Marine Micropaleontology 69 (2008) 173–192

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Marine Micropaleontology

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Middle Eocene–late Oligocene climate variability: Calcareous nannofossilresponse at Kerguelen Plateau, Site 748

G. Villa a,⁎, C. Fioroni b, L. Pea a, S. Bohaty c, D. Persico a

a Dipartimento di Scienze della Terra, Università di Parma, Viale Usberti, 157A, 43100 Parma, Italyb Dipartimento di Scienze della Terra, Università di Modena e Reggio Emilia, L.go S. Eufemia,19, 41100 Modena, Italyc Earth and Planetary Sciences Department, University of California—Santa Cruz, Santa Cruz, California, 95064, USA

a r t i c l e i n f o

⁎ Corresponding author. Tel.: +39 0521 905370; faxE-mail address: giuliana.villa@unipr.it (G. Villa).

0377-8398/$ – see front matter © 2008 Elsevier B.V.doi:10.1016/j.marmicro.2008.07.006

a b s t r a c t

Article history:Received 27 February 2008Received in revised form 23 July 2008Accepted 25 July 2008

A major deterioration in global climate occurred through the Eocene–Oligocene time interval,characterized by long-term cooling in both terrestrial and marine environments. During thislong-termcooling trend, however, recent studies havedocumented several short-livedwarmingand cooling phases. In order to further investigate high-latitude climate during these events, wedeveloped a high-resolution calcareous nannofossil record from ODP Site 748 Hole B for theinterval spanning the late middle Eocene to the late Oligocene (~42 to 26 Ma). The primarygoals of this study were to construct a detailed biostratigraphic record and to use nannofossilassemblage variations to interpret short-term changes in surface-water temperature andnutrient conditions. The principal nannofossil assemblage variations are identified using atemperate-warm-water taxa index (Twwt), fromwhich three warming and five cooling eventsare identified within the middle Eocene to the earliest Oligocene interval. Among these climatictrends, the cooling event at ~39 Ma (Cooling Event B) is recorded here for the first time.Variations in fine-fraction δ18O values at Site 748 are associatedwith changes in the Twwt index,supporting the idea that significant short-term variability in surface-water conditions occurredin the Kerguelen Plateau area during the middle and late Eocene. Furthermore, ODP Site 748calcareous nannofossil paleoecology confirms the utility of these microfossils forbiostratigraphic, paleoclimatic, and paleoceanographic reconstructions at Southern Oceansites during the Paleogene.

© 2008 Elsevier B.V. All rights reserved.

Keywords:EoceneOligoceneNannofossilsStable isotopesPaleoclimatology

1. Introduction

The Eocene Epoch (~55 to 34 Ma) was characterized by adramatic transition in global climate from a warm, ice-free“greenhouse” world to a cool “icehouse” world with signifi-cant glaciation in the polar regions. The Eocene beganwith anextreme, rapid warming event during the Paleocene/EoceneThermal Maximum (PETM) (e.g. Kennett and Stott, 1991),followed bya sustained period of globalwarmth in the earliestEocene (~55 to 50 Ma), known as the Early Eocene ClimaticOptimum (EECO) (Zachos et al., 2001). Following this early

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Eocene greenhouse period, a long-term cooling trend ensued,culminating in the widespread glaciation of Antarctica at theEocene/Oligocene boundary (~34 Ma) during the Oi-1 event(Miller et al., 1987; Zachos et al., 1996; Lear et al., 2000; Zachoset al., 2001). A substantial long-term decrease in global tem-peratures is interpreted through the middle and late Eocene,with cooling of up to 7 °C in deep waters and high-latitudesurface waters (Miller et al., 1987; Zachos et al., 2001).

Long-term Eocene cooling that occurred from the EECOto the Oi-1 event was not entirely monotonic or stepwise.Rather, intervals of both rapid warming and cooling have beendocumented in middle and late Eocene deep-sea records.Specifically, a significant warming anomaly was initiallyidentified at approximately 41 Ma in high southern latitudedrillcores (Barrera and Huber, 1993; Diester-Haass and Zahn,

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1996). Subsequentworkhas documented this transientwarm-ing event in both high-resolution fine-fraction and benthicforaminiferal stable isotope records from several SouthernOcean sites (ODP Site 689, 738 and 748) (Bohaty and Zachos,2003). These records show a sharp decline in δ18O values(~1‰), corresponding to a temperature increase of 4 °C of bothsurface and intermediate deep waters on both the KerguelenPlateau (Indian sector of the Southern Ocean) and Maud Rise(Atlantic sector of the Southern Ocean). This prominent eventis designated as theMiddle Eocene Climatic Optimum (MECO)and is interpreted to represent an important climatic reversalin the midst of long-term cooling through the middle to lateEocene (Bohaty and Zachos, 2003).

Recent studies on Paleogene calcareous nannofossil paleoe-cology have been completed in several time intervals and keyareas of the Southern Ocean (e.g. Wei and Wise, 1990a; Weiet al., 1992; Bralower, 2002; Persico and Villa, 2004; Villa andPersico, 2006) and mid-latitude oceans (Agnini et al., 2006;Gibbs et al., 2006). These studies have confirmed the importantrole of these microfossils for paleoclimatic and paleoceano-graphic reconstructions in the Paleogene. High-resolution cal-careous nannofossil records, however, have not been generatedwithin the middle and late Eocene interval at Southern Oceansites. Therefore, in order to further investigate both the long-term paleoceanographic evolution through this interval, as

Fig. 1. Location map o

well as transient climate events, we have developed a near-continuous record of middle Eocene to late Oligocene calcar-eous nannofossil assemblages from ODP Hole 748B (KerguelenPlateau) (Fig. 1). The primary aims of this study are to obtaina high-resolution biostratigraphic record (Fig. 2), and, mostimportantly, to evaluate diversity and abundance patterns ofcalcareous nannofossil assemblages at high latitudes throughthe greenhouse to icehouse transition. We have aimed to testwhether episodes of climatic change are manifested in seasurface settings of the Southern Ocean and, therefore, reflectedin calcareous nannofossil assemblage variations. In turn, wehave utilized the quantitative record of assemblage fluctua-tions developed in this study for paleoclimatic reconstruc-tions through both long-term and short-term climatic eventswithin the late middle Eocene to late Oligocene interval (~42to 26 Ma).

2. Materials and methods

Site 748 was drilled during Ocean Drilling Program (ODP)Leg 120 (Wise et al., 1992) and is located on the SouthernKerguelen Plateau in the southern Indian Ocean (~58°S) atwater depth of 1291 m (Fig. 1). The Eocene and Oligocenesediments at Site 748 were deposited at relatively shallowpaleodepths (~600–900 m), well above the carbonate com-

f ODP Site748.

Fig. 2. Calcareous nannofossil biostratigraphy of ODP Hole 748B from core 9 to core 20. Southern Ocean biozonation modified fromWei and Thierstein (1991) andadditional bioevents proposed in this work, correlated with standard zonations of Martini (1971) and Okada and Bukry (1980). LO: Lowest Occurrence; LCO: LowestConsistent Occurrence; HO: Highest Occurrence; HCO: Highest Consistent Occurrence.

