The mode and timing of windward reef-island ...

Geomorphology 318 (2018) 320–335

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The mode and timing of windward reef-island accretion in relation with Holocene sea-level change: A case study from Takapoto Atoll, French Polynesia L.F. Montaggioni a,⁎, B. Salvat b, A. Aubanel c, A. Eisenhauer d, B. Martin-Garin a a

Aix Marseille Univ, CNRS, IRD, Coll France, CEREGE, Marseille, France PSL-EPHE (École pratique des hautes études)-CNRS-UPVD, USR 3278, Labex Corail, Université de Perpignan, 66000 Perpignan, France c Environmental Consultant, BP 2038, Papeete 98713, Tahiti, French Polynesia d GEOMAR, Helmholtz-Zentrum für Ozeanforschung Kiel, Kiel, Germany b

a r t i c l e

i n f o

Article history: Received 16 January 2018 Received in revised form 19 June 2018 Accepted 24 June 2018 Available online 05 July 2018 Keywords: Atoll islet development Sea level Holocene Tuamotu islands

a b s t r a c t Takapoto Atoll (northern Tuamotu Islands, French Polynesia, Central Pacific) was selected as a test area to clarify the conditions of atoll island accretion in relation to mid- to late-Holocene sea-level changes. Surveys were conducted along two distinct cross-island profiles, on the windward coast of the atoll. In addition, the stratigraphy of an oceanfacing islet was described from an excavation in order to reconstruct the successive island accretionary stages. At both sites, the basement of the atoll-rims consists of conglomerate pavements on which lie shingle ridges, reaching 4 m in elevation. Stratigraphic analysis of the excavated ridge reveals alternation of gravelly sandsupported to gravel-dominated sediments. The chronology of island accretion is based on dating of 41 U/Th surface and excavated coral specimens. Ridge initiation occurred from about 1000 yr BP when sea level was close to its present position, shingle deposits progressively prograded from the lagoon margins oceanwards and were partially cemented at their bases. Cementation may have increased the resistance of the islets to erosion. As a result, some island lands accumulated and have persisted over the last millennium. The modern gross island morphology was acquired during the last 500 years. This model can be considered to be of regional value for the northern Tuamotu islands, adjusted for local thermal subsidence, hydroisostasy and/or lithospheric flexure. Compared to some other Indo-Pacific reef islands, island initiation at Takapoto appears to be have been delayed by 2 to 4 millennia, probably in response to retardation in the reef catching-up with mid-Holocene sea level. Dating of individual coarse-grained coral clasts allowed the major wave-surge events that have hit Takapoto to be identified for the last millennium. The use of gravels results in the identification of a greater number of medium-energy surge impacts, when compared with megaclast-based records. The frequency of storm events identified is consistent with that derived from historical observations; severe storms have a very low frequency of occurrence – one to two events per century on average. © 2018 Elsevier B.V. All rights reserved.

1. Introduction Atolls are mid-ocean, ring-shaped reefal islands, comprising a forereef extending below the low tide mark, a narrow reef rim and an internal lagoon. From the reef edge lagoonwards, reef rims generally comprise a succession of an exposed algal ridge, a reef flat colonized by scattered coral communities, and islands – or rather islets due to their limited size – usually consisting of unlithified to firmly lithified skeletal sands and coral rubble (Woodroffe and Biribo, 2011), locally overlying antecedent conglomerate pavements or platforms (Montaggioni, 2011). Known as motu in the Pacific, in storm-prone areas coarsegrained reef islets develop mostly on the windward sides of atoll rims ⁎ Corresponding author. E-mail address: [email protected] (L.F. Montaggioni).

https://doi.org/10.1016/j.geomorph.2018.06.015 0169-555X/© 2018 Elsevier B.V. All rights reserved.

as shingle ridges, culminating at elevations rarely exceeding 4 m above mean sea level (amsl). By contrast, along leeward rim areas, lowerlying sandy cays are the dominant forms. A transverse profile across most windward atoll rims shows a similar topography, with a prominent steep seaward ridge, locally interrupted by shallow swales and ponds in its central part. Behind the seaward ridge there is generally a gentle lagoonward-dipping beach ridge (Chevalier et al., 1979; Woodroffe et al., 1999; Woodroffe and Biribo, 2011). Atoll reef islets in the Indo-Pacific are interpreted as depositional landforms formed during the last few millennia in relation to changes in sea level (Pirazzoli and Montaggioni, 1986; Dickinson, 1999; Woodroffe et al., 1999; Kench et al., 2005; Woodroffe, 2005; Barry et al., 2007; Kench et al., 2014a, 2014b). The interaction between reef growth and the rate of sea-level fluctuations has been assumed to be a fundamental driver on their initiation and accretion (Perry et al., 2011). However, the

L.F. Montaggioni et al. / Geomorphology 318 (2018) 320–335

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Fig. 1. A: Location map of Takapoto Atoll in the Tuamotu Islands (French Polynesia, central Pacific). B: Google Earth map of Takapoto Atoll showing location of areas (boxes labelled as C and D) from which were selected the two studied morphological profiles, and location of Fig. 2C and D: aerial views showing the location of the two morphological profiles: northeastern (see Fig. 3) and southeastern (see Fig. 4) profiles.

eustatic regime under which they have accreted remains questionable. The initiation and development of these bodies may have been triggered by a post-highstand sea-level fall (Woodroffe et al., 1999; Dickinson, 2009; Kench et al., 2014a, 2014b; Yasukochi et al., 2014), by a rise in sea level, or by stillstands higher than the present datum (McLean et al., 1978; Stoddart et al., 1978; Woodroffe and Morrison, 2001; Kench et al., 2014a, 2014b; Yamano et al., 2014). This diversity of views strongly suggests that reef islets are able to form at a variety of sea-level stages (Kench, 2014). As emphasized by several authors (Holthus et al., 1992; Woodroffe et al., 1999; Barry et al., 2007; Yamano et al., 2007; Woodroffe, 2008;

Andréfoüet et al., 2012; Yamamoto and Esteban, 2014; Buadromo et al., 2016), reef islets on atolls are regarded as particularly vulnerable to the effects of the global rise in sea level in response to the current warming trend, a view supported by the IPCC (Nurse et al., 2014). However, in such a context, the future of low-lying coral islands is controversial. There are those who consider that these could be submerged (Yamano et al., 2007; Dickinson, 2009; Connell, 2013; Hubbard et al., 2014; Woodruff et al., 2013; Storlazzi et al., 2015), and others who have suggested that they will persist, migrating laterally (Webb and Kench, 2010; Rankey, 2011; Biribo and Woodroffe, 2013; Ford, 2012, 2013; Le Cozannet et al., 2014; Pala, 2014; Ford and Kench, 2015;

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Fig. 2. Aerial view of a typical islet from the northeastern, windward area of Takapoto Atoll. This ridge is situated some 500 m south of the northeastern test profile. The geomorphological units identified from reef-front to lagoon are indicated (Photo courtesy Samuel Étienne).

Kench et al., 2015; McLean and Kench, 2015; Duvat and Pillet, 2017; Duvat et al., 2017). In any case, catastrophic events such as storms, cyclones and tsunamis have a significant impact on reef islands, including both construction and erosion through associated surges (Woodroffe et al., 1999). Generally, storms and cyclones tend to construct motu, shingle ridges of coral boulders including reef megaclasts (McKee, 1959; Stoddart, 1971; Baines and McLean, 1974; Bourrouilh-Le Jan and Talandier, 1985; Pirazzoli et al., 1988; Scoffin, 1993; Terry et al., 2013; Lau et al., 2016) from the adjacent forereef zones (Laboute, 1985; Harmelin-Vivien and Stoddart, 1985; Harmelin-Vivien and Laboute, 1986; Done, 1992; Harmelin-Vivien, 1994). As emphasized by Yamano et al. (2007), an understanding of the present vulnerability of atoll islands to ongoing sea-level rise requires a detailed reconstruction of island development. Although attention has been paid to both qualitative and quantitative modelling of reef-islet accretion on atoll rims (Barry et al., 2007), the relative timing of the

successive accretionary events responsible for island formation is poorly constrained, particularly on French Polynesian atolls. The present paper aims to examine the morphology and chronostratigraphy of the deposits from the eastern side of the Takapoto Atoll rim in order to define the scenario and timing of reef island accretion. Such a reconstruction is of prime importance – and should help in understanding and anticipating the evolution of atoll rims and in particular assessing their vulnerability to sealevel rise and to expected increasing cyclone intensity (Lau et al., 2016). 2. Study site 2.1. Location and morphology of the atoll rim Takapoto is one of the northern atolls in the Tuamotu Archipelago, located between 14°33″–14°43″ south and 145°08″–145°16″ west (Fig. 1). The Tuamotu archipelago extending over 1500 km along a

Fig. 3. Northeastern profile, windward side of Takapoto Atoll (see Fig. 1C for location), showing the different geomorphic zones from the outer reef slope to the lagoon (see Fig. 7 for closeup views of the main units). The locations and ages of the U/Th dated coral samples (BAT, TAK) collected from the surface of deposits are given in Table 1. OCP = outermost conglomerate pavements; ICP = innermost conglomerate pavements; n.d. = not detectable.


