Reef-island accretion and soil development on Makin, Kiribati, central Pacific

The late Holocene accretionary history of reef islands on Makin, the northernmost table reef of the Gilbert atoll chain, western Kiribati, has been reconstructed based on conventional radiocarbon dating of coral shingle and bulk sand samples, and AMS radiocarbon dating of individual grains. Makin is geomorphologically and sedimentologically simple, comprising one main island with only a minor residual lagoon, and composed of sand dominated by the reef-flat foraminifera Calcarina spengleri, Amphistegina lobifera and Baculogypsina sphaerulata. Deposition commenced in mid platform around 2500 years ago; in situ fossil coral (Heliopora) dated at 2400±80 years BP indicates that sea level was 0.4–0.5 m above present at that time. Progradation occurred progressively to westward at a relatively constant rate of 200–300 m ka−1. However, progradation was interrupted on the eastern side by lagoon encapsulation around 1400 years ago. The soil and vegetation characteristics are uniform over most of the reef islands as a result of clearance and the planting and maintenance of coconuts, and do not reflect reef-island depositional history. Remnant stands of Pisonia forest, with an associated phosphatic soil, are not related to any particular geomorphological feature, or period of deposition. The reef islands are continuing to accrete as a result of production of foraminifera on the reef flat.