Carbonate Sediment Connectivity on Coral Reef Platforms and its Relation to the Evolution of Small Sand Islands: Mamanuca Island Group, Fiji

Abstract

The morphology and evolution of small reef islands is a widely researched aspect of coral reef research. Many of these studies have however considered small islands as separate entities to their surrounding reefs, with the linkage between reefs and islands often overlooked. In the Mamanuca Island Group, Fiji, many small sand islands have formed on platform reefs that have no lagoon and lack distinct windward leeward zonation. These islands therefore provide an ideal location to investigate reef-island sediment connectivity and its role in evolution of small sand islands. 10 reef islands were geomorphically mapped and surveyed from the reef platform to the island surface. 108 sediment samples were also collected from the surface and sub- surface of 10 reef islands and analysed for sediment composition and texture. Furthermore, 13 radiometric dates were obtained for surface and sub surface samples through bulk sand radiocarbon and component specific AMS dating techniques. The islands range in size from 0.04-4.68ha and adopt four main forms; unvegetated sub tidal gravel cays, unvegetated sand cays above MSL, fully vegetated sand cays above MSL and one high basalt island. Reef depths are similar across these platforms, varying between 0.15-8m below MSL between islands and a mainland fringing reef. Sediment is primarily medium-coarse well-sorted sand, and dominated by coral, Halimeda, coralline algae and foraminifera. There is little zonation in sediment composition across the reef flat on smaller reefs (<20ha) within the group related to the small distance of transport across the reef flats. On larger islands, multivariate statistical analysis revealed that a decrease in sediment linkage means that minor zonation occurs between surface zones. Surface sediment on these islands was also chronologically constrained by six radiocarbon dates, showing recent late Holocene sediment production (<637 Cal BP) with minimal reworking evident. Sub surface samples were taken from six auger holes on the two largest islands, and differ compositionally from surface samples showing an abundance of Halimeda. Composition was similar to the beaches on these islands indicating a similar suite of processes is evident on the contemporary island reef flats and as the island accreted. Seven bulk sand and component specific dates on the two largest islands showed that island evolution in the Mamanucas occurred within the late Holocene (<2168 Cal BP), with subsequent vertical and lateral growth of the islands from the island core. A conceptual model was created to show the relationship between island evolution and sediment connectivity in the Mamanuca Island Group. Islands are initially formed as small gravel cays below MSL and develop quickly to unvegetated and vegetated sand cays in relation to high sediment connectivity. The final stages involve cay stabilization and a reduction in sediment connectivity as the island reaches a growth limit and reef areas increase. The co-existence of each phase in the Mamanucas is related to different underlying reef morphologies, sediment production and energy regimes between reefs.