The evolution of a Holocene fringing reef and island: reefal environmental sequence and sea level change in Tonaki Island, the Central Ryukyus

Abstract

Within the lndo-Pacific region Holocene reef development over tlie last 6000 yBP has occurred during a near stable sea level period. In particular, development of reef flat and related features have been associated with the stillstand. This is illustrated by the Holocene evolution of a reef in the Japanese Ryukyu Islands. A continuous reef structure, 8 m in thickness and 630 n~ in length, was observed from a fresh excavation in a modern fringing reef in the western part of Tonaki Island of the central Ryukyu Islands. Documented by 34 radiocarbon ages, the reef first reached a relative sea level of ca. 1 m above the present level at about 5200 yBP by growth of branching Acropora thickets, and by accumulation of angular clasts of tabular Acropora at the landward side. Storm features occur within all the observed reef structure and suggest that the reef has been continuously affected by high-energy events for at least 5500 yBP. The topographic outline of the modern fringing reef was formed at an early stage of sea level stillstands affected by wind, climate, and substrate topography. Delayed closure of the 'Holocene high energy window' resulted in vigorous reef growth on the landward side. The reef flat accreted seaward about 400 m during the last 4500 yBP with the development of spurs and grooves. 'The growth environment shifted from a sheltered to a wave-affected condition during the seaward accretion of the reef flat. Reduced colony sizes of tabular Acropora and decreased upward reef growth rates also occurred at about this time. With the seaward accretion, the shoreward grooves became isolated and infilled by rounded clasts. Radiocarbon age from an early archeological feature ('Touma and Oshiro 1979) indicates that the formation of a Holocene tombolo is closely tied to sea level fall around 3500 yBP. This sea level fall and sufficient development of wave resistant structures at the reef edge, provided shore protection and contributed to tombolo stability. Department of Physical Geography, Faculty of Education, Okayama University, Okayama 700 Japan. Department of Geography, Faculty of Science, Tokyo Metropolitan University, Minan~i-Osawa, Ilachioji 192-03 Japan. Laboratory of Geography, College of Education, University of the Ryukyus, Okinawa 903-01 Japan. Department of Geography, Kansai Gaikokugo liniversity, Makikata, Osaka 573 Japan. Department of Geography, Komazawa University, Setagaya 154 Japan.