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Early Oligocene cooling reflected by the dinoflagellate cyst Svalbardella cooksoniae

Katarzyna K. Śliwińska ⁎, Claus Heilmann-ClausenDepartment of Earth Sciences, Aarhus University, Høegh-Guldbergs Gade 2, DK-8000 Aarhus C, Denmark

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

Article history:Received 13 September 2010Received in revised form 31 January 2011Accepted 20 February 2011Available online 26 February 2011

Keywords:Dinoflagellate cystsEastern North SeaEarly OligoceneOi1a cooling eventPaleoclimateSea level changeSvalbardella cooksoniaeAreosphaeridium diktyoplokum

The early Oligocene interval in several boreholes from the eastern North Sea and Denmark has been studiedfor dinoflagellate cysts. The cold water dinoflagellate cyst Svalbardella cooksoniae was recorded in a narrowlowermost Oligocene interval in all offshore sections. A critical evaluation of previous records of Svalbardellacooksoniae reveals that it is present in the same narrow interval of Chron 12r, close to the NP21/NP22boundary in many high and mid latitude Northern Hemisphere sections, ranging from the Greenland Sea inthe north to Italy in the south. The Svalbardella cooksoniae interval is closely associated with the LO of thedinoflagellate cyst Areosphaeridium diktyoplokum. The age of the Svalbardella cooksoniae event indicates that itis synchronous with the Oi1a oxygen isotope maximum of Pekar and Miller (1996) and Pekar et al. (2002). Inthe Danish land area the Svalbardella cooksoniae interval and hence the Oi1a event is shown to coincide withan unconformity. A regional comparison shows that synchronous unconformities are present also in Belgiumand New Jersey. A slightly older regional unconformity occurring on both sides of the North Atlantic coincideswith the Oi1 oxygen isotope maximum of Pekar and Miller (1996). The finding of a sedimentary packageseparated by two successive regional unconformities is interpreted to reflect a significant, temporary eustaticsea level rise, and hence a melting phase on the East Antarctic ice sheet between the Oi1 and Oi1aglacioeustatic sea level falls.The paleogeographic distribution and lower frequency of Svalbardella during the earliest Oligocene Oi1aevent, as compared with the mid Oligocene Oi2b Svalbardella spp. event, suggest that the Oi1a glacial episodewas less severe than the Oi2b glaciation. This is in agreement with recent ice volume estimates for the twoevents.

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1. Introduction

The global benthic foraminifera isotope record reveals two strongcooling events at the beginning of the Oligocene, as indicated by theOi1 and Oi1a oxygen isotope (δ18O) maxima of Pekar et al. (2002; andreferences therein). The early Oligocene cooling reflects the firstmajor Antarctic glaciation and is the most distinctive cooling phase inthe Paleogene (Zachos et al., 2001) The Oi1 cooling event is dated tomagnetochron C13n and the slightly younger Oi1a event is dated tothe early part of Chron C12r (Pekar et al., 2002). Based on data fromoffshore New Jersey, Pekar and Miller (1996) linked the Oi1 eventwith a significant global eustatic sea level fall and the TA4.3/TA4.4third order sequence boundary of Haq et al. (1987). Pekar et al. (2001)showed that also the Oi1a oxygen maximum coincides with anunconformity. Both unconformities were therefore interpreted aseffects of glacioeustatic sea-level falls.

An important earliest Oligocene dinoflagellate event of theNorthern Hemisphere is the LO (last occurrence) of Areosphaeridium

diktyoplokum, which has been used for recognition of the Eocene–Oligocene boundary and for interregional correlation (e.g., Schiøler,2005; Van Mourik and Brinkhuis, 2005). Van Mourik and Brinkhuis(2005) suggested that in Central Italy the Oi1 cooling event is syn-chronous with the LO of A. diktyoplokum.

In the southern North Sea Basin the approximate stratigraphicposition of the early Oligocene cooling was constrained on basis ofoxygen isotope data by De Man et al. (2004). According to theseauthors, the lowermost part of the Oligocene (mid nannofossil zoneNP21, below the LO of Areosphaeridium diktyoplokum) was depositedunderwarm conditionswhereas somewhat younger sediments, abovethe base of NP23, were deposited under a colder climate. Furthermore,in the same area, an unconformity at the NP21/22 transition wasrecognized by Vandenberghe et al. (2003) as the TA4.3/TA4.4sequence boundary. De Man et al. (2004) suggested that the coolingevent coincided with the unconformity level.

The arctic dinoflagellate Svalbardella is generally considered as areliable cold water indicator (e.g., Head and Norris, 1989; Sluijs et al.,2005). Episodes of southward migration have therefore been inter-preted to indicate cooling events. The best documented southwardmigration event took place in the mid Oligocene (Van Simaeys et al.,2005a). The mid Oligocene Svalbardella interval was shown by VanSimaeys et al. (2005a) to be related to the Oi2b cooling event (Pekar

Palaeogeography, Palaeoclimatology, Palaeoecology 305 (2011) 138–149

⁎ Corresponding author.E-mail address: kasia.sliwinska@geo.au.dk (K.K. Śliwińska).

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et al., 2002 and references therein) known as the ‘Oligocene GlacialMaximum’ (Van Simaeys et al., 2005a).

The purpose of the present paper is to investigate widespreadearliest Oligocene occurrences of Svalbardella in the NorthernHemisphere, near the time when Areosphaeridium diktyoplokumbecame extinct. We furthermore investigate the timing and relation(if any) between this widespread dinoflagellate event and the oxygenisotope cooling events. For these purposes we studied the dinoflagel-late cyst distribution in three commercial wells from the easternNorth Sea and a cored research borehole from onshore Denmark (11/10-1, Mona-1, Nini-1, Horn-1; Fig. 1, Table 1). Published dinoflagellatedata from a number of high and mid latitude sections in the NorthernAtlantic region and Europe (Fig. 1A) were critically evaluated, in partreinterpreted, and compared with our findings.

