Kubota et al. Progress in Earth and Planetary Science
https://doi.org/10.1186/s40645-020-00395-3(2021) 8:29

RESEARCH ARTICLE

Progress in Earth andPlanetary Science

Open Access

Paleoceanography of the northwestern
Pacific across the Early–Middle Pleistocene
boundary (Marine Isotope Stages 20–18)
Yoshimi Kubota1* , Yuki Haneda2 , Koji Kameo3, Takuya Itaki2 , Hiroki Hayashi4 , Kizuku Shikoku4,
Kentaro Izumi5 , Martin J. Head6 , Yusuke Suganuma7,8 and Makoto Okada9

Abstract
The fluctuating position of the boundary between the Kuroshio (warm) and Oyashio (cold) currents in the midlatitude western North Pacific affects both heat transport and air–ocean interactions and has significant
consequences for the East Asian climate. We reconstruct the paleoceanography of Marine Isotope Stages (MIS) 20–
18, MIS 19 being one of the closest astronomical analogues to the present interglacial, through multiple proxies
including microfossil assemblage data, planktonic foraminiferal isotopes (δ18O and δ13C), and foraminiferal Mg/Cabased temperature records, from the Chiba composite section (CbCS) exposed on the Boso Peninsula, east-central
Japan. Principal component analysis (PCA) is used to capture dominant patterns of the temporal variation in thesemarine records, and shows that the relative abundances of calcareous nannofossil and radiolarian taxa areconsistent with the water mass types inferred from geochemical proxies. The leading mode (36.3% of total
variance) mirrors variation in the terrestrial East Asian winter monsoon (EAWM), reflecting seasonal trends
dominated by the winter monsoon system. In the CbCS, this mode is interpreted as reflecting the interplay
between the warm Kuroshio and cold Oyashio waters, which is likely related to the latitudinal shift of thesubtropical–subarctic gyre boundary in the North Pacific. The second mode (15.4% of total variance) is closely
related to subsurface conditions. The leading mode indicates that MIS 19b and 19a are represented by millennialscale stadial/interstadial oscillations. Northerly positions for the gyre boundary during late MIS 19c, the interstadialsof MIS 19a, and early MIS 18 are inferred from the leading mode, which is consistent with a weak EAWM andconsequent mild winter climate in East Asia. Nonetheless, the northerly positions for the gyre boundary during late
MIS 19c and early MIS 19a were not associated with subsurface warming presumably due to the suppressed gyre
circulation itself caused by the weak Aleutian Low. Intermittent southerly positions for the gyre boundary areinferred for the stadials of MIS 19b and 19a. Regional sea surface temperature (SST) comparisons in the western
North Pacific reveal that the moderate SSTs during MIS 19a through early MIS 18 were restricted to the mid- tohigh latitudes, influenced by the weak EAWM. Comparison between MIS 20–18 and MIS 2–1 suggests that glacial
MIS 20 and 18 had significantly milder winters than MIS 2, likely related to the relatively weak EAWM.

* Correspondence: yoshimi@kahaku.go.jp
1
Department of Geology and Paleontology, National Museum of Nature andScience, 4-1-1, Amakubo, Tsukuba, Ibaraki 305-0005, Japan
Full list of author information is available at the end of the article© The Author(s). 2021 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License,
which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give
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1 Introduction
Pronounced glacial–interglacial oscillations characterize
the Quaternary Period. The Early–Middle Pleistocenetransition (EMPT) spanning ~ 1.4–0.4 Ma (Ruddiman etal. 1986; Head and Gibbard 2015a) saw a fundamental
and progressive shift from 41 ky to quasi-100 ky cyclicity
along with an increase in global ice volume during glacials and a decrease in the relative duration of interglacials compared to glacial stages (Lisiecki and Raymo2007). Orbital controls on Earth’s climate, namely precession, obliquity, and eccentricity, change latitudinaland seasonal insolation and have modulated the climate
system throughout the Quaternary (Cronin 2010). Marine Isotope Stage (MIS) 19, occurring at ~ 790–761 ka
within the middle of the EMPT, along with MIS 11(424–374 ka), are the closest analogues for the presentinterglacial in terms of astronomical and paleoclimatic
signatures (Tzedakis et al. 2009; Yin and Berger 2011;
Tzedakis et al. 2012; Head and Gibbard 2015a; Head inrevision). Therefore, paleoceanographic records for MIS
19 offer a crucial understanding of the natural variabilityof our modern climate in the absence of anthropogenic
CO2 emissions, and so provide a baseline needed to predict future climate change.
The early phase of the EMPT (~ 1.4–0.7 Ma) is characterized by increasingly pronounced global and regional
climate changes such as severe glaciations (e.g., Clark et
al. 2006; Elderfield et al. 2010; Head and Gibbard 2015a),evolution of the tropical sea surface temperature (SST)
pattern (de Garidel-Thoron et al. 2005; Dyez and Ravelo2014), and changes in the Asian monsoon and vegetation(Sun et al. 2006, 2019). A reconstruction of SSTs in thetropical Pacific inferred that the zonal east–west SST contrast increased during ~ 1.2–0.8 Ma, resulting in a shift
from El Niño-like to more La Niña-like conditions (deGaridel-Thoron et al. 2005; Dyez and Ravelo 2014). In
Asia, a series of studies on the Chinese Loess Plateau revealed that the amplitude and cyclicity of the Asian monsoon and vegetation had changed during the ~ 1.2–0.7 Ma
interval (Sun et al. 2006, 2019). This evidence suggeststhat developments during the early EMPT involved dynamic circulation changes including an intensification of
the Walker circulation and trade winds (Dyez and Ravelo2014) and probably enhancement of the Asian monsoon
system (Han et al. 2012).
The Kuroshio is a north flowing boundary current in
the western North Pacific and represents one of the
Earth’s critical heat engines, playing a crucial role intransporting heat poleward (Talley 2013). The Oyashio,
another North Pacific western boundary current but inthe subarctic gyre, carries abundant nutrients and its
fluctuations impact primary production along the coastof the northern Japanese archipelago. The evolution ofthese two current systems is therefore of significant

interest. Marine records of calcareous microfossils aresparse for the western North Pacific where their preservation is often poor owing to the shallow carbonate
compensation depth. On the other hand, long-term environmental changes during the early EMPT have been
investigated using diatoms (Sancetta and Silvestri 1986)
and radiolarians (Matsuzaki et al. 2015), which are relatively well preserved in the North Pacific, although thetemporal resolution of these studies is low. The early
EMPT in the western North Pacific (~ 1.3–0.7 Ma) is
characterized by warm and oligotrophic conditions
based on the radiolarian fauna (Matsuzaki et al. 2015),
suggesting the northward migration of the Kuroshio
Current probably in response to the intensification of
the East Asian summer monsoon and associated enhanced trade winds (Han et al. 2012; Matsuzaki et al.
2015). Together with the strong Kuroshio Current during the early EMPT, the radiolarian record suggests thatthe subarctic Oyashio Current migrated southward(Matsuzaki et al. 2015), which is consistent with enhanced cold conditions in the higher latitudes of theNorth Pacific (Sancetta and Silvestri 1986; Morley and
Dworetzky 1991; Matul et al. 2009; Zhang et al. 2014).
However, it remains unclear how these environmental
changes were linked to the regional-scale re-organizationof the circulation system (e.g., Walker Circulation andAsian monsoon systems) due to the lack of highresolution data.
The Chiba composite section (CbCS) including the
Chiba section itself (35° 17′ 39.6"N, 140° 08′ 47.6" E to
35° 17′ 36.9" N, 140° 08′ 47.2" E; Suganuma et al. 2021)
exposed on the Boso Peninsula, central Japan, and occurring within the Kokumoto Formation of the Kazusa
Group (Kazaoka et al. 2015), represents a high-resolution
continuous sedimentary succession deposited on the shelfedge to continental slope at depths greater than 200 m(Nishida et al. 2016). This succession contains excellently
preserved calcareous microfossils (Kameo et al. 2020;
Suganuma et al. 2018), and its favorable nature has permitted the establishment of a centennial- to millennialscale benthic foraminiferal oxygen isotope (δ18O) stratigraphy (Okada et al. 2017; Suganuma et al. 2018; Haneda etal. 2020a). The boundary between the warm Kuroshio andcold Oyashio currents is presently situated just east of the
Boso Peninsula (Qiu 2001). During the late Quaternary,
the position of the gyre boundary between the subtropical
Kuroshio and subarctic Oyashio currents was oscillating
at orbital (Moore et al. 1980; Thompson and Shackleton
1980; Chinzei et al. 1987; Oba et al. 2006) and millennial(Yamamoto et al. 2005) time scales, causing significant
changes in SST and nutrient levels in the mixed zone
(Oba et al. 2006; Yamamoto et al. 2005). Multiple
paleoceanographic proxies obtained from the CbCS
should, therefore, provide insights into the Kuroshio

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and Oyashio currents variability during a significant
interval (MIS 20–18) at an ideal location.
The CbCS unambiguously records the Matuyama–
Brunhes (M–B) paleomagnetic polarity boundary (Suganuma et al. 2015) which is the primary chronologicaldatum for the Lower–Middle Pleistocene Subseries
boundary (Head et al. 2008; Head and Gibbard 2015b;
Gibbard and Head 2020). The boundary occurs immediately above the regionally widespread and prominent
Ontake-Byakubi-E (Byk-E) tephra bed (Suganuma et al.
2015; Hyodo et al. 2016; Takeshita et al. 2016; Okada et al.
2017; Haneda et al. 2020b). The combination of the M–B
boundary, the marine isotope stratigraphy, and the
Byk-E tephra bed allows paleoenvironmental proxies
within the CbCS to be correlated at local, regional,
and global scales. The Global boundary Stratotype
Section and Point for the Middle Pleistocene Subseries and Chibanian Stage is placed at the base of the
Byk-E tephra bed at the Chiba section (Head, 2019;
Suganuma et al. 2021).
In this study, we synthesize paleoceanographic recordsof the CbCS, including microfossil fauna and flora, foraminiferal isotope (δ18O and δ13C), and Mg/Ca-based
temperature records, to achieve an objective understanding of marine paleoenvironmental changes characterizing the late MIS 20 – early MIS 18 interval. We thencompare the CbCS records with published SST reconstructions at lower and higher latitudes in the western
North Pacific and with terrestrial records in East Asia, toexplore paleoenvironmental links between the CbCS andregional to global climate oscillations. In addition to the
published CbCS data in Okada et al. (2017), Suganuma
et al. (2018), and Haneda et al. (2020a), we newly reporton an Mg/Ca-based temperature record for Globigerina
bulloides and on high-resolution δ13C data for G. bulloides
and Globorotalia inflata.

