Holocene variability of the Kuroshio Current in the OkinawaTrough, northwestern Paci¢c Ocean

Zhimin Jian a;c;*, Pinxian Wang a, Yoshiki Saito b, Jiliang Wang a,Uwe P£aumann c, Tadamichi Oba d, Xinrong Cheng a

a Laboratory of Marine Geology, Tongji University, Shanghai 200092, Chinab Marine Geology Department, Geological Survey of Japan, Higashi 1-1-3, Tsukuba 305, Japan

c Institut fu«r Geowissenschaften, Universita«t Kiel, D-24118 Kiel, Germanyd Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060, Japan

Received 26 April 2000; received in revised form 23 October 2000; accepted 25 October 2000

Abstract

Cores 255 and B-3GC from the southern and northern Okinawa Trough, respectively, were studied for determiningthe variability of the Kuroshio Current on centennial scales during the Holocene based on detailed AMS 14C dates,stable isotope, and planktonic foraminiferal distribution. The main flow of the Kuroshio Current was inferred to re-enter the Okinawa Trough at V7.3 calendar (cal.) kyr BP, leading to abrupt changes in sedimentation rate, remarkablyincreased abundance of the Kuroshio Current indicator Pulleniatina obliquiloculata, increased sea surface temperature(SST) and depth of thermocline (DOT). During V4.6^2.7 cal. kyr BP, the abundance of P. obliquiloculata sharplydecreased, corresponding to a decreases in SST and DOT, implying that the influence of the Kuroshio Currentweakened at that time, possibly as a result of the intensified winter monsoon. Significantly, the Kuroshio Currentproxies (e.g. the difference in SST between the southern and northern Okinawa Trough) display periodicities of V1500yr and V700^800 yr (ascribed to the second harmonic of the 1500 yr cycle in oceanic thermohaline circulation) duringthe Holocene. The Holocene events (at V0.6, 1.7, 3.3, 4.6, 5.9, 8.1, 9.4 cal. kyr BP) in the Okinawa Trough appear to bethe most recent manifestation of the millennial scale climate cycle, recorded in the North Atlantic, Arabian Sea marinesediment cores and Greenland ice cores, suggesting global climatic tele-connections. ß 2000 Elsevier Science B.V. Allrights reserved.

Keywords: Kuroshio; sea-surface temperature; Foraminifera; stable isotopes; Holocene; Okinawa Trough

1. Introduction

The Kuroshio Current originates from theNorth Equatorial Current in the western Paci¢c,

and carries warm and saline water £owing east ofTaiwan and entering the Okinawa Trough to-wards the north. Its main axis is in the area ofstrongest heat exchange between sea and atmos-phere in the western Paci¢c, strongly in£uencingthe East Asian climate, upper-ocean thermalstructure and distribution of marine sediments inthis region [1,2]. Changes in the Kuroshio Currentduring the late Quaternary, especially since the

0012-821X / 00 / $ ^ see front matter ß 2000 Elsevier Science B.V. All rights reserved.PII: S 0 0 1 2 - 8 2 1 X ( 0 0 ) 0 0 3 2 1 - 6

* Corresponding author. Present address: Institut fu«r Geo-wissenschaften, Universita«t Kiel, D-24118 Kiel, Germany.Fax: +49-431-8804376; E-mail : zj@gpi.uni-kiel.de

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last glacial stage, have been studied by Chineseand Japanese researchers [3^12]. Previous studiesrevealed that during the last glacial maximum theKuroshio Current was not present in the OkinawaTrough but shifted to a position east of the Ryu-kyu Islands [4,8,11]. During the Holocene atV7.5^6.0 kyr BP, the main axis of the KuroshioCurrent re-entered the Okinawa Trough [5,8].Since then, the path of the Kuroshio Currenthas shifted several times [3,8,9]. The late Holocenecooling event at V4.0^2.0 kyr BP, re£ected by theremarkable decrease in the abundance of theplanktonic foraminifer Pulleniatina obliquiloculata(the Kuroshio Current indicator species) [6^8,11,12], suggested that the Kuroshio Currentweakened or shifted to the Paci¢c at that time[8]. However, there are vigorous disputes aboutthe changes in path and strength of the KuroshioCurrent during the Holocene [8^11], and fewhigh-resolution paleoceanographic records withdetailed age controls.

For this reason, we selected cores 255 and B-3GC from the southern and northern OkinawaTrough, respectively, and attempted to determinethe variability of the Kuroshio Current on centen-nial scale during the Holocene based on detailedAMS 14C datings, stable isotopes, and planktonicforaminiferal distributions.

