Original article

Refinements of calcareous nannofossil biostratigraphy at theMiocene/Pliocene Boundary in the Mediterranean region§

Précisions de la biostratigraphie à nannofossiles calcaires à la limiteMiocène/Pliocène dans la région Méditerranéenne

Agata Di Stefano *, Gioconda SturialeDipartimento di Scienze Geologiche, University of Catania, C.so Italia 57, 95129 Catania, Italy

Received 11 December 2008; accepted 8 June 2009

Available online 30 October 2009

Geobios 43 (2010) 5–20

Abstract

The high-resolution quantitative study of the calcareous nannofossil assemblages in two Mediterranean deep-sea successions (ODP Sites969B and 975B) encompassing the Miocene/Pliocene boundary allows the recognition of a set of reliable bioevents useful to detecting the base ofthe Zanclean stage. The results have been successfully compared with two on-land sections (Cava Serredi, Tuscany, and Montepetra borehole,Marche Region, Central Italy). This study confirms that Ceratolithus acutus and Triquetrorhabdulus rugosus, the markers traditionally used foridentifying the Miocene/Pliocene Boundary are very rare in the Mediterranean area and cannot be used for biostratigraphic correlation.Conversely, the absence interval (paracme) of Reticulofenestra pseudoumbilicus and the distribution range of a new species belonging to theReticulofenestra genus (Reticulofenestra zancleana nov. sp.) show high biostratigraphical potential. The position of the new biohorizons hasbeen compared to those of the planktonic foraminifers events, and correlated to the CaCO3 cycles reconstructed for the two sections. On the basisof these new nannofossil bioevents, Rio et al.’s (1990) MNN12 biozone is subdivided into three subzones, thus improving the biostratigraphicresolution of the Early Pliocene.# 2009 Elsevier Masson SAS. All rights reserved.

Keywords: Calcareous nannofossil; Biostratigraphy; Zanclean (Early Pliocene); Mediterranean region

Résumé

L’étude quantitative à haute résolution des associations de nannofossiles calcaires de deux successions méditerranéennes de mer profonde (SitesODP 969B et 975B) traversant la limite Miocène/Pliocène, a permis de reconnaître un groupe de bioévénements fiables, utiles pour la détection duZancléen basal. Les résultats ont été comparés avec succès à ceux de deux coupes affleurantes (Cava Serredi, Toscane, et le forage de Montepetra,Région Marche, Italie Centrale). Notre étude confirme que Ceratolithus acutus et Triquetrorhabdulus rugosus, marqueurs classiquement utiliséspour l’identification de la limite Miocène/Pliocène, sont très rares en Méditerranée et ne peuvent pas être retenus pour les corrélationsbiostratigraphiques. Par contre, l’intervalle d’absence (paracmé) de Reticulofenestra pseudoumbilicus et l’intervalle de distribution d’une nouvelleespèce, Reticulofenestra zancleana nov. sp., montrent un haut potentiel biostratigraphique. La comparaison de la position relative des biohorizonsdétectés par rapport aux cycles du CaCO3 et aux événements à foraminifères, montre qu’ils sont synchrones dans la région Méditerranéenne. Sur labase de ces nouveaux bio-événements à nannofossiles, la biozone MNN12 de Rio et al. (1990) est partagée en trois sous-zones, améliorant ainsi larésolution biostratigraphique du Pliocène Inférieur.# 2009 Elsevier Masson SAS. Tous droits réservés.

Mots clés : Nannofossiles calcaires ; Biostratigraphie ; Zancléen (Pliocène Inférieur) ; Région Méditerranéenne

§ Corresponding editor: Fabienne Giraud.* Corresponding author.

E-mail address: distefan@unict.it (A. Di Stefano).

0016-6995/$ – see front matter # 2009 Elsevier Masson SAS. All rights reserveddoi:10.1016/j.geobios.2009.06.007

1. Introduction

The Global Stratotype Section and Point (GSSP) of theMiocene/Pliocene Boundary (M/P B) was proposed by Hilgenand Langereis (1993) and ratified by Van Couvering et al.

.

A. Di Stefano, G. Sturiale / Geobios 43 (2010) 5–206

(2000) at the base of the Trubi sequence in the Eraclea Minoasection (Southern Sicily), five precessional cycles below theThvera magnetic event (Lourens et al., 1996) and astrochro-nologically calibrated at 5.332 Ma (Lourens et al., 2004). TheM/P B in the Mediterranean area coincides with the end of thewell known ‘‘Messinian Salinity Crisis’’ (Hsü et al., 1973,1978; see Rouchy and Caruso, 2006; Roveri et al., 2008 forwide and up-to-date reviews) and it is physically identified by asharp lithological change from evaporitic or continentaldeposits (‘‘Lago-Mare’’, Orszag-Sperber, 2006 and referencestherein) to marine open-sea pelagic marls (‘‘Trubi’’ Fm inSicily).

The partially or totally (according to different models)interrupted connections between the Mediterranean and theAtlantic Ocean during the Late Miocene and the closure of theconnection between the Mediterranean and the Indian Oceansince the Early-Middle Miocene (Rögl and Steininger, 1984;Rögl, 2001) are responsible for the establishment in the paleo-Mediterranean region of a distinct biogeographic province,persisting through the Pliocene-Pleistocene time interval(Thunell, 1979; Berggren, 1984). As a consequence, the‘‘standard’’ biozones (Martini, 1971; Okada and Bukry, 1980),established in oceanic areas, are of low applicability in theMediterranean region, and regional calcareous nannofossilbiostratigraphic schemes have been proposed by differentauthors (Bukry, 1973; Schmidt, 1973; Bizon and Müller, 1977,1978; Müller, 1978, 1985; Ellis, 1979; Raffi and Rio, 1979;Driever, 1981, 1988; Backman et al., 1983; Rio et al., 1984).Previous studies on the Early Pliocene nannofossil biostrati-graphy of the Mediterranean were summarized by Rio et al.(1990) who first introduced quantitative analyses for definingthe boundaries of a zonal scheme, in a detailed study of ODPSite 653 (Cornaglia Basin, West of Sardinia, Tyrrhenian Sea).These authors pointed out the difficulty of recognizing, in theMediterranean successions, the biohorizons usually adopted forapproximating the M/P B. Three important nannofossil eventsoccur in the word ocean close to the M/P B and are very wellconstrained outside the Mediterranean: the Last Occurrence(LO) of Discoaster quinqueramus, the First Occurrence (FO) ofCeratolithus acutus, and the LO of Triquetrorhabdulusrugosus, respectively astronomically calibrated at 5.54, 5.345and 5.279 Ma (Raffi et al., 2006).

