Kurokawa et al. Progress in Earth and Planetary Science
https://doi.org/10.1186/s40645-018-0250-1(2019) 6:2

Progress in Earth andPlanetary Science

RESEARCH ARTICLE

Open AccessCyclostratigraphy of the Late Miocene to
Pliocene sediments at IODP sites U1425
and U1430 in the Japan Sea andpaleoceanographic implications
Shunsuke Kurokawa1, Ryuji Tada1, Kenji M. Matsuzaki1,2*, Tomohisa Irino3 and Lofi Johanna4

Abstract
High-resolution studies of sedimentary sequences have reconstructed paleoceanographic changes in the Atlantic
and southern and equatorial Pacific Oceans during the Late Miocene and Pliocene, but comparable analyses arelacking for the North Pacific Ocean. However, continuous samples of hemipelagic sequences covering this timeinterval were obtained at Integrated Ocean Drilling Program sites U1425 and U1430 in the Japan Sea during
expedition 346. Because the paleoceanography of the Japan Sea was sensitive to glacio-eustatic sea-level changes,
changes in the sediment record are manifested as cyclic lithological changes. By using a gamma ray attenuation(GRA) density, which reflects biogenic silica content, as an indicator of sea-level changes, we developed an
independent orbitally tuned chronology at each site to construct high-resolution, high-precision age models for thetime span of 11.8 to 1.45 Ma. First, the 405-kyr-filtered GRA profile at each site was tuned to 405-kyr-filtered orbital
eccentricity. Then, using the 405-kyr-tuned age model, the 100-kyr-filtered GRA profile was tuned to shorteccentricity cycles to adjust the 405-kyr-tuned age models. We used the resulting age models to revise the timeintervals of previously published lithological units and paleoceanographic stages for the Japan Sea. Our profiles
based on physical properties are a good match to the oxygen isotope record, except for 6.6–3.6 Ma, a time intervalwith small-amplitude fluctuations in the oxygen isotope record.
Keywords: IODP, Japan Sea, U1425, U1430, Pliocene, Late Miocene, Orbital tuning, Sea level

Introduction
The Late Miocene to Pliocene time encompassed the expansion and stabilization of the East Antarctic ice sheet,the onset of northern hemisphere glaciation and the intensification of the Asian Monsoon system (e.g., Zachos
et al. 2001; De Vleeschouwer et al. 2017, Tada et al.
2016; Betzler et al. 2018; Clift 2017; Holbourn et al.
2018). Paleoceanographic changes during this period
have been reconstructed at high resolution on the basisof deep-sea sediments mainly from the Atlantic andsouthern and equatorial Pacific oceans (e.g., Bell et al.
* Correspondence: km.matsuzaki@aori.u-tokyo.ac.jp
1
Department of Earth and Planetary Science, Graduate School of Science,
The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
2
Present Address Atmosphere and Ocean Research Institute, The Universityof Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8564, Japan
Full list of author information is available at the end of the article

2014, 2015; Westerhold et al. 2005; Holbourn et al.
2005, 2007, 2013a, 2013b, 2018; Drury et al. 2016). This
research has shown that changes in Miocene and Pliocene climate and oceanography were closely associatedwith orbital-scale changes in ice volume and the carbon
cycle (Holbourn et al. 2013a, 2013b, 2018; Drury et al.
2016). However, there are few continuous, long-term
sedimentary sequences of this age suitable forhigh-resolution analyses from the northwest Pacific
Ocean because the calcium carbonate compensation
depth was shallow in the area at that time (e.g., Berger
1967; Berger et al. 1982). For that reason, paleoceanographic conditions of the Pacific Ocean during thisperiod are not well understood.
The Japan Sea has been a semi-enclosed marginal sea
since at least the Early Miocene (Iijima and Tada 1990).
Its connection to the Pacific Ocean through relatively© The Author(s). 2019 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.

(2019) 6:2

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shallow straits has made the Japan Sea sensitive toglacio-eustatic sea-level changes, which are manifested
as cyclic lithological changes in its sedimentary records(Tada 1994). Therefore, it appears feasible to construct ahigh-resolution and high-precision age model by the useof cyclostratigraphy based on lithological variations, as
inferred from gamma ray data, in hemipelagic sedimentsfrom the Japan Sea.
In this study, we constructed an orbitally tuned age
model for the hemipelagic sediments of the Japan Sea
during the period 11.8–1.45 Ma, reconstructed temporalchanges in the physical properties of cores, and compared them with global and regional benthic oxygen isotope records. We utilized core data from Integrated
Ocean Drilling Program (IODP) sites U1425 and U1430
in the central and south-central Japan Sea, recovered
during expedition 346 (Tada et al. 2015b, 2015c). We
used a revised version of the stratigraphic record derived
from the cores at each site (Irino et al. 2018) combinedwith both shipboard and newly published biostratigraphic data (Kamikuri et al. 2017) to constructhigh-resolution, high-precision age models at the twosites.
Physical properties measured at high resolution during
the cruise were utilized to correlate the sediment records
between holes and between coring sites. These physicalproperties were also used to support orbital tuning because sedimentary cycles were clearly evident in thehigh-resolution record (e.g., Lourens et al. 2005; Tian etal. 2008).

Kurokawa et al. Progress in Earth and Planetary Science

Oceanographic and geological setting

The Japan Sea, with an area exceeding 106 km2 and amean depth of 1667 m, is connected to neighboring seas
and the Pacific Ocean through narrow and shallow
straits (Fig. 1). The Tsushima Warm Current (TWC),
the only current flowing into the sea, controls its
oceanographic conditions (Gamo et al. 2014). The TWC

Fig. 1 A bathymetric map of the Japan Sea modified from Tada et al. (2015b, 2015c). Red circles show the locations of the studied sites U1425
and U1430 drilled during IODP exp. 346 and white circles indicate the drilling sites during ODP leg. 127/128. Warm currents flowed into theJapan Sea are also shown as red arrows. FBTWC, SBTWC, and TBTWC indicate the first, second, and third branch of the Tsushima warm current(TWC), respectively

(2019) 6:2

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splits into three branches as it enters the Japan Sea(Hase et al. 1999). The first branch originates in the eastern channel of Tsushima Strait and flows along the west
coast of Honshu Island. The second branch originates in
the western channel of Tsushima Strait and flows initially along the southern Korean Peninsula, then along
the shelf edge to the west of and generally parallel to thefirst branch. However, this branch is also strongly influenced by eddies generated by baroclinic instability (Haseet al. 1999). The third branch originates in the western
channel of Tshushima Strait, flows northward along the
Korean Peninsula, and then turns to the east around 36°
N and forms the Subpolar Front (Kim and Yoon 1999).
In the northwestern Japan Sea, cooling and freezing
caused by the winter monsoon increases the subsurface
water density (Talley et al. 2003), which forms the deep
Japan Sea Proper Water, characterized by its high dissolved oxygen content (Gamo et al. 1986). However,oceanographic conditions have been significantly different in the past, and the sediments of the Japan Sea
give evidence of the basin’s strong sensitivity to regional and global climatic and oceanographic changes(Tada 1994).
The Japan Sea first opened approximately 28 Ma, andthe deposition of hemipelagic sediments began around
20 Ma (Tamaki et al. 1992; Tada 1994). In their revised
lithostratigraphy of Japan Sea hemipelagic sediments obtained during Ocean Drilling Program (ODP) leg 127,
Tada and Iijima (1992) divided the middle Miocene to
Holocene sedimentary sequence into six lithologicalunits. Of these, unit 4 (15.5–10.5 Ma) is composed ofgray, highly siliceous rocks and has two subdivisions.
Subunit 4B (15.5–14 Ma) is composed of claystone withoccasional parallel laminations and flattened horizontal
burrows, and subunit 4A (14–10.5 Ma) consists of alternating decimeter-scale layers of dark chert and lightclaystone, the claystone being more strongly bioturbatedthan the chert. Unit 3 (10.5–6 Ma) is characterized by alternating decimeter-scale layers of non-bioturbated dark
diatomaceous silty clay and bioturbated light silty clay.
Unit 2 (6–2.5 Ma) is composed of heavily bioturbated,homogeneous greenish diatom ooze and diatomaceous
clay. Unit 1 (2.5–0 Ma) is composed of alternating centimeter- to meter-scale layers of dark and light clay tosilty clay that are biosiliceous or biocalcareous to various
degrees (Tada and Iijima 1992; Tada 1994).
Tada (1994) reconstructed the circulation of thepaleo-Japan Sea from oceanographic information (surface and bottom water conditions, calcium carbonate
compensation depth, surface productivity, channel position, and sill depth) and divided the evolution of theJapan Sea into five stages corresponding to the deposition of the five uppermost lithological units of Tada andIijima (1992). During stages 2 and 3 (15.5–6.5 Ma),

