Paleoceanographic Changes of Sea of Japan During 3.5-0.8 Ma
Akihisa Kitamura
Shizuoka University, Shizuoka 422-8529, Japan

ABSTRACTOn the basis of fossil record from marine deposits in and around the Sea of Japan, the environment during 3.5–0.8 Ma is divided into two stages. During the old stage (3.5–1.71 Ma (stage
60/59)), cold surface water prevailed, except for the short term inflow (< 41 ky duration) of warm
Tsushima Current at 3.5, 3.2, 2.9, 2.4 and 1.9 Ma. During the young stage (1.71–0.8 Ma), the Tsushima Current flowed into the sea at every interglacial highstand. This change was caused by
opening of the southern channel due to crustal stretching in the northern Okinawa Trough. The
onset of inflow of the Tsushima Current caused intrusion of warm- water organisms into the Sea
of Japan during interglacial stages and barrier to migration of land organisms between East Asia
and Japan. In summary, the environments and organisms in and around the Sea of Japan since
3.5 Ma have been strongly controlled by the development of southern channel (1.71 and 1.35 Ma)
and global climate changes (glacial/interglacial cycles).
Keywords: Sea of Japan, Warm Tsushima current, Fossil record, Shallow-marine deposits, Pliocene, Pleistocene

INTRODUCTION
The warm Tsushima Current supplies a large
quantity of heat to the Sea of Japan as well as IndoPacific marine organisms. The current also acts toisolate the terrestrial organisms of Japan from thoseof Asia. Therefore, it probably had a strong influence on the Pliocene-Pleistocene paleoclimate, paleoenvironment, and ecosystem within and aroundthe Sea of Japan. In this paper, I review paleoceanographic changes within the Sea of Japan during theperiod from 3.5 to 0.8 Ma. First, I will introduce“the recent oceanographic condition of the Sea ofJapan”. Second, I discuss a “proxy for the warm
Tsushima Current”; third, “cyclic changes in shallow marine environments during the early Pleistocene”; and, fourth, “paleoceanographic changes
within the Sea of Japan during the period from 3.5
to 0.8 Ma.
e-mail: seakita@ipc.shizuoka.ac.jp

OCEANOGRAPHIC CONDITION
OF THE SEA OF JAPANThe Sea of Japan is a semi-enclosed marginal sea
and is connected to the East China Sea to the south
via the Tsushima Strait, to the Pacific Ocean throughthe Tsugaru Strait, and to the Sea of Okhotsk to thenorth via the Soya and Mamiya straits. All of thesestraits have water depths of less than 140 m, similar
to the sea level at the last glacial maximum at
20,000 years ago, when the Sea of Japan was isolated from the Pacific Ocean. Consequently, at thistime the salinity of the surface water was relatively
low because of the more precipitation relative toevaporation over the sea [1].
The warm Tsushima Current, a branch of thewarm Kuroshio Current, flows into the Sea of Japan
today. Based on the stratigraphic data of warm-water planktonic foraminifers, the Tsushima Current
first began to flow into the Sea of Japan in this way