2. Geological setting

The studied sections are situated in the central and eastern NorthSea Basin (Figs. 1, 2). During the Eocene hemipelagic clays and marlswere deposited in this area. The beginning of the Oligocene marks animportant change of the depositional environment with the onset oflarge, southward prograding, clinoforms. These sediments sourcedfrom southern Norway (Fig. 1B) and the progradations were inter-preted as indicating an uplift of western Fennoscandia (Michelsen

et al., 1998). This view is, however, disputed by others, who assumethat the changed pattern of sedimentation was caused by theinstallation of a cooler climate (Huuse et al., 2001; Nielsen et al.,2009). A switch to siliciclastic clinoforms during the early Oligocenewas observed in globally widespread shelf areas (Bartek et al., 1991;Pekar et al., 2000; Lavier et al., 2001) and therefore points toward aglobal factor, such as cooling of climate.

The 11/10-1 well is situated in the Norwegian-Danish Basin, closeto the border between the Norwegian and the Danish North Seasectors. The well is located on the western flank of the Horn Graben,close to the Kreps Fault Zone (Fig. 1B). Seismic sections indicate thatthe well is near an early Oligocene depocentre (Danielsen et al., 1997).

The Nini-1 well is situated in the central-northern part of theDanish sector of the North Sea, within the mid-Oligocene depocentre(Danielsen et al., 1997; Śliwińska et al., 2010).

The Mona-1 well is situated in the Central Graben, in a distalposition relative to the Oligocene sediment source.

The cored Horn-1 research borehole is situated onshore Denmark,in the eastern part of the Norwegian-Danish Basin (Fig. 1).

3. Materials and methods

The only available sources for dating of the early Oligocene NorthSea sections, are ditch cuttings and rare side wall cores from industrial

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Fig. 1. (A) Map showing locations of the sites included in this study. The solid line marks the position of the cross-section shown in Fig. 4. (B) Map of the eastern part of the North SeaBasin (eastern North Sea and Denmark) showing the most important geological structures and the location of four offshore wells and an onshore borehole included in the presentstudy. Modified from Danielsen et al. (1997).

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wells. In samples prepared from ditch cuttings datings are generallyrestricted to last occurrences (LO's). The determination of in situ firstoccurrences (FO's) of microfossils is uncertain due to the possibility ofdown hole contamination (caving) invariably associated with ditchcutting samples. However, in a few instances, e.g. the FO ofChiropteridium spp., it was possible to indicate a plausible positionof the event.

For establishing the correlation between the wells wewere using adistinctive seismic reflector and associated distinctive upwardsincrease in gamma-ray values representing the top of the MajorSequence Stratigraphic Unit 3 of Michelsen et al. (1998), also knownas the “top Eocene” (Huuse and Clausen, 2001). The reflector has beendated to the mid part of NP21 (Michelsen et al., 1998) and can berecognized in all studied wells from the eastern North Sea.

The Nini-1 well was drilled by Dong E&P in 2000. Washed anddried ditch cutting samples were provided by the Geological Survey ofDenmark and Greenland (GEUS) and processed at the Department ofEarth Sciences, Aarhus University. Samples were processed using astandard palynological preparation technique including treatmentwith HCl, HF and HNO3. Details of the method are described in Woodet al. (1996). Samples were sieved on 20 μm sieves. Slides for light-microscopy were made in gelatine-jelly. For the purpose of theproject, six samples were examined. Preservation of the dinoflagellatecysts was good. The “top Eocene” reflector is located at ca. 1650 m(Ole Rønø Clausen, pers. comm.).

The 11/10-01 well was drilled by Philips Petroleum CompanyNorway in 1969. Palynological slides from the well were prepared atthe Geological Survey of Denmark and Greenland (GEUS) andprovided for the present study by the Norwegian PetroleumDirectorate (NPD). Seven palynological samples were studied. Pres-ervation of the dinoflagellate cysts was good. The “top Eocene”reflector is located at ca. 1024 m (Danielsen et al., 1997).

The Mona-1 well was carried out by Chevron in 1983. A first semi-quantitative study of the dinoflagellate cysts (Heilmann-Clausen,

1992, unpublished) was based on side-wall core and ditch cuttingsamples. These samples were processed at GEUS. Only three samples(at 2316.48 m, 2346.96 m and 2377.44 m) cover the interval of ourinterest. Therefore, for improving the resolution of the section, fouradditional samples (from ditch cutting material) were processed atthe Department of Earth Sciences, Aarhus University. Palynologicalslides used by Heilmann-Clausen in 1992 were re-studied. Preserva-tion of the dinoflagellate cysts was good tomoderate. The top ofMajorSequence Stratigraphic Unit 3 (“top Eocene”) is located at 2358.8 m(Danielsen et al., 1997).

All depths in the North Sea wells are given in meters below kellybushing.

The Horn-1 borehole was drilled in 2001–2002 by the Departmentof Earth Sciences, Aarhus University. Samples were prepared at theDepartment of Earth Sciences, Aarhus University. 15 samples wereexamined for the present study. Preservation of the dinoflagellatecysts was very good. An unconformity corresponding to the “topEocene” reflector is recognized at 101.55 m.

Microscopy on all samples was performed in plain transmittedlight. In each sample at least 200 dinoflagellate cysts were countedand the remaining area of the counted slide was searched for rarespecies.