2. Oceanographic settings

The most striking oceanographic feature in theOkinawa Trough is the Kuroshio Current, which£ows northeastward along the edge of the conti-nental shelf and then leaves the Okinawa Troughthrough the Tokara Strait (Fig. 1). A branch ofthe Kuroshio Current known as the TsushimaCurrent runs northward from the OkinawaTrough into the Sea of Japan through the Tsu-shima Strait [1,2]. The Okinawa Trough as thesoutheastern part of the East China Sea (ECS)is occupied by the open-sea water mass withhigh temperature and deep thermocline due tothe in£uence of the Kuroshio Current. The con-tinent shelf in the northwestern ECS is dominatedby a coastal water mass with low temperature anda shallow thermocline. From the northwest to the

southeast in the ECS, the sea surface tempera-tures (SST) are 9^23³C in winter and 26^29³C insummer; the sea water temperatures at a depth of100 m (shortened as SWT) are 13^22³C in winterand 15^25³C in summer; the annual depths ofthermocline (DOT) are V10^275 m [13].

The two gravity cores 255 (123³07PE, 25³12PN,water depth 1575 m) and B-3GC (128³31PE,31³29PN, water depth 555 m) used in this studywere taken from the continental slope of southernand northern Okinawa Trough, respectively (Fig.1). Core 255 is beneath the main axis of the Kur-oshio Current and close to the area where theKuroshio Current changes direction o¡ northeast-ern Taiwan. The SSTs at this site are 23.2³C inwinter and 29.1³C in summer, the SWTs are21.9³C in winter and 22.8³C in summer, and theannual DOT is V200 m. Core B-3GC, taken dur-ing the BO 94-20 cruise of R/V Bosei-Maru in1994, is just east of the Tsushima Current, amain branch of the Kuroshio Current. The SSTsat this site are 18.5³C in winter and 28.5³C insummer, the SWTs are 16.9³C in winter and20.5³C in summer, and the annual DOT isV160 m [13]. Therefore, the SSTs, SWTs andDOTs at both sites are quite di¡erent, with de-creasing values from south to north in the Okina-wa Trough.

Fig. 1. Location of two cores used in this study. Shaded ar-rows show the Kuroshio Current and its branches.

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3. Materials and methods

The sediments in cores 255 (655 cm long) andB-3GC (218 cm long) consist of dark gray clayand silty clay. A total of 170 samples were ana-lyzed for planktonic foraminifera, with 5^10 cmsampling interval for core 255 and 2.5 cm for coreB-3GC. Planktonic foraminifera were picked onlyfrom the size fraction v 150 Wm, identi¢ed [14^16]and counted. When they were abundant, the sam-ple was split using a bi-separation method to yielda subsample containing at least 300 specimens.Determination of carbonate content in the samplewas made using the conventional acid-digestionmethod with the analytical precision of 6 1.0%.

On the basis of the census data, the relativeabundance of each planktonic foraminiferal spe-cies was computed. We estimated SSTs using twodi¡erent planktonic foraminiferal transfer func-tions. The standard errors of the linear transfer

function FP-12E [14] are 2.48³C for winter SSTand 1.46³C for summer SST, while those of theSIMMAX-28 formula using modern analog tech-nique [15] are 1.27³C and 0.45³C, respectively. Asplanktonic foraminifera inhabit the upper fewhundred meters of the water column, the SIM-MAX-28 technique can also be used to estimateSWT [15]. As for the estimation of the DOT, weused the planktonic foraminiferal transfer func-tion of the tropical Paci¢c Ocean, which has astandard error of 22 m and additional 5 m oferror [16].

About 20 mg of planktonic foraminifera Neo-globoquadrina dutertrei shells ( = 150 Wm) werepicked from 28 samples in core 255 and 22 sam-ples in core B-3GC, and measured for AMS 14Cdatings at the Beta Analyses Co., USA. All themeasured AMS 14C ages were calibrated to calen-dar (cal.) scale using the relationships between the14C and U/Th time-scale [17] (Table 1). The two

Table 1AMS 14C age data of cores 255 and B-3GC

Core Depth 14C age Calibrated age Core Depth 14C age Calibrated age(cm) (yr BP) (yr BP) (cm) (yr BP) (yr BP)

255 75^85 2930 þ 60 2599 255 585^595 18 910 þ 70 21 833255 120^130 3 760 þ 60 3 657 255 615^625 18 640 þ 70 21 515255 140^150 3 630 þ 60 3 491 255 635^642 18 540 þ 70 21 397255 160^170 4 170 þ 50 4 181255 180^190 4 440 þ 60 4 222 B-3GC 0^8 610 þ 60 265255 200^210 4 600 þ 60 4 722 B-3GC 8^18 750 þ 70 350255 220^230 4 820 þ 60 4993 B-3GC 18^28 1 780 þ 60 1 248255 230^240 5 010 þ 60 5 225 B-3GC 28^38 2 790 þ 60 2 424255 240^250 5 060 þ 60 5 286 B-3GC 38^48 3 060 þ 60 2 763255 260^270 5 440 þ 60 5 738 B-3GC 48^58 3 540 þ 60 3 375255 280^290 5 760 þ 60 6 108 B-3GC 58^68 4 020 þ 50 3 990255 310^320 6 260 þ 60 6 669 B-3GC 68^78 4 330 þ 60 4 383255 330^340 7 230 þ 60 7 702 B-3GC 78^88 4 490 þ 50 4 284255 350^360 6 820 þ 60 7 272 B-3GC 88^98 5 350 þ 60 5 632255 365^375 9 170 þ 110 9 768 B-3GC 98^108 5 890 þ 70 6 256255 375^380 9 420 þ 60 10 063 B-3GC 108^118 6 530 þ 60 6 962255 385^395 10 540 þ 60 11 762 B-3GC 118^128 7 190 þ 60 7 660255 400^410 11 900 þ 60 13 427 B-3GC 128^138 7 380 þ 60 7 857255 420^430 10 720 þ 60 11 983 B-3GC 138^148 7 370 þ 60 7 847255 440^420 10 680 þ 60 11 934 B-3GC 148^158 7 740 þ 60 8 230255 460^470 15 220 þ 60 17 442 B-3GC 158^168 8 100 þ 60 8 603255 480^490 19 290 þ 70 22 281 B-3GC 168^178 8 630 þ 70 9 165255 515^525 19 300 þ 70 22 292 B-3GC 178^188 8 890 þ 60 9 420255 525^535 19 070 þ 70 22 022 B-3GC 188^198 9 070 þ 60 9 653255 540^550 19 330 þ 90 22 328 B-3GC 198^208 9 550 þ 60 10 222255 560^570 18 940 þ 60 21 869 B-3GC 208^218 9 810 þ 60 10 551