west-north-west to east-south-east volcanic chain across the central Pacific, consists of 77 low-lying reef atolls. Takapoto is an atoll with a total surface area of 74 km2, about 17 km long and 5.5 km wide, and an outer rim of 23 km2. The atoll islets studied are either elongate or crescentshaped, oriented NE–SW (Fig. 2), locally separated by inter-islet channels – called hoa. The rim is approximately 350 m wide. It encloses a lagoon N45 m deep (Chevalier et al., 1979; Salvat, 1981; Salvat and Richard, 1985). According to Andréfouët et al. (2000), 55% of the rim area is occupied by coconut plantations. The outer part of the atoll rim is a living reef system, subdivided into three zones, from the sea inwards (Chevalier et al., 1979; Salvat and Richard, 1985; Kühlmann and Chevalier, 1986; Montaggioni, 1981, personal observation). The outer reef zone, varies from 0 to about 10 m deep in the upper part, and comprises a spur-and-groove system, followed oceanwards between 10 and 20 m depth by a gently-dipping terrace, approximately 100 m wide, interpreted as the top of the last interglacial reef. Below this, a subvertical drop-off falls to depths N60 m. The emergent, inner parts of the spurs are capped by an algal ridge, about 20 m wide, reaching 0.20 to 0.50 m above low tide level. Behind the algal-ridge line, a reef-flat zone, b100 m wide, is colonized by scattered coral heads and encrusted by coralline algae. The innermost parts bear the remains of conglomerate pavements and beach-rock slabs. The central part of the atoll rim is occupied by an almost continuous emergent island, only interrupted by shallow inter-islet channels, shoaling locally at low tide on the northwestern and northeastern sides of the atoll. As on most Tuamotu islands, the emerged areas of the Takapoto rim consist of two superimposed sedimentary units (Fig. 2): 1. Conglomerate pavements outcropping just behind the reef-flat zone consist of coarse-grained skeletal shingle, storm-derived from the adjacent upper outer reef zone, and rising 0.50 to 0.60 m above low tide level (Chevalier et al., 1979; Salvat, 1981; Salvat and Richard, 1985; Montaggioni and Pirazzoli, 1984; Pirazzoli and Montaggioni, 1986). Previous radiocarbon dating of two coral clasts from conglomerates on Takapoto provided ages of 1320 ± 80 and 5020 ± 140 cal. years BP. These conglomerates were cemented within both phreatic and vadose marine diagenetic environments (Montaggioni and Pirazzoli, 1984). 2. Overlying unconsolidated skeletal sediments, range from pebbles, and gravel to coarse sands, locally rich in foraminifera. These form islets, 100 to 400 m wide, along the northeastern and southwestern rim margins, where cyclone-generated waves converge, and form metre-thick ridges, composed predominantly of reworked coral colonies. Locally, on the innermost margin of the atoll rim, there are gently-dipping beach-rock slabs (Chevalier et al., 1979). Conglomerate pavements appear sporadically beneath the unconsolidated deposits bordering the lagoon.

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2.2. Compositions of proximal coral communities The scattered coral on reef-flat and upper outer-reef zones, to about 30 m depth have been quantitatively analysed by Chevalier and Denizot (1979), Bouchon (1983) and Kühlmann and Chevalier (1986). The latter described coral communities of the windward eastern areas. In reef-flat settings, the surfaces are almost totally bare of coral, only a few massive (Porites) and encrusting forms (Millepora) are present. The algal ridge is almost devoid of corals. The outer-reef zone, from the surface to about 5 m deep, is colonized by a community dominated by robust branching and encrusting corals, including Pocillopora and Montipora, with an average cover rate of 90%. Coral coverage remains high from 5 to 30 m depth, with populations mostly consisting of Porites, Montipora, Millepora and Pocillopora. Approaching 30 m depth, coral coverage decreases to 15% and at 40 m depth is b5%. As noted by Bouchon (1983) and Kühlmann and Chevalier (1986), Pocillopora verrucosa is common on the outer slope. According to Harmelin-Vivien and Laboute (1986), the coral communities on the reef flat and upper, low-angle, fore-reef zones to depths of about 20 m, are the main sources of detritus nourishing island ridges. Coral colonies from deeper zones are generally shed downslope when broken by cyclone-generated waves. 2.3. Climate As in other parts of French Polynesian, the climate in the northern Tuamotu is tropical, warm and humid. The rainy, austral summer lasts from November to April and the relatively cooler, and dryer, austral winter from May to October. In the northeastern atolls, air temperatures range between 23 and 30 °C. Annual rainfall averages 1500 mm. Humidity is high (around 80%) throughout the year. Tides are a twice daily regime and microtidal, averaging 0.5 m in amplitude and reaching a maximum of 0.70 m at spring tide. The regional climate system is driven by the trade winds and by the El Niño-Southern Oscillation (ENSO) which regulates tropical to extratropical storms (Andréfoüet et al., 2012). As in all Tuamotu islands, Takapoto is influenced by the south-east trade-winds, blowing from the ENE sectors for 70% of the year and from the SE for 20%. In the austral winter, sea conditions are controlled by strong trade-winds and southern swells, resulting in a high energy wave regime. Periods of low wave heights (around 1.5 m) and calm occur between periods of moderate to high swells (2.5 m). During the austral summer, the wave regime is dominated by low wave heights (Andréfoüet et al., 2012). Trade-winds are significantly less active and generate moderate-energy wave conditions, locally disturbed by storm events initiated either by ENSO-related cyclones or by storms originating from northern latitudes (Andréfoüet et al., 2012). The cyclone season usually extends from November to April (Laurent and Varney, 2014). Most of the cyclones affecting French Polynesia occur during ENSO events. The eastward migration of warmer

Fig. 4. Southeastern profile, windward side of Takapoto Atoll (see Fig. 1D for location) showing the different geomorphic zones from the outer reef slope to the middle part of the islet and the location of the excavation site. Ages of coral samples (BAT) from local beach-rocks slabs are provided in Table 1.

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Fig. 5. Cross-section of the excavation site, southeastern profile, windward of Takapoto Atoll (see Figs. 1D and 4 for location). The stratigraphic column shows the successive superimposed sedimentary units from base to top: conglomerate pavement; Unit 1, composed of indurated to unconsolidated sandy gravel rich in foraminifera; Unit 2 composed of gravels that include an interbedded soil horizon; Unit 3 is of gravel. Numbers 1 to 7 refer to stations at which close-up pictures were taken. The white circles indicate the locations of U/Th dated coral samples. Adjacent columns list: mean size (in cm) of gravels at different stratigraphic levels, field numbers (BAT) of dated coral samples and corresponding ages related to 2017 CE (see Table 1 for details of dates).

sea surface temperatures during El Niño episodes promotes the displacement of tropical storms further east into the Tuamotu region (Larrue and Chiron, 2010). The cyclone track in French Polynesia is typified by a tapering channel between the Society and the Austral Islands through which 70% of the cyclone tracks pass (Larrue and Chiron, 2010). Little is known about extreme wave hazard due to the short and partial historical record of tropical cyclones and tsunamis, and the relative scarcity of such events in the region over the last century. Lau et al.

(2016) provided a comprehensive survey of known cyclones and their effects in historical times. A total of about 24 cyclones were registered in the Tuamotu in the past 192 years from 1822 to 2014. The northwestern Tuamotu islands are occasionally affected by tropical storms and cyclones (4–10 per century; Dupon, 1987) with maximum wind gusts up to 150 km/h and swells higher than 6 m. Since the beginning of the 20th century, Takapoto has been hit by four cyclones (Duvat et al., 2017 and references therein). Over periods of several centuries, the number of


325

Table 1 Uranium/Thorium ages of coral samples from Takapoto Atoll. Details include field sample (BAT, TAK) numbers, laboratory sample numbers indicating the year of U/Th analysis (2016, 2017), sample locations on the northeastern (NE) and southeastern (SE) profiles and at the excavation site (Exc SE) relative to present mean sea level (msl), taxonomic identification of coral samples, geographical coordinates, ages and isotopic composition with statistical errors (two standard deviations of the mean). BR = beach-rock SR = superficial layer of shingle ridges; CP = conglomerate pavements. Samples BAT 103, 105 and 108 were collected by G. Haumani, TAKBS 1 to 4 by V. Duvat, all other samples by B. Salvat and A. Aubanel ou BS et AA. Sample ID

Analysis ID

Sample material

Coral identification

Longitude (W)

Latitude (S)

Age

±

238

U (ppm)

±

230

Th (ppt)

±

BAT041 BAT042 BAT043 BAT044 BAT045 BAT048 BAT051 BAT053 BAT054 BAT056 BAT057 BAT058 BAT059 BAT063 BAT064 BAT065 BAT066 BAT067 BAT068 BAT071 BAT072 BAT074 BAT075 BAT076 BAT077 BAT079 BAT080 BAT082 BAT083 BAT085 BAT086 BAT089 BAT090 BAT103 BAT104 TAK5D TAKBS1 TAKBS2 TAKBS3 TAKBS4

