Itaki et al.
Progress in Earth and Planetary Science(2022) 9:5
https://doi.org/10.1186/s40645-021-00465-0

RESEARCH ARTICLE

Progress in Earth andPlanetary Science

Open AccessMillennial‑scale oscillations in the Kuroshio–
Oyashio boundary during MIS 19 basedon the radiolarian record from the Chiba
composite section, central Japan
Takuya Itaki1* , Sakura Utsuki1,2, Yuki Haneda1,3, Kentaro Izumi2, Yoshimi Kubota4, Yusuke Suganuma3,5 andMakoto Okada6

Abstract
Marine isotope stage (MIS) 19 is considered to be the best orbital analog for the present interglacial. Consequently,
clarifying the climatic features of this period can provide us with insights regarding a natural baseline for assessing
future climate changes. A high-resolution radiolarian record from 800 to 750 ka (MIS 20 to MIS 18) was examined fromthe Chiba composite section (CbCS) of the Kokumoto Formation, including the Global Boundary Stratotype Section
and Point for the lower–middle Pleistocene boundary on the Boso Peninsula on the Pacific side of central Japan.
Millennial-scale oscillations in the Kuroshio warm and Oyashio cold currents were revealed by the Tr index, which isestimated using a simple equation based on radiolarian assemblages. The estimated Tr values ranged between 0.1
and 0.8 for MIS 18 through MIS 19, with minimum and maximum values corresponding to values observed off present
day Aomori (41°N) and the Boso Peninsula (35°N), respectively. The observed patterns tended to be synchronous withthe total radiolarian abundance associated with their production. Multiple maxima in radiolarian abundance occurred
during periods of the Oyashio expanded mode before 785 ka and during periods of Kuroshio extension after 785 ka
in MIS 19. Such increases in radiolarian abundance with the Kuroshio extension during MIS 19 are likely related to
improvements in nutrient and photic environments with the development of a two-layer structure along the Kuroshio–Oyashio boundary zone. A similar pattern of millennial-scale climatic changes was also recognized in a precipitation record from the Sulmona Basin in central Italy, suggesting a close relationship with the CbCS record as a result ofa large-scale climate system similar to the Arctic Oscillation in the northern hemisphere.
Keywords: Paleoceanography, Lower-Middle Pleistocene GSSP, Interglacial, MIS 19, Tr index, Teleconnection
1 Introduction
Marine Isotope Stage (MIS) 19 is almost at the midpoint of a transition between progressive increases in theamplitude of climate oscillations with a shift in periodicity from a 41-ky rhythm to a quasi-100 ky rhythm that
occurred 1400–400 ka, known as the Early–Middle*Correspondence: t-itaki@aist.go.jp
1
Geological Survey of Japan/AIST, Institute of Geoinformation
and Geology, 1‑1‑1 Hihashi, Tsukuba, Ibaraki 305‑8567, Japan
Full list of author information is available at the end of the article

Pleistocene transition (EMPT) (e.g., Head and Gibbard 2015). Global climatic changes through the EMPT,
including MIS 19, were modulated by astronomical
forcing and led to changes in the intensity of the Atlantic meridional overturning circulation (AMOC) (Hodell
et al. 2008; Kleiven et al. 2011) and global sea levels(Elderfield et al. 2012). Furthermore, MIS 19 has been
intensively studied because it is the best orbital analog forthe present interglacial (e.g., Tzedakis et al. 2012). MIS 19
is characterized by a reduced-amplitude 400-ky eccentricity cycle and consequent suppression of precessional© The Author(s) 2022. 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 appropriate credit to theoriginal author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images orother third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line
to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory
regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of thislicence, visit http://​creat​iveco​mmons.​org/​licen​ses/​by/4.​0/.

Itaki et al. Progress in Earth and Planetary Science(2022) 9:5

forcing. Although this orbital configuration is similar to
MIS 11 and the present interglacial (MIS 1), only MIS 19
and 1 show an obliquity peak and the precession minimum in-phase (Tzedakis 2010). Therefore, it is important to obtain a detailed understanding of the climaticfeatures of this period, as doing so could provide insights
for establishing a natural baseline for assessing futureclimate changes (e.g., Tzedakis et al. 2012; Giaccio et al.
2015; Sánchez Goñi et al. 2016; Suganuma et al. 2018;
Head 2021).
High-resolution paleoclimatic records during MIS 19
have been documented for the North Atlantic (Kleiven
et al. 2011; Ferretti et al. 2015; Sánchez Goñi et al. 2016),
Indian Ocean (Valet et al. 2014), southern Europe (Giaccio et al. 2015; Simon et al. 2017; Nomade et al. 2019;Regattieri et al. 2019), Japan (Hyodo et al. 2017; Suganuma et al. 2018; Haneda et al. 2020), Lake El’gygytgyn(Wennrich et al. 2014), Lake Baikal (Prokopenko et al.
2006), and Antarctica (Jouzel et al. 2007). The amplitudes
and timing of millennial-scale variations in these paleoclimatic records tend to be synchronized among areas,suggesting the existence of a global climatic teleconnection pattern (Tzedakis et al. 2012; Nomade et al. 2019;
Haneda et al. 2020). In order to clarify the mechanisms
underlying the development of such a pattern, performing high-resolution reconstructions of the paleoenvironment in various areas of the world is necessary.
The Kazusa Group, which is composed of Pleistocene
deep-sea deposits, is continuously distributed on the
Boso Peninsula on the Pacific side of central Japan (e.g.,
Kazaoka et al. 2015). The area is unique in that, despite
their relatively young age, the deep-sea outcrops are
exposed on land as a result of active uplifting. Therefore,
the sections comprising this group are well suited forstudies of Pleistocene environmental changes. Furthermore, the Chiba composite section (CbCS) of the Kokumoto Formation in this group is well exposed and has acontinuous marine sedimentary record across MIS 19(Fig. 1), which has been extensively investigated (GSSP
Proposal Group 2019; Suganuma et al. 2015; Nishida
et al. 2016; Okada et al. 2017; Simon et al. 2019; Haneda
et al. 2020; Izumi et al. 2020; Kameo et al. 2020; Kubota
et al. 2021). The CbCS has been ratified by the International Commission on Stratigraphy as a Global Boundary
Stratotype Section and Point (GSSP) for the Lower–Middle Pleistocene boundary.
At present, the offshore area near the Boso Peninsula
is influenced by both the Kuroshio warm and Oyashio
cold currents and, therefore, it is expected to be sensitive to both glacial-interglacial cycles and even small
climatic changes in East Asia. Suganuma et al. (2018) discussed the close relationship between climatic changes
in this area and climatic dynamics in East Asia based onPage 2 of 14

