Lucien F Montaggioni
email: montaggioni@cerege.fr

Antoine Collin

Dorothée James

Bernard Salvat

Bertrand Martin-Garin

Gilles Siu

Marguerite Taiarui

Yannick Chancerelle

Morphology of fore-reef slopes and terraces, Takapoto Atoll (Tuamotu Archipelago, French Polynesia, central Pacific): The tectonic, sea-level and coral-growth control

Keywords: submerged terraces, fore-reef slopes, Pleistocene high stands, atoll, central Pacific

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subsurface chronostratigraphy of drilled reef sequences from some other Pacific atolls, the terraces are interpreted as remnants of reef tracts formed during Pleistocene interglacial, high sea-stands, respectively during MIS substage 5e (about -125 ka), MIS substages 7a (about -195 ka), 7c (about -210 ka), 7e (about -240 ka) and 9e (-330 ka). After depth correction of ground lowering during periods of emergence and subsidence effects, terrace elevations were expected to indicate eustatic sea-level positions at each identified high sea-stand, at approximately 7 m (MIS 5e) and 2.2 m (MIS 7a) above present sea level and 9.5 m (MIS 7c), 11 m (MIS 7e) and 8.6 m (MIS 9e) below present sea level. During the post-glacial sea-level rise, from about 9.5 ky, the studied portions of fore-reef slopes were re-flooded. It was assumed that coralgal deposits in the form of thin veneers, coral heads, outer-reef crest-like reliefs and pinnacles have simply overlapped pre-existing structures.

Locally, the fore-reef zones are carved by trenchs, i.e. incipient detachment faults, and scars, i.e. imprints of former landslides.

Similarly, some drop-offs may be regarded as fault walls. The prevailing direction of faulting-collapsing surfaces is consistent with the SW-NE orientation of the main axis of the atoll and submarine ridges rising up from the north-western floor of the Tuamotu Plateau. Analysis of the composition and distribution of coral communities to -30 m depth revealed high, downslope-increasing coral coverage (24 % to 67 % on

Introduction

Coral reefs, typical ecosystems of many tropical coastlines, are known to be reliable indicators of climate, environmental (see Montaggioni and Braithwaite, 2009;Phinney et al., 2013 for reviews) and relative sea-level changes (see reviews by Montaggioni and Braithwaite, 2009;[START_REF] Murray-Wallace | Quaternary sealevel changes[END_REF][START_REF] Woodroffe | Coral reefs and sea-level change[END_REF][START_REF] Camoin | Coral reef response to Quaternary sea-level and environmental changes: state-ofthe art[END_REF]. Intensive submarine exploration has given evidence that reefs have developed at times of lower sea levels, and at present located along continental and island shelves at different depths. A number of submerged reefs were described in the Pacific Ocean (Australia: [START_REF] Abbey | Geomorphology of submerged reefs on the shelf edge of the Great Barrier Reef: the influence of oscillating Pleistocene sea-levels[END_REF]Webster et al., 2009;[START_REF] Linklater | Submerged fossil reefs discovered beyond the limit of modern reef growth in the Pacific Ocean[END_REF]-French Polynesia: Montaggioni et al., 1987;[START_REF] Cabioch | Successive reef depositional events along the Marquesas foreslopes (French Polynesia) since 26 ka[END_REF][START_REF] Camoin | Reef response to sea-level and environmental changes during the last deglaciation: Integrated Ocean Drilling Program Expedition 310, Tahiti Sea Level[END_REF][START_REF] Seard | Impact of environmental parameters on coral reef development and drowning: forward modelling of the last deglacial reefs from T ahiti (French Polynesia[END_REF]-Hawaii: Faichney et al., 2009, 2010 and references herein -Papua New Guinea: Webster et al., 2004b;Ryukyus: Arai et al., 2016), in the Indian Ocean [START_REF] Dullo | Morphology and sediments of the fore-slopes of Mayotte, Comoro Islands: direct observations from a submersible[END_REF][START_REF] Rovere | Submerged reef terraces in the Maldivian Archipelao (Indian Ocean)[END_REF]and references herein) and the western Atlantic (Lightly et al., 1978;[START_REF] James | The seaward margin of Belize Barrier and Atoll Reefs[END_REF][START_REF] Blanchon | Discovery of submerged relic reef and shoreline off Grand Cayman: further evidence for an early Holocene jump in sea level[END_REF][START_REF] Khanna | Coralgal reef morphology records punctuated sea-level rise during the last deglaciation[END_REF]and references herein). Submerged reefs have proven to provide valuable information on relative sea-level and tectonic histories (Webster et al., 2004a[START_REF] Webster | Dro wned coralline algal dominated deposits off Lanai, Hawaii; carbonate accretion and vertical tectonics over the last 30 ka[END_REF][START_REF] Cabioch | Successive reef depositional events along the Marquesas foreslopes (French Polynesia) since 26 ka[END_REF][START_REF] Faichney | Unravaling the tilting history of the submerged reefs surrounding Oahu and the Maui_Nui Complex, Hawaii[END_REF][START_REF] Abbey | Geomorphology of submerged reefs on the shelf edge of the Great Barrier Reef: the influence of oscillating Pleistocene sea-levels[END_REF][START_REF] Rovere | Submerged reef terraces in the Maldivian Archipelao (Indian Ocean)[END_REF] and subaerial erosional patterns as well [START_REF] Purdy | Origin of atoll lagoons[END_REF].

The origin of submerged reefs and associated topographic anomalies -terraces, breaks-in-slope, pinnacles and steps , has been intensively discussed (Hopley, 2006;[START_REF] Beaman | New evidence for dro wned shelf edge reefs in the Great Barrier Reef, Australia[END_REF][START_REF] Abbey | Geomorphology of submerged reefs on the shelf edge of the Great Barrier Reef: the influence of oscillating Pleistocene sea-levels[END_REF][START_REF] Rovere | Submerged reef terraces in the Maldivian Archipelao (Indian Ocean)[END_REF]. Reef tracts were speculated to form during either the last deglacial sea-level rise (-20 ka to -8 ka) or early, during late to mid-Pleistocene variations of sea level. They may be multi-generational, resulting from stacked successive growth and erosional episodes over several cycles of submergence and sub-aerial exposure, finally overlain by last deglacial deposits. Especially, terraces are interpretated to be constructional in origin or to have been shaped by erosive wave action during sea-level seastands lower than present. Similarly, steps may result from seaward accretional processes during stillstands, step and dropoff recession during low stands or faulting. Contrary to shelf slopes of continents and high volcanic islands, those of atolls have been little explored. Earlier bathymetric surveys using single beam echo-sounding have been conducted from the end of the forties on a number of Pacific atolls [START_REF] Ladd | Deep drilling on Mid way Atoll[END_REF][START_REF] Winterer | Bathymetry and and regional tectonic setting of the Line Islands Chain[END_REF][START_REF] Smith | Bathymetric, seismic and alternative sand and gravel resource surveys, Tarawa Atoll, Kiribati[END_REF][START_REF] Wong | Gridded bathymetric data for the Republic of the Marshall Islands[END_REF].

Recently, investigations using shallow-water multibeam mapping, multispectral satellite-, optical-and acoustic-based imagery off several atolls from Pacific US overseas territories, provided new detailed insights into shelf morphology (for instance, see [START_REF] Poppenga | Evaluating the potential for near-shore bathymetry on the Majuro Atoll, Republic of the Marshall Islands, using Landsat 8 and WorldView-3 Imagery[END_REF]. In French Polynesia, shallow-water (0 to -30 m) bathymetric investigations were initiated at Moruroa [START_REF] Chevalier | Etude géomorphologique et bionomique de l'atoll de Mururoa (Tuamotu)[END_REF]).. At Takapoto, the first underwater surveys were carried out by [START_REF] Kühlmann | The corals (Scleractinians and Hydrocorals) of the Takapoto Atoll, Tuamotu Islands: ecological aspects[END_REF]. A series of terraces and breaks-in-slope occur at different depths to about -40 m downslope. From satellitederived near-shore bathymetry, [START_REF] Collin | Understanding interactions between shoreline changes and reef slope morphometry on Takapoto Atoll (French Polynesia)[END_REF] provided the first high-resolution outer-slope morphometry of the eastern side of Takapoto.

In addition to geomorphology, the characterisation of coral communities living along the upper fore-reef slopes is an important step in terms of paleo-water depth reconstructions.

The earlier works on the compositional and distributional patterns of coral communities at Takapoto were carried out by Chevalier and Denizot (1979), [START_REF] Kühlmann | Zusammensetzung und ökologie von Tiefwasser-Korallenassoziationen[END_REF], [START_REF] Bouchon | Les peuplements de scléractiniaires de l'atoll de Takapoto (Polynésie Française)[END_REF] and [START_REF] Kühlmann | The corals (Scleractinians and Hydrocorals) of the Takapoto Atoll, Tuamotu Islands: ecological aspects[END_REF].

In this study, are presented both morphological and biological datasets from the foreslopes of Takapoto Atoll in the Tuamotu Archipelago (French Polynesia, south-central Pacific). Analysis

Environmental setting

Geological background

French Polynesia lies in the south-central Pacific and extends over about 5 500 000 km 2 , from 134°28'W to 154°40'W and from 7°50'S to 27°S (Fig. 1a). Its sea floor includes a broad zone, the so-called 'Pacific Superswell' which is 250 to about 1000 m shallower compared to the depth predicted for its age by classical seafloor subsidence models [START_REF] Mcnutt | The South Pacific Superswell[END_REF][START_REF] Mcnutt | Superswells[END_REF][START_REF] Adam | Geodynamic modeling of the South Pacific superswell[END_REF]. It is typified by a great number of volcanoes, aseismic volcanic plateaus and seamounts, aligned along south-east to north-west linear (N115°E) trends, consistent with the predicted direction of absolute motion of the Pacific plate [START_REF] Duncan | Pacific plate motion recorded by linear volcanic chains[END_REF]. By contrast, the underlying oceanic crust exhibits structural features, including south-west to north-east-oriented, transform faults and magnetic lineations [START_REF] Patriat | Deep crustal structure of the Tuamotu plateau and Tahiti (French Polynesia) based on seismic refraction data[END_REF] and volcanic ridges [START_REF] Jordahl | Seafloor topography and morphology of the Superswell Region[END_REF], and thus, oblique to the hot-spot-generated island trails [START_REF] Ito | Crustal structure of the Tuamotu Plateau, 15°S, and implications for its origin[END_REF] .

The Tuamotu Plateau, part of the oceanic superswell, encompasses an area of approximately 1500 km long and 800 km large (Fig. 1b). It does not exceed 2000 m in bathymetry and is composed of basaltic flows, postulated to have erupted near the East Pacific spreading centre. The plateau is known to be aged of 70 [START_REF] Okal | A model for the plate tectonic evolution of the east-central Pacific based on Seasat investigations[END_REF] to less than 50 million years [START_REF] Schlanger | Geology and Geochronology of the Line Islands[END_REF].