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pensation depth. The predominant lithologies within theEocene interval at this site are foraminiferal nannofossil oozesand diatom nannofossil oozes, and, as such, are well-suitedfor a calcareous microfossil study.

We analyzed calcareousnannofossils fromCores 748B-9H to20H (~66 to 181mbsf) taking samples from u-channels studiedby Roberts et al. (2003). Samples were taken every 8–10 cm,with the exception of two intervals: from 180.96 to 177.16mbsf,every 20 cm, and from 104.79 to 95.30mbsf, every 30 cm. From133.88 to 130.71 mbsf the core was not sampled because offragmentation of sediments in the u-channel in this shortinterval. All sampleswere preparedusing the settling techniquedescribed by de Kaenel and Villa (1996), which assures auniform and homogeneous distribution of the nannofossils.Calcareous nannofossils were examined under crossed-polarized light, transmitted light, and phase-contrast light at1250×magnification. Quantitative analyses were performed bycounting at least 500 specimens on each slide and convertingthe number of index species normalized to a prefixed area(1 mm2). This technique allowed a detailed evaluation of thebiostratigraphic signal and conversion of the abundances topercentages, used to estimate the paleoecological significanceof the assemblage variations. Specimens were counted only ifmore than one half of an individualwas observed. Specimens ofZygrablithus bijugatus were broken in many samples, and,therefore, two halves of this taxon were counted as onespecimen in the abundance totals.

In this study, we present the combined range of resultsfrom ODP Site 748, joining the data set obtained in thepresent study with the late Oligocene results previouslypresented by Villa and Persico (2006). The combined data setincludes results from a total of 957 samples, spanning atime interval from the middle Eocene to the late Oligocene(Appendix A). This long high-resolution dataset allowsprecise evaluation of the stratigraphic position of importantbioevents and the paleoecological relationships betweentaxa, as well as a detailed examination of the assemblageresponse to paleoclimatic events.

In the Eocene–Oligocene section of Hole 748B, calcareousnannofossil abundance varies from common to abundant(Fig. 3), and preservation ranges from good to moderate.Z. bijugatus, a nonresistant to dissolution species (Wind andWise, 1978), is always present and well preserved, yet signsof dissolution were observed in the central area of somespecimens of Chiasmolithus spp. and Reticulofenestra reticu-lata. Discoaster spp. are often characterized by diageneticovergrowth, and, thus, most specimens within this genuscould not be identified to species level.

The calcareous nannofossil assemblage record from Hole748B is compared and interpreted along with the fine-fractionstable isotope records (δ13C and δ18O) from the middle Eoceneto the upper Oligocene interval of Hole 748B. Previously pub-lished results from Bohaty and Zachos (2003) are combinedhere with new high-resolution data (~10 cm sample spacing inmost intervals) from Cores 748B-12H, 13H, 17H, 18H, and 20H.Additional low-resolution analyses were also carried out onsamples from Cores 748B-8H, 9H,10H, and 11H. This combineddata set represents a nearly continuous, high-resolution recordfrom ~43 to 29 Ma, with a low-resolution record for the upperOligocene interval (~29 to 25 Ma). The complete fine-fractionstable isotope dataset for Hole 748B is compiled in Appendix B.

The fine-fraction material for stable isotope analysis wasobtained by disaggregating a small piece of bulk sample(~0.2 cm3) in deionized water and wet-sieving through 10 μmnylon mesh. Microscopic examination of several smear slidesof the fine-fraction residues indicates that the samples arepredominantly composed of nannofossils b12 μm in diameter,although most samples contained a minor fraction (b5%) ofnon-nannofossil carbonate. The mass contribution of non-nannofossil material (e.g. foraminiferal fragments), however,is considered to be a very minor component. Stable isotopeanalysis of the samples was performed using VG Prism andOptima mass spectrometers in the light stable laboratory atthe University of California, Santa Cruz. NBS-19 and Atlantis IIstandards, in addition to an in-house CarraraMarble standard,were included in all sample runs. All values are reportedrelative to the Vienna PeeDee Belemnite (VPDB) standard, andanalytical precision is estimated at 0.04‰ (1σ) for δ13C and0.06‰ (1σ) for δ18O. Approximately 15% of the samples werereplicated on separate sample runs.

3. Age model and nannofossil biostratigraphy

The age model for the Oligocene section of Hole 748B wasconstructed primarily using the magnetostratigraphy and tiepoints identified by Roberts et al. (2003). A reliable magnetos-tratigraphy, however, is not available for the Eocene interval ofHole 748B (Roberts et al., 2003).Within this lower interval, thefine-fraction stable isotope recordswere correlated to Site 689(Maud Rise, Atlantic sector of the Southern Ocean) usingunique features of the stable isotope records that could becalibrated with the magnetostratigraphic record available atthis site (Florindo and Roberts, 2005). The ages applied to themagnetostratigraphic (reversal) events at both Site 689 andSite 748 are taken from the astronomically-tuned calibrationsof Pälike et al. (2006, Appendix). Age calibrations for im-portant nannofossil bioevents recognized at Site 748 are listedin Table 1; age assignments inferred in previous studies arerecalibrated with respect to the geomagnetic polarity time-scale of Berggren et al. (1995), and listed in Table 2.

Previous studies of calcareous nannofossils at Site 748were performed on low-resolution sample sets by Wei et al.(1992) and Aubry (1992a). The high-resolution samplingperformed in the present study allowed refinement of theposition of the bioevents (Table 1) recognized in these pre-vious studies, as well as several additional bioevents.

The nannofossil biozonation for the Southern Ocean asdefinedbyWei andWise (1990b) andWei andThierstein (1991)is applied to the Hole 748B section (Fig. 2). The study intervalspans from the R. umbilicus (pars) Zone to the R. bisecta (pars)Zone, which corresponds to the CP 14a (pars) Zone to the CP 19(pars) Zone of Okada and Bukry (1980), respectively (Fig. 2). Inthis section, we identified 22 bioevents (Table 1) delineated asLowest Occurrence (LO), Highest Occurrence (HO), LowestConsistent Occurrence (LCO) and Highest Consistent Occur-rence (HCO) of index species, according to Raffi et al. (2006). Inthe following a brief discussion of 14 key biohorizons,whichweregard as most essential for the subdivision of the Eocene–Oligocene interval in the Southern Ocean, is given:

(1) LO of R. reticulata. The LO of R. reticulata is identified at171.15mbsf (Fig. 3), at about the same position indicated

Fig. 3. Calcareous nannofossil total abundance and abundance patterns of selected nannofossil species at ODP Hole 748B, expressed as number of specimens per mm2, are plotted against depth, chronostratigraphy, andmagnetostratigraphy (Roberts et al., 2003) correlated to the geochronological time scale (Pälike et al., 2006). Biostratigraphic events are indicated with arrows. The grey area indicates an interval with no biostratigraphic data.

