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Age calibration of cyst and acritarch events in the Pliocene–Pleistocene of the eastern North Atlantic (DSDP Hole 610A)

Stijn De Schepper1,* and Martin J. Head2 1Cambridge Quaternary, Department of Geography, University of Cambridge, Downing Place, Cambridge CB2 3EN, United ; email: [email protected] 2Department of Earth Sciences, Brock University, 500 Glenridge Avenue, St. Catharines, Ontario L2S 3A1, Canada email: [email protected]

ABSTRACT: An independently calibrated record of dinoflagellate cyst and acritarch events is presented for the Early Pliocene through Middle Pleistocene (ca. 4.0–0.5 Ma) of eastern North Atlantic Deep Sea Drilling Project (DSDP) Hole 610A. A new age model is estab- lished for this hole and tied to marine isotope stratigraphy and magnetostratigraphy back to 3.6 Ma. New data on the range of the calcar- eous nannofossil Reticulofenestra pseudoumbilicus indicate that the base of this hole is about 1.0 Myr younger than previously thought. A diverse dinoflagellate cyst and acritarch record allows the significant highest and/or lowest occurrences of 19 dinoflagellate cyst and seven acritarch taxa to be recognised in Hole 610A and calibrated to the latest astronomically-tuned Neogene time scale (ATNTS 2004) via our new age model. Comparing records across the North Atlantic and Mediterranean reveals near-synchronous highest occur- rences of the dinoflagellate cysts Ataxiodinium confusum (2.63–2.65 Ma), Invertocysta lacrymosa (2.72–2.74 Ma in the eastern and central North Atlantic and Mediterranean) and Impagidinium solidum (ca. 3.15–3.17 Ma), and the acritarch rockhallensis (ca. 3.83–3.88 Ma). Highest occurrences of the dinoflagellate cyst Batiacasphaera minuta/micropapillata (3.83–ca. 3.7 Ma) and acritarch Cymatiosphaera latisepta (2.49–2.63 Ma) also provide useful markers for correlation. A precise stratigraphy for Hole 610A allows us to evaluate the impact of paleoceanographic and climatic events on the dino- flagellate cyst record. Climatic and oceanographic reorganizations associated with the onset of Northern Hemisphere glaciation appear responsible for the disappearance of many species between 2.8 and 2.6 Ma. The lowest occurrence of Impagidinium cantabrigiense (1.86 Ma) in the Olduvai Subchron is one of the few good biostratigraphic markers for the uppermost Gelasian in Hole 610A.

INTRODUCTION 611, northern North Atlantic (Mudie 1987), DSDP Hole 603C, Organic-walled dinoflagellate cysts and acritarchs exhibit rela- western North Atlantic (Head and Norris 2003; M.J.H., unpub- tively high species diversity in Pliocene–Pleistocene deposits of lished data), and ODP sites 642–644, Norwegian Sea (Mudie the high-latitude North Atlantic and adjacent seas (Mudie 1987; 1989). de Vernal and Mudie 1989a, b; Mudie 1989; Smelror 1999; Piasecki 2003). This diversity suggests the potential for detailed DSDP Hole 610A (53º13.297'N, 18º53.213'W; water depth, and reliable biostratigraphic correlations within a region critical 2417m) is located in the sub-polar environment of the eastern for understanding the development of northern hemisphere cli- North Atlantic, on the Feni Drift at the SW edge of the Rockall mate. Planktonic and calcareous nannofossils are Trough, a deep NE-SW orientated deep depression (up to traditionally used for Plio-Pleistocene in the 3500m depth) on the European continental shelf west of Ireland low-latitude North Atlantic (e.g. Martini 1971; Berggren et al. (text-fig. 1). The hole penetrated a 201m-thick uninterrupted 1995), but their diversity diminishes progressively at higher lat- succession of Quaternary through Early Pliocene age. The li- itudes and this inevitably hinders their biostratigraphic utility thology consists of interbedded calcareous mud and foram- (Perch-Nielsen 1985; Spiegler and Jansen 1989; Weaver 1987). iniferal–nannofossil ooze for the upper part of the hole, and Diatoms show high diversity and abundance in high-latitude more homogenous nannofossil ooze below (Shipboard Scien- deposits of the North Atlantic, but their delicate opaline silica tific Party 1987). This single hole was chosen for study because frustules are prone to dissolution (Barron 1993), unlike the or- of its excellent core recovery (95%), absence of hiatuses and ganic walls of dinoflagellate cysts and acritarchs. To explore relatively high sedimentation rates (5cm/kyr; Shipboard Scien- the biostratigraphic potential of dinoflagellate cysts and acri- tific Party 1987), and its detailed and independent age control tarchs for the Plio–Pleistocene of higher northern latitudes, it is based on magnetostratigraphy throughout (Clement and Robin- necessary to document those sites that have excellent independ- son 1987) and marine isotope stratigraphy for the time interval ent chronostratigraphy. Currently, this has been accomplished between 3.6 and 2.4 Ma (Kleiven et al. 2002). In evaluating our for only a few Pliocene–Early Pleistocene sites: DSDP Hole data from Hole 610A, we have found it necessary to establish a new age model for the lower part of the hole.

*Present address of corresponding author: Stijn De Schepper, Fachbereich-5, Geowissenschaften, Universität Bremen, Postfach 330 The principal aim of this study is to identify biostratigraphic 440, D-28334, Germany. Tel.: +49 421 218 3974. Fax: +49 421 218 events from the dinoflagellate cyst and acritarch record of Hole 4451. Email: [email protected] 610A, compare these events across the North Atlantic and adja- stratigraphy, vol. 5, no. 2, pp. 137–161, text-figures 1–7, plates 1–2, table 1, 2008 137 S. De Schepper and M. J. Head: Age calibration of dinoflagellate cyst and acritarch events, Pliocene–Pleistocene of the eastern North Atlantic

cent seas for synchroneity, and evaluate the impact of pale- Full preparation details are given in De Schepper (2006) and De oceanographic and/or paleoclimatic events. Schepper and Head (2008).

Throughout this study, the astronomically-tuned Neogene time Microscope slides were scanned with a Leica DMLB micro- scale ATNTS 2004 (Lourens et al. 2005) is used, and the litera- scope along non-overlapping traverses under a 40× objective, ture has been updated to this time scale where possible. Where and palynomorphs were counted until at least 300 dinoflagellate ages for marine isotope stages are cited, we have used the cysts had been enumerated. The remainder of the slide was ex- LR2004 time scale (Lisiecki and Raymo 2005), and updated the amined for rare species using a 20× objective. A JEOL 6400 numerical ages of the bioevents in Versteegh (1997). Datums Scanning Electron Microscope (SEM) was used on selected for Hole 610A are extracted from De Schepper (2006). Taxo- samples, and images were acquired digitally using Noran Van- nomic nomenclature for dinoflagellate cysts and acritarchs fol- tage software. lows Head and Norris (2003), Fensome and Williams (2004 and references therein) and De Schepper and Head (2008). All taxa representing biostratigraphic events in Hole 610A are illustrated on Plates 1 and 2, and full authorial citations for taxa are given in Table 1. GEOLOGICAL AND PALEOCEANOGRAPHIC SETTING Abrupt deepening of the Rockall Basin (of about 1000m; AGE MODEL Scrutton 1986) occurred during the Late Eocene to Oligocene as the result of rapid differential subsidence (Stoker 1997; The Baldauf et al. (1987) age model Stoker et al. 2005 and references therein; Praeg et al. 2005). The age model for Hole 610A (Baldauf et al. 1987) was based This event outpaced sedimentation leaving a deep-water basin exclusively on the magnetostratigraphic interpretation of Clem- that remains today (Stoker 1997; Stoker et al. 2005). From the ent and Robinson (1987). This model was calibrated to the Oligocene–Miocene, deep bottom currents influenced sedimen- Berggren et al. (1985) Neogene time scale and gave an extrapo- tation and formed sediment drifts (Stoker 1997, McDonnel and lated age of 4.48–4.60 Ma for the base of the hole (Weaver and Shannon 2001, Stoker et al. 2005). Only by the mid-Miocene Clement 1987). When recalibrating that age model to the had the Wyville-Thomson Ridge subsided enough to ensure the ATNTS 2004, the bottom of the hole (ca. 201 mbsf) would cor- overflow of Norwegian Sea Deep Water (Stoker 1997; Stoker respond to a maximum age of ca. 5.0 Ma (text-fig. 3, dotted et al. 2001, Stoker et al. 2005). Poore et al. (2006) indicated an line). age of 12 Ma for the onset of North Atlantic Deep Water over- flow related to the submergence of the Iceland-Faeroe Ridge. The sedimentation rate curve of Baldauf et al. (1987) is linear at Since then, the Rockall Trough has undergone no major topo- around 5 cm/kyr for much of Hole 610A, but displays unusual graphic changes. In the Late Pliocene and Pleistocene, sedi- variations between 1 and 5 cm/kyr in the lowermost 50m of the mentation changed from foraminiferal–nannofossil ooze to an hole (text-fig. 3, dotted line). There is no evidence from the li- ooze intercalated with calcareous mud (Shipboard Scientific thology or seismic stratigraphy (Shipboard Scientific Party Party 1987), and major prograding wedges appeared (Stoker et 1987; Masson and Kidd 1987) to suggest an unconformity in al. 2001, 2005). Hole 610A, or that changes in bottom currents might be respon- sible for the apparent variations in sedimentation rate. This has With the exception of glacial episodes, it can be assumed that compelled us to re-evaluate the biostratigraphic (planktonic oceanic conditions similar to those of today have prevailed foraminifer, calcareous nannofossil, and dinoflagellate cyst) since the mid-Miocene. The present day paleoceanography of and magnetostratigraphic evidence for Hole 610A. the Rockall Trough is complex, involving warm, saline and northward flowing surface water currents as the North Atlantic Towards an improved age model Current (NAC), the European Shelf Edge Current (CSC; C10 unconformity Hansen and Østerhus 2000; text-fig. 1), and the Eastern North Atlantic Water (ENAW; Holliday et al. 2000). Below 1200m An unconformity (C10) has been identified in the Lower Plio- water depth, Labrador Sea Water (LSW) enters the trough from cene of the Rockall Trough (Stoker et al. 2001). It has an ero- the south, but due to topographic constraints it re-circulates in sional character, suggesting the result of increased erosion by an anti-clockwise or cyclonic gyre (text-fig. 1), whereas Arctic bottom currents and tilting of shelf areas associated with re- Intermediate Water and Norwegian Sea Deep Water enter the gional plate re-organisation (Stoker et al. 2001, 2005; Praeg et Rockall Trough from the north (Ellet et al. 1986; Dickson and al. 2005). Although it was not initially reported from the logs or Kidd 1987). seismic profiles of DSDP Site 610, Stoker et al. (2001) attrib- uted a change in seismic character (based on Masson and Kidd MATERIALS AND METHODS 1986, fig. 6) to the C10 reflector between cores 9 and 10 in Hole 610. The interval represented by these cores was mostly not The present study is based on 102 samples from the Lower Plio- reached in Hole 610A (Shipboard Scientific Party 1987, p. 364), cene through lowermost Middle Pleistocene of DSDP Hole and there is no evidence that the unconformity is represented in 610A (text-fig. 2), spanning the interval between 199.11 and this hole. In the Rockall Trough area, this unconformity has 28.71 mbsf (= meters below seafloor). One sample from each been dated between 4.5 and 3.85 Ma (Stoker et al. 2001; Stoker section of core was analysed between sections 610A-21-6 and et al. 2005). The deposits at the base of Hole 610A are therefore 610A-4-1, resulting in an average sampling interval of ca. younger than 4.5–3.85 Ma, and much younger than the 4.9–5.0 1.5m. Ma inferred from the age model of Baldauf et al. (1987).

