Sedimentary facies and Holocene depositional processes of Laura Island, Majuro Atoll

GEOMOR-04750; No of Pages 9 Geomorphology xxx (2014) xxx–xxx

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Sedimentary facies and Holocene depositional processes of Laura Island, Majuro Atoll Toru Yasukochi a,1, Hajime Kayanne a,⁎, Toru Yamaguchi b, Hiroya Yamano c a b c

Department of Earth and Planetary Science, The University of Tokyo, Tokyo 113-0033, Japan Department of Ethnology and Archaeology, Keio University, Tokyo 108-834, Japan National Institute for Environmental Studies, Tsukuba, Ibaraki 305-8506, Japan

a r t i c l e

i n f o

Article history: Received 31 August 2012 Received in revised form 30 March 2014 Accepted 3 April 2014 Available online xxxx Keywords: Atoll Reef island Sediment Foraminifera Spine ratio

a b s t r a c t The depositional processes that formed Laura Island, Majuro Atoll, Marshall Islands, were reconstructed based on a facies analysis of island sediments and spine ratios, and radiocarbon ages of foraminifera. Sedimentary facies were analyzed from trenches and drill cores excavated on the island and its adjacent reef flat. Depositional ages were obtained using benthic foraminifera (Calcarina) whose spines had not been abraded. The facies were classified into two types: gravelly and sandy. The initial sediments of these sites consisted of gravelly facies in the lower horizon and sandy facies in the upper horizon. Their ages were approximately 2000 cal BP and coincident with the onset of a 1.1-m decline in regional relative sea level, which enabled deposition of the gravelly facies. Half of the sand fraction of the sediment was composed of larger benthic foraminifera. The spine ratio showed that their supply source on the reef flat was located oceanside of the island. The supply source appears to have been caused by the relative sea-level fall. This indicates that the studied island was formed by a relative reduction in wave energy and enhanced foraminiferal supply, both of which were triggered by the late Holocene relative sea-level fall. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Atoll islands are small, low lying, and composed largely of unconsolidated Holocene calcareous sand and gravel that has been deposited on their coral rims over the past few thousand years (Stoddart and Steers, 1977; Mclean and Woodroffe, 1994; Woodroffe et al., 1999; Perry et al., 2011). Thus they are highly vulnerable to the impacts of climate change and sea level rise anticipated as a result of global warming (Roy and Connell, 1991). Approximately 500 atolls exist worldwide, most of which are in the Pacific Ocean. Understanding how these islands have formed in the past will allow insights into reef-island responses to the anticipated sea-level rise. Previous studies have reported that a relative sea-level fall occurred after the mid-Holocene highstand in most coral reef regions of the IndoPacific (Grossman et al., 1998). The amplitude of the relative sea-level fall varied by location from about 0.5 m to more than 2 m. The beginning of the relative sea-level fall ranged from ca. 3000 to 1500 years BP (Dickinson, 2003). The falling of relative sea level after the midHolocene highstand is regarded as an important factor in producing atoll islands (Wiens, 1962; Schofield, 1977; Dickinson, 1999, 2004).

⁎ Corresponding author at: Department of Earth and Planetary Science, The University of Tokyo, Hongo, Tokyo 113-0033, Japan. Tel.: +81 5841 4573. E-mail address: [email protected] (H. Kayanne). 1 Present address: JX Nippon Oil and Gas Exploration Corporation, Tokyo, Japan.

However, these studies were not based on the actual depositional ages of the islands. Radiometric dating of reef islands has indicated that most islands began to form at the time of the mid-Holocene relative sea-level fall, but some others did not. Little Makin Island (a table reef in Kiribati) was formed by foraminifer-rich sand at around 2500 years BP, when the sea level was at least 0.4–0.5 m higher than at present. The atoll prograded continuously as the sea level fell (Woodroffe and Morrison, 2001). West Island in the Cocos (Keeling) Islands began to form at 4000–3000 years BP and experienced a 0.6-m fall of relative sea level after 3300–2500 years BP (Woodroffe et al., 1999). In the Marshall Islands, a relative sea-level fall occurred after 2000 years BP, when the formation of Majuro Atoll was initiated (Kayanne et al., 2011). Weisler et al. (2012) reported ages of 3100–2930 years BP for Utrok Island (Utrok Atoll) and Kaven Island (Maloelap Atoll), which suggested that island formation was initiated independently of the sea-level fall. Warraber Island (a platform reef) in Torres Strait, Australia, began to form around 3000 years ago, lagging ca. 3000 years behind the onset of sea-level fall from a 0.8-m highstand around 5800 years ago (Woodroffe et al., 2000; Woodroffe, 2002; Woodroffe et al., 2007). Kench et al. (2005) showed that atoll islands on South Maalhosmadulu Atoll in the Indian Ocean initiated accumulation with Halimeda-rich sand around 4000 years BP, with a stable or rising sea level. Atoll island formation has been examined in terms of the accretion history of the islands in relation to sea level changes. However, the dates of detrital sand may include time lags between the death of the

