Sagawa et al. Progress in Earth and Planetary Science (2018) 5:18
DOI 10.1186/s40645-018-0168-7

Progress in Earth andPlanetary Science

RESEARCH ARTICLE

Open AccessIntegrated tephrostratigraphy and stable
isotope stratigraphy in the Japan Sea andEast China Sea using IODP Sites U1426,
U1427, and U1429, Expedition 346 Asian
MonsoonTakuya Sagawa1* , Yoshitaka Nagahashi2, Yasufumi Satoguchi3, Ann Holbourn4, Takuya Itaki5,
Stephen J. Gallagher6, Mariem Saavedra-Pellitero7, Ken Ikehara5, Tomohisa Irino8 and Ryuji Tada9

Abstract
Integrated Ocean Drilling Program Expedition 346 “Asian Monsoon” obtained sediment successions at seven sites in
the Japan Sea (Sites U1422–U1427 and U1430) and at two closely located sites in the northern East China Sea (Sites
U1428 and U1429). The Quaternary sediments of the Japan Sea are characterized by centimeter- to decimeter-scale
dark–light alternations at all sites deeper than 500 m water depth. The sedimentary records from these sites allow
an investigation of the regional environmental response to global climate change, including changes in the Asian
Monsoon and eustatic sea level. However, the discontinuous occurrence of calcareous microfossils in the deep-sea
sediments and their distinct isotope signature that deviates from standard marine δ18O records do not permit thedevelopment of a detailed stable isotope stratigraphy for Japan Sea sediments. Here, we present thetephrostratigraphy for the two southernmost sites drilled in the Japan Sea (Sites U1426 and U1427) and for one site
drilled in the East China Sea (Site U1429) along with the benthic δ18O isotope stratigraphy for the shallower Site
U1427 and the East China Sea Site U1429. Eighteen tephra layers can be correlated between sites using the majorelement composition and morphology of volcanic glass shards, and the compositions of grains and heavy minerals.
Tephra correlations show that negative δ18O peaks in the Japan Sea correspond to positive glacial maxima peaks inthe East China Sea. Using this integrated stratigraphic approach, we establish an orbital-scale age model at Site
U1427 for the past 1.1 Myr. The correlation of tephra layers between the shallower Site U1427 (330 m below sea
level: mbsl) and the deeper Site U1426 (903 mbsl) in the southern Japan Sea provides the opportunity for further
age constraints. Our results show that alternations in sediment color at Sites U1426 and U1427 can be correlated
for the past 1.1 Myr with minor exceptions. Thus, the stable isotope stratigraphy established at the shallower Site
U1427 can be correlated to Site U1426, and in turn to all sites drilled during Expedition 346, based on correlationsof dark–light layering.
Keywords: Japan Sea, East China Sea, Tephrostratigraphy, Stable isotope stratigraphy, Integrated Ocean Drilling Program,
Expedition 346, Inter-site correlation* Correspondence: tsagawa@staff.kanazawa-u.ac.jp1
Institute of Science and Engineering, Kanazawa University, Kakuma-machi,
Kanazawa, Ishikawa 920-1192, Japan
Full list of author information is available at the end of the article© The Author(s). 2018 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0
International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, andreproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to
the Creative Commons license, and indicate if changes were made.

Sagawa et al. Progress in Earth and Planetary Science (2018) 5:18

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Introduction
Integrated Ocean Drilling Program (IODP) Expedition
346 “Asian Monsoon” drilled seven sites in the Japan Sea(JS) and two closely located sites in the northern East
China Sea (ECS) (Fig. 1). The main scientific goal of thisexpedition was to unravel the evolution and variability of
the East Asian monsoon over multiple timescales. The JS
drilling sites spanned both latitudinal and depth transects
in order to explore linkages between atmospheric andoceanic processes, along the edge of the Eurasian continent, the Korean Peninsula, and the Japanese islands, andto investigate regional responses to far-field forcing. However, detailed correlation of the suite of cores obtained
during Expedition 346 is a pre-requisite for any regional
paleoclimatic and paleoceanographic reconstructions.
The late Quaternary deep-sea sediment in the JS ischaracterized by centimeter- to decimeter-scale alternations
124°E

128°

of dark and light layers (Tada et al. 1992 1999; Nakajima etal. 1996), with the darker sediment occasionally showing
fine laminations. The alternation of color was caused bychanges in deep-water oxygenation in sediments recoveredfrom depths > 500 m below sea level (mbsl) (Ikehara et al.
1994). Dark–light color alternations are present at six ofthe seven sites drilled during Expedition 346 (Sites
U1422–U1426 and U1430), and they occupy water
depths of 990–3430 mbsl, whereas the succession at
Site U1427 on the outer margin of a marginal terrace(~ 330 mbsl) shows gradual and subtle color changes(Fig. 1). Previous studies have demonstrated that the dark–light layers in sediments from deeper sites can be correlated
across the JS (e.g., Tada et al. 1992; Nakajima et al. 1996).
This is also the case for the six deeper sites of Expedition
346, which are readily correlated over the past 1.5 Myr(Tada et al. 2018). Due to its tectonic setting, the JS and

132°

136°

140°

144°

Soya
Strait

44°
N

U1422

U1423

Japan Sea
U1424

40°

Tsugaru
StraitHakkodacaldera

U1425

U1430

U1426/MD01-2407
e

idg

iR

Ok

36°

U1427/PC3
MD01-2421
Sanbe
volcanoTsushima
StraitShishimuta
calderaAso caldera

32°

East
China
Sea

U1428

U1429

Kakuto caldera
Aira caldera
Ata caldera

North Pacific

Kikai caldera

Fig. 1 Locations of sites drilled during IODP Expedition 346 in the Japan Sea and East China Sea. The source volcanoes of some of the regional
tephra layers are shown

Sagawa et al. Progress in Earth and Planetary Science (2018) 5:18

northern ECS sediments contain a large number of tephra
layers that were erupted from volcanoes on the Japaneseislands and the Korean Peninsula (Ikehara 2015). Tephra
deposition is geologically synchronous; therefore, tephra
layers provide an excellent way to precisely correlate sediment successions in the JS and the ECS.
The Pliocene and Pleistocene tephrostratigraphy andtephrochronology of the Japanese islands were summarized by Machida (1999), Tamura and Yamazaki (2010),
and Satoguchi and Nagahashi (2012). Major- and traceelement compositions of volcanic glass shards of themore regionally widespread tephra layers were investigated by Nagahashi et al. (2004, 2007, 2015). Theseelement compositions of tephra layers have been used to
identify and correlate the layers (e.g., Nagahashi et al.
2004). Tephra layers in JS sediments were originally described by Pouclet and Scott (1992), who summarized
their temporal distribution in the sediments of ODP Leg
127/128. These tephra layers were subsequently investigated to understand the nature of volcanic activity(Nakagawa et al. 1994) in order to identify the sources
and to establish relationships with oxygen isotope stratigraphy for the past 400 kyr (Shirai et al. 1997). The
stratigraphy and petrography of Upper Pleistocene marine tephras from the JS and the North Pacific Ocean
were summarized by Ikehara (2015) and Aoki (2008),
respectively. However, the long-term tephrostratigraphyof JS sediments, its correlation with known widespread
tephra layers, and the tephrochronology for the past
several million years have not been established to date.
If the tephrostratigraphy of JS sediments is known,
comparison of environmental changes between the Japanese islands and the Japan Sea will be possible. Here, we
provide correlations between tephra layers that are interbedded in sediment successions at Sites U1426, U1427, andU1429 with regionally occurring tephras in order to determine isochronous horizons for these sites.
The generation of orbital-scale age models for the JS
sediments is complicated by the sporadic occurrence
and variable preservation of calcareous microfossils,
which hinders the construction of a detailed oxygenisotope stratigraphy. Furthermore, the JS carbonate compensation depth (CCD) has varied from < 1000 to >3000 m over the past 30 kyr (Oba et al. 1991), and the
JS oxygen isotope signal is different from that of theopen ocean, as it is a semi-enclosed environment (e.g.,
Oba et al. 1991; Kido et al. 2007; Yokoyama et al. 2007).
The JS is connected to the North Pacific Ocean and adjacent marginal seas (ECS and Okhotsk Sea) only by
shallow and narrow straits (Fig. 1). Even the sill depthsof the deepest connections to the open ocean, theTsushima and Tsugaru Straits, are only ~ 130 m, while
the sill depths in the Soya Strait and the Mamiya Strait
are ~ 55 and ~ 15 m, respectively. Therefore, glacial–

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interglacial sea-level change has had a marked influence on
seawater exchange through the straits. For example, during
the last glacial maximum (LGM), the JS was almost isolated
from the surrounding seas, resulting in planktonic δ18O
minima that are interpreted as a signal of low-salinity surface water (Oba et al. 1991).
The shallow Site U1427 is an important site for Expedition 346 site correlation for two main reasons. First, as
it is in the southern part of the JS, the sediment succession at Site U1427 contains tephra layers that are alsopresent at the ECS sites. These tephra layers can be used
to constrain the JS δ18O stratigraphy and to correlatethe δ18O stratigraphy of the JS to the ECS, the latterwith an open-ocean δ18O signal. Second, the sediment at
this site contains calcareous microfossils throughout thesuccession, except for two intervals (Tada et al. 2015a),thereby allowing high-resolution δ18O analyses. Onceinter-site correlation between Site U1427 and the deeper
JS sites is achieved, the U1427 δ18O stratigraphy can beprojected to the deeper sites, where the preservation offoraminifera is poor.
Objectives of this study

