国立科博専報，(51), pp. 109–123 , 2016 年 3 月 28 日
Mem. Natl. Mus. Nat. Sci., Tokyo, (51), pp. 109–123, March 28, 2016

Miocene Floral Changes in the Circum-Japan Sea Areas—Their Implications in the Climatic Changes and the Time of Japan Sea Opening
Boris I. Pavlyutkin1, Atsushi Yabe*2, Vladimir V. Golozoubov1 and Ludmila F. Simanenko1
1

Far Eastern Geological Institute, Far Eastern Branch, Russian Academy of Sciences,
Prospect 100-letiya Vladivostoka 159, Vladivostok 690022, Russia
2
Department of Geology and Paleontology, National Museum of Nature and Science,
4–1–1 Amakubo, Tsukuba, Ibaraki 305–0005, Japan*E-mail: yabeatsu@kahaku.go.jp

Abstract. The Miocene plant megafossil assemblages from the circum-Japan Sea areas, including southern Primorye, Korea, and Japan, were reviewed in terms of ﬂoristic composition andphysiognomic characters. These attributes were used to understand the vegetation history during
the formation of the Sea of Japan with its effects on ﬂoristic changes in each area. There was atemperature increase from the early early Miocene to the middle Miocene with a peak at the early–
middle Miocene boundary, and subsequent temperature drop in the late Miocene in all ﬂoral successions. The early Miocene assemblages from each area are similar to one another, but the number of common species between those from the southern Primorye and Japanese Islands decreased
since the middle Miocene, suggesting an inﬂuence of geographic separation of these areas possibly
due to the rifting of the Sea of Japan. The degree of temperature increase was larger in the southern part (the Japanese Islands and southern Korean Peninsula), whereas it was much smaller in thenorthern part (southern Primorye and northern Korean Peninsula). Effects of cold and warm water
currents both toward the north and south of the Sea of Japan were suggested for ﬂoristic differentiation of these areas.
Key words: Plant megafossil assemblages, Forest vegetation, Paleoclimate, Circum-Japan Sea
areas, Miocene

Introduction
Extant forest vegetation in southern Primorye
and the Japanese Islands show quite different
pictures, even though they are situated at similar
latitudes (Fig. 1). Southern Primorye is coveredby a mixture of conifers and deciduous angiosperms (Mixed Coniferous and Broad-leaved forest), which developed under cooler-temperateconditions with low mean annual temperatures(Krestov, 2003). Although this forest type is sporadically distributed in the mountains in the Japanese Islands, most parts of the Japanese Islands
are covered by those dominated by a mixture of
deciduous and evergreen angiosperms thatchange their composition latitudinally and altitudinally.

Forest vegetation of these areas and their differences are severely controlled by the monsoonal climate (Asian summer and winter monsoons) and the prevalence of warm and cold
oceanic currents (Fig. 1). The summer monsoon
brings high precipitation in these areas, while thewinter monsoon brings strong cold and dryness
to the continental side. Because of the effect of
the Tsushima warm current that transports humid
air to the Sea of Japan, a strong northwesterly
from the continent brings heavy snow to theJapan Sea-side of the Japanese Islands (Vitvitsky,1980), which is considered to be critical fordeveloping the beech forest in Japan (Hara,
2010).
Modern geographic conﬁgurations of theseareas are thought to have primarily formed dur-

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Boris I. Pavlyutkin et al.

Fig. 1. Geographic conﬁgurations of the study area with plant megafossil localities treated in the study. Warmand cold oceanic currents around the Japanese Islands were shown.

ing the Miocene due to the rifting of the Sea ofJapan. A number of geologic evidences support
connections between Japan and the Eurasian continent before the rifting (Golozoubov, 2006). In
fact, ﬂoristic compositions of Paleogene ﬂoras inthese areas are quite similar to one another andplenty of common species exist (Pavlyutkin &Petrenko, 2010). Therefore, the onset of ﬂoristic
differentiation in these areas should be highly
dependent on the formation of the Sea of Japan.
The rifting and opening of the Japan Sea basin
have been considered to occur at a short timeinterval during the mid-Miocene (Otofuji, 1996).
However, recent revision of the tectonic development of the marginal Eurasian continent indicates a much longer phenomenon initiated in the
Paleogene (Golozoubov et al., 2007, 2009, 2012;

Kano et al., 2007).
To better understand the history of ﬂoristicchange in the circum-Japan Sea areas, a numberof studies have been carried out (Baikovskaya,
1974; Klimova, 1983; Ablaev & Vassiliev, 1988).
However, the ages of the plant-bearing deposits
in southern Primorye were controversial (Pavlyutkin et al., 1985) and this hampered the detailed
discussion about the time and diversiﬁcation ofeach ﬂora in light of the Japan Sea formation.
Some of the present authors have worked on thegeology and plant fossil assemblages in southern
Primorye and revised the previously establishedgeologic framework based on detailed stratigraphic correlation and newly obtained radiometric ages (Pavlyutkin & Petrenko, 2010).
In this study, we compare ﬂoristic data

