1

Identification of tsunami deposits using a combination of radiometric dating and oxygen-2isotope profiles of articulated bivalves

3

A. Kitamuraa, b, M. Itoa, S. Sakaic, Y. Yokoyamad, Y. Miyairid

4aFaculty of Science, Shizuoka University, 836 Oya Suruga-ku, Shizuoka 422-8529, Japan

5b

Center for Integrated Research and Education of Natural hazards, Shizuoka University, 836

6Oya, Suruga-ku, Shizuoka 422-8529, Japan

7

c

8

Yokosuka, Kanagawa 237-0061, Japan

9

d

10

Institute of Biogeochemistry, Japan Agency for Marine-Earth Science and Technology,

Atmosphere and Ocean Research Institute, University of Tokyo, 5-1-5 Kashiwanoha,

Kashiwa, Chiba 277-8564, Japan

11
12

ABSTRACT

13

Four tsunami deposits (deposits I–IV) have been identified on Ishigaki Island, southwest

14

Japan. The youngest tsunami deposit (deposit I) was caused by the Meiwa tsunami, which

15

occurred on 24 April AD 1771, as described in reliable historical documents. Two well-16

preserved specimens of articulated marine bivalve were collected from the youngest tsunami

17

deposit (deposit I) and an additional two from the second-youngest tsunami deposit (deposit

18

II; 920–620 cal. yr BP). The shells were tightly closed and empty inside. No encrusting

19

epifauna or evidence of erosion was observed on the inner or outer shell surfaces. In each20

tsunami deposit, the 14C ages of the shells are nearly identical. The mode of occurrence and21

coincidence of ages mean that these shells were transported and buried alive by tsunamis. We

22analyzed the oxygen-isotope (18O) profiles of these bivalves to determine the seasons of

23

their death, which provides clues to the seasonal timing of tsunamis. Tsunami deposits I and24

II were formed during spring and fall, respectively. The former supports the proposal that25

tsunami deposit I was caused by the 1771 Meiwa tsunami and provides regional radiocarbon

26

reservoir age for the late 1700s; the latter provides a chronological constraint on the27

identification of tsunami deposit II. Thus, a combination of radiometric dating and 18O

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profiles of articulated bivalves derived from tsunami deposits provides important chronologic

29

constraints for examining paleo-tsunami events.

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Keywords: tsunami deposits; oxygen isotope profiles; articulated bivalves; Ryukyu Trench

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1. Introduction

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Over the last two decades, the 2004 Indian Ocean earthquake (Mw 9.1) and the 2011 off

35

the Pacific coast of Tohoku earthquake (Mw 9.0) caused mega-tsunamis that resulted in36

approximately 220,000 and 20,000 deaths, respectively. Both tsunamis transported sandy

37

sediments from beaches and beach ridges, and deposited sand sheets across broad coastal

38

lowland regions (Moore et al., 2006; Naruse et al., 2010; Goto et al., 2011; Shishikura et al.,39

2012; Takashimizu et al., 2012). Age dating of ancient sandy tsunami deposits is essential for40

determining the recurrence interval and size of mega-tsunamis over timescales of several

41

thousand years (e.g., Minoura et al., 2001; Monecke et al., 2008; Shishikura et al., 2010;42

Sawai et al., 2012; Kitamura et al., 2013; Kitamura, 2016). Depositional ages of tsunami

43

deposits are commonly based on 14C dating. However, in case of tsunami events with a44

recurrence interval of 200 years, such as those occurring in the Nankai trough (Ando, 1975),

45

the Kuril trench (Satake et al., 2005), and south-central Chile (Cisternas et al., 2005), the46

analytical error of the dated samples (especially marine specimens) are generally too large to47

identify tsunamis. Using a combination of 14C dating and oxygen-isotope (18O) profiles of

48

articulated bivalves derived from tsunami deposits, it is possible to determine both the49absolute age of a tsunami and the season of the year in which it occurred (Kingston, 2016).

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Articulated bivalves are targeted in such analyses, as they are assumed to have undergone

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transport and burial alive as a result of a tsunami (Kingston, 2016).