Accretion of the north-west atoll-bearing volcanoes is attributed to the passage of two hotspots 50-30 million years ago as they migrated beneath the Pacific plate. However, the morphology and orientation of a number of linear volcanic features onto the plateau were controlled by tectonic structures of the underlying lithosphere. [START_REF] Ito | Crustal structure of the Tuamotu Plateau, 15°S, and implications for its origin[END_REF]. Carbonate deposition over the oldest volcanoes is likely to have begun as early as 49 million years [START_REF] Jackson | Regional syntheses, Line Islands chain, Tuamotu Island chain and Manihiki Plateau, central Pacific Ocean. Init Repts[END_REF].

Location of the study area.

Located between 14°33'S-14°43'S and 145°08'W-145°16'W, Takapoto Atoll rises from the northwestern portion of the Tuamotu Plateau (Fig. 1b). Its total surface area is 74 km², about 17 km long and 5.5 km wide. The atoll is elongate, oriented NE-SW, with an approximately 350 m wide rim enclosing a -45 m-deep lagoon. The outer part of the atoll rim is a living reef system, subdivided into three zones, from the sea inwards (Chevalier et al., 1979;[START_REF] Salvat | Geomorphology and marine ecology of the Takapoto Atoll[END_REF][START_REF] Salvat | Takapoto Atoll, Tuamotu Archipelago[END_REF][START_REF] Laboute | Evaluation des dégats causés par les passages des cyclones de 1982-1983 en Polynésie Française sur les pentes externes des atolls de Tikehau et de Takapoto (Archipel des Tuamotu)[END_REF][START_REF] Kühlmann | The corals (Scleractinians and Hydrocorals) of the Takapoto Atoll, Tuamotu Islands: ecological aspects[END_REF]Montaggioni, 1981, personal observation). The outer reef zone comprises a spur-and-groove system, extending downwards to about 6-10 m, followed by sub-horizontal to gently sloping terraces to depths of 10-15 m, previously interpreted as the top of the last interglacial high-stand reef [START_REF] Montaggioni | The mode and timing of windward reef-island accretion in relation with Holocene sea-level change: a case study from Takapoto Atoll, French Polynesia[END_REF]. Below this, low gradient to sub-vertical slopes are [START_REF] Andréfoüet | Island shadow effect and the wave climate of the Western Tuamotu Archipelago (French Polynesia) inferred from altimetry and numerical model data[END_REF].

The eastward migration of warmer sea surface temperatures during ENSO episodes promotes the displacement of tropical storms further east into the Tuamotu region [START_REF] Larrue | Les îles de Polynésie Française face à l'aléa cyclonique[END_REF].

Little is known about frequency of extreme wave hazards, i.e. tropical cyclone-and tsunami-generated. Over periods of several centuries, the number of cyclonic events has probably not exceeded 2-3 per century. The effects of cyclones and storms in shaping atoll morphology at the long-term scale is poorly documented [START_REF] Montaggioni | The mode and timing of windward reef-island accretion in relation with Holocene sea-level change: a case study from Takapoto Atoll, French Polynesia[END_REF] onto atoll surfaces [START_REF] Harmelin-Vivien | Catastrophic impact of hurricanes on atoll outer reef slopes in the Tuamotu (French Polynesia)[END_REF]. Similarly, there is little information about the frequency of tsunami occurrences in the Tuamotu. Based on numerical modelling and direct observations, the Tuamotu islands seem not to have significantly suffered from tsunami impacts compared to some high-volcanic islands in French Polynesia [START_REF] Sladen | Evaluation of far-field tsunami hazard in French Polynesia based on historical and numerical simulations[END_REF].

Methods and data

Acquisition and analysis of geomorphic data

Pleiades-1 satellite imagery was collected over Takapoto Atoll on September, 22 2016 at 20 h 01 min 03 sec (UTC). Orbiting at 694 km altitude, Pleiades-1 sensor provides four spectral wavebands (blue: 430 nm-550 nm, green: 490 nm-610 nm, red: 600 nm-720 nm, and infrared: 750 nm-950 nm) at 2 m pixel size. The processed imagery totaled 72 294 672 pixels in the form of 9759 lines and 7408 columns. Given the sunsensor-scene trigonometry, a geometric correction was necessary to project multispectral imagery in UTM 6 South within RGPF local datum. For the sake of standardization and further comparison, a radiometric correction was carried out. It firstly converted top-of-atmosphere (TOA) digital number into TOA radiance using calibration factors, secondly TOA radiance into down-of-atmosphere (DOA) radiance by compensating for atmosphere absorption, and thirdly DOA radiance into DOA reflectance by sun irradiance normalization.

In parallel, a sonar campaign, aiming at calibrating-validating a depth model, was conducted, from September, 25 to 26, 2016, to acquire water depth soundings over both lagoon and ocean parts (Fig. 2a). Fieldwork week was close enough to imagery date to be confident with an absence of bathymetric disturbances between two periods. An array of 276 082 geolocated measurements was compiled across 104.4 km over 16 h 44 min recordings. The dataset ranged from 0.54 m to 180.85 m with an average of 9.90 m ± 10.02 m. The sonar device, installed on a fishboat, was a 200 kHz echosounder Lowrance HD7 leveraging a 10 Hz GPS unit. Water level driven by tide was corrected using the nearest French Marine gauge in Rangiroa atoll.

The bathymetry has been extracted based on the optical law, showing that electro-magnetic radiation is absorbed by water in respect to the wavelength. Pleiades-1 infrared waveband enabled to discriminate land from sea areas, owing to the strong water absorption of the infrared. Three remaining visible Pleiades-1 wavebands (blue, green and red) were used to elucidate the bathymetry based on the ratio transform [START_REF] Stumpf | Determination of water depth with high-resolution satellite imagery over variable bottom types[END_REF]: [START_REF] Li | High precision mass spectrometric uranium-series dating of cave deposits and implications for paleoclimate studies[END_REF] where a0 is the intercept for a null water depth (0 m), a1 is the modeled slope converting the ratio to absolute bathymetry, Rw is the DOA reflectance related to the waveband λi, and R∞ is the DOA reflectance above no-bottom water. Both intercept and slope stemmed from soundings, whose values were re-adjusted by an offset linked with the tide level observed at the satellite collection time.

A very strong agreement between waterborne sonar observations and spaceborne modeled depths was reached (R 2 = 0.95, RMSE = 0.17 m, see [START_REF] Collin | Understanding interactions between shoreline changes and reef slope morphometry on Takapoto Atoll (French Polynesia)[END_REF] for more details). Since the model accuracy declined with depth, a truncature at 38 m was applied. The formula, underlying the model, was thereafter implemented to every pixel, in order to build a digital depth model spanning the entire scene captured by the satellite.

Finally, a set of 41 bathymetric profiles was selected (Fig. 2b) and visually inspected to define the type and spatial distribution

1 )) ( ) ( ln( )) ( ) ( ln( a R R R R a Z j j w i i w -         - - = ∞ ∞ λ λ λ λ
of morphological features imaged on the atoll shelf slopes.

Especially, criteria used to define terraces include the identification of flat-topped to low-gradient surfaces and major breaks-in-slope across the island shelves. The geomorphic features are classified according to their sizes, from larger-scale (hectometres to kilometres) to finer-scale (metres to decametres). The geomorphological classification used in this study is adapted from previously published descriptions and in common usage (see [START_REF] Beaman | New evidence for dro wned shelf edge reefs in the Great Barrier Reef, Australia[END_REF]Addey et al., 2011 for reviews). Table 1 and figures 2b and 3 present the different geomorphic features.

Shelf-slope inclination and size, elevation and width of the different geomorphic features were manually measured from the recorded transects. To measure terrace width, slope extension and inclination, and graphically compare the depth ranges of the successive terrace levels, positive reliefs, such as pinnacles and coral heads, were masked.

Acquisition and analysis of biologic data

The study site is located at the south end of the atoll, close to the village of Fakatopatere and profile T23. Digital images of the quadrats were acquired using a SLR Nikon D300s body camera with aquatica housing and equipped with a Nikkor 15 mm (AF 15 mm f / 2.8) lens and two SB 105 Nikonos strobes.

Pictures were taken at -5 m, -10 m, -15 m, -20 m, -25 m, and -30 m depth running downslope. At each depth site, 4 pictures were taken perpendicular to the bottom surface with a frequency of 1 picture every 30 seconds by a scuba diver swimming at low speed (1 m per 5 sec). Reef bottom to camera lens distance was approximately 5 m. Digital pictures were processed on computer screen using the point intercept method (50 points-picture). Frequency of coral/non-coral substrate was assessed on each picture by identifying the benthic category present under each point. Except in some cases, coral taxa were identified at the genus level. Amount of living coral cover was obtained by calculating the proportion of points occupied by living coral colonies against the total number of points in a given processed surface area.

Results

Morphometry of fore-reef features.

Visual examination of the 41 bathymetric profiles (T1 to T41) revealed significant variability in distribution, shape and size of fore-reef features.

Gross morphology

Six morphologically distinct sectors can be delineated around the atoll, based on seaward extension and mean slope gradients of the bathymetric profiles (Figs. 4 and5). There is locally an atypical zone at the mid-south-eastern side (T17, T18) along which a gently dipping (25°-35°) slope ends in drop-offs not higher than 7 m (Fig. 4a and5e). The zone between the reef crest and the 38 m isobath extends for a length of 120 m to 210 m. Pinnacles are common, with heights of 2 m to 4.80 m.

Terrace levels.

The morphology of the Takapoto outer-reef slopes is typified by several sets (or levels) of terraces. These lie at relatively recurrent depths from the sea surface to about 38 m and have a relatively spatial continuity. Seven morphologically distinct levels of terrace (TL1 to TL7) were recognized (Fig. 7).

Terraces TL1

As in most of reef zones facing the open ocean, the outer reef crest extends into a seaward dipping terrace, which is the top surface of the spur-and-groove system (Fig. 8). Locally, this terrace is missing or poorly developed, ending in abruptly dipping (50°-76°) or vertical drop-offs, especially along the north-eastern, eastern (windward) and south-western (leeward) sites. This means that the development of the spur-and-groove system is not directly dependent upon the location and orientation of the foreslopes, since locally lacking from both some windward and leeward sectors. The main driving factor seems to the slope angle; spurs and grooves are usually restricted to more gently dipping slopes, irrespective of exposure to winds and waves. At its seaward end, the depth of TL1 edges range between 2 m and 7 m, with an average depth of 5.24 m ± 2.0 m.

Terraces TL 2

This terrace, identified from 34 profiles, is found at the average depth of 10.1 m ± 2.4 m (Figs. 6,8). It is missing from narrow, steeply dipping slopes and drop-offs (profiles T6 to T10, T22, T23, T27 to T30). The surfaces seem generally to have a bumpy morphology generated by thick and widespread coral layers or metre-high coral heads, locally oriented perpendicular to the terrace margins. In most cases, the seaward margin of TL2 ends in a steep break-in-slope. Another point to note is the presence of outer reef-crest-like morphologies, less than 2 m high, at the seaward margin of the terraces.

Terraces TL 3

On 28 profiles, a third set of terraces is identified at an average depth of 14.7 m ± 3.8 m. These terraces are missing from a number of transects because of local strong steepness (T4, T8, T9, T10, T20, T26 to T33, T37, T39 to T41).

Terraces TL4

Another level of terraces is encountered at an average depth of 20.3 m ± 2.3 m on 20 out of the 41 profiles. Yet, the strong steepness of the fore-reef slopes makes the mid-fore-reef zone deeper than 20 m not accessible, as seen on T21 to T30, T33, T34, T35, T36, T39 to T41. Despite high coral cover rate, the occurrence of TL4 can be inferred from T17 to T20 based on sharp changes in slope.