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cores used in this study are the best-dated high-resolution cores in this region until now.

Oxygen and carbon stable isotope measure-ments were performed on the planktonic foramin-ifera Globigerinoides ruber and P. obliquiloculatausing a Finnigan-MAT 252 mass spectrometer atthe Laboratory of Marine Geology, Tongji Uni-versity for core 255, and using a Finnigan-MAT251 at the Graduate School of EnvironmentalEarth Science, Hokkaido University for core B-3GC. The analytical precision expressed as 1 s forNBS-19 standard carbonate was 0.06x. Theaverage di¡erence of duplicate foraminiferal anal-yses is V0.12x for oxygen and 0.09x for car-bon.

A menu-driven PC program SPECTRUM [18]was used to study the Holocene variability of fourproxies for the strength of the Kuroshio Current.Compared to the widely used Blackman^Turkeyapproach for spectral analysis, the advantage ofSPECTRUM is the avoidance of any interpola-tion of the time series. In this study, the timeseries was divided into two 50% overlappingWelch-overlapped-segment-averaging (WOSA)segments which were linearly detrended. Assum-ing that at least two full cycles are observed with-in each WOSA segment, the lowest reliable fre-quency in this study is 0.0003121/yr,corresponding to the period of 3200 yr. The spec-tral peaks with lower frequency than this limit arediscarded in the study.

4. Results

4.1. Sedimentation rates and terrigenous input

The lower part below 465 cm in core 255 con-tains to a certain extent the reworked sedimentsfrom turbidite or down-slope transport [8] (Table1). The eight age data below 465 cm becomeyounger with increasing depth in the core (Fig.2). Fortunately, the sediment in both cores 255and B-3GC is continuous and free of turbiditeduring the Holocene, resulting in sample resolu-tions of 185 yr and 130 yr in the two cores, re-spectively. The sedimentation rate of core 255greatly changed at depth of 355 cm at V7.3 cal.

kyr BP and suddenly increased from 8.1 cm/kyr to59.4 cm/kyr. The sedimentation rate of core B-3GC changed at depth of 113 cm at V7.0 cal.kyr BP but suddenly decreased from 27.9 cm/kyrto 15.2 cm/kyr with a slight increase during theperiod of 4.6^2.4 cal. kyr BP. In fact, this tremen-dous change in sedimentation rate was caused bythe sedimentation of non-carbonate materials(Fig. 2), which are mainly composed of terrige-nous materials, except for volcanic ash at somelevels.

4.2. Indicator species of the Kuroshio Current

In the surface sediments of the ECS, P. obliqui-

Fig. 2. Results of AMS 14C dating and the changes of sedi-mentation rate. Black area represents the sedimentation rateof non-carbonate materials, while white area indicates that ofcarbonate.

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loculata is very abundant (exceeds 10% of theplanktonic foraminiferal fauna) beneath themain axis of the Kuroshio Current [19] and henceis regarded as the indicator species of the Kuro-shio Current [6^8,11,12,20]. In core 255 of thesouthern Okinawa Trough, P. obliquiloculata re-markably increased in abundance from V7.3 cal.kyr BP (Fig. 3), marking the re-entering of themain axis of the Kuroshio Current into the Oki-nawa Trough through the sea area o¡ northeastTaiwan during the Holocene [5,8]. This remark-able increase in the abundance of P. obliquilocu-

lata at V7.3 cal. kyr BP was not so distinct incore B-3GC. But since V7.3 cal. kyr BP, theabundance curves of P. obliquiloculata can be cor-related almost peak by peak between the twocores (Fig. 3). The abundance of P. obliquilocula-ta, in particular, abruptly decreased again during4.6^2.8 cal. kyr BP in core 255 and during 4.5^2.7cal. kyr BP in core B-3GC, which represents theso-called P. obliquiloculata minimum event [6^8,11]. Our new data display that there was a mi-nor increase in the abundance of P. obliquilocula-ta in the middle of this event (Fig. 3).

