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Palynological analyses of Eocene to Oligocene sediments from DSDP Site 338,Outer Vøring Plateau

James S. Eldrett a,⁎, Ian C. Harding b

a Shell UK Ltd., 1 Altens Farm Road, Aberdeen, AB12 3FY, UKb School of Ocean and Earth Science, Southampton Oceanography Centre, University of Southampton, European Way, Southampton, SO14 3ZH, UK

a b s t r a c ta r t i c l e i n f o

Article history:Received 26 March 2008Received in revised form 28 September 2009Accepted 5 October 2009

Keywords:palynologyEocene–OligoceneVøring PlateauNorwegian–Greenland Seapalaeoenvironment

Against the background of the profound global climatic shift from greenhouse to icehouse conditions duringthe Eocene–Oligocene transition, major geographic and oceanographic changes were taking place in theNorwegian–Greenland Sea region. The Vøring Plateau was a prominent structural feature which influencedthe evolution of water mass circulation in the Nordic seas, and we present detailed palaeoenvironmentalreconstructions of this structure. New palynological results suggest that shallow water inner-neriticenvironments were developed across parts of the Vøring Plateau during early Eocene times, with terrestrialand brackish water palynomorphs indicating that both basement highs to the north, and the crestal part ofthe Vøring Escarpment, may have been emergent. A transition from marginal-marine to open marineconditions occurred around 44 Ma ago, with the complete subsidence of the Vøring Plateau below sea level,facilitating inter-basinal surface water circulation and promoted a significant increase in photic zone fertility.Carbon sequestration associated with such enhanced productivity in the late Eocene Nordic seas may havecontributed to declining Cenozoic atmospheric carbon dioxide levels, thence to declining globaltemperatures and the development of limited Northern Hemisphere continental ice on Greenland in thelatest Eocene.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

A critical phase in Earth history occurred during the Eocene andOligocene epochs (~55 to ~30 million years ago), with an array ofgeological records indicating a profound shift in global climate duringthis interval (Lear et al. 2000; Zachos et al. 2001; Coxall et al. 2005;Ivany et al. 2006; Schouten et al. 2008). The shift from a state that waslargely free of polar ice caps to one in which ice sheets approachedtheir modern size on Antarctica (DeConto and Pollard, 2003), also sawthe development of some continental ice on Greenland (Eldrett et al.,2007). Moreover, climate modelling studies have demonstrated thatdecreasing greenhouse gas concentrations, modulated by orbitalparameters were primarily responsible for this climatic deterioration(DeConto and Pollard, 2003; Pagani et al., 2005). However, it has alsobeen argued that the opening or closing of various marine gatewaysand shifts in surface and deepwater currents throughout the Cenozoicmay have impacted on the latitudinal distribution of heat andmoisture, influencing regional-scale climate in the polar high-latitudes (i.e. Lawver and Gahagan, 2003, Huber et al., 2004; Stickleyet al., 2004). In particular, significant geographic, oceanographic and

environmental changes occurred during the early Cenozoic in theNorwegian–Greenland Sea region, namely the separation of thecontinental masses of Eurasia and Greenland, and the subsequentsubmergence of land bridges which had acted as important barriers tothe exchange of surface and deep waters between the Norwegian–Greenland Sea, the Arctic Ocean and the North Atlantic (Eldholm et al.,1994). The Vøring Plateau was a prominent structural feature of theNorwegian continental margin, which acted as one such barrier,influencing the Cenozoic evolution of water mass circulation in theeastern Norwegian–Greenland Sea (Laberg et al. 2005).

Structurally, the Vøring Plateau can be divided into two parts(inner and outer) by the Vøring Plateau Escarpment (Talwani et al.1976; Laberg et al. 2005; see Fig. 1). Post-Palaeocene thermal coolingof the lithosphere resulted in differential regional subsidence, causingthe outer part of the plateau to subside faster than the inner portions(Skogseid and Eldholm, 1989; Walker et al. 1997). Moreover, duringthe Eocene, the Vøring Marginal High was elevated both with respectto the growing ocean basin to the west, and to the more easterlyVøring Basin, and parts of the plateau may even have been emergentat this time (Eldholm et al. 1989). Interpretations of multichannelseismic data and core sediments have suggested that ocean current-influenced sedimentation prevailed on the outer plateau slope duringthe Eocene, while shallow marine conditions prevailed on the innerparts (Laberg et al. 2005). The latter authors suggest that it was notuntil the late Miocene to early Pliocene that fully hemipelagic

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⁎ Corresponding author. Tel.: +44 1224881217; fax: +44 1224 882383.E-mail addresses: james.eldrett@shell.com (J.S. Eldrett), ich@noc.soton.ac.uk

(I.C. Harding).

0377-8398/$ – see front matter © 2009 Elsevier B.V. All rights reserved.doi:10.1016/j.marmicro.2009.10.004

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sedimentation was established on the plateau, when the subsidenceof the entire Vøring structure below sea level reduced topographiccontrol on circulation (Laberg et al. 2005).

