Reconstruction of paleoceanographic conditions in the northwestern Pacific Ocean over the last 500 kyr based on calcareous nannofossil and planktic foraminiferal assemblages

Marine Micropaleontology 96–97 (2012) 29–37

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Research paper

Reconstruction of paleoceanographic conditions in the northwestern Pacific Ocean over the last 500 kyr based on calcareous nannofossil and planktic foraminiferal assemblages Shun Chiyonobu ⁎, Yuko Mori, Motoyoshi Oda Institute of Geology and Paleontology, Graduate School of Science, Tohoku University, 6-3 Aramaki Aoba, Sendai 980-8578, Japan

a r t i c l e

i n f o

Article history: Received 6 January 2012 Received in revised form 18 July 2012 Accepted 28 July 2012 Keywords: Calcareous nannofossil Planktic foraminifera Kuroshio Extension Shatsky Rise East Asian Monsoon

a b s t r a c t Calcareous nannofossil and planktic foraminiferal assemblages from ODP Hole 1210A in the northwestern Pacific Ocean were used to reconstruct surface-water conditions for the past 500 kyr. Stratigraphic control was provided by calcareous nannofossil events that are thought to be synchronous over a broad range of latitudes. Calcareous nannofossil and planktic foraminiferal assemblages and abundance patterns indicate the unlikelihood of long term (Milankovitch-scale) latitudinal shifts of the Kuroshio Extension over the last 500 kyr and illustrate two successive surface water-mass states, one that prevailed prior to 300 ka and one that existed after 300 ka. The relative abundance of very small placoliths and the absolute abundance of the upper photic zone (UPZ) coccolith species decrease abruptly at approximately 300 ka. The relative abundance of the lower photic zone (LPZ) species Florisphaera profunda greatly increases at the same time, although intervals during which the relative abundance of this taxon is very low or absent also occur prior to 300 ka. The absolute abundance of planktic foraminifera gradually increased after the 300-ka boundary, including peaks of Globoconella inflata. These assemblage and abundance changes suggest significant modifications to the surface water-mass structure. Surface water was weakly stratified prior to 300 ka, but alternated between intensely stratified and vertically mixed after 300 ka. Changes in the surface water-mass structure suggest an intensification of the East Asian summer and winter monsoon after 300 ka. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The Shatsky Rise, located under the subtropical Kuroshio water of the northwestern Pacific Ocean, forms an important oceanographic boundary. Previous studies have illustrated that shifting of major climate zones in the Quaternary significantly affected conditions in this region (Thompson and Shackleton, 1980; Thompson, 1981; Leinen et al., 1986; Hovan et al., 1991; Kawahata et al., 2000; Yamane, 2003). The subarctic front and transitional zone migrated southward during marine isotope stage (MIS) 2 and MIS 6, as determined from variations in the coiling ratio of Neogloboquadrina pachyderma (Thompson and Shackleton, 1980). Hovan et al. (1991) noted that mass accumulation rates of eolian dust and calcium carbonate in core V21-146 from Shatsky Rise were high during glacial periods. The mass accumulation rate (MAR) of calcium carbonate was also high during glacial periods after 300 ka and interglacial periods prior to 300 ka. Hovan et al. (1991) link this change in calcium carbonate MAR to a change in eolian grain size ca. 300 ka, with the older pattern characterized by lower-frequency ⁎ Corresponding author at: Research Institute of Innovative Technology for the Earth, 9-2 Kizugawadai, Kizugawa, Kyoto 619-0292, Japan. Tel.: + 81 774 75 2312; fax: + 81 774 75 2313. E-mail address: [email protected] (S. Chiyonobu). 0377-8398/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.marmicro.2012.07.002

and higher-amplitude fluctuations relative to the post-300 ka pattern. More recently, overlapping δ13C values from shallow- and deepdwelling planktic foraminifera have indicated that core site S-2 on the Shatsky Rise (Fig. 1) has been under the sustained influence of subtropical Kuroshio water for the past 540 kyr, with the greatest influence of this current prior to 260 ka (Yamane, 2003). Numerous core studies have investigated the variations of calcareous nannofossil and planktic foraminifera assemblages in response to oceanographic changes. Calcareous nannofossils and planktic foraminifera are the most important producers of calcareous sediment in the ocean (e.g., Young, 1994; Schiebel, 2002) and are sensitive to changes in the vertical structure of near-surface water (Tsuchihashi and Oda, 2001; Oda and Yamasaki, 2005; Chiyonobu, 2009). Geographic and temporal variations in calcareous nannoplankton and planktic foraminiferal assemblages are controlled by environmental changes in surface waters, and thus these organisms are useful tools in reconstructing paleoceanographic and vertical water-mass structure. This study examines the relationship between changes in calcareous nannofossils and planktic foraminiferal assemblages and that between microfossil assemblages and paleoceanographic conditions through analyses of microfossils recovered from Ocean Drilling Program (ODP) Hole 1210A located on Shatsky Rise beneath the

