reorganization of the upper ocean circulation in the mid-holocene in the northeastern atlanticthis...

Reorganization of the upper ocean circulation inthe mid-Holocene in the northeastern Atlantic1,2

Sandrine Solignac, Michael Grelaud, Anne de Vernal, Jacques Giraudeau,Matthias Moros, I. Nicholas McCave, and Babette Hoogakker

Abstract: A micropaleontological investigation was conducted on two sediment cores from the Reykjanes Ridge (RR;core LO09-14; 59812.30’N, 31805.94’W) and the Faroe–Shetland Channel (FSC; core HM03-133-25; 60806.55’N,06804.18’W) to document hydrographical changes of the North Atlantic Current (NAC) during the Holocene. Dinocyst andcoccolith assemblages were analyzed, and quantitative reconstructions of sea surface temperatures (SSTs) and sea surfacesalinities (SSSs) were conducted based on dinocyst assemblages. Both proxies suggest a major reorganization of surfacecirculation patterns in the northeastern North Atlantic between 7 and 5.4 ka BP. At both sites, SSSs before 6.5–7 ka BPwere lower than during the mid-late Holocene, suggesting dispersal of meltwater through the NAC. Long term trends ofSSTs, however, show higher than present summer SSTs on the RR from 9.3 to *6 ka BP, and lower than present SSTs inthe FSC until ca. 5.4 ka BP. The contrasted SST trends at the two sites suggest that decreasing summer insolation was notthe only forcing behind hydrographical changes in the region. Decoupling of the NAC and the Slope Current (SC), whichboth influence the FSC, is proposed as a possible mechanism. We hypothesize that a strong NAC during the early to mid-dle Holocene resulted in a SST increase on the RR and decrease in the FSC. Inversely, a weaker NAC after 5–6 ka BP,leading to decreased SSTs on the RR, would have enhanced the relative contribution of the warmer, saltier SC in the FSC,thus resulting in a regional SST and SSS increase.

Resume : Une etude micropaleontologique a ete effectuee sur deux carottes de sediments prelevees de la ride de Rey-kjanes (RR – carotte LO09-14; 59812,30’N, 31805,94’O) et du chenal Faroe–Shetland (FSC – carotte HM03-133-25;60806,55’N, 06804,18’O) afin de documenter les changements hydrographiques du courant de l’Atlantique Nord au coursde l’Holocene. Des assemblages de dinokystes et de coccolithes ont ete analyses et des reconstructions quantitatives detemperature « SST » et de salinite « SSS » a la surface de la mer ont ete effectuees basees sur les assemblages de dino-kystes. Les deux indicateurs suggerent une reorganisation majeure des patrons de circulation a la surface dans le nord-estde l’ocean Atlantique Nord entre 7 et 5,4 milliers d’annees avant le present (ka BP). Aux deux sites, les SSS avant 6,5–7 ka BP etaient inferieures a celles durant l’Holocene moyen/tardif, suggerant une dispersion de l’eau de fonte par le cou-rant de l’Atlantique Nord. Les tendances a long terme des SST montrent toutefois des SST estivales superieures aux SSTactuelles dans la carotte RR de 9,3 a *6 ka BP et des SST inferieures aux SST actuelles dans la carotte FSC jusqu’a envi-ron 5,4 ka BP. Les differentes tendances des SST aux deux sites suggerent que l’ensoleillement decroissant en ete ne soitpas le seul forcage responsable des changements hydrographiques dans la region. Le decouplage du NAC et du courant depente « SC », qui influent tous deux sur le FSC, est propose en tant que mecanisme possible. Nous posons l’hypothesequ’un fort courant de l’Atlantique Nord a l’Holocene precoce/moyen a conduit a une augmentation des SST a la ride deReykjanes et a une diminution au chenal Faroe–Shetland. Inversement, un courant de l’Atlantique Nord plus faible, poste-rieur a 5–6 ka BP, conduisant a une SST reduite sur la RR, aurait rehausse la contribution du courant de pente, plus chaudet plus salin, dans le chenal Faroe–Shetland, ce qui aurait augmente la SST et la SSS regionales.

[Traduit par la Redaction]

Received 24 January 2008. Accepted 29 October 2008. Published on the NRC Research Press Web site at cjes.nrc.ca on 20 January2009.

Paper handled by Associate Editor P. Hollings.

S. Solignac3 and A. de Vernal. GEOTOP, Universite du Quebec a Montreal, C.P. 8888, Succursale Centre-ville, Montreal,QC H3C 3P8, Canada.M. Grelaud. CEREGE, Europole de l’Arbois, 13545 Aix-en-Provence, France.J. Giraudeau. Environnements et Paleoenvironnements Oceaniques, UMR CNRS 5805, Universite Bordeaux 1, Avenue des Facultes,33405 Talence, France.M. Moros. Bjerknes Centre for Climate Research, Allegaten 55, Bergen, 5007, Norway, and Institut fur Ostseeforschung, Warnemunde,Germany.I.N. McCave and B. Hoogakker. Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, UK.

1This article is one of a series of papers published in this Special Issue on the theme Polar Climate Stability Network.2GEOTOP Publication 2009-0002.3Corresponding author (e-mail: [email protected]).

