Calcareous plankton and geochemistry from the ODP site 1209B in the NW Pacific Ocean (Shatsky Rise):...

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Calcareous plankton and geochemistry from the ODP site 1209B in the NW PacificOcean (Shatsky Rise): New data to interpret calcite dissolution and paleoproductivitychanges of the last 450 ka

Manuela Bordiga a, Luc Beaufort b, Miriam Cobianchi a, Claudia Lupi a,⁎, Nicoletta Mancin a, Valeria Luciani c,Nicola Pelosi d, Mario Sprovieri e

a Dipartimento di Scienze della Terra e dell'Ambiente, Università degli Studi di Pavia, Via Ferrata 1, 27100 Pavia, Italyb CEREGE, CNRS-Universités Aix-Marseille, Avenue Louis Philibert BP 80, 13545 Aix en Provence, Francec Dipartimento di Scienze della Terra, Università degli Studi di Ferrara, Via Saragat 1, 44100 Ferrara, Italyd Istituto Ambiente Marino Costiero (IAMC) - sede di Napoli, Calata Porta di Massa (Interno Porto di Napoli), Napoli, Italye Istituto Ambiente Marino Costiero (IAMC) - CNR, Via del Mare 3, 91021 Torretta Granitola (Trapani), Italy

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

Article history:Received 3 July 2012Received in revised form 6 December 2012Accepted 12 December 2012Available online 28 December 2012

Keywords:Calcareous nannofossilsPlanktonic foraminiferaShastky RiseDissolution indicesPrimary productivity indices

The high-resolution, multi-proxy investigation of microfossil and isotopic data from Shatsky Rise (ODP Site1209B, NW Pacific Ocean) is presented to evaluate the potential of calcareous nannofossil assemblages asdissolution and paleoproductivity proxies over the last 450 ka.To identify the best nannofossil index to evaluate dissolution (in particular, under polarized light microscope),we calculate and compare the different nannofossil and planktonic foraminiferal dissolution-indices from ouroriginal dataset. The results demonstrate that the most reliable and reproducible nannofossil dissolution indexis the Nannofossil Dissolution Index (NDI) proposed by Marino et al. (2009), particularly for records prior to250 ka.The NDI data from the studied Site 1209B represent evidence of preservation maxima mainly during degla-ciations, whereas dissolution peaks are recorded at the onset of glacial phases or during severe interglacials.These fluctuations are demonstrated to be basin-wide features in the Pacific. The synchronous timing in thefluctuations of the preservation indices, which consistently lagged behind the oxygen isotope cycles, clearlydemonstrates the basin-wide changes in the ocean chemistry during the glacial–interglacial transitions. TheMid-Brunhes Dissolution Event, which was recorded in the Western Pacific at depths below the lysocline atthe Marine Isotope Stage (MIS) 11, is not detectable at our relatively shallower site.At the studied site, the intervals of high productivity generally coincide with the time of good preservationand light carbon-isotope values and vice versa. Therefore, carbonate undersaturation and changes in oceanchemistry (carbonate ion concentration) rather than the variations in the organic carbon flux appear tohave controlled the pattern of CaCO3 preservation.In addition to the characterization of the dissolution proxies, changes in the calcareous nannofossil assem-blages were used to evaluate the primary productivity fluctuations at the mid-latitudes of the NW Pacificover the last 450 ka. The results highlight a general decrease of paleoproductivity during the entire timeperiod as well as shorter glacial/interglacial fluctuations from the base-core upwards. We interpret thesefeatures to be variations in the thermocline and nutricline dynamics related to the northward migration ofthe Kuroshio Extension, which was triggered by the Mid-Brunhes Event and may have caused deepeningof the thermocline/nutricline at the site. The spectral and wavelet analyses performed on the microfossil da-tabase prove that the variations in paleoproductivity and carbonate dissolution over the last 450 ka were pri-marily driven by the glacial–interglacial variability (100 ka periodicity) and by the obliquity-controlledchanges.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

The evaluation of primary productivity in the surface waters andcalcite preservation in the deep sea sediments are very importantfor estimating the ocean buffering mechanism that regulates atmo-spheric carbon dioxide because the biotic pump removes CO2 from

Palaeogeography, Palaeoclimatology, Palaeoecology 371 (2013) 93–108

⁎ Corresponding author. Tel.: +39 0382985895; fax: +39 0382985890.E-mail address: [email protected] (C. Lupi).

0031-0182/$ – see front matter © 2012 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.palaeo.2012.12.021

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the surface oceans and the deep-sea carbonates contain the largestreservoir of CO2. Carbonate dissolution mainly occurs below thelysocline due to the undersaturation of the deep and pore waters.However, according to some authors, approximately 25% of CaCO3

produced in the surface waters undergo dissolution at depthsshallower than the lysocline (Milliman, 1993; Milliman et al., 1999;Schiebel et al., 2007) in response to the degradation of organic carbongenerating more acidic seafloor and interstitial waters during times ofenhanced productivity (Emerson and Bender, 1981; Weaver et al.,1998).

A set of proxies that are well constrained and correlated to eachother is a fundamental tool for understanding the current and pastmarine carbon cycle; among these proxies, of particular importanceare the calcite dissolution and the primary productivity indices.

Several dissolution indices have been proposed to indicate pastchanges in the deep-sea carbonate ion concentration. Some of theseindices are based on calcareous micro-organisms, particularly theplanktonic foraminifera (e.g., Dittert and Henrich, 2000; Conan etal., 2002; Mekik et al., 2002, 2010; Loubere and Chellappa, 2008).The ratio of dissolution prone to dissolution resistant planktonic fora-miniferal shells (Berger, 1970) or the ratio of the foraminiferal frag-ments to whole shells (Peterson and Prell, 1985; Wu and Berger,1989) is extensively used for dissolution evaluation. Furthermore,primary productivity has been estimated using various geochemicaland micropaleontological methods such as the planktonic and ben-thic carbon isotope gradients (Sarnthein and Winn, 1990) as well asindices based on various microfossil taxa, such as the nannofossil spe-cies, Florisphaera profunda (Beaufort et al., 2010). To obtain detailedquantitative data on the F. profunda abundance, the classic light mi-croscope analyses are combined with the SYRACO program, which isan automatic nannofossil recognition method (Beaufort and Dollfus,2004).

Calcareous nannoplankton, which is marine phytoplankton, playsan important role in the carbon cycle because it is an important com-ponent of the biological pump. The calcareous nannoplankton usesCO2 to build its soft tissues, which sink to the bottom of the seaafter its death. Moreover, the calcareous nannoplankton utilizesboth dissolved inorganic carbon and calcium to build its tests.

Several dissolution and paleoproductivity indices have been re-cently proposed on the basis of calcareous nannofossils althoughthere is no agreement on their reliability. Moreover, the planktonicforaminiferal shells are more sensitive to dissolution than coccoliths(Chiu and Broecker, 2008), and methods to identify deep-sea dissolu-tion are relatively better constrained.