WPWP to the mid-latitude western North Pacific. Further downstream, the Kuroshio Current enters thedeep Shikoku Basin from the East China Sea throughthe Tokara Strait and flows northeastward until 140° E
and 35° N (Qiu 2001). After diverging from the Japanese coast, the Kuroshio Current turns eastward andis renamed the Kuroshio Extension (Qiu 2001).
Regarding seasonal variations for the Kuroshio Current,
volume transport is typically higher in July/August and isreduced in October in the East China Sea (Kawabe 1988;
Andres et al. 2008). A branch of the Kuroshio Current, theRyukyu Current, a subsurface-intensified current, flows on
the forearc side of the Ryukyu Islands, although its mean
volume transport is much less than for the Kuroshio
Current, and has large temporal and spatial variability(Andres et al. 2008, 2015). The Kuroshio Current
strengthens from a mean downstream transport of about
15 Sv east of Luzon to 24 ± 0.9 Sv in the East China Sea(Andres et al. 2015). The Kuroshio further strengthens to
65 ± 6 Sv off Shikoku, fed in part by the Ryukyu Current(Zhu et al. 2003; Andres et al. 2015). South of Japan, theKuroshio path meanders with a 2–10 year cyclicity (Kawabe
1995). Seasonal variation in the Kuroshio volume transport
is highly variable south of Japan (~ 133.5° E), varying withthe year of observation (Book 2002; Kakinoki et al. 2008).
Interannual to decadal variation in the Kuroshio Current's
volume transport is controlled by wind stress curl over the
Pacific (Andres et al. 2011). Wind stress curl is induced by
the trade winds and westerlies over the central North Pacific, and Rossby waves propagate variations in the windstress far westward (Deser et al. 1999). The clockwise windstress curl is related to the strength of the Aleutian Low(AL) and westerlies on a decadal timescale (Hanawa andKamada 2001).
Water masses of the Kuroshio Current are characterized by a salinity maximum at depths of 100–250 m anda salinity minimum at 500–700 m, with the main
thermocline occurring between them (Fig. 1c and d;
Nitani 1972; Masuzawa 1972). The water in the salinity
maximum zone originates from North Pacific Tropical
Water (NPTW), formed at the sea surface by strong
evaporation at 20–30° N in the central region of theNorth Pacific (Oka and Kawabe 1998; Katsura et al.
2013). NPTW subducts from the surface to a depth of
200 m in the southern part of the subtropical gyre, andis then advected westward by the North Equatorial
Current and poleward by the Kuroshio (Tsuchiya 1968).
Variations in the NPTW are linked to wind strength
over the subtropical gyre (Shuto 1996), with wind intensification leading to an increase in the transport of
NPTW (Suga et al. 2000). Remnants of NPTW in the
Kuroshio are recognizable as a subsurface salinity maximum and presumably contribute to the formation of
the broad subsurface salinity maximum that is typical in

Kubota et al. Progress in Earth and Planetary Science

1.1 Modern oceanographic setting

The North Pacific is largely divided into two main
circulatory systems, the subtropical and subarctic gyres(Qiu 2001) (Fig. 1). The subtropical gyre is maintained by
the trade winds in the tropics and the westerlies at midlatitudes (Qiu 2001). The trade winds drive equatorial surface current systems westward in the tropical Pacific,
resulting in a large warm area (> 28 °C) distributed in thewestern tropical Pacific (De Deckker 2016). Known as
the western Pacific Warm Pool (WPWP), it serves as
the heat engine of the globe (De Deckker 2016).
The Kuroshio Current branches from the North Equatorial Current generally at 12–13° N off the Philippines
in the northern part of the WPWP, flowing northwards
where it enters the East China Sea from the east of
Taiwan (Qiu 2001). The Kuroshio Current, therefore,transports a large quantity of heat and salt from theKubota et al. Progress in Earth and Planetary ScienceFig. 1 (See legend on next page.)(2021) 8:29

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Kubota et al. Progress in Earth and Planetary Science(2021) 8:29

Page 5 of 24(See figure on previous page.)
Fig. 1 The current system in the western North Pacific and latitudinal water properties, showing a regional current systems (Qiu 2001) and thelocation of the Chiba composite section (CbCS) b the two North Pacific gyres based on Qiu (2001) and core site locations. Latitudinal c
temperature (Locarnini et al. 2013), d salinity (Zweng et al. 2013) profiles in the Kuroshio and Kuroshio–Oyashio mixed zone obtained from the
red line in panel a

the northwestern part of the subtropical gyre (Suga et al.
2000).
The subarctic gyre in the North Pacific drives anotherwestern boundary current, the Oyashio Current (Qiu
2001). The Oyashio Current originates from the East
Kamchatka Current in the Sea of Okhotsk but has different water properties including high levels of dissolved
oxygen in the upper 700 m. The total volume transportof the Oyashio reaches 20–30 Sv in winter and springand decreases to 3–4 Sv in summer and fall, which isconsistent with the annual rhythm of the wind-driven
subtropical gyre. The Oyashio branches into two paths:
one turns east at 42° N southeast of Hokkaido, while theother continues southward along the margin of Japan as
far as 38.7° N. Below the surface of the Oyashio Current,
North Pacific Intermediate Water (NPIW) extends tointermediate depths (300–800 m) that are well-definedby a salinity minimum (e.g., Talley 1993; Yasuda 1997;
Yasuda 2004). The NPIW originates from the Okhotsk
Sea Intermediate Water (~ 200–1000 m deep), which ischaracterized by low temperatures (1.0–1.5 °C), lowsalinity (33.5–33.7), and high oxygen content (3.3 to 4.9mL/L) (Itoh et al. 2003). The Okhotsk Sea Intermediate
Water is formed by Western Subarctic Pacific Water
that is modified by surface cooling and freshening in the
Okhotsk Sea and dense shelf water produced by brine
rejection during the growth of sea ice along the northern
shelves of the Okhotsk Sea (Yamamoto et al. 2002). The
NPIW is mainly formed along the Kuroshio Extension(Hiroe et al. 2002) and possesses a salinity range of
33.9–34.1 (Bostock et al. 2010). The low salinity NPIW
reflects the dominance of precipitation in the subpolar
North Pacific. Thus, it can be regarded as the lowsalinity counterpart of the high salinity NPTW (Sugaet al. 2000).
At present, the Kuroshio Current’s warm and saline
water marginally reaches the CbCS (Fig. 1a–d). The
Kuroshio and Oyashio currents mix in the area between
35 and 42° N, forming steep latitudinal SST and sea surface salinity (SSS) gradients (Fig. 1c and d). The annual
SST in the Kuroshio region (~ 34° N) is ~ 22 °C, dropping to 16 °C in the Kuroshio–Oyashio mixed zone (~38° N) (Fig. 1b and c). Similarly, annual SSS in the Kuroshio region is 34.5, whereas in the mixed zone it is 34.1
(Fig. 1d). Accordingly, sea surface oxygen isotopes(δ18Ow) indicate + 0.1 to − 0.2‰ (VSMOW) in the Kuroshio region, and − 0.75‰ (VSMOW) near the Oyashio
Front (~ 40° N) (δ18Ow = 0.521, SSS = − 17.955; Obaet al. 2006). These SST and SSS latitudinal gradientsaffect foraminiferal calcite δ18O in the Kuroshio–Oyashio
mixed zone as follows. Foraminiferal δ18O changes
by + 0.32‰/°N (− 1.5 °C/°N) due to the effect of SST(− 0.21‰ °C−1, Bemis et al. 1998) and only by − 0.05‰/°N(− 0.1 in salinity/°N) due to SSS (Oba et al. 2006) between
34° N and 38° N (Fig. 1c and d).
AL activity during the winter dominantly impacts
upper oceanic states in the North Pacific (Sugimoto andHanawa 2009). Variations in SSTs in the Kuroshio–Oyashio mixed zone and the magnitude of the subtropicaland subpolar gyres are primarily caused by AL activity(Latif and Barnett 1996; Ishi and Hanawa 2005; Yasudaand Sakurai 2006). When the location of the AL shifts tothe east (west) in its strengthening (weakening) phase,westerlies strengthen (weaken) in association with anenhancement (diminishment) of both subtropical andsubarctic gyres (Sugimoto and Hanawa 2009). This longitudinal shift occurs on an interdecadal timescale,whereas the latitudinal change of the AL shifts on a timescale of about ten years (Sugimoto and Hanawa 2009).
The latitudinal shift is independent of the variation in intensity of the AL and causes a meridional shift of thegyre boundary (Sugimoto and Hanawa 2009). Accordingly, SSTs in the Kuroshio–Oyashio mixed zone andKuroshio Extension area change in association withthe latitudinal shift of the AL and the westerlies(Sugimoto and Hanawa 2009). An enhanced AL istightly coupled with an intensified East Asian wintermonsoon (EAWM) (Chang et al. 2006). On an interannual scale, the EAWM tends to be weaker in El
Niño years and stronger in La Niña years (e.g., Zhang
et al. 1996).
In terms of nutrients, Kuroshio water is relatively
oligotrophic resulting in low biological productivity inthe downwelling-dominant subtropical gyre, whereas the
Oyashio is rich in nutrients fed by the upwellingdominated subarctic gyre in the north (Qiu 2001).
Therefore, nutrient levels in the upper part of the watercolumn in the Kuroshio–Oyashio mixed zone arecontrolled by either water mass advected into the area.

2 Methods
2.1 Chiba composite section and age model

The Chiba composite section (CbCS) comprises the
Urajiro, Yanagawa, Yoro River, Yoro-Tabuchi, andKokusabata sections (Suganuma et al. 2021). We used
the age model of Suganuma et al. (2018), this being

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the latest age model and based on the composite benthic δ18O stratigraphy astronomically tuned to the sea
level proxy curve of Ocean Drilling Program (ODP)
Site 1123 off New Zealand (Elderfield et al. 2012).
This chronology is supported by the U-Pb radiometricage of the Byk-E tephra (Suganuma et al. 2015). The
age model uncertainty is thought to be ca. 5 ka as inferred from the chronologic uncertainty of 4 ka inthe LR04 stack used as a target curve by Elderfield
et al. (2012) (Lisiecki and Raymo 2005) plus anotherca. 1 ka of uncertainty in our tuning to the Elderfield
et al. (2012) record (Suganuma et al. 2021).

parts but microstructures such as the spine bases are wellpreserved. The roughened surfaces probably representslight damage by dissolution. However, the very low incidence of apparent fragmentation of the foraminiferal tests
for the CbCS indicates that the extent of dissolution was
minimal.
The δ13C measurements were carried out in two laboratories. For G. inflata, the YT, TB, and TB2 samples
were analyzed using a Finnigan-MAT253 isotope mass
spectrometer coupled with a Kiel IV carbonate preparation device at the Department of Geology andPaleontology, National Museum of Nature and Science(NMNS), Tsukuba, Japan. The KG, YG, YN and YW
samples were mainly analyzed using a GV Instruments
IsoPrime with the Multicarb preparation system at theCenter for Advanced Marine Research, Kochi University(KU), Kochi, Japan. We did not recognize any systematicdifferences in the results of the two laboratories forG. inflata. For G. bulloides, all samples were analyzed at
the NMNS. It is known that the δ13C of G. bulloides
varies with its size (Bemis et al. 2000; Birch et al. 2013). As
we used two size fractions for G. bulloides, we found thatthe smaller size (YT, TB, TB2) yielded 0.27‰ lower
values, on average, than the larger size (YN, YW, YG, KG)(Additional file 5). Thus, we subtracted the offset of
0.27‰ from the YN and YW samples in the composite
data. International standards NBS-19 and CO-1 were used
to calibrate the measured isotopic values to the Vienna
Pee Dee Belemnite (VPDB) standard. The Porites coralstandard JCp-1 was utilized as the laboratory standards.
The standard deviation of the δ13C measurements was
0.014‰ based on 376 measurements of the NBS-19
standard at the NMNS, and 0.025‰ based on 52 measurements of the CO-1 standard at KU. The composite datafor δ13C include the YT, TB, TB2, YW samples, andSample YN08.