Miocene ﬂoral changes in the circum-Japan Sea Areas

obtained from the circum-Japan Sea areas including southern Primorye, the Korean Peninsula,and the Japanese Islands, based on the latest ﬂoristic and geologic studies in order to show thevegetation history and climate during the Miocene. We discuss time and possible effects of the
Sea of Japan formation on the vegetation differentiation between the continent and the JapaneseIslands.
Materials and methods
Regional ﬂoral data from southern Primorye,
the Korean Peninsula, and Japan (mainly fromthe Sea of Japan side) were compared ﬂoristically and vegetationally. For climate analysis, weused a simple linear regression analysis termedthe Leaf Margin Analysis (LMA: Wolfe, 1979;
Wing & Greenwood, 1993) to obtain quantitative
data on mean annual temperature (MAT). Thismethod is based on the relationship between the
MAT of a given area and the percentage ofwoody dicotyledons with a smooth leaf margin in
a given ﬂora under humid to mesic conditions(Wolfe, 1979; Wing & Greenwood, 1993).
Among published regression equations, we used
the one provided by Wolfe (1979), which has
been developed based on East Asian vegetation.
Errors for the analysis were calculated based on
Wilf (1997). Classiﬁcation of forest vegetation
was followed after Wolfe (1979).
Floral data
Southern Primorye
The Miocene ﬂoral succession of southern Primorye has been a matter for continuous debates(Baikovskaya, 1974). Klimova (1983) revised
the former stratigraphic scheme and recognized
two horizons, namely the Sineutesovsky (thelower–middle Miocene) and the Ust-Suifunsky(the upper Miocene). At that moment, the ʻMioceneʼ Ust Davydovsky Formation was rejectedfrom the Miocene records and was newly compared with the Eocene–Oligocene Harutori Formation in Hokkaido (Klimova, 1983). Cenozoic

111

ﬂoras from Primorye were principally comparedwith those of Japan and Sakhalin, however thecurrent status for the age of Japanese ﬂoras has
not been critically cited in the scheme. One ofthe present authors (BP) has worked on the
Cenozoic stratigraphy and ﬂoral succession ofsouthern Primorye and concluded that some ofthe ʻMioceneʼ ﬂoras accepted by Klimova (1983)
and other authors (e.g., Ablaev & Vassiliev,
1998) should be assigned to the early Oligocene
based on ﬂoristic comparison with Paleogene
ﬂoras in Japan (Pavlyutkin & Petrenko, 2010).
Consequently, they recognized four Miocene ﬂoral horizons: Sine-Utesovsky (lower lower Miocene), Nezhinsky (upper lower Miocene), Novokachalinsky (middle Miocene), and Ust-Suifunsky(upper Miocene).
The Sine-Utesovsky horizon is represented bya plant assemblage from the Sine-Utesovsky Formation in the Ivanovsky depression, southwest of
Vladivostok. The Sine-Utesovsky horizon ischaracterized by deciduous angiosperms with asmall amount of conifers: Metasequoia occidentalis, Sequoia langsdorﬁi, Kadzura sp., Ulmus
pseudolongifolia, Zelkova elongata, Fagus evenensis, F. palaeocrenata, Quercus yoshiokaensis,
Betula mitai, Alnus usyuensis, A. prenepalensis,
A. duensis, A. schmalhausenii, Carpinus subyedoensis, Tilia hommashinichii, Cladrastis
chaney, Hemitrapa borealis, Acer protomiyabei,and Alangium kryshtofovichii (Pavlyutkin,unpublished data). No evergreen angiosperm has
been recorded so far. The Sine-Utesovsky horizon is comparable to the earliest Miocene Aniaitype ﬂoras of Japan (Huzioka, 1964; Tanai, 1961)
because they share many common species aswell as physiognomic characters (Pavlyutkin &Petrenko, 2010). The age of the Sine-Utesovsky
Formation is assumed to be the earliest Miocene
based on a K–Ar age of 22.0±1.0 Ma from theoverlying basalt rock (Popov et al., in Pavlyutkin& Petrenko, 2010).
The Nezhino Flora that typiﬁes the late early
Miocene Nezhinskii horizon was recently
described by Pavlyutkin et al. (2012) from theNezhinsky Formation in the Pavlovsky depres-112

Boris I. Pavlyutkin et al.

sion. Its age was estimated between 20.9–
17.1 Ma based on radiometric dating. There were
127 species described from the formation that arecharacterized by temperate angiosperms belonging to the family Ulmaceae, Betulaceae, Fagaceae, Salicaceae, Fabaceae, and Sapindaceae,with minor amounts of warm-temperate elements, such as Davidia, Nyssa, Idesia, andEurya. Some species that are the same or close to
the elements of the Daijima-type ﬂoras of Japan(Tanai, 1961), which range from the late early toearly middle Miocene, were also recognized:
Pinus miocenica, Liquidambar europaea, Ulmus
carpinoides, Castanea miomollissima.
The Novokachalinsky horizon is represented
by the Khanka Flora from the western coast of
Lake Khanka near the border between Russia
and China (Pavlyutkin, 2005). The ﬂora is characterized by the predominance of Fagus chankaica with subordinate elements like Metasequoia
disticha, Taxodium dubium, Sorbus iljinskajae,
Zelkova elongata, Acer rotundatum, and Aesculus iljinskajae. This ﬂora is comprised of 165
species belonging to 82 genera and 39 families,
and it is dominated by deciduous angiosperms.
No evergreen species has been recognized fromthe ﬂora. Many common genera exist between
the Miocene ﬂoras of Primorye and Japan; however, the percentage of endemic species is high inthe Khanka Flora.
The type section of the Ust-Suifunsky horizon
is located in the Pushkinsky depression near
Vladivostok. The Ust-Suifunsky Formation andits correlatives contain several fossil plants common to the lower Novokachalinsky horizon (Pavlyutkin, 2002). But it is characterized by thedominance of Ulmus, Cercidiphyllum, Populus,
and Salix, as well as by the rarity of Fagus andconifers in its lower part, although Fagus dominate in the upper part. Fission-track ages
obtained at the type section indicate a late Miocene age of the ﬂora (10.8±1.1 and 11.8±0.9
Ma: Pavlyutkin et al., 1985).
No age data is available for the Novokachalinsk horizon but we assumed it to be middle
Miocene based on stratigraphic and ﬂoristic cor-