To verify the significance and the reliability of the calcareousnannofossil dissolution and the productivity indices, we used newmicropaleontological and geochemical data from the last 450 ka sed-imentary record of the core ODP Leg 198-1209B. This core was recov-ered at a 2387 mwater depth from the Shatsky Rise in the NW PacificOcean (Fig. 1). The Pacific Ocean, in recent as well as ancient times,has played an important role in the thermohaline circulation, and ithas also affected the earth's climate in several ways, such as throughthe El Niño-Southern Oscillation, the Asian Monsoons, the local cur-rent (Oyashio and Kuroshio currents) and/or its ability in transferringcarbon dioxide from/to the deep ocean to/from the atmosphere (Qiu,2001; Zhang et al., 2007). Therefore, the evaluation of the dissolutionand productivity variations through time in the NW Pacific could helpin the reconstruction of the fluctuation of the saturation state of thedeep waters and in the identification of the important changes inthe ocean chemistry, which in turn, have influenced the role of thisocean sector in the carbon cycle, and therefore, in the climate systemduring the last 450 ka.

The main goals of the paper are as follows: (i) to evaluate and cor-relate the different nannofossil dissolution-indices with the indicesthat are largely used based on planktonic foraminifera to identifythe best nannofossil dissolution index (particularly in polarized light

microscope); (ii) to present the paleoproductivity indices inferredby calcareous nannofossils and geochemical data to evaluate thepaleoproductivity changes at the site over the last 450 ka; and(iii) to interpret the relationship between the dissolution, primary pro-ductivity and glacial–interglacial cycles during this significant intervalby integrating the methods based on the two major components ofthe calcareous plankton, on the stable carbon and oxygen isotope dataand on the spectral andwavelet analyses of the dissolution and produc-tivity time series.

2. Regional setting

The ODP core 1209B (32° 39.1081′ N and 158° 30.3564′ E) wasdrilled at a water depth of 2387 m on the Shatsky Rise, which is a pla-teau located in the NW Pacific Ocean 1600 km eastward from Japan(Fig. 1A). The area is sensitive to climate changes because it is locatedin a transitional zone between the subtropical and the subarctic gyres(Thompson and Shackleton, 1980; Thompson, 1981; Sancetta andSilvestri, 1986; Haug et al., 1995; Thunell and Mortyn, 1995; Kawahataet al., 1999). Currently, the surface oceanography is dominated by theinflow of the subtropical warm and oligotrophic Kuroshio Current fromthe south, and by the inflow of the subarctic, cold and eutrophic OyashioCurrent from the north (Kawahata et al., 1999; Qiu, 2001; Kawahata andOhshima, 2002).

The subtropical Kuroshio Current is the western boundary currentof the northern hemisphere of the Pacific Ocean; it originates fromthe North Equatorial Current and continues northward east of Japan.Near 35° N, the flow of the Kuroshio Current shifts eastward, asKuroshio Extension, persisting for approximately 1200 km into theopen Pacific environment. The northern margin of the Shatsky Riseis located just below the Kuroshio Extension (Fig. 1A).

The subarctic inflow is dominated by the Oyashio Current, whichis a subpolar gyre and runs southward from the east coast of Kamchatkato Japan. Here the Oyashio Current is opposed by the Kuroshio Current;the two currents maintain their own frontal system (Polar Front)along the Kuroshio Extension, producing eddies and intense upwell-ing. The climatic conditions and primary productivity on the ShatskyRise are strongly influenced by the relative positions of these twocurrents.

At 300–600 m and up to 1000 m of water depth below theKuroshio Extension, the North Pacific Intermediate Water (NPIW)originates from the subduction along the Polar Front. Subsequently,it injects itself into the subtropical gyres (Reid, 1965), producingstrong vertical gradients in temperature and salinity, with an averagesalinity of ~34‰ at approximately 1000 m depth (Fig. 1B) (Braloweret al., 2002).

Below 2000 m, the Pacific waters attain the uniform properties ofthe Pacific Deep Waters. Currently, access of the Arctic Bottom Wateris prevented by the shallow Bering Strait (Warren, 1983; Keigwin,1987; Broecker, 1995). Therefore, the deep basins of the PacificOcean are filled from the southwest by the Deep Western BoundaryCurrent with a mixture of both the Antarctic Bottom Water and theNorth Atlantic Deep Water (Millero, 1996). The deep water mass,which is called the Circumpolar DeepWater (CPDW), is characterizedby cold, saline, oxygen-rich and silicate-poor waters (Owens andWarren, 2001). The regional calcite lysocline and compensationdepth (CCD) are recorded at 2900 m (Vincent, 1975) and from 4000to 4500 m (Berger and Winterer, 1974), respectively.

3. Material and methods

3.1. The ODP core 1209B

The ODP core 1209B was recovered from the Southern High of theShatsky Rise, which is the most elevated and central sector of the area(Fig. 1A). This site displays the most complete stratigraphic sequence

94 M. Bordiga et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 371 (2013) 93–108


among those cored in this area. The entire core consists of 297.6 m ofnannofossil ooze, clayey nannofossil ooze and nannofossil ooze withclay and covers a stratigraphic interval fromUpper Cretaceous to Recent(Bralower et al., 2002). Minor components throughout the sequenceinclude foraminiferal, diatom and radiolarian oozes.

The present study involves the upper 6 mbsf (meters below seafloor) of the succession consisting largely of nannofossil ooze andnannofossil ooze with clay.

3.2. Calcareous nannofossil analyses

3.2.1. Light microscope (LM) analysis: sample preparation and countingmethod

A total of 59 samples were analyzed for their calcareous nannofossilcontent (every 10 cm down core) from 6 mbsf to the core-top. Thesamples were prepared using the settling technique described by deKaenel and Villa (1996). The settled slides were analyzed under a polar-ized light microscope at 1000× magnification. The species abundancewas quantified; for each sample, all the specimens were counted infive fields of view (corresponding to 0.035 mm2). The counts were suc-cessively converted from N/mm2 to percentage relative abundance ofeach of the identified species of calcareous nannofossils. Both thewhole and separated shields of Calcidiscus leptoporus coccoliths werecounted to quantify one of the preservation indices discussed in thispaper.

Taxonomy of the calcareous nannofossils follows Hine andWeaver (1998), Young (1998), Young et al. (2003) and the referencecontained in the web-site http://nannotax.org (edited by Young et al.,2011).

3.2.2. The SYRACO program: sample preparation and analysesThe 59 samples were re-prepared using the standard smear slide

technique (Backman and Shackleton, 1983; Bown and Young, 1998)and subsequently analyzed through a polarized optical microscopethat is used for the automatic scanning of slides under cross-polarized light. This additional sample preparation was required toanalyze the samples with the SYRACO program. In fact, the automaticscanning microscope works well with smear slides, which have alsobeen employed by other authors for performing similar analyses(e.g., Beaufort et al., 2010). The microscope stage motions and focusare computer-controlled. For each sample, 200 images were acquiredby a 2-megapixel Spot Insight (diagnostic instrument) camera. Thecoccoliths were automatically detected by software developed inC++ at CEREGE, namely, SYRACO2 (SYstème de ReconnaissanceAutomatique de COccolithes, version 2; Dollfus and Beaufort, 1999;Beaufort and Dollfus, 2004). This program is based on an artificialneural network (ANN) and is able to recognize unique patterns. Weused SYRACO in two steps as described in Beaufort and Dollfus(2004). In the first step, SYRACO extracted all the coccolith speciesusing a primary ANN. Subsequently, in the second step, the software

Fig. 1. A) Location of the site ODP 198-1209B on the Shatsky Rise. Current trajectory of the Kuroshio Extension and Oyashio Current as well as the position of the Subtropical and theSubarctic Fronts are reported (modified from Kawahata and Ohshima, 2002). B) Topographic and hydrographic profile of salinity from Ocean Data View (Schlitzer, 2012, http://odv.awi.de) and the major water masses (Reid, 1965; Owens and Warren, 2001). The location of the site ODP 198-1209B (2387 m depth) is also indicated. NPIW: North Pacific Inter-mediate Water; CPDW: Circumpolar Deep Water; SAF: Subarctic Front; STF: Subtropical Front.