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Can. J. Earth Sci. 45: 1417–1433 (2008) doi:10.1139/E08-061 # 2008 NRC Canada

Introduction

The flow of Atlantic waters to the high latitudes of the NorthAtlantic plays a major role in the heat transfer to the pole, reg-ulating the climate of western Europe as well as influencingconvection processes through which deep water formation oc-curs. It is thus essential to understand how warm, saline watersupplies to the Nordic seas and the Arctic Ocean vary throughtime and what the consequences of these variations are.

Many studies have shown decreasing sea surface temper-atures (SSTs) during the Holocene, commonly attributed toinsolation changes (e.g., Marchal et al. 2002). However, pre-vious studies (e.g., Solignac et al. 2004, 2006; de Vernal andHillaire-Marcel 2006) show that hydrographical changes inthe mid- and high latitudes of the North Atlantic were notuniform with respect to their amplitude and spatial extentand suggest that a better understanding of the various com-ponents of the oceanic circulation is needed.

Here, we present a Holocene record from the Faroe–Shetland Channel (FSC), which is one of the pathways forpoleward Atlantic water fluxes. Two micropaleontologicaltracers, coccoliths and dinoflagellate cysts (dinocysts), wereanalyzed to identify changes in the surface hydrography ofthe FSC. Comparison with a record from the ReykjanesRidge (RR), southwest of Iceland, led to the documentationof the spatial patterns of the changes in sea-surface condi-tions and permitted us to propose possible mechanisms re-sponsible for them.

The distribution of the various species of dinoflagellatesand coccolithophores is strongly influenced by hydrographi-cal parameters such as SST and sea surface salinity (SSS),and a good correspondence is observed between the charac-teristics of the overlying water masses and dinocyst or coc-colith assemblages preserved in the sediment (e.g., Eide1990; Baumann et al. 2000; de Vernal and Marret 2007 andreferences therein). Coccolith and dinocyst assemblages maysuffer from specific drawbacks, such as calcite dissolution incoccoliths (Samtleben and Schroder 1992) or the higher sen-sitivity to oxidation of some dinocyst species (Zonneveld etal. 2001). Analyzing both proxies therefore allows us to dis-criminate somewhat between ecological signals and tapho-nomic processes and thus to obtain a more accurate pictureof the paleoceanography of the Holocene in the northeasternAtlantic.

Oceanographic settingThe study area lies under the influence of poleward-

flowing Atlantic waters, which are warm and saline com-pared with the cold and fresher waters from the ArcticOcean. However, here, it is essential to highlight the differ-ences among the various water masses comprised under thegeneral term ‘‘Atlantic waters.’’

The Gulf Stream splits into a northern and a southernbranch at *558W (Reverdin et al. 2003), and its northernbranch eventually splits up as well. A first limb turns northbefore crossing the mid-Atlantic Ridge (MAR; Holliday etal. 2006) and flows along the western flank of the RR in theIrminger Sea, forming the Irminger Current (IC; Pollard et

al. 2004). As it continues towards Greenland, the IC makesone of the northern limbs of the North Atlantic SubpolarGyre (Fig. 1). The upper water masses in the subpolar gyreare referred to as Subarctic Intermediate Water (SAIW),4which has its source in the polar Labrador Current (LC; Pol-lard et al. 2004). SAIW is typically characterized by astrong, shallow, seasonal salinity-driven stratification and isrelatively cold and fresh. It extends eastward to *328W at558N–568N and farther east (258W–268W) southward (at538N), forming the Subarctic Front with warmer, more salinewaters from the North Atlantic Current (NAC; Fig. 1; Read2001; Pollard et al. 2004; Holliday et al. 2006).

The remainder of the northern branch of the Gulf Streamflows eastward along the 508N parallel as the NAC (Read2001), crossing the North Atlantic Basin. Despite some dis-crepancies among authors, there is a general agreement on a‘‘multi-branch’’ structure of the NAC. The northernmostNAC branch (Fig. 1) coincides with the Subarctic Front(Pollard et al. 2004; Brambilla 2006). The southern NACbranch partly recirculates in the Iceland Basin (Brambilla2006) and carries a water mass referred to as Western NorthAtlantic Water (WNAW), which is seasonally highly strati-fied, warm, and saline (Read 2001). Part of the southernbranch then flows south along the eastern flank of the RR,and eventually both northern and southern NAC branchescross the MAR again towards the west and join the north-ward path of the IC along the western flank of the RR(Fig. 1; Pollard et al. 2004).

Another part of the southern branch of the NAC continuestowards the Iceland–Faroe Gap and becomes the slightlyfresher and colder Modified North Atlantic Water (MNAW)in the vicinity of the Iceland–Faroe Gap. Part of it thenflows east-southeast as the Faroe Current (FC) and reachesthe FSC (Fig. 1).

A third branch of the NAC flows northeastward into thesouthern Rockall Trough and then on Rockall Plateau andHatton Bank (Fig. 1). This branch is referred to as theRockall–Hatton branch and carries WNAW (Pollard et al.2004). In addition, a major difference compared with theIceland Basin characterizes the Rockall Trough and theFSC; here another type of water mass influences the region(Hansen and Østerhus 2000; Pollard et al. 2004). The SlopeCurrent (SC) originates in the intergyre region in the Bay ofBiscay (Read 2001) and flows into the Rockall Trough allthe way to the FSC (Hansen and Østerhus 2000; Pollard etal. 2004; Fig. 1). It carries Eastern North Atlantic Water(ENAW; Pollard et al. 1996, 2004; Read 2001), which issignificantly warmer, more saline, and much less stratifiedthan waters derived from the NAC. The upper water massin the FSC therefore results from the mixing of colder,fresher MNAW or WNAW from the Rockall–Hatton Branchof the NAC on the Faroese side, and warmer, more salineENAW from the Scottish slope (Hansen and Østerhus 2000;Orvik and Niiler 2002).

