Were glacial iceberg surges in the North Atlantic triggered by climatic warming?

Were glacial iceberg surges in the North Atlantic triggered byclimatic warming?

M. Moros a;�, A. Kuijpers b, I. Snowball c, S. Lassen b, D. Ba«ckstro«m d,F. Gingele a, J. McManus e

a Baltic Sea Research Institute, Seestrasse 15, 18119 Rostock, Germanyb Geological Survey of Denmark and Greenland, �ster Voldgade 10, DK-1350 Copenhagen K, Denmark

c Quaternary Geology, University of Lund, Tornava«gen 13, Lund, SE-22363, Swedend Earth Science Center, Gothenburg, S-41381, Sweden

e Woods Hole Oceanographic Institution (WHOI), Department of Geology and Geophysics, Clark 121, Woods Hole, MA 02543-1050,USA

Received 19 June 2001; received in revised form 30 July 2002; accepted 27 September 2002

Abstract

High-resolution physical, mineralogical, sedimentological and micropalaeontological studies were carried out onNorth Atlantic cores from the Reykjanes Ridge at 59‡N and from the region southwest of the Faeroe Islands. All coresites are situated along the pathway of Iceland^Scotland Overflow Water (ISOW) and the various parametersmeasured display similar features. Previously identified carbonate oscillations [Keigwin and Jones (1994) J. Geophys.Res., 99, 12397^12410] in the time span back to the Marine Isotope Stage 5^4 transition and Late Glacial lithic events[Bond and Lotti (1995) Science, 267, 1005^1010], such as the Heinrich ice-rafting events, are all represented in the corerecords. Long-term trends and higher-frequency changes in ISOW intensity were reconstructed on the basis of variousindependent proxy records. The long-term trends in circulation match theoretical orbitally forced insolation changes.Our observed links between ice-rafted detritus (IRD) input, variations in sea surface temperature (SST) andcirculation at greater depth point to the need to re-examine the origin of IRD events. We suggest that these eventsmay have been triggered by enhanced, partly sub-surface, heat transport to the north. Enhanced northward heattransport may have caused bottom melting of floating outlet glaciers and ice shelves, leading to increased icebergdischarge and ice sheet destabilization. This discharge resulted in lower SST’s and a lower temperature overGreenland. Thus, as shown by our records, this scenario implies a temporary de-coupling of surface processes andcirculation at greater depth. A key feature is the occurrence of a saw-tooth pattern in the marine data, which is similarto the Greenland ice core records. Moreover, the ‘warming’ theory of IRD events would explain the observed ‘out-of-phase’ relationship between the Greenland and Antarctic ice core records and also the rapid establishment of highertemperatures over Greenland immediately after the cold phases (stadials) of the DansgaardÔeschger cycles.E 2002 Elsevier Science B.V. All rights reserved.

0025-3227 / 02 / $ ^ see front matter E 2002 Elsevier Science B.V. All rights reserved.PII: S 0 0 2 5 - 3 2 2 7 ( 0 2 ) 0 0 5 9 2 - 3

* Corresponding author. Present address: Bjerknes Centre for Climate Research, Allegaten 55, N-5007 Bergen, Norway.Tel. : +47 555 89829; Fax: +47 555 84330.

E-mail addresses: [email protected] (M. Moros), [email protected] (A. Kuijpers), [email protected](I. Snowball), [email protected] (S. Lassen), [email protected] (D. Ba«ckstro«m), [email protected] (F. Gingele),[email protected] (J. McManus).

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Marine Geology 192 (2002) 393^417

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Keywords: North Atlantic; magnetic susceptibility; Heinrich events; ice-rafting detritus; palaeocurrents

1. Introduction

Over the past decade geological evidence fromthe Southern Hemisphere has arisen to support ade-coupling (‘out-of-phase’) of Northern andSouthern Hemisphere climate systems during theLast GlacialÎnterglacial transition (e.g. Blunier etal., 1998). Although still debated, the Greenlandand Antarctic ice core data indicate that this tran-sition occurred in the Southern Hemisphere 1^2.5kyr before the Northern Hemisphere. The polarrecords also show a similar phase shift for higher-frequency glacial climate changes, i.e. the so-called DansgaardÔeschger (DÔ) events, whichwere particularly pronounced during Marine Iso-tope Stage (MIS) 3 (e.g. Blunier and Brook, 2001;Kanfoush et al., 2000).Millennial changes in Late Quaternary sea-sur-

face temperatures (SST’s) of the North Atlantichave been linked to atmospheric DÔ tempera-ture variations over Greenland (e.g. Bond et al.,1993; Bond and Lotti, 1995; Fronval et al., 1995;Keigwin and Jones, 1994; McManus et al., 1994;Oppo et al., 1997). These oceanographic changeswere associated with iceberg discharge eventsfrom the surrounding ice sheets. Episodes oflarge-scale iceberg discharge, so-called Heinrichevents (Heinrich, 1988) marked the end of longercooling cycles of the ice core records (Bond et al.,1993; Broecker, 1994). Higher-frequency episodesof North Atlantic iceberg discharges (Bond andLotti, 1995; Dokken and Jansen, 1999; Elliot etal., 1998; Fronval and Jansen, 1996; Lacksche-witz et al., 1998) are linked to rapid warm^coldoscillations with a return period of about 1.5 kyr(e.g. van Kreveld et al., 2000), and are believed torepresent the DÔ cycles found in the ice cores(Dansgaard et al., 1993; Grootes and Stuiver,1997; Mayewski et al., 1997). A puzzling featureof the Heinrich events and the DÔ cycles hasbeen a conspicuous abrupt warming, which ap-pears to have occurred immediately after the ice-berg surges reached a maximum. The mecha-nism(s) that forced ice-rafted detritus (IRD)events is (are) still poorly understood: internal

ice sheet instabilities, ocean circulation changes,sea-level £uctuations, variations in solar parame-ters, and ice-load induced earthquakes have allbeen postulated as possible triggers or forcing fac-tors behind these events (reviewed by Alley andClark, 1999). Geological data pose a further fun-damental question, which remains to be an-swered: why did meltwater pulses frequently pre-cede peak IRD events (Funder et al., 1998; Jonesand Keigwin, 1988; Nam, 1997; Nam and Stein,1999; Norgaard-Pedersen, 1997; Sarnthein et al.,1995; Stein et al., 1996, 1994a,b; van Kreveld etal., 2000)?In general, the North Atlantic plays an impor-

tant role in the global ocean conveyor belt system,which has regulated the climate in Europe andfurther a¢eld (Broecker, 1991). A major compo-nent of the thermohaline circulation is the north-ward transport of warm, saline surface waters.Through convective processes in the Nordic Seasthis northwards energy transport is balanced by asoutherly return £ow of cold, dense waters intothe North Atlantic basins. In order to understandnatural palaeoclimate variability and the underly-ing forcing mechanisms, it is crucial to reconstructLate Quaternary changes in the surface and thedeep water conditions. Late Quaternary changesin deep water production in the North Atlantichave been previously established by the N

13Cand N

18O (Labeyrie et al., 1995; Vidal et al.,1998) values of benthic foraminifera (Chapmanand Shackleton, 1998; Keigwin and Boyle,1999; Oppo and Lehman, 1995), as well as thegeneral benthic faunal assemblage (Lassen et al.,1999; Rasmussen et al., 1996b). Further proxiesrelated to the lithic components concern bulksediment magnetic properties (Kissel et al., 1999,1998; Moros et al., 1997), the Sr and Nd isotopesignature of the lithic particles (Revel et al., 1996),the clay mineral assemblage (Fagel et al., 2001,1997; Gehrke et al., 1996; Grousset and Chesse-let, 1986) and parameters that characterise thegrain-size distribution (e.g. Hall and McCave,2000; Haskell and Johnson, 1991; McCave etal., 1995a; Prins et al., 2002).

