changes in calcareous nannofossil assemblages during the mid-pleistocene revolution

Marine Micropaleontology 69 (2008) 70–90

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Marine Micropaleontology

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Changes in calcareous nannofossil assemblages during theMid-Pleistocene Revolution

Maria Marino a,⁎, Patrizia Maiorano a, Fabrizio Lirer b

a Dipartimento di Geologia e Geofisica, Università di Bari, Via E. Orabona, 4-70125 Bari, Italyb Istituto Ambiente Marino Costiero (IAMC)-CNR sede di Napoli, Calata Porta di Massa, 80133 Napoli, Italy

a r t i c l e i n f o

⁎ Corresponding author. Tel.: +39 80 544 3454; fax:E-mail address: [email protected] (M. Marino).

0377-8398/$ – see front matter © 2008 Elsevier B.V.doi:10.1016/j.marmicro.2007.11.010

a b s t r a c t

Article history:Accepted 19 November 2007

The distribution of calcareous nannofossils are quantitatively analysed in high-resolution deep-sea core samples from North Atlantic DSDP Site 607 and Eastern Mediterranean ODP Site 967,between marine oxygen isotope stage (MIS) 35 and 15, in order to observe the response ofcalcareous nannofossils to the Mid-Pleistocene Revolution (MPR). Nannofossil abundancepatterns showmajor variations throughMIS 25–22 and distinct higher amplitude changes fromMIS 21 upward, sometime clearly correlatable to glacial–interglacial cycles. High amplitude,short-term, fluctuations of small placoliths and F. profunda characterize the interval throughMIS 24 to 22/21, suggesting strong instability in nutricline dynamics and surface waterproductivity. Principal Component Analysis and Shannon–Weaver indices (diversity anddominance) highlight significant shifts, mainly related to variations in the characteristics ofsurfacewaters. In both sites aminimum in diversity is recorded atMIS 22/21, corresponding to amaximum in the dominance of Reticulofenestra spp. at Site 607 and of Pseudoemiliania lacunosaat Site 967. Above MIS 21 the increase in abundance of various taxa (C. leptoporus,Umbilicosphaera spp., Syracosphaera spp., Rhabdosphaera spp., Oolithotus spp.) and a generalincrease in diversity are related to a distinct trend toward more stable and oligotrophic surfacewaters in contrast to the higher productivity and less stratified surface waters of the EarlyPleistocene.In addition, power spectral analysis was performed on calcareous nannofossil abundances,Shannon Index diversity values and Factor 1 of Principal Component Analysis. These showsignificant peaks in the obliquity (41-kyr) and eccentricity (100-kyr) frequency bands,indicating that the quasi-periodic oscillations in the climate proxy data are controlled bythese orbital parameters. A relationship between changes in abundance of calcareousnannofossils and production of glacial North Atlantic Deep Water is inferred.

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Keywords:Calcareous nannofossilsDSDP Site 607ODP Site 967Mid-Pleistocene RevolutionPaleoecologyPaleoceanography

1. Introduction

The Mid-Pleistocene Revolution (MPR) represents theperiod when the climate-forcing signal in the geologicalrecord changed from the dominant 41-kyr cyclicity ofobliquity to the 100-kyr cyclicity of eccentricity (Berger andJansen, 1994). It is also considered a transitional and gradualprocess (Mid-Pleistocene Transition, Berger et al., 1993;Mudelsee and Stattegger, 1997), characterized by global

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changes in the climate system, including lower globaltemperatures, increased global ice volume, and lower sea-surface temperatures (Shackleton et al., 1990). The timing ofthe transition is still under debate, being variously interpretedas starting as early as 1500 kyr and ending as late as 600 kyr(Prell, 1982; Ruddiman et al., 1989; Mudelsee and Schulz,1997; Rutherford and D'Hondt, 2000; Raymo et al., 2004).During this important climate transition significant modifica-tions in the global thermohaline circulation occurred, invol-ving changes in the production of North Atlantic Deep Water(NADW), an oxygenated and nutrient-poor water mass (Oppoand Lehman, 1993), which plays a vital role in the ventilation

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71M. Marino et al. / Marine Micropaleontology 69 (2008) 70–90

of the world's deep ocean. Significantly weaker formation ofNADW occurred during the Mid-Pleistocene glaciations: thishas been associated with an increase of global aridity and adecrease of atmospheric CO2 (Raymo et al., 1997; Shackleton,2000), during glacial times (Barnola et al., 1987; Jouzel et al.,1993), in turn related to decreased deep oceanic circulation,and enhanced biological activity (Broecker, 1982; Knox andMcElroy, 1984; Broecker and Peng, 1989; Sigman and Boyle,2000). Weaker NADW production may have caused dramaticstagnation of deep-water circulation since MIS 24 (Schmiederet al., 2000) and chemical stratification of water masses, asrevealed by vertical benthic δ13C gradients in the NorthAtlantic (Oppo et al., 1995; Raymo et al., 1997; Flower et al.,2000) and in the Pacific and Indian oceans (Keigwin, 1987;Duplessy et al., 1988; Kallen et al., 1988). The δ13C valuesduring glacial periods were lower in the deeper waters(N2500 m) than in upper waters (Boyle, 1988, 1990; Curryet al., 1988; Raymo et al., 1997), possibly due to tectonicprocesses (Raymo et al., 1988; Raymo, 1994) and/or to thenorthward expansion of the deep Southern Ocean Water,which has low δ13C values and higher nutrient content(Raymo et al., 1990; Oppo and Lehman, 1993; Bertram et al.,1995; Kleiven et al., 2003). Variations in the mode of oceaniccirculation during glacial–interglacial periods modified thevertical nutrient distribution (Boyle, 1988) and affected thebenthic and planktic assemblages. In particular, phytoplank-ton primary productivity changed as a result of variablenutrient availability during glacials and interglacials, playinga critical role via the biological pump on sequestration ofcarbon in the deep ocean and so on atmospheric CO2 levelsand climate variations (Knox andMcElroy,1984; Broecker andPeng, 1989; Sigman and Boyle, 2000).

Fig. 1. Pattern of major deep-water currents as indicated by arro

The response of marine phytoplankton communities tothe Mid-Pleistocene environmental fluctuations is poorlyknown. Coccolithophores are a major constituent of modernmarine phytoplankton; they are abundant and widespread inthe oceans with many taxa sensitive to changes in theproperties of the surface water masses such as temperature,salinity, turbidity and nutrient content. Moreover, coccolitho-phores play key roles in biogeochemical cycles, thus affectingthe surface ocean CO2 balance. Despite the increasing docu-mentation of the ecological preferences of modern cocco-lithophores, this group has been little used for Pleistocenepaleoclimatic and paleoceanographic reconstruction whencompared with other plankton groups.

This study examines changes in calcareous nannofossils inorder to determine their response to global climate variationduring the Mid-Pleistocene Revolution. The analyses focus onthe interval belonging to the Mid-Pleistocene Transitionaccording to Raymo et al. (2004), between about 1200 and600 kyr, from MIS 35 to 15, in high-quality and temporallywell-constrained deep-sea cores at the North Atlantic DSDPSite 607 and Mediterranean ODP Site 967. Site 607, recoveredat 3427 m on the western flank of the Mid-Atlantic Ridge andlocated at the core depth of NADW today (Fig. 1), is ideallylocated to record the relative strength of NADW productionduring the Pleistocene (Raymo et al., 1990). Site 967,recovered at 2564 m, is situated in the eastern Mediterranean(Fig. 2), a basin that was affected by modifications inMediterranean thermohaline circulation during the Pleisto-cene (Rossignol-Strick et al., 1982; Rossignol-Strick,1983). Thestudy of these two sites allows us to compare the response ofthe calcareous nannofossil assemblage in a semi-enclosedbasin and in the open ocean during global paleoclimatic

ws and location of Site 607 in the North Atlantic Ocean.

