Marine Micropaleontology 74 (2010) 108–118

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Marine Micropaleontology

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Paleoceanographic evolution of North Pacific surface water off Japan during the past150,000 years

Itaru Koizumi a,⁎, Hirofumi Yamamoto b

a Hokkaido University, Japanb Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Japan

⁎ Corresponding author. Atsubetsu-kita 3-5-18-2, AtsJapan.

E-mail address: itaru@sci.hokudai.ac.jp (I. Koizumi).

0377-8398/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.marmicro.2010.01.003

a b s t r a c t

a r t i c l e i n f o

Article history:Received 19 November 2009Received in revised form 24 January 2010Accepted 28 January 2010

Keywords:Td′ (the ratio of warm- and cold-waterdiatoms)-derived annual SST (°C)Wavelet analysisLast interglacial periodKuroshio–Kuroshio ExtensionOyashioTsugaru Warm CurrentEarth's orbital parametersEl Niño–Southern Oscillation (ENSO)

Hydrographic variability in the Mixed Water Region of the Northwest Pacific Ocean at latitudes 35°–40°N,between the Kuroshio Extension and Oyashio Front, causes complex upwelling, leading to large primaryproductivity and thus great fishery resources. We reconstructed the periodicity of the variability in NorthPacific Intermediate Water upwelling and surface ocean hydrography based on the high-resolution analysisof diatom assemblages in seven cores, representing the last 150,000 years. We derived annual sea surfacetemperatures (SSTs) through a diatom-based proxy (Td′). The Td′-derived annual SSTs (°C) are controlled byorbital forcing, and show a reversed saw-tooth in southern cores, in contrast to a normal saw-tooth patternin the northern cores. Oceanic diatom abundances along the northern margin of the Mixed Water Region aretwice times as high as beneath the axis of the Kuroshio Extension, and fluctuated in a revised saw-toothpattern with higher overall abundances interglacials. After the last deglaciation, annual SSTs declinedmarkedly during Heinrich and Bond events in the northern North Atlantic, when ice-rafted detritustransported by icebergs was abundant. Wavelet analyses of the record of oceanic diatom abundances showsignificant variability at 2.0-kyr, 2 to 5.6-kyr and 3.2 to 9.6-kyr periods. Wavelet analyses of the annual SSTrecords show significant periodicity at 1.4 to 2.6-kyr, 3.3 to 4.0-kyr, 7.2 to 12.8-kyr cycles.

ubetsu-ku, Sapporo 004-0073,

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© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Surface water-masses around the Japanese Islands show distinctsurface flow patterns, as documented by their physical properties,especially temperature (Fig. 1; Kawai, 1972; Yasuda et al., 1996;Masujima et al., 2003). Understanding the current patterns of thewaters to the East of Tohoku, the northern part of Honshu Island, iscrucial for understanding the complex hydrographic relationsbetween waters in the subtropical and the subarctic North Pacific.The ocean waters in the northwest Pacific, off the East coast of Japan,can be divided into three areas: the warmer Kuroshio Area to theSouth (with the Kuroshio Warm Core and the Kuroshio Extension toits East), the Mixed Water Region even further to the East and offshore between 35° and 40°N, and the colder Oyashio Area to the North(Fig. 1; see Yasuda et al., 1996 for detail).

The most complicated hydrographic conditions occur in the MixedWater Region between the Kuroshio Front to the North of the KuroshioExtension and the Oyashio Front, at latitude 35° to 40°N. In this region,numerous meanders and eddies occur, especially along the northernboundary of the Kuroshio Extension (Fig. 1). The complex hydrographyin the MixedWater Region causes local upwelling due to the isopycnal

mixing between the two waters with a vertically different salinity andvelocity structure (Yasuda et al., 1996), thus high phytoplanktonproductivity which leads to abundant fishery resources. North PacificIntermediate Water (NPIW), characterized by a salinity minimum atdepths of 300–800 m with a density range centered at 26.8 σθ (Talleyet al., 1995), is newly formed along the Kuroshio Extension by isopycnalmixing of low-salinity Oyashio water and old saline NPIW originatingfrom the Kuroshio (Yasuda et al., 1996). The low-salinity waters, whichoriginate from ventilated waters in the Okhotsk Sea, in the First andSecondOyashio Intrusionsmeetwith the TsugaruWarmCurrent (TsWCin Fig. 1) waters originated in the Tsushima Warm Current, flowingnorthwards along the eastern side of the Japan Sea, and the Kuroshiowarm core rings and are strongly modified. The modified, but still low-salinitywatersmeetwith theKuroshio Extension and formtheKuroshioFront (Fig. 1; Kawai, 1972; Yasuda et al., 1996).Waterswith a salinity of33.9–34.1 psu are distributed almost zonally at 35°–42°N from 150°E to170°W (Talley et al., 1995; Yasuda et al., 1996). The Oyashio water,which is CO2-rich surface waters in the subpolar gyre, transport intoNPIW is thought to be important for isolating anthropogenic CO2 (ex-CO2) into the intermediate depth of the subtropical gyre in the NorthPacific (Tsunogai et al., 1993).