the results of various proxies from the CbCS. Recently,
more detailed millennial-scale studies from this section
have been conducted on oxygen isotopes in foraminifera (Haneda et al. 2020), the geochemical record (Izumi
et al. 2021), and calcareous nanofossils (Kameo et al.
2020). Kubota et al. (2021) discussed the dominant patterns in temporal variations in marine conditions usinga principal component analysis (PCA) based on multiple
proxies from geochemical and microfossil assemblages in
the CbCS. The findings of these studies showed that millennial-scale changes in a variety of palaeoceanographic
records from the CbCS likely reflect fluctuations in the
Kuroshio–Oyashio boundary around the Boso peninsula
and that these fluctuations may be linked to global teleconnection patterns.
Radiolarians, a marine plankton group with opaline(siliceous) skeletons, are preserved as microfossils in
deep-sea sediments and are widely used as a paleoceanographic proxy. Radiolarian fossils are widespread in sediments of the North Pacific, even below the carbonate
compensation depth (CCD) where calcareous microfossils are poorly represented. Consequently, the relation
between radiolarian faunal distribution and oceanic conditions has been examined more extensively than it hasfor other microfossils (e.g., Nigrini 1970; Matsuzaki and
Itaki 2017). Examination of this microfossil group is well
suited for detecting changes in the Kuroshio–Oyashio
front that are closely related to past climatic changes
around the Boso Peninsula (e.g., Chinzei et al. 1987; Yasudomi et al. 2014). Furthermore, radiolarian fossils canbe used as a proxy for not only surface environments, butalso for deep water masses because of differences in habitat preference among species (e.g., Matsuzaki and Itaki2017). The radiolarian fossil record is thus a robust paleoceanographic tool in studies of the CbCS.
A total of 36 radiolarian species and species groups
have been reported for the CbCS (Motoyama et al. 2017),
and radiolarian assemblages related to climatic changes
have been reported by Suganuma et al. (2018); however,
the inclusion of few species meant that their results wereconsidered to be low-resolution and preliminary. In orderto provide more accurate interpretations of paleoceanographic phenomena, more detailed changes in radiolarian assemblages and their abundance are required.
Therefore, in this study, we conducted a high-resolution
analysis of millennial-scale changes in the Kuroshio–
Oyashio system during MIS 19 based on radiolarian fossil records from the CbCS.

2 Methods
A total of 236 radiolarian samples, including 195 newsamples and 41 samples that were used for preliminary
analysis by Suganuma et al. (2018), were analyzed in thisItaki et al. Progress in Earth and Planetary Science(2022) 9:5(a)Page 3 of 14

45˚N

t

n
rre

o

i
sh

Cu St. 50N

40˚N

t

a

im
sh

nt

rre

Ku

130˚E

150˚E

170˚W

Kuroshio Extension
Currentihros

Kagoshima

Ku

130˚E

135˚E

nt

rre

u
oC

15

MD01-2421

Pacific Ocean

140˚E

145˚E

150˚E

10
5
0
°C

o
Yor

(c)

170˚E

nt

Tokyo

Wakayama

30˚N

25

20

Polar Fro

Boso Peninsula

30˚N

hi

s
ro

Ts

35˚N

Kuroshio Extenion Current

Oy

St. B

Sendai

FukushimauuoC

Aomori

en

rr
Cu

oC
shi

a

Japan Sea

40˚N

St. 40N

ent

urr

Aleutian Low 5
50˚N

ya St. KNOT
Subarctic Current

O

30

(b)

R.

35°25′

.

su R

Obit

ific nc
Pa ceaO
Kokusabata
Isumi R.

R
Yoro
.
Obit
su R

mi

R.
0

5 km
River
Fault
Chiba composite
section

Ko
ito

R.

Kd38

K

d3

8

.

Isu

Kazusa Group

Chiba & Yoro-Tabuchi

Shimosa G.

Holocene

Yanagawa & Urajiro

Kd38

35°20′

Kd38 tephra

Kioroshi F.
– Jizodo F.
35°15′
Kongochi F.
Kasamori F.
Mandano F.
Chonan F.
Ichijiku F.
Kakinokidai F.
Kokumoto F.
Umegase F.
Otadai F.
Kiwada F.
Ohara F.
35°10′Namihana F.
Katsuura F.
Kurotaki F.

Pre-Kazusa G.
140°0′

140°5′

140°10′140°15′140°20′140°25′140°30′Fig. 1 Maps showing a major ocean currents around Japan, b summer sea-surface temperatures around the Boso Peninsula, and c simplified
geological map (Kazaoka et al. 2015). Red stars in (c) indicate locations of the Urajiro, Yanagawa, Yoro River, Yoro-Tabuchi, and Kokusabata sections
that comprise the Chiba composite section (CbCS)

study (Additional file 1: Tables S1 and Additional file 2:
Table S2). Of the 195 new samples, Kubota et al. (2021)
used a subset of 63 samples for the radiolarian dataset intheir study. Sampling locations, stratigraphic framework,and age models are based on Suganuma et al. (2018) and
Haneda et al. (2020) (Additional file 3: Fig. S1).
Freeze-dried samples were weighed and wet sieved
through 45 µm meshes, and two types of slides wereItaki et al. Progress in Earth and Planetary Science(2022) 9:5

Page 4 of 14

prepared to quantify abundance (Q-slide) and for faunalanalysis (F-slide) using standard techniques described
in Itaki et al. (2018). The level of preservation of theradiolarian fossils was judged as being good (majorityof specimens complete, with no or minor dissolution,
recrystallization, and/or breakage), moderate (minor, butcommon dissolution, with a small amount of breakage of
specimens) or poor (strong dissolution, recrystallization,or breakage, many specimens unidentifiable).
The total number (abundance) of radiolarians in 1 g ofdry sediment was estimated using the following equation:

Total radiolarian abundance (individuals/g)= total number of individuals on Q - slide
× 200/sample weight (g)The total radiolarian abundance was estimated for
193 of the 195 samples that were newly collected in thisstudy; two samples were excluded from the analysis due
to problems with Q-slide preparation.
For 153 of the 195 new samples, the relative abundance (percentage of total assemblage) of the species wasestimated by counting and identifying more than 200
individuals on the F-slide. However, when radiolarian
individuals were scarce, as many as could be identified
were counted (Additional file 1: Table S1). Radiolarians were observed under an optical microscope at 40×to 200× magnification. Identifications were performed
using a taxonomic framework adapted from Itaki (2009),
Matsuzaki and Itaki (2017), and Matsuzaki et al. (2020).
Photomicrographs of radiolarians from the CbCS are
illustrated in Motoyama et al. (2017).
The Tr value, which is a radiolarian-based climate
index originally proposed by Nigrini (1970), was calculated using the following equation:

