# Dephasing of islet building between leeward and windward reef-rim sides, Bora Bora Island, Society Archipelago, central South Pacific

Lucien F. Montaggioni, Bernard Salvat, Edwige Pons-Branchu, Bertrand Martin-Garin, Gilbert Poli, Marie-Thérèse Vénec-Peyré, David Lecchini,

Gaston Tong Sang

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## ► To cite this version:

Lucien F. Montaggioni, Bernard Salvat, Edwige Pons-Branchu, Bertrand Martin-Garin, Gilbert Poli, et al.. Dephasing of islet building between leeward and windward reef-rim sides, Bora Bora Island, Society Archipelago, central South Pacific. Quaternary International, 2025, 719, pp.109638. 10.1016/j.quaint.2024.109638. hal-04896572

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1 Dephasing of islet building between leeward and windward reef-rim sides, Bora Bora

2 Island, Society Archipelago, central South Pacific

3 Lucien F. Montaggioni a, Bernard Salvat b, Edwige Pons-Branchu c, Bertrand Martin-Garin a*,

4 Gilbert Poli d, Marie-Thérèse Vénec-Peyré e, David Lecchini f, Gaston Tong Sang g

5 a Aix Marseille Univ, CNRS, IRD, INRAE, Coll France, CEREGE, 13331 Marseille,

6 France

7 b PSL-École Pratique des Hautes Etudes, UAR 3278, EPHE, CNRS, UPVD, Perpignan,

8 France

9 c LSCE/IPSL, CEA-CNRS-UVSQ, Paris-Saclay, 91198 Gif-sur-Yvette, France

10 d Environmental Consultant 75016 Paris, France

11 e CR2P, Centre de recherche en paléontologie, MNHN-CNRS-Sorbonne Univ, 8 Rue

12 Buffon 75005 Paris, France

13 f PSL University, EPHE-UPVD-CNRS, UAR 3278 CRIOBE, 98729 Moorea, French

14 Polynesia and Laboratoire d'Excellence "CORAIL", Perpignan, France

15 g Town Hall, Vaitape, 98730 Bora Bora, French Polynesia

16 * Corresponding author.

17 E-mail address: bertrand.martin-garin@univ-amu.fr (B. Martin-Garin).

18 Abstract

19 Defining the timing and accretional mode of low-lying, reef-rim islets over the recent past is a
20 prerequisite for better understanding their future dynamical behaviour in response to rapid rise
21 in sea level and increasing storminess. In high island settings where reliefs locally act as
22 protection against meteorological hazards, establishing the level of vulnerability in the face of
23 global warming remains an important issue. At Bora Bora, a high volcanic island (French
24 Polynesia), internal lithostratigraphy of two selected islets – Motu Tevairoa, leeward, west-
25 north-western side; Motu Tofari, windward, east-north-eastern side – from the barrier-reef rim
26 was reconstructed from four excavated cross-sections. Motu Tevairoa exhibits at the ocean-
27 facing shoreline, pebble-and sand-supported facies abruptly grading into sand-dominated facies
28 inwards. Motu Tofari is dominated by boulder-pebble facies along the oceanic border. U/Th
29 dating was conducted from a collection of 48 coral clasts, in order to reconstruct the timing of
30 islet building. Motu Tevairoa appears to have started to develop by about 2,300–2,200 cal. yr
31 BP from a central islet depocentre laterally extending over time. At the ocean-facing settings,
32 deposition was initiated by about 1,600 cal. yr BP. By contrast, on Motu Tofari, deposition
33 along the oceanic shoreline, occurred as soon as 3,100 cal. yr BP. A time lag of approximately
1,000 years seems to separate islet accretion between the windward and leeward sides. This strongly suggests that winter storms in the southern hemisphere and those generated from an easterly direction in the northern hemisphere, mid latitudes have played a major role in periodically supplying the north-eastern rim in coral detritus while the western side was occasionally nourished by distant-source swells derived from the north-western sectors. In both settings, the main phases of islet building occurred during the late Holocene sea-level drop. This questions the ability of islets at Bora Bora to adapt to a rapid rise in sea level in the near future.

**Keywords:** Reef-rim islet development; Climate change; Storminess; Society Archipelago; South Pacific

# 1. Introduction

In the face of climate disturbance, low-lying coastal areas are postulated to experience the effects of rising sea level and marine extreme hazard events in the next decades (Horton et al., 2020; IPCC, 2022). In the tropics, low-lying coral reef-rim islets – *motu*, in the Polynesian language – are common features on low (e.g. atolls) and mid-ocean high islands in the Indian and Pacific Oceans. Reef islets rarely exceed 3 to 5 metres in elevation (Gabrié and Salvat, 1985). Low islet relief would promote instability (Woodroffe, 2008; Yamano et al., 2007), resulting in alteration of their physical integrity due to rapid rise in sea level and increasing storminess. Previous studies on the physical islet dynamics over the last decades in diverse tropical areas revealed that there is no univoqual response model of islet adaptation to environmental changes (Kench et al., 2005, 2014a, b; Kench and Brander, 2006; Duvat, 2019; Duvat and Pillet, 2017, Duvat et al., 2017a,b; Masselink et al., 2020; Xu et al., 2023). This is due to the fact that, according to their geographic location, low-lying island systems are exposed to an array of climatic boundary conditions, including those prone to and sheltered from cyclones. These boundaries can change within individual islands or island groups. Predictions about the future islet behaviour include maintenance and/or extension (Webb and Kench, 2010; Ford, 2013; Le Cozannet et al., 2014; Pala, 2014; Ford and Kench, 2015; McLean and Kench, 2015; Duvat et al., 2017a; Duvat, 2019; Kennedy, 2024), submersion to partial erosion (Yamano et al., 2007; Dickinson, 2009; Connell, 2013; Hamylton and Puotinen, 2014; Hubbard et al., 2014; Woodruff et al., 2013; Storlazzi et al., 2015). However, as already emphasized by Kennedy (2024), the scenarios about coral-reef islet evolution during the last decades cannot
be used as guidelines for predicting the future behaviour of these sedimentary bodies since sea level could rise at higher rates not observed in modern times and with a significant increase in the intensity of cyclogenesis.

One of the critical issues is to identify the conditions that have governed islet accretion between and within tropical islands over the last millennia and centuries (Woodroffe et al., 2007; Woodroffe and Biribo, 2011; Montaggioni et al., 2019, 2023a,b). Reef-rim islets have formed when sea level was rising or higher than present mean sea level (pmsl) (Stoddart et al., 1978; Woodroffe and Morrison, 2001; Kench et al., 2014a, 2014b; Yamano et al., 2014) or falling (Woodroffe et al., 1999; Dickinson, 2009; Kench et al., 2014a, 2014b; Yasukochi et al., 2014). For instance, in a number of atolls in the Tuamotu (French Polynesia), islets have settled and dominantly developed throughout the falling course of sea level over the late Holocene (Montaggioni et al., 2019, 2023a,b). These controversial findings indicate that the future sea level rise could be a dominant control of islet evolution or not according to island location relative to regional basin geodynamics and climate.