The diatom records from seven cores in this hydrographicallycomplex area off northeast Japan show the Pleistocene oscillationsbetween warm and cold regimes, and documented the two glacial–interglacial cycles over the past 150,000 years (Koizumi et al., 2004;

Fig. 1.Map showing the location sites of seven cores used in this paper. Major current systems and the generalized distribution of surface water-masses around the Japanese Islandsare also indicated. ECSCW = East China Sea Coastal Water = green color. Tsushima Warm Current and TsWC = Tsugaru Warm Current = orange color. Kuroshio Warm Core andKuroshio Extension Warm Current = red color. Subarctic water mass = blue color. Arrows indicate flow of the current. W = warm-water, C = cold-water (Kawai, 1991).

109I. Koizumi, H. Yamamoto / Marine Micropaleontology 74 (2010) 108–118

Koizumi and Yamamoto, 2005, 2007, 2008). In the cores MD01-2421,MR02-03-2, and MR99-04-3 taken from the area beneath theKuroshio–Kuroshio Extension, warm-water diatoms are predominantin littoral–neritic association, because the warm Kuroshio Currentflows northeast along the coast of southwest Honshu Island. Cold-water diatoms are predominant in the oceanic association foundoutside the Kuroshio Extension, in offshore areas where thesouthward Oyashio Intrusions flow from the Bering and OkhotskSeas (Koizumi and Yamamoto, 2008) (Fig. 1).

Diatom abundances are higher in the near-shore than in the off-shore areas, and near-shore diatom assemblages were affected moreseriously by paleo-environmental changes. The abundances ofreworked, extinct diatoms were higher during glacial periods, butoverall diatom abundances were higher in interglacial periods, as wasthe temperature as reflected in the diatom-based proxy Td′ (Koizumiand Yamamoto, 2008). The oceanic diatom abundance showspronounced fluctuations at orbital periodicities (23-kyr and 41-kyr),and the Td′ values show fluctuations at periodicities of 60-kyr, 30-kyr,and 23-kyr (Koizumi et al., 2004). It is considered that thehydrographic variability of upwelling and interaction betweensubarctic and subtropical waters due to eustatic sea-level changingby fluctuations at orbital periodicities plays an important role in theprimary production of oceanic diatoms.

In this paper we synthesize the fluctuations in diatom associationsand annual paleo-sea surface temperatures (paleo-SSTs) (°C) derivedfrom Td′ ratio (Koizumi, 2008) in the northwestern Pacific Ocean offnortheastern Japan over the past 150,000 years, as well as waveletanalysis for the Td′-derived annual SSTs (°C) and oceanic diatomabundances.

2. Core materials and sediment age mode

We used 7 cores, one of which is presently located below theKuroshio Warm Core (MD01-2421), one below the Kuroshio

Extension (MR02-03-2), one below the Mixed Water Region(MR99-04-3), one below the First (ODP Hole 1150A) and two belowthe Second Oyashio Intrusions (MR00-05-2 and MR99-04-2), and onebelow the Tsugaru Warm Current (MR97-04-1). Age models for thecore sediments are based on a combination of tephrachronology,isotope stratigraphy, paleomagnetic data, and AMS 14C dates, asshown in Table 1 and Fig. 2. Latitude, longitude and water depth arepresented in Table 1, together with the length of the full cores and thestudied sections.

The sedimentation rates were highest in cores MD01-2421 andODP Hole 1150A. Core MD 01-2421 was taken by the R/V MarionDufresne on a slightly convex sub-marine plateau in the continentalslope off Inubo-zaki, central Japan, and is presently under theKuroshio Warm Current. This core (total length 4479 cm) has anoverall sedimentation rate of 31.7 cm/kyr, and a more than twice ashigh rate (67.1 cm/kyr) for its Holocene part (Fig. 2). We studied theupper 4522 cm of core MD01-2421, consisting of olive gray,homogeneous silty clay with intercalated thin layers of fine-grainedsand and volcanic ash (Koizumi et al., 2004). Details of the age modelare shown in Table 1 (see also Oba et al., 2006; Yamamoto, per. comm.,2007).

Ocean Drilling Program (ODP) Hole 1150A was drilled on a deep-sea terrace on the landward side of the Japan Trench, and has asedimentation rate of 37.4/kyr in its upper 2680 cm. The upper2000 cm of sediment consists of interbedded diatomaceous ooze andclay (Sacks et al., 2000). Three marker tephra layers occur in thePleistocene sequence: the Towada–Hachinohe (To–H) tephra at505 cm (14.83 ka), the Shikotsu–Daiichi (Spfa-1) tephra at 1507 cm(39.80 ka), and the Aso-4 tephra at 2681 cm (88.00 ka) (Motoyamaet al., 2004). The diatom assemblages from ODP Hole 1150A weredescribed in Koizumi and Sakamoto (2003).

All other studied cores have sedimentation rates ranging from 13to 4 cm/kyr. We studied the core MR (R/V Mirai) 02-03-2, presentlyunder the Kuroshio Extension, composed of dark olive colored clay to

Table 1Chronostratigraphic framework (depth vs. calendar age) in seven cores around the Japanese Islands.

