Toki et al.
Progress in Earth and Planetary Science(2022) 9:59
https://doi.org/10.1186/s40645-022-00519-x

Progress in Earth andPlanetary Science

Open Access

RESEARCH ARTICLE

Sr isotopic ratios of hydrothermal fluids
from the Okinawa Trough and the implicationsof variation in fluid–sediment interactions
Tomohiro Toki1,2* , Tasuku Nohara1, Yoshiaki Urata1, Ryuichi Shinjo2,3 , Shuko Hokakubo‑Watanabe3,
Jun‑ichiro Ishibashi4   and Shinsuke Kawagucci5,6   

Abstract
Sr isotope ratios of hydrothermal fluids were observed at five sediment-associated sites in the Okinawa Trough toinvestigate the diversity of subseafloor fluid–rock–sediment interactions. The estimated 87Sr/86Sr ratios of the hydro‑
thermal endmember fluids at the five sites were all higher than those at the sediment-starved sites. The endmember
Sr isotopic ratios of hydrothermal fluids were diverse within the Okinawa Trough, ranging from 0.7077 at the Iheya
North Knoll site to 0.712 at the Yonaguni Knoll IV site. To our knowledge, 0.712 is the highest value reported to date forseafloor hydrothermal fluids. This variation is likely attributable to the relative contributions of multiple subseafloor Sr
reservoirs, which are 87Sr-poor volcanic rock and 87Sr-rich hemipelagic sediments containing clay minerals of terrestrial
origin. These data support a model based on the carbon isotope ratio of C
­ H4, which indicates whether volcanic rocks
or terrestrial sediments are distributed in the high-temperature reaction zone of the hydrothermal system.
Key points• The Sr isotopic ratios of hydrothermal fluids from the Okinawa Trough are exceptionally high.
• The enrichment of 87Sr at Yonaguni Knoll IV cannot be explained without the influence of terrestrial clay minerals in sediments.
• High Sr isotopic ratios are a common feature of sediment-associated hydrothermal systems.

Keywords: Hydrothermal system, Sr isotopic ratio, Okinawa Trough, Solid phase, Clay mineral

1 Introduction
Deep-sea hydrothermal activity in sedimentary basins(Von Damm et al. 1985b; Butterfield et al. 1994; Kawagucci 2015; Baumberger et al. 2016) involves fluid
chemistry that differs from sediment-starved site activity (Von Damm et al. 1985a; Bonatti 1975) because of*Correspondence: toki@sci.u-ryukyu.ac.jp
1
Department of Chemistry, Biology and Marine Science, Faculty of Science,
University of the Ryukyus, 1 Senbaru, Nishihara, Okinawa 903‑0213, Japan
Full list of author information is available at the end of the article

hydrothermal fluid–sediment interactions. As deep-sea
sediments consist of various materials, such as organic
matter, clay minerals, and biogenic carbonate, constituents of these compounds are released and characterize the fluid chemistry at sediment-associated sites. For
example, strontium (Sr), boron (B), and lithium (Li) arereleased into fluids through high-temperature alterationof sedimentary clay minerals (Araoka et al. 2016; Chibaet al. 1993; Yamaoka et al. 2015; You and Gieskes 2001).
Furthermore, methane ­(CH4) and ammonia (­NH4+)
enrichment of fluids at sediment-associated sites is© The Author(s) 2022. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which
permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to theoriginal author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images orother third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line
to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory
regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of thislicence, visit http://​creat​iveco​mmons.​org/​licen​ses/​by/4.​0/.

Toki et al. Progress in Earth and Planetary Science(2022) 9:59

considered to be a result of sedimentary organic matterdecomposition (Ishibashi et al. 1995; Gamo et al. 1991).
Isotopic ratios are powerful geochemical tools for tracing the origins and fates of environmental constituents.
Recent studies have reported significant variations in­CH4 isotopic ratios in sediment-associated fluids (e.g.,
Kawagucci et al. 2013). This variation strongly suggests
two styles of fluid–sediment interaction on the subseafloor. Kawagucci et al. (2011) hypothesized that microbes
inhabiting sedimentary environments in low-temperature fluid recharge zones generate 13C-depleted ­CH4,
whereas thermal decomposition of sedimentary organic
matter in high-temperature fluid discharge zones generates 13C-rich ­CH4. However, this model can only beapplied when sediments contain sufficient carbon. Somesediments lack carbon; hence, variation in carbon isotopes cannot be applied to assess the style of subsurface
sediment-fluid interactions.
87
Sr/86Sr ratios are widely used as tracers to identify theorigin of Sr, because terrestrial (continental) and marine(volcanic) Sr reservoirs have distinct signatures (Chibaet al. 1993; Zhang et al. 2022, 2020; Noguchi et al. 2011).
The Sr isotopic ratios of hydrothermal endmember fluids
occurring at sediment-starved vent sites exhibit values
within a specific range on a global scale (0.7029–0.7058;
n = 87; Diehl and Bach 2020). In contrast, the Sr isotopic
ratios of hydrothermal endmember fluids in the Okinawa
Trough (0.7089 for Izena Hole: Chiba et al. 1992; 0.7100
for Minami-Ensei Knoll: Chiba et al. 1993; Kawagucci
et al. 2013), a sediment-covered backarc basin containing
more than 20 hydrothermal vent sites (e.g., Kawagucci2015), have been found to be higher than those of hydrothermal endmember fluids occurring at sediment-poorvent sites. The high Sr isotopic ratios are likely related
to subseafloor fluid–sediment interactions, because Sr
is present in sedimentary organic matter, clay minerals,
and biogenic carbonate (e.g., Yoshimura et al. 2020). This
study systematically discusses the causes of the diversityof hydrothermal endmember fluids that exhibit high Sr
isotopic ratios or Sr isotopic ratios comparable to thoseof surrounding volcanic rocks and contributes to ourunderstanding of the mechanisms that produce suchdiversity.
In the present study, we determined the Sr isotopic
ratios in addition to the major and trace element concentrations in hydrothermal fluids collected from five vent

Page 2 of 19

sites in the Okinawa Trough, particularly in the central
and southern regions. The Sr isotope ratios of the ventfluids at the five sites were considerably higher thanthose reported previously for sediment-poor seafloorhydrothermal fluids. The possible mechanism generating this high Sr isotopic ratio at the Okinawa Trough was
explored based on the Sr isotopic ratios of hydrothermal
fluids from the Izena Hole and Minami-Ensei Knoll in the
middle Okinawa Trough (MOT), as reported previously(Chiba et al. 1992, 1993).

2 Geological setting
The Okinawa Trough is a backarc basin formed by subduction of the Philippine Sea plate beneath the Eurasian
Plate (Lee et al. 1980; Fig. 1a). The trench-arc-back-arc
system is ~ 1200 km long; the northern end extends tothe west of Kyushu, and the southern end extends tonortheastern Taiwan (Letouzey and Kimura 1986). Particles transported from Eurasia and Taiwan Island were
deposited in the Okinawa Trough (Dou et al. 2010). The
Okinawa Trough is divided into northern, middle, andsouthern regions bordered by the deep-sea canyon of theTokara and Kerama gaps (Fig. 1b). The sediment layers in
the northern and southern regions of the trough are 8 km
and ~ 2 km thick, respectively (Sibuet et al. 1987).
The Minami-Ensei Knoll is located at the bottom of
the MOT at a water depth of ~ 1200 m, with a specificheight of 600 m and a diameter of ~ 10 km (Chiba et al.
1993; Kawagucci et al. 2013). The top of the knoll has four
depressions, among which only the “C depression” exhibits submarine hydrothermal activity, venting hydrothermal fluids at temperatures of up to 278 °C (Chiba et al.
1993; Kawagucci et al. 2013). Studies of multichannel
seismic profiles and P-wave velocity models in the surrounding area have reported that the MOT is coveredby a 2–3-km-thick Shimajiri Formation (Nishizawa et al.
2019). Ishibashi et al. (2015) speculated that the MinamiEnsei Knoll intrudes through the thick sediments of the
Shimajiri Group of the Okinawa Trough. Kawagucci et al.
(2013) and Kawagucci (2015) inferred that hydrocarbon
decomposition occurs through ­CH4 formation and interacts with sediments outside the knoll on the recharging
side of the reaction zone.
The Iheya North Knoll is a large volcanic complex witha specific height of ~ 130 m and diameter of ~ 10 km andis located at the bottom of the MOT at a water depth(See figure on next page.)
Fig. 1 a Location of the Okinawa Trough. The box denotes the area shown in Fig. 1b. b Locations of the hydrothermal fields in the Okinawa Trough(stars). Triangles indicate Quaternary volcanoes. Bathymetric contours are indicated every 200 m (thin lines) and 1000 m (thick lines). Dashed lines
represent depth contours (100 and 200 km) in the Wadati-Benioff zone (Letouzey and Kimura 1986; Pezzopane and Wesnousky 1989). Solid circles
indicate the volcanic rock sampling sites: red for those in Shinjo et al. (1999), blue for those in Shinjo and Kato (2000), and yellow for those in thisstudy. MOT and SOT represent the middle Okinawa Trough and southern Okinawa Trough, respectively. Boundaries between NOT and MOT and
between MOT and SOT are indicated by light blue dashed lines

Toki et al. Progress in Earth and Planetary Science(2022) 9:59

Page 3 of 19

30

km
0

100

200

28

26

122˚
Fig. 1 (See legend on previous page.)