387-17 372-17 398-17 409-17 404-17 400-17 384-17 370-17 386-17 368-17 373-17 402-17 401-17 403-17 375-17 765-17 405-17 381-17 385-17 369-17 367-17 764-17 366-17 388-17 382-17 394-17 378-17 379-17 766-17 376-17 377-17 383-17 380-17 811-17 812-17 491-16 829-16 830-16 831-16 832-16

CP N–E transect BR N–E transect BR N–E transect SR N–E transect SR N–E transect SR N–E transect SR N–E transect SR N–E transect SR N–E transect CP N–E transect Exc S–E 4.0 m surface Exc S–E 4.0 m surface Exc S–E 4.0 m surface Exc S–E 3.40 m Exc S–E 3.40 m Exc S–E 3.40 m Exc S–E 2.66 m Exc S–E 2.66 m Exc S–E 2.66 m Exc S–E 2.15 m Exc S–E 2.15 m Exc S–E 2.15 m Exc S–E 1.0 m Exc S–E 1.0 m Exc S–E 1.0 m Exc S–E 0.30 m Exc S–E 0.30 m Exc S–E 0.30 m CP Exc S–E top CP Exc S–E top CP Exc S–E top BR S–E transect BR S–E transect CP N–E transect lagoon CP N–E transect lagoon Boulder close to N–E SR N–E transect SR N–E transect SR N–E transect SR N–E transect

Dipsastrea cf. speciosa Indeterminate coral Favites sp. Cyphastrea serailia Cyphastrea serailia Goniastrea stelligera Dipsastrea sp. Acropora sp. Acropora sp. Indeterminate coral Dipsastraea speciosa Dipsastraea speciosa Dipsastraea speciosa Pocillopora eydouxi Pocillopora eydouxi Pocillopora eydouxi Pocillopora eydouxi Pocillopora eydouxi Pocillopora eydouxi Pocillopora eydouxi Pocillopora eydouxi Pocillopora eydouxi Pocillopora eydouxi Pocillopora eydouxi Pocillopora eydouxi Pocillopora eydouxi Pocillopora eydouxi Pocillopora eydouxi Indeterminate coral Indeterminate coral Indeterminate coral Indeterminate coral Indeterminate coral Indeterminate coral Indeterminate coral Porites sp. Dipsastraea speciosa Dipsastraea speciosa Dipsastraea speciosa Dipsastraea speciosa

145°08′30.92 145°08′27.71 145°08′27.71 145°08′29.17 145°08′29.17 145°08′33.09 145°08′33.09 145°08′32.00 145°08′32.00 145°08′51.30 145°12′46.62 145°12′46.62 145°12′46.62 145°12′46.62 145°12′46.62 145°12′46.62 145°12′46.62 145°12′46.62 145°12′46.62 145°12′46.62 145°12′46.62 145°12′46.62 145°12′46.62 145°12′46.62 145°12′46.62 145°12′46.62 145°12′46.62 145°12′46.62 145°12′46.62 145°12′46.62 145°12′46.62 145°12′45.64 145°12′45.65 145°08′51.30 145°08′50.32 145°08′57.81 145°08′47.06 145°08′47.06 145°08′47.06 145°08′47.06

14°34′42.17 14°34′42.83 14°34′42.84 14°34′42.35 14°34′42.35 14°34′42.06 14°34′42.07 14°34′42.31 14°34′42.31 14°35′18.14 14°40′42.94 14°40′42.94 14°40′42.94 14°40′42.94 14°40′42.94 14°40′42.94 14°40′42.94 14°40′42.94 14°40′42.94 14°40′42.94 14°40′42.94 14°40′42.94 14°40′42.94 14°40′42.94 14°40′42.94 14°40′42.94 14°40′42.94 14°40′42.94 14°40′42.94 14°40′42.94 14°40′42.94 14°40′43.73 14°40′43.73 14°35′18.14 14°35′15.07 14°33′26.39 14°33′39.85 14°33′39.85 14°33′39.85 14°33′39.85

1478 294 75 64 n.d. 400 505 236 423 6523 501 1781 348 566 438 597 837 454 496 1097 719 561 1135 2796 717 7705 1631 935 1005 1374 1111 872 48 2237 2758 369 111 500 181 453

31 20 29 26 n.d. 36 30 26 20 54 29 38 27 23 24 3 35 25 30 33 25 3 31 34 31 120 59 31 4 30 24 22 20 10 15 56 7 12 11 10

2.330 3.061 2.087 2.120 n.d. 1.867 2.300 2.282 3.064 2.173 2.128 1.892 2.177 2.725 2.654 2.022 1.862 2.773 2.155 2.181 2.429 1.911 2.103 2.307 2.042 0.689 1.134 2.227 0.917 2.199 3.115 3.263 2.921 2.232 1.447 1.872 1.940 2.425 2.349 2.755

0.003 0.005 0.002 0.002 n.d. 0.004 0.004 0.003 0.007 0.003 0.004 0.003 0.003 0.006 0.004 0.004 0.002 0.005 0.003 0.003 0.004 0.003 0.003 0.002 0.002 0.001 0.001 0.005 0.001 0.001 0.005 0.004 0.003 0.003 0.003 0.005 0.004 0.006 0.006 0.007

0.594 0.156 0.027 0.024 n.d. 0.148 0.203 0.094 0.225 2.388 0.185 0.585 0.133 0.267 0.201 0.209 0.270 0.218 0.186 0.421 0.302 0.187 0.422 1.112 0.253 0.898 0.333 0.360 0.160 0.521 0.604 0.492 0.025 0.863 0.688 0.120 0.038 0.211 0.078 0.217

0.011 0.010 0.010 0.009 n.d. 0.010 0.011 0.010 0.010 0.010 0.010 0.010 0.010 0.010 0.010 0.000 0.010 0.011 0.011 0.011 0.009 0.000 0.010 0.010 0.010 0.010 0.010 0.010 0.000 0.010 0.010 0.011 0.010 0.000 0.000 0.017 0.002 0.004 0.004 0.004

n.d. = not detectable; act. ratio = activity ratio. Recommendations of Dutton et al. (2017) were followed for the presentation of U/Th age data. All statistical errors are two standard deviations of the mean (2σ mean). All samples have been corrected for initial 230Th by using a 230Th/232T hactivity ratio of 0.66 ± 0.25. Non-reported data consist of 230Th/232Th ratios which became negative due to background corrections. 238 U Concentrations are not corrected for the background. Dutton, A. et al. Data reporting standards for publication of U-series data for geochronology and timescale assessment in the earth sciences. Quat. Geochronol. 39, 142–149 (2017). Fietzke, J., Liebetrau, V., Eisenhauer, A. & Dullo, C. Determination of uraniumisotope ratios by multi-static MIC-ICP-MS: Method and implementation for precise U-and Th-series isotope measurements. J. Anal. At. Spectrom. 20, 395–401 (2005).

cyclonic events in the northwestern Tuamotu has probably not exceeded 2–3 per century (Canavesio, pers.comm.). The cyclonic hazard in the region is typified by a very low frequency of events, but displays “crisis” phases, mainly related to ENSO episodes, during which several cyclonic events may occur within short periods. Gaps between extreme wave events of longer than 100 years are not uncommon. Cyclone hazard records reveal a 50-year return period for waves exceeding 12 m high (Canavesio, 2014). There is little information about the effects of cyclones and storms in shaping atoll morphology. In 1906, on Anaa Atoll, cyclone-generated waves eroded the western side of the island over a distance of N300 m. During cyclone Orama, in February 1983, the village on Takapoto was flooded by 4–5 m high waves and outer-reef coral communities were partly destroyed. Laboute (1985) noted that 50–100% of living corals were destroyed along the eastern side of the atoll during this event. As in the records of cyclone activity, the historical assessment of tsunamis in the Tuamotu is very fragmented (Etienne, 2012). Archaeological data suggest that since the 16th century the Tuamotu has

experienced less than ten tsunamis, generating 0.3–1.9 m-high waves (Lau et al., 2016). Based on numerical modelling and direct observations, the Tuamotu islands seem not to have been significantly affected by tsunami waves compared to other high-volcanic islands in French Polynesia (Sladen et al., 2007). Reconstructions of current absolute sea-level changes in the tropical Pacific over a 60-year period (1950–2009) indicate that sea level rose by 2.5 ± 0.5 mm per year (Becker et al., 2012). This value is higher than the mean global absolute sea-level rise (1.2–1.8 mm per year) estimated for the 20th century. 3. Materials and methods 3.1. Field survey Detailed topography was surveyed at two sites, along transects from the northeastern and southeastern sides of the reef rim (Fig. 1). From the reef front inwards, the northeastern transect lies

326

232


Th (ppb)

±

0.147 0.013 0.064 0.018 n.d. 5.895 0.897 0.300 0.253 2.009 0.323 1.680 0.496 0.051 0.080 0.094 0.047 0.225 0.370 2.720 0.200 0.313 2.874 2.061 0.029 2.579 4.694 0.183 0.283 0.619 2.559 0.077 0.277 0.260 0.516 n.d. 0.143 0.021 1.169 0.082