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Please cite this article as: Yasukochi, T., et al., Sedimentary facies and Holocene depositional processes of Laura Island, Majuro Atoll, Geomorphology (2014), http://dx.doi.org/10.1016/j.geomorph.2014.04.017

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aged organisms and their deposition, thus leading to misinterpretation of the depositional history. The depositional process must also be considered to better understand atoll island formation. Sediment transport is essential to deposition in reef islands (Perry et al., 2011). Wave energy, sediment transport, and reef island geomorphology on coral reef flats have been modeled (Kench and Cowell, 2000; Brander et al., 2004; Kench and Brander, 2006). However, the results are difficult to apply directly to Holocene reef-island formation without reconstructing sediment production and relative sea-level change. Barry et al. (2007) developed a numerical model of atoll island development using the island volume and sediment production as parameters. However, their model did not include the history of deposition considering sediment supply and relative sea-level changes. To interpret the depositional processes of island sediment, we used the spine ratio of foraminifera. Akiyama (1979) was the first to use the spine ratio to reconstruct sand transportation on a modern reef at Yoron Island in southwest Japan. The approach was based on the idea that Baculogypsina sphaerulata spines are vulnerable to abrasion, and the degree of abrasion seems to be proportional to the distance of transport between the source and the deposition area. Yamanouchi (1988) used the spine ratios of B. sphaerulata and Calcarina spengleri to reconstruct the origin and transport of sediments on a modern reef in Ishigaki and Ikei Islands in southwest Japan. These studies showed that Baculogypsina and Calcarina are useful as tracers of sand transportation, where these foraminifera are abundant. More recently, Ford and Kench (2012) conducted a tumbling barrel experiment to examine the durability of the skeletal remains of coral reefs, including B. sphaerulata, and found rapid abrasion of its spines, followed by a slower abrasion of its main body. Dawson (2012) used the ages and preservation levels of B. sphaerulata to examine the depositional processes on Raine Reef in the Great Barrier Reef. They showed that the surface sediments of the reef island have a rapid turn-over time. The spine ratio technique has never been applied to Holocene sediments, but we expect that it will be useful for reconstructing the processes of atoll island deposition. Recently, Yamaguchi et al. (2009) and Kayanne et al. (2011) reported the human settlement history of Laura Island on the western edge of Majuro Atoll within a framework of island formation history. The island was formed 2000 years BP, coincident with a fall in sea level, and was colonized by people shortly after its emergence. However, the sediment and sedimentary processes of the island had not previously been studied in detail. In this paper, on the basis of the same excavations and radiocarbon dates, we analyzed the sedimentary facies of island sediments and the spine ratio (degree of abrasion) of foraminifera compared to modern coastal sediments, to better understanding of the island depositional process in terms of sediment source, transportation and accumulation under the falling sea level. 2. Study site Majuro Atoll, located at 171°11′E, 7°6′N, is the capital of the Republic of the Marshall Islands. The atoll is 40 km east to west and about 10 km north to south (Fig. 1). The main passage between the lagoon and the outer ocean is located in the center of the northern atoll rim. The islands on the south rim are connected by causeways, which are collectively called Long Island (Xue, 2001). The land portion of the atoll varies in width from less than 0.2 km to more than 1 km, and the average elevation is only a few meters above mean sea level (Anthony et al., 1989). Most reef flats are intertidal and are subaerially exposed at low tide (Yamano et al., 2006a). The tide is semidiurnal and has a range of approximately 1.8 m (Yamano et al., 2006b). Majuro is subject to east to northeast trade winds during the winter. It is not situated in the typhoon belt, but typhoon damage occurred in 1905 and 1918 (Spennemann, 1996). Biogeographical studies of foraminifera showed the presence of Calcarina and absence of Baculogypsina in some atolls in the Marshall Islands (Todd, 1960).

