Quaternary Research 74 (2010) 91–99

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Millennial-scale vegetation changes during the last 40,000 yr based on a pollenrecord from Lake Biwa, Japan

Ryoma Hayashi a,⁎, Hikaru Takahara b, Akira Hayashida c, Keiji Takemura d

a Graduate School of Agriculture, Kyoto Prefectural University, 1-5 Hangi-cho, Shimogamo, Sakyo-ku, Kyoto, Kyoto 606-8522, Japanb Graduate School of Life and Environmental Sciences, Kyoto Prefectural University, 1-5 Hangi-cho, Shimogamo, Sakyo-ku, Kyoto, Kyoto 606-8522, Japanc Department of Environmental Systems Science, Doshisha University, 1-3 Tatara Miyakodani, Kyotanabe, Kyoto 610-0321, Japand Institute for Geothermal Sciences, Graduate School of Science, Kyoto University, Noguchibaru, Beppu, Oita 874-0903, Japan

⁎ Corresponding author. Fax: +81 75 703 5683.E-mail address: hayashiryoma@gmail.com (R. Hayas

0033-5894/$ – see front matter © 2010 University of Wdoi:10.1016/j.yqres.2010.04.008

a b s t r a c t

a r t i c l e i n f o

Article history:Received 10 August 2009Available online 1 June 2010

Keywords:D–O cyclesEast Asian monsoonLake BiwaMillennial-scale vegetation changePollen recordCharcoal record

A high-resolution pollen record covering the last 40,000 yr (BIW95-4) from Lake Biwa, western Japan, showsregional vegetation responses to millennial-scale climate changes. From 40 to 30 ka, Cryptomeria japonicawas dominant around the lake among pinaceous conifers and deciduous broad-leaved trees. During thisperiod, fluctuations of C. japonica are correlated with Dansgaard–Oeschger (D–O) cycles recognized from theanhysteretic remanent magnetization (ARM) record. Increases in the abundance of this taxon may have beencaused by wetter summer conditions influenced by the East Asian monsoon or increased snowfall on the Seaof Japan side of the Japanese archipelago. Between 29 and 14 ka, pinaceous conifer forests mainly composedof Pinus subgenus Haploxylon, Tsuga, and Picea trees developed. At approximately 23 ka, Picea trees increasedin abundance as ARM values decreased. This expansion of Picea trees has been correlated with Heinrich event(HE) 2 in the North Atlantic. At about 14 ka, the distribution of broad-leaved forest (mainly composed ofdeciduous oaks) began to expand after D–O 1. Evidence of significant vegetation change related to the abruptYounger Dryas cooling event has not been found.

© 2010 University of Washington. Published by Elsevier Inc. All rights reserved.

Introduction

Millennial-scale climate oscillations during the last glacial period[Dansgaard–Oeschger (D–O) cycles] were first identified in ice coresfrom Greenland (Dansgaard et al., 1993). Proxy records fromnumerous ice and sediment (both terrestrial and marine) coreshave also indicated millennial-scale climate oscillations in otherregions around the world (Voelker, 2002). Furthermore, speleothemrecords from China have indicated correlations of millennial-scalemonsoon fluctuations with D–O cycles in the Greenland ice cores(Wang et al., 2001, 2008; Yuan et al., 2004). Sánchez Goñi et al. (2008)suggested similar patterns of enhancement of Mediterranean climateand the Asian summer monsoon regime during precession minima,likely related to a summer atmospheric teleconnection patternbetween the two regions. However, key questions still remain aboutthe transmission of the millennial-scale variability in the NorthAtlantic to the East Asian region and the possible atmospheric andoceanic mechanisms involved. Answering those questions requires adetailed understanding of the impacts of millennial-scale climateoscillations in the East Asian region.

hi).

ashington. Published by Elsevier I

Climate of Japanese archipelago is strongly affected by the EastAsian monsoon and ocean currents in the Northwestern Pacific(Fukui, 1977). Thus, paleoenvironmental records around Japan aregood indicators for atmospheric and oceanic changes in the East Asianregion. Recently, millennial-scale climate oscillations around theJapanese archipelago have been suggested by both terrestrial andmarine core records, including the Sea of Japan (Wang and Oba, 1998;Tada et al., 1999; Irino and Tada, 2002; Nagashima et al., 2007), LakeBiwa (Kuwae et al., 2004; Yamada, 2004; Iwamoto and Inouchi, 2007;Hayashida et al., 2007), and Lake Nojiri (Kumon et al., 2003).

