Spatial and temporal variability of surface water in the ...

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Marine Micropaleontology 49 (2003) 335^364 www.elsevier.com/locate/marmicro

Spatial and temporal variability of surface water in the Kuroshio source region, Paci¢c Ocean, over the past 21,000 years: evidence from planktonic foraminifera Yurika Ujiie¤ a; , Hiroshi Ujiie¤ b , Asahiko Taira c , Toshio Nakamura d , Kazumasa Oguri e a

c d

Ocean Research Institute, University of Tokyo, Minami-dai 1-15-1, Nakano-ku, Tokyo 164-8639, Japan b Izumi-cho 1156-4-338, Tachikawa Cite, Tokyo 190-0015, Japan Center for Deep Earth Exploration, Japan Marine Science and Technology Center, Natsushima-cho 2-15, Yokosuka 237-0061, Japan Tandetron AMS 14 C Dating Laboratory, Center for Chronological Research, Nagoya University, Chikusa, Nagoya 464-8602, Japan e Institute for Frontier Research on Earth Evolution, Japan Marine Science and Technology Center, Natsushima-cho 2-15, Yokosuka 237-0061, Japan Received 2 December 2002; received in revised form 11 April 2003; accepted 18 April 2003

Abstract The Kuroshio Current is the major western boundary current of the North Pacific Ocean and has had a large impact on surface water character and climate change in the northwestern Pacific region. The Kuroshio Current becomes a distinctive surface flow in the Ryukyu Arc region after diverging from the North Equatorial Current and passing through the Okinawa Trough. Therefore, the Ryukyu Arc area can be called the Kuroshio source region. We reconstructed post-21-ka time^space changes in surface water masses in the Ryukyu Arc region using 15 piston cores which were dated by planktonic N18 O stratigraphy and AMS 14 C ages. Our analysis utilized spatial and temporal changes in planktonic foraminiferal assemblages which were classified into the Kuroshio, Subtropical, Coastal, and Cold water groups on the basis of modern faunal distributions in the study region. These results indicate that the Kuroshio Current and adjacent surface water masses experienced major changes during: (1) the Last Glacial Maximum (LGM), and (2) the so-called Pulleniatina minimum event (PME) from V4,500 to 3,000 yr BP. The Kuroshio LGM event corresponds to severe global cooling and is marked by decreases in planktonic N18 O values and estimated sea-surface temperature (SST) with the dominance of the Cold water group of planktonic foraminifera. Cooling within the Kuroshio source region was enhanced during the LGM event because the Kuroshio Current was forced eastward due to the formation of a land bridge between Taiwan and the southern Ryukyu Arc which prohibited its flow into the Okinawa Trough. Except for the severe reduction and disappearance of the Pulleniatina group, no clear cooling signal was identified during the PME based on N18 O values, estimated SST values and variations in the composition of planktonic foraminiferal faunas. The PME assemblages are marked by high abundances of Neogloboquadrina dutertrei, a distinctive Kuroshio type species, along with other species assigned to the Coastal and Central water groups. Subtle ecological differences exist between Pulleniatina obliquiloculata and N. dutertrei; i.e. P. obliquiloculata exhibits lower rates of reproduction under conditions of lower primary productivity in the central Equatorial Pacific Ocean. El Nin‹o-like conditions in the Equatorial Pacific Ocean result in lower rates

* Corresponding author. Fax: +81-3-5351-6438.

E-mail address: [email protected] (Y. Ujiie¤).

0377-8398 / 03 / $ ^ see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0377-8398(03)00062-8

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Y. Ujiie¤ et al. / Marine Micropaleontology 49 (2003) 335^364

of surface transport in the Kuroshio Current. In turn, this response triggers lower rates of primary productivity in central equatorial surface waters as well as in the upstream Kuroshio source region, ultimately resulting in a lower abundance of P. obliquiloculata. Thus, we interpret the PME as a possible proxy signal of El Nin‹o-like conditions and enhancement of the El Nin‹o Southern Oscillation climate system after the PME in the tropical and sub-tropical Pacific Ocean. D 2003 Elsevier B.V. All rights reserved. Keywords: Kuroshio Current; sea surface water mass; planktonic foraminiferal assemblage; Last Glacial; Pulleniatina minimum event

1. Introduction The extent of cool tropical surface water in the Paci¢c Ocean during the glacial periods remains a controversial topic. However, it is clear that cooling would have signi¢cantly a¡ected a variety of important oceanic and atmospheric processes and patterns as emphasized by Andreasen and Ravelo (1997). This study focuses on apparent changes in sea surface temperature (SST) and circulation within the source region of the Kuroshio Current in the western Paci¢c Ocean during: (1) the Last Glacial period, and (2) the later so-called ‘Pulleniatina minimum event’ (PME). The Kuroshio Current represents the major western boundary current of the North Paci¢c Ocean and variations in its character and behavior have immediate consequences for the ocean and climate processes in the entire northwestern Paci¢c region. Recently, several authors have suggested that SST in the marginal seas of the western Paci¢c Ocean cooled by V5.0‡C during the Last Glacial period (e.g. Moore et al., 1980; Linsley et al., 1985; Thunell et al., 1994; Martinez et al., 1997; P£aumann and Jian, 1999). The latter value stands in contrast to the estimated V2.0‡C variation in SST thought to have characterized the open tropical Paci¢c Ocean during the Last Glacial period (e.g. Moore et al., 1980; CLIMAP Project Members, 1981; Ohkohchi et al., 1994). The di¡erences in cooling of SST in marginal seas vs. the open ocean likely re£ect the higher sensitivity of marginal seas to global cooling and the associated large scale changes in atmospheric and ocean circulation. The results of our study demonstrate that changing con¢gurations (e.g. geography) of marginal seas in the western Paci¢c region during the Last Glacial Maximum (LGM)

also represent an important factor which forced variations in the character and circulation of the adjacent Paci¢c Ocean. In particular, Ujiie¤ et al. (1991) and Ahagon et al. (1993) indicate that the path of the Kuroshio Current was forced to migrate eastward during the LGM due to the emergence of the Taiwan^ Ryukyu land bridge, a major geographic barrier separating the East China Sea from the open Paci¢c Ocean. An analysis of stable isotopes and frequency variations of the Pulleniatina group in 17 piston cores from the Ryukyu Arc region further details this critical change in the course of the Kuroshio Current (Ujiie¤ and Ujiie¤, 1999). In addition, a brief period of Holocene change in the Kuroshio Current, the PME, has been identi¢ed in this same area by Jian et al. (1996), Li et al. (1997) and Ujiie¤ and Ujiie¤ (1999). Most recently, Ujiie¤ and Ujiie¤ (2000) used factor analysis to compare surface water mass properties (e.g. temperature, salinity and chrolophyll) with planktonic foraminiferal assemblages in 52 surface sediment samples collected in the Ryukyu Arc region. Their works resulted in planktonic foraminiferal assemblages being classi¢ed into four groups (Kuroshio, Subtropical, Coastal and Cold water groups). In this report, we use the planktonic foraminiferal groupings of Ujiie¤ and Ujiie¤ (2000) along with a more detailed analysis of planktonic foraminiferal assemblages to establish variations in SST within the Kuroshio Current and surrounding water masses during the post-V20-kyr period.

2. Geographic and oceanographic setting The Ryukyu Arc region is located in the north-

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Fig. 1. (a) Location of surface currents and water masses in the northwestern Paci¢c Ocean. (b) Location of piston cores used in this study and main path of the Kuroshio Current in the Ryukyu Arc region. Bathymetric contours: 200, 1000 and 3000 m, respectively.

