Deglacial impact of the Scandinavian Ice Sheet on the North Atlantic climate system Francesco Muschitiello
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Deglacial impact of the Scandinavian Ice Sheet on the North Atlantic climate system Francesco Muschitiello
Deglacial impact of the Scandinavian Ice Sheet on the NorthAtlanticclimatesystem FrancescoMuschitiello ©FrancescoMuschitiello,StockholmUniversity2016 ISBN978-91-7649-368-7 CoverpicturebyBjörnEriksson, PrintedbyHolmbergs,Malmö2016 Distributor:DepartmentofGeologicalSciences TomyfriendRoberto……. ὑστέρῳδὲχρόνῳσεισμῶνἐξαισίωνκαὶκατακλυσμῶνγενομένων͵μιᾶςἡμέραςκαὶνυκτὸς χαλεπῆςἐπελθούσης͵τότεπαρ΄ὑμῖνμάχιμονπᾶνἁθρόονἔδυκατὰγῆς͵ἥτεἈτλαντὶςνῆσος ὡσαύτωςκατὰτῆςθαλάττηςδῦσαἠφανίσθη Plato(Timaeus) Abstract ThelongwarmingtransitionfromtheLastIceAgeintothepresentInterglacialperiod,the last deglaciation, holds the key to our understanding of future abrupt climate change. In the last decades, a great effort has been put into deciphering the linkage between freshwater fluxes from melting ice sheets and rapid shifts in global ocean-atmospheric circulation that characterized this puzzling climate period. In particular, the regional expressions of climate change in response to freshwater forcing are still largely unresolved. This projects aims at evaluating the environmental, hydro-climatic and oceanographic responseintheEasternNorthAtlanticdomaintofreshwaterfluxesfromtheScandinavian IceSheetduringthelastdeglaciation(∼19,000-11,000yearsago).Theresultspresentedin this thesis involve an overview of the regional representations of climate change across rapid climatic transitions and provide the groundwork to better understand spatial and temporalpropagationsofpastatmosphericandoceanperturbations. Specifically, this thesis comprises i) a comparison of pollenstratigraphic records from densely 14C dated lake sediment sequences, which provides insight into the regional sensitivityofNorthEuropeanvegetationtofreshwaterforcingintheNordicSeasaround theonsetoftheYoungerDryasstadial(∼12,900yearsago);ii)areconstructionofNorth Europeanhydro-climate,which,togetherwithtransientclimatesimulations,shedlighton themechanismsandregionalityofclimateshortlypriortothetransitionintotheYounger Dryasstadial;iii)studiesofa∼1250-yearlongglacialvarvechronology,whichprovidesan accuratetimingforthesuddendrainageofproglacialfreshwaterstoredintheformericedammed Baltic Ice Lake into the North Atlantic Ocean; iv) a 5000-year long terrestrialmarinereconstructionofEasternNorthAtlantichydro-climateandoceanographicchanges that clarifies the hitherto elusive relationship between freshwater forcing and the transient behaviour of the North Atlantic overturning circulation system. The results presented in this thesis provide new important temporal constraints on the events that punctuatedthelastdeglaciationinNorthernEurope,andgiveaclearerunderstandingof theocean–atmosphere–ice-sheetfeedbacksthatwereatworkintheNorthAtlantic.This increasesourunderstandingofhowtheEarthclimatesystemfunctionsinmoreextreme situations. Svensksammanfattning Den långa, successivt varmare övergångsperioden, avbruten av flera kalla episoder, från den senaste istiden in i den nuvarande interglaciala värmeperioden, dvs den senaste deglaciationen/isavsmältningen, har ledtrådar till vår förståelse av framtida abrupta klimatförändringar. Under de senaste årtiondena har stora ansträngningar gjorts för att dechiffrera kopplingar mellan sötvattenpulser från smältande inlandsisar och snabba förändringar i den globala ocean-atmosfäriska cirkulationen, vilket var kännetecknande fördennadelvisgåtfullaklimatperiod.Specielltärderegionalaklimatyttringarnaavstora sötvattenflödenettolöstproblem. Syftet med detta projekt har varit att utvärdera den miljömässiga, hydroklimatiska och oceanografiska responsen i östra Nordatlanten på stora sötvattenflöden från den Skandinaviskainlandsisenunderdensenastedeglaciationen,ca19,000till11,000årföre nutid. Resultaten i avhandlingen innefattar en översikt av hur det regionala klimatet påverkasvidsnabbaklimatiskaövergångsperioderochutgördärmedettunderlagföratt bättre förstå hur störningar i dåtidens atmosfär och ocean kunde spridas, både rumsligt ochtidsmässigt. Mer specifikt innefattar denna avhandling, i) en jämförelse av olika pollenstratigrafiskt undersökta och noggrant 14C daterade sjösediment, vilka ger inblick i den regionala vegetationens sensitivitet för sötvattenflöden till de Nordiska haven i samband med att denkallayngredryasperiodeninleddesförca12,900årsedan;ii)enrekonstruktionav det nordvästeuropeiska hydroklimatet för ca 13,000 år sedan, vilket i kombination med transientaklimatsimuleringarklarläggerklimatetsmekanismerochregionalitetstraxföre övergångentillyngredryas;iii)undersökningaravenca1250årlångkronologibaserad påglacialalervarv,vilkengerenexaktålderfördenplötsligadräneringen/tappningenav sötvatten från den proglaciala Baltiska Issjön ut till Nordatlanten; iv) en 5000 år lång terrester-marin rekonstruktion av östra Nordatlantens hydroklimat och oceanografiska förändringar,vilkenklargördethittillsgäckandeförhållandetmellansötvattenflödenoch de transienta processerna i Nordatlantens djupvattenbildning, den s.k. termohalina cirkulationen.Resultateniavhandlingengernyaviktigatidsmässigabegränsningarförde händelser som ideligen störde och avbröt utvecklingen i samband med den senaste deglaciationen i Nordeuropa. Detta ger en ökad insikt i den dåvarande Nordatlantens oceaniska,atmosfäriskaochglacialaåterkopplingsmekanismer,vilketökarförståelsenför hurjordensklimatsystemfungerarundermerextremaförhållanden. Listofpapersandauthorcontributions This thesis consists of an overview of the main aims of this PhD project, the employed methodologicalapproach,andsummariesofthekeyresults.Theappendiceslistedbelow arealsoincluded.PaperI,IIandIIIhavebeenpublishedinthejournalsindicatedandare reprintedunderpermissionoftherespectivepublishers.PaperIVisamanuscript. I. Muschitiello, F.andWohlfarth,B.,2015.Time-transgressiveenvironmentalshifts across Northern Europe at the onset of the Younger Dryas. Quaternary Science Reviews109,49-56. II. Muschitiello, F., Pausata, F.S.R., Watson, J.E., Smittenberg, R.H., Salih, A.A.M., Brooks, S.J., Whitehouse, N.J. Karlatou-Charalampopoulou, A., Wohlfarth, B., 2015. FennoscandianfreshwatercontrolonGreenlandhydroclimateshiftsattheonsetof theYoungerDryas.NatureCommunications6:8939. III. Muschitiello, F., Lea, J., Greenwood, S.L., Nick, F.M., Brunnberg, L., MacLeod, A., Wohlfarth,B.,2015.TimingofthefirstdrainageoftheBalticIceLakesynchronous withtheonsetofGreenlandStadial1.Boreas10.1111/bor.12155. IV. Muschitiello, F., Dokken, M.T., Väliranta, Björck, S., M., Davies, M.S., Luoto, T., Schenk,F.,Smittenberg,R.H.,Reimer,P.J.,Wohlfarth,B.NorthAtlanticoverturning andclimateresponsetomeltwaterforcingduringthelastdeglaciation. PaperI:F.M.conceivedthestudy,wasthemaincontributorintermsofanalyses,wrotethe initial version of the paper and made the figures. B.W contributed with writing and interpretationoftheresults. Paper II: F.M. conceived the study, performed isotope and geochemical analyses on lake sedimentcores,interpretedtheproxydata,performedstatisticalanalysis,wrotetheinitial version of the paper and made the figures. F.S.R.P. analysed climate model output and contributed to the interpretation of the proxy data. J.E.W. performed the chironomid analysis. R.H.S. contributed to biomarker data evaluation. A.A.M.S. analysed the air back trajectory data. S.J.B. and N.J.W. contributed to the temperature data evaluation. A.K.C. performed the pollen analysis. B.W. led the fieldwork campaign, subsampling and identification of samples for terrestrial 14C analysis, provided insight into regional paleoenvironment and acquired financial support. All authors contributed with interpretationoftheresultsandeditingofthemanuscript. PaperIII:F.M.conceivedthestudy,performedgeochemicalanalysesofthesedimentsand statistical analyses, wrote the initial version of the paper and made the figures. J.M.L. designed and performed the ice-flow model analysis. F.M and J.M.L. led the fieldwork campaignforthenewsedimentcoresandinterpretedthegeochemicalresults.S.L.G.was the main contributor of Figure 1 and helped with the interpretation of the regional paleogeography.F.M.N.providedtheiceflowmodel.L.B.providedtheclayvarvedataset fromSandfjärden.A.M.contributedtovarvedataevaluation.B.W.providedtheclayvarve data sets for Östergötland, varve thickness data and IRD counts, insight into regional paleoenvironment and acquired financial support. All authors contributed with interpretationoftheresultsandeditingofthemanuscript. Paper IV: F.M. conceived the study, performed isotope and geochemical analyses on lake sedimentcores,interpretedtheproxydata,performedstatisticalanalysis,wrotetheinitial version of the paper and made the figures. T.M.D. performed the marine multi-proxy analyses and provided the marine 14C data; M.V. contributed with the macrofossil data; R.H.S.contributedtobiomarkerdataevaluation;S.B.,S.M.D,T.L.,F.S.,andP.J.R.helpedwith interpretationofproxydata.B.W.ledthefieldworkcampaignfortheterrestrialstudysite, subsamplingandidentificationofsamplesforterrestrial14Canalysis,providedinsightinto regional paleoenvironment and acquired financial support. All authors contributed with interpretationoftheresultsandeditingofthemanuscript. Thefollowingpapersarenotincludedasapartofthisthesis: 1. Muschitiello, F., Zhang, Q., Sundqvist, H.S., Davies, F.J., Renssen, H., 2015. Arctic climate response to the termination of the African Humid Period. Quaternary ScienceReviews125,91-97. 2. Muschitiello, F., Andersson, A., Wohlfarth, B., Smittenberg, R.H., 2015. The C20 highlybranchedisoprenoidbiomarker–anewdiatom-sourcedproxyforsummer trophicconditions?OrganicGeochemistry81,27-33. 3. Davies,F.J.,Renssen,H.,Blascheck,M.,Muschitiello,F.,2015.TheimpactofSahara desertificationonArcticcoolingduringtheHolocene.ClimateofthePast11,571586. 4. Steinthorsdottir,M.,deBoer,A.,Oliver,I.C.,Muschitiello,F.,Blaauw,M.,Wohlfarth, B., 2015. Response to: Comment on “Synchronous records of pCO2 and Δ14C suggestrapid,ocean-derivedpCO2fluctuationsattheonsetoftheYoungerDryas”. QuaternaryScienceReviews107,270-273. 5. Steinthorsdottir, M., de Boer, A., Oliver, I.C., Muschitiello, F., Blaauw, M., Reimer, P.J.,andWohlfarth,B.,2014.SynchronousrecordsofpCO2andΔ14Csuggestrapid, ocean-derived pCO2 fluctuations at the onset of the Younger Dryas. Quaternary ScienceReviews99,84-96. 6. Muschitiello, F.,Wohlfarth,B.,Schwark,L.,Sturm,C.,Hammarlund,D.,2013.New evidence of Holocene atmospheric circulation dynamics based on lake sediments fromsouthernSweden:alinktotheSiberianHigh.QuaternaryScienceReviews77, 113-124. Tableofcontents 1.Introduction....................................................................................................................................................................................................................................1 2.Thesisobjectivesandkeyresults................................................................................................................................................................4 3.Investigationarea..................................................................................................................................................................................................................5 3.1.Background...........................................................................................................................................................................................................................5 3.2.Previouswork....................................................................................................................................................................................................................6 4.Materials,methodsandapplications.....................................................................................................................................................7 4.1.Sampling....................................................................................................................................................................................................................................7 4.2.14Cdating..................................................................................................................................................................................................................................8 4.2.1.Bayesianage-depthmodelling................................................................................................................................................11 4.2.2.Reservoirageestimation................................................................................................................................................................12 4.3.Synchronizationofclimaterecords................................................................................................................................................13 4.4.Hydrogen-isotopiccompositionoflipidbiomarkerand