Terraces TL5

The relevant terraces are found from 21 sites at an average depth of -24 m ± 2 m. These are missing from the transects T6, T13 à T17, T26 to T41.

Terraces TL6

These terraces are inferred from seven profiles showing gently shelving surfaces (T3, T11, T14 to T17, T20, and T23) and located at an average depth of 30.5 m ± 3 m.

Terraces TL7

The deepest terrace recorded is at 38 m, only from T17 and T18. Examining patterns among profiles show similarities and contrasts in the number and width of terraces. In order to determine the number of terraces within a given width range, a frequency histogram was generated (Fig. 9). Among the 112 terraces identified from the 41 profiles, most have widths less than 20 m.

Furthermore, the width of terraces appears to be partly correlated to the length of profiles (Fig. 10) which is slopegradient dependent. Where profile lengths do not exceed 80 m, most terrace widths are less than 8 m. On profiles as long as 100 m-110 m, a number of terraces can be as wide as 15 m to 20 m. The profiles longer than 130 m have one-quarter of their terraces, 25 m to 45 m wide.

Pinnacles

Steep-sided, high reliefs, found on terraces, were interpretated as isolated pinnacles. These are widepread along gently inclined slopes (profiles T1 to T6), but less common on steeply dipping slopes. Their elevation and diameter range from about 2.30 m to 15 m and 2 m to 12 m respectively. However, it is neccessary to remain cautious about inferring the possible lateral extent of a pinnacle line from transects.

Drop-offs, trenchs, and scars

Drop-offs of 11 m to 13 m high are encountered at several shelf locations (T13, T19, T21, T26, T27, T32, T37) at different depth ranges, from -9 m to -35 m deep. Trenchs are observed on profiles T24 and T31 (Fig. 5d), in the form of 6 m to 17 mdeep indentations, at depths of -6 to -12 m and -12.5 to -29.5 m respectively. Concave scars, interpreted as collapsed margin portions, are found at the south-eastern side and along the western side of the atoll (Fig. 2a).

Quantitative assessment of coral communities

Living coral cover increases downslope from 24 %-25 % at depths of -5 m and -10 m to 42 %, 55 %, 67 %, and 50 % at depths of -15 m, -20 m, -25 m, and -30 m respectively (Table 2, Fig. 11). Robust-branching Pocillopora (mainly, P. verrucosa) is the dominant genus at the upper fore-reef settings, representing 10 % to 26 % of coral coverage, from -5 m to -10 m deep. At depths of -25 m to -30 m, pocilloporids do not exceed 2 %-4 %. Sub-massive to platelike Porites (P. rus) becomes the prominent coral form from 15 m deep, locally covering 50 % to up to 85 % of the substrate. Other scleractinian corals belong to Acropora, Porites cf. lobata, Leptastrea, and Montipora, each of them not exceeding 2 % to 4 % of the total coral cover. It is noteworthy that along steeply dipping slopes and drop-offs, modern coral buildups have significantly re-shaped antecent substrate morphology (Fig. 12)

Discussion

Mode and timing of terrace formation

As pointed out by [START_REF] Rovere | Submerged reef terraces in the Maldivian Archipelao (Indian Ocean)[END_REF], in absence of radiochronological constraints, the age and formational patterns of marine terraces remain speculative. However, different possible scenarios for their mode and timing of formation can be discussed, especially since two processes, respectively constructional or erosional, may have been involved in terrace shaping, either prior to the last glacial maximum (-23 ka to -20 ka-interval) over the past 300 ky or afterwards, during the last deglaciation.

From a contructional point of view, most submerged terraces identified in this study, especially those from gently sloping shelves (profiles T1 to T5, T17, T25, T35) may have developed according to one of the two following models. First, these may result from backstepping of reef bodies developed during the last deglacial rising sea level or previous deglaciation times. In these cases, reef-related terraces would have been generated by punctuated sea-level jump events that have forced reef bodies to drown and backstep upslope through time, rather than develop upwards from a gradual and continuous sea-level rise.

Secondly, the terraces may have been shaped during one of high sea-stands that have occurred over the late to mid-Pleistocene. In all scenarios, terraces represent the still visible, outermost surfaces of reef edges of former reef tracts.

From an erosional point of view, submerged terraces may result from marine planation during episodes of slowing in sea-level rise or stillstands during the last deglaciation. In this scenario, terraces would be wave-cut platforms, as suggested in the Maldives for the last deglaciation [START_REF] Rufin-Soler | Submarine morphology in the Maldives and Holocene sea-level rise[END_REF][START_REF] Green | Geomorphic and stratigraphic signals of postglacial meltwater pulses on continental shelves[END_REF].

At Takapoto, as everywhere in French Polynesia, during the post-glacial transgressive phase, sea was at a level corresponding to the deepest isobath recorded in the profiles (-38 m) by -9.5 ka. Sea level continued to rise at a rapid and relatively constant rate of 10 mm • y -1 before reaching its present position by about -6 ka [START_REF] Bard | Deglacial sea-level record from Tahiti corals and the timing of global meltwater discharge[END_REF][START_REF] Bard | Deglacial meltwater pulse 1B and Younger Dryas sea-levels revisited with boreholes at Tahiti[END_REF].

Slowly rising sea level is known to provide the much-needed accommodation space for enhanced vertical reef accretion while lateral accretion is dependent of the relative time spent at sea level. According to the mean vertical reef accretion rates (4-5 mm • y -1 ) estimated for the early Holocene (see [START_REF] Dullo | Coral growth and reef growth: a brief review[END_REF][START_REF] Montaggioni | History of Indo-Pacific coral reef systems since the last deglaciation: development patterns and controlling factors[END_REF][START_REF] Hubbard | The Changing Face of Reef Building[END_REF]Gischler and Hudson, 2019, for reviews), the rate of sea-level rise was most likely too high to allow coral growth to maintain pace with sea level or to prograde oceanwards. As observed on the mid-shelf slopes at Tahiti [START_REF] Seard | Impact of environmental parameters on coral reef development and drowning: forward modelling of the last deglacial reefs from T ahiti (French Polynesia[END_REF], during the postglacial transgression, corals have rather built isolated, straightwalled pinnacles or thin veeners covering the Pleistocene antecedent topography than typical, flat-topped fringing reefs.

A similar scheme was described from Mayotte Island (Comoros, Western Indian Ocean) by [START_REF] Dullo | Morphology and sediments of the fore-slopes of Mayotte, Comoro Islands: direct observations from a submersible[END_REF]. By analogy, at Takapoto, the major morphological characteristics of the fore-reef slopes are assumed to dominantly reflect those of the antecedent substrate upon which coral buildups have grown more recently. Accordingly, the core of most terraced structures is probably late to mid Pleistocene in age. The heigth of steps is probably indicative of the thickness of the related reef unit. Additionally, a sea-level rise at the rate of 10 mm • y -1 appears inappropriate to allow shelf slopes to be abrased or excavated. Similarly, during the penultimate (from MIS 6b to MIS 5e) and oldest (MIS 10 to MIS 9, MIS 8 to MIS 7) deglaciation episodes, sea level rose at rates similar to or higher than those recorded from the last deglaciation [START_REF] Siddall | Eustatic sea-level during past interglacials[END_REF], precluding any significant constructional, abrasional or excavational event to have occurred. Accordingly, the identified terraces and associated reef units were assumed to have formed during high sea stands. Some terraces possess outer crest-like reliefs of 0.5-1.5 m in elevation and 3-6 m in width at the seaward margin (for intance, see Fig. 5a,b). Given their specific location on top of the outermost part of submerged reef edges, these features are interpreted to be isolated coral heads or laterally extending, continuous ridge-like, coralgal buildups developed at windward, high-energy settings during the Holocene course of sea-level. Contrary to most of pinnacles described along the shelf slopes at Takapoto, their upward development would have been drastically interrupted in response to the rapid sea-level rise. In some way, these are « give-up » reefal features in the sense of [START_REF] Neumann | Reef response of sea level rise: keep-up, catch-up or give-up[END_REF]. Morphologically speaking, these buildups are similar to windward, high-energy, coralline algal ridges [START_REF] Adey | Algal ridges of the Caribbean Sea and West Indies[END_REF][START_REF] Adey | Coralline algae as indicators of sea level[END_REF]Montaggioni and Braithwaite, 2009), typical of Pacific atoll rims (Chevalier et al., 1969), particularly at Takapoto (Chevalier et al., 1979;[START_REF] Chevalier | Eude géomorphologique de l'atoll fermé de Taiaro[END_REF]. Submerged terraces with raised coralgal ridges at their outer extent are known from a number of Pacific reef sites: Australian Great Barrier Reef [START_REF] Beaman | New evidence for dro wned shelf edge reefs in the Great Barrier Reef, Australia[END_REF][START_REF] Abbey | Geomorphology of submerged reefs on the shelf edge of the Great Barrier Reef: the influence of oscillating Pleistocene sea-levels[END_REF][START_REF] Webster | Response of the Great Barrier Reef to sea-level and environmental changes over the past 30,000 years[END_REF], Hawaii (Webster, 2004a) and Papua New Guinea (Webster et al., 2004b[START_REF] Webster | Coral reef evolution on rapidly subsiding margins[END_REF], Ryukyu Islands [START_REF] Arai | A newly discovered submerged reef on the Miyako-Sone platform, Ryukyu Island Arc, northwestern Pacific[END_REF] and the Marquesas [START_REF] Cabioch | Successive reef depositional events along the Marquesas foreslopes (French Polynesia) since 26 ka[END_REF]. These terraces are thought primarily to be remnants of coralgal piles developed at the seaward margins of incipient fringing reefs during the postglacial times, but not capable of keeping pace with rising sea level.

At Takapoto, terraces TL2 exhibit locally low, coral-built ridges oriented perpendicular to the reef-edge line and alternating with elongated, sand-filled grooves, similar to spurs-and-groove reliefs (Fig. 11b and Montaggioni, personal observations, 1981). Accordingly, TL2 are regarded as outerreef edge features. By analogy, the terraces identified downslope (TL3 to TL7) may be also interpreted to be remnants of paleo-reef-edge surfaces.

Along both gently inclined terraces and shelf slopes, pinnacles are common (for instance, see Fig. 5a-f). These features were also described on a number of submerged tropical shelves, mostly on the Australian Great Barrier Reef where they are usually associated with modern juvenile reefs [START_REF] Hopley | The Geomorphology of the Great Barrier Reef: Quaternary Development of Coral Reefs[END_REF].

According to [START_REF] Abbey | Geomorphology of submerged reefs on the shelf edge of the Great Barrier Reef: the influence of oscillating Pleistocene sea-levels[END_REF], during the last deglaciation, terraces would not be ideal settings for extensive vertical coral accretion, due to their narrowness. Similarly, at Takapoto, during the final course of the rising sea level, from about 9.5 ka, and despite the subsequent increase in accommodation space, coral growth was unable to produce, flat-topped, aggrading reefs.

