Sedimentology and formation of lagoonal platform reef islands in Huvadhoo Atoll, Maldives

Abstract

Sediment samples from the reef platform and island cores were collected from two lagoonal reef islands in Huvadhoo Atoll, Maldives, in order to investigate island evolution and building processes in a region where detailed sedimentary and chronology data do not yet exist. The chronostratigraphy of the study islands was resolved using sedimentary data from field samples, combined with radiocarbon ages of sediments and ground penetrating radar records of the island subsurface. Further, this study is the first to apply scanning electron microscopy and elemental mapping in the evaluation of single constituent dating of Halimeda spp. in order to investigate its suitability for inferring depositional chronology on reef islands. Chronostratigraphic results indicate that the lagoonal reef islands on Huvadhoo Atoll formed around 3000 years ago and reached their current dimensions by 350 cal yr BP. A three-phase model of island evolution was developed in relation to mid-Holocene sea-level rise, which presents the first incidence of island building over multiple phases of sea-level change in the Indian Ocean. Medium-scale (decadal) shoreline sediment redistribution identified from morphological field mapping and satellite imagery provide clues to the building processes that occurred during island evolution, and suggest frequent reworking of island sediments over the course of island building. Similar compositional properties of reef flat and island sediments suggest a reef-island link and that the study islands are still being maintained by their surrounding reefs at present. The formation of the lagoonal reef islands in Huvadhoo Atoll during various stages of sea-level change, along with the potential for shoreline adjustment and evidence of contemporary reef connectivity, suggest enhanced resilience of Maldivian reef islands in response to projected sea-level rise.