Directory UMM :Data Elmu:jurnal:O:Organic Geochemistry:Vol32.Issue1.2001:
Organic Geochemistry 32 (2001) 57±67
www.elsevier.nl/locate/orggeochem
Alkenone-sea surface temperatures in the Japan Sea
over the past 36 kyr: warm temperatures at the
last glacial maximum
R. Ishiwatari a,*, M. Houtatsu a, H. Okada b
a
Department of Chemistry, Graduate School of Science, Tokyo Metropolitan University, Hachioji, Tokyo 192-03, Japan
b
Department of Earth and Planetary Science, Graduate School of Science, Hokkaido University, Sapporo 060, Japan
Received 4 January 2000; accepted 11 October 2000
(returned to author for revision 27 April 2000)
Abstract
The Japan Sea experienced bottom water anoxia at the last glacial maximum (LGM) since it is surrounded by four
shallow straits, the sill depths of which are close to, or shallower than, the drop in sea level (120 m) that occurred
then. A distinctive negative d18O excursion of planktonic foraminifera also took place during the LGM. This excursion
has been interpreted from foraminiferal data as recording a drop in the paleosalinity of surface waters on the
assumption of a constant low sea surface temperatures between 34 and 11 ka. We present here a pro®le of alkenonebased sea surface temperatures (alkenone-SSTs) over the past 36 kyr. Our results suggest that SSTs during the LGM
were much higher than those previously assumed. After considering the factors that might aect estimation of alkenone-SSTs and comparisons of core-top alkenone-SSTs values with values for modern seawater we conclude that the
higher alkenone-SSTs during the LGM are reliable and reasonable. These warm SSTs were probably caused by
radiative equilibrium associated with the development of stable water strati®cation in the Japan Sea during the LGM.
# 2001 Elsevier Science Ltd. All rights reserved.
0
Keywords: Sea surface temperature; Alkenones; Uk37 index; Paleoceanography; Japan Sea; Marine sediments
1. Introduction
During the late Quaternary, the Japan Sea, which is a
semi-isolated marginal sea located between Japan and
the Asian continent and connected to the Paci®c Ocean
through shallow straits with sill depths of less than 130
m, experienced signi®cant changes in sea-level (e.g. Oba
et al., 1991, 1995; Ishiwatari et al., 1994, 1999; Tada et
al., 1999). During the last glacial interval, the Japan Sea
became almost isolated from the open ocean as sea-level
dropped by approximately 120 m at the last glacial
maximum (LGM).
* Corresponding author at present address: 3-16-11 Takaidonishi, Suginami, Tokyo 168-0071, Japan. Tel.: +81-3-58417707; fax: +81-3-3334-7035.
E-mail address: ryoshi@rc4.so- net.ne.jp (R. Ishiwatari).
Oba and his coworkers discovered a negative excursion in oxygen isotopic composition of planktonic foraminifera at site L-3 (Oki ridge) in the Japan Sea, where
the d18O became gradually lighter by 2.7% between 33
and 17.5 ka, the latter corresponding to the end of the
LGM [19±18 ka (calendar age) is used for the LGM
around the Japan Sea (Oba et al., 1995)], and then shifted
to heavier values by 3.3% until 1.2 ka (Oba et al., 1991;
1995). Oba et al. (1991) assumed that the negative d18O
excursion of planktonic foraminifera during the 33±17.5
ka period was caused only by the decrease in paleosalinity of surface seawater. They estimated that the paleosalinity dropped from 33 to 20 during this interval,
using a temperature-d18O relationship of foraminifera
(cited in Section 4.1.2.) and assuming that the paleo-SST
remained constant at 4.41 C, together with assumptions about the d18O values of seawater and freshwater for
0146-6380/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.
PII: S0146-6380(00)00151-0
58
R. Ishiwatari et al. / Organic Geochemistry 32 (2001) 57±67
mixing (Matsui et al., 1998). A paleosalinity of 20 is 14
less than the modern level.
However, their conclusion was based on an interpretation of oxygen isotope analysis of planktonic and benthic
foraminifera. No paleo-SSTs have been measured to date.
Nevertheless, the model of depleted paleosalinity and lowSSTs during the LGM seems accepted in the scienti®c
community (e.g. Keigwin and Gorbarenko, 1992; Crusius
et al., 1999), although CLIMAP estimates give higher
SSTs than 4 C (16±20 C for August and 4-6 C for February) around site L-3 at the LGM (CLIMAP Project
Members, 1981). Therefore, it was important to evaluate
the scenario of depleted paleosalinity in the Japan Sea
during the LGM by measuring paleo-SSTs. We felt that
0
the UK
37 method might provide a useful contribution for
estimating paleo-SSTs in the Japan Sea. In Ishiwatari et
al. (1999) we reported alkenone-based SSTs of up to
18 C in the Japan Sea at approximately 17.5 ka. The SST
of 18 C is much higher than SST previously assumed by
Oba et al. (1991) and Matsui et al. (1998). However, we
failed to measure alkenone-based SSTs in sediments older
than 17.5 ka, because the abundance of alkenones in the
sediment core (core L-3: described later) was too low to be
measured, and hence we could not discuss the changes
in paleo-SSTs during the LGM in the Japan Sea.
