Chronology and paleoenvironments during the late

Chronology and paleoenvironments during the late Weichseliandeglaciation of the southwest Iceland shelf

ANNE JENNINGS, JAMES SYVITSKI, LINDA GERSON, KARL GRO¨ NVOLD, ASLAUG GEIRSDOTTIR,JORUNN HARDARDOTTIR, JOHN ANDREWS AND SVEINUNG HAGEN

Jennings, A., Syvitski, J., Gerson, L., Gro¨nvold, K., Geirsdo´ttir, A., Hardardo´ttir, J., Andrews, J. & Hagen, S.2000 (September): Chronology and paleoenvironments during the late Weichselian deglaciation of the south-west Iceland shelf.Boreas,Vol. 29, pp. 167–183. Oslo. ISSN 0300-9483.

Foraminifera, sedimentology, and tephra geochemistry in core 93030-006 LCF from the southwestern Icelandshelf were used to reconstruct paleoenvironments between 12.7 and 9.414C ka BP. Seismic-reflection profilesplace the core in glacial-marine and marine sediments within one meter of the underlying glacial till. Forami-nifers in the earliest glacial-marine sediments provide a record of ice-distal conditions and immigration ofslope species onto the shelf in association with warm Atlantic water. Meltwater increased during the Allerødunder a weakened Atlantic water influence. Arctic conditions began by 11.1414C ka BP with an abrupt in-crease in meltwater and near exclusion of boreal fauna from the shelf. Meltwater diminished in the earlyYounger Dryas, coinciding with sea-surface cooling between 11.14 and 10.514C ka BP. A slight warming re-corded in the uppermost glacial-marine sediments was interrupted by an inferred jo¨kulhlaup event emanatingfrom glacier ice on the Western Volcanic Zone. Retreat of the ice margin from the sea sometime betweenc.10.3 and 9.9414C ka BP coincided with this event. The onset of postglacial marine sedimentation occurredalong with increasing evidence of Atlantic waterc. 9.9414C ka BP and was interrupted by a short-lived Pre-boreal cooling of the Irminger Currentc. 9.91 14C ka BP. Conditions similar to those today were establishedby 9.714C ka BP.

Anne Jennings (e-mail: [email protected]), James Syvitski, Linda Gerson and John Andrews, Cam-pus Box 450, INSTAAR and Department of Geological Sciences, University of Colorado, Boulder, CO 80309-0450 USA; Karl Gro¨nvold, Nordic Volcanological Institute, Grensa´svegur 50, 108 Reykjavı´k, Iceland; AslaugGeirsdottir, Department of Geosciences, University of Iceland, 101 Reykjavı´k, Iceland; Jorunn Hardardottir,Science Institute, University of Iceland, Dunhagi 3, 107 Reykjavı´k, Iceland; Sveinung Hagen, Institute of Geol-ogy and Biology, University of Tromsø, 9037 Tromsø, Norway; received 25th May 1999, accepted 9th Febru-ary 2000

Iceland is located in a climatically sensitive geographicposition relative to atmospheric circulation and thepresent-day marine arctic and polar fronts (Fig. 1).Large changes in the positions of the marine polar frontmapped for the last deglaciation (Ruddiman & Macin-tyre 1981) have also been observed in the latestPleistocene to Holocene sediments preserved in Iceland(Rundgren 1995; Ingo´lfssonet al.1997). The extent andactivity of glaciers in Iceland from the Last GlacialMaximum (LGM) toc.9 ka were affected by changes inthe position of the polar front (e.g. Ingo´lfsson &Norddahl 1994). The lack of radiocarbon dates onland during the LGM indicates that Iceland was largelyice covered, and that the ice-sheet margins weresomewhere offshore. The ice extent during the LGMis often assumed to have been near the shelf-slopebreak, although there is no firm, dated evidence of thisassumption (Denton & Hughes 1981; Ingo´lfsson &Norddahl 1994; Andrewset al. 2000). Radiocarbondating of uplifted glacial-marine deposits and lakesediments indicates a stepwise retreat of the ice marginsfrom the contemporaneous periphery of Iceland fromc.12.7 toc. 9.7 14C ka BP (Ingolfsson & Norddahl 1994and references therein; Geirsdo´ttir et al.1997). Ingolfs-son & Norddahl (1994) suggested that deglaciation

began sometime before 13 ka BP in response to climaticamelioration or sea-level rise, but the evidence for theearliest part of the deglaciation must be locatedoffshore. Thus, there is a gap in the information aboutthe glacial history of Iceland between the LGM and 12.714C ka BP, the earliest evidence of the ice margin onland. In addition, there is a lack of continuous, well-dated, high-resolution records of the changes inenvironmental conditions during deglaciation.

The purpose of this article is to present a continuoushigh-resolution paleoenvironmental record of deglacia-tion of the southwest Iceland shelf between 12.7 and 9.414C ka BP (Fig. 1) based on data collected on cruiseHU93030 of the research vesselC.S.S. Hudson. Thesedata include high-resolution seismic reflection profilestied to sedimentological, tephra, and foraminiferalanalyses of radiocarbon-dated core 93030-006 LCF(Long Coring Facility), a giant piston core fromJokuldjup, Faxaflo´i Bay (Fig. 1).

Physical setting

Jokuldjup is a bathymetric depression in outer Faxaflo´iBay, southwest Iceland shelf (Fig. 1). The Irminger

Current,a branchof theNorth Atlantic Current,carrieswarm, saline Atlantic water over the site (Fig. 1).Temperatureandsalinitymeasurementsin Faxafloi Bayshowawell-mixedwatercolumnwith salinitiesof 35%andtemperaturesbelow50m of 7°C (Helgadottir 1984;Aspreyet al. 1994).

Faxafloi Bay is bounded to the south by theReykjanesPeninsulaand to the north by the Snae-fellsnesPeninsula(Fig. 1). The ReykjanesPeninsulaforms part of the WesternVolcanic Zone (WVZ) ofIceland, which is characterizedby active fissurevolcanismof tholeiitic affinities (Jakobsson& Peters1979).The SnaefellsnesPeninsula,on the otherhand,forms a part of a volcanic zone, which was activemainly during the Pliocene and early to middlePleistocene(Fig. 2; Sæmundsson1979;Lacasseet al.1996).The Katla subglacialvolcanicsystem,which islocated within the EasternVolcanic Zone (EVZ) ofIceland,hasbeenshownto be the sourceof both thebasalticandrhyolitic componentsof the VeddeAsh, amajor componentof Ash ZoneI in the North Atlantic(Lacasseet al. 1995;Gronvold et al. 1995),which waseruptedin c. 10.3ka BP (Bard et al. 1994;Birks et al.1996).

Previouswork on the southwestIcelandshelf

Helgadottir (1984) presentedthe first study of marine

coreson the southwestIcelandshelf.Sheanalyzedthesedimentologyand foraminiferal stratigraphyof fivegravity cores, from a depth-transectin Jokuldjup,Faxafloi Bay. At the time of her study, acceleratormassspectrometry(AMS) dating was in its earliestphases,so no radiocarbondateswere available.How-ever,recentAMS datingof thesecoresindicatesthattheoldestrecordextendsto 10.6 ka BP (Helgadottir pers.comm.1999).The sedimentsin thesecoresarepebblymudwith anarcticfaunalassemblageprior to c. 10.5kaBP,changingto bioturbatedmudwith aborealfaunaby9.3 ka BP. Syvitski et al. (1999)interpretedthehistoryof glaciationof thesouthwestIcelandshelfusinghigh-resolutionseismic reflection profiles collectedduringC.S.S.Hudsoncruise 93030, and Hagen(1995) pre-sented a paleoceanographicstudy of the Holocenesedimentsfrom Jokuldjup using cores93030-006TC(trigger core) and the upper 2.8m of the 93030-006LCF.

