Gallagher et al. Progress in Earth and Planetary Science (2015) 2:17
DOI 10.1186/s40645-015-0045-6

REVIEW

Open Access

The Pliocene to recent history of theKuroshio and Tsushima Currents:
a multi-proxy approach
Stephen J. Gallagher1*, Akihisa Kitamura2, Yasufumi Iryu3, Takuya Itaki4, Itaru Koizumi5,6 and Peter W. Hoiles1,7
Abstract
The Kuroshio Current is a major western boundary current controlled by the North Pacific Gyre. It brings warm
subtropical waters from the Indo-Pacific Warm Pool to Japan exerting a major control on Asian climate. The
Tsushima Current is a Kuroshio offshoot transporting warm water into the Japan Sea. Various proxies are used todetermine the paleohistory of these currents. Sedimentological proxies such as reefs, bedforms, sediment sourceand sorting reveal paleocurrent strength and latitude. Proxies such as coral and mollusc assemblages reveal past
shelfal current activity. Microfossil assemblages and organic/inorganic geochemical analyses determine paleo- sea
surface temperature and salinity histories. Transportation of tropical palynomorphs and migrations of Indo-Pacificspecies to Japanese waters also reveal paleocurrent activity. The stratigraphic distribution of these proxies suggests
the Kuroshio Current reached its present latitude (35 °N) by ~3 Ma when temperatures were 1 to 2 °C lower thanpresent. At this time a weak Tsushima Current broke through Tsushima Strait entering the Japan Sea. Similar
oceanic conditions persisted until ~2 Ma when crustal stretching deepened the Tsushima Strait allowing inflow
during every interglacial. The onset of stronger interglacial/glacial cycles ~1 Ma was associated with increased
North Pacific Gyre and Kuroshio Current intensity. This triggered Ryukyu Reef expansion when reefs reached theirpresent latitude (~31 °N), thereafter the reef front advanced (~31 °N) and retreated (~25 °N) with each cycle.
Foraminiferal proxy data suggests eastward deflection of the Kuroshio Current from its present path at 24 °N into
the Pacific Ocean due to East Taiwan Channel restriction during the Last Glacial Maximum. Subsequently Kuroshio
flow resumed its present trajectory during the Holocene. Ocean modeling and geochemical proxies show that theKuroshio Current path may have been similar during glacials and interglacials, however the glacial mode of thiscurrent remains controversial. Paleohistorical studies form important analogues for current behavior with futureclimate change, however, there are insufficient studies at present in the region that may be used for this purpose.
Modeling of the response of the Kuroshio Current to future global warming reveals that current velocity may
increase by up to 0.3 m/sec associated with a northward migration of the Kuroshio Extension.
Keywords: Kuroshio Current; Tsushima Current; Pliocene; Pleistocene; Holocene; Paleoceanographic proxies;
Biogeography; Microfossils; Macrofossils; Geochemistry* Correspondence: sjgall@unimelb.edu.au
1
School of Earth Sciences, The University of Melbourne, Melbourne, Victoria
3010, Australia
Full list of author information is available at the end of the article© 2015 Gallagher et al. This is an Open Access article distributed under the terms of the Creative Commons Attribution License(http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium,provided the original work is properly credited. The Creative Commons Public Domain Dedication waiver (http://
creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.

Gallagher et al. Progress in Earth and Planetary Science (2015) 2:17

Review
Boundary currents and ocean gyres are important elements in heat transport on Earth; they fundamentallycontrol the Earth’s climate. The Kuroshio Current is amajor western boundary current in the western Pacific
Ocean; it plays a key role in distributing heat from thetropics to the mid-latitudes and has been an important
control on the climate of Northeast Asian for millions ofyears. This paper reviews evidence for the Pliocene torecent history of the Kuroshio Current and the Tsushima
Current.
Oceanographic setting

The Kuroshio meaning “black stream” Current plays a
key role in north Pacific circulation (Barkley 1970). Its“blackness” is related to the fact that it is derived fromnutrient- and sediment-deficient subtropical North
Pacific waters (Qiu 2001). Even though it occupies lessthan 0.1 % of the area of that ocean, it transports between 20 and 130 Sverdrups (1 Sverdrup (Sv) is 106 m3/s)
of warm (16 to 28 °C) relatively high salinity water at
speeds up to 130 cm/s (Qiu 2001; Wei 2006; Hsin et al.
2008). It travels northwards via a narrow band (<100 km
wide and <1 km thick) that ultimately ends in the northern Pacific Ocean (Fig. 1). It plays a key role in modulating
the climate of the northwest Pacific Ocean as it advects
significant heat from the Indo-Pacific Warm Pool to theregion forming the western arm of the North Pacific Gyre
(Sawada and Handa 1998).

Page 2 of 23

The Kuroshio Current can be divided geographicallyinto three regions: (1) the source region upstream of
Tokara Strait, (2) south of Japan, and (3) the downstream Kuroshio Extension (Qiu 2001). It also bifurcates
and travels into the Japan Sea via the Tsushima Strait.
1. The source region: The Kuroshio Current is anoffshoot of the North Equatorial Current thatoriginates east of the Philippines at around 12°–13 °N(Fig. 1) where flow volume reaches 19 to 30 Sv (Qiu
2001). Occasionally, it intrudes into the Luzon Strait(21 °N, 121 °E) south of Taiwan; thereafter, it crosses
the Yonaguni Depression via the East Taiwan Channel
into the East China Sea (Fig. 2) and follows the edgeof the continental shelf in the Okinawa Trough withflow volumes ranging from 22 to 27 Sv (Qiu 2001). It
flows along the western part of the Okinawa Troughand leaves the continental slope via the Tokara Strait(Fig. 2) reaching the southern island of Japan at ~30 °N.
Here, the current bifurcates with one main branchpassing through the Tsushima Strait (where it feeds
the Tsushima Current in the Japan Sea, Fig. 3) and theother flowing northwards into the northwest Pacific
Ocean.
2. South of Japan: as it crosses the Tokara Strait,
Kuroshio flow volume increases from 30 to 55 Sv
where the current is represented by the meanannual sea surface 16 °C isotherm (Qiu 2001). The
net increase in flow rate may be due to anticyclonic

Fig. 1 The surface oceanography of the Northwest Pacific Ocean adapted from Gallagher et al. (2009). The 200 m shelf edge bathymetric contour
is indicated. The mean annual isotherm values are from Locarnini et al. (2006). The North Pacific currents are from Inoue (1989), Tomczak andGodfrey (1994), Qiu (2001), and Tada et al. (2014) ME Mindanao Eddy, HE Halmahera Eddy

Gallagher et al. Progress in Earth and Planetary Science (2015) 2:17

Page 3 of 23

Fig. 2 The southern path of the Kuroshio Current showing the inferred extent of the Ryukyu Reefs in glacial and interglacial periods (adaptedfrom Iryu et al. 2006)

gyre to its south or an occasional northeast flowing
current. It follows several different meander paths(Qiu 2001) south of Honshu Island (Fig. 3).
3. Downstream Kuroshio Extension: At 35 °N and 140 °E,
the Kuroshio leaves the Japanese coast and flows
eastward into the North Pacific Ocean as theKuroshio Extension (Fig. 1). Flow volume in thisextension can reach up to 130 Sv due to the existenceof strong recirculation gyres to its south (Qiu 2001).
The Tsushima Current: The warm saline waters of theKuroshio Current mix with freshwater in the East China
Sea south of Japan (Qiu 2001) before flowing into theJapan Sea via the Tsushima Strait as the Tsushima Current(Figs. 1 and 3). The Japan Sea is a semi-enclosed marginal

sea with an area of average depth of 1350 m and area of
1,000,000 km2. The sea is connected to the East China Sea
through the Tsushima Strait, to the Pacific Ocean via the
Tsugaru Strait, and to the Okhotsk Sea through the Soyaand Mamiya straits (Fig. 3). All straits are narrower than
160 km and shallower than 130 m deep. Today, the only
oceanic water flowing into the Japan Sea is the warm
Tsushima Current. The current in the Tsushima Strait
reaches surface velocities ranging from 0.3 to 0.4 m/s (upto 1 m/s during storms, Katoh et al. 1996) and flows
northward along the western coast of Honshu Island. The
volume of flow in the Tsushima Strait ranges from 2.6 to
1.1 Sv (Takikawa and Yoon 2005). Since the majority of
the current flows out through the Tsugaru Strait to the
Pacific Ocean, sea surface temperature (SST) decreases

Gallagher et al. Progress in Earth and Planetary Science (2015) 2:17

Page 4 of 23

Fig. 3 The path of the Tsushima and Kuroshio Current and Extension around Japan. Tsushima current nomenclature (TWC-1 to 3) is from Tada
et al. (2014). The position of the Kuroshio meanders is adapted from Kubo et al. (2004). Base map adapted from General Bathymetric Chart of the
Oceans (GEBCO) www.gebco.net. DSDP Site 302 is indicated BP Boso Peninsula, JJ Jeju Island, JO Joetsu, KI/IR Kozu-shima Island/Izu Ridge, MS
Mamiya Strait, OK Oki Ridge/ODP Site 798, OI Oshima Island, OS Osami Strait, SI Sado Island, SS Soya Strait, TK Tohoku, TSG Tsuguru Strait

significantly around the strait. This is represented by amarine biogeographical boundary southwest coast of
Hokkaido (Horikoshi 1962).
North of Tsugaru Strait, the remainder of the Tsushima
Current flows northward along the western coast of
Hokkaido Island (Fig. 3). A part of the current flows out
through the Soya Strait to the Okhotsk Sea, while the remainder reaches the northern Japan Sea and becomes
dense enough to sink to the bottom due to cooling andfreezing (Suda 1932; Moriyasu 1972; Gamo and Horibe1983). The sunken water mass is known as the Japan
Sea Proper Water (JSPW) and has a low temperature(0.1–0.3 °C), low salinity (34), and high dissolved oxygen

concentration (>210 μmol/kg). The deep-water region
below the Tsushima Current is occupied by cold-water
masses such as Japan Sea Intermediate Water (JSIW)
and JSPW (Suda 1932; Moriyasu 1972). The thermocline
between the Tsushima Current and JSIW is at about
150–160 m depth on the west side of Hokkaido. Benthic
and nekton faunas change at this depth (Ogata 1972;
Nishimura 1973, 1974; Tsuchida and Hayashi 1994;
Kojima et al. 2001; Iguchi et al. 2007).
Proxies for the Kuroshio and Tsushima Currents