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M. Moros et al. /Marine Geology 192 (2002) 393^417394

In this paper we present a series of high-reso-lution physical, mineralogical and sedimentologi-cal records which extend to the transition betweenMIS 5a and MIS 4 of cores taken from theReykjanes Ridge (at 59‡N) and the Faeroe area.Based on various independent proxy records wedemonstrate that millennial changes in the sedi-ment composition of both studied areas can beascribed to changing circulation and to variationsin iceberg discharge. The stratigraphic relation-ships between the independent palaeoceano-graphic proxies are discussed in terms of changesalong the pathway of the Iceland^Scotland Over-£ow Water (ISOW). Furthermore, possible link-ages between surface water conditions (icebergdischarge and SST variations) and processes atgreater depth are proposed. The results of ourstudy may point to the need to re-examine themechanism responsible for IRD events, whichmay play a major role in controlling the reported‘out-of-phase’ relationship between Southern andNorthern Hemisphere ice core records.

1.1. Oceanographic setting of the coring sites

The £ow of cold water masses from the NordicSeas over the Greenland^Scotland Ridge ismainly concentrated in the Denmark Strait andthe Faeroe^Shetland Channel (and Faeroe BankChannel) gateway. Combined with water massesoriginating from the Labrador Sea, these cold anddense over£ow water masses are volumetricallyimportant components of the North AtlanticDeep Water (NADW) and constitute the lowerlimb of a global thermohaline circulation system.Relatively warm and saline North Atlantic Cur-rent (surface) waters are advected into the NordicSeas (e.g. Hansen and Rsterhus, 2000), complet-ing the £ow in the upper limb of the global cir-culation system.For the purpose of our study, it is stressed that

any fundamental reduction or increase of theover£ow activity is linked to a weakening orstrengthening of the northward ocean heat trans-port into the Nordic Seas, respectively. Thesetransport changes are also seen in palaeoceano-graphic records from the main over£ow passages(e.g. Kuijpers et al., 1998a; Rasmussen et al.,

1996a,b). In the Faeroe^Shetland over£ow gate-way, cold and dense over£ow waters with a lowersalinity (S6 35.0) normally extend from 400^500m water depth to the bottom of the Faeroe^Shet-land Channel (e.g. Hansen and Rsterhus, 2000).South of this channel the Wyville^ThomsonRidge forms a barrier for most of the south-£ow-ing over£ow waters and over£ow of the ridge isonly intermittent. Under such conditions, part ofthe latter over£ow water masses continues tomove south of the ridge in a westerly directionfollowing the contours of the rise of the FaeroeBank (core site ENAM94-09) and Bill BaileyBank (Kuijpers et al., 1998b).After exiting the Faeroe Bank Channel, the

main over£ow waters experience mixing with sur-rounding warmer Atlantic water, resulting in atemperature rise from slightly below 0‡C to about2‡C west of the Faeroe Bank Channel outlet (coresite ENAM-30). These water masses £ow furtherwestward, mainly concentrated along the south£ank of the Iceland^Faeroe Ridge. Over£ow ofthe Iceland^Faeroe Ridge is concentrated in thearea immediately east of Iceland, but also west ofthe Faeroe Islands over£ow of the ridge has beenobserved, although at strongly varying rates(Hansen and Rsterhus, 2000). These water massesthat originally have been remotely produced aresubject to signi¢cant mixing with other watermasses in the region of the ridge (van Aken andBecker, 1996; van Aken and de Boer, 1995; vanAken and Eisma, 1987). Within this context, it isstressed that for the ease of the reader, in thefollowing sections over£ow waters will be referredto as ISOW, although admixtures of various dif-ferent water masses exist. South of Iceland, ISOWis de£ected to a more southerly direction £owingparallel to the axis of the Reykjanes Ridge, wherethe current strength and direction are steered bytopography (van Aken and de Boer, 1995). Atwater depths exceeding about 1200 m at this sec-tion of the ISOW pathway (Reykjanes Ridge coresites ; 1200^1800 m water depth) interaction oc-curs between ISOW and Labrador Sea Water. Amore detailed description of the hydrography nearthe Reykjanes Ridge coring site area has beengiven by Bianchi and McCave (2000). At thesouthern extremity of the Iceland Basin, lower

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M. Moros et al. /Marine Geology 192 (2002) 393^417 395

ISOW £ows into the western North Atlantic basinvia the Charlie^Gibbs Fracture Zone and turnsnorth again along the west £ank of the ReykjanesRidge.Studies on benthic foraminiferal assemblages

and their isotopic signature of sediments re-coredin the Reykjanes Ridge area in 1997 demonstratedthat surface and deep circulation during the inter-stadials of MIS 3 to MIS 2 was similar to themodern and Holocene circulation with in£uenceof the convection in the Nordic Seas and gener-ation of NADW, hence of ISOW (Rasmussen etal., in press). This conclusion agrees well withprevious studies of the interstadial palaeoceanog-raphy in the southeastern Norwegian Sea (Ras-mussen et al., 1996b). Convection and modern-like deep water formation in the Nordic Seas dur-ing interstadials are also indicated by high N

13C

values in benthic foraminifera in a core from theVUring Plateau (Dokken and Jansen, 1999).

2. Methods

Eighteen gravity cores (GC) from the Reyk-janes Ridge at 59‡N, 31‡W were retrieved duringcruise SO82 in 1992 with research vessel (R/V)Sonne and during cruise LO09 in 1993 with R/VProf. Logachev. At some stations, additional giantgravity cores (GGC) were recovered. In this paperwe focus on the data obtained from 7 GC’s and4 GGC’s (Fig. 1; Table 1). Cores ENAM-30 andENAM94-09, from the Faeroe area, were col-lected using a piston corer during ENAM cruisesin 1993 and 1994, respectively, with R/V Pelagia(Table 1).

Fig. 1. Location of the study areas, bathymetry and bottom water circulation (arrows) in the North Atlantic between 50 and65‡N. Abbreviation: ISOW, Iceland^Scotland Over£ow Water. The inset shows the core locations and bathymetry of the Reyk-janes Ridge area. For core positions, see Table 1.

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GRAPE density and bulk magnetic susceptibil-ity (MS) were measured on the Reykjanes Ridgecores using a GEOTEK multi-sensor-core-logger.The MS was measured with a Bartington MS2FPoint Sensor. MS from ENAM94-09 was deter-mined using a hand-held Microkappa KT-5 mag-netometer. Discrete contiguous oriented sampleswere taken with 2U2U2-cm plastic boxes fromReykjanes Ridge cores and ENAM-30. To obtaininformation about changes in the magnetic grain-size, initial mass speci¢c magnetic susceptibility(Min), anhysteretic remanent magnetisation(ARM) and magnetic hysteresis loops were mea-sured. A detailed description of the mineral mag-netic methods can be found in Snowball and Mo-ros (in press). The use of magnetic parameters inpalaeoceanographic studies has been widely dis-cussed, e.g. Verosub and Roberts (1995); Waldenet al. (1999) and Stoner and Andrews (1999).The bulk mineralogy (quartz content and

quartz/plagioclase ratio^Q/Plag ratio) was deter-mined by X-ray di¡raction (PHILLIPS device,