This study aims to (1) establish a tephrostratigraphy forthe past 1.1 Myr based on tephra layers interbedded in
the sediment successions of Sites U1426, U1427, andU1429; (2) construct age models for Sites U1427 andU1429; and (3) correlate the sediment succession of theshallow JS Site U1427 with the deeper JS Site U1426. To
achieve these objectives, we investigate the petrography
and major-element chemistry of tephra layers within thesediment successions at these three sites, analyze benthic
foraminiferal oxygen isotopes at Sites U1427 and U1429,
and analyze radiolarian, planktonic foraminiferal, andcalcareous nannofossil assemblages at Site U1427. By
using tephras as the primary correlating horizons, weare also able to correlate sediment color variations atshallow- and deep-water sites in the JS.
Geographical and oceanographical setting

The JS and ECS are marginal seas located along theeastern margin of the Eurasian continent (Fig. 1). The JS
is connected with the North Pacific Ocean, Okhotsk Sea,and East China Sea. Although the maximum water
depth in the JS is 3700 m, the straits connecting to
adjacent seas are shallow (< 130 m), meaning that intermediate and deep waters in the JS are separated fromthe adjacent seas. The only surface water flowing into
the JS is the Tsushima Warm Current (TWC) flowing
from the ECS through the Tsushima Strait. Since the
TWC originates from the Kuroshio Current in the subtropical North Pacific Ocean and mixes with nutrient-richcoastal water derived from the Yangtze River in the ECS,

Sagawa et al. Progress in Earth and Planetary Science (2018) 5:18

this surface current supplies heat, salt, and nutrients to
the JS.
During the LGM, the sill depths of the two deepest
straits were ~ 10 m (Matsui et al. 1998). Thus, surface
water exchange through the straits was limited, and
TWC inflow was restricted, resulting in reduced salt
supply from the ECS. The surface water freshening is recorded in the planktonic foraminiferal δ18O data of
deep-sea sediments across the JS (Oba et al. 1991).
Therefore, the oxygen isotope stratigraphy in the JS iscomplicated by glacio-eustatic effects on surface salinity.
Since carbonate δ18O values reflect both temperatureand seawater δ18O, the JS planktonic foraminifera δ18O
values reflect two kinds of minima on a glacial–interglacial time scale: (1) during interglacial maxima (warmconditions) and (2) during glacial maxima (low seawaterδ18O; i.e., low salinity). Therefore, to correlate with marine isotope stages, one should interpret planktonic δ18O
data along with temperature information derived from,for example, planktonic microfossil assemblages (Kido etal. 2007). In this study, we focus on the benthic foraminiferal δ18O record in the shallow Site U1427 (Fig. 1).
The water depth of this site is ~ 330 mbsl, well below
the modern thermocline, which remains at ~ 100–250 m
water depth throughout the year (World Ocean Atlas
2013 (Locarnini et al. 2013)). Thus, the warm signal, attributed to intrusion of the TWC during interglacial
maxima, is not expected to be recorded in the benthic
δ18O values, thereby eliminating δ18O minima induced
by warm temperatures during the interglacial maxima.
Glacio-eustatic sea-level change also influenced the ecological environment in the JS. Since the sea-level drop
during glacial periods restricted the inflow of the TWC,
assemblages of planktonic microfossils during glacial andinterglacial periods differed markedly (Oba et al. 1991),
and subtropical species were present only during interglacials. Therefore, combining records of benthic δ18O andTWC planktonic microfossil species at Site U1427 allows
glacials to be distinguished from interglacials, which canbe used to establish an orbital-scale age model.

Methods/Experimental
Sediment cores

Site U1426 is in the south-central part of the JS, near the
top of the Oki Ridge (37°2.00′N, 134°48.00′E, 903 mbsl)(Fig. 1). The sediment succession of the upper 120 m ischaracterized by decimeter-scale color banding with alternations of dark and lighter brown layers (Fig. 2a). The
organic-rich darker layers generally display a sharp base,whereas the upward transition from dark to light layers is
gradational and commonly strongly bioturbated (Tada et al.
2015b). Some darker layers are thinly laminated. Calcareous
microfossil preservation is relatively good in the upper partof the succession at this site compared with deeper sites,

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although foraminifera are poorly preserved or absent insome intervals (Tada et al. 2015b). Several tephra layers
occur in the succession.
Site U1427 is located in the south-central part of the
JS on the outer margin of the marginal terrace off Tottori (35°57.92′N, 134°26.06′E, 330 mbsl), ~ 110 km
south of Site U1426 (Fig. 1). The sediment is dominatedby heavily bioturbated and homogeneous clayey silt.
Faint laminations are present in the intervals at 162–
177, 225–246, and 346–372 m (U1427_patched_CCSF-D_rev20170310, CCSF: core composite depth below seafloor, see below for explanation of the “patched” depthscale). Centimeter-thick tephra layers occur frequently
throughout the succession. Owing to its proximity toSite U1426, it is likely that the same tephra layers arepresent at both sites. The sediment at this site differsfrom that of deeper JS sites, including Site U1426, as itlacks dark–light color banding. Instead, the sedimentshows gradual meter-scale color changes accompanied
by lithological changes (Fig. 2a). The sediment contains
well-preserved foraminifera throughout, except for thetwo laminated intervals in the upper part of the core
(Tada et al. 2015a). Sites U1427 and U1426 are close to
the first and second branch of the TWC, respectively.
Site U1429 is in the southern part of the Danjo Basinin the northwestern part of the ECS (31°37.04′N, 128°59.85′E, 732 mbsl). The upper ~ 180 m of the succession
is characterized by calcareous-nannofossil-rich clay with
diatoms and foraminifera (Tada et al. 2015c). The sediment is heavily bioturbated throughout and shows subtle
meter-scale color variability (Fig. 2a). Tephra layers ofvarious thickness, ranging from centimeter-scale to > 0.5 m,
are present throughout the succession.
The shipboard composite sections, so-called “splice sections,” of Sites U1426 and U1427 (Tada et al. 2015a,
2015b) were revised post-cruise based on re-examinationof close-up section photographs; the splice of Site U1429
is from Tada et al. (2015c). During this re-evaluation, astratigraphical gap and repeated successions were foundin the original shipboard splice (Irino et al. 2018). Detailed
inter-hole correlation and revision of the shipboard splices
for Sites U1426 and U1427 are presented by Irino et al.
(2018). The depth-scale for the revised splice section isexpressed as “patched_CCSF-D” in this study. The depthscale for Sites U1426 and U1427 used in this study are thelatest versions, “U1426_patched_CCSF-D_rev20150519”
and “U1427_patched_CCSF-D_rev20170310,” respectively,
unless noted otherwise.
Shipboard measurements

Sediment cores recovered during the expedition were analyzed using a Whole-Round Multisensor Logger (WRMSL;
measuring P wave velocity, density, and magnetic susceptibility), Natural Gamma Radiation Logger (NGRL), Section

Sagawa et al. Progress in Earth and Planetary Science (2018) 5:18

Page 5 of 24abFig. 2 a Composite images and color profiles of the studied intervals of Sites U1426, U1427, and U1429. Shipboard line-scan images of thesurfaces of split sediment cores. Brightness and contrast of the images are enhanced to highlight color changes. Positions of regional tephras andcorrelative layers in the successions are indicated by magenta lines. b Depth relationships of correlative tephra layers between sites

Half Imaging Logger (SHIL), and Section Half Multisensory
Logger (SHMSL). For details of each method, please see theMethods chapter of the Proceedings volume (Tada et al.
2015d). The SHIL captures continuous high-resolution images of the archive half-surface. Images were collectedshortly after core splitting and surface scraping to avoid
color changes resulting from excessive sediment drying andoxidation of the surface. High-resolution color data (red,green, and blue) were acquired by digital color imaging. Reflectance of visible light and magnetic susceptibility (MS)were measured on the surfaces of sediment cores, whichwere covered with clear plastic wrap, on the SHMSL withan Ocean Optics USB4000 spectrophotometer and a

Bartington Instruments MS2E point sensor, respectively.
These measurements were made at 1 cm spacing to provide
high-resolution records. L*, a*, and b* were calculated fromthe obtained reflectance spectra.
TephraWe analyzed 56 samples from Site U1426, 56 samples
from Site U1427, and 28 samples from Site U1429.
Occurrences of tephra layers were recorded during shipboard visual core descriptions. Recognition of tephra
layers was based on color, sedimentary structures, andcomponents. The samples were ultrasonicated toremove clay contaminants and then separated into fourSagawa et al. Progress in Earth and Planetary Science (2018) 5:18

grain-size fractions using 32, 63, and 250 μm sieves. The
major-element composition of volcanic glass shards wasanalyzed using scanning electron microscope–energy
dispersive spectroscopy (SEM–EDS, Jeol JSM-6510LV+Oxford INCA x-Act EDS system) at Fukushima
University, Japan. Synthetic and natural mineral standards were used primarily for quantitative calibrations.
Compositions of eight major elements were further recalibrated by sub-standards, the compositions of whichwere determined by X-ray fluorescence (XRF) analysis(Nagahashi et al. 2003). In addition, the mineralogicalcomposition, the morphology and relative compositionsof volcanic glass shards, and the heavy mineral composition (specific gravity 2.8) were investigated for grains in
the 63–250 μm size fraction of samples collected fromkey correlative horizons. Images of glass morphology
were obtained using a low-vacuum SEM (Hitachi Miniscope TM-1000).
Oxygen isotope analyses