Tr = Xw/(Xw + Xt + Xc)
where Xw, Xt and Xc are the number of warm-, temperate- and cold-water radiolarian species and species
groups, respectively.
Yanagisawa (1993) demonstrated that a similar climatic index based on diatoms (Td value) does not havethe same sensitively to sea-surface temperatures in all
oceanic regions; however, it can be used as a tracer for
two different water masses originating from the warmand cold currents within the transitional zone of thecurrents. The same logic can be applied to the Tr value.
For example, as shown in Fig. 2, latitudinal variation in
the Tr value based on the dataset of Matsuzaki and Itaki(2017) shows that the polar front (Tr = ca. 0.5), definedas the latitudinal zone showing the steepest Tr variation,is located at around 37°N off present-day Fukushima,
and the Boso Peninsula is located within the transitional

50

45
Subarctic

43

40

Aomori35

Transition

Sendai

Latitude (°N)

30

Fukushima
CbCS (Boso Peninsula)

25
Subtropical

15

39
37
35

Wakayama

20

41

33

Kagoshima

Latitude (°N)

4531

Tropical

10

29

5

27

0

25
0.0

0.2

0.4
0.6
Tr value

0.8

1.0

0.0

0.2

0.4

0.6
0.8
Tr value

1.0

Fig. 2 Plot showing the latitudinal distribution of Tr values in the northwestern Pacific based on the dataset from Matsuzaki and Itaki (2017). Similarplots for each species or species groups used for the Tr estimation are shown in Additional file 5: Fig. S3

Itaki et al. Progress in Earth and Planetary Science(2022) 9:5

zone of the Tr value due to mixing of the Kuroshio andOyashio currents. On the other hand, values above 0.8
and below 0.2 show gradual slopes latitudinally, indicating that sensitivity to changes in water mass is low. Suganuma et al. (2018) applied Tr values to their preliminary
results for 41 samples from the CbCS and obtained values
in the sensitive interval between 0.2 and 0.8. Therefore,
this index can be used for assessing the relative contributions of the Kuroshio and Oyashio currents to the CbCS.
The data used to estimate Tr values in this study and thatof Suganuma et al. (2018) are given in Additional file 2:
Table S2.

3 Results
Radiolarian fossils were observed in all of the analyzed
samples. Preservation levels were moderate to good,and no modifications to the assemblage and/or abundance were observed. The number of radiolarian fossils
in each sample ranged between 57 and 363 individuals(203 ± 74.7, average ± standard deviation).
Figure 3 shows the temporal changes in total radiolarian abundance (individuals/g) and relative abundance(percentage of total assemblage) for 11 species/species
groups in the new dataset from the CbCS. The total abundance ranged from 90 to 1220 individuals/g, and periodic
fluctuations at 2400-, 2900-, and 5800-year intervals wereobserved over the period analyzed in the study (Fig. 3a,
Additional file 6: Fig. S4). Multiple maxima and minima
in the total radiolarian abundance tended to be synchronized with the absolute abundance (individuals/g) of
several major species or species groups; however, these
relationships were not significant for the relative abundance of the assemblage (%) (Fig. 3b–n).
In this high-resolution analysis using 153 samples, a
total of 30 species and species groups were identified(Additional file 1: Table S1). The most dominant radiolarian taxon was Tetrapyle circularis/fruticosa group
sensu Matsuzaki et al. (2020), followed by Spongodiscus resurgens Ehrenberg, Larcopyle buetchlii Dreyer,
Didymocyrtis tetrathalamus (Haeckel) group (including young form), and Lithomelissa setosa Jørgensen.
As minor radiolarians, the Dictyocoryne profunda
Ehrenberg / Dictyocoryne truncatum (Ehrenberg)
group (including their young form), Stylochlamydium
venustum (Bailey), Amphirhopalum ypsilon Haeckel,
Druppatractus irregularis Popofsky, the Dictyocoryne
elegans (Ehrenberg)/Dictyocoryne muelleri (Haeckel)
group (including their young form), the Phorticium
pylonium (Haeckel)/polycladum Tan and Tchang group,and Lithelius minor Jørgensen and Cycladophora davisiana Ehrenberg were also observed. Changes in theabundance of radiolarian fauna relating to associatedwith water masses were as follows.

Page 5 of 14

Subtropical water fauna: the T. circularis/fruticosa group, the D. tetrathalamus group, the D.
truncatum/profunda group, the A. ypsilon, P. pylonium/polycladum group, and the D. elegans/muelleri groupare all subtropical species that are characteristic of theKuroshio region where sea-surface temperatures range
between 20 and 29 °C according to Matsuzaki and Itaki(2017). Radiolarians in this group accounted for 5–45%
of the assemblage in the CbCS and their abundance
tended to be high during MIS 19c to 19b and low in MIS
19a and MIS 20, except for the P. pylonium /polycladum
group, which showed low abundance values during MIS
19c (Fig. 3b–g). Based on the results of a sediment trap
study at Station B in a coastal region of the Oyashio in
the western subarctic Pacific, Itaki et al. (2008) reported
that these subtropical radiolarians are dominant in summer (see Fig. 1b).
Mixed water fauna: L. buetschlii, S. resurgens, and S.
venustum accounted for 9–36% of radiolarians in the
CbCS and tended to show higher abundance in MIS
19b and MIS 20 (Fig. 3h–j). Matsuzaki and Itaki (2017)assigned these radiolarians to the “warm-temperate species” group, which are found in the mixing area of theKuroshio and Oyashio waters between 16 and 23 °C. Sediment trap records at Station B in a coastal region of the
Oyashio showed that flux peaks occur during spring for
L. buetschlii and S. venustum, and in winter and springfor S. resurgens (Itaki et al. 2008).
Subarctic coastal water fauna: L. setosa accounted
for 0–19% of radiolarians in the CbCS, with abundance
increasing significantly during MIS 19a and MIS 20(Fig. 3k). Two short-period maxima also occurred in MIS
19; one in MIS 19b and the other in the middle of MIS
19c. This species is commonly encountered at temperatures ranging from 12 to 18 °C in the coastal regions of
the Oyashio (Matsuzaki and Itaki, 2017) and in Norwegian fjords (Bjørklund, 1974). Further, this species was
dominant in the assemblage from autumn to winter at
Station B in the coastal region of the Oyashio (Itaki et al.
2008). The cold and low-salinity coastal Oyashio Current,
which flows southwestward along the southeastern coastof Hokkaido in winter and spring, may originate frommelting sea-ice in the Sea of Okhotsk (Kono et al. 2004).
Although Matsuzaki and Itaki (2017) categorized
another group of “subarctic species” comprising species
such as Ceratospyris borealis Bailey and Antarctissa? sp.
1 Nimmergut and Abelmann which are adapted to subarctic waters with temperatures of 7–12 °C, these taxa
were not encountered in the CbCS, suggesting that themainstream of the Oyashio Current flowing offshore has
no influence on the faunal composition of this group.
Deep-dwelling taxa: D. irregularis, L. minor, andC. davisiana are known to be species that are mainly