Fig. 3. Down-core variations in (a) the abundance of P. obliquiloculata (horizontal bars represent 95% con¢dence intervals), (b)FP-12E-derived SST and (c) its communality, (d) SIMMAX-28-derived SST and (e) its similarity. Horizontal bars for SSTs repre-sent the error estimates. Vertical dashed lines show the modern winter and summer SSTs at the core sites. The P. obliquiloculataminimum event is shaded.

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4.3. SST

The estimated Holocene SSTs of core 255 fromFP-12E and SIMMAX-28 are 19.7^26.6³C and21.5^26.6³C for winter, 27.8^29.5³C and 28.2^29.1³C for summer, respectively, while those ofcore B-3GC are 17.6^22.1³C and 18.4^26.0³Cfor winter, 27.3^28.9³C and 27.2^28.9³C forsummer. The di¡erences between the average Ho-locene SSTs and the present day SSTs at both sitesare less than the standard errors of estimation[14,15], except for the average Holocene SIM-MAX-28-derived winter SST (21.8³C) in core B-3GC. The average communality from FP-12E andsimilarity from SIMMAX-28 are higher than 0.85,while the amplitudes of £uctuations in winter andsummer SSTs are much greater than their stan-dard errors (Fig. 3), indicating the estimated SSTsfrom both transfer functions are quite reliable.

As shown in Fig. 3, both FP-12E-derived andSIMMAX-28-derived SSTs displayed the sametrend and the SSTs in core 255 were higher thanthose in core B-3GC. The winter SST in bothcores remarkably increased at V7.5^7.3 cal. kyrBP, in parallel with the increase in abundance ofP. obliquiloculata. Prominent minima in winterSST and less evident minima in summer SST pro-vide striking signals of Holocene climatic changein the Okinawa Trough at approximately 9.5^8.0,4.6^2.6, and 1.5^0 cal. kyr BP, i.e. roughly every3000 yr (Fig. 3). This intra-Holocene cycle ofV3000 yr is similar to the climate cycles foundin the SST record of the South China Sea (SCS)[21], and also in the Arabian Sea [22] and ice coresfrom Greenland [23,24]. The most outstandingminimum between 4.6 and 2.3 cal. kyr BP waswidely observed in Holocene sediments of thewestern Paci¢c marginal seas [6,11], known asthe `P. obliquiloculata minimum event' [6]. In ad-dition, the latest minimum in winter SST at V0.2cal. kyr BP in core 255 coincided with the LittleIce Age and its amplitude of decrease (V1³C) wasequal to that recorded in the Sargasso Sea [25].

4.4. Upper water thermal structure

The planktonic foraminifera G. ruber andP. obliquiloculata are thought to re£ect condi-

tions within the mixed layer and near 150 m(within the bottom of the mixed layer or inthe uppermost thermocline, depending on thethickness of the mixed layer), respectively[26]. Thus, if the thickness of the mixed layeris reduced and the DOT decreases, we ex-pect increased vN18OP: obliquiloculata3G: ruber andvN13CP: obliquiloculata3G: ruber (di¡erence between G.ruber and P. obliquiloculata), but if the thicknessof the mixed layer is very thick and the DOTincreases, we expect similar records for the twospecies and decreased vN18OP: obliquiloculata3G: ruber

and vN13CP: obliquiloculata3G: ruber. In cores 255 andB-3GC, the two species displayed a generally de-creasing trend in N18O during the transition fromthe last deglaciation to the postglacial period (Fig.4). The vN18OP: obliquiloculata3G: ruber obviously de-creased since V7.0 cal. kyr BP and increasedagain during 4.3^2.5 cal. kyr BP in core B-3GC.This means that the DOT increased since V7.0cal. kyr BP and decreased again during the periodof 4.3^2.5 cal. kyr BP in the northern OkinawaTrough. However, this phenomenon was notpresent in core 255, though the minor increasein vN18OP: obliquiloculata3G: ruber during 3.7^2.8 cal.kyr BP might be ascribed to the decrease in theDOT (Fig. 4). For the records of N13C, the great-est change occurred at V7.2 cal. kyr BP in coreB-3GC. Since then, the N13C records of the twospecies abruptly converged, with much decreasedvN13CP: obliquiloculata3G: ruber, supporting the increasein the DOT. But in core 255, the N13C records ofthe two species converged throughout the Holo-cene (Fig. 4), indicating that the thickness of themixed layer was very thick and the DOT was deepat this site. Therefore, during the Holocene the£uctuations in the DOT were smaller in the south-ern than in the northern Okinawa Trough.

