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Holocene fringing reef development around Bora Bora is controlled by variations in accommodation space (as a function of sea-level and antecedent topography) and exposure to waves and currents. Subsidence ranged from 0 to 0·11 m kyr−1, and did not create significant accommodation space. A windward fringing reef started to grow 8·7 kyr bp, retrograded towards the coast over a Pleistocene fringing reef until ca 6·0 kyr bp, and then prograded towards the lagoon after sea-level had reached its present level. The retrograding portion of the reef is dominated by corals, calcareous algae and microbialite frameworks; the prograding portion is largely detrital. The reef is up to 13·5 m thick and accreted vertically with an average rate of 3·12 m kyr−1. Lateral growth amounts to 13·3 m kyr−1. Reef corals are dominated by an inner Pocillopora assemblage and an outer Acropora assemblage. Both assemblages comprise thick crusts of coralline algae. Palaeobathymetry suggests deposition in 0 to 10 m depth. An underlying Pleistocene fringing reef formed during the sea-level highstand of Marine Isotope Stage 5e, and is also characterized by the occurrence of corals, coralline algal crusts and microbialites. A previously investigated, leeward fringing reef started to form contemporaneously (8·78 kyr bp), but is thicker (up to 20 m) and solely prograded throughout the Holocene. A shallow Pocillopora assemblage and a deeper water Montipora assemblage were identified, but detrital facies dominate. At the Holocene reef base, only basalt was recovered. The Holocene windward–leeward differences are a consequence of less accommodation space on the eastern island side that eventually led to a more complex reef architecture. As a result of higher rates of exposure and flushing, the reef framework on the windward island side is more abundant and experienced stronger cementation. In the Pleistocene, the environmental conditions on the leeward island side were presumably unfavourable for fringing reef growth.

In light of the global sea-level rise and climate change of the 21th century, it is important to look back into the recent past in order to understand what the future might hold. A multi-proxy data set was compiled to evaluate the influence of geomorphological and environmental factors, such as antecedent topography, subsidence, sea level and climate, on reef, sand apron and lagoon development in modern carbonate platforms through the Holocene. Therefore, a combination of remote sensing and morphological data from 122 modern carbonate platforms and atolls in the Atlantic, Indian and Pacific Oceans were conducted, along with a case study from the oceanic (Darwinian) barrier-reef system of Bora Bora, French Polynesia, South Pacific. The influence of antecedent topography and platform size as factors controlling Holocene sand apron development and extension in modern atolls and carbonate platforms is hypothesized. Antecedent topography describes the elevation and relief of the underlying Pleistocene topography (karst) and determines the distance from the sea floor to the rising postglacial sea level. Maximum lagoon depth and marginal reef thickness, when available in literature, were used as proxies for antecedent topography. Sand apron proportions of 122 atolls and carbonate platforms from the Atlantic, Indian and Pacific Oceans were quantified and correlated to maximum lagoon depth, total platform area and marginal reef thickness. This study shows that sand apron proportions increase with decreasing lagoon depths. Sand apron proportions also increase with decreasing platform area. The interaction of antecedent topography and Holocene sea-level rise is responsible for variations in accommodation space and at least determines the extension of the lateral expansion of sand aprons. In general, sand apron formation started when marginal reefs approached relative sea level. Spatial and regional variations in sea-level history let sand apron formation start earlier in the Indo-Pacific region (transgressive-regressive) than in the Western Atlantic Ocean (transgressive). The influence of sea level, antecedent topography and subsidence of a volcanic island on late Quaternary reef development was evaluated based on six rotary core transects on the barrier and fringing reefs of Bora Bora. This study was designed to revalue the Darwinian model, the subsidence theory of reef development, which genetically connects fringing reef, barrier reef and atoll development by continuous subsidence of the volcanic basement. Postglacial sea-level rise, and to a minor degree subsidence, were identified as major factors controlling Holocene reef development in that they have created accommodation space and controlled reef architecture. Antecedent topography was also an important factor because the Holocene barrier reef is located on a Pleistocene barrier reef forming a topographic high. Pleistocene soil and basalt formed the pedestal of the fringing reef. Uranium-Thorium dating shows that barrier and fringing reefs developed contemporaneously during the Holocene. In the barrier–reef lagoon of Bora Bora, the influence of environmental factors, such as sea level and climate, tsunamis and tropical cyclones controlling Holocene sediment dynamics was evaluated based on sedimentological, paleontological, geochronological and geochemical data. The lagoonal succession comprises mixed carbonate-siliciclastic sediments overlying peat and Pleistocene soil. The multi-proxy data set