Material and methods

Two core records from the northeastern Atlantic wereused in this study. The westernmost one, LO09-14

4 Water mass denominations vary among authors; here we use those provided by Read (2001).

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(59812’N, 31805.94’W, 1493 m water depth), is located onthe RR, southwest of Iceland (Fig. 1). This record is a com-posite made out of four cores, i.e., box core LO09-14 LBC,giant gravity core LO09-14 GGC, gravity core LO09-14 GC(all 3 retrieved at the same site), as well as neighbouring

core DS97-2P (58856’33N, 30824’59W, 1685 m waterdepth), which was used to fill the gap from 1.2 to 2.2 kaBP between the LO09-14 LBC and the LO09-14 GGC (Gre-laud 2004; Moros et al. 2004). The easternmost site, HM03-133-25 (60806’N, 06804.18’W, 1156 m water depth), is lo-

Fig. 1. (a) Trajectories of the main ocean surface currents (EGC, East Greenland Current; FC, Faroe Current; GS, Gulf Stream; IC, IrmingerCurrent; LC, Labrador Current; NAC-N, northern branch of the North Atlantic Current; NAC-RH: North Atlantic Current Rockall–Hattonbranch; NAC-S, southern branch of the North Atlantic Current; NwC-E, eastern branch of the Norwegian Current; NwC-W: western branchof the Norwegian Current; SC, Slope Current; WGC, West Greenland Current) and location of the regions discussed in the text (FSC,Faroe–Shetland Channel; HB, Hatton Banks; IFG, Iceland–Faroe Gap; RP: Rockall Plateau; RR, Reykjanes Ridge; RT, Rockall Trough).Dotted arrows represent cold currents and solid arrows represent warm currents. The broken line near the Reykjanes Ridge corresponds withthe Subarctic Front (SAF). (b) Location of cores HM03-133-25 and LO09-14 and other cores discussed in the text.

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cated at the junction of the Faroe–Shetland and Faroe Bankchannels, in the pathway of the Norwegian Sea Deep Wateroverflow current (Masson et al. 2004), south of the Faroe Is-lands (Fig. 1).

The LO09-14 site thus lies in the vicinity of the subarcticfront, the position of which controls the relative contribu-tions of SAIW and WNAW. The HM03-133-25 site, at thesouthern end of the FSC, is influenced by ENAW that flows,along with a varying amount of WNAW, through the Rock-all Trough and over Wyville–Thompson Ridge.

As a result, SSTs are lower on the RR (10.9 and 6.8 8C insummer and winter, respectively) than in the FSC (11.6 and8.4 8C; National Oceanographic Data Center (NODC) 2001).The same goes for SSSs (34.97 in summer and 35.14 in win-ter on the RR versus 35.29 and 35.28 in the FSC; NODC2001).

ChronologiesAges on the three LO09-14 cores were calculated from

34 AMS 14C dates measured on the foraminifer species Glo-bigerina bulloides, and core DS97-2P was dated with20 AMS 14C ages measured on planktonic foraminifera (fordetails, see Andersen et al. (2004), Moros et al. (2004), andPrins et al. (2001)). Sedimentation rates on all cores allow atemporal resolution varying from <10 to *150 years with asample spacing of 1 cm.

The chronology for core HM03-133-25 is based on10 AMS 14C dates. The 14C ages were converted into calen-dar years using the 5.0.1 version of the CALIB software ofStuiver and Reimer (1993), with a reservoir age of 400 years.A spline fit was used to convert depths into calendar ages.Sedimentation rates vary from 18 to 86 cm�ka–1, and tempo-ral resolution of our analyses from *47 to 395 years (I.N.McCave, personal communication).

Micropaleontological tracersThe assemblages of two microfossil groups, coccoliths

and dinoflagellate cysts (or dinocysts), were analyzed onboth cores and on the same samples to allow direct compar-ison of the two proxies.

Sediment sample preparations for coccoliths were con-ducted using standardized techniques detailed in Andrewsand Giraudeau (2003). Coccoliths were identified with anoptical microscope at 1250� magnification and at the spe-cies level, except for Syracosphaera spp., which might in-clude more than one species. In core HM03-133-25, thedistinction was made between two morphotypes of Emilia-nia huxleyi according to the degree of calcification of theircentral grid as identified by Beaufort and Heussner (2001,see also references therein). These morphotypes are here-after referred to as open (slightly calcified) and closed(highly calcified) E. huxleyi.

Samples were treated for dinocyst population analyses fol-lowing standard techniques described in de Vernal et al.(1999). Ideally, 300 dinocysts should be counted for eachsample to get statistically significant results. However, verylittle sediment was left for palynological treatments in coreLO09-14, and it was often not possible to reach 300 individ-uals, especially in the 3–6 ka BP interval. Here we used onlythe samples for which >50 dinocysts were counted for quan-titative reconstructions. Dinocysts were identified according

to the taxonomic nomenclature of Rochon et al. (1999) andHead et al. (2001) on a transmitted light microscope at500� magnification. Heterotrophic brown cysts such asBrigantedinium spp., Quinquecuspis spp., or Lejeunecystaspp. had to be grouped in the Protoperidinioid class whenunidentifiable. When properly oriented on the slide and easyto recognize, they were identified at the species level.

