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Sedimentological and chronological aspects of the Younger Dryas – transition record in southern Finland and northern Baltic

Outi hyttinen

ACADEMIC DISSERTATION To be presented, with the permission of the Faculty of Science of the University of Helsinki, for public examination in auditorium E204, Physicum, Kumpula Campus, on June 15th 2012, at 12 noon.

Department of Geosciences and Geography A19 / Helsinki 2012 © Outi Hyttinen (synopsis and Paper IV) © Elsevier (Paper I) © Taylor & Francis (Paper II) © SAGE Publications (Paper III)

Cover photo: Oivonoja clay pit in Koria, near Kouvola.

Author´s address: Outi Hyttinen Department of Geosciences and Geography P.O.Box 64 00014 University of Helsinki Finland [email protected]

Supervised by: Professor Veli-Pekka Salonen Department of Geosciences and Geography University of Helsinki

PhD Anu Kaakinen Department of Geosciences and Geography University of Helsinki

Co-Supervised by: Docent Aarno Kotilainen Geological Survey of Finland

Reviewed by: Professor Juha Pekka Lunkka Department of Geology, University of Oulu

PhD, University Lecturer Tiit Hang Institute of Geology, Tartu University

Opponent: Associate Professor Mark Johnson Department of Sciences, University of Gothenburg

ISSN 1798-7911 ISBN 978-952-10-6323-7 (pbk.) ISBN 978-952-10-6324-4 (PDF) http://ethesis.helsinki.fi

Unigrafia Helsinki 2012 Hyttinen O., 2012. Sedimentological and chronological aspects of the Younger Dryas – Holocene transition record in southern Finland and northern Baltic. Unigrafia. Helsinki. 38 pages and 3 figures.

Abstract

In this study, different types of sediments depo­ In offshore environment (water depth > 40 sited in the Baltic Sea Basin in Southern Finland m), the falling water level triggered debris flows, and the Gulf of Finland before and after the Bal- which eroded and redeposited older, varved sedi- tic Ice Lake (BIL) drainage were examined. The ments creating a distinct deformation unit. In the aim was to gain a better understanding of changes northern Baltic proper and Gulf of Finland, up in sedimentation in offshore, shallow water and to 4 m thick, discontinuous deposits of homoge- onshore beach environments, to provide an inde- neous clay bearing traces of rotational slump and pendent age control for the drainage event, and deformation were deposited. In Jokela, where the to test the applicability of dendrochronological original zero varve was first described by Saura- cross-correlation methods to varve clay data. The mo (1923), a homogeneous clay unit containing study consisted of acoustic sounding data from deformed sandy pods and layers was observed. offshore, one offshore marine sediment core, six This unit corresponds to a zero varve which was outcrops related to the BIL/Yoldia Sea transition formed as a consequence of a sudden water level sediments, and a digitized version of original drop in the BIL. In shallow water environment varve measurements by Sauramo. (water depth < 40 m), deposition of varved sed- In the Baltic basin area, the drainage of the iments ceased and, as a result of rapid regres- BIL occurred close to end of the Younger Dryas sion, shore processes started to operate, during cold event. This sudden 25–28 m fall in water which progressive marine terraces were formed. level had originally been chosen as the zero da- The first signs of saline incursion into southern tum for the Finnish varve clay chronology, but Finnish area were ca 100 varve after the the "key horizon" concept was not developed fur- BIL drainage. On newly emerged land, exposed ther and its chronostratigraphical connection and sediments were prone to wind erosion. The oc- importance remain unclear. Since the early 20th currence of massive cover sands, "Lammi loess", century annually laminated, or varved, sediments in the Second Salpausselkä area has traditionally have been used succesfully in constructing Late been attributed to rapid dust-storm type of de- - Holocene ice retreat chronologies position. However, there are also indications of and in dating ice marginal formations. This also non-aeolian origin of these deposits. applies to Finland. Correlating varve chronnolo- Morphologically well-defined coastal terra­ gies across the Salpausselkä zone is difficult, due ces are related to the oldest Yoldia Sea level (YI), to slow ice retreat rates and ice front oscillations which was developed after the BIL drainage. One during the Younger Dryas period. Therefore, the of these terraces within the First Salpausselkä older part of the chronology, which pre-dates the zone was dated by optically stimulated lumi- drainage event, is only loosely connected to Ho- nescence (OSL) method. This is the first direct locene varve series. YI-level date in Finland and it yielded ages of Department of Geosciences and Geography A19

111 200–1 400 ± 2 700 years. The finding high- a debris-flow unit and in the shallow water sedi- lights the potential of shore terraces in verifying ment as the change into non-annual deposition Lateglacial – early Holocene varve chronology in rhythm. Finland. Another approach to strengthen the de- -After the BIL drainage, freshwater condi- glaciation chronology could be applying to clay tions prevailed in the early Yoldia sea phase for varve data the statistical methods used in den- at least ca 100–200 years. drochronology. This approach separates the lo- -The first OSL-dating of oldest Yoldia Sea cal variation within sedimentary basin from the terrace (YI) in Finland gave ages of 11 200 and climatic signal, and enables varve correlations 11 400 ± 2 700 years. This gives also a mini- over longer distances. mum age to the BIL drainage. The main findings can be summarized as -Although drainage facies makes a good following: stratigraphic marker horizon, the erosion and re- -The BIL drainage was a basin-wide sedi­ deposition related to drainage event combined mentological event, leading to the deposition of with a possible hiatus in sedimentation make a distinct drainage varve facies expressed in the drainage varve facies problematic as a chro- annually laminated glaciolacustrine sediment as nostratigraphic clay varve key horizon.

4 Tiivistelmä

Työssä tutkittiin sedimenttejä, jotka ovat ker- Työn tavoitteena oli ymmärtää veden pinnan rostuneet Itämeren altaaseen Etelä-Suomen ja laskun aiheuttamia muutoksia altaan kerrostumis­ Suomenlahden alueella Baltian jääjärven muut- olosuhteissa sekä saada lustosavikronologiasta tuessa Yoldiamereksi noin 11 600 vuotta sit- riippumaton ikä erityyppisille pinnanlaskuun liit- ten. Tuolloin Skandinavian mannerjäätikkö tyville sedimenttimuodostumille optisesti stimu- peitti vielä suuria maa-alueita: jäätikön reuna loidulla luminesenssimenetelmällä (OSL). OSL oli Suomen alueella Toisen Salpausselän ko- perustuu sedimenttirakeiden "nollautumiseen" hdalla. Baltian jääjärvi peitti lukuun ottamatta auringonvalossa. Mittaamalla rakeista kuumen- korkeimpia huippuja lähes kaiken jään alta pal- nettaessa vapautuvan signaalin voimakkuus jastuneen maan, ennen kuin jääjärvestä avautui voidaan arvioida milloin rae on viimeksi altistu- yhteys valtamereen Billingenin alueella Ruot- nut auringon säteilylle ennen kerrostumistaan eli sissa. Näin jääjärven pinta laski 25–28 m muu- hautautumistaan. Lisäksi Sauramon lustokrono- taman vuoden aikana, kunnes vakiintui senhet- logiaan haluttiin kokeilla puulustotutkimukses- kisen valtameren pinnan tasoon. Tämä paljasti sa rutiininomaisesti käytettyjä tilastollisia mene- uusia maa-alueita veden alta ja myös mahdollis- telmiä, joilla pyritään vähentämään paikallisten ti satunnaisten suolapulssien tulon Itämeren al- vaihtelujen merkitystä kronologiassa. taan keskiosiin. Jotkut suolapulsseista päätyivät­ Tilastollisten menetelmien soveltaminen lus- Suomen rannikolle saakka. Baltian jääjärven pin- toaineistoon osoittautui lupaavaksi, niiden avul- nan lasku liittyy ajallisesti kylmän ilmastovai- la pystytään mahdollisesti kytkemään entistä heen, Nuoremman Dryaksen, loppumiseen ja kauempana sijaitsevia kerrostumispaikkoja toi- lämpimän jakson, Holoseenin, alkuun. siinsa. Erityisen ongelmallinen jakso Suomen Etelä-Suomen ja Suomenlahden alueella lustokronologiassa on Baltian jääjärven pur- Baltian jääjärvivaihetta ja Yoldiameren alkua kautumista edeltävän ja seuraavan ajanjakson luonnehtivat­ niinsanotut lustosavet, joissa muu- kytkeminen toisiinsa. Tämä liittyy muutoksiin tokset mineraaliaineksen raekoossa kuvastavat sedimentaatiossa. vuoden­aikojen vaihtelua. Vertaamalla raekoon Syvässä vedessä pinnanlasku todennäköi­ muutoksia eri alueilla toisiinsa voidaan saada sesti laukaisi rinteillä massaliikuntoja, veden ja selville jopa vuoden tarkkuudella, kuinka kau- sedimentin tiheitä seoksia, jotka alas vyöryes- an jään reunalla on kestänyt vetäytyä paikas- sään kuluttivat ja uudelleen kerrostivat van- ta toiseen. Tähän Gerard De Geerin Ruotsissa hempia lustosedimenttejä. Myös pienempiä ai- 1900-luvun alussa kehittämään metodiin ja Matti neksen romahduksia esiintyi. Tämän jälkeen olo- Sauramon laajaan kenttätyöhön perustuu suurel- suhteet altaassa rauhoittuivat ja lustokerrostumia ta osin käsitys viimeisimmän jääkauden loppu- alkoi taas syntyä. Tutkimusaineiston perusteella vaiheista Etelä-Suomen alueella. Sauramon lus- ei voida ottaa kantaa siihen, kuinka kauan al- tosavikronologian nollavuotena on pidetty juuri taan kerrostumisolosuhteiden rauhoittuminen edellä mainittua Baltian jääjärven purkautumis- kesti, ja näin ollen lustokronologian eri osien ta, joka näkyy epätavallisen paksuna vuosiker- kytkeminen toisiinsa on edelleen varmentamat- rostumana. ta. Suolaisen veden pulssien saapuminen tutki-

5 Department of Geosciences and Geography A19 musalueelle kesti lustokerrostumien perusteella Veden alta paljastunut maa-aines joutui voi- vähintään 100 vuotta, suolaisuuden lisääntymi­ makkaan tuulieroosion kohteeksi. Ensimmäisen nen näkyy selkeänä muutoksena lustojen raken- ja Toisen Salpausselän välisellä alueella on tavat- teessa. Edellä mainitun kaltainen kerrostumis­ tu laajalti hienojakoista hiekka- ja silttivaltaista historia on havaittavissa muun muassa Tuusulan ainesta, joka ohuena kerroksena verhoaa maan- Jokelassa, josta Sauramo kuvasi nollalustonsa. pintaa. Tämä ns. lössi tai peittohiekka on tulkittu Matalammassa vedessä lustosedimenttien tähän intensiivisen tuulivaikutuksen jaksoon liit- kerrostuminen loppui veden pinnan laskun tyväksi kerrostumaksi. Lähtökohtaisesti tällaisen myötä, ja rantavoimat alkoivat vaikuttaa ker- aineksen pitäisi olla hyvin nollautunutta ja antaa rostumiseen voimakkaasti. Tähän liittyy muun luotettavia ikämäärityksiä. OSL-ajoituksen toi­ muassa rantaterassien syntyä ja aineksen raekoon mimattomuus tässä kerrostumassa kyseenalais- yleistä karkenemista. Matalamman veden ker- taa Salpausselkien välisten hiekkakerrostumien rostumat antoivat vaihtelevia OSL-ikiä. Lahden pelkän eolisen luonteen. Renkomäestä saatu Suomen ensimmäinen OSL- Yhteenvetona voidaan sanoa, että Baltian ikä Yoldiavaiheen rantaterassille oli 11 200–11 jääjärven purkautuminen jätti jälkeensä tunnus­ 400 ± 2 700 vuotta. Tämä viittaisi siihen, että omaisen joukon sedimenttikerrostumia, jotka rantaterassien järjestelmällinen OSL-ajoittami­ voidaan löytää koko altaan alueelta. Näin ollen nen voisi tuoda uutta tietoa Itämeren altaan his- tapahtuma on jättänyt jälkeensä hyvin ajoitetun toriasta. Toisaalta osa matalammankaan veden merkkihorisontin. Tämä horisontti sopii kuiten- kerrostumista ei ollut saanut riittävästi auringon- kin huonosti lustosavikronologian pohjaksi, sillä valoa sedimenttirakeiden nollautumiseen. tapahtumaan liittyy laajalti eroosiota, uudelleen- kerrostumista ja ajanjakso, jolloin kerrostumista ei ole tapahtunut.