Cores MD01-2421 MR02-03-2 MR99-04-3 ODP 1150A MR97-04-1 MR00-05-2 MR99-04-2 References

Latitude (°N) 36 36.00 37.50 39.18 40.56 40.00 40.08

Longitude (°E) 141 146.50 152.00 143.33 142.93 146.00 149.85

Water depth (m) 224 5712.0 5848.0 2681.0 1520.0 5177.0 5608.0

Core length (cmbsf) 4582.0 1818.5 1876.0 72260.0 683.8 1988.0 1813.0

Studied length (cmbsf) 4522.0 1300.0 970.3 2680.0 683.8 1380.0 950.0

Age controls (cal kyr BP) (cmbsf) (cmbsf) (cmbsf) (cmbsf) (cmbsf) (cmbsf) (cmbsf)

AMS14C 0.31 8 Oba et al. (2006)AMS14C 0.54 57 Yamamoto (per. comm. 07)AMS14C 0.96 106 Yamamoto (per. comm. 07)AMS14C 1.79 158 Oba et al. (2006)AMS14C 2.38 207 Yamamoto (per. comm. 07)AMS14C 2.79 258 Yamamoto (per. comm. 07)AMS14C 3.62 308 Oba et al. (2006)AMS14C 4.12 358 Yamamoto (per. comm. 07)AMS14C 4.65 408 Yamamoto (per. comm. 07)AMS14C 5.57 456 Oba et al. (2006)AMS14C 5.95 506 Yamamoto (per. comm. 07)AMS14C 6.76 556 Yamamoto (per. comm. 07)AMS14C 7.57 606 Oba et al. (2006)AMS14C 8.47 657 Yamamoto (per. comm. 07)AMS14C 9.29 707 Yamamoto (per. comm. 07)AMS14C 12.01 806 Oba et al. (2006)MIS 2.0 12.05 38.2 Martinson et al. (1987)AMS14C mean 12.90 827 Oba et al. (2006)To–H tephra 14.83 505.0 221.0 77.5 Yamamoto and Aoki (2002)MIS 2.2 17.85 54.4 Martinson et al. (1987)AMS14C 21.11 1003 Oba et al. (2006)MIS 3.0 24.11 64.7 Martinson et al. (1987)AT tephra 29.37 1210 Miyairi et al. (2004)To–Of tephra 31.27 540.5 297.0 Aoki et al. (2000)AMS14C 37.51 1305 Oba et al. (2006)Spfa-1 tephra 39.80 321.5 1507.0 661.0 392.0 Aoki et al. (2000)AMS14C mean 43.33 1458 Oba et al. (2006)MIS 3.3 50.21 247.7 Martinson et al. (1987)MIS 3.3/3.31 51.57 1776 Martinson et al. (1987)SINT800 54.00 379 Guyodo and Valet (1999)MIS 4.0 58.96 2005 267.7 Martinson et al. (1987)MIS 4.22 65.22 2085 Martinson et al. (1987)MIS 5.0 73.91 2427 344.7 Martinson et al. (1987)MIS 5.1 79.25 2775 365.7 Martinson et al. (1987)MIS 5.1/5.2 85.22 2827 Martinson et al. (1987)Aso-4 tephra 88.00 535.0 2681.0 1151.5 679.0 Matsumoto et al. (1991)MIS 5.2 90.95 2960 410.4 Martinson et al. (1987)MIS 5.2/5.3 94.06 3117 Martinson et al. (1987)MIS 5.3 99.38 3328 448.4 Martinson et al. (1987)MIS 5.33/5.4 105.57 3413 Martinson et al. (1987)Kc–Hb 110.73 1237.5 Yamamoto and Aoki (2002)MIS 5.4 110.79 3471 491.4 Martinson et al. (1987)MIS 5.4/5.51 117.30 3648 Martinson et al. (1987)MIS 5.5 122.56 4027 552.7 Martinson et al. (1987)MIS 6.0 129.84 4241 598.8 Martinson et al. (1987)SINT800 130.00 813 Guyodo and Valet (1999)MIS 6.1 132.81 4329 Martinson et al. (1987)MIS 6.2 135.10 634.0 Martinson et al. (1987)MIS 6.3 141.33 4479 Martinson et al. (1987)SINT800 146.00 884 Guyodo and Valet (1999)MIS 6.4 152.58 674.0 Martinson et al. (1987)MIS 6.41 161.34 698.0 Martinson et al. (1987)SINT800 190.00 1259 Guyodo and Valet (1999)

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silty clay with ash layers and yellowish green bands. The age model isbased on the correlation between the paleomagnetic intensity(NRM15/ARM15) of the core and the global changes in intensity ofthe Earth's magnetic field SINT800 (Guyodo and Valet, 1999; Koizumiet al., 2003).

We used the upper 970 cm of coreMR99-04-3, presently under theSecond Oyashio Intrusion, and consisting of an alternation of grayisholive and olive black colored clay with dark greenish gray laminatedlayers, burrows and ash layers. The core contains two marker tephralayers (Aoki et al., 2000), the Spfa-1 tephra and the Aso-4 tephra.

Three piston cores are located along a transect running east-to-west at about 40°N, off the Sanriku coast. The first of these, coreMR97-04-1, is presently under the Tsushima Warm Current, andcomposed of olive gray colored homogeneous silty clay withintercalated thin layers of fine- and medium-grained sand or volcanicash and a thin laminated clay (Yamamoto, 1999). The marine isotopeevents were recognized in the record of benthic foraminiferal tests(Table 1; Oba et al., 1999; Koizumi et al., 2006). Of the second core inthe transect (core MR00-05-2, presently under the Second OyashioIntrusion), we studied the upper 1380 cm, composed of gray olive

Fig. 2. Depth (m) vs. age (cal kyr BP) models for seven cores. The stage boundaries ofthe Marine Isotope Stage (Martinson et al., 1987) are also indicated.