Previous studies suggest that planktonic fora-miniferal species abundance is controlled byupper water temperature and nutrient gradients.When the DOT shoals, the mixed layer-dwellingspecies (G. ruber, Globigerinoides sacculifer, andGlobigerinita glutinata) decrease, while the ther-mocline-dwelling species (P. obliquiloculata, Glo-borotalia menardii, Globorotalia in£ata, and N. du-tertrei) increase in abundance [27]. In cores 255and B-3GC, the mixed layer-dwelling species in-

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creased in abundance after V7.2 cal. kyr BP,while the thermocline-dwelling species displayedan opposite trend (Fig. 5), indicating that theDOT decreased during this period. This inferencecan be con¢rmed by changes of the DOT esti-mated using planktonic foraminiferal transferfunction. The average Holocene DOTs in cores255 (203 m) and B-3GC (176 m) agree with theirmodern annual DOT [13]. Moreover, the averagecommunalities are 0.90 and 0.88 in cores 255 andB-3GC, indicating that this transfer function caninterpret V95% of the total variance in the ob-served faunal information. Generally, the DOT

was relatively deeper but more stable in core255 than in core B-3GC (Fig. 5). The majorchange of the DOT in core 255 occurred atV7.2 cal. kyr BP, with increased DOT after it.But in core B-3GC the major change occurred atV4.3^2.5 cal. kyr BP with obviously decreasedDOT, which corresponded to the increase in thevN18OP: obliquiloculata3G: ruber. It is noticeable that thedi¡erence (vT0ÿ100 m) between the winter SST andSWT remarkably decreased during the period ofV4.3^2.5 cal. kyr BP in both cores (Fig. 5), im-plying that the winter mixing of upper waterabove 100 m strengthened while the DOT de-

Fig. 4. Oxygen and carbon isotopic records of cores 255 and B-3GC: N18O (a) and N13C (b) of G. ruber and P. obliquiloculata ;(c) di¡erence in N18O (vN18OP: obliquiloculata3G: ruber) and (d) N13C (vN13CP: obliquiloculata3G: ruber) between P. obliquiloculata and G. ruber.The P. obliquiloculata minimum event is shaded.

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creased during the P. obliquiloculata minimumevent.

5. Discussion

There are a lot of di¡erent opinions concerningthe timing of the re-entry of the main axis of theKuroshio Current into the Okinawa Trough[5,8,10] and the cause of the P. obliquiloculataminimum events [6,11,12] during the Holocene,

but previous studies lacked detailed age controls.In this study, we deduced for the ¢rst time pastchanges in the Kuroshio Current on a centurytime-scale from two well-dated cores from thesouthern and northern Okinawa Trough. AtV7.3 cal. kyr BP, the SST and DOT abruptlyincreased to above modern values in both thesouthern and northern Okinawa Trough. Thischange was accompanied by the remarkably in-creased abundance of the Kuroshio Current indi-cator P. obliquiloculata. This tremendous change

Fig. 5. Planktonic foraminiferal indicators for the DOT: (a) the estimated DOT using transfer function (horizontal bars representthe error estimates; vertical dashed lines show the modern DOT value) and (b) its communality; (c) relative abundance of mixedlayer-dwelling species and (d) of thermocline-dwelling species (horizontal bars represent 95% con¢dence intervals); and (e) di¡er-ence (vT0ÿ100 m) in SIMMAX-28-derived winter SWT. The P. obliquiloculata minimum event is shaded.

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suggests that the main axis of the Kuroshio Cur-rent with high SST and deep DOT re-entered theOkinawa Trough at that time. Since then, thecoastal water has retreated to the continent shelfand the Kuroshio Current has dominated theOkinawa Trough. The Kuroshio Current £owedo¡ northeast Taiwan and brought signi¢cantamounts of terrigenous materials, hence greatlyincreasing the sedimentation rate of core 255.Since V7.3 cal. kyr BP the main branch of theKuroshio Current, the Tsushima Current, £owedwest of core B-3GC and this strengthened warmcurrent prevented terrigenous materials from theChinese mainland to reach the northern OkinawaTrough, hence leading to a decrease in sedimenta-tion rate. Previous studies revealed that theTsushima Current strengthened and re-enteredthe Sea of Japan at V8.0 cal. kyr BP [28]. Re-cently, Sawada and Handa reported the remark-ably strengthened Kuroshio Current at V7.1 cal.kyr BP in the northwestern Paci¢c o¡ central Ja-pan [9], supporting our inference. This increasedin£uence of the warm Kuroshio Current was alsorecorded in a change to more open-ocean, highersalinity environments since V7.5 cal. kyr BP inthe Yellow Sea [29], and even documented in theChangjiang River delta [30]. Moreover, the re-en-tering of the main axis of the Kuroshio Currentinto the Okinawa Trough at V7.3 cal. kyr BPalmost coincided with the initiation of a stablewarm and humid phase in Chinese climate drivenby the intensi¢ed summer monsoon [31]. In fact,boreal summer insolation reached a maximumduring the middle Holocene, when North Paci¢csubtropical high pressure must also have peaked[32]. The intensi¢cation of the subtropical highpressure may have resulted in the culmination ofthe East Asian monsoon due to the increasedland^sea contrast during summer [33], and alsostrengthened the Kuroshio Current [9], which, inturn, led to the migration of its main axis into theOkinawa Trough.