Therefore, as an extension of the previous study, this
study was initiated to: (i) check the reproducibility of
alkenone-SSTs in the Japan Sea by measuring alkenones
at another site, (ii) present data on alkenone-derived SSTs
of the Japan Sea over the last 36 kyr with the discussion of
the reliability of the alkenone-method in the light of previous studies, and (iii) present some new ideas on the
history of oceanographic conditions of the Japan Sea.
2. Materials and methods
2.1. Sediment samples
Two sediment cores Ð GH93, KI-5 (core length 302
cm; core bottom age 18 ka; ``KI-5'' is used hereafter)
which was taken at 39 34.070 N, 139 26.280 E (water
depth 754 m) in 1993, and KT94-15, PC-9 (core length
500 cm; core bottom age 38 ka; ``PC-9'' is used hereafter) which was taken at 39 34.360 N, 139 24.410 E (o
Akita city: water depth 807 m) in 1994 Ð were used for
this study. Analytical data from a sediment core KH-793, L-3 (core length 235 cm) taken from the top of the
Oki Ridge (37 03.50 N, 134 42.60 E; water depth 935 m;
``L-3'' is used hereafter) in 1979 (Oba et al., 1991) was
used for reference. The sampling locations of cores KI-5
and PC-9 and core L-3 are shown in Fig. 1.
The lithologies of the KI-5 and PC-9 sediment core have
been described by Okumura et al. (1996). Brie¯y, the upper
part of these cores consists of olive gray to greenish claysilt, indicating that the sediments were deposited under
Fig. 1. Sampling locations of cores GH93, KI-5, KT94-15, PC9 and core KH-79-3, L-3 in the Japan Sea.
oxic conditions. A thinly laminated layer (TL-1) is present
at 192±201 cm depth in core KI-5, and at 129±137 cm in
core PC-9. The top of a second thick laminated layer called
TL-2 is observed at 272 cm depth in KI-5, and at 215 cm in
PC-9. These two laminated layers (TL-1 and TL-2) correspond well to those commonly observed in cores taken
from the Japan Sea (Ikehara et al., 1994; Ishiwatari et al.,
1994,1999; Oba et al., 1991). In the case of core L-3, TL-1
is observed at 115 cm and the top of TL-2 at 160 cm
depth, respectively. These two laminated layers indicate
highly anoxic conditions in the bottom sediments.
The age model for core L-3 has been described by
Ishiwatari et al. (1999). Brie¯y, it was established by
using well-known volcanic ash layers (K-Ah, U-Oki,
and AT) which occur in the core. Their ages are 7300
calendar (cal) years B.P. for K-Ah (80±76 cm depth in
core L-3), 10,750 cal years B.P. for U-Oki (94±86 cm depth
in core L-3) and 24,500 14C years B.P. for AT. AMS-14C
ages of eight sections (planktonic foraminifera) were
determined by Oba et al. (1995). Calendar ages were calculated from radiocarbon ages using the data set
obtained for Lake Suigetsu (Kitagawa and van der
Plicht, 1998). The age model for cores KI-5 and PC-9
was established by correlating the volcanic ash layers as
well as the two laminated layers (TL-1: 11.3 ka and the
top of TL-2: 17.5 ka) with those in core L-3.
Subsamples were taken from these cores. Subsamples
were 50 mm thick (corresponding to 550 yr) with some
exceptions at volcanic layers for core L-3, 60 mm (500
yr) for core PC-9, and 20 mm for core KI-5 (125 yr),
respectively. Wet subsamples were used for lipid analysis for cores PC-9 and KI-5, while freeze-dried subsamples were used for core L-3, since the latter core has
been stored at room temperatures for a long time after
R. Ishiwatari et al. / Organic Geochemistry 32 (2001) 57±67
collection and was almost in an air-dried state. No
appreciable eect of sample storage at room temperatures on Uk37' values has been reported (Sikes et al.,
1991), nor is it seen in core L-3 (described later).
2.2. Total organic carbon and total nitrogen analysis
Subsamples taken from these cores were freeze-dried
and ground to powder for total organic carbon (TOC) and
total nitrogen (TN: total organic nitrogen plus ammonium
nitrogen) analysis. Duplicate determination used HCltreated sediment samples (100 mg) analysed by dry
combustion using a Fison NA1500-NCS elemental analyzer. The relative standard deviation of these measurements was better than 10% for both TOC and TN.
