Shirota et al.
Progress in Earth and Planetary Science(2021) 8:66
https://doi.org/10.1186/s40645-021-00456-1

PAPER WITH FULL DATA ATTACHED

Progress in Earth andPlanetary Science

Open Access

Changes in surface water masses
in the northern East China Sea since the Last
Glacial Maximum based on diatom assemblages
Keiko Shirota1, Yusuke Okazaki1* , Susumu Konno2, Yosuke Miyairi3, Yusuke Yokoyama3,4,5,6,7 andYoshimi Kubota8

Abstract
We investigated diatom assemblages for the last 19 ka in a piston core KY07-04 PC01 collected from the northern
Okinawa Trough, East China Sea. Most of the diatom taxa in the Core KY07-04 PC01 were subtropical to temperatemarine species throughout the core. Pronounced turnover of major diatom groups from periphytic to planktonic taxa
occurred during deglaciation between 17 and 14 ka ago. During the Last Glacial Maximum (LGM) to early deglaciation, the representative taxa were Paralia spp., a periphytic diatom abundant in continental shelves of the modern
East China Sea covering the Chinese Coastal Water. Relative abundances of Paralia spp. rapidly decreased during
15–14 ka ago, corresponding to sea-level rise, displaced by planktonic taxa such as Thalassionema. Thalassionema
nitzschioides varieties living in subtropical water increased since 12 ka and have reached the present level at 8 ka ago,suggesting strengthening the influences of Kuroshio water in the northern East China Sea during the early to middle
Holocene. Warm planktonic diatom taxa such as Nitzschia bicapitata also gradually increased from 12 to 8 ka ago. Near
the core top, a sudden increase in N. bicapitata might relate to the intensifying Kuroshio during the latest Holocene.
Keywords: Diatoms, East China Sea, Sediment core, Last Glacial Maximum, Holocene, Water mass, Chinese coastal
water, Kuroshio
1 Introduction
The East China Sea is a marginal sea of the western North
Pacific bordered by the Kyushu, the Ryukyu Islands, andTaiwan. The East China Sea connects with the JapanSea via the Tsushima Strait (~ 130 m sill depth) and theSouth China Sea via the Taiwan Strait (less than 100 m
in depths). The East China Sea is composed of two areas:
continental shelves (< 200 m water depth) in the northwest, including the Yellow Sea, and the Okinawa Trough(1000–2200 m water depth) in the southeast (Fig. 1).
The Yellow River (Huanghe) and the Yangtze River*Correspondence: okazaki.yusuke.886@m.kyushu-u.ac.jp
1
Department of Earth and Planetary Sciences, Graduate Schoolof Science, Kyushu University, 744 Motooka, Nishi‑ku, Fukuoka 819‑0395,
Japan
Full list of author information is available at the end of the article(Changjiang) discharge into the East China Sea, providing much fresh water and suspended particles. The Yellow River is the second largest river globally in suspended
sediment load (Milliman and Meade 1983). The Yangtze
River supplies about 80% of the total discharge of freshwater from rivers flowing into the East China Sea, playing a critical role in sea-surface salinity distribution of thecontinental shelves of the East China Sea (Ichikawa andBeardsley 2002).
Bathymetry constrains the distribution of surface
water masses and oceanic fronts in the East China Sea(Ichikawa and Beardsley 2002). The Kuroshio is a western boundary current of the North Pacific, playing anessential role in poleward heat transport (Qiu 2019). The
Kuroshio flows northward and enters the East China Sea
through the channel between Taiwan and Yonaguni-Jima.
In the East China Sea, the Kuroshio path follows closely© The Author(s) 2021. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which
permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to theoriginal author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images orother third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line
to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory
regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of thislicence, visit http://​creat​iveco​mmons.​org/​licen​ses/​by/4.​0/.

Shirota et al. Progress in Earth and Planetary Science(2021) 8:66

along the steep continental slope and changes the direction to the east around ~ 30°N, flowing out to the Philippine Sea through the Tokara Strait. As a result, warm,
saline, and nutrient-depleted Kuroshio water occupies
the southern and central Okinawa Trough. The Tsushima
Warm Current flows northward and enters the Japan Sea
through the Tsushima Strait. The Taiwan Warm Current flowing northward through the Taiwan Strait andonshore Kuroshio intrusion across the East China Sea
shelf break is the source of the Tsushima Warm Current(Isobe 2008). This warm water prevails in the northern
Okinawa Trough. The Chinese Coastal Current flows
southward along the Chinese coast on the continental
shelves of the western East China Sea, mixing the Yangtze River discharge (Ichikawa and Beardsley 2002). Thus,
cold, less saline, and nutrient-rich Chinese Coastal water
distributes in the continental shelf regions of the East
China Sea.
The East China Sea has experienced marked geographic
changes with the sea-level rise since the LGM. During the
LGM between 21 and 17 ka ago, sea-level was estimatedto be − 125 to − 130 m (Yokoyama et al. 2018), leading
to aerial exposure of most East China Sea continental
shelves, closure of the Taiwan Strait, and constrictionof the Tsushima Strait. Such major geographic changes
must have led to changes in the distribution and circulation of surface water in the East China Sea. There is a lineof evidence for low sea-surface temperature (SST) widely
observed over the glacial East China Sea (Ijiri et al. 2005;
Sun et al. 2005; Chang et al. 2009a; Yu et al. 2009; Chenet al. 2010; Kubota et al. 2010, 2015b; Xu et al. 2013; Ujiié
et al. 2016; Matsuzaki et al. 2019), expansion of the Chinese Coastal Water (Xu and Oda 1999), advancement ofriver mouths of the Yellow River (Xu et al. 2014; Zheng
et al. 2016; Zhao et al. 2017, 2018; Beny et al. 2018) andthe Yangtze River (Dou et al. 2010a, b, 2012). Most previous studies indicated weakened the Kuroshio Current during the LGM. At the same time, Kawahata andOhshima (2004) suggested an intrusion of the Kuroshio
Current into the northern East China Sea during the
LGM based on the occurrence of pollen Phyllocladus,celery pines originated in New Guinea and Southeast
Asia. The glacial path is controversial as reviewed byGallagher et al. (2015) whether the southward migration
occurred (Ujiié et al. 2003; Su and Wei 2005; Kao et al.
2006; Dou et al. 2010a, b, 2012, 2016; Lim et al. 2017; Li