In the Mediterranean, the LO of D. quinqueramus is notdocumented because of the essentially non-marine character ofthe deposits during that time interval. Furthermore, thepresence of D. quinqueramus is a controversial point. Raffiet al. (2003) state that typical specimens of this species and ofthe codistributed D. berggrenii are absent, and only atypicalspecimens occur. Conversely, the presence of typical specimensof D. quinqueramus is documented in the Mediterranean area(Mazzei, 1985; Cipollari and Cosentino, 1995; Triantaphyllouet al., 1999; Iaccarino et al., 2008) but their discontinuousdistribution and the low frequencies do not allow to defineuseful biohorizons based on this species. The record of C.acutus and T. rugosus is very scattered, and they are seldomreported in the literature. In particular Cita and Gartner (1973)reported C. acutus in the basal Zanclean of the Capo Rossello

section, whereas the presence of T. rugosus was not mentioned.On the contrary Rio et al. (1984) found a few T. rugosus in thefirst 6 m of the basal Zanclean (again in the Capo Rossellosection; see also Sgarrella et al., 1999), but they did not find anyC. acutus. The LO of C. acutus appears in Okada and Bukry’sscheme as secondary events for identifying the boundarybetween CN10b and CN10c zones, but it was not adopted in theRio et al.’s scheme, due to the absence of the species at ODPSite 653.

More recent nannofossil biostratigraphic studies based onland sections involve the Early Pliocene of the Capo Rossello-Eraclea Minoa (Sicily) and Roccella Jonica-Capo Spartivento(Calabria) sections (Di Stefano et al., 1996; Sgarrella et al.,1997, 1999), and the Pissouri section (Cyprus Island; DiStefano et al., 1999). Further data derive from the biostrati-graphic studies carried out on the sections drilled during ODPLegs 160 (Castradori, 1998; Di Stefano, 1998; Spezzaferriet al., 1998; Staerker, 1998) and 161 (Siesser and de Kaenel,1999), summarized in Iaccarino et al. (1999). The presence ofvery rare C. acutus is mentioned by Castradori (1998) only inone sample in the basal Zanclean sediment of Site 969Bsuccession (Eastern Mediterranean), and very rare specimens ofT. rugosus were found by Di Stefano et al. (1996) in the CapoSpartivento section, only after ‘‘long-lasting’’ searches. The LOof T. rugosus was identified at Pissouri section, where only onebadly preserved specimens of C. acutus was found (Di Stefanoet al., 1999). According to Siesser and de Kaenel (1999: p. 225),C. acutus occurs too infrequently in the Mediterranean to beused for reliable biostratigraphy. In disagreement with most ofthe existing literature, Popescu et al. (2007) report an almostcontinuous presence of T. rugosus and C. acutus in theMaccarone section (Marche Region, Central Italy).

In this paper we present a set of calcareous nannofossilevents, astrochronologically calibrated, obtained through thehigh resolution bio- and cyclostratigraphic study of LowerPliocene deep-sea successions (Sites 969B and 975B),compared with two on-land sections. The aims are:

� p

roviding new biostratigraphic tools for the recognition of theM/P B in the Mediterranean region; � im proving the biostratigraphic resolution of the existing

zonal scheme in the considered interval;

� e stablishing correspondences between oceanic and Mediter-

ranean Lower Pliocene successions.

2. Studied successions and methods

ODP Site 969B (33850.469’N, 24852.978’E) is located onthe Mediterranean Ridge, between the Ionian (to the West) andthe Levantine (to the East) basins, some 100 km south of Crete(Fig. 1), at a water depth of 2213.6 m (Shipboard ScientificParty, 1996a). The M/P B (97.17 mbsf) corresponds to a sharptransition from structureless calcareous clays to bioturbatedpale brown-grey nannofossil ooze. About 17 m of the Site 969Bsuccession were investigated (from just above the M/P B,sample 969B-11H-6, 125 cm, 97.15 mbsf, to sample 969B-10H-1, 145 cm, 80.35 mbsf). The investigated interval is

Fig. 1. Location map of Site ODP-160-969B (Eastern Mediterranean) and Site ODP-161-975B (Western Mediterranean).

A. Di Stefano, G. Sturiale / Geobios 43 (2010) 5–20 7

lithologically characterized by thirteen sapropels layers, whichrange in thickness from 5 to 15 cm occurring along theinvestigated section. Ninety-one samples were collected for thepresent study at a distance of 20 cm each.

Site 975B (38853.785’N, 4830.596’E) is located between theBalearic Islands and the South Balearic Basin (Fig. 1) at2421 m water depth (Shipboard Scientific Party, 1996b). About25 m of sediment cores were sampled, from just above the M/PB at 305.22 mbsf (sample 975B-33X-2, 125 cm, 305.15 mbsf)to 279.85 mbsf (sample 975B-30X-5, 25 cm). A total of 140samples were, collected every 20 cm and analyzed.

For each sample, a smear slide was prepared followingstandard methodology and analyzed with a Zeiss Axioscopemicroscope under magnification of 1000 � and 1250 � .Optical adhesive (Norland #61) was used as a mountingmedium for all of the smear slides which were exsiccated underultraviolet light. The greater resolution provided by a TescanVega\\LMU Scanning Electron Microscope (equipped with anEDAX Neptune XM4 60 microanalysis working in energydispersive spectrometry), was used to describe the speciesReticulofenestra zancleana nov. sp. (see Appendix A).