Standard palynological preparation techniques were applied, Calcareous nannofossils including HCl and HF, and sieving at 10µm. No oxidation or al- kali treatments were used. Samples 610A-21-6, 18–24cm to The highest common occurrence of Reticulofenestra pseudo- 610A-8-1, 81–86cm received brief (30–40 second) ultrasound. umbilicus in Hole 610A was reported at 4.37 Ma (Takayama

138 Stratigraphy, vol. 5, no. 2, 2008

TEXT-FIGURE 1 Location of DSDP Hole 610A in the North Atlantic, together with other DSDP, ODP and IODP sites in the North Atlantic and adjoining seas, and the Singa section of southern Italy, as mentioned in the text. Light grey areas in the eastern North Atlantic represent shelves (water depth < 500m). Solid ar- rows represent warm surface water currents, dotted arrows cold bottom water currents. AIW = Antarctic Intermediate Water, CSC = European Shelf Edge Current (= Continental Slope Current), ENAW = Eastern North Atlantic Water, LSW = Labrador Sea Water, NAC = North Atlantic Current, NADW = North Atlantic Deep Water, NSDW = Norwegian Sea Deep Water, WTR = Wyville Thomson Ridge. and Sato 1987; Kameo and Takayama 1999; J. Young, pers. Lourens et al. 2005; ODP Site 926 in Gibbs et al. 2005) to as old comm.) using the age model of Baldauf et al. (1987), implying as 3.7 Ma (Eastern Mediterranean in Lourens et al. 2005; ODP an anomalously low position of this datum compared with other Site 662 in Gibbs et al. 2005). sites in the North Atlantic (e.g. 3.70, 3.79 and 3.84 Ma in Lourens et al. 2005; 3.81–3.82 Ma in Gibbs et al. 2005; 3.85 Ma Indirect evidence for the maximum age of Hole 610A is based in Shipboard Scientific Party 2005). Moreover, the highest oc- on the absence of the nannofossil genus Amaurolithus, which currence (HO) of Sphenolithus abies shows a similar pattern: has a HO at 4.50 Ma in the North Atlantic (Shipboard Scientific 4.10–4.16 Ma in Hole 610A (Takayama and Sato 1987), but Party 2005) and at 4.56 Ma in the South China Sea (Jian et al. elsewhere between 3.50 and 3.65 Ma (DSDP Sites 606–609, 2003). This would suggest that the base of Hole 610A is youn- 611 in Takayama and Sato 1987; ODP Leg 111 and 138 in ger than ca. 4.50 Ma. Indeed, Takayama and Sato (1987) in their

139 S. De Schepper and M. J. Head: Age calibration of dinoflagellate cyst and acritarch events, Pliocene–Pleistocene of the eastern North Atlantic

original study assigned the base of Hole 610A to nannofossil Magnetostratigraphy zone NN15, which is younger than the recalibrated age model of Baldauf et al. (1987) and consistent with our new age model. The upper 150m of the hole has an unambiguous paleomagnetic signal: the important polarity changes are recognized (Clement Planktonic foraminifera and Robinson 1987), and for the Piacenzian and Gelasian stages the age model is corroborated by marine isotope stratigraphy Globorotalia margaritae was recorded only in the lowermost (Kleiven et al. 2002). For the interval below 150m (Zanclean sample of Hole 610A at 201 mbsf (Sample 610A-21, CC; Stage), Clement and Robinson (1987) recognised the potential Weaver 1987). This HO datum corresponds to 4.48–4.60 Ma by problems of a weak paleomagnetic signal. The magnetisation extrapolation of the Baldauf et al. (1987) age model (Weaver intensities of samples in the lower part of Hole 610A are partic- 1987; Weaver and Clement 1987, tab. 1). Recalibrated to the ularly low at around 10-6 A/m (source data available at ATNTS 2004, this corresponds to 4.88–5.00 Ma in Hole 610A, http://www.ngdc.noaa.gov/mgg/geology/dsdp/data/94/ which is considerably older than any other recorded HO for this 610A/dsed_mag.txt). Moreover, these data result from a mag- species: around 3.84 Ma from several ODP and DSDP sites in netic signal that may not have been completely cleared of over- Weaver and Bergsten (1991), 3.929–3.967 Ma in the eastern prints (B. Clement, written comm. 2005). equatorial Atlantic (Chapman et al. 1998), 3.58 Ma in Berggren et al. (1995), 3.71 Ma at DSDP Site 606, 3.91 Ma at DSDP Site New age model 607, 4.52–4.56 Ma at DSDP Site 609 and 4.11–4.13 Ma at DSDP Site 611 (Weaver and Clement 1987, tab. 1; ATNTS The new age model for Hole 610A (solid line in text-fig. 3) uses 2004). Also, several tropical sites show older ages for the HO the magnetostratigraphy of Clement and Robinson (1987) from (but less than ca. 4.3 Ma, recalibrated to ATNTS 2004, at the Gilbert/Gauss to Matuyama/Brunhes Chron boundaries. DSDP Sites 660, 661, 662 and 664; Weaver and Raymo 1989). This model is supported by the calcareous nannofossil age The HO of Globorotalia margaritae in the North Atlantic model (dashed line in text-fig. 3) from the present day to the therefore ranges between 4.5 and 3.6 Ma, causing the presence Pleistocene (until ca. 1.8 Ma). The data points of the calcareous of this species in the base of Hole 610A to conflict with the nannofossil model are from Chapman and Chepstow-Lusty Baldauf et al. (1987) age model. (1997), Shipboard Scientific Party (2002), Jian et al. (2003), Lourens et al. (2005) and Shipboard Scientific Party (2005). Be- low ca. 1.8 Ma, a small offset between the magnetostratigraphic and nannofossil models can be attributed to the low sampling The absence of Reticulatosphaera actinocoronata from Hole resolution of Takayama and Sato (1987) but possibly also to the 610A (De Schepper 2006) is significant because this species is effect of the relatively high latitude of Site 610 on the HO of the characteristic of middle and lower Zanclean deposits elsewhere Discoaster species. For the entire Piacenzian Stage, marine iso- in the North Atlantic region. Louwye et al. (2004) reviewed the tope stratigraphy is available and is underpinned by the records of Reticulatosphaera actinocoronata in the North At- magnetostratigraphy, providing especially precise ages for this lantic and considered the HO to be between 4.7 and 4.4 Ma. The interval (ca. 3.6–2.6 Ma; Kleiven et al. 2002). This marine iso- LO of Corrudinium devernaliae appears not to have been tope stratigraphy (text-fig. 4) is established by visually calibrat- reached in Hole 610A, the lowermost sample containing this ing the benthic foraminiferal dataset of Kleiven et al. (2002) to species in moderate abundance. Its LO has a magnetostrati- the LR04 stack (Lisiecki and Raymo 2005), thereby updating graphically calibrated age of 4.66 Ma in western North Atlantic the interpretation of Kleiven et al. (2002). Correlation with the DSDP Hole 603C (Head and Norris 2003; M.J.H., unpublished LR04 stack proves difficult between 3.40 and 3.60 Ma, but two data) and of 4.86 Ma in central North Atlantic DSDP Hole 607 tie-points from marine isotopes in that interval were used for the (Sample 607-22-6, 110–112cm; S.D.S., unpublished data). Its age model (text-fig. 3). LO in Labrador Sea ODP Site 646 is placed tentatively within nannofossil zone NN12 (as Corrudinium sp. I in de Vernal and Given the incompatibility between the magnetostratigraphy, Mudie 1989b; Knüttel et al. 1989) which is dated at between calcareous nannofossil, dinoflagellate cyst and planktonic 5.55 and 5.05 Ma. The LO of Leiosphaeridia rockhallensis was foraminifer biostratigraphy in the lower part of the hole as dis- apparently likewise not reached in Hole 610A. In Hole 603C, cussed above, the age model is here redrawn. For the Zanclean this datum occurs within Subchron C3n.1r (Head and Norris Stage, the HO of R. pseudoumbilicus at 3.82 Ma (Gibbs et al. 2003) and has a magnetically calibrated age of 4.37 Ma 2005) is considered the most reliable event, and has been chosen (M.J.H., unpublished data). The lowest observed occurrence of as a tie-point. The curve is then extrapolated linearly from the Impagidinium solidum near the base of Hole 610A is likely Gilbert/Gauss reversal through the biostratigraphic event of R. younger than ca. 4.0 Ma. In the Mediterranean, the LO of this pseudoumbilicus to the bottom of the hole. species was observed near the top of nannofossil zone NN12–NN13 (late Zanclean) of the Trubi Formation in Sicily, Use of age model to calibrate biostratigraphic events, and accuracy Italy (Versteegh and Zevenboom 1995). In western North At- assessment lantic DSDP Hole 603C, the LO is observed within Subchron C2Ar and has a magnetostratigraphically calibrated age of 4.02 The new age model (text-fig. 3) has been used to calculate nu- Ma (M.J.H., unpublished data). merical ages by interpolation between tie-points. Sample depths were correlated to the ATNTS 2004 (Lourens et al. 2005) via Therefore, the base of Hole 610A is likely to be younger than the age model, a method also used by Eldrett et al. (2004), to about 4.4 Ma based on the presence of Leiosphaeridia provide absolute ages for each sample and bioevent. rockhallensis and possibly also the absence of Reticulato- sphaera actinocoronata. Sample 610A-20-6, 35–40cm, which However, there are inaccuracies associated with the construc- is about 11m above the base of the hole, is likely to be younger tion of the age model. Paleomagnetic reversal boundaries are than about 4.0 Ma based on the presence of Impagidinium placed at the midpoint between two samples of differing mag- solidum. netic signal, resulting in uncertainty that can be as much as half

140 Stratigraphy, vol. 5, no. 2, 2008

TEXT-FIGURE 2 Stratigraphic ranges of selected dinoflagellate cyst and acritarch taxa in DSDP Hole 610A. Also shown are magnetic polarity for Hole 610A (Clement and Robinson 1987), calcareous nannofossil zonation (Takayama and Sato 1987), planktonic foraminifer zonation (Weaver and Clement 1987), and dia- tom zonation (Baldauf 1987). Abbreviations for subchrons are as for text-figure 3. Grey shading in the magnetostratigraphic column indicates uncertain polarity between bounding samples used by Clement and Robinson (1987, p. 650) with respect to the position of our samples.