Most of the previous studies have focused on the accretional history of atolls, regarded as the most vulnerable tropical coastal areas in the face of ongoing climate disturbance. The aim of this paper is to define the timing and accretional mode of reef-rim islets from Bora Bora, a high volcanic island in the Society Archipelago (central South Pacific). Contrary to atolls, reliefs of high islands locally act as protection against meteorological hazards. Defining the boundary conditions of islet development in such settings over the recent past may help to better understand the future response of islet systems in the face of climate change.

## 2. Environmental setting

### 2.1. *Island geomorphology and reef ecology*

Bora Bora is located in the Society Island Group in French Polynesia, central South Pacific, at 16°30' south and 151°44' west (Fig. 1A). Geomorphologically regarded as an almost atoll, that is consisting of a lagoon surrounded by a reef rim, with significant remains of high land in its central part (Fig. 1B), the high island appears in the form of a residual volcanic caldera, about 30 km², partly collapsed and widely opened westwards, culminating at 727 m (Mount Otekamu). The volcanic edifice was active from about 3.45 to 3.10 million years before present, dominantly erupting alkali basaltic lavas and breccia (Blais et al., 2000; Guillou et al., 2005). The coastline of the high island is partly lined by fringing reefs, 50 m to 150 m wide, and
separated from an outer barrier reef system, by a lagoon, ~40 m in maximum depth. The barrier reef is about 45 km in circumference, 2 km in maximum width and 70 km² in area. The outer barrier-reef flat is predominantly colonized by acroporid and pocilloporid coral colonies. The barrier-reef rim exhibits a number of emerged islets (motu), composed of gravelly to sandy sediments, mainly located along its north-western to south-eastern sides. Islets are totally lacking at the south-western and southern rim areas. The reef rim is locally interrupted by shallow channels (hoa), particularly in its north-north-western part, allowing sea water exchanges between the open ocean and the lagoon. Water depth in the innermost barrier-reef zones (sand aprons) is 3 to 5 m. Islets preferentially occupy north to eastern reef-rim areas where they form linear and continuous strips, approximately 300 m wide. By contrast, along the north-western side, motu occur as scattered units. There, Motu Tevairoa appears to be one of the largest, isolated islet, 2,000 m long, 1,400 m wide, in the Society Archipelago. The motu rest locally on conglomerate platforms as observed on most low-lying coral reef islets in French Polynesia (Montaggioni et al., 2021).

Ecological monitoring conducted on the outer barrier-reef edges and fringing reefs (Lecchini et al., 2021) indicated that living coral coverage ranges from 28% to 30%. Dead coral colonies represented 39% to 42% of total substrate surfaces. The dominant forms were *Porites* – 39–78% of the total substrate surfaces – in all zones. *Acropora* was only present on reef edges, *Pavona* on fringing reef flats, *Pocillopora* on patch reefs and barrier-reef flats.

Based on drilling operations, the fringing reef units, about 20 m thick, were revealed to have started to grow by about 8,700–9,300 cal. yr BP, from volcanic and paleosoil substrates (Gischler et al., 2016). The inner zones are mainly made up of pocilloporids, with minor amounts of acroporids, while the outer ones are dominated by acroporids (Gischler et al., 2019). Initiated at about 10,000 cal. yr BP, the Holocene pile of the outer barrier-reef then developed coevally with the fringing reefs. Up to 30 m thick, it is resting on a Late Pleistocene reef unit, dated to about 117,000 cal. yr BP (Late Interglacial stage) (Gischler et al., 2016). The coral assemblages are predominantly composed of robust branching acroporids in the uppermost sections while the lower ones consist of massive and columnar agaricids. After sea level reached and outpassed its present position by about 6,000 cal. yr BP, sediment transport pathways were created between the outer barrier reef crest and the lagoon, resulting in deposition of a sand carbonate apron system, predominantly composed of coarse-grained detritus from 6,000 to 3,000 cal. yr BP (Gischler et al., 2016; Isaack et al., 2016). At the same time, finer sediments
were transported and retained in the lagoon. From 4,500 to 500 cal. yr BP, significant changes in grain-size in the extracted lagoonal sequences indicated that relatively repeated sediment reworking and redeposition events have occurred, probably controlled by cyclone activity (Isaack et al., 2016). In the lagoonal sediments, three foraminiferal assemblages were identified: a *Textularia*-rich assemblage in the deep central basin, a porcelaneous assemblage (miliolids) together with *Cymbaloporetta* in the outer shallower zones and an *Elphidium*-dominated assemblage in the inner embayment areas (Parker and Gischler, 2021).

## 2.2. *Climate, oceanography and marine hazard events*

The Society Archipelago is experiencing a tropical humid climate, with a very marked annual cycle. The regional climate system is both driven by the trade-winds coming from the eastern sectors and by El Niño-Southern Oscillation (ENSO) which regulates tropical to extra-tropical storms (Andréfoüet et al., 2012). The warm, rainy, austral summer lasts from November to April and the relatively cooler and dryer, austral winter from May to October. The climate is typified by heavy rainfall during the warm season, but low rainfall during the cool season. Rainfall approximates 2,000 mm/year. The recorded highest annual rainfall value is 2,682 mm in 1987 and the lowest 1,155 mm in 1976. These events are mainly governed by a general flux from the north to north-west sectors, typical of the South Pacific Convergence Zone. In addition, the high island relief substantially disturbs the wind regime and, therefore, acts as a significant control of rainfall. As a result, the island coastlines exposed to the trade-winds experience heavier rainfall than the leeward, protected ones. Monthly air-temperatures vary from 24 °C to 28 °C, with an annual mean value of 27.4 °C (Gabrié and Salvat, 1985). At Bora Bora, during the 1981–2017 interval, maximum and minimum monthly temperature was 31.1 °C and 24.8 °C respectively. Sea-surface temperatures in the lagoon at Bora Bora vary from 23 °C to 27 °C (Guilcher et al., 1969). The tide regime is microtidal and semi-diurnal, reaching maximum amplitude of 0.20 m at spring tide.

At the regional scale, the prevailing wind regime relates to trade winds that are active throughout the year at a mean speed of 25 km/h. Westerly winds, from north-west to south-west, blow occasionally, mainly controlled by tropical climate disturbances affecting French Polynesia, especially during the austral summer, and by mid-to high latitude disturbances during the austral winter. At Bora Bora (Fig. 2A, B), the mean annual wind speed is 5.1 m/s (18.4 km/h). The monthly wind speed is maximum in August (5.6 m/s) and minimum in March (4.8 m/s). Winds blowing at speeds lower than 2 m/s (7.2 km/h) represent 7.8 % of the yearly
wind activity. The winds blow preferentially from the east (65.5%), more strongly during the cool season. Westerly to northerly winds are more active (21.6%) during the hot season (Laurent and Maamaatuaiahutapu, 2019).