which correspond to the deposition of units 4 and 3, respectively, the surface water was predominantly cool andthe deep water fluctuated between suboxic and anoxic(Tada 1991, 1994). Tada (1991) interpreted cyclic
meter-scale lithological changes and apparently annual
parallel laminations in the Onnagawa Formation innortheastern Honshu (equivalent to subunit 4A) toargue that terrigenous input fluctuated periodically due
to orbitally driven sea-level oscillations. During stage 4(6.5–2.5 Ma), corresponding to the deposition of unit 2,
cold and oxic surface water flowed into the Japan Sea
from the north and formed deep water. The consequent
upwelling resulted in higher productivity in the surface
water and greater deposition of biogenic silica. Stage 5(2.5–0 Ma), corresponding to the deposition of unit 1,
was characterized by large and frequent oscillations ofoceanographic conditions. During interglacial sea-levelhighstands, the TWC was strong and resulted in the
production of deep water within the Japan Sea, such thatthe bottom water became hyperoxic. During glacial lowstands, the Japan Sea became isolated and the TWC was
restricted, leading to density stratification beneath the
low-salinity surface water and the development of euxinic bottom water (Tada 1994).

Kurokawa et al. Progress in Earth and Planetary Science

Methods/Experimental
Sites and materials

IODP site U1425 (39°29.44′ N, 134°26.55′ E, water
depth 1909 m) is on a terrace in a northeast-southwest-oriented graben in the middle of Yamato Bank, and site
U1430 (37°54.16′ N, 131°32.25′ E, water depth 1072 m)
is about 300 km to the southwest on the southern upper
slope of the eastern South Korean Plateau (Tada et al.
2015a, 2015c) (Fig. 1). The location of site U1425 is now
under the influence of the third branch of the TWC at
the Subpolar Front, and site U1430 is slightly south of
the Subpolar Front (Hase et al. 1999). Miocene sediments were recovered at these two sites during expedition 346 (Tada et al. 2015b, 2015c).
The sedimentary sequence at site U1425 covers thetime interval from the Late Miocene to the Holocene(Fig. 2a) without any obvious hiatus and is dominated byclay, silty clay, diatomaceous ooze, and siliceous claystone (Tada et al. 2015b). The sequence has been divided
into three lithological units (units I to III) on the basisof sediment composition, in particular, the abundance ofbiosiliceous material (Tada et al. 2015a). At site U1430,
the sedimentary sequence extends from the middle Miocene to the Holocene (Fig. 2b) and has been divided intolithological units I to IV (Tada et al. 2015c). Between 7
Ma and approximately 4.5–5 Ma, sedimentation was extremely slow or absent at this site (Tada et al. 2015c;
Kamikuri et al. 2017).

Kurokawa et al. Progress in Earth and Planetary Science(2019) 6:2

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Fig. 2 Age-depth plot of sites U1425 and U1430. Biostratigraphic age control points are shown as symbols. Symbols with parentheses arebiostratigraphic datums that are considered as less reliable. Detail explanation is given in the text. 405- and 100-kyr correlation points arealso shown (red and blue circles). a U1425, b U1430

MethodCorrelation with logging data

Continuous sedimentary records are required to apply
cyclostratigraphy to sedimentary sequences. IODP expedition 346 drilled two or more holes at sites U1425

and U1430, from which a continuous Miocene–Pliocene
stratigraphic record, or splice, was assembled for eachsite. During the expedition, the cores at each site werestratigraphically correlated onboard using physical properties such as natural gamma radiation (NGR), gamma(2019) 6:2

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ray attenuation (GRA), and color reflectance (Tada et al.
2015b, 2015c). On the shore, core photographs were examined to identify small faults and disturbances that
could not have been identified in the physical property
profiles. The initial splice was revised on the basis ofseveral new tie points within core intervals corresponding to lithological units I and III to correct miscorrelations and avoid intervals containing sponge plugs andother discontinuities (detailed in Irino et al. 2018). This
study adopted the revised splices, in which the updated
core composite depth below sea floor (CCSF-D) is referred to as the revised CCSF-D in this paper, and allcore depths referred to hereafter are revised CCSF-D
depths. The revised CCSF-D for site U1425 is CCSF-D
patched rev20170309 in Irino et al. (2018), and the revised CCSF-D for site U1430 is CCSF-D patched
rev20160113 in Irino et al. (2018). To check the validityof these revised splices, we correlated the revised composite NGR data with the hostile standard gamma ray(HSGR) logging data from sites U1425 and U1430.
Downhole logs are unaffected by core expansion because
they were acquired in situ, and their records are perfectly continuous with depth. For this reason, core-log
integration helps in identifying miscorrelations that may
be present (e.g., deMenocal et al. 1992; Lofi et al. 2016).

In our comparison of the revised NGR splices andthe downhole HSGR logs (acquired in hole B at eachsite), we found strong correlations at the meter scale(Fig. 3). This confirms the validity of the revised
splice as there were no significant core gaps deeper
than ~ 80 m at sites U1425 and U1430. Correlation
points between NGR and HSGR at sites U1425 andU1430 are listed in Tables 1 and 2. Correlation points
occur every ~ 5 m in the record from both sites. This
resolution ensured that no orbital eccentricity cycles
were missing.
It should be noted that gamma ray attenuation by thesteel hole casing precluded such a comparison in theupper ~ 80 m at both sites. However, three holes were
drilled in this depth range at both sites, such that the revised splices were considered reliable despite the lack of
core-log integration.