0.020 0.019 0.020 0.018 n.d. 0.038 0.021 0.019 0.018 0.020 0.019 0.019 0.019 0.018 0.019 0.019 0.019 0.021 0.020 0.021 0.018 0.018 0.019 0.018 0.019 0.019 0.024 0.020 0.019 0.018 0.020 0.020 0.018 0.018 0.019 n.d. 0.012 0.013 0.019 0.013

[238U/232Th] act. ratio

±

49,151 713,697 100,483 357,451 n.d. 980 7936 23,573 37,516 3348 20,393 3485 13,595 166,100 102,622 66,761 121,649 38,122 18,027 2482 37,589 18,874 2265 3466 214,566 827 748 37,610 10,032 10,992 3767 131,734 32,649 26,563 8675 n.d. 41,894 357,774 6222 104,282

6656 1,031,427 30,788 342,455 n.d. 7 184 1458 2681 33 1176 41 509 58,733 24,669 13,613 49,971 3497 982 19 3345 1108 16 31 138,750 6 4 4004 665 321 31 34,593 2176 1873 312 n.d. 3410 223,025 101 16,740

[230Th/232Th] act. ratio

±

757.0 2193.0 79.6 241.4 n.d. 4.7 42.4 58.7 166.4 222.3 107.4 65.1 50.2 982.9 470.4 417.1 1065.4 180.9 94.0 28.9 281.9 111.5 27.4 100.9 1606.8 65.1 13.3 366.9 106.0 157.2 44.2 1198.9 16.9 620.1 249.1 n.d. 49.6 1876.2 12.5 496.3

103.4 3172.6 39.0 250.0 n.d. 0.3 2.5 7.1 13.8 2.5 8.4 1.4 4.2 349.3 115.6 85.0 439.5 18.9 7.4 0.8 26.6 6.5 0.7 1.3 1041.1 0.9 0.4 40.5 7.0 5.4 0.8 315.9 6.7 43.7 8.9 n.d. 5.0 1170.0 0.7 80.2

[230Th/238U] act. ratio

±

0.0154 0.0031 0.0008 0.0007 n.d. 0.0048 0.0053 0.0025 0.0044 0.0664 0.0053 0.0187 0.0037 0.0059 0.0046 0.0062 0.0088 0.0047 0.0052 0.0117 0.0075 0.0059 0.0121 0.0291 0.0075 0.0787 0.0177 0.0098 0.0106 0.0143 0.0117 0.0091 0.0005 0.0233 0.0287 0.0039 0.0012 0.0052 0.0020 0.0048

0.0003 0.0002 0.0003 0.0003 n.d. 0.0003 0.0003 0.0003 0.0002 0.0003 0.0003 0.0003 0.0003 0.0002 0.0002 0.0000 0.0003 0.0002 0.0003 0.0003 0.0002 0.0000 0.0003 0.0003 0.0003 0.0009 0.0005 0.0003 0.0000 0.0003 0.0002 0.0002 0.0002 0.0000 0.0001 0.0006 0.0001 0.0001 0.0001 0.0001

between 14°34′43.29″South, 145°08′24.96″West, and 14°34′40.94″ South, 145°08′38.98″West (Fig. 3). The southeastern transect lies between 14°40′45.32″South, 145°12′43.46″West and 14°40′41.90″ South, 145°12′47.97″West (Fig. 4). Detailed levelling was conducted using a conventional automatic level. Each reference point along the profiles was positioned using DGPS coordinates. Elevations were measured by reference to present-day mean sea level (msl). In addition to topographic surveys, subsurface stratigraphy at the southeastern study site was described from an excavation (Fig. 5) dug by backhoe from the top of the shingle ridge to low-tide level. Field grain-size analysis of coarser detrital material used the UddenWentworth classification (Terry and Goff, 2014) as measured within square-metre quadrats. Laboratory analyses include petrographic descriptions of the lithified sediment samples from thin-sections. Coral pebbles were collected for uranium-series dating at both sites. Taxonomic identification of dated coral specimens was made at the specific level by Michel Pichon.

[230Th/238U] corrected

±

0.0154 0.0031 0.0008 0.0007 n.d. 0.0042 0.0053 0.0025 0.0044 0.0662 0.0052 0.0185 0.0036 0.0059 0.0046 0.0062 0.0088 0.0047 0.0052 0.0114 0.0075 0.0059 0.0118 0.0289 0.0075 0.0779 0.0169 0.0097 0.0105 0.0142 0.0116 0.0091 0.0005 0.0233 0.0286 0.0039 0.0012 0.0052 0.0019 0.0048

0.0003 0.0002 0.0003 0.0003 n.d. 0.0004 0.0003 0.0003 0.0002 0.0003 0.0003 0.0003 0.0003 0.0002 0.0002 0.0000 0.0003 0.0002 0.0003 0.0003 0.0002 0.0000 0.0003 0.0003 0.0003 0.0009 0.0006 0.0003 0.0000 0.0003 0.0002 0.0002 0.0002 0.0000 0.0001 0.0006 0.0001 0.0001 0.0001 0.0001

[234U/238U] act. ratio

±

1.143 1.140 1.144 1.142 n.d. 1.139 1.140 1.139 1.140 1.139 1.142 1.142 1.143 1.142 1.142 1.143 1.145 1.137 1.140 1.140 1.138 1.145 1.144 1.143 1.142 1.141 1.139 1.141 1.144 1.138 1.140 1.142 1.140 1.148 1.146 1.146 1.151 1.148 1.151 1.149

0.004 0.004 0.003 0.004 n.d. 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.003 0.005 0.004 0.003 0.004 0.005 0.003 0.004 0.003 0.003 0.004 0.003 0.004 0.004 0.003 0.004 0.003 0.003 0.004 0.003 0.004 0.003 0.004 0.006 0.004 0.006 0.006 0.004

[234U/238U] act. ratio

±

1.143 1.140 1.144 1.142 n.d. 1.139 1.140 1.139 1.141 1.142 1.142 1.143 1.143 1.143 1.142 1.143 1.145 1.138 1.141 1.140 1.138 1.146 1.144 1.144 1.142 1.144 1.140 1.141 1.145 1.138 1.140 1.142 1.140 1.149 1.147 1.146 1.151 1.148 1.151 1.149

0.004 0.004 0.003 0.004 n.d. 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.003 0.005 0.004 0.003 0.004 0.005 0.003 0.004 0.003 0.003 0.004 0.003 0.004 0.004 0.003 0.004 0.003 0.003 0.004 0.003 0.004 0.003 0.004 0.006 0.004 0.006 0.006 0.004

0

3.2. Radiometric dating procedures A total of 41 coral samples, labelled BAT (according to Bernard Salvat, collector) and TAK (according to Virginie Duvat, collector) (Table 1), were dated using the uranium-series (230Th/234U ratio) method to chronologically constrain the history of island deposition. Among them, 13 surface samples were taken from each of the geomorphic zones identified (beach-rocks, shingle ridges, conglomerate platforms) at the northeastern test site. 21 dates were obtained from the various stratigraphic units identified in the faces of the excavation. The list of dated corals includes two surface samples extracted from beach-rock slabs close to the excavation site. In addition, four surface coral specimens labelled TAKBS1 to TAKBS4 (Duvat, pers. comm.) collected from a sector a little further north than the northeastern test site were incorporated into the present study. Finally, a surface fragment of an emerged Porites microatoll from a hoa on the western coast was dated to determine sea level during its latest growth phase. As emphasized by Montaggioni (1979), Woodroffe et al. (1999, 2007)


and Kench et al. (2014a, 2014b), using reworked, detrital corals as markers for reconstructing depositional chronology of reef-derived material requires special caution because an indefinite period has elapsed between the time of death of the dated coral colony, transport, breakdown and time of deposition and stabilization. To attempt to overcome this uncertainty, 3 samples from each stratigraphic unit at the southeastern excavation site were dated. In a given layer, the age of the youngest dated coral clast is necessarily the closest to the time at which the layer has deposited. Accordingly, the youngest age relates to the maximum age of sediment deposition. 230 Th/234U measurements of coral ages were performed at the GEOMAR Institute, Kiel, Germany. Separation of uranium and thorium from the sample matrix was achieved using Eichrom-UTEVA resin, following previously published methods (Fietzke et al., 2005; Blanchon et al., 2009; Douville et al., 2010). Determination of uranium and thorium isotope ratios (230Th/234U) used multi-ion-counting inductively coupled plasma mass spectroscopy (MC-ICP-MS) following the method of Fietzke et al. (2005). The ages were calculated using the half-lives published by Cheng et al. (2000). For isotope dilution measurements, a combined 233U/236U/229Th spike was used with stock solutions calibrated for concentration using NISTSRM 3164 (U) and NIST-SRM 3159 (Th) as combi-spike, calibrated against CRM-145 uranium standard solution (formerly known as NBL-112A) for uranium isotope composition and against a secular equilibrium standard (HU-1, uranium ore solution) for the precise determination of 230Th/234U activity ratios. Whole-procedure blank values of this sample set were measured between 0.5 and 1 pg for thorium and between 10 and 20 pg for uranium. Both values are in the range typical of this method and the laboratory (Fietzke et al., 2005). Table 1 summarizes all measured uranium and thorium data and the calculated U/Th ages. For uranium and thorium isotope analysis only samples with no detectable traces of calcite were used for measurements. The possible presence of calcite was identified from X-ray diffractometry analyses. The data show that 238U concentrations vary between 3.2626 ± 0.0040 ppm (BAT 89) and 0.6895 ± 0.0011 ppm (BAT 79) with a mean 238U concentration of 2.106 ppm. The concentrations of 232Th vary from 5.895 ±