Itaki et al. Progress in Earth and Planetary Science

MIS

19a

19b

(2022) 9:5

19c

20

(ind. /g)

0.8

0
6
5

40

0.4
0.2

(a) Total radiolarian abundance

(%)

40

(b) Tetrapyle circularis / fruticosa group

30
20
10
0
10

(c) Dctyocoryne truncatum / profunda group

50

8

40

6

30

4

20
10

2

0

0

100

(d) Didymocyrtis tetrathalamus group

16

80

12

60

8

40

4

20
0
40
35
30
25
20
15
10
5
0
40
35
30
25
20
15
10
5
0

0
4

(e) Dictyocoryne elegans / muelleri group32
1(f) Amphirhopalum ypsilon

4
3
2
1

30

(g) Phorticium pylonium / polycladum group

25
20
15
10
5
0
760

765

770

775

780

Age (ka)

785

790

MIS
(ind. /g)
70
60
50
40
30
20
10
0
350
300
250
200
150
100
50
0
40
35
30
25
20
15
10
5
0
140
120
100
80
60
40
20
0
70
60
50
40
30
20
10
0
50

0.6

1400
1200
1000
800
600
400
200
0
350
300
250
200
150
100
50
0
60

Page 6 of 14

795

19a

19b

19c

20
(%)
20

(h) Larcopyle buetschlii

15
10
5
0
35
30
25
20
15
10
5
0
6

(i) Spongodiscus resurgens(j) Stylochlamydium venustum

5
4
3
2
1
0
25
20
15

(k) Lithomelissa setosa

10
5
0
8
7
6
5
4
3
2
1
0
7
6
5
4
3
2
1
0

(l) Druppatractus irregularis(m) Lithelius minor

30
20
10

0

0

5

25

4

20

3

15

2

10

1

5

0

0

3

(n) Cycladophora davisiana

2
1
0
760

765

770

775

780

785

790

795

Age (ka)

Fig. 3 Variations in absolute abundance (individuals/g) of total radiolarians (a) and 13 species and species groups (b–n) for 153 samples newly
examined in this study (solid lines with dots). Relative abundances (percent) of the total assemblage for each species are also shown as palebold lines. Reddish brown (b–g), green (h–j) and blue (k) indicate groups related to the Kuroshio water, mixed water and coastal Oyashio water,
respectively. Deep-dwelling radiolarians are shown as black lines (l–n). The Tr curves (red lines) are overlapped in panels (a) and (k). Intervals of thetotal radiolarian maxima are shown as yellow vertical bands

Itaki et al. Progress in Earth and Planetary Science(2022) 9:5

adapted to waters deeper than 200 m in the North Pacific(e.g., Matsuzaki et al. 2020). Interestingly, the occurrence patterns of these species differed from each other(Fig. 3l–n). The abundance of D. irregularis was relatively high during MIS 19b to MIS 19c and low in MIS
19a and MIS 20. Lithelius minor shows similar variationsin abundance to the mixed-water fauna, with higher values observed in MIS 19b and MIS 20. Although C. davisiana is a minor species that accounts for less than 3%of the assemblage, its abundance increased in MIS 19a
and MIS 19b (Fig. 3n). According to Matsuzaki and Itaki(2017), D. irregularis and L. minor are both deep dwellers that are adapted to mid-latitude regions in the North
Pacific, while C. davisiana is found in intermediate todeep waters that originate from higher latitudes. Other
deep dwelling taxa, such as Actinomma leptodermum(Jørgensen), Cornutella profunda Ehrenberg, Sponguruscylindricus Haeckel were also recognized in the CbCS;
however, their abundance was low.
The Tr value was estimated based on the seven indicator species/species groups shown in Additional file 1:
Table S2, which were also mentioned in Suganuma
et al. (2018); these species/groups included the subtropical water (warm) group (T. circularis/fruticosa
group, D. tetrathalamus group, and D. truncatum /profunda group), the subarctic coastal water (cold) group(L. setosa), and the mixed water (temperate) group (L.
buetschlii, S. resurgens and S. venustum). Figure 4 shows
the temporal variations in the Tr value obtained for the
194 samples and the total radiolarian abundance in 234
samples estimated in this study and in Suganuma et al.
(2018), together with the oxygen isotope ratio for planktonic foraminifera (Haneda et al. 2020). The radiolarian-based summer SST obtained from a new dataset
comprising 153 samples is also shown in this figure.
The Tr value fluctuated considerably between 0.1 and
0.8 from MIS 20 to MIS 18, likely reflecting oscillations in
the Kuroshio–Oyashio boundary throughout this period(Fig. 4). High Tr values are indicative of a large contribution from the Kuroshio Current, while low values reflect
the influence of the Oyashio Current and Oyashio coastal
waters. The range in Tr values (0.1–0.8) recorded in the
CbCS corresponds to latitudes from approximately 41°N
(off present-day Aomori Prefecture) to 31°N (off present-day Kagoshima Prefecture) (Fig. 2). The current Tr
value near the Boso Peninsula (CbCS) is approximately
0.6. Generally, higher values of between 0.4 and 0.8 withmillennial-scale oscillations (periodicity 2.9 kyr, Additional file 6: Fig. S4) were observed during most of MIS
19c to 19b, and values below 0.4 were observed in MIS 20
and mid-MIS 19a. The multiple maxima in the Tr curve(shaded intervals in Fig. 4) tended to correspond with

Page 7 of 14

maxima and minima in total radiolarian abundance afterand before 785 ka, respectively.