141 S. De Schepper and M. J. Head: Age calibration of dinoflagellate cyst and acritarch events, Pliocene–Pleistocene of the eastern North Atlantic

TABLE 1 the age difference between these samples. For Hole 610A, the Dinoflagellate cyst and acritarch taxa for which datums are used in the distance between two samples across a magnetic reversal used present study. in the new age model varies from 1.50m to 3.60m, and averages DINOFLAGELLATE CYSTS 1.80m (Clement and Robinson 1987). With a sedimentation rate of 5 cm/kyr, this accounts for an error of 15–36 kyr (average 18 Amiculosphaera umbraculum Harland 1979 Plate 1, figs. 4–5 kyr) on the age of the magnetic reversals (see also Weaver and Clement 1987). The marine isotope stratigraphy is based on Ataxiodinium confusum Versteegh and Zevenboom in Versteegh 1995 samples different from those analyzed for dinoflagellate cysts, Plate 1, figs. 1–3 and so it has been necessary to interpolate ages from the marine Barssidinium graminosum Lentin, Fensome and Williams 1994 isotope samples. With the close sampling interval of Kleiven et Plate 2, fig. 31 al. (2002; average of ca. 20cm) and the sedimentation rate of 5 Batiacasphaera minuta (Matsuoka 1983) Matsuoka and Head 1992 cm/kyr, this may account for an error of 2 kyr on the estimated Plate 1, figs. 6–8 age of the dinoflagellate cyst samples. Corrudinium devernaliae Head and Norris 2003 Plate 1, figs. 11–13 Bioturbation has been identified in the hole, especially at lithological boundaries (Shipboard Scientific Party 1987), and Edwardsiella sexispinosa Versteegh and Zevenboom in Versteegh 1995 Plate 1, figs. 9–10 gives a maximum error of 5.4 kyr too old and 8.8 kyr too young for the recorded HO or LO compared to the true events Habibacysta tectata Head, Norris and Mudie 1989 (Ruddiman and Glover 1972; Weaver and Clement 1987; Plate 2, figs. 9–10, 14–15 Versteegh 1997). Impagidinium cantabrigiense De Schepper and Head 2008 Plate 2, figs. 11–13 The greatest error between the true and recorded HO and LO of Impagidinium solidum Versteegh and Zevenboom in Versteegh 1995 a species is likely to be caused by the sampling resolution, Plate 1, figs. 16–18 which is equal to the sample distance in real time (see Versteegh Invertocysta lacrymosa Edwards 1984 1997). This distance for Hole 610A is between 62 and 296cm Plate 1, figs. 14–15 (average 158cm). With a sedimentation rate of 5 cm/kyr, this corresponds to a temporal error on the bio-events of between Invertocysta tabulata Edwards 1984 12.4 and 59.2 kyrs, with an average of 21.6 kyr. For example, Plate 1, figs. 19–20 the recorded age of 3.974 Ma for a HO has an error of 0.032 Ma Operculodinium janduchenei Head, Norris and Mudie 1989 due to the sampling resolution, meaning that the true HO could Plate 1, figs. 21–23 be as young as 3.942 Ma. Operculodinium tegillatum Head 1997 Plate 1, figs. 26–28 RESULTS Operculodinium? eirikianum Head, Norris and Mudie 1989 emend. Head 1997 var. eirikianum (autonym) Recognizing biostratigraphic events Plate 1, figs. 24–25 Reworking appears limited in Hole 610A, and does not substan- Operculodinium? eirikianum Head, Norris and Mudie 1989 emend. Head tially complicate the dinoflagellate cyst biostratigraphy. The 1997 var. crebrum De Schepper and Head 2008 well-defined stratigraphic range tops seen in selected species in Plate 2, figs. 32–33 Hole 610A, and the scarcity of specimens higher in the se- Pyxidinopsis tuberculata Versteegh and Zevenboom in Versteegh 1995 quence of species that are abundant within well-defined ranges Plate 1, figs. 29–30 lower in the hole (e.g. Batiacasphaera minuta, Operculodinium Pyxidinopsis vesiculata Head and Norris 2003 tegillatum), together imply that reworking of Plio–Pleistocene Plate 2, figs. 1–3 sediments is relatively unimportant. Spiniferites sp. A We distinguish between highest occurrence (HO), highest com- Plate 2, figs. 6–8 mon occurrence (HCO) and highest persistent occurrence Tectatodinium pellitum Wall 1967 (HPO). The HO is the highest in-situ occurrence of a species in Plate 2, figs. 4–5 the hole. The HPO represents the highest continuous occurrence ACRITARCHS (i.e. in successive samples) of the species in the hole, even where such occurrences are marked by few specimens. Sporadic Algal cyst type 1 of Head (1996) occurrences above a HPO might represent reworking. A HCO Plate 2, fig. 16 marks the highest sample in which a particular species is con- Cymatiosphaera latisepta De Schepper and Head 2008 spicuously abundant, although specimens will occur above this Plate 2, fig. 30 level in much lower numbers. In the case of species that typi- Cymatiosphaera? invaginata Head, Norris and Mudie 1989 cally occur in high abundance, and where the probability of re- Plate 2, figs. 20–21 working is thus greatest, a HCO might represent the highest Lavradosphaera crista De Schepper and Head 2008 in-situ occurrence of the species in question, with specimens Plate 2, figs. 22–24 above this level being reworked. Lavradosphaera lucifer De Schepper and Head 2008 Biostratigraphic events of dinoflagellate cysts and acritarchs Plate 2, figs. 25–27 at DSDP Hole 610A Lavradosphaera sp. 1 Plate 2, figs. 17–19 The palynological analysis of 102 samples, from sections 610A-21-6 to 610A-4-1, spans the time interval between ca. 4.0 Leiosphaeridia rockhallensis Head in Head and Norris 2003 Ma and 0.53 Ma. Raw counts are listed in text-figure 2. Ages for Plate 2, figs. 28–29 the events are based on the new age model and illustrated in

142 Stratigraphy, vol. 5, no. 2, 2008 igraphy. Below the t/Gauss Chron calcareous nannofossil ³ ly on magnetostratigraphy, and position of samples used in this study, ° y shading indicates the variation possible due to the range on ce with sedimentation rate. The dashed line shows the calcareous lithology (Shipboard Scientific Party 1987), diatom zonation (Baldauf 1987). Abbreviations for subchrons: JAR = Jaramillo, OLD ¯ µ reinterpreted magnetic polarity in Hole 610A (this study), core, ² 1987, p. 650), marine isotope stratigraphy and calcareous nannofossil strat Hole 610A (solid line), using tie-points from magnetostratigraphy (Gilber ® d to the ATNTS 2004 (Lourens et al. 2005). Tie-points are based exclusive species given by the Shipboard Scientific Party (2005, fig. 4). Gre section (in grey shading, coring gaps), ­ Discoaster (using Gibbs et al. 2005), the curve is extrapolated linearly in accordan planktonic foraminifer zonation (Weaver and Clement 1987), depth (mbsf), ´ ¬ , with error bars for their position in the hole; see Clement and Robinson ² the interpretation of Clement and Robinson (1987). New age model for DSDP ± Reticulofenestra pseudoumbilicus magnetic polarity in Hole 610A (Clement and Robinson 1987) used by Baldauf et al. (1987), = Olduvai, REU = Réunion, KAE = Kaena, MAM = Mammoth, COC = Cochiti, NUN = Nunivak, SID = Sidufjall. zonation (Takayama and Sato 1987), ± TEXT-FIGURE 3 Dotted line: age model of Baldauf et al. (1987) for DSDP Hole 610A, recalibrate correspond to boundary and above nannofossil age model, based onthe ages HOs of of HOs these of species. Symbols: tie-point of

143 S. De Schepper and M. J. Head: Age calibration of dinoflagellate cyst and acritarch events, Pliocene–Pleistocene of the eastern North Atlantic

text-figure 5 and summarized in text-figures 6 and 7. Hole cluding papillae (as in the holotype) and microreticulation 610A biostratigraphic events have been placed within a re- (Stover 1977). Specimens in Hole 610A are exclusively gional framework by comparing with ODP sites 642–646, 911, microreticulate, but to facilitate comparison with records else- 963, and 986, DSDP holes 400, 603C, 607, IODP Site 1308 and where, both species are considered together. In western North the Singa section of Calabria, southern Italy. The locations of Atlantic DSDP Hole 603C, Batiacasphaera minuta has a HO these sites are shown in text-figure 1. within Subchron C2Ar and a magnetostratigraphically cali- brated age of 3.74 Ma (M.J.H., unpublished data). In Labrador Zanclean Stage (Early Pliocene) – Dinoflagellate cysts Sea ODP Hole 646B, the HPO and HCO are close to the calcar- eous nannofossil zone NN15/NN16 boundary (ca. 3.7 Ma; as HO Pyxidinopsis vesiculata Batiacasphaera sphaerica, in de Vernal and Mudie 1989b; Occurrence and age: Sample 610A-21-2, 35-40cm, 193.28 Knüttel et al. 1989), with sporadic higher occurrences through mbsf; 3.92 Ma. Persistent in low abundances in the lowermost zone NN16 probably representing reworking. part of the hole. A rare occurrence in the Piacenzian is attrib- uted to reworking. Kuhlmann et al. (2006) recorded the HO of B. minuta near the Gauss/Matuyama Chron boundary in the Dutch sector of the Calibration: Subchron C2Ar, calcareous nannofossil zone southern North Sea. However, specimens were found only in NN15, planktonic foraminifer zone PL3–6, diatom zone low abundances in a few samples (Kuhlmann 2004, appendix, Nitzschia jouseae. tab. 1), suggesting that they are reworked. In the Pliocene North Sea deposits of Belgium, this species occurs in situ only in the Discussion: This species has been found previously only in Kattendijk Formation (5.0 to 4.7–4.4 Ma), with records in the western North Atlantic DSDP Hole 603C where it ranges up- Oorderen Sands Member of the Lillo Formation (3.7–2.7 Ma) wards into the Nunivak Subchron (C3n.2n, 4.631–4.493 Ma) being attributed to reworking (Louwye et al. 2004; De Schepper and has a magnetostratigraphically calibrated HO of 4.54 Ma 2006; De Schepper et al., in press). (Head and Norris 2003; M.J.H., unpublished data). HO Corrudinium devernaliae LO Tectatodinium pellitum Occurrence and age: Sample 610A-21-1, 24–30cm, 191.67 Occurrence and age: Sample 610A-20-4, 16–22cm, 186.49 mbsf; 3.90 Ma. This is an abrupt event in the lowermost part of mbsf; 3.83 Ma. Almost continuously present in low abundance the hole. A single rare occurrence higher in the Zanclean (Sam- from this sample upwards to its HPO in the upper Piacenzian. ple 610A-20-1, 31–36cm) is attributed to reworking. Calibration: Subchron C2Ar, near the calcareous nannofossil Calibration: Subchron C2Ar, calcareous nannofossil zone zone NN15/NN16 boundary, planktonic foraminifer zone NN15, planktonic foraminifer zone PL3–6, diatom zone PL3–6, diatom zone Nitzschia jouseae. Nitzschia jouseae. Discussion: The extant neritic Tectatodinium pellitum has a Discussion: In western North Atlantic DSDP Hole 603C, the range extending back to the Early Paleocene (Head and HO of this species has a magnetostratigraphically calibrated Nøhr-Hansen 1999), and its absence from the lowest part of age of 4.11 Ma (Head and Norris 2003; M.J.H., unpublished Hole 610A is therefore caused by ecological exclusion or data). In Labrador Sea ODP Hole 646B, the HPO is in the upper changes in transport from the shelf. part of nannofossil zone NN15 and dated at ca. 3.9 Ma (as Corrudinium sp. I in de Vernal and Mudie 1989b; Knüttel et al. HO Operculodinium tegillatum 1989), with rare and sporadic higher occurrences probably rep- resenting reworking (Head and Norris 2003). Rare and sporadic Occurrence and age: Sample 610A-19-4, 62–67cm, 177.35 occurrences in the Upper Pliocene of ODP Site 647 in the Lab- mbsf; 3.71 Ma. Dominant in some samples, but variable in rador Sea (as Corrudinium sp. I in de Vernal and Mudie 1989b) abundance. Single occurrences in the Piacenzian are attributed probably also represent reworking (Head and Norris 2003). A to reworking. specimen was recorded from northern North Atlantic DSDP Site 611 (as Corrudinium harlandii in Mudie 1987, pl. 3, figs. Calibration: Subchron C2Ar, calcareous nannofossil zone 9a and 9b) in calcareous nannofossil zone NN14–NN15, plank- NN16, planktonic foraminifer zone PL3–6, diatom zone tonic foraminifer zone PL3–5, and the Nitzschia jouseae diatom Nitzschia jouseae. zone. Discussion: In western North Atlantic DSDP Hole 603C, this HO Batiacasphaera minuta/micropapillata species has a HCO at a magnetostratigraphically calibrated age Occurrence and age: Sample 610A-20-4, 16–22cm, 186.49 of 3.98 Ma, but is consistently present in low numbers to its HO mbsf; 3.83 Ma. Abundances are higher in the lower part of the at 3.59 Ma (M.J.H., unpublished data). Judged within the con- hole. Single specimens in samples 610A-19-4, 62–67cm and text of Hole 610A, it is possible that reworking has extended the 610A-14-2, 64–69cm are considered reworked. HO of this species in Hole 603C. In Labrador Sea ODP Hole 646B, this species has a HPO close to the calcareous Calibration: Subchron C2Ar, near the calcareous nannofossil nannofossil zone NN15/NN16 boundary (ca. 3.7 Ma) (as zone NN15/NN16 boundary, planktonic foraminifer zone Operculodinium crassum in de Vernal and Mudie 1989b; PL3–6, diatom zone Nitzschia jouseae. Knüttel et al. 1989). Discussion: A conceptual overlap exists between Batiaca- In Belgium, O. tegillatum is restricted to the Kattendijk Forma- sphaera minuta and Batiacasphaera micropapillata, the former tion (5.0 to 4.7–4.4 Ma; Louwye et al. 2004; De Schepper et al., having an exclusively microreticulate ornament (Matsuoka and in press) and in eastern England it is restricted to the Coralline Head 1992) and the latter bearing various ornament types in- Crag Formation (tentatively 3.8–3.3 Ma in Head 1997, 1998).