The radiocarbon ages of fossil microatolls on a lagoon-side reef flat of Long Island were about 2000 cal BP, the same as the oldest age of the island sediment. Their elevation was 1.1 m above that of present-day living corals, indicating a 1.1-m highstand of the sea until 2000 years BP (Kayanne et al., 2011). A relative sea-level fall in the late Holocene was also reported in nearby atolls (Enewetak Atoll, Tracey and Ladd, 1974; Buddemeier et al., 1975; and Bikini Atoll, Tracey and Ladd, 1974). Laura Island is the largest island in Majuro Atoll and is located on the west rim of the atoll. This crescent-shaped island is about 1.2 km wide at the widest point between the lagoonward and oceanward coastlines (Figs. 1B and 2). Anthony and Peterson (1987) collected three continuous core samples to a maximum depth of 24.4 m at Laura Island. They found that Laura is underlain by sediment that is mainly composed of foraminifera, Halimeda, and coral, and considered that the sediment was deposited during the Holocene marine transgression, although a depositional age structure was not presented.

3. Materials and methods 3.1. Facies analysis A cross-section topographic survey of Laura Island and the reef flat was obtained using an autolevel (Fig. 2). Seven trenches along the transversal survey line (Lr-4, Lr-18, Lr-2-8, Lr-TP2′, Lr-Mij, Lr-48, Lr-55) and six trenches (Lr-S0, Lr-S1, Lr-S2, Lr-N1, Lr-N2, Lr-N3) along a longitudinal line on Laura Island were excavated (Fig. 2). The size of each trench was 2 m by 1 m, and the depth was 1 to 2 m. The facies and dates were described by Kayanne et al. (2011). The detailed sediment composition and structure will be described in this study. The grain size, components, and sedimentary structure were observed and sampled at 20- or 30-cm intervals from the wall of each trench. The samples were sieved, and sand-sized samples with grain sizes of − 1.0–0.0 ϕ (very coarse), 0.0–1.0 ϕ (coarse), and 1.0–2.0 ϕ (medium) were impregnated with epoxy resin and thin-sectioned. A minimum of 200 grains from each size fraction were identified under a petrographic microscope (Boss and Liddell, 1987). The identified component groups were foraminifera (Calcarina, Amphistegina, Soritidae, and others), coral, Halimeda, coralline algae, shell, and others. The overall sand composition of each sample was estimated from the composition of each fraction multiplied by the proportion of that fraction in the total sample weight. The fractions under 2.0 ϕ (usually very minor) were assumed to have the same origin as the remainder of the sand sample (Collen and Garton, 2004). Four cores were obtained by Kayanne et al. (2011) using a portable core sampler (Geoact Co. Ltd., Kitami, Japan) from the reef on the ocean side of Laura along the transversal survey line (Lr-I, Lr-II, Lr-III, Lr-IV; Fig. 2). The depths of the cores were 1 to 2 m. The identified component groups in the core samples were coral, coralline algae, and cemented composite bioclasts.

3.2. Spine ratio Surface sediments from 10 sites along the coastline around Laura and three northern reefs in Laura were sampled and sieved (Fig. 2). Larger benthic foraminifera Calcarina were abundant in a fraction of 0.0–1.0 ϕ in these samples. We classified the Calcarina tests of each sample into three grades according to Yamanouchi (1988). Grade A had complete spines, Grade B had some spines, and Grade C had no spines at all (Fig. 3). The number in each grade was counted for at least 1000 tests of Calcarina. In this study, Grade A was very rare because Calcarina tests were well abraded. We therefore summed grades A and B, and the residual spine ratio (percentage), the number of Grade A plus Grade B per total number of Calcarina tests (%) was calculated. We also obtained the spine ratio from the trench sediment samples.

Please cite this article as: Yasukochi, T., et al., Sedimentary facies and Holocene depositional processes of Laura Island, Majuro Atoll, Geomorphology (2014), http://dx.doi.org/10.1016/j.geomorph.2014.04.017

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A 30

0

30

Atoll locations

B

Fig. 1. A: Global distribution of atolls with locations of study sites and other islands mentioned in the text, reproduced from Shimazaki et al. (2006). Atoll location is based on ReefBase (http://www.reefbase.org). B: IKONOS satellite image of Majuro Atoll and location of Laura Island.

