Holocene changes in sea level and coastal environments on Rarotonga, Cook Islands, South Pacific Ocean

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The Holocene coastal plain of the island of Rarotonga, characterized by beach ridges and swamps, is the best developed of the Cook Islands group which lies in eastern Polynesia. Using geomorphic mapping, excavation and coring, and 14C dating of these coastal deposits, we have reconstructed a sea-level curve and drawn maps of shoreline development for the middle to late Holocene. Our sea-level curve shows that sea level attained a position similar to that at present by 6000-6500 cal. BP, then gradually rose to a level of c. + 1.5 m above present sea level by c. 4500 cal. BP. Highstands, possibly with fluctuations to lower levels, occurred during the interval c. 4500 to c. 800-500 cal. BP. Sea level has fallen to its present level since c. 800-500 cal. BP. The sea-level curve is similar to curves obtained elsewhere in Polynesia. Hydroisostatic movement may have been responsible for the relative sea-level change before c. 800-500 cal. BP, whereas the sea-level fall since then is in agreement with records from western Polynesia that attribute such a fall to climatic change. The coastal plain of Rarotonga began to emerge c. 4500 cal. BP, and prograded to the present coastline, most conspicuously on the northeastern coast. The formation of such a flat area on the coast associated with these mid-to late Holocene shoreline developments is likely to have influenced the pattern of human settlement in Rarotonga.