327

0.038 ppb (BAT 48) to 0.013 ± 0.001 ppb (BAT 42) with an average value of 0.99 ppb. Both the measured 232Th and 238U values are in the typical range for young corals from oceanic islands (cf. Chen et al., 1991). The δ234U(T) values (Table 1) are highest for sample BAT 103 of 1.149 ± 0.003 and lowest for sample BAT 67, 1.137 ± 0.005. It is obvious that most of the δ234U(T) values fall within statistical uncertainties in the range of the presently most precise δ234U seawater value of 146.8 ± 0.1 (Andersen et al., 2010). Hence, all data can be considered to be robust and reliable. Calculated U/Th ages for the coral samples vary between 7705 ± 120 (conglomerate platform) to modern (beach-rock). These probably reflect the age range of coral detritus forming the atoll islets all around Takapoto. The average age uncertainty for the coral ages is of the order of ±24.2 yr, corresponding to an age-dependent uncertainty between 0.25 and 34% of the calculated coral age. Radiocarbon ages from the literature or still unpublished were calibrated to calendar ages with two sigma error ranges using the IntCal09 and Marine09 radiocarbon calibration programme (Reimer et al., 2013). 4. Results 4.1. Surface morphology and sedimentary facies The two surveyed profiles of Takapoto reveal that the islets form steep ridges rising from the reef-flat and conglomerate–platform surfaces. The width of the reef-flat zone is approximately 100 m at the northeastern and 77 m at the southeastern site. The vegetated island surfaces reach elevations of 3.50–4 m above msl. Both profiles show ocean-facing steeper, erosional scarps whereas the lagoonward sides (2 m high) are typified by gentle slopes. The northeastern profile is about 340 m long and has a mean elevation of about 2.5 m above msl (Fig. 3). It is typified by the occurrence of a double ridge, running parallel to the coast. Along this profile, from the oceanfacing shoreline inwards, the surface can be divided into a series of geomorphic zones (Fig. 6). Behind the reef-flat zone, the shoreline is bordered by extensive conglomeratic beach-rock outcrops, ranging between

Fig. 6. Plot of U/Th dated coral samples versus their respective topographic and/or stratigraphic location at the northeastern and southeastern sites (see Figs. 1, 3–5 for sample locations). Ages are expressed in years BP and/or years AD. Note trends in decreasing ages from lagoon oceanwards along the northeastern (NE) profile and from base to top at the southeastern (SE) excavation site. Numbers refer to field BAT samples (see Table 1 for details of dates) — br = beach-rocks.

328


Fig. 7. A to F: close-up pictures of the main different geomorphic zones from the northeastern profile, windward of Takapoto Atoll (see Figs. 1C and 3 for location). Ages of the U/Th dated coral samples (BAT) are indicated; n.d. = not detectable.

0.5 and 0.80 m above msl, and followed by a narrow swale. Inwards, there is a ridge, composed of boulders and pebbles (mean diameter 100 mm), about 45 m wide and flat-topped, that grades steeply upward (30° to 45°) to an elevation of 3.50 m above msl. Further inland, there is a ponded area backing the inner flank of the ridge. The inner margin of the pond is bounded by an almost continuous conglomerate platform 0.40–0.50 m above msl. The central part of the islet is occupied by a second gravelly ridge, about 170 m wide, reaching a maximum elevation of 4 m above msl. The ocean-facing flank forms a steep scarp (about 40° - dipping) while the lagoon-facing side is gently dipping (b10°). Sands dominate at the inward ridge foot, but the flanks consist of coarse to medium pebbles (10–15 mm diameter). The profile ends in a low-lying sandy to gravelly, symmetrical ridge, not exceeding 1 m in height, and perched above a conglomerate platform.

The emergent southeastern rim is about 400 m wide. Only the outermost 100 m of this, behind the reef-flat zone, was surveyed in order to describe the modern morphological frame with which the stratigraphy of the excavation is compared (Fig. 4). The islet surface is densely vegetated, particularly in central to the inner parts. Immediately landward of the reef-flat zone, beach-rock occurs. Further inland, there is a gravelly ridge that starts with a scarp, dipping at 30–45° and rises inland to a maximum elevation of 4.0 m above msl. At the excavation site, the ridge is 3.70 m high and mainly composed of particles of about 20 mm in mean diameter. Approximately 90% of the coral clasts are from pocilloporids, with acroporids and a few faviids. Granule-sized and sand size particles mostly include fragments of corals, molluscs, coralline algae and larger benthic foraminifera, with subordinate amounts of echinoderms, bryozoans and encrusting foraminifera. Overlying a


conglomerate platform about 0.30 m-thick, the ridge from the base to the top includes three sedimentary units (Fig. 5): – Unit 1 (Fig. 5, Stations 6 and 7) is a gravelly sand floatstone, about 1.90 m thick, between 0.30 and 2.20 m above msl. The coarser fraction in this consists of cobbles to pebbles from 30–60 mm mean size. They locally form planar cross-bedding that dips gently (b10°) seawards. Two subunits are present. The lower 0.60 m is indurated, and the upper 1.30 m is typically unconsolidated. The entire unit is graded, fining-upwards. The sand is moderately well-sorted, with a mean size of 0.29 mm. Composition varies between size classes. Grains N1.50 mm are up to 70% soritid foraminifera, with about 25% coral fragments associated with gastropods, bivalves, amphisteginid foraminifera and coralline algae. The 1.50–0.50 mm class comprises up to 60% Amphistegina lessoni and 20% mollusc fragments, together with corals, coralline algae and echinoderms. The dominant components in the 0.50–0.25 mm fraction, are corals, coralline algae and molluscs; with subordinate amphisteginids (b10%), encrusting foraminifera, soritids, echinoderms and bryozoans. Fractions smaller than 0.25 mm are dominated by coralline algae, molluscan and coral detritus. – Unit 2 (Fig. 5, Stations 2 to 5). The boundary between Units 1 and 2 is 2.20 m above msl. Unit 2 is about 1.60 m thick, and graveldominated, extending from 2.20–3.90 m above msl. It forms two subunits, based on grain-size, expressing a coarsening-upward trend. The lower subunit from 2.20–2.60 m above msl, is mostly pebbles ranging from 50 mm maximum size, 20 mm mean size and b10 mm minimum size. The upper subunit consists of relatively well-sorted, cobbles and pebbles, with mean diameters of 50 mm. The most striking feature is a lack of granule- to sand-sized grains. A 0.10 m thick sandy soil, rich in fragments of roots, is present within Unit 2 (Fig. 5, Station 3), between 3.20 and 3.30 m above msl. – Unit 3 (Fig. 5, Station 1) Is a superficial layer, about 0.10 m thick, that caps the ridge at an elevation of 4 m above msl. It consists mainly of boulders (maximum 500 × 250 mm; mean 100–250 mm) mixed with cobbles of 150 mm diameter and pebbles of 30–50 mm. The shapes and surface textures of coral fragments are significant. In both sites they are well-rounded and polished, reflecting intense rolling and reworking before final deposition. 4.2. The chronology of island accretion Radiometric dating of the surface coral fragments from the northeastern transect reveals ages ranging between modern (BAT45) and 505 yr BP (BAT51) (Figs. 3, 7). Two pebbles collected from the outer ridge yielded