Page 2 of 17

et al. 2018; Ma et al. 2021) or the path remained similar
to the modern position (Lee et al. 2013; Zheng et al. 2016;
Vogt‐Vincent and Mitarai 2020).
Diatoms are unicellular photosynthetic eukaryotes
producing biogenic opal frustules found in nearly all
marine and freshwater habitats. They are one of themost major primary producers playing essential rolesin the carbon and silicon cycles of the ocean (Smol andStoermer 2010). A great diversity of species is living bysensitively responding to the aquatic environmental
changes. In the modern East China Sea, characteristic diatom assemblages are dwelling in response to thewater masses changes and the with pronounced seasonality (Asaoka 1975, 1980; Oh and Koh 1995; Furuya et al.
1996; Chiang et al. 1999; Ishikawa and Furuya 2004; Guoet al. 2014; Li et al. 2014; Chen et al. 2019b). Although
the East China Sea is befitting for reconstructing changes
in surface water masses, a few paleoceanographic studies
employing diatom assemblages are available. Chang et al.
(2009b) presented a diatom assemblage record in Core
MD01-2404 from the central part of the Okinawa Troughfor the past 100 ka at millennial timescales. The diatom
record in Core MD01-2404 seemed to be synchronouswith the millennial-scale climate changes in the high-latitude Northern Hemisphere during the glacial-deglacial
periods. Tanimura et al. (2002) employed a periphytic
diatom, Paralia sulcata, in Core KH82-4 St-14 from thenorthern Okinawa Trough to discuss the expansion andretreat of the Chinese Coastal Water since the LGM withcentennial timescales. They found abundant Paralia sulcata during the LGM to early glacial termination, whichsuggested significant expansion of the Chinese Coastal
Water during then. Li et al. (2012, 2015) investigated diatom assemblages in Core MD05-2908 from the southern
Okinawa Trough at decadal resolutions for the past six
ka. These studies have revealed that diatom assemblages
are helpful to reconstruct past surface environmental
changes of the East China Sea. However, a high-resolution whole assemblage record since the LGM has not
yet been available in the East China Sea. Unfortunately,
these previous studies (both modern and paleo) presented minimal taxonomic information and photos using
a light microscope (LM). Only Li et al. (2014) presented
scanning electron microscope (SEM) images for several
Thalassiosira taxa.

(See figure on next page.)
Fig. 1 a Bathymetric map showing the location of Core KY07-04 PC01, MD98-2195 (yellow star), A7, and DGKS9604 (yellow circle) in the East China
Sea. Dotted lines indicate 100 m water depth. Surface water masses (Kuroshio, Chinese coastal water, Changjiang River Diluted Water (CRDW), andTsushima Warm Current (TSWC)) are shown. b Lower panels show the annual mean of sea-surface temperature (SST) and c sea-surface salinity (SSS)
based on the World Ocean Atlas 2018 (Locarnini et al. 2018; Zweng et al. 2018)

Shirota et al. Progress in Earth and Planetary Science(2021) 8:66

Page 3 of 17(a)Japan Sea

lowlYellow
Sea

TSWC

a

CRDW

n

Ta

iw

an

St

ra

O

it

kin

aw

a

East China A7
Sea

Tokara Straitough

DGKS9604

Tr

Ya

K

KY0704 PC01
MD98-2195

er

at

er

v

Ri

hio

s
uro

lw

e
gtz

sta

er

Ts
us
hi
m

a
co

Paleo Yellow Riv

St

se

ra

it

ine

Ch

Ye

r

ve

Ri

Philippine Sea
South China
Sea

(b)

30

40°N SST (°C)

(c)
40°N

35

SSS

34

25
35°N

35°N
33

20
30°N

30°N

32

15

120°E
Fig. 1 (See legend on previous page.)

125°E

130°E

25°N

Ocean Data View

Ocean Data View

25°N

10
120°E

125°E

130°E

31

30

Shirota et al. Progress in Earth and Planetary Science(2021) 8:66

We present the whole diatom assemblages record andtheir LM and SEM images in Core KY07-04 PC01 fromthe northern Okinawa Trough since the LGM at centennial timescales. Available high-resolution geochemical
proxy records such as oxygen isotopes and Mg/Ca SST
in Core KY07-04 PC01 allow us to directly compare thediatom assemblage and the geochemical dataset (Kubota
et al. 2010, 2015b). The purpose of this study is to clarifychanges in surface water masses such as the Kuroshio
Current and the Chinese Coastal Water from the semiclosed LGM to the present via deglacial sea-level rise.

2 Materials and methods
2.1 Core KY07‑04 PC01 and its age model

A piston core sample KY07-04 PC01 was obtained froma water depth of 758 m on the northern slope of the
Okinawa Trough, East China Sea (31° 38.35ʹ N, 128°
56.64ʹ E) during the R/V Kaiyo KY07-04 cruise in February 2007 (Fig. 1). Core KY07-04 PC01 consists ofhomogeneous olive to gray silty clay with two ash layers

Page 4 of 17(Kubota et al. 2010). The widespread tephra identified
the upper ash layer between 408.5 and 488.5 cm in core
depth as the Kikai‐Akahoya (K-Ah; erupted at 7.3 ka ago).
The age model of KY07-04 PC01 was established basedon 18 radiocarbon dating of planktic foraminiferal shells
and tephrochronology by Kubota et al. (2010). This study
revised the Core KY07-04 PC01 age model with 26 radiocarbon dating of planktic foraminiferal shells (Table 1).
New radiocarbon dates for eight samples during the
middle to late Holocene interval were measured by the
National Electrostatics Corporation 250 kV single-stage
accelerator mass spectrometry at Atmosphere and Ocean
Research Institute, The University of Tokyo (Yokoyama
et al. 2019). We conducted radiocarbon calibration toconvert radiocarbon age to calendar age using CALIB 8.2(Stuiver et al. 2021) with the Marine20 calibration curve(Heaton et al. 2020). Local marine reservoir age (ΔR) was
estimated as − 200 ± 100 yr from 33 data from the East
China Sea coast (Konishi et al. 1982; Kong and Lee 2005;
Hideshima et al. 2001; Southon et al. 2002; Yoneda et al.

Table 1 Age control points of Core KY07-04 PC01 by radiocarbon ages of planktic foraminifera and K-Ah tephra chronology
Core depth (cm)14

C age (yr BP)

Error (yr)Calendar age (yr BP)

Error (yr, ± 2σ)