paleo-hydrologicalapplication..............................................................................................................................................................14 4.5.LipidextractionandδDanalysis.........................................................................................................................................................17 5.SummariesofPaperI-IV.........................................................................................................................................................................................17 5.1.PaperI.......................................................................................................................................................................................................................................17 5.2.PaperII....................................................................................................................................................................................................................................19 5.3.PaperIII..................................................................................................................................................................................................................................21 5.4.PaperIV..................................................................................................................................................................................................................................22 6.Terrestrial-marineproxycomparison............................................................................................................................................24 7.Currentworkandunpublisheddata...................................................................................................................................................30 7.1.ImpactoftheScandinavianIceSheetonregionalclimateusinga spatiallyhigh-resolutionclimatemodel...................................................................................................................................30 7.2.SensitivityoftheScandinavianiceSheettovolcanicforcing.....................................................................30 7.3.Unpublisheddatasets........................................................................................................................................................................................31 8.Futurework.................................................................................................................................................................................................................................31 Acknowledgements.................................................................................................................................................................................................................34 References.............................................................................................................................................................................................................................................36 F. Muschitiello 1.Introduction Characterisingtheimpactofmeltingicesheetsontheglobalclimatesysteminresponse toglobalwarmingrequiresacomprehensiveunderstandingoftheinterplaybetweenthe cryosphere,oceansandatmosphereatregionalscales.Specifically,asthestabilityofthe GreenlandIceSheetandothersourcesoffreshwaterstoredovernorthernhigh-latitude continental regions are under threat (Fig. 1) (Moon et al., 2012; Shepherd et al., 2012; Hannaetal.,2013),largeuncertaintiesarecastuponthefateoftheAtlanticMeridional OverturningCirculation(AMOC)–acriticalcomponentoftheEarth’ssystem. In the North Atlantic Ocean – the key centre of action of the AMOC – warm and saline surface waters carried from the subtropical sector rapidly cool and sink. The process releases heat to the atmosphere with substantial impacts on hydro-climate and temperaturesoverlargeregions,andmorecriticallyoverWesternandNorthernEurope. Shifts in atmospheric circulation patterns are thus of central concern in the debate surroundingthetransientbehaviouroftheAMOC,astheycanleadtoextremeweather eventsovershorttimescalesduetothemovementoffronts,diversionofRossbywaves, or persistent atmospheric patterns. Therefore, future changes in ocean thermohaline properties owing to increasing meltwater discharge have the potential to drive significant shifts in regional climates with profound consequences for ecosystems and societies. The diagnostic ability to understand and predict climate change can be aided by knowledgeofpastclimateanalogues.Forinstance,thetransitionfromtheLastIceAgeto thepresentwarmInterglacial,thelastdeglaciation(∼19,000-11,000yearsago),provides an ideal natural laboratory to decipher the physical mechanisms behind rapid climate change. The last deglaciation was a critical period of climate shifts during which every component of the Earth’s system underwent numerous abrupt and rapid large-scale changes(Fig.2)(Dentonetal.,2010;Clarketal.,2012). Figure1.a,Globalanomalyofmean2mairtemperaturechange(T2m)estimatedwiththe fullsetofGCMsfromtheCMIP5database.Valuesaresimulatedwithrespectto1970-1999 for experiments of the historical period (grey, 41 models), and the Representative ConcentrationPathways(RCP)4.5(blue;42models)andRCP8.5(red;40models)scenarios (Mossetal.,2010).Thefullensemblemeansaredisplayedasthicklines;verticalbarsrefer to±1σforthereferenceperiod2071-2100.b,Sameas(a)butfortheArctic,definedasthe region north of 60° N. c, Same as a but for the Greenland Ice Sheet (GIS) defined as the regioncovering60-85°Nand20-70°Wandapplyingaland/seamaskfromeachGCMto confinetheanalysistothelandarea.MassbalancetermsoftheGISarealsoshown(Boxand Colgan,2013).Dataarepresentedascumulativeanomaliesrelativetothereferenceperiod 1840-1900. 1 Deglacial impact of the Scandinavian Ice Sheet on the North Atlantic climate system 2 F. Muschitiello Figure 2. (previous page) Regionalandglobal climaterecordsofthelastdeglaciation.a, Mapofborealsummerinsolationanomaliesrelativetopresent(Laskaretal.,2004);b,δ18O valuesfromtheNorthGreenlandIceCoreProject(NGRIP;Rasmussenetal.,2006);c,Atlantic MeridionalOverturningCirculationproxyreconstructionfromthesubtropicalNorthAtlantic (McManus et al., 2004) presented with both analytical and chronological errors (±2σ); d, atmospheric CH4 from the West Antarctic Ice Sheet Divide ice core (WDC; Marcott et al., 2014);e,CO2concentrationfromtheWDCicecorewitherrormargins(±2σ);f,hemispheric proxy-based temperature stacks (Shakun et al., 2012); g, observed relative global sea-level change (Lambeck et al., 2014); h, δ18O values from WDC (WAIS Divide Project Members, 2013). LGM, Last Glacial Maximum; OD, Oldest Dryas; B/A, Bølling-Allerød; YD, Younger Dryas,Hol,Holocene. TheseshiftsweremostprominentlyexpressedintheNorthAtlanticregion(Björcketal., 1996;Loweetal.,2008;Steffensenetal.,2008),butwithloweramplitudesintheSouthern Hemisphere(Fig.2)(Barkeretal.,2009;Stennietal.,2011;Shakunetal.,2012).Thetwo longestandcoldestclimatereversalsintheNorthernHemisphere,arecommonlytermed YoungerDryas(YD;∼12,900-11,700yearsago)andOldestDryas(>14,700yearsBP),and theinformalnameforthewarmerinterstadialpriortotheYDisBølling-Allerød(14,70012,900 years ago), in reference to earlier pollen-stratigraphic work in Scandinavia (Iversen,1954;Mangerudetal.,1974;Wohlfarth,1996). Theexplanationfortheoccurrenceofmultiplewarmandcoldintervalsattheendofthe Last Ice Age, when northern summer insolation was steadily increasing, has presented a majorchallengeforthepaleoclimatecommunity.Muchresearchduringthepastdecades has therefore been placed on multi-proxy analyses of terrestrial, marine and ice core archives and on correlations between the different archives to detect and quantify the impactofthesedramaticclimaticshifts.However,precisecorrelationswere-andstillare- difficultduetointrinsiclimitationswithdatingtechniquesandtheinsufficientresolution ofexistingrecords(Laneetal.,2013;Blockleyetal.,2014;Rasmussenetal.,2014a). Greenlandicecorerecordshoweverstandoutinthisrespectandhavethereforeplayeda pivotalroleindiscussingtheunderlyingcausesofabruptclimatevariability.Theice-core datasetsrecordpastclimaticchangesinamultitudeofatmosphericproxies(Steffensenet al.,2008);provideacontinuousannualchronologythroughouttheLastInterglacial-Glacial cycleintheNorthAtlanticregion(Rasmussenetal.,2014b);andcanbesynchronizedto Antarcticicecoresusingmethanemeasurementsandvolcanicaerosolsignatures(Blunier et al., 1998; Buizert et al., 2015; Sigl et al., 2015). Such synchronization allows a direct correlationandcomparisonofNorthernandSouthernHemisphereclimaticchanges.This has led to the hypothesis of the Atlantic bipolar seesaw mechanisms, which attributes a large role to the AMOC in triggering abrupt global climate shifts, through an asymmetric inter-hemispherictemperatureforcing(Broecker,1998; Knuttietal.,2004;Barkeretal., 2009; Stenni et al., 2011; Cvijanovic et al., 2013). Critically, this mechanism, which influenced climate in the Atlantic and neighbouring sectors, had a greater impact in northernhigh-tomid-latitudesduringthelastdeglaciation(Shakunetal.,2012). North Atlantic marine reconstructions compellingly show that the AMOC system underwentlargeperturbationsduringthelastdeglaciation(McManusetal.,2004;Roberts et al., 2010). However, it remains an open question as to whether sudden AMOC instabilities were a response to continental meltwater discharge from the North Atlantic seaboard(Duplessyetal.,1992;Bard,2000;Clarketal.,2001),tochangesinregionalsea 3 Deglacial impact of the Scandinavian Ice Sheet on the North Atlantic climate system ice distribution (Bradley and England, 2008), to an intrinsic threshold behaviour of the coupledatmospheric-icesheetsystem(Zhangetal.,2014),ortoanintertwinementofthe factorscitedabove.Moreimportantly,itislargelyunclearhowfreshwaterperturbations mediated direct and indirect effects on regional shifts in climate and atmospheric circulation. Furthermore, the temporal expressions of continental climate responses relativetooceancirculationchangesintheNorthAtlanticarestillpoorlyresolved,thereby limiting our understanding of the true driving mechanisms and the direction of physical eventsassociatedwithrapidclimatechange. 2.Thesisobjectivesandkeyresults Onewaytoexploretheissuesbroachedaboveistoimproveexistingregionalproxy-record chronologies to create a broad spatial network of well-constrained climate reconstructions. On the other hand, a means to directly investigate the mechanisms driving the coupled ocean-atmosphere system is to generate isotope records of precipitation from lake sediments, as the physical properties of precipitation associated withregionalhydro-climatepatternsareexpectedtorespondwithoutdelaytolarge-scale shifts in ocean and atmospheric circulation. If analysed at adequate resolution and supportedbyprecisechronologies,lakesedimentisotopeanalysishasthusthepotentialto provide temporal climate reconstructions that deliver information on both the hydrographicandhydrologicalsystem. In this thesis project I provide: i) a North European perspective on freshwater-driven environmental, climatic and hydrological changes during the last deglaciation; ii) an improvedunderstandingoftheocean–atmosphere–ice-sheetfeedbacksandmechanisms at work in the Nordic Seas; and iii) better chronological constraints on key hydrological andhydrographiceventsthatoccurredduringthelastdeglaciation.Altogether,thisstudy discloses new research directions to generate more reliable marine chronologies in the North Atlantic, thus aiding in the comparison of marine and terrestrial climatic reconstructions. Explicitly, I have applied geochronological models to a large set of marine and lake sedimentrecordstoassignprecisetemporalconstraintstopastclimaticevents;generated high-resolutionisotopeandothergeochemicaldatasetsfortwolakesedimentsequences to reconstruct the deglacial hydro-climate; and assembled an extensive data set comprising all available North European quantitative paleoclimatic reconstructions spanning the last deglaciation. This overall approach was complemented with climate modelsimulations.ThemostsignificantresultsofthisPhDworkare: 1 - The establishment of precise geo-chronologies for eight key terrestrial and marine sedimentary records from Northern Europe and the Nordic Seas. I have reconstructed a new deglacial 14C reservoir age record for the Nordic Seas, which constitutes the best temporallyresolvedrecordofitskindandcanserveasafuturechronologicalbenchmark forNorthAtlanticpaleoclimatereconstructions. 2 - The construction of a new 1250-year long glacial varve chronology, which tracks the annualrecessionoftheScandinavianIceSheetinsouthernSweden.Thevarvechronology wasplacedonanabsolutetimescale,andbyusinggeochemicalanalysesIidentifiedthe first catastrophic drainage of the Baltic Ice Lake with an unprecedented precision (±2 years). 4 F. Muschitiello 3 - The generation of the first North European hydro-climate reconstructions based on isotope analyses (δD) of specific molecular compounds from southern Swedish lake sedimentsspanningthelastdeglaciation. 