To achieve a better understanding of the palaeoenvironmentalconditions and subsidence history of the Vøring Plateau during thePalaeogene, we conducted quantitative micropalaeontological analy-ses of organic debris (palynofacies) and dinoflagellate cyst (dinocyst)assemblages extracted from sediments from Deep Sea Drilling Project(DSDP) Site 338 (Fig. 1). Palynomacerals comprise both terrestrial andmarine components and are thus ideal for palaeoenvironmentalreconstructions as they can distinguish between (and identifycontributions from) both terrestrial and marine sources. In addition,while foraminiferal studies in the Norwegian–Greenland Sea arelimited by variable recovery due to dissolution and environmentalexclusion, Eocene dinocysts are abundant and extremely diversethroughout this region, and are known to be good indicators of surfacewater conditions (i.e. sea-surface temperature and productivity: Sluijset al., 2005).

Manum (1976) provided the first Palaeogene dinocyst zonation forthe Norwegian–Greenland Sea based on Site 338, and although hisstudy was of relatively low resolution (i.e. one sample every 9 m), hedescribed many new taxa. Manum (1976) also conducted basicpalynofacies analyses that provided an insight into the palaeoenvir-onmental history of the Vøring Plateau. Eldrett et al. (2004) presenteda new study of the dinocysts from Site 338, supplemented by OceanDrilling Program (ODP) sites 643 and 913, identified additional age-diagnostic species from the Norwegian–Greenland Sea, and therebyestablished a more detailed Eocene–Oligocene dinocyst stratigraphyfor the region. Eldrett et al. (2004) also provided the first robustmagnetic polarity stratigraphy for sites 338, 643 and 913, thuspermitting independent age control and direct calibration of theNorwegian–Greenland Sea dinocyst biostratigraphy to the GlobalPolarity Timescale. The latter study provides the chronostratigraphicbasis for the palaeoenvironmental interpretation and regionalcorrelation of Norwegian–Greenland Sea localities presented in thispaper. The age–depth model used for Site 338 is presented in Fig. 2,with the main age-diagnostic events listed in Table 1.

2. Materials and methods

2.1. Site background

Sediments from Site 338 were recovered during DSDP Leg 38,which cored lower Eocene to Quaternary sediments overlying basalticbasement from the northern flank of the Vøring Plateau (Fig. 1). Wehave analysed Eocene to Oligocene sediments assigned to the fourlithological units shown in Fig. 2 (Talwani et al., 1976). The deepestsediments overlying basaltic basement were assigned to lithologicalunit 3C, which are almost exclusively brownish-grey sandy mud withvariable sand content. In smear slides, these coarse, terrestriallyderived sediments were shown to contain a high percentage of lithicfragments and quartz grains (Talwani et al., 1976). Despite poorrecovery, the sediments assigned to lithological unit 3B are comprisedmainly of terrigenous muds, which are locally sandy and calcareous.Mottling is extensive, and bioturbation includes Chondrites and Hel-minthoida burrows in the majority of cores (Talwani et al., 1976).Lithological unit 3A is comprised of glauconitic sandy mud. Moreover,these sediments contain rounded lithic grains comprised of alteredtrachytic lava, together with grains of magnetite, pyroxene, amphi-bole and subhedral plagioclase, suggesting derivation from apredominantly igneous source (Talwani et al., 1976). The presenceof angular clay clasts and upward coarsening sequences point towardturbiditic processes as the main transport mechanism. The sedimentsassigned to lithological unit 2D are comprised mainly of muddydiatomaceous oozes, with common Zoophycos burrows.

2.2. Palynological methods

Fifty-nine samples from the Eocene to lower Oligocene sedimentsfrom DSDP Hole 338 (Fig. 1) were processed at the School of Oceanand Earth Science (SOES), University of Southampton, UK, usingstandard palynological techniques (as outlined below).

Samples (10–15 g) were demineralised using cold hydrochloric (30%HCl) and hydrofluoric (60% HF) acids. Lycopodium spore tablets wereadded according to the method of Stockmarr (1971) to facilitate theestimation of cyst concentrations. Many residues required no furthertreatment after this stage, andwere then air-driedonglass coverslips andmounted on glass microscope slides using Elvacite. Those samplescontaining pyrite or amorphous organic matter (AOM) were eitheroxidised in concentrated nitric acid (70% HNO3) or subjected to a fewseconds in a tuneable ultrasonic probe. Many samples contained sparseassemblages with residual heavy minerals, and thus heavy liquidseparation using sodium polytungstate (specific gravity=2) was usedto concentrate thepalynomorphs. Sieves of 10μmmesh sizewereused toconcentrate the remaining residues, which were also mounted usingElvacite. We have used the taxonomic nomenclature of Fensome andWilliams (2004) and converted the age–depth model for Site 338 fromthe Berggren et al. (1995) timescale (Eldrett et al. 2004) to the Gradsteinet al. (2004) timescale. Slides were scanned under a stereo-binocularmicroscope and approximately 300 particles counted in each residue toenable quantitative analysis of the palynofacies assemblages. In addition,a separate count for dinocysts was undertaken; however, as somesamples contained low concentrations of dinocysts (being dominatedeither by terrestrial material or heavy minerals) it was not alwayspossible to produce a statistically significant dinocyst count. Thereforethe dinocyst analysis was restricted to a subset of forty-nine sampleswhich had moderate to good recovery, enabling ~300 specimens to becounted per residue. The entire slide was then scanned in order toidentify rare age or environmentally diagnostic dinocyst species.

