Lagoonal reef island formation in Huvadhoo atoll, Maldives, highlights marked temporal variations in island building across the archipelago

Abstract

Coral reef islands provide the only pedestals of human habitation in mid-ocean atoll archipelagoes. Understanding the environmental controls on island formation is critical for evaluating the future response of islands to environmental change and sea-level rise. This study examines the evolution of lagoonal reef islands in Huvadhoo atoll, southern Maldives. Using extensive ground penetrating radar (GPR) imaging of island sediment structure, combined with comprehensive analysis of island morphology, sedimentology, and 49 radiometric dates, detailed reconstructions of the evolution of two lagoonal islands is presented. A four-stage model shows a complex history of island formation across various stages of sea-level change during the late-Holocene: 1) lagoon infill occurring from ~3000–2000 years ago during a period of sea-level fall from the mid-Holocene highstand; 2) emergence of the island core through vertical aggradation during the latter stages of sea-level fall between ~2000 and 1500 yBP; 3) rapid lateral expansion of the islands between 1500 and 500 yBP coincident with the availability of sand-size sediments and during a period of sea-level oscillation; and, 4) subsequent minor expansion and modification of island shorelines through alongshore reworking of island sediments over the past 500 years. Results provide evidence of marked differences in the timing and mode of island formation within Huvadhoo atoll. Significantly, the formation of the lagoonal islands post-dates the formation of islands on the atoll rim by 1000–2000 years. Furthermore, the formation of sand islands over infilled shallow lagoons contrasts the formation of most atoll rim islands through the sequential deposition of storm-generated rubble deposited directly across reef surfaces that have reached their vertical growth limit. Results of lagoonal island formation in the southern Maldives provide comparative data for the northern Maldives. While the sequence of island formation follows the faro infill model identified in the northern archipelago, lagoonal islands in the south appear to have formed 2000 years after those in the north. This difference is likely to reflect lagged reef growth response from deeper foundations in the southern archipelago.