coherent patterns of ice-rafted debris deposits in the ... · coherent patterns of ice-rafted...

Coherent patterns of ice-rafted debris deposits in theNordic regions during the last glacial (10^60 ka)

Mary Elliot a;b;*, Laurent Labeyrie b;c, Trond Dokken d, Sandrine Manthe e

a LDEO, Rt. 9W, Palisades, NY 10964, USAb LSCE, Av. de la Terrasse, 91198 Gif sur Yvette, France

c Universite Paris-Sud, Departement des Sciences de la Terre, Orsay, Franced UNIS, P.O. Box 156, 9170 Longyearbyen, Norway

e DGO, Av. des Facultes, 33405 Talence Cedex, France

Received 7 March 2001; accepted 23 October 2001

Abstract

We have synchronized records of ice-rafted rock debris deposits of three sediment cores from the Norwegian Sea andthe Irminger Basin during the last glacial period from 10 to 50 ka by combining the use of radiocarbon dates andadjustments of physical properties. Our synchronized records indicate that layers rich in ice-rafted debris were depositedthroughout the Nordic regions at times near to synchronous with the major collapses of the Laurentide ice sheet: duringthe Heinrich events. There are also millennial-scale, coherent and near to synchronous deposits of ice-rafted rock debrisinto the Norwegian Sea related to repetitive changes of the flux of icebergs from the Fenno-Scandinavian. Thecorrelation with the cold phases of the Dansgaard^Oeschger temperature record points to a close coupling betweenatmospheric temperature oscillations and variations of iceberg fluxes into the Norwegian Sea during the last glacial.Variations in atmospheric circulation patterns bringing moisture supply to high latitudes and the distribution of thismoisture over the different Northern Hemisphere coastal ice sheets and ice shelves could be controlling both the timingof ice sheet advances and the flux of iceberg to the open ocean. ß 2001 Elsevier Science B.V. All rights reserved.

Keywords: Heinrich events; ice sheets; Norwegian Sea; magnetic susceptibility

1. Introduction

Numerous archives such as ocean sediment de-posits and ice core records have given evidence ofabrupt, large amplitude and global climate reor-ganizations, which occur every 1^3000 yr, e.g.[1,2]. These studies point to the existence of an

internally driven mode of climate change, whichoperates on millennial-scale and persists throughglacial and interglacial periods [2,3]. During thelast glacial, this mode of climate variability is ex-pressed by large amplitude atmospheric temper-ature oscillations recorded in the Greenland icecores [1], the Dansgaard^Oeschger (DO) oscilla-tions. During the same period, from 10 to 60 ka,sediment records from the Norwegian Sea [4^6]and from the northern North Atlantic [7,8] giveevidence of repetitive millennial-scale variationsof ice-rafted rock debris (IRD) deposits. These

0012-821X / 01 / $ ^ see front matter ß 2001 Elsevier Science B.V. All rights reserved.PII: S 0 0 1 2 - 8 2 1 X ( 0 1 ) 0 0 5 6 1 - 1

* Corresponding author. Tel. : +1-845-365-8768;Fax: +1-845-365-8165.

E-mail address: [email protected] (M. Elliot).

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coarse rock fragments were transported to theopen ocean by drifting icebergs. Some of theseIRD layers, known as the Heinrich events (HE)[9,10], occur every 5^7 ka and correspond to ma-jor collapses of the Laurentide and other North-ern Hemisphere ice sheets [7,11^13]. HE aremarked by drastic modi¢cations of sub-polarNorth Atlantic sea surface hydrology and deep-water circulation: lowered sea surface salinity, re-duced sea surface temperature (SST) [14] and ¢-nally a reduced formation or even a shut down ofNorth Atlantic Deep Waters (NADW) in theNorth Atlantic Ocean [15^17]. More detailed rec-ords revealed the existence of high frequency pe-riods of IRD deposits, intercalated between theHE. These IRD layers (or detrital events (DE)to distinguish them from the HE) showed notabledi¡erences in petrology re£ecting a di¡erentsource of icebergs [7]. They were associated withcold SST in the sub-polar North Atlantic [8,17^19]. However, the negative planktonic N18Oanomalies associated with these IRD layers wereobserved only at high latitudes in the NorwegianSea and close to Iceland and were of smaller am-plitude [4,5,20]. Similarly, the reduction of deep-water formation during these events was smallerthan those observed during the HE [17]. Thenorthward migration of southern source deepwaters appears to a¡ect only the deep-watermasses below 4000 m [21].

