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THE EEMIAN INTERGLACIAL AT SOKLI, NORTHERN FINLAND

Anna Plikk

The Eemian Interglacial at Sokli, northern Finland

A multi-proxy environmental and climatic reconstruction based on a 9 m long lacustrine sediment sequence

Anna Plikk ©Anna Plikk, Stockholm University 2018

ISBN print 978-91-7797-159-7 ISBN PDF 978-91-7797-160-3 ISSN 1653-7211

Printed in Sweden by Universitetsservice US-AB, Stockholm 2018 Distributor: Department of Physical Geography, Stockholm University Abstract

Understanding the fundamentals of the Earth’s climate system during the Quaternary Period (the past ca 2.6 million years) is of imminent interest in light of the reported recent climatic changes. The Eemian Interglacial (ca 129,000-116,000 years ago) is the last period before the present interglacial (the Holocene, the last ca 11,700 years) when temperatures were as warm as or warmer than today and may serve as an analogue to future climatic warming. The Eemian is a relatively well-studied period, but many aspects of the interglacial climatic dynamics still remain unresolved. One reason for this is the chronological uncertainties of many records from this time period; another is the relatively uneven distribution of palaeoarchives. Especially from high latitude areas, terrestrial records from the Eemian are few due to the impact of glacial erosion during the Weichselian Glacial (ca 116,000-11,700 years ago). This thesis is based on data from Sokli, N Finland, where cold-based conditions of the Weichselian ice sheets and non-typical bedrock conditions have allowed preservation of an unusually thick Eemian lacustrine deposit in stratigraphic sequence with overlying Weichselian and Holocene deposits. The main purpose of this thesis is to contribute to the knowledge of climate variability in Fennoscandia and the North Atlantic region during the Eemian, and it involves a qualitative multi-proxy reconstruction of the Eemian lake and its development as well as a quantitative reconstruction of the regional Eemian July temperature evolution using subfossil chironomids. Attention is paid also to the identification and characterization of short-term climatic events in the Sokli record. The main contribution of this study is to increase the spatial coverage of Eemian palaeoclimatological data. Specific results include the description of the lake development, identifying phases where non-climatic variables likely affect the quantitative chironomid-based temperature reconstruction, as well as qualitative and quantitative reconstructions of climatic features such as changes in seasonality and temperature. Five major lake phases are distinguished. A glaciolacustrine phase is followed by an early Eemian lake phase characterized by short mixing periods and rising nutrient levels. The following early mid-Eemian lake phase is characterized by a prolonged open water season and high productivity and possibly corresponds to the Eemian climatic optimum. The late mid-Eemian lake phase is characterized by infilling and eutrophic conditions while the late Eemian lake phase reflects shallow, dynamic and cooling conditions. The qualitative study suggests a shift from a higher seasonality in the early Eemian towards more oceanic conditions during the mid-Eemian, while chironomid-inferred July temperatures indicate that near present-day temperatures prevailed at Sokli already from the beginning of the interglacial and that optimum temperatures were only moderately (ca + 1°C) higher than at present. These results suggest that the climatic development at Sokli followed that of central Europe during the Eemian. Two events characterized by colder and more arid conditions are identified, and can be tentatively correlated to meltwater events registered in marine records from the Nordic seas and subpolar north Atlantic, possibly reflecting the effect that meltwater pulses and related disruptions in the thermohaline circulation had on adjacent continental areas.

Sammanfattning

Mot bakgrund av de pågående och förestående klimatförändringarna ökar vikten av en förbättrad för- ståelse av klimatsystemet och dess naturliga variation under kvartärtiden (de senaste 2.6 miljoner åren). För att studera klimatvariationer under perioder utom räckvidd för historiska data eller instru- mentella mätserier krävs olika typer av klimatarkiv såsom iskärnor eller sediment från hav eller sjöar där s.k. proxydata (organismer, kemiska ämnen, sedimentologiska faktorer etc. vars sammansättning eller utformning speglar en klimat- eller miljösignal) finns bevarat. Den senaste mellanistiden, Eem (ca 129 000 – 116 000 år sedan) utgör den närmaste perioden innan den nuvarande mellanistiden (holocen, de senaste ca 11 700 åren) då det globala klimatet var lika varmt som, eller varmare än, idag. Även om Eem inte är en perfekt motsvarighet till holocen när det gäller t.ex. graden av solinstrålning (en viktig drivkraft bakom klimatförändringar under kvartärtiden) eller till framtida klimatutveckling eftersom halten av växthusgaser i atmosfären höll sig på förindust- riella nivåer under Eem och aldrig nådde de nivåer vi ser idag, så utgör Eem inom klimatforskningen en sorts försöksperiod för att studera klimatologiska processer och mekanismer i en varmare värld samt för att testa och utvärdera klimatmodeller. Detta eftersom Eem är en relativt välstuderad period, med en stor mängd proxydata insamlade. Från de områden som varit periodvis nedisade under den istid som följde på Eem (Weichselistiden ca 116 000-11 700 år sedan) har däremot väldigt lite material bevarats p.g.a. omfattande glacial erosion. Detta gäller t.ex. landområden belägna på höga latituder i den norra hemisfären, som Kanada, NV Sibirien och Skandinavien där mycket få långa arkiv från Eem bevarats. I denna avhandling presenteras material från Sokli (norra Finland) där en nio meter lång sjösedi- ment-sekvens har återfunnits i stratigrafisk följd med överliggande sediment från Weichselistiden och holocen. Sedimenten har bevarats p.g.a. sitt skyddade läge i en djup vittringshåla i den relativt lättero- derade lokala berggrunden samt p.g.a. det geografiska läget i isdelarzonen där bottenfrusna förhållan- den hos de Skandinaviska inlandsisarna lett till en förhållandevis låg erosion. Sedimentsekvensen från Sokli är unik för området till följd av sin mäktighet samt eftersom den har blivit daterad till Eem med absoluta dateringsmetoder. Det huvudsakliga syftet med denna avhandling är att bidra till en ökad kunskap om klimatet i Skan- dinavien och området kring norra Nordatlanten under Eem. Arbetet med avhandlingen har ingått som en del i ett forskningssamarbete med fokus på den långa sedimentsekvensen från Eem från Sokli, med docent Karin Helmens som projektledare. Forskningssamarbetet har lett till att en mängd proxydata kan sättas samman (s.k. multi-proxy studie) för att rekonstruera den lokala samt regionala miljö- och klimatutvecklingen. Denna avhandling fokuserar på en kvalitativ (icke kvantifierbar) rekonstruktion av den forna sjön och dess utveckling med avseende både på lokala icke-klimatrelaterade förändringar och klimatrelaterade (långsiktiga och kortsiktiga) förändringar, samt en kvantitativ rekonstruktion av sommartemperaturer baserad på subfossila fjädermyggslarver (chironomider). Den kvalitativa rekonstruktionen baseras på biologiska data (pollen, sporer, makrofossil, diatoméer (kiselalger), mikrofossil av icke-pollen typ (t.ex. grönalger och akineter från cyanobakterier)) samt sedimentologiska data (li- tostratigrafi, halten organiskt material, C/N (kol/kväve)-ratio, halten biogent kisel, geokemi baserad på röntgenfluorescens (XRF)-analys) och ligger bl.a. till grund för utvärdering av den kvantitativa rekonstruktionen. Ett viktigt bidrag från denna avhandling är att öka den geografiska täckningen av paleoklimatolo- giska data från Eem. Baserat på resultaten från den kvalitativa studien kan sjöutvecklingen delas in i fem olika faser som reflekterar både storskaliga klimatvariationer och lokala processer. Det som börjar som en isdämd sjö med hög minerogen turbiditet övergår efter troligtvis bara några hundra år till en relativt djup sjö präglad av korta cirkulationsperioder, klart vatten och ökande näringsgrad. Sedan följer en fas med hög produktivitet och en förlängd period av isfria förhållanden, troligtvis förknippad med det interglaciala klimatoptimumet. Under följande fas blir sjön allt grundare till följd av sedimentation, och mer produktiv. Den sista fasen präglas av variationer i vattendjup och temperatur, med indikationer på ett begynnande kallare klimat. Två kortare klimatfluktuationer karaktäriserade av kalla och torra förhållanden kan också urskiljas dels under den tidiga delen av Eeminterglacialen dels i initi- alskedet till etableringen av kallare förhållanden i slutet av interglacialen. Dessa fluktuationer har eventuellt sina motsvarigheter i fluktuationer detekterade i marina sedimentkärnor från norra Nordat- lanten (Nordiska havet) samt Labradorhavet och som relaterats till plötsliga inflöden av sötvatten från smältande inlandsisar. Registreringen av sådana fluktuationer i terrestra arkiv indikerar den effekt smältande isar kan ha på omgivande landområden. Resultaten från den kvantitativa studien pekar också på att klimatutvecklingen under tidig Eem i Sokli gick från ett mer kontinentalt klimat mot ett mer maritimt präglat klimat, vilket motsvarar utvecklingen i centrala Europa och följer förändringarna i solinstrålning under Eem mot en minskad sommarinstrålning och ökad vinterinstrålning. Temperaturrekonstruktionen från Sokli visar en relativt jämn temperaturutveckling under Eem, med temperaturer nära de nuvarande redan från början av interglacialen. Med hjälp av den kvalitativa rekonstruktionen kan avsnitt när temperaturrekonstruktionen är mindre tillförlitlig identifieras. Under sådana avsnitt är det troligt att andra faktorer än temperatur påverkat chironomidsammansättningen. Den chironomidbaserade temperaturrekonstruktionen visar på en relativt liten temperaturskillnad (+1 °C) från dagens temperatur (även under klimatoptimumet), vilket ligger nära de temperaturskillnader som rekonstruerats för centrala Europa (+1-2 °C). En temperaturskillnad på bara ca 1 grad är däremot mycket lägre än de temperaturskillnader som rekonstruerats utifrån andra arkiv på höga (nordliga) latituder där temperaturer upp till 9 °C över dagens har rekonstruerats. Den förhållandevis modesta temperaturökningen i norra Skandinavien under Eem kan möjligen förklaras med att Skandinavien (och Europa) i och med Golfströmmen, som var aktiv också under Eem, hålls varmare än medel för latituden och därmed inte värms lika mycket som andra områden på höga latituder när en högre inkommande mängd solenergi omfördelas över jorden.

List of Papers

This doctoral thesis consists of a summary and the following papers, which are referred to by their Roman numerals in the text.

I Helmens KF, Salonen JS, Plikk A, Engels S, Väliranta M, Kylander M, Brendryen J, Renssen H. 2015. Major cooling intersecting peak Eemian Interglacial warmth in northern Europe. Quaternary Science Reviews 122:293-299

II Plikk A, Helmens KF, Fernández-Fernández M, Kylander M, Löwemark L, Risberg J, Salo- nen JS, Väliranta M, Weckström J. 2016. Development of an Eemian (MIS 5e) Interglacial palaeolake at Sokli (N Finland) inferred using multiple proxies. Palaeogeography, Palaeocli- matology, Palaeoecology 63:11-26

III Kylander M, Plikk A, Rydberg J, Löwemark L, Salonen JS, Fernández-Fernández M, Hel- mens KF. 2018. New insights from XRF core scanning data into boreal lake ontogeny during the Eemian (Marine Isotope Stage 5e) at Sokli, northeast Finland. Quaternary Research 89:352-364

IV Plikk A, Engels S, Luoto TP, Nazarova L, Salonen JS, Helmens KF. Chironomid-based temperature reconstruction for the Eemian Interglacial (MIS 5e) at Sokli, northeast Finland. Sub- mitted to Journal of Paleolimnology

V Plikk A, Risberg J, Helmens KF. Diatom assemblages from an Eemian palaeolake in northern Europe with morphological observations of rare Aulacoseira sp. resting spores. Manu- script

Contributions by myself and co-authors: My main supervisor and co-author KH organized the field sampling performed by the Geological Sur- vey of Finland in 2010, and initiated the project. I have performed the diatom, chironomid and biogeochemical (organic carbon, C/N) analysis used in Papers I (chironomids by SE), II, III, IV and V, and performed XRF-data processing. Paper II is based on multiple proxies analysed by me, SS, MV, MFF, MK, LL and NK which I compiled and interpreted with input from all co-authors. I lead the writing of papers II, IV and V and constructed figures and tables (XRF-table by MK) and performed the chironomid-based temperature reconstructions. Ideas were developed by KH (Paper I), MK (Paper III) after discussions with me and KH, me (Paper II, IV and V) after discussions with KH (Paper II), and with SE (IV). For Paper I and III, I contributed with interpretation of diatom data and discussion. All co- authors are acknowledged for reading and commenting on the manuscripts.

Co-authors: Karin Helmens (KH), Stefan Engels (SE), Jan Risberg (JR), Malin Kylander (MK), Ludvig Löwemark (LL), Sakari Salonen (SS), Minna Väliranta (MV), María Fernández-Fernández (MFF), Jan Weck- ström (JW), Niina Kuosmanen (NK), Tomi Luoto (TL), Larisa Nazarova (LN) Contents

1. Introduction ...... 1 1.1. Objective and scope of the study ...... 3

2. Background ...... 4 2.1. The Last Interglacial ...... 4 2.1.1. Definitions and chronology of the Last Interglacial ...... 4 2.1.2. Last Interglacial climate development ...... 6 2.1.3. The Last Interglacial in Fennoscandia ...... 8 2.2. Climate influences and reconstructions ...... 11 2.2.1. Regional climatic influences ...... 11 2.2.2. Palaeoclimatic reconstructions ...... 11

3. Site description ...... 13 3.1. The Sokli site ...... 13 3. 2. Stratigraphic setting and chronology ...... 14

4. Methods ...... 16 4.1. Coring and subsampling ...... 16 4.2. Biological methods ...... 17 4.2.1. Diatom analysis ...... 17 4.2.2. Chironomid analysis ...... 17 4.2.3. Pollen, spores and non-pollen palynomorph (NPP) analysis ...... 18 4.2.4. Macrofossil analysis ...... 18 4.3. Geochemical methods ...... 19 4.3.1. X-ray fluorescence (XRF) core scanning ...... 19 4.3.2. Carbon and nitrogen content ...... 19 4.3.3. Fourier-transform infrared spectroscopy (FTIRS) ...... 19

5. Results - Summary of papers ...... 21 5.1. Paper I ...... 21 5.2. Paper II ...... 21 5.3. Paper III ...... 22 5.4. Paper IV ...... 23 5.5. Paper V ...... 24

6. Discussion ...... 25 6.1. Characteristics and development of the Sokli Eemian palaeolake ...... 25 6.2. The chironomid-based temperature reconstruction from Sokli ...... 29 6.2.1. The influence of calibration dataset properties ...... 29 6.2.2. The influence of confounding variables ...... 30 6.2.3. Temperature development at Sokli ...... 31 6.3. Chronology, regional correlation and palaeoclimatologic implications ...... 31