Based on detailed AMS 14C datings from thisstudy, the P. obliquiloculata minimum event oc-curred at V4.6^2.7 cal. kyr BP in the OkinawaTrough, very close to previous estimates (V4.5^3.0 cal. kyr BP) [11]. During this period, theabundance of P. obliquiloculata remarkably de-

creased, in parallel to the decrease in the SSTand DOT, indicating that the in£uence of theKuroshio Current decreased. This pattern mightbe related to (1) a decrease in the strength of theKuroshio Current itself, (2) a shift of the mainaxis of the Kuroshio Current towards the Paci¢cOcean, and (3) the intensi¢cation of the wintermonsoon and associated climatic cooling at thistime interval. Ujiie and Ujiie recently proposedthe formation of a barrier between Taiwan andthe southern Ryukyu Arc as a potential causefor the P. obliquiloculata minimum event [11].However, the sill depths (V800 m) between Tai-wan and the Ryukyu Arc hardly convince us thatthere was such a great tectonic change within thelate Holocene. Based on the oxygen isotopicchanges of planktonic foraminifera in two coreso¡ eastern Taiwan, the Kuroshio axis had shiftedto the east at about 4.0 cal. kyr BP [3]. However,the DOT changed little in the southern OkinawaTrough during this period, as shown by stableisotopes and species composition of planktonicforaminifera in core 255 (Figs. 4 and 5). There-fore, the P. obliquiloculata minimum event wasnot caused by the eastward shift of the Kuroshioaxis, because the upper water thermal structureat site 255 would have greatly changed in suchcase. According to a millennial time-scale analysison the Kuroshio Current, Sawada and Handa didnot recognize such a change during the P. obliqui-loculata minimum event [9]. Our new data alsodo not support the hypothesis that the strengthof the Kuroshio Current continuously weakenedduring the almost 2000 yr from V4.6 to 2.7 cal.kyr BP (see below). Moreover, any explana-tion using the Kuroshio Current can hardly beapplied to areas away from the Kuroshio Current,such as the southern SCS where this event oc-curred [34].

In the Okinawa Trough, the reduced anomaliesbetween the winter SST and SWT (Fig. 5) duringV4.3^2.5 cal. kyr BP suggest that vertical mixingthen strengthened, probably a result of the inten-si¢ed winter monsoon. Similar changes weredocumented as a climatic cooling occurring sinceV3.8 kyr BP in the Yangtze River delta [30], anda decrease of annual temperature and increase ofaridity since V4.1 cal. kyr BP in the coast of

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Bohai Bay [35]. Based on the spatial distributionand N13C composition of P. obliquiloculata in theSCS, Wang et al. also speculated a probable linkof the P. obliquiloculata minimum event to anincrease in winter monsoon [21]. In fact, therehas been a worldwide climatic cooling sinceV4.0 ka BP, known as `Neoglacial' in Europe[36]. In Japan, there were two cold stages at2446 BCV2267 BC (Middle/Late Jomon coldstage) and 866 BCV398 BC (Latest Jomon coldstage) [37]. In China, there was an environmentaldeterioration from about 4 ka BP [31], with adrier and colder climate around 2.8 ka BP knownas Zhouhan cold period [38]. It is very interestingthat a similar P. obliquiloculata minimum eventwas found in the marine oxygen isotope stage 5ein the western SCS (Baoqi Huang, personal com-munication). This event paralleled precisely ashort-lived and marked weakening of the NorthAtlantic Deep Water ventilation o¡ northwestAfrica around 4.6 cal. kyr BP, which was alsofound at 122.5 cal. kyr BP within stage 5e [39].

Therefore, the P. obliquiloculata minimum eventin the Okinawa Trough is not a regional but glob-al event which was not con¢ned to the Holocenebut occurred in the previous interglacial stage,suggesting a joint atmospheric and/or thermoha-line forcing of the ocean circulation.

In order to determine the variability of theKuroshio Current during the Holocene, we triedto use four proxies for the strength of the Ku-roshio Current in this study. In the modern Oki-nawa Trough, the SST and SWT decrease alongthe Kuroshio Current (its in£uence reaches sev-eral hundred meters below the sea surface [2])from south to north. If the strength of the Ku-roshio Current decreases, we expect increased dif-ferences in the SST and SWT between the south-ern and northern Okinawa Trough. If the salinitydoes not change greatly, the di¡erence in the N18Oof G. ruber and P. obliquiloculata between thesouthern and northern Okinawa Trough, repre-senting the di¡erence in sea surface and subsur-face temperature, should increase along with the

Fig. 6. Down-core variations in the proxies of the Kuroshio Current: the di¡erence between core 255 and B-3GC in (a) annualFP-12E-derived SST (vSST), (b) N18O of G. ruber (vN18OG: ruber), (c) annual SIMMAX-28-derived SWT (vSWT), and (d) N18O ofP. obliquiloculata (vN18OP: obliquiloculata). Shaded bars are approximate group peaks of the Holocene events, taking into account allof the records.

