on

ater. J

an Sngeowethe srs thater, werns

that the intermediate water at 200 m depth during interglacial stager probae inflow

After 1.3 Ma, the intermediate water at 200 m depth became cooler in a step-wise manner. This cooling

the warm Tsushima Current, which enters via Tsushima Strait

mean annual sea surfacea has increased by between

in predicting ecologicalresponses to future change (e.g. Faunmap Working Group,

marine organisms to environmental changes in the Japan Seahas been studied based on the radiolarian fauna with middle-late Pleistocene fossil records preserved in deep-sea sediments

JOURNAL OF QUATERNARY SCIENCE (2009) 24(8) 880889Copyright 2009 John Wiley & Sons, Ltd.Published online 17 September 2009 in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/jqs.1315* Correspondence to: A. Kitamura, Institute of Geosciences, Shizuoka University,Shizuoka 422-8529, Japan.and makes its way northward along the western coast ofHonshu Island (Fig. 1). The deep region below the TsushimaCurrent is occupied by cold-water masses such as Japan/EastSea Intermediate Water (JESIW) and Japan Sea Proper Water(JSPW) (Suda, 1932; Moriyasu, 1972). The main thermoclinebetween the Tsushima Current and JESIW is found at about150160 m depth off Sanin and Hokuriku areas (Fig. 1). At thisdepth, benthic and nekton faunas change from warm-water tocold-water elements (e.g. Ogata, 1972; Nishimura, 1973,1974; Tsuchida and Hayashi, 1994; Kojima et al., 2001; Iguchiet al., 2007). Therefore, changes in water temperature of theJESIW would have a strong influence on the distribution ofoffshore fauna along the outer continental shelf of Japan.

1996; Jackson and Overpeck, 2000; Davis and Shaw, 2001). Inundertaking studies of biotic responses to climate change indifferent regions, it is important that appropriate fossil recordsare selected in terms of the degree of similarity of topographicfeatures and climate conditions. The fossil record after 1.7 Ma issuitable for investigations of the ecology of the Japan Sea,because the Tsushima Current flowed into the Japan Sea duringevery interglacial highstand within this period (Kitamura et al.,2001; Kitamura and Kimoto, 2006).

In terms of shallow-water fauna in the Japan Sea, Kitamuraet al. (2000), Kitamura and Ubukata (2003) and Kitamura(2004) examined the responses of molluscan species to rapidwarming during 10 stage transitions that occurred betweenMarine Isotope Stages (MIS) 50 and 28. The response of deep-The Japan Sea is a semi-enclosed marginal sea with an area ofapproximately 1 000 000 km2 and average depth of 1350 m.The sea is connected to the East China Sea through theTsushima Strait, to the Pacific Ocean via the Tsugaru Strait, andto the Sea of Okhotsk through the Soya and Mamiya Straits.These straits are narrow and shallower than 130 m deep (Fig. 1).

1.21 0.48C (southern area) and 1.64 0.48C (central area)(Japan Metrological Agency, 2007). The water temperaturewithin the upper 500 m has also shown a warming trend (of0.10.58C) over this period (Kim et al., 2001, 2008). Thiswarming may have influenced the ecosystem of deep-seaorganisms (Danovaro et al., 2004).

Knowledge of past biotic responses to Quaternary climateKEYWORDS: Japan Sea; Globorotalia inflata; intermediate water; Tsushima Current; Asian monsoon.

Introduction

At present, the only oceanic water flowing into the Japan Sea is

Over the past 100 a, thetemperature within the Japan Se

change may prove instructivesouthern strait. Copyright # 2009 John Wiley & Sons, Ltd.can be explained by an intensification of thermohaline circulation in the Japan Sea associated withstrengthening of the East Asian winter monsoon and an increase in salinity due to enlargement of thewarmest during the Quaternary. This warm intermediate watethe low-salinity nature of the Tsushima Current due to the largEarly Pleistocene evolutiIntermediate WaterAKIHISA KITAMURA*

