1Long-term changes in sediment type and cavernicolous bivalve assemblages in

2

Daidokutsu submarine cave, Okinawa Islands: evidence from a new core extending

3

over the past 7,000 years

4

N. Yamamotoa, A. Kitamuraa,*, A. Ohmoria, Y. Morishimaa, T. Toyofukub, S. Ohashic

5

a

6

E-mail: seakita@ipc.shizuoka.ac.jp

Institute of Geosciences, Shizuoka University, Shizuoka 422-8529, Japan

7b

Japan Agency for Marine-Earth Science and Technology, Yokosuka 237-0061, Japan

8c

Kaiyo Planning Co. Ltd., 1732-12, Maeda, Urasoe, Okinawa, 901-2102, Japan1

9

Abstract We analyzed sediments and cavernicolous bivalve assemblages within cored10

sediments recovered from the Daidokutsu submarine limestone cave, which occurs in

11

Pleistocene limestone on the fore-reef slope of Ie Island, off Okinawa, Japan. We found12

sand-sized brown grains within sediments aged 7,000-6,000 cal BP, and identified the13

grains as debris derived from a red soil layer developed in coastal zones around the14

Island. Temporal changes in the abundance of the debris suggest that water flow in the

15

cave weakened at about 6,500 cal BP. Temporal changes in the cevernicolous bivalve

16

assemblages indicate that the environmental conditions of the innermost cave area have

17

gradually spread to the cave entrance over the past 5,000 years. These findings indicate

18that the filling of cavities within the reefal foundations of the cave has continued over19

time, resulting in a progressive decrease in the flux of water between the interior and20

exterior parts of the cave during at least the past 6,500 years. This indicates in turn a

21

long-term decline in the functioning of the cryptic community within the reef as a22

source of the nutrients that support coral reef growth. Our findings regarding the23

distribution of debris from the red soil layer strongly support the occurrence of a24

mid-Holocene high-energy window, which represents a period of high wave energy

25

prior to the formation of a reefal barrier.

26
27

Keywords

28

Environmental change

Coral reef・Submarine cave sediments・Cavernicolous bivalves・Soil・

29

2

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Introduction

31
32

Cavities in coral reefs are produced in a number of ways, including reef framework

33

growth and the dissolution of limestone (Kobluk 1988). Such cavities constitute34

between 30 and 75% of the bulk volume of coral reefs (Garret et al. 1971; Ginsburg

35

1983) and between 60 and 75% of the total available reef surface (Jackson et al. 1971;

36

Logan et al. 1984). The large area of reef made up of cavities is inhabited by sponges,

37

bryozoans, brachiopods and other organisms. The biomass of the cryptic communities

38

has the potential to extend beyond the reef surface (Hutchings 1974; Brock and Brock

39

1977; Meesters et al. 1991).

40

Previous studies have addressed the evolutionary significance of cryptic

41

communities (Jackson et al. 1971; Jackson and Winston 1982; Gili et al. 1986;

42

Harmelin 1997; Wörheide 1998; Kano and Kase 2000, 2008; Kano et al. 2002;

43

Motchurova-Dekova et al. 2002; Ubukata et al. 2009) and the fluxes of various

44materials (e.g., dissolved oxygen, dissolved inorganic matter, dissolved organic matter,45

and bacterioplankton) in reef cavities (Fichez 1990a, 1990b, 1991; Richter et al. 2001;

46

Scheffers et al. 2004; Van Duyl et al. 2006; De Goeij and Van Duyl 2007). However, we

47

are unaware of any published studies that have searched for millennia-scale variations

48

in the community of organisms in reef cavities, with the exception of Kitamura et al.

49(2007a).

50

Kitamura et al. (2007a) examined cavernicolous bivalve assemblages in surface and51

cored sediments within Daidokutsu submarine cave (29 m water depth) on the fore-reef

52

slope of Ie Island (26º43’N, 127º50’E) (Fig. 1), Okinawa, Japan. The cave is developed

53

in Pleistocene Ryukyu Limestone, having formed by the movement of underground54

water during periods of low sea level (Hayami and Kase 1993). The bivalve species3

55

living in the innermost area of the cave have become increasingly dominant over the56

past 5,000 years, while those living near the entrance of the cave have declined in

57

abundance over this period (Kitamura et al. 2007a). This finding indicates that the58

environmental conditions of the innermost cave area have progressively extended to the59

entrance of the cave.