Principal components analyses (PCA) were performed oneach of the two dinocyst records, as well as on each of thetwo coccolith records, to highlight the major shifts in the as-semblages.

Quantitative hydrographical parameter reconstructionsThe best analogue technique was applied (e.g., Guiot

1990) on the dinocyst assemblages for quantitative recon-structions of sea-surface conditions based on the similaritybetween modern and fossil dinocyst spectra. It allows thequantitative reconstruction of SSTs and SSSs in winter andsummer (de Vernal et al. 2001, 2005).

The relative abundances of taxa were logarithmically trans-formed to give more weight to secondary taxa that thrive inmore specific ecological niches than the ubiquitous dominanttaxa. The modern dinocyst assemblage database used herecomprises the 1075 reference sites from the northern NorthHemisphere presented in de Vernal et al. (2005) and addi-tional ones (Kielt 2007; Ladouceur 2007; Radi et al. 2007;Pospelova et al. 2008; Richerol et al. in press), bringing thetotal number of sites to 1189. Hydrographical data (SSTs andSSSs, in winter and summer) at 0 m of water depth, averagedover the period 1900–2001 (NODC 2001), were assigned toeach of these reference sites. More details on this statisticalapproach can be found in de Vernal et al. (2005).

ResultsDinocyst and coccolith assemblages in cores HM03-133-

25 and LO09-14 depict strong fluctuations that reveal im-portant changes in the surface hydrography of the northeast-ern Atlantic during the Holocene (Fig. 2). Here we focus onthe main changes as depicted by the first principal compo-nents (PC) extracted from the dinocyst and coccolith recordsand by the quantitative, dinocyst-based reconstructions(Figs. 3, 4).

HM03-133-25, Faroe–Shetland ChannelThe first two PCs of the coccolith record explain 38.9%

and 33.0% of the variance, respectively (Fig. 3a). In otherwords, they are almost equally important in summarizingthe variability of the assemblages, and together they capturethe major variations, which are indicated by the PC curvesshifting from positive to negative values or vice versa. Incontrast, the first PC (PC 1) of the dinocyst record explainsa higher (44.1%) proportion of the variance, while the sec-ond one (PC 2, 12.6%) results from second-order variations.Here we retained the first two components of each record,which reflect the main fluctuations. The interesting aspectto note is the excellent correspondence between dinocystPC 1 and coccolith PC 2, and, to a lesser extent, betweendinocyst PC 2 and coccolith PC 1 (Fig. 3a), showing thatthe two plankton groups varied in a parallel way.

Several distinct phases can be distinguished based on the


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PCs (Fig. 3a; Table 1). The Holocene in the FSC starts witha short thermal optimum as shown by the presence of dino-cyst species Spiniferites mirabilis from 9.8 to 9.4 ka BP andhigh percentages (up to 20%) of coccolith Gephyrocapsamuellerae (Fig. 2a). Currently, S. mirabilis is a subtropicalto temperate taxon that is generally not encountered in re-gions with summer SSTs < 12 8C. In the North Atlantic, itis observed in the Mediterranean Sea, on the shelf off Portu-

gal and France, and on the southern Rockall Plateau (Ro-chon et al. 1999). Similarly, G. muellerae does notconstitute >10% of modern assemblages from this region(Ziveri et al. 2004), which is consistent with the thermal op-timum depicted by S. mirabilis. Accordingly, the highest re-constructed SSTs of the entire Holocene are observed before9.4 ka BP, along with the lowest SSSs and the strongest sea-sonality (Fig. 4a).

Fig. 3. (a) Eigenvalues of the first principal components extracted from the dinocyst (black) and coccolith (grey) records of core HM03-133-25. (b) Eigenvalues of the first principal components extracted from the dinocyst (black) and coccolith (grey) records of core LO09-14.Broken lines link data points in an interval of sparse quantitative results.


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At 9.4 ka BP, S. mirabilis decreases, followed by a dropof G. muellerae and C. leptoporus *8.6 ka BP (Fig. 2a).Currently, coccolith assemblages comprising almost only E.huxleyi and lower percentages of C. pelagicus are observedin the Norwegian Sea (Ziveri et al. 2004; Solignac et al.2008) in colder waters than those of the core locations. Thisis in agreement with the decrease of warm taxon S. mirabilisand with the reconstructed SSTs, which drop at that timewhile SSSs increase (Fig. 4a).

At 7 ka BP, the strongest change in the dinocyst recordis related to the drop of the relative abundance of Nema-tosphaeropsis labyrinthus (Figs. 2a, 3a; Table 1). In the coc-colith record, closed E. huxleyi increases significantly at*6.8 ka and C. leptoporus increases from 6.5 ka BP on.Dandonneau et al. (2006) observed a shift from G.muellerae-dominated to closed E. huxleyi-dominated assemb-lages along a transect crossing the NAC, as the water masscharacteristics change from temperate to subtropical, sug-gesting a preference for warmer and (or) saltier waters forclosed E. huxleyi. Similarly, higher relative abundances ofC. leptoporus are observed offshore Portugal and Francecompared with higher latitudes (Ziveri et al. 2004; Solignacet al. 2008). Its reappearance after 6.5 ka BP could then indi-cate generally warmer conditions. At 7 ka BP, summer SSSsincrease, and both SSTs and SSSs increase drastically at5.4 ka BP.

