Late Holocene deposition of coral reef boulders and cyclonic activity in Tuamotu–Gambier Islands (French Polynesia): an overview

Predictions about future extreme marine hazard events in the tropical South Pacific require, as a prerequisite, a robust knowledge of similar ancient event locations and frequency over the last millennia. Among sedimentary archives, coral reef boulders, i.e., pieces of forereef rocky spurs, are regarded as reliable proxies for reconstructing such past hydrodynamic episodes. Based on island geocentred mapping, the distributional pattern of boulder sites in the 77 Tuamotu-Gambier Islands (French Polynesia) shows that 84 % of them are located within the WNW to ESE rim sectors, broadly falling into line with the distribution of atoll rim islets (76 %). Only 24 islands are devoid of boulder deposits most likely because of their geographical setting or reduced exposure to cyclone trajectories. Morphometric analysis of 123 boulder corals dated by 14 C or U/Th methods from 23 atolls in the Tuamotu archipelago allows both the mode and timing of boulder emplacement to be clarified. Boulder detachment and transport are identified as resulting from strong cyclone-generated rather than tsunamigenic swells. Boulder weights range from 2 to up to 600 tons and displacement distances of 5 to 376 m. Flow velocities calculated to extract and lift forereef spur fragments onto reef top surfaces range between 7.2 m/s (25.9 km/h) to 17.6 m/s (63.3 km/h). Boulder sliding and rolling motions onto reef top surfaces are estimated to have operated at speeds of 1.7–5.5 (6.1–19.0 km/h) and 2.1-10.2 m/s (7.5–36.7 km/h) respectively. These hydrodynamic thresholds are consistent with boulder emplacement by high intensity (category 4–5) cyclone events. The dated boulder record indicates that deposition occurred over the last four millennia, with two major periods of intensified activity from about 2200 to 1500 cal yr BP (200 BC to 500 AD, Roman Warming Period) and from about 600 cal yr BP (1400 AD, Little Ice Age) to the present respectively. Spectral (Rayleigh) periodogram analysis of the dated boulder dataset suggests a recurrent clustering of major boulder-emplacement episodes on millennial timescales. Comparisons with sedimentary records from some other tropical to equatorial Pacific sites indicate that cyclonic activity in the Tuamotu–Gambier were sometimes in phase or sometimes out-of-phase with records from other tropical Pacific regions, most likely in response to multi-centennial to millennial variability in ENSO dynamics and shifts in the South Pacific Convergence Zone (SPCZ).