2.3. Alkenone and alkenoate determination
Free lipids were extracted by ultrasonication of wet
samples (ca. 4 g) in 50 ml Pyrex glass centrifuge tubes
containing benzene/methanol (6:4 v/v). The extracts
were collected after centrifugation (10 min, 3000 rpm),
and saponi®ed with 5% NaOH-methanol. Distilled
water was added to the solvent extracts after their transfer to separatory funnels and the neutrals were extracted
with n-hexane/diethyl ether (9:1 v/v). The neutrals were
fractionated on a silica gel column (silica gel: Mallinckrodt Inc. 100 mesh; column size: 505 mm i.d.).
Long-chain alkenones were obtained as the third (benzene) eluate after hexane (1st eluate containing saturated hydrocarbons) and hexane/benzene (2nd eluate
containing unsaturated and/or aromatic hydrocarbons)
elutions. The 1st and 2nd eluates were stored for later
study. The third eluates were evaporated to small
volumes and analyzed by gas chromatography (GC) or
gas chromatography/mass spectrometry (GC/MS).
GC analysis of alkenones used an HP 5890 instrument
®tted with a splitless injector and a DB-5 fused-silica
capillary column (60 m0.32 mm i.d.). The oven temperature was programmed from 50 to 150 C at 30 C
minÿ1 and then to 325 C at 3 C minÿ1. Quantitation was
achieved by integration of FID peak areas; 1-docosene
was used as a reference (injection) standard. GC/MS analysis used a Finnigan INCOS 50 MS/Varian 3400 GC, with
analytical conditions similar to those for GC analysis.
Alkenone-SSTs were calculated in this study using the
0
empirical equation given by Prahl et al. (1988): Uk37 =0.034
k0
T ( C)+0.039, where U37 =[C37:2]/[C37:2+C37:3].
Alkenoates were analyzed by a similar procedure used
for the alkenones except for elimination of the NaOH±
methanol saponi®cation.
2.4. Micropaleontological analysis
Coccolith assemblages were analyzed on smear
slides using a polarizing microscope with 1250 times
59
magni®cation. The abundance of coccoliths was measured only qualitatively, and ranked into four categories:
very rare, rare, few and common. For the samples containing rare to common coccoliths, 200 specimens were
identi®ed to species or genus level, and their numbers
were recorded. Occurrences of Florisphaera profunda and
reworked specimens were counted separately.
3. Results
3.1. TOC and alkenone/alkenoate analytical data
The vertical pro®les for TOC concentrations were
useful for correlating the sediment cores L-3, PC-9 and
KI-5 (Fig. 2). The TOC pro®les for cores PC-5 and KI-5
were very similar, even though the two cores were taken
in dierent years (1994 and 1993). Since the sampling
location for core PC-9 is close to that for core KI-5, this
result indicates the good reproducibility of TOC pro®les
in this region. TOC concentrations in core PC-9 slightly
decreased upward from 500 cm (37 ka) to 357 cm (28 ka),
became uniform until 214 cm (17.5 ka), and then
increased upward. At 128.5 cm (11.3 ka: TL-1 layer),
TOC concentrations exhibited a maximum and then
decreased again slightly followed by nearly constant
values. This vertical pro®le matches well that in core L-3.
In both cores L-3 and PC-9, a considerable amount of
C37 alkenones was detected in the Holocene section (C37
conc.: 1.70.3 mg gÿ1 for L-3, 1.20.7 mg gÿ1 for PC-9).
For the sections below the top of TL-2 (17.5 ka), however, the abundance of alkenones in core L-3 was extremely low (C37 conc.: 5) below 6 C, and moreover, the increase of %37:4
corresponds to the decrease in salinity (1.5) for the
Nordic sea core.
If SSTs were steady at 4 C and paleosalinity
decreased to 22 in the Japan Sea during the 33±17.5 ka
interval as claimed by Matsui et al. (1998), and if the
relationship between %37:4 and salinity found by
Rosell-Mele (1998) is valid for the Japan Sea, then a
high %37:4 content should be observed for the sediment
samples in the 33±17.5 ka interval. The temporal variation in %37:4 over the last 36 kyr is displayed in Fig. 6.
Evidently, no high %37:4 values for core PC-9 are
observed in the 33-17.5 ka interval, suggesting that the
SST cannot be so low. In the light of the ®ndings of
Rosell-Mele (1998), SSTs at site PC-9 must have been at
least higher than 6 C.
4.2.4. Other uncertainties
There are still several possible uncertainties disturbing
the estimation of accurate SSTs. It is known that longchain alkenones preserved in sediments are synthesized
essentially at the depths of highest primary production
(Bentaleb et al., 1999; Weaver et al., 1999). However,
there exists a possibility that the growth temperatures at
which the C37 alkenones were produced might have
been in¯uenced by changes in the glacial±interglacial
R. Ishiwatari et al. / Organic Geochemistry 32 (2001) 57±67
65
became anoxic, leading to the formation of laminated sediment layers of TL-2. Evidence for
anoxic deep waters was presented in Section 4.2.2.