Sediment samples for benthic foraminiferal isotope analysis
at Site U1427 were washed using a 63 μm sieve, and theresidues were dried at 40 °C in an oven overnight. Uvigerina spp. and Cassidulina spp. were picked from the 250–
355 μm fraction of the dried residues. Picked foraminiferal
shells were gently crushed using two glass plates under the
microscope. The fragments were ultrasonicated several
times with Milli-Q and methanol. The δ18O values of foraminifera in samples from Site U1427 were measured witha Thermo Finnigan MAT253 mass spectrometer connected
to a Kiel III carbonate device at the Kochi Core Center,
Japan. The isotope results were calibrated to the Vienna
Pee Dee belemnite (VPDB) using the National Institute ofStandards and Technology (NIST: Gaithersburg, Maryland)
standard NBS 19. The analytical precision was better than
± 0.06‰.
Sediment samples from Site U1429 were dried in anoven at 40 °C, wet-sieved at 63 μm, and dried at 40 °C.
Between three and seven well-preserved tests of Uvigerina
spp. were selected for analysis, except for a few samples inwhich Uvigerina was rare or absent, in which case
Bulimina spp. or Cibicidoides spp. were selected. Tests
were broken into large fragments, cleaned in alcohol in an
ultrasonic bath, and then dried at 40 °C. Replicate measurements of Site U1429 samples and core tops from several ocean basins (Holbourn et al. unpublished data)
indicate no significant offset between the δ18O values ofUvigerina and Bulimina. The δ18O values of Cibicidoides
were adjusted by adding 0.71‰ to the original measurements to compensate for the offset between Uvigerinaand Cibicidoides (Holbourn et al., unpublished core-topdata). Measurements of benthic foraminiferal δ18O at
Site U1429 were made using a Finnigan MAT251
mass spectrometer at the Leibniz Laboratory, Kiel

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University, Germany. The instrument is coupled on-line toa Carbo-Kiel Device (type I). Samples were reacted by individual acid addition (99% H3PO4 at 73 °C). Standard external error was better than ± 0.08‰ for δ18O. Results were
calibrated using the NIST carbonate isotope standards NBS
20, 19, and 18, and are reported on the VPDB scale.
Microfossil analysis

A total of 455 samples from Site U1427 were processed forradiolarian analysis (at ~ 1.5 m intervals). Of these, 270
samples were from the upper 350 m, which was the depth
range targeted in this study. Detailed analytical methods
and results for all samples analyzed are provided by Itaki etal. 2018. Based on previous studies of JS sediment cores,
the radiolarian taxon, Tetrapyle octacantha group, occurredcommonly during interglacial periods and only sporadicallyduring glacial periods over the past 640 kyr (Itaki 2007;
Itaki et al. 2007). In this study, we considered the following
taxa as TWC indicators: the Tetrapyle octacantha group,
Dictyocoryne spp., Didymocyrtis tetrathalamus, Euchitonia
spp., and Spongaster tetras. These taxa are common in
the subtropical regions today, and their distribution in
the JS surface sediment is closely related to the inflowof the TWC (Motoyama et al. 2016). These TWC indicators are expressed as numbers of individuals pergram of dry sediment.
A total of 231 samples (at ~ 1 m intervals) were processed for foraminiferal paleoenvironmental analyses. Sediment was washed with hydrogen peroxide over a 63 μm
wire mesh sieve. Dried residues were divided into fractions
using a microsplitter, and a minimum of 100 individuals(typically > 200 tests) were identified and counted in the >150 μm size fraction. Four subtropical planktonic foraminifera species (Globigerinoides ruber, Pulleniatina obliquiloculata, Globorotalia menardii, and Globorotalia tumida)
are indicators of the TWC, along with the percentage ofright-coiling Neogloboquadrina pachyderma (Neogloboquadrina incompta; sensu Darling et al. 2006) (Oba et al. 1991;
Domitsu and Oda 2005; Gallagher et al. 2015). The %TWC
species and %N. incompta are expressed as percentages of
the total planktonic foraminifera assemblage.
Sixty samples were processed for coccolith analysis (at~ 3 m intervals). Sediment samples were freeze-dried
and prepared following combined dilution and filtering(Andruleit 1996), which enabled estimates of the absolute numbers of coccoliths per gram of dry sediment.
Between 61 and 129 mg of dry bulk sediment was suspended in buffered water and treated ultrasonically for~ 30 s. Samples were wet-split using a rotary sample divider, and 1/100 of the original solution was filteredthrough polycarbonate filters (0.45 μm pore size) with avacuum pump. The filters were dried in an oven at ~ 40 °C for at least 24 h and stored. A small part of each filter
was cut out, fixed on an aluminum stub, and sputtered

Sagawa et al. Progress in Earth and Planetary Science (2018) 5:18

Page 7 of 24

Fig. 3 Example of the Harker diagrams for correlative tephra samples from Sites U1426 (NK270), U1427 (NK193), U1429 (NK232), and the Aso-1
tephra in Lake Biwa sedimentwith gold/palladium. Following a counting routine, a minimum of 400 coccoliths per sample were identified andclassified under SEM at × 3000 magnification. However, ina few cases, this minimum number was not reached. Identification of coccoliths followed Young et al. (2003) andthe electronic guide Nannotax 3 (http://www.mikrotax.org/Nannotax3/). TWC indicators included the taxa Calcidiscus leptoporus, Gephyrocapsa oceanica, andHelicosphaera carteri (Muza 1992).

Results and discussion
Correlation of tephra layers between sites and withwidespread tephra layers

The following screening procedure was applied for detecting correlative tephra between sites.
1) The concentrations of four major elements (Al2O3,
FeO, CaO, and K2O) in volcanic glass shards
were compared between sites, and samples witha consistent composition within two standard deviations (2σ) for each element were selected. We used
these four oxides for the first screening as they are
more accurately quantified and suitable for thediscrimination of tephras than other elements.
2) All major-element compositions were plotted against
SiO2 (i.e., Harker diagrams; Harker 1909), excluding
data that differed between sites by more than 1σ.
3) If potentially correlative tephra layers passed thetwo previous screening steps but occurred in aninconsistent stratigraphic position in the successions between sites, they were eliminated fromthe list of correlative horizons.
4) Finally, the correlations were carefully assessed on
the basis of lithofacies, mineralogical composition,
the morphology and composition of volcanic glass
shards, and heavy mineral composition.
Only tephras that passed all steps above were identified as correlative tephra layers. Furthermore, all tephra
layers investigated in this study were examined to determine whether they are correlated to well-known widespread tephra layers of the Japanese islands, using theprocedure noted above.
The well-known and widespread tephra Aso-1, which
was deposited at ~ 0.25 Ma, can be correlated betweensites. The average major-element compositions of volcanic

Sagawa et al. Progress in Earth and Planetary Science (2018) 5:18

glass shards for NK270 at Site U1426, NK193 at Site
U1427, and NK232 at Site U1429 show good agreement
within 1σ (Additional file 1: Table S1). The SiO2 contentsof these samples are 67–69 wt% and the relative compositions of elements plot in similar areas on Harker diagrams(Fig. 3). The mineralogical compositions are also consistent, with the samples comprising mainly volcanic glass
shards and heavy minerals that are mainly orthopyroxene
and clinopyroxene with minor hornblende and apatite(Additional file 2: Table S2). The morphologies of glass
shards are platy and intermediate (platy and pumice) types(Fig. 4 and Additional file 2: Table S2). Based on the data
above, NK270, NK193, and NK232 are correlated witheach other. The major-element compositions of volcanic
glass shards, grain composition, heavy minerals, and glass
morphology of these tephra samples are similar to theBT61 tephra in the Takashima-oki sediment core from
Lake Biwa (Yoshikawa and Inouchi 1993; Nagahashi et al.
2004) (Figs. 3 and 4; Additional files 1 and 2: Tables S1and S2). The BT61 tephra is a co-ignimbrite ash (Sparks
and Walker 1977) that was associated with the Aso-1
pyroclastic flow deposit erupted from the Aso caldera inPage 8 of 24

the southern part of Kyushu Island (see Fig. 1 for the location of the Aso caldera). Furthermore, the stratigraphic
positions of these samples are consistent with the stable
isotope stratigraphy. Therefore, these three tephra layers
correlate to the Aso-1 tephra.
There are 18 tephra layers that can be correlated betweensites or to well-known widespread tephra layers of the past
1.1 Ma (Tables 1, S1, and S2). These are the Kikai-Akahoya(K-Ah), Sakurajima-Satsuma (Sz-S), Aira-Tanzawa (AT),
Aso-4, Kikai-Tozurahara (K-Tz), Ata, Sanbe-Kisuki (SK),Aso-3, Ata-Torihama (Ata-Th), Aso-1, Kakuto (Kkt),
Kobayashi-Kasamori (Kb-Ks), Hakkoda-Kokumoto (HkdKu), and Shishimuta-Pink (Ss-Pink), from youngest to oldest (Table 1). All of the tephra layers, with the exception of
three tephras (Sz-S, SK, and Hkd-Ku), are co-ignimbrite
ash associated with large-scale pyroclastic flow eruptions
on Kyushu island. Sz-S and SK are Plinian pumice-fall deposits from Sakurajima volcano in southern Kyushu andSanbe volcano in the San-in area (Fig. 1). Hkd-Ku is a coignimbrite ash associated with a large-scale pyroclastic flow
eruption of Hakkoda Caldera in northern Honshu Island.
Figure 2b shows the depth relationship of tephra layers at

NK270 (U1426)

NK193 (U1427)

NK232 (U1429)

Aso-1 (Lake Biwa)

Fig. 4 Scanning electron images of volcanic glass shards of the Aso-1 tephra at Sites U1426, U1427, U1429, and Lake Biwa

Sagawa et al. Progress in Earth and Planetary Science (2018) 5:18

Page 9 of 24

Table 1 Tephra layers identified at Sites U1426, U1427, and U1429
U1429

U1427
Sample no.

U1426

Sample no.

Depth (m CCSF-D)

Depth (m, Patched_
CCSF-D_rev20170310)

NK454

2.97

NK457

6.14

NK458

18.06

NK191

10.69

NK224

48.26

NK491

37.54

NK225

52.68

NK226

56.23

Sample no.