95M. Bordiga et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 371 (2013) 93–108


used specific ANNs trained to recognize individual species and sepa-rate coccoliths from unwanted objects. During the final step, imagesof the coccoliths were extracted and imported into species-specificimage files.

The goal of SYRACO is to classify the objects into classes. The clas-ses may be coccolith species, groups of coccolith species, sub-speciesof coccolith or non-coccolith objects. In this study, coccoliths weresubdivided into classes that grouped coccoliths with similar shapesand sizes: Emi (shape similar to Emiliania huxleyi), Gee (similar toGephyrocapsa ericsonii), Gem (similar to Gephyrocapsa muellerae),Geo (similar to Gephyrocapsa oceanica), FP (similar to Florisphaeraprofunda), and Sys (similar to Syracosphaera spp.). The program alsoreturns the number of individuals recognized for each class, which al-lows the calculation of the relative percentage of each individual.Furthermore, the specimens detected in each sample are then charac-terized by data concerning their average length, mass (pg) and area.These morphometric measurements were obtained from the ANN se-lected images using the software described in Beaufort et al. (2008)and with the principals described in Beaufort (2005).

The SYRACO software has the advantage of collecting a largeamount of images, and therefore, fields of view in a fairly short timewith respect to an operator. During routine work, the SYRACO soft-ware can recognize approximately 96% of the coccoliths belongingto 11 Pleistocene taxa (Beaufort and Dollfus, 2004). Moreover, thisprogram is able to correct the error derived from the misidentificationof non-coccoliths as coccoliths by the statistical estimation of the in-vaders in a sample. Moreover, we have considered that the SYRACOsystem is not able to identify the bridge in the coccolith centralarea that is detectable only by using a rotating stage (Grelaud etal., 2009). Therefore, some small species belonging to the genusGephyrocapsa are not easily recognizable by the SYRACO software.Nevertheless, in the formula used for calculating paleoproductivity(Table 1), identification of the different species of the small sizedGephyrocapsa is not required. Conversely, Florisphaera profunda,which is the main paleoproductivity indicator (Table 1), is fairlywell identified by the SYRACO software because of its unique shapeand its birefringence features. Nevertheless, human verification ofthe images collected by the automatic microscope was performed toconfirm the species component of the SYRACO groups.

3.3. Calcareous nannofossil dissolution indices

Several indices have been proposed in the literature to evaluatepreservation based on both the calcareous nannofossil and planktonic

foraminiferal assemblages (Table 1). Most of the calcareousnannofossil indices are derived from two basic indices that were pro-posed several years ago. These indices consider the entire range ofcoccoliths from Calcidiscus leptoporus, a dissolution-resistant speciesbuilt of larger and strongly calcified coccoliths (Schneidermann,1977), with respect to its single shield (Matsuoka, 1990) or to moredelicate coccoliths of other taxa (Dittert et al., 1999).

Subsequently, Marino et al. (2009) proposed the NannofossilDissolution Index (NDI) that considers the ratio between the proxi-mal and the distal shield abundances of Calcidiscus specimens withrespect to the entire placoliths of the same taxon (Table 1). Thisindex attains high values for excellent preservation. More recently,Amore et al. (2012) proposed another dissolution index, i.e., DI, basedon the ratio of easily dissolved to more resistant forms.

Dittert et al. (1999) compare the abundance of Emiliania huxleyi,which produces very delicate coccoliths, and the abundance ofCalcidiscus leptoporus proposing the E. huxleyi–C. leptoporus dissolu-tion index (CEX) (Table 1). For this latter index, high values are indica-tive of good assemblage preservation. The CEX validation was obtainedby comparison with a dissolution index based on the foraminiferal testultrastructure by Dittert et al. (1999). The CEX index was subsequentlymodified by Boeckel and Baumann (2004) by including other fragilespecies, such asGephyrocapsa ericsonii (CEX′), to improve the investiga-tion of the late Quaternary, when E. huxleyiwas not as abundant as it iscurrently (Table 1).

Because it is very difficult to recognize Emiliania huxleyi under alight microscope, Lόpez-Otálvaro et al. (2009) used CEX′ but withthe addition of small Reticulofenestra, a taxon also represented bydelicate species. For this study, however, we did not consider the sug-gestion of Lόpez-Otálvaro et al. (2009) because the percentage ofReticulofenestra was below 1% of the assemblages. For this reasonand also because our observations are based on a light microscopedataset, we calculated different dissolution indices (NDI, CEX, CEX′)from those proposed in the literature. Additionally, we partially mod-ified the CEX and CEX′ indices by including the species Gephyrocapsaaperta (Table 1; CEX+G. aperta and CEX′+G. aperta). Emilianiahuxleyi and the small Gephyrocapsa belong to the family of theNoelaerhabdaceae whose members have been subjected to substan-tial phylogenetic development since the early Pliocene (Thiersteinet al., 1977). They show similar coccolith size and structure, andtherefore, in our opinion they might display similar dissolution be-havior. To validate and correlate the different preservation indiceswe plotted them in pairs and then we calculated their Pearson'scorrelation coefficients (r).

Table 1Summary and formulas of the main nannofossil and foraminiferal dissolution and productivity proxies used in the text. For abbreviations see text.

Preservation indices

Index Formula Authors

Nannofossil NDI % entire placoliths C: leptoporus% entireþ% distalþ% proximal placoliths C: leptoporus Marino et al. (2009)

CEX % E: huxleyi% E: huxleyiþ% C: leptoporus Dittert et al. (1999)

CEX′ % E: huxleyiþ% Gephyrocapsa% E: huxleyiþ% G: ericsoniiþ% C: leptoporus Boeckel and Baumann (2004)

CEX (+ G. aperta) % E: huxleyiþ% G: aperta% E: huxleyiþ% G: apertaþ% C: leptoporus This work

CEX′ (+ G. aperta) % E: huxleyiþ% G: ericsoniiþ% G: aperta% E: huxleyiþ% G: ericsoniiþ% G: apertaþ% C: leptoporus This work

Foraminifera FI% number of fragments=8number of fragments=8ð Þþnumber of whole Hayward et al. (2004)

MFI% 100×{Dm/(Dm+Wm)} Mekik et al. (2002, 2010)TFI% 100×{Dt/(Dt+Wt)} Loubere and Chellappa (2008)

Productivity indices

Index Formula Authors

Nannofossil %Fp 100×FP/(FP+SMP+GEO) Beaufort et al. (2010)Foraminifera (Δ δ13Cp–b) δ13Cp–δ13Cb Sarnthein and Winn (1990)



3.4. Planktonic foraminiferal preparation and dissolution indices

The 59 samples collected for the analysis of planktonic foraminif-era, were prepared following standard procedures. Approximately3 cm3 of dried sediment was washed and sieved through two sieveswith meshes of 150 and 63 μm. The washed residuals were thendried in an oven at approximately 40 °C.