The aim of this paper is to understand betterthe relationship between ice sheet dynamics andthe millennial-scale climate £uctuations duringglacial periods. Our approach is to precisely testthe timing of periods of increased iceberg £uxesfrom the di¡erent Northern Hemisphere ice sheetsto the open ocean and the abruptness of thechanges in £uxes. Two models can be proposed.The ¢rst one would be characterized by a more orless constant £ux of iceberg, from the di¡erentoutlets of continental ice sheets, to the openocean. SST would then control the distributionof drifting icebergs and IRD deposits, in the Nor-dic regions and in the sub-polar North Atlantic.The timing of IRD deposits in areas where SSTvaried little would be stochastic and reveal nostructure that could thereafter be correlated toother climate proxies, such as that suggested by

[22]. A second model would be characterized bycoherent and repetitive variations of the £ux oficebergs from the continental ice sheets. In thiscase we should be able to trace synchronous andwell-de¢ned layers rich in IRD from the sourcearea into the open ocean. In this paper, wepresent a detailed study of the relationship be-tween the Nordic regions' continental ice sheetshistory (£uxes of icebergs and timing of thechanges in £uxes) during the last glacial. Wehave synchronized three records of IRD depositsin the Norwegian Sea and in the Irminger Basinusing their physical parameters. We were thusable to closely compare the timing of the IRDdeposits at each location. We further comparedthe timing of the IRD deposits in this area withthe timing of the HE in the sub-polar North At-lantic and with the DO temperature £uctuationsrecorded in Greenland.

2. Material and results

This study compares high-resolution records ofplanktonic foraminifera (Neogloboquadrina pachy-derma s.) N18O and N13C, coarse (s 150 Wm) rockfragment abundance or ice-rafted debris andwhole core magnetic susceptibility (MS) obtainedfrom sediment cores located in strategic areasacross the Nordic regions (Fig. 1; Table 1). TheMS records have previously been presented in de-tail in [23,24].

We use records of IRD and MS of core MD95-2010, located o¡ the coast of Norway [6]. IRDcounts were performed every 1^0.5 cm corre-sponding to an average resolution of V55^60yr. Cores MD 95-2009 (this study) and ENAM93-21 [25] have been obtained at the same loca-tion close to the Faeroe Islands. Measurements ofMS and N. pachyderma s. N18O and N13C wereobtained from core MD 95-2009 and have beenused for the validation step (see Section 4.1). N.pachyderma s. N18O and N13C measurements wereobtained every 10 cm corresponding to an averageresolution of 650^700 yr. This 23.95 m long corespans the past glacial^interglacial cycle, but wepresent only the upper 15.95 m which covers thepast 80 ka (see Fig. 2). We use the MS and IRD

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M. Elliot et al. / Earth and Planetary Science Letters 194 (2001) 151^163152

records of the twin core ENAM 93-21 [25] tocompare the timing of the IRD deposits in theNorwegian Sea (see Section 4.2). IRD countswere performed with an average interval of 4 cmcorresponding to an average resolution of 125 yrand are presented in [4,25]. The MS records ofthese twin cores show identical records (see Figs.2 and 3). These Norwegian Sea sites were chosenas they will preferentially record the past historyof the Fenno-Scandinavian ice sheet (FIS). This issupported by the proximity of these sites to theFIS but also by analyses of the lithic fraction ofnearby core ODP 644 [26].