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colored siliceous clay, mottled with abundant glassy ash layers. Thefollowing tephra layers were identified: the Kutcharo–Haboro (Kc–Hb) tephra (110.73 ka), the Aso-4 tephra, the Spfa-1 tephra, theTowada–Ofudo (To–Of) tephra (31.27 ka) and the To–H tephra(Yamamoto and Kanamatsu, 2001; Yamamoto and Aoki, 2002). Ofthe third core in the transect, core MR99-04-2, we studied the upper950 cm, mainly composed of dark olive gray colored clay withabundant glassy ash layers, recognized as the Aso-4 tephra, the Spfa-1tephra, the To–Of tephra and the To–H tephra (Yamamoto et al., 2000;Aoki et al., 2000).

3. Method of study

The average sampling interval in core MD01-2421 is 20 cmcorresponding to 600 years throughout the core. The samplinginterval in four MR cores is 10 cm, corresponding to ∼1000–1700 years, but in core MR97-04-1 10 cm corresponds to 3100 yearsin average. We sample the Holocene at 5 cm intervals in cores MR99-04-3, MR97-04-1 and MR99-04-2. The average sampling interval inODP Hole 1150A is 25 cm corresponding to 813 years throughout thecore.

The procedures of slide-preparation, microscopic examination andpaleoceanographic analyses followKoizumi et al. (2004). Recently, theequations for the estimation of annual paleo-SSTs (°C) based on Td′ratio around Japanese Islands were calibrated in surface sedimentsamples versus the measured mean annual sea surface temperatures(Koizumi, 2008), andweused the calibration as presented in this studyto derived Td′-derived annual paleo-SSTs (°C) over the past150,000 years.

Wavelet analyses were performed to evaluate the time series ofthe values in the band ratio Td′-derived annual paleo-SSTs (°C) andoceanic diatom abundances at century–millennial time-scales. Weused a Morlet wavelet with a wave number of six to calculate thewavelet spectra and to identify large scaled changes in variancewithin selected frequency bands through time. The wavelet softwareprovided by Torrence and Compo (1998) are available at URL: http://

paos.colorado.edu/research/wavelet/. The time series were paddedwith zero, and parameters x interval=0.25, start scale=2, scalewidth=0.1 and mother wavelet=Morlet. Significance levels wereset at 10%, and a red-noise (autoregressive lag 1) background wasestimated from the alpha parameter described in Torrence and Compo(1998).

4. Results and discussion

4.1. Extinct diatoms

The relative abundances of extinct diatoms are generally less than10% of the total diatom abundances in cores MR02-03-2 off the Jobancoast, and MR99-04-3 and MR99-04-2 off the Sanriku coast located inthe Tohoku Area (Figs. 1 and 3). However, higher abundance peaks inthe largefluctuations are seen in coresODPHole 1150AandMR97-04-1,located on the continental slope, and core MR00-05-2 in the MixedWater Region between the warm-water eddy and Second OyashioIntrusion. Such extinct diatoms are reworked and/or displaced, derivedfrom the sea-floor when it was exposed, and they include rareoccurrences of freshwater and littoral diatoms (Tables A1–A7). Theabundances of such reworked extinct diatomswere higher during timesof lowered sea-level, i.e., glacial periods. The relative abundancesgenerally decreased after ∼20–15 ka, when sea-level started to riseduring deglaciation.

The dominant extinct speciesMelosira albicans, is accompanied byPseudopodosira elegans throughout the cores. These two species wereoriginally described from the Pliocene in Sakhalin and Kamtschatica(Sheshukova-Poretzkaya, 1964), and are common in the PlioceneTatsunokuchi Formation in Fukushima and Miyagi Prefectures, innortheast Japan (Koizumi, 1972, 1973b). However, according to thebiostratigraphic data presented for DSDP (Deep Sea Drilling Project)(Koizumi, 1973a) and ODP (Koizumi, 1992) sites at middle-highlatitudes in the North Pacific Ocean, their last occurrences are in thePleistocene Proboschia (=Rhizosolenia) curvirostris Zone at ∼1.24–0.30 Ma. And also they occur regularly throughout the Pleistocene toHolocene sediments recovered by piston coring in the North PacificOcean (Koizumi et al., 2001, 2004; Shimada and Hasegawa, 2001).

4.2. Relative abundances of diatoms of the littoral–neritic association

The relative abundances of littoral–neritic association are almost amirror image of those of oceanic association (Fig. 3). Three cores takenfrom the area beneath the Kuroshio–Kuroshio Extension off the Jobancoast contain abundant littoral–neritic diatoms, reaching to ∼25–50%of the total diatom abundances. However, they aremuch less commonin the four cores off the Sanriku coast in the Mixed Water Region. TheKuroshio flowing toward the northeast along the near shore ofsouthwest Honshu transports the littoral–neritic diatoms into thesouthern margin of the Mixed Water Region.