After 5.4 ka BP, SSTs and SSSs remain rather stable forthe rest of the Holocene, apart from two short cooling eventsat 4 ka BP and after 1 ka BP (Fig. 4a). These events rely onfew data points and should be interpreted with care, but de-creasing SSTs around these dates are indicated by droppingpercentages of C. leptoporus (Fig. 2a) as well. Rather, it isthe stable values around these events that are somewhat atodds with the variability observed in the dinocyst and cocco-

Table 1. Species scores on the first two components (PC 1 andPC 2) of the principal component analyses performed on the coc-colith and dinocyst records of cores HM03-133-25 and LO09-14.

HM03-133-25

Species PC 1 (38.9%) PC 2 (33.0%)Calcidiscus leptoporus 0.8614 –0.1057Coccolithus pelagicus 0.7833 –0.2349Closed Emiliania huxleyi –0.1189 –0.7853Open Emiliania huxleyi –0.9538 0.0532Gephyrocapsa muellerae 0.2248 0.9186Syracosphaera spp. –0.0386 –0.6714

Species PC 1 (44.1%) PC 2 (12.6%)Bitectatodinium tepikiense 0.5134 0.0223Brigantedinium spp. 0.6141 0.2362Cysts of Pentapharsodinium

dalei0.2931 –0.3528

Cysts of Polykrikos schwartzii 0.3096 0.2737Echinidinium spp. 0.3495 0.2985Impagidinium sphaericum –0.2575 0.3836Nematosphaeropsis labyrinthus –0.8605 –0.4106Operculodinium centrocarpum 0.3059 0.4162Protoperidinioids 0.4968 0.4024Quinquecuspis concreta 0.1946 0.1882Selenopemphix nephroides –0.0681 0.5488Selenopemphix quanta 0.5095 –0.0784Spiniferites elongatus –0.5326 0.5266Spiniferites mirabilis –0.6505 0.4775Spiniferites ramosus –0.9165 0.1344Trinovantedinium applanatum 0.5389 –0.5023

Rare species (<1%) not taken into account in the PCA*Ataxodinium choaneCysts of Polykrikos kofoidiiDubridinium spp.Impagidinium aculeatumImpagidinium pallidumImpagidinium paradoxumImpagidinium spp.Islandinium minutumSpiniferites membranaceusSpiniferites spp.Votadinium calvum

LO09-14

Species PC 1 (34.0%) PC 2 (26.2%)

Calcidiscus leptoporus –0.0791 –0.5281Coccolithus pelagicus 0.7067 –0.4897Emiliania huxleyi 0.6125 0.6252Gephyrocapsa muellerae –0.8499 –0.1776Gephyrocapsa oceanica –0.6561 0.4334Syracosphaera spp. –0.0822 –0.6654Species PC 1 (32.3%) PC 2 (17.1%)

Bitectatodinium tepikiense 0.5016 –0.1294Brigantedinium spp. 0.5866 0.6909Cysts of Pentapharsodinium

dalei–0.4451 –0.5948

Cysts of Polykrikos kofoidii 0.0853 0.0126Cysts of Polykrikos schwartzii 0.5709 –0.1257Echinidinium spp. 0.7063 –0.175Impagidinium aculeatum –0.0965 0.0771

Table 1 (concluded).

Species PC 1 (32.3%) PC 2 (17.1%)

Impagidinium pallidum 0.0165 –0.1448Impagidinium paradoxum –0.0062 –0.1161Impagidinium sphaericum –0.0898 –0.3894Impagidinium spp. 0.0805 –0.0587Islandinium minutum –0.036 –0.0247Nematosphaeropsis labyrinthus –0.7138 –0.2318Operculodinium centrocarpum 0.1742 –0.1581Protoperidinioids 0.3418 –0.0816Quinquecuspis concreta 0.0598 0.2151Selenopemphix nephroides 0.072 0.0255Selenopemphix quanta 0.803 –0.3115Spiniferites elongatus –0.3852 0.0131Spiniferites mirabilis –0.6222 0.5892Spiniferites ramosus –0.4377 –0.4909Spiniferites spp. –0.3712 0.2726Trinovantedinium applanatum 0.7076 –0.2209

Rare species (<1%) not taken into account in the PCA*Dubridinium spp.Impagidinium patulumOperculodinium janducheneiVotadinium calvum

*These rare species do not generally occur together in the same samples,and where they do, make up <2% of the assemblage.


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lith assemblages (Figs. 2a, 3a). Also, there is a slight offsetbetween the reconstructed SST and SSS values at 0.3 ka BP(Fig. 4a) and modern values (NODC 2001). This may bedue to a lack of modern assemblages in the Faroe–ShetlandChannel (de Vernal et al. 2005). As a result, many of theselected best analogues come from the Norwegian Sea, in-ducing a slight bias. In the late Holocene, many of the sam-ples share the same analogues, yielding a rather flat curveand not fully reflecting the subtle variations in dinocyst as-semblages. Absolute values of sea-surface condition recon-structions after 5.4 ka BP should thus be taken with care.However, warmer SSTs in the mid- to late Holocene aresupported by the greater proportions of closed E. huxleyiand C. leptoporus in the coccolith record. Hence, althoughour dinocyst-based reconstructions might lack detail after5.4 ka BP, SSTs were still on average higher than duringthe first half of the Holocene.