4. Dinoflagellate key events in the earliest Oligocene

The most widely recognized dinoflagellate cyst event near theEocene–Oligocene transition in the North Atlantic-European region isthe LO of Areosphaeridium diktyoplokum (Plate 1). A study from highlatitudes was carried by Eldrett et al. (2004), who interpreted the LOof A. diktyoplokum to occur in Chron C13 and dated the event to 33.6–33.4 Ma (Eldrett et al., 2004: p. 116, 118). In mid latitudes (centralItaly) the event is recorded in the lower part of Zone P18, in themiddlepart of Zone NP21, and in the upper part of Chron C13n (Van Mourikand Brinkhuis, 2005; Hyland et al., 2009; Pross et al., 2010). The event

Table 1Record of the five most important dinoflagellate cysts in the examined samples of the studied North Sea wells. The table shows the number of counted specimens.

Depth (m) belowkelly bushing

Areosphaeridiumdiktyoplokum

Chiropteridiumspp.

Glaphyrocystasemitecta

Spiniferitesmanumii

Svalbardellacooksoniae

Total number ofcounted cyst

Type ofsample

Nini‐1 1560 R 35.0 1.0 1.0 206.0 DC1590 R 25.0 o 0.5 203.0 DC1610 1.0 o 2.0 1.0 206.0 DC1620 o 0.5 o 204.5 DC1630 5.0 1.5 219.0 DC1640 2.0 1.5 1.0 o 202.5 DC

Mona‐1 2316.5 5.0 o 201.0 SWC2322.6 6.0 o o 221.0 DC2328.7 2.0 1.5 1.0 202.5 DC2334.8 1.0 1.0 2.5 209.0 DC2340.9 3.0 2.0 206.5 DC2347.0 1.0 210.0 DC2377.4 2.5 1 204.5 SWC

11/10‐1 579.1 128.5 314.0 DC609.7 4.5 4.5 238.5 DC624.8 24.0 1.0 232.5 DC640.1 41.0 1.0 260.0 DC670.6 24.0 1.0 278.0 DC701.4 38.5 327.5 DC731.5 42.0 339.5 DC762.0 24.0 305.5 DC792.5 11.5 269.0 DC823.0 6.0 o 261.0 DC853.4 1.0 2.0 o 218.5 DC883.9 1.0 o 3.0 o 236.0 DC914.4 o 1.0 3.0 227.5 DC

o—present outside counting area.R—reworked specimen.DC—ditch cutting sample.SWC—side wall core.

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was dated to 33.3 Ma byWilliams et al.(2004). According to Pross et al.(2010) the LO of Areosphaeridium diktyoplokum in high latitudes isobserved slightly earlier than in the lower latitude Italian settings. Inthe southern North Sea Basin, however, the LO of A. diktyoplokumoccurs in the upper part of NP21 (Van Simaeys et al., 2005b). In theeastern North Sea Basin the LO of A. diktyoplokum occurs in NP21(Köthe, 1990; Michelsen et al., 1998) and seems to occur in the lowerpart of Chron C12r (see Discussion). The demise of Areosphaeridiumdiktyoplokum coincides with an unconformity in the Danish land area(Heilmann-Clausen, 1995; Michelsen et al., 1998).

Another important earliest Oligocene dinoflagellate cyst event isthe FO of Chiropteridium spp. (C. lobospinosum and C. galea) (Plate 1).In the Greenland Sea (ODP Hole 913B) their FO is above the LO ofA. diktyoplokum in an interval of reverse polarity (Eldrett et al., 2004;Fig. 4A). The event is indirectly calibrated with the NP21/22 boundary(Eldrett et al., 2004). In the North Sea Basin, the FO of Chiropteridiumspp. is recorded in the uppermost part of NP22 (Köthe, 1990, 2003;Van Simaeys et al., 2005b). In central Italy Chiropteridium spp. firstappear later, in the NP23 Zone (Biffi and Manum, 1988) or even inNP24 (Coccioni et al., 2008; Pross et al., 2010). These observationsindicate that the event is diachronous between the Greenland Sea,

North Sea Basin and Italy, and the species were migrating southduring the period from the earliest Rupelian to the mid-Rupelian.Within the North Sea Basin, however, the FO of Chiropteridium spp. isconsidered to be synchronous.

A third stratigraphic marker for the earliest Rupelian is Spiniferitesmanumii (Plate 1). As shown by Lund (2002) Spiniferites manumiioccurs in the Norwegian Sea (ODP Site 643), the North Sea and NWBelgium in a synchronous and short (ca. 400 Ka) stratigraphicinterval. In the Rupelian stratotype area the interval is bracketed bytwo septarian layers (S30 and S40) and dated there to Chron C12r andthe NP23 Zone (Steurbaut, 1986; Lagrou et al., 2004). The study byEldrett et al. (2004) confirmed that S. manumii occurs in ChronozoneC12r in the Greenland Sea (ODP Site 913B, cf. their Figs. 4 and 5 andonline dataset).

Finally, the LO of Glaphyrocysta semitecta and the FO of Wetzeliellagochtii recorded in some of the studied sections (Monte Cagnero,Massicore, 11/10-1, Nini-1, ODP Hole 642A and the Kallo well) wereused as supplementary markers.

5. Results

5.1. Well 11/10-1

An interval of rare Svalbardella cooksoniae is present in threesamples, at 823.0 m, 853.4 m and 883.9 m, respectively (Fig. 3,Table 1, Plate 1). At 883.9 m the LO of Areosphaeridium diktyoplokum isrecognized. The LO of Glaphyrocysta semitecta is observed at 853.4 m.The FO of presumed in situ Chiropteridium occurs at 762.0 m, which isthe lowest common occurrence of the genus. Spiniferites manumii wasfound in samples at 609.7 m, 624.8 m and 640.1 m.