7. Conclusions ...... 35

8. Future perspectives ...... 36 Financial support ...... 37 Acknowledgements ...... 37 References ...... 38

1. Introduction

Future projections of climate change suggest that global temperatures will increase by 0.5-5 °C in the coming century, with more pronounced warming occurring in the Arctic due to the effect of various feedback mechanisms amplifying polar warming (IPCC 2013). The Last Interglacial (LIG) (the Eemian in Europe, roughly equivalent to Marine Isotope Stage (MIS) 5e) dated to ca 129-116 thousand years (hereafter ka) ago (Govin et al. 2015) is the most recent period prior to the present interglacial when climate was as warm as or warmer than today, with global average annual temperatures estimated to have been about 1-2 °C warmer than present, and high latitude temperatures at least 2 °C (and up to 9 °C ) warmer than present (CAPE-Last Interglacial Project Members 2006, Francis et al. 2006, Turney and Jones 2010). Although the LIG is a non-perfect analogue to the Holocene in that orbital insolation, an important climatic driver, differs distinctly between the two interglacials and greenhouse gas values were near pre-industrial (Bakker et al. 2014), the changes in insolation follow similar trajectories throughout the two interglacials and the pattern and magnitude of reconstructed temperatures for the LIG resembles those projected for the future. Thus the LIG is generally seen as a test-bed for climate dynamics in a warmer-than present world and paleoclimate information from this period is important e.g. for validation of climate models (Bakker et al. 2014, Capron et al. 2014). As it stands, the LIG is also the most recent record of a full interglacial period and as such serves as a reference for natural climate variability during all phases of an interglacial, from the deglaciation to the thermal maximum to the climatic instability associated with the onset of glaciation. Reconstructions of the LIG climate based on ice cores and marine cores from the North Atlantic region or on compilations of terrestrial pollen sequences from Europe have indicated relatively stable conditions throughout the interglacial (Zagwijn et al. 1996, Shackleton et al. 2003, Kühl et al. 2007, Brewer et al. 2008, NEEM-members 2013). There is, however, a growing body of evidence for quite substantial LIG climatic variability, expressed in the registration of short-term climatic events especially in records with higher resolution (Fronval and Jansen 1996, Björck et al. 2000, Rioual and Mac- kay 2005, Oppo et al 2006, Rioual et al. 2007, Cochoud et al. 2009, Bauch et al. 2011, Milner et al. 2013, Galaasen et al. 2014, Regattieri et al. 2014, Irvali et al. 2016). These findings point to the effect of abrupt shifts in oceanic circulation with far felt climatic implications. The occurrence of such events in a warmer world, and the potential driving mechanisms are important to resolve in light of future warming. However, due to the general lack of independent absolute chronological controls of records from this period and the relatively uneven distribution of sites, the timing, geographical extent and expression of events remains uncertain, thus complicating interpretation of causative factors. The high latitude Arctic and subarctic north Atlantic region constitute a dynamic and sensitive part of the Earth system, being involved in feedbacks associated with ice sheet and sea ice dynamics, and changes in strength and stability of the Atlantic meridional overturning circulation (AMOC) and local wind and pressure systems (Bauch 2013). Through application of a common chronology to marine records from the North Atlantic Ocean temporal offsets in temperature trends between latitudes have been suggested (Rasmussen et al. 2003, Govin et al. 2012, Capron et al. 2014). Records from the Nor- dic seas (the Norwegian, Iceland and Greenland seas) register lower and variable sea surface temperatures (SSTs) during the early interglacial which has been related to input of fresh meltwater from melting ice sheets, affecting the extent and stability of the AMOC (Risebrobakken et al. 2006, 2007, Bauch and Erlenkeuser 2008, Bauch et al. 2011, 2012, van Nieuwenhove et al. 2011, Bauch 2013, Zhurav- leva et al. 2017), and possibly also the climate on adjacent continents (Jones et al. 2017) during the early interglacial. Continuous high latitude terrestrial records from the LIG are sparse due to the impact of glacial erosion and thus we know relatively little about the climate evolution in the high latitude continental areas and how these were affected by e.g. the dynamic changes of the AMOC as registered in marine cores. In the present project, the climatic (temperature, seasonality) and environmental changes recorded in LIG sediments from the high latitude site Sokli in northeast Finland (Fig. 1) have been investigated, in order to increase the knowledge of LIG climate dynamics in this sparsely studied re-

1 gion. The record from Sokli is unusual in that it contains a long lacustrine sediment sequence which has been confidently dated to the Eemian, preserved in sequence with Weichselian stadial and interstadial, and Holocene, deposits (Helmens et al. 2000, 2007a, Alexanderson et al. 2008, Helmens 2014). The sediment record has been preserved from glacial erosion due to the location in a deep weathering depression in the relatively soft rocks of the Sokli Carbonatite Massif in combination with regionally limited glacial erosion related to low ice-velocities in the ice-divide zone and frozen bed conditions of the Fennoscandian ice sheet (Kleman et al. 1999, Johansson et al. 2011). The high- latitude position of the Sokli site near the northernmost extent of the AMOC makes it likely to respond sensitively to changes in the North Atlantic thermohaline circulation.

Figure 1. Location of study site (red) and sites referred to in the text and figures (black, 1-39), and main present sea surface ocean circulation pattern in the North Atlantic region. NAC=North Atlantic Current, NwAC=Norwegian Atlantic Current, NCC=Norwegian Coastal Current, EGC=East Greenland Current, IC=Irminger Current, WGC=West Greenland Current, BC=Baffin Current. Extent of the Saalian ice sheets modified after Colleoni (2009). 1-Lake CF8 (Axford et al. 2011), 2-Fog and Brother of Fog Lakes (Francis et al. 2006), 3-NEEM ice core (NEEM community members 2013), 4-NGRIP ice core (North Greenland Ice Core Project members 2004), 5-MD03-2664 Labrador Sea (Galaasen et al. 2014, Irvali et al. 2016), 6-PS1243 Nordic seas (Bauch et al. 2012, Zhuravleva et al. 2017), 7-ODP-980 North Atlantic (Oppo et al. 2006), 8-Leveäniemi (Lundqvist 1971, Robertsson 2000), 9-Tepsankumpu (Hirvas 1991, Saarnisto 1999), 10-Paloseljänoja (Hirvas 1991), 11-Ostrobothnia (compilation) (Eriksson 1993), 12-Prangli (Liivrand 1991), 13-Plasumi (Kalnina 2001), 14- !"#$%$%&'$"+$"%":'; -S Sweden (compilation) (Robertsson 2000), 16-Fjøsanger (Mangerud et al. 1979, 1981), 17-Hollerup (Björck et al. 2000), 18-Bispingen (Müller 1974), 19-the Amersfoort and Amster- dam basins (Harting 1874, 1875, Zagwijn 1996), 20-Solniki (Kupryjanowicz et al. 2016), 21- Han-sur-Lesse Cave (Vaanstenberge et al. 2016), 22-Gröbern (Kühl et al. 2007), 23-Bourgeois-Delunay Cave (Cochoud et al. 2009), 24-La Grande Pile (Woillard 1978), 25-Entrische Kirche and Spannagel Cave, Austrian Alps (Spötl et al. 2007, Meyer et al. 2008), 26-Ribains Maar (de Bealieu and Reille 1992, Rioual et al. 2007), 27-MD95-2042 Iberian margin, North Atlantic (Shackleton et al. 2002, 2003, Sánchez-Goñi et al. 1999, 2000, 2005), 28- Corchia Cave (Drysdale et al. 2005, 2007, 2009), 29-Tana che Urla Cave (Regattieri et al. 2014), 30-Lago Grande di Monticchio (Brauer et al. 2007), 31-Ioannina (Tzedakis et al. 2003), 32-Tenaghi Philippon (Milner et al. 2013), 33-Site 975, Mediterranean Sea (Kandiano et al. 2014), 34-Pequiin Cave (Bar-Matthews et al. 2003), 35-Bol’shoy Lyakhovsky Island (Kienast et al. 2008), 36- Lake Baikal (Rioual and Mackay 2005), 37-Dongge Cave (Yuan et al. 2004, Kelly et al. 2006), 38-Sanbao Cave (Wang et al. 2007), 39-Hulu Cave (Cheng et al. 2006).

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1.1. Objective and scope of the study

The aim of this PhD-thesis has been to contribute to the knowledge of climate variability in Fen- noscandia and the North Atlantic region during the Eemian Interglacial. The thesis work is done as part of a research collaboration focusing on the long Eemian record from Sokli, with Associate Profes- sor Karin Helmens as project leader. Working in a group enables the use of multiple proxies to interpret the environmental and climatic changes recorded in the sediment sequence. The focus of this thesis has been on aquatic organisms, diatoms and chironomid larvae, both as a basis for qualitative investigations of local environment and climatic changes and as a basis for quantitative reconstructions of temperature changes. The long record from the high latitude Sokli site may provide detailed insights into how the temperature evolution and especially the dynamic early Eemian in the northern North Atlantic region as seen in the marine records, was reflected on land. The use of multiple proxies and high resolution studies enables the identification of events that may be expressed in different ways depending on proxy. We rely on both pollen- and chironomid-based July temperature reconstructions based on the transfer function approach from the Sokli site. Temperature reconstructions based on the transfer function approach is a valuable tool in palaeoclimatology but has limitations. To evaluate the influence of local non-climatic processes affecting the composition of the biological assemblages, and to get a picture of the sedimentary environment, one important part of this thesis is to make a qualitative interpretation of the lake-environment and development that can be used as a basis for evaluation of the quantitative temperature reconstructions. The chironomid-based temperature reconstruction presented in this thesis is important as a complement to and a form of internal validation of, the pollen-based reconstructions from the Sokli Eemian sequence performed within the working group, in a region where comparisons with other records from the studied interval is difficult.

Specific objectives that have been addressed in this thesis:

Reconstruction of the Eemian lake and its development, discriminating between influences from local non-climatic processes (e.g. infilling) and regional climate-related processes on both long-term and short-term scales at Sokli

Quantitative reconstruction of the Eemian July temperature evolution at Sokli using subfossil chironomids

Identification and characterization of short-term climatic events in the Sokli record

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2. Background

During the Quaternary Period the Earth has experienced a series of shifts between glacial and interglacial climate modes. The primary pacers for these glacial-interglacial cycles are the Milankovitch cycles of orbital forcing. Periodic shifts in the Earth’s orbit around the sun and in the tilt and direction of the Earth’s axis affect the intensity of summer insolation received at the surface and induce climate change. Various feedback mechanisms involving all parts of the climate system act as further control- lers on the pace and magnitude of the climate changes. The main forcing mechanism behind climate and ice volume changes over the last glacial-interglacial cycles (~0.6 million years (Ma)) is the combination of the 41 ka obliquity and the 23 ka precession cycles, and the 100 ka eccentricity cycle acts as a reinforcer of the combined effect of these two mechanisms at glacial/interglacial transitions (Rud- diman 2003, 2004, Oldfield 2005).

2.1. The Last Interglacial

2.1.1. Definitions and chronology of the Last Interglacial The LIG appears as a period of exceptionally warm conditions in the Antarctic EPICA Dome C ice core record of the past 800 ka (Masson- 18O values in marine sediment records (reflecting ice volume and water temperature changes) (Martinson et al. 1987, Shackleton et al. 2003, Lisiecki and Raymo 2005) and as a distinct period of forest succession in European pollen records (Zagwijn 1996, Kukla et al. 1997, Müller and Sánchez-Goñi 2007). It is characterized by global temperatures 1-2 °C above present, atmospheric greenhouse gas concentrations similar to pre-industrial levels and sea levels 6-9 m above present (Kopp et al. 2009, Dutton and Lambeck 2012, Dutton et al. 2015) (Fig. 2). The LIG is referred to as the Eemian Interglacial in western central European pollen records and the type area is the Amsterdam-Amersfoort area, the Netherlands (Harting 1874, 1875). The Eemian is defined as a period of distinct forest succession and generally warm depositional conditions and is characterized by remarkably uniform vegetation development across central Europe (Zagwijn 1996). In the marine oxygen isotope stratigraphy (Emiliani 1955, Shackleton 1969) the LIG is termed the Marine Isotope Stage (MIS) 5e and encompasses the range of light isotope values extending between the midpoint of transitions into and out of the climatic optimum conditions. A similar definition is adopted in ice core oxygen isotope stratigraphies, though encompassing the range of heavier isotope values. Pollen analysis performed on marine cores have shown that the boundaries of the MIS 5e (as defined from oxygen isotopes) and the Eemian (as defined from pollen) is not synchronous (Sánchez- Goñi et al. 1999), and how the LIG is defined in different records using different proxies may have implications for comparisons between records (i.e. timing of events) but has shown to differ between studies (Govin et al. 2015). The duration and timing of the LIG at different sites and in different records remain a key uncer- tainty in studies of the LIG. The problem of finding applicable radiometric dating techniques to most LIG records has resulted in a lack of absolute and independent age models from this time period. Most chronologies thus rely on indirect approaches such as tuning to orbital parameters (e.g. for marine records) (Martinson et al. 1987, Lisiecki and Raymo 2005), modelling in combination with absolute age markers/orbital tuning (e.g. ice core records) (Govin et al. 2015) or alignment to an absolutely dated reference record assuming that a certain (dated) climatic event, such as the distinct rise in temperature at the beginning of the interglacial, is regionally synchronous (e.g. pollen records). The use of different alignment strategies and reference records in different studies, as well as different definitions of the term Last Interglacial, adds to the difficulties in comparing palaeorecords from different regions and proxies (Govin et al. 2015) and assessing leads and lags in the climate system. The use of radio

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Figure 2. Context of the Eemian (LIG, MIS 5e) in climatic indicators for the past 150 ka: (a) June insolation at 65°N (Laskar ":'; > ?:'@&"##""QX"'>"%:[$@18]^$X$"@&$'%#_'`{| @ }:{|XQ["+$@18O @|%@"%:+':$|%'%## ':{|XQ["+$@$X|:|Q$@:"{Q"+':~+"@';@~;':"#+|{+|{:["|X:|&$@"@|+" }%:'+@:$@' (Petit et al. 1999). (e) Atmospheric CO2 concentrations from Antarctic ice cores (Lüthi et al. 2008). (f) Atmos- pheric CH4 concentration from Dome C, Antarctica (Loulergue et al. 2008). (g) Relative sea level reconstructed from benthic foraminiferal oxygen isotope records (Waelbroeck et al. 2002). (h) Forest succession at La Grande Pile (Woillard 1978), selected taxa (after Helmens 2014) aligned to MIS events (this study). metric dating techniques (e.g. U/Th dating of speleothems) for dating events or phases (e.g. end of deglacial transition, peak interglacial conditions, onset of glacial inception), are in this respect highly important. The few terrestrial records that exist containing yearly laminated sediments (e.g. Müller 1974, Brauer et al. 2007) are also of great importance. On the basis of annual laminae counting of a partly laminated lake sediment sequence complement- ed by extrapolation in non-laminated parts, Müller (1974) estimated a duration of the Eemian at Bisp- ingen, northern Germany, of ca 11 ka. The Bispingen chronology could however not be anchored due to a lack of continuous laminations up to the present. In contrast, in southern Europe the record from Lago Grande di Monticchio, Italy, containing laminated sediments from the entire Eemian and partly laminated sediments up to the present, indicates a duration of the Eemian of ca 17.7 ka (127.2-109.5 ka). This longer duration in southern Europe is in agreement with other estimates from this region, for example; the duration of ca 15.5 ka (127.3-111.8 ka) suggested by Tzedakis et al. (2003) on the basis of orbital tuning of a long terrestrial pollen record from Ioannina, Greece, and the duration of ca 17 ka (129±1-112±0.8 ka) based on speleothem and multi-proxy data from north-west Italy (Drysdale et al. 2005, 2007). The differences between the estimates for central and southern Europe is suggested to reflect the effect of a climatic cooling near the end of the interglacial leading to a southward retreat of the forest limit, a reduction of thermophilous trees and opening of woodlands in northern Europe while thermophilic trees remained for several millennia in southern Europe (Tzedakis 2003, Müller and Kukla 2004). This cooling is reflected in several records at about 119-116 ka (North Greenland Ice Core Project members 2004, Brauer et al. 2007, Spötl et al. 2007, Meyer et al. 2008, NEEM community members 2013). In general it is considered that gradual cooling led down to a relatively short period around 108 ka when stadia/glacial conditions are established in most records (Govin et al. 2015) (Ta- ble 1). Several records with independent chronologies reflect an onset and subsequent peak of LIG warm conditions between 129 and 127 ka (Govin et al. 2015) (Table 1). However, there seems to be a significant lag in the establishment of interglacial warmth in marine records from the high latitude North Atlantic region (Rasmussen et al. 2003, Bauch et al. 2012, Govin et al. 2012). This delayed warming of the Nordic seas in relation to the North Atlantic has been associated with deglacial processes continuing into the early Eemian in the Nordic seas (Rasmussen et al. 2003, Risebrobakken et al. 2006,

5

Bauch et al. 2012, Zhuravleva et al. 2017). During this early period alternating cold-warm events in the Nordic seas have been related to fluctuations in the strength of the AMOC associated with ice- sheet meltwater discharges (Risebrobakken et al. 2006, Van Nieuwenhoove et al. 2011, Bauch et al. 2012). The role of the Greenland ice sheet versus the continental Laurentide and Eurasian ice sheets in the temperature evolution of the Nordic seas during this period remains unclear (Zhuravleva et al. 2017). In contrast, the onset of the terrestrial Eemian has generally been assumed to have been fairly synchronous across Europe, with a delay in the establishment of woodlands of only a few centuries, similar to the development during the onset of the Holocene (Zagwijn 1996, Tzedakis 2003). Howev- er, a recent study from Denmark suggested that the terrestrial Eemian in Fennoscandia mimicked the sea-surface temperature trend of the Nordic seas, with a delayed warming and substantially cooler conditions continuing ca 3000 years into the interglacial (Jones et al. 2017).

2.1.2. Last Interglacial climate development The main driver of long-term climate variations during the Quaternary Period is changes in the Earth’s orbit around the sun (Milankovitch 1941). The response of the climate system to this orbital forcing is moderated by a variety of feedbacks involving the atmosphere, ocean, sea-ice, land-ice and vegetation. While the LIG experienced similar atmospheric greenhouse gas concentrations to the pre-industrial Holocene, the orbital configuration was different. Stronger eccentricity and a different phasing of obliquity and precession resulted in more pronounced changes in latitudinal, annual and seasonal insolation over the LIG compared to the Holocene (Bakker et al. 2014). July 65°N insolation during the

Table 1. Ages of palaeoclimatic events registered in continental records with an independent (absolute) chronology. Age errors (±) are related to absolute dating uncertainties of the used time scales. The record from Green- land (Rasmussen et al. 2014) is based on a synchronization of the NGRIP, GRIP and GISP2-ice core records.