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Table 1Summary of positions of calcareous nannofossil biohorizons at ODP Hole748B

Event Depth(mbsf)

Core, section,interval (cm)

Ma (Berggrenet al., 1995)

Ma (Pälikeet al., 2006)

HO C. altus 71.60 9-4, 53–54 26.2 25.58HO C. oamaruensis 104.84 13-1, 29–30 31.21 31.17HCO R. umbilicus 105.94 13-1, 139–140 31.52 31.51HO I. recurvus 109.01 13-3, 149–150 32.46 32.49LO C. altus 112.95 13-6, 80–81 33.30 33.31HO R. oamaruensis 115.86 14-2, 21–22 33.97LO R. oamaruensis 125.20 15-2, 1–2 35.54LCO I. recurvus 127.48 15-3, 80–81 35.77HO R. reticulata 128.35 15-4, 30–31 35.92HO C. cf. altus 130.35 15-5, 80–81 36.23HO N. dubius 134.34 16-1, 129–130 36.70LO R. bisecta 143.90 17-1, 120–121 37.59LO I. recurvus 148.25 17-4, 110–111 37.98LCO C. oamaruensis 149.25 17-5, 60–61 38.06HO C. solitus 149.45 17-5, 90–91 38.08HCO C. solitus 151.35 17-6, 140–141 38.34LO C. oamaruensis 155.01 18-2, 136–138 38.80LO C. cf. altus 155.81 18-3, 66–68 38.88HCO E. formosa 157.11 18-4, 56–58 39.02HCO Discoaster spp. 165.99 19-3, 143–145 40.08HO R. clatrata 170.01 19-6, 95–97 40.56LO R. reticulata 171.15 19-CC, 7–9 40.69 (hiatus)

LO = Lowest Occurrence, HO = Highest Occurrence, LCO = Lowest ConsistentOccurrence, HCO = Highest Consistent Occurrence.

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byWei et al. (1992). The presence of a hiatus is inferred at171.16 mbsf, at the core break between Cores 748B-19Hand 20H, recognizable from the distribution of severalnannofossil taxa and from abrupt changes in the δ18Oand δ13C records. Therefore, the age of this bioevent(40.69 Ma) should be considered with a degree of cau-tion (Table 1).

(2) HO of R. clatrata. This event is recorded at 170.01 mbsf(Fig. 3) with a calibrated age of 40.56 Ma (Table 1). Therange of this species at Site 748 corresponds to the rangeof R. onusta in Wei et al. (1992). Because the speciesrecognized here show the features of R. clatrata, asdescribed by Müller (1970), we therefore believe that

Table 2Biostratigraphic events published ages

Event Wei andThierstein (1991)Site 744

Wei and Wise (1992) Marino and Fl(2002a,b) Site

HO C. altus 26.1 25.8HCO R. umbilicus 31.3 31.3 late C12rHO C. oamaruensisLO C. abisectus 31.3 Late C12rHO I. recurvus 31.8 32.3 Early C12rLO C. altusHO R. oamaruensis 33.7 33.7 34.0LO R. oamaruensis 35.4 35.4 35.36HO R. reticulata 36.1 36.1LO R. bisecta C18r (40.13–41LO I. recurvus 36.0 36.0 36.0HO C. solitus 38.4 Reported as nLO C. oamaruensis 38.0 early C17rHCO E. formosa 32.8LO R. reticulata 41.2 41.2

Revised ages for nannofossil datums with respect to the geomagnetic polarity time

our R. clatrata corresponds to R. onusta of Wei et al.(1992).

(3) HCO of Discoaster spp. The abrupt drop in abundanceof Discoaster spp. at 166.00 mbsf is interpreted as aHighest Common Occurrence (HCO) datum for thisgenus (Fig. 3). This event has an age of 40.08 Ma(Table 1). Compared to the distribution of Eocenerosette-shaped discoasters at lower latitudes, theirexclusion at Site 748 occurs much earlier and is herethought to be indicative of a paleoclimatic signal (i.e.an environmentally-controlled local disappearance).Wei and Wise (1990a), Persico and Villa (2004), andArney and Wise (2003) recorded the absence of dis-coasters in theupperEocenesediments atMaudRise andKerguelen Plateau; the latter authors also noted thatdiscoaster abundance and diversity at Leg 183 are dra-matically reduced during the middle Eocene.

(4) HCO of Ericsonia formosa. As observed in the discoastergroup, a similar middle to high-latitude diachroneity isnoted for the HCO of E. formosa. This event occurs at157.11 mbsf in the middle Eocene section, below theHO of Chiasmolithus solitus, and not in the earliestOligocene (Fig. 3) as documented at middle latitudesites (Martini, 1971).Wei andWise (1990b) noted a similar earlier extinctionof E. formosa at high-latitude Sites 689 and 690 onMaud Rise. As suggested by Aubry (1992a) andBerggren et al. (1995), this bioevent is clearly diachro-nous between mid and high-latitude sites. The HCOs ofboth discoasters and E. formosa are also in agreementwith the data documented at Site 738 (unpublisheddata) and are considered regional paleoecologicalbioevents. The age assigned here to HCO of E. formosaat Site 748 is 39.02 Ma (Table 1).

(5) HO of C. solitus and LO of C. oamaruensis. In thebiozonation of Wei and Thierstein (1991), the HOof C. solitus is reported to occur below the LO ofC. oamaruensis. However, at some sites this relativeposition is not observed, and diachrony betweenhigh and mid-latitude sites has also been identified

ores1090

Mc Gonigal andDi Stefano (2002)Sites 1123–1124

Arney andWise (2003)Site 1138

Wei(2004)

Persico andVilla (2004)Sites 689 744

26.1 26.1 26.1532.3 31.3 31.40–31.4

33.91–33.9731.1

32.6 32.3 32.34–32.5533.18–33.19

33.7 33.91–33.7135.4 35.8 35.78

35.0 35.9.257) 38.0 38.0

36.0 36.0 36.0 36.1–35.8ot reliable 40.3 37.9 38.2

37.0 37.032.8

41.2

scale of Berggren et al. (1995).

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(Aubry, 1992a; Wei et al., 1992; Marino and Flores,2002a) (Table 2). In addition, poor preservation of thecentral area and taxonomic problems (Marino andFlores, 2002a) may lead to misinterpretation ofthe stratigraphic range of these species. At Site 748,the HO of C. solitus (149.45 mbsf, 38.08 Ma) clearlyoccurs above the LO of C. oamaruensis (155.01 mbsf,38.80 Ma) (Fig. 3 and Table 1). One possible reason forthis discrepancy may be linked to the difference in thequantitative methods used in the different studies. Inthe present study, a detailed quantitative analysisreveals the presence of an interval containing rarespecimens of C. oamaruensis below its LCO; conse-quently, the LO of this taxon is placed at the base ofthis interval of rare occurrence. Additionally, rarespecimens of C. solitus are also consistently notedabove the HCO of this taxon. As such, the abundancepatterns of these two species (Fig. 3) clearly show thatthe LCO of C. oamaruensis (149.25 mbsf) and HCO ofC. solitus (151.35 mbsf) likely correspond to the eventsoriginally identified in the previous biozonations. Theages of these events at Site 748 are calibrated at38.06 Ma and 38.34 Ma, respectively (Table 1). There-fore, considering the HCO of C. solitus and LCO ofC. oamaruensis at Site 748 as the bottom and the top ofthe Discoaster saipanensis Zone respectively, we canrecognize this biozone, although it is very reduced inthickness (Fig. 2) compared to previous studies (e.g.Wei and Thierstein, 1991). In addition, the zonalboundary (top of the Discoaster saipanensis Zone) islocated below the middle/late Eocene boundary; thissituation has been demonstrated also at Sites 690 and689 (Florindo and Roberts, 2005) and Site 738 (unpub-lished data).