A. Kitamuraat about 9,000 years before present [1, 2]. The current enters the sea via Tsushima Strait and flows
northward along the western coast of Honshu Island, with the majority of the current flowing out tothe Pacific Ocean via the Tsugaru Strait. The remainder of the current reaches the northern shoresof the Sea of Japan and becomes dense enough tosink to the bottom due to cooling and freezing during winter. This sunken water mass is the Japan Sea
Proper Water (JSPW), which has a low temperatureand high concentration of dissolved oxygen. The
water temperature within the intermediate zone of
the Sea of Japan is significantly lower than that of
the Pacific. This remarkable zone of low temperature is due to the JSPW.
PROXY FOR THE WARM TSUSHIMACURRENTThe history of the Tsushima Current is recordedin the stratigraphic distribution of warm-water taxa
within the Sea of Japan. On the basis of the stratigraphic distribution of diatoms from deep-sea sediment cores, warm-water species appeared at 3.5 Ma[3]. However, warm-water diatoms are not a goodproxy for the Tsushima Current because they occurred during the late stage of the last glacial period, oxygen isotope stage 2 [4]. As noted above,
surface water salinity was very low within the Sea
of Japan during the last glacial period [1]. Such alow salinity has an influence on foraminiferal δ18O
values and alkenone thermometry [1, 5]. However,
we do not yet understand the degree of this effectbecause we have yet to determine the salinity at thistime. Consequently, there is some debate regarding
estimates of the sea surface temperature during thelast glacial period. Many recent studies have estimated the sea surface temperatures in the northwestern Pacific and the East China Sea, reporting thattemperatures during the last glacial period wereabout 4°C lower than those of the present day [6, 7].
Thus many workers thought that the sea surface
temperature within the Sea of Japan during the lastglacial period was significantly lower than currentday temperatures. In summary, there is problem inusing diatoms as a proxy for the warm Tsushima
Current.
In contrast, warm-water planktonic foraminifers
and molluscs have not been described from the lastglacial period [1, 5], suggesting that they are moreaccurate proxies for the warm Tsushima Current
than diatoms. But further research is required todemonstrate the utility of these species as proxies

for the current. As noted above, the cool JSPW occupies the deeper part of the Sea of Japan, below
the Tsushima Current. In the area off southwesternJapan, the thermocline between the two water masses is located at about 150 m water depth. The depthof the thermocline exerts a strong control on thebathymetric range of warm-water benthos [8]. Even
during times when the Tsushima Current flowed into
the Sea of Japan, it was possible that warm-watermolluscs were absent from a given sediment if it
was deposited below the thermocline. The depth of
the thermocline becomes shallower northward. Thus,
the number of species of Indo-Pacific molluscs decreases gradually northward, with no species at allobserved off Oshima Peninsula, Hokkaido. In termsof identifying the stratigraphic distribution of warmwater molluscs, the sedimentary records preservedin southern areas of the Sea of Japan and in shallow
water are suitable for detailed examinations of temporal changes in the Tsushima Current. Warm-waterplanktonic foraminifera are also useful in this regard.
The distribution of planktonic foraminifera isstrongly controlled by the surface sea-water temperature. Moreover, they are short-lived and respondrapidly to environmental change; in comparison,adult molluscs are relatively long-lived. Co-workers
and I proposed that warm-water Globigerinoides ruber is the most suitable planktonic foraminifera species as a proxy for the warm Tsushima Current [9].
This is because it is the shallowest dwelling species
and it remains in surface waters throughout its entire life cycle [10]. The lower limit of the temperature tolerance of G. ruber is 19°C, meaning that thepresent-day population is unlikely to survive winterwithin the Sea of Japan.
Recently, planktonic foraminiferal assemblages
from 51 surface sediment samples were described[11]. I and co-worker examined their data and identified two patterns in terms of the distribution of G.
ruber [12]. The first pattern is that the species decreases in abundance from inshore to offshore areas(Fig. 1). The second pattern is that its relative abundance decreases northward, being absent in the area
off northern Hokkaido. This pattern is very clear
when considering sediment samples collected fromwater depths shallower than two hundred meters(Fig. 1). Thus, the utility of G. ruber as the proxy
for the Tsushima Current requires the same attention
as that required by warm-water molluscs. There exists an additional special problem
encountered in examining the stratigraphic distribution of G. ruber. Because their size is less thanPaleoceanography of Japan Sea During 3.5-0.8 Ma

Fig. 1 (A) Locality map of fossil records and the sampling location of planktonic foraminiferal assemblages reported by
Domitsu and Oda (2005). (B) Relationship between relative abundance of planktonic foraminifera Globigerinoides ruber andwater depth of the sampling points. (C) Relationship between relative abundance of G. ruber and annual mean sea surface
temperature (Japan Oceanographic Data Center; http://www.jodc.go.jp/service_j.htm). The relative abundance of G. ruber
was calculated from data presented by Domitsu and Oda (2005). Numbers are the sampling location numbers.

