Kameo et al. Progress in Earth and Planetary Science
https://doi.org/10.1186/s40645-020-00355-x(2020) 7:36

Progress in Earth andPlanetary Science

RESEARCH ARTICLE

Open Access

Calcareous nannofossil biostratigraphy of
the Lower–Middle Pleistocene boundary of
the GSSP, Chiba composite section in the
Kokumoto Formation, Kazusa Group,
central Japan, and implications for seasurface environmental changes
Koji Kameo1*, Yoshimi Kubota2, Yuki Haneda3, Yusuke Suganuma4,5 and Makoto Okada6

Abstract
The Chiba composite section (CbCS), in the middle of the Boso Peninsula in central Japan, was ratified as the
Global Boundary Stratotype Section and Point (GSSP) for the Lower–Middle Pleistocene boundary, accompanied by
the Matuyama–Brunhes (M–B) paleomagnetic polarity boundary in January 2020. This study examined thecalcareous nannofossil biostratigraphy of the CbCS to describe potential nannofossil events and discuss sea-surface
environments around the M–B paleomagnetic polarity boundary. There are no clear biohorizons at the M–B
paleomagnetic polarity boundary, although a temporary disappearance of Gephyrocapsa specimens (≥ 5 μm in
diameter), an important calcareous nannofossil genus in the Pleistocene, occurs just above the Lower–Middle
Pleistocene boundary. Although this is a characteristic event around the M–B paleomagnetic polarity boundary, it isunclear whether the event is globally traceable.
Changes in the environmental proxy taxa of calcareous nannofossils in the CbCS revealed that sea-surface
environments were driven by glacial-interglacial and millennial-scale climate forces. The time-transgressive changeof the Tn value, a calcareous nannofossil temperature index, is mostly concordant with the planktonic foraminiferal
oxygen isotope fluctuation. Abundant occurrences of a warm-water species, Umbilicosphaera spp., indicate that theKuroshio Current was strong after ~ 783 ka. Even the strong influence of the Kuroshio Current, cooling events
related to southward movements of the Kuroshio Front occurred every 10,000 years based on the presence of acold-water taxon, Coccolithus pelagicus. Additionally, the inflow of coastal waters strengthened after ~ 778 ka
because of abundant occurrences of Helicosphaera spp. Millennial-scale sea-surface changes were also inferred fromdetected floral fluctuations of less than 10,000 years.
Keywords: Calcareous nannofossils, Chiba composite section, Lower–Middle Pleistocene boundary, M–B
paleomagnetic polarity boundary, Gephyrocapsa, Kuroshio Current* Correspondence: kameo@faculty.chiba-u.jp
1
Department of Earth Sciences, Faculty of Science, Chiba University, 1-33,
Yayoi, Inage, Chiba, Chiba 263-8522, Japan
Full list of author information is available at the end of the article© The Author(s). 2020 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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(2020) 7:36

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Introduction
Calcareous nannofossils are major phytoplankton fossils that are used to determine the geologic age ofstrata and to infer past marine environmental
changes. Many calcareous nannofossil biohorizons,corresponding to the appearance and disappearance ofspecies, are known because calcareous nannofossils
have rapidly evolved since the Late Triassic (e.g.,
Bown et al. 2004). Comparative studies between thesebiohorizons and other chronostratigraphic data havebeen used to construct a biochronological framework.
The framework of the Neogene–Quaternary biohorizons exists in the eastern equatorial Pacific, the western equatorial Atlantic, and the Mediterranean(summarized by Raffi et al. 2006). However, usefulbiohorizons and their relationship to marine isotope
stages have not yet been established in the North Pacific deep-sea cores because of the rarity of calcareous
sediments caused by a shallow calcite compensation
depth (CCD) (e.g., Pälike et al. 2012) from the Neogene to the Quaternary.
The marine Kazusa Group is a Japanese Pleistocene
unit on the Boso Peninsula (central Pacific Japanese
coast; Fig. 1) that yields abundant microfossils including
pollen and spores, planktonic and benthic foraminifers,
calcareous nannofossils, diatoms, and radiolarians (e.g.,
Oda 1977; Sato et al. 1988). These microfossils havebeen used to determine the age of the Group and thepaleoceanographic conditions. However, the time resolution of these studies was relatively low (i.e., a minimum of tens of kyrs). Therefore, further
biochronostratigraphic analyses of the formations in theGroup are required to obtain suitable Quaternary age
models and to discuss past climatic changes around the
NW Pacific.
This paper reviews previous studies of the calcareous
nannofossil biostratigraphy of the Kazusa Group anddiscusses the calcareous nannofossil assemblages in the
Chiba composite section (CbCS), which is the ratified
Global Boundary Stratotype Section and Point (GSSP)
for the Lower–Middle Pleistocene boundary. Furthermore, detailed assemblage data of the calcareous nannofossils
acrossthe
Matuyama–Brunhes(M–B)
paleomagnetic polarity boundary are described, and thevariability of calcareous nannofossils related to paleoceanographic changes in sea-surface waters is discussed. It
is expected that the GSSP for the Lower–Middle Pleistocene boundary should provide good climatic information
for understanding recent climatic changes because theboundary is situated within the interglacial Marine Isotope Stage (MIS) 19 (e.g., Head and Gibbard 2005). The
orbital configuration in MIS 19 is considered to be similar to the current interglacial MIS 1 (e.g., Tzedakis et al.
2012);
thus,
orbitally
inducedclimatic
andpaleoceanographic changes within MIS 19 should be investigated to clarify their effects on the recent marine
biosphere. Calcareous nannoplankton, one of the mostimportant marine primary producers, is a suitable tracer
for environmental changes which occurred at the sea
surface, especially within the photic zone.

Kameo et al. Progress in Earth and Planetary Science

Geological setting and chronostratigraphic analyses of
the Kazusa Group

The Lower to Middle Pleistocene Kazusa Group is composed of thick siliciclastic sediments in the Boso Peninsula on the Pacific side of the Japanese islands (e.g., Ito
1998; Kazaoka et al. 2015; Ito et al. 2016). The basin
where the Kazusa Group was deposited developed as aforearc basin in response to the northward or northwestward subduction of the Pacific Plate against the Japaneseislands (e.g., Seno and Takano 1989). The Group consists of deep-sea to shallow-shelf sediments and is divided into ten formations on the eastern part of thePeninsula; namely, in chronological order from oldest to
youngest: the Kurotaki, Katsuura, Namihana, Ohara,
Kiwada, Otadai, Umegase, Kokumoto, Kakinokidai, Chonan, and Kasamori Formations (Fig. 1; e.g., Suzuki et al.
1995; Kazaoka et al. 2015). Shallowing-upward successions and deepening successions eastward of the basin
are recognized in the Group, suggesting a northeastward
paleocurrent direction (e.g., Ito et al. 2016). Generally,
all formations are composed of mostly continuous, undeformed sediments; however, in the eastern part of the
Boso Peninsula, the sequence is disrupted by NNESSW-trending faults.
On the Yoro River, in the middle part of the Boso
Peninsula, a thick succession of the Kazusa Group ispresent (Figs. 1 and 2). Several chronostratigraphic
and chronological studies have been carried out inthis sequence. Planktonic foraminifera (Oda 1977),
calcareous nannofossils (Takayama 1973; Sato et al.
1988), diatoms (Cherepanova et al. 2002), magnetostratigraphic (Niitsuma 1971; Niitsuma 1976), andδ18O (Okada and Niitsuma 1989; Pickering et al.
1999; Tsuji et al. 2005) studies estimate the age ofthe Group to span ca. 2.4–0.45 Ma (see summary by
Ito et al. 2016). Many interbedded tephra layers allow
the correlation of different facies among the formations not only within the Group but also in other formations in Japan (e.g., Satoguchi and Nagahashi2012). Sedimentological analyses revealed the depositional environments of the formations, and sequencestratigraphic analysis was applied to clarify their formation processes and relationship to sea-level changes(e.g., Ito and Katsura 1992). As a result, glacioeustatic
variability during the Quaternary has primarily controlled the deposition of the formations of the Kazusa
Group, even in the active margin setting (Ito 1992).