Modern biogeography and sedimentologic patterns in
the Northwest Pacific Ocean are often directly related toGallagher et al. Progress in Earth and Planetary Science (2015) 2:17

the Kuroshio and Tsushima Current flow. Here, we
focus primarily on the sedimentary features and biogeographic patterns that are likely to be preserved in thefossil record allowing the geological history of theseoceanic features to be determined. We include the following fossilizable groups: macrofossils—corals and molluscs (bivalves and gastropods) and microfossils—
diatoms, radiolarians, nannoplankton, ostracods, palynomorphs, and foraminifera. We add a summary of various
geochemical proxies that may be used to determine“paleo” temperature and salinity of sea water that indicate Kuroshio and Tsushima Current activity in the past.
Sedimentary features
Bedforms and sediment sorting

The high-velocity Kuroshio Current (up to 130 cm/s)
creates characteristic directional bedforms and wellsorted sand deposits in the shelfal regions (<400 m
depth) around Japan. Sand streamers, sand waves, current lineations, and megaripples show ocean current directions south of Kyushu in the Osumi Strait (OS: Fig. 3:
Ikehara 1989). Near Kozu-shima Island on the Izu Ridge(KI/IR: Fig. 3), Kuroshio Current velocities range from
0.3 to 0.5 m/s (up to 3 m/s) causing directional bedforms to form sand dune fields at depths between 200
and 400 m (Kubo et al. 2004). Elsewhere on the Pacificside of Japan, medium to large subaqueous sand dunes
are propagated by oceanic currents at depths typicallyless than 100 m (Ikehara and Kinoshita 1994). In theJapan Sea, the southern branch of Tsushima Current
generates directional asymmetric bedforms several meters high in shelf areas (<70 m water depth) in the
Tsushima Strait (Fig. 3: Nishida and Ikehara 2013).

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Islands (Yamamoto et al. 2006) leading to a highly diverse hermatypic coral faunas in the region (Veron 1992;
Veron and Minchin 1992). Typically, the present global
latitudinal limit of coral reef development is constrainedby the 18 °C winter maximum sea surface isotherm(Kleypas et al. 1999). However, small-scaled reefs havebeen found 2° north of this isotherm in the Tsushima
Strait at 33° 48′ N representing the highest latitude reefs
in the world (Yamano et al. 2001, 2012). Hermatypiccoral diversity decreases northwards from over 400 species around Yaeyama Islands near Taiwan at ~24 °N(where the Kuroshio Current originates) to 24 species at~35 °N (Tateyama, Boso Peninsula) where the Kuroshio
Extension meets the Polar Front (Fig. 3; Veron 1992;
Veron and Minchin 1992; Tsuchiya et al. 2004). Twentyfour species (Table 1) are present along the full path of
the “main” Kuroshio Current (Veron 1992; Tsuchiya
et al. 2004). Four species of corals are present in the
Tsushima Strait (Table 1; Yamano et al. 2001, 2012).
Faunal and floral biogeography

This section outlines modern biogeographic indicators(Tables 1 and 2) of the Kuroshio and Tsushima currents.
Macrofossils

Most macroscopic faunal elements such as benthic molluscs and corals dominate the shelfal regions (<200 m)of the Northwest Pacific Ocean; as such, their geologicalutility as paleocurrent indices is primarily limited to
strata formed at these shallow depths. The key IndoPacific coral species that indicate the Kuroshio Current
have been described in the “Coral reefs” section; this section reviews the modern molluscan biography in thearea.

Transported palynomorphs

Transported palynomorph assemblages may be used todetect past Kuroshio Current activity. The tropical
podocarp genus Phyllocladus is not present in Japan
today; however, its presence in the fossil record off
Kyushu suggests transportation by the Kuroshio
Current from the Indo-Pacific Warm Pool (Kawahata
and Ohshima 2004). Fluctuations in abundance of boreal sourced conifer pollen versus Ephedra from the
Asian continent and other flora transported by the
Kuroshio from the East China Sea have been used todetermine paleolatitudinal variations in the Kuroshio
Extension (Kawahata and Ohshima 2002); however,these Ephedra may also be windblown.
Coral reefs

The Kuroshio Current transports coral larvae and heatfrom the tropics to the southern half of mainland Japan(Iryu et al. 2006). It extends luxuriant coral reef development to ~31 °N (south of Kyushu, Fig. 2) in the Ryukyu

Molluscs (bivalves and gastropods) The planktic
janthinid gastropod genus Hartungia is a well-known
Kuroshio Current indicator (Tomida and Kitao 2002;
Tomida et al. 2013). Several species are present along
the full path of the “main” Kuroshio Current (Table 1)
including Stellaria exutum, Cucullaea labiata, Jupiteria
confuse, Cryptopecten vesiculosus, Cycladicama cumingi,
Paphia amabilis, Paphia schnelliana, and Venus foveolata (Nobuhara 1993a, b). Molluscan species that areused as a proxy for the Tsushima Current are taxa thatlive in the area south of the southwestern Hokkaido, andlive at the area shallower than 150–160 m depth off
San-in and Hokuriku (Fig. 3) including Zeuxis castus,
Stellaria exutum, Jupiteria gordonis, Crenulilimopsis
oblonga, Cycladicama cumingi, and Paphia schnelliana(Kitamura and Ubukata 2003). These species are present
in the Pacific coast area south of Boso Peninsula at
35 °N (BP: Fig. 3). Cold-water species (Acila insignis,
Yoldia notabilis, Glycymeris yessoensis, Felaniella usta,

Gallagher et al. Progress in Earth and Planetary Science (2015) 2:17

Table 1 Biogeographic proxies for the Kuroshio and Tsushima
Currents
Kuroshio Current indicators

Tsushima Current indicators

Microfossils
Planktic foraminifers

Planktic foraminifers

Page 6 of 23

Table 1 Biogeographic proxies for the Kuroshio and Tsushima
Currents (Continued)
Lithelius minor

Monozonium pachystylum

Spongaster tetrars

Octopyle stenozona

Spongosphaera streptcantha

Pseudocubus obeliscus

Beella digitata

Globigerinoides ruber

Stylodictya arachnia

Spongosphaera streptcantha

Globigerinella calida

Neogloboquadrina
pachyderma (Dextral)

Stylodictya multispina

Tetrapyle octacantha

Globigerinoides ruber

Pulleniatina obliquiloculata

Molluscs (gastropods and bivalves)

Globigerinoides sacculifer
Globigernioides conglobatus
Globorotalia hirsuta
Globorotalia menardii
Pulleniatina obliquiloculata
Sphaeroidinella dehiscens s.l.
Indo-Pacific benthic foraminifers

Indo-Pacific benthic foraminifers

Asterorotalia gaimardii

Asterorotalia gaimardii

Asterorotalia spp.

Asterorotalia spp.

Heterolepa margaritiferus

Heterolepa margaritiferus

Nannoplankton

Calcidiscus leptoporous

Calcidiscus leptoporous

Ceratolithos cristatus

Gephyrocapsa oceanica

Florisphaera profunda/profunda +Emiliania huxlei + Geophyrocapsa
oceanica (F-EG ratio)

Helicosphaera carteri

Jupiteria gordonis

Hartungia spp.

Paphia schnelliana

Jupiteria confuse

Stellaria exutum

Paphia amabilis

Zeuxis castus

Paphia schnelliana
Stellaria exutum
Venus foveolata
CoralsAcropora pruinosa

Cyphastrea spp.

Acropora solitaryensis

Echinophyllia spp.

Alveopora japonica

Favia speciosa

Blastomussa wellsi

Hydnophora spp.

Cyphastrea chalcidicum
Cyphastrea microphthalma

Rhabdosphaera clavigera

Cyphastrea serailia

Thoracosphaera spp.

Echinophyllia apera

Umbellosphaeara spp.

Euphyllia ancora

Umbilicosphaera siboga foliacea

Favia speciosa
Diatoms (see Table 2)

Ostracods

Goniapora columna
Goniapora lobata

Argiloecia hanaii

Bradylyea

Cytheropteron rhombea

Cytheropteron miurense

Cytheropteron uchioi

Hirsutocythere? hanaii
KritheMacrocypris decola
Neonesidea oligodentata s.l.
Schizocythere kishinouyei

Radiolaria

Cycladicama cumingi

Cycloseris cyclolites

Oolithus fragilis

Neonesidea oligodentata

Cycladicama cumingi

Cyloseris hexagonalis

Helicosphaera carteri

Munseyella japonica

Crenulilimopsis oblonga

Cucullaea labiata

Caulastreae tumida

Gephyrocapsa oceanica

Diatoms (see Table 2)

Molluscs (gastropods and bivalves)

Cryptopecten vesiculosus

CoralsPseudorotalia indopacifica
Nannoplankton

Macrofossils

Radiolaria

Acrosphaera spinosa

Dictocoryne profunda

Dictocoryne profunda

Dictocoryne truncatum

Dictocoryne truncatum

Didymocyrtis tetrathalamus

Didymocyrtis tetrathalamus

Euchitonia flucata

Hydnophora exesa
Leptastreae purpurea
Leptoseris mycetoseroides
Oulastrea crispata
Pavona varians
Pleisastrea versipora
Porites heronensis
Psammocora profundacellaPsammocora superficialis
Sourced from Chinzei et al. (1987), Veron (1992), Kuroyanagi and Kawahata(2004), Tsuchiya et al. (2004), Yamano et al. (2001, 2012), Gallagher et al.
(2009), Hoiles et al. (2012), and Tada et al. (2015). A list of diatom proxies is
included in Table 2

Gallagher et al. Progress in Earth and Planetary Science (2015) 2:17

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Table 2 Diatom proxies for sea surface temperature in the northwest PacificTd′
Warm-water species