A. Kitamura

one millimeter, they are too small to be examined in
outcrop with the naked eye. Consequently, sampling
effects must be taken into account when evaluating
the stratigraphic distribution of the species.
CYCLIC CHANGES IN SHALLOWMARINE ENVIRONMENTS
DURING THE EARLY PLEISTOCENEIn this chapter, I review cyclic changes in bothmolluscs and planktonic foraminifer within depositional sequences in the early Pleistocene Omma Formation. At its type section, the Omma Formation is
up to 220 m thick, and has been divided into lower,
middle, and upper parts (Fig. 2) [13, 14]. The lower
and middle parts are composed of fourteen depositional sequences that were deposited at inner- toouter-shelf depths. Five depositional sequences are
identified in the upper part, deposited at alluvial
plain to inner-shelf depths [15]. The basement ofbasal shell beds corresponds to sequence boundary.
In each depositional sequence, the lithofacies
present are, in ascending order, a basal shell bed, awell-sorted fine sandstone, a muddy fine- to veryfine sandstone, and a well-sorted fine sandstone. The
depositional sequences contain in situ molluscan
fossils, although X-ray diffraction studies reveal diagenetic calcite derived from the aragonite shells. It
is highly likely that this diagenetic alteration also
affected planktonic foraminifers. Thus we did notgeochemically analyze fossils within the Omma Formation.
The molluscan fauna changes over time fromcold-water, upper-sublittoral species to warm-water,lower-sublittoral species, followed again by coldwater, upper-sublittoral species during the depositionof the depositional sequence. Warm-water species
are Indo-Pacific elements that live in the area southof Tsugaru Strait. The cold-water molluscan faunaconsists of living and extinct species. Most of living
species inhabit in the area north of Tsugaru Strait,
while the extinct species are endemic to the Sea ofJapan, including Clinocardium fastosum and Turritella saishuensis. Since fossils records are incomplete,
we have not understood the process of extinction of
the endemic species.
The systematic nature of changes observed in themolluscan fossil associations within individual depositional sequences indicates that the oceanic conditions changed in parallel with fluctuations in water
depth, such that increases in water depth corresponded to periods of warming of the marine climate. Thus it is thought that these changes in water

depth and oceanic conditions were related to glacioeustasy with a period of the 41,000 years, corresponding to the period of orbital obliquity [14].
Subsequently, co-workers and I have correlated
nineteen depositional sequences with oxygen isotope
stages 56 to 21.3, based on a combination of sequence stratigraphic and biostratigraphic and magnetostratigraphic data [16]. The data reveal that the
Tsushima Current flowed into the Sea of Japan at
every interglacial highstand, except for oxygen isotope stages 25, 23, and 21.3, over the period from
1.6 to 0.8 Ma (MIS 56 to 20) (Fig. 2) [16]. The dissolution of shells means that molluscs and planktonic foraminifer cannot be identified for these stages.
Contemporaneous fossil records have not been detected around the Sea of Japan.
The relative abundance of warm-water Globigerinoides ruber shows cyclic changes that are consistent with those of warm-water molluscs (Fig. 2).
The repeated appearance of warm-water molluscs
and G. ruber shows that they were introduced into
the Sea of Japan with the Tsushima Current and locally died out during succeeding glacial periods [9].
PALEOCEANOGRAPHIC CHANGESWITHIN THE SEA OF JAPANDURING THE PERIOD BETWEEN
3.5 AND 0.8 MA
Plio-Pleistocene marine sediments are well exposed along the Sea of Japan coast along northeastern Japan. Since these strata contain abundantmolluscs and planktonic foraminifera, a number ofworkers have examined the stratigraphic distributions of these organisms. In order to reconstruct thehistory of the
Tsushima Current in the period before and after
the interval represented by fossils within the Omma
Formation, I and co-worker compiled fossil records
according to the following criteria: 1. the identification of many datum planes; and 2. the existence ofdata for both molluscs and planktonic foraminifers.
Several strata meet these criteria, including the late
Pliocene Yabuta Formation and the Plio-Pleistocene
Junicho Formation (Fig. 3) [16]. To help fill existing
gaps in the data, we are currently adding fossil records from the Omma Formation in Toyama Prefecture, Plio-Pleistocene strata in the Joetsu area, andthe late Pliocene Kuwae Formation. But we have
yet to obtain fossils records over the period fromthe last occurrence of Neodenticula koizumi to thebase of the Olduvai Subchron. In addition, marine
deposits are not found on land for periods younger

Paleoceanography of Japan Sea During 3.5-0.8 Ma

Fig. 2 Columnar section of the Omma Formation at its type section, showing stratigraphic distribution of selected molluscan fossils and the relative abundance of Globigerinoides ruber.