Foraminiferal species

2.5–12.5

185

75

Bomb 14C

Bomb 14C

N.dutertrei

2.5–12.5

265

115

Bomb 14C

Bomb 14C

G.bulloides

50–60

1040

50

670

226

N.dutertrei

78–79.75

1643

49

1236

266

G. sacculifer, P. obliquiloculata, N. dutertrei

98.5–103.5

2955

80

2793

357

N.dutertrei

102.25–104.75

2429

53

2142

320

G. sacculifer, P. obliquiloculata, N. dutertrei

124.75–127.25

2812

48

2610

293

G. sacculifer, P. obliquiloculata, N. dutertrei

151–156

2955

50

2796

321

N.dutertrei

173.5–176

3331

37

3239

299

N. dutertrei

201–206

3635

50

3637

309

N.dutertrei

223.5–226

4147

89

4292

391

G. sacculifer, P. obliquiloculata, N. dutertrei

251–256

4550

80

4822

376

N.dutertrei

273.5–276

4985

53

5313

304

G. sacculifer, P. obliquiloculata, N. dutertrei

300–302.5

5325

55

5716

279

N.dutertrei

322.5–325

5878

44

6304

285

G. sacculifer, P. obliquiloculata, N. dutertrei

365–367.5

6688

65

7180

280

G. sacculifer, N. dutertrei

7252

56

K-Ah tephra top

409

7252

56

K-Ah tephra bottom

492–494.5

489
6960

60

7452

251

N.dutertrei

492–502

7000

90

7502

286

G.bulloides

639.5–642

9040

75

9841

338

N.dutertrei

781–783.5

10,475145

11,856559

N.dutertrei

925.5–935.5

11,50070

13,016270

N.dutertrei

925.5–935.5

11,70080

13,214300

G.bulloides

1068.5–1071

13,250105

15,371428

N.dutertrei

1240–1247.5

14,74080

17,295398

N.dutertrei

1403.5–1408.5

15,745105

18,434344

N.dutertrei

1398.5–1408.5

15,975105

18,637378

G.bulloides

Shirota et al. Progress in Earth and Planetary Science(2021) 8:66

2007; Hirabayashi et al. 2017)) in 14CHRONO Marine20
reservoir correction database. The K-Ah tephra age was7252 ± 56 yr, estimated from Lake Suigetsu varve sediment(McLean et al. 2018). Based on the radiocarbon and K-Ah
tephra dates, the revised age model of Core KY07-04 PC01
was established (Table 1; Fig. 2). The revised age model isconsistent with the previous one by Kubota et al. (2010).
Its continuous sedimentation with a high sedimentation
rate of ~ 80 cm ­kyr−1 ensures Core KY07-04 PC01 for centennial to millennial scales paleoceanographic study.
2.2 Microscopic observation for diatoms

Core KY07-04 PC01 samples were sliced at 2.5 cm intervals and stored in a refrigerator. A total of 72 subsamples
taken at ~ 20 cm intervals were taken for diatom assemblage work and freeze-dried with an EYELA FDU-1200
freeze-dryer. Approximately 1 mg of the freeze-driedsample was treated with 25 mL of 30% hydrogen peroxide at hydrogen peroxide on a hot plate with top plate
temperature set to 100 °C for about 1 h to digest organic
matter. After the reaction was complete, MilliporeElix water was added to a total volume of 200 mL andallowed to stand for 8 h. The removal of supernatant wasremoved, and Millipore-Elix water was added again. The
following is a procedure of slides for LM observation.
The pretreated suspended solution of each sediment
sample was drawn by 2 mL plastic pipette, dropped on
cover glass (24 × 40 mm), and dried on a hot plate with
top plate temperature set to 40–50 °C. Pleurax (Mount

Calendar age (ka ago)
0

0

2

4

6

8

10

12

14

16

18

20

200

Core depth (cm)400

K-Ah tephra

600
800
1000
1200
1400

Fig. 2 The age-depth plot of Core KY07-04 PC01. Open circles
denote radiocarbon dates (calibrated to calendar age) of planktic
foraminifera with error bars (± 1σ). Orange vertical line indicates K-Ah
tephra stratum. The K-Ah tephra datum (7252 ± 56 yr ago) is from
Lake Suigetsu varve sediment (McLean et al. 2018)

Page 5 of 17

Media, Wako Pure Chemical Industries, Tokyo, Japan)
was dropped on the cover glass and mounted on theglass slide at 120 °C. LM observations were performed at
magnifications of × 1000 using Olympus BX53 and BX50
bright-field microscopes to identify and count diatoms inCore KY07-04 PC01 samples. Counting of diatom valves
was performed as follows: (1) for centric diatoms, a valve
with a central part was counted as one valve; and (2) for
pennate diatoms, a valve with an apical part was counted
as 0.5 valves. More than 200 diatom valves were counted
for diatom assemblage analyses of Core KY07-04 PC01
samples (Additional file 1: Tables S1 and S2).
Sample preparation for SEM observation is describedas follows. The pretreated suspended solution of eachsediment sample was filtered through an Advantec membrane filter (0.45 µm pore size, 47 mm diameter), rinsed
with Millipore-Elix water, and dried. The filter samples
were cut into 6 mm × 6 mm squares, mounted onto brass
stubs with carbon tape, and coated with O­ sO4 using an
osmium coater (Neoc-STB; Meiwafosis Co. Ltd., Tokyo,
Japan). A field-emission scanning electron microscope(FE-SEM, JMS-7001F; JEOL Ltd., Akishima, Japan) wasused for detailed taxonomic work of the diatoms. The
acceleration voltage was set at 5 kV.
Identification and counting were conducted at the species level as much as possible. Images of observed major
diatom taxa are shown in Additional file 2: Plates 1–6 inthe supplement. We counted specimens belonging to thegenus Paralia as Paralia spp. Paralia spp. include several
species such as P. sulcata, P. fenestrata, P. longispina, P.
allisonii, P. crawfordii, P. ehrmanii, and P. obscura (Hasle
and Syvertsen 1997; Sawai et al. 2005; Konno and Jordan
2008; MacGillivary and Kaczmarska 2012, 2013). The
morphology of encountered Paralia specimens in Core
KY07-04 PC01 often had intermediate features between
the species and was hard to identify by LM observation.
Genus Thalassionema was one of the most abundant diatoms in Core KY07-04 PC01 samples. This study classified Thalassionema into six species, four varieties, andtwo morphological types (Table 2; Additional file 2: Plate
4–6) following to Hasle and Syvertsen (1997), Tanimura(1999), Hasle (2001), Tanimura et al. (2007) and Akiba
et al. (2014). The diatom taxa were grouped by their lifestyle (periphytic, planktonic), habitats (freshwater, brackish, marine), and geographic distributions (coastal, cold,warm) based on Round et al. (1990), Hasle and Syvertsen(1997), and Koizumi (2008) (Table 2).

3 Results
One hundred twenty-five diatom taxa were observed inCore KY07-04 PC01 samples (Table 2). Paralia, Thalassionema, and Nitzschia were the most abundant diatom
genus in Core KY07-04 PC01 samples (Fig. 3). Paralia

Shirota et al. Progress in Earth and Planetary Science(2021) 8:66

Page 6 of 17

Table 2 Diatom taxa identified in Core KY07-04 PC01 with their ecological distributions
Diatom taxa

Categories

Achnanthes spp.

Periphytic

Actinocyclus cf. octonarius Ehrenberg

Marine, Coastal

Actinocyclus spp.

Marine, Coastal

Actinoptychus senarius (Ehrenberg) Ehrenberg

Marine, Coastal

Actinoptychus cf. splendens (Shadbolt) Ralfs

Marine, Coastal

Actinoptychus spp.

Marine, Coastal

Alveus marinus (Grunow) Kaczmarska and Fryxell

Planktonic, Marine, Warm

Amphora spp.

Periphytic

Ardissonea cf. robusta (Ralfs ex Pritchard) De Notaris

Periphytic, Brackish/Marine, Coastal

Asterolampra sp.

Planktonic, Marine, Warm

Asteromphalus arachne (Brébisson) Ralfs

Planktonic, Marine, Warm

Asteromphalus flabellatus (Brébisson) Greville

Planktonic, Marine, Warm

Asteromphalus sp.

Planktonic, Marine, Warm

Aulacoseira spp.