6 Acknowledgements

The idea for the work came from prof. Veli-Pekka Prof.emer. Bo Strömberg was kind to lend Salonen. I´ve learned immensly from him during his coring equipment. He and prof.emer. Joa- all stages of this project, from fieldwork to pub- kim Donner were willing to share their valuable lishing and everything in-between. Without his knowledge of varved clays and the Baltic Ice enthusiasm, knowledge and help this work would Lake drainage. I wish to express my gratitude not have been possible. It has been a joy to work to prof. Philip Gibbard for fruitful discussions under the guidance of Dr. Anu Kaakinen. She and interest he has shown towards this work. generously shared her acute knowledge of sedi- This work was carried out at the Division mentology both at the office and in the field. Time of Geology, Department of the Geosciences and and advice were always granted, along with good Geography, University of Helsinki. Three cheers company. Doc. Aarno Kotilainen introduced me for all my past and present co-workers and col- to the wonderful realm of marine geology and leagues for creating such a warm and friendly hidden secrets of the Baltic Basin. His insight atmosphere! Especially I would like to thank Dr. and enthusiasm has helped me a lot. Seija Kultti for mentoring and friendship, and I sincerely thank my excellent and thorough Dr. Esa Heilimo, Dr. Aku Heinonen, Emilia Ko- pre-examiners prof. Juha Pekka Lunkka and PhD sonen, Dr. Mia Kotilainen, Dr. Frauke Kubisch- Tiit Hang. Their constructive and helpful com- ta, Dr. Matti Kurhila, Paula Niinikoski and Eli- ments and reviews improved the quality of thesis. na Sahlstedt for excellent discussions, support Helsinki University Foundation, Finnish and company. Nothing in office and laboratory Graduate School of Geology, and University of would have worked without Mikko Haaramo, Helsinki, Department of Geosciences and Ge- Tuija Vaahtojärvi, Juhani Virkanen and Kirsi- ography are gratefully acknowledged for fund- Marja Äyräs. ing this work. No man is an island. I express many thanks I warmly thank my co-authors Kari Eskola, to my friends for all the joyful and relaxing mo- and Doc. Samuli Helama, who shared their ments and for keeping things in perspective. My knowledge and skills with me and also helped mother Katri, my siblings Juha-Matti, Mika and in the field. Suvi, and their families have offered unparallel The collaboration and help of Marine Ge- support and care during years, of which I will ology Group in Geological Survey of Finland always be indebted. Laura and Malviina, without and the crew of R/V Aranda is greatly appreciat- you I would simply be a lesser person. ed. Microtomography pictures were taken at the Department of Physics, University of Helsinki.

7 Department of Geosciences and Geography A19

Contents

Abstract 3 Tiivistelmä 5 Acknowledgements 7 List of original publications 9 Abbreviations 10 List of figures 10 1. Introduction 11 1.1. Aims of the study 13 2. Geological background 13 2.1. Geology of the study area 13 2.2. Deglaciation chronology from Alleröd to Holocene (13 900–11 500 cal yr BP) 15 2.3. Characteristics of the present Baltic Sea 16 2.4. Characteristics of the Baltic Sea in the past 16 2.4.1. The BIL (14 000–11 550 cal yr BP) 17 2.4.2. The Yoldia Sea (11 550–10 700 cal yr BP) 18 2.4.3. The Ancylus Lake (10 700– ca 9 800 cal yr BP) 18 2.4.4. The Litorina Sea (ca 9 800 cal yr BP- present) 18 3. Material and methods 19 3.1. Acoustic sounding data (Paper I) 19 3.2. Sedimentology (Paper II) 19 3.3. Application of dendrochronological cross correlation methods in a clay varve study (Paper III) 19 3.4. OSL datings (Paper IV) 20 4. Results 21 4.1. Acoustic sounding data reflecting properties of BIL/Yoldia Sea transition sediments (Paper I) 21 4.2. Sedimentological properties of BIL/Yoldia Sea transition sediments (Paper II) 21 4.3. Applying the dendrochronological cross-correlation methods in varve clay series (Paper III) 22 4.4. Chronological aspects of the BIL/Yoldia Sea transition: testing the OSL-method to drainage sediments (Paper IV) 23 5. Discussion 23 5.1. Sediment characteristics indicating the BIL/Yoldia Sea transition 23 5.1.1. Drainage unit 24 5.1.2. Varve thickness and colour 25 5.2. Time of arrival and duration of saline water conditions in the study area during the Yoldia Sea 26 5.3. Paleoseismicity as a possible explanation for the drainage varve 27 5.4. The possible reasons for problems in connecting Sauramos varve series to the absolute chronology 28 5.5. Future prospects: trace fossils 29 6. Conclusions 31 References 31

8 List of original publications

This thesis is based on the following publications:

I Hyttinen, O., Kotilainen, A. & Salonen, V.-P. (2011). Acoustic evidence of a Baltic Ice lake drainage debrite in the northern Baltic Sea. Marine Geology 284, 139–148.

II Hyttinen, O., Salonen, V.-P. & Kaakinen, A. (2011). Depositional evidence of water-level changes of the Baltic Ice Lake in southern Finland during the Younger Dryas/Holocene transition. GFF 133, 77–88.

III Helama, S., Hyttinen, O. & Salonen, V.-P. (2012). Varve archives re-explored to assess Late Weichselian proglacial sedimentary chronologies. Progress in Physical Geography. 36, 187–208.

IV Hyttinen, O., Eskola, K., Kaakinen, A. & Salonen, V.-P. Exploring the applicability of OSL-age determinations to sediments related to drainage of the Baltic Ice Lake in south- ern Finland. Manuscript.

The publications are referred to in the text by their roman numerals.

Author´s contribution to the publications: I The study was planned by A. Kotilainen and V.-P. Salonen. The data classification and in- terpretation was conducted by O. Hyttinen. The manuscript was prepared by O.Hyttinen and article jointly written by all authors.

II The study was planned by V.-P. Salonen and O. Hyttinen. The fieldwork was conducted and data interpreted by O. Hyttinen and V.-P. Salonen. The manuscript was prepared by O. Hyttinen and article jointly written by all authors.

III The study was planned and conducted and manuscript written and by S. Helama. The ar- ticle was commented and contributed by O. Hyttinen and V.-P. Salonen.

IV The study was planned by O. Hyttinen and V.-P. Salonen. Field work and sampling was contributed by all authors. The laboratory work and age determination was done by K. Eskola. The data interpretation and manuscript preparation was done by O. Hyttinen, V.-P. Salonen and K. Eskola, article was jointly written by all authors.

9 Department of Geosciences and Geography A19

Abbreviations

AMS Accelerated mass spectrometry BIL Baltic Ice Lake BP Before present BSB Baltic Sea Basin GSSP Global Stratotype Section and Point OSL Optically Stimulated Luminescence psu practical salinity unit SIS Scandinavian Ice Sheet TOC Total organic carbon

List of figures

Fig. 1. A paper slip containing Sauramo´s original varve thickness measuremets (left) and a metal box filled with varved sediment (right), page 12 Fig. 2. Late Weichselian deglaciation lines and study sites, page 14 Fig. 3. The generalised model of depositional conditions in the study area before and after the BIL drainage, page 30

10 1. Introduction et al., 1999; Litt et al., 2001) have been suggest- ed as European auxiliary stratotype of the Pleis- tocene - Holocene boundary Global Stratotype Annually laminated sediments or varved sedi­ Section and Point (GSSP) (Walker et al. 2009). ments are important archives of seasonally The varve counting method itself has re- changing sedimentation conditions. These rhyth- mained remarkably similar to that used by De mic changes produce horizontally bedded layers, Geer. Like in dendrochronology, thickness varia- or laminae, with alternating composition and tex- tions of varves are often measured directly from ture, such as clastic varves composed mainly of the wall of the clay pit, or from the fresh surface silt and clay, or organoclastic varves with organic of a sediment core (Fig. 1). The thickness of one and clastic layers. Clastic varves form couplets, clastic varve is the distance from the bottom of where coarser or more minerogenic sediment is coarser lamina to the upper contact of the asso- deposited during the spring and summer, and ciated clay lamina. Thickness variations between finer or more biogenic material in the winter- sites are visually compared and different local- time. As changes in grain size and total organic ities are connected based on similar trends in carbon (TOC) are strongly related to climate by the variability. Also image processing programs, temperature and evaporation, varved sediments which utilize digitized grayscale tone variations may yield valuable climatic information. In the to quantify varve thicknesses, have been used early 20th century a Swedish geologist, Gerard especially in studies of organic varves, often De Geer, proposed a theory of the annual ori- combined with magnetic susceptibility measure- gin of clastic, laminated sediments and he man- ments or geochemical data (e.g. Lindeberg & aged to correlate varve series taken from differ- Ringberg, 1999; Ojala & Francus, 2002). ent sites on the basis of trends in varve thick- Besides providing useful material for dating ness variations (De Geer, 1912). His clay varve and reconstructing ice-margin positions, varved chronology and its application as - at its best- a sediments have the potential to record past precise dating method has shown its applicabil- changes in lake water-levels. So-called drain- ity e.g. in studying Late Pleistocene and Early age varves, or varves related to water-level fall Holocene ice sheet retreat and ice margin po- in the basin, may preserve sediment structures sitions in Sweden (De Geer, 1912, 1940; Ca- typical of debrites or turbidites, or they can be to, 1987; Strömberg, 1994), Finland (Sauramo, homogeneous, clay-rich units, often containing 1918, 1923; Niemelä 1971; Strömberg, 1990, eroded and/or redeposited older material. Nu- 2005) and Estonia (Hang, 2003; Kalm, 2006). merous examples from glacial lakes in Scandina- In North America, the New England varve chro- via (e.g. De Geer, 1912; Sauramo, 1923; Nilsson, nology (Antevs, 1922, 1928; Verbosub, 1979a,b; 1968), Europe (e.g. Hang, 2003; Gruszka, 2007; Ridge, 2004) has been used to trace movements Murton et al., 2009; Putkinen et al., 2011) and of the Laurentide Ice Sheet. Varved sediments North America (e.g. Johnson et al., 1999; Breck- have also been used as a calibration aid for ra- enridge, 2007; Lajeunesse & StOnge, 2008; Roy diocarbon datings, especially when crossing ra- et al., 2011) have shown the importance of varved diocarbon age plateaux (e.g. Goslar et al., 1995; lake sediment records in reconstructing water- Litt et al., 2003). An indicator of the importance level changes. In the Baltic Sea Basin (BSB) of certain varve series as palaeoclimatic archives, area, the drainage of the Baltic Ice Lake (BIL), is that varved lake sediments in Germany (Brauer which occurred close to the Younger Dryas -

11 Department of Geosciences and Geography A19

Fig. 1 An example of Sauramo´s varve thickness measuremets (left). Pencil marks indicate varve boundaries. A metal box filled with varved clay sediment (right). Sauramo used these boxes to take samples to be later measured in the lab.