Cold-water species

Actinocyclus ellipticus Grunow

Actinocyclus curvatulus JanischA. elongatus Grunow

A. ochotensis Jousé

Alveus marinus (Grunow) Kaczmarska & Fryxell

Asteromphalus hyalinus Karsten

Asterolampra marylandica Ehrenberg

A. robustus Castracane

Asteromphalus arachne (Brebisson) Ralfs

Bacterosira fragilis Gran

A. flabellatus (Brebisson) Greville

Chaetoceros furcellatus Bailey

A. imbricatus Wallich

Coscinodiscus marginatus Ehrenberg

A. pettersonii (Kolbe) Thorrington-Smith

C. oculus-iridis Ehrenberg

A. sarcophagus Wallich

Fragilariopsis cylindrus (Grunow) Krieger

Azpeitia africanus (Janisch) Fryxell & Watkins

F. oceanica (Cleve) Hasle

A. nodulifera (Schmidt) Fryxell & Sims

Neodenticula seminae (Simonsen & Kanaya)
Akiba & Yanagisawa

A. tabularis (Grunow) Fryxell & Sims

Porosira glacilis (Grunow) Jorgensen

Fragilariopsis doliolus (Wallich) Medlin & Sims

Rhizosolenia hebetata (Bailey) Gran

Hemidiscus cuneiformis Wallich

Thalassiosira gravida Cleve

Nitzschia interruptestriata Simonsen

T. hyalina (Grunow) Gran

N. kolaczekii Grunow

T. kryophila (Grunow) Joergensen

Planktoniella sol (Wallich) Schütt

T. nordenskioldii Cleve

Pseudosolenia calcar-avis (Schültze) Sundstrom

T. trifulta Fryxell

Rhizosolenia acuminata (Peragallo) Gran
R. bergonii Peragallo
R. hebetata (Bailey) Gran f. semispina (Hersen) Gran
R. imbricata Brightwell
Roperia tessellata (Roper) Grunow
Thalassiosira leptopus (Grunow) Hasle & Fryxell
T. oestrupii (Osterfeld) Proshkina-Lavrenko
Twt
Xw of Twt

Xt of Twt

Xc of Twt

Warm-water species without Thalassiosira oestrupii of Td′

Coscinodiscus radiatus Ehrenberg

Cold-water species of Td′

Fragilariopsis fossilis (Frenguelli) Medlin & Sims

Thalassionema nitzschioides Grunow s.l.

Actinocyclus oculatus Jousé

F. reinholdii (Kanaya emend Barron & Baldauf) Zielinski& Gersonde

Thalassiosira oestrupii (Osterfeld)
Proshkina-Lavrenko

Neodenticula kamtschatica (Zabelina)
Akiba & Yanagisawa

Nitzschia jouseae Burckle

N. koizumii Akiba & Yanagisawa

N. miocenica Burckle

Proboscia barboi (Brun) Jordan & Priddle

Rhizosolenia praebergonii Mukhina

P. curvirostris (Jousé) Jordan & Priddle

Thalassiosira convexa Mukhina

Thalassiosira nidulus (Tempère & Brun) Jousé

T. miocenica Schrader
T. praecovexa Burckle
The list includes diatoms used to calculate Td′ = SST (Koizumi 2008) and the “paleoclimate curve” Twt (Barron 1992)

Megangulus zyonoensis, and Mercenaria stimpsoni; Kitamura et al. 1994) are those living in the area north of
the southern Hokkaido and/or deeper than 150–160 m
depth off Sanin and Hokuriku (Fig. 3). These species arepresent north of 35 °N on the Pacific coast.

Microfossils

Microscopic floral and faunal elements are abundant
in deep and shallow water sediment around Japanand the East China Sea. Similar to the mollusc andcoral assemblages, most benthic microbiota (benthic

Gallagher et al. Progress in Earth and Planetary Science (2015) 2:17

foraminifera and ostracods) are confined to the sea
bed in shelfal regions (<200 m) whereas other planktic
forms (diatoms, radiolarians, nannoplankton, planktic
foraminifera, and ostracods) are widely distributedin oceanic and shelfal sediment and are often key
fossil Kuroshio or Tsushima Current indices (Tables 1
and 2).
Diatoms

Diatom paleoceanographic proxies have been statistically determined by comparing the distribution of diatom species in the surface sediments to factors suchas primary production, SSTs (°C), salinity, and otherphysical–chemical parameters in modern surface waters (Kawai 1972, 1991; Yasuda et al. 1996; Masujima
et al. 2003). The diatom temperature (Td) ratio was
proposed by Kanaya and Koizumi (1966) to estimate
sea surface water temperatures: Td = [Xw/(Xw + Xc)] ×100, where Xw is the frequency of warm-water species
and Xc is that of cold-water species. Td ranges invalue from 0 to 100. Going from the subarctic totropical regions, Td becomes systematically larger,
showing a positive correlation with sea surface watertemperature over a given geographical site. Koizumi
et al. (2004) redefined the Td ratio as Td′ ratio, applied strictly to oceanic holoplanktonic association: Td′ = [(Xw + XW)/(Xw + XW + Xc + XC)] × 100,
where
Xw and Xc, excluding Biddulphia (=Odontella) aurita,
as originally proposed by Kanaya and Koizumi (1966)
and XW and XC are supplemental species. In order tocalibrate the Td′ ratio for paleo-temperatures, Koizumi (2008) performed regression analysis between
the Td′ ratio (Table 2) in 123 surface sediment samples around the Japanese Islands in the northwest Pacific and the mean annual SSTs (°C) at the core sites.
Equations to derived annual SST (°C) from Td′ ratio
are different for the Tohoku area (y = 6.5711 × Td′0.273,
R = 0.89946) than for the Japan Sea (y = 5.4069 × Td′
26841, R = 0.89088). The annual paleo-SSTs (°C) in the Tohoku area were in general higher than those in the JapanSea despite lower Td′ ratio because the warm-water species Fragilariopsis doliolus is abundant only in the Tsushima Warm Current in the Japan Sea. Td′derived
annual paleo-SSTs (°C) agree with the δ18O of planktonic
foraminiferal and UK’37 (an organic biomarker, see geochemical proxies below)-derived summer paleo-SSTs (°C)
at sites off central Japan. Td′-SSTs (°C) fluctuate on
centennial-to-millennial timescale, indicating a strong andregular flow of the Kuroshio–Kuroshio Extension andTsushima Current during the Plio–Pleistocene. The Td′-SSTs (°C) in the Tohoku area off the northeast part ofHonshu Island (Fig. 3) are generally higher than those in
the Japan Sea despite lower Td′ values because the warmwater species Fragilariopsis doliolus is abundant only inPage 8 of 23

the Tsushima Current in the Japan Sea. Those fluctuations
are synchronous with abrupt climate events reported fromthe different paleoclimatic proxy records in many regionsof the Northern Hemisphere (Tada et al. 1999; Koizumi and
Sakamoto 2010; Koizumi and Yamamoto 2011). The Twt
ratio or “paleoclimate curve” was created to interpret the
Pliocene paleoclimatic changes in the northwest Pacific
Ocean (Barron 1992). In the equation Twt = (Xw + 0.5Xt)/(Xc + Xt + Xw), Xw is the total number of subtropical totropical (warm-water) species. Xt is the total number ofwarm transitional species (Thalassionema nitzschioides,
Thalassiosira oestrupii, and Coscinodiscus radiatus) and
Xc is the total number of subarctic to arctic (cold-water)
species. Xw and Xc include such extinct Pliocene species
as Fragilariopsis fossilis, Fragilariopsis reinholdii, Nitzschia
jouseae, and Thalassiosira convexa as Xw, and Neodenticula kamtschatica and Neodenticula koizumii as Xc. Koizumi (Koizumi and Sakamoto 2012, 2013) added
associated species and created the Td′ ratio of Koizumi(2008). This included additional extinct Pliocene species tothose listed by Barron (1992) such as Nitzschia miocenica,
Rhizosolenia praebergonii, Thalassiosira miocenica, andThalassiosira praeconvexa as Xw and Actinocyclus oculatus, Proboscia curvirostris, and Thalassiosira nidulus as
Xc. Thalassiosira oestrupii is converted from warm-water
species in Td′ ratio to Xt in Twt because T. oestrupii prefers a relatively cooler environment (e.g., Ren et al. 2014).

Radiolarians

Tropical and subtropical radiolarian species in the North
Pacific Ocean (Nigrini 1970; Lombari and Boden 1985;
Yamauchi 1986; Motoyama and Nishimura 2005; Kamikuri et al. 2008) and the Japan Sea (Itaki 2003;
Motoyama et al. submitted) are Kuroshio and Tsushima
Current indicators. Nigrini (1970) reported distributionsof 37 radiolarian species in 83 Holocene sediments
widely collected from the North Pacific and classified
into 6 recurrent groups which reflect surface circulation
and upper water masses. Furthermore, she proposed Tr
value as a climatic index based on cold, transition, andwarm groups defined as the following equation:
Tr ¼ Xt=ðXt þ Xc þ XwÞ  100
Where Xw, Xt, and Xc indicate relative abundance of
warm, transition, and cold water groups, respectively.
Yamauchi (1986) analyzed radiolarian assemblages insurface sediments from the northwestern Pacific along
the Kuroshio and Oyashio Current paths and recognized
three faunal groups relating to tropical, subtropical, andsubarctic waters. He also proposed a climatic index SR
value as the following equation based on each fauna;Gallagher et al. Progress in Earth and Planetary Science (2015) 2:17