However, vegetation responses in Japan to the millennial-scaleclimate oscillations during the last glacial period are still poorlyunderstood. Although vegetation changes during the Younger Dryas(YD) were suggested by pollen records from Lake Suigetsu in westernJapan (Nakagawa et al., 2003, 2005; Yasuda et al., 2004) and theKenbuchi Basin on Japan's northern island of Hokkaido (Igarashi et al.,1993; Igarashi, 1996), such high-resolution pollen records indicatingmillennial-scale vegetation change during the last glacial period are rarein Japan. Pollen recordswith an accurate agemodel and a high samplingresolution from the last glacial period are indispensable for under-standing vegetation responses to abrupt climate changes in Japan.

In Lake Biwa, continuous clay sediment for the last 430,000 yr exists(Meyers et al., 1993). Although several pollen records from Lake Biwahave been published (e.g., Fuji, 1984; Miyoshi et al., 1999), those pollenrecords have too low resolution for discussing abrupt vegetation

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92 R. Hayashi et al. / Quaternary Research 74 (2010) 91–99

changes. In this study, we present a 250 yr-resolution pollen recordfrom the BIW95-4 piston core, Lake Biwa, western Japan. In addition,analysis ofmicroscopic charcoal indicates the influence of fire events onthe regional vegetation. Hayashida et al. (2007) suggested thatvariations of the anhysteretic remanent magnetization (ARM) in theBIW95-4 core, which represents the amount of fine magnetic particles,were associated with higher precipitation rates related to the increasedfluxof theparticles fromthewatershed andprobably correlatedwithD–O cycles recorded in the Greenland ice cores. The pollen record of thisstudy provides additional information about the interpretations ofenvironmental changes in the D–O cycles around Lake Biwa. Wecompare the pollen record to the environmental magnetic record, thespeleothemrecords fromHuluCave in China (Wanget al., 2001) and theoxygen isotope record of the Greenland ice core (North Greenland IceCore ProjectMembers, 2004), and discuss vegetation responses in Japanto millennial-scale fluctuations of the East Asian monsoon and D–Ocycles in the North Atlantic region.

Study site

Lake Biwa is located at an altitude of 85 m in the Omi Basin,western Japan. The basin measures 30 km from east to west and50 km from north to south, and is surrounded by mountains ofapproximately 1000 m. Lake Biwa is the largest lake in Japan with asurface area of 670 km2, a catchment area of 3174 km2, and amaximum water depth of 104 m. Tephrochronological and paleocli-matological studies have revealed that continuous clay sedimentationhas occurred in Lake Biwa for the last 430,000 yr (Meyers et al., 1993).The BIW95-4 core was drilled in 1995 off Shirahige Cape on thewestern shore of Lake Biwa (35°15′ N, 136°03′ E) at a water depth of67 m (Takemura et al., 2000) (Fig. 1).

In the Japanese islands, the summer East Asian monsoon provideswarm and humid conditions, whereas winter monsoons and theTsushima Current within the Sea of Japan bring winter snowfallespecially in the coastal areas of the Sea of Japan, and these climateconditions also strongly affect vegetation composition and patterns(Maekawa, 1974). The Hikone Climatological Station on the easternshore of Lake Biwa has recorded a mean annual temperature in thearea of 14.4 °C and mean annual precipitation of 1617.9 mm (JapanMeteorological Agency, 2001). During the winter, more snow falls inthe northern part of Lake Biwa due to the influence of the Sea of Japan.

Climax vegetation around Lake Biwa is warm temperate evergreenbroad-leaved forest mainly composed of evergreen oaks (Quercus

Figure 1. Location map for Lake Biwa and the BIW95-4 core. a) Location of Lake Biwa and Huwater depth of Lake Biwa, based on data from the Lake Biwa Environmental Research Insti

subgenus Cyclobalanopsis) and Castanopsis. Above 700 m, the forest iscool temperate deciduous broad-leaved, composed of beech (Faguscrenata) and deciduous oaks (Quercus subgenus Lepidobalanus).Cryptomeria japonica is also distributed in the cool temperate forestsof snowy regions in the coastal areas of the Sea of Japan. Thedistribution of C. japonica today indicates annual precipitation above2000 mm, and both winter snowfall and summer rainfall areimportant for the growth of this tree along the coastal areas of theSea of Japan (Hayashi, 1960). Also, as suggested by studies of buriedforest, C. japonica dominated lowland peat bogs and alluvial fansacross the region during the late Holocene (Fujii, 1965; Takahara andTakeoka, 1990).