3.3. Radiocarbon ages Sixteen radiocarbon ages for foraminifera (Calcarina gaudichaudii) from the trench sediments and in situ coral samples from drill cores were already reported by Kayanne et al. (2011) and are shown in Table 1. The radiocarbon ages were corrected for the isotopic fraction and calibrated to cal BP by OxCal (ver. 3.10; Ramsey, 2005). Marine04 (Hughen et al., 2004) was used for the calibration curve, and the local reservoir effect was (ΔR) = − 35 ± 25 according to Kayanne et al. (2011). We dated Calcarina tests that still had some spines. Ford and Kench (2012) found relatively less durability of the skeletal remains of calcareous algae and foraminifera based on tumbling barrel experiments. Thus, Calcarina tests with spines would be suitable for dating to infer the age of formation of the facies in which they are included, as these foraminifera could have been transported from their original habitat by currents soon after their death (Yamano et al., 2001; Weisler et al., 2012). 4. Results 4.1. Topography and sedimentary facies Ocean-side and lagoon-side ridges were recognized reaching up to 3 m above the MSL, whereas the central basin was about 1.5 m above the

MSL (Fig. 4). The ocean-side ridge consisted of coral gravels. The upper 1 m of the lagoon-side ridge was a sand dune formed by fine sand, underlain by a beach deposit with foraminifer-rich sand. The elevation of the reef flat on the ocean side was about 0.7 m below the MSL, with the reef emergent at spring low tide. The lagoon shoreline is devoid of reef. The beach grades steeply to 5 m water depth. Reef drill cores were composed of cemented bioclasts, which contained coral gravel and in situ coral, coralline algae, shell, echinoderms, and benthic foraminifera (Calcarina and Amphistegina). Corals were composed of robust-branching Pocillopora sp., Heliopora, and Acropora spp. In the northwest Pacific, the dominant robust-branching (corymbose) corals are represented by Acropora and Pocillopora spp., which have contributed to framework growth (Hongo and Kayanne, 2011). Therefore, the facies of the cores were considered to represent shallow water reef flats. Island sediment from the trenches was classified into gravelly and sandy facies. Gravelly facies underlie the lower horizon of the central part of the island (S1, S2, Lr-2-8, N1, N2). They consist of massive, matrix (very coarse or coarse sand) supported, subrounded flat coral pebble to cobble gravel. The gravel size was smaller in S1, S2, and Lr 2–8 (~1 to 3 cm in diameter) and larger in N1 and N2 (Fig. 5a; occasionally larger than 10 cm in diameter). Pebbles and granules were occasionally horizontally stratified in S1 (Fig. 5b). Imbrication was not observed, except for a lagoonward imbrication in the lower gravelly facies of S1 (Fig. 5c). Normal grading was observed in the massive sandy facies of

Please cite this article as: Yasukochi, T., et al., Sedimentary facies and Holocene depositional processes of Laura Island, Majuro Atoll, Geomorphology (2014), http://dx.doi.org/10.1016/j.geomorph.2014.04.017

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the lower and upper horizons of all trenches except N1 and lower horizon of N2. Calcarina was the dominant foraminifera in samples (70– 90%). Soritidae was more abundant in the upper horizon (sandy facies) than the lower horizon (gravelly facies) in many trenches (S1, S2, TP2′, N1, N2). 4.2. Spine ratios Fig. 6 shows spine ratios of the surface sediments along the coast and subsurface sediment from the trenches at Laura. Spine ratios were high on the northern reef surface of Laura (56%, 26%, and 39%). Living foraminifera were also observed abundantly on the northern reef. The ratio in the ocean coastline was higher at the west corner of Laura (19%), where the reef rock was exposed along the coast, and on the northern oceanside coastline (12%). The spine ratio decreased toward the southern lagoon-side coastline of Laura. The spine ratios at the southern oceanside coastline were also low (0 and 1%). The spine ratios of trench sediments were generally similar to those of coastal surface sediment. The spine ratio was relatively high in the north of the island (24% at N3 and 16% at N2), low in the central and lagoon-ridge trenches (0–4% at N2, Lr-55, Lr-2-8, TP2′, Lr-4, S1, and S2), and slightly higher in the ocean-ridge trench (10% at Lr-55). The ratio of the southern neck (S0) was slightly higher (7%) than that of the coastal sediment. 4.3. Chronology

Fig. 2. Study sites on Laura Island. Trenches Lr-4, 18, 23, TP2′, 48, and 55 and boring cores were excavated by Kayanne et al. (2011).

N1, S1, and S2 (Fig. 5d). Sandy facies formed the upper horizon, which was composed of massive, sorted very coarse to coarse sand. The sandy facies in N2 only had planer cross-beds formed with foraminiferal assemblages, trending lagoonward with a dip angle of ~8° (Fig. 5e). The boundaries of the gravelly and sandy facies were clear in Lr-2-8, N1, and N2, but gradual in S1 and S2. Gravelly facies without a sandy facies covering were observed in the south neck (S0). Trenches TP2′ and Lr-18 were sandy with normal grading. The upper horizon of the north tip (N3) had planer cross beds trending oceanward, in contrast to the lagoonward trending planer cross beds in the upper horizon of N2. The sediments of the ocean side of the island (Lr-55, Lr-48, Lr-Mij) contained larger coral gravel, whereas those of the lagoon side (Lr-4) contained no gravel but had planer cross beds trending lagoonward in the upper horizon. Larger benthic foraminifera, mainly Calcarina, Amphistegina, and Soritidae, were the dominant components of sand sediment in both