LO09-14, Reykjanes RidgeOn the RR, a first phase is observed from 10.3 to 9–9.4 ka

BP, with the highest percentages of coccolith G. muellerae(Fig. 2b). G. muellerae then decreases while dinocyst S. ra-mosus increases, followed by warm taxon S. mirabilis, whichindicates the onset of the thermal optimum as seen in thequantitative reconstructions (Fig. 4b). The second transitionoccurs *8 ka BP when percentages of S. mirabilis drop andwarm coccoliths C. leptoporus and G. oceanica decrease, in-dicating a climate cooling until *6.5 ka BP as shown also bythe gradual SST decrease (Fig. 4b). The strongest change inboth coccolith and dinocyst assemblages is recorded aroundthis date as illustrated by their first PCs (32.3% and 34.0%of the variance, respectively; Fig. 3b). It corresponds with anincrease of cold C. pelagicus in the coccolith record and withthe disappearance of warm taxon S. mirabilis and a drop ofN. labyrinthus in the dinocyst record (Fig. 2b; Table 1) as inthe Faroe-Shetland Channel. N. labyrinthus is presentlyfound in various environments in the world ocean and ap-pears to be adapted to a wide range of hydrological parame-ters. In the North Atlantic, however, it reaches its highestrelative abundance in the subpolar gyre in the Labrador andIrminger seas (Rochon et al. 1999), i.e., in slightly colderand fresher water with a seasonal, salinity-driven stratifica-tion (Read 2001). Boessenkool et al. (2001), in a study on di-nocyst assemblages from the Greenland coast, hypothesizedthat N. labyrinthus might be better adapted to stratified con-ditions compared with turbulence-tolerant Operculodiniumcentrocarpum. Generally higher percentages of N. labyrin-thus in the first half of the Holocene might thus be related tolower SSSs associated with more stratified upper waters,rather than a lower SST signal, especially as the concomitantpresence of warm water S. mirabilis, higher percentages ofsubtropical to temperate G. oceanica (Ziveri et al. 2004),and the absence of cold water thriving coccolith C. pelagicusin core LO09-14 argue against cooler conditions during thisinterval. Accordingly, quantitative reconstructions yieldlower SSSs until 5.9 ka BP.

After *6.5 ka BP, secondary assemblage changes are lessmarked than in the FSC. Reconstructions of sea-surface con-ditions obviously suffer from the lack of data mentioned ear-lier for the interval from 6 to 3 ka BP, during whichcoccolith assemblages are quite stable apart from a single

peak of C. leptoporus. The lowest percentages of warm G.oceanica are recorded during this interval, suggesting rathercold conditions. The second PC (PC 2) of the coccolith as-semblages depicts a shift from 3 to 1.4 ka BP due to a slightincrease of C. leptoporus (Fig. 2b), which corresponds witha SST increase (Fig. 4b). At 1.4 ka BP, G. muellerae and C.leptoporus are partly replaced by C. pelagicus. These shiftsdo not stand out clearly in the PCA performed on the dino-cyst assemblages, although an increase of Brigantediniumspp., T. applanatum and Echinidinium spp. at 1.4 ka BP isobserved (Figs. 2b, 3b). A weak cooling is recorded after1.4 ka BP, which could correspond to the increase in C. pe-lagicus (Fig. 4b).

Discussion

Reorganization of eastern North Atlantic surfacehydrography in the mid-Holocene

The most prominent changes depicted by the principalcomponent analyses performed on coccolith and dinocyst as-semblages from both cores occurred between 7 and 5.4 kaBP, showing a large scale reorganization of the North Atlan-tic oceanic system, at least in the surface layers. It is consis-tent with previous studies suggesting major mid-Holoceneclimate changes (e.g., Steig 1999). This time interval corre-sponds with two important changes in terms of climate forc-ing (e.g., Kaplan and Wolfe 2006): the last remnants of theLaurentide ice sheet disappeared *6 ka BP (Dyke et al.2003), supplying the ocean with fresh meltwaters until then(Fairbanks 1989); and summer insolation, which peaked at10 ka BP, reached its steepest decline rates after 6 ka BP(Berger and Loutre 1991).

Meltwater influence on the North Atlantic hydrographyIn both records, percentages of N. labyrinthus were rela-

tively high until *6.0–6.5 ka BP (Fig. 2). As mentionedearlier in the paper, this could be an indication of morestratified waters, at least seasonally, due to generally lowerSSSs (Fig. 4b). The mid-Holocene decrease in N. labyrin-thus is consistent with other records from the Iceland Basin,namely cores MD95-2015 (Eynaud et al. 2004), MD99-2254(Solignac et al. 2004), HU91-045-072, and HU91-045-080(A. de Vernal, unpublished data; see Fig. 1 for localities).Interestingly, the early to mid-Holocene low SSSs also cor-respond to the highest contributions of the EGC–WGC dia-tom assemblage analyzed on the same core (Andersen et al.2004). Currently, this assemblage develops in cold, fresherwaters from southeast Greenland and the Labrador Sea (An-dersen et al. 2004), which bears some resemblance with thedistribution of N. labyrinthus. It might be that, rather thanbeing a temperature signal, the EGC–WGC diatom assem-blage in this core reflects the lower salinity of the water col-umn.