Depth (m, Patched_CCSF-D_
rev20150519)

Tephraname(1.51)*

K-Ah

3.92

AT

(10.28)*

Aso-4

Sz-S
NK268

K-Tz
Ata
NK192

41.23

SK
*

(16.39)Aso-3(26.84)*

Ata-Th

28.63

Aso-1

NK227

69.83

NK341

126.40

NK299

84.67

NK232

140.09

NK193

97.70

NK234

164.59NK194

126.74

NK235

181.30NK195

149.77

NK168

38.57

Unknown-a

NK197

196.86

NK169

50.45

Kb-Ks

NK199

228.70

NK274

60.00

Unknown-b

NK204

298.37

NK280

69.79

Hkd-Ku

NK174

77.92

SayamaNK208

354.53

NK175

81.38

Unknown-c

NK312

421.80NK176

102.12

Ss-Pink

NK270

Kkt

*

Depths are calculated by interpolation of correlation tie points with MD01-2407

the three sites. The data suggest that sedimentation at all
three sites was continuous and that the sedimentation rates
at Sites U1426 and U1429 were 0.23 and 1.2 times higher
than at Site U1427, respectively.
Oxygen isotopes at Sites U1427 and U1429

Figure 5a shows the benthic and planktonic foraminiferal δ18O records of core KR07-12 PC3, which was retrieved near Site U1427. The age model of this core isbased on radiocarbon measurements of planktonic foraminifera from 16 horizons and the location of the AT
tephra layer. The amplitude of the negative excursion ofplanktonic foraminiferal δ18O at the LGM is similar tothat reported from deeper sites in previous studies (e.g.,
Oba et al. 1991; Yokoyama et al. 2007). There is no significant offset between the δ18O values of Uvigerina spp.
and Cassidulina spp. The LGM benthic and planktonic
foraminiferal isotope records show low δ18O values, suggesting that the benthic foraminiferal δ18O curve at Site
U1427/PC3 records surface-freshening events at glacial
maxima. The peak of freshening occurred when sea level
was lowest (Yokoyama et al. 2000; Mix et al. 2001).
Benthic foraminiferal δ18O values in the upper 350 m ofcore from Site U1427 vary between 1.3 and 5.0‰
(Fig. 5b). Samples are barren of benthic foraminifera at
160–180 and 224–245 m. Several negative peaks arepresent, five of which (at 6, 50, 132, 180, and 246 m)

have relatively high amplitudes of ~ 2‰, whereas otherpeaks show smaller amplitudes. The benthic isotope andsediment color (b*) records show similar patterns, with
δ18O minima corresponding to b* minima (Fig. 5b).
The δ18O record of ECS Site U1429 is shown in Fig. 6b.
The δ18O values vary between 2.3 and 4.0‰, representing three glacial–interglacial cycles. The age model for
Site U1429 was derived by correlating the benthic δ18O
record with the LR04 global stack δ18O curve (Lisiecki
and Raymo 2005) and by integrating tephra layers forwhich the eruptive ages are known. The tie points with
LR04 are shown in Fig. 6b by red broken lines and arelisted in Table 2. We used the ages of the following four
tephra layers: K-Ah (7.2 ± 0.1 ka: Smith et al. 2013); Sz-S(~ 13 ka: Okuno 2002); AT (30.0 ± 0.2 ka: Smith et al.
2013); and Aso-4 (87.0 ± 0.3 ka: Aoki 2008). The K-Ah
and AT tephras are widely found in marine, lacustrine,
and terrestrial sediments. The stratigraphic positions ofthese tephras relative to the global oxygen isotope curve
are well established (e.g., Oba et al. 1991; Ijiri et al. 2005;
Domitsu and Oda 2006; Kubota et al. 2010). Smith et al.
(2013) estimated the eruption ages of K-Ah and AT
using tephra layers preserved in the sediment of Lake
Suigetsu, the chronology of which has been establishedby varve counting with high-resolution accelerator mass
spectrometry radiocarbon measurements (Bronk Ramsey
et al. 2012). Radiocarbon measurements of soil just

Sagawa et al. Progress in Earth and Planetary Science (2018) 5:18

Page 10 of 24abFig. 5 δ18O records of a KR07-12 PC3 and b Site U1427. Red triangles in a indicate the positions of planktonic foraminiferal radiocarbon dates.
The gray line in b shows the sediment color profile b* at Site U1427. The vertical lines in a and b denote tephra horizons

beneath the Sz-S tephra yielded an age of ~ 13 cal. kyr
BP (Okuno 2002). This age is consistent with the δ18O
stratigraphy of marine core MD98-2195 (Moriwaki et al.
2011). Aoki (2008) established the stratigraphic positionof the Aso-4 tephra relative to the benthic foraminiferal
δ18O record of marine core MD01-2421 off central
Japan in the Pacific Ocean. These results are in line withthe δ18O stratigraphy of Site U1429 (Fig. 6b). The average sedimentation rate at Site U1429 is ~ 48 cm/kyr(Fig. 6c), and the age at the base of the site is ~ 385 ka.
The error in the correlation with LR04 is estimated tobe ± 2.5 kyr based on the average time resolution of thebenthic δ18O record. The nomenclature of marine isotope stages (MISs) and substages used in this study follows Railsback et al. (2015). The benthic foraminiferal
δ18O data at Sites U1429 and U1427 are available in
Additional files 3 and 4: Table S3 and S4, respectively.
Correlation of Sites U1427 and U1429

Six tephra layers are present at Sites U1427 and U1429(Table 1 and Fig. 2). Five of them are identified as AT,
Aso-4, Ata-Th, Aso-1, and Kkt (from youngest tooldest), whereas the source volcano and eruption age ofthe lowest tephra, tentatively called “unknown-a” in thisstudy, are not known. These tephra layers allow the JS
and ECS benthic δ18O records to be constrained (Fig. 6).
The unknown-a tephra at the bottom of Site U1429 isnear the MIS 11/10 boundary. The Kkt tephra is near
the end of MIS 10 at Site U1429 and at the end of thelight δ18O peak at 132 m depth at Site U1427, suggesting that this δ18O peak is equivalent to MIS 10a. Twotephra layers, Aso-1 and Ata-Th, are present before andafter the MIS 8 maximum at Site U1429, respectively;
therefore, the small peak at ~ 90 m at Site U1427 is
identified as MIS 8. Two negative peaks at ~ 58 and50 m at Site U1427, between Aso-4 and Ata-Th, arecorrelated to MIS 6c and 6a. The AT tephra, which wasformed just prior to the LGM, has already been described. Assuming that δ18O minima correspond to thelowest sea levels, then the δ18O minima at Site U1427
during glacial periods may be correlated to δ18O maxima at Site U1429. The tie points for this correlation
are shown in Fig. 6 (blue dotted lines) and listed inTable 3.

Sagawa et al. Progress in Earth and Planetary Science (2018) 5:18

160
unknown aKkt

Ata-Th

AT

2.0

140

U1427

3.0

Benthic

18

O (‰ VPDB)

a 1.0

Depth (m, U1427_Patched_CCSF-D_rev20170310)
40
60
80
100
120
Aso-1

20

Aso-4

0

Page 11 of 24

4.0

5.0
6

10

8

U1429
3.0

Benthic

18

O (‰ VPDB)

b 2.0 MIS 2

4.0

LR04

Sed. Rate(cm/kyr)

K-Ah

c

Sz-S

5.0

80
40
0
0

50

100

150

200
Age (ka)

250

300

350

400

Fig. 6 Comparison of benthic foraminiferal δ18O records at Sites U1427 (a) and U1429 (b). Red dotted lines in b indicate age control points
correlating U1429 δ18O measurements and the LR04 stack. c Sedimentation rate at Site U1429. The positions of two tephra layers, K-Ah and Sz-S,
used to constrain the Site U1429 age model are shown by vertical lines (see text for details)

Age model of Site U1427

The age model of the sediment succession at Site U1427
was constructed by combining age constraints from benthic foraminiferal δ18O measurements and tephrostratigraphy, the relative abundance of subtropical microfossils,
and radiolarian biostratigraphy (Figs. 7 and 8). A summaryof this approach is provided in Table 4.
As discussed above, the negative peaks in benthic foraminiferal δ18O values at Site U1427 correlate to glacial maxima in the upper 160 m, equivalent to the past 400 kyr(Fig. 6). Given that eustatic change is a major cause of negative δ18O excursions (e.g., Oba et al. 1991; Kido et al. 2007),such a correlation is likely to be also applicable to glacial
stages older than 400 ka. Two distinct negative excursions,at ~ 180 and ~ 246 m, are present below 160 m (Fig. 7b).
The upper excursion occurs prior to MIS 10 and therefore

is equivalent to MIS 12. The lower excursion corresponds
to MIS 16, not to MIS 14. This interpretation is based on
sea-level and ice-volume estimates which suggest that the
sea-level fall during MIS 14 was less than that during MIS
16, when the ice volume was as large as during MIS 12(Bintanja et al. 2005; Elderfield et al. 2012). Therefore, thesea-level drop at MIS 14 is likely to have resulted in less intense surface freshening in the absence of a strong negative
δ18O excursion. The two zones that lack benthic foraminifera at 160–180 and 224–245 m occur just above negative
excursions and correspond to intervals of laminated sediment. The shipboard data show that all microfossil groups,
except diatoms, are rare or absent in these intervals (Tadaet al. 2015a). Taking into account the higher abundance offreshwater diatom species in these intervals (Tada et al.
2015a), we suggest that surface water freshening during

Sagawa et al. Progress in Earth and Planetary Science (2018) 5:18

Page 12 of 24

Table 2 Correlation tie points between Sites U1429 and LR04

Table 3 Correlation tie points between Sites U1427 and U1429

U1429 depth (m, CCSF-D)

Age (kyr)U1429 depth(m, CCSF-D)

Event

2

U1427 depth (m, U1427_
Patched_CCSF-D_
rev20170310)