a

b

Radiocarbon ages are shown in the column sections in Fig. 4. The upper 1 m of the coral reef flat on the ocean side was dated to 4300–1900 cal BP, indicating that the reef caught up with the sea level and may have accreted horizontally during 4300–1900 cal BP. The oldest dates were obtained from the lower gravelly facies in the lower sections of Lr-TP2′, Lr-2-8, N1, and S1, with a narrow age range of 2280–1900 cal BP. Sand facies above the gravel facies in these trenches were dated to 2090–1770 cal BP. Younger radiocarbon ages of 1520–1290 cal BP were obtained from trenches S0, N2, and N3. The dates from the ocean ridge (Lr-55) and the lagoon-side ridge (Lr-4) were youngest: 880–750 and 695–605 cal BP, respectively. In each trench, the depositional age was relatively similar vertically, indicating that the vertical sedimentation at each trench point was abrupt. In plan view the central north–south axis was first formed at one time during 2200–2000 cal BP. Then, the axis stretched to the north (N2, N3) and south (S0). Finally, the ocean ridge (Lr-55) and the beach ridge (Lr-4) were formed. 5. Discussion 5.1. Modern sediment transport along the coast of Laura As shown in Fig. 6, the spine ratio is higher in the modern northern coastline of Laura, and gradually declines toward the south except at

c

Fig. 3. Tests of Calcarina that have: a) complete spines (grade A), b) some spines (grade B), and c) no spines at all (grade C). Spines are considered to be abraded in proportion to the distance that they are transported from the supply source (Yamanouchi, 1988). The size of each foraminifer is about 1 mm.

Please cite this article as: Yasukochi, T., et al., Sedimentary facies and Holocene depositional processes of Laura Island, Majuro Atoll, Geomorphology (2014), http://dx.doi.org/10.1016/j.geomorph.2014.04.017

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Table 1 List of radiocarbon dates. ΔR = −35 ± 25. Material

δ13C (‰)

Conventional 14C age (year BP)

Years cal BP (1σ calibration)

Foraminifera sand from trenches (Fig. 4) Lr-TP2-30 Beta-191715 Lr-TP2-105 Beta-191712 Lr-2-8-145 TERRA-102207b35 Lr-2-8-240 TERRA-102207b36 Lr-55-95 Beta-191713 Lr-4-105 Beta-191710 N1-140 Beta-210285 N1-80 Beta-210284 N2-80 Beta-218012 N3-150 Beta-218013 S0-140 Beta-218011 S1-140 Beta-210287 S1-60 Beta-210286

Foraminifera (Calcarina gaudichaudii) Foraminifera (C. gaudichaudii) Foraminifera (C. gaudichaudii) Foraminifera (C. gaudichaudii) Foraminifera (C. gaudichaudii) Foraminifera (C. gaudichaudii) Foraminifera (C. gaudichaudii) Foraminifera (C. gaudichaudii) Foraminifera (C. gaudichaudii) Foraminifera (C. gaudichaudii) Foraminifera (C. gaudichaudii) Foraminifera (C. gaudichaudii) Foraminifera (C. gaudichaudii)

−2.7 −4.5 −0.5 −7.0 −2.8 −2.5 −3.2 −3.0 −2.3 −2.2 −3.3 −3.2 −2.4

2350 2380 2365 2495 1230 1050 2320 2200 1880 1790 1820 2420 2350

2090–1940 2130–1970 2100–1970 2280–2140 880–750 695–605 2050–1900 1910–1770 1520–1390 1410–1290 1470–1330 2170–2010 2090–1940

Coral from drill core (Fig. 4) LrI-14 Beta-191716 LrII-5 Beta-191717 LrIII-1 Beta-191718

Coral (Heliopora coerulea) Coral (Acropora branching type) Coral (Acropora corymbose type)

+4.6 −0.3 +1.5

4080 ± 70 3580 ± 100 2330 ± 60

Sample no.

Lab. code

the west tip of the island. Along the coast of the lagoon side, little production of Calcarina is expected because of greater water depth (Hohenegger, 2004). In fact, few living Calcarina were observed on the lagoon side of the island (Fujita et al., 2009). Therefore, Calcarina tests on the modern coast of the lagoon side are likely to have been transported from the northern reef of Laura. Along the coast of the ocean side of Laura, there is a reef that emerges in low tides. The intertidal ocean reef flat provides a suitable habitat for foraminiferal growth and sand production (Hohenegger, 2004). Indeed, high spine ratios on the coast of the west tip of Laura indicate a foraminiferal habitat nearby.