60

According to Kobluk (1988), most cavities in reefs are eventually filled with61

surface-derived sediments, cement, debris, and in situ skeletons of the cryptic

62

community. On this basis, Kitamura et al. (2007a) concluded that the progressive

63

infilling of cavities at Daidokutsu cave has resulted in a decrease in the flux of water64

between the interior and exterior parts of the cave, leading in turn to an increase in the65

spatial extent of the zone of nutritional deficiency. However, the stratigraphic

66

distribution of cavernicolous bivalve assemblages within the cave requires additional

67

consideration, as the possibility exists that the lowest part of the cored sediments

68

consists of sediment gravity flow deposits (22 cm thick), which would have modified

69

the species composition in the sediments (see below). To extend Kitamura et al.’s70(2007a) study, we examined the temporal distribution of both cavernicolous bivalves

71

and sediment within Daidokutsu cave during the past 7,000 years, based on an

72

examination of newly obtained sediment cores.

73
74

Study area

75
76

Cave conditions and sediments

77

Ie island is approximately 23 km2 in area and is covered by the Pleistocene Ryukyu

78

Limestone, with a fringing reef close to Pleistocene limestone sea cliffs that are79

undercut at the mean high-water level (Fig. 1). The surface of the fringing reef is4

80

largely flat and is exposed at ebb tide (mean tidal range, 1.8 m). The reef crest is81

located roughly 200 m from the shoreline, and 20 m from the entrance to Daidokutsu

82

cave.

83

Daidokutsu cave is situated on the northeastern coast of Ie Island. The cave’s

84

entrance lies at approximately 19 m below sea level (Fig. 1), and is roughly 2 m high85

and 18 m wide (Hayami and Kase 1992). The cave is 40 m long, dark inside, and86

deepens abruptly inward to its deepest point at 29 m below sea level. Water temperature

87

within the cave ranges from 30°C (August) to 20°C (February), and seasonal trends in88

water temperature are similar to those at 30 m water depth in the open ocean around89

Okinawa (Kitamura et al. 2007b). During sediment sampling, we found air bubbles

90

rising from the sea floor above the cave, indicating that water is exchanged between the91

water masses inside and outside the cave via the cave entrance and via numerous

92

passages in the surrounding porous substrates.

93

Based on the sea-level curves of Fairbanks (1989), Bard et al. (1996), and Toscano

94

and Macintyre (2003), Daidokutsu cave is considered to have become submerged at ca.

95

9,000 yr BP, with the entrance having become completely submerged at about 8,000 yr

96

BP. Sea level had attained its present level by about 7,000 yr BP.

97

The floor of the cave is covered by calcareous sediments (Kitamura et al. 2003).

98

Kitamura et al. (2007a) divided the surface sediments in the cave into three facies (Fig.

992): Facies 1 is gray calcareous sand that occurs at sites close to the entrance, Facies 2 is100

gray calcareous mud that occurs in the inner part of the cave, and Facies 3 is calcareous

101

sand containing the skeletons of partly encrusted coralline sponges, which occurs in

102

patches within Facies 2.

103

Kitamura et al. (2007a) collected and examined cored sediment samples from the104

central part (core 06) and the innermost part (core 04) of Daidokutsu cave (Figs. 2, 3).
5105Core 06 (148 cm thick) can be divided into a lower part, consisting of yellow

106

calcareous mud (> 22 cm long), and an upper part consisting of gray calcareous mud

107(approximately 126 cm thick) (Fig. 3). The sharp boundary that separates the two parts

108

is estimated to represent an age of 5,450 cal BP (Kitamura et al. 2007a). Based on the109

relationship between sediment depth and 14C data, the gray calcareous mud is110

interpreted to have been deposited continuously at a sedimentation rate of 21.1

111

cm/1,000 years. The 14C ages of samples obtained from the lower part of the core fall112

within a narrow range from 5,980 to 5,060 cal BP, showing a non-linear relationship113

with depth (Fig. 3). Accordingly, Kitamura et al. (2007a) considered the possibility that114

the sediment in the lower part originated as a sediment gravity flow. Although there

115

exist differences in color and sedimentation rate between the yellow and gray116

calcareous muds, the continuous deposition of mud-sized particles throughout the117

sequence indicates that still-water conditions prevailed for 5,000 years.

118
119

Bivalve assemblages

120

Hayami and Kase (1992, 1993, 1996) and Kase and Hayami (1992) studied

121

cavernicolous bivalves in tropical west-Pacific coral reefs, including Daidokutsu cave.