329

ages of modern (BAT45) to 64 yr BP (BAT44), while five pebbles from the inner ridge provide the oldest ages on the transect, between 236 yr BP (BAT53) and 505 yr BP (BAT51), indicating an incremental pattern of development with increasing age of the ridges from the shoreline landwards, Two samples taken from the beach-rock and adjacent unconsolidated gravel in front of the outer ridge give ages of 75 yr BP (BAT43) and 294 yr BP (BAT42). Dating results indicate that deposition of the surface layer, from the shoreline to the inner ridge, occurred within the last 500 years. Coral fragments from the conglomerate platforms have ages of 1478, 2237, 2758, and 6523 yr BP (BAT41, 103, 104, 56), and are of increasing age from the outer parts of the islet to the lagoon margin. Dating of the material extracted from the excavation site (Figs. 5, 7, 8) indicates that deposition and stabilization of the basal conglomerate platform could not have occurred there prior to 1000 yr BP, according to the youngest date obtained (1005 yr ± 4 yr BP, BAT83). Deposition of the gravellysand supported unit (Unit 1), overlying the conglomerate platform, began approximately 900 years ago and ended around 550–500 years ago as suggested by the dates of 935 ± 31 yr (BAT82) and 561 ± 30 yr BP (BAT74). The overlying gravelly unit (Unit 2) was deposited between 454 yr ± 25 yr (BAT67) and 438 yr ± 24 yr BP (BAT64). The surface layer (Unit 3, with BAT57, 58 and 59) was deposited 500–350 years ago. Ages decrease from base to top of the sedimentary sequence (Fig. 8). The two samples from beach-rock on the shore of the southeastern transect were dated at 48 (BAT90) and 872 (BAT89) yr BP (Fig. 4). A detailed analysis of U/Th dating results over the past 2000 years allows the distribution of dated coral samples to be defined (Fig. 7). With the ages expressed in AD centuries, the pattern is as follows: one dated sample from the 3rd, 4th, 7th, 10th, 19th and 21st centuries, two from the 9th, 11th, 12th, 13th, and 18th centuries, three from the 15th and 17th centuries and four from the 20th century. There is no record from the 1st, 2nd, 5th, 6th, 8th and 14th centuries. However, this is more likely to mean that coral sampling failed to identify these time periods rather than reflect a lack of hazard events. Taking into account possible age overlaps between samples, as identified by U/Th error ranges, comparison of the age database with the historical record of the impacts of natural hazards on the Tuamotu may identify wavesurge events. Discrepancies in the record from century to century may result from differences in cyclone activity or in sampling, due to increasing preservation of and/or access to younger coral clasts. Based on the analysis of the recently completed database of cyclone activity and impacts in French Polynesia from the 19th to 21st centuries (Laurent and Varney, 2014; Lau et al., 2016), an attempt was made to link dated samples to specific cyclonic events in the Tuamotu. Samples TAKBS2 and TAKBS4 with ages of 1511 ± 12 and 1558 ± 10 yr respectively, are contemporary with deposition of two reef megaclasts on northeastern coasts of the atoll (Etienne et al., in preparation). The age of sample TAKBS3 at 1833 ± 12 yr encompasses two cyclonic events, in December

Fig. 8. Plot of U/Th ages versus elevation (relative to present mean sea level) of coral samples from the excavation site, southeastern profile, windward coast, Takapoto Atoll. Black circles refer to shingle (storm) ridge deposits; white circles refer to underlying conglomerate pavements. Note the significant trend in increasing ages with increasing depth suggesting a good coherency of the coral age records.

330


1831 and December 1843, in western French Polynesia. Sample TAKBS1 dated as 1904 ± 7 may relate to the 1903 January cyclone, one of the strongest known to have impacted the area. Atmospheric pressure varied from 978 to 944 hPa, indicating that this cyclone falls into category 2–3 according to the Saffir-Simpson scale. Sample BAT 43, dated as 1941 ± 29 yr may relate to the January 1958 event, the only one registered in the Tuamotu for the first half of the 20th century since 1903. Sample BAT 44 dated as 1952 ± 26 yr may have been removed by either the January 1958 or January 1970 cyclones, typified by pressures of 992 and 944.3 hPa respectively (category 1). Sample BAT 90 dated as 1969 ± 20 yr may have been generated by the January 1970, December 1977 (Tessa) or February 1983 (Orama) cyclones known to have affected the northern and eastern Tuamotu islands. 5. Discussion 5.1. Dating coral gravel as a record of islet accretion: validity and limits In a number of previous studies (Woodroffe et al., 1999, 2007), the pattern of radiometric ages of individual skeletal components (corals, molluscs, foraminifera) has been considered to reflect short transport and rapid stabilization of the biological components post-mortem. Nevertheless, the worn surfaces and sub-spherical shapes of almost all coral boulders and pebbles strongly suggest that they experienced long and repeated movements prior to stabilization. In addition, the great disparities in component-ages within a same bed in the different sedimentary bodies (conglomerates, ridges and beach-rocks) clearly show that time lapses of several centuries may occur between coral death and definitive sediment stabilization, implying several cycles of reworking, mixing and re-sedimentation. For instance, Duvat and Pillet (2017) showed that former storm ridges on the windward shore of Takapoto were partly reworked in response to Tropical Cyclone Orama (Category 3, 1983), and received significant volumes of “fresh” sediment, locally up to 62% of islet volume. In the basal conglomerates on the lagoon margins, periods of up to 4000 years separates deposition of individual coral bioclasts. The longest interval for coral shingle deposition was recorded by the sand-supported gravel (Unit 1 of the excavation site), extending to 6155 years. Shorter intervals are found in the upper part of Unit 3, with a duration of 161 years, and on the northeastern transect in superficial ridge deposits (65–105 yr). Fine-grained biogenic detritus (especially foraminifera) is a better indicator of time of deposition than corals, in reef areas with prolific foraminiferal production (Woodroffe and Morrison, 2001). However, in Takapoto dense foraminiferal accumulations are restricted to the excavated Unit 1. Moreover, while suitable to be dated by radiocarbon techniques, these organisms are particularly unsuited to U/Th dating due to very low concentrations in uranium (Henderson and O'Nions, 1995). The use of corals therefore appears to be a suitable alternative to reconstruct a relatively suitable depositional history of atoll islands. The good coherency of age distribution across the northeastern profile (Fig. 6) and in the excavation sequence (Figs. 6, 8) indicates that there are reproducible patterns of age structure identifiable even if dating coral detritus is regarded as the least consistent in terms of pattern and a poor indicator of time of deposition (Woodroffe et al., 1999, 2007). The youngest U/Th ages represent the maximum age in terms of deposition. 5.2. Age of islet accretion Radiometric dating of the cemented conglomerate pavements demonstrates that most were deposited during the mid to late Holocene (Pirazzoli and Montaggioni, 1986; Chivas et al., 1986; Yu et al., 2012). It is instructive to note that the ages of corals embedded in these tend to increase from the shoreline lagoonwards. Previous findings at Takapoto are consistent with this interpretation (Pirazzoli and Montaggioni, 1986). Basal coral conglomerates in the inner parts of the southeastern reef-flat zone were dated at 1560 and 1320 radiocarbon yr BP (1109 ± 150 and

863 ± 170 cal. year BP). By contrast, a conglomerate platform at a similar elevation from the eastern lagoonal margin provided an age of 5020 radiocarbon yr BP (5305 ± 345 cal. year BP). This may indicate that progradation played a significant role in the formation of the isletbearing conglomerate platforms. Prior to island initiation, storm deposits were transported from the reef front lagoonwards across the reef-flat zone, to be deposited along the lagoon margins. Through time, and prior to stabilization and cementation, coral detritus accumulated between the lagoon and the reef front, continuing and progressively overlying and reducing the area of the then reef-flat zone from the lagoon seawards. In this scenario, stabilization and cementation of the conglomerate is believed to have occurred after approximately 1000 yr BP. If correct, this supports the idea that the islets of Takapoto, in at least the southeastern area, formed recently, with the onset of deposition at around 1000 yr BP followed by a development period of about 700 years. Their modern morphological characteristics were probably acquired in the 17th century. The youngest ages obtained from dating of the successive sediment layers in the excavation site; decrease progressively from base to top, from 947, 726, 568, 503, 444, and 353 yr BP (Fig. 8). Similarly, the progressive increase in age of coral fragments with distance inland along the northeastern profile is an argument in favour of a lateral extension of the islets from the lagoon towards the ocean (i.e. prograding ridge system) consistent with the “oceanward accretion” model described by Woodroffe et al. (1999). The chronology of the accretionary scenario at Takapoto is very similar to that established on Tepuka Island, Funafuti Atoll (Kench et al., 2014a). Tepuka started to grow at around 1100 yr BP extending over a 500-year period. Islet development on both Takapoto and Tepuka appears to have occurred significantly later than in a number of other sites throughout the Indo-Pacific province. For example, on Jabat Island (Marshall Islands, central Pacific), island accretion occurred from 4800 to 4000 yr BP (Kench et al., 2014b). Similarly, on both the Maldive Islands and Bewick Cay (northern Great Barrier Reef), islets started to develop around 5000 years ago on emerging reef-flats that formed between 6000 and 5000 yr BP (Kench et al., 2005, 2012). The position of sea level relative to reef surfaces could not be a control on atoll-island establishment and development. Island formation may have occurred during lower or higher stillstands, and during rising or falling sea levels. The relative delay in island initiation and development at Takapoto compared to that in other areas may reflect a greater time required for reef surfaces to ‘catch up’ with sea level. Indeed, in the three sites mentioned above, island-bearing reef-flats formed between 6000 and 5000 yr BP. Due to a lack of reef drilling investigations in the northern Tuamotu, it is not known when atoll rims approximated to midHolocene sea level (Hallmann et al., 2018). On Mururoa Atoll (southern Tuamotu), reef-flats seem to have grown close to sea level by 4000–3000 yr BP (Camoin et al., 2001). On the nearby Fangataufa Atoll, dating of drilled coral framework from the reef-flat yielded radiocarbon ages of 4230 ± 120 yr (4315 ± 340 yr calendar year BP) and 4030 ± 130 yr (4054 ± 351 yr calendar year BP) at depths of 4.1 and 1.1 m below msl respectively (Montaggioni, unpublished), strongly suggesting that reef-flats had not caught up with sea level before 4000–3500 yr BP. A similar evolutionary scheme can be drawn from the Society reef systems, reef-flat surfaces developed close to late Holocene sea level at around 3000 yr BP (Pirazzoli and Montaggioni, 1988; Montaggioni et al., 1997). This means that most French Polynesian reef-flats are unlikely to have approximated late Holocene sea level before 3000 yr BP. Consequently, a period of about 2000 years, from 3000 to 1000 yr BP, separated the latest phase of reef-flat aggradation from the beginning of island deposition, at least at the excavation site. Based on the ages of coral fragments extracted from conglomerates in several sites at Takapoto, the major phase of deposition of the gravel beds, finally forming conglomerate platforms, i.e. the foundations of the atoll islands, is interpreted as having occurred between 3000–2500 and 1000 yr BP. Combined with the return period of storm events, the positions and elevations of storm ridges relative to sea level at the time of deposition control the extent to which cementation of ridges can operate. Montaggioni and Pirazzoli (1984)