Kameo et al. Progress in Earth and Planetary Science(2020) 7:36

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Fig. 1 Oceanographic conditions around the Japanese islands (a) and the location of the studied area (b). a Major surface currents around theJapanese islands based on Brown et al. (2001), Qiu (2001), and Yasuda (2003). Sea-surface temperature from the World Ocean Atlas 2013
(Locarnini et al. 2013) illustrated by Ocean Data View software (Schlitzer 2020). TSC: Tsushima Current, TGC: Tsugaru Current, KE: Kuroshio
Extension. Detailed locations of the studied areas are shown in Fig. 2. b The generalized geological map of the Boso Peninsula is based on thatcreated by Tokuhashi and Watanabe (2008). Stratigraphy in the Group distributed in the eastern part of the Peninsula is illustrated in Fig. 3

The Chiba Composite Section (CbCS) is in the
middle part of the Kokumoto Formation, which is
the upper formation of the Group (e.g., Suzuki et al.
1995). This formation is mainly composed of
massive mudstones interbedded with sandstones, indicating deposition in shelf-edge and slope environments (e.g., Ito 1992). No evidence of hiatuses or
unconformities is present in the CbCS (Nishida
et al. 2016).

Oceanographic conditions on the Pacific side of theJapanese islands

Oceanic waters flowing around the Pacific side ofJapanese islands are composed of two main surface
currents, the warm Kuroshio and the cold Oyashio
currents, which are northeast- and southwest-flowing
surface currents, respectively (Fig. 1). The Kuroshio
Current is warm, with high-salinity surface water. It
is the western boundary current of the subtropicalKameo et al. Progress in Earth and Planetary Science(2020) 7:36

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Fig. 2 Locations of the studied areas, along the Yoro River, Yanagawa, Urajiro, Chiba section, and Kokusabara construct the Chiba composite
section (CbCS). The basemap is from 1:50,000 “Ootaki” topographic map published by the Geospatial Information Authority of Japan (GSI).
Detailed locations are shown in Suganuma et al. (2018). Samples shown in BSCH, BSKD, BSKM, BSUS, BSOD, YORG, and HRSG were used by Satoet al. (1988) and Sato (1989). The location of “the JNOC TR-3” is also shown

gyre in the North Pacific Ocean (e.g., Qiu 2001;
Yasuda 2003) and flows northward from the westernequatorial Pacific region. This current is less than
100 km wide and approximately 1000-m deep

(Gallagher et al. 2015) and flows along the Pacificcoast of the western part of the Japanese islands andbecomes an eastward flow—the Kuroshio Extension—after separating from the coast (Fig. 1).

(2020) 7:36

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The Oyashio Current is the southwestern extension of
the East Kamchatka Current, which is itself the westernextension of the Western Subarctic Gyre from the North
Pacific (e.g., Qiu 2001; Yasuda 2003). The Oyashio
Current originates in the subarctic region. A mixed
water regime between the Kuroshio and Oyashio currents is present off the coast of northeastern Japan, andthe boundary between the two currents is either the
Kuroshio Front or the Subarctic Front (Qiu 2001). In themixed water region, the Oyashio Current generally flows
beneath the warmer Kuroshio Current; generating complex warm and cold eddies in the surface waters (e.g.,
Unoki and Kubota 1996; Yasuda 2003). These eddies
bring eutrophic surface waters to the mixed water region, whereas the Kuroshio Current is oligotrophic. Regional variations in both currents strongly influence themarine environments and organisms around the NW
Pacific because physical and chemical conditions, suchas sea-surface temperature, salinity, and nutrient richness, are strongly controlled by the strength of both currents (e.g., Kimura et al. 2000).
The Tsugaru Current is another surface water mass
along the Pacific coast of Japan, which is a southwardcurrent flowing along the NE Japanese coast (Fig. 1; e.g.,
Qiu 2001; Gallagher et al. 2018; Yasuda 2003). This
current flows between the coast and the Oyashio
Current and originates in the Japan Sea as the Tsushima
Current, which branches from the Kuroshio Current
around the eastern part of the East China Sea off thewest coast of the Tokara Islands, small islands to thesouth of Kyushu (Fig. 1). The Tsushima Current enters
the Japan Sea through the shallow (< 140 m) Tsushima
Strait (Fig. 1). It then passes through the Tsugaru
Strait—a shallow (< 130 m) strait between Hokkaido andHonshu, and finally flows southward along the Pacificside of the Japanese islands (Gallagher et al. 2018;
Yasuda 2003).

Quaternary sediment cores (Takayama and Sato 1987;
Sato and Takayama 1992). Around the same time, similar Quaternary nannofossil biohorizons have been proposed by several authors, such as Raffi et al. (1993) andMatsuoka and Okada (1989). Sato et al. (1988) and Sato
(1989) dated the formations in the Kazusa Group along
the Yoro River in order to locate the Plio-Pleistocene
boundary in the uppermost Japanese Quaternary sequences (Table S1). Subsequently, Sato et al. (1999) revised the calcareous nannofossil biostratigraphy detectedin the Kazusa Group. Samples used in these papers wereobtained along the Yoro, Heizo, and Hirasawa Rivers(Fig. 2), and the eastern part of the Boso Peninsula. Tsuji
et al. (2005) established a high-resolution chronostratigraphy based on oxygen isotope stratigraphy, magnetostratigraphy, and calcareous nannofossil biostratigraphy
in portions of the Otadai and Umegase Formations in
the JNOC TR-3 sediment core (Fig. 2). Based on thesestudies, the following ten datums (= biohorizons) arerecognized in the Kazusa Group (Fig. 3):