(6) LO and LCO of Isthmolithus recurvus. At Sites 689 and744, Persico and Villa (2004) detected the LO ofI. recurvus at 36.10 Ma and at 35.80 Ma, respectively,above the HO of R. reticulata, in general agreementwith previous age assignments and stratigraphicrelationships for these bioevents (Table 2). In contrastto these studies, we detected rare, but incontestablespecimens of I. recurvus in Hole 748B at 148.25 mbsf(Fig. 4), below the HO of R. reticulata (at 128.35 mbsf,with an age of 35.92 Ma). The LO of I. recurvus at Site748 has an approximate age of 37.98 Ma (Table 1). Asimilar early occurrence was indicated in Chron C17nby Backman (1987) at Site 523 in the Southern AtlanticOcean, but it was interpreted as downhole contamina-tion. Recent data on Eocene Alpine Italian sectionsprovide a similar result (Rio D., pers. com., 2006). Thisearly occurrence in such different latitudinal andoceanographic settings cannot be explained by meansof contamination at both sites.The LCO of I. recurvus occurs at 127.48 mbsf in Hole748B and is used here to mark the base of theI. recurvus Zone. If the LCO is time-trangressive, thebase of the biozone could be diachronous at differentlatitudes. The age of this event at Site 748 has beenestimated at 35.77 Ma (Table 1). These observationsunderline the importance of high-resolution quanti-tative analyses.

(7) LO of R. bisecta. The LO of R. bisecta occurs at 143.90mbsfin Hole 748B (Fig. 4) with an age of 37.59Ma (Table 1). Inaccordance with the original description ((Hay et al.,1966) Roth, 1970), we only include in this taxon speci-mens b10 μm, distinguishing it from R. stavensis(N10 μm). Thus, it is difficult to compare our data withthe LO reported by Jovane et al. (2007) at 38 Ma, whichincludes specimens of R. bisecta N10 μm (= R. stavensis).The age assignments for this event reported by thedifferent authors are given in Table 2.

(8) HO of Neococcolithes dubius. The HO of N. dubius occursin the Hole 748B section at 134.34 mbsf (Fig. 4), with acalibrated age of 36.70 Ma (Table 1). We consider thisdatum as a possible additional bioevent within thelate Eocene, in general agreement with the results ofMarino and Flores (2002b), Wei and Wise (1990a) andMadile and Monechi (1991) who have reported thisevent within CP 15a Zone.

(9) LO and HO of R. oamaruensis. The LO (125.20 mbsf) andthe HO (115.86 mbsf) of R. oamaruensis in Hole 748B(Fig. 4) are important bioevents in the biozonationused for the Southern Ocean, due to low abundance orcomplete absence of discoasters in the late Eocene atall Southern Ocean sites. The LO of R. oamaruensis iscalibratedat35.54Ma (Table 1). TheHOofR. oamaruensisis used to identify the Eocene/Oligocene boundary,and usually occurs at the top of Chron C13r (Table 2).Although the 748B section is most likely condensedwithin an interval containing ice-rafted debris and themagnetostratigraphy is unreliable below C13n (Robertset al., 2003), an approximate age of 33.97Ma is assignedto the HO of R. oamaruensis at this site (Table 1).

(10) HCO of R. umbilicus. The HCO of R. umbilicus occurs at105.94 mbsf (Fig. 4), calibrated at 31.51 Ma, whichis comparable with previous age assignments for thisbioevent (Table 2).

(11) HO of C. altus. This bioevent has been matter ofdiscussion between several authors, with reported agesranging from the middle Eocene (see Firth and Wise,1992) to the early Oligocene (Perch-Nielsen, 1985; deKaenel and Villa, 1996). In Hole 748B, we identify theLO of C. altus at 112.95 mbsf within the lowermostOligocene, and the age of this bioevent is calibrated at33.31Ma.Webelieve that thedifferentproposedages forthe LO of C. altusmay be due to the occurrence of similarspecies that could be confused with C. altus. In fact, in arestricted upper Eocene interval of Hole 748B (from155.81 to 130.35 mbsf; Fig. 4), we detected specimenswith morphological features similar to C. altus, markedhere as cf., but distinct enough to suggest that this formrepresents a species different from C. altus. Therefore,a detailed study on the biometry of the C. solitus–oamaruensis–altus group was undertaken to determinethe real stratigraphic distribution ofC. altus and apossiblephylogenesis of C. altus from C. oamaruensis, C. solitus, orC. expansus (Persico and Villa, 2008).

4. Paleoecology

Although there is uncertainty in assigning environmentalpreferences to extinct nannofossil taxa, there is a general

Fig. 4. Abundance patterns of selected nannofossil species at ODP Hole 748B, expressed as number of specimens per mm2, are plotted against depth, chronostratigraphy, and magnetostratigraphy (Roberts et al., 2003)correlated to the geochronological time scale (Pälike et al., 2006). Biostratigraphic events are indicated with arrows. The grey area indicates an interval with no biostratigraphic data.

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consensus that some nannofossil species can be interpretedas reflecting distinct paleoecological conditions, thus beingindicative of climatic and oceanographic changes. For thePaleogene, numerous papers have focused on calcareousnannofossil paleoecology, in particular across the Paleocene–Eocene boundary and within the earliest Oligocene interval.For the Paleocene–Eocene boundary interval, several authorshave proposed a link between selected nannofossil speciesabundance and oceanographic changes occurred during thePETM (e.g. Bralower, 2002; Kahn and Aubry, 2004; Tremoladaand Bralower, 2004; Gibbs et al., 2006; Agnini et al., 2006;Jiang and Wise, 2006, 2007).

The earliest Oligocene interval, which is interpreted as atime of widespread glaciation of Antarctica, is another crucialtime for climatic changes and has been investigated for theresponse of nannofossil assemblage variations (Wei andWise,1990a; Persico and Villa, 2004), resulting in a profoundchange at high latitudes from temperate to cool dominatedassemblages.

In this work, we consider an extended middle Eocene tolate Oligocene interval from ~42 to 26 Ma at Site 748. Ourapproach is to consider paleoecological preferences of therecorded species according to literature (Table 3) and thendirectly compare the nannofossil abundance patterns withfine-fraction stable isotope records (Figs. 5–7). The mainnannofossil abundance variations are assumed to represent apaleoecological response and an adaptation to paleoclimaticand/or trophic changes.

In the following discussion, a brief explanation of themostpaleoecologically-indicative taxa is given, and a summaryoverview of previous paleoecological assignments is providedin Table 3.