Kurokawa et al. Progress in Earth and Planetary Science

Chronology

To produce an orbitally tuned age model, we used the
shipboard biostratigraphic data described in Tada et al.
(2015b, 2015c) for sites U1425 and U1430 and additional
radiolarian datums from Kamikuri et al. (2017).
The biostratigraphic age model for site U1425 extended from 9.3 Ma, the oldest age constraint within theFig. 3 Correlation between compoSite NGR (black) and HSGR (blue) by downhole logging. Orange lines show the correlation points. Core-log
correlation for site U1425: a 110–140 m, b 200–230 m, and c 310–340 m revised CCSF-D. Core-log correlation for site U1430: d 100–130 m,
e 140–170 m, and f 210–240 m revised CCSF-D. All correlation points at both sites are listed in Tables 1 and 2

Kurokawa et al. Progress in Earth and Planetary Science(2019) 6:2

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Table 1 Correlation points between revised composite core depth below seafloor (revised CCSF-D) and wireline log matched depth
below sea floor (WMSF) at site U1425
Site U1425
Hole

Core

Type

Sect

Depth from top of the section (cm)Depth CSF-A (m)Depth revised CCSF-D (m)Depth WMSF (m)

D

12

H

3

30

102.6

108.285103.024

D

13

H

2

70

106.2

111.849

107.2352

B

14

H

2

110

108.6

113.872

109.3408

B

14

H

4

130

111.83117.102110.8448

B

14

H

6

50

114.03

119.302

112.8

D

14

H

2

90

116.1

122.168

114.6048

B

17

H

2

150

120.8

125.111116.7104

D

15

H

4

30

123.2

128.43120.9216

D

15

H

5

130

125.7

130.930

125.4336

D

16

H

1

110

129

135.421129.344

D

16

H

4

30

132.72139.141132.6528

D

16

H

6

90

136.32142.741135.9616

B

21

H

4

30

141.4

146.968140.9248

B

21

H

6

50

144.6

150.168

144.384

D

18

H

5

70

146.1

152.474

146.1888

B

22

H

3

50

149.6

156.228

149.1968

B

22

H

5

70

152.8

159.428

152.656

D

19

H

4

130

154.7

162.399154.912

B

23

H

3

90

159.5

166.468

158.672

B

23

H

6

50

163.6

170.568

162.1312

D

20

H

5

110

165.5

173.492

164.8384

B

24

H

3

90

169

176.800

169.0496

D

21

H

3

50

171.4

179.491

171.1552

D

21

H

6

10

175.5

183.591

175.5168

B

25

H

4

90

180

187.840

179.1264

D

22

H

4

130

183.2

192.039183.1872

B

26

H

2

110

186.7

196.105

187.0976

D

23

H

3

50

190.4

199.085190.4064

D

23

H

6

90

195.3

203.985

195.2192

B

27

H

4

50

198.6

207.082

198.3776

B

27

H

5

110

200.7

209.182

200.4832

D

24

H

4

90

201.8

211.196202.1376

B

28

H

2

110

205.7

214.978

205.4464

B

28

H

3

90

207

216.278

206.1984

D

25

H

4

50

210.6

220.883

211.0112

B

29

H

3

130

215.9

225.473215.3728

B

29

H

6

70

219.8

229.373219.1328

B

30

H

3

30

224.4

234.212223.7952

B

30

H

5

30

227.4

237.212226.9536

D

28

H

5

30

230.9

240.128

229.5104

B

32

H

2

70

232.8

243.086

231.1648

D

29

H

2

110

236.7

246.37235.5264

Kurokawa et al. Progress in Earth and Planetary Science(2019) 6:2

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Table 1 Correlation points between revised composite core depth below seafloor (revised CCSF-D) and wireline log matched depth
below sea floor (WMSF) at site U1425 (Continued)
Site U1425
Hole

Core

Type

Sect

Depth from top of the section (cm)Depth CSF-A (m)Depth revised CCSF-D (m)Depth WMSF (m)

B

33

H

3

130

238.6

249.122237.7824

B

33

H

5

90

241.2

251.722240.4896

D

30

H

5

50

245.3

255.969245.7536

B

34

H

4

30

248.6

259.059248.4608

B

34

H

5

110

250.9

261.359

250.8672

D

31

H

4

10

252.9

264.151253.5744

B

35

H

3

70

257

267.448256.5824

B

35

H

5

110

260.4

270.848259.2896

D

32

H

3

130

262.1

273.966

262.2976

D

32

H

6

70

266

277.866

266.0576

B

37

H

2

110

270.1

281.540

269.968

B

38

H

1

70

272.9

283.901

272.8256

D

35

H

2

110

276.7

288.839277.0368

B

39

H

4

30

281.33292.896

280.0448

D

37

H

1

50

284

297.418

284.7072

D

37

H

4

150

288.49301.908

288.016

D

37

H

7

50

291.17304.588

290.1216

D

38

H

2

110

294.6

308.184294.3328

D

39

H

2

90

299.1

313.181297.9424

B

44

H

3

110

303.1

317.19303.9584

D

40

H

3

110

305.5

321.36307.1168

D

41

H

3

10

309.2

324.355

310.7264

B

47

H

1

130

314.4

327.223

313.8848

B

47

H

3

70

316.8

329.623317.0432

D

43

H

3

30

318.8

333.363318.9984

D

43

H

5

110

322.6

337.163322.3072

B

49

H

3

30

325.8

340.342325.3152

splice, to 1.45 Ma, the end of the orbitally tuned cyclostratigraphic age model for site U1425 (Tada et al. 2018).
All biostratigraphic datums used are plotted in Fig. 2and listed in Additional file 1. The following datums
were discarded. The last occurrence (LO) and first occurrence (FO) of the planktonic foraminifer Globorotalia ikebei were determined by its rare occurrence in
only one sample, and its presence was attributed toreworking (Tada et al. 2015b). The LO of the diatom
Thalassiosira schraderi had a relatively large age uncertainty and was not used (Tada et al. 2015b). The LO of
the radiolarian Hexacontium parviakitaensis in theJapan Sea was estimated as 2.87–2.90 Ma by Kamikuri
et al. (2017), who placed this datum at 86.99 m, whereas
the trend from our estimated sedimentation rates suggested that the LO of H. parviakitaensis should be at
81.01 m, and we could not rule out reworking. Finally,

the FO of the radiolarian Cycladophora sphaeris(=Cycladophora sakaii in Kamikuri et al. 2004, 2007)
has age uncertainties of ~ 0.5 Myr between the eastern
North Pacific, western North Pacific, and the Bering
Sea (Kamikuri et al. 2004, 2007).
The biostratigraphic age model for site U1430 extended from 11.8 to 1.45 Ma (Fig. 2b). The LO of Hexacontium parviakitaensis was excluded for consistency
with site U1425. The linear sedimentation rates (LSR)
for the stratigraphic interval corresponding to 5 to ~1.45 Ma were too low (~ 1 cm/kyr) to detect meaningful
orbital-scale changes in GRA, and the radiolarian datums and the occurrence of a glauconite layer suggest ahiatus during the time interval from 7 to 5 Ma (Kamikuri et al. 2017; Matsuzaki et al. 2018; Tada et al. 2015c).
For this reason, we conducted an orbital tuning only for11.8 to 7.3 Ma.

Kurokawa et al. Progress in Earth and Planetary Science(2019) 6:2

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Table 2 Correlation points between revised composite core depth below seafloor (revised CCSF-D) and wireline log matched depth
below sea floor (WMSF) at site U1430
Site U1430
Hole

Core

Type

Sect

Depth from top of the section (cm)Depth CSF-A (m)Depth revised CCSF-D (m)Depth WMSF (m)