Material and methods

Lithology and chronology

The BIW95-4 sediment core is 14.45 m long and consists ofhomogeneous gray clay with black to dark gray seams, coaly layers,and vivianite (Takemura et al., 2000). Comparison with magneticsusceptibility data from another core from the same site has revealedthat the top 1.23 m of the core is missing (Takemura et al., 2000).Widespread tephra layers such as the Kawagodaira (Kg), Kikai-Akahoya (K-Ah), Ulreung-Oki (U-Oki), Sakate, Daisen-Higashi Daisen(DHg), Daisen-Sasaganaru (DSs), and Aira-Tn (AT) layers wereidentified in the BIW95-4 core (Takemura et al., 2000) (Table 1). Inaddition, 10 AMS (Accelerator Mass Spectrometry) radiocarbon dateswere measured and calibrated using INTCAL98 (Stuiver et al., 1998)and the Lake Suigetsu calibration curve (Kitagawa and Van der Plicht,1998) (Table 1). Following Hayashida et al. (2007), an age model forthe BIW95-4 core has been constructed by cubic spline interpolationof the tephra and radiocarbon dates excluding the ages provided bythe dating of bulk (TOC: total organic carbon) samples (Fig. 2). Theage for the bottom of the core was extrapolated from the age curve.Based on this age model, the BIW95-4 core extends back toapproximately 40,000 yr with an average sedimentation rate of40 cm/ka.

Pollen analysis

Sediment samples for pollen analysis were taken from the BIW95-4 core at 10 cm intervals yielding a resolution of 250 yr based on theaverage sedimentation rate. A total of 141 samples of 1 cm3 volume

lu Cave, China (Wang et al., 2001); b) drill site of the BIW95-4 core. Contours show thetute.

Table 1Widespread tephra and radiocarbon ages of the BIW95-4 core (after Hayashida et al., 2007).

Composite depth (m) Material Tephra Lab number 14C age (yr BP) 1 sigma Calendar age (ka BP)

1.65 Leaf HGr-995 2431 110 2.541.86 Volcanic ash Kawagodaira 2.952.81 Leaf HGr-1008 4910 120 5.673.21 Volcanic ash K-Ah 6300 7.254.13 Volcanic ash U-Oki 10.196.83 Volcanic ash Sakate 18.737.71 Leaf HGr-1009 17,770 150 21.159.98 TOC HGr-1117 22,200 400 25.8010.90 TOC HGr-1113 25,200 500 28.6011.76 Volcanic ash AT 28.7812.00 TOC HGr-1114 27,670 600 31.5012.75 TOC HGr-1115 30,200 800 32.3013.46 Leaf HGr-1010 29,900 750 32.0014.00 Leaf HGr-1001 32,500 1400 33.9014.66 Leaf HGr-1011 N37,500

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were processed using standard palynological techniques includingpotassium hydroxide (KOH), 180 µm sieving, hydrofluoric acid (HF),and acetolysis (Faegri et al., 1989). For the estimation of pollenconcentrations and accumulation rates, we added 17×104 plasticmicrospheres to each sample (Ogden, 1986). Pollen samples weremounted in silicon oil and examined with a transmitted lightmicroscope at 400× magnification. At least 200 tree pollen grainswere counted for each sample. Percentages of pollen and spore taxawere calculated based on the tree pollen sum. Because highfrequencies of Alnus pollen were considered to derive from Alnusthickets on the bogs in Japan, this taxon was excluded from the treepollen sum. The pollen diagram was divided into local pollenassemblage zones using the constrained incremental sum of squares(CONISS) method (Grimm, 1987) based on percentages of the treepollen.

Charcoal analysis

In this study only microscopic charcoal was analyzed becauselarger fragments were virtually absent on the 180 µm sieves duringprocessing of the pollen samples. To assess microscopic charcoalcontent, 72 of the pollen slides were studied at approximately 20-cmintervals, providing a 500-yr sampling resolution. Charcoal particles(opaque, angular, and black fragments) were identified using atransmitted light microscope at 200× magnification (Whitlock andLarsen, 2001). To estimate the charcoal accumulation rate, at least 200plastic microspheres in each pollen slide were counted with the

Figure 2. Age model for the BIW95-4 core (after Hayashida et al., 2007).

charcoal particles. Following MacDonald et al. (1991) and Tinner et al.(1998), the areas of charcoal particles larger than 75 µm2 weremeasured using NIH Image software (http://www.rsb.info.nih.gov/nih-image/). Charcoal accumulation rates were calculated using theaverage sedimentation rate from the BIW95-4 core age model.