Quantitative analyses of the nannofossil assemblages wereobtained through three different counting methodologies (Rioet al., 1990 and references therein). Total assemblage composi-tion (C1) was evaluated counting 500 specimens (> 3 mm),which guarantee to involve species displaying percentageshigher than 1%. The second method (C2) consists in counting anindex species within a predetermined number of taxonomicallyrelated forms. This method was applied to Discoaster (30–50specimens), helicoliths (50 specimens) and Reticulofenestragreater than 3 mm (100 specimens). The third method (C3),consisting in counting an index species within a fixed area of theslide, was used for detecting stratigraphically significant but veryrare taxa (ceratolithids and triquetrorhabdulids). A minimum of

100 fields of view (roughly corresponding to 5 mm2), havingapproximately the same density, was analyzed and abundance ofthe taxa has been reported as number per mm2.

For each sample from both Sites 969B and 975B, thecalcium carbonate content has been evaluated using a De AstisCalcimeter (gas-volumetric method), to highlight the litholo-gical cyclicity of the two sections.

3. Results and discussion

3.1. Calcareous nannofossil biostratigraphy at ODP Sites969B and 975B

Calcareous nannofossils are generally abundant and welldiversified in both sections, with a good degree of preservation.A total of 16 taxa were identified. The results of the C1 methodare reported in Figs. 2 and 3.

At Site 969B the nannofossil assemblages are dominated by‘‘small’’ Reticulofenestra spp. (3–7 mm) and ‘‘small’’ Dictyo-coccites spp. (3–7 mm); the latter are very rare in the uppermostpart of the section. Calcidiscus spp., Discoaster spp.,Geminilithella rotula, Helicosphaera spp. and Sphenolithusspp. show wide fluctuations reaching maxima percentages of40%. Rhabdosphaera, Syracosphaera spp. and Umbilico-sphaera spp. range from 0 to 10%. Abundances of Coccolithuspelagicus, Pontosphaera spp. and Scyphosphaera spp. neverexceed 5%. Reticulofenestra pseudoumbilicus (> 7 mm,according to Fornaciari et al., 1996) is virtually absent in thelower part of the section. The same trend is presented by largeDictyococcites spp. (> 7 mm, also including Reticulofenestraspp. > 7 mm not referred to R. pseudoumbilicus) but with lower(up to 10%) percentages. Similar results are observed at Site975B, with some differences: C. pelagicus, traditionallyinterpreted as a cold water proxy (Okada and McIntyre,

Fig. 2. Results of the C1 method (total assemblage composition evaluated within 500 specimens > 3 mm) at Site 969B.

Fig. 3. Results of the C1 method (total assemblage composition evaluated within 500 specimens > 3 mm) at Site 975B.

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1979; Winter et al., 1994) displays higher percentages (up to30%), whereas warm-temperate genera such as Discoaster spp.,G. rotula and Umbilicosphaera spp., have average frequenciesof 2–3%, lower that at Site 969B. These differences couldreflect the different location of the two sites, considering theactual oceanographic condition for the Mediterranean Sea, withthe eastern part of the basin being characterized by warm and

oligotrophic surface waters whereas in the western part thesurface waters are colder and richer in nutrients (Béthoux et al.,1999; Pinardi and Masetti, 2000). Recent studies (e.g., Morigiet al., 2007) indicate that similar differences that reflect on thedistribution of plankton community, occurred in the Medi-terranean during the Late Miocene. Our results show that thesepaleoceanographic conditions probably persisted in eastern and

A. Di Stefano, G. Sturiale / Geobios 43 (2010) 5–20 9

western Mediterranean also in the Early Pliocene, after theMessinian Salinity Crisis.

A circular morphotype of Reticulofenestra has beenobserved in the lowermost part of the Site 969B succession,in agreement with Castradori (1998) who first reported of an‘‘unusual morphotype of Reticulofenestra sp. with circularoutline’’ (p. 118) as abundant throughout Sections 160-969B-11H-5 and 11H-6 and in Sections 160-967A-13H-2 through13H-4. Similar specimens were observed in the earliestZanclean samples of Capo Spartivento (Southern Italy) andPissouri sections (Cyprus; Di Stefano et al., 1999). For the lackof any report of this species in the literature, which seems to berestricted to Early Pliocene, we suggest to describe it as a newspecies Reticulofenestra zancleana nov. sp. (see Appendix A).

C2 results (Figs. 4 and 5) highlight the presence of R.zancleana nov. sp. also in the lower sampled interval of Site975B, where it reaches the maximum percentage of 15% withinthe Reticulofenestra population; higher percentages (up to 40%)are observed at Site 969B. R. pseudoumbilicus specimens arepresent at the very base of both sections and become very rare justabove, delineating a paracme interval. Within helicoliths, H.carteri (not reported in Figs. 4 and 5) is the dominant species. H.intermedia occurs continuously but in low frequencies in thelower part of the sections, and shows a scattered distributionabove. H. sellii discontinuously occurs along the sections,increasing in abundance above; its FCO level is recorded only atSite 975B. Among Discoasters, D. pentaradiatus, D. surculus

Fig. 4. Results of the C2 method (index species evaluated within a maximumtaxonomically related forms) at Site 969B.

and D. brouweri are the dominant species, all showing highvariability in abundance. D. adamanteus and D. intercalarisreach maximum percentages of 40% whilst D. asymmetricus issporadically present along the sections. D. triradiatus and D.tamalis were recorded in very low frequencies at Site 969B. D.variabilis is abundant in the lower parts of the two sections,reaching maxima percentages of 80% at Site 969B and of 50% atSite 975B, and sharply decreasing in the upper intervals.