4.1. Discoaster

Discoaster spp. are usually considered warm-water taxaand adapted to oligotrophic conditions (Table 3). In Hole 748B,the discoaster HCO falls in the sample immediately above theminimum δ18O values within the middle Eocene section(Fig. 5), considered the interval of maximum temperatureduring the MECO event. This event is interpreted as thewarmest interval of the entire late middle to late Eoceneinterval (Bohaty and Zachos, 2003). Following this event, theδ18O record shows a general tendency toward positive values,indicating a long-term cooling trend persisting up to the earlylate Eocene (Fig. 5). It is noteworthy that at Site 748 thediscoaster group disappears in the middle Eocene (40.08 Ma),as previously reported in other Southern Ocean sites (Weiand Wise, 1990a, Wei et al., 1992; Arney and Wise, 2003).This stratigraphic distribution is also confirmed at Site 738(unpublished data), where the discoaster HCO is detected (at94.00 mbsf) close to the peak of the MECO event, i.e. muchearlier than the extinction of rosette-shaped discoasters thatoccurs near the E/O boundary (~34 Ma) in lower latitudesections (Miller et al., 2008; Pearson et al., 2008). Thisdisappearance may have been influenced by a SST coolingtrend, as indicated by the δ18O data, and thus represents thebiogeographic exclusion of rosette-shaped discoasters fromSouthern Ocean following the MECO event. In support of thishypothesis, discoasters were not detected in the late Eoceneinterval at Sites 689 and 744 (Persico and Villa, 2004). On the

other hand, discoasters are also thought to have been in-fluenced by the nutrient regime of surface-waters, preferringoligotrophic conditions (Table 3). In fact, they are more abun-dant from 180 to 172mbsf, where fine-fraction δ13C values arelow (Fig. 5), possibly suggesting a link to long-term change innutrient conditions through the late middle Eocene interval.Above this level, discoasters decrease as δ13C values increase(Fig. 5), which suggests that increased fine-fraction δ13Cvalues may be indicative of augmented nutrient availability insurface waters. However, although nutrient conditions mayhave played a role in discoaster abundance in the middleEocene, we believe that the progressive decrease in SSTs inthe Southern Ocean during this interval was most likely theprimary factor in the local disappearance of the discoastergroup at Southern Ocean sites in the late middle Eocene.

4.2. Sphenolithus moriformis

Sphenolithus moriformis is the only representative of thegenus Sphenolithus observed in the nannofossil assemblagerecord from Hole 748B. Its abundance is higher during theMECO interval, and it broadly mirrors the long-term profile ofthe fine-fraction δ18O curve, decreasing toward the Eocene–Oligocene boundary and being nearly absent through most ofthe Oligocene following the Oi-1 event (Fig. 5). It re-occursagain in the late Oligocene, during a time interval interpretedas characterized by warmer surface waters (Villa and Persico,2006; Pekar et al., 2006; Zachos et al., 2001). Sphenolithus isconsidered to be an indicator of oligotrophic, warm-waterconditions (Aubry, 1998; Bralower, 2002; Gibbs et al., 2004).Gibbs et al. (2006) and Agnini et al. (2006) infer a majornutrient control over temperature during the PETM, and con-clude that paleofertility is the primary factor controlling thedistribution and abundance of this taxon. Therefore, the long-term decline of Sphenolithus at Site 748 may indicate achange from oligotrophic to eutrophic or mesotrophic condi-tions through the late Eocene to the early Oligocene interval.

4.3. Ericsonia formosa

The genus Ericsonia is thought to have thrived in warm-waters in the Paleogene (Haq and Lohmann, 1976; Wei andWise, 1990a; Aubry, 1992b; Kelly et al., 1996; Bralower, 2002).Agnini et al. (2006), studying the Venetian Pre-Alps, recog-nized an acme of Ericsonia during the PETM, and consider itas a warm and eutrophic taxon. In Hole 748B, E. formosa isabundant in the middle Eocene interval from 180 to 160 mbsfprior to and during the MECO interval (Fig. 5). Therefore, wealso consider this species as a warm-water indicator. Asdiscussed above for the discoaster HCO, the HCO of E. formosain the middle Eocene (Fig. 3) can be considered an ecologicalbioevent in the Southern Ocean, influenced by a gradual SSTdecrease that occurred at the end of themiddle Eocene (~39 to37 Ma).

4.4. Neococcolithes dubius

Neococcolithes dubius is here considered a temperate-waterindicator (Fig. 6). This taxon is abundant within the MECOinterval in Hole 748B but also persists with high abundanceafter the MECO interval. This stratigraphic distribution may

Table 3Published paleoecological preferences of selected nannofossil species

Species Authors

Bukry(1973)

Wei andWise(1990a,b)

Wei andThierstein(1991)

Aubry(1992a,b)

Firth andWise (1992)

Wei et al.(1992)

Kelly et al.(1996)

Monechiet al. (2000)

Bralower(2002)

Kahn andAubry(2004)

Persicoand Villa(2004)

Tremoladaand Bralower(2004)

Agniniet al.(2006)

Gibbset al.(2006)

Villa andPersico(2006)

Presentwork

Chiasmolithusspp.

Temperate-cool

Cool-cold Warm(Large Ch.),cold(Small Ch.)

Eutrophiccold

Cold Cool Eutrophic(C. oamaruensis)

Cooleutrophic

Cool Cooleutrophic

Cool Cool

E. formosa Warm-temperate

Warm Warm Warm Warmoligotrophic

Warm Warmoligotrophic

Warm,probablyoligotrophic

Warmoligotrophic

C. pelagicus Warm Temperate Temperate TemperateC. floridanus Temperate-

coolEutrophictemperate-cold

Eutrophic No-tempaffinity

Discoasterspp.

Oligotrophicwarm

Warm Warmoligotrophic

Oligotrophic Warmoligotrophic

Warmoligotrophic

Warm,probablyoligotrophic

Warmoligotrophic

Warmoligotrophic

I. recurvus Cool Cool TemperateR. bisecta Warm-

temperateTemperate Warm Temperate Temperate Temperate

R. daviesi Cold Cool Cool Cool Cool CoolR. oamaruensis CoolR. reticulata Warm-

temperateNot warm Warm

mesotrophicCool Oligotrophic

R. samodurovi Temperate Oligotrophic–mesotrophicwarm

Temperate Temperate

R. umbilicus Temperate Oligotrophic–mesotrophicwarm

Cool(R. hillae)

Temperate Temperate

S. moriformis Warm Warm Warm Warmoligotrophic

Warm Warmoligotrophic

Warmoligotrophic

Warm Warmoligotrophic

Z. bijugatus Oligotrophic Near-shore Warmoligotrophic

Warmoligotrophic

Cooleutrophic

Oligotrophic Warm Temperateeutrophic

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Fig. 5. Percentage distribution of warm-water taxa, δ18O and δ13C data are plotted against depth, magnetostratigraphy (Roberts et al., 2003) and chronostratigraphy and correlated to the geochronological time scale (Pälikeet al., 2006). The grey area indicates an interval with no biostratigraphic data.

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Fig. 6. Percentage distribution of temperate-water taxa, δ18O and δ13C data are plotted against depth, magnetostratigraphy (Roberts et al., 2003) and chronostratigraphy and correlated to the geochronological time scale(Pälike et al., 2006). The grey area indicates an interval with no biostratigraphic data.

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Fig. 7. Percentage distribution of cool-water taxa (R. daviesi and Chiasmolithus spp.) plotted against depth, δ18O record (left). R. reticulata plotted against δ13C record (right). On the left: chronostratigraphy andmagnetostratigraphy (Roberts et al., 2003) of ODP Hole 748B correlated to the geochronological time scale (Pälike et al., 2006).The grey area indicates an interval with no biostratigraphic data.