± ± ± ± ± ± ± ± ± ± ± ± ±

40 50 36 33 40 40 40 40 40 40 40 40 40

4300–4060 3640–3380 2080–1900

Nevertheless, the spine ratio along the ocean-side coast gradually declines toward the south, except at the west tip, suggesting a north to south trend of sand transportation surrounding the modern coast of Laura. Low spine ratios along the southern part of the ocean side and lagoonal coast are also likely to reflect the alongshore transport of sediment from the east along Long Island. Rosti (1990) and Xue (2001) pointed out that longshore sediment transport on Long Island is westward and controlled by the prevailing wind direction. On the lagoon side of Long Island, few living Calcarina were observed. Thus

West

East

Lr-IV Legend

Lr-III

Soil Other/Indeterminate Coral Fragments Flat Lamination Shell Fine Sand Medium Sand Coralline algae Coarse Sand Very Coarse Sand

Lr-I

Lr-II

Foraminifera Other Coral

Soritidae

Calcarina Amphistegina

Halimeda

Gravel

N2 N3

North

N1

S2 Lr-2-8

S1 S0

South

Fig. 4. Columnar sections, component of sand in each horizon, component of foraminifera in each sand sample, and radiocarbon age (cal. cal BP), across: a) the west–east transect and b) the north–south transect. Refer to Figs. 2 and 4 for the trench sites. Note the fining-upward sequence and the ages (ca. 2000 cal BP) in the central part of the island (S1, S2, Lr-2-3, N1, N2). Foraminifera (mainly Calcarina) are the main component of the sand.

Please cite this article as: Yasukochi, T., et al., Sedimentary facies and Holocene depositional processes of Laura Island, Majuro Atoll, Geomorphology (2014), http://dx.doi.org/10.1016/j.geomorph.2014.04.017

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a

b

c

d e

f

g

Fig. 5. a: Pebble and cobble, lower part of Laura N2; b: horizontally stratified granule and pebble, lower part of Laura S1; c: faint imbrication, lower part of Laura S1 (south facing wall); d: massive sand, middle part of S2; e: planar cross-bed, upper part of Laura N2; f: gravelly shore at the northern tip of Laura. The dark color of coral gravels is because of the algae on their surface, g: planar beds of Calcarina-rich layer observed on a modern beach on Laura.

foraminifera sand must have first been washed from the ocean side to the lagoon side and then transported along the lagoon-side shore. Long Island might have been cut by channels in the past (Xue, 2001), which provided conduits for sand transfer from ocean to lagoon. Sand

was subsequently transported westward along the lagoon-side coast of Long Island to Laura Island. The long distance of transportation should have abraded the tests and contributed to the low spine ratio.

Please cite this article as: Yasukochi, T., et al., Sedimentary facies and Holocene depositional processes of Laura Island, Majuro Atoll, Geomorphology (2014), http://dx.doi.org/10.1016/j.geomorph.2014.04.017

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Fig. 6. Spine ratio (%) from trenches and modern surface sediment. The ratio from both trench and modern surface sediment in the northern part shows a relatively higher value, indicating that the sediment supply source was at the northern reef and the sediment was transported from the source. See text for further description.

5.2. Sediment transport and depositional process of Laura Radiocarbon ages demonstrate that the reef crest of the ocean side of Laura had caught up with the sea level by 3600 cal BP. During the reef growth, the lagoon side was filled with bioclastic sediment containing coral, Halimeda, and foraminifera sand (Anthony et al., 1989), which provided the basement of the island. Reef-building corals do not grow above the lowest low water (LLW) level. Therefore the reef framework in 3600 cal BP around Laura should have been no higher than the LLW at that time. After the formation of the reef framework, the 1.1-m relative sea level fall during the past 2000 years exposed significant parts of the reef framework, which then emerged above LLW (Fig. 4). Larger benthic foraminifera observed commonly in the island sediment (C. gaudichaudii, Amphistegina lobifera, and Soritidae) are shallowwater species that can live abundantly on the reef flat and back reef, which are subaerially exposed during low tides (Hallock, 1984; Hohenegger, 2004; Fujita et al., 2006). Therefore, the relative sea-level fall at 2000 cal BP provided an optimum habitat for these foraminifera on the reef, resulting in the supply of foraminiferal tests for island formation. Similar foraminiferal production as a result of relative sea-level falls has been reported in the Ryukyu Islands (Japan) and the Green Island on the Great Barrier Reef (Yamano et al., 2000, 2001). The spine ratio of the central sedimentary body of Laura, which accumulated since ca. 2000 cal BP, is as low as that of the modern lagoon