C3 method has been applied to emphasize the presence ofrare, but stratigraphically significant forms such as thosebelonging to ceratolithid group. Amaurolithus species arerelatively abundant at Site 969B (maximum value of 40specimens per mm2) with specimens of A. delicatus, A. primus,A. tricorniculatus, and intergrade forms between A. delicatusand A. primus (Fig. 6). Similar distributions were observed atSite 975B, but the taxa show lower abundance (maximum of sixspecimens per mm2; Fig. 7). Ceratolithus are generally rare,particularly at Site 975B, where very few specimens of C.acutus are recorded in two samples. At Site 969B, C. acutusdiscontinuously occurs in the lowermost interval (withmaximum value of five specimens per mm2). Rare specimensof C. larrymayeri and C. rugosus were also observed in theintermediate part of the section, whilst C. rugosus occurs in theupper part. Finally, despite its low frequencies (maximum valueof 10 specimens per mm2), the LO of T. rugosus is recognizableat Site 969B, differently from Site 975B, where this species isfound in four samples only, in extremely low abundance.

of 50 [Discoaster and helicoliths] and 100 [Reticulofenestra] specimens, of

Fig. 5. Results of the C2 method (index species evaluated within a maximum of 50, [Discoaster and helicoliths], and 100, [Reticulofenestra] specimens, oftaxonomically related forms) at Site 975B.

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3.2. Calcium Carbonate cycles at ODP Sites 969B and 975B

Since the pioneering article by Hilgen (1987) it is well statedthat Early Pliocene pelagic sediments of the Mediterranean areaare characterised by peculiar small-scale fluctuations of theCaCO3 content which define lithological cycles correlable withprecession and eccentricity cycles (Hilgen, 1991). Results ofour analyses have been expressed throughout the graphs shownin Fig. 8. At Site 969B the calcium carbonate content variesfrom a minimum of 10.7% to a maximum of 89.8%, depictingsharp lithological cycles. The tract between 88.35 and 84.45mbsf is characterized by less ample fluctuations in the CaCO3

content that varies from 69.3 to 85.9%. At Site 975B clear andsharp fluctuations of the carbonate content have been recorded,with values ranging from 40.10% to 64.40% for most of thesection. The general trend indicates a gradual decreasing of thecalcium carbonate content that culminates in the uppermostpart where values range from 33% to 40%.

Following Hilgen’s theory, minima of calcium carbonatecontent have been correlated to minima of precessional cycles,starting from the base of cycle 1 that coincides with the M/P B(5.332 Ma), and lithological cycles have been coded accordingto Lourens et al. (1996). The cyclostratigraphic reconstructionsare constrained by the position of the following bioevents,available for Sites 969B and 975B, whose position with respectto lithological cycles is well stated as synchronous in severalMediterranean sections (Table 1):

� a

n acme interval of Sphaeroidinellopsis, from an averagedepth of 96.44 to 93.01 mbsf at Site 969B and from 303.96 to301.50 mbsf at Site 975B, from cycle 2 to 6;

� th

e FCO of Globorotalia margaritae at an average depth of89.82 mbsf (Site 969B) and 297.18 mbsf (Site 975B), incycle 10; � th e end of the paracme of R. pseudoumbilicus at an average

depth of 86.95 mbsf (Site 969B) and 292.8 mbsf (Site 975B),in cycle 14.

According to the proposed reconstruction, the top of Site969B succession is correlated to cycle 21, that represents adouble cycle, as well as the subsequent cycle 22, in the Rossellocomposite section (Lourens et al., 1996). Conversely in thestudy sections, the intervals corresponding to cycles 21 and 22(well detectable at Site 975B) distinctly correlate to twoprecession cycles, 466–464 and 462–460, respectively. The topof Site 975B succession has been attributed to cycle 34. Theabsence of the G. puncticulata FO level recognized in cycle 35in the Capo Rossello area (Sgarrella et al., 1999) strengths theproposed reconstruction.

4. Calcareous nannofossil events in Early Pliocenesediments of the Mediterranean area: comparisonbetween the investigated deep-sea sequences andselected on-land sections

Comparison of quantitative data between the two investi-gated sections (Fig. 9) permits to test the biostratigraphicpotential of selected taxa and to evaluate their applicabilitywithin the Mediterranean region. The position of the detectedbioevents has been compared with the lithological cycles,correlated in turn to the precession cycles, to obtainastrochronological ages.

Fig. 6. Results of the C3 method (number of ceratolithids and triquetrorhabdulids evaluated within a minimum of 100 fields of view, reported as n/mm2) at Site 969B.

A. Di Stefano, G. Sturiale / Geobios 43 (2010) 5–20 11

The results have also been compared with those obtained intwo on-land sections, encompassing the M/P B, investigatedthrough similar analysis methodologies (high resolutionquantitative biostratigraphic analyses and cyclostratigraphy)to verify the stratigraphic value of the recognised bio-events indifferent sedimentation and geodynamic contexts. We chose theCava Serredi section (43829034.9900N, 10827028.8200E; Rifor-giato et al., 2006; Fig. 10), located in the Volterra Basin (TuscanApennine, Central Italy), made up of marly clays sedimentsreferable to an upper epi-bathyal environment (inner neriticzone), and the Montepetra borehole (4385502000N, 1281005500E;Gennari et al., 2008; Fig. 11) drilled within the Sapignosyncline (Umbro-Marchigiano Apennine, Central Italy), thatrepresents a semi-closed wedge-top basin of the northwesternApenninic foredeep. For the Monte Petra borehole magnetos-tratigraphic data are also available.