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western region of the Paci¢c Ocean (Fig. 1a). The Ryukyu Arc proper separates the East China Sea from the open Paci¢c Ocean and extends V1200 km between Taiwan and the island of Kyushu, Japan (Fig. 1b). The East China Sea includes a broad continental shelf and the adjacent Okinawa Trough which reaches water depths of more than 2000 m. The Ryukyu Arc can be subdivided geographically and geologically into northern, central and southern sections by the Tokara Strait, Kerama Gap and Yonaguni Depression, respectively (Ujiie¤ and Nishimura, 1992). In terms of general oceanographic setting, the Ryukyu Arc region belongs to the gyre margin of the Western Paci¢c Central water (Fig. 1a). The Kuroshio Current diverges from the North Equatorial Current (NEC) in the area east of the Philippine Islands and forms a western boundary current which £ows along the gyre margin (Nitani, 1972). The upper waters (0^500 m) of this boundary current £ow northward into the Okinawa Trough through the Yonaguni Depression and pass along the outer edge of the continental shelf of the East China Sea forming the main track of the Kuroshio Current (Fig. 1b). In the northern Okinawa Trough, the Kuroshio water mixes with fresh water delivered by the Huanghe and Yangtze rivers which emerge from Chinese mainland (Katoh et al., 1996). After passing through the

Okinawa Trough, the Kuroshio Current turns eastward and exits the trough through the Tokara Strait into the Paci¢c Ocean. The current £ows northeastward along the southern margin of the Japanese island of Kyushu, thence northward along the eastern margins of Shikoku and Honshu, Japan, until it meets the southward £owing Oyashio Cold Current at about 37‡N (Fig. 1a). A narrow branch of the Kuroshio Current £ows northward from the East China Sea into the Japan Sea through the Danjo Basin and the Tsushima Strait forming the warm Tsushima Current.

3. Materials For this study, we chose 11 piston cores from a set of 17 piston cores originally studied by Ujiie¤ and Ujiie¤ (1999) and used an additional four cores with the goal of detailing down-core changes in planktonic foraminiferal assemblages in the Ryukyu Arc region during the post-LGM (e.g. the last 21,000 years). Locations and water depths of the 15 cores studied are listed in Table 1 with locations shown in Fig. 1b. Six piston cores collected in the Okinawa Trough area were located directly under the main track of the Kuroshio Current along the edge of the continental shelf (cores RN96-PC1, RN93-PC6, RN93-PC4, MD98-

Table 1 Location, water depth and core lengths of piston cores used in this study Number

Core number

Latitude

Longtitude

Water depth (m)

Core length (cm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

RN88-PC5 RN92-PC3 RN92-PC4 RN93-PC1 RN93-PC3 RN93-PC4 RN93-PC6 RN93-PC8 RN93-PC12 RN94-PC3 RN94-PC6 RN95-PC1 RN95-PC3 RN96-PC1 MD982193

25‡15.5PN 28‡52.8PN 31‡40.2PN 28‡34.4PN 27‡41.5PN 26‡33.4PN 25‡41.3PN 24‡33.6PN 24‡01.3PN 30‡55.7PN 29‡45.0PN 32‡04.8PN 30‡49.8PN 24‡58.5PN 27‡23.7PN

125‡09.5PE 130‡40.1PE 128‡43.1PE 127‡12.8PE 126‡25.5PE 125‡40.4PE 124‡22.3PE 123‡44.9PE 124‡25.9PE 131‡51.1PE 131‡26.0PE 128‡59.7PE 128‡10.1PE 122‡56.1PE 126‡16.3PE

2051 2510 710 1022 1292 1440 1849 1561 2160 1536 3031 676 500 1676 1614

451 318 548 512 402 414 496 372 393 409 346 470 437 486 3747

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2193, RN93-PC3, and RN93-PC1). Two of the cores were located close to the Arc proper (cores RN93-PC8 and RN88-PC5) with another three cores collected from the Danjo Basin under the branch of the Kuroshio Current which ultimately forms the Tsushima Current (cores RN95-PC3, RN92-PC4 and RN95-PC1). Three cores were obtained from the northern Ryukyu Trench slope in the area where the Kuroshio Current £ows eastward out of the Okinawa Trough (cores RN92PC3, RN94-PC6 and RN94-PC3). Finally, one core was located on the southern Ryukyu Trench slope in an area under the direct in£uence of the Paci¢c waters (core RN93-PC12). The lithology of the 15 cores generally consists of homogeneous silty clay or clay with few burrows and thin ash or sand layers (Ujiie¤ et al., 1997a). However, core RN95-PC1 contains an exceptionally thick sand layer between V248 and 360 cm. In addition, many sand layers are intercalated with clay units in the middle part of core MD98-2193 at a core depth of V10^27 m. Only the upper V10 m of core MD98-2193 was used for this study, thus avoiding sand layers.

4. Methods and taxonomic treatment 4.1. Methods Stable oxygen and carbon isotope ratios (N18 O and N13 C vs. PeeDee Belemnite, respectively) were measured using V40 specimens of planktonic foraminifera Globigerinoides sacculifer larger than 250 Wm. Two centimeter-thick sediment samples were collected at 10-cm intervals in each core. We used specimens of Globigerinoides ruber for our measurements in core RN88-PC5 due to insu⁄cient numbers of G. sacculifer. We measured the isotope ratios using a Finnigan MAT delta E mass spectrometer at the University of the Ryukyus except for samples from cores RN95-PC3 and MD98-2193 which were measured using a Finnigan MAT 251 at Hokkaido University. Measurements displayed an external reproducibility better than S 0.05x for N18 O and S 0.03x for N13 C. In order to establish age control points in our

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cores, Accelerator Mass Spectrometer (AMS) 14 C ages were measured based on analysis of V500 specimens of Globigerinoides sacculifer or Neogloboquadrina dutertrei at 60 stratigraphic horizons using the Tandetron Accelerator Mass Spectrometer in the Dating and Materials Research Center of Nagoya University. Table 2 lists the measured AMS 14 C ages along with their calendar ages which were calibrated using CALIB 4.3 of Stuiver and Reimer (1993) about the period between 460 and 20,760 yr BP. In this study, we use the timescale of calendar ages. All sediment samples were washed through a 63-Wm screen to recover the planktonic foraminiferal assemblage at 10-cm intervals for each core. The dried residues were subsequently divided into aliquots convenient for picking of specimens. We randomly picked, identi¢ed and counted V150 specimens of planktonic foraminifera larger than 149 Wm in each sample following the procedures described by Ujiie¤ and Ujiie¤ (2000). Down-core changes in estimated SSTs were based on analysis of variations in the composition of the planktonic foraminiferal assemblages. There are two techniques for the estimation of SST using faunal data; the transfer function method and the modern analog technique (MAT). An early study by Thompson (1981) applied the transfer function (FP-12E) to estimate changes in SST in the northwestern Paci¢c Ocean. However, a later study by Prell (1985) demonstrated that the MAT is more sensitive. Moreover, the MAT approach appears to be less in£uenced by the e¡ect of carbonate dissolution (Thunell et al., 1994). Unfortunately, the data base of Prell (1985) contains few core-tops from the Paci¢c Ocean, particularly in the vicinity of the Ryukyu Arc. Therefore, we added the detailed data of this study area presented by Ujiie¤ and Ujiie¤ (2000) to the data base. In the data set, the best analog was chosen from the 10 best analogs using measurements of the squared chord distance. In this study, we discarded the reported occurrence of the so-called ‘P^D intergrade’ (e.g. specimens presumably displaying an intermediate morphology between Neogloboquadrina pachyderma and N. dutertrei) of Prell (1985), because both species can be clearly distinguished.

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Table 2 AMS 14 C ages and calendar ages used in this study Core number Depth (cm) RN88-PC5

RN92-PC3

RN92-PC4

RN93-PC1

RN93-PC3

RN93-PC4

RN93-PC6

RN93-PC8

RN93-PC12 RN94-PC3

RN94-PC6 RN95-PC1

RN95-PC3

RN96-PC1

110^114 130^134 340^344 350^354 44^48 136^140 196^200 296^300 226^236 296^306 408^414 486^496 50^52 300^303 370^371 20^24 100^104 270^274 98^100 160^163 210^213 350^351 70^74 240^242 460^462 0^4 80^84 300^304 28^30 136^138 24^26 56^58 186^188 326^328 190^194 20^22 246^250 330^332 0^4 50^54 120^124 60^64 150^154 170^174 270^274 340^344 380^384 460^464

AMS 14 C age (yr BP) 3 920 5 090 8 720 8 400 1460 4200 6000 6800 3 430 4 150 5 620 6 730 2 430 9 900 9 810 960 4 010 12 500 4 460 7 430 9 460 11 760 2 190 3 880 9 350 720 4 570 12 980 3 360 13 420 2 990 6 120 14 250 s 37 670 12 190 9 610 s 35 980 s 34 750 3 190 11 170 15 370 3 280 4 430 4 490 6 340 7 620 10 840 10 850

Table 2 (Continued). Core number Depth

Error Calibrated age (cm) (yr BP) 110 130 180 140 90 100 150 120 110 90 90 100 80 130 90 70 80 130 80 90 100 120 100 80 180 120 90 120 80 110 100 140 110 110 100