showed that, at Takapoto, petrographic analysis of the conglomerate platforms indicate that early marine cementation occurred to an elevation of at least 0.60 m above msl. This is consistent with observations at the excavation site, where only the lowermost 0.60 m of the sequence is affected by marine cementation; cements occur in the form of grain-contact bridges, reflecting a subtle stage of induration. 5.3. Mode of islet accretion: the role of hazard events The grain-size attributes of bioclastic deposits are assumed to reflect sediment settling velocity in relation to the carrying capacity of waves and currents (for instance, see Cuttler et al., 2016). Despite the local occurrence of sand-size material in the sedimentary sequence at the excavation site (Fig. 5), the fact that the ridges are predominantly gravels indicates that the main phases of islet accretion have not operated under fair weather conditions. Indeed, natural hazards, including storms, cyclones and tsunamis, are known to play an important role in the formation and shaping of islands (Woodroffe, 2008 and references therein). More specifically, cyclones, functioning as constructional agents, have moved large blocks and finer-grained detrital material from adjacent reef sources across atoll rims. The morphological and stratigraphical attributes of shingle ridges provide valuable information on the frequency and intensity of the wave surges impacting islands (Nott, 2011). The calculated vertical accretion rate of the ridge at the excavation site averages about 5.0 mm/yr. Such a rate is very similar to mean vertical rates of most Holocene coral reef frameworks (Dullo, 2005; Montaggioni, 2005). However, it is well known that sedimentary sequences are truncated records of the depositional history of a given environment (Sadler, 1981, 1999). Lines of evidence indicate that catastrophic hydrodynamic events are poorly or incompletely preserved in atoll-rim ridge and beach stratigraphies. This reflects the intense reworking and migration of reef-derived detritus lagoonwards by storms and tsunamis (Lau et al., 2016). Indeed, studies of cyclone-generated deposits along atoll rims show that shingle ridges can accrete or be destroyed during a single extreme event. For instance, during Tropical Cyclone Bebe in 1971, a 3.5 m-high ridge accumulated on Funafuti Atoll (central Pacific) (Maragos et al., 1973). A contrasting situation was experienced by the northeastern coast of Anaa Atoll (Tuamotu) in 1906 where cyclonegenerated waves destroyed the southern portion of the motu over a length of 300 m (Canavesio, 2014). Thus, a given ridge may represent a single event or several diachronous deposits, according to the frequency and intensity of extreme energy events in the region. Establishing the number of storm units contained by any one ridge may be biased by sampling including incomplete age determination (Nott, 2011). However, based on the dense age dataset from the excavation site (Figs. 5 and 8), the stratigraphic sequence on Takapoto may be interpreted as built by at least four storm events, the two older occurring before 500 yr BP (16th century) and the two younger between 500 and 350 yr BP (16th and 17th centuries). Each event may have been separated from that following by an interval of non-deposition as long as 100 years although this may also have incorporated damaging episodes. The rate of ridge accretion is partly dependent on the number of extreme events affecting the zone with time and also rates of coral population recovery. The Pocillopora-dominated coral populations in the upper outer-reef zone, the main source of coral gravel for the Takapoto ridges (Harmelin-Vivien and Laboute, 1986), are able to regenerate after 20 years in high-energy settings, significantly less than the apparent return period of ridge formation events of about 100 years. The role of tsunami-generated surges in the Tuamotu is difficult establish because such events are poorly documented. Six tsunamis are inferred in the Tuamotu since the 16th century and seven in other French Polynesian areas since the end of the first millennium (Lau et al., 2016) with wave heights of b2 m. This agrees with the interpretations of Sladen et al. (2007) that assigned a moderate hazard level to the Tuamotu region, regarding it as liable to experience tsunami wave run-ups b2 m. Thus, it is doubtful that the significant volumes of

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coarse-grained coral detritus accumulated onto atoll rims were generated by tsunamis. 5.4. Comparison with cyclone records from megaclasts Large boulders (megaclasts) are mostly derived from the spurs forming the upper parts of the outer-reef frameworks and from coastal beach-rock slabs thrown onto adjacent reef flats and atoll islets by high energy wave events (Bourrouilh-Le Jan and Talandier, 1985; HarmelinVivien and Laboute, 1986). Megaclasts have been tentatively used as proxies for registering extreme hazard phenomena, especially tropical cyclone activity in the Tuamotu Archipelago over the last millennium (Toomey et al., 2013; Lau et al., 2016). Comparison between megaclastrelated records and our data suggest that cyclone activity was relatively high in the Tuamotu and especially Takapoto between the 11th and 17th centuries, with an average of two extreme wave events per century (Toomey et al., 2013; Etienne et al., in preparation). Special focus falls on the 16th century in which three to five cyclone events have been identified. By contrast, while the interval between 1250 and 1305 is regarded as a period of increasing storminess in the central Pacific related to the Medieval Climatic Optimum (Nunn, 2007; Lau et al., 2016), there is no record for the 14th century event in our database. On both Makemo and Takapoto, five to nine extreme wave events may have occurred between 1200 CE and 1900 CE that are consistent with our findings. However, the absence of megaclasts on Makemo Atoll (Lau et al., 2016) indicates that in the Tuamotu the 280-year period from the mid-15th century to the beginning of 18th century was typified by reduced storm activity, reflecting cooler sea surface temperatures triggered by the Little Ice Age. Such cooling and consequent reduction in cyclone genesis might reflect increasing, but short-lived volcanic activity (Korty et al., 2012). The view of a reduced storminess contrasts with the data presented here, showing that, from the 15th to 18th centuries, 8 to 12 high-energy events hit the area. Furthermore, the relative warming between the beginning of the 18th and the mid-19th century, is expected to have been accompanied by a stormy climatic interlude, as recorded on Makemo (3–5 events over the 17th and 19th centuries) but apparently not recorded at Takapoto (Etienne et al., in preparation). However, our results are in total agreement with the data derived from megaclast dating at Makemo. In the present study 3–5 storm surges were registered by gravels during the same time span. Although megaclasts on both Makemo and Takapoto record 5–9 extreme events between 1200 and 1900 yr BP our dating of corals has captured 11–16 high-energy events. Such a discrepancy between the records from megaclasts and gravels is likely to indicate that, contrary to megaclast production, coral detritus can be produced by storms and cyclones of only moderate intensity (category 1). Indeed, it is noteworthy that, in contrast to coral gravels, megaclasts primarily record the largest and most intense hydrodynamic events but not smaller less intense ones (Nott, 2011). As a consequence, consulting both databases should promote a better understanding of the frequency and intensity of cyclone events. In the northern Tuamotu, it appears from coral records that the mean number of high-energy events per century ranges between one and two with a recurrence interval of 50 to 100 years. One striking feature is the lack of sampled megaclasts older than 1900– 2500 yr BP (Lau et al., 2016; Etienne et al., in preparation). The fact that during the mid-Holocene, at the peak of the cyclone season in the southern hemisphere, the top-of-atmosphere (TOA) solar radiation was lower than at present (Korty et al., 2012) might be an explanation for a reduction in storminess at that time. An alternative view is that most midHolocene mega-blocks are at present trapped in recent storm ridges. 5.5. A regional model of atoll-island development Several models of atoll-island deposition in the Indo-Pacific have been published over the last two decades. These reveal that an interplay of substrate elevation and sea level at the time of island initiation has been critical to island formation and sustainability. Dickinson (2004)