Measurements of MS, N. pachyderma s. N18Oand N13C and IRD have been obtained fromcore SU 90-24 located o¡ the Greenland coast.IRD and N. pachyderma s. N18O and N13C weremeasured every 2 cm corresponding to a resolu-

tion of 125 yr. This site was chosen as it willpreferentially record changes of iceberg £uxesfrom both the local ice sheets and the out£owof icebergs from the Nordic regions into theNorth Atlantic Ocean. In core SU 90-24, theIRD layers are associated with an increase in vol-canic glass originating from Iceland [20]. Smallquantities of detrital carbonate, observed duringthe later phase of the HE [27], appear to originatefrom the Laurentide ice sheet and could have beentransported from the Arctic Ocean, via the EastGreenland current (EGC), or from the south viathe Irminger current (IC).

3. Age models and synchronizing method

Numerous 14C dates were measured frommono-speci¢c samples of N. pachyderma s. oneach core (see Table 1) which provide an absolutetimeframe to our study. However, the errors as-sociated with 14C dating of ocean sediment coresare at least 1000 yr at 25^30 ka, due to the inher-ent error of the method, a lack of constraints onreservoir ages [28], and the calibration methods[29]. These errors lead to subjective correlationsof climate records, which exhibit millennial-scalevariability in distant geographical regions. Ourapproach has been to adjust and re¢ne the 14Cage models by using the physical properties ofthe studied sediment cores. MS records of sedi-ment cores from the northern North AtlanticOcean and Norwegian Sea along the modernpath of the NADW show patterns of variations,which closely match the DO temperature oscilla-tions. This correlation was ¢rst identi¢ed by[4,25]. Further studies of benthic foraminiferal as-

Table 1Core locations, number of accelerator mass spectrometry (AMS) 14C dates for each core, average sedimentation rates

Core Lat. Long. Depth Number 14C dates Mean sedimentation rates Reference(³N) (³E) (m) (cm/ka)

SU 90-24 62³04 337³02 2100 32 15 [17]MD 95-2010 66³41 4³33 1230 28 40^12 [6]MD 95-2009 62³44 3³59 1020 16 23^20 This studyENAM 93-21 62³44 3³59 1020 13 23^20 [4]

The 14C dates of cores SU 90-24, ENAM 93-21 and MD 95-2010 are presented in [4,6,20].

Fig. 1. Position of cores used in this study. The path ofmodern NADW is indicated by the large dark gray line, sur-face currents, the IC and EGC, are indicated by the thinlines.

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M. Elliot et al. / Earth and Planetary Science Letters 194 (2001) 151^163 153

semblages [25], grain size distribution [30], mag-netic properties [24], have since shown that thesevariations of MS appear to be caused by changesin the intensity of deep boundary currents alongthe modern NADW path. Although variations inMS do not appear linearly related to changes inthe ventilation rates of NADW [17], di¡erent

modes of deep-water formation have been shownto occur with the changes of MS [6]. Low valuesof MS are associated with smaller magnetic min-eral grain sizes and correspond to decreased in-tensity of bottom water currents, conversely highMS values are associated with larger magneticmineral grain sizes and increased intensity of bot-

Fig. 2. Comparison of (A) MS (measured using U-Channel devise, see [23,24] for detail), (B) N13C and (C) N18O records of N. pa-chyderma s. in the 200^250 Wm size range of cores SU 90-24 and MD 95-2009 from 10 to 80 ka. The (D) age di¡erence betweenthe initial age model, built using the 14C dates and the adjusted age model, is indicated in ka. The age model of core MD 95-2009 has been adjusted to ¢t the MS record of core SU 90-24. The age model of core SU 90-24 presented here has been slightlymodi¢ed from the previous one [20]. The ages of the HE, represented by the dark gray bands, were adjusted within error of 14Cdates, to ¢t the average age of the HE estimated from a series of seven cores across the North Atlantic Ocean, e.g. Table 3. Thegood match of N13C and N18O records of N. pachyderma s. at both locations is used to test the validity of the use of MS as atool to adjust age models in this region.