Event Date (ka) Dating method Location Reference Establishment of 130-129 U-Th, speleothems Austrian Alps Spötl et al. 2007 interglacial 129 U-Th, speleothems China Yuan et al. 2004, Wang et al. conditions 2008, Cheng et al. 2006, Kelly et al. 2006 129.4±1.8 Ice modelling and age markers Antarctica Jouzel et al. 2007, Govin et max) al. 2015 129±1.5 U-Th on speleothems (2005) Corchia, Italy Drysdale et al. 2005, 2009 128.5±1.9 Ice modelling and age markers (CH4 Antarctica Loulergue et al. 2008, Govin max) et al. 2015 128±1 U-Th, speleothems SW France Cochoud et al. 2009 128 U-Th, Speleothems Israel Bar-Matthews 2003 127.2 Laminated lake sediments Italy Brauer et al. 2007 Peak interglacial 128.6±0.7-127.3±0.8 U-Th, speleothems Corchia, Italy Drysdale et al. 2005 conditions 127.9-126.3 U-Th, speleothems SW France Cochoud et al. 2009 127.2-124.55 Laminated lake sediments Italy Brauer et al. 2007 127-124 U-Th, speleothems Austrian Alps Meyer et al. 2008 Beginning of 121±1 U-Th, speleothems SW France Cochoud et al. 2009 glacial inception 120.7±1 U-Th, speleothems China Kelly et al. 2006 120.3±1.7 Ice modelling and age markers Antarctica Jouzel et al. 2007, Govin et max) al. 2015 120-119 U-Th, speleothems Israel Bar-Matthews 2003 119.6±0.6 U-Th, speleothems China Yuan et al. 2004 120.8-118.5 U-Th, speleothems Austrian Alps Spötl et al. 2007 119-117 U-Th, speleothems Austrian Alps Meyer et al. 2008 119.1 Layer counting and ice modelling Greenland Rasmussen et al. 2014 (120) 117.3 U-Th, speleothems Belgium Vaanstenberge et al. 2016 115.8 Laminated lake sediments Italy Brauer et al. 2007

Establishment of 112.7 Ice modelling and age markers (CH4) Antarctica Loulergue et al. 2008, Govin glacial condi- et al. 2015 tions 112±0.8 U-Th, speleothems Corsica, Italy Drysdale et al. 2007 110.6 Layer counting and ice modelling Greenland Rasmussen et al. 2014 109.5 Laminated lake sediments Italy Brauer et al. 2007 107.8 ¡¡¢ Antarctica Jouzel et al. 2007, Govin et al. 2015

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LIG peaked at 128 ka (Berger et al. 2007) and was above Holocene maximum values between approximately 129-123 ka (Masson-Delmotte et al. 2013). The penultimate (Saalian Glacial) glacial maximum is dated to approximately 140-135 ka (Drys- dale et al. 2009, Thomas et al. 2009, Govin et al. 2015) after which a transitional phase took place until the onset of interglacial conditions at around 129 (127) ka in most records (Govin et al. 2015, Table 1). This transitional phase is associated with Heinrich Event 11 (H11) and massive collapse of northern hemisphere ice shelves and release of icebergs, an initial spread of forests in southern Europe and early signs of warming in European speleothems (Spötl et al. 2007, Drysdale et al. 2009). The stronger insolation during the early LIG compared to the Holocene has been used to explain the relatively rapid retreat of ice sheets and subsequent spread of forests across Europe, probably reinforced by marine inundation across northern Europe and western Russia during the early LIG (Lambeck et al. 2006). Based on glacial rebound modelling, Lambeck et al. (2006) reconstructed the retreat of the Eurasian ice sheet margin towards the northeast across Fennoscandia towards the Kara Sea before the onset of the LIG climatic optimum. Immediately after deglaciation and before rebound, marine waters are estimated to have penetrated into the Baltic and inundated near shore areas up to 89 m above present sea level along the eastern Baltic Sea coast (Ostrobothnia). On the northeast Kola Peninsula marine waters are thought to have reached 150 m a.p.s.l. (Lambeck et al. 2006) and for a short time period (during the climatic optimum as established from pollen-data) Fennoscandia was an island, isolated from the European continent (Funder et al. 2002). Warmest conditions in Europe are generally reconstructed for the earliest phase of the interglacial; at Lago Grande di Monticchio warmest annual temperatures are reconstructed from the beginning of the Eemian (127.2 -124.6 ka) and are tentatively correlated to the warmest July temperatures reconstructed to have occurred during the first 2 ka in northern Europe (Zagwijn 1996), consistent with the strong summer insolation during this time (Brauer et al. 2007). There are also indications that the early LIG saw decreasing continentality in the form of increasing winter temperatures during this early phase, until the spread of Carpinus across the European continent suggests a pronounced shift to more oceanic conditions with slightly lower summer temperatures and substantially warmer winter temperatures. A decrease in seasonality over the early-mid interglacial has been related partly to decreasing July insolation and a subsequent small increase in winter insolation (Zagwijn 1996), but has also been related to the propagation of Atlantic waters into the Nordic seas and the onset of “Holocene-like” oceanic circulation after the cessation of the early LIG meltwater input into the Nordic seas (Rasmus- sen et al. 2003). As noted already, the onset of cooling at ca 119-116 ka marks the start of a period of marked diachroneity in European records at the end of the LIG (Tzedakis 2003). The forcing behind the climatic changes during this period is hypothesized to be partly related to decreasing summer insolation (Tzedakis 2003) but several studies also highlight the influence of various feedback processes during this interval set about by the decreasing insolation, involving expanding land and sea ice cover with associated albedo feedbacks, increased freshwater (iceberg) input into the subpolar north Atlantic and associated changes in ocean circulation (e.g. strengthening of the AMOC into the Nordic seas supplying moisture for glacier growth) (Kukla et al. 1997, Müller and Kukla 2004, Risebrobakken et al. 2007, Brewer et al. 2008, Mokeddem et al. 2014, Irvali et al. 2016). High latitude (Arctic) pre-Holocene terrestrial records are often fragmented, and only few long and continuous records that quantify temperature changes during the LIG exist from this region. Tempera- ture reconstructions from northern Greenland, Arctic Canada and northeast Siberia indicate that optimum temperatures in the Arctic during the LIG were substantially higher than the global mean (up to 9 °C warmer than present) (Francis et al. 2006, Kienast et al. 2008, Axford et al. 2011, NEEM community members 2013). Due to the lack of independent age control, comparisons with other records and detailed interpretation of the climatic evolution are complicated. In Arctic Canada, full interglacial warmth is reconstructed from the onset of interglacial deposition, interrupted by a cold reversal and later followed by a decline in temperatures (Axford et al. 2011) and at Greenland warmest temperatures are inferred from the early part of the interglacial (NEEM community members 2013). The climate of the LIG has been considered generally stable (Litt et al. 1996, Zagwijn 1996, Kühl and Litt 2003, Kühl et al. 2007) but a number of studies point towards the existence of short term climatic (cooler/drier) events both at the onset of the interglacial and around glacial inception (Fronval and Jansen 1996, Seidenkrantz and Knudsen 1997, Müller et al. 2005, Rioual et al. 2007, Cochoud et al. 2009, Axford et al. 2011). Several marine records register at least two events occurring slightly

7 before or during the onset of cooling around 119-116 ka and one major event occurring at ca 126 ka, shortly after the onset of interglacial conditions (Oppo et al. 2006, Mokeddem et al. 2014, Irvali et al. 2016, Zhuravleva et al. 2017). Due to dating-uncertainties, assuming synchrony with events on land is difficult, but also in terrestrial records late interglacial cold and/or arid events have been registered in e.g. Denmark and Poland (Björck et al. 2000, Kupryjanowicz et al. 2016) and early events in Denmark and France (Björck et al. 2000, Rioual et al. 2007). Recently records from the Mediterranean region (Milner et al. 2013, Kandiano et al. 2014, Regattieri et al. 2017) that pick up arid events both at the end and beginning of the interglacial, seemingly synchronous to events registered in North Atlan- tic/Nordic seas records, suggest a linkage between Mediterranean hydrology and high-latitude processes.

2.1.3. The Last Interglacial in Fennoscandia Regional pollen assemblage zonation schemes from various parts of central mainland Europe show similar biostratigraphical development over the LIG (Zagwijn 1996), with a pre-temperate stage dominated by Betula and Pinus forests, followed by an early temperate stage with warm temperate decidu- ous Quercetum-mixtum forests, succeeded by Carpinus and Abies forests of the late temperate stage, and finally a post-temperate stage once again dominated by Pinus and Betula forests (Table 2). These forested stages separate stages dominated by steppic or subarctic species, e.g. Artemisia, of the lateglacial and stadial phases of preceding and following glacials. Due to glacial erosion most Fennoscandian interglacial deposits are fragmentary and it is only recently, with the introduction of new dating methods, that deposits with more certainty can be ascribed to a particular interglacial period. Pollen analysis has been the main method used to separate interglacial from interstadial deposits and to correlate the interglacial sediments with e.g. the Eemian of northwestern Europe. In southern Sweden, central Finland and southwestern Norway interglacial deposits with pollen sequences possible to correlate to the Eemian sequences of central Europe, Den- mark and the Baltic countries have been found (Mangerud et al. 1979, 1981, Berglund and Lagerlund 1981, Påsse et al. 1988, Påsse 1992, Robertsson 2000, Eriksson 1993, Salonen et al. 2008, Lemdahl et al. 2013). Robertsson (2000) has made a generalized pollen diagram for the Eemian deposits in southern Sweden, distinguishing a pre-temperate Betula and Pinus dominated phase, an early temperate phase dominated by Corylus and Quercus, a late temperate phase dominated by Carpinus and Picea and finally a post-temperate phase dominated by Pinus and Picea (Table 2). This general development can be correlated to the Eemian pollen sequences from Denmark (Andersen 1965, Björck et al. 2000) and northern Estonia, Lithuania and Latvia (Liivrand 1991, Kalnina 2001, Šerin et al. 2014), corresponding also with the development registered in the pollen diagrams from central Europe (e.g. Zagwijn 1996). From central Finland a general biostratigraphic development has been constructed by Eriksson (1993) based on a compilation of different deposits. The vegetation development principally resembles that of southern Sweden, with the exception that Carpinus is less important (Table 2). In northern Sweden and Finland the ice-sheets of the last glacial period were partly cold based and affected the morphology of the landscape to a lesser extent than further south (Hirvas 1991, Kleman et al. 1999). The most complete Eemian pollen sequences in Sweden and Finland are found here in the northern parts. These are the sites Leveäniemi in Swedish Lapland and Tepsankumpu, Paloseljänoja and Sokli in Finnish Lapland, which have been correlated with the Eemian on the basis of pollen analysis (all three) but also with luminescence dating (Sokli) (Lundqvist 1971, Ilvonen 1973, Forsström 1990, Hirvas 1991, Robertsson 1997, Saarnisto et al. 1999, Helmens et al. 2000, Helmens et al. 2007a, Alexanderson et al. 2008). The correlation of the vegetation development of these interglacial sequences with Eemian sites in central Europe is problematic due to the distance and the differing climatic regimes which affect the composition of the vegetation. As an example, thermophilous elements such as the mixed oak forest are generally missing from the pollen flora of the northerly sites. At Leveäniemi, four local pollen assemblage zones have been distinguished (Lundqvist 1971, Rob- ertsson 1997). A first pre-temperate substage is dominated by Betula and Juniperus, then an early- temperate substage is characterized by Pinus and Betula, a late temperate substage by Pinus and Picea and a post-temperate substage by Betula (Table 2). Presumably long-distance transported pollen grains of Carpinus occurring in the late-temperate substage and Quercus and Corylus in all substages possi-

8 bly reflect the spread of these species towards the north, although not reaching the Leveäniemi area (Robertsson 1997). The vegetation history reflected at Leveäniemi can be correlated with the interglacial sequences from Tepsankumpu and Paloseljänoja in northern Finland (Hirvas 1991, Saarnisto et al. 1999). Four local pollen assemblage zones are distinguished also here (Table 2). The vegetation development is characterized by an evolution from a pre-temperate stage dominated by Betula, to an early temperate stage dominated by Betula and Pinus, a late-temperate substage characterized by a peak in Picea, and finally to a post-temperate stage dominated by Betula. The main difference is that in the Finnish flora the late-temperate period is characterized also by the presence of Larix, which is absent from the Swedish Eemian deposits (Saarnisto et al. 1999). Also, Betula remains dominant throughout the record at Tepsankumpu, and the correlation with the Eemian is not entirely proven (Helmens 2014). The pollen flora of the deposit at Sokli was first studied by Ilvonen (1973) in connection with drilling carried out during carbonatite prospecting in the Sokli mire. Ilvonen (1973) reported a deposit consisting of extensive diatomite which contained a pollen flora indicating a climate warmer than the present. The deposit was thus interpreted as interglacial and correlated with the Eemian. A more detailed pollen analysis of the deposit was made by Forsström (1990), but the sequence studied was affected by hiatuses. In 2000, Helmens et al. produced a pollen analysis based on two cores from the Sokli mire, one covering the upper part of the interglacial deposit and one the lower. They distinguish four pollen assemblage zones similar to those observed at Tepsankumpu and Paloseljänoja, however with a lower relative abundance of Betula. The vegetation development reflects an interglacial cycle with an early stage dominated by Betula, passing into one dominated by Pinus, followed by an optimum stage dominated by Pinus but with the presence of Picea and Larix and then the return to colder conditions reflected in a stage once again dominated by Betula (Helmens et al. 2000) (Table 2). From northern Fennoscandia, Eemian temperatures have been reconstructed at Leveäniemi using insect (Coleoptera) and plant remains, indicating that optimum climatic conditions were characterized by only slightly higher than present July temperatures (+1 °C) and a more continental climate, similar to that of central Finland today (Lundqvist 1971).

9

d Picea (Larix- (Larix- Picea Corylus- Quercus- Carpinus) Sokli Sokli Pinus Pinus (Betula- Picea- Quercus) (Helmens et et (Helmens 2000)al. (Paloseljänoja) Picea (Larix- Picea Carpinus) Betula-Pinus (Quercus- Corylus) (Saarnisto et al. al. et (Saarnisto 1999) northern Sweden (Leveäniemi) an (Leveäniemi) Sweden northern Zagwijn (1996). Danish zone desig- zone Danish (1996). Zagwijn southern Norway, Lithuania, Latvia, Lithuania, Norway, southern Picea (Pinus- Picea Carpinus- Corylus) Pinus Pinus (Betula- Corylus- Quercus) 2000) Leveäniemi Tepsankumpu species for each zone are reported, other other reported, each zone are for species Quercus- Quercus- Pinus- Ulmus Picea (Corylus- (Corylus- Picea (Carpinus) Quercus Quercus (Ulmus) C Finland (Compilation) (Eriksson 1993) (Robertsson Betula-Pinus Betula Betula Betula Betula Quercus- Pinus- Corylus (Ulmus) Carpinus Carpinus (Carpinus) Corylus-Tilia Corylus-Tilia Corylus- Corylus- Quercus- Ulmus Estonia Estonia (Prangli) (Liivrand 1991) Carpinus- Corylus-Tilia Pinus Pinus Ulmus- Corylus- Quercus (Pinus) (Taxus- Picea- Corylus) Picea Picea Picea Tilia Corylus- Quercus Latvia Latvia (Plasumi) (Kalnina 2001) Quercus- Quercus- Ulmus (Pinus) Corylus-Tilia Corylus-Tilia Corylus- Corylus (Ulmus) Carpinus Carpinus Carpinus (Carpinus) (Šerien et al. 2014) (Medininkai) Lithuania s distinguished in central Europe, Denmark, southern Sweden, Sweden, southern Denmark, Europe, central in distinguished s Pinus- Quercus Corylus- Quercus S Norway (Kråkenes) et (Mangerud 1979)al. between zones of A and B is provided. The most characteristic characteristic most The provided. is A and B of zones between Corylus- Querqus (Ulmus) Carpinus Carpinus (Picea) S Sweden (Compilation) (Roberstsson 2000)

Pinus- Pinus- Fraxinus - Ulmus Tilia-Taxus Quercus (Ulmus) (Picea) (Picea) 4 Corylus- Andersen 1965) (Hollerup) Denmark Denmark ted within brackets. brackets. ted within Pinus-Quercus Pinus-Quercus Tilia) E2a Pinus-Ulmus 2 Pinus (Ulmus) Pinus Betula-Pinus Pinus-Betula Pinus Pinus-Betula Pinus Betula-Pinus Pinus-Betula E3b E3b E3a Quercus-Corylus E2b Quercus 3 Quercus- -- E4b Taxus (Corylus- E4a Corylus (Quercus) Corylus- (Amsterdam Amersfoort) (Zagwijn 1996) (Björck et al. 2000,