Fig. 2. Hydrographic section through the Mediterranean Sea and location of Site 967 (from Emeis and Sakamoto, 1998, modified). MIW = MediterraneanIntermediate Water; EMDW = Eastern Mediterranean Deep Water; WMDW = Western Mediterranean Deep Water.

72 M. Marino et al. / Marine Micropaleontology 69 (2008) 70–90

changes. Diversity indices were calculated, and PrincipalComponent Analysis were performed on the quantitative dataset, in order to improve the interpretation of nannofossilabundance changes and enhance knowledge of the ecologicalpreferences of nannofossil taxa. Finally, spectral analysis hasbeen performed on the nannofossil abundance variations inorder to extract possible Milankovitch periodicities.

2. Oceanographic setting

NADW formation is tied to the flow of warm saline surfacewaters to the North Atlantic and Nordic Seas in the GulfStream and the associated North Atlantic Drift. The NADWforms north of 60° (Fig. 1), mainly in the Greenland andLabrador Seas (Kellogg, 1987; Hay, 1993), as surface waterscool and increase in salinity due to evaporation and heat loss,thus creating a dense water mass that moves southward atdepth (N2 km) carrying saline water to the South Atlantic andother ocean basins. Three components of NADW are distin-guishable: the upper Labrador Sea Water enters the NADWbetween 1600 and 2500 m depth; the lower Denmark StraitOverflow Water is cold and relatively fresh flowing below3500 m in the NADW. The third component (N-E AtlanticDeep Water) originates in the Greenland Sea, leaving thebasin between Iceland and Scotland; it is warmer and saltybecause it entrains higher density Mediterranean water andLabrador Sea Water, and sinks between 2500 and 3500 in theNADW. This thermohaline circulation, involving the move-ment of warm salty water to high latitude of North Atlantic,the formation of NADWand the exit of water masses from theNorth Atlantic, is considered a conveyor belt whose strengthvaried on glacial and interglacial time scales during thePleistocene (Broecker and Denton, 1989; Broecker, 1991;

Imbrie et al., 1993). At the present day, the nutrient-poorand oxygenated NADW (Oppo and Lehman, 1993) fills thedeep northern and tropical Atlantic, whereas, during the lastglacial maximum (LGM), this deep-water mass was replacedby nutrient-rich (high δ13C) Southern Ocean Water (SOW)(Boyle and Keigwin, 1982, 1987; Oppo and Faibanks, 1987;Duplessy et al., 1988; Curry et al., 1988; Oppo and Lehman,1993; Oppo et al., 1995). Many studies have suggested thatNADW was still produced during Pleistocene glaciations butreached depths lower than 2 km (Curry et al., 1988; Oppo andLehman, 1993; Zhan et al., 1997; Venz et al., 1999; Kleivenet al., 2003), thus producing a water mass stratification asrevealed by benthic foraminifera bathymetric δ13C gradient(Flower et al., 2000). Within this framework, Site 607 lies inwater that is predominantly NADW today.

The semi-enclosed Mediterranean basin exchanges water,salt, heat, and nutrients with the North Atlantic Ocean. Thenutrient-rich and low salinity Atlantic surface waters floweastward through the Gibraltar Strait (Fig. 2). Flowing east-wards, the surface waters increase in salinity due to strongevaporation, and decrease in nutrient content due to consum-ption by primary producers. Winter cooling and evaporationof surface waters combine to form dense MediterraneanIntermediate Water (MIW), which spreads between 150 and600 m below the surface layer (Wust, 1961; Malanotte-Rizzoliand Hecht, 1988). MIW flows westward increasing its nutri-ent content and finally exits the Strait of Gibraltar belowthe surface Atlantic water. Ventilation of deeper waters ofthe basin (Eastern Mediterranean Deep Water and WesternMediterranean Deep Water) is ensured by dense thermally-driven waters forming in the Gulf of Lions and in the Aegeanand Ionian seas. The thermohaline Mediterranean circulationwas modified during Pleistocene monsoonal maxima driven by


astronomical variations (precessional minima and eccentricitymaxima), thus determining unventilated or anoxic deep condi-tions and sapropel deposition (Rossignol-Strick et al., 1982;Rossignol-Strick,1983). The investigated Site 967 is located in anarea bounding the formation area of IntermediateWatermasses(Fig. 2).

3. Age models

The age models adopted here utilise orbitally tuned targets.The age model of Raymo et al. (2004) was used at Site 607.In this age model δ18O data from the benthic foraminifera Ci-bicidoides and Uvigerina of Ruddiman et al. (1989), have beentuned to the Shackleton et al. (1990) target record. The datahave been linearly interpolated to a uniform 3-kyr spacing andsmoothedwith a five-point runningmean (Raymo et al., 2004).This provides a clear pattern of global climate changes in theδ18O record and enables an easier comparison with the mainnannofossil assemblage variations. Glacial stages were labelledaccording to Ruddiman et al. (1989).

The age model for Site 967 is from Kroon et al. (1998) andwas established by a combination of astronomical calibrationof the sapropel record and the stable oxygen isotope stra-tigraphy. The planktonic δ18O record at Site 967 documentsglobal climate changes; it shows large amplitude fluctuationsmainly caused by variations in surface water temperatureand salinity, predominantly influenced by eccentricity, obli-quity, and precessional related orbital fluctuations (Kroonet al., 1998). The age models adopted in the present study arealmost equivalent between the two different sections andare well comparable with the recent isotope chronology ofLourens (2004). Main references of studied sites are shown inTable 1.

4. Sampling and analyses

A total of 263 samples were analysed in the interval acrossMIS 35 to 15. According to the different sedimentation rates,ranging from about 25–40m/Ma at both sites, sample spacingvaried between 10 and 20 cm, in order to obtain approxi-mately one sample per 2–5 kyr. Frequently, the samples usedfor oxygen isotope analysis (Ruddiman et al., 1989; Kroonet al., 1998) have also been investigated in this study.