Faulting and submarine landslides

The present data reveal that, at both windward and leeward settings, except in its north extremity and mid-south-east, part the Takapoto rim is bounded by steep fore-reef slopes, with gradients up to 45° or vertical submarine cliffs. There are no significant differences in the overall fore-reef morphology between windward and leeward sides of the atoll. The steepest fore-reef slopes were found as well along the western, leeward coast as the eastern, windward one (Figs. 4 and5). This suggests that the swell regime from the east is not the main driver of foreslope evolution.

The atoll seaward margins are nearly linear as opposed to most of the curved high volcanic island margins found in the nearby Society and Marquesas archipelagos, thereby suggesting structural failure. Faulting control in shelf shaping is further supported by both the presence of vertical drop-offs, higher than 10 m, and trenchs, 6 m to 17 m deep, cutting through the upper parts of the shelf slopes. The trenchs are considered to be incipient, normal faults. Similar features were reported from Tikehau (Faure and[START_REF] Faure | Formations récifales 1: Définitions des unités récifales et distribution des principaux peuplements de scléractiniaires. In: L'atoll de Tikehau (Archipel des Tuamotu, Polynésie française)[END_REF]Moruroa (Chevelier et al., 1969). The overall tectonic evolution of the Tuamotu plateau, appears to be partly driven by extensional processes in relation to major, regional fracture zones. The fault distributional patterns of nearby, high-carbonate Makatea Island [START_REF] Montaggioni | Makatea Island[END_REF] are quite similar, oriented in a south-west to north-east (35°N-50°E) direction. It is noteworthy that the lagoons of Takapoto and other atolls of the outer Northern most Tuamotu island-belt (Takaroa, Ahe, Manihi, Raroia, Takume), together with that of southermost Amanu atoll, are aligned according the same direction. These atolls are superimposed upon ridges, that are expected to have been formed from erupting fissures influenced by the underlying lithosphere fractures [START_REF] Jordahl | Seafloor topography and morphology of the Superswell Region[END_REF]. The dominant fault system affecting the western and eastern shelf slopes at Takapoto is clearly oriented parallell to the atoll and ridge alignment. By contrast, the island shelves at the north end, which developed outwards at right-angles to the SW-NE fault system, have probably escaped large-scale collapsing, thus explaining their unusual morphology, i.e. large seaward development, high density of pinnacles and lack of high escarpments.

Scars in the normally convex seaward margins of atolls and reef barriers as well were usually interpretated as resulting from submarine landslides generated by slope failure [START_REF] Fairbridge | Landslide patterns on oceanic volcanoes and atolls[END_REF][START_REF] Keating | Structural failure and drowning of Johnston Atoll, central Pacific Basin[END_REF][START_REF] Keating | Side-scan sonar images of submarine landslides on the flanks of atolls and guyots[END_REF][START_REF] Keating | Island failures and associated tsunami hazards[END_REF][START_REF] Riyaz | Reef slope failure in the northeastern corner of Malé, Maldives[END_REF]Terry andGoff, 2012, 2017;[START_REF] Webster | Submarine landslides on the Great Barrier Reef shelf edge and upper slope: a mechanism for generating tsunamis on the north-east Australian coast?[END_REF][START_REF] Poupardin | The 1979 submarine landslinegenerated tsunami in Mururoa, French Polynesia[END_REF][START_REF] Puga-Bernabéu | Gloria Knolls slide: a prominent submarine landslide complex on the Great Barrier Reef margin of north-eastern Australia[END_REF]. As emphazised by [START_REF] Terry | Comment on 'Late Cenozoic sea level and the rise of modern rimmed atolls[END_REF], submarine landslides can significantly modify gross atoll morphology. In addition, giant landslides can generate tsunamis [START_REF] Webster | Submarine landslides on the Great Barrier Reef shelf edge and upper slope: a mechanism for generating tsunamis on the north-east Australian coast?[END_REF] and subsequent, partial submersion of atoll rims [START_REF] Poupardin | The 1979 submarine landslinegenerated tsunami in Mururoa, French Polynesia[END_REF]. Although rare, especially in the Tuamotu (Sladen et al. 2017), landslide-generated tsunamis can contribute to atoll shaping and island formation.

Significance of terraces in terms of sea level

In absence of a chronological frame, as a first approximation, the submerged terraces identified at Takapoto may refer to higher or lower Pleistocene sea-stands relative to present sea level. Hovewer, over the last 500,000 years, sea level during interstadials and glacial, low stands were systematically at depths greater than -30 m (see Murray-Wallace and Woodroffe, 2014 for reviews), especially at atoll-subsiding sites [START_REF] Camoin | A 300,000-yr coral reef record of sea-level changes, Mururoa Atoll (Tuamotu Archipelago, French Polynesia)[END_REF]. Accordingly, at Takapoto, all the terrace sets were assumed to refer to interglacial, high-sea stands.

Morphological and chronostratigraphical correlations between

Takapoto and some other atolls used as reference sites in the Pacific can be attempted to help attributing a given terrace level to a specific highstand. All reference cores used herein were extracted from outer reef-flat settings (Fig. 13).

On Moruroa Atoll, there is an upper, gently sloping (10°) terrace, about 60 m wide, extending from about -4 m to -10 m deep. It is interrupted by a break-in-slope followed by a steeper (25°) mid-shelf topography (Chevalier et al., 1969). Similar morphologies were described from Enewetak and Bikini Atolls, Marshall Islands [START_REF] Tracey | Quaternary history of Eniwetok and Bikini atolls, Marshall Islands[END_REF], Tarawa, Gilbert Islands [START_REF] Marshall | Holocene growth of a mid-Pacific atoll: Tarawa, Kiribati[END_REF] and Yongle Atoll, Xisha Islands, South China Sea [START_REF] Guozhong | Sedimentary characteristics of coral reefs in the northern part of the South China Sea[END_REF]. The breakin-slope at approximately -10 m deep was regarded as superficially translating the presence of an unconformity, i.e. subaerial exposure surface. This surface is encountered beneath the modern outer reef-flat and reef-edge zones at depths of -10 m to -14 m at Enewetak [START_REF] Tracey | Quaternary history of Eniwetok and Bikini atolls, Marshall Islands[END_REF][START_REF] Lincoln | Atoll stratigraphy as record of sea level changes: problems and prospects[END_REF]Ristvet et al., 1977;[START_REF] Szabo | Age of subsurface stratigraphic intervals in the Quaternary of Enewetak Atoll, Marshall Islands[END_REF], -8 m to -10 m at Bikini [START_REF] Emery | Geology of Bikini and nearby atolls[END_REF][START_REF] Tracey | Quaternary history of Eniwetok and Bikini atolls, Marshall Islands[END_REF], -11 m to -17 m at Tarawa [START_REF] Marshall | Holocene growth of a mid-Pacific atoll: Tarawa, Kiribati[END_REF], and -16 m to -17.3 m at Yonge Atoll [START_REF] Jian | Meteoric diagenesis in Pleistocene reef limestones of Xisha Islands, China[END_REF].

Uranium-series dating of coral material collected above and below this unconformity revealed that this surface separates an overlying Holocene reef unit not older than 9 ka, from a late interglacial (MIS 5e) unit, dated at -120 ka ± 10 ka to -134 ka ± 5 ka, as observed in Moruroa, Enewetak, Bikini, and Tarawa (Fig. 13). The thickness of the Holocene reef sequences ranges approximately between 12 m and 17 m. At Takapoto, the uppermost reef unit, spur-shaped, approximates a maximun thickness of 10 m. By analogy with the reference atolls, it is considered to be Holocene in age, probably not older than 9 ka.

On the basis of morphological and structural similarities between the upper parts of outer-edge zones among atolls, terraces TL2 were thougth to be geomorphic analogs of the unconformity surfaces atop of MIS 5e reef units. During the relevant high sea-level episode (-128 ka to -116 ka; [START_REF] Muhs | Evidence for the timing and duration of the last interglacial period from the high-precision uraniumseries ages of corals on tectonically stable coastlines[END_REF], sea level was between 5.5 m to 9 m above psl [START_REF] Dutton | Ice volume and sea level during the last interglacial[END_REF]. Based on statistical modelling, there would have a 95 % confidence level that global sea level, irrespective of local controls, peaked at 6.6 m above psl and a 67 % probability that sea level was higher than 8 m above psl during MIS 5e, but it was unlikely to have been higher than 9.4 m [START_REF] Kopp | Probablistic assessment of sea level during the last interglacial stage[END_REF].

A second subaerial exposure surface was found in the same reference cores at Moruroa, Enewetak, Bikini and Yongle, at depths of -18 m to -22 m, -22 m to -23 m, -24 m to -27 m, and 30 m respectively (Fig. 13). The reef sequence capped by this surface yielded U-Th ages averaging -215 ka at Moruroa [START_REF] Camoin | A 300,000-yr coral reef record of sea-level changes, Mururoa Atoll (Tuamotu Archipelago, French Polynesia)[END_REF] thus indicating that the relevant reefs have developed during the penultimate interglacial (MIS 7) high stand. No date is available from other atoll sites. On these atolls, the depth of the second unconformity relative to psl is locally greater than that of terraces TL3 (14.7 m ± 3.8 m), but closer to the level of terraces TL4 (20.3 m ± 2.3 m) and TL 5 (24 m ± 2 m). It is obvious that attempting to link the terraced surfaces TL3, TL4, and TL5 to MIS 7 substages result in significant uncertainties, in part, induced by differences in local erosional and tectonic patterns (see sections 4 and 5 stage).

Among the reference atolls, the total thickness of reef units deposited during the entire MIS 7 stage is similar, varying from 11 m to 15 m. At Takapoto, the thickness of the intervals delineated by TL3-TL4, TL4-TL5, and TL5-TL6 range between 5 m and 7 m only. Such thickness values appear to be too low to account for the entire MIS 7 depositional history of Takapoto when compared to other atoll sites and with regard to the thermal subsidence constraints of the atoll (see section 5 below). On the contrary, integrating the whole TL3-TL6 depth interval (average = 16 m) provides a greater reliance on depositional patterns for the entire MIS 7 episode in the studied area. Furthermore, while there is often an inability to distinguish one substage to another within the penultimate and older interglacials from cored reef sequences [START_REF] Camoin | A 300,000-yr coral reef record of sea-level changes, Mururoa Atoll (Tuamotu Archipelago, French Polynesia)[END_REF][START_REF] Montaggioni | Quaternary atoll development: New insights from the two-dimensionam stratigraphic forward modeling of Mururoa Island (Central Pacific)[END_REF], identification of distinct sealevel pauses from a series of closely stepped terraces seems achievable if assuming that terrace formation is the result of successive backstepping events during successive high-stand substages. Each terrace may correspond to the outer-edge surface of one single backstepped reef tract as observed in a number of reef provinces worldwide (Barbados: [START_REF] Fairbanks | A 17,000-year glacio-eustatic sea level record: influence of glacial melting rates on the Younger Dryas event and deep-ocean circulation[END_REF]the Marquesas: Cabioch et al., 2008-Hawaii: Sanborn et al., 2010-Texas: Khann et al., 2017-Great Barrier Reef, Australia: Webster et al., 2018). Accordingly, the best interpretative scenarios for TL3, TL4, and TL5 are the following. These terraces may correlate respectively with MIS 7a (-201.5 ka to -189.7 ka), MIS 7c highstand (-217.2 ka to -206 ka) and MIS 7e (-248.9 ka to -231 ka) (Figs. 13 and14). [START_REF] Buigues | Geology and hydrology of Mururoa and Fangataufa, French Polynesia[END_REF]. This unit is regarded as the lateral equivalent of reef-margin deposits recovered from oblique boreholes and dated at -332 ka ± 12 ka (MIS 9e substage) by [START_REF] Camoin | A 300,000-yr coral reef record of sea-level changes, Mururoa Atoll (Tuamotu Archipelago, French Polynesia)[END_REF]. On Enewetak and Bikini, a third unconformity was penetrated at 36.2-42.5 m (Ristvet et al., 1977;[START_REF] Szabo | Age of subsurface stratigraphic intervals in the Quaternary of Enewetak Atoll, Marshall Islands[END_REF], and at 49 m below psl [START_REF] Tracey | Quaternary history of Eniwetok and Bikini atolls, Marshall Islands[END_REF] respectively. In the latter site, a coral sample from the underlying interval gave an age older than 400 ka, apparently inconsistent with local stratigraphy. To maintain consistency with the overall development history of Takapoto submerged terraces, TL6 are approximated to refer to MIS 9e.