Co KK-radiation). For the Q/Plag ratios the in-tensities of the 4.26 AX quartz and the 3.21 AX feld-spar re£ections were used (details in Moros et al.,submitted). The calcite contents of the ReykjanesRidge cores and ENAM-30 were calculated fromthe X-ray measurements using a CaCO3 X-raycounts calibration curve based on the 3.03 AX cal-cite re£ection. CaCO3 was measured on represen-tative samples of every core with an infrared an-alyser ELTRA Metalyt 500CS after treating thesediments with 50% H3PO4.The physical grain-size distribution of the frac-

tion 6 63 Wm was determined with a GALAICIS-1 laser particle-sizer. The relative abundancedata of the polar foraminifera N. pachyderma (s)and IRD countings on the grain-size fractions 150 Wm of core SO82-05GGC were taken fromvan Kreveld et al. (2000). Smectite contents of theclay fraction (6 2 Wm) are from Gehrke et al.(1996). Settling velocity distribution measure-ments of the fraction 63^1000 Wm of core LO09-18GC were carried out using an advanced settling

Table 1Positions and water depths of the studied cores

Core Location Water depth Position

ENAM-30 61‡33.945PN 1224 Faeroe area10‡15.714PW

ENAM94-09 60‡20.14PN 1286 Faeroe area9‡25.99PW

LO09-18GC 58‡58.043PN 1471 southeastern Reykjanes Ridge £ank30‡40.989PW

SO82-04GC 59‡05.757PN 1503 southeastern Reykjanes Ridge £ank30‡28.730PW

SO82-07GC/GGC 59‡00.737PN 1580 southeastern Reykjanes Ridge £ank30‡36.156PW

LO09-23GGC 59‡01.827PN 1417 northwestern Reykjanes Ridge £ank31‡06.876PW

LO09-20GC 59‡04.935PN 1394 northwestern Reykjanes Ridge £ank30‡58.619PW

SO82-05GGC 59‡11.175PN 1416 northwestern Reykjanes Ridge £ank30‡54.399PW

SO82-02GGC 59‡21.465PN 1730 northwestern Reykjanes Ridge £ank31‡05.089PW

SO82-03GC 59‡19.597PN 1774 northwestern Reykjanes Ridge £ank31‡08.352PW



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tube system, MacroGranometer1. For age con-trol, accelerator mass spectrometry (AMS) 14Cdatings for ENAM-30 were taken from Ba«ck-stro«m (1998), for ENAM94-09 from Lassen etal. (2002) and for Reykjanes Ridge cores fromMoros et al. (1997) and Lackschewitz et al.(1998). The AMS 14C datings of the ReykjanesRidge cores are shown in Fig. 2a on appropriateplaces in the MS record of LO09-18GC. Oxygenisotope analyses of ENAM-30 were carried out onN. pachyderma (s) at the WHOI stable isotopelaboratory, using a Finnigan MAT255 mass spec-trometer (Ba«ckstro«m, 1998). The N

18O and N13C

values of the benthic foraminifera Cibicides £etche-ri of ENAM94-09 were measured at the Bergenstable isotope laboratory (Lassen et al., 2002).

3. Stratigraphy

The patterns of the GRAPE density, MS (Fig.2a), Q/Plag ratio (Moros et al., submitted) andcalcite (Fig. 2b) records in all Reykjanes Ridgecores as well as in core ENAM-30 from the Faer-oe area display similar features, which makes itpossible to correlate these cores. AMS 14C dat-

Fig. 2. (a) MS and (b) calcite vs. composite depth records of the Reykjanes Ridge and Faeroe ENAM-30 cores. Horizontal barsindicate the positions of the Ashes II and I, the Heinrich layers (H) and the Younger Dryas (YD). The MS spikes in (a) are la-belled with the corresponding ice core interstadials (IS). The positions of the BUlling (B)/AllerUd (A) interstadials are marked andthe obtained AMS 14C ages (3400 yr reservoir corrected) are integrated in (a). The position of an additional dark basaltic ashlayer is marked by a horizontal dashed grey line. In (b), the calcite spikes are labelled with the corresponding ice core IS and thecalcite record of GPC-5 from Bermuda Rise is presented. Calcite spikes in GPC-5 are numbered (italic letters) according to Keig-win and Jones (1994).

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ings, the presence of Ashes I and II, as well as theoxygen isotope data support the correlation (seeLackschewitz et al., 1998; Lackschewitz andWallrabe-Adams, 1997; Moros et al., 1997). Forall cores a composite depth down to the transitionbetween MIS 5a and MIS 4 was thus constructedand this is used in selected ¢gures.Ashes I and II were not found in ENAM-30.

However, these ashes were observed as very thinlayers in the nearby ENAM-33 (Kuijpers et al.,1998b) and ENAM93-21 cores (Rasmussen et al.,1996b), which show MS features that can be cor-related to ENAM-30 and the Reykjanes Ridgecores. Interestingly, Ash II displays distinctly ¢nermagnetic grain-sizes, which indicates an excellentcorrelation tool (Snowball and Moros, in press ;

Figs. 7 and 8). An additional datum level is adark basaltic ash layer, which was AMS 14C datedto 23.3 kyr BP in core ENAM-30 (Ba«ckstro«m,1998) and to 23.6 kyr BP in Reykjanes Ridgecore SO82-05GGC (van Kreveld et al., 2000).The basaltic ash layer is magnetically character-ised by a high S-ratio in ENAM-30, which indi-cates a relatively high ratio of haematite to mag-netite (Ba«ckstro«m, 1998). This ash layer was alsoobserved in core ENAM93-21 (T. Rasmussen,pers. commun.).The positive peaks in the MS records (e.g. Fig.

2a) between H5 and H2 were labelled with thecorresponding DÔ IS numbers of the Greenlandice core records according to Dokken and Jansen(1999), Kissel et al. (1999, 1998), Moros et al.

Fig. 3. Log settling velocity curves (right) of four LO09-18GC samples (fraction s 63 Wm). Samples were taken from sedimentsdeposited during IS 12ÎS 11 and lithic event k, respectively. Note the changing shape of the prominent spike in the settling ve-locity records.

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(1997), and Rasmussen et al. (1996b). The num-bering of the MS spikes with the appropriateGISP2 IS 21-15 is based on the stratigraphic po-sition of Ash II and the transition between MIS5a and MIS 4. Ash II is placed near IS 15 in theGISP2 ice core (Zielinski et al., 1997). Thus, thetwo strong MS spikes between Ash II and theMIS 5a^MIS 4 transition can be assigned toGISP2 IS 20 and 19. Alternatively, it is possiblethat no MIS 4 sediments were recovered by thesecores. However, such a hiatus is highly unlikelybecause relatively high sediment accumulationrates existed during MIS 4 (low current strengthduring MIS 4, see below). The stratigraphic posi-tion of H6 (in the ice core between IS 18 and 17)resulted from the positions of IS 20 and 19 de-scribed above. Further age control to support thecorrelation of the marine sediments to the Green-land ice core records is given by the Laschampgeomagnetic excursion (Kissel et al., 1999; Lajet al., 2000) which has been found in severalReykjanes Ridge cores at MS peaks correspond-ing to IS 10 (Snowball and Moros, in press ; addi-tional unpublished data).

4. Proxy records used

Here we describe the proxy methods used torecord changes in the surface water (varying ice-berg input and SST changes) and deep water con-ditions (relative changes in the bottom waterstrength by direct and indirect evidences).