2.3. Palynofacies analyses

Palynofacies analyses of the insoluble organic matter in sedimentsprovide a tool for palaeoenvironmental reconstruction as it provides alink between conditions in the marine and terrestrial realms. Themain palaeoenvironmental interpretations based on palynofaciesassemblages are based primarily on those developed by Tyson(1995). The palynofacies scheme applied to the studied sediments ispresented in Table 2.

2.4. Dinocyst palaeoecology

Broad ecological groupings of Palaeogene dinocyst taxa have beendefinedbyvarious authors using a variety of approaches (see overview inSluijs et al. (2005)), and it is these groups, in conjunction with thedinocyst palaeoecological model outlined in Pross and Brinkhuis (2005)that are used in the following interpretations. The composition of themain dinocyst eco-groups employed are presented in Table 3.

3. Results

We present the results of palynofacies assemblage analyses compris-ing absolute abundances of specific palynofacies components (Fig. 3) andpercentage abundances and ratios of selected palynological groups(Fig. 4). Themost important dinocyst species/genera and eco-groups (cf.Pross and Brinkhuis, 2005), are presented in the form of absolute (Figs. 5and 6) and relative abundances (Figs. 7,8), respectively. Five palynolog-ical assemblage zones (NS1–NS5;NS=Nordic Sea)havebeendefinedbycompositional changes in the palynofacies and dinocyst assemblages (asdescribed below), the majority coincident with lithological variationswithin the cored succession.

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Fig. 2. Age–depth plot of Site 338 against the timescale of Gradstein et al. (2004). Dinocyst species abbreviations used here are defined in Table 1. Palaeomagnetic key: normalpolarity = black; reversed polarity = white, uncertain data = crossed hatched.

Fig. 1. Bathymetric and structural map of the Norwegian–Greenland Sea. CJFZ: Central Jan Mayen Fracture Zone; EJFZ: East Jan Mayen Fracture Zone. Inset: Vøring Plateau mapindicating positions of the Vøring Plateau escarpment, Vøring marginal high and Vøring Basin, in addition to DSDP/ODP drilling sites.

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3.1. Palynological Assemblage Zone NS1 (400.18–319.65 mbsf; 51.1Ma–49.0 Ma)

The palynofacies assemblages in this zone (Figs. 3 and 4) arecharacterised by the uncommon relative (4% of particulate organicmatter) and absolute (~2600 g−1) abundance of dinocysts, asassemblages are predominantly composed of terrestrially derivedmaterial (~96% of particulate organic matter), of which the phytoclastcomponent is the most abundant (75% of particulate organic matter).This assemblage zone is also characterised by the frequent occurrenceof sporomorphs (20% of particulate organic matter, 13,100 g−1), andfungal spores (1961 g−1), which peak at 380.62 mbsf (7839 g−1), butbecome less abundant up-section.

The dinocyst assemblages (Figs. 5–8) in this zone are comprisedmainly of the Glaphyrocysta–Areoligera eco-group (~20% of dinocysts;1844 g−1), with additional members of the Wetzeliellaceae and Ho-motryblium eco-groups, albeit in low relative (~10% and b3%respectively) and low absolute (b850 g−1 and b50 g−1 respectively)abundance. Although the Spiniferites group is generally rare through-out this interval, it does increase in absolute abundance (62–400 g−1)near the top of this assemblage zone.

3.2. Palynological Assemblage Zone NS2 (319.65–297.24 mbsf;49.0–48.6 Ma)

The palynofacies assemblages from this interval are characterised byincreasing relative abundance of thepalynomorphcomponent from25%of total particulate organic matter at the base of this interval to almost67% in the upper part of the zone (Fig. 4). This trend largely reflects theincrease in the relative abundance of plankton (from 9% up to 41% ofparticulate organic matter; Fig. 4). In particular, the base of thisassemblage zone is defined by the increase in the absolute abundance ofdinocysts to 18,444 g−1 at 319.65 mbsf (from 3278 g−1 in AssemblageZone NS1 beneath), followed by a continued high abundance ofdinocysts to the top of Assemblage Zone NS2 (~8600 g−1; Fig. 3).Despite the continued decrease in terrestrially derivedmaterial relativeto marine palynomorphs throughout NS2 (terrestrial debris to marinepalynomorph ratio=0.88 to 0.53: Fig. 4), a significant proportion of thepalynofacies assemblage is comprised of sporomorph and phytoclastcomponents. In particular, there is continued high absolute abundanceof woody material (32,764 g−1) and cuticle (~10% of phytoclasts;4118 g−1; see Supplementary Dataset 1).

Within NS2, the dinocyst assemblages contain specimens of theImpagidinium eco-groupbetween317.44 mbsf–314.18 mbsf (Figs. 5–8).However, slightly up-section (310.14 mbsf–304.18 mbsf), represen-tatives of the Impagidinium eco-group are absent and there is a largeincrease in the relative (up to 60%) and absolute (up to 6390 g−1)

Table 1Dinocyst datum events.