200 100 E1 Betula-Pinus 1 Betula Betula Betula Betula-Pinus Betula- 450 450 1000/ 1200 700 ±2000 ±2000 E6b E6a ±4000 Pinus (Carpinus) Picea E5 6 Carpinus (Pinus) Picea (Pinus) Picea 7 Pinus Picea 5 Carpinus Pinus Picea Pinus Pinus Pinus Pinus Pinus Betula Betula Betula A Duration C Europe (Müller 1974) B stonia and central Finland. (B) Compilation of regional Eemian pollen assemblage zones distinguished in northern Fennoscandia; Fennoscandia; northern in distinguished zones assemblage pollen Eemian regional of Compilation (B) Finland. central and stonia

important species are repor are species important northern Finland (Tepsankumpu, Paloseljänoja and Sokli). Central European zone designation and duration after (Müller 1974) and 1974) (Müller after duration and designation zone European Central Sokli). and Paloseljänoja (Tepsankumpu, Finland northern correlation no compilation In this (1965). Andersen after nation Table 2. (A) Compilation of regional Eemian pollen assemblage zone assemblage pollen Eemian regional of Compilation 2. (A) Table E

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2.2. Climate influences and reconstructions

2.2.1. Regional climatic influences The present-day climate of Europe is to a large extent affected by the proximity to the northernmost branch of the AMOC, the North Atlantic Current, raising (winter) temperatures in especially northwestern Europe several degrees above the latitudinal average (Barry and Chorley 2010). This current is mainly driven by the prevailing southwesterlies which also transfer warm oceanic air masses into Europe. The climate becomes progressively more continental towards the east of Europe, with the transition being more abrupt in Fennoscandia due to the barrier formed by the S-N extending Fen- noscandian mountain chain against the westerly winds. The strength of the westerlies is affected by the north Atlantic pressure cells of the North Atlantic Oscillation (NAO); the Azores high and the Iceland- ic low. When the pressure cells are well developed (in a positive phase) with pronounced high and low pressure the westerlies are strong with their strongest expression in the winter season. Strong westerlies bring mild and wet winters and cool summers to western/northwestern Europe and drier conditions to southern Europe/the Mediterranean, while temperatures above Greenland and Baffin Bay are cooler. The westerlies cross the Baltic before reaching Finland, and mainly affect the southern and southwestern parts of the country with warm, maritime conditions. Counteracting the warm/wet influence of the westerlies is the Siberian high, which when it extends towards the west (weak westerlies) brings dry and cold winter conditions and high summer temperatures to Europe (Barry and Chorley 2010). Fur- thermore, cold arctic air can penetrate further southward when the westerlies are weak. The NAO shows no cyclicity in variability but seems to vary over relatively short time-scales. Time-series of NAO variability extending back to the late 19th century shows decadal to half century-scale periodicity of the different modes (Barry and Chorley 2010). In contrast, the North Atlantic Current is part of the global conveyor belt, which completes a full cycle in approximately 1500 years. Any change in this circulation may have the possibility to promote climate changes lasting several hundred or thousand years. Such changes may be initiated by input of fresh water e.g. through melting glacier ice or sea ice transport into the North Atlantic, lowering the salinity of the surface waters and thus disrupting the oceanic convection (Barry and Chorley 2010).

2.2.2. Palaeoclimatic reconstructions Lake sediments provide natural archives of past environmental or climatic changes for periods beyond the instrumental record. Fossils preserved in lake sediments as well as composition and properties of the sediment themselves have been vastly used in Quaternary palaeoecology to study climate and environmental changes since the development of the discipline (Birks et al. 2010). The development in the 1970s of quantitative methods to reconstruct past climates (Imbrie and Kipp 1971) enabled the use of quantitative estimates as inputs to, or more often for evaluation of the performance of climate models. One of the most common reconstruction methods is the calibration (or transfer) function approach. As with other palaeoecological approaches, this method relies on the prin- ciple of uniformitarianism (Rymer 1978); that modern-day taxon-environment relationships have not changed over the time studied, and thus can be applied to reconstruct past conditions. The calibration- function approach is based on a space-for-time substitution and involves i) the construction of a modern calibration dataset where data on the distribution and abundance of organisms related to present- day environmental variables is gathered ii) compilation of a fossil dataset (e.g. from a sediment core) describing variations in distribution and abundance of organisms over time and iii) using the present- day taxon-environment relationship to reconstruct the past environment based on the fossil assemblage composition (Birks and Seppä 2004) The calibration function approach relies on a number of basic assumptions e.g. i) that taxa are sys- tematically related to their environment, ii) that the climatic variable to be reconstructed is linearly (or monotonically) related to an ecologically important determinant in the system of interest, iii) that the fossil and calibration set taxa are the same biological entities and that their relationship to the environmental variable has not changed over time, iv) the mathematical model used can adequately model the species responses and produce accurate and unbiased reconstructions and v) that environmental

11 variables other than the one of interest have had negligible influence on the fossil assemblage or that the joint distribution of the variables has not changed over time (Birks and Seppä 2004, Birks et al. 2010). Over the years it has been repeatedly emphasized that violation of any of these basic principles are likely to have important (negative) implications for the final reconstruction (Birks 1995, Birks et al. 2010, Velle et al. 2010, Juggins and Birks 2012, Juggins 2013). As the composition of most aquatic assemblages found in lake sediments is generally influenced by a number of variables (e.g. in lake- variables such as pH, depth, nutrients, temperature, habitat availability; climatic variables such as precipitation, seasonality, degree days etc.) the assumption that other variables have had negligible influence is in most quantitative reconstructions never met, and it is a risk that the joint distribution may also have changed (Birks et al. 2010). A key concern when performing quantitative environmental reconstructions is thus critical examination and validation of the resulting reconstruction. This can be aimed at, at least, by careful evaluation against independent climate records from other sites but also against climate reconstructions based on different proxies from the same site, and by performing a multi-proxy qualitative study of the site in order to identify possible confining variables (i.e. local non-climatic factors affecting the composition of the assemblage used for reconstruction) (Birks and Birks 2006, Birks et al. 2010). The performance of the numerical model applied can be evaluated, but performance statistics is only a rough guide to the reliability of the reconstruction as it does not show if any of the basic principles are vio- lated. Also the quality of the modern calibration dataset used has important implications for the resulting reconstruction (Birks and Birks 2006, Birks et al. 2010). Important requirements for optimal calibration set development is that the modern data represent the range of possible past climatic variables, the modern data is from the same sedimentary environment as the fossil data, that the modern sites are evenly distributed along the environmental gradient to be reconstructed and that the full range of a taxon’s distribution along the environmental gradient is represented (i.e. is not truncated) (ter Braak and Looman 1986, ter Braak 1995, Birks et al. 2010). For periods in the distant past from which records are scarce, careful site selection in order to account for confining variables and similarities with the modern data is often not possible. For ancient records, such as the Eemian record from Sokli, where the lake itself is not preserved, a qualitative multi-proxy analysis can thus contribute with important information about the depositional environment against which the quantitative reconstruction can be evaluated.

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3. Site description

3.1. The Sokli site

Sokli (67°48’N, 29°18’E) is situated in northeastern Finland, close to the Russian border. Here, a ca 2 km2 large sedimentation basin containing approximately 30 m thick sediment deposits originating from the late Saalian/early Eemian to the Holocene was found in connection with carbonatite prospecting carried out in the 1970’s (Ilvonen 1973). The sedimentation basin is formed as a weathering or glacially carved depression in a Paleozoic carbonate-rich magma intrusion (the Sokli Carbonatite Mas- sif) that penetrated through the surrounding crystalline Precambrian Shield approximately 365 Ma ago (Vartiainen 1980). The easily eroded carbonatite of the intrusion provided a setting where sediments deposited during the Late Pleistocene could be preserved from subsequent glacial erosion. The Sokli Carbonatite Massif belongs to the westernmost part of the Kola Alkaline Province and consists of a circular ca 5 km in diameter core (surface elevation ca 220 m a.s.l.) of highly weathered carbonatite with surficial formation of phosphorus rich phoscorites, surrounded by a hilly ring of fenites reaching up to ca 320 m a.s.l (Fig. 3). The massif is crossed by a NE-SW trending fracture zone along which sedimentation basins have formed (Vartiainen 1980, Talvitie et al.1981). A mire, the Sokli aapa, occu- pies the center of the massif and is crossed by the small river Soklioja which flows from east to west into the bigger Nuorttii River, in turn draining into the Barents Sea at the coast of the Kola Peninsula to the northeast of Sokli. The vegetation around Sokli is characterized by northern boreal forest dominated by Betula, Picea and Pinus, and extensive mires with a typical vegetation of Sphagnum spp, Rubus chamaemorus, Ericaceae, Betula nana, Salix spp and Carex spp. The highest peaks in the area (ca 500 m a.s.l.) at approximately 20 km distance from Sokli are above the altitudinal tree-line and covered with tundra vegetation. The northern limit of conifers is 100-150 km north of Sokli and the

Figure 3. (A) The environmental setting of the Sokli Eemian palaeolake, eastern Finnish Lapland, and the coring site of this study (2010/2 and 4) and previous studies referred to in this text (B-series, 900, 901, 902, 905) (Hel- mens et al. 2000, 2007). Geomorphology after Talvitie et al. (1981) and Johansson 1995, bedrock outline after Vartiainen (1980). (B) Location of the Kola Alkaline Province. Deglacial Weichselian ice-divide after Johansson (1995).

13 tree-line limit (birch) 300 km. The present (1971-2000) climate at Sokli is cold boreal with a mean annual temperature of 1°C, a July mean of 13°C and a February mean of -14°C. Precipitation averages at 500-550 mm/year (Drebs et al. 2002). The ice cover period of lakes in the area is between October and the end of May.

3. 2. Stratigraphic setting and chronology

Thick, in situ, occurrences of preglacial weathered bedrock are common in central Finnish Lapland and around Sokli (Hirvas 1991). The extent of these occurrences is associated with the extent of the ice divide zone of the Weichselian glaciations which crosses central Finnish Lapland. Low ice- velocities and frozen bed conditions within this zone helped preserving weathered bedrock and Pleis- tocene sediments (Hirvas 1991, Kleman et al. 1999, Johansson 2011). Preserved sequential till beds (Till bed I-VI, from youngest to oldest) representing six different ice flow stages have been recorded from Lapland (Hirvas 1991). The uppermost three are thought to have been deposited during the Weichselian as these are found overlying sediments correlated with the Eemian stage, and the fourth is associated with the Saalian glaciation (Hirvas 1991, Johansson 2011). Till bed II has been interpreted as representing the youngest flow stage of glacier ice covering the entire Lapland, while till bed I has been associated with local deposition at the oscillating ice front during the last deglaciation (Hirvas 1991). In a compilation of studies of the glaciation in the Eurasian Arctic, Svendsen et al. (2004) described four major glaciations from the last 160 ka in the Barents Sea region. The oldest is associated with the Saalian glaciation (> 130 ka) when ice covered all of Fennoscandia, this is followed by a less extensive early Weichselian glaciation (ca 90-80 ka) when ice covered the Scandinavian mountain range, most of Norway, central to northern Sweden and Finnish Lapland. This glaciation possibly corresponds to Till bed III found in Finnish Lapland and MIS 5b (Helmens et al. 2000, Svendsen et al. 2004). A third glaciation covering all of Fennoscandia, but not reaching as far as the Saalian glaciation, is thought to have occurred during the Middle Weichselian (MIS 4, 60-50 ka). Till beds from this glaciation are not mentioned in the study of Hirvas (1991) but might be associated with the so called “old northern till” found in the eastern part of central Finnish Lapland, close to Sokli (Johans- son 1995) and thought to correspond to till beds in Ostrobothnia and southern Finland (Salonen et al. 2008). The fourth major glaciation, when the ice once again covered all of Fennoscandia, corresponds to the Late Weichselian glacial maximum (ca 20-15 ka) and can be associated with till bed II and I of the Finnish Lapland till stratigraphy (Hirvas 1991). The last deglaciation of eastern Finnish Lapland is dated to ca 10,600 cal. BP (Johansson and Kujansuu 2005, Shala et al. 2014). Till-covered organic beds (interglacial, interstadial) are many in the area but they are generally thin and found in secondary position (e.g. Hirvas 1991, Salonen et al. 2008). The first drilling at Sokli was performed in the early 1970’s. The sediments retrieved were analyzed for pollen (Ilvonen 1973), and lithologically described. The results showed a small sedimentary basin filled with interlayering deposits of till, sand and silt, and a horizon of diatom gyttja containing a pollen flora corresponding to an interglacial, presumably the Eemian (Ilvonen 1973). A second drilling resulted in a more detailed palynological analysis of the Eemian deposit and the overlying silty sediments (Forsström 1990). The sediments were retrieved by auger from the margin of the basin and showed evidence of hiatuses within and above the Eemian sequence. The first cores collected by percussion drilling also contained coring gaps (Johansson and Räsänen 1994) but indicated that the complete Sokli sequence consisted of interlayered organic and minerogenic beds. The first more complete sedimentary sequence was cored in 1996 (Helmens et al. 2000) with a piston corer. Lithological descriptions and results from palynological and chronological analysis were presented by Helmens et al. (2000). Bracketing thermoluminescence (TL) and infrared stimulated luminescence (IRSL) ages of 180-150 ka and 110±20 ka support the correlation of the basal organic sediments with the Eemian Interglacial. The Eemian sediments are overlain by fluvial gravels and sands followed by sandy gyttja associated with the stadial and interstadial MIS 5d and 5c respectively, before the first till layer of three in the sequence appears. Each till layer is followed by fining upward sediments thought to represent deglaciation. The first till layer is associated with Till bed III of Hirvas (1991) and MIS 5b and is followed by

14 a sandy gyttja deposit associated with the Odderade interstadial, or MIS 5a. The second till layer that overlies the gyttja is associated with the “old northern” till. This is associated with MIS 4 and followed by laminated clays and silts deposited under interstadial glaciolacustrine conditions during early MIS 3. The last till layer is associated with Till bed II and I of Hirvas (1991) and interpreted as deposited during late MIS 3 or MIS 2 (Helmens et al. 2000, 2007a). The chronology for the Sokli sequence is based on TL, IRSL, optically stimulated luminescence (OSL) dating on quartz using SAR-protocol and 14C-dating of terrestrial plant macrofossils (Helmens et al. 2000, Alexanderson et al. 2008) (Fig. 4). Detailed multi-proxy and quantitative climate reconstructions have been performed on material from the nearly continuous Sokli B-series core, retrieved from the central part of the basin in 2002 (e.g. Helmens et al. 2007a, 2007b, 2009, 2012, Bos et al. 2009, Väliranta et al. 2009, Engels et al. 2008, 2010, Helmens and Engels 2010). The LIG section of the Sokli B-series core contained hiatuses and the core did not reach the base of the diatom gyttja. Therefore, in 2010, the Sokli 2010/4-core was collected. The studies presented in this thesis are based on down core analysis of the LIG and penultimate deglacial sequence from this core. A summary of the methods and analysis employed is intro- duced below.

Figure 4. (A) Lithology of the Sokli 2010/2 and 4 cores from the central Sokli basin. *The absolute age control and correlation with the marine oxygen-isotope stratigraphy are based on investigations of the Sokli B-series and 900/902 cores (Helmens et al. 2000, 2007, Alexanderson 2008). (B) Lithology of the penultimate deglacial/Eemian sequence investigated in the present study. Left profile: retrieved cores (numbered I to XIII), right profile: corrected for compression (see text).

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4. Methods

4.1. Coring and subsampling

Coring was performed by percussion drilling in the winter of 2010 by the Geological Survey of Fin- land. Sediment cores of 1 m length and 5 cm diameter were collected using a flow-through coring device. The open core head allowed contamination (sand and gravel) from the sides of the drill-hole to enter the sampling tubes. This had to be pushed out by the collected sediment, leading to various degrees of compression of the Eemian gyttja (Fig. 4). The contact between the Eemian gyttja and the contamination was usually sharp and the contamination could be avoided when subsampling. Sample depths had to be recalculated based on the degree of compression and the compressed core sections are shown along-side the lithological column in figure 4. In total, 12 m of sediment forming the basis of this study was recovered, including 9 m of Eemian diatom-rich gyttja overlying 3 m of minerogenic glaciolacustrine sediment. The cores were split lengthwise, lithologically described, scanned using XRF core scanning and subsampled in 2010/2011. The 9 m of diatom gyttja was sampled at 1 cm interval (for diatom, chironomid and geochemical analysis) and 2 cm interval (for pollen and macrofossil analysis), respectively. The underlying 3 m of minerogenic sediments were sampled at lower resolution with respect to sedimentary boundaries. A lithological description of the sampled sediment sequence is found in Table 3.