The high mean annual SSTs (18±19 C) may have
occurred under heavily strati®ed surface seawater
where thermal energy due to solar radiation is
trapped in shallow waters. A similar phenomenon
of radiative equilibrium is observed in brackish
lakes (Yoshimura, 1976).
According to Oba et al. (1991), Thalassiosira lacustris,
a diatom species common in the less saline waters of
coastal areas, became abundant in the sedimentary sections during the 32±17.5 ka interval. This suggests that
paleosalinity was lower in this interval. Therefore, a
realistic scenario for environmental changes in the
Japan Sea is that both SST rose and paleosalinity fell
during this interval. Further studies are needed to
develop a method of extracting signals of paleosalinity
from those preserved in the Japan Sea sediments.
Fig. 6. Variations in %37:4 values in core PC-9 in the Japan
Sea. Shaded areas indicate laminated sediments.
climatic regime or by changing oceanographic conditions caused by severely strati®ed surface seawaters. As
discussed by Herbert et al. (1995), if the season of maximal production of haptophyte microalgae were to shift
with climatic regime, it would produce alkenone-SSTs
anomalies. Although it is dicult to rule out such
potential anomalies, we have no evidence to suggest that
they are important here.
4.3. Environmental changes in the Japan Sea
0
Notwithstanding the possibility that Uk37 values are
controlled by unknown factor(s), we conclude that the
temporal variations in mean annual SST over the past
36 kyr, as shown in Fig. 5, are reasonable. Thus we
conclude that the following environmental changes
occurred in the Japan Sea over the past 36 kyr.
. 36±17.5 ka: During the interval 36-17.5 ka, the
water column of the Japan Sea became strati®ed
as sea-level dropped. As vertical mixing and
upward transport of nutrients became restricted,
deep waters [deeper than 400 m from the sea
surface as suggested by Ikehara et al. (1994)]
. 17.5-8 ka: Between 17.5 and 12 ka, the SST
decreased and then increased between 12 and 9 ka
at sites PC-9 and L-3. The decrease in SSTs is
interpreted to have been caused by the onset of
vertical mixing of seawater by the in¯ow of seawater from the north (Oyashio Current: Oba et
al., 1991), or the south (Keigwin and Gorbarenko,
1992). The vertical mixing of seawater led to the
disappearance of anoxic deep water. This is shown
by the positive shift in d13C values of diploptene
from ÿ57 to ÿ25% (Yamada et al., 1997), the
disappearance of laminae.
The lowering of SST during 17±12 ka is qualitatively
supported by the diatom fossil analysis where the ratio of
warm-current species (Pseudoeunotia doliolus, Paralia sulcata, Thalassiosira oestrupii) to total species [warm-current region sp.+cold-current region sp. (Denticulopsis
seminae, Thalassiosira trifulta, Thalassiosira nordenskioldii)] dropped from about 0.8±0.9 at 17 ka to 0.4±0.5 at 12
ka for core C-3 taken at the site L-3 (Koizumi, 1984). This
result also demonstrates that the constant 4 C model of
Matsui et al. (1998) at the LGM is not realistic.
The SST rise in the 12±11 ka interval were probably
caused by the in¯ow of warm seawater from the south
through the Tsushima Strait (Oba et al., 1991).
. 8±0 ka. The present oceanographic regime became
established at about 8 ka, and conditions since
then have been similar to what is observed today.
5. Conclusions
A marked negative d18O excursion of planktonic foraminifera during the LGM observed in the Japan Sea
66
R. Ishiwatari et al. / Organic Geochemistry 32 (2001) 57±67
sediment cores has been interpreted as recording a drop
in paleosalinity of surface waters on the assumption of
constant low sea surface temperatures between 34 and
11 ka. This assumption has now been checked and our
results for alkenone-SSTs over the past 36 kyr at Sites
L-3, PC-9 and KI-5 indicate that SSTs during the LGM
were much higher than those previously assumed.
Comparisons of core-top alkenone-SSTs values with
values for modern seawater in the Japan Sea suggest
that SSTs estimated from alkenones are reliable. The
warm SSTs were probably the result of equilibrium of
thermal energy due to solar radiation in strati®ed surface seawaters under conditions where the Japan Sea
was largely isolated from the open ocean when sea level
was lowest at the LGM. Physical modeling of the warm
SSTs of the Japan Sea at the LGM, and magnitudes of
paleosalinity change remain to be studied.
Acknowledgements
We are grateful for Drs. D. Brincat and J. Volkman
who carefully read this manuscript for improvement.
We thank Drs. T.D. Herbert and E. Gonzalez for their
critical reviews of the manuscript and constructive
comments. We also thank Dr. T. Oba for discussions
during the revision of the manuscript. This work was
conducted as an international collaborative study on
paleoceanography of the Japan Sea (Chief scientist: T.