5.2. The Nini-1 well

In the Nini-1 well, Svalbardella cooksoniae was recognized in threesamples (Fig. 3, Table 1, Plate 1). Svalbardella cooksoniaemakes up 0.5%of the assemblage at 1610 m, and was observed (outside the countedarea) in samples at 1620 mand 1640m (Table 1). The FO of Svalbardellacooksoniae at 1640 m coincides with the LO of Areosphaeridiumdiktyoplokum. The last consistent occurrence of Glaphyrocysta semitectawas foundat1630 m(Table 1). The LOof Spiniferitesmanumiiand the FOof presumed in situ Chiropteridium spp. were found at 1560m and1590 m, respectively.

5.3. The Mona-1 well

In Mona-1 a short interval of Svalbardella cooksoniae can berecognized (Fig. 3, Table 1, Plate 1). The interval is closely bracketedby the LO of Areosphaeridium diktyoplokum at 2377.4 m and the LO ofSpiniferitesmanumii at 2316.5 m (Table 1). However, the observed LO ofAreosphaeridium diktyoplokum may be below the real LO, since nosampleswere available between the “top Eocene” at ca. 2358 m and thefirst Oligocene sample at 2347 m (Fig. 3). The top of the Svalbardellacooksoniae interval is observed above the presumed FO of in situChiropteridium spp., which is recorded at 2340.9 m.

5.4. The Horn-1 borehole

The lowermost sample in the studied part of the Horn-1 bore-hole contains a dinoflagellate assemblage of early Bartonian age.Areosphaeridiumdiktyoplokum is persistently present between101.25 mand 68.0 m. The FO of Chiropteridium spp. (C. lobospinosum and C. galea)is found at 67.5 m, immediately above the LO of A. diktyoplokum. Theevents are separated by an unconformity at 67.75 m (cf. Section 6.3).

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6. Discussion

6.1. Comparison with previous early Oligocene records of Svalbardellacooksoniae

An early Oligocene interval of Svalbardella cooksoniae has beenrecognized in previous studies from various regions of Europe and theNorth Atlantic Ocean (Table 2). These observations will briefly bediscussed in the following.

6.1.1. The Greenland SeaThe section in ODP Hole 913B from the Greenland Sea (Fig. 1A)

provides the most northerly record of Svalbardella cooksoniae. Aninterval between 425 and 575 m was dated with dinoflagellate cystsby Firth (1996). Eldrett et al. (2004) re-examined Firth's material andsupplemented the study with an analysis of additional samples andmagnetostratigraphic analysis. Relevant data are shown in Fig. 4.

Eldrett et al. (2004, Table 1) showed the LO of A. diktyoplokum inHole 913B to be in a sample at 452.8 m. According to their Fig. 4, thissample, falls in an interval of reverse polarity interpreted asrepresenting Chron C12r. However, the last continuous occurrenceof A. diktyoplokum was observed at 463.07 m (online dataset). At452.8 m only one specimen of Areosphaeridium diktyoplokum wasrecorded, co-occurring with Eatonicysta ursulae, an early Lutetianmarker species that must have been reworked. This suggests thatA. diktyoplokum at that depth may also have been reworked. Wetherefore interpret the LO of in situ A. diktyoplokum to be in thesample at 463.07 m. This sample was taken just above a polarity shiftfrom normal to reverse according to Eldrett et al. (2004; Fig. 4A).

The FO's of Chiropteridium spp. and Spiniferites manumii (recordedas Spiniferites sp. 1) were recognized at 453.59 m by Eldrett et al.(2004), who assigned this sample to Chron C13n in their Table 1.However, the sample at 453.59 m is clearly from a reverse polarityzone according to Figs. 4A and 5 in Eldrett et al. (2004).

Plate 1. Photographed specimens are localized in the slides with “England Finder” coordinates. Scale bars=50 μm. Scale bar in Fig. 1 applies also to Figs. 2–4. 1. Svalbardellacooksoniae high focus, borehole 11/10-1, 853.4 m, sample 853.4 Ø′/4, coordinates Q14. 2. Svalbardella cooksoniae low focus, borehole 11/10-1, 853.4 m, sample 853.4 Ø′/4,coordinates Q14. 3. Svalbardella cooksoniae high focus, borehole Nini-1, 1610 m, sample K-32 F-1, coordinates S12. 4. Svalbardella cooksoniae high focus, borehole Mona-1, 2322.58 m,sample K-126 F-1, coordinates F50-1. Scale bar in Fig. 5 applies also to Figs. 6 and 7. 5. Spiniferites manumii, borehole 11/10-1, 609.7 m, sample 609.7 OD, coordinates J19-4. 6.Chiropteridium galea, borehole Nini-1, 1560 m, sample K-34 F-1, coordinates F15-3. 7. Areosphaeridium diktyoplokum, borehole Nini-1, 1640 m, sample K-124F-1.

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Due to the discrepancies between the data presented in Fig. 4A andTable 1 of Eldrett et al. (2004), their identification of the polaritychrons is questionable. For this reason, our Fig. 5 shows their polarityzones without any chronozone assessment.

Svalbardella cooksoniae was observed in many samples recordedbetween 541.34 and 463.07 m, and the FO is dated to middle EoceneC19r (Eldrett et al., 2004). However, a clear and distinctive acme of S.cooksoniae is recorded in two samples at 464.3 and 463.07 m. The

sample at 463.07 m yielded 6.15% S. cooksoniae and also the LO of insitu Areosphaeridium diktyoplokum.