Table 3. Lithological description of the Sokli Eemian palaeolake sediment sequence. Depth (m) Sediment 16.00-16.20 Sandy gyttja 16.20-16.58 10 cm thick layers of brown gyttja with small yellowish spots intercalated with 2-3 cm thick layers of homogeneous sand 16.58-16.75 Brown gyttja 16.75-17.07 Light greyish-reddish sporadically spotted gyttja 17.07-17.33 Brown gyttja with yellowish spots 17.33-17.41 Silty gyttja 17.41-19.50 Brown gyttja (Black laminations seen during coring at ca. 18.30-17.41) 19.50-19.86 Dark brown gyttja 19.86-20.50 Vaguely laminated light brown gyttja 20.50-20.73 Gyttja, contaminated/disturbed during coring 20.73-21.00 Vaguely laminated reddish gyttja 21.00-21.50 Laminated dark brown gyttja with thin yellowish laminations. 1 cm thick dark laminations every 5-10 cm. In the upper part of the unit the thin laminae becomes slightly distorted 21.50-23.24 Partly laminated brown gyttja with thin yellowish laminations (Homogeneous gyttja at 22.75-22.25 and 21.88-21.50) 23.24-23.33 Laminated silty gyttja 23.33-23.41 Silty gyttja 23.41-25.00 Partly laminated brown gyttja with thin yellowish laminations (Homogeneous gyttja at 24.80-24.46, 24.02-23.88 and 23.57-23.41) 25.00-25.10 Silt 25.10-25.20 Sandy silt 25.20-26.50 Laminated fine and medium sand 26.50-26.70 Fining upwards medium-fine sand 26.70-27.00 Coarse grained sand 27.00-27.15 Laminated fine and medium sand 27.15-27.35 Fining upwards medium-fine sand 27.35-28.35 Medium sand 28.35-28.50 Laminated fine and medium sand

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4.2. Biological methods

4.2.1. Diatom analysis Diatoms are microscopic unicellular algae with shells (frustules) surrounding the cell, made of rigid amorphous silica, which makes them resistant to decomposition. More than 10,000 (possibly up to 200,000) worldwide occurring freshwater species are found in a wide variety of aquatic environments (Battarbee et al. 2003, Vanormelingen et al. 2008). In lacustrine settings they can occur in benthic (epiphytic, epipelic, epilithic or epipsammic), planktonic or tychoplanktonic (i.e. either benthic or planktonic) habitats, but they may also be found in streams (rheophilic) or semiaquatic habitats or soils (aerophilic). Ornamentation and shell shape enables identification of the diatoms to species or subspe- cies level, and data on habitat preferences allows ecological grouping of diatom assemblages/taxa (Battarbee et al. 2003). Diatoms are generally well preserved in lake sediments and register a variety of limnological factors (e.g. pH, nutrient levels, conductivity, habitat changes, and light availability) that can be related to various limnological and environmental parameters such as water depth, climate- and catchment development, and thermal- and mixing dynamics. One of the dominant controls on diatom assemblage composition is water-column mixing, which regulates nutrient supply to the epi- limnion (Bradbury 1988, Kilham et al. 1996). In colder regions ice cover duration affecting the length of the spring overturn and ice-free season and primary production, has further proven to be a major determinant for the lake diatom assemblage composition (Weckström et al. 2014). Samples for diatom analysis were prepared following standard procedures for siliceous microfossils (Battarbee et al. 2003). In total, 100 samples were analyzed with a light microscope using oil immer- sion optics at ×1000 magnification, and a minimum of 400 valves were identified for each sample. Identification was carried out to the lowest possible taxonomic level following standard floras (Krammer and Lange-Bertalot 1991, 2008a, b, 2010a, b) in consultation with previous studies on the Sokli and Finnish Lapland flora (Helmens et al. 2009, 2012, Mölder and Tynni 1967, 1968, 1969, 1970, 1971, 1972, 1973, Tynni 1975, 1976, 1978, 1980, 1982). Detailed taxonomic descriptions were based on analyses using an environmental scanning electron microscope (ESEM) (Philips XL-30- ESEM-FEG SEM, Department of Geological Sciences Stockholm University). Other siliceous microfossils e.g. chrysophyte cysts and phytoliths were counted but not identified. Diatom percentages were calculated from the sum of all diatoms.

4.2.2. Chironomid analysis Chironomids (non-biting midges; Arthropoda: Insecta: Diptera: Chironomidae) is the most common family among insects preserved in lake sediments and includes more than 5000 species worldwide (Brooks et al. 2007). The chitinous larval remains, mainly head capsules, are generally well preserved in lake sediments and retain features possible to identify at least to the genus level. Most chironomids are aquatic in lakes and streams although some occur in semi-terrestrial or brackish to saline aquatic habitats. The chironomids pass four larval stages of which the first is planktonic. In further stages the most common habitat is benthic or epiphytic. Chironomids are a common and long used proxy in palaeoecological studies (Brundin 1949, Thienemann 1954) and they have been increasingly used as palaeoclimatic indicators following the development of chironomid-based transfer functions for reconstruction of mean July air temperature (Brooks 2006, Walker and Cwynar 2006). Chironomid-based transfer functions have proven to work well for reconstructing major climatic fluctuations during the last glacial/interglacial transition (Brooks 2006, Heiri et al. 2014; Brooks and Langdon 2014). However, it has been argued that chironomid-based transfer functions work less well in recording smaller scale temperature changes such as those occurring during the warm interval of an interglacial (Brooks 2006, Velle et al. 2010). Temperature has a direct effect on emergence, growth, respiration and development of the individual insect but also controls chironomid distribution by indirect effects on e.g. lake nutrient status, stratification regime/oxygen state and composition of littoral vegetation (Eggermont and Heiri 2012, Dickson and Walker 2015). Variance in any of these factors unrelated to temperature may compromise a reconstruction based on chironomids. The impact of confounding variables is a general problem with the transfer function approach, but it implies that result-

17 ing reconstruction needs to be evaluated against other palaeoecological data (Juggins and Birks 2012, Juggins 2013). In total, 74 samples were analysed for chironomids using standard methods (Brooks et al. 2007). Between 0.29-4.17 g of wet sediment/sample was treated with HNO3, and heated over water bath for 20 minutes. The sample was then sieved through 100 μm mesh, and the residue put in a Bogorov sort- er (Gannon 1971). Chironomid head capsules were handpicked using fine forceps under ×2.5/×6 magnification (stereo microscope), air-dried and mounted on a cover slip using Euparal. At least 50 head capsules per sample were counted (Quinlan and Smol 2001) (minimum counting sum was 56.5 in the lowermost sample, otherwise counting sums ranged between 62 and 179.5). Identification was made to highest possible taxonomic resolution under ×40 magnification using a light microscope. Identification mainly followed Brooks et al. (2007). The reconstruction of July temperatures was based on the weighted-average partial least squares (WA-PLS) calibration function method (ter Braak and Juggins 1993), in which the reconstructed environmental variable is the weighted average of the optima of the taxa in a fossil sample. In chironomid analysis WA-PLS has become the standard method, since it has a strong ecological and statistical basis and performs well under non-analogue situations and on data that is closed and contains many zero values (Birks et al. 2010). Three different calibration datasets; a Norwegian (Brooks and Birks 2001, 2004), Finnish (Olander et al. 1999, Luoto et al. 2009, 2014a, b, 2016) and northern Russian (Naza- rova et al. 2008, 2011, 2015) are used for reconstructing temperatures and compared. These are all from boreal-subarctic tundra areas and have a good representation of the most common fossil taxa. They however cover slightly different temperature ranges which may affect the inferred temperatures (Engels et al. 2014).

4.2.3. Pollen, spores and non-pollen palynomorph (NPP) analysis Pollen analysis is the principal technique for studying past vegetation dynamics. Pollen grains and spores are part of the reproductive system of plants, ferns and mosses (Bennett and Willis 2003). Pol- len grains and spores are found abundantly in lake sediments, the outer walls of the grains/spores are resistant to decomposition and their morphological characteristics allow identification to family or genus level and sometimes even species level, making them suitable for palaeoecological studies. Non-pollen palynomorphs encompass all those microfossils that are preserved in pollen preparation that are not pollen or spores (van Geel 2003). This includes e.g. algae, eggs of flatworms, and cyano- bacteria, and many still not identified “Types” distinguished by morphological description and classi- fication. Many of these unidentified types can still be used as palaeoenvironmental indicators, with ecological information based on co-occurrence with other identified taxa and environmental parameters (Korde and Zhuze 1966, van Geel 1986). Analysis of plant microfossils (pollen, spores, NPPs) has been performed on the entire 12 m of Eemian/glaciolacustrine sediments. An initial low resolution (2- 10 cm interval) analysis by Dr Sakari Salonen Helsinki University, Finland, was followed by a high resolution (1 cm interval) analysis by Dr Sakari Salonen and Niina Kuosmanen, Czech University of Life Sciences Prague, Czech Republic. Samples were prepared using KOH, sieving (212 μm mesh), Na-pyrophosphate, acetolysis and treatment with a bromoform heavy-liquid to remove silicate. The residue was mounted in glycerol and analyzed using a light microscope. Minimum counting sum was 220 and mean 392. Pollen, spores and NPP percentages were calculated from the sum of all terrestrial pollen and spore taxa. Conifer stomata were additionally counted from the pollen slides. The data is used both to infer mean July and January air temperature variations and to reconstruct the nature and development of the terrestrial, wetland and aquatic ecosystem. The results of the high resolution analysis including July and January temperature reconstructions are currently in revision (Salonen et al. in revision).

4.2.4. Macrofossil analysis The study of plant macrofossils, e.g. seeds, fruits, spores and vegetative parts of plants large enough to be seen by the naked eye, is a long-used technique to infer past vegetation and environmental as well as climatic (e.g. through the indicator species approach) conditions and is a valuable complement to pollen analysis (e.g. Jessen and Milthers 1928, Birks 2003). A low-resolution macrofossil analysis of

18 the entire 12 m of Eemian/glaciolacustrine sediments was performed by Dr. Minna Väliranta, Helsinki University, Finland. The results from the macrofossil analysis can be used to infer minimum mean July temperatures to complement temperature data attained from chironomid and pollen analyses (Iversen 1954, Kolstrup 1980, Väliranta et al. 2009) and additionally provide information about local environment and processes within the lake and catchment e.g. proximity of the coring site to the shore and indications of water depth. Samples for macrofossil analysis were prepared from subsamples of mainly ca 5 cm3. The sediment was sieved using a 100 )m mesh under running water and the residue examined using stereo and high-magnification light microscopes.

4.3. Geochemical methods

4.3.1. X-ray fluorescence (XRF) core scanning Prior to subsampling, all cores were scanned at the SLAM Lab at the Department of Geological Sci- ences, Stockholm University, using an Itrax XRF core scanner from Cox Analytical Systems (Gothen- burg, Sweden) in collaboration with Associate Professors Malin Kylander and Ludvig Löwemark. The Itrax core scanner non-destructively collects high-resolution elemental composition data through XRF analysis, as well as optical and X-radiographic images of the cores (Croudace et al. 2006). XRF analysis provides a record of down-core geochemical variation. The chemical record is influenced by a variety of factors and processes within the lake and the catchment, such as source rock composition, weathering and depositional/post-depositional processes, which in turn might be related to changes in climate, and elemental data can be used to infer changes in e.g. minerogenic input, redox processes and lake levels (Kylander et al. 2011). The cores were scanned in 2010 (Eemian gyttja) and 2012 (minerogenic lowest 3 m) at 2.5-mm resolution using a molybdenum tube for heavier elements. Based on analytical performance (counting statistics), reliable data was acquired for Si, S, K, Ca, Ti, Mn, Fe, Rb, Sr and Zr. All data were normalized to the (incoherent+coherent) scattering to remove various instrumental effects and then smoothed using a 5-point running mean to capture the main shifts. A Principal Component Analysis (PCA), elemental correlations and elemental ratios were used to identify elements that showed a similar behavior and therefore were likely controlled by the same process.

4.3.2. Carbon and nitrogen content The C/N ratio of organic material, as determined by elemental analysis, is a widely used method to discriminate between different sources of sedimentary organic matter. Organic matter with a lacustrine (algal) origin typically has atomic C/N ratios between 4 and 10, whereas organic matter with a terrestrial origin typically has C/N ratios above 20 (Meyers and Teranes 2001). The organic carbon content can be used as a crude measure for organic production. Total organic carbon (TOC wt%) and total nitrogen (TN wt%) content was measured on freeze dried and homogenized sediment with a Carlo Erba NC2500 Elemental Analyzer at the Department of Geological Sciences, Stockholm University. Duplicate samples were treated with HCl (10 %) to account for carbonates in the sediment. Samples for loss-on-ignition (LOI wt%) were prepared following Heiri et al. (2001) and organic and carbonate content was calculated after combustion in 550°C for 4 hours and in 950°C for 2 hours respectively. LOI and C/N analysis was performed at 10 cm intervals in the gyttja sequence except for the depth intervals 23.49-23.20 and 17.52-17.28 where analysis was performed at 2-4 cm intervals.

4.3.3. Fourier-transform infrared spectroscopy (FTIRS) FTIRS is a method to quantitatively infer compound concentrations in sediments which is especially suited for long sediment sequences and a limited amount of material (Vogel et al. 2008, Rosén et al. 2010). With FTIRS, the spectral signatures of all the sediment compounds in a sample are retrieved for every sample analyzed (Rosén et al. 2010). A calibration model based on a comparison of convention- ally measured concentrations of a compound of interest and the FTIR spectra in the same sample is then used to quantitatively infer the amount of that compound from the retrieved FTIR spectra. FTIRS

19 has been used to quantify e.g. biogenic silica (BSi), total organic carbon (TOC), total inorganic carbon (TIC) and total nitrogen (TN) in lake sediments (Vogel et al. 2008, Rosén et al. 2010, Meyer-Jacob et al. 2014), and is used in this study to quantify BSi. A set of 900 1 cc sub-samples from the Eemian diatom gyttja has been analyzed using FTIRS at the Department of Ecology and Environmental Sci- ence, Umeå University, Sweden, by Dr. Peter Rosén and MSc María Fernández- Fernández (Fernán- dez Fernández 2011). The analysis was made using a VERTEX 70 FTIR spectrometer (Bruker Optics) on freeze-dried homogenized samples. All spectra were pretreated in order to minimize the spectral variation not related to the sample properties. Thirty samples were selected for conventional measurements and used for validation of the developed models. Concentrations were measured with conventional techniques at the Alfred Wegener Institute for Polar and Marine Research.

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5. Results - Summary of papers

5.1. Paper I: Major cooling intersecting peak Eemian Interglacial warmth in northern Europe

This paper focuses on the Tunturi cooling event, a millennial scale cooling event intersecting Eemian Interglacial warmth at Sokli, established from pollen- and chironomid-based quantitative July temperature reconstructions and supported by qualitative data from multiple proxies (chironomids, pollen, diatoms, geochemistry). The stability of the Eemian climate in the north Atlantic region has been under debate for some time. Several continental records from the Eemian have suggested that the Eemian was a rather stable period, which is in sharp contrast to recent, high latitude marine records from the subpolar North Atlantic and the Nordic seas indicating that the Eemian Interglacial was interrupted by major cooling events possibly connected to abrupt changes in the AMOC. Considering that the warmer-than-present Eemian can be seen as an (non-perfect) analogue to a future warmer climate, the pos- sibilities and reasons for such abrupt changes in the AMOC and the effects this has on the adjacent continents is essential to study. The Tunturi cooling event is characterized by an abrupt summer cooling on the order of 2-4 °C, a change to a more open vegetation cover and increased soil erosion and possibly a prolonged lake ice- cover period. Comparisons between records from the Eemian are complicated by the lack of absolutely dated age control markers in most records. In this paper we correlated the Tunturi cooling-event with an event of similar magnitude and structure recorded in a marine core from the Norwegian Sea, dated to ca 120 ka BP (Bauch et al. 2011) and a short but distinct reduction in North Atlantic deep- water formation and sea-surface temperature cooling recorded in marine core MD03-2264 from the Eirik Drift, Labrador Sea (Galaasen et al. 2014). The correlation with these events and not with an earlier major event registered at the Eirik Drift, dated to ca 126 ka, is based on the assumption that Sokli was still covered by the Fennoscandian ice sheet at the time of the earlier event as suggested by cold conditions and IRD input in the Norwegian Sea. In a later paper (paper IV), we revise this chronological alignment based on new evidence. In this paper the cooling is suggested to have been caused by intensive melting of the Greenland Ice Sheet resulting in a reduction in the strength of the AMOC. Other evidence of short term climatic events during the Eemian is registered in the Norwegian, Greenland and North Seas. However, there is still a lack of evidence of widespread cooling during this interval. This may be related either to time resolution problematics or to a limited geographical impact of the event (i.e. confined to the northern North Atlantic region). Despite these uncertainties, the existence in the Eemian record from Sokli of a cooling event interrupting warm interglacial conditions illustrates the potential for climatic instability in a warmer than present world and the impact abrupt changes in the North Atlantic Ocean circulatory system could have.