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Before the occurrence of the 2011 off the Pacific coast of Tohoku earthquake, the largest

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tsunami recorded in Japan was the Meiwa tsunami of 24 April AD 1771. The tsunami struck

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Ishigaki and Miyako Islands along the southern Ryukyu Trench (Fig. 1), and resulted in

55~12,000 deaths (Kawana and Nakata, 1994). Based on historical records, the run-up heights

56of this tsunami were up to 27 m (Kawana and Nakata, 1994; Goto et al., 2010). That event57

and earlier tsunamis left numerous tsunami boulders in island areas (Kato and Kimura, 1983;

58

Kawana and Nakata, 1994). Examination of tsunami boulders has provided information about

59

the recurrence intervals of large tsunamis, but no information about tsunami size (Goto et al.,60

2010; Araoka et al., 2013). This is because tsunami run-up heights cannot be reconstructed

61

from the distribution of tsunami boulders, although they can be determined from the landward62

margins of sandy tsunami deposits (MacInnes et al., 2009; Abe et al., 2012). However, sandy

63

tsunami deposits have not been found on Ishigaki and neighboring islands.

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Ando et al. (2018) identified four tsunami deposits (I–IV) in an excavated trench on

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Ishigaki Island, southwest Japan (Fig. 2). Although there is no terrestrial organic material

66(e.g., seeds, tree branches, or plant debris) in he tsunami deposits, well-preserved molluscs

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and corals were collected for radiocarbon dating. 14C dating showed that deposits I–IV were68

deposited after 248 cal. yr BP, at 920–620 cal. yr BP, at 1670–1250 cal. yr BP, and at 2700–

692280 to 1670–1250 cal. yr BP, respectively. Ando et al. (2018) also concluded that deposit I

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was caused by the AD 1771 Meiwa tsunami.

71

During the trench survey of Ando et al. (2018), the first author collected well-preserved,

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articulated marine bivalves, two specimens from deposit I and two from deposit II. The shells

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were tightly closed and empty inside. The inner and outer shell surfaces did not exhibit

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encrusting epifauna or evidence of abrasion and dissolution. The 14C ages of the two shells

75

from each tsunami deposit are nearly identical to one another (Fig. 2). Ando et al. (2018)76

therefore concluded that the shells were transported and buried alive by tsunamis. A similar

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interpretation was made by Reinhardt et al. (2006) and Donato et al. (2008), who concluded

78

that the presence of allochthonous articulated bivalves in tsunami deposits in Israel or Oman

79

indicates transport of live specimens over a large distance. In these previous studies, the80

premise is that these articulated shells should would all die at the same time, vs other81

processes that maybe more gradual (i.e. changes in water conditions).

82

In Ishigaki Island, Suzuki et al. (2008) identified the emerged massive Porites coral

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boulders using a combination of 14C dating and 18O profiles. But, bivalves in tsunami

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deposits are advantageous, because live transport (articulated specimens) can be recognized85

versus gastropods, foraminifera and corals (Donato et al., 2008).

86
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In the present study, we analyze the 18O profiles of these specimens to reconstruct theseasonal timing of the two tsunamis in Ishigaki Island.

8889

2. Study site

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The trench site is located at a farm on Ibaruma, Ishigaki Island (Fig. 1). The elevation of91

the trench site is approximately 3–10 m above sea level. The reef located off the study area is921320 m wide and consists of a reef crest, a reef pavement, and a shallow lagoon that is <4.093

m deep (Hongo and Kayanne, 2009). Based on the sea surface temperature (SST) recorded in94Ishigaki Port from 1914 to 2000 (Mishima et al., 2010), the seasonal SST ranges from 19–

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20°C (February) to 29–30°C (late August to early September). Abe et al. (2009) produced

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continuous 18O records of sea surface water (18Osw) for Ishigaki Port from 1998 to 2004.

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The annual mean value is 0.22 ‰, with a range of −0.1‰ to 0.4‰.