4.1. Surface water conditions

4.1.1. Iceberg input-identi¢cation of Heinrich andfurther lithic eventsAs previously reported for Reykjanes Ridge

sediments (Moros et al., 1997) and for other coresfrom the Norwegian Sea and North Atlantic out-side the Ruddiman IRD belt (e.g. Dokken andJansen, 1999; Kissel et al., 1999, 1998; Rasmus-sen et al., 1996b), the Heinrich events and furtherIRD layers display relatively low MS values com-pared to IS sediments. Based on the earlier pub-lished characteristics for Heinrich events and fur-ther lithic layers in Reykjanes Ridge sediments, a

common stratigraphy has already been establishedfor the Heinrich (H4^H1) events, the lithic eventscorrelated to DÔ cycles 12^2 in the Greenlandice core N18O record and the Younger Dryas (YD)stadial (Moros et al., 1997). Besides MS andGRAPE density, an extremely valuable parameterfor the identi¢cation of ice-rafting events is thequartz content and also the Q/Plag ratio (Moroset al., submitted). In Fig. 7 the correlation be-tween IRD countings (van Kreveld et al., 2000)and the mineralogical Q/Plag ratio of core SO82-05GGC is shown. Here, based on a combinationof the patterns of the multiple proxies, and al-ready published literature (e.g. Kissel et al.,1999; van Kreveld et al., 2000) H6, H5 and addi-tional lithic events have been identi¢ed. The lithicevents between H4 and H1 were labelled hâ ac-cording to Bond and Lotti (1995). We extend thenumbering of lithic events by using the Q/Plagratio, GRAPE density and MS records back tothe transition between MIS 5a and MIS 4 withnî (Figs. 5^7; Moros et al., submitted). Most ofthe lithic events recorded at the Reykjanes Ridgecores have been also found in ENAM-30 by usingthe same proxies (Figs. 2a and 5).We further investigated how the IRD content

a¡ects the rather complex GRAPE density pa-rameter. GRAPE density changes mainly resultfrom variations in the porosity (water content)and re£ect changes in the sediment grain-size dis-tribution. Here we can ignore the impact of thegrain density on the GRAPE density because ofits relatively low variability (authors’ unpublisheddata). During phases of enhanced ice-raftingpoorly sorted detrital material with a bimodalgrain-size distribution accumulated. These sedi-ments have a high proportion of coarse grainedmaterial s 63 Wm, but also a very ¢ne grainedmaterial tail. Phases with low IRD input are char-acterised by sediments with a unimodal grain-sizedistribution in the bulk carbonate-free fraction.The poor sorting causes the observed higherGRAPE densities (via lower porosity) in theIRD layers. In addition, data obtained from set-tling velocity measurements of the bulk grain-sizefraction s 63 Wm (Fig. 3) strongly support thisearlier interpretation (Moros et al., 1997). TheGRAPE density linked to the lithic event k is

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presented here. The accompanying IS 12 and IS11 show typically GRAPE density minima. It isclear that the sand fraction of the sediments de-posited during the IS display a unimodal settlingvelocity distribution with a well-developed spike(due to foraminifera). The distribution s 63 Wmappears well-sorted. A completely di¡erent pic-ture is shown by the settling velocity curvesfrom samples of the GRAPE density spike. Thedistribution is typically bimodal with the coarserand heavier IRD grains on one hand and a less-developed spike, with higher contents of relatively¢ner-grained or lighter particles, on the otherhand (Fig. 3). Thus, poorer sorting of the bulkcoarser sediment fractions is observed in materialdeposited during Heinrich and other IRD events.Obviously, the GRAPE density parameter is

determined by surface water processes (via vary-ing IRD and foraminifera input) and by deepwater processes (via changing grain-size distribu-tion in 6 63 Wm).

4.1.2. Calcite recordsThe calculation of the calcite content by using

the 3.03 AX X-ray counts allows us to discuss the‘pure’ calcite content, which mainly re£ects theamount of calcareous foraminifera in the sedi-ments. Reykjanes Ridge sediments with high cal-cite contents are generally characterised by a rel-atively low percentage abundance of N. pachy-derma (s) (Fig. 7), which indicates warm surfacewater conditions.Owing to a strong correlation to the N. pachy-

derma (s) records the calcite content seems to be auseful proxy for changing surface water condi-tions, at least during MIS 3. The impact of dilu-tion by detritus and/or winnowing e¡ects by cur-rents on the calcite content is low, which isobvious when comparing the calcite and N. pa-chyderma (s) records with the ISOW £ow intensityvariation records (Fig. 7).

4.2. Reconstruction of variations in the ISOW £owintensity

Direct physical evidence for changing bottomwater strength is provided by magnetic grain-sizevariations and the detrital grain-size distribution.

Indirect evidences for ISOW intensity £uctuationsand/or higher weathering activity on Iceland areprovided by parameters that are sensitive to theamount of basaltic Iceland-derived material in thesediments and to their spatial sedimentation pat-tern.

4.2.1. Direct evidence by magnetic and detritalgrain-size distribution studiesMagnetic parameters have successfully been

used by e.g. Kissel et al. (1999, 1998) to recon-struct NADW production changes in the NorthAtlantic during MIS 3. Snowball and Moros (inpress) showed that titanomagnetite grain-size var-iations, as determined by magnetic hysteresisproperties, can be interpreted in terms of bottomwater £ow intensity on the Reykjanes Ridge.These titanomagnetite grain-size variations arebest expressed by the ratio MRS/MS, where highervalues indicate ¢ner magnetic grains and lowerbottom water £ow intensities.In the case of detrital grain-size distribution, it

has been shown that it is necessary to carefullystudy the distribution within the 6 63 Wm frac-tion. Our investigations on several cores from theReykjanes Ridge and the Faeroe area have shownthat the most useful grain-size parameters for theidenti¢cation of relative changing bottom water£ow intensities are: the Mean grain-size valueswithin the size fractions 0.5^10 Wm (here pre-sented as Mean 6 10 Wm) and 0.5^20 Wm (notshown here) for the Reykjanes Ridge sedimentsand the content of the size fraction 1^3 Wm(here presented as content 1^3 Wm) of the totalgrain-size fraction 6 63 Wm for the Faeroe areasediments. The most suitable grain-size parametermay vary between sites with di¡erent sedimentsources, accumulation rates and hydrodynamicconditions. Note that there is a strong correlationbetween the titanomagnetite magnetic grain-sizeand ‘normal’ grain-size records, as illustrated inFig. 6 (authors’ unpublished data of furthercores). Small di¡erences between these recordscan occur, possibly due to the di¡erential in£u-ence of IRD peaks with respect to these two in-dependent methods (Snowball and Moros, inpress). The long-term trends recorded by theMean 6 10 Wm and content 1^3 Wm parameters

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are also observed in the sortable silt size (10^63Wm) records calculated from our laser particle-si-zer measurements. In the grain-size range s 5 Wmlaser sizers provide results comparable to thoseobtained by the widely used Sedigraph (e.g. Bian-chi and McCave, 1999; Hall and McCave, 2000;McCave et al., 1995a,b). However, the implica-tions of the various parameters (Mean 6 10 Wmand content 1^3 Wm on the one hand and sortablesilt on the other hand) di¡er signi¢cantly in rec-ord parts which are strongly in£uenced by ice-rafted material (authors’ unpublished data). Ourobservation is in accordance with new ¢ndings ofPrins et al. (2002) who, based on end-membermodelling, question the use of the sortable siltsize parameter as bottom current proxy especiallywhen IRD in£uence is strong. Prins et al. (2002)argue for the use of ¢ne fractions (less than 20Wm) for relative bottom current changes in thestudy area (we stress that we only discuss relative

£ow changes). Fine grain fractions of clay havealso been used as a proxy of relatively changingbottom water energy (Hall and McCave, 2000).