SR ¼

Σ3k¼1 ðS k ⋅nk Þ
Σ3k¼1 nkWhere
k = 1: subarctic faunak = 2: subtropical faunak = 3: tropical fauna
nk: number of specimens for each group
The SR value shows a close correlation with annual
mean temperature at 50 to 200 m water depths.
In the northern part of the East China Sea, Changet al. (2003) distinguished three radiolarian assemblages
related to oceanographic patterns in the region. The
mixed water assemblage is dominated by Tetrapyle circularis, Tetrapyle quadriloba, and Didymocyrtis tetrathalamus tetrathalamus and restricted to shelfal water. The
Kuroshio Water assemblage is dominated by Lithelius
minor, Dictyocoryne profunda, Stylodictya multispina,
Acrosphaera spinosa, Dictyocoryne truncatum, Spongaster tetrars, Stylodictya arachnia, and Didymocyrtis tetrathalamus tetrathalamus, controlled by the Kuroshio
Surface Water. And the transition assemblage dominatedby Tetrapyle quadriloba and Monozonium pachystylum isassociated with the Tsushima Current water.
Radiolarian faunal composition in the Japan Sea, which
is characterized by deep-dwelling species associated with
the JSPW and subtropical surface water species, is significantly different from assemblages of the adjacent
mid-latitude North Pacific Ocean within the same latitudinal range (Itaki 2003; Motoyama et al. submitted).
Motoyama et al. (submitted) reported that the observed
assemblages in the sea include some subtropical elements, but many of the major temperate and subarctic
elements of the North Pacific are excluded from the region. Because the Japan Sea is connected to the adjacentseas and ocean only via shallow straits, its surface waters
are dominated by relatively warm waters derived fromthe East China Sea via the Tsushima Current, and theentry of oceanic waters from the adjacent North Pacific
Ocean and the Sea of Okhotsk is effectively blocked.
This semi-enclosed geography is most likely responsible
for the dominance of certain subtropical surface dwellers
and for the near-absence of transitional and cool-water
species from corresponding latitudes of the North
Pacific.
Living radiolarians in the Japan Sea have been examined using plankton tows, and most of warm-water species are recognized in shallower depths (Itaki 2003;
Ishitani and Takahashi 2007; Kurihara and Matsuoka
2010; Itaki et al. 2010). In the Tsushima Strait, radiolarian assemblages are similar with those in the East China
Sea, while it is different from those in the northern
Japan Sea (Itaki et al. 2010). According to Kurihara andMatsuoka (2010) who discussed seasonal variations ofPage 9 of 23

radiolarians in plankton samples from near the Sado
Island (Fig. 3) of the eastern Japan Sea, warm-water species including Tetrapyle octacantha, Spongosphaera
streptcantha, Pseudocubus obeliscus, and Dictyocoryne
profunda group dominate the radiolarian assemblage
from September to November. This result is consistent
with results from a sediment trap experiment deployedin the Tsugaru area (Fig. 3), where the Tsushima Current
flows into the northwest Pacific via the Tsugaru Strait(Itaki et al. 2008). These results imply that the warmwater species in the Japan Sea increase their abundance
during summer to autumn associated with a stronger
Tsushima Current.
Nannoplankton

The Florisphaera profunda/(F. profunda + Emiliania
huxlei + Gephyrocapsa oceanica) (F-EG ratio) is used asa Kuroshio Current and nearshore proxy (Su and Wei
2005). Additional Kuroshio taxa include Calcidiscus leptoporous, Ceratolithus cristatus, Helicosphaera carteri,
Oolithus fragilis, Rhabdosphaera clavigera, Thoracosphaera spp., and Umbellosphaera spp. (Chinzei et al.
1987; Su and Wei 2005). Key warm Tsushima Current
indices in the Japan Sea include: G. oceanica, C. leptoporous, and H. carteri (Muza 1992).
Ostracods

Ikeya and Cronin (1993) analyzed modern ostracod assemblages around Japan and identified biogeographicallysignificant taxa (Table 1). Typical Kuroshio Current assemblages can be divided into two: a Southwest Honshu,
Kyushu assemblage with Neonesidea oligodentata, andan East China Sea assemblage with Cytheropteron rhombea, Cytheropteron uchioi, Argiloecia hanaii, and Munseyella japonica. Tsushima Current indicators include
Krithe, Hirsutocythere hanaii, Cytheropteron miurense,and Bradlyea. Modern ostracods biofacies analyses in
the Tsushima Strait and southwestern Japan Sea reveals thata distinctive biota (Biofacies B) migrates into the sea fromthe East China Sea via the Tsushima Current (Tanaka
2008). Key species include in Biofacies B Neonesidea oligodentata s.l., Schizocythere kishinouyei, Cytheropteron miurense, and Macrocypris decola.
Planktic foraminifera

Modern tropical and subtropical planktic foraminifera
inhabit typically Kuroshio waters. One particular diagnostic taxon: Pulleniatina obliquiloculata is used extensively in paleoceanographic analyses (Li et al. 1997; Xu
and Oda 1999; Ujiié and Ujiié 1999; Jian et al. 2000; Ujiié
et al. 2003). Other species include Globigerinoides ruber,
Globigerinoides sacculifer, Globigerinoides conglobatus,
Globigerinella calida, Globorotalia hirsuta, Globorotalia
menardii, and Sphaeroidinella dehiscens (Chinzei et al.

Gallagher et al. Progress in Earth and Planetary Science (2015) 2:17

1987). The subtropical species Globigerinoides ruber
migrates into the Japan Sea via the Tsushima Current(Kitamura et al. 2001; Domitsu and Oda 2006) and is a
key Tsushima Current indicator. Globigerinoides ruber isthe shallowest dwelling species that stays in surface waters during its life cycle (Fairbanks et al. 1982). Its lower
limit of temperature tolerance is 19 °C (Hemleben et al.
1989). The switch from sinistrally coiled (colder) Neogloboquadrina pachyderma to dextral (warmer) coiled forms
is also used to interpret Tsushima Current influence (Obaet al. 1991).
Benthic foraminifera

Biogeographic analyses shows that the following IndoPacific Warm Pool benthic foraminiferal taxa: Pseudorotalia indopacifica, Asterorotalia gaimardii, Asterorotalia
spp., and Heterolepa margaritiferus migrate via the
Kuroshio Current to shelfal regions in the East China
Sea and around Japan (Gallagher et al. 2009). Two ofthese taxa (Asterorotalia gaimardii and Heterolepa margaritiferus) migrate into the southern shallow regions of
the Japan Sea (Gallagher et al. 2009; Hoiles et al. 2012)
via the Tsushima Current.
Geochemical proxies

Various geochemical indicators may be used to determine temperature and salinity variability in the surface
ocean affected by the Kuroshio and Tsushima currents.
The three techniques most commonly used are organic
biomarkers, carbon/oxygen isotope analyses, and elemental proxies from calcareous microfossils (Mg/Ca):
Organic biomarkers

The UK’37 index is a proxy used to reconstruct sea surface
temperatures (Rosell-Melé and McClymont 2007). This
index is based on the relative abundance of di-unsaturated
alkenones (C37:2) and tri-unsaturated alkenone (C37:3) inthe sediment, and UK’37 stands for “unsaturated ketones
with 37 carbon atoms”. Alkenones are abundant in marine
sediment and produced primarily by phytoplankton (Haptophyceae including the nannoplankton Emiliania huxleyi) living in the euphotic zone; this proxy has beencalibrated using a worldwide core top database (RosellMelé and McClymont 2007). The TEX86 index (TetraEther index of tetraethers with 86 carbon atoms) was
proposed by Schouten et al. (2002) as seawater temperature index. It is based on the number of cyclopentane
rings in glycol diakuyl glycerol tetraethers (GDGT’s) in themembrane lipids of archaebacteria such as Crenoarcheota
in the upper 100 m of the water column (Rosell-Melé and
McClymont 2007). Calibrations of TEX86 analyses oflipids in seabed sediment suggest that temperature estimates strongly correlate mean annual sea surface temperatures (Schouten et al. 2002).

Page 10 of 23

Oxygen and carbon isotope analyses

Carbon and oxygen isotope analyses of calcareous foraminifera are routinely used in paleoceanographic analyses (see review in Ravelo and Hillaire-Marcel 2007).
Typically, the ratios of stable oxygen and carbon isotopes are measured in foraminiferal carbonate expressedas δ18O and δ13C values in units per mil relative to
VPDB (Vienna Pee Dee Belemnite). Higher δ18O values
are caused by increased seawater evaporation (higher
salinity), lower temperatures, and ice volume expansion(glacials) whereas lower values may reflect fresh water
input, higher temperatures, and ice volume reduction(interglacials). Planktic foraminiferal δ18O values reflect conditions in the upper 200 m of the water column whereas benthic foraminiferal δ18O values seabed
conditions.
Elemental proxies from calcareous microfossils (Mg/Ca)

While oxygen isotope analyses are often used fortemperature estimates, δ18O values vary due to salinity
and ice volume effects. Another independent measure of
seawater temperature is the Mg/Ca of marine calcitepreserved in foraminiferal and ostracod shells (see review in Rosenthal 2007). As the substitution of Mg incalcite is endothermic, then Mg/Ca will reflect increasing temperatures. Estimates suggest that values ofaround 3 % Mg will substitute per °C (Rosenthal et al.
1997). Similar to oxygen isotope analyses, Mg/Ca ofplanktic foraminifera reflect sea surface paleotemperatures whereas Mg/Ca in ostracods and benthic foraminifera are used to determine seabed temperatures.
Paleosalinity and past ice volume variability can be discerned using paired Mg/Ca and δ18Ocalcite (Rosenthal
2007).
Geological history of the Kuroshio and Tsushima Currents

During the Neogene, the Kuroshio and Tsushima currents waxed and waned with the opening of gateways(the East Taiwan Channel and Tsushima Strait) due totectonic and/or climate and sea level variability. This has
resulted in various interpretations of the paleogeographic extent and intensity variability of the Kuroshio
Current. The timing of the breakthrough of the Tsushima
Current has also been investigated in detail. This section
reviews the geological history of these currents and how
they evolved through the Neogene. A summary of the key
sections revealing Kuroshio and Tsushima Current history
is in Table 3.
1. Pliocene to Middle Pleistocene history of the Kuroshio
Current

In contrast to the many studies of the history of the late
Pleistocene to Holocene Kuroshio Current, pre-latePleistocene research is more limited (Table 3). The pre-Gallagher et al. Progress in Earth and Planetary Science (2015) 2:17

Page 11 of 23

Table 3 A list of key stratigraphic sections in the northwest Pacific reviewed in this paper that reveal the 5 million year to recent
history of the Kuroshio and Tsushima Currents
Summary of the key sections
1. Pliocene to Middle Pleistocene history of the Kuroshio Current
Oldest (Ma)Youngest (Ma)Group/formation

Location

Proxy/proxies

Author9

2.5

Shimajiri Group

Okinawa-jima

Nannofossils

Imai et al. 2013

5.3

3.5

Shimajiri Group

Okinawa-jima

Nannofossils

Imai et al. 201353

Miyazaki GroupKyushuMolluscs

Tomida and Kitao 2002;
Tomida et al. 2013

Pliocene

Pleistocene

East and West Japan

Ostracoda

Ikeya and Cronin 1993

Pliocene

Early Pleistocene

Pacific side of Japan

Molluscs

Tsuchi and Shuto 1984;
Ogasawara 1994

30Chinen Formation

Okinawa-jimaBenthic Foraminifera

Gallagher et al. 2009

3.58

0.78

Seagipo Formation

2.8

2.5

1.65

0

ca. 700 ky

ca. 700 ky

144 ky150 kyJeju island

Benthic Foraminifera

Kang et al. 2010

KyushuOstracod/Planktic
foraminifera

Iwatani et al. 2012

Ryukyu Group

Ryukyu Islands

Coral reefs/corals

Yamamoto et al. 2006; Iryu et al. 2006;
Montaggioni et al. 2011

Kazusa GroupWest coast Japan

Sand ridge facies
analyses

Ito and Horikawa 2000

0

Off central Japan,
PacficRadiolarians

Yasudomi et al. 2014

0TohokuDiatoms

Koizumi and Yamamoto 2011

Location

Proxy/proxies

Author

2. Late Pleistocene to Holocene Kuroshio Current
OldestYoungest

Section type

K’