Event Key Hole 338

Depth(mbsf)

Chron Age(Ma)

LO Adnatosphaeridium vittatum Av 288.33 C21n 45.6LO Areoligera medusettiformis Am 287.35 C20r 44.6LO Areoligera tauloma At 260.60 C18n.2n 39.2LO Areosphaeridium diktyoplokum Ad 253.84 C13n 33.2FO Areosphaeridium ebdonii Ae 293.38 C21r 48.4LO Areosphaeridium ebdonii 261.25 C18r 39.6LO Areosphaeridium michoudii Ami 260.60 C18n.2n 39.2LO Batiacasphaera compta Bc 453.84 C13n 33.2FO Cerebrocysta magna Cm 293.38 C21r 48.4LO Cerebrocysta magna 286.74 C20r 44.7LO Cereodinium depressum Cd 277.16 C19r 41.4LO Charlesdowniea tenuivirgula Ct 262.00 C18r 39.6LO Charlesdowniea columna Cc 295.52 C21r 48.4FO Chiropteridium galea Cg 253.84 C12r–C13n 33.4FO Chiropteridium lobospinosum Cl 253.84 C12r–C13n 33.4LO Cordosphaeridium funiculatum Cf 257.21 No data –

LO Diphyes colligerum Dc 257.75 C18n.1r 39.1FO Diphyes ficusoides Df 320.40 C22n 49.2LO Diphyes ficusoides 287.35 C20r 44.6FO Distatodinium ellipticum De 268.41 C19n 40.6FO Dracodinium pachydermum Dp 320.40 C22n 49.2LO Dracodinium pachydermum 289.75 C21n 46.5LO Eatonicysta ursulae Eu 291.91 C21r 48.5FO Enneadocysta arcuata Ea 268.41 C19n 40.6LO Glaphyrocysta ordinate Go 258.30 C18n.2n 38.2FO Heteraulacacysta porosa Hp 268.41 C19n 40.4LO Heteraulacacysta porosa 257.75 No data –

LO Hystrichosphaeropsis costae Hc 290.65 C21n 46.6LO Hystrichostrogylon clausenii Hcl 288.90 C21n 46.3LO Melitasphaeridium pseudorecurvatum Mp 257.21 C13r 33.8FO Phthanoperidinium distinctum Pd 267.67 C19n 40.5LO Phthanoperidinium distinctum 260.60 C18.2n 39.2▪▪Phthanoperidinium geminatum Pg 278.07 C19r–C20n 41.5▪Phthanoperidinium geminatum 257.21 C18n.1r 39.1FO Phthanoperidinium regalis-clithridium Pc 287.35 C20r–C21n 45.3LO Phthanoperidinium regalis-clithridium 278.59 C19r–C20n 41.5FO Rhombodinium rhomboideum Rr 263.90 C18n.2n 39.3LO Rhombodinium rhomboideum 263.40 C18n.1r 39.2LO Rottnestia borussica Rb 257.21 C18n.1r 39.0FO Svalbardella cooksoniae Sc 268.41 C19r 41.3LO Svalbardella cooksoniae 257.21 C13r 33.8FO Wetzeliella ovalis Wo 267.67 C19n 40.5LO Wetzeliella ovalis 260.60 C18n.2n 39.2

Abbreviations used in Fig. 2 can be found in the column labelled ‘Key’; mbsf=meters belowsea floor. Ages are presented with reference to the timescale of Gradstein et al (2004).

Table 2The palynofacies classification scheme.

Group Category Sub-category

Marine palynomorphs Dinoflagellate cysts Chorate cystsProximate cystsProximo-chorate cysts

Other marine palynomorphs AcritarchsPrasinophytesOther algal remains

Terrestrial palynomorphs Pollen Bisaccate gymnospermsAngiosperm pollen

Spores Trilete sporesMonolete spores

Fungal remains Fruiting bodiesHyphae

Structured organic matter CuticleBlack wood Lath, equantBrown wood Lath, equant

other Membranous phytoclastsDegraded particulate matterUnidentified particlesLycopodium spike*

Table 3The dinocyst eco-group classification scheme after Pross and Brinkhuis (2005).

Eco-group Eco-group compositions

Cribroperidinium Cribroperidinium spp.Spiniferites Spiniferites spp.

Achomosphaera spp.Senegalinium Senegalinium spp.Glaphyrocysta–Areoligera Glaphyrocysta spp.

Areoligera spp.Cleistosphaeridium spp.

Cordosphaeridium CordosphaeridiumDeflandrea Deflandrea spp.Wetzeliellaceae Wetzeliellaceae

Cerodinium spp.Phthanoperidinium Phthanoperidinium spp.Impagadinium Impagadinium spp.

Nematosphaeropsis spp.Homotryblium Homotryblium spp.Thalassiphora Thalassiphora spp.