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tom water currents [24]. Our working hypothesisis based on these results and we consider that thevariations of MS are forced by one common fac-tor at each location, changes in the intensity ofdeep boundary currents. We further hypothesizethat the variations of MS, given the high degreeof similarity of the records, oscillate near to syn-chronously with both the north^south oscillationsof the Polar Front in the sub-polar North Atlan-tic and with the DO atmospheric temperature os-cillations. We cannot preclude the existence ofsmall leads or lags of perhaps decades to 100 yr

given some small di¡erences in the MS records atthe di¡erent sites and the overall accuracy withwhich our correlations are obtained. The sedimentcore age models were thus initially built usingradiocarbon dates down to around 35 ka, oxygenisotope stratigraphy and any additional regionalstratigraphic markers, such as ash layers. We thenused the MS records as a tool to adjust age mod-els and synchronize records of three sedimentcores from the Nordic region. Core SU 90-24, inwhich the positions of HE 1^5 have been identi-¢ed [20], was taken as the reference core.

Fig. 3. Comparison of the MS records (A) and coarse lithic content expressed as number of grains (s 150 Wm) per gram of drysediment of the Norwegian Sea cores MD 95-2010 (B), ENAM 93-21 (C). The dark gray bands represent the positions of theHE, the light bands correspond to the positions of the minimum MS values intercalated between the HE.

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4. Results

4.1. Validation

Our ¢rst step has been to compare records ofMS and N. pachyderma s. N18O and N13C of coresSU 90-24 and core MD 95-2009. We have chosento use core MD 95-2009 as its record spans thelast V80 ka while core ENAM 93-21, studied athigher temporal resolution, spans only the lastV60 ka. Both core SU 90-24 and core MD 95-2009 have numerous 14C dates (Tables 1 and 2). Acommon stratigraphic time-scale was obtained byadjusting the preliminary age model of core MD95-2009, based on 14C dates, through the correla-tion of MS records (Fig. 2). We observe a goodmatch of the MS records from 10 to 80 ka. Theage shifts applied the initial age model and arewithin the 2c error bars of the 14C control pointsof core MD 95-2009 (Table 2) except for threecases at around 24, 27 and 46 ka (Fig. 2). How-ever, the age di¡erences between the 14C controlpoints and the adjusted age model are less than500 yr, which is close to the analytical uncer-tainty. The comparison of the N18O and N13C re-cords exhibits a good agreement of the major iso-topic transitions and there is a clear synchronismof some of the detailed low N18O events, corre-

sponding to the meltwater input during the HE(Fig. 2). The timing of both the long-term trendsand many of the detailed events observed in theN13C N. pachyderma s. records, a proxy for sub-surface ventilation [31], also match well at bothlocations. This step illustrates the good agreementbetween isotope stratigraphy and the use of mag-netic parameters as a correlating tool to adjustage models of the Nordic region's sediment cores.

4.2. Timing of the IRD events in theNorwegian Sea

We have applied the same technique to syn-chronize the IRD records of cores MD 95-2010and ENAM 93-21. MS records from these twoNorwegian Sea sediment cores can be correlatedin great detail, particularly from 20 to 50 ka (Fig.3). During this period high values of MS havebeen numbered 3^12 by correlation to DO inter-stadial events 3^12, respectively. From 14 to 20ka, no adjustments were done to initial age mod-els using the MS records, as there are notable mis-matches between the two records (Fig. 3). Thiscould be the result of the shift of the location ofdeep-water formation during the last glacial max-imum (LGM), which occurred south of Iceland[32,33]. Using these synchronize records, the am-plitude and the timing of the deposit of IRDlayers at the two locations are very similar. There

Table 2AMS 14C dates of core MD 95-2009 obtained from mono-speci¢c samples of planktonic foraminifera N. pachyderma s.