relations (Fig. 2).
Korean Peninsula
Miocene strata bearing plant fossils are well
exposed in the Pohang Bay area, North Gyeongsang Province, South Korea. Classical localities
are also known in the Myongchong–Kilchu district in North Hamgyong Province and Tongchon
in Gangwon (Kangwon) Province, North Korea(Huzioka, 1972).
Three levels of plant-bearing strata have beenrecognized in the Pohang Bay area. The lower
most stratum is the lacustrine Geumgwangdong
Formation of the Jianggi Group, which yields theearly early Miocene Jianggi Flora (Huzioka,
1972; Paik et al., 2012). The Yeonil Flora wasreported from the marine Duho Formation of theupper most part of the Pohang Group, that
unconformably overlies the Jianggi Group(Huzioka, 1972). Palynoﬂoral assemblage of theformation was reported by Yamanoi (1992b).
The Jianggi and Yeonil ﬂoras consist of 36 and35 species, respectively. More than 70% of
Jianggi species are common to the Aniai-type
ﬂoras, while ca. 65% of the Yeonil species are
known in the Daijima-type Noroshi (Ishida,1970) and Notonakajima (Matsuo, 1963) ﬂoras
in Noto Peninsula, Japan. Thus, they were compared to the Aniai-type and Daijima-type ﬂoras,
respectively (Huzioka, 1972).
Jeong et al. (2004, 2009) reported two fossil
wood assemblages from the Sinjeongri Formation that overly the Geumgwangdong Formationin the Jianggi Group. They are characterized by
an appearance of some wood species known in
the Daijima-type wood assemblage (Terada,
1998), such as Taxodioxylon cunninghamioides,
Camellia japonoxylum, Distrylium chiharuhirayae, and Wataria miocenica, in association
with the ones known in the Aniai-type woodassemblage (Terada, 1998), Fagus hondoensis.
Leaf fossils are dominated by Fagus antipoﬁi (K.
Uemura, personal communication with AY).
Therefore, Lim et al. (2010) concluded that theseassemblages should be transitional ones between
two ﬂoral types.

Miocene ﬂoral changes in the circum-Japan Sea Areas

The Jianggi Group is dated by Ar/Ar ages of
extrusive (19.4±2.1 Ma) and intrusive (16.8±
2.2 Ma) rocks; both indicate an early Miocene
age for this group (Paik et al., 2012). The marine
Duho Formation is assumed to be late middle
Miocene based on dinoﬂagellate cysts (14–12 Ma: Yun et al., 1997) and a paleomagnetic
study (14.5–11.5 Ma: Kim et al., 1993).
Two plant bearing strata have been recognizedin the Miocene deposits in the Myongchong–Kilchu district (Huzioka, 1972). The Yongdong
Flora from the Yongdong Formation is composedof 21 species of deciduous angiosperms andconifers; all of its components are common to the
Jianggi Flora. The Hamjindong Flora wasreported from the marine Hamjindong Formationin the Myeoncheon Group, which unconformably
overlies the Yongdong Group. It consists of 45
species belonging to 31 genera and 23 families,
and it is dominated by deciduous angiosperms
and deciduous and evergreen conifers. Although
it also resembles the Jianggi Flora or the Aniaitype ﬂoras of Japan, it is comparable to the Daijima-type ﬂoras due to the occurrence of someindex species as Quercus miovariabilis, Liquidambar miosinica, and Parrotia fagifolia(Huzioka, 1972). The diatom assemblage fromthe Hamjindong Formation was correlated to theupper part of the Denticulopsis hyalina zone to
the D. dimopha zone (14–9.5 Ma) (Koizumi,1988), suggesting a late middle to early late Miocene age.
A Miocene plant assemblage from the freshwater deposits in Tongchon, Kangwon Province,
referred to as the Tongchon Flora, consists of 20
species of deciduous angiosperms and conifers.
No evergreen plants were recognized. Most of its
components are common to the Aniai-type ﬂoras(Huzioka, 1972), but it was also compared with
the Daijima-type ﬂoras with the occurrence of
the key species, Comptonia naumanni. Although
we have no data for the age of this plant-bearing
stratum at the moment, we herewith follow
Huziokaʼs (1972) opinion that it was situatedbetween the Jianggi and Yeonil ﬂoras based on
its composition.