144 Stratigraphy, vol. 5, no. 2, 2008

TEXT-FIGURE 4 Marine isotope stratigraphy for DSDP Hole 610A between 3.6 and 2.4 Ma, based on the original benthic foraminiferal isotope data of Kleiven et al. (2002), but recalibrated to the LR04 global stack (Lisiecki and Raymo 2005; curve offset to right to aid comparison). Grey shading indicates glacial inter- vals. Dots on the time axis represent samples analysed for palynology. Dinoflagellate cyst and acritarch events are given for the interval between MIS KM6 and MIS 95. A key to taxon abbreviations in the rectangles is given in text-figure 5. To the right are shown major global paleoceanographic and paleoclimatic events during this time interval, with abbreviations as follows: CAS = Central American Seaways, IRD = ice-rafted debris, NADW = North Atlantic deep water, NGS = Norwegian–Greenland Sea, NHG = Northern Hemisphere glaciation, THC = thermohaline circulation.

LO and HCO Spiniferites sp. A bifurcate again. Tabulation expressed by low sutural crests and Occurrence and age: LO in Sample 610A-19-1, 78–83cm, precingular archeopyle (3’’). Operculum free. 173.01 mbsf; 3.65 Ma. HCO in Sample 610A-18-6, 3–8cm, 170.17 mbsf; 3.61 Ma. This characteristic species has a short Zanclean Stage (Early Pliocene) – Acritarchs acme at the top of the Zanclean. HO Leiosphaeridia rockhallensis Calibration: LO and HCO in Subchron C2Ar, calcareous Occurrence and age: Sample 610A-20-4, 16–22cm, 186.49 nannofossil zone NN16, planktonic foraminifer zone PL3–6, mbsf; 3.83 Ma. Rare but persistent, and biostratigraphically the diatom zone Nitzschia jouseae. most useful acritarch in the lower part of the hole. Its LO was not identified in Hole 610A and must therefore predate ca. 4.0 Discussion: This species occurs rarely and sporadically up to Ma. Sample 610A-16-2, 35–38cm (2.93 Ma). It is uncertain whether these occurrences above the HCO are due to reworking. Calibration: Subchron C2Ar, near the calcareous nannofossil zone NN15/NN16 boundary, planktonic foraminifer zone Morphological description: Subspherical to ovoidal spiniferate PL3–6, diatom zone Nitzschia jouseae. cyst, yellow to light brown in color, with a columellate wall structure that appears densely and coarsely granulate in plan Discussion: In western North Atlantic DSDP Hole 603C, this view. Gonal and intergonal processes are hollow, long, wider at acritarch has a well-defined and magnetostratigraphically cali- base than top, and bear bifurcate and trifurcate process tips that brated age range from 4.37 to 3.88 Ma (Head and Norris 2003;

145 S. De Schepper and M. J. Head: Age calibration of dinoflagellate cyst and acritarch events, Pliocene–Pleistocene of the eastern North Atlantic

M.J.H., unpublished data), which appears to be consistent with stratigraphically calibrated age of 3.17 Ma (M.J.H., unpub- its range in Hole 610A. lished data; Figs. 6 and 7). The HO of this species was observed also in calcareous nannofossil zone NN16–NN17 (undifferenti- HO Lavradosphaera lucifer ated, early Piacenzian) of the Trubi Formation in Sicily, Italy Occurrence and age: Sample 610A-18-5, 80–86cm, 169.43 (Versteegh and Zevenboom 1995). mbsf; 3.60 Ma. Occurrence is sporadic but appears to be re- stricted to the Zanclean (5.33–3.60 Ma). Specimens in two HO Operculodinium janduchenei Piacenzian samples are considered reworked. Occurrence and age: Sample 610A-16-1, 33–38cm, 143.76 mbsf; 2.90 Ma. This species occurs only in low numbers. Calibration: Near the subchron C2An.3n/C2Ar boundary, cal- careous nannofossil zone NN16, planktonic foraminifer zone Calibration: Subchron C2An.1n, MIS G15, calcareous nanno- PL3–6, diatom zone Nitzschia jouseae. fossil zone NN16, planktonic foraminifer zone PL3–6, diatom Discussion: This species has been reported from across the zone Nitzschia jouseae. North Atlantic region (De Schepper and Head 2008). In western North Atlantic DSDP Hole 603C, it has a HO in Subchron Discussion: In central North Atlantic DSDP Hole 607, this spe- C2An.3n (lower Piacenzian) with a magnetostratigraphically cies occurs as high as MIS 91 (2.34 Ma; Versteegh 1997); and calibrated age of 3.47 Ma (M.J.H., unpublished data). in northern North Atlantic DSDP Hole 611, its HO is at the top of the Gauss Subchron (as Operculodinium sp. of Piasecki, in Mudie 1987). Piacenzian Stage (Middle Pliocene) – Dinoflagellate cysts LO Operculodinium? eirikianum var. crebrum LO Pyxidinopsis tuberculata Occurrence and age: Sample 610A-18-1, 81–86cm, 163.44 Occurrence and age: Sample 610A-15-5, 63–68cm, 140.46 mbsf; 3.33 Ma. mbsf; 2.84 Ma. Present in low abundances in six of the nine samples within the interval from Sample 610A-15-5, 63–68cm Calibration: Near the subchron C2An.3n/C2An.2r boundary, to 610A-14-3, 124–129 cm. MIS MG1, calcareous nannofossil zone NN16, planktonic foraminifer zone PL3–6, diatom zone Nitzschia jouseae. Calibration: Middle of Subchron C2An.1n, MIS G11/G12, cal- careous nannofossil zone NN16, planktonic foraminifer zone Discussion: This variety is restricted to the Piacenzian of this PL3–6, near the boundary between diatom zones Nitzschia hole, and has not been reported elsewhere (De Schepper and jouseae and N. marina. Head 2008). The HO is not clearly defined in Hole 610A (text-fig. 2). Discussion: This species has a LO in the Lower Miocene (Aquitanian) of the Mediterranean (Versteegh and Zevenboom HO Edwardsiella sexispinosa 1995), and a specimen was illustrated from the upper Lower Occurrence and age: Sample 610A-17-3, 81–87cm, 156.84 Miocene of Norwegian Sea ODP Hole 643A (as Pyxidiella sp. 1 mbsf; 3.20 Ma. Edwardsiella sexispinosa disappears abruptly; a of Mudie 1987, in Mudie 1992, pl. 4, fig. 4). single occurrence of a poorly preserved specimen encountered outside the regular counts in Sample 610A-16-1, 33–38cm is HO Invertocysta lacrymosa considered reworked. Occurrence and age: Sample 610A-15-1, 123–128cm, 135.06 Calibration: Subchron C2An.2n, MIS KM5, calcareous mbsf; 2.74 Ma. Occurs persistently in low to moderate numbers nannofossil zone NN16, planktonic foraminifer zone PL3–6, from the base of the hole to the highest sample, where a maxi- diatom zone Nitzschia jouseae. mum abundance of 7.2% was recorded. Discussion: In western North Atlantic DSDP Hole 603C, the Calibration: Upper part of Subchron C2An.1n, MIS G7 (111), HO is observed within Subchron C2An.1n, and has a magneto- near the calcareous nannofossil zone NN16/NN17 boundary, stratigraphically calibrated age of 2.81 Ma (M.J.H., unpub- planktonic foraminifer zone PL3–6, diatom zone Nitzschia ma- lished data). M. Smelror (written comm. 2007) cited a range top rina. in the Norwegian–Greenland Sea questionably at 3.1 Ma. Versteegh and Zevenboom (1995) cited a Chattian (calcareous Discussion: This species has a HO in MIS 110 at 2.72 Ma in nannofossil zone NP25) through middle Piacenzian (middle of central North Atlantic DSDP Hole 607/607A (Versteegh 1997), zone NN16) range for this species in southern Italy. in MIS 111 at 2.74 Ma in the Singa section of southern Italy (Versteegh 1997), and at ca. 2.84 Ma (Versteegh 1997) in Bay HO Impagidinium solidum of Biscay DSDP Hole 400A (as Thalassiphora delicata in Occurrence and age: Sample 610A-17-2, 108–114cm, 155.61 Harland 1979). Off Svalbard, in ODP Hole 986D, it has a HO mbsf; 3.15 Ma. This species occurs only in low numbers, but is within the Matuyama Chron, close to ca. 2.6 Ma (Smelror persistent between Samples 610A-18-6, 3–8cm and 610A-17-2, 1999). Its HO is estimated at ca. 2.8 Ma in Labrador Sea ODP 108–114cm, below which it occurs only sporadically. Hole 646B (de Vernal and Mudie 1989b; Knüttel et al. 1989) and questionably at 2.8 Ma in the Norwegian–Greenland Sea Calibration: Subchron C2An.2n, MIS KM2/KM3, calcareous (M. Smelror, pers. comm.). Climatic deterioration appears to nannofossil zone NN16, planktonic foraminifer zone PL3–6, explain the disappearance of this species from the North Atlan- diatom zone Nitzschia jouseae. tic region (Versteegh 1997). In western North Atlantic DSDP Hole 603C, a HPO is observed in Subchron C2An.1n, and has a Discussion: In western North Atlantic DSDP Hole 603C, this magnetostratigraphically calibrated age of 2.81 Ma (M.J. Head, species has a HO within Subchron C2An.2n and a magneto- unpublished data).

146 Stratigraphy, vol. 5, no. 2, 2008

TEXT-FIGURE 5 Age diagnostic events of the dinoflagellate cysts and acritarchs in DSDP Hole 610A, plotted on the new age model. Inset: ages (Ma) of the LO (= lowest occurrence), HO (= highest occurrence), HCO (= highest common occurrence) and HPO (= highest persistent occurrence) of each taxon. Symbols ¬ to ° and abbreviations for subchrons are as for text-figure 3. Symbol ± reinterpreted magnetic polarity in Hole 610A (this study), ² calcareous nannofossil zonation (Takayama and Sato 1987), ³ planktonic foraminifer zonation (Weaver and Clement 1987), ´ diatom zonation (Baldauf 1987). Grey shading in the magnetostratigraphic column indicates uncertain polarity between bounding samples used by Clement and Robinson (1987, p. 650) with respect to the position of our samples.