Basedon the interpretationof seismicprofiles, ice-marginal depositshave beenreportedat the southernand southwesternshelf edge, and at intermediatepositionsin Faxafloi Bay, as shownin Fig. 2 (Egloff& Johnson1979; Helgadottir 1984). Farther to thenorth, Olafsdottir (1975) mapped a 100-km longbathymetric ridge near the shelf-slopebreak on theoutermarginof theLatra Bank(Fig. 2). This featureisassumedto be an endmoraine,markingthe maximumWeichselianice marginat about18 ka BP (Olafsdottir

Fig. 1. Left: Map showinglocation of Icelandrelative to surfacecurrentsand sea-icelimits (Hurdle 1986).Right: Map of southwesternIcelandshowingthebathymetryof theshelf; thevolcaniczonesaredemarcatedby grayhatchedlines,with WVZ andEVZ denotingWes-tern andEasternVolcanic Zones.Volcanic centersandassociatedfissureswarmsare shownasopencircles.The locationsof Katla (K),SnaefellsnesPeninsula(S), ReykjanesPeninsula(R) andcore93030-006LCF arealsoshown.

168 AnneJenningset al. BOREAS29 (2000)

1975; Einarsson& Albertsson1988; Ingolfsson1991;Syvitski et al. 1999).Syvitski et al. (1999) suggestedthatamorainein Kolluall Trough,slightly northof core93030-006LCF marksa stillstandin theoverall retreatof ice from the last glacial maximumposition(Fig. 1).None of the offshore moraine-inferredaccumulationshasbeendirectly dated(cf. Andrewset al. 2000).

Sea-levelstudieshave been used to constrain thehistoryof deglaciationaroundIceland(e.g.Rundgrenetal. 1997).Thehighestmarinelimit variesasa resultofdiffering glacial loads.A limit of 110m in southernIcelandis associatedwith theYoungerDryas/PreborealageBudi morainal complex (Hjartarson& Ingolfsson1988; Geirsdottir et al. 1997; Ingolfsson et al. 1997;Fig. 2). The marinelimit decreasesto between60 and80m in westernIceland(Ingolfssonetal. 1995).A localmarinelimit of 80m in westernIcelandhasbeenrelatedto a glacialadvanceto theoutercoastduringtheOlderDryas,the Skipanesevent(Ingolfsson1988;Fig. 2). Aregional marine limit of 60m in westernand south-westernIcelanddatesto retreatfrom a YoungerDryasreadvance,which Ingolfsson (1988) refers to as theSkorholtsmelarevent.A rapid relative fall in sealevelfrom 40toÿ25m in Faxafloi Bay(Thors& Helgadottir1991)datedsometimebetween10.3and9 kaBPatteststo thevery rapid isostaticreboundduring andafter the

deglaciation(Ingolfsson et al. 1997). However,sedi-mentstudieswithin LakeHestvatn(42m a.s.l.)andtheBudi morainal complex in the southernlowlands ofIcelandindicatethatthelowlandsweresubmergeduntilapproximately9.0 ka BP, basedon the transitionfrommarinesedimentto lacustrinesedimentatÿ27m withinLake Hestvatn(Geirsdottir et al. 1997). Jokulhlaupsplayeda major role in thefinal collapseof the ice fromtheBudi morainecomplex(Geirsdottir et al. 1997).

Thecorein this study,93030-006LCF, lies spatiallyandtemporallybetweenthepotentialLGM positiononthe outershelf and the YoungerDryas/PreborealBudimorainecomplexon land, and thus shouldrecordtheevents and environmental conditions that occurredbetweenthe formationof thesefeatures.

Materialsandmethods

Huntec and Airgun seismic reflection records andsedimentcoreswereamongthe typesof datathatwerecollectedonthe1993C.S.S.Hudsoncruise(Aspreyetal.1993).Detailsconcerningthegeophysicaldataaregivenin Syvitski et al. (1999).A seriesof different typesofcoreswastakenatasinglecoringsitein Faxafloi Bay inorder to ensurecollection of the completesediment

Fig. 2. Map of southwesternIcelandshowingbathymetry,icemarginalfeatureson theshelf,OlderDryas(OD), YoungerDryas(YD) andPreboreal(PB) icemarginsandthemaximummarinelimit (demarcatedby grayhatchedlines) between10.3and9.6 kaBP. The locationof core93030-006LCF andthe seismicprofilein Fig. 3 arealsoshown.

BOREAS29 (2000) LateWeichseliandeglaciation,SWIcelandshelf 169

section,includinga box core,Lehighcore,anda LongCoringFacility (LCF) core,which comprisesaninstru-mentedpistoncorerandatriggercorer.Core93030-006LCF was collected from 254m water depth at64°17.06'N, and 24°12.42'W. The 13.17-mlong corewassplit in half longitudinallyduringthecruise.Thehalfto be archived was visually describedfor color andsedimentarystructures(Hein,in Aspreyetal. 1993),andphotographed.The working half of the core wasimmediatelysampledfor bulkdensityat20-cmintervals.Foraminiferalandsedimentsubsamplesspanninga2-cminterval were taken every 20cm, exceptwhere litho-facieschangesoccurred;in thesecases,samplesweretaken above and below the lithofacies change.Thearchive half of the core was X-rayed at the BedfordInstituteof OceanographyCoreRepository.

Foraminiferal samplesfrom 93030-006LCF werepreparedat the Instituteof Arctic andAlpine Research(INSTAAR) using the methods detailed elsewhere(Jenningset al. 1998).Benthicandplanktonicforami-niferawerepickedfrom the>125mm fractionandwereidentified using a stereomicroscope.Benthic speciespercentagesand planktonic and benthic foraminiferalabundanceswerecalculated.Climatic tolerancesof thebenthicspeciesweredeterminedfrom previouswork onthe Iceland shelf (Helgadottir 1984; Jenningset al.1994)andelsewherein thenorthernNorth Atlantic (cf.Knudsenet al. 1996). Planktonic foraminifera wereidentified to species,but thereweregenerallytoo fewpresent in the benthic foraminiferal splits to allowpercentagesto be calculated.However, in 12 of thesamples,asmanyas100planktonicforaminiferscouldhavebeenobtainedby examiningtheentiresample,andin the uppermosttwo samplesof the LCF core, therewereabundantplanktonicforaminifers.

Sedimentological analyses including grain size,carbonatecontent,organic carboncontent,and mass-magneticsusceptibilitywere completedat INSTAAR.The grain-size distributions of the samples weremeasuredusinga combinationof dry sieving(Ro-tap)for particlesizesgreaterthan44mm, andsedimentation(Sedigraph5000DParticleSizeAnalyzer)for particlessmaller than 44mm. The carbonate content wasmeasuredusing the Chittick apparatus(Dreimanis1962).

Tephrashardsfrom a tephralayer at 2.78m in thecorewereisolatedfrom thebulk>125mm sandfractionprepared for foraminiferal analysis. The chemicalcompositionof 47 shardswasanalyzedby microprobeat theNordic VolcanologicalInstitute,Iceland.

AMS radiocarbondates were obtained using theUniversity of Arizona Tandem Accelerator MassSpectrometer.All reportedageswerecorrectedfor anassumedmarinereservoirageof 400years,in ordertobe consistentwith most other Icelandic studies(e.g.Sveinbjornsdottir et al. 1993).However,this correctionundoubtedlyvariedduringthelastdeglaciation(Bardetal. 1994).

