Oxygen and carbon isotopic variation in co-existing larger foraminifera from a reef flat at Akajima, Okinawa, Japan

The larger foraminiferal assemblage on the reef flat at Akajima (Ryukyu Island Arc) is comprised of Marginopora kudakajimaensis, Amphisorus hemprichii, Sorites orbiculus, Peneroplis planatus, Heterostegina depressa, Neorotalia calcar, Calcarina gaudichaudii, Baculogypsina sphaerulata and Amphistegina lessonii. The oxygen and carbon isotopic variations in several specimens of each species, living under similar conditions, were examined. Symbiosis had little to no effect on oxygen isotopic composition of the bulk carbonate of the analysed species. The isotopic analysis along the ontogenetic stages indicates near-equilibrium precipitation of oxygen isotopes in the early life stages and, therefore, the initial chambers should be microsampled in isotopic analyses for a better estimate of palaeotemperature. The order of depletion of δ13C parallels the order of accretion of calcium carbonate by the foraminiferal species. A poor correlation between δ18O and δ13C and a large variation in δ18O and δ13C with size of the shell are the characteristic features of the symbiont-bearing benthic foraminifera inhabiting reef flat environments. In a deeper water species oxygen isotopic values vary little with size of the shell.