Redetermination of Chronology of Aldabra Atoll by ²³⁰Th/²³⁴U Dating

Abstract

THE elevated coral atoll of Aldabra provides a unique opportunity to study evolution and biological process because of its undisturbed nature^1,2. Interpretations of the geomorphology by Stoddart et al.³ have now been superseded by a detailed geological survey in 1969 by Braithwaite, Kennedy and Taylor (manuscript in preparation), who discovered a complex stratigraphy involving both marine and terrestrial deposits. Most of the present land area, however, is formed from two extensive marine deposits. The lower and older of these consist largely of a fine calcarenite characterized by abundant calcareous red algae, but with few corals and molluscs. Most originally aragonitic skeletons are recrystallized. The deposit rises to about 2 m, above present sea level, and is separated by an erosion surface and sometimes terrestrial deposits from an extensive upper limestone consisting of coarse calcarenites to calcrudites with abundant well preserved corals and molluscs. This deposit occupies most of the outer rim of the atoll and rises to around 8 m above present sea level. Subsequent marine erosion has cut an ill-defined terrace at 8 m and a well-marked one at 4 m.