HO Barssidinium graminosum calibrated age of 2.81 Ma (M.J.H., unpublished data). Occurrence and age: Sample 610A-15-1, 123–128cm, 135.06 Barssidinium spp. have a HO in MIS 101 at 2.55 Ma in the mbsf; 2.74 Ma. Singa section of southern Italy (Versteegh 1997), but are re- corded only sporadically after 2.65 Ma. Calibration: Upper part of Subchron C2An.1n, MIS G7 (111), near the calcareous nannofossil zone NN16/NN17 boundary, HPO Tectatodinium pellitum planktonic foraminifer zone PL3–6, diatom zone Nitzschia ma- rina. Occurrence and age: Sample 610A-15-1, 123–128cm, 135.06 mbsf; 2.74 Ma. Above this datum, rare specimens were re- Discussion: Barssidinium graminosum is recorded infrequently corded sporadically in the Gelasian and Lower Pleistocene. in the literature. In western North Atlantic DSDP Hole 603C, Barssidinium cf. graminosum has a HPO within the upper Calibration: Upper part of Subchron C2An.1n, MIS G7 (111), Piacenzian in Chron C2An.1n at a magnetostratigraphically near the calcareous nannofossil zone NN16/NN17 boundary,

147 S. De Schepper and M. J. Head: Age calibration of dinoflagellate cyst and acritarch events, Pliocene–Pleistocene of the eastern North Atlantic

planktonic foraminifer zone PL3–6, diatom zone Nitzschia ma- timing coincides approximately with the onset of Northern rina. Hemisphere glaciation (=NHG). In western North Atlantic DSDP Hole 603C, the HO is within Subchron C2r and has a Discussion: T. pellitum is an extant species occurring today in magnetostratigraphically calibrated age of 2.49 Ma (M.J.H., un- warmer parts of the North Atlantic region (Wall and Dale 1967, published data). 1968). It disappeared from the southern North Sea after about 2.7 Ma, presumably as the result of Northern Hemisphere cool- Piacenzian Stage (Middle Pliocene) – Acritarchs ing (Head 1998), but evidently returned during the Eemian Stage of the Late Pleistocene as it thrived in the Baltic Sea at LO Cymatiosphaera latisepta this time (Head et al. 2005; Head 2007). Occurrence and age: Sample 610A-18-1, 81–86cm, 163.44 HO Ataxiodinium confusum mbsf; 3.33 Ma. Present always in low numbers but fairly persis- tent throughout its range in Hole 610A, which is restricted to the Occurrence and age: Sample 610A-14-4, 36–41cm, 129.09 Piacenzian. mbsf; 2.63 Ma. Calibration: Near the subchron C2An.3n/C2An.2r boundary, Calibration: Upper part of Subchron C2An.1n, MIS G1 (105), MIS MG2, calcareous nannofossil zone NN16, planktonic near the calcareous nannofossil zone NN17/NN18 boundary, foraminifer zone PL3–6, diatom zone Nitzschia jouseae. planktonic foraminifer zone PL3–6, diatom zone Nitzschia ma- rina. Discussion: In Labrador Sea ODP Hole 646B, this species has an isolated LO in calcareous nannofossil zone NN16 (Zan- Discussion: In both the Singa section (S. Italy) and the central clean), and a lowest persistent occurrence in the upper part of North Atlantic DSDP Site 607, the HO is recorded near the top zone NN16 (as Nematosphaeropsis sp. I in de Vernal and Mudie of nannofossil zone NN16, in MIS 106 (G2) at 2.65 Ma 1989b; Knüttel et al. 1989; Clement et al. 1989). In western (Versteegh and Zevenboom 1995; Versteegh 1997). In western North Atlantic DSDP Hole 603C, this species has a LO within North Atlantic DSDP Hole 603C, this species has a HO near the Subchron C2An.3n, and a magnetostratigraphically calibrated top of Subchron C2An.1n and a magnetostratigraphically cali- age of 3.59 Ma (De Schepper and Head 2008; M.J.H., unpub- brated age of 2.63 Ma (M.J.H., unpublished data). lished data). HO Operculodinium? eirikianum var. eirikianum LO Lavradosphaera sp. 1 Occurrence and age: Sample 610A-14-3, 124–129cm, 128.47 Occurrence and age: Sample 610A-15-5, 63–68cm, 140.46 mbsf; 2.62 Ma. Present usually in low to moderate numbers, mbsf; 2.84 Ma. reaching 13% of the total assemblage in Sample 610A-15-2, 35–41cm. A single isolated specimen in Sample 610A-12-3, Calibration: Subchron C2An.1n, MIS G11/G12, calcareous 83–88cm is possibly reworked. nannofossil zone NN16, planktonic foraminifer zone PL3–6, near the boundary between diatom zones Nitzschia jouseae and Calibration: Upper part of Subchron C2An.1n, MIS 104/105, N. marina. near the calcareous nannofossil zone NN17/NN18 boundary, planktonic foraminifer zone PL3–6, diatom zone Nitzschia ma- Morphological description: Small subspherical to ovoidal rina. palynomorph, with a thick wall deeply incised by U-shaped channels, giving the palynomorph a fossulate appearance. Discussion: This species ranges upwards at least to MIS 91 Channels often interconnect, and vary in depth and may almost (2.34 Ma) in central North Atlantic DSDP Site 607 (Versteegh reach the innermost wall layer. They also vary in width (2.6 to 1997), and a single specimen was reported from the St. Erth 5.6µm on one specimen) and length. Muri (the areas of wall be- Beds of SW England, which are dated at ca. 2.0 Ma (Head tween channels) are flat-topped, have a spongy to cancellous in- 1993, 1999). Operculodinium? eirikianum is considered a ternal structure, and are of even height over entire vesicle. The cold-intolerant species (Head 1997). Its disappearance in Hole pylome is large and polygonal, and the operculum free. Maxi- 610A seems related to the onset of North Atlantic glaciation. mum diameter of vesicle (incl. muri), 20.5(23.6)27.0µm; wall The HPO of Operculodinium? eirikianum var. eirikianum in thickness, 2.6(3.5)4.8µm. Twenty-two specimens measured. western North Atlantic DSDP Hole 603C is recorded near the base of Subchron C2An.3n, and has a magnetostratigraphically Discussion: Lavradosphaera sp.1 differs from Lavradosphaera constrained age of 3.59 Ma (M.J.H., unpublished data). crista in not possessing large polygonal lumina, and from Lavradosphaera lucifer in lacking a spiked appearance. This HO Pyxidinopsis tuberculata species has not been reported previously. Occurrence and age: Sample 610A-14-3, 124–129cm, 128.47 HO Lavradosphaera sp. 1 mbsf; 2.62 Ma. Restricted to an interval between Samples 610A-15-5, 63–68cm and 610A-14-3, 124–129cm, occurring in Occurrence and age: Sample 610A-15-2, 35–41cm, 135.68 most samples but in low numbers. mbsf; 2.75 Ma. Calibration: Upper part of Subchron C2An.1n, MIS 104/105, Calibration: Subchron C2An.1n, MIS G7/G8 (111/112), calcar- near the calcareous nannofossil zone NN17/NN18 boundary, eous nannofossil zone NN16, planktonic foraminifer zone planktonic foraminifer zone PL3–6, diatom zone Nitzschia ma- PL3–6, diatom zone Nitzschia marina. rina. HCO Algal cyst type 1 of Head (1996) Discussion: Versteegh (1997) reported the HO of Pyxidinopsis Occurrence and age: Sample 610A-15-1, 123–128cm, 135.06 tuberculata at MIS 100 (2.52 Ma) both in central North Atlantic mbsf; 2.74 Ma. Above this datum, the species occurs rarely in DSDP Hole 607, and in the Singa section of southern Italy. This the subjacent Sample 610A-14-6, 37–42cm (2.69 Ma), and in

148 Stratigraphy, vol. 5, no. 2, 2008

TEXT-FIGURE 6 Comparison of selected dinoflagellate cyst and acritarch biostratigraphic events (abbreviations HO, HCO, HPO, LO: see text-fig. 5) across the NorthAt- lantic and adjacent seas. Species abbreviations used: see inset text-figure 5. Datums from Versteegh (1997) are updated to the time scale of Lisiecki and Raymo (2005). Those of de Vernal and Mudie (1989b) are based on the Hole 646B calcareous nannofossil biostratigraphy of Knüttel et al. (1989) and magnetostratigraphy of Clement et al. (1989). Datums for the Norwegian–Greenland Sea are based on a compilation of sites from ODP legs 104, 151 and 162 (M. Smelror, unpublished data). * = HPO of Habibacysta tectata from ODP Hole 911A (Matthiessen and Brenner 1996).

149 S. De Schepper and M. J. Head: Age calibration of dinoflagellate cyst and acritarch events, Pliocene–Pleistocene of the eastern North Atlantic

two widely separated samples in the Gelasian, with a HO at Calibration: HPO in Subchron C2An.1n, MIS G7 (111), near 610A-12-2, 123–128cm (2.14 Ma). the calcareous nannofossil zone NN16/NN17 boundary, plank- tonic foraminiferal zone PL3–6, diatom zone Nitzschia marina. Calibration: Subchron C2An.1n, MIS G7 (111), near the cal- HO in Subchron C2n, calcareous nannofossil zone NN19, careous nannofossil zone NN16/NN17 boundary, planktonic planktonic foraminiferal zone N22, diatom zone Nitzschia foraminifer zone PL3–6, diatom zone Nitzschia marina. reinholdii.

Discussion: This palynomorph was first identified (as Spini- Discussion: In western North Atlantic DSDP Hole 603C, this ferites sp. 1 in Head 1993) from the St. Erth Beds, which were species has two acme zones in the upper Pliocene: acme zone 1, deposited just prior to the Olduvai Subchron at ca. 2.0 Ma which has a top at 2.81 Ma, and acme zone 2, which has a top at (Head 1993 and reference therein). It has been recorded in mod- 1.83 Ma. The top of acme zone 1 approximately equates with erate numbers from the Gelasian of the northern North Sea the 2.74 Ma HPO in Hole 610A, and the top of acme zone 2 ap- (Head et al. 2004), where its associations with particular pears to equate with the 1.82 Ma HO in Hole 610A (M.J.H., un- dinoflagellate cysts suggests tolerance of cool conditions. The published data). In Labrador Sea ODP Hole 646B, this species record of this species in eastern England begins with the occurs in persistently high numbers until about 15m below the Coralline Crag Formation (tentatively 3.8–3.3 Ma in Head Gauss/Matuyama Chron boundary (as Cymatiosphaera sp. in de 1997, 1998), and younger records are given for the Ludham Vernal and Mudie 1989b; Knüttel et al. 1989; Clement et al. borehole from 2.4 Ma to Early Pleistocene? (Head 1996; Head 1989), but occurs sporadically and in lower numbers up to the 1998). In Belgium, this species is recorded in the Kattendijk Middle Pleistocene MIS 12/11 boundary (ca. 0.427 Ma) (de Formation (5.0 to 4.7–4.4 Ma in Louwye et al. 2004; De Vernal et al. 1992, text-fig. 7). Schepper et al., in press) and the Oorderen Sands Member of the Lillo Formation (3.7–2.7 Ma in Louwye et al. 2004; De HO Cymatiosphaera latisepta Schepper et al., in press). Although this species is apparently Occurrence and age: Sample 610A-14-3, 124–129cm, 128.47 cool tolerant, the HPO in Hole 610A seems related to mbsf; 2.62 Ma. paleoclimatic changes associated with the onset of the NHG. HCO and HO Lavradosphaera crista Calibration: Upper part of Subchron C2An.1n, MIS 104/105, near the calcareous nannofossil zone NN17/NN18 boundary, Occurrence and age: HCO in Sample 610A-16-4, 112–117cm, planktonic foraminifer zone PL3–6, diatom zone Nitzschia ma- 149.05 mbsf; 3.00 Ma. Abundant and persistent in the lower rina. Piacenzian, but rare and sporadic up to its HO in Sample 610A-14-5, 78–83cm, 131.01 mbsf; 2.67 Ma (upper Pia- Discussion: In Labrador Sea ODP Hole 646B, this species has a cenzian). It is uncertain whether these higher records represent well-defined HO within calcareous nannofossil zone NN17– reworking, and therefore the HO is considered provisional. NN18 (undifferentiated) and close to the Gauss/Matuyama Chron boundary at ca. 2.6 Ma (as Nematosphaeropsis sp. I in de Calibration: HCO in lower part of Subchron C2An.1n, MIS Vernal and Mudie 1989b; Knüttel et al. 1989; Clement et al. G19/G20, calcareous nannofossil zone NN16, planktonic 1989). In western North Atlantic DSDP Hole 603C, its HO is Nitzschia jouseae foraminifer zone PL3–6, diatom zone .HOin just below the Gauss/Matuyama Chron boundary, and has a upper part of Subchron C2An.1n, MIS G3 (107), calcareous magnetostratigraphically calibrated age of 2.63 Ma (De nannofossil zone NN17, planktonic foraminifer zone PL3–6, Schepper and Head 2008; M.J.H., unpublished data). In central Nitzschia marina diatom zone . North Atlantic DSDP Hole 607, it has a HO in MIS 98, at 2.49 Discussion: In Labrador Sea ODP Hole 646B, this species has a Ma within the lower Gelasian (as Nematosphaeropsis sp. I in well-defined HO about 20m below the Gauss/Matuyama Chron Versteegh 1997). The HO of this species in the North Atlantic (as I in de Vernal and Mudie 1989b; Knüttel et al. appears to be a good marker for the uppermost Piacenzian–low- 1989; Clement et al. 1989). In the Norwegian Sea it has a HO ermost Gelasian. near the top of the Gauss Chron (as Platycystidia sp. 1 in Mudie 1989). In western North Atlantic DSDP Hole 603C, it has a Gelasian Stage (Late Pliocene) – Dinoflagellate cysts magnetostratigraphically calibrated HCO at 3.05 Ma with spo- radic and rare higher occurrences possibly representing rework- HO Invertocysta tabulata ing (De Schepper and Head 2008; M.J.H., unpublished data). Occurrence and age: Sample 610A-13-7, 4–10cm, 123.67 mbsf; 2.51 Ma. Occurs fairly persistently and always in low HPO and HO Cymatiosphaera? invaginata numbers throughout its range in Hole 610A. Occurrence and age: HPO in Sample 610A-15-1, 123–128cm, 135.06 mbsf; 2.74 Ma. HO in Sample 610A-10-7, 5–8cm, Calibration: Subchron C2r.2r, MIS 99, calcareous nannofossil 94.87 mbsf; 1.82 Ma. A single specimen in Sample 610A-10-4 zone NN18, planktonic foraminifer zone PL3–6, diatom zone 23–28cm is possibly reworked. Nitzschia marina.