Holocene transition, is an excellent example of the correlation with the help of a stratigraphi- a basin-wide event depositing a drainage varve. cal marker sequence consisting of varves with This event horizon was originally chosen as the limestone fragments (Strömberg, 1990). The zero varve of Finnish varve chronology, as well Swedish varve chronology covers over 14 000 as a boundary between the Gotiglacial, and the varve years, and it has been connected to calen- Finiglacial: the former meaning an intermediate dar years: varves from Ångermanälv river val- stage after the maximum extent of the Scandi- ley have been connected both to the present time navian Ice Sheet (SIS) when ice retreated from and to De Geer´s chronology (Lidén, 1938; Cato, central Scania to the Fennoscandian moraines, 1987). Later it has been revised multiple times and the latter meaning the final deglaciation in (e.g. Strömberg, 1985; Cato, 1987; Strömberg, the Early Holocene in Sweden (De Geer, 1912, 1989, 1994; Ringberg, 1991; Brunnberg, 1995), 1940). but radiocarbon datings and local revisions in- A major problem in the study of glacial dicate that there are still errors in it (e.g. Björck varved deposits in Finland is the lack of pre- et al., 1996; Wohlfarth, 1996; Wohlfarth et al., cise age control. The Finnish varve chronolo- 1997, 1998; Andrén et al., 1999; Wohlfarth & gy can be considered a floating one. It is based Possnert, 2000). Altogether 875 years seem to be on Sauramo´s extensive work in southern Fin- missing from the postglacial part of the Swed- land (Sauramo, 1918, 1923) and the core part ish varve chronology (Andrén et al., 2002). In of the chronology covers ca 2 100 varve years. comparison, the New England varve chronology Revisions of the chronology have suggested an is floating: it consists of two parts which cover earlier formation of the Salpausselkä ridges and ca 4 000 and ca 1 900 years, and it is calibrat- a longer interval between the formation of the ed based on radiocarbon ages (e.g. Ridge et al., 1st and 2nd Salpausselkäs (Niemelä, 1971), or 1999; Ridge & Larsen, 1990; Ridge 2004) and an estimation of 75 missing varve years from cosmogenic isotopes (Balco & Schaefer, 2006). the postglacial part of the chronology (Ström- The applicability of micropaleontology is berg, 1990, 2005). The Finnish varve chronolo- typically limited when determining the age of gy has tentatively been connected to its Swedish sediments deposited soon after the ice sheet re- counterpart by correlating measurements from treat because they are practically barren of mi- both sides of the Baltic Sea and strengthening cro- and macrofossils and have a very low con-

12 tent of organic matter, both of which also restrict perioid of drainage, and the varve chronology the use of the AMS 14C method (Ignatius et al., is the best tool available for developing a time 1981). The reservoir effect, i.e. the redeposition scale for deglaciation sediments. The aim of this of older carbon and delay in mixing of surface study was threefold: (i) to acquire a more de- and deep waters, in the BSB area is problem- tailed picture on how the BIL/Yoldia Sea transi- atic because it has most likely varied over time tion can be observed in terms of sedimentology (Hedenström & Possnert, 2001), bringing thus both in clay exposures representing shallow wa- additional uncertainty in bulk sediment datings. ter deposition and in deep water submerged sedi- There is little study on this particular aspect. The ments, i.e. what type of sediment was deposited palaeomagnetic secular variation record in south- after the drainage, what controlled the deposition ern Finland does not yet cover the time period in and how traceable those sediments are over lon- question (Ojala & Alenius, 2005). Therefore, the ger distances, (ii) to obtain an age for the drain- lateglacial deglaciation chronology in Finland age event which would be independent of the is based largely on geomorphology and varve varve chronology and would clarify the Finnish chronology. It is suggested that between 12 800 varve chronology, and (iii) to use cross-correla- and 11 590 yr BP, the ice margin retreated from tion methods and statistically validate their ap- northern Estonia to the 2nd Salpausselkä, which plicability to Sauramo’s varve chronology, and also resulted the formation of the Salpausselkäs to give additional knowledge of the limitations (Saarnisto & Saarinen, 2001; Kalm, 2006). In and possibilities related to varve correlations. Sweden, the BIL drainage has been dated to 11 550 cal yr BP (e.g. Andrén et al., 2002), as well as 2. Geological background the end of the Younger Dryas at 11 500 cal yr BP (e.g. Wohlfarth et al., 2008, Donner 2010). This places the drainage of the BIL and the transition 2.1. Geology of the study area to the early Yoldia Sea in a chronological context. The study area is situated at southern Finland and the Gulf of Finland (Fig. 2). It belongs to the 1.1. Aims of the study Fennoscandian shield, which consists mostly of There is a demand for targeted sedimento- Paleoproterozoic rocks (shales, migmatites, vol- logical and chronological studies on Baltic Sea canic rocks and granitoids) formed 1.82–1.93­ Ga sediments, because the strata are chronostrati- ago during the , Mesopro- graphically important. They relate to the most terozoic (1.65-1.2 Ga old) rapakivi granites, an- dramatic event in the recent geological history ortosites and diabases, as well as (1.4–1.2 of the Baltic Sea area and have an important Ga) sandstones on the western coast extending connection to the Younger Dryas – Holocene further into the Gulf of Bothnia (Winterhalter transition. Despite its significance, the sedimen- et al., 1981; Koistinen et al., 2001). Addition- tological evidence of the BIL drainage in south- ally, small provinces of Paleozoic mudstones, ern Finland area has not been adequately studied. sandstones, conglomerates and carbonates can be The focus of this study was on the drainage found on the western coast. The Gulf of Finland of the BIL and the beginning of the early Yoldia lies at the southern margin of the Fennoscan- Sea phase. The study of this event is important dian shield, which dips under Palaeozoic sed- because the weakest points of the Finnish varve imentary rocks. Major tectonic features in the chronology are found in the parts covering the area include faults, fractures and lineaments,

13 Department of Geosciences and Geography A19 like the Teissure-Tornquist Zone, a major frac- ing the Palaeozoic , sedimentation was rath- ture zone on the southwestern border of the East er continuous and the area was subsiding, the European Platform, or the Kökar–Hanko–Hel- Mesozoic and Cenozoic eras being dominated sinki shear zone and the Åland–Paldiski–Pskov by non-deposition, which was partly interrupt- shear zone (Koistinen et al., 2001). Some tec- ed by recurrent marine transgressions (Šliaupa tonic zones have been active several times since & Hoth, 2011). the . At present seismicity in the area is A relatively thin layer of glacial Quaternary relatively low: according to the North-Europe- sediments covers the bedrock in southern Fin- an earthquake database (http://www.helsinki.fi/ land. The crystalline bedrock in Fennoscandia geo/seismo/bulletiinit/index.html), ca 50 earth- has been relatively resistant against glacial ero- quakes, with magnitudes mostly of <3, have sion and the landscape is typically dominated been observed in the study area, while, e.g., the by mega-scale scouring features. The average northern half of Swedish eastcoast has undergone thickness of minerogenic sediments is estimated much more of seismic activity. Isostatic land up- as ca 8 m, which was interpreted to equal a 7-m lift in the study area is 2-4 mm/y (Ekman, 1996). lowering of bedrock by glacial erosion during the Extensive intrusions of rapakivi granites Quaternary (Okko, 1964). Since the last glacia- and associated igneous rocks between 1.67 and tion, the rate of bedrock weathering in southern 1.45 Ga (Haapala & Rämö, 1992) reactivated Finland has been estimated to have been 1–3 the crust. The BSB was developed at 1.4–1.2 cm (Tanner, 1938). Ga (Korja et al., 2001). During the Late Edi- In the area south of the 1st Salpausselkä, the acaran-Early Cambrian sub-periods, the break- bedrock is usually covered by a till layer depos- up of the Rodinia established a ited by the Late Weichselian ice sheet. This lay- passive continental margin basin and there was er varies in thickness and is missing in places. a marine transgression, resulting in the deposition The till can be of consolidated lodgement-type of quartzitic sandstones, siltstones and shales, or more loose melt-out type. In many places the such as the blue Cambrian clays in Estonia. Dur- uppermost part of the till layer was later modi-

LAMMI RENKOMÄKI IHALAINEN Finland N KORIA SANTALA 50 km JOKELA

Sweden 2nd Salpausselkä (11 600 yr BP) 1st Salpausselkä (12 600 yr BP) Russia Central Swedish End Moraines MGML-2010-4

OSL sample sites Estonia Clay-pit sites Ice margin positions

Levene (13 400 yr BP) Acoustic study lines Palivere (12 700 yr BP) Pandivere-Neva (13 300 yr BP) Marine core site Sauramo´s varve site lines

Fig. 2 Deglaciation lines and study sites. The ice margin positions are based on Lundqvist & Wohlfarth (2001), Saarnisto & Saarinen (2001) and Vassiljev et al. (2011).

