New insights into the Holocene development history of a Pacific, low-lying coral reef island: Takapoto Atoll, French Polynesia

Abstract

Low-lying atoll islands are known to be made up of unconsolidated, coral-rich detritus, together with high amounts of foraminifera, derived from both outer-reef and lagoonal environments. While regarded as highly vulnerable to ongoing global changes, these islands are poorly constrained in term of development history. Herein is presented a detailed chronostratigraphic study of Tuamotu atoll islands (French Polynesia, south central Pacific) based on analyses of sedimentary sequences through seven excavations along two transects across both windward and leeward rim areas around Takapoto Atoll. The island accretionary chronology is supported by radiometric dating of 62 coral and molluscan clasts. The sequences range between 3.80 m and 1.20 m in thickness, from the oceanic shoreline lagoonwards. Four lithofacies were identified from sediment composition and texture: a coral (pocilloporid)-rich, gravel-supported, preferentially located in the outermost rim areas; a coral-rich, gravelly sand-supported, locally interbedded with gravel-supported units; a foraminifera (amphisteginid)-rich, sand-dominated, mainly located at the central and innermost rim settings; and an organic-rich, sand facies atop of some sequences. A model of atoll islet formation is drawn up in relation to mid to late Holocene sea-level changes. The foundations of islets (motus), namely conglomerate platforms, started to form with deposition of patchy, rubble spreads over the upper reef-rim surfaces from ca 4,500 yr BP as sea level was about 0.80 m above its present mean level. On these platforms, islets started to accrete not before ca 2,300 yr BP, from isolated depocentres located midway between outer-reef and lagoon margins. At that time, sea level at about +0.60 m above present mean sea level was starting to slowly decrease to its present position. The major growth phases occurred in a context of continued sea-level fall. Islets continued to accrete through concentric ridges mainly until the last 300 years. Accretion was dominantly driven by low-frequency, high-energy wave-surge events. From dating of coral clasts, a number of one to two events by century were identified as having apparently contributed to island formation at Takapoto. Regionally predicted increase in the rates of sea-level rise may have negative impacts on such islands since these have evolved under conditions of falling sea level.