Planktonic, Freshwater

Azpeitia nodulifera (Schmidt) Fryxell and Sims

Planktonic, Marine, Warm

Azpeitia spp.

Planktonic, Marine

Bacillaria spp.

Periphytic

Bacteriastrum cf. delicatulum Cleve

Planktonic, Marine

Bacteriastrum cf. hyalinum Lauder

Planktonic, Marine

Bacteriastrum spp.

Planktonic, Marine

Biddulphia spp.

Marine, Coastal

Campylosira sp.

Periphytic, Marine, Coastal

Catenula cf. adhaerens (Mereschkowsky) Mereschkowsky

Periphytic, Marine, Coastal

Chaetoceros spp.

Planktonic, Marine

Climacodium frauenfeldianum Grunow

Planktonic, Marine, Warm

Cocconeis scutellum Ehrenberg

Periphytic, Marine, Coastal

Cocconeis spp.

Periphytic

Coscinodiscus marginatus Ehrenberg

Planktonic, Marine, Cold

Coscinodiscus radiatus Ehrenberg

Planktonic, Marine

Coscinodiscus spp.

Planktonic, Marine

Cyclostephanos spp.

Planktonic, Freshwater

Cyclotella choctawhatcheeana Prasad

Planktonic, Brackish/Marine, Coastal

Cyclotella litoralis Lange and Syvertsen

Planktonic, Brackish/Marine, Coastal

Cyclotella striata (Kützing) Grunow

Planktonic, Brackish/Marine, Coastal

Cyclotella stylorum Brightwell

Planktonic, Brackish/Marine, Coastal, Warm

Cyclotella spp.

Planktonic, Coastal

Cymatosira lorenziana Grunow

Periphytic, Marine, Coastal, Warm

Cymatosira spp.

Periphytic, Marine, Coastal

Cymatotheca spp.

MarineCymbella spp.

Periphytic, Freshwater

Delphineis australis (Petit) Watanabe, Tanaka, Reid, Kumada and Nagumo

Periphytic, Marine, Coastal

Delphineis minutissima (Hustedt) Simonsen

Periphytic, Marine, Coastal

Delphineis surirella (Ehrenberg) Andrews

Periphytic, Marine, Coastal

Delphineis spp.

Periphytic, Marine, Coastal

Denticula antillarum Cleve

Periphytic, Coastal

Diatoma spp.

Periphytic, Freshwater

Dimeregramma spp.

Marine, Coastal

Diploneis cf. crabro (Ehrenberg) Ehrenberg

Periphytic, Marine, Coastal

Diploneis cf. suborbicularis (Gregory) Cleve

Periphytic, Marine, Coastal

Shirota et al. Progress in Earth and Planetary Science(2021) 8:66

Page 7 of 17

Table 2 (continued)
Diatom taxa

Categories

Diploneis spp.

Periphytic, Coastal

Discostella sp.

Planktonic, Freshwater

Ditylum brightwellii (West) Grunow

Planktonic, Marine, Coastal

Eucampia spp.

Planktonic, Marine

Eunotia spp.

Periphytic, Freshwater

Fallacia spp.

Periphytic

Fragilaria spp.

Periphytic, Freshwater

Fragilariopsis doliolus (Wallich) Medlin and Sims

Planktonic, Marine, Warm

Glyphodesmis spp.

Periphytic, Marine, Coastal

Grammatophora spp.

Periphytic, Marine, Coastal

Gyrosigma spp.

Periphytic, Brackish/Marine, Coastal

Hemiaulus sp.

Planktonic, Marine

Hemidiscus cf. cuneiformis Wallich

Planktonic, Marine, Warm

Hyalodiscus spp.

Periphytic, Marine, Coastal

Hydrosera spp.

Periphytic, Brackish/Marine, Coastal

Lioloma spp.

Planktonic, Marine

Lyrella spp.

Periphytic, Marine, Coastal

Mastogloia sp.

Periphytic

Melosira sp.

Periphytic

Minidiscus spp.

Planktonic, Marine, Coastal

Navicula spp.

Periphytic

Neodelphineis indica (Taylor) Tanimura

Planktonic, Marine, Warm

Neodelphineis pelagica Takano

Planktonic, Marine, Warm

Nitzschia bicapitata Cleve

Planktonic, Marine, Warm

Nitzschia capuluspalae Simonsen

Planktonic, Marine, Warm

Nitzschia interruptestriata Simonsen

Planktonic, Marine, Warm

Nitzschia cf. sicula (Castracane) Hustedt

Planktonic, Marine, Warm

Nitzschia spp.

Planktonic

Odontella spp.

Planktonic, Marine

Opephora spp.

Periphytic, Marine, Coastal

Paralia cf. sulcata (Ehrenberg) Cleve

Periphytic, Marine, Coastal

Paralia spp.

Periphytic, Marine, Coastal

Plagiogramma spp.

Periphytic, Marine, Coastal

Plagiogrammopsis cf. vanheurckii (Grunow) Hasle, Stosch and Syvertsen

Periphytic, Marine, Coastal

Pleurosigma spp.
Porosira spp.

Planktonic, Marine

Proboscia spp.

Planktonic, Marine

Psammodictyon panduriforme (Gregory) Mann

Periphytic, Marine, Coastal

Psammodictyon spp.

Periphytic, Marine, Coastal

Pseudotriceratium spp.

Planktonic, Marine

Rhaphoneis spp.

Periphytic, Marine, Coastal

Rhizosolenia bergonii Peragallo

Planktonic, Marine, Warm

Rhizosolenia setigera Brightwell

Planktonic, Marine

Rhizosolenia styliformis Brightwell

Planktonic, Marine

Rhizosolenia spp.

Planktonic, Marine

Rhopalodia sp.

Periphytic

Roperia tesselata (Roper) Grunow ex Pelletan

Planktonic, Marine

Shionodiscus oestrupii (Ostenfeld) Alverson, Kang and Theriot

Planktonic, Marine, Warm

Shionodiscus trifultus (Fryxell) Alverson, Kang and Theriot

Planktonic, Marine, ColdShirota et al. Progress in Earth and Planetary Science(2021) 8:66

Page 8 of 17

Table 2 (continued)
Diatom taxa

Categories

Shionodiscus spp.

Planktonic, Marine

Skeletonema sp.

Planktonic, Marine, Coastal

Staurosira spp.

Freshwater

Surirella spp.

Periphytic

Synedra spp.

Periphytic, Freshwater

Tabellaria spp.

Periphytic, Freshwater

Thalassionema bacillare (Heiden) Kolbe

Planktonic, Marine, Warm

Thalassionema frauenfeldii (Grunow) Tempère and Peragallo sensu lato

Planktonic, Marine

Thalassionema frauenfeldii (Grunow) Tempère and Peragallo sensu stricto

Planktonic, Marine

Thalassionema nitzschioides (Grunow) Mereschkowsky

Planktonic, Marine

Thalassionema nitzschioides var. incurvatum Heiden

Planktonic, Marine, Warm

Thalassionema nitzschioides var. inflatum Heiden

Planktonic, Marine, Warm

Thalassionema nitzschioides var. parvum Heiden

Planktonic, Marine, Warm

Thalassionema nitzschioides varieties

Planktonic, Marine, Warm

Thalassionema pseudonitzschioides (Schuette and Schrader) Hasle

Planktonic, Marine

Thalassionema synedriforme (Greville) Hasle

Planktonic, Marine, Warm

Thalassionema umitakae Akiba et Tanimura

Planktonic, Marine, Cold

Thalassionema spp.