Results and discussions

Overview of vegetation and fire history for the last 40,000 yr aroundLake Biwa

The BIW95-4 pollen record (Fig. 3) is divided into three localpollen assemblage zones (zones A–B), which date to 40–29, 29–12.5and 12.5–2 ka. Those zones correspond roughly to MIS 3 to 1,although the transition from zone B to zone C is younger than the MIS2–1 boundary (Lisiecki and Raymo, 2005). In this section, orbital-scalevegetation changes around Lake Biwa are summarized below. Thewell-dated pollen record of BIW95-4 core provides reliable ageestimates for the vegetation changes, which improve previous studiesof pollen records from Lake Biwa (e.g., Fuji, 1984; Miyoshi et al.,1999).

40–29 ka (zone A)From approximately 40 to 29 ka, temperate conifer forests

composed mainly of C. japonica grew around Lake Biwa withdeciduous oaks (Quercus subgenus Lepidobalanus) and pinaceousconifers (for example, Pinus subgenus Haploxylon, Tsuga, and Piceatrees). The accumulation rate of microscopic charcoal is relatively low,especially in the lower part of zone A. During this period, Cryptomeriapollen fluctuated four times. In contrast, pinaceous conifers (mainlyPinus subgenus Haploxylon and Tsuga trees) increased in abundanceduring the periods when C. japonica decreased. In particular, a coldclimate phase at approximately 40 ka is implied by an increase inPicea pollen abundance coincident with low percentage values ofCryptomeria pollen.

A mixed forest of temperate conifers, deciduous broad-leavedtrees, and pinaceous conifers was established in the latter part of MIS3, which is consistent with pollen records from the Kurota Lowland onthe coast of the Sea of Japan (Takahara and Kitagawa, 2000) and theKamiyoshi Basin located inland (Takahara et al., 2000) near Lake Biwa.However, the main taxa of temperate conifers present at Lake Biwaand the Kamiyoshi Basin were different. Cupressaceae trees domi-nated the Kamiyoshi Basin, in contrast to Lake Biwa and the KurotaLowland where C. japonica dominated in the latter half of MIS 3.Additionally, the abundance of F. crenata in the Kurota Lowland washigher than at Lake Biwa. Takahara et al. (2000) suggested that thesevegetation patterns during MIS 3 may be attributable to a gradient in

Figure 3. Summary pollen diagram and microscopic charcoal accumulation rate data for the BIW95-4 core. Pollen percentages are based on the sum of total tree pollen.

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snow conditions, with slightly dry, light snowfall inland and wet,heavy snowfall along the Sea of Japan.

29–12.5 ka (zone B)While C. japonica began to decrease in abundance at approxi-

mately 31 ka, Pinus subgenus Haploxylon and Tsuga trees began toincrease and became dominant after 29 ka above the AT tephra layer.Subsequently, vegetation around Lake Biwa during MIS 2 wascharacterized by the growth of pinaceous conifer forests composedmainly of Pinus subgenus Haploxylon, Tsuga, and Picea trees.Percentages of Abies pollen from the BIW95-4 core during this periodare less than those from two other sediment cores from Lake Biwa(Fuji, 1984; Miyoshi et al., 1999). During MIS 2, deciduous broad-leaved trees, such as Betula and Quercus, were also present, andrelatively high values of upland herb taxa (mainly Poaceae andArtemisia) indicate that forests were moderately open or discontin-uous. Microscopic charcoal accumulation rates are relatively lowduring MIS 2.

Throughout MIS 2, Pinus subgenus Haploxylon and Tsuga pollendominate in the BIW95-4 core. Tsukada (1983, 1985) used pollen andplant macrofossil records to infer that temperate coniferous forestswere distributed across lowland western Japan at the last glacialmaximum. The occurrence of plant macrofossils found at 50 maltitude at Tado-Cho, approximately 50 km east of Lake Biwa (Minakiand Matsuba, 1985), and the pollen assemblage composition duringMIS 2 in the BIW95-4 core, suggest the dominance of temperateconifers such as Pinus koraiensis and Tsuga sieboldii associated withdeciduous broad-leaved trees in the lowland area surrounding LakeBiwa. Furthermore, Picea pollen increased in abundance after 26 kaand became dominant between 25 and 22 ka, suggesting theexpansion of spruce forests around Lake Biwa in a colder and drierclimate.

