Abstract:
Fundamental knowledge about the change and dynamics, and what thresholds drive sediment accumulation in tropical reef settings are poor. Little is also known about how they may respond to the higher and stormier seas that are predicted in an enhanced greenhouse world. Niue's rocky shore setting and the regular occurrence of small isolated pocket-beaches provides an ideal environment to investigate key factors that drive beaches to accumulate or erode within a tropical reef setting. Niue is the largest uplifted coral atoll in the world, covering an area of 200 km^2 and rising to 70 m above sea level. The island is characterised by a series of Pleistocene reef terraces with distinct platforms forming at the base at approximate mean sea level. Lateral reef
growth at sea level is juxtaposed with landward retreat of the limestone cliffs leading to the
formation of shore platforms. Geomorphological surveys of 9 sites revealed a combined reef platform width of up to 150 m with the widest section found on the leeward side of the island on the north western coast and the narrowest (<30 m) being located on the more exposed south eastern coast. Therefore, their distribution is likely related to the energy environment around the island. Beaches up to 12 m wide and 50 m long are only found in protected coves along the
shoreline. Their development is determined by platform width, with beaches only occurring in areas where platform width is more than 60 m. While distance from the reef crest played a role in dissipating wave energy across the platform therefore reducing beach erosion, beach stability is reliant the morphology of the underlying ramp on the landward edge of the platform. Beaches increased in width at higher elevations therefore implying that a higher ramp can effectively reduce the amount of wave energy reaching the landward edge of the beach resulting in the accumulation of sediment. Composition analysis of 51 samples reveal that the Niuean beaches are largely composed of unconsolidated bioclastic sand and gravels derived from the surrounding reef platform. They are characterised by an assemblage of chlorozoan carbonates typical of tropical areas, in which coral and coralline algae are prominent (>50%) except on the north western
platforms (Hio and Tuapa) where foraminifera is the key component. Radiocarbon dating further indicates the youth of these beaches returning modern ages for reef flat microatolls as well as the beach sand itself. These sedimentary environments on Niue are therefore intrinsically linked to the platform biota and their preservation also dependent on the frequency of cyclones. The fast recovery of the foraminifera-rich north western beaches following Tropical Cyclone Heta (2004) is an indication that the
foraminifera community can re-establish quicker after cyclones. This therefore confirms that the beaches are highly dynamic, and build out or erode during alternated calm and stormy conditions. The close links between beach accumulation and their biotic communities will be strongly affected by human-induced climate change, likely leading to the beaches becoming more ephemeral in the future.
