Significance of emerged Holocene corals around Ovalau and Moturiki islands, Fiji, southwest Pacific

Abstract

Abstract The volcanic islands Ovalau and Moturiki in central Fiji have been subsiding very slowly for most of the past few thousand years and thus have potential for helping define Holocene eustatic changes in this otherwise tectonically-active region. Dates from fossil microatolls (Porites spp.) emerged above the modern reef flats around these islands indicate such changes. Ten dates from around Ovalau are presented. They indicate a general fall of sea level from a high 1.6 m above its present level within the period 6000–3000 yr BP. The dates are readily divisible into discrete vertical groups which may indicate the inability of some reefs to reach Lowest Astronomical Tide level (as they commonly do) where freshwater plumes enter the ocean. Alternatively, the vertical separation of dates may indicate rapid transgressive–regressive oscillations. The distribution of dates is also characterized by a temporal hiatus of about 1000 years around 4200 yr BP which can be explained as a result of (a) either a rapid sea-level fall around this time, (b) a series of prolonged low sea-level episodes associated with an increased frequency of El Nino events, or (c) increased nearshore deposition of terrigenous sediment perhaps associated with an increased tropical-cyclone frequency or early human impact.