3.Investigationarea 3.1.Background One of the central aims of this study is to investigate deglacial atmospheric circulation dynamics over Northern Europe by using lake sediment stable isotope analyses. In NorthernEuropethisinformationisscarceandmainlyreliesonbulksedimentaryorganic matter(Ahlbergetal.,1996;O’Connelletal.,1999;Jonesetal.,2002;Marshalletal.,2002; Diefendorfetal.,2006),whichmakesitdifficulttoextractclimatefactorsfromthenoiseof lakeendogenicprocesses(LengandHenderson,2013). InsouthernSweden,someoftheproblemsassociatedwithstableisotopeanalysesonbulk sedimentary carbonates have been circumvented by using isotope measurements on specificcarbonatecomponentsoflakesediments,suchasmolluscshells,ostracodvalves, andalgaeencrustations(HammarlundandKeen,1994;HammarlundandLemdahl,1994; Hammarlund,1999;Hammarlundetal.,1999).However,theserecordsarefragmentaryor supported by poor chronological frameworks. Therefore, to fill this gap, new well-dated isotopic records from southern Sweden based on lipid biomarker compounds were generated (Fig. 3). The records were obtained from two lake sediment sequences, HässeldalaPort(56°16’N;15°03’E,40ma.s.l.)andAtteköpsMosse(56°23’N;12°51’E, 180 m a.s.l.), located along the south-eastern and south-western coast of Sweden, respectively(Fig.3).Thesesitesaretodaysmallpeatbogs,butcontainedlakesduringthe lastdeglaciation. Stableisotopesignaturesonlacustrinelipidcompoundshavebeensuccessfullyappliedto reconstructpaleo-hydrologicalprocessesassociatedwiththelastdeglaciationinWestern Europe (Rach et al., 2014). Precisely, these isotope records have allowed reconstructing regional shifts in precipitation patterns and water vapour availability, providing knowledgeonthedynamicsofNorthAtlanticstormtracksandthephysicalcharacteristics ofNorthAtlanticOceanwaters. Southern Sweden is an excellent area to employ these novel isotope proxies, as here hydro-climate shifts scale in a linear fashion with upwind, near-field oceanographic metrics(Fig.4a,b).Indeed,seasurfacetemperatures(SST)primarilycontroltheamount of moisture delivered to the region by regulating the flux of moisture released from the seawater to the atmosphere (Fig. 4a). Under modern conditions, the amount of precipitationinsouthernSwedenistightlylinkedwiththeprevailingwesterlywindsthat pick up moisture from the North Sea and the Skagerrak-Kattegat basin, which constitute the main source of moisture for precipitation (Gustafsson et al., 2010). By contrast, precipitation is less abundant during an anticyclonic regime (Fig. 4b) and moist air is generally transported from the Baltic Sea only under exceptionally warm surface water conditions(Gustafssonetal.,2010). During the last deglaciation, the region was located south of the Scandinavian Ice Sheet marginanddownwindofitsprimarydrainageroute.Therefore,itislikelythatmeltwater dischargefromtheicesheettotheNorthSearesultedinlowerSSTsandfresherwaters. This implies a lower water-to-air moisture uptake at times of increased meltwater outflow, with relatively depleted isotope signatures of seawater as the moisture source 5 Deglacial impact of the Scandinavian Ice Sheet on the North Atlantic climate system became fresher. Consequently, hydro-climate records on land are likely to capture the meltwater signal as this instantaneously propagates downwind in the form of drier air reachingthecoastalareaandlighterstableisotopicsignaturesofprecipitation. Inturn,terrestrialhydro-climateproxyrecordsfromsouthernSwedenarenotonlyuseful toreconstructregionalhydrologyandprecipitationpatterns,butpotentiallyidealtobetter understandtheregionalcouplingbetweenice-sheet,oceanandatmosphere.Furthermore, theseindirectfreshwaterreconstructionscanbenefitofrobustandatmospheric-based14C chronologies obtained from terrestrial macrofossils, which circumvent the intrinsic chronologicaluncertaintiesassociatedwithmarinereconstructions. Figure3.Corelocationsthatformpartofthisthesisandmainsitesdiscussedinthetext.Red circled dots indicate sites where new data were generated within the framework of this thesis,i.e.AtteköpsMosse(ATK),HässeldalaPort(HÄ),marinecoreMD99-2284.Greendots indicate sites used for comparative analysis and/or re-evaluation of available data. 1, Sluggan Bog; 2, marine core HM79-6/4; 3, Meerfelder Maar; 4, Kvaltjern; 5, Kråkenes; 6, Kulturmyra; 7, Lake Gammelmose; 8; Lake Madtjärn. The green rectangle in south-eastern SwedenhighlightstheareaofinvestigationdealtwithinPaperIII.Thebluelinedenotesthe approximate extension of the former Baltic Ice Lake (Björck et al., 1996) during the Late Allerød (Riede et al., 2011). The white line indicates the estimated ice margin for the ScandinavianIceSheetlimitat13,000yearsBP(Hughesetal.,2015). 3.2.Previouswork The ancient lake of Hässeldala Port is located in Blekinge, southern Sweden (Fig. 3) and filled in during the Early Holocene. The site is today a small peat bog covering ∼20 m2. Complete sediment sequences of variable depth have been retrieved and analysed using differentproxies(Daviesetal.,2003,2004;Wohlfarthetal.,2006;Kylanderetal.,2013; Steinthorsdottiretal.,2013;Ampeletal.,2015;Muschitielloetal.,2015a). 6 F. Muschitiello The basin contains a sedimentary sequence that covers the period between the Late Bølling and Early Holocene pollen zone (∼14,500-9500 years ago). The sediments have beenextensivelystudiedoverthelastdecadeusingavarietyofbiologicalandgeochemical proxies.Thetephro-chronologicalframeworkofthesitewasfirstestablishedbyDavieset al. (2004, 2003). The pollen- and litho-stratigraphy was established by Wohlfarth et al. (2006).Morerecently,sedimentgeochemistry(Kylanderetal.,2013),fossilleafstomata (Steinthorsdottir et al., 2013), diatom (Ampel et al., 2013), and biomarker records (Muschitielloetal.,2015a)havebeeninvestigated. TheancientlakeofAtteköpsMosseislocatedinsouthwesternSwedenclosetotheborder betweentheprovincesofSkåneandHalland(Fig.3).Itisasmallbasinthatfilledinduring the Early Holocene. The basin, which today is a peat bog covering ∼200 m2, has a full deglacialandHolocenestratigraphicsequence(from∼16,000yearsagotopresent)(Veres, 2001).ThesitehasbeenpreviouslyinvestigatedbyVeres(2001),whoestablishedalithostratigraphy, analysed the sediments for loss-on-ignition, grain-size, magnetic susceptibilityand14Cdating. Figure 4. a, Modern moisture source distribution and transport to the main study area. Summer (JJA) correlation between specific humidity in southern Sweden (averaged across 56-57°N and 12-16°W; HadCRUH) and sea-surface temperature over the adjacent seas relativetotheperiod1974-2003(contour;HadSST1).Theyellowlinedelimitstheareawhere the correlation is 95% CI. b, Summer (JJA) relationship of blocking circulation versus precipitation(CRU-TS3.23)insouthernSweden(asdefinedina).Blockingcirculationishere characterized as a pressure index defined by the 850 hPa atmospheric pressure difference (Trenberth’s NH) between 10°E and 40°W at 65°N. A positive blocking index indicates northward flow and negative values southward flow. Thick lines represent the 10-year movingaverages.Precipitationdataisexpressedasananomalyrelativetotheperiod19742003 and presented on a reverse axis. Northward flow negatively correlates with precipitation anomalies in the study area (R2= 0.50). The yellow dashed line indicates the total observed freshwater (FW) storage anomaly of the Norwegian Sea relative to the observationalperiod(Glessmeretal.,2014). 4.Materials,methodsandapplications 4.1.Sampling AtHässeldalaPort,anumberofcoreshavebeencollectedovertheyearsandthepresent isotopeanalysesrefertoCore5.Thesedimentsequenceswerecollectedin2011usinga 7 Deglacial impact of the Scandinavian Ice Sheet on the North Atlantic climate system Russian corer (10 cm diameter, 1 m length) with 0.5 m overlap between the successive cores. The lithology of Core 5 is shown in Table 1. Core 5 was not only sub-sampled for stableisotopeanalysesbutalsoforloss-on-ignition,fossil-leafstomatal,diatoms,and 14C dating(Steinthorsdottiretal.,2013;Ampeletal.,2015). ForthesiteofAtteköpsMosse,sedimentsequenceswerecollectedin2010usingaRussian corer (7.5 and 5 cm diameter, 1 m length) with 0.5 m overlap between the successive cores. The cores were first scanned for X-ray fluorescence analyses and the lithologies were then described (Table 2). Based on these, a composite stratigraphy was created, whichwasthebasisforfurthersub-sampling.Sub-samplesweretakenforloss-on-ignition, carbon and nitrogen, chironomid, biomarker, ancient DNA, and tephra and 14C dating. Table 1 – LithostratigraphicdescriptionoftheHässeldalasedimentsuccession.Sedimentunitsarenumbered accordingtothereferencelithostratigraphyofWohlfarthetal.(2006). Description Unit Depth(cm) 12-11 270.5-303.5 10 303.5-308.5 Darkbrowngyttja 9 308.5-322.5 Browngyttja 8a 322.5-332.5 Lightbrownalgaegyttjaclay/clayeyalgaegyttja 8b 332.5-334.5 Brownclayeysiltyalgaegyttja 8c 334.5-338.5 Lightbrownalgaegyttjaclayorclayeyalgaegyttja 7a 338.5-341.5 Bioturbatedzone;mixoftheupperlightbrownlayerandthelower darkbrowngyttja 7b-6 341.5-348.5 Mediumbrownclayeyalgaegyttja;visibleplantmacros 5 348.5-358.5 Mediumbrowngyttjaclay/claygyttja 3a 358.5-362.5 Brownclayeyalgaegyttja 3b 362.5-364.5 Lightbrown-yellowishsiltyclay 2 364.5-369.5 Darkbrownpeatygyttjaorgyttjapeat Yellowish-beigesiltyclay 4.2.14Cdating Radiocarbon(14C)datingisthemostwidelyusedtechniquetoinferthedown-coreageof lake and marine sedimentary records. 14C atoms are produced in the upper atmosphere, wherecosmicraysleadtothecollisionoffreeneutronswithnitrogenatoms(14N)causing adisplacementofprotonsthatturns 14Nin 14C. 14Catomsareoxidisedtocarbondioxide (14CO2), mixed in the atmosphere and oceans, and taken up by living organisms via metabolicactivity.Upondeathoftheorganism,CO2uptakeceases,whereastheradioactive 14Cslowlydecaysintothestableelement14N(Libby,1952). Lake and marine sediments generally contain a certain amount of organic carbon in the formoffossilmaterial(plantmacroremainsandforaminifera,respectively,amongmany others), which can be dated by the radiocarbon method. This method allows measuring the decay of 14C from the time of an organism’s death until present, i.e. the time of measurement(Libby,1952). The atmospheric 14C content varies over time due to changes in production rates and owing to carbon exchange between different reservoirs, e.g. the oceans, atmosphere, biosphere, and cryosphere. Therefore, to estimate the absolute age associated with a 14C measurement,itisnecessarynotonlytoaccountfortherelateddecayfactor,butalsofor 8 F. Muschitiello changes in past atmospheric 14C levels. This requires the use of a calibration curve, whereby the atmospheric 14C content can be directly equated to a calendar age (Stuiver andKra,1986;StuiverandBraziunas,1993). TheinternationallyratifiedradiocarboncalibrationcurveIntCal13(Reimeretal.,2013)is a calibration data set based on several absolutely dated records that have incorporated carbon from the atmosphere at the time of formation. For the last deglaciation, the IntCal13 curve is based on a number of independent records (Fig. 5a). After 13,900 calibratedyearsbefore1950AD(hereaftercal.yearsBP),thecalibrationcurveisdefined by precise dendrochronological measurements (Hua et al., 2009; Friedrich et al., 2004; Kromeretal.,2004).Priorto13,900cal.yearsBPthecurverelieson 14Cmeasurements frommarinesediments(Bardetal.,2013;Hughenetal.,2006,2004),corals(Durandetal., 2013; Fairbanks et al., 2005; Bard et al., 1990), speleothems (Southon et al., 2012; Hoffmannetal.,2010;Becketal.,2001),andvarvedlakesediments(BronkRamseyetal., 2012). Table2–LithostratigraphicdescriptionoftheAtteköpsmossesedimentsuccession. Description Unit Depth(cm) 9 402-462 8 462-510 Darkbrownfinedetritusgyttja 7e 510-511.5 Browntogreyishsiltygyttjalayer 7d 511.5-542 Mediumbrownsiltygyttja/algaegyttja 7c 542-549 Brownsilty/algaegyttja 7b 549-582.5 Darkbrownsiltyalgaegyttja 7a 582.5-590 Brownsiltygyttja/algaegyttja 6b 590-613.5 Brownsiltygyttja 6a 613.5-625 Siltydarkgreyclayeygyttja 5f 625-627.5 Brownish-greyclayeysilt 5e 627.5-629 Darkbrownclayeysilt 5d 629-655 Brownish-greyclayeysilt 5c 655-664 Brownish-orangeclayeysilt 5b 664-673 Brown-greyishsilt 5a 673-682 Darkbrownsilt 4 682-702 Alternatinglayerswithdarkbrownmossesandgreyish-brown clayeysilt/siltclay 3d 702-704 Greysilt 3c 704-708 Greycoarsesand 3b 708-710 Greysandysiltwithmosses 3a 710-714.5 Greyfinesand 2 714.5-724 Alternatinglayersofdarkbrownmossesandgreyish-brownclayey silt 1e 724-728 Brownish-greyclayeysilt 1d 728-731 Darkgreyfinesandlayer 1c 731-740 Greyclayeysilt 1b 740-744.5 Greysiltwiththinsandlayers(1mm) 1a 744.5-750 Darkbrowncoarsedetritusgyttjaorpeat Greyfinesandlayers 9 Deglacial impact of the Scandinavian Ice Sheet on the North Atlantic climate system Figure5.RadiocarboncalibrationandBayesianage-depthmodeling.a,Deglacialportionof the IntCal13 radiocarbon calibration curve; raw data composing the related database (Reimeretal.,2013)iscolorcodedbythetypeofproxyandexpressedwitherrors(±2σ).b, Exampleofacalibrationofaradiocarbondate.Theredprobabilitydistributionrepresents the measured 14C value of the