55 ky

0

Core

South of Japan

U

LGM

0

Core

Ryukyu Arc

Planktic foraminifera/
oxygen isotopes

37index

Sawada and Handa,1998
Ujiié and Ujiié 1999; Jian et al. 2000;
Ujiié et al. 2003

10.5 ky0Core

East China Sea

Mg/Ca and C/O analyses

Lin et al. 2006: Kubota et al. 2010

9 ky

0

Submarine cave

Okinawa-jima

δ18O micro-bivalves

Yamamoto et al. 2010;
Kitamura et al. 2013

6.5 ka BP

5.0 ka BP

East coast Japan

Molluscs

Matsushima 1984

LGM

9.5 ka BP

ODP Leg 195,
Site 1202

Detrital sediment andsorting analyses

Dieckmann et al. 2008

13 ky

0

ODP Leg 195,
Site 1202

Nannofossils

Su and Wei 2005; Wei 2006

LGM

0

Core

Okinawa TroughMg/Ca; d18O data;
ocean modeling

Kao et al. 2006; Lee et al. 2013

LGM

0

Core

Hesse Rise,
Pacific Ocean

Palynoflora

Kawahata and Ohshima 2002

LGM

0

Core

Northwestern
Pacific Ocean

Radiolarians

Yamamoto et al. 2004

LGM

0

Core

Northeastern East
China Sea

Palynoflora

Kawahata and Ohshima 2004

3. Pliocene to Middle Pleistocene history of the Tsushima Current
Oldest (Ma)Youngest (Ma)Group/Formation

Location

Proxy/proxies

Author4

2.2

Mita Formation

JoetsuMolluscs

Amano et al. 2008

3.5

1.7

See Fig. 4

JoetsuMolluscs/Planktic
foraminifera

Kitamura et al. 1999; Kitamura andKimoto 2006

1.7

0.8

Omma Formation

JoetsuMolluscs/Planktic
foraminifera

Kitamura et al. 1999; Kitamura andKimoto 2006

1.6

1

Omma Formation

JoetsuBenthic foraminifera

Hoiles et al. 2012

Gallagher et al. Progress in Earth and Planetary Science (2015) 2:17

Page 12 of 23

Table 3 A list of key stratigraphic sections in the northwest Pacific reviewed in this paper that reveal the 5 million year to recent
history of the Kuroshio and Tsushima Currents (Continued)
0.8

0VariousCentral andnorthern Japan

Molluscs

Omura 1980; Matsuura 1985;
Nishimura 1996; Chiba and Sato 2014

1.7

0.8

Oki Ridge

ODP Site 798

Planktic foraminifera

Kheradyar 1992

3.4

130 ka

1.7
Mid Pleist

Late Pleist

VariousJapan Sea

Diatoms

Koizumi 2013

Core

DSDP Site 302

Radiolarians

Kamikuri and Motoyama 2007

Core

Japan Sea

Radiolarians

Morley et al. 1986; Itaki 2001, 2007;
Itaki et al. 2007

Section type

Location

Proxy/proxies

AuthorShelf outcrop

Japan Sea offsouthwestern Japan

Molluscs

Chiji et al. 1981

4. Late Pleistocene to Holocene Tsushima Current
OldestYoungest

yrs

yrs

16,00011,000

9300

5000

8610

Core

Off Sado Island

Molluscs

Kitamura et al. 2011

Outcrop

Tsushima Islands

Molluscs

Saito et al. 1995

7200

900

Outcrop

Coastal area along
Hokkaido

Molluscs

Kito et al. 1998: Matsushima 2010

9300

0

Core

Japan Sea

Planktic foraminifera

Domitsu and Oda 2006

8300

6800

Core

Off the coast of Sanin

Planktic foraminifera

Takata et al. 2006

10,0008000

CoreJapan Sea

Radiolarians

Itaki 2001

early Pleistocene pathway of the Kuroshio Current is uncertain because tectonic evolution related to the openingof the Okinawa Trough and paleogeography of the
Ryukyu Islands have not been well delineated. Seismic
records suggest that the backarc rifting of the southernand northern Okinawa Trough was likely initiated in theearly Pleistocene and in the late Miocene (Park et al.
1998; Gungor et al. 2012). Although the molecular biological data show that Taiwan and the southern Ryukyus
were separated at 6–3 Ma (e.g., Koizumi et al. 2014), thisage does not necessarily indicate the timing of inflow of
the Kuroshio Current into the back-arc of the Ryukyu
Islands. Extensive distribution of the Shimajiri Group,
consisting mainly of fine siliciclastics derived from theEurasian Continent, and its correlative sediments aroundthe Ryukyu Islands, the Okinawa Trough, and East
China Sea Shelf clearly indicate that the Kuroshio
Current did not enter the back-arc side during the deposition of the group (~9–2.5 Ma). Once the Kuroshio
Current flowed into the back-arc side, the Ryukyu Island
became free from siliciclastic sedimentation. The paleoceanographic changes induced by stretching of the
Okinawa Trough triggered carbonate sedimentation at~1.4–2.0 Ma (e.g., Iryu et al. 2006), which is represented
by that of upper Chinen Formation (shelf carbonate) and
the Ryukyu Group (coral reef and associated shelf carbonates). Calcareous nannofossil assemblages of the
Shimajiri Group in Okinawa-jima (~9–2.5 Ma), deposited in the forearc basin, record Kuroshio Currentrelated paleoceanographic variability. Eutrophism (highnutrients) prevailed from 5.3 to 3.5 Ma followed byoligotrophic (low-nutrient Kuroshio Current) conditions(Imai et al. 2013). This variability is also related to local
tectonic movement.
The Kuroshio Current may have reached the Pacificcoast of Japan by ~5.0 Ma with the migration of thetropical planktic gastropod species Janthina (Hartungia)
typica in the Miyazaki Group in Kyushu (Tomida andKitao 2002; Tomida et al. 2013). In addition, an ingressionof strong Kuroshio flow is suggested ~3.0 Ma (Tomida
et al. 2013).
Ikeya and Cronin (1993) analyzed Pliocene to Pleistocene fossil ostracods assemblages from strata on the east
and west side of Japan and conclude that temperatures
were 1–2 °C cooler during the Pliocene. However, theseauthors did not specifically identify “fossil” evidence ofthe Kuroshio and Tsushima Current activity.
The warm temperate to subtropical Kakegawa mollusc
fauna typifies Pliocene to early Pleistocene strata on the
Pacific Ocean side of Japan. This fauna includes the species: Anadara suzukii, Anadara castellata, Amussipecten
praesignis, Amusium pleusonectens, Siphonalia declivis,
Umbonium suchiense, Turritella perterbra, Venericardia
panda, and Glycymeris nakamurai (Tsuchi and Shuto
1984; Ogasawara 1994). This biota is comparable to themodern Kuroshio fauna. The Indo-Pacific gastropod
species Makiyamaia subdeclivis migrated to the Ryukyu
Islands at the start of the Pliocene, thereafter reaching
the south island of Japan by ~3.0 Ma (Tsuchi and Shuto
1984; Shuto 1990) suggesting that Kuroshio Current

Gallagher et al. Progress in Earth and Planetary Science (2015) 2:17

finally reached its present geographic extent during thistime.
Indo-Pacific benthic foraminifera Pseudorotalia indopacifica, Asterorotalia gaimardii, and Heterolepa margaritiferus (Gallagher et al. 2009) are present in Pliocene
to Recent strata in Japan (35 to 39 °N). Heterolepa margaritiferus first occurs in Okinawa in latest Pliocene
strata younger than ~3.0 Ma. The distribution of A. gaimardii, H. margaritiferus, and P. indopacifica on thewest coast of Japan at 35 °N suggests that the biogeographic realm of the Kuroshio Current (Gallagher et al.
2009) reached its present latitude by the latest Pliocene(~3.0 Ma). Similarly, Tsuchi (1997) and Ogasawara (1994,
2002) used Japanese molluscan biogeography (the Kakegawa
mollusc fauna) to suggest Kuroshio Current intensification
at ~3.0 Ma. Maier-Reimer et al. (1990) suggested that theincrease in intensity of the North Pacific Gyre and related
Kuroshio Current after 3.0 million years was related to theonset of the Northern Hemisphere Glaciation and theclosure of the Central American seaway.
Benthic foraminiferal analyses of the Saegwipo Formation in Jeju Island in the East China Sea (JJ: Fig. 3) suggest fluctuating warm (Kuroshio) and cool phases from3.58 to 0.78 Ma (Kang et al. 2010). After 0.78 Ma, warm
Kuroshio Current waters dominated.
Ostracod and planktic foraminiferal analyses reveal
cooling in a 2.8 to 2.5 Ma section on the west coast of
Kyushu (Iwatani et al. 2012). Modern analog technique(MAT) reveals that Kuroshio influence decreased significantly across the Pliocene–Pleistocene boundary(~2.6 Ma) associated with global glacio-eustatic intensification (Iwatani et al. 2012).
The presence of Pleistocene fossil reefs in the Ryukyu
Islands is directly related to Kuroshio Current intensity.
The region was typified by a coral-bearing “ramp” from1.65 Ma (Montaggioni et al. 2011); thereafter, the Ryukyu
reefs expanded to their present latitudinal limit (Fig. 2)
from 1.1 to 0.8 Ma (Yamamoto et al. 2006). This diachroneity and reef expansion may be related to a step-wise increase in Kuroshio Current intensity related to themagnitude of glacio-eustatic variability through the Middle Pleistocene Transition (Gallagher et al. 2014). In thepast, the Ryukyu Reef front migrated to more southerly
latitudes in glacial periods (~24 °N) and migrated north(~31 °N) during interglacials (Fig. 2: Iryu et al. 2006).
Facies analyses of a 700 ka sand ridge complex in the
Kazusa Group on the west coast of Japan at 35 °N revealed strong paleo-Kuroshio activity (Ito and Horikawa
2000). Strong millennial-scale variations in current
strength were interpreted to be related to global climate
variable from the Middle to Late Pleistocene.
Radiolarians assemblages were analyzed in a 144 kyr
sediment record off central Japan, where the Kuroshio
Current diverts into the northwestern Pacific Ocean atPage 13 of 23~36 °N (Yasudomi et al. 2014). Kuroshio species (all collosphaerids except Acrosphaera arktios, Didymocyrtis
tetrathalamus, Euchitonia spp., Dictyocoryne spp., Octopyle stenozona–Tetrapyle octacantha) increased in abundance during MIS-1, 5a, 5c, and 5e. Yasudomi et al.
(2014) also use the Tr value (the ratio of warm-water tothe total of warm- and cool-water radiolarian species inan assemblage) to determine the varying dominancewarm Kuroshio and cool Oyashio Currents in this region. Their analyses showed that a transitional surface
water mass was replaced by a subtropical water mass between 131 and 125 ka.
Diatom analyses of a 150,000-year-old record in the
Tohoku area reveal orbital scale glacial–interglacial environmental variability (Koizumi and Yamamoto 2010).
High Td′-derived SSTs (°C) are estimated for MIS 5e
with MIS 5e temperatures ~5 °C higher than present innearshore regions and only ~1.5 °C higher in offshore
regions. However, Td′-SSTs (°C) decline markedly in
MIS 6 and at the MIS 6/5 boundary and event 5.2 in
MIS 5b, correlating to Heinrich events 1–6 (Heinrich1988). Td′-SSTs (°C) decrease during the Younger Dryas(YD) due to weakening Kuroshio–Kuroshio Extension
intensity and are 7–9 °C lower than present-day values
in the Tohoku area (Koizumi 2008). The early Holocene
is transitional period characterized by a long-term increasing trend of temperatures with several cooling
events, while the middle Holocene coincides with the
Holocene hypsithermal period and is 1–2 °C warmer
than the earlier and later period. The late Holocene neoglacial period is marked by a generally decreasing trendof the Td′-SSTs (°C), reflecting the decreased solar insolation in the Northern Hemisphere summer (Koizumi
and Sakamoto 2010).
2. Late Pleistocene to Holocene Kuroshio Current