4 Discussion
4.1 Millennial‑scale fluctuations

The observation of Tr values below 0.4 in MIS 20 andmid-MIS 19a imply that the CbCS was located underconditions similar to those of the present day Oyashio
current north of 37°N (off present-day Fukushima Prefecture). On the other hand, distinct millennial-scale fluctuations in Tr values of between 0.4 and 0.7 (using three
moving averages in Fig. 4) during MIS 19 clearly indicate
the occurrence of Kuroshio/Oyashio oscillations. The
lowest value corresponds to the polar front, while thehighest value corresponds to values around 34°N (off present-day Wakayama Prefecture).
Although some discrepancies in the low-resolution
record are recognized in MIS 20 and MIS 18, the observation of low and high variations in Tr values in thehigh-resolution record interval during MIS 20 and
MIS 19, respectively, is generally consistent with largescale changes in the oxygen isotope ratio of planktonic
foraminifera (e.g., Globigerina bulloides d’Orbigny(δ18OGb) and Globorotalia inflata (d’Orbigny) (δ18OGi)),
which corresponds to the glacial-interglacial cycles
reported by Haneda et al. (2020) (Fig. 4a). Such similarities in the fluctuations of δ18O and Tr imply the existenceof similar oceanic conditions, such as temperature andsalinity. Since the radiolarians that are typically used toestimate Tr are usually distributed from the surface tothe subsurface (ca. 0–200 m) regions of the water column, the Tr values tended to be more consistent with thegeneral δ18O patterns obtained from G. bulloides and G.
inflata, indicating that these species inhabited the surface
and subsurface regions, respectively. The Tn values (nannofossil temperature index) that are characteristic of thesurface conditions in the CbCS show a similar pattern to
the large-scale changes in Tr values (Kameo et al. 2020).
However, in terms of millennial-scale changes withrelatively small amplitudes, the Tr curve did not show
any correspondence with δ18O values during early MIS
19c. For example, the Tr peaks seen during early MIS19c
were not observed in the δ18O record. The changes inforaminifera δ18O are associated with changes in temperature and δ18O of seawater (mainly temperature in
the case of the CbCS, according to Kubota et al. 2021),
while the Tr variations might also be related to other conditions, such as salinity, nutrients, and water mass structure. In the Kuroshio–Oyashio boundary zone, changes
in the water structure and their effect on nutrient conditions are complicated, and it is possible that such changes
could account for the inconsistencies observed in δ18O

Itaki et al. Progress in Earth and Planetary Science(2022) 9:5

Page 8 of 14

Age (ka)
750

760

MIS

18

770

19a

780

19b

790

19c

800

20
(‰)
-1.00

Globigerina bulloides(a) Oxygen isotope ratio

Average of δ18O Gb& δ18O Gi

0.00
Range of
δ18O Gb&
δ18O Gi

1.00

Globorotalia inflata

2.00
0.8

(b) Tr index

Kuroshio

Modern

0.6

Polar
front
0.4
(ind./g)
1400

Oyashio

1200
1000

0.2

(c) Total radiolarian abundance
0.0

800
600
400
200
0

Maxima with Kuroshio mode
750

760

770

780

Maxima with Oyashio mode
790

800

Age (ka)
Fig. 4 Comparisons among paleoceanographic proxies: a Oxygen isotope record (δ18O) of planktonic and benthic foraminifera (Haneda et al.
2020). Pale blue indicates the range of values for Globigerina bulloides (δ18OGb, upper limit) and Globorotalia inflata (δ18OGi, lower limit) from the
Yoro River and Yoro-Tabuchi sections. Dark blue solid line indicates average of δ18OGb and δ18OGi. Dashed lines with solid circles and × symbols
indicate δ18OGb and δ18OGi from the Kokusabata, Yanagawa and Urajiro sections, and the dark-blue dashed line without dots shows their average.
b Variations of the Tr index. Pale red line indicates original plots, and their three-point moving average is shown as a red line for the high-resolution
interval. c Total radiolarian abundance changes. Yellow line shows the original plots and the dark line indicates moving average over three points.
Gray bands indicate intervals of multiple maxima of the Tr curve for the high-resolution interval

records in the high-resolution record. In other words, Tr
can be regarded as an index for a variety of conditions
that include surface/subsurface temperatures, nutrient

conditions, and fluctuations in the water structure of theKuroshio–Oyashio confluence.

Itaki et al. Progress in Earth and Planetary Science(2022) 9:5

Inconsistencies between Tr and δ18O during early MIS
19c could also be attributed to differences in the ecology of the radiolarians used to estimate the Tr index.
Although the radiolarian fauna of the Kuroshio Current
show generally similar patterns, the detailed patterns ofeach species group are sometimes different; for example,
while a peak in the abundance of the D. tetrathalamus
group that was observed at the MIS 19/MIS 20 boundary corresponded with a peak in Tr values, the D. truncatum / profunda and T. circularis/fruticosa groups
showed a decrease in abundance at that time. According
to the results of a principal component analysis (PCA)based on multiple proxies from geochemical and microfossil assemblages in the CbCS (Kubota et al. 2021), thePC1 and PC2 components reflect surface and subsurface
conditions, respectively, and PC3 is related to stratification structure of the upper waters. For example, theabundance of L. setosa, which is a coastal species of the
Oyashio Current and is included in PC1, was closely
related to the δ18O variations. For the warm-water radiolarians, the abundance of the D. tetrathalamus group
showed a relatively strong correlation with PC1. PC2,
which included the D. truncatum / profunda group,
shows close relation to the G. inflata Mg/Ca ratio as aproxy of the subsurface water temperature. PC3 is correlated with differences in the δ13C and δ18O values
between G. bulloides and G. inflata, which were causedby the water column having a stratified structure. The
radiolarian species S. resurgens and the T. circularis/fruticosa group show high loadings for PC3. Significant Tr
variations during early MIS 19c, which is inconsistent
with δ18O values, are probably associated with the subsurface and stratification conditions explained by PC2
and PC3 of small relation to the δ18O variations.
Another possible reason for the inconsistencies
observed between the Tr and δ18O variations during
early MIS 19c could be due to seasonal factors. Based on
the findings of sediment trap experiments at Station B incoastal areas of the Oyashio Current, the contribution of
the abundance of radiolarians in the Kuroshio group to
the Tr index during MIS 19 increases during the summer
to autumn (Itaki et al. 2008). On the other hand, δ18O
and geochemical records of the planktonic foraminifera
G. bulloides and G. inflata are interpreted as reflecting
peaks in winter to spring in the Kuroshio–Oyashio mixedzone (Kubota et al. 2021). The utility of using the δ18O
of G. bulloides as a winter proxy has been demonstrated
by Sagawa et al. (2013) using sediment traps at station B.
Therefore, multiple increases in the Tr values during MIS
19c were possibly associated with Kuroshio extension
during summer to autumn, and such signals were notrecognized/ in the winter-spring records of planktonic
δ18O.