14 fied by littoral processes, which transported fine In southwestern Sweden, more or less con- sediments further away and thus enriched the tinuous Younger Dryas moraines form a 100- gravel and sand content in sediment. The till is km wide zone. These end moraine ridges were overlaid by glaciofluvial sand and gravel, which mostly formed within marine environments and was usually deposited as eskers, deltas, subaquat- consist of sorted sediments and diamictons. To- ic fans or beach sands. The Salpausselkä end wards the southeast the morphology of the depos- moraines dominate the landscape in southern its changes: continuous ridges become divided Finland. They were deposited during Younger into multiple ridges and finally into hummocky Dryas and consist of till, gravel and sand. The moraine landscape in south-central Sweden. In area south of the Salpausselkäs was subaquatic southeastern Sweden there are only a few topo- after deglaciation, until dry land was exposed graphically visible ice-marginal ridges south of by isostatic land uplift. Therefore, a veneer of the Younger Dryas end moraine zone (Lundqvist fine sediments deposited during various stages & Wohlfarth, 2001). of the Baltic Sea covers all but the highest hills. In Finland, deglaciated areas were immedi- ately covered with the waters of the BIL and the 2.2. Deglaciation chronology topographically well-defined 1st and 2nd Sal- from Alleröd to Holocene pausselkä end moraines were deposited during (13 900–11 500 yr BP) the Younger Dryas. The Palivere ice marginal After 16 000 cal yr BP a rapid deglaciation of moraine system (12 675 varve years BP; Hang SIS started in the southern BSB. In Sweden, the & Sandgren, 1996, see Fig. 2 for location) in retreat was relatively slow along the west coast, northern Estonia is the closest well-developed while large areas in the central highland and on ice terminal position predating the deposition of the southeastern and eastern coast were rapid- the 1st Salpausselkä. According to Kalm (2006), ly deglaciated between 15 000 and 14 400 cal the formation of Palivere end moraine zone, the yr BP, with stagnant ice remaining in the high- deglaciation of the Gulf of Finland and the for- er elevated areas until 13 900 cal yr BP (Lun- mation of the1st Salpausselkä end moraine zone dqvist & Wohlfarth, 2001; Houmark-Nielsen & took 400–425 years at the most. Hang (1997) Kjær, 2003). Around 13 400 cal yr BP the ice suggests that the deglaciation of the western part had retreated to the Levene end moraine area of the Karelian Isthmus, below the level of the (Lundqvist & Wohlfarth, 2001; see Fig. 2 for BIL, took ca 450 varve years. Saarnisto & Saa- location), while the northern Baltic proper, the rinen (2001) give an age of ca 12 250 cal yr BP Gulf of Finland and southern Finland were still for the 1st Salpausselkä and ca 11 600 cal yr BP covered by SIS. The Younger Dryas cold event for the 2nd Salpausselkä, based on the varve clay (12 650 - 11 500 cal yr BP) slowed down the measurements and paleomagnetism. Using the retreat of SIS. The Younger Dryas end moraines 10Be-method, Tschudi et al. (2000) dated the 1st around Fennoscandia in Finland, Sweden and Salpausselkä to 11 930±950 yr BP (with consid- Norway and in northwest Russia stand witness eration of erosion) and Rinterknecht et al. (2004) to a marked cooling of the climate, re-advances gave an error-weighted mean age of 12 400±700 and stillstands of the ice margin. The moraines yr BP for the 1st Salpausselkä. consist of till and glaciofluvial material, and are At the end of the Younger Dryas the mean deposited in the ice marginal zone on land or in July temperature rose by 4–10°C (Renssen & a marine or glaciolacustrine basin. Isarin, 2001; Wohlfarth et al., 2004) and the SIS

15 Department of Geosciences and Geography A19

started to retreat rapidly. In southern Sweden the 2.4. Characteristics of the ice recession rate was 75-100 m/ (Ringberg, Baltic Sea in the past 1991) before the Younger Dryas, from 20-50 m/ At the beginning of the Pleistocene, the Bal- year to 0 m/year (stillstand) during the Younger tic Sea depression was occupied by the north- Dryas (Brunnberg, 1995), and 200 m/year dur- east-southwest running Baltic stream (Gibbard, ing the Early Holocene (Brunnberg, 1995). In 1988). Marine sediments related to the Holstein­ Finland, the recession rates have been of simi- ian interglacial are found in the Baltic Sea ar- lar magnitudes (Sauramo, 1923; Niemelä, 1971). ea and adjacent regions (Marks & Pavlovskaya, 2003), and there are several sites known with 2.3. Characteristics of the marine deposits from the Eemian interglacial present Baltic Sea (Ikonen & Ekman, 2001; Miettinen et al., 2002). The Baltic Sea is a brackish epeiric (or inland) During the Eemian interglacial (130–115 ka sea, a semi-enclosed water body consisting of BP), lacustrine conditions prevailed for ca 300 a few deeper basins separated by bedrock sills. years before the marine phase (Kristensen & The main sub-basins are the Bothnian Bay, the Knudsen, 2006). After the lacustrine phase, the Bothnian Sea, the Gulf of Finland, the Gulf of BSB Baltic was connected to the Barents Sea via Riga and the Baltic proper. The average depth the Karelian area during the first ca 2–2.5 ka, as is 52 m, and the maximum depth 459 m (Seif- the Saalian ice sheet had caused a deep glacio- ert & Kayser, 1995). Dense, saline water flows isostatic crustal anomaly. The Eemian Baltic Sea in from the southwest, from Skagerrak/Kattegat, had a strong west-east temperature and salinity and fresh water is supplied by large rivers in the gradient: warmer and more saline surface waters north. This creates a south-north salinity gradi- in the western BSB and lower salinity and colder ent in the basin. The surface salinity is ca 8–10 bottom water in the eastern BSB, possibly creat- practical salinity units (psu) in the southern Bal- ing strong salinity stratification and hypoxic bot- tic, ca 7–8 psu in the Baltic proper and ca 3–5 tom conditions (Andrén et al., 2011). After ca 6 psu in the Gulf of Finland and the Bothnian Sea ka the level of the Eemian Baltic Sea level fell (Matt­häus, 2006). and its salinity decreased (Eiríksson et al., 2006; The brackish surface water is separated from Kristensen & Knudsen, 2006). more saline bottom waters by a permanent halo- It is likely that the early Weichselian glacia- cline which can be found at a water depth of tions did not affect the central and southern Baltic 30–40 m in the Arkona Basin, at 40–60 m in Sea area during the Weichselian stage (Roberts- the Bornholm Basin, and at 70–80 m in the Got- son et al., 2005). The first Baltic glacial event land Basin and the Landsort Deep (Kullenberg, occurred during the Mid-Weichselian (Svendsen 1981; Matthäus, 2006). The Bothnian Sea and, in et al., 2004; Houmark-Nielsen, 2007; Salonen particular, the Bothnian Bay have practically no et al., 2008; Larsen et al., 2009), and freshwa- haloclines and as a result of less variable inflow ter lakes covered the central and southern BSB conditions compared to other areas the Both- (Andrén et al., 2011). Before the Last Glacial nian Sea and Bay are better ventilated (Stige- Maximum ice extent ca 18 000–17 000 cal yr brandt, 2001). BP in the southeastern sector of SIS (Lunkka et al., 2001) and ca 22 000 cal yr BP in the west (Mangerud, 2004), there might have been two

16 major ice advances reaching southwest Baltic at the end of the Younger Dryas, the retreat of (Houmark-Nielsen & Kjær, 2003). The present the ice margin accelerated. This led to a rapid state of the basin is linked closely to the retreat drainage of the BIL at Mt. Billingen at around of the Late Weichselian SIS. A thorough review 11 550 cal yr BP (Björck & Digerfeldt, 1984; of the recent history of the Baltic was compiled Strömberg, 1992; Andrén et al., 2002). The water by Björck (1995) and updated by Björck (2008) level in the BIL dropped 25 m in Sweden (Svens- and Andrén et al. (2011). In the following, the son, 1991; Björck, 1995, Jakobsson et al. 2007) main emphasis is given on describing the BIL and 27–28 m in Finland (Donner, 1951, 1978) and Yoldia Sea phases with only a short overview within only 1–2 years, and 7 000–8 000 km3 of of the Ancylus Lake and Litorina Sea stages. water were released into the ocean (Jakobsson et al., 2007). This outburst of water created large 2.4.1. The BIL (14 000–11 550 cal yrs BP) drainage sediment fans west of Mt. Billingen It has been assumed that the level of melt- (e.g. Strömberg, 1992; Johnson et al., 2010). On waters in the ice free areas of the BSB equalled the Swedish west coast the outburst sediments the contemporary sea level at around 16 000 cal have been detected in marine cores (e.g. Cato et yr BP. The BIL started to dam up at ca 14 000 al., 1982; Bergsten, 1994; Bodén et al., 1997). cal yr BP (Andrén et al., 2011) as the uplift of The rapidly melting ice sheet in south-central the threshold in the Öresund area lifted the Baltic Sweden and the isostatically depressed areas in Basin above sea level.. At ca 13 000 cal yr BP, south central Sweden later allowed the incursion the ice margin was in south-central Sweden. Due of marine waters towards the east. to the glacioisostatic depression, the altitude of Shorelines formed during the BIL in south- this area was considerably lower than the thresh- ern Finland are named as BI, BII and BIII. BI is old in the Öresund region. Therefore water was the oldest, BII occurs 5 m below BI, while BIII, dammed up 5–10 m above the ocean level. The the youngest shoreline recognized in the Salpaus- deglaciation of the area around Mt. Billingen led selkä zone occurs ca 5 m below BII. BIII is in- to the first, poorly documented, drainage of the terpreted to represent the conditions just prior the BIL. After the drainage, the new pathway for the BIL/Y drainage. In the older literature, a certain waters was towards the west and this continued "g-level" predating the BI level is mentioned (e.g. for 300–400 years (Björck, 1995). Around 12 Donner, 1978; Glückert 1995). This g-level con- 800 cal yr BP, at the beginning of the Younger sisted of glaciofluvial plateaus situated at 25 m Dryas cold period, the re-advancing ice front fi- lower altitudes than BI. The interpretation was nally closed the connection between the sea in that a gradual transgression had prevailed during the west and the Baltic in the east (Andrén et al., the early BIL phase. The concept of g-level was 2011). Meanwhile, the former threshold in the generally abandoned after Fyfe (1990) proved Öresund had risen even higher above sea level that these sediment formations were deposited so the BIL level started to dam up until it was subaqueously and therefore do not indicate ac- 25 m above ocean level. tual water level. Due to different land uplift rates The deep basins of the freshwater BIL were and ice retreat patterns, the BI and BIII shorelines characterized by oxic conditions and low organic formed in the 1st and 2nd Salpausselkä zone are productivity (Andrén et al., 2002). During this currently between ca 160 m and 100 m asl (Don- period, glacial varved and non-varved clays were ner, 1978; Eronen & Haila, 1990). The highest deposited. As the climate started to get warmer shorelines are found in the Lahti region and their