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abundance of the Glaphyrocysta–Areoligera eco-group, mainly com-prised of the species Areoligera medusettiformis. This abundanceevent is widespread at the same stratigraphic level in the North Sea(Bujak and Mudge, 1994), Hampshire Basin (Eaton, 1969; 1976) andthe Rockall Plateau (Costa and Downie, 1979). Increases are also seen

in the relative abundance (17% of dinocysts) and absolute abundance(3049 g−1) of the Wetzeliellaceae eco-group, in addition to anincrease in Senegalinium spp. (15–47%; 922–4123 g−1). Representa-tives of the Homotryblium eco-group are absent in this assemblagezone.

Fig. 3. Absolute abundance plots of main palynofacies groups for Site 338. a. Depth panel; mbsf =meters below seafloor; b. Lithological unit as defined by Talwani et al. (1976). Corelithology; symbols defined in Fig. 2. c. Age model: the chronology for Site 338 is based on direct correlation to the geomagnetic polarity timescale. d. Absolute abundance of selectedpalynofacies groups.

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3.3. Palynological Assemblage Zone NS3 (297.24–286.20 mbsf;48.6–44.0 Ma)

Assemblages in this zone are characterised by the continued step-wise increase in the planktonic component of the palynofacies

assemblages, reaching a maximum of ~50% of particulate organicmatter, of which dinocysts remain the most abundant (18,623 g−1;Figs. 3 and 4). Conversely, the sporomorph component of theassemblages decreases from an initial high of 25% to only 13% oftotal particulate organic matter at the top of this assemblage zone,

Fig. 4. Selected palynofacies parameters for Site 338. Columns a–c as in Fig. 3. d. Selected palynofacies parameters.

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reflected in a low sporomorph tomarine palynomorph ratio (0.25; seeSupplementary Dataset 1). The decrease in sporomorphs is particu-larly notable in the angiosperm pollen component, which decreases inabsolute abundance from 5218 g−1 at 297.24 mbsf to b100 g−1 at286.20 mbsf, while absolute abundance of both spores (301 g−1) andfungal remains (80 g−1) stays very low. However, high absoluteabundance (5068 g−1) of the glochidia and massulae of the hydro-

pterid fern Azolla spp., uniquely occur at 293.38 mbsf (see Supple-mentary Dataset 1). Absolute abundance of woody material decreasesthroughout this zone (from 3753 g−1 to 1269 g−1), essentially theresult of a decrease in brown wood phytoclasts (see SupplementaryDataset 1).

The dinocyst assemblages in this zone are characterised by thehigh absolute (max. 8340 g−1) and relative (max. 44% of dinocysts)

Fig. 5. Absolute abundance plots of selected dinocyst eco-groups for Site 338 (pars). Columns a–c as in Fig. 3. d. Absolute abundance plots of selected dinocyst eco-groups.

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abundance of the Spiniferites eco-group (Figs. 5–8). Moreover, there isan increase in abundance of the Wetzeliellaceae eco-group (b1–3% ofdinocysts; 17–556 g−1), an initial increase in the Senegalinium eco-group (1–38% of dinocysts; 75–1988 g−1), while the Phthanoperidinium eco-group is mainly comprised of forms with compoundarcheopyles such as P. regalis. In addition, the relative and absoluteabundances of the Thalassiphora eco-group also increases in thisinterval, reaching maximum abundances of 17% of dinocysts and~830 g−1, respectively. These assemblages are also characterised bythe consistent occurrence, albeit in very low relative abundances (b2%of dinocysts; b~250 g−1), of the Impagidinium eco-group.

3.4. Palynological Assemblage Zone NS4 (286.20–257.21 mbsf;44.0–33.9 Ma)

The palynofacies assemblages in this zone (Figs. 3 and 4) arecharacterised by a very high relative (~68% of total particulate organicmatter) and high absolute (average: 15,781 g−1) abundance ofplankton, of which dinocysts are the dominant component, reaching

a maximum of 27,988 g−1 at 278.59 mbsf (Fig. 3). Conversely, woodymaterial is relatively rare (9% of total particulate organic matter), withvery low absolute abundances of both black wood (average: 92 g−1)and brown wood (average: 712 g−1; see Supplementary Dataset 1).

The dinocyst assemblages in this zone are characterised by theabsolute and relative increase in the Phthanoperidinium eco-group,primarily P. geminatum, which demonstrates two abundance peaks,reaching maximum absolute abundances of 2300 g−1 (278.59 mbsf)and ~2200 g−1 (262.00 mbsf), respectively (Figs. 5–8). Whilst theImpagidinium eco-group of dinocysts display a low but still significantrelative abundance (b8%), they do show an increased absoluteabundance (max. 1690 g−1).

3.5. Palynological Assemblage Zone NS5 (257.21–247.80 mbsf;33.9–~33.0 Ma)

This assemblage zone is dominated by the planktonic componentof the palynofacies assemblage (99% of total palynomorphs; 79% ofparticulate organic matter; Fig. 4). In particular, dinocysts are the

Fig. 6. Absolute abundance plots of selected dinocyst eco-groups for Site 338 (cont.). Columns a–c as in Fig. 3. d. Absolute abundance plots of selected dinocyst eco-groups.

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most numerically abundant component in this zone, with an absoluteabundance of 6940 g−1 (Fig. 3). However, the upper part of thestudied interval (253.84–247.80 mbsf) is barren of all palynologicalmaterial.