5.2. Paper II: Development of an Eemian (MIS 5e) Interglacial palaeolake at Sokli (N Finland) inferred using multiple proxies

This paper is a detailed multi-proxy study of the development of the Sokli Eemian palaeolake. Several proxies expected to respond both to processes active in the catchment (erosion, vegetation development, land cover changes) and in the lake (changes in pH, water depth, nutrient levels) is combined to get an as coherent picture as possible of what the characteristics of the lake was and what processes shaped the composition of the assemblages in the lake at different times and, in a further step, to distinguish if any of these processes could be related to climate. The motives were several: i) this is an

21 ancient lake, i.e. the lake basin itself is not preserved and thus little is known about the depositional environment ii) the depositional environment is important to consider when planning and interpreting quantitative climate reconstructions based on e.g. pollen and chironomids as it can affect both taphonomy and composition of the assemblages used in the reconstruction iii) it is also important to distinguish the effect of processes that are related to local non-climatic changes in the environment, i.e. infilling, slumps, re-direction of streams, that could lead to confusing signals in a quantitative temperature reconstruction and iv) many of the changes in the lake and catchment can be related to climate and thus the multitude of data from this lake could be used also to make a qualitative reconstruction of long-term and short-term climatic changes during the studied interval. Biological proxies (diatoms, macrofossils, pollen/spores and NPPs) were used together with biogeochemical proxies (itrax-derived XRF data of the geochemical composition of the sediments, FTIRS- inferred BSi content, LOI measurements of organic carbon content and C/N data). Additionally, the pollen-based temperature reconstruction from paper I was included as a reference for general temperature development. Based on the changes in these proxies the development of the Sokli Eemian palaeolake was divided into five major lake phases (L1-L5) and two events (E1 and E2) distinguished in Paper I, including the Tunturi cooling event (E1). The results indicate that the Sokli Eemian palaeolake began as a glacially-fed lake characterized by low nutrient levels and high input of minerogenic material (L1). The surrounding catchment was covered by pioneering open birch vegetation. This phase probably represents a relatively short time interval of rapid sediment accumulation during deglaciation. With the transition to L2, a stable interglacial lacustrine environment is being established. This phase is interpreted as the period of the most continental conditions at Sokli. The multi-proxy record indicates relatively warm summers, long and stable stratification periods and short mixing periods. Through the phase, nutrient levels increase as well as the length of the mixing periods as continentality decreases and conditions get milder. The phase ends abrupt with the Tunturi cooling event (E1). During the event, the multiple proxies reflect not only a more open vegetation, soil erosion and cooling (Paper I), but also more arid conditions leading to lower lake levels and less stable summer stratification. Following the Tunturi event, the productivity of L3 was high as a result of stable summer stratification and mild winters that led to a prolonged open water season promoting internal loading of nutrients. This phase may correspond to the climatic optimum. During L4 the climatic interpretation of the limnological data is complicated by decreasing water depths as a consequence of infilling which leads to less stable summer stratification and rising nutrient levels. As the lake gets shallower towards the end of L4 there is an increased influence of wetland- associated taxa as well as of cold-indicating taxa. The second cooling event, E2, is expressed as a sud- den decrease in water depth reflecting increased aridity, and cooler, less productive conditions. Fol- lowing this, in L5, there is a short return to conditions prior to the event, indicating a slight climatic recovery, before cooling is continued. Changing hydrological conditions leads to the deposition of repeated sand layers and final infilling of the lake. The temperature development at Sokli seems to follow a similar trajectory as other European records, mainly reflected in the evidence of gradually decreasing continentality and relatively stable, warm conditions. Superimposed on the long-term trend, however, is the expression of two climatic events characterized by cooler/more arid conditions, with likely counterparts in other records from the Eemian.

5.3. Paper III: New insights from XRF core scanning data into boreal lake ontogeny during the Eemian (Marine Isotope Stage 5e) at Sokli, northeast Finland

In this paper the wealth of information from the detailed XRF core scanning data is utilized in combination with previously published qualitative multi-proxy biotic (diatoms, pollen, NPP) and biogeochemical (BSi, TOC) data to explore in detail the influence of processes such as changes in sediment source and influx, nutrient and base cation supply, chemical weathering and biological productivity during the different phases of lake development. The pattern of development, or ontogeny, of the Sokli

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Eemian palaeolake is compared with that of Holocene lakes in boreal regions, to distinguish the impact of bedrock composition on the Sokli palaeolake. Principal Component Analysis (PCA) is used to identify the main processes driving the changes in the dataset. Three components together explain 82% of the variance in the elemental data. PC 1 is represented by Zr, Ti, K, Rb, Ca and Si which are all part of the crystal structure of common minerals and are thus transported detritally to the lake. This PC is also correlated with visible silt and sand layers in the stratigraphy and is interpreted as indicating input of (coarse-grained) minerogenic material. PC2 is driven by variations in S, Fe, Mn and Ca. These elements are all common in the Sokli Carbon- atite Massif (SCM) and can be transported to the lake either as particulate material, in dissolved form or bound to organic matter. Even though these elements are often seen as indicators of productivity and anoxia, we interpret their common signature here as indicating deposition of detrital material originating from SCM rock types. This is partly because under reducing (anoxic) conditions Fe and Mn ions would show opposite patterns, which is not the case here. PC3 is associated with Sr and Ca. Both these minerals can enter the system detritally, in dissolution or absorbed to organic matter. We attrib- ute the delivery of Sr and Ca to the sediment by precipitation or absorption to organic matter and interpret variations in Sr/PC3 as a proxy for chemical weathering. Based on the above interpretations we trace the input of 1) detrital material from sources beyond the SCM, 2) detrital input from sources within the SCM and 3) chemical weathering throughout the Eemian at Sokli. In general, the lake evolution was characterized by relatively stable minerogenic input. Decreasing minerogenic input during the early Eemian reflect stabilization of the catchment in the form of soil and vegetation development, and during the cold/arid events peaks in coarser (PC1) minerogenic input reflect the influence of erosion of the catchment. Chemical weathering (PC3) increases in the mid-Eemian lake phases which may have added to the productive conditions in the lake. The abundant and continual supply of base cations and nutrients provided by the SCM helped to maintain a relatively alkaline and productive lake throughout lake development, separating its development from the common trajectory of boreal lakes situated on acidic bedrock towards increasing oligotrophy and acidity as cations are leached from the catchment soils.

5.4. Paper IV: Chironomid-based temperature reconstruction for the Eemian Interglacial (MIS 5e) at Sokli, northeast Finland

In this study a chironomid-based July temperature reconstruction based on chironomid data from the Sokli Eemian sequence is performed. To better understand the drivers of climate dynamics in a warmer than present climate, spatially distributed data is required. However, from previously glaciated high- latitude regions continuous high resolution temperature records from the entire Eemian are sparse. Therefore, in this study, the temperature development at the high latitude Sokli site is investigated, and temperatures are reconstructed across the beginning, optimum and end of interglacial conditions at Sokli. In this paper an updated age model for the Sokli Eemian sequence compared to Paper I is used. This model is based on an assumption of a rapid deglaciation due to high insolation and thus a near synchronous Eemian climate evolution across northern Europe and makes use of supporting stalagmite dating (Salonen et al. in revision). A transfer function approach applying a WAPLS regression is used to reconstruct July temperatures. Three independent chironomid calibration datasets (Norwegian, Finnish and northern Russian) are used to evaluate the effect of dataset properties on the final reconstruction, and qualitative data from previous studies (Paper II and III) are further used to distinguish the influence of confounding variables on the final reconstruction. The temperature reconstructions based on three different calibration datasets show large similarities in trends. However the mean values of the reconstructions differ, which we relate to differences in the length and range of the temperature gradient covered by the modern datasets and the distribution of lakes along the gradient. In the upper part of the record, qualitative data indicate shallowing and increasingly productive conditions. This part of the record is when inferred temperatures are highest and the reconstructions show most similarities, suggesting a significant influence of rising nutrient lev-

23 els/DOC as confounding variables driving the reconstruction. Since most of the Sokli Eemian chironomid taxa are warm-indicating taxa, it is concluded that the Russian (and then the Finnish) calibration dataset(s), expanding further on the warm side of the temperature gradient, may give the most likely representation of the optima of the Sokli fossil taxa. The reconstruction based on the Russian calibration set was deemed more successful as it may also better capture optima of cold indicating taxa. The chironomid-based temperature reconstruction using the Russian calibration dataset shows a similar trend as the most recent pollen-based temperature reconstruction from Sokli (Salonen et al. in revision) however with slightly lower (0-1.5 °C) mean temperatures, which may be related to different temperature ranges covered by the different calibration datasets. Our results indicate that warm, near present-day temperatures prevailed at Sokli already from the beginning of the interglacial, contrary to what was suggested by a recent chironomid-based temperature reconstruction from Denmark (Jones et al. 2017), and that temperatures increased only slightly during the optimum phase. These results suggest that the temperature development at Sokli followed that of central Europe during the Last Interglacial and that polar amplification was less pronounced than in other high latitude (Arctic) areas. The two cooling events, the Tunturi (E1) and the Värriö (E2) events, occurring in the early interglacial and at the beginning of glacial inception, can be tentatively correlated to events registered in marine records from the Nordic seas and subpolar north Atlantic, reflecting possibly the effect meltwater events and disruptions in the thermohaline circulation had on adjacent continental areas.

5.5. Paper V: Diatom assemblages from an Eemian palaeolake in northern Europe with morphological observations of rare Aulacoseira sp. resting spores

In this study we present the diatom flora of the Sokli Eemian palaeolake with photomicrographs of the most common planktonic taxa and detailed morphological descriptions and photomicrographs of rare resting spores belonging to the Aulacoseira islandica-skvortzowii group, not reported from recent material in Europe. Detailed description of specimen not found in the present regional flora may be useful for further studies on past distribution and evolution of taxa. Furthermore changes in distribution and evolution may be related to changing climatic conditions. Therefore, in this study we present a detailed description of the resting spores, discuss possible related and similar taxa and other occurrences of resting spores, and reflect on what climatic conditions may have led to the presence and following absence of Aulacoseira sp. resting spores in the Sokli Eemian palaeolake and the region. Our identification at Sokli of resting spores belonging to the A. islandica-skvortzowii group, presently found in Lake Baikal and in lakes in the Lake Baikal rift zone, and the registration of similar resting spores in other Finnish interglacial and interstadial deposits, support the view that this species group (or spore formation as a trait) may have had a more widespread distribution in the past. It is possible that a past more widespread distribution can be related to the more continental climatic conditions that prevailed during the early Eemian and Weichselian interstadials.

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6. Discussion

The main goal of this project was to contribute to the knowledge of Eemian Interglacial climate development in continental Fennoscandia and the northern North Atlantic region, considering general climatic trends and the existence and expression of climatic events. To achieve this, careful evaluation and interpretation of the depositional site and qualitative and quantitative reconstructions of the non- climatic and climatic changes have been undertaken. In order to consider the climate data in relation to other sites the chronological frame for the site has also been studied. The results from the multi-proxy studies of the Sokli Eemian palaeolake include; i) description of the characteristics of the Sokli palaeolake; ii) identification of the main non-climatic drivers influencing the aquatic assemblages; iii) identification of possible climate related changes in the aquatic assemblage; iv) reconstruction of long-term climatic trends and identification of climatic events in the Sokli Eemian sequence; v) evaluation of uncertainties associated with the temperature reconstruction based on the Sokli chironomid data; vi) description of rare diatom taxa occurring in the Sokli deposit and possible climatic and evolutionary correlations and vii) chronological correlation of the Sokli sequence with other records from Europe and from the North Atlantic Ocean.

6.1. Characteristics and development of the Sokli Eemian palaeolake

The multi-proxy record from the Sokli Eemian palaeolake indicate that the lake was characterised by relatively nutrient rich and alkaline conditions throughout most of its existence, which is especially evident in the dominance of high nutrient (phosphorus) favoured alkaliphilic diatoms (Paper II). The particular bedrock conditions at Sokli, with its crust of highly weathered carbonatite and phoscorite (Sorjonen-Ward and Luukkonen 2005), seem to have resulted in a relatively constant phosphorus supply and high alkalinity over the interglacial with implications for the aquatic assemblage of the Sokli palaeolake. Thus, the changes in the diatom community is less influenced by catchment processes affecting DOC and nutrient export (leaching), which is a common driver of diatom assemblage changes in boreal regions on acidic bedrock (Fritz and Anderson 2013), but rather reflects changes in nutrient dynamics associated with the strength of stratification and duration of water-column mixing (Paper II, III). As a result, changes in the diatom community have a high potential of reflecting direct climatic forcing affecting e.g. the length of ice-cover and stability of summer stratification. However, our qualitative interpretations of the data also show that the lake experienced substantial shallowing as a consequence of infilling over the course of the interglacial, and as changes in water depth affect stratification and mixing dynamics the climate signal from diatoms becomes obscured. Based on the qualitative interpretation of the multi-proxy data (Paper I, II) five main lake phases and two cool/arid events, summarized in Table 4, are distinguished. Two distinctly different sedimentary environments characterize the studied sequence; the high energy, turbid, glacial-lake setting represented by the lowermost minerogenic sediments of L1, likely deposited over a relatively short time period during deglaciation, and the gradually infilling lake setting represented by the thick sequence of diatom-rich gyttja (L2-L5). Results from Paper II indicate that the Sokli palaeolake changed from a relatively deep (down to ca 10 m), clear and stably stratified dimictic lake with a pronounced spring peak in internal phosphorous loading (L2-L3), to a shallow turbid and eutrophic lake (L4-L5) with less stable stratification, possibly shorter periods of summer stratification and more regular recycling of nutrients throughout the growing season. This change seems dominantly to be related to decreasing water levels, as it is characterized by pronounced changes in the aquatic assemblage while there is no significant change in catchment vegetation. During the deeper phase (L2), the lake was likely characterised by a relatively small

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Table 4. Description of lake phases and events identified in the Sokli Eemian palaeolake record and corresponding chironomid zones. The chronology is established by linear interpolation between tie points assuming constant sedimentation rates, and approximate age is visually estimated. Tie points are based on alignment with absolutely dated speleothem records assuming synchronicity in regional climatic changes.

