Atoll islets reshaping processes induced by extreme cyclonic waves and calm weather. Evidence from two centuries of megablocks emergences and coral rubble displacements in Anaa, Rangiroa and Tikehau atolls (western Tuamotu archipelago- French Polynesia)

Understanding the islets morphodynamics of polynesian atolls requires a study of their evolution over several decades. These low-lying islands are exposed to highly variable ocean/weather conditions, especially high energy events. This study is based on a diachronic analysis of aerial photographs, in the field measurements and the U/Th dating of megablocks in three atolls in the western part of the Tuamotu archipelago. It presents an investigation of these dynamics from the 19th century until now. This period makes it possible to record the movements of sediments accumulated during decades of calm weather but also to observe the impact of moderate cyclones such as that of 1983. This article also highlights that exceptional cyclones such as those of 1822 and 1906 can dislocate the coral reef and move up the famous Rangiroa megablock whose volume has been significantly refined by photogrammetry at 369 m(3). In Rangiroa and Anaa, we have identified three blocks weighting >500 tons that were put in place by these cyclones.