Results

Seismicstratigraphy

Airgun profiles in Jokuldjup indicate over 80m ofsedimentsabove volcanic bedrock. Syvitski et al.(1999)documentedthreeintervalsof ice-contactsedi-mentsand glacial-marinemuds within this sequence,suggestingat leastthreeglacial advance/retreatcyclesin the basin. The higher resolution Huntec profilesprovidedetail of theuppermostcycle (Fig. 3).

Acoustically unstratified sedimentswith irregulartopography,interpretedas ice-contactsediments,areconformablyoverlain by 13m of glacial-marinesedi-ments(Fig. 3). Theglacial-marineunit is characterizedby distinctive low to mediumtone,weak to moderateacousticstratification,anda conformablebeddingstylein which basal topography underlying the unit istranslatedthroughinternalreflectorsto theunit surface.

A two-partunit overliestheglacial-marineunit andiscalledthe ‘transitionalunit’ (Fig. 3). The lower partofthetransitionalunit comprisesa reflectorthatconformsto the surfaceof the underlying glacial-marineunit.This reflector’s coherency and strength graduallyweaken into the deeper water of the trough. Theoverlying part of the transitionalunit hasan irregularsurfaceandthickensto up to 6 m in thedeeperareasoftheshelftrough.It hasmoderatetoneandlacksinternalacousticstratification.The acousticcharacteristicsoftheupperpartof thetransitionalunit stronglyresemblethecharacteristicsof debrisflows.Core93030-006LCFpenetratesthe transitionalunit in a locationwhereit isvery thin. In the corethis correlatesto a 0.76-mthick,tephra-bearing,sandyandpebblyinterval consistingoftwo distinct sedimenttypes.

The transitionalunit is overlain by an acousticallystratifiedunit with moderatelateral thicknessvariationthat representspostglacialmarine sediments(Fig. 3).The postglacial unit consists of moderately weak,closely spacedreflectors of medium tone. The unitonlaps onto the margins of the shelf trough. Theexposedseafloor of the postglacialunit has circularpockmarks,80 to 100m in diameter,as observedinhigh-resolutionsidescansonarimages.The pockmarksmaybeactivesitesof gasor waterescape.

Lithostratigraphy

Stratigraphiccorrelationof the other cores(box core,Lehighcoreandtriggercore)takenat thesamesiteandcomparisonwith the Huntec data indicate that core93030-006LCF collectedsedimentsto within onemeterof the contactbetweenglacial-marineand ice-contactsediments,but that it bypassedtheupper3.20m of thesedimentsection(Fig. 3). Thus,the 13.17-mlong core93030-006 representsfrom 3.20 to 16.37m of theHuntec profile across the site (Fig. 3). The upper1.10m of sedimentin 93030-006LCF was disturbed


during coring and was not sampled.Therefore,ouranalyses on the core begin at 1.10m, which isequivalentto 4.30m belowtheseafloor.Thelithofacieslog andsedimentologicaldataarepresentedin Fig. 4.

13.17–10.7m, Fmd: Bioturbated gray (5Y on theMunsell color chart)clayeysilt with small (2 to 8 mmdiameter)dropstones.10.7–3.02m, Fsd:Bioturbatedgray(5Y on theMunsellcolor chart)clayeysilt with small (2 to 8 mm diameter)dropstonesandindistinct stratification.

Thesetwo lithofacies correspondwith the glacial-

marine seismic unit interpretedon the Huntec high-resolutionseismicprofile. No colorlessplateybubble-wall tephrashardswereobserved,butbrowntephrawasabundantin the sandfraction. The ice-rafteddetritus(IRD) content, defined as the weight percentof the>1 mm grains,is quitelow throughoutthis interval,butrises above9 m depth in the core, and reachespeakpercentagesbetween7 and 5 m (Fig. 4). The totalcarbon percentagesfollow a similar trend with aninterval of higher percentagesbetween9 and 5 m,possiblyreflectingadecreasein sedimentationrateoverthis interval (Syvitski et al. 1990).

Fig. 3. Huntecprofile (upper)andinterpretationof the Huntecprofile (lower) in Jokuldjup showingplacementof 93030-006LCF withinthe profile (PM = postglacial marine unit; GM = glacial-marineunit in the core); pm= circular pock marks, 80–100m in diameter;df = debrisflow depositthickeninginto deeperwater;tr = transitionalreflectorweakeninginto deeperwaterandmarkedby 30%sandcon-tent.


Fig. 4. Lithostratigraphiclog showing lithofacies units, seismicunits (SU, with GM = glacial marine unit; T = transitionalunit; PM = postglacialmarine unit), grain size, weightpercentageof >1 mm grainsinterpretedasice rafteddetritus(IRD), massmagneticsusceptiblity(MS), total carbon,calciteandradiocarbondates(seeTable1). Thedatesusedin thefinal chronologyare representedas black dots.Thoseeliminatedfrom the final chronology(seetext) are shownas gray dots.Sedimentationratescalculatedbetweendatedlevelsare indicatedalong the lines connectingthe dates.Lithofaciescodes:Fs= stratifiedmud; Fb= bioturbatedmud; Ssd= stratifiedsandwith dropstones;Fsd= stratifiedmud with drop-stones;Fmd= massivemudwith dropstones.

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3.02–2.71m, Ssd: Stratified, dropstone-bearing sand.The sandfraction consistsof both basaltic(black andbrown) and rhyolitic (colorless platey bubblewall)tephra shards and black basaltic rock fragments.Stratificationis pronouncedandburrowsareabsent.A0.5-cm thick sand layer occurs at 2.96m. Largedropstonesof up to 3 cm in diameterareconcentratedbetween2.88 and 2.79m. A sandysilt (32.4% sand)tephra-bearinglayer lies between2.78and2.71m.

2.71–2.26m, Fb: bioturbatedsandysilt andclayeysilt,with relatively high sand content between12.1 and17.2%. Sandfraction consistsof both basaltic(blackandbrown) andrhyolitic (colorlessplateybubblewall)tephrashardsandblackbasalticrock fragments.IRD israreor absent.

These two sandy sediment lithofacies, Ssd and Fb,correspondwith the two-part transitionalseismicunitthat separatesthe glacial-marinesedimentsfrom thepostglacialmarineunit on the Huntecprofile (Fig. 3).Bothunitshavehighcontentsof volcanicash,expressedin thehighsandpercentages,but thedifferencesin IRDcontentandstratificationsuggestthat theoriginsof thetwo subfaciesaredifferent. The lower subfacies(Ssd)correspondswith the lower unit of the transitionalseismic units. The overlying massive, tephra-richsubfacies(Fb) correspondswith thedebris-flowunit.

2.26–1.10m, Fs:Massiveto crudelystratifieddark-grayclayey silt and silt with some burrowing structures.Dropstoneswereobservedonly at 1.63m.

This lithofaciescorrespondswith the lower part of thepostglacialmarineseismicunit.

Tephraanalysis

Chemical analysesof tephra shardsfrom the tephralayerat2.78m, thestratified,sandy,pebblytephralayer(Ssd), showed three distinct chemical compositions(Fig. 5; Table 2). Two of thesecomponents,basaltic(basic)andrhyolitic (silicic) glassshards,havechemi-

cal compositionsconsistentwith the end membersoftheVeddeAsh(Norddahl& Haflidason1992;Gronvoldet al. 1995). These grains constitute the earliestobservedoccurrenceof colorless,platey, bubblewallshardsin the core, suggestingthat the glacial-marinesediments below 3.02m probably were depositedbefore the eruption of the Vedde Ash from Katla,southernIceland,atabout10.3kaBP(Bardet al. 1994;Mangerudet al. 1984). A third chemical componentdistinguished at 2.78m is basaltic grains, with acompositionsimilar to the WVZ in southwestIceland(Fig. 2).