Lake Chiro Depth (m) Approx. age Characteristics phase zone (ka) L1 28.5-25.0 130-129.6 Deglacial phase Glacial lake, deep, high minerogenic turbidity, low productivity L2 Ch1 25.0-23.45 129.6-127.5 Decreasing continentality, warming climate Clear, relatively deep (down to ca 10 m) waters, oligo-mesotrophic, increasing nutrient levels and length of open-water season, stable summer stratification Tunturi 23.45-22.55 127.5-126.5 Cold arid conditions, open forest event (E1) Increased catchment erosion (minerogenic input), breakdown of summer stratification (decreased lake levels/increased wind stress) L3 Ch2 22.55-20.85 126.5-124 Warm temperatures, less continental conditions, climatic optimum Reduced lake ice-cover, long open-water season, stable summer stratification, high productivity L4 Ch3 20.85-17.43 124-118.5 Decreasing summer temperatures Decreasing water depths, less stable summer stratification, increasing productivity, increased infilling and overgrowth Värriö Ch4 17.43-17.05 119.5-119 More arid and possibly cool conditions event (E2) Low lake levels, decreased productivity, longer ice-cover period

L5 17.05-16.00 119-117.5 Short warming followed by cooling Infilling eutrophic conditions followed by disturbed conditions with deposition of sand layers, increases in cold-indicating taxa area of greater depth and clear conditions as reflected in a high abundance of littoral elements (benthic diatoms, littoral macrophytes) (Paper II), and in the chironomid assemblage which is dominated by littoral taxa (Paper IV). Additionally, low oxygen conditions in the hypolimnion (as indicated by lami nations) may have led to unfavourable conditions for the development of profundal chironomid taxa. Nevertheless, the preservation of laminations and the composition of the planktonic taxa suggest deeper water conditions prevailed during this phase. The development of the aquatic system during L2 is affected by primary succession and apart from climate amelioration reflects the development of catchment vegetation and stabilization of catchment soils. With warming temperatures and vegetation development, chemical weathering and productivity in the lake increases (Paper II, III). During this lake phase we interpret our data as reflecting the most continental climatic conditions i.e. characterized by warm summers, stable stratification periods, long winter/ice cover and short mixing periods. This is reflected in the relatively low abundance and diversity of spring/autumn blooming planktonic diatom taxa and the presence of distinct laminations and benthic diatoms favoured by long ice cover and cooler conditions. Continentality gradually decreases during L2, and with the transition to L3 increasing oceanic conditions is evident in extended mixing periods as reflected in the dominance and succession of spring blooming planktonic diatoms and the appearance of autumn blooming planktonic diatoms (Paper II) (Fig. 5). This shift is related mainly to warming winters and a shorter period of lake ice cover. Such conditions may explain the final disappearance of Aulacoseira islandica-skvortzowii resting spores, associated with pronounced continental climatic conditions (Paper V). The presence only of resting spores and not of vegetative cells of this thinly silicified diatom taxon indicates the effect of tapho- nomic processes, e.g. dissolution and recycling of silica (Ryves et al. 2013), affecting the diatom assemblage of the Sokli Eemian palaeolake. Dissolution is inevitable in diatom records, but should be kept in mind. In general, the diatoms in the Sokli palaeolake are well preserved and very small and thinly silicified species can be observed, although their number may actually have been greater. Both the chironomids and diatoms reflect milder but also more nutrient rich conditions during L3 (Paper II, IV). As productivity and temperature are generally (naturally) related in the modern chironomid calibration sets (e.g. Luoto 2011) it is important to distinguish if rising nutrient levels in the pal- aeodata are related to warmer conditions or mainly related to changes in non-climatic factors (e.g. infilling and nutrient focusing). In the Sokli palaeolake we see indications of warming conditions during L3 (i.e. extended length of growing season, earlier ice-out, increased chemical weathering) while

26 the impact of water depth changes is not yet apparent (Paper II, III). However, it is possible that water depths were already decreasing, though not yet to the critical threshold inducing major changes in nutrient circulation dynamics and species assemblages. Therefore it is important to consider indications of warming temperatures unrelated to nutrient increase, as may be indicated by the increase within this phase of Pseudochironomus, a warm chironomid taxon associated dominantly with mesotrophic conditions (Brooks et al. 2007). In the upper half of the record (L4 and L5) there is a distinct shift in the aquatic assemblage which we relate to a shift in thermal dynamics as a result of infilling (Fig. 5). Mixing is still pronounced and nutrient levels are high but the importance of diatom species favoured by a high phosphorus load during early spring decreases and the changes are thus related to a less stable summer stratification and hypolimnetic phosphorus build-up following the decrease in lake levels. Throughout the upper part of the record there is an increase in various aquatic taxa associated with shallow and eutrophic conditions. Also the light climate within the lake may have changed, which is possibly reflected in the chironomid assemblage as the lake goes from a clear-water phase (L2, L3) with relative dominance of submerged macrophytes and chironomids associated with submerged macrophytes e.g. Corynocera ambigua (Brodersen and Lindegaard 1999), into a phase (L4, L5) dominated by floating leaved macrophytes and chironomids tolerant of high biological turbidity (i.e. phytoplankton blooms) and low- light conditions (Fig. 6). In studies of shallow unstratified or eutrophic stratified lakes from Denmark, Brodersen and Quinlan (2006) saw how eutrophication associated with lowering water levels could lead to an abrupt shift from a clear-water state with submerged macrophytes to a turbid state with phytoplankton dominance. A similar development may be reflected in the Sokli palaeolake. The periodic shifts back to dominance of C. ambigua in the upper part of the record (late L5 and the Värriö cooling event) likely reflect lowered productivity due to climatic cooling. The increasingly shallow conditions are accompanied by an increased input of dissolved organic carbon (DOC). This increase could be related to an increase in the marginal areas (wetland expansion), but may possibly also reflect the expansion of Picea in the catchment. In the upper part of the record (L5) cooling conditions are suggested by the appearance of cold indicating diatom (e.g. Staurosirella lapponica) and chironomid taxa (e.g. Tanytarsus lugens) (Fig. 6). The XRF data seemed to register many of the main processes described by the aquatic and sedi- mentological data in paper II. In Paper III, PC 1 is associated with Zr, Ti, K, Rb, Ca and Si and visible lithological boundaries, and related to minerogenic input of coarser grained sediment. PC2 is associated with Fe, S, Mn and Ca and is related to input of minerogenic material from the SCM (Paper III). PC2 forms distinct peaks in L2, late L3 and early L5 where the sediments were visibly laminated (in L5 these laminations were only visible directly after coring, indicting rapid oxidation of laminae- forming components), and may thus reflect an association with laminae formation and preservation. Unfortunately detailed μ-XRF studies of the laminations could not be performed (the sediment was too brittle for preparation). In Paper II we reasoned that laminations was formed by precipitation of Fe hydroxides during spring mixing or FeS during stratification periods as the presence of laminations coincided with peaks in Fe and S in the geochemical record. However, in Paper III this was found unlikely since Fe and Mn were correlated to the same PC (had similar behaviour) although under reducing conditions Mn oxides should be rapidly reduced and removed from the sediments. Ojala et al. (2013) however, show that Fe oxides and Fe-Mn-Ca-Si compounds constitute the most important inorganic geochemical fraction in the dark winter laminae in the laminated sediments of lake Nautajärvi, Finland, where also manganese and iron sulphides were found. Thus, the interpretation of PC2 in Pa- per III may describe just part of the factors influencing the behaviour of this PC (i.e. the origin of material from the SCM), but may leave out important factors influencing the depositional and preserva- tional conditions (e.g. stratification, anoxia) involved in shaping the behaviour of PC2. Possibly, detailed μ-XRF studies and microscopic examination of the sediments could have contributed to the understanding of such processes in the Sokli Eemian palaeolake.

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temperature (13°C) indicated indicated (13°C) temperature ö (E2) cooling events (Paper I, (Paper cooling events ö (E2) (grey boxes) representing boot- representing boxes) (grey mian palaeolake, including dia- including palaeolake, mian robustifying iteration) (red line) and present-day mean July and present-day line) (red iteration) robustifying igure 5. Summary of the qualitativeigure Summary multi-proxy 5. of the Ee Sokli of developmentof reconstructed study the (as the III) in Paper and II F tom, pollen, non-pollen palynomorph and geochemical data. Grey bars indicate the two cooling events; the Tunturi (E1) and Värri (E1) Tunturi the events; cooling two the indicate bars Grey data. geochemical and palynomorph non-pollen pollen, tom, limits error with presented is I Paper from reconstruction temperature July mean pollen-based The revision). in al. et Salonen one 0.05, (span smoother a LOWESS errors, standard strap-estimated as a reference (dotted red Modified line). from Paper II.

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6.2. The chironomid-based temperature reconstruction from Sokli

6.2.1. The influence of calibration dataset properties Three different chironomid-climate calibration datasets (Norwegian, Finnish and northern Russian) were used in the chironomid-based July temperature reconstruction (Paper IV), to evaluate the effect of calibration dataset selection on the temperature reconstruction. The datasets are all from boreal- arctic biomes and have similar performance and representability of dominant fossil taxa, and may all be used to reconstruct temperatures at Sokli. The datasets however differ in aspects such as the length and range of the temperature gradient covered, distribution of lakes along the temperature gradient, degree of continentality and other confounding variables, which may affect the reconstruction out- come (ter Braak and Looman 1986, Heiri et al. 2011, Engels et al. 2014). Engels et al. (2014) applied previous versions of the three calibration datasets to the MIS3, MIS 5d-c and Holocene Sokli sequences and showed how the shorter gradients of the Finnish and Russian calibration datasets impacted on taxon-specific estimates of temperature optima and tolerance, and how they may have influenced the quantitative temperature reconstructions. The Russian and Finnish datasets used in the present study have both been extended towards the cold end (compared to the versions used by Engels et al. 2014), likely improving their performance. Generally, selecting a calibration dataset from the same region as your fossil site would be favourable to minimise differences in e.g. climate regime and bedrock conditions that could affect the assemblages (Engels et al. 2014). The bedrock at Sokli is unusual for the area and also the climate regime can be expected to have been quite different (i.e. more continental during the early Eemian and highly oceanic during the latter part), but also to have varied across the interglacial as a consequence of changing insolation pattern. Therefore, the selection of calibration datset becomes less dependent on geographical proximity. The more easterly located northern Russian calibration dataset (Nazarova et al. 2015) was included in the study as it covers a long temperature range, including substantially higher (lower) temperatures than the Norwegian (Finnish) datasets, which may improve the representation of the distribution of the fossil data. This calibration dataset however encompass a range of other climate-related variables such as winter temperatures, continentality, precipitation, permafrost extent which may affect the chironomid community to (yet) unknown extent. The results from Paper IV show that while the trends in inferred temperatures are similar, the composition of the calibration dataset has a profound impact on mean temperatures of the reconstruction. The length and range of the temperature gradient, the lake distribution along the gradient as well as other confining variables (e.g. water depth) all have the potential to influence the estimated optima of the individual taxa. The most common fossil taxa have similar distributions along the temperature gradient in all three datasets if put on the same temperature scale, lending support to the estimated weighted average (WA) optima of the individual taxa. However, the Norwegian calibration set shows generally lower WA optima for the most common (warm-water) taxa, indicating that the lower maximum temperature covered by this calibration set may limit the estimation of optima. The temperature distribution of deep (cool) water taxa are not well represented by the Russian or Finnish calibration sets as these predominantly include shallow lakes. However, for the Eemian palaeolake where deep water taxa are rare, this may be of lesser importance. In fact, it was found (Paper IV) that in the Rus- sian calibration set, the temperature distribution of shallow water species may be better represented due the dominance of shallow lakes across the temperature gradient (Nazarova et al. 2015). Thus, in Paper IV we argue for the selection of the Russian dataset as the most suitable for reconstructing Eemian temperature variability at Sokli.

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Figure 6. Chironomid-inferred July temperatures reconstructed from three different calibration datasets: the Norwegian (green) (Brooks and Birks 2001, 2004), northern Russian (red) (Nazarova et al. 2008, 2011, 2015) and Finnish (blue) (Olander et al. 1999, Luoto 2009, Luoto et al. 2014a,b, 2016) and main features of lake development distinguished from the combined proxy (chironomid, diatom, pollen, non-pollen-palynomorphs) data. Striped area marks the period when factors other than temperature may have influenced the temperature reconstruction (Paper IV). (Pediastrum bor v. ps – Pediastrum boryanum v. pseudoglabrum)

6.2.2. The influence of confounding variables Productivity and warm temperatures naturally correlate, which is also reflected in modern calibration datasets (Brodersen and Anderson 2002, Heiri et al. 2011, Luoto 2011). This suggests that, in a naturally eutrophic, but cooler, lake the taxa may reflect erroneously high temperatures. The Sokli palaeolake chironomid assemblage is dominated by taxa that are common in boreal lakes in northern Finland today (Luoto 2013). It is not dominated by highly eutrophic and particularly warm water taxa, in fact the most dominant taxa are relatively evenly distributed across the temperature (and nutrient) gradient and the main characteristic for the assemblage seems to be the predominance of littoral taxa. In a study of eutrophic but oxygen rich high arctic lakes Stewart et al. (2013, 2014) showed that chironomids responded to nutrient enrichment by increasing abundances following the greater food availability, but not by changing species assemblages. Only when oxygen conditions and habitat structure were affected could changes in the chironomid assemblage composition be seen. In the Sokli Eemian palaeolake, eutrophic taxa increase during L3, but the qualitative multi-proxy data indicate that this productivity increase is climate related (Paper II). However, with the transition to L4 and especially to the very shallow conditions in the upper part of the phase it is likely that decreasing lake levels and associated increased productivity, in combination with increased DOC, is primarily driving the changes in the chironomid assemblage through changing habitat structure (shading) and oxygen levels. In the early part of the phase however, no distinct effect on inferred temperatures can be seen (Fig. 6).

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6.2.3. Temperature development at Sokli Our chironomid-based July temperature reconstruction (Paper IV) suggests temperatures remained close to present throughout the studied sequence. These results are similar to the most recent pollen based reconstruction for the Eemian at Sokli (Salonen et al. in revision), where temperatures are between 0-1.5 °C higher throughout the sequence. This difference may be related to the range of temperatures covered by the different calibration datasets with the pollen dataset extending further on the warm end. The studied sequence covers mainly the warm phase of the interglacial. The relatively low increase in optimum temperatures compared to present contrast that from other high latitude sites (Francis et al. 2006, Kienast et al. 2008, Axford et al. 2011, NEEM community members 2013) and more resembles the optimum temperatures estimated for central Europe (1-2 °C above present) (Aalbersberg and Litt « ¬« ¬ ¬ on the Fennoscandian climate, already raising temperatures above the latitudinal average, and indicates that warming of high latitudes is not uniform, but functions as an evening-out of temperature gradients. Parts of the record where non-climatic variables (DOC and morphogenic eutrophication) may have obscured the chironomid-based temperature reconstruction were identified (Paper II and IV). Thus, in the upper part of L4 and in L5 the chironomid-based reconstruction should be interpreted with caution. However, periodic increases in oligo/mesotrophic taxa reflecting periods of cooler and more oxygen rich conditions in L5 (Ch4), suggest that temperature, through its effect on productivity and/or stability of stratification, could still be driving changes in the assemblage (Fig. 6). Two distinct climate events are picked up in the Sokli record. Based on the multi-proxy data, the Tunturi event (E1) is characterized by lower lake levels, increased erosion and less stable thermal summer stratification thought to reflect cooler more arid conditions and more open vegetation (Paper II, III). The event is marked by an initial ca 1.5 °C drop in chironomid-inferred temperatures and a slight increase in oligo-mesotrophic chironomid taxa (Paper IV). Like the Tunturi event, the late Värriö (Salonen et al. in revision) event (E2), is characterized by more arid and cool conditions resulting in decreased lake levels and productivity and possibly also an extended period of lake ice cover. The event is more pronounced in the aquatic proxies than the Tunturi event, probably due to the over- all shallow nature of the waters in this part of the Eemian sequence, increasing the sensitivity to lake level changes.

6.3. Chronology, regional correlation and palaeoclimatologic implications

In Paper I it is assumed that at Sokli, being situated in high latitude northern Fennoscandia, interglacial conditions only established after the termination of deglacial meltwater input into the Nordic seas. The latter is evidenced in pronounced ice-rafted debris (IRD) content at the base of the marine Eemian sequences and low reconstructed SSTs, reflecting unstable influx of Atlantic water into the Nordic seas and possibly ongoing deglaciation of the Fennoscandian ice sheet (Rasmussen et al. 2003, van Nieuwenhove et al. 2011, Bauch et al. 2011, 2012). Based on alignment with isotope records, deglacial input into the Nordic seas is estimated to have ended only at ca 125 ka, and the Tunturi event was thus aligned to an event of similar magnitude dated to 120 ka (Bauch et al. 2011, 2012). Salonen et al. (in revision) and Paper IV, however, take distance from this age model. Instead it is assumed that the warming at Sokli followed the European mainland and southern Fennoscandia. This assumption is based on indications of a rapid deglaciation induced by high summer insolation and subsequent rapid warming across the European continent after ca 130 ka (Lambeck et al. 2006, Govin et al. 2015, Salo- nen et al. in revision). The end of the Sokli Eemian record is set at 117.5 ka, in association with the first substantial cooling and decoupling of temperature records between central and southern Europe dated in central European stalagmites (Meyer et al. 2008, Vansteenberge et al. 2016). The Sokli chronology (Fig. 7) is based on linear interpolation between tie points (130 respectively 117.5 ka, tied to