Oba) partly supported by Japan Society for the Promotion of Science.
Associate EditorÐJ.K. Volkman
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www.elsevier.nl/locate/orggeochem
Alkenone-sea surface temperatures in the Japan Sea
over the past 36 kyr: warm temperatures at the
last glacial maximum
R. Ishiwatari a,*, M. Houtatsu a, H. Okada b
a
Department of Chemistry, Graduate School of Science, Tokyo Metropolitan University, Hachioji, Tokyo 192-03, Japan
b
Department of Earth and Planetary Science, Graduate School of Science, Hokkaido University, Sapporo 060, Japan
Received 4 January 2000; accepted 11 October 2000
(returned to author for revision 27 April 2000)
Abstract
The Japan Sea experienced bottom water anoxia at the last glacial maximum (LGM) since it is surrounded by four
shallow straits, the sill depths of which are close to, or shallower than, the drop in sea level (120 m) that occurred
then. A distinctive negative d18O excursion of planktonic foraminifera also took place during the LGM. This excursion
has been interpreted from foraminiferal data as recording a drop in the paleosalinity of surface waters on the
assumption of a constant low sea surface temperatures between 34 and 11 ka. We present here a pro®le of alkenonebased sea surface temperatures (alkenone-SSTs) over the past 36 kyr. Our results suggest that SSTs during the LGM
were much higher than those previously assumed. After considering the factors that might aect estimation of alkenone-SSTs and comparisons of core-top alkenone-SSTs values with values for modern seawater we conclude that the
higher alkenone-SSTs during the LGM are reliable and reasonable. These warm SSTs were probably caused by
radiative equilibrium associated with the development of stable water strati®cation in the Japan Sea during the LGM.
# 2001 Elsevier Science Ltd. All rights reserved.
0
Keywords: Sea surface temperature; Alkenones; Uk37 index; Paleoceanography; Japan Sea; Marine sediments
1. Introduction
During the late Quaternary, the Japan Sea, which is a
semi-isolated marginal sea located between Japan and
the Asian continent and connected to the Paci®c Ocean
through shallow straits with sill depths of less than 130
m, experienced signi®cant changes in sea-level (e.g. Oba
et al., 1991, 1995; Ishiwatari et al., 1994, 1999; Tada et
al., 1999). During the last glacial interval, the Japan Sea
became almost isolated from the open ocean as sea-level
dropped by approximately 120 m at the last glacial
maximum (LGM).
* Corresponding author at present address: 3-16-11 Takaidonishi, Suginami, Tokyo 168-0071, Japan. Tel.: +81-3-58417707; fax: +81-3-3334-7035.
E-mail address: ryoshi@rc4.so- net.ne.jp (R. Ishiwatari).
Oba and his coworkers discovered a negative excursion in oxygen isotopic composition of planktonic foraminifera at site L-3 (Oki ridge) in the Japan Sea, where
the d18O became gradually lighter by 2.7% between 33
and 17.5 ka, the latter corresponding to the end of the
LGM [19±18 ka (calendar age) is used for the LGM
around the Japan Sea (Oba et al., 1995)], and then shifted
to heavier values by 3.3% until 1.2 ka (Oba et al., 1991;
1995). Oba et al. (1991) assumed that the negative d18O
excursion of planktonic foraminifera during the 33±17.5
ka period was caused only by the decrease in paleosalinity of surface seawater. They estimated that the paleosalinity dropped from 33 to 20 during this interval,
using a temperature-d18O relationship of foraminifera
(cited in Section 4.1.2.) and assuming that the paleo-SST
remained constant at 4.41 C, together with assumptions about the d18O values of seawater and freshwater for
0146-6380/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.
PII: S0146-6380(00)00151-0
58
R. Ishiwatari et al. / Organic Geochemistry 32 (2001) 57±67
mixing (Matsui et al., 1998). A paleosalinity of 20 is 14
less than the modern level.
However, their conclusion was based on an interpretation of oxygen isotope analysis of planktonic and benthic
foraminifera. No paleo-SSTs have been measured to date.
Nevertheless, the model of depleted paleosalinity and lowSSTs during the LGM seems accepted in the scienti®c
community (e.g. Keigwin and Gorbarenko, 1992; Crusius
et al., 1999), although CLIMAP estimates give higher
SSTs than 4 C (16±20 C for August and 4-6 C for February) around site L-3 at the LGM (CLIMAP Project
Members, 1981). Therefore, it was important to evaluate
the scenario of depleted paleosalinity in the Japan Sea
during the LGM by measuring paleo-SSTs. We felt that
0
the UK
37 method might provide a useful contribution for
estimating paleo-SSTs in the Japan Sea. In Ishiwatari et
al. (1999) we reported alkenone-based SSTs of up to
18 C in the Japan Sea at approximately 17.5 ka. The SST
of 18 C is much higher than SST previously assumed by
Oba et al. (1991) and Matsui et al. (1998). However, we
failed to measure alkenone-based SSTs in sediments older
than 17.5 ka, because the abundance of alkenones in the
sediment core (core L-3: described later) was too low to be
measured, and hence we could not discuss the changes
in paleo-SSTs during the LGM in the Japan Sea.
