Lagoonal reef accretion and holocene sea-level history from three atolls in the Cook Islands, Central South Pacific

^ 1 GRAY, S.C. and HEIN, J.R., 2005. Lagoonal reef accretion and holocene sea-level history from three atolls in the Cook Islands, entral South Pacific. Journal of Coastal Research, SI(42), 253-264. West Palm Beach (Florida). ISSN 0749 0208. Radiocarbon ages of corals from cores collected at nine drill sites in the lagoons of three atolls (Pukapuka, Rakahanga, Aitutaki, Cook Islands) provide a history of lagoon sedimentation in response to Holocene sea-level rise and stabili zation. Holocene lagoonal reefs were established between 8700 and 7800 years B.P. on 130,000-200,000 year-old reef platforms that are presently 7 to 22 m below the floor of the lagoons. Comparison of radiocarbon ages of the deepest corals to published sea-level curves indicate that Holocene reefs colonized these substrates rapidly (<~500 years) after lagoon flooding, in water depths of less than 8 m. Subsequently, reef growth lagged behind sea-level rise until the outer reef rims reached sea level between 5000 and 4000 years B.P. Average vertical sediment accretion rates for the Holocene in the lagoons varied by location (83 ? 2 to 278 ? 8 cm/ka) and decreased through the Holocene in six of seven drill holes as the lagoons shallowed and became enclosed by the outer reef. A sample from an emergent (<0.5 m above present mean tide) reef on Rakahanga is 4610 ? 100 years old, which may indicate a higher middle Holocene relative sea level on Rakahanga. Coral growth in Rakahanga lagoon ceased less than 2000 years ago, but was prolific in the early to middle Holocene. The timing and pattern of Holocene reef development exhibited in the Cook Islands is consistent with other oceanic islands. An assessment of the response of reef development to sea-level change during the Holocene provides a baseline to predict how future sea-level changes may affect the morphology of modern reefs. ADDITIONAL INDEX WORDS: Reef growth, accretion rates, 14C ages.