30 50 60 120 90 80 100 120 150 130

3 886 5 454 9 077 8 908 981 4 277 6 384 7 307 3 318 4 220 5 985 7 252 2 062 10 732 10 447 540 3 998 14 066 4 618 7 875 10 265 13 169 1 796 3 834 10 004 362 4 805 15 046 3 221 15 549 2 748 6 542 16 502 out of range 13 547 10 301 out of range out of range 2 972 12 824 17 788 3 114 4 568 4 685 6 785 8 047 12 164 12 174

MD982193

0 101.24 200 300 392.5 500 600 698.36 800 900 1000 1100

AMS 14 C age (yr BP) 2 020 3 500 4 300 6 910 10 420 11 270 13 580 14 440 18 540 19 970 25 390 27 710

Error Calibrated age (yr BP) 160 220 90 120 210 150 240 350 300 240 330 460

1 567 3 371 4 410 7 417 11 343 12 881 15 726 16 720 21 438 23 084 out of range out of range

4.2. Taxonomic treatment In all 43 species of planktonic foraminifera were identi¢ed in this study as listed in Table 3 with 39 of these taxa previously illustrated in Ujiie¤ and Ujiie¤’s (2000) study of Recent species in the Ryukyu Arc region. Orbulina bilobata, Globigerinita iota, Turborotalia tosaensis and Globorotalia viola were additionally recognized in the lower parts of some of the cores used and are illustrated on Plate I. However, the latter four taxa were not included in our quantitative faunal analysis as they only occur in very low abundance. For purposes of faunal analysis, some morphological species were united into single counting groups as follows: (1) the Pulleniatina group, mainly composed of Pulleniatina obliquiloculata (Parker and Jones), with a few specimens resembling Pulleniatina ¢nalis Banner and Blow, and Pulleniatina okinawaensis Natori; (2) Globigerina foliata Bolli, tentatively included with Globigerina bulloides s.s. ; and (3) Globigerinoides cyclostomus (Galloway and Wissler) and Globigerinoides pyramidalis (van der Broeck), included in the Globigerinoides ruber group. Ujiie¤ and Ujiie¤ (2000) distinguished relationships between surface water masses and modern planktonic foraminiferal faunas in the Ryukyu Arc region using multivariate faunal analysis of assemblages in 52 surface sediment samples. Ten of the 39 species recognized by Ujiie¤ and Ujiie¤ (2000) were included in an ‘Others’ category for

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Table 3 Planktonic foraminifera identi¢ed in this study and composition of counting group used in analysis Identi¢ed taxon

Code number

Counting group

Globorotalia menardii (Parker, Jones and Brady) Globorotalia tumida (Brady) Globorotalia viola Blow Neogloboquadrina dutertrei (d’Orbigny) Neogloboquadrina incompta (Cifelli) Neogloboquadrina pachyderma (Ehrenberg) Truncorotalia truncatulinoides (d’Orbigny) Turborotalia hirsuta (d’Orbigny) Turborotalia scitula (Brady) Turborotalia bermudezi (Rogl and Bolli) Turborotalia in£ata (d’Orbigny) Turborotalia crassaformis (Galloway and Wissler) Turborotalia tosaensis Takayanagi and Saito Pulleniatina okinawaensis Natori Pulleniatina obliquiloculata (Parker and Jones) Pulleniatina ¢nalis Tenuitellinata angustiumbilicata (Bolli) Tenuitella £eisheri Li Globigerinita glutinata (Egger) Globigerinita uvula (Ehrenberg) Globigerinita iota Parker Candeina nitida Ehrenberg Globoquadrina? cf. G. conglomerata (Schwager) Globorotaloides hexagona (Natland) Beella digitata (Brady) Globigerina bulloides d’Orbigny Globigerina foliata Bolli Globigerina falconensis Blow Globigerina quinqueloba Natland Globigerinella aequilateralis (Brady) Globigerinella sp. Globigerinella calida (Parker) Globigerinoides conglobata (Brady) Globigerinoides cyclostomus (Galloway and Wissler) Globigerinoides pyramidalis (van der Broeck) Globigerinoides ruber (d’Orbigny) Globigeinoides sacculifer (Brady) Globigerinoides tenellus Parker Globoturborotalia rubescens (Hofker) Sphaeroidinella dehiscens (Parker and Jones) Orbulina universa d’Orbigny Orbulina bilobata (d’Orbigny) Hastigerina parapelagica Saito and Thompson Hastigerina pelagica (d’Orbigny)

1 2 3 4 5 6 7 8 9 10 11

Globorotalia menardii Globorotalia tumida Globorotalia viola Neogloboquadrina dutertrei Neogloboquadrina incompta Neogloboquadrina pachyderma Truncorotalia truncatulinoides Turborotalia hirsuta Turborotalia scitula Turborotalia bermudezi Turborotalia in£ata group

12 13

Turborotalia tosaensis Pulleniatina group

14 15 16 17 18 19 20 21 22 23

Tenuitellinata angustiumbilicata Tenuitella £eisheri Globigerinita glutinata Globigerinita uvula Globigerinita iota Candeina nitida Globoquadrina? cf. G. conglomerata Globorotaloides hexagona Beella digitata Globigerina bulloides

24 25 26 27 28 29 30

Globigerina falconensis Globigerina quinqueloba Globigerinella aequilateralis Globigerinella sp. Globigerinella calida Globigerinoides conglobata Globigerinoides ruber group

31 32 33 34 35 36 37 38

Globigeinoides sacculifer Globigerinoides tenellus Globoturborotalia rubescens Sphaeroidinella dehiscens Orbulina universa Orbulina bilobata Hastigerina parapelagica Hastigerina pelagica

purposes of quantitative analysis because of their rare and sporadic occurrence (e.g. less than 1% of any given assemblage on average); these species include Tenuitellinata angustiumbilicata (Bolli), Turborotalia bermudezi (Rogl and Bolli), Tenuitella £eisheri Li, Candeina nitida Ehrenberg, Globo-

quadrina? cf. G. conglomerata (Schwager), Globorotaloides hexagona (Natland), Globigerinella sp., Globoturborotalia rubescens (Hofker), and Hastigerina pelagica (d’Orbigny). Thus, 27 species and species groups were ultimately subjected to cluster and factor analyses to determine faunal

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composition. Each factor was compared with key oceanographic parameters (temperature, salinity and chlorophyll). In particular, loadings in R-mode factor analysis allowed taxa to be classi¢ed into four groups, A through D (Table 4). Group A consists of warm water taxa and those taxa susceptible to carbonate dissolution including the Globigerinoides ruber species group, G. sacculifer, G. tenellus, Globigerinella aequilateralis and Globigerina falconensis. Group B includes the Pulleniatina group, Neogloboquadrina dutertrei, Globorotalia tumida and G. menardii, all of which are dissolution resistant and also characterize the Kuroshio Current water. Group C is composed of dextral coiling morphs of Neogloboquadrina pachyderma, N. incompta and Turborotalia in£ata, all indicative of ‘relatively cold’ water within the subtropical study region. Group D is composed of Globigerina bulloides and Globigerinella calida which likely re£ect the in£uence or invasion of coastal water (Table 4).

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Table 4 Four faunal groups of planktonic foraminifera classi¢ed according to factor loadings in the Q-mode cluster analysis on modern planktonic assemblages of the Ryukyu Arc region (after Ujiie¤ and Ujiie¤, 2000) Group A, Subtropical water Globigerinoides ruber group Globigerinoides sacculifer Globigerinella aequilateralis Globigerina falconenesis Globigerinoides tenellus Group B, Kuroshio water Pulleniatina group Neogloboquadrina dutertrei Globorotalia tumida Globorotalia menardii Group C, Cold water Neogloboquadrina pachyderma Neogloboquadrina incompta Turborotalia in£ata group Sphaeroidinella dehiscens Group D, Coastal water Globigerina bulloides Globigerinella calida

5. Planktonic N18 O £uctuation and SST Planktonic N18 O values in 12 cores located in slope areas of the Okinawa Trough and Ryukyu

Plate I. SEM micrographs of species found at some depth in the piston cores discussed in this study (for Recent planktonic foraminifera found in surface sediment samples from the study region, see Ujiie¤ and Ujiie¤, 2000). All specimens illustrated will be stored at the Department of Geology, National Science Museum, Tokyo, together with occurrence charts. White bar = 100 mm

1. 2,3. 4. 5. 6. 7,8.

9,10.

11.