radiocarbon date. The grey-colored area indicates the calibrated probability distribution of the radiocarbon date using the IntCal13 calibration curve(yellow).Thethickblacklinesshowthecalibratedagerangesthatencompassthe95% CIofthemeasured 14Cdate.Thedashedandsolidthinlinesillustratetheintersectionofthe errors(±2σ)andthemeanofthe 14Cage,andthecalibrationcurve,respectively.c,Example of a depositional process with sediment progressively accumulating over time (yellow columns).Blackdotsshowchangesinaccumulationrateandtheblacklinereflectsthe“true” age-depthhistoryofthesedimentaryrecord.Theredtrianglesrepresentradiocarbon-dated samplesandtheredlinereflectsatentativeage-depthrelationbasedonlinearinterpolation between the chronological constraints. d, Construction of a probabilistic age-depth model (seetextfordetails). Calibrationtotheatmospheric 14Ccurve(Fig.5a,b)isahighlysuitablemethodtoconvert the 14C age of samples from terrestrial organisms to absolute age. For marine samples, however,the 14Ccontentreflectsthe 14CO2dissolvedintheocean.Oceansaredepletedin their 14C content relative to that of the atmosphere. This results in an apparent 14C age differencebetweentheoceanwaterandthecontemporaneousatmosphere,whichisalso referred to as radiocarbon reservoir age (Stuiver and Braziunas, 1993). During the last deglaciation,themagnitudeofthereservoiragevariedovertimeandspaceprimarilyasa function of the strength in the rate of ocean ventilation and terrestrial freshwater discharge(Waelbroecketal.,2001;Björcketal.,2003;Bondeviketal.,2006;Thompsonet al.,2011).Giventhatthelastdeglaciationwascharacterizedbylargeandrapidchangesin oceancirculation(e.g.McManusetal.,2004),thetimingofwhichcanonlybeconstrained 10 F. Muschitiello by means of marine radiocarbon chronologies, a regional assessment of changes in reservoirageintheNorthAtlanticisstillachallengingendeavor. Radiocarbon dating and calibration are not only the first step towards establishing the age-depth relationship for lake and marine sedimentary sequences. The second step to inferareliableage-depthrelationshipinvolvesprobabilisticage-depthmodeling.Thiscan bebasedonBayesianstatistics,anapproachthathasbeenwidelyappliedinthisthesis.In the following, the method and concepts behind Bayesian age-depth modeling are discussed.Anoutlineofthemethodandcalculationstogeneratethemarine 14Creservoir agerecordpresentedinthisthesisisalsoprovided. 4.2.1.Bayesianage-depthmodeling Bayesian age-depth modeling has become increasingly popular in the last decades to reconstruct accumulation histories of radiocarbon-dated geological records (Buck et al., 1991; Blaauw and Christen, 2005; Parnell et al., 2008; Bronk Ramsey, 2008). Bayesian statistics combine data and prior information to infer the posterior distributions, i.e. predictive probability distributions that are conditional on the observed data. Bayesian depositionalmodelsarethusconstructedbasedupontheavailableagemeasurementsand usingexplicitpriorparameterconstraintssuchas–forinstance–positiveaccumulation, meanaccumulationratesandmeanaccumulationratevariance.Inadepositionalcontext, the approach aims at mathematically finding a representative set of possible ages associated with each depth interval in a sedimentary record. The full mathematical formalism for the model elaboration can be found in Blaauw and Christen (2005, 2011) andBronkRamsey(2008). The model operates via a Markov Chain Monte Carlo (MCMC) sampling method, which simulatesadistributionofpossiblesolutions(Gilksetal.,1996),withaprobabilitythatisa productofthepriordistributionofeachparameterandthelikelihoodprobabilitiesofthe observed data. Therefore, the resulting posterior distributions are a probabilistic representationofthedepositionalhistoryofthesedimentaryrecordthatfullyaccountsfor theavailableagemeasurements. InBayesianagemodels,radiocarbondatesaretreatedintheircalibratedform(Fig.5a,b). Thecalibrationofa14CdateiwithvalueRianduncertaintyδRiisobtainedviacomparison tothecalibrationcurve,whichprovidesacontinuousestimationofthe 14Cageovertime R(ts) and the associated uncertainty δR(ts). The agreement between the 14C age and the calibrationcurveateachpointintime,ts,canbeexpressedasalikelihood,Pi(ts). exp − 𝑃i 𝑡s ∝ (𝑅i − 𝑅(𝑡s))! 2(δ𝑅i! + δ𝑅 𝑡s ! ) ! δ𝑅i + δ𝑅 𝑡s (1) ! This provides the calendar age probability distribution of a calibrated 14C date (Fig. 5b, greypatch). To simulatetheposteriordistributionswithMCMC,themodelisgenerallydrivenby the Metropolis-Hastings sampler algorithm (Metropolis et al., 1953; Hastings, 1970). The 11 Deglacial impact of the Scandinavian Ice Sheet on the North Atlantic climate system algorithm is used to obtain a candidate matrix Y from a probability density function referred to as proposal distribution si(yi). The candidate point Y is then used as the new statetofthechainwithaprobabilitygivenbyα: 𝑅 t = 𝑃!"#$% (𝑌! ) 𝑃!"#$% (𝑌!!! ) withα = min 1, 𝑅 t . (2) IntheeventthecandidatematrixYisrejected,thechainstaysatthesamepointandthe stateissettoXt=Xt-1.ThecandidatematrixYisacceptedif 𝑈 0,1 ≤ α (3) with𝑈 0,1 beingauniformrandomnumberbetween0and1.Whenα=0allcandidate matricesareaccepted,whereaswhenα=1onlycandidateswithprobabilityequalto1are accepted.Inasimplifiedconfiguration,themodelprobabilitycanbedefinedbythreemain contributions–time(T),depth(D)andaccumulationrateconstraints(Z). 𝑃!"#$% = 𝑃! . 𝑃! . 𝑃! (4) The full mathematical specification for each of the probabilities can be found in Bronk Ramsey(2008).Themodelisguidedbyascoregivenintermsofthenegativelogarithmof theposteriorprobability,whichprovidesanestimateofthemodelperformance,whilethe convergence of the MCMC chain around the ‘true’ parameter values is monitored by acceptedPmodelvalues.ForeachstepoftheMCMCchain(typicallymorethan106iterations intotal)severalageestimationsforeveryinputandinterpolateddeptharegenerated(Fig. 5d). This allows to estimate age probability distributions for every given depth interval, andfortheassociatedconfidenceintervals. 4.2.2.Reservoirageestimation MarineradiocarbonreservoiragesareexpressedasR(t)andΔR(t)(StuiverandBraziunas, 1993). R(t) is defined as the departure of a measured marine 14C age, 14CM(t), from the corresponding contemporaneous atmospheric 14C age, 14CATM(t), on the IntCal13 calibration curve (Reimer et al., 2013) at the calibrated age t of deposition of the 14C samplematerial(equation5). 𝑅 𝑡! = 14C M 𝑡! − 14C ATM 𝑡! (5) Thecalibratedagetcanbeinferredfroma 14Cdatefromaterrestrialsampleobtainedat the same depth as the marine sample, or more accessibly using age output from a depositionalmodel,asdescribedabove(e.g.PaperIV). 12 F. Muschitiello Ontheotherhand,ΔRisdefinedasthedepartureofameasuredmarine 14Cage, 14CM(t), from the corresponding contemporaneous global marine 14C age, 14CMAR(t), on the Marine13calibrationcurve(Reimeretal.,2013)(equation6). Δ𝑅 𝑡! = 14C M 𝑡! − 14C MAR 𝑡! (6) The parameters 14CM, 14CATM, and 14CMAR are accompanied by errors, which are normally distributed, i.e. the measured radiocarbon mean value and the associated uncertainty. Calibrated ages ti can be obtained using MCMC output from an age-depth model and probabilitydensityfunctionscanbecalculatedforeachR(t)andΔR(t)values(e.g.Olsenet al.,2009,2014). 4.3.Synchronizationofclimaterecords A meticulous stratigraphic alignment of proxy data from sediment cores is essential to accurately transfer information across records recovered from the same sedimentary basin (e.g. Paper II). The alignment can also be applied to records located within a confined region, thereby allowing synchronization of less well-dated records to more robustlydatedreconstructions.Thisreliesontheassumptionthatclimateconditionswere similar throughout the region and that the resolution of at least one of the records is coarser than the timing required for a climate event to propagate between the core locations (e.g. Lane et al., 2013). In particular, the latter application can be particularly helpfulinrefiningmarinechronologies(e.g.PaperIV),whicharegenerallypronetolarge uncertaintiesassociatedwithoften-unknownregionalreservoiragecorrections. The stratigraphic alignment of sediment sequences is commonly pursued through an interactiveadjustmentoftimeseries,whichconsistsofmanuallylinkinguser-definedtie points(Björcketal.,2003;Austinetal.,2011).Thisapproachisqualitativeandproneto subjectivity,withlimitedreproducibility.Italsoimpliesassumingconstantsedimentation ratesbetweentiepoints,whichinvolveserrorsthataredifficulttoassess. However, over the last two decades a number of deterministic algorithms have been developed to automate the alignment process (e.g. Lisiecki, 2002; Lisiecki and Herbert, 2007; Malinverno, 2013; Lin et al., 2014). Such approaches involve deformation of the entirety of one proxy record onto a reference time series, thus allowing tuning multiple sedimentary records in a more flexible fashion, which accounts for uneven compaction and/orexpansionofsedimentsovertime. Thestratigraphicalignmentalgorithmusedinthisthesis(AnderssonandMuschitiello,in preparation) was largely inspired by the work of Malinverno (2013). The algorithm is driven by a Markov-Hasting MCMC method (similar to that adopted for age modelling), wherethealgorithmjumpsfromthecurrentstatetothenextstatebasedonastochastic perturbationofthecurrentstateintroducedatarandompositioninthedepth/agescaleof thereferenceproxyrecord.Ifthestochasticperturbationimprovesthefitbetweenthetwo proxytimeseries–asmonitoredbyaprobabilitycriterion–thealgorithmacceptsthenew state,wheretheacceptancecriterionisdescribedinequation(2)and(3).Thegoodnessof thefit(F)oftheperturbedstaterelativetothatoftheunperturbedstateiscomputedas: 13 Deglacial impact of the Scandinavian Ice Sheet on the North Atlantic climate system 𝐹= ! !!! 𝑌! (𝑖)!"#$%#&'$()* − 𝑌! 𝑖 𝑛 ! !"#$%& (7) wherenarethenumberofoverlappingdatapointsandYA(i)linearisthelinearinterpolation ofthereferenceproxyrecordA,fittingthedepth/agescaleoftheperturbedrecordB.To account for the finite differences between the reference and fitted proxy time series, an analogousfit,fitFD,isalsocomputed.Thefitsareassumedtobenormallydistributed(with standarddeviationσ)andthetotalprobabilityforthenewstate(i)isexpressedas: 𝑃 𝑖 = 𝑒 !!"#(!)/!! ! 2𝜋𝜎 ! ∙ 𝑒 !!"#!" (!)/!! 2𝜋𝜎 ! ! (8) whereσistheparameterthatdefinestheexpectedsimilaritybetweenthetimeseries. TheMCMCchainistypicallyrunformorethan106iterations.Thealgorithmisrelatively fast and provides a robust method to find an optimal fit between two proxy time series alsoaccountingfortheirlocalvariability(e.g.PaperIIandPaperIV).Afullaccountofthe mathematical formulation associated with the algorithm will be presented elsewhere (AnderssonandMuschitiello,inpreparation). 4.4.Hydrogen-isotopiccompositionoflipidbiomarkerandpaleo-hydrological application At the molecular level, the analysis of organic matter preserved in sedimentary records can provide much more detailed environmental information as compared to bulk sedimentarygeochemistry.Forinstance,specificmolecularcompoundscanberelatedto particularprecursororganismsand/orgroupoforganismstherebyprovidinginsightsinto theprevailingenvironmentalconditionswithindefinitebiologicalsystemsandecological niches (e.g. Meyers and Ishiwatari, 1993; Didyk et al., 1978). This class of molecular compoundsisalsoknownas‘biomarkers’. In lake sediment studies, biomarker analysis on organic matter from photosynthesizing organisms allows for the separation of aquatic and terrestrial sedimentary components, which enables examining environmental conditions in the lake and the surrounding catchment.Aswateristhemainsourceofhydrogenforphotosynthesizingorganisms,the hydrogen-isotopic composition (δD) of sedimentary lipid biomarkers has emerged as a powerfultoolinthestudyofancientenvironmentsandclimates(EstepandHoering,1980; Sternberg,1988). Amongthemostroutinelyusedlipidbiomarkersarethen-alkanehydrocarbons,whichare ubiquitous constituents of biological systems and excellently preserved in sedimentary records spanning a variety of geological time scales (Eglinton and Eglinton, 2008). Both the membranes of algae and aquatic plants, and the cuticular waxes of higher terrestrial plant leaves contain large amounts of n-alkanes, and specifically short-chain and longchainn-alkanes,respectively(e.g.Fickenetal.,2000).TheδDvaluesoftheseaquaticand 14 F. Muschitiello terrestrial n-alkanes are generally offset from, and highly correlated with, the δD composition of the source water used by the precursor organisms following hydrogentransfer reactions, i.e. intracellular water for algae and aquatic plants, and leaf water for terrestrialplants(Fig.6)(e.g.Sessionsetal.,1999;Sachseetal.,2004).Thisistheresultof a number of environmental parameters (e.g. precipitation amount and source, temperature,relativehumidity)andphysiologicalprocessesinvolvingintracellularwater (e.g. leaf physiology, salinity, light intensity, biosynthetic pathway) that control the isotopicfractionationbetweenhydrogeninthesourcewaterandintheorganiclipids,also knownasnetorapparentfractionation(Sachseetal.,2012). Althoughmanyoftheseprocessesarestillamatterofstudy,theδDcompositionoflipid biomarkers from lacustrine sediments has rapidly become an established proxy to reconstructpaleo-hydrologicalconditionsandδDofprecipitation(e.g.Aichneretal.,2010; Rachetal.,2014).Assuch,δDvaluesofshort-chainn-alkanes(n-C17-23)fromaquaticalgae andsubmergedplantscollectedfromlake-surfacesedimentsalongclimaticgradientshave revealedahighcorrelationwithlake-waterδDvalues(Fig.6)(Huangetal.,2004;Sachse et al., 2004, 2006). Analogously, δD values of long-chain n-alkanes (n-C27-31) from leaf waxesofterrestrialplantsextractedfromlake-surfacesedimentsalongclimaticgradients arehighlycorrelatedwithprecipitationδDvalues(Fig.6a,b)(Huangetal.,2004;Sachseet al.,2004;Garcinetal.,2012). ThisfindingsuggestsagoodpreservationofthesourcewaterδDsignalwherebytemporal andspatial(withinthecatchment)integrationmayreducethevariabilityassociatedwith individual biological sources and specific processes. However, even though both aquatic and terrestrial plants undergo isotopic fractionation, which depends on the specific biosynthetic pathway, the net or apparent fractionation of the δD values of terrestrial nalkanes is strongly affected by two additional fractionation steps: soil-water evaporation and leaf-water transpiration processes (Fig. 7) (e.g. Sachse et al., 2006, 2012). These parameters,whicharecontrolledbyplantanatomyconditions,relativehumidityandsoil moisture availability, can be framed into a mechanistic Craig-Gordon model (Craig and Gordon,1965)foropenwaterbodieswhereleaf-waterevaporativeenrichment(ΔDe)can beexpressedas: ΔD! = 𝜀 ! + 𝜀! + (ΔD! − 𝜀! ) 𝑒! 