113

Japan
Miocene ﬂoras in the Japanese Islands areclassiﬁed into three types: the Aniai-type, Daijima-type, and Mitoku-type ﬂoras in ascending
stratigraphic order (Tanai, 1961, 1992).
The Aniai-type ﬂora of the early early Miocene age (Tanai, 1961; Huzioka, 1964) in central
and northern Japan is characterized by the predominance of deciduous angiosperms and deciduous and evergreen conifers with representative
elements of Fagaceae, Betulaceae, Ulmaceae,
Juglandaceae, Salicaceae, and Sapindaceae. In
terms of physiognomic characters, it correspondswith the Mixed Northern Hardwood forest thatdevelops under cold-temperate climate conditions (Tanai, 1992).
The Daijima-type ﬂora of late early to earliest
middle Miocene (Tanai, 1961, 1992; Huzioka,
1963) has been recognized throughout the Japanese Islands. The strata bearing the Daijima-type
ﬂora unconformably covers those bearing theAniai-type and it often contains marine organisms in its upper part or conformably overlying
beds. These fossils, represented by the ArcidPotamid molluscan fauna, clearly indicate an
inﬂuence from the warm oceanic climate. Mostof the Japanese Islands were inundated bymarine conditions during the latter part of the
middle Miocene, and thus, few fossil assemblages have been recognized.
The ﬂoras in Honshu are characterized by a
mixture of deciduous and evergreen angiosperms
that correspond to the Notophyllous Broadleaved Evergreen forest indicating warm-temperate climate conditions. Latitudinal and altitudinaldifferences of ﬂoristic composition have beenrecognized in the Daijima-type ﬂoras (Tanai &Uemura, 1988). Those from southern Hokkaido
and northern most Honshu were compared to theMixed Broad-leaved Deciduous forest (Wolfe &Tanai, 1980). Moreover, earlier part of the Daijima-type ﬂora in Honshu is also enriched intemperate elements. They were compared to theMixed Broad-leaved Deciduous or Mixed Broadleaved Evergreen and Deciduous forest (Yabe,
2008a, b). They can be recognized due to the114

Boris I. Pavlyutkin et al.

occurrence of key species of deciduous angiosperms: Comptonia naumanni, Quercus miovariabilis, Liquidambar miosinica, and Parrotia pristina.
The Mitoku-type ﬂora of the late Miocene–
Pliocene age (Tanai, 1961) is characterized
by the predominance of beech species (Fagus
stuxbergii and F. paleojaponica) in association
with various deciduous dicotyledons, conifers,
and a few evergreen dicotyledons. Constituentsof this type of ﬂoras are more modernized thanprevious ones and their composition also resembles those in modern vegetation.
Temporal ﬂoristic changes that occurred during the Miocene in Japan were revealed to haveoccurred at the same time as changes in climate
conditions during that time interval (Kano &Yanagisawa, 1989; Tanai, 1992; Yabe, 2008b).
Based on accumulated radiometric ages andstratigraphic revisions, the boundary age of theAniai-type and Daijima-type ﬂoras is considered
to be approximately 20 Ma (Yabe, 2008b). The
transition between the Daijima-type and Mitokutype ﬂoras are not well understood because
plant-bearing terrestrial strata are scarce in Japan
during the middle Miocene. Some plant fossil

assemblages from marine strata have beenrecorded previously (Kawase & Koike, 2001;
Uemura et al., 2006); however, their compositions were skewed due to taphonomic effects inmany respects. Based on a palynological study ofoffshore deposits in the Sea of Japan, Yamanoi(1992a) proposed that the Daijima-type pollen
assemblage (NP-2) was traceable until 13 Ma andthe Mitoku-type assemblage (NP-4) appearedsince 7 Ma. Yamanoi (1989) deﬁned the zone
NP-3 as transitional vegetation between the Daijima-type and Mitoku-type ﬂoras. However,recently obtained radiometric ages suggested thatsome assemblages that are comparable with the
Mitoku-type ﬂora had appeared much earlier(i.e., Yagii Floras: 10 Ma, Kobayashi et al., 2011;
Pinus-trifolia bed: 12 Ma, Tsukagoshi, 2011).
Although the appearance of the Mitoku-type
ﬂora might differ site-wise, we tentatively
assigned the boundary age between the Daijimatype and Mitoku-type ﬂoras to the middle–late
Miocene boundary. The correlation of each ﬂora
from the three different areas is shown in Fig. 2.

Fig. 2. Correlation of Miocene ﬂoras in the southern Primorye, North and South Korea and the Japanese Islands.
NP: palynoﬂoral zonation proposed by Yamanoi (1992a). MNCO: mid-Neogene climatic optimum. The upper
limit of the Daijima-type ﬂoras in Japan was followed after Yamanoi (1992a). It can be extended toward theend of the middle Miocene (allow) based on the correlation with the Yeonil and Hamjindong ﬂoras in Korea.
For further details see main text.

