The mode and timing of windward reef-island accretion in relation with Holocene sea-level change: A case study from Takapoto Atoll, French Polynesia

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