Due to its geographic location, Bora Bora receives mainly trade-wind and distant-source swells from the south-east sectors. Usually, trade-wind swells are 2 m high and have periods not exceeding 10 s. During the austral summer, swells derive from the north-western sectors, in relation with mid-latitude, north-hemisphere depressions (Fig. 2C). During the winter, depressions from the south-west and the south (Fig. 2D) generate strong and long swells with periods higher than 12 s and mean heights of up to 4 m (Laurent and Maamaatuaiahutapu, 2019).

In French Polynesia, the cyclonic season usually occurs from November to April, with a large annual variability mainly related to ENSO activity. The eastward migration of warmer sea surface temperatures during El Niño episodes promotes the displacement of tropical storms further east into the Tuamotu region. The cyclone track pattern 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). Tropical depressions and cyclones develop during the hot season as the Intertropical Convergence Zone and the South Pacific Convergence Zone move southwestwards. Over the 1981–2010 span, 71% of the identified depressions occurred from December to February. Cyclogenesis was the highest in February (40% of events). The cyclonic activity varies significantly from year to year.

Since the beginning of the 20th century to 2010, eight cyclones have passed near Bora Bora: February 1935; January 1959; March 1961; *Emma*, March 1970, typified by wind speed > 120 km/h and air pressure of 970 hPa; *Robert*, April 1977, wind speed: 137 km/h; *Reva*, March 1983, wind speed: 185 km/h, air pressure: 954 hPa, wave height > 6 m; *Wasa*, December 1991, wind speed: 200 km/h, pressure: 938 hPa; *Osea*, November 1997, winds: 135 km/h, pressure: 950 hPa, wave height: 6–7 m. However, there is no information about the degree of disturbance to islet morphology (Laurent and Varney, 2014). Similarly, little is known about the extreme wave hazard events that have hit the considered area during the historical times. Only three cyclones were reported for the 19th century (December 1843, January 1856, February 1865). The cyclonic hazard events in the region is typified by a very low frequency. The cyclone records reveal a 50-year return period for wave exceeding 12 m high (Canavesio, 2014).
The frequency and effects of tsunamis in the Society Islands are relatively well documented at the historical time scales (Talandier, 1980; Sladen et al., 2007). French Polynesia is periodically hit by tsunami-generated waves. Bora Bora is regarded as less exposed to tsunami hazards than the nearby islands of Tahiti, Moorea and Huahine (Schindelé et al., 2006). Wave run-ups ranging from 1 m to less than 3 m high were postulated to have hit the island (Sladen et al., 2007). Given reefs are acting as protecting barriers, the effects are limited to an increase in sea level, without crashing waves, resulting in flooding of coastal flats.

Absolute sea-level in the tropical Pacific over the 1950–2009 were shown to have risen at a rate of 2.5 ± 0.5 mm/yr, that is higher than the mean global absolute sea-level rise (1.2–1.8 mm/yr) estimated for the 20th century (Becker et al., 2012). Recent predictions indicate that sea level would rise at rates close to 8 mm/yr by the end of the century (Horton et al., 2020).

## 3. Material and methods

### 3.1. Field and laboratory work

Analysis of internal stratigraphy was conducted from excavated cross-sections at western and eastern reef-rim settings (Fig. 1B), respectively on Motu Tevairoa at the leeward side (Fig. 1C), and on Motu Tofari at the windward side (Fig. 1D). First, detailed topographic surveys were conducted at different preliminarily selected sites on both islets. The excavated sites were selected so as to an overview of the sedimentary sequences from islet key-zones, respectively ocean border, mid part and lagoon side, in order to attempt reconstructing the successive building phases of the islets. On Motu Tevairoa, Site T1 is located at the ocean-facing margin – 16°27′56.91″ south, 151°46′51.42″ west –, Site T2 in the central part – 16°28′27.72″ south, 151°46′38.26″ west – and Site T3 at the lagoon border – 16°28′46.46,45″ south, 151°46′30.31″ west. On Motu Tofari, Saint Regis Hotel, it was possible to excavate in one location only, restricted to the ocean side because the inner areas have been severely reworked since hotel landscaping works. Site SR is located at the ocean-facing margin – 16°29′07.67″ south, 151°41′34.40″ west. DGPS coordinates of each site were obtained and its elevation was measured using a conventional automatic level by reference to present mean sea level (pmsl), assuming a conservative error of ± 0.10 m. After topographic surveys, excavations, about 4 m long and 3 m width, were dug using a backhoe at each selected site, from the islet top to the underlying conglomerate pavement surface (T1) or to the water-table level (T2 and T3).

For each individual sediment layer, grain-size was determined and the respective amount of the
different grain-size classes (boulder, pebble, sand) was estimated using a visual chart. based on the Udden-Wentworth grain-size scale (Terry and Goff, 2014). Among the biological components in the sand-grained fractions, only the composition of foraminiferal assemblages was identified. Foraminiferal tests were counted in fractions coarser than 120 µm from three selected samples, respectively collected at Sites T1, T2 and T3 (Motu Tevairoa). Counting of foraminiferal tests in each sample was performed from sand volumes of 5 cm³. Foraminiferal tests were taxonomically identified at the family or generic levels. This analysis was conducted in order to attempt defining the sources from which the sand material has originated. At each excavated sites, from successive layers, coral clasts were sampled in order to be radiometrically dated.

## 3.2. Uranium-Thorium dating procedures

Chironostratigraphy of excavated islet cross-sections was reconstructed from uranium-series dating of 48 coral specimens collected along the walls and the bottom of excavations, and respectively labelled BG (collection in 2021) and Bora (collection in 2022). Material preparation for dating started by cutting of millimetric subsamples from originally collected coral clasts. The most clean and pristine subsamples, devoid of apparent exogenous contaminant and diagenetic alteration were selected under optical microscope. Confirmation of the pristine state of the selected subsamples is given finally by their isotopic signature similar to that of sea water. Then, these subsamples were rinsed with mQ water and ultra-sonicated several times. After adding a triple <sup>229</sup>Th <sup>233</sup>U <sup>236</sup>U spike in a Teflon beaker, samples (from 150 to 350 mg) were dissolved with diluted HCl. The U–Th separation and purification were performed after coprecipitation with Fe(OH)<sub>3</sub>, on 0.6 ml columns filled with U-TEVA and prefilter resins, in nitric media – see Pons-Branchu et al., 2022 for details. The U and Th isotopic compositions were analyzed at the Laboratoire des sciences du climat et de l'environnement (LSCE, France), on a Multi-Collector inductively coupled plasma source mass spectrometer (MC-ICPMS) Thermo Scientific™ Neptune Plus fitted with a desolvating introduction system (aridus II), and a jet interface. For mass fractionation correction, we used an exponential mass fractionation law – normalized to natural <sup>238</sup>U/<sup>235</sup>U isotopic ratio – and standard – sample bracketing. More details on the analytical procedure – chemistry and MCEICPMS analysis – can be found in Pons-Branchu et al. (2022). After corrections for peak tailing, hydrate interference and chemical blanks, <sup>230</sup>Th/<sup>234</sup>U ages were calculated from measured atomic ratios through iterative age estimation using the <sup>230</sup>Th, <sup>234</sup>U, and <sup>238</sup>U decay constants of Cheng et al. (2013) and Jaffey et al. (1971). Ages are expressed in calendar years
(cal. yr) BP corrected for detrital thorium, by reference to 2023, i.e. the year of sample analysis.
In order to make age results more easily comparable to previous research, particularly that based
on radiocarbon measurements, ages are also given in conventional years BP by reference to
1950 (see Table S1, Supplementary material).