Institute of Geosciences, Shizuoka University, Shizuoka, Japan

Kitamura, A. 2009. Early Pleistocene evolution of the Japan Sea Intermediate W

Received 20 March 2008; Revised 20 September 2008; Accepted 29 May 2009

ABSTRACT: The Early Pleistocene fossil record of the Japforaminifer Globorotalia inflata succeeded in expanding its raJapan Sea during interglacial stages of the Early Pleistocene; hsurface sediment samples from the southwestern-most part ofinflata and the water mass structure of the Japan Sea, it appeamainly controlled by the low temperature of the intermediate wamass of Japan Sea Proper Water. The temporal and spatial pattE-mail: seakita@ipc.shizuoka.ac.jpof the Japan Sea

. Quaternary Sci., Vol. 24. pp. 880889. ISSN 0267-8179.

ea reveals that the planktonicfrom the East China Sea to the

ver, the species is only found inea. Based on the ecology of G.t its geographical distribution ishich is caused by the cool-water

of G. inflata occurrence indicates between 1.46 and 1.3 Ma wasbly resulted from the influence ofof East China Sea Coastal Water.(Itaki et al., 2004, 2007; Itaki, 2007). The results of these studies

EARLY PLEISTOCENE EVOLUTION OF THE JAPAN SEA INTERMEDIATE WATER 881reveal that the water temperature of JESIW during pastinterglacial periods was no warmer than that of the present day.

This study examines the evolution of intermediate waterduring interglacial stages of the Early Pleistocene, based on acompilation of fossil records from shallow- and deep-seasediments. Based on these data, it is proposed that intermediatewater during some of the interglacial stages from 1.7 to 0.9 Mawas warmer than that of the present day. Therefore, analyses ofthe fossil and sediment records of these periods provide keyinsights in terms of predicting future changes in theenvironment of the Japan Sea arising from global warming.

Data sources

Planktonic foraminiferal data used in this study were collectedfrom shallow-sea sediments of the Omma and Setanaformations, and deep-sea sediments recovered from ODP Site798.

Figure 1 Map of the Japan Sea and surrounding region, showing the locatioPliocene to Early Pleistocene palaeogeography of the Hokuriku district (area othe sampling locations of planktonic foraminiferal assemblages reported by

Copyright 2009 John Wiley & Sons, Ltd.Omma Formation

The Omma Formation is exposed around Kanazawa City, alongthe western coast of central Japan (Figs 1 and 2). The formation isup to 220 m thick at its type section along the Saikawa River atOkuwa, and has been divided into lower, middle and upper parts(Fig. 3) (Kitamura et al., 1994, 2000). The lower and middle partsconsist of 14 depositional sequences that represent inner- toouter-shelf environments (Kitamura, 1998), while the upper partcontains five depositional sequences associated with alluvialplain to inner-shelf environments (Kitamura and Kawagoe,2006). Each depositional sequence consists of the following fourlithofacies (in ascending order): a basal shell bed, a well-sortedfine sandstone, a muddy fine to very fine sandstone, and a well-sorted fine sandstone. The basement of each basal shell bedcorresponds to the sequence boundary and ravinement surface(Kitamura, 1998).

During the period of deposition of each sequence, themolluscan fauna changed over time from cold-water, upper-sublittoral species to warm-water, lower-sublittoral species,followed by a return to cold-water, upper-sublittoral species

ns of the fossil records used in this study. The inset map shows the lateutlined by the rectangle in the main map). The numbered dots representUjiie and Ujiie (2000) and Domitsu and Oda (2005)

J. Quaternary Sci., Vol. 24(8) 880889 (2009)DOI: 10.1002/jqs

is assigned to one of interglacial stages 27, 25 or 23.