U1429 Age (ka)

0.82
2.97

7.3

0.00

0.0

0.0

Core top

6.14

13

6.01

9.40

18.0

MIS 2

9.40

18

10.69

18.06

30.0

AT

18.06

30

37.54

48.26

87.0

Aso-4

21.48

38

50.78

71.34

140.0

MIS 6a

27.85

45

57.84

81.00

155.0

MIS 6c

34.55

55

84.67

126.40242.0

Ata-Th

43.14

75

88.51

130.00

252.0

MIS 8

45.99

82

97.70

140.09

270.2

Aso-1

48.26

87

126.74164.59

339.2

Kkt

51.00

91

131.00167.18346.0

MIS 1053.11

91149.77181.30

383.2

61.18

109

Unknown-atephra

64.56

123

66.24

130

71.34

140

81.00

155

87.14

167

90.75

174

97.04

185

103.46201

110.29217

119.40237

122.00242

126.40242

130.00252

139.58269

145.70283

160.72329

166.04343

these two glacial maxima was too extreme for some marine
groups.
The trend of benthic foraminiferal δ18O variations at
Site U1427 closely follows the sediment color reflectanceb* data (Fig. 5b). All negative excursions in δ18O values
correspond to b* minima. The b* variation shows a negative correlation with natural gamma radiation (NGR) andMS trends and a positive correlation with the relative
abundance of biogenic components such as calcareous
nannofossils (figure F13 in Tada et al. 2015a). Since NGR
and MS values reflect terrigenous clastic content andshow a negative correlation with biogenic component
abundance, b* is interpreted to reflect the ratio of biogenic
to terrigenous input. As Site U1427 is on the outer marginof the marginal terrace (~ 330 mbsl), terrigenous input at
this site is likely to be strongly controlled by sea-level variability. Therefore, δ18O values and b* should exhibit similar variations on glacial–interglacial time scales. However,two b* minima are present at ~ 30 m and ~ 70 m that are
not associated with a negative δ18O excursion (Fig. 5b).
The sediment in these intervals is characterized by black
to very dark greenish-gray clayey silt with heavy bioturbation, with an appearance that is not typical of glacial
maxima (Tada et al. 2015a). Nevertheless, assuming that
sea-level falls at glacial maxima are typically accompanied
by b* minima, then the MIS 12 and 16 glacial maxima areat 167.2 and 232.0 m, respectively (Fig. 7 and Table 4). In
the absence of a stable-isotope record below 350 m, we
have correlated b* minima to glacial maxima, with MIS
24–22 at 345–370 m and MIS 26 at 382 m.
The regional tephra layers also constrain the age of
sediment in this section. We used four tephra layers(AT, Aso-4, Hkd-Ku, and Ss-Pink) for which the relative
positions in the stable isotope stratigraphy are known.
Pickering et al. (1999) established a stable isotope stratigraphy and magnetostratigraphy for the deep-marine
successions of the middle Kazusa Group in the Yoro
River section in southeast Japan. The stratigraphic
position of the Ku1 tephra, which is also known as the
Hkd-Ku tephra (Satoguchi and Nagahashi 2012), is at
the base of MIS 18, slightly above the Brunhes–
Matuyama boundary (Pickering et al. 1999). Based on
this, the two minor negative δ18O peaks at 284.7 and295.4 m, above the Hkd-Ku tephra layer, correspond to
MIS 18, and the δ18O minimum below Hkd-Ku at
314.1 m is the MIS 20 maximum. The Ss-Pink lies in the
Jaramillo Subchron in lacustrine deposits of the Osakaand Kazusa groups (Torii et al. 1974; Hayashida et al.

Sagawa et al. Progress in Earth and Planetary Science (2018) 5:18

Page 13 of 24

a

b

c
d

e

Fig. 7 Summary correlation of Site U1427 records with the LR04 stack. a LR04 benthic isotope stack. b U1427 benthic foraminiferal δ18O values
and sediment color b* variation. Abundance of TWC taxa of three planktonic groups, c calcareous nannofossils, d planktonic foraminifera, and eradiolaria. Background color indicates correlation of glacial and interglacial maxima, which are defined by benthic δ18O measurements and TWC
indicators, respectively. Red lines indicate age constraints from well-known regional tephra layers

1996). High-resolution chronostratigraphic analyses suggest that the stratigraphic position of the O7 tephra in
the Kazusa Group, which is also known as the Ss-Pink
tephra (Satoguchi and Nagahashi 2012), is in the earlypart of MIS 30, and the age of this tephra is ~ 1.050–
1.056 Ma (Tsuji et al. 2005).
To further constrain the stratigraphy, we used ingressionsof subtropical TWC species of planktonic microfossils tocalibrate interglacial maxima. The relative abundance of
TWC indicators of these three microfossil groups (calcareous nannofossils, planktonic foraminifera, and radiolarians) generally show good correlations (Fig. 7c–e). The
broader peaks of calcareous nannofossil abundance are dueto the lower resolution of this part of the study. The higher
abundance is only observed in calcareous nannofossils at
12–17, ~ 32, and 57–63 m (Fig. 7c), possibly due to nutrient
availability as well as differences in habitat depths between
the various microfossil proxies. Calcareous nannoplankton
are limited to the photic zone due to photosynthesis,
whereas zooplankton dwell in a wide range of depths (e.g.,
Fairbanks et al. 1980; Itaki 2003; Itaki et al. 2003; Kuroyanagi
and Kawahata 2004). Global sea-level records suggest thatthe amount of sea-level lowering at glacial maxima wascomparable to the sill depth of the modern Tsushima Strait(Siddall et al. 2003; Rohling et al. 2014). In other words, thesea level at other periods was sufficiently high to allow

Sagawa et al. Progress in Earth and Planetary Science (2018) 5:18

0

200

400

Page 14 of 24

Age (ka)
600

800

1000

0

Sed.Rate(cm/kyr)

Depth (m, U1427_Patched_CCSF-D_rev20170310)

a

100

200

300

tephrabenthic 18O & b*
TWC indicators
Bio-event

400

100
80
60
40
20
0

b

0

200

400

600
Age (ka)

800

1000

Fig. 8 Age–depth relationship at Site U1427 (a) and sedimentation rate (b)surface water to flow in through the strait. Given that the
TWC indicators of zooplankton groups show peaks withshort durations just after glacial maxima, substantial inflowof the TWC only occurred during interglacial maxima.
Therefore, maxima in calcareous nannofossils that are notpresent in the other microfossil groups could suggest thatthe sea level was not as high as at the present day and thatthe depth of the Tsushima Strait was limited to only a few
tens of meters, preventing zooplankton from migrating
through the strait. Therefore, we interpret the peaks of zooplankton just after the glacial negative δ18O excursions asinterglacial maxima (Fig. 7). All interglacials except for MIS
17 show higher abundances of subtropical species
compared with glacial periods. The lack of TWC indicators during MIS 17 might relate to environmental
factors outside of the JS; e.g., the northwestern Pacificregion and ECS. Radiolarian assemblages in coresfrom the Kuroshio region yield a relatively low abundance of low- to middle-latitude species during the peakof MIS 17 compared with other interglacial maxima (Matsuzaki et al. 2015), implying that subtropical water masses
had less influence on the ECS at this time compared with

Sagawa et al. Progress in Earth and Planetary Science (2018) 5:18

Page 15 of 24

Table 4 Age control points for the succession at Site U1427
U1427 Depth (m, Patched_CCSF-D_
rev20170310)

Age (ka)

SourceDatumsReference00Core top

6.01

18

Benthic δ18O

10.69

MIS 2

Lisiecki and Raymo (2005)

30.0

TephraAT

Smith et al. (2013)

20.98

54

Bio-event

LO Lychnocanoma sakaii

Itaki et al. (2007)

36.09

85

Bio-event

LO Amphimelissa setosa

Itaki et al. (2007)

37.54

87.0

TephraAso-4

Aoki (2008)

45.10

123

TWC microfossil

MIS 5eLisiecki and Raymo (2005)

50.78

140

Benthic δ O

MIS 6aLisiecki and Raymo (2005)

57.84

155

Benthic δ18O

MIS 6cLisiecki and Raymo (2005)

76.67

217

TWC microfossil

MIS 7cLisiecki and Raymo (2005)

84.67

242

Tephra (tied to U1429)

Ata-Th88.51

252

Benthic δ18O

MIS 8a

97.68

270.2

Tephra (tied to U1429)

Aso-1

108.55285

TWC microfossil

MIS 9aLisiecki and Raymo (2005)

121.27329

TWC microfossil

MIS 9eLisiecki and Raymo (2005)

126.74339.2

Tephra (tied to U1429)

Kkt

131.00346

Benthic δ18O

MIS 10a

153.05406

TWC microfossil

MIS 11c

Lisiecki and Raymo (2005)

167.2

433

b*

MIS 12aLisiecki and Raymo (2005)

191.57491

TWC microfossil

MIS 13aLisiecki and Raymo (2005)

198.94525

TWC microfossil

MIS 13c

Lisiecki and Raymo (2005)

209.58575

TWC microfossil

MIS 15aLisiecki and Raymo (2005)

220.58612

TWC microfossil

MIS 15eLisiecki and Raymo (2005)

232.0

630

b*

MIS 16aLisiecki and Raymo (2005)

284.65718

Benthic δ18O

MIS 18a

Lisiecki and Raymo (2005)

295.37750

Benthic δ18O

MIS 18e

Lisiecki and Raymo (2005)

298.37756

Tephra

Hkd-KuPickering et al. (1999)

309.27780

TWC microfossil

MIS 19c

Lisiecki and Raymo (2005)

314.11

798

Benthic δ18O

MIS 20b

Lisiecki and Raymo (2005)