Simple smear slides were prepared from unprocessedsamples according to standard technique (Bown and Young,1998) and analysed under a polarized light microscope at amagnification of 1000×. Quantitative data were collected bycounting about 300 nannofossils N4 μm in size. Nannofossils

Table 1Studied sites, their location and related reference

Studied sites Reference

Leg/site Location, latitude, longitude Previous calcareousnannofossil studies

DSDP 94/Site 607 Eastern Atlantic, 41° N, 33° W Takayama and Sato (1987Raffi et al. (1993)Wei (1993)Raffi (2002)Maiorano and Marino (20

ODP 160/Site 967 Eastern Mediterranean, 34° N, 32° E Raffi (2002)Maiorano and Marino (20

b4 μm in size were not included in these counts, since smallplacoliths always dominate the assemblages and soabundancesof the other taxa would have not been well represented.Additionally, in order to evaluate abundances of Florisphaeraprofunda and small placoliths b4 μm in size, a supplementarycount of 300 specimens of the total nannofossil assemblageswas performed, as suggested by Matsuoka and Okada (1989)and Castradori (1993). The abundance of each taxon has beenplotted as a percentage of the selective count. Quantitativeabundances of calcareous nannofossils at the same sites can bealso found inMaiorano andMarino (2004), where only selectedmarker species have been considered per unit area. Numericaldata of the N4 µm assemblage were performed using PAST(PAleontology STatistic) software (Hammer et al., 2001), inorder to estimate the Shannon–Weaver index (diversity anddominance). In addition Principal Component Analysis wascarried out on this assemblage, using SPSS software version 10.The measure of diversity is based on a count on genus level, oron species when a sole species is recorded within a genus(Pseudoemiliania lacunosa, Calcidiscus leptoporus, Coccolithuspelagicus s.l., Neosphaera coccolithomorpha, Braarudosphaerabigelowii); this allows comparison of homogeneous taxonomicunits in order to identify the main changes in the relativerichness and dominance of the assemblages. Patterns ofdiversity and of the factor with maximum variance have beenplotted and interpreted in terms of paleoclimatic and paleo-ceanographic changes. The ecological preferences of theobserved taxa are discussed in Appendix A.

The quantitative data of selected calcareous nannofossilspecies, the PCA Factor with maximum variance and Shan-non–Weaver Index fluctuations were processed by spectralanalysis using the AnalySeries program of Paillard et al.(1996). The spectral analysis and the filtering (Gaussian band-pass filters) procedures are based on the standard approach ofJenkins and Watt (1968) and Weedon (1991), which make itpossible to highlight the harmonic structure of the obtainedsignal and offers the opportunity to filter the original timeseries in selected frequency bands. This technique wasdeveloped to identify, in climate-sensitive records, cyclicalternations correlatable with variations of the same orderrecognised in the astronomic target curves.

5. Results

5.1. Changes in the nannofossil abundances

The studied interval corresponds to MIS 35 to 15 acrossthe small Gephyrocapsa and P. lacunosa zones. Quantitative

Oxygen isotopestratigraphy

Sapropel stratigraphy–insolation cycle

Age model

) Ruddiman et al. (1989) / Raymo et al. (2004)

04)Kroon et al. (1998) Kroon et al. (1998) Kroon et al. (1998)

04)

Fig. 3. Abundance patterns of selected calcareous nannofossils at Site 607 and correlationwith δ18O stratigraphy. Shaded bands represent glacial cycles. Quantitative data are % abundances in a count of 300 total nannofossilsN4 μm in size. Last common occurrence (LCO) of R. asanoi and re-entry of medium Gephyrocapsa (reemG = first occurrence of Gephyrocapsa omega N4 μm) are indicated according to Maiorano and Marino (2004) and thisstudy.

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Fig. 4. Abundance patterns of selected calcareous nannofossils at Site 607 and correlationwith δ18O stratigraphy. Shaded bands represent glacial cycles. Quantitative data are % abundances in a count of 300 total nannofossilsN4 μm in size. Last common occurrence (LCO) of R. asanoi and re-entry of medium Gephyrocapsa (reemG = first occurrence of Gephyrocapsa omega N4 μm) are indicated according to Maiorano and Marino (2004) and thisstudy.

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abundances of the most significant calcareous nannofossilsthrough the succession of glacial and interglacial intervals areshown in Figs. 3–8.

In both sections the major component of the calcareousnannofossil assemblage is represented by placoliths b4 μm(Figs. 5 and 8). At Site 607 (Figs. 3–5), among the nannofossilsN4 μm, Reticulofenestra spp., medium Gephyrocapsa, P.lacunosa are the most abundant taxa. Other common taxaare C. leptoporus and Helicosphaera spp. Abundances generallylower than 10% are recorded for Umbilicosphaera spp., Syra-cosphaera spp., Rhabdosphaera spp., Oolithotus spp. On theother hand F. profunda, C. pelagicus s.l., and Pontosphaera spp.are minor components of the nannofossil assemblage havingabundances consistently lower than 7%. Calciosolenia spp.,Scyphosphaera spp., N. coccolithomorpha, and Thoracosphaeraspp. are very rare (percentage lower than 1%) or absent andtherefore their abundance patterns are not shown.

At Site 967 (Figs. 6–8) the calcareous nannofossil assem-blage is mostly represented by the extinct species P. lacunosa,medium Gephyrocapsa, F. profunda, Rhabdosphaera spp., andSyracosphaera spp. Additional common taxa are Reticulofe-nestra spp., Umbilicosphaera spp., C. leptoporus, Calciosoleniaspp., Helicosphaera spp. Minor components are Pontosphaeraspp., C. pelagicus s.l., Oolithotus spp. A few taxa such as Cera-tolithus spp., Scyphosphaera spp. and B. bigelowii are very rareand scattered in the assemblage.

Fig. 5. Abundance patterns of small placoliths and Florisphaera profunda at Site 607Quantitative data are % abundances on 300 specimens of the total nannofossil asseGephyrocapsa (reemG = first occurrence of Gephyrocapsa omega N4 μm) are indicate

In both sections some of the taxa show short-termvariations, sometimes clearly related to glacial–interglacialcycles. The pattern of medium Gephyrocapsa observed aboveits well-known temporary disappearance interval, ending instage 29 at Site 607 (this study) and 25 at Site 967 (Maioranoand Marino, 2004), generally shows a positive correlationwith interglacial stages (Figs. 3 and 6) and particularly withMIS 25, 23, 21, and 19.

At Site 607 C. leptoporus, Umbilicosphaera spp., Syraco-sphaera spp., Rhabdosphaera spp., and Oolithotus spp., havehigher abundances during interglacial stages, especially in theupper part of the record, MIS 21–15 (Fig. 3). These taxa do notclearly show a relation with oxygen isotope fluctuations atSite 967 (Fig. 6), with the exclusion of Umbilicosphaera spp.which shows positive correlation with interglacial episodes,particularly in the interval MIS 21–15, when the taxon isrecorded in higher relative abundance. Syracosphaera spp.and Rhabdosphaera spp. are more abundant taxa in theMediterranean site (Fig. 6), with percentages up to 30–40%,than in the Atlantic section (percentages lower than 5–8%)(Fig. 3).

The abundance of Helicosphaera spp. does not show a clearrelation to glacial–interglacial stages at either site (Figs. 4and 7). At Site 607 a sharp increase is noted from MIS 21upward (Fig. 4). Helicosphaera spp. are more abundant in theMediterranean section with percentages generally of about

and correlation with δ18O stratigraphy. Shaded bands represent glacial cyclesmblage. Last common occurrence (LCO) of R. asanoi and re-entry of mediumd according to Maiorano and Marino (2004) and this study.

.

Fig. 6. Abundance patterns of selected calcareous nannofossils at Site 967 and correlationwith δ18O and sapropel stratigraphy. Shaded bands represent glacial cycles. Quantitative data are % abundances in a count of 300 totalnannofossils N4 μm in size. Last common occurrence (LCO) of R. asanoi and re-entry of medium Gephyrocapsa (reemG = first occurrence of Gephyrocapsa omega N4 μm) are indicated according to Maiorano andMarino (2004).