4.1. Reticulofenestra zancleana nov. sp.

R. zancleana nov. sp. occurs almost continuously within theReticulofenestra population, from the M/P B up to cycle 7,slightly pre-dating the FCO of G. margaritae. This peculiar

Reticulofenestra is not present in Late Miocene sediments(Iaccarino et al., 2008) and cannot be confused with R. rotariaTheodoridis (see Appendix A). Its FO virtually coincides withthe M/P B. Similar distribution range was observed at MontePetra borehole and Cava Serredi sections. Rare specimens canoccur at higher stratigraphic levels (the highest occurrence hasbeen recorded in cycle 22 at Site 975B). The Last CommonOccurrence (LCO) of R. zancleana nov. sp., above which thespecies is discontinuously present in low percentages (< 5%),occurs in cycle 7 in all the investigated sections. In our opinionthis is an easily traceable biohorizon, therefore it can be usefulfor stratigraphic correlations.

4.2. Ceratolithus acutus

As discussed before, C. acutus, whose FO marks the CN10a/CN10b boundary (Okada and Bukry, 1980) and considered asone of the ‘‘standard’’ bioevent for recognizing the M/P B, isvery rare in the Mediterranean area. Our results confirm thisassessment, and show that Ceratolithus is rare within thenannofossil assemblages at Site 969B and 975B (< 1% of thetotal assemblages). At Site 969B, C. acutus shows a

Fig. 7. Results of the C3 method (number of ceratolithids and triquetrorhabdulids evaluated within a minimum of 100 fields of view, reported as n/mm2) at Site 975B.

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discontinuous distribution pattern with low abundances; its FOis recorded very close to the M/P B (base of cycle 1; Fig. 9). AtSite 975B, the species is extremely rare as well, and occurswithin cycle 4. No C. acutus specimens were found at MontePetra borehole and Cava Serredi section. The FO of C. acutus inthe Mediterranean region is confirmed to be a poorlyreproducible event, useless for approximating the M/P B andfor stratigraphic correlation.

4.3. Triquetrorhabdulus rugosus

The T. rugosus LO is considered an additional event torecognize the CN10a/CN10b subzonal boundary (Okada andBukry, 1980) and a ‘‘classical’’ bioevent for identifying the M/PB. Our results show the almost continuous presence of thespecies in the lowermost part of Site 969B (Fig. 6). Accordingto our cyclostratigraphic reconstruction, the LO of the speciesfalls in cycle 5 (5.253 Ma), in good agreement with previousfindings elsewhere in the Mediterranean region (Rio et al.,1984; Sgarrella et al., 1999; Di Stefano et al., 1999) and inoceanic areas (5.279 Ma, Raffi et al., 2006). At Site 975B, thespecies is extremely rare and scattered, and it is totally absent

both at Monte Petra and Cava Serredi. Our results confirm theunreliability of this biohorizon for stratigraphic correlationwithin the Mediterranean region.

4.4. Reticulofenestra pseudoumbilicus

The quantitative distribution patterns obtained for R.pseudoumbilicus in the studied sections (Figs. 4, 5, 10 and11) clearly evidence a short interval where the taxon is absent orextremely rare, starting from just above the base of thePliocene.

Di Stefano et al. (1996) first identified this interval at CapoSpartivento-Roccella Jonica section, and observed it at Site 653(Rio et al., 1990: p. 523, Fig. 5). Similar results were obtained atSite 969E (Di Stefano, 1998), in Pissouri section (Di Stefanoet al., 1999) and Capo Rossello section (Sgarrella et al., 1999).The range of R. pseudoumbilicus paracme interval waspreviously constrained between cycle 6 (base of paracmeinterval) and cycle 14 (top of paracme interval). Our resultsindicate that the base of R. pseudoumbilicus paracme is not asynchronous event because it was recorded in differentpositions in the different sections, with respect to the planktonic

Fig. 8. CaCO3 cycles at Sites 969B and 975B. ‘‘A’’ to ‘‘N’’ indicate the position of nannofossil (this study) and planktonic foraminifer events (after Iaccarino et al.,1999).

A. Di Stefano, G. Sturiale / Geobios 43 (2010) 5–20 13

foraminiferal events and CaCO3 cycles. In fact, at Sites 969Band 975B this event seems to fall within cycles 5 and 4,respectively (Fig. 9). At Cava Serredi the species is abundantduring cycles 1 and 2, and then it is discontinuously distributedin low frequencies up to cycle 6 (Fig. 10). At Monte Petra R.pseudoumbilicus is abundant very close to the M/P B (cycles 1and 2) and decreases up to cycle 6 (Fig. 11). Above this level thespecies shows a discontinuous distribution probably due toreworking, considering the terrigenous nature of the MontePetra borehole sediments. Our results indicate that the base ofthe R. pseudoumbilicus paracme is a clear and useful horizonthat shows a slight diachroneity, ranging from cycle 2 to cycle 6.Instead, the end of the paracme is a sharp event that abruptlyoccurs in correspondence to cycle 14 (above the FCO of G.margaritae) at Sites 969B and 975B, in agreement with theliterature data (Table 1). We indicate the beginning and the endof this interval as P‘‘P’’B and P‘‘P’’E respectively, adding letter‘‘P’’ to distinguish it from another paracme interval of R.pseudoumbilicus occurring in the Late Miocene (Raffi et al.,2003).

4.5. Helicosphaera intermedia

Castradori (1998) observed the consistent presence of H.intermedia (a typical Miocene species) in the basal Zanclean of

Holes 969B and 967A and pointed out the biostratigraphicpotential of this species. Siesser and de Kaenel (1999)suggested to use the last consistent occurrence of H. intermediafor roughly approximate the NN12/NN13 boundary in theMediterranean. At Site 969B this species is almost continu-ously present in the Early Zanclean up to cycle 6 (Figs. 4 and 9),with maximum percentage of 15% of the total Helicosphaera;then it is rare and discontinuously distributed up to cycle 19. AtSite 975B H. intermedia shows slightly different distributionand abundance (Figs. 5 and 9), as well as at Cava Serredisection and Monte Petra borehole where it shows significantfrequencies only in proximity of the M/P B (Figs. 10 and 11).Therefore, data from all the investigated sections indicate thatH. intermedia does not provide a clear biohorizon.