Planktonic, Marine

Thalassiosira eccentrica (Ehrenberg) Cleve

Planktonic, Marine

Thalassiosira cf. leptopus (Grunow) Hasle and Fryxell

Planktonic, Marine

Thalassiosira lineata Jousé

Planktonic, Marine, Warm

Thalassiosira nordenskioeldii Cleve

Planktonic, Marine, Cold

Thalassiosira spp.

Planktonic, Marine

Thalassiothrix spathulata Hasle

Planktonic, Marine, Warm

Thalassiothrix spp.

Planktonic, Marine

Trachyneis sp.

Periphytic, Marine, Coastal

Tryblionella spp.

Periphytic(periphytic) was the dominant genus before 14 ka ago,
whereas Thalassionema and Nitzschia (planktonic) wereabundant after 14 ka ago.
At the species level, downcore profiles of diatom taxa
with greater than 4% at least in a sample were shown(Fig. 3). Paralia spp., periphytic coastal diatom taxa,
was representative in Core KY07-04 PC01 samples
before 14 ka ago, exceeding 30% during the LGM andearly deglaciation from 19 to 15 ka ago (Fig. 3). Anotherperiphytic coastal diatom species, Delphineis surirella,
showed a similar pattern with Paralia spp., with highrelative abundances before 16 ka ago (Fig. 3). Relative
abundances of Nitzschia bicapitata, a major planktonic
warm species, were high from the deglacial period to
middle Holocene and the core top of Core KY07-04 PC01
(Fig. 3). Thalassionema species and varieties presented
various downcore profile patterns in Core KY07-04 PC01

(Fig. 3). Morphological varieties of Thalassionema nitzschioides were persistently high percentages throughout
the core. Among the T. nitzschioides varieties, three varieties (var. incurvatum, var. inflatum, and var. parvum)
increased during the middle-late Holocene. On the other
hand, Thalassionema frauenfeldii sensu stricto (s.s.) and
Thalassionema synedriforme were relatively high during
the deglacial period.
Figure 4 shows diatom groups by habitats (freshwater and brackish) and geographic distributions (coastal,cold, warm) in Core KY07-04 PC01. During the LGM,
periphytic diatoms, mostly Paralia spp., were predominant, whereas planktonic diatoms were the minority.
During the deglacial pronounced turnover between 18
and 14 ka ago, Planktonic diatoms became the majority
in Core KY07-04 PC01. With the turnover, coastal diatoms decreased significantly. Note that contributions of0Thalassionema
pseudonitzschioides

Thalassiosira eccentrica

0 10 20

Rhizosolenia styliformis

0 10 0 10 0 10 0 10 0 10 0 10 0 10

Thalassionema nitzschioides
var. nitzschioides

Thalassionema frauenfeldii s.sThalassionema frauenfeldii s.lThalassionema umitakae

Thalassionema synedriforme

Thalassionema nitzschioides
var. parvum

Thalassionema nitzschioides
var. inflatum

80 0 10 0 10 0 10 0 10 20 30 0 10 0 10 0 10

Thalassionema nitzschioides
var. incurvatum

Thalassionema bacillare

Page 9 of 17

Thalassiosira lineata

Shionodiscus oestrupii

60(2021) 8:66

Nitzschia bicapitata

40

Ditylum brightwellii

20

Cyclotella striata

0Delphineis surirella

Paralia spp.

Shirota et al. Progress in Earth and Planetary Science

0 10 0 10 0 10

2

Calendar age (ka ago)

4
6
8
10
12
14
16
18
20
Periphytic

Planktonic

Fig. 3 Downcore profiles of relative abundances (%) of major diatom taxa with greater than 4% at least in a sample of Core KY07-04 PC01 groupedby the geographic distributions: coastal (light green); warm (orange); blue (cold); and non-specific geographic distribution (grey) (Table 2). Yellow
triangles and a red diamond on the right denote radiocarbon dates and K-Ah tephra, respectively

freshwater and brackish water diatoms have been small
in total assemblage even during the LGM and early
deglaciation (Fig. 4). Marine diatoms were the majority
in the diatom assemblages in Core KY07-04 PC01 since
the LGM. Planktonic warm water diatom group gradually
increased from 18 to 12 ka ago, occupying ~ 40% for thelast 12 ka.

4 Discussion
4.1 Diatom assemblages as paleoenvironmental indicators

Throughout the core, marine diatoms were dominant,suggesting that the northern Okinawa Trough has beena marine environment and limited fresh/brackish water
influences even during the LGM. This is consistent withthe previous study reporting the continuous occurrencesof planktic foraminifera (Xu and Oda 1999; Ijiri et al.
2005) and radiolarians (Matsuzaki et al. 2019) during the
LGM in the northern East China Sea. Periphytic diatom
Paralia spp. were dominant in the Core KY0704 PC01
during the LGM. Previous studies in the modern and past
East China Sea referred to as Paralia sulcata, equivalent

to Paralia spp. in the present study (Asaoka 1980; Oh
and Koh 1995; Furuya et al. 1996; Tanimura et al. 2002;
Chang et al. 2009b; Ishikawa and Furuya 2004; Guo et al.
2014; Yuan et al. 2014; Chen et al. 2019b). Paralia spp.
distributes on muddy sediments of the Japanese coastwith salinity ranging from 11 to 34 (Sawai et al. 2005). In
the modern East China Sea and the Yellow Sea, Paralia
spp. dwells on the continental shelves as a representative coastal water indicator throughout the year (Asaoka
1980; Furuya et al. 1996; Ishikawa and Furuya 2004; Guoet al. 2014; Yuan et al. 2014; Chen et al. 2019b), including the eutrophic coastal area of the Yellow Sea (Di et al.
2013; Liu et al. 2013). Asaoka (1980) revealed that Paralia
spp. distributions in summer were almost synchronouswith suspended mud deposited on the central part of East
China Sea continental shelves. Furuya et al. (1996) andIshikawa and Furuya (2004) reported dominant Paralia
spp. in the inner shelf area (< 70 m water depth) by active
vertical mixing in winter and spring, respectively. Guo
et al. (2014) presented the seasonality of diatom assemblages and the abundances in the continental shelf area

Shirota et al. Progress in Earth and Planetary Science(2021) 8:66

Freshwater Brakish(%)(%)

Coastal
(%)

0

0

10

0

10

0

20

40

Page 10 of 17

Warm
(%)
60

80

0

20

Cold
(%)
40

60

0

10

2

Calendar age (ka ago)