Depth Age 2c error(cm) (ka (3400))

0 4.61 0.1450 8.24 0.2

120 10.53 0.2280 14.19 0.26330 15.02 0.26360 16.32 0.28390 18.36 0.32430 21.36 0.4510 23.96 0.46540 25.78 0.5580 27.53 0.58620 28.78 0.62740 31.68 0.8840 34.16 1951 42 2.2

1041 43.1 2.4

Table 3Average age and duration of the HE 1^5 calculated fromseven ocean sediment cores across the North Atlantic (SU90-24, SU 9016, V2381 and ODP 609, e.g. [7] ; Na 87-22 andSU 90-08 [14,16] and CH69-K09, e.g. [19])

Age S.D. Duration S.D.(ka) (ka) (ka) (ka)

H1 base 15.1 0.7 1.6 0.9top 13.4 0.3 1.6 0.9

H2 base 22.1 0.8 1.7 0.9top 20.4 0.1 1.7 0.9

H3 base 27.4 1.6 1.3 1.1top 26.1 0.6 1.3 1.1

H4 base 34.9 1.1 0.8 0.4top 33.9 0.7 0.8 0.4

These ages are presented in 14C ka and have been deter-mined using the same method as that presented in [20].These ages have been obtained by assuming a constant reser-voir age of 400 yr.

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is a comparable overall increase in IRD contentduring the LGM between 22 and 13 ka and lowIRD content at both locations between 22 and 26ka and between 11 and 13 ka.

Two patterns of shaded boxes are used in Fig.3. The darker boxes mark the timing of the HE asde¢ned by the average age and duration of HEestimated from seven sediment cores across theNorth Atlantic Ocean (Table 3) using the sameapproach as in [20]. The shaded box correspond-ing to H5 occurs during minimum MS values pri-

or to MS event 12. IRD increase at both loca-tions, although the base of H5 in core ENAM93-21 is not very well-de¢ned. The H4 period isalso associated with minimum MS values, prior toMS event 8, and an increase in IRD at both lo-cations. In detail the initiation of H4 seems tooccur during MS event 9. The H3 period is asso-ciated with low MS values prior to MS event 4.Only core MD 95-2010 shows a distinct increasein IRD, which occurs towards the end of the in-terval de¢ned by the shaded box. An increase in

Fig. 4. Comparison of the MS records of core SU 90-24 (bold) and the composite MS record of the Norwegian Sea (thin lines)(A) and coarse lithic content expressed as number of grains (s 150 Wm) per gram of dry sediment of the Norwegian Sea compo-site record (B) and core SU 90-24 (C). The dark and light gray bands highlight the same periods of time as in Fig. 3.

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MS values can also be observed between MSevents 5 and 4, during the H3 period, and appearssimilar to the warming numbered 4b [8] and ob-served in North Atlantic Ocean SST records[8,17]. H1 also occurs during a minimum MSand is associated with high concentrations ofIRD. Finally, H2 is associated with high concen-trations of IRD, although they appear to increasegradually. However, this is the only HE which isnot associated with low values of MS.

The lighter bands highlight the periods of mini-mum MS, between the HE, correlated to the othercold stadial periods. Between 26 and 50 ka eachshaded area is associated with an increased con-centration of IRD at each location suggesting thatmost of these distinct layers of IRD appear tohave been deposited simultaneously. Core MD95-2010 shows the most striking correlation be-tween the low MS values and the high IRD con-tent, which was previously documented [6]. Thesupplementary layers observed in core ENAM93-21 are often de¢ned by single peaks (Fig. 3)and could be related to a local signature fromthe Faeroe Islands.

4.3. Timing of the IRD events in theNorwegian Sea and in the Irminger Basin

To compare these records with that of core SU90-24, we constructed an average stacked Norwe-gian Sea record (Fig. 4). This composite recordwas obtained by stacking the two records andapplying a three point running average. It hasthe e¡ect of highlighting parts of the recordswhich have a common signal and smoothingthose that do not. In doing so, the individualpeaks of IRD observed in core ENAM 93-21are smoothed and the noise is reduced. Here againthe MS records can be correlated with great pre-cision from 50 to 22 ka and notable mis-matchescan be observed during LGM. The comparison ofthe timing and the amplitude of the IRD recordsindicates a good general agreement although theIRD layers observed in the Irminger Basin arebetter de¢ned and smaller in amplitude. Duringthe LGM, high values of IRD content can beobserved at all the sites. There are low values ofIRD content at all locations between 22 and 25 ka

and between 11 and 13 ka. We used the sameshading as those used in Fig. 3. Our synchronizedrecords show that the periods of time correspond-ing to the HE (the darker shaded boxes) are as-sociated with increased abundance of IRD at alllocations. In detail, the initial increase of IRDduring H4 occurs during MS event 9, and theIRD layer which occurs during the H3 intervalseems to occur, in both records, in the later phaseof the low MS event prior to MS event 4. Finally,H2 does not occur during a minimum MS valueas observed in the Norwegian Sea records.