## 4. Results

### 4.1. Islet lithostratigraphy and internal structure

Examination of the sections within the excavations reveals that, on the north-west, leeward rim side, at the Motu Tevairoa sites (Fig. 3), the total thickness of islet deposits ranges from 2.60 m at the ocean-facing margin (Site T1), 1.60 m in the mid part (Site T2) to 2.00 m at the lagoon edge (Site T3). The maximum thickness of the sedimentary sequences is 3.60 m above conglomerates at the SR site, on Motu Tofari, at the north-north-east reef-rim side (Fig. 4).

The sequences are composed usually of unconsolidated homogeneous material through their entire thickness except those at the ocean-facing (T1) site on Motu Tevairoa. There, the section consists of alternations of single layers ranging from about 0.20 m to 0.50 m thick (Fig. 5A). These layers refer to sand-supported (Fig. 5B) and pebble-supported (Fig. 5C), pebble-rich layers. Across this motu, from the ocean-facing shore (Site T1) landwards (Sites T2, T3), sediment grain size decreases markedly. The coral pebble detrital facies, typical of the oceanward border, changes into sand-dominated facies inwards, at the mid and lagoonward motu settings, where which pebbles and granules do not exceed 20% of the total sediment volumes (Figs. 5D, 6A,B). Larger pebbles are 100 mm in mean diameter. These consists mainly of rounded pieces of poritids, pocilloporids and merulinids. Sand-sized deposits contain larger foraminiferal tests, associated with molluscan, coral and coralline fragments. The composition of foraminiferal assemblages varies slightly across the islet sites. Amphisteginids are dominating in the three sites, ranging from 49% of the total assemblage at Site T1, 44% at Site T2 to 32% at Site T3 (Table 1). The amount of soritids and miliolids varies from 16% to 19% and from 10%, 16% to 19% respectively from the ocean-facing zone inwards. While missing at Sites T1 and T2, *Cymbaloporetta* is strictly found at the lagoon margin (Site T3) with 19% of the total foraminiferal percentage. By contrast, *Anomalinella* (*A. rostrata*) tests, missing at Site T3, typify Sites T1 and T2, with respective amounts of 16% and 10% (Table 1).

At the north-north-east, windward, ocean-facing SR site (Motu Tofari), the sequence consists of boulder-pebble-dominated deposits (Fig. 7A), mixed with sands occupying about 10-15% of the total volume (Fig. 7B, C, D). The mean diameter of boulders reaches 150 to 200 mm. Some
288 largest boulders are entire colonies of massive poritids and merulinids or pieces of pocilloporids
289 and beach-rock plates.

290 The uppermost beds at Sites T2, T3 (Motu Tevairoa), and SR (Motu Tofari) refer to organic-
291 rich, black-coloured, root-bearing, sandy soils, varying from 0.20 to 0.60 m in thickness.

292 4.2. Islet building chronology

293 On Motu Tevairoa, all dated coral clasts extracted from the excavated sections are younger than
294 2,660 cal. yr BP. The age distributional pattern differs from site to site (Figs. 3, 8). The oldest
295 ages are recorded from the excavated site close to the central part of the islet (Site T2); the ages
296 range between 2,654 ± 17 (Sample Bora 3) and 2,015 ± 13 cal. yr BP (Sample Bora 11). Such
297 an age clustering clearly indicates that the motu centre has accreted within a time interval not
298 exceeding 600 years and has probably remained stabilized by the development of plant cover.

299 The coral clast dated at 2,015 cal. yr BP was collected from the lowermost layer of the sequence.
300 This strongly suggests that this date relates to the very earliest initiation of the islet.

301 At Site T1, the ages are grouped between 1,600 and 30 cal. yr BP, as expressed by Samples BG
302 72 (1,608 ± 13 cal. yr BP) and BG 59 (31 ± 5 cal. yr BP). At Site T3, deposition appears to have
303 occurred between about the last 800 years, as suggested by Samples Bora 32 (793 ± 11 cal. yr
304 BP) and Bora 28 (503 ± 5 cal. yr BP) extracted from the base and the top of the excavation
305 respectively. The samples collected close to the top of excavations T2 and T3, respectively
306 dated at 2,614 ± 33 cal. yr (Bora 2) and 503 ± 5 cal. yr BP (Bora 28) are indicative of a relative
307 stability of the central and lagoon-facing areas of the motu over the last centuries to millennia.
308 However, the occurrence of older samples in reversed position in both T2 (Bora 7: 2,159 ± 12
309 cal. yr BP; Bora 4: 2,251 ±19 cal. yr BP; Bora 3: 2,654 ± 17 cal. yr BP; Bora 2: 2,614 ± 33 cal.
310 yr BP) and T3 (Bora 19: 1,068 ± 8 cal. yr BP; Bora 22: 1,318 ± 13 cal. yr BP) sequences gives
311 evidence of some brief episodes of islet reshaping within the same time span. By contrast, the
312 ocean-facing edge (Site T1) appears clearly to have been built over the last 1,600–1,500 years
313 and frequently reworked. Indeed, the lower parts of the sequence, about 1.5 m thick, were
314 deposited between 1,608 ± 5 (BG 72) and 1,406 ± 9 cal. yr (BG 65), a period of about 200
315 years. The uppermost section was deposited and experienced occasional reworking over the last
316 two centuries and recent decades; the oldest sample returned an age of 202 ± 10 cal. yr BP (BG
317 62) and the other samples range from 74 ± 7 (BG 63) and 31 ± 5 cal. yr BP (BG 59). As a
318 summary, the time of Motu Tevairoa initiation is not definitively inferred due to intense
319 sediment reworking. The earliest building stages are postulated to have occurred about 2,300–
320 2,200 years ago.
321 Islet accretion on the windward, north-north-east, reef-rim side (Motu Tofari, Site SR) started
322 significantly earlier than along the north-western rim side (Figs. 4, 8). In this area, islet initiation
323 occurred about 3,100 years ago, according to the ages of the lowermost clast samples (BG 51:
324 3,130 ± 15 cal. yr BP; BG 42: 3,049 ± 70 cal. yr BP; BG 37: 3,186 ± 14 cal. yr BP) that was
325 about 1,500 years earlier than at the leeward ocean-facing T1 site. Surprisingly, within Site SR,
326 the clasts deposited just above the basal 3,000–3,100 yr BP dated coral samples provided ages
327 markedly younger, ranging between 1,900 and 1,400 cal. yr BP (BG 35: 1,929 ± 14 cal. yr BP;
328 BG 31: 1,758 ± 7 cal. yr BP; BG 30: 1,850 ± 12 cal. yr BP; BG 27: 1,462 ± 10 cal. yr BP) and
329 clustered within a narrow range of about 500 years. This suggests that a time lag of more than
330 1,000 years occurred between the earliest depositional events and the following ones or severe
331 reworking occurred between 3,000 and 1,900 cal. yr BP. The uppermost 1 m deposits were
332 emplaced approximately over the last 600 years and appear to be relatively stable over the last
333 400 years.