The belonging of terraces TL7 to a given MIS stage is facing considerable uncertainty and therefore is not discussed.

Effects of subaerial erosion

Another prerequisite to define a paleo-relative sea-level position from given terrace levels is quantifying the impact of subaerial erosion on the morphological evolution of terraces during times of emergence. From estimates of the amount of reef rocks destroyed over the last 120 ky, the rates of carbonate ground lowering from some Pacific atolls and reliefs were thought to range between 0.011 mm • y -1 and 0.202 mm • y -1 , with mean rates of 0.11 mm • y -1 [START_REF] Paulay | A simulation model of reef island morphology: the effects of sea level fluctuations, growth, subsidence and erosion[END_REF]. These values are similar to those calculated (0.12 mm • y -1 on average) by [START_REF] Purdy | Origin of atoll lagoons[END_REF] from Enewetak Atoll for the same time span in response to local rainfall regime. Since present-day annual rainfall at Takapoto is around 1500 mm • y -1 , quite similar to that recorded at Enewetak [START_REF] Purdy | Origin of atoll lagoons[END_REF], a mean value of 0.12 mm • y -1 is taken as the maximum solution rate during Pleistocene low sea stands in the north-western Tuamotu.

Subsidence

In the Tuamotu, the thermal sinking rate of the underlying oceanic crust is lower than the usual decaying exponential rate (about 0.32 mm • y -1 -0.35 mm • y -1 : [START_REF] Menard | Elevation and subsidence of oceanic crust[END_REF][START_REF] Korenaga | Subsidence of normal oceanic lithosphere, apparent thermal expansivity, and sea floor flattening[END_REF] as a function of increasing age as for most of oceanic volcanic domains [START_REF] Patriat | Deep crustal structure of the Tuamotu plateau and Tahiti (French Polynesia) based on seismic refraction data[END_REF]. The Tuamotu plateau. In relation to its age (47 to 55 million years old), the Tuamotu plateau is considered to be near isostatic equilibrium since it was emplaced [START_REF] Patriat | Deep crustal structure of the Tuamotu plateau and Tahiti (French Polynesia) based on seismic refraction data[END_REF]. Moreover, for the north-western Tuamotu islands, earth dynamic topography models predicted about 2-3 m fall since the last 125 ky, which is included in the subsidence signal [START_REF] Austermann | Detection of a dynamic topography signal in last interglacial sea-level records[END_REF].

Quantitative constraints on the possible rate of subsidence at Takapoto can be provided on the basis of the following assumptions: [START_REF] Li | High precision mass spectrometric uranium-series dating of cave deposits and implications for paleoclimate studies[END_REF] subsidence is expected to approximate linearity for the past 300 ky; (2) TL2-related reef unit formed during MIS 5e high stand; (3) at that time, sea level peaked at +7 m above psl (Rovere et al., 2016); (4) the rate of sub-aerial erosion averaged 0.12 mm • y -1 ; (5) the time span during which MIS 5e reef was exposed lasted 106.5 ky, from 116 ka to 9.5 ka, i.e. from the termination of the MIS 5e substage to the time of re-flooding of the shelf slope from the deepest detected isobath. As a result, the ground lowering of the TL2-related reef pile during emergence can be estimated at around 12 m. If correct, given TL2 is now at about 10 m deep, the subsidence amplitude would be about 5 m. The calculated subsidence rate is 0.047 mm • y -1 for the past 125 ky. This rate is in complete accord with that estimated (0.045 mm • y -1 ) from Pukapuka and Rakahanga Atolls [START_REF] Gray | Geochronology and subsurface stratigraphy of Pukapuka and Rakahanga atolls, Cook Islands: late Quaternary reef growth and sea level history[END_REF], the origin of which is linked to the formation of the Manihiki Plateau as old as the Tuamotu basement [START_REF] Mahoney | An Isotopic Survey of Pacific Oceanic Plateaus: Implications for their Nature and Origin[END_REF]. Similar thermal subsidence rates, around 0.035 mm • y -1 to 0.050 mm • y -1 were inferred from other mid-plate low-lying carbonate islands originated during the early Cenozoic [START_REF] Schlanger | Post-Eocene subsidence of the Marshall Islands recorded by drowned atolls on Harrie and Sylvania guyots[END_REF][START_REF] Quinn | The history of post-Miocene sea level change: inferences from stratigraphic modeling of Enewatok Atoll[END_REF][START_REF] Gischler | Late Quaternary reef growth and sea level in the Maldives (Indian Ocean)[END_REF].

Identification of paleo-relative high sea-levels

Paleo-sea-level reconstructions over the last 300 ky were attempted to be achieved on the basis of the vertical distribution of terrace levels at Takapoto (Table 3). The rates of sub-aerial erosion (0.12 mm • y -1 ) and thermal subsidence (0.047 mm • y -1 ) previously determined were used in the calculations, assuming that similar environmental and tectonic conditions have prevailed during all considered high sea stands.

The position of a paleosea level at a given high stand (SL) was calculated as follows: SL = S + E -TL where S is the product of the amount of time during which a given terraced-reef tract has subsided and of the subsidence rate, subsidence operating as soon as reef growth has initiated; E is the produit of the amount of time during which a given reef pile has been emerged and undergone ground lowering and of the erosion rate, considering that duration of emergence is the time interval between the termination of a given high stand and the start of the following re-flooding and subsequent overlapping by a new reef generation. TL is the present depth at which a given terrace is found (Table 3).

Terraces TL3, as remains of MIS 7a reef surfaces, would have suffered a ground lowering of 7.40 m and a drowning of 9.50 m since they have formed, resulting in a deepening of -16.90 m relative to their initial position. Based on its present location at an average depth of 14.70 m ± 3.8 m, the paleo-sea level is estimated to have been at 2.2 m ± 3.8 m above psl. Regarding terraces TL4, as referred to MIS 7c, ground-lowering and subsidence amplitudes were assessed to be 1.20 m and 9.60 m respectively. The resultant drop did not exceed 10.80 m while these features are found at present at -20.30 m ± 2.3 m. This means that the related past sea level was at 9.50 m± 2. The past sea-level positions derived from fore-reef morphometry at Takapoto appear to be in good agreement with some predicted on the basis of different proxies in different seas (Fig. 14). Regarding the elevations at which sea level peaked during the MIS 7 substages, reconstructions gave evidence for only two prominent high sea levels, MIS 7a and 7e, close to or higher than present, between -12 m and +4 m relative to psl. These findings are based on U/Th dating of speleothems in Bahamian caves [START_REF] Li | High precision mass spectrometric uranium-series dating of cave deposits and implications for paleoclimate studies[END_REF][START_REF] Lundberg | Late Pleistocene sea level change in the Bahamas from mass spectrometric U-series dating of submerged speleothem[END_REF], open-system uranium-series ages [START_REF] Thompson | An open-system model for U-series age determinations of fossil corals[END_REF][START_REF] Thompson | Open-system coral ages reveal persistent suborbital sea-level cycles[END_REF] and ESR ages [START_REF] Schellmann | A revised morpho-and chronostratigraphy of the late and middle Pleistocene coral reef terraces on southern Barbados (West Indies)[END_REF] of corals collected from uplifted reef terraces at Barbados, stratigraphic and radiometric constraints in the Bahamas and Bermuda [START_REF] Hearty | Sea-level high stand chronology from stable carbonate platforms (Bermuda and Bahamas)[END_REF], paired Mg/Ca and oxygene isotope measurements of planktonic foraminifera cooled in a core from the Cocos Ridge [START_REF] Lea | Reconstructing a 350 history of sea level using planktonic Mg/Ca and oxygen isotope record from a Cocos Ridge core[END_REF], sea water oxygen isotope in the Red Sea [START_REF] Siddall | Sealevel fluctuations during the last glacial cycle[END_REF] and coupling of the Red Sea sea-level signal with Asian monsoon records [START_REF] Grant | Sea-level variability over five glacial cycles[END_REF]. By contrast, past sea-level reconstructions derived from oxygene isotope analysis of speleothems [START_REF] Bard | Sea-level during the penultimate interglacial period based on a submerged stalagmite from Argentarola Cave (Italy)[END_REF][START_REF] Dutton | Phasing and amplitude of sea-level and cimate change during the penultimate interglacial[END_REF] would indicate that sea level during the MIS 7 stage maintained continuously below psl, at a depth of around -18 m. Similar results, suggesting that paleo-sea level during the penultimate interglacial stage has fluctuated between 10 m and 17 m below psl, were gained from the analysis of inclined reef cores at Moruroa [START_REF] Camoin | A 300,000-yr coral reef record of sea-level changes, Mururoa Atoll (Tuamotu Archipelago, French Polynesia)[END_REF]. During the MIS 9 stage, sea level is inferred to have peaked at levels close to present [START_REF] Siddall | Eustatic sea level during past interglacials[END_REF][START_REF] Rohling | Antarctic temperature and global sea level closely coupled over the past five glacial cycles[END_REF][START_REF] Murray-Wallace | Quaternary sealevel changes[END_REF].

Comparison with the composition of coral communities I in some other Tuamotu and Pacific atolls

Compositional and distributional patterns of corals along outerreef slopes from atolls, are known to be controlled, not only by bathymetry and water agitation, but also by reef orientation, and slope gradient (Wells, 1956;[START_REF] Kühlmann | Zusammensetzung und ökologie von Tiefwasser-Korallenassoziationen[END_REF][START_REF] Bouchon | Les peuplements de scléractiniaires de l'atoll de Takapoto (Polynésie Française)[END_REF][START_REF] Faure | Formations récifales 1: Définitions des unités récifales et distribution des principaux peuplements de scléractiniaires. In: L'atoll de Tikehau (Archipel des Tuamotu, Polynésie française)[END_REF][START_REF] Laboute | Evaluation des dégats causés par les passages des cyclones de 1982-1983 en Polynésie Française sur les pentes externes des atolls de Tikehau et de Takapoto (Archipel des Tuamotu)[END_REF][START_REF] Kühlmann | The corals (Scleractinians and Hydrocorals) of the Takapoto Atoll, Tuamotu Islands: ecological aspects[END_REF][START_REF] Maragos | Description of reefs and corals for the 1988 protected area survey of the northern Marshall Islands[END_REF]. At Takapoto, as observed in both windward and leeward sides, Pocillopora remains dominating within the upper -10 m to -15 m along gently sloping spurs (TL1), and terraces TL2, while Porites achieves prominence from -15 m to -30 m, along steeper to sub-vertical slopes [START_REF] Bouchon | Les peuplements de scléractiniaires de l'atoll de Takapoto (Polynésie Française)[END_REF][START_REF] Kühlmann | The corals (Scleractinians and Hydrocorals) of the Takapoto Atoll, Tuamotu Islands: ecological aspects[END_REF].