TEXT-FIGURE 7 ® Summary of dinoflagellate cyst and acritarch biostratigraphic events (abbreviations HO, HCO, HPO, LO: see text-fig. 5) calibrated to magnetostratigraphy, calcareous nannofossil, planktonic foraminifer and diatom biostratigraphy, and marine isotope stratigraphy (MIS) for DSDP Hole 610A, their estimated ages, and comparison with other sites. References for these other sites are as follows: 1, Head and Norris (2003); 2, M.J.H., unpub- lished data; 3, de Vernal and Mudie (1989b); 4, Knüttel et al. (1989); 5, Versteegh and Zevenboom (1995); 6, M. Smelror, unpublished data; 7, Versteegh (1997, updated to the time scale of Lisiecki and Raymo 2005); 8, Mudie (1987); 9, Harland (1979); 10, Head (1993); 11, Head (1999); 12, A. de Vernal (pers. commun.); 13, Brinkhuis et al. (2003); 14, Head (1998); 15, De Schepper and Head (2008); 16, Clement et al. (1989).

150 Stratigraphy, vol. 5, no. 2, 2008

151 S. De Schepper and M. J. Head: Age calibration of dinoflagellate cyst and acritarch events, Pliocene–Pleistocene of the eastern North Atlantic

Discussion: Versteegh (1997) placed the HO of this species at HPO Habibacysta tectata 2.40 Ma, in MIS 94, at central North Atlantic DSDP Site 607. In the Singa section of southern Italy, this species disappears in Occurrence and age: Sample 610A-11-7, 2–5cm, 104.44 mbsf; MIS 97 at ca. 2.46 Ma (Versteegh 1997). In western North At- 2.08 Ma. lantic DSDP Hole 603C, its HO is within Subchron C2An.1n and has a magnetostratigraphically calibrated age of 2.69 Ma Calibration: Subchron C2r.1r, calcareous nannofossil zone (M.J.H., unpublished data). NN18, planktonic foraminifer zone PL3–6, diatom zone Nitzschia marina.

PLATE 1 All pictures are taken in bright field illumination unless otherwise indicated. The sample and slide number and England Finder ref- erence are given sequentially for each specimen. Various magnifications.

1–3 Ataxiodinium confusum Versteegh and Zevenboom. larger than endoblast. Periblast length, 76µm. (15) Sample 610A-18-1, 11–13cm, slide 18.1a(1), M37/4. Sample 610A-15-5, 63–68cm, slide 15.5(1), C23/0. Dorsal view of (1) dorsal surface showing 1P Dorsoventral view at mid-focus, with endo-archeo- archeopyle (3’’), (2) mid-focus on wall circum- pyle and operculum visible, and peri-archeopyle cavation, and (3) ventral surface. Note the faintly or- opening slightly larger than endoblast. Periblast namented cyst wall. Periblast maximum diameter, length, 68µm. 40µm. 16–18 Impagidinium solidum Versteegh and Zevenboom. 4–5 Amiculosphaera umbraculum Harland. Note charac- Sample 610A-18-1, 81–86cm, slide 18.1(1), O46/0. teristic funnel-shaped apical connection between Dorsal view of (16) dorsal surface, (17) mid-focus endo- and periblast, and contact between endo- and showing the thick wall, and (18) ventral surface. periblast at antapex. (4) Sample 610A-17-5, Length (excluding crests), 54µm; septa height, 1.4µm; 98–100cm, slide 17.5d(1), F48/1. Ventral view with wall thickness, 2.1µm. focus on 1P endo-archeopyle (3’’). Periblast length, 70µm; periblast width, 61µm, endoblast length, 47µm, 19–20 Invertocysta tabulata Edwards. (19) Sample 610A- periblast width, 35µm. (5) Sample 610A-17-5, 20-3, 31–36cm, slide 20.3 (1), G35/0. Ventral view at 98–100cm, slide 17.5d(1), D24/1. Dorsal view. mid-focus. Periblast length, 82µm. (20) Sample Periblast length, 65µm, periblast width, 59µm, 610A-15-6, 83–85cm, SEM Sample 15.6. SEM pic- endoblast length, 40µm, periblast width, 22µm. ture. Ventral view. Periblast length, 97µm. 6–8 Batiacasphaera minuta (Matsuoka). Sample 610A- 21–23 Operculodinium janduchenei Head, Norris and 21-3, 39–44cm, slide 21.3(1), Q41/4. Antapical view Mudie. Sample 610A-17-6, 94–96cm, slide 17.6g(1), of (6) antapical surface, (7) mid-focus, and (8) apical Z23/2. Dorsal view of (21) dorsal surface, (22) surface with archeopyle. Maximum diameter, 28µm; mid-focus, and (23) ventral surface. Central body wall thickness, 1.0µm. length (excluding processes), 27µm. 9–10 Edwardsiella sexispinosa Versteegh and Zevenboom. 24–25 Operculodinium? eirikianum Head, Norris and Mudie (9) Sample 610A-21-4, 35–41cm, slide 21.4(1), F8/0. var. eirikianum (autonym). Sample 610A-18-1, View uncertain, clearly showing the six processes. 42–44cm, slide 18.1c (1), L49/0. View uncertain, (24) Central body maximum diameter, 32µm, process upper focus on microreticulate central body wall, and length, ca. 45µm. (10) Sample 610A-17-6, 111– (25) mid-focus revealing solid processes. Central 117cm, slide 17.6j(1), N22/1. Equatorial view with body maximum diameter, 36µm. focus on four of the six processes, also showing apical 26–28 Operculodinium tegillatum Head. Sample 610A- and antapical horns. Central body length, 51µm; cen- 21-2, 35–40cm, slide 21.2(1), F38/2. Ventral view of tral body width, 48µm; process length, 35–40µm. (26) ventral surface, (27) mid-focus and (28) dorsal 11–13 Corrudinium devernaliae Head and Norris. Sample surface. Central body maximum diameter, 33µm. 607-21-5, 33–39cm, slide 21.5, G43/1. Dorsal view of 29–30 Pyxidinopsis tuberculata Versteegh and Zevenboom. (11) dorsal surface, (12) mid-focus, and (13) ventral Sample 610A-14-4, 36–41cm, slide 14.4(1), K49/3. surface. Central body length (excluding crests), Dorsal view of (29) dorsal surface, and (30) ventral 29µm; crest height, 1.7µm. surface showing circular to elongated solid ridges. 14–15 Invertocysta lacrymosa Edwards. (14) Sample 610A- Central body length, 41µm, wall thickness, 1.0µm, 20-5, 32–38cm, slide 20.5(1), R34/0. Dorsoventral process length, 1.7–2.2µm. view at mid-focus, showing peri-archeopyle slightly

152 Stijn De Schepper and Martin J. Head Plate 1

stratigraphy, vol. 5, no. 2, 2008 153 S. De Schepper and M. J. Head: Age calibration of dinoflagellate cyst and acritarch events, Pliocene–Pleistocene of the eastern North Atlantic

Discussion: It is not known whether five rare and isolated oc- graphic events calibrated to the astronomically-tuned time currences of this species extending to 0.760 Ma (Middle Pleis- scale. DSDP Hole 603C (Head and Norris 2003; M.J.H., unpub- tocene) in Hole 610A represent reworking. In the Arctic Ocean lished data) has magnetostratigraphic control, but the relatively ODP Hole 911A, a HPO was recorded at ca. 2.0 Ma, with occa- wide sampling intervals limit precise identification of sional specimens occurring up to about 1.1 Ma (Matthiessen biostratigraphic events. This hampers detailed east–west com- and Brenner 1996). In Norwegian Sea DSDP Hole 642B, this parison across the North Atlantic. Age estimates from Labrador species occurs sporadically as high as MIS 19 at the base of the Sea ODP Hole 646B (de Vernal and Mudie 1989b; Knüttel et al. Brunhes Chron in the earliest Middle Pleistocene (as 1989; Clement et al. 1989) and the Norwegian–Greenland Sea sp. 1 in Mudie 1989). In western North Atlantic DSDP Hole (M. Smelror, written comm. 2007) are based on limited 603C, this species occurs persistently to the base of Subchron magnetostratigraphy and on calcareous nannofossil stratigra- C1r.3r, with a magnetostratigraphically calibrated age of 1.77 phy. In spite of these limitations, the approximate comparisons Ma (M.J.H., unpublished data). given below (see also text-figs. 6 and 7) identify those more promising species for future biostratigraphic investigation. LO Impagidinium cantabrigiense Occurrence and age: Sample 610A-11-1, 49–54cm, 95.92 Near-synchronous last appearances mbsf; 1.86 Ma. Sporadic and rare in the lower part of its range, Ataxiodinium confusum disappears in MIS G1 (105) at 2.63 Ma but becoming persistent onwards through the upper Lower in the eastern North Atlantic and at 2.63 Ma in the western Pleistocene (from Sample 610A-7-6, 71–73cm) and Middle North Atlantic, and only slightly earlier in MIS G2 (106) at 2.65 Pleistocene. Ma both in the central North Atlantic and Mediterranean. Calibration: Subchron C2n, calcareous nannofossil zone NN19, near the boundary between planktonic foraminifer zones The disappearance of Invertocysta lacrymosa is synchronous PL3–6 and N22, near the boundary between diatom zones within 20 kyr in the eastern North Atlantic and the Singa section Nitzschia reinholdii and Nitzschia marina. of southern Italy (both MIS 111, 2.74 Ma), and central North Atlantic (MIS 110, 2.72 Ma) (text-figs. 6 and 7). Two other lo- Discussion: This species is known also from the Pleistocene of calities with less accurate age control record the event slightly ODP Site 963 in the Mediterranean (M. Papanikolaou, pers. earlier: in the Labrador Sea and Norwegian–Greenland Sea the comm.), although neither range top nor base were identified. It disappearance is tentatively estimated at ca. 2.8 Ma. The HPO has also been recorded from IODP Site U1308, where its HO is in the western North Atlantic occurs at 2.81 Ma. The apparent tentatively placed in MIS 11 and the LO is estimated at around earlier disappearances in the Labrador Sea and Norwe- 1.9–2.0 Ma, based on calcareous nannofossil stratigraphy (A. gian–Greenland Sea could be due to earlier climatic cooling as- de Vernal, pers. comm.). sociated with onset of NHG at these higher latitudes.