A

11

H

4

10

93.7

98.93378

94.1832

B

11

H

3

30

97.1

102.0074

96.6216

B

11

H

5

70

100.5

105.4074

100.1268

A

12

H

3

70

102.3

108.2382

102.87

B

12

H

2

70

105.5

110.9596

105.6132

B

12

H

4

10

107.9

113.3596

107.442

A

13

H

2

90

110.5

117.2062

111.4044

B

13

H

1

110

113.9

120.0207

114.1476

B

13

H

4

70

118

124.1207

117.8052

B

13

H

6

30

120.6

126.7207120.396

A

14

H

4

50

122.6

129.7123

122.9868

B

14

H

3

70

126

132.7575

125.73

B

14

H

6

30

130.1

136.8575

129.9972

A

15

H

4

70

132.3

139.8281

132.8928

B

15

H

2

130

134.6

141.9267

134.5692

B

15

H

5

50

138.3

145.6267

137.922

A

16

H

3

50

140.1

148.4069

140.8176

B

16

H

3

50

144.8

152.5529

144.78

B

16

H

5

50

147.8

155.5529

147.828

A

17

H

3

110

150.2

159.1428

150.7236

B

17

H

2

10

152.4

160.4806

152.0952

B

17

H

3

130

155.1

163.1806

154.8384

B

17

H

6

10

158.4

166.4806

157.5816

A

18

H

3

110

159.7

168.7812

159.8676

B

18

H

3

30

163.6

172.4537

163.2204

B

18

H

4

70

165.5

174.3537

164.7444

B

18

H

6

10

167.9

176.7537

167.3352

A

19

H

4

10

169.7

179.3117

169.6212

B

19

H

3

90

173.7

182.7136

173.8884

B

19

H

5

50

176.3

185.3136

176.022

A

20

H

2

110

177.2

187.4457

177.8508

B

20

H

1

70

180

189.7139

180.1368

B

20

H

3

110

183.4

193.1139

183.1848

B

20

H

5

50

185.8

195.5139

185.4708

A

21

H

4

10

188.1

199.0353

188.8236

B

21

H

2

50

190.8

202.5076

190.6524

B

21

H

4

10

193.4

205.1076

193.0908

A

22

H

3

150

197.5

209.5986

197.358

A

22

H

6

30

200.8

212.8986

201.168

B

22

H

4

90

203.7

215.3605

203.3016

B

22

H

6

130

207.1

218.7605

206.6544

A

23

H

5

30

208.5

220.9178

208.9404

Kurokawa et al. Progress in Earth and Planetary Science(2019) 6:2

Page 9 of 20

Table 2 Correlation points between revised composite core depth below seafloor (revised CCSF-D) and wireline log matched depth
below sea floor (WMSF) at site U1430 (Continued)
Site U1430
Hole

Core

Type

Sect

Depth from top of the section (cm)Depth CSF-A (m)Depth revised CCSF-D (m)Depth WMSF (m)

B

23

H

3

70

211.5

223.5968

211.3788

B

23

H

5

90

214.7

226.7968

214.4268

A

24

H

4

10

216.3

229.1107216.7128

B

24

H

3

130

221.6

233.4764

221.1324

B

24

H

5

130

224.6

236.4764

224.3328

A

25

H

4

50

226.2

238.7943

226.0092

A

25

H

5

130

228.5

241.0943

228.6

B

25

H

4

110

232.4

244.2598

231.8004

A

26

H

2

50

232.7

246.2652

233.7816

B

26

H

3

70

238.3

250.4556

237.8964

Cyclostratigraphy

Quaternary sediments in the Japan Sea contain alternating dark- and light-colored layers that were synchronously deposited on the millennial scale, providing a toolfor high-resolution correlation (Tada et al. 2018). An age
model for the upper part of unit I (1.45–0 Ma) wasestablished at site U1424 by Tada et al. (2018) by correlating the onboard GRA record of Tada et al. (2015b,
2015c) (regarded as a proxy of sea level) with the LR04
oxygen isotope (δ18O) sequence of Lisiecki and Raymo(2005), which allows ages of each of the dark layers atsite U1424 to be extended to other sites. The orbitally
tuned age model proposed here thus applies to the sedimentary sequence older than 1.45 Ma.
The Japan Sea sediments are composed of biosiliceous
clay to silty clay (Tada et al. 2015b, 2015c). In biosiliceous sediments, GRA reflects the ratio of detritus to diatoms and decreases as diatom content increases because
diatom frustules have high internal porosity (Tada andIijima 1983). Diatom abundance is strongly linked to sea
level because the surface productivity of the Japan Sea iscontrolled by the influx of nutrients through Tsushima
Strait, which in turn has been modulated by glacio-eustatic sea-level changes (Tada et al. 2018). GRA likewise
increases as the input of detritus increases. During
sea-level lowstands, for instance, more terrigenous sediment should reach the study sites in the Japan Sea as the
exposed area of the paleo-Japanese islands increases andthe distance from the shoreline decreases, and the consequent increase in the mass accumulation rate of detritus (Tada 1991) results in higher GRA values. Thus,
GRA can be used as a sea-level proxy.
To use GRA as a proxy, noise was first removed fromthe composite onboard GRA data (Tada et al. 2015b,
2015c). GRA values below the specific gravity of seawater (1.02 g cm− 3) were discarded, as were data corresponding to tephra and cracked intervals. The filtered

GRA profiles for sites U1425 and U1430 are presented
in Additional file 2. We next performed a spectral analysis of the filtered GRA data to assess their fit to ourtuned biostratigraphic age model using REDFIT software(Schulz and Mudelsee 2002). We generated Gaussian-filtered GRA time-series for both sites and assessed theircorrelation with temporal variations of Earth’s orbital parameters, including long-eccentricity cycles (405 kyr)and short-eccentricity cycles (100 kyr) calculated by Laskar et al. (2004). For the Gaussian band-pass filtering,
we used AnalySeries 2.0.8 software (Paillard et al. 1996)
to sample evenly spaced data points from unevenly
spaced original data. Band-pass filters were centered at
0.0025 kyr−1 (0.0005 kyr−1 bandwidth) and at 0.01 kyr−1(0.003 kyr−1 bandwidth) to extract 405- and 100-kyr cycles, respectively. Tuning assumed no phase lag between
GRA variability and orbital parameters, because the response time of an ice sheet is not known in this timeinterval (Holbourn et al. 2007).
To further improve our orbitally tuned age model, we
tuned the GRA data from site U1425 to the oxygen isotope record, which is sensitive to sea-level fluctuation.
The site U1425 record was chosen for its continuity andfor its wide range of variability, which aided the tuning
process. We tuned these data to the LR04 oxygen isotope record of Lisiecki and Raymo (2005) for 5.3–1.4 Ma
and the benthic oxygen isotope record of Holbourn etal. (2018) from ODP site 1146 in the South China Sea
for 10–5.2 Ma.

Results: orbital tuning
A critical part of orbital tuning is the choice of the initial
tie point. For site U1425, we used the dark layer no. 21–
2 projection tie points of Tada et al. (2018), which are at
1.45 Ma at 54.79-m-core depth. These have an error
margin of about 4 kyr, inherited from the LR04 age
model used as a tuning target. This error is negligible(2019) 6:2