326.75822

TWC microfossil

MIS 21a

Lisiecki and Raymo (2005)

332.13836

TWC microfossil

MIS 21c

Lisiecki and Raymo (2005)

343.66858

TWC microfossil

MIS 21 g

Lisiecki and Raymo (2005)

352.29874b*

MIS 22

Lisiecki and Raymo (2005)

380.23954

TWC microfossil

MIS 25eLisiecki and Raymo (2005)

382.30964

b*

MIS 26

Lisiecki and Raymo (2005)

395.08996

TWC microfossil

MIS 28b

Lisiecki and Raymo (2005)

407.441023

TWC microfossil

MIS 29

Lisiecki and Raymo (2005)

18

Lisiecki and Raymo (2005)

Lisiecki and Raymo (2005)

421.801053

TephraSs-Pink

Tsuji et al. (2005)

424.151072

TWC microfossil

MIS 31

Lisiecki and Raymo (2005)

other interglacials. The correlation horizons between
TWC microfossil data and LR04 are listed in Table 4.
Four datum levels of radiolarian marker species from
Site U1427 were described by Itaki et al. 2018, and threeof them are within the period considered in the present
study. The last occurrences (LOs) of Lychnocanomma
sakaii (Morley and Nigrini 1995) and Amphimelissa
setosa (Cleve) are well-known biohorizons in the

K-Ah

Sz-S

AT

Aso-4
K-Tz

Ata

SK

Aso-3
Ata-Th
Aso-1
Kkt

Kb-Ks
Hkd-KuSs-Pink

Kikai-Akahoya

Sakurajima-Satsuma

Aira-Tanzawa

Aso-4

Kikai-Tozurahara

Ata

Sanbe-Kisuki

Aso-3

Ata-Torihama

Aso-1

KakutoKobayashi-Kasamori

Hakkouda-Kokumoto

Shishimuta-Pink

±5

ka

± 30
± 50

990
1040

ka

ka

ka
ka

ka

ka

ka

ka

ka

ka

ka

ka

ka

ka

ka

ka

ka

ca. 650
± 160

ka BP
ka

650

± 80

± 10

340
620

± 80

± 14

± 40

±6

360

266

240

123

± 20
± 14

100
110

± 27
±3

100
108

± 26
± 7.1

98
96.7

±7

27
89

± 0.27
±4

25.12

±2

13.4

26

ka

± 1.3

13.8
ka

cal ka BP

ka±2

ca. 12.8

7.7

Unit
ka BP

Error

ca. 6.5

Age

C radiocarbon dating method, TL thermoluminescence dating, FT fission track dating, RTL red thermoluminescence dating, K-Ar Potassium-Argon dating*14

Abbreviation

Name of widespread tephra

Table 5 Compilation of age estimates for the regional tephra layers of this study

FT

K-Ar

FT

K-Ar

FT

K-Ar

FT

K-Ar

FT

K-Ar

TL

FT

K-Ar

FT

RTL

FT

K-Ar

TL

TL

14C

TL

TL

14C

TL

14C

Method*Danhara et al. 1994

Uto and Suto 1985

Hirosaki City 2001

Muraoka 1991

Danhara 1995

Machida and Arai 2003

Danhara 1995

Matsumoto et al. 1991

Danhara 1995

Matsumoto et al. 1991

Shitaoka et al. 2009

Kimura et al. 1999

Matsumoto and Ui 1997

Danhara 1995

Miyairi 2007

Danhara 1995

Matsumoto 1996

Nagatomo et al. 2005

Nagatomo et al. 2005

Miyairi et al. 2004

Shitaoka et al. 2016

Shitaoka et al. 2016

Okuno 2002

Nagatomo et al. 2005

Okuno et al. 2013

References

Sagawa et al. Progress in Earth and Planetary Science (2018) 5:18
Page 16 of 24

Sagawa et al. Progress in Earth and Planetary Science (2018) 5:18

subarctic Pacific, and their estimated ages in the Japan
Sea are 54 and 85 ka, respectively (Itaki et al. 2007).
These datums are present at Site U1427 between two
widespread tephras, AT and Aso-4. The LO of Schizodiscus japonicus Matsuzaki et al. (2014) is the same eventas the LO of Spongodiscus sp. reported from the North
Pacific Ocean, the Bering Sea, and the Sea of Okhotsk.
In the Sea of Okhotsk, this event occurs during MIS 9(287 ka) (Matul et al. 2002) or MIS 8c (272 ka) (Matulet al. 2009). The depth of this event at Site U1427 is between 112.05 and 112.77 m, which is prior to the Aso-1
tephra (272 ka) and the peak of subtropical species at
MIS 9a (285 ka). Therefore, the age of ~ 287 ka proposed by Matul et al. (2002) appears to be likely. However, if we use the 287 ka age for this datum, thesedimentation rates rapidly increase by a factor of ~ 10
across this datum, which does not appear to be geologically plausible. This may relate to a large error in the age
data of Matul et al. (2002). Therefore, this datum is notincluded in the age model (Table 4).
The potential sources of error in the age model of Site
U1427 with respect to LR04 are uncertainties in the correlation of benthic foraminiferal δ18O measurements
and TWC indicators with LR04, and age estimations ofwidespread tephras and radiolarian biohorizons. The
correlation of δ18O values and TWC indicators with
LR04 would generate errors of a few thousands of years
due to the resolution of the records. The errors of age
estimation for tephra layers used in this study are lessthan ± 1 kyr for AT and Aso-4 (Aoki 2008; Smith et al.
2013), and ± 3 kyr for Hkd-Ku and Ss-Pink (Pickering
et al. 1999; Tsuji et al. 2005). The ages of biohorizons
are calibrated by JS planktonic foraminiferal δ18O
values of MD01-2407 (Itaki et al. 2007), but the errors onthese values are unknown. We estimated the errors ofthese biohorizons as ± 3 kyr, which includes the uncertainty of the age model of MD01-2407 (Kido et al. 2007).
Including these sources of error, we estimate the uncertainty of the age model of Site U1427 to be ± 5 kyr forthe period 0–400 ka and ± 10 kyr for the period 400–
1000 ka.
Age estimation of regional tephra layers using the newly
established chronology

Fifteen out of eighteen correlative tephra layers were identified as regional tephra layers. The ages of regional tephra
layers have been previously established by several techniques using eruption products near a source volcano or
regional deposits (Table 5). The tephrochronology of
Pliocene to Pleistocene strata around the Japanese islands
was developed using these techniques combined withbiostratigraphy, magnetostratigraphy, and oxygen isotope
stratigraphy (Tamura and Yamazaki 2010; Satoguchi andNagahashi 2012). As discussed above, we only used six

Page 17 of 24

widespread tephra layers as age constraints, where theirrelative stratigraphic positions with respect to the oxygenisotope stratigraphy are known. In this section, we discuss
the sedimentation ages of tephra layers that are not used asage constraints, by comparing the ages estimated in thisstudy with those reported in previous studies. The ages estimated in this study were calculated by a simple linear
interpolation method.
The K-Tz tephra, which is widespread in central andwestern Japan, was deposited just after MIS 5c (Machida
and Arai 2003). The estimated age based on U1429 δ18O
stratigraphy is 91 ka, which corresponds to MIS 5b. Thisestimate is within error of the results of fission-track(FT) analyses (98 ± 26 ka, Danhara 1995) and the redthermoluminescence method (93.7 ± 7.1 ka, Miyairi 2007)(Table 5).
The stratigraphic position of the Ata tephra in marine
sediment was previously reported to be between MIS 5d
and 5c, with an estimated age of 103–107 ka (Oba 1991).
Our estimate is 98 ka, which is younger than the value
suggested by Oba (1991) and the K–Ar age of pyroclastic
flow deposits (108 ± 3 ka, Matsumoto and Ui 1997).
Chun et al. (2004) reported the Aso-3 tephra layer inmarine sediment cores from the Ulleung Basin, in a bioturbated mud just above a dark finely laminated layer
deposited during MIS 6a. Nagahashi et al. (2004) reported the Aso-3 tephra from an interval with an estimated age of 133 ka below MIS 5e in marine clay incores from the Osaka coast. The stratigraphic positionof Aso-3 at Site U1429 agrees with these results, and theestimated age of ~ 137 ka is also consistent with previous studies.
The Ata-Th tephra layer is prior to MIS 7e. In thepresent study, its age is estimated to be 242 ka. Thisagrees with its estimated age in lacustrine sediment from
Lake Biwa (238 ka, Yoshikawa and Kuwae 2001) and FT
analyses (240 ± 40 ka, Danhara 1995).
The Aso-1 tephra lies beneath the maximum δ18O
value in MIS 8 (Fig. 6). We suggest that this δ18O maximum correlates to MIS 8c rather than 8a, which is the
MIS 8 maximum in the LR04 stack, as there is anotherpositive peak at Site U1429 just below 7e (Fig. 6). The
estimated age of Aso-1 in this study is 270 ka, which iswithin error of a K–Ar date (266 ± 14 ka, Matsumoto etal. 1991) and is older than the age estimated from Lake
Biwa (249 ka, Yoshikawa and Kuwae 2001). There is adiscrepancy of ~ 21 kyr between Site U1429 and Lake
Biwa, possibly due to the large error in the Lake Biwa
age estimation, as discussed later.
The Kkt tephra is present near the end of MIS 10 at
Site U1429 (Fig. 6), and its age is estimated to be 339 ka,
consistent with that from Lake Biwa (334 ka, Yoshikawa
and Kuwae 2001) and radiometric data (Danhara 1995;
Machida and Arai 2003) (Table 5).