12.5–2 ka (zone C)After 14 ka, Pinus subgenus Haploxylon, Tsuga, Abies, and Picea

trees decreased in abundance and deciduous broad-leaved trees,mainly composed of deciduous oaks, began to increase. Vegetationchange at the beginning of MIS 1 correlates with decreasing quartzand increasing TOC content in the BIW95-4 core (Yamada, 2004).Quartz content is considered a proxy for aeolian dust flux influencedby the intensity of the Asian winter monsoon and westerlies. TOCcontent is used as a proxy for primary biological production, and inturn for precipitation (Yamada, 2004). Aquatic productivity isconsidered to increase due to a greater input of nutrients fromincreased soil erosion when the rate of precipitation rises (Meyers etal., 1993). At about 12.5 ka, pinaceous conifers were virtually absentaround Lake Biwa. Analyses of the varved record from Lake Suigetsusuggest that deciduous broad-leaved trees began to increase inabundance after 15 ka, and pinaceous conifers almost disappeared at14.8 ka along the coast of the Sea of Japan (Yasuda et al., 2004).

Deciduous broad-leaved forest, mainly composed of deciduousoaks (Quercus subgenus Lepidobalanus), became dominant aroundLake Biwa at approximately 12 ka. However, abundance of F. crenatawas limited around Lake Biwa in this period, in contrast to an increasein abundance during the same period along the coast of the Sea ofJapan (Takahara and Takeoka, 1992; Yasuda et al., 2004). It suggestsdrier conditions particularly during the winter around Lake Biwa,because highwinter precipitation is themost influential climate factordetermining the present distribution of F. crenata (Matsui et al., 2004).Additionally, an increase in the accumulation rate of microscopiccharcoal suggests that fire events occurred during this period. Anincrease in fire events during the early Holocene has also beendiscussed by Inoue et al. (2001) based on a charcoal record of LakeBiwa sediment throughout the last 130 ka. The increase in fire eventsmay have influenced the subsequent development of deciduous oakforests around Lake Biwa, as fire is an important disturbance agent for

the development of oaks in the warm temperate deciduous and cooltemperate mixed broadleaf/conifer forests of monsoon Asia (Naka-shizuka and Iida, 1995).

C. japonica began to increase gradually after 12 ka, while bothcharcoal accumulation rates and the abundance of deciduous oaksdeclined after 10 ka. After 8 ka, below the K-Ah tephra layer, Quercussubgenus Cyclobalanopsis (evergreen oaks) pollen began to increasein abundance in the core, and between 6 and 3 ka forests around LakeBiwa featured both evergreen oaks and C. japonica. C. japonica becamedominant after 3 ka. Higher microscopic charcoal accumulation ratesduring the late Holocene suggest increasing fire activity, likely relatedto human activity after the Jomon period (Inoue et al., 2001).However, the change in fire regime does not appear to have alteredvegetation composition before about 2000 cal yr BP.

Vegetation responses to millennial-scale climate changes during the last40,000 yr around Lake Biwa

In this study, we compared the pollen record of the BIW95-4 corewith the ARM record (Hayashida et al., 2007) to investigate vegetationresponses to abrupt climate change in the Lake Biwa region (Fig. 4).On the basis of the age model and ARM variations of the BIW95-4 core(Hayashida et al., 2007), we compared the vegetation history aroundLake Biwa with millennial-scale climate changes recorded in theGreenland ice cores (North Greenland Ice Core Project Members,2004) and the speleothem record of Hulu Cave, China (Wang et al.,2001) (Fig. 5).

The later part of MIS 3Variations in the pollen record between 40 and 30 ka coincidewith

fluctuations in the ARM record. Cryptomeria pollen exhibits aparticularly strong correlation, with high abundance values shortlyafter the ARM peaks representing D–O 5 to 8 (Hayashida et al., 2007).However, all peaks in Cryptomeria pollen abundance during thisperiod occur later than the peaks in the ARM record. In contrast,pinaceous conifer pollen (for example, Pinus subgenus Haploxylon andTsuga) increases in abundance when ARM values are relatively low.Picea pollen also increases in abundance in the lower part of theBIW95-4 core. However, Betula pollen has a similar pattern to thepinaceous conifers, and other deciduous broad-leaved trees do notshow a clear correspondence with the ARM variation.