4.6. Discoaster variabilis

Driever (1988) observed a distinguishable shift in abundanceof D. variabilis in the Lower Pliocene sediments from differentMediterranean sections. Staerker (1998) recorded a sharpdecrease in abundance of D. variabilis in ODP Leg 160sedimentary sections, suggesting its usefulness as a biostrati-graphic marker for the Early Pliocene. Our results indicate thatthe decrease in abundance of D. variabilis is clearlydiachronous at Sites 969B and 975B (Fig. 9), and in the two

Tabl

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A. Di Stefano, G. Sturiale / Geobios 43 (2010) 5–2014

on-land sections (Figs. 10 and 11). Therefore this event is notsuitable for improving biostratigraphic classification andcorrelation.

4.7. Ceratolithus rugosus and Amaurolithus spp.

The FO of C. rugosus, marking the NN12/NN13 boundary inMartini’s (1971) scheme, was not used in Rio et al.’s (1990)Mediterranean biostratigraphic scheme, due to the scarcity ofthis species in the Mediterranean area. Previously, Raffi and Rio(1979) proposed to subdivide the NN12-NN13 interval usingthe end of continuous presence of Amaurolithus spp.(coincident with the final exit of the rare A. primus and A.tricorniculatus) instead of the C. rugosus FO. Our data confirmthe unreliability of C. rugosus as biostratigraphic marker inMediterranean. C. rugosus was recorded as rare only at Site969B, in which the astrochronological age assigned to its FO(cycle 17, 4.974 Ma) is in reasonably good agreement with theage obtained in ocean for the same horizon (5.12–5.04 Ma,Raffi et al., 2006). Also, the LOs of A. primus and A.tricorniculatus, which at Site 975B slightly follow thecontinuous occurrence of H. sellii, are confirmed to beunreliable for the very low frequency and scattered occurrencerecorded. Nevertheless, the astrochronological age obtained forthis event in Mediterranean (cycle 34, 4.552 Ma) is inagreement with that recorded in oceanic areas (4.50 Ma,Raffi et al., 2006).

4.8. Helicosphaera sellii

Rio et al. (1990) used the FO of H. sellii to subdivide thelong interval corresponding to the NN12-NN13 biozones. Ourresults indicate that the species is absent just above the M/P Bbut it shows discontinuous low frequencies below its commonand continuous occurrence, recorded at Site 975B in theuppermost part of the section, at the top of cycle 31 (4.62 Ma),in good agreement with previous results (base of cycle 32 atCapo Rossello borehole, Sgarrella et al., 1999). Recent studies(Morigi et al., 2007; Iaccarino et al., 2008) revealed thepresence of typical H. sellii in Late Miocene Mediterraneansections, thus confirming previous literature (Haq, 1973; Perch-Nielsen, 1985) that consider the FAD or the FO of the species tobe a Late Miocene event. Anyway the FCO of this species,occurring in the Early Pliocene, is an easily detectable event,undoubtedly useful for correlation in the Mediterranean region.

5. Subdivision of the MNN12 biozone: proposal forLower Pliocene higher resolution calcareous nannofossilbiostratigraphy

According to the most recent nannofossil biostratigraphicscheme available for the Mediterranean region (Raffi and Rio,1979, emend. Rio et al., 1990) all the sections considered in thepresent study fall within the MNN12 (A. tricorniculatus)biozone. The base of the subsequent MNN13 (C. rugosus)biozone has been recognised only in the uppermost part of theSite 975B succession.

Fig. 9. Comparison of the Early Zanclean nannofossil biohorizons detected at Sites 969B and 975B. The CaCO3 cycles have also been reported. ‘‘A’’ to ‘‘N’’ indicatethe position of nannofossil (this study) and planktonic foraminifer events (after Iaccarino et al., 1999).

A. Di Stefano, G. Sturiale / Geobios 43 (2010) 5–20 15

The well-known scarcity of the ceratolithid group in the EarlyPliocene Mediterranean successions, confirmed in the presentstudy, results in the difficulty of recognising the MNN12 zone. Infact, following the original authors’ definition, characteristic ofthis biozone is ‘‘...the substantial presence of genus Amaur-olithus, however, not dominant (simply more numerous than inthe adjacent intervals). . .’’ (Raffi and Rio, 1979, p. 146).Furthermore, the nannofossil assemblage characterizing the baseof the biozone ‘‘...differs only slightly from that present in thepre-evaporitic Messinian. Discriminant elements are the absenceof D. quinqueramus and the presence . . . of rare specimens of A.tricorniculatus...’’ (Raffi and Rio, 1979, p. 146). Recent studieson Messinian pre-evaporitic deposits (Iaccarino et al., 2008)clearly indicate that the Amaurolithus genus is rather abundantespecially in sapropel layers and on the contrary D. quinquer-amus is discontinuously distributed in low percentages.Considering in addition that C. acutus and T. rugosus onlyoccur in very well preserved deep-sea successions (e.g., Site969B) and only after long-searching research, it is oftenvery hardto discriminate Lower Pliocene (Zanclean) from Upper Miocene(Messinian) sediments in the Mediterranean region.

For these reasons we suggest to use the presence of theeasily recognisable R. zancleana nov. sp. for characterizingthe assemblage of the lowermost Pliocene sediments in theMediterranean area (basal part of the MNN12 biozone) and touse the end of the R. pseudoumbilicus paracme, whichappears to be a synchronous event, to subdivide the above-

cited biozone and increase biostratigraphic resolution(Fig. 12).

5.1. Amaurolithus tricorniculatus Interval Zone (MNN12)

Authors: Raffi and Rio (1979), emend. Rio et al. (1990),emend. present paper.

Definition and age: lower boundary: reestablishment ofopen marine condition after the Messinian Salinity Crisis; M/PB (5.332 Ma); FO of Reticulofenestra zancleana (cycle 1,5.332 Ma); top boundary: FCO of Helicosphaera sellii (cycle31, 4.620 Ma).