17 Department of Geosciences and Geography A19 altitude decreases towards the east. In Estonia, clays were deposited in front of the receding ice five BIL levels are thought to have been formed margin, while non-varved clays were formed in between 13 300 and 11 500 cal yr BP (Vassiliev more distal positions (Ignatius, 1958). In south- et al. 2011). They indicate a gradual regression ern Finland, the oldest YI shoreline between ca between 13 300 and 12 700 cal yr BP (levels 120 and 145 m asl marks the highest water lev- A1 and A2), small changes in water depth but el of the Yoldia Sea (Donner, 1978; Eronen & considerable rearrangements of Haila, 1990). drainage systems around 12 200 cal yr BP (level BI) and water level regression until 11 500 cal yr 2.4.3. The Ancylus Lake BP (levels BII and BIII) (Vassiljev et al., 2011). (10 700–ca 9 800 cal yr BP) In Sweden, the highest BIL shorelines prior to Glacioisostatic land uplift was rapid in south the drainage are currently between ca 160 m and central Sweden and the gradual shallowing of 10 m asl (Svensson, 1989), the highest altitudes outlets forced the outflow to the straits in the are found in the Stockholm region and decrease Vänern area. This eventually led to the Ancylus towards the south. Lake phase of the BSB. Transgression occurred in the area south of the outlets (Björck, 1995) in 2.4.2. The Yoldia Sea the Vänern area while regression took place in (11 550–10 700 cal yrs BP) the area north of the outlets. Homogeneous or The Yoldia Sea phase of the BSB was es- laminated, grey clayey sediment, poor in organic tablished after the drainage of the BIL to the material, was deposited during this freshwater contemporary sea level. The Yoldia Sea phase phase. Laminated fine sand and clay-rich rhyth- can be divided into three sub-phases (Svensson, mites together with dropstone structures were 1989).The first and third sub-phases character- deposited in a proximal glaciolacustrine setting ized by a very low salinity were interrupted by next to the ice margin. a sub-phase characterized by brackish condi- tions which lasted 200-300 years (Wastegård 2.4.4. The Litorina Sea et al., 1995; Andrén et al., 2002). Saline water (ca 9 800 cal yr BP –present) reached as far as the southern Baltic (Björck et The early Litorina Sea was practically non- al., 1990; Andrén et al., 2000a; Andrén et al., saline, until around 8 500 cal yr BPa strong 2007) and Finland (Heinsalu, 2001; Strömberg, and rapid spread of saline influence occurred 2005; Heinsalu & Veski, 2007). The highest sa- throughout the BSB. This is seen in the sedi- linity values existed in low-lying areas between ment record as a change in sediment type and Lake Vänern and Stockholm in central Sweden as an increasing amount of marine diatoms. As (Schoning, 2001). The dominant sedimentary a result, the organic content increased and green- deposits from the Yoldia Sea phase consist of ish grey gyttja and gyttja clay started to deposit. organic-poor (silty) clays, often varved in and The highest salinity in the Baltic Sea was reached north of the Gotland Basin (Andrén et al., 2002). 6 000 years ago (Andrén et al., 2011). The rea- Rapid land uplift in south-central Sweden led to son for the decreasing salinity since then can be the shallowing of the connecting sounds and the related to increased precipitation, lower summer Yoldia Sea phase came to an end. During the re- temperatures and/or restricted inflow of Atlantic treat of the ice sheet in the Baltic Basin, varved water into the BSB (Björck, 2008).

18 3. Material and methods 3.2. Sedimentology (Paper II) Two open-face clay exposures in southern Fin- land, south of the 1st Salpausselkä end moraine 3.1. Acoustic sounding data (Paper I) (Fig. 2), were chosen for sedimentological analy- The acoustic sounding was targeted to collect ses to study lithological indications of the drain- systematic and up-to-date information on off- age event in the selected sections. The site in shore-marine sequences representing the transi- Jokela had been previously examined by Sau- tion from ice proximal varved clays to homo- ramo (1918, 1923), Niemelä (1971) and Donner geneous postglacial clays (Ignatius et al., 1981; (1995) and was therefore known to represent the Åker et al., 1988). Acoustic records from the BIL/Yoldia Sea transition in an accessible sec- northern Baltic proper and the Gulf of Finland tion. Furthermore, it provided a possibility to were collected onboard R/V Aranda during compare varve thickness measurements with the 2006–2007 by the Marine Geology Group of older data. The section in Koria presented in this the Geological Survey of Finland (GTK) (Fig. study was chosen after preliminary field studies 2). A MD DSS sonar system (Meridata Finland on lithological characteristics of the varves. The Ltd), operating in the pinger mode at the fre- sections were logged in the summers of 2009 quency of 12 KHz was used. Approximately and 2010 by describing sediment properties like 1 200 km of high-resolution acoustic data were grain size, structures, contacts between laminae, analysed and interpreted as a desktop study, us- beds and units, and finally dividing the series into ing the Meridata MDPS postprocessing system. lithostratigraphical units. Colour was defined in the data were examined in detail and soft-sedi- the field on natural moist sediment using Munsell ment structures were classified by their appear- ColorTM soil charts. In the Koria section, a foil ance and occurrence of erosion and disturbance corer (Strömberg, 1989) was used to extend the structures. This information was combined with profile downwards from the base of the pit. The the bathymetric data. varve record was measured in the field by mark- To validate the acoustic profile interpretation, ing the varve limits on a paper tape directly from a 535-cm-long sediment core MGML-2010-4 the cleaned outcrop or core surface. Rhythmites (Fig. 2) was retrieved in June 2010 on R/V Aran- were measured as distances from the top of one da using a 90 mm diameter piston-corer. The core clay lamina to the top of the next lamina above was cut into sections onboard, and kept refrig- to an accuracy of 1 mm. This was repeated us- erated before sediment description, subsampling ing digital photographs (Jokela) and an additional and analysis. The halved and trimmed sediment sediment profile (Koria). core was described in the laboratory, and the sedi- ment surface was measured at intervals of 0.5 3.3. Application of cm for magnetic susceptibility, using a Barting- dendrochronological cross ton MS2E1. Plastic containers were used in sub- correlation methods in a clay varve study (Paper III) sampling for microtomography X-ray images of the core. The X-ray samples were scanned with To construct a varve chronology, several varve a μCT scanner nanotom® in the Department of thickness measurements from separate loca- Physics, University of Helsinki. tions are correlated. Correlation is based on vi-

19 Department of Geosciences and Geography A19 sual comparison of variations in thickness, like directly to calendar years. In this respect, the in dendrochronological studies. In addition to OSL dating method and OSL ages differ from that, statistical methods used routinely in den- 14C ages, which give an age of organic material drochronology to remove noise - non-climatic in sediment and need to be calibrated for a cor- signal or local variations - from the dataset were responding age in calendar years. However, to applied. This approach was tested on an existing obtain correct OSL ages, the grains must have clay-varve dataset. The varve measurements by been exposed to sunlight long enough before de- Sauramo (1918) were digitized by measuring the position (i.e. to bleach well). Examples of sedi- varve thicknesses published and transferring the ments potentially datable in glacial environments results in a database. The digitized series were are shallow water glacial-lake beach sediments all located south of the 1st Salpausselkä (Fig. (e.g. Mangerud et al., 2001; Murray & Olley, 2), and the material consists of 47 individual 2002; Fuchs & Owen, 2008) or subaerially trans- varve series with lengths between 16 and 291 ported aeolian sediments (e.g. Koster, 2005; Rob- increments, covering a total time period of 867 erts, 2008). To calculate the age in optical dating varve years. the dose rate, i.e. the background radiation per The data series of individual varve diagrams unit of time, needs to be calculated. This factor were processed by detrending, pre-whitening and is caused by natural radioactive elements in the averaging methods adopted from tree-ring stud- soil and by cosmic radiation. The other factor ies. The original varve dates (Sauramo, 1918, needed in the optical dating age equation is the 1923) were re-examined using the treated, di- equivalent dose, i.e. the estimate of the amount mensionless series. First, each series was corre- of radiation a grain has been exposed to since it lated with all other series in their suggested tem- was last bleached. poral positions; additionally, each sample was The BIL drainage created and deposited a series lagged forward and backward in time to deter- of well-defined sediment units containing sand. mine whether offsetting the time-series would Four sites, Santala, Lammi, Ihalainen and Ren- yield higher visual and statistical correlation. In komäki (Fig. 2), were selected for studying sedi- the case of higher correlation in a new position, ments and sampling for OSL-age determinations. the number of years lagged was taken as an in- The chosen deposits had been interpreted to be dication of the number of offsetting varve years. connected either with the BIL drainage, like in As an additional step, the varve series were di- Ihalainen (Rainio, 1993), or to have been depos- vided into overlapping segments before the lag ited soon after it, as in Santala, Lammi (Gibbard, analysis. Identification of the segment in which 1977; Rainio, 1997), Ihalainen (Rainio, 1993) a correlation substantially drops was used to lo- and Renkomäki (Okko, 1962). The selected sites cate the year of a dating error. Subsequently, the form a transect through the basin: from 30-m t-value was calculated between the individual se- deep water (Santala) to ca 10-m deep water (Ih- ries and the average of all other series. alainen), and further to littoral (Renkomäki and Ihalainen) and aeolian (Lammi) sedimentary en- 3.4. OSL datings (Paper IV) vironments. Optically stimulated luminescence (OSL) is a On the study sites, sedimentological obser- dating method which under favorable conditions vations included grain-size, fabric, structure and can give an absolute age of the deposition and colour of the sediment. After description, OSL- burial of mineral grains. OSL years correspond samples were collected from freshly cleaned ver-

20 tical sediment surfaces in clay- and sand pits. preted as a high-energy flow event, correspond- Opaque copper tubes, 28 mm in diameter and ing with the BIL drainage event and gravity flow 30 cm in length, were hammered into the se- deposits triggered by the base-level fall. The de- lected bed, sealed and stored in the dark. Site formations were stratigraphically related to BIL coordinates and altitudes were measured with a and Yoldia Sea clays, occurring throughout the GPS, and the altitudes were checked against top- study area. The process generating debrites was ographic maps (1:20 000). The gamma dose rate destructive, controlled mostly by sea-floor to- was measured in sample locations using a por- pography and redeposited older sediment. The table spectrometer. Quartz grains from the sam- distance to the glacier margin has not controlled ples were analysed at the Laboratory of Chronol- the type i and type ii deformations. Deformations ogy of the Finnish Museum of Natural History, in unit III (iii) closely resemble the gravity-flow University of Helsinki using the SAR protocol. deposits described in the Archipelago Sea and the Bothnian Sea as described by Kotilainen & Hutri 4. Results (2004) and Virtasalo et al. (2007), or in progla- cial sedimentary basins elsewhere (e.g. St-Onge et al., 2004). These deformations were possibly 4.1. Acoustic sounding data triggered by neotectonics, but this cannot be as- reflecting properties of BIL/Yoldia sociated with a single event throughout the basin. Sea transition sediments (Paper I) Based on the acoustic data, five acoustic units 4.2. Sedimentological properties (I-V) and three deformation horizons (i, ii and of BIL/Yoldia Sea transition (Paper II) iii) were defined. The examination of the gravity sediments core lithologies confirmed the interpretations of Two sections studied for sedimentological pur- acoustic units II and III. Acoustic unit I was sub- poses represented a continuous varve record, stratum, acoustic unit II represented ice-proximal with a total of 447 (Koria) and 450 (Jokela) BIL varves, and acoustic unit III recorded the rhythmic laminae deposited in the water depth transition into and the actual Yoldia Sea phase. of ca 40–70 m. The studied series record a distal Unit IV represented the lacustrine sulphidic clay- glaciolacustrine environment during the Younger gyttja of the Ancylus Lake (Ignatius, 1958) and Dryas cold event when the ice margin was about unit V organic-rich Litorina Sea and/or the mod- 20–30 km away. Measured varve thickness var- ern Baltic Sea sediments (Ignatius, 1958). ied between 1 and 69 mm. Two pronounced clay The oldest deformation horizon (i) was iden- colour changes were observed in sections. The tified in the lower part of acoustic unit II and first change was sharp, from reddish clay lami- indicated horizontal sliding and compacting of nae to grey clay laminae, while red-hued clay sediment, possibly relating to early diagenesis being a typical feature for rhythmites associated and consolidation of the sediment (Virtasalo et with the BIL in Finland and Sweden. The red al., 2007) or readvances and stillstands of ice colour is caused by redox-variations (e.g. John- sheet (St-Onge et al., 2008). Faulting, rip-up mud son & Ståhl 2010). Another pronounced colour clasts and complete or partial mixing of the stra- change in the varve series is the sharp transition ta ( deformation horizon ii) in the upper part of from thin, grey varves to thick, clay-rich brown acoustic unit II and the lower part of acoustic unit varves, marking the arrival of saline water and III were seen in the studied core. This was inter- the beginning of the saline Yoldia Sea phase, as

