Sand cay evolution on reef platforms, Mamanuca Islands, Fiji

The sedimentary link between reef islands and their surrounding platforms is critical to understand if the morphological response of these systems to future boundary condition changes such as associated with human-induced climate change is to be understood. The sand cays of the Mamanuca Islands in Fiji are an ideal system to investigate this link between reef and the islands that have developed on them. The Mamanuca's are a relatively simple sedimentary system being characterised by oval shaped platform reefs with a relatively uniform and horizontal reef crest and flat. Nine islands were investigated within the group through GPS and auto level surveying, with associated surface and subsurface sediment sampling. The islands are composed of a typical chlorozoan sediment assemblage which is well mixed across each reef and island. This mixing is related to the high transport capacity across the reef platforms with waves able to move sediment for at least 50% of the tidal cycle. Radiocarbon dating indicates the islands are young, with modern ages returned on sand from the reef flat and ages of 500–600 years CalBP from the island beaches. Dating from the centre of the islands at 2 m depth indicate island formation was occurring from at least 2260–2110 years CalBP. The young age of the sediments combined with the high transport potential suggests each cay is linked to the contemporary reefs that the islands are developed on. It is inferred that the islands in the Mamanuca Group initially started as a subtidal gravel accumulation. Once wave energy is dissipated enough intertidal sands start to accumulate and eventually the islands reach a critical size where vegetation may colonise. The contemporary development of the reef islands in the Mamanuca's and their links to sediment production on the reef flat suggests that they may be able to adjust their morphology to future environmental conditions.