Remarks: this zone is essentially coincident with the Rio etal.’s (1990) homonymous one. The FO of R. zancleana nov. sp.has been added as lower zonal boundary. The upper boundary ismarked by the FCO of H. sellii. On the basis of the resultspresented here we propose to subdivide the MNN12 Zone intothree subzones (Fig. 12), easily recognisable on the basis of therestricted distribution range of R. zancleana nov. sp. and theparacme interval of R. pseudoumbilicus.

5.2. Reticulofenestra zancleana Partial Range Subzone(MNN12a)

Authors: Di Stefano and Sturiale, present paper.Definition and age: lower boundary: reestablishment of

open marine condition after the MSC; M/P B (5.332 Ma); FO of

Fig. 10. Quantitative distribution pattern of selected species at Cava Serredi section (Tuscany; after Riforgiato et al., 2006). Foram events as specified in Fig. 8.

A. Di Stefano, G. Sturiale / Geobios 43 (2010) 5–2016

Reticulofenestra zancleana (base of cycle 1, 5.332 Ma); topboundary: LCO of Reticulofenestra zancleana (cycle 7,5.199 Ma).

Remarks: this subzone corresponds to the lowermost partof the MNN12 zone. It is mainly characterized by thecommon presence (up to 35% within the Reticulofenes-tra > 3 mm population) of the nominal species. The topboundary slightly post-dates the LO of T. rugosus, detectablein cycle 5 in well preserved Mediterranean deep-seasuccessions. Rare specimens of C. acutus may occur. R.pseudoumbilicus is well represented at the very base of thissubzone. D. brouweri, D. variabilis, D. pentaradiatus and D.surculus dominate within the discoasterids; D. asymmetricussporadically occurs. H. intermedia is present with maximumfrequencies of 15%. Amaurolithus genus is mainly repre-sented by A. primus, A. delicatus and intergrade formsbetween these two morphotypes. Several planktonic for-aminiferal events occur within this subzone: two peculiarpeaks (‘‘shifts’’) of left coiled N. acostaensis and the acmeinterval of Sphaerodinellopsis spp.

5.3. Reticulofenestra zancleana – Reticulofenestra

pseudoumbilicus Interval Subzone (MNN12b)

Authors: Di Stefano and Sturiale, present paper.Definition and age: lower boundary: LCO of Reticulofe-

nestra zancleana (cycle 7, 5.199 Ma); top boundary: paracme

‘‘P’’ end of Reticulofenestra pseudoumbilicus (cycle 14,5.004 Ma).

Remarks: this subzone roughly corresponds to theintermediate part of the MNN12 Zone. R. zancleana nov. sp.is discontinuously present in low frequencies and R.pseudoumbilicus is virtually absent. Within the discoasteridspopulation, D. brouweri and D. pentaradiatus are the dominantspecies, and D. variabilis drastically reduces its abundance; D.asymmetricus sporadically occurs. Helicosphaera intermediashows a discontinuous distribution and very low percentages(generally lower than 5%); A. delicatus and A. tricorniculatusoccur. In well preserved Mediterranean deep-sea successions C.acutus sporadically occurs. The FO of the planktonicforaminifer G. margaritae is recorded within this subzone.

5.4. Reticulofenestra pseudoumbilicus – Helicosphaera

sellii Interval Subzone (MNN12c)

Authors: Di Stefano and Sturiale, present paper.Definition and age: lower boundary: paracme ‘‘P’’ end of

Reticulofenestra pseudoumbilicus (cycle 14, 5.004 Ma); topboundary: FCO of Helicosphaera sellii (cycle 31, 4.620 Ma).

Remarks: this subzone corresponds to the upper part of theMNN12 Zone. R. pseudoumbilicus represents the dominantspecies within the Reticulofenestra (> 3 mm) population. D.brouweri, D. surculus and D. pentaradiatus are the dominantspecies within the discoasterids population; D. asymmetricus

Fig. 11. Quantitative distribution pattern of selected species at Monte Petraborehole (Marche Region; modified after Gennari et al., 2008). Foram events asspecified in Fig. 8.

Fig. 12. Refined nannofossil biostratigraphic scheme proposed for the Early Pliocenwith planktonic foraminifer events (on the right) are based on data reported by Ia

A. Di Stefano, G. Sturiale / Geobios 43 (2010) 5–20 17

and very rare specimens of D. tamalis sporadically occur. H.sellii is present, showing a discontinuous distribution and lowpercentages (generally lower than 10%). The lower boundariesslightly precede the FO of C. rugosus, recorded in cycle 17 inwell preserved Mediterranean deep-sea successions. The topboundary slightly predates the LO of A. primus + A.tricorniculatus and the FO of the planktonic foraminiferGloborotalia puncticulata.

6. Concluding remarks

The calcareous nannofossil data, collected through quanti-tative analyses of the Early Pliocene record of ODP Sites 969B(Eastern Mediterranean) and 975B (Western Mediterranean) inthe interval from the M/P B (5.332 Ma) up to the common andcontinuous occurrence of H. sellii (4.62 Ma; MNN12 biozone),provide a detailed biostratigraphic classification. The lowapplicability, in Mediterranean nannofossil biostratigraphy, ofbioevents traditionally used to approximate the M/P B (C.acutus FO and T. rugosus LO) is confirmed by the present data.Quantitative analyses provided the distribution pattern ofdifferent taxa of the nannofossil assemblages in order tohighlight the stratigraphic potential of additional events thatcould be used for approximating the M/P B and as correlationtools within the Mediterranean region. The CaCO3 cyclesobtained for both sites, correlated with the precessional cycles,provided astrochronological ages for the detected bioevents.

e of the Mediterranean area (MNN12 Biozone of Rio et al.’s, 1990). Correlationccarino et al. (1999), astronomically calibrated in the present study.