21 Department of Geosciences and Geography A19 already interpreted by Sauramo (1918). The cur- Before the arrival of saline water to the study rent strata could therefore be correlated with sedi- area, a transitional varve series (unit 5 containing ment facies previously described in Finland (Sau- 120 varves in Koria and 190 in Jokela) was de- ramo 1918, 1923; Niemelä 1971; Rainio 1993; posited in the study area. Previous studies have Strömberg 2005) and Sweden (e.g. Strömberg, suggested a transition interval of 240-400 years 1992; Brunnberg, 1995; Andrén et al., 2002). in Finland. The facies change from thin grey The lithology in the both sequences studied winter layers to thick brown winter layers in the was divided into 6 units starting from the lower- uppermost unit 6 has been described from many most unit 1. Units 1, 3, 5 and 6 were laminated, places in southwest Finland and associated with 2 and 4 were deformation beds. Starting from the arrival of brackish water (Sauramo, 1918, the lowermost unit 1 and continuing in unit 3, 1923; Niemelä, 1971; Strömberg, 1990, 2005). It the reddish-clay clastic varve series in Jokela is likely that the saline water incursion was a dia- (315 varves) and Koria (275 varves) reflect sta- chronous event controlled by bottom topography, ble annual sedimentation in a distal BIL glacio- currents and the widening of the outlet in Swe- lacustrine environment. Sedimentation resulted den (Heinsalu, 2001; Andrén et al., 2002). This mainly from suspension, interrupted by meltwa- renders the event unsuitable for precise varve- ter-derived pulses of coarser material (Brunn- chronological correlation or dating. berg, 1995; Ringberg & Erlström, 1999). Two deformation horizons, units 2 and 4, interrupt the 4.3. Applying the varved sediment series. Both can be associated dendrochronological cross- with a water level fall, or the older one (unit 2) correlation methods in varve clay series (Paper III) could be a local slumping event. This unit was deposited ca 120 varve years before the assumed The applicability of statistical methods used in BIL drainage. It is worth noting that neither Sau- dendrochronological studies was tested to varve ramo (1918, 1923) nor Niemelä (1971) do report records using Sauramo´s (1918) varve measure- such a unit/bed. It is possible that before the BIL ments from southern Finland. The data series drainage, the water level was gradually lowering of individual varve-thickness diagrams were de- as a result of isostatic change, rather than drop- trended, prewhitened and averaged. It was as- ping in steps. This, too, points more towards unit sumed, that the effects of changing deposition 2 originating from local slumping. mode, the retreating glacier margin, the circula- The younger deformation unit (unit 4) tion processes in the pro-glacial basin and the showed many signs of reworking and deforma- local depositional variations would thus be re- tion. A very similar unit could be found in all moved from the data. The methods used in the Niemelä´s (1971) cores south of the 1st Salpaus- study indicated that Sauramo´s connections be- selkä, in eastern Finland (Rainio, 1993) and in- tween varve series were valid. In addition to this, Lammi, between the 1st and the 2nd Salpaus- it was seen that the series having 80 or more selkäs (Gibbard, 1977). Therefore, unit 4 was in- varves have a higher potential to be unambigu- terpreted as the BIL drainage bed, i.e. as the orig- ously cross-dated than shorter series. It was pos- inal ''zero varve'' described by Sauramo (1918). sible to separate two types of varve-thickness di- The water-level drop reactivated sediments de- agrams: those having regional chronological im- posited previously, which generated mud flows portance due to good correlativity over a distance (Vesajoki, 1982; Johnson et al., 1999). of more than 20 km distance and those show-

22 ing sub-regional correlativity over a distance less It is extremely important to understand the than 20 km. The former type can be regarded as genesis of the sandy units sampled in order to ob- suitable material for constructing geochronology. tain potentially optimal samples cannot be over- The latter type has a low geochronological va- estimated. The results of this study indicate that lidity but a more local sedimentological impor- traditional morphology- and altitude-based clas- tance. Dendrochronological methods provided sifications of shoreline features or land uplift his- information needed for realistic correlations to tory studies could benefit from OSL-dating. The be made between two or more varve records and results also suggest that most of the BIL drain- these methods can be considered potentially use- age-related sediments are/were unsuitable for ful in constructing varve series. OSL-dating. However, the single–grain method could improve the results, as many of the sam- 4.4. Chronological aspects of ples which yielded too old an age consisted of the BIL/Yoldia Sea transition: grains from several age populations. testing the OSL-method to drainage sediments (Paper IV) 5. Discussion The Renkomäki shore terrace was the only ma- terial out of 4 sequences studied in southern Fin- land that gave an expected age (11 400 and 11 5.1. Sediment characteristics 200 ±2 700 yr BP), being thus the first direct age indicating the BIL/ determination for the oldest Yoldia Sea shoreline Yoldia Sea transition in Finland. The other samples were interpreted Sediments deposited in southern Finland and as only partially bleached or as representing a northern Baltic during the BIL and early Yoldia mixture of sediments from different age genera- Sea phases contain little of microfossils (e.g. Åk- tions. The mixture of different age generations er et al, 1988; Andrén et al., 2002; Subetto et al., may result in a well-defined dose distribution but 2002; Veski et al., 2005). The cold and turbid yielding too old an age, indicating recycling of water column was dark due to suspended sedi- older material. In case of Santala, this explains ment, allowing only reduced light penetration, the clear maximum in dose measurements but and in consequence, the TOC values remained > 16 000 yr BP age. The Lammi samples could very low. Therefore, the use of pollen, forami- have been deposited very quickly during a dust- niferal or diatom assemblages is not reliable for storm type of aeolian activity. During transporta- defining the BIL/Yoldia Sea transition in the sedi- tion, a thick cloud of suspended particles would ment record. In the study area, the climatic condi- have shielded sunlight very efficiently and in- tions before 11 650 yr BP were cold and dry with hibited proper bleaching of the grains. It is also open vegetation ("steppe tundra") (Heikkilä & possible, that final deposition took place in shal- Seppä, 2003; Wohlfarth et al., 2007; Spiridonov low water, which was very rich in suspended et al. 2007; Amon et al., 2011). The proximity sediment material. This further prevented grains of the ice margin and large, cold water mass from bleaching. For Ihalainen area, the evidence would have influenced atmospheric circulation of sedimentary environment was inconclusive, regionally (Wohlfarth et al., 2007). East of the but the deposits were clearly of Mid-Holocene study area, BIL extended into the Lake Lado- or younger age, indicating prevailing erosion af- ga Basin (Björck, 1995; Saarnisto & Saarinen, ter the BIL drainage. 2001). South of the study area, on the Blekinge

23 Department of Geosciences and Geography A19 coast, southeastern Sweden, scarce aquatic mi- in Jokela and Koria (Paper II). Niemelä (1971) cro- and macrofossils and very low carbon con- mentioned thick varves and a darker hue of the tent indicate oligotrophic conditions and mosaic sediment, but according to him the upper part of regional vegetation in the basin during the BIL/ his unit (thickness ca 3.30 m) was not varved. Yoldia Sea transition (Yu et al., 2005). Based on sedimentological observations in Sauramo (1923) described four different ho- this work and previous studies, three main fea- rizons from the Jokela (Jokela and Kolsa brick- tures indicate a change in the sedimentary en- yards) section in southern Finland. The low- vironment during the BIL/Y transition. These ermost horizon aSs ("ante-Salpausselkä") was features are the drainage unit itself, the deposi- composed of relatively thick varves, up to 10 tion of thick varves following the drainage, and cm. Upwards in this series, the varves got thin- the colour change from reddish to non-reddish ner and the material finer. In the description giv- clay, observed in the sediment record close to the en in Paper II this horizon corresponds to unit 1 drainage horizon. Sauramo (1923) connected the in the Jokela and Koria sections: parallel lami- exceptionally thick varve with the drainage of nated and ripple laminated fines with slicken- the BIL, and the thick, dark grey-brown varves sides and dropstones. Sauramo´s (1923) second of the uppermost unit with the arrival of saline horizon ISs ("1st Salpausselkä"), was 1.25 m water to the study area. This study confirms these thick and the varve thickness was 1.5-0.3 cm. interpretations and widens the spatial coverage The sediment consisted of silt and reddish clay, of the evidence by introducing a new locality in and the varves were difficult to distinguish in Koria and by connecting offshore drainage fa- their natural state of humidity. This was also cies to onshore sites. confirmed by Niemelä (1971). This horizon ISs included an exceptionally thick varve contain- 5.1.1. Drainage unit ing sand and gravel (Sauramo, 1923; Niemelä, Niemelä (1971) detected the drainage unit in all 1971; Donner, 1995). In the current study this his sediment cores and sections south of the 1st horizon corresponds to units 3 and 4 in Jokela Salpausselkä. The sediment in the drainage unit and Koria: parallel laminated and ripple laminat- observed in his core records had slight varia- ed fines and massive clay with deformed sand tions in structure and grain size. As presented layers (Paper II). Sauramo’s third horizon, iSs in Paper I, it was also possible to identify the ("inter-Salpausselkä"), was 1.25 m thick and the drainage unit in the sediments of the offshore varve thickness varied between 1.5 and 0.4 cm. Baltic Sea basins. According to Nilsson (1968), The varve boundaries were sharp and no mention the BIL drainage varve he observed in Sweden was made of the red hue of clay. Niemelä (1971) was very similar to the drainage unit described agreed with Sauramo´s observations. This hori- in the current study. Therefore, it seems appropri- zon is consistent with unit 5 in Jokela and Ko- ate to state that the drainage unit is widespread ria: ripple laminated and parallel laminated fines and exists in a variety of sedimentary records, with occasional normal grading, and a more grey but its physical properties depend on the local clay hue (Paper II). Sauramo’s (1923) uppermost sedimentary environment. horizon, IISs ("2nd Salpausselkä") consisted of In deep water (offshore zone), the drainage thick and dark varves with thin silt parts. The unit is discontinuous. Where present, it may be thickness of the unit was not given. The proper- up to a few meters thick and deposited by de- ties described by Sauramo are also seen in unit 6 bris flows, which eroded and deformed older