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3. Samples and methods

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The bivalve specimens analyzed in this study are Fragum loochooanum and F. unedo for101

deposit I and Regozara flavum and Mactra maculata for deposit II. As the thickness of the102

outer shell layer of F. unedo was too thin to collect powdered carbonate samples, we did not

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perform an 18O analysis on this specimen. Both F. loochooanum and R. flavum inhabit sandy

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substrata in the lower intertidal zone to a depth of 20 m (Okutani, 2000). M. maculata lives in105

the lower intertidal zone to a depth of 30 m (Okutani, 2000).

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The specimens were embedded directly in polyester resin without any chemical treatment,107

and cut along the axis of maximum growth. The sections were ground with 1200 SiC grit.

108

Powdered carbonate samples were collected from the outer shell layer using an automated109

Micromill sampler at the Japan Agency for Marine-Earth Science and Technology

110(JAMSTEC), Yokosuka, Japan (Sakai and Kodan, 2011). Individual milling steps ranged

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between ca. 550 and 1000 m perpendicular to the direction of growth. Each milling yielded112

40–120 mg of aragonite powder. The powdered carbonate samples received no additional

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thermal or chemical treatment prior to 18O analysis. The sample was analyzed using a mass114

spectrometer (IsoPrime, Micromass) at JAMSTEC. Individual samples were reacted with

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100% phosphoric acid at 90°C. Oxygen and carbon isotope values of shell carbonate (18Oshell116

and 13Cshell) are reported relative to the Peedee Belemnite (PDB), and the analytical

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precision (1) was better than ±0.1‰ in all cases. X-ray diffractometer analysis revealed that118

the shells consist entirely of aragonite.

119

No previous study has investigated the lifespans of F. loochooanum, R. flavum and M.

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maculate, however, the lifespans of closely related species are 5 years (Fragum fragum), 5

121

years (Acrosterigma burchardi), and 12 years (Mactra chinensis), (Limpanont et al., 2010;

122

Moss et al., 2016). Thus, the lifespans of the study species may reach 5 years.

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124

4. Results

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The18Oshell values of F. loochooanum from deposit I are between −0.23‰ and −1.46‰,

126

and become heavier and then lighter toward the distal end (Fig. 3). Except for one

127

exceptionally light δ18O value (−4.73‰), 18Oshell of R. flavum from deposit II ranges from128

−3.04‰ to −1.79‰, and exhibits two cycles of becoming lighter and then heavier toward the129shell margin (Fig. 3). The18Oshell values of M. maculata from deposit II are −2.70‰ to

130−1.21‰, becoming lighter and then heavier toward the distal region (Fig. 3).

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5. Discussion

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It is widely accepted that biogenic aragonite is precipitated at or very near 18O

134

equilibrium with the ambient water, and that deviations from the 18O equilibrium can be

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accounted for by species-specific vital effects operating during the precipitation of the136biogenic aragonite (Kim et al., 2007). To accurately reconstruct the water temperature from137

the 18O compositions of F. loochooanum, R. flavum, and M. maculata, temperature138

equations (18Oshell-derived temperatures) for each studied species must be used. As equations

139

have not been published for the species used in the present study, we applied 18Oshell-derived

140

temperatures using six published temperature equations for abiogenic and biogenetic

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aragonite (Dunbar and Wefer, 1984; Grossman and Ku, 1986; Patterson et al., 1993; Carré et142al., 2005; Kim et al., 2007; Aubert et al., 2009; Fig. 3). Based on data presented by Abe et al.

143(2009), we used a 18Osw value of 0.22‰ for the study area.

144

For the most part, the 18Oshell-derived temperatures based on five previously published145

equations fall within the same temperature range (Fig. 3). However, in the cases of R. flavum

146

and M. maculata, most values obtained using the equation of Grossman and Ku (1986) are147

higher than the maximum SST. Thus, we excluded this equation in our discussion, along with148

the exceptionally light δ18O value (−4.73‰) of R. flavum.