Miocene ﬂoral changes in the circum-Japan Sea Areas

Correlation and climate analysis
Floristic correlation
On the basis of the stratigraphic correlation ofeach area in association with additional radiometric and biostratigraphic ages, we recognized
four Miocene phytolevels in the circum-Japan
Sea areas. They were deﬁned here as Stages I to
IV.
Stage I is represented by the Aniai-type ﬂorasof Japan and its equivalents (Jianggi and Yongdong ﬂoras) from the earliest Miocene age. The
Sinij Utes Flora was also compared to this stage(Pavlyutkin & Petrenko, 2010). The ﬂoristic
composition of fossil assemblages of this stage
are similar to one another, and they have been
compared to the Mixed Northern Hardwood forest, which indicate cool-temperate climatic conditions (Wolfe, 1979).
Stage II is represented by the Nezhino Floraand the lower part of the Daijima-type ﬂoras,
which were dated between 20–16 Ma. The Sinjeongri and Tongchong ﬂoras from the Korean
Peninsula can be correlated to this stage. Florasof this stage, especially in the Japanese Islands,
are characterized by an appearance of many thermophilic species in its latter part and are comparable to the Notophyllous Broad-leaved Evergreen forest. Whilst the earlier part of this stage
can be correlated to the Mixed Broad-leaved
Deciduous or Mixed Broad-leaved Evergreen
and Deciduous forest. The occurrence of pollen
grains from mangrove plants, which are though
to be the elements of tropical–subtropical conditions, has been recorded in central and northeast
Japan during the transition between Stage II and
III (Yamanoi et al., 1980, 2008, 2010, 2011;
Saito et al., 1995). Some common species exist
among these assemblages, though the total number of it is smaller in the Tongchong and Nezhino
ﬂoras in continental Asia (Huzioka, 1972; Pavlyutkin et al., 2012).
Stage III is represented by the Khanka Flora.
The Hamjindong and Yeonil ﬂoras in the Korean
Peninsula, and the upper part of the Daijima-type
ﬂoras were compared stratigraphically. The age

115

range of this stage is assumed to be 16–12 Ma.
Most assemblages other than the Khanka Flora
were found in marine deposits. Although only a
few fossil assemblages are available in this stagein Japan, the ﬂoristic similarities between the
Daijima-type Stage II assemblage and the Yeonil
Flora conform to a prevalence of the Daijimatype ﬂoras until the latter part of the middle Miocene (Stage III). Fossil assemblages from thisstage are characterized by a mixture of thermophilic and temperate elements. The Yeonil ﬂora is
assignable to the Notophyllous Broad-leaved
Evergreen forest like that in the previous stage,
while the Hamjindong and Khanka ﬂoras,
enriched more in temperate elements, are comparable to the Mixed Broad-leaved Deciduous forest.
Stage IV is represented by the Ust-Suifunsky
horizon and is comparable with the Mitoku-type
ﬂoras. They are dominated more by temperateelements than those of the preceding stage,
although the Ust-Suifun Flora show unique features distinct from the Mitoku-type ﬂoras inJapan. Latitudinal ﬂoristic changes are muchclearer in this time period and the assemblages
from central Japan can be compared to the Mixed
Mesophytic forest, while those in the lowland
vegetation in northeast Honshu and southern Primorye are comparable to the Mixed Northern
Hardwood forest.
Climate analysis
Miocene plant fossil assemblages mentioned
in the previous section always contain notophyllclass leaves, indicating a mesic to humid condition. Therefore, changes in ﬂoristic composition
are considered to be principally controlled bychanges in temperature values. Thus, we analyzed each stage assemblage in terms of the temperature change during the Miocene. For Stage I,
II, and III assemblages from the JapaneseIslands, those from Akita (Aniai and Daijima
ﬂoras), southwest Hokkaido (Kaminokuni and
Yoshioka ﬂoras), and Noto (Noroshi and Notonakajima ﬂoras) were selected for study. In order toexamine the Stage IV assemblage, we added

Boris I. Pavlyutkin et al.

116
Table 1.

Quantitative estimates for Mean Annual Temperature based on Leaf Margin Analysis (Wolfe, 1979).

Area

Flora

Southern Primorye Nezhino*1

Korean Peninsula

Japanese Islands

Stage # of species # of entires
II

94

11

E

MAT

Error

Reference

11.70

4.72

1.01

Pavlyutkin et al. (2012)

Khanka

III

99

17

17.17

6.40

1.16

Pavlyutkin (2005)

Ust-Suifun L*2
Ust-Suifun U

IV
IV

24
82

3
11

12.50
13.41

4.97
5.25

2.07
1.15

Pavlyutkin (2002)
Pavlyutkin (2002)

Jianggi*3

I

31

8

25.81

9.04

2.40

Huzioka (1972)

Yeonil*4
Hamjindong*5

III
III

27
38

9
6

33.33 11.34
15.79 5.97

2.78
1.81

Huzioka (1972)
Huzioka (1972)

Kaminokuni*6
Aniai*7

I
I

32
37

2
6

6.25
16.22

3.05
6.10

1.31
1.85

Tanai and Suzuki (1963)
Huzioka (1964)

Yoshioka*8
Utto*9Noroshi*10

II
II
II

57
69
61

11
25
25

19.30 7.05
36.23 12.23
40.98 13.68

1.60
1.77
1.93

Tanai and Suzuki (1963)
Huzioka (1963)
Ishida (1970)

Notonakajima*11

II–III28

1139.29 13.16

2.82

Matsuo (1963)

Shanabuchi*12
Yagii*14
Onbara*15

IV
IV
IV

59
47
39

8
15
7

13.56 5.29
31.91 10.91
17.95 6.63

1.36
2.08
1.88

Tanai and Suzuki (1965)
Horiuchi (1996)
Uemura (1986)