24 laminated fine sediments (Paper I). The drainage bidites are products of one sedimentation event unit is sharply overlaid by undeformed crudely and grading is associated with waning flow. In laminated sediment. In shallow water (shore- varve formation there are two separate mecha- face zone), the drainage unit is laterally rela- nisms, suspension settling in winter and depo- tively continuous, with rather consistent physi- sition from over- and interflows, underflows or cal properties even over a distance of 100 km. surge currents in summer. It is a homogeneous clay unit with deformed Summer deposition from inter- and over- sand and gravel layers some tens of centimetres flows is often seen as a sharp contact between thick (Paper II). The lower contact of the drain- couplets, but the situation is more complicated age unit is, at least in Koria, erosional, and the in the case of underflows. Underflows can be re- upper contact is sharp. The sediment sections garded as density-driven turbidity currents, and in Ihalainen and Santala (Paper IV) represent their dispersal pattern is controlled by topogra- the transition from shallow water to the shore phy. Therefore, a summer layer deposited by an environment and it seems that the water level underflow may have a more diffuse or graded drop caused post-depositional convoluted bed- contact to the overlying winter layer, and thick- ding just below the drainage unit. In Ihalainen ness variations in the summer layer reflect both and Santala, ripple and parallel laminated sand sediment dispersal patterns and short-term vari- and silt were deposited after the drainage. The ation in sedimentation. The Coriolis effect and seasonal rhythmic cyclicity controlled no more wind-driven currents also distribute sediment to sediment deposition, but sediment transportation the lake bottom, for example katabatic winds in and sedimentation were subject to local wave the vicinity of glaciers drive surface water away and current processes within the shallow water from the ice margin. A compensational coun- shore environment (Paper IV). tercurrent due to underflow and surface current driven in the opposite direction by katabatic wind 5.1.2. Varve thickness and colour has been reported from glacier-fed lakes. This In a sequence of rhythmically laminated sedi- was seen as enhancing turbulence in the bottom ments, it is important to be able to verify the an- zone near subaqueous sill (Chikita et al., 1996). nual vs. non-annual origin of couplets. Turbidity The laminations in the Jokela and Koria currents can deposit rhythmites in a timescale of sediments, as described in Paper II, can be re- minutes to hours. To distinguish between varves garded as varves. Sedimentation during the BIL and turbidites in glacial lakes, Smith & Ashley has been relatively regular both in winter and (1985) suggested the following criteria: a gra- in summer, and slump-generated turbidity cur- dational transition within a couplet, variation in rents have been rare. The BIL/Yoldia Sea tran- both silt and clay layer thickness and normal sition unit is clearly of a non-varved origin, and grading throughout the whole couplet are typi- thus should not be considered one thick varve in cal of turbidites. Varves exhibit a sharp contact varve thickness measurements. During the early between the layers within a couplet, clay layers Yoldia Sea phase, varves were again deposited whose thickness remains rather constant, more in an environment possibly promoting stronger variation in silt layer thickness and, relatively underflows, seen as more frequent occurrence of often, graded clay layers but not graded silt lay- rippled lamination. In the beginning of the saline ers. Turbidites can be triggered, e.g., by slump- Yoldia Sea phase, the clay laminae of the cou- ing of sediment or river floods. Therefore, tur- plets are considerably thicker than prior to that

25 Department of Geosciences and Geography A19 phase but still exhibit a consistent change and 2001; Strömberg, 2005), but it started to influ- the couplets have sharp inner contacts. There- ence sedimentation in different parts of the basin fore they can be regarded as varves. In general, at different times. varve thickness measurements from Jokela and The following pattern for saline water to Koria show a typical upwards-thinning pattern of the Yoldia Sea basin was proposed by Andrén glaciolacustrine varves, if the uppermost thicker & Sohlenius (1995). As the denser saline water varves deposited in saline water are excluded. flowed into the northwestern Baltic proper, it fol- This pattern is commonly associated with the lowed the bottom topography and occupied first increasing distance to the ice-margin (Paper III). the deepest parts of the BSB. Fresh meltwater The varved BIL sediments have a reddish hue emerging from the ice front was cool and sed- indicating oxidation of iron compounds (Saura- iment-laden, so it also moved as a bottom cur- mo, 1923; Strömberg, 2005; Johnson & Ståhl, rent. When these two flows met, saline inflow 2010). The colour change from reddish to grey- was forced to take a southward direction along ish which was described above, is not related to the east coast of Sweden. Andrén & Sohlenius bedrock composition, but redox-conditions in the (1995) also suggested, that within the Yoldia Sea, sediment. The -rich meltwater can keep there was a similar 15-year periodicity for the the sediment which contains little of organic mat- intrusion of saline water as there is in the present ter oxidized if there is a steady support of sus- Baltic (Stigebrandt, 1987), estimating that each pended matter The greyish transition varves bear saline pulse could have reached ca 3 km further indications of reduced conditions, for which one north if the ice recession rate in the area was ap- good explanation is the arrival of brackish wa- proximately 200 m/year. More marine conditions ter and the formation of a permanent halocline, seem to have prevailed in the Bornholm Basin resulting in anoxia at the bottom (Andrén et al. than in the Gotland Basin (Andrén et al., 2000b). 2002). An increase in sedimentation rate and or- The time from the final drainage of the BIL to ganic carbon content can also create a reductive the first marine incursion in Sweden is between environment. A similar change from red to grey ca 200 (Nilsson, 1968) and ca 300 varve years or brown clay in varved sediments has been re- (Brunnberg, 1995), which is somewhat more ported from e.g. Lake Agassiz (Anderson, 2011) than the results summarized in Paper II indi- and Lake Superior (Breckenridge, 2007). In the cate. However, it is still unclear, for how long latter case, at least, the change is related to the time the varve deposition was interrupted in shal- bedrock source of material, rather than to redox- lower parts of the basin, like in Jokela and Koria conditions. (Paper II). The duration of the Yoldia Sea saline phase has been defined as having been ca 200 5.2. Time of arrival and duration clay varve years (Strömberg, 1989), 150 years of saline water conditions in the (diatoms; Heinsalu, 2001), 120 clay-varve years study area during the Yoldia Sea (mineral magnetic measurements; Andrén and The study of various varve sequences from Sohlenius, 1995) or 60 clay-varve years (fossil southern Sweden, southern Finland and the content; Andrén and Sohlenius, 1995). Accord- northern Baltic proper has made it obvious that ing to Raukas (1994), the study area is close to brackish influence was neither synchronous nor the furthermost eastern salinity limit of the Yoldia instantaneous (e.g. Sauramo, 1923; Andrén & Sea. The sections studied do cover only the be- Sohlenius, 1995; Brunnberg, 1995; Heinsalu, ginning of the saline sub-phase and therefore

26 do not yield a new age estimation. If the varia- seismic collapse of lake margins which initiated tion in the duration of the saline phase is more debris flows which in turn triggered distal turbi- than one hundred years, it is not unrealistic to dites. Osleger et al. (2009) suggested that for the assume that the results concerning the arrival of latter, sediment material potentially derives from saline water presented in this study are reliable. lake walls and bottom sediments, thus indicating mixing of older sediments. They also suggested 5.3. Paleoseismicity as a possible that seismically induced turbidites do not neces- explanation for the drainage varve sarily cross tectonic structures in the lake basin. One possible explanation for the clay-rich, ho- Guyard et al. (2011) described a Mass Wasting mogeneous deposit with sandy interlayers (unit Deposit (MWD), consisting of folded and re- 4 or deformation type ii, as described in Joke- worked glacigenic material from the Pingualuit la and Koria sections, Paper II, and the marine crater lake. The MWD was suggested to origi- core, Paper I) could be a seismically triggered nate either from slope instability induced by rapid gravity flow. Small-scale sliding and slumping drainage or a paleoseismic event . From the crater in clay rich sediments is a regular phenomenon. lake El´gygytgyn, several Late Quaternary mass Kohv et al. (2009) were able to differentiate three movements were reported (Juschus et al., 2009). groups of modern landslides in Estonia. The larg- Mass-flow induced debrites showed lateral het- est slides were retrogressive slide complexes de- erogeneity, signs of erosion and the occurrence veloped in the glaciolacustrine clay. The second of displaced but intact sediment fragments. Here group included slides in marine sand and silt, also, either lake-level fluctuations or earthquakes triggered by additional shear stress generated by were suggested as the initiators of debris flows. groundwater flow in the slope, a situation some- Two types of rapidly deposited layers (RDL) what comparable to the basel-level fall in a basin. whose thickness varied in the cm-to-m scale, The third group consisted of small landslides at were described from a fjord overdeepened by the river bank, that were actually the first stage glacial erosion (St-Onge et al., 2004). The first in developing retrogressive landslide complex- type had fining-upward beds with a sandy base. es, triggered by fluvial activity decreasing the These were interpreted as Bouma-type turbi- stability of slopes in the varved clay. The criti- dites, which were originated from earthquake- cal slope angle to place fine grained sediment triggered terrestrial and/or submarine slides in the state of instability is rather small. In the which had transformed into a debris flow and Estonian glaciolacustrine clay it was estimated then to a turbidity current (e.g., Piper et al., 1999). at ≥10° and in fine-grained marine sands at 20° The second type had a fining-upward and then (Kohv et al. 2009). This seems to support the a coarsening-upward unit followed by a fining- idea of a linkage between the drainage debrite upward unit. This group was interpreted as the and topographical lows, as postulated in Paper I. result of initial earthquake shaking, followed by Several studies of lake deposits have docu- a hyperpycnal turbidity current generated by the mented sediment units which had been initiated flood (Mulder et al., 2002). by seismic activity, base-level changes or flood- Palaeoseismicity has been suggested as the ing. Oselger et al. (2009) found that the high- causing mechanism of the 4–6 m thick turbidite- volume runoff of rivers was a trigger for most like layer found in the same stratigraphical po- of the cm-scale turbidites in Lake Tahoe. An- sition in the Gulf of Bothnia, Archipelago Sea other depositing mechanism was found to be a and in the northern Baltic proper (Virtasalo et