334 4.3. *Islet accretion rates*

335 The accretion rates of reef-rim islets can be derived from mean ages of the dated coral clasts.
336 At Motu Tevairoa, vertical mean accretion rates are estimated to ranging between 16 cm/100
337 yr at Site T1 to 25 cm/100 yr at Sites T2 and T3. These values take into account necessarily
338 both building and eroding phases since the islet deposits are likely to be occasionally removed
339 by subsequent storm events. The lower value recorded at the ocean-facing T1 site is interpreted
340 as having suffered more frequent reworking events than at the inward T2 and T3 sites. If
341 partitioning the sequences in distinct sections based on apparently homogeneous bed units,
342 deposition can be regarded as having occurring at maximum rates of about 50 to 75 cm/100 yr.
343 Lateral accretion from the initial depocentre expected to have formed in the mid parts of the
344 motu (close to Site T2) towards the ocean is estimated to have operated at a mean rate of 30
345 m/100 yr. A rate of about 15m/100 yr may be estimated from the lateral islet development
346 between the depocentre and the lagoon margins.

347 On Motu Tofari, at the ocean-facing ridge (Site SR), vertical accretion rates would average 12
348 cm/100 yr. From 3,000 to 1,500 cal. yr BP, deposition occurred at a rate of 13 cm/100 yr. From
349 1,500 cal. yr BP to present, the accretion rate is thought to have not exceeded 10 cm/100 yr.
350 Comparison of vertical mean rates obtained from the leeward and windward, outer rim sides at
351 Bora Bora indicates that accretion occurred at rates apparently five times lower along the
352 windward rim. This could be due to the fact that the windward reef-rim areas have been more
353 frequently exposed to disturbance and reworking than the leeward sides.
354 **5. Discussion**

355 *5. *Timing and mode of islet building*

356 Across Motu Tevairoa, the distributional pattern of isochrons (Fig. 9) indicates that the oldest 357 deposited sequence is that located in the middle part of the rim (Site T2). This suggests that the 358 motu started to accrete from presumably isolated, patchy locations, regarded as depocentres, 359 situated approximately at a mid distance between the ocean-facing reef-rim shore and the 360 lagoon borders. Site T2, situated at around 500 m from about the modern ocean- and lagoon- 361 facing beaches, is considered to be one of the earlier depocentres formed onto the north-west 362 reef-rim areas, from which the proto-islet has continued to laterally grow both oceanwards and 363 lagoonwards. As extending laterally over the reef-rim surfaces, adjacent depocentres have 364 finally to form a continuous and large islet. However, there is no direct evidence that the entire 365 motu under study has resulted from the binding of a series of isolated, nearby loci. Observations 366 of the present-day physiography of the small, isolated islets located just north of Motu Tevairoa 367 reveal that these are separated by narrow channels of water exchange (*hoa*). Similar channels 368 may have locally functioned during the building of the proto-islet. The final phases of motu 369 accretion could have resulted in a complete infilling of *hoa* interspaces. Site T2 is inferred to 370 have occurred at the location of an original depocentre that was initiated between approximately 371 2,300 and 2,200 cal. yr BP. From about 2,000 to 1,000 cal. yr BP, the islet was in the final 372 phases of building. Over the last 1,000 years, the islet has experienced partial accretion and 373 occasional reshaping of its ocean- and lagoon-facing margins. Surprisingly, through Motu 374 Tevairoa, while accretion started from its mid part, there is a decreasing grain-size trend from 375 the ocean side lagoonwards. This probably means that, due to the distance between the sediment 376 sources, i.e. the adjacent foreereef benthic communities and the depocentres, wave transport 377 capacity during storm events allowed only finer, sand-grained material to be deposited in the 378 middle and inner parts of the reef rim areas. At the same times, the coarser-grained sediments 379 remained blocked along the outer parts of the reef rim, thus progressively contributing to the 380 islet expansion on the ocean side.

381 On Motu Tofari, located at the windward reef-rim side, the mid and lagoon-facing parts of the 382 motu have not been excavated due to severe anthropogenic disturbance. Only, the ocean-facing 383 islet edge were excavated at Site SR. allowing accretional history to be postulated (Fig. 10). 384 First, the windward, north-east islets may have started to accrete from initial depocentres 385 centered in the mid rim areas, continued to grow laterally oceanwards and lagoonwards, as 386 observed on the leeward, north-western rim side at Bora Bora or on both rim sides on Takapoto
Atoll, Tuamotu (Montaggioni et al., 2019). Secondly, Motu Tofari may have developed from depocentres located along the ocean-facing rim margin, then prograding inwards, as described at South-Marutea Atoll, Tuamotu (Montaggioni et al., 2023a). Another alternative is motu development relatively coevally throughout the rim areas, as observed along the windward, north-east rim at Fakarava Atoll (Montaggioni et al., 2023b). Regardless of the evolution at Motu Tofari, deposition is interpreted to have started about 1,000 years earlier than along the leeward, north-west rim zones. Accordingly, in the studied areas from Bora Bora, the timing of islet building differs from those reported from other locations. For instance, at Takapoto, islets began to develop around 2,500 years ago on both rim sides (Montaggioni et al., 2019). Similarly, at South-Marutea, although it started as soon as 5,000 yr BP, islet initiation occurred coevally on protected and exposed coastlines (Montaggioni et al., 2023a). At Bora Bora, such a dephasing in islet accretion between the two rim sides could be expected to result from variability in intensity and frequency of marine hazard events between windward and leeward rim sides. Coral clasts ripped from adjacent reef-flat and outer-slope communities is known to be projected onto reef-rim surfaces by both high-energy – cyclones, winter storms, strong tropical and distant-source depressions – and moderate-energy events (Canavesio, 2014, 2019; Canavesio et al., 2023; Duvat et al., 2020). Swells generated by both winter trade-winds and subtropical, mid latitude depressions generated in the northern hemisphere, are postulated to have hit directly the eastern shorelines. These marine hazards regularly supply reef-rim surfaces in coral detritus. By contrast, subtropical storms coming from the north-western sectors and those from the south and south-western sectors as well, although more frequent throughout the year, would have a limited control on clast supply along the north-western coastline due to its orientation. This may explain retardation in islet initiation at the leeward reef-rim sittings.