Page 9 of 14

Millennial-scale Kuroshio–Oyashio fluctuations during the interglacial stage have also been reported fromother interglacial periods. Holocene 1500-year cycles
in SST fluctuations have been reported based on the
Uk’37 index (Isono et al. 2009) and diatom fossil assemblages (Koizumi, 2008) from a marine sedimentary core
(MD01-2421) collected from the western Pacific at 36°N
near the Boso Peninsula. In those studies, the amplitudeof the fluctuation in water temperature was estimated to
be about 1 °C. Yasudomi et al. (2014), who conducted adetailed examination of the radiolarians of the last interglacial period (MIS-5e) using the same core, showedthat the Tr value fluctuated with a 500-year cycle. The Tr
values in their study (0.6–0.9) were slightly higher thanresults from the CbCS in this study, implying that thewater temperatures in the Holocene and MIS-5e werelikely higher and showed smaller fluctuations than thosein MIS 19.
4.2 Relation of water structure to radiolarian ecology

Changes in water masses affect not only radiolarianassemblages, but also their productivity. It is generally
considered that the abundance of radiolarians preservedin sediments reflects the productivity of the water column; however, other factors can also affect the distribution of radiolarian fossils in sediments, including dilution
by terrigenous material and dissolution of radiolarianskeletons during diagenesis. In this study, we assumed
that radiolarian abundance is correlated with productivity, because the effects of extreme dissolution anddilution are likely to be small given the minor changes
observed in the dissolution index and sedimentary facies.
Below, we discuss changes in productivity under different
radiolarian ecological settings in the Kuroshio–Oyashio
boundary zone observed in the CbCS.
Radiolarians in the oceanic ecosystem occupy thetrophic level of secondary producers. In terms of feeding
strategies, radiolarian taxa can be herbivorous, omnivorous, or employ symbiotic photosynthetic algae or
cyanobacteria. Consequently, changes in total radiolarian abundance in the CbCS could be reflected by factorsthat affected their feeding ecology. According to Zhang
et al. (2018), who examined living radiolarians in the
North Pacific epifluorescence microscopy, 37 radiolarian taxa living in the Kuroshio region, such as the T. circularis/fruticosa group and the D. tetrathalamus group,
depend on symbiosis with photosynthetic algae for theirnutrition. On the other hand, L. setosa, S. resurgens andS. venustum, which are distributed in the Oyashio region,
do not have symbiotic photosynthetic algae, suggestingthat these species are either herbivorous or omnivorous.
Thus, factors that affect radiolarian production are likely

Itaki et al. Progress in Earth and Planetary Science(2022) 9:5

to be reflected in the trophic structure of radiolarians in
specific water mass environments.
The question is what factors can be used to infer millennial-scale variations of total radiolarian abundance in
the CbCS. According to Okazaki et al. (2005), who collected radiolarians in time-series experiments using sediment traps deployed at three stations (50 N, KNOT, and40 N) in the northwestern Pacific (see Fig. 1a for locations), the correlation coefficients between total radiolarian flux and total mass flux (closely related to diatomflux) were relatively high at Station 40 N in the Kuroshio–
Oyashio Extension (r = 0.68) compared to those sampled
at the other two subarctic stations (KNOT: r = 0.43 and50 N: r = 0.44). Their findings suggested that the relationship between radiolarian production and primary production is not straightforward, primarily because of thewide vertical distribution and various trophic patterns
observed in radiolarians.
The millennial-scale maxima in total radiolarian abundance tends to coincide with warmer and cooler intervals that appeared after and before 785 ka, respectively(Fig. 4). The timing of this shift corresponds to a change
from Oyashio-dominant conditions to Kuroshio-dominant conditions. In other words, increases in radiolarian
productivity occurred during the period when Tr was
below 0.5 before 785 ka, and during the period when Tr
was above 0.6 after 785 ka. The Tr value of 0.5 that distinguishes the Kuroshio- and Oyashio-dominant conditions is defined by a steep latitudinal gradient in Tr
values, indicating the existence of a boundary at that time
between the two water masses off present-day Fukushima (Fig. 2). This region corresponds to the polar frontal zone, which is characterized by having a summer SST
of approximately 23 °C and nutrient-rich Oyashio waters
distributed to the north (Fig. 1b). The high radiolarianabundance observed during cooler intervals at the end of
MIS 20 and early MIS 19 is likely to have been affectedby the high nutrient supply from the Oyashio water witha southward shift in the polar front (Oyashio expanded
mode, Fig. 5a). This southward shift in the polar front,
in turn, probably occurred in response to the southward shift of the westerly jet with the intensification of
the winter Aleutian Low, as discussed in Suganuma et al.
(2018), Haneda et al. (2020), and Kubota et al. (2021).
On the other hand, the multiple maxima observed inradiolarian productivity during the warm MIS 19 tended
to correspond with Tr maxima above 0.6, here definedas the Kuroshio expanded mode. The Kuroshio neutralmode is defined as Tr values between 0.5 and 0.6 without any corresponding maxima in radiolarian productivity after 785 ka. The millennial-scale fluctuations in
the Tr value and radiolarian productivity during MIS
19 can be explained by changes in the water structure

Page 10 of 14(a) Oyashio
expandedmodeRadiolarian
production

South

North

herbivorousor omnivorous(b) Kuroshio
neutralmode

Nutrient poor
Kuroshio water

Nutrient rich
Oyashio water(c) Kuroshio
expandedmode

algal symbiont

CbCS
Fig. 5 Schematics of oceanographic conditions indicating a Oyashio
expanded mode, likely present in late MIS 20 and characterized byhigh radiolarian productivity, and b neutral mode in MIS 19, when
the Kuroshio–Oyashio layered structure was contracted, and csame conditions, but with Kuroshio extended mode when the layer
structure was extended

associated with oscillations in the Kuroshio neutral andexpanded modes (Fig. 5b, c). Nishibe et al. (2015) proposed that high primary productivity can be attributed
to the development of favorable conditions for photosynthesis when the warm Kuroshio water overlays thecold Oyashio water. In addition, the areal extent of theKuroshio–Oyashio layered structure varies from year toyear, and this interannual variability affects primary productivity, making it possible to relate these variations tointer-decadal shifts in the climate regime of the North(2022) 9:5