Similar trends were reported from the nearby Tikehau Atoll [START_REF] Faure | Formations récifales 1: Définitions des unités récifales et distribution des principaux peuplements de scléractiniaires. In: L'atoll de Tikehau (Archipel des Tuamotu, Polynésie française)[END_REF].

Coral cover rates from other Pacific atolls are poorly documented [START_REF] Wells | Recent corals of the Marshall Islands[END_REF][START_REF] Maragos | Description of reefs and corals for the 1988 protected area survey of the northern Marshall Islands[END_REF]. The only detailed, quantitative coral censuses of seaward outer margins were carried out on Mili and Rongelap Atolls (Marshall Islands, central Pacific) at different depths between -5 m and -20 m [START_REF] Pinca | Community analysis of reef habitats in the Marshall Islands[END_REF]. Porites is by far the most abundant genus (14-62 %) in both leeward and windward sites. Surprisingly, robust branching Pocillopora, 12 % to up to 20 % in cover rates, are restricted to lagoons, apparently missing from opensea environments. By contrast, on some other Marshall Islands (Bikini, northern atolls), as on Takapoto, Pocillopora species are amongst the most common outer-reef dwellers, especially in the upper fore-reef parts [START_REF] Wells | Recent corals of the Marshall Islands[END_REF][START_REF] Maragos | Description of reefs and corals for the 1988 protected area survey of the northern Marshall Islands[END_REF].

These observations indicate there can have marked, withinregion to inter-regional differences in coral composition, maybe due to. competitive interactions between coral groups [START_REF] Lenihan | Influence of corallivory, competition, and habitat structure on coral community shifts[END_REF]. Anyway, in the study areas, coral growths contribute intensively to shaping of upper fore-reef slopes to depths greater than 40 m.

The present study confirmed the reliability of robust-branching Pocillopora verrucosa as a relative sea-level marker as previously demonstrated in some Indo-Pacific reef sites (Montaggioni and Faure, 1997;Montaggioni et al., 1997;[START_REF] Cabioch | Reef coralgal assemblages as recorders of paleobathymetry and sea level changes in the Indo-Pacific province[END_REF][START_REF] Camoin | A 300,000-yr coral reef record of sea-level changes, Mururoa Atoll (Tuamotu Archipelago, French Polynesia)[END_REF]. Composite P. verrucosa-dominated frameworks appeared to be restricted to the 0-10 m depth range.

Conclusions

Detailed geomorphological analysis of satellite-based bathymetric profiles obtained from fore-reef zones all around Takapoto Atoll, using spaceborne imagery, together with direct underwater observations of fine-scale geomorphic features and coral community composition, allows the following conclusions to be drawn. However, along upper fore-reef zones, coral growths appear to play a key role in controlling superficial morphogenesis.

Table captions

Table 1. Geomorphic features identified along the fore-reef slopes of Takapoto Atoll.

  of satellite-based datasets gains insight into the detailed spatial distribution of submerged geomorphic features. Scuba-diving investigations reveal richness and cover rate of coral populations. The aims of this study are to: (1) identify the main geomorphic features present on the shelf slopes and quantitatively describe their morphology; (2) define the depth distributional trends of these features, especially those of terraces; (3) explore scenarios of their possible nature, mode and timing of formation, based on chronostratigraphic comparisons with other atoll sites; (4) determine the main depth-related compositional and distributional attributes of coral communities.

  developing to depths greater than 60 m. The emergent, innermost parts of the spurs are capped by an algal crest, about 20 m wide, reaching 0.20 m to 0.50 m above low tide level. 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. Air temperatures vary between 23 °C and 30 °C. Annual rainfall averages 1500 mm. Tides are a twice daily regime and microtidal, 0.5 m in average amplitude and 0.70 m in maximum at spring tide. 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 for 20 % from the SE sector. In the austral winter, 2.5-m high swells are generated by strong trade-winds. During the austral summer, trade-winds are significantly less active and generate moderateenergy wave conditions, locally disturbed by storm events initiated either by ENSO-related cyclones or by storms originating from northern latitudes

  0

  The north-end sector (T1 to T5) appears to be the widest with deepest points at 30 m to 36 m, at a distance of 143 m to 190 m from the modern reef-crest line (Fig.4a and 5a). It has relatively gently dipping slopes (23° to 38°) and numerous sub-horizontal terraces, 7 m to 45 m wide. Usually, terraces terminate in steps, 1.80 m to 10 m high. There is a prominent line of pinnacles, with elevations of 2.70 m to 17 m. The eastern sector (T11 to T16) exhibits steeply dipping slopes (45°-58°) punctuated by narrow (less than 15 m) terraces ending in steps, 2 m to 15 m high (Figs. 4b and 5b). The deepest points are at 30 m to 37 m, 80 m to 110 m far from the modern reef-crest line. Pinnacles are usually rare or scattered, and not exceed 3.50 m in average elevation. Coral heads are occasional. The south-end sector (T19 to T25) is characterized by steeply dipping slopes (45°-65°) frequently ending in vertical dropoffs (Figs. 4c and 5c). The lowest depths recorded range between 26 m to 36 m, at the distance of 70 m to 135 m from the modern reef-crest line. Terrace width varies between 3 m and 15 m. Step height ranges from 2 m to 17 m. Pinnacles are absent or uncommon. The north-western sector (T31 to T41) exhibits a gradient of 45° to up to 70°. The deepest points recorded range between 6 m and 24 m, at 80 m to 105 m far from the reef front line. There are successive, narrow terraces (2 m-15 m wide) and low steps (1.80 m-12 m). Pinnacles and coral heads are occasional.The steepest foreslopes are located in the north-eastern and south-western sectors of the atoll (T6 to T10 and T26 to T30, respectively) (Fig.4e and 5e). The mean slope angle varies between 64° and up to 75° (Fig.6). Along the north-eastern margin, the base of the profiles does not exceed depths of 26 m to 36 m, at the distance of 40 m to 55 m from the modern reefcrest line. Along the south-western side, the deepest detected isobath ranges between 9.5 m and 14 m only, at a distance of 40 m-50 m from the modern outer reef-crest line. In both sectors, the slope is interrupted by narrow terraces, usually less than 7 m wide ending in low steps and drop-offs, ranging from 2 m to 15 m in elevation. The height of pinnacles does not exceed 8.5 m. Coral heads are common.

Few

  coastal landforms referring to MIS 9 (-335.6 ka to -316.8 ka), especially in reef provinces, are documented, most likely due to lower preservation potential and reduced accessibility (Murray-Wallace and Woodroffe, 2014). At Moruroa, a third solution unconformity was recognized in vertical cores between 29 m to 32 m below psl. It caps a 7-11 m thick, reef unit

  3 m below psl. Two alternative scenarios can account for the evolutionary history of MIS 7e reefs represented by terraces TL5. In the case where this reef generation was overlapped by younger MIS 7c reefs, the time of emergence has not exceeded 11 ky and the thickness of the reef pile released by solution ranged around 1.30 m. If sea level was lower, MIS 7e reefs may have remained exposed over 31 ky. Accordingly, ground lowering of MIS 7e deposits would reach 3.70 m. In both situations, subsidence was 10.30 m. The resulting terrace deepening ranges between 13.0 m and 15.40 m. Given terraces TL5 are now located at a depth of -24 m ± 2 m, the MIS 7e sea level may have been at approximately 11 m or 8.60 m ± 2 m below psl. The morphological evolution of reefs TL6 is summarized as follows. Sub-aerial erosion would have removed 9.20 m while sinking motion was 15.80 m in amplitude. Finally, terraces TL6 were deepened by 25 m. With respect to their present depth location, the corresponding reef tops were believed to have originally cropped out at a paleo-sea level between 5.80 m ± 3 m below psl.

1 .

 1 Six distinct fore-reef sectors were delineated based on seaward extension to the detected slope depths and mean slope gradients. The lowest gradient (< 30°-35°) and most extended (close to 200 m) sectors are situated at the northern atoll end and along a short portion of the mid-south-eastern side. It exhibits wide terraces and low steps. The north-eastern sector displays highest gradient (68° on average) and narrowest (90 m) slopes and narrow terraces. The remaining eastern and northwestern sectors, together with south-western and southern ends have steep slopes (52° to 57° on average), extending seawards over 90 m to 100 m on average, with narrow terraces and high steps.

2.

  Among the described geomorphic features, of specific interest are terraces, outer-reef crest-like reliefs, pinnacles, steps and drop-offs, scars and trenches to reconstruct the development history of fore-reef zones. Seven levels of submerged terraces were identified at depths averaging -5 m, -10 m, -15 m, -20 m, -24 m, -30.5 m, and -38 m respectively. These range from about 45 m to 2 m in width. The shallowest terrace level refers to the present-day surfaces of spurs. In lack of any radiometric dating, the deeper terrace levels were postulated to be remnants of a series of Pleistocene reef tracts, severely altered by sub-aerial erosion during periods of emergence, and finally overlapped by coral veneers during the post-glacial marine transgression, from about 9.5 ky to present. This scenario is in great part based on comparisons with subsurface reef chronostratigraphy from several Pacific atolls. The rates of sub-aerial erosion and linear subsidence were estimated at 0.12 mm • y -1 and 0.047 mm • y -1 respectively from comparisons with calculated surface lowering of Pacific atolls and usually accepted sea-level position at 125 ka. After depth correction of ground lowering and of subsidence effects, the interglacial high sea-stands during which each reef tract has formed and the position of the corresponding sea level were tentatively defined. The 10-m, 14.5m, 20-m, 24-m and 30.5-m terraces were expected to relate to the MIS 5e, 7a, 7c, 7e and 9e respectively, despite substantial uncertainties on elevation estimates. Outer reef crest-like features, less than 2 m high, occur frequently at the seaward edge of terraces. Similarly, steep-walled pinnacles are commun at different places and depths on terraces and slopes. Both were considered to be remains of coralgal buildups, unable to compensate with the post-glacial sea-level rise. Steps and drop-offs, a few metres to more than 15 m high, are presumably remains of the foreslopes of preexisting reef tracts or preserved scars of partially collapsed fore-reef slopes. Trenchs cutting the fore-reef zone were found locally and represent detachment fissures. The faces of the western and eastern fore-reef slopes are aligned in south-west to north-east direction, consistent with the main axis orientation of the atoll. This direction is that of atoll-bearing, submarine volcanic ridges discovered in the north-western part of the Tuamotu plateau, generated from and shaped by underlying lithospheric fractures. 3. Compositional and distributional patterns of coral populations at the study site are in agreement with previous investigations in the Tuamotu atolls. Robust branching Pocillopora forms appear to be the main builders on the uppermost, gently dipping parts of forereef zones, to the depth of about 10 m. This explains why this species is regarded as diagnostic in terms of paleo-bathymetry reconstruction. Pocilloporids are replaced by sub-massive to plate-shaped Porites, profusely growing from -15 m to -35 m deep at least. Concerning biogeographic distribution of corals across the central Pacific, there are noticeable differences between the Marshall and Tuamotu islands, probably controlled mainly by local ecological constraints.