Therefore, as an extension of the previous study, this
study was initiated to: (i) check the reproducibility of
alkenone-SSTs in the Japan Sea by measuring alkenones
at another site, (ii) present data on alkenone-derived SSTs
of the Japan Sea over the last 36 kyr with the discussion of
the reliability of the alkenone-method in the light of previous studies, and (iii) present some new ideas on the
history of oceanographic conditions of the Japan Sea.
2. Materials and methods
2.1. Sediment samples
Two sediment cores Ð GH93, KI-5 (core length 302
cm; core bottom age 18 ka; ``KI-5'' is used hereafter)
which was taken at 39 34.070 N, 139 26.280 E (water
depth 754 m) in 1993, and KT94-15, PC-9 (core length
500 cm; core bottom age 38 ka; ``PC-9'' is used hereafter) which was taken at 39 34.360 N, 139 24.410 E (o
Akita city: water depth 807 m) in 1994 Ð were used for
this study. Analytical data from a sediment core KH-793, L-3 (core length 235 cm) taken from the top of the
Oki Ridge (37 03.50 N, 134 42.60 E; water depth 935 m;
``L-3'' is used hereafter) in 1979 (Oba et al., 1991) was
used for reference. The sampling locations of cores KI-5
and PC-9 and core L-3 are shown in Fig. 1.
The lithologies of the KI-5 and PC-9 sediment core have
been described by Okumura et al. (1996). Brie¯y, the upper
part of these cores consists of olive gray to greenish claysilt, indicating that the sediments were deposited under
Fig. 1. Sampling locations of cores GH93, KI-5, KT94-15, PC9 and core KH-79-3, L-3 in the Japan Sea.
oxic conditions. A thinly laminated layer (TL-1) is present
at 192±201 cm depth in core KI-5, and at 129±137 cm in
core PC-9. The top of a second thick laminated layer called
TL-2 is observed at 272 cm depth in KI-5, and at 215 cm in
PC-9. These two laminated layers (TL-1 and TL-2) correspond well to those commonly observed in cores taken
from the Japan Sea (Ikehara et al., 1994; Ishiwatari et al.,
1994,1999; Oba et al., 1991). In the case of core L-3, TL-1
is observed at 115 cm and the top of TL-2 at 160 cm
depth, respectively. These two laminated layers indicate
highly anoxic conditions in the bottom sediments.
The age model for core L-3 has been described by
Ishiwatari et al. (1999). Brie¯y, it was established by
using well-known volcanic ash layers (K-Ah, U-Oki,
and AT) which occur in the core. Their ages are 7300
calendar (cal) years B.P. for K-Ah (80±76 cm depth in
core L-3), 10,750 cal years B.P. for U-Oki (94±86 cm depth
in core L-3) and 24,500 14C years B.P. for AT. AMS-14C
ages of eight sections (planktonic foraminifera) were
determined by Oba et al. (1995). Calendar ages were calculated from radiocarbon ages using the data set
obtained for Lake Suigetsu (Kitagawa and van der
Plicht, 1998). The age model for cores KI-5 and PC-9
was established by correlating the volcanic ash layers as
well as the two laminated layers (TL-1: 11.3 ka and the
top of TL-2: 17.5 ka) with those in core L-3.
Subsamples were taken from these cores. Subsamples
were 50 mm thick (corresponding to 550 yr) with some
exceptions at volcanic layers for core L-3, 60 mm (500
yr) for core PC-9, and 20 mm for core KI-5 (125 yr),
respectively. Wet subsamples were used for lipid analysis for cores PC-9 and KI-5, while freeze-dried subsamples were used for core L-3, since the latter core has
been stored at room temperatures for a long time after
R. Ishiwatari et al. / Organic Geochemistry 32 (2001) 57±67
collection and was almost in an air-dried state. No
appreciable eect of sample storage at room temperatures on Uk37' values has been reported (Sikes et al.,
1991), nor is it seen in core L-3 (described later).
2.2. Total organic carbon and total nitrogen analysis
Subsamples taken from these cores were freeze-dried
and ground to powder for total organic carbon (TOC) and
total nitrogen (TN: total organic nitrogen plus ammonium
nitrogen) analysis. Duplicate determination used HCltreated sediment samples (100 mg) analysed by dry
combustion using a Fison NA1500-NCS elemental analyzer. The relative standard deviation of these measurements was better than 10% for both TOC and TN.