The relative abundances of species of the littoral–neritic associationgradually decrease overall between 150 ka and 15 ka, with gentlefluctuations of several kyr duration. The abundances increased againduring the Holocene period. In the littoral–neritic association, thewarm-water species such asHemiaulus sinensis, Odontella reticulum andThalassionema nitzschioides var. parva are more abundant than cold-water ones in coresMR02-03-2 andMR99-4-3, taken off the Joban coast,andMR99-04-2, from off the Sanriku coast (Tables A1–A7). The relativeabundances of warm-water and cold-water species in the littoral–neritic association are almost even in cores MD01-2421 and ODP Hole1150A, located in a near-shore environment except for the time after8 ka. In contrast, the relative abundances of cold-water species areslightly higher than those of warm-water ones in coreMR97-04-1, nowlocated at the southern end of the First Oyashio Intrusion off the Sanrikucoast. Sudden peaks in abundance of the exclusively littoral–neriticcold-water species Odontella aurita occur from ∼8 ka on. The relative

Fig. 3. Chronostratigraphic variations (%) of the relative abundances (valves/200 valves) of extinct, littoral–neritic and oceanic diatoms over the last 150,000 years in seven cores inthe Tohoku Area, off northeast Japan. High abundances of extinct diatoms are correlated within the lowered sea-level phase estimated from normalized δ18O curve (Martinson et al.,1987) in each core recovered from the area near shore. White triangles on the left side of each column indicate the age control points for age model.

112 I. Koizumi, H. Yamamoto / Marine Micropaleontology 74 (2010) 108–118

abundances of O. aurita during the last 8 ka in the near-shore cores aretwice as high as in the cores recovered off shore. This increase in theabundance of O. aurita suggests that the Oyashio Current movedsouthwards along the eastern margin of Hokkaido and Honshu Islands(Koizumi et al., 2006). It therefore indicates that the modernhydrographic situation of the Tohoku Area originated about8000 years ago.

4.3. Relative abundances of diatom of the oceanic association

The relative abundances of oceanic association increased as thelocations of the cores become further offshore, with large fluctuations(Fig. 3). In three cores from off the Joban coast, the relative abundancesat ∼60-kyr periods are following secondary and smaller fluctuations at∼20-kyr periods (Koizumi et al., 2004). In the four cores off the Sanrikucoast, large fluctuations occurred at periods of ∼40-kyr, with secondaryand smaller fluctuations at ∼20-kyr periods. The peaks in abundance ofoceanic diatoms are correlated with each other, occurring in theinterglacial phases throughout the cores (Fig. 3).

The relative abundances of warm-water species in oceanicassociation are slightly over than those of cold-water species in theinterglacial phases of MIS 5e and 1 (Holocene epoch) in the cores inthe Kuroshi–Kuroshio Extension area. The abundances of warm-waterand cold-water species are almost even in cores MD01-2421, MR02-03-2 and MR99-04-3, located in the warm-water tongue along theKuroshio Extension off the Joban coast (Figs. 1 and 4). In four corestaken off the Sanriku coast, the abundances of warm-water speciesslightly increase with distance away from the near-shore area (Fig. 4).

Cold-water species in the northeastern cores taken off the Sanrikucoast dominate over warm-water ones, with large fluctuationsthroughout the cores. It suggests that the cores near shore are locatedwithin the First Oyashio Intrusion, but the cores off shore wereaffected by the warm-water eddies detached from the KuroshioExtension. The repeating ∼40-kyr periods in high relative abundancesof cold-water species are correlated between the cores. In these cores,the relative abundances of cold-water species decreased markedly at145 ka, 115–105 ka, 65 ka, 25 ka, and 8 ka. At these levels, the relative

abundances of such cold-water species as Fragilariopsis cyclindrus,Fragilariopsis oceanica, and Thalassiosira gravida decrease, whereasthose of warm-water species such as Azpeitia nodulifera, Azpeitiatabularia, Fragilariopsis doliolus, and Roperia tesselata generallyincreased (Tables A1–A7).

T. gravida (=T. antarctica) is associated with sea-ice in coastalwaters of arctic and boreal seas (Shiga and Koizumi, 2000), so that itsrelative abundances decreased as the cored area becomes further offshore when sea-level rose. The large fluctuations at ∼40-kyr periodswith secondary and smaller fluctuations at ∼20-kyr periods typicallyoccur throughout the near-shore cores. The peaks in abundances arein glacial phases.

F. doliolus is one of the most common plankton species insubtropical–tropical areas in the World Ocean (Hasle, 1976), and itsrelative abundances increase during interglacial MIS 5e and 1. Theincrease of this species in the cores from off-shore areas suggests thatwarm-water eddies reached the core location.

4.4. Annual SST (°C) derived from Td′ ratios

In core MD01-2421, located at the northern margin of the KuroshioWarm Core (Figs. 1 and 6), the Td′-derived annual SSTs (°C) (red)generally agree with alkenone Uk′37-based summer SSTs (°C) (green;Yamamoto et al., 2004, 2005; Yamamoto, per. comm., 2007) and withthe patterns of δ18O variation in benthic foraminiferal tests (purple;Uvigerina spp. and Bulimina aculeata) (Oba and Murayama, 2004; Obaet al., 2006) over the last 144,000 years (Fig. 5). The diatom SSTs are lessthan the alkenone SSTs and δ18O curve at the peakwarming duringMIS5e and 1.DuringMIS 4 andbetween lateMIS4 to earlyMIS3, thediatomSSTs exceed those suggested by alkenone and δ18O.