shows variations in grain-size, total organic carbon (proxy for primary productivity), Ca and Cl element intensities (proxies for carbonate availability and lagoonal salinity) during the mid-late Holocene. These patterns could result from event sedimentation during storms and correlate to event deposits found in nearby Tahaa, probably induced by elevated cyclone activity. Accordingly, elevated erosion and runoff from the volcanic island and lower lagoonal salinity would be a result of rainfall during repeated cyclone landfall. However, Ti/Ca and Fe/Ca ratios as proxies for terrigenous sediment delivery peaked out in the early Holocene and declined since the mid-Holocene. Benthic foraminifera assemblages do not indicate reef-to-lagoon transport. Alternatively, higher and sustained hydrodynamic energy is probably induced by stronger trade winds and a higher-than-present sea level during the mid-late Holocene. The increase in mid-late Holocene sediment dynamics within the back-reef lagoon is supposed to display sediment-load shedding of sand aprons due to the oversteepening of slopes at sand apron/lagoon edges during their progradation rather than an increase in tropical storm activity during that time. The influence of sea-level and climate changes on sediment import, composition and distribution in the Bora Bora lagoon during the Holocene is validated. Lagoonal facies succession comprises siderite-rich marly wackestones, foraminifera-siderite wackestones, mollusk-foraminifera marly packstones and mollusk-rich wackestones during the early-mid Holocene, and mudstones since the mid-late Holocene. During the early Holocene, enhanced weathering and iron input from the volcanic island due to wetter climate conditions led to the formation of siderite within the lagoonal sediments. The geochemical composition of these siderites shows that precipitation was driven by microbial activity and iron reduction in the presence of dissolved bicarbonate. Chemical substitutions at grain margins illustrate changes in the oxidation state and probably reflect changes in pore water chemistry due to sea-level rise and climate change (rainfall). In the late Holocene, sediment transport into the lagoon is hampered by motus on the windward side of the lagoon, which led to early submarine lithification within the lagoon.

Five mixed carbonate-siliciclastic sedimentary facies were identified in the barrier-reef lagoon of Bora Bora using microfacies and statistical analyses of 70 sediment samples taken at high resolution from two vibrocores. Facies and facies successions were interpreted with respect to Holocene sea-level and climate changes. The windward lagoon core is characterized by sideritic marly wackestones and foraminifera-sideritic wackestones, deposited around 7700 years BP (years before present) during the early-mid Holocene transgression. At that time, ex- tensive weathering and erosion of iron-bearing minerals from the volcanic island, due to a wetter climate, were expressed in the formation of synsedimentary siderite in lagoonal sediments. The enrichment in δ18O (+0.32 to +0.54‰) in the siderite grains indicates marine to mixed marine-meteoric conditions during precipitation. Siderite formation resulted from microbial degradation of organic material, indicated by depleted δ13C values (−13.61 to −14.48‰) that led to reducing conditions in lagoonal sediments, and resulted in iron reduction in the presence of dissolved bicarbonate. The chemical compositions of the siderites changes upcore, from rela- tively high Fe (91–95 mol%) and low Mn (5–6 mol%) at the core base to relatively low Fe (83–88 mol%) and high Mn (11–16 mol%) at the core top. The substitutions of Fe by Mn, Ca and Mg at grain margins illustrate changes in pore-water chemistry towards more oxygenated conditions and reflect sea-level rise and elevated rainfall during the early-mid Holocene. A drier climate during the mid-late Holocene was accompanied by re- duced iron input and the proportion of siderite decreased, approaching zero in the upper section of the core. In the leeward lagoon core, siderite is again common in the lower section, and decreases in abundance upcore. Mollusc-foraminifera marly packstones and mollusc wackestones accumulated ~5400–3500 years BP during the mid-Holocene in the windward core. Early in this period rotalid and miliolid foraminifera dominated. These are tolerant of environmental stress such as changes in water quality, nutrients or salinity. From the mid-Holocene to the present, textularid foraminifera are common in both cores, and indicate normal marine lagoonal conditions. Since the mid-late Holocene sea-level highstand and fall to modern level, mudstones have dominated in both cores. During the last 1000 years coral fragments have increased in abundance in the windward lagoon, pre- sumably as a result of lagoonward progradation of fringing reefs in the mid-late Holocene. Since the late Holocene, motus on the windward side of Bora Bora have hampered sediment transport and lagoonward pro- gradation of sand aprons. Increasing numbers of peloids, largely hardened faecal pellets, in the windward core in the last 1000 years may reflect early submarine lithification within the lagoon. Our study shows that during the Holocene, sea-level and climate change have influenced sediment import, composition and distribution in the Bora Bora lagoon. The sensitive response of the environment to external changes demonstrates the potential of tropical reef lagoons as archives of climate and sea-level changes.