2.3. Alkenone and alkenoate determination
Free lipids were extracted by ultrasonication of wet
samples (ca. 4 g) in 50 ml Pyrex glass centrifuge tubes
containing benzene/methanol (6:4 v/v). The extracts
were collected after centrifugation (10 min, 3000 rpm),
and saponi®ed with 5% NaOH-methanol. Distilled
water was added to the solvent extracts after their transfer to separatory funnels and the neutrals were extracted
with n-hexane/diethyl ether (9:1 v/v). The neutrals were
fractionated on a silica gel column (silica gel: Mallinckrodt Inc. 100 mesh; column size: 505 mm i.d.).
Long-chain alkenones were obtained as the third (benzene) eluate after hexane (1st eluate containing saturated hydrocarbons) and hexane/benzene (2nd eluate
containing unsaturated and/or aromatic hydrocarbons)
elutions. The 1st and 2nd eluates were stored for later
study. The third eluates were evaporated to small
volumes and analyzed by gas chromatography (GC) or
gas chromatography/mass spectrometry (GC/MS).
GC analysis of alkenones used an HP 5890 instrument
®tted with a splitless injector and a DB-5 fused-silica
capillary column (60 m0.32 mm i.d.). The oven temperature was programmed from 50 to 150 C at 30 C
minÿ1 and then to 325 C at 3 C minÿ1. Quantitation was
achieved by integration of FID peak areas; 1-docosene
was used as a reference (injection) standard. GC/MS analysis used a Finnigan INCOS 50 MS/Varian 3400 GC, with
analytical conditions similar to those for GC analysis.
Alkenone-SSTs were calculated in this study using the
0
empirical equation given by Prahl et al. (1988): Uk37 =0.034
k0
T ( C)+0.039, where U37 =[C37:2]/[C37:2+C37:3].
Alkenoates were analyzed by a similar procedure used
for the alkenones except for elimination of the NaOH±
methanol saponi®cation.
2.4. Micropaleontological analysis
Coccolith assemblages were analyzed on smear
slides using a polarizing microscope with 1250 times
59
magni®cation. The abundance of coccoliths was measured only qualitatively, and ranked into four categories:
very rare, rare, few and common. For the samples containing rare to common coccoliths, 200 specimens were
identi®ed to species or genus level, and their numbers
were recorded. Occurrences of Florisphaera profunda and
reworked specimens were counted separately.
3. Results
3.1. TOC and alkenone/alkenoate analytical data
The vertical pro®les for TOC concentrations were
useful for correlating the sediment cores L-3, PC-9 and
KI-5 (Fig. 2). The TOC pro®les for cores PC-5 and KI-5
were very similar, even though the two cores were taken
in dierent years (1994 and 1993). Since the sampling
location for core PC-9 is close to that for core KI-5, this
result indicates the good reproducibility of TOC pro®les
in this region. TOC concentrations in core PC-9 slightly
decreased upward from 500 cm (37 ka) to 357 cm (28 ka),
became uniform until 214 cm (17.5 ka), and then
increased upward. At 128.5 cm (11.3 ka: TL-1 layer),
TOC concentrations exhibited a maximum and then
decreased again slightly followed by nearly constant
values. This vertical pro®le matches well that in core L-3.
In both cores L-3 and PC-9, a considerable amount of
C37 alkenones was detected in the Holocene section (C37
conc.: 1.70.3 mg gÿ1 for L-3, 1.20.7 mg gÿ1 for PC-9).
For the sections below the top of TL-2 (17.5 ka), however, the abundance of alkenones in core L-3 was extremely low (C37 conc.: 5) below 6 C, and moreover, the increase of %37:4
corresponds to the decrease in salinity (1.5) for the
Nordic sea core.
If SSTs were steady at 4 C and paleosalinity
decreased to 22 in the Japan Sea during the 33±17.5 ka
interval as claimed by Matsui et al. (1998), and if the
relationship between %37:4 and salinity found by
Rosell-Mele (1998) is valid for the Japan Sea, then a
high %37:4 content should be observed for the sediment
samples in the 33±17.5 ka interval. The temporal variation in %37:4 over the last 36 kyr is displayed in Fig. 6.
Evidently, no high %37:4 values for core PC-9 are
observed in the 33-17.5 ka interval, suggesting that the
SST cannot be so low. In the light of the ®ndings of
Rosell-Mele (1998), SSTs at site PC-9 must have been at
least higher than 6 C.
4.2.4. Other uncertainties
There are still several possible uncertainties disturbing
the estimation of accurate SSTs. It is known that longchain alkenones preserved in sediments are synthesized
essentially at the depths of highest primary production
(Bentaleb et al., 1999; Weaver et al., 1999). However,
there exists a possibility that the growth temperatures at
which the C37 alkenones were produced might have
been in¯uenced by changes in the glacial±interglacial
R. Ishiwatari et al. / Organic Geochemistry 32 (2001) 57±67
65
became anoxic, leading to the formation of laminated sediment layers of TL-2. Evidence for
anoxic deep waters was presented in Section 4.2.2.