30

S. Chiyonobu et al. / Marine Micropaleontology 96–97 (2012) 29–37

60oN

Oyashio SAF

o

45 N SAB KBF

V21-146 S-2

30oN

20oN

Shatsky Rise

Hole 1210A

KE

STF

135oE

150oE

165oE

180oE

Fig. 1. Locations of ODP Hole 1210A, used in this study, core S-2 (Yamane, 2003), and core V21-146 (Thompson and Shackleton, 1980), referenced in this report. Schematic illustration of the surface current and water-mass structure in the Kuroshio–Oyashio transition area (Yasuda, 2003). SAF: Subarctic Front, SAB: Subarctic Boundary, KBF: Kuroshio Bifurcation Front, KE: Kuroshio Extension, STF: Subtropical Front.

track of the Kuroshio (Fig. 1). On the basis of these relationships, we have reconstructed the paleoceanography and paleoclimate of the northwestern Pacific Ocean over the past 500 kyr. 2. Oceanographic setting Surface and subsurface current structures in the Kuroshio– Oyashio transition area (northwestern Pacific region) are illustrated in Fig. 1, based on Yasuda (2003). The Kuroshio south of Japan and the Kuroshio Extension (KE) east of Japan are the western and northern boundary currents of the North Pacific subtropical gyre. In the Kuroshio–Oyashio transition area, several fronts separate water masses or circulation regimes (Yasuda, 2003). The Subarctic Front (SAF) is defined by the 4 °C isotherm at 100-m water depth (Favorite et al., 1976). Subarctic surface water can drift southward across the SAF (the Ekman drift) and cover the surface in the Transition Domain between the SAF and Subarctic Boundary (SAB), which is a near-surface salinity front south of the SAF (Yasuda, 2003). The Kuroshio Bifurcation Front (KBF) is defined by the 6–8 °C isotherm at 300-m water depth (Mizuno and White, 1983). The Kuroshio Extension bifurcates near Shatsky Rise, a topographic feature located around 158°E, and the northward branch extends to about 40°N (Mizuno and White, 1983). The northern part of Shatsky Rise is directly beneath the KE. The site of Hole 1210A is located between the KE and the Subtropical Front (STF: Fig. 1). 3. Material and methods Hole 1210A was 242.4 m deep, and Core 198-1210 A-1H through 27H consists of alternating layers of nannofossil ooze and clayey foraminiferal ooze (Shipboard Scientific Party, 2002). Samples were taken from Core 198-1210A-1H-1, 16–20 cm, and 198-1210A-2H-2, 148–150 cm, at approximately 25-cm intervals. Thirty-five samples

were examined for calcareous nannofossils, and 24 samples were examined for planktic foraminifera. Nannofossil assemblages from all sites were prepared using a standard method (Chiyonobu et al., 2006). For each sediment sample, 0.04 g of dried material was ground and homogenized with a mortar and pestle, placed into a small beaker, and diluted with 50 mL of water to form a suspension. Then, 0.5 mL of suspension was placed on a cover slip (18 mm × 18 mm) with a pipette, and the sample was dried on a hotplate at 45 °C. Samples were then mounted on a microslide using Norland Optical Adhesive. The prepared material was analyzed under a binocular polarizing microscope with an oil-immersion objective at a magnification of 1500. For each sample, 200 nannofossil specimens (excluding Florisphaera profunda) were counted at random to determine their relative abundances. At the same time, the number of nannofossils present along a straight transect was counted to calculate their absolute abundance. F. profunda specimens were counted separately from the other coccoliths to clarify relative abundances of the upper photic zone (UPZ) and the lower photic zone (LPZ) species. Preservation of coccoliths from Hole 1210A is variable. Most of the samples show good to moderate preservation. Differential dissolution is commonly evident on the rims of Gephyrocapsa and Emiliania placoliths. Small specimens of Emiliania huxleyi and small Gephyrocapsa specimens that have lost their bridges are difficult to distinguish from one another using light microscopy. In such cases, scanning electron microscopy was used to identify species. Preservation quality was assessed using three categories: good (G) for little or no evidence of dissolution and/or overgrowth, moderate (M) for minor dissolution or crystal overgrowth, and poor (P) for strong dissolution or crystal overgrowth, in which case many specimens are unidentifiable. Reworked calcareous nannofossils were not documented in any of the samples. Samples analyzed for planktic foraminifers were weighed dry and washed over 63-μm sieves. The residues were dried in an incubator at 60 °C, weighed dry, and then split into several aliquots to reduce the total number of planktic foraminiferal individuals. In each sample, the faunal composition of planktic foraminifers was determined based on a minimum count of 200 individuals larger than 125 μm in diameter. Planktic foraminiferal tests were well preserved. The abundance of dissolution-resistant species (RSP, after Cullen and Prell, 1984) was consistently below 30%, indicating that the composition of planktic foraminiferal faunas was minimally modified by dissolution. Dry sample weight was divided by the number of aliquots, and the total number of planktic foraminifera per unit volume of sediment was calculated.

