Holocene sea-level change and tectonic uplift deduced from raised reef terraces, Kikai-jima, Ryukyu Islands, Japan

Kikai-jima (Kikai Island) is surrounded by four Holocene raised coral reef terraces, which are thought to be an offlapping sequence of reef deposits caused by combined effects of seismic uplift and Holocene sea-level change. Many studies in this region have investigated Holocene sea-level changes and reef growth, but there are relatively few in which reliable sea-level indicators are given. We have found that Pocillopora verrucosa, one of the most abundant coral species on the upper-reef slopes of fringing reefs in the Ryukyus, has its peak abundance at a depth of 1.5 m. Therefore, this species is considered ideal for the analysis of relative sea-level change and can be used as a dipstick for the Holocene reef deposits in this area. Based on the distribution of P. verrucosa on the four Holocene raised terraces, we calculate relative paleo-mean sea levels to be 10.8–11.1 and 8.5–8.9 m for Terrace I, 5.0–5.3 m for Terrace II, 4.0–4.3 m for Terrace III and 1.9–2.5 m for Terrace IV. These results, combined with hitherto known and newly measured radiometric dates (103 total), clearly show that the four terraces formed in response to repeated seismic uplifts at 6.3, 4.1, 3.1 and 1.4 ka, and that sea level was higher than present between 7.0 and 6.3 ka.