[H-CG30] [EJ] 堆積・侵食・地形発達プロセスから読み取る地球表層環境変動

[HCG30-02] 星砂の磨耗度と放射性炭素年代に基づく瀬底島海浜堆積物の年代と堆積過程

*伊藤 真裕子¹、森 愛²、本郷 宙軌²、浅海 竜司²、宮入 陽介³、横山 祐典³、藤田 和彦² (1.琉球大学大学院理工学研究科物質地球科学専攻、2.琉球大学理学部物質地球科学科、3.東京大学大気海洋研究所)

キーワード：有孔虫、サンゴ礁、海浜堆積物、放射性炭素年代、摩耗度

Topographic changes of coral-reef beaches are predicted to occur due to sea-level rise associated with global warming. Beach sediments are mainly composed of skeletal fragments and shells produced by calcifying organisms, which are sensitive to environment changes. However, the production age, transport time and depositional age of beach sediments have not yet been fully understood. Here we show the age and depositional process of beach sediments around Sesoko Island (Okinawa, Japan), based on the abrasion grades and radiocarbon (¹⁴C) ages of Baculogypsina (star sand; Foraminifera). Results of abrasion-grade analysis showed that well-preserved tests of Baculogypsina (a pristine test with most spines remained) became fewer from the reef flat toward the beach, where abraded tests with no spines were found abundantly. Results of ¹⁴C dating showed that all Baculogypsina ages were after ca. 1300 cal AD, while most of coral fragment ages were younger (ca. 1700 cal AD-Modern) than Baculogypsina ages. Baculogypsina ages generally became older from the reef flat toward the beach. Baculogypsina ages in beach sediments were younger in the north side than the south side. These results suggest that the production of Baculogypsina tests increased after 1300 cal AD. This is likely caused by the formation of a reef flat (i.e. the increase of foraminiferal habitats) related to a relative sea-level fall at late Holocene. After transported from a reef flat, Baculogypsina tests were deposited into a beach mainly from the north to the south by nearshore currents. The presence of modern coral fragments brought by tidal waves and typhoons suggest that beach formation continues until present.