The vertical accretion rates of both the windward and leeward sequences at Bora Bora, averaging 10 to 50 cm per century, are closed to values estimated from other French Polynesian sites (Takapoto: 10–50 cm; South-Marutea: 5–20 cm).

## 5.2. Island building and sea level changes

In the Society Archipelago, the sea level course over the last millennia is well constrained (Pirazzoli and Montaggioni, 1988; Botella, 2015; Hallmann et al., 2018, 2020). Sea level reached its present position by 6,500 cal. yr BP and continued to rise to +0.80 m by about 4,500 cal. yr BP. It remained relatively stable at this elevation until 3,500 cal. yr BP, and then gradually declined to its present position.
At Bora Bora, the depositional episodes of islets at both windward and leeward rim settings occurred over the last 3,000 years as sea level from about +0.80 m above pmsl to its present position. Such a scenario may suggest that islet accretion would have been promoted by such gradual sea level drop (Fig. 11) as observed elsewhere – The Maldives, Western Indian Ocean (Liang et al., 2022). However, in other French Polynesian locations (South-Marutea: Montaggioni et al., 2023; Rangiroa: Montaggioni et al., 2022), islet accretion was locally active during the rise in sea level from 6,000 to 4,000 cal. yr BP and continued during the following high stand (4,100–3,500 cal. yr BP). This gives evidence that, at Bora Bora and nearby islands and atolls, islet initiation and development have occurred throughout the mid to late Holocene sea-level course, irrespective of the position of sea level. This confirms the fact that sea-level changes are not key controls of islet accretion in French Polynesia, as suggested previously (Montaggioni et al., 2019, 2023a,b). However, islets emplaced during the phases of sea-level fall as observed at Bora Bora are expected to experience partial disturbance owing to the predicted rise in sea level in the next decades, especially if the rates in sea level rise will be higher than 8 mm/yr in the considered region (Botella, 2015).

The source of skeletal detritus deposited across the leeward reef-rim surfaces can be partly identified, due to distinct foraminiferal assemblages in the sandy fractions (Venec-Peyré, 1991). While amphisteginids, soritids and miliolids appeared to be ubiquitous forms, *Anomalinella rostrata* typifies the ocean-facing side (Site T1) and the mid part (Site T2). By contrast, *Cymbaloporetta* appears to be typical of the lagoon margin (Site T3), instead of *Anomalinella*. Differences in the compositional pattern of foraminifera indicate that the outer islet areas were mainly supplied in fine-grained detritus from reef-flat and forereef zones, while sediment nourishment of the innermost islet parts derived from the lagoon. Canavesio (2019) showed that distant-source swells from high latitude storms can cause significant rises in the lagoon water level, resulting in major submersion of reef-rims. During such events, lagoon apron sediments can be reworked and finer fractions can be easily transported and deposited along the lagoon edges.

### 5.3. Conceptual model of motu formation

As previously shown on Takapoto Atoll (Montaggioni et al., 2019), the motu accretional scheme identified on Motu Tevairoa refers to as the *central core* model defined by Woodroffe et al., (1999) in which motu development has initiated from initial deposit loci (depocentres) located in the mid parts of reef-rim areas and from which deposits have extended towards the ocean and lagoon borders (Fig. 12). A number of atoll-islet building models have been already
described from some cyclone-prone or not-prone reef settings, based on reconstruction of low-lying, reef-rim and lagoon-islet development history in the Indian and Pacific oceans (Stoddart et al., 1978; Chivas et al., 1986; Richmond et al., 1992; McLean and Woodroffe, 1994; Woodroffe et al., 1999; Kench et al., 2005, 2014a,b, 2020; Barry et al., 2007; Yamano et al., 2014; Kench and Mann, 2017). The accretional model defined on Bora Bora is in accordance with Dickinson's views (2004) and Kappelmann et al. (2024) pointing out that coral-reef islet building occurred during a gradual sea level fall during the mid to late Holocene. By contrast, in other tropical Indian and Pacific areas, differing models of low-lying, coral islet development were described, in which main accretion occurred during marine transgressive to high-stand periods (Kench et al., 2005, 2014; East et al., 2018; Kench, 2025 for review). In any case, long-term atoll-islet stability is assumed to be directly driven by sea level changes. Indeed, as formed in different sea-level contexts, i.e. transgressive, regressive or at higher than present, low-lying coral reef islets will be going to respond differentially to the predicted sea-level rise. Those accreted during marine regressive events may be altered by rising sea levels, especially if sea level is expected to rise at rates up to 8 mm/yr as predicted for French Polynesia (Botella, 2015), storminess to increase in intensity while vertical reef-growth rates not to exceed 4-5 mm/yr on average (Perry et al., 2018) in the next decades. Similarly, the present islet development history agrees with the findings from McLean and Woodroffe (1994), Richmond (1992), Woodroffe et al. (1999). These authors suggested that the presence of hard basements close to or at sea level, such as conglomerate platforms, is a prerequisite for islet development.

In French Polynesia, incipient depocentres can have settled in different locations of reef-rim surfaces, from the ocean-facing margin (Montaggioni, 2023a) inwards or homogeneously distributed over the entire rim surfaces (Montaggioni et al., 2023b). This means that motu formation has occurred in different hydrodynamic contexts, mainly controlled by frequency, intensity and flow direction of storm surges.

## 6. Conclusions

Chronostratigraphic examination of excavated sections from Motu Tevairoa and Motu Tofari provides insights into the timing and mode of reef-rim islet accretion at Bora Bora Island. As previously shown on Tuamotu atolls, there is a clear decreasing grain-size trend from the ocean-facing reef-rim sides towards the lagoon margins. Along the ocean-facing shorelines, very coarse detritus have deposited: boulder-pebble dominated at the windward, north-eastern Motu Tofari and pebble-dominated at the leeward, north-western Motu Tevairoa. The dominance of boulders at the windward island coastline may reflect high frequency of winter storms rather than cyclone activity. At the leeward side, pebble-dominated sediments change
inwards into sand-supported, then sand-dominated. Coarser-grained detritus are composed of
entire coral colonies (poritids, pocilloporids) and fragmented beach-rock flagstones. Sandy
deposits allow the outer and inner islet zones to be differentiated based on the composition of
foraminiferal assemblages. Sediments from the outer and middle parts of Motu Tevairoa
contain are rich in *Anomalinella*, while the lagoon margins are typified by high amounts of
*Cymbaloporetta*. The thickness of the studied motu sections ranges between about 3–2,50 m at
the outer parts, but does not exceed 1,50 m in the mid parts.