Palaeoceanographic information is commonly obtained from

882 JOURNAL OF QUATERNARY SCIENCE(Kitamura et al., 1994). This pattern indicates that the oceanicconditions changed in parallel with fluctuations in water depth,such that increases in water depth corresponded to periods ofwarming of the marine climate (Fig. 4). Thus it has been proposedthat these depositional sequences were related to glacio-eustasywith a period of 41 ka, corresponding to the period of orbitalobliquity (Kitamura et al., 1994). Subsequently, Kitamura and

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Figure 2 Correlation of fossil records in Early Pleistocene sedimentsalong the western coast of Honshu and Hokkaido IslandsKimoto (2006) correlated 19 depositional sequences with MIS 56to 21.3, based on a combination of sequence stratigraphic,biostratigraphic and magnetostratigraphic data (Fig. 3). Withineach depositional sequence, the relative abundance of warm-water Globigerinoides ruber, which is the most suitableplanktonic foraminifera species as aproxy for the warmTsushimaCurrent, shows cyclical changes consistent with those of warm-water molluscs (Kitamura et al., 2001) (Fig. 3).

Five depositional sequences have been identified within theOmma Formation at Oyabe, Toyama Prefecture (Takata, 2000)(Figs. 1, 2 and 4). These sequences are numbered successivelyfrom 1 to 5 in ascending stratigraphic order, and are correlatedwith MIS 60 to 50 (Kitamura et al., 2001). Takata (2000)documented the planktonic foraminiferal assemblages in thesedepositional sequences. Kitamura et al. (2001) examinedthe stratigraphic distribution of G. ruber within the earlyPleistocene Omma Formation and other Pliocene formationsdistributed along the Japan Sea coast, and proposed that theTsushima Current flowed into the Japan Sea during everyinterglacial highstand from 1.7 Ma. This result was subsequentlyconfirmed by Kitamura and Kimoto (2006). The fossil records ofMIS 25, 23 and 21.3 have not been detected within the OmmaFormation for the period from 1.6 to 0.8 Ma (MIS 56 to 20),because dissolution prevents the identification of calcareousfossils.

Setana Formation

Nojo and Suzuki (1999) examined planktonic foraminiferaassemblages within the Setana Formation in Hokkaido, Japan

Globorotalia inflata

Copyright 2009 John Wiley & Sons, Ltd.G. inflata lives beneath the transitional zone formed by theconvergence of the warm, high-salinity Kuroshio Current andthe cold, low-salinity Subarctic Water Mass in the westernNorth Pacific (Thompson, 1981; Xu and Oda, 1995), andmigrates downward to 200 m water depth during the course ofits life cycle (Hemleben et al., 1989). During the cold season,many studies have reported this species in subsurface water ofthe North Pacific, off the coast of Japan (Oba and Hattori, 1992;Tsuchihashi and Oda, 2001; Eguchi et al., 2003; Oda andYamasaki, 2005; Oba et al., 2006). G. inflata has been reportedfrom surface sediments off Shimokita Peninsula (coredeep-sea sediments. Although many sediment cores have beencollected within the Japan Sea, there exists only one detailedanalysis of the stratigraphic distribution of planktonic for-aminifers during the Pleistocene. Kheradyar (1992) examined133 samples from ODP Site 798 at a depth of 900 m at OkiRidge, located 150 km off the coast of the Japanese Islands(Fig. 1). The sampling intervals were 30 ka for the period 1.71.4 Ma and 13 ka after 1.4 Ma. Individuals of G. ruber wereidentified within 14 samples (Fig. 3). In the interval from 1.7 to0.8 Ma, equal to the entire Early Pleistocene, only six horizonsyield G. ruber. The frequency of occurrence is significantlylower than that within the Omma Formation (Fig. 3). Thisdiscrepancy might reflect the low resolution of the samplingintervals, especially the interval from 1.7 to 1.4 Ma, or the lowpreservation of carbonate shells during interglacial periods. Forexample, Kheradyar (1992) reported that 18 samples at site 798are devoid of planktonic foraminifera (Fig. 3). Oba et al. (1991)noted that the calcium carbonate compensation depth (CCD)was shallower than 1000 m during the last deglacial period.This shallow CCD resulted from oxic bottom conditionsassociated with a strong inflow of the Tsushima Current andsubsequent enhancement of JSPW production with a highdissolved-oxygen level. In a similar way, carbonate preser-vation may have been poor in the sediments during many ofthe interglacial periods. Among planktonic foraminifers, theshell of G. ruber is relatively more subject to dissolution(Thompson, 1981; Hemleben et al., 1989), meaning thatit may have been preferentially dissolved. In summary,the calcareous fossil records of shallow marine deposits inthe Japan Sea may be suitable for identifying early Pleistoceneinterglacial stages.

