Recent macroids on the Kikai-jima shelf, Central Ryukyu Islands, Japan

Sandy and gravelly carbonate sediments found off Kikai-jima, southern Japan, a coral reef-related island shelf, represent the northernmost sub-tropical, carbonate deposits in the Central Ryukyu Islands (Ryukyus). On the Kikai-jima shelf, at water depths of 61 to 105m, these sediments are characterized by macroid pavements. Since the abundance of very small and of exceptionally large macroids may indicate specific hydrodynamic controls regarding constraints on growth and taphonomy, the detailed analysis of recent and fossil macroid pavements is meaningful ecologically and environmentally. Macroids, ranging in size from ca 25 to 130mm in diameter, are spheroidal and sub-spheroidal in shape and consist mainly of the encrusting foraminifer Acervulina inhaerens and subordinate thin encrusting and lumpy coralline algae. Accessory components include bryozoans, serpulids and, to a lesser extent, encrusting arborescent foraminifera (Homotrema and Miniacina). Low sedimentation rates and occasional movement due to current action are indicated by sizes, shapes and growth-forms of the studied macroids, the Entobia-Gastrochaenolites-Trypanites-Maeandropolydora ichnocoenosis and the 'Bioerosion Index' for coated grains (introduced herein). The deep-water tidally induced current energy was sufficient to maintain multi-directional growth (spheroidal shapes) of the larger macroids and to initiate macroid growth using the diverse biogenic remnants as nuclei. The asymmetrical inner arrangement suggests possible periods of stability for the macroids. The residence time of the coated grain in its original environment determines the size and morphology of the macroid and the selection of coating organisms. The composition of the coating community is mainly a consequence of component growth rates in relation to turnover time and residence time. Long-term studies are needed to assess the spatial and temporal resolution of present-day encrusting communities across biogeographic provinces and shelf to slope regions.