124˚

126˚

128˚

24
130˚

Toki et al. Progress in Earth and Planetary Science(2022) 9:59

of ~ 1100 m. Three hydrothermally active zones have been
described: the Original, Natsu, and Aki sites (Monmaet al. 1996; Kasaya et al. 2015; Nakamura et al. 2015). The
Original site was drilled in the Integrated Ocean Drilling Program (IODP) Expedition 331 (Takai et al. 2011;
Yeats et al. 2017), whereas the CK14-04 and CK16-01
cruises drilled the Aki site in expeditions 907 (Takai
et al. 2015) and 908 (Kumagai et al. 2017), respectively.
Based on the results of drilling the Original site, the seafloor in this region is covered by low-permeability hydrothermally altered clay mineral layers with a thicknessof tens of meters. Mineral layers confine the horizontal
hydrothermal flow (Miyoshi et al. 2015; Takai et al. 2011;
Masaki et al. 2011). The surrounding trough floor is covered by the 2–3-km-thick Shimajiri Formation, and theknoll itself has no significant cover (Ishibashi et al. 2015).
Kawagucci et al. (2011) reported that the contribution of
microbial ­CH4 to hydrothermal fluids is large. Microbial­CH4 is assumed to be produced in the sedimentary layers of the recharge zone surrounding the knoll body andincorporated into hydrothermal fluids (Kawagucci et al.
2011; Kawagucci 2015). The Pb isotopic ratios in galena
collected from hydrothermal vent chimneys suggest thatthe sediment influence on hydrothermal fluids at theIheya North Knoll is lower than that at the Izena Hole(Totsuka et al. 2019).
The Izena Hole is also located in the MOT ~ 300 m
above the surrounding seafloor at a depth of 1610 m, andit has a rectangular caldera of approximately 6 km × 4 km
on the western slope of a collapsed knoll body. The Izena
Hole hydrothermal field has two hydrothermal sites:
the HAKUREI and JADE sites (Ishibashi et al. 2014).
Both sites have distinct fluid compositions (Ishibashi
et al. 2014) and thus, were treated as individual sites
in this study. The JADE site is located on the northeast
wall of the caldera, and the HAKUREI site is situated ina depression on the southwest side of the caldera floor(Ishibashi et al. 2014; Kawagucci et al. 2010; Halbach
et al. 1989). During the CK16-05 cruise (Exp. 909), thearea around the HAKUREI site was drilled, and pumice was found several tens of meters below the seafloor.
Additionally, hydrothermally altered clay minerals are
distributed up to 180 m below the seafloor (Nozaki et al.
2021). The carbon isotopic ratios of C
­ H4 in the JADE andHAKUREI hydrothermal fluids suggest that both microbial and thermogenic C­ H4 were supplied in the recharge
area (Kawagucci et al. 2010; Kawagucci 2015). However,even if hydrogen is produced in the recharge zone during
thermogenesis, it is decomposed by magmatic volatiles
from oxic silicic magma in the reaction zone (Kawagucci
et al. 2013). The reason for the hydrogen enrichment onlyat the HAKUREI site could be the influence of sediment
cover around the discharge zone, and organic matter inPage 4 of 19

the sediment could be decomposed to generate hydrogen(Ishibashi et al. 2014). This theory also explains ethane
enrichment only at the HAKUREI site.
The Yonaguni Knoll IV is located at the foot of thewestern slope of the Yonaguni Knoll in the southern Okinawa Trough (SOT) at a water depth of 1385–
1336 m. Hydrothermal fluids emanate from nearly 10 mhigh chimneys, and the surrounding area is covered bymuddy sediments (Suzuki et al. 2008). Studies of seismic
structures in the vicinity have reported that the SOT iscovered with 2–3-km-thick submarine sediments (Nishizawa et al. 2019). The origin of C
­ H4 is based on the thermogenic decomposition of organic matter, and the highhydrogen concentration suggests that the knoll body,with little organic matter, acts as a recharge zone, withthe thermal breaking of organic matter in the sediments
around the vent (Kawagucci 2015).

3 Samples and sample preparation
3.1 Sample collection

Fluid samples were collected from the Iheya North Knoll(during the KY11-02, NT11-16, and NT12-06 cruises),Izena Hole (during the NT10-17 cruise at the JADE site
and NT10-17 and NT11-15 cruises at the HAKUREI site),
Hatoma Knoll (during the YK07-04, NT07-12, NT08-13,
and NT09-11 cruises), and Yonaguni Knoll IV (during
the YK03-05 cruise). KY, NT, and YK in the cruise names
indicate the vessels KAIYO, NATSUSHIMA, and YOKOSUKA, respectively, all of which are owned by the Japan
Agency for Marine-Earth Science and Technology (JAMSTEC); the two-digit numbers preceding and following
the hyphen indicate the post-2000 year and the serial
number of the voyage, respectively. The sample IDs provided information on the cruise, dive, sampler type, andsample number (Table 1). The samplers used included
the water and hydrothermal fluid Atsuryoku Tight Sampler (WHATS) (Tsunogai et al. 2003; Saegusa et al. 2006;
Miyazaki et al. 2017), Ti-syringe sampler (Von Dammet al. 1985a), Cheap-WHATS sampler (Kawagucci et al.
2016), vacuum sampler (Sedwick et al. 1994), bag sampler(a 2-L plastic bag connected to an impeller pump), andNiskin sampler. The samplers were loaded onto the JAMSTEC ROV Hyper Dolphin and DSV Shinkai 6500. The
sampling locations (latitude, longitude, and depth) andthe maximum temperature of the fluid observed during
sampling are shown in Table 1.
After the samples were brought on board, the liquid
was immediately distributed to a polyethylene bottle for
pH measurement, followed by a polyethylene syringewith a mixed cellulose ester membrane filter (mesh size:
0.45 μm) as described by Toki et al. (2016). The samples
were filtered and divided into two polyethylene bottles
for onboard ­NH4+ concentration analysis and laboratory27º47.413′