122

The authors reported about 60 species of cavernicolous bivalves that share the123following characteristics: 1) reduced adult size (usually less than 5 mm in length); 2)

124

unusually large prodissoconch I and absence of prodissoconch II in many species,

125

indicating non-planktotrophic development; and 3) persistent denticles of the126

provinculum are retained until the adult stage in many pteriomorph species, indicating

127

significant paedomorphosis by progenesis. According to Kase and Hayami (1992) and128

Hayami and Kase (1996), these features reflect a common adaptive strategy toward the129

oligotrophic condition and low predation pressure of cave habitats.
6

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Eight bivalve species are dominant in the surface sediments within Daidokutsu cave.

131

Kitamura et al. (2007a) divided these species into three groups based on their spatial

132

distribution (Fig. 4). The first group (Cosa kinjoi and Parvamussium crypticum) mainly

133

inhabits the innermost cave area, whereas the second group (Cosa waikikia and Hiatella134

sp. aff. H. orientalis) is found near the entrance, and the third group (Chlamydella

135

tenuissima, Cyclopecten ryukyuensis, and Carditella iejimensis) shows no distinct

136

distribution pattern. Carditella iejimensis appears to be endemic to caves of Ie Island,

137

while the others are found on many isolated islands in tropical west-Pacific coral reefs

138(Hayami and Kase 1996).

139

The first group showed a rapid increase in abundance immediately above the140boundary between the yellow and gray calcareous muds (ca. 5,450 cal BP) in the141

central area (core 06; Fig. 3). It is possible that the stratigraphic distribution of this142

group was influenced by the mixing of shells transported by a sediment gravity flow.

143

The species of the second group, along with Chlamydella tenuissima, show a decrease144

in abundance at about 2,200 cal BP in the central part of the cave (core 06), and the145

relative abundances of these species are very low in the innermost part of the cave (core14604; Fig. 3) over the past 3,000 years. We observed no significant changes in the147

temporal or spatial distribution of Cyclopecten ryukyuensis and Carditella iejimensis.

148

These findings indicate that the cave environment has remained largely unchanged for149

the past 5,000 years.

150
151

Methods

152
153

We sampled the cave sediment by hand using a coring tube of 6 cm in diameter and154

233 cm in length (core 19; Figs. 2, 5). The core did not reach basement. The sediment7

155

core was first split and described, and then sliced into 1 cm thick samples for the156

measurement of mud and carbonate content. We picked and counted all micro-bivalves

157

from the >0.5 mm fraction of each sample. Mud content was determined using

158

standard sieves. The carbonate content of 500 mg of sediment was determined by

159

gentle removal with 10% acetic acid.

160

Kitamura et al. (2006) reported a pumice-bearing sediment layer (Daidokutsu

161

pumice) within the gray calcareous mud. The main period of deposition of this layer is162

estimated to have occurred from 1,990 to 1,710 cal BP (Kitamura et al. 2006). The

163

pumice layer can be identified by its relatively low carbonate content, thereby enabling164

its use as a reference time surface.

165

The

obtainedbivalves

show

excellent

preservation

despite

most

being

166

disarticulated: the prodissoconch can be observed in many individuals. These factors

167

indicate that the shells represent a life assemblage. Taxonomic identifications are based168

on Hayami and Kase (1993). For disarticulated shells, a separate valve was counted as

169

one individual. The stratigraphic distribution of each species is expressed as the density

170

of individuals (numbers per gram).

171

The radiocarbon ages of three well-preserved mollusc shells were determined by

172

Beta Analytic Inc., using accelerator mass spectrometry (Table 1). Calibrated age

173

ranges were calculated according to Method A of Stuiver et al. (1998) after applying a

174

local correction for the northwestern Pacific of 355 years (∆R = 35 ± 25) (Hideshima et175

al. 2001).

176

In this study, we discovered sand-sized brown grains (debris derived from a red soil

177

layer; see below) in the lower portion of core 19. We observed these grains in thin178

sections viewed under a polarizing microscope, and analyzed their mineral composition179

by powder X-ray diffractometry (XRD) using a Rigaku RINT2500 V housed at
8

180

Shizuoka University, Japan. The major element compositions of 10 brown grains were181

analyzed using a non-destructive energy dispersive X-ray fluorescence (XRF) analyzer

182(TATSCAN-F1) housed at the Japan Agency for Marine-Earth Science and Technology

183(JAMSTEC). The chemical compositions were determined at a tube voltage of 30kV

184

and the diameter of the collimater was 400µm. We picked and counted all brown grains

185

from the 0.5-1.0 mm and 1.0-2.0 mm fractions of samples collected at 1 cm intervals

186

within core 19. In the same way, we determined the stratigraphic distribution of brown187

grains in cores 04 and 06. We also picked and counted brown grains from the 0.5-1.0

188

mm and 1.0-2.0 mm fractions of a sample of reef-slope sediment and six samples of189

surface sediment from within the cave (Figs. 1, 2).