A. Di Stefano, G. Sturiale / Geobios 43 (2010) 5–2018

The reliability of the additional biohorizons has been tested intwo on-land sections (Monte Petra borehole and Cava Serredisection, Central Italy) and with data from existing literature. Ofparticular interest are:

� th

e distribution pattern of R. zancleana nov. sp., continuouslypresent from the M/P B up to cycle 7; � th e absence interval (‘‘paracme’’) of R. pseudoumbilicus

(here indicated as paracme ‘‘P’’ to differentiate it from theMiocene one), with a moderately diachronous onset, placedfrom cycle 2 to 6, and an end occurring synchronously withincycle 14.

The restricted stratigraphic range of R. zancleana nov. sp.and the base of the R. pseudoumbilicus paracme interval seemreliable events for approximating the M/P B in Mediterranean,useful to evaluate the completeness of the earliest Zancleansuccessions. Furthermore, the LCO of R. zancleana nov. sp. andthe paracme end of R. pseudoumbilicus can be used tosubdivide the MNN12 biozone and permit to increase, togetherwith the planktonic foraminifer events, the biostratigraphicresolution in the Lower Pliocene of the Mediterranean region.

Two ‘‘oceanic’’ events, the LO of T. rugosus and the FO of C.rugosus, have been recorded at Site 969B within cycle 5 and 17,respectively, in good agreement with the oceanic areas. The firstevent slightly pre-dates the LCO of R. zancleana nov. sp., and thesecond immediately follows the Pliocene paracme end of R.pseudoumbilicus, thus representing useful correlation tools forEarly Pliocene Mediterranean and oceanic successions.

Acknowledgments

Financial support for this study was provided by PRA(University of Catania) grants to Agata Di Stefano and by‘‘Dottorato di ricerca in Evoluzione geologica di orogeni di tipomediterraneo’’ grants (University of Catania) to GiocondaSturiale. ODP is warmly thanked for providing the Sites 969Band 975B samples used for the present study. Antonio Torre andSimone Mineo are thanked for performing the CaCO3 contentanalyses with the precious help of Claudia Magrini andFederica Riforgiato (Dipartimento Scienze della Terra, Uni-versita di Siena). Marco Viccaro is thanked for the kind supportin the SEM analyses and Carmelo Ferlito for the ‘‘Frenchlanguage consultancy’’.

We are grateful to Isabella Raffi, Maria Triantaphyllou andFabienne Giraud for the careful review of the manuscript andtheir valuable suggestions.

Appendix A. New Early Pliocene species ofReticulofenestra Hay, Mohler and Wade, 1966, from theMediterranean region

A.1. Introduction

In the quantitative analyses carried out on the LowerPliocene nannofossil assemblages yielded in deep-sea sedi-ments of ODP Sites 969B (Eastern Mediterranean) and 975B

(Western Mediterranean), a peculiar Reticulofenestra morpho-type was observed, characterized by a circular outline and athick and bright cycle of inner tube elements (‘‘collar’’).Previously, authors reported an ‘‘unusual morphotype ofReticulofenestra sp. with circular outline’’ (ODP Site 969B,Castradori, 1998: p. 118), and a ‘‘circular morphotypebelonging to Reticulofenestra population’’ (Pissouri [Cyprus]and Capo Spartivento [Southern Italy] sections; Di Stefanoet al., 1999: pp. 135–137). This easily detectable Reticulofe-nestra is here described as the new species Reticulofenestrazancleana. It is characterized by peculiar morphologicalfeatures and has a restricted stratigraphic range (Figs. 3 and6) astrochronologically constrained (Fig. 8). The same specieshas been recorded in two on-land sections outcropping incentral Italy (Monte Petra borehole, Marche Region; CavaSerredi section, Tuscany) showing the same stratigraphic range(Figs. 10 and 11).

A.2. Taxonomy

All type material is stored at the Department of GeologicalScience, University of Catania, Italy.

Genus Reticulofenestra Hay, Mohler and Wade, 1966Reticulofenestra zancleana Di Stefano and Sturiale, nov. sp.Fig. 13.Diagnosis: small/medium-sized placolith with circular

outline, very bright thick inner tube elements and wide centralopening. It is composed of two shields, the proximal one notvisible at distal view. In SEM view the outline is circular and theouter margin serrated. The central opening area is circular andspans up to half of the total diameter. The inner tube thatprotrudes distally is composed of imbricated elements andextends for about half of the distal shield. In crossed polarizedlight the central opening area appears square and the distalshield is less birifrangent than the central collar, which is verywell evident also in parallel light.

Differentiation: it differs from the Late Miocene R. rotariaTheodoridis by its thick cycle of inner tube elements (‘‘collar’’)and the less wide central opening. It cannot be confused withPseudoemiliania lacunosa (Middle Pliocene-Pleistocene).

Size: Diameter 5–8 mm.Derivation of name: from the Zanclean Stage, due to its

restricted stratigraphic distribution range for lacking the‘‘fringed’’ outline.

Occurrence: the FO of the species virtually coincides withthe M/P B and it is distributed within the MNN12 biozone(Early Pliocene, Zanclean). It is commonly present at the baseof the above cited zone, defining the homonymous subzoneMNN12a (present study), ranging from cycle 1 to 7; it isdiscontinuously present within the MNN12b subzone (cycles 8-14); highest occurrence in the intermediate part of subzoneMNN12c (cycle 22).

Holotype: Fig. 13(1): sample ODP Site 969B, 11H-6-25 cm.

Type locality: ODP Site 969B, Eastern Mediterranean(33850.469’N, 24852.978’E).

Fig. 13. Reticulofenestra zancleana Di Stefano and Sturiale, nov. sp. 1. Holotype, sample ODP-968B-11H-6, 25 cm, SEM distal view. 2. Paratype, sample ODP-968B-11H-6, 145 cm, SEM distal view. 3–6. Paratypes: sample ODP-968B-11H-6, 25 cm, crossed polarized (3, 5) and parallel light (4, 6).

A. Di Stefano, G. Sturiale / Geobios 43 (2010) 5–20 19

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