4. Taxonomic note on calcareous nannofossils and planktic foraminifers Most of the species identified in this study belong to the family Noelaerhabdaceae (including the genera Emiliania, Pseudoemiliania, Reticulofenestra, and Gephyrocapsa) (Thierstein et al., 1977; Raffi et al., 1993; Wei, 1993; Flores et al., 1999; Flores et al., 2000). Gephyrocapsid taxonomy is based on reports by Raffi et al. (1993), Flores et al. (2000), Takahashi and Okada (2000), and Baumann and Freitag (2004). The genus Gephyrocapsa was first divided into Gephyrocapsa caribbeanica, Gephyrocapsa oceanica, Gephyrocapsa muellerae, and other forms, and the latter were subdivided into two taxonomic groups based on coccolith length, i.e., small (2.5–3.5 μm) and very small (b 2.5 μm). We also classified Gephyrocapsa spp. based on the bridge angle: >45° (small Gephyrocapsa spp. and G. oceanica), 0°–25° (G. muellerae), and 25°–45° (G. caribbeanica; Table 1). Species-level taxonomic criteria of planktic foraminifera follow Saito et al. (1981) and Kennett and Srinivasan (1983), except for N. pachyderma and Neogloboquadrina incompta, which follow Oda and Domitsu (2009).

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31

Table 1 Terminology for Gephyrocapsa spp. used in this study and equivalents of other authors (mainly adopted from Baumann and Freitag, 2004). This study

Gephyrocapsa spp. (very small)

Gephyrocapsa spp. (small)

Gephyrocapsa muellerae

Gephyrocapsa caribbeanica

Gephyrocapsa oceanica

Coccolith size Bridge angle Other authors Bollmann (1997) Flores et al. (2000) Takahashi and Okada (2000)

b2.5 μm 0°–90°

2.5–3.5 μm >45°

2.5–4 μm 0°–25°

>3.5 μm 25°–45°

>3.5 μm >45°

G. minuta small Gephyrocapsa small Gephyrocapsa

G. cold G. muellerae medium Gephyrocapsa low angle (cold)

G. oligotrophic/G. transitional G. caribbeanica G. caribbeanica

Baumann and Freitag (2004)

G. ericsonii/G. aperta

G. equatorial G. oceanica medium Gephyrocapsa high/intermediate angle (warm) G. caribbeanica

G. muellerae

G. caribbeanica

G. large/G. equatorial G. oceanica medium/large Gephyrocapsa high/intermediate angle (warm) G. oceanica

5. Results 5.1. Age model for Hole 1210A Pleistocene calcareous nannofossil datums, as proposed by Sato and Takayama (1992), Jordan et al. (1996), and Raffi et al. (2006), were identified in Hole 1210A. The absolute ages of the datums of the northwestern Pacific region were provided by Sato et al. (2009) and Domitsu et al. (2011); these were correlated with those of marine isotope stages (Lisiecki and Raymo, 2005). Some age-diagnostic species are found in Hole 1210A, and nannofossil datums can be detected (Table 2). Gephyrocapsa parallela is observed continuously above Sample 1210A 3H-1W 50–52 cm (15.9 m below the seafloor; mbsf), and no specimens of Reticulofenestra asanoi have been observed above Sample 2H–7W 50–52 cm (15.4 mbsf). Thus, the studied interval can be correlated with the section above the last occurrence (LO) of R. asanoi. Moreover, Pseudoemiliania lacunosa is found below Sample 1210A 2H–1W 148–150 cm; the LO of this species, which correlates to 451 ka (MIS 12), is placed between 7.15 and 7.38 mbsf. The first occurrence (FO) of E. huxleyi, which occurs at the end of MIS 8 at approximately 250 ka, is found at Sample 1210A 1H-3 100–102 (4.00 mbsf) and 125–127 cm (4.25 mbsf) consistent with the continuous occurrence of this species in the upper part of the core. The relative abundance of E. huxleyi increases between 0.75 and 1.25 mbsf. This abundance change has been correlated to ca. 40 ka within MIS 3 (e.g. Jordan et al., 1996). Yamane (2003) reported the same two calcareous nannofossil datums (FO E. huxleyi and LO P. lacunosa) from among the calcareous nannofossil datums of the S-2 core located about 1° north of Hole 1210A and calculated linear sedimentation rates. These constraints indicate that Hole 1210A contains a continuous sedimentary succession back to 509 ka. 5.2. Calcareous nannofossils 5.2.1. Nannofossil assemblages Fourteen genera and 20 species of calcareous nannofossils were identified in sediments from Hole 1210A. Nannofossil assemblages are characterized by an abundance of very small specimens (b 2.5 μm) of

Gephyrocapsa throughout the succession (Fig. 2). Very small Gephyrocapsa specimens are particularly abundant before approximately 300 ka and between approximately 130 and 190 ka. Calcidiscus leptoporus and Coccolithus pelagicus are consistently present throughout the succession and increase in abundance after 300 ka (Fig. 2). G. caribbeanica was abundant before ca. 300 ka (Fig. 2). G. oceanica and small specimens (2.5– 3.5 μm; bar angle >60°) of Gephyrocapsa are present throughout the examined interval (Fig. 2). The upper part of the succession is dominated by E. huxleyi (Fig. 2). Helicosphaera carteri, Rhabdosphaera clavigera, Umbellosphaera spp., and Umbilicosphaera sibogae are present throughout the succession (Fig. 2). Reticulofenestra spp. (b 2.5 μm and >2.5 μm) are consistently present throughout the succession. G. muellerae is very rare throughout the succession (less than 1%). 5.2.2. Relative abundance of coccoliths Based on the recent biogeography of coccolithophores, two groups were identified and their relative abundances compared: very small coccoliths (b2.5 μm Gephyrocapsa and Reticulofenestra) and warmwater species. Drastic changes in the abundance of very small coccoliths are evident in this core. Sediment deposited prior to ca. 300 ka is characterized by abundant very small coccoliths (average 50%; Fig. 3), but the average abundance of these coccoliths decreases to less than 50% after ca. 300 ka. G. caribbeanica is significantly less abundant from ca. 300 ka to the present, as are the very small coccoliths. Above approximately 120 ka, E. huxleyi abruptly increases, which is matched by a decrease in the very small coccoliths. After ca. 300 ka, the abundance of C. pelagicus and C. leptoporus increases to 10%, as does the abundance of F. profunda, in contrast to the declining abundance of very small coccoliths (Figs. 2, 3). 5.2.3. Absolute abundance of calcareous nannofossils Sediment from Hole 1210A contains coccoliths in abundance and exhibits remarkable changes in the relative abundance of coccolith taxa (Fig. 3). Samples from below ca. 300 ka contain a high absolute abundance of UPZ species, which average approximately 2.5 × 10 10 specimens/m 2/kyr, whereas above ca. 300 ka, the mean abundance decreases to 5 × 10 9 specimens/m 2/kyr (Fig. 3a). The absolute abundance of very small coccoliths also decreases from 1 × 10 10