The low SSSs until *6 ka BP might correspond to theinfluence of meltwaters from the Laurentide ice sheet.Whereas sites in the vicinity of the ice sheet, such as in theLabrador Sea (e.g., core HU91-045-094P, Fig. 1; de Vernaland Hillaire-Marcel 2006) experienced direct meltwater ef-fects resulting in a much more dramatic SSS decrease in theearly Holocene, distant sites in the path of the NAC couldhave been influenced in a more subtle way. Meltwaters


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from the Laurentide ice sheet eventually joined the LC,which mixes with the Gulf Stream at the southern boundaryof the subpolar gyre (Fig. 1) and contributes to the SubarcticIntermediate Water eventually subducting under the NACand influencing its properties through mixing processes (Pol-lard et al. 2004). Reduced salinity in the LC would thushave an effect on the salinity of the NAC. In core HM03-133-25, where the NAC has a somewhat lesser influence,SSSs in the early Holocene were higher than on the RR,and the shift to even higher values in the late Holocene wasmore gradual (Fig. 4a).

Summer insolation decrease during the HoloceneThe decrease of summer insolation throughout the Holo-

cene (Berger and Loutre 1991) is thought to be responsiblefor the onset of the Neoglaciation *5.7 ka BP in numerousrecords (Marchal et al. 2002 and references therein).

The influence of the high summer insolation during theearly Holocene is visible in the SST record of core LO09-14on the RR, which depicts a thermal optimum from 9.4 to*8 ka BP, and in the high percentages of temperate–subtropical coccolith G. oceanica. Although the drop in

SSTs from 8 to 6.5 ka BP is much sharper than the gradualdecrease of summer insolation (Berger and Loutre 1991), itis consistent with increasing percentages of cold coccolith C.pelagicus (Fig. 2b), alkenone data obtained on the same core(Moros et al. 2004; Fig. 5), as well as other records indicatinga thermal maximum ending *6.5 ka BP in this region (e.g.,Giraudeau et al. 2000; de Vernal and Hillaire-Marcel 2006;see also Kaufman et al. 2004 and references therein).

In the FSC, however, SSTs do not depict the same trend.The SST optimum, seen in both the coccolith and dinocystrecords, occurred earlier than on the RR, which is in agree-ment with the study of Kaufman et al. (2004) on the timingof the Holocene thermal maximum and with sedimentologi-cal and dinocyst studies showing that modern-like oceaniccirculation started as early as the end of the Younger Dryasin the region (Stoker et al. 1989; Howe et al. 1998). Thethermal optimum then stopped *9 ka BP and SSTs re-mained generally low until 5.4 ka BP, after which theyrose. The relatively warm conditions observed during themid-late Holocene in the FSC suggest that the effects of di-minishing insolation were not straightforward and did notgenerate a homogeneous cooling.

Fig. 5. Comparison of dinocyst-based (black curve, three-point running mean) and alkenone-derived (grey curve, Moros et al. 2004) SSTrecords of core LO09-14. Note that different scales were used for the two records. Horizontal lines are the modern SST values at this site(NODC 2001). Broken lines link data points in an interval of sparse quantitative results.


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Possible mechanisms of hydrographical variations duringthe Holocene

Changes in sea-surface parameters can result from differ-ences in the relative contribution of the various currents in-fluencing a given region. Notably, Holliday (2003) showed

that SST and SSS variations in the Rockall Trough are pri-marily explained by the degree of mixing between theWNAW-carrying NAC and the ENAW-carrying SC. Hence,a stronger influence of WNAW relative to ENAW would re-sult in colder, fresher waters in the FSC.

Fig. 6. Comparison of dinocyst-based SSTs (black curve, three-point running mean) of core HM03-133-25 with percentages of hematite-stained grains of core VM29-191, Rockall Trough (grey curve, Bond et al. 2001). Note the inverse scale for the hematite-stained grains.

Fig. 7. Comparison of dinocyst-based SSTs (black curve, three-point running mean) of core HM03-133-25 with foraminifera-based SSTs ofcore MD95-2011, Voring Plateau (grey curve, Risebrobakken et al. 2003). Note the different scales used for the two records.


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The lower SSTs and SSSs recorded from 9.4 to 5.4 ka BPin core HM03-133-25 might then reflect a stronger flow ofNAC through the Rockall Channel and (or) through theIceland–Faroe Gap and the FC. Hence, despite the high in-solation at the beginning of the Holocene, a weakened rela-tive contribution of the SC, once the NAC reached its fullstrength at *9.4 ka BP (as indicated by the start of the ther-mal optimum in core LO09-14), led to colder and freshersea-surface conditions.