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side. According to the modern profile of the spine ratio described in Section 5.1., the low value (1–4%) of the ratio indicates that the sediment of the central body of the island was transported a long distance from its supply source. The ratio is high at the northern sedimentary body, which was deposited around 1300 cal BP, showing the foraminiferal habitat to the north of the island. The sand was transported from the north of Laura or from the south along the lagoon-side coast of Long Island, similar to the modern process. A higher spine ratio in the western sedimentary body also shows that a supply source existed on the western reef at ca. 750 cal BP, as on the modern reefs. Slightly higher spine ratios in the southern sedimentary body, however, suggest that another foraminiferal habitat used to exist in the southern reef of Laura or Long Island. We presume that the southern habitat disappeared during the formation of Long Island, which also halted the sand transportation from the ocean to the lagoon. Sand production and transportation firstly increased by emergence of reef flat, but then decreased by island formation. Coral gravel is abundant in the lower horizon of the trenches and similar in size and roundness to that in the ocean-side ridge (Fig. 5f). Coral gravels of the same geomorphic unit on the ocean-side ridge of Funafuti Atoll, Tuvalu, were transported by hurricane waves (Maragos et al., 1973). Therefore, the gravelly facies may have been deposited under a high-energy environment. The timing of deposition was ca. 2000 cal BP. Assuming the intensity of the incident ocean swell had been constant, the energy on the reef would have been higher where the high-tide water depth was greater (Kench and Brander, 2006). During the sea-level highstand before 2000 cal BP, the wave energy passing over the reef flat would have been higher due to greater water depths. Coral gravels, Halimeda and foraminifera sand passed over the reef to fill up the lagoon-side depression, which formed the basis of the island, together with a rigid framework of coral reef flats on the ocean side (Anthony et al., 1989). However, the surface of the gravel, as well as the coral reef flat, would never have reached above sea level under the high energy condition. The subsequent relative sea-level fall lowered the wave energy over the reef flat and enabled the gravel to stay on the reef even under high tides, which resulted in the initial formation of the island. Once the initial gravelly facies were deposited, sand-sized grains with low settling velocity were deposited under low-energy shallow water above the initial deposit. The same lamination of Calcarina tests as is observed in the upper horizon of some trenches is commonly observed on sandy beaches, especially on the lagoon side (Fig. 5g), which suggests a low-energy depositional environment. Soritidae tests have a low settling velocity due to its shape and porosity (Maiklem, 1968). It is therefore moved easily (Collen and Garton, 2004), which may explain the higher content of Soritidae tests in the upper horizon of the island sediment. Calibrated radiocarbon ages were around 2000 cal BP at both the lower and the upper horizons of the central sedimentary body of the island (Fig. 4). This indicates that the initial deposition of gravel and sand described above occurred on the central body of the island with a size of 2 to 3 km from north to south, rapidly within 200 years (almost the range of error of the radiocarbon age) at ca. 2000 cal BP. Once the central core of the island was formed, the island accreted both lagoonward and oceanward, with a decelerating or regular accretion after initiation as a central core in the models in Woodroffe et al. (1999). This study also reconstructed longitudinal accretion of the island, which began earlier (ca. 1300 cal BP) than lateral accretion. Because of the existence of the island, wave energy should have declined on the lagoon side, which resulted in deposition of sandy sediment. Coarser sediments were deposited on the ocean-side coast, getting closer to the reef crest as the island accreted oceanward (Figs. 4 and 7). However, we did not date the sediments between the central body and the ocean and lagoon-side ridges. Thus it cannot be determined whether the accretion occurred continuously or sporadically from our data.

Please cite this article as: Yasukochi, T., et al., Sedimentary facies and Holocene depositional processes of Laura Island, Majuro Atoll, Geomorphology (2014), http://dx.doi.org/10.1016/j.geomorph.2014.04.017

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T. Yasukochi et al. / Geomorphology xxx (2014) xxx–xxx

Stage I (3500-2000 cal. BP)

Stage II (2000 cal. BP)

Stage III (2000-650 cal. BP)

Modern S.L.

Fig. 7. Schematic of depositional process that formed Laura Island. Owing to a relative sea level fall from ca. 2000 cal BP, wave energy declined so that gravels were able to stay on the reef, and living foraminifera increased on the shallow reef flat. See text for further explanation.