27º47.420′

27º47.420′

27º47.420′

27º47.416′

27º47.416′

27º14.938′

27º14.858′

27º14.841′

25º16.177′

25º16.177′

Original

Original

Original

Original

Original

Original

HAKUREIHAKUREIHAKUREI

JADE

JADE

GusukuKY11-02
D1249
W-3

NT11-16
D1315
W-1

NT11-16
D1315
W-2

NT11-16
D1315
W-3

NT12-06
D1356
W-2

NT12-06
D1356
W-4

NT10-17
D1191
W-B

NT11-15
D1313
W-3

NT11-15
D1313
W-4

NT10-17
D1184
W-A

NT10-17
D1185
B-1

YK07-04
D999 A

HatomaKnoll
123° 50.471′ 1474

127º04.786′ 1340

127º04.786′ 1340

127º03.995′ 1623

127º03.962′ 1617

127º04.134′ 1593

126º53.890′ 1035

126º53.890′ 1035

126º54.043′ 1057

126º54.043′ 1057

126º54.043′ 1057

126º54.048′ 1048

126º53.803′ 978

126º53.861′ 1028

m

Depth

5.21

4.88

5.51

‒
‒

5.10

4.75

5.43

4.62

4.69

5.11

4.95

337

‒

582

598

246

388

435

447

457

460

‒

600

444

444

431

430

423

451

456

454

mmol/
kg

Na

591

596

572

571

574

576

‒

5.46

584

‒

316

251

179

248

309

309

306

306

306

311

309

571

mmol/
kg

Cl‒

‒

25 °C

℃
310

pH

Tmax

3.3

4.7

9.1

2.2

5.8

5.0

0.9

0.3

0.1

0.1

0.2

0.2

1.0

0.1

mmol/
kg

Mg

11.2

19.5

19.8

22.4

22.1

22.2

23.0

23.1

21.1

21.1

20.9

22.0

21.4

22.0

mmol/
kg

Ca

45.5

52.9

56.1

64.3

62.9

64.5

66.4

66.6

70.4

70.0

69.5

74.2

73.4

74.6

mmol/
kg

K

8.0

3.4

3.5

4.3

4.3

3.7

1.9

1.9

1.9

2.0

1.9

1.8

2.1

2.0

mmol/
kg

NH4+

B

2.5

2.8

2.9

3.5

3.4

3.5

2.0

2.0

1.9

1.9

1.8

2.0

2.0

2.0

mmol/
kg

Sr

56.1

101.7

103.4

129.4

122.6

121.7

81.8

82.0

76.6

76.4

75.6

79.7

77.6

79.3

µmol/
kg

Sr/86Sr[Sr]EHF

5.79 × ­10–2

0.70852

0.708910.70895

8.77 × ­10–2
4.58 × ­10–2

0.709200.70769

1.14 × ­10–2

1.67 × ­10–2

0.70764

3.68 × ­10–3

0.709230.70770

1.48 × ­10–3

4.75 × ­10–2

0.70769

1.49 × ­10–3

0.709240.70769

2.21 × ­10–3

4.11 × ­10–2

0.70768

0.70765

2.00 × ­10–3

1.30 × ­10–2

0.000018

0.000011

0.000011

0.000011

0.000008

0.000013

0.000009

0.000012

0.000013

0.000013

0.000014

0.000015

0.000015

52.7

99.3

98.8

129.1

121.2

120.4

81.1

81.8

76.5

76.3

75.5

79.6

76.9

79.2

0.000014

Error

1.52 × ­10–3
0.70765

87

µmol/
kg

Mg/Sr
mmol/
µmol

(2022) 9:59

24° 51.522′

27º47.454′

Original

KY11-02
D1247
W-4

Izena
Hole

27º47.410’

Original

KY11-02
D1247
W-2

E

N

Iheya
North
Knoll

Longitude

Latitude

Site

SampleID

Field ID

Table 1 Chemical and isotopic compositions of hydrothermal fluids from Okinawa Trough in this study

Toki et al. Progress in Earth and Planetary Science
Page 5 of 19

Field ID

24° 51.522′

24° 51.488′

24° 51.488′

24° 51.488′

24° 51.496′

24° 51.564′

24° 51.509′

24° 51.464′

24° 51.464′

24° 51.498′

24° 51.498′

GusukuGusukuGusukuGusukuGusukuGusukuGusukuGusuku

BSW

GusukuIri

Iri

GusukuGusukuYK07-04
D999
W-3

YK07-04
D999 B-2

YK07-04
D999 B-3

YK07-04
D1000 AYK07-04
D1001 AYK07-04
D1001
W-1

YK07-04
D1001
W-2

NT07-12
D706
W-1

NT07-12
D707 V-2

NT07-12
D709
W-2

NT07-12
D709
W-3

NT07-12
D709
W-4

NT08-13
D866
W-1

NT08-13D866
W-4

24° 51.522′

24° 51.522′

24° 51.522′

24° 51.522′

Gusukum

Depth123° 50.479′ 1474

123° 50.479′ 1474

123° 50.380′ 1532

123° 50.380′ 1532

123° 50.466′ 1477

123° 50.387′ 1527

123° 50.467′ 1474

123° 50.468′ 1469

123° 50.468′ 1469

123° 50.468′ 1469

123° 50.471′ 1473

123° 50.471′ 1474

123° 50.471′ 1474

123° 50.471′ 1474

123° 50.471′ 1474

E

N

YK07-04
D999
W-2

Longitude

Latitude

Site

SampleID

Table 1 (continued)