The development of pinaceous conifer forests composed of Pinussubgenus Haploxylon, Tsuga, and Picea trees at approximately 40 ka,which indicate a cold and dry climate around Lake Biwa, can becorrelated with the stadial period of HE 4. Fluctuations in vegetationcomposition after HE 4 can be correlated to D–O 8 to 5. Thedistribution of C. japonica expanded during the interstadials, whereasthe abundance of pinaceous conifers increased together with uplandherbs during the stadials of the D–O cycles. The expansion of C.japonica during the interstadials around Lake Biwa indicates climatewarming and an increase in precipitation affected by the strongsummer East Asian monsoon, which is supported by the ARM recordof Lake Biwa (Hayashida et al., 2007) and the Hulu Cave speleothemrecords (Wang et al., 2001). In addition, increase of winter snowfall onthe Sea of Japan side of the Japanese archipelago, likely related to themillennial-scale environmental changes of the Sea of Japan suggestedby Tada et al. (1999), may have affected the abundance of C. japonicabecause this tree is widespread and locally abundant in the cooltemperate zone of the snowy regions today (Hirayama and Sakimoto,2003). C. japonica can regenerate by layering from a bent branch orstem influenced by snow pressure, which is an adaptive regenerationstrategy in snowy regions (Hirayama and Sakimoto, 2003). Incontrast, the increase of pinaceous conifers together with uplandherbs during the stadials indicates a cold and dry climate around LakeBiwa at those times.

Figure 4. Comparison of major pollen records (this study) and the anhysteretic remanent magnetization (ARM) record (Hayashida et al., 2007) from the BIW95-4 core.

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However, the interstadials at about 31 and 32 ka from the LakeBiwa record are younger than D–O 5 and 6 in the Greenland andHulu Cave records. In this study, the Lake Suigetsu calibration curve(Kitagawa and Van der Plicht, 1998) was used for the calibration ofradiocarbon ages from the BIW95-4 core because it was based onJapanese terrestrial sediments near Lake Biwa. Hughen et al. (2004)indicated that the Lake Suigetsu record showed younger calendarages than the Cariaco record and the Bahama record. Therefore, theinterstadials from the Lake Biwa record at approximately 31 and32 ka may be older and correlate with D–O 5 and 6. Based on theCalPal-2007 calibration curve (Danzeglocke et al., 2009), thecalibrated radiocarbon age at 13.46 m of this core, just beforethe peak of Cryptomeria pollen likely correlated with D–O 6, is34,090±690 cal yr BP, and estimated ages of those interstadialsbecome comparable to D–O 5 and 6 in the Greenland and Hulu Caverecords.

The high-resolution pollen record from the BIW95-4 core indicatesthat the peaks in abundance of Cryptomeria pollen occurred later thanthose of the ARM record between 40 and 30 ka (Figs. 4 and 5). At thepeaks of Cryptomeria pollen abundance, C. japonica might increase onpeat bogs and alluvial fans in lowland formed by the increasingprecipitation at the interstadials suggested by the ARM record,because the development of C. japonica forests on such locationswere recognized in the Holocene records (Fujii, 1965; Takahara andTakeoka, 1990). After the development of those new favorablelocations for C. japonica, it likely took time for C. japonica forests todevelop there. This is a possible reason for the time lags between thepeaks of Cryptomeria pollen and the ARM record. However, otherpossibilities can be considered by additional higher resolution recordsin the future.

MIS 2During MIS 2, the abundance of pinaceous conifer pollen appears

to correlate with the ARM variations. At the ARM peak at approx-imately 26 ka, Cupressaceae-type pollen is relatively abundant.Subsequently, Picea pollen begins to increase in abundance andpeaks in the period between approximately 24 and 23 ka when ARMvalues decrease. After approximately 23 ka, coincident with the ARMpeak, Picea pollen begins to decrease in abundance and Pinussubgenus Haploxylon pollen increases, along with minor increases ofCryptomeria and Cupressaceae-type pollen.

No significant peaks in the ARM record from the BIW95-4 corewere recognized for correlation with D–O 4 and 3. Tada et al. (1999)indicated that the dark layer in the Sea of Japan core is comparable tothe D–O 4 event found above the AT tephra layer. This relationshipbetween D–O 4 and the AT tephra layer in the Sea of Japan coresuggests that the decrease of the tree/upland herb pollen ratio andCryptomeria pollen between 29 and 28 ka may be comparable withthe stadial period of HE 3. In addition, the increase of the deciduousbroad-leaved/pinaceous pollen ratio with a small peak in Cryptomeriapollen abundance at approximately 27 ka may correlate with D–O 4.Additional vegetation and climate records are necessary to discernfurther detailed correlation between these vegetation changes and D–O 4 around Japan, as the vegetation records discussed here may havebeen affected by disturbances after the eruption of the AT tephra at29 ka.

