Distribution of Larger Foraminifera in the Reef Sediments of Akajima, Okinawa, Japan

Abstract

This paper examines the bathymetnc distribution of larger foraminiferal species on the reef flat and reef slope of Akajima Island, Iocated near Okinawa in the Ryukyu island arc of Japan, It also estimates the total skeletal carbonates contributed by larger foraminifeia in the reef sediments of this island. Three distinct assemblages are recognized with increasing water-depths: (i) Marginopora vertebralis and Baculogypsina sphaerulata are confined to reef flat, occurring at a water depth of 1.5 m (ii) Peneraplis planatus and Neorotalia calcar occur within 30 m bathymetry followed down depth by (iii) Operculina ammonoides, Nummulites venosus, Alveolinella quoyi and Baculagypsinoides spinosus between 30 and 60 m. Cycloclypeus is the deepest dwelling genus, making its first appearance at 60 m. Larger foraminifera contribute about 5.8% of the sediments accumulated on reef flat and reef slope of Akajima, which is significantly higher than the global average. There is considerable variation in relative contributions by miliolids and rotaliids on reef flat and reef slope. On the reef flat, miliolids contribute 64% of the total skeletal carbonates produced by larger foraminifera, whereas on reef slope it contributes only about 5%.