Orbulina bilobata d’Orbigny (core RN93-PC8, 170^172 cm in core depth). This taxon is here included in the Orbulina universa population. Candeina nitida d’Orbigny (RN93-PC8, 70^72 cm); sutural supplementary pores distinct since the young stage; (a) spiral side, (b) opposite side views. Turborotalia crassaformis (Galloway and Wissler) (RN93-PC12, 180^182 cm); typical specimens not occurring in the upper portion of cores; (a) spiral side, (b) apertural-lateral side, (c) umbilical side views. Globigerinita iota Parker (RN94-PC6, 310^312 cm); (a) spiral side, (b) opposite side views. Globigerinella calida Parker (RN96-PC1, 0^2 cm); a full-grown specimen like this one never occurs in the surface sediment samples; (a) evolute side, (b) apertural-lateral views. Globorotalia viola Blow (RN93-PC8, 210^212 cm, and RN93-PC8, 270^272 cm, respectively); rarely found in the deeper portion of the study cores; peripheral keel is not well developed; (a) spiral side, (b) umbilical side, (c) apertural-lateral side views. Hastigerina parapelagica Saito and Thompson (RN96-PC1, 220^222 cm, and RN92-PC4, 60^62 cm); full-grown specimens like these were not found in the surface sediment samples; (a) spiral evolute side, (b) apertural, lateral side, (c) involute side views. Beella digitata (RN95-PC3, 2^4 cm); large abnormal form.

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Fig. 3. Time-series changes in planktonic N18 O values united in four areas (the southern Okinawa Trough, central Okinawa Trough, Danjo Basin and Ryukyu Trench slope). Area de¢ned by black arrows indicates the LGM (21^16 ka); hatched area representing V11^8.5 ka de¢nes the period of rapid warming.

Trench decrease toward the Recent (e.g. toward core-tops), likely re£ecting post-glacial warming (Fig. 2a,b). Alternatively, N18 O values increase in the oldest portions of six of the cores which penetrated sediments representing the Last Glacial period (cores RN93-PC8, MD98-2193 and RN93PC3 from the Okinawa Trough and cores RN93-PC12, RN94-PC6 and RN94-PC3 from the Ryukyu Trench slope). The AMS 14 C ages in the 12 cores generally correspond with ages estimated from the patterns of the N18 O values, except for anomalous patterns recorded in the lower parts of cores RN93-PC1 and RN88-PC5

(Fig. 2a). The N18 O values in the lower part of core RN88-PC5 are lower than those in other cores and exhibit higher £uctuation. We attribute the di¡erence to the fact that we used Globigerinoides ruber, a shallower dwelling species rather than G. sacculifer for N18 O measurement in this core. Thus, the N18 O values may possibly re£ect seasonal changes within shallow water including possible changes in salinity. The planktonic N18 O curve recorded in core RN96-PC1 suggests that a hiatus is present at V345 cm. The presence of a hiatus at this horizon is also indicated by the AMS 14 C dates.

Fig. 2. Down-core £uctuations in planktonic N18 O values and frequency of the Pulleniatina group, showing calendar ages (arrows) and AMS 14 C ages (italic) in the Okinawa Trough (a), and the Danjo Basin and Ryukyu Trench slope (b).

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In contrast to the similar N18 O patterns recorded in the 12 cores noted above, £uctuations in three cores from the Danjo Basin yield di¡erent curves (Fig. 2b). Sediments younger than V3 ka are missing in core RN95-PC3. In addition, core RN92-PC4 represents shorter sediment of the Late Holocene due to a higher rate of sedimentation recorded in this core. The N18 O values of the upper core £uctuate over a range of V3.0x, although evidence of fresh water input is lacking. Sediments penetrated by core RN95-PC1 reach Marine Isotope Stage (MIS) 6 due to the exceptionally low rate of sedimentation at this site (V5 cm/ka). MIS 1 is only represented by 30 cm in this core. When £uctuations in planktonic N18 O values in all cores are expressed in terms of time-series changes (Fig. 3), it is apparent that the majority of curves express a similar trend. The most typical case is represented by core MD98-2193 which also has the highest time-resolution among all cores studied. Heavy N18 O values (between 0 and 0.5x) were recognized during the LGM from 21 to 16 ka, and followed by a decrease in values at V11 ka, an event interpreted to represent the BXllingÂllerXd chronozones. During the Early Holocene from V11 ka to 8.5 ka, values subsequently decreased even more steeply. Estimated variations in summer (SSTs) and winter (SSTw) sea surface temperatures are illustrated on Fig. 4a,b, along with £uctuations in planktonic N18 O values. Note that SSTs displays a small ( 6 2‡C) di¡erence between MIS 2 and the post-glacial period, whereas SSTw displays a difference of 5‡C. Holocene SSTw oscillates frequently within a narrow range, particularly in the 10 cores from the Okinawa Trough and Danjo Basin, although simultaneous periodicity has not been established between these curves. In contrast, no change in SST was estimated for the LGM period because the best analog for the LGM was derived from Prell’s (1985) core-top

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data from core RC12-138. Although the latter core was collected o¡shore of east Shikoku and directly beneath the track of the Kuroshio Current (33‡00PN, 134‡09PE), reported compositions of the planktonic foraminifera in core-top samples do not correlate well with data from our cores. In order to detect small £uctuations in SST during the LGM in this study, it was necessary to obtain detailed data from the larger area of the northwestern Paci¢c Ocean. After V16 ka, post-glacial warming of planktonic N18 O values and SST are recognized at V11 ka in the Okinawa Trough area as evidenced in cores RN96-PC1 (V12 ka), RN93-PC8 (V13 ka), RN93-PC4 (V13^12 ka), MD98-2193 (V13 ka), RN93-PC3 (V12^11.5 ka) (Fig. 4a). These trends represent signals of the BXllingÂllerXd interstadial. Before the steep deglaciation, SST decreased in the same cores during a short period between V12.5 and 10 ka. This cooling represents the Younger Dryas (YD) event, although the signal did not clearly appear in the planktonic N18 O curves. Similar records are reported from the Okinawa Trough (Li et al., 2001), South China Sea (Wang et al., 1999) and Sulu Sea (Garidel-Thoron et al., 2001). In fact, Wang et al. (1999) and Pelejero et al. (1999) interpreted the small variations in planktonic N18 O values around YD time as re£ecting variations in salinity rather than SST, according to the estimations by the Uk37 alkenone index. In the period after V8 ka, the modern warm conditions established in the Okinawa Trough. However, a distinct increase in SST was delayed until V7 ka in the Danjo Basin (core RN95-PC3) and in the northern Ryukyu Trench slope area (cores RN94-PC6 and RN94-PC3). In contrast, the warming started at V12 ka in the southern Ryukyu Trench slope (core RN93-PC12) (Fig. 4b). The delayed increase of SST in the latter area expresses both the manner of post-glacial warming as well as the gradual development of

Fig. 4. Time-series changes of SSTs and SSTw compared with planktonic N18 O values in the Okinawa Trough (a), and the Danjo Basin and Ryukyu Trench slope (b). The hatched area marks the PME, whereas the dotted area marks a period of cold temperature including the LGM. Legend similar to those of the following ¢gures unless noted otherwise. The broad arrow highlights a distinct decrease of SSTw.

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Fig. 5. Time-series changes in the relative abundances of the four planktonic foraminiferal groups (Kuroshio, Subtropical, Cold and Coastal water groups) in the southern Okinawa Trough (a), central Okinawa Trough (b), Ryukyu Trench slope (c), and Danjo Basin (d). The white arrow marks the cold period immediately following the LGM.

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349

Fig. 5 (Continued).

out£ow of the main Kuroshio Current through the Okinawa Trough.