4.2.2. Indirect evidence by changing input ofIceland-derived material and their spatialsedimentation patternsThe amount of Iceland-derived basaltic materi-

al in the sediments determines the plagioclase(here the 3.21 AX plagioclase; Moros et al., sub-mitted) and smectite contents (smectite/illite ratio;Fagel et al., 1997; Fagel et al., 2001; Gehrke etal., 1996; Grousset et al., 1982), and titanomag-netite concentration (Kissel et al., 1999; Snowballand Moros, in press). Thus, sediments depositedduring phases of enhanced ISOW £ow display ingeneral higher titanomagnetite contents (higherMS) and higher smectite/illite ratios in the clayfraction compared to those sediments that accu-mulated during phases of reduced over£ow inten-

Fig. 4. Q/Plag ratio and Min vs. composite depth records of selected cores from the northwestern and southeastern ReykjanesRidge £ank as well as the ridge crest. In addition, the sedimentation rates of the cores calculated for the time spans betweenH4^H3 and H2^H1 are shown. The positions of the Heinrich layers (H) and the Last Glacial Maximum (LGM) are marked.The reference lines of the bar charts were placed to 0.7 in the Q/Plag ratio and to 2.0 in the Min record in order to show the dif-ferences in the spatial distribution pattern of the parameters.

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sity. Further indication that the Iceland area andthe Iceland^Scotland Ridge are major sources ofReykjanes Ridge/Faeroe area sediments comesfrom Sr^Nd isotope measurements (Revel et al.,1996).Support for the hypothesis that varying ISOW

£ow via the e⁄ciency of transport of Iceland-de-rived material determines the record features isprovided by the spatial distribution pattern ofthe MS values and mineralogical features. Herewe focus on the spatial variability in the litholog-ical properties of the Reykjanes Ridge cores. Twoareas can be separated on the basis of these prop-erties, i.e. the southeastern and northwestern£anks (Fig. 1). Compared to the northwestern£ank, the sediments from the southeastern £ankgenerally have lower calcite contents, lower Q/Plag ratios (Fig. 4), higher Min (Fig. 4), lowerGRAPE densities and higher sediment accumula-tion rates. These data suggest a stronger in£uenceof the ISOW on the sediment composition of thesoutheastern £ank, which agrees well with the ex-isting circulation system and recent sedimentationpattern (Gehrke et al., 1996). Thus, if only iceberginput (the source of quartz) controls the arealdistribution of Q/Plag, the ratios would be equallydistributed over the whole area during IRDevents. However, the generally lower baseline lev-els on the southeastern £ank, particularly duringthe iceberg-free stages, re£ect the stronger impactof the ISOW suspended load with plagioclase(and titanomagnetite) derived from Iceland andIceland^Scotland Ridge (Fig. 4). A similar obser-vation was made by Ruddiman and Bowles(1976). The ridge crest (Fig. 1) in ca. 900 m waterdepth forms a strong barrier to the ISOW becauseit towers more than 800 m above the ridge £anks(ca. 1770 water depth).Our conclusions are also supported by the ¢nd-

ing that during times of reduced bottom currentintensity (see below) the di¡erences between thebackground levels of the parameters of both ridge£anks were smoothed but not totally levelled out.This feature is, for example, evident when com-paring the lateral Q/Plag ratio pattern of the ex-treme cold, but relative stable Last Glacial Max-imum (LGM) phase, with the pattern of the veryunstable MIS 3 period (between H5 and H3).

5. Results

5.1. Surface water conditions

A widely discussed phenomenon in the NorthAtlantic is the strongly varying iceberg input. Ice-berg discharges have via their meltwater signi¢-cant impacts on SST and atmospheric tempera-ture. However, the causes of these discharges,which are recorded as IRD peaks, are still poorlyunderstood. The most striking feature in our ice-berg rafting records is their often saw-tooth shape(Figs. 2b, 5^8). This saw-tooth shape indicatesthat iceberg discharges increased steadily as partof a long-term process, rather than occurring asabrupt events. On the other hand, the saw-toothpatterns indicate that the IRD input rapidly ter-minated. The amplitude and shape of the short-term lithic events seems to be linked to insolation(July 65‡N) changes (Figs. 7 and 8). StrongestIRD events occurred during phases of increasinginsolation and when insolation is at an intermedi-ate and relatively stable level.The calcite events observed at Bermuda Rise

(Keigwin and Boyle, 1999; Keigwin and Jones,1994) have been identi¢ed in our records (Fig.2b). IS 14^5 are clearly indicated in the marinecalcite and N. pachyderma (s) records (Figs. 2band 7). Reykjanes Ridge and Faeroe area sedi-ments deposited during IS 4 and IS 3 do nothave increased calcite contents. However, theseIS are clearly documented in the coccolith (Lack-schewitz et al., 1998) and in the N. pachyderma (s)records with spikes and minima, respectively(Figs. 5 and 7). In general, MIS 4 and MIS 2are characterised by low calcite contents. Onlysmall £uctuations were observed in MIS 4. Bycontrast, in MIS 2 thin (ca. 1^2 cm) calcite-rich(up to 40%) layers were found to possess the sameforaminiferal characteristics as the calcite spikesin MIS 3 (Fig. 7). Supported by AMS 14C dat-ings, two maxima and the corresponding minimain the calcite record of MIS 2^MIS 1 can belinked to the BUlling and AllerUd interstadialsand the Older Dryas (OD) and YD stadials, re-spectively (Fig. 6). The observed spikes in the cal-cite record of ENAM-30 are not as distinct as inthe Reykjanes Ridge cores (Fig. 2b).

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The trends in the variation of the amplitudeof the calcite spikes from IS 14 to IS 5 can belinked to the changes in the duration and am-plitude of the IS documented in Greenland icecores, as well as to the insolation level (Fig. 7).Thus, highest calcite contents are observed in IS14 sediments, decreasing amounts from IS 14to IS 9, then relatively higher values in IS 8and 7 sediments, ¢nally followed by decreasinglevels towards IS 5. One may argue that these

changes are only related to the long-termtrends in ISOW strength (in terms of winnowinge¡ects). However, it is noteworthy that IS 14 to 9also display a decreasing trend in the abun-dance record of N. pachyderma (s) and IS 8 isalso prominent. Interestingly, similar features areobserved in the Bermuda Rise calcite records (Fig.2b). As a general feature, a saw-tooth pattern isshown by the N. pachyderma (s) data between IS4b to 3.

Fig. 5. Q/Plag ratio, MS, content 1^3 Wm, calcite, N. pachyderma (s), N18O records vs. depth for ENAM-30. Vertical bars indicatethe position of the Heinrich events and small letters the observed additional IRD events. Corresponding Greenland ice core inter-stadials (IS) are indicated as well. Note the saw-tooth shape of distinct Q/Plag ratio spikes. As in Reykjanes Ridge, cores IS 4and 3 are clearly shown in the N. pachyderma (s) but not in the calcite record (arrows). The dashed arrow indicates the ISOW£ow intensity increase towards the Holocene, which started well before peak H1.