The dinocyst assemblages (Figs. 5–8) characterising this zone aresimilar in composition to that described for Assemblage Zone NS4, butbetween 257.21 and 253.84 mbsf, the Spiniferites eco-group is presentin common relative (22% of dinocysts) and absolute (1661 g−1)

Fig. 7. Relative abundance plots of selected dinocyst eco-groups for Site 338 (pars). Columns a–c as in Fig. 3. d. Relative abundance plots of selected dinocyst eco-groups.

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abundances, although representatives of the Impagidinium eco-groupare present, they are rare (b1% of dinocysts; ~25 g−1).

4. Discussion

4.1. Early Eocene (Ypresian). Age: 51.1–48.6 Ma (400.18–297.24 mbsf;Assemblage Zone NS1–NS2)

Palynologically, the lower Eocene bioturbated clastic sediments(Units 3B and 3C) are characterised by palynofacies assemblagesdominated by terrestrially derived material, which together with raremarine dinocysts are indicative of a marginal-marine depositionalenvironment. This is confirmed by the pre-49 Ma dinocyst assemblages

being comprised of restrictedmarine taxa, suchas representatives of theHomotryblium eco-group. In the latest Ypresian (49.0–48.6 Ma) theHomotryblium eco-group disappears, and the dinocyst assemblages aredominated by the Glaphyrocysta–Areoligera eco-group, reflecting amore neritic influence on the Vøring Plateau. This interpretation is alsosupported by nanoplankton assemblages dominated by the shallowwater species Imperiaster obscurus (Müller, 1976). However, a brackishwater influence is also indicated by the occurrence of the SenegaliniumandWetzeliellaceae dinocyst eco-groups in addition to the sporomorphassemblages being dominated by non-saccate forms, which areprimarily transported by fluvial mechanisms (Tyson, 1995). Thus wesuggest that during the early Eocene, the Vøring Plateau depositionalenvironment experienced a transition from restricted marine to more

Fig. 8. Relative abundance plots of selected dinocyst eco-groups for Site 338 (cont.). Columns a–c as in Fig. 3. d. Relative abundance plots of selected dinocyst eco-groups.

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neritic conditionswith a developingbrackishwater influence by theendof the early Eocene (~49 Ma).

The presence of basaltic grains throughout this terrestriallyderived sequence is consistent with the palynological interpretation,indicating that a nearby basaltic sourcewas being eroded andmaterialtransported to the site of deposition. Skogseid and Eldholm (1989)also suggest that part of the Vøring Plateau may have been emergentduring the Palaeogene, and that differential regional subsidencecaused the outer part of the plateau to subside faster than the innerparts. Based on one-way seismic isopach maps (Caston, 1976;Skogseid and Eldholm, 1985), the sediment is therefore likely tohave been sourced from erosion of basement highs to the north of theVøring Plateau escarpment and the emergent crest of the escarpmentitself (Fig. 9).

In Assemblage Zone NS2, representatives of the Homotrybliumeco-group are absent while the dominance of terrestrially derivedmaterial gradually declines, an observation also noted by Manum(1976). This indicates a greater marine influence towards the end ofthe early Eocene – early Mid Eocene. Moreover, the sporadicoccurrence of representatives of the Impagidinium eco-groupthroughout the early Eocene suggests an intermittent oceanicinfluence on the Vøring Plateau, reflecting local subsidence and/orrandom excursions of oceanic water masses, or the flooding of theplateau during eustatic sea level highs.

4.2. Mid Eocene (Lutetian). 48.6–39.1 Ma (297.24–257.21 mbsf;Assemblage Zone NS3–NS4)

The lower part (297.24–286.2 mbsf) of the middle Eocenesequence at Site 338 is characterised by the consistent occurrence oftaxa belonging to the Impagidinium eco-group, albeit in very lowabsolute and relative abundances. Species of the genera Impagidiniumand Nematosphaeropsis appear only on the oceanic side of the coastal/oceanic transition in all the major modern oceans, such that the merepresence of these taxa may be used as a reliable indicator of theinfluence of oceanic waters (Wall et al., 1977; Dale, 1996; seediscussion in Pross and Brinkhuis, 2005), as any resuspension ortransport of dinocyst assemblages is dominantly in an offshoredirection. Additional studies from Quaternary and Cenozoic sedi-ments further support the contention that this eco-group is a goodindicator of oceanic conditions (De Vernal and Mudie, 1989;Versteegh, 1994, and Brinkhuis and Biffi, 1993; Brinkhuis, 1994;Zevenboom, 1995; Peeters et al., 1998, respectively). Thus thepresence of low numbers of the Impagidinium eco-group in thesesediments suggests continued oceanic influence on the VøringPlateau, and that neritic taxa such as Spiniferites, Areoligera spp. andHomotryblium spp., rather than being the result of shifting oceanicand neritic water masses are probably allochthonous, being trans-ported from a shallow shelf environment to the site of deposition. This

Fig. 9. Schematic reconstruction of the Vøring Plateau and palaeonvironmental conditions during the Eocene–Oligocene. a. early to mid Eocene (51.1–44 Myr); the Vøring Plateauwas partially emergent, but differential subsidence also resulted in areas characterised by an inner neritic environment with brackish surface water conditions resulting fromenhanced continental runoff. The deeper basins were stratified. b. late mid Eocene (44–39.1 Myr); the Vøring Plateau became fully submerged due to subsidence resulting in theintroduction of terrestrially derived nutrients into the photic zone and enhanced surface water productivity. In addition, upwelling on the Vøring margins due to invigorated oceancirculation and mixing of the water column contributed to the regionally fertile surface waters which may have contributed to the drawdown of CO2 in the late Eocene.

