The Recent Carbonate Sediments of Palmyra Atoll, Northern Line Islands, Central Pacific Ocean
Palmyra Atoll is an isolated carbonate reef system located approximately 1600 km south of Hawaii in the northern Line Islands, central Pacific Ocean. Sediment samples from the lagoons and tidal zones were analyzed for grainsize and composition, and the results used to compile detailed maps and interpret the environments and lithofacies present. A distinct grainsize distribution was observed forming concentric bands ranging from coarse gravel rubble on the outer reef through to finer material in the interior of the atoll in the deep lagoons, where peloidal muds prevail. Five lithologic facies have been identified and typical sediments are poorly sorted and near-symmetrical in their grainsize distribution. On average, sediments are medium sand. A distinct chlorozoan assemblage was observed with coral and calcareous red algal fragments forming half of the sediment, with varying amounts of molluscs, Halimeda and foraminifera being the lesser major constituents. Lagoonal and tidal sediments showed little variation in composition between locations and lacked clear compositional zonation, characteristic of other larger atolls of the Pacific. Palmyra Atoll is unique in that it has had little human intervention for the last sixty years and as a result uninhibited natural processes are occurring. It is also unique in that it displays relatively deep for its size (<55 m), steep-sided compartmentalized lagoons that have abundant fine material (upward of 70% silt or finer), a feature not commonly observed at other Pacific atolls. This fine material has been identified as a peloidal mud and its mode and rate of deposition may be partly controlled by the abundant zooplankton in the lagoons. Recent sediments of Palmyra Atoll are almost entirely carbonate, originating from reef organisms inhabiting the atoll. The only other material is small amounts of siliceous sponge skeletons.