Table 2 Calcareous nannofossil events and absolute ages in Hole 1210A, based on Sato et al. (2009). FO: first occurrence; LO: last occurrence. Calcareous nannofossil events

Upper (age)

Lower (age)

Middle (age)

Error (age)

Sample top

Sample bottom

FO Emiliania huxleyi

264.64

265.47

265.055

0.415

LO Pseudoemiliania lacunosa LO Reticulofenestra asanoi FO Gephyrocapsa parallela

450.61

452.2

451.405

0.795

852.42

854.05

853.235

0.815

987.34

987.98

987.66

0.32

1210A 1H–3W 100–102 1210A 2H–1W 125–127 1210A 2H–7W 50–52 1210A 3H–1W 50–52

1210A 1H–3W 125–127 1210A 2H–1W 148–150 1210A 3H–1 W 50–52 1210A 3H–2W 50–52

top (mbsf)

bottom (mbsf)

middle (mbsf)

error (m)

4

4.25

4.125

0.125

7.15

7.38

7.265

0.115

15.4

15.9

15.65

0.25

15.9

17.4

16.65

0.75

40

0

20

0

20

0

40

0

20

40

0

20

40

20

40

60

0

no sa (% e ) (> ti c 2. ul 5 of µm en ) ( es % tra R ) s (< eti pp 2. cul . 5 of µm en ) ( es % tra R ) s .c pp la . vi ge ra ( % U ) m be llo sp ha er U a .s sp ib p. og (% ae ) (% ) N (re um co lat be cc ive r o ol t f F ith o . s 20 pr of 0 of un ot da he rt ax a)

P. 20

0

R

cu la

ar

te

ri

(%

)

. 0

.c

an ce .o G

20

H

ic

a

an be ib ar .c G

40 60

G (< ep 2. hyr 5 o µm ca ) ( psa % s ) pp

ic

) % i( ey xl hu E.

40

(%

a

) (% us ic ag el .p C

20

)

(%

) (% s ru po to ep .l C 0

G (2 ep ba .5 hy r > - 3 roc 45 .5 µ ap )o ( m; sa s % pp ) .

S. Chiyonobu et al. / Marine Micropaleontology 96–97 (2012) 29–37

)

32

20

0

20

0

20

0

20

0

20

0

20

0

20

40

60

80

0

100

Age (kyr)

200

A 300

400

B 500

Fig. 2. Relative abundance (%) of calcareous nannofossil species in ODP Hole 1210A. Note that calcareous nannofossil datums are shown on the extreme right. A: First occurrence of Emiliania huxleyi, B: Last occurrence of Pseudoemiliania lacunosa.

specimens/m 2/kyr to 3 × 10 9 specimens/m2/kyr at ca. 300 ka (Fig. 3e; dashed line). The subordinate taxa (G. caribbeanica, C. pelagicus, and F. profunda) exhibit weak variations, in contrast to the dominant taxon, E. huxleyi, which increases in absolute abundance above ca. 125 ka (average 2 × 10 9 specimens/m 2/kyr; Fig. 3b, d, f). 5.3. Planktic foraminifera 5.3.1. Planktic foraminiferal assemblages The planktic foraminiferal assemblage in sediment from Hole 1210A contains 40 species belonging to 12 genera. N. incompta is dominant throughout the succession, contributing approximately 50% of specimens before ca. 300 ka. Globigerina bulloides and Globoconella inflata are also prominent throughout the succession; G. inflata is particularly abundant in the upper part of the core (Fig. 4). Globigerinoides ruber, Globigerina falconensis, Globigerinita glutinata, Globorotalia truncatulinoides, Neogloboquadrina dutertrei, Globigerinita iota, and Orbulina universa are present throughout the examined interval, and G. bulloides increases slightly toward the top of the core. In general, N. incompta, G. inflata, G. bulloides, and G. glutinata represent up to 70% of the total planktic foraminiferal assemblage in most samples (Fig. 4). 5.3.2. Relative abundance of planktic foraminifera Stratigraphic changes in the four most abundant species and in the rare species G. ruber, which is oceanographically important, are illustrated in detail in Fig. 4. N. incompta exhibits an average relative abundance of approximately 30–40% from 500 ka to 300 ka, but decreases to approximately 10–30% from ca. 300 ka to 0 ka. The relative abundance of G. inflata varies inversely with the abundance of N. incompta; its average abundance is b20% prior to ca. 300 ka but increases to >20% after ca. 300 ka. In general, the relative abundance of G. bulloides is approximately 5–10%, but increases slightly after 300 ka. The distribution of G. glutinata shows little variation in relative abundance (1–15%) and resembles that of G. bulloides. The abundance of G. ruber is very low in this core, averaging approximately 5% and exhibiting constant occurrence without abundance peaks (Fig. 4).