The whole >150 μm fraction was quantitatively analyzed througha stereoscopic microscope to determine three different dissolutionindices: the fragmentation index (FI%), the Globorotalia menardiifragmentation index (MFI%) and the Globorotalia truncatulinoidesfragmentation index (TFI%).

The FI percentage was estimated using the formula of Le andShackleton (1992) that was partly modified by Hayward et al. (2004)and assuming that one fragment is a planktonic foraminiferal portionwith less than three whole chambers (Table 1). The high FI% valuescorrespond to poor carbonate preservation at the sea floor.

The MFI% was calculated following the formula of Mekik et al.(2002; 2010) where the MFI% represents the relative percentage ofthe number of damaged specimens (Dm) to the number of wholeundamaged (Wm) plus damaged specimens of Globorotalia menardiiwithin a sediment aliquot (in this case, the whole and not partial>150 μm fraction) (Table 1). The high MFI% values correspond topoor carbonate preservation at the sea floor.

The TFI% has been proposed by Loubere and Chellappa (2008) asa promising mid to higher latitude quantitative proxy for carbonate dis-solution. The authors demonstrated that Globorotalia truncatulinoidesfragments follow a linear and progressive fragmentation response to in-creased degrees of bottomwater undersaturation, in a manner very sim-ilar to that experienced by Globorotalia menardii. Similar to the MFI%, theTFI% was calculated as the relative percentage of the number of damagedspecimens (Dt) to the number of whole undamaged (Wt) plus damagedspecimens of G. truncatulinoides in a sediment aliquot (in this case thewhole and not partial >150 μm fraction) (Table 1). The high TFI% valuescorrespond to poor carbonate preservation at the sea floor.

For both the MFI% and TFI% indices, species fragmentation wasquantified by grouping the shells into 4 categories, namely, A, B, Cand D: A—intact (whole), B—holed, C—greater than half a shell andD—less than half a shell. The count was performed only on particlesthat could be unambiguously identified as Globorotalia menardii andGloborotalia truncatulinoides species.

3.5. Productivity indices

The relative abundance of Florisphaera profunda has been frequentlyused to evaluate primary productivity fluctuations (Table 1). This spe-cies inhabits the lower photic zone (Okada and Honjo, 1973; Okadaand McIntyre, 1977; Molfino and McIntyre, 1990), and changes in itsabundance patterns have been explained in terms of variations in thethermocline–nutricline dynamics, water turbidity and primary produc-tivity (Molfino and McIntyre, 1990; Ahagon et al., 1993; Castradori,1993; Jordan et al., 1996; Beaufort et al., 1997; Ziveri and Thunell,2000; Fernando et al., 2007; Zhang et al., 2007; Incarbona et al., 2008;Liu et al., 2008; López-Otálvaro et al., 2008). Where the nutricline isdeep, the total primary productivity is low and the nannoplankton as-semblage is dominated by F. profunda. Conversely, when high surfacewater productivity increases in response to a shallower nutricline or up-welling events, the relative abundance of F. profunda decreases.

In thiswork, we used the Florisphaera profunda index (%Fp) proposedby Beaufort et al. (2010):

%Fp ¼ 100� FP= FPþ SMPþ GEOð Þ;

where FP, SMP and GEO are the number of F. profunda, small placoliths(essentially Emiliania huxleyi Emi, with Gephyrocapsa ericsonii Gee),and Gephyrocapsa oceanica, respectively.

In the studied core, the counting performed by the SYRACO softwaresubdivided the assemblages principally into theGephyrocapsa, Emilianiaand Florisphaera genera, which represent 90% of the coccolithophoreassemblages. Therefore, the %Fp was calculated using only these taxaas in Beaufort et al. (2010). High values of %Fp correspond to a wellstratifiedwater column, deep thermocline, low turbidity and lowprimaryproductivity.

Finally, the difference between planktonic and benthic foraminiferalcarbon isotope values (Δδ13Cp–b) (Sarnthein and Winn, 1990) wasused as a further semi-quantitative paleoproductivity proxy. It providesinformation about the surface to deep water δ13C gradient and thestrength of the biological pump, thus reflecting surface paleoproductivity(e.g., Zhang et al., 2007).

The carbon isotope analyses were carried out on isolated specimensof Globoconella inflata, Uvigerina peregrina and Cibicides wuellerstorfi(see the section below for more details), respectively. High values ofΔδ13Cp–b indicate high productivity.

3.6. Stable isotope analysis

The oxygen and carbon isotope analyses were carried out on isolatedspecimens of both the planktonic foraminiferal species Globoconellainflata and the endobenthic species Uvigerina peregrina from the>150 μm fraction. In the samples where U. peregrina was absent, thebenthic species Cibicides wuellerstorfi was isolated and analyzed tocomplete the isotopic record.

A total of 58 samples for the planktonic (from 0 to 6 mbsf) and 38for the benthic foraminifera (from 0 to 3.7 mbsf) were analyzed withan average sampling interval of 10 cm. Samples were ultrasonicatedand heated at 370 °C under vacuum, prior to being measured by theautomated continuous flow carbonate preparation GasBenchII device(Spötl and Vennemann, 2003) and a ThermoElectron Delta Plus XPmass spectrometer at the IAMC-CNR (Naples, Italy) isotope geochem-istry laboratory. Acidification of the samples was performed at 50 °C.An internal standard (Carrara Marble with δ18O=−2.43 vs. VPDBand δ13C=2.43 vs. VPDB) was run every 6 samples and the NBS19 in-ternational standard, after every 30 samples. The standard deviationsof δ18O and δ13C isotope measures were estimated on the basis of ~20repeated samples. The oxygen isotope data derived from Cibicideswuellerstorfi were calibrated through the method of McCave et al.(2008) after adding a 0.64 factor to uniform the C. wuellerstorfi tothe Uvigerina peregrina values. Moreover, to account for the ‘vitaleffect’, in the infaunal species U. peregrina the δ13C values wereadjusted by +0.9‰ (δ13CCib-equivalent) (Shackleton and Hall, 1984).These isotope data were reported in per mil (‰) relative to theVPDB standard.

3.7. Carbonate content

The carbonate content of the 59 studied samples was measuredthrough a simple determination in 0.50 g of sediment (Müller andGastner, 1971). The CaCO3 in each sample was determined by measur-ing the increase in gas pressure caused by acidifying each dried sedi-ment sample in a closed vessel. The volume of the produced gas wasmeasured in a graduated burette and converted to carbonate mass inrelation with the volume of CO2 produced by a known mass of pureCaCO3. The CaCO3 content was expressed as percentage weight (%wt).

3.8. Spectral analysis

To identify the significant frequencies and their evolution withtime in the ODP 1209B nannofossil dissolution and primary produc-tivity signals (NDI, FI%, %Fp), we performed the following analysis:

i) spectral analysis for unequally sampled signals using the REDFITsoftware (Schulz and Mudelsee, 2002); and



ii) evolutionary power spectra using Foster's wavelet analysis algorithm(Foster, 1996a, b, c).