Spatial and temporal distribution ofOcean Drilling Project (ODP) Site 798(Fig. 1), and reported the species compositions of 15 samplesfrom a section that spans the first appearance datum ofGephyrocapsa parallela (the authors used 0.95 Ma, whichcorresponds to 1.045 0.025 Ma in the timescale of Bergeret al., 1994) and the last appearance datum of Reticulofenestraasanoi (the authors used 0.83 Ma, which corresponds to0.899 0.025 Ma in the timescale of Berger et al., 1994)(Fig. 2). Based on these data, the authors identified G. ruber infour horizons (Fig. 5). Because Nojo and Suzuki (1999)considered that the studied section represents a singledepositional sequence, the depositional age of the fossil recordJ. Quaternary Sci., Vol. 24(8) 880889 (2009)DOI: 10.1002/jqs

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conglomeratesandstonemudstoneparallel laminationcross laminationhummocky cross strat.

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Copyright 2009 John Wiley & Sons, Ltd. J. Quaternary Sci., Vol. 24(8) 880889 (2009)DOI: 10.1002/jqs

EARLY PLEISTOCENE EVOLUTION OF THE JAPAN SEA INTERMEDIATE WATER 883

MD01-2409), in the northwestern North Pacific (Kuroyanagiet al., 2006) (Fig. 1). To the best of my knowledge, there are noreports of G. inflata from areas north of Shimokita; therefore, itappears that this area marks the northern limit of the species.

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Figure 4 Stratigraphic distribution of the planktonic foraminifer species Globin the Oyabe, Okuwa and Yuhidera areas, showing reconstructed water de

LegendMolluscs

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Figure 5 Stratigraphic distribution of the planktonic foraminifer speciesGlobigerinoides ruber and Globorotalia inflata within the Setana For-mation at the Soebetsu section. Modified from Nojo and Suzuki (1999)

Copyright 2009 John Wiley & Sons, Ltd.Based on data from the Japan Oceanographic Data Center(2007) (18 18 grid cells), the mean monthly minimumtemperature off the Shimokita area from 0 to 300 m depth is458C (Fig. 6). This indicates that individuals of G. inflata aredifficult to continue to live and reproduce in a water mass witha mean monthly minimum temperature below 458C.

AlthoughG. inflata occurs in the East China Sea (Fig. 1) (Ujiieand Ujiie, 2000), it has not been reported from surfacesediments, sediment traps, plankton tows or pumping withinthe Japan Sea (Ichikura and Ujiie, 1976; Park and Shin, 1998;Kuroyanagi and Kawahata, 2004). Kitamura et al. (2001)interpreted that its absence from the Japan Sea is related to theshallow depth of the Tsushima Strait (130 m) rather than watertemperature. On this basis, the authors proposed that theoccurrence of G. inflata can be used as an indicator of the

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igerinoides ruber and Globorotalia inflata within the Omma Formationpths at the time of depositionexistence of a southern strait with a water depth greater thanthat of the present day (130 m); however, G. inflata wasrecently found in surface sediments of the southwestern-mostJapan Sea (Domitsu and Oda, 2005) (Fig. 1), therebydemonstrating that the present Tsushima Strait does not actas a barrier to the migration of G. inflata into the Japan Sea.