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5.2. ISOW £ow intensity changes: long-termtrends and higher-frequency oscillations

Long-term trends in the proxy records, whichare related to changing ISOW £ow intensity areas follows (Figs. 6^8): decreasing intensity at theMIS 5a^MIS 4 transition, a relatively low £owlevel during MIS 4, which is followed by a steadyincrease towards the early part of MIS 3. Notethat the latter increase started before H6. Thisintensi¢cation peaked after IS 14 (as indicatedby spikes in the calcite and N. pachyderma (s)records; Fig. 7). From this highest £ow level ageneral decreasing trend, interrupted by a short-term intensi¢cation around IS 8, is observed. Low£ow speed is found during the LGM, which iscomparable to conditions from MIS 4. Our mag-netic parameters, mineralogical and grain-size dis-tribution data from the Reykjanes Ridge clearlyshow that the transition to the Holocene is char-acterised by continuously increasing £ow speed(Figs. 5^8). It is noteworthy that this latterISOW intensi¢cation started well before peakH1, AMS 14C dated to 15.9, 16.0 and 16.2 kyrBP in SO82-04GGC, LO09-20GC (Lackschewitzet al., 1998; Moros et al., 1997) and SO82-05GGC (van Kreveld et al., 2000), respectively.Data from ENAM94-09 support this early inter-mediate water signal (Fig. 6b). The latter dataindicate changes towards a lower nutrient level,salinity and oxygen content in the intermediatewater mass at this site (Lassen et al., 2002), whichare consistent with the initial £ow intensi¢cationrecorded at the Reykjanes Ridge. Only a weakreduction in bottom water circulation was ob-

served in relation to H1, or the YD. A similarobservation has been made in connection withH6. This event occurred after a strong initial in-crease in the intermediate water £ow intensity.In general, the long-term trend in ISOW £ow

intensity in the time span between the MIS 5a^MIS 4 transition and the Holocene has the shapeof the July insolation record at 65‡N. This link isobvious when looking at the average values of theparameters shown in Figs. 7 and 8. The proposedlink is typi¢ed by the above mentioned two initial£ow pulses that preceded H1 and H6. The timingof the early ISOW signals ¢ts well with strongintensi¢cations of NADW production recordedat 40‡N (Chapman and Shackleton, 1998), o¡Portugal (Shackleton, 2001; Shackleton et al.,2000; Zahn et al., 1997) and enhanced thermoha-line circulation in the Southern Ocean (Brathauerand Abelmann, 1999; Charles et al., 1996; How-ard and Prell, 1994; Labeyrie, 1996). Moreover, itis coeval with rising surface temperature overAntarctica (Fig. 7; Blunier and Brook, 2001; Blu-nier et al., 1998).Superimposed on the long-term trend are high-

frequency oscillations which might be linked tothe DÔ events (Figs. 5^7). Increasing ISOW£ow intensities in the H3 to H2 section are obvi-ously related to IS 4^2 (Figs. 5^7). The amplitudeof the high-frequency oscillations may be relatedto insolation changes. For example, short-termvariations in the ISOW speed have been identi¢edduring the last deglaciation phase with the ODand YD cold events (lower £ow speed) and theBUlling and AllerUd warm episodes but with avery low-amplitude (Fig. 6). Similarly, the ex-

Fig. 6. (a) Calcite, Min, Mrs/Ms and MARM/Min ratio, Mean 6 10 Wm, Q/Plag ratio and GRAPE density vs. depth records ofLO09-18GC. Marked are the positions of the AMS 14C datings (in 14C kyr BP reservoir corrected) in (a) and (b) in italic letters.The ages in boxes indicate the onset of the ISOW intensi¢cation, which lasted until mid-Holocene times (dashed arrow). This in-tensi¢cation, as also indicated by arrows in (c, Reykjanes Ridge cores) and (d, Faeroe Area), started well before the peak of H1.An early intermediate water signal preceding H1, which started at 15.8 14C kyr BP, is also shown in ENAM94-09 core (b) fromthe Faeroe area by virtue of changing N

18O and N13C values of the benthic foraminifera Cibicides £etcheri (Lassen et al., 2002).

Note the distinct saw-tooth pattern in the various proxy records (e.g. grey areas). Greenland ice core IS numbers are integratedin the calcite, Min and MARM/Min ratio records. Vertical grey bars mark the positions of the Younger Dryas (YD) and the Heinrichlayers (H4^H1), and small letters mark the other lithic events (Q/Plag ratio record) according to Bond and Lotti (1995). Arrowsindicate the corresponding signals in the various proxy records (e.g. rel. IS 8 maximum in SST, re£ected by high calcite contents,corresponds to strongest IRD event h). The positions of the Older Dryas (OD)/YD episodes, and the warm BUlling (B)/AllerUd(A) interstadials are indicated.

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pected IS 18^15 £uctuations are totally obscuredby the long-term ISOW intensi¢cation trend fromMIS 4 to IS 14 (Figs. 7 and 8). Well-pronouncedhigh amplitude £uctuations were observed in thetime span between H4 and H3 when the insola-tion changed only moderate. Here decreasedISOW £ow speed is recorded during peak IRDevents (also during the time interval between IS14 and H4).

6. Discussion

6.1. Coupling and de-coupling of surface andbottom water processes

When discussing links between surface anddeep water processes three major factors mustbe taken into account: (1) SST changes, (2) £uc-tuations in IRD input (meltwater), and (3) varia-tions in deep water (ISOW) £ow intensity. Due tothe higher stratigraphic resolution and more ac-curate and precise chronologies available, we ¢rstfocus on the last deglaciation. As quoted above,the steady increase in the ISOW £ow intensitytowards the Holocene started ca. 1.5 kyr beforepeak H1 and it was not signi¢cantly a¡ected bythe YD (Figs. 5^7). The latter fact has also beenhighlighted by other studies (Fagel et al., 1997;Veum et al., 1992) and recently observed o¡ Por-tugal (Shackleton, 2001). Thus, intensi¢cation ofISOW £ow started at ca. 16 kyr BP (ca. 19 cal kyrBP) and was apparently not a¡ected for longerperiods by regional (surface water) processes,such as the YD. In fact, the ISOW intensi¢cationappears to have been governed by orbitally forcedinsolation changes. Time lags between the re-

sponses of Earth’s climate system to these insola-tion changes have previously been discussed (Im-brie et al., 1993). The timing, organisation andreorganisation of the ISOW £ow is in accordancewith a proposed ‘super-fjord’ process during thelast deglaciation (Berger and Jansen, 1995) andthe ¢rst deglaciation meltwater signal in the Nor-dic Seas. The latter, Termination 1a, was dated to15.8 kyr BP (14C age) in the Greenland Sea (Nam,1997), to 15.0 kyr BP in the Fram Strait (Jonesand Keigwin, 1988), to 15.7 kyr BP in theAmundsen Sea (Stein et al., 1994a) and to 16.1kyr BP in the deep sea north of Iceland (Koc andJansen, 1994). The ¢rst deglaciation signal of theNorth Atlantic, prior to H1, is in accordance withbenthic foraminifera evidence o¡ Portugal(Shackleton, 2001; Shackleton et al., 2000; Zahnet al., 1997) and clay mineral studies in the Lab-rador Sea (Fagel et al., 1997). In addition, grain-size studies of deep water cores from the RockallTrough region, northeast Atlantic, report a simi-larly NADW re-activation (McCave et al., 1995a).Alkenone based SST reconstructions in the sub-tropical (Chapman et al., 1996) and northeastNorth Atlantic (Madureira et al., 1997) point totemperature rises that started at ca. 18^19 cal kyrBP and 21.5 cal kyr BP, respectively. New SSTdata from Feni Drift provide strong support foran initial enhancement of North Atlantic convey-or activity prior to H1 (Lagerklint and Wright,1999). Recent work in the western tropical Atlan-tic has documented a similar signi¢cant warmingfor H1 (Ru«hlemann et al., 1999).We stress that the early re-activation of the

ISOW £ow at the Reykjanes Ridge at 59‡N iscoeval with early Southern Hemisphere deglacia-tion and enhanced thermohaline circulation in the