325

10

70

75

320

4

322

255

322

‒

‒

‒

‒

457

‒
‒

5.67
‒

186

388

367

254

457

204

279

288

315

400

437

382

271

307

mmol/
kg

Na

479

‒

‒

5.71

356

5.44

542

‒

‒

7.38

394

414

493

537

484

361

‒

mmol/
kg

Cl‒

5.38

5.61

6.34

‒

‒

‒
‒

5.41

323

‒

25 °C

℃
321

pH

Tmax

0.8

52.3

31.0

26.4

1.8

52.8

0.2

8.8

2.8

10.5

39.8

46.8

34.7

8.4

20.5

mmol/
kg

Mg

9.9

9.8

12.1

12.2

13.5

10.3

12.4

10.8

7.0

13.2

10.3

8.9

9.6

10.5

9.4

mmol/
kg

Ca

35.6

10.3

28.1

29.5

49.0

10.6

42.9

45.5

55.4

48.7

18.0

13.7

28.4

43.2

30.5

mmol/
kg

K

0.004

0.1

2.7

‒

2.0

39.8

87.5

71.2

‒

0.4

69.1

53.3

81.4

49.2

51.6

57.0

62.7

81.7

83.2

73.6

54.2

60.4

µmol/
kg

Sr

‒

‒

‒

‒
6.2

‒

2.3

3.0

2.5

0.7

0.4

1.0

2.4

1.6

mmol/
kg

B

‒

‒

7.2

5.7

1.1

‒

2.2

6.8

‒

mmol/
kg

NH4+

1.93 × ­10–2

5.97 × ­10–1

4.35 × ­10–1

3.82 × ­10–1

3.33 × ­10–2

6.48 × ­10–1

4.95 × ­10–3

0.70856

0.709140.70886

0.708800.708540.709170.70857

0.708540.70850

4.86 × ­10–2
1.70 × ­10–1

0.70868

0.709070.709210.70900

1.67 × ­10–1

4.87 × ­10–1

5.62 × ­10–1

1.54 × ­10–1

0.000018

0.000014

0.000014

0.000014

0.000017

0.000015

0.000017

0.000022

0.000016

0.000013

0.000013

0.000017

0.000017

38.9

‒

‒

37.0

51.4

‒

49.0

41.4

54.2

51.7

‒

‒

‒

44.7

0.70866

1.54 × ­10–1

0.000017

34.9[Sr]EHF

0.708860.000032

Error

3.40 × ­10–1

Sr/86Sr
µmol/
kg

87

mmol/
µmol

Mg/Sr

Toki et al. Progress in Earth and Planetary Science(2022) 9:59
Page 6 of 19

Field ID

24° 51.480′

24° 51.514′

24° 51.514′

24° 51.505′

24° 51.468′

24° 51.609′

24° 51.594′

24° 51.594′

24° 51.512′

24° 51.522′

24° 51.489′

Agari

Agari

GusukuGusukuGusuku

Agari

BSW

Chura

Chura

GusukuGusukuAgari

GusukuGusukuGusukuNT08-13
D869 V-1

NT08-13
D869 V-2

NT08-13
D869
W-2

NT08-13
D869
W-4

NT08-13
D870
W-1

NT08-13D870
W-4

NT08-13
D871 V-1

NT08-13
D871
W-1

NT08-13
D871
W-3

NT08-13
D872 V-1

NT08-13
D872
W-1

NT08-13
D872
W-4

NT08-13
D873 V-1

NT08-13
D873 V-2

NT08-13
D873 V-3

123° 50.466′ 1472

123° 50.450′ 1483

123° 50.450′ 1483

123° 50.504′ 1479

123° 50.467′ 1476

123° 50.477′ 1474

123° 50.392′ 1504

123° 50.392′ 1504

123° 50.350′ 1452

123° 50.515′ 1479

123° 50.483′ 1475

123° 50.468′ 1475

123° 50.468′ 1475

123° 50.503′ 1478

123° 50.503′ 1478

123° 50.476′ 1470

m

Depth

539
435
‒
‒

7.49
‒
‒
‒

‒

‒

180
‒

549

526

‒
‒

‒

‒

‒

180

‒

‒

‒

89

178

‒

‒

‒

9

206

535

6.29

‒

522

538

425

5.47

5.71

‒

‒

‒

8

8

‒

301

‒

mmol/
kg

Cl‒

‒

25 °C

℃
7

pH

Tmax

426

453

409

425

210

421

369

345

454

424

454

451

454

321

281

419

mmol/
kg

Na

45.1

50.9

44.2

47.8

1.6

46.8

32.3

18.4

52.8

48.6

52.5

51.9

52.3

11.4

0.8

47.4

mmol/
kg

Mg

13.8

10.1

16.7

9.5

11.7

9.8

11.3

13.4

10.2

9.7

10.1

10.4

10.1

15.3

16.8

9.6

mmol/
kg

Ca

16.2

10.2

12.4

9.5

39.4

10.0

24.2

39.7

10.0

9.3

9.9

10.1

10.1

46.9

53.3

9.5

mmol/
kg

K

0.9

N.D

‒

‒

‒

‒

‒

4.3

0.005

‒

0.01

0.1

0.1

4.8

‒

N.D

mmol/
kg

NH4+

0.7

0.4

0.5

0.4

2.2

0.4

1.2

2.0

0.4

0.4

0.4

0.4

0.4

2.5

2.9

0.4

mmol/
kg

B

97.8

89.1

110.1

83.7

48.3

85.2

76.2

71.6

89.6

83.8

89.2

92.0

89.0

70.0

68.1

83.6

µmol/
kg

Sr

Sr/86Sr

0.709175.64 × ­10–1

4.61 × ­10–1

5.72 × ­10–1

4.02 × ­10–1

5.71 × ­10–1

3.26 × ­10–2

5.50 × ­10–1

4.24 × ­10–1

2.56 × ­10–1

5.89 × ­10–1

5.80 × ­10–1

0.709020.709150.709020.709160.70859

0.709160.70899

0.708780.709170.709210.709140.709225.87 × ­10–1

5.88 × ­10–1

0.708700.708540.7091487

1.63 × ­10–1

1.23 × ­10–2

5.68 × ­10–1

mmol/
µmol

Mg/Sr

0.000019

0.000018

0.000018

0.000016

0.000021

0.000015

0.000015

0.000017

0.000013

0.000022

0.000019

0.000015

0.000015

0.000015

0.000016

0.000014

Error

‒

‒

‒

‒

46.5

‒

‒

52.5

‒

‒

‒

‒

‒

59.0

67.4

‒

[Sr]EHFµmol/
kg(2022) 9:59

24° 51.501′

24° 51.493′

24° 51.493′

24° 51.480′

24° 51.501′

GusukuE

N

NT08-13
D867
W-2

Longitude

Latitude

Site

SampleID

Table 1 (continued)

Toki et al. Progress in Earth and Planetary Science
Page 7 of 19

Plume

Plume

Tiger

Abyss

Tiger

Tiger

Tiger

Tiger

YK03-05
D762-N1

YK03-05
D762-N2

YK03-05
D762-W1

YK03-05
D762-BYK03-05
D763-W2

YK03-05
D763-W3

YK03-05
D763-W4

YK03-05
D763-B‒: No data

Tiger

24° 51.487′

GusukuNT09-11
D1037
W-3

YK03-05
D760-B24° 51.511′

GusukuNT09-11D1035
W-1

24° 50.885′

24° 50.885′

24° 50.885′

24° 50.885′

24° 50.787′

24° 50.897′

24° 50.905′

24° 50.905′

24° 50.897′

24° 50.891′

24° 51.510′

GusukuNT08-13
D873
W-1

Tiger

24° 51.521′

Gusukum

Depth

122° 42.014′ 1361

122° 42.014′ 1361

122° 42.014′ 1361

122° 42.014′ 1361

122° 42.038′ 1379

122° 42.020′ 1365

122° 42.033′ 1358

122° 42.033′ 1358

122° 42.025′ 1362

122° 42.035′ 1362

123° 50.463′ 1472

123° 50.473′ 1473

123° 50.467′ 1477

123° 50.467′ 1477

E

N

NT08-13
D873 V-4

Longitude

Latitude

Site

SampleID

Yonaguni YK03-05
D759-W1
Knoll IV

Field ID

Table 1 (continued)

‒

‒

4.84
‒

‒

4.74

4.91

5.30

‒

6.95

210

210

210

‒

‒

‒

6.55

5.05

‒
‒

5.29

5.16

5.49

323

320

320

6.35

25 °C

℃

‒

pH

Tmax

583

583

589

600

547

606

553

563

525

550

391

312

536

366

mmol/
kg

Cl‒

‒

49.4

30.7

32.1

‒
465

22.0

45.9

18.4

49.0

48.0

40.7

49.2

1.0

2.0

51.5

14.9

mmol/
kg

Mg

454

456

‒

437

437

‒

450

287

239

449

279

mmol/
kg

Na

10.6

15.6

10.3

17.8

9.6

20.5

8.9

8.9

10.0

9.4

16.0

12.2

10.0

13.5

mmol/
kg

Ca

10.1

43.7

41.5

58.0

14.4

64.4

9.3

9.4

17.3

9.9

47.4

41.8

9.9

33.3

mmol/
kg

K

‒

3.7

3.3

5.0

1.0

5.9

N.D

0.04

‒

0.1

6.2

6.6

0.004

5.7

mmol/
kg

NH4+

0.4

1.9

1.8

2.5

0.7

2.8

0.3

0.3

0.8

0.4

3.0

2.6

0.4

1.7

mmol/
kg

B

87.3

97.0

63.8

98.8

84.1

105.3

79.2

79.0

58.7

81.6

62.4

49.3

88.2

70.0

µmol/
kg

Sr

Sr/86Sr

0.70855

0.709221.60 × ­10–2
6.02 × ­10–1

0.709290.71053

3.17 × ­10–1
5.66 × ­10–1

0.710540.71104

2.23 × ­10–1
5.03 × ­10–1

0.70935

0.71126

0.709170.70920

5.45 × ­10–1

1.74 × ­10–1

6.18 × ­10–1

6.08 × ­10–1

0.70961

0.70852

4.00 × ­10–2

6.94 × ­10–1

0.70916

5.83 × ­10–1

0.000010

0.000011

0.000016

0.000014

0.000013

0.000012

0.000010

0.000010

0.000011

0.000007

0.000016

0.000016

0.000016

‒

‒

‒

83.8

‒

95.3

‒

‒

‒

‒

61.5

47.1

‒

54.9

[Sr]EHF

2.13 × ­10–1

0.000014

Error

µmol/
kg
0.70878

87

mmol/
µmol

Mg/Sr

Toki et al. Progress in Earth and Planetary Science(2022) 9:59
Page 8 of 19

Toki et al. Progress in Earth and Planetary Science(2022) 9:59

­Cl– concentration analysis and for on-land cation (Na,Mg, Ca, K, B, and Sr) concentrations and Sr isotopic ratio
analyses. The samples in polyethylene bottles for cation
concentration and Sr isotopic ratio analyses were further
acidified (pH < 2) with 3 M ­HNO3 and transported to thelaboratory.
3.2 Analytical procedure

Analyses of various chemical components of the fluid
were performed as previously described (Toki et al.
2016). The pH of the fluid was analyzed using a glass
electrode at room temperature, ~ 20 to 25 °C. Fluid C­ l–concentrations were analyzed by Mohr titration, and values were determined using standard seawater (IAPSO)
to within 1% precision. Fluid N­ H4+ concentrations wereanalyzed using the colorimetric method, and a calibration curve was constructed using an ammonium chloride
solution to determine the concentration. One sample wasmeasured three times for each analysis to determine theanalytical precision. Analytical precision was within 1%.
Cations in the fluid were analyzed using inductively coupled plasma optical emission spectroscopy (ICP-OES;
Model 5100, Agilent Technology, Santa Clara, CA, USA)at Kyushu University. Concentrations were determinedusing standard solutions prepared by mixing standard
reagents for atomic absorption with an analytical precision within 3%.
Sr was separated from fluid samples with a Sr-Spec
resin (mesh size: 50–100 µm; Eichrom Industries), as
described by Pin et al. (1994). A 0.15-mL aliquot wasplaced into a precleaned vial made of perfluoroalkoxy
alkane, dried at ~ 100 °C and then, dissolved in 0.15 mL
2 M ­HNO3 (EL grade). A mini-column with a 0.1 mL
Sr-Spec resin bed was first washed with 1 mL ultrapure
water (Resistivity at 25 °C:18.2 MΩ·cm) and then, conditioned with 0.5 mL 2 M H­ NO3. Next, 0.15 mL of thesample solution was loaded onto the column, and the
major ions were washed out with 0.15 mL 2 M ­HNO3.
Barium was eluted with 1.2 mL of 8 M ­HNO3. Next, we
washed the column with 0.4 mL 2 M ­HNO3 and eluted
the Sr fraction on the column by adding 1 mL of 0.05 M­HNO3. The Sr fraction was collected in a precleaned vial
made of perfluoroalkoxy alkane and dried at ~ 100 °C on
a hot plate.
The Sr isotopic ratio (87Sr/86Sr) of the samples from the
Hatoma Knoll in the SOT was measured using thermal
ionization mass spectrometry (MAT262; Finnigan). The
dried eluent was dissolved in 2 µL of 1 M ­H3PO4, andthe solution was loaded onto a Ta filament. The drop washeated and transferred to a thermal ionization mass spectrometer. The measured 87Sr/86Sr ratio was normalized
to an 86Sr/88Sr value of 0.1194. We obtained an averagevalue of 0.710235 ± 0.000028 (1σ, n = 9) for the National