Kubota et al. Progress in Earth and Planetary Science

2.2 Carbon isotope analysis

We here present a high temporal resolution δ13C record
for planktonic foraminifers G. bulloides and G. inflatafrom the CbCS, which was analyzed together with thealready published δ18O records in Okada et al. (2017),
Suganuma et al. (2018), and Haneda et al. (2020a). The
temporal resolution of the composite data set is 170 years
for G. bulloides (n = 309) and 160 years for G. inflata (n =336). The sampling interval is 35 cm on average. The
horizons analyzed are listed in Additional files 1 and 3.
The samples, all siltstones, were disaggregated with sodium sulfate and/or using a SELFRAG high voltage pulse
fragmentation system installed at the National Instituteof Polar Research (NIPR) in Tokyo, Japan. The nonmagnetic fraction, including foraminifers, was then extracted from the siltstone samples using an isodynamic
separator at Ibaraki University. We manually picked
foraminiferal tests from the non-magnetic fraction foreach sample. For each sample, where possible, we selected more than 20 individuals of G. inflata with a test
diameter greater than 250 μm for the δ13C analysis. We
used G. bulloides from the > 250 μm size fraction for the
Yanagawa (sample ID: YN and YG), Urajiro (sample ID:
YW), and Kokusabata (sample ID: KG) sections, and
150–250 μm size fraction for the Yoro River (sample ID:
TB and YT) and Yoro-Tabuchi (sample ID: TB2) sections due to the low abundance of this species (Additional file 1). It was occasionally necessary to use fewer
than 20 individuals in samples where abundances of thisspecies were low.
The preservation of foraminifers was generally good to
excellent with very slight fragmentation (Suganuma et al.
2018). Preservation was assessed under the stereomicroscope and SEM (Pearson and Burgess 2008). The
foraminiferal tests were translucent and “glassy” understereomicroscopy except for thickly crusted G. inflata,
indicating no apparent diagenetic features (i.e., dissolution,
overgrowth, and recrystallization). The finer-scale structures of typical foraminiferal tests for G. bulloides wereobserved under SEM (Additional file 4). The SEM photographs indicate that the surfaces are slightly rough in some

2.3 Mg/Ca analysis

For Mg/Ca analysis, we used G. bulloides from the> 250 μm size fraction for 1.0-m and 0.2-m spaced
samplings for the Yanagawa (YN), Urajiro (YW), andKokusabata (KG) sections (Additional file 1). These foraminiferal tests had been collected previously for isotope
analysis. Mg/Ca analysis was conducted on 42 samples
from the CbCS (Additional files 1, 2 and 3). The temporal
resolution achieved on average was 1.2 kyr, although thedata are sparse for most of MIS 19 due to the low abundance of G. bulloides from this size fraction. The samples
were cleaned using the “reductive” cleaning protocols ofBoyle and Keigwin (1987), which includes rinsing withultrapure water and methanol, reduction, oxidation, andleaching with 0.001M HNO3 in this order (Barker et al.
2003; Martin and Lea 2002). After these cleaning steps,

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the foraminiferal samples were dissolved in ~ 1 mL of 0.3
M HNO3.
Mg/Ca analysis was performed using a Finnigan
ELEMENT XR sector-field inductively coupled plasma
mass spectrometer at the Mutsu Institute for Oceanography, Japan Agency for Marine-Earth Science andTechnology. Isotopes of three elements (24Mg, 44Ca,
48
Ca, and 55Mn) were analyzed using Sc as the internal
standard (Uchida et al. 2008; Kubota et al. 2010, 2019).
The measurement was carried out in medium resolution mode (m/Dm = 4000). Mn/Ca was routinely
monitored to evaluate the effectiveness of the removalof the Mn-oxide coating by the cleaning procedure. We
used the SPEX Claritas PPT® single standard solution to
make standard solutions. Four standard solutions, used
to determine the concentration of each element, wereprepared by successive dilutions of a multi-elemental
stock standard solution (Ca, Mg, Mn) to match theconcentrations of Ca (~ 0.04–5 ppm) and Mg (~ 0.08–
10 ppb), and Mn (~ 0.02–2 ppb). To all samples andstandard solutions, an Sc solution was added as an internal standard to adjust the Sc concentration to 1 ppb.
We conducted replicate measurements of working standards (Ca ~ 1 ppm) every six samples or fewer for eachrun. The relative standard deviation (RSD) of Mg/Ca
for 24 replicate measurements of the working standard
was 1.1% for five runs, which is equivalent to ~ 0.1 °C
in Mg/Ca-based temperature reconstruction. Mn/Ca
values on average were 40 μmol/mol, and no values
exceeded 104 μmol/mol, indicating negligible Mn-oxide
contamination.
Mg/Ca values for G. bulloides were converted to atemperature scale using the calibration of Mashiotta
et al. (1999) as follows:

and Florisphaera account for 90% of the total calcareous
nannofossils, obscuring the rare taxa that characterizetypical marine environments (Suganuma et al. 2018). We
therefore used the results of an additional 100 counts
for subordinate taxa after 200 counts for major taxa
(Kameo et al. 2020). The relative abundances of C.
leptoporus, C. pelagicus braarudii, C. pelagicus pelagicus, Umbilicosphaera spp., and Helicosphaera spp.
are expressed as those of the subordinate taxa,
whereas F. profunda abundances are based on the
total nannofossil count.
For the radiolarians, we used six taxa, Tetrapyle spp.,
Spongodiscus resurgens, Lithomelissa setosa, Didymocyrtis spp., Dictyocoryne spp., and Cycladophora davisiana;
and for the present study we present 63 horizons adding
to the data published in Suganuma et al. (2018) (Additional file 3). Planktonic foraminiferal assemblage data
(Suganuma et al. 2018) were not included in PCA since
their temporal resolution was much lower than for calcareous nannofossils and radiolarians.
The original micropaleontological data used in thisstudy are expressed as relative abundances (e.g., percentage of the taxon to the total fauna/flora). The
sum of the relative abundance data for each sampling horizon is therefore necessarily constant(100%), violating underlying assumptions used with
PCA such as the independence of each dataset(Aitchison 1982, 1986; Kucera and Malmgren 1998).
Log-ratio transformation of the raw percentage data
has been proposed to avoid the mathematical problems caused by this constant-sum constraint for geological data (Aitchison 1982, 2003). The effect of theconstant-sum restriction on covariance and correlation matrices is removed when the relative abundances are expressed as logarithms of ratios, where
the denominator is the geometric mean of the percentages in each sample (Aitchison 1982, 1986;
Kucera and Malmgren 1998). In this study, the raw
percentage data were transformed into log-ratio data
following the method proposed by Aitchison (1982)
and Kucera and Malmgren (1998). Zero elements ineach data set were replaced by one-half of the lowest
value of the data set (Aitchison 1982). Strictly speaking, the relative abundances of subordinate taxa forcalcareous nannofossils are not equivalent to thoseof major taxa (e.g., F. profunda). However, the temporal variation of the subordinate taxa was comparable to that of F. profunda as ultimately standardized.
The temporal resolutions of calcareous nannofossil
and radiolarian records are 0.7 kyr and 0.4 kyr, respectively. Prior to PCA, the CbCS records with different time-resolutions and different variances werestandardized and linearly interpolated every 100 years
from 751.8 to 800.2 ka using MATLAB ® (version R2020a).

Kubota et al. Progress in Earth and Planetary Science

Mg=Ca ¼ 0:474 expð0:107T Þ
where T represents temperature.
2.4 Principal component analysis
2.4.1 Floral and faunal records

We conducted principal component analysis (PCA) on
multi-proxy records of the CbCS, comprising floral,
faunal, and geochemical data. For PCA, we selected taxa,
listed in Table 1, that reflect particular water mass types,
in this case the Kuroshio and Oyashio currents. For calcareous nannofossils, we used six taxa, Calcidiscus leptoporus, Coccolithus pelagicus braarudii, Coccolithus
pelagicus pelagicus, Umbilicosphaera spp., Helicosphaeraspp., and Florisphaera profunda (Suganuma et al. 2018;
Kameo et al. 2020). The relative abundance of F. profunda was based on a count of 200 specimens of thesmear slide (Suganuma et al. 2018; Kameo et al. 2020).
However, the genera Gephyrocapsa, Pseudoemiliania,

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Table 1 The results of principal component analysis (PCA) on the Chiba composite section records. The percentage of the total
variance explained and correlation coefficient (loading) for each mode are presented according to taxa
Principal component mode
PC1

PC2

PC3

PC4

PC5

PC6

8.2

6.5

5.1

Percentage of the total variance explained
36.3

15.4

9.7

Microfossil group

Taxa

Radiolarians

Tetrapyle spp.

-0.14

0.14

0.34

0.15

0.49

0.12

Spongodiscus resurgens

0.07

0.25

0.42

-0.05

0.16

0.33

Lithomelissa setosa

0.27

-0.35

-0.07

-0.08

0.19

-0.43

Didymocyrtis spp.

-0.21

0.01

-0.16

0.25

-0.36

-0.28

Calcareous nannofossils

Geochemical proxies

Principal component coefficients (loadings)

Dictyocoryne spp.

-0.17

0.43

-0.01

0.13

-0.01

0.01

Cycladophora davisiana

0.00

-0.16

-0.22

-0.34

-0.32

0.59

Calcidiscus leptoporus

-0.20

-0.16

-0.25

0.50

0.06

0.19

Coccolithus pelagicus braarudii

0.34

0.11

-0.09

-0.23

-0.10

-0.04

Coccolithus pelagicus pelagicus

0.05

0.24

0.02

-0.47

0.04

-0.18

Umbilicosphaera spp.

-0.35

0.10

0.08

0.00

-0.14

-0.06

Helicosphaera spp.