The “REDFIT” power spectrum program, based on a modifiedversion of the Lomb–Scargle periodogram (Lomb, 1976; Scargle,1982), was used to search the possible periodicities recorded inour non-uniformly sampled and red-noised signals. To model thered-noise automatically, the REDFIT program produces first-orderautoregressive (AR1) signals with sampling times and characteristictime-scales matching those of the real data without requiring interpo-lation. The estimated AR1 model, transformed from the time domaininto the frequency domain and compared with the spectrum of thesignal, allowed the estimation of the upper confidence interval of theAR1 noise for various significance levels based on an x2 distributiontest-hypothesis.

The wavelet analysis is most likely the best recent solution toanalyze non-stationary signals (frequency changes along time) thatsolve the signal-cutting problem. The wavelet is shifted along the sig-nal and the spectrum is calculated for every position. This process isrepeated several times with a slightly shorter (or longer) waveletfor each new cycle. In the end, the result is a collection of time-frequency representations of the signal, all with different resolutions(multiresolution analysis).

To handle the irregularly sampled signals, we need an extension ofthe classic wavelet formalism. Such extensions were developed byFoster (1996c), which defines the Weighted Wavelet transform(WWZ) and the Weighted Wavelet Amplitudes (WWA) as a suitableweighted projection of the three basic functions (real and imaginarypart of the Morlet wavelet and a constant), orthogonalized by rotatingthe matrix of their scalar products. Furthermore, the author also usedthe statistical F-tests to distinguish between the periodic componentsand a noisy background signal.

4. Results

4.1. Stable isotopes and age model

The oxygen and carbon isotope curves, calibrated through pub-lished (Lupi et al., 2012) and novel biostratigraphic data, weretuned to the orbitally calibrated reference stack LR04 of Lisiecki andRaymo (2005) (Fig. 2), with the AnalySeries program (Paillard et al.,

1996). The tie points used were few nannofossil biostratigraphicdata (cross-over Gephyrocapsa caribbeanica and Emiliania huxleyi;LO of E. huxleyi) and the mid-points of the glacial–interglacial phasesof each 100-ka cycle (Lisiecki and Raymo, 2005).

The obtained age model (Fig. 2) demonstrates a continuous sedi-mentary succession for the uppermost 6 m of the site, which extendsfrom the late Holocene, Marine Isotope Stage (MIS) 1 to MIS 12 atb450 ka. The sedimentation rate remains relatively constant throughoutthe core with an average sediment accumulation rate of 1.36 cm/ka. Thesampling resolution of the record is 7.36 ka.

The δ18O values measured on the planktonic specimens (δ18Op)demonstrate 3.0 to 0‰ excursions from the base to the top of the re-cord with short-term fluctuations at the glacial–interglacial scale(Fig. 2). The oxygen isotope curve measured on benthic specimens(δ18Ob) is available from MIS 8 to MIS 1 and does not show a markedlong-term excursion. Instead, the record demonstrates only short-termfluctuations ranging from 3.5 to 5.9‰, which correspond to the glacial/interglacial variability with theminima centered atMIS 7 and 1 and themaxima at MIS 8, 6 and 2, respectively.

The carbon isotope values measured on the planktonic specimens(δ13Cp) demonstrate a 1.2 to 0‰ negative excursion from MIS 12 toMIS 7 followed by a rapid positive shift, which reaches averages of1.0‰ at MIS 5. A further maximum is documented at MIS 9, after alightening of the carbon isotope values at MIS 11. In addition to theorbital scale variability, the benthic δ13C provides evidence of along-term positive excursion from the base to the top of the studiedrecord with a mid-term minimum at MIS 8 and a maximum at MIS4/3 transition, respectively.

4.2. Dissolution indices

4.2.1. Calcareous nannofossilsA high nannofossil total abundance, with a mean value of

18,447 N/mm2, is documented in the lower portion of the studied inter-val betweenMIS 12 andMIS 9 (Fig. 3). FromMIS 8 up to the core-top, thenannofossil abundance displays a marked decrease, reaching a meanvalue of 6200 N/mm2. The curve of the entire Calcidiscus leptoporuscoccoliths displays high amplitude variations from MIS 8 upward, withhigher values during the glacials. Conversely, the occurrence of separatecoccoliths appearsmore frequent during some interglacials (MIS 7,MIS 5and MIS 1).

Fig. 2. Age model for the site ODP 198-1209B, based on calcareous nannofossil biostratigraphy (Lupi et al., 2012), magnetostratigraphy (Bralower et al., 2002) and stable oxygenisotope statigraphy (planktonic and benthic foraminifera, this work). The oxygen isotope records are tuned to the orbitally calibrated reference δ18O benthic stack LR04 of Lisieckiand Raymo (2005). Circles identify the tie points, selected glacial–interglacial transition of each 100 ka cycles (Lisiecki and Raymo, 2005) and selected nannofossil events. Theplanktonic and benthic carbon isotope records, plotted against the core age model, are also reported. Numbers are for glacial–interglacial marine isotope stages. Glacial phasesare shaded.



The relative abundances of the species demonstrated in Figs. 3 and 4are the same as those used in the dissolution index formulas (Table 1).Among these, only Emiliania huxleyi is not distributed throughout the

entire studied interval because the distribution is stratigraphicallycontrolled. The mid-term pattern of E. huxleyi is most likely evolutivelycontrolled because it increases in abundance from MIS 4 when the

Fig. 3. ODP 198-1209B nannofossil total abundance and relative abundances of selected species. Planktonic and benthic oxygen records and δ18Ob stack LR04 (Lisiecki and Raymo,2005) are reported as reference. Numbers indicate glacial–interglacial marine isotope stages; glacials are shaded.

Fig. 4. Light microscope (LM) and Scanning Electron Microscope (SEM) micrographs of calcareous nannofossil species cited in the text (2, 3, 4, 6, 8, 10 and 12). 1. Calcidiscusleptoporus, 3.4 mbsf, SEM. 2. C. leptoporus, proximal shield, 1.1 mbsf, LM Cross Nicols (XN). 3. C. leptoporus, distal shield, 1.1 mbsf, LM XN. 4. C. leptoporus, entire shield,1.1 mbsf, LM XN. 5. Emiliania huxleyi, 0.3 mbsf, SEM. 6. E. huxleyi, 1.4 mbsf, LM XN. 7. Gephyrocapsa aperta, 1.8 mbsf, SEM. 8. G. aperta, 1.8 mbsf, LM XN. 9. Gephyrocapsa ericsonii,4.4 mbsf, SEM. 10. G. ericsonii, 4.4 mbsf, LM XN. 11. Florisphaera profunda, 0.3 mbsf, SEM. 12. F. profunda, 1.2 mbsf, LM XN.



E. huxleyi/Gephyrocapsa cross-over is documented. The fluctuations inabundance of E. huxleyi do not show clear relationships with theglacial–interglacial cycles. Gephyrocapsa ericsonii displays a high abun-dance during the interglacials up to MIS 9 and during the glacials fromMIS 8 upward, and Gephyrocapsa aperta is rare up to MIS 6, where itabruptly increases, recording the highest values during MIS 6, 4 and 2.