Table1. AMS radiocarbondatesfrom 93030-006LCF.

Depth(cm) Sampleno. Reportedage Reservoircorrectedage Speciesdated

1.10–1.12 AA-15701 9825� 95 9425� 95 Melonisbarleeanus1,2

1.63–1.65 AA-15702 10310� 90 9910� 90 M. barleeanus1,2

2.22–2.23 AA-15703 10335� 95 9935� 95 Mixed benthicspecies1,2

2.52 AA-27262 10430� 60 10030� 60 N. labradorica1,2

2.78 AA-20732 11730� 305* 11330� 305* N. labradorica& I. helenae2

4.70 AA-20733 12130� 290* 11730� 290* N. labradorica& E. excavatumf. clavata2

6.22 AA-19915 11830� 125* 11430� 125* Ophiuroidossicles2

6.25 AA-12897 11535� 85 11135� 85 E. excavatumf. clavata2

9.55 AA-20734 12410� 165 12010� 165 Ophioroidossicles2

11.75 AA-20735 12690� 195 12290� 195 N. labradorica2

12.35 AA-20736 12810� 205 12410� 205 E. excavatumf. clavata2

13.17,cc** AA-12896 13105� 85 12705� 85 N. labradorica2

1 Hagen(1995);2 Manley& Jennings(1996);* Dateexcludedfrom the final 14C chronology;** Corecutter.

Fig. 5. Microprobecompositionaldata of tephrashardsat 2.78min 93030-006LCF (Table2) comparedwith the compositionof thebasaltic and rhyolitic grains in GRIP ice core (Gronvold et al.1995).


Chronologyandsedimentationrates

Twelve AMS radiocarbon dates were obtained onbenthic foraminifera (n = 10) and echinoid ossicles(n = 2) (Hagen1995;Manley& Jennings1996)(Table1). Three of the dateswere stratigraphicallyreversedcomparedto underlyingdatesandthereforethesethreedateswere excludedfrom the final chronology(Table1). Given the offsets in the radiocarbontime-scaleduring the deglaciation,causedby changesin oceancirculation and radiocarbonproduction, we did notcalibrateour radiocarbondatesto a calendartime scale(e.g. Bjorck et al. 1996). This approachis consistentwith other deglacialand early Holocenechronologieson Iceland(e.g.Ingolfssonetal. 1997;Geirsdottir etal.1997).Thefirst observationof VeddeAshat thebaseoflithofaciesSsd,thetransitionalunit betweentheglacial-marine and postglacial seismic units, provides amaximumagefor lithofaciesSsdof 10.3ka BP (Birkset al. 1996). We considerthis to be a maximum agebecausewe recognizethat theVeddeAsh in lithofaciesSsd may not representa primary ash-fall deposit.Prevailingwindsduringthetime of theVeddeeruptionmay have excluded the delivery of Vedde Ash toFaxafloi Bay by ashfall, andtheVeddeAsh mayhavebeendeliveredlater by otherprocessessuchasraftingon disintegratingglacierice (cf. Larsenet al. 1998).A14C dateon N. labradorica and Islandiella spp.withinlithofaciesSsdat2.78m wasstratigraphicallyreversed,indicating reworkingof older materialsduring deposi-tion of lithofaciesSsd.Thedateof 10.03� 60kaBPon

N. labradorica from 2.52m in lithofaciesFb is not themost conservativeupperageconstrainton lithofaciesSsdbecausealthoughit is notstratigraphicallyinverted,it is within sediments interpreted as debris flow.Therefore,the upperageusedfor lithofaciesSsdandFb (the two lithofacieswithin the transitionalseismicunit) is thedateof 9.94� 95 ka BP, at 2.22to 2.23m,only 3 cm abovelithofaciesFb. Therefore,lithofaciesSsd(andthe transitionalseismicunit) musthavebeendepositedsometimebetween10.3 and 9.94 ka BP.Sample ages in the core were calculatedby linearextrapolationbetweendatedlevels,and we assigntheageof 10.3ka BP to thebaseof lithofaciesSsd.We donot attempt an estimateof the errors involved withlinear extrapolationas describedin Andrews et al.(1999).

Sedimentationrateswere calculatedbetweendatedlevels in centimetersper radiocarbonyears,and arethereforesubject to errors from radiocarbonplateauxand potential changesin the marine reservoir age.Sedimentationratesrise from 0.28cm/yr at thebaseofthecoreto highervaluesbetween12.4and12.0ka BP(0.50 to 0.79cm/yr), and intermediatevalues(0.38 to0.39cm/yr) between9.55 and 3.02m (Fig. 4). Withtheserapid sedimentationrates,20-cmsamplinginter-vals in the glacial-marinesedimentsrepresentbetween25 and72 14C years.

Using a maximumageof 10.3 ka BP at the baseoflithofacies Ssd and a date of 9.94 ka BP abovelithofaciesFb, sedimentsin the transitionalunit weredeposited with an average sedimentation rate of

Table2. Microprobecompositionalanalysisof tephrain Core93030-006LCF, 2.78m.

SiO2 TiO2 Al2O3 FeOtotal MnO MgO CaO Na2O K2O P2O5 Total

Vedde/Skogar,rhyolitic grains[25] 71.1 0.27 13.7 3.8 0.13 0.25 1.38 5.44 3.3 0 99.37Vedde/Skogar,basalticgrains[5] 47.2 4.54 12.8 14.6 0.23 5.03 10.2 2.87 0.59 0.55 98.6Fissurevolcanics;basaltgrains 51.2 1.65 12.9 12.3 0.21 6.2 11.4 2.44 0.18 0.05 98.52

51.4 1.51 13.3 12.4 0.26 6.54 11.7 2.2 0.03 0.08 99.3351.4 1.4 13.2 12.4 0.22 6.68 11.6 2.25 0.03 0.09 99.2550.4 1.65 13.6 10.2 0.2 6.76 14.2 1.94 0 0 99.0649.5 1.47 14 11.2 0.2 6.8 12.5 2.17 0.11 0.03 97.9850.4 1.31 13.6 11.8 0.23 7.05 12.3 2.04 0.02 0.09 98.849.6 1.49 13.9 11.3 0.19 7.18 12.5 1.94 0.11 0.01 98.250.1 1.46 13.4 11.3 0.22 7.3 12.6 2.38 0.12 0.15 98.9550.3 1.54 13.8 11.6 0.21 7.32 12.6 2.08 0.1 0.07 99.649.1 1.61 14.1 11.5 0.2 7.42 12.5 2.07 0.09 0.01 98.4948.9 1.63 13.3 11.1 0.16 7.65 12.4 2.09 0.09 0.01 97.4149.1 1.53 14.2 11 0.21 7.72 12.4 2.1 0.11 0.06 98.4149 1.53 15 11 0.15 7.78 12.5 2.05 0.07 0.05 99.0448.9 1.55 13.3 11.3 0.21 7.83 12.5 2.12 0.08 0.04 97.7549.8 1.58 14.1 11.3 0.19 7.92 12.4 2.17 0.13 0.09 99.7548.6 1.67 14.3 10.9 0.18 7.99 12.3 2.2 0.06 0 98.249.7 1.63 15.2 11.1 0.17 8.13 12.4 1.96 0.1 0.06 11.42

Vedde/Skogar 71.7 0.31 13.5 3.93 0.11 0.2 1.14 5.3 3.34 0 99.53(Gronvold et al. 1995) 48.2 4.23 11.8 14.1 0.2 4.54 9.31 3.2 0.9 0 96.48


0.22cm/yr. However, the sedimentarystructuresandthehigh,variablesandcontentssuggestthat therewereprobablylarge changesin sedimentationrate over thetime interval representedby the transitional seismicunit.