Age (ka)
750

0.8

760

770

780

790

-9.0

-8.0

Dry

0.6

0.4

Kuroshio

0.2
750

800

760

-7.0

770

780

Age (ka)790

δ18 O (‰VPDB)

Recent studies have demonstrated that paleoclimate andpaleoceanographic changes recorded in the CbCS werelinked to global-scale variations in climate (Suganuma
et al. 2018; Haneda et al. 2020; Kubota et al. 2021). In thissection, the possible relationship between the variationin Tr values in the CbCS and changes in paleoclimate
records from southern Europe due to the Arctic Oscillation (AO) is discussed.
As discussed in the preceding section, the correspondence between millennial-scale maxima in radiolarian
productivity and Tr peaks during the Kuroshio mode
after 785 ka were likely related to the development of theKuroshio–Oyashio layered structure. On the other hand,
fluctuations in the abundance of L. setosa are considered to be influenced by the East Asian Winter Monsoon(EAWM), which, according to the PCA results of Kubota
et al. (2021), is affected by the intensity of the Aleutian
Low. In actuality, L. setosa records information during
the winter season according to its seasonal productivity in the coastal Oyashio region (Itaki et al. 2008). Significant increases in L. setosa abundance during MIS 19a,
MIS 19b, and MIS 20 are coincident with decreases of the
Tr values, suggesting that the influence of the EAWM isstrong during the Oyashio expanded mode. Two minor
increases in L. setosa abundance at 781 ka and 784 ka
tended to overlap with high Tr values. Such a condition
could be attributed to Kuroshio transport increasing
when the Aleutian Low intensifies (Deser et al. 1999).
These dynamics may also explain the inconsistencies
observed between Tr values and planktonic δ18O during MIS 19c, when the Tr maxima were associated withKuroshio expansion during summer to autumn, whileplanktonic δ18O records were associated with winterspring signals.
Millennial-scale climatic changes observed during
MIS 19 have also been reported for the northern Atlantic Ocean (Kleiven et al. 2011) and central Europe (Giaccio et al. 2015; Sánchez Goñi et al. 2016; Regattieri et al.
2019). Figure 6 shows a possible correlation between

Chiba Conposite
Section

4.3 Possible teleconnection

climate records for the Sulmona Basin in central Italy(Giaccio et al. 2015; Regattieri et al. 2019) and the CbCS
in Japan (this study). Oxygen isotope records for endogenic calcite from the Sulmona Basin can be used as aproxy for precipitation amount, with lower values reflecting increased precipitation and higher values reflecting a
reduction in precipitation.
Synchronous trends in each of these regions during
MIS 19 suggest the existence of a large-scale teleconnection over the northern hemisphere, not unlike theeffect of the Arctic Oscillation (AO) (or North Atlantic
Oscillation, NAO) on the intensity of the winter Aleutian Low (Tompson and Wallace 1998). It is considered that the wet (dry) conditions in southern Europe
during MIS 19 were caused by the southern (northern) shift of westerly storm tracks during the negative(positive) phase of the AO/NAO (Regattieri et al. 2019).
On the other hand, the Kuroshio transport increases(decreases) with the intensification of the Aleutian Low
during the negative (positive) AO (Deser et al. 1999),and such a situation is expected to have occurred during MIS 19, based on the radiolarian record discussed
above. As shown in Fig. 6, a consistent relationship isseen between paleo-records of humidity in central Italy
and the Kuroshio expansion in the CbCS during MIS
19. This hypothesis does not contradict the teleconnection mechanism proposed by Haneda et al. (2020),
in which millennial-scale latitudinal oscillations of theKuroshio Extension Front (KEF) during the late MIS 19

Sulmona Basin

Pacific (Nishikawa et al. 2016). Favorable conditions forphotosynthesis are also required by radiolarians withalgal symbionts in the Kuroshio region, such as the T. circularis/fruticosa group (Zhang et al. 2017), which is themost important radiolarian group in terms total radiolarian abundance in the CbCS. In addition, the abundance
variations in radiolarians with algal symbionts are correlated with the stratification component (PC3) of Kubota
et al. (2021). Therefore, the millennial-scale variations
observed in radiolarian productivity in the CbCS during
MIS 19 are likely the result of the expansion and retreatof the Kuroshio–Oyashio layered structure (Fig. 5b, c).

Page 11 of 14

Tr index

Itaki et al. Progress in Earth and Planetary Science

-6.0

800

Fig. 6 Probable correlations of paleo climate records between the
Sulmona Basin in Italy (Giaccio et al. 2015; Regattieri et al. 2019) andthe CbCS in Japan (this study)

Itaki et al. Progress in Earth and Planetary Science(2022) 9:5

and MIS 20–19 transition were associated with Atlantic
meridional overturning circulation (AMOC) disruption
due to freshwater discharges into the North Atlantic.

5 Conclusions
Radiolarian fossil assemblages were examined using
236 samples from the CbCS including the Lower–Middle Pleistocene GSSP in the Kokumoto Formation of
the Kazusa Group, central Japan. Based on a high-resolution record of abundance changes in total radiolarians
and 11 species or species groups, paleoceanographic
changes during MIS 20 to MIS 18 (800–750 ka) werediscussed.
The Tr index, which can be used as a proxy for the
relative contributions of the Kuroshio and Oyashio currents based on radiolarian assemblages, indicated theoccurrence of millennial-scale oscillations in the Kuroshio–Oyashio boundary. The Tr value ranged between
approximately 0.2 and 0.8, which corresponds to the values off present day Aomori (41°N) and the Boso Peninsula (35°N). Except for a period during early MIS 19, theTr variations tended to be consistent with those of δ18O
in planktonic foraminifera associated with temperature.
Such inconsistencies between Tr and δ18O are likely
related to differences in radiolarian ecology caused byfactors such as temperature, habitat depth, stratification,and seasonal changes in production.
Multiple maxima and minima in total radiolarian production with millennial-scale periodicity during MIS
19 tended to increase with the development of the layered Kuroshio–Oyashio structure during the Kuroshio
expanded mode. This is likely related to the expansion offavorable conditions for photosynthesis, such as the lightenvironment, with warm Kuroshio water overlaying thecold Oyashio water.
Millennial-scale fluctuations in the Kuroshio–Oyashio
boundary during MIS 19 can be correlated with European climate changes (Giaccio et al. 2015; Sánchez Goñi
et al. 2016; Regattieri et al. 2019), and these fluctuations
are thought to have emerged as a result of atmosphericcirculation teleconnection over a wide area (e.g., Arctic
Oscillations). However, the number of high-resolution
analyses of MIS 19 is limited, and it is expected thatdetailed teleconnection mechanisms will be elucidated bycomparing analysis results over wide geographic areas.
Abbreviations
AO: Arctic oscillations; Ca/Ti: Calcium/titanium ratio; CbCS: Chiba composite
section; GSSP: Global boundary stratotype section and point; M–B boundary:Matuyama–Brunhes boundary; MIS: Marine isotope stage; NAO: North Atlantic
oscillations; PC: Principal component; PCA: Principal component analysis; SST:
Sea-surface temperature; TOC: Total organic carbon; δ18O: Oxygen isotope
ratio.