𝑒! (9) ε+isthetemperature-dependentliquid-to-vapourequilibriumfractionationatthewaterairinterface,εkisthekineticfractionationduringdiffusionofvapourfromtheintracellular spaceintheleaftotheatmosphere,ΔDvistheisotopicenrichment/depletionofvapourin the atmosphere relative to the source water, and ea/ei is the leaf-to-air vapour pressure ratio, which is an expression of relative humidity, leaf temperature and air temperature (Sachseetal.,2012). Soil-water and leaf-water evapotranspiration are poorly understood owing to the large numberofbiologicalunknownsandintegrationstepsassociatedwiththenetorapparent fractionation, and due to the lack of experimental culture-based studies. Although an empirical understanding of all the processes behind the net or apparent fractionation in higherplantsmayhelptopavethewayforquantitativeapplicationstopaleo-hydrological reconstructions, at present, the isotopic difference between terrestrial and aquatic n- 15 Deglacial impact of the Scandinavian Ice Sheet on the North Atlantic climate system alkanes can only be employed as a qualitative indicator for reconstructing changes in catchment evapotranspiration (Fig. 7). However, before using lipid biomarker δD values for paleo-hydrological interpretations, it is advisable to characterize the ancient environment using a multi-proxy approach. In particular, vegetation reconstructions basedonpollenandmacrofossilinformationcanhelptofactoroutoraccountforpossible biologicalshiftsthusallowingdisentanglingofclimaticversusphysiologicaleffectsonleafwaterδD. Figure 6.Relationshipbetweensource-waterδDandn-alkaneδDvaluesfromlake-surface sediments. The relationship is presented for long-chain n-alkanes (n-C29) (a) and short-tomid-chain n-alkanes (n-C17, n-C23) (b, c). The δD values of n-C23 alkanes of sediments are compared to those from their primary biological source, Potamogeton plants, from the respective lake sediments (data from Aichner et al. (2010)). δD values are given with their ±1σ uncertainty for replicate measurements (lipids and lake water), and errors are calculated from precipitation or obtained from the Atomic Energy Agency GNIP database. Errorbarspresentedincreflectthe±2σuncertainty.ModifiedafterSachseetal.(2012). Figure 7. Conceptual overviewofthehydrogen-isotopicrelationshipbetweensourcewater and sedimentary n-alkanes of aquatic and terrestrial plants (not to scale). The red dot illustrates a hypothetical mixture of water pools within the leaf, constituting the ultimate hydrogensourceforlipidbiosynthesis.εbio,biosynthetichydrogen-isotopicfractionation;εl/w, isotopicfractionationbetweenlipidsandsourcewater.ModifiedfromSachseetal.(2012). 16 F. Muschitiello 4.5.LipidextractionandδDanalysis Lipidextractionwasperformedonfreeze-driedsedimentsamples(2-8cm3)viasonication withdichloromethane:methanol(9:1)for20minutes,andsubsequentcentrifugation.This was repeated three times and supernatants were combined at each step. Aliphatic hydrocarbon fractions were isolated from the total lipid extract using silica gel columns (5%deactivated)thatwereelutedwithpurehexane.Thesaturatedhydrocarbonfraction was desulphurized by elution through 10% AgNO3-impregnated silica gel using pure hexaneaseluent.Saturatedhydrocarbonfractionswereanalysedbygaschromatography- mass spectrometry for identification and quantification, using a Shimadzu QP2010 Ultra. Shortandlong-chain(typicallyC19toC33)n-alkaneswereidentifiedbasedonmassspectra from the literature and retention times. The concentration of individual compounds was estimatedbasedonthecomparisonofpeakareasrelativetothatofsqualane,usedasan internalstandardaddedtothesamplesbeforelipidextraction. Isotope ratios R (R = D/H with 2H or D for deuterium and 1H or H for protium) are expressed as δD values in per mil (‰), which reflect the relative deviation of R in the samplefromastandard(ViennaStandardMeanOceanWaterwithδD=0‰). δD = 𝑅!"#$%& − 𝑅!"#$%#&% . 1000 𝑅!"#$%#&% (10) δDvaluesweredeterminedusingaThermoFinniganDeltaXLmassspectrometerandall analyses were performed in triplicate. A standard mixture of n-alkanes with known δD composition (mix A4, provided by A. Schimmelmann, Indiana University, USA) was run severaltimesdailytocalibratethemeasuredδDvalueofareferencegasusedtobracket all analyses. Only sample values that were characterised in the isotope-ratio mass spectrometerchromatogramsbybaselineseparatedpeaksandwereofhighenoughpeak sizetofallwithinthelinearityrangeoftheinstrumentwereusedfordatainterpretation. 5.SummariesofPaperI-IV 5.1. Paper I - On the timing of environmental shifts across Northern Europe at the Allerød-YoungerDryastransition TheAllerød-YoungerDryas(AL-YD)transitionisthelastmajorlarge-scaleclimateshiftto severe cold conditions before the start of the present warm interglacial. Thanks to the disposal of a large number of terrestrial proxy reconstructions supported by reasonably good chronologies, the AL-YD transition constitutes an excellent workplace to explore leadsandlagsinresponsetorapidclimatechange. The onset of the YD in Northern Europe is defined by a distinct shift in pollen and macrofossilrecordsfromlakesedimentsequences.Thepollen-stratigraphicboundaryhas long been used as a common regional chronological constraint (Mangerud et al., 1974; Wohlfarth, 1996; Björck et al., 1998; Lowe et al., 2008) in Northern Europe, since it was assumedthatitreflectsarapidandsynchronousresponseoftheregionalvegetationtothe cooling related with the YD. However, at fine chronological resolution, climate records show that even though climate events may have been abrupt at a local scale, they can spreadinatime-transgressivefashionoverwidergeographicalscales(Laneetal.,2013). 17 Deglacial impact of the Scandinavian Ice Sheet on the North Atlantic climate system Similarly, environmental changes do not necessarily respond linearly to climate shifts (Claussenetal.,2013;Rachetal.,2014). Therefore, if differences in the vegetation response time between sites existed, these wouldnotbefullycapturedinlow-resolutionrecords.Furthermore,priortothearrivalof thelatestradiocarboncalibrationcurveIntCal13(Reimeretal.,2013),theoccurrenceofa longradiocarbonageplateauaround13,000-12,800cal.yearsBP(Reimeretal.,2009)had madeitdifficulttoreasonablynarrowdowntheageuncertaintyofthepollen-stratigraphic AL-YDboundary. The new radiocarbon calibration curve (Fig. 5a) has significantly improved the accuracy around the aforementioned radiocarbon age plateau, which is now constrained by treering 14Cdata(Huaetal.,2009).Thus,thenewcalibrationcurveofferstheopportunityto re-examinesomeofthemostdenselydatedNorthEuropeanterrestrialchronologies. In Paper I, the radiocarbon chronologies of four key sites spanning the AL-YD transition wererevisitedandcomparedbyconsistentlyconstructingnewBayesianage-depthmodels foreachsedimentaryrecord.Thechronologiesofthefoursites–LakeKråkenes(Birkset al.,2000),LakeMadtjärn(Björcketal.,1996),LakeGammelmose(Andresenetal.,2000), andSlugganBog(Loweetal.,2004)–areunderpinnedbyalargenumberofAMS14Cdates derived from terrestrial plant macrofossils and the local AL-YD pollen stratigraphic boundary is finely constrained in terms of sampling resolution. The age-depth models were produced using two different routines, OxCal (Bronk Ramsey, 2010) and Bacon (Blaauw and Christen, 2011) after calibration with the IntCal13 data set (Reimer et al., 2013). The results show a clear, geographically consistent and diachronous signal of vegetation changes over Northern Europe, with an early AL-YD transition at ∼13,10012,900cal.yearsBPintheBritishIslesregionandDenmark,andalaterAL-YDtransition at ∼12,750-12,600 cal. years BP in southern Sweden and western Norway (Fig. 8). It is hypothesized that the early transition was associated with regional cooling owing to increased freshwater outflow into the Nordic Seas from the southern margin of the Scandinavian Ice Sheet. By contrast, the second phase was probably brought about by large-scale cooling caused by a widespread climate reorganization associated with a southwarddiversionoftheNorthAtlanticwesterlywindbelt(Braueretal.,2008;Rachet al.,2014). Apotentialdownsideofthisstudyisthattheresultsrelyheavilyontheoriginaldefinition of the pollen zone boundary at each site. For instance, new vegetation reconstructions from the site of Hasselø in Denmark suggest that here the AL-YD transition was concomitant with that recorded at the Swedish and Norwegian sites (Mortensen et al., 2015).WhilethesenewresultsdisagreewithourconclusionofanearlyAL-YDvegetation shiftinDenmark,theageuncertaintiesthataccompanythelocalpollenzoneboundaryat Hasseløprecludeanyconclusivesayonthismatter. Chronologicalassessmentsofanumberofindependenttemperaturereconstructionsfrom British sites point however at an early phase of cooling starting as early as ∼13,100 cal. years BP (Elias and Matthews, 2014). This evidence strongly argues in favour of potentially early vegetation shifts in the British Isles, thus supporting our results and interpretations. Paper I highlights the importance of establishing robust and coherent chronologies, but also shows some of the limitations of relying on pollen data solely for climate reconstructions. 18 F. Muschitiello Figure 8. Allerød-Younger Dryas age estimates inferredfromNorthEuropeanpollenstratigraphies. K, Kråkenes; M, Madtjärn; HÄ, Hässeldala Port; LG, Lake Gammelmose; SB, Sluggan Bog; MFM, Meerfelder Maar. Age estimates for the onset of Greenland Interstadial 1a (GI-1a) and Greenland Stadial 1 (GS-1) are expressedontheIntCal13time scale after synchronization with the Greenland Ice CoreChronology2005(Muscheleretal.,2014). 5.2.PaperII-NorthAtlantichydro-climatepatternsaroundthestartoftheYounger Dryasstadial A number of studies have shown that hydrological and climate shifts spread rather uniformlyacrosstheNorthAtlanticdomainattheonsetoftheYDcoldperiod(Grafenstein etal.,1999;BirksandAmmann,2000;Rachetal.,2014;Bartoloméetal.,2015),resulting insimilarsignsofchangesacrossEuropewithrespecttothoserecordedinGreenlandicecore proxies. This has generally been attributed to changes in sea-ice coverage over the NorthAtlantic,whichcausedmid-latitudestormtrackstodivertacrossawideregion,thus creating a tele-connective mechanism that linked the Greenland and European climate systems(Braueretal.,2008). Unsurprisingly, this conclusion has somewhat provided a justification for aligning European climate records to ice-core isotope stratigraphies(e.g. Grafenstein et al., 1999; Bakke et al., 2009; Lang et al., 2010). Such alignments have mainly been applied when available chronologies were not sufficiently reliable to allow for an accurate temporal comparison with events recorded in Greenland ice cores. This however has limited our ability to investigate whether regional atmospheric patterns across the North Atlantic werebroadlyconsistentornotduringthelastdeglaciation. PaperIIaddressessomeoftheseissuesbystudyingnewwell-datedhydro-climate(δDon lipid biomarkers) and temperature (chironomid inferred) data from a lake sediment record from southern Sweden (Hässeldala Port – Fig. 3). The records were examined relative to hydro-climate events recorded in Greenland ice cores. The chronological comparisonwasalsofacilitatedbytherecentsynchronizationofthe 14Candice-coretime scalesusingthecommoncosmogenicisotopevariationsintree-ringandice-corerecords (Muscheleretal.,2014).Finally,theproxy-basedreconstructionswerecoupledtoclimate model simulations in order to investigate the ocean and atmosphere parameters responsiblefortheobservedspatialhydro-climatepatterns. Theproxyrecordsindicateprogressivelydrierandcoldersummerconditionsinsouthern