Uranium-series dating of insular phosphorite from Ebon atoll, Micronesia

Abstract

THE uranium-series method of absolute age determination has been widely applied to the radiometric geochronology of fossil coral reefs and associated problems of palaeoclimatology^1–3. Attempts have also been made to extend this method to marine phosphorites, leading to a better definition of the environmental conditions prevailing at the time of their formation^4–8. Although the occurrence of phosphorite deposits on many islands in the central Pacific and Indian Ocean is well documented⁹, no absolute ages for any of these deposits have been available. This is regrettable because the widely accepted hypothesis⁹ for the origin of these insular phosphorites by chemical interaction of bird guano with the underlying reef limestones is based largely on circumstantial rather than direct evidence. We report here the first attempt to determine the age of both the apatite component as well as that of the associated coral material in a phosphorite deposit from Ebon atoll in Micronesia, in an effort to explore the feasibility of dating insular phosphorite deposits by the uranium series method and thus to relate the time of phosphate deposition to known environmental conditions in a given area.