4
6
8
10
12
14

YD
MWP-1A

16
18
20

Fig. 4 Downcore profiles of diatom environmental indicators: relative abundances (%) of freshwater (light blue); brackish (cyan); coastal (green);warm (orange); and cold (blue) in Core KY07-04 PC01 (Table 2). Gray hatches indicate MWP-A and YD periodsof the East China Sea. Based on their study, Paralia spp.
was one of the most abundant diatom taxa in spring, fall,
and winter. On the other hand, the occurrence of Paralia
spp. was low in summer. These modern observations
have clarified that abundant Paralia spp. occurrences in
the nutrient-rich shelf water of the East China Sea withtemperatures ranging from 11 to 20 °C and salinity from33.5 to 34.5. This water-mass property corresponds to thewater mass properties of Chinese coastal water (Ichikawa
and Beardsley 2002). Paralia spp. is transported with suspended mud from the coastal area/continental shelves tothe northeastern East China Sea.
Tanimura et al. (2002) investigated Paralia spp. abundances in KH82-4 St-14 piston core (31° 44.4ʹ N, 129°02.1ʹ E, 740 m water depth) adjacent to KY07-04 PC01.
They found abundant Paralia spp. occurrences from 19
to 13 cal ka BP (16–10 ka BP in 14C age shown in Tanimura et al. (2002), suggesting a significant expansion ofChinese coastal water sea-level drop during the LGM
to early deglaciation. A similar but much smaller (up to20%) abundance peak during the LGM to early deglaciation was observed at Core MD01-2404 in the southwestern Okinawa Trough (Chang et al. 2009b). Delphineis
surirella, a periphytic coastal diatom species that dwellin the subtidal zone (Zong and Horton 1999; Sawai
et al. 2009), showed relatively high abundances during
19–14 ka ago, presenting a similar pattern with Paralia
spp. (Fig. 3). These consistently support the expansion of Chinese coastal water during the LGM to early
deglaciation.

In Core KY-07-04 PC01, planktonic diatom Thalassionema was the dominant diatom genus since 14 kaago (Fig. 3). This study classified Thalassionema into
six species, four varieties, and two morphological types(Table 2; Fig. 3) following the previous studies (Hasle
and Syvertsen 1997; Tanimura 1999; Hasle 2001; Tanimura et al. 2007; Akiba et al. 2014). Thalassionema
nitzschioides is known as a cosmopolitan planktonic species (Hasle 2001). In the East China Sea, occurrences ofThalassionema nitzschioides have been reported fromthe Yellow Sea, the continental shelves, and the Okinawa
Trough (Asaoka 1975, 1980; Oh and Koh 1995; Furuya
et al. 1996; Chiang et al. 1999; Ishikawa and Furuya 2004;
Guo et al. 2014). In particular, Thalassionema nitzschioides was abundant near the shelf break (Furuya et al.,
1996; Chiang et al., 1999; Ishikawa and Furuya, 2004; Guoet al., 2014).
Tanimura et al. (2007) indicated that each Thalassionema nitzschioides species and varieties showed specific geographical distributions in the North Pacific: T.
nitzschioides var. nitzschioides and T. pseudonitzschioides
in the Subarctic Gyre; T. nitzschioides var. incurvatum,var. inflatum, and var. parvum in the Subtropical Gyre.
Tanimura (1999) proposed a ratio of three T. nitzschioides varieties (var. incurvatum, var. inflatum, andvar. Parvum) in Thalassionema nitzschioides complex(hereafter refers as T-ratio) as an index for Kuroshio
intensity based on sediment trap experiments in the
Kuroshio path, western North Pacific. Tanimura (1999)
exhibited temporal changes in T-ratio and distance from

Shirota et al. Progress in Earth and Planetary Science(2021) 8:66

the Kuroshio (the shortest distance from the trap station to the Kuroshio path). When Kuroshio migrated to
the north, the sediment trap station was covered withsubtropical water where the three T. nitzschioides varieties were abundant. In Core KY-07-04 PC01, T-ratio has
increased since 12 ka ago and has reached its present
level since 8 ka ago (Fig. 5), which is consistent with thetimings of northward migration of Kuroshio in the western North Pacific (Takemoto and Oda 1997; Tanimura
1999; Yamamoto et al. 2005) and intensification of Tsushima Warm Current flowing into the Japan Sea (Gallagher
et al. 2015; Yokoyama et al. 2006), respectively. A typicaldark color layer in many deep-sea sediment cores fromthe Japan Sea with high organic carbon contents, called
TL1, is seen when the Tsushima Warm Current intrusion was re-established since the LGM (Oba et al. 1991;
Tada et al. 1999; Yokoyama et al. 2006). Thus, the T-ratio

Sea-surface temperatue
(°C)
20

24

28

20

24

Page 11 of 17

increase suggests strengthening the influence of Kuroshio Current water in the northern East China Sea.
Tanimura et al. (2007) recognized nine species, varieties, and morphological types of Thalassionema
nitzschioides. Akiba et al. (2014) described the Nd morphological type of Thalassionema nitzschioides sensu
lato (s.l.) as a new species Thalassionema umitakae. The
distribution of T. umitakae in the North Pacific is mainlyin the subarctic region but a few occurrences in the subarctic boundary of the western North Pacific (Tanimura
et al. 2007). Abundant occurrences of T. umitakae werereported in the glacial Japan Sea (Akiba et al. 2014). In
Core KY07-04 PC01, T. umitakae occurred throughout the core since the LGM with no pronounced trend(Fig. 3), implying no significantly cold surface water covered in the northern Okinawa Trough even during the
LGM.

Warm
(%)

δ18Oresidue(‰, VSMOW)
28

-1

0

0

1

0

20

40

Thalassionema
varieties index
60

0

0.2

0.4

0.6

2
4

Calendar age (ka ago)

6
8
10
12

YD

14

MWP-1A

16
18
20

A7
DGKS9604

KY0704-PC01
MD98-2195

KY0704-PC01

KY0704-PC01

KY0704-PC01

Fig. 5 Sea-surface temperature records based on planktic foraminiferal Mg/Ca (red) and alkenones in the central Okinawa Trough: Core A7 (Sunet al. 2005) and Core DGKS9604 (Yu et al. 2009); and the northern Okinawa Trough: Core KY07-04 PC01 (Kubota et al. 2010, 2015b) and Core
MD98-2195 (Ijiri et al. 2005) as well as residual δ18O as a salinity proxy in Core KY07-04 PC01(Kubota et al. 2010, 2015a, b). SST and residual δ18O
at Core KY07-04 PC01 and the errors (± 1σ) were derived by Paleo-Seawater Uncertainty Solver (PSU Solver) (Thirumalai et al. 2016) with themethodology of Kubota et al. (2019). Downcore profiles of relative abundances (%) of warm diatom indicators and Thalassionema variety index inCore KY07-04 PC01 were also shown. Gray hatches indicate MWP-A and YD periods. Horizontal broken lines denote pronounced decreases in warm
diatom indicators during Holocene

