Changes in Molluscan Assemblages and Sediment Type in the Outer Shelf of theJapan Sea Since 13,000 Years BP
Author(s): Akihisa Kitamura, Ken Ikehara, Hajime Katayama and Ayako Koshino
Source: Paleontological Research, 15(1):37-42. 2011.
Published By: The Palaeontological Society of Japan
DOI: http://dx.doi.org/10.2517/1342-8144-15.1.037
URL: http://www.bioone.org/doi/full/10.2517/1342-8144-15.1.037

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Paleontological Research, vol. 15, no. 1, pp. 37–42, April 30, 2011
c by the Palaeontological Society of Japan⃝
doi:10.2517/1342-8144-15.1.037

Changes in molluscan assemblages and sediment type in theouter shelf of the Japan Sea since 13,000 years BP
AKIHISA KITAMURA1 , KEN IKEHARA2 , HAJIME KATAYAMA2 AND AYAKO KOSHINO1
1 Institute of Geosciences, Shizuoka University, Shizuoka 422-8529, Japan (e-mail: seakita@ipc.shizuoka.ac.jp)
2 National Institute of Advanced Industrial Science and Technology, Institute for Marine Resources and Environment, GSJ, Tsukuba,

Ibaraki 305-8567, Japan
Received July 15, 2010; Revised manuscript accepted February 20, 2011
Abstract. No previous studies have investigated changes in the offshore molluscan assemblages of the JapanSea during the last deglacial period. We examined the sedimentary facies and the stratigraphic distribution ofmolluscs in cored sediments collected at 132 m water depth off Sado Island, Japan. The ages of 13 specimens ofmolluscan shells were determined by 14 C dating. Moreover, we analyzed the relative abundance of the planktonic
foraminifer Globigerinoides ruber, which is a proxy of the Tsushima Current. The results show that the coredsediments were deposited from 13,574 ± 121 cal yr BP at a water depth that increased over time from about 50
m to 132 m. Three sedimentary facies are recognized in the cored sediment, in ascending stratigraphic order;
coquina sediments, massive silt, and massive ﬁne sand yielding many molluscan shells. The change in substrate
was caused mainly by variability in the input of terrigenous deposits at 10,640–9,300 cal yr BP (the transition
from coquina to silt) and at 8,860–5,180 cal yr BP (the transition from silt to sand).14 C data show that themolluscan species Porterius dalli, Cyclocardia ferruginea, Tridonta alaskensis and Puncturella nobilis were livingin the Japan Sea before the initial inﬂow of the Tsushima Current at 9,300 cal yr BP. The modern molluscan
fauna in the study area, which is dominated by Limopsis belcheri and Crenulilimopsis oblonga, may have become
established by at least 5,180 cal yr BP.
Key words: Japan Sea, last deglacial period, offshore molluscs, planktonic foraminifera

Introduction
The Japan Sea is a semienclosed marginal sea with an areaof approximately 1,000,000 km2 and average depth of 1,350
m. The sea is connected to the East China Sea through the
Tsushima Strait, to the Paciﬁc Ocean via the Tsugaru Strait,
and to the Sea of Okhotsk through the Soya and Mamiya
straits. These straits are narrow and shallower than 130 m
deep. At present, the only oceanic water ﬂowing into theJapan Sea is the warm Tsushima Current, which enters via
the Tsushima Strait and makes its way northward along thewest coast of Honshu Island (Figure 1). The deep region
below the Tsushima Current is occupied by the cold-water
Japan Sea Proper Water (JSPW) (Suda, 1932; Moriyasu,
1972). The main thermocline between these two water
masses is found at about 150–160 m depth off the Hokuriku
and Niigata areas (e.g., Ogata, 1972; Nishimura, 1973, 1974).
Many workers have investigated temporal changes in thefossil assemblage of offshore microorganisms within theJapan Sea since the Last Glacial Maximum (LGM; 25,000–

20,000 yr BP) (e.g., Oba et al., 1991, 1995; Keigwin andGorbarenko, 1992; Gorbarenko and Southon, 2000; Takei
et al., 2002; Ikehara, 2003; Itaki et al., 2004; Domitsu andOda, 2006, 2008; Yokoyama et al., 2007). From the LGM
to 17,500 cal yr BP, the Japan Sea was nearly isolated fromthe surrounding seas because of the global sea-level drop,
and was covered with low-salinity water. From 17,500 to
11,500 cal yr BP, as sea levels rose, the cold Oyashio Current ﬂowed into the Japan Sea through the Tsugaru Strait in
the north (Oba et al., 1991). According to Domitsu and Oda(2006), Globigerinoides ruber has inhabited the Japan Sea
from 9,300 cal yr BP to the present day, indicating that thewarm Tsushima Current began to ﬂow into the sea at 9,300
cal yr BP. Modern surface conditions in the southern Japan
Sea were established at approximately 6,900 cal BP (Domitsu
and Oda, 2008). Such drastic environmental changes may
also have caused marked changes in the species compositionsof offshore molluscan fauna, although few studies have investigated changes in the fossil assemblage of offshore molluscs
within the Japan Sea since the LGM.