Thesedimentationratesin lithofaciesFs,thebaseofthe postglacialmarine seismicunit, rise to very highvaluesup to 2.34cm/yr in the early Preborealchron-ozone.The high ratesmust reflect the influenceof theradiocarbonplateau,but thereis independentevidencefor rapid sedimentationincluding the presenceof IRDanda short-livedreturnto glacial-marineforaminiferalfauna.Thesedimentationratefalls to 0.11cm/yr in theuppermostpart of thecore,soonafter 9.9 ka BP.

Foraminiferal biostratigraphy

Seventy-twobenthicand four planktonicspecieswereidentifiedin thecore.Eight informal assemblagezoneswere defined basedon changesin the benthic fora-miniferal faunas.Theseassemblagechangesprovideimportantinformationon the paleoenvironmentalcon-ditionsduring thedeglaciation(Figs6 and7; Table3).In theglacial-marineunit (AssemblageZones1 through5), the two common glacial-marine species in thenorthernhemisphere,CassidulinareniformeandElphi-dium excavatumf. clavata, dominatethe foraminiferalfaunas,supportingtheacousticandlithofaciesdatathatsuggesttheseareglacial-marinesediments(Fig. 6).

AssemblageZone 1, Glacial-Marine Unit: 13.10–11.35m (12.68–12.24ka BP).

Thebenthicfaunacontainsapronouncedcomponentofspecieswith borealand slope-dwellingenvironmentalsignificance,including Pullenia osloensis,Cassidulinaneoteretis, and Stainforthiaconcava. P. osloensisandC. neoteretisarerareor absentfor theremainderof thecore, only returning at very low percentagesin thepostglacial sediments.C. neoteretishas been recog-nized as an early postglacial immigrant into shelftroughsin theBarentsSea,whereit hasbeenassociatedwith the inflow of Atlantic-derivedwaters(Lubinski etal. 1996;Hald & Aspeli 1997).E. excavatumf. clavatarisesfrom low valuesof c.10%to apronouncedpeakofalmost70% at about12.4ka BP. The E. excavatumf.clavata peak coincides with the first increase insedimentationrateat about12.4ka BP.Theconcentra-tion of benthicforaminiferais relativelylow throughoutthe interval. The ratio of the Arctic planktonicforaminifer, Neogloboquadrinapachydermasinistralto thetotal planktonicfaunafluctuateswidely, with thehighestratioscorrespondingto therisein E. excavatumf. clavata.

AssemblageZone 1 is interpreted to reflect arelatively strongpresenceof Irminger Current duringtheBølling chronozonecoincidentwith rapiddeglacia-tion of the shelf. Conditions shifted from a strongAtlantic water influence,reflectedin particularby the

presenceof Boreal speciescommon in the IrmingerCurrenttoday(Helgadottir 1984;Jenningset al. 1994),to a strongglacial meltwaterinfluenceasindicatedbythe dominanceof E. excavatumf. clavata (Hald et al.1994) during an interval of increasedsedimentationrate,and back to a stronginfluenceof Atlantic water.Large swingsin the ratio of Arctic to total planktonicfaunasuggestfluctuatingnear-surfaceoceanconditions.

AssemblageZone 2, Glacial-Marine Unit: 11.35–9.55m (12.24–12.01ka BP).

The distinguishingcharacteristicsof this assemblagezonearea pronouncedrise in E. excavatumf. clavatapercentagesand benthic foraminiferal concentrations.This zonecoincideswith an interval of rapid sedimen-tation ratefrom 12.29to 12.01ka BP. The endof thisrapid sedimentationinterval is markedby the abruptdecreasein N. labradorica, decreasedE. excavatumf.clavata, and relative increasesin C. reniforme,Cibi-cides lobatulus, Astrononion gallowayi and Melonisbarleeanus. Theratio of N. pachydermasinistralto thetotal planktonicfaunafluctuateswidely.

AssemblageZone2, representstheendof theBøllingchronozone.Thedominanceof E. excavatumf. clavatareflectsincreasedinfluenceof glacierice on themarineenvironmentproducingmeltwaterand increasedsedi-mentation.Atlantic water continuedto influence theshelf, as is shownby the presenceof Boreal species.Continuedfluctuationsin the ratio of Arctic to totalplanktonic fauna suggestvariable near-surfaceoceanconditions.

AssemblageZone3, Glacial-MarineUnit: 9.55–6.25m(12.01–11.14ka BP).

The benthicassemblageis dominatedby C. reniformeuntil 7.55m (11.48 ka BP) when E. excavatumf.clavata rises to high percentages.The glacial-marinespeciespercentagestogetherrise as high as 94% at6.25m (c. 11.14ka BP). A. gallowayiandC. lobatuluspercentagesalso rise over this interval, whereasN.labradorica percentagesare very low. Boreal species,M. barleeanus, andS. concavaco-occurfairly consis-tently until 6.25m (11.14 ka BP) when thesespeciesessentiallydisappearfrom theassemblages.TheratioofN. pachydermasinistral to the total planktonic faunadeclinesover this interval.

AssemblageZone3, coincideswith the Older Dryasand Allerød chronozones.The dominance of C.reniforme, the rise in total carbonvaluesandIRD andthe slower sedimentationrate reflect distal glacial-marine conditions with diminished meltwater (cf.Polyak & Mikhailov 1996). Boreal speciesand otherindicatorsof the influx of warm salineAtlantic watersshowa weakvarying influenceof the IrmingerCurrentalong the shelf. The overall decreasein the ratio ofArctic to total planktonicforaminiferssuggestswarm-ing of surfacewatersduring this zone.


Fig. 6. Foraminiferalspeciesdataagainstlithofacies,foraminiferalzonation,andreservoir-correctedradiocarbondatesretainedfor final chronology.Speciesincludedin Borealspeciesarelisted in Table3. The graybox representsthe VeddeAsh-bearingsedimentsof the transitionalunit.

176A

nn

eJe

nn

ing

seta

l.B

OR

EA

S29

(2000)

AssemblageZone 4, Glacial-Marine Unit: 6.25–3.70cm (11.14–10.48ka BP).

Over this interval, percentagesof C. lobatulus, A.gallowayi, and the arctic shelf species Islandiellahelenaeand I. norcrossirise. E. excavatumf. clavatadecreasesfrom valuescloseto 60%at 5.50m to valuesas low as 10% at 3.03m. The trend in C. reniformepercentagesis the oppositeto that for E. excavatumf.clavata, rising from valuesas low as 10% to valuesclose to 60% at 3.03m. Although N. labradoricaappearsto be important over this interval, the strati-graphicallyreversedradiocarbondateat 4.70m, whichrelied mainly on N. labradorica, suggeststhat thisspeciesis probablyreworkedbetween5.30and3.70m.The ratio of N. pachydermasinistral to the totalplanktonic fauna fluctuateswidely, but rises overalltowardhigh values.

AssemblageZone4 coincideswith thefirst half of theYounger Dryas chronozone.The abundant glacial-marine fauna, low percentagesor absenceof Borealfauna, rising Arctic to total planktonic foraminiferal

ratios and the continuedhigh IRD reflect the onsetofcooler oceanconditionswith continuedglacial influ-ence.

AssemblageZone5, Glacial-MarineUnit: 3.70–3.02m(10.48–10.3ka BP).

Coincidingwith the declinein E. excavatumf. clavataandthe rise in C. reniforme, the percentagesof borealspecies,M. barleeanus, andS.concavaincrease.Overthis sameinterval, IRD contentsin thecorefall to verylow percentages(Fig. 4). The ratio N. pachydermasinistral to the total planktonicfaunais high over thiszone, but shows a slight decline to lower valuescorrespondingwith therisein theborealbenthicspeciespercentages.