Sagawa et al. Progress in Earth and Planetary Science (2018) 5:18

a 10

0

100

Page 18 of 24

U1427 CCSF-D_Patched_rev20170310 (m)
200
300

400

B value

20

30

Ss-Pnkunkonwn c

Hkd-Kuunkonwn b

Kb-Ks

unkonwn a

Aso-1

U1427
U1426

1

60

0.5
0
-0.5
Laminated interval

Correlation coefficientb

Ata-Th

AT

50

Aso-4

40

-1
0

100

200
300
U1427 CCSF-D_Patched_rev20170310 (m)400

Fig. 9 a Comparison of sediment color profiles at Sites U1427 (red) and U1426 (blue) plotted on the U1427 depth scale. Note that the axis of B
values is inverted. Control points by correlative tephra layers are shown by vertical black lines. Additional control points from color change
correlation are shown by red lines. For additional details, see Additional file 8: Figure S2. b Moving correlation coefficient of two color profiles,
calculated for a 20 m window with width indicated by orange bar. Intervals of laminated sediment at Site U1427 are shown at the base of thelower panel

The difference in estimated ages between Site U1429
and Lake Biwa is greater for Aso-1 (~ 21 kyr) and smaller
for Ata-Th and Kkt (~ 5 kyr). The age estimate for Lake
Biwa sediment is based on a change in pollen assemblage,
which shows alternations of warm-temperate and coolboreal species in response to glacial–interglacial cycles(Miyoshi et al. 1999). Ata-Th and Kkt are present at transitions from cool to warm periods, and these can be correlated to terminations. However, the Aso-1 tephra occurs
within a period of cool-boreal conditions, equivalent to
MIS 8. As the duration of the MIS 8 cold period (~ 30 kyr)is longer than that of terminations (< 8 kyr), the age estimate of Aso-1 may have a greater uncertainty than that of
Ata-Th and Kkt.
The ages of the SK and Kb-Ks tephra layers, which are
not present at Site U1429, are based on the Site U1427
age model. The SK tephra layer at 41.23 m depth at Site
U1427 is ~ 4 m below the Aso-4 tephra (Fig. 5b). The
estimated age of the SK tephra is 102 ka in this study,
which agrees with radiogenic data (100 ± 20 ka, Kimura
et al. 1999; 110 ± 14 ka, Shitaoka et al. 2009) (Table 5).

The Kb-Ks tephra layer at 196.86 m is within MIS 13c/
13b (Fig. 7), and the estimated age of 516 ka is muchyounger than that derived from FT analyses (620 ± 80 ka:
Danhara 1995). Shirai (2001) interpreted the stratigraphic position of the Kb-Ks tephra at ODP Sites 794
and 797 relative to changes in diatom abundance. The
diatom abundance in the deep-sea sediment of the JS isconsidered to be related to sea-level change (Tada et al.
1992, 1999), which controls nutrient input through the
Tsushima Strait. Therefore, variations in diatom abundance follow the δ18O curve, enabling estimates of sediment age. The age of 520 ± 20 ka (MIS 13c) estimatedby Shirai (2001) shows good agreement with the resultof this study (516 ka).
In this study, three equivalent tephra layers are present
between sites (tentatively called unknown-a, unknown-b,
and unknown-c), which were not correlated to regional
tephras. The unknown-a tephra is found at three sites,
and the unknown-b and unknown-c tephras occur onlyat Sites U1426 and U1427. The ages of these tephra
layers were estimated using the age model of Site

Sagawa et al. Progress in Earth and Planetary Science (2018) 5:18

Page 19 of 24

a

b

c

d

Fig. 10 a LR04 oxygen isotope stack. b Color variation at Sites U1426 and U1427 over the past 1.0 Myr. Note that the axis of B values is inverted.
Control points by correlative tephra layers are shown by black triangles. Additional control points by color correlation are shown by red triangles.
c Color variations at Site U1426 plotted on a U1427-based age model (blue) and a U1424-based age model (orange). d Age difference between
U1427-based and U1424-based age estimates for the past 1.0 Myr

U1427. The age of the unknown-a tephra layer is 383 ka,
above the distinct peaks of TWC indicators of MIS 11c(Fig. 7). The ages of the unknown-b and unknown-ctephra layers are estimated to be 625 and 880 ka, respectively. Although there is no benthic foraminiferal
δ18O record for these intervals, both tephra layers correspond to b* minima, suggesting that they were erupted
during the glacial maxima of MIS 16 and 22. The stratigraphic positions of these three unidentified tephra
layers are in MIS 11, 16, and 22, respectively, and they
may be useful for correlating sediment successions atother locations.

Correlation of Sites U1426 and U1427

Eight tephra layers are present at Sites U1426 andU1427 (Table 1 and Fig. 2). Although the widespread
tephra layers of K-Ah, Aso-4, Aso-3, and Ata-Th are notpresent at Site U1426, Kido et al. (2007) reported thesetephras in core MD01-2427, 10 km away. Since the twosites are adjacent and the color variations of the sediments are similar (Additional file 5: Figure S1), thestratigraphic positions of the tephra layers in core
MD01-2407 can be correlated to Site U1426 using
sediment color profiles (Additional file 6: Table S5).
Therefore, two more widespread tephra layers, Aso-4

Sagawa et al. Progress in Earth and Planetary Science (2018) 5:18

and Ata-Th, can also be used to correlate features between Sites U1426 and U1427.
We attempted to correlate the color variations of sediment at Sites U1426 and U1427 using tephra layers as
primary correlation horizons. As noted above, thedecimeter-scale dark and light color alternations at Site
U1426 are distinctive, whereas color variability at Site
U1427 is less obvious and more gradual. To highlightcolor changes at Site U1427, the brightness and contrast
in photographs of sediment surfaces were enhanced(Fig. 2). These enhanced photographs clearly show
meter-scale color variations at Site U1427.
The color variations of sediments in deeper-water sites
have been well documented (Tada et al. 1992; Kido et al.
2007). The fact that dark–light alternation is commonly observed and correlated in the JS indicates that it reflects abasin-wide phenomenon (e.g., Nakajima et al. 1996). Previous studies have shown that the brightness (L*) at deeper
sites correlates well to the total organic carbon (TOC) content, except during glacial maxima (Tada et al. 1992; Kido
et al. 2007), when finely laminated dark layers were deposited under euxinic conditions due to density stratificationin the JS, and TOC was relatively low. Black and Anderson(2017) showed that the L* and B values of Site U1427 correlate well with the TOC content. Therefore, TOC seems tobe the major component contributing to the dark sedimentcolor at deeper and shallower sites, suggesting that correlation using color profiles is reasonable.
Figure 9 shows the B values of RGB correlation between Sites U1426 and U1427. These cores are correlated using shipboard line-scan images of the sedimentsurface and RGB data at 0.05 mm intervals, and a 3 cm
moving average of 5 mm resolution data for Site U1426
and a 10 cm moving average of 5 mm resolution data
for Site U1427. The overall variations in RGB reflect
sediment brightness. Therefore, similar to L*, RGB appears to reflect TOC. Ten tephra layers were used as
primary tie points in Fig. 9, and additional tie points
were selected based on B value trends (Additional file 7:
Table S6). A detailed comparison of the color profiles at
the two sites is shown in Additional file 8: Figure S2.
Overall, the color variations appear to correlate well (seethe running correlation coefficient calculated using awindow with 20 m width in Fig. 9b). The average correlation coefficient over the whole interval is ~ 0.4. Five intervals at ~ 130, 160, 220–265, 345–370, and 395–425 m do
not correlate (Fig. 9). Based on our age model, these five intervals correspond to the glacial periods of MIS 10, 12, 16,
22, and 28–30 (Fig. 7). Three glacial periods (MIS 12, 16,
and 22) correspond to faintly laminated intervals, where
benthic foraminifera are sparse or absent (Tada et al.
2015a), suggesting that intermediate water properties weresignificantly different during these periods compared withother times. The poor correlation between shallow- andPage 20 of 24

deep-sediment colors during glacial periods is likely due tothe fact that the process of dark layer formation during glacial maxima differed from the process that operated duringother periods. During glacial maxima, thick dark layers withfine laminations were caused by stagnant euxinic deep
water due to a strong density stratification, which was notaccompanied by high TOC. Ikehara et al. (1994) suggested
that the thick dark layer deposited during the LGM only
occurs in water depths greater than 500 m, suggesting thatthe redox boundary was below ~ 500 m and that environments below and above this depth were quite different during glacial maxima compared with other periods.
We constructed an age model for Site U1427 andattempted to correlate the color profiles between Sites
U1427 and U1426. Although the correlation of the twosites is not always significant, ten correlative tephra
layers provide robust ties at least for the glacial–interglacial intervals. The depth-scale of Site U1426 was converted to an age scale via correlation with Site U1427. The
B value variations at Sites U1426 and U1427 over the past
1.0 Myr are shown in Fig. 10b. The B values show orbitalscale variations at both shallow and intermediate depths
in the JS, where sediment colors are lighter during glacials
and darker during interglacials. Given that the sedimentcolor reflects the TOC content of the sediment, the overall
pattern suggests that productivity was higher during interglacials, probably due to an increased input of nutrientsfrom the ECS during periods of high sea level in contrast
to glacial periods, when the JS was almost entirely cut off
from the ECS (Tada et al. 1999).
Evaluation of age models

Combining several kinds of age constraints enabled us
to establish the orbital-scale age model of the shallow
Site U1427, and this chronology was transferred to thedeeper Site U1426 by correlating sediment color profiles(Fig. 9). The sediments at the six deeper sites (U1422–
U1426 and U1430) drilled during Expedition 346 arecharacterized by dark–light alternations. As noted above,the sporadic occurrence of foraminiferal tests made itdifficult to establish a stable isotope stratigraphy for the
deeper sites. Therefore, age estimation for deeper sites
was attempted using a different approach (Tada et al.
2018). To evaluate and understand the limitations of thevarious approaches, we compared two independent age
models that are based on different concepts.
Tada et al. (2018) derived an age model for Site U1424
based on the magnetostratigraphy, tephrochronology,
and tuning of physical properties to the LR04 stack.
Physical properties, such as gamma ray attenuation(GRA), bulk density, and NGR, in deeper cores are considered to reflect diatom content because of their highinternal porosity (Tada and Iijima 1983) and the effect ofdilution of detrital materials. The diatom content in theSagawa et al. Progress in Earth and Planetary Science (2018) 5:18