Akihisa Kitamura et al.

200

m

38

o

JT-25

Oy
ma
shi
Tsu

rre
Cu

m

Sea of Japan

100

ash

40°N

200

io

m

kaid

Hok

nt

Sado
Niigata

Tsushima
Strait

shio
Kuro

100m

50 km

Northwest Pacific

30°N
130 °E

140 °E

Figure 1. Regional map of the Japan Sea. Location of core JT-25.

Emery et al. (1971) and Chiji et al. (1981) reported thecold-water molluscs Mytilus coruscus (11,050 ± 600 yr BP;
Emery et al., 1971), Macoma calcarea (15,740 ± 400 yr BP;
Emery et al., 1971), Mercenaria stimpsoni (13,130 ± 280 yr
BP; Chiji et al., 1981) and Patinopecten yessoensis (16,340± 420 yr BP; Chiji et al., 1981) from the continental shelf
within the Japan Sea off southwestern Japan. Other thanM. coruscus, these species are not found in this area today,
but live off western Hokkaido (Okutani, 2000). On the other
hand, little is known of the faunal changes in offshore molluscs following the initiation of inﬂow of the Tsushima Current at 9,300 cal yr BP. To reconstruct temporal changes in
the offshore molluscan fauna and sedimentary environment
during the last deglacial period, we analyzed sedimentary facies and the stratigraphic distribution of molluscs and planktonic foraminifers in a sediment core recovered from the continental shelf of the Japan Sea.
Study area
A gravity core (JT-25) was collected in 132 m water
depth at a site located 7 km north of Sado Island, Japan(38◦ 23.33′ N, 138◦ 30.57′ E) during Cruise GH89-2 of the R/V
Hakurei-Maru of the Geological Survey of Japan in May–
July, 1989 (Figure 1). The continental shelf around Sado Island is very narrow (Figure 1). Ito (1989) reported that Modiolus margaritaceus, Crenulilimopsis oblonga, Parvamussium
intuscostuma, Limopsis belcheri, Acila divaricata and Striodentalium rhabdotum are the dominant molluscan species
around Sado Island at water depths of 100–300 m.
The core tube is 6 cm in diameter. The lowermost sediment in the core is semiconsolidated massive clay (Nakajima
et al., 1995). Given that the lowest sea level during the LGM(25,000–20,000 yr BP) is estimated to have been −130 to−140 m (Yokoyama et al., 2000, 2001), the study area atthis time was located between coastal low-lying land and theshoreface.
Methods
The sediment core (92 cm thick) was split and described
before picking and counting molluscs. Taxonomic identiﬁcations of mollusc shells are based on Okutani (2000). For
disarticulated shells, separated valves were counted (Table 1).
The radiocarbon ages of 13 mollusc shells were determinedby Beta Analytic Inc., using accelerator mass spectrometry(Table 2). Calibrated age ranges were calculated using an
online radiocarbon calibration program Calib 6.0 program(http://calib.qub.ac.uk/). 14 C ages are expressed in years before present (years BP), where the present is AD 1950.
Mud content and the relative abundance of the planktonic
foraminifer G. ruber were examined in 1-cm-thick samples
at 2.5-cm intervals from the middle and upper parts (40–10
cm and 10–0 cm depths, respectively) of the core (but notfrom the lower part because of time averaging; see below).
Each sample was washed through a 63-µm-mesh sieve anddried at 70◦ C. The dried >63-µm fractions were sieved again
through a 125-µm-mesh sieve, and the >125-µm fractions
were divided into aliquots for picking. For assemblage analysis, at least 200 specimens (>125 µm) were picked. Wethen identiﬁed and counted G. ruber from each sample. The
species is most suitable as an index of the Tsushima Current (Kitamura et al., 2001). The utility of this index has
been strongly supported by a study of the distribution ofplanktonic foraminiferal species from surface sediments of
the Japan Sea (Domitsu and Oda, 2005).
Results
The sediment core is divided into lower, middle, and upperparts. The lower part (40–92 cm depth) consists of upwardﬁning coquina sediments (Figure 2) that include many bio-Offshore molluscs in Japan Sea since 13 ka