Kameo et al. Progress in Earth and Planetary Science

Previous studies on calcareous nannofossil
biostratigraphy of the Kazusa Group

Several biostratigraphic studies on calcareous nannofossils from Japanese strata have been conducted since
Takayama (1967) first described calcareous nannofossils
from the Neogene and Quaternary sections in the southern Kanto region of Japan. In the Boso Peninsula,
Takayama (1973) studied the calcareous microfossil biostratigraphy of the uppermost Cenozoic formations on
the Japanese islands and from a sediment core taken
near the northwestern region of the Pacific Ocean anddescribed the biohorizons of planktonic foraminifera andcalcareous nannofossils. In 1987, Takayama and Satoestablished 11 calcareous nannofossil datums (biohorizons) composed of the first and last occurrences of calcareous nannofossil taxa using the North Atlantic(1) The LAD (last appearance datum: equivalent to thelast occurrence or highest occurrence) of Discoasterbrouweri in the middle part of the Ohara Formation(Sato et al. 1988). The numerical age for thisbiohorizon is 1.93 Ma by GTS2012 scaling(Anthonissen and Ogg 2012 in Gradstein et al.
2012). This biohorizon is in the eastern part of the
Boso Peninsula; however, it is not in the Yoro River.
(2) The FAD (first appearance datum: equivalent to thefirst occurrence or lowest occurrence) ofGephyrocapsa caribbeanica above the Kd38 tephrain the lower part of the Kiwada Formation (Satoet al. 1988). The biohorizon is situated just above
the magnetochron C2n, the Olduvai
Subchronozone (Niitsuma 1976). This biohorizoncorresponds to the first occurrence of medium
Gephyrocapsa spp. The biohorizon is correlatedwith MIS 59 (Wei 1993; Berger et al. 1994) or atransition of MIS 58/59 (Lourens et al. 2004;
Anthonissen and Ogg 2012). The numerical age is
1.73 Ma (Anthonissen and Ogg 2012).
(3) The FAD of Gephyrocapsa oceanica is just below
the Kd25 tephra in the Kiwada Formation (Satoet al. 1988, 1999). The LAD of Calcidiscus
macintyrei also occurs in the same horizon. The
FAD of G. oceanica was defined by Takayama andSato (1988) and Sato and Takayama (1992) and issituated into MIS 57 in the Ontong Java Plateau(Berger et al., 1994). No age data by GTS2012
scaling for the biohorizon has been calculated.
(4) Large forms of Gephyrocapsa specimens first appear
close to the Kd19 tephra in the Kiwada Formation(Sato et al. 1988, 1999). They are composed of large

Kameo et al. Progress in Earth and Planetary Science(2020) 7:36

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Fig. 3 Calcareous nannofossil biostratigraphy by Sato et al. (1988). The relative abundance of main nannofossil species (from Sato et al. 1988; Sato
1989) is also shown. A dotted line in the Chonan Formation indicates reworked specimens. The numerical ages of the biohorizons (Anthonissen
and Ogg 2012) are also shown. The last occurrence (LAD) of Pseudoemiliania lacunosa are situated above the section

specimens, which are over 6 μm in diameter withan oblique bridge to the short axis (Takayama andSato, 1987). On the other hand, some authors
treated large specimens over 5.5 μm in diameter aslarge Gephyrocapsa (e.g., Raffi et al. 1993). The firstoccurrence of Gephyrocapsa over 6 μm in diametercorresponds to MIS 50 in the Ontong Java Plateau(Berger et al. 1994) and MIS 45 in the North
Atlantic Ocean (Sato et al. 2009). Its firstoccurrence seems to be slightly diachronous assame as that of specimens over 5.5 μm in diameter(e.g., Wei 1993; Raffi et al. 2006). The numericalage of the biohorizon based on the GTS2012
scaling has not been estimated.

(5) Helicosphaera sellii disappears prior to the Kd8
tephra in the upper part of the Kiwada Formation(Sato et al. 1988, 1999). Lourens et al. (2004) andAnthonissen and Ogg (2012) indicated that thebiohorizon corresponds to MIS 49 in theeastern equatorial Pacific. This biohorizon is
also situated into MIS 47 in the Ontong Java
Plateau (Berger et al. 1994). However, it seems
to be diachronous (e.g., Lourens et al. 2004)
because the biohorizon is situated in MIS 37/
38 boundary in the North Atlantic (Sato et al.
2009). The numerical age of this biohorizon is
calculated at 1.34 Ma (Anthonissen and Ogg2012).

(2020) 7:36

Page 7 of 17(6) The LAD of large forms of Gephyrocapsa is in theupper part of the Kiwada Formation (Sato et al.
1988, 1999). The stratigraphic position of thebiohorizon, however, could shift due to therelatively low-resolution analysis of nannofossils
across the Kiwada and Otadai Formation boundary.
Corresponding oxygen isotope stage is MIS 37 (e.g.,
Berger et al. 1994; Lourens et al. 2004; Anthonissen
and Ogg, 2012). However, the biohorizon is slightly
later in the North Atlantic, in MIS 35 (Sato et al.
2009). Its numerical age is 1.24 Ma (Anthonissen
and Ogg 2012).
(7) The FAD of Reticulofenestra asanoi was initiallydescribed below the O22 tephra in the Otadai
Formation (Sato et al. 1988). Later, its stratigraphic
range was extended below the O23 tephra (Satoet al. 1999). The biohorizon is correlated with MIS
35 (e.g., Wei 1993; Berger et al. 1994; Raffi et al.
2002) or MIS 34 in the North Atlantic (Sato et al.
2009). The age of this biohorizon is 1.14 Ma(Anthonissen and Ogg 2012).
(8) Gephyrocapsa parallela first appears in theuppermost Otadai Formation. The first occurrenceof this species is considered to be the reentrance of
medium Gephyrocapsa specimens, usually over
4 μm in diameter (Raffi 2002), with a parallel bridgeto the short axis. Larger forms (> 5.5 or ≥ 6 μm in
diameter) of this species are sometimes found above
its first occurrence, but they differ from large formsof Gephyrocapsa due to having a parallel bridge tothe short axis. The biohorizon is between thetephras O1 and O7. A high-resolution chronologicalanalysis of the JNOC TR-3 core—near Kamishiki,
Otaki, in the north of Kazusa-Nakano (Fig. 2), by
Tsuji et al. (2005) revealed that this biohorizon is in
the middle part of the core, between tephras TR-3B
and TR-3C. The first occurrence corresponds to
MIS 29 (Anthonissen and Ogg 2012) or MIS 30(Berger et al. 1994; Tsuji et al. 2005) but slightly
later stages in the North Atlantic (e.g., Sato et al.
2009; Wei 1993). This biohorizon was also recognized within the magnetochron C1r.1n (Jaramillo
Subchronozone) (Berger et al. 1994; Tsuji et al.
2005). The age of this biohorizon is 1.04 Ma(Anthonissen and Ogg 2012).
(9) The LAD of R. asanoi is below the tephra U4 in the
Umegase Formation (Sato et al. 1999). Thisbiohorizon can be correlated with MIS 22 and/or
MIS 23 (e.g., Wei 1993; Berger et al. 1994; Raffiet al. 2002). The numerical age is 0.91 Ma(Anthonissen and Ogg 2012).

situated below the LAD of Pseudoemiliania lacunosa(0.44 Ma; Anthonissen and Ogg 2012).