Page 9 of 19

Institute of Standards and Technology (NIST) SRM-987
standard.
The 87Sr/86Sr ratios of the other samples were measured
via multiple collector-inductively coupled plasma-mass
spectrometry (Neptune Plus) with a solution sample of
200 ppb Sr prepared with 0.3 M ­HNO3. The values werestandardized to NIST SRM-987 (87Sr/86Sr = 0.71025). The
precision is the standard deviation of the measured SRM987 sample: 0.710252 ± 0.000025 (2σ; n = 16).
3.3 Endmember calculation

The sample may inevitably be mixed with seawater fromthe sampling inlet, or the hydrothermal fluids may be
combined with seawater below the seafloor before venting. Therefore, it is customary to determine the chemical composition of pure hydrothermal fluids by assumingthat they do not contain Mg (Von Damm 1995). This isbecause laboratory experiments have shown that whenbasalt, andesite, and rhyolite react with seawater, mostof the Mg is removed from the seawater (Bischoff andDickson 1975; Mottl and Holland 1978; Seyfried Jr andBischoff 1981; Hajash and Chandler 1982; Shiraki et al.
1987). The endmember Sr concentration for each sample
was estimated using the following equations:[Mg]sample = 1−f × [Mg]EHF + f × [Mg]SW (1)[Sr]sample = 1−f × [Sr]EHF + f × [Sr]SW(2)

where subscripts indicate the sample, endmemberhydrothermal fluid (EHF), and seawater (SW). ­[Mg]EHF,­[Mg]SW, and ­[Sr]SW were 0, 53 mmol/kg, and 87 µmol/
kg, respectively (Von Damm et al. 1985a); therefore,­[Sr]EHF can be calculated from the analytical results of the­[Mg]sample and [Sr] sample. Because Mg-poor hydrothermal fluids are obtained from most hydrothermal systems(Table 1; Fig. 2), pure hydrothermal fluids are assumed
to be Mg-free to obtain the endmembers. However, thisstudy did not receive Mg-free hydrothermal fluids fromthe Yonaguni Knoll IV hydrothermal system. Suzuki
et al. (2008) also collected samples from the Yonaguni
Knoll IV hydrothermal system and obtained almost Mgfree hydrothermal fluids. Therefore, even in the Yonaguni Knoll IV hydrothermal system, pure hydrothermal
water was assumed to be Mg-free, and endmembers wereobtained. Only by standardization will it be possible tocompare hydrothermal waters and chemical compositions worldwide (Von Damm 1995).
To estimate the 87Sr/86Sr ratio of the pure hydrothermal fluid at each sampling site, the 87Sr/86Sr ratio in thefluid sample was plotted against the ­[Mg]sample/[Sr]sample
ratio (Fig. 3). In this plot, mixing between pure fluid andseawater is represented by a straight line. The Mg-freeToki et al. Progress in Earth and Planetary Science(2022) 9:59

Fig. 2 Relationship between Mg and Sr concentrations inhydrothermal fluids from the Iheya North Knoll, HAKUREI, JADE,
Hatoma Knoll, and Yonaguni Knoll IV sites in the Okinawa Trough.
Light blue stars indicate the value of seawater. Red lines areregression lines through all data and seawater for each site. No suchline was drawn for the Yonaguni Knoll IV because of poor fit

Page 10 of 19

entrainment, we limited the discussion to samples with
Mg < 30 mmol/kg (Table 1).
The Sr isotopic ratios of the fluid samples collected
at Iheya North Knoll, HAKUREI, JADE, Hatoma Knoll,
and Yonaguni Knoll IV were 0.70764–0.70770, 0.70920–
0.70924, 0.70891–0.70895, 0.70850–0.70922, and
0.70917–0.71126, respectively (Table 1). The 87Sr/86Sr
ratios of the pure hydrothermal fluid at each hydrothermal site were estimated based on the 87Sr/86Sr–[Sr]/[Mg] plot (see Sect. 3.3, Endmember calculation) and arelisted in Table 2. The estimated Sr isotopic ratios of theendmembers ranged from 0.7077 (Iheya North Knoll) to
0.7120 (Yonaguni Knoll IV). The endmember Sr isotopic
ratio of the hydrothermal fluid at Yonaguni Knoll IV was
the highest among all Sr isotopic ratios reported to datefrom hydrothermal sites worldwide (Table 2 and Fig. 4).
The endmember Sr isotopic ratios differed between the
HAKUREI (0.7092) and JADE (0.7089) sites. However,both sites are located at the Izena Hole (Ishibashi et al.
2014). The Sr isotopic ratios of the endmembers did notcorrelate with the Sr concentrations of the endmembers.
The Okinawa Trough exhibited no geographic trends in
Sr isotopic ratio (Fig. 1 and Table 2).

5 Discussion
5.1 Sr isotopic ratios of venting fluids and subseafloor
reservoirs

Fig. 3 Relationship between Mg/Sr and 87Sr/86Sr in hydrothermal
fluids from the Iheya North Knoll, HAKUREI, JADE, Hatoma Knoll, andYonaguni Knoll IV sites in the Okinawa Trough. For an explanation of
insets, refer to Fig. 2

y-intercept of the regression line for a hydrothermal site
represents the 87Sr/86Sr ratio of the pure hydrothermal
fluid for the hydrothermal site (Albarède et al. 1981).

4 Results
Table 1 lists the analytical results, including the Mg
and Sr concentrations and the 87Sr/86Sr ratios. The
endmember Sr concentrations of each sample were
calculated using Eq. (1) and (2), which yielded concentrations of 75.5–81.8 µmol/kg (Iheya North Knoll),
120.3–129.1 µmol/kg (HAKUREI), 98.7–99.3 µmol/kg(JADE), 34.7–67.4 µmol/kg (Hatoma Knoll), and 83.6–
95.1 µmol/kg (Yonaguni Knoll IV) (Table 1). As the error
for endmember extrapolation increases with seawater

To understand the diverse fluid–sediment interactions through interpretation of the Sr isotopic ratios, we
compared the Sr isotopic ratios of hydrothermal fluids
observed to date with those of representative subseafloor
Sr reservoirs worldwide (Fig. 4), classified based on tectonic setting and sediment occurrence at the sites. The Sr
isotopic ratios of hydrothermal fluids in sediment-starved
mid-ocean ridge systems fell within a specific range of
0.7030–0.7042 at EPR21° N and EPR13° N (Albarède et al.
1981; Michard et al. 1984), Kairei (Gamo et al. 2001), andthe Kane Fracture Zone and the Endeavor Segment of
the Juan de Fuca Ridge (Campbell et al. 1988; Jamieson
et al. 2016). The Sr isotopic ratios of sediment-starved

Table 2 End-members of Sr isotope ratios in hydrothermal fluids
from hydrothermal fields in Okinawa TroughField ID

Site

87

HAKUREI

0.7092JADE

0.7089Iheya North Knoll
Izena Hole

Sr/86Sr

0.7077

Hatoma Knoll

0.7085

Yanaguni Knoll IV

0.7120Minami-Ensei Knoll

0.7100Toki et al. Progress in Earth and Planetary Science(2022) 9:59

Page 11 of 19

Fig. 4 Endmember Sr isotopic ratios of hydrothermal fluids from the Okinawa Trough and sites worldwide. Black represents data from MOR
hydrothermal systems, blue represents data from backarc systems, and red represents data from arc volcanoes. Open symbols indicate
sediment-starved systems, and closed symbols indicate sediment-associated systems. The Sr isotopic ratios of solids (MORB, volcanic rocks fromthe Okinawa Trough, and sediments from the East China Sea) are also shown. A light blue star and dash-dotted line indicate the Sr isotopic ratios ofseawaterbackarc systems were somewhat higher and within a certain range (0.7036–0.7056) in the Lau Basin (Mottl et al.
2011), Manus Basin (Reeves et al. 2011), North Fiji Basin(Grimaud et al. 1991), and Mariana Trough (Kusakabe
et al. 1990). The Sr isotopic ratios of sediment-associated
MOR (Mid-ocean ridge) systems are 0.7043 in the Middle Valley (Butterfield et al. 1994), 0.7056 in the Guaymas
Basin in the Gulf of California (Piepgras and Wasserburg
1985), 0.7081 at Loki’s Castle on the Arctic MOR (Baumberger et al. 2016), and 0.7099 at the Escanaba Trough of
the Juan de Fuca Ridge (Campbell et al. 1994), which are
relatively high compared with those of sediment-starvedhydrothermal systems. Sr isotopic ratios of volcanic rocks
were 0.704–0.706 in the Okinawa Trough (Appendix)
and 0.7023–0.7030 at the MOR (MOR basalt, MORB;White et al. 1987). The Sr isotopic ratios of bulk sedimenton the continental shelf of the East China Sea were virtually identical to those of bulk sediment in the Okinawa
Trough (0.713–0.719; Asahara et al. 1995).