SwedenduringthefewcenturiesprecedingthestartoftheYDstadial(∼13,100-12,880cal. yearsBP)asopposedtowetterand/orwarmerconditionsobservedinGreenland(Fig.9a). AstheδDrecordsareexpectedtoqualitativelytracktherateoffreshwaterdischargefrom the southern margin of the Scandinavian Ice Sheet to the adjacent North and Norwegian Seas(seesection2)–themainmoisturesourceofprecipitation–itissuggestedthatthis period coincides with increased Scandinavian Ice Sheet meltwater forcing in the Nordic Seas. 19 Deglacial impact of the Scandinavian Ice Sheet on the North Atlantic climate system Figure 9. Reconstructed and modeled climate change under Scandinavian Ice Sheet freshwater forcing prior to the onset of the Younger Dryas stadial. a, GRIP snow accumulation record (Johnsen et al., 1995) compared to Hässeldala catchment evapotranspirationreconstruction.Recordsarepresentedwithshadingindicatingempirical 68% uncertainty bounds based on both analytical and age-model errors. Greenland stratigraphic events (Rasmussen et al., 2006) and Hässeldala’s pollen zones are also displayed. b, Modelled summer changes (JJA) in sea-level pressure preceding the abrupt cooling associated with the Younger Dryas stadial. The area delimited in red shows the locationwherefreshwaterforcingwasprescribed.Significancelevelsareindicatedbyblack stippling(95%CI). This hypothesis was tested by using a transient climate simulation performed with a coupledatmosphere-oceanmodel(Liuetal.,2009;Heetal.,2013).Precisely,theregional climate response to a weak freshening (0.011 Sv) of the North and Norwegian Seas was investigated.Thefreshwaterforcinggeneratesasea-levelpressure(SLP)dipoleacrossthe North Atlantic, with relatively higher SLP over Northern Europe and lower SLP over Greenland (Fig. 9b). In the model, the dipole is a direct expression of increased sea-ice coverintheNorwegianandBarentsSeasresultingfromthefreshwaterinput. This physical mechanism is capable to fully explain the divergent hydro-climate and temperatureshiftsrecordedshortlypriortotheonsetoftheYDstadialbothinsouthern Sweden and Greenland. Interestingly, the hydro-climate dipole in the model is bound to thepresenceoffreshwateranomaliesintheeasternsectoroftheNordicSeas,whichare necessarytoaccountfortheshiftsobservedinthereconstructions.Infact,thedipoleisnot simulatedwhenfreshwaterisreleasedfromNorthAmericansources.Moreover,themodel results compellingly support the interpretation of the δD records in terms of regional meltwaterforcing. Inaddition,theseresultsimplythataligningNorthEuropeanrecordstoGreenlandclimate signals is not a viable option if we are to understand leads-lags and spatial patterns of climateresponsetofreshwaterforcing. Inconclusion,thisstudyhighlightsapreviouslyunrecognizedsensitivityofNorthAtlantic hydro-climate to Scandinavian Ice Sheet meltwater forcing. More importantly, it shows thatScandinavianIceSheetmeltwaterdischargetotheNordicSeascancontrolthetiming and signs of the isotopic shifts registered in Greenland ice cores shortly prior to the YD 20 F. Muschitiello stadial. This is a potentially valid mechanism to explain the early vegetation shifts and dropintemperaturesobservedintheBritishIslesandoutlinedinPaperI. 5.3. Paper III - Glacial varve evidence for a catastrophic outburst of meltwater synchronouswiththeonsetoftheYoungerDryasstadial Since the late 80’s, when Wallace Broecker and colleagues (Broecker et al., 1989) hypothesizedthatacatastrophicmeltwaterpulsefromtheNorthAmericancontinentwas themaintriggeringmechanismfortheonsetoftheYD,thesearchforthefloodpathway hasbeenoneofthemostcontroversialtopicsinpaleoclimatesciences. Yet,despitethatthedrainageorfloodhypothesisisatpresenttheclassicalexplanationfor thestartoftheYD,thereconstructedtimingofthecatastrophicmeltwateroutburstfrom the Laurentide Ice Sheet is still a matter of debate. Reconstructed ages for this event (Lowelletal.,2005;Fisheretal.,2009;Murtonetal.,2010;NotandHillaire-Marcel,2012; Breckenridge, 2015) are systematically too young or too uncertain with respect to the hydro-climate shifts that mark the onset of the YD stadial as observed in Greenland icecorerecords,whereitisreferredtoasGreenlandStadial1(GS-1;12,846±69iceyearsat 1σ–Rasmussenetal.,2006).Thiscastssomedoubtsonthecausalrelationshipbetween the drainage of proglacial lakes in North America and major shifts in atmospheric and oceancirculationattheonsetoftheYDstadial. TheScandinavianIceSheet,ontheothersideoftheNorthAtlanticOcean,isperhapsoneof themostoverlookeddriversofdeglacialclimatechange.DuringtheLateAL(∼13,000cal. years BP), the ice front was located south of the south central Swedish lowland area (Björck,1995;LundqvistandWohlfarth,2000;Hughesetal.,2015)andtheBalticIceLake (Fig.3)wasdammedup.Rapidrecessionoftheicemarginduringthisperiodatthewater dividenearMt.Billingen(Fig.3)generatedaspillwaysystemthatconnectedtheBalticIce Laketotheseainthewest,whichresultedinanabrupt5-10mloweringoftheBalticIce Lake(Björck,1995;Björcketal.,1996). The Baltic Ice Lake drainage has long been a contentious issue. However, new evidence now confirms that a catastrophic outflow of meltwater actually took place near Mt. Billingen (Swärd et al., 2015). However, these reconstructions lack a precise chronology thatallowspinningdowntheexactageofthisevent. Paper III constrains the timing of the Late AL drainage of the Baltic Ice Lake by reevaluating an annually resolved glacial varve chronology from southeastern Sweden (Wohlfarth et al., 1998). The new composite 1257-year long varve chronology (∼13,20012,00 cal. years BP) is based on 57 records and provides insights into the timing of ice recession and depositional events within the Baltic Ice Lake. In addition, the chronology wasplacedonanabsolutetimescaleusingtheVeddeAshvolcanicmarkerandnew14Cage modelling. This allowed comparing for the first time the melting history of the ScandinavianIceSheettotheGreenlandice-coreandradiocarbontimescales(Rasmussen etal.,2006;Reimeretal.,2013)withhighaccuracyandresolution. Geochemicalandsedimentologicalanalysesoftheglacialvarverecordsindicatesarapid change in sedimentation regime and a long-lasting disappearance of ice-rafted debris in theBalticIceLake,respectively,whichcoincidedwiththestartofGS-1.Thisdepositional eventtookplaceat12,847±2years(1σ)ontheice-coretimescaleandat12,876±22cal. years (1σ) on the IntCal13 time scale. The event occurred 726 ± 2 years after the depositionoftheVeddeAshascomparedto725±6yearsforthestartofGS-1inice-core 21 Deglacial impact of the Scandinavian Ice Sheet on the North Atlantic climate system records. A simplified ice-sheet model indicates that the change in sedimentation regime and especially the drop in ice-rafted debris transport can be explained by ice-margin stabilizationinresponsetoalargedropintheBalticIceLakewaterlevel. Figure 10. Radiocarbon calibrated age of the first drainageoftheBalticIceLakeinferredfromawigglematching model underpinning the glacial varve chronology from Östergötland (red). The related probability is compared to the calibrated age of all available 14Cdatesthatindirectlyconstrainthisevent. Dates from Blekinge and Arkona Basin refer to the timing of a major lowering of the Baltic Ice Lake. DatesfromHunnebergconstrainthetimingwhenthe spillwayatMt.Billingenbecameicefree.Thebluebar indicatestheageprobabilityforthestartofGreenland Stadial 1 (GS-1) on the IntCal13 time scale. A list of referencestotherespectivedatesispresentedinPaper II. Resultsfromasystematicre-calibrationofallavailable 14Cdatesthatconstrainthetiming ofdeglaciationattheMt.BillingenoutletandtherelatedBalticIceLakewaterleveldrop (Muschitiello et al., 2015b, 2015c) argue in favour of our hypothesis (Fig. 10). A mechanism,onlyhintedatinPaperII,isalsoproposed,forwhichacatastrophicoutflowof meltwater may have induced excess sea ice in the Norwegian and Barents Seas, which recirculatedintothesubpolarNorthAtlanticgyre.Itissuggestedthatsea-icerecirculation in the Nordic Seas can cause the coupled atmosphere-ocean system to cross thresholds beyond which a stadial climate regime is triggered. Critically, this is a robust feature in Earth-SystemandGeneralCirculationmodels(Fig.11)(Drijfhoutetal.,2013;Lehneretal., 2013) and thus provides a plausible physical mechanism for the inception of sustained coldclimateeventsofthepast. 5.4.PaperIV–DeglacialAMOCandmeltwaterforcingintheNordicSeas:theocean perspective Thecommonexplanationfortheinceptionofsustainedcoldstadialperiodsduringthelast deglaciation involves an abrupt weakening of the AMOC via freshwater forcing in the North Atlantic Ocean (McManus et al., 2004; Praetorius et al., 2008; Clark et al., 2012). However, paleoceanographic evidence for these abrupt ocean circulation changes and their relationship with freshwater forcing remains elusive. Moreover, the representation offreshwatersources,timingandmagnitudevariesbetweenclimatemodels(cf.Zhanget al.,2014),asdoesthesensitivityofthesimulatedAMOCtofreshwaterperturbations(Fig. 12)(e.g.Rahmstorfetal.,2005). Amongthemostcontroversialissuesthatarisewhensimulatingpastclimatechangeusing proxy-based reconstructions, is that climate models require unrealistically large freshwater perturbations and that the longevity of the simulated stadial is entirely dependent upon the duration of the applied freshwater forcing (Ganopolski and Rahmstorf, 2001; Knutti et al., 2004). This implies that the stability of the AMOC is 22 F. Muschitiello systematically overestimated in climate models (Hofmann and Rahmstorf, 2009), precludingthecomprehensionofpossiblebi-stableregimesoftheoverturningcirculation system. Figure11.AbruptsouthwardprogressionoftheNorthAtlanticsea-icemargininresponseto enhanced sea-ice production in the Barents as simulated with the EC-Earth climate model under Pre-Industrial boundary conditions. a, Averaged annual sea-ice anomalies for model years430-450relativetotheclimatologyfortheyears200-400(justpriortothecoldevent). b,asin(a)butfortheaveragedannualanomalyfortheyears450-550,duringthepeakof the cold event. Values are expressed as a fraction of 1. c, Atlantic Meridional Overturning Circulation time series in sverdrups (Sv= 106 m3 s-1). The black line shows the maximum overturningandtheredlinetheoverturningstrengthat36°Natdeeperdepth(1600m).The coloredareasshowtheperiodsofintegrationfor(a)and(b)aswellasthereferenceperiod usedtoestimatetheanomalies.ModifiedafterDrijfhoutetal.(2013). Figure 12. Sensitivity of the North Atlantic thermo-haline circulation to freshwater forcing. The panel shows hysteresis curves found in coupled3-Dglobaloceanmodels.Circlesindicate the present-day climate state of each model. ModifiedfromRahmstorfetal.(2005). 23 Deglacial impact of the Scandinavian Ice Sheet on the North Atlantic climate system Therefore,littleisknownonthetransientbehaviouroftheAMOCanditstruesensitivity tofreshwaterforcingduringthelastdeglaciation.Furthermore,duetolargeuncertainties with the marine reservoir effect, it is still unclear whether shifts to a weak-state of the AMOCarethetriggeroramereresponsetochangesassociatedwithothercomponentsof theclimatesystem,infirstinstanceseaiceandatmosphericcirculation. InPaperIV,thedeglacialhistoryoftheupperlimboftheAMOC–theNorthAtlanticinflow totheNordicSeas–isreconstructedusingSSTandδ18OrecordsfrommarinecoreMD992284 (Fig. 3, 13), which is located at the gateway for transport of oceanic heat flux to northern latitudes. By synchronizing the upwind SST signal to downwind hydro-climate records (δD on lipid biomarkers) from the terrestrial site of Atteköps Mosse, it was possible to provide core MD99-2284 with a precise atmospheric-based 14C chronology. Moreover,weinfermeltwaterdischargefromtheScandinavianIceSheettotheadjacent NordicSeasbyreconstructingtheregionalevolutionofthemarine 14Creservoirage(Fig. 13), a proxy indicating the contribution of continental freshwater containing dissolved inorganiccarbonwithlow14Cactivity. The reconstructions indicate a substantially unaltered strength of the North Atlantic InflowandAMOCthroughoutthewarminterstadialphase(∼14,700-12,900cal.yearsBP). This is surprising as the marine records suggest a significant and variable outflow of freshwater from the Scandinavian Ice Sheet (Fig. 13), with a total ice-melt discharge estimatedat∼2.8±0.3msea-levelequivalent(Hughesetal.,2015). Bycontrast,synchronouslywiththestartofGS-1,theinflowofsalinesubtropicalwaters criticallydecreasestogetherwithamajorweakeningoftheAMOC(Fig.13).Thisimpliesa tight coupling between ocean and atmospheric perturbations within the North Atlantic system. Thepreviousstudy(PaperIII)providesevidenceforacatastrophicdrainageofmeltwater fromtheScandinavianIceSheetandaplausiblephysicalmechanisminvolvinginjectionof extraseaiceintothesubpolargyre.InthelightoftheapparentinertiaoftheAMOCsystem tolong-termfreshwaterfluxes,itislikelythattheabruptshifttoaweakAMOCmodeat the onset of GS-1 was the result of sea-ice–wind feedbacks rather than changes in buoyancy forcing. In conclusion, despite sudden meltwater pulses from ice-dammed continental lakes deliver only a fraction of freshwater to the oceans as compared to the amount that decaying ice sheets can release over millennia altogether, the associated feedbackshaveacrucialimpactontheAMOCmeanstateandcanlikelypushthesystem overitstippingpoint.Thisnewframeworkforunderstandingrapidclimatemodeshiftsis acriticalbenchmarkfordesigningfutureclimatemodelexperiments. 