Early Pleistocene – Dinoflagellate cysts The disappearance of Impagidinium solidum has been recorded with precision only in the eastern and western North Atlantic, HO Amiculosphaera umbraculum but appears synchronous at around 3.15–3.17 Ma. Occurrence and age: Sample 610A-8-7, 19–24cm, 75.82 mbsf; 1.44 Ma. This is the only species with a clearly defined HO in Batiacasphaera minuta/micropapillata disappears from the the Pleistocene of Hole 610A. Below this datum, it is present in eastern North Atlantic at 3.83 Ma and from the western North almost all samples, in varying abundances, down to the base of Atlantic at 3.74 Ma, and has a last common appearance in the the hole. Labrador Sea at around 3.7 Ma. The discrepancy between the eastern and western North Atlantic may be caused by Calibration: Subchron C1r.2r–C1r.3r, calcareous nannofossil paleoenvironmental differences between these two sites, or per- zone NN19, planktonic foraminifer zone N22, diatom zone haps by reworking in Hole 603C: the species dominates in this Nitzschia reinholdii. hole at 3.98 Ma, but becomes progressively less common up-hole (M.J.H., unpublished data). Discussion: This species has been reported up to about 1.80 Ma (earliest Pleistocene) in eastern England (Head 1998), and has a Disappearance of the acritarch Leiosphaeridium rockhallensis HPO in the Olduvai Subchron in western North Atlantic ODP is at 3.83 Ma in the eastern North Atlantic and at 3.88 Ma in the Hole 603C (M.J.H., unpublished data). It extends into the western North Atlantic. This small and superficially indistinct Lower Pleistocene both of the Bay of Biscay DSDP Hole 400A species was not reported from the Labrador Sea or central North (Harland 1979) and the Norwegian Sea ODP Hole 644A Atlantic or from the Singa section of southern Italy. (Mudie 1989). Asynchronous last appearances In the mid-latitude Southern Hemisphere, west of Tasmania (ODP Hole 1168A), it has a HO in Subchron C1r.3r at 1.6 Ma The disappearance of Invertocysta tabulata falls at MIS 99 (Brinkhuis et al. 2003). (2.51 Ma) in the eastern North Atlantic, MIS 97 (2.46 Ma) in the Singa section of southern Italy, and MIS 94 (2.40 Ma) in the DISCUSSION central North Atlantic. In the western North Atlantic, it disap- pears from the record at 2.69 Ma. Given the wide range of last Comparing events across the North Atlantic and adjacent appearances (2.69–2.40 Ma) between the sites, a clima- basins tic/oceanographic cause seems most likely. An early disappear- Only for eastern North Atlantic DSDP Hole 610A (this study), ance in the western North Atlantic might reflect the influence of central North Atlantic DSDP Holes 607/607A, and the Singa cold-water water outflow from the Labrador Sea, with later dis- section of southern Italy (Versteegh 1997; updated to the LR04 appearances occurring in areas (central North Atlantic and time scale of Lisiecki and Raymo 2005) are Pliocene biostrati- Mediterranean) where warmer waters prevailed longer.

154 Stratigraphy, vol. 5, no. 2, 2008

Pyxidinopsis vesiculata, Corrudinium devernaliae, Operculo- Impagidinium cantabrigiense first appears at 1.86 Ma in eastern dinium tegillatum, and Operculodinium? eirikianum var. eiriki- North Atlantic DSDP Hole 610A. This species has also been re- anum also disappear earlier in the western North Atlantic than ported at central North Atlantic IODP Site U1308, its first ap- elsewhere. Pyxidinopsis vesiculata, Operculodinium tegillatum pearance tentatively placed at ca. 1.9–2.0 Ma (A. de Vernal, and Corrudinium devernaliae have their HOs in the lower part pers. comm.). The first appearance of Impagidinium of Hole 610A, where the age model is based on extrapolation cantabrigiense may therefore prove to be a good marker for the and possibly less accurate. However, divergence in the timing latest Gelasian in the central and eastern North Atlantic. of disappearances between sites is large (up to 600 ka), and therefore cannot be explained only by inaccurate dating and Climatically influenced stratigraphic events between 3.6 and sampling resolution. The disappearances seem to reflect local 2.4 Ma or regional oceanographic/climatic factors. The present sampling resolution in DSDP Hole 610A is too low Two other species show a different pattern: Edwardsiella to attribute events to a particular isotope stage (see Section sexispinosa disappears considerably earlier in the eastern North 4.2.7). However, the effect of oceanographic/climatic changes Atlantic (3.20 Ma) and in the Norwegian–Greenland Sea (3.1? between 3.6 and 2.4 Ma on the disappearances of certain species Ma) than in the western North Atlantic (2.81 Ma) (text-figs. 6 is especially evident around the onset of the NHG (text-fig. 4). and 7). Pyxidinopsis tuberculata disappears from the eastern North Atlantic at 2.62 Ma, from the Singa section of southern Although the Greenland ice-sheet was expanding between 3.5 Italy and the central North Atlantic at 2.52 Ma, and from the and 3.0 Ma and NHG progressively intensified (Maslin et al. western North Atlantic at ca. 2.49 Ma, again showing a prema- 1998), the period before 3.0 Ma is characterised by stronger ture disappearance in Hole 610A. thermohaline circulation (THC) and enhanced ventilation of the deep Atlantic (Raymo et al. 1996; Ravelo and Andreasen 2000). Tectatodinium pellitum disappears from eastern North Atlantic Between 3.6 and 2.8 Ma, Edwardsiella sexispinosa, Impagi- Hole 610A at 2.74 Ma, with the exception of rare and sporadic dinium solidum, Operculodinium janduchenei and Lavrado- appearances in the Gelasian and Early Pleistocene. This is an sphaera crista all have their last appearances (text-fig. 4). Both extant neritic species presently with a south-temperate to tropi- Edwardsiella sexispinosa and Impagidinium solidum survived cal distribution in the North Atlantic (Wall and Dale 1967 1968; the major cool event of MIS M2 (Lisiecki and Raymo 2005; Head 1998). Its abrupt disappearance from Hole 610A in the Stein et al. 2006), and their HOs appear to coincide with the end late Piacenzian can be attributed to deteriorating climatic condi- of strong abyssal flow at Hole 610A (Hassold et al. 2006). How- tions in the region. ever, direct links with climatic change are not clear from our re- cords. From 3.0 Ma onwards, the polar front shifted southwards Cymatiosphaera latisepta has comparable disappearances in from the Norwegian–Greenland Sea (NGS) towards the North the Labrador Sea (ca. 2.6 Ma), eastern North Atlantic (2.62 Atlantic (Henrich et al. 2002), and is likely expressed in the in- Ma), and western North Atlantic (2.63 Ma). However, in the creasing number of dinoflagellate cyst disappearances. Al- central North Atlantic, it disappears at 2.49 Ma in the early though enhanced THC and increased SST are recorded in the Gelasian. At least two pronounced acmes of Cymatiosphaera? area between 2.95 and 2.81 (Bartoli et al. 2005, 2006), the HO invaginata have been detected in the later Pliocene of both the of Operculodinium janduchenei might be related to the glacial western and eastern North Atlantic. The first ends abruptly at MIS G14, which is more strongly expressed in Hole 610A 2.74 Ma in Hole 610A and a little earlier, at 2.81 Ma, in Hole (Kleiven et al. 2002) than in the global stack of Lisiecki and 603C. This event appears also to be reflected in Labrador Sea Raymo (2005). ODP Hole 646B, occurring 10–20m below the top of the Gauss Chron (as Cymatiosphaera sp. in de Vernal and Mudie 1989b; Clement et al. 1989). The second acme represents a brief (single A further eight HOs, two HPOs, and one HCO are recorded be- sample) but significant increase in abundance within the upper tween 2.8 and 2.6 Ma, are related to climatic and oceanographic part of the Olduvai Subchron both in Hole 610A and Hole reorganisation at the onset of the NHG (text-fig. 4). During this 603C, at 1.82–1.83 Ma. Cymatiosphaera? invaginata seems to interval, the polar front progressively moved southward into the be sensitive to climatic/oceanographic changes within the North Atlantic, while the formation of NADW in the Norwe- higher latitudes of the North Atlantic, but high-resolution re- gian–Greenland Sea was reduced or even terminated (Henrich cords are needed to establish precise correlations between sites et al. 2002). During MIS G4–G6 (ca. 2.74 Ma) there occurs gla- and elucidate those paleoenvironmental factors controlling its ciation in the European and Asian Arctic (e.g. Maslin et al. abundance. 1998; Bartoli et al. 2005, 2006), an increase in IRD in the North Atlantic (e.g. Kleiven et al. 2002; Ravelo et al. 2004), a reduc- First appearances tion in North Atlantic thermohaline overturn rates (Ravelo et al. 2004), and a change in orbital parameters (e.g. Haug and The first appearances of Tectatodinium pellitum (3.83 Ma) and Tiedemann 1998; Maslin et al. 1998; Ravelo et al. 2004). These Pyxidinopsis tuberculata (2.84 Ma) in eastern North Atlantic major paleoceanographic and paleoclimatic reorganisations ap- Hole 610A evidently represent local events, as these species pear to explain the disappearances of Invertocysta lacrymosa, elsewhere range down into the Paleocene and Miocene, respec- Barssidinium graminosum, and Lavradosphaera sp. 1, and the tively. temporary disappearances of Tectatodinium pellitum, Algal cyst type 1, and Cymatiosphaera invaginata just prior to MIS The first appearance of Cymatiosphaera latisepta in the eastern G4–G6. Shortly afterwards, the disappearances of a further four North Atlantic (3.33 Ma), if precisely detected, is delayed rela- species (Ataxiodinium confusum, Operculodinium? eirikianum, tive to that of the western North Atlantic (3.59 Ma) by 260 kyr, Pyxidinopsis tuberculata and Cymatiosphaera latisepta)just again suggesting that local paleoenvironmental events are re- prior to or possibly within MIS 104 (text-fig. 4), are very likely sponsible for the arrival of this species at DSDP Site 610. attributable to the same processes.