A. Kitamura

Fig. 3 Correlation of fossil records in the Late Pliocene to Early Pleistocene sediments along the western coast of Honshu Island. Location of fossil records are shown in Fig. 1.

than 0.8 Ma, except for marine terrace deposits thatformed during the last interglacial period (Fig. 3).
Warm-water molluscan fossils are reported fromthese deposits, indicating that the warm Tsushima
Current flowed during oxygen isotope stage 5 [17,18].
Most of the molluscan species found within the
Yabuta, Junicho, and Kuwae Formations are currently living below 150 m water depth within the Sea ofJapan [19–22]. Thus, any reconstruction of the inflow of the Tsushima Current depends on the stratigraphic distribution of the warm-water planktonic
foraminifer G. ruber. The average sampling intervalof several thousands to ten thousands years [23–25]
is sufficient to detect interglacial to glacial cycles
with a period of 41,000 years. However, deposition-

al sequences have not been identified in these formations because of their depth of deposition. In
addition, the time intervals between identified datums are in the order of two hundred thousand years(Fig. 3), representing a much lower temporal resolution than that in the Omma Formation.
Fig. 4 shows warm-water fossil records and demonstrates that the history of the warm Tsushima Current in the period between 3.5 and 0.8 Ma can bedivided into two intervals. In the interval between
3.5 and 1.71 Ma, the current flowed periodicallyinto the Sea of Japan at 3.2, 2.9, 2.4, and 1.9 Ma.
Comparing these ages with δ18O stratigraphy, thethree older events may correlate with KM5 or 3,
G17 or 15 and isotope stages 95 or 93, and isotope
stage 69, respectively. During the second interval,

Paleoceanography of Japan Sea During 3.5-0.8 Ma

Fig. 4 (A) Stratigraphic distribution of maximum relative abundance of Globigerinoides ruber at each interglacial stage. Reconstruction of the southern strait between the Sea of Japan and East China Sea at common condition during 3.5–1.7 Ma(B), major sea-level highstand during 3.5–1.7 Ma (C) and interglacial stages during 1.7–0.8 Ma (D).

between 1.71 and 0.8 Ma, the Tsushima Current
flowed into the Sea of Japan at every interglacial
highstand, except for MIS 25, 23, and 21.3.
During the Pliocene, between about 4 and 2.7 Ma,
many regions of the northern hemisphere experienced a warm climate, including Japan [26–28]. In
addition, sea levels during the interglacial periods
between 3.3 and 2.5 Ma were up to 50 m higher
than the present-day level [29]. Thus, a drasticchange in the frequency of inflow of the Tsushima
Current occurred with the formation of the southernstrait (Fig. 4). This event was caused by crustal
stretching in the northern Okinawa Trough associated with a change in the convergence direction of thePhilippine Sea plate [9].
CONCLUSIONS
On the basis of the fossil record preserved in marine deposits found in and around the Sea of Japan,

the marine environment over the period from 3.5 to
0.8 Ma can be divided into two stages. Before theformation of the southern strait, the sea was only
connected to external waters via the northern strait,
except for periodic inflows of the Tsushima Current.
The sea was cold and relative stable. Since the formation of the southern strait at 1.7 million years, theenvironment of the Sea of Japan and organisms
within the sea have been strongly influenced by glacial to interglacial cycles and the condition of thesouthern strait.
ACKNOWLEDGEMENTS
I am very much thankful to Prof. Shunsuke F.
Mawatari and Dr. Hiroshi Nishi for encouraging me
to write this paper.

A. Kitamura



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