6.1.2. The Norwegian SeaODP Hole 642A is situated in the Norwegian Sea NE of Iceland

(Fig. 1A). An interval between 70.28 and 556.71 m was dated withdinoflagellate cysts by Manum et al. (1989). The borehole wasrestudied by Eldrett et al. (2004), who reexamined the palynologicalsamples and supplemented the analysis with magnetostratigraphy.The LO of Areosphaeridium diktyoplokumwas reinterpreted to occur at481.0 m, and a new marker, the FO of Wetzeliella gochtii at 478.0, wasadded. Positions of the dinoflagellate cyst events are shown in Fig. 4,based on Eldrett et al. (2004). According to Eldrett et al. (2004, Fig. 4)the magnetostratigraphic calibration of dinoflagellate cyst eventsabove the base of Chron C17n (at ca. 483.5 m) is tentative. Themagnetostratigraphy is therefore not included in our study.

Svalbardella cooksoniae is present only in one sample at 468.4 m(Fig. 4), where it is common (between 2% and 25%) according toManum et al. (1989). The occurrence of S. cooksoniae is bracketed bythe LO of Areosphaeridium diktyoplokum below, and the FO's ofSpiniferites manumii and Chiropteridium spp. at 463.2 m.

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Fig. 3. The Svalbardella cooksoniae interval in the central and eastern North Sea. The colored interval indicates the presence of Svalbardella cooksoniae. Positions of wells are shown inFigs. 1B and 2. Depths are in meters. Dashed line, FO (first occurrence) of in situ Chiropteridium spp.; solid lines, LO (last occurrences).

Table 2Previously published Northern Hemisphere sections with early Oligocene Svalbardellarecords.

– The Greenland Sea: ODP Hole 913B (Firth, 1996; Eldrett et al., 2004)– The Norwegian Sea: wells 6610/2-1S, 6609/11-1, 6407/9-2 and 6407/9-5

(Eidvin et al., 2007), ODP Sile 642A ( Manum et al., 1989)– The North Sea: well Alma-1X (Schiøler, 2005)– The Belgian Basin: the Kallo borehole (De Coninck, 1999)– Southernmost Poland, the Flysch Carpathians: the Leluchów outcrop (Gedl, 2004)– Central Italy: the Monte Cagnero outcrop (Brinkhuis and Biffi, 1993)

the Massicore borehole (Van Mourik and Brinkhuis, 2005)

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DSDP Hole 338 is located on the Norwegian shelf (Fig. 1A). It wasfirst studied by Manum (1976) and subsequently by Eldrett et al.(2004), who supplemented the dinoflagellate cyst biostratigraphywithmagnetostratigraphic data. However, the polarity of the intervalcovering the Eocene–Oligocene transition is uncertain.

The FO of Chiropteridium galea is recorded in the uppermostsample (253.84 m) studied by Eldrett et al. (2004). The LO of in situAreosphaeridium diktyoplokum is difficult to indicate as the species hasan almost continuous record through the upper part of the studiedsection. Svalbardella cooksoniae is present from 276.11 m to 257.21 m.The Eocene–Oligocene boundary coincides with an unconformity at257.75 m according to Eldrett and Harding (2009). The unconformityprecludes an unquestionable chronozone assessment of the sedi-ments preserved above it, and the section will not be further dealtwith in the present study.

On the inner continental shelf of the Norwegian Sea a number ofwells (Fig. 2) were studied by Eidvin et al. (2007), who described aSvalbardella cooksoniae Zone of earliest Rupelian age (Fig. 5). The topof the zone was marked by the LO of Svalbardella cooksoniae and thebase of the zone by the LO of Areosphaeridium diktyoplokum. Thedistribution of Svalbardella cooksoniae in the wells is shown in Fig. 5.The abundance of S. cooksoniae is generally 0–5%, except in the 6610/2-1S well, where an acme of 5–20% was observed at 1020 m. In 6610/2-1S the top of the S. cooksoniae interval was dated with strontiumisotopes to 32.2 Ma. The early Oligocene section is expanded in the6610/2-1S well, but the Eocene–Oligocene transition coincides herewith an unconformity. The 6609/11-1 well has a continuous sedi-mentary record from Upper Eocene to Lower Miocene according toEidvin et al. (2007), with the LO of Areosphaeridium diktyoplokum at1385 m, the LO of Svalbardella cooksoniae at 1375 m and the FO ofChiropteridium lobospinosum at 1365 m. In wells 6407/9-2 and 6407/9-5 the LO of S. cooksoniae is recorded at 880 m and 860 m, respectively.

6.1.3. The North SeaThe Alma-1X well is situated in the central North Sea, south of the

wells studied by us (Fig. 1). A detailed biostratigraphic study of thedinoflagellate cysts was made by Schiøler (2005). Svalbardella

cooksoniae was recorded in two samples, at 1844.0 m and 1853.2 m(Fig. 3). The LO of Areosphaeridium diktyoplokum was observed at1853.2 m and the LO of Spiniferites manumii at 1825.8 m.

6.1.4. Continental EuropeA detailed dinoflagellate cyst study of the Kallo borehole from the

Rupelian stratotype area in Belgium, i.e., southern North Sea Basin(Fig. 1A) was carried by De Coninck (1999). The Kallo borehole wascored and therefore both thefirst and lastoccurrences ofmicrofossils canbe indicated here. Calcareous nannofossils were studied by Steurbaut(1986).