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Fig. 8. Distribution of temperate-water taxa, cool-water taxa and warm-water taxa plotted against depth, chronostratigraphy, magnetostratigraphy (Roberts et al.,2003) correlated to the geochronological time scale (Pälike et al., 2006) and fine-fraction δ18O record, from ODP Hole 748B.

186 G. Villa et al. / Marine Micropaleontology 69 (2008) 173–192

suggest that SST cooling following the MECO was insufficientto exclude this taxon from Southern Ocean waters until~37 Ma, when a stronger cooling occurred (see Section 5.5).The abundance distribution of N. dubius in Hole 748B mayalso reflect a nutrient control on its occurrence. This idea issupported by the generally high values observed in the δ13Ccurve in the interval corresponding to the higher abundanceof N. dubius (174–152 mbsf).

4.5. Zygrablithus bijugatus

There is currently disagreement about the main factorscontrolling Z. bijugatus abundance during the Paleogene. Thistaxon has been interpreted as both a warm (Bralower, 2002;Kahn and Aubry, 2004) and cool-water taxon (Tremolada andBralower, 2004), thriving in shallow (Monechi et al., 2000) ordeep photic habitats (Aubry, 1998; Bralower, 2002; Stoll et al.,2007), with preference for eutrophic (Tremolada and Bralower,2004) or oligotrophic conditions (Aubry, 1998; Bralower, 2002;Agnini et al., 2006; Gibbs et al., 2006). In the Hole 748B section,this taxon shows a general decrease in abundance from the base

of the studied interval up to the E/O boundary (Fig. 6),suggesting a temperate-water preference for this species.

4.6. Reticulofenestra umbilicus group

This group, which includes R. umbilicus and R. samodurovi,has been associated with temperate-water conditions in pre-vious studies (Table 3). This ecological preference is generallysupported by the observed decline in abundance through theEocene–Oligocene boundary interval at Site 748 (Fig. 6). A peakin abundance is observed at about 120 mbsf (34.8 Ma) duringthe late Eocene, when alternating rapid SST fluctuations havebeen recognized at Sites 689 and 744 (Persico and Villa, 2004).

4.7. Coccolithus pelagicus group

This group includes both Coccolithus pelagicus and Cocco-lithus eopelagicus and shows a marked decrease in abundanceat the Eocene–Oligocene boundary. We consider this grouphaving a temperate-water preference in the Paleogene of theSouthern Ocean (Fig. 6), as previously suggested by other

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authors (Table 3). This ecological association stands in contrastto the cold-temperature preference of the living specimens(Ziveri et al., 2004). Haq and Lohmann (1976) infer an evo-lution in its ecological preference through time to explain thisdiscrepancy.

4.8. Reticulofenestra bisecta group

This group, which includes Reticulofenestra bisecta andReticulofenestra stavensis, decreases in abundance through theEocene–Oligocene boundary interval and increases again inthe late Oligocene. As such, it is regarded as a temperate-watergroup (Fig. 6; Table 3). An interval of increased abundance isalso recorded between ~137 and 130 mbsf (~36.9 to 36.1 Ma)in the upper Eocene section of Hole 784B, which correspondsto an interval of lower δ18O values, thus suggesting an intervalof warmer SSTs.

4.9. Reticulofenestra daviesi group

The R. daviesi group includes Reticulofenestra spp. between5 and 8 µm and has been considered a cool-water taxon in allprevious studies (Table 3). In Hole 748B, the abundance curveof R. daviesi is noticeably in phase and inversely correlatedwith the fine-fraction δ18O record (Fig. 7), confirming theprevious paleoecological preference assignment. A significantincrease in abundance of this species over a short interval nearthe E/O boundary indicates a direct response to the initiationof the Oi-1 event, as observed by Persico and Villa (2004).Above this level, R. daviesi becomes a major component of theassemblage throughoutmost of the Oligocene interval (Fig. 7).

4.10. Chiasmolithus spp.

The Chiasmolithus spp. group (C. altus, C. oamaruensis,C. solitus, C. expansus, Chiasmolithus sp.) abundance curveshows a gradual increase from the late middle Eocene to thelate Oligocene, with a clear decrease during the MECO eventand an increase in correspondence to the Oi-1 event. There-fore, we consider this group as indicative of cool-water con-ditions (Fig. 7), in agreement with previous ecologicalassignments (Table 3).

4.11. Reticulofenestra reticulata

Reticulofenestra reticulata has been considered as depen-dent on both surface-water temperature and fertility condi-tions (Table 3). The abundance curve of R. reticulata obtainedin this study from Hole 748B shows an increase between 160and 135 mbsf (Fig. 7), which roughly corresponds to aninterval of decreased fine-fraction δ13C values, suggesting anoligotrophic preference for this species. Wei et al. (1992),based on the comparison of the distribution of this speciesbetween high and low latitudes, attribute a preference forcool waters, as it is more abundant at high-latitude sites.

4.12. Isthmolithus recurvus

In Hole 748B, I. recurvus is rare and its distribution doesnot allow any paleoecological consideration. Therefore,I. recurvus has not been included in any specific paleoecolo-

gical group in this study. Previous studies have interpreted atemperature dependence for this taxon, with a preferencetowards cool (Wei et al., 1992; Monechi et al., 2000) ortemperate waters (Persico and Villa, 2004; Villa and Persico,2006).

5. Discussion

Based on the Hole 748B results discussed above, it isevident that calcareous nannoplankton experienced signifi-cant assemblagefluctuations in response to climatic variationsthat occurred during the middle Eocene to late Oligocene inthe Indian sector of the Southern Ocean. In order to furtherassess paleoceanographic variability through this time inter-val, we have subdivided the nannofossil taxa into majorpaleoecological groups. Taxa with similar inferred SST pre-ferences have been grouped, thus obtaining warm-water taxa(Discoaster spp., E. formosa, S. moriformis), temperate-watertaxa (R. umbilicus group, C. pelagicus group, R. bisecta group,N. dubius, Z. bijugatus), and cool-water taxa (R. daviesi, Chias-molithus spp.) curves (Fig. 8). A curve based on the abundanceof R. reticulata is also plotted to indicate possible trophicvariations, where increased abundance of this taxon maycorrespond to lower nutrient levels (Fig. 7). Additionally, atemperate-warm-water taxa index (Twwt) calculated as [(tem-perate+warm) / (temperate+warm+cool)]⁎100 is plottedagainst the isotopic curves (Fig. 9).

During the middle Eocene to the late Oligocene interval,several paleoclimatic events have been previously identifiedwithin both paleontological and geochemical data sets,including the MECO warming event at ~40 Ma (Bohaty andZachos, 2003; Jovane et al., 2007), the late Eocene warminginterval at ~36 Ma (Bohaty and Zachos, 2003), the late Eocenecooling event at ~35 Ma (Vonhof et al., 2000; Bohaty andZachos, 2003), the Oi-1 event at ~34 Ma (Miller et al., 1991;Zachos et al., 1996; Coxall et al., 2005; Wei et al., 1992; Aubry,1992b; Persico and Villa, 2004), and a warming episode in thelate Oligocene at ~26 Ma (Miller et al., 1987; Zachos et al.,2001; Villa and Persico, 2006; Pekar et al., 2006). The longrecord of nannofossil data collected at high resolution fromHole 748B allows us to examine each of these events indetail, as well as infer broad climatic trends. Overall, thenannofossil record shows a gradual stepwise cooling trendalong the studied time interval. This trend, however, is punc-tuated by several important short-term climatic warmingevents. Starting from the base of the section we describebelow these events in detail (Fig. 9). In addition to the short-term warming episodes, the prominent cooling events arealso highlighted and labelled with letters A, B, and C.