Nishimura (1974) suggested that the Japan Sea is home torelatively few species of deep-sea organisms compared with thePacific Ocean or Sea of Okhotsk, and concluded that theexceptionally low temperatures of the deep interior of the JapanSea prevent successful colonisation by deep-sea organismsfrom other seas. This interpretation is supported by the findingsof Terazaki (1993) and Itaki (2007). As noted above, G. inflatamigrates to depths below 200 m during its life cycles(Hemleben et al., 1989). Based on Japan Oceanographic DataCenter data, the intermediate water at 200 m depth in the JapanSea is about 58C throughout the year, close to wintertime watertemperature at 200 m depth off the Shimokita area (Fig. 6). Onthis basis, it appears that the low temperatures of theintermediate water may be an important factor in limitingexpansion of G. inflata to the Japan Sea, and that G. inflatashells in surface sediments of the southwestern-most Japan Searepresent a population extirpated immediately after passagethrough the Tsushima Strait.

Kitamura et al. (2001) reported the planktonic foraminiferG. inflata from sediments of interglacial stages 5941 (except

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A BShimokitafor 49) and 29 in the fossil record of the Omma Formation(Figs 3 and 4). Moreover, they concluded that the speciesimmigrated through the southern strait during an interglacialhighstand in sea level, as there exists a positive correlationbetween the abundance of this species and that of G. ruber.Nojo and Suzuki (1999) reported individuals of G. inflata fromfive horizons within sediments that correlate with one ofinterglacial stages 27, 25 or 23 (Fig. 5). It is likely that G.inflata entered the Japan Sea at stages 25 or 23, as Kitamuraet al. (2001) failed to identify the species from sediments ofstage 27.

In an analysis of samples recovered from ODP Site 798,Kheradyar (1992) identified G. inflata in eight horizons rangingfrom 150 to 128 m below the sediment surface (Fig. 3). Basedon the ages of stratigraphic datums reported by Berger et al.(1994), the ages of these horizons are estimated to be 1.461.3 Ma, corresponding to interglacial stages 4741. G. inflatahas not been recognised from other middle PleistoceneHolocene deep-sea sediments (Oba et al., 1991; Domitsu andOda, 2006; Takata et al., 2006; Kido et al., 2007).

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Variations in the temperature of the EarlyPleistocene JESIW

Based on the above fossil records, the temporal and spatialdistribution of G. inflata is here divided into four stages. DuringStage I (1.71.46 Ma), G. inflata intruded into coastal areas ofthe Japan Sea during almost every interglacial stage, but did notreach the Oki Ridge. During Stage II (1.461.3 Ma), thedistribution of G. inflata extended to the Oki Ridge duringinterglacial stages 47, 45, 43 and 41. During Stage III (1.30.9 Ma), colonisation was limited to stage 29 and one of stages25 or 23, without reaching the Oki Ridge. During Stage IV(0.9 Ma to present), G. inflata failed to extend its range into theJapan Sea, except for the southwestern-most area.

The continuous occurrence of G. inflata has been reportedfrom Quaternary sediments in the western Pacific Ocean off theJapanese Islands (e.g. Igarashi, 1994; Pickering et al., 1999;

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J. Quaternary Sci., Vol. 24(8) 880889 (2009)DOI: 10.1002/jqs

cooling of water masses from south of the subpolar front; and (b)convection via sea ice formation and brine rejection in thenorth and west of the Japan Sea. High wintertime temperatures

886 JOURNAL OF QUATERNARY SCIENCElead to a decrease in thermally driven convection and brinerejection. Likewise, low surface salinity, and hence low waterdensity, reduces the efficiency of thermally driven convectionand enhances the formation of brine rejection. This occursbecause reduced surface salinity leads to an increase in thefreezing point of seawater, thereby enhancing ice formation(Postlethwaite et al., 2005). It therefore appears that surfacesalinity and wintertime temperature were important factors indetermining the water temperature of Early Pleistoceneintermediate water via changes in the ventilation rates of JSPW.