Fig. 7. IRD counting (van Kreveld et al., 2000), Q/Plag ratio, Mean 6 10 Wm, Mrs/Ms, calcite and N. pachyderma (s) (van Kre-veld et al., 2000) vs. depth records of core SO82-05GGC. For comparison, the GISP2 (Grootes and Stuiver, 1997) and BYRD(Blunier and Brook, 2001) N18O ice core records are also presented. IS number 4b indicates a short-term warming identi¢ed inSO82-05GGC (van Kreveld et al., 2000) and in Greenland ice cores (Dansgaard et al., 1993; Grootes et al., 1993). Lines showthe correlation between the ice core and the N. pachyderma (s) records. Note the similar saw-tooth shape of the marine Q/Plagratio and the ice core record. Vertical arrows show corresponding Mean 6 10 Wm and Q/Plag ratio (IRD) maxima. Additionallyintegrated in the GISP2 ice core record is the July insolation curve at 65‡N (Berger and Loutre, 1991; dark grey dashed line). Adashed line marks the position of Ash II. Two strong ISOW intensi¢cations, which are obviously linked to strong insolation in-creases, are indicated by dashed arrows. Note that these intensi¢cations start well before peak H6 and H1 (at 19.2 cal kyr BP âge in the box), respectively. Furthermore, an initial temperature increase is also observed in the GISP2 ice core record (arrow).

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Southern Ocean. Our records indicate that sub-surface level £ow in the Northern Atlantic inten-si¢ed at the same time as surface waters remainedrelatively cold and relatively iceberg-free. For rea-sons of continuity and mass balance, we have toassume that the early intensi¢cation of SouthernOcean thermohaline circulation and warming atlower latitudes implies re-activation of North At-lantic THC, at least at a sub-surface level. A sim-ilar observation was recently recorded for Termi-nation II (Bauch et al., 2000). Furthermore, alsoShackleton et al. (2000) observed a temporarilyde-coupling of surface and deep water processeso¡ Portugal in benthic and planktonic N

18O re-cords. The above-mentioned ¢ndings are basedon several independent proxy records and allowus to speculate on the origin of H1 and other IRDevents. We pose two questions, which might beparticularly crucial for understanding the appar-ent ‘out-of-phase’ relationship between Antarcticaand Greenland. First, was H1 triggered by en-hanced heat transport to the higher latitudesand hence a ‘warming-event’? Second, did themeltwater originating from iceberg discharge ob-scure the enhanced northward heat transport asseen in SST’s? If the answers to these questionsare positive, then the reconstructed scenario is oftemporarily de-coupling between surface and deepwater processes in the North Atlantic at the sametime as colder temperatures prevailed over Green-land. Note that an initial temperature increaseover Greenland is also observed at ca. 21^22 calkyr BP (Fig. 7).Based on our records a similar scenario can

apply to H2. Note again the stepwise increasingamplitudes of the spikes in the N. pachyderma (s)record in the time span between H3 and H2 ^ asaw-tooth shape on an average level (Fig. 7). Alsothe ISOW £ow intensity and IRD proxy recordsdisplay a well-developed saw-tooth pattern overthe time interval focused on here. Interestingly,the IRD saw-tooth started later than the ISOWsaw-tooth and the intermediate water £ow shutdown slightly before peak H2 (Figs. 6^8). A sim-ilar scenario is observed when looking at IS 8(Figs. 6^8). Note the overlapping saw-tooths be-tween the IRD and intermediate water £ow inten-sity records. Early SST decreases, registered in

palaeoecological data sets by the increased abun-dance of N. pachyderma (s), might be due to thecold meltwater originating from icebergs ^ where-as simultaneously ISOW £ow intensity is still in-creasing. Note that van Kreveld et al. (2000) re-ported: ‘The majority of IRD peaks occur nearthe end of low-SST stadials’ in relation to melt-water input. Various other studies at northernhigh latitudes report signi¢cant meltwater pulsesbefore peak IRD events, e.g. H1 (Funder et al.,1998; Jones and Keigwin, 1988; Nam, 1997; Namand Stein, 1999; Norgaard-Pedersen, 1997; Sarnt-hein et al., 1995; Stein et al., 1996, 1994a,b).In most MIS 3^MIS 2 reconstructions the

ISOW £ow intensity shuts down before IRD in-put peaks. This is consistent with the ¢ndings ofPrins et al. (2002) who observed low ISOW £owspeed during peak IRD events. However, whenlooking carefully at their records the ISOW shutsdown after the iceberg discharge already in-creased. Note again, if insolation changes seemto be the strongest force behind ISOW £ow inten-sity changes (e.g. before H6 and H1) no signi¢-cant, only weak, reductions in the bottom water£ow have been observed, which are related to thehigher-frequency lithic, hence meltwater events.The IRD threshhold level seems to be deter-

mined by two processes that are potentially linkedto insolation changes: (1) the heat £ux to thenorth, i.e. thermohaline circulation intensity, and(2) the available ice sheet cover before the ISOW£ow intensi¢ed. The processes leading to the saw-tooth patterns displayed by the various proxy re-cords must be understood to determine the originof IRD events. Our hypothetical links between theenhanced northward heat-transport, increasingmeltwater input and cold SST could explain avariety of other features observed in the palaeo-records, e.g. those in the IS 14^12 time interval.Here strong ISOW £ow is observed at the sametime as cold SST’s existed and relatively cold at-mospheric temperatures prevailed over Green-land.Insolation changes could explain several fea-

tures of the records. When insolation (July65‡N) is stable at a high or low level, or isstrongly increasing, the amplitudes of the high-frequency oscillations are small (older part of

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Fig. 8. MS and smectite content (in clay fraction 6 2 Wm, Gehrke et al., 1996) records of three Reykjanes Ridge cores vs. depth.Mrs/Ms of core SO82-07GGC, plus Q/Plag ratio records are also shown. Vertical grey bars mark the positions of the Heinrichlayers (H6^H1), dashed lines the position of Ash II. Note that the smectite contents and the MS values (average peak values)started to increase and the magnetic Mrs/Ms ratio to decrease well before H1 (dashed arrows). The data point to a more or lesssteady increase of the content of Iceland-derived material in the sediments, most probably caused by the steady increase inISOW £ow which started well before H1 and lasted until mid-Holocene times. A further strong increase in over£ow intensity isobserved before H6 (dashed arrow). The latter intensi¢cation and the increase, obviously related to the last deglaciation, seem tobe coherent with strong insolation increases in July at 65‡N. The general patterns of the various proxy records that re£ect ISOW£ow intensity changes are quite similar to that of the insolation curve. Note that large amplitude £uctuations in the ISOW inten-sity occurred during the time span between H4 and H3 (DÔ cycles 8^5) when insolation is on a stable moderate level.