5.2 Relationships of Sr isotopic ratios with representative
chemical and isotopic compositions of hydrothermal
fluidsTo assess the co-variation between Sr isotopic ratios andother chemical and isotopic components from the global
vent dataset, we plotted several representative parameters of the endmember hydrothermal fluids against
the Sr isotopic ratios (Fig. 5a‒g). No clear correlation
between Sr/Cl and Sr isotopic ratios was detected among
global sediment-associated sites, including the Okinawa
Trough sites (Fig. 5a), whereas Sr/Cl ratios varied even
within the sediment-starved sites. This finding indicatesthat the Sr isotopic ratios of sediment-associated sites are
determined not by a single Sr reservoir or the degree of
Sr leaching from a reservoir but by multiple Sr reservoirs.
Moreover, no clear correlation was observed between the
Ca/Cl and Sr isotopic ratios in the global hydrothermal
fluids (Fig. 5b). Reeves et al. (2010) conducted a seriesof observations in the Manus backarc basin. They foundToki et al. Progress in Earth and Planetary Science(2022) 9:59

Page 12 of 19

Fig. 5 Relationships between endmember Sr isotopic ratios in the Okinawa Trough hydrothermal fluids in this study and the concentration ratiosof a Sr/Cl, b Ca/Cl, c K/Cl, d B/Cl, e ­NH4+, f ­CH4, and g ­H2

Toki et al. Progress in Earth and Planetary Science(2022) 9:59

that the Ca/Cl ratio variation among hydrothermal fluids
in this basin is related to the host rock type. For example,
the Vienna Wood site is basalt-hosted with a Ca/Cl ratioof approximately 0.12. In contrast, the felsic rock-hosted
sites have Ca/Cl ratios lower than 0.06 (Fig. 5b). The relatively low Ca/Cl ratios and variable Sr isotopic ratios in
the Okinawa Trough fluids may be attributable to a lower
contribution from the Ca-rich mafic rock and comparable contributions from multiple Sr reservoirs (Table 3).
Similarly, the large variation in the Ca/Cl ratio but constant Sr isotopic ratios similar to the mid-ocean ridge
basalt (MORB) value in sediment-starved hydrothermal
fluids may be explained by Ca leaching from plagioclase
in basalt (partly substituted by Sr) via albitization (Rosenbauer et al. 1988; Von Damm and Bischoff 1987). No correlation was observed between the K/Cl and Sr isotopic
ratios in the hydrothermal fluids analyzed (Fig. 5b and c).
Because felsic rocks are more enriched in K than mafic
rocks (Sakai et al. 1990), felsic rock-hosted hydrothermal
systems, such as the Okinawa Trough and Manus Basin(Reeves et al. 2011), have higher K/Cl ratios than basalthosted hydrothermal systems (Fig. 5c). The K/Cl ratios at
the sediment-associated sites were not always high, andwe found no clear correlation between the K/Cl and Sr
isotopic ratios. This finding suggests that the Sr isotopic
ratios in hydrothermal fluids are regulated by factors
other than the host rock type (Fig. 5a‒c).
The B, N­ H4+, ­CH4, and ­H2 concentrations were higher
in fluids at sediment-associated hydrothermal sites andwere relatively strongly correlated with Sr isotopic ratios(Fig. 5d‒g), suggesting that 87Sr-rich Sr reservoirs in thefluids were mainly derived from the same B, ­NH4+, ­CH4,and ­H2 reservoir sources at sediment-associated hydrothermal sites. ­NH4+, ­CH4, and ­H2 in fluids are thought
to be generated by the decomposition of sedimentaryorganic matter, represented by ­(CH2O)106 ­(NH3)16 ­H3PO4(Redfield 1934), as follows (Piepgras and Wasserburg
1985):

Page 13 of 19

suspended particles, including organic particles (Spivack et al. 1987). Sedimentary B is bound to the lattice ofclay minerals during burial (Williams et al. 2001) and isreleased after exposure to high temperatures (> 300 °C;
You and Gieskes 2001). Although B enrichment in fluids
at sediment-associated sites is not directly related to thedecomposition of organic matter (unlike C­ H4 and N­ H3),
sedimentary B and its release at high temperatures are
responsible for the abundance of B in fluids.
5.3 Virtually complete removal of seawater‑derived Srsuggested from sediment‑starved system observations

The Sr isotopic ratios of hydrothermal endmember ventfluids were comparable to those of major subseafloor Sr
reservoirs, such as volcanic rocks, recharged seawater,and bulk sediment (Fig. 4). Thus, the Sr isotopic ratiosof the hydrothermal endmember fluids can be explainedby the Sr influx/efflux of these reservoirs during subseafloor hydrothermal-fluid circulation. The Sr isotopic
ratios of hydrothermal fluids in the sediment-starved
MOR and backarc systems were within a certain range(0.7034–0.7056), and the Sr isotopic ratios of hydrothermal endmember fluids in the sediment-starved setting
were much lower than the seawater value (0.7092) andclose to the value for volcanic rocks (0.7023–0.7056). The
low Sr isotopic ratios and consistency between fluids andvolcanic rocks demonstrate that virtually all seawaterderived 87Sr recharged into the subseafloor was removed,
and 87Sr-depleted Sr was leached from volcanic rocks
into the fluid. This observation-based model is consistent with experimental simulation studies on hydrothermal fluid–mineral interactions, in which Sr in the liquid
phase (i.e., recharged seawater) is removed throughco-precipitation with anhydrite when heated to 200 °C(Albarède et al. 1981; Wilckens et al. 2019), whereas Sr
in some minerals (e.g., from volcanic rock and sediment)is released into fluids at temperatures above 200 °C (Wei

(CH2 O)106 (NH3 )16 H3 PO4 + 106H2 O → 106CO2 + 212H2 + 16NH3 + H3 PO4 ,(3)(CH2 O)106 (NH3 )16 H3 PO4 + 106H2 O → 53CO2 + 53CH4 + 16NH3 + H3 PO4(4)These ideal equations assume a reaction that generates­CO2 and an intermediate product, ­H2 (Eq. 1), followed bymethanogenesis with 4:1 H­ 2:CO2 stoichiometry (Eq. 4).
In these equations, the ­NH3 generated was equilibrated
with water as follows:−
NH3 + H2 O ↔ NH+
4 + OH (5).
B in the fluid is considered to originate from B in thesediment. Sedimentary B is generally abundant becauseof its accumulation by adsorption onto the surfaces of

2007; Berndt et al. 1988; James et al. 2003). The removalof seawater-derived Sr through co-precipitation withanhydrite is also expected in sediment-associated hightemperature hydrothermal fluids because of its sulfatedepleted nature at temperatures higher than 200 °C(Albarède et al. 1981; Wilckens et al. 2019). This suggeststhat Sr derived from recharged seawater accounts for anegligible proportion of Sr in the hydrothermal vent fluids in the Okinawa Trough.