6.Terrestrial-marineproxycomparison To evaluate the regional significance of the hydro-climate records generated within this thesisproject,theδDvaluesderivedfromtheaquaticcomponents(δDaq)oflakesediments from Hässeldala Port (HÄ) and Atteköps Mosse (ATK) are here compared to each other andotherregionalmarinerecords. In Paper II and III it was argued that, at least prior to the onset of the Holocene, the ΔδDterrestrial-aquaticvaluesfromHÄandATKrecordsweremainlydependentuponthelocal hydrological conditions rather than upon changes in local vegetation. Across the key climatictransitionsdiscussedinthisthesis,e.g.attheonsetofALandYD,theΔδDterrestrial- 24 F. Muschitiello Figure 13. a, Sea-surface temperatures (SST) and b, near-pycnocline ice-volume corrected seawater δ18O from marine core MD99-2284 compared to Atlantic Meridional Overturning Circulationproxyreconstruction(McManusetal.,2004).Shadingsreflectthe68%CIbased on both analytical and chronological errors. c, Reconstructed North Atlantic surface ocean reservoir 14Cages(ΔR).Whitedotsrefertore-evaluated 14Cdatafromthewesterncoastof Norway (Bondevik et al., 2006) and red dots refer to 14C data obtained from marine core MD99-2284. The shading reflects uncertainties in the reconstruction based on dating and measurementerrors(95%CI).DarkershadingindicatesmorelikelyΔRvalues.Themeanis shown as a red line. The white arrow shows the present-day ΔR (Bondevik et al., 2006). d, Varve-(red)andradiocarbon-based(blue)ageestimates(±1σ)forthedrainageoftheBaltic Ice Lake (Muschitiello et al., 2015b, 2015c). e, Volume evolution of the Eurasian ice sheets expressedinmsea-levelequivalent(Hughesetal.,2015).SIS,ScandinavianIceSheet;SBKIS, Svalbard, Barents and Kara Sea Ice Sheet; BIIS, British-Irish Ice Sheet. Greenland stratigraphic events are displayed on the IntCal13 time scale. Colored bars show the two majorphasesofsurface-watercoolingintheNorwegianSea. 25 Deglacial impact of the Scandinavian Ice Sheet on the North Atlantic climate system aquaticrecordsindicatehydrologicalshiftstowardsrelativelydrierconditionswithrespect to the preceding period. These hydrological shifts would have enhanced evaporative deuteriumenrichmentoflake-waterandthusofδDaq.However,theδDaqrecordsfromHÄ andATKshowlargeisotopicshiftstowardsmorenegativevaluesatthestartofbothAL andYD,andYD,respectively.Thisimpliesthatlocalhydrologicalprocessesoperatedinthe opposite direction of the observed shifts in δDaq and were likely not a primary factor in controllingtheδDcompositionoflake-water.Therefore,theδDaqrecordsshouldreflect,to alargeextent,changesintheisotopiccompositionoftheprecipitationsource,whichare primarilyrelatedtothesurfacehydrographyoftheeasternsectoroftheNordicSeas(see section3.1.). The δDaq signals, which only overlap for ∼2,000 years (∼14,000-12,000 cal. years BP), display a systematic offset but also some differences, with HÄ showing a larger negative shiftinδDaqvaluesatthetransitioninto,andduringthefirsthalfoftheYDrelativetoATK (Fig.14). TheδDaqvaluesatHÄare∼30-70‰lowerthanthoseatATK(∼60-100‰whencompared withHÄ’sδDrecordfromC21alkanes).ThesystematicδDoffsetbetweenthetworecords may arise from three possible reasons or from a combination of these: the Rayleigh rainout effect (Gat, 1996); differences in the δD composition of the moisture source contributing precipitation to each site; differences in the length of the thawing season between the eastern and western coast. In southern Sweden, the first case is a common phenomenon,wherebytheisotopicratiosofprecipitationdecreaseastheairmassesmove eastwardsfromthewestcoast(Jonssonetal.,2010).Forinstance,instrumentaldatashow that the present-day annual difference in δD values of precipitation across Northern Europe(e.g.fromtheBritishIslestotheBalticcountries)rangesbetween-65and-110‰ (Bowen,2003). Astothesecondreason,itisplausiblethataportionoftheprecipitationdeliveredtoHÄ– especially in summer when the circulation was more anticylonic (Muschitiello et al., 2015c) – was inherited from the leeward side of the Scandinavian Ice Sheet. Here, the BalticIceLakeconstitutedamoisturesourcecharacterisedbyheavilydeuterium-depleted meltwater as opposed to the North Sea seawater on the windward side of the ice sheet. This could explain systematically lower δD values of precipitation integrated in HÄ’s sedimentswithrespecttoATK.However,theisotopicsignalincorporatedbytheaquatic vegetation reflects the δD composition of the lake-water during the growing season, i.e. latespring/earlysummer(Sachseetal.,2004).Itwasdemonstratedthattheearlysummer isotope signatures of lake-water in southern Sweden are strongly affected by the replenishment of the local aquifer by meltwater associated with winter and spring snowfall(Muschitielloetal.,2013). It is therefore likely that δD values recorded in HÄ’s sediments are reminiscent of the isotopiccompositionoftheprecipitationcarriedbythedominantwesterlywindsduring thecoldseason,whenthecirculationwaszonal.Altogether,thiswouldmeanthattheδD offsetobservedbetweenATKandHÄduringthelastdeglaciationisprobablymorerelated to the distillation of moisture transported across southern Sweden from the sea to the inlandratherthantoasignalfromeasternsources. Another possible scenario involves relatively shorter summers at HÄ relative to ATK. In fact,heretheproximitytothecoldBalticIceLakewaterbodymayhaveledtoalonger 26 F. Muschitiello Figure 14. a, Schematic upper circulation of the North Sea and location of marine coring sitesJM99-1200(EbbesenandHald,2004),MD99-2284(thisthesis),HM79-6/4(Karpuzand Jansen, 1992), and terrestrial sites Atteköps Mosse, and Hässeldala Port (this thesis). b, ComparisonbetweenterrestrialδDrecordsofprecipitation,annualsea-surfacetemperature (SST) and spring sea-ice reconstructions from the Norwegian Sea. The data set of marine coreMD99-2284wassynchronizedtoAtteköp’stimescaleasexplainedinPaperIV.Theage models of core JM99-1200 (Ebbesen and Hald, 2004) and HM79-6/4 (Karpuz and Jansen, 1992) were established using the available 14C dates corrected for variations in regional reservoirageasreconstructedinPaperIV.(Captioncontinuesonpage29) 27 Deglacial impact of the Scandinavian Ice Sheet on the North Atlantic climate system thawing season and colder summers as compared to the western coast of Sweden. This may have postponed and lengthened the inflow of deuterium-depleted snowmelt in the lake discussed above, thereby resulting in relatively lower δDaq signature at HÄ. Such hypothesis is supported by chironomid-inferred summer temperature records, which indicate a minimum of 1-2 °C colder summers at HÄ (Muschitiello et al., 2015c) than at ATK (not shown) during ∼14,000-12,000 cal. years BP. In particular, variable summer temperature differences could also provide an explanation for the observed transient discrepanciesinδDbetweenthetwosites. FurthercluescanalsobedrawnbycomparingtheδDaqrecordstootherregionalmarine proxyreconstructions,sinceweexpectthehydro-climatedatasetstoalsodependonthe seasurfacetemperatureconditionsatthemarinesourceofprecipitation.Inthefollowing, three precisely dated marine sediment records from the Norwegian Sea are introduced (Fig.14a,b):coreJM99-1200(EbbesenandHald,2004),coreMD99-2284(PaperIV),and coreHM79-6/4(KarpuzandJansen,1992). ThechronologyofMD99-2284isbasedonthesynchronizationtoATK’sδDrecordsandit hasbeendiscussedinPaperIV.TheradiocarbonchronologyofJM99-1200andHM79-6/4 have here been revisited by using a Bayesian age-depth model (Bacon), the Marine13 calibration curve, and by applying marine reservoir correction factors according to the reconstructionpresentedinPaperIV.TheSSTreconstructionsfromcoreJM99-1200and MD99-2284arebasedonforaminiferaassemblagesandindicatessub-surfaceconditions. Bycontrast,theSSTreconstructionfromHM79-6/4isbasedondiatomassemblagesthat yield temperature conditions at a shallower depth (in the photic zone). The core JM991200 has also been studied for biomarker-based reconstructions of sea ice conditions (Cabedo-Sanzetal.,2012). Ainterestingfeaturethatarisesfromthecomparisonbetweentheterrestrialandmarine records is that the δD record from ATK tracks the sub-surface water temperature signal reasonably well (Fig. 14b), which provided the foundations for the synchronization discussedinPaperIV.Bycontrast,theshiftsinδDvaluesfromHÄsedimentsappeartobe in better agreement with variations in seasonal sea-ice cover and surface water temperatures in the Norwegian Sea at the start and throughout the YD (Fig. 14b). Especially,thepre-YDcoolingstartingat∼13,000cal.yearsBP,whichhasbeenidentified in the HÄ isotopic records (Paper II), is evident in the marine records, whereby a shift towardsnear-permanentsea-iceconditions(Cabedo-Sanzetal.,2012)andcolderSSTsare initiatedafewcenturiesearlierthanthestartofGS-1(Fig.14b). Thequestionarisesastowhyhydro-climateproxiesfromATKaremoresensitivetosea sub-surface conditions, while records from HÄ respond to surface dynamics. During the deglaciation,ATKwaslocatedclosetothecoastattheheadofalongfjordsystem,whichis today known as the Skagerrak-Kattegat (Fig. 3). Modern high-latitude fjords, such as in Greenland or in Scandinavia, are highly stratified, with warm subtropical waters flowing beneathashallowbrackishorfreshwaterlayer (Stigebrandt,1981;Straneoetal.,2010). Thesurfacewatersareconstantlyreplenishedthroughoutthefjordsandatthehead,the deeper seawater can be entrained in the shallow fresher layer owing to the turbulence inducedbytheactionofwindandbyrunofffromland(Stigebrandt,1981).Thiscanresult instrongmixingandupwellingofsub-surfacewatersattheheadofthefjord. The presence of the Scandinavian Ice Sheet during the last deglaciation would have promotedtheoccurrenceofdescendingkatabaticwinds,sweepingoffseaice,icebergsand ultimately freshwater from the northern coasts of the Skagerrak. Moreover, a stronger 28 F. Muschitiello anticyclonic circulation regime over the ice sheet during summer (Muschitiello et al., 2015c) may have been more conducive to steer meltwater westwards, out from the Skagerrak-Kattegat complex, into the Norwegian Sea and eventually northwards (Fig. 14a). In particular, this paleo-hydrographic flow associated with a stronger summer meridional circulation is consistent with the spatial pattern recorded with instrumental data(Fig.4b). Suchapatternofcirculationwithinthefjord,withfreshoutflowatthesurfacebalancedby saltier, sub-surface inflow and vertical mixing at the head, can explain why the δD signatures of precipitation at ATK track the sub-surface water temperature. Bearing in mind that the δD signal at ATK is broadly consistent with other independent lacustrine δ18O records from the south-western coast of Sweden (Hammarlund and Keen, 1994; Hammarlund and Lemdahl, 1994), this implies that the marine moisture source for ATK andthesurroundingareawaspredominantlyassociatedwiththeheadoftheSkagerrakKattegat fjord. Conversely at HÄ, further to the east, the δD signatures of precipitation probablyintegratedaseasurfacesignaloverarelativelywiderregion. The mechanism involving the integration of vertical physical properties of different seawater masses by precipitation in two independent terrestrial records would not just provideanadditionalexplanationforthetransientdifferencesbetweenδDvaluesatATK andHÄ.ItcouldalsoprovideafurtherexplanationfortheobservedoffsetinδDvaluesof precipitation discussed above. Since the moisture carried to ATK was inherited from relatively closer marine waters (i.e. relatively warmer and more saline), the associated isotopic signatures were more enriched in D as compared to HÄ, where the moisture originatedfromabroaderandoverallfreshersource. Localversusregionalhydro-climatesensitivitybetweenthetwoSwedishsiteswouldalso clarifywhyreconstructedsummertemperaturesatATK(chironomid-inferred;notshown) donotagreewiththerecordfromHÄ(Muschitielloetal.,2015c)orotherNorthEuropean deglacialtemperaturerecords(Heirietal.,2007;EliasandMatthews,2014).Incontrast, thetemperaturerecordfromHÄ(Muschitielloetal.,2015c)isingoodagreementwiththe summer temperature history in the British Isles (Elias and Matthews, 2014), thus supportingtheregionalsignificanceoftheclimaterecordsgeneratedatthislattersite. Figure14(continued).TheSSTrecordsfromcoreJM99-1200andMD99-2284arebasedon foraminiferaassemblagecountsandreflectsub-surfaceconditions,whereastherecordfrom core HM79-6/4 is based on diatoms and reflects surface conditions. Note the general agreement between the two sub-surface temperature records. The sea-ice reconstruction (Cabedo-Sanz et al., 2012) was obtained from the same sedimentary record of core JM991200 and is based on the PBIP25 index, defined as the abundance of the biomarker brassicasterol versus IP25. Greenland stratigraphic events according to the IntCal13 time scale(Muscheleretal.,2014)arealsodisplayed.IsotopeandSSTrecordsarepresentedwith barsindicating±1σerrorassociatedwithbothanalyticalandchronologicaluncertainty.The age uncertainties of the sea-ice record are the same as for the SST reconstruction of core JM99-1200. The ages for the Vedde Ash and Saksunarvatn tephra used to construct the chronologies were based on age-modeling results from Lohne et al. (2013). The interval characterisedbyfrequentsea-icecoverandlowSSTsintheNorwegianSeaishighlightedin grey. 29 Deglacial impact of the Scandinavian Ice Sheet on the North Atlantic climate system Inconclusion,thiscomparativeanalysisbringsupthecomplexityofinterpretingδDpaleorecordsasafunctionofhydro-climatefactorsandhighlightstheneedtocoupleempirical reconstructionswithisotope-enabledclimatemodelsthatcanhelpaccountingforshiftsin theprecipitationmoisturesource. 