Page 10 of 20

for our purposes, and for site U1425, we assumed thatthe proposed depth-age model has no lag. It was moredifficult to define the age model for site U1430 becauseof the presence of the hiatus. Following Kamikuri et al.
(2017), we assumed that the hiatus ended at the FO of
the radiolarian Lychnocanoma parallelipes, at 80.057 m(Irino et al. 2018). The FO of L. parallelipes is 7.2–7.4
Ma (Kamikuri et al. 2017); therefore, we assumed that
the FO of this species is at 7.3 ± 0.1 Ma. This implies
that the proposed depth-age model for site U1430 has
an error margin of 0.1 Myr.
The results of our spectral analysis show that the GRA
data at both sites have a characteristic cyclicity that canbe correlated with orbital cycles. At site U1425, peaks
around 0.0025 and 0.01 that are significant above the
80% confidence level correspond to the long andshort-eccentricity cycles, respectively, and peaks around0.024 may be attributable to obliquity cycles (Fig. 4a). At
site U1430, the power spectrum also shows a clear peak
at 0.0025, a peak at 0.0057 that corresponds to the long
obliquity cycle (174 kyr), and peaks around 0.024 that
correspond to the 41-kyr obliquity cycle, although ashort-eccentricity peak is absent (Fig. 4b). In sum, theGRA data displayed orbital signals at both sites, which
we used to construct orbitally tuned age models.
We constructed an age model for each site in two
steps. First, 405-kyr-filtered GRA profiles from our biostratigraphic age model were tuned to the long (405 kyr)eccentricity cycle. Because this cycle has a stable periodicity over the last 250 Myr, it is well suited for cyclostratigraphy (Hinnov 2000; Laskar et al. 2004; Hinnov and
Hilgen 2012). Second, we tuned the resulting GRA profiles to the short-eccentricity cycle by correlating eccentricity minima with 100-kyr-filtered GRA maxima under
the assumption that sea-level lowstands occur during eccentricity minima that increase the influx of detritus to
the Japan Sea and thus raise GRA values, as explained
previously.
The 405-kyr eccentricity cycle is relatively weak inbenthic oxygen isotope records younger than about 5
Ma (Zachos et al. 2001; de Boer et al. 2014), althoughboth 405-kyr and 100-kyr eccentricity cycles are clear inbenthic carbon and oxygen isotope records from the
Oligocene and Miocene (Pälike et al. 2006; Liebrand etal. 2011, 2016, 2017; Holbourn et al. 2013a, 2013b; Beddow et al. 2016). Accordingly, we did not use the405-kyr eccentricity cycle as a tuning target after 5.0 Ma
at site U1425. The plot of 405-kyr-filtered GRA records
based on the biostratigraphic age model and thelong-eccentricity cycle from 9.3 to 5 Ma at site U1425
shows that the two records are nearly a half-wavelength
out of phase (Fig. 5a). We tuned the GRA maxima to
the nearest eccentricity minima to yield the result shownin Fig. 5b. There are 10 correlated cycles in this timeinterval. The mismatch in amplitudes between the GRA
signals and eccentricity from 6.2 to 7.5 Ma (Fig. 5b)means that amplitude modulation of GRA by thelong-eccentricity cycle is not established for the 9.3–5
Ma interval.
Using an age model based on the long-eccentricitytuned age model for 9.3–5 Ma and the biostratigraphic
model for 5–1.45 Ma, we conducted short-eccentricity
tuning to correlate the GRA record and the short-eccentricity (100 kyr) cycle. Both GRA and eccentricity were filtered with a 100-kyr filter using Gaussian band-passfiltering (see the “Method” section), and GRA maxima
were correlated with eccentricity minima (Fig. 6a and b).
The amplitudes of the two records were well matched except from 6.6 to 3.6 Ma, when the overall amplitude of
GRA was small regardless of eccentricity (Fig. 6b). The
depths and ages of the 405- and 100-kyr correlation points
for site U1425 are listed in Tables 3 and 4 and plotted inFig. 2a, which shows that orbitally tuned correlation
points are consistent with the biological datums.
We applied the same procedure to build an orbitally
tuned age model for the 11.8–7.3 Ma interval at site
U1430, first correlating the 405-kyr-filtered GRA maxima with long-eccentricity minima (Fig. 5c and d), thenrefining the age model by correlating peaks in the
100-kyr-filtered GRA and short-eccentricity records(Fig. 6c and d). The 405-kyr and 100-kyr correlation
points are listed in Tables 3 and 4 and plotted against
depth in Fig. 2b.
We checked the validity of the tuned results by comparing their power spectra to the power spectra of thebiostratigraphic age models (Fig. 4a and f ). At site
U1425, GRA peaks after short-eccentricity tuning became stronger at 405, 100, and 41 kyr and exceeded the
95% confidence level, although other peaks were littlechanged (Fig. 4c and e). At site U1430, cycles of long eccentricity and obliquity were stronger after orbital tuning (Fig. 4b and d). Thus, we are confident that our
two-step orbital tuning improved the accuracy of the age
models at both sites because the orbital signals of the
GRA power spectra were stronger across all orbital
peaks (Fig. 4). In addition, we tuned the GRA record to
the oxygen isotope record at site U1425 to check thevalidity of our assumption that GRA is sensitive to
sea-level changes and to check the validity of the orbital
tuning results. The results, given in Additional files 3, 4,
5, 6, 7, 8, 9, 10, 11, 12, and 13, indicate that our orbitally
tuned age model is consistent with the δ18O-tuned age
model with a lag no greater than 40 kyr, or less than halfof an eccentricity cycle.
Although the GRA power spectra before orbital tuning
had peaks attributable to obliquity and precession, we
did not conduct tuning with respect to obliquity or precession because the obliquity and precession cycles have

Kurokawa et al. Progress in Earth and Planetary ScienceKurokawa et al. Progress in Earth and Planetary Science(2019) 6:2

Page 11 of 20

Fig. 4 Spectral analysis results of the GRA time-series GRA. Gray shades indicate the orbital signals. l.E = long eccentricity, s.E = short eccentricity,
O = obliquity, P = precession. a U1425 biostratigraphy based age. b U1430 biostratigraphy based age. c U1425 long-eccentricity-tuned age. d
U1430 long-eccentricity-tuned age. e U1425 short-eccentricity-tuned age, f U1430 short-eccentricity-tuned age

relatively high uncertainties owing to effects of tidal dissipation and changes in dynamical ellipticity, and because GRA amplitudes were too small during some timeintervals.

Discussion
Comparison of physical properties with sea-level change

After adjusting our GRA records on the basis of ourshort-eccentricity-tuned age model, we compared them

to a previously published sea-level record. Miller et al.
(2005) compiled two high-resolution benthic oxygen isotope records covering the last 9 Myr from ODP site 846
in the equatorial Pacific (Shackleton et al. 1995) andODP site 982 in the northern North Atlantic (Hodell et
al. 2001) and converted them to a record of eustatic sea
level, using a calibration ratio of 0.1‰/10 m. In this record, sea-level fluctuations over glacial-interglacial cycles
were roughly 40 m before ~ 6 Ma, gradually increased toKurokawa et al. Progress in Earth and Planetary Science(2019) 6:2

Page 12 of 20

Fig. 5 Correlation between 405-kyr-filtered GRA and 405-kyr-filtered eccentricity. a, c The 405-kyr-filtered GRA (blue) based on biostratigraphybased age model at sites U1425 and U1430. Eccentricity is from Laskar et al. (2004) and band-pass filtered at 405-kyr (red) (negative values areuppermost). Biostratigraphic age control points (radiolarians) are shown as purple squares. b, d The 405-kyr-filtered GRA (blue) based on longeccentricity (405-kyr)-tuned age model at sites U1425 and U1430. The 405-kyr-filtered GRA maxima are tuned to 405-kyr-filtered eccentricity minima