Based on an analysis of the magnetic susceptibility and grainsize of loesspalaeosol and red-clay sequences on the ChineseLoess Plateau, Sun et al. (2006) proposed that the East Asianwinter and summer monsoons intensified at 1.25 Ma. Koizumiand Ikeda (1997) reported a southward shift in the subarcticsurface water mass at about 1.3 Ma (the revised age for thisevent is 1.298 Ma, based on the timescale of Berger et al.,1994), based on an analysis of the diatom record at ODP Site797, located 300 km off the coast of the Japanese Islands in2862.2 m water depth. It is possible that these events arecontemporaneous within the uncertainty of the respective agemodels. If so, these findings indicate that wintertime tempera-tures became cooler from 1.3 to 1.25 Ma, meaning that bothKameo et al., 2006; Kitamura et al., 2008). In addition, anoxicconditions within intermediate water have not been identifiedfrom sediments deposited in the Japan Sea during interglacialperiods (Kido et al., 2007). Based on these observations, itappears that the temporal distribution of G. inflata in the JapanSea is governed neither by the absence of G. inflata in theKuroshio Current region off Japan nor low levels of dissolvedoxygen in deep-sea regions of the Japan Sea. As an alternativeexplanation, I propose that water temperature at 200 m depth isan important factor in determining whether G. inflata is able tosuccessfully colonise the Japan Sea. As noted above, it appearsto be difficult for the species to continue to live and reproducein a water mass below a mean monthly minimum temperatureof 458C. If this is right, it is likely that the water temperature at200 m depth in coastal areas within the Japan Sea exceeded 458C during interglacial stage 59 (beginning of Stage I). DuringStage II, intermediate water at 200 m depth may have exceeded458C in both coastal areas off the Japanese Islands and at theOki Ridge. During Stage III, it seems that the intermediate waterremained below 458C during most periods, but reached thistemperature in coastal areas during at least two interglacialstages. During Stage IV, the intermediate water at 200 m depthmay have remained below 458C.

Causes of Pleistocene changes in thetemperature of intermediate water

As noted above, Kim et al. (2001) reported a clear warming inthe Japan Sea below 500 m water depth from 1969 to 1996.Based on a comparison of potential temperatures taken in thewestern Japan Basin during 1996 and 2007, Kim et al. (2008)reported that the potential temperature at 5001000 m waterdepth has increased by about 0.058C since 1996. According toKim et al. (2001), the warming is associated with changes indeep-water structures in the area, associated with decreasedbottom-water formation and increased intermediate-waterformation in recent years.

Postlethwaite et al. (2005) proposed that JSPW is formed by atleast two significant ventilation mechanisms: (a) convection viaCopyright 2009 John Wiley & Sons, Ltd.thermally driven convection and the formation of brinerejection were weak during Stages I and II.

Shortly after the formation of the southern strait of the JapanSea at 1.7 Ma (Kitamura et al., 2001), the strait may have beennarrow and shallow, restricting the inflow volume of theTsushima Current such that the supply of fresh water around thesouthern strait was probably greater than that of today. If thisinterpretation is correct, a reduction in the salinity of theTsushima Current would have restricted thermally drivenconvection, resulting in relatively warm intermediate water at200 m depth in coastal areas during Stage I.

The intermediate water mass was warmest during Stage II.According to Kitamura and Kimoto (2007), sea surface watersduring interglacial stages 47 and 43 were significantly warmerthan during other interglacial stages of the Early Pleistocene.Because these periods correspond to high boreal summerinsolation, when eccentricity-modulated precession extremeswere aligned with obliquity maxima, Kitamura and Kimoto(2007) suggested that orbital-induced changes in the strength ofthe subtropical North Pacific Ocean gyre acted to intensify theTsushima Current. There exists the possibility that a relativelywarm current restricted the formation of deep-sea water and ledto the formation of warm intermediate water; however, thisexplanation may be insufficient. Kitamura et al. (1997)examined molluscan and planktonic foraminiferal fauna withinthe Omma Formation at the Yuhidera section (Fig. 1), andidentified a molluscan assemblage (Transitional II-1 Subasso-ciation) that lived immediately below the thermocline duringinterglacial 47. Based on sedimentary features and oxygenisotope records from deep-sea cores, this depth is estimated tohave been about 110 m (Kitamura et al., 1997). The upperdepth of the present-day main thermocline is located at depthsof 150160 m off the Hokuriku area (Ogata, 1972; Nishimura,1973). Based on data from the Japan Oceanographic DataCenter (2007) (18 18 grid cells), the summer and wintertemperatures at the uppermost part of the thermocline in thisarea are 898C (March) and 108C (August), respectively (Fig. 6,Line EF). Thus water temperatures at about 110 m depth duringMIS 47 were lower than present-day temperatures by 18C(March) and 348C (August), meaning that the total heattransport by the Tsushima Current was lower than that of today.The palaeoceanographic condition does not support theproposal that a relatively warm current restricted the formationof deep-sea water.