149

The 18Oshell-derived temperatures of F. loochooanum in tsunami deposit I decreased and150

then increased toward the margin of the shell (Fig. 3). The range (3.5–5.9°C) corresponds to151

35%–59% of a temperature range of 10°C. In the case of the minimum 18Oshell-derived

152

temperature, except for the value (17.1°C) obtained using the equation of Kim et al. (2007),

153

the results are equal to (19.1–20.7°C) or slightly higher than (22.7°C) the minimum monthly154

SST (19–20°C). In contrast, the highest 18Oshell-derived temperature (23.0–26.2°C) is

155

inferred to be at the shell margin, corresponding to the spring SST. Thus, we deduce that the156

18Oshell-derived temperatures of F. loochooanum correspond to the change from fall of the157

previous year to spring, and that the time of death was spring. This result confirms that158

deposit I was related to the Meiwa tsunami of 24 April AD 1771, and provides information of159

region specific reservoir age (R).

160

Ando et al. (2018) estimated 14C ages of F. loochooanum and F. unedo using ΔR = −36 ±161

78. The value was determined from coral of Ishigaki Island for AD 1947–1950 by Hirabayahi

162

et al. (2017). Given that the calendar ages of the two shells are AD 1771, R of samples are163

calculated as −38 ± 19 and −111 ± 29 years, respectively. Hirabayahi et al. (2017) reported

164

that seasonal-scale fluctuations of R at AD 1947–1950 range from −136 ± 42 to 62 ± 50.

165

Thus, we think that the difference in R of two shells may be explained by difference of

166

periods of the 14C dated portion and seasonal-scale fluctuations of R.

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The 18Oshell-derived temperatures of R. flavum in tsunami deposit II show two cycles. The

168

maximum 18Oshell-derived temperatures are equal to (29.3–30.7°C) or slightly higher than169

(31.1–31.8°C) the maximum monthly SST (29–30°C), while the minimum 18Oshell-derived

170

temperature (24.6–27.2°C) is 4–7°C higher than the minimum monthly SST (19–20°C). This

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discrepancy in the minimum temperatures implies a slowdown or cessation of shell growth172

between fall and spring. We therefore propose that the shell portion of R. flavum formed over173

a period of 2 years, and that the time of death was fall because the last 18Oshell-derived

174

temperature is slightly lower than the midpoint of the maximum and minimum 18Oshell-

175

derived temperatures.

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The 18Oshell-derived temperatures of M. maculata in tsunami deposit II increased and then177

decreased toward the margin of the shell (Fig. 3). The maximum 18Oshell-derived

178

temperatures are equal to (29.2–30.3°C) or slightly lower than (28.1°C) the maximum

179

monthly SST (29–30°C). The minimum 18Oshell-derived temperature (21.8–25.5°C) was

180

obtained from the sample at the shell margin, and is 2–6°C higher than the minimum monthly181

SST (19–20°C). We conclude that the shell portion of M. maculata formed from spring of the182

previous year to fall, and that the animal died during fall. The difference in 18Oshell-derived

183

temperature between R. flavum and M. maculate in tsunami deposit II is explained by184

difference of species-specific vital effects and/or different habitats.

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In summary, the 18Oshell profiles of R. flavum and M. maculata indicate that tsunami

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deposit II was caused by a tsunami in the fall of a year during the period 920–620 cal. yr BP.

187

From the historical document Miyakojima-Kyuki (“Old Records of Miyako Island”), which188

was written at about AD 1748 and did not mention the years in which events occurred,

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Shimoji (2007) inferred that large tsunamis struck the Miyako Island around the 14th century

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and in the late 15th century. Our results cannot be used to assess the validity of this191

interpretation of historical documents, although they provide important information for

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studies of tsunamis in the south Ryukyu islands during the 14th and15th centuries.

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6. Conclusions

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This study analyzed the 18O profiles of two well-preserved and articulated marine

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bivalves from each of sandy tsunami deposits I and II on Ishigaki Island along the southern

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Ryukyu Trench. Our results show that shell death in deposits I and II occurred during spring198

and fall, respectively. The season of shell death in tsunami deposit I supports that the deposit

199

was generated by the 1771 Meiwa tsunami, which therefore provides a region-specific200

radiocarbon reservoir age. The season of shell death in tsunami deposit II provides a201

constraint on the tsunami that formed the deposit. A combination of taphonomic study,

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radiometric dating, and analysis of the 18O profiles of bivalve shells derived from tsunami

203

deposits provides important chronologic constraints on the tsunami events that formed the204

deposits.