Kameo et al. Progress in Earth and Planetary Science

Based on Sato et al. (1988), the Chonan Formation, an
uppermost formation of the Kazusa Group, can be

Biogeography of modern calcareous nannoplanktons
from the Japanese islands

The ecological preferences of several nannofossil taxa in
the CbCS were mainly interpreted from previous ecologic and biogeographic studies of calcareous nannoplankton around the Japanese islands (e.g., Tanaka 1991;
Hagino 1997). Tanaka (1991) examined calcareous
nannofossil species in 437 surface sediments from the
Pacific Ocean, the Japan Sea, and the East China Sea,
and clarified 10 biotopes based on the Q-mode cluster
analysis of nannofossil assemblages. The distribution
patterns of coccolith thanatocoenosis follow changes inthe environmental conditions of surface currents around
the Japanese islands, and several diagnostic nannoplankton taxa are recognized in specific surface currents andwithin oceanic areas. Aizawa et al. (2004) summarized
the ecology and biogeography of the main Quaternary
calcareous nannoplankton species in order to evaluate
calcareous nannofossil assemblages during the last 144
ka in a piston core off the Kashima coast, north of the
Boso Peninsula. Ecological information on these modern
calcareous nannoplankton species in other oceans enables us to discuss paleoceanographic changes based on
calcareous nannofossils.
A nannofossil tracer of the Kuroshio Current (Fig. 1)
is Umbilicosphaera spp. because extant Umbilicosphaera
sibogae is dominant in the main route of the Kuroshio
Current (Tanaka 1991). Recently, Pleistocene Umbilicosphaera was classified into several species (e.g., Younget al. 2017). U. sibogae consists of two species, U. sibogae, and Umbilicosphaera foliosa; however, this study
follows the definition used by Tanaka (1991) and treats
them uniformly as U. sibogae.
Common Coccolithus pelagicus in thanatocoenosis isassociated with cooler surface waters from the Kuroshio(Subarctic) Front and the mixed water region off thenortheastern coast of the Japanese islands (Tanaka
1991). This taxon may include C. pelagicus pelagicus
and C. pelagicus braarudii. Based on their ecologicalpreferences in the Atlantic Ocean (e.g., Cachão andMoita 2000), C. pelagicus pelagicus corresponds tocolder conditions than C. pelagicus braarudii (Narciso
et al. 2006). C. pelagicus braarudii is common in Atlantic upwelling areas, where surface waters are cool andeutrophic. Based on Aizawa et al. (2004), C. pelagicus isabundant in surface waters under low-temperature andhigh-nutrient conditions.
Tanaka (1991) suggested that the dominance of Helicosphaera spp. in thanatocoenosis is typical of coastal
water from the East China Sea. This taxon is present
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which is off the southwestern coast of the Japaneseislands. Tanaka (1997) indicated that this taxon occurredabundantly in sediment trap samples from the continental shelf slope in the East China Sea. Aizawa et al. (2004)
summarized that the presence of this taxon may presumably be affected by nutrient levels.
A lower photic taxon, Florisphaera profunda, has beenutilized to reconstruct nutrient dynamics around theequatorial regions (e.g., Molfino and McIntyre 1990),
water transparency, and/or turbidity (surface stratification) in the NW Pacific (Ahagon et al., 1993). According
to Tanaka (1991), F. profunda occurs in more stable,
stratified, warm offshore waters and gradually increases
in number farther from the Japanese coast (Tanaka
1991). Another offshore taxon, Calcidiscus leptoporus, ismore abundant to the east of the Izu-Ogasawara Ridge,
in the northeastern part of the Pacific Ocean (Tanaka
1991). There has been no relationship established between the occurrence of this species and the Kuroshio
and/or Oyashio currents.
Rhadbdosphaera clavigera, Syracosphaera spp., Calciosolenia spp., and Umbellosphaera spp. are observed
mainly south of the Kuroshio Front (Tanaka 1991), although these are considered as very rare taxa in the
nannoplankton thanatocoenosis.

counts. According to Suganuma et al. (2018), specimensof Gephyrocapsa, Pseudoemiliania, and Florisphaera
comprise approximately 90% of the total flora in the section. Thus, the frequencies of other rare taxa related tosea-surface conditions are obscured. In this study, weapplied additional counting of subordinate taxa (Okada
and Wells 1997) after 200 counts. The subordinate taxa
in this study are nannofossil specimens except for major
taxa (Gephyrocapsa, Pseudoemiliania, and Florisphaera).
Permanent slides were made following the simplified
method of Flores and Sierro (1997) to obtain the countdata for subordinate taxa. The overall preservation of
calcareous nannofossils in each sample was visually evaluated using the following criteria modified after that of
Takayama and Sato (1987): good (i.e., most fossils havelittle or no evidence of dissolution and/or overgrowth),moderate (i.e., fossil specimens are partly etched, but all
taxa can be identified), and poor (i.e., most specimens
are etched, and it can be difficult to identify eachspecies).
To avoid taxonomic problems, Gephyrocapsa specimens were subdivided into several morphotypes basedon the maximum diameter of the coccolith. At the sametime, a bridge angle to the diameter was also observed.
The maximum diameter of a specimen was measured at
1 μm intervals using an eyepiece graticule under a lightmicroscope. Based on the measurements, Gephyrocapsa
specimens were subdivided into four categories (< 3 μm,
3–4 μm, 4–5 μm, and ≥ 5 μm). Most Gephyrocapsa species over 4 μm in diameter in the CbCS are likely to be
Gephyrocapsa parallela or Gephyrocapsa omega, with aparallel bridge to the short axis.
This study basically follows the age model of the CbCS
established by Suganuma et al. (2018), which combines
oxygen isotope stratigraphy and magnetostratigraphy.
Oxygen isotope data of planktonic and benthic foraminifera revised by Haneda et al. (2020) were also used inthis study. Marine isotope stages MIS 20 to MIS 18 wererecognized, and high-resolution oxygen isotope records
enabled us to identify three substages within MIS 19(MIS 19c–19a). A chronologic uncertainty of the age
model is approximately 4 kyr (Haneda et al. 2020).
In order to estimate sea-surface temperatures during
the deposition of the CbCS, we calculated the nannofossil temperature index (Tn value) proposed by Aizawa
et al. (2004). Tn values were estimated by using the relative abundances of subordinate counts, as mentioned
above. Tn was estimated as follows:
Tn = warm taxa/(warm taxa + cold taxa) × 100
Originally, warm taxa were considered as Kuroshio
taxa (Aizawa et al. 2004). These are composed of Umbilicosphaera spp., Rhabdosphaera clavigera, Oolithotus
spp., Syracosphaera spp., and Umbellosphaera spp.,
which are all recognized in the Kuroshio region (Tanaka

