Lagoonal facies, ages, and sedimentation in three atolls in the Pacific

In order to investigate Pacific atoll lagoonal facies and temporal evolution, we collected and analyzed surface and core sediments from three Pacific atoll lagoons of various sizes and depths (Kayangel of Palau Islands, Enewetak and Majuro of the Marshall Islands). The lagoons were divided into three facies: Calcarina facies, Calcarina–Heterostegina mixed facies and Heterostegina facies. These were based on the presence or absence of the tests of two larger benthic foraminifera, Calcarina and Heterostegina, as indicators of reef flat and deep lagoon environments, respectively. The Calcarina facies is composed of allochthonous reef-derived materials, the Calcarina–Heterostegina mixed facies is a mixture of reef-derived and in situ lagoonal materials, and the Heterostegina facies is composed mainly of in situ lagoonal materials. As both foraminifera are widely distributed in the Western to Central Pacific region, the facies classification presented in this study can be applicable to other Pacific atolls. Based on the results of radiocarbon dating on samples obtained from cores at Majuro lagoon, the accumulation rates of the lagoonal sediments were estimated as 0.65 m kyr−1. Calcium carbonate production was calculated on the basis of the accumulation rates and density of sediments to be 710 g CaCO3 m−2 yr−1.