Sawada and Handa (1998) used the UK’37 index to determine variability in Kuroshio flow intensity in 55,000year-old marine sediment from south of Japan (33° to
31 °N). These authors relate the strong variability inthese data to trade wind variability associated withchanges in subtropic North Pacific high pressure.
Planktic foraminiferal assemblage data and oxygen isotope data are used to interpret the 20,000-year history of
the Kuroshio Current in the Ryukyu Arc (Ujiié and Ujiié
1999; Ujiié et al. 2003; Jian et al. 2000). These authorsand others (Li et al. 1997; Xu and Oda 1999) use Pulleniatina as a Kuroshio Current index. Jian et al. (2000)
and Ujiié et al. (2003) interpret Pulleniatina minima
events (PMEs) in the strata of this region to indicate
times of reduced Kuroshio Current intensity since theLGM (Last Glacial Maximum, 22,000 years ago). Jianet al. (2000) suggests that the Kuroshio Current weakened from 4.6 to 2.7 cal. kyr BP possibly related toGallagher et al. Progress in Earth and Planetary Science (2015) 2:17

increased East Asian monsoon intensity and that Kuroshio intensity oscillated on a millennial scale with periodicities ranging from ca. 1500 to 800 years due toocean thermohaline circulation variability. Other factors
influencing Kuroshio Current intensity include variations in monsoonal forcing and freshwater input (Kaoet al. 2006). The reduced Kuroshio Current influence inthe region has also been attributed to the emergence ofa land bridge between the Ryukyu Arc and Taiwan (theEast Taiwan Channel: Fig. 2) that periodically deflected
the Kuroshio Current to turn east at ~24 °N from during
the early Holocene and the LGM (Ujiié et al. 1991,
2003). However, molecular biologic studies on reptiles
and amphibians with limited mobility capability amongisolated islands suggest that such a land bridge may nothave existed (Ota 1998).
In addition, Lin et al. (2006) and Kubota et al. (2010)detected no change in SST or sea surface salinity during
Pulleniatina minima, based on the Mg/Ca-derived temperatures using planktonic foraminifer from the East
China Sea. Yamamoto et al. (2010) analyzed δ18O of
micro-bivalve from a submarine cave off the Okinawa
Islands, Japan, and also detected no climate or hydrological anomaly variability associated with Pulleniatina
minima. Kitamura et al. (2013) estimated that a meanannual surface water temperature was about 1 °C higher
than during the Medieval Warm Period (900–1100 AD)
and the Middle Holocene Climatic Optimum (9–5.5 kaBP) and that the recent warming is likely exceptional
during the past 7000 years around Okinawa Islands,
based on δ18O of micro-bivalve from a submarine cave.
Matsushima (1984) reported that tropical molluscan
species, which are presently living south of Kyushu(~30 °N) including Rhinoclavis kochi, Barbatia fusca,
Saccostrea mordax, Periglypta chemnitzi, Dendostrea
paulucciae, and Meropesta capillacea that migrated to~36 °N 6.5–5.0 ka BP, indicating timing of the enhancement and northward expansion of the Kuroshio Current.
Detrital sediment and sorting analyses of Ocean Drilling Program (ODP) Leg 195, Site 1202 (Fig. 2) suggestthat Kuroshio inflow did not impinge on the Okinawa
Trough during the LGM when fluvial siliciclastics from
Yangtze River northwest of Taiwan dominated (Diekmann
et al. 2008). The presence of sorted silt from northeast
Taiwan suggests it re-entered the region after 12.2 ka BP.
The current intensified from 11.2 to 9.5 ka BP as a response to rapid sea level rise (Diekmann et al. 2008).
A 13,000-year record of calcareous nannofossil assemblages from ODP Leg 195, Site 1202 (Fig. 2) reveals
Kuroshio Current variability (Su and Wei 2005; Wei
2006). These authors use F-EG ratio as a proxy for the
Kuroshio Current. Their analyses revealed significant
floral change at 9 ka. Prior to 9 ka, Emiliania huxleyi
and Gephyrocapsa spp. dominated, whereas, after 9 ka,

Page 14 of 23

deep-dwelling Florisphaera profunda become common
and Gephyrocapsa rare. This change is interpreted to indicate the reentry of the Kuroshio Current into thesouthern Okinawa Trough during the early Holocene.
Other studies used ocean models and geochemical
data (Mg/Ca and δ18O data) to document Kuroshio
Current variability since the LGM (Kao et al. 2006; Leeet al. 2013). Kao et al. (2006) used a 3D ocean model todetermine the effect of sea level and changing geomorphology on the Kuroshio Current in the Okinawa
Trough. Their modeling suggests that as sea level fell, it
reduced Kuroshio transport volume and changed the nature of the Kuroshio Current meanders. Lee et al. (2013)combined new oxygen isotope data with ocean modelingto suggest that the LGM and present path of theKuroshio Current were similar and that glacial/interglacial
sea level variability does not significantly alter the temperature, salinity, and path of the current. This contrasts
to previous studies above (Ujiié et al. 2003) where theKuroshio Current is interpreted to be deflected away fromthe Okinawa Trough into the Pacific at 24 °N due East
Taiwan Channel closure during the LGM (Fig. 2).
If the Kuroshio Current was deflected at 22 °N into
Pacific Ocean during the LGM (Ujiié et al. 2003), thiswould have caused the Kuroshio Extension to shiftsouthward by at least 10° (Kawahata and Ohshima2002). However, minor shifts (~3°) to the south in thelatitudinal position of this extension have been interpreted in the central Pacific Ocean using palynofloral
data (Hesse Rise; 34° 54.25′ N, 179° 42.18′ E) during glacial periods (Kawahata and Ohshima 2002). Furthermore, calculated SR values (the SST index of Yamauchi1986) for late Pleistocene to Holocene radiolarian assemblages in a 30 °N to 40 °N north–south transect of
sediment cores collected from off Japan in the northwestern Pacific Ocean also suggests a minor southerly shift of~3° of Kuroshio Extension during LGM (Yamamoto et al.
2004).
The podocarp genus Phyllocladus is not present inJapan today; however, it is present in the Indonesian archipelago, and therefore, its presence in the northeasternpart of the East China Sea (west of the Kyushu Island)
during the LGM suggests transportation by the Kuroshio
Current (Kawahata and Ohshima 2004).
3. Pliocene to Middle Pleistocene history of the Tsushima
Current

Pliocene-early Pleistocene marine strata are well exposed
along the coastal region of the Japanese Islands and yield
abundant fossil molluscs (Kaseno and Matsuura 1965;
Ogasawara 1977; Matsuura 1985; Chinzei 1986; Kanazawa
1990; Kitamura and Kondo 1990; Matsui 1990; Suzuki
and Akamatsu 1994). Kitamura and Kimoto (2007) summarized the stratigraphic distribution of molluscan fossils