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MIS 3 and MIS 2, last deglaciation) ^ obviouslydue to less in£uence of the secondary, but poorlyunderstood, mechanism which is responsible forthe proposed 1.5-kyr cycle. Increasing insolationleads to, via ISOW intensi¢cation, strong IRDdischarge as found for H6 and H1 (also H11,unpublished data). Obviously during these phasesinsolation is the strongest forcer of ISOW £owintensity. Thus, for instance, the 1.5 kyr cyclehas no strong impact on YD £ow conditions.Note in that context the decreasing amplitude ofthe cold events during the last deglaciation (H1-YD-PBO-8.2 event) which was due to the shrink-ing ice-cover. Within this context we note that IS18^15 cannot be distinguished in our records.When insolation is at a stable, medium level, oronly slightly changes (e.g. time span between H4and H3), or decreases, the in£uence of the 1.5-kyrcycle on the environmental conditions appears tobe strongest. This is in accordance with recent¢ndings by Schulz (2002) who investigated the1470-yr ice core cycle in detail.Substantial support for the Heinrich ‘warming’

idea comes from recent studies (Grousset et al.,2000; Scourse et al., 2000), which demonstratethat the smaller European ice sheets calved beforethe Heinrich (i.e. H2 and H1) iceberg armadasstarted. Furthermore, Andrews et al. (1998) arguefor the same ‘warming’ hypothesis. Their Ba⁄nBay detrital carbonate layers, which might belinked to North Atlantic Heinrich events, weredeposited during IS recorded in Greenland icecore records. Hiscott et al. (2001) observed a co-variance between SST and ice-rafting from 60 to20 kyr in the Labrador Sea. Stein et al. (1996),Funder et al. (1998) and Nam (1997) interpretedtheir meltwater signals before H11 and H1 as theresult of warm water incursions and possiblyundermelting (sub-surface). Moreover, it hasbeen reported that the benthic foraminiferal as-semblages (notably of stadial and not interstadialperiods) are dominated by species with an a⁄nityfor relatively warm intermediate water (Rasmus-sen et al., 1996a). It is likely that this warm At-lantic water initiated large-scale bottom meltingand destabilisation of marine ice shelves and largeoutlet glaciers of the European Barents Sea. Sucha process would also apply to the Greenland and

Laurentide ice sheets. Within this context it isimportant to note that recent studies have indi-cated the existence of probably more than 1-km-thick glacial ice shelves in the Arctic (Polyak etal., 2001). Thus, as also evident from the studiesby Bauch et al. (2001), it seems that sub-surfaceheat transport must be taken into account. Obser-vations of recent iceberg discharge processes andice-rafting (Reeh et al., 1999), plus studies of lateHolocene environmental changes in East Green-land fjord and coastal waters (Jennings andWeiner, 1996) show that the transport and depo-sition of iceberg-derived IRD increased duringrelatively warm periods. The latter study demon-strated an increase of IRD production in periodsof enhanced Atlantic Intermediate Water £ow rel-ative to Polar Water in the East Greenland Cur-rent.

6.2. Is the saw-tooth pattern a key issue whencomparing marine and Greenland ice core records?

There appears to be a link between the saw-tooth pattern of the marine records and theN18O records of Greenland ice cores (Fig. 7). Ifthe idea that the IRD events are ‘warming’ events,i.e. they are triggered by enhanced (partly sub-surface) heat transport to the North, is accepted,then no further process needs to be postulated toexplain the sudden return to warmer temperaturesover Greenland immediately after the IRD events.Due to the enhanced heat transport to the northand subsequent gradually increasing iceberg calv-ing, there is a steady decrease in SST (due tomeltwater input) also a¡ecting the atmospherictemperature over Greenland. This initial en-hanced northward heat transport is inferredfrom a variety of sediment core parameters con-trolled by ISOW £ow. In other words, the NorthAtlantic surface and intermediate water and shal-lower sub-surface processes were temporarily de-coupled due to a negative feedback response toinitial warming, at the same time as colder tem-peratures were established over the North Atlan-tic region and Greenland. After the iceberg dis-charge rate fell below threshold values, thetemperature over Greenland could rapidly riseto higher, more ‘normal’ values. ‘Normal’ values

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re£ect the basic long-term heat transport level,re£ected in the intermediate water £ow intensity,which seems to be orbitally forced. ISOW £owconditions are only strongly a¡ected by the ice-berg melting processes under conditions when so-lar insolation forcing is dominated by the second-ary forcing mechanism, the 1.5 kyr cycle.Our proposed IRD ‘warming’ mechanism could

explain the ‘out-of-phase’ relationship betweenthe Greenland and Antarctic ice cores records(Fig. 7; Blunier and Brook, 2001; Broecker,1998; Charles et al., 1996; Kanfoush et al.,2000) and the delays between warming in the tem-perature over Greenland (expressed in N

18O rec-ords of Greenland ice cores) and insolation in-creases in July 65‡N (Fig. 7, e.g. last deglaciation;time interval IS 19 and IS 17 due to IRD eventH6). Note again, that initial increasing tempera-tures are also observed in the GISP2 ice core atca. 21 kyr BP (Fig. 7). Strong warmings in Ant-arctica and strong ISOW intensi¢cations are re-corded before prominent North Atlantic IRDevents, i.e. for H6^H4, H2, H1 (Fig. 7; Blunierand Brook, 2001).

7. Conclusion

It has been shown that the multi-proxy sedi-ment data sets from the Reykjanes Ridge (at59‡N) as well as from the Faeroe area (at 61‡N)display similar temporal features that can be spa-tially correlated. The well-known Heinrich eventsH4^H1 and the higher-frequency lithic events de-scribed by Bond and Lotti (1995) have been ob-served in our records. They were identi¢ed ac-cording to typical characteristics: relatively lowmagnetic susceptibility, high GRAPE densities,high quartz/plagioclase ratios, high ice-rafted de-tritus contents and high percentage abundance ofthe polar N. pachyderma (s). In the cores fromboth studied areas the calcite events reported byKeigwin and Jones (1994), which represent theinterstadials IS 16^5 of the Greenland ice corerecords, have been found and their occurrenceindicates at least their large-scale regional charac-ter. The long-term trends in the ISOW £ow inten-sity and circulation changes, which were recon-

structed on the basis of various independentproxy records, appear to be strongly controlledby orbitally forced insolation changes. The ob-served features and de-coupling of surface anddeep water processes point to the need to re-eval-uate the origin of the ice-rafted detritus events.Our data strongly suggest that these events mayhave been triggered by enhanced, partly sub-sur-face, heat transport to the North. A key feature inthe palaeo-records seems to be the occurrence ofsimilar saw-tooth patterns in the marine as well asin the Greenland ice core records. When acceptingthe idea that the North Atlantic IRD events are‘warming events’, which resulted in regional lowerSST and hence lower atmospheric temperatureover Greenland, the observed ‘out-of-phase’ rela-tionship between the Greenland and Antarctic icecore records can be explained. Furthermore, the‘warming’ theory of ice-rafted detritus eventscould explain the rapid recovery to higher temper-atures over Greenland after the cold phases of theDansgaardÔeschger cycles, the delays betweenthe Greenland ice core records and insolation(July 65‡N) changes, the di¡erent magnitude ofice-rafted detritus events (massive H11, H4, H1and those related to DansgaardÔeschger cycles8^5) and the H6 and H3 ‘observation problems’in North Atlantic sediments. We note again thatwhile some details of our proposed mechanism forIRD events may remain unresolved (are partlyspeculative), the high-resolution data presentedhere appear to have major implications for theunderstanding of the widely discussed origin ofthe North Atlantic ice-rafted detritus events.

Acknowledgements

We are grateful to Klas Lackschewitz, ColliWallrabe-Adams, Shirley van Kreveld, Tine Ras-mussen and Svante Bjo«rck who helped in manyfruitful discussions. Colli Wallrabe-Adams is alsothanked for performing the settling velocity mea-surements at Geomar in Kiel. Ru«diger Stein,Herve Chamley and an anonymous reviewer arethanked for their helpful comments. We are grate-ful to Shirley van Kreveld for providing N. pachy-derma (s) and IRD data from core SO82-05GGC

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and to Lloyd Keigwin for providing the calcitedata of GPC-5. This study was supported (M.Moros) by the German Deutsche Forschungsge-meinschaft (Project GI324/1-1).

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