The distribution of dinoflagellates indicates that the section includesa nearly complete lowermost Rupelian succession. Svalbardella sp. wasrecorded at 106.0 m (Fig. 4) and constitutes 0.2–0.7% of the organicwalled phytoplankton. Svalbardella sp. of De Coninck (1999) resemblesSvalbardella cooksoniae from the North Sea wells. The FO of Wetzeliellagochtii was recorded in the same sample. The LO of Areosphaeridiumdiktyoplokum was recorded at 109.5 m. Two other significant dinofla-gellate events are the LOofGlaphyrocysta semitecta at 98.0 m and the FOof Chiropteridium galea (recorded as C. mespilanum) at 94.0 m.

The calcareous nannofossils are present only in the uppermost partof the profile. According to Steurbaut (1986) the interval between96.0 m and 100.0 m belongs to NP22.

In southernmost Poland (Fig. 1A), in a Paratethyan setting, theEocene–Oligocene transition is exposed in the Leluchów outcrop(Fig. 4). Dinoflagellate cysts, planktonic foraminifera and calcareousnannoplankton were studied in the section (Gedl, 2004 and referencestherein).

Svalbardella sp. is present in two samples (Fig. 4). A singlespecimen of Svalbardella sp. was recorded in a sample yielding the LOof Areosphaeridium diktyoplokum. Two samples higher, 2–5 specimensof Svalbardella sp. were found. Another dinoflagellate event, whichenables correlation with sections from the North Sea Basin and theBelgian section, is the FO of Wetzeliella gochtii.

InCentral Italy, in a northern Tethyan setting, Svalbardella cooksoniaewas described from the Lower Oligocene in two sections (Fig. 6). Inthe Monte Cagnero outcrop Svalbardella cooksoniae was recorded by

5

10A.diktyoplokum

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>5% Svalbardella cooksoniae

Fig. 4. The Svalbardella cooksoniae interval in the most important Northern Hemisphere sections. The positions of the sites are shown in Fig. 1A. Magnetostratigraphy in ODP Hole913B after Fig. 4a in Eldrett et al. (2004). For the LO of Areosphaeridium diktyoplokum in ODP Hole 913B, see Discussion. Barren samples from ODP Hole 913B are not shown in thefigure. Symbols as in Fig. 3.

144 K.K. Śliwińska, C. Heilmann-Clausen / Palaeogeography, Palaeoclimatology, Palaeoecology 305 (2011) 138–149

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Brinkhuis and Biffi (1993) in two samples, at 111.6 m and 111.0 m,above the LO of Areosphaeridium diktyoplokum (103.0 m) and LO ofGlaphyrocysta semitecta (107.0 m). Svalbardella cooksoniae makes upless than 0.5% of the counted flora in the two samples (Brinkhuis andBiffi, 1993). The section was restudied by Hyland et al. (2009) with anew set of samples for integrated biostratigraphy, magnetostratigraphyand cyclostratigraphy. The Oi-1 event, based on an upwards shifttowards higher values of calciumcarbonate,waspositioned in the lowerpart of Chron C13n. According to Hyland et al. (2009) the results of thedinoflagellate cyst studymimic the earlier results obtained by Brinkhuisand Biffi (1993). Discrepancies between the depth scales of the twopublicationsand the lackof a distribution chart for dinoflagellate cysts inHyland et al. (2009) preclude precise positioning of the Svalbardellacooksoniae interval in the profile presented by Hyland et al. (2009).

In a borehole section (Massicore), situated nearMassignano, a singleoccurrence of Svalbardella cooksoniae was recorded at 7.11 m by VanMourik andBrinkhuis (2005). S. cooksoniaemakes up0.5%of the organicwalled phytoplankton in the sample. The event is observed above the LO

of Areosphaeridium diktyoplokum (10.5 m), and below the LO ofGlaphyrocysta semitecta (6.70 m). Threeotherdinoflagellate cyst events:the FO and the LO of Gelatia inflata and the LO of Phthanoperidiniumcomatum, were recognized and used for correlation between the twoItalian sections, as shown on Fig. 7.

6.2. Biostratigraphical positioning of the event and significance forrevision of the Eocene–Oligocene boundary

In all the sections the base of the Svalbardella cooksoniae intervaleither coincideswith the LOofAreosphaeridiumdiktyoplokum (Site 913B,11/10-1, Nini-1, Alma-1X, Leluchów) or occurs shortly above it (642A,6609/11-1, Kallo, Central Italy). The base of the Svalbardella cooksoniaeinflux is thus closely associated with the LO of A. diktyoplokum. In all thesections the Svalbardella cooksoniae interval predates the FO ofChiropteridium spp. (with the Mona-1 well as a possible exception, buthere the FO of Chiropteridium spp. is localized with uncertainty, seeabove) and/or the LO of Spiniferitesmanumii.The interval of S. cooksoniae

Areo

liger

a se

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Eidvin et al., 2007

Ear

ly O

ligoc

ene

Fig. 5. The Svalbardella cooksoniae interval in the eastern Norwegian Sea based on Figs. A4, A5, A11 and A13 in Eidvin et al. (2007). The colored interval indicates the presence ofSvalbardella cooksoniae. Symbols like in Figs. 3 and 4.

145K.K. Śliwińska, C. Heilmann-Clausen / Palaeogeography, Palaeoclimatology, Palaeoecology 305 (2011) 138–149

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is thus closely bracketed by these events. The biostratigraphic position isconstrained by additional dinoflagellate cyst events in some of thesections, namely the LO of Glaphyrocysta semitecta, the FO and LO ofGelatia inflata, the FO of Wetzeliella gochtii, the LO of Areoligerasemicirculata and the LO of Phthanoperidinium comatum.