Assemblage Zone 5 reflects decreasing glacialinfluenceduring the latter half of the YoungerDryaschronozone.Shifts in dominancefrom E. excavatumf. clavata to C. reniforme indicate changesto highersalinity conditions and diminished glacial meltwater(Hald & Steinsund1996).Slight warmingof the shelfbeganby 3.70m (10.48kaBP)markedby theslight rise

Fig. 7. Summaryfigure showing key environmentalproxies of glacial influenceand Atlantic water inflow in 93030-006LCF againstradiocarbonyearsandchronozones(Mangerud1974).Theserecordsarecomparedwith the local glacial recordin Borgarfjordur showinga time-distancediagramof glacieradvancesandretreatsfrom the fjords to the outercoast(Ingolfsson1988).IRD asindicatedby weight% of >1 mm grains;C. neoteretisasevidenceof early immigrationof slope-dwellingspeciesafter deglaciation;Atlantic water influenceasshownby foraminiferal speciesindicative of Atlantic water carriedin the Irminger Current(Boreal speciesas in Fig. 6, plus M. bar-leeanus,P. osloensis,and S. concava); turbid glacial meltwaterindicatedby E. excavatumf. clavata; cold, relatively highersalinity gla-cial-marineconditionsindicatedby C. reniforme; andcold seasurfaceconditionsindicatedby increasingratio of Arctic (N. pachydermasinistral) to the total planktonicforams.SU= seismicunit (PM = postglacialmarineunit; T = transitionalunit; GM = glacial-marineunit)andFZ = foraminiferalzonation.


in Borealbenthicspeciesandotherbenthicwarm-waterindicators.Theratio of Arctic to total planktonicfaunais very high during this zone, showing only a slightdecreasethat correspondsto the rise in Borealbenthicfauna,perhapssuggestinga stratifiedwatercolumn.

AssemblageZone 6, Transitional Unit: 3.02–2.26m(10.3–9.95ka BP).

The samplesfrom lithofaciesSsdandFb showa verymarkedchangein thebenthicforaminiferalfaunafromthe underlyingpebbly,glacial-marinemud. I. helenaeandI. norcrossi(up to 65%)andN. labradorica (up to59%)dominatethesamples,andthetwo glacial-marinespecies,C. reniforme and E. excavatumf. clavata,comprise19% of the fauna,at most. The ratio of N.pachydermasinistralto thetotalplanktonicfaunais 1.0,and the planktonic foraminiferal abundancesare verylow.

The stratigraphicallyinverted age at 2.78m on N.labradorica and Islandiella sppsuggeststhat muchofthe faunain lithofaciesSsdis reworked.However,the

stratigraphically consistent radiocarbon date on N.labradorica at 2.52m in lithofaciesFb indicatesthat asimilar cold, Arctic benthic fauna occupiedthe shelfneartheendof theYoungerDryaschronozone.

AssemblageZone 7, Postglacial Unit: 2.23–1.63m(9.94–9.91ka BP).

In the earliestpostglacialsediments,percentagesof E.excavatumf. clavata, Boreal species,M. barleeanus,and S. concava all begin to rise. At 1.63m, E.excavatumf. clavata and C. reniforme increasewhilethe Boreal speciesdecrease.The ratio N. pachydermasinistral to the total planktonicfaunafollows the trendin thebenthicspecies,with anearlydeclinein theratiofollowed by a well-definedrise.

In AssemblageZone7, theincreasein Borealspeciesandthedecreasein theratioof Arctic to totalplanktonicforaminifers showsthat Atlantic water carried in theIrminger Current becamea strongerinfluenceon theshelf during the earliest Preboreal chronozone.Aresurgenceof glacial-marinespeciescoinciding with

Table 3. Foraminiferalspecieslist including the dominant taxa that comprisethe ‘Boreal’ speciesin Fig. 6. The faunal province andpaleoenvironmentalsignificancceof eachtaxonis given from modernIcelandshelf foraminiferalassemblagedataandthe distributionsofspecieswithin the easternNorth Atlantic faunalprovinces(seeHelgadottir 1984;Knudsenet al. 1996;Jenningset al. 1994).

FaunalProvince(paleoenvironmentalinterpretation)

Benthicforaminiferalspecies

Boreal(Atlantic waterinIrminger Current) Transitional

Arctic(glacial marine) Arctic Indifferent

Angulogerinaangulosa(Williamson 1858) xAstrononiongallowayi Loeblich & Tappan1953 xBuccellafrigida calida Cushman& Cole 1930 xBulimina marginatad’Orbigny 1826 xCassidulinalaevigatad’Orbigny 1826 xCassidulinaneoteretisSeidenkrantz1995 xCassidulinaobtusaWilliamson 1858 xCassidulinareniformeNørvang1945 xCassidulinoidesbradyi (Norman1880) xCibicidesbertheloti(d’Orbigny 1839) xCibicideslobatulus(Walker andJacob1798) xElphidiumexcavatum(Terquem)f. clavataCushman1930 xElphidiummagellanicum(Heron-Allen& Earland1932) xHyalineabalthica (Schroeter1783) xIslandiella helenaeFeyling-Hanssen& Buzas1976 xIslandiella norcrossi(Cushman1933) xMelonisbarleeanus(Williamson) xNonionellaturgida (Williamson 1858) xNonionellairidea Heron-Allen& Earland1932 xNonionellinalabradorica (Dawson1860) xPullenia osloensisFeyling-Hanssen1954 xStainforthiaconcava(Hoglund1947) xStainforthiafusiformis(Williamson 1858) x

Planktonicforaminiferalspecies

Globigerinabulloidesd’Orbigny xNeogloboquadrina pachyderma(Ehrenberg1861)dextral xNeogloboquadrina pachyderma(Ehrenberg1861)sinistral xNeogloboquadrina dutertrei (d’Orbigny) asP–D* intergrade xTurborotaliaquinqueloba(Natland1838) x

* P–Dintergradeis undifferentiatedN. pachydermaandN. dutertrei.


an apparentincreasein sedimentationratemay reflecteitherreworkingof glacial-marinesedimentsduringtheearly postglacialperiod, or a return of glacial-marineconditions during the early Preboreal. The slightincreasein IRD supportsthe interpretationof a partialreturn to glacial-marineconditions,but is not conclu-sive.

AssemblageZone 8, Postglacial Unit: 1.62–1.10m(9.91–9.4ka BP).

Borealspeciesrise to 87%, the ratio of Arctic to totalplanktonic fauna declines,and glacial-marinespeciesdisappear from the shelf. Benthic and planktonicforaminiferal concentrationsrise dramatically. Thisassemblagemarkstheonsetof fully postglacialmarineconditions with a strong influence of warm Atlanticwaterin the Irminger Currenton theshelf between9.9and9.7 ka BP.

Discussion

Origin of the transitionalunit; sedimentaryevidenceof a jokulhlaup

Thesedimentaryorigin of thetwo-parttransitionalunitthat separatesthe glacial-marineand postglacialsedi-mentsis importantin the interpretationof thedeglacia-tion of thesouthwesternIcelandshelf.Multiple linesofevidence (stratigraphical, seismic, sedimentological,geochemical,chronological,biostratigraphical)suggestthat this unit representsa marked change in thesedimentaryenvironment,possiblyacatastrophicevent.