The Td′-derived annual SSTs (°C) range from 9.7 °C at 128.5 ka (earlyMIS5e) to 21.6 °C at 122.7 ka (middleMIS5e) and∼53.4 ka (earlyMIS 3).SSTs (°C) decreased markedly at the six times: 128.5 ka (early MIS 5e),87.7 ka (MIS 5b), 46.4 ka and40.5 ka (MIS 3), 18.1 ka (MIS 2), and12.1 ka(the MIS2/1 boundary) (Fig. 5). These times correspond in age with theNorth Atlantic Heinrich events (Heinrich, 1988), characterized byenhanced delivery of ice-rafted detritus by icebergs from the Laurentide

Fig. 4. Chronostratigraphic variations (%) of oceanic warm- and cold-water diatom species over the last 150,000 years in seven cores in the Tohoku Area, off northeast Japan. The relativeabundances of oceanic cold-water species are slightly over than those ofwarm-water species in the cores located in theKuroshio–Kuroshio Extensionoff the Joban coast, but they dominate overthose of warm-water ones with large fluctuations throughout cores the cores off the Sanriku coast.White triangles on the left side of each column indicate the age control points for agemodel.

Fig. 5. The comparison of paleo-SSTs (°C) basedon the Td′ ratio (red), alkenone-derivedUk′37(green; Yamamoto et al., 2004, 2005;Yamamoto, per. comm., 2007) and δ18O of benthic

foraminiferal tests (purple; Oba andMurayama, 2004; Oba et al., 2006) in core MD01-2421.Red arrows indicate warm time, and blue arrows show cool time.

113I. Koizumi, H. Yamamoto / Marine Micropaleontology 74 (2010) 108–118

and Fenno-Scandinavian ice sheets (Chapman et al., 2000), and withglacial marine isotope events in the orbitally based chronostratigraphy(Martinson et al., 1987). Coeval cooling events can be recognized in allcores from the Tohoku Area (Fig. 6).

The SSTs (°C) rise in the middle of interglacial periods MIS 5e and 1.Relatively high Td′-derived SSTs (°C) occur atmiddleMIS5e, theMIS5d/5c boundary,MIS 5b, lateMIS 5a, theMIS 5a/4 boundary to beginning 4,3 andmiddleMIS 1, all interglacial time periods (Fig. 5). The SSTs (°C) inMIS 5e of the last interglacial period are ∼5 °C higher than the value atthe top of cores (corresponding to the Recent) in the near-shore cores,but the difference decreases to ∼1.5 °C in the off-shore cores.

Spectral analysis of Td′ ratios in core MD01-2421 indicates thatduring the last 150 kyr there is pronounced variability at 60-kyr, 30-kyr (similar to that of the long-term variation of El Niño–SouthernOscillation (ENSO), Clement et al., 1999) and 23-kyr periodicities(Koizumi et al., 2004). Variations in equatorial productivity reflectprecession-controlled, long-term ENSO variations in the East–Westthermocline slope in the Indo-Pacific (Beaufort et al., 2001), and alsothe middle latitude North Pacific margins (Yamamoto et al., 2005).

Wavelet analyses were performed on the records of Td′-derivedannual SSTs (°C) of cores MD01-2421 under the Kuroshio Warm Core,and core MR97-04-1 at the outer margin of the TsugaruWarm Current.The fluctuations in annual SSTs (°C) in core MD01-2421 over the130,000 years since MIS 5e occur in four intervals: 130–100 ka, 90–53 ka, 50–20 ka, and 20–0 ka (Fig. 7). These four fluctuations show areversed saw-tooth pattern, and in each saw-tooth of ∼30-kyr periodsthere are in a change from shorter cycles to longer ones. From 130 to115 ka on MIS 5e, fluctuations became longer, with their periodincreasing from 3.8-kyr to 10.2-kyr, to 15.9-kyr, and to 24.1-kyr. Duringthe Holocene, fluctuations of 1.4-kyr and 4.0-kyr were recognized.

The annual SSTs (°C) fluctuated at 118-kyr periods correspondingto the eccentricity cycle, the 48-kyr periods during 140–45 kaintervals due to the orbital-obliquity (tilt) cycles, and 20 to 25-kyr

Fig. 6. Chronostratigraphic variations of the Td′-derived annual paleo-SSTs (°C) in seven cores in the Tohoku Area off northeast Japan. The high Td′-derived annual paleo-SSTs (°C) inmiddle MIS 5e (Marine Isotope Event 5.5) and 1 of the interglacial phase are correlated among cores. The remarkable decline of paleo-SSTs (°C) recognized in seven cores arecorrespond to the ages of the Heinrich events 1–6 in the northern North Atlantic (Heinrich, 1988) and for the Marine Isotope Event 5.2 in MIS 5b (Martinson et al., 1987). Whitetriangles on the left side of each column indicate the age control points for age model.