-0.20

-0.37

0.16

0.08

0.02

0.02

Florisphaera profunda

-0.33

-0.04

0.11

-0.28

0.11

-0.21

G. bulloides δ13C

-0.26

-0.16

-0.33

-0.23

0.37

0.01

G. bulloides δ18O

0.32

-0.08

-0.04

0.15

0.30

0.03

G. inflata δ C

-0.27

-0.35

0.21

-0.20

0.05

-0.09

G. inflata δ18O

0.39

-0.15

0.14

0.20

0.00

-0.01

13

G. inflata Mg/Ca

-0.02

0.35

-0.09

0.09

-0.05

-0.35

δ13C difference (Δδ13Cinf-bul)

-0.01

-0.19

0.54

0.03

-0.32

-0.09

δ18O difference (Δδ18Oinf-bul)

0.08

-0.07

0.18

0.04

-0.30

-0.04

2.4.2 Geochemical records

We used the composite records of oxygen and carbon
isotopes (δ18O and δ13C) for G. bulloides and G. inflata,
the differences in δ18O and δ13C between the two species, and the Mg/Ca-based temperature reconstructionsof G. inflata (Table 1) in PCA. Among them, the δ13C
records are newly published in this study, and the δ18O
and Mg/Ca data for G. inflata are published in Haneda
et al. (2020a) and Suganuma et al. (2018) (Additional file3). We excluded the Mg/Ca data for G. bulloides from
PCA because the temporal resolution is low especiallyduring MIS 19c and the following interstadials owing to
the low abundance of this species within the examinedinterval.
We used the published Mg/Ca values of G. inflata(Suganuma et al. 2018) and converted these totemperature using the following species-specific equation for G. inflata by Anand et al. (2003):
Mg=Ca ¼ 0:56 expð0:058T Þ

where T denotes temperature. We chose the equation
above instead of that by Anand et al. (2003) as used in
Suganuma et al. (2018) because it shows realistic temperatures for the Late Holocene (Sagawa et al. 2011).
Note that the process of standardization for the Mg/Cabased temperature before PCA leads to the same PCA
results as other Mg/Ca-temperature equations proposed
elsewhere that give a range of absolute temperature
values. The geochemical proxies presented were standardized before PCA. As with the faunal and floral data,
the age was linearly interpolated every 100 years from751.8 to 800.2 ka. PCA was performed in MATLAB®
(version R2020a) using the “pca” function.

3 Results
3.1 Mg/Ca-temperature reconstruction for G. bulloides

The Mg/Ca values for G. bulloides range between 1.51
and 4.62 mmol/mol: 10.8–21.8 °C on the temperaturescale (Fig. 2a and Additional file 3). The Mg/Ca
temperature shows relatively high values (~ 19 °C) at~800 ka, decreasing to the lowest values (~ 11 °C) duringKubota et al. Progress in Earth and Planetary Science(2021) 8:29

Page 9 of 24

Fig. 2 Marine records of the Chiba composite section (CbCS) showing a δ18O (red) and Mg/Ca-based winter temperature (green) for G. bulloides,b δ18O (purple) and Mg/Ca-based winter temperature (green) for G. inflata (Suganuma et al. 2018; Haneda et al. 2020a), c δ13C for G. bulloides(blue) and for G. inflata (orange), d Δδ13Cinf-bul (yellow) and Δδ18Oinf-bul (black), e, f microfossil relative abundances for e radiolarians (Suganuma
et al. 2018 and this study), f calcareous nannofossils (Kameo et al. 2020), g benthic δ18O in the CbCS (Suganuma et al. 2018), h comparison to
global mean δ18O of water (δ18Ow) of Elderfield et al. (2012) (blue) and the benthic δ18O stack of Lisiecki and Raymo (2005) (black). Younger
Dryas-type cooling events during Termination IX and distinct millennial events in δ18O of G. bulloides during MIS 19a are highlighted in gray andyellow. Arrows in Fig. 3c represent negative peaks in δ13C of G. inflata mentioned in the text. Figure 2 illustrates the data from 801 to 751 ka.

termination IX. Subsequently, the temperature increased
to ~ 19 °C in late MIS 19c and maintained high values
until MIS 18. As mentioned above, the temporal resolution is low for MIS 19.
3.2 Carbon isotopes

The average and standard deviation of the composite δ13C for G. inflata (δ13Cinf) over the entire succession
from 801.13 to 747.56 ka (n = 336) is 0.65‰ and0.26‰, respectively. δ13Cinf varies between − 0.15 and1.11‰ (Additional file 3), showing low values from MIS
20 to early MIS 19c and high values in MIS 19b throughearly MIS 18, and a decreasing trend toward the middleof MIS 18 (Fig. 2c). Three distinctive negative peaks
were found in late MIS 19a with an amplitude of
0.3–0.4‰.
The average and standard deviation of the composite δ13C for G. bulloides (δ13Cbul) over the entire

succession from 801.13 to 747.56 ka (n = 309) is −1.63‰ and 0.28‰, respectively. δ13Cbul values are lower
than for δ13Cinf, varying between − 2.55 and − 0.82‰(Additional file 3). The temporal trend in δ13Cbul is different from that in δ13Cinf, showing low values in MIS 20
through early MIS 19c (Fig. 2c). δ13Cbul increases during
MIS 19c, reaches highest values during MIS 19b, and decreases thereafter. Short-period oscillations are superimposed on these long-term variations. Two negative δ13Cbul
peaks at 764.5 ka and 760.0 ka in MIS 19a correlate with
two of three distinctive negative δ13Cinf peaks, but the following δ13Cinf peak seem not to be associated with theδ13Cbul record.
3.3 PCA results

The PCA results indicate that the leading mode (PC1)carries 36.3% of the total variance, characterized by highloadings in the δ18O records for G. inflata (δ18Oinf) and G.

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bulloides (δ18Obul), the calcareous nannofossil C. pelagicus
braarudii, and the radiolarian L. setosa (Table 1). Temporal variation in PC1, the best representative of theδ18O data, exhibits a glacial–interglacial cycle, showing
high scores in MIS 20 and a decreasing trend at
Termination IX (Fig. 3a). PC1 shows lowest scores in
middle to late MIS 19c and increases subsequently inassociation with short-period variations. Althoughmillennial-scale fluctuations superimposed on orbitalscale variability are also prominent in the planktonic
δ18Obul and δ18Oinf records during Termination IX,
MIS 19b, and MIS 19a (Suganuma et al. 2018; Haneda
et al. 2020a), these fluctuations are not fully expressedin PC1 (Fig. 3a). In particular, three negative peaks areprominent in both the δ18Obul and δ18Oinf records during MIS 19a (763–757 ka) but they are marginally seenin PC1. In contrast, a prominent positive peak in PC1 at~772 ka in MIS 19b coincides with the positive peakin δ18Obul. A positive peak in PC1 at ~761 ka in MIS
19a is also prominent and correlated to δ18Obul
and δ18Oinf. PC1 also shows negative loadings in thecalcareous nannofossil taxa Umbilicosphaera spp. andF. profunda and in δ13Cinf and δ13Cbul, in descendingorder (Table 1).
The second mode (PC2) carries 15.4% of the total variance and is characterized by high loadings in the Mg/
Ca-temperature for G. inflata and the radiolarian Dictyocoryne spp. (Table 1). Negative loadings are found inδ13Cinf, the calcareous nannofossil Helicosphaera spp.,
and radiolarian L. setosa. PC2 exhibits mostly negativescores in MIS 20, reaches highest scores in early MIS
19c, and shows a decreasing trend in MIS 19c throughMIS 19a (Fig. 3b). This rapid transition following theearlier peak in MIS 19c contrasts with the more gradualchanges seen in PC1. The Mg/Ca-temperature of G.
inflata and the δ13Cinf record oscillate greatly during late
MIS 19a and in MIS 18, but the millennial-scale variations of PC2 exhibit more similarity to δ13Cinf than to
the Mg/Ca-temperature of G. inflata.
The third mode (PC3) contributes 9.7% of the total
variance and is dominated by variations in the radiolarian S. resurgens, Tetrapyle spp., and Δδ13Cinf-bul (Table1). The calcareous nannofossil C. leptoporus, radiolarian
C. davisiana, and δ13Cbul all show negative loadings for

Kubota et al. Progress in Earth and Planetary ScienceFig. 3 Results of principal component analysis (PCA) compared to related geochemical proxies in the Chiba composite section, for a leading
mode (PC1), b second mode (PC2), and c third mode (PC3). The geochemical proxies presented are standardized. Highlighted events are thesame as for Fig. 2

(2021) 8:29

Page 11 of 24

PC3. PC3 does not display a typical glacial–interglacial
pattern but oscillates greatly on the millennial tomulti-millennial scale over the entire examined interval(Fig. 3c).

local salinity and global ice volume) with small speciesspecific disequilibrium factors (< 1‰, Rohling and
Cooke 1999). Suganuma et al. (2018) posited for the
CbCS that a difference in δ18O between G. inflata andG. bulloides (δ18Oinf – δ18Obul = Δδ18Oinf-bul) reflected avertical density (representing dominantly temperature)gradient between subsurface and surface waters and thata large Δδ18Oinf-bul reflected stratification of the ocean.
The foraminiferal δ13C is also determined by equilibrium fractionation and disequilibrium factors, but its interpretation is complicated by a highly variable ambient
seawater δ13C and disequilibrium factors that are notwell known (Rohling and Cooke 1999). Off east-central
Japan today, the δ13C of the dissolved inorganic carbon(DIC, hereafter δ13CDIC) in the ocean is primarily determined by fluctuations between Kuroshio and Oyashio
waters (Oba et al. 2006) that control nutrients at the surface, local primary productivity, and export production(Rohling and Cooke 1999). Since the δ13CDIC gradient in
the water column between the surface and subsurface is
steep, changes in the depth of calcification may cause
deviations in the quantity of δ13C observed in foraminiferal calcite at the studied site (Oba et al. 2006). At thesurface, the δ13CDIC is higher in the Oyashio Current region (~ 2‰) than in the Kuroshio Current region (~1‰) (Oba et al. 2006). This surface δ13CDIC gradient iscaused by high primary productivity in the Oyashio
Current, where phytoplankton preferentially utilize 12C
during photosynthesis. Generally, δ13CDIC has highvalues at the surface that diminish with depth owing to
the remineralization of organic matter in the water column. In the Oyashio region, the δ13CDIC for the subsurface (100–400 m) rapidly decreases from ~ 0 to ~− 0.7‰
with depth, while in the Kuroshio region it decreases
from ~ 0.8 to ~ 0.5‰ (Oba et al. 2006).
The δ13C disequilibrium is species (or specimen) specific and related to the (1) photosynthetic activity ofsymbionts, (2) incorporation of metabolic CO2, (3)growth rate, and (4) carbonate chemistry in the ambient
waters (Rohling and Cooke 1999). The non-spinose species are less affected by these biological fractionation effects because of their lack of symbionts (Ravelo andFairbanks 1995). Ravelo and Fairbanks (1995) suggested
that disequilibrium fractionations (kinetic and biologicalfractionation effects) vary as a function of temperature
based on a positive δ13C–δ18O relationship in G. inflatafrom core-top samples in the tropical Atlantic. Globigerina bulloides is a spinose but non-symbiont-bearing species and has the potential to record in situ δ13CDIC. The
δ13Cbul varies as a function of temperature; decreasing
0.11‰/°C as more δ13C-depleted respired CO2 is incorporated into shell carbon at a higher metabolic rate
(Bemis et al. 2000). This metabolic-related temperatureeffect is mainly seen in opportunistic species, such as G.