According the U/Th dating results, at the leeward rim side, Motu Tevairoa, seems to have
started to develop by about 2,300–2,200 cal. yr BP from a nodal, depocentre located close to
the middle part of the islet from which it has extended laterally oceanwardsand lagoonwards,
finally reaching the present-day ocean shoreline not prior to 1,600 cal. yr BP. By contrast, the
development of Motu Tofari, at the windward rim side, was initiated at least 1,000 years earlier,
from 3,100 cal. yr BP. Such a time lag in islet initiation between the windward and leeward rim
areas has to be questioned. Winter storms dominantly coming from the south-eastern sectors
could have regularly supplied the eastern island coasts in reefal detritus. By contrast, the
western reef rim areas were only occasionally provisioned, mainly from distant-source swells
generated from the north-eastern sectors in the north-hemisphere. This seems to be supported
by the total lack of detrital islets along the western to south rim areas, contrary to the eastern
ones typified by a continuous islet strip.

It is noteworthy that islet deposition at both windward and leeward rim sides occurred during
the mid to late Holocene sea-level drop. Accordingly, the predicted sea level rise at rates
exceeding those reached during the last millennia could unbalance islet shoreline dynamics and
thus promote physical disturbance and reshaping, in the near future, especially in a context of
increasing cyclone intensity.

## 509 Author contributions

510 LFM designed research, conducted field work and sampling collection at Bora Bora with BS
511 and DL. EPB performed U-Th and isotope Sr dating analyses and interpretations. BMG
512 participated in the data interpretations, designed and provided illustrations. LFM wrote the
513 manuscript with input from EPB, BS and BMG.

Conceptualization: LFM, BS, BMG; methodology (field work): BS, LFM, GP, MTVP, DL;
(laboratory analysis): LFM; (U/Th dating): EPB; validation: LFM; BS; formal analysis: LFM,
BS; writing – original draft preparation: LFM, BMG, BS; writing-review and editing: LFM,
517 BMG, BS, GP; graphics: BMG; supervision: BS, LFM, BMG; project administration and
518 funding acquisition: BS. Administrative authorization: GTS. All authors agree with to the
519 published version of the manuscript.

520 **Data availability**

521 Statement: the data that support the findings of this study are available from the corresponding
522 author.

523 **Declaration of competing interest**

524 The authors declare to have no competing interest.

525 **Acknowledgements**

526 The authors warmly thank the following personalities and bodies for their logistic support or
527 for giving permission to access the investigated sites: Vaea Buchin, owner of the parcel located
528 at the ocean-side site (Site T1, Motu Tevairoa); Rémy Bouchet, Bora Bora by Pearl Resorts
529 (Sites T2, T3, Motu Tevairoa); the Director of the Saint Regis Bora Bora Resort - Site SR from
530 Motu Tofari -; the staff of the Bora Bora Town Hall, Ia Vai Ma Noa Bora bora and the
531 *Polynésienne des Eaux* agency. The authors also thank Arnaud Dapoigny for assistance for
532 measurements by MC-IPCMS and Aleximo Progam for preparation of coral samples before
533 dating. The comments and corrections by David Kennedy (University of Melbourne, Australia)
534 and two other reviewers have significantly improved the manuscript.

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749 **Figure captions**

750 Figure 1. (A) Location map of Bora Bora Island in the Society Archipelago, French Polynesia,
751 central South Pacific; (B) Aerial view of Bora Bora Island (Google Earth Image), showing the
752 location of the studied reef-rim islets. (C) Motu Tevairoa, leeward, north-north-west. (D) Motu
753 Tofari, windward, north-north-east. The stars indicate the location of the excavated islet sites.

754 Figure 2. Average, seasonal directions and speeds of winds and swells; winds recorded from
755 1981 to 2017, meteorological station, Bora Bora airport; swell parameters reconstructed from
756 analysis of satellite imagery and numerical modelling, from 1981 to 2010. (A) Winds, summer,
757 hot season. (B) Winds: winter, cool season. (C) Swells: summer hot season. (D) Swells: winter,
758 cool season. Modified from Laurent and Maamaatuaiahutapu (2019).

759 Figure 3. Topographic profile across Motu Tevairoa, at the leeward, north-west reef-rim side,
760 Bora Bora Island, with location and lithostratigraphy of the three studied excavated sites – T1:
761 ocean side; T2: central part, close to the heliport; T3: lagoon side. The stratigraphic position
762 and ages of U/Th dated coral clasts are given. See figure 1B, C for location of excavated sites.
763 Ages are given in calendar years BP relative to 2023. See Table S1 (Supplementary material)
764 for the values of error ranges on U/Th dates. WL: water table surface.

765 Figure 4. Topographic profile across Motu Tofari, at the windward, north-north-east reef-rim
766 side, Bora Bora Island, with location and lithostratigraphy of the studied excavated site – SR:
ocean side. The stratigraphic position and ages of U/Th dated coral clasts are given. See figure 1B, D for location of the excavated sites. Ages are given in calendar years BP relative to 2023.
767
768
769 See Table S1 (Supplementary material) for the values of error ranges on U/Th dates.

Figure 5. Motu Tevairoa, at the leeward, north-west reef-rim side, Bora Bora Island: internal structure and lithology of the excavated sites. (A) Ocean side: Site T1, general view, showing the dominance of sand-supported material at base and pebble-dominated at top. (B) Close-up of sand-supported, pebble-rich deposits at the lower part of the excavated section. (C) Close-up of sand- to pebble-supported layers at the upper part of the excavated section. (D) Central part: Site T2, general view of the excavation, showing sand-dominated sediments overlain by organic-rich, brown sands, and the water table surface at a depth of about 0.60 m from the motu top surface.
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771
772
773
774
775
776
777

Figure 6. Motu Tevairoa, at the leeward, north-west reef-rim side, Bora Bora Island: internal structure and lithology at the lagoon-side excavation (Site T3). (A) Close-up of the lower section, showing sand-dominated deposits. (B) Close-up of the upper section, showing sand material overlain by an organic-rich, sandy layer.
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779
780
781

Figure 7. Motu Tofari, at the windward, north-east reef-rim side, Bora Bora Island: internal structure and lithology of the excavation at Site SR. (A) General view, showing alternations of coarse-grained layers. (B) Close-up of the lowermost part of the section, composed of coral boulders and pebbles. (C) Close-up of the intermediate, boulder-rich bed, partly sand-supported. (D) close-up of the uppermost part of the section, composed of boulder to pebble-dominated deposits covered with organic-rich soil.
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783
784
785
786
787

Figure 8. Plot of coral clast ages versus topographic and stratigraphic locations within the excavated motu sites – see figures 1, 3, and 4 for location – note the decreasing clast age trend from base to top within both ocean-side excavations. By contrast, the clast age distribution in the excavated central site (T2) tends to be reverse, expressing a partial reworking of sediments. Ages are given in calendar years relative to 2023. Symbols: green circles (Motu Tofari, windward, north-eastern reef-rim, Site SR); yellow circles (Motu Tevairoa, leeward, north-western reef-rim, Site T1); yellow crosses (Motu Tevairoa, leeward, north-western reef-rim, Site T2); yellow triangles (Motu Tevairoa, leeward, north-western reef-rim, Site T3).
788
789
790
791
792
793
794
795

Figure 9. Cross-section through Motu Tevairoa, north-west reef-rim, Bora Bora Island, showing the successive accretional episodes. The dashed lines refer to the isochrons, expressing in years
796
797
BP and are defined from the time intervals between the U/Th dated coral clast samples. Note that the position of the 2,000 yr isochron within the islet body is not definitively inferred, due to intense sediment reworking. Location of the dated clasts collected from the underlying conglomerate platform is indicated. See Table S1 (Supplementary material) for the values of error ranges on U/Th dates.