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periods (precession cycles) during 145–120 ka and 40–0 ka intervals.The 60 to 108-kyr periods are interpreted as the intervals of sea-levelchanges due to glacial–interglacial cycles since 145 ka. The shorterperiods (1.4-kyr, 2.1-kyr and 3.3 to 4.0-kyr) in variability of the annual

Fig. 7. Thewavelet analysis for thevaluesof theband ratio Td′-derivedannual paleo-SSTs (°C) in cothe shape of revised saw-tooth changing from shorter cycles to longer ones in each period of ∼confidence level (power significance). The contour levels are chosen for 75%, 50%, 25% and 5% of thcone of influence, where zero padding has reduced the variance. c. The global wavelet power specassuming the same significance level and background as in b.

SSTs (°C) are also recognized between intervals of 60–0 ka (Fig. 7).The period of 1.4 to 2.1-kyr correspond with the Heinrich events(Heinrich, 1988), Dansgaard–Oeschger cycles (Alley, 1998), fluctua-tions of residual Δ14C record with variance centered on ∼2-kyr

reMD01-2421. a. Time series ofpaleo-SSTs (°C) in coreMD01-2421. Thefluctuations indicate30-kyr since 130,000 years. b. Wavelet power spectrum. Solid black line indicates the 10%ewavelet power.More intense colors indicate higher power. The cross-hatched region is thetrum (black line). The dashed line indicates the significance for the global wavelet spectrum,

115I. Koizumi, H. Yamamoto / Marine Micropaleontology 74 (2010) 108–118

(Stuiver et al., 1991), and the cycle of ∼2-kyr interval in the variabilityof El Niño/Southern Oscillation activity (Moy et al., 2002). The cyclesof 4-kyr periods are twice of 2-kyr periods.

The annual SSTs (°C) over the last 150,000 years in northern coreMR97-04-1 fluctuated in a curve similar to a saw-tooth, at ∼40-kyrperiods (Fig. 8). The lower frequencies in 96 to 112-kyr and 7.2 to12.8-kyr periods corresponding to the Bond cycles (Alley, 1998) canbe recognized since 150 ka. The SSTs' fluctuation at a periodicity ∼32-kyr occurs in 75–105 ka and 44–0 ka intervals, and corresponds totheir periodicity of the long-term variation of the El Niño–SouthernOscillation (ENSO) (Clement et al., 1999). The 16.0 to 20.8-kyrperiodicity corresponding to the precession cycles were recognized in150–115 ka and 10–0 ka intervals. Fluctuations at a periodicity of 1.6-kyr and 2.6-kyr are recognized during 134–113 ka, 82–70 ka, 50–28 ka, and 25–0 ka intervals (Fig. 8b).

These observations imply that the paleo-hydrography of theTohoku Area off northeast Japan corresponded to global environmen-tal changes on an orbital time scale.

4.5. Oceanic diatom abundance

The oceanic diatom abundances (×107 oceanic diatom valves per1 g of dried sediment) were calculated in order to exclude diatomsthat were not part of the local ecosystem, including as such extinct(reworked) forms, freshwater, and littoral–neritic diatom species.Oceanic diatom abundances generally were higher during interglacialthan during glacials (Fig. 9). They were higher in the cores recoveredfrom near shore, off the Sanriku coast, where diatoms have a highproductivity due to mixing of the Tsugaru Warm Current or KuroshioExtension, and the cold Oyashio Intrusion in the northwesternmarginof the Mixed Water Region (Fig. 1). Very sharp and short-termincreases in abundance occurred at 135.1 ka, 93.4 ka and 41.2 ka incore MR97-04-1. Larger term fluctuations (with superimposed

Fig. 8. Thewavelet analysis for the values of the band ratio annual Td′-derived paleo-SSTs (°C)indicate the form of saw-tooth at ∼40-kyr periods since 150,000 years. b. Wavelet power scontour levels are chosen for 75%, 50%, 25% and 5% of the wavelet power. More intense colopadding has reduced the variance. c. The global wavelet power spectrum (black line). The dasignificance level and background as in b.

secondary, smaller fluctuations) occurred at 84.5 ka, 52–45 ka,36.0 ka, and 10.6–2 ka in core ODP Hole 1150A. Large decreases inabundance occurred at ∼65.0 ka and ∼22.0 ka in all seven cores.

The periodicity in the variability of the oceanic diatom abundancesis mainly at 41-kyr, corresponding to the obliquity (tilt) band, and 23-kyr, corresponding to the precession band (Koizumi and Yamamoto,2008). Vertical and lateral mixing between cold and warm waters byglacio-eustatic sea-level changing from glacial to interglacial periodsdue to the orbital forcing should be promoted to primary productionof oceanic diatoms.

The pattern of fluctuations of oceanic diatom abundance in coreMR97-04-1 shows the revised saw-tooth pattern over the last150,000 years, changing from lower abundance variations at shortercycles, to higher values in longer cycles at 132 ka, 100 ka, 70 ka and30 ka, i.e., in ∼30-kyr periods (Fig. 10a). Wavelet analysis indicatesthat the fluctuations may be subdivided into four intervals: from 145to 112 ka, from 105 to 70 ka, from 55 to 28 ka, and 18–0 ka. The lowfrequencies of 96 to 112-kyr and 44 to 64-kyr periods oscillate since150,000 years. Fluctuations at 24-kyr periodicity occurred in theinterval between 145 and 90 ka (Fig. 10b). Shorter fluctuations of∼2.0-kyr periods occurred in the interval from 131 to 128 ka, whereasfluctuations at a periodicity of 3.2 to 9.6-kyr periods can be recognizedin the interval between 48.0 and 34 ka. Fluctuations at periodicities of2 to 5.6-kyr periods were recognized during 1.8–0 ka intervals.

