HOLOCENE-EMERGED CORAL REEFS AND SEA-LEVEL CHANGES IN THE RYUKYU ISLANDS

The numerous coral reefs in the Ryukyu Islands provide detailed evidence of the relationship between reef development and late Holocene sea-level changes. The islands belong to the tectonically active island arc and have experienced many destructive earthquakes in the historical period. It is, therefore, assumed that the Holocene reefs have been terraced at various elevations, due to sudden uplift associated with major earthquakes, at various sites during the period of the post-glacial eustatic sea-level changes. In this paper, the authors have attempted to restore the late Holocene sea-level curve as follows. First, cross-profiles of various emerged reefs were constructed, and fossil corals were collected for 14C dating (Figs. 2, 3, 5, 7, 8, 10 and 11). Secondly, 58 fossil corals were dated by the 14C method (Table 1), and relative sea-level curves were constructed for Kodakarajima, Takarajima and Kikaijima islands (Figs. 13, 14 and 15). An attempt was then made to restore the late Holocene sea-level curve, corrected for intermittent seismic uplifts, using the Kikaijima relative sea-level curve (Fig. 15). The amounts of intermittent seismic uplift were calculated using a constant long-term up-lift rate (1.87m/1, 000yrs.) which minimized the difference between pre- and post-uplift sea-levels, the rate being calculated by least squares methods. The restored sea-level curve (Fig. 15) resembles the Shepard curve, which shows a gradual rise of the sea-level towards the present sea-level, rather than the Fairbridge curve, which oscillates considerably during the past 6, 000 years including several stands higher than the present.