JS sediment is thought to be strongly related to eustatic
sea-level changes, which constrained nutrient influx into
the sea as well as subsequent diatom production (Tadaet al. 1992, 1999). Therefore, GRA and NGR data reflectsea-level changes and are useful in identifying marine
isotope stages. Correlation of Site U1424 to the LR04
stack was achieved by relating the variability in physicalproperties to glacial maxima over the past 3 Myr (Tadaet al. 2018). Tada et al. (2018) also correlated the six
deeper sites using distinct, relatively thick dark layers,magnetostratigraphy, and tephra layers. As a result, 64
horizons can be correlated for the past 1.5 Myr. This
inter-site correlation enables us to transfer the age modelof Site U1424 to all deeper sites, including Site U1426.
Thus, two independent age estimations are available forSite U1426, based on the correlation of dark layers withSite U1424 and the correlation of color variations withSite U1427. Figure 10c–d compares age models and age
differences between these approaches. The age differenceis calculated by subtracting U1424-based ages from
U1427-based ages, and this is plotted against the U1427based age model. Positive values indicate that the U1424based age model leads the U1427-based model.
The age estimates for Site U1426 obtained using two different methods show good agreement for the past 300 kyr(Fig. 10), when more tie points of color variations at Sites
U1426 and U1427 are available compared with the older intervals (red triangles in Fig. 10b). Large discrepancies (~ 20kyr) are present at MIS 11, 13–12, 15, and 22. The discrepancies during interglacial stages (MIS 11 and 15) are probably due to the lack of age control points for interglacials at
Site U1424. Since the correlation of physical properties between Sites U1424 and LR04 is based on glacial peaks, ages
of interglacial maxima were calculated by interpolation
using a constant sedimentation rate (Tada et al. 2018). In
contrast, the interglacials at Site U1427 are well constrainedusing the high abundance of TWC taxa (Fig. 7). The poorcorrelation during interglacials would be improved if theconstraints provided by TWC indicators were available forthe deeper sites. The age discrepancies during glacial stages(MIS 13–12 and 22) are probably due to lack of color correlation tie points between Sites U1426 and U1427. As discussed above, the sediments of MIS 12, 16, and 22 are
faintly laminated at Site U1427 and differ markedly fromthe equivalent sediments at Site U1426 (Fig. 9). Furthermore, the interval of MIS 13–12 is characterized by largechanges in calcium carbonate content at deeper sites (Kidoet al. 2007; Khim et al. 2014), which are not present at theshallower Site U1427. Therefore, no tie points are determined for these glacial periods (Fig. 10b). However, there
are correlative tephra layers at MIS 22 (unknown-c) and 16(unknown-a). Therefore, age discrepancies during these glacial periods are attributed not to poor correlation between
Sites U1426 and U1427, but to correlation between LR04

Page 21 of 24

and Site U1424 and/or Site U1427. Further investigation isneeded to improve the age estimation of these glacial periods. Although there is up to a 20 kyr age difference, thegeneral agreement of the two independent age models implies that the orbital-scale variations in sediment brightness
at both shallow and deep sites, which mainly reflect TOC
content, were controlled by eustatic sea-level changes.

Conclusions
Two IODP Expedition 346 sites in the Japan Sea and thenorthern East China Sea are correlated on a glacio-eustatic
orbital scale for the first time. Detailed morphological, mineralogical, and elemental-composition analyses of tephras
enhance the chronostratigraphic basis of this correlation.
Eighteen tephra layers were identified that can be correlated
between sites or to other well-known tephras. The results
suggest that glacial surface water in the Japan Sea wasmuch fresher than that in the East China Sea. Several
microfossil proxy records show strong variability that is directly related to glacial–interglacial sea-level change. The
age model for the upper part of Site U1427 suggests thatthe top 430 m is younger than 1100 ka. Despite the lack of
distinct dark–light alternations, the subtle color variations
at the shallow Site U1427 generally correlate to color variations at the deeper Site U1426 on an orbital scale, and perhaps on the millennial scale in the 120 kyr interval. The
agreement of age estimates based on two different methods
supports the age models derived for Sites U1427 andU1424, and the inter-site correlation of JS sites. The intersite correlation makes it possible to compare proxy records of latitudinally and vertically distributed Expedition
346 Sites in the Japan Sea and to compare between theJapan Sea and the East China Sea.
Additional files
Additional file 1: Table S1. Major element compositions of volcanic
glass shards of tephra layers investigated in this study and well-known
regional tephra layers. (XLSX 27 kb)
Additional file 2: Table S2. Petrographic property of tephra layers
investigated in this study and well-known regional tephra layers. (XLSX
20 kb)
Additional file 3: Table S3. Benthic foraminiferal δ18O results at Site
U1429. (XLSX 13 kb)
Additional file 4: Table S4. Benthic foraminiferal δ18O results at Site
U1427. (XLSX 15 kb)
Additional file 5: Figure S1. Correlation of sediment color profiles at
two adjacent Sites U1426 and MD01-2407. (ZIP 553 kb)
Additional file 6: Table S5. Correlation tie points at Sites U1426 andMD01-2407. (XLSX 12 kb)
Additional file 7: Table S6. Correlation tie points at Sites U1426 andU1427. (XLSX 11 kb)
Additional file 8: Figure S2. Correlation of sediment color profiles at
Sites U1426 and U1427. Control poits by correlative tephra layers areshown by black lines. Additional control points are shown by green lines.
(TIFF 339 kb)

Sagawa et al. Progress in Earth and Planetary Science (2018) 5:18

Abbreviations
CCD: Carbonate compensation depth; CCSF: Core composite depth below
seafloor; ECS: East China Sea; EDS: Energy Dispersive Spectroscopy; FT: Fission
track; GRA: Gamma-ray attenuation; IODP: Integrated Ocean Drilling Program;
JS: Japan Sea; LGM: Last glacial maximum; LO: Last occurrence; mbsl: Meters
below sea level; MIS: Marine isotope stage; MS: Magnetic susceptibility;
NGR: Natural gamma radiation; NGRL: Natural gamma radiation logger;
NIST: National Institute of Standards and Technology; SEM: Scanning electron
microscope; SHIL: Section Half Imaging Logger; SHMSL: Section Half
Multisensor Logger; TOC: Total organic carbon; TWC: Tsushima Warm
Current; VPDB: Vienna Pee Dee Belemnite; WRMSL: Whole-Round Multisensor
Logger; XRF: X-ray fluorescenceAcknowledgements
We are grateful to Yasufumi Iryu and Peter Clift (journal editors) and threeanonymous reviewers for their constructive comments on the manuscript.
We would like to show our great appreciation to all of the scientists,
technicians, and crew of D/V JOIDES Resolution during IODP Expedition 346.
This research used samples and data provided by the IODP. TS, T Itaki, T
Irino, KI, and RT would like to thank J-DESC for financial aid supporting the
IODP shipboard scientists. TS thanks Yusuke Okazaki for financial support,
Reika Nakamura and Kiyoka Fujino for sample preparation, and Takuya
Matsuzaki and technical staff at the Kochi Core Center for their assistance withisotope measurements. This study was performed under the cooperative
research program of the Center for Advanced Marine Core Research, Kochi
University (14B045, 15A024, 15B021). SJG acknowledges the support of the
ANZIC IODP office and the ARC Basin Genesis Hub.
Funding
This work was partly supported by the IODP Exp. 346 After Cruise Research
Program, JAMSTEC and by “Collaborative Study of Long-term Changes in the
Asian Monsoon” of Program for Advancing Strategic International Networks
to Accelerate the Circulation of Talented Researchers, Japan Society for the
Promotion of Science. AH gratefully acknowledges institutional funding fromChristian-Albrechts-University.
Authors’ contributions
TS, T Irino, and RT proposed the topics. YN, YS, and KI conducted tephra
analyses and correlation. TS and AH conducted stable isotope
measurements. T Itaki, SJG, and MSP conducted microfossil assemblage
analyses. All authors contributed to the writing of the manuscript and read,
discussed, and approved the final version of the manuscript.
Competing interests
The authors declare that they have no competing interests.

Publisher’s NoteSpringer Nature remains neutral with regard to jurisdictional claims inpublished maps and institutional affiliations.
Author details
1
Institute of Science and Engineering, Kanazawa University, Kakuma-machi,
Kanazawa, Ishikawa 920-1192, Japan. 2Faculty of Symbiotic Systems Science,
Fukushima University, 1 Kanayagawa, Fukushima, Fukushima 960-1296, Japan.
3
Lake Biwa Museum, 1091 Oroshimo, Kusatsu, Shiga 525-0001, Japan.
4
Institute of Geosciences, Christian-Albrechts-University, Ludewig-Meyn-Str.
10, 24118 Kiel, Germany. 5Geological Survey of Japan, National Institute ofAdvanced Industrial Science and Technology, 1-1-1 Higashi, Tsukuba, Ibaraki
305-8567, Japan. 6School of Earth Sciences, The University of Melbourne,
Melbourne, Victoria 3010, Australia. 7Department of Geosciences, FB5,
University of Bremen, Klagenfurter Strasse, 28359 Bremen, Germany. 8Facultyof Environmental Earth Science, Hokkaido University, N10W5, Sapporo,
Hokkaido 060-0810, Japan. 9Department of Earth and Planetary Science, The
University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.
Received: 30 August 2017 Accepted: 8 January 2018

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