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Fig. 7. Abundance patterns of selected calcareous nannofossils at Site 967 and correlationwith δ18O and sapropel stratigraphy. Shaded bands represent glacial cycles. Quantitative data are % abundances in a count of 300 totalnannofossils N4 μm in size. Last common occurrence (LCO) of R. asanoi and re-entry of medium Gephyrocapsa (reemG = first occurrence of Gephyrocapsa omega N4 μm) are indicated according to Maiorano andMarino (2004).

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Fig. 8. Abundance patterns of small placoliths and Florisphaera profunda at Site 967 and correlation with δ18O and sapropel stratigraphy. Shaded bands representglacial cycles. Quantitative data are % abundances on 300 specimens of the total nannofossils assemblage. Last common occurrence (LCO) of R. asanoi and re-entryof medium Gephyrocapsa (reemG = first occurrence of Gephyrocapsa omega N4 μm) are indicated according to Maiorano and Marino (2004).


15–20% and with peaks reaching 60%; in the Atlantic site thegenus has percentages mainly lower than 5%, with a fewpeaks up to 20%.

The pattern of the extinct P. lacunosa, at Site 607, revealshigh abundances during the glacialsMIS 24, 20 and 16, showingthe highest values during MIS 16 (Fig. 4). On the other hand, inthe Mediterranean section, where the species is more abun-dant, several abundance peaks of the species are recorded in thewarmer periods with the highest abundance at the bottom ofMIS 21 (Fig. 7). At Site 607, Reticulofenestra spp. (R. minutula, R.asanoi, Reticulofenestra sp., see Appendix A) are the majorcomponentof the assemblage in the lower coresof the site up tobottom of MIS 21 (Fig. 4), when an abrupt decrease is recorded.This pattern is also observed from MIS 23–22 to MIS 18 at Site967 (Fig. 7).

The patterns of Calciosolenia spp. and Pontosphaera spp.do not show apparent relationship with glacial–interglacialstages at either site. Calciosolenia spp. are relatively moreabundant in the Mediterranean site (Fig. 6) than in theAtlantic section. C. pelagicus s.l. is mainly represented by C.pelagicus braarudii in the Atlantic site (Fig. 4), showingdistinct peaks in abundance in the cold glacials MIS 18 and16. In the Mediterranean section the taxon primarily consistsof C. pelagicus pelagicus while C. pelagicus braarudii mainlyoccurs at MIS 16 (Fig. 7). C. pelagicus azorinus is virtuallyabsent at both sites. The small placolith group (essentiallyGephyrocapsa spp. b4 µm) shows abundance fluctuations at

both sites (Figs. 5 and 8). Strong short-term shifts in theabundance characterize the upper MIS 25 to MIS 22/21. Nosimple relationship can be observed with respect to glacial–interglacial cycles, even though small placoliths seem toincrease in several interglacial episodes.

Although F. profunda is rare in the calcareous nannofossilassemblage at the Atlantic site, important short-term fluctua-tions characterize MIS 24 to MIS 22/21 (Fig. 5); at Site 967significant shifts are observed through MIS 22 and 21 (Fig. 8).A general opposite trend can be observed between thepatterns of small placoliths and F. profunda at the Atlanticsite. This negative correlation is also observed at Site 967,mainly through MIS 36–26.

5.2. Measure of diversity and Principal Component Analysis

At Site 607, a distinct change in the Shannon–Weaver index(H) occurs at the MIS 22/21 transition (Fig. 9): a minimum inthe diversity is observed at MIS 22, and a sharp increase isrecorded fromMIS 21 upward. At Site 967, a sharp decrease indiversity characterizes the bottom ofMIS 21 (Fig.10), followedby a recovery within MIS 21, testifying a reasonable similaritywith theAtlantic site. The pattern of diversity is different in thetwo sections: at Site 607 the index has slightly lower valuesand a more distinct trend through time; it fluctuates between0.8 and 1.6 belowMIS 22, and between 1.6 and 2 aboveMIS 22(Fig. 9). At Site 967 it consistently ranges between 1.2 and 2,

Fig. 9. Correlationbetweenδ18O stratigraphy, pattern of diversity (H), dominance (D) and score plot of Factor 1 at Site 607. Shadedbands represent glacial cycles. Last commonoccurrence (LCO) ofR. asanoi and re-entry ofmediumGephyrocapsa (reemG=first occurrence ofGephyrocapsa omegaN4 μm)are indicated according toMaiorano andMarino(2004) and this study.


throughout the section (Fig. 10). The trend of dominancereflects changes in diversity in both sections; maximumdominance values, recorded at minimum diversity in thenannofossil assemblage at MIS 22/21, correspond to dom-inance of Reticulofenestra spp. at Site 607, and of P. lacunosa atSite 967.

Principal Component Analysis allows an interpretation ofthe complex patterns of nannofossil changes at the Mid-Pleistocene Transition. At both sites only Factor 1 is discussed,since Factor 2 has a very low variance (b14%) and therefore isof uncertain interpretation. At Site 607 the first component(Factor 1) accounts for 23% of the variance and it is positivelyloaded mainly by Umbilicosphaera spp. and C. leptoporus withnegative loadings for Reticulofenestra spp. (Table 2). Factor 1seems todiscriminatebetween taxa (seeAppendixA) indicativeof warm, oligotrophic and stratified water masses (Umbilico-sphaera spp. and C. leptoporus), and of mixed and highproductive cool waters (Reticulofenestra spp.). The Factor 1score plot (Fig. 9) shows short fluctuations below MIS 22 andsignificantly more positive values, together with high ampli-tude changes, from MIS 21 upwards. This pattern seemsunrelated to the LCO of Reticulofenestra asanoi and to the dropin abundance of Reticulofenestra sp. at MIS 23 (Maiorano andMarino, 2004), well below the general decrease of Reticulofe-nestra spp. and the more positive values of Factor 1 recordedfrom MIS 21. In addition, the pattern of Factor 1 has a positivecorrelationwith the pattern of diversity (r=0.91), with positivevalues of Factor 1 corresponding to increase in diversity. At Site967, Factor 1 (Fig. 10) accounts for 22% of the total variance andis positively loaded by Syracosphaera spp. and Rhabdosphaera

spp. and negatively by P. lacunosa (Table 2). The former taxaseem to benefit fromwarm and stratified waters (Appendix A).The ecology of the extinct species P. lacunosa is unknown;however it belongs to placolith-bearing taxa, which in themodern assemblages are adapted to eutrophic conditions(Young, 1994). This may suggest that the negative loadings ofFactor 1 reflect high productivity and mixing of surface waters.Although the pattern of Factor 1 shows significant short-termfluctuations, a distinct trend is not visible through time atSite 967.

5.3. Dissolution of calcareous nannofossil assemblages

Although dissolution is more intense during glacialperiods in both the Atlantic Ocean (Curry and Lohman,1986; Meyers and Diester-Haas, 1987; Diester-Haas andRothe, 1987; deMenocal et al., 1997) and the MediterraneanSea (Vázquez and Zamarreño, 1993), the calcareous nanno-fossil preservation, which generally varies from moderate togood at both sites, does not show clear differences indissolution status between glacial and interglacial stages.This could be supported by the pattern of Shannon–Weaverindex, which does not show minima during the glacialperiods, as would be expected from a dissolution effect,with the exception of MIS 22 or 22/21. The minimum indiversity occurring at these times is also accompanied by adecrease of dissolution-resistant coccoliths, such as mediumGephyrocapsa and C. leptoporus (Figs. 3 and 6), and by adominance of eutrophic taxa (P. lacunosa and Reticulofenestraspp.) (Figs. 4 and 7) and therefore it may be interpreted as due

Fig. 10. Correlation between δ18O stratigraphy, pattern of diversity (H), dominance (D) and score plot of Factor 1 at Site 967. Shaded bands represent glacial cycles.Last common occurrence (LCO) of R. asanoi and re-entry of medium Gephyrocapsa (reemG = first occurrence of Gephyrocapsa omega N4 μm) are indicated accordingto Maiorano and Marino (2004).