39

Table 1. List of molluscan species in cored sediments. A: articulated shells, D: disarticulated shells. Data on the modern bathymetric range andsubstrate are from Higo et al. (1999) and Okutani (2000).
Bathymetricrange (m)Species

Lower

Part
HorizonSubstrate

Puncturella nobilis

tidal zone-85

rubbleHomalopoma granuliferum

50-300coarse sand and stones

Nuculana (Nuculana) pernula pernuloides

20-400fine sandy mud

Nuculana (Thestyleda) yokoyamai

50-450fine sand and mud

Porterius dalli

subtidal-300

rocks and boulders

Limopsis belcheri

100-800

sandy mud

Limopsis uwadokoi

10-400sandy mudCrenulilimopsis oblonga

20-200sand and fine sandy mud

Cyclocardia ferruginea

35-400sand

Tridonta alaskensis

50-200fine sand and sandy mud

Keenaea samarangae

50-300sandy mudAnisocorbula venusta

tidal zone-200

sandHiatella orientalis

tidal zone-300

root of seaweed or rocks

Myadora japonica

10-990fine sand and mud

1

2

3

Middle
4

5

6

Upper
7

8

2
1
A0/D2

A0/D1

A0/D1

A1/D0

A0/D1
A4/D6
A0/D2
A0/D2
A0/D1

A0/D1

A0/D2

A0/D1
A0/D2

A0/D1

A2/D25

A0/D1

A0/D1
A0/D1

Tridonta sp.

A0/D1

A0/D1

A0/D1

A0/D1

A0/D1

A0/D2
A0/D1

U: upper part
M: middle part
L: lower part0

HorizonCyclocardia ferruginea
Porterius dalli
Puncturella nobilis
Nuculana yokoyamai
Crenulilimopsis oblonga
Hiatella orientalis
Nuculana pernuloides
Limopsis uwadokoi
Limopsis belcheri
Astarte alaskensis
Anisocorbula venusta
Homalopoma granuliferum
Keenaea samarangae
Myadora japonica

selected molluscan species
sea level (m)
-100

20

cal years(kyr BP)

mudcontent (%)

density ofGlobigerinoides planktonic forams(numbers/g) 5
ruber (%)

8

U

M

10

7
6
5
4
3

50

0

2
1

semi-consolidated clay

50

100 0

5

10 0

1
2
X104

Age
(kyr BP)

L

clay

100
cm

0

0

1
2-4
5-10
11-20
0

5

10 15

Sea-level reconstruction
coral reef data (Tahiti)
coral reef data (Barbados)
coral reef data (Barbados)
coral reef data (New Guinea)
KL11 high-resolution resampling

sand

Figure 2. Columnar section of the cored sediment, showing stratigraphic changes in molluscan species, mud content, relative abundance of theplanktonic foraminifer Globigerinoides ruber, and the number density of planktonic foraminifers, along with depositional rates inferred from the 14 C agesof molluscs and a sea-level reconstruction (Siddall et al., 2003).

clastic fragments such as coralline algae, bryozoans, andmolluscs. Eleven molluscan species including Porterius dalli,
Cyclocardia ferruginea, Tridonta alaskensis (Figure 3) andPuncturella nobilis are identiﬁed from the lower part (Hori-zons 1 to 5); all bivalves are disarticulated and weakly
abraded, indicating that the shells may be transported after
death for any distance from their original habitats. However, the assemblage shows no evidence of the postmortem

40

Akihisa Kitamura et al.

Table 2. Results of 14 C dating of molluscs. All samples were analyzed by accelerator mass spectrometry (Beta-Analytic Corporation).
Laboratory
numberSpecies

Preservation

14

Measured
C age (BP)

