Holocene sea level dynamics drive formation of a large atoll island in the central Indian Ocean

Abstract

Abstract The impact of global environmental change on coral reef islands is uncertain with few studies having resolved the critical controls on island formation and change. Based on detailed, topographic surveys, sediment analysis and radiometric dating, we present a multi-phase model of the formation of a large inhabited reef island in the central Indian Ocean in response to Holocene sea level dynamics and transformations in reef development and reef ecology. The initial phase of island building occurred later than elsewhere reported in the Maldives, 2500–2100 years ago during the mid-Holocene sea level highstand. The island was able to support habitation shortly thereafter. Subsequent island expansion occurred as a consequence of relative sea-level fall and emergence of the reef platform that forced a transition in reef flat ecology and dominant sediment producers that contributed to island accumulation to the southwest. Small-scale sea level oscillations (± 0.8 m) over the past two millennia have driven periods of island accumulation, that intermittently reactivate geomorphic processes around the island shoreline. Significantly, the multiple phases of island development have occurred while the island has been continuously inhabited, demonstrating the adaptive capacity of the island community to multiple phases of change. Results highlight the complexity of reef island development and indicate that future physical trajectories will vary depending on not only relative sea level change, but how such changes modify water depth and wave regimes across reef surfaces, and changes in dominant sediment producers able to contribute to island building. This interrelationship differs between sites within and between reef regions and may account for currently expanding islands on reef surfaces.