It is noteworthy that the Pleistocene diatom record at ODPSite 797 (Koizumi and Ikeda, 1997) (Figs 1 and 3) reveals high-frequency oscillations in the occurrence of the diatom Paraliasulcata between 1.57 and 1.38 Ma (72.567.4 m below thesediment surface; Fig. 3). This species, a low-salinity coastal-water diatom, is regarded as an indicator of the East China SeaCoastal Water (ECSCW) (Tanimura, 1981; Koizumi, 1989).These ages were estimated from the mean accumulation rate,which was calibrated using the ages of stratigraphic datumspresented by Berger et al. (1994), as Koizumi and Ikeda (1997)used the geomagnetic polarity timescales presented by Candeand Kent (1992). Considering the resolution of the individualage models, the period of high abundance of P. sulcata is veryclose to Stage II. P. sulcata is found in surface sediments in thesouthwestern part of the Japan Sea, consistent with theobservation that a small amount of the ECSCW is carried intothe Japan Sea with the Tsushima Current along the margin of theKorean Peninsula (Tada et al., 1999). On this basis, the highabundance of P. sulcata indicates that the inflow volume of theECSCW during Stage II was greater than that at other stages.

The Changjiang River can be considered the most importantsource of fresh water for the ECSCW (Chen et al., 1994). TheChangjiang diluted water (CDW) extends northeastward to theJ. Quaternary Sci., Vol. 24(8) 880889 (2009)DOI: 10.1002/jqs

enIkehara for encouraging me to write this paper. I thank two anonymousreviewers and Dr Takuya Itaki for comments that improved the manu-script.

ll

Ca

ChwCR

Dac

DaQ

water mass. Paleontological Research 9: 255270.la-ce

from planktic foraminiferal assemblages and stable isotope records.Marine Micropaleontology 61: 155170.

EARLY PLEISTOCENE EVOLUTION OF THE JAPAN SEA INTERMEDIATE WATER 887vicinity of Cheju Island in summer (Hu, 1994), and causes areduction in the salinity of the surface layer of the TsushimaCurrent (Ogawa, 1983). It is possible that the summertime lowsalinity of surface water exceeds 22 psu, as G. ruber toleratessalinities of 2249 psu (Bijima et al., 1990).

In winter, when the northeasterly monsoon is predominant,the CDW occurs as a narrow band against the coast of China tothe southwest (Hu, 1994); consequently, the salinity of thewintertime surface layer of the Tsushima Current is higher thanthat of the summertime current. As noted above, G. inflata isabundant in the North Pacific off Japan during the cold season.It is therefore considered that G. inflata could immigrate intothe Japan Sea during the winter season. In this context, I thinkthat the summertime low-salinity nature of the TsushimaCurrent is due to the large inflow of ECSCW. This may becaused by two factors. Firstly, the large inflow of ECSCWresulted from geographic features around the southern strait.The river mouth of the Changjiang River might have shifted tothe nearest strait during Stage II. Secondly, precipitationincreased around the Changjiang River drainage basin andsouthern strait. It is widely known that variations in the EastAsian summer monsoon followed trends in the NorthernHemisphere summer insolation during the Holocene (Dykoskiet al., 2005; Sun et al., 2005; Wang et al., 2005; Morimotoet al., 2007). Thus it is possible that an increase in precipitationresulted from an intensification of the East Asian summermonsoon. As noted above, an increase in the sea surfacetemperature of the Kuroshio Current was associated with highsummertime boreal insolation during interglacial stages 47 and43 (Kitamura and Kimoto, 2007). Therefore I believe that low-salinity and warm wintertime surface water caused thereduction of thermally driven convection and brine rejection.Consequently, the enrichment of intermediate water formationcaused the warmest condition of intermediate water at 200 mdepth during interglacial stages between 1.46 and 1.3 Maduring the Quaternary.