Shirota et al. Progress in Earth and Planetary Science(2021) 8:66

Thalassionema frauenfeldii was relatively abundant in
the western margin of the North Pacific (Tanimura et al.
2007). In this study, we counted T. frauenfeldii as separated the two morphological types T. frauenfeldii s.l. andT. frauenfeldii s.s. Simonsen (1974) described Thalassiothrix frauenfeldii (= Thalassionema frauenfeldii s.l.),
T. frauenfeldii s.l. is reported from the open ocean of thewestern North Pacific (Hasle 2001; Tanimura et al. 2007).
On the other hand, Thalassionema frauenfeldii s.s. islikely to be a coastal species because Hallegraeff (1986)
and Sar et al. (2007) collected this from the Argentine
and Korean coasts. In Core KY07-04 PC01, T. frauenfeldii s.l. gradually increased since the LGM, whereas T.
frauenfeldii s.s. was relatively high abundances during theLGM to the deglacial period with a gentle peak centeredat ~ 12 ka ago (Fig. 3). The overall trend suggests a transition from semi-closed to open ocean environments during the deglacial period.
Warm water taxa dwelling in the open ocean gradually
increased since the LGM and reached ~ 30% at 8 ka ago(Fig. 3). Among them, Nitzschia bicapitata is the major
open-ocean species abundant in tropical to temperate
oceans (Fryxell 2000), including in the western Pacificwarm pool in the western equatorial Pacific with > 28 °C
sea-surface temperature (Kobayashi and Takahashi 2002)
and Kuroshio region (Tanimura 1992). In Core KY07-04
PC01, relative abundances of N. bicapitata were highbetween 12 and 5 ka ago, with a rapid increase near
the core top (Fig. 3). Li et al. (2012, 2015) also reported
a high percentage of N. bicapitata for the last 400 ka inCore MD05-2908 from the southern Okinawa Troughoff northeast of Taiwan. Because of the restricted N.
bicapitata distribution in the open ocean, abundant N.
bicapitata suggests strengthening Kuroshio inflow into
the East China Sea. During the early to middle Holocene,
Kuroshio intensified due to strengthening the Subtropical Gyre by precessional forcing (Isono et al. 2009; Yamamoto 2009).
4.2 Changes in surface water mass in the East China Sea
since the LGM

Diatom assemblages dominated by Paralia spp. in Core
KY07-04 PC01 indicate intensified Chinese coastal waterwhereas weak Kuroshio water in the northern East China
Sea during the LGM. Planktic foraminiferal assemblage
records also suggested enhanced coastal water during
the LGM (Xu and Oda 1999; Ijiri et al. 2005). Reconstructed SSTs during the LGM were ranging from 19 to
22 °C in the northern East China Sea based on planktic foraminiferal Mg/Ca (Kubota et al. 2010; Ujiié et al.
2016), alkenone (Ijiri et al. 2005), and radiolarian transfer function (Matsuzaki et al. 2019) (Fig. 5). In the central and southern Okinawa Trough, reconstructed SSTs

Page 12 of 17

during the LGM were between 21 and 24 °C (Sun et al.
2005; Chang et al. 2009a; Yu et al. 2009; Chen et al. 2010;
Xu et al. 2013) (Fig. 5). These SSTs were 2–5 °C lower
than the present ones but still within temperate to subtropical ranges, consistent with continuous occurrencesof warm open-water diatom taxa in Core KY07-04 PC01
since the LGM. In addition, the regional high-resolution climate model study by (Vogt‐Vincent and Mitarai2020) supports scenario-based proxy records. Kuroshio
kept flowing into the Okinawa Trough during the LGM
but significantly weakened. However, Kuroshio’s pathand intensity during the LGM varied depending on themodels. During the LGM, the Kuroshio path migrated
to the south of the Ryukyu Islands suggested by a model(Kao et al. 2006). On the contrary, a simulation indicatesstronger Kuroshio flowing into the Okinawa Trough (Leeet al. 2013).
Expansion of the nutrient-rich Chinese coastal water
led to enhanced productivity in the glacial East China
Sea (Ijiri et al. 2005; Chang et al. 2009a; Kubota et al.
2017; Matsuzaki et al. 2019). Dominant Paralia spp.
in Core KY07-04 PC01 as an indicator of nutrient-richcoastal water (McQuoid and Nordberg 2003; Di et al.
2013; Liu et al. 2013) demonstrated high productivity inthe glacial northern East China Sea. The Chinese coastal
water prevailing in the northern East China Sea was anadvancement of the river mouth of the Yellow River withsea-level drop during the LGM. Clay mineral compositions and Sr-Nd isotopes of the detrital matter revealed
an ample detrital supply from the old Yellow River tothe northern East China Sea during the LGM (Xu et al.
2014; Zhao et al. 2017, 2018; Beny et al. 2018). Tsushima
Warm Current (TSWC) gradually intensified since 16 kaago, elevated at 11 ka, and fully evolved at ~ 7 ka after
the K-Ah eruption in the northern Okinawa Trough (Xuet al. 2014; Zhao et al. 2017). These timings were consistent with the increasing patterns of %warm diatoms andT-ratio (Fig. 5). In the central Okinawa Trough, the Yangtze River played a critical role in detrital supply (Diekmann et al. 2008; Dou et al. 2010a, b, 2012).
Relative abundances of Paralia spp., periphytic diatom
taxa, in Core KY07-04 PC01 gradually decreased since theLGM, subsequently rapidly decreased from 15 to 14 ka ago(Fig. 3). One of the most pronounced deglacial events is
meltwater pulse 1A (MWP-1A) between 14.6 and 14.3 ka
ago when relative sea-level rose at Tahiti, Barbados, andSunda Shelf about 10–20 m from − 110 to − 90 m (Deschamps et al. 2012). Due to the East China Sea location as
categorized as a far-field (i.e., sites far away from past andpresent ice-covered regions (Yokoyama and Purcell 2021), a
similar magnitude of relative sea-level rise is expected. With
the rapid sea-level rise during MWP-1A, river mouths of
the Yellow River and the Yangtze River must have retreated.