At approximately 23 ka, Picea pollen increases in abundancewith adecrease in the ARM record in the BIW95-4 core. This increase in Piceapollen suggests expansion of boreal conifer forests around Lake Biwa,as Picea trees are mainly distributed in the boreal zone at present. Inaddition, Pinus subgenus Haploxylon pollen is considered to imply the

Figure 5. Comparison of the BIW95-4 core records with the Hulu Cave record and the Greenland ice core record. Millennial-scale fluctuations in the BIW95-4 core correlated to D–Oevents and Heinrich events are indicated by dark gray and light gray lines, respectively. Dashed lines show possible correlations to D–O events and Heinrich events. a) The NorthGreenland Ice Core Project (NGRIP) oxygen isotope curve from Greenland. Numbers show Greenland interstadials (North Greenland Ice Core Project Members, 2004); b) the oxygenisotope curve of the Hulu Cave stalagmites. Numbers show Chinese interstadials (gray line, stalagmite PD; black line, stalagmite MSD) (Wang et al., 2001); c) the ARM susceptibilityfrom the BIW95-4 core (Hayashida et al., 2007); d) tree/upland herb pollen ratio from the BIW95-4 core; e) Pinus subgenus Haploxylon/Picea pollen ratio from the BIW95-4 core inzone B where Picea pollen appeared continuously; f) deciduous broad-leaved tree without the Betula/pinaceous conifer pollen ratio from the BIW95-4 core; g) Cryptomeria pollenpercentage from the BIW95-4 core.

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presence of the temperate conifer P. koraiensis based on plantmacrofossil data (Minaki and Matsuba, 1985). Therefore, the Pinussubgenus Haploxylon/Picea pollen ratio indicates the proportion oftemperate conifers to boreal conifers around Lake Biwa. The trend ofthis ratio correlates with the HE 2 pattern in the Greenland ice coresand the Hulu Cave speleothem. During HE 2, the abundance of borealconifers (Picea) may have increased under the colder and driercondition. Furthermore, the tree/upland herb pollen ratio decreaseswith the ARM record and the Hulu Cave record at approximately16 ka. The increase in abundance of upland herbs may indicate acolder or drier climate compared with HE 1, although it may suggestjust local vegetation changes such as the increase of reed at the lakeshore wetland. During MIS 2, more significant millennial-scalevegetation changes are recognized in the cold periods of Heinrichevents than in the warming periods of D–O events. The northern limitof the polar front in summer is considered to have been locatedwithinthe Seto Inland Sea, to the south of Lake Biwa, during MIS 2 (Toyodaand Naruse, 2002). Therefore, the regional climate at Lake Biwawouldhave been strongly influenced by the winter East Asian monsoonwithout the summer East Asian monsoon influence, and significantmillennial-scale climate change occurred during cold periods.

The beginning of MIS 1At the beginning of MIS 1, pinaceous conifer pollen starts to

decrease in abundance following the small ARM peak at approxi-mately 14 ka, while deciduous tree pollen, mainly composed ofQuercus subgenus Lepidobalanus, increases. Subsequently, Quercussubgenus Lepidobalanus pollen becomes dominant and pinaceousconifer pollen is virtually absent after the ARM peak at approximately12 ka. Hayashida et al. (2007) suggested that evidence of the YD wasnot observed in the ARM record between 15 and 10 ka, in contrast tothe Greenland ice core record. In this study of the BIW95-4 core, therewas also no evidence from the pollen record of significant vegetationchange related to abrupt cooling at the beginning of the YD.

