Pol len an a lyt i cal stud ies of lake and bog sed i - ments. Striae, 38: 1–119.

WALKER M. J. C., BJÖRCK S., LOWE J. J., CWYNAR L. C., JOHNSEN S., KNUDSEN K.-L., WOHLFARTH B. and INTIMATE GROUP (1999) — Iso to pic ‘events’ in the GRIP core: a stratotype for the Late Pleis to cene. Quat. Sc. Rev., 18: 1143–1150.

WASTEGDRD S., ANDRÉN T., SOHLENIUS G. and SANDGREN P.

(1995) — Dif fer ent phases of the Yoldia Sea in the north-west ern Bal - tic Proper. Quat. Int., 27: 121–129.

WASTEGDRD S., BJÖRCK J. and RISBERG J. (1998) — Deglaciation, shore dis place ment and early-Ho lo cene veg e ta tion his tory in east ern mid dle Swe den. Ho lo cene, 8: 433–441.

WASTEGDRD S. and SCHONING K. (1997) — Cal car e ous fos sils and ra dio car bon dat ing of the sa line phase of the Yoldia Sea stage. GFF, 119: 245–248.

WHITTAKER A., COPE J. C. W., COWIE J. W., GIBBONS W., HAILWOOD E. A., HOUSE M. R., JENKINS D. G., RAWSON P. F., RUSHTON A. W. A., SMITH D. G., THOMAS A. T. and WIMBELDON W. A. (1991) — A guide to strati graphi cal pro ce dure.

J. Geol. Soc., Lon don, 148: 813–824.

WOHLFARTH B., BJÖRCK S., CATO I. and POSSNERT G. (1997) — A new mid dle Ho lo cene varve di a gram from the river Cngermanälven, north ern Swe den: in di ca tions for a pos si ble er ror in the Ho lo cene varve chro nol ogy. Boreas, 26: 347–353.

WOHLFARTH B., BJÖRCK S. and POSSNERT G. (1995) — The Swed - ish Time Scale — a po ten tial cal i bra tion tool for the ra dio car bon time scale dur ing the Late Weichselian. Ra dio car bon, 37: 347–360.