Kubota et al. Progress in Earth and Planetary Science

4 Discussion
4.1 Interpretation of geochemical proxies

The planktonic foraminifer G. bulloides is most abundantat depths shallower than 100 m in the Pacific Ocean off theJapanese archipelago (Arikawa 1983; Kuroyanagi and Kawahata 2004; Oba and Hattori 1992). This species tends to beabundant in winter in the Okinawa Trough (Yamasaki andOda 2003; Yamasaki et al. 2010; Xu et al. 2005) and Ryukyu
Trench (Xu et al. 2005), but its flux depends on the positionof the Kuroshio Current in the Okinawa Trough (Xu et al.
2005). In contrast, large fluxes of G. bulloides have been
observed in spring (April–May) and autumn (October–November) in the subarctic region (Itou 2000; Kuroyanagi etal. 2002) and Kuroshio–Oyashio mixed zone (Oda andYamasaki 2005). Nevertheless, the flux of G. bulloides depends greatly on the meanders of the Kuroshio Current
and associated nutrient supply in the Kuroshio–Oyashio
mixed zone (Oda and Yamasaki 2005). Thus, Oda andYamasaki (2005) concluded that G. bulloides attains its
maximum abundance at times of greatest food availability.
Although the highest flux of G. bulloides would be anticipated during the phytoplankton blooming season, the reconstructed Mg/Ca-SST of G. bulloides for the late
Holocene was ~ 3 °C lower than the alkenone-based SST
that represents a blooming season in early summer (Sagawa
et al. 2006; Yamamoto et al. 2005). Given that G. bulloides
is a surface-dwelling species, its calcification depth is likely
similar to the phototrophic haptophyte algae that produce
the alkenones. We therefore interpret G. bulloides abundances as reflecting the early months of the blooming season (i.e., late winter through spring) in the Kuroshio–
Oyashio mixed zone.
The planktonic foraminifer G. inflata is a subsurface
dweller and typifies the central water mass near thenorthern margin of the Kuroshio Current (Coulbourn etal. 1980; Vincent and Berger 1981). In the western North
Pacific, its calcification depth toward the south is deep(~ 300–400 m at 28° N; Sagawa et al. 2011) while slightly
shallower (> 100 m) in the Kuroshio–Oyashio mixedzone at 35° N (Oba and Hattori 1992). Globorotalia
inflata prefers dwelling in a constant temperature at
subsurface depths (100–400 m) during the cold season
in the Kuroshio–Oyashio mixed zone (Oda and Yamasaki 2005). We therefore interpret the geochemical proxies of G. inflata to reflect the subsurface environment
with a bias toward the winter season (Suganuma et al.
2018).
The foraminiferal δ18O is primarily determined bytemperature and the δ18O of the ambient water (e.g.,(2021) 8:29

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bulloides, which have short life cycles and likely havehigh metabolic rates throughout life (Birch et al. 2013).
The δ13Cbul in the CbCS shows lower values than that of
G. inflata, which is consistent with the previous study by
Oba et al. (2006).
At the studied site, the low SST of the Oyashio
Current is expected to have increased the δ13Cbul because of the temperature effect on the foraminiferal calcite δ13C (Bemis et al. 2000). On the other hand,concerning the change in the surface δ13CDIC between
the Kuroshio and Oyashio water regions, the influenceof the Oyashio is expected to have increased the δ13Cbul(Oba et al. 2006). Indeed, the δ13Cbul off east-central
Japan exhibited higher values during the Last Glacial
Maximum when the Oyashio influence became greater(Oba et al. 2006). In contrast, the δ13CDIC gradient between the Kuroshio and Oyashio water regions in thesubsurface is opposite to that of the surface: the δ13CDICof the Oyashio waters is lower than the Kuroshio(Oba et al. 2006). However, the low temperature in
the Oyashio waters would increase the δ13Cinf (Ravelo
and Fairbanks 1995), and therefore the temperatureeffect acts to diminish the latitudinal δ13CDIC gradientin the Kuroshio–Oyashio mixed zone.
The vertical gradient in foraminiferal δ13C (δ13C of G.
inflata – δ13C of G. bulloides = Δδ13Cinf-bul), as proposed by Oba et al. (2006), is another measure ofwhether the Kuroshio or Oyashio waters were dominant
in terms of nutrient levels and productivity. As foraminiferal δ13C includes a global δ13CDIC change for the entire
ocean during glacial–interglacial cycles, the subtracting
process proposed by Oba et al. (2006) excludes theglobal signal, and therefore the Δδ13Cinf-bul indicates
local δ13CDIC change. The vertical gradient in δ13CDIC
between subsurface and surface waters is greater in
the Oyashio waters than the Kuroshio waters (Oba et
al. 2006). Given that there is a ~ 2‰ offset between
the surface δ13CDIC and δ13Cbul, the foraminiferal
Δδ13Cinf-bul would be smaller in the Oyashio waters
and larger in the Kuroshio waters (Oba et al. 2006).
Indeed, the Δδ13Cinf-bul was smaller during glacials (MIS
2 and MIS 6) and larger during interglacials (MIS 1
and MIS 5) in the Kuroshio-Oyashio mixed zone
(Oba et al. 2006).

δ18O records at the CbCS; both benthic and planktonic
δ18O values show pronounced glacial–interglacial cycles
from MIS 20 to 18 (Fig. 2a, b, and g). Particularly, Suganuma
et al. (2018) mentioned that the warmer SSTs during thelater part of MIS 19, as inferred from the low δ18Obul, most
likely correspond to the northward shift of the Kuroshio–
Oyashio boundary, supported by stratified conditions
based on the increased F. profunda and Δδ18Oinf-bul.
The temporal variation in PC1 resembles variation
shown by the δ18O (positive loading) and δ13C (negative
loading) among the geochemical proxies and tends todisplay negative scores from late MIS 19c to MIS 18(Fig. 3a). The calcareous nannofossils F. profunda andUmbilicosphaera spp., negatively correlated to the score
for PC1, are the Kuroshio- (or tropical water) related
taxa (Table 1 and Fig. 4). Umbilicosphaera spp. areabundant today in the Kuroshio axis, and F. profunda isabundant in warm waters of the central Pacific (Tanaka
1991). The radiolarian taxon Didymocyrtis spp., whichcorrelates negatively but weakly with PC1 (Table 1, Fig.
4a and b), is also the typical dominant taxon in thewarm shallow waters of the North Equatorial Current(Matsuzaki and Itaki 2017). The temporal variations inDidymocyrtis spp. agree well with those of the calcareous
nannofossils F. profunda and Umbilicosphaera spp.
(Fig. 4b). The radiolarians Tetrapyle spp. and Dictyocoryne
spp., which are subtropical taxa characteristic of theKuroshio Current region (Matsuzaki and Itaki 2017),
exhibit negative loadings, although these are relatively
weak.
The calcareous nannofossil C. pelagicus is found in the
Kuroshio–Oyashio mixed zone off the northeastern
coast of the Japanese archipelago, and is especially abundant in the northern part (Tanaka 1991). Coccolithus
pelagicus in Tanaka (1991) may include C. pelagicus
braarudii and C. pelagicus pelagicus, whereas we separate C. pelagicus braarudii from C. pelagicus pelagicus(Kameo et al. 2020). Although these two subspecies havetheir ecological preferences in the subpolar Atlantic
Ocean (i.e., C. pelagicus pelagicus prefers colder waters
than C. pelagicus braarudii) (Narciso et al. 2006), we
treat them together as a typical representative of cooler
surface waters in this study based on the observation by
Tanaka (1991). Thus, we interpret C. pelagicus braarudii
as a representative of the Oyashio or mixed waters at
the CbCS. The PCA result indicates that C. pelagicus
braarudii and the radiolarian L. setosa show positive
loadings for PC1 (Table 1, Fig. 4c). The radiolarian L.
setosa is a subarctic to arctic coastal species, and absent
offshore (Matsuzaki and Itaki 2017). These consistentfaunal and floral signatures indicate that PC1 is related
to the shallow waters of the Kuroshio or Oyashio/
mixed waters. Therefore, together with the planktonic δ18 O, we interpret PC1 as an indicator of the

Kubota et al. Progress in Earth and Planetary Science

4.2 Multi-proxy approach for the Chiba composite section

The published high-resolution geochemical and relatively high-resolution faunal and floral records(Suganuma et al. 2018), in addition to planktonic
δ13C data presented in this study, provide an excellent opportunity to examine inter-proxy relationships andenhance our understanding of the paleoceanography basedon robust evidence from multiple proxies. Suganuma et al.
(2018) overviewed temporal variations in the foraminiferal

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Page 13 of 24

Fig. 4 Results of principal component analysis (PCA) compared to microfossil relative abundances and geochemical proxies. a Scores for PC1,
PC2, and PC3. b, c Standardized microfossil relative abundances for PC1, d for PC2, and e for PC3, but the y axis for C. leptoporus is reversed.
Highlighted events are the same as Fig. 2

relative contributions of the Kuroshio and Oyashio/mixed
waters; the negative scores of PC1 representing the dominance of the shallow Kuroshio waters and vice versa.
PC2 nearly parallels the Mg/Ca of G. inflata andshows highest values during early MIS 19c (Fig. 3b). The
subsurface temperature signal, likely coupled with thevariation in the subsurface warm water mass (NPTW)
probably transported by the Kuroshio Current, is therefore closely linked to PC2. The negative loading inδ13Cinf is also high. The negative correlation between

Mg/Ca-temperature and δ13Cinf suggests that thetemperature effect is not negligible on the δ13Cinf.
Among the radiolarian taxa, a high loading for PC2 isfound in Dictyocoryne spp. (Table 1). The temporal variation in Dictyocoryne spp. has an obvious similarity tothat in the subsurface temperature (Fig. 4d). Among the
genus Dictyocoryne, D. profunda and D. truncatum arewidely found in the equatorial Pacific and the Kuroshio
region of the western North Pacific, but have greatest
abundances in the equatorial region (Matsuzaki et al.