Prior to depositionof thetransitionalunit, glacial icehaddirectly influencedthemarineenvironment;afteritsdeposition,thedirectinfluenceof glacialicewaslargelyabsent.Interpretationof the seismicprofiles suggeststhatthetransitionalunit wasdepositedin two phases:anearly phase combining rain-out of sediments andsedimentgravity flows, followed by a phaseof debrisflow deposition.In thecore,the two phasesinterpretedfrom seismicprofiles are representedby two distinctlithofacies;a lower, stronglystratified,very sandyunitwith large dropstones,and an overlying unstratified,bioturbated,sandyunit lacking coarseIRD.

The high sandcontentof the transitionalunit is aconsequenceof an abruptinflux of basaltic(basic)andrhyolitic (silicic) glassshardsof VeddeAsh aswell asbasalticgrainsderivedfrom thefissurevolcanicsof theWVZ (Fig. 5). The concentrationof the VeddeAsh inthe sand indicates that the transitional unit cannotsimply representwinnowing of the shelf. The strati-graphicallyreversedradiocarbondatewithin lithofaciesSsd indicatesreworking of older foraminifera duringdepositionof the lower subunit.

Oneinterpretationof theorigin of thetransitionalunitwhich is consistentwith all of theevidencepresentedisthat it representssedimentsdepositedas a result of a

catastrophicglacialoutburstflood (jokulhlaup)beneaththe glacier over the WVZ. Suchan eventcould havebrokenup themarineicemarginandcausedit to retreatontoland,removingits directinfluencefrom themarineenvironment.During theearlierphaseof suchanevent,aburstof sediment-ladenmeltwaterandicebergswouldhave carried abundant sediments, including tephrashards,in suspensionandin turbidity currentsonto theshelf.Previouslydepositedshelfsedimentswould havebeenentrainedas well, as evidencedby the stratigra-phically reverseddate within the lower subunit. Asubsequentphaseof debris flow depositionprobablyimmediatelyfollowed the initial event.

Although the Veddetephracomprisescloseto 50%of thetephragrainsin thetransitionalunit, a jokulhlaupfrom thesubglacialKatla volcanois anunlikely originfor thisdeposit.Faxafloi Bay is topographicallyisolatedfrom Katla; a jokulhlaupfrom Katla would travelto thesouthratherthanto thewestinto Faxafloi Bay.A morelikely explanationis that VeddeAsh wasdepositedasair-fall tephraontothesurfaceof theglacialice thatwasdraining into Faxafloi. Coincidentvolcanic activity inthe fissurevolcanic zone produceda jokulhlaup thatcarried fissure volcanic materials into Faxafloi in acatastrophic event. Vedde Ash would have beenpassivelytransportedon the ice during suchan event.Lacasseetal. (1996)correlatedvoluminousignimbritesfound in SouthIcelandto the VeddeAsh eruptionandindicated that this eruption may have generatedpyroclastic flows and jokulhlaups that initiated sedi-ment-gravityflows into the deepsea,suggestingthatsucheventshavebeenreportedduring approximatelythesametime interval.

Thereareuncertaintiesin the timing of the inferredjokulhlaup. The rhyolitic grains constitute the firstoccurrenceVeddeAsh, which hasbeendatedto 10.3ka BP (Bard et al. 1994).However,we cannotprovethat thedelivery of VeddeAsh to Faxafloi Bay wasbyash fall from the primary eruption. The date on N.labradorica from the overlying lithofacies Fb of10.03� 60 ka BP is too young to representprimarydepositionof VeddeAsh. In addition,lithofaciesFb isinterpretedto correspondto the debrisflow depositinthe transitional seismic unit. It should have beendepositedrapidly and should contain reworkedsedi-mentsaswell. Therefore,the mostconservativeupperageestimateof the transitionalunit is derivedfrom thedate of 9.94� 95 ka BP, 3 cm above the boundarybetween the transitional unit and the postglacialsediments.Without additional radiocarbondatesfromnew cores,we cannotfurther constrainthe ageof thisevent. We can say only that it must have occurredbetween10.3and9.94ka BP.

The agemodel for 93030-006LCF is basedon theassumptionthat lithofacies Ssd is contemporaneouswith the VeddeAsh. This assumptionis supportedbytheobservationthatsynchronous,or verycloselyspacedin time volcaniceruptionsin theWVZ andKatla area


reasonablescenario.Increasedvolcanicactivity duringthe lastdeglaciationon Icelandhasbeencloselylinkedwith pressurereleaseon magmachambersresultingfrom diminishingglacial loads(e.g.Sejrupet al. 1989;Sjøholmet al. 1991;Sigvaldasonet al. 1992).Further-more, sedimentfrom Lake Hestvatnin SouthIcelanddatedfrom the end of the YoungerDryaschronozoneinclude both the Vedde Ash and coevally formedtephras that are chemically affiliated with intenseeruptioneventswithin the WVZ (Hardardottir et al. inpress).

Onsetof deglaciationof theshelf

Givenits stratigraphicpositionof only about1 m aboveice-contactsediments,the basaldateof 12.7ka BP oncore 93030-006LCF puts a close constrainton thetiming of deglaciationof Jokuldjup. The basalfaunalzoneof the coredid not havea typical ‘ice proximal’assemblage.Boreal species and the slope-dwellingspecies,C. neoteretiswere common, reflecting boththe presenceof the Irminger Currentduring the initialdeglaciationandimmigrationof C. neoteretisonto therecentlydeglaciatedshelf.Slight isostaticdepressionofthe shelf may havecontributedto the penetrationof adeeperwatermassimmediatelyaftertheiceretreat.Theglacial-marinespeciesC. reniforme occurredin rela-tively low numbers,but a largepercentagespikeof E.excavatumf. clavatapunctuatesthe interval, reflectingthe influence of turbid meltwater delivered by theretreatingice (Hald et al. 1994).

Otherstudiesshowthat the ice marginhadretreatedto coastalareasandreadvancedto neartheperipheryoflandafter retreatfrom theLGM configuration(Ingolfs-son1988).Syvitskiet al. (1999)notedthatdeglaciationmusthaveproceededextremelyrapidly, becausedatesin raisedmarine sedimentsin Borgarfjordur indicatethatinitial deglaciationtherewasaccomplishedby 12.5ka BP.To accomplishsuchrapid ice retreat,calvingofthe ice margin must havebeenthe dominantablationmode.

Although the observations in the core can beexplainedby rapid deglaciationby calving, they aretroublesomefor another reason. Most other arcticshelves had begun to retreat from their maximumLGM positions by c. 14.5 ka BP (e.g. Andrews &Tedesco1993;Polyaket al. 1995).Directly acrosstheDenmarkStrait, the GreenlandIce Sheethadretreatedrapidly from its maximumpositionon theshelfedgebyc. 14.5 ka BP (Stein 1996; Smith et al. 1998), butretreatedmoreslowly from the inner shelf.

We do not know whetherthe retreatdocumentedinJokuldjup representscontinuous retreat from a lateWeichselian margin near the shelf break, or if itrepresentsretreatfrom an intermediateposition land-wardof theshelfbreak.If theformerweretrue,thentheice sheetwould havestayedat the shelf edgevery latecomparedto othermargins.Morainesmappedby Egloff

& Johnson(1979)at intermediatepositionson the SWIcelandshelf (Fig. 2) mayhaveformedat theLGM, orthey may representstillstands or slight readvancesduring thedeglaciation.

Paleoenvironments12.7–9.4ka BP

Core93030-006LCF providesa ‘continuous’recordofIceland shelf paleoenvironmentsduring the Last Ter-mination, between12.7 and 9.4 ka BP. In Fig. 7 wesummarizethe interpretationsof the environmentalconditionsin Jokuldjup between12.7 and 9.4 ka BPby showing the variations in the key environmentalproxiesagainstradiocarbonageandregionalandlocalclimatic andglacial events.