190
191

Results

192
193

The sediment in core 19 shows a fining-upward trend, and is divided into a lower gray194

calcareous sand (233-178 cm depth) and an upper gray calcareous mud (178-0 cm

195

depth) (Fig. 5). Between 233 and 192 cm depth, the carbonate content increases upward196

from 88 to 94%. Except for the horizon from 52 to 28 cm depth, the carbonate content

197

between 192 and 0 cm depth is consistent at 95-97%. Between 52 and 28 cm depth, the198

carbonate content decreases to 84% (Fig. 5). We regard this horizon to represent the199

deposit containing the Daidokutsu pumice (deposited between 1,990 and 1,710 cal BP).

200

Table 1 lists the radiocarbon ages obtained for mollusc shells; 14C ages are plotted

201

in Fig. 5. The age of a bivalve shell from the base of core 19 is 7,170 ± 45 cal BP,

202

indicating that the cored sediments preserve a record of fossils and sedimentation over203

the past 7,000 years. Based on 14C age data and the stratigraphic position of the204

Daidokutsu pumice, we estimate a sedimentation rate of 41.7 cm/1,000 years between
9

205

150 and 233 cm depth, and a rate of 29.8 cm/1,000 years between 0 and 150 cm depth

206(Fig. 5). This change in sedimentation rate at 150 cm depth was synchronous with the207change in deposition from calcareous sand to calcareous mud. An age-depth model

208

presented in this study reveals that pale yellow calcareous mud in the lower part of core209

6 is heteropic with gray calcareous mud in core 19 (Figs. 3, 5), although we cannot

210

explain the cause of the change in sediment color at ca. 5,450 cal BP.

211

Observations of thin sections reveal that the sand-sized brown grains are also brown

212

internally (Supplementary Fig. 1). The grains were not dissolved by 10% acetic acid,213

and XRD analysis reveals that they contain quartz (Fig. 6). XRF analysis reveals that214

the grains consist mainly of Si, Al, and Fe (Table 2). Based on these observations, we

215

interpret the brown grains to be detritus derived from a red soil layer that is widely

216

distributed across Ie Island (Tokashiki 1993; Vuai et al. 2003; Mkadam et al. 2006).

217

Debris with a diameter of 1.0-2.0 mm was only found in the lower part of core 19 (Fig.

218

5). The number density of these grains (numbers per gram) decreases upward from219

0.6/g at 233 cm depth to 0-0.1/g at 203 cm depth. The number density of debris with a220

diameter of 0.5-1.0 mm shows an upward decrease from 10/g at 233 cm depth to 0-1/g

221

at 180 cm depth (Fig. 5). This upper stratigraphic limit of small-sized debris (0.5-1.0

222

mm in diameter) derived from the red soil layer coincides with the boundary between

223

calcareous sand and calcareous mud (178 cm depth), and coincides with an upward

224

increase in carbonate content over the interval between 233 and 192 cm depth. The

225

debris from the red soil layer is recognized within surface sediment at three sites (d-f)

226of the six sites analyzed in the cave, although at number densities less than 0.08/g

227(Table 3). We found no debris at the other three sites (a-c) in the cave, in the reef-slope

228

sample (Table 3), or in core 04. The debris occurs sporadically in core 06 (Fig. 3).

229

The cavernicolous bivalve fauna within core 19 is dominated by Huxleyia
10

230

cavernicola, Bentharca tenuis, Cosa kinjoi, Cosa waikikia, Parvamussium crypticum,

231

Cyclopecten ryukyuensis, Chlamydella tenuissima, Carditella iejimensis, and Hiatella

232

sp. aff. H. orientalis (Fig. 5, Supplementary Table 1). Species of the first group (Cosa233

kinjoi and Parvamussium crypticum) occur continuously above 158 and 130 cm depth,234

respectively. In contrast, species of the second group (Cosa waikikia and Hiatella sp.