The dissolution index (NDI) provided by the calcareous nannofossildata (Fig. 5A) displays short term fluctuations through the record.Higher values (good preservation) frequently occur during some glacialphases, such as MIS 10, MIS 8 and MIS 6, as well as in some interglacialphases, such as MIS 11 and the upper part of both MIS 5 and MIS 3. A

high dissolution rate is mainly recorded at the upper part of the inter-glacials (top: MIS 11, MIS 9, MIS 7, MIS 5).

The CEX and CEX′ indices (high values for excellent preservation,Fig. 5A) are calculated from the Lowest Occurrence (LO) of E. huxleyi(MIS 8) up to the top-core. These indices show a similar trend relatedto the glacial–interglacial cycles, both recording better preservationduring the glacials. However, the indices display a dissimilar recordin the lowermost portion of the interval where CEX′ documents betterpreservation compared to CEX.

The CEX and CEX′ with Gephyrocapsa aperta (Fig. 5A; Table 1) dis-play rather similar trends, furnishing higher values principally during

Fig. 5. A) Calcareous nannofossil dissolution indices as reported in Table 1. B) Planktonic foraminiferal dissolution indices (FI%, MFI% and TFI%) as reported in Table 1 and NDI andCaCO3 contents. Planktonic and benthic oxygen records and δ18Ob stack LR04 (Lisiecki and Raymo, 2005) are reported as reference. Numbers indicate glacial–interglacial marineisotope stages; glacials are shaded.



the last glacial intervals, which is in agreement with the aforemen-tioned indices.

4.2.2. Planktonic foraminiferaThe foraminiferal dissolution indices show short-term fluctuations

with implications that are very similar to those recorded by the NDI(Fig. 5B). The FI% curve displays higher values (bad preservation)mostly during interglacials (particularly MIS 9, MIS 7 and MIS 5)and in the lowermost part of the glacial MIS 8 (Fig. 5B). The best pres-ervation likely prevails within the glacials (especially MIS 10, upperpart of MIS 8 and MIS 6) with the only exception of MIS 11.

The MFI% curve exhibits positive peaks during the interglacials(Fig. 5B), with wider fluctuations from MIS 7 upwards, whereas inthe lower part of the core, the MFI% records higher mean values butlower variability. Unfortunately, the TFI% curve suffers from the dis-continuous occurrence of Globorotalia truncatulinoides, especially inthe lower portion of the core, preventing the establishment of aclear relationship with the G–I cycles.

The CaCO3 content (Fig. 5B) is high throughout the core. Consider-ing the long-term trend, the highest values are almost constantlyapproximately 70–80% and occur from bottom-core up to MIS 9,whereas the upper part records percentage close to 60% with a mini-mum of ~40% at MIS 7. Regarding the G–I cycle, the fluctuations of theCaCO3 curve are faint and are mainly not in phase with the G–Is,except for the interval from MIS 8 to MIS 6, which demonstrateshigher values at the interglacials.

4.2.3. Search for the most reliable calcareous nannofossil dissolutionindex for polarized light microscope

To determine the reliability of the various calcareous nannofossildissolution indices and their reproducibility in the NW PacificOcean, the correlation coefficients and the widely applied indicesbased on planktonic foraminifera (FI%, MFI%, TFI%) are provided(Fig. 6).

The good qualitative fit among the nannofossil NDI and the forami-niferal FI% andMFI% indices described previously is corroborated by thestrong correlation values, namely, of −0.76 and −0.66, respectively(Fig. 6A). The TFI% was proposed by Loubere and Chellappa (2008) asa promising mid to higher latitude quantitative proxy of carbonatedissolution. The TFI% does not demonstrate significant correlationwith the NDI (r=−0.13), most likely as a consequence of the discon-tinuous occurrence of Globorotalia truncatulinoides throughout the re-cord (Fig. 5B).

Weak negative correlation is detectable between CEX and FI%(r=−0.32; Fig. 6B), whereas significant correlation does not existbetween CEX′ and FI% (r=0.16; Fig. 6B). Moreover, in the lattercase, the coefficient value becomes positive, which is inexplicable be-cause the two indices must be negatively correlated. Therefore, theuse of the CEX′ index, as proposed for the Atlantic Ocean by Boeckeland Baumann (2004), seems problematic for this area of the NWPacific Ocean.

The same results emerge from the correlation between CEX andCEX′ and MFI%; specifically, there is low correlation between CEXand MFI% (r=−0.29; Fig. 6B) and extremely low positive correlationbetween CEX′ and MFI% (r=0.06; Fig. 6B). A possible explanationcould be the rarity or absence, particularly in the upper part of the re-cord, of Gephyrocapsa ericsonii that is less than 4% in abundance.

The addition of Gephyrocapsa aperta in the CEX and CEX′ improvesthe correlation coefficients because G. aperta reaches values of 40% inabundance in the upper portion of the core. The index CEX+G. apertadisplays negative correlation coefficients of −0.40 and −0.49 withFI% and MFI%, respectively (Fig. 6B); CEX′+G. aperta is correlatedwith FI% (−0.31) and MFI% (−0.41) (Fig. 6B).

This evidence makes NDI the most reliable and reproduciblenannofossil dissolution proxy for this area. Another important advantageof NDI is that it can be easily estimated at the LM. The CEX and CEX′,

conversely, have a limited applicability to the more ancient recordwhen E. huxleyi is absent, in addition they are carefully estimable bySEM analysis.

4.3. Primary productivity indices

Among the SYRACO groups, the Emi curve (Fig. 7) displayspercentages higher than those detected under light microscope forE. huxleyi. The high values of Emi, recorded at the bottom of the suc-cession up to MIS 8, which is before the LO of the species, are due tothe very abundant specimens of small Gephyrocapsa caribbeanica.After an abrupt decrease, the Emi percentage increases again, fromMIS 6 upward, with some fluctuations. The abundance peaks aremost likely caused by Gephyrocapsa aperta. The Gee group recordshigh abundances during the entire interval. Furthermore, this groupincludes mostly small G. caribbeanica, other small Gephyrocapsa andfragments of other types. The high values of Gem, from bottom upand to the end of MIS 8, are also due to the high abundance of smallG. caribbeanica, whereas after MIS 8, the Gem decreases abruptlybecause it includes mainly Gephyrocapsa muellerae. The Geo group israre from the bottom up to MIS 8, and subsequently, it peaks at 19%with two peaks at MIS 7 and 5. The Geo group is composed ofGephyrocapsa oceanica and some coccolith fragments. Finally, FP dis-plays substantially low values until the mid-portion of MIS 8, afterwhich, it increases in abundance particularly in the interglacial inter-vals from MIS 7 to MIS 3.

The %Fp index curve (Fig. 7) displays values that were less than25% for almost the entire record, even if a slight positive long-termtrend is detectable. In addition, the %Fp index shows two maximacentered, in the upper portion of the MIS 7 and in the lower portionof the MIS 5.

The Δδ13C p–b curve (Fig. 7) displays a negative correlation value(r=−0.53) with %Fp, indicating that the two proxies record thesame paleoproductivity fluctuations, although they are derived fromtwo different and independent sets of data. Particularly, accordingto the isotope gradient, paleoproductivity is higher in the lower por-tion of the record up two MIS 8, and subsequently, it decreases withtwo minima at MIS 7 and MIS 5.