Toki et al. Progress in Earth and Planetary Science(2022) 9:59

5.4 Possible contribution of 87Sr‑rich terrestrial minerals
in sediment to Sr isotopic ratios of hydrothermal fluidin the Okinawa TroughBecause the Sr isotopic ratios of volcanic rocks in the
Okinawa Trough are low and fall within a specific range(Appendix), the high Sr isotopic ratios of fluids at the
Okinawa Trough sites (Fig. 4) are attributable to reactions with 87Sr-rich sediments. To understand how sedimentary Sr reservoirs contribute to the venting fluid at
sediment-associated sites, we considered the compositional characteristics of the Okinawa Trough sediments
and the possible Sr isotopic ratios of each Sr-bearing
component.
The sediments of the Okinawa Trough contain several
Sr-bearing components, such as organic matter originating from primary production in the ocean surface layer,
biological skeletons, and clastic debris weathered fromthe continental crust. Organic matter occupies < 10%of the bulk sediment, and the Sr content of sedimentary organic matter is approximately 10 ppm (Gao et al.
2008). The Sr content in the organic matter can be as low
as ppb in bulk sediments and is negligible as a Sr sourcefor hydrothermal fluids (Gao et al. 2008). The 88Sr/86Sr
ratios of hydrothermal fluids suggest a significant contribution of Sr derived from biogenic carbonates to hydrothermal fluid Sr (Yoshimura et al. 2020). The Sr isotopic
ratio of sedimentary biogenic carbonate produced byforaminifera or coccoliths in seawater columns is identical to that of seawater at the time of formation (Stevenson et al. 2009). Throughout the history of the Okinawa
Trough, the variation in seawater 87Sr/86Sr isotopic ratios
has remained virtually constant at 0.7092 (Glasby andNotsu 2003; Elderfield and Schultz 1996; McArthur et al.
2001; Mokadem et al. 2015). Some data in the hydrothermal fluid at Yonaguni Knoll IV fall into the seawater
value, not on the simple mixing line between the 87Srpoor endmember and seawater (Figs. 2 and 3), which may
be because of the contribution of sedimentary biogenic
carbonate.
Because of their terrestrial origin, sedimentary clay
minerals and quartz in the Okinawa Trough have high87
Sr/86Sr ratios. Clay minerals and quartz deposited in
the Okinawa Trough are transported in large quantities
from Eurasia via the Yangtze and Yellow Rivers (Yuanet al. 2008). The Sr isotopic ratios of bulk terrestrial sandson the Loess Plateau, one of the largest mineral sources
for rivers, ranged from 0.71418 to 0.72045 (Yokoo et al.
2004). Specifically, the Sr isotopic ratios in the clay mineral- and quartz-bearing fractions, estimated by processing bulk sand with HCl, were 0.71379–0.71563 and0.71867–0.72157, respectively (Yokoo et al. 2004). The
Sr contents of clay minerals and quartz in the Okinawa
Trough have been found to be 100–300 ppm andPage 14 of 19

100–150 ppm, respectively (Yokoo et al. 2004). Because
high 87Sr/86Sr isotopic ratios (0.71418–0.72045) of sedimentary clay minerals and quartz were expected in the
Okinawa Trough, the 87Sr/86Sr isotopic ratio of 0.7120 at
the Yonaguni Knoll IV site cannot be explained without
the Sr contribution from these terrestrial components.
The 87Sr/86Sr isotopic ratios observed in this study suggest that in addition to volcanic rocks, sedimentary biogenic carbonate, terrestrial clay minerals, and quartz are
likely to be Sr sources for the fluid.
5.5 Involvement of sediment in the high‑temperature
reaction zone

The higher concentrations of B, N
­ H4+, ­CH4, and ­H2 inhydrothermal fluids among sediment-associated systems, including the Okinawa Trough sites, suggest the
involvement of sediments in the high-temperature reaction zone. To explain the variation in Sr isotopes among
the hydrothermal fluids, we developed a simple hypothesis (Fig. 6). The subseafloor hydrothermal fluid loses
seawater-derived Sr during heating by anhydrite precipitation. It monotonously accumulates Sr through Sr leaching from the solid phase. Sr removal, excluding anhydrite
precipitation, did not occur until venting. In this hypothesis, the critical parameter is the temperature of thefluid–sediment interaction. For example, at the Yonaguni IV site, which is characterized by high Sr isotopic
ratios, high-temperature (> 150 °C; Wei 2007) fluid–sediment interactions should be substantial because of theintense leaching of 87Sr-rich components in sedimentary
clay minerals and quartz. In the Iheya North Knoll and
Hatoma Knoll, characterized by relatively low Sr isotopic
ratios in hydrothermal systems in the Okinawa Trough,high-temperature fluid–sediment interactions are absent,
and both sedimentary and volcanic rock Sr reservoirs
play comparable roles in controlling the vent Sr isotopic
ratios.
Our hypothesis can be incorporated into a previously developed model that describes how sedimentaryorganic matter interacts with subseafloor hydrothermal fluid from the perspective of carbon isotopic ratios
in ­CH4 (Fig. 6; Kawagucci et al. 2011, 2013; Kawagucci 2015; Toki et al. 2016). According to the model, the13
C-rich thermogenic ­CH4 and 87Sr-rich terrestrial Sr
in the Yonaguni IV and Minami-Ensei sites indicate asubstantial contribution to the thermal decompositionof sedimentary organic matter (> 100 °C) in addition tohigh-temperature leaching of Sr from clay minerals andquartz, representing a high-temperature fluid–sedimentinteraction in the subseafloor reaction zone (Fig. 6a). In
contrast, the model also indicated that the 13C-depleted
biogenic ­CH4 signature and 87Sr-poor volcanic Sr signature found at the Iheya North Knoll and Hatoma Knoll

Toki et al. Progress in Earth and Planetary Science(2022) 9:59

Page 15 of 19

Fig. 6 Schematic diagram showing two types of hydrothermal circulation in the Okinawa Trough (modified after Kawagucci 2015). Arrows indicatehydrothermal fluid circulation, with color representing the temperature of hydrothermal fluid. a Hydrothermal circulation in contact with sediments
at high temperatures and volcanic rocks at moderate temperatures (> 150 °C). Fluid is in contact with sediments at high temperatures, and thesediments supply Sr (e.g., Yonaguni Knoll IV). b Hydrothermal circulation in contact with sediments at low temperatures and volcanic rocks athigh temperatures. Sr from volcanic rocks is supplied to hydrothermal fluids (e.g., Iheya North Knoll and Hatoma Knoll). Thus, C­ H4 is provided bythermogenic ­CH4 at high temperatures (a) and microbial sources at low temperatures (b)sites suggest metabolic degradation of sedimentaryorganic matter in the cool fluid recharge zone (< 122 °C),
with a supply of volcanic rock-derived Sr following theremoval of Sr associated with anhydrite precipitation
below 150 °C (Fig. 6b). In this case, 87Sr-poor Sr generated through high-temperature fluid–rock interactionsin the reaction and discharge zones is likely the Sr source
for the venting fluid. Thus, the model provides a reasonable explanation for the observed isotopic compositionsof carbon and Sr in the hydrothermal fluids of sedimentassociated systems.

6 Conclusions
Hydrothermal fluids were collected from the Okinawa
Trough, and their Sr isotopic ratios were measured. The
Sr isotopic ratios of hydrothermal fluids from Yonaguni
Knoll IV were extremely high, in agreement with previously published results from the Minami-Ensei Knoll.
Such 87Sr-rich isotopic ratios of hydrothermal fluids
might be a common feature of sediment-associatedhydrothermal systems. These results are consistent witha recent model describing how sedimentary organic matter interacts with fluid within a subseafloor hydrothermal
fluid circulation developed from the perspective of the
carbon isotopic ratios of C
­ H4. In this model, sediments
are distributed around the reaction zone at the Yonaguni
Knoll IV and Minami-Ensei sites, where the hydrothermal fluids bear 13C-rich thermogenic ­CH4 and 87Sr-richterrestrial Sr. However, in the Iheya North Knoll and
Hatoma Knoll sites, where the hydrothermal fluids bear

13

C-depleted biogenic ­CH4 and 87Sr-poor volcanic Sr,
sediments are distributed around the recharge zone.
Thus, both tracers are an advantageous way to determine
whether fluids interact with sediments in the hydrothermal reaction zone. To further justify the model’s ability
to clarify how subseafloor fluid–sediment interactions
affect fluid chemistry, exploring the isotope systematics
of multiple elements such as B and Li will be helpful.