235

aff. H. orientalis) show an upward decrease in abundance from about 72 cm depth. The

236

stratigraphic distribution of Chlamydella tenuissima and Bentharca tenuis is similar to237

that of the second group. Huxleyia cavernicola shows an upward decrease in abundance238

from about 150 cm depth. Both Cyclopecten ryukyuensis and Carditella iejimensis

239

occur continuously throughout the cored sediments (Fig. 5).

240
241

Discussion242

Debris from the red soil layer

243

In core 19, debris from the red soil layer is commonly observed in the horizon from the244bottom of the core to about 180 cm depth. The age range of this interval is estimated to245

be 7,000 to 6,000 cal BP, based on our age model (Fig. 5). In contrast, the debris is only

246

rarely observed in the younger sediments within core 19, in other cores (04 and 06), and247

in surface sediments sampled from within the cave, and is absent from reef-slope

248

sediments (Table 3). Consequently, we consider that the stratigraphic distribution of249

debris from the red soil layer indicates that the washing of debris from coastal paleosols

250

became weaker, and nearly terminated, during the period leading up to 6,000 cal BP.

251

Kawana and Kan (2002) reconstructed the Holocene development of coral reefs

252

around southern Okinawa Island, located about 70 km south of the present study area,

253

based on an analysis of drill cores. The authors documented that reef began to grow254

upward upon basement at about 7,600 yr BP, under 10-20 m water depth, and reached
11

255

the present sea surface at 6,000-5,500 yr BP, and then developed seaward via accretion

256of the forereef. At the time when a similar pattern of coral reef development occurred

257

around Ie Island, termination of the supply of debris from the red soil layer coincided258with the establishment of the reef crest/flat zones (when the reef reached the present sea

259

level). The observed changes in supply of debris from the red soil layer are therefore

260

considered to be consistent with the existence of a high-energy window―a concept first261

proposed by Neumann (1972) and revisited by Hopley (1984). During the262

mid-Holocene, in areas where the rate of sea-level rise outstripped the ability of newly

263

settled coral communities to grow vertically, a period of high wave energy may have264

occurred prior to the formation of a reefal barrier. The high-energy window may have265

remained open from the first complete submergence of the antecedent foundations until266

the moment at which modern reef flats began to develop, between about 8,000 and

267

6,000 yr BP (Neumann 1972; Hopley 1984; Montaggioni 2005). Under such conditions,268

the enhanced hydrodynamic energy may have resulted in the rapid winnowing of finer269

particles from paleosols and terrigenous sediments deposited on inner shelves and270

coastal zones. From 6,000 yr BP, the window progressively closed as reef tops reached271

sea level, which stabilized around its present position, and reef flats became sufficiently

272

extended laterally. Consequently, the supply of detritus was terminated. It appears that273

the stratigraphic distribution of debris from the red soil layer provides direct evidence274

of the occurrence of a mid-Holocene high-energy window.

275

It is noteworthy that the decline in the abundance of larger debris (1.0-2.0 mm in276

diameter) preceded that of smaller debris (0.5-1.0 mm in diameter) by 550 years (Fig.

277

5). This difference is well explained by a weakening in water flow within the cave,278

rather than changes in sorting processes outside the cave.

279
12

280

Long-term changes in cavernicolous bivalve assemblages

281

The continuous occurrence of Cyclopecten ryukyuensis and Carditella iejimensis282

indicates that the cave environment in Daidokutsu has persisted for the past 7,000 years.

283

The progressive decrease in abundance of species of the second group (Cosa waikikia

284

and Hiatella sp. aff. H. orientalis), as well as Chlamydella tenuissima and Bentharca285

tenuis, is consistent with the findings of Kitamura et al. (2007a). Our data indicate that286

species of the first group (Cosa kinjoi and Parvamussium crypticum) were absent from287

core 19 at depths between 223 and 160 cm, although other species occurred

288

continuously throughout this interval. This finding confirms Kitamura et al’s (2007a)289

tentative conclusion that spiecies of the first group living in the innermost part of the290

cave became dominant over the past 5,000 years. In addition, we found that Huxleyia

291

cavernicola shows an upward decrease in abundance from a depth of about 150 cm,

292

corresponding to about 5,130 cal BP. The bivalve species described above are293

suspension feeders, whereas Huxleyia cavernicola is thought to be a deposit feeder

294(Hayami and Kase 1993). This species is very rare in the surface sediments in the cave295

(Fig. 4). Thus, the temporal changes in species abundance reveal a decrease in the296amount of organic matter in cave sediment at about 5,130 cal BP.