Gallagher et al. Progress in Earth and Planetary Science (2015) 2:17

in the Pliocene to lower Pleistocene marine strata along
the Japan Sea coastal area (Amano et al. 1987, 1988, 1990,
2000; Amano and Kanno 1991; Arai et al. 1991, 1997;
Cronin et al. 1994; Miwa et al. 2004) and the lower Pleistocene Omma Formation in the Hokuriku area (HO: Fig. 3:
Kitamura and Kondo 1990; Kitamura 1991, 1994; Kitamura
et al. 1994; Kitamura and Kawagoe 2006). Amano et al.
(2008) documented Tsushima Current warm-water molluscan species and other cold-water species, which lived
shallower than 100–120 m depth, in 12 horizons between
tuff beds dated from 4 to 2.2 Ma in the Mita Formation in
Yatsuo-machi of Toyama City near Joetsu (JO: Fig. 3). Sequence stratigraphic analysis was carried out on the
Omma Formation (Kitamura 1998; Kitamura et al. 2001;
Kitamura and Kawagoe 2006), and this provided an astronomically tuned a fossil record. The Omma Formation is
up to 220 m thick and consists of 19 depositional sequences representing inner- to outer-shelf environmentsthat are correlated to oxygen isotope stages (MIS) 56 to21.3 (Kitamura 1998; Kitamura and Kimoto 2006). During
the deposition of each sequence, the molluscan faunachanged from cold-water, upper-sublittoral species towarm-water (Tsushima Current), lower-sublittoral species,
followed by a return to cold-water, upper-sublittoral species (Kitamura et al. 1994). Molluscan fossils are notpresent in MIS 25, 23, and 21.3, due to shell dissolution(Kitamura and Kawagoe 2006).
Based on stratigraphic distribution of warm-water species, Kitamura and Kimoto (2006) divided the history of
the warm Tsushima Current from 3.5 to 0.8 Ma into twointervals (Fig. 4). The first interval from 3.5 to 1.7 Ma,
the current periodically flowed into the Japan Sea at 3.2,
2.9, 2.4, and 1.9 Ma (MIS 69). Comparing these ages
with δ18O stratigraphy (Shackleton et al. 1995), the threeolder events may correlate with KM5 or 3, G17 or 15,
and MIS 95 or 93, respectively. The second interval corresponds to a time when the Tsushima Current flowed
during every interglacial highstand from 1.7 to 0.8 Ma(MIS 59–20), except for MIS 25, 23, and 21.3 (Fig. 4).
The present southern channel in the Tsushima Strait(Fig. 3) is interpreted to have formed by MIS 59(1.7 Ma) as the Tsushima Current began to flow during
each interglacial period (Kitamura et al. 2001).
Since sea levels in the interglacial periods at 3.3 to
2.5 Ma were up to 50 m higher than at present (Dwyer
et al. 1995), the altitude of the southern part of the JapanSea during 3.5 to 1.7 Ma was about 50 m above thepresent-day sea level. The altitude of the area might havedecreased by 1.7 Ma. According to Kamata and Kodama(1999), crustal rotation and incipient rifting has also occurred in South Kyushu and the northern Okinawa
Trough since 2 Ma. The authors interpreted that these
events were caused by the transition to the westward
convergence of the Philippine Sea plate. On the basis ofPage 15 of 23

timing and nature, Kitamura et al. (2001) assume that achange in the convergence direction of the Philippine
Sea plate led to deepening and/or widening of the
Tsushima Strait. It should be noted that the timing ofthese tectonically induced paleocenographic changes areroughly coeval with those in the Central and South
Ryukyus, which triggered carbonate sedimentation. Thesesuggest that the East China Sea and the Japan Sea became
more influenced by the Kuroshio and Tsushima currents,
respectively, from 1.5–2.0 Ma onwards due to the stretching of the Okinawa Trough and the deepening of the
Tsushima Strait.
Inner- to outer-shelf environments in the southern
Japan Sea may have been unsuitable for cold and theTsushima Current molluscs from MIS 48 to 47, 44 to 43,
and 32 to 31 (Kitamura et al. 2000; Kitamura 2004).
These deglaciation MIS periods correspond to periods
when eccentricity-modulated precession extremes werealigned with obliquity maxima and coincided with thethree high peak of July solar insolation at 65 °N (495,
493, and 500 W/m2) between MIS 50 and 26. This implies that anomalously high seasonality induced byorbital-insolation cycles may have played an importantrole in establishment of non-analog molluscan fauna in
the early Pleistocene Japan Sea. Kitamura (2004) usedsummer insolation data (Berger 1978) to suggest thatsuch non-analog fauna would not established itself again
in the next 150,000 years, while present global and/or
regional climate systems are maintained.
Marine molluscan fossil records younger than 0.8 Ma
are common in central and northern Japan (Omura
1980; Matsuura 1985; Nishimura 1996; Shirai and Tada
2000; Chiba and Sato 2014). These records show that
the Tsushima Current flowed during MIS 9, 7, and 5(Fig. 4).
Planktic foraminifera also reveal the long-term historyof the Tsushima Current (Fig. 4). Kheradyar (1992) analyzed Pleistocene planktic distribution in the strata of
ODP Site 798 at Oki Ridge (OK: Fig. 3). The samplingintervals were 30,000 years for strata ranging from 1.7 to
1.4 Ma and sample interval of 13,000 years in strata
younger than 1.4 Ma. Only six horizons yield G. ruber
from 1.7 to 0.8 Ma at Site 798, whereas in this interval,
this species is present in 17 interglacial stages in the
Omma Formation (Kitamura and Kimoto 2006) (Fig. 4).
This discrepancy can be explained by either the low
resolution of the sampling intervals, especially the interval from 1.7 to 1.4 Ma, or poor carbonate preservation
during interglacial periods. In the Japan Sea, the calcium
carbonate compensation depth (CCD) was less than
1,000 m during the last deglacial period (Oba et al.
1991). The shallower CCD was caused by oxic bottom
conditions associated with a strong Tsushima Current
inflow followed by increased JSPW production leadingGallagher et al. Progress in Earth and Planetary Science (2015) 2:17

Fig. 4 (See legend on next page.)

Page 16 of 23

Gallagher et al. Progress in Earth and Planetary Science (2015) 2:17

Page 17 of 23(See figure on previous page.)
Fig. 4 (i) The distribution of G. ruber and Tsushima Current molluscs in interglacials of the Omma Formation (adapted from Kitamura and Kimoto2006) correlated to the LR04 benthic stack of Lisiecki and Raymo (2005). The red dots (1.6 to 1 Ma) are interglacials where migratory Indo-PacificWarm Pool benthic foraminifera are present (Hoiles et al. 2012). Red dots at 0 and ~3 Ma are Warm Pool benthic foram ingressions described in
Gallagher et al. (2009). Reconstructions of the paleogeography of the Tsushima Strait after 1.6 Ma (ii) and prior to 1.7 Ma (iii) adapted from
Kitamura and Kimoto (2006)

to a high dissolved oxygen level. These oceanic conditions
may have caused poor carbonate preservation during
many of the interglacial periods over the last 0.64 Ma(Kido et al. 2006).
The influx of G. ruber is at least 0.40 m lower than thelowest occurrence of Tsushima Current molluscs in thedepositional sequence of the Omma Formation (Kitamura
et al. 1999). This suggests that changes in bottom wateron the upper continental shelf lagged behind changes insurface water associated with the influx of the Tsushima
Current.
The stratigraphic distribution of Pliocene to early
Pleistocene planktonic foraminifera is similar to that of
the molluscan fossils (Kitamura and Kimoto 2006) andshowing: (1) the Tsushima Current flowed at 3.2 Ma(MIS KM 5 or 3), 2.9 Ma (MIS G 17 or 15), and 1.9 Ma(MIS 59), in the 3.5–1.7 Ma interval; and (2) the current
flowed at every interglacial highstand, except for MIS 25,
23, and 21.3, in the 1.71–0.8 Ma interval (Fig. 4). Miwa
et al. (2004) analyzed planktonic foraminifers in 84 samples with 1–10 m sampling intervals in the Yabuta Formation, near Joestu (3.5–2.4 Ma). The authors identified one
G. ruber horizon at 3.2 Ma (Fig. 4), confirming that the influence of the Tsushima Current was limited at narrowcoastal area based on the molluscan fossil records.
The relative abundance of G. ruber in MIS 47, 43, and
31 is higher than in other interglacial stages from MIS
50 to 26 (1.5–1.0 Ma) in the early Pleistocene Omma
Formation (Fig. 4: Kitamura and Kimoto 2007). As noted
above, these interglacial stages coincided with the threehigh peak of July solar insolation at 65 °N between MIS
50 and 26.
The relative abundance of G. ruber at MIS 21.5
reaches 46 % in the Omma Formation and is higher thanduring MIS 47, 43, and 31 (Hoiles et al. 2012). Compared to the modern data of Domitsu and Oda (2006),
this value is 40 % greater than today in the southernmost
area of the Japan Sea. In ODP Site 798 (Oki Ridge:
Fig. 3), the highest relative abundance of the species(6 %) is between the top of Jaramillo Subchron and thebase of Brunhes Chron (Kheradyar 1992). The relative
abundance of G. ruber is only 2 to 3 % in surface sediment from sites 150 km south of ODP Site 798. Boreal
summer insolation at MIS 21.5 was lower than those of
MIS 47, 43, and 31 (Berger 1992). In the LR04 δ18O global isotope stack (Lisiecki and Raymo 2005), values arehigher during this time compared to values during MIS

47, 43, 31, 11, 9, 5, and 1 (Fig. 4). Thus, the high relative
abundance of G. ruber may be explained by its migration
into the Japan Sea from the East China Sea rather thanglobal climate change. Since G. ruber is a shallowdwelling species, it is likely that wide and shallow Tsushima
Strait significantly facilitated its preferential migration. Sea
level during MIS 22 was lowest than those at any othertime between MIS 56 and 21 (Bintanja et al. 2005;
Kitamura and Kawagoe 2006; Elderfield et al. 2012;
Rohling et al. 2014; Kitamura, in press). During the sealevel lowstand, the southern channel may have widened
by erosion. After MIS 21, the deepening of the channel associated with erosion impeded the migration of G. ruber.
Kheradyar (1992) documented seven G. ruber horizons
indicating Tsushima Current flow in 0.8 Ma and younger
strata at ODP Site 798. However, these data cannot becorrelated to marine isotope stages, due to the lack of awell-constrained chrono- and biostratigraphy.
Analyses of benthic foraminifera in the Omma Formation in the Japan Sea reveals common Indo-Pacific migratory species: Asterorotalia gaimardii, Asterorotalia
concinna, Asterorotalia milletti, and Asterorotalia yabei
during MIS 47 around 1.4 Ma (Hoiles et al. 2012). Thesetaxa occur intermittently during most interglacial highstand periods from stages 55 to 29 showing a similar
pattern to the planktic foraminifera and molluscan fossil
assemblages described above. This is strong corroborating evidence for Tsushima inflow into the Japan Sea during each interglacial between 1.6 and 1.0 Ma (Hoiles
et al. 2012).
Low-frequency and high-amplitude variations in diatom Twt and Td′-SSTs (°C) occur between 3.4 and
2.8 Ma in the Japan Sea (Koizumi and Yamamoto, inpress), when Twt gradually increases due to a decreasingin cold-water species. Tsushima Current molluscs arepresent intermittently during this period (Fig. 4). After
2.8 Ma, the Twt ratio decreases markedly due to coolingnear the Pliocene/Pleistocene boundary ~2.6 Ma. Twt
and Td′-SSTs (°C) increase from 2.6 to 2.0 Ma due to alarge decrease of cold-water species. The change fromwarm-water species to cold-water species denotes cooling from 1.19 to 1.17 Ma and 0.68 to 0.66 Ma during
MIS 37–36 and MIS 17–16 when Tsushima Current
molluscs and foraminifera are absent. Strong variations in
cold-water species abundance occurs during the Jaramillo
Subchron (MIS 30–26). The occurrence of warm-water
species from 70–130 ka correlates to MIS 5 (Termination