(2021) 8:29

Page 14 of 24

2016, 2020; Matsuzaki and Itaki 2017). Dictyocoryne muelleri is also abundant in the surface Kuroshio water but
found even at the subsurface (100–200 m) and intermediate (500–700 m) depths in the East China Sea (Matsuzaki
et al. 2016). An increase in Dictyocoryne spp. at the CbCS,
therefore, suggests the more significant transport by the
Kuroshio Current from the equatorial/subtropical region.
Given that the subsurface temperature contributes significantly to PC2, we further suggest that Dictyocoryne spp. isassociated with the subsurface Kuroshio water, which isconsistent with the depth habitat of D. muelleri.
In contrast, the loading for δ18Obul is low in PC2, suggesting that the surface temperature does not contributeto PC2; PC2 is uniquely linked to the subsurface
temperature. Here, Helicosphaera spp. mainly consists of
Helicospharea carteri, Helicosphaera hyalina, and Helicosphaera pavimentum with rare Helicosphaera wallichii(Kameo et al. 2020). This taxon is abundant in the East
China Sea and also found in coastal areas along thesouthern part (< 36° N) of the Japanese archipelago(Tanaka 1991). The coastal taxa (L. setosa and Helicosphaera spp.) exhibit negative loadings for PC2,
reflecting the characteristics of the coastal waters(cool and less saline) in contrast to the subsurface waterof the Kuroshio Current (warm and saline). The decreasing trend in PC2 during middle and late MIS 19c coincides with a lowering of the subsurface temperature,suggesting less influence of NPTW. This oceanic change
may be favorable to the coastal taxa as the relative abundances of Helicosphaera spp. and L. setosa begins to increase during middle and late MIS 19c.
The temporal variation in PC3 lacks a typical glacial–interglacial pattern and is correlated withΔδ13Cinf-bul. According to the interpretation of Oba
et al. (2006), a high Δδ13Cinf-bul is led by a stratified
ocean that is closely associated with the more significant influence of the Kuroshio Current. Our results
indicate that a high Δδ13Cinf-bul mostly corresponds toa high Δδ18Oinf-bul (Fig. 2d), being consistent with thenutrient levels inferred by Δδ18Oinf-bul. The highΔδ18Oinf-bul is interpreted as a stratified water column
and vice versa (Suganuma et al. 2018). Concerning
the faunal data, PC3 indicates a relatively high correlation to the radiolarian S. resurgens. High abundancesof this species are typically characteristic of the mixture of subtropical and subarctic waters carried by the
Kuroshio and Oyashio currents where they converge andsuggest nutrient-rich waters (Matsuzaki and Itaki 2017).
However, the temporal variation of S. resurgens is in opposition to the nutrient levels suggested by the Δδ13Cinf-bul
record. Although S. resurgens is a transitional species
abundant in a broad temperature zone between 16 and 23°C (Matsuzaki and Itaki 2017), we are inclined to the interpretation that this species is better adapted to theoligotrophic waters of the Kuroshio Current than the
nutrient-rich Oyashio waters in the Kuroshio-Oyashio
mixed zone. On the other hand, the calcareous nannofossil C. leptoporus favors offshore waters far from the coastof the Japanese archipelago, as with F. profunda, but ismore abundant toward the north (26–38° N) (Tanaka
1991). The temporal variation of this species is opposite
to those of Δδ13Cinf-bul and S. resurgens (Fig. 4e), suggesting that C. leptoporus is more abundant in
nutrient-rich (low Δδ13Cinf-bul) waters. The contrast
between C. leptoporus and S. resurgens and their relationship to the Δδ13Cinf-bul record appears contradictory, but implies an inverse water mass preference interms of nutrient status. However, we note that thetemperature factor cannot be negligible in the CbCS,
as mentioned above for δ13Cinf, suggesting that theΔδ13Cinf-bul signal is a mixture of nutrient levels andthe vertical temperature gradient.
A Younger Dryas-type cooling event during Termination IX, reflected in the δ18Obul record as pronounced
positive peaks at around 788 ka and 791 ka (Suganuma
et al. 2018; Haneda et al. 2020a), is well expressed in
PC3 (Fig. 3c). A rapid increase in PC3 at ~787 ka coincides with PC2 and shows a distinct positive peak earlyin MIS 19c (Fig. 3b). Similar rises are seen in several
stratification indices (Δδ18Obull-benthic, Haneda et al.
2020a; Δδ18Oinf-bul and Δδ13Cinf-bul, this study; Fig. 2d),suggesting stratification and probably reduced nutrient
levels after the second Younger Dryas-type cooling
event. These changes indicate a rapid reorganization of
the water column structure along with subsurface warming approaching the interglacial climate of MIS 19.
Given that the relative abundances of Dictyocoryne spp.
and S. resurgens mark rapid rises respectively (Fig. 4d
and e), these taxa could have responded swiftly to thesignificant oceanographic changes following the Younger
Dryas-type cooling event.
PC3 exhibits millennial-scale variability across the entire
interval of study, with short-period variations mostly positively correlated to both Δδ13Cinf-bul and Δδ18Oinf-bul except
for a 765–752 ka interval from MIS 19a to MIS 18 (Fig. 2 d
and Fig. 3c). During the exceptional interval (765–752 ka)
mentioned above, a high Δδ13Cinf-bul is correlated to a low
Δδ18Oinf-bul, which is not merely explained by a KuroshioOyashio oscillation. In particular, three prominent negativeδ18Obul peaks, at 763 ka, 760 ka, and 757 ka, which also coincide with the low Δδ13Cinf-bul, are apparent as negativescores in PC3. These opposing Δδ13C–Δδ18O relationships
suggest a complicated mechanism underlying the Δδ13Cinf-bul record.
In summary, the two modes, PC1 and PC2, generally follow glacial–interglacial cyclicity on an orbital scale, confirming that the global climate rhythm dominantly controls
half of the total variance (PC1 + PC2 = 51.7%) of the multi-

Kubota et al. Progress in Earth and Planetary Science(2021) 8:29

Page 15 of 24

proxy records in the CbCS. The relationships between therelative abundances of individual taxa and water mass
characteristics are generally consistent among the microfossil groups and geochemical proxies (Figs. 3 and 4).

AL in the North Pacific, plays a crucial role in determining
the winter wind strength and associated North Pacificgyres. These seasonal differences operate on an orbital
scale where they are clearly archived in the Chinese
loess plateau during MIS 20–18 (Hao et al. 2012).
The late Quaternary magnetic susceptibility records,
the proxy for the EASM, in the Chinese Loess Plateau
are synchronous with the global benthic δ18O stack(Hao et al. 2012) (Fig. 5g and h). In contrast, the grainsize records for Chinese loess, the proxy for the EAWM,
generally lack the “saw-tooth” shape typical of the marine δ18O curve during MIS 19–18 (Fig. 5f and h), implying that global climate (i.e., global ice volume,
atmospheric CO2 concentration) has not been a dominant controlling factor on the EAWM (Hao et al. 2012).
In particular, middle MIS 19c through MIS 18 are characterized by a pronounced weak EAWM (Hao et al. 2012).
These previous studies proposed that this weak EAWM,
probably associated with a weak Siberian High, would be

Kubota et al. Progress in Earth and Planetary Science

4.3 Comparison among east Asian regional records andclimatological interpretation

The East Asian monsoon system is generated by seasonal thermal contrasts between the Eurasian continentand the Indo-Pacific Ocean, producing seasonal reversals
in the prevailing wind and precipitation that result inwet summers and dry winters (Wang 2006). Humidity inEast Asia is therefore mainly controlled by the East
Asian summer monsoon (EASM) system whose variation
can be recorded in Asian terrestrial proxies such as themagnetic susceptibility record of the Chinese Loess
Plateau (Guo et al. 2009). Meanwhile, the EAWM, generated during winter by contrasts in atmospheric pressure
between the Siberian High on the Eurasian continent andFig. 5 Western Pacific sea-surface temperature (SST) records for a subarctic ODP Site 882 based on alkenones (Martínez-García et al. 2010), b the
Chiba Composite Section (CbCS) based on G. bulloides δ18O (Suganuma et al. 2018; Haneda et al. 2020a) and Mg/Ca-based temperature (thisstudy), c ODP Site 1146 in the South China Sea based on alkenones (Herbert et al. 2010), and d tropical ODP Site 871 based on Mg/Ca (Dyez andRavelo 2014); in comparison with e principal component scores for the CbCS, f, g proxies from the Yimaguan and Luochuan sections of theChinese Loess Plateau showing f stacked grain size and g stacked magnetic susceptibility records (Hao et al. 2012), h benthic δ18Ow of Elderfield
et al. (2012) (blue) and benthic δ18O stack of Lisiecki and Raymo (2005) (orange)(2021) 8:29

Page 16 of 24

caused by the moderate summer insolation minima (Haoet al. 2012) or the enhanced winter insolation in the
Northern Hemisphere (Suganuma et al. 2018).
As discussed above, PC1 is a measure of the influence ofthe Kuroshio Current at the CbCS. We further interpret variations in PC1 to be coupled with the latitude of the boundary between the subtropical and subarctic gyres in the
North Pacific (Yamamoto et al. 2005; Suganuma et al. 2018)
because surface taxa of the Kuroshio and Oyashio/mixed
water and the planktonic δ18O dominantly contribute toPC1. The temporal variation pattern in PC1 is more similar
to the EAWM proxy than the EASM proxy in terms of thegradual transition during the termination and the long duration of the warmest interglacial conditions (Fig. 5e–g).
These similarities between PC1 and the EAWM proxy suggest that the EAWM exerts influence on many of the marine proxies of the CbCS. Because the wind-stress curl, whichis the primary driver of the North Pacific gyres, is greater
during winter than summer, it is reasonable to consider thatwintertime climate signals are actively recorded in the discussed marine proxies of the CbCS. However, PC1 shows along-term increasing trend after the peak of MIS 19c. This
trend is similar to global ice volume and the EASM proxy,suggesting that PC1 is subject to the global signal and
EASM as well as EAWM in the earlier in MIS 19. Although
the mostly negative scores for PC1 infer a northerly position
for the Kuroshio–Oyashio boundary during late MIS 19c
through early MIS 18, millennial-scale variations are superimposed on the long-term trend. MIS 19b and 19a are especially represented by distinct millennial-scale oscillations.
Apparent positive peaks in PC1 at ~772 ka and ~761 ka correlate with positive peaks in planktonic δ18O and to decreases in the relative abundances of Umbilicosphaera spp.
and Didymocyrtis spp., and to increases in L. setosa (Fig.
4), indicating that the millennial-scale oscillations in
PC1 during MIS 19b and 19a are closely linked to
Kuroshio–Oyashio variations. Thus, the Kuroshio–Oyashio boundary likely shifts southwards when the scores forPC1 shift to positive values (stadials), whereas the Kuroshio water dominates and the gyre boundary shifts northwards when the scores for PC1 become negative(interstadials). The stadial at ~772 ka in MIS 19b and at~761 ka in MIS 19a are predominant in the CbCS
records. In contrast to the high-resolution records from theCbCS, however, millennial-scale variations are not clearlyobservable in the Chinese monsoon proxies probably because of their relatively low temporal resolution.
As PC2 represents the subsurface, as discussed above,it further helps to understand the water column structure in the Kuroshio–Oyashio mixed zone. The temporal
pattern in PC2 begins to deviate from PC1 from early
MIS 19c (Fig. 5e), suggesting a change in subsurface
water that is independent of the ocean surface. Today,
increases in the salinity of NPTW, the subsurface watermass in the Kuroshio Current, are associated with theintensification of the AL and result in an accelerated
subtropical gyre (Suga et al. 2000). As the Kuroshio
Current carries NPTW into the mid-latitudes, the subsurface temperature in the Kuroshio–Oyashio mixed region is presumably affected by the intensity of thesubtropical gyre; relaxation of the subtropical gyre may
be associated with subsurface cooling and freshening
and vice versa. The reduced gyre circulation may occur
independently of the north–south shift of the gyreboundary (PC1), as suggested by modern observations(Sugimoto and Hanawa 2009), potentially explaining thedifference in the temporal variations between PC1 andPC2. Thus, we infer that the northerly shift in the gyreboundary during late MIS 19c and early MIS 19a after
the stadial at ~772 ka was not associated with subsurface
warming due to the suppressed gyre circulation itself
caused by the weak AL. This interpretation is consistent
with a decrease in the tropical Dictyocoryne spp. thatwould be transported by the Kuroshio Current: thistaxon is therefore a good indicator of the gyre circulation. In
contrast, the millennial-scale variations in PC2 tend to coincide with changes in PC1 during late MIS 19a, suggestinga mechanism different from the previous period.
Furthermore, an increase in the production and advection of NPIW might have contributed to subsurface
cooling during mid-MIS 19c. As was suggested by Itaki
and Ikehara (2004), the production of the Okhotsk Sea
Intermediate Water that contributes to NPIW is coupledtightly with winter warmth and significantly increased
during a warm interval of the Holocene. We infer thatmoderate winters from mid-MIS 19c led to an increase
in NPIW advection into the subsurface layer of theKuroshio–Oyashio mixed zone.