27 Department of Geosciences and Geography A19 al., 2006; Hutri et al., 2007). In the Bothnian Sea position of melt-water inflow into the lake wa- there were no observations of a similar type of ter column, lake stratification, the relative densi- layer (Hutri et al., 2007). All the observations that ties of lake water and melt water, ice rafting, the could be stratigraphically correlated were formed extent of ice cover, and slope stability (Ashley, between the glacial distal varves and the “lower 1975, 1989; Smith & Ashley, 1985). Ancylus Lake” sediments. They were found to Some factors controlling the varve thick- become younger towards the northern parts of the ness variations are more directly linked to cli- study area and the thickness of the layer was also mate, such as temperature (e.g., Desloges, 1994; increasing towards the contemporary ice margin. Leonard, 1985) and precipitation (e.g., Leemann It was proposed by Virtasalo et al. (2006, 2007), & Niessen 1994; Lotter & Birks 1997). Some that the normal faulting, slumping and debris- factors are more likely to be related to intraba- flow deposition are time-transgressive within the sinal processes. As stated by Ringberg (1991), direction of the ice-margin retreat and are relat- Strömberg (1994) and Brunnberg (1995), based ed to the post-glacial seismic activity, resulting on studies in Sweden, varves are difficult to cor- from the time-transgressive reactivation of old relate when the retreat of ice margin was slow, it bedrock fracture zones. Possible contributors are was stagnant or there were oscillations and read- also ice-berg scouring and/or gravitational failure vances. This is best explained by irregular melt- of oversteepened depositional slopes. water discharge, including e.g. more overland Based on sedimentological properties, such stream discharge. Varves which were deposited as the absence of typical turbidite sediment struc- after the retreat rates increased at the beginning tures, lack of an upward-fining character and the of the Holocene were easier and more reliable to erosion and redeposition of older sediment, the correlate. A similar problem is found in Finnish drainage unit (deformation horizon ii and unit varve chronology (Strömberg, 1990). This re- 4) described in Paper I and Paper II is not a flects a change from more transitory melt-water turbidite, but a mass-flow induced debrite. Cur- streams to stable drainage patterns, seen, e.g., as rently there are no unambiguous sedimentolog- long eskers chains and large glaciofluvial depos- ical criteria to distinguish seismically triggered its (Brunnberg, 1995). debrites from debrites triggered by a base-level The amount of sediment material present in fall. In Paper I, the drainage unit was associat- the lake also contributes to regional thickness ed with basin topography and found in a fixed variations Young et al. (2000) suggested that au- stratigraphic context from various sedimentolog- tocorrelation in clay varve series originates from ical settings. This strongly favours the base-level ''a slow response time'', meaning that if in one fall hypothesis. year a considerable amount of sediment mate- rial is present in the lake, part of that material 5.4. The possible reasons is going to be deposited during the following for problems in connecting winter. If only a small amount of winter mate- Sauramo´s varve series to rial is present, its contribution to the next year’s the absolute chronology winter layer will be minor. This could be the sit- When different variables affecting sedimentation uation during colder climatic conditions, when in the glaciolacustrine environment are consid- there would be less melt water and the ice cover ered, the list includes at least the proximity to ice, in the lake would probably be more extensive the number and direction of sediment sources, the and last longer. An example of variation in melt

28 season inflow peaks was described in the Mir- shore terraces show most potential for OSL age ror Lake sedimentary record as the frequent oc- determination purposes. currence of two-silt-unit varves (Smith, 1978; Dating of the drainage deposits within the Tomkins & Lamoureux, 2005). This suggested BSB is challenging.The maximum ice extent of that past sedimentation has been characterized the SIS during the Late Weichselian was reached by distinct snow and glacial melt phases, thus later in the east, and deglaciation was more rap- potentially controlling varve thickness and struc- id and occurred earlier in the east (Saarnisto & ture variations. Based on the results from Paper Lunkka, 2004).This is one source of error when III, local variations could be removed from varve correlating varve records in different parts of the thickness diagrams, thereby increasing the cor- BSB. Also the erosion and redeposition, pos- relativity to regional (> 20 km distance) scale. As sible hiatuses in sedimentation and, in general, the tested part of chronology includes also varves the very variable nature of sediments deposited which were deposited during Younger Dryas, the during the BIL/Yoldia Sea transition, as depicted application of the technique might provide a tool in Figure 3, has to be taken into account when to cross problematic periods of slow ice retreat correlating and dating the drainage deposits. This and stillstand of the ice margin. study has shown that drainage varves cannot be In many glacial subenvironments, sediment used as varve-chronological markers. The un- grains do not necessary bleach completely. In- known amount of erosion and the duration of complete bleaching results in too old OSL ages, hiatus might explain why the attempts to con- as experienced in Paper IV and in several studies nect varve diagrams depicting the BIL/Yoldia from glacigenic sediments (e.g. Gemmell, 1999; Sea transition in different places have not suc- Richards, 2000; Lukas et al., 2007, Raukas et al. ceeded. However, as a basin-wide marker ho- 2010). Other possible sources of error are biotur- rizon drainage deposits appear to work better. bation (animal activity and roots), redeposition of sediment, changes in sediment water content and 5.5. Future prospects: trace fossils different properties of mineral grains (Murray et One possible means to get more detailed in- al., 2002; Murray & Funder, 2003, Duller, 2004). formation on the environments and age of the There was no evidence that these other factors BIL drainage is studying the contemporary fau- would have significantly affected the age deter- nal assemblages. Although a glacial lake has mination of the BIL/Yoldia Sea transition sedi- been a harsh environment for living organisms ments. The fact that only a small proportion of and microfossils are rare, it most likely was not the grains in an aliquot emit a signal, means that without life. Trace fossils have been reported different stages of bleaching can contaminate the from several Pleistocene glacial lake deposits, age signal (Duller et al., 2000; Alexanderson et closest examples being from Finland (Gibbard, al., 2008). One method to control the occurrence 1977), Sweden (Högbom, 1893, 1915; Anders- of grains of incomplete bleaching is the statisti- son, 1897) and Lithuania (Uchman et al., 2009). cal analysis of the small-aliquot or single-grain Holocene trace fossils have been studied close to data (Wallinga, 2002; Duller, 2006). Further OSL the area studied here, by Virtasalo et al. (2006, investigations of the BIL/Yoldia Sea transition 2011) and by Uchman & Kumpulainen (2011). deposits using single grain measurements might In older studies, there are reports of chirono- give better defined results also for studies on sites mid trails from Sweden (Högbom, 1893; Ander- described in Paper IV. Sofar, the BIL/Yoldia Sea sson, 1897) and Finland (Andersson, 1897), as

29 Department of Geosciences and Geography A19

Fig. 3. Sedimentological reconstruction of depositional settings prior to and after the BIL/Y transition Upper picture: study sites depicted before BIL drainage. Lower picture: situation just after the drainage. well as crustacean, annelid and possible fish trac- were seen as a Planolites-dominated assemblage. es from Sweden (Högbom, 1915). In addition, Uchman & Kumpulainen (2011) studied Early insect larval tracks from varved sediments relat- Holocene proglacial lake sediments from Swe- ed to the BIL/Yoldia Sea transition from Finland den, which had deposited during the late Yoldia were described by Gibbard (1977). The trace fos- Sea phase or early Ancylus Lake phase, ca 11 sil distribution in a Lithuanian Pleistocene pro- 000 cal yr BP. They interpreted traces as Mermia glacial lake indicated that food was distributed ichnofacies, and suggested that the ecosystem patchily and that summers and transitional peri- was strongly affected by the local sedimentation ods were the most active times of trace making processes, arthropods and fishes living in the ba- and winters were less active or inactive (Uchman sin, rather than glacially derived melt water and et al., 2009). In studies of Holocene trace fossil position of the glacier margin. assemblages from the Archipelago Sea, Virtasalo Uchman et al. (2009) suggested that a rel- et al. (2006) noted a succession in trace fossil atively uniform trace fossil assemblage, fit- assemblage. Glaciolacustrine rhythmites depos- ting generally to Mermia ichnofacies (Buatois ited ca 11 300 cal yr BP, characterized by low & Mángano, 1995), can be seen in Pleistocene organic content, were found to be barren of trace glacial lakes in several regions. It is composed fossils. During post-glacial lacustrine conditions, of arthropod trackways, grazing traces and fish Palaeophycus and Arenicolites trace assemblag- traces. The level of oxygenation, availability of es reflected domicile -based activities. Later on food or other factors controlled the change of increasing salinity, increasing sediment organic ichnoassemblage rather than change in the wa- content and decreasing sea floor oxygen content ter depth. The ecological development model for

30 two New England glacial lakes by Benner et al. well-dated event. (2009) fits well in this idea. In the Benner et al. • Lag in the arrival of brackish water could be (2009) succession, stage I represented a simple interpreted as an undefined time period be- benthic system with pioneer invertebrates such cause of prevailing erosion or non-deposition as nematodes, oligochaetes, ostracods and chi- in the basin after the drainage. ronomids living on the lake bottom. These pio- • The early Yoldia Sea phase is characterized in neers could have been transmitted passively by the foreshore and shoreface environment by insects and birds, or actively. The initial arrival shallow water formations and well-developed of fish was seen in stage II. To survive, fish pio- shore terraces. The first YI OSL-date (11 200 neers would have needed an established benthos and 11 400 ±2700 yrs) was obtained from in the lake. The stage III trace fossils indicated Renkomäki shore terrace, thus proving the crustacean and a more abundant fish population. potential of the method and the material for Stage IV represented an environment with a shift dating the BIL/Yoldia Sea transition. from a glacial water source to a non-glacial wa- • Statistical approaches, like the methods rou- ter-source with increasing velocity of undercur- tinely used in dendrochronology, combined rents. This meant less silt and clay in water and with sedimentological observations were- made it possible for fishes to breed. proven to offer new possibilities in the con- It seems that there is potential to investigate struction and revision of varve chronologies. trace fossils in varved sediments in southern Fin- • The BIL/Yoldia Sea transition represents a land in order to gain more specific information change in sedimentary environment which on the facies change at the BIL/Yoldia Sea transi- is potentially reflected in contemporary ich- tion. Furthermore, trace fossil assemblages could nofacies assemblages, thus giving new per- provide information on small-scale saline pulses, spectives in studies of Late Weichselian pro- increase -decrease in the amount of suspended glacial environments. sediment and bottom oxia-anoxia. References 6. Conclusions Åker, K., Eriksson, B., Grönlund, T. & Kankainen, T. 1988. Sediment stratigraphy in the northern Gulf • The BIL drainage triggered topographically of Finland. Geological Survey of Finland, Special controlled debris flows in deep-water areas Paper 6, 101–117. Alexanderson, H., Johnsen, T., Wohlfarth, B., Näslund, of the BSB. While drainage deposits occur J.-O. & Stroeven, A. 2008. Applying the optically more sporadically in the Gulf of Finland and stimulated luminescence (OSL) technique to date northern Baltic proper, the drainage unit can the glacial history of southern Sweden. Reports from the Department of Physical Geography and be traced over long (~ 100 km) distances in Quaternary Geology, Stockholm University 4. 33 the area close to the 1st Salpausselkä. pp. Amon, L., Veski, S., Heinsalu, A. & Saarse, L. 2011. • The offshore drainage-related sediments Timing of Lateglacial vegetation dynamics and were deposited in an environment unsuitable respective palaeoenvironmental conditions in for OSL-dating, but they display a well-de- southern Estonia: evidence from the sediment re- cord of Lake Nakri. Journal of Quaternary Science fined facies change both in relation to water- 27, 169–180. level fall and the arrival of saline water. The Anderson, T.W. 2011. Evidence from Nipawin Bay in Frobisher Lake, Saskatchewan, for three highstand former can be used as a chronostratigraphi- and three lowstand lake phases between 9 and 10 cal horizon, since it was formed by a single,

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