However, in contrast to the Norwegian Sea re-cords, the lighter shaded boxes, which highlightthe stadial events between the HE, are not system-atically associated with an increase in IRD atboth locations. The DE do not appear to be char-acterized by synchronous increases of iceberg£uxes into the Norwegian Sea and in the IrmingerBasin suggesting a more complex pattern (Fig. 4).The close correlation of MS records, particularlybetween 32 and 46 ka, rules out the possibility ofinaccurate adjustments.

5. Discussion

5.1. The HE and the DE

The evidence presented in this study shows thatIRD concentrations increased near to synchro-nously at all three locations in the Nordic regionsduring the HE. The only notable exception is theabsence of IRD during H3 in core ENAM 93-21.Our observations thus strongly favor the existenceof coherent responses of the Northern Hemi-sphere's ice sheets (Nordic regions and Lauren-tide) during these events. The presence of Icelan-dic material at the base of the HE in the sub-polarNorth Atlantic [7,34] suggests that the ¢rst ice-bergs to arrive at these latitudes originated atleast partly from the Nordic regions. There is stillsu¤cient uncertainty in the timing of the IRDdeposits in the Nordic regions to conceive thatthese may precede the Laurentide ice sheet collap-ses by decades to hundreds of years. Studies ofboth the petrologic composition of IRD withinthe IRD layers and sedimentation £ux estimates

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would be necessary to precisely test the timing ofthe increased iceberg surging from Laurentide ver-sus the Nordic regions ice sheets.

The sediment records o¡ the Norwegian coastand close to the Faeroe Islands also suggest co-herent and synchronous increases of iceberg £uxesfrom the FIS on millennial-scale. In this area theabundance of planktonic foraminifera remains sa-turated in polar species N. pachyderma s. duringthe studied period making it unlikely that the dis-tribution of icebergs was controlled by water tem-peratures, e.g. [4,5,20]. Furthermore, the record ofcore MD 95-2010 located o¡ the Scandinaviancoasts close to the glacial ice sheet outlets' posi-tions shows well-de¢ned IRD layers, with sharpbase and top. These observations do not favor thepersistence or stochastic iceberg delivery from theFIS to the open ocean during the studied period,except perhaps during the LGM. Core ENAM93-21 does show a certain number of isolatedevents, which could be due to local ice sheet in-stabilities and could also re£ect the stochastic be-havior of continental ice sheet such as that de-scribed by [22]. But the overall result, whichshows near to synchronous increases of IRD dur-ing times of minimum MS, favors the existence ofcoherent and synchronous increases of iceberg£uxes from the FIS on millennial time-scales.

A striking feature of these records is the oppo-sition between the HE characterized by near tosynchronous deposits of IRD at all locations,and the inter-HE where only the Norwegian Searecords reveal synchronous increases of IRD. HEcharacterized by the sudden and large-scale ice-berg surges from the Laurentide ice sheet there-fore appear as more catastrophic and globalevents, most probably enhanced by signi¢cantchanges in sea level [35,36]. Inversely, the DE ap-pear to operate mainly in the Nordic regions andpresent a more complex regional signature.