5.1. Eocene cooling event A

From 41.6 to 41.3 Ma (174–172 mbsf), prior to the MECOevent and immediately before a short hiatus, the Twwt indexsuggests a sharp decrease in surface-water temperature. Thisbrief cooling event is also recognized in the fine-fraction δ18Orecord but only the initial phase of the cooling episode is ob-served because the section is truncated by a hiatus at171.16 mbsf. Tripati et al. (2005) recognized a cooling episodeat this time and attributed it to an early glacial phase in the latemiddle Eocene, which has been considered synchronous in

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both hemispheres by Tripati et al. (2008). Edgar et al. (2007)also recognized, at ~41.6 Ma, a strong positive shift in δ18Oisotope curve from Site ODP 1260 (Demerara Rise), but theysuggest, however, that most of this signal is linked to a coolingof surface and bottomwaters of the Atlantic Ocean, and not toice growth, thus excluding the presence of large ice sheets inthe Northern Hemisphere at this time. Our nannofossil datafrom Site 748 support this interpretation, indicating that asignificant component of the positive δ18O shift is related tocooling of Southern Ocean surface waters.

5.2. The MECO warming event

In general, there is evidence of an interval of warmer sur-face waters from ~40.7 to 39.1 Ma at Site 748, within the latemiddle Eocene interval that includes the MECO event. TheMECO event itself is associated with a higher percentage oftemperate-water taxa and an increase of warm-water taxa.Nevertheless, the latter (e.g. discoasters) do not show a posi-tive increase exactly at the peak of theMECO, but just below it.Immediately above the peak of the MECO event, however, asharp decrease in the abundance of warm-water taxa occurs(Figs. 8 and 9).

Fig. 9. Summary of the main paleoclimatic events evidenced by the Twwt indexrecord, plotted against age (Pälike et al., 2006) and chronostratigraphy. Dark grey banenlarged area of the interval around the MECO event, showing that calcareous nannothe cooling event B.

The rapidity and magnitude of warming phase duringthe MECO imply that this event affected the Southern Oceanbiological communities. In spite of this, our data show thatduring the warming peak indicated by the δ18O (i.e. at~40.0 Ma), the temperate-water-taxa are dominant and onlyminor variations of the other groups are recognizable. Incomparison, the nannofossil response to the MECO event isnot as marked as the profound turnover described during thePETM (Bralower, 2002; Agnini et al., 2006; Gibbs et al., 2006)or in associationwith the Oi-1 event (Persico and Villa, 2004).The relative lack of assemblage variation during the peakwarmingof theMECO event could possibly be a function of theabsolute range and magnitude of warming, interpreted fromthe δ18O record as indicative of ~4 °C temperature increase, i.e.from 10° to 14 °C through the entire MECO event (Bohaty andZachos, 2003). It is possible that the nannofossil taxa presentat Site 748 were not sensitive to this range of SST variation,which is outside of the critical temperature range definedbetween 2 °C and 8 °C (Persico et al., 2006).

Although we suggest that nannofossil behavior during theMECOmight be the result of SST warming, the possibility thatother factors, such as nutrient availability, water massstratification and/or surface current changes, may have also

([(temperate+warm)/(temperate+warm+cool)]⁎100) and fine-fraction δ18Ods indicatewarmer events, light grey bands showcooler events. On the left, thefossils register the warming event slightly before the δ18O record, followed by

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influenced the assemblage variation cannot be ruled out. Aswe have previously discussed, for example, the decline ofDiscoaster spp. abundance through the MECO interval couldindicate a change from an oligotrophic to a more eutrophicregime through the course of the warming event.

5.2.1. The age of the MECOThe age of this event was proposed by Bohaty and Zachos

(2003) at 41.5 Ma on the basis of the biochronology derivedtime scale of the LO of R. reticulata, which has a calibrated ageof 42 Ma (Berggren et al., 1995). However, the Southern Oceansites studied by Bohaty and Zachos (2003) do not have a goodmagnetostratigraphic record in this interval, so direct orprecise calibration of the bioevents is not possible.

Recently, Jovane et al. (2007) revised the magnetostrati-graphy of the Contessa section (Italy), where they preciselydated a positive shift of the bulk δ13C curve and correlated it tothe MECO event, occurring in Chron C18n2n, at about 40 Ma,using the Berggren et al. (1995) time scale. They documentedthe LO of R. reticulata at a level 5 m below the positive shift inδ13C, in C18r at about 41 Ma; they therefore suggest that thebioevent is diachronous at different latitudes.

In Hole 748B, we recorded an analogous stratigraphicposition of the LO of R. reticulata to that observed in theContessa section, occurring 7m below the positive shift of theδ13C. Assuming an accumulation rate of ~9 m/m.y. in theMECO interval of Site 748, the LO of R. reticulata has an age of~40.7 Ma. We thus suggest that the LO of R. reticulata hassimilar ages at high latitudes and in the northernmid-latitudeContessa section.

This age assignment sheds doubts on the reliability of themagnetostratigraphic signal in the Kerguelen Plateau sites forthis time interval, as mentioned by Jovane et al. (2007) andRoberts et al. (2003). Therefore, the original age calibration ofthe LO of R. reticulata is most likely misinterpreted (Pospichaland Wise, 1990).

5.3. Eocene cooling event B

Immediately following the MECO event a cooling episodeat ~39 Ma is indicated by a drop in the Twwt index (Fig. 9),which marks the end of the late middle Eocene warmingphase. This pronounced cooling event detected in the Twwtindex, recorded here for the first time, does not correspondto a related increase in fine-fraction δ18O values, which showa more gradual, long-term trend toward positive values. Ingeneral, the δ18O record between ~39 and 37 Ma indicatesgradual cooling, while the Twwt index shows severalprominent cycles. The nannofossil assemblage variation inthis interval is most likely related to instability in SouthernOcean SST conditions, but it is unclear why there is no directrelationship to changes in the fine-fraction δ18O record.

5.4. Middle/late Eocene cooling event C

The brief decrease of the Twwt index at ~37 Ma is inter-preted as a cooling episode that occurred near the middle/late Eocene boundary. It coincides with a rapid positive shiftin fine-fraction δ18O values (Fig. 9), which represents thepeak of a gradual cooling that follows the MECO warmingevent.

5.5. Late Eocene warming interval

From 36.26 to 36.0 Ma, an increase in the Twwt indexcorresponds to a decrease in fine-fraction δ18O values (Fig. 9).This event corresponds to the late Eocene warming eventinterpreted by Bohaty and Zachos (2003).