Kameo et al. Progress in Earth and Planetary Science

Materials and methods
We used mudstone samples from Suganuma et al.
(2018) from the CbCS of the Kokumoto Formation on
the Yoro River (Fig. 2) to investigate potential bioeventsofcalcareous
nannofossils
acrossthe
M–B
paleomagnetic polarity boundary and environmental
changes in detail. Previous studies (Sato et al. 1988,
1999) suggest that there is no characteristic change inthe occurrence of nannofossil species across this boundary (Fig. 3). The interval is between the last occurrencesof Reticulofenestra asanoi and Pseudoemiliania lacunosa,
and previous studies in other oceans (e.g., Raffi et al.
2006) have no apparent diagnostic biohorizons.
The analyzed intervals, including the tephra Byk-E(Takeshita et al., 2016), are in the middle part of the
Kokumoto Formation. The formation mainly contains
massive mudstones with thin sandstones and occasional
mudstone/sandstone interbeds. This study adds 12 samples to those presented by Suganuma et al. (2018); thetotal number of samples was 75. The time resolution inthis study is less than one thousand years, on average.
Typical smear slides and permanent slides were prepared and observed using a light-polarized microscope(Olympus BX51) at ×1500 magnification. Smear slides
were used to estimate the relative abundance of eachcalcareous nannofossil species based on the count of 200
specimens per slide. Additional transects of each smear
slide were observed to detect rare species outside 200

(2020) 7:36

Page 9 of 171991). On the other hand, cold nannofossil taxa consistof Coccolithus pelagicus pelagicus and C. pelagicus
braarudii. As mentioned above, Tanaka (1991) did notdivide C. pelagicus into two subspecies and consideredC. pelagicus as an Oyashio taxon. However, a few occurrences of this species were recognized just south of theKuroshio Front, indicating the presence of locally lowtemperature and nutrient-rich areas near the Front.
Considering geographic distributions and ecologicalpreferences in other oceans, they are likely to be C. pelagicus braarudii.

and 5, Table S2); comprising 90% of the total flora in thesection. Small forms of Gephyrocapsa (< 3 μm in longer
diameter) commonly reached over 70% of the total flora
below sample TB3-43 in the lower part. This taxon, however, decreased from samples TB3-38 to TB14 andreached an average abundance of 45% above sample
TB10.
In contrast, Gephyrocapsa specimens (from 3 to 4 μm
in diameter) slightly increased above sample TB3-38 andreplaced the smaller forms. The specimens over 5 μm in
diameter were sporadically observed in the section; however, they were absent, in TB2-44 to KG30 samples between tephra Byk-A and an unnamed tephra layer in the
middle of the Kokusabata section (Fig. 4). Within thelower part of this interval (TB2-44 to KG34), specimensfrom 4 to 5 μm in diameter were less numerous thanthose in other intervals. The interval with minor Gephyrocapsa (4–5 μm in diameter) and absent Gephyrocapsa
over 5 μm in diameter corresponded to ~ 770–760 ka(Fig. 6). Most of the Gephyrocapsa (≥ 4 μm in diameter)
specimens in the CbCS had a bridge parallel to the short

Kameo et al. Progress in Earth and Planetary Science

Results
Calcareous nannofossil assemblages of the Chiba
composite section

The middle part of the Kokumoto Formation generally
contained well preserved, abundant calcareous nannofossils, while some samples yielded fewer nannofossil specimens in the upper part of the section. Dominant
Gephyrocapsa, Pseudoemiliania, and Florisphaera characterized the calcareous nannofossil assemblages (Figs. 4

Fig. 4 Stratigraphic distributions and relative abundances of main calcareous nannofossil taxa in the CbCS. Sample locations and lithologies weremodified from Suganuma et al. (2018) and Haneda et al. (2020). Magnetic reversals by Suganuma et al. (2015) and Okada et al. (2017) are also shownKameo et al. Progress in Earth and Planetary Science(2020) 7:36

Page 10 of 17

Fig. 5 Main calcareous nannofossils in the CbCS. a crossed nicols. b Without nicols. (1) Calcidiscus leptoporus (Murray & Blackman) Loeblich &Tappan, Sample TB2-16. (2) Coccolithus pelagicus braarudii (Gaarder) Geisen et al., Sample TB2-16, 3. Coccolithus pelagicus pelagicus (Wallich)
Schiller, Sample TB2-16. (4) Gephyrocapsa parallela Hay & Beaudry or Gephyrocapsa omega Bukry, Sample TB3-15. (5) Pseudoemiliania lacunosa
Kamptner ex Gartner, Sample TB3-15. (6) Umbilicosphaera sibogae (Weber-van Bosse) Gaarder, Sample TB2-16. (7) Helicosphaera carteri (Wallich)
Kamptner, Sample TB3-15. (8) Florisphaera profunda Okada & Honjo, Sample TB3-15. Scale bar is 5 μm

axis of the specimen; thus, they were named Gephyrocapsa parallela or Gephyrocapsa omega.
Florisphaera profunda, another common species, typically replaced small forms of Gephyrocapsa above samples
TB3-38 (Fig. 4). Here, the relative abundance of the taxon
was over 15%, whereas its abundance was 5% or less below
this level. Pseudoemiliania lacunosa, a common species in
the Pliocene–Pleistocene sediments, was present throughout this section and became more abundant above TB18
(approximately 4% on average). Umbilicospheara sibogae
increased in samples above TB3-38, although its average
percentage was approximately 2% of the total flora. Helicosphaera spp., Calcidiscus leptoporus, and Coccolithus pelagicus are considered key taxa typical of the uppermost
part of the Neogene and Quaternary and were also present
in the CbCS with an average continuous presence of ~ 1–

2%. Thus, their variability in abundance was not revealed
in standard counts.
Occurrence of environmental indicators

F. profunda, one of the most common species in the
nannofossil assemblages, is a lower photic taxon in lowto middle-latitude oceans (Fig. 4). This taxon became
abundant above sample TB3-30, ~ 787 ka (Fig. 7). As apossible Kuroshio water taxon, Umbilicosphaera spp.
(mostly U. sibogae) was also abundant within subordinate taxa at almost the same time when its average abundance changed from ~ 20 to 40 % (Table S3). This taxon
was most abundant from ~ 787 to ~ 776 ka (TB65–
TB18) (Fig. 7). Subsequently, its abundance slightly decreased until ~ 769 ka (TB2-48), before increasing again.
Short-term variabilities of this taxon from ten thousand to a