Kubota et al. Progress in Earth and Planetary Science

4.4 Northwestern Pacific during MIS 20–18 andcomparison to MIS 2–1

To capture the regional temperature structure in thewestern North Pacific during MIS 20–18 and its relation
to the global and Asian regional climate signals, we compare the published SST records between 5° N and 50° N
to the newly reported Mg/Ca-SST (G. bulloides) record
for the CbCS, although the data treated here do notcontain the glacial maximum for MIS 18 (Table 2; Fig. 5
a–d and Fig. 6). SST records for MIS 20–18 have beenreported for the WPWP (Mg/Ca of Globigerinoides
ruber from Ocean Drilling Program [ODP] Site 871;
Dyez and Ravelo 2014), South China Sea (alkenones
from ODP Site 1146; Herbert et al. 2010), and the subarctic (alkenones from ODP Site 882; Martínez-García etal. 2010) (Figs. 5a, c, and d). The benthic δ18O and magnetic susceptibility data used for their age models are illustrated in Additional file 6 for comparison. The benthic
δ18O stratigraphy combined with magnetostratigraphy

50.35Latitude Proxy
(ºN)

44.53

50.35

ODP 882 167.58

Longitude(ºE)

125.83117.42141.78141.82145.00Site

MD982181

ODP
1144

MD012421

MD012420

MD012412

ODP 882 167.58

36.07

36.03

20.05

6.30

35.28

140.13

CbCS

MartínezGarcía etal. (2010)

Haradaet al.
(2006)

Sagawaet al.
(2006)

Yamamoto
et al. (2005)

Wei et al.
(2007)

Stott et al.
(2002)

9.6

18.8

24.1

28.6

Average SST (ºC)

8.8

18.4

24.1

28.1

3.0

15.2

23.7

27.9

0.5

0.4

0.4

9.0

11.8

16.3

19.1

26.3

29.5

7.4

12.2

7.2

16.0

23.9

28.1

1.1

2.1

0.4

0.3

MIS
19c

Standard deviation of SST (ºC)

1.0

0.3

0.3

1.6

0.8

1.9

0.8

1.0

0.7

2.4

2.0

2.5

1.4

1.1

1.4

MIS 1
MIS 2 andMIS 1
MIS 2 and
(0–11 ka) deglaciation (0–11 ka) deglaciation(11–29 ka)(11–29 ka)

9.8

17.7

25.4

29.1

MIS 19a,19b

MIS 18MIS 20(787.5–801.0*ka)

MIS 18MIS 19a,(747.0*– 19b
756.9 ka) (756.9–
773.9 ka)
MIS 19c(773.9–787.5ka)Standard deviation of SST (ºC)

Average SST (ºC)

3.1

0.3

0.2

MIS
20

1

5

8

6

3

4

10

5

2

136

27

16

21

196

4

113

41

61

47

93

MIS 1
MIS 2 and
(0–11 ka) deglaciation(11–29 ka)

1

16

11

5

MIS19a, 19b MIS19c MIS
20

Number of data for averaging

1

17

7

4

MIS18

Number of data for averaging(2021) 8:29

Alkenone

AlkenoneMg/Ca(Globigerina
bulloides)

Mashiotta
et al.
(1999)

Mg/Ca
Nürnberg(Globigerinoides et al.
sacculifer)(2000)

Alkenone

MartínezGarcía etal. (2010)

This study

Herbertet al.
(2010)

Mg/Ca
References
Calibration

Mashiotta
et al.
(1999)

Mg/Ca
Nürnberg(Globigerinoides et al.
ruber)(1996)

AlkenoneMg/Ca(Globigerina
bulloides)

Alkenone

19.46

116.27

ODP
1146

Dyez andRavelo(2014)

Mg/Ca
References
Calibration

Mg/Ca
Dyez and(Globigerinoides Ravelo
ruber)(2013)

Latitude Proxy
(ºN)

5.55

Longitude(ºE)

ODP 871 172.35

Site

Table 2 Western Pacific SST records compared in this study for MIS 2–1 and MIS 20–18 (Fig. 7). *Ages for inception of MIS 20 and termination of MIS 18 are based on the oldest
and youngest ages of our data

Kubota et al. Progress in Earth and Planetary Science
Page 17 of 24

Kubota et al. Progress in Earth and Planetary Science(2021) 8:29

Page 18 of 24

Fig. 6 A comparison between MIS 2–1 and MIS 20–18 with bottom axis for the age for MIS 2–1 and top axis for the age for MIS 20–18. a
Northern Hemisphere summer insolation at 65° N for MIS 2–1 (blue) and MIS 20–18 (black), b, c proxies from the Yimaguan and Luochuan
sections of the Chinese Loess Plateau showing b stacked grain size (EAWM) and c stacked magnetic susceptibility (EASM) (Hao et al. 2012) for MIS
2–1 (blue) and MIS 20–18 (black), d, e the planktonic foraminiferal (G. bulloides), d δ18O, and e Mg/Ca-based SST in the Kuroshio–Oyashio mixedzone for MIS 2–1 in MD01-2420 (blue, from Sagawa et al. 2006) and MD01-2421 for δ18O only (dark blue, from Oba et al. 2006), and MIS 20–18(black, from the CbCS)

enable us to compare the SST records within an uncertainty of ca. 5 ka (Suganuma et al. 2021).
The SSTs at all sites increased from MIS 20 to 19c,
which coincides with a decreasing trend in the global benthic δ18O stack (Fig. 5), implying that the Pacific SSTs
were subject to global glacial–interglacial changes (i.e., atmospheric CO2 concentration forcing and global ice volume) during Termination IX. However, SSTs dropped
relatively rapidly after the peak at low-latitude ODP Site
871 and ODP Site 1146, whereas SSTs remained higheven into MIS 18 at the mid-latitude CbCS and ODP 882.
The mid-latitude SST pattern is similar to the EAWM
record, suggesting the strong influence of EAWM variation on the mid-latitudes after MIS 19b. This contrast in
SST patterns between low- and mid-latitudes suggests thatthe low-latitude SST is less sensitive to the EAWM anddominantly controlled by global climate drivers such as atmospheric CO2 concentration.

Comparing MIS 20–18 with MIS 2–1 provides an
excellent opportunity to investigate natural climate
variability and constrain predictions of future climatechange. Insolation and marine records of the CbCS
and the cores MD01-2421 (36° 01.4′ N, 141° 46.8′ E,2224 m water depth) and MD01-2420 (36º 04' N,
141º 49' E, 2101 m water depth) are compared inFig. 6. Sites MD01-2421 and MD01-2420 (hereafterMDs) are located in the Kuroshio–Oyashio mixed
zone, ~ 100 km north of the CbCS with a modern
annual SST ~ 1.5 °C (~ 0.4‰ on the δ18O scale)colder than at the latitude of the CbCS (Fig. 1b andc). Sites MDs are therefore expected to have recorded somewhat cooler conditions than the CbCS
but broadly similar oceanographic information in thepast. Comparisons reveal that the δ18O and Mg/CaSST records of G. bulloides in the CbCS for MIS 19
are similar to those of MDs for MIS 1 (Fig. 6 d and(2021) 8:29

Page 19 of 24

Fig. 7, Table 2). However, the δ18Obul in the CbCS
during MIS 20 is ~ 2‰ lower than for MDs during
MIS 2 (Fig. 6d), which is not entirely accounted forin the global ice volume difference (< 0.4‰ based on
Bintanja et al. 2005; Elderfield et al. 2012). The Mg/
Ca-SST record also indicates an extremely low
temperature (~ 7 °C) during MIS 2 (Table 2). The
high δ18Obul during MIS 2 is therefore attributed to
the low SST. This is not simply explained by the latitudinal distance from the CbCS to the MD sites, as
the modern winter SST gradient between the two locations is only ~ 1.5 °C (Fig. 7).
In contrast to the Mg/Ca-SST reconstruction at site
MD01-2420 (Sagawa et al. 2006), alkenone-based SSTs
show a drop of only 3 °C on average during MIS 2 at site
MD01-2421 (Table 2) (Yamamoto et al. 2005). The SST
difference between the two proxies probably stems fromthe seasonality, and the foraminiferal proxies likely capture information on the colder seasons unlike the alkenones as mentioned earlier. The low SSTs derived fromG. bulloides therefore suggest severe winters during
MIS 2. Severe cooling during MIS 2 is also suggested by
the calcareous nannofossil assemblage-based temperaturereconstruction (Tn), characterized by the higher abundance of C. pelagicus during MIS 2 (Aizawa et al. 2004).
Although the calcareous nannofossil assemblages tend to
shift dramatically within narrow transitional areas such as

the Kuroshio–Oyashio mixed zone, C. pelagicus occupied~ 60% of the subordinate taxa during MIS 2 (Aizawa et al.
2004), whereas C. pelagicus pelagicus and C. pelagicus
braarudii accounted for only 40% of the subordinate taxa
during MIS 20 (Kameo et al. 2020).
To understand the mechanisms underlying the significant difference between MIS 20, MIS 18, and MIS 2, we
further examine the western North Pacific SST recordby comparing MIS 20 and MIS 18 with MIS 2, and MIS
19 with MIS 1 (Fig. 7, Table 2). Although the SST at
ODP Site 882 at 50°N indicate a dramatic cooling during
MIS 20, this should be treated with caution because it isbased on a single data point (Table 2). The cooling during MIS 20 is comparable to MIS 2 at low latitudes.
However, the Mg/Ca proxies for MD01-2421 during
MIS 2 are exceptional in the Kuroshio–Oyashio mixed
zone, suggesting significantly milder winters for MIS 20
than for MIS 2. A mild winter climate, inferred from aweak EAWM, has also been presented from Chinese
Loess grain-size records during MIS 20 (Hao et al. 2012;
Sun et al. 2019). Hao et al. (2012) argued that glaciations
during MIS 20 and 18 were initiated by very weak precessional insolation minima followed by warm summer
conditions unfavorable to Northern Hemisphere icesheet growth at the inception of these glacial periods. As
a result, ice and snow accumulation were suppressed on
the Eurasian continent, leading to a weak Siberian High

