琉球列島南部の石西礁湖における完新世サンゴ礁の掘削によるボーリングコアの記載

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Ryukyu Islands are an active island arc on the island shelf in high latitude between the Ryukyu Trench and the Okinawa Trough. They are surrounded mainly by fringing reefs, except for barrier reefs in the Sekisei Lagoon between Ishigaki and Iriomote Islands in the southern Ryukyus and off the eastern coast of Kume Island in the central Ryukyus. In order to clarify development of the Sekisei Barrier Reef, drilling of 3 cores through the Holocene reefs was carried out to reach the basement rock of the Pleistocene Ryukyu Limestone with about 20m in depth. Two cores (B-1, B-2) are drilled through the barrier reefs, and one core (B-3) through the patch reef in the inner lagoon. The Holocene reef in the Sekisei Lagoon commenced to have formed about 7630 ^14C yrs BP, which is almost same as those at the reef in the western Kume Island and at the Gushichan reef in the southern Okinawa Island in the central Ryukyus, although depth of the basal reefs in these areas is about 10m. Mean upward growth velocity of reefal sediment in each core shows 3.6 m/ 1000 yrs (B-1), 4.7 m/ 1000 yrs (B-2), and 8.9 m/ 1000 yrs (B-3). Inner reef at the B-3 point started to develop on the deepest basement and the catch-up type reef grew rapidly upward mainly with branching corals. Upward development of the reef finally reached the sea level about 5600 ^14C yrs BP, and formed the patch reef which was the first reef-phase in the Sekisei Lagoon. The outer reefs at the B-1 and B-2 points grew upward with almost same grow rate of reefal sediments as those in the Ryukyus and other fringing reefs in the world. Upward development of the outer reefs was late to reach the sea level about 3720～3620 ^14C yrs BP, and formed barrier reefs showing the second reef-phase in the Sekisei Lagoon.
琉球大学教育学部の論文

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菅浩伸
岡山大

琉球列島南部の石西礁湖における完新世サンゴ礁の掘削によるボーリングコアの記載

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