Kameo et al. Progress in Earth and Planetary Science(2020) 7:36

Page 11 of 17

Fig. 6 Age plots of the principal calcareous nannofossil taxa, Gephyrocapsa. This taxon is subdivided into size groups of < 3 μm, 3–4 μm, 4–5 μm,
and ≥ 5 μm in diameter. The oxygen isotope curves of the CbCS (Haneda et al. 2020; Suganuma et al. 2018), and abundance of medium-sized
Gephyrocapsa in the North Atlantic (Site 607) and the Mediterranean (Site 967) by Maiorano and Marino (2004) are also shown for comparison
with size variations of Gephyrocapsa. Thin and thick lines of oxygen isotopes (Okada et al. 2017; Suganuma et al. 2018; Haneda et al. 2020) aremeasured data and 1000 year moving averages, respectively

few thousand years were also present. As a temperate andoffshore taxon, C. leptoporus had a relative abundance within
subordinate taxa of 9–40%. This taxon occurred prior to ~787 ka (TB65) and rapidly decreased at ~ 786 ka (TB61).
Subsequently, a gradual increase of this taxon was observed
after ~ 776 ka (TB14). Fluctuations in the abundance of thistaxon with different cycles were recognized on the scale oftwenty thousand and a few thousand years.
C. pelagicus braarudii, a cool and nutrient-rich watertaxon, was abundant before ~ 783 ka (TB50) and from ~764 to ~ 762 ka (KG38–KG32). This subspecies predominated when Umbilicosphaera spp. decreased, except forthe period from ~ 787–784 ka (TB3-38–TB54) andreached a 29% maximum within subordinate counts at
791 ka (YN12). At the same time, C. pelagicus pelagicus(a colder-water taxon) was also recognized but generally
had fewer occurrences than C. pelagicus braarudii. C.
pelagicus pelagicus was occasionally present approximately every ten thousand years (~ 801–795, ~ 787–
783, ~ 772, ~ 764–761, and ~ 753 ka) (Fig. 7) and wasmost abundant around ~ 772 ka (TB2-16).
Helicosphaera spp., another abundant and characteristic
taxon within the subordinate species, increased before ~ 787
ka (TB3-30) and after ~ 778 ka (TB26) (Fig. 7). Helicosphaera spp. mainly consisted of Helicospharea carteri, Helicosphaera hyalina, and Helicosphaera pavimentum with rare

Helicosphaera wallichii. Average values before ~ 787 ka andafter ~ 778 ka were 16% and 24% in the subordinate taxon,
respectively. Between these intervals, corresponding to theearly part of MIS 19c, fewer occurrences—approximately
10% on average—were recorded. The sequential pattern of
this taxon was dissimilar to that of other calcareous nannofossil specimens. Small fluctuations with a duration of several
thousand years of this taxon were also observed throughout
the studied intervals. Other nannofossil taxa in subordinate
counts included Rhabdosphaera clavigera, Syracosphaera
pulchra, Oolithotus fragilis, and other minor and extremelyrare species (see Table S3).
The Tn value, which is the calcareous nannofossil
temperature index, shows warm-cold fluctuation within
the CbCS (Fig. 7). Tn values were relatively low before ~783 ka, after which higher values were recorded. The
fluctuation of Tn values was similar to that of the oxygen isotope of a planktonic species, Globigerina bulloides, in that Tn values were characteristically low at ~772 and ~ 763–761 ka.

Discussion
Possible bioevent around the M–B paleomagnetic polarity
boundary at the Chiba composite section

There are no apparent useful nannofossil bioevents in
the CbCS across the M–B paleomagnetic polarity

Kameo et al. Progress in Earth and Planetary Science(2020) 7:36

Page 12 of 17

Fig. 7 Age plots of sea-surface proxy taxa of calcareous nannofossils along the Pacific side of the Japanese islands. Inferred surface environments
are based on calcareous nannofossil thanatocoenosis distributions by Tanaka (1991). The Tn value, a calcareous nannofossil temperature index, isalso shown. For comparison with floral data, oxygen isotope data of the CbCS (Okada et al. 2017; Suganuma et al. 2018; Haneda et al. 2020) andTr values of radiolarian fossils (Suganuma et al. 2018) are also shown. Thin and thick lines of oxygen isotopes are measured data and 1000 year
moving averages, respectively (Haneda et al. 2020). Purple arrows indicate cold horizons. Light-blue hatches are comparatively cold intervals

boundary (e.g., Raffi et al. 1993; Takayama and Sato
1987). This makes it difficult to constrain the M–B
paleomagnetic polarity boundary with calcareous
nannofossils.
In the Quaternary strata, the significance of size variations of Gephyrocapsa has been recognized elsewherearound the world (e.g., Rio et al. 1990). Appearances anddisappearances of Gephyrocapsa of different sizes andmorphologic characters can be correlated globally andused to date marine sediments. Matsuoka and Okada(1990) demonstrated the size and morphological changesof Gephyrocapsa species in the Quaternary strata of the
Indian Ocean and recognized several morphotypes. The
first and last occurrences of these morphotypes are global and have been utilized as biohorizons. Across theM–B boundary, Gephyrocapsa specimens are typified byabundant smaller forms (< 4 μm) and medium to largesized specimens with a higher bridge angle (nearly parallel to the short axis) called Gephyrocapsa omega (Bukry
1973), Gephyrocapsa parallela (Hay and Beaudry, 1973),
Gephyrocapsa sp. 3 (Rio 1982), and/or Gephyrocapsa sp.
C (Matsuoka and Okada, 1990; Hagino et al. 2011). In
the CbCS, specimens over 4 μm in diameter correspondto these forms. In the middle part of the section, fewer
medium-sized specimens (4–5 μm) and no specimens
over 5 μm in diameter occurred (Figs. 4 and 6), which
represents characteristic changes of Gephyrocapsa in the
CbCS.
Maiorano et al. (2004) described the calcareous nannofossil biostratigraphy of the Lower to Middle Pleistocene
in the Montalbano Jonico section and a Mediterranean
Ocean Drilling Project (ODP) site (Site 964). Maiorano
and Marino (2004) also provided quantitative data on
the temporal and spatial distributions of Gephyrocapsa
and Reticulofenestra from the Early to Middle Pleistocene, including across the M–B paleomagnetic polarity
boundary in the North Atlantic Ocean (Site 607) and theMediterranean (Sites 964, 967, and 977). These studies
revealed temporal disappearances of medium-sized
Gephyrocapsa (4–5.5 μm in size) around MIS 22 (td1)and MIS 20–19 (td2). Based on these patterns, both
temporal disappearances are characteristic, but they
seem to be diachronous (Maiorano and Marino 2004).
In the CbCS, no intervals with a complete absence of
medium-sized specimens (4–5 μm) were observed. However, there is a characteristic interval with fewer

(2020) 7:36

Page 13 of 17

occurrences of medium-sized Gephyrocapsa (4–5 μm)
and a temporal disappearance of Gephyrocapsa specimens over 5 μm in diameter. This interval in the CbCS
may correspond to a later part of td2 (Maiorano andMarino 2004). One possibility is that the end of td2
might be correlated with the top of the absent intervalof Gephyrocapsa over 5 μm in diameter at ~ 760 ka because of re-occurrence of them at the same time (Fig. 6).
The presence of Gephyrocapsa over 5 μm in diameterin the CbCS suggests that this horizon corresponds tothe latest occurrence of Gephyrocapsa sp. C identified by
Matsuoka and Okada (1990) and Hagino et al. (2011).
Across the M–B paleomagnetic polarity boundary, between the tops of Reticulofenestra asanoi (0.889 Ma) and
Pseudoemiliania lanunosa (0.443 Ma), Gephyrocapsa sp.
C (a medium to large Gephyrocapsa specimen with acomparatively high angle bridge nearly parallel to theshorter axis), disappeared. Although the exact timing of
the last occurrence of Gephyrocapsa sp. C has not been
determined, Gephyrocapsa specimens (≥ 5 μm) only occurred just prior to the disappearance of Gephyrocapsa
sp. C in the upper part of the Pleistocene of the Indian
Ocean (Hagino et al. 2011). Gephyrocapsa specimens
over 5 μm in diameter in the lower CbCS may correlatewith the uppermost occurrence of Gephyrocapsa sp. C.
To correlate these Gephyrocapsa events identified in the
CbCS with other oceans, studies on size variations andmorphologic changes of Gephyrocapsa must be conducted across the M–B paleomagnetic polarity
boundary.