5.6. Vonhof cooling event

This cooling event, which was previously-recognized at Site689B (Vonhof et al., 2000) and subsequently confirmed byBohaty and Zachos (2003), is recorded in the nannofossilassemblage record fromHole 748B. The cooling signal recordedbyoxygen isotopesbetween35.5 and35.25Ma corresponds to asharp decrease in the Twwt index (Fig. 9).

5.7. Latest Eocene climate instability

From 35.5 to 34.1 Ma the Twwt index shows evidence of adynamic and cyclic signal (Fig. 9). During this time interval, thesame trend of the Twwt index has been clearly identified atSites 689 and 744 (Persico and Villa, 2004), suggesting climateinstability, with alternations of relatively cool and relativelytemperate conditions, preceding the Oi-1 event. Within thissame interval, Jovane et al. (2006) documented a cyclic signalin the magnetic susceptibility record from the mid-latitudeMassignano section (Italy), interpreted to be related to orbitalforcing. Further studies on calcareous nannofossils at South-ern Ocean Sites 748, 689 and 744, focusing in detail on thistime interval, could possibly confirm a similar astronomicalpacing and correlation to the Massignano section.

5.8. Earliest Oligocene cooling and the Oi-1 event

Just above the Eocene/Oligocene boundary (33.79 Ma) atthe base of Chron C13n (33.705 Ma), a remarkable changein the Twwt index marks a sudden and profound coolingevent. This dramatic change in nannofossil assemblagesoccurs exactly at the same level as the rapid positive shift ofthe δ18O curve, confirming the significance of this paleocli-matic event. The same agewas deduced from the Twt index atODP Site 689 at Maud Rise and Site 744 in Kerguelen Plateau(Persico and Villa, 2004). This shift of the Twwt index is muchmore evident than at the MECO warming event and is con-sidered to reflect the response to a pronounced decrease in SSTbelow 8°C (within the nannoplankton critical temperatureinterval) and increased nutrient levels. The augmentednutrient availability in the earliest Oligocene in the SouthernOcean could possibly have triggered an increase in primaryproductivity, also observable in the total abundance curve(Fig. 3), inducing a decrease in atmospheric CO2, which likelyturned out to be a positive feedback for the cooling event.

5.9. Oligocene

From the has early to the early late Oligocene (~34.0 to26.3 Ma), nannofossil assemblages at Site 748 indicate coolsurface waters, with the coolest conditions in the early lateOligocene interval from ~28.5 to 26.3 Ma. Following this coolphase, an increase in temperate-water taxa begins at ~26 Ma(73 mbsf) (Fig. 8), indicating a SST warming episode in the

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Southern Ocean during the late Oligocene. As discussed byVilla and Persico (2006), it is stillmatter of debatewhether thedecrease in deep-sea benthic δ18O values in the latestOligocene (Zachos et al., 2001) is tied to warm saline watermasses originating in the Atlantic Ocean and flowing in thewhole Southern Ocean (Pekar et al., 2006), or if warming,coupled with the collapse of Antarctic ice sheet, globallyaffected deep water masses (Zachos et al., 2001). Thenannofossil assemblage records from both Sites 689 and 748indirectly contribute to this debate and suggest that thesurface waters in the southern high latitudes warmedsignificantly during the latest Oligocene. This observationmay further suggest that at least some component of theobserved decrease in benthic δ18O valuesmay in fact be due todeep-sea warming.

6. Conclusions

In this study, detailed quantitative nannofossil assemblageanalysis of ODPHole 748B sediments has enabled an improvedSouthern Ocean biostratigraphic scheme for the middleEocene to late Oligocene interval. The ages of previously-recognized events are refined, and several additional bioe-vents are identified. These calibrations will be useful for fur-ther chronostratigraphic correlations between different sitesin the circum-Antarctic region.

Within the improved biostratigraphic framework, we haveassigned the nannofossil species to different paleoecologicalgroups, with respect to the SST and nutrient availability, de-duced also by comparison with the δ18O and δ13C data, andcreated a temperate-warm-water taxa (Twwt) index. Varia-tions in the Twwt index through time are inferred to beprimarily indicative of SST variations in response to climatechanges. The fine-fraction δ18O and the nannofossil assem-blage records give a similar picture for the long-term climatictrends from the middle Eocene to the late Oligocene, allowingus to recognize five cooling events within this time interval(Fig. 9). Among them, the cooling event at about 39 Ma,indicated as cooling event B, is reported here for the first time.

TheMECOevent at 40Marepresents the lastmajorwarmingevent of the Eocene. The termination of the MECO event isassociated with the regional exclusion of rosette-shapeddiscoasters from the Southern Ocean, most likely due to rapidcooling following the warming event. In the long intervalfollowing theMECOevent,we register a continuousprogressivecooling trend,which intensified at 37Ma (event C) and at 35Ma(Vonhof event). Brief interruptions in this overall cooling trendare noted during two intervals of SST instability in the latemiddle Eocene, and in the latest Eocene, respectively. Furtherinvestigation of the apparent cyclic signal in the latest Eoceneinterval, as previously suggested by Jovane et al. (2006) for theMassignano section, is recommendable.

In the early Oligocene, changes in calcareous nanno-fossil assemblages are closely associated with Oi-1 eventrecorded in the δ18O records, indicating cooling of South-ern Ocean surface waters in conjunction with growth ofthe East Antarctic ice sheet. This event is followed by ageneral indication of cool surface waters throughout theOligocene from nannofossil assemblages. This long-termcooling pattern ends abruptly during a distinct late Oligocenewarming phase from 26.5 Ma to the top of the section, con-

sidered to fall within the latest Oligocene (Villa and Persico,2006).

In this work, we confirm that calcareous nannofossils area valuable tool for paleoclimatic reconstructions in high-latitude settings during the Paleogene. At Site 748, nanno-fossil assemblage variation, interpreted from a paleoecolo-gical perspective, shows similar long-term climatic trends asthe fine-fraction δ18O data within the middle Eocene to lateOligocene interval. There are, however, some inconsistenciesin the finer-scale details, particularly with regard to the peakof the MECO event and the cooling event B, in which the tworecords seem to present a slightly different behaviour. Thereason for these discrepancies may be in part related to anenhanced response of nannoplankton to changes in SST,nutrient conditions or stratification. Alternatively, the tem-perature signal deduced from the δ18O record is obscured byice-volume or local salinity changes. Numerous short-termfluctuations in the Twwt index within the cooling events at~37 and ~35 Ma are also not evident in the fine-fraction δ18Orecord (Fig. 9). These intervals of surface-water instabilityinterpreted from the nannofossil assemblage data mayrepresent a finer response of a more complex picture thatis not clearly identified in the stable isotope records alone.These events should be investigated in future high-resolu-tion work at other sites.

Acknowledgements

We are grateful to A. Roberts for kindly making availablethe samples from u-channels at the National OceanographyCentre, Southampton, UK. Careful reviews by S.W. Wise andan anonymous reviewer helped to improve the manuscript.This research was supported by COFIN 2005 to I. Premoli Silvaand NSF Polar Programs grant OPP-0338337 to J.C. Zachos andM.L. Delaney.

Appendix A. Supplementary data

Supplementarydata associatedwith this article canbe found,in the online version, at doi:10.1016/j.marmicro.2008.07.006.

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