Holocene coral reef development under windward and leeward locations at Ishigaki Island, Ryukyu Islands, Japan

Abstract

Coral reef growth is largely controlled by the wave energy associated with ocean swells and local winds. This growth has responded to changes in wave conditions that evolved in concert with Holocene sea-level changes. We analyzed Holocene facies of the Ibaruma and Fukido reefs of Ishigaki Island, southern Ryukyu Islands, in the context of temporal and spatial changes in wave conditions by comparing present-day coral assemblages and internal (fossil) reef facies. Four major coral assemblages that developed under different wave-energy conditions were recognized. Six cores recorded a Holocene reef sequence consisting of four sedimentary facies that differed in the relative volumes of four coral taxa: Acropora digitifera and Acropora hyacinthus, and lesser amounts of Favia stelligera and Montastrea curta. Holocene windward reef growth began at 7820 cal. years BP after flooding of the Pleistocene reef limestone surface; the ensuing reef growth kept up with the rapid sea-level rise. A vertical growth rate of 7.5 m/1000 years during reef nucleation and early growth stages decreased to 3.5 m/1000 years during the main stage of reef framework development. The uppermost part of the windward reef consisted of a very robust framework, unlike the open framework of the leeward reef. Ages indicated that the uppermost leeward reef was ca. 3000 years younger than the uppermost windward reef. These variations in facies development and timing were caused by differences in local wind-driven wave conditions.