Figure 10. Cross-section through the ocean-facing side at Motu Tofari, north-east reef-rim, Bora Bora Island, showing the successive accretional episodes. The dashed lines refer to isochrons, expressing in years BP and are defined from the time intervals between the U/Th dated coral clast samples. Location of the dated clasts collected from the underlying conglomerate platform are indicated. See Table S1 (Supplementary material) for the values of error ranges on U/Th dates.

Figure 11. Age distribution of U/Th dated coral clasts extracted from the four excavated sites (T1, T2, T3, SR) through Motu Tevairoa (leeward, north-west rim side) and Motu Tofari (windward, north-east rim side), Bora Bora Island, relative to sea level change over the past 6,500 years in French Polynesia. Clasts ages are grouped by 500-yr increments. Note that sediment deposition throughout Motu Tevairoa occurred over the last 2,500 years, while at Motu Tofari, the ocean-facing section has deposited earlier, between 3,500 and 3,000 cal. yr BP.

Figure 12. Partial planimetric reconstruction of Motu Tevairoa, leeward, north-west rim side, Bora Bora Island, showing the episodes of islet accretion over the last 2,500–2,300 years. The isochrons delineate the postulated extension of surface areas during the successive islet growing phases. Interrogation points refer to the eastern areas of the motu in which there is no record.

Table 1. Composition of foraminiferal assemblages in 3 selected sand-grained samples, Motu Tevairoa, north-western reef-rim, Bora Bora Island. The samples were taken from each excavated site respectively: 1) at the ocean side (Site T1), BG 67 was collected from a layer, 1.10 m above the underlying conglomerate platform (postulated age of deposition: 1,450n – 1,400 cal. yr BP); 2) at the central part (Site T2), B 14 was extracted from the bottom of the excavation, 1.60 m below the islet surface (postulated age of deposition: about 2,200 cal. yr BP); 3) at the lagoon margin (Site T3), B 33 were collected from a layer, 0.40 m above the underlying platform (postulated age of deposition: 700 cal. yr BP). Counting of foraminiferal tests in each sample was performed from sand volumes of 5 cm³. The relative percentage of
foraminiferal forms was established from sand volumes of 5 cm³. Foraminiferal tests are identified at the family to generic levels. *Anomalinella* belongs to the *rostrata* species. The miscellaneous types include *Spirillina*, *Homotrema*, *Gypsina*, *Poroeponides*, and *Textularia*.

832

833 **Supplementary material**

Table S1. Uranium–Thorium data of coral samples from excavated islet sites, Bora Bora Island. Are given successively laboratory sample codes, field sample (Bora, BG) numbers, uranium and thorium contents, isotopic composition with statistical errors – two standard deviations of the mean – and ages expressed both relative to the year of dating analysis (2023) and to 1950. Uranium and thorium contents and isotopic ratios. δ²³⁴U_M = {[(²³⁴U/²³⁸U)sample / (²³⁴U/²³⁸U)eq]−1} × 1000, where (²³⁴U/²³⁸U)sample is the measured atomic ratio and (²³⁴U/²³⁸U)eq is the atomic ratio at secular equilibrium. δ²³⁴U(T) is the initial value and is calculated by the equation: δ²³⁴U(0) = δ²³⁴Umeans exp(λ²³⁴t), where t is the age and λ²³⁴ is the decay constant for ²³⁴U. All ages were corrected for authigenic ²³⁰Th using (²³⁰Th/²³²Th) of the detrital fraction = 10 ± 5.

834

835

836

837

838

839

840

841

842

843
<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /></figure>
A

<figure><img src="image_7.png" /></figure>

B

<figure><img src="image_8.png" /></figure>

D

<figure><img src="image_9.png" /></figure>

C

<figure><img src="image_10.png" /></figure>
NNW

BORA BORA

SSE

MOTU TEVAIROA
(LEEWARD, NORTH-WEST)

<figure><img src="image_11.png" /></figure>

Present
mean
sea level

Organic-rich material

Pebble-supported

Sand-supported

Sand-dominated

Conglomerates

Ages (years BP)
# BORA BORA

## MOTU TOFARI
(WINDWARD, NORTH-EAST)

WNW

ESE

<figure><img src="image_12.png" /></figure>
<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /></figure>
<figure><img src="image_17.png" /></figure>

<figure><img src="image_18.png" /></figure>
<figure><img src="image_19.png" /><figcaption>B</figcaption></figure>

<figure><img src="image_20.png" /><figcaption>C</figcaption></figure>

<figure><img src="image_21.png" /><figcaption>D</figcaption></figure>

<figure><img src="image_22.png" /></figure>
MOTU TOFARI

Winward, NE

Ocean side
SR

<figure><img src="image_23.png" /></figure>

MOTU TEVAIROA

Leeward, NW

Lagoon
side
T3

Samples Bora

<figure><img src="image_24.png" /></figure>

Centre
T2

Samples Bora

<figure><img src="image_25.png" /></figure>

Ocean side
T1

Samples BG

<figure><img src="image_26.png" /></figure>

Conglomerate
platforms

<figure><img src="image_27.png" /></figure>

Elevation (meters relative to
海平面)
BORA BORA

NNW

SSE

MOTU TEVAIROA
(LEEWARD, NORTH-WEST)

LAGON

OCEAN

Elevation (m)
OUTER REEF SLOPE
ALGAL CREST
REEF-FLAT

<figure><img src="image_28.png" /></figure>

INNER TALUS

Present
mean
sea level

Conglomerate platform

200 m

Ages (years BP)

Isochrons

3478

3222

3515
BORA BORA

MOTU TOFARI
(WINDWARD, NORTH-EAST)

WNW

ESE

<figure><img src="image_29.png" /></figure>
sediments to redox N

<figure><img src="image_30.png" /></figure>

Age BP

<figure><img src="image_31.png" /></figure>
<figure><img src="image_32.png" /></figure>

<figure><img src="image_33.png" /></figure>

Excavation site

Isochrons (years BP)