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interpretation is also supported by the occurrence of intermittentturbiditic Bouma sequences in the basal middle Eocene sediments atSite 338, which display angular clay rip-up clasts and coarsening-upward sequences. In addition, the continued clastic sedimentation inthe basal middle Eocene (lithological unit 3A) may reflect the erosionand delivery of material from the Vøring escarpment and associatedmarginal highs, which had recently become submerged.

A single sample (293.38 mbsf) from the basal middle Eocenesediments contains massulae of the free-floating fern Azolla spp.,comprising up to 20% of the total palynofacies assemblage. It has beensuggested that the influx of Azolla spp. in the Nordic Seas reflects anallochthonous freshwater assemblage transported by freshwaterspill-overs originating from the Arctic Ocean. The evidence for thiscontention comes from Integrated Ocean Drilling Progam (IODP) Leg302, which recovered super-abundant Azollamassulae from the ArcticOcean which were inferred to be of the same age (Brinkhuis et al.,2006). The occurrence of representatives of the Senegalinium eco-group, indicative of freshwater to brackish conditions (Pross andBrinkhuis 2005; Sluijs et al. 2005) supports this interpretation.However, Azolla occurs in turbidites at Site 338, which raises thepossibility that the Azollamassulae and other taxa (i.e. representativesof the Senegalinium eco-group) may have been brought in by masstransport of material originating from the shelf, rather than (orperhaps in addition to) direct longer distance surface transportationfrom the Arctic Ocean.

The latemid Eocene (286.2–257.21 mbsf; 44.0–39.1 Ma; Lutetian) ischaracterised by diatom oozes and a high peridinioid:gonyaulacoiddinocyst ratio (P:G ratio; see Supplementary Dataset 2), with absoluteabundance increases in Phthanoperidinium geminatum and Deflandreaspp.. Although extinct, the latter taxon is known to characterise areas ofhigh primary productivity related to high diatom abundance andincreased nutrient availability (Brinkhuis et al., 1992; Pross andBrinkhuis 2005; Sluijs et al. 2005). Indeed, despite the partial diageneticdissolution of silica in the deep-basins of the Norwegian–Greenland Sea(e.g. ODP site 913; Scherer and Koç, 1996), the peaks in Phthanoperidi-nium spp. and Deflandrea spp. are accompanied by increased accumu-lation and preservation of diatom-rich sediments (Firth, 1996).Moreover, at Site 338 the diatom assemblage is comprised of Chaeto-ceros spp. resting spores (Suto, 2006), which are indicative of upwellingconditions, thus further strengthening the contention that thesesediments underlay fertile ocean waters supporting enhanced produc-tivity. The enhanced marine surface water productivity reported heremay have been caused by the introduction to the photic zone ofnutrients derived from the final drowning of the Vøring Plateau as itsubsided below sea level. Additionally the enhanced productivity mayalso be the result of upwelling conditions becoming established on theVøring margin (Fig. 9).

The occurrence of prasinophyte algae and higher abundances ofthe Thalassiphora eco-group argue for at least seasonal stratification ofthe water column and lowered oxygen conditions in bottom waters,deriving from a high biological oxygen demand created by theenhanced photic zone productivity (see Pross, 2001). Enhancedsurface water productivity in the late Eocene Nordic seas, and inparticular along the recently submerged Vøring Plateau, would haveincreased biological drawdown of carbon dioxide and sequestration oforganic carbon into marine sediments under conditions of loweredbottom water oxygenation, which may have contributed to decliningCenozoic atmospheric carbon dioxide levels (Pagani et al., 2005;Lowenstein and Demicco, 2006) Although we cannot accuratelyquantify the amount of organic carbon sequestration across theVøring Plateau in the late Eocene, it may have played a role inpromoting high-latitude cooling (Zanazzi et al. 2007; Schouten et al.2008) and the development of Northern Hemisphere ice during thelatest Eocene (Moran et al. 2006; Eldrett et al. 2007).