Shirota et al. Progress in Earth and Planetary Science(2021) 8:66

The supporting evidence is by records of clay mineral compositions and Sr-Nd isotopes of detrital matter, suggesting
switch of the source region from the river to the continental shelves (Dou et al. 2010a, b, 2012; Xu et al. 2014; Zhao
et al. 2017; Hu et al. 2020). Gradual increases in warm
diatoms, T-ratio, and Mg/Ca SST in Core KY07-04 PC01
indicate a gradual intensification of Kuroshio during thedeglacial period. During the Younger Dryas between 12.9
and 11.6 ka ago (Cheng et al. 2020), warm diatoms kept
increasing, whereas Mg/Ca SST was relatively low (Fig. 5).
In the northern Okinawa Trough, Kuroshio intensified between 12 and 5 ka ago, reaching its maximum at~ 8 ka ago based on diatom taxa living in warm, open
water (Fig. 5). Multi-proxy records also indicate theintensified Kuroshio during late deglacial to middle Holocene, i.e., SST rise (Ijiri et al. 2005; Kubota et al. 2010,2015b; Ruan et al. 2015; Zhao et al. 2015), deepening of
the thermocline (Li et al. 2020), planktic foraminiferal
assemblage (Xu and Oda 1999; Ijiri et al. 2005), floral
changes of calcareous nannoplankton assemblage (Suand Wei 2005), sortable silt (Diekmann et al. 2008), terrigenous input from Taiwan (Dou et al. 2010a, b, 2012,2016; Xu et al. 2014, 2019; Zheng et al. 2016; Zhao et al.
2017; Hu et al. 2020), organic carbon and reactive phosphorus (Shao et al. 2016; Li et al. 2018), mercury concentration (Lim et al. 2017), and redox sensitive metals(Zou et al. 2020). Warm diatoms showed several rapid
decreases based on the direct comparison between diatom assemblage and Mg/Ca SST in Core KY07-04 PC01.
Some of these events accompany Mg/Ca SST drops
during the Holocene (Fig. 5). An event at 7 ka ago was
the most prominent. Subsequently, Kuroshio intensity
might have weakened during the late Holocene due to its
southward migration by precessional forcing controllingthe strength of the North Pacific subtropical gyre (Isonoet al. 2009; Yamamoto 2009). However, a sudden increasein N. bicapitata, representative diatom species dwellingin the open ocean with high SST, was observed near thecore top of KY07-04 PC01 (Fig. 3). High-resolution diatom assemblage record during the late Holocene in thesouthern Okinawa Trough showed high abundances ofN. bicapitata for the last ~ 400 yrs (Li et al. 2015, 2012).
Such high abundances of N. bicapitata over the Okinawa
Trough is consistent with the finding of intensification of
Kuroshio during the latest Holocene (Zhang et al. 2019),
implying persistent poleward heat transport regardlessof the weakening of the Atlantic Meridional Overturning Circulation (Summerhayes and Zalasiewicz 2018).
Alternatively, the high N. bicapitata abundances might
be related to the response to the global warming (Chenet al. 2019a) and southward course shift of the Yellow
River discharged into the Yellow Sea between 1546 and1855 A.D. (Saito and Yang 1995).

Page 13 of 17

5 Conclusions
We conducted microscopic observations for diatom
assemblages since the LGM in Core KY07-04 PC01 fromthe northern Okinawa Trough, East China Sea. The produced diatom assemblage data are the first centennialscale resolution since the LGM in the East China Sea with
SEM and LM photographs of the representative taxa.
Most of the diatoms found in Core KY07-04 PC01 weremarine taxa throughout the core, with a few freshwater/brackish water taxa. There was a substantial turnover ofmajor diatom groups from periphytic to planktonic taxa
during deglaciation between 17 and 14 ka ago. During
the LGM and early deglaciation, the representative periphytic taxa were Paralia spp., abundant in the modern
continental shelf region of the East China Sea covered by
the Chinese coastal water. The advancement of the Yellow
River mouth with a sea-level drop during the LGM was
the plausible reason for the prevailing Chinese coastal
water in the northern East China Sea. Rapid decrease in
the relative abundances of Paralia spp. during 15–14 ka
ago corresponded to MWP-1A, a major deglacial eventwith the rapid sea-level rise during 14.6–14.3 ka ago.
Planktonic cold diatom taxa were less than 10% throughout the core, suggesting the oceanic environment in thenorthern Okinawa Trough has been temperate to subtropical even during the LGM. Planktonic diatom Thalassionema was the dominant diatom genus since 14 ka ago.
A ratio of three T. nitzschioides varieties (var. incurvatum,var. inflatum, and var. parvum) in Thalassionema nitzschioides complex (T-ratio) increased since 12 ka ago andhas reached its present level since 8 ka ago, responding
to strengthening and northward migration of Kuroshio
during early to middle Holocene by precessional forcing. Warm planktonic diatom taxa also showed a gradualincrease from 12 to 8 ka ago. A sudden increase in N.
bicapitata, warm open ocean species, was found near thecore top, which might be related to the intensifying Kuroshio during the latest Holocene.
Abbreviations
CRDW: Changjiang River Diluted Water; LGM: Last Glacial Maximum; LM: Light
microscope; MWP: Meltwater pulse; SEM: Scanning electron microscope; SSS:
Sea-surface salinity; SST: Sea-surface temperature; TSWC: Tsushima Warm Current; YD: Younger Dryas.

Supplementary Information
The online version contains supplementary material available at https://​doi.​
org/​10.​1186/​s40645-​021-​00456-1.
Additional file 1: Tables S1 and S2. Counts of diatom valves and relative
abundance (%) of diatom taxa in Core KY07-04 PC01.
Additional file 2: Plates 1-6. LM and SEM images of major diatom taxa inCore KY07-04 PC01.

Shirota et al. Progress in Earth and Planetary Science(2021) 8:66

Acknowledgements
We sincerely thank the scientists, technicians, captain, and crew of the KY0704 Cruise by R/V Kaiyo, Japan Agency for Marine-Earth Science and Technology, for their efforts in obtaining sediment samples. Mr. Kazuhiko Shimada of
Kyushu University supported the FE-SEM operation. We also thank Dr. Fumio
Akiba of Diatom Minilab Akiba, Co. Ltd. for his helpful suggestions on diatom
taxonomy.
Authors’ contributions
YO designed the study, analyzed the data, and wrote the manuscript. KS carried out microscopic observation and analyzed the data with technical support from SK. YM and YY conducted radiocarbon measurements. YK collected
sediment core sample and analyzed Mg/Ca of foraminifera. All authors readand approved the final manuscript.
Funding
This work was supported by JSPS KAKENHI Grant Numbers 15K13585,
16H04832, 16H04069, 21K12222 to YO, and 20H00193 to YY.
Availability of data and material
The datasets supporting the conclusions of this article are included within thearticle and its additional files.

Declarations
Competing interests
The authors declare that they have no competing interest.
Author details
1
Department of Earth and Planetary Sciences, Graduate School of Science,
Kyushu University, 744 Motooka, Nishi‑ku, Fukuoka 819‑0395, Japan. 2 Marine
Works Japan Ltd, 2‑15 Natsushima‑cho, Yokosuka 237‑0061, Japan. 3 Atmosphere and Ocean Research Institute, The University of Tokyo, 5‑1‑5 Kashiwanoha, Kashiwa, Chiba 277‑8564, Japan. 4 Department of Earth and Planetary
Science, The University of Tokyo, 7‑3‑1 Hongo, Bunkyo‑ku, Tokyo 113‑0033,
Japan. 5 Graduate Program Onn Environmental Sciences, The Universityof Tokyo, 3‑8‑1 Komaba, Meguro‑ku, Tokyo 153‑8902, Japan. 6 Biogeochemistry
Research Center, Japan Agency for Marine-Earth Science and Technology,
2‑15 Natsushima‑cho, Yokosuka 237‑0061, Japan. 7 Research School of Physics,
The Australian National University, Canberra, ACT​2601, Australia. 8 National
Museum of Nature and Science, 4‑1‑1, Amakubo, Tsukuba, Ibaraki 305‑0005,
Japan.
Received: 17 August 2021 Accepted: 10 November 2021

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