Late Holocene eustatic sea-level changes deduced from geomorphological features and their 14C dates in the Ryukyu Islands, Japan

Abstract

This paper deduces late Holocene eustatic sea-level changes from 14C dates and present elevations of almost all Holocene raised coral reefs in the Ryukyu Islands (an active island arc), without the assumption of a constant uplift rate. Stable relative sea levels, are restored by using sea-level indicators, such as raised coral reef terraces, fossil coral, wave-cut notches, and beach rock. The oldest and the youngest 14C ages of calcium carbonate fossils, collected from a depositional terrace-surface, show their formative period. When all relative sea-level changes of thirteen islands are compared, the Ryukyu Islands form three groups according to the oldest age in the formative period of the Holocene raised reef terraces in each island, that is 6500 yr B.P., 5000 yr B.P., and 3500 yr B.P. The youngest of the terrace-formative periods in the three island groups correspond with each other. One eustasy explains the seismic upheaval and the formative process of the terrace-surfaces of the three island groups. The eustatic sea-level trend in the Ryukyu Islands can be summarized as follows. From 6500 to 3500 yr B.P., the sea level continued to rise generally. Around 6500 yr B.P., 5000 to 3500 yr B.P., and 3500 to 1700 yr B.P., the rates of sea-level rise became slower. The sea level between 3500 and 1700 yr B.P. was stable and it has been the highest in the Holocene. At that time its height was less than 1.0 m above the present sea level. The Holocene uplift rate of each island is estimated to be half to three times as great as that of the last 125 000 years. The uplift rates of the three island groups in the late Holocene are inferred to have been decreasing exponentially. The crustal movement and the distribution of raised coral reefs are further discussed here.