Table 2 .Fig. 1 .Fig. 2 .

 212 Fig. 1. (a) Location map of Takapoto Atoll in the Tuamotu

Fig. 3 .

 3 Fig. 3. Virtual bathymetric profile showing the different

Fig. 4 .

 4 Fig. 4. Plots of bathymetric profiles from the six fore-reef

Fig. 5 .

 5 Fig. 5. Selected bathymetric profiles from each delineated fore-

Fig. 6 .

 6 Fig. 6. 3-D reconstruction of a portion of the WSW side at

Fig. 7 .

 7 Fig. 7. Depth location of terraces identified from each

Fig. 8 .

 8 Fig. 8. Google Earth Pro aerial view of a portion of the eastern

Fig. 9 .

 9 Fig. 9. Relative frequency and percentage of terraces plotted by

Fig. 10 .

 10 Fig. 10. Plot of terrace width as a function of the detected

Fig. 11 .

 11 Fig. 11. Close-up pictures of coral communities at different depths from -5 m to -30 m, fore-reef slope, Fakatopatere area,

Fig. 12 .

 12 Fig. 12. Wide-angle view of drop-offs between -28 m and -32 m, showing dense cover of Porites rus.

Fig. 13 .

 13 Fig. 13. Depth ranges of terrace levels (TL1 to TL6) identified

Fig. 14 .

 14 Fig. 14. Eustatic sea level curve for the last 350 ky (from Grant

Acknowledgments

The present work was supported by the French national research agency (CNRS) under the STORISK research project (N o ANR-15-CE03-0003) coordinated by Virginie Duvat. Field work has benefited from logistical support and help from the research station of the Office of Marine Resources in Takapoto, Gaby Maiti Haumani. Bathymetric, satellite and echo-sounder-based records were provided by AC and treated by DJ. Data on the composition and distribution of coral communities were provided by GS and MT, and exploited by YC. LM, the corresponding author, analysed

and interpreted the data, and wrote the manuscript with imput from the co-authors. Figures were conceived by LM and BM-G.


Sophie Catteau-Jonard

Ph.D.; Anne Brunel
email: nanneb@hotmail.fr

M.D.; Agathe Dumont
email: agathe.dumont@chru-lille.fr

Geoffroy Robin
email: geoffroy.robin@chru-lille.fr

Pascal Pigny
email: pascal.pigny@chru-lille.fr

Didier Dewailly
email: didier.dewailly@chru-lille.fr

M.D Robin ; Geoffroy

M.D.; Pascal Pigny³

Sophie Catteau

Serum FSH level is lower in dysovulating than in ovulating non-PCOS obese women, independently of body mass index

Keywords: Obese women, ovulation disorder, FSH, BMI, leptin

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Résumé

Objectif : La prévalence des troubles de l'ovulation (TO) est 3 fois plus élevée chez les femmes obèses que chez les femmes de poids normal. La plupart de ces TO sont liés à un syndrome des ovaires polykystiques concomitant (SOPK), mais l'obésité peut à elle seule provoquer un TO, à travers des mécanismes qui restent mal documentés. En effet, la littérature sur les femmes obèses sans SOPK avec TO est très faible. Le but de notre travail était d'étudier une population de femmes obèses dysovulantes sans SOPK afin de mieux préciser le mécanisme de la dysovulation. Matériels et méthodes : Cette étude observationnelle rétrospective de femmes obèses infertiles sans SOPK a comparé un groupe témoin sans TO (n = 45) à celui avec TO (n = 30) (groupe TO). Les caractéristiques cliniques, hormonales et échographiques ont été recueillies entre les jours 2 et 5 du cycle. Les femmes âgées de plus de 37 ans ainsi que celles ayant des ovaires multifolliculaires ou des troubles hormonaux ont été exclues. Résultats : L'indice de masse corporelle (IMC) était significativement plus élevé dans le groupe TO, tout comme le tour de taille et les taux sériques d'insuline et de leptine. À l'inverse, leur taux d'hormone folliculo-stimulante (FSH) était significativement plus bas. Après ajustement pour l'IMC, seul le taux de FSH sérique restait significativement différent entre les 2 groupes. L'analyse discriminante suggérait que la FSH pouvait avoir un effet beaucoup plus important sur la dysovulation que l'IMC. Conclusion : Un faible taux de FSH sérique pourrait contribuer aux troubles de l'ovulation chez certaines femmes obèses, indépendamment de l'IMC. Le mécanisme physiopathologique de ce résultat et son impact sur les stratégies thérapeutiques doivent être clarifiés.

Mots-clés :

Femmes obèses ; dysovulation ; FSH ; IMC ; leptine

BACKGROUNG

According to the World Health Organization in 2014, around 1.9 billion people over 18 years of age were overweight, 600 million of them were obese, that is, 13% of the world's adult population [START_REF]Obesity and overweight fact sheet[END_REF]. Obesity is a real problem of modern society and is a major public health concern. Its frequency is steadily increasing: the world's obese population has more than doubled since 1980 [START_REF]Obesity and overweight fact sheet[END_REF]. According to a national epidemiological study on overweight and obesity in 2012, the prevalence of obesity in women of reproductive age in France was 6% for those aged 18 to 24 years and 11.1% of those 25 to 34 [START_REF] Obepi-Roche | enquête épidémiologique de référence sur l'évolution de l'obésité et du surpoids en France[END_REF].

The negative impact of obesity on spontaneous fertility is well documented [START_REF] Brewer | The Adverse Effects of Obesity on Conception and Implantation[END_REF][START_REF] Best | Obesity and fertility[END_REF]. Obese women face a risk of infertility up to three times higher than women with normal body mass index (BMI) [START_REF] Rich-Edwards | Adolescent Body Mass Index and Infertility Caused by Ovulatory Disorder[END_REF]. Time to conception is 3 months longer in overweight women and 9 months longer in obese women than in normal-weight women [START_REF] Law | Obesity and Time to Pregnancy[END_REF]. Obesity is also associated with a higher risk of failure of assisted reproductive technologies [START_REF] Maheshwari | Effect of Overweight and Obesity on Assisted Reproductive Technology--a Systematic Review[END_REF].

The negative impact of obesity on fertility occurs at different levels. Hormonal and inflammatory disorders due to obesity may impair oocyte quality and maturation, and endometrial quality, as well as aggravate the manifestations of polycystic ovary syndrome (PCOS) [START_REF] Brewer | The Adverse Effects of Obesity on Conception and Implantation[END_REF][START_REF] Broughton | Obesity and Female Infertility: Potential Mediators of Obesity's Impact[END_REF]. Obese women have a risk of ovulation disorders (OD) three times higher than normal-weight women [START_REF] Hartz | The Association of Obesity with Infertility and Related Menstural Abnormalities in Women[END_REF], mostly due to PCOS.

Although the impact of obesity on male fertility has been studied extensively [START_REF] Alves | Male Fertility and Obesity: Are Ghrelin, Leptin and Glucagon-like Peptide-1 Pharmacologically Relevant?[END_REF][START_REF] Chambers | The Impact of Obesity on Male Fertility[END_REF][START_REF] Davidson | Deleterious Effects of Obesity upon the Hormonal and Molecular Mechanisms Controlling Spermatogenesis and Male Fertility[END_REF], data about cycle disorders in obese women with no disorders other than overweight are still sparse. Most studies do not differentiate between obese women with or without PCOS. Thus it is still unclear how obesity alone causes OD. The objective of this work was to compare 2 groups of obese women without PCOS, PCOM (polycystic ovarian morphology), or other hormonal issues: one group with OD (n=30) and the other without OD (n=45).

METHODS

This descriptive, retrospective study analyzed data collected from the department of Endocrine Gynecology at our University Hospital between 2011 and 2014. It compares two groups of infertile obese women without PCOS: a group with OD (OD group, n=30) and another without OD (control group, n=45).

All women had previously consented to the use of their clinical, laboratory, and ultrasound data for research purposes. This study was approved by the Institutional Review Board of the University Hospital.

POPULATION

Seventy-five obese women (BMI > 30 kg/m2) were included. Exclusion criteria were: age > 37 years (to prevent confounding by potential diminished ovarian reserve), premature ovarian failure (amenorrhea > 4 months and repeated basal FSH >25 IU/L) [START_REF] Eshre) Guideline Group On | ESHRE Guideline: Management of Women with Premature Ovarian Insufficiency[END_REF], PCOS (defined by the modified Rotterdam classification [START_REF] Dewailly | Diagnosis of Polycystic Ovary Syndrome (PCOS): Revisiting the Threshold Values of Follicle Count on Ultrasound and of the Serum AMH Level for the Definition of Polycystic Ovaries[END_REF]), hyperprolactinemia, severe thyroid dysfunction, Cushing syndrome, follicle number per ovary (FNPO) > 18 (threshold values for PCOM/PCOS, according to our in-house threshold) [START_REF] Dewailly | Diagnosis of Polycystic Ovary Syndrome (PCOS): Revisiting the Threshold Values of Follicle Count on Ultrasound and of the Serum AMH Level for the Definition of Polycystic Ovaries[END_REF].

The OD group included 30 obese women with OD, which was defined by: short cycles (<25 days) or long cycles (>35 days) or spaniomenorrhea (<8 cycles per year) or primary or secondary amenorrhea.

The control group included 45 obese women with regular cycles (25-35 days), normal ovarian assessment (laboratory and ultrasound), and with tubal or male infertility.

METHODS

All women underwent a clinical examination between cycle days 2 and 5 (spontaneous cycle or triggered with a progesterone challenge test). Blood samples were collected on the same day for assays. Blood tests included the radioimmunoassay of serum LH, FSH, estradiol, AMH, androstenedione, DHAS (serum dehydroepiandrosterone sulfate), testosterone, sex hormone-binding protein, 17-hydroxyprogesterone, leptin, and insulin, as previously described [START_REF] Dewailly | Oligoanovulation with Polycystic Ovaries but Not Overt Hyperandrogenism[END_REF][START_REF] Dewailly | Reconciling the Definitions of Polycystic Ovary Syndrome: The Ovarian Follicle Number and Serum Anti-Müllerian Hormone Concentrations Aggregate with the Markers of Hyperandrogenism[END_REF]. Serum AMH levels were measured with the enzyme immunoassay AMH-EIA (ref A16507) from Beckman Coulter Immunotech (Villepinte, France), with an analytical range between 0 and 150 ng/ml and a functional sensitivity of 0.4 ng/ml. A pelvic ultrasound was performed at the same time with a Voluson E8 Expert (GE Systems) with vaginal probe frequency of 5-9 MHz. The ovarian area and FNPO (follicles between 2-9 mm in diameter) were measured as previously described [START_REF] Jonard | Ultrasound Examination of Polycystic Ovaries: Is It Worth Counting the Follicles?[END_REF].