4.4. Dissolution and primary productivity time series analyses

Results of the wavelet and spectral analyses can be summarized asfollows (Fig. 8):

- a 100 ka orbital eccentricity is not recorded in the NDI signal;conversely, a clear peak appears at 40-ka most likely related to theobliquity cycle. Furthermore, a strong signal at 30 ka periodicity isobserved along the whole investigated interval.

- The FI% signal records the 100 ka orbital eccentricity reasonablywell, although it is also associated with a stronger 67 ka periodicityrelated to unknown forcing processes. In addition, in the range of200–350 ka, a peak with a central periodicity of 30 ka is detectable.

- The %Fp signal shows a marked 100 ka orbital periodicity peak, wellrecognizable along almost the record. From 130 to 250 ka, the 33 kaperiod is detectable in parts of the records.

5. Discussion

5.1. Interpreting climate-forced dissolution and primary productivity

Carbonate preservation in marine sediments has long been used toinfer patterns in the ocean circulation, dissolution and/or biogenicproduction.

Data shown in Fig. 9 offer an opportunity to speculate on the car-bonate dissolution and primary productivity during the last 450 ka.



5.1.1. Carbonate dissolutionThe comparisons of the dissolution curve with the δ18Op and δ18Ob

curves display a variability related to the G–I cycles. Specifically, intensedissolution occurs mainly during some interglacials and during sometransitions from interglacial to glacial periods, whereas increasedpreservation occurs during almost all the deglaciations. These patternsof carbonate variability (Indo-Pacific type, Hodell et al., 2001) are dem-onstrated to be basin-wide in the Pacific (e.g., Farrell and Prell, 1989,1991; Wu et al., 1991; Zhang et al., 2007), including the prominentpreservation spike at 130 ka and the marked dissolution event in theHolocene (Farrell and Prell, 1991; Medina-Elizalde, 2007; Lalicata andLea, 2011). In particular, there is a precise correspondence with ourcase study of the maximum dissolution event over the last 550 kaduring MIS 8 (Zhang et al., 2007).

Therefore, the synchronous timing in the fluctuations of the pres-ervation indices, that consistently lagged behind the oxygen isotopecycles, demonstrates basin-wide changes in the ocean chemistry atthe glacial–interglacial transitions. Such large dissolution during thebuild-up phases implies a CO2 consuming mechanism, and the re-verse mechanism occurs during the carbonate preservation phasesobserved during the deglaciations. These effects, which inhibit theballasting effect of the ocean, act as positive feedback on climatecooling or warming.

The Mid-Brunhes Dissolution Event (Hodell et al., 2000; Wang etal., 2003; Barker et al., 2006), which should be recorded at the MIS11, is not detectable at our site. Anderson et al. (2008) noted thatthis severe dissolution event occurred in the Western Pacific at depthsbelow the lysocline (Wu et al., 1991), and it cannot be recognizedabove the lysocline using foraminiferal fragment ratios.

5.1.2. Changes in paleoproductivityWe relate the long-term record of paleoproductivity decreasing

(slight positive trend of %Fp index, Fig. 9) to a long-term migrationtoward north of the northern oceanic fronts that occurred at the

end of the Mid-Brunhes Event (MBE). The northward migration of theKuroshio Extension caused a deepening of the thermocline/nutriclineat the site, whichwas progressively replaced by less productive southernsurface waters.

At mid-term, the consistent high productivity (low values of the%Fp index, Fig. 9) fromMIS 12 to MIS 9 indicates high nutrient supply,and the primary productivity was most likely related to a shallownutricline/thermocline. We suppose that these high-nutrient condi-tions of the surface waters are related to a southern position of thefrontal system (Subarctic Front), confined from the Kuroshio Extensionto the south and the Oyashio Current to north, capable of producingeddies and intense upwelling. At present, climatic conditions andprimary productivity on the Shatsky Rise are strongly influenced bythe relative positions of these two currents (Kawahata et al., 1999;Qiu, 2001; Kawahata and Ohshima, 2002).

From MIS 8 and even more from the MIS 8/7 transition, the strongdecrease of paleoproductivity, culminating in the upper portion of theinterglacial MIS 7, could be linked to the northern position of the oce-anic fronts and to a zonal circulation most likely related to the warmMIS 7 interglacial phase. Consequently, the site appears to be locatedsouth of the Kuroshio Extension in a subtropical, oligotrophic regimecharacterized by stratified water column and deeper thermocline/nutricline, favoring the deep dwelling species, such as Florisphaeraprofunda, which experienced its abundance maximum (Fig. 9).

The extreme glacial conditions of the MIS 6, marked by a new in-crease in the primary productivity and shallowing of the thermocline–nutricline (low abundance of Florisphaera profunda), produced a newsouthward migration of the frontal system; the subsequent return ofthe Kuroshio Extension and of an oligotrophic regime over our site ismost likely related to the substage MIS 5e, one of the warmest periodsof the last one million years. Finally, from MIS 5 to the core top, anocean configuration similar to that recorded duringMBE is documented.

The Δδ13Cp–b curve generally mirrors the %Fp trend negatively,even though it documents an episode of productivity maximum

Fig. 7. Calcareous nannofossil abundance patterns based on the SYRACO counting method and carbon isotope gradient (Δδ13C p–b). See text for the SYRACO class (Emi, Gee, Gem,FP). Planktonic and benthic oxygen records and δ18Ob stack LR04 (Lisiecki and Raymo, 2005) are reported as reference. Numbers indicate glacial–interglacial marine isotope stages;glacials are shaded.

Fig. 6. Scatter plots of calcareous nannofossil and planktonic foraminiferal preservation indices calculated and discussed in the text. A) Pearson's correlation coefficients (r) betweenNDI and foraminiferal indices and among all foraminiferal indices; B) Pearson's correlation coefficients (r) between calcareous nannofossil and foraminiferal indices.



during MIS 8 and the two marked drops at MIS 6/5 and MIS 4/3 tran-sitions that %Fp does not record. The gradient peak within the MIS8 has also been observed by Zhang et al. (2007) in the Western Equa-torial Pacific.

Lόpez-Otálvaro et al. (2008) document similar paleoproductivityfluctuations in the Eastern Equatorial Pacific for the last 560 ka andrelate these fluctuations to the influence of El Niño-like events. Con-versely, Zhang et al. (2007) speculate that mostly after 280 ka,

Fig. 8. Power spectral and wavelet analyses of NDI, FI% and %Fp records. Brown and black lines in Lomb–Scargle power spectra indicate the 95% and 80% confidence level, respectively.Red line indicates the AR (1) theoretical red noise spectrum (Schulz andMudelsee, 2002). In wavelet spectrum, brown and black lines indicate the 95% and 80% confidence level, respectively.



thermocline dynamics were not a primary control of biological produc-tivity for the Western Pacific but that nutrients can be brought into thephotic zone by other mechanisms such as lateral currents, fluvial pro-cesses orwind transport. In particular for the Shatsky Rise region, the au-thors postulate eolian dust fluxes directly from the Chinese–Mongoliandust source regions, most likely during some very arid glacials, such asMIS 6. We are in general agreement with this interpretation, from MIS8 upwards, where the drops in paleoproductivity at MIS 7, MIS 5 andMIS 4/3 transition (Fig. 9) correspond to the minimum values of theeolian flux reported by Zhang et al. (2007). This finding seems to testifya superimposition of the wind dynamics on the long term deepening ofthe thermocline/nutricline, as suggested by the Florisphaera profundaabundance (Barker et al., 2006).