The distribution of abundant Deflandrea spp and the synchronousPhthanoperidinium spp. “bloom events”, which are recorded in a

variety of DSDP/ODP sites (Eldrett et al. 2004) suggests a (geograph-ically) connected surface water circulation in the Nordic seas duringthe mid Eocene, compared to the more restricted basin circulation inthe early Eocene (Eldholm et al. 1994; Laberg et al. 2005). Thisinterpretation is supported at Site 338 by the transition fromprimarily terrestrially derived to pelagic, diatomaceous sedimentation(lithological unit 3A to 2D), reflecting the final subsidence of theVøring Plateau, which therefore allowed the establishment of surfacewater circulation across the Vøring structure. This surface watercirculation across the subsiding Vøring Pleateau is reflected by thesignificant increase in representatives of the oceanic dinocyst Impa-gadinium eco-group, and a marked decrease in sediment accumula-tion rates (2.8 cm/kyr in the Ypresian to b0.5 cm/kyr in the Lutetian;see Fig. 2), indicating a change in sediment source from continental toprimarily hemipelagic. Dinocyst assemblages from the Norwegian–Greenland (Eldrett et al. 2004) and North Seas (Bujak and Mudge,1994) and Denmark were compared by Heilmann-Clausen and VanSimaeys (2005) and shown to have strong similarities. Indeed, similardinocyst assemblages are noted in middle Eocene sediments fromother regions, such as the west of Greenland (Nøhr-Hansen, 2003);the Labrador Sea (Head and Norris, 1989; Eldrett pers obs; 2009),Rockall Plateau (Costa and Downie, 1979), southern England (Bujaket al. 1980) and the Arctic (Sangiorgi et al. 2008). Therefore thefindings of Heilmann-Clausen and Van Simaeys (2005) and thepresent study not only argue against palaeo-provincialism withinNordic seas, but also indicate the existence of at least surface waterconnections extending from the North Atlantic across the Greenland–Iceland Scotland Ridge probably via shallow water channels in thewestern (Denmark Strait) and eastern parts (Shetland Channel) of theridge, to the Arctic Ocean via a shallow water channel in the FramStrait (Fig. 9).

4.3. Mid to Late Eocene (Lutetian–Rupelian). 39.1–33.9 Ma

Enhanced surface water circulation across the submerged VøringPlateau is inferred during the Late Eocene from the identification of astratigraphic hiatus at Site 338 (~257.75 mbsf), which spans an 8 Myperiod from the base of the Bartonian in the mid Eocene to the base ofthe Rupelian in the early Oligocene (Fig. 2). The hiatus at Site 338partially overlaps with a 3.5 My hiatus at Site 643A, located down-slope of Site 338 (Eldrett et al., 2004), and with coeval hiatuses in theNorth Sea Basin (Neal et al. 1994; Michelsen et al. 1998), whichsuggest an underlying regional cause. The distribution of hiatuses inthe North Atlantic region has been associated with glacioeustaticlowering of sea level and intensification of deep water circulation dueto high-latitude cooling during late Eocene to Oligocene times(Kennett, 1977; Keller et al. 1987; Miller et al., 1987; Miller, 1992;Abreu and Anderson, 1998; Zachos et al., 2001). The age of thisregional unconformity is consistent with evidence in the earlyOligocene for the onset of North Atlantic Deep Water (NADW)formation (Davies et al., 2001), and coincides with a major seismicreflector at sites 338 and 643, separating the Eocene Vd seismic unitfrom the Oligocene Vc seismic unit (Laberg et al., 2005).

4.4. Early Oligocene. 33.9–33 Ma (257.21–247.80 mbsf; AssemblageZone NS5)

During the early Oligocene, Site 338 was still under oceanicinfluence as indicated by the low abundance of terrestrial materialand the continued presence of Impagidinium spp.. The absence of allpalynological material at the top of the studied section, preventsdetailed palaeoenvironmental reconstructions, but points towards theoccurrence of well oxygenated, corrosive bottom waters during themid early Oligocene.

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5. Conclusions

Palynological results confirm that the Vøring Plateauwas a shallowwater inner-neritic environment during early Eocene times. Thepresence of terrestrially derived and brackish water palynomorphsindicate that, due to local variations in subsidence, both the basementhighs to the north of the Vøring Plateau escarpment and the crestalpart of the escarpment itself may have been emergent at this time andactively contributing material to Site 338. Middle Eocene sedimentsdemonstrate the transition from marginal-marine to fully marineconditions on the Vøring Plateau, with subsidence below sea leveloccurring at ~44 Ma, facilitating inter-basinal surface water circula-tion and promoting enhanced photic zone fertility. Enhancedproductivity in the late Eocene Nordic seas, including that along therecently submerged Vøring Plateau, may thus have contributed todeclining Cenozoic atmospheric carbon dioxide levels (Pagani et al.,2005; Lowenstein and Demicco, 2006) and thus played a role inpromoting high-latitude cooling (Zanazzi et al. 2007; Schouten et al.2008) and development of ice in the Northern Hemisphere during thelatest Eocene (Moran et al. 2006; Eldrett et al. 2007).

Acknowledgements

This research used samples provided by the Ocean Drilling Program(ODP). ODP was sponsored by the U.S. National Science Foundation(NSF) and participating countries under management of Joint Ocean-ographic Institutions (JOI), Inc..Wealso thankS. Akbari for palynologicalprocessing and to the manuscript reviewers Prof. Henk Brinkuis(Utrecht University) and Prof. Claus Heilmann-Clausen for their helpfulsuggestions, which have greatly improved the final manuscript.

We also thank the American Association of Stratigraphic Palynol-ogists (AASP), The Geological Society of London, The Micropalaeon-tological Society (TMS) and the Natural Environment ResearchCouncil (NERC) for funding this research.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.marmicro.2009.10.004.

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