Table 2Loadings of calcareous nannofossil taxa on the first principal component atSite 607 and Site 967

Matrix of component Site 607 Matrix component Site 967

Factor 1 Factor 1

P. lacunosa 0.344 P. lacunosa −0.727Medium Gephyrocapsa 0.451 Medium Gephyrocapsa −0.151Helicosphaera spp. 0.579 Helicosphaera spp. 0.536Reticulofenestra spp. −0.887 Reticulofenestra spp. −0.574C. leptoporus 0.668 C. leptoporus 0.484Syracosphaera spp. 0.400 Syracosphaera spp. 0.721Umbilicosphaera spp. 0.777 Umbilicosphaera spp. 0.035Rhabdosphaera spp. 0.264 Rhabdosphaera spp. 0.671Calciosolenia spp. 0.372 Calciosolenia spp. 0.096Pontosphaera spp. −0.035 Pontosphaera spp. 0.545Oolithotus spp. 0.517 Oolithotus spp. 0.205N. coccolithomorpha 0.443 Scyphosphaera spp. 0.596Scyphosphaera spp. 0.314 B. bigelowii 0.201C. pelagicus pelagicus −0.066 Ceratolithus spp. 0.223C. pelagicus braarudii 0.230 C. pelagicus pelagicus 0.535

C. pelagicus braarudii 0.387Method: Principal ComponentAnalysis

Method: Principal ComponentAnalysis


to a primary ecological response of the nannofossil assem-blage rather than to dissolution.

5.4. Spectral analyses

In order to understand the short-term fluctuationsoccurring in the calcareous nannofossil assemblages, interms of astronomical forcing and paleoceanographic changesduring the Middle Pleistocene Revolution, power spectralanalysis and filtering procedures have been attempted. Fewdata are available on orbital periodicity in the calcareousnannofossil record during the Pleistocene, and they mainlyfocus on F. profunda, revealing Milankovitch periodicities(Molfino and McIntyre, 1990; Bassinot et al., 1997; Beaufortet al., 2003). The power spectral analysis (calculated accord-ing to the Blackman–Tukey method with a Tukey window),suggests that the fluctuations observed in most of the taxa (C.leptoporus, F. profunda, P. lacunosa, medium Gephyrocapsa,Syracosphaera spp. and Rhabdosphaera spp.), Factor 1 andShannon–Weaver Index from both sites (Fig. 11), yieldsignificant peaks mainly in the obliquity (41-kyr) frequencybands. The 100-kyr periodicity is also present in the C.leptoporus, P. lacunosa and medium Gephyrocapsa powerspectra. F. profunda and C. leptoporus are the only two recordsshowing significant spectral variance occurring at thefrequency of 23-kyr at Site 967 (Fig. 11). These results indicatethat the quasi-periodic oscillations observed in the variationsof calcareous nannofossil assemblage and characteristics ofsurfacewaters (Factor 1) are largely controlled by these orbital

parameters. However, other distinct peaks in the spectrado not correspond to the primary Milankovitch frequencies(Fig. 11).

Since wide disagreement exists in defining when thepossible transition from 41-kyr forcing up to 100-kyr couldhave occurred, floating spectral analyses were performed in

Fig. 11. Power spectral analysis in the whole time record on selected calcareous nannofossil species (C. leptoporus, F. profunda, P. lacunosa, medium Gephyrocapsa,Syracosphaera spp. and Rhabdosphaera spp.), Factor 1 and Shannon Index data. Spectral densities are normalised and plotted on a linear scale. Thin and thick linesrepresent the same datum for Site 607 and Site 967, respectively. The grey bands represent the classical Milankovitch periodicities. Confidence Interval=95%.


several time windows, on selected climate-sensitive recordsfrom both sites. The results seem to suggest that ~1200–~850 kyr and ~850–~575 kyr time intervals represent the twopossible windows where the 41-kyr–100-kyr transition couldoccur. In the present study we report only data from site 967where this change seems almost clear in few taxa (Fig. 12):the power spectra of P. lacunosa, C. leptoporus and Syraco-sphaera spp. data performed in the two selected timewindows 1200–850 kyr and 850–575 kyr, revealed significantpeaks in the 41-kyr and 100-kyr frequency bands, respec-tively (Fig. 12). Finally Gaussian band-pass filtering has beenapplied to extract the 41-kyr frequency component fromselected calcareous nannofossils, Factor 1 and Shannon Indextime series and to compare them with the stable isotopestratigraphy (Fig. 13). Visual comparison of the time seriessuggests a general good relationship between the obliquitycomponent of the climate proxies, in terms of ecologicalsignature, and the standard δ18O target curves for Site 607and Site 967. The main discrepancies are present in the

Mediterranean Site 967 where the calcareous nannofossiltaxa are not completely in agreement with the δ18O record inthe whole studied time interval. This may be related to anadditional non-astronomical forcing which controls theclimatic system (regional component).

6. Discussion

The quantitative analyses point out few differences in thetaxonomic composition of the calcareous nannofossil assem-blage between the Atlantic and the Mediterranean sites. Syr-acosphaera spp. and Rhabdosphaera spp. are significantlymore abundant in the Mediterranean section, which may be aresponse to warmer and more oligotrophic condition in thesurface waters; this observation is in agreement with thehigher abundances of these extant taxa in the EasternMediterranean (Knappertsbusch, 1993). Moreover, higherdiversity values and higher abundance of F. profunda at Site967 also suggest the more oligotrophic aspect of the eastern

Fig. 12. Power spectral analysis performed on calcareous nannofossils speciesP. lacunosa, C. leptoporus, Syracosphaera spp., from Site 967, using two timewindows (575–850 kyr thick line and 850–1200 kyr thin line). The greybands represent the classical Milankovitch periodicities. ConfidenceInterval=95%.


Mediterranean Sea with respect to Atlantic water massesduring the Pleistocene. Although there are no data on theecological preference of the extinct P. lacunosa, and only fewdata are known on the ecology of Calciosolenia spp., thesignificantly higher abundance and stronger fluctuations ofthese taxa in the Mediterranean site may suggest a possiblerelation with higher variability in salinity and nutrientcontent, due to the changeable paleoceanographic conditionsduring Pleistocene rhythmic sapropel deposition in theeastern Mediterranean. In fact, peaks in abundance of Cal-ciosolenia spp. and increase of P. lacunosa have been recentlyrecorded within sapropel layers (Negri et al., 2003; Floreset al., 2005).

