-
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:
[email protected] 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
0
1
2
1
23
4
Ma
Omma F.Okuwasection
Omma F.Yuhiderasection
Omma F.Oyabesection
Setana F.Soebetsusection
56
78
1. Matuyama/Brunhes boundary 0.790+0.005 Ma2. LAD
Reticulofenestra asanoi 0.889+0.025 Ma3. FAD Gephyrocapsa parallela
1.045+0.025 Ma4. LAD Gephyrocapsa (large) 1.243+0.03 Ma5. LAD
Helicosphaera sellii 1.451+0.025 Ma6. FAD Gephyrocapsa (large)
1.515+0.025 Ma7. FAD Gephyrocapsa oceanica 1.664+0.025 Ma8. Top of
the Olduvai Subchron 1.783+0.001 MaFAD: first appearance datumLAD:
last appearance datum
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
-
UtatsuyamaFormation
11
10
U5
U1
U3
U2
sand
9
8
76
543
2
1
L-1
L-2
L-3
0
10 m
1010
U4
SaikawaFormation
O1
O3
O2
Meg
angu
lus
zyo
noen
sis
Mer
cena
ria s
timps
oni
Aci
la i
nsig
nis
Yol
dia
nota
bilis
Jupi
teria
con
fusa
Oliv
a m
uste
lina
molluscan fossil
molluscan fossil
Meg
angu
lus
zyon
oens
isFe
lani
ella
ust
aA
cila
insi
gnis
Clin
ocar
dium
fast
osum
Turr
itella
sai
shue
nsis
Yol
dia
nota
bilis
Cyc
ladi
cam
a cu
min
giS
tella
ria e
xutu
mP
aphi
a sc
hnel
liana
Jupi
teria
gor
doni
s
low
er p
art
mid
dle
part
uppe
r par
t
1
2
3
4
5
6
7
Omma FormationOkuwa section
0 40(%)
rela
tive
abun
danc
e of
Glo
bige
rinoi
des
rube
r
Site 60718O ( )
25
29
33
35
41
45
47
49
53
55
57
24
26
28
30
34
36
38
40
42
44
48
50
52
54
23
27
32
37
39
43
46
51
56
21.3
21.5
31
22
200.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.7
AgeMa
5.0 4.0 3.0
(I)
0 20
rela
tive
abun
danc
e of
Glo
boro
talia
infla
ta
(%)
Datum planes1: younger than LAD Reticulofenestra asanoi 0.899
0.025Ma2: FAD Gephyrocapsa parallela 1.045 0.025Ma3: Top of
Jaramillo 0.992 0.01Ma4: Base of Jaramillo 1.073 0.01Ma5: LAD
Gephyrocapsa (large) 1.243 0.03Ma6: LAD Helicosphaera sellii 1.451
0.025Ma7: FAD Gephyrocapsa (large) 1.515 0.025MaMagnetic polarity
B: Brunhes; J: Jaramillo; O: Olduvai(I) Takayama et al. (1988);
(II)Total number of specimens counted in each sample (unit
less)
conglomeratesandstonemudstoneparallel laminationcross
laminationhummocky cross strat.
shell bedshell fossilstuffcold-water speciescold-water and
extinct specieswarm-water species
00
ODP Site 798
1.8
0.7
rela
tive
abun
danc
e of
Glo
bige
rinoi
des
rube
r
abun
danc
e of
Par
alia
sul
cata
0
rela
tive
abun
danc
e of
Glo
boro
talia
infla
ta
Mag
netic
pol
arity
Dep
th (m
eter
bel
ow s
edim
ent s
urfa
ce)
(%)(%)0 400
ODPSite 797
Dep
th (m
eter
s be
low
sed
imen
t sur
face
)
(II)
J
O
B
Stage
IV
III
II
I
Figure 3 Comparison of fossil records from the Omma Formation at
its type section (Takayama et al., 1988; Kitamura et al., 2001;
Kitamura andKimoto, 2006) with ODP Sites 797 (Koizumi and Ikeda,
1997) and 798 (Kheradyar, 1992). Biostratigraphic datum horizons in
the Omma Formation areafter Takayama et al. (1988) and Sato and
Takayama (1992); magnetostratigraphic data are from Kitamura et al.
(1994); the timescale for the oxygenisotope record at DSDP Site 607
(Ruddiman et al., 1989) and ages of biostratigraphic datum horizons
and magnetic polarity changes are based on thechronology of Berger
et al. (1994). L-1 to L-3, 1-11 and U1-U5: depositional sequence
numbers (Kitamura et al., 1994, 2001; Kitamura and Kawagoe,2006).
FAD, first appearance datum; LAD, last appearance datum
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.
Okuwa sG. ruber
11
109
8
7
6543
2
1
L-1
L-3
0 1 01
sand
0 25(%)A
0
10 m
sand
1
4
23
5
Oyabe section(Takata, 2000)
G. ruber G. inflata
0 100 20(%)
water depth
(%)0 100 200
(m)
L-2
0
10 m
B
A: FAD Gephyrocapsa oceanica1.664 0.025Ma
B: FAD Gephyrocapsa (large)1.515 0.025Ma
Figure 4 Stratigraphic distribution of the planktonic
foraminifer species Globin the Oyabe, Okuwa and Yuhidera areas,
showing reconstructed water de
LegendMolluscs
silty sandstone
fine sandstone
884 JOURNAL OF QUATERNARY SCIENCEX
X
pebbly sandstone
siltstonesampling horizons which are not detected of Gr. inflata
and Gs. ruber
XX
X
X
X
XX
Gr. inflata
Gr. inflataGs. ruber
Gr. inflata
Gr. inflataGs. ruber
Gr. inflata
50cmGs. ruber
Gs. ruber
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
Yuhidera sectionG. ruber G. inflata
0 20(%) 0 20(%)
sand
water depth
(m)
20-3
050
-60
100-
120
200
4
23
0
10 m0
G. inflatawater depth
(m)
0
20-3
050
-60
100-
1200 20(%)
ection
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
J. Quaternary Sci., Vol. 24(8) 880889 (2009)DOI: 10.1002/jqs
-
30)w
ater
dep
th (m
)
15
EARLY PLEISTOCENE EVOLUTION OF THE JAPAN SEA INTERMEDIATE WATER
88501020305075
100125150200250300
01020305075
wat
er d
epth
(m)
th (m
)
15
15
5
2520
10
Pacific Ocean (August)
Pacific Ocean (March)
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).
100125150200250300
wat
er d
ep
5
10
A BBoso
01020305075
100125150200250300
5
10
5
FE
Japan Sea (March)
Japan Sea (August)
01020305075
100125150200250300
wat
er d
epth
(m)
10
20
25
15
FE
Figure 6 Distribution of temperature in the modern Pacific Ocean
and JOceanographic Data Center (2007)
Copyright 2009 John Wiley & Sons, Ltd.5075pt
h (m
1001020305075
100125150200250300
01020
15
20
25
10
5
Japan Sea (March)
Japan Sea (August)C DDiscussion
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;
100125150200250300
wat
er d
e5
C D
Tsus
him
ast
rait
Noto
A
B
C
D
798E F
Boso
Noto
Tsush
ima
strait
500 km
no data
Shimokita
apan Sea off the Japanese Islands. Modern data are from the
Japan
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
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variations inplanktonic foraminifera at three sediment traps in the
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