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Acknowledgements

207

We acknowledge constructive reviews from anonymous reviewers and suggestions by the208

Editor, Prof. Shu Gao, for improvements. We thank Dr. Masataka Ando and Toshiyuki

209

Chinen for our survey on Ishigaki Island. This study was supported by Grants-in-Aid

210(17H02972) awarded by the Japan Society for the Promotion of Science and the Integrated

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Center for Education and Research of Natural Hazards, Shizuoka University.

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213

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Heian period AD 869 Jogan tsunami. AFREC News, 16, 1−10 (in Japanese)316

Shishikura, M., Fujiwara, O., Sawai, Y., Namegaya, Y., Tanigawa, K., 2012. Inland-limit of the317

tsunami deposit associated with the 2011 off-Tohoku earthquake in the Sendai and

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Ishinomaki Plains, Northeastern Japan. Ann. Rep. Active Fault and Paleoearthquake Res. 12,319

45–61 (in Japanese with English abstract)

320

Suzuki, A., Yokoyama, Y., Kan, H., Minoshima, K., Matsuzaki, H., Hamanaka, N., Kawahata,

321

H., 2008. Identification of 1771 Meiwa Tsunami deposits using a combination of322

radiocarbon dating and oxygen isotope microprofiling of emerged massive Porites boulders.

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Quat Geochronol. 3, 226–234.

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Takashimizu, Y., Urabe, A., Suzuki, K., Sato, Y., 2012. Deposition by the 2011 Tohoku-oki

325tsunami on coastal lowland controlled by beach ridges near Sendai, Japan. Sediment. Geol.

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282, 124–141.

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Figure captions

329

Fig. 1. (a) Plate boundaries around the southern Ryukyu Islands (after Ando et al., 2009). EU:

330

Eurasia Plate; NA: North American Plate; PC: Pacific Plate; PS: Philippine Sea Plate; MY:331

Miyako Island. 1: epicenter of the 2011 Tohoku-oki earthquake (Mw 9.0). (b) Ishigaki Island,332

showing the location of the study area and the tsunami run-up heights (m) in 1771 on Ishigaki

333

island (after Goto et al., 2010). (c) Photograph of the study trench.

334
335

Fig. 2 (a) Columnar sections of the study trench, modified from Ando et al. (2018). (b) 14C

336

age probability distributions (cal. yr BP) showing 95% error bars for studied shell samples

337(Ando et al., 2018). The ages were translated into a calendar timescale using the program

338

OxCal4.1 (Brank Ramsey, 2009), based on comparisons with the IntCal13 data (Reimer et al.,3392013), after applying a local marine correction for the Ishigaki area of ΔR = −36 ± 78

340

(Hirabayahi et al., 2017). (c) Mode of occurrence of Regozara flavum.

341
342

Fig. 3 Records (left panel) of 18Oshell and 13Cshell versus distance from the ventral margin of343

the shell. Note that the 18Oshell axis has been inverted. Reconstructed temperature record344(right panel) based on 18Oshell. Fragum loochooanum (upper panel) was collected from345

tsunami deposit I. Regozara flavum and Mactra maculate (middle and lower panels) were346

collected from tsunami deposit II.

347

a

Jap
an
Trenc
h

40°N

EU

h
ug
ro
i T
a
nk
Na

30°N

Tr
en
ch

yu
uk
Ry

130°E

15-20

24°33’ N

trench
11

PC

MY

b

120°E

1 8

c

Ishigaki 10
Island24°21’ N

PS

c. Trenchb. Ishigaki Island

NA

8

9 12

22 22-32

500 km140°E

124°06’ E

124°21’ E

Epicenter

348

349

Fig. 1

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