MAT: Mean Annual Temperature; E: Percentage of woody species with smooth leaf margin. MATs and their errors
were calculated after equations shown in Wolfe (1979) and Wilfe (1997), respectively. Uste-Suifunsk Flora was divided
in the lower (L) and upper (U) ﬂoras according to Pavlyutkin (2002). The age of each ﬂora was based on the following
literatures: *1, 20.9–17.1 Ma, Pavlyutkin et al., 2012; *2, 11.8–10.8 Ma, Pavlyutkin et al., 1985; *3, 20.9–17.1 Ma, Paik
et al., 2012; *4, 14.5–11.5 Ma, Jung and Lee, 2009; *5, 14–9.5 Ma, Koizumi, 1988; *6, 24–23 Ma, Ganzawa, 1992; *7,
24.0–19.4 Ma, Kano et al., 2012; *8, 18–17 Ma, Suzuki et al., 1992; *9, 18.2–16.4 Ma, Yoshikawa et al., 2002; *10,
19–18 Ma, Yoshikawa et al., 2002; *11, 16.3–15.7 Ma, Yoshikawa et al., 2002; *12, 8.2–6.3 Ma, Yahata and Nishido,
1995; *13, ca. 4 Ma, Uemura, personal communication in 2015; *14, 10 Ma, Kobayashi et al., 2011; *15, 6 Ma, Kanoand Nakano, 1985.

those from central (Yagii Flora) and westernJapan (Onbara Flora) in order to understand thetemporal changes in each area as well as its latitudinal difference (Table 1). Most of the estimated values are concordant with physiognomic
characters of each assemblage, but others show
distinctly lower temperatures. This is probably
because LMA tends to be inﬂuenced by the number of angiosperm species and because deciduous
species were highly divided into morphospecies
in some fossil assemblages.
The LMA for ﬂoras in Japan indicates a clear
trend of temperature increase from Stages I to II.
There was a four or six degrees increase in temperature observed in southwest Hokkaido (Kaminokuni to Yoshioka ﬂoras) and northeast Honshu(Aniai to Utto ﬂoras). The Stage I Jianggi Flora
in South Korea show a similar temperature value
with that in Japan. The Stage II–III transition,
represented by the Notonakajima Flora indicates
one of the warmest conditions, while the StageIII Yeonil Flora indicates a slightly cooler condition than the preceding one. The Stage IV assemblages indicate cooler conditions and a latitudinaltemperature gradient is observable from Hokkaido (Shanabuchi Flora) to Central Japan (Yagii
Flora). The Onbara Flora yielded in the intramountain basin of western Japan represents asimilar temperature condition as Northeast Honshu.
In southern Primorye, the Stage II Nezhino
Flora indicate a mean annual temperature of lessthan 5 degrees, which is similar to the Aniai-type
ﬂoras in northeast Honshu or southwest Hokkaido in Japan. The Stage III Khanka Flora
together with the Hamjindong Flora in North
Korea indicate slightly warmer conditions but aremuch lower than that indicated by the Yeonil
Flora in South Korea. During Stage IV, the UstSuifun ﬂoras indicate lower temperature than the
Khanka Flora and specify similar values estimated in the assemblage from northern Hok-

Miocene ﬂoral changes in the circum-Japan Sea Areas

kaido.
Discussion
Miocene ﬂoral changes in the circum-Japan
Sea areas
Floristic comparisons among southern Primorye, Korea, and Japan clearly show similarities
and differences in composition of each ﬂora andtheir ﬂoral changes among these areas.
A synchronized pattern of ﬂoristic changes canbe recognized in Japan and in the southern partof the Korean Peninsula. Namely, climate warming started prior to 20 Ma and became obvious
near the transition between Stages II and III,
when mangrove swamps developed in many
places in the Japanese Islands. A similar patternof ﬂoristic change has been shown to occur during the early Miocene on the Paciﬁc-side ofnortheast Japan (Yabe, 2008b). Terrestrial vegetation change in that area is synchronized withshallow marine climate and two warming episodes were recorded there at around 20 Ma and
16 Ma (Yabe, 2008b). The transition between
Stages II and III has been known as ʻthe midNeogene climatic optimum (MNCO)ʼ (Tsuchi,1986), and it lasted from ca. 16.5 to 15 Ma, whenwarm oceanic climate prevailed throughout thecoastal areas in East Asia. In Japan, mollusks
typical of ʻtropical–subtropicalʼ marine conditions expanded their distribution further north(Ogasawara & Nagasawa, 1992). Pollen grainsfrom mangrove plants were also recorded fromthese strata in the Japanese Islands (Yamanoi etal., 2011). The earlier part of the Stage II assemblage is known to occur in the Ito-o Formation in
Fukui Prefecture, central Japan, where ﬂoristic
composition is more temperate and comparablewith the Mixed Broad-leaved Deciduous forest(Yabe, 2008a).
After a short warming period, the climate
became slightly colder, which is represented by
the presence of Yeonil Flora. Yamanoi (1992a)
distinguished palynoﬂoras of this time period aszone NP-3. This assemblage represents a cooler
climate than zone NP-2 with the common occur-