Gallagher et al. Progress in Earth and Planetary Science (2015) 2:17II), however, a peak of warm-water diatom species is notpresent during MIS 11 (Termination V).
Subtropical (Tsushima Current) radiolarian species
such as Tetrapyle octacantha, Dictocoryne profunda, andDidymocyrtis tetrathalamus are present in strata younger than 1.7 Ma at DSDP site 302 (~40 °N) in the Japan
Sea (Kamikuri and Motoyama 2007) coinciding with the
influx of the subtropical planktonic foraminifera Globigerina ruber described above (Kitamura and Kimoto2006).
Significant glacial–interglacial variability in radiolarianassemblages also occur in middle to late Pleistocene
strata in the Japan Sea (Morley et al. 1986; Itaki 2001,
2007; Itaki et al. 2007). One particular Tsushima Current
indicator, Tetrapyle octacantha group, increase in abundance during interglacial periods whereas it is absent orrare during the glacial periods due to Tsushima Current
restriction.
4. Late Pleistocene to Holocene Tsushima Current

Cold-water molluscs Mytilus coruscus (11,050 ± 600 years
BP; Emery et al. 1971), Macoma calcarea (15,740 ±400 years BP; Emery et al. 1971), Mercenaria stimpsoni(13,130 ± 280 years BP; Chiji et al. 1981), and Patinopecten yessoensis (16,340 ± 420 years BP; Chiji et al. 1981)
are present on the continental shelf in the Japan Sea offsouthwestern Japan. Fossil records of cored sediments at
132 m water depth off Sado Island, Japan (SI: Fig. 3),
show that the molluscs Porterius dalli, Cyclocardia ferruginea, Tridonta alaskensis, and Puncturella nobilis were
present prior to the initial inflow of the Tsushima
Current at 9300 cal. BP (Kitamura et al. 2011). Subsequently, the modern molluscan fauna (dominated byLimopsis belcheri and Crenulilimopsis oblonga) first occurs
at 5180 cal. BP in the study area. However, it is possiblethat the development of this modern molluscan fauna may
relate to a reduction in terrigenous sediment influx andnot directly related to Tsushima Current activity. A marked
reduction in terrigenous sediment supply may be causedby enhanced vegetation and restabilized mountain slopesin the sediment source areas (Nakajima and Itaki 2007).
The oldest Tsushima Current molluscs from the latest
Pleistocene–Holocene coastal plains of the Japan Sea are
8610 ± 100 year BP old Anodontia stearnsiana in incisedvalley fill on the Tsushima Islands (Saito et al. 1995) (Fig. 3).
Tsushima Current molluscs with ages ranging from 7200
to 5000 years BP, 4200 to 3200 years BP, 2500 to 2300 years
BP, and 1000 to 900 years BP are also present in the
coastal area along Hokkaido ~43 °N (Kito et al. 1998;
Matsushima 2010). This suggests periodic intensificationof the northward inflow of the Tsushima Current.
G. ruber is present in 9300 cal. BP to recent strata incore from the Japan Sea suggesting continuous Tsushima
Current inflow from this time (Domitsu and Oda 2006).

Page 18 of 23

These authors suggest that modern surface oceanic conditions in the southern Japan Sea had been established
by 7300 cal. BP. Takata et al. (2006) used planktic foraminiferal assemblages to interpret periods of enhanced
Tsushima Current from 8300–8000 and 7300–6800 years
BP off Sanin (Fig. 3). The latter period corresponds withmolluscan data that suggest Tsushima Current intensification 7200 to 5000 years BP (Matsushima 2010).
Warm-water radiolarians including Tetrapyle octacantha,
Octopyle stenozona, Dictocoryne profunda, Dictocoryne
truncatum, Didymocyrtis tetrathalamus, and Euchitonia
flucata were rare during the last glacial period, and theirinitial migration via the Tsushima Current occurred at
10 ka and became abundant after 8 ka (Itaki 2001).
5. Kuroshio Current and Extension change with futureclimate change

The studies reviewed above reveal that current intensityand position has varied markedly over millions of yearwith changing greenhouse (Pliocene) to icehouse (Pleistocene) conditions. These studies may improve our understanding of current behavior with future climatechange; however, there are insufficient millennial andannular fossil proxy studies in the Northwest Pacific thathave the resolution required to directly compare withstudies of modern oceanographic variability with climatechange. In addition, there is limited atmospheric andocean modeling of the response of the Kuroshio Current
and Extension to future global warming (Sakamoto et al.
2005; Wu et al. 2012) suggesting that the current velocity will increase by up to 0.3 m/s (i.e., it may double inspeed) as the North Pacific gyre intensifies. Furthermore,
the position of the Kuroshio Extension has moved froman average position of ~34 °N northward to ~34.5 °N
with global warming over the last century (Wu et al.
2012).

Conclusions
The Kuroshio Current is a major warm western boundary current in the western North Pacific. On the westside of the north Pacific Gyre, it plays a key role in distributing heat from the tropics to the pole and has beena major control on the climate of northeast Asian for
millions of years. It bifurcates into the Japan Sea as the
Tsushima Current and moderates Japanese climate. The
geological history of these currents has been extensively
investigated using various proxies. Sedimentological proxies such as reef, bedforms, palynological and sedimentsource distribution, and sediment sorting reveal current
intensity/velocity and paleolatitude. Fossil coral and mollusc assemblage analyses provide strong evidence of theeffect of these currents on paleoshelfal regions. Microfossils (planktic/benthic foraminifera, diatoms, radiolarians,
and coccolith) assemblage variations are directly related toGallagher et al. Progress in Earth and Planetary Science (2015) 2:17

the strength and extent of past Kuroshio and Tsushima
currents. Assemblage data combined with organic biomarker (UK’37, TEX86), stable isotope (δ18O), and elemental (Mg/Ca) analyses are used to determine paleo-sea
surface temperature and salinity histories in primarily
oceanic archives. The transportation of tropical terrestrial
palynomorphs and migrations of Indo-Pacific invertebrate
macro- and microscopic species to Japanese waters reveal
the paleocurrent intensity and latitudinal extent.
A review of stratigraphic distribution of these proxies
suggests that the Kuroshio Current reached its present
latitude (35 °N) by ~3 Ma when temperatures were 1 to
2 °C lower than present around Japan. At around thistime, a weak Tsushima Current broke through Tsushima
Strait entering the Japan Sea. Unlike Tsushima pathways
today that extend into the center of the Japan Sea, early
Tsushima Current was confined to coastal regions ofJapan when sea level was up to 40 m higher thanpresent. Limited stratigraphic data for the next 1 Ma
suggests similar oceanic conditions with variable coolingand warm condition persisted until ~2 Ma. After 2 Ma,
crustal stretching in the Okinawa Trough deepened
Tsushima Strait facilitating inflow during every successive interglacial. The onset of stronger interglacial/glacial
cycles at ~1 Ma was associated with increased North
Pacific Gyre and Kuroshio Current intensity. This triggered Ryukyu Reef expansion, and these reefs reachedtheir present latitude (~31 °N); thereafter, the reef front
advanced (~31 °N) and retreated (~25 °N) with each successive cycle. Planktic foraminiferal proxy data (Pulleniatina minima events) from the south Okinawa Troughsuggests eastward deflection of the Kuroshio Current
from its present path at 24 °N into the Pacific Ocean
due to East Taiwan Channel restriction during the Last
Glacial Maximum. This deflection may have shifted theKuroshio Extension and Polar Front by 10 °S southward
to ~24 °N. Subsequently, Kuroshio flow resumed its
present trajectory during the Holocene. However, ocean
modeling and geochemical analyses (paired δ18O, δ13C,
Mg/Ca analyses) of Late Pleistocene strata in the Okinawa
Trough and East China Sea show that the Kuroshio
Current path did not change between glacial and interglacial periods. Furthermore, analyses of radiolarian and terrestrial palynomorph assemblages in marine strata in thecentral North Pacific shows that the glacial/interglacial
latitudinal position of Kuroshio Extension/Polar Front
only shifted ~3° southward during glacial maxima.
Our synthesis shows that Kuroshio and Tsushima
Current intensity and paleolatitude has varied significantly
over millions of years with changing greenhouse (Pliocene)
to icehouse (Pleistocene) conditions. Limited atmosphericand ocean modeling of the response of the Kuroshio
Current and Extension to future global warming suggeststhat the Kuroshio flow velocity will increase by 0.3 m/s as

Page 19 of 23

the North Pacific gyre intensifies. Furthermore, the position of the Kuroshio Extension has shifted northwards by
0.5° with global warming over the last century. Further
paleoceanographic analyses and modeling of the currents
are required if we are to improve our understanding of
how these modern oceanographic features of the northwest Pacific will respond to future climate change.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
SG proposed the review topic. All authors collaborated with the corresponding
author in the construction of manuscript. All authors read and approved thefinal manuscript.
Acknowledgements
We would like to thank Ryuji Tada for inviting this submission. The detailed
comments of and Ken Ikehara and two anonymous reviewers improved thetext. We thank Marc Humblet for comments on the “Coral reef” section. SG was
supported by ARC LP0219312 and an ANZIC IODP analytical grant (2012). YI waspartly financially supported by the Japan Society for the Promotion of Science(JSPS) KAKENHI (Grant-In-Aid for Scientific Research) Grant Number 25247083.
Author details
1
School of Earth Sciences, The University of Melbourne, Melbourne, Victoria
3010, Australia. 2Institute of Geosciences, Shizuoka University, Shizuoka
422-8529, Japan. 3Institute of Geology and Paleontology, Graduate School of
Science, Tohoku University, Aobayama, Sendai 980-8578, Japan. 4Geological
Survey of Japan, AIST, Marine Geology Research Group/Institute of Geology
and Geoinformation, Tsukuba Central 7, 1-1-1 Higashi, Tsukuba, Ibaraki
305-8567, Japan. 5Division of Earth System Science, Hokkaido University,
Sapporo, Hokkaido Prefecture 060-0808, Japan. 6Atsubetsu-kita 3-5-18-2,
Atsubetsu-ku, Sapporo 004-0073, Japan. 7Present address: The DownUnder
GeoSolutions Group, Level 3, 76 Kings Park Road, West Perth, WA 6005,
Australia.
Received: 28 August 2014 Accepted: 6 May 2015

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