5.3. Application to other atoll islands Whereas Akiyama (1979) and Yamanouchi (1988) used spine ratios to explore modern sediment transport on reefs, we used the spine ratio to reconstruct the origin and transportation of the initial depositional processes of an atoll island; this approach was found to be useful as a tracer of sand transportation for both modern and geological sediment. Component-specific radiocarbon dating with Calcarina that still have spines enabled three-dimensional reconstruction of the formation history of Laura. Our method of spine ratios for reconstruction of sediment transport and radiocarbon dating of Calcarina are applicable to other reef islands where Calcarina is abundant on the reef. Occurrences of Calcarina in recent beach sands have been reported in most of the islands in the western Pacific (Todd, 1960). Even where Calcarina does not occur, Baculogypsina can be used for spine ratio and radiocarbon dating. Laboratory tumbling experiments showed that B. sphaerulata lost its spines within 200 hours of tumbling, and the abrasion rate slowed markedly after 500 hours (Ford and Kench, 2012). The experimental results suggested that foraminifera sand with spines should have been transported shortly after its production. Dawson (2012) classified three levels of preservation for B. sphaerulata tests: pristine, moderately abraded, and severely abraded. These correspond to our classification of Grade A (complete spines), Grade B (some spines), and Grade C (no spines at all), respectively. Dawson (2012) observed that only 0.5% of B. sphaerulata sand was pristine in the beach sediment of Raine Island in the Great Barrier Reef, and a high ratio of pristine tests was only observed in the growth habitat. Such results are consistent with our observation that little sand was of Grade A in the beach and trench sediment. We used the foraminifera species Calcarina in this study, but general abrasion features of spines are common between Baculogypsina and Calcarina. Moderately and severely abraded levels were observed in 15.7% and 83.7% B. sphaerulata sand, respectively. The production area of the foraminifera is an average of 1 km from the beach in Laura Island, which is also consistent with our observation of the high spine ratio in the northern part of the island. This island is close to or 1 km apart from the foraminifera habitat.

Spine ratios and three-dimensional reconstruction of island accumulation showed that the initial sediment of Laura Island was transported to areas where the wave energy was low enough for deposition, even though the sediment supply source was distant from the island. The initial deposition of Laura occurred along the wide part of the reef rim (probably the center or lagoon side of the rim). Other physical conditions, such as the dominant direction of waves and swells, tidal ranges, and ecological conditions such as the dominant sand-forming creatures will also affect atoll island deposition processes. Further studies of sedimentary facies of other atoll islands are needed to confirm and generalize the formation process and response of reef islands. Acknowledgements This research was financially supported by the Environment Research and Technology Development Fund, Ministry of the Environment, Japan (Projects B15 and A-0805). An IKONOS image of Majuro was provided by SOPAC. We are grateful to Yoshihiro Tsuji for his help with the identification of calcareous sand, Hiromune Yokoki for helpful discussions, and others who supported our work, including John Bungitak, Caleb McClennen, Andrew Finlay, Royal Caesar, Kazuhiko Fujita, Masao Watanabe, Hiroshi Takagi, and Yoichi Ide. References Akiyama, Y., 1979. Breakdown process of “Star Sand” as an index of littoral drift in shallow lagoon of the recent coral reef, Yoron Island, Japan. Geophys. Sci. (Chiri-Kagaku) 31, 33–39 (in Japanese, with English Abstr.). Anthony, S.S., Peterson, F.L., 1987. Carbonate geochemistry and hydrology relationships, Laura Majuro Atoll, Marshall Islands. Technical Report, 172. Univ. Hawaii, Water Resources Research Center (77 pp.). Anthony, S.S., Peterson, F.L., MacKenzie, F.T., Hamlin, S.N., 1989. Geohydrology of the Laura fresh-water lens, Majuro atoll: a hydrogeochemical approach. Geol. Soc. Am. Bull. 101, 1066–1075. Barry, S.J., Cowell, P.J., Woodroffe, C.D., 2007. A morphodynamic model of reef-island development on atolls. Sediment. Geol. 197, 47–63. Boss, S.K., Liddell, W.D., 1987. Patterns of sediment composition of Jamaican fringing reef facies. Sedimentology 34, 77–87. Brander, R.W., Kench, P.S., Hart, D., 2004. Spatial and temporal variations in wave characteristics across a reef platform, Warraber Island, Torres Strait, Australia. Mar. Geol. 207, 169–184.

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Please cite this article as: Yasukochi, T., et al., Sedimentary facies and Holocene depositional processes of Laura Island, Majuro Atoll, Geomorphology (2014), http://dx.doi.org/10.1016/j.geomorph.2014.04.017