Holocene fringing reefs and sea-level change in Mangaia Island, Southern Cook Islands

Abstract

Holocene relative sea levels in oceanic islands that are situated sufficiently far from glaciated regions provide basic information for the study of the melting histories of the continental ice sheets and of the rheological structure of the Earth. We have studied Mangaia Island, South Cooks, as one such oceanic island located in the middle Pacific Ocean. In addition to the usual geological and geomorphological observations of geomorphic features associated with former shorelines, we have used a portable drilling sampler for shallow borings of coral reefs. Geological and geomorphological studies of the coast of Mangaia Island have revealed that there was a higher sea level than the present in the mid-Holocene. The heights and ages of emerged microatolls on the emerged bench indicate that the sea reached a maximum level of +1.7 m around 4000-3400 yr B.P., and then emergence is considered to have occurred between 3400 and 2900 yr B.P. At the same time, the reef crest formed in the period from 5000 to 3400 yr B.P. emerged above the sea and suffered erosion. As a result of this change in sea level, upward reef growth from the fore reef slope began to form a new reef crest by 2000 yr B.P. to seaward of the old reef crest. The reef margin of Mangaia grew outward intermittently with seaward jump of the reef front, in accordance with a fall of sea level, in the late Holocene. The presence of fossil reef crests or eroded algal ridges on modern reef flats is a rather common feature in the South Pacific. The geomorphic development of such features on reef flats can be explained by the effect of the late Holocene fall in sea level.