5.3.3. Absolute abundance of planktic foraminifera Total planktic foraminiferal abundance in sediment from Hole 1210A is high, and distinct variations are present (Fig. 4). The absolute abundance of planktic foraminifera is between approximately 5 × 10 4 specimens/m 2/kyr and 7 × 10 5 specimens/m2/kyr, increasing gradually after ca. 300 ka and suddenly during 200–125 ka. The absolute abundance of N. incompta exhibits peaks from ca. 300 to 250 ka, ca. 250 to 190 ka, and ca. 100 to 60 ka, with values ranging from approximately 3 × 104 specimens/m 2/kyr to 2 × 105 specimens/ m 2/kyr. The absolute abundance of G. inflata also changes from 1 × 104 specimens/m2/kyr to 1.5 × 105 specimens/m2/kyr, with a rapid increase after 200 ka. The absolute abundance pattern of G. bulloides is similar to that of G. inflata, varying between approximately 1 × 104 specimens/ m 2/kyr and 1.5 × 10 5 specimens/m 2/kyr, with peaks at ca. 190 to 125 ka. G. glutinata abundance ranges from approximately 1 × 104 specimens/m2/kyr to 5 × 105 specimens/m 2/kyr. 6. Discussion 6.1. Geographic distribution of calcareous nannofossils and planktic foraminifera Several authors have found that the angle between the long axis and the bridge of Gephyrocapsa is related to temperature (e.g., Bollmann, 1997). We also classified very small Gephyrocapsa spp., which indicate warm, cold, and intermediate temperatures (Table 1). Specimens smaller than 2.5 μm are mainly represented by Gephyrocapsa spp. and Reticulofenestra spp. (Takahashi and Okada, 2000). Warm surface waters at low latitudes to the south of Shatsky Rise were affected by the KE and STF (Fig. 1) especially in summer (Yasuda et al., 2000), indicated by abundant Gephyrocapsa species with a high bridge-angle >45° (Bollmann, 1997; Hagino et al., 2000, 2005). In this study, R. clavigera and U. sibogae also are included among the warm-water species of the subtropical western Pacific Ocean (Okada and Honjo, 1973; Tanaka, 1991). The assemblages in samples from Hole 1210A, which spans a 500-kyr interval, are characterized

S. Chiyonobu et al. / Marine Micropaleontology 96–97 (2012) 29–37

Absolute abundance of Relative abundance of UPZ coccoliths C. pelagicus (%) (x1010 specimens/m2/kyr) 0 1 3 2 0 20 40 60 80 0

Relative abundance of Relative abundance of warm water speceis (%) E. huxleyi (%) 0

20

40

60

80

0

20

40

60

80

33

Relative abundance of very small coccoliths (< 2.5 µm) (%) 0 20 40 60 80

Number of F. profunda (relative to 200 coccoliths of other taxa) 0 20 40 60 80

100

Age (kyr)

200

300

400

500

(a)

(b)

(c)

(d)

(e)

(f)

0 1 2 0 1 2 0 1 2 2 0 1 0 2 1 Absolute abundance Absolute abundance Absolute abundance Absolute abundance Absolute abundance of warm water species of C. pelagicus of E. huxleyi of F. profunda of very small coccoliths (x1010 specimens/m2/kyr) (x1010 specimens/m2/kyr) (x1010 specimens/m2/kyr) (x1010 specimens/m2/kyr) (x1010 specimens/m2/kyr) Fig. 3. Relative abundance of major calcareous nannofossil species. (a) Absolute abundance of UPZ coccoliths, (b) relative abundance (solid line) and absolute abundance (dashed line) of Coccolithus pelagicus, (c) relative abundance (solid line) and absolute abundance (dashed line) of warm-water species of calcareous nannofossils, (d) relative abundance (solid line) and absolute abundance (dashed line) of Emiliania huxleyi, (e) relative abundance (solid line) and absolute abundance (dashed line) of very small (b2.5 μm) coccoliths, (f) relative to 200 coccoliths of other taxa (solid line) and absolute abundance (dashed line) of Florisphaera profunda.