The high mean annual SSTs (18±19 C) may have
occurred under heavily strati®ed surface seawater
where thermal energy due to solar radiation is
trapped in shallow waters. A similar phenomenon
of radiative equilibrium is observed in brackish
lakes (Yoshimura, 1976).
According to Oba et al. (1991), Thalassiosira lacustris,
a diatom species common in the less saline waters of
coastal areas, became abundant in the sedimentary sections during the 32±17.5 ka interval. This suggests that
paleosalinity was lower in this interval. Therefore, a
realistic scenario for environmental changes in the
Japan Sea is that both SST rose and paleosalinity fell
during this interval. Further studies are needed to
develop a method of extracting signals of paleosalinity
from those preserved in the Japan Sea sediments.
Fig. 6. Variations in %37:4 values in core PC-9 in the Japan
Sea. Shaded areas indicate laminated sediments.
climatic regime or by changing oceanographic conditions caused by severely strati®ed surface seawaters. As
discussed by Herbert et al. (1995), if the season of maximal production of haptophyte microalgae were to shift
with climatic regime, it would produce alkenone-SSTs
anomalies. Although it is dicult to rule out such
potential anomalies, we have no evidence to suggest that
they are important here.
4.3. Environmental changes in the Japan Sea
0
Notwithstanding the possibility that Uk37 values are
controlled by unknown factor(s), we conclude that the
temporal variations in mean annual SST over the past
36 kyr, as shown in Fig. 5, are reasonable. Thus we
conclude that the following environmental changes
occurred in the Japan Sea over the past 36 kyr.
. 36±17.5 ka: During the interval 36-17.5 ka, the
water column of the Japan Sea became strati®ed
as sea-level dropped. As vertical mixing and
upward transport of nutrients became restricted,
deep waters [deeper than 400 m from the sea
surface as suggested by Ikehara et al. (1994)]
. 17.5-8 ka: Between 17.5 and 12 ka, the SST
decreased and then increased between 12 and 9 ka
at sites PC-9 and L-3. The decrease in SSTs is
interpreted to have been caused by the onset of
vertical mixing of seawater by the in¯ow of seawater from the north (Oyashio Current: Oba et
al., 1991), or the south (Keigwin and Gorbarenko,
1992). The vertical mixing of seawater led to the
disappearance of anoxic deep water. This is shown
by the positive shift in d13C values of diploptene
from ÿ57 to ÿ25% (Yamada et al., 1997), the
disappearance of laminae.
The lowering of SST during 17±12 ka is qualitatively
supported by the diatom fossil analysis where the ratio of
warm-current species (Pseudoeunotia doliolus, Paralia sulcata, Thalassiosira oestrupii) to total species [warm-current region sp.+cold-current region sp. (Denticulopsis
seminae, Thalassiosira trifulta, Thalassiosira nordenskioldii)] dropped from about 0.8±0.9 at 17 ka to 0.4±0.5 at 12
ka for core C-3 taken at the site L-3 (Koizumi, 1984). This
result also demonstrates that the constant 4 C model of
Matsui et al. (1998) at the LGM is not realistic.
The SST rise in the 12±11 ka interval were probably
caused by the in¯ow of warm seawater from the south
through the Tsushima Strait (Oba et al., 1991).
. 8±0 ka. The present oceanographic regime became
established at about 8 ka, and conditions since
then have been similar to what is observed today.
5. Conclusions
A marked negative d18O excursion of planktonic foraminifera during the LGM observed in the Japan Sea
66
R. Ishiwatari et al. / Organic Geochemistry 32 (2001) 57±67
sediment cores has been interpreted as recording a drop
in paleosalinity of surface waters on the assumption of
constant low sea surface temperatures between 34 and
11 ka. This assumption has now been checked and our
results for alkenone-SSTs over the past 36 kyr at Sites
L-3, PC-9 and KI-5 indicate that SSTs during the LGM
were much higher than those previously assumed.
Comparisons of core-top alkenone-SSTs values with
values for modern seawater in the Japan Sea suggest
that SSTs estimated from alkenones are reliable. The
warm SSTs were probably the result of equilibrium of
thermal energy due to solar radiation in strati®ed surface seawaters under conditions where the Japan Sea
was largely isolated from the open ocean when sea level
was lowest at the LGM. Physical modeling of the warm
SSTs of the Japan Sea at the LGM, and magnitudes of
paleosalinity change remain to be studied.
Acknowledgements
We are grateful for Drs. D. Brincat and J. Volkman
who carefully read this manuscript for improvement.
We thank Drs. T.D. Herbert and E. Gonzalez for their
critical reviews of the manuscript and constructive
comments. We also thank Dr. T. Oba for discussions
during the revision of the manuscript. This work was
conducted as an international collaborative study on
paleoceanography of the Japan Sea (Chief scientist: T.
Oba) partly supported by Japan Society for the Promotion of Science.
Associate EditorÐJ.K. Volkman
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