297

As noted above, our new data indicate that the water flow in the cave weakened at298

about 6,500 cal BP, suggesting in turn a decrease in the exchange of water between the299interior and exterior of the cave, and a consequent decline in nutrient levels within the300

cave. However, the observed changes in cavernicolous bivalve assemblages occurred

301

after termination of the supply of debris from the red soil layer, at about 6,000 cal BP.

302

This finding indicates that the population dynamics of Cosa waikikia, Hiatella sp. aff.

303

H. orientalis, Chlamydella tenuissima, Bentharca tenuis and Huxleyia cavernicola were304

not significantly influenced by the decline in nutrient supply at about 6,500 cal BP.
13

305

However, it appears that the ongoing decline in nutrient supply resulted in an expansion306

of the distribution of Cosa kinjoi and Parvamussium crypticum, thereby preventing the307

five species listed above from inhabiting the inner part of the cave.

308

The large extent of the cryptic habitat and extensive cover of encrusting organisms

309

within reef cavities provide a potentially important interface in the exchange of material

310

between cavities and the overlying water column (e.g., Tribble et al. 1988; Richter and

311

Wunsch 1999). Coral cavities are sinks of bacterioplankton and dissolved organic

312

carbon, and a source of NOx and PO43- (Van Duyl et al. 2006; De Goeij and Van Duyl313

2007; De Goeij et al. 2008). Our results show that, at least in terms of the coral cryptic

314

community within cavities developed in the reefal foundations, a decline in the

315

functioning of the cryptic community as a source of nutrients to the coral reef over at

316

least the past 6,500 years.

317
318

Acknowledgments We gratefully acknowledge K. Konishi, T. Kase and T. Ubukata for

319

their advice. Thanks are also due to Shigemitsu Kinjyo, Koushin Yasumura for the320

preparation of samples. We thank A. Beu and an anonymous reviewer, whose comments

321

and suggestions improved the original manuscript. We thank A. Stallard for improving

322

the English in the manuscript. This study was funded by Grants-in-Aid 16340159 and

323

19540492 awarded by the Japan Society for Promotion of Science, The Japan Science

324

Society, a Sasakawa Scientific Research Grant, and a Grants-in-Aid from the Fukada

325

Geological Institute.

326
327

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Figure captions
Figure 1 Location map of the submarine Daidokutsu cave on Ie Island, off Okinawa
Island, Japan.

457
458

Figure 2 Map showing water depth and the locations of sampling points of surface459

sediments (a-f) and cored sediments. Also shown is the distribution of surface

460

sediment facies (see text for details).

461
462

Figure 3 Columnar sections of submarine-cave sediment within cores 04 and 06,

463

showing stratigraphic changes in mud and carbonate content, abundance of debris

464

derived from a red soil layer, the dominant cavernicolous bivalve species, and465

depositional rates, as inferred from the 14C ages of molluscs (modified after

466

Kitamura et al. 2007a).

467
468

Figure 4 Spatial distributions of selected cavernicolous bivalve species in Daidokutsu

469

cave (modified after Kitamura et al. 2007a). Percentages are given relative to the470

total number of individuals of all bivalve species. The locations of a-f are shown in

471

Fig. 2.

472
473

Figure 5 Columnar sections of submarine-cave sediment within core 19, showing

474

stratigraphic changes in mud and carbonate content, abundance of debris derived

475

from a red soil layer, the dominant cavernicolous bivalve species, and depositional476

rates, as inferred from the 14C ages of mollusks.

477
478

Figure 6 Diffractograms of brown grains (debris derived from a red soil layer) found
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479

within sediment from Daidokutsu cave. Peaks labelled “1” correspond to those for

480

quartz.

481
482
483

Table 1 Results of 14C dating of molluscs. All samples were analyzed by accelerator
mass spectrometry.

484
485
486

Table 2 Concentrations of major elements (wt%) within 10 grains of debris derived
from a red soil layer, as measured by non-destructive X-ray fluorescence.

487
488

Table 3 Grain distribution (wt%) and number density of debris derived from a red soil

489

layer (number/g) in a sample of reef-slope sediment and six samples of surface

490

sediment from within Daidokutsu cave.

491
492
493

Electronic Supplementary Material Figure 1 Photomicrographs of debris derived from a
red soil layer (plane-polarized light).

494
495
496

Electronic Supplementary Material Table 1 List of cavernicolous bivalve species incore 19. A: articulated shells, R: right valve, L: left valve.

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