After 1.27 Ma, the intermediate water became cooler in asouthward direction from the northern Japan Sea, as indicatedby the contrasting stratigraphic distribution of G. inflata inshallow- and deep-sea records (Fig. 3). As noted above, thestrengthening winter monsoonal winds and southward shift inthe subarctic water mass took place at 1.31.25 Ma. Thusdecreasing wintertime temperature due to strengthening wintermonsoon might cause enhancement of thermally drivenconvection and brine rejection. As a result, the exceptionallycool temperatures of the intermediate water, which is also thepresent-day condition, became established during everyinterglacial stage after about 0.9 Ma. This cooling is alsorelated to the increasing amplitude of glacialinterglacialcycles and enlargement of the southern strait. The former factorcontributed to the strong cooling of the entire water mass of theJapan Sea during relatively intense glacial periods, therebymoderating the degree of warming during the followinginterglacial periods such that G. inflata was unable to expandits range into the Japan Sea. The latter factor led to an increasein the volume of ocean water moving from the East China Sea tothe Japan Sea (increasing salinity), and promoted formation ofJSPW.

As noted above, the mean annual sea surface temperaturewithin the Japan Sea has increased by between 1.21 0.48C(southern area) and 1.64 0.48C (central area) over the past100 a (Japan Metrological Agency, 2007). Recently, Takaya-nagi et al. (2008) reported that the salinity of summertimesurface water in the area within the East China Sea alonglatitude 31328 N decreased by 2.8 over the past 50 a. Theauthors proposed that this reduction in salinity was caused byan increase in influx from the Changjiang River. ThisCopyright 2009 John Wiley & Sons, Ltd.Dykoski CA, Edwards RL, Cheng H, Yuan DX, Cai YJ, Zhang ML, Lin YS,Qing JM, An ZS, Revenaugh J. 2005. A high-resolution, absolute-dated Holocene and deglacial Asian monsoon record from DonggeCave, China. Earth and Planetary Science Letters 233: 7186.

Eguchi ON, Ujiie H, Kawahata H, Taira A. 2003. Seasonal variations inplanktonic foraminifera at three sediment traps in the Subarctic,Domitsu H, Oda M. 2006. Linkages between surface and deep circutions in the southern Japan Sea during the last 27,000 years: evidenDomsed7: 1391713951.en C, Beardsley RC, Limeburner R, Kim K. 1994. Comparison of

inter and summer hydrographic observations in the Yellow and Easthina Sea and adjacent Kuroshio during1986. Continental Shelfesearch 14: 909929.

novaro R, DellAnno A, Pusceddu A. 2004. Biodiversity response tolimate change in a warm deep sea. Ecology Letters 7: 821828.vis MB, Shaw RG. 2001. Range shifts and adaptive responses touaternary climate change. Science 292: 673679.itsu H, Oda M. 2005. Japan Sea planktic foraminifera in surfaceiments: geographical distribution and relationships to surfacethe9imits for growth and survival of some planktonic foraminifers inaboratory cultures. Journal of Foraminiferal Research 20: 95116.nde SC, Kent DV. 1992. A new geomagnetic polarity time scale for

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1. The temporal and spatial patterns of the distribution of G.inflata indicate that the intermediate water at 200 m depth inthe Japan Sea warmed with the initiation of inflow of theTsushima Current at every interglacial stage since 1.7 Ma,and was warmest between 1.46 and 1.3 Ma. Cooling ofintermediate water has occurred since 1.3 Ma. The inter-mediate water became exceptionally cold, which is also thepresent-day condition, during every interglacial stage afterabout 0.9 Ma.

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