about 100 m by 1.5 Ma, then remained at that amplitudeuntil the present (Miller et al. 2005).
For this comparison, we compiled the GRA dataset
from both sites, then resampled it and the sea-level record of Miller et al. (2005) at 100-kyr intervals between 9
and 1.45 Ma. The result (Fig. 7) shows a fair correlation(r = 0.63) between the two datasets for this period. Between 6.6 and 3.6 Ma, the GRA amplitudes were small
and the estimated sea levels were mostly restricted between 10 and − 20 m. These lower values of estimatedsea-level changes may be reflected in the low amplitudeof the GRA dataset, and the correlation between GRA
and estimated sea-level amplitude was slightly stronger
when we excluded the data from 6.6 to 3.6 Ma (r = 0.65).
DeVleeschouwer et al. (2017) compiled a high-resolution
benthic oxygen isotope record for the entire Neogene andQuaternary from multiple drill sites in the Atlantic, easternequatorial Pacific, and South China Sea. We used thisrecord to compare the variabilities of NGR and GRA atsites U1425 and U1430 with the oxygen isotope record ofbenthic foraminifers since 12 Ma. The GRA and NGR data
were well correlated with the benthic oxygen isotope record
before 8 Ma and after 3.3 Ma (Fig. 8), including the last
1.45 Ma (Tada et al. 2018); however, the correlation was unclear between 8 and 3.3 Ma, when both datasets hadorbital-scale variations of small amplitude.
In the benthic oxygen isotope record of Miller et al.
(1991), excursions Mi-5 to Mi-7 occurred during thetime interval covered by the Japan Sea sediments examined in this study. In their orbitally tuned age model
based on Fe signals in South Atlantic shallow marine
sediments, Westerhold et al. (2005) recognized the Mi-5
and Mi-6 events at 11.7 and 10.7–10.4 Ma, respectively.
At site U1430, we recognized GRA and NGR maxima
corresponding to these two events at 11.7 and 10.6 Ma,
respectively (Fig. 8). The Mi-6 event is also clearlyKurokawa et al. Progress in Earth and Planetary Science(2019) 6:2

Page 13 of 20

Fig. 6 Correlation between 100-kyr-filtered GRA and 100-kyr-filtered eccentricity. a, c The 100-kyr-filtered GRA (blue) based on long-eccentricitytuned age model at sites U1425 and U1430. Eccentricity is filtered at 100-kyr (red). b, d The 100-kyr-filtered GRA (blue) based on short-eccentricity(100-kyr)-tuned age model at sites U1425 and U1430. The 100-kyr-filtered GRA maxima are tuned to 100-kyr-filtered eccentricity minima

evident as a cooling step in the benthic oxygen isotope
record from ODP site 1146 in the South China Sea (Holbourn et al. 2013a). Although the age of the Mi-7 event
is uncertain because the oxygen isotope signal has manysmall maxima between 9.6 and 9.0 Ma (Andersson andJansen 2003; Westerhold et al. 2005), GRA and NGR
maxima in the Japan Sea sediments are compatible withtwo candidate ages proposed by Westerhold et al.
(2005), 9.4 and 8.7 Ma (Fig. 8). The close correspondence between physical properties and sea-level changes
such as the Mi events tends to confirm the accuracy ofshort-eccentricity-tuned age models. In addition, ourresults support the possibility that the amplitude ofsea-level changes can be estimated from GRA data independent of oxygen isotope values.
Correlation between sites U1425 and U1430

Figure 8 shows the temporal variations of NGR, GRA,
and LSR for the last 12 Myr at sites U1425 and U1430.
The sedimentary record at site U1425 extends back to
9.3 Ma without any stratigraphic gaps, whereas the sedimentary record at site U1430 from 11.8 to 0 Ma is interrupted by a probable hiatus from ~ 7.3 to 4.5–5 Ma.
Thus, comparisons of the two records are limited to theTable 3 405-kyr correlation points for sites U1425 and U1430
Site U1425

Site U1430

405-kyr tie point
Depth revised CCSF-D (m)Age (Ma)

Depth revised CCSF-D (m)Age (Ma)180.44

5.253

106.1497.685

196.925.661

120.4438.087

212.606.067

134.8818.494

228.62

6.469

147.9148.899

246.486.869

163.9999.303

261.72

7.272

181.4529.705

276.297.677

191.255

10.112292.758.085

200.029

10.524309.348.493

211.634

10.934325.728.901

223.129

11.337

Kurokawa et al. Progress in Earth and Planetary Science(2019) 6:2

Page 14 of 20

Table 4 100-kyr correlation points for sites U1425 and U1430
Site U1425

Site U1430

100-kyr tie point
Depth revised CCSF-D (m)Age (Ma)

Depth revised CCSF-D (m)Age (Ma)

55.9461.516

84.7907.391

59.2071.601

93.4187.483

62.382

1.686

100.3197.578

65.415

1.789

106.3787.649

69.0481.892

110.9047.737

72.2801.985

114.716

7.842

75.5132.071

117.860

7.948

78.2022.170

120.4338.045

80.4622.276

123.903

8.131

83.1802.381

127.088

8.229

85.9832.456

131.8898.336

89.0442.542

137.126

8.526

91.9332.639

140.3178.617

94.5652.730

143.508

8.722

97.7122.816

147.1748.914

102.718

3.001

152.2269.008

106.037

3.106

156.4419.110

108.8693.206

160.0699.214

111.472

3.293

163.7509.309

114.104

3.387

168.2659.389

116.4783.490

174.0249.566

118.8243.588

180.3659.651

121.8563.677

183.2059.737

125.318

3.777

185.4979.837

128.579

3.883

188.660

10.043131.297

3.984

191.300

10.129133.986

4.075

192.841

10.219143.123

4.273

195.09610.322

146.3134.377

197.265

10.424149.360

4.455

199.348

10.510153.0164.540

201.551

10.596155.8834.636

204.01610.701

159.360

4.728

206.292

10.806

162.550

4.815

209.250

10.903166.0274.900

212.137

10.992

169.6115.000

216.724

11.092173.052

5.106

219.361

11.197176.3145.207

222.113

11.298179.8625.292

227.669

11.368183.5655.381

234.373

11.465

187.5245.484

240.310

11.569191.8615.585

247.013

11.669Kurokawa et al. Progress in Earth and Planetary Science(2019) 6:2

Page 15 of 20

Table 4 100-kyr correlation points for sites U1425 and U1430 (Continued)
Site U1425

Site U1430

100-kyr tie point
Depth revised CCSF-D (m)Age (Ma)

Depth revised CCSF-D (m)Age (Ma)

197.275

5.666

253.81211.756201.0985.754

204.8505.859

208.7465.963

212.0646.059

215.100

6.151

219.194

6.252

222.8416.355

226.0796.450

229.7366.532

233.7836.628

241.000

6.728

245.840

6.821

249.5766.910

252.9657.008

256.108

7.113

258.615

7.213

261.3347.302

264.3437.389

267.5697.483

270.5597.578

274.020

7.649

277.2497.737

280.9437.842

284.6367.948

288.4438.045

292.5528.131

297.1818.229

303.1418.336

307.3938.441

311.5808.526

315.8568.617

319.5328.722

323.3218.823

327.455

8.914

333.3499.008

339.879

9.110

345.889

9.214

periods 9.3–7.3 and 5–1.5 Ma (Fig. 8). We therefore
compiled NGR, GRA, and LSR time-series using the age
model of Tada et al. (2018) for the uppermost part ofboth sites, biostratigraphic data for the 5.0–1.5 Ma

interval at site U1430, and our short-eccentricity-tuned
age models (Fig. 2).
In general, temporal changes in GRA and NGR
were well correlated between the two sites from 3.5

Kurokawa et al. Progress in Earth and Planetary Science(2019) 6:2

Fig. 7 Relationship between GRA and estimated sea level. GRA data
are from sites U1425 and U1430 and resampled every 100 kyr. Sea
level is converted from oxygen isotope values using a calibration
ratio of 0.1‰/10 m (Miller et al. 2005). Estimated sea level is also
evenly sampled every 100 kyr. Regression line is applied to the alldata during the last 9 Myr. Red circles are the data from 6.6 to 3.6 Ma

to 0 Ma. From 5.3 to 3.5 Ma, GRA and NGR values werehigher at site U1430 than at site U1425 (Fig. 8), a difference that can be explained by the fact that the sediment atsite U1430 is less diatomaceous or more tuffaceous thanat U1425 (Tada et al. 2015b, 2015c). From 9.3 to 7.3 Ma,Page 16 of 20