5.2. Interaction between carbonate dissolution and paleoproductivity

Among the different mechanisms invoked to explain the G–Icarbonate dissolution changes, together with the lysocline depthor the deep-water circulation patterns, there are also fluctuationsin the primary productivity (e.g., Arrhenius, 1988).

As mentioned above, the transitory shallowing of the lysocline,indicated by enhanced carbonate dissolution, has been related tothe high-productivity events of the surface waters. These eventswould have induced organic matter remineralization and consequentundersaturation of carbonates. However, in this work a clear inverserelationship between the productivity and carbonate preservation isnot apparent. On the contrary, selected intervals of high productivitycoincide with time of good preservation, such as at MIS 6, ashighlighted by the NDI and FI% data (Fig. 9). Moreover, the compari-son of the δ13CCib-equiv curve with the dissolution curves shows oppo-site trends; the intervals with the best carbonate preservationcoincide with light carbon-isotope values and vice versa. We proposethat these features occurred because carbonate preservation did notallow the weighting of carbon, which instead occurred when themaximum dissolution of carbonates consumed the CO2 of the seafloor. Our inference is supported by Anderson et al. (2008) andLoubere and Chellappa (2008), who studied the carbonate preserva-tion in the middle to high south Atlantic and equatorial Pacific andconcluded that carbonate undersaturation and changes in the oceanchemistry (carbonate ion concentration) rather than variations in or-ganic carbon flux have controlled the pattern of CaCO3 preservation.

Currently, the studied site lies at −2387 m depth, well above theregional lysocline estimated to be at ca. 2900 m depth, but we cannot

exclude the possibility that during some warm interglacials, such asMIS 5 and MIS 7, the elevated CO2 pressure could have temporarilycaused an increase of the lysocline.

At the mid-latitude of our site, the variation in primary productiv-ity (%Fp index) and carbonate dissolution are primarily caused by theglacial–interglacial variability (100 ka periodicity) and obliquity or-bital cycles (41 ka periodicity), as evidenced by the spectral andwavelet analyses. The spectral peak at 30 ka, though less markedwith respect to the previous ones, is one of the most discussednon-orbital periodicities recorded in the marine climate proxies, es-pecially in the Pacific region (Pisias and Rea, 1998; Beaufort et al.,2001; Kershaw et al., 2003) and in the Indian Ocean (Pisias andMix, 1997). In particular, Mix et al. (1995) and Elkibbi and Rial(2001) interpreted the 30 ka periodicity as a non-linear coupling be-tween the 100 ka and 41 ka cycles (1/41+1/100=1/30), and there-fore suggested that it was not directly controlled by the ‘standard’orbital forcing. Beaufort et al. (2001) also document significantpeaks at 30 ka in the productivity signal of the Equatorial PacificOcean. These authors assumed that these spectral peaks reflect themodulation phase of precession in the Equatorial Pacific Pool relatedto the Boreal Monsoon and ENSO-like dynamics. It is likely that theoccurrence of the 30 ka cycle in the productivity and carbonate disso-lution signals of the Shatsky Rise reflects this type of periodicity. Un-fortunately, our record does not permit a reliable interpretation of thecycles at a resolution smaller than the glacial/interglacial intervals.

6. Summary and conclusions

The high-resolution, multi-proxy investigation of calcareous micro-fossil and isotopic data from the Shatsky Rise (western North Pacific,ODP Site 1209B) represents a useful case-history to verify the potentialof calcareous nannofossil assemblages as reliable dissolution andpaleoproductivity proxies over the last 450 ka.

The main results of our study can be summarized as follows:

(1) The comparison and correlation among the nannofossil andplanktonic foraminiferal dissolution indices allow us to identifythe NDI proposed byMarino et al. (2009) as the best nannofossilindex to evaluate the dissolution under a polarized light micro-scope, particularly for records prior to 250 ka.

(2) The NDI data provide evidence for the preservation maximaduring deglaciations and dissolution peaks at the onset of glacialphases or during severe interglacials. These fluctuations are

Fig. 9. Nannofossil and planktonic foraminiferal (NDI, FI% andMFI%) preservation indices, nannofossil and geochemical productivity proxies (percentage of Florisphaera profunda, Δδ13C p–b)and CaCO3 content. Planktonic and benthic oxygen records and δ18Ob stack LR04 (Lisiecki and Raymo, 2005) are reported as reference. Numbers indicate glacial–interglacial marine isotopestages; and glacials are shaded.



demonstrated to be recorded basin-wide in the Pacific (Farrelland Prell, 1991; Medina-Elizalde, 2007; Lalicata and Lea, 2011).The Mid-Brunhes Dissolution Event, recorded in the WesternPacific during MIS 11, is not detectable at our relativelyshallower site.

(3) Intervals of high productivity usually occur during times ofgood preservation and light carbon-isotope values on thesea floor and vice versa. These features can be interpreted asresponses to the lack of carbon weighting, which happenedwhen the increased dissolution of carbonates consumes theCO2 on the sea floor. Therefore, carbonate undersaturationand changes in the ocean chemistry, rather than variationsin organic carbon flux, appear to have controlled the patternof CaCO3 preservation. Today, the site ODP 198-1209B lies at awater depth of 2387 m, well above the regional lysocline atca. 2900 m depth. However, we cannot exclude the possibilitythat during some warm interglacials (MIS 5 and MIS 7), the ele-vated CO2 pressure could have temporarily caused a rising of thelysocline.

(4) Over the last 450 ka we observe a general decrease in thepaleoproductivity coupled with shorter glacial/interglacial fluc-tuations. This evidence was related to the variations in the ther-mocline and nutricline dynamics that are possibly controlled bymechanisms such as wind transport, lateral currents or fluvialprocesses that could act at the mid-latitudes. The coupled dy-namic of the Kuroshio Extension and the eolian dust fluxescould have directly influenced the primary productivity in theShatsky Rise region.

(5) The spectral andwavelet analyses performed on themulti-proxydataset demonstrate that the variations in the paleoproductivityand carbonate dissolution were primarily driven by theglacial–interglacial variability (100 ka periodicity) and theobliquity-controlled changes. Nevertheless, we cannot ex-clude the possibility that the 30-ka periodicity recorded inthe productivity and carbonate dissolution signals could reflecta combination of both the Boreal Monsoon and ENSO-like dy-namics as previously supposed for the Equatorial area (Beaufortet al., 2001).

Acknowledgments

We thank the Ocean Drilling Program that furnished samples toM.C. and allowed the study of the Site 1209B. This study comesfrom the Ph.D. project of M.B. and was financially supported by aFAR grant to M.C. (Pavia University). We thank Thierry Corrège, Editorin chief of the journal, and two anonymous reviewers for your sugges-tions which have greatly improved the manuscript. Finally, we reallythank the “Laboratorio Arvedi” of Pavia for the SEM images.

Appendix A. Supplementary data

Supplementary data associated with this article can be found in theonline version, at http://dx.doi.org/10.1016/j.palaeo.2012.12.021. Thesedata include Google maps of themost important areas described in thisarticle.

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