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decreasing strength of the Kuroshio Current.Thus, we computed the di¡erence in annual FP-12E-derived SST (vSST), annual SIMMAX-28-derived SWT (vSWT), N18O of G. ruber(vN18OG: ruber), and N18O of P. obliquiloculata(vN18OP: obliquiloculata) between cores 255 and B-3GC, after interpolation to 100 yr interval accord-ing to the time resolution of this study.

Taking into account the four proxies, therewere seven group peaks around V0.6, 1.7, 3.3,4.6, 5.9, 8.1 and 9.4 cal. kyr BP representing peri-ods with weakened Kuroshio Current (Fig. 6).Hence, in contrast to the conventional view, theKuroshio Current must have experienced a seriesof abrupt reorganizations during the Holocene.For the four proxies, the group peaks were clearerand their amplitudes were greater after V7.3 cal.kyr BP than before. This change probably repre-sented the re-entering of the main axis of the Ku-roshio Current into the Okinawa Trough. Duringthe P. obliquiloculata event, there were two re-markable decreases in the strength of the Kuro-shio Current at around 4.6 and 3.3 cal. kyr BP.The recent four events at V4.6, 3.3, 1.7 and 0.7cal. kyr BP were correlated with the Middle/LateJomon cold stage, Latest Jomon cold stage, Ko-fun (AD 240^AD 732) cold stage and Little IceAge recorded in Japan [37]. In Chinese history,the Zhouhan cold period around 2.8 ka BP [38]may correspond to the event at V3.3 cal. kyr BPin the Okinawa Trough. Climate in China wascoldest in the fourth century. In AD 366 the Bo-hai Bay froze during successive 3 yr and an armyof 3000^4000 persons with wagons were able tocross over the ice. The average temperature atthat time was 2V4³C lower than today [40] andthis cold period was comparable in time to theevent at V1.7 cal. kyr BP in the OkinawaTrough. Climate in China became suddenly coldagain in the 13th century, corresponding to theevent at V0.6 cal. kyr BP in the OkinawaTrough. The Taihu Lake near Shanghai frozethickly in AD 1329. The Yellow River also frozein the ¢rst half of the 14th century [40]. It isplausible that the Holocene events associatedwith the weakened Kuroshio Current reducedthe transport of heat from the tropics to northernmid-latitudes and, hence, cooled down the tem-

perature of adjacent lands, recorded in Chineseand Japanese histories [37,40].

In the North Atlantic, ice-rafted debris eventsexhibit a distinct pacing on millennial scales dur-ing the Holocene, with peaks at V1.4, 2.8, 4.2,5.9, 8.1 and 9.4 cal. kyr BP [41]. These Holoceneclimatic events were almost simultaneous withthose observed herein, except that the latest eventat V0.6 cal. kyr BP in the Okinawa Trough wasabsent from the North Atlantic records becausecore-tops slightly predate that event. However, inthe Sargasso Sea, SSTs were V1³C cooler atV0.4 cal. kyr BP and 1.7 cal. kyr BP than today[25]. The two cooling events were comparable intime to the last two events in the OkinawaTrough. The most recent Holocene event atV0.6 cal. kyr BP in the Okinawa Trough waspossibly related to the Little Ice Age. However,the worldwide cold event at V8.2^7.8 cal. kyr BPwas not so pronounced in the Okinawa Trough asin the North Atlantic [41,42] and in Greenland icecores [23,24]. Due to the low sedimentation ratebefore V7.3 cal. kyr BP in core 255 and the dis-turbance of volcanic ash around 8.0 cal. kyr BP incore B-3GC, more high-resolution cores areneeded to understand the response to the coldevent at V8.2^7.8 cal. kyr BP in the OkinawaTrough. Nevertheless, concentrations of sea saltand terrestrial dust increased in Greenland icecores during the period 0^600, 2400^3100, 5000^6100 and 7800^8800 yr ago [43]. Recently, deMe-nocal et al. showed a succession of Holocene cool-ing events centered at 0.35^0.80, 1.9, 3.0, 4.6, 6.0,8.0, and 10.2 cal. kyr BP from a core o¡ subtrop-ical Africa in the North Atlantic [44], which weresynchronous with Holocene events in the Okina-wa Trough within the dating uncertainty. Ourdata suggest that a tight coupling mechanismundergoing recurring shifts on short, millennialscales operates in atmospheric and ocean circula-tion between the Paci¢c and Atlantic Oceans.

For the Holocene events, the mean pacing is1450 þ 454 yr in the Okinawa Trough, which isstatistically similar to the 1374 þ 414 yr recordedin the North Atlantic [41]. Spectral analyses revealthat both the vSST and vN18OG: ruber display astrong periodicity of V1500 yr and weak perio-dicities of V600^700 yr, while the vSWT and