13
C
(‰)

Conventional
14
C age (BP)Calibrated age
cal BP

Beta-134201 Limopsis belcheri

Articulated shell

4370 ± 703.44840 ± 70

5147 ± 106

Beta-134202 Limopsis belcheri

Articulated shell

4410 ± 70

3.54880 ± 70

5182 ± 104

Nuculana (Thestyleda)
yokoyamai

Articulated shell

7860 ± 40

1.18290 ± 408862 ± 82

Beta-134199 Cyclocardia ferruginea

Disarticulated shell

11550 ± 802.812010 ± 80

13399 ± 84

Beta-134198 Limopsis uwadokoi

Disarticulated shell

9310 ± 70

3.49780 ± 70

10641± 90

Beta-134197 Cyclocardia ferruginea

Disarticulated shell

11100 ± 100

2.711560 ± 100

13030 ± 124

Beta-133917 Tridonta alaskensis

Disarticulated shell

11120 ± 40

2.911580 ± 40

13101 ± 50

Beta-133916 Puncturella nobilis

Disarticulated shell

10270 ± 40

2.710730 ± 40

12128 ± 93

Beta-133915 Cyclocardia ferruginea

Disarticulated shell

11510 ± 40

2.911970 ± 40

13392 ± 58

Beta-126557 Cyclocardia ferruginea

Disarticulated shell

10110 ± 90

2.110550 ± 90

11789 ± 176

Beta-133914 Porterius dalli

Disarticulated shell

11400 ± 50

2.611860 ± 50

13323 ± 57

Beta-133913 Pectinoidea

Fracture

11390 ± 40

2.111840 ± 40

13307 ± 54

Disarticulated shell

11710 ± 100

0.812130 ± 100

13574 ± 121

Beta-134200

Beta-126558 Cyclocardia ferruginea

106 to 5,182 ± 104 cal yr BP (Table 2), possibly indicating
winnowing of the surface sediment due to penetration of thecore. Because most of the bivalves of the middle and upperparts are well preserved, these fossils are considered to be
indigenous.
Globigerinoides ruber occurs in all samples from the middle and upper parts (Figure 2). Planktonic foraminifera are
more numerous in the upper part than in the middle part (Figure 2).
Figure 3. A. Limopsis belcheri. B. Tridonta alaskensis. Scalebar, 1 cm.

transport of species from other habitats, such as the intertidal
zone. All the molluscs are found in the present-day surface
sediments of the study area (Ito, 1985, 1989). Consequently,
we consider that the molluscan fossils in the lower part of thecore are indigenous. The 14 C ages of 10 molluscan samples
from the lower part range from 13,574 ± 121 to 10,641 ±90 cal yr BP (Figure 2, Table 2). A reverse age relationshipfound in many samples reveals signiﬁcant time averaging of
the fossils due to either bioturbation or resedimentation.
The middle part (10–40 cm depth) consists of massive siltthat contains a disarticulated shell of C. oblonga and an articulated shell of Nuculana (Thestyleda) yokoyamai (Horizons 6
and 7) that yield a 14 C age of 8,862 ± 82 cal yr BP (Figure 2,
Table 2).
The upper part (0–10 cm depth) is characterized by ﬁnesand that contains many molluscan shells, including C. oblonga and L. belcheri (Horizon 8) (Figures 2, 3). The 14 C
ages of two specimens of well preserved and articulated shell
valves of L. belcheri fall within a narrow range from 5,147 ±Discussion and conclusion
Temporal changes in sedimentary environment
The 14 C ages of shell samples indicate that the cored sediments were deposited from 13,574 ± 121 cal yr BP. Basedon the sea-level curve proposed by Siddall et al. (2003) (Figure 2), water depth during deposition of the cored sediment
increased over time from about 50 m to 132 m (the present
depth). The upward-ﬁning trend in the coquina sediments
can be explained by a reduction in hydrodynamic energy associated with sea-level rise.
The change in sedimentary facies from coquina sediments(lower part of the core) to massive silt (middle part) indicatesan increase in the input of ﬁne terrigenous sediment. Thechange in sedimentary facies from the middle to upper partsis characterized by a decrease in the mud content and an increase in the relative abundance of molluscs and planktonic
foraminifera (Figure 2), suggesting a reduction in the supply of terrigenous muddy particles to the study area during
deposition of the upper part.
The youngest 14 C age obtained for shell samples in thelower part of the core is 10,641 ± 90 cal yr BP. The sediments of the middle part contain G. ruber, which has inhab-

