MID- AND LATE HOLOCENE CHANGES IN SEA LEVEL AND SHORELINE ON RAROTONGA, COOK ISLANDS

Holocene coastal plain characterised by beach ridges and swamps, which provide excellent data on sea level and shoreline, is most developed on Rarotonga of the Cook Islands in the south Pacific. We present here a model for mid- and late Holocene changes in sea level and shoreline for Rarotonga based on micro-landforms and surface deposits in the coastal plain, using air-photographs, level-equipment, power shovel, hand-borer and radiocarbon dates as well as general field survey. Sea-level change obtained is as follows: Sea level attained near-present levels c. 6000 14C BP, following early Holocene rapid sea-level rise. A gradual rise occurred until c. 4000 14C BP, when +1.5 m maximum height was attained. Sea level was essentially stable c. 4000 to c. 600 14C BP. Since c. 600 14C BP sea level has fallen to the present-day level. The sea-level curve is similar to others obtained in Polynesia. Hydro-isostatic movement is plausible to be responsible for this highstand, while recent sea-level fall since c. 600 14C BP is likely related to climatic change as discussed in western Polynesia. Shoreline changes have been greatly dominated by the sea-level change. Marine transgression attained most inland points c. 5000 - 6000 14C BP when sea level ceased rising rapidly and reached near-present sea levels. The shoreline began to prograde c. 4000 14C BP, which matches the culmination of sea-level rise. Coastal plain has expanded to the present coast conspicuously on northeastern coast, providing low flat area most suitable for human settlement. Post 600 BP sea-level fall resulted in slight emergence of the coastal plain, which had widely developed. This mid- to late Holocene coastal evolution helps to properly evaluate its relation to human occupation history as well as early human settlement in the Cook Islands.