by an abundance of warm-water species (approximately 25% average). Warm-water species abundance also decreases, from 5×109 specimens/ m2/kyr to 2×109 specimens/m2/kyr at ca. 300 ka (Fig. 3c; dashed line). The Neogloboquadrina group is dominant in modern planktic foraminiferal assemblages from surface sediments (Takemoto and Oda, 1997) and in plankton net (Kuroyanagi and Kawahata, 2004) and sediment trap (Oda and Yamasaki, 2005) samples around Japan. The coiling direction of N. pachyderma has been widely used as one of the most useful paleoceanographic proxies of ocean temperature, which is thought to control the coiling direction of this single species. However, morphologic characteristics and geographical distribution patterns suggest that N. pachyderma can be divided into two distinct species with opposite coiling direction preferences, one in the Atlantic (Darling et al., 2006; Kucera, 2007) and the other in the northwest Pacific (Oda and Domitsu, 2009). In the northwest Pacific, N. pachyderma inhabits the Oyashio water and subarctic water mass and has dominantly sinistral shells, with rarely dextral forms (Takemoto and Oda, 1997; Eguchi et al., 2003; Oda and Domitsu, 2009: given as sinistral N. pachyderma). On the other hand, the dominantly dextrally coiled N. incompta, with rarely sinistral shells, thrives in (1) the perturbed area located between the Oyashio and the Kuroshio fronts (Takemoto and Oda, 1997; Oda and Domitsu, 2009) and (2) the transitional zone (close to the Kuroshio

bifurcation; Fig. 1) between the SAF and the KE (Eguchi et al., 2003: given as dextral N. pachyderma). These two species contribute to the reconstruction of northwest Pacific paleoceanography. The subtropical mode water and water mass within the meandering breadth of the KE are inhabited by G. inflata and G. ruber (Tsuchihashi and Oda, 2001; Eguchi et al., 2003). 6.2. Displacement of the Kuroshio Extension Coccolith and planktic foraminiferal assemblages from Hole 1210A at Shatsky Rise are characterized by relatively high species diversity and an abundance of cosmopolitan or nutrient-indicator coccolith species, such as very small placoliths. Coccolith species that prefer warmer, temperate water, such as G. oceanica, exhibit relatively high abundances. Sediment from Hole 1210A yielded abundant warm-water coccolith species throughout the succession (Fig. 3). They averaged 30% in abundance and peaked at more than 40% after ca. 300 ka. G. muellerae and E. huxleyi occurred from about MIS 8 (Samtleben, 1980). G. muellerae is dominant toward higher latitudes and lower sea-surface temperatures (Pujos, 1988; Flores et al., 1997; Bollmann et al., 1998). However, G. muellerae has very low relative and absolute abundances (lower than 1%) in Hole 1210A.

absolute abundance of planktic foraminifera (x105 specimens/m2/kyr) 0 2 4 6 8 0

relative abundance of N. incompta (%) 0

20

40

60

relative abundance of G. inflata (%) 0

20

40

60

0

20

40

relative abundance of G. bulloides (%) 0

20

40

60

r/ G abu .r n ub d a er n c (% e o ) f

S. Chiyonobu et al. / Marine Micropaleontology 96–97 (2012) 29–37

r/ G abu .g n lu d a ti n n c at e a of (% )

34

0

20

100

Age (kyr)

200

300

400

500

0 1 2 0 1 2 0 1 2 0 1 absolute abundance of absolute abundance of a/ a of absolute abundance of G. incompta G. inflata G. glutinata G. bulloides (x105 specimens/m2/kyr) (x105 spec./m2/kyr) (x105 spec./m2/kyr) (x105 spec./m2/kyr) Fig. 4. Total planktic foraminifera numbers (absolute abundance of planktic foraminifera) and relative abundance (%; solid lines) and absolute abundance (dashed lines) of major planktic foraminifera species at ODP Hole 1210A.

This suggests that oceanic conditions in the Shatsky Rise region may not have deviated from those affected by the KE. Moreover, the relative abundance of G. ruber and Globigerinoides sacculifer are stable (approximately 10%) throughout the succession (Fig. 4), suggesting that the Shatsky Rise core site was under the sustained influence of a warm-water current or mass that indicates a correlation between the KE and KBF zones for the past 500 kyr. Yamane (2003), who reported δ 13C values for shallow- and deep-dwelling planktic foraminifera, also indicated that the Shatsky Rise area has been under the influence of subtropical Kuroshio water for the past 540 kyr, with the greatest influence of this current prior to 260 ka, based on overlapping δ 13C values. Thus, latitudinal shifts of the KE (Fig. 1) may not have been significant over the last 500 kyr. 6.3. Establishment of the modern water-mass structure of Shatsky Rise An abundance of very small placoliths (Gephyrocapsa spp. and Reticulofenestra spp. b 2.5 μm) is generally interpreted as indicating

relatively high productivity or high nutrient supply (Wells and Okada, 1996; Okada and Wells, 1997). These taxa proliferate when nutrients are abundant and their coccoliths are used as a proxy for water-column nutrient concentration (Takahashi and Okada, 2000). The abundance of these taxa in sediment from Hole 1210A indicates that nutrients were supplied to the euphotic zone in this interval. Most coccolithophores live in the UPZ above ca. 100-m water depth. However, modern F. profunda inhabits the LPZ in tropical and subtropical environments (Okada and Honjo, 1973). Molfino and McIntyre (1990a, 1990b) linked the abundance of F. profunda to changes in the nutricline/thermocline position. The abundance of F. profunda is related to seawater transparency (Ahagon et al., 1993), increased water temperatures in summer/fall (Cortés et al., 2001), and well stratified sea-surface waters. From ca. 500 to 300 ka at Hole 1210A, the absolute abundance of coccoliths, a high relative and absolute abundance of very small placoliths, and a low relative and absolute abundance of F. profunda are evident (Fig. 3). These conditions suggest that the sea-surface conditions above Shatsky Rise prior to ca. 300 ka were steady and characterized by a shallow nutricline and a thermocline that reached the UPZ, creating