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Fig. 9. Evolutionary model of island development at Takapoto Atoll. A: by about 6000–5500 yr BP, at a time of sea-level stabilization close to modern (present) mean sea level (msl), the reef rim consisted of a late Pleistocene to mid-Holocene carbonate pile submerged at depths of 5–6 m relative to msl. The reef-rim top caught up with sea level during the late Holocene, by approximately 3000 yr BP. B: From 3000 to 2000 yr BP, sea level rose by about 1 m above msl. Coral clasts from reef flats and proximal upper reef slopes were displaced and deposited in innermost reef-rim settings and along the lagoonal slopes. C: From 2000 to 1200 yr BP, sea level dropped by about 1 m, promoting cementation of the earliest bioclastic deposits while accumulation of coral gravels continued on the outermost reef-rim areas, progressively prograding oceanwards. D: Cementation of shingle layers was complete by about 1000 yr BP, resulting in stable conglomerate pavements. From this time, as sea level remained close to msl, fresh and reworked coral detritus began to be deposited along lagoonal margins, resulting in the formation of the earliest emergent shingle ridges and islets on the inner parts of the reef rim. E: During the last centuries, shingle ridges continued to develop oceanwards and formed an almost continuous network of islets. Reef growth was restricted to reef front and reef slopes, periodically supplying ridges with coral clasts.

claimed out that a gradual fall in sea level may promote island formation, largely because a late Holocene fall in sea level was detected in French Polynesia. McLean and Woodroffe (1994), Richmond (1992), Woodroffe et al. (1999) all developed models that need hard substrates (reef-flats, conglomerate platforms) close to or at sea level as a prerequisite for island accretion, implying that long-term island stability is mainly controlled by sea level changes. For example, on West Island,

Cocos (Keeling) Islands in the eastern Indian Ocean, islets formed when sea level reached a height close to present position (Woodroffe and McLean, 1994). By contrast, models of island development established in the Maldives in the Indian Ocean (Kench et al., 2005), and the Marshall Islands (Kench et al., 2014b), Kiribati (Woodroffe and Morrison, 2001), and the Great Barrier Reef in the Pacific (Kench et al., 2012) indicate that the main phases of building occurred when


sea level was 0.50 to 2.50 m above its present position or in the final stages of rising. Morphological and chronostratigraphical analyses of the windward rim at Takapoto, suggest a three-step sequence of island development (Fig. 8). This model can be considered to be of regional value for the northern Tuamotu islands, adjusted for local thermal subsidence, hydroisostasy and/or lithospheric flexure. (Fig. 9.) – First step: Mid-to Late Holocene sea-level rise. Following the Late Pleistocene deglaciation, in the Tuamotus and in the Society archipelago, where sea level reached its present position around 6000–5500 yr BP (Pirazzoli and Montaggioni, 1985, 1986, 1988; Bard et al., 1996; Camoin et al., 2001; Rashid et al., 2014; Hallmann et al., 2018), reef-rim surfaces remained at depths of 5–6 m relative to msl before they accreted vertically to approximate sea level. – Second step: deposition of shingle layers. Sea level continued rising to elevations of around +0.80 to +1 m relative to msl from about 4500 yr BP (Pirazzoli and Montaggioni, 1988; Hallmann et al., 2018). Reef-rim surfaces stabilized close to sea level from about 3000 yr BP. From that time, storm activity has resulted in transport and accumulation of coarse coral debris across rim surfaces. Because atoll rims are narrow, a large proportion of reef-derived detritus was transported across them to be deposited on the inner lagoon margins, as shown by dating of the conglomerates. – Third step: The seaward progradation and incipient cementation of the gravel layers. Since about 2000 yr BP, sea level has gradually fallen, reaching its modern position by approximately 1200 yr BP. Gravel layers have progressively covered (prograded across) reefrim surfaces from the lagoon seawards. Gravel deposits reworked by breaking waves and surges passing over were spread to form superficial layers, resulting in flat beds b1 m thick, suggesting that they were exposed during low tide. As demonstrated by Montaggioni and Pirazzoli (1984), cementation of these beds began as sea level fell to about 0.50 m. The emerging upper sections of these deposits were periodically submerged during high tide, allowing percolating waters to generate marine vadose cements. The water-saturated lower sections were affected by phreatic cementation. – Fourth step: The formation of conglomerate pavements and earlier accretionary phases of atoll-islets. The final stabilization and cementation of gravel layers resulted in the formation of conglomerate pavements that have been probably complete by around 1000 yr BP, when sea level dropped to its present position. The subsequent supply of sediment from adjacent reef zones by high-energy events began to accumulate lagoonwards over the conglomerate substrate, following the same progradation model as that describing formation of conglomerate platforms. – Five step: Later accretionary phases of atoll-islets. Shingle ridges have formed progressively from the lagoon towards the outer shorelines over the last millennium. Modern islet forms were acquired about 500 to 300 years ago. Reef growth is restricted to outer reef zones that periodically supply islets with coral detritus. Contrary to a number of previous island-building scenarios, this model indicates that islands have accreted when conglomeratic foundations were emergent and sea level was at its present position.

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from the inner part of the rim seawards. A similar pattern typifies the formation of conglomerate platforms. The stratigraphic attributes and time resolution of the excavated sediments do not permit reconstruction of the successive depositional and erosional events that have determined the modern morphology of the storm ridges. The 700 yr-long history is concealed in a b4 m-thick sequence. The pattern of sea-level changes since the mid-Holocene is relatively well constrained in the Tuamotu, permitting the construction of a consistent evolutionary scheme of atoll-rim development in relation to sea level (Hallmann et al., 2018). While studies on atoll-island accretion elsewhere reveal that the position of sea level relative to reef surfaces has no influence on the amount of deposited material and duration of deposition, the time at which reef surfaces reached sea level is of great significance. The present study may suggest that the delay in island initiation is directly driven by the availability of hard substrates serving as foundations. Furthermore, the presence of conglomerate pavements is not a prerequisite for island initiation as observed in a number of tropical areas. Rather, the amount of time necessary for conglomerate genesis only postpones the starting time of island development by about 2000 yr. Finally, the development of windward islets at Takapoto seems to have started at the beginning of the last millennium, and thus these are among the youngest throughout the Indo-Pacific. Using coral boulder-based ages to reconstruct hazard events appears to have been as effective as dating of megaclasts, but in a different way. While megaclasts are known to reliably record cyclone events, dating of smaller clasts gives direct access to lower energy events. Our findings show that combining both methods will improve the identification of surge events in tropical seas. The list of identified storm surges derived from dating boulders is very close to that established from historical observations (averaging one or two per century over the last millennium). The cementation of the lower ridge components observed at the excavation site is critical for long term island maintenance, allowing ridges to better resist surge attack and periodic displacement of sediment. Although the height of sea level with respect to reef substrates does not appear to have been a control on island accretion over the mid to late Holocene, in the near future, increasing sea level, combined with increasing cyclone activity may result in more powerful storm surges than those presently observed. Forecasts on multidecadal scales that indicate limited adjustments of shorelines or simpler-shaping of islands may express reality over the short to medium term as sea level will not rise in excess of a few tens of centimetres above the present position. However, they may underestimate the erosion potential faced by islands in the next century and further into the future. Numerical simulations, based on shallow-water hydrodynamic models anticipate dramatic changes of island shorelines over the next century in response to rising sea level and new erosional regimes (Shope et al., 2016, 2017). Mann et al. (2016) rightly claimed that studies have to cover different time-scales with short- to long-term records of both constructional and erosional processes. A key issue therefore is if changing patterns of sediment production will occur in relation to the rise in sea level (Perry et al., 2011). Acknowledgements

6. Conclusions As pointed out by Woodroffe et al. (1999), radiometric dating of individual coral clasts can be a reliable tool to define a time frame in which island building took place, despite the extended period of storage for some dated coral clasts prior to final stabilization. Increasing the number of coral samples to be dated from a same sedimentary layer can prevent or at least reduce age reversals and inconsistency. This is supported by the coherence of coral-based ages provided here, systematically increasing from the ocean-facing zones landward and from base to top within ridges; reversal dates are effectively absent. The distribution of ages from surface samples strongly suggests that deposition occurred

This work was supported by the French National 791 Research Agency (CNRS) under the STORISK research project (N° ANR-15-CE03-0003). Field work has benefited from logistical support and help from the research station of the Office of Marine Resources in Takapoto, Gaby Maiti Haumani. The authors warmly thank the following colleagues for their contributions: Norbert Faarii for engineering the excavation in the south eastern area of the atoll, Virginie Duvat and Valentin Pillet, Université de La Rochelle, for providing the topographic profiles of the northeastern site and maps for field works, Virginie Duvat for providing samples collected in 2016, Michel Pichon, Museum of Tropical Queensland, Townsville, for identification of coral samples; Fabien Morat and Peter Esteve, Université

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de Perpignan, for helping in computerizing maps and tables. Thanks to the inhabitants of the atoll for having kindly transmitted their local knowledge. Field work and sampling have been conducted from the end of February to early March 2017 by BS and AA U/Th dating was performed by AE; LM, the corresponding author was in charge of drafting the article in collaboration with co-authors; figures were conceived by LM and BM-G. Thanks are also given to Tyler Goepfert for performing chemical preparation and measurement of U- and Th-data. This work is dedicated to the memory of Paolo Pirazzoli who conducted outstanding research on the Holocene sea-level history in French Polynesia. We are also greatly indebted to the three anonymous reviewers and editor for their constructive comments and suggestions which have significantly improved our manuscript. References Andersen, M.B., Stirling, C.H., Zimmermann, B., Halliday, A.N., 2010. Precise determination of the open ocean 234U/238U composition. 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