After 5–6 ka BP, the influence of the NAC on the FSCweakened and was compensated by a stronger relative con-tribution of the warmer, more saline SC. Interestingly, a re-cord from farther south in the Rockall Trough (core VM29-191; Bond et al. 2001; Fig. 1) depicts generally lower per-centages of ice-rafted debris after 5.2 ka BP, suggestingwarmer conditions (Bond et al. 2001). This record agreesquite well with ours (Fig. 6). In addition, foraminifera-basedSSTs (Risebrobakken et al. 2003) from core MD95-2011, lo-cated in the eastern Norwegian Sea under the most ENAW-influenced branch of the Norwegian Current (Poulain et al.1996; Hansen and Østerhus 2000), depict generally warmerconditions in the mid- to late Holocene (Fig. 7). Hence, itseems that several sites in the path of ENAW-influencedcurrents were characterized by increasing SSTs from themid-Holocene onwards.

Comparison with today’s climate brings to light possiblemechanisms that might cause such changes. Notably, atmos-pheric circulation variability can have significant effects onoceanic circulation. Today, the leading mode of atmosphericvariability in the North Atlantic region is the North AtlanticOscillation (NAO; Hurrell 1995). Many studies have focusedon the present-day effects of the NAO on oceanic circula-tion. This led several authors to suggest that the changes inthe hydrography of the North Atlantic during the Holocenemight have been the result of a long-term modulation of theNAO (e.g., Rimbu et al. 2003). However, much disagree-ment on whether the Holocene witnessed a shift from adominance of positive to negative NAO-like atmosphericpatterns or vice versa is apparent in the literature (e.g.,Tremblay et al. 1997; Risebrobakken et al. 2003).

Nilsen et al. (2003, 2004), based on model experiments,inferred a negative correlation between the poleward flowsof Atlantic waters in the Iceland–Faroe Gap and the FSC.They suggested that an increase of the NAO index createsstronger winds that result in a decreased inflow in theIceland–Faroe Gap and increased inflow in the FSC. It isnot clear, however, whether the enhanced flow in the FSCimplies a higher contribution of ENAW and thus higherSSTs and SSSs. Care should also be taken as local climaticfeatures can override the NAO signature. In the FSC, Holli-day (2003) studied the effect of the NAO on the amount ofWNAW entering the Rockall Trough and thus fresheningthe water masses and stated that no constant, direct relation-ship between the Rockall Trough water masses and theNAO is observed.

Other studies focused on the recent variations of the sub-polar gyre strength as observed by Hakkinen and Rhines(2004) from altimetry data. These variations do not seem tobe linked to wind stress changes associated with the NAO.Hatun et al. (2005) related them to variations of SSSs in themid-latitude North Atlantic and showed that when the sub-

polar gyre is strong, its influence on the NAC is high,whereas the influence of the subtropical gyre is low. Thiswould be consistent with decreasing SSTs on the RR after6.5 ka BP due to the NAC carrying a greater proportion ofSAIW relative to WNAW. The second effect of the strengthof the subpolar gyre is to control the influence of the rela-tively fresh NAC in the Rockall Trough. When the gyre isstrong, the influence of the NAC decreases, making the rel-ative contribution of the SC stronger. This, again, would beconsistent with our data. A similar pattern of subpolar gyrestrength associated with decoupled Atlantic water-carryingcurrents could have characterized the eastern North Atlanticduring the Holocene, although on a longer time scale.

ConclusionComparison of the dinocyst and coccolith records from

the RR and the FSC shows a remarkable agreement betweenthe two proxies, supporting further the interpretation of as-semblage variations in terms of ecological changes affectingthe upper water column. Our records unequivocally indicatea major reorganization of the northeastern North Atlanticsystem in the mid-Holocene between 7 and 5.4 ka BP. Thistime interval corresponds with the end of meltwater suppliesfrom decaying ice sheets (Dyke et al. 2003) and with thesummer insolation decrease getting steeper (Berger andLoutre 1991). Quantitative reconstructions of SSSs yield rel-atively low values until 6.5–7 ka BP at both sites, suggest-ing that meltwater supplies might have had a significantinfluence, probably through the NAC. In contrast, SSTstrends are very different at the two sites.

The RR record depicts an early to mid-Holocene thermaloptimum consistent with previous studies and interpreted asthe result of a high summer insolation. The gradual decreasein SSTs from 8 ka BP onwards was much sharper than thegradual insolation decrease, suggesting a nonlinear responseof the ocean. This is even clearer in the FSC, where, after ashort and very early thermal optimum (10.0–9.4 ka BP),SSTs remained relatively cold until 5.4 ka BP when a slightincrease is recorded. A possible mechanism to explain thistrend involves a variable influence of the poleward Atlanticwater-carrying currents in the area. Our study site today liesunder the influence of the NAC and the warmer, saltier SC.A stronger NAC during the first half of the Holocene couldhave resulted in a weaker relative contribution of the SC andthus lower SSTs and SSSs.

AcknowledgementsThis study is a contribution of the Polar Climate Stability

Network (PCNS) supported by the Canadian Foundation ofClimate and Atmospheric Science (CFCAS). The authorsalso acknowledge financial support from the Natural Scien-ces and Engineering Research Council of Canada (NSERC),the Fonds quebecois de la recherche sur la nature et les tech-nologies (FQRNT), the French Institut National des Sciencesde l’Univers/Conseil National de la Recherche Scientifique(INSU/CNRS) (Project ‘‘TARDHOL’’) and the FrenchAgence Nationale de la Recherche (ANR Project ‘‘PICC’’).Thanks are due to the two anonymous reviewers and Asso-ciate Editor Peter Hollings for their comments on the manu-script.


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reorganization of the upper ocean circulation in the mid-holocene in the northeastern atlanticthis...

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