31 the pollen zonation) assuming a constant sedimentation rate between these points (Salonen et al. in revision). Recent advancements in correlation of marine cores and the application of a common alignment with the AICC2012 ice core chronology (Bazin et al. 2013, Veres et al. 2013) suggest that substantial melting of the ice sheets and the massive IRD input associated with meltwater event H11 took place just before the rapid rise in Antarctic CH4 and the subsequent rapid northern hemisphere warming (Govin et al. 2012, Capron et al. 2014). The alignment of marine records onto the AICC2012 chronology is based on the assumption that surface water temperature changes in the subantarctic (subarctic) oceans occurred simultaneously with air temperature changes over Antarctica (Greenland), and that air temperature changes above Greenland are synchronous with abrupt methane increases recorded in Antarctic ice cores. Such a correlation is observed during the last glacial period and Termination I (the last deglaciation) (Govin et al. 2012, Capron et al. 2014). Sharp methane increases are associated with the glacial terminations of the last 800 ka and is assumed to reflect the methane emissions by boreal wetlands resulting from warming and ice sheet retreat (Loulergue et al. 2008). The distinct rise in CH4 in Antarctic ice cores at the beginning of the LIG is dated in the AICC2012 chronology to 128.5 ka (Govin et al. 2015), and the transition to interglacial (warm) conditions is recorded in independently dated speleothem records from the European Alps at ca 130/129 ka (Spötl et al. 2007, Cochoud et al. 2009, Drysdale et al. 2009) (Table 1). In the following comparison of the Sokli data with marine records we focus on using records that have been aligned to the common (AICC2012) chronology (Fig. 7). Govin et al. (2015) applied this approach to the MD95-2042 marine core (Shackleton et al. 2002) from the Iberian margin (Fig. 1) by ¡¡¬ ¢ 18O (reflecting local SST) to the methane rise. Peak values are then obtained ca 128 ka BP. The MD95-2042 core is one of the few marine cores containing a pollen stratigraphy thus allowing for a correlation of the European pollen stratigraphy with the marine rec- ¬¬¢ 18O is followed by approximately 1 kyr by the peak in forested conditions representing optimum interglacial conditions at ca 127 ka. This date is very similar to the timing of peak interglacial conditions as recorded from Lago Grande di Monticchio and also conforms well with the timing calculated by Müller (1974) of the last interglacial peak warmth, occurring between 1-1.5 kyr after the beginning of forest expansion. In the Nordic seas, fluctuations in planktonic 18O and IRD have been related to meltwater events associated with the deglaciation (Risebrobakken et al. 2006, Van Nieuwenhove et al. 2011, Bauch 2013, Bauch et al. 2012). In a recent publication by Zhuravleva et al. (2017) the AICC2012 chronology has been applied to the southern Nordic seas core PS1243 using several tie points. A period of early warm conditions in the Nordic seas are suggested by decreases in cold water planktonic foraminifera (Nps %) registered in core PS1243 (Fig. 7). However, other records from the Nordic seas indicate ongoing input of meltwater (partly from the Greenland ice sheet) during this early phase of the interglacial, which Zhuravleva et al. (2017) consider still belong to the main deglaciation. Based on the revised chronology (Salonen et al. in revision), the Sokli data show relatively stable near present-day summer temperatures (Paper IV) already from the beginning of the interglacial. The multi-proxy data indicate that seasonality begins to decrease already prior to the Tunturi cooling event but the main shift to more oceanic conditions occurs after the event (Paper II). Pollen-based climatic reconstructions from central and western Europe similarly show more continental conditions during the early Eemian (Zagwijn 1996, Brauer et al. 2007, Kühl et al. 2007, Brewer et al. 2008). The continentality trend seems to follow changes in insolation during the early and early middle part of the Eemian. It may be noted that the shift to more oceanic conditions at Sokli occurs approximately 3 ka after the onset of interglacial conditions, which is similar to pollen records from central Europe reflecting a shift to more oceanic conditions (i.e. the transition to the Carpinus phase) 3 ka after the onset of interglacial conditions and to the transition from dry to wetter conditions at Lago Grande di Mon- ticchio (3 ka after the onset of interglacial conditions). This would agree (within age uncertainties) with the suggestion by Rasmussen et al. (2003) that the shift to more oceanic conditions in European records can be related to the establishment of proper interglacial AMOC circulation and warmer SSTs in the Nordic seas which is dated to approximately 3-4 ka after the interglacial transition or to ca 125.5 ka (Rasmussen et al. 2003, Risebrobakken et al. 2006, Govin et al. 2012, Bauch et al.2012, Zhuravleva et al. 2017).

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Figure 7. The Sokli temperature reconstructions compared to other last interglacial records. (a) PS1243 Nordic seas: ice rafted detritus (IRD)/g (>250μm); Neogloboquadrina pachyderma (sinistral) % (Bauch 2013, Zhurav- leva et al. 2017). (b) MD03-2664 Labrador Sea: Modern Analogue Technique inferred winter SSTs (Irvali et al. 2016). (c) Sokli N Finland: chironomid-inferred July temperatures (red) (Paper IV); pollen-inferred July temperatures (black) (Helmens et al. 2015). (d) NEEM ice core Greenland: fixed-elevation change of temperature !@|{{~%$:`{"{>"+X " |+@[$':';` XQ";"|:["{18O profiles from stalagmites CC5, CC7 and CC28 (Drysdale et al. 2007, 2009). (f) Lago Grande di Monticchio (LGdM) Italy: percentage of temperate (Quercus) tree pollen (Brauer et al. 2007). (g) MD95-2042 North Atlantic (Iberian margin): percentage of tem- Q"+':":+""Q|;;"%Q;'%&:|%$@18O profile (Shackleton et al. 2002). (h) ODP980 North Atlantic: SSTs; IRD/g (>250μm) (Oppo et al. 2006). (i) North Atlantic cold events H11, C26, C25, C24 (Oppo et al. 2006, Bauch and

Kandiano 2007, Govin et al. 2015). (j) EDC ice core Antarctica: CH4 concentration (Loulergue et al. 2008, Govin et al. 2015). (a), (b) on AICC2012 time-scale after Zhuravleva et al. (2017); (c), (e), (f) on their individual timescales; (d), (g), (i), (j) on AICC2012 after Govin et al. (2015); (h) on AICC2012 after Capron et al. (2014) (SST) and Govin et al. (2015) (IRD). Densely dotted line ties together the establishment of interglacial conditions (rapid warming) in the different records. Dotted line ties together (tentatively) the onset of an early interglacial cold event as defined in the individual records. Upper thin dotted line ties together the onset of peak MIS 5d conditions in the different records. MIS and terrestrial stages according to Shackleton et al. (2002) with boundaries at their original placement along the isotope stratigraphy of MD95-2042, thin dotted line marks the onset of cooling (glacial inception/end of last interglacial) as noted in many records (see Table 1).

Records from the Nordic seas and the subpolar North Atlantic show evidence of cooling and freshwater events intersecting interglacial conditions during the LIG (Bauch et al. 2011, Galaasen et al. 2014, Irvali et al. 2016, Zhuravleva et al. 2017). The first and most prominent of these at ca 126.5 ka may be related to a meltwater event off the East Greenland margin possibly in combination with an outburst flood from the Laurentide ice sheet, affecting SSTs in the western Nordic seas and the Labra- dor Sea as well as the central Nordic seas (core PS1243) (Zhuravleva et al. 2017). The occurrence at Sokli of a cooling event (the Tunturi event) of similar magnitude as the above mentioned, interrupting warm conditions a few millennia after the establishment of interglacial conditions may well reflect a continental equivalent to this(these) cooling event(s). Based on the palaeolimnological record (Paper II) it was concluded that this event was characterized at Sokli not only by cooling but also by increased aridity. Indications of relatively low magnitude possibly cold but essentially arid events intersecting warm interglacial conditions can be found in records from Europe e.g. at the beginning of the Carpinus expansion at Ribains Maar and Hollerup (Björck et al 2000, Rioual et al. 2007) or following the Eemian thermal maximum at ca 124.5-124.1 ka at Lago Grande di Monticchio (Brauer et al. 2007) and at 126.7-125.6 ka in north Italian speleothems (Cochoud et al. 2009, Drysdale et al. 2009, Regat- tieri et al. 2014). However, there are also records where no distinct early/mid Eemian events are seen (e.g. Litt et al. 1996, Tzedakis et al. 2003, Kühl et al. 2007). The Tunturi event, dated to ca 127.5- 126.5 ka, is characterized by a relatively short and sharp initial cooling and lake level lowering that is evident in most of the studied proxies from Sokli, followed by a longer less prominent part. It is uncer-

33 tain to what extent the lack of evidence of a cooling in many records is due to a limited geographical extent of the climatic impact, to resolution, or to different sensitivities of the records and proxies themselves. Regarding the second cold/arid Värriö event, the uncertain chronological constraint of the upper part of the Eemian sequence at Sokli makes correlations to similar events in other records highly tenta- tive. However, similar short term, relatively low magnitude cool and arid events does seem to be picked up in several records, both marine and terrestrial (e.g. Björck et al. 2000, Oppo et al. 2006, Rioual et al. 2007, Bauch et al. 2012, Mokeddem et al. 2014, Irvali et al. 2016, Kupryjanowicz et al. 2016, Zhuravleva et al. 2017). The Värriö event at Sokli occurs prior to the main cooling at the Sokli site and is thus likely preceding the marked ocean cooling event C26 (Oppo et al. 2006) associated with glacial inception. It is likely that cooler and drier conditions during glacial inception led to the final drying up of the Sokli palaeolake, ending the Eemian record at Sokli. The results from Sokli suggest that cooling events registered in the Nordic seas and the subpolar north Atlantic records are also registered in the Sokli record, reflecting the sensitivity of north Europe- an climate to changes in the ocean circulatory system even under warm interglacial conditions.

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7. Conclusions

This study has contributed to the spatial coverage of Eemian Interglacial palaeolimnological and palaeoclimatological data, filling a gap in data from the terrestrial high latitude north At- lantic region. Through multi-proxy qualitative analysis, the evolution of the lake regarding long-term site-specific changes and climatic evolution, and short-term events, has been described. The lake development was driven by climate but also infilling and decreasing water depths affecting circulation/stratification regime and nutrient and oxygen status of the lake. The lake developed from a relatively deep and stably stratified lake into a shallow lake with only weak summer stratification. Deposition in the lake probably ended following complete infilling and overgrowth. The alkaline and phosphorus-rich bedrock added to elevated nutrient levels and relatively constant high pH in the lake, aiding climatic interpretations of the qualitative data as catchment- related/succession processes (leaching, decreasing pH) that otherwise may have been strong drivers of the aquatic (diatom) assemblage, were less important. Seasonality was most pronounced in the early Eemian and subsequently decreased, mainly as a result of winter warming. This development is similar to the general climatic development recorded at the European continent and reflects the changing seasonality of incoming solar ra- diation over the interglacial, and possibly also changes in the AMOC (establishment of Holo- cene-like conditions). The chironomid-inferred July temperatures follow a similar trend as pollen-inferred temperatures from Sokli, although the chironomid-inferred temperatures are lower. Our results indicate that warm, near present-day July temperatures prevailed at Sokli already from the beginning of the interglacial and that temperatures, though possibly slightly underestimated, were only moderately higher (ca +1°C) than present-day temperatures at Sokli. These results suggest that the temperature development at Sokli followed that of central Europe during the last interglacial and that polar amplification was less pronounced than in other high latitude areas. Based on the qualitative data, parts of the record where non-climatic-induced oxygen deficien- cy (due to high DOC/morphogenic eutrophication) may obscure the chironomid-based temperature reconstruction were identified. Furthermore, the effect of variations in e.g. the length and range of the temperature gradient covered by the modern calibration dataset, as well as of the distribution of lakes along the gradient on the mean values of the reconstructions was evaluated and used as a base for choosing the modern calibration set deemed most suitable to capture the temperature optima of the Sokli Eemian fossil taxa. Two climatic events characterized by cooler and more arid conditions have been identified in the early interglacial and prior to glacial inception. The events can be tentatively correlated to events registered in marine records from the Nordic seas and subpolar north Atlantic, reflecting the effect meltwater pulses and disruptions in the thermohaline circulation had on adjacent continental areas. Diatom resting spores possibly belonging to the A. islandica-skvortzowii group, not found in recent material from Europe, were identified. The presence of these spores in the Sokli sediments as well as in other interglacial and interstadial deposits in Finland, and the present-day distribution of similar spore forming Aulacoseira in the highly continental Baikal Rift zone suggests that this group (or trait) was more widespread in the past and that climatic conditions such as decreasing seasonality, may have led to the final disappearance of this group (trait) from Finland.

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8. Future perspectives

Multi-proxy studies including NPPs are relatively under-represented in palaeoclimatic studies. I found that including NPPs could partly provide supporting, additional evidence (e.g. of nutrient enrichment and decreasing water depths) to the diatom data, but it also added to the interpretation of the seasonal dynamics (i.e. changes in thermal/stratification regime) in the lake. Including more of the lake biota in an analysis gives a fuller understanding of the processes and dynamics in the lake, and periods of bloom and dormancy (in relation to nutrient availability) are important to consider in order to interpret climate-related changes in a lake. This is not new, but it is a direction which I would like to explore if I was to continue working with palaeolimnology and palaeoclimatology. The reconstructed summer temperatures were in general quite stable throughout the Eemian at Sok- li, while winter or early spring temperatures and humidity/precipitation (expressed through changes in lake ice-cover period) seem to have changed quite substantially both at Sokli (Plikk et al. 2016, Salo- nen et al. in revision) and at other Eemian sites (e.g. Kühl et al. 2007, Brewer et al. 2008). Further work on the Sokli Eemian record (since it is dated and already has multitude of data this is actually encouraged) could focus on identifying changes in winter temperature and seasonality, and precipitation (humidity) variations. Reconstructions of moisture availability and relatively humidity could be accomplished by the use of analysis of isotopes on lipid biomarkers from the Sokli sediments (Sachse et al. 2012). For time-periods beyond the reach of instrumental data, quantitative reconstructions are a necessary means to obtain quantitative data to be used e.g. for input in climate models. One way of validating quantitative reconstructions is by comparisons to other climate reconstructions from the same area. This cannot be done for the Sokli Eemian record to any major extent simply due to the lack of data from the region. Additionally, the lack of independently dated records in northern Fennoscandia ham- pers comparison and in a first step dating (e.g. OSL-dating) of other important records such as the Tepsankumpu deposit could be performed. The understanding of the dynamics behind an event such as the Tunturi-event increases if its geographical extent can be traced. As noted, the Tunturi event is more pronounced in the pollen-data than in the aquatic assemblage, while the opposite is true for the Värriö event, thus similar events may most likely be recorded in multi-proxy records. Future studies may be aimed at compiling a record of the spatial and temporal distribution of similar arid/cold events to see if there is a pattern, which may then possibly be related to climatic drivers (e.g. changes in the strength of the westerlies associated with changes in SST across the northern North Atlantic). The compilation of calibration datasets involves hard work, collecting vast amounts of data across large regions without knowing before-hand the quality of the data. The results from Paper IV highlight the need to put more emphasis on (chironomid) calibration set composition, and to collect, as far as possible, information about a range of environmental variables when sampling a lake and focus on increasing the sample distribution along, and range of, the sampled gradient. The ongoing collection of data from lakes may in the future give the possibility to assemble a calibration set based on lakes similar to the palaeolake regarding non-climatic features (e.g. shallow/deep) across a range of temperature gradients to get as good representation of the fossil assemblage as possible.

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Financial support

This PhD-project was funded by the Swedish Nuclear Fuel and Waste Management Company (SKB) and the Department of Physical Geography, Stockholm University. Additional funding was provided by the Bolin Centre for Climate Research. I would also like to acknowledge the Bolin Centre for Cli- mate Research, Gerard De Geer fund, L&E Kinanders stiftelse, Lagrelius fund, Stiftelsen Längmanska kulturfonden for the financial support I received throughout this project.

Acknowledgements

First and foremost I would like to thank my supervisor Karin Helmens for putting her trust in me, for her support and guidance, and for introducing me to a group of talented and helpful Finnish research- ers (Sakari Salonen, Jan Weckström, Minna Väliranta, Tomi Luoto) and to Stefan Engels, my ever helpful and intelligent co-supervisor. Thank you Stefan for your constructive comments and discrete but great improvements to my texts, and for always being available and quick to answer when I’ve asked for help. I am very grateful also to my co-supervisor Jan Risberg; you always help and you are an inspiration with your positive attitude. Thanks also to Peter Kuhry for discussions and constructive comments regarding the thesis outline and planning. I would like to thank all my co-authors who have surprised me with their rapid responses and comments to questions and manuscripts, thank you for sharing your expertise and experience and for your encouraging comments. I am happy to have met so many friendly and generous people when I’ve been at excursions, cours- es and conferences and within research in general – I would especially like to thank Steve Brooks, Angela Self, Tomi Luoto and Larisa Nazarova for sharing their data, Alison MacLeod for offering help with preparation of thin slides for μ-XRF, and Fumiko, Annika and Francois for taking me with them to the Japanese country side. Thanks to Christine Lane and Stefan, who let me stay over on my visit to Britain. Thank you Jenny Sjöström and Christoffer Hemmingsson who helped me with water samples, and Christos Katrantsiotis for finalizing the first chironomid-diagram for me when I was on parental leave. Thank you everyone who offered their time to help with statistics; Elsa, Adam, Jessica, Stefan, Sakari and Richard Telford, and thanks to Mark Edlund for guidance regarding resting spores. Thanks also to Heike Sigmund and Caspar Meyer Jacobs for C/N analysis and FTIRS data-treatment. Thanks to Emelie for dealing with the party-arrangements. Janne and Sven, you have made me smile (and go crazy), and Malin Kylander has been a great support. Jonas and Jens deserve a special thanks for giving me extra energy and inspiration the last year. My lovely friends from the start of my studies in geology, Marina, Therese and Emelie who all have finished their PhDs by now – you have been an inspiration and support. I am grateful to have been surrounded by so many talented, friendly, helpful and fun colleagues here at the department, Susanna deserves a special word here – thank you for being so calm and always sorting everything out. Finally, I would like to thank my family; my parents and parents in law for taking care of sick chil- dren, picking up from kindergarten, even preparing dinner for us during these last stressful months when the colds just kept coming; Jon – thank you for pulling the load these last months, and finally, thank you kids for forcing me to forget about work as soon as I enter the door at home.

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