STATISTICS

Continuous variables had a non-Gaussian distribution and were compared with the non-parametric Mann Whitney test. Covariance analysis and discriminant analysis were also carried out to study the confounding effect of BMI. Data analysis was performed with SPSS software (Statistical Package for the Social Sciences, version 22.0, Chicago II, USA). Results were considered significant when p < 0.05.

RESULTS

Table 1 reports the clinical, laboratory, and ultrasound characteristics of the 2 groups. BMI, waist circumference, insulin and leptin levels were significantly higher in the OD group than in the control group, and FSH levels significantly lower. After covariance analysis to adjust for BMI, only serum FSH values were significantly lower in the OD group than in controls (P<0.001). The other differences were no longer significant.

Discriminant analysis, with OD as a binary (i.e., yes/no) dependent variable and BMI, FSH, leptin, insulin and waist circumference as independent variables yielded two significant models: the first included only FSH and the second FSH and BMI. The first model accounted, for 36% of the total variance of OD, and the second for 44%.

DISCUSSION

To our knowledge, this study is the first to report significantly lower FSH levels in non PCOS obese women with, compared to without, OD, in infertile couples. Those women had also a higher BMI, which appeared to be independently associated with OD according to our discriminant analysis. The effect of BMI was, nonetheless, much weaker than that of FSH, as the model including both parameters increased the portion of the total variance of OD only slightly (+8%) over that of the model including only FSH (36%). Our analysis thus uncovered a relative FSH deficiency in obese women with OD that may contribute to OD independently of BMI and other markers of obesity.

This finding disagrees with experimental data from rodents. In obese murine models, OD has been linked to hypogonadotropic hypogonadism, mediated by hyperleptinemia [START_REF] Tortoriello | Increased Expression of Hypothalamic Leptin Receptor and Adiponectin Accompany Resistance to Dietary-Induced Obesity and Infertility in Female C57BL/6J Mice[END_REF][START_REF] Tortoriello | Dietary-Induced Obesity and Hypothalamic Infertility in Female DBA/2J Mice[END_REF]. Leptin is a satiety hormone, secreted by adipocytes [START_REF] Cunningham | Leptin's Actions on the Reproductive Axis: Perspectives and Mechanisms[END_REF][START_REF] Messinis | Leptin in Human Reproduction[END_REF].

It affects reproductive functions via ovarian and hypothalamic receptors [START_REF] Tong | Central Leptin Regulation of Obesity and Fertility[END_REF]. Obesity is associated with high serum leptin levels, which dysregulate GnRH secretion [START_REF] Clément | A Mutation in the Human Leptin Receptor Gene Causes Obesity and Pituitary Dysfunction[END_REF][START_REF] Moschos | Leptin and Reproduction: A Review[END_REF] and abnormalities in ovarian steroidogenesis [START_REF] Agarwal | Leptin Antagonizes the Insulin-like Growth Factor-I Augmentation of Steroidogenesis in Granulosa and Theca Cells of the Human Ovary[END_REF] and folliculogenesis [START_REF] Duggal | The in Vivo and in Vitro Effects of Exogenous Leptin on Ovulation in the Rat[END_REF]. In obese rats, hyperleptinemia increases the secretion of neuropeptide Y, which in turn decreases pulsatile GnRH secretion [START_REF] Khorram | Release of hypothalamic neuropeptide Y and effects of exogenous NPY on the release of hypothalamic GnRH and pituitary gonadotropins in intact and ovariectomized does in vitro[END_REF] and leads to hypogonadotropic hypogonadism (low FSH and LH levels) [START_REF] Tortoriello | Increased Expression of Hypothalamic Leptin Receptor and Adiponectin Accompany Resistance to Dietary-Induced Obesity and Infertility in Female C57BL/6J Mice[END_REF]. In our study, however, the OD and non-OD groups did not differ for LH or leptin levels after adjustment for BMI.

Obesity is also associated with higher concentration of adipokines and inflammatory factors, such as adiponectin, interleukin 6, PAI1 (plasminogen activator inhibitor type 1), and TNFα [START_REF] Mcgown | Adipose Tissue as an Endocrine Organ[END_REF]. Some authors report that these factors negatively affect gonadotropin secretion or ovulation or both [START_REF] Gosman | Obesity and the Role of Gut and Adipose Hormones in Female Reproduction[END_REF][START_REF] Rivier | Cytokines Act within the Brain to Inhibit Luteinizing Hormone Secretion and Ovulation in the Rat[END_REF][START_REF] Al-Safi | Estradiol Priming Improves Gonadotrope Sensitivity and Pro-Inflammatory Cytokines in Obese Women[END_REF]. According to Wang [START_REF] Wang | Altered Circulating Inflammatory Cytokines Are Associated with Anovulatory Polycystic Ovary Syndrome (PCOS) Women Resistant to Clomiphene Citrate Treatment[END_REF], women resistant to clomiphene citrate have higher levels of C-reactive protein and chemokine than women sensitive to it. Further studies are needed to confirm and explain the role of inflammatory factors in the OD of some obese women.

The difference in BMI does not by itself explain the difference in FSH levels between the 2 groups, as demonstrated by the discriminant analysis. BMI and FSH each independently explain OD. Previous studies have already shown significantly lower FSH levels in PCOS patients with OD, even in lean subjects [START_REF] Desforges-Bullet | Increased Anti-Müllerian Hormone and Decreased FSH Levels in Follicular Fluid Obtained in Women with Polycystic Ovaries at the Time of Follicle Puncture for in Vitro Fertilization[END_REF][START_REF] Homburg | The relationship of serum anti-Mullerian hormone with polycystic ovarian morphology and polycystic ovary syndrome: a prospective cohort study[END_REF]. This low concentration is still not completely understood. Some authors have suggested that a pathological increase in the frequency of GnRH pulses leads to higher levels of LH than FSH [START_REF] Marshall | Gonadotropin-Releasing Hormone Pulses: Regulators of Gonadotropin Synthesis and Ovulatory Cycles[END_REF]. This relative deficit of FSH might thus attenuate the aromatization of androgens to estrogens and thereby inhibit follicular maturation and ovulation [START_REF] Solorzano | Neuroendocrine Dysfunction in Polycystic Ovary Syndrome[END_REF]. This subtle decrease in FSH might partially explain the OD. We can go further and hypothesize that low FSH levels might no longer be able to counteract folliculogenesis-inhibiting factors, such as AMH [START_REF] Durlinger | Regulation of Ovarian Function: The Role of Anti-Müllerian Hormone[END_REF], which could cause defective selection of the dominant follicle and thus result in OD. In this situation, the addition of exogenous FSH or anti-estrogens should restore ovulation in the same way as in patients with PCOS. The underlying problem is the management and safety of pregnancy in situations of high obesity. In addition, it is possible that the usual doses of the therapies are to be increased in these patients with a larger volume of distribution.

FSH levels may thus be the cause of OD in obese women with OD.

Conversely, however, low FSH levels might be a consequence of OD. Because no corpus luteum is developed, no luteolysis occurs. Accordingly, there is no decrease in inhibin A and estradiol levels, no intercyclic elevation of FSH, and therefore a low basal FSH level (cycle days 2-5) [START_REF] Buffet | Regulation of the Human Menstrual Cycle[END_REF].

The women were very carefully selected to be PCOM-free. A weakness of this study, however, is the small number of women included in the groups. The number of obese women with OD unrelated to PCOS is small. The control group had a lower BMI, because it was difficult to find similarly overweight women without OD. Is OD inevitable above a certain BMI? In our experience, no patient over BMI 41 ovulates correctly, although we have not constructed a ROC curve to verify the cutoff. Another potential bias might be the prescription of the progesterone challenge test to the women who did not ovulate spontaneously, mainly to women from the OD group.

Although dydrogesterone has no or a very small antigonadotropic activity [START_REF] Christin-Maitre | Pharmacodynamics of Follicle Stimulating Hormone (FSH) in Postmenopausal Women during Pulsed Estrogen Therapy: Evidence That FSH Release and Synthesis Are Controlled by Distinct Pathways[END_REF], we cannot rule out its potential responsibility for a small artificial decrease in basal FSH levels.

CONCLUSIONS:

In obese women without PCOS and with OD unrelated to PCOS, low serum FSH level may contribute to OD independently of BMI. This result, unreported until now, requires further investigation to confirm and explore the pathophysiological mechanism of this finding and its impact on therapeutic strategies.

ETHICAL STATEMENT, COMPLIANCE WITH ETHICAL STANDARDS:



  Funding: no funding was received  Conflict of Interest: The authors declare that they have no conflict of interest. Ethical approval: This study was approved by the Institutional Review Board of the University Hospital (DEC 16-185). Informed Consent: All women had previously consented to the use of their clinical, laboratory, and ultrasound data for research purposes. Availability of data and material: The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.

TABLE 1 :

 1

		Group with ovulation disorders (n=30) MEDIAN (5 th -95th	Control group (n=45) MEDIAN (5 th -95th percentile)	P*	P** with adjustment for BMI
		percentile)
	Age (years)	29.5 (18.65-36.90)	30 (22.3-34.7)	1.000	NA
	BMI (kg/m²)	40.75 (30.26-54.25)	33.00 (30.00-40.5)	<0.0001	NA
	Waist circumference (cm)	112 (87.85 -146.15)	98 (83-121.75)	<0.0001	0.916
	Hirsutism	0 (0-5)	0 (0-8.9)	1.000	NA
	(Ferriman & Gallway score)
	Estradiol (pg/mL)	36 (21.55-60.05)	34 (18.60-59.50)	1.000	NA
	FSH (IU/L)	4.4 (2.61-6.27)	5.60 (3.75-8.54)	<0.0001	<0.001
	LH (IU/L)	2.45 (1.10-6.46)	2.70 (1.30-5.64)	1.000	NA
	AMH (pmol/L)	18.85 (8.4-42.89)	19.6 (10.06-33.35)	1.000	NA
	Testosterone (ng/mL)	0.26 (0.11-0.57)	0.21 (0.07-0.40)	1.000	NA
	Androstenedione (ng/mL)	0.92 (0.32-2.88)	1.15 (0.56-1.75)	1.000	NA
	DHAS (µmol/L)	3.55 (1.57-8.53)	3.80 (1.83-8.01)	1.000	NA
	17 OH-progesterone	0.33 (0.13-0.95)	0.41(0.12-0.80)	0.423	NA
	(ng/mL)
	Sex-binding protein	25.45 (11.70-49.25)	28.65 (14.2-60.23)	1.000	NA
	(nmol/L)
	Insulin (mIU/L)	9.45 (3.71-34.31)	6.1 (1.4-17.66)	0.030	0.623
	Leptin (ng/mL)	66.1 (19.2-166.9)	49.9 (28.7-129.2)	0.033	0.914
	Right ovarian area (cm²)	4.38 (2.66-6.87)	3.68 (2.63-5.20)	0.273	NA
	Left ovarian area (cm²)	3.31 (1.74-5.76)	3.38 (2.21-5.01)	1.000	NA
	Follicle number per ovary	10.25 (0.00-17.10)	8.75 (6.00-13.43)	1.000	NA

Legend: Clinical, laboratory, and ultrasound characteristics of the groups. *Comparison between the 2 groups with non-parametric Mann Whitney test. **Covariance analysis to adjust the comparison for BMI.