5.2. Linking Nordic regions ice sheet history andDO events

Based on the correlation between MS recordsand atmospheric temperatures, we can further in-vestigate the relationship between the ice raftingevents and the DO temperature oscillations. H3, 4

and 5 all occur during low values of MS, prior toMS events 4, 8 and 12, respectively (Figs. 3 and4). The timing of the HE, when compared to theice core record, is thus in agreement with previouscorrelations, which were based on SST records inthe North Atlantic Ocean, e.g. [8,18,19]. In detailthe initiation of H4, at the base of the unusuallysmall interstadial event 9, suggests that this eventwas perhaps able to perturb the system forcing itrapidly back into a cold mode. H3 also occursduring a period of minimum MS phase but thedeposit of IRD in the Nordic regions appears tolag the initiation of the associated cool stadial.

In the Norwegian Sea, low MS values (i.e. coldstadial) are always associated with an increasedIRD deposit with no apparent lead of lag, provid-ing evidence of a close relationship between var-iations of the £ux of icebergs from the FIS andthe cold phases of the DO events. We have nodirect means to de¢ne these changes of iceberg£uxes as catastrophic `surges' due to internal icesheet dynamics or as increases of iceberg deliveryin response to climate forcing. However, the evi-dence given from studies of ice sheet dynamicsdoes not easily explain how the large continentalice sheets could grow and retreat in such a rapid,repetitive and coherent manner [37,38]. We postu-late that the ice shelves and coastal ice sheets,which bordered the FIS, may be more sensitiveto climate forcing and could respond instantane-ously to changes in atmospheric temperatures andmoisture supply. Evidence from benthic foramini-feral records in this area shows alternate modes ofdeep-water formation. Open-ocean deep-waterformation in the Norwegian Sea is associatedwith the warm interstadial, whereas brine forma-tion seems to prevail during the cold stadials [6].Deep-water formation by brine processes operatestoday around the ice shelves of Antarctica. Thesechanges in modes of deep-water formation couldthus be related to the growth and retreat of thenearby ice shelves.

Furthermore, the IRD records at all locationsexhibit a change in regime between 20 and 13 ka.Indeed, around the LGM period, concentrationsof IRD are particularly high and the absence ofwell-de¢ned layers of IRD re£ects most probablya more stochastic behavior of the FIS. It is also

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during this period that the continental ice sheetsreach their maximum extent and the FIS wouldhave advance over the continental slope [39] mak-ing it unlikely for ice shelves to develop. It is alsoat this time that the amplitude of the temperatureoscillations is reduced in Greenland. These obser-vations make it plausible that ice shelves couldhave played an important role in the DO temper-ature oscillations. The close relationship betweenthe Norwegian Sea sediment records and the DOevents could also be due to an input of icebergsand low saline waters directly into the area ofdeep-water formation. The input of icebergs intothis area could subsequently provide the smallsalinity perturbations able to perturb the glacialclimate [40], but they do not appear to lead to acomplete shut down of deep-water formation, asobserved during the HE [17].

In the Irminger Basin, the positions of the DEwith respect to the MS-DO events are more ran-dom (Fig. 4). Some IRD layers are particularlysmall in amplitude and could bias the statisticalsigni¢cance of some of these `events'. However,we postulate that when the quantity of rock frag-ments rises above a critical level and presents sim-ilar quantities as those observed during the HE,they can be used to de¢ne a period of increasediceberg £ux. Some DE, such as the event ataround 40 ka, in the Irminger Basin thus appearto occur during the warm interstadials, corre-sponding to an increased iceberg £ux to theopen ocean occurring independently from that ob-served in the Norwegian Sea (Fig. 4). Further-more, sediment core records from the ReykjaneRidge, east of core SU 90-24, reveal higher per-centages of coarse fraction during low MS values[30]. In that area, IRD were deposited duringtimes of cold SST, associated with a decreasedintensity of the warm IC [8]. It is possible thatcore SU 90-24 is recording a local phenomenon.The di¡erent paleoceanographic setting could ac-count for the observed di¡erences in IRD signals.While core SU 90-24 will mainly re£ect variationsof the EGC during the glacial period, sites closeto the Reykjanes Ridge record the variability ofthe IC associated with the north^south shifts ofthe Polar Front. These apparent mis-matches inthe timing of the IRD deposits at the two loca-

tions question the possibility that the icebergsbearing the IRD particles observed at the Reyk-jane were from the East Greenland Ice Sheet.To address this issue further records obtainedfrom sediment cores north of the position ofcore SU 90-24 would be necessary. Perhaps thelocal ice sheets did not respond similarly to thecold stadials, or the marine-based and coastal icesheets were able to react independently on such arapid millennial-scale in the eastern and westernNordic regions. These observations would be inagreement with the hypothesis that the millennial-scale variations of iceberg £uxes, when comparedto the HE, could be related to processes whichoperate mainly in the coastal regions within theNordic regions.