Appendix
Data from volcanic rocks.
For our analyses, we used previously reported data
(Shinjo et al. 1999; Shinjo and Kato 2000) and rock samples collected from dredges and dives (using remotely
operated vehicles and submersibles) of the SOT during
several cruises. Table 3 lists the locations and identitiesof the sampling points, which are plotted as solid yellow circles in Fig. 1b. Analytical procedures were basedon previously described methods (Pin et al. 1994). Before
collecting measurements, we rinsed the rocks in an ultrasonic bath and repeatedly washed them in hot ultrapure
water for 1–3 weeks to eliminate the influence of seawater. The rock samples were crushed to powder using anagate mill before leaching in hot 6 M HCl (EL grade).
After separation, the measurements were collected using
thermal ionization mass spectrometry (MAT262; Finnigan). The dried eluent was dissolved in 1 M ­H3PO4 (2µL), loaded onto a tantalum filament, and next, the solution was dried with a current. The 87Sr/86Sr values wereToki et al. Progress in Earth and Planetary Science(2022) 9:59

Page 16 of 19

Table 3 Rock sample identities and Sr isotope ratios in this study
CruiseYK00-06

SampleLatitudeLongitude

Depth

N

E

m

87

Sr/86Sr

Error

Type

562–3

25° 14.26′

124° 52.89′

1913

0.704565

18

Basaltic andesite

562-8

25° 14.17′124° 52.84′

1924

0.704133

16

Basalt562-1025° 13.85′

124° 52.41′

1898

0.704706

10

Basaltic andesite

569-2

25° 14.42′

124° 52.52′

2085

0.704780

28

Basalt569-3

25° 14.34′

124° 52.82′

1996

0.704741

18

Andesite

2117

0.704026

16

Basalt563-2

25° 15.82′

124° 24.62′

564-2

24° 59.41′

124° 45.84′1703

0.704429

18

Basaltic andesite

NT00-06 Leg 1

460-1

24° 57.83′

124° 12.15′1613

0.704542

16

Dacite460-6

24° 58.34′

124° 13.49′1514

0.704116

16

DaciteN95-06956,232

24° 51.00′

124° 05.00′

1633

0.704572

16

RhyoliteN96-07

9,651,411

24° 46.00′

123° 59.00′

1861

0.705506

16

AndesiteNT00-06 Leg 1

1178-1

24° 42.61′

123° 06.05′

1493

0.704518

16

Basalt

YK00-06
KH-02-1 Leg 3
NT00-06 Leg 1

YK00-06
KH-02-1 Leg 3

1178-224° 42.58′

123° 06.00′

1439

0.704528

18

Basaltic andesite

1177-424° 56.98′

123° 11.86′1757

0.704545

18

Basalt1177-524° 57.00′

123° 12.03′

1749

0.704616

18

Basaltic andesite

1902

0.704616

18

Rhyolite

559-1

24° 56.24′

123° 06.37′

559-6

24° 55.36′

123° 05.91′

1640

0.704334

34

RhyoliteD6-4

24° 51.97′

122° 06.70′

1620

0.704125

18

Basalt459-1

24° 48.85′

122° 56.93′

1542

0.706055

14

Dacite459-3

24° 48.01′

122° 57.00′

1514

0.706077

16

Andesite

1176-524° 51.73′

122° 43.58′

1367

0.70551218

Rhyolite

1176-724° 51.89′

122° 43.18′1156

0.705552

16

Rhyolite

560-4

24° 51.28′

122° 41.96′1284

0.705606

18

Rhyolite

560-7

24° 51.54′122° 42.08′1186

0.705587

14

RhyoliteD5-5

24° 54.29′

122° 44.94′1265

0.705596

18

RhyoliteD5-5X

24° 54.29′

122° 44.94′1265

0.704502

18

Basaltic andesite

normalized using the average value (0.71023 ± 0.00060)
of the NBS987 standard material (National Bureau ofStandards).
Table 3 lists the analytical results for the rock samples.
The values ranged from 0.704026 to 0.706077, indicating that they were more enriched in 87Sr than MORB
(0.7023–0.7030; White et al. 1987). These rock samples were more radiogenic than basalts collected fromthe middle to southern regions of the Okinawa Trough(0.703686–0.704620; n = 15; Shinjo et al. 1999). Volcanic
rocks collected west of 123°E had a higher 87Sr/86Sr ratio
than those collected from the MOT (0.704018–0.704884;
n = 14; Shinjo and Kato 2000). However, volcanic rocks
east of 123°E had values comparable to those of volcanic
rocks from the MOT (Table 3). Based on these integrated data, the 87Sr/86Sr ratio of the volcanic rocks in the
Okinawa Trough was found to be 0.7030–0.7049 (boldred bar in Fig. 3).

Abbreviations
CK: Drilling vessel Chikyu; DSV: Deep-submergence vehicle; EPR: East Pacific
Rise; IAPSO: International Association for the Physical Sciences of the Oceans;
ICP-OES: Inductively coupled plasma optical emission spectroscopy; IODP:
Integrated Ocean Drilling Program; JAMSTEC: Japan Agency for Marine-Earth
Science and Technology; KY: Research vessel Kaiyo; MOR: Mid-ocean ridge;
MORB: Mid-ocean ridge basalt; MOT: Middle Okinawa Trough; NBS: National
Bureau of Standards; NIST: National Institute of Standards and Technol‑ogy; NOT: Northern Okinawa Trough; NT: Research vessel Natsushima; ROV:
Remotely operated vehicle; SOT: Southern Okinawa Trough; SRM: Standard
reference material; SUGAR​: Super-cutting-edge Grand and Advanced Research
Program; WHATS: Water and Hydrothermal Fluid Atsuryoku Tight Sampler;
X-star: Institute for Extra-cutting-edge Science and Technology Avant-Garde
Research; YK: Research vessel Yokosuka.
Acknowledgements
The authors express their heartfelt gratitude to the professors and staff ofthe University of the Ryukyus for their examination of this work, which hasenhanced the quality of the manuscript. We would like to thank Editage(www.​edita​ge.​com) for English language editing.
Author contributions
TT conceived the study, obtained funding, and conducted the sampling,
analysis, data processing, and manuscript writing and revision. TN conductedToki et al. Progress in Earth and Planetary Science(2022) 9:59

the analysis and data processing and wrote the first draft of the manuscript.
YU conducted the analysis and data processing and wrote the first draft of themanuscript. RS obtained funding, conducted the analysis and data processing,
and wrote and revised the manuscript. SH-W analyzed and processed the data
and wrote the first draft of the manuscript. JI conceived the study, obtained
funding, conducted the sampling and analysis, and revised the manuscript.
SK conceived the study, conducted sampling, and revised the manuscript. All
authors read and approved the final manuscript.
Funding
This study was funded by the Council for Science, Technology, and Innovation(CSTI) cross-ministerial Strategic Innovation Promotion Program (SIP), Okinawa
Research Core for Highly Innovative Discipline Science for Marine Science(ORCHIDS), Japan Science and Technology Agency (JST) Research Institute of
Science and Technology for Society (RISTEX) grant (Grant Number: JPM‑
JRX19IA), Japan Society for the Promotion of Science (JSPS) KAKENHI grants(Grant Numbers: JP18H03733 and JP20H04315), and The Research Institute for
Humanity and Nature (RIHN) LINKAGE project (No. 14200145).
Availability of data and materials
All newly obtained data from this study are presented in the manuscript, anddata sharing is not applicable.

Declarations
Competing interests
The authors declare that they have no competing interests.
Author details
1
Department of Chemistry, Biology and Marine Science, Faculty of Science,
University of the Ryukyus, 1 Senbaru, Nishihara, Okinawa 903‑0213, Japan.
2
Research Institute for Humanity and Nature, 457‑4 Motoyama, Kamigamo,
Kita‑Ku, Kyoto 603‑8047, Japan. 3 Department of Physics and Earth Sciences,
Faculty of Science, University of the Ryukyus, 1 Senbaru, Nishihara, Okinawa
903‑0213, Japan. 4 Kobe Ocean‑Bottom Exploration Center, Kobe University,
5‑1‑1 Fukaeminamimachi, Higashinada‑Ku, Kobe 658‑0022, Japan. 5 Super‑Cut‑ting‑Edge Grand and Advanced Research (SUGAR) Program, Institutefor Extra‑Cutting‑Edge Science and Technology Avant‑Garde Research (X‑Star),
Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2‑15
Natsushima‑Cho, Yokosuka 237‑0061, Japan. 6 Research Institute for Global
Change (RIGC), Japan Agency for Marine-Earth Science and Technology (JAM‑
STEC), 2‑15 Natsushima‑Cho, Yokosuka 237‑0061, Japan.
Received: 14 March 2022 Accepted: 22 October 2022

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