Gephyrocapsa during MIS 20 (Figs. 4 and 6) also supports this interpretation, as an abundance of smaller
forms generally demonstrates a high nutrient supply inthe upper photic zone (e.g., Okada and Wells 1997;
Chiyonobu et al. 2012).
Around 790 ka, extreme decreases in a lower photic
taxon, F. profunda, indicate the collapse of sea-surface
stratification (Fig. 7) because this species is more likely
to be abundant in stable surface conditions. At the sametime, increases in small Gephyrocapsa (< 3 μm) (Fig. 4)
and C. pelagicus braarudii also indicate a turbulent
photic zone, resulting in cool and nutrient-rich sea surface conditions. This suddenly disturbed horizon corresponds to the Ydt1 (Younger Dryas-type event 1:
Suganuma et al., 2018; Haneda et al., 2020) which is acharacteristic cooling event at the latest MIS 20. Another cooling event, called Ydt2, was not clearly detected, even though there was a small reduction in theTn value around 788 Ka (Fig. 7).
Umbilicosphaera spp. became abundant around 787 ka
and persisted until 776 ka, suggesting that the strong ingression of the Kuroshio Current during MIS 19 was associated with a northward retreat of the Kuroshio Front.
This interval almost corresponds to the full-interglacialconditions in MIS 19 (785.0–775.1 ka; Suganuma et al.
2018) when the Boso Peninsula was strongly influenced
by southern surface waters and the Kuroshio Current.
However, Tn values suggest the beginning of warmingof sea-surface waters was during early MIS 19c, around~ 783 ka. The Tr value, a sea-surface temperature index
calculated by radiolarians (Suganuma et al. 2018), also
supports the timing of warming. During 787–783 ka, cooccurrences of Kuroshio water taxa and cool-water taxa
(Fig. 7) suggest that the study area was possibly situatedaround the Kuroshio Front and seasonal movements of
the Front around the CbCS may have largely fluctuated.
In the early MIS 19c, F. profunda and C. leptoporus increased in abundance, although F. profunda began to increase earlier than C. leptoporus. Recently, F. profunda
was found to have mainly lived outside the Kuroshio
Current, to the south of 32–34° N (Tanaka 1991). Conversely, C. leptoporus is relatively abundant in the eastern part of the offshore areas, from 142° E. Thus, agradual increase in both species signifies the stronger influence of offshore waters and the relative deepening of
the basin.
After ~ 783 ka, the Kuroshio Current was common
throughout the study area because of the abundant occurrences of warm-water taxa. Moreover, the gradual increase and common occurrences of Helicosphaera spp.
after ~ 778 ka suggest the influence of coastal waters.
This taxon is abundant in the East China Sea and coastal
areas along the Pacific side of Japanese islands (Tanaka
1991; Tanaka 1997) and may relate to the inflow of fresh

Kameo et al. Progress in Earth and Planetary ScienceSea-surface environmental changes based on calcareous
nannofossils

The stratigraphic distribution of certain major nannofossil taxa and their time-series variability revealed that seasurface conditions, including surface currents, changed
during the deposition (Fig. 7).
Prior to 788 ka, during MIS 20, warm-water taxa, Florisphaera profunda, and Umbilicosphaera spp., were notcommon. Instead, the abundance of Cocccolithus pelagicus braarudii with C. pelagicus pelagicus, a colder, subarctic taxon (e.g., Cachão and Moita 2000), suggests coldand eutrophic surface conditions (Fig. 7). C. pelagicus
deposited within surface sediments around the Japaneseislands may include both subspecies (Tanaka, 1991), andthe dominant area of C. pelagicus coincides with the distribution areas of the cool mixed water region and thecold Oyashio Current from Sanriku to Hokkaido coast.
The continuous occurrence of C. pelagicus suggests thatthe influence of Kuroshio water was weak during MIS
20 and that cold and eutrophic waters from the north of
the Kuroshio Front invaded the Kazusa Basin. Tn values
are low during this interval, indicating low sea-surface
temperatures. The presence of small forms of(2020) 7:36

Page 14 of 17

coastal waters (Tanaka 1991). In the CbCS, however,Umbilicosphaera spp. and F. profunda were still common (Fig. 7) at the same time. Their occurrences suggestthat the influence of coastal water near the coast of theJapanese islands may have increased along with the
Kuroshio Current after the full interglacial during MIS
19. In Japan Sea, subtropical Tsushima Warm Current
radiolarian species were rapidly decreased in HoleU1427 (Sagawa et al. 2018). Sagawa et al. (2018) suggested that shallowing the Tsushima strait due to lowering sea-level prevented the Tsushima Warm Current
from flowing into Japan Sea after interglacial maxima. It
has possibly influenced on common occurrences of theKuroshio Current nannofossil taxa and increases of acoastal nannofossil taxon at the CbCS.
Although the study area has been under the strong
Kuroshio Current regime, climatic fluctuations shorter
than the glacial-interglacial cycle were found for eachtaxon. In particular, strong Kuroshio Current and comparatively cooler water regimes were regularly repeated(Fig. 7). C. pelagicus pelagicus and C. pelagicus braarudii
seem to occur approximately every 10,000 years at ~801–795 ka, ~ 787–783 ka, ~ 772 ka, ~ 764–761 ka, and~ 753 ka. Preliminary spectrum analysis of C. pelagicus(Fig. S1) also shows approximately 10,000 periodicity
throughout the section, indicating cyclic southward invasions of the Kuroshio Front. These cold invasions mostly
coincide with the larger oxygen isotope values of
surface-dwelling Globigerina bulloides. On the basis of
the occurrences of C. pelagicus pelagicus, the durations
and influences of cold waters seem to be shorter andweaker, respectively. Haneda et al. (2020) suggested
millennial-scale oceanographic variations, possibly influenced by North Atlantic climate variability and tropicalinsolation. They indicated that the millennial-scale oscillation during MIS 19b–19a corresponds to four stadialinterstadial cycles (Fig. 7; Haneda et al. 2020). These patterns indicate that the oscillatory movement of theKuroshio Front may have controlled abrupt, millennialscale increases and decreases in surface temperature.
Two stadial periods (S1 and S3 by Haneda et al., 2020)
correspond to these cooler intervals indicated by calcareous nannofossils at ~ 772 ka and ~ 764–761 ka, respectively. However, other climatic changes (stadial periods
S2 and S4) indicated by oxygen isotope data were only
shown by small reductions in Tn values (Fig. 7). Further
examination of a higher-resolution analysis of calcareous
nannofossils would be needed in order to discuss
millennial-scale changes in surface waters.