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vN18OP: obliquiloculata show consistently strong perio-dicity of V700^800 yr with an additional perio-dicity of V2600 yr for the vSWT (Fig. 7). Theperiodicity of V2600 yr is comparable to theroughly 3000 yr cycle identi¢ed in the local SSTof the Okinawa Trough (Fig. 3) and the SCS [21],and also to the climatic cycles found from Indianmonsoon sediment records in the Arabian Sea[22]. As for the V1500 yr cycle, it had been foundin the Arabian Sea [22], North Atlantic [41,45]and Greenland ice cores [41,43] and attributedto changes in oceanic thermohaline circulation.The V700^800 yr cycle amounts to half the1500 yr cycle recently described in the northernSCS [21], possibly re£ecting a second harmonic ofthe 1500 yr cycle. Before the Holocene there werea series of Dansgaard^Oeschger (D/O) events on

millennial scales in the last glacial stage, found inthe North Atlantic [46], Arabian Sea [22] andGreenland ice cores [41,45]. Recently this glacialmillennial scale climatic cycle was described in theSCS [21], the Sea of Japan [33] adjacent to theOkinawa Trough, and in Chinese loess [47]. Thereis no statistical di¡erence between pacings of theHolocene climate events and the rapid climateshifts that dominated the last glaciation. Appar-ently, the Holocene events in the Okinawa Troughare the most recent manifestation of a pervasive,at least quasiperiodic, millennial scale climatecycle operating independently of the glacial^inter-glacial climate state [41].

Much of the evidence for the millennial scaleevents has come from studies of the North Atlan-tic region [41,44,45]. The newly discovered envi-ronmental events in the Paci¢c [21,48] are mostfrequently ascribed to a tele-connection with theNorth Atlantic, while very little attention hasbeen paid to the tele-connection within the Pacif-ic. In fact, the source area of the Kuroshio Cur-rent is the Western Paci¢c Warm Pool (WPWP)[49], which plays an important role in global cli-mate changes through El Nin¬o-Southern Oscilla-tion phenomena. Therefore, further work isneeded for understanding the tele-connections be-tween the Kuroshio Current, the WPWP and theAtlantic high- and low-latitude climate during theHolocene and the complex links in tele-connec-tions.

6. Conclusions

Past changes in the Kuroshio Current were de-duced from two well-dated sediment records withcentennial scale resolution from the southern andnorthern Okinawa Trough. Based on multi-prox-ies, we draw the following conclusions regardingthe variability of the Kuroshio Current during theHolocene.

(1) Based on changes in sedimentation rate,striking increase in the abundance of the Kuro-shio Current indicator P. obliquiloculata, in SST,and in DOT in cores 255 and B-3GC, we deducethat the Kuroshio Current re-entered the Okina-wa Trough at V7.3 cal. kyr BP. However, during

Fig. 7. Spectral analysis of the four proxies of the KuroshioCurrent for the Holocene (settings: OFAC = 4.0; HIFAC =1.0; Nseg = 2; Hanning window; see [17] for details). Num-bers above peaks denote respective periods. Cross in lowerleft corner marks 6 dB bandwidth (horizontal) and 80% con-¢dence interval (vertical). Horizontal dashed line denotes theaverage value of the spectrum and is a rough estimate for awhite noise component in the time series. Considering onlythose parts of spectral peaks above this level gives an estima-tion of their corresponding variance contribution.

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V4.6^2.7 cal. kyr BP (that is, the P. obliquilocu-lata minimum event), the abundance of P. obliqui-loculata sharply decreased, in parallel to a de-crease in SST and DOT. This indicates that thein£uence of the Kuroshio Current was reduced,and the in£uence of the coastal water wasstrengthened at that time, possibly related to theintensi¢ed East Asian winter monsoon.

(2) The high-frequency oscillations of the Ku-roshio Current during the Holocene in the Okina-wa Trough center at periodicities of V1500 yrand V700^800 yr. The V700^800 yr cycle mayform a second harmonic of the V1500 yr cycle,possibly related to the oceanic thermohaline cir-culation. Taking into account all the proxies forthe strength of the Kuroshio Current (e.g. thedi¡erence in SST between the southern and north-ern Okinawa Trough), the Holocene events atV0.6, 1.7, 3.3, 4.6, 5.9, 6.9, 8.1, 9.4 cal. kyr BPare associated with the weakened Kuroshio Cur-rent in the Okinawa Trough. These events werewell-correlated to those recently described for theNorth Atlantic, Arabian Sea and SCS marinesediment cores, and Greenland ice cores, suggest-ing global climatic tele-connections. Compared toglacial D/O events, the Holocene events in theOkinawa Trough appear to be the most recentmanifestation of millennial scale climate cyclesoperating independently of the glacial^interglacialclimate cycle.

Acknowledgements

We would like to thank R. Chen for providingthe samples of core 255; N. Araki, S. Sato, M.Hamada, Y. Kato, H. Katayama and the crew ofR/V Bosei-Maru for their support in samplingcore B-3GC; and A. Holbourn for improvingthe English. We are indebted to G. Bond andW. Howard for their critical reviewing of themanuscript. This work was funded by the Pro-gram for the Young Scienti¢c and TechnologicalStars of Shanghai (Grant 98QG14043) and theNational Nature Science Foundation of China(Grant 49999560), and also done as part of theMarginal Sea Flux Experiment in the West Paci¢c(MASFLEX) supported by Special Coordination

Funds for Promoting Science and Technology ofthe Science and Technology Agency of Japan.[FA]

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