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Fig. 5. Schematic illustration of different scenarios describing the changing surface water and climatic conditions, as follows: (a) summer monsoon (strongly stratified) after 300 ka, (b) winter monsoon (vertical mixing) after 300 ka, Note: arrows show the winter monsoon wind at the Kuroshio–Oyashio transition area, and (c) weakly stratified surface condition prior to 300 ka. The illustrations are cross-sectional diagrams of the northwestern Pacific Ocean.

high nutrient conditions and possibly leading to coccolithophore blooming. A possible explanation for such a water-mass structure prior to approximately 300 ka is that a weak thermocline and a weak nutricline were both within the UPZ (~50 m), probably in spring or autumn. In addition, Yamane (2003) reported that the carbonate concentration at Shatsky Rise was very high between 560 and 260 ka. The high relative abundance of very small placoliths and G. caribbeanica and the absolute abundance of coccoliths indicate increased carbonate production in the ocean. Planktic foraminiferal assemblages prior to ca. 300 ka are characterized by a high relative abundance of N. incompta, which prefers the well-stratified conditions and deep chlorophyll maxima that develop in summer (Schiebel et al., 2001). The abundance of N. incompta prior to

ca. 300 ka suggests that a distinct thermocline and stratified conditions developed during this interval (Fig. 5). In contrast, the water mass over Shatsky Rise after ca. 300 ka was very different from the weakly stratified conditions of the earlier time interval and was characterized by obvious alternation between times of more stratified conditions and times of vertical mixing. These characteristics are suggested by the decreasing abundance of very small placoliths, the declining absolute abundance of UPZ coccoliths, the increasing relative abundance of E. huxleyi, and the increasing relative abundance of F. profunda (Fig. 3). The relative abundance of C. pelagicus, which is traditionally considered a cold-water indicator (McIntyre and Bé, 1967; Haq, 1980; Winter et al., 1994), also increased during this

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interval. Cortés et al. (2001) showed that E. huxleyi reaches its highest densities in spring and autumn, F. profunda increases in summer, and coccolithophores have low population densities in winter. Paleoceanographic conditions during this interval were characterized by the development of a clearly stratified thermocline and seasonal vertical mixing in the photic zone. Moreover, there is an increase in the relative abundance of G. inflata, a decrease in the relative abundance of N. incompta, and an increase in the flux of planktic foraminifera (Fig. 4). G. inflata prefers vertical mixing in winter in the northwest Pacific (Tsuchihashi and Oda, 2001; Oda and Yamasaki, 2005). In general, foraminifers and coccoliths indicate that the surface water column after ca. 300 ka was characterized by pronounced seasonality, and less seasonality from 500 to 300 kyr (Fig. 5). Since ca. 500 ka, two distinct and successive surface water-mass conditions are indicated by floral and faunal patterns in the sediments from Hole 1210A. Up to 300 ka, the absolute abundance of coccoliths and planktic foraminifera was high. However, after 300 ka, coccolith abundance decreased as planktic foraminiferal abundance increased. This suggests the influence of either a weak thermocline or an influx of nutrients to the sea surface through seasonal stratification, followed by vertical mixing, or a combination of these factors. Consequently, the modern type of water mass structure at Shatsky Rise was established after 300 ka. The stratification of the North Pacific Ocean was characterized by the development of seasonality, which have been dominated by warmer late-summer and autumn and colder winter and spring sea-surface temperatures (Haug et al., 2005). The calcareous nannofossil and planktic foraminiferal results from the Shatsky Rise indicate the possible increase in the temperature difference between the summer and winter in the northwestern Pacific region. Assemblages of calcareous nannoplankton and planktic foraminifera adapted successfully to modern oceanic conditions at this time (Fig. 5). Several of the proxy indicators used in this study indicate a change in surface water-mass conditions around 500 ka. Many studies have suggested a global climatic event prior to approximately 300 ka (Janecek and Rea, 1985; Jansen et al., 1986; Hovan et al., 1991; Prokopenko et al., 2002; Hao and Guo, 2005; Suganuma et al., 2009) that was expressed in various climatic subsystems. Prokopenko et al. (2002) reported that glaciations during MIS 14 and MIS 12 were not as extensive as in other Pleistocene glacials recorded in Lake Baikal, Siberia. Hao and Guo (2005) showed that there was an intensification of the East Asian summer monsoon after 500 ka, based on eolian deposits on the Chinese Loess Plateau. Suganuma et al. (2009) suggested a significant intensification of the South Asian summer monsoon from 500 to 300 ka and a slight weakening after 300 ka, based on enhanced magnetite input and hematite and maghemite contributions. The coccolith and planktic foraminiferal data of this study, which show the water-mass structure to be weakly stratified, suggest the presence of the East Asian summer monsoon prior to 300 ka. 7. Conclusions Sediment from Hole 1210A, located on Shatsky Rise under the modern position of the Kuroshio Extension, provides excellent material for use in the paleoceanographic reconstruction of the northwestern Pacific region for the past 500 kyr. An age model for this hole was constructed using calcareous nannofossil biostratigraphy, and shows continuous sedimentation for the last 500 kyr. Calcareous nannofossil and planktic foraminiferal assemblages and abundance patterns illustrate two successive surface water-mass states, one that prevailed prior to ca. 300 ka and one that existed after ca. 300 ka. The relative abundance of very small placoliths and the absolute abundance of UPZ coccolith species decrease abruptly at approximately 300 ka. The relative abundance of the LPZ species F. profunda greatly