Based on the age model of the BIW95-4 core, the development ofdeciduous broad-leaved forest mainly composed of deciduous oaksafter approximately 14 ka correlates with D–O 1 in the Greenland icecores. However, there is no indication of significant vegetation changein the BIW95-4 core record related to the abrupt cooling event of theYD. During the YD, deciduous oaks became dominant at approxi-mately 12 ka around Lake Biwa. In contrast, the Lake Suigetsu pollenrecord from the coast of the Sea of Japan, about 30 km north of LakeBiwa, did show a vegetation response to the YD (Nakagawa et al.,2003, 2005; Yasuda et al., 2004). During the interstadial periodcorrelated with D–O 1, the distribution of deciduous broad-leavedforest (mainly composed of deciduous oaks) rapidly expanded andpinaceous conifers decreased in abundance around Lake Suigetsu after15 ka (Nakagawa et al., 2003, 2005; Yasuda et al., 2004). Subsequent-ly, F. crenata increased in abundance around Lake Suigetsu during thestadial period of the YD (Nakagawa et al., 2003, 2005; Yasuda et al.,2004). Yasuda et al. (2004) suggested that the onset of cold wintersand increased snowfall caused the increase in abundance of F. crenatain the YD. Furthermore, Nakagawa et al. (2006), using quantitativeclimate reconstruction techniques from modern analogs, suggestedthat the abrupt climate change of the YD caused lower temperaturesand higher precipitation during the winter in Japan, whereasconsiderably less change occurred during the summer.

Differences in vegetation between the coast of the Sea of Japan andinland during the YDwere probably caused by contrasting amounts ofprecipitation during the winter. Whereas the strong winter East Asianmonsoon (influenced by the Sea of Japan) brought higher precipita-tion as snowfall during the YD along the coast, conditions were stillslightly dry around Lake Biwa because of the lower snowfall. Inaddition, fire events occurring around Lake Biwa at the beginning ofMIS 1 may also have influenced vegetation differences between theinland regions and the coast of the Sea of Japan. Future investigation of

charcoal records along the coast is necessary to understand theinfluence of fire on the vegetation around Lake Biwa during the YD.

Although F. crenata increased a little in abundance around LakeSuigetsu (Nakagawa et al., 2003, 2005; Yasuda et al., 2004), the impactof climate change during YD on vegetation around this region wasweak, and no pinaceous conifers, which were dominated during MIS2, increased again on either the coast of the Sea of Japan or inland area.Especially around Lake Biwa, deciduous broad-leaved forests mainlycomposed of deciduous oaks were undisturbed by the climate changeduring YD.

Conclusions

A 250 yr-resolution pollen record indicates millennial-scale veg-etation change throughout the last 40,000 yr around Lake Biwa inwestern Japan. Between 40 and 30 ka, fluctuations in abundance of C.japonica are correlated with D–O 8 to 5 recorded in the ARM variationof the BIW95-4 core (Hayashida et al., 2007) and the Greenland icecores (North Greenland Ice Core Project Members, 2004). Increases inabundance of C. japonicamay have been due to the summer East Asianmonsoon recorded in the speleothem record of Hulu Cave, China(Wang et al., 2001), or the increase of winter snowfall on the Sea ofJapan side of the Japanese archipelago. At approximately 23 ka, alongwith a decrease in the ARM values (Hayashida et al., 2007), Picea treesincreased in abundance within the pinaceous conifer forests. Theexpansion of Picea trees is correlated with HE 2. In addition, anincrease in the amount of upland herbs around Lake Biwa may becorrelated with HE 1. Whereas pinaceous conifer forest decreased indistribution, deciduous broad-leaved forest (mainly composed ofdeciduous oaks) began to increase in the Lake Biwa region after 14 ka.Vegetation change during this period correlates with D–O 1. However,evidence of significant vegetation change related to the abrupt YDcooling event is not found in the BIW95-4 core.

Although high-resolution pollen records indicating millennial-scale vegetation change during the last glacial period have been rarearound East Asia, Takahara et al. (in press) suggested regionalvegetation responses to D–O cycles during the last glacial from theEast Asian Islands of Taiwan, Japan and Sakhalin. The results of thisstudy has clarified detailed millennial-scale vegetation changesduring the last 40 ka and improved the understanding of relationshipswith millennial-scale monsoon changes in China and D–O cyclesaround the North Atlantic region. Additional high-resolution pollenrecords of well-dated sediments samples around the East Asian regionand comparisons with pollen records from the North Atlantic regionprovide good information for understanding the teleconnectionpattern of the millennial-scale climate variability between the NorthAtlantic and East Asian regions.

Acknowledgments

We thank K. Yamada for discussions about the TOC and aeolianquartz records of the BIW95-4 core. We also thank M.F. Sánchez Goñiand G. Jiménez-Moreno for their helpful comments on the submittedmanuscript. This study was supported by a Grant-in-Aid for JapanSociety for the Promotion of Science (JSPS) Fellows (20·10357; R.Hayashi) and Grants-in-Aid for Scientific Research from JSPS of Japan(16380109; H. Takahara).

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