Despite the different composition of calcareous nanno-fossil assemblages between the two sections, the variationsthrough time seem to reflect modifications in the character-istics of water masses in both sites, as evidenced byabundance patterns of selected taxa, pattern of Factor 1 anddiversity. These variations are mainly controlled by theobliquity 41 kyr periodicity and therefore primarily reflectclimate signal and glacial–interglacial cycles. Few taxa (C.leptoporus, P. lacunosa, Syracosphaera spp.) record the 41–100 kyr periodicity transition at around 850 kyr, at MIS 22/21.

Obliquity-related variations in the benthic δ18O recordsfrom the open ocean have been interpreted to reflect changes

in global ice volume (e.g. Shackleton et al., 1990, 1995; Raymoet al., 1989; Tiedemann et al., 1994). Obliquity-relatedvariations in the Mediterranean δ18O record have been inter-preted in a similar way (Lourens and Hilgen, 1997), but in thiscase the signal is amplified due to regional changes in sea-surface salinity and/or temperature (Hilgen et al., 1993).According to these data the strong power at the 41-kyrperiodicity, occurring in the calcareous nannofossil variationsfrom Site 967, is consistent with the assumption that theeastern Mediterranean climate (during the Pleistocene) wasconsiderably affected by changes in northern hemisphereclimate (Lourens et al., 1996a,b; Kroon et al., 1998 andreferences therein), when Arctic ice sheets expanded rapidly.

Trends in the abundance of selected taxa, observed in theopen ocean Atlantic Site 607, have been of major importancefor unravelling significant paleoceanographic changes withrespect to the semi-enclosed Mediterranean Sea. The lowamplitude fluctuations noted in most of the patterns, throughMIS 35–22, and the distinct higher amplitude variations fromMIS 21 upwards (Figs. 3–4), can be associated with the majorchanges inphysical and chemical features of thewatermasses,related to the shift toward more intense glacial conditions(Ruddiman et al., 1989). More unstable conditions in surfacewaters are already known during the acme interval of smallGephyrocapsa (small Gephyrocapsa Zone, MIS 38/37–29/25),which has been suggested as characterized by high produc-tivity and poorly stratified surfacewaters (Gartner et al.,1987).In the studied sections the high amplitude, short-term, shiftsof small placoliths and F. profunda (Figs. 5 and 8) occurringthrough MIS 24 to 22/21 indicate strong instability in thenutricline dynamics and in productivity of surfacewaters. Thispattern may be the beginning of significant changes in thecalcareous nannofossil assemblage as a response to the shifttowards much larger north hemisphere ice sheet at the Mid-Pleistocene Transition, at around 920 kyr (Ruddiman et al.,1989; Berger and Jansen,1994;Mudelsee and Schulz,1997). Atthis transition, the amplitude of sea levelfluctuations in glacialcycles increased up to 33m (Prell,1982; Ruddiman et al.,1989)and nutrient supply also increased from land and continentalshelves (Kitamura and Kawagoe, 2006), leading to higherprimary productivity (Broecker, 1982).

The minimum in diversity recorded in both sites at MIS 22and 22/21 (Figs. 9–10) and the corresponding dominance ofReticulofenestra spp. at Site 607 and of P. lacunosa at Site 967,may reflect r-selected life strategies in eutrophic conditions(Young, 1994). A positive relation between enhanced stabilityof water masses, an increase in diversity and switch to k-selection life strategies is already well known (Dodd andStanton,1981; Hallock,1987; Aubry,1992; Brand,1994; Young,1994; Bownet al., 2004). The dominance of taxa atMIS 22/21 isrecorded just above the minimum values in Atlantic δ13Crecorded in the benthic foraminiferal tests at the base of MIS22 (Kleiven et al., 2003; Raymo et al., 2004), possibly due alsoto the higher flux of organic matter at the sediment–waterinterface, during a major sea level fall or to the northwardexpansion of the deep Southern Ocean Water.

From MIS 21 upwards, the significant increase of warmoligotrophic taxa, mainly during interglacial cycles, and ageneral increase in diversity, seem to indicate the onset ofmore stable and stratified surface waters. This change in thenannofossil assemblage seems to support the hypothesis of a

Fig. 13. Comparison of the filtered 41-kyr components in selected calcareous nannofossil taxa (grey dotted line) at Site 607 (A) and at Site 967 (B), superimposed onthe original calcareous nannofossil data (solid line) and the δ18O stratigraphy (solid line in the leftmost column).


weaker production or suppression of North Atlantic DeepWater during glacials (Raymo et al., 1990; Raymo et al., 1997;Venz et al.,1999), whichmay have been responsible for amorestratifiedwater column, lower upwelling rate and oligotrophicsurface waters.

7. Concluding remarks

Calcareous nannofossil assemblages in the studied coresshow strong changes in abundance patterns that may reflectmodifications in surface waters. Although the abundances ofseveral taxa co-vary with glacial or interglacial cycles,particularly at Site 607, the Principal Component Analyses

suggest that in both analysed sections the primary change(Factor 1) in the assemblage is related to stratification andnutrient content of surface waters. The quasi-periodicoscillations in the paleoenvironmental proxy data arecontrolled by orbital cyclicity, especially obliquity (41-kyr).

The beginning of the calcareous nannofossil changes isobserved through MIS 24/22, and is marked by high am-plitude fluctuations in the abundance of small placoliths andF. profunda suggesting strong instability in the nutriclinedynamics. This is probably related to the transition from thedominant 41-kyr cyclicity to the 100-kyr cyclicity. Distinctchanges in siliceous and calcareous plankton compositionhave been also recorded during the MPR, at MIS 24–22 (Wang


and Abelmann, 2002; Zheng et al., 2005), as due to significantmodification in the surface water masses. At MIS 22/21, aminimum in diversity in both the sites and dominance ofReticulofenestra spp. at Site 607 and of P. lacunosa at Site 967are recorded, and are interpreted as r-selected life strategiesin more eutrophic conditions. From MIS 21 upwards, calcar-eous nannofossil changes suggest a trend towards morestable, oligotrophic and stratified surface waters which maybe related toweaker production of glacial NADW. This trend isclearer in the Atlantic section than in the Mediterranean site;it cannot be excluded that within the semi-enclosed Medi-terranean basin the interaction between several factors(salinity, nutrients, temperature, turbidity) and the rhythmicpaleoceanographic changes related to Pleistocene sapropeldeposition, may have played a more complex role in theabundance pattern of calcareous nannofossils. Preliminaryresults suggest that variations of few calcareous nannofossiltaxa at Site 967 recorded a change from the dominant 41-kyrto 100-kyr periodicity from about 850 kyr upwards.

Acknowledgements

The authors wish to thank the Ocean Drilling Program forproviding samples of the investigated sites. Careful reviews ofH. Kinkel, J. Young, J.A. Flores and an anonymous reviewer aregreatly acknowledged. This research was financially sup-ported by Fondi di Ateneo, Univerisità di Bari, N. Ciaranfi, 2004.

Appendix A. Ecological notes

In this section we review the available data on theautecology and paleoecological significance of the identifiedtaxa and compare these observations with the results of thepresent study.