decreases at the same time, although intervals during which the relative abundance of this taxon is very low or absent also occur prior to ca. 300 ka. The absolute abundance of planktic foraminifera gradually increases after ca. 300 ka, and the relative abundance of G. inflata also increases, exhibiting peaks after ca. 300 ka. The abundance changes in the calcareous plankton assemblage over the last 500 kyr, suggesting significant modifications to surface water-mass structure, and unlikelihood of long term (Milankovitchscale) latitudinal shifts of the Kuroshio Extension. Surface water was weakly stratified prior to ca. 300 ka but alternated between intensely stratified and vertically mixed after ca. 300 ka. The modern type of water-mass structure at Shatsky Rise became established at approximately 300 ka, and assemblages of calcareous nannoplankton and planktic foraminifers evolved a range of adaptations to modern oceanic conditions at this time. Changes in the surface water-mass structure suggest intensification of the East Asian summer and winter monsoon after approximately 300 ka. Acknowledgments We are grateful to Dr. Ralf Schiebel, Dr. James C. Ingle, Jr., and Dr. Richard W. Jordan for insightful comments regarding this work. Samples from ODP Leg 198 Hole 1210A were provided by IODP USIO at Texas A & M University. This work was supported by a Grant-in-Aid for Scientific Research (A) JSPS No. 17204043 to M. Oda, by the Global Century Center of Excellence program “Education and Research Center for Dynamic Earth” of Tohoku University, and by the Research Institute of Innovative Technology for the Earth (RITE) and Ministry of Economy, Trade and Industry (METI). Appendix A. Supplementary data Supplementary data to this article can be found online at http:// dx.doi.org/10.1016/j.marmicro.2012.07.002. References Ahagon, N., Tanaka, Y., Ujiie, H., 1993. Florisphaera profunda, a possible nannoplankton indicator of late Quaternary changes in sea-water turbidity at the northwestern margin of the Pacific. Marine Micropaleontology 22, 255–273. Baumann, K.-H., Freitag, T., 2004. Pleistocene fluctuations in the northern Benguela Current system as revealed by coccolith assemblages. Marine Micropaleontology 52, 195–215. Bollmann, J., 1997. Morphology and biogeography of the genus Gephyrocapsa coccoliths in Holocene sediments. Marine Micropaleontology 29, 319–350. Bollmann, L., Baumann, K.-H., Thierstein, H.R., 1998. Global dominance of Gephyrocapsa coccoliths in the Late Pleistocene: selective dissolution, evolution or global environment change. Paleoceanography 13, 517–529. Chiyonobu, S., 2009. Upwelling strength and water mass structure changes in the equatorial Pacific Ocean during the last 550 kyr as recorded by calcareous nannofossil assemblages (in Japanese with English abstract). Fossils 86, 34–44. Chiyonobu, S., Sato, T., Narikiyo, R., Yamasaki, M., 2006. Floral changes in calcareous nannofossils and their paleoceanographic significance in the equatorial Pacific Ocean during the last 500,000 years. Island Arc 15, 476–482. Cortés, M.Y., Bollmann, J., Thierstein, H.R., 2001. Coccolithophore ecology at the HOT station ALOHA, Hawaii. Deep-Sea Research II 48, 1957–1981. Cullen, J.L., Prell, W.L., 1984. Planktonic foraminifera of the Northern Indian Ocean: distribution and preservation in surface sediments. Marine Micropaleontology 9, 1–52. Darling, K.F., Kucera, C., Kroon, D., Wade, C.M., 2006. A resolution for the coiling direction paradox in Neogloboquadrina pachyderma. Paleoceanography 21, PA2011 http://dx.doi.org/10.1029/2005PA00189. Domitsu, H., Uchida, J., Ogane, K., Dobuchi, N., Sato, T., Ikehara, M., Nishi, H., Hasegawa, S., Oda, M., 2011. Stratigraphic relationships between the last occurrence of Neogloboquadrina inglei and marine isotope stages in the northwest Pacific D/V Chikyu Expedition 902, Hole C9001C. News Letters on Stratigraphy 44, 113–122. Eguchi, N.O., Ujiie, H., Kawahata, H., Taira, A., 2003. Seasonal variations in planktonic foraminifera at three sediment traps in the subarctic, transition and subtropical zones of the central North Pacific Ocean. Marine Micropaleontology 48, 149–163. Favorite, F., Dodimead, A.J., Nasu, K., 1976. Oceanography of the subarctic Pacific region, 1960–71. Int. North Pacific Fisheries Comm., 33. 187 pp. Flores, J.A., Sierro, F.J., Franés, G., Vázquez, A., Zamarreno, I., 1997. The last 100, 000 years in the western Mediterranean: sea surface water and frontal dynamics as revealed by coccolithophores. Marine Micropaleontology 29, 351–366.

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