155 S. De Schepper and M. J. Head: Age calibration of dinoflagellate cyst and acritarch events, Pliocene–Pleistocene of the eastern North Atlantic

Invertocysta tabulata survived the first cooling phases, but fur- original age model (Baldauf et al. 1987) in two respects: 1) the ther glaciation of the Northern Hemisphere, i.e. glaciation of Piacenzian interval is fine-tuned using the marine isotope stra- NE America (Maslin et al. 1998) and a shift from restricted to tigraphy of Kleiven et al. (2002) for Hole 610A calibrated to the more expansive ice-sheets (Raymo 1994), explain its demise at global stack of Lisiecki and Raymo (2005), and 2) the lower ca. 2.51 Ma (MIS 99). part of the hole is re-dated using new information on the HOs of several groups including the calcareous nanno- CONCLUSIONS fosssil species Reticulofenestra pseudoumbilicus and Spheno- The age model of DSDP Hole 610A is revised and calibrated to lithus abies. The base of Hole 610A, previously dated at ca. the most recent geomagnetic and astronomically-tuned time 4.9–5.0 Ma (Weaver and Clement 1987, updated using ATNTS scale (ATNTS 2004, Lourens et al. 2005). It differs from the 2004), is now considered to be ca. 4.0 Ma.

PLATE 2 All pictures are taken in bright field illumination unless otherwise indicated. The sample and slide number and England Finder ref- erence are given sequentially for each specimen. Various magnifications.

1–3 Pyxidinopsis vesiculata Head and Norris. Sample 20–21 Cymatiosphaera? invaginata Head et al. Sample 610A-21-5, 33–39cm, slide 21.5(1), M14/0. Dorsal 610A-17-1, 53–55cm, SEM Sample 17.1. View of view of (1) dorsal surface, (2) mid-focus, and (3) ven- pylome. Diameter (including crests), 24µm. Sample tral surface. Cyst length, 27µm; wall thickness, 610A-17-1, 53–55cm, SEM Sample 17.1. View of 0.9µm. pylome. Diameter (including crests), 28µm. 4–5 Tectatodinium pellitum Wall. Sample 610A-17-6, 22–24 Lavradosphaera crista De Schepper and Head. (22) 44–46cm, slide 17.6d(2), C33/4. Dorsal view of (4) Sample 610A-17-4, 66–68cm, SEM Sample 17.4. dorsal surface and (5) mid-focus. Cyst maximum di- Apical view showing pylome. Maximum diameter ameter, 46µm; wall thickness, 3.6µm. (including crests), 20µm. (23–24) Holotype. Sample 610A-17-6, 132–134cm, slide 17.6j(1), A32/1. 6–8 Spiniferites sp. A. Sample 610A-18-6, 3–8cm, slide Oblique apical view at (23) mid-focus and (24) lower 18.6(1), D38/2. Ventral view of (6) ventral surface re- focus. Maximum diameter (including crests), 23µm. vealing a columellate wall structure, (7) mid-focus, and (8) dorsal surface, showing the archeopyle. Cen- 25–27 Lavradosphaera lucifer De Schepper and Head. (25) tral body length, 42µm; wall thickness, 1.2µm; maxi- Sample 610A-17-4, 66–68cm, SEM Sample 17.4. mum process length, 13µm. Apical view showing pylome. Maximum diameter (including crests), 24µm. (26–27) Holotype. Sample 9–10, Habibacysta tectata Head, Norris and Mudie. (9, 10, 610A-19-5, 111–116cm, slide 19.5(1), C18/0. Apical 14–15 14) Sample 610A-12-5, 110–115cm, slide 12-5(1), view showing (26) mid focus and (27) antapical sur- F21/0. Ventral view of (9) ventral surface with focus face. Maximum diameter (including crests), 26µm. on wall ornament, (10) mid-focus and (14) dorsal sur- face showing archeopyle with rounded angles. Maxi- 28–29 Leiosphaeridia rockhallensis Head. Holotype. Sam- mum diameter, 40µm; wall thickness, 2.1µm. (15) ple 610A-21-5, 33–39cm, slide 21.5(1), D19/0. Un- Sample 610A-17-1, 53–55cm, SEM Sample 17.1. certain view at two slightly lower foci. Maximum Dorsal view. Length, 31µm. diameter, 22µm; wall thickness, 3.0µm. 11–13 Impagidinium cantabrigiense De Schepper and Head. 30 Cymatiosphaera latisepta De Schepper and Head. Holotype. Sample 610A-5-5, 52–54cm, slide 5.5(2), Holotype. Sample 610A-15-3, 108–113cm, slide H44/0. Dorsal view of (11) dorsal surface, showing 1P 15.3(1), D11/1. Uncertain view at mid-focus. Central archeopyle (3’’), (12) mid-focus, and (13) ventral sur- body maximum diameter, 18µm. face. Central body length (excluding crests), 27µm. 31 Barssidinium graminosum Lentin, Fensome and Wil- 16 Algal cyst type 1 of Head (1996). Sample 610A-15-4, liams. Sample 610A-18-5, 80–86cm, slide 18.5(1), 113–115cm, SEM Sample 15.4. Uncertain view. Q24/0. View uncertain. Central body length, 52µm; Height of photomicrograph, 21.5µm. average process length, 15µm. 17–19 Lavradosphaera sp. 1. (17–18) Sample 610A-15-5, 32–33 Operculodinium? eirikianum var. crebrum De 63–68cm, slide 15.5(1), N4/1. Uncertain view. Maxi- Schepper and Head. Holotype. Sample 610A-17-5, mum diameter (incl. muri), 27µm; average wall thick- 109–114cm, slide 17.5(1), O21/4. Dorsal view show- ness, 3.9µm. (19) Sample 610A-15-5, 63–68cm, slide ing (32) dorsal surface and (33) mid-focus. Central 15.5(1), J39/0. Uncertain view, lower focus on body length, 32µm; wall thickness, 3.0µm. pylome. Maximum diameter (incl. muri), 24.7µm; av- erage wall thickness, 3.1µm.

156 Stijn De Schepper and Martin J. Head Plate 2

stratigraphy, vol. 5, no. 2, 2008 157 S. De Schepper and M. J. Head: Age calibration of dinoflagellate cyst and acritarch events, Pliocene–Pleistocene of the eastern North Atlantic

Based on the palynological analyses of 102 samples spanning BARRON, J.A., 1993. Diatoms. In: Lipps, J.H., Ed., Fossil the time interval between ca. 4.0 Ma and 0.53 Ma in DSDP and , 155–167. Cambridge, MA: Blackwell Scientific Publi- Hole 610A, a total of 32 dinoflagellate cyst and acritarch cations. biostratigraphic events have been identified and compared with BARTOLI, G., SARNTHEIN, M., WEINELT, M., ERLENKEUSER, other sites in the North Atlantic and adjacent seas. Of these H., GARBE-SCHÖNBERG, D. and LEA, D.W., 2005. Final closure events, only seven are first appearances, including that of of Panama and the onset of northern hemisphere glaciation. Earth Impagidinium cantabrigiense which may prove a good marker and Planetary Science Letters, 237: 33–44. for the latest Gelasian in the central and eastern North Atlantic. Species disappearances within Hole 610A reflect a general BARTOLI, G., SARNTHEIN, M. AND WEINELT, M., 2006. Late global cooling trend through the later Pliocene and Pleistocene. Pliocene millenial-scale climate variability in the northern North At- Species with relatively synchronous HOs in the North Atlantic lantic prior to and after the onset of Northern Hemisphere glaciation. region are Ataxiodinium confusum (2.63–2.65 Ma), Paleoceanography, 21: PA4205, doi:10.1029/2005PA001185. Invertocysta lacrymosa (2.72–2.74 Ma in the eastern and cen- tral North Atlantic and Mediterranean), Impagidinium solidum BERGGREN, W.A., KENT, D.V. and VAN COUVERING, J.A., 1985. Neogene geochronology and chronostratigraphy. In: Snelling, N.J., (3.15–3.17 Ma), and Leiosphaeridium rockhallensis (3.83–3.88 Ed., The chronology of the geological record, 211–250. London: Ma); and to a lesser extent Cymatiosphaera latisepta Geological Society of London Memoir. (2.49–2.63 Ma) and Batiacasphaera minuta/micropapillata (3.83–ca. 3.7 Ma). Other species events are evidently BERGGREN, W.A., HILGEN, F.J., LANGEREIS, C.G., KENT, D.V., diachronous across the North Atlantic region and presumably OBRADOVICH, J.D., RAFFI, I., RAYMO, M.E. and SHACKLE- related to regional paleoenvironmental conditions. TON, N.J., 1995. Late Neogene chronology: New perspectives in high-resolution stratigraphy. Geological Society of America Bulletin, Many species either disappeared or strongly declined in num- 107: 1272–1287. bers between 2.8 and 2.6 Ma, apparently in response to climatic and oceanographic reorganisations at the onset of Northern BRINKHUIS, H., MUNSTERMAN, D.K., SENGERS, S., SLUIJS, A., Hemisphere glaciation. Invertocysta tabulata survived the ini- WARNAAR, J. and WILLIAMS, G.L., 2003. Late Eocene–Quater- tial cold phases only to disappear during the more extensive nary dinoflagellate cysts from ODP Site 1168, off Western Tasma- nia. In: Exon, N.F., Kennett, J.P., Malone, M.J. Eds., Proceedings of glaciation of the earliest Gelasian. the Ocean Drilling Program, Scientific Results, 189, 1–36. College Station, Texas: Ocean Drilling Program. ACKNOWLEDGMENTS doi:10.2973/odp.proc.sr.189.105.2003. This contribution is based on the doctoral research of S.D.S. who is grateful to the Gates Cambridge Trust for the award of a CHAPMAN, M.R. and CHEPSTOW-LUSTY, A., 1997. Late Pliocene Gates Cambridge Scholarship (University of Cambridge), and climatic change and the global extinction of the discoasters: an inde- additional funding from the Dudley Stamp Memorial Trust pendent assessment using oxygen isotope records. Palaeogeogra- (Royal Society) and Philip Lake Fund (Department of Geogra- phy, Palaeoclimatology, Palaeoecology, 134: 109–125. phy, University of Cambridge). S.D.S. also welcomes funding CHAPMAN, M.R., FUNNELL, B.M. and WEAVER, P.E.E., 1998. Iso- from the Deutsche Forschungsgemeinschaft (International lation, extinction and migration within Late Pliocene populations of Graduate College “Proxies in Earth History”, EUROPROX, the planktonic foraminiferal lineage Globorotalia (Globoconella)in University of Bremen). M.J.H. acknowledges support from the the North Atlantic. Marine , 33: 203–222. Palynology Oil Company Consortium (Unocal, Phillips, Amoco, Statoil, Norsk Hydro, and Elf-Aquitaine) for the initial CLEMENT, B.M. and ROBINSON, F., 1987. The magnetostratigraphy research on Hole 610A, and more recently a Natural Sciences of Leg 94 sediments. In: Ruddiman, W.F., Kidd, R.B., Thomas, E., and Engineering Research Council of Canada discovery grant. Baldauf, J.G., Clement, B.M., Dolan, J.F. et al., Eds., Deep Sea Drill- We thank ODP for providing the samples. Maria Papanikolaou, ing Project, Initial Reports, 94, 635–650. Washington D.C.: U.S. Anne de Vernal and Morten Smelror kindly made unpublished Government Printing Office. data available. Kikki Kleiven is thanked for providing the raw oxygen isotope data; Jeremy Young for his help with the CLEMENT, B.M., HALL, F.J. and JARRARD, R.D., 1989. The nannofossils. The comments of Stephen Louwye, Jens magnetostratigraphy of Ocean Drilling Program Leg 105 sediments. In: Srivastava, S.P., Arthur, M.A., Clement, B.M., Aksu, A.E., Matthiessen and Ellen Thomas helped considerably to improve Baldauf, J.G. et al., Eds., Proceedings of the Ocean Drilling Pro- the final version of this publication. gram, Scientific Results, 105, 583–595. College Station, Texas: Ocean Drilling Program. REFERENCES BALDAUF, J.G., 1987. Diatom biostratigraphy of the middle- and DALE, B., 1996. Dinoflagellate cyst ecology: modelling and geological high-latitude North Atlantic Ocean, Deep Sea Drilling Project Leg applications. In: Jansonius, J., McGregor, D.C., Eds., Palynology: 94. 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