Carbonate Lagoon and Beach Sediments of Tarawa Atoll, Gilbert Islands

A large, shallow lagoon, almost isolated from the open ocean, distinguishes Tarawa from other atolls where carbonate sedimentation has been investigated. Passes through the atoll reef are lacking except along the western periphery. Size distribution statistics (median grain size, coefficients of sorting and skewness) were obtained for 57 sediment samples from Tarawa Lagoon and 12 samples from the fringing reef flats. Samples we subdivided into 10 separate grain size fractions and the mineralogy of each fraction was determined. Component analysis of the larger grains was undertaken to identify the different sources of carbonate particles. Lime muds are accumulating in shallow water (6-10 m) over a large area of the southern-eastern lagoon. Evidence provided by mineralogy, carbon isotopic composition, and electron microscopy indicates that the fines consist of a mixture of algae-secreted needles and particles derived from the physical and biological breakdown of skeletal detritus. The abundance of carbonate muds in shallow water is attributed to the absence of current winnowing.