117

rence of Fagus and the paucity of evergreen
Quercus. Cooling probably started just after theStage II–III transition as represented by the
Tatsugoroshi Flora from the marine deposits
dated 15.9–15.7 Ma, which contain elements ofboth the Daijima-type and Aniai-type ﬂoras(Uemura et al., 2006).
In terms of marine conditions, molluscs typical to the temperate oceanic conditions ﬂourishedafter MNCO (Ogasawara et al., 2008). Molluscan fossils co-occurring with the Yeonil Flora(i.e., Yeonil Fauna) contains both deep sea andshallow marine mollusks and the latter indicate awarm-temperate marine climate (Ogasawara,
personal communication with AY). The expansion of temperate molluscs was considered tohave started around 14 Ma (Suzuki, 2003).
Stage IV, represented by the Mitoku-type
ﬂoras, indicates a more temperate climate condition. Equivalent pollen zone NP-4 is characterized by the dominance of Fagus and Taxodiaceae
and a sharp decline of Carya and Liquidambar
pollen grains, indicating much cooler climate
conditions. It probably represents a cooling trend
known to occur in the Late Miocene and thereafter (Zachos et al., 2001).
Floral succession recognized in North Korea
and southern Primorye, on the other hand, are notconcordant with that in Japan and South Korea.
Although there is a trend of climate warmingfrom Stages II to III and probably between
Stages I and II, Miocene climate changes inthose areas appear to be gentle. Stage II Nezhino
and Stage III Khanka ﬂoras are both enriched intemperate elements, and the reconstructed temperature value also supports a cool-temperate climate similar to the Stage I assemblage in Japanand South Korea or the Stage II Yoshioka Florain southwest Hokkaido. The Stage IV Ust-Suifun
assemblages also show temperate aspects. However, the absence of beech species as well as thepaucity of conifers probably indicates slightly
colder and dryer conditions than the preceding
one. Therefore, a distinct warming period was
not recorded on the continental side.
Floristic similarities between the continental

118

Boris I. Pavlyutkin et al.

side and the Japan side started to be disrupted
prior to Stage III. Floras in Stage III and IV insouthern Primorye consist more of endemic elements. Moreover, a strong difference in ﬂoristic
composition between the Yeonil and Hamjindong
ﬂoras were observed, even though they are foundin coeval strata. The Tongchon Flora, geographically located between the above two assemblages, is also enriched in temperate elements.
Thus, this may represent a latitudinal temperature gradient during that time. However, the latitudinal temperature gradient estimated based on
the Daijima-type ﬂoras of Honshu Island inJapan is much lower (Tanai, 1991). Besides, theﬂoristic gap was detected around northernmost
Honshu (Tanai & Uemura, 1988). Consideringthat there is no tectonic distinction among theseareas, the inﬂuence of extra-terrestrial effects
should be considered.
Based on marine diatom assemblages from thesediments in the Sea of Japan, Tsoy & Shastina(1999) reported the prevalence of warm-water
low-latitude species in the southeast part of the
Sea of Japan, which is distinct from those in thenorthwestern part, where cold-water species
dominated. Similarly, molluscan assemblage
from the Hamjindong Formation is dominated byboreal genera (Yoldia, Portlandia, Mya) with therare occurrence of subtropical genera (Macoma)(Ablaev et al., 1990). It shows a strong contrast
not only with the Ryeongryukdong fauna (Makiyama, 1936) from the underlying Ryeongryukdong Formation but also with those from theDuho Formation. Therefore, the inﬂuence of thecold current, which resembles the modern day
cold current, i.e., the Liman Cold Current, on
northernmost Honshu, Hokkaido, and the coastal
area of the continent, and the effect of a warm
oceanic current on Honshu and the southern
Korean Peninsula were suggested as major contributing factors that helped to develop the terrestrial vegetation of each area.
Cause of the coastal-inland distinctions
The contrast between the ﬂoral assemblages
northwest (southern Primorye and North Korea)

and southeast (Japanese Islands and South
Korea) of the Sea of Japan became evident from
Stage III and were more obvious in Stage IV
assemblages. This can be explained by the geographic separation of the Japanese Islands due tothe opening of the Sea of Japan since the early–
middle Miocene and subsequent diversiﬁcationof local ﬂora. Pavlyutkin & Golozoubov (2010)
inferred the effect of the winter monsoon on thisﬂoral contrast in association with a strong warm
oceanic current that came northeastward acrossthe Sea of Japan since MNCO. The onset of
monsoonal climate occurred as early as the late
Eocene (Wang et al., 2013). Most scientists,
however, conﬁrmed that the intensiﬁcation of amonsoonal climate occurred later than the Late
Miocene (Jacques et al., 2012). If the diversity of
Khanka Flora (165 species) and the dominanceof beech species in the assemblage are taken into
considerations, the effect of monsoonal climate,i.e., low mean annual temperature, high summer
precipitation and low winter precipitation, seems
unlikely. Rather, it supports a more humid andrelatively warmer conditions than modern day
Primorye, which is in line with the suggestionthat is based on physiognomic analysis. Jacqueset al. (2012) discussed that the xeric vegetationin northeast China started to develop when thewinter monsoon became stronger during the late
Miocene to Pliocene. Interestingly, there is nostrong sign of a mid-Neogene climatic optimum
that can be recognized in the interior of China.
Therefore, peculiar features of the Khanka Floracan probably be explained by the distance fromthe coast. Whether the Stage IV Ust-Suifun ﬂoras
represent the effect of intensiﬁed monsoonal climate needs to be tested based on further study
because pollen grains of beech species are abundant in the upper horizon of this stage.
Implications for the period of Japan Sea formation
The rifting and opening of the Sea of Japanbasin caused a key geographic barrier between
continental East Asia and Japan. It may have further inﬂuenced the regional climatic conditions