C. leptoporus has been described as being characteristic oftropical to subtropical oligotrophic warmwater masses and ofupper and middle photic zones (McIntyre and Bè, 1967;McIntyre et al., 1970; Okada and Honjo, 1973; Blasco et al.,1980; Klejine, 1993; Winter et al., 1994; Flores et al., 1999;Baumann et al., 2004; Ziveri et al., 2004). In the easternMediterranean, co-variance of C. leptoporus and H. carteri hasbeen observed (Ziveri et al., 2000). In addition, someecological preference for eutrophic environments have beeninferred (Roth and Berger, 1975; Fincham and Winter, 1989;Andruleit and Rogalla, 2002; Flores et al., 2003). C. leptoporusis one of the most dissolution-resistant species (Schneider-mann, 1977) and the increase of its abundance in more fertileareas has been supposed to be related to selective dissolutionoccurring in organic-carbon rich sediments (Baumann et al.,2004). C. leptoporus has been shown to consist of differentsub-taxa (Knappertsbusch et al., 1997; Quinn et al., 2004),although their distribution is rather complex (Ziveri et al.,2004). No subdivision into sub-taxa was attempted in thisstudy. However, it is noteworthy that C. leptoporus is mainlyrepresented by specimens N8 μm in size, belonging to thelarge morphotype of Knappertsbusch et al. (1997). In thestudied sections, the ecological preferences of C. leptoporusare related to warmer, oligotrophic and stratified surfacewaters; the species loads positively on Factor 1 together withSyracosphaera spp. and Rhabdosphaera spp. at Site 967, andwith Umbilicosphaera spp. at Site 607.

Calciosolenia spp. The ecological affinity of calciosolenidshas not been extensively-documented. They have beendescribed as coastal or shelf taxa (Reid et al., 1978; Banse,1994; Andruleit and Rogalla, 2002; Malinverno et al., 2003),living in the middle photic zone (Jordan and Chamberlain,1997; Malinverno et al., 2003). Peaks in abundance of Cal-ciosolenia spp. have been recorded within Messinian sapropellayers (Flores et al., 2005). The taxon was significantly moreabundant in Site 967 than in the Atlantic Site 607; this patternmay have a relation with sapropel deposition according toFlores et al. (2005), and probablywith higher nutrient contentand turbidity of surface water, as suggested by Banse (1994)and Andruleit et al. (2003).

C. pelagicus s.l. has been often considered a cold-watertaxon (McIntyre and Bè, 1967; Roth and Coulbourn, 1982;Samtleben and Bickert, 1990; Samtleben et al., 1995;Andruleit, 1997). High nutrient concentration and moredynamic conditions from several upwelling areas have beenalso related to this taxon (Blasco et al., 1980; Giraudeau andBaley, 1995; Cachao and Moita, 2000; Boeckel and Baumann,2004). Recently, Baumann et al. (2000) and Geisen et al.(2002) documented the existence of two extant subspecies: C.pelagicus ssp. pelagicus (b10 μm long) and C. pelagicus ssp.braarudii (N10 μm long), the former being a subartic taxon andthe latter a temperate form, related to upwelling conditions(Baumann, 1995; Baumann et al., 2000; Cachao and Moita,2000; Parente et al., 2004; Narciso et al., 2006). More recentlya third subspecies C. pelagicus ssp. azorinus (N14 μm long) hasbeen differentiated (Parente et al., 2004) and related to theinfluence of water masses from the Azores front. In bothstudied sites these taxa are consistently rare, so they areclearly outside their primary distribution area. Nonetheless,the increase in abundances of C. pelagicus ssp. braarudiiduring MIS 16 (sites 607 and 967) and 18 (Site 607) maysuggest an ecological preference of the larger form for cold-water masses during the Mid-Pleistocene. The high abun-dance values of F. profunda during MIS 16 and 18 seems toexclude the occurrence of upwelling conditions.

F. profunda inhabits the lower photic zone (Okada andHonjo, 1973; Okada and McIntyre, 1977; Molfino andMcIntyre, 1990). Its distribution seems to be limited to waterswarmer than 10 °C (Okada and Honjo, 1973; Okada andMcIntyre 1979). Changes in the abundance patterns of thespecies have been explained in terms of changes in thermo-cline and nutricline dynamics and in primary productivity(Molfino and McIntyre, 1990; Castradori, 1993; Jordan et al.,1996; Beaufort et al., 1997; Ziveri and Thunell, 2000). Lowabundances of F. profunda are generally considered to be anindication of relatively high surface water productivity due toa shallow nutricline or upwelling. Higher values of the smallplacolith/F. profunda ratio have been used as an indication ofhigher productivity in the upper photic zone (Beaufort et al.,1997; Takahashi and Okada, 2000; Flores et al., 2000;Colmenero-Hidalgo et al., 2004). Hence, the higher abun-dance of F. profunda at Site 967 is likely to be a response to themore oligotrophic surface water in the eastern Mediterraneanwith respect to the Atlantic ocean; the anti-correlation of thespecies with abundance fluctuations of small placoliths inboth the Atlantic and Mediterranean sections, supports theinterpretation that the species is an indicator of deepnutricline and low productivity in surface waters.


Medium Gephyrocapsa. In the study sites medium Ge-phyrocapsa (N4 μm) are mainly composed of G. omega, whoseecological preferences are poorly known: a preference forwarm and low salinity waters and for high nutrient conditionshas been inferred in Pleistocene sediments (Maiorano andMarino, 2004). Data from the recent record indicate a warmwater preference for gephyrocapsids having high angle bridge(McIntyre et al., 1970; Bollmann, 1997; Takahashi and Okada,2000). Comparable environmental preferences are known forGephyrocapsa oceanica Kamptner (N3 μm); the species hasproved to be a warm water taxon both in the recent (Okadaand Honjo, 1973; Honjo and Okada, 1974; Geitzenauer et al.,1976; Okada and McIntyre, 1979; Kleijne et al., 1989;Giraudeau, 1992; Jordan and Chamberlain, 1997; Jordan andWinter, 2000; Hagino et al., 2000; Findlay and Giraudeau,2000; Di Stefano and Incarbona, 2004) and in the Pleistocenerecord (Thierstein et al., 1977; Gartner, 1988; Flores et al.,1999; Sprovieri et al., 2003). It prefers high nutrient contents(Winter, 1982; Mitchell-Innes and Winter, 1987; Gartner,1988; Houghton and Guptha, 1991; Kinkel et al., 2000; Corteset al., 2001; Andruleit and Rogalla, 2002; Boeckel andBaumann, 2004) and low salinity (Tanaka, 1991; Klejine,1993; Knappertsbusch, 1993; Jordan and Winter, 2000; DiStefano and Incarbona, 2004). G. oceanica is also considered alow latitude upwelling taxon (Mitchell-Innes and Winter,1987; Ziveri et al., 1995; Broerse et al., 2000; Andruleit et al.,2000). However, different ecological requirements are knownfor G. oceanica: high abundances are recorded in the upperPleistocene cold stages (Henriksson, 2000; Kinkel et al.,2000), and in stable waters (Gartner et al., 1987). G. oceanicatype 1 (large central area) of Hagino et al. (2000) may indicatewell-stratified oligotrophic waters. Moreover, coastal watersand neritic sea environments can be dominated by G. oceanica(Okada and Honjo, 1975; Zang and Siesser, 1986; Jordan et al.,1996; Cortes et al., 2001). In the studied sections mediumGephyrocapsa shows a positive relation with interglacialstages, suggesting a preference for warm surface waters.The lower abundance of this taxon in the eastern Mediterra-nean site suggests a preference for lower salinity and highernutrient conditions.