The Bølling chronozone,was an overall warminterval, during which the recently deglaciatedshelfwas under the influenceof a strong flow of Atlanticwater in the Irminger Current.The stronginfluenceofthe IrmingerCurrentwasinterruptedbetween12.6and12.4 ka BP and 12.2 and 12.0 ka BP by glacialmeltwater influx. The Kolluall moraineoffshoremaybecorrelativewith thesefeatures,but it is asyetundated(Syvitski et al. 1999).The timing of increasedglacialconditionsbetween12.2 and 12 ka BP precedestheOlderDryas-agedSkipanesglacialeventin Borgarfjor-dur (beginningby 12 ka BP andculminatingat 11.7kaBP).However,theseeventsoverlapif atwo-sigmaerroron thedatesis used.

Between12.0 and 11.1 ka BP, spanningthe OlderDryas and Allerød chronozones,cooler conditionsreflect distal glacial-marineconditions with a weak,sporadic influence of the Irminger Current and aprogressivewarmingof surfaceor near-surfacewaters.The glacial meltwater influencebeganto increaseat11.4kaBP.Theoccurrenceof glacial-marinesedimentswith boreal-arctic to arctic transitional molluscanfaunasin Borgarfjordur during theAllerød chronozonec. 11.7 ka BP and 11 ka BP (Ingolfsson 1988) isconsistentwith the paleoenvironmentsin core 93030-006 LCF. Stratigraphy,chronologyandfaunalcontentof theFossvogurmarinesedimentsindicateatemporaryexpansionof glaciers into marine environmentwithelevatedrelativesealevel in theReykjavık regionneartheendof theAllerød chronozone.

Core93030-006LCF showsinitiation of theYoungerDryascoolingat 11.14ka BP. Theglacial influenceonthemarineenvironmentdiminished,butcoolconditionspersisted,with a slight influence,or absenceof Atlanticwater in the Irminger Currentbetween11.14and10.5ka BP. This cold interval correspondswell with theSkorholtsmelarevent(11 to 10.3ka BP). Theseresultsare also consistentwith data from Fossvogur,SWIceland, where glacial-marinesedimentswere beingdepositedduring the late Allerød/earlyYoungerDryas(Sveinbjornsdottir et al. 1993;Geirsdottir & Eirıksson1994). Geirsdottir & Eiriksson (1994) concludedthatduringtheYoungerDryas(at leastfrom 11 to 10.55ky


BP accordingto the list of datesin Sveinbjorndottir etal. 1993)the sedimentsreflect increaseddistancefromthe sedimentsource(i.e. glacier front) and continuedmarinetransgression.

From10.5to 10.3ka BP, theglacial influenceon theshelf, including glacial meltwater, diminished. Thewater column was stratified, with warmer Atlanticwaters penetratingonto shelf again near the seabed,butsurfaceconditionsremainingcold.Thetiming of theonsetof subsurfacewarmingat 10.5 ka BP is derivedfrom the assumptionthat thefirst occurrenceof VeddeAshin thecoreis contemporaneouswith eruptionof thetephra.If this tephrawere deliveredcloserto 10.0 kaBP, thenthe onsetof warmingwould be later aswell,which would be moreconsistentwith other recordsofthe durationof YoungerDryascooling indicating thatthe Vedde Ash was erupted during a cold interval(Haflidasonetal. 1995;Hald& Aspeli1997).However,raisedmarine recordsfrom Iceland show glacier iceretreatduringthe latterhalf of theYoungerDryas(e.g.Ingolfsson1988;Geirsdottir & Eiriksson1994)with aboreal–arcticmolluscanfauna inhabiting the inshoreareas(Eirıkssonetal. 1997).Eirıkssonetal. (1997)alsonoted an early warming during the Younger Dryas,whichtheyattributedto anearlyinflux of warmAtlanticwater.However,in the 93030-006LCF, the warmingappearsto haveterminatedduring the inferredJokulh-laupeventin thevery latestpartof theYoungerDryas.In addition, recent work by Eiriksson et al. (2000)showsa subsurfacewarming at the sametime on thenorthernIcelandshelf.

Between10.3 and 9.94 ka BP, a jokulhlaup eventbeneaththe glacier on the WVZ is inferred to havebrokenup themarineicemarginandcausedit to retreatontoland,removingits directinfluencefrom themarineenvironment.Arctic conditionsexistedontheshelfafterthe jokulhlaup, as indicated by the dominance ofIslandiella spp.andArctic to total planktonicratiosof1.0 (Figs6, 7).

During the early Preboreal chronozone,Atlanticwatercarriedin the Irminger Currentincreasedon theshelf, coinciding with a partial return to loweredsalinity, glacial-marine conditions. On the basis ofisotopeanalysesand planktonic foraminiferal assem-blagedatacenteredoverthesameintervalin the93030-006 LCF, Hagen(1995) reporteda 6°C cooling in theIrminger Current. The concurrenceof the planktonicand benthic data strengthensthe evidence for aPreborealcooling on the Iceland shelf. Geirsdottir etal. (1997) indicate rapid accumulationof sedimentsalong the southern Iceland coast during the earlyPreborealin responseto the retreatof glacial ice fromthe Budi morainal complex. Ice retreat from thePreborealice margin could be recordedin FaxafloiBay, but thedatafrom thecorearenot conclusive.

The onsetof postglacialmarine conditionswith astronginfluenceof warmAtlantic waterin theIrmingerCurrenton theshelfbeganby 9.7 ka BP.

Conclusions

1. Jokuldjup was deglaciatedat c. 12.7 ka BP. Therapid deglaciationwas probably accomplishedbycalving of the ice margin.

2. Between12.7 and11.1 ka BP, the shelf wasunderthe competinginfluencesof Atlantic water inflowandinflux of sediment-ladenglacialmeltwaterfromanicemarginterminatingon theinnershelfor in thefjords.

3. YoungerDryas cooling was initiated at c. 11.1 kaBP, as reflectedby little or no influenceof warmAtlantic water.Accordingto thechronologyadoptedin thisarticle,coolingcontinueduntil at least10.5kaBP, whensubsurfacepenetrationof Atlantic watersproduceda slight renewedwarming.

4. The end of glacial-marinesedimentationcoincideswith the first occurrenceof Vedde Ash within atransitionalunit, interpretedasa jokulhlaupdeposit.The inferredjokulhlaupwould haveemanatedfrombeneaththeglacierover theWVZ andis envisionedasplaying a strongrole in the retreatof the glacierice from the marineenvironment.Given the uncer-tainties in dating the transitional unit, this eventoccurredsometimebetweenc. 10.3and9.94ka BP.

5. The onset of postglacial marine sedimentationoccurredalongwith increasingevidenceof Atlanticwater c. 9.94 14C ka BP and was interruptedby ashort-livedPreborealcoolingof theIrmingerCurrentc. 9.91 14C ka BP. Postglacialmarine conditionscharacterizedby astronginfluenceof warmAtlanticwaterin the Irminger Currentbeganby 9.7 ka BP.

Acknowledgements. – The researchpresentedin this article wassupportedby USNationalScienceFoundationgrantATM-9531397.Thedatafrom thecorewill besubmittedto theNOAA PaleoclimateDatabase.We greatly appreciatethe assistanceof R. Kihl, whocarried out the sedimentologicalanalyses,and N. Weiner, whohelpedwith the faunal analyses.W. CunninghampreparedFig. 4.M. Smithprovidedthelithofacieslog. Discussionswith M. Hald,G.Helgadottir andO. Ingolfssonwerevery helpful.K. L. Knudsenandan anonymousreviewerofferedhelpful criticism of the manuscript.

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Chronology and paleoenvironments during the late