GRA and NGR curves at the two sites were similar. The
close correspondence in physical properties suggests that
paleoceanographic conditions were similar at sites U1425
and U1430.
During the hiatus interval at site U1430 from 7.3
to 5.3 Ma, there were no significant changes in physical properties and LSR at site U1425 (Fig. 8) or in
LSRs at ODP sites 794, 795, and 797 also located in
the Japan Sea (Tada 1994). The Late Miocene hiatuses have been reported from some on-land sections in northeastern Japan (Watanabe 1994), butthese correspond to the diatom datums 5D, 6A, and
6B (Akiba 1986; Yanagisawa and Akiba 1998; Tada etal. 2015a, 2015c), which range in age from 9.3 to
7.6 Ma. The hiatus at site U1430 coincides in depthwith a major regional unconformity on seismic profiles (MB3 unconformity in Horozal et al. 2017) that
extends over much of the eastern South Korea plateau where site U1430 was drilled. The origin of thisunconformity is unclear. Watanabe (1994) and Horozal et al. (2017) attributed it to erosion by bottom
currents or a brief episode of tectonic uplift. Whatever its origin, this hiatus must reflect a local,
basin-scale event because there are no drasticchanges in LSRs in this time interval in other JapanSea cores.

Fig. 8 Comparison of NGR, GRA, and LSR with composite benthic isotope curve from DeVleeschouwer et al. (2017) (purple). Vertical green lines
indicate good correlation between physical properties and oxygen isotope, while yellow lines are indicated that correlation is unclear. Marine
isotope stage number from Lisiecki and Raymo (2005) and Miller et al. (1991) is also shown in the top panel. Variations at site U1425 are shownas blue and U1430 as red. Lithological unit (Tada et al. 2015b, 2015c) and paleoceanographic condition stage number (Tada 1994) are also shownand that positions are revised in this study. Parallel lamination positions are shown as blue bars at site U1425 and red bars at site U1430

Kurokawa et al. Progress in Earth and Planetary Science(2019) 6:2

Revision of lithological divisions at sites U1425 and
U1430

Tada et al. (2015b, 2015c) defined a set of lithologicalunits in Japan Sea sediments, of which the latest four (Ito IV) and their subunits are represented in the sediments treated in this study. The revised GRA, NGR, andlogging profiles and the revised age model constructedin this study lead us to propose revisions of the boundaries of these units and subunits.
Tada et al. (2015b, 2015c) stated that unit I is characterized by decimeter-scale intervals of alternating lightand dark sediment and that the boundary between subunit IA and the underlying subunit IB is distinguishedby a downward decrease in the frequency of these alternations. They placed the base of subunit IB at 50.7 m atsite U1425 and at 46.9 m at Site U1430; however, thischange in frequency occurs deeper, at 54.8 m at site
U1425 and 50.7 m at site U1430, as shown by lithological observations as well as NGR and GRA data
(Fig. 8). The same discrepancy is also observed at other
IODP expedition 346 sites in the Japan Sea (Tada et al.
2018; Irino et al. 2018). The age of the sediment at theserevised boundary positions is 1.45 Ma (Tada et al. 2018).
The NGR and GRA profiles in subunit IA (Fig. 8) display large, suborbital-scale fluctuations after 1.45 Ma
that correspond to the color banding in the sediments.
The base of subunit IB is defined by a downward change
in sediment color and increase in diatom content (Tadaet al. 2015b, 2015c). The increase in diatom content corresponds to a clear decrease in NGR at 90.71 m at site
U1425, ~ 1.5 m shallower than the depth specified by
Tada et al. (2015b, 2015c), and 61.14 m at site U1430,
the same depth specified in the original definition. The
age of this boundary is 2.61 Ma according to our revised
age model (Fig. 8). During deposition of unit IB, NGR
and GRA fluctuated strongly, as they did in unit IA, but
at a lower frequency than in unit IA.
In unit II, the boundary between subunits IIA and IIB
is defined by a downward increase in diatom content,
which is recorded as a downward decrease in NGR or
GRA at 137.34 m at site U1425 (4.14 Ma in our age
model) and 74.42 m at site U1430 (4.04 Ma in our age
model). NGR data do not show a clear downward decrease at 74.42 m at site U1430, but this probably reflects the proximity of an NGR peak from a glauconite
layer (e.g., Tada et al. 2015c) at ~ 78.8–79.7 m. The ageof the boundary differs slightly between the two sites;
however, the age of the upper hiatus at site U1430 isbased on a biodatum, which has an error of about ±0.1 Myr. Accordingly, the boundary positions are consistent between the sites.
Tada et al. (2015b, 2015c) distinguished unit III fromunit II by its lower diatom content and darker sedimentcolor, which is reflected in higher values of NGR andPage 17 of 20

GRA (Fig. 8). The boundary between units II and III(specifically, subunit IIB and subunit IIIA) is at 263.25 m
at site U1425 and 84.81 m at site U1430, and its age is
7.36–7.39 Ma according to our age model (Fig. 8). Subunit IIIA is characterized by higher NGR and GRA
values, with fluctuations of greater amplitude, than unit
II reflecting the increased contrast between alternating
light and dark layers.
In summary, our analysis shows that the revised lower
boundaries of the lithological units of Tada et al. (2015b,
2015c) correspond to the following ages: 1.45 Ma forsubunit IA, 2.61 Ma for subunit IB, 4.04–4.14 Ma forsubunit IIA, and 7.36–7.39 Ma for subunit IIB.
Lithostratigraphic correlation of Japan Sea sites andrefinement of paleoceanographic stages

The criteria for defining lithological units were basicallythe same for ODP leg 127/128 and IODP expedition
346. According to Tada and Iijima (1992), unit 1 in ODP
leg 127/128 sediments is characterized by decimeter- tometer-scale alternations of dark and light silty clay toclay and is divided into subunits 1A and 1B on the basisof the frequency of dark layers and the intensity of bioturbation. Unit 2 is composed of clayey diatom ooze anddiatomaceous clay that is heavily bioturbated and mottled, with alternating dark and light layers and a generaldarkening of sediments near its base. This unit is not
subdivided. Unit 3 is characterized by decimeter- tometer-scale alternating light and dark layers, the latterbeing faintly laminated, and by a relatively high organiccarbon content.
Tada (1994) compiled the paleoceanographic data of
ODP leg 127/128 and paleogeographic data on the Japanese islands by Iijima and Tada (1990) to reconstruct the
paleoceanographic evolution of the Japan Sea. The proposed stages of this history correspond to the lithologicalunits of Tada and Iijima (1992). Given that correspondence, paleoceanographic stages 5, 4, and 3 of Tada
(1994) are shown in Fig. 8 as coeval with lithologicalunits I, II, and III, respectively.
At sites U1425 and U1430, the boundary between
units III and II is based on the increase in diatom content and decrease in terrigenous content and is assigned
an age of ~ 7.4 Ma. Tada (1994) described a dramatic increase in biogenic silica content around 8 Ma at the
ODP leg 127/128 sites, which is probably correlated withthis boundary. According to Tada (1994), during paleoceanographic stage 3, bottom water conditions fluctuated between anoxic and suboxic in association witheustatic sea-level changes. This history is consistent withthe sporadic occurrence of parallel laminations in lithological subunit IIIA at sites U1425 and U1430, whichsuggests the suppression of bioturbation due to low oxygen levels (Fig. 8). The difference in the ages of parallel(2019) 6:2