Helicosphaera spp. In the study sites Helicosphaera spp. ismainly represented by H. carteri. The genus is reported as acoastal water taxon (Okada, 1992; Giraudeau, 1992; Ziveriet al., 1995) with a preference for low nutrient and hightemperature waters (Haidar and Thierstein, 1997). H. carteri isknown to have affinities for warm water (McIntyre and Bè,1967; Gard and Backman, 1990; Brand, 1994; Baumann et al.,2005) and moderately elevated nutrient levels (Ziveri et al.,1995; Ziveri et al., 2000; Andruleit and Rogalla, 2002; Findlayand Giraudeau, 2002; Ziveri et al., 2004; Baumann et al.,2005). It seems to prefer also oligotrophic waters (Giuntaet al., 2003) and peaks of the species are recorded withinsapropel layers (Muller, 1985; Castradori, 1993; Negri et al.,1999; Negri and Giunta, 2001; Negri et al., 2003). Highabundances of H. carteri have been recorded during highproductivity episodes (Pujos, 1992; Flores et al., 1995) and inupwelling regions (Estrada, 1978; Giraudeau, 1992). Signifi-cant peaks of H. carteri are also observed during glacials 2–4(Flores et al., 1999). Lower salinity and turbid water pref-erences are indicated by Colmenero-Hidalgo et al. (2004). Thepaleoecological meaning of Helicosphaera carteri is therefore

highly variable. In the present study, the taxon is moreabundant in the eastern Mediterranean and shows a moredistinct pattern at Site 607 increasing in abundance fromMIS21 upward, together with several warm and oligotrophic taxa.

Oolithotus spp. The species O. fragilis and O. antillarum arereported as both lower and middle photic zone taxa (Okadaand McIntyre, 1977; Winter et al., 1994; Takahashi and Okada,2000). Oolithotus fragilis has been described as a warm waterspecies, with a preference for moderately high nutrient con-ditions (Roth and Coulbourn, 1982); Oolithotus antillarumseems to showaffinity to high nutrient concentrations (Zeltner,2000; Andruleit and Rogalla, 2002). The genus shows a positiverelation to interglacial stages, mainly at Site 607, suggesting awarmwater preference.

Small placoliths. In the studied sites, this group is mostlyrepresented by small Gephyrocapsa and rare Reticulofenestraspp. Small specimens of these genera are considered oppor-tunistic taxa of the upper photic zone (Gartner et al., 1987;Gartner, 1988; Okada and Wells, 1997), whose dominances insurfacewaters indicate upwelling areas (Gartner, 1988; Okadaand Wells, 1997; Takahashi and Okada, 2000) or othereutrophic conditions (Gartner et al., 1987; Gartner, 1988;Takahashi and Okada, 2000; Colmenero-Hidalgo et al., 2004;Flores et al., 2005). The extant species R. parvula is known tobe abundant in upwelling areas (Okada and Honjo, 1973) andduring late Quaternary high fertility periods (Biekart, 1989;Okada and Wells, 1997). Dominances of small placolithshave been recorded as coincident with decreases in diversity(Gartner et al., 1987; Okada and Wells, 1997). Small Gephyr-ocapsa are recorded as indicative of higher temperature insurface water (Colmenero-Hidalgo et al., 2004), and as abun-dant during interglacial stages (Flores et al., 1999; Henriksson,2000) or shortly after shifts towards significant glaciations(Gartner et al., 1987). High nutrient and lower temperatureconditions are also inferred for the small Gephyrocapsa eco-logical preference (Gartner, 1988). Moreover, the abundancechanges of small Gephyrocapsa and of small reticulofenestridshave been related to fluctuations of stability or stress in theenvironment and their increase indicates poorer stratificationof surface waters (Gartner et al., 1987). At the studied sites,small placoliths show a negative relationship with the abun-dance of F. profunda, mainly at Site 607, and higher abundancefluctuations through MIS 25–22/21, supporting ecologicalpreference of these taxa for high productivity and instabilityof surface waters.

P. lacunosa is an extinct placolith-bearing taxon, whichseems to prefer more eutrophic conditions and unstable en-vironment. (See Factor 1); its higher abundance in theMediterranean site may be interpreted as related to highervariability in salinity and nutrient content of surface waters.

Reticulofenestra spp. In the study sites Reticulofenestra spp.(N4 μm) include R. asanoi, Reticulofenestra sp. (sensuMaioranoand Marino, 2004) and R. minutula. The ecology of these taxais unknown; a preference for lower salinity water has beeninferred for R. asanoi and Reticulofenestra sp. in the earlyPleistocene record (Maiorano and Marino, 2004). Reticulofe-nestra spp. are placolith-bearing taxa, which, in the modernassemblage, dominate the coastal and upwelling areas andreflect r-selected life strategies in eutrophic conditions(Young, 1994). In the Messinian sediments, Flores et al.(2005) have related abundant R. pseudoumbilicus (N5 μm) to


eutrophic and turbulent conditions. In the studied sections,the ecological preferences for more eutrophic and turbulentconditions are inferred for Reticulofenestra spp. (Factor 1),according to recent data from Flores et al. (2005).

Rhabdosphaera spp. are mainly represented by R. claviger.This group is abundant in warm and low nutrient environ-ments (McIntyre et al., 1972; Roth and Berger, 1975; Pujos,1992; Haidar and Thierstein, 1997; Jordan and Winter, 2000;Sprovieri et al., 2003; Baumann et al., 2004). R. claviger lives inthe upper photic zone (Honjo and Okada, 1974; Winter et al.,1994; Takahashi and Okada, 2000). Data collected in thepresent study for this taxon are in agreement with prefer-ences for warmer, stratified and oligotrophic water masses.This pattern is clearer at the Mediterranean site where thetaxon is more abundant (Factor 1).

Syracosphaera spp., mainly Syracosphaera histrica and S.pulchra, are considered as warm and oligotrophic taxa(McIntyre et al., 1972; Roth and Coulbourn, 1982; Pujos,1992; Haidar and Thierstein, 1997; Jordan et al., 1996; Floreset al., 1999; Findlay and Giraudeau, 2000; Baumann et al.,2004; Ziveri et al., 2004). Positive relations between someSyracosphaera spp. and eutrophic environments have, how-ever, also been inferred (Estrada, 1978; Giraudeau, 1992), aswell as with lower salinity of surface water and increase interrigenous input (Weaver and Pujol, 1988; Flores et al., 1997;Colmenero-Hidalgo et al., 2004). The ecological preference ofthe taxa at both Site 607 and Site 967 appears to be for warm,stratified and oligotrophic waters (Factor 1).

Umbilicosphaera spp. Most of the dataset on this grouprefer to U. sibogae s.l. and its ecological preference appearsambiguous. It has been reported as a warmwater (Okada andMcIntyre,1977; Okada andMcIntyre,1979; Brand,1994;Wellsand Okada, 1996; Flores et al., 1999) and oligotrophic taxon(Giraudeau, 1992; Ziveri et al., 2004). However a positiverelation with moderate to high productive waters has beenalso described (Roth and Berger, 1975; Andruleit et al., 2000;Flores et al., 2003). According tomore recent data (Ziveri et al.,2004), Umbilicosphaera sibogae var. sibogae mostly occursunder oligotrophic conditions while U. sibogae var. foliosaprefers mesotrophic waters. In the studied sections theabundance pattern of Umbilicosphaera spp. (mainly U. sibogaevar. sibogae) indicates preference for warmer, oligotrophicand stratified surface waters.

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