5. Summary and conclusions

1. Reworked and/or displaced extinct diatoms derived from theseafloor dominantly occurred at the lower sea-levels of glacialperiods, in the near shore, northern part of the Mixed WaterRegion.

2. The relative abundances of the littoral–neritic association diatomspecies are almost a mirror image of those of oceanic association.

in coreMR97-04-1. a. Time series of paleo-SSTs (°C) in coreMR97-04-1. The fluctuationspectrum. Solid black line indicates the 10% confidence level (power significance). Thers indicate higher power. The cross-hatched region is the cone of influence, where zeroshed line indicates the significance for the global wavelet spectrum, assuming the same

Fig. 9. Chronostratigraphic variations of oceanic diatom abundances (×107/g of dried sediment) in seven cores in the Tohoku Area, off northeast Japan. Diatom abundances increasein the interglacial phase, and in the cores recovered from near shore off the Sanriku coast. The fluctuations show the shape of revised saw-tooth changing from smaller abundances atshorter cycles to high values longer cycles at ∼30-kyr periods. White triangles on the left side of each column indicate the age control points for age model.

116 I. Koizumi, H. Yamamoto / Marine Micropaleontology 74 (2010) 108–118

These abundances are higher in the cores beneath the Kuroshio–Kuroshio Extension than in the cores under the northern margin ofthe Mixed Water Region, because the Kuroshio flowing northeastalong the near shore of southwest Honshu transports littoral–neritic diatoms into the southern margin of the Mixed Water

Fig. 10. The wavelet analysis for the values of the band ratio oceanic diatom abundance in cspectrum. Solid black line indicates the 10% confidence level (power significance). The contoindicate higher power. The cross-hatched region is the cone of influence, where zero padddashed line indicates the significance for the global wavelet spectrum, assuming the same

Region. The peaks of high abundances of oceanic association arecorrelated with each other in the interglacial phases in all cores.

3. The relative abundances of oceanic warm-water diatoms increasedin the cores located in the warm-water tongues and eddies alongthe Kuroshio Extension. On the other hand, the relative abundances

ore MR97-04-1. a. Time series of paleo-SSTs (°C) in core MR97-04-1. b. Wavelet powerur levels are chosen for 75%, 50%, 25% and 5% of the wavelet power. More intense colorsing has reduced the variance. c. The global wavelet power spectrum (black line). Thesignificance level and background as in b.

117I. Koizumi, H. Yamamoto / Marine Micropaleontology 74 (2010) 108–118

of oceanic cold-water diatoms were higher than those of warm-water diatoms in the northern cores of the Mixed Water Region.

4. The Td′-derived annual SSTs (°C) are in agreement with thealkenone Uk′37-based summer SSTs (°C) and the oxygen isotopecurve of benthic foraminiferal tests over the last 150,000 years incore MD01-2421.

5. The curve of the annual SSTs (°C) over 150,000 years showsfluctuations in the shape of a reversed saw-tooth in southern cores,but in the shape of a saw-tooth in northern cores. Wavelet analysesof the annual SSTs (°C) records clearly describe the shorter periodsof variability of 1.4 to 2.6-kyr, 3.3 to 4.0-kyr, and 7.2 to 12.8-kyr.

6. The oceanic diatom abundances in the northern margin of theMixed Water Region are twice those in cores beneath the streamaxis of the Kuroshio Extension, because streaky structures owing tomixing of Oyashio Intrusions and warm-water eddies along theKuroshio Extension developed in the northern margin area.

7. The wavelet analyses of oceanic diatom abundances over the last150,000 years show oscillation in the low frequencies of 96 to 112-kyr and 44 to 64-kyr periods, and in the higher frequencies of ∼2.0-kyr, 2 to 5.6-kyr and 3.2 to 9.6-kyr periods.

Acknowledgements

We thank Prof. Hodaka Kawahata of the University of Tokyo forproviding the samples from MD01-2421 of the IMAGES VII cruiseWEPAMA in 2001. We thank also Drs. Takuya Itaki of the NationalInstitute of Advanced Industrial Science and Technology (AIST),Kazuho Fujine of the Integrated Ocean Drilling Program (IODP) andKana Nagashima of the Japan Agency for Marine-Earth Science andTechnology (JAMSTEC) for subdividing sub-samples and providingconcerning research data. We gratefully acknowledge Drs. TomohisaIrino of Hokkaido University for his spectral analysis, and MasanobuYamamoto of Hokkaido University for offering the data on agemodels.We acknowledge with special thanks to Dr. John A. Barron of the USGSat Menlo Park for reviewing critically and suggestions for improvingan earlier draft. We also special thank to Prof. Ellen Thomas of YaleUniversity, the regional editor of Marine Micropaleontology, forcritical reviews and for making English of the manuscript better.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.marmicro.2010.01.003.

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