Offshore molluscs in Japan Sea since 13 ka
ited the Japan Sea since 9,300 cal yr BP (Domitsu and Oda,
2006) (Figure 2). Consequently, the age of the boundary between the lower and middle parts is estimated to be between
10,641 ± 90 and 9,300 cal yr BP. The age of the boundary
between the middle and upper parts is between 8,862 ± 82
and 5,182 ± 104 cal yr BP, based on the 14 C ages of shellsamples.
Based on the sea-level curve proposed by Siddall et al.
(2003) (Figure 2) and the depositional ages of the three partsof the core, water depth during deposition of the lower, middle, and upper parts is estimated to have been 50 m to 90–100
m, 90–100 m to the present depth (132 m), and the present
depth, respectively. These ﬁnding indicate that the input ofﬁne terrigenous deposits increased from 10,640 to 9,300 cal
yr BP, when sea level rose rapidly, but decreased from 8,860
to 5,180 cal yr BP, when sea level was relatively stable or
slowly rising. It is well known that the input of terrigenous
deposits to offshore areas shows a decrease during times ofrelatively rapid sea-level rise. Thus, the changes in the supplyof terrigenous sediment to the study area cannot be explainedby sea-level change.
Nakajima and Itaki (2007) examined the temporal variability of the coarse fraction percentage, and the thickness
and frequency of turbidite beds for the past 70 ka along
the Toyama Deep-Sea Channel, located 50 km west of thepresent study area. The results show that turbidite deposition
increased during the period from 18 to 8 ka, reﬂecting an
increase in the input of terrigenous sediments from the Honshu mainland associated with enhanced precipitation resulting from intensiﬁcation of the summer monsoon. In addition,
the authors reported a period of intense turbidite depositionin the Toyama Deep-Sea Channel at ca. 10 ka. The authors
concluded that enhanced vegetation and restabilized mountain slopes resulted in reduced sediment input to the channel
system since 8 ka.
As noted above, in the present study area, the input of ﬁne
terrigenous deposits increased at 10,640 to 9,300 cal yr BP
and decreased at 8,860 to 5,180 cal yr BP. These trends
are consistent with those reported for the Toyama Deep-Sea
Channel system, indicating that changes in the supply of terrigenous deposits in the present study area were also caused
by varying sediment input associated with changing conditions in the sediment source areas. If this interpretation iscorrect, the temporal changes in sedimentation within the
study area would have occurred over the entire outer shelfof the Japan Sea off Central Japan.
Temporal changes in molluscan fauna
It is widely accepted that the main factors that limit the distribution of offshore molluscs are substrate, water depth andwater temperature. The three sedimentary facies recognizedin the present study contain markedly different species compositions (Figure 2, Table 1), indicating that the changes in41

substrate had a strong inﬂuence on temporal changes in themolluscan fauna. As noted above, the changes in substrate
were caused by the varying input of ﬁne terrigenous deposits
associated with changing conditions in the sediment sourceareas. These ﬁndings imply a possibility that the molluscan
faunal change recorded in the study area occurred over theentire outer shelf of the Japan Sea off Central Japan.
As noted above, water depth during deposition of thelower, middle, and upper parts of the core was 50 m to 90–
100 m, 90–100 m to the present depth (132 m), and thepresent depth, respectively. Based on the recent distributionof molluscs (Table 1), all of the species contained in the coresediments, except for L. belcheri, can live at the range of water depths encountered in the study area since 13,000 cal yr
BP (50–132 m depth). The bathymetric range of L. belcheri
is 100 to 800 m, meaning that even if the species had lived in
the Japan Sea before 10,640 cal yr BP, the water depth during
deposition of the lower part would have been too shallow forthis species.
The initiation of inﬂow of the Tsushima Current at 9,300
cal yr BP caused warming of the bottom water temperatureand resulted in the establishment of the modern molluscan
fauna. The 14 C ages of shell samples show that P. dalli, C.
ferruginea, T. alaskensis and P. nobilis lived in the Japan Sea
before 9,300 cal yr BP, prior to the inﬂow of the Tsushima
Current. Our results also show that the modern molluscan
fauna, which is dominated by L. belcheri and C. oblonga, wasestablished by at least 5,180 cal year BP, at least within the
study area. The water depth during deposition of the middle
part of the core is similar to that during deposition of the upper part. The stratigraphic distribution of G. ruber indicatesthat the Tsushima Current ﬂowed into the study area during
deposition of the middle and upper parts. Consequently, we
believe that there was no signiﬁcant difference in bottom water temperature between these periods. Based on the recent
distribution of molluscs, the substrate during deposition of
the middle part of the core was not unsuitable for all four ofthe bivalve species found in the upper part (Table 1). Therefore, it is possible that the development of the modern molluscan fauna was caused by a concentration of shells associated with a marked reduction in terrigenous sediment supply
rather than by colonization of the study area.
Acknowledgments
We thank A. Stallard for improving the English in
the manuscript. We also thank two anonymous reviewers, whose comments and suggestions improved the original manuscript. This study was funded by Grants-in-Aid
16340159, 111691106 and 11833018 from the Japan Society
for the Promotion of Science, and by the Saneyoshi Scholarship Foundation.

42

Akihisa Kitamura et al.
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