It has been suggested (VanMourik and Brinkhuis, 2005) tomove theEocene–Oligocene boundary upwards so as to coincide with the LO ofA. diktyoplokum, to reflect themajor coolingwithin the currently definedOligocene. Our findings of a Svalbardella interval closely associatedwiththis event in widespread sections adds weight to the importance of theLO of A. diktyoplokum and thus supports the suggested revision of theboundary.

6.3. The Svalbardella cooksoniae interval as an indicator of the Oi1acooling event

The correlation of the offshore Nini-1 well with the Horn-1 borehole(Fig. 7) shows that the Svalbardella cooksoniae event occurred withinChron C12r. Occurrence of the event in Chron C12r is confirmed by themagnetostratigraphic data in the Massicore section (Van Mourik andBrinkhuis, 2005). Calcareous nannofossil data from the Kallo borehole,the Leluchów profile (Fig. 4) and the Monte Cagnero section (Fig. 6)indicate that the event took place during the late NP21 Zone (MonteCagnero) and before the endof theNP22Zone (Kallo). ChronC12r spansthis biostratigraphic interval, according to biomagnetostratigraphiccalibrations in Berggren et al. (1995) and Luterbacher et al. (2004).

Gelatia inflata

Gelatia inflata

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P18NP21 5

10Areosphaeridium diktyoplokum

Phthanoperidinium comatum

Glaphyrocysta semitecta Wetzeliella gochtii

Monte CagneroBrinkhuis& Biffi, 1993

Massicore Van Mourik & Brinkhuis

2005

C13n

C12r

?

Fig. 6. The Svalbardella cooksoniae interval in Central Italy: Monte Cagnero andMassicore sections. Symbols as in Fig. 3.

Areosphaeridium diktyoplokum

TOP EOCENE

Köthe(1990; 2003)

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onshoreDenmark P

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th

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Grundfør Mb.

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SøvindMarl Fm.

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.

Horn-1onshore Denmark

Fig. 7. Correlation between the North Sea (Nini-1 well) and Danish onshore area (Horn-1 borehole). Lithological units onshore Denmark after Heilmann-Clausen (2006).Magnetostratigraphy after Śliwińska et al. (2011). Symbols as in Figs. 3–5. Abbreviations: Eoc. = Eocene, Bart. = Bartonian.

146 K.K. Śliwińska, C. Heilmann-Clausen / Palaeogeography, Palaeoclimatology, Palaeoecology 305 (2011) 138–149

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As shown in publications of Pekar andMiller (1996) and Pekar et al.(2001, 2002), two oxygen isotope maxima, Oi1 and Oi1a, occurredduring the early Oligocene. The Oi1amaximum is calibrated against theearly part of ChronC12r and the latest part ofNP21anddated to32.8 Ma(Pekar andMiller, 1996). The Svalbardella cooksoniaemigration event inthe Northern Atlantic/Western Tethyan realm therefore seems to besynchronous with the Oi1a cooling event and hence to be useful as anindicator for this cooling phase.

6.4. Sea level change associated with the Svalbardella cooksoniae event

In the eastern North Sea Basin, the LO of Areosphaeridiumdiktyoplokum and the FO of Chiropteridium spp. are separated by aninterval which constitutes the D13 Zone of Köthe (1990) and the NSO-2Zone of Van Simaeys et al. (2005b), (Fig. 7). In Danish onshore Rupeliansections, the LO of Areosphaeridium diktyoplokum coincides with anunconformity (Michelsen et al., 1998). In the most complete of theseonshore sections, e.g., Horn-1 (Fig. 7), the D13/NSO-2 Zone is missing.The overlying unit (Linde Clay, Fig. 7) is referred to NP22 in the Linde-1borehole (Thomsen, 1995). The Svalbardella bearing interval recordedoffshore (Nini-1, Mona-1, Alma-1X and 11/10-1) belongs in the D13/NSO-2 Zone and thus coincides with the onshore unconformity (Fig. 7).

The presence throughout the Danish land area of this unconformityindicates a significant sea level fall. A similar development is seen in thesouthern part of theNorth SeaBasinwhere anunconformity of the sameage occurs between strata referred to NP21 and NP22, respectively(Vandenberghe et al., 2003). These authors recognized the unconfor-mity as the TA4.3/TA4.4 sequence boundary and also suggested it tocoincide with the Oi1a event. An oxygen isotope study in the same arearevealed a ~1.5‰ increase in δ18O values across the unconformity (DeMan et al., 2004), supporting its coincidence with a cooling event.

6.5. Wider implications of the early Oligocene Svalbardella cooksoniaeinterval

Calibrations of apparent sea level changes to oxygen isotope valuesindicate the buildup of an East Antarctic ice sheet from near zero duringthe late Eocene to levels 20–25% higher than today during the earliestOligocene, and a corresponding apparent sea level fall equivalent to80 m (Pekar and Christie-Blick, 2008). The Oi1a unconformity in theNorth Sea Basin and its counterpart in the New Jersey continentalmargin (Pekar et al., 2001), are clearly related to ice cap dynamics onAntarctica and suggest a strong expansion of the ice sheet at the timeconstrained by these unconformities.