Kubota et al. Progress in Earth and Planetary ScienceFig. 7 Latitudinal sea-surface temperature (SST) plot of the western North Pacific listed in Table 2 for MIS 20, 19c, 19b–19a, 18, 2, 1, and present(Locarnini et al. 2013). Filled and open symbols represent alkenone- and Mg/Ca-based SST records, respectively. Error bars are standard deviation
values listed in Table 2

(2021) 8:29

Page 20 of 24

and associated weak EAWM winds (Hao et al. 2012).
The noticeable warm SSTs during MIS 18 at the CbCS
is also in line with the weak EAWM.
There is no conclusive evidence for the mean state of
the global climate, including global ice volume and atmospheric CO2 concentration, for MIS 20. The global
ice volume increased during glacials after the EMPT, but
the timing of the beginning and end of the EMPT
depends on the criteria used for its definition (e.g., Clark
et al. 2006; Elderfield et al. 2010; Head and Gibbard
2015a). Based on bottom water δ18O reconstruction andits spectral power in the southwest Pacific, changes associated with the EMPT ended ~ 650 ka (Elderfield et al.
2012). A review of sea-level proxies and reconstructions
for the Pleistocene (Rohling et al. 2014) has shown
significant uncertainties or discrepancies depending on theregions used for the reconstructions: − 70 to − 80 m for
MIS 20 based on the Mediterranean Sea (Rohling et al.
2014) and North Atlantic (Sosdian and Rosenthal 2009) but~− 120 m based on the southwest Pacific Ocean (Elderfield
et al. 2012). Thus, there is presently no conclusive evidence
that clearly explains mild winter temperatures during MIS
20 from the global ice volume perspective. Nor are there reliable atmospheric CO2 reconstructions at high temporal
resolution for MIS 20, although a transient model simulates
similar CO2 levels to MIS 2 for MIS 20 (Willeit et al. 2019).
In contrast, the mild glacial condition for MIS 18, suggestedby the sea-level proxies (Elderfield et al. 2012; Rohling et al.
2014), may be associated with the warm SST at the CbCS
and weak EAWM.
Concerning the climate and ocean variability during
MIS 19 and MIS 18, Suganuma et al. (2018) argued thatthe small thermal contrast between Siberia and the northwestern Pacific Ocean and resulting weak EAWM led to
the northward migration of the gyre boundary and a morestratified near-surface water column during the later partof MIS 19 after the stadial at ~772 ka. PC3 of our study
shows mostly positive scores during MIS 19a and 18, inferring stratification associated with reduced nutrient supply due to less vertical mixing of the ocean. The generaltrend in PC3 agrees with the interpretation of Suganuma
et al. (2018) for MIS 19a and 18. However, we note thatthe northward migration of the gyre boundary (negativescores for PC1) and the reduced nutrient state (positivescores for PC3) are not correlated on millennial-scale (stadial–interstadial oscillations) during late MIS 19a probably
due to the complex effects of temperature and δ13CDIC on
foraminiferal δ13C at the CbCS. The monsoon climate reconstruction throughout the Pleistocene reveals that thecommon temporal pattern of winter monsoon climate isseen during MIS 19–18 and MIS 11–10, where insolation
forcing is similar to the modern interglacial (Hao et
al. 2012). In these winter monsoon records, mild
winters persisted even after summer/annual climate andatmospheric CO2 levels declined toward the following
glacial state (Hao et al. 2012). During these interglacials,
the onset of winter cooling in East Asia lagged thesummer/annual climate by 30–40 kyr. This pattern isalso seen in the CbCS records (PC1 and related proxies)(Figs. 2, 3, and 4), likely indicating that the marine environment in the Kuroshio-Oyashio mixed zone is subject tothe East Asian winter climate system for MIS 19–18.

Kubota et al. Progress in Earth and Planetary Science

5 Conclusions
Microfossil assemblage and geochemical (δ18O, δ13C, and
Mg/Ca) data collected at high temporal resolution fromthe CbCS have been subjected to PCA. Results indicatethat the leading mode, PC1, which carries 36.3% of thetotal variance, likely represents oceanic conditions driven
by winter climate in the western North Pacific. The temporal variations in the leading mode are closely linked to
the EAWM but are also subject to the global signal. The
second mode, PC2, which carries 15.4% of the total variance, is well correlated to ocean subsurface conditions in
the CbCS. The leading mode is dominated by the relative
contributions of the Kuroshio and Oyashio currents, as
was discussed in Suganuma et al. (2018). We further
interpret the leading mode to be closely related to thenorth–south shift of the Pacific gyre boundary, and thesecond mode associated with the intensity of the subtropical gyre itself. However, the strength of the subtropicalgyre as represented by the subsurface temperature in the
CbCS record (the second mode) will have varied independently from the latitudinal shift of the gyre boundary,especially during MIS 19c–b. Radiolarian and calcareous
nannofossil assemblages are concordant, showing similarwater mass types. Our analysis reveals two aspects (surface
and subsurface) of the variabilities in the marine records
at the CbCS associated with the global and Asian regional
climate changes. Millennial-scale variations are superimposed on the long-term trend; the Kuroshio–Oyashio
boundary probably shifts southwards during the stadials
and northerly during the interstadials.
Glacial MIS 20 had significantly more mild winters
than MIS 2. For the interglacial MIS 19 and following
glacial MIS 18, the onset of winter cooling in East Asia
lagged the summer/annual climate by 30–40 kyr, whichis also seen in the leading mode of the CbCS marine
proxies. Stratification is also suggested during MIS 19a
through 18, leading to a suppressed nutrient supply due
to reduced mixing.
6 Supplementary Information
The online version contains supplementary material available at https://doi.
org/10.1186/s40645-020-00395-3.
Additional file 1: Fig. S1. Sampling horizons for the Chiba composite
section (CbCS).

Kubota et al. Progress in Earth and Planetary Science(2021) 8:29

Page 21 of 24

Received: 22 October 2019 Accepted: 30 November 2020

Additional file 2. Text for Table S1.
Additional file 3: Table S1. Microfossil, Mg/Ca, δ O, and δ C datafrom the CbCS.
18

13

Additional file 4: Fig. S2. SEM photographs of the planktonic
foraminifer G. bulloides from sample ID TB2-13. The foraminiferal tests
were coated with Au for SEM. a) Spiral side (3 kV, spot size = 40), b) internal test wall (15 kV, spot size = 40), c) and d) external test surface (15kV, spot size = 40) showing spine bases.
Additional file 5: Fig. S3. Comparison between NMNS and KU for δ13C
of G. inflata and G. bulloides.
Additional file 6: Fig. S4. Supplement figure for Ocean Drilling
Program (ODP) Sites 882, 1146, and 871.

Abbreviations
CbCS: Chiba composite section; MIS: Marine Isotope Stage; SST: Sea-surface
temperature; AL: Aleutian low; EAWM: East Asian winter monsoon;
EASM: East Asian summer monsoon; WPWP: Western Pacific warm pool;
δ13Cinf: δ13C for G. inflata; δ13Cbul: δ13C for G. bulloides; δ18Oinf: δ18O for G.
inflata; δ18Obul: δ18O for G. bulloides; Δδ18Oinf-bul: Difference between δ18Oinfand δ18Obul; Δδ13Cinf-bul: Difference between δ13Cinf and δ13Cbul
Acknowledgements
Katsunori Kimoto, Hideki Yamamoto, and Takamitsu Omura are thanked fortheir technical advice and support of the Mg/Ca analysis; and Kanako
Mantoku, Mika Yagishita and Taeko Umeno for their assistance in our
experiments. We are grateful to the journal editor and reviewers for theirhelpful comments on the manuscript.
Authors’ contributions
MJH, YS, and MO proposed the topic to YK, and YK conceived and designed
the manuscript. YK and YH compiled and analyzed the records. KK, TI, HH,
and KS processed the microfossil records and helped in analytical methods
and their interpretation. The authors read and improved the final manuscript.
Funding
This work was supported by JSPS grant JPMXS05R2900001. YK was partly
funded by projects “Rebuilding of micropaleontological reference centercollections” and “Chemical stratigraphy and dating” at the National Museumof Nature and Science for Mg/Ca and isotope analyses. YH was funded by aGeoscience Research Grant from the Tokyo Geographical Society and a
Sasakawa Scientific Research Grant from the Japan Science Society (29-625).
MJH acknowledges support from a Natural Sciences and Engineering
Research Council of Canada Discovery Grant.
Availability of data and materials
The datasets supporting the conclusions of this article are included within
the article and its additional files.
Competing interests
The authors declare that they have no competing interest.
Author details
1
Department of Geology and Paleontology, National Museum of Nature andScience, 4-1-1, Amakubo, Tsukuba, Ibaraki 305-0005, Japan. 2Geological
Survey of Japan, National Institute of Advanced Industrial Science andTechnology, Tsukuba Central 7, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567,
Japan. 3Department of Earth Sciences, Faculty of Science, Chiba University,
1-33, Yayoi, Inage, Chiba, Chiba 263-8522, Japan. 4Interdisciplinary Graduate
School of Science and Engineering, Shimane University, Nishikawatsu-Cho
1060, Matsue, Shimane 690-8504, Japan. 5Faculty & Graduate School of
Education, Chiba University, 1-33 Yayoi-Cho, Inage-Ku, Chiba, Chiba 263-8522,
Japan. 6Department of Earth Sciences, Brock University, 1812 Sir Isaac Brock
Way, St. Catharines, ON L2S 3A1, Canada. 7National Institute of Polar
Research, 10-3, Midori-Cho, Tachikawa, Tokyo 190-8518, Japan. 8Departmentof Polar Science, School of Multidisciplinary Sciences, The Graduate
University for Advanced Studies (SOKENDAI), Hayama, Japan. 9Department ofEarth Sciences, Ibaraki University, 2-2-1, Bunkyo, Mito, Ibaraki 310-8512, Japan.

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