Page 18 of 20

laminations between the two sites may reflect the position of euxinic or anoxic water masses. The presence ofparallel laminations only at site U1425 in the upper partof unit IIIA signifies euxinic or anoxic conditions in thebottom water there and oxic or suboxic conditions atsite U1430, whereas the presence of parallel laminations
only at site U1430 in the lower part of unit IIIA (~ 9.0Ma) may indicate the development of an oxygen minimum zone within the Japan Sea, because site U1430 wasat the appropriate paleo-water depth. Another line ofevidence that an oxygen minimum zone developed or
expanded in the Japan Sea around 8 Ma is radiolarian
data indicating the presence of North Pacific typedeep-water species at that paleo-water depth (Matsuzaki
et al. 2018).
Paleoceanographic stage 4 (7.4–2.61 Ma) corresponds
to lithological unit II, which is characterized by relatively
low values of GRA and NGR at Site U1425. Tada (1994)
suggested that during this stage, terrigenous input was
diluted by diatoms due to higher productivity caused byenhanced production of deep water and consequent
upwelling.
During paleoceanographic stage 5 (2.61–0 Ma), NGR
and GRA fluctuated strongly at millennial to orbital
scales, interpreted as the result of millennial-scalechanges in the East Asian summer monsoon superimposed on glacio-eustatic sea-level changes (Tada et al.
2018). The deposition of lithological unit I is thought tohave been initiated by a drop in sea level caused by theonset of northern hemisphere glaciation (Lisiecki andRaymo 2005; Tada et al. 2018).

correlating the lithological units at sites U1425 andU1430 with those at the sites drilled during ODP leg
127/128, we were able to refine the timing of changes inpaleoceanographic conditions on the basis of ourshort-eccentricity-tuned age model. As a result, we revise the boundary ages of paleoceanographic stages proposed by Tada (1994), and their timings are consistent
with other records.

Kurokawa et al. Progress in Earth and Planetary Science

Conclusions
Long-term composite sedimentary records from IODP
sites U1425 and U1430 in the Japan Sea were interpretedusing cyclostratigraphy to establish high-resolution age
models for the time interval between 11.8 and 1.45 Ma.
We used GRA to measure detrital cycles in the sediments, then tuned the GRA profiles to long (405 kyr)and short (100 kyr) eccentricity cycles. In the resulting
orbitally tuned age model, NGR and GRA are strongly
correlated with the estimated sea-level changes since 9
Ma. Although the changes in physical properties andbenthic oxygen isotope ratios were relatively small before 3 Ma, they are shown here to coincide withlarge-scale sea-level changes such as Mi events. Using
the short-eccentricity-tuned age model, we compared
the physical properties and LSRs at sites U1425 andU1430 and found that the GRA and NGR profiles arestrongly correlated where they overlap, suggesting that
paleoceanographic conditions were similar at the twosites, the most notable difference being in paleoceanographic conditions during the deposition of subunit IIIA
as signified by the occurrence of parallel laminations. By

Additional files
Additional file 1: All biostratigraphic datums used. (XLSX 234 kb)
Additional file 2: Filtered GRA profiles for sites U1425 and U1430.
(XLSX 19 kb)
Additional file 3: GRA data tuned to LR04 oxygen isotopes records ofLisiecki and Raymo (2005) : 1.4-2.5 Ma. (JPG 2.54 mb)
Additional file 4: GRA data tuned to LR04 oxygen isotopes records ofLisiecki and Raymo (2005) : 2.4-3.5 Ma. (JPG 1.99 mb)
Additional file 5: GRA data tuned to LR04 oxygen isotopes records ofLisiecki and Raymo (2005) : 3.4-4.5 Ma. (JPG 1.97 mb)
Additional file 6: GRA data tuned to LR04 oxygen isotopes records ofLisiecki and Raymo (2005) : 4.4-5.5 Ma. (JPG 1.87 mb)
Additional file 7: GRA data tuned to oxygen isotopes records ofHolbourn et al. (2018) : 5.4-6.5 Ma. (JPG 1.58 mb)
Additional file 8: GRA data tuned to oxygen isotopes records ofHolbourn et al. (2018) : 6.4-7.5 Ma. (JPG 1.64 mb)
Additional file 9: GRA data tuned to oxygen isotopes records ofHolbourn et al. (2018) : 7.4-8.5 Ma. (JPG 2.53 mb)
Additional file 10: GRA data tuned to oxygen isotopes records ofHolbourn et al. (2018) : 8.4-9.3 Ma. (JPG 1.67 mb)
Additional file 11: Tie points obtained from oxygen isotopes tuning.
(XLSX 38 kb)
Additional file 12: Comparison of orbital tuning results with oxygenisotopes tuning results. (JPG 851 kb)
Additional file 13: 1.45 to 9.3 Ma Oxygen isotope tuned GRA and NGR
records. (JPG 2.32 kb)

Abbreviations
CCSF: Core composite depth below sea floor; FO: First occurrence;
GRA: Gamma ray attenuation; HSGR: Hostile standard gamma ray;
IODP: Integrated Ocean Drilling Program; LO: Last occurrence; LSR: Linear
sedimentation rate; NGR: Natural gamma radiation; ODP: Ocean Drilling
Program; TWC: Tsushima warm current; WMSF: Wireline log matched depth
sea floor
Acknowledgements
We thank the technicians and scientists of IODP Expedition 346 for theirefforts to acquire the shipboard data that were used in this work. We thankDr. Martin Ziegler and Dr. Andrew C.G. Henderson, whose suggestions helped
to greatly improve the manuscript. We also thank the Japanese shipboard
scientists of Expedition 346 for constructive suggestions and comments.
Funding
This work was supported by a grant to the IODP Expedition 346 After Cruise
Research Program, JAMSTEC, awarded to TR and IT, and by Japan Society for
the Promotion of Science (JSPS) KAKENHI Grant Number 16H01765 awarded
to TR. Work by MKM was partly financed by JSPS Research Fellowshipfor Young Scientist grant 16J02142.
Availability of data and materials
The filtered GRA data used in this study are available in Additional file 2.

Kurokawa et al. Progress in Earth and Planetary Science(2019) 6:2

Authors’ contributions
All data were collected by expedition 346 scientists during the IODP
expedition 346 cruise. KS conducted the orbital tuning of the GRA data andwrote a draft of the manuscript. TR proposed the topic and conceived anddesigned the study. MKM helped interpret the biostratigraphy, helped inrevising the manuscript, and conducted the oxygen isotope tuning of the
GRA data provided in the supplementary files. TI and LJ helped in thecorrelation of core and logging data. SK drew the figures and wrote a draftof the manuscript. All authors read and approved the final manuscript.
Authors’ information
KS is a student of TR. TR was co-chief scientist of IODP Expedition 346. IT and
LJ were shipboard scientists of IODP Expedition 346. MKM was a JSPS
Postdoctoral researcher under TR, now assistant professor at the University ofTokyo.
Competing interests
The authors declare that they have no competing interests.

Publisher’s NoteSpringer Nature remains neutral with regard to jurisdictional claims in publishedmaps and institutional affiliations.
Author details
1
Department of Earth and Planetary Science, Graduate School of Science,
The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.
2
Present Address Atmosphere and Ocean Research Institute, The Universityof Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8564, Japan. 3Graduate
School of Environmental Earth Science, Hokkaido University, Sapporo
060-0810, Japan. 4Géosciences Montpellier, Université Montpellier II, Place
Eugène Bataillon, 34095 Montpellier Cedex 5, France.
Received: 28 September 2017 Accepted: 12 December 2018

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