One explanation for the observed di¡erencescould be di¡erences of ice accumulation rates,which would particularly a¡ect the coastal icesheet and ice shelves in the eastern and westernsectors of the Nordic regions. Such a pattern re-calls today's asymmetric distribution of precipita-tion and temperature anomaly patterns observedover Greenland and Scandinavia and closely re-lated to di¡erent modes of North Atlantic oscil-lation [41]. If this atmospheric circulation phe-nomenon exhibits preferential modes ofoperation (amplitude and frequency) during thedi¡erent phases of the DO oscillations, perhapsthis could lead to an enhanced asymmetric distri-bution of moisture across the Nordic regions sub-sequently leading to di¡erent timing of ice sheetadvances.

6. Concluding remarks

We were able to precisely compare the timingand the variations of iceberg £uxes from the dif-ferent Nordic regions' ice sheets during the lastglacial by building accurate age models and syn-chronizing ocean sediment records from distantareas. Results support the existence of near tosynchronous and coherent periods of increasediceberg £uxes to the Nordic seas. The HE areassociated with near to synchronous deposits ofIRD in the Norwegian Sea and in the IrmingerBasin and occur during times of minimum MS

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except for H2. Based on the correlation betweenMS and the ice core records H3, H4 and H5 occurprior to DO events 4, 8, and 12 in agreement withprevious observations. This result favors the exis-tence of coherent response of the di¡erent North-ern Hemisphere continental ice sheets perhaps en-hanced by small changes in sea level due to thelarge contribution of the Laurentide ice sheet.During the inter-Heinrich intervals, coherent andsynchronous increases of iceberg £uxes from theFIS can be observed in the Norwegian Sea sedi-ment cores. These well-de¢ned layers rich in IRDclose to the glaciers' outlets occur during mini-mum MS values correlated to cold stadial events.Our results favor the existence of coherent andrepetitive advances of the FIS on millennial-scale.

These observations provide evidence that repet-itive variations of iceberg £uxes into the Norwe-gian Sea are closely related to the DO tempera-ture oscillations recorded in Greenland. Theicebergs could originate from the more sensitiveareas of the continental ice sheet : the coastal icesheets and marine-based ice shelves bordering theFIS. The Norwegian sector of the Nordic regionsappears more sensitive, responding instantane-ously to changes in atmospheric temperaturesand moisture supply during the glacial. Finally,di¡erences in moisture supply could also accountfor the observed di¡erences in the Irminger Basinand in the Norwegian Sea.

Acknowledgements

This research was supported by the NOAAPostdoctoral Program in Climate and GlobalChange, administered by the University Corpora-tion for Atmospheric Research. Basic supportfrom the CEA-CNRS to the LSCE, programs Ge-osciences marines, PNEDC (INSU), ATP, EU en-vironmental program, IFRTP and the IMAGESprogram. The isotopic data were obtained in Gifsur Yvette; B. Lecoat, D. Dole and J. Tessierwere in charge of the measurements. H. Leclaire'ssupport and advice for micropaleontology were ofgreat help. We also acknowledge M. Arnold andN. Tisnerat, in charge of the accelerator massspectrometry, where the 14C dates have been ob-

tained. The role of IFREMER and Genavir inoperating the coring cruises Paleocinat 1 and Im-ages 1 of the French R/V Le suroit and the Mari-on Dufresne is acknowledged. The paper wasgreatly improved thanks to numerous discussionswith Gerard Bond. I also wish to thank MarcChapman and an anonymous for the quality oftheir reviews. This is LDEO contribution 6265.[EB]

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