nannofossil index for the Lower–Middle Pleistocene
boundary and to consider environmental changes insea surface conditions along the southwest Pacificside of the Japanese islands around the M–B
paleomagnetic polarity boundary. This paper reviews
previous calcareous nannofossil investigations and introduces the detailed calcareous nannofossil biostratigraphy of the CbCS, as follows. In addition, local
bioevents and floral changes associated with oceanographic conditions were found using high-resolution
analysis of calcareous nannofossils:

Kameo et al. Progress in Earth and Planetary Science

Conclusions
This study examined calcareous nannofossil assemblages of the CbCS of the Kokumoto Formation of
the Kazusa Group in order to extract the calcareous

1. Previous results of the calcareous nannofossil
biostratigraphy of the Kazusa Group (e.g., Sato et al.
1988, 1999) and resultant nannofossil biohorizons(datums) were summarized. Quaternary calcareous
nannofossil biohorizons established in the North
Atlantic (Takayama and Sato 1987; Sato and
Takayama 1992) were applied in the Group. Similarbiohorizons were also found in other Quaternary
sequences (e.g., Raffi et al. 1993), and thus, theworldwide utility of the nannofossil biohorizons wasdemonstrated.
2. Based on high-resolution calcareous nannofossil
biostratigraphy of the CbCS, there are no apparent bioevents of calcareous nannofossil species
around the M–B paleomagnetic polarity boundary. Gephyrocapsa specimen (≥ 5 μm in diameter), with a higher bridge angle (nearly parallelto the short axis), may have temporarily disappeared from ~ 770–760 ka, just after the M–B
paleomagnetic polarity boundary appears in thesequence. This event could be related to thetemporary disappearance of medium-sized
Gephyrocapsa in MIS 20–19 (td2: Maiorano andMarino 2004). Moreover, the presence of large
specimens of Gephyrocapsa corresponds to thelatest occurrence of Gephyrocapsa sp. C (Matsuoka and Okada, 1990). Further investigation of
calcareous nannofossils around the M–B
paleomagnetic polarity boundary will be needed
because it is unclear if this event can be traceable to other areas.
3. Floral changes in the environmental proxy taxa
of calcareous nannofossils enable us to inferchanges in surface currents around the M–B
paleomagnetic polarity boundary. Before 787 ka,
the influence of the Kuroshio Current was weakaround the Kazusa Basin, and cool and eutrophic
conditions were inferred because of the presenceof Coccolithus pelagicus. Generally, abundantoccurrences of Umbilicosphaera spp. andFlorisphaera profunda indicate that the Kuroshio
Current affected the Kazusa Basin after 787 ka.

Kameo et al. Progress in Earth and Planetary Science(2020) 7:36

After ~ 783 ka, the Kuroshio Current wascommon until MIS 18, but sea-surface temperatures fluctuate shorter than the glacial-interglacial
cycle. Moreover, periodic southward invasions of
the Kuroshio Front occurred approximately every
10,000 years based on the presence of a coldwater species. Shorter-term fluctuations of flora
were also observed and correspond to millennialscale environmental changes.

Supplementary information

Page 15 of 17

Author details
Department of Earth Sciences, Faculty of Science, Chiba University, 1-33,
Yayoi, Inage, Chiba, Chiba 263-8522, Japan. 2Department of Geology andPaleontology, National Museum of Nature and Science, 4-1-1, Amakubo,
Tsukuba, Ibaraki 305-0005, Japan. 3Geological Survey of Japan, National
Institute of Advanced Industrial Science and Technology, 1-1-1 Central 7,
Higashi, Tsukuba, Ibaraki 305-8567, Japan. 4National Institute of Polar
Research, 10-3, Midori-cho, Tachikawa, Tokyo 190-8518, Japan. 5Departmentof Polar Science, School of Multidisciplinary Sciences, The Graduate
University for Advanced Studies (SOKENDAI), 10-3, Midori-cho, Tachikawa,Tokyo 190-8518, Japan. 6Department of Earth Sciences, Ibaraki University,
2-2-1, Bunkyo, Mito, Ibaraki 310-8512, Japan.
1

Received: 22 September 2019 Accepted: 19 July 2020

Supplementary information accompanies this paper at https://doi.org/10.
1186/s40645-020-00355-x.
Additional file 1: Table S1. Counts of calcareous nannofossils of the
Kazusa Group by Sato (1989).
Additional file 2: Table S2. Counts of overall calcareous nannofossils of
the Chiba composite section (This study and Suganuma et al. 2018).
Additional file 3: Table S3. Counts of subordinate taxa of calcareous
nannofossils of the Chiba composite section.
Additional file 4: Fig. S1. Spectral power of Coccolithus pelagicus basedon REDFIT method (Schulz and Mudelsee 2002) using the PAST4 software(Hammer et al. 2001). A blue dotted line shows 95% confidence level anda red dotted line indicates the first-order autoregressive (AR1) red noisemodel fit. A periodic change of 11 kyr is characteristically extracted fromsequential occurrence of C. pelagicus in the CbCS.
Abbreviations
CbCS: Chiba composite section; FAD: First appearance datum; GSSP: Global
Boundary Stratotype Section and Point; LAD: Last appearance datum; M-B
paleomagnetic polarity boundary: Matuyama–Brunhes paleomagnetic
polarity boundary
Acknowledgements
We would like to thank Mr. Kento Watanabe, who co-operated with KK in
conducting the preliminary studies of the calcareous nannofossils of the
CbCS. Mr. Daisuke Kuwano at Chiba University gave us important advice forspectral analysis of nannofossil assemblages. We would also like to thank all
those who assisted us in this project from the Tabuchi district, Ichihara City,
Chiba Prefecture, and Ministry of Education. We wish to thank Prof. Dr. Stephen J. Gallagher, anonymous reviewers, and Dr. Takuya Sagawa for theirconstructive and useful comments, which significantly improved our manuscript. We would like to thank Editage (www.editage.com) for English language editing.
Authors’ contributions
KK conducted an overall investigation of calcareous nannofossils including
sample treatments, counting, interpretation of data, writing of themanuscript, drawing of figures and tables, and creation of other additional
files. YK, YH, YS, and MO assisted with the data interpretation for
paleoceanography. YH and MO also take responsibility for thelithostratigraphy, the identification of samples, and their stratigraphic
locations. YH supported the drawing of the figures related tomagnetostratigraphy and oxygen isotopes. The authors read and approved
the final manuscript.
Funding
JSPS KAKENHI Grant Number 18K03771 supported this work. MO was partly
funded by JSPS KAKENHI, Grant Number 16H04068 and 19H00710.
Availability of data and materials
The counting data of calcareous nannofossils of this article are included
within the article and its additional files.
Competing interests
The authors have no competing interests to declare.

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