Page 12 of 14

Supplementary Information
The online version contains supplementary material available at https://​doi.​
org/​10.​1186/​s40645-​021-​00465-0.
Additional file 1: Table S1. Occurrence list of radiolarians from the CbCS
that are newly reported in this study.
Additional file 2: Table S2. Relative abundance (%) of radiolarian indicator, Tr values, and total radiolarian abundance from the CbCS. Dataset wascompiled for this study and 41 preliminary results from Suganuma et al.
(2018, green hatches with italic characters). Because three samples (YN-07,
YN-09, YN-05) reported by Suganuma et al. (2018) are stratigraphicallyoverlapped with newly analyzed samples, they were excluded from thedataset.
Additional file 3: Fig. S1. (Lithologic column). Lithologic column andsampling horizon of the CbCS, modified from Suganuma et al. (2018).
Stratigraphic correlation between the Urajiro, Yanagawa, Yoro River,Yoro-Tabuchi, and Kokusabata based on lithology and tephra beds areindicated by black dashed and red solid lines. The red solid lines especiallyindicate correlation points for reconstructing the age model of the TB-2
core (Additional file 4: Fig. S2).
Additional file 4: Fig. S2. Temporal profile of low field magnetic susceptibility and Ca/Ti. Black solid lines and circles are data from the CbCS
(Okada et al. 2017; Simon et al. 2019; Izumi et al. 2021). Light blue lines
are data from the TB-2 core (Hyodo et al. 2016, 2017), which was dated
using the latest age model of the CbCS by Suganuma et al. (2018). The
age model of the TB-2 core, which was drilled at ~ 200 m northeast fromthe Chiba section (Hyodo et al. 2016), was obtained by tuning the diatom
assemblage record from Osaka Bay (Hyodo and Kitaba 2016; Hyodo
et al. 2017), whereas the chronology of the CbCS is based on tuning thebenthic foraminifera oxygen isotope record to an astronomically dated
sea level record from ODP 1123 (Elderfield et al. 2012; Suganuma et al.
2018). The difference in chronology hinders comparisons of paleoenvironmental records between the CbCS and TB-2 core. To resolve this issue, weconverted the depth scale of the TB-2 core to the age scale of the CbCS
using the latest age model by Suganuma et al. (2018) based on correlating
six tephra beds and one upper limit of the mud crust bed (Additional
file 3: Fig. S1). Resultant temporal profiles of Ca/Ti ratio and low-fieldmagnetic susceptibility of the TB-2 core show excellently synchronizedvariation with those of the CbCS (this S. Fig.; Hyodo et al. 2016, 2017;
Okada et al. 2017; Simon et al. 2019; Izumi et al. 2021). This indicates thatthe tephrostratigraphic and lithologic correlation allows us to compare
the paleoenvironmental records between the CbCS and TB-2 core.
Additional file 5: Fig. S3. Plot showing the latitudinal distribution ofradiolarian species or species groups used to estimate the Tr index in thenorthwestern Pacific based on the dataset from Matsuzaki and Itaki (2017).
Additional file 6: Fig. S4. Results of spectral analysis (simple periodogram) for δ18O of G. bulloides (18O_Gb), Tr index, and total radiolarian
analysis using the PAST statistical software package (Hammer et al. 2001).
The three-point moving average data was processed through the regularinterpolation for spacing and a bandpass filter from 0.03 to 0.4.
Acknowledgements
We express our sincere gratitude to all members of the GSSP proposal for
Chiba section samples. Noritoshi Suzuki provided useful advice regarding
radiolarian ecology. We are also grateful to Hitomi Yamazaki for assistance with
conducting laboratory experiments.
Authors’ contributions
TI conceived the experiments and wrote the manuscript. TI and SU conducted
the experiments. YH, KI, and YK contributed to determining geochemical
proxies and their interpretation. YH, YS, and MO contributed to stratigraphic
correlation and interpretation of datasets. YH, YS, and MO conducted geological surveys and sampling. All authors agree to submission of the manuscript.

Itaki et al. Progress in Earth and Planetary Science(2022) 9:5

Author information
TI is a senior researcher at the Geological Survey of Japan (GSJ), AIST, Japan;
SU was a Master course student at Chiba University and an AIST research assistant, Japan (2018–2020); YH is a post-doctoral fellow at the GSJ, AIST, Japan;
KI is an associate professor at Chiba University, Japan; YK is a researcher at the
National Museum of Nature and Science, Japan; YS is an associate professor at
NIPR, Japan; MO is a professor at Ibaraki University, Japan.
Funding
This work was supported by JSPS KAKENHI Grant Numbers 16H04068 and
19H00710 (MO).
Availability of data and materials
The datasets supporting the conclusions of this paper are available as additional files (Additional file 1: Table S1).

Declarations
Competing interests
The authors declare that they have no competing interests.
Author details
1
Geological Survey of Japan/AIST, Institute of Geoinformation and Geology, 1‑1‑1 Hihashi, Tsukuba, Ibaraki 305‑8567, Japan. 2 Faculty and Graduate
School of Education, Chiba University, 1‑33 Yayoi‑cho, Inage‑ku, Chibashi,
Chiba 263‑8522, Japan. 3 National Institute of Polar Research, 10‑3 Midori‑cho,Tachikawa, Tokyo 190‑8518, Japan. 4 Department of Geology and Paleontology,
National Museum of Nature and Science, 4‑1‑1 Amakubo, Tsukuba, Ibaraki
305‑0005, Japan. 5 Department of Polar Science, School of Multidisciplinary
Sciences, The Graduate University for Advanced Studies (SOKENDAI), Hayama,
Japan. 6 Department of Earth Sciences, Ibaraki University, 2‑2‑1 Bunkyo, Mito,
Ibaraki 310‑8512, Japan.
Received: 22 March 2021 Accepted: 22 December 2021

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