7.Currentworkandunpublisheddata 7.1.ImpactoftheScandinavianIceSheetonregionalclimateusingaspatiallyhighresolutionclimatemodel The on-going work related to the research presented inthis thesis focuses on the use of spatially high-resolution climate model simulations (using CESM1.0.5) to investigate YD summer climate over Europe (Schenk et al., in preparation). The model simulations are employed to understand the importance of northward heat transport and ocean-toatmosphereheatfluxovertheNorthAtlanticunderaweakAMOCregime,suchasduring the YD (McManus et al., 2004). The main motivation of this study is to shed light on the elusive regional climate impact of a cold North Atlantic Ocean during a period with high and increasing insolation forcing (Fig. 1). Moreover, the simulations, which include new data-calibrated ice-sheet model reconstructions, are being compared to an extensive Europeandatasetofproxy-basedquantitativetemperaturerecords(chironomids,aquatic pollen,terrestrialplantmacrofossils). Preliminaryresults(Schenketal.,inpreparation)suggestthat,converselytoearliercoarse resolution climate simulations of the YD (e.g. Renssen et al., 2015), the competition betweenacoldoceanandhighorbitalforcingresultsinwarmersummerconditionsover Eurasia,withtheexceptionofcoastalandhighelevationsites(Fig.15a).Inspiteof10°C colderSSTsintheNorthAtlantic,thepresenceoftheScandinavianIceSheetsignificantly impactstheregionalatmosphericflowpreventingcoldwesterlywindsfromtheAtlanticto penetrateinland,resultinginnortherlyflowovertheNordicSeasandincreasedblocking circulation over the ice sheet. In turn, this circulation pattern induces a high radiative balance of surface energy fluxes, thus explaining the summer warming in continental Europe,whereasthecoldoceanhasagreaterinfluencealongthecoastalregions(Fig.15a). The plant macrofossil-based temperature reconstructions from the proxy compilation (MinnaVälirantaandMaijaHeikkilä–UniversityofHelsinki)arebroadlyconsistentwith the pattern of warming observed in the simulations (Schenk et al., in preparation). However, chironomid-based reconstructions show summer cooling during the YD. The simulationssuggestthatthiscanbeexplainedbyaprogressiveshorteningofthegrowing season in summer (Fig. 15b). In particular, a longer and colder spring can postpone meltingoflakeice,whichinturncoolslakesevenduringsummer.Hence,thissnowmelt processmayhaveimportantimplicationsfortheinterpretationoflake-sedimentbiological proxies. 7.2.SensitivityoftheScandinavianiceSheettovolcanicforcing Further on-going work surrounds the comparison of the new annual glacial-varve chronologypresentedinPaperIIIwithGreenlandglaciochemicalrecordsattheendofthe lastdeglaciation.Theglacialvarvechronology(Muschitielloetal.,2015b),whichhasbeen synchronizedtotheGreenlandIceCoreChronology2005(Rasmussenetal.,2006)viathe common Vedde Ash time marker, offers for the first time the opportunity to compare 30 F. Muschitiello meltingratevariationsoftheScandinavianIceSheettoice-corevolcanicrecordsatannual resolution. The comparison shows that years characterised by anomalous Scandinavian Ice Sheet melting coincide in time with volcanic eruptions as recorded in ice-core aerosol loading records(Fig.16)(Zielinskietal.,1996).Byusingoutputfromclimatemodelsimulations (Jungclaus et al., 2010), it is shown that explosive volcanic eruptions can generate an instantaneous climate response in the North Atlantic, which results in a substantial decreaseinseasonalprecipitation. Theseresultsmaysuggestthatice-sheetmeltanomaliesidentifiedinthevarverecordare potentially a result of snow-albedo feedbacks that lowered the reflectance of bare ice under reduced snow accumulation conditions, a mechanism particularly efficient in drivingice-masslossinmodernglaciersandicesheets(Francouetal.,2003;VanTrichtet al.,2016). Thisanalysismayprovidethefirstevidenceforthesensitivityofcontinentalicemassesto volcanicforcingduringiceageterminations.Thishasimportantimplicationswithrespect to the tremendous amount of meltwater trapped by recessing ice sheets and its pivotal roleonrapidclimatechange. 7.3.Unpublisheddatasets InadditiontotheproxyrecordsandmodelsimulationspublishedinPaperIIandIVand presented in this report, there is a large number of unaccounted data and model output behindthisproject.InthefollowingIoutlinesomeoftheunpublishedproxydatasetsthat weregeneratedduringmyPhD. Hässeldala’ssedimentswerethoroughlyinvestigatedforisotopeandbiomarkeranalysis. Unpublished data comprise δD records from C20 Highly Branched Isoprenoids, δ18O on cellulose,d-excess,andδ13Conasuiteofn-alkanes. UnpublisheddatafromAtteköp’ssedimentscompriseX-rayfluorescencedata,δDrecords from C20 Highly Branched Isoprenoids and chironomid-based temperature records (investigator: Tomi P. Luoto – University of Helsinki) and plant macrofossil-based temperaturerecords(MinnaVäliranta–UniversityofHelsinki). 8.Futurework Future work will primarily focus on a set of sensitivity climate experiments using the CCSM3 model (Frederik Schenk – Stockholm University). The simulations will aim at a betterunderstandingofphysicalprocessesbehindtheoccurrenceofan“earlycooling”in NorthernEuropeandahydro-climatedipoleacrosstheNorthAtlanticduringtheLateAL, asobservedinPaperIandIIofthisthesis,respectively. Thesensitivityexperimentswillbedesignedbyprescribingdifferentamountsandratesof freshwater forcing from the Laurentide Ice Sheet and Scandinavian Ice Sheet. Additional sensitivityexperimentswillberuntobetterexaminetheimpactofseaiceanomaliesinthe NorwegianSeaandBarentsSeaduringthesameperiod(FrancescoPausata–Stockholm University). Time slice experiments (12,000 versus 13,000 years BP) will also be run with the ECHAMisomodel(JesperSjolte–LundUniversity)toallowforaisotopeproxy-model 31 Deglacial impact of the Scandinavian Ice Sheet on the North Atlantic climate system Figure 15. a, Modeled July surface air temperature anomaly between the Younger Drays stadialandtheprecedingwarmAllerødinterstadial(12,000minus13,000modelyearsBP). b,Sameas(a)withsummertemperatureanomaliesfromproxydataoverlain.c,Sameas(a) for Growing Season Length. Positive (negative) values in (c) indicate a longer (shorter) growing season of land vegetation. The start (end) of the growing season is defined as the periodcharacterizedbysixstraightdayswithatemperatureabove(below)5.5°C(Nemani etal.,2003).Significancelevelsareindicatedbyblackstippling(95%CI).DatafromSchenk etal.(inpreparation). comparisonatthetransitionintotheYDstadial. Finally, using the state-of-the-art, high resolution (1/6°, ∼18 km), coupled ocean sea-ice circulationmodelMITgcm(AlanCondron–MassachusettsInstitutesofTechnology),aset oftransientsimulationswillbeperformedtoresolvethecirculationoftheoceanandsea iceassociatedwiththedrainageoftheBalticIceLakeattheonsetoftheYDstadial.These 32 F. Muschitiello simulations will shed light on the trajectories of meltwater at a resolution 10-15 times higherthanotherGCMsandhelptounderstandtheroleoftheBalticIceLakedrainageon AMOCstability. Future proxy analyses will focus on generating new isotope records from southern Scandinavia that allow extending the terrestrial-marine synchronization discussed in Paper IV and thus the chronology of marine core MD99-2284 into the Holocene. If this attemptturnsouttobesuccessful,bydatingnew 14CsamplefromcoreMD99-2284,itwill be possible to extend the North Atlantic marine reservoir age record into the Early Holocene. Figure 16. Comparison between the new Late Glacial varve-clay chronology from Östergötland and Greenland ice-core volcanic records. a, Varve thickness standardized anomalies of the portion of the varve chronology composed of 55 overlapping varve diagrams(Muschitielloetal.,2015b).b,VolcanicsignalrecordedinGISP2(SO42-)andNGRIP (H+)icecores.VolcanicaerosolsulphatesdepositedattheGISP2sitearepresentedbothas absolute values (orange) and as flux (red). The GISP2 record was synchronized to the GreenlandIceCoreChronology2005timescaleviacommonvolcanicmarkers(Rasmussenet al., 2007, 2008). c, Results from Monte Carlo significance tests of synchronicity between exceptionallythickvarveyearsandvolcanicevents.Intheleft-handpanelsynchronicitywas tested using 1,000 permutations of the varve thickness anomalies. In the right-hand panel synchronicity was tested using 1,000 individual realizations of the varve thickness record withsimilarrednoisespectralcharacteristics.Thegreenareaindicatestheregionabovethe 95th percentile and the red stars indicate the estimated agreement (%) between varve anomaliesandvolcanicevents.Exceptionallythickvarveyearscorrespondingtoanomaliesin atmospheric sulphate loading and the related volcanic event are also indicated. The ‘b2k’ conventionoftheGreenlandIceCoreChronology2005ishereconvertedintoBP(1950years AD). 33 Acknowledgements Funding for this project was provided by the Swedish Nuclear Waste Management Company(SKB).AdditionalfundingwasprovidedforvariousactivitiesbytheBolinCentre for Climate Research and the Department of Geological Sciences, Stockholm University. I wouldalsoliketoacknowledgefinancialsupportthroughINTIMATEcost-action. Alongwithinstitutions,therearepeople.Firstandforemost,Iwouldliketothankmyteam of advisors. Barbara, like the last deglaciation, our relationship has been a bumpy road withalotofupsanddowns.IamawareIhaven’tbeentheeasieststudenttodealwithand due to my temper and my over-enthusiasm I caused you, to put it mildly, quite some annoyance.Still,youalwaysremainedprofessional,supportedme,advisedmewisely,and eventually even indulged to some of my wacky ideas. You have also been my harshest criticandreviewer.Ibelievethishasmademeamorethoroughresearcherandcertainlya betterwriter.NowIknowthatifIcansellyouanidea,well,thenIcansellittoanyone.I willalwaysbegratefultoyouforlettingmeplaywiththemudinyoursandbox,whichhas madetheselastfouryearsoneofthemostfantasticexperienceofmylife. Thank you Rienk for passing the “art” forward to me, for the passionate training and discussionsinsideandoutsidethelaboratoryrooms.Isotopesruleandwillalwaysdo! A great thanks to my parents for their love and the ceaseless support throughout my elevenlongyearsasauniversitystudent.Mom,IpromiseI’llfindajobnow. I would also like to thank another very special group of people: the Saarikoskis. Sure enough,youarelikeasecondfamilytome.Thankyouforallthelove(andtheawesome breakfasts). Thanks to all my friends. I apologize I cannot name you all here one by one. However, I must mention some leading figures. Robert, I could have not possibly accomplished this PhDwithoutyou.Youhavebeenagreatsourceofmotivationformywork.Thankyoufor all the English synonyms, the “sandwiches”, the endless party nights and all the (mis)adventures we have been through together. Please, forgive me for the noise in Norway. Thank you James for never declining an invitation to the pub. With you I had some of the most exciting (and productive) science talks in front of a pint that I’ve ever had. Cat, Lisa, Patrik, you have been true friends and definitely a constant that have accompaniedmethroughoutthisjourney.Thankyouforallthefunyoubrought. Among my colleagues, there are a number of awesome characters that I would like to acknowledge.Innoparticularorder:thehard-workingandrelentlessguysfromthefourth floor,Hugsy,Reuby,Cliff,littleAlex,andBarbarella;myofficematesNatalia,Pedro,Francis, Liselott,andthebeautifulprincessMoo;andofcoursethesilicafriendsWimandPatrick. OutsidethedepartmentIhavealsobeenfortunatetomakesomegreatfriendswhohave remindedmethatlifeisnotjustaboutpaleo-problems.ThankyouJustine,Elissa,Matilda, Friedman,Nico,Jan,Riccard,JosefandMarco. Onalesshumannote,Imustthankanumberofanthropomorphicdeitiesfortheconstant presencewhenthingswentwrong,aswellassushiandnachosforthenecessaryboostof nutrientsthathelpedmetodeliverthejob. To these folks and the rest of the departmental staff: this time would not have been the same without you. A special thank goes to Jane, Carina, Anna and Heike for the help in gettingmylaboratoryworkmovingdayafterday.Thankyouforcopingwithanannoying andattimescrankyItalianwhodoesnotalwaysfeelcomfortableinalabcoat. 34 IwouldliketothankSvanteBjörckfortheinspirationandfruitfuldiscussionsthroughthe last years (and also for writing my Swedish abstract!). A huge thanks to Eve for always havinganopendoorandagoodpieceofadvice.I’malsoverygratefultoMartinJakobsson, who granted a young and unskilled limnologist with the opportunity to experience an unforgettableresearchcruiseacrosstheArcticOcean. 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