Miocene ﬂoral changes in the circum-Japan Sea Areas

119

Fig. 3. Dynamics of the Sea of Japan opening (modiﬁed and supplemented after Golozoubov, 2006). A, the early
Paleogene (ca. 50 Ma). B, the late early Miocene (ca. 20 Ma). C, the early middle Miocene (ca. 15 Ma). 1: PreCenozoic terranes. 2: areas of the newly formed oceanic crust. 3: the Okhotsk Sea Plate. 4: the Paciﬁc Plate. 5:
the axial part of the Izu–Bonin arc. 6: strike-slip fault zones, including HSF (Hokkaido–Sakhalin faults), TnF(Tanakura fault), FMF (Fossa-Magna faults), TsF (Tsushima fault). 7: extension axes. 8: directions of plate
displacements. 9: subduction zones. 10: directions of block rotation. Letters in circles show underwater rises:
Ya, Yamato; EK, East-Korean; Kr, Krishtofovich; K, Kita-Oki.

and lead to the distinct vegetation of each area. A
detailed correlation of Miocene ﬂoras in thisstudy revealed that its effect became obvious
during Stage III.
The Japan Sea basin was formed by the clockwise rotation of southwest Japan and counterclockwise rotation of northeast Japan during theshort time period between the late early to early
middle Miocene (i.e., 15–17 Ma: Otofuji, 1996).
Jolivet & Tamaki (1992) discussed that the opening of the Sea of Japan could have started ca. 25
million years ago based on the oldest sedimentary rocks studied during deep-water drilling in
the newly formed depressions of the Sea of Japan(up to 20 Ma). Earlier rifting has often been proposed (Kano et al., 2002, 2007). In order toaccept that the pull-apart sedimentary basins,
which are located along the Strait of Tatar (Golozoubov et al., 2012), Sikhote-Alin (Golozoubov
et al., 2007, 2009), south of the Korean Peninsula (Martynov et al., 2006), were formed simul-

taneously with the major depressions of the Sea
of Japan, the origin and development of the basin
could be as early as the early Eocene, as long as
it is not younger than 50 Ma.
However, late Paleogene ﬂoras in Primorye
are quite similar to those found in the JapaneseIslands. The two areas have many common species and climate change trends inferred from theﬂora in these areas are parallel to one another(Tanai & Uemura, 1991a, b, 1994; Ablaev, 2000;
Pavlyutkin, 2007), so that a geographic distinction between these areas during that time isunlikely. Thus, two major stages were distinguished in the course of the development of the
Sea of Japan basin. The ﬁrst stage was settled
when the Japanese Islands separated from themainland. A series of pull-apart intra-continental
basins were formed (Fig. 3A & B). The separate
blocks between the basins were close to eachother, as well as to the continental side and to theJapan side. This may provide the preservation of

Boris I. Pavlyutkin et al.

120

ﬂoristic links. It is worthy to note that fragments,
which have broken off the continent during their
movement, did not undergo signiﬁcant rotationof each block at this time period. At the second
stage, the further rather fast drift of Japanoccurred within the limits of a so-called twodoor model by Otofuji (1996) with considerable
anticlockwise rotation of northeast Japan and theclockwise rotation of southwest Japan (Fig. 3B &C). Since this time period, Japan and the continent have been separated by vast water spaces
and it does not only provide geographic separation but also climatic differentiation between theisland and the continental sides of the Sea ofJapan, which leads to the distinction of forest
vegetation in these areas.
Acknowledgments
We would like to express sincere gratitude to
Dr Kazumi Yokoyama for his advice and help
with the ﬁeld survey. Thanks are also due to Dr
Kazuhiko Uemura for his discussion and reading
earlier versions of the manuscript. Dr Kenshiro
Ogasawara gave suggestions on the molluscan
fossils from the Korean Peninsula.
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中新世における環日本海植物化石群の変化―古気候変化と日本海拡大の影響に注目してPavlyutkin B. I.・矢 部

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淳・V. V. Golozoubov・L. F. Simanenko

ロシア沿海州南部，韓半島および日本列島に分布する中新世植物化石群集を層位的に対比し，同地域における中新世の森林植生変化と推測される古気候変化を明らかにするとともに，日本海の拡大による植生への影響を議論した．全地域において前期中新世から前期／中期中新世境界への温暖化とその後の寒冷化の傾向が確認できたが，南部と北部とで温暖化の規模に違いがあることが明らかとなった．また，中期中新世以降，各地の化石群集に地域性が現れ，後期中新世にさらに顕著となった．これらの現象は日本海の拡大にともなう地理的分断の影響に加え，大陸沿岸を洗う寒流と日本列島沿岸を流れる暖流の影響をうけた可能性が推測された．