Themost complete of the New Jersey sections of Pekar et al. (2001)includes two separate earliest Oligoceneunconformities reflecting twosea level falls associated with the Oi1 and Oi1a cooling, respectively. Asimilar situation with two separate unconformities can be documen-ted in the North Sea Basin. A first regional unconformity is the "topEocene" unconformity overlain by the first prograding clinoformsrecorded by Michelsen et al. (1998) as their Major Unit 3/4 boundary.The intra NP21 age of this unconformity (Michelsen et al., 1998) showsthat it is synchronous with the Oi1 event. The second unconformity issynchronouswith theOi1a cooling event and is recorded in the easternNorth Sea Basin (Fig. 7) as discussed in Section 6.4. The presence of asedimentary succession between the two unconformities on bothsides of theNorthAtlantic suggests an eustatic sea level rise, and hencea melting phase on the East Antarctic ice sheet between the Oi1 andOi1a oxygen isotope maxima.

Numerous models (i.e. DeConto and Pollard, 2003; Pollard andDeConto, 2005;DeConto et al., 2008) suggest that a drop in atmosphericpCO2 acted as a trigger for the onset of the icehouse climate at thebeginning of theOligocene. The atmospheric pCO2 values across themidEocene–early Oligocene transition were rather unstable (Pagani et al.,2005). The first drop below the threshold value for Antarctic glaciationcoincided with favorable orbital parameters which initiated the rapid

expansion of the East Antarctic ice sheet close to the Eocene–Oligoceneboundary (Pearson et al., 2009) and led to the Oi1 oxygen isotopemaximum. The second oldest Oligocene oxygen isotope maximumrepresenting the Oi1a cooling event also coincided with an eccentricityminimum and low amplitude obliquity (cycle 81Ol-C12r), and like anyother major late Paleogene glaciations (Oi-1, Oi2b, and M-1), followedstrong periodicities of 405 k.y. and 1.2 m.y. (Pälike et al., 2006).

The mid Oligocene was characterized by a more stable atmo-spheric pCO2 (Pagani et al., 2005) and ice caps on East Antarctica wereconsistently present for almost 4 Ma and were 30% larger than thepresent day East Antarctic ice sheet (Pekar and Christie-Blick, 2008).

The genus Svalbardella is knownmainly from the middle Eocene toOligocene of northern high latitudes (Manum, 1960; Eldrett et al.,2004). The first appearance of Svalbardella in C19r (Eldrett et al., 2004)is subsequent to a gradual global cooling (Bohaty and Zachos, 2003).The paleogeographic distribution shows the affinity of the genus withcolder Arctic surface waters (Head and Norris, 1989; Van Simaeyset al., 2005a).

The widespread records of Svalbardella cooksoniae, shown in thepresent study to be associated with the Oi1a event, resembles the midOligocene Svalbardella spp. event which was linked with the heaviestOligocene oxygen isotope values, i.e., the Oi2b cooling event (VanSimaeys et al., 2005a).

Except in themostnortherly sections (GreenlandSea andNorwegianSea) Svalbardella was less abundant during the Oi1a event than it wasduring the mid Oligocene Oi2b event, in which Svalbardella reached amaximum of 8–10% of total dinoflagellate cysts in Italy (Van Simaeyset al., 2005a). During the Oi2b event Svalbardellamigrated down to theSouthern Pacific Ocean (Van Simaeys et al., 2005a). In contrast, duringthe early Oligocene Oi1a event, Svalbardella has only been recorded inthe Northern Hemisphere. This distribution confirms the climaticscenario for the strongest cooling within the mid Oligocene (VanSimaeys et al., 2005a; Pälike et al., 2006) and suggests that the secondstrongest cooling occurred during the earliest Oligocene Oi1a event.

7. Conclusions

We show that the cold water dinoflagellate cyst Svalbardellacooksoniae is present in a short interval in the earliest Oligocene inmany localities within the Northern Atlantic and Western Tethyanrealms. The base of the interval occurs near the LO of Areosphaeridiumdiktyoplokum and the top is below the FO of Chiropteridium spp.Combined evidence places the Svalbardella cooksoniae event in theearly part of Chron C12r and within the late NP21-NP22 interval, andthe occurrence is therefore connected with the Oi1a oxygen isotopemaximum. Previous suggestions to redefine the Eocene–Oligoceneboundary to coincide with the LO of Areosphaeridium diktyoplokum issupported by the finding of Svalbardella cooksoniae close to the LO ofAreosphaeridium diktyoplokum in many sections.

Unconformities in Denmark, Belgium and New Jersey show that asignificant eustatic sea level fall coincided with the Oi1a coolingphase. Slightly older unconformities in the eastern North Sea Basinand New Jersey correlate with the Oi1 cooling phase. The presence oftwo successive regional unconformities indicates that a significant,but short lived, sea level rise took place between the Oi1 and Oi1acooling events.

The Oi1a Svalbardella cooksoniae event resembles the midOligocene Oi2b Svalbardella spp. event. However, the lower abun-dance of Svalbardella and a paleogeographical distribution restrictedto the Northern Hemisphere suggest that the early Oligocene coolingwas less severe.

Acknowledgments

Kirsten Rosendal, Department of Earth Sciences (Aarhus University),prepared the samples and Bartosz Goledowski, Department of Earth

147K.K. Śliwińska, C. Heilmann-Clausen / Palaeogeography, Palaeoclimatology, Palaeoecology 305 (2011) 138–149

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Sciences (Aarhus University), made an outline for Fig. 1B. Katarzyna K.Śliwińska acknowledges financial support for her PhD project from theAarhus University (Denmark) and Fur Museum (Denmark). TheNorwegian Petroleum Directorate (NPD) is acknowledged for placingpalynological preparations from the 11/10-1 well at our disposal. Thefirst author thanksMikkel Jürgens for inspiringdiscussions and criticismof the first version of the manuscript. Robert Knox (British GeologicalSurvey) and a second anonymous reviewer are thanked for theirthoughtful and constructive suggestions and corrections which sub-stantially improved the manuscript.

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