6. Time-series £uctuations of planktonic foraminiferal assemblages

time-series £uctuations and spatial-temporal variations of the four assemblage groups implying variations in oceanographic condition in the Ryukyu Arc region. 6.1. Changes of the Kuroshio water group (Group B)

As noted earlier, Ujiie¤ and Ujiie¤ (2000) analyzed the relationships between Recent planktonic foraminifera and the key oceanographic parameters in the Ryukyu Arc region and classi¢ed assemblages into the Subtropical, Kuroshio, Coastal, and Cold water groups (e.g. groups A^D). These same groupings are utilized in our analysis of fossil assemblages in this study. The following sections summarize the results of our analysis of

The Pulleniatina group (Table 3) is a typical component of Group B of Ujiie¤ and Ujiie¤ (2000) and the most characteristic faunal group of Kuroshio water. Ujiie¤ and Ujiie¤ (1999) noted that the Pulleniatina group rapidly increased in frequency toward the top of most cores analyzed in the Ryukyu Arc region along with decreasing post-glacial N18 O values. In the southern Okinawa Trough

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Fig. 5 (Continued).

samples, the post-glacial relative abundance of this group reached more than V20%. In contrast, the abundance of the Pulleniatina group declined to less than V10% during the same period in the Danjo Basin samples implying intensi¢ed northward £ow of the Kuroshio Current (Fig. 2a,b). A similarly low abundance ( 6 10%) of this group was recognized in the southern Ryukyu Trench slope (core RN93-PC12) because this site is located outside of the main path of the Kuroshio Current. The frequency pattern of the Pulleniatina group is marked by two distinct decreases over the past V40,000 years in this study area, i.e. during the Last Glacial period and the period from V4.5 to

3.0 ka. The signi¢cant decrease associated with the LGM has been ascribed to the blocking of the £ow of the Kuroshio Current during the LGM by the exposure of a land bridge connecting Taiwan and the Ryukyu Arc (Ujiie¤ et al., 1991; Ahagon et al., 1993; Ujiie¤ and Ujiie¤, 1999). The decrease of the Pulleniatina group began from V4.5 ka and continued to 3.0 ka (AMS 14 C dating by Ujiie¤ and Ujiie¤, 1999 and this study). This short span has been termed the PME by Jian et al. (1996). The disappearance of the Pulleniatina group during the Last Glacial period was associated with a distinct increase of N18 O values and decreases in SST in the Okinawa Trough and Danjo Basin. In contrast, the plank-

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tonic N18 O values recorded during the PME period did not change (Fig. 2a,b) and also SST did not signi¢cantly decrease (Fig. 4a,b). The PME was not recognized in cores RN95-PC1 and RN95-PC3 due to the low sedimentation rate and time-resolution in these cores. The PME is absent in core RN93-PC12 due to its location on the southern Ryukyu Trench slope, outside the main path of the Kuroshio Current. Thus, the PME marks a signi¢cant change in the Kuroshio water. The relative abundance of Group B appears to decrease during the LGM and PME (Fig. 5a^d). However, variations in the abundance of Group B are not as sensitive an indicator as those of the Pulleniatina group. Group B contains three other species in addition to the Pulleniatina group, including Neogloboquadrina dutertrei, Globorotalia menardii and G. tumida (Table 4). In general, the abundance of N. dutertrei is comparable to those of the Pulleniatina group (Fig. 6a,b). It is especially noteworthy that the abundance of this taxon remains relatively high during the LGM and does not decrease during the PME with only slight decreases in abundance toward the Recent. The di¡erence in abundance patterns between the Pulleniatina group and N. dutertrei suggests a di¡erence in habitat of these two taxa. N. dutertrei is characteristic of the Kuroshio Current water and also of the subtropical gyre margin (Thompson, 1981). Our data indicate that N. dutertrei thrived or even increased during both the LGM and PME (Fig. 6a,b), e¡ectively ¢lling the niche of the Pulleniatina group. 6.2. Changes of the Subtropical water group (Group A) Group A represents a eurythermal group of species characterizing the subtropical water. The down-core variations in the frequency of Group A parallel those in the N18 O curve in almost every core (Fig. 5a^d). During the Last Glacial period, Group A even decreased in cores from the Danjo Basin in which the abundance of Group A was generally lower than in other cores (Fig. 5c). The exception to this trend in core RN94-PC6 is ascribed to its location within the lysocline and the

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fact that species comprising Group A are susceptible to dissolution (Ujiie¤ and Ujiie¤, 2000). Group A displays an inverse relationship with the £uctuations of Group B in cores collected from the southern Okinawa Trough as highlighted by trends in cores RN88-PC5 and RN93PC8 from the Ryukyu Arc side (Fig. 5a). The same set of trends can be seen in cores RN94PC3 and RN92-PC3 (Fig. 5d). The alternating increases in abundance of Groups A and B are interpreted as re£ecting shifts in the boundary between the Kuroshio and subtropical water. 6.3. Changes of the Cold water group (Group C) Group C consists mainly of Neogloboquadrina pachyderma (dextral), N. incompta and Turborotalia in£ata, and is considered to be characteristic of relatively cooler water within the generally subtropical setting of the study area (Ujiie¤ and Ujiie¤, 2000). Although T. in£ata characterizes the Central water mass of the North Paci¢c Ocean (Bradshaw, 1959; Thompson, 1981), this water mass is cooler than Kuroshio water. Group C displays relatively high abundances in all cores during the Last Glacial period, particularly from V18 to V16 ka and also increased at V12 ka in contrast to the heavier N18 O values according to the YD (Fig. 5a^d). In addition, the frequency £uctuation of Group C displays a trend reversal to the frequency pattern of Group A in cores RN94PC3, RN94-PC6, RN92-PC3 and RN93-PC12 from the Ryukyu Trench slope (Fig. 5d). The abundance of Group C in the Okinawa Trough cores decreased after 11 ka and almost disappeared in younger sediments approaching core-tops, a trend which occurred in concert with upward decreasing N18 O values (Fig. 5a,b). Although members of Group C never disappeared near core-tops from the Danjo Basin (Fig. 5c) and Paci¢c Ocean (Fig. 5d), dextral Neogloboquadrina pachyderma and N. incompta are decreasing. N. pachyderma was abundant prior to V11 ka, especially during the LGM. In contrast, Turborotalia in£ata increased during the post-glacial period in the Danjo Basin and northern Ryukyu Trench slope, despite the fact that this species is a member of Group C (Fig. 7). In addition, T. in-

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Fig. 6. Time-series changes in the relative abundances of the Pulleniatina group and Neogloboquarina dutertrei, both taxa belonging to Group B (Kuroshio water), in the Okinawa Trough (a), and the Danjo Basin and Ryukyu Trench slope (b).

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353

Fig. 7. Time-series changes in the relative abundances of the three species which compose Group C, in the Danjo Basin and Ryukyu Trench slope.

£ata increased in frequency during the PME within core RN92-PC4 from the Danjo Basin and cores RN94-PC3 and RN92-PC3 from the northern Ryukyu Trench slope, implying that the Central water mass invaded this area during this time. 6.4. Changes of the Coastal water group (Group D) Frequency £uctuations of Group D are inverse to those of the Kuroshio group (Group B) in all cores (Fig. 5a^d) as exempli¢ed in central Okinawa Trough cores (RN93-PC4, MD98-2193, RN93-PC3, and RN93-PC1). In particular, there was a distinct increase in Group D during the PME (Fig. 5b). This group is characteristic of coastal water and the edge of the continental shelf (Ujiie¤ and Ujiie¤, 2000) and is dominated by Glo-

bigerina bulloides, a species correlated with continental shelf water by Xu and Oda (1999). Thus, increases in the frequency of Group D are thought to express the spread of coastal water within the Okinawa Trough (particularly in its northern area) during the PME when the Kuroshio Current (Group B) diminished. Fig. 8 displays time-series £uctuations in occurrences of Globigerina quinqueloba, Neogloboquadrina pachyderma and Group D. G. quinqueloba is considered to be the most characteristic species of coastal water by Takemoto and Oda (1997) and Xu and Oda (1999). This species was not included in any faunal grouping utilized by Ujiie¤ and Ujiie¤ (2000) due to its relatively uncommon occurrence in their study area as a whole. However, signi¢cant occurrences of G. quinqueloba occur during the LGM in cores located near the

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Fig. 8. Time-series changes in the relative abundances of Globigerina quinqueloba, Neogloboquadrina pachyderma and Group D (Coastal water group), in the Okinawa Trough, the Danjo Basin and Ryukyu Trench slope. Note the enlarged scale for G. quinqueloba.

outer continental shelf in the Danjo Basin and central Okinawa Trough. The occurrence of G. quinqueloba becomes more distinct toward the northeast and remains signi¢cant through the post-glacial period in the Danjo Basin cores. The highly £uctuating occurrences of G. quinqueloba, Group D and N18 O values in core RN92PC4 from the Danjo Basin may in fact re£ect the unstable £ow of Huanghe River water into this area after V7.5 ka. In contrast, low frequencies of G. quinqueloba were recorded in cores RN94-PC3 and RN92-PC3 from the northern Ryukyu Trench slope. Frequency patterns of Globigerina quinqueloba and those of Group D in the Okinawa Trough during the LGM likely re£ect the seaward exten-

sion of the continental coastline into this area during this period. In addition, it is also important to note that G. quinqueloba disappeared from this study area during the PME but was present during the LGM. 6.5. Spatial distribution of the four groups at six time planes In order to clarify the space^time distribution of planktonic foraminiferal assemblages we present spatial distribution patterns for the four groups (A^D) and ‘Others’ along six time planes over the past V21,000 calendar years (Fig. 9a^f). Group B, representing Kuroshio water, is subdivided into the Pulleniatina group and others, the

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355

356


main component of which is Neogloboquadrina dutertrei, referred to simply as N. dutertrei. Group C, representing Cold water, is subdivided into Turborotalia in£ata and Cold water species, respectively. The Pulleniatina group almost disappeared from beneath the Kuroshio Current area during the PME (Fig. 9b), whereas Neogloboquadrina dutertrei and Group D (Coastal water-type) were more abundant in the Okinawa Trough than during the present (Fig. 9a). In addition, a remarkable increase of Turborotalia in£ata was recorded in cores from the Danjo Basin and northern Ryukyu Trench slope. Warm climate conditions prevailed in the Ryukyu Arc region throughout the Holocene except during the PME. The hypsithermal event (Fig. 9c) at V6,600 yr BP is marked by foraminiferal distributions which are essentially equivalent to modern distributions in this area. Especially, the Pulleniatina group prevailed over the entire study area. The relative abundance of Cold water-type species (Group C) was low during the PME in contrast to the high abundance of this group during the LGM (compare Fig. 9b,f). Species included in ‘Others’ also display inverse patterns during the LGM and PME. Species placed in ‘Others’ include taxa which occur in abundances too low to be used for individual statistical analysis. Thus, the total abundance of ‘Others’ re£ects species diversity of this area with low diversity during cold periods including the LGM and high during the PME and present. These latter trends are supported by £uctuations in N18 O values (Fig. 2a,b). However, Group D species representing Coastal water prevailed in the Okinawa Trough region during the LGM and also PME, i.e. periods marked by the diminished in£uence of the Kuroshio Current. Turborotalia in£ata, representing Central water, occurs in signi¢cant abundances in the Tokara Strait and northern Okinawa Trough during the PME (Fig. 9b). Therefore, Central water covered

the northern trough through the strait during this period. Cold water-type species (Group C) and dextral Neogloboquadrina dutertrei decreased in abundance in the study area from V21,000 to 16,000 yr BP (Last Glacial period) accompanied by the disappearance of the Pulleniatina group (Fig. 9e,f). Group C species were most dominant at V16,500 yr BP in the Okinawa Trough rather than at V21,000 yr BP, a pattern corresponding to the increase of N18 O values (Fig. 3). Cold water elements gradually decreased from V16,500 to 12,000 yr BP (Fig. 9d,e) whereas the Pulleniatina group and Group A (Subtropical water-type) increased. The abundance of Group D (Coastal water-type) gradually decreased in the central and southern Okinawa Trough during the same period.

7. Discussion Planktonic foraminiferal distributions indicate that the drastic changes of the Kuroshio Current and surrounding surface water masses occurred twice during the past V21,000 years including the Last Glacial period and the shorter PME from V4,500 to 3,000 yr BP. However, there are signi¢cant di¡erences between the £uctuations of planktonic N18 O values during these two periods; values clearly decreased during the former period whereas they did not exhibit signi¢cant change during the PME (Fig. 2a,b). 7.1. Paleoceanography during the Last Glacial period Two important components of paleoceanographic changed in the Ryukyu Arc region during the Last Glacial period. One is the change of surface water character in the tropical and subtropical areas representing response to global cooling. The other involves changes in the circulation of surface water imposed by changes in the geogra-

Fig. 9. Spatial distributions of the four groups and ‘Others’ in terms of their relative abundance at present (a), PME at V4,000 yr BP (b), V6,600 yr BP (c), V12,000 yr BP (d), V16,500 yr BP (e), and V21,000 yr BP (f), respectively.

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phy of the region related to variations in sea level, especially changes in the adjacent marginal seas. Large scale changes in major surface current systems during the LGM have been proposed for several areas. For example, Lynch-Stieglitz et al. (1999) estimated that the Gulf Stream, the western boundary current of the North Atlantic Ocean, was relatively weak (about 2/3) during the LGM based on interpreted changes in the density of the water column deduced from benthic N18 O values in cores from the Florida Strait. In addition, Vink et al. (2001) proposed a southward migration of the NEC in the Atlantic Ocean during the LGM as the product of a southward migration of the trade wind belt and compression of the tropical climatic zone. However, application of this case to the apparent shifts in the Kuroshio Current cannot be made because no direct evidence of such a change in circulation in the North Paci¢c Ocean has been put forward. The lower sea level during the LGM is known to have imposed major changes in the geography of the continental margin and insular areas of the marginal northwestern Paci¢c region. Oba (1988) estimated that sea level lowered by 127 S 30 m in the East China Sea during the LGM. In addition, Ujiie¤ et al. (1991) and Ujiie¤ and Ujiie¤ (1999) suggested the formation of a land bridge connecting the central and southern Ryukyu Arc with Taiwan during this same period. At present, the Kuroshio Current enters the Okinawa Trough across a sill with a present maximum water depth of 800 m located near Yonaguni Island in the southern Ryukyu Arc. The Yonaguni sill is thought to have formed by tectonic subsidence of the Ryukyu^ Taiwan land bridge at V10,000 yr BP as a product of northward arcârc collision between the Ryukyu and Luzon Arcs, a process driven by spreading in the southern Okinawa Trough (Ujiie¤, 1994; Hsu and Sibuet, 1995; Ujiie¤ et al., 1997b). An obvious hiatus dated between V12,000 and 8,000 yr BP was found in core RN96-PC1 collected near the Yonaguni sill and the Okinawa Trough entrance of the Kuroshio Current (Fig. 2a). This hiatus likely records erosion by a fully developed £ow of the Kuroshio Current into the Okinawa Trough after V11,000 yr BP during the course of subsidence of the Yonaguni sill. A

357

80.5-cm long core (RN96-PC2) collected on the westerly ridge of the southern Ryukyu Arc is heterogeneous in character with sand-mottles containing silt and irregular shaped sand layers (Ujiie¤ et al., 1997a). These characteristics are thought to re£ect post-Holocene neotectonic movement which occurred in southern Ryukyu Arc. Independent of the land bridge problem, Tanimura (1999) proposed that a northward shift of the Kuroshio Current took place after V11 ka based on analysis of down-core changes in the distribution of varieties of the diatom Thalassionema nitzschioides in cores o¡ Honshu, Japan. T. nitzschioides is abundant in the warm saline waters of the North Paci¢c subtropical gyre and Tanimura (1999) used sediment trap data and variations within this species to estimate the position of the current. Any £ow prohibited from entering the Okinawa Trough would be turned eastward

Fig. 10. Locations of the main path or track of the Kuroshio Current at present and during the LGM. The shaded area and associated coastline depict the geography during a period of lowered sea level (ca. 200 m, LGM).

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around the southern tip of the Ryukyu Arc (Fig. 10), although Tanimura (1999) did not present a precise location of the ‘ancient’ Kuroshio Current. Several studies have indicated that the SST of the tropical Paci¢c Ocean remained similar to the present or was slightly cooler (less than 2‡C) during the LGM (Moore et al., 1980; CLIMAP Project Members, 1981; Ohkohchi et al., 1994). As the Kuroshio Current originates from the NEC, it might be expected that the SST of the Kuroshio Current hardly changed during the LGM. However, the di¡erence between the LGM and present N18 O values in the Ryukyu Arc region was calculated to be V1.5^1.8x (Fig. 2a,b). The changes in N18 O values that can be attributed to ice volume decrease during the post-LGM period was estimated to be 1.2x by Fairbanks (1989). If the ice-volume e¡ect is subtracted from the glacialînterglacial di¡erence between planktonic N18 O values, the residual value should represent an upper limit of the contribution attributable to corresponding change in SST where a 0.2x di¡erence in N18 O values corresponds to a 1.0‡C di¡erence in temperature. Applying these assumptions, we estimated the vSST between the LGM and present in the Kuroshio source region as V3.0‡C based on data from four cores from the Paci¢c Ocean side of the Ryukyu Arc where the in£uences of low salinity continental shelf water can be discounted. Di¡erences in the LGM and modern SST in the marginal area of the western Paci¢c Ocean have been estimated to range between 3.0 and 6.0‡C based on planktonic SST and N18 O values (e.g. Moore et al., 1980; Linsley et al., 1985; Thunell et al., 1994; Martinez et al., 1997; P£aumann and Jian, 1999). These latter values preclude signi¢cant latitudinal heat transfer and support heat circulation via a shifting of the entire Kuroshio Current system. Although analysis of changes in planktonic foraminiferal fauna has been used to estimate variations in SST, the relationships between faunal changes and hydrographic variables associated with vertical convection in the surface and intermediate layers of the ocean must also be considered. Wind-driven mixing or upwelling of cold nutrient-rich intermediate water could have

caused cooling in marginal areas of the western Paci¢c Ocean (Linsley et al., 1985; Huang et al., 1997; Wang et al., 1999), in turn triggering increased primary productivity (Kawahata, 1999; Garidel-Thoron et al., 2001). For example, Wang et al. (1999) have proposed that strengthened winter monsoon e¡ects occurred in the South China Sea during the LGM. Based upon analyses of key hydrographic parameters (e.g. temperature, salinity, nutrients as well as £uvial and eolian sediment supply), the latter authors considered cyclic changes of the winter/summer monsoon and concluded that the predominant monsoon was caused by variations in solar radiation in the northern hemisphere. This interpretation explains why vSSTw exhibits a larger difference than vSSTs in not only the South China Sea but also the Ryukyu Arc region represented in our study (Fig. 4a,b). Moreover, it is possible that the monsoon-driven upwelling preceded cooling and subsequent changes of the thermocline. Interestingly, a high abundance of Neogloboquadrina dutertrei, a species thought to be associated with strong mixing or upwelling (Chen and Prell, 1998; Chen et al., 1998; Martinez et al., 1998), is present within LGM sediments in the Ryukyu Arc region. However, we interpret the LGM trend of N. dutertrei as re£ecting the presence of gyre margin water rather than due to strong vertical mixing, based on associated evidence of a strati¢ed water column as explained below. This study revealed signi¢cant occurrences of the Coastal water group (particularly Globigerina bulloides) and the Cold water group during the LGM (Figs. 5a^d and 9e,f) implying a slight but important expansion of coastal water over the Okinawa Trough during this period, particularly in the northern part of the area. In addition, Globigerina quinqueloba also appeared in signi¢cant abundance during the LGM in the Danjo Basin and the northern Okinawa Trough (Fig. 8). G. bulloides and G. quinqueloba both prefer relatively low temperature and salinity and represent members of the subarctic and transitional faunas of Bradshaw (1959). G. bulloides is also commonly associated with upwelling zones in the subtropical water according to Be¤ (1977). In particular, G. bul-

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loides and G. quinqueloba were shown to have a preference for low temperature water which was mixed with river water in the East China Sea based on studies of surface sediment samples by Xu and Oda (1999). Decreases in the abundance of these two species toward core-tops in the Okinawa Trough (Fig. 8) suggest that fresh and low temperature water retreated from this area after V10 ka due to the strengthened £ow of the Kuroshio Current. Faunal evidence in core RN95PC3 indicates that a fresh water in£uenced continued to V8.5 ka in the Danjo Basin. Recently, Tanimura et al. (2002) reported the changes in the occurrence of the benthic diatom Paralia sulcata over the last V30 kyrs in cores from the Danjo Basin and south o¡ Shikoku Island which forms an extension area of the Kuroshio Current. The abundance of this species distinctly increased in the Danjo Basin core from 16 to 9 ka including the LGM, whereas it disappeared in the Shikoku core at V13 ka. These results suggest that P. sulcata frustles were not carried from the Okinawa Trough through the Tokara Strait to the Paci¢c Ocean because of no £owing of the Kuroshio Current, although the coastal water extended seaward. 7.2. The Pulleniatina minimum event After V11 ka, planktonic N18 O values in the Okinawa Trough display small short-time oscillations ( S 0.5x to S 1.0x) while the SSTw values display similar oscillations (Fig. 4a,b). These oscillations may re£ect fresh water input into the Trough. However, Ujiie¤ et al. (2001) noted that the input of terrigenous materials from continental rivers constantly decreased in post-LGM time, based upon analysis of organic matter (i.e. organic C/N ratio, C28/C16 fatty acid ratio, lignin phenol content) in six cores from the edge of the Okinawa Trough. These results suggest a gradual decrease in fresh water input to the Okinawa Trough after the LGM. In marked contrast to the general pattern for Holocene warming, the PME represents a major change in oceanographic conditions in the Kuroshio source region from V4,500 to 3,000 yr BP. The PME has been interpreted as a re£ection

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of a short cooling period by Jian et al. (1996), based upon analysis of SST and thought to be equivalent to the ‘Neo-glacial period’ of Denton and Karlen (1973). However, Jian et al. (1996) estimated changes in SST using the transfer function FP-12E, which is changed by composition of planktonic foraminiferal assemblages. This latter program includes Pulleniatina obliquiloculata as a prominent proxy of warm water. Therefore, when P. obliquiloculata decreases in abundance, calculated SST values necessarily decrease. Ujiie¤ and Ujiie¤ (1999) and Li et al. (2001) rejected this presumed cooling in the Ryukyu Arc region during the PME, based on a lack of change in planktonic N18 O values in samples representing this event. For this study, we re-calculated the SST values using the revised MAT data. This analysis yielded relatively stable SST values throughout 11 cores with the exception of cores RN96-PC1, RN93PC4, RN93-PC1 and RN94-PC3, with SSTw decreasing by at most V4‡C during the PME (Fig. 4a,b). In addition, abundance of the Cold water group of planktonic foraminifera did not increase during the PME except in core RN94-PC3 collected from the northernmost site in the study area (Fig. 5a^d). Moreover, the composition of the Cold water group displaying increases in core RN94-PC3 is dominated by Turborotalia in£ata, a species characteristic of the Central water. These faunal and isotopic patterns indicate that the Kuroshio Current weakened as a boundary current during the PME with minimal £ow of the current allowing Coastal water to invade the Okinawa Trough and Central water to move into the northern Ryukyu Arc region (Fig. 9b). In contrast to the PME, the distribution of the Cold water group is much clearer during the LGM (Fig. 9b vs. Fig. 9d^f). Neogloboquadrina dutertrei, a species characteristic of Kuroshio water, increased during the PME when abundance of the Pulleniatina group decreased (Fig. 6a,b). Both taxa inhabit the thermocline layer in the Atlantic Ocean (Ravelo et al., 1990) and have maximum abundances at a depth of V120 m in the Sargasso Sea (Be¤, 1960). Similar habitats for both taxa were also observed during a plankton-net tow study in the Equatorial Paci¢c Ocean (Watkins et al., 1998). However,

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N. dutertrei is also characteristic of the gyre margin assemblage whereas the Pulleniatina group is essentially limited to equatorial waters (Thompson, 1981). Assuming that Pulleniatina obliquiloculata is transported from the equatorial area to higher latitudes via the Kuroshio Current, any signi¢cant decrease in strength of this current should result in lower occurrences of this group in our study area, in contrast to increases of N. dutertrei. In addition, Watkins et al. (1998) found slightly di¡erent distributions of N. dutertrei and P. obliquiloculata in relation to primary productivity based on a plankton-net tow study in

the central Equatorial Paci¢c Ocean (Fig. 11a). These authors found that P. obliquiloculata prefers areas of higher primary productivity. At present, essentially continuous upwelling of nutrient-rich water provides a suitable habitat for P. obliquiloculata in the central Equatorial Paci¢c (Fig. 11b). However, if equatorial upwelling is diminished or becomes weaker due to a deepening of the thermocline, primary productivity and the abundance of P. obliquiloculata should decrease. The PME was recognized in the South China Sea (Jian et al., 1996; Wang et al., 1999; P£aumann and Jian, 1999) and probably recognized in

Fig. 11. (a) Variations in standing stocks of Pulleniatina obliquiloculata and Neogloboquadrina dutertrei tests in relation to the integrated primary production in the Equatorial Paci¢c Ocean (modi¢ed after Watkins et al., 1998). P. obliquiloculata is more dominant in the environment of enough primary productivity (more than V0.40 mmol/cm2 /d in integrated primary productivity) supplied with strong tropical upwelling (1), whereas the standing stock of N. dutertrei increases below this boundary of integrated primary productivity caused by diminished tropical upwelling (2). (b) Comparison of the surface circulation of the Equatorial Paci¢c Ocean, particularly the western Paci¢c Ocean, between a normal year (1) and an El Nin‹o year (2). In a normal year, the bifurcation point of the NEC exists at a more southern latitude and the nutrient-rich water is supplied to the central Equatorial Paci¢c associated with the upwelling alike the condition shown in (a,1). Thus, the Kuroshio Current has a maximum transport. In contrast, during the PME the eastward extension of the Western Paci¢c Warm Pool blocks the nutrient-rich water toward the central Equatorial Paci¢c alike the condition shown in (a,2) and the NEC bifurcation point moves to the northernmost area. Thus, the transport of the Kuroshio Current becomes minimal and the habitat of P. obliquiloculata becomes limited.

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the NEC area by Boltovskoy (1990). Wang et al. (1999) and P£aumann and Jian (1999) suggested that the PME was not a product of cooling based upon SST values estimated using the Uk37 alkenone index and the transfer function ‘SIMMAX28’ modi¢ed by adding data from the South China Sea. Thus, the PME should be considered as a response to a change in the sub-surface character of the Equatorial Paci¢c Ocean. When considering change from tropical to subtropical circulation in the northwestern Paci¢c Ocean, it is important to understand the associated changes in volume of geostrophic transport within the major surface currents involved in this process. Based on oceanographic observation, several research groups have noted that the transport within the Kuroshio Current correlates with the bifurcation position of the NEC in the western Equatorial Paci¢c Ocean (Qiu and Lukas, 1996; Metzger and Hurlburt, 1996). At the point of bifurcation, the NEC is diverted into the northern £ow (the Kuroshio Current) and southern £ow (the Mindanao Current) o¡ the Philippines (Fig. 11b). The Kuroshio Current has minimum transport in the fall or prior to El Nin‹o conditions when the NEC bifurcates at the northernmost latitude (Qiu and Lukas, 1996). Therefore, the El Nin‹o-like environment during the PME was likely preceded by minimum transport of the Kuroshio Current. The latter condition would have been accompanied by a deeper thermocline causing lower rate of primary productivity in subsurface water of the central Equatorial Paci¢c Ocean and a poor habitat for Pulleniatina obliquiloculata. However, Akitomo et al. (1996) and Kawabe (2001) reported that the transport of the Kuroshio Current increased during El Nin‹o conditions with intensi¢cation of the trade wind. Although it is necessary to ascertain long-term relationships between the Kuroshio Current and El Nin‹o circulation, our data suggest that the PME was associated with an El Nin‹o-like climate mode. Recent reports indicate that the periodicity of the modern El Nin‹o system in the Equatorial Paci¢c Ocean was probably initiated after V5 ka, based upon analyses of sub-marine core, lake sediments and modeling (Rodbell et al., 1999;

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Clement et al., 2000; Koutavas et al., 2002). In particular, Koutavas et al. (2002) indicate that the strongest east^west gradient of SST in the Equatorial Paci¢c Ocean occurred during the Mid-Holocene (V5^8 kyr BP), coincident with minimum seasonality of insolation and subsequent cooling. Koutavas et al. (2002) link this La Nin‹a-like pattern, enhanced SST gradient and strengthened trade winds to a short period of cooling V6,000 yr BP. Moreover, Barron et al. (in press) presented precisely the transition from the La Nin‹a-like pattern to the modern El Nin‹o-Southern Oscillation (ENSO) system at ca. 3.2 ka at ODP site 1019 o¡ southern Oregon. From 8.2 to 3.2 ka, they estimated that the alkenone SSTs became 1‡C cooler and the California Current stronger with enhanced upwelling, which was supported by a decrease of the Central gyre diatom Pseudoeunotia doliolus, and an increase of the upwelling diatom Thalassionema nitzschioides. However, this marine condition changed to the modern system at ca. 3.2 ka (i.e. after the PME). In accordance with this change, pollen analysis suggests that the continental condition became highly seasonal like the present one from ca. 5.2 to 3.5 ka, as evidenced by the steady increase of coastal redwood and alder. Therefore, the present ENSO pattern may have started immediately after the Mid-Holocene cooling event in the Equatorial Paci¢c Ocean. In other words, the PME may signal the initiation of the typical or modern ENSO system.

8. Conclusions The major conclusions of our analysis can be summarized as follows. (1) The Kuroshio Current was prevented from £owing into the Okinawa Trough during the LGM by a land bridge formed between Taiwan and the southern Ryukyu Arc as a product of the associated low stand of sea level. This geographic condition a¡ected heat transport within the Kuroshio source region leading to lowered planktonic N18 O and SST values and the appearance of Cold water species, despite only minimal variations in SST in the tropical Paci¢c Ocean during

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the LGM. During this same period, Coastal water species spread over the Okinawa Trough, in particular from the northern part, as the Kuroshio Current retreated from this area. (2) Planktonic N18 O and SST records indicate that the Last Glacial period ended at V13 ka with distinct deglaciation beginning V11 ka. Based on the increased presence of the Kuroshio Current faunal group and decreasing abundances of the Cold and Coastal water groups, the Kuroshio Current began to re-enter the Okinawa Trough during the post-13-ka period as sea level rose. Subsequently, portions of Taiwan and the southern Ryukyu Arc area tectonically subsided and eroded to increasing depths allowing full £ow of the Kuroshio Current into the Okinawa Trough. A prominent hiatus at V11 ka in core RN96-PC1 from the southern Okinawa Trough may be related to this event. (3) The in£uence of the Kuroshio Current declined in the study region during the PME between V4.5 to 3 ka and Coastal and Central water planktonic foraminiferal groups spread over the Okinawa Trough in some extent. In contrast to the LGM patterns, the abundance of the Cold water group did not increase during the PME and, except for a single core in the northernmost part of the study area, SST values remained relatively constant. Thus, no cooling of surface water was recognized in the study region during PME time. (4) Impact of the PME has been recognized outside the Kuroshio source region including the South China Sea and probably within the Equatorial Paci¢c Ocean. Thus, the PME was likely a response to a broad scale oceanographic^climatic process or mechanism. (5) Evidence developed during this study suggesting that the PME is associated with enhancement of the El Nin‹o-ENSO climate mode in the tropical Paci¢c region. Our analysis revealed that Neogloboquadrina dutertrei became abundant in the study region during the same period that the Pulleniatina group diminished or disappeared. Because N. dutertrei characterizes not only the Kuroshio water but also water of the subtropical gyre margin, segregation of these two taxa of differing habitats likely occurred in response to mi-

gration of the Kuroshio Current. P. obliquiloculata £ourishes under conditions of higher primary productivity, within the thermocline zone and strengthened tropical upwelling in the Equatorial Paci¢c Ocean (Watkins et al., 1998). A deeping of the thermocline during the PME would have reduced primary productivity, creating a habitat unstable for P. obliquiloculata. Decreasing transport within the Kuroshio Current during the PME is suggested by faunal evidence of the other surface water masses within the Kuroshio source region during this period. All of these patterns collectively suggest that the PME is associated with the beginning of the typical ENSO system in the Equatorial Paci¢c Ocean following a sustained period marked by the La Nin‹a mode (V5^8 ka). However, this hypothesis should be ascertained by further study on more precise ecology of these species, particularly through plankton-net tow study.

Acknowledgements This work was mainly carried out by Y.U. for the obtaining of a Ph.D. Thesis at the Ocean Research Institute, University of Tokyo, under the supervision of A.T. We like to thank Tadamichi Oba and Michiyo Shimamura of Hokkaido University for facilitating the stable isotope measurement of two cores. Eiji Matsumoto, Nagoya University, ¢nancially supported the AMS 14 C age dating of Core MD98-2193 by K.O. We express our appreciation to Hodaka Kawahata, Geological Survey of Japan, and Franck Bassinot, LSCE, CNRS, for o¡ering the chance to Y.U. to join the IMAGES IV cruise. Tomonori Ono, University of the Ryukyus, greatly assisted us on board and during laboratory works. We acknowledge the captains and crews of the R/V Nagasaki-maru and Marion Dufresne for their co-operation aboard. We are much indebted to James C. Ingle, Jr., Stanford University, CA, for his thorough and critical review of the ¢rst